PHARMACOLOGY AND PHARMACOTHERAPEUTICS TWENTY-FOURTH EDITION
R.S. SATOSKAR, M.B.B.S., B.Sc. (Med.), Ph.D. (Sheffield) Formerly, Professor and Head, Department of Pharmacology, Seth G.S. Medical College; T.N. Medical College L.T.M. Medical College and Associate in Clinical Medicine, K.E.M. Hospital, Mumbai
NIRMALA N. REGE, M.D., Ph.D., D.N.B. (Clin. Pharm) Professor and Head, Department of Pharmacology and Therapeutics, Seth G.S. Medical College and K.E.M. Hospital, Mumbai
S.D. BHANDARKAR, M.D., F.R.C.P. (Edin), F.R.C.P. (Glasgow) Formerly, Hon. Professor and Head, Department of Endocrinology, and Associate in Clinical Pharmacology, Seth G.S. Medical College and K.E.M. Hospital, Mumbai
Table of Contents Cover image Title page Copyright Dosage Note Preface to the Twenty-fourth Edition Preface to the First Edition Some Abbreviations Used in the Text Dedication
Section I: General Pharmacology Chapter 1: General Considerations and Pharmacokinetics The Nature and Sources of Drugs Sources of Drug Information Routes of Drug Administration and Dosage Forms Absorption and Bioavailability Distribution of a Drug Fate of a Drug Drug Excretion Plasma Half-life and its Significance Methods of Prolonging the Duration of Action of a Drug Special Drug Delivery Systems
Chapter 2: Pharmacodynamics – Drug Receptor Interaction; Adverse Drug Reactions Site of Drug Action Structure Activity Relationship (SAR) Mechanism of Drug Action Drug Receptors Dose Response Relationship Adverse Drugs Reactions (ADR) Drug Allergy Manifestations of ADR Treatment of Acute Drug Poisoning
Chapter 3: Principles of Drug Prescribing; Factors Modifying the Effects of a Drug; and Drug Interactions Drug Prescribing P-drug (Personal drug) Concept Essential Drugs Orphan Drugs Factors Modifying the Effects of a Drug Drug Interactions
Chapter 4: Drug Invention; New Drug Development; and Drug Assay Animal Toxicity Studies Clinical Evaluation Drug Assay
Section II: Drugs Acting on the Central Nervous System Chapter 5: General Considerations Chapter 6: Aliphatic Alcohols Acute Alcohol Intoxication Chronic Alcoholism Alcohol Dependence
Chapter 7: General Anaesthetics Inhalational General Anaesthetics Volatile Liquid Anaesthetics Gaseous Anaesthetics Non-Volatile General Anaesthetics Neuroleptanalgesia Pre-anaesthetic Medication Drugs Administered During Anaesthesia
Chapter 8: Sedatives, Hypnotics and Pharmacotherapy of Sleep Disorders Benzodiazepines Non-benzodiazepine, Benzodiazepine-receptor Agonists Barbiturates Alcohols Aldehydes Melatonin Recepter Agonist Orexin Receptor Antagonists Miscellaneous Pharmacotherapy of Insomnia and Other Sleep Disorders
Chapter 9: Drugs Effective in Seizure Disorders Types of Epilepsy Anti-epileptic Drugs (AED) General Principles of Management of Epilepsy Epilepsy and Pregnancy Drug Therapy of Epilepsy
Chapter 10: Opioid Analgesics and Antagonists Opium Alkaloids Semisynthetic Derivatives of Natural Opium Alkaloids Synthetic Morphine Substitutes Opioid Antagonists
Chapter 11: Analgesic-Antipyretics and Nonsteroidal Anti-inflammatory Drugs (NSAIDs) Salicylates Para-Aminophenol Derivatives Indoles and Related Drugs Heterocyclic Arylacetic Acid Derivatives Propionic Acid Derivatives Fenamates Oxicams Preferential and Selective COX-2 Inhibitors Pharmacotherapy of Pain NSAID and Renal Damage
Chapter 12: Central Nervous System Stimulants Stimulants of the Cerebral Cortex Stimulants of the Brain Stem and Medullary Centres Stimulants of the Spinal Cord Reflex Stimulants of the Central Nervous System
Chapter 13: Psychopharmacology - 1: Introduction, Antipsychotic Drugs and Pharmacotherapy of Major Psychotic Disorders Antipsychotic Drugs Atypical Antipsychotics (Second Generation) Management of Schizophrenia Manic Depressive Psychosis – Management
Chapter 14: Psychopharmacology - 2: Anxiolytics, Antidepressants and Mood Modifying Agents Treatment of Anxiety Disorders Antidepressant Drugs Monoamine Oxidase Inhibitors (MAOI) Tricyclic Antidepressants (TCA) Selective Serotonin (5-HT) Reuptake Inhibitors (SSRI) Treatment of Major Depression Mood Stabilisers
Psychomotor Stimulants Psychotogenic Drugs Drug Induced Psychiatric Syndromes
Chapter 15: Drug Therapy of Parkinsonism and Other Neurodegenerative Disorders Drug Therapy Management of Parkinsonism Drug Therapy of Other Extrapyramidal Syndromes Drug-induced Extrapyramidal Reactions (EPR) Motor Neuron Disease (MND) Drug Therapy Drugs and Memory Multiple Sclerosis (MS)
Section III: Local Anaesthetics Chapter 16: Cocaine, Procaine and Other Synthetic Local Anaesthetics
Section IV: Autonomic Nervous System Chapter 17: General Considerations Distribution of Parasympathetic Nervous System Distribution of Sympathetic Nervous System Neurohumoral Transmission Neurohumoral Transmitters Neurotransmitter Uptake Mechanisms and Drugs
Chapter 18: Adrenergic Agonists and Antagonists Catecholamines Noncatecholamines Noncatecholamines Mainly Used as Vasopressor Agents Nasal Decongestants
Anorectic Sympathomimetic Drugs Miscellaneous Compounds Sympathetic Blocking Drugs Adrenergic Receptor Blockers
Chapter 19: Cholinergic Drugs Esters of Choline Cholinomimetic Alkaloids Cholinesterase Inhibitors Organophosphorus Compound (OPC) Poisoning
Chapter 20: Muscarinic Receptor Blocking Drugs; Pharmacotherapy of Bladder Dysfunction Belladonna Alkaloids Synthetic and Semisynthetic Atropine Substitutes Bladder Dysfunction – Pharmacology
Chapter 21: Ganglion Stimulating and Blocking Drugs GanglionBlocking Agents
Chapter 22: Skeletal Muscle Relaxants Centrally Acting Skeletal Muscle Relaxants Peripherally Acting Skeletal Muscle Relaxants Drugs Acting Directly on Skeletal Muscle
Section V: Other Biogenic Amines and Polypeptides Chapter 23: Histamine and Antihistaminic Drugs Histamine, Anaphylaxis and Allergy H1 Receptor Antagonists H2 Receptor Antagonists H3 and H4 Receptors
Chapter 24: 5-Hydroxytryptamine (Serotonin), its Agonists and Antagonists; and Treatment of Migraine Pharmacotherapy of Migraine
Chapter 25: Angiotensin, Kinins, Leukotrienes, Prostaglandins and Cytokines Kinins Leukotrienes (LTs) Prostaglandins (PGs) Cytokines
Section VI: Drugs Used in Respiratory Disorders Chapter 26: Pharmacotherapy of Cough Pharyngeal Demulcents Expectorants Central Cough Suppressants Peripherally Acting Agents Other Antitussives Mucolytic Agents
Chapter 27: Pharmacotherapy of Bronchial Asthma, COPD and Rhinitis Drug Therapy During an Acute Attack Prevention of Acute Attacks Leukotriene Modifiers Treatment of Chronic Persistent Asthma Severe Acute Asthma (Status Asthmaticus) - Treatment COPD - Management Treatment of Acute Respiratory Failure Surfactants and the Respiratory Distress Syndrome Drug Therapy of Rhinitis
Section VII: Cardiovascular Drugs Chapter 28: Pharmacotherapy of Cardiac Arrhythmias Atrial fibrillation: Management Drugs Used in the Treatment of Heart-Block
Chapter 29: Pharmacotherapy of Angina Pectoris, Acute MI and Peripheral Vascular Diseases Methods of Assessing Coronary Circulation Angina Pectoris Drugs Used in Angina Pectoris Potassium Channel Activators Antiplatelet Drugs Cytoprotectives Treatment of Angina Pectoris Acute Myocardial Infarction: Management Drugs Used in the Treatment of Peripheral Vascular Disorders
Chapter 30: Pharmacotherapy of Hypertension, Pulmonary Hypertension and Orthostatic Hypotension Drugs Acting Centrally Ganglionic Blocking Agents Adrenergic Neuron Blockers Catecholamine Depletors Adrenergic Receptor Blockers Vasodilator Drugs Potassium Channel Activators Renin Inhibitors Angiotensin Converting Enzyme Inhibitors (ACEI) Angiotensin Receptor Blockers (ARB) Aldosterone Antagonist Thiazides as Antihypertensives Miscellaneous Drugs
Hypertension – Therapy Principles of Drug Therapy Choice of Drug Therapy Use of Drug Combinations Management with Drugs Hypertension in the Elderly Hypertension in Pregnancy Supportive Treatment of Hypertension Treatment of Hypertensive Crises Drug Induced Hypertension Pulmonary Hypertension Orthostatic hypotension
Chapter 31: Pharmacotherapy of Heart Failure ACEI and Vasodilators in Heart Failure Beta-adrenergic Blocking Agents in Heart Failure Diuretics in Heart Failure Aldosterone Antagonists in Heart Failure Digitalis and Other Inotropic Agents in Heart Failure Management of Acute LVF Nonpharmacological Treatment of Heart Failure
Chapter 32: Pharmacotherapy of Shock Whole Blood, Plasma and Plasma Fractions Colloidal Plasma Expanders Crystalloid Plasma Expanders Cardiovascular Drugs in Shock Treatment of Shock
Section VIII: Drugs Acting on Blood and Blood Forming Organs
Chapter 33: Drugs and Blood Coagulation Antiplatelet Agents Fast Acting Anticoagulants Direct Thrombin Inhibitors Slow Acting Anticoagulants In Vitro Anticoagulants Fibrinolytic Agents Hemostatic Agents Management of Acute Variceal Bleeding Sclerosing Agents
Chapter 34: Drugs Effective in Iron Deficiency and Other Related Anemias Iron Metabolism Treatment of Iron Poisoning Adjuvants to Iron Therapy Erythropoietin (EPO) and Anemia
Chapter 35: Drugs Effective in Megaloblastic Anemias and Neutropenia Drugs Used in Neutropenia Drugs used in Thrombocytopenia
Chapter 36: Drug-Induced Blood Dyscrasias
Section IX: Water, Electrolytes and Drugs Affecting Renal Functions Chapter 37: Water, Sodium, Potassium and Hydrion Metabolism Water Metabolism Sodium Metabolism Sodium Depletion Sodium Excess Potassium Metabolism
Potassium Depletion and Hypokalemia Hyperkalemia and Potassium Excess Acidosis and Alkalosis Acidosis Alkalosis
Chapter 38: Nutritional Supplementation Therapy Nutritional Requirements in Healthy Adults Alterations in Nutritional Requirements in Acute Illness Sequelae of Malnutrition Assessment of Nutritional Status Nutritional Supplementation: Aims and Indications Enteral Nutrition Parenteral Nutrition Total Parenteral Nutrition (TPN)
Chapter 39: Diuretic and Anti-Diuretic Drugs Diuretics Osmotic Diuretics Xanthines as Diuretics Carbonic Anhydrase Inhibitors Benzothiadiazines (Thiazides) Loop Diuretics Potassium Sparing Diuretics Management of Edema Complications of Diuretic Therapy Acute Renal Failure (ARF) Anti-diuretic Agents Vasopressin Receptor Antagonists Drugs and Nephrotoxicity
Section X: Drugs Used in Disorders of the Gastrointestinal Tract
Chapter 40: Appetite Stimulants, Digestants, Antiflatulents, Appetite Suppressants and Hypolipidemic Agents Digestants Antiflatulents and Carminatives Anorexiants and Treatment of Obesity Atherosclerosis and Hyperlipidemia Antihyperlipidemic Drugs
Chapter 41: Emetics, Drug Therapy of Vomiting, Vertigo and Diarrhoea Emetics Drug Therapy of Vomiting Therapy of Vertigo and Dizziness Drug Therapy of Diarrhoea Chronic Diarrhoea
Chapter 42: Pharmacotherapy of Constipation Anthraquinone Group Irritant Oils Miscellaneous Stimulant Laxatives Osmotic Laxatives Bulk Laxatives Emollient Laxatives 5-HT4 Receptor Agonists Chloride Channel Activators Constipation and Laxatives Treatment of Hemorrhoids and Anal Fissure
Chapter 43: Pharmacotherapy of Peptic Ulcer Disease Gastric Antacids Non-Systemic Antacids Systemic Antacids Antacid Therapy Antisecretory Agents
Mucosal Protective Drugs Management of Peptic Ulcer
Section XI: Oxytocics and Uterine Relaxants Chapter 44: Pharmacology of Ergot Alkaloids, Oxytocin, other Oxytocics and Uterine Relaxants Ergot Alkaloids Oxytocin Other Oxytocics Uterine Relaxants (Tocolytics)
Section XII: Chemotherapy Chapter 45: Sulfonamides, Trimethoprim, Cotrimoxazole, Nitrofurans and Quinolones Sulfonamides Inflammatory Bowel Disease-Drug Therapy Cotrimoxazole Nitrofurans Quinolones
Chapter 46: Penicillins and Other Antibiotics Effective Mainly Against Gram Positive Organisms Semisynthetic Penicillins Beta lactamase Inhibitors Macrolides Lincosamide Antibiotics Glycopeptide Antibiotics Miscellaneous Antibiotics Streptogramins Oxazolidinones
Chapter 47: Aminoglycosides and Other Antibiotics Effective Mainly Against Gram Negative Organisms Non-aminoglycoside Agents Monobactams
Chapter 48: Antibiotics Effective Against Both Gram Positive and Gram Negative Organisms Cephalosporins Carbapenems Rifamycins Drug Therapy of Bacterial Meningitis
Chapter 49: Tetracyclines and Chloramphenicol Tetracyclines Semisynthetic Tetracyclines Chloramphenicol Pharmacotherapy of Bacillary Dysentery Pharmacotherapy of Chronic Bronchitis
Chapter 50: Antifungal Agents Azole Derivatives Echinocandins
Chapter 51: General Principles of Chemotherapy of Infections Selection of Antimicrobial Agent Antimicrobial Combinations Antimicrobial Prophylaxis Microbial Drug Resistance Dangers of Antimicrobial Therapy Misuse of Antimicrobial agents
Chapter 52: Chemotherapy of Urinary Tract Infections Drug Therapy of UTI Treatment of Lower UTI
Treatment of Upper UTI Antimicrobial Prophylaxis
Chapter 53: Chemotherapy of Sexually Transmitted Diseases Drug Therapy of Syphilis Drug Therapy of Gonorrhea Drug Therapy of Non-Gonococcal Urethritis (NGU) Drug Therapy of Lymphogranuloma Venereum Drug Therapy of Chancroid Drug Therapy of Granuloma Inguinale Vaginitis – Drug Therapy Drug Therapy of Viral STD Drug Therapy of HIV STD – Prophylaxis
Chapter 54: Chemotherapy of Tuberculosis First Line Drugs Second Line Drugs Third Line Drugs Newer Drugs Management of Pulmonary Tuberculosis MDR Tuberculosis Management Other Forms of Tuberculosis Glucocorticoids in Tuberculosis Chemoprophylaxis of Tuberculosis Nontuberculous Mycobacterial Infections Tuberculosis and HIV Infection
Chapter 55: Chemotherapy of Leprosy Drugs Used in Leprosy Management of Leprosy
Chapter 56: Chemotherapy of Malaria Cinchona Alkaloids
4-Aminoquinolines 8-Aminoquinolines Quinoline Methanol Phenanthrene Methanol Biguanides Diaminopyrimidines Artemisinin Compounds Antimicrobials Management of Malaria
Chapter 57: Chemotherapy of Amoebiasis Imidazole Derivatives Quinoline Derivatives Emetine Group Antiamoebic Antibiotics Miscellaneous Agents Management of Amoebiasis
Chapter 58: Chemotherapy of Other Protozoal Infections Visceral Leishmaniasis or Kala-azar Oriental Sore American Mucocutaneous Leishmaniasis Trypanosomiasis Toxoplasmosis Trichomoniasis Giardiasis
Chapter 59: Chemotherapy of Viral Infections Anti-herpes and anti-CMV Drugs Anti-hepatitis B Drugs Anti-hepatitis C Drugs Anti-influenza Drugs Interferons
Chapter 60: Chemotherapy of Helminthiases Drug Therapy of Roundworms Drug Therapy of Hookworms Drugs Therapy of Pinworms Drug Therapy of Strongyloidiasis Drug Therapy of Trichuriasis (Whipworm) Drug Therapy of Filariasis Drug Therapy of Guinea Worm Drug Therapy of Tapeworms Drug Therapy of Schistosomiasis Drug Therapy of Flukes
Chapter 61: Chemotherapy of Malignancy Alkylating Agents Antimetabolites Radioactive Isotopes Cytotoxic Antibiotics Antimitotic Natural Products Hormones and Anti-Hormonal Drugs Miscellaneous Agents Tyrosine Kinase Inhibitors and Monoclonal Antibodies (mAb) Biological Response Modifiers Proteasome Inhibitors Drug Therapy of Malignant Diseases
Chapter 62: Antiseptics, Disinfectants and Insecticides Physical Agents Acids and Alkalies Alcohols Aldehydes Surfactants Phenols and Related Compounds Halogens and Halogen Containing Compounds
Oxidising Agents Dyes Heavy Metals Gases Choice of Method of Sterilisation and Disinfection Insecticides
Section XIII: Drugs Used in Endocrine Disorders Chapter 63: Anterior Pituitary Hormones Endocrine Physiology-Introduction The Pituitary Gland
Chapter 64: Thyroid Hormones and Antithyroid Drugs Treatment of Myxedema Coma Iodine and the Endemic Goitre Antithyroid Drugs Pharmacotherapy of Hyperthyroidism Thyrotoxic Crisis – Treatment Iodine-Containing Contrast Media
Chapter 65: Pancreatic Hormones, Antidiabetic Drugs and Pharmacotherapy of Diabetes Mellitus Non-Insulin Antidiabetic Drugs Oral Antidiabetic Agents Parenteral Non-Insulin Antidiabetic Agents Pharmacotherapy of Diabetes Mellitus Management of Emergencies in the Diabetic Hyperglycemic Agents Sweetening Agents
Chapter 66: Adrenal Cortical Steroids Adrenocorticotropin
Hormones of the Adrenal Cortex Glucocorticoids and Mineralocorticoids Adrenal Function Tests
Chapter 67: Gonadotropins, Estrogens and Progestins Gonadotropins Placental Hormones Estrogens Progesterone and Other Progestins
Chapter 68: Antifertility Agents and Ovulation Inducing Drugs Estrogen-Progestin Combination Pill Progestins Alone as Contraceptives Antiestrogenic Agents Injectable Contraceptives Antiprogestins Postcoital Contraception Male Contraception Medical Termination of Pregnancy (MTP) Ovulation Inducing Drugs Antigonadotropic Compounds
Chapter 69: Androgens, Anabolic Steroids and Antiandrogens Erectile Dysfunction Anabolic Steroids Antiandrogens Management of Hirsutism Benign Prostatic Hyperplasia
Chapter 70: Calcium, Phosphorus, Fluoride and Magnesium Metabolism; Parathyroid Hormone and Vitamin D; Treatment of Osteoporosis Phosphorus Metabolism Osteoporosis – Management Fluoride
Magnesium Metabolism
Section XIV: Drugs Used in Common Skin and Eye Disorders Chapter 71: Pharmacotherapy of Common Skin Disorders and Skin Protectives Principles of Drug Application Vehicles and Formulations Choice of Preparation Keratolytic Agents Drug Therapy of Bacterial and Viral Skin Infections Drug Therapy of Fungal Skin Infections Dandruff and Seborrhoeic Dermatitis Drug Therapy of Scabies and Pediculosis Drug Therapy of Acne Vulgaris Drug Therapy of Allergic Skin Disorders Drug Therapy of Psoriasis Drug Therapy of Alopecia Drugs Affecting Skin Pigmentation Sunscreens and Barrier Preparations Cosmetics, Tooth Powders and Dermal Fillers Anhidrotics and Deodorants Drug-Induced Skin Disorders
Chapter 72: Ocular Pharmacology Antimicrobial Agents Anti-inflammatory Drugs Glucocorticoids in the Eye Antihistaminics and Mast Cell Stabilisers Mydriatics and Miotics Drug Therapy of Glaucoma Immunosuppressives and Antimitotic Drugs Local Anaesthetics
Diagnostic Agents Miscellaneous Agents Drug Induced Ocular Toxicity
Section XV: Immunopharmacology Chapter 73: General Considerations: Vaccines and Antisera Active Immunisation Passive Immunisation Drug Therapy of Scorpion Sting
Chapter 74: Immunoglobulins, Monoclonal antibodies, Immunosuppressants and Immunomodulators Monoclonal Antibodies (mAb) in Therapeutics Immunosuppressants Immunostimulants and Immunomodulators
Section XVI: Miscellaneous Chapter 75: Pharmacotherapy of Gout, Rheumatoid Arthritis and Osteoarthritis Drugs Used During Acute Stage Drugs Used in Long Term Therapy of Gout Xanthine Oxidase Inhibitors Uricosuric Drugs Pharmacotherapy of Rheumatoid Arthritis (RA) Osteoarthritis (OA)
Chapter 76: Metals and Their Antagonists Heavy Metal Antagonists
Chapter 77: Gases: Therapeutic and Toxic Therapeutic Gases
Toxic Gases
Chapter 78: Enzymes in Therapy Chapter 79: Vitamins and Antioxidants Fat Soluble Vitamins Water Soluble Vitamins Antioxidants
Chapter 80: Drugs, Pregnancy and the Infant Pharmacokinetics During Pregnancy Drugs, the Fetus and the Newborn Effects of Drugs on Pregnancy Teratogenicity Drug-Prescribing During Pregnancy Breast Feeding and Drugs
APPENDIX: Guide to Further Reading Index
Copyright Reed Elsevier India Pvt. Ltd. Registered Office: 818, 8th Floor, Indraprakash Building, 21, Barakhamba Road, New Delhi 110001 Corporate Office: 14th Floor, Building No. 10B, DLF Cyber City, Phase II, Gurgaon-122 002, Haryana, India Popular Prakashan Pvt. Ltd. Registered Office: 301, Mahalaxmi Chambers, 22, Bhulabhai Desai Road, Mumbai - 400 026
Pharmacology and Pharmacotherapeutics, 24e, R.S. Satoskar, Nirmala N. Rege, S.D. Bhandarkar Copyright © 2015 by Dr. R.S. Satoskar First Edition, 1969; Russian Edition, 1985; Twentieth Edition, 2007; Reprinted 2007, 2008, July 2008; Twenty-first Edition, 2009; Reprinted May 2010, December 2010; Twenty-second Edition, 2011; Reprinted July 2012; Twenty-third Edition, 2013; Reprinted September 2014; Twenty-fourth Edition 2015 All rights reserved. ISBN: 978-81-312-4361-9 e-Book ISBN: 978-81-312-4371-8 This Twenty-Fourth edition of Pharmacology and Pharmacotherapeutics by R.S. Satoskar, Nirmala N. Rege, S.D. Bhandarkar is co-published by arrangement with Reed Elsevier India Pvt. Ltd. and Popular Prakashan Pvt. Ltd. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher ’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Author (other than as may be noted herein).
Not ice
Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the co-publishers nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a result of any actual or alleged libelous statements, infringement of intellectual property or privacy rights or matter of product liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. Please consult full prescribing information before issuing prescription for any product mentioned in this publication. Printed in India by Thomson Press, Faridabad.
Dosage Note Pharmacology and drug therapy, like medicine, is an everchanging science. As new information accumulates, changes in drug therapy are inevitable. The authors and publishers of this book have taken special care to provide drug information and dosage schedules, that are generally in accord with the accepted standards at the time of publication. However, it may be noted that in view of the possibility of human error or advances in medicinal sciences, such therapeutic approaches are liable to change. Neither the authors nor the publishers nor any other party who has been involved in the publication of this work warrants that the information contained herein is in every respect accurate and complete. We urge the readers to confirm the information with other sources and refer to the manufacturers’ recommendation for all dosages, especially for the new drugs and for those used infrequently in clinical practice.
Preface to the Twenty-fourth Edition The textbook of Pharmacology and Pharmacotherapeutics undergoes continuous update every 2 years to maintain its reputation as an authentic source of unbiased and reliable information about drugs and their uses in therapeutics. This revised 24th edition, apart from routine update has undergone some major changes. For example, two chapters viz. antipsychotics and antidepressants have been introduced instead of one single chapter on psychopharmacology. We feel that this change makes individual chapter more concise and focused on the drug groups used to treat diverse disease conditions. In addition, a section has been introduced on immunopharmacology, which includes the basics of immunology and the drugs and biologicals, which have prophylactic and therapeutic role in modulating immune status. This is a relevant change considering the recent advances in the field of immunotherapy. With the rapid advances in molecular biology and pathophysiology of diseases, several old concepts have undergone revision leading to changes in therapeutic approaches and this will continue in future. Further, observations on variations in dose response in various ethnic groups and even in individuals from the same group (Pharmacogenomics) have provided ample examples that point out the need for careful selection of available drug regimens. This is particularly important while treating chronic ailments such as asthma, diabetes mellitus, hypertension, mental illness and using drugs with narrow therapeutic index. Hence continuous updating of knowledge has become a necessity even more than before. It has become easier to retrieve drug information within no time with the advent of information technology but the application of this information in clinical practice is a real challenge. This book while giving the recent advances in drug therapy also discusses the various options available for managing the patient in a given situation. Therapeutics is as much as an art as science. In this era of computerization and data analysis, there is a tendency to standardize the drug treatment and draw flowcharts and algorithms based on safety and efficacy profile of the drug in different population. Such data and charts are certainly useful as general guidelines but it must be remembered that drug treatment has to be ‘tailor made’ for each patient taking into consideration the several factors including the availability and affordability of the recommended drugs. Working of human body is highly complex and several factors are involved in modulating its working – some known and many still unknown. Doctor ’s experience and wisdom besides the knowledge is more important in making the therapeutic decisions that will benefit the patient in a given situation. In the last decade or so, we have seen many new drugs getting withdrawn from the market within a short period after their introduction due to recognition of adverse effects. Taking cue from this scenario, one should be more cautious while selecting any ‘new’ drug in preference to the older established therapy, as we know that the ‘new’ drugs may have marginal benefits, inadequate safety data and are often introduced hastily with tall claims. Temptation of using multiple drugs with a hope of achieving quicker cure without rationale could be disastrous. Such therapy increases the chances of undesirable drug interactions and toxicity.
We thank Dr. Manjunath TA and Dr Rajendraprasad Rao, our residents, for their help in literature search. Dr. Manjunath helped also in manuscript revision. We acknowledge Dr. Rajendraprasad for drawing additional figures for this edition. We thank Mr. Harsha Bhatkal, Publisher, Popular Prakashan Pvt. Ltd. and his team for their excellent co-operation in bringing out this edition. As always, we welcome constructive criticism and suggestions from our readers. Mumbai May 2015 R.S. Satoskar, Nirmala N. Rege and S.D. Bhandarkar
Preface to the First Edition Pharmacology has undergone phenomenal growth during the last twenty years and drug therapy now forms a major aspect of therapeutics. So far, pharmacology was traditionally associated with the study of drugs in dogs, cats and rats, while therapeutics or clinical application was regarded as an entirely independent and a mystical skill. Now it is generally agreed that the major object of teaching pharmacology to the medical students is to provide a rational basis for choosing and using drugs skilfully to relieve patients’ ailments. This is becoming more and more important as the practising doctor is now confronted with so called “newer drugs” at such a great pace that even a full-time pharmacologist sometimes finds it difficult to keep abreast of their merits and demerits. It is highly desirable, therefore, that students of pharmacology should be educated to develop a critical outlook towards various drugs, as they are introduced. This means that book on pharmacology meant for medical students should not only give detailed account of various pharmacological actions but should also furnish a critical appraisal of their present day use in therapeutics. We have attempted in this book to combine these two important aspects. In addition, an outline of experimental evaluation of drugs in animals and man is also provided. While doing this, it was thought essential to give the relevant information from other disciplines like physiology, pathology and clinical medicine. This, though a repetition to a certain extent, is no doubt useful to understand the basis of rational therapeutics. After all, pharmacology is in some respects a bridge between basic medical sciences on one hand and clinical medicine on the other. Most of the presently available text books, except a few classics written by many authors, fail to achieve this goal. It is a common experience of those who teach pharmacology in this country to find it difficult to recommend one single book to the undergraduate medical students. Many books which give excellent information about pharmacological actions treat the therapeutics very cursorily while others that give delightful therapeutics probably assume that students know most of the basic pharmacology. It is not practicable to recommend routinely the classical multi-author books to undergraduate students, as they have many other subjects to go through, which are equally important and advanced. This book is written to fill up this gap between a big book and a concise, less informative work, so that students will get all the necessary information by reading one book. While doing this, obviously we have to restrict the size of the book, lest it would be unwieldy and defeat the very purpose for which it is written. In order to achieve this, history and chemistry are reduced considerably while diagrams are included strictly to facilitate the understanding of the subject. The coverage is given according to the importance of the subject in therapeutics. Wherever multiple drugs of similar type are available, only the important prototype is discussed in detail, while others have received only a brief mention. Although the book is written mainly for undergraduate medical students, it will also prove useful to post-graduates and practising doctors. The therapeutics part includes many details so that the book would continue to be useful even after passing pharmacology and is expected to serve as a pharmacotherapeutic reference work. The big multi-author books on this subject are no doubt excellent and authoritative but are not
easily accessible to practising doctors in an emergency. In such circumstances this book should find its use. The book is not written ‘with an eye on examination’ but it is the hope of the authors that by reading this book students would develop an attitude of thinking towards newer drugs which are many times made to appear like “therapeutic marvels”. It is not expected that undergraduate students should ‘cram’ this book and try to remember everything that is given. It is neither possible nor necessary. They are expected to learn the basic pharmacology of the drugs in common clinical use and their rational application in therapeutics. However, the authors will feel rewarded, if students can grasp the ideology and spirit behind presentation of this book. Drug therapy related to tropical problems is emphasized; this topic is often dismissed summarily in other works of this size. Proprietary names are included wherever necessary so that their pharmacological identity is recognised by the reader. No detailed reference list is given as this would have added many more pages. Instead, the books, reviews, symposia and monographs referred to are enlisted at the end of the book. The enthusiastic reader may refer to these for a more extensive reference list. Preparations included in the Indian Pharmacopoeia are marked as I.P. and those included in British Pharmacopoeia are listed in a separate list at the end. It is not possible to present a book of such a size without generous help of others and the authors are deeply grateful to their many colleagues at Seth G.S. Medical College and Lokmanya Tilak Municipal Medical College, Bombay. Particularly, the help rendered by Drs. B. S. Kulkarni, S. M. Chittal, S. V. Gokhale, M. G. Wagh, C. H. Kewalramani, Mr. N. K. Dadkar, Mr. V. S. Jathar, Dr. S. M. Karandikar and Miss P. Mirwankar is gratefully acknowledged. We also would like to express our grateful thanks to many authors and publishers who promptly conceded our requests and granted permission to reproduce certain tables and diagrams, as indicated in the text. We are greatly indebted to Dr. A. F. Golwalla, Hon. Professor of Medicine, Seth G. S. Medical College and K.E.M. Hospital, Bombay for his encouragement and permission to reproduce E.C.G. records. Finally, our thanks are due to Popular Prakashan and Popular Press (Bom.) Pvt. Ltd., who as publishers and printers respectively are responsible for delivering this book in your hand expeditiously. Bombay, November, 1968 R.S. Satoskar, A.K. Kale and S.D. Bhandarkar
Some Abbreviations Used in the Text ACD: Anaemia of chronic disease AChE: Acetylcholinesterase AD: Alcohol dehydrogenase ADHD: Attention deficit hyperactivity disorder ANC: Acid neutralizing capacity ANF/P: Atrial natriuretic factor/peptide APP: Acute phase proteins ARDS: Adult respiratory distress syndrome ATA: Atmosphere absolute AUC: Area under curve AVP: Arginine vasopressin BMD: Bone mineral density BMI: Body mass index BNP: (human) Brain type natriuretic peptide BPH: Benign prostatic hypertrophy CDK: Cyclin-dependent kinases CFS: Chronic fatigue syndrome CLDII: Continuous, low dose, insulin infusion CMRNG: Chromosomally mediated resistant N.gonorrhoeae CPDA: Citrate-phosphate-dextrose-adenine CPPD: Calcium pyrophosphate dihydrate DES: Daytime excessive sleepiness DUB: Dysfunctional uterine bleeding DVT: Deep vein thrombosis EAA: Excitatory amino acids
EDRF: Endothelium derived relaxing factor EGFR: Epidermal growth factor receptor EPR: Extrapyramidal reaction ET: Endothelin FAD: Flavin adenine dinucleotide FADH2: Reduced flavine adenine dinucleotide FMN: Flavin mononucleotide GERD: Gastroesophageal reflux disease GLUT: Glucose transporter GM-CSF: Granulocyte/macrophage colony stimulating factor GPCR: G-protein-coupled receptors GRA: Glucocorticoid-remediable aldosteronism HRT: Hormone replacement therapy HVA: Homovanillic acid IBD: Inflammatory bowel disease IBS: Irritable bowel syndrome IGF: Insulin like growth factor IL: Interleukin ILA: Insulin like activity IRI: Immunoreactive insulin IRMA: Immunoradiometric acid IRS: Insulin receptor substrate LES: Lower esophageal sphincter LTP: Long term potentiation LVEF: Left ventricular ejection fraction MAC: Mycobacterial avium complex MAC: Minimum alveolar concentration
MAP: Muscle action potential MDI: Metered dose inhaler MDR: Multi drug resistant MHPG: 3 methoxy-4-hydroxy phenol glycol MIC: Minimum inhibitory concentration MND: Motor neurone disease MRP: Multidrug resistance associated protein NAD: Nicotnamide adenine dinucleotide NADH: Reduced nicotinamide adenine dinucleotide NADP: Nicotinamide adenine dinucleotide phosphate NADPH: Reduced nicotinamide adenine dinucleotide phosphate NANC: Non-adrenergic, non-cholinergic NAP: Nerve action potential NARES: Non-allergic, non-infectious rhinitis with eosinophilia NEP: Neutral endopeptidase NGU: Non gonococcal urethritis NK: Natural killer NK: Neurokinins NMDA: N-methyl-D-aspartate OTC: Over-the-counter PAOP: Pulmonary artery occlusive pressure PAT: Paroxysmal atrial tachycardia PDE: Phospodiasterase POMC: Pro-opio-melanocortin PPD: Purified protein derivative PPNG: Penicillinase producing N. gonorrhoeae PRA: Plasma renin activity
PUFA: Polyunsaturated fatty acids PVT: Paroxysmal ventricular tachycardia RIMA: Reversible inhibitor of MAO RPCFT: Reiter protein complement fixation test SAM: s-adenosyl methionine SNpc: Substatia nigra pars compacta SNRI: Serotonin norepinephrine reuptake inhibitor SR: Sarcoplasmic reticulum SSKI: Saturated solution of potassium iodide SSRI: Selective serotonin reuptake inhibitors TDM: Therapeutic drug monitoring TI: Therapeutic index TSS: Toxic shock syndrome TSST-1: Toxic shock syndrome toxin-1 UDPG: Uridine diphospho- glucose VMA: Vanilylmandelic acid VMR: Vasomotor rhinitis VREF: Vancomycin resistant enterococcus faecium
Dedication “Good health is not just the absence of disease. It is the positive dynamic energy state in which internal organs work in perfect harmony and concord; and external behaviour is smooth and relaxed.”
— Swami Vivekananda
S E C T IO N I
General Pharmacology OUT LINE Chapter 1: General Considerations and Pharmacokinetics Chapter 2: Pharmacodynamics – Drug Receptor Interaction; Adverse Drug Reactions Chapter 3: Principles of Drug Prescribing; Factors Modifying the Effects of a Drug; and Drug Interactions Chapter 4: Drug Invention; New Drug Development; and Drug Assay
1
General Considerations and Pharmacokinetics Illness has been man’s heritage from the beginning of his existence, and the search for remedies to combat it is perhaps equally old. The world’s oldest known therapeutic writings come from India and China. The earliest Indian records are the Vedas. Although there are medical descriptions in Rigveda (3000 B.C.), Ayurveda, the science of life advocates various medicinal preparations of herbal and mineral origin. These are presented in ancient treatise Charaka samhita, Sushruta samhita and Vagbhata. The original Ayurvedic materia medica was later superseded to some extent by the alchemic or chemical substances at about the beginning of Christian era. The Chinese materia medica ‘Pan Tsao’ was probably written in 2735 B.C. and contained many plant and metallic preparations and a few animal products. The earliest sources of Western medicine come from Egypt and the two kingdoms of Assyria and Babylonia. The ‘Papyri’ were the first written account of medical experiences from Egypt, and date back to 1900 B.C. The papyrus discovered by George Ebers in 1872 A.D. mentions about 700 herbal remedies, including opium. A Babylonian clay tablet (700 B.C.) mentions about 300 drugs. Modern medicine is considered to date from Hippocrates, a Greek physician (450 B.C.), who for the first time introduced the concept of disease as a pathologic process and tried to organise the science of medicine on the basis of observation, analysis and deduction. Hippocratic practice did not include extensive use of drugs, probably because he did not believe in shotgun or magical remedies, but instead recommended judicious use of simple and efficacious drugs. Till the beginning of the 19th century, the treatment of diseases included such obnoxious remedies as flesh, excreta and blood of various animals along with metal and plant preparations. James Gregory (1753-1821) was responsible for popularising heroic treatment consisting of blood letting, large doses of emetics and drastic purgatives, often with disastrous results. Such treatment without any rational basis was labelled Allopathy (the other suffering), a term which is still wrongly applied to the system of modern scientific medicine, as opposed to Homeopathy (similar suffering). The concept of Homeopathy was first introduced in the early 19th century by Hannemann who thought that “like cures like, and dilution potentiates the action of drugs.” Homeopathy outlines the therapy for various ailments with drugs in very high dilutions. The word Pharmacology is derived from two Greek words Pharmacon (an active principle) and logos (a discourse or treatise). It is the science that deals with drugs. Development of modern pharmacology as a science is fairly recent and probably started taking shape following the introduction of experimental procedures in animals by Francois Magendie (1783-1855) and Claude Bernard (1813-1878). Spectacular developments in physiology, organic chemistry and molecular biology have greatly accelerated the advances in pharmacology. In turn, pharmacology has helped to elucidate many basic physiological and pathological mechanisms in health and disease. Pharmacology consists of detailed study of drugs, particularly their actions on living animals, organs and tissues. The actions may be beneficial or harmful. The object of
pharmacology is mainly to provide such scientific data, using which one can choose a drug treatment of proven efficacy and safety from the various options available, to suit the patient. Pharmacology includes allied topics such as: Pharmacognosy is the science of identification of drugs from natural sources. Pharmacy is the science of identification, selection, preservation, standardisation, compounding and dispensing of medicinal substances. Clinical pharmacy is the science of drug formulations, their stability, shelf life, handling and also education of the patient about compliance and counselling him on how to take the medication, and monitoring for errors in drug therapy. The clinical pharmacist optimises the patient care with the help of the physician. Pharmacokinetics is a study of absorption, distribution, metabolism and excretion of drugs, and their relationship to pharmacologic response (what the body does to a drug). Pharmacodynamics is the quantitative study of the effects of drugs (what the drug does to the body). Such studies elucidate the site and mechanism of drug action (Fig. 1.1).
FIG. 1.1 Pharmacokinetics and Pharmacodynamics
Therapeutics is a branch of medicine concerned with prevention and cure of disease or relief of symptoms. The word means to care for, to tend to or to nurse. It involves not only scientific knowledge and judgement (obtained from the books) but also skills, wisdom and the sense of responsibility. Toxicology is the science of poisons which includes detection and knowledge about the nature and effects of poisons as well as treatment of poisoning. Poisons are substances that cause harmful, dangerous or fatal symptoms in animals and humans; many drugs in large doses act as poisons. Chemotherapy is concerned with the effect of drugs upon microorganisms and parasites, living and multiplying in a living organism. It now includes the drug treatment of cancer as well.
Pharmacoepidemiology is the study of the use and effects of drugs in large number of people. It helps to gain further insight into the efficacy and safety of new drugs after they are released for community use. Such studies are essentially observational, case-control and cohort studies e.g. relationship of smoking or OC pills to cancer. Pharmacoeconomics is the analysis of the cost of drug therapy and its benefits to the health care system and the society. It examines the quantitative relations between the cost and the benefit (cost-benefit-analysis). Pharmacovigilance is the process of identifying and responding to the issues of drug safety through the detection of drug effects, usually adverse. It is related to the surveillance of drugs once they are released for use in the community and relies on voluntary reporting, prescription monitoring, medical records and statistical studies in the population. Pharmacogenetics is the study of genetic basis for variations in drug metabolism and response in humans. It deals with identifying inherited variations mediated through single gene. Pharmacogenomics is the science that examines genomic variability to assess its effects on the drug response of humans (see later in this Chapter), microbes (antimicrobials) and tumours (anticancer drugs); and explores the ways these variations can be used to predict the patient’s response, either good or poor, to the drug. This may help in the development of target-specific personalized and safer drugs. Pharmacometrics is a science that deals with evolving a quantitative relationship between exposure to the drug (pharmacokinetics) and its response (pharmacodynamics), derived by constructing mathematical models based on few observations. Such models are used in simulations (PKPD modeling) to predict the response or the dose in situations not dealt with earlier e.g. deriving dose for patient with renal dysfunction or having undesirable effect to a given dose. The word drug is derived from the French word ‘drogue’, a dry herb. A drug is defined as any substance used for the purpose of diagnosis, prevention, relief or cure of a disease in man or animals. According to WHO, “A drug is any substance or product that is used or intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient.”
The Nature and Sources of Drugs The various sources of drugs are: I Mineral: e.g. Liquid paraffin, Magnesium sulfate, Kaolin and Aluminium trisilicate. II Animal: e.g. Insulin, Heparin, Gonadotropins, and Antitoxic sera. III Plant: e.g. Morphine, Digoxin, Quinine, Atropine and Reserpine. Plants contain mixtures of several chemical constituents (Table 1.1), which vary from plant to plant. Some of them (active principles) are responsible for pharmacological effects e.g. the alkaloids like morphine in opium and atropine in Atropa belladonna respectively; the glycoside digoxin in Digitalis purpurea; and phytoestrogens like genistein or daidzein in soy preparations. The other phytochemicals in plants may enhance pharmacological effects or may impart stability to active principles or improve their pharmacokinetics and serve as adjuvants. Table 1.1 Plant constituents
IV Micro-organisms: Bacteria and Fungi, isolated from soil, are important sources of antibacterial substances, e.g., Penicillin. V Synthetic: e.g. Non-steroidal antiinflammatory drugs, Hypnotics, Anticancer drugs and ACE inhibitors. Majority of the drugs currently used in therapeutics are synthetic. VI Genetic engineering (DNA recombinant technology), e.g. Insulin and Hepatitis B Vaccine. VII Biologicals: In the last decade many biological agents have also become available for therapeutic purposes. This is a heterogeneous group and includes complex protein molecules that interact with cytokines or cell surface markers e.g. growth factors, monoclonal antibodies, cytokines. They are developed using molecular biology techniques. Apart from drugs, gene based therapy and stem cell therapy are available today. Gene based therapy: The developments in biotechnology, including recombinant DNA technology, have made it possible to synthesise short nucleotide sequences (genes). These are responsible for the in-vivo synthesis of proteins critical in certain metabolic pathways.
They can be introduced into human beings for therapeutic purposes. The object of gene therapy is to introduce functional genetic material into mammalian cells to replace or supplement the activity of defective genes. A variety of diseases are due to inherited deficiencies of single genes; examples are thalassemia, phenylketonuria and cystic fibrosis. In these conditions, serious metabolic disturbances result because of either deficiency or faulty chemical composition of the protein product of the abnormal gene; or accumulation of precursors which cannot be metabolised further because of the enzyme deficiency. This metabolic abnormality can be corrected if the synthetic gene is delivered to the target tissue(s). In contrast, the purpose of gene therapy in acquired diseases such as cancer is to add new molecular function(s) capable of altering the course of the disease, or to block an existing function. Finally, gene-transfer-mediated vaccination is applicable to both infectious and noninfectious diseases. Gene based therapy is in its infancy and many technological and ethical problems remain to be solved. Stem cell therapy: Recently stem cells (either embryonic or adult pleuripotent cells) have been used as a therapeutic approach for regeneration and proliferation of functional cells in the body e.g. in myocardial infarction, osteoarthritis and diabetes mellitus. This approach has great potential in therapeutics but much remains to be done. Nanomedicines: These are synthesised using nanotechnology. The latter is defined as the intentional design, characterisation, production and applications of materials, structures, devices and systems by controlling their size and shape in the nanoscale range (1 to 100 nm). Nanomedicines are close to biological molecules in size and have very high surface/volume ratio. Their outer surface or their core can be loaded with chemicals (either a metal or an organic substance); and can be administered either intralesionally (for cancer) or IV (for therapeutic purpose or as contrast agent for diagnostic imaging). They are also being studied for in vitro measurement of molecules of interest in biological materials for diagnostic purpose. The nanotechnology is in its infancy; and the long term in vivo safety of these agents is still to be confirmed.
Sources of Drug Information It is essential to select drugs for treating a disease in rational manner – based on logical thinking supported by comprehensive and objective information. The sources that provide information about the drug are of 3 types: (1) Primary information sources include original research publications in the journals, reports of clinical drug trials and pharmacological research and serve as basic foundation to provide factual data. (2) Secondary information sources are derived from the primary and include review articles, meta-analyses and compilations of published articles or their parts done by bibliographic, abstracting, or indexing services like Medline, Current Contents, International Pharmaceutical Abstracts, Index Medicus, Excerpta Medica. (3) Tertiary information sources are documents written by individuals or groups and are often peer reviewed. These include formulary, standard treatment guidelines, textbooks, general reference books, drug bulletins, The WHO Model Lists of Essential Drugs and drug compendia. Depending on the information needed, one may select an appropriate source. Pharmacopoeia, is an official code containing a selected list of the established drugs and medicinal preparations with descriptions of their physical properties and tests for their identity, purity and potency. Pharmacopoeia defines the standards which these preparations must meet, and may mention their average doses for an adult. Examples are the Indian Pharmacopoeia (IP), the British Pharmacopoeia (BP), the United States Pharmacopoeia (USP) and the European Pharmacopoeia. They have legal standing from the point of view of drug regulatory authority. A formulary is not a regulatory document but provides information about the available drugs, based on original and reputed drug information sources as well as experts’ recommendations. It provides up-to-date guidance to prescribers and aids rational prescribing and dispensing drugs e.g., WHO Model Formulary, the British National Formulary (BNF) and the National Formulary of India (NF). British Pharmaceutical Codex is published by Council of Pharmaceutical Society of Great Britain. It gives information about drugs, other pharmaceutical substances and formulated products. Further, it provides standards for identification and purity for a range of substances and materials for which standards are not provided by the BP. Martindale The Complete Drug Reference is a compendium, published periodically over the last 120 years. It is prepared by the Royal Pharmaceutical society of Great Britain and provides unbiased information on drugs and medicines used throughout the world, including their trade names and manufacturers’ contact information. It also includes plant drugs, diagnostic agents, radiopharmaceuticals, pharmaceutical excipients, toxins and poisons. It provides synopses of treatments for diseases. The drug information is backed by references, systematic reviews, and meta-analyses or evidence based guidelines. It thus serves as an excellent source of drug information. Different brands of same medicine are marketed by different manufacturers. Information about the brands, their formulations, strengths, cost, dose, and precautions, adverse effects, contraindications is available in publications such as the Physicians Desk Reference (PDR), the Indian Drug Review (IDR), the Monthly Index of Medical specialities
(MIMS) or CIMS India website.
Routes of Drug Administration and Dosage Forms Drugs may be applied locally, or may be administered orally and or by injection. Local application: Dusting powder, paste, lotion, drops, ointment or plaster exert action at the site of application (topical action). Drugs may also be administered locally in the following forms: bougie for urethra, pessary for vagina, inhalers for bronchi, and suppository for the vagina and rectum. Drugs used as aqueous solutions for local effects on mucous membranes are likely to be absorbed and may produce adverse systemic effects. In case of corneal application, the drug may penetrate into the anterior chamber and affect the ciliary muscle e.g. cocaine. Similarly, during irrigation or spraying of the nose, a compound may find its way into the middle ear through the eustachian tube. Lipoid pneumonia following aspiration of an oily solution into the respiratory tract has been reported. Administration of a medicament in a liquid form into the rectum is called enema. Enemata are of two types: • Evacuant enema: e.g. soap water enema. The aim is to remove the faecal matter and the flatus. The water stimulates the rectum by distension while soap acts as a lubricant/softener. The quantity of fluid administered is about 600 ml. It is useful in treating selected cases of constipation. It is also administered before surgical operations, delivery and radiological investigation of the GI tract. • Retention enema: The fluid containing the drug is retained in the rectum for local action as with prednisolone enema for ulcerative colitis; or it may act systemically after absorption through the mucous membrane. The quantity of fluid administered is usually 100-120 ml. For systemic absorption by transrectal route, see later. Oral or Enteral route: This is the most commonly employed route for drug administration. Its advantages are: • Convenient and safe • Economical • Complications of parenteral therapy are avoided. However, it has the following disadvantages: • The onset of drug action is tardy. • Irritant and unpalatable drugs cannot be administered by this route. • The absorption of certain drugs can be irregular or negligible e.g. aminoglycosides. • The route may not be useful in the presence of vomiting and diarrhoea. • The route cannot be employed in an unconscious or uncooperative patient nor in an emergency; and • Drugs likely to be destroyed by digestive juices cannot be administered orally e.g. insulin and enzymes for systemic action. Further, certain drugs like testosterone though absorbed, get inactivated in the intestinal wall and the liver (first pass metabolism) and only a small portion reaches the systemic circulation. Tablets or capsules are often made more acceptable by various types of coating such as synthetic resins, gums, sugars, plasticizers, polyhydric alcohols, waxes, colouring agents and flavouring agents. Certain precautions should be taken during oral administration of drugs. Capsules and
tablets should be washed down with a glass of water with the patient in upright posture, either sitting or standing, as this enhances the passage into the stomach and permits rapid dissolution. Giving drugs orally to a recumbent patient should be avoided, if possible, especially in the case of drugs which can damage the esophageal mucosa e.g. tetracyclines, iron salts, slow release potassium preparations and alendronate. Enteric coated tablets: Sometimes, tablets are coated with cellulose-acetate-phthalate, gluten and anionic co-polymers of methacrylic acid and its esters. These substances resist the acid juice of the stomach but permit disintegration in the intestinal alkaline juices. Enteric coating is done to: • Prevent gastric irritation and destruction of the drug in the stomach. • Achieve the desired concentration of the drug in the small intestine; and • Retard the absorption of the drug. If the coating is very hard, a tablet may pass out without being dissolved in the GI tract (Fig.1.2 A and 1.2 B).
FIG. 1.2 Radiographs (A&B) showing intact tablets in the GI tract. Arrows indicate presence of tablets which are not dissolved.
The conventional oral dosage forms serve only the purpose of introducing specific amounts of drug into the body. They do little to maintain uniform body drug concentration. Further, in order to produce a therapeutic concentration at the site of action one has to administer much larger quantities of the drug which can cause adverse reactions. Additional disadvantages include the need for frequent dosing, problems related to concomitant food intake and long term patient compliance. Controlled release (CR) and time release preparations (Timsules, Spansules) release the drug over an extended period of time. Such preparations have the particles of drug covered with coatings which dissolve at different time intervals. The coating which dissolves early releases an amount of the drug which establishes its action quickly; the coating which dissolves more slowly ensures a slow release of the remainder of the drug, thus providing uniform medication over a prolonged period.
Parenteral routes: Routes of administration other than the alimentary tract (the enteron) are called parenteral. Its advantages are: • They can be employed in an unconscious or an uncooperative patient. • They can be employed in cases of vomiting and diarrhoea and in patients unable to swallow. • Drugs which might irritate the stomach or which are not absorbed orally can be administered. • They avoid drug modification by the alimentary juices and liver enzymes; and • Rapid action and accuracy of dose are ensured. The disadvantages are that they are: • Inconvenient for use, self medication being difficult. • Less safe, and liable to cause infection if proper care is not exercised. • Likely to injure important structures such as nerves and arteries; and • More expensive. The parenteral routes are: I Inhalation: Drugs may be administered by this route, using: • Pressurised, metered dose aerosols, e.g. salbutamol and beclomethasone. • Dry powders from inhalers activated by patient’s inhalation, e.g., salbutamol; or • Oxygen or compressed air driven nebulised solutions, e.g., salbutamol. • Gases, e.g., general anaesthetics, vapours of volatile oils. Drugs given by inhalation produce rapid effects. Thus, nicotine, morphine and tetrahydrocanabinol are rapidly absorbed following the inhalation of tobacco, opium or marijuana smoke, respectively. Drugs go directly to the left side of the heart through the pulmonary veins and may produce cardiac toxicity. Local irritation may cause bronchospasm and increase the respiratory tract secretion. II Injections: Injection given by any routes need strict aseptic technique. Intradermal (ID): This is given into the layers of the skin e.g. BCG vaccine. Only a small quantity can be administered by this route and the injection may be painful. It is also employed for testing drug sensitivity. Subcutaneous (SC): Only non-irritant substances can be injected by this route. The commonest drug used by this route is insulin. The drug absorption is slower than with IM route. However, the action is sustained and uniform. Absorption by this route is unreliable in shock. Subcutaneous drug implants can act as ‘depot’ therapy e.g. testosterone implants. In pediatric practice, saline is sometimes injected SC in large quantities (hypodermoclysis). Drug absorption from the subcutaneous area can be enhanced by the addition of the enzyme hyaluronidase (Chapter 78). Intramuscular (IM): In addition to soluble substances, mild irritants, suspensions and colloids can be injected by this route. The rate of absorption is reasonably uniform and the onset of action is rapid. The rate of absorption is faster from deltoid and vastus lateralis than from gluteus maximus. The volume of injection should not exceed 10 ml. However, IM absorption is not always faster than oral absorption e.g. diazepam, hydrocortisone, digoxin and phenytoin are absorbed more slowly IM than orally. Drugs injected IM, especially irritants may:
• Cause local pain and even necrosis e.g. quinine, iron, and paraldehyde. • Damage the nerve causing severe pain and even paresis of the muscles supplied by it. IM injection should not be made into the buttock until the child starts to walk, as the gluteus maximus is very tiny till the child starts to walk; the lateral aspect of the thigh should be used. Intravenous (IV) : Drugs given directly into a vein produce very rapid action, and the desired blood concentration can be obtained rapidly with a smaller dose. Titration of the dose is possible. A drug may be injected IV: (a) As a bolus e.g. furosemide; (b) Over 5-10 minutes, diluted in 10-20 ml of isotonic glucose or saline e.g. aminophylline or (c) In an infusion in 50-100 ml or larger in volume. An infusion is employed to: (i) Slow the administration of the drug to avoid toxicity e.g., morphine; (ii) Maintain a constant plasma level of the drug e.g. insulin or dopamine; and (iii) Administer large volumes either rapidly or over prolonged periods of time e.g. fluids in shock or dehydration. The disadvantages of this route are: • Once the drug is administered by this route, its action cannot be halted. • Local irritation can lead to phlebitis (Chapter 38). • Self-medication is difficult; and • Extravasation of certain substances may cause irritation and even sloughing (Table 1.2). Table 1.2 Irritants on IV administration
Possible damage due to vasopressors may be reduced by local infiltration of phentolamine.
Precautions during IV therapy: • Before injecting, ensure that the needle and the syringe are airfree, and that the needle is in the vein. Irritating solutions should be administered by piggybacking into a running IV drip or through a central line. If they are administered through a peripheral vein, the IV site should be rotated at regular intervals. If extravasation occurs, attempts to aspirate the extravasated substance through the c should be made before removing the cannula. • The injection should be given slowly in case of certain drugs such as iron and aminophylline, as a sudden high blood concentration may be dangerous; and • Only the minimum quantity required to elicit a particular effect should be injected. Intra-arterial drug administration produces a sudden high concentration in arterial blood and hence, may be harmful locally or damaging to tissues supplied by the artery. This route is used in diagnostic studies, such as angiograms, and in embolisation therapy. Certain antimalignancy compounds are administered by regional intra-arterial infusion in localised malignancies. Intrathecal administration involves introduction of drugs such as spinal anaesthetics
into the subarachnoid space. The drugs act directly on the CNS. This route also is convenient for producing high local concentrations in the subarachnoid space e.g. certain antibiotics and antimalignancy drugs. Lignocaine is used extradurally to produce anaesthesia for pelvic surgery. The epidural use of morphine for analgesia is described in Chapter 10. Intraperitoneal: This route has been sometimes used in infants for giving fluids like glucose saline, as the peritoneum offers a large surface for absorption. It is also used for peritoneal dialysis. Intraosseous (intramedullary) cannulation: Drugs injected into the bone marrow of the iliac crest or the tibia (using a special gadget) are rapidly absorbed into the circulation. Adrenaline injected in this manner is of great help in adults in severe shock, and those with sudden cardiac arrest, and no immediate access to a vein. In infants and children, drugs such as adrenaline and dopamine have been used in this manner for resuscitation in acute, life-threatening situations when standard venous access methods have failed. Intra-articular and Intra-lesional: Certain drugs are administered directly into a joint for the treatment of local conditions. This ensures a high local concentration of the drug e.g. hydrocortisone acetate in the treatment of rheumatoid arthritis. Repeated injections may damage the joint. Glucocorticoids and local anaesthetics have been administered intralesionally into painful and tender spots. Intracavernosal and Transurethral: See Chapter 69. III Transcutaneous/Transdermal: Iontophoresis: In this procedure, a galvanic current allows the penetration of drugs applied to the skin into the deeper tissues. Salicylates have been used by this method. Inunction: Certain drugs when rubbed into the skin (inunction) can get absorbed and produce systemic effects e.g. nitroglycerin ointment in angina pectoris and NSAIDs for sprain. Certain potent glucocorticoids when applied to skin lesions for local effects may get absorbed and cause systemic adverse effects. Jet injection (Dermojet): This needleless method involves the transcutaneous introduction of a drug by means of a high velocity jet produced through a micro-fine orifice. It is used for giving insulin or mass vaccination. Transdermal delivery system: It is available as an adhesive unit to deliver drugs slowly through skin producing prolonged systemic effect e.g. scopolamine for prevention of motion sickness. Estraderm is a self-adhesive, TTS-releasing predetermined quantities of estradiol per 24 hours, for a period of 3-4 days. It must be emphasised that percutaneous absorption of topically applied drugs is greater in infants and children, particularly in prematures and if the skin is burnt or excoriated. This can enhance drug toxicity. IV Trans-mucosal: Sublingual administration: A tablet containing a medicament is placed under the tongue and is allowed to dissolve in the mouth. The active agent thus gets absorbed through the buccal mucous membrane directly into the systemic circulation. Drugs commonly administered by sublingual route are nitroglycerin in angina pectoris and buprenorphine as an analgesic. Some other drugs which may be administered sublingually are nifedipine, diazepam, lorazepam, domperidone, ondansetron and ergotamine. Advantages of this route are shown in Table 1.3.
Table 1.3 Advantages of sublingual route
Trans-nasal route: DAVP, a synthetic analogue of vasopressin, GnRH agonist (nafarelin) and calcitonin are the drugs administered by this route. A toxic substance should not be administered by this route, as it may directly reach the brain along with lymphatic channels. Also see status epilepticus in Chapter 9. Trans-rectal route: The rectum has rich blood and lymph supply, and drugs can cross the rectal mucosa like the other lipid membranes; thus, unionised and lipid soluble substances are readily absorbed. The portion absorbed from the upper rectal mucosa is carried by the superior haemorrhoidal vein into the portal circulation whereas that absorbed from the lower rectum enters directly into the systemic circulation via the middle and inferior haemorrhoidal veins. Thus approximately 50% of drug absorbed by the rectum bypasses the liver. In addition, CYP3A4 is absent in the lower intestine. Hence, chances of first pass metabolism are lower than after the oral dose. Examples of drugs that can be given rectally are indomethacin in rheumatoid arthritis, aminophylline for bronchospasm, and diazepam for status epilepticus. The advantages of this route are: • Gastric irritation is avoided. • Duration of action can be controlled, if suitable solvent is used. • Avoids first pass metabolism. • Convenient route for long term care of geriatric and terminally ill patients. • Administration of a rectal suppository or a capsule is a simple procedure which can be undertaken by unskilled persons and by the patient himself. The disadvantages are: • Incomplete and erratic absorption, and • Irritation of rectal mucosa by drugs. Endotracheal route: In patients who have an indwelling endotracheal tube, certain drugs (e.g. adrenaline, atropine, diazepam, naloxone lignocaine) can be administered by this route for an immediate effect. The drug is diluted in 5-10 ml of isotonic saline before administration. For optimal drug effect, computerised, miniature, syringe pumps are now available for: • Continuous administration of insulin and nitroglycerine; and • Intermittent, pulsed administration of GnRH. A drug may exert different effects when given by different routes. Thus, oral magnesium sulfate acts as saline laxative. When injected, it is a depressant of the CNS and acts as an anti-convulsant. On the other hand, hypertonic magnesium sulfate, given as a retention enema, can be used to reduce cerebral edema.
Absorption and Bioavailability Absorption is the process by which drugs enter the systemic circulation. Absorption of a drug from various sites, its movement among various body compartments and its distribution within the cell are all determined by the properties of the drug and those of biological membranes in the body. For understanding the drug absorption, drugs can be divided into three groups: (1) Those that do not ionise, are non-polar and lipid soluble and hence, are easily diffusible. (2) Those that always get ionised, are water soluble polar (lipid insoluble); and almost nondiffusible; and (3) Those that are partly ionised and partly non-ionised and hence partly water soluble and partly lipid soluble. Weakly acidic drugs remain unionised at acidic pH; whereas weakly basic drugs remain unionised at alkaline pH. Molecules, including drug molecules, cross a biological membrane by: (1) Simple or passive diffusion is the commonest means by which a drug gets absorbed and distributed in the tissues. In this process, the drug molecules move across the cell membrane, in proportion to their concentration, from higher to lower concentration. Cellular energy is not required and the system does not become saturated. Passive diffusion may be either lipid diffusion or aqueous diffusion. • Lipid diffusion: Drug molecules of lipid soluble (non-ionised) drugs dissolve in cell lipid membrane, and are rapidly transported across it to the other side of the membrane, down the concentration gradient. Lipid/aqueous partition coefficient of a drug decides the rate of absorption. In case of weak acids and weak bases, the absorption is dependent upon the pH of the medium. • Aqueous diffusion: Most biological membranes are relatively permeable to water. In the epithelial membrane of the gut, the cells are joined by tight epithelial junctions, and water passes through the cells rather than between them; this bulk transport of water carries with it water-soluble substances of small molecular weight (less than 700 daltons) such as urea and alcohol. In contrast, water is transported by filtration between the cells of the capillary endothelium except in the CNS. In the CNS, tight junctions create a blood-brain barrier. (2) Transport using transmembrane transporters: Transporters are proteins located across the mucosal cells of the intestine (enterocytes), hepatocytes, renal tubules and in the capillary endothelium of vital organs. They pick up an endogenous substance or a drug at one face of the cell and release it at another. They may be: (i) Solute carriers (SLC) which are either facilitatory transporters for nonionic solutes or ion-coupled secondary active transporters for neurotransmitter reuptake (Chapter 17); or (ii) ATP binding cassettes (ABC) family Membrane transporters are functionally of two types: (a) Uptake transporters which allow the transport of organic anions and cations into the cells; and (b) Efflux transporters which allow the transport of agents only out of the cells, even against high concentration gradient. Many efflux-active transporters belong to ABC family,
and utilise energy obtained from hydrolysis of ATP e.g. P-glycoprotein family. PGlycoprotein is expressed on both, the enterocytes and hepatocytes, where they serve as an efflux pump. It is also present at the BBB. Transmembrane transport occurs: • By facilitated diffusion in which an SLC spanning the cell membrane moves molecules down the chemical/concentration gradient; or moves cations into a negatively charged area, or anions into a positively charged area (i.e. down the electrical gradient). Examples are transport of glucose and amino acids into cells. • By active transport which requires input of energy: (a) Where a carrier protein moves molecules uphill against the chemical or electrical gradient e.g. iodide transport into the thyroid follicular cells against concentration gradient. Active, carrier mediated transport is also important in the case of molecules whose movement across a membrane would otherwise be unacceptably slow e.g. endogenous hormones, metabolites, neurotransmitters and nutrients, and drugs which structurally resemble them e.g. folic acid antagonists and hormonal analogues. (b) Via ion channels in the cell membrane (Chapter 2). Examples are inorganic ions such as Na+, K+, Ca++ and Cl–, and the ionised fraction of ionisable drugs. (c) By endocytosis, comprising: (i) Receptor mediated endocytosis e.g. insulin and LDL. (ii) Pinocytosis (cell drinking) where endogenous substances (e.g. immunoglobulins in the small intestine of the neonate) as well as large, highly charged drug molecules are engulfed by the cell membrane e.g. lipid droplets. Majority of the drugs, being lipid soluble, are absorbed by passive diffusion; a few by other modes. As a rule, drugs which are neither lipid soluble nor water soluble e.g. barium sulfate, are not absorbed from the gut. Information regarding the rate of absorption of a drug is necessary: • To determine the frequency of its administration. • To ascertain the duration of effective action; and • To predict the onset of desired or undesired effects of the drug. The time between the administration of a drug and the development of response is known as the biological lag. The route of administration determines the biological lag. Oral absorption mostly occurs in the upper GI tract. Drugs given orally may be inactive systemically because of: • Enzymatic degradation of polypeptides within the lumen of the GI tract e.g. insulin, adrenocorticotropic hormone (ACTH). • Poor absorption from the GI tract e.g. aminoglycoside antibiotics; or • Inactivation by first pass metabolism. Bioavailability of a drug is defined as the amount or percentage of an active drug that is absorbed from a given dosage form following its non-vascular administration, and reaches systemic circulation, to be available at the desired site of action. When the drug is given IV, the bioavailability is 100 %. Single dose bioavailability test involves an analysis of plasma or serum concentration of the drug at various time intervals after its oral administration and plotting a serum concentration time curve (Fig.1.3). The area under such a curve (AUC) provides
information about the extent (amount of drug absorbed) and the rate of absorption as well as the time required (Tmax) to achieve the maximum concentration (Cmax). The bioavailability ‘F’ is determined by comparing the AUC after oral administration of a drug with the AUC after IV administration of the same dose of the drug.
FIG. 1.3 The plasma drug level curves following oral administration of three formulations of the same basic drug. MTC = minimum toxic concentration, MEC = minimum effective concentration. The pink area indicates the therapeutic range. For formulation A and B, the areas under the curves are identical. However, formulation A would produce quick onset and short duration of action compared to formulation B whose effect would last much longer. Formulation C gives inadequate plasma levels and is, therefore, likely to be therapeutically ineffective.
Drug formulations are considered to be pharmaceutically equivalent if they contain the same active ingredients, and are identical in strength, concentration and dosage forms. The pharmaceutically equivalent drug formulations are considered to be bioequivalent when the rates and extent of bioavailability of the active ingredients in the two formulations do not differ significantly when tested by standard procedures. Such substances are likely to be therapeutically equivalent. Bioavailabilty is reduced if absorption is reduced or if the drug is metabolized before getting into the circulation. Factors affecting drug absorption and its bioavailability are listed in Table 1.4.
Table 1.4 Factors affecting oral drug absorption and its bioavailability
I Physical properties: • Physical state: Liquids are absorbed better than solids and crystalloids are absorbed better than colloids. • Lipid or water solubility: Drugs in aqueous solution mix more readily than those in oily solution with the aqueous phase at the absorption site, and hence are absorbed faster. However, at the cell surface, the lipid soluble drugs penetrate into the cell more rapidly than the water soluble drugs. Bile salts emulsify the fat soluble vitamins A and D in the small intestine and assist their absorption. II Dosage forms: • Particle size: The particle size of sparingly soluble drugs can affect their absorption. A tablet that contains large aggregates of the drug may not disintegrate even on prolonged contact with gastric and intestinal juices and hence, may be poorly absorbed. Small particle size is important for absorption of corticosteroids, antibiotics like chloramphenicol and griseofulvin, certain oral anticoagulants and spironolactone. By reducing the particle size, the dosage of the active drug can be reduced without lowering its efficacy. On the other hand, for an antihelminthic such as bephenium hydroxynaphthoate, the particle size should be large enough to reduce its absorption. Particle size is of no consequence in the case of freely water soluble drugs. • Disintegration time and dissolution rate: The effect of the physical factors is commonly evaluated by determining: (i) The disintegration time which measures the rate of break up of the tablet or the capsule into the drug granules; and (ii) The dissolution rate which is the rate at which the drug goes into solution. The disintegration time of a tablet is a poor measure of the bioavailability of the contained drug. This is because, in addition to disintegration time and particle size, other factors such as crystalline form (polymorphism), saturation solubility and solvation can modify the bioavailability of a drug. The dissolution rate is perhaps a better parameter. • Formulation: The method of formulation can markedly influence the drug absorption and thus determine its bioavailability. Usually, substances like lactose, sucrose, starch and calcium phosphate or lactate are used as inert diluents in formulating powders or tablets. Such fillers may not be totally inert but may affect the absorption as well as stability of the medicament. Thus, calcium and magnesium ions reduce the absorption of tetracyclines, while calcium phosphate used as a diluent for calciferol has caused calcium toxicity, when given in large doses. Replacement of calcium phosphate by lactose made a marked difference in the efficacy of a reformulated phenytoin preparation. A faulty formulation can render a useful drug therapeutically useless. The study of the influence of
formulation on the therapeutic activity of drugs is known as biopharmaceutics. III Physiological factors: • Ionisation: The mucosal lining of the GI tract is impermeable to ionised form of weak organic acids and weak organic bases. At the body pH, most drugs exist in two forms: (1) an unionised component, predominantly lipid soluble; and (2) an ionised, water soluble component. The unionised fraction can cross the cell membrane rapidly. The amount of the drug which crosses the gut wall is determined by the gradient of its concentration between the lumen of the gut and the portal venous blood. If the plasma concentration of a drug present in a free, unionised form, is rapidly reduced by binding with plasma proteins, its absorption from the gut lumen is enhanced. • pH of the GI fluid and the blood: Weakly acidic drugs are rapidly absorbed from the stomach as they exist in the acidic medium of the stomach in an unionised form. They act rapidly on oral administration e.g., salicylates and barbiturates. However, most of the weakly acidic drugs are also absorbed from the duodenum because of their solubility in the alkaline medium and the large absorbing surface area. Weakly basic drugs are not absorbed until they reach the alkaline environment of the small intestine. The alkaline environment, in which the drugs exist in an unionised form, facilitates their absorption. Their actions are delayed when administered orally e.g. pethidine and ephedrine. At the pH values found in the intestine, the strongly acidic or basic drugs are highly ionised and hence, they are poorly absorbed. Aminoglycosides are strong bases and hence, their absorption from GI tract is poor. • GI transit time: The presence of food, and the volume, viscosity and tonicity of the gastric contents can influence drug absorption by altering the gastric emptying time. Rapid absorption occurs if the drug is given on an empty stomach. Table 1.5 shows the effect of food on the GI absorption of drugs. Table 1.5 The effect of food on drug absorption
Increased peristaltic activity, as in diarrhoea, reduces the drug absorption. Anticholinergic drugs, which prolong gastric emptying time, also impair absorption of drugs. • Enterohepatic cycling: involves drug excretion into the intestine after its absorption, followed by its reabsorption. This increases the bioavailability of a drug, e.g., morphine. • Area of the absorbing surface and local circulation: Drugs are absorbed better from the small intestine than from the stomach because of the larger surface area of the former. Reduction in the absorbing surface following major GI resection, reduces the drug absorption. Increased vascularity can increase absorption. • First pass elimination: The bioavailability of certain drugs is reduced by rapid metabolic degradation during the first passage through the gut wall (isoprenaline) or the liver
(propranolol). The other examples are opioids, beta-adrenergic blockers, progesterone, isosorbide dinitrate etc. • Presence of other agents: Vitamin C enhances the absorption of oral iron, while phytates retard it. The absorption of fat-soluble vitamins is reduced in the presence of liquid paraffin, whereas cholesterol absorption is reduced by sitosterol. Calcium, present in milk and in antacids, forms insoluble complexes with the tetracyclines and reduces absorption. IV Pharmacogenetic factors: See later. V Disease states: Structural changes in the GI mucous membrane result in malabsorption syndrome. Gastrointestinal mucosal edema significantly depresses the absorption of drugs such as hydrochlorothiazide in patients with congestive heart failure. Absorption and first pass metabolism may be affected in conditions like thyrotoxicosis, achlorhydria, cirrhosis of the liver and biliary obstruction. The only valid tests of bioavailability of a drug preparation are: • The levels of the drug in biological fluids such as plasma, urine and saliva; and • An objectively measurable parameter of its therapeutic efficacy, e.g. heart rate and BP. Therapeutically, bioavailability is more important in the case of drugs with a narrow therapeutic index e.g. digoxin and aminophylline.
Distribution of a Drug After absorption, a drug enters or passes through the several body fluid compartments depending upon its physicochemical properties (Table 1.6). They are: Table 1.6 Distribution of drugs in different body compartments Body compartment • Total body water (60%) • Extracellular space (20%) • Intravascular space (5%) • Body fat (2–5% in men; 10–13% in women) • Bones (12–15%)
Types of drugs S mall, water soluble molec ules suc h as alc ohol and antipyrine. Large water soluble molec ules suc h as mannitol. Very large, strongly protein-bound molec ules suc h as heparin. Highly lipid soluble molec ules suc h as DDT and thiopentone. Fluoride and lead.
• Plasma • Interstitial fluid compartment • Transcellular fluid compartment, e.g., fluids in the GI tract, bronchi, CSF • Intracellular fluid compartment The apparent volume of distribution (Vd) is defined as the volume into which the total amount of a drug in the body appears to be uniformly distributed. It is calculated as the total amount of drug in the body divided by the concentration of the drug in the plasma at zero time. For many drugs, (Vd) is constant over a wide dosage range.
Some drugs pass into the cells whereas others are distributed extracellularly. However, a drug can penetrate into and exist in more than one compartment. The rate of passage of a drug through a membrane is dependent upon the pH of the body compartment and the dissociation constant (pK) of the drug. pK is the pH at which the nonionised and ionised drug concentrations are equal. Nonionised, lipid soluble drugs (the vast majority) readily cross membranes and are distributed throughout the body; they have large volumes of distribution. On the other hand, drugs which are highly protein bound (e.g. warfarin) or ionized (gentamicin) remain largely within the vascular compartment and have very low volumes of distribution. A drug with Vd = 16 litres is likely to be distributed in ECF water, which includes plasma and interstitial fluid. Where the Vd exceeds the total volume of body water (42 litres), there is a possibility of a drug accumulating in a tissue e.g. Vd for digoxin is 420 litres as it accumulates in the skeletal muscles. Chloroquine exhibits Vd of 13000 litres as it is concentrated in the liver. Such drugs cannot be easily removed by dialysis. Plasma concentration of a drug: This depends upon the drug’s • Rate of absorption • Distribution
• Metabolism; and • Excretion After absorption, the drug circulates in the blood either in the free form or bound to plasma proteins-either albumin or alpha-acid-glycoprotein. These proteins are termed as acceptors. Albumin is the main binding protein for many endogenous substances and drugs. The fraction bound to protein usually falls as the total concentration of the drug increases and the binding sites get saturated. Table 1.7 lists the effects of protein binding on drugs. Table 1.7 Effects of protein binding on drugs
Binding of drugs to plasma proteins assists absorption. Diffusion across the intestinal wall continues as long as the concentration within the gut exceeds that of the unbound fraction in the portal capillaries. Protein binding: • Acts as a temporary ‘store’ of a drug and tends to prevent large fluctuations in concentration of unbound drug in the body fluids. • Reduces diffusion of the drug into the cells and thereby delays its metabolic degradation e.g. 90 % of long-acting sulfonamides and 98% of warfarin circulate in bound form whereas protein binding is negligible with ethosuximide and amoxycillin. • Reduces the amount of drug available for filtration at the glomeruli and hence delays its excretion. • Reduces the drug clearance. • Reduces concentrations of free drug to be available for desirable effect e.g. highly protein bound sulfonamides like sulfadoxine may have too low concentration in interstitial fluid, CSF and tissue cells to combat dangerous infections. While prescribing a new drug such as an antibiotic claimed to achieve higher and prolonged plasma concentration than a previously available drug, one should ascertain the degree of protein binding. With extensively protein bound drugs, the therapeutic activity may be low. The extent of drug binding depends on the binding protein concentration in the plasma. Thus, in pregnancy, the protein bound fraction of thyroxine increases due to a rise in the concentration of the specific binding protein in the plasma. Conversely, in hypoproteinaemia, there is a rise in the free fraction due to low plasma albumin levels; the therapeutic dose required may thus be smaller. Administration of drugs which get bound to the same binding sites on plasma proteins may result in a sudden increase in the free concentration of one of them, possibly to a dangerous level. Thus, if a patient, stabilised on the anticoagulant warfarin takes salicylates in addition, a sudden increase in free concentration of warfarin due to its displacement from the binding sites by salicylates may result in haemorrhage (Chapter 3).
Drug storage: The concentration of a drug in certain tissues such as fat and liver after a single dose may persist even when its plasma concentration has decreased to low or undetectable levels. Thus, the hepatic concentration of mepacrine within 4 hours after its oral administration is 200 times that of the plasma level. This concentration may reach a very high level on chronic administration. Iodine is similarly concentrated in the thyroid tissue. Membrane transporters are involved in drug targeting to a specific tissue e.g. metformin concentrates in the liver with the help of SLC type influx transporters. Many lipid soluble drugs are stored in the body fat depots e.g. on IV administration, 70% thiopentone is taken up by the body fat from which it is released slowly. Because of such storage, repeated exposure to certain chemicals (e.g. DDT), even in small doses, may lead to chronic toxicity. Although termination of drug effects mainly occurs due to biotransformation and excretion, it may also result from redistribution of the drug from its site of action into other tissues or sites. Placental transfer: The passage of drugs through the placenta into the fetal circulation is determined by the properties of the drug, the properties of the placenta and the altered maternal blood levels due to changing pharmacokinetics in pregnancy. There are a number of influx transporters in placenta making the placental barrier imperfect. The effect on the fetus is determined by its gestational age (Chapter 80). Blood-Brain Barrier (BBB): The composition of the CSF and the extracellular (EC) fluid in the brain differs significantly from that of the plasma. A specialised system not only maintains the special composition of brain fluid in the face of fluctuating composition of the plasma, but also protects the brain from toxic substances. Unlike in the capillaries of the peripheral circulation, the endothelial cells of the brain capillaries do not permit bulk passage of water and solutes between the endothelial cells which are joined to each other by continuous tight junctions. Only what can pass through the endothelial cells is allowed to pass (diffusion barrier). Epithelial cells of the choroid plexus also have tight junctions and constitute the Blood- CSF-Barrier. These barriers allow free passage of lipid soluble drugs such as diazepam. Ionisable organic molecules (which many drugs are) are largely denied such passage from blood into the brain. However, substances can pass freely between CSF in the subarachnoid space and the ECF in the brain; hence, drugs such as antibiotics introduced into the CSF can enter the brain ECF in adequate concentration. Drugs may penetrate into the brain using uptake transporters for endogenous substances and nutrients. Apart from the anatomical features, functional BBB and blood-CSF-barrier involve membrane transporters such as p-glycoproteins (MDR1) and organic anion transporting polypeptides (OATP) which extrude a large number of structurally diverse drugs and protect the brain from their adverse effects. The metabolites of brain neurotransmitters and organic ions are extruded into the CSF at the level of the choroid plexus by mechanisms that are similar to those in the renal tubules. Membrane transporters in the choroid plexus actively secrete drugs from the CSF into the blood. Drugs and endogenous metabolites, irrespective of their molecular size and lipid solubility, exit with bulk flow of the CSF through the subarachnoid villi into the venous sinuses in the brain. The classical BBB, however, is absent in certain areas of the brain such as the pineal gland and the area postrema of the fourth ventricle. These areas act as brain sensors for
changes in the composition of the plasma. The BBB becomes less efficient in the presence of inflammatory diseases such as meningitis and encephalitis. Parenterally administered antibiotics can then reach therapeutic concentrations in the brain EC fluid. When the infection is controlled and the inflammation subsides, the BBB tends to be restored. As that may happen while viable microorganisms persist in the CSF, drug dosage should not be reduced till the CSF is sterilised.
Fate of a Drug The changes that a drug (foreign substance to body- xenobiotic) undergoes in the body and its ultimate elimination are considered as the fate of the drug. Alteration of a drug within a living organism is known as bio-transformation. After absorption, drugs could undergo three possible fates: • Metabolic transformation by enzymes: which may be microsomal, cytosolic or mitochondrial. The metabolism of drugs usually: (1) Inactivates an active drug; or (2) Activate an inactive drug (prodrug); or (3) Generate active metabolite(s) of an active drug. (Table 1.8). Table 1.8 Effects of biotransformation on drugs
• Spontaneous change into other substances without the intervention of enzymes e.g. the anti-cancer drug mechlorethamine (a prodrug) changes spontaneously into the active ethyleniminium cations at the slightly alkaline pH of the plasma; or similar inactivation of the muscle relaxant atracurium (Hofmann reaction). • Excreted unchanged: If a drug is already highly polar and water soluble, it is not metabolised and gets excreted as such, e.g. aminoglycosides. There are many tissues which can metabolise drugs, but by far the most active tissue per unit weight is the liver. The enzymes which metabolise drugs are distinct from those which function in the intermediary metabolism. Hepatic microsomal enzymes: These enzymes are located in the liver microsomes which form a part of the smooth membrane of the endoplasmic reticulum of the hepatic cells. Among these enzymes are those which catalyse a variety of oxidative and reductive reactions e.g., superfamilies of enzymes-cytochrome P 450 (CYP), flavin containing monooxygenases (FMO) and epoxide hydroxylases (EH) as well as some phase II enzymes like esterases, amidases, glucuronyl transferases. Microsomal enzyme systems are accessible only to substances with a high oil/water partition coefficient. These enzymes alter drugs to make them more polar and water soluble, so that they can be excreted by the kidneys. Animal species vary not only in the kinds of microsomal enzymes they possess but also in their quantitative distribution. • CYPs are involved in the metabolism of many dietary and xenobiotic compounds (Chapter 3) and in synthesis of endogenous agents (e.g. steroids, bile acids from cholesterol). CYP450 is so named because it absorbs light maximally at 450 nm. A drug bound to cytochrome P450 may be either oxidised or reduced. There are many isozymes of the enzyme CYP450, each of which is encoded by a separate gene; 50 are functionally active. Variations in their gene structure explain the differences in the drug metabolism among
different individuals and ethnic groups. The naming of the isozymes follows an orderly pattern e.g. in the name CYP3A4, 3 stands for the family, A for the subfamily, and 4 for the chromosome encoding gene. CYP3A4 is involved in the metabolism of several drugs, followed by CYP2D6. The other important isoenzymes are CYP2C9, CYP2C19 and CYP1A2. • FMOs are minor contributors to drug metabolism. H2 receptor antagonists, clozapine, itopride are metabolised by them. The metabolites are benign and cause no drug-drug interaction. • EH deactivates potentially toxic metabolites produced by CYPs e.g. carbamazepine 10,11 epoxide, an active metabolite of the carbamazepine is inactivated by microsomal EH. Non-microsomal enzymes: Drugs are also metabolised by non-microsomal enzymes, present in liver, plasma and tissues including placenta and even by those present in the intestinal micro-organisms (microfloral enzymes) e.g. MAO, alcohol dehydrogenase, xanthine oxidase. The xenobiotic enzyme reactions involved in metabolic transformations are: Phase I (Non-synthetic): • Oxidation • Reduction • Hydrolysis; and Phase II (Synthetic): • Synthesis (conjugation or transfer reactions). Phase I reactions: Oxidation, reduction and hydrolysis introduce polar groups such as hydroxyl, amino, sulfhydryl and carboxy into drugs which are consequently made water soluble and pharmacologically less active. Thus, metabolism of drugs is essentially a detoxification process. However, during the initial stages of metabolism of certain drugs, active and even toxic compounds may be produced. Thus, parathion, an insecticide, is quite inactive in itself but is converted in the body to paraxone, the active toxic compound; similarly, imipramine, an antidepressant drug is bio-transformed into an active compound desipramine; cyclophosphamide, sulindac and enalapril are activated by oxidation (Table 1.8). Oxidation: A drug may be oxidised by more than one mechanism and for the same drug this may differ in different species of animals. The reactions include: • Microsomal oxidation which involves: (i) Hydroxylation, wherein hydroxyl group is introduced into the drug molecule e.g. conversion of salicylic acid to gentisic acid; or (ii) Dealkylation, wherein an alkyl group is removed e.g. conversion of phenacetin to the active compound p-acetaminophenol; or (iii) Deamination, wherein an amino group is removed e.g. conversion of amphetamine to benzyl-methyl-ketone. • Non-microsomal oxidation: e.g. ethyl alcohol is oxidised to carbon dioxide and water. Methyl alcohol is oxidised to toxic formic acid and formaldehyde. • Mitochondrial oxidation: A mitochondrial enzyme monoamine oxidase (MAO) causes oxidative deamination of substances like adrenaline, 5-HT and tyramine. Reduction: Many halogenated compounds and nitrated aromatic compounds are
reduced by the microsomal enzymes e.g. halothane and chloramphenicol; drugs like chloral hydrate, disulfiram and nitrites are reduced by non-microsomal enzymes. Hydrolysis: This is usually carried out by enzymes ‘carboxy esterases’ that hydrolyse (split with addition of water) the esters and amide containing compounds. These enzymes are microsomal, non-microsomal and microfloral in origin. They are usually of low specificity and exhibit considerable species variation. Drugs like pethidine, procaine, acetylcholine, diacetylmorphine, atropine, neostigmine and phenytoin, are hydrolysed by esterases. Digitalis glycosides are rendered inactive by hydrolysis. Methanamine mandelate, an urinary antiseptic, is hydrolysed in the urinary tract, at an acid pH, to formaldehyde and ammonia. Phase II reactions: Conjugation or transfer reaction: This is a synthetic process by which a drug or its metabolite is combined with an endogenous substance, resulting in various conjugates such as glucuronides, ethereal sulphates, methylated compounds and amino acid conjugates. Conjugation invariably results in inactivation of the compound. After such inactivation, large molecules are eliminated in the bile whereas smaller molecules are excreted in the urine. Glucuronides are produced by the combination of a hydroxyl, carboxyl or amino group of drug molecule with glucuronic acid. Compounds like morphine, paraamino benzoic acid (PABA), stilboesterol, salicylic acid and phenol are excreted mainly in the form of glucuronides. Ethereal sulphates are produced by the combination of sulphate and hydroxyl or amino group. A classical example of amino acid conjugation is the combination of benzoic acid with glycine to form hippuric acid. A drug may be metabolised and inactivated by more than one successive reaction e.g. progesterone is first reduced to pregnanediol which is then conjugated; chloramphenicol is similarly reduced and then conjugated. In practice, patients may differ in their response to a standard dose of a drug. This is largely due to variations in the rate of drug metabolism among individuals. Factors affecting drug metabolism are listed in Table 1.9. The major factors are: genetic, environmental and disease related. Table 1.9 Factors affecting drug metabolism
The ability of the microsomal enzymes to metabolise drugs is poor in premature infants and neonates as compared to adults. Hence, the liver of a premature infant is unable to conjugate chloramphenicol to the same extent as in adults, resulting in very high serum concentration of chloramphenicol causing toxicity. Undernutrition also depresses the functional capacity of these enzyme systems and this should be borne in mind particularly
in countries where undernutrition is common. The ability of the diseased liver to metabolise drugs diminishes. Drugs like pethidine and morphine which are metabolised in the liver may thus have an unusually prolonged action in hepatic cirrhosis. Reduction in hepatic blood flow in shock and congestive heart failure can cause marked reduction in the metabolic degradation of lignocaine. Certain agents such as ethanol, barbiturates, on repeated administration, stimulate the synthesis of microsomal enzyme system. This is called enzyme induction. It takes 2-3 weeks to induce enzymes. Once the enzymes are induced they metabolise drugs which are their substrates, more rapidly. Thus, exposure to the insecticide DDT accelerates the biotransformation of drugs, leading to their faster elimination. Enzyme induction also occurs to a limited extent in the kidney, lung, skin and gut. For enzyme inhibition, see Chapter 2.
Drug Excretion Drugs, except the volatile general anaesthetics, and metabolites of drugs are usually excreted by a route other than that of absorption. The important channels are: Kidneys : The processes which determine the elimination of a drug in the urine are: • Passive glomerular filtration: Only the unbound fraction of unionized drugs is filtered at the glomerulus; but they are reabsorbed by diffusion back from the tubular lumen into the cells lining the tubules. Thus, ultimately a very small amount of the drug appears in the urine. Ionised drugs which are poorly absorbed are excreted almost entirely by glomerular filtration and are not reabsorbed. • Active tubular secretion: Many weak acids (anionic substances) and weak bases (cationic substances) are actively secreted by proximal tubules by carrier-mediated systems involving transporters such as p-glycoprotein and the multidrug-resistance-associated protein type 2 (MRP2). These transporters are also responsible for excretion of conjugated metabolites of drugs (Table 1.10). Table 1.10 Some drugs secreted by proximal tubule into urine
Tubular secretion of weak organic acids such as penicillin can be blocked by probenecid, and their half-life can be prolonged. Secretion of weak bases by renal tubules can also be blocked but the blocking agents are too toxic for any therapeutic utility. • Passive renal tubular reabsorption: Passive diffusion is a bidirectional process and drugs may diffuse across the tubules in either direction depending upon the drug concentration, lipid solubility and the pH e.g. salicylates. The pH of the urine influences the excretion of certain weak acids and weak bases. Thus, weak acids are quickly eliminated in an alkaline urine e.g. barbiturates and salicylates; while weak bases are rapidly excreted in an acidic urine e.g. pethidine and amphetamine. On the other hand, the action of these substances in the body can be prolonged if the urinary pH is not favourable for their excretion. The tubular reabsorption of weak acids is minimum when the urine is alkaline because a large portion of these compounds is ionised in an alkaline medium. Similar is the case with weak bases in acid urine. Elimination of weak acids and weak bases can thus be accelerated by: • Maintaining a high rate of urine flow by the use of diuretics; and • Adjusting the urinary pH. In the presence of renal damage, the ability of the kidney to excrete drugs is impaired.
This might result in unacceptable high blood levels and prolonged drug action with normal doses. Great care must, therefore, be exercised when drugs like aminoglycosides or coumarin anticoagulants are used in the presence of kidney damage. Similarly, potassium salts may produce dangerous hyperkalemia if the kidney function is impaired. Some other drugs, the dose of which must be adjusted in renal failure are: (a) Only in severe renal failure: Co-trimoxazole, carbenicillin, cefotaxime, metronidazole and fluoroquinolones; and (b) Even in mild renal failure: Cephalexin, ethambutol, amphotericin B, acyclovir and flucytosine. Protein binding reduces the amount of the drug available for filtration at the glomerulus but protein bound drugs may still be available for secretion by the proximal renal tubules, e.g., phenylbutazone. This is because the bound form of the drug is released from its combination with plasma proteins when the plasma concentration of the free form of the drug is lowered. Since drugs, metabolites and toxins are concentrated in the kidneys during their excretion, this organ is frequently the site of drug-induced renal toxicity. Lungs: Volatile general anaesthetics and drugs like paraldehyde and alcohol are partially excreted by the lungs. Their presence can be recognised by the odour they impart to the breath. Bile: Transport systems similar to those in the kidneys are present in the hepatocytes which actively secrete drugs and their metabolites into the bile. Drugs such as phenolphthalein, doxycycline and cefoperazone appear in high concentrations in the bile. Such drugs may get repeatedly reabsorbed from the intestine and re-excreted in bile, thereby exerting a prolonged action (enterohepatic circulation). Intestines: Drugs and their metabolites can be actively secreted from the systemic circulation into the intestinal lumen using transporters such as p-glycoprotein present in the enterocytes. Further, drugs can passively diffuse from the blood into the intestinal lumen, depending on their pK and the luminal pH. Laxatives like cascara and senna, which act on the large bowel are partly excreted into that area from the blood stream, after their absorption from the small intestine. Heavy metals are also excreted through the intestine and can produce intestinal ulceration. Skin: Arsenic and heavy metals like mercury are excreted in small quantities through the skin. Arsenic gets incorporated in the hair follicles on prolonged administration. This phenomenon is used for detection of arsenic poisoning. Saliva and milk: Certain drugs like iodides and metallic salts are excreted in the saliva. Lead compounds deposited as lead sulfide produce a blue line on the gums. Excessive salivation is a frequent symptom of chronic, heavy metal poisoning. Secretion of drugs in milk is discussed in Chapter 80.
Plasma Half-life and its Significance Information about the time course of drug absorption, distribution and elimination (Pharmacokinetics) can be expressed in mathematical terms. Pharmacokinetic parameters aid in the selection and adjustment of drug dose schedules. However, they are not a substitute for, but rather a supplement to, clinical monitoring and judgement. These parameters include bioavailability, Vd, half-life (t½) and clearance. Significance of bioavailability and Vd is discussed earlier. Clearance and dose determine the magnitude of the steady state. Drugs are eliminated from the body by: • First order kinetics; or • Zero order kinetics. Elimination of most drugs occurs exponentially (first order kinetics) i.e. a constant fraction of the drug in the body disappears in each equal interval of time. Thus, following a single IV dose, the plasma concentration of the drug falls exponentially (Fig 1.4a). That is, the drug is removed from the body not at a constant rate but at a rate proportional to its plasma concentration. In the case of an exponentially eliminated drug, a plot of the log of concentration against time gives almost a straight line. The rate of an exponential process may be expressed either:
FIG. 1.4(A) Exponential curves of plasma concentration of a drug following oral and IV administration. The slope is independent of the route of administration (First order kinetics)
(a) in terms of its rate constant (K) which expresses the fractional change per unit of time, or (b) in terms of its half time (t½), the time required for 50% completion of the process (elimination half-life, plasma half-life). Half-life of a drug can be computed using
following formula:
With drugs whose elimination is exponential, the elimination half-life is independent of the dose, the route of administration and the plasma concentration (C). It depends on Vd as well as on the metabolism and the renal excretion (t½ = 0.693 x Vd/C). However, the actual quantity of the drug removed per unit time is smaller at lower plasma concentrations and larger at higher plasma concentrations. Further it should be noted that t½ during long term oral administration of a drug may be different from that after a single IV dose. Reduced clearance of the drug due to disease is expected to prolong biological half-life and the drug effect. This reciprocal relationship is valid only as long as the Vd of the drug does not change. Simple calculation shows that 93.75% of the drug is eliminated in four half lives. Since more than four half lives are required for complete exponential elimination, repeated dosing intervals shorter than this leads to drug accumulation. Thus a drug administered in equal doses, intermittently, at constant time intervals, will accumulate exponentially to a plateau plasma level (steady state). The concentration at steady state(SS) = Dose rate/Clearance. After the plateau is reached, drug elimination equals drug absorption during the dose interval (Fig. 1.4b). The time taken to attain the steady state depends only upon on its half-life (t½). It takes five elimination half-lives for the drug to reach the plateau level in the plasma. Thus, in practice, dobutamine with t½ of 2 minutes, given by infusion reaches a steady state in 10 minutes, whereas it takes 7.5 days for oral diazepam (t½ 36 hr).
FIG. 1.4(B) Rise of drug concentration to a plateau (steady state) level during repeated oral administration of a constant dose. Peaks are the high points of the fluctuations whereas troughs are the low points of the fluctuations.
The drug concentration maintained during the steady state is directly proportional to both elimination t½ and the quantity of the drug given per unit time (as dose/dosage interval). Dose of a drug: It is the specific amount of medication to be taken at a given time. The average daily amount of a drug that is actually prescribed is termed as prescribed daily dose (PDD). It is based on the pharmacokinetic considerations and also varies with patient characteristics and severity of the disease (Chapter 3). To compare drug use across different countries or different health care facilities in a given country, a measurement unit has been introduced by WHO, which is termed as defined daily dose (DDD). DDD is an assumed average maintenance dose per day of a drug used for its main indication in adults. This standard is used in pharmacoepidemiological studies to measure drug consumption in a given population. It does not necessarily reflect the recommended or prescribed daily dose. Dose determination: Sometimes, for achieving the therapeutic drug concentration, a loading dose of the drug is administered, followed by maintenance doses. The loading dose is defined as the one or series of doses given initially and are higher than the subsequent doses. It is administered for achieving the desired plasma concentration rapidly. A loading dose is a product of Vd and desired plasma concentration. Its disadvantage is the possible toxic effect caused by large initial dose particularly in sensitive individuals. The subsequent doses are required for maintaining a steady state plasma drug concentration in the therapeutic range. In order to prevent unduly high plasma levels of a drug when its elimination is reduced in a patient with impaired hepatic, renal or cardiovascular function, the maintenance dose must be reduced. This may be achieved either by reducing each individual dose or by lengthening the dosage interval in proportion to the increase in the biological half-life. Dosing frquency: When a drug with rapid absorption or short biological half-life is administered repeatedly in the same dose at fixed, long intervals of time, marked fluctuations in the plasma concentration (during plateau state) may occur between the doses. These fluctuations can be reduced by giving the drug at shorter intervals, say by giving half doses at half intervals. Thus, during levodopa therapy, steady plasma levels and
a steady clinical response can be maintained only by giving the drug in at least four divided doses per day. Insulin with a very short t½ of a few minutes is best administered by a continuous, IV infusion for maximum efficacy in diabetic coma. On the other hand, in drugs with long half-lives these fluctuations in the plasma concentration at plateau are less marked and hence these drugs may be administered at longer intervals. Thus, for maintenance therapy, digoxin, thyroxine and chlorpropamide may be given once a day to maintain a steady response. In the case of some drugs (human growth hormone and propranolol), the pharmacological effects may in fact last much longer than is suggested by their t½. With some drugs (e.g. allopurinol) this may be due to the formation of an active metabolite (e.g. oxypurinol) with a long t½. Such drugs can therefore be given at much longer intervals than their t½ would indicate. Benzylpenicillin, although its t½ is short (30 minutes), is effective in a six hourly dosage regimen. This is so because it is possible to give the drug in such large doses that the lowest concentration achieved in such a regimen is far in excess of the minimum effective concentration, because of a wide margin of safety. With certain drugs such as phenytoin, alcohol, dicoumarol, probenecid, oral propranolol, and large doses of salicylates the elimination is exponential with lower dosage levels, but when the dose exceeds a certain critical level, the eliminating mechanisms get saturated and then a fixed quantity of the drug is eliminated per unit time. This is called dose dependent elimination or saturation kinetics or zero order kinetics. With such drugs, an increase in the dose can cause an increase in the biological half-life and a disproportionate increase in the plasma level. This can result in drug toxicity (Fig. 1.5).
FIG. 1.5 Comparison of non-saturation and saturation kinetics for drugs, given orally, 12 hourly. Fig. A shows plasma concentration curves of a drug that follows non-saturation kinetics (first order). Note that the steady state plasma concentration is directly proportional to the dose. Fig. B shows curves for a drug that follows saturation kinetics (zero order) e.g. phenytoin. Note that no steady state is reached with high doses and that even a small increase in the dose results in a disproportionately large (? toxic) plasma concentration.
The concentrations of many drugs at their site(s) of action are in equilibrium with their plasma concentration. Hence, the therapeutic response to such drugs correlates better with plateau plasma levels than with dosages. Variations in plasma levels following similar doses (on weight basis) observed in different subjects are due to variations in the rates of pharmacokinetic processes either due to disease condition or genetic differences. With drugs such as theophylline and phenytoin, there may be as much as 3-5 fold variation in the plasma concentration achieved in different individuals given similar doses. Further, drug concentration in certain tissues may persist even when the plasma drug concentration is low or undetectable after stopping the drug, thus giving a prolongation of the effect e.g. digoxin in the ventricular muscle. Therapeutic Drug Monitoring (TDM): In practice, routine measurement of plasma drug levels is cumbersome, expensive and impractical. However, with drugs whose therapeutic index is distinctly related to plasma levels, TDM may be useful e.g. (1) For guiding the effective therapy and reducing the risk of ADR (Table 1.11). Table 1.11 Some drugs with low therapeutic index and high rate of ADR
(2) For treating drug poisoning; and (3) For checking patient’s compliance. The timing of the blood sampling should be soon after the dose or immediately prior to the next dose. TDM is not required for drugs whose dose can be correlated with clearly measurable indices such as BP, blood sugar, urine volume or blood coagulation parameters. Similarly, it is of no value when the plasma drug concentration correlates poorly or not at all with the drug effect as with anticancer drugs, tricyclic antidepressants and benzodiazepines (multiple active metabolites).
Methods of Prolonging the Duration of Action of a Drug The drug action can be prolonged by: • Retarding drug absorption. • Retarding drug metabolism in the liver. • Retarding renal excretion of the drug. • Using compounds which are highly protein bound; and • Modifying the molecular structure. Retarding drug absorption: Oral absorption of a drug can be retarded by administering it on full stomach or by giving it in various coated formulations e.g. sustained release preparations of nitroglycerine. Such formulation, however, does not always prolong the action of a drug (Also, see ‘Routes of Drug Administration and Dosage Form’ earlier). Absorption of a drug after parenteral administration can be retarded by: • Reduction in the vascularity of the absorbing surface: This can be achieved by administration of a vasoconstrictor along with the drug, e.g., adrenaline with procaine. • Reduction in the solubility of the drug: This can be achieved by combining the drug with a compound having poor water solubility or giving the drug in a suspension form. Thus, penicillin is combined with procaine, a compound with poor water solubility. • Administration of the drug in oily solution or in combination with beeswax: e.g. pitressin tannate in oil. Mixing of the drug with a water repellent like aluminium monostearate also delays the absorption, as in the case of penicillin with aluminium monostearate. • Combination of the drug with a protein from which it is released slowly, e.g., protamine zinc insulin. • Esterification: Steroidal sex hormones such as testosterone and estrogens, when esterified with carboxylic acids, give compounds such as benzoate, propionate, enanthate and cypionate which are absorbed slowly. • Pegylation i.e. combination with polyethylene glycol (PEG) e.g., interferons. • Depot preparations (e.g., DOCA in Addison’s disease) or of steroid filled silastic capsules (e.g., progestogens for contraception). • Long acting dermatological preparations such as nitroglycerine ointment and transdermal discs; transdermal patches of estradiol, scopolamine and glucocorticoids. Retarding drug metabolism: The hepatic microsomal enzyme systems concerned with metabolism may be inhibited by certain drugs, such as monoamine oxidase inhibitors leading to prolonged drug action. The action of levodopa can be extended by combining it with a dopa decarboxylase inhibitor which inhibits its metabolism in the blood. However, inhibition of biotransformation may alter the milieu interior of the body by delaying the inactivation of endogenous products like the steroid hormones. Retarding renal excretion of the drug: Drug excretion by glomerular filtration cannot be blocked or slowed without producing harmful effects on the kidney, but the tubular secretion of certain drugs can be blocked by using compounds which share the same tubular secretory pathway. Thus, probenecid is used to reduce the penicillin excretion (Chapter 46). Increased protein binding of the drug in the plasma: Long-acting sulfonamide sulfa-
methoxypyridazine is bound to plasma proteins more extensively than short-acting sulfadiazine. Suramin, used in the treatment of trypanosomiasis, is extensively bound to the plasma proteins. Such drugs have prolonged action. Drugs that are sequestered in the adipose tissue (such as quinestrol, a cyclopentyl ester of estradiol) have a prolonged action.
Special Drug Delivery Systems Various special drug delivery systems which incorporate drugs in a dosage form that releases the medication at a predetermined site or at a predetermined rate, over an extended period of time from a single application, have been developed. Some of them are: • Devices for slow, prolonged release of a drug for topical action such as ocusert, progestasert and drug-eluting stents. • A device for rapid delivery of anti-convulsant lorazepam to the CNS (Chapter 9). • Prodrugs • Targeted delivery systems; and • Liposomes Ocusert, when placed under the eyelid, delivers a steady flow of pilocarpine round the clock for seven days without causing any discomfort, and avoiding the need for repeated eye drops. Progestasert, an intrauterine contraceptive device, produces controlled release of minute quantities of progesterone within the uterus for a year. Drug-eluting stents (DES): Such stents consist of a metallic stent backbone covered with a polymer, containing a drug (sirolimus or paclitaxel). The drug is gradually released over the next 14-30 days and modifies the local healing response within the stented artery. Used during coronary angioplasty and stenting, they help to reduce the incidence of restenosis. Prodrug is an inactive chemical compound that, after administration, undergoes biotransformation to the pharmacologically active drug. Such prodrugs may overcome the barriers limiting the usefulness of a drug. These barriers could be in: • The pharmaceutical phase; or • The pharmacokinetic phase For example, chloramphenicol palmitate is preferred in paediatric practice because of its less bitter taste. Since dopamine does not cross the BBB 1-dopa is used to treat Parkinson’s disease, to increase the bioavailability of dopamine in the CNS. Altering the polarity of ampicillin by esterifying ampicillin to form talampicillin improves its bioavailability. Prodrugs may also be used to achieve longer duration of action e.g. esters of antipsychotic phenothiazines like fluphenazine. Another important use of prodrugs is to provide site-specific delivery of drugs. Thus, methenamine is a prodrug for formaldehyde; it is converted to formaldehyde and ammonia at the acidic urinary pH and acts as a urinary tract antiseptic. Targeted delivery of anti-cancer drugs using monoclonal antibodies against cancer cell antigens is one of the innovations in drug delivery systems. These antibodies ‘home’ in on the cancer cells and deliver lethal concentrations of the drug selectively to the cancer tissue. Liposomes, another vehicle for targeted drug delivery, are concentric, spherical shells of phospholipids in a watery medium, into which drugs are incorporated. They are administered by the IV route. Drugs which have been administered via liposomes are anticancer drugs (daunorubicin and doxorubicin), antifungal drug (amphotericin B) and the antibiotic gentamicin. A drug administered in a specific dosage form via an appropriate route undergoes all the four pharmacokinetic processes viz. absorption, distribution, metabolism and excretion. These are important determinants of the drug concentration in systemic circulation, which
in turn determines its concentration at the site of action (target concentration). Its pharmacological effects are proportional to the target concentration, which constitutes pharmacodynamics of the drug.
2
Pharmacodynamics – Drug Receptor Interaction; Adverse Drug Reactions Pharmacological and biochemical effects of a drug occur when it reaches the site of action in the body. They may be the effects desired by the clinician treating the patient or may be undesirable and sometimes toxic. The site of drug action or where a drug acts, and the mechanism of drug action or how a drug acts, are the two fundamental and complex problems in pharmacodynamics. Understanding pharmacodynamics is essential as it provides the basis not only for rational selection of drugs but also for their judicious use to produce the maximum benefit with minimum risk.
Site of Drug Action Generalising about the site of drug action is easy and a tentative conclusion can be arrived at by the process of elimination; but the precise determination of the specific site and the mechanism of action of the drug is often difficult. A drug may act: • Locally i.e. at the point of application e.g. glucocorticoid ointment for skin lesion; counterirritants such as methyl salicylate; gastric antacids; or • Systemically i.e. after absorption into systemic circulation, (a) During passage through the body e.g. osmotic diuretics; or (b) By reaching an effective concentration in a particular tissue (general anaesthetics in the brain or diuretics in the kidneys) or in a cell type (e.g. anticancer drugs within the cancer cells or antibiotics within microbes). Methods for localisation of site of action of drugs: • Anatomical and physiological: These surgical procedures isolate the organ or tissues at different levels by using excision or ablation techniques. Various parts of the CNS or other organs are sequentially exposed, effects of drugs by local application are observed and their disappearance confirmed after ablation, to locate the precise site of action. Such techniques have been used to locate the site of action of emetics and antiemetics. • Biochemical localisation: The enzyme systems can be isolated in functional condition by means of in vivo and in vitro techniques and the actions of drugs can be conveniently studied e.g. physostigmine and diisopropyl fluorophosphate (DFP) on cholinesterase. • Pharmacological localisation: If a drug produces a fall in blood pressure and if this is prevented by prior administration of an antihistaminic, it can be concluded that the drug probably acts in the same place and by the same mechanism as histamine. Use of such blockers may also suggest the probable site of action of drugs e.g. muscarinic receptors for cholinergic drugs using atropine as a blocker. • Tracer techniques: In these procedures, the drug (ligand) is labelled with a radionuclide (tracer). The commonly used radioactive labels are 14C, 3H, and 35S. The tracer technique is potentially the most accurate one in determining the distribution and the site of action of drugs, but it is difficult to differentiate between the drug and its metabolites and this may create difficulties in interpreting the results.
Structure Activity Relationship (SAR) The activity of a drug is intimately related to its chemical structure. Knowledge about the chemical structure of a drug is useful for: • Synthesis of new compounds with more specific actions and fewer adverse reactions, • Synthesis of competitive antagonists and • Understanding the mechanism of drug action. Following are the examples that emphasise the importance of certain chemical groups for the drug action and also give some idea about their mechanism. I Synthesis of new compounds: New compounds or drug substitutes may be designed for the following purposes: • To increase or decrease the duration of action of the original drug or to get a more potent compound: (i) Procaine, a local anesthetic, when administered IV, reduces the cardiac rate and excitability. However, it is rapidly hydrolysed in the plasma and hence, its cardiac action is too transient. A compound structurally similar to procaine but resistant to hydrolysis, procainamide, has a longer duration of action and is used to treat cardiac arrhythmias. (ii) Atropine, when instilled into the eye, produces dilatation of the pupil (mydriasis) and also paralyses the accommodation (cycloplegia). However, these effects persists for about a week. The substitute homatropine produces mydriasis and cycloplegia that last for 24 hours. (iii) Ranitidine, an H2 receptor blocker, was developed by modifying the structure of cimetidine, and was found to be more potent with longer duration of action. • To restrict the drug action to a particular system of the body: Chlorpromazine possesses a host of pharmacological actions such as antipsychotic, anticholinergic, sedative and hypotensive. By structural modifications of the chlorpromazine molecule, compounds have been synthesised which have a more potent anti-psychotic effect but lesser sedative and hypotensive effects e.g. trifluoperazine. • To reduce the adverse reactions, and other disadvantages associated with the drugs: (i) Nicotinic acid used in the treatment of pellagra may produce itching and flushing of the skin and sometimes a fall in BP. A related compound nicotinamide has the same efficacy against pellagra but does not produce itching or flushing. (ii) Benzyl penicillin, given orally, is inactivated by gastric acid. Penicillins have been synthesised which are gastric acid resistant and hence can be given orally e.g. phenoxymethyl penicillin. Staphylococci develop resistance to benzyl penicillin fairly fast. Hence, penicillinase resistant penicillin, cloxacillin, was synthesised. II Synthesis of competitive antagonists: (i) Para-amino benzoic acid (PABA) is an essential growth factor for several microorganisms. Paramino salicylic acid (PAS) which shows a structural similarity to PABA, acts by competing with it for the uptake by certain bacteria. Non-availability of PABA arrests the multiplication of the bacteria. (ii) The respiratory depressant action of morphine can be antagonised by the structurally similar compound nalorphine (Chapter 10). III Understanding the mechanism of drug action:
(i) Adrenaline stimulates both the alpha and the beta adrenergic receptors. A related compound, isoprenaline, selectively stimulates the beta adrenergic receptors while a very closely similar compound dichloroisopropylarterenol (DCI) blocks the beta adrenergic receptors. The difference in mechanism of action is due to chemical structure. (ii) The drug chlorpromazine (a phenothiazine) is a tranquillizer used in psychotic agitational disorders. A structurally similar compound imipramine (iminodibenzyl derivative), on the other hand, is an antidepressant and is used as a mood elevator. Chirality: The word chirality comes from the Greek word ‘cheir ’ meaning the hand. Many commonly used drugs (atenolol, ibuprofen, warfarin) exist as racemates i.e. 50 : 50 mixtures of non-superimposable, right handed and left handed, mirror image stereoisomers (enantiomers). The left handed molecules fit only the left handed receptors and the same is true of the right handed molecules (chiral receptors). The two types of molecules may have either the same pharmacological action with different intensities on account of pharmacokinetic differences e.g. verapamil; or entirely different actions e.g. thalidomide in which the l-thalidomide is a potent hypnotic whereas the d-thalidomide is a potent teratogen. This fact, unknown then, accounted for the thalidomide disaster in pregnant women in the 1960s. Several drugs have now been developed as single enantiomers in the interest of selective and uniform pharmacological action and reduced risk of adverse reactions; examples are l-atenolol, s-omeprazole, s-zopiclone, and lsalbutamol.
Mechanism of Drug Action The terms drug action and drug effects often are used as synonyms. However, the drug action always precedes the drug effects. Drug action is the initial interaction of a drug with cells at the site of action; the resultant physiological and biochemical consequences are the drug effects. The drug action and drug effects depend upon the drug concentration achieved at the site of action, which is determined by: • Absorption of the drug after oral or parenteral administration • First pass metabolism • Distribution • Biotransformation • Excretion • Tissue affinity, e.g., ultra short-acting barbiturates like thiopental are mainly concentrated in the central nervous system; and • Condition of the body or the milieu interior e.g. iron is absorbed more rapidly in individuals with iron deficiency anaemia. A drug may act by virtue of its: I Physical properties: • Colour: A pleasant colour may exert a psychological effect, e.g., tincture of cardamom. • Physical mass: Compounds like agar and psyllium seeds absorb water when administered orally and swell in size. This initiates peristalsis and exerts a laxative effect. • Smell: Volatile oils like peppermint oil are used to mask the unpleasant smell of mixtures. • Taste: Compounds with a bitter taste reflexly increase the flow of hydrochloric acid in the stomach and improve the appetite. • Osmolality: Osmotic diuretics like mannitol, osmotic purgatives like magnesium sulfate. • Adsorption: Kaolin and activated charcoal in the treatment of diarrhoea, and poisoning. • Soothing-demulcent: Syrups as pharyngeal demulcents in the treatment of cough (Chapter 26); calamine lotion in eczema (Chapter 71). • Reduction in surface tension: Cationic surfactants like cetrimide for cleaning the skin. • Electrical charge: Heparin, a strongly acidic compound, exerts its anticoagulant effect by virtue of its negative charge (Chapter 33). • Radioactivity: 131I in the treatment of hyperthyroidism (Chapter 64). • Radio-opacity: Barium sulphate as ‘barium meal’, organic iodine compounds for the visualisation of the urinary and biliary tracts. II Chemical properties: • Acidity or alkalinity: Antacids in the treatment of peptic ulcer (Chapter 43). • Chelation: The chelating agent forms a ring structure with the molecules of lead, copper and other metals. This compound is non-toxic, water soluble and is excreted in the urine (Chapter 76). III Ability to modulate body function regulators: These involve: • Neurotransmitters (NT), hormones, growth factors: Drugs may resemble neurotransmitters, hormones or growth factors, bind to the specialised constituents of cells and mimic or oppose their actions.
Drugs may be used as replacement when the production of endogenous substances decreases. Replacement finds an important application in the treatment of hormone deficiencies e.g., insulin in diabetes mellitus, thyroxine in hypothyroidism, and hydrocortisone in Addison’s disease. Drugs may interfere with NT uptake e.g. Serotonin reuptake transporter (SERT) is a target for the antidepressant fluoxetine which inhibits serotonin reuptake and increases its concentration in the synapse. Noradrenaline is taken up by neurons with the help of membrane transporters of the SLC type (Chapter 1). These transporters can be targets for psychotropic drugs. • Enzymes: Drugs may act by either increasing the rate of enzymatic reactions in the body (enzyme stimulation) or decreasing such rate (enzyme inhibition). Enzyme stimulation by drugs, which are foreign substances, is unusual; it occurs commonly with endogenous substances such as hormones, e.g. adrenaline stimulates adenylyl cyclase. Apparent enhancement in enzyme activity by drugs is due to enzyme induction (stimulation of synthesis of the enzyme). Enzyme inhibition with drugs is either nonspecific, e.g. denaturing by alcohol or heavy metals; or specific, e.g. inhibition of cholinesterase by physostigmine, inhibition of carbonic anhydrase by acetazolamide and that of angiotensin converting enzyme (ACE) by enalapril. • Transport processes: Various transport processes such as Na+K+ ATPase pump, Ca++ channels, K+ channels and Na+-Ca++ exchange regulate the ionic concentrations of the cells and control the cell functions. Drugs may bind to the proteins subserving these transport processes and alter the activity of cells. For example, digitalis binds to and inhibits Na+- K+ ATPase pump; verapamil blocks the calcium channel; while minoxidil opens the K+ channel. • Structural proteins: Drugs may also bind to structural proteins of the body. For example, colchicine used to treat gout binds to tubulin of inflammatory cells and prevents their chemotaxis. Cyclosporine, an immuno-suppressant binds to immunophillins. • Other cell constituents such as cell wall and DNA: Many antiviral and anticancer drugs are structural analogs of nucleic acids and compete with them to get incorporated in cellular RNA or DNA, interfering with cellular division, e.g. folic acid antagonist methotrexate. Penicillin interferes with cell wall synthesis. The above mentioned regulators serve as receptors for the drug. For details, see later. Types of drug effects: Drugs may produce their effects by: • Stimulation • Depression • Irritation • Antimicrobial effects; and • Modification of the immune status It must be emphasised that a drug produces only a quantitative and not a qualitative change in the function of the target organ. Stimulation: Increase in the activity of specialised cells is called stimulation. Excessive stimulation may ultimately lead to depression. A drug may specifically stimulate certain portions of a particular system but depress others e.g. morphine stimulates the vagus and the oculomotor nuclei and the CTZ but depresses the respiratory and the cough centres.
Depression: Decrease in the activity of specialised cells is called depression. Quinidine depresses the myocardium while barbiturates depress the central nervous system. Irritation: The term irritation indicates that a drug produces adverse effects on the growth, nutrition and morphology of living tissues. Irritation is a nonspecific phenomenon that can occur in all the tissues. It produces changes in the cellular structure and can produce inflammation, corrosion and necrosis of cells. Heavy metals like mercury and silver are irritants. Mild irritation may have therapeutic utility e.g. senna and cascara stimulate the mucosal cells of the gut and act as laxatives. The cellular changes produced are: (a) Astringent effect (precipitation of proteins): This may sometimes be beneficial. The irritant, however, may dissolve the precipitated proteins resulting in deeper penetration of the irritant and causing more extensive tissue damage. This effect is called as corrosive effect. Many strong acids and alkalis exert a corrosive effect. (b) Dehydration; and (c) Cytotoxic action (damage to the cell wall or the nucleus) e.g. anticancer drugs. When an irritant agent is applied locally to the skin to relieve deep seated pain, it is referred to as counterirritant. Volatile oils like turpentine oil are often used in this fashion. The counterirritant is applied to the skin situated over the organ responsible for pain. (a) It stimulates the sensory nerve endings in the skin and the afferent impulses are relayed in the cerebrospinal axis to efferent vasomotor fibres supplying the internal organ. Thus, the increased circulation to the skin has its counterpart in the deep integumental structures and viscera innervated from the same segmental level of the central nervous system. (b) In addition, the sensory impulses emanating from the skin may interfere with the transmission of pain impulses coming from the viscera and may even produce their partial or complete exclusion by occupying the final common sensory pathway (Gating theory, Chapter 10). The vasodilatation and blockade of pain impulses may explain the relief of deep seated pain. Antimicrobial effects: Drugs are used for prevention, arrest and eradication of infections; they act specifically on the causative organisms e.g. antibiotics (Chapter 45). Modification of immune status: Vaccines, sera and certain other agents (levamisole and corticosteroids) act by altering (enhancing or depressing) the immune status (Chapters 73 and 74).
Drug Receptors The receptors are specific protein macromolecules in the cell membrane, the cytosol or the nucleus. The term receptors is reserved for these protein macromolecules with functional correlates, but not for proteins such as binding proteins which have no functional correlates. Numerous receptors for hormones, neurotransmitters and drugs have been identified, purified, cloned and their structure has been determined. Many drugs (ligands) bind to (i) Receptors for the endogenous substances, (ii) Enzymes, or (iii) Other constituents, which serve as receptors for the drugs. Such ligand binding alters enzyme activity, changes permeability to ions, leads to conformational change or introduces genetic material in the nucleus. The receptor serves a dual function: • It acts as a recognition molecule for specific ligand/s (molecules which bind to it) from among numerous molecular species present in the fluid that bathes the cell; and • It initiates biochemical reactions which transmit the signal from the ligand to proteins in the cell membrane and within the cell (post-receptor events); such events amplify the original signal by a cascade effect and regulate the function of the cell. The use of transgenic mice in whom one of the receptors has been either knocked out or overexpressed has helped in advancing our knowledge of physiology and pathology greatly. The receptor, its cellular target and the intermediary molecules (transporters) if any, are designated as receptor-effector system (signal transduction system). Biologically, the receptor is generally inactive till a ligand binds to it; only then it gets ‘activated’ and triggers the post-receptor events. An exception occurs when the receptor in a target endocrine gland undergoes constitutive activation due to a genetic mutation (Chapter 63). In such a case, the target gland overproduces its hormone, and a disease state sets in e.g. an overfunctioning, solitary, thyroid nodule ultimately becomes a toxic nodule (Plummer ’s disease). The principal parameters which characterise the interaction between a ligand and a receptor are selectivity and affinity. Selectivity of binding of drugs to receptors depends on their physico-chemical structure. Affinity is a measure of the ‘strength’ of binding between the drug and its receptor and is defined by a constant in the binding relationship between the drug and the receptor. The ability of the drug to elicit a response after its interaction with the receptor is termed as the intrinsic activity or the efficacy of the drug. The biological response to a drug is also regulated by alteration in the receptor number and affinity. An agonist is a drug which initiates pharmacological action after binding to the receptors (same site or other allosteric site). It is a drug with high affinity for the receptor and also high intrinsic activity. An antagonist is a drug which also binds to the receptors but does not elicit a pharmacological action; it causes receptor blockade. An antagonist, therefore, has the same affinity for the receptor as the agonist but its intrinsic activity is poor. Partial agonist is a drug with affinity equal to or less than that of the agonist but less intrinsic activity. Such a drug, no matter how high its concentration, will not produce the full response which the tissue is capable of. Further, a partial agonist, because of its ability
to occupy receptors, diminishes the action of an agonist when the two are used simultaneously. In the case of opioids (Chapter 10), which act on several types of receptors, some act as agonists or partial agonists on one type of receptor but as antagonists on another type of receptor, e.g. pentazocine and nalbuphine act as agonists on k receptors but as antagonists on µ receptors. Such drugs are called mixed agonist-antagonists. Sometimes, after combination with certain receptors, a drug may produce actions opposite to those produced by a pure agonist (affinity but negative efficacy). Thus, betacarbolines through interaction with benzodiazepine binding sites on GABAA receptors produce anxiety, arousal and increase in muscle tone, actions opposite to those produced by diazepam. Such drugs are called inverse agonists. It has been also proposed that a receptor exists in two conformational states, active (Ra), and inactive (Ri) which are in equilibrium (Two state receptor model theory). The relative affinity of a drug for any of these two conformations shifts the equilibrium towards either Ra state or Ri state. Thus a drug having higher affinity for the Ra activates the receptor and is termed as a full agonist. The one having moderate affinity for the Ra displays intermediate intensity of effects and is a partial agonist. A drug that binds with equal affinity to both the conformations, Ra and Ri, does not alter the equilibrium and acts as a competitive antagonist while the one with selective preferential affinity for Ri produces an effect opposite to that of a full agonist and is an inverse agonist. Multiple receptor ‘types’ and ‘subtypes’ for a ligand are common. A neurotransmitter may activate several receptor types, e.g. dopamine and adrenaline have five receptors each, histamine has four, while acetylcholine has seven. Increasing concentrations of an agonist evoke a progressively increasing tissue response until the maximum response is reached. If another drug that acts on the same receptor system produces quantitatively different maximum response, then its efficacy (intrinsic activity) must differ. Hence, difference in maximum responses can form the basis for comparing intrinsic activities of drugs. In practice, this means that drug ‘X’ produces a therapeutic effect larger than the maximum effect produced by drug ‘Y’; e.g., as a diuretic furosemide is more efficacious than hydrochlorothiazide (Fig. 2.1b).
FIG. 2.1 Relative potency and relative efficacy of two agonist drugs X and Y (See text).
The term potency of the drug, on the other hand, means that weight for weight, drug ‘X’ has a greater effect than drug ‘Y’; the maximum effect obtainable is, however, similar; for example prednisolone is more ‘potent’ than hydrocortisone (Fig. 2.1a). Some agents are so potent that only a few molecules have to interact with their receptors to induce a massive response. This obviously needs an amplifier system. The simplest amplifier unit is an enzyme molecule that is activated by a drug molecule (active principle) and then converts several substrate molecules into product molecules. If such units are coupled, the product molecules in their turn activate a second enzyme, and so on. Most hormones and neurotransmitters exert their effects without entering the cell. They interact with specific receptors which are coupled to various effector or amplifier systems responsible for generating internal signals or second messengers which initiate a further sequence of enzyme reactions. In such amplifier system model, the active agent needs to activate only a fraction of its receptors to obtain maximal response from the effector system. Thus, there is a spare capacity for the specific receptors. Spare receptors are qualitatively similar to the occupied receptors and are available for action. Thus, adrenaline can elicit the maximum cardiac inotropic response even when 90% of the cardiac ß1 adrenergic receptors are occupied by relatively irreversible antagonists. This indicates that the cardiac tissue possesses a large number of spare beta1 receptors. Receptor-mediated responses to drugs and hormones often become blunted with time during continued exposure to the drug/hormone. This phenomenon is termed desensitisation, refractoriness or tolerance. Thus, repeated administration of ephedrine and adrenergic agonists in bronchial asthma causes a reduction in therapeutic response. Desensitisation is usually reversible, and involves phosphorylation. The density (number) of receptors on cells, their occupancy and their capacity to respond (efficacy) can change in response to the specific binding molecules, (agonist or antagonist), such as autocoids, or hormones or drugs. The cell can increase or decrease the number of receptors in response to a given signalling molecule. Cells do this to maintain the overall homeostasis. When they increase the number of receptors, it is called upregulation while when they decrease the number of receptors to become less sensitive to certain molecues it is designated as down- regulation. Such balancing act between up-
regulation and down-regulation is the characteristic of physiological homeostatic patterns. Down-regulation is thus a process where the actual number of receptors present in the cell/tissue decreases; this occurs more slowly than desensitisation and is less rapidly reversible. It involves net degradation of the cell receptors. Prolonged high concentration of α2 adrenergic agonist clonidine used to treat hypertension, causes reduction in the number of central α2 adreno-receptors available for activation. When the drug is suddenly withdrawn, the number of central α2 receptors is too small for endogenous agonist (noradrenaline) to produce their effective stimulation. This results in sudden rise of BP due to stimulation of peripheral vascular α1 and cardiac β receptors. In contrast, the continued occupation of cell receptors by antagonists may increase the number of cell receptors (up-regulation). Such receptors become accessible to the endogenous agnosist. When the antagonist is stopped suddenly, the stimulation of incresed number of receptors causes an exaggerated response to agonist. Thus, chronic administration of beta-adrenergic blocker, propranolol, is accompanied by increase in the number of beta-adrenergic receptors. Its sudden withdrawal in a patient with ischemic heart disease makes him more susceptible to the effects of endogenous noradrenaline and may precipitate angina. Types of receptors (Table 2.1 and Fig. 2.2): Table 2.1 The main types of receptors
ANF = Atrial Natriuretic Factor cAMP= cyclic AMP. cGMP= cyclic GMP n = Nicotinic m = Muscarinic H = Histaminic ACh = Acetylcholine.
FIG. 2.2 Types of receptor-effector linkage. 1 = Ligand gated ion channel (ionotropic receptors); 2 = G-protein coupled receptor (Metabotropic); 3 = Kinase-linked receptors; 4 = Nuclear receptors; R = receptor; G = G-proteins; E = Enzyme; L = Ligand (Modified from Pharmacology by Rang HP et al, 5th ed, Churchill Livingstone, 2003)
• Type 1: Ion channel linked (Ionotropic). • Type 2: G-protein-coupled (GPCR) (Metabotropic). • Type 3: Protein kinase-linked; and • Type 4: Receptors that regulate DNA (gene) transcription. Ion channel linked receptors are cell membrane spanning proteins. Agents binding with them open a transmembrane channel and permit ions to cross the membrane phospholipid bilayer. Which ions flow and what voltage changes occur as a consequence depend upon the type of channel. Thus, opening of the nicotinic receptor channel permits sodium ions to cross the membrane into the cell and cause depolarisation of the membrane. On the other hand, the gamma-aminobutyric acid (GABA) receptor channel allows chloride ions to permeate into the cell, and hyperpolarises the cell membrane. Opening of the potassium channels allows potassium ions to leak out of the cell and thus hyperpolarises the cell membrane e.g. sulfonylurea receptor. Many drugs (phenytoin and benzodiazepines) act by modifying the function of receptor channels. G-proteins or guanine nucleotide binding proteins are a specific class of proteins that are coupled to certain receptors and are involved in the regulation of secondary messengers. GPCR are found on all cell types and are abundant in the brain and the gut. They are either stimulatory (Gs) or inhibitory (Gi) in action. The G in the name refers to guanosine diphosphate or triphosphate. ACh and GABA can activate ion channels as well as GPCR. A ligand binding to GPCR promotes binding of GTP to G-proteins. The activated G-proteins in turn activate effector systems such as enzymes (adenylyl cyclase and phospholipase) and ion channels (Ca++ & K+). The second messengers for such actions are: (a) intracytoplasmic calcium ion concentration; (b) cyclic AMP; and (c) inositol 1, 3, 5-triphosphate (IP3) and diacylglycerol (DAG) released from the phospholipid in the cell membrane. In some instances, these three are interlinked. The classic examples of GPCR (Fig. 2.2) are the adrenergic β1 and α2 receptors and dopamine receptors. Thus:
(1) The extracellular ligand binds to a cell-surface GPCR; which in turn, (2) Activates G-protein located on the cytoplasmic face of the plasma membrane. (3) The activated G-protein alters the activity of the effector element such as the adenylyl cyclase enzyme (or an ion channel). (4) Adenylyl cyclase converts intracellular ATP to cyclic AMP, the second messenger. Many drugs such as opiates act on GPCR either as agonists or as antagonists. Similarly, peptides (eg beta endorphins), acetylcholine (muscarinic actions) and biogenic amines (5HT) act by binding to GPCR. Nearly 65% of the drugs act via GPCRs. Protein kinase linked receptors (Fig 2.2), the third family of cell surface receptors, are enzymes like tyrosine kinases. They serve as receptors for insulin and epidermal growth factor. Tyrosine kinases activate themselves by autophosphorylation after the hormone binds to them. The autophosphorylated tyrosine kinase then phosphorylates intracellular proteins on the tyrosine residues. Apart from membrane linked enzymes, certain nonenzyme entities also serve as receptors for cytokines. Membrane bound- and intracellular guanylyl cyclase serves as receptors for natriuretic peptides and nitric oxide (NO) respectively. Nuclear receptors for steroids are present in the cytoplasm; those for thyroid hormones are present in the nuclear chromatin. These receptors (Fig. 2.2), after activation by hormone binding, act on the genetic material in the nucleus to initiate or inhibit the process of transcription. It must be emphasised that a given agent may activate more than one type of receptors.
Dose Response Relationship Wide quantitative variations in drug responses can occur between different species and within the same species under different conditions. Methods have, therefore, been devised to study the phenomenon of variation in pharmacological drug response and to minimise the errors of prediction in therapeutic use of drugs. Each drug has a characteristic dose response curve for a specified set of conditions, but in general, the dose response curve conforms to the S-shaped or sigmoid type, or to segments of the sigmoid. The magnitude of the drug effect is a function of the dose administered. Two basic types of dose effect relationship have been observed: (i) Graded or quantitative dose-response relationship; and (ii) Quantal or all or none dose-response relationship Graded or quantitative dose-response relationship: This type of relationship relates the size of the response in a single biological unit to the dose of the drug. As the dose administered to a single subject or discrete organ or tissue is increased, the pharmacological response also increases in graded fashion provided the dose has exceeded some critical level called the threshold dose (Fig. 2.3). The graded dose-response relation is partially a reflection of the extent of occupancy of the receptors by the drug. Since an entire dose response relationship is determined from one animal, the curve does not tell us about the degree of biological variation inherent in a population.
FIG. 2.3 Effect of graded dose of histamine on isolated guinea-pig ileum.
The degree of response produced by increasing doses of a drug eventually reaches a steady level, termed as the ceiling response, and the dose with which it is obtained is the ceiling dose. If the dose exceeds the ceiling dose, there is no further increase in the therapeutic effect. In fact, such a dose may provoke different and possibly undesirable responses. The ceiling dose allows us to compare the therapeutic efficacy of various compounds. Fig. 2.4 a shows a ‘dose versus response’ curve whereas Fig. 2.4 b shows the same data in the form of a ‘log-dose versus response’ curve. The latter is particularly useful for the comparison of various compounds.
FIG. 2.4(A) Dose-response relationship curve from the data in Fig. 2.3
FIG. 2.4(B) Same dose-response relationship plotted on logarithmic scale.
Quantal or all or none dose-response relationship: In contrast to graded responses, the quantal responses are all or none. The quantal curve shows the frequency with which any dose of a drug evokes a stated, fixed (all or none) pharmacological response in a subject population. It is, therefore, essentially a frequency distribution of the responders (actual numbers or percentage of the total number of subjects) to different doses of the drug. Each subject is categorised as responding or non-responding, according to a prior decided criterion of response. While studying an anti-epileptic drug in animals, each animal is classified as responding (seizure-free) or not responding at a specified time after the drug treatment. Obviously, sensitive animals will respond to smaller doses while some will be
resistant and need very large doses. Usually, the sensitivity of animals to different doses is distributed normally with respect to the logarithm of the dose. Thus, for a given drug, if log dose is plotted on the horizontal axis and the % responding to the various dose levels is plotted on the vertical axis, a Gaussian (normal) distribution is obtained (Fig. 2.5). The curve represents the distribution of sensitivity of a group of animals to the given drug. In this figure about 10 % of the animals in a given population remain seizure-free at a dose level of log dose ‘0’, while another 10 % do not respond until the dose is increased to log dose ‘2’. Majority of the animals, however, respond at doses between ‘0.5’ and ‘1.5’ on the log scale. The same data, plotted as the cumulative number of animals that responded against log dose, would give an S shaped cumulative frequency curve. For a given dose of a drug, a cumulative frequency curve gives the per cent of animals responding to that dose and to lower doses.
FIG. 2.5 Quantal dose response curve
The quantal dose response curve, however, is not always exactly symmetrical or bellshaped but may show ‘skewing’ or ‘truncation’. This shows that besides polygenic random variation, non-random but inter-coupled events like other actions of the drug and experimental limitations influence the quantal dose response curve. Further, there are drugs whose clearance in living animals segregates into distinct groups because of drug biotransformation controlled by a single gene. A bimodal quantal log-dose response curve may be obtained in such cases. The median lethal dose or LD50 : This is the dose (mg/kg) which would be expected to kill one-half of an unlimited population of the same species and strain. The median effective dose or ED50 : This is the dose (mg/kg) which produces a desired response in 50 per cent of the test population. Therapeutic index (TI) : It is an approximate assessment of the safety of the drug. It is expressed as the ratio of the median lethal dose to the median effective dose:
The margin of safety is the difference between the therapeutic and the lethal doses. As the drug metabolism varies from species to species, the TI would also vary. Therapeutic index supplies reliable information when both the LD50 and ED50 are determined for the same strain of a given species. ED50 can be obtained from either quantal or graded dose response curves. As LD50 cannot be worked out in humans, the formula for TI in humans can be restated as:
The larger the TI, the safer is the drug. For safe therapeutic application of a compound, its TI must be more than one. Such drugs have very little dose-related toxicity. Thus, penicillin has a very high TI while it is much smaller for digoxin, aminophylline and lidocaine. In practice, no drug produces only a single effect but has a spectrum of effects. Further, a drug may be selective in one respect but nonselective in another. Thus, although antihistaminics selectively block histamine actions, most of them cause significant sedation. For therapeutic purposes, selectivity of a drug effect is clearly one of its more important properties. Thus depending upon its effect, a drug may have many therapeutic indices. The margin of safety of aspirin when used for headache is far greater than its margin of safety for the relief of arthritic pain or in rheumatic fever. This is because the latter use requires much larger doses. In clinical practice, there is often a need to use two or more drugs concurrently. The resultant effect may vary depending on the combination used. There may be: (1) Additive effect (Summation): When the total pharmacological action of two or more drugs administered together is equivalent to the sum of their individual pharmacological actions (1+1=2), the phenomenon is termed as an additive effect e.g. combination of aspirin and paracetamol in the treatment of pain and fever. (2) Synergism: Facilitation of a pharmacological response by the concomitant use of two or more drugs is called drug synergism. The word synergism is derived from the two Greek words, ergo (work) and syn (with) and indicates a pharmacologic co-operation. This co-operation usually results in a total effect greater than the sum of their independent actions (1+1>2), e.g. codeine and aspirin for pain; hydrochlorothiazide and atenolol for hypertension. If the synergism results in prolongation of action of one of the drugs, it is termed time synergism, e.g. procaine and adrenaline combination increases the duration of action of
procaine. The term potentiation is often loosely employed for synergism and should be avoided, as the word ‘potentiate’ means ‘to endow with power ’, which no drug is really capable of achieving. (3) Antagonism: The phenomenon of opposing actions of two drugs on the same physiological system is termed as drug antagonism. It can be: • Chemical • Competitive (reversible) • Non-competitive (irreversible) • Physiological Chemical antagonism: The biological activity of a drug can be reduced or abolished by a chemical reaction with another agent e.g. heparin and protamine, BAL and arsenic. Competitive or reversible antagonism: When the agonist and the antagonist compete for the primary binding site on the same receptors, it is designated as competitive antagonism. The extent to which the antagonist opposes the pharmacological action of the agonist will be decided by the relative numbers of receptors occupied by the two compounds. Competitive antagonism can be overcome by increasing the concentration of the agonist at the receptor site, e.g. acetylcholine and atropine antagonism at muscarinic receptors. If the concentration of acetylcholine at the receptor level is increased by the administration of an anticholinesterase, the blockade produced by atropine can be reversed (reversible antagonism). Similar is the case between noradrenaline and prazosin, an alpha-adrenergic blocking agent. A competitive antagonist shifts the dose response curve to the right. The maximal response to agonist is, however, not impaired (Fig. 2.6a).
FIG. 2.6 (a) Characteristic of competitive antagonism between noradrenaline and an antagonist. Black dots denote the control curve. Subsequent curves are drawn with increasing concentrations of the antagonist. Note that the curves are almost parallel and maximum response could be obtained with noradrenaline on each occasion. (b) Characteristic of non-competitive antagonism. Note the decrease in maximum response to acetylcholine following the increase in antagonist concentration.
Non-competitive antagonism: In this type of antagonism an antagonist inactivates the receptor (R) so that an effective complex with the agonist cannot be formed, irrespective of the concentration of the agonist. This may happen in various ways: (a) The antagonist might combine with R at the same site, but the combination is so firm that even higher agonist concentration cannot displace it (irreversible). (b) The antagonist binds to an alloasteric site so as to prevent the expected characteristic biologic response to the agonist; or (c) The antagonist might itself induce a certain change in R so that the reactivity of the binding site where agonist should interact is reduced or abolished e.g. noradrenaline and phenoxybenzamine on vascular smooth muscle; acetylcholine and decamethonium at the neuro-muscular junction. Although the agonist curve shifts to the right, the slope is reduced and the maximum response diminishes. The extent of inhibition produced depends on the characteristics of the antagonist; the agonist has no influence upon the degree of antagonism or its reversibility (Fig. 2.6b). Physiological antagonism: This term is sometimes used where a drug reverses the effects of another drug by acting on different receptors, e.g., adrenaline given in histamine reaction. Sometimes the term ‘functional antagonism’ is used to represent the interaction of two agonists that act independently of each other but cause opposite effects, e.g., acetylcholine and adrenaline. Importance of drug antagonism: An understanding of drug antagonism is important in: • Correcting adverse effects of drug; e.g., chlorpromazine induced extrapyramidal reactions which are due to cholinergic activation can be countered by benzhexol (Chapter 13). • Treating drug poisoning; e.g., morphine with naloxone, organophosphorus compounds with atropine; and
• Predicting drug combinations which would reduce drug efficacy, e.g., the penicillin and tetracycline combination is inferior to penicillin alone in pneumococcal meningitis.
Adverse Drugs Reactions (ADR) The administration of a drug may result in the development of: • Side effects • Untoward effects • Toxic effects; or • Allergic and idiosyncratic effects. Side effects: Side effects are in fact pharmacological effects produced with therapeutic dose of the drug, e.g., dryness of mouth with atropine. These effects are neither harmful nor damaging and recovery is quick on stopping the drug or reducing the dose. Side effects which might be troublesome in a particular situation may be useful under other circumstances. Thus, dryness of mouth is undesirable when a person suffering from intestinal colic is given atropine, but it is useful during preanaesthetic medication. Untoward effects: Untoward effects also develop with therapeutic dose of a drug, but are undesirable and, if severe, may necessitate the cessation of treatment, e.g. resistant staphylococcal diarrhoea following tetracycline therapy and potassium loss due to diuretic drugs. Toxic effects: These are seen usually when a drug is administered repeatedly and/or in large doses. Toxic effects always cause tissue damage to induce signs and symptoms (toxidromes). The recovery takes longer time and may be partial or impossible. Drug toxicity is usually the primary attribute of a drug and is dose dependent, e.g., respiratory depression with morphine; hepatotoxicity due to paracetamol. It is important to remember while treating the toxidromes that the kinetics of the drug changes in toxic doses (toxikinetics). According to WHO, adverse drug reaction (ADR) is defined as “any response to a drug that is noxious and unintended and that occurs at doses used in man for the prophylaxis, diagnosis or therapy of disease or for modification of physiological function”. It excludes adverse reactions due to drug overdose (poisoning), drug abuse and therapeutic errors. The term drug intolerance, used commonly, literally means ‘failure to tolerate’ and can be used to describe any type of adverse drug reaction (Fig 2.7).
FIG. 2.7 Classification of drug intolerance
ADR could be either local (irritation, necrosis or thrombophlebitis) or systemic. The basic cause of an ADR may be discernible (pharmaceutical, pharmacokinetic or
pharmacodynamic) or may be unknown. ADRs are generally classified into Type A (Augmented); Type B (Bizarre); Type C (Chronic) Type D (Delayed) and Type E (End of dose). The differences between Type A and Type B ADRs are listed in Table 2.2. Type C ADRs (e.g. analgesic nephropathy) are both time and dose related whereas Type D ADRs (e.g. carcinogenesis) are only time related. Type E ADR are seen after sudden stoppage of drug (e.g. beta blockers or clonidine). Table 2.2 Comparison between Type A and Type B ADR Nature of ADR Mechanism P harmacologically predictable Dose dependent Incidence and morbidity * Mortality * Treatment
Type A Augmented/normal response Hyper -response Largely yes Yes High
Type B Totally abnormal, bizarre response Genetic , immunologic al/unknown No
Low Adjust dose
High S top the drug
No Low
*
= In the community
Type A ADR, also known as augmented (quantitative) ADR, are largely predictable on the basis of the known pharmacological actions of a drug and usually are dose related. They are extension of the pharmacological effects e.g., insulin hypoglycemia, or an effect due to an action of the drug at another site (e.g., anticholinergic effects of phenothiazines). Type B ADR are also known as bizarre (qualitative) ADR. The symptoms and signs observed are different from those expected from the known pharmacological actions of the drug and are not dose-related, unpredictable effects. Their mechanism is sometimes known (genetic or immunological) but may often be unknown. Idiosyncrasy is a Type B ADR wherein the abnormal response to a drug is either due to a genetic or unknown mechanism. Thus, sometimes genetically determined absence or reduced activity of certain enzyme(s) in an individual is responsible for the ADR. Drugs like primaquine, salicylates and sulfonamides cause haemolysis in individuals whose RBC lack the enzyme G6PD. In many cases, however, the cause of the idiosyncrasy is unknown e.g. chloramphenicol-induced aplastic anemia. In general, all unusual, idiosyncratic reactions should be considered genetically determined until proved otherwise. As against idiosyncrasy, allergy always has immunological basis.
Drug Allergy The word allergy is derived from Greek words “allos” meaning altered and “ergos” meaning energy. Most of the drugs/sera used in therapeutics are capable of causing allergic or hypersensitivity reactions. They may be mild or very severe like anaphylaxis and have immunological basis. They occur in individuals who have been sensitised following the prior administration of the same drug or structurally similar drug. To understand how drugs cause immunologically mediated reactions, it is necessary to know some basic immunological concepts. These are described in Chapter 73. Generally, proteins with molecular weight of over 5,000 daltons administered IM/IV, readily stimulate the production of antibodies. Peptides of molecular weight less than 5,000 daltons are less immunogenic. Non-protein compounds such as drugs can become immunogenic after chemical coupling to a carrier protein in the body and produce antibodies that can react with the drug which thus behaves as a hapten. A hapten is a substance which is antigenic in the sense that it reacts with an antibody but itself is incapable of stimulating antibody production unless combined with a carrier protein. Drug allergy differs from drug toxicity in many ways. The lesion produced by the former is lower in incidence and is unpredictable; prior exposure to the drug may cause sensitisation. The lesion is dose independent and rash, fever, eosinophilia and blood dyscrasias can occur. ‘Hypersensitivity’ or ‘allergic’ reactions can occur, when an individual sensitised to an antigen (e.g. drug), again comes in contact with the same antigen. The resulting tissuedamaging reactions are: (1) Type I (Immediate Hypersensitivity) reactions: IgE mediated (a) Allergic reaction and (b) Anaphylaxis. A single injection of egg albumin into a guinea pig has no obvious effect. However, antibodies to this protein are formed and the animal is sensitised. A repeat injection of egg albumin in such an animal causes a violent reaction called anaphylaxis. The animal gets asphyxiated from bronchospasm, the blood pressure falls due to vasodilatation and death occurs within a few minutes. The antigen reacts with a specific class of antibody, reaginic antibodies (IgE), bound to the surface of mast cells and basophils. This interaction causes degranulation of mast cells and basophils with massive liberation of histamine and other mediators of immediate hyper-response, leading to anaphylaxis. Mediators released are: • Those that increase vascular permeability and contract smooth muscles, e.g., histamine, PAF, SRS-A, bradykinin. • Those that are chemotactic for or activate other pro-inflammatory cells, e.g., leukotriene B4, eosinophil and neutrophil chemotactic factors. • Those that modulate the release of other mediators, e.g., bradykinin, PAF, prostaglandins; • Those which cause termination of the inflammatory response. Under physiological conditions, mast cell triggering forms a vital part of the acute inflammatory defence reaction. Different species vary in their response. Thus, anaphylaxis can be readily induced in the guinea pig, much less easily in the rabbit and the least readily in the rat. In humans, death is usually due to laryngeal edema, a feature unique to man, bronchospasm leading to
asphyxia, or vasodilatation with circulatory collapse. A similar systemic reaction can occur in a sensitized human subject following a repeat injection of a drug like penicillin or antitoxic serum. This is the phenomenon of systemic anaphylaxis. Anaphylactoid (pseudo-allergic) reactions mimicking anaphylactic shock sometimes occur after oral administration of aspirin (Chapter 11) and after IV administration of iodine containing diagnostic contrasts (Chapter 64). They are not immunological in nature. As compared to systemic anaphylaxis, local anaphylactic reactions (atopic allergy) to extrinsic antigens (allergens) such as pollen, animal danders, mites in house dust, and absorbed foodstuffs occur more frequently in man (Chapter 71). Combination of the allergen with cell bound IgE antibody in the bronchial tree, the nasal mucosa or the skin releases mediators of anaphylaxis giving rise to localized reactions such as, asthma, rhinitis (Chapter 27) or urticaria (Chapter 23). The offending antigen can be identified by intradermal prick test. There is a strong familial tendency. The symptoms of atopic allergy are to a certain extent controllable with antihistaminic drugs and other mediator antagonists (see Chapter 23). Courses of antigen injection may desensitise by forming blocking IgG or IgA antibodies, or by turning off IgE production. Figure 2.8 summarises the therapeutic approaches to atopic allergy.
FIG. 2.8 Atopic allergy and targets for therapy
(2) Type II (Cytotoxic) reaction: In this case, the IgG and IgM antibodies formed bind to an antigen present on the cell surface and promote their destruction by lysis/phagocytosis by polymorphs and macrophages or by non-adherent lymphoid killer cells through an extracellular mechanism. Transfusion reactions, anti-D antibodies in Rhesus incompatibility and antibodies to kidney glomerular basement membrane are examples of this type of reaction. Methyldopa-induced hemolysis is a Type II reaction. (3) Type III (Immune Complex Mediated) reaction: The union of soluble antigen with IgG antibody in vivo forms immune complexes which may ultimately cause histamine release, activation of kinin system and aggregation of platelets resulting in microthrombi, small vessel damage, and further release of vasoactive amines. The attracted polymorphs release tissue-damaging enzymes on contact with the complex. In case of high levels of circulating antibodies, the antigen is precipitated near the site of its entry into the body. The reaction in the skin is characterised by erythema, edema and cellular infiltration, maximal at 3-8 hours (Arthus reaction). When the antigen is relatively in excess, soluble complexes formed circulate in the body and are deposited at preferred sites such as the skin, the joints, the renal glomeruli and the choroid plexus. This type of reaction is manifested as serum sickness following the injection of large quantities of foreign protein e.g. horse serum; skin reactions of SLE (systemic lupus erythematosus) induced with isoniazid or hydrallazine; and vasculitis with sulfonamides. Intrapulmonary Arthus type reactions to exogenous, inhaled antigens is responsible for
many hypersensitivity disorders such as farmer ’s lung; such reactions are often provoked by the local release of antigens from infective organisms. Thus, chemotherapy may cause an abrupt release of microbial antigens, producing dramatic immune complex mediated reactions such as erythema nodosum leprosum in lepromatous leprosy and the Jarisch-Herxheimer reaction in syphilitics treated with penicillin. (4) Type IV (Cell Mediated or Delayed Hypersensitivity) reaction: In this case, Tlymphocytes carrying a specific receptor on their surface are activated by the antigen to release certain active factors. This phenomenon is observed in contact dermatitis and in diseases caused by chemicals, dusts, mycobacteria, chlamydia, fungi and helminths, and in the rejection of transplants. Inflammatory reactions initiated by mononuclear lymphocytes and not by antibody alone are called delayed hypersensitivity reactions. The word delayed indicates the secondary cellular response appearing at 48-72 hours after the antigen exposure. In contrast, the immediate hypersensitivity response seen within 12 hours of the antigen challenge, is initiated by basophil mediated reactions (Type I) and by preformed antibodies (Types II and III). A typical delayed hypersensitivity response is observed in the Mantoux reaction following the intradermal tuberculin injection, wherein an indurated and erythematous reaction occurs within 48 hours. Delayed type of reactions are often observed: (a) With drugs that are capable of binding to body constituents and forming new antigens e.g. sulfonamides and penicillin; (b) Following insect bites; and (c) Following contact with certain plants or food in sensitised individuals. Desensitisation: This term is used to describe two different processes. (a) In one case the second dose of antigen fails to evoke any response in a sensitised preparation. This may be due to exhaustion of antibody or of an essential enzyme system activated by the antigen-antibody reaction. Examples are penicillin and insulin allergies. (b) The second type of desensitisation is the one which is carried out in therapeutic practice. In this case, a course of graded injections of an antigen is given to a hypersensitive patient in order to render him less allergic to the antigen. This is believed to be due to formation of blocking antibodies (See Chapter 27). The patient must be informed about the drugs he is allergic to, in order to prevent such reactions in the future.
Manifestations of ADR Drugs can cause ADR related to almost all tissues and organs. Some important ADR are: I Gastro-intestinal: Several drugs cause anorexia, nausea, vomiting (e.g. metronidazole, chloroquine), diarrhoea/constipation (e.g. iron salts, morphine). Aspirin and other NSAID can cause gastric ulceration and even bleeding. II Haemopoietic: It ranges from anaemia to blood dyscrasias like leucopenia, agranulocytosis, aplastic anaemia and thrombocytopenia. The reduction in clotting factors may lead to haemorrhages (Chapter 36). III Hepatocellular: Drugs can damage the liver: • By direct action, either themselves or more commonly through their metabolites (e.g. paracetamol and tetracycline). The latent period between exposure and liver injury is usually short. Such hepatotoxicity is predictable, dose related and can be demonstrated in animals; other organs such as the kidney may also be affected; or • Due to idiosyncratic reaction. This hepatotoxicity is infrequent, unpredictable, not dose related and has no animal model. It may occur at any time during or shortly after exposure to the drug. Sometimes extrahepatic general allergic manifestations may be present such as rash, arthralgias, fever, leukocytosis, and eosinophilia. However, in most cases toxic metabolites that damage liver cells directly are responsible e.g. halothane hepatitis and isoniazid hepatotoxicity. The direct toxicity exhibits morphologic changes which are characteristic for individual agent e.g. carbon tetrachloride causes a centrilobular zonal necrosis; Amanita phalloides produce massive hepatic necrosis while tetracyclines induce microvesicular fat deposits. As against this, the idiosyncratic hepatotoxic reactions may cause more variable picture. Non-specific hepatitis (isoniazid, pyrazinamide, co-trimoxazole, phenytoin, halothane), bridging hepatic necrosis (e.g. methyldopa), cholestasis interfering with biliary secretion causing hyper-bilirubinemia (certain anabolic steroids), cholestatic hepatitis (chlorpromazine, amoxicillin-clavulanic acid, oxacillin, erythromycin estolate) and sclerosing cholangitis (floxuridine). A rare but serious form of long-lasting cholestasis is the vanishing bile duct syndrome in which drugs cause destruction of intrahepatic biliary ductules. For example, carbamazepine, chlorpromazine, haloperidol, amitryptiline and azathioprine. Drugs given for HIV (e.g. zidovudine, protease inhibitors) can cause mitochondrial hepatotoxicity in form of steatohepatitis. Chemotherapeutic agents like cyclophosphamide, melphalan, busulfan affect hepatic sinusoidal lining cells to induce venoocclusive disease. Oral contraceptive induced cholestasis appears to be genetically determined. Patients with genetic absence of CYP2D6 also experience hepatotoxicity with desipramine, propranolol, and quinidine. In case of some drugs such as methyldopa, sodium valproate, and isoniasid both the mechanisms can operate. The hepatic effects of potentially hepatotoxic drugs are generally monitored by periodic measurement of SGPT. However, mild, transient, nonprogressive increase in SGPT can be seen with isoniazid, valproate, phenytoin, and statins. IV Cardiac: Drugs can cause cardiotoxicity directly. Thus, they may precipitate arrhythmias or even cardiac arrest (e.g. digoxin, quinine, aminophylline, flecainide). Fenfluramine is
known to cause valvular fibrosis (Chapter 40). V Renal: Drugs can cause albuminuria, hematuria and even tubular necrosis. Nephrotoxicity is either direct or immunologically mediated (Chapter 39). VI Abnormalities of taste and smell: Drugs are known to produce abnormalities of taste and smell sensations: hypogeusia is a decrease in taste acuity; ageusia is total loss of ability to recognise taste; dysgeusia is distortion of taste sensation; Hyposmia, anosmia and dysosmia represent the corresponding abnormalities of the sense of smell. A patient with hyposmia or anosmia may have decreased ability for perception of the flavour of food. Some drugs associated with the above abnormalities are: d-penicillamine, pyrazinamide, captopril, methimazole, biguanides, l-dopa and bromocriptine. VII Ocular toxicity: See Chapter 72. VIII Ototoxicity: Some topical preparations and certain drugs (aminoglycosides) can cause ototoxicity and impair hearing (Table 2.3). Table 2.3 Drugs causing ototoxicity
IX Dermal: The skin is a common target organ for various allergic and photosensitivity reactions. Anticancer drugs can cause hair loss. Metalloids like arsenic and heavy metals like mercury are secreted in the sweat and can produce exfoliative skin rashes (Chapter 71). X Electrolyte disturbances: Diuretics like thiazides and furosemide may produce hyponatremia or hypokalemia. NSAID may cause sodium retention and edema. XI Endocrine disturbances: Chlorpromazine may produce menstrual irregularities, galactorrhoea and amenorrhoea. Combination oral contraceptives may arrest lactation in nursing mothers. Glucocorticoids depress the synthesis of ACTH and endogenous cortisol. Abrupt withdrawal of these compounds may, therefore, precipitate acute hypocorticisolism (Addisonian crisis) while vigorous therapy may cause Cushing’s syndrome. XII Infertility and sexual impotence: (Chapters 68 and 69). XIII Behavioural and CNS: Compounds like amphetamine may cause disorientation, confusion and inability to concentrate; glucocorticoids may produce euphoria, restlessness and psychosis; and benzodiazepines may cause anterograde amnesia (Chapter 8). For behavioral teratogenicity, see Chapter 80. Ethambutol can cause optic neuritis while dystonic reactions can occur following phenothiazines. XIV Carcinogenesis: Estrogens exacerbate mammary carcinoma in females. Increased risk of developing endometrial cancer has been reported in women receiving prolonged estrogen therapy without concomitant progestogen. XV Teratogenicity: A teratogen is an agent which can cause a fetal physical malformation and behavioural effects when maternal administration results in significant exposure during organogenesis (18th to 60th days of fetal life). After that period, during the remainder of pregnancy, exposure to fetotoxic drugs may cause functional disability or an
alteration in growth of the organs/fetus but no physical defects. The word teratogenicity is derived from the Greek word teratos which means monster. The sedative ‘thalidomide’, prescribed to pregnant women with morning sickness, was found to produce various types of developmental anomalies in the newborns. The commonest anomalies were ‘amelia’ or total absence of limbs, and ‘phocomelia’ or absence of one or more limbs (Fig. 2.9).
FIG. 2.9 Limb abnormalities (Seal limbs) following thalidomide administration in the mother. (With courtesy of Dr. R. A. Pfeiffer from the Univ - Kinderkilinic, Munster. Direktor: Prof. Dr. H. Mai.)
XVI Unmasking and exacerbation of disease: Drugs can exacerbate an already existing disease or unmask a latent condition, e.g. glucocorticoids unmask latent diabetes and may exacerbate an existing peptic ulcer. Isoniazid may unmask latent epilepsy. XVII Production of a disease (Iatrogenic disease): Sometimes, drugs themselves may produce certain pathological syndromes. The diseases produced as a result of therapeutic measures are known as iatrogenic diseases, the Greek word iatros meaning “physician”. Thus, glucocorticoid therapy can precipitate hypertension, congestive heart failure and Cushing’s syndrome. Glucocorticoids, aspirin and indomethacin may precipitate perforation of duodenal ulcer. Repeated doses of NSAID can cause renal damage. XVIII Immunosuppression (Chapter 74) XIX ADR due to drug interaction: This can occur when two or more drugs are administered concurrently (Chapter 3). XX Adverse reactions precipitated by abrupt drug withdrawal: Abrupt cessation of administration of several groups of drugs after prolonged use can cause: • Resurgence/rebound of underlying disease • A typical ‘withdrawal syndrome’; or • Symptoms as a result of physiological adaptation e.g. pituitary suppression by glucocorticoids. The withdrawal syndromes after abrupt stoppage of agents such as alcohol, CNS depressants, anti-epileptics, clonidine and nitrates are described in the respective chapters. With such drugs, the cessation of therapy must be gradual, with small decrements and under close medical supervision. Sometimes, substituting a drug with a longer half life (e.g. methadone in patients taking morphine) is helpful. ADR and p-glycoproteins: Membrane efflux transporters like p-glycoproteins extrude
the noxious, naturally occurring substances and drugs from the cells of the vital organs. In species of mice genetically deficient in these cell membrane proteins, the toxicity of vinblastine (an anticancer drug) and ivermectin (an antifilarial drug) is markedly increased. They also serve as protective mechanisms against xenobiotics. In practice, a clinician must be aware of the toxicity of the drug he uses.
Treatment of Acute Drug Poisoning Drug poisoning could be accidental, homicidal or suicidal. Table 2.4 outlines the principles of treatment of acute poisoning. Table 2.4 Principles of treatment of acute poisoning
• Removal of the poison: In a conscious patient, vomiting can be induced by tickling the back of the pharynx. Ipecac syrup (not fluid extract) 15-30 ml (adult dose) followed by 200 ml of water is useful in inducing vomiting in many cases including infants. Ipecac may be repeated after 20 minutes, if needed. If there is no emesis, gastric lavage should be carried out to remove the ipecac. Induction of emesis is contraindicated if: (a) The patient is stuporous, delirious or comatose; is in shock; is getting convulsions; has an inadequate gag reflex; or (b) He has ingested a corrosive poison, a CNS stimulant, or a petroleum distillate such as kerosene. Though salt and water can induce vomiting it may cause hypernatremia. In the hospital, the ingested poison can be removed by gastric lavage. Absorption of a poison from the GI tract can be reduced in certain cases by using activated charcoal (Carbomix) in the dose of 50 g (25 g in children). Activated charcoal adsorbs most of the drugs and poisons except alkalies, arsenic, lithium carbonate, cyanide, mineral acids and ferrous sulfate. Charcoal should not be administered concurrently with ipecac or a specific antidote as it may adsorb them and render them ineffective. As most poisons do not dissociate from activated charcoal if it is present in excess, there is no need to remove the charcoal from the GI tract. If activated charcoal is not available Universal Antidote may be substituted. It consists of 2 parts of powdered charcoal with 1 part of tannic acid and 1 part of magnesium oxide. Activated charcoal is particularly useful for reducing the absorption of alkaloids from the gut. A homemade substitute for the universal antidote is two parts of burnt toast, one part of strong tea and one part of milk of magnesia. Repeated oral doses of activated charcoal (50 g initially, followed by 25 g every 4 hours, in adults) enhance the elimination of some drugs which are excreted back into the GI tract after they are absorbed; e.g. aspirin, carbamazepine, barbiturates, phenytoin and theophylline. Oral administration of polyethylene glycol (PEG) may be used for total bowel clearance in case of poisoning (Chapter 42). • Elimination of the poison can be enhanced: (a) By increasing the urine output with diuretics like mannitol, furosemide; (b) By adjusting the PH of urine: weakly acidic drugs such as phenobarbitone (Chapter
8) and salicylates (Chapter 11) are excreted faster in an alkaline urine; and (c) By dialysis or hemoperfusion • Administration of specific antidote (Table 2.5). More examples are given in other chapters. Table 2.5 Antidotes for emergency treatment of certain poisonings P oison P aracetamol Theophylline Mercury, arsenic and copper Atropine and other antimuscarinics Carbon monoxide Cyanide Methyl alcohol and ethylene glycol Lead Nitrites Opioids Benzodiazepines Organophosphate compounds Warfarin Beta-adrenergic stimulants
Antidote N-ac etylc ysteine Propranolol BAL Physostigmine Oxygen Dic obalt edetate; amyl nitrite followed by sodium nitrite, then sodium thiosulfate Ethyl alc ohol; 4-Methyl pyrazole Calc ium disodium edetate; dimerc aptosuc c inic ac id Methylene blue Naloxone Flumazenil Atropine; pralidoxime Vitamin K1 Propranolol
• Supportive treatment includes maintenance of a patent airway, assisted mechanical ventilation, maintenance of BP by fluids and vasopressor agents, nutrition by intravenous glucose and prevention of secondary infection. Self-medication is often an important cause of drug poisoning. This is particularly true of commonly used and ‘available over the counter’ drugs like fever and pain remedies (paracetamol) and vitamin D preparations. The other common agents to cause self-poisoning are ingestion of adulterated alcohol (methyl alcohol) and insecticides.
3
Principles of Drug Prescribing; Factors Modifying the Effects of a Drug; and Drug Interactions Modern therapeutics is not just an art but is more of a science. It is now more and more dominated by evidence-based medicine (EBM) that is defined by Dr. David Sackett as the “the conscientious, explicit and judicious use of current, best evidence in making decisions about the care of individual patients.” Thus, it is essential for the physician, before prescribing any therapy, to examine objectively the available evidence for patient care resulting from systematic research, and to integrate the same with individual clinical expertise. In practice, the treatment of a sick person includes many aspects, and administration of drugs is one of them. In certain patients, drugs are of the greatest importance while in others they have less important role to play. In all situations, the doctor-patient relationship is of prime importance. “Words have the potential to ‘heal’ or to ‘sicken’. Words that bring expectations are interventions on their own. Words will not ‘cure’ but they can affect attitudes and emotions, and ultimately body sensations; the right words can lead a patient to optimism, whereas the wrong words can produce despair.”
Drug Prescribing A practitioner who prescribes drugs must know: • Natural course of the disease he is treating. • Pharmacological actions and toxicity of a drug he uses. • Reasons for choosing a particular drug. • Methods of assessing drug efficacy and safety; and • The possible interactions when several drugs are administered concurrently. Drugs should be prescribed only when: • There is a clear indication for them; and • The benefit to be expected from them to the patient outweighs the possible harm, immediate or remote. Whenever possible, drugs should be prescribed by their official (i.e. generic or nonproprietary) names rather than by proprietary names. A proprietary name is a trade name applied to a particular formulation by its manufacturer. Drugs sold under non-proprietary or generic names are cheaper than those sold under proprietary names. Life saving drugs may be an exception; this is because sometimes the generic formulations for some reason may not come up to the same standards of quality as branded drug formulations unless there is a strict quality control. New entrants to the drug market are invariably costly; however, many costly preparations sold in the market are neither new nor necessarily better than older, established, inexpensive preparations. In fact, established drugs are often introduced in the market under various brand names, either alone or often in various combinations and are sold at a fancy price, purely with effective advertisement and modern sales promotion techniques. It should be remembered that real wonder drugs are rarely born and do not require much advertisement; an important principle of marketing behind advertisement and sales promotion is to create a demand where real need does not exist. Practitioners, therefore, should always have a critical outlook towards accepting a new remedy. Many times, drugs are marketed without adequate and reliable clinical trials and sometimes with excessive claims regarding their properties and superiority over the established remedies. It is surprising that such products are often prescribed even though there is neither reliable evidence of their merit nor their safety. How you use a drug is often more important than which drug you prescribe. Proper use requires familiarity with both therapeutic and toxic effects of drugs. This is difficult if one switches from one drug to another frequently. Usually, it is beneficial to be slow in accepting any new agent (Be neither the first nor the last to start a new drug). In practice, the initial choice of the drug and the dose regimen will depend upon the correct diagnosis, the severity of the disease and the presence of complication/associated disease. In addition, the risk/benefit and cost/efficacy ratios of the drugs selected should also be taken into account. It is unfortunate that doctors often over-prescribe drugs for trivial complaints. The reasons for this are not clear. Although this has some relation to increased demand by patients for drugs, the major fault probably lies with the medical profession. Many busy practitioners find it difficult to keep in touch with the current literature and are easily persuaded by the promotional techniques used by the pharmaceutical industry. Others
probably would like to impress their patients by their ‘most up-to-date’ knowledge about ‘the latest drugs’, while some may not even bother about what preparation they prescribe and what drugs it contains. Majority of them are neither aware of nor do they bother to know about the cost of the preparation. Dosing during therapy: As experience accumulates, physicians find that smaller doses work equally well, and are safer and more economical. It is well known that individuals vary in their response to drugs. The importance of pharmacogenomics is now being increasingly recognised. New drugs are often introduced at a dose that will be effective in about 90% of the target population, because this is known to help market penetration. Doses are also partly determined by an irrational preference for round numbers. A sizeable minority (30%) are likely to be needlessly overdosed by following doses recommended during early marketing period of a new drug. (PDD; Chapter 2), which may cause adverse effects in some. This should be borne in mind while prescribing a drug, especially a recently marketed one. Dose searching should continue even after a drug is marketed; atenolol, a beta blocker, used initially in the dose of 100 mg daily, in the treatment of hypertension has been found to be equally effective in the dose of 50 mg daily. Various formulae used formerly to calculate the dose give only gross estimation. They are based on the average body weight of 70 kg. In many countries average body weight is less than 70 kg. Further it is now established that quantitative response to drug differs in different population e.g. Americans, Asian- Americans need much smaller doses of psychotropic and antihypertensive drugs than Caucasians. A wise general policy is “Start low, go slow”, particularly in children and the elderly, except perhaps during an emergency. It is important to note that elderly (> 65 years) may suffer from dementia; they may forget to take the drugs or swallow more amount. This makes it mandatory to monitor compliance. Herbal remedies: There is no objection to prescribing proven plant preparations. Several drugs used in allopathic medicine have been derived from plants. However, while prescribing such heavily promoted herbal remedies, the physician should keep in mind the following: • These preparations are difficult to standardise. • Tall claims are made about their efficacy without adequate scientific data. • Herbal remedies previously thought to be innocuous are known to be potentially toxic. The best example of this is the Chinese herbal teas which are now known to be hepatotoxic. • The regulations by Drug Controller General of India (DCGI), applicable to the modern drugs, are not applicable to them. If not processed properly, herbo-mineral formulations (bhasmas) can lead to toxicity. Their safety during pregnancy is unknown. • The herbal combinations offered and the doses recommended are not supported by any scientific data. • It is almost impossible to test for the presence of the ingredients mentioned in the package insert/container label. • They are not always as cheap as claimed. Some of the so called ‘herbal medications’ have been found to be adulterated with modern drugs such as glucocorticoids, diclofenac, benzodiazepines, phenytoin, statins and sildenafil.
Fixed-dose combinations: Use of rational drug combinations is helpful. They may: • Be convenient and improve compliance; • Enhance the efficacy of therapy, e.g antituberculosis therapy, where they prevent drug resistance; and combined estrogen-progesterone contraceptive pills which demonstrate synergy; or • Reduce drug toxicity, e.g. levodopa-carbidopa combination. However, combinations of drugs such as antimicrobials expose the patient to additional toxicity, and sometimes may even reduce the effectiveness of therapy. Further, it is difficult to adjust the dose of the individual drugs. Such drug formulations are always expensive and not necessarily more effective. Hence, a combination should not be prescribed unless there is reason to consider that the patient needs all the drugs in the formulation and that the doses are appropriate and will need no individual adjustment. Often, such combinations: (1) Contain inadequate dose of the main ingredient e.g. ampicillin and cloxacillin (250 mg of each). (2) Contain a second agent of doubtful efficacy e.g. serratiopeptidase added to paracetamol or NSAID. (3) Have ingredients that may exert only additive effects e.g. ibuprofen and paracetamol. (4) Are not supported by scientific evidence and may be irrational; and (5) Are costly. Table 3.1 outlines the principles of prescription writing. The drugs prescribed should be the most suitable, the least expensive and easily available. Prescribing “fancy and expensive tonics” to patients who can hardly afford two meals a day is also unethical. It is mandatory to explain to the patient what to expect from the drug(s) prescribed, and their possible ADRs. Table 3.1 Principles of prescription writing
Some abbreviations used in prescription orders are: od = once a day;/d = per day; bid = twice a day; tid = thrice a day; qid = four times a day; q as in q4hr = 4 hourly; HS = at night; sos = as required e.g. a sleeping pill (Chapter 8); prn (pro re nata) = an extra dose as required, in addition to the basal orders e.g. an analgesic (Chapter 11).
It is not enough simply to note the failure of drug therapy and the adverse reactions produced. If one has to make any purposeful decision about the future use of the drug, it is necessary to know more about it. What is the cause of the failure? How useful is the drug usually? Is it commonly used? For what condition is it usually given? What good does it do? Is the risk in its use worth the benefits expected? Can therapy be improved? This constitutes a therapeutic audit and would help the doctor in using drugs in future cases
more rationally and effectively. Pharmacoeconomics: Not only the poor countries but even the rich nations are now finding it difficult to control the ever-rising cost of medical care. For several decades, this book has been consistently emphasising that while selecting a drug, its cost-effectiveness (getting the maximum benefit at the minimal cost) and cost-benefit ratio should be taken into consideration. While trying to prolong the patient’s life, ensuring that his/her quality of life is also enhanced is equally important. Hence, while selecting a drug it is the doctor ’s duty to keep in mind its real need, cost and affordability to the patient. How many practitioners know that the cost of the same drug promoted under different brand names may vary as much as 5-10 times? Dermal preparations containing the newer potent steroids are far more expensive than equally effective, well established, older preparations. This approach will help more prudent deployment of the available resources for better medical care in the society at large. Patient compliance: Except when hospitalised, patients are responsible for taking their own drugs. Often, there is a discrepancy between what is prescribed and what the patient actually takes. The reasons for non-compliance are: • Complexity of the regimen (several drugs to be taken several times a day). • Cost. • Adverse reactions. • Poor motivation. • Length of therapy and • Natural disinclination to take injections. The drug regimen should be as simple as it can be kept: as few drugs as possible and once or twice a day administration, if permissible. Cost being a major consideration in long-term treatment (e.g of hypertension), only cost-effective drugs should be prescribed; this often means prescribing drugs by their generic names. The patient’s motivation may be improved by personal contact and constant reminders that “drugs do not work unless you take them”. A sympathetic discussion about the difficulties of drugs prescribed and about the possible adverse effects is likely to have a salutary effect on compliance. On the other hand, mere distribution of printed leaflets about drugs prescribed, diet etc, may educate the patient but is less effective in improving patient compliance. Moreover they are useless in an illiterate population. Occasionally, it is necessary to take into confidence the patient’s care takers, particularly with old or mentally disturbed patients. Although, a surprise check on urine or plasma level of the drug or its metabolite helps to detect defaulters, it may be impracticable. Drug ‘generations’: In recent times, newer drugs belonging to an existing group are often promoted as 2nd and 3rd ‘generation’ drugs, implying that they are universally superior to the older members of the group. Often, ‘first generation’ means that these drugs have been available for many years while second generation drugs are made available recently. The differences are usually minor and mostly in pharmacokinetics. The ‘first generation’ drugs may still be the drugs of choice in selected indications. For example: cephalosporins of the first generations are (among cephalosporins) the preferred drugs in Gram positive infections. Drugs expiry date: Do the drugs become useless after the expiry date mentioned by the manufacturers? No. Drug companies, because of certain legal compulsions and liability
concerns, will not advocate such use after the expiry date mentioned on the package which is usually 2-3 years from the date of manufacture. Shelf-life of a drug is the time where a given product, stored under reasonable conditions, is expected to remain stable (> 90% of potency). Most of the drugs remain stable for a long time beyond their expiry dates. According to Medical Letter (Volume 44, 93, 2002), “There are virtually no reports of toxicity from degradation products of outdated drugs (except tetracycline). How much of their potency they retain varies with the drug and the storage conditions, especially humidity, but many drugs stored under reasonable conditions retain 90% of their potency for at least 5 years after the expiration date on the label, and sometimes much longer ”. However, one has to be more careful in case of biological drugs (Chapter 74). Spurious drugs: Spurious drugs present a serious health problem. In some countries, counterfeit medicines may constitute 20-50% of the available products. They may comprise low quality manufacture of correct ingredients; wrong/undisclosed ingredients; adulteration; insufficient quantity of ingredients; false labelling; or no active ingredient at all. Examples are glucocorticoids added to herbal medicines for asthma and arthritis; and turmeric dispensed as tetracycline. For detection of counterfeit drugs, one needs a good infrastructure for vigilance and for enforcement of drug regulations.
P-drug (Personal drug) Concept In practice, the physician usually needs 50-60 drugs routinely to treat common ailments. Pdrugs are the drugs one chooses for prescribing regularly and with which one becomes familiar. Such a list would spare the physician repeated search for a better drug from among the many available. Guidelines for choosing P-drugs are outlined in Table 3.2. Table 3.2 Steps for selecting P-drugs
Sometimes, although a drug may be very efficacious it may not be suitable in a given patient because of other patient-related factors such as renal/hepatic damage, diabetes mellitus, pregnancy, lactation or drug allergy. The cost of treatment and, especially, the cost/benefit ratio of a drug or a dosage form is also a major selection criterion. Many patients may have to be treated with a less-than-ideal drug which has to be accepted because of the unaffordable cost or non-availability of the ideal drug. Personal treatment (P-treatment) and P-drugs are not identical. Not every P-treatment includes a P-drug, e.g. life style modifications in obesity. As with P-drugs, the choice of Ptreatment is guided by the therapeutic objectives in a given patient, and depends upon efficacy, safety, suitability and cost of the treatment. With the availability of increasing number of drugs and formulations, and the flood of promotional material, it is desirable to prepare one’s own pocket P-drug formulary. It should contain not only names of P-drugs but also dosage form, dosage, frequency and duration of administration of the drugs; patient–teaching material about the drugs; and patient monitoring information for doctor ’s own use. Of course, one has to keep oneself well informed about the merits and demerits of new drugs, and revise one’s P-drug list from time to time.
Essential Drugs As per WHO definition: “Essential drugs are those drugs that satisfy the health care needs of the majority of the population. They should, therefore, be available at all times in adequate amounts and in appropriate dosage forms, at a price the individual and the community can afford.” The list of such drugs is presented at two levels. (1) The core list which indicates the minimum drug needs for a basic health care system, listing the most cost-effective drugs for priority conditions; and (2) The complementary list which consists of drugs for priority diseases that are costeffective but not necessarily affordable, or may need specialised health care facilities. This list also includes essential drugs for less frequent diseases. The section on reserve antiinfective agents could thus be integrated into the complementary list. This concept has been advocated for use in hospitals and has many advantages: • It improves the practitioner’s knowledge of the drugs he is prescribing and hence the quality of prescribing. • It reduces the cost of patient care. The drugs from such selected lists should be made available at all times, even at the remotest places in the country. Obviously, the list will differ according to factors such as local disease pattern, disease incidence, available infrastructure and the financial resources at disposal. Genetic, environmental and demographic factors have also to be taken into account. Only those drugs which are of proven value, relatively safe and cost-effective are included. Such lists need revision from time to time. Fixed-dose combinations are acceptable only when the dosage of each ingredient meets the requirements of a defined population group, and the combination has advantage over a single compound in therapeutic effects, safety and compliance e.g. antituberculosis drugs. Many countries have now prepared essential drug lists based on their priorities. Of course, it is not correct to say that drugs not included in such lists are all ‘nonessential’ and hence not required. Some of them are indeed useful for special indications under specific circumstances. Many anti-cancer drugs not appearing in the ‘Essential Drugs List’ are important in special circumstances.
Orphan Drugs Orphan drugs are drugs meant for the diagnosis, prevention or treatment of rare diseases. They are not easily available because their manufacture is not commercially-viable for various reasons which include: • Their limited demand • Enormous cost of production • Non-patentability of the drug; and • The complex and costly procedure for establishing the efficacy and safety of the drug, and for the governmental approval process. Some examples of orphan drugs are: certain anticancer and antiviral drugs; certain antiparasitic drugs (pentamidine) and drugs used in the treatment of rare genetic enzyme deficiencies. Further, one should also remember that some of the older drugs, which had been proven useful for many years and have high cost-effectiveness for routine use, have now become “orphan drugs” because their commercial production is no longer profitable e.g. cyanocobalamine. Drugs such as chlorpropamide, reserpine and diloxanide furoate are not easily available. They have become “the victims of myth (that newer drugs are necessarily superior), mastermarketing and fashionable prescribing.”
Factors Modifying the Effects of a Drug Individuals differ both in the degree and the character of the response that a drug may elicit. The doses of official preparation of drugs are, therefore, always expressed in the form of a range which gives the therapeutic effect in majority of subjects. The important factors which influence the effect of a drug are: I Body weight: The average dose is mentioned either in terms of mg. per kg. body weight or as the total single dose for an adult weighing 50-100 kg. The smaller dose should be used in Indians whose average weight is about 50 kg. However, dose expressed in this fashion may not apply in cases of excessively obese individuals or those suffering from edema, dehydration or emaciation. Nutritional factors can sometimes alter drug metabolising capacity and this should be kept in mind in undernourished patients. II Age and sex: Children may not react to all drugs in the same manner as young adults. The pharmacokinetics changes with age. Thus, gastric emptying is prolonged and gastric pH fluctuates in neonates and infants. Their liver capacity to metabolise drugs is low, renal function is less well developed and proportion of body water is higher in the infants. The metabolic clearance of drugs such as chloramphenicol, barbiturates, pethidine, salicylates, sulfonamides, diazepam and aminoglycosides is less in infants than in adults. The pharmacodynamics of drugs in children may differ from that in adults as well. Drugs may show unique effects in children, not seen in adults. Thus, some substances may disturb the patterns of growth and development that occur only during particular periods of life e.g. tetracyclines and glucocorticoids in children. Metoclopramide and other dopamine antagonists produce acute dystonic reactions more often in children and adolescents than in adults. With few exceptions, drugs are more active and more toxic in the newborn than in adults. The doses of antidiphtheria serum and antitetanus serum, however, are not modified by age. The pediatric doses are expressed in terms of body weight (mg/kg per dose or per day) or in terms of body surface area (mg/sq.mt. per dose or per day) (Table 3.3). In practice,
Table 3.3 Determination of children’s doses from adult doses on the basis of body surface area* Weight (kg) Approx. surface 2 4 6 8 10 15 20 25 30 35 40 45 50 55
area in square meters Approx. percentage of adult dose 0.15 9 0.25 14 0.33 19 0.40 23 0.46 27 0.63 36 0.80 46 0.95 55 1.08 62 1.20 70 1.30 75 1.40 81 1.51 87 1.58 91
**
A.M.A. Drug Evaluation 1973 (2nd Ed.) *
Based on Done, A.K.: “Drugs for Children” in Modell, W. (Ed) Drugs of Choice 1972–73. St. Louis : The C.V. Mosby Co., 1972.
**
Based on average adult surface area of 1.73 sq meters.
it is better to rely on a handy reference book than on one’s memory or above calculations (Chapter 80) during neonatal and pediatric prescribing. Adult doses should be used in children over 55 kg of weight and in those who have achieved puberty. Old people also present problems in dosage adjustment and this may vary widely with different people. While prescribing the drugs for elderly (particularly after 60 yrs), it is importnant to remember that in the elderly the body fat may increase while total water and lean body mass decrease. Further, plasma albumin concentration may be lower. As age advances, liver and kidney mass and blood flow decreases so that the metabolic inactivation and renal excreation of drugs slows down. In the elderly, the serum creatinine level may be within normal range even though the creatinine clearance is markedly reduced. These changes demand modification of dosage regimen. Pharmacodynamically, the elderly are more sensitive to CNS drugs such as neuroleptics, hypnotics, sedatives and respiration depressants. The response to drug acting on β adrenergic receptors is attenuated while possibility of postural hypotinsion with antihypertensive is increased due to reduced sensitivity of baroreceptors. It is known that incidence of ADR rises with age. Hence, it is important to select proper (less toxic) drugs, dose regimen and formulation while prescribing for the elderly. Often, elderly suffer from senile dementia which make it necessary to monitor compliance. Also remember that in the elderly, “stopping a drug is equally important as starting it.” Central depressants such as hypnotics and tranquillisers may produce confusional states or falls in the elderly. Some drugs needing special care in old people are listed in Table 3.4.
Table 3.4 Some drugs to be used with special care in the elderly
In general, the elderly patients should be prescribed as few drugs as possible, preferably those with less serious ADR, for short periods. Detailed, previous, drug history should be elicited before prescribing drugs to the elderly. Gastric metabolism of alcohol is lower in women than in men, which is responsible for gender related differences in blood alcohol levels. III Pregnancy and lactation: Special care should be exercised when drugs are prescribed during pregnancy and lactation. • Pregnancy: Drugs which may stimulate the uterine smooth muscle are contraindicated during pregnancy. Further, many drugs administered to the mother are capable of delaying the onset of labour or of crossing the placenta and affecting the fetus (Chapter 80). • Lactation: See Chapter 80. IV Diet, Tobacco, Alcohol and Environment: Medicines are usually taken after a meal to reduce the risk of gastric irritation, nausea and vomiting. Food, however, can have significant effect on the pharmacokinetics of drugs (Table 1.5). Generally, food depresses the rate and the extent of drug absorption. Drugs may be given on empty stomach (i) to prevent mixing with the foodstuffs e.g. the anthelmintics, (ii) to get an immediate action e.g. drugs used for motion sickness, and (iii) to prevent drug inactivation in the stomach, e.g., penicillin V. Tetracyclines form insoluble chelates with aluminium, calcium and magnesium salts, which reduces their absorption. Captopril, digoxin, thyroxine sodium and rifampicin are examples of drugs better absorbed on empty stomach. The dose of a hypnotic required to produce sleep during daytime is higher than that required to produce sleep at night. High altitude with low barometric pressure diminishes the capacity of the body to oxidize drugs and this may precipitate drug toxicity. Polycyclic hydrocarbons present in cigarette smoke and hydrocarbon pesticides such as DDT induce hepatic microsomal CYP enzymes. This accelerates the biodegradation of several drugs. Alcohol modifies the response to many drugs. It also induces hepatic enzymes and causes rapid metabolism of certain drugs. On the other hand, hepatic injury due to alcohol can enhance response to drugs (Chapter 6). V Route of administration: Intravenous doses of the drugs are usually smaller than oral doses, particularly in case of drugs which are incompletely absorbed orally e.g. digoxin. The onset of drug action is quicker with the IV route; this might enhance the chances of drug toxicity.
VI Psychological factors: The personality of the physician may influence the drug effect considerably, particularly if the drug is intended for use in a psychosomatic disorder. Inert dosage forms called placebos are known to produce therapeutic benefit in conditions like angina pectoris and bronchial asthma (Chapter 4). The personality of the patient and the physician may also modify the drug effect. The dose of chlorpromazine required to produce tranquillisation in an otherwise normal individual is 50 to 100 mg/day. However, the same drug has to be administered in the dose of 500 to 1000 mg/day to achieve the quietening effect in highly agitated schizophrenic patients. VII Genetic factors: Variations in the human genome determine the genetically mediated differences in drug metabolism and response (pharmacogenomics). The latter are usually due to defective/deficient enzyme systems responsible for inactivating the drug. Often, this results in drug accumulation and toxicity. Patients with hereditary disorders of intermediary metabolism such as diabetes mellitus rarely show a disturbance in the metabolism of drugs and other foreign compounds. This is because the microsomal enzyme system, involved in the metabolism of drugs, does not participate to a significant extent in the intermediary metabolism. Some examples of genetic variations are: • Acetylation and hydroxylation of drugs: The rate of acetylation of INH, dapsone, hydralazine and some sulfonamides is controlled by an autosomal recessive gene and the dosage of these drugs depends upon the acetylator status of individuals. Similarly, slow hydroxylators are liable to exaggerated responses (excessive beta blockade with metoprolol) and to drug toxicity (lactic acidosis with phenformin). • Plasma cholinesterase (Pseudocholinesterase): Some persons inherit a modified type of esterase (atypical pseudocholinesterase) that is less efficient than the normal enzyme in hydrolysing the drug succinylcholine. Such people may develop prolonged respiratory paralysis even with a therapeutic dose of succinylcholine. • Phenytoin hydroxylation: Certain individuals are unable to p-hydroxylate diphenylhydantoin and develop marked toxicity, during phenytoin therapy of epilepsy. • Hepatic CYP2D6 enzyme variation: Tricyclic antidepressants (TCA) are mostly metabolised by CYP2D6. Patients with mental depression exhibiting slow metabolism need much smaller doses of TCA than the fast metabolisers. On the other hand, failure to respond to TCA is common in ultra-fast metabolisers. • Erythrocyte diaphorase: The enzyme erythrocyte NAD-diaphorase protects the erythrocytes by reducing methemoglobin to hemoglobin. Individuals with a hereditary deficiency of this enzyme are likely to develop methemoglobinemia after administration of drugs such as sulfonamides and nitrites. • Glucose-6-phosphate dehydrogenase (G6PD): Primaquine and certain other drugs cause hemolysis in individuals with a deficiency of G6PD (Chapter 36). • Miscellaneous: These include inherited abnormal drug response such as: (i) Resistance to coumarin anti-coagulants. (ii) Chinese patients tend to respond to lower doses of propranolol than do the Western patients although the metabolism of propranolol is significantly faster in the Chinese. Genetic variations in the activity of alcohol dehydrogenase and aldehyde dehydrogenase
among various ethnic groups have been reported. About 90% of Whites have in their liver a form of alcohol dehydrogenase which metabolises alcohol in vitro more slowly than the corresponding liver enzyme in 90% of Orientals. In 50% of Asians inactive form of aldehyde dehydrogenase is observed due to mutation. As a result they have higher levels of acetaldehyde following alcohol ingestion, which causes facial flush and other intense responses. (iii) Barbiturates markedly enhance the activity of the hepatic enzyme delta-aminolevulinic acid synthetase leading to a marked rise in the rate of porphobilinogen synthesis. This precipitates an acute attack of porphyria in susceptible individuals. (iv) Precipitation of severe hyperpyrexia, muscle rigidity, hyperkalemic cardiac arrest and death (malignant hyperthermia, an autosomal dominant condition) by anaesthetics such as halothane, methoxyflurane and cyclopropane. VIII Metabolic disturbances: Changes in water-electrolyte and acid-base balance, body temperature and other physiological parameters may modify the effects of drugs e.g. (i) Aspirin reduces body temperature only in the presence of pyrexia. (ii) The vasoconstrictor effect of noradrenaline is reduced in the presence of metabolic acidosis. (iii) The absorption of iron from the gut is maximum if the individual has an iron deficiency anemia. Hypokalemia can enhance digoxin cardiotoxicity. IX Presence of disease (Comorbidity): Drugs like morphine and chlorpromazine may produce unusually prolonged effect in cirrhotic patients. Antibiotics like streptomycin and kanamycin, excreted mainly by the kidneys may prove toxic if the kidney function is impaired. In myxedema, morphine acts for a much longer time because of the low rate of oxidation. In congestive heart failure, the clearance of lignocaine may diminish by 50%. Pulmonary and GI disease may also alter pharmacokinetics. X Cumulation: If a drug is excreted slowly, its repeated administration may build up a sufficiently high concentration in the body to produce toxicity e.g. digoxin, emetine and heavy metals. Sometimes, a cumulative effect is desired e.g. with phenytoin in the treatment of epilepsy. Most often, however, it is undesirable. Substances like lead can remain deposited in bones without producing toxic effects. This is called passive cumulation; toxic manifestations occur as soon as it is released into the blood. To avoid cumulation: (a) One must know if the drug is eliminated slowly or rapidly. (b) Stop the drug administration at the appearance of the first warning symptom. (c) Select carefully the form in which the drug is to be administered; and (d) Check liver and kidney function before and during drug administration, as even an otherwise non-cumulative drug would produce cumulation in the presence of hepatic and renal damage. XI Other drugs and chemicals: Previous or concurrent therapy with certain drugs may result in stimulation or inhibition of the metabolism of other drugs. Both tobacco smoke and alcohol consumption induce CYP450 liver enzymes. They accelerate the metabolism of a number of drugs,
leading to a reduction in their therapeutic effects (see later). XII Additive effect (Summation): See Chapter 2. XIII Synergism: See Chapter 2. XIV Antagonism: See Chapter 2. XV Drug tolerance: When a large dose of a drug is required to elicit an effect ordinarily produced by its normal therapeutic dose, the phenomenon is termed as drug tolerance. Drug tolerance is of two types: (I) True tolerance: This is seen on both oral and parenteral administration of a drug and can be: (a) Natural or (b) Acquired. (a) Natural tolerance: This is seen in various animal species and also among the various human races. It includes: • Species tolerance: Certain animal species can tolerate certain drugs in quantities lethal to man, e.g., some rabbits can tolerate large quantities of belladonna. This is attributed to the enzyme atropine esterase in their liver and plasma, which rapidly detoxifies belladonna. • Racial tolerance: A solution of ephedrine instilled into the conjunctival sac of the Caucasians produces prompt dilatation of the pupil but in Negroes it may not produce any dilatation. (b) Acquired tolerance results only on repeated administration of a drug and may take weeks or months to develop e.g opiates, barbiturates, nitrates and xanthines. Tolerance is sometimes desirable, e.g., barbiturates, when used in the treatment of epilepsy, produce tolerance for their soporific but not for their antiepileptic effect. Generally however, tolerance is undesirable. • Tissue tolerance: In this type, the development of tolerance is confined to certain effects or to certain systems, e.g., morphine produces tolerance for its euphoriant effect, but the pupils and the GI tract do not become tolerant. Thus, the same dose of morphine invariably produces pinpoint pupils and constipation but may fail to produce euphoria. • Cross tolerance: If an individual initially develops tolerance to a drug belonging to a particular group, he also shows tolerance to other drugs from the same group. This phenomenon is known as cross tolerance e.g. that between alcohol and the general anaesthetics like ether. (II) Apparent or pseudotolerance: The feudal kings, much worried about poisons, were often in the habit of taking small doses of arsenic by mouth. This apparently rendered them immune to oral arsenic but poisoning could occur if any other route was chosen. This tolerance is probably due to the local changes developed by the GI tract which prevent the poison from getting absorbed from the gut. Mechanism of development of tolerance: Tolerance can be: (a) Pharmacokinetic (Dispositional); or (b) Pharmacodynamic (Functional) Dispositional tolerance is due to changes in drug pharmacokinetics leading to decreased intensity and duration of contact between a given drug and the target tissue. Thus, the barbiturates after repeated administration enhance their own degradation by inducing hepatic microsomal enzyme systems. In many tumours p- Glycoprotein transporters pump out the administered anti-cancer drugs and make them resistant to therapy.
Functional tolerance is probably due to changes in the properties and functions of the target tissue, that make them less sensitive to a given drug concentration. Thus, it is associated with some cellular changes. With some drugs, this may be related to a decrease in drug receptors (down-regulation). With compounds like morphine, alcohol and barbiturates, it has been demonstrated that the cells of the CNS, which usually develop tolerance to these drugs, become capable of normal physiological functions in the presence of high concentrations of these drugs. The adaptive mechanisms involved are not clearly understood. Tachyphylaxis: Tolerance to drugs as described above usually takes some time to develop. However, with certain drugs like ephedrine, tyramine, amphetamine and 5hydroxytryptamine, tolerance may appear within a few minutes in isolated preparations as well as in the intact animals. Thus, if any of these drugs is administered repeatedly, at very short intervals, the pharmacological response elicited decreases progressively. This phenomenon is known as tachyphylaxis or acute tolerance (Fig 3.1). Repeated doses of ephedrine at short intervals, in the treatment of bronchial asthma, may produce diminishing response.
FIG. 3.1 Effect of repeated doses of tyramine on BP in anaesthetised dog. Black dots denote administration of the same dose. Note the decreasing response.
Various mechanisms are responsible for the appearance of tachyphylaxis. With tyramine, it is due to depletion of the noradrenaline stores from the sympathetic nerves. However, with sympathomimetics like ephedrine and amphetamine, tachyphylaxis can occur without appreciable depletion of the noradrenaline stores. Tachyphylaxis probably can occur if the drug dissociates slowly from its binding to the receptor, thus continuing receptor blockade while losing its intrinsic activity and hence its pharmacological effect. Tachyphylaxis with isoprenaline is accompanied by a decline in beta receptor number while receptor affinity for the agonist remains unaltered. With other drugs, however, tachyphylaxis is probably due to some unidentified ‘adaptive response’ of the tissue concerned. XVI Drug dependence: Repeated administration of certain drugs may induce a habit and dependence. If the habit forming agent is not made available to the habitué, he develops withdrawal symptoms characterised by psychic/physical disturbances like headache, restlessness and emotional upset and/or convulsions and vasomotor collapse. WHO has defined drug dependence as “a state, psychic and sometimes also physical, resulting from the interaction between a living organism and a drug, characterised by behavioural and other
responses that always include a compulsion to take the drug on a continuous or periodic basis in order to experience its psychic effects and sometimes to avoid the discomfort of its absence. Tolerance may or may not be present. A person may be dependent on more than one drug”. Withdrawal of a drug can precipitate a drug-withdrawal syndrome. Drug dependence is of three types (Table 3.5): Table 3.5 Important drugs known to cause dependence
• Opiate or morphine type: Morphine and its congeners like codeine, dihydromorphinone and heroin. Synthetic morphine substitutes such as meperidine (pethidine) and its congeners, methadone and its congeners, morphinan compounds, pentazocine and diphenoxylate • Alcohol-barbiturate type: Ethyl alcohol, barbiturates, paraldehyde, chloral hydrate, meprobamate, benzodiazepines and methaqualone. • Nicotine (tobacco). II Drugs that cause definite psychic but mild or questionable physical dependence: • Opiate antagonist type: Morphine antagonists like nalorphine; morphinan antagonists like levallorphan; • Amphetamine type: Amphetamine, methamphetamine and phenmetrazine. Piperidines like methylphenidate and pipradol. III Drugs that cause only psychic dependence: Cocaine, LSD, psilocybin, mescaline, cannabis (marihuana, hashish), caffeine (coffee, tea). IV Volatile substances: Glue, nail varnish, petrol, paint solvents, hair spray etc. -->
(a) Psychological; (b) Physical; and (c) Combined A condition in which a drug produces “a feeling of satisfaction and a psychic drive that require periodic or continuous administration of the drug to produce pleasure or to avoid discomfort”, is called psychic dependence. In case of physical dependence, the body “achieves” an adaptive state that manifests itself by intense physical disturbances when the drug is withdrawn (withdrawal syndrome). The term ‘addiction’ used formerly to denote the phenomenon involving both psychic and physical dependence on drugs, is currently designated as drug abuse. Its characteristics include: • An overpowering desire (compulsion) to continue taking the drug in spite of knowing its harmful effects. • A tendency to increase the dose; and • A high tendency to withdrawal symptoms. Compulsive drug use is commonly but not necessarily associated with the development of tolerance and physical dependence. There are some drugs where tolerance and physical dependence develop after chronic use; but they are not self administered nor used compulsively. Man has long sought ways of enhancing his pleasure and of easing his discomfort. Various agents are consumed to achieve this goal. Commonly used beverages like tea and coffee stimulate the CNS and are capable of producing drug dependence but this is not necessarily harmful in itself. Tobacco (nicotine) on the other hand is a dependenceproducing agent capable of causing physical harm to the user although it produces relatively little stimulation of the CNS. Most of the drugs used by the addicts have predominantly CNS effects. Such drugs as opiates, barbiturates, alcohol and cocaine all produce sense of well-being in the user. This is termed euphoria and contributes considerably to the development of dependence. Stimulation of the CNS probably plays an important role. However, when exposed to these drugs under similar environmental influences, all the recipients do not develop dependence. It is not clear why some individuals stop after initial experimentation. Others continue drug use but do not become dependent and still others become compulsive drug users or addicts. But many of those who develop dependence may have some psychological
problems. Besides the user ’s personality, availability of the drug plays an important part in the development of drug dependence. Thus, for the development of drug dependence, both the ‘seed’ and the ‘soil’ are required. A potential addict may start and continue taking a dependence inducing drug: • Following its medicinal use. • To achieve a sense of relief from stresses and tension of life. • To satisfy curiosity about drug effects. • To achieve a sense of belonging, to be ‘accepted’ by others in the group. • To express hostility or independence. • To have pleasurable (euphoric), new, thrilling or even dangerous experiences. • To gain an improved understanding or creativity; to escape from reality and to have a dreamy state. Drugs of abuse are usually taken orally, and sometimes IV or by inhalation. The latter routes give much higher plasma concentrations, which are associated with a feeling of ‘kick’ or ‘flash’ resembling an orgasm. Drugs may be abused continuously e.g. heroin, or intermittently to produce short term euphoric effects during rave parties e.g. cocaine or cannabis. The experience achieved by the individual under the influence of the drug is so impressive that he develops a craving for it and finds it difficult to give it up. These drugs themselves are powerful reinforcers. Hence, they are also called masterful drugs. Drugs that cause serious disability of functioning normally by inducing psychic/physical dependence are called Hard Drugs e.g. cocaine and heroin. Factors which appear to facilitate the initiation of drug abuse are given in Table 3.6. Table 3.6 Factors which facilitate initiation of drug abuse
Mechanism of drug dependence: Exact mechanism is not known. Alteration of the cellular metabolism of the CNS is a prime factor in the development of dependence (Chapter 6). It appears that these drugs affect the glutaminergic and dopaminergic transmission in amygdala and ventral striatum of the forebrain. Other systems may become tolerant to the drug but only the CNS is capable of developing physical dependence. Withdrawal of the drug produces distorted homeostasis leading to the development of a withdrawal syndrome or an abstinence syndrome. The withdrawal syndrome may vary from a mild to a severe one, sometimes resulting in fatality. The symptoms of withdrawal syndrome are usually characterised by rebound effects in those same physiological systems that were initially modified by the drugs. Thus, general CNS depressants, on withdrawal, cause hyper-excitation while withdrawal of central stimulant amphetamine produces weakness, lack of energy, hyperphagia and depression. Withdrawal symptoms due to drugs with long half-lives e.g. phenobarbitone are usually less severe but more
protracted. Drug dependence, once developed, is difficult to treat. To achieve any success, complete co-operation of the individual is vital. Table 3.7 summarises the principles of treatment of dependence. The details are described in the respective chapters. Table 3.7 Principles of treatment of drug dependence
It is highly desirable that drugs which are likely to be administered over a prolonged period should be screened for their dependence liability in animals.
Drug Interactions Drug interactions may result from the use of two or more drugs. This may lead to enhanced or diminished effect that may be useful or harmful. The useful drug interaction is illustrated by synergistic combinations of drugs such as antibiotics or antihypertensives. Harmful drug interactions are, unfortunately, more numerous and are discussed below. A new symptom appearing during treatment with a drug may be due to the disease or the drug. This can be perplexing enough. But, if the patient is receiving several drugs and two or more drugs are capable of causing the same new symptom as the underlying disease, or if two drugs in concert elicit symptoms that would not otherwise appear, the physician is in a quandary. In this situation, he may attribute the new symptom wrongly to the disease itself or to idiosyncrasy to one of the drugs, instead of recognising it as a drug interaction. The incidence of ADR rises with the number of drugs used. A physician who uses multiple drugs must, therefore, be constantly alert to the possibility of drug interaction. Further, he must be aware of both ‘risky drugs’ (Table 3.8) and ‘vulnerable patients’ (Table 3.9) in this respect. Table 3.8 Risky drugs
Table 3.9 Vulnerable patients
It is absolutely essential that when a patient’s clinical condition changes, particularly if he is severely ill or elderly, all drug treatment should be reviewed as a matter of course. Drug interactions may occur either outside the body or in the body. I Drug interactions outside the body: The most glaring examples of this are seen when several drugs are mixed in an IV infusion. One or more of the drugs may get inactivated or even precipitated. This is often attributable to changes in the pH of the solutions. The following are some of the examples:
• Use of wrong vehicle for infusion: No drug should ever be added to blood, plasma, amino acid solutions, fat emulsions (which tend to crack), sodium bicarbonate solution, mannitol solution (from which mannitol tends to crystallise) and to heparin infusion. An infusion set should be discarded after administration of blood and not used for other infusion fluids.
Highly acidic solutions (pH may be as low as 3.5) such as dextrose or fructose are unsuitable as vehicles for sodium or potassium salts of weakly acidic drugs such as phenytoin, barbiturates, methicillin and novobiocin, as the latter tend to get precipitated at this pH. Drugs such as benzyl penicillin, ampicillin, heparin and aminophylline are unstable at the pH of these solutions. Dextrose solution is, however, eminently suitable for infusing noradrenaline which is stable at the acidic pH. Isotonic saline is slightly acidic or neutral and is a suitable vehicle for most drugs with the exception of noradrenaline. If noradrenaline has to be infused in isotonic saline, vitamin C should be added to the infusion. Most antibiotics become unstable and deteriorate in large volumes of fluid. Erythromycin lactobionate is unstable in electrolyte solutions but may be diluted with 5% dextrose solution. Amphotericin B should be diluted with 5% dextrose of a pH recommended by the manufacturer, but not with saline. • Addition of drugs to an infusion: Drugs should be added to infusion containers only when constant plasma concentrations are needed or when the administration of a more concentrated solution would be harmful. No more than one drug should be added to any infusion container and the components should be compatible. Solutions should be mixed thoroughly and checked for absence of particulate matter before use. The container should be labelled with the patient’s name and the quantity of the additive and with the date and time of the addition. Infusion should be examined from time to time for cloudiness, crystallisation or change of colour; if any sign of deterioration is observed, discontinue the infusion. Phenytoin, phenothiazines, vitamin B complex (± vitamin C), amphotericin, sulfadiazine and furosemide should not be mixed with any other drug in solution. Beta-lactam antibiotics should not be mixed with any proteinaceous material for fear of forming immunogenic and allergenic conjugates. The following undergo loss of potency when added to large volume infusions e.g. ampicillin in glucose or lactate containing infusions; mustine hydrochloride in isotonic saline; gentamicin-carbenicillin added to the same infusion. Also see Chapters 1 and 38 for IV infusions. II Drug interactions in the body can be: (a) Pharmacokinetic, one drug affecting the absorption, distribution, transport, metabolism or excretion of another drug; or (b) Pharmacodynamic, one drug altering the pharmacological action of another drug. Drug absorption: • Drugs given orally can interact in the gut to form complexes which may not be absorbed. Thus, calcium, magnesium, aluminium and iron salts interfere with absorption of tetracycline and of prednisolone. Sucralfate reduces the bioavailability of phenytoin. Such interactions can be avoided by separating the administration of the two drugs by at least two hours. • A drug altering the gastric pH can alter the solubility of another agent and thus may influence its absorption e.g. sodium bicarbonate reduces the absorption of tetracycline. Drugs can also affect the absorption by modifying the gut motility and gastric emptying. Antimuscarinic drugs and opioids can slow down the absorption of other drugs by delaying gastric emptying.
• Sorbitol accelerates the gastrointestinal absorption of paracetamol. • A few women taking low dose, combination oral contraceptives may be put to risk of pregnancy by the concurrent administration of a broad spectrum antibiotic (ampicillin or tetracycline). By reducing the bacterial flora in the intestines, these antibiotics disrupt the deconjugation and hence re-absorption of the steroids secreted into the intestine. Drug distribution: Some drugs are bound strongly to plasma proteins and remain pharmacologically inactive. Certain groups of drugs seem to share a limited number of protein binding sites and can be displaced from them by each other. This results in an increase in the unbound and pharmacologically active form of one of the drugs leading to toxicity. However, this type of drug interaction is clinically significant only with drugs which are extensively (> 90%) protein bound, have a small apparent volume of distribution (Vd) and have effects proportional to their concentration. e.g. (i) Clofibrate can displace warfarin sodium from the binding sites, leading to a bleeding tendency. (ii) Salicylates can displace tolbutamide from the binding sites, leading to hypoglycemic coma. Drug transport: Guanethidine and the related adrenergic neurone blocking drugs are actively transported into adrenergic neuron by the same transport system that is responsible for noradrenaline uptake into the neuron. This system is inhibited by the antidepressant imipramine, which interferes with the antihypertensive activity of guanethidine. Drug metabolism: • Stimulation: The synthesis of the drug-metabolising microsomal enzymes is enhanced (enzyme induction) by a number of commonly used drugs, insecticides and polycyclic hydrocarbons (Table 3.10). This reduces the efficacy and increases the therapeutic dose of the drugs metabolised by the microsomal enzymes. It should be noted that different inducers are relatively selective for certain families of CYP450 enzymes. Table 3.10 Drug-induced acceleration of metabolism of drugs Inducer Barbiturates
Drugs whose metabolism is accelerated (victim drugs) Barbiturates, oral Antic oagulants, S teroids (oral c ontrac eptives, testosterone, gluc oc ortic oids), vitamins (D and K), Thyroxine, Phenylbutazone, Phenytoin, Griseofulvin, Chloramphenic ol, Theophylline. P henytoin Gluc oc ortic oids, Vitamin D, Theophylline. Griseofulvin Warfarin Rifampicin Oral c ontrac eptives, Gluc oc ortic oids, Metoprolol, Propranolol. Carbamazepine Vitamin D.
Dicophane and gamma benzene hexachloride are powerful inducers of drug-metabolising microsomal enzymes. Hence, research on drug metabolism could be misleading if the animal quarters are sprayed with either of these insecticides. Nicotine is also a powerful enzyme inducer.
• Inhibition: Inhibition of the metabolism of one drug by another may lead to toxicity of the former. See Table 3.11.
Table 3.11 Drug-induced inhibition of metabolism of drugs Inhibitor Allopurinol Disulfiram Isoniazid Chloram-phenicol Cimetidine Fluoxetine Erythromycin Ketoconazole P henylbutazone Ethanol Valproate Grape fruit juice *
Drugs whose metabolism is inhibited (victim drugs) Tolbutamide, Methotrexate, Probenec id. Alc ohol, Tolbutamide, Warfarin, Phenytoin. Gluc oc ortic oids, Oral Contrac eptives, Carbamazepine, Phenytoin. Tolbutamide, Probenec id, Phenytoin. Warfarin, Diazepam. Warfarin, Phenytoin, some benzodiazepines. Amiodarone, Digoxin, Antipsyc hotic s, Warfarin, Theophylline. Cyc losporin, Astemizole, Terfenadine. Phenytoin, Tolbutamide. Methanol (benefic ial effec t) Lamotrigine. Cyc losporin, Terfenadine and most Calc ium c hannel bloc kers.
Many drugs inhibit the p-hydroxylation of phenytoin, increasing the chance of phenytoin toxicity. They include dicoumarol, isoniazid, disulfiram, chloramphenicol and methylphenidate. *
Contains psoralen.
Clinically significant drug interactions involving CYP family are shown in Table 3.12. Table 3.12 Enzymes of CYP family with examples of substrates and inhibitors
*
Absent in 20–30% Asians, who require low dose of substrate drugs.
**
Induced by tobacco smoking.
Drug excretion: This can be facilitated or interfered with by certain drug combinations. Thus, the excretion of weakly acidic drugs like sulfonamides, salicylates and barbiturates can be enhanced by making the urine alkaline. Probenecid inhibits the tubular secretion of penicillin, indomethacin and riboflavine. Quinidine, verapamil and amiodarone can double the plasma digoxin concentration. Receptor site (Pharmacodynamic interaction): In this case, drugs acting on the same receptor site or at different active receptors may enhance or decrease the response, e.g., tubocurarine and aminoglycoside antibiotics may accentuate the block at the neuromuscular junction; marked CNS depression is caused by concurrent administration of morphine and barbiturates. Several examples of pharmaco- dynamic drug interactions are described in respective chapters. Changes in electrolyte and fluid balance: Drugs that cause potassium depletion may potentiate the effects of digitalis and non-depolarising muscle relaxants, but antagonize the anti-arrhythmic action of lignocaine, quinidine and procainamide.
Interactions among chemotherapeutic agents: Injudicious combinations of chemotherapeutic agents may prove harmful in therapeutics (Chapter 51). Many of the drug interactions reported in the literature (especially those which are due to competitive binding to the same plasma protein) may not be clinically significant. Therefore, one should be careful in distinguishing between drug interactions which are clinically significant and those which are not. Unfortunately, drug interactions are not always predictable from animal studies. Hence, the physician should always be wary of including too many drugs in the prescription so as to minimise this danger. Further, the physician should always enquire about alcohol and tobacco consumption by the patient. Numerous drug interactions between alcohol and other drugs have been reported (Chapter 6).
4
Drug Invention; New Drug Development; and Drug Assay A new drug is defined as a new substance of chemical, biological or biotechnological origin for which adequate data is not available for the regulatory authority to judge its efficacy and safety for the proposed claim. Until recently, many drugs were invented largely by trial and error, and often by chance. In the 20th century, drug development involved sequential screening of synthetic chemicals or extracts of biological material in isolated animal organs, followed by their testing in whole animals. In late 1950s, radioligand binding assays were developed, which enabled the scientists to study the interaction of compounds with receptors and then select the ones with the best fit for studying their activity. In the late 20th century, combinatorial chemistry changed the scenario. This new technique involves computer based molecular modeling of compounds and can produce a large number of compounds with best fit to the receptor. Advances in cell biology and receptor technology have helped in developing screening systems. It involves automated, micro techniques to screen innumerable compounds in a day. One of the popular techniques today is High Content Screening (HCS), in which in vitro or cell-based assays are used to screen large numbers of compounds for their effects on cellular or subcellular targets. New areas have been developed to collect, store and interpret the information generated this way viz. bioin-formatics, chemoinformatics and functional genomics. Thus, with the help of Computer-Aided Drug Design (CADD) and HCS, ‘hits and leads’ are invented. A “Hit” is a molecule with confirmed structure, confirmed activity in primary throughput screening and a good profile in secondary assay; whereas a “Lead” is a hit series with proven structure activity relationship both in vitro and in vivo. The leads are then tested by the pharmacological methods to determine their efficacy and safety and finally in humans by clinical pharmacological studies. Ethics Committee approval is mandatory before conducting such studies in animals or humans. Prior to any clinical evaluation, the investigator should obtain reasonably clear answers to three important questions: • Is the data from animal studies adequate? • What is the probable risk involved in giving the drug to humans? Is it worth the risk? • Is there any need for a new drug in the disease under consideration and if so does the new remedy seem promising? The objectives of animal studies are outlined in Table 4.1.
Table 4.1 Objectives of animal studies
Animal Toxicity Studies In order to assess the safety of a drug, various toxicity studies are carried out in animals such as mice, rats, guinea pigs, dogs and monkeys, under varying conditions of drug administration. The detailed account of such studies is beyond the scope of this book. The important tests include: • Systemic toxicity studies (i) with a single dose; and (ii) with repeated doses. • Local toxicity studies • Specialised toxicity studies including tests for male fertility; female reproduction and fetal developmental toxicity; allergenicity/hypersensitivity; genotoxicity and carcinogenicity. I Systemic toxicity studies: (a) Single dose toxicity studies: The main object of a single dose study is to determine minimum lethal dose (MLD), maximum tolerated dose (MTD) and if possible, the target organ of toxicity. In these studies a drug is tested for the effects of a single dose. Graded doses are given in two rodent species (mice and rats of both sexes), using the same route as that intended for humans. One additional route is used in one of the species to ensure systemic absorption of the drug. Animals are observed for mortality for up to 14 days (72 hours if the administration is parenteral). Detailed observations are made of the effects of the drug on important physiological functions and body weight. Microscopic examination of grossly affected organ is carried out. LD10 and LD50 should preferably be estimated. (b) Repeated dose toxicity studies: These studies are also carried out in at least two species, of which one should be non-rodent. After the initial dose-ranging studies for MTD, the final systemic toxicity study (FSTS) is carried out. Three doses are selected: (1) the highest dose having observable toxicity; (2) mid-dose causing some symptoms but no gross toxicity or death; and (3) the lowest dose free of toxicity and comparable to the intended therapeutic dose or its multiple. A control group treated with vehicle is a must for comparison. Selected doses are administered depending on the duration of intended use in humans and the phase of the proposed clinical trial. The route of drug administration should be the same as that proposed for human use. Parameters for safety include cage side observations for eye changes, loss of fur, behavioural and physiological observations, body weight changes, food/water intake, blood chemistry, hematology and examination of organs. The sites of injections are inspected for gross and microscopic changes. ECG and fundoscopy are done in non-rodent species. Sometimes, ‘high-dose reversal study’ is included wherein animals are studied after stopping the treatment or after recovery from signs of toxicity. II Local toxicity studies: The drug is applied to an appropriate site, e.g. skin, vagina or cornea to determine local effects in suitable species (guinea pigs or rabbits). If the drug gets absorbed from such site, appropriate systemic toxicity studies will also be required. III Specialised toxicity studies: (a) Male fertility studies detect effects of a drug on structure and functions of male reproductive organs. (b) Female reproduction and fetal developmental toxicity studies include observations on the mating behaviour, progress of gestation, parturition, health during pregnancy and in
post-partum period. Ability of the drug to induce fetal malformations and/or death in utero (i.e. teratogenicity) when given throughout organogenesis is looked for. Perinatal toxicity study is undertaken if the drug is to be given to pregnant or nursing mothers for long periods or where possibility of adverse effects on fetal development is high; the drug is administered throughout the last trimester. These studies are carried out in one rodent species (preferably rat) and one non-rodent species (rabbit). (c) Allergenicity/hypersensitivity tests are carried out in guinea pigs to determine the minimum irritant dose and the effect of a challenge after sensitisation. (d) Genotoxicity tests: Certain drugs are known to produce genetic abnormalities. As genes are bearers of hereditary information, abnormal genes may produce various types of overt and covert abnormalities in the subsequent generations. These are in vivo and in vitro tests conducted to detect genetic damage, if any. (e) Carcinogenicity studies detect the ability of a drug to induce malignancy. They are performed for all drugs that are expected to be used clinically for more than 6 months, as well as for those which are intended to be used frequently in an intermittent manner. Generation of pharmacokinetic data during the toxicity studies (toxicokinetic data) helps to relate doses and systemic exposures achieved with toxicological findings. Although animal studies provide analogies and serve as useful experimental models, the administration of a biologically active agent to human beings is associated with an element of risk which cannot be predicted by even the most careful and exhaustive animal studies. Hence, the drug has to be carefully evaluated by human experiments for its safety and efficacy before it is accepted for therapeutic use. A new drug with a completely novel structure and with novel pharmacological action is rarely born. It is often discovered by chance observation (serendipity) as in the case of penicillin, antidiabetic sulfonylureas and thiazide diuretics. Most of the new drugs prepared are related to, or are very similar to, already known drugs. Obviously, the benefit offered by such imitative, me-too agents to the patient is generally small; and sometimes the new drug may even be worse than the well tried parent compound. The clinical studies with such imitative drugs are justifiable only if the other established drugs are far from ideal and if animal studies indicate distinct advantages over the parent compound in clearly defined terms. Effectiveness of the drug in very small doses, in itself, is not a justification for human studies, when the toxic effects are known to run parallel with ‘higher potency’. Such a preparation would be difficult to dispense and difficulties in regulating its dosage may be enormous. A clinical trial is justified if the new drug is shown to be more efficacious or at least potentially less toxic without significant reduction in its therapeutic efficacy. A drug with a completely new structure and/or a new mode of action, if found reasonably safe in animal studies, certainly deserves to be studied clinically. Human pharmacological studies with such a compound with particular reference to its mode of action and its actions on various systems of the body may eventually lead to formulation of a new potentially useful structure. A new compound with different chemical structure is likely to produce more novel actions than a closely related imitative compound and hence, it certainly deserves clinical studies. Interpretation of animal data: No tests on animals, however meticulous and prolonged, can ever prove with absolute certainty the efficacy or safety of a new drug in man. Animal
pharmacological studies would only indicate the probable beneficial and toxic effects that may be expected during human trials; one must weigh the probable benefits against possible harm that could occur. Almost every drug with biological activity will produce some adverse effects. Drugs that are claimed to be absolutely harmless are also likely to be therapeutically useless. Although animal studies can eliminate obvious toxicity, certain serious adverse reactions such as allergy, neuropsychological toxicity and blood dyscrasias are difficult to detect in animals. It is known that pharmacokinetics and pharmacodynamics of a drug differ both qualitatively and quantitatively in different species. A drug found highly effective in animals may be totally ineffective in humans because of differences in genomic profiles. Thus, penicillin, a potent and reasonably safe antibiotic used in humans, can produce fatal toxicity in guinea pigs even in small doses. Further, the animals used in toxicity studies are not necessarily suffering from the disease for which the drug would be used in humans. Hence, such studies should be carried out in many species and, if possible, in both healthy and diseased animals. In toxicological experiments, high doses of the drug should be used to bring out the possible toxic effects, and subacute and chronic toxicity studies in animals must always precede the chronic administration of the drug in man. Such studies in animals help the physician to understand and treat the effects of over-dosage of drugs in man. Subjective responses like nausea, headache, tinnitus, weakness and loss of libido due to a new drug are difficult to discover by animal studies. Such effects, if severe, could considerably reduce the therapeutic utility of the drug. The relative usefulness of animal studies would be decided by the relevance of animal tests to the human condition, where the drug would be used. Thus, the animal studies regarding a potential diuretic or hypotensive or an antibacterial drug will provide more useful information than those regarding drugs supposed to be effective in human mental diseases, since similar conditions cannot be produced experimentally in animals. While deciding the dose to be administered for the first time in humans, it should be remembered that dose in smaller animals tends to be larger (on mg/kg basis) than that in larger animals, and dosage schedule based on weight basis in animals, should never be applied to human studies. If possible, the pharmacological actions of a new drug in humans should be controlled by measuring blood levels guided by similar estimations done in different species of animals. It has been suggested that during the first cautious trial in humans, about 1/10th to 1/5th of the predicted effective dose should be administered and then increased gradually.
Clinical Evaluation A clinical investigator who has an adequate background in interpreting animal studies and who studies drugs cautiously and critically in humans, with continuing analysis of the result achieved is called a Clinical Pharmacologist. Clinical Pharmacology involves the study of various aspects of pharmacodynamics and pharmacokinetics in humans, both in health and in disease; and, therefore, is a discipline which is a part of medicine as a whole. It helps in defining guidelines for rational drug prescribing, and includes studies on pharmacoeconomics, pharmacovigilance and pharmacoepidemiology. The Clinical Pharmacologist should be a combination of a pharmacologist and a clinician. Clinical Pharmacologists can be clinicians working in the disciplines of medicine, therapeutics, cardiology, anaesthesiology or surgery, and having interest in pharmacology. Obviously, one should restrict such work to the field of one’s specialisation where one has the requisite experience and full knowledge of the ever increasing advances in the subject. This is essential because the safety and effectiveness of new drugs solely depend on his strict, unbiased and uncompromising adherence to the highest scientific standards. More than the equipment and finances, it is the honesty, commitment and calibre of the investigator involved that matters the most. It is essential to have information about the chemical and pharmaceutical properties (such as solubility and stability) of a new product before it is evaluated clinically. Such studies need to be approved by the institutional Ethics Committee and statutory authority. Further, prior informed consent of both, volunteers and patients is mandatory. Healthy subjects with risk factors for an ADR e.g. age less than 18 years, old age, reproductive age in women, history/family history of drug allergy, organ dysfunction and so on, are generally excluded from the drug trials. “A risk factor is a personal attribute (age and gender), an aspect of behaviour or lifestyle, environmental exposure, or inborn/inherited characteristic. On the basis of epidemiologic evidence, it is known to be associated with a health-related condition considered important to prevent”. Phases of drug development: Phase 1 studies (Human pharmacology): are carried out at a few selected centres. The main aim is to obtain precise information from the smallest number of volunteers in the minimum possible time. Both subjective and objective evaluation is done along with relevant laboratory studies. Once the safety is proved in volunteer studies, further studies are carried out in pathological states which the new drug is expected to modify. The prime requisites for such an evaluation are: • Uniformity of subjects with respect to age, sex, nutritional status and so on. • Precise diagnosis in the case of patients. • A clear index of response relevant to the therapeutic objective. The clinical investigator must decide how far it is ethical to withhold a known treatment for the sake of trial with a new drug and whether any additional ancillary therapy is needed. If other known drugs are also to be administered simultaneously for some reasons, it is necessary to watch for drug interactions. In certain fields like endocrinology or infectious diseases, where the etiopathology is precisely known, the clinicopharmacological studies with the new compound could be
precise, quantitative and predictable e.g. penicillin in pneumonia and thyroxine in hypothyroidism. But such information about the disease is often not available and the natural history of the disease is such that the correct assessment of a new drug becomes difficult as in atherosclerosis, psychiatric disorders and rheumatoid arthritis. Clinical assessment is also difficult in case of drugs expected to produce subjective relief of symptoms such as pain, anorexia and sleeplessness. Hence, controlled clinical trials are absolutely necessary to prove or disprove the therapeutic usefulness of the drug as well as for comparing it with the previously established drug. These evaluations are carried out in Phase 2 and Phase 3 studies. Phase 2 studies (Therapeutic exploration): In this phase, clinical evaluation is carried out in patients to explore efficacy, and to determine dose regimen (dose finding) for the next phase. Phase 3 studies (Therapeutic confirmation): These studies include controlled clinical trials. A controlled trial may be defined as one where a new drug is compared with the previously established therapy or placebo therapy, under standardised conditions. It is designed to ensure that the comparisons made are precise, informative and as convincing as possible. Such studies are mainly conducted in two ways: • Where drug is given to one group and results are compared with those in the other group (parallel design); and • Where the drug therapy is alternated with control therapy (crossover design) either with a placebo or with the previously established drug, in the same patient. In the first method, patients are allocated to various groups. In order to balance the groups and to avoid any bias, allocation should be carried out by randomisation by a person who is completely unaware of the therapy allotted to an individual group. One of the groups gets the new treatment while the other receives control therapy in a similar form and in similar way as the drug under evaluation. When both the evaluating clinician and the subject are unaware of the nature of the drug being administered, the procedure is called a double blind controlled trial. It effectively reduces the influence of extratherapeutic factors. In case any laboratory investigations are involved, the specimens should be submitted to the laboratory under a code number. The treatments are decoded only after the trial is over. In the second method, the patient acts as his own control. This reduces the chances of erroneous results due to individual variation amongst the patients. Allocation of the patient to new therapy first or to control treatment first is decided by randomisation and the evaluating clinician should not know the sequence of drug administration before completion of the trial. As in the first method such a study can be made a double blind controlled trial. Finally, the results of such controlled trials at a few centres are confirmed by broad multicentric clinical trials at many centres before the drug is recommended for general use. Phase 4 studies (Post marketing Surveillance): Since ADRs continue to occur even after a new drug is released for use in the community, careful pharmacovigilance must continue to avoid tragedies like that which occurred following the use of thalidomide. For this purpose, pharmacovigilance centres (Chapter 1) are established in various parts of the country, to whom the various ADR observed in practice are reported by the practitioners.
In turn, these centres will alert the doctors about any unusual and previously unknown ADR with the drug. It is during this phase, that astute observation by practising doctors leads not only to early detection of ADR but also to recognition of unanticipated additional benefits which result in additional, secondary uses for drugs e.g. beta blockers for glaucoma and minoxidil for baldness. The characteristics of the various phases of the clinical trial are summarised in Table 4.2. Table 4.2 Characteristics of various phases of the clinical trial
Phase 0 studies recently recommended by US FDA for some drugs, are exploratory, firstinhuman studies. A very small number of patients (10–15) are given sub-therapeutic microdoses of the drug for approximately one week. Such doses are less likely to be toxic but are pharmacologically active to exhibit effects on molecular target in an assay system. Sometimes they are used to study the suitability of pharmacokinetic properties. Such early data helps to make suitable modifications or to select only those candidates, which are capable of producing the effect as observed in the pre-clinical studies. This approach is cost-saving as early weeding out of ineffective agents is possible. It also increases efficiency of drug development programme by allowing effective drugs to enter further clinical studies. The drugs to be tested in Phase 0 should have wider therapeutic index, a known target and validated biomarker with assay system. Role of Placebo: Placebo is a Latin term which means “I may please you”. The placebo effect is an effect attributable to a medicament as a procedure, and is not due to any specific
pharmacodynamic property of the substance for the condition being treated. Placebo effect may be defined as “how the patient’s perception of treatment influences his/her response”. Placebos are used: • During the clinical trial, to eliminate the possibility that the benefit of the drug is solely due to chance; and • As therapeutic agents that work psychologically. The placebo helps to (i) Differentiate the drug effects from natural fluctuations in a disease like asthma; and (ii) Sort out the real drug-related ADR from those simply due to the procedure. A placebo is usually an inert substance like starch or lactose. Even when an active drug is used, its placebo effect often comforts the patient long before the drug is effective. It is well known that the patient as well as his relatives get some immediate relief as soon as the doctor ’s medicine is administered, irrespective of its drug content. This is because of their faith in the doctor that things will go well in his hands. Placebos can often produce relief of subjective symptoms associated with psychological disturbances. This includes relief from anxiety, headache, pain, insomnia and breathlessness. Hence, placebos are often employed in the treatment of certain diseases where the psychic element is suspected to be responsible for subjective (psychosomatic) symptoms. Objective responses such as increase or decrease in neutrophils and eosinophils may sometimes be seen with placebos. When administered for its therapeutic effects, the placebo preparation : • Must appear to be relevant to the illness. • Must be harmless; and • Should preferably conform to the patient’s expectations. To be effective, the ‘potency’ of the preparation must be shown by some signs such as strong taste, a colourful capsule or a tablet of odd shape and sometimes even by obvious but harmless side effect like coloured urine. During clinical trials, placebos are used to eliminate the effect of bias of the physician and the patient, particularly in evaluating a drug claimed to be effective in conditions such as bronchial asthma, angina pectoris, pain and psychiatric disorders. In such cases the placebo should be indistinguishable from the active medicament in physical properties like colour, smell, taste and dosage form. Placebo effect may be modified by: • Personality of the physician: Reassurance and optimistic outlook often achieve a better effect. “The doctor himself must inspire confidence. It is difficult to define this quality. It does not lie so much in what is said as in the doctor ’s shape and bearing, and in those instinctive signs whereby one animal unknowingly conveys its mood to another. Some have it and some do not. In this respect, the hospital specialist is in an easier position than the GP because he is backed by a temple of healing, which is clearly nearer the seat of power than a wayside shrine. Since few doctors are good enough actors to simulate the confidence they do not have, it often happens that one who is kind and credulous is a better healer than another who is informed and critical. Placebo reactions go faster when both parties have faith and in this respect knowledge is an inhibitor ”. • Personality of the patient: Some individuals are amenable to suggestions. Such people are termed placebo reactors, and since a placebo acts by suggestion, they derive benefit
from the use of placebos. Neurotics are great placebo reactors while depressed or psychotic subjects are usually resistant. Individuals who are of conservative, suspicious, or sceptical nature are not likely to benefit from placebos. Such negative reactors, on the contrary, may become worse as per their ‘own expectations’. A strong negative reactor may even take a pride in saying that he or she is “allergic to all drugs”. • Form of administration: It is not surprising that the greatest placebo effect (as high as 81%) is achieved with injections. This may perhaps explain the preference for the use of injections by the practitioners! Colour, taste, presence or absence of stress are other factors which modify placebo effect. Like active drugs, placebos can produce certain adverse subjective reactions such as drowsiness, headache, dryness of mouth, fatigue, insomnia, constipation, impotence, difficulty in concentrating and a ‘drugged feeling’. An abstinence syndrome, which responds to injection of normal saline, has been described after prolonged placebo therapy. Much of the routine treatment such as vitamins, tranquillisers and tonics, prescribed by the doctors often acts as a placebo for themselves too! Many physicians cannot “bear to think they are doing nothing; so they, like their patients, are willing to believe. They persuade themselves or are persuaded of the virtues of their treatment”. Interpretation of clinical data: After the completion of the clinical trial, the results are subjected to statistical analysis. If the difference between the two groups of treatment is so large that the probability of its occurrence simply by chance is less than 5 times in 100 (p10 mg/dl), it is metabolised at a constant rate which, is independent of blood alcohol concentration (Zero Order Kinetics). As NAD is consumed during the degradation of alcohol, the limited availability of hepatic NAD sets the upper limit on the rate of alcohol metabolism, and changes the first order kinetics to zero order kinetics. Another hepatic enzyme, CYP2E1 (microsomal ethanol oxidating system) contributes to alcohol metabolism only at high alcohol concentration (100 mg/dl). It is induced by chronic alcohol intake. Small amounts are excreted by the kidneys, sweat and lungs. Increased production of toxic metabolites such as acetaldehyde, NADH, Acetyl CoA can cause organ damage in chronic alcoholics. Fomepizole inhibits alcohol dehydrogenase (see below) whereas certain drugs like disulfram, metronidazole, trimethoprim and cefotetan inhibit aldehyde dehydrogenase. Is alcohol a food? One gm of alcohol gives 7.1 calories in the form of acetate. Although it can spare carbohydrates, alcohol is not a food, because it has no protein, vitamin or any other nutrients (‘empty calories’). Chronic alcoholics who do not eat properly suffer from nutritional deficiencies. Therapeutic uses of ethyl alcohol: It is a solvent for various active ingredients. It is sometimes used: • In the symptomatic treatment of fever because of its cooling effect on skin. • In the prevention of bed sores as it hardens the skin. • As an antiseptic (70%) • As an appetiser (10%).
• In methyl alcohol poisoning (see later). • To wash out phenol in cases of accidental skin contamination; and • By local injection to destroy a nerve as in trigeminal neuralgia.
Acute Alcohol Intoxication Chemical analysis of blood and urine can give some idea about the degree of intoxication in an individual who has ingested alcohol (Table 6.1). However, it is difficult to associate a particular blood alcohol concentration with a specific degree of impairment. Consumption of alcohol before driving is hazardous. Table 6.1 Correlation of blood alcohol levels with behavioural changes Blood concentration (mg per 100 ml) Behavioural changes < 50 Not signific ant 50–100 Feeling of exaltation, talkativeness 100–200 Emotionally unstable, motor inc oordination, nystagmus 200–300 S taggering, loss of self c ontrol 300–400 S tupor-dead drunk 400–500 Coma, anaesthetic effec t > 500 Respiratory arrest and death
Interpretation Not under the influenc e of alc ohol but may affec t the reac tion time. Possibly under the influenc e Probably under the influenc e Definitely under the influenc e Grossly intoxic ated
After death, the degradation of alcohol ceases and the brain and blood levels of alcohol remain constant for sometime. Hence, post-mortem samples of blood can give reliable estimates about the degree of intoxication at the time of death. Approximately 600 ml of pure alcohol can produce a fatal effect in an individual of 70 kg body weight. Coma after an acute alcoholic bout may be due to the CNS depression, head injury or severe hypoglycemia which especially occurs in fasting individuals. Hypomagnesemia is common. Some individuals may develop severe acute hepatitis. Death due to acute alcohol poisoning is, however, uncommon. The treatment consists of: • General nursing care • Maintenance of vital functions • Thiamine 100 mg (bolus) IV; and • Glucose 50%, 50 ml IV, for hypoglycemia. • Magnesium sulfate 2-4 g IV over 1-2 hours. If the acutely intoxicated patient is not comatose but only rowdy, careful use of a sedative such as a benzodiazepine is indicated.
Chronic Alcoholism “At risk” (heavy) drinking is defined by National Institute on Alcohol Abuse and Alcoholism (NIAAA) as consuming greater than 14 drinks per week or more than 4 drinks per occasion for men, and greater than 7 drinks per week or more than 3 drinks per occasion for women. A ‘drink’ is that amount of any alcoholic beverage which contains 14 g of alcohol. Although all those who drink alcohol are not addicts, repeated ingestion can lead to dependence. Alcoholism is a polygenic disorder with both genetic and environmental determinants, in which the subject continues drinking in spite of adverse medical or social consequences. The intensity of craving is such that the ‘desire to drink’ remains uncontrollable and is the major interest in life. This leads to alcohol abuse which creates many social and occupational problems. Nearly 50% of Asians have abnormal inactive enzyme aldehyde dehydrogenase, which causes higher levels of acetaldehyde following alcohol intake; thus resulting in severe nausea, vomiting. This may decrease the risk of excessive drinking. Though precise mechanism for alcohol dependence is not known, up- regulation of NMDA receptors and voltage sensitive Ca++ channels occurs. This is associated with downregulation of GABAA mediated responses. Alcohol addicts are liable to suffer from: • Neuropsychiatric syndromes such as cognitive problems like dementia, Korsakoff ’s psychosis, hallucinosis, suicidal tendencies and Wernicke’s encephalopathy. • Nutritional deficiencies such as polyneuritis due to thiamine deficiency and anemia. Organ damage leading to hepatic cirrhosis, chronic pancreatitis, cardiomyopathy and optic nerve degeneration. Apart from history and the obvious physical and mental deterioration, alcohol addiction can be diagnosed by using the CAGE questionnaire. Withdrawal syndrome: Sudden withdrawal of alcohol in alcoholics leads to withdrawal syndrome (Table 6.2). Delirium tremens, though rare, is the most severe component of the abstinence syndrome which develops some days after sudden withdrawal of alcohol in chronic alcoholics. The symptoms consist of restlessness, insomnia, tremors, hallucinations generally involving great fear, delirium, autonomic instability and even convulsions. Table 6.2 Alcohol withdrawal syndrome
The treatment of withdrawal syndrome is outlined in Table 6.3.
Table 6.3 Symptomatic treatment of alcohol withdrawal syndrome
Alcohol Dependence Treatment of alcohol dependence consists of: (i) Detoxification which aims at providing safe withdrawal to make the patient alcohol-free (ii) Support therapy to prevent relapses and to assist the patient in maintaining abstinence (iii) Rehabilitation which includes (a) psychotherapy; and (b) institutional therapy The major part of the treatment includes education, counseling and cognitivebehavioral therapy; drugs form only a small part. Detoxification: Following a thorough physical examination for existing complications (such as liver failure, GI bleeding, nutritional deficiencies) in an alcoholic who wishes to stop alcohol drinking, alcohol is withdrawn. Adequate nutrition and oral multiple B vitamins, including 50–100 mg of thiamine daily is instituted. However, IV fluids are not necessary unless dehydration signs or history of recent bleeding, vomiting, or diarrhea are present. Glucose is administered IV if there is hypoglycemia. Benzodiazepines, particularly the long acting ones, such as diazepam (10 mg) or chlordiazepoxide (25-50 mg), considered as the substituting drugs for alcohol, are the treatment of choice. They are given orally every 4–6 h on the first day, with tapering of doses over the next 5 days. Abatement of symptoms like anxiety and agitation occurs within 3-5 days of therapy but regaining normal sleep pattern may take several months. Lorazepam and oxazepam are preferred in the elderly and those with liver failure. Every 4 hrly dosing of these drugs is needed to avoid plasma level fluctuation that may precipitate seizures. Carbamazepine, an antiepileptic drug (Chapter 9), is an effective alternative to benzodiazepines in treating mild to moderate withdrawal symptoms. It prevents seizures and does not cause respiratory depression nor impairs memory. It is relatively safe. Sometimes delirium tremens is precipitated (see earlier). BDZ in high doses are effective to decrease agitation and preventing seizures but they do not change confusion state. Clonidine 0.1-0.4 mg qid, or Atenolol 50-100 mg daily may be used to control autonomic hyperactivity. Use of antipsychotics like haloperidol or olanzapine in this condition lacks evidence for efficacy. They may not exacerbate confusion but may increase the risk of seizures. They should not be used for mild withdrawal symptoms. Support therapy: Anti-craving drugs help in preventing relapses and support complete abstinence during rehabilitation. They are classified as: I Opioid antagonists, e.g., Naltrexone, Nalmefene II NMDA receptor antagonist: e.g., Acamprosate III Aversion drugs, e.g., Disulfiram, IV Dopaminergic antagonists, e.g., Tiapride (experimental); and V Supporting drugs, e.g., Lithium, Carbamazepine, Topiramate The pharmacological basis for use of these drugs lies in the neurobiology of dependence. During alcohol dependence there is excessive activation of dopamine-rich ventral tegmental reward system which gives the feeling of pleasure or reward by stimulating nucleus accumbens (NAc) and the prefrontal cortex. The GABA neurons are inhibitory for dopaminergic system. But, opioid mu receptors present on GABA nerve terminal on activation inhibit GABA release resulting into increased activity of
dopaminergic neurons. NALTREXONE: Naltrexone and the other mu opioid antagonists (Chapter 10) free the inhibitory GABA and block dopamine release in the NAc. Naltrexone, 50-100 mg once daily, reduces craving and urge to drink alcohol and thus maintains the abstinence. It has to be given for 12-16 weeks or more after detoxification. Once-a-month injection of naltrexone is available. The most common side effect of naltrexone is nausea. It may cause dose dependent hepatotoxicity. Disulfiram and naltrexone should not be combined as both are hepatotoxic. Nalmefene, an analogue of naloxone with a greater bioavailability has longer duration of action. It does not cause dose dependent hepatotoxicity. ACAMPROSATE: This analogue of GABA, acts as an agonist at the GABAA receptors, and as a weak antagonist at the NMDA receptors. It reduces voluntary alcohol consumption, and craving that occur in alcohol withdrawal and early abstinence. It is as effective as naltrexone and can be combined with it. It is largely excreted by the kidneys. Adverse effects are usually mild and include diarrhoea, rash and headache. The usual dose is two enteric coated tablets (333 mg each) thrice daily. DISULFIRAM (Tetraethylthiuram disulphide): This drug is used to make alcohol consumption an unpleasant experience so that the person gives up drinking. Therapy is initiated after ensuring that alcohol has not been consumed for at least 12 hours. Treatment is initiated with 500 mg as a single daily dose for 1-2 weeks, followed by 125-250 mg OD as the maintenance dose. After a week’s therapy, even a small amount of alcohol produces toxic reactions, such as flushing, perspiration, palpitation, marked nausea, vomiting and fall of BP (antabuse reaction). The patient realises that while on this drug he cannot tolerate even small amounts of alcohol and abstains from drinking. Severe violent reactions can occur even with the first dose and hence, this treatment should be carried out in a hospital. However, the drug hardly improves the success rate of continued abstinence nor does it delay the resumption of drinking. Mechanism of action: The drug inhibits aldehyde dehydrogenase and blocks the oxidation of acetaldehyde (Fig. 6.1). This raises the blood level of toxic acetaldehyde, which directly causes nausea, vomiting, dizziness, headache and acts on the cardiovascular system. In addition, disulfiram also inhibits dopamine beta oxidase and thus interferes with the synthesis of NA. Adverse reactions: It can cause drowsiness, nausea, headache, cramps, fatiguability, a metallic taste in the mouth and acidosis. It can also cause depression, psychosis and peripheral neuropathy. It is hepatotoxic. It is contraindicated in: • Hepatic and circulatory diseases. • Uncontrolled diabetes mellitus. • Obvious personality changes in alcoholics. Drug Interactions: Disulfiram inhibits metabolic degradation of warfarin, theophylline, benzodiazepines, carbamazepine, tricyclic anti-depressants and phenytoin. The first and the last interactions can be hazardous. LITHIUM: Oral lithium carbonate acts as a mood stabiliser and reduces alcohol consumption. It may be used as an adjunct to treat alcohol withdrawal (Chapter 14).
An antiemetic ondansetron, a 5HT3 receptor antagonist, is effective in early-onset alcoholics by reducing the desire to drink but has no effect on its pharmacokinetics. Topiramate, an antiepileptic is also claimed to have anticraving effect. Among the various drugs for the treatment of alcohol dependence, the evidence for efficacy is strongest for naltrexone and acamprosate. A combination of 2 or more treatment modalities is useful rather than using a single drug. • Psychotherapy: by a sympathetic doctor can often give rewarding results during early stages. Dependence is reversible, if the addict realises that his drinking has become a problem to him. Complete co-operation by the patient is very essential and he should be explained that indulgence in even small quantities of alcohol again would lead to a relapse. Rehabilitation: In all alcoholics, the possibility of an underlying psychiatric disorder such as schizophrenia, anxiety disorder, panic disorder or depression must be considered and treated. • Institutional therapy: Psychotherapy and drug therapy can be reinforced by institutional therapy (Alcoholics Anonymous, AA) where the patient sees for himself the exalcoholics who have become abstainers and are living a happy life. This helps to boost the patient’s morale. A religious and spiritual approach is also helpful. Certain abnormalities of personality develop in very chronic cases which make the treatment far more difficult. Thus, in Korsakoff ’s psychosis there is a marked impairment of memory, disorientation in space, impaired physical capacity and diminution of will power. In such cases the results of aversion therapy with drugs are generally disappointing. Drug interactions: Numerous drug interactions between alcohol and other drugs have been described (Table 6.4). This should be borne in mind while prescribing these drugs to alcoholics. Table 6.4 Interactions of ethyl alcohol with other drugs
METHYL ALCOHOL (Methanol): This is used as a solvent and is added in a 5% concentration to denature ethyl alcohol. It is not used therapeutically but often misused for adulteration of ethyl alcohol. Its absorption and distribution are similar to those of ethyl alcohol; the rate of metabolism, however, is very slow. It is mainly oxidised to formaldehyde and subsequently to formic acid, which is toxic. The latter reaction is folate dependent and causes folate depletion. Pharmacological actions: Following its ingestion, initial symptoms resemble those of ethyl alcohol ingestion. The later symptoms are due to: • CNS depression. • Acidosis following the production of formic and other organic acids. • Toxic effects of formaldehyde and formic acid on the retinal cells.
The symptoms may be delayed, particularly if ethanol is also consumed simultaneously. Usually, headache, vertigo, nausea, vomiting severe abdominal pain, dyspnoea and motor restlessness occur. Bradycardia has a bad prognostic significance. Coma can develop very rapidly, followed by death. Death is usually preceded by blindness. However, total blindness could occur with as little as 15 ml of methyl alcohol while ingestion of 30 ml is fatal. Many deaths have been reported following the ingestion of methyl alcohol or alcoholic brew adulterated with methyl alcohol for its ethyl alcohol like effects. Treatment of toxicity: This is summarised in Table 6.5. It is directed mainly at supporting respiration and rapid correction of acidosis by IV sodium bicarbonate (Chapter 37). The development of blindness is enhanced by acidosis. Methyl alcohol is oxidised slowly and hence, acidosis can recur even after adequate initial alkali administration. It is necessary, therefore, to observe the patient closely for several days. Hypokalemia, if present, needs correction; so also the maintenance of adequate nutrition and water and electrolyte balance. The patient’s eyes should be protected from strong light. Specific therapy aims at suppression of metabolism of methanol by alcohol dehydrogenase and enhancement of its removal by hemodialysis. Table 6.5 Treatment of methanol poisoning
(a) Ethyl alcohol 10% IV; 0.6 g/kg loading dose, followed by 10 g/hour infusion, in adults (b) Fomepizole: 100 mg, diluted with 250 ml of isotonic saline and infused slowly over 45 min. • Promote metabolic degradation of formate:
Folinic acid 1mg/kg (Max. 50 mg) IV, together with folic acid 1mg/kg IV, 4 hourly for 6 doses. • Diuretics, urine alkalinisation • Hemodialysis in severe case • Maintenance of nutrition
-->
The infusion of ethyl alcohol has been recommended on the basis that it has higher affinity for alcohol dehydrogenase than methanol. Hence it slows down the oxidation of
methyl alcohol by competing for the same metabolic pathway. Ethanol administration can be life saving if, for some reasons, alkali therapy is delayed. Folic acid can accelerate metabolism of formate to CO2 and H2O. FOMEPIZOLE: This 4-methyl pyrazole is useful in the treatement of poisoning with methyl alcohol and EG (see below). It inhibits alcohol dehydrogenase rapidly and effect remains prolonged. It is well tolerated. Headache, nausea, dizziness and sometimes, minor allergic reactions may occur. Its use appears to be more effective and safe than ethyl alcohol but fomepizole is expensive. ETHYLENE GLYCOL (EG): This compound is used as an antifreeze in automobile radiators and as industrial solvent. The therapeutic misuse of industrial glycerol contaminated with EG, causes fatal nephrotoxicity. The initial transient excitation is followed by CNS depression. The acid metabolites and NADH induced lactic acidosis (Fig. 6.2) cause metabolic acidosis; while oxalate and other intermediates which are nephrotoxic and can cause tubular necrosis and acute renal failure. Ingestion of about 100 ml of EG, if untreated, can be fatal.
FIG. 6.2 Hepatic metabolism of ethylene glycol
Treatment of EG poisoning: It is similar to that of methanol poisoning (Table 6.5). Additionally, pyridoxine should be administered IV in the dose of 100 mg daily, to promote the conversion of glyoxalic acid to glycine. Methyl alcohol and EG are rapidly cleared by the kidneys if the renal function is normal. Diuretics, urinary alkalinisation and early hemodialysis (which removes EG from the blood) may be helpful.
7
General Anaesthetics General anaesthetics are the agents which bring about loss of all modalities of sensation, particularly pain (analgesia), inhibition of autonomic reflexes, relaxation of skeletal muscle, amnesia and a reversible loss of consciousness. Nitrous oxide was discovered by Priestley in 1776 but it was used for the first time in 1844 by Horace Wells, a dentist in Hartford, USA for a painless tooth extraction. Morton, a second year medical student, in 1846 successfully showed the use of ether as a general anaesthetic in the first classic demonstration held in the operating room of the Massachusetts General Hospital, Boston, USA. Since then, anaesthesiology has progressed considerably and many safer agents are now available for use. These agents can be classified as: I Inhalational general anaesthetics: • Volatile liquids: Diethyl ether; Ethyl chloride; Trichloroethylene; Halothane; Enflurane; Isoflurane, Desflurane, Sevoflurane • Gases: Nitrous oxide. II Intravenous general anaesthetics: • Ultra short acting barbiturates: Thiopental sodium; Methohexital. • Propofol. • Benzodiazepines: Midazolam, Diazepam • Phencyclidine derivative: Ketamine • Opioid analgesics: Morphine, Fentanyl, Sufentanil • Miscellaneous: Etomidate, Dexmedetomidine Mechanisms of action: Although the general anaesthetics are capable of depressing all the functional elements of the CNS including the spinal cord, their clinical effects vary, depending upon their chemical structure, and their ability to localise within CNS and interact with target sites. Thus, specific cortical areas exhibit different sensitivities to inhalational and IV anaesthetics. Inhibition of motor response to pain, such as that caused by surgical incision, is primarily mediated by the spinal cord. By inhibiting the spinal cord activity, the general anaesthetics decrease the transmission of noxious stimuli ascending from the spinal cord to the brain, and thereby decrease supra-spinal arousal. They also modify the descending signals and cause immobilisation. Above the spinal cord, general anaesthetics globally depress blood flow and glucose metabolism and selectively alter neuro-transmission in multiple supra-spinal regions, making those areas electrically silent. Their behavioural and physiological effects, including hypnosis and amnesia, are mediated by the midbrain, reticular activating system, thalamus, pons, amygdala and hippocampus. Although there is no definitive evidence for specific targets for general anaesthetics, it is postulated that they affect these areas. Tuberomamillary nucleus, a GABA modulated region of hypothalamus that is linked with the sleep states, is also implicated. Current evidence suggests that molecular targets for anaesthetic agents are primarily proteins of the neuronal ligand gated ion channels. The major channels associated with nicotinic receptors, 5-HT3, GABAA, NMDA receptors are the binding sites for anaesthetic agents. Though the specific receptors vary from anaesthetic to anaesthetic (e.g. nicotinic receptors and NMDA
for nitrous oxide; GABAA for most IV anaesthetics; and NMDA for ketamine), in general, these agents inhibit the release of the pre-synaptic excitatory neurotransmitters. They also alter the post-synaptic responsiveness to the released neurotransmitters by increasing the activity of the inhibitory ion channels in the post-synaptic receptors and by enhancing inhibitory neurotransmission within the CNS. Specific behavioural effects of an anaesthetic are attributable to the selectivity of ion channel it acts on.
Inhalational General Anaesthetics The factors which control the transfer of an inhalation anaesthetic agent from the alveoli to the blood, and from there into the brain are: • Its solubility in blood. • Its density: The lighter the gas, the faster does it diffuse into and out of the tissues. • Its partial pressure in the anaesthetic mixture (solubility in blood is inversely proportional to the partial pressure), in the arterial and mixed venous blood, and in the tissues; and • The rate of blood flow through the lungs and the tissues. The lower the solubility of an anaesthetic agent in blood, the more rapid is the rise in partial pressure in arterial blood e.g. nitrous oxide, desflurane, sevoflurane have low blood solubility but they rapidly reach high arterial tensions. This leads to rapid equilibrium with brain producing rapid induction. The highly soluble anaesthetic agents with high blood/gas partition coefficient such as diethyl ether show slow induction, slow recovery and slow change in depth of anaesthesia. The higher the concentration of an anaesthetic agent in the inspired gas mixture, the greater will be rise in partial pressure in the blood. Moderately soluble agents like halothane enflurane and isoflurane can be used at higher concentration to achieve rapid induction of anesthesia, which can then be maintained with lower concentration. In addition, with higher rate and depth of ventilation, the rise in partial ressure of anaesthetic agents with moderate to high blood solubility is high. Ventilation changes however, do not influence the speed of induction of low solubility agents. The general anaesthetics have a low margin of safety and the therapeutic indices vary from 2 to 4. It is also difficult to estimate the dose accurately. The anaesthesiologists describe the measure of potency of inhalation anaesthetic agents in terms of Minimum Alveolar Concentration (MAC) of the anaesthetic, at one atmospheric pressure, that produces ablation of movement in response to the surgical incision. Usually, 0.5 to 2 MAC are required for adequate anaesthesia. The MAC required for halothane and enflurane are 0.75 and 1.68, respectively, while that for nitrous oxide is 105. Thus, nitrous oxide is a poor anaesthetic, when used alone. Methods of administration of inhalational general anaesthetics: • Open method: This is a simple method of administering a volatile anaesthetic. A simple mask like Schimmelbusch mask covered with six to ten layers of gauze, which does not fit the contour of the face is held on the face and an anaesthetic like ether, or ethyl chloride is poured on it in drops. The anaesthetic vapour, diluted with air, is inhaled through the gap between the mask and the face. The method does not need any anaesthesia apparatus. There is no rebreathing. This is also called an ‘open drop’ method. • Semi-open method: This method is similar to open method but the dilution with air is prevented by using either a well-fitting mask like Ogston’s mask or layers of gauze between face and the mask. A small carbon dioxide build-up occurs with this method. • Semi-closed method: This method allows some rebreathing of the anaesthetic drug with the help of a reservoir but in addition, part of the volume of each succeeding inspiration is a new portion from an anaesthetic mixture. This method involves accumulation and
rebreathing of carbon dioxide. • Closed method: This method employs the chemical agent soda lime to absorb the carbon dioxide present in the expired air. It requires the use of a special apparatus but is particularly useful when the anaesthetic agent is potentially explosive. Stages of Anaesthesia: Guedel, in 1920, referring mainly to the anaesthetic ether, outlined the four stages of general anaesthesia and divided the third stage of surgical anaesthesia into four planes (Fig. 7.1). These stages can be distinctly discerned with the majority of the volatile general anaesthetics. The stages are:
FIG. 7.1 The signs and stages of ether anaesthesia
• Stage I: Stage of analgesia • Stage II: Stage of delirium • Stage III: Stage of surgical anaesthesia • Stage IV: Stage of respiratory paralysis. I Stage of Analgesia: This stage stretches from the beginning of inhalation of anaesthetic to loss of consciousness. It is manifested as a sensation of remoteness, falling, suffocation or as visual or auditory aberrations. A feeling of warmth is experienced by some individuals. Analgesia is produced in this stage before the loss of consciousness. Minor surgical procedures such as incision of an abscess, dental extraction and obstetrical manoeuvres can be carried out successfully during this stage. However, it is difficult to maintain it for a long time. In the later stage I, amnesia is observed. The patient rapidly loses consciousness to pass into the second stage, the stage of delirium. II Stage of Delirium or Excitement: This stage extends from the loss of consciousness to the beginning of surgical anaesthesia. It may be associated with excitement, shouting, increased muscular activity,
breath holding, tachypnoea and hyperventilation. Some of these manifestations are due to release of the lower centres from the inhibitory control of higher centres as a result of cortical depression. The pupils may dilate and marked hypertension and tachycardia may develop, due to release of adrenaline. Struggling, increased tone of the skeletal muscles, retching and vomiting are the undesirable features of this stage. They can be minimised by proper pre-anaesthetic medication and rapidly increasing concentration of anaesthetic agents. III Stage of Surgical Anaesthesia: This stage is characterised by regular respiration, a gradual loss of reflexes, and relaxation of the skeletal muscles. Reflex activity is lost. This stage is usually employed for surgical intervention and is divided into four planes (Fig. 7.1). Plane i: The pupils dilate progressively with the depth of anaesthesia. The respiration is full, regular, deep and of thoracoabdominal character. The BP and the pulse rate are normal. The skeletal muscles are incompletely relaxed. The lid reflex, swallowing, retching and vomiting get abolished. The corneal reflex is present but the conjunctival reflex is lost. The loss of pharyngeal reflex in the middle of this plane enables the anaesthesiologist to pass a pharyngeal airway. Plane ii: The respiratory excursions are regular but the amplitude is diminished. Muscle relaxation is adequate. Reflexes arising from the larynx are also abolished and endotracheal intubation can be performed. Plane iii: This is characterised by the beginning of asynchrony between the thoracic and the abdominal respiratory movements. The BP begins to fall, the intercostal muscles are gradually paralysed and the respiration assumes an increasingly abdominal character. The pupillary light reflex and the corneal reflex are lost. The muscle relaxation is essentially complete. Plane iv: The paralysis of the intercostal muscles is complete, the pupils are dilated, do not respond to light, the muscles are flaccid and the BP is low. The secretions are completely abolished. IV Stage of Respiratory Paralysis: This stage is characterised by severe depression of the vital medullary centres, leading to vasomotor collapse and respiratory arrest. Circulatory and respiratory support is a must. The stages described here may differ considerably with different anaesthetic agents. Halothane produces hypotension much more readily than ether. Pupillary dilatation is insignificant with halothane. With ether the skin becomes pale, cold and wet in preparalytic stage while with halothane, it is warm and dry until the development of marked hypotension. Preanaesthetic medication with the opioid analgesics, atropine and the use of skeletal muscle relaxants also modifies the signs of the anaesthesia and may interfere with the proper assessment of the depth of anaesthesia. In modern anaesthetic practice, the stages are never discerned separately because a combination of agents (balanced anaesthesia) is used. Usually a rapid and smooth induction is achieved with the help of an intravenous agent such as thiopental or propofol; the anaesthesia is maintained with inhalational drugs like halothane or nitrous oxide-oxygen plus a volatile anaesthetic like isoflurane. Such combinations are termed as balanced anaesthesia techniques, which utilise optimally favourable properties of anaesthetic agents minimising their adverse effects.
Inadequate anaesthesia is indicated by: • Signs of ANS overactivity, such as tachycardia, rise of BP, sweating and lacrimation. • Grimacing; or • Other muscle activity. Surgical anaesthesia is indicated by: • Loss of eyelash (lid) reflex; and • Development of rhythmic respiration. Deep anaesthesia is suggested by : • Depression of respiration. • Hypotension; and • Asystole. Associated blood loss and hypoxia can aggravate the situation. They must be avoided. Computer assisted EEG based indices are now used to monitor cerebral functions and to decide anaesthetic requirements.
Volatile Liquid Anaesthetics The volatile general anaesthetics which are liquid at room temperature are all extremely potent but relatively more soluble in blood, cell water and fat; hence, both induction and recovery with these agents are slower than that with the gaseous general anaesthetics. DIETHYL ETHER: Ether (Fig 7.2) is a colourless, volatile liquid with a pungent odour and with the boiling point 350C. Anaesthetic ether contains 96-98% diethyl ether. Ether vapour is irritating. Ether, when exposed to air, moisture or light may form ether peroxides or acetic aldehyde, which are irritant. To avoid this conversion, ether is marketed in sealed containers or amber coloured bottles covered with black paper.
FIG 7.2 Ether Halothane
Ether is inflammable and mixtures of ether with air, nitrous oxide or oxygen may explode over the entire range of anaesthetically useful concentrations. A concentration of 10-15% of ether in the inspired air is usually required for induction while a concentration of 4 to 5% ensures a satisfactory maintenance of anaesthesia in plane iii (Fig. 7.1); with a concentration of more than 7% respiratory failure may develop. Ether rubbed into the skin produces local vasodilatation with a sense of warmth and pain (rubefacient action). It dissolves the sebaceous secretion and, in the form of etheral soap, is used as a cleansing agent. It is also used as a solvent. Absorption, fate and excretion: Only a minor portion of ether is oxidised within the body; 85 to 90% is eliminated through lungs and the remainder through the skin, urine, milk and sweat. Ether crosses the placental barrier and reaches comparable concentrations in the foetal blood. Advantages: • Can be administered without complicated apparatus and air can be used as a diluent and source of oxygen. • Can be used during an emergency without pre-anaesthetic medication. • Has a wide margin of safety. • Excellent analgesic. • Curarimimetic and hence causes satisfactory muscle relaxation. • Reflex stimulant of respiration and bronchodilator. • Less likely to precipitate cardiac arrhythmias. • Little hepato-/nephro-toxicity. • Can be used during delivery; and • Economical Disadvantages: • Inflammable and explosive, and therefore potentially hazardous; cautery cannot be used. • Induction is slow and may be stormy; and recovery slow. • Irritant and may cause nausea/vomiting. Increase in salivary and bronchial secretion may
cause cough/laryngeal spasm. • Rarely may cause cardiac arrest. • May cause convulsion, especially in children; and • Exhibits cross tolerance with ethyl alcohol. HALOTHANE: It is a fluorinated volatile anaesthetic with structural similarity to chloroform (Fig. 7.2). It is a heavy, colourless liquid with a characteristic sweet, fruity odour. Halothane readily attacks most of the metals including stainless steel, brass and copper and may also affect the rubber elements of the anaesthetic equipment. Halothane produces loss of consciousness in a concentration of 2 to 3% in oxygen vapour and the anaesthesia can be maintained by using 1 to 2% of halothane vapour with oxygen and nitrous oxide. Absorption, fate and excretion: About 60-80% of halothane is eliminated unchanged through lungs in the first 24 hours. About 20% appears to be retained in the body and is probably metabolised. Advantages: • Non-inflammable and non-irritant to the respiratory tract; hence not unpleasant for induction. It has a fruity odour. • Potent anaesthetic with speedy induction and recovery. • Inhibits pharyngeal and laryngeal reflexes, making tracheal intubation easy. It does not cause laryngospasm, bronchospasm or coughing but in fact causes bronchodilatation. • Postoperative vomiting infrequent. • Can induce controlled hypotension and a bloodless field; to be used only by experts for this purpose. Disadvantages: • Special apparatus is necessary. • Inadequate muscular relaxation for abdominal surgery. Slow post-operative recovery. • Poor analgesic. • Depresses respiration. • Can raise intracranial tension due to cerebral vasodilatation. • Causes hypotension by direct depression of myocardium, and sensitizes the heart to catecholamines, causing cardiac arrhythmias. This is its major drawback. • Post-operative recovery of mental function is slow. Shivering during recovery is common. • Induces hepatocellular microsomal enzyme. Rarely, it may cause delayed allergic hepatic necrosis due to the toxic metabolite, trifluoroacetyl chloride. • Can cause malignant hyperthermia in susceptible individuals. ENFLURANE: This halogenated volatile liquid anaesthetic is chemically 2-chloro 1, 1, 2trifluoroethyl difluoromethyl ether. It is non-inflammable, with mild, sweet odour and boils at 570C. Chemically it is very stable. About 80% of enflurane is excreted unchanged by lungs and only 2-5% is metabolised by liver. Anaesthesia produced by enflurane resembles closely that produced by halothane, except that the muscular relaxation is better and tachypnoea is uncommon. It causes hypotension and it potentiates the activity of nondepolarising muscle relaxants. Further, it depresses myocardial contraction force and sensitises the heart to the actions of catecholamines. Hence it can cause cardiac arrhythmias. Like halothane, it causes
bronchodilatation. The compound could produce seizures and involuntary motor activity during deep anaesthesia. Hence, it is relatively contraindicated in patients with epilepsy and brain lesions. Compared to halothane, liver damage is rare. ISOFLURANE: This volatile liquid with pungent odour is related to enflurane. Its pharmacological properties are similar to those of halothane but it is less liable to cause hypotension. The drug is metabolised only to the extent of 0.2%. Advantages: • Physically stable and non-inflammable. • Rapid induction and recovery of anaesthesia. • Bronchodilator. • Good muscle relaxant. • Potent coronary vasodilator. Does not affect renal blood flow. • Less likely to sensitise the myocardium to adrenaline; and • Hepatotoxicity rare. Isoflurane is considered by many as the “near ideal” anaesthetic. It is the preferred agent in neurosurgery. Disadvantages: • Pungent and respiratory irritant. • Causes peripheral arterial vasodilatation and can cause coronary steal; and • It does not protect against local ischemia. Desflurane has properties similar to those of isoflurane. It is extremely volatile, with pungent odour. It has rapid onset of action and rapid recovery. It may cause cough, salivation and bronchospasm. Hence, it is not preferred for induction of anaesthesia. Sevoflurane is non-irritating to the respiratory tract and can be used for induction of anaesthesia in children as it is pleasant to inhale. ETHYL CHLORIDE: Ethyl chloride is a nonirritating, highly volatile and inflammable liquid. The vapour has a characteristic but not unpleasant odour. When sprayed on the skin rapidly, it produces transient paralysis of cutaneous sensory nerve endings and local anaesthesia. The local anaesthetic effect lasts from a few seconds to a minute, hence only very minor operations such as incision of an abscess can be carried out. Though inhaled ethyl chloride induces anaesthesia quickly, the margin of safety is narrow. It is now rarely used. TRICHLOROETHYLENE: This clear, colourless liquid with a characteristic odour is non-irritant and non-inflammable. Trichloroethylene is a potent analgesic with a rapid onset of action. However, muscular relaxation is inadequate. It sensitises the myocardium to adrenaline. It was used as a selfmedication analgesic during labour in the form of intermittent inhalation. It is no more used.
Gaseous Anaesthetics NITROUS OXIDE: Nitrous oxide is a colourless, inorganic gas with a sweet taste. It does not undergo significant decomposition in the body. Nitrous oxide, if administered along with air, produces a stage of excitement and delirium and also produces amnesia. Hence the name “laughing, gas”. It is commonly used together with oxygen and other agents such as isoflurane. Nitrous oxide produces analgesia when inhaled in the concentration of 35 to 40% with air. Loss of consciousness occurs with the concentration of 65 to 70% and plane i of surgical anaesthesia can be reached with an 80:20 mixture of nitrous oxide and oxygen. Further increase in the concentration of the anaesthetic agent produces hypoxia. Nitrous oxide has no serious, deleterious effects on circulation, respiration, liver and kidneys, and it is probably the safest anaesthetic agent. Absorption, fate and excretion: Nitrous oxide is not altered within the body and is carried in the form of a physical solution in the blood. It is rapidly eliminated through lungs within 2 to 5 minutes after its withdrawal. Advantages: • Non-inflammable and non-irritant. • Rapid induction and recovery. • Analgesic in subanaesthetic concentration; and • Nausea and vomiting are uncommon. Disadvantages: • Not a potent anaesthetic agent by itself; must be supplemented with another preanaesthetic agent or a muscle relaxant. • Excitement may be violent. • CO2 accumulation and hypoxia may develop during prolonged use. • Diffusion hypoxia develops at the time of discontinuation of nitrous oxide and is dangerous in patients with low cardiopulmonary reserve. This can be prevented by administration of 100% oxygen while discontinuing nitrous oxide. • A special apparatus is required. • An increase in spontaneous abortions has been reported in the wives of male dentists and in female chairside, dentists’ assistants. • Any closed gas-filled space tends to expand during administration of nitrous oxide. It is, therefore, contraindicated in patients with collections of air in the pleural, pericardial or peritoneal cavities; intestinal obstruction; occlusion of the middle ear; chronic obstructive airway disease; or emphysema. It is also contraindicated in patients who have recently undergone pneumoencephalography. • May cause hallucinations; female patients might allege sexual misbehaviour by the doctor. This is also seen after propofol anaesthesia. Therapeutic uses: It may be used for tooth extraction, obstetric analgesia, and during painful procedures such as changing dressings in burns patients, cleaning and debridement of wounds and cauterisation. When nitrous oxide is given in high concentration (70-80%) with another potent inhalational anaesthetic like halothane it facilitates delivery of the latter to blood at a higher rate and helps in achieving faster induction. This effect is termed as second gas
effect. On the other hand, when nitrous oxide is discontinued, a large amount of nitrous oxide rapidly diffuses into alveoli from the blood owing to its low blood solubility. This dilutes the alveolar air causing a drop in the partial pressure of oxygen in alveoli. The gas also dilutes alveolar carbon dioxide causing decreased drive for ventilation. During this time, breathing room air results in hypoxia, which is termed as diffusion hypoxia. This hypoxia can be prevented by administration of 100% oxygen 5-10 min after discontinuing nitrous oxide. Other uses: Measurement of coronary/cerebral blood flow by Fick’s principle. Xenon: is a rapidly acting potent inhalational anaesthetic agent, which is very expensive. It is a non-competitive antagonist of NMDA receptor and agonist of TREK channel (twopore K+ channels). It is well tolerated even by elderly. Since modern surgery makes increasing use of electronic devices, inflammable and/or explosive anaesthetics like ether are now considered obsolete in many advanced countries. Open ether, however, is still being used in many developing countries and is considered as a relatively safe anaesthetic despite its inflammable and explosive nature, particularly when a qualified anaesthetist and anaesthetic equipment are not available.
Non-Volatile General Anaesthetics Ultra short acting barbiturates: The ultra short acting barbiturates administered IV to produce general anaesthesia are the thiobarbiturates (thiopental, thiamytal and thiobarbitone) and the methylated oxy-barbiturates (hexobarbitone and methohexitone) (Chapter 8). The compound employed most commonly are thiopental and methohexitone. Methohexitone is twice as potent as thiopental and shorter acting. THIOPENTAL: Thiopental sodium is readily soluble in water but the solution deteriorates on keeping. The clinically used solution is intensely alkaline with a pH varying from 10.5 to 11. High alkalinity can cause local irritation and venous thrombosis. Given IV, it rapidly induces hypnosis and anaesthesia–without analgesia. Anaesthetic action: The induction is very quick and pleasant. Consciousness is lost first, then the reflex activity and muscle tone and lastly, the vital medullary centres are depressed. Pupils react to light and remain contracted in light hypnosis. The corneal reflex remains active until deep anaesthesia is achieved. Cerebral blood flow and cerebral metabolic rate are reduced and there is a marked reduction of intracranial tension. It also reduces cerebral metabolism. It is, therfore, a choice for patients with cerebral swelling. A fairly reliable sign of an adequate induction by thiopental is the absence of the eyelid reflex. Presence of swallowing, phonation and reflex movements of eyes during anaesthesia indicate need for further injection. Though the reflexes return in 10-30 minutes, after stoppage the patient remains disoriented for several hours and hence, must not be left alone (See below). Absorption, fate and excretion: The very short duration of action is attributed to its high lipid solubility. The rapid metabolism of the drug by liver may also contribute to its short duration of action. With successive doses, body fat depots get saturated with the drug. Slow release of the stored drug back into the plasma is responsible for the prolonged recovery and continuation of drowsiness observed after the cessation of anaesthesia. Thiobarbiturates readily cross the placental barrier and appear in breast milk. Advantages: • Non-explosive and non-irritant; easy to administer. • Very rapid and smooth induction, and rapid recovery after small doses. • Quiet respiration; no cardiac arrhythmias; and • Nausea, vomiting, excitement and post-operative complications are infrequent. Disadvantages: • Poor analgesic and muscle relaxant. • Conducting anaesthesia and judging its depth are difficult as the usual stages of anaesthesia are not discernible. • Pharyngeal and laryngeal reflexes persist, permitting occurrence of coughing, apnoea, laryngospasm and bronchospasm. Hence, equipment for controlled ventilation must be available. • Depression of respiratory centre. • Depresses myocardium and vasomotor centre and produces hypotension. It must never be given to a patient who is sitting such as in a dental chair. • Highly irritant if extravasated; nerve palsy and limb gangrene reported. • Relaxes gastroesophageal sphincter, causing silent regurgitation; and
• Can precipitate acute attack in patients with acute intermittent porphyria. Barbiturate anaesthesia is to be used with great caution in the presence of hepatic and/or renal damage, in shock, in airway obstruction, in individuals with a past history of bronchial asthma or severe cardiovascular disease. Therapeutic uses: • For induction of general anaesthesia. • As anaesthetic agent for operations of short duration e.g. fracture reduction, dilatation and curettage, laryngoscopy and bronchoscopy. Methohexitone is preferred during ECT as it increases ictal activity. • As an anaesthetic in patients with history of malignant hyperthermia, head enjury and brain tumor. • As anticonvulsant in the emergency treatment of intractable seizures. Preparations: (i) Thiopental sodium 0.5 to 1.0 g powder. It is used as a freshly prepared, 2.5% solution for IV anaesthesia. (ii) Methohexitone: Twice as potent as thiopental and shorter acting. It is used as 1% solution. PROPOFOL: This IV anaesthetic causes rapid induction and rapid recovery with small hangover effect. It is generally used for sedation, induction and maintenance of general anaesthesia and for brief ambulatory procedures. It causes dose-dependent cortical depression and is an anticonvulsant. It is largely (88%) metabolised by the liver and partly cleared by the other mechanisms. Advantages: • It has specific anti-emetic action and is less likely to cause bronchospasm. • It has no effect on the hypoxic, pulmonary vascular reflexes. • Because of quick and pleasant recovery from anaesthesia, it is now preferred to thiopental for intubation, endoscopy and day-care surgery. • Is safe during pregnancy. • It may be combined with an ultra-short acting opioid such as remifentanil. Disadvantages: • Narrow therapeutic window. • Excess dose can cause greater sedation, myocardial and respiratory depressant effects than other IV agents. • Prologed IV infusion can lead to cumulation with sedation and acidosis. This results in propofol infusion syndrome seen in patients with head injury. • The emulsion formulations are painful. Pretreatment/mixing with lignocaine can reduce the pain. Fospropofol, a water soluble prodrug, is claimed to cause less pain on IV injection. It is hydrolysed by endothelial alkaline phosphatase to propofol. MIDAZOLAM: This short acting benzodiazepine (Chapter 8) is used either IM or IV for sedation and anaesthesia. It is also used as premedication or as anesthetic adjuvant due to its sedative, anxiolytic and amnestic properties. Its onset of action is rapid and t½ is 2-4 hours. The IV dose is 2.5 to 7.5 mg; the usual IM dose is 5 mg. With IV administration of midazolam, the same precautions are required as with IV diazepam. It is water soluble and less irritant to the veins than diazepam (Chapter 8). It has also been used by SC infusion
(by a syringe pump) as an anticonvulsant. It is a relatively safe drug. Benzodiazepines do not produce true general anaesthesia, as the awareness about surrounding remains, so also the mobility; hence surgical procedures are difficult to carry out under BDZ anaesthesia given alone. KETAMINE: This agent is chemically related to phencyclidine and acts as an antagonist at NMDA receptors. It probably acts on the cerebral cortex, particularly the limbic system. It has analgesic property in subnarcotic doses, and light anaesthesia usually does not cause depression of the protective pharyngeal and laryngeal reflexes. It is a potent bronchodilator. Given IV, it is quick acting although the onset of action is slower than that of thiopental. Following a single dose, it induces a state of dissociative anaesthesia characterised by complete analgesia combined with amnesia and catatonia, with or without loss of consciousness. The patient can open his eyes and can obey instructions. Respiratory support in not needed. Analgesia lasts for about 40 minutes whereas anaesthesia lasts for about 15 minutes due to rapid redistribution. Anaesthesia can be induced by both IM (5-10 mg/kg) and IV (1-2 mg/kg) routes. A low dose 0.1-0.25 mg/kg IV produces adequate analgesia and is an alternative to opioids to minimise respiratory depression. The drug increases the BP, heart rate and cardiac output by activating central sympathetic system and preventing peripheral reuptake of NA. It can be used in patients in shock. However, it should be avoided in patients with ischemic heart disease. Disadvantages: • It sometimes causes nystagmus, involuntary movements and hypertonus. • It may cause delirium, hallucinations and colourful dreams during induction and recovery, especially in adults. Diazepam, midazolam or propofol given prior to ketamine can prevent these disturbances. • Rarely, laryngospasm may occur; salivation may be troublesome. • Muscular relaxation is inadequate. • It increases intraocular and intracranial pressures. • It is a drug of abuse (Date Rape Drug). It can be used as an inducing agent but, its use in low dose, in combination with other anaesthetic agents like propofol is preferred. It is of choice in poor-risk elderly, children and patients with asthma. It is used for short-lasting diagnostic procedures like cardiac catheterisation and bronchoscopy, for dressing of burns, forceps delivery, breech extraction, manual removal of the placenta and dental work. It is not used: • In patients suffering from hypertension, cardiac decompensation or a cerebrovascular accident. • For surgery of the pharynx, larynx or bronchi. • In abdominal surgery, as it relieves visceral pain poorly. • In thyrotoxic patients, in whom it may cause rise in blood pressure. • In pregnant women at term, because of its oxytocic activity. However, it may be used during caesarian section as it causes less fetal and neonatal depression. • During operations on the eye, as it causes a rise in the intra-ocular pressure; and • In the presence of psychiatric disorders such as acute psychosis and schizophrenia.
Barbiturates and diazepam are chemically incompatible with ketamine. They should never be administered from the same syringe or via the same infusion set. A topical formulation of ketamine is available for neuropathic pain. ETOMIDATE: This drug, a carboxylated imidazole, has potent hypnotic and anaesthetic properties. A single IV dose of 300 mcg/kg produces loss of consciousness within 10 seconds and a state of anaesthesia, followed by sleep. Recovery is rapid and complete due to redistribution. Cardiovascular and respiratory depression are minimal. In fact, cardiovascular stability during and after induction is considered to be a major advantage of etomidate; hence it is preferred in elderly patients prone to hemodynamic instablility and those with poor cardiovascular reserve. The drug, however, commonly causes pain on injection, myoclonus and post-operative nausea and vomiting. Further, it inhibits steroidogenesis resulting in suppression of adrenocortical stress response. It is primarily used for induction, along with opioid analgesics as etomidate has no analgesic effect. Opioids help during endotracheal intubation and reduce involuntary muscle movements.
Neuroleptanalgesia Neuroleptics (antipsychotics) are a group of drugs which induce a state of apathy and mental detachment in which the patient is mildly sedated and uncaring about his surroundings. These compounds are used in the treatment of major psychoses and are discussed in detail in Chapter 13. Neuroleptanalgesia is a method of IV anaesthesia which combines the use of a neuroleptic drug with an opioid analgesic drug. Such a combination produces a state which differs from the classical general anesthesia in that the subject is conscious and is able to co-operate during the operative procedure. The most favoured combination in clinical practice is that of the neuroleptic droperidol and the analgesic fentanyl. DROPERIDOL: This is a butyrophenone derivative like haloperidol. Its pharmacological actions are similar to those of chlorpromazine (Chapter 13); but it is short acting (2-3 hours) and more potent than haloperidol. Apart from typical behavioural effect of calming, droperidol also has antiemetic and alpha-adrenergic blocking (adrenolytic) actions. Like all neuroleptic drugs it can produce extrapyramidal reactions. FENTANYL : This drug belongs to the group of 4-acylanilino piperidines. It is a morphine-like opioid analgesic (Chapter 10) used exclusively as a supplementary analgesic in inducing general anaesthesia. Like morphine, it suppresses the respiratory and cough centres and causes nausea, vomiting and miosis. It is 100 times more potent than morphine. However, its action is of shorter duration. Given IM or IV (2-20 mcg/kg), it rapidly produces profound analgesia lasting for about 30 minutes. These actions can be antagonised by naloxone. Droperidol 2.5 mg and fentanyl citrate 50 mcg in 1 ml, given IV, causes complete analgesia and amnesia sufficient for surgical procedures without marked hypnosis. The onset of anaesthesia is slow. Major advantages of this procedure are: • Smooth onset and rapid post-anaesthetic recovery. • Less danger of hypotension and other circulatory disturbances. • Suppression of vomiting and coughing. • Continued analgesia in postoperative period. • Availability of patient’s co-operation during the operative procedures such as eye, oral and orthopaedic surgery, angiocardiography, myelography and bronchoscopy. Since the combination does not disturb the cardiovascular dynamics, it is claimed to be very useful in old people and in ‘poor risk’ cases. Further, the combination can be used to induce anaesthesia which can then be continued with other general anaesthetic agents like nitrous oxide-oxygen mixture and muscle relaxants. Adverse reactions: They are due to toxicity of individual drugs. They include hallucinations, mental depression, extrapyramidal disturbances due to droperidol and respiratory depression due to fentanyl. The latter may be marked and assisted, controlled ventilation is necessary. As compared to droperidol, fentanyl has a shorter duration of analgesic action (30 minutes) and supplementary doses of fentanyl (1 mcg/kg) may be needed after 20 minutes. Fentanyl, alfentanil and sufentanil are sometimes used as IV analgesics in short operations because of their brief duration of action. They can be used as co-inducing
agents. Remifentanil, a synthetic opioid, has rapid onset of action and is metabolised rapidly by esterases in the plasma and muscles. As a result, its duration of action is extremely short with less respiratory depression. High dose of morphine can be used with benzodiazepines to achieve anaesthesia in patients for cadiac surgery when circulatory reserve is limited. Rational Use of Anaesthetic Agents: Selection of appropriate general anaesthetic agent depends upon drug-related factors and the host-related factors. The former include characteristics of the drug, its pharmacokinetic features, effects on the homeostasis and ADR. Host-related factors include the procedure the patient is undergoing, time required for the same, patient’s characteristics like age, co-morbid conditions and concomitant medications. Careful attention needs to be paid to minimise adverse effects of the drugs, maintain homeostasis and facilitate smooth post-anaesthetic recovery. Minor surgical and diagnostic procedures can be performed safely under judicious use of sedation based techniques without using general anaesthesia. Many protocols are available to achieve analgesia and deep sedation using various agents in combination. For example, in monitored anaesthesia care technique, midazolam IV is used as premedication followed by propofol infusion for deep sedation and opioid analgesic/ketamine for analgesia. Conscious anaesthesia technique employs use of small doses of IV anaesthetics like propofol and midazolam along with opioid analgesics to produce pain and anxiety- free altered level of consciousness. In this state, no respiratory support is needed and patient can respond to commands. This technique can also be used in ICU for patients under stress and on mechanical ventilators by using additional agents like muscle relaxants and dexmedetomidine. Deep sedation technique produces a state like light GA from which patient cannot be aroused easily. The protective reflexes are lost and surgical stimuli do not elicit any verbal responses. Thiopental, midazolam, propofol, ketamine and potent opioid analgesics given IV can be used for this purpose.
Pre-anaesthetic Medication Pre-anaesthetic medication is the term applied to drugs used prior to the administration of an anaesthetic agent, with the object of making anaesthesia safer and more agreeable to the patient. The reasons for such medication are: • For sedation, to reduce anxiety and apprehension without producing much drowsiness. • To obtain an additive or synergistic effect so that induction could be smooth and rapid and the dose of the general anaesthetic could be reduced. • To counteract certain adverse effects of the anaesthetic drug used such as salivation, bradycardia and vomiting. • To relieve pre- and post-operative pain. • To suppress respiratory secretions and to reduce reflex excitability. There is no single drug which can achieve all these objectives and hence usually a combination of drugs is used. It must be emphasised, however, that factors other than drugs can favourably affect preoperative psychological preparation; and a preoperative visit by the anaesthesiologist and a sympathetic discussion with the patient about the events of the next day have a high therapeutic value. The drugs commonly used for preanaesthetic medication are: (1) Opioid analgesics such as morphine (10-15 mg IM), pethidine (50-100 mg IM) and buprenorphine (300 mcg IM), are generally employed for their sedative and analgesic properties before major surgery. Epidural and intrathecal routes allow low dose to produce analgesia with less systemic side effects. Buprenorphine has longer duration of action than morphine and pethidine (Chapter 10). They also reduce the amount of general anaesthetic required. For fentanyl and congeners, see earlier. Disadvantages: • They may depress respiration and may cause respiratory arrest. Further, drugs like morphine increase the tone of smooth muscles such as bronchial muscles. In emphysema or in kyphoscoliosis where the pulmonary reserve is already low, use of opioids may precipitate pulmonary insufficiency. • They may cause vasomotor depression, and may decrease the ability of circulation to respond to stress. They often delay the awakening as their clinical effect lasts for 4-6 hours. • Morphine may induce vomiting and cause antidiuresis. • Pethidine by its vagolytic action may produce tachycardia. • Both these drugs are histamine liberators. (2) Sedatives and tranquillisers: Benzodiazepines (midazolam diazepam, lorazepam) are preferred because of their safety, muscle relaxant property and less respiratory depression (Chapter 8). They also provide amnesia. Diazepam in dose of 5 to 20 mg. has been most widely used. It is active orally and can also be given parenterally, though its action is less predictable by the latter route. Other tranquilliser compounds used belong to phenothiazine. Phenothiazines possess sedative, antiarrhythmic, antiemetic and antihistaminic properties. Phenothiazines commonly employed are promethazine and trimeprazine. They can be given orally as well as parenterally (Chapter 13). Phenothiazines and benzodiazepines should not be combined with opioids particularly in patients with respiratory insufficiency.
Dexmedetomidine: This imidazole derivative causes analgesia and sedation with very little respiratory depression by its central and peripheral α2 adrenergic receptor agonist action. Its main adverse effects are hypotension and bradycardia. It is used IV for short term sedation of critically ill adults and as anaesthetic adjunct. (3) Antimuscarinic drugs: Atropine (600 mcg IM) is generally combined with morphine to block the vagal actions so as to reduce salivary and respiratory secretions and to prevent parasympathetically induced reflex hypotension and bradycardia. It may thus lessen the possibility of cardiac arrhythmias during the induction stage. Due to blockade of cardiac vagal action, atropine may produce tachycardia. However, with newer anaesthetics, atropine is less commonly used. Instead, a synthetic long acting quaternary amine, such as glycopyrrolate is now the preferred anti-muscarinic agent because of its less central actions and less tendency to cause excessive tachycardia (Chapter 20). (4) Antiemetics: The commonly used phenothiazines such as promethazine and trimeprazine have antiemetic properties and thus may help to prevent the post-operative nausea and vomiting. This advantage should, however, be weighed against the possible hypotension following these drugs. 5HT3 antagonist, ondansetron is also used. Other drugs used are cyclizine, 50 mg., trimethobenzamide 200 mg and benzquinamide 25-50 mg. (5) Other drugs: In addition to the use of above mentioned drugs, proper pre-evaluation and specific premedication are needed in patients with special problems such as chronic lung disease, emphysema, ischemic heart disease, diabetes mellitus, hypertension, undernutrition and in debilitated and old people. Antibiotic prophylaxis may be needed (Chapter 51). The risk of stopping long-term medication before surgery is often greater than the risk of continuing it during surgery. This applies particularly to glucocorticoids, analgesics, antiparkinsonian drugs, anti-glaucoma drugs, and thyroid or antithyroid drugs. On the other hand, it is advisable to discontinue combined oral contraceptive pills 4 weeks before major surgery; monoamine-oxidase inhibitors 2 weeks before surgery; warfarin 3-5 days before surgery, aspirin 7 days before surgery and lithium 2 days before surgery.
Drugs Administered During Anaesthesia These are: • Skeletal muscle relaxants like succinyl choline and curarimimetics to achieve good muscle relaxation (Chapter 22). • A very short acting ganglion blocking agent like trimethaphan camphor sulfonate or sodium nitroprusside to produce controlled hypotension (Chapter 30). • Drugs administered to counter the anaesthetic complications e.g. vasopressor agents (such as methoxamine or phenylephrine) to correct hypotension, antiarrhythmics to correct cardiac arrhythmias and anticonvulsants. The prophylactic administration of supplementary glucocorticoids to patients receiving steroid therapy or those with a history of such therapy within two years prior to surgery is necessary to avoid serious hypotension and shock during surgery. Antibiotics like streptomycin and neomycin (aminoglycosides) have neuro-muscular blocking action and hence, can produce skeletal muscle paralysis when instilled into pleural or peritoneal cavity during anaesthesia; these drugs can also potentiate the actions of curarimimetic, skeletal muscle relaxants (Chapter 47). Patients on beta-adrenergic blockers tend to develop hypotension more often following certain anaesthetic agents. Malignant hyperthermia is a serious but rare complication of general anaesthesia (see Chapter 22).
8
Sedatives, Hypnotics and Pharmacotherapy of Sleep Disorders Physiologically, sleep is regarded as absence of wakefulness, where the responses to environmental stimuli are greatly reduced. But, in fact, it is an active state, related to definite anatomic structures as well to several neurotransmitters and biogenic amines. Yet, its exact mechanism is not known. The determinants of natural sleep are many but the most important regulator is probably a “central pacemaker ” (or the biological clock) in the ventrolateral preoptic (VLPO) hypothalamus. The circadian system maintains normal daily sleep- awake cycle and the feeling of freshness; REM sleep is modulated by circadian system. It is associated with increased melatonin secretion by the pineal gland, lowering of body core temperature and increased plasma concentration of cortisol. Physiologically, the melatonin concentration is high during night (sleep period) while daytime concentration is very low. Its nocturnal high plasma concentration correlates with increased sleep propensity, reduced body temperature and decreased alertness. Melatonin acts mainly on two receptors MT1 and MT2 which are found in hypothalamus, hippocampus, cerebellum and other parts of the brain. It is involved in sleep-wake cycle and thermoregulation. It plays an important role in the circadian timings system (chronobiotic actions). An appropriately timed administration of exogenous melatonin increases sleep propensity, reduces sleep latency, decreases alertness and lowers core body temperature. We all need sleep. It is believed that restoration of natural balance among the neuronal centres in the brain takes place chiefly during sleep, and the association between sleep and growth in the early years of life is generally accepted. Based on electrophysiological studies (EEG, electromyogram and electro-oculogram), sleep has been classified into two types (Fig 8.1):
FIG. 8.1 Sleep cycle and EEG changes in a normal young adult. Shaded areas denote REM sleep. CPS: cycles per second
• Non Rapid Eye Movement sleep (NREM); and • Rapid Eye Movement sleep (REM). While falling asleep, one passes sequentially through stages 1, 2, 3 and 4 of NREM sleep. After about 90-120 minutes of NREM sleep, REM sleep occurs, lasting for 5-30 minutes. The NREM-REM cycle repeats 4-5 times during the night, with progressive lengthening of the REM bouts until one awakens from REM (not NREM) sleep in the morning. In general, NERM stages 1 and 2 constitute 50-60%, NREM stages 3 and 4 (slow wave) 15 to 25% while REM 20-25% of total sleep in young adults. Slow wave sleep is prominent in children and decreases with the age so that it may even be absent in healthy old people. There are important differences between NREM and REM sleep. They are: • NREM sleep is very peaceful with preponderance of the parasympathetic activity and diminution of the metabolic rate, heart rate, cardiac output and peripheral vascular resistance. Dreaming is infrequent and the dreams are rarely recalled on awakening. On the other hand, the sympathetic activity predominates during REM sleep. The sleep is not so restful; 75% of the dreams occur in this type of sleep; the dreams tend to be more vivid, bizarre and often sexual, with erections occurring in the males. Dreams are accompanied by appropriate cardiovascular responses to the perceived dream activities such as running or escaping. As a result, the heart rate, BP, cardiac output, peripheral vascular resistance, small airway resistance and metabolic rate rise markedly. Short periods of central apnoea may occur during REM. • During NREM sleep, the EEG shows alpha rhythm together with sleep spindles. The sleep becomes deeper during the four stages of NREM sleep and it becomes progressively less easy to awaken a person from this type of sleep. On the other hand, during REM sleep the EEG resembles that of an awake and alert person (it shows a beta rhythm) and the brain is highly active with increased oxygen consumption. In spite of this, it is difficult to awaken a person from REM sleep. • There is no eyeball movement during NREM sleep, whereas the eyeballs move rapidly
and jerkily during REM sleep; hence the name. • Muscle tone diminishes progressively during NREM sleep, but the muscles which hold the chin up and keep the middle respiratory passages open are active. By contrast, all voluntary muscles except extraocular muscles are profoundly flaccid during REM sleep. • In general, prolactin secretion is increased during sleep; Growth hormone secretion occurs during stages 3 and 4 of NREM sleep. The notion that sleep is a uniformly quiescent and peaceful state, and therefore devoid of stress, is not correct. Both NREM and REM types of sleep expose a person to different types of stress. For example, relative hypotension during stage 4 of NREM sleep accounts for some of the ischemic cerebrovascular strokes during sleep. On the other hand, the extreme hypotonia of the small muscles that hold the middle respiratory passages open can lead to obstructive sleep apnoea during REM sleep. Further, the rise in the cardiovascular parameters due to catecholamine secretion during REM sleep can lead to hypertensive-hypoxic cardiovascular events. A normal person spends about one-third of his life in sleep. Adequate sleep is a necessity of life. In practice, many individuals complain of lack of sleep, insomnia. A hypnotic drug is one which produces sleep resembling natural sleep. A sedative, on the other hand, is a drug that reduces excitement, calms the patient, and is commonly used as an anxiolytic. Hypnotics and sedatives both depress the CNS, the difference being quantitative. Classification of hypnotics: All hypnotics act on different subunits of (GABA) receptors (Chapters 5 and 9). They are classified as: I Selective, benzodiazepine, GABAA receptor agonists: a. Benzodiazepines e.g. Diazepam, Oxazepam, Lorazepam etc. b. Non-benzodiazepines e.g. Zopiclone, Zolpidem, Zaleplon (Z drugs) II Non-selective, non-benzodiazepine GABAA receptor agonist: a. Barbiturates e.g. Phenobarbitone, Pentobarbitone. b. Non-barbiturates e.g. Chloral hydrate, Paraldehyde III Melatonin receptor agonist: Ramelteon IV Orexin receptor antagonist: Suvorexant V Miscellaneous: a. Sedative antihistaminics e.g. Diphenhydramine, Promethazine b. Tricyclic antidepressants e.g. Doxepin Drugs like morphine and pethidine, besides acting as opioid analgesics, also possess hypnotic property. Hence, they are grouped as Anodyne hypnotics. However, they should not be used as hypnotics in the absence of severe pain (Chapter 10).
Benzodiazepines BENZODIAZEPINES (BDZ): These compounds (Fig 8.2) have largely replaced the barbiturates as hypnotics (see later).
FIG. 8.2 1,4-Benzodiazepine Diazepam nucleus
Mechanism of action: GABA, the most potent inhibitory transmitter in the CNS controls the state of neuronal excitability. It acts by binding to the neuronal GABAA receptor (Fig. 8.3) and opens the chloride channels. BDZ bind selectively to subunits of the GABAA receptors, a site distinct from that of GABA or barbiturates binding site, and is designated as benzodiazepine binding site. They modulate allosterically GABA binding. Thus, they increase the frequency of chloride channel opening and the chloride ion concentration in the neuron. This causes hyperpolarisation of the neuronal membrane, making it more difficult for the excitatory neurotransmitters to depolarise the cell. They enhance the effectiveness of GABA by lowering the concentration of GABA required for opening the chloride channels.
FIG. 8.3 GABAA - BDZ - Barbiturate binding sites on GABAA receptor. R: Binding site, Z-agents: Zolpidem etc Note: Selective BDZ - GABAA receptor agonist acts through GABAA receptor. Barbiturates in therapeutic doses directly open chloride channel.
Although all the BDZ have similar pharmacological properties, they differ in their selectivity and vary in their clinical usefulness owing to the existence of multiple subtypes of BDZ binding sites in the CNS. Thus, the antispasticity effect appears to involve the GABAA receptors in the brain stem and the spinal cord, whereas the sedative and
anxiolytic actions are localised to the limbic system. Pharmacological actions: Benzodiazepines act as: • Anxiolytics (See Chapter 14) • Sedative-Hypnotics • Anticonvulsants; and • Muscle relaxants Sedative-hypnotic action: In small doses, BDZ produce relief from anxiety and in larger doses, they induce sleep. All benzodiazepines are qualitatively similar in their effects on the important sleep parameters. Thus, they decrease the time to onset of sleep, prolong stage 2 of sleep, and shorten stages 3 and 4 and REM sleep. The total sleep time is increased with diminished awakenings. Clinically, BDZ are preferred as hypnotics because they: • Induce sleep which is more refreshing and with fewer hangover symptoms such as drowsiness, dysphoria, and mental or motor depression. • Preserve near–normal sleep, remain effective as hypnotics for longer periods of time, and cause less rebound of REM sleep after withdrawal. • Can induce sleep even in the presence of pain and do not cause hyperalgesia. • Do not exert significant action on respiration and CVS even in large doses. Hence its use in patients having asthma is not contraindicated. • Have higher therapeutic index; and • Cause fewer drug interactions and have less potential for drug abuse. However, all can induce anterograde amnesia, where there is impairment of memory for events following the drug administration. Muscle relaxant and anti-convulsant actions: They have a central muscle relaxant action (Chapter 22). They increase the seizure threshold and act as anticonvulsants. Anaesthetic action: (Chapter 7) Miscellaneous actions: IV injections can dilate coronaries. They may lower the BP and decrease the respiratory rate. Absorption, fate and excretion: The various BDZ differ from each other in their pharmacokinetic characteristics. Given orally, diazepam and chlorazepate are most rapidly and completely absorbed from the proximal small intestine; prazepam and oxazepam are the least rapidly absorbed; flurazepam and lorazepam fall in between these two groups. The absorption of chlordiazepoxide and diazepam given IM is slow, incomplete and erratic. The only BDZ with reliable absorption from the IM site are lorazepam and midazolam. Rectal route for diazepam is generally used in the convulsing patient. The duration of action following a single dose depends upon the rate and extent of drug distribution and of metabolic degradation. BDZ are metabolised by hepatic microsomal CYP3A4 and 2C19 enzymes, and hepatic damage prolongs their action. The t½ is prolonged in subjects over 60 years age and in infants due to reduced hepatic clearance; hence, dosage should be reduced under such conditions. Depending on their elimination t½ they can be grouped as shown in Table 8.1.
Table 8.1 Benzodiazepine derivatives and oral doses
Δ Use half the dose in old people. *
These compounds are metabolised to clinically important active metabolites with elimination half-life values ranging between 36 and 200 hrs. **
In divided doses.
***
For IV use, inject 5 mg directly and slowly.
Some of the BDZ are biotransformed to clinically active metabolites, some of them with longer half-life than the parent compound. Thus, desmethyldiazepam (t½ 36-200 hours), a major metabolite, plays an important role in the clinical effects of chlordiazepoxide, diazepam, prazepam and clorazepate. Clorazepate and prazepam are in fact pro-drugs and reach the systemic circulation only as desmethyldiazepam. Flurazepam is converted to the active metabolite desalkylflurazepam. Multiple dose therapy with such drugs leads to accumulation of the long half-life, active metabolites, resulting in prolongation of the effect, and may cause unwanted daytime sedation. However, it should be noted that the clinical drug effects do not necessarily increase in direct proportion to plasma concentration because of development of tolerance. Because of long half-life, clinically significant amounts of chlordiazepoxide, diazepam or desmethyl-diazepam may persist in the blood and in the body for many days/weeks after termination of prolonged therapy. This could be beneficial in anxiety state as it prevents the rapid return of anxiety and delays the development of withdrawl symptoms. Oxazepam and lorazepam lack active metabolites and are preferred in the elderly. Preparations and doses: See Table 8.1. Adverse reactions: Benzodiazepines in general are well tolerated. The common side effects are due to dose related depression of CNS: drowsiness, lethargy and ataxia. They also cause impairment of visual-motor coordination, behavioural changes, daytime
sedation, and anterograde amnesia. The drugs may occasionally produce personality changes and may cause a paradoxical increase in hostility, irritability and anxiety especially in the elderly. They should be used cautiously in the presence of respiratory, liver and cardiac diseases. Rarely, BDZ cause leucopenia, allergy, photosensitisation, vertigo, headache, impaired sexual function and menstrual irregularities. Patients develop tolerance to the sedative (but not to the anxiolytic) action, as well as physical and psychic dependence. Withdrawal syndrome includes insomnia, agitation and rarely convulsions. The withdrawal symptoms are more intense following the discontinuation of shorter acting BDZ than of longer acting BDZ. The treatment is similar to that of barbiturate dependence (see later). Administration of BDZ to the mother before delivery can cause apnoeic spells, reluctance to feed, hypotonia and hypothermia in the newborn (floppy baby syndrome). Drug interactions: They enhance the effects of CNS depressants such as alcohol, barbiturates and amitriptyline. Microsomal enzyme inhibitors like cimetidine and isoniazid retard the elimination of diazepam. However, serious drug interactions are rare. Therapeutic uses: • As hypnotics (see later) • In anxiety states (Chapter 14). • During withdrawal of alcohol (Chapter 6) • As skeletal muscle relaxants (Chapter 22). • As anticonvulsants (Chapter 9). • As pre-anaesthetic and anaesthetic medication: Midazolam which has short t½ is used as IV anaesthetic agent (Chapter 7). FLUMAZENIL an imidazo-benzodiazepine, binds competitively to BDZ binding sites and antagonises the actions of BDZ. It does not block the pharmacologic effects of GABA or all GABA-mimetics. Given alone, it has minimal effect on the CNS. The drug can cause withdrawal syndrome in patients dependent on BDZ. Clinically, it rapidly reverses the effects of BDZ and facilitates the return of consciousness within 5-15 minutes in patients with BDZ poisoning. Given orally, it is rapidly absorbed and has a high hepatic clearance. It is metabolised in the liver and little is excreted unchanged. It is given IV initially in the dose of 0.2 mg over 30 seconds, and followed by 0.3 mg every minute to a total of 3 mg. It can also be given by IV infusion. The duration of action of a single dose is 30-60 minutes. Therapeutics uses: • In BDZ poisoning, and • For reversal of sedative effect of a BDZ administered during general anaesthesia, a diagnostic or therapeutic procedure. It is contraindicated in patient with seizure disorders, raised intracranial pressure as after severe head injury, and those on tricyclic anti-depressants.
Non-benzodiazepine, Benzodiazepine-receptor Agonists Non-benzodiazepines like zolpidem, zaleplon, zopiclone and eszopiclone (Z agents) are the newer sedative hypnotic agents which have varied chemical structures (Table 8.2). They bind selectively to a subset of BDZ binding sites and enhance the effect of GABA. Their characteristic features are: Table 8.2 Comparison of Eszopiclone, zolpidem and zaleplon
± = Very mild effect; + = Mild effect
• As sedative-hypnotics, they are as effective as BDZ and provide normal architecture of sleep. • They are rapid in onset and have short duration of action. Zaleplon is perhaps the best (even with middle-of-night use) because of its ultra-short t½. • Their ADR profile is similar to BDZ. They have adverse effects on memory and cognitive function. They often cause headache, and impair next morning driving performance. • They are metabolised in the liver. Zopiclone has active metabolites. • They lack anixiolytic, anticonvulsant and muscle relaxant properties. • Their effects can be antagonised by flumazenil. • Habituation, drug abuse and withdrawal symptoms have been reported. Hence they should be used in low doses than recommended eg. 5 mg of zolpidem instead of 10mg. Zaleplon is effective in reducing sleep latency and therefore used in insomnia with difficulty in falling asleep. But it does not decrease premature awakening nor increase total sleep time. Zopiclone, with longer t½, can be used for initiating and maintaining sleep. Zolpidem also decreases nocturnal arousal. Eszopiclone is the S-isomer of zopiclone and has similar actions. Its plasma t½ is 6 hours. In general, they appear to have only marginal advantage over the short acting BDZ.
Barbiturates BARBITURATES are the derivatives of barbituric acid which is a condensation product of urea with malonic acid. Barbituric acid itself is devoid of any hypnotic activity (Fig. 8.4).
FIG. 8.4 Synthesis of barbituric acid
Conventionally, they are divided according to their duration of action as: I Long acting (8 hours or more) e.g. Phenobarbitone. II Intermediate acting (4 to 8 hours) e.g. Amylobarbitone, Butobarbitone, and Pentobarbitone. III Short acting (less than 4 hours) e.g. Secobarbitone (quinalbarbitone). IV Ultra short acting (in minutes): e.g. Thiopentone. Replacement of the oxygen attached to C2 by sulphur enhances markedly the lipid solubility of an ultra-short acting barbiturate. Because of rapid onset and short duration of action, they are used for IV anaesthesia, (Chapter 7). Mechanism of action: Barbiturates cause reversible depression of all excitable tissues, the CNS being exquisitely sensitive. They bind to beta subunit of the inhibitory GABAA receptor, a site distinct from the site at which BDZ bind (Fig. 8.3). At lower doses, they enhance the action of GABA whereas in therapeutic doses they open the chloride channels directly. They also inhibit the excitatory AMPA-glutasmate receptors. Pharmacological actions: Barbiturates are less selective than BDZ and act at multiple sites in the CNS. Central Nervous System: Barbiturates depress the CNS in a dose-dependent manner. • Sedation and hypnosis: Phenobarbitone given in small doses acts as a daytime sedative. Larger doses produce sleep. The sleep resembles natural sleep. However, they: (a) Cause more reduction in duration of REM sleep and number of cycles than BDZ (b) Exhibit hangover (residual sedation and headache on waking) more than with BDZ. (c) Cannot induce sleep in presence of pain, (whereas BDZ can) and, (d) Cause rebound increase in REM sleep on sudden withdrawal; this can lead to increased dreaming and nightmares during the withdrawal period, especially in addicts. Further, (e) Hypnotic doses of barbiturates produce motor incoordination, ataxia. Although barbiturates reduce anxiety, they may cause distortion of judgement and may impair vigilance and attention to external stimuli. In old people and children, barbiturates occasionally produce dysphoria or excitement and a state
of confusion. • Anaesthetic effect: Thiobarbiturates IV produce general anaesthesia (Chapter 7). • Anticonvulsant and antiepileptic effect: Barbiturates administered in anaesthetic doses inhibit or abolish drug induced convulsions and those due to epilepsy and tetanus. Phenobarbitone and mephobarbitone have a selective antiepileptic action (Chapter 9). • Respiration: Respiration is normally maintained as a result of: (i) A neurogenic drive originating in the reticular activating system. (ii) A chemical drive depending upon the concentration of carbon dioxide and pH of the arterial blood which directly modify the activity of the medullary respiratory centre; and (iii) A hypoxic drive mediated through the carotid and the aortic body chemoreceptors. Barbiturates cause dose-dependent depression of the respiratory centre. With toxic doses, the respiration is maintained mainly by the ‘hypoxic drive’. A further increase in the barbiturate concentration abolishes the hypoxic drive and also causes a direct paralysis of the medullary centre. • Spinal cord: Both the polysynaptic and the monosynaptic reflexes of the spinal cord are depressed by barbiturates. Cardiovascular system: Toxic doses produce a sustained hypotension as a result of (i) direct depression of the myocardium and the vasomotor centre, and (ii) hypoxia. Kidney: Barbiturate anaesthesia results in reduction of urinary output as a result of decrease in the GFR, and stimulation of secretion of ADH. Acute barbiturate poisoning is often associated with oliguria largely due to severe hypotension. Liver: In patients intolerant of barbiturates, hepatic involvement may occur along with dermatitis and damage to other organs. Barbiturates exert various actions on certain liver enzymes: • On acute administration, barbiturates combine with various subtypes of CYP450 enzymes and competitively inhibit the metabolism of drugs and endogenous steroids. • Chronic administration causes induction of hepatic microsomal enzymes (Chapters 1, 3) leading to increased inactivation of certain drugs, including barbiturates themselves. This may explain the phenomenon of tolerance to barbiturates. • They induce delta-amino-levulinic acid (ALA) synthetase, a mitochondrial enzyme, and aldehyde dehydrogenase, a cytoplasmic enzyme. Increase in ALA synthetase results in an increase in ALA and porphobilinogen synthesis. In patients suffering from acute intermittent porphyria, barbiturates may precipitate a severe attack resulting in paralysis and even death. • Phenobarbitone increases the hepatic glucuronyl transferase and the bilirubin-binding Y-protein and stimulates the metabolism of bilirubin. Absorption, fate and excretion: Barbiturates are weak acids and maximum absorption occurs from the stomach where the barbiturates exist in an unionised form. Given orally, sodium salts are uniformly and rapidly absorbed but because of their extreme alkalinity, they may cause epigastric distress. Absorption also occurs from the intestine and the rectum. The barbiturates are distributed in all tissues and body fluids. They readily cross the placental barrier and small amounts may be secreted in milk. The factors which affect the distribution and fate of various barbiturates are their: • Lipid solubility
• Degree of protein binding, and • Extent of ionisation The short acting barbiturates are highly soluble lipid. Hence, these compounds have a rapid onset of action. They are more rapidly metabolised, but at the same time tend to get completely reabsorbed by the kidney tubules. Barbiturates exist in the plasma in an ionised and a non-ionised form. An increase in pH (alkalinisation) of blood and urine increases the ionisation of the barbiturates causing their efflux from the tissues such as brain into the plasma. The ionised form does not cross the biological membranes and is excreted in urine. All barbiturates are metabolised in the liver. The inactive metabolites are conjugated with glucuronic acid and are excreted in the urine. In the case of phenobarbitone, however, 25-30% of the dose is excreted unchanged. Preparations and dosage: (i) Phenobarbitone as an antiepileptic: See Chapter 9. (ii) Amylobarbitone 50 mg tablets. (iii) Butobarbitone 100 mg tablets. (iv) Secobarbitone 100 mg tablets. (v) Thiopental sodium as an IV anaesthetic: See Chapter 7. Adverse reactions: • Intolerance: They may cause excitement (with hypnotic doses) headache, nausea, vomiting, diarrhoea and lassitude. Occasionally, barbiturates themselves may produce paroxysmal pain. • Allergic reactions include urticaria, angioneurotic edema, other skin reactions, agranulocytosis and thrombocytopenic purpura. • Anemia: Prolonged phenobarbitone therapy may produce megaloblastic anemia which responds to folic acid (Chapter 36). • Depression of fetal respiration: Barbiturates, if administered to a woman during labour, may depress the foetal respiration. • Porphyria: Barbiturate administration may precipitate an attack of acute intermittent hepatic porphyria. • Drug automatism: When a barbiturate is employed as a hypnotic, because of confusion and amnesia, a patient may repeatedly take the barbiturate at night and poison himself. This phenomenon is known as drug automatism. • Tolerance: Repeated administration of barbiturates causes tolerance to their sedative and hypnotic actions. It can be attributed to: (i) Increased hepatic inactivation and (ii) Adaptation of the neuronal tissue to the drug. Barbiturate addicts often show cross tolerance to other general CNS depressants such as general anaesthetics. However, tolerance to the hypnotic effect of barbiturates fails to modify their lethal dose significantly. Acquired barbiturate tolerance usually disappears completely within 1 to 2 weeks of abstinence. • Drug dependence: Repeated ingestion of barbiturates causes drug dependence. The manifestations of chronic barbiturate intoxication are thick slurred speech, ataxia, impaired reflexes, hypotonia, nystagmus and difficulty in accommodation. The nutrition is usually unimpaired. Barbiturate withdrawal symptoms are summarised in Table 8.3.
Table 8.3 Barbiturate withdrawal symptoms
The treatment of barbiturate dependence is purely symptomatic. Generally, the withdrawal should be gradual, over 10 days to 3 weeks, depending upon the severity of the dependence. If desired, replacement could be made with a hypnotic such as chlordiazepoxide 50 mg., or diazepam 10 mg. Drug interactions: See Table 8.4. Table 8.4 Important drug interactions of barbiturates
Therapeutic uses: • As anticonvulsants: Barbiturates have been used to control convulsions in eclampsia and status epilepticus. They have now been replaced by benzodiazepines. For the use of phenobarbitone in epilepsy, see Chapter 9. • General anaesthesia: See Chapter 7. • Psychiatric uses: Amylobarbitone, pentobarbitone and thiopentone are employed by IV route to produce a state of deep sedation in which the cortical inhibitions are abolished. This may bring forth the suppressed psychic disturbances; the patient becomes more communicative and amenable to suggestions. This procedure of narcoanalysis, amytal interview (lie detection test) may be useful. • Neonatal Jaundice: Phenobarbitone stimulates the liver to produce glucuronyl transferase, the enzyme essential for metabolism of bilirubin. It is, therefore, used to treat certain types of neonatal jaundice. They are no more recommended as hypnotics. Acute barbiturate poisoning : Acute barbiturate poisoning causes marked CNS depression, particularly the respiration, and a peripheral circulatory collapse. The frequent and often fatal complications are atelectasis, pulmonary edema and bronchopneumonia or acute renal shutdown. Treatment of acute barbiturate poisoning: The severity of barbiturate poisoning is assessed by clinical signs prior to treatment and correlates well with plasma levels of barbiturate. Presence of reflexes, response to painful stimuli and maintenance of BP and of respiration without external assistance indicate better prognosis, while cases showing deep coma with absent reflexes, respiratory depression and cardiovascular collapse have a high mortality. Table 8.5 summarises the principles of management of acute barbiturate poisoning.
Table 8.5 Principles of management of acute barbiturate poisoning
• Gastric lavage: If the patient is conscious and less than four hours have elapsed since ingestion, vomiting may be induced with syrup of ipecac or concentrated salt solution. If the patient is unconscious, simple aspiration of the gastric contents is helpful if carried out within four hours of barbiturate ingestion. In comatose patients, endotracheal intubation should precede gastric intubation to prevent aspiration. • Adequate tissue oxygenation: Adequate ventilation is important. If the respiration is not much affected, oxygen can be given by a nasal catheter. Endotracheal intubation is performed when spontaneous respiration is inadequate and also to remove secretions. If assisted ventilation is required for more than 24 hours, tracheostomy is usually performed. Frequent monitoring of blood gases and blood pH is helpful. Respiratory physiotherapy minimises lung complications. • Forced diuresis: Barbiturate excretion can be enhanced by increasing the urinary flow by using diuretics like mannitol and furosemide. Mannitol, an osmotic diuretic, is given IV, initially in the dose of 100-120 ml of 25% solution. Subsequently, a sustained infusion of 5% mannitol alternately in a litre of normal saline and a litre of 5% dextrose is administered at the rate of 500 ml per hour for next 3 hours. The infusion is thereafter adjusted depending upon urine output and the state of hydration. Potassium chloride (10 to 20 mEq) is added to each litre according to serum chemistry, and alkalinisation with sodium bicarbonate may be conveniently carried out through the drip. An average urine volume of 10-12 litres in 24 hours (a flow rate of 8-10 ml per minute) is considered as satisfactory diuresis. The dose of mannitol should not be more than 20 g per hour. Diuresis is terminated on awakening. Alternatively, furosemide is used in the dose of 20 mg along with 500 ml of 1.2% sodium bicarbonate and one litre of 5% dextrose IV in the first hour. The urine flow should be above 5 ml per minute at the end of the hour. If it is not, furosemide should be given IV in large doses (upto 500 mg per 24 hours); it is essential to monitor the serum chemistry, central venous pressure and urine output. Forced diuresis is most useful in poisoning due to phenobarbitone, barbitone and allobarbitone, but not in poisoning due to other barbiturates which are more protein bound and are less ionised at the achievable urine pH. Shock, cardiac failure and renal impairment are absolute contraindications to forced diuresis. It must be noted that forced diuresis is a potentially dangerous procedure and should only be considered for those patients who have taken phenobarbitone in such doses that they are unlikely to survive with supportive therapy alone. It is not a substitute for the intensive supportive therapy as outlined above as most of the deaths are because of failure to maintain adequate tissue oxygenation. • Intravenous fluids: Fluids must be given in sufficient quantity as an adjuvant to forced
diuresis, in order to prevent dehydration and for maintaining the blood volume. Normal saline with dextrose is employed for this purpose. If hypotension does not respond to replacement by fluids (Chapter 32), vasopressor agents like dopamine may be used. Overloading of the circulation should be avoided. • Alkalinisation of the urine: This increases the excretion of long acting barbiturates, such as phenobarbitone; it has no significant effect on the renal elimination of short acting barbiturates. Sodium bicarbonate 3.75g (45 mEq) as 50ml of a 7.5% solution may be added to every litre of fluid intended for IV administration. The urinary pH should be maintained between 7.5 and 8.5. • Prophylactic antibiotics: These should not be used on routinely but may be necessary in those requiring tracheostomy or catheterisation. • Dialysis and hemoperfusion: Elimination of barbiturates from the body can be hastened by peritoneal dialysis, charcoal hemoperfusion and hemodialysis. All are more effective in removing long acting barbiturates than short acting ones. In general, peritoneal dialysis is more suitable than forced diuresis in patients who have severe cardiac and renal impairment. Hemodialysis is about forty times more effective than forced diuresis in promoting barbiturate elimination. Indications for hemodialysis are outlined in Table 8.6. Charcoal hemoperfusion is now considered superior to peritoneal dialysis and hemodialysis for the same purpose. Table 8.6 Indications for hemodialysis in acute barbiturate poisoning
Alcohols ETHANOL: Taken at bedtime, ethyl alcohol may act as a mild sedative. However, it cannot be recommended as a hypnotic as small doses may produce excitement; further the diuresis induced by it may interrupt sleep. In addition, there is the danger of drug dependence. CHLORAL HYDRATE AND TRICHLOROETHANOL: Oral chloral hydrate and its active metabolite trichloroethanol act as hypnotics. Their mechanism of action is similar to that of barbiturates. In small doses, it causes sedation. A slightly larger dose (0.5-1 g) at bed time results in sleep. Once a popular hypnotic, particularly for children, it is now rarely used. Adverse reactions: The common adverse reactions are allergic skin rash and epigastric pain, nausea and vomiting due to gastric irritation. Chloral hydrate produces an additive effect with ethyl alcohol. The drug should be avoided in the presence of marked hepatic, cardiac or renal damage, peptic ulcer, oesophagitis or gastritis. Preparations and dosage: (i) Chloral hydrate: Dose 0.5 to 2.0 g. In children, 30-50 mg/kg as syrup to a maximum of 1g Infants may be given 50 to 75 mg per dose. It is banned in India.
Aldehydes PARALDEHYDE: Paraldehyde is a colourless and inflammable liquid with a characteristic odour and an acidic taste. It has hypnotic and anticonvulsant properties. Oral paraldehyde induces sleep within 15 to 30 minutes, which lasts for 6 to 8 hours. Hangover is uncommon. Therapeutic doses of paraldehyde have no deleterious effects on the respiratory and the vasomotor centres. However, it crosses the placental barrier and may delay the breathing in the newborn. The drug is mainly metabolised in the liver. About 1128% is excreted unchanged through the lung. Adverse reactions: Paraldehyde is irritant to the mucosa and given IM, it may cause tissue necrosis. The drug decomposes to acetic acid and acetaldehyde in the presence of light and heat; and death may result from administration of old paraldehyde. Hence, paraldehyde stored for more than 6 months should not be used. It may produce excitement and delirium in the presence of pain. It is excreted in breath and imparts an unpleasant odour to it. Tolerance and addiction to paraldehyde are rare. Alcoholics exhibit cross tolerance to paraldehyde. It can dissolve plastics and hence should not be injected with a plastic syringe. Preparation and dosage: (i) Inj. paraldehyde 5 to 10 ml administered deep IM in the buttock. When the larger dose is being administered, it is divided between two sites to minimise local irritation. The drug does not support the growth of micro-organisms and may be used as such in an emergency. The dose in children is 0.2 ml/kg. (ii) It can also be given rectally in the dose of 15-30 ml diluted with three parts of a vegetable oil. Therapeutic uses: It is used IM as an anticonvulsant in status epilepticus, tetanus and eclampsia. Its IV use is not recommended as it may cause violent coughing and pulmonary edema. The drug should not be administered per rectum in patients with inflammatory lesions of the bowel. It should be avoided in the presence of severe impairment of hepatic and pulmonary function. Though it is now rarely used as a hypnotic, it is an useful and safe anticonvulsant during emergency.
Melatonin Recepter Agonist MELATONIN: The sleep regulating hormone, melatonin has been used to prevent jet lag and to induce post-travel sleep in air travellers. Jet lag causes daytime drowsiness, insomnia, frequent awakenings, anxiety and GI upset. Melatonin reduces these symptoms and aids the person sleeping post-travel. The doses prescribed are 5-8 mg on the evening of departure and 1-3 nights after arrival at the new destination. Given orally as 0.3–10 mg at night, it may help to improve onset, duration and quality of sleep in patients aged over 55 years with insomnia. It increases REM sleep. It is generally well tolerated. It may cause day time drowsiness, fatigue, dizziness, headache and irritability. As it has an inhibitory action on the pituitary LH and testicular aromatase enzymes, it should not be used in pregnancy. It is also avoided in nursing mothers because it inhibits prolactin release. Ramelteon and tasimelteon are the newer analogues of melatonin. Ramelteon is used in transient as well as chronic insomnia especially for sleep onset problems. It does not cause rebound insomnia. Adverse reactions are similar to those of melatonin. It has an active metabolite. It is contraindicated in liver failure and those taking inhibitors of CYP2C9 and CYP1A2.
Orexin Receptor Antagonists SUVOREXANT: This drug promotes sleep by preventing orexin neuropeptide from binding to their receptors. Normally signaling of orexin neuropeptide sustains wakefulness while orexin neuron remains silent during sleep. Given orally, it has long half life and high protein binding. Next day somnolence is common and cataplexy like symptoms can occur. It needs further evaluation.
Miscellaneous In addition to the drugs described above, which have a primary hypnotic action, various other drugs have hypnotic action as their prominent side effect. These are listed in Table 8.7. Table 8.7 Drugs with sedation as a prominent side effect
H1 receptor antagonists diphenhydramine and promethazine are sometimes preferred as hypnotics in pediatric practice. They can cause anti-cholinergic side effects. Tricyclide antidepressants like amitriptyline and doxepin are useful in patients with mental depression. They have negligible abuse potential (Chapter 14). Some drugs used as hypnotics (barbiturates, phenothiazine antihistaminics and chlorpromazine) may cause insomnia as an idiosyncratic reaction. Dexmedetomidine: See Chapter 7. Older sedative-hypnotics such as ethinamate, glutethimide, methyprylon, meprobamate and inorganic bromides are now rarely used. Methaqualone, once used as a hypnotic, is extensively misused as a drug of abuse. It has no therapeutic application.
Pharmacotherapy of Insomnia and Other Sleep Disorders Ability to go to sleep is a very personal attribute and people are either ‘good sleepers’ or ‘poor sleepers’. The latter, on the whole, take longer to fall asleep, sleep less, awaken more often have less REM and stage 4 NREM sleep, and have higher physiological arousal (heart rate, body temperature) than good sleepers. At the extreme end of the spectrum of poor sleepers is the person who sleeps through the whole night in several cat naps instead of sleeping continuously. Reduced total sleep time without any subjective or objective consequences suggests that the subject may be a physiologically short sleeper. As with all other things in life, most people learn to live with their own sleeping pattern. Length of total daily sleep in normal individuals varies between 4-10 hrs (average 7 hrs). The duration decreases in the elderly (average < 6 hrs.) Clinically, sleep disorders manifest as: (1) Insomnia (2) Hypersomnia (excessive daytime sleep) (3) Parasomnia (nightmare, sleep walking etc.); and (4) Miscellaneous e.g. (a) Circadian rhythm disorders (disturbed sleep schedule) and (b) Restless leg syndrome (RLS). Insomnia: As per classification of American Sleep-Disorders Association, insomnia is defined as “a repeated difficulty with sleep initiation, duration, consolidation or quality that occurs despite adequate time and opportunity for sleep and results in some form of day time impairment and lasting for at least one month”. In practice, insomnia is said to be present when an individual complains of inability to fall or stay asleep, of reduction in the total sleep period, of sleep disturbed by nightmares or of sleep that does not refresh. Insomnia can be (a) acute or (b) chronic. Chronic insomnia can be primary or secondary. Transient (no more than three nights) and short-term (Acute) insomnia (less than about 3 weeks) may occur in the absence of disease and is then due to stress caused by reactions to life changes, environmental factors, grief, job demands, travelling through time zones etc. Other than this, acute insomnia may be due to physical discomfort such as pain, dyspnoea, cough, fever, nocturnal myoclonus or psychiatric causes such as anxiety. It may occasionally be induced by drugs, such as adrenergic agonists and aminophylline. Chronic insomnia by definition lasts for at least 3 weeks and needs detailed evaluation. A two week sleep diary and an interview with the sleep-partner may be useful. It is a complex process and rarely benefits from hypnotics alone. Persistent insomnia is both a risk factor in and a precursor of mood disorders; it is associated with increased risk of automobile accidents, increased alcohol consumption and daytime sleepiness. Insomnia secondary to pain, fever, dyspnoea or myalgia, usually responds to appropriate treatment of the cause. If organic disease is responsible for insomnia (such as COPD, GERD, hyperthyroidism), it should be treated. Sudden fearful awakening with palpitation and sweating should arouse the suspicion of an associated major disorder such as IHD, hypoglycemia or severe anxiety state. The presence of dyspnoea in such a patient may indicate early heart failure which should be ruled out. About 30-60% of the patients with chronic insomnia have a recognisable psychiatric illness such as chronic anxiety, depression or psychosis. These should be looked for and treated. Difficulty in
staying asleep is a frequent complaint of depressed patients. This is associated with marked decrease in stage 4 of NREM sleep. It is not benefitted by hypnotics, and treatment of depression is of prime importance (Chapter 14). Rational therapy of insomnia depends upon the accurate diagnosis, and its precipitating and perpetuating factors. These can be easily identified in subjects with acute insomnia. Non-pharmacological therapy: In patients with ‘primary’ insomnia, a definite cause cannot be ascertained. In almost 30% of such cases, simple, non-pharmacological measures (Table 8.8) may help the subjects to establish good sleep habits. Such measures involve basic sleep hygiene consisting of (a) cognitive therapy given by specialist and (b) behavioural therapy which could be advised by the practitioners. Table 8.8 Non-pharmacological (sleep hygiene) measures for treating insomnia
Behavioural strategies include bed time restrictions, stimulus control therapy and relaxation and education about sleep hygiene. Bed time restriction involves reduction in time spent in bed closely to match actual time spent asleep. This simple procedure is useful for people who spent lot of time in bed but are not sleeping. Stimulus control involves instructions related to reassociate bed and bedroom with sleep and reestablish a regular sleep pattern. In chronic primary insomnia drugs are prescribed only as last resort because although they may provide a short time improvement, such benefits are not persistent. Drug therapy: It is empirical and gives only symptomatic relief. Pharmacologically, it is impossible to clearly separate sedative, antianxiety and hypnotic drugs. It would appear that with most drugs belonging to these classes, the desired effect can be produced by an appropriately adjusted dose. In therapeutic doses, most ‘hypnotic’ drugs have similar actions; they • Decrease the latent period of sleep. • Increase the total sleep time. • Decrease the awake time and the awakenings; and • Reduce the period of REM sleep. There are more similarities than differences among the various hypnotic drugs. The differences are, however, of practical importance. They are: (a) Rapidity of action. (b) Duration of action. (c) Differences in the degree of suppression of REM sleep; and (d) Adverse reactions, especially liability to produce hangover, respiratory depression,
dependence and impairment of cognitive function. Table 8.9 outlines the principles of drug therapy of insomnia. Transient insomnia can be helped by a short course for 2-3 nights. Table 8.9 Principles of drug therapy of insomnia
Although claims have been made for superiority of one drug over another based on differential actions on the sleep stages, clinical criteria of efficacy in alleviating a particular sleeping problem (difficulty in falling asleep, frequent awakenings, short duration of sleep or “unrefreshing” sleep) are more useful. BDZ receptor agonists because of their flexible pharmacokinetics, efficacy, tolerability and safety are to be preferred. If an elderly person has difficulty in falling asleep (sleep onset insomnia), a drug that is rapidly effective such as a short acting BDZ should be administered 20-30 minutes before the usual bedtime. For persons with difficulty in maintenance of sleep, longer acting drugs like lorazepam or temazepam may be preferred. Long half-life BDZ such as diazepam and flurazepam are preferred when day-time sedation is also desired. Newer Z agents are not superior to BDZ and offer only marginal advantage. Certain antidepressants with hypnotic activity such as amitriptyline, doxepin and trazodone can be substituted for BDZ. They have an advantage that they do not cause dependence and drug abuse, but may have other side effects such as anticholinergic effects. They are potent REM sleep suppressants (Chapter 14). Sudden withdrawal of REM suppressants causes a sharp increase in the REM sleep. Such a ‘rebound’ in an anxious patient is often associated with increased dreaming, nightmares, restlessness, insomnia and even fits. Some patients may, therefore, continue the hypnotic to avoid these reactions, thus leading to drug dependence. Eventually, a chronic state of intoxication ensues with tremor and confusion during day and insomnia at night. Unlike the drugs mentioned above, antipsychotic phenothiazines are poor hypnotics. It is, therefore, rational to prescribe a conventional hypnotic to treat uncorrected insomnia in a patient on antipsychotics. Whichever hypnotic is chosen, it should be used initially in a small dose and increased only if absolutely necessary. Once a good night’s sleep is obtained, attempts should be made to omit the drug for a few nights. The drug should be used to condition the patient to sleep better and should not be allowed to make a slave out of him. He should be explained that he can now sleep well without the drug and that an occasional night of imperfect sleep will
do not harm. Limitations of hypnotics: The major drawback of all hypnotics is the ‘hangover ’. All of them impair performance the next day. Even smaller amounts used as daytime sedative can impair social judgement and performance. Patients should be warned not only about the possible interactions of hypnotics with alcohol and other drugs, but also about the possibility of impaired performance such as car driving the next day. Rarely, these drugs can cause: (a) Severe hypersensitivity reactions including anaphylaxis and/or angioedema; and (b) Hazardous sleep-related activities such as sleep driving, telephoning while asleep, and preparing and eating food while asleep. It is unlikely that a potent hypnotic will not cause a hangover and will be free from dependence liability. However, the newer benzodiazepines with short t½ such as lorazepam and the non-benzodiazepines are relatively safer. Often, a patient taking hypnotics also takes other drugs simultaneously. Such combinations can be sometimes dangerous. Thus, MAO inhibitors taken to relieve mental depression may lead to slow inactivation of other depressant drugs, giving rise to serious toxicity (Chapter 14). They should be used cautiously in presence of drug abuse. Sedatives and hypnotics are indicated in various situations in children but they should not be employed as a substitute for other important measures such as discussion with the parents about their children’s behavioural problems and the importance of change in parental attitudes. Routine use of hypnotics for conditions like tics, nightmares, breathholding attacks, masturbation, aggressiveness, fears and school phobias, and head banging is not considered justifiable. The more rational approach in all such cases is to discuss the psychological problem with the parents and counselling. Hypersomnia: Its treatment is discussed in Chapter 14. Sleepwalking, night terrors and nightmares are designated as parasomnias. Sleepwalking and night terrors are the mild and severe manifestations of parasomnia, occurring about 1-3 hours after the onset of sleep, when stage 3 and 4 sleep is more prevalent. The disorders are idiopathic when they begin in childhood and benefit from (a) safety precautions and (b) the use of drugs like diazepam and flurazepam which suppress stages 3 and 4 of sleep. An organic cause such as a brain tumour must be ruled out in adult patients. Nightmares, commonly known as bad dreams, occur during REM sleep. They may occur in normal children and in children with fever. Other environmental causes should be looked for. The best way to handle them is to avoid terrifying stories, movies and frightening TV programmes. When they occur for the first time in adult life, depression is an important cause, and such depressed patients may be at increased risk for suicide. They are treated with REM suppressants such as tricyclic antidepressants (Chapter 14). The cause of restless leg syndrome (RLS) is not known. Clinically it responds to some extent to gabapentin and dopamine agonists, pramipexole and ropinirole (Chapter 15). Often, a clinician is tempted to prescribe a hypnotic readily under pressure from patients, relatives, nursing staff or himself. In the long run, this attitude may cause more harm to the patient than good. It is useful to remember that: • Detailed history must be taken to rule out causes such as severe anxiety or depression; and consumption of drugs (Table 8.10), excessive tea, coffee and colas.
Table 8.10 Drug induced insomnia
• Periodic loss of sleep in itself is not harmful and therefore, does not require treatment with drugs. Professionals like doctors, nurses and seamen who also lose sleep on and off remain resilient and healthy. • The clinician should not have a negative approach to what lies behind the presented symptom of sleeplessness, and hypnotics must not be prescribed readily ‘on demand’. • One should be critical in repeating the ‘sleeping pill’ prescription and try to avoid its continuation. • No hypnotic is safe, all can cause harm and none is effective in helping patients with problems underlying their insomnia. Special caution is necessary in patients with respiratory diseases, suicidal tendencies and history of drug dependence. The risk of falls and fractures increases especially in the elderly patients using hypnotics and other psychotherapeutic agents. • Moreover some of these drugs lose their effectiveness on repeated administration. • The difficulties of stopping the hypnotic in chronic users could be enormous. They should be tapered off slowly. • Effectiveness and safety are still the main considerations in choosing a hypnotic drug. • Chronic insomnia usually needs long term strategy for its management, mainly with cognitive, behavioural therapy and cautious use of drugs to a limited extent. • Hypnotics should not be used in patients with sleep apnoea. It is important to remember that some drugs can cause insomnia. (Table 8.10). Insomnia in the elderly: The commonest causes of insomnia in this population are agerelated changes in the sleep cycle and daytime napping. One should always look for causes like medical illnesses, loneliness, depression, anxiety, dementia and loss of family support. The patient should understand that improvement rather than total relief of insomnia is an achievable goal. Because of reduced body water, renal and hepatic function and increase in body fat, the pharmacological profile of hypnotics may be altered in the elderly, with prolongation of their half life (Chapter 1). The following points are important while treating insomnia in this age group: (a) Restriction of fluids in the evening. (b) Smaller than usual doses of short acting BDZ which are the hypnotics of choice (c) If BDZ are not tolerated, use zolpidem and zaleplon. (d) Avoid sedative antihistaminics as they are liable to cause delirium and antimuscarinic side effects; and (e) Think about possible psychiatric problems particularly depression and treat it if present. (f) Help to resolve socioeconomic problems. Often change in behaviour of the other family
members towards the patient can produce remarkable effects.
9
Drugs Effective in Seizure Disorders Seizure is a sudden and transient episodes associated with abnormal excessive electrical discharges from a group of CNS neurons. The rapid, rhythmic, and synchronous firing may occur due to epilepsy or due to a systemic disorder such as hypoglycemia or hypocalcemia, or an intracranial/severe systemic infection. Drugs can also induce seizures (See later). Epilepsy is a collective term for a group of chronic disorders characterised by recurrent seizures associated with disturbance of consciousness and/or a characteristic body movement (convulsion), and sometimes autonomic hyperactivity. In case of other seizures, there generally is no environmental or physiological trigger such as emotions, exercise, flashing lights or loud music immediately preceding the seizure. They are self limited and are called Non Epileptic Seizures (NES). Their main treatment comprises that of the cause. Drugs used in the treatment of seizure disorders can be divided into : I Anticonvulsants: drugs which are used to abolish seizures (antiseizure) and II Antiepileptics: drugs which are administered prophylactically to prevent seizures. Anti-seizure drugs can also be classified as: • Centrally acting e.g. General anaesthetics, Diazepam, Paraldehyde, Barbiturates (Chapters 7, 8) and specific antiepileptics. • Acting mainly on the spinal cord e.g. Mephenesin (Chapter 21). • Peripheral skeletal muscle relaxants e.g. d-Tubocurarine and Succinylcholine (Chapter 22).
Types of Epilepsy In practice, the drug treatment of epilepsy is guided by the nature of seizures. Table 9.1 shows the currently used classification of seizures, based on history, clinical findings, EEG recording and imaging studies. Thus, the seizures can be divided into two broad groups. (1) Generalised seizures; and (2) Focal seizures. Table 9.1 Types of seizures
Absence seizures (Petit mal): Typical, Atypical Tonic-clonic seizures (Grand mal) Tonic seizures Atonic seizures Myoclonic seizures Infantile spasms II Focal seizures III Focal, generalised or unclear epileptic spasms -->
The drugs used for all focal seizures are generally the same; whereas in the case of the generalised seizures, they depend upon the type of seizure in the different subgroups. I Generalised seizures. These are due to (a) mutations in Ca++ channels or (b) changes in the neuronal network. • Absence seizure (Petit mal): It consists of sudden impairment of consciousness without convulsive movement and without loss of postural control. The patient appears to go blank for less than 30 seconds and there may be accompanying fluttering of eyelids or small chewing movements. Awareness of the surroundings is regained quickly at the end of an attack, and the patient may not even know that one has occurred. The EEG is diagnostic with diffuse, bilaterally synchronous 3 per second wave and spike discharges. Absence seizures almost always begin in childhood. The child may outgrow these seizures. Typical idiopathic absence seizures respond well to drug treatment. In children with underlying brain disease, absence seizures may co-exist with other types of generalised seizures.
• Tonic-Clonic seizures are accompanied by a generalised abnormality in the EEG (grand mal or major epilepsy). They are characterised by sudden loss of consciousness without any warning (aura), followed by generalised tonic, and finally clonic convulsive movements lasting for 1-2 min. This is followed by a period of headache, drowsiness and sleep. The attack may be accompanied by tongue biting, frothing and incontinence. • Tonic seizure : As above but without clonic phase. • Atonic seizure (Drop attack) : Such a seizure consists of sudden loss of postural tone, without accompanying tonic or clonic movements. The head may drop for a few seconds or the child may drop to the floor without any apparent cause. Such seizures reflect diffuse brain damage and may be a manifestation of secondary generalised epilepsy. • Myoclonic seizure : This is a sudden, brief, repetitive muscle contraction involving a body part or the whole body. In the latter case, there is violent fall without loss of consciousness. Myoclonic seizures may occur by themselves or coexist with other seizures. The EEG changes are characteristic. II Focal Seizures originate in a localised area of the brain (usually medial temporal lobe or inferior frontal lobe) with a localised focus of EEG abnormality, and may or may not become generalised. The manifestations depend on the brain region or regions involved. The interictal EEG is either normal or shows epileptiform spikes but EEG during sezure is non-localising. In adults, the commonest form of epilepsy is focal epilepsy wherein the commonest associated lesion is in hippocampal sclerosis. • Focal seizures without cognitive impairment: Focal seizures can cause motor, sensory, autonomic or psychic symptoms without impairment of cognition. The patient is conscious and is aware of the event which lasts for a few seconds to a few minutes. (a) Motor: This begins as recurrent contractions of a particular muscle group, e.g. thumb, toe or angle of mouth and may spread to involve contiguous areas. The voluntary control is lost. These are the visible manifestations of epileptic focus in the motor cortex. (b) A variety of subjective symptoms may be experienced by the patient: sensory (numbness or parasthesiae limited to one part of the body); olfactory; gustatory; auditory; vertiginous; autonomic (flushing, sweating); or psychic such as deja-vu and dreamy state or unwarranted fear or anger. These seizures are associated with highly localised abnormal discharge which spreads widely into a limbic system. • Focal seizures with dyscognitive features: These consist of an aura (unusual smell, sudden intense emotional feelings), followed by impaired consciousness (for 30 sec to an hour). The ictal phase begins with repetitive motor activity such as lip smacking, swallowing or aimless wandering or unconscious performance of highly skilled activities such as car-driving (automatisms) or motionless stare. There is amnesia for the entire period of the seizure or a postictal aphasia. Only 70-80% of the seizures arise from the temporal lobe. Hence, although the terms psychomotor, temporal and limbic have been used synonymously, all such seizures are not the same. Focal slowing or sharp wave activity or both, on the EEG, provides confirmation. • Focal seizures leading to generalised seizures: These start as focal seizures and develop into one of the generalised seizures by spreading to cerebral hemispheres. Focal seizures arising from a focus in the frontal lobe tend to become generalised. Such a seizure may be followed by postictal neurological deficit (Todd’s paralysis).
Distingushing between primarily generalised seizure and focal changing to generalised seizure is important as the choice of drugs differs for both. III Epileptic spasms are seen in neonates and infants and may be due to immature CNS. There occurs flexion or extension of proximal muscles, including truncal muscles for brief period. The EEG shows diffuse, giant slow waves with a background of irregular, multifocal spikes and sharp waves. Status epilepticus (SE): Prolonged seizures (more than 5-10 minutes) or repetitive seizures (of any variety) without recovery of consciousness between attacks comprise SE. When tonic-clonic seizures go into SE, the situation can be life threatening and is a medical emergency (see later). The above description applies to a classification of the ‘seizures’. The classification of ‘epilepsies’, on the other hand, must also take into account seizure types, etiology, age of onset, genetic factors, EEG findings, associated neurologic defects, imaging results, response to treatment and prognosis. Defining a specific epilepsy syndrome may be more helpful in judging the prognosis and in selecting the drug treatment rather than taking into consideration only the seizure characteristics. In about 10% of persons with true seizures, multiple EEG studies reveal no abnormalities. Therefore, a normal EEG does not rule out a seizure disorder in a person with a diagnostic clinical picture. Neurophysiology: John Hughlings Jackson postulated about a century ago that epileptic seizures were caused by “occasional, sudden, excessive, rapid, local discharges from the gray matter ”. Modern electrophysiology has confirmed this. Depending on the neurotransmitter released, the brain neurons are grouped as excitatory and inhibitory. The primary inhibitory transmitter in the brain is GABA whereas the excitatory transmitter is mostly the amino acid glutamate. GABA acts on the GABA receptors, and glutamate acts through the N-methyl-D aspartate (NMDA) and non-NMDA receptors (Chapter 5). Activation of these receptors modifies various voltage-gated Na+, K+, Ca++ and Cl− ion channels and excites or inhibits the neuron. An action potential is an all or none phenomenon; once the threshold is reached, the action potential fires. This cellular event is associated with influx of Na+ into and efflux of K+ out of the neuronal cell. Normally, the neurons fire action potentials singly or in short runs; and the excitability is kept under control by powerful inhibitory influences. Pathophysiology: It involves two, importantly related events: (i) Hyper-excitability: is the abnormal responsiveness of the neurons to an excitatory input, leading to multiple discharges. Chronic hyperexcitability can result from a number of mechanisms. (ii) Hypersynchrony refers to the recruitment of a large number of nearby neurons to an abnormal firing mode. Thus, epilepsy is a network phenomenon involving the participation of many neurons firing simultaneously. The characteristic patho-physiologic event in a seizure is believed to be paroxysmal depolarization shift (PDS) of neuronal membrane potential and associated burst discharge. PDS are represented by interictal (between seizures) spikes i.e. sharp waveforms in the EEG of epileptic patients and help to localise epileptic focus. Excitatory neurotransmitters are probably involved in the initiation and spread of the seizure discharge, and the inhibitory transmitter GABA is responsible for its termination.
The normal brain contains billions of neurons which ‘fire’ asynchronously (i.e. at different times). Inhibitory feedback loops in the normal brain regulate the frequency of firing of individual neurons and prevent synchronisation. When such inhibitory feedback is defective, a large number of cells in a given area of the brain fire at the same time (i.e. they synchronize) and produce a self-regenerating electrical impulse. Such an area constitutes an epileptic focus. Such foci may be cortical or subcortical. They may discharge intermittently, only to be shown up on the gross surface EEG, but may not cause symptoms. Factors which may trigger the abnormal focus or permit the spread of activity to the normal brain include hyperventilation, alkalosis, hypoglycemia, overhydration, hypocalcemia, overeating, and emotional stress. Spread of the abnormal electrical activity to the normal brain tissue causes a generalised seizure. The clinical type of seizure is independent of the brain pathology but is determined by the site of the abnormal focus. The response to treatment correlates best with the site of the focus. Experimentally, drugs with a potential anti-epileptic activity are assessed against seizures induced in mice by (1) injecting medullary stimulants or (2) by applying a maximal electrical shock. The chemical commonly used to produce seizures is pentylenetetrazol. Drugs which antagonize leptazol seizures are generally useful in petit mal. Drugs likely to be effective in grand mal epilepsy usually confer protection against electrically induced seizures. A model for human focal epilepsy is that produced in animals by “Kindling”. This consists of delivery of brief localised trains of electrical stimuli to an area of the brain, repeatedly, at about 24 hour intervals. After a time, generalised motor seizures are regularly elicited during such electrical stimulation. Eventually, spontaneous, recurring seizures start occurring; such ‘kindled’ animals are very sensitive to a variety of chemical and sensory convulsive stimuli. Models of status epilepticus have also been developed using chemical agents like kainic acid or pilocarpine, or sustained electrical stimulation. Destruction of hippocampal neurons has been reported in these models, as in humans suffering from either epilepsy following febrile convulsions or severe limbic seizures who exhibit hippocampal sclerosis.
Anti-epileptic Drugs (AED) Currently used AED are basically anti-seizure agents but whether they prevent epileptogenesis is uncertain. AED can be classified according to their mechanism of action Table 9.2. They act by preventing the generation and/or spread of the seizure. Drugs for focal-seizure inhibit mainly the voltage-activated Na+ channels, while anti-petit mal seizure drugs inhibit voltage-activated Ca++ channels. The agents modulating GABA transmission are effective against partial and tonic-clonic seizures. Table 9.2 AED classified according to their mechanism of action
GABA: (a) Acting through GABA-related receptors: Barbiturates, Benzodiazepines. (b) By releasing GABA from neuronal endings: Gabapentin. (c) By inhibiting GABA transaminase: Sodium valproate, Vigabatrin. (d) By inhibiting neuronal reuptake of GABA: Tiagabine. • Decrease release of excitatory neurotransmitter glutamate: Lamotrigine • Miscellaneous: Levetiracetam, Acetazolamide.
-->
Note: A given drug may act through multiple mechanisms.
I Hydantoin derivatives: DIPHENYLHYDANTOIN (Phenytoin sodium): This drug, introduced by Merritt and Putnam in 1938, is still an important drug in the treatment of epilepsy with the exception of petit mal and myoclonic seizures. It is structurally related to barbiturates (Fig 9.1).
FIG. 9.1 Diphenylhydantoin
Mechanism of action: The drug slows the recovery of voltage-dependent Na+ channels, resulting in decreased permeability to sodium ions (stabilising effect) of all neuronal membranes including the peripheral nerves as well as other non-excitable and excitable membranes. Reduction in the neuronal Na+ concentration causes: (a) Reduction in paroxysmal depolarization shift (PDS). (b) Decreases post-tetanic potentiation (PTP) which is responsible for the spread of the seizure activity. The PTP is an enhancement of synaptic transmission following repeated tetanic, high frequency stimulation of the presynaptic fibres; and (c) Inhibits the spread of seizure discharges in the brain and shortens the duration of afterdischarge. In patients in whom it is effective, the generalised abnormality in EEG disappears but the abnormal focal electrical activity persists. High concentrations of phenytoin also augment brain level of GABA, 5-HT and homovanillic acid. This may contribute to its toxicity. Pharmacological actions: CNS actions: It exerts a selective anti-epileptic action (see above) without causing drowsiness. The onset of action is slow even on IV injection but the action persists for a considerable time after cessation of therapy. Cardiovascular actions: It has a cell membrane stabilising effect on the myocardium (Chapter 28). Absorption, fate and excretion: Phenytoin is slowly and variably absorbed from the gut, with plasma peak level at 3-12 hours after ingestion. In plasma, it is 70-95% albumin bound and is metabolised mainly by parahydroxylation in the liver. The drug is concentrated in bile and is reabsorbed from the intestine as parahydroxyphenol. In individuals deficient in the liver parahydroxylase, toxicity can occur even with small doses. At plasma concentration below 10 mcg/ml, (sub-therapeutic), elimination is exponential and the plasma t½ is about 24 hours. As it approaches the therapeutic concentration of 20 mcg/ml, its metabolism becomes saturated; it then exhibits dose dependent (Zero order) elimination and the plasma concentration rises disproportionately to the dose increment; the elimination t½ increases. Hence, the dose increments must be smaller with increasing dosage. About 94% of a single dose is excreted in urine within 48 hours. On chronic medication, the drug disappears from the plasma within 3 days after stopping the treatment because it induces hepatic microsomal enzymes. Adverse reactions: Within the therapeutic range of plasma phenytoin level (10-20 mcg/ml) the drug is usually well tolerated. With higher plasma levels(> 20 mcg/ml), the half-life increases to 35 hours or longer. When this happens, even a slight increase in the dose can cause increased toxicity. However, the toxicity disappears equally quickly on reducing the daily dose. Death due to phenytoin is rare.
• Intolerance: Urticarial, scarlatiniform and measles-like skin rashes may occur. They may be accompanied by lymphadenopathy, hepatomegaly and jaundice. • Central nervous system: Mild toxicity consists of drowsiness, fatigue, headache and confusion. Larger doses can cause a vestibulo-cerebellar syndrome characterised by vertigo, ataxia, nystagmus and dysarthria. Ocular pain with blurring of vision, delusions, hallucinations and other psychotic episodes are sometimes encountered. Rarely, cognitive impairment, behavioural changes may occur. Peripheral neuropathy has been reported particularly in old people. These effects are dose related and are reversible. Ataxia, however, may occasionally persist for long periods. • Gastrointestinal tract: Alkalinity of the drug causes nausea and vomiting which can be prevented by taking the daily dose in divided portions, after meals, with plenty of water. • Face and gums: Hyperplasia and hypertrophy of the gums with edema and bleeding occur in approximately 15% of patients. It is not related to the dose of phenytoin. Scrupulous dental hygiene but not vitamin C can prevent the gingival hyperplasia. In most cases, the gums return to normal within a year after discontinuation of the drug. Long term phenytoin therapy sometimes causes hypertrophy of the facial subcutaneous tissue, hypertrichosis and coarsening of facial features (Phenytoin facies). • Endocrine effects: Hypertrichosis is seen especially in children. Less commonly, hyperglycemia and osteomalacia have been observed. • Enzyme induction: Phenytoin is a potent inducer of hepatic microsomal enzymes. It accelerates its own metabolism and that of other drugs such as vitamin D, folate, glucocorticoids, gonadal steroids, thyroxine, OC pills, doxycycline, warfarin and carbamazepine thereby reducing their therapeutic efficacy. Thus, it may cause osteomalacia and megaloblastic anemia. • Teratogenicity: Hydantoins, administered during the first trimester of pregnancy, can cause fetal hydantoin syndrome. It comprises midline hypoplasia, ptosis, wide mouth, inner epicanthic folds, short neck, mild webbing of the neck, short phalanges and hypoplastic nails. Some children may develop congenital heart defects, microcephaly and mental subnormality. • Miscellaneous: Rarely, blood dyscrasias including aplastic anemia and agranulocytosis, may occur. Appearance of LE cells and methaemoglobinaemia can also occur. Rapid IV administration can cause cardiovascular collapse and/or severe CNS depression. Drug interactions: See Table 9.3. Table 9.3 Drug interactions of phenytoin
Preparations and dosage: Phenytoin sodium tablets 50 and 100 mg and IV preparation 50 mg/ml. IV dose should not exceed 25-50 mg per minute. It should not be given IM because of its poor absorption from this site.
Bioavailability of phenytoin may differ with different brands and the patients should be advised to use the same brand all the time. Fosphenytoin sodium, a prodrug of phenytoin, can be given IV more rapidly; it causes fewer reactions; it can also be given IM. Therapeutic uses: • Grand mal epilepsy: It abolishes grand mal seizures in nearly 60% of the patients and reduces their frequency in another 15 to 20%. • Focal seizures: Phenytoin is preferred to phenobarbitone in this type of seizures. The drug often controls but does not completely abolish the seizure activity. It is also occasionally useful in infantile spasm. • Status epilepticus (see later). • Cardiac arrhythmias: Chapter 28. • Miscellaneous: It has been used with some success in certain types of neuralgia e.g. trigeminal neuralgia, in diabetic neuropathy with dysasthesias (Chapter 11) and in chorea. II Barbiturates: PHENOBARBITONE: Phenobarbitone is discussed in Chapter 8. It is concentrated in the epileptic focus. Its antiepileptic activity is similar to that of diphenylhydantoin but in addition it raises the seizure threshold. It has a different mechanism of action (Chapter 8). Hence, it can be combined with phenytoin in the treatment of resistant grand mal, focal cortical seizures and hypsarrhythmia. The daily dose varies from 60 to 180 mg in divided doses or as a single dose at night. The main advantages of phenobarbitone are: • It is well tolerated by most patients. • Its half life is long, permitting single-dose-a-day therapy, with better compliance. • Therapeutic drug monitoring is usually not necessary. • It is cost-effective. Phenobarbitone is of limited value in temporal lobe epilepsy and may aggravate petit mal seizures. It may produce excitement or hyperactivity in children and in old people. Because of its long half life, it takes 2-3 weeks to reach a steady therapeutic plasma level. Adverse reactions: Drowsiness, lethargy and depression are common. However, they tend to abate after a few weeks of treatment Nystagmus and ataxia are seen with larger doses. Other long term adverse effects include memory loss, irritability and hematologic changes. Sometimes, connective tissue abnormalities such as frozen shoulder and Dupuytren’s contractures may occur. It is also teratogenic. Convulsions following phenobarbitone withdrawal are difficult to control with phenytoin. Hence, while switching over from phenobarbitone to phenytoin, the dose of phenobarbitone is tapered off gradually and that of phenytoin increased slowly, till the latter drug fully takes over. Therapeutic uses: • Grand mal epilepsy (See later). • Status epilepticus Mephobarbitone and primidone are prodrugs and their antiepileptic effect is due to their active metabolite phenobarbitone. Primidone is also used in the treatment of essential tremor resistant to propranolol.
III Iminostilbenes: CARBAMAZEPINE is a tricyclic (iminostilbene) compound (Fig 9.2) with structural resemblance to the antidepressant, imipramine.
FIG. 9.2 Carbamazepine
Pharmacological actions: Its mechanism of action is similar to that of phenytoin but it is claimed to cause less cognitive impairment. Carbamazepine is also useful in the treatment of trigeminal neuralgia, a condition characterised by paroxysms of intense stabbing pain within the distribution of trigeminal nerve, without sensory loss or other evidence of organic disease of the nerve. This condition, because of its paroxysmal nature, tendency to relapse and partial response to phenytoin, has been regarded as a type of epilepsy. Carbamazepine is remarkably specific for trigeminal neuralgia and probably for the related syndrome of glossopharyngeal neuralgia. It is also effective in the deafferentiation pain in diabetic neuropathy, cancer and multiple sclerosis. Absorption, fate and excretion: Oral absorption is slow; overall bioavailability however, approaches 90%. It is metabolised by the liver (98%). Children metabolise the drug faster than adults. The plasma half-life, initially 24-36 hours, falls to around 12 hours on chronic dosing because of autoinduction. It is a potent hepatic microsomal enzyme inducer and accelerates its own metabolism as well as that of many other lipid soluble drugs. Valproate inhibits its metabolism. Adverse reactions: The drug is usually well tolerated. However, it can cause nausea, anorexia, giddiness, vomiting, ataxia, mental confusion and skin rash. Diplopia and blurred vision may occur, making driving dangerous. The rare but serious toxic effects reported include obstructive jaundice, peripheral neuritis, agranulocytosis, thrombocytopenia and aplastic anaemia. Long term use of carbamazepine may cause fluid retention and insidious development of sluggishness, both mental and physical. The loss of physical and mental drive can be so gradual that the patient and the family may wrongly attribute it to the normal process of ageing. It is a minor teratogen. Drug interactions: See Table 9.4. Table 9.4 Drug interactions of carbamazepine
Preparations and dosage: It is available as 100, 200 and 400 mg tablets and as 400 mg
controlled release (CR) tablets. The initial dose is 100 mg bid, gradually increased to 6001200 mg per day in divided doses in temporal lobe epilepsy and to 400-800 mg in neuralgias. Therapeutic uses: • Grand mal and focal seizures where it is used singly or in combination. • Trigeminal neuralgia: see above. • Deafferentiation pain in various disorders (Chapters 10, 11). • Diabetes insipidus of pituitary origin, where it stimulates ADH release (Chapter 39). • As an alternative to lithium carbonate in the management of manic-depressive psychosis and as an adjunct in the treatment of drug resistant schizophrenia (Chapter 13). • In the treatment of alcohol withdrawal syndrome (Chapter 6). Oxcarbazepine, a prodrug, has similar activity and therapeutic uses as carbamazepine but it is more expensive. Its active metabolite is a s- isomer, eslicarbazepine, which is also available as prodrug, eslicarbazepine acetate. However, it is used only for focal seizures as an add on therapy to be given as single dose. Both, oxcarbazepine and eslicarbazepine are selective inducers of cytochrome isoenzyme that metabolises estrogens. IV Succinimides: ETHOSUXIMIDE: This is the most frequently used succinimide (Fig. 9.3).
FIG. 9.3 Ethosuximide
Mechanism of action: The drug reduces the low threshold calcium currents (T currents) in the thalamic neurons which are responsible for the generation of the absence seizures. Pharmacological actions: It is effective only in petit mal epilepsy. It does not induce liver enzymes and monitoring of blood levels is not required. Absorption, fate and excretion: It is completely absorbed from the GI tract and is present in the plasma mostly in the free form. About 20% is excreted unchanged in the urine and the rest is metabolised by the liver. Adverse reactions: These comprise anorexia, nausea, vomiting, drowsiness, dizziness and occasionally parkinsonism. Skin rashes, blood dyscrasias, SLE and psychic disturbances may rarely occur. It can unmask grand mal epilepsy. Preparations and dosage: It is available as 250 mg capsules and as a syrup (250 mg per 5 ml). The usual starting dose is 250 mg per day in children, increased by 250 mg at weekly intervals till the seizures are controlled. A daily dose of 750-1000 mg (generally given as a single dose) is not exceeded. Therapeutic uses: It is the drug of choice in petit mal epilepsy. Additional drug(s) are needed to control associated or unmasked grand mal epilepsy. V Valproic acid: SODIUM VALPROATE: Chemically sodium valproate is sodium dipropyl acetate (Fig.
9.4).
FIG. 9.4 Sodium valproate
Mechanism of action: This is a broad spectrum antiepileptic which probably acts at multiple sites. Its actions are similar to that of both ethosuximide and phenytoin. Thus, it: (i) Inhibits the T type Ca++ current (ii) Delays the recovery of the inactivated Na+ channels (iii) Inhibits GABA transaminase, thus increasing the GABA activity (Fig. 9.5).
FIG. 9.5 Antiepileptic drugs acting on GABA metabolism: (1) Vigabatrin inhibits GABA – T (2) Tiagabine blocks GABA reuptake (3) Valproate inhibits GABA - T and stimulate GAD (4) Barbiturates activate GABA receptors (5) Benzodiazepines facilitate GABA action
Pharmacological actions: In petit mal, it is as effective as ethosuximide. However, it is more liable to cause GI adverse effects than ethosuximide. In patients with both petit mal and grand mal seizures, sodium valproate may be the drug of choice as it is able to control both types of seizures. It is also used in myoclonic seizures and with variable success in akinetic seizures and infantile spasms. It is not effective in cortical focal epilepsy nor in temporal lobe epilepsy. It is claimed that it does not alter the patient’s behaviour, alertness and cognitive function, and therefore does not impair learning ability and performance. It is well tolerated by the elderly and is the drug of choice in them. Absorption, fate and excretion: Sodium valproate is rapidly and almost completely absorbed after oral administration. Eighty to 95% of plasma valproate is protein bound.
More than 90% is metabolised in the liver. Adverse reactions: The main adverse reactions are nausea and vomiting. It increases appetite and may cause weight gain. Dose related hair loss may occur. Hypoalbuminemia is common and hepatotoxicity is its major drawback. Hence it is advisable to do baseline hepatic function studies before starting sodium valproate. The other infrequent adverse effects are sedation, ataxia, incoordination, thrombocytopenia and pancreatitis. Sodium valproate inhibits platelet aggregation, although this is unlikely to be of clinical significance unless the patient is also on other drugs that affect coagulation. It is teratogenic, and spina bifida is associated with its use during pregnancy. Sodium valproate does not induce hepatic microsomal enzymes. In fact it is an inhibitor of these enzymes. Thus it inhibits its own metabolism and that of lamotrigine, phenobarbitone, phenytoin and carbamazepine, and may enhance their toxicity. Preparations and dosage: Sodium valproate 100 and 200 mg tablets; syrup 200 mg per 5 ml Therapy is initiated 10 mg/kg/day in two divided doses. This is increased by 5 to 10 mg/kg/day at weekly intervals upto 20-30 mg/kg/day. Doses as high as 60 mg/kg/day have been used. Therapeutic uses: • Petit mal seizures. • Combined grand mal and petit mal seizures. • Myoclonic epilepsy. • Focal epilepsy. • Manic depressive psychosis (Chapter 13) • As prophylactic in febrile convulsion VI GABA transaminase inhibitor: VIGABATRIN (gamma-vinyl GABA): This drug acts as an irreversible GABA transaminase inhibitor (Fig. 9.5), thereby leading to increased concentration of brain GABA. It is not metabolised but is excreted unchanged in the urine. Its use is restricted to epilepsy not satisfactorily controlled by other drugs. The adverse effects are weight gain, drowsiness, depression, memory disturbances, diplopia and constriction of visual fields. Hence, visual field testing is mandatory. Attacks of acute behavioral changes in some patients is a major disadvantage and the drug should be avoided in patients with mental illness. Vigabatrin is particularly useful in infantile spasm. It worsens absence seizures and myoclonic seizures. VII GABA re-uptake inhibitor: TIAGABINE: This rationally designed nipecotic acid analogue selectively inhibits the neuronal and astrocytic re-uptake of GABA, and thus increases in synaptic GABA concentration. Orally, it is almost completely absorbed. Food delays its absorption. The drug is extensively metabolised in the liver and has a half-life of 7 to 9 hours. Common adverse reactions include headache, dizziness, somnolence and tremor. The drug is mainly used as an “add on” drug for the treatment of partial seizures, with or without secondary generalisation in adolescents and adults. It is usually administered in a dose of 4 to 12 mg tid. VIII GABA agonists: GABAPENTIN: This is a GABA molecule bound to a lipophilic cyclohexane ring.
However, it does not mimic GABA. Its precise mechanism of action is not known. It binds to α2δ subunit of voltage gated calcium channels and probably increases the release of GABA. The drug is well absorbed orally; is not protein bound; does not get metabolised and is excreted unchanged in the urine. It has a plasma half life of 5-9 hours. Concurrent use of gabapentin does not affect the blood levels of other antiepileptic drugs. It usually causes mild to moderate somnolence, dizziness, ataxia, fatigue, edema, blurred vision, vertigo which can interfere with activities like driving. Tolerance develops to these effects within 2 weeks. Gabapentin is used in combination with other drugs, in partial seizures with or without secondary generalisation, resistant to other drug therapy. It is administered in the dose of 100 mg tid, increased gradually to 900 to 1200 mg per day. Gabapentin-encarbil is an extended release formulation. Pregabalin: This drug, related chemically to gabapentin, has similar mechanism of action and uses. It is excreted unchanged in the urine. The adverse effects are similar to those of gabapentin. It does not interact with the other antiepileptic drugs. It is used as an add-on drug in the treatment of focal epilepsy. Both the drugs have also been used in the treatment of migraine, deafferentiation pain such as post-herpetic neuralgia and diabetic neuropathy, restless leg syndrome and in bipolar disorders with variable benefits. They are expensive. IX Benzodiazepines: Benzodiazepines, in general, are anti-convulsants but not antiepileptics. They increase the effectiveness of the inhibitory neuro-transmitter GABA. (Chapter 8 and 14). Benzodiazepines are not a good choice for the long term treatment of epilepsy because: (a) Tolerance can develop and seizures may recur within few months. (b) Drowsiness and ataxia can occur; and (c) Status epilepticus on abrupt cessation of these drugs is difficult to treat. DIAZEPAM: When given IV or rectally, it can be life saving in status epilepticus and is the treatment of choice in this condition as well as in non-epileptic seizures. Because of its high lipid solubility and good penetration into the brain, it has a very rapid onset of action. Midazolam and lorazepam are used as anticonvulsants in emergency such as status epilepticus. CLONAZEPAM: This drug is useful in the treatment of petit mal, myoclonic seizures and infantile spasms. In petit mal, it is used in patients who do not respond to ethosuximide and sodium valproate and not as the primary drug. Tolerance develops and breakthrough seizures may occur after 1 to 2 months of therapy. It has also been used as an adjunct to phenobarbitone and phenytoin in the treatment of resistant grand mal. The serious adverse effects are mainly neurological and comprise drowsiness, ataxia, personality changes, slurred speech, tremor, vertigo and confusion. They are dose-related. Skin eruption, anemia, leucopenia and thrombocytopenia have been reported. It is liable to cause respiratory depression and to increase the salivary and bronchial secretions. The other ADR involve the cardiovascular, the GI and the genitourinary systems. Tolerance is known to occur and psychic and physical dependence have been reported. Clonazepam is available as 0.5 mg tablets and as 1 mg/ml ampoules for IV injection. Therapy is initiated in adults and in children over 10 years of age with oral
administration of 0.5 mg twice a day; the dose is gradually increased to maximum of 4-8 mg per day. CLOBAZAM: It has actions and disadvantages similar to those of diazepam but has less sedative effects. It is used as an adjunct in treatment of epilepsy. Clobazam is claimed to be useful in short courses in patients in whom seizures occur in clusters. X Miscellaneous: Lamotrigine, topiramate, levetiracetam, zonisamide and lacosamide are considered as broad spectrum as they are useful in both, focal and generalised seizures. They are mainly used as add-on drugs. LAMOTRIGINE: This phenyl-triazine compound acts like phenytoin on Na+ channels. It also inhibits the release of the excitatory amino acid glutamate. Given orally, it is almost completely absorbed and is eliminated mainly by hepatic metabolism. Its t½ is 24 hours. The drug can cause skin rash, nausea, vomiting, diplopia, ataxia, Stevens-Johnson syndrome and DIC. It is used as an add-on drug in patients with resistant focal and secondarily generalised seizures. Due to its membrane stabilising action, it is used in the treatment of deafferentiation pain. It is used in the dose of 50 mg bid, increased gradually to 200 mg per day. Valproic acid inhibits its metabolism and hence in patients taking valproate, the dose of lamotrigine is reduced to 25 mg on alternate days. TOPIRAMATE: This drug acts similarly as phenytoin. In addition, it also has some GABA receptor enhancing and weak glutamate receptor inhibiting activity. Topiramate is used as monotherapy for focal and primary generalized seizures and also as antiobesity drug. The adverse effects are mainly neuropsychological viz. dizziness, drowsiness, psychomotor slowing, difficulty in concentrating, confusion, ataxia, depression, acute myopia, glaucoma and raised intracranial tension. It is claimed to be useful in chronic alcohol addicts as an anti-craving drug and also as antiobesity drug. LEVETIRACETAM: This drug, a pyrolidone derivative, is structurally related to the older nootropic drug piracetam. It is effective in the kindling animal model but has no effect on the electroshock or pentylenetetrazole induced seizures. It binds to synaptic vesicular protein (SV2A) but how this modifies release of GABA and glutamate is not clear. Given orally, it is absorbed rapidly and completely. It is not bound to plasma proteins. It is excreted (70-80%) unchanged in the urine. Its plasma t½ is 6-8 hours. Adverse reactions are mild and include drowsiness, asthenia and dizziness. Rare, important side effects are emotional lability, agitation and nervousness. Hence, it is wise to monitor carefully patients prone to psychiatric disturbances. It is used as a add-on drug to treat refractory myoclonic, or focal seizures and uncontrolled generalised tonic- clonic seizures. Zonisamide: This sulfonamide derivative inhibits T type Ca++ currents and like phenytoin delays the recovery of the inactivated Na+ channels. Given orally, it has a long t ½ (1-3 days) and is mainly excreted in the urine. The ADR reactions are somnolence, ataxia, anorexia, and fatigue; rarely, renal calculi develop. Lacosamide: This new AED, related to aminoacid serine, acts by enhancing slow inactivation of Na+ channels. It is given orally and IV. It is metabolised in the liver and 30% is excreted unchanged. The adverse effects include dose dependent dizziness, headache, fatigue, ataxia and vomiting. It may also cause sedation, tremor and diplopia. It may cause prolongation of PR interval. Its efficacy for partial onset seizures is similar to other drugs
and is used as add-on drug in resistant cases. Rufinamide, a triazole derivative, has been shown to reduce tonic-atonic seizure frequency by enhancing slow inactivation of voltage gated Na+ channels. It is used as an adjunct in treatment of seizures in children with Lennox-Gastaut syndrome. Ezogabine: This potassium channel facilitator reduces the degree of depolarisation needed to open the K+ channels in the neurons. As a result, K+ channels open faster and stay open for a longer time. This slows the repetitive firing of impulses by the neurons. It may be used as adjunctive therapy for focal seizures. The adverse effects include dizziness, somnolence, fatigue, confusion, vertigo, tremors, disturbances in gait, attention and memory; blurred vision, dysarthria and euphoria. Weight gain, psychotic symptoms, QTc interval prolongation and suicidal tendencies have also been reported. When administered concomittantly with phenytoin/carbamazepine its serum concentration decreases. Perampanel: This is the first non-competitive antagonist of AMPA receptors on postsynaptic neurons and inhibits AMPA dependent calcium entry into the neurons. It is metabolized by CYP3A4 followed by glucuronidation. The adverse effects include dizziness, somnolence, vertigo, ataxia, aggression, euphoria, blurred vision, weight gain, fatigue and dysarthria. Parampenal can cause serious, life threatening psychiatric, behavioral adverse effects including homicidal ideation. It is not recommended in patients with severe renal or hepatic impairment. This is used as adjunct therapy for focal seizures. It is not recommended for children less than 12 years. Acetazolamide: The anti-epileptic activity of this diuretic is correlated with its carbonic anhydrase inhibitory activity (Chapter 39). As an adjunct, it is occasionally effective in resistant petit mal and grand mal epilepsies.
General Principles of Management of Epilepsy Seizures are a symptom of an underlying disorder: genetic, traumatic, metabolic, inflammatory, drug induced or due to drug withdrawal. Although treatment of the cause can cure seizures, this may not be possible for all. Epilepsy should be considered as an illness and not a social stigma. As long as an epileptic is willing to be careful and to take the treatment continuously under supervision, he should be given a fair chance in finding a job for himself. Occupations involving driving of vehicles and working with machines, near a water front or at heights are not suitable for epileptics. Likewise, swimming is a forbidden sport. Within these limits, a well controlled epileptic may be a good employee if he knows his limitations. About 65-70% of epileptics are found to have an underlying brain lesion (secondary epilepsy), and idiopathic epilepsy accounts for the remaining cases. Secondary epilepsy is not heritable. When a person with idiopathic epilepsy marries a non-epileptic, the chance of transmission to an offspring is about 2%. When two persons with idiopathic epilepsy marry, the chance is about 60-70%. The rational management of epilepsy needs an accurate evaluation of the epileptic syndrome. Currently, there is no drug cure for epilepsy. The current aim is to achieve prolonged seizurefree periods with lowest risk of drug toxicity, permitting the patient to lead as full a life as his/her capabilities permit. Complete control of symptoms may not always be possible. Antiepileptic drugs are continued even if they prevent the seizures only partly. Drugs, used regularly, abolish seizures completely in 60-80% of the patients and reduce their frequency in another 10-20%. This is usually achieved without producing intolerable adverse effects. Patients can be restored to a full working life, making social rehabilitation possible. Occasionally, these drugs will suppress the abnormal electrical activity and after therapy for years, may produce complete clinical cure in a few cases. This is especially true in patients with petit mal epilepsy. To achieve best results with drugs, the following must be carefully observed: • Proper initial evaluation is necessary to rule out other neurological events (e.g. syncope) that might be mistaken for seizures; to ascertain if a single seizure was precipitated by a reversible abnormality (e.g., hypoglycemia); and to determine if a structural (e.g., brain tumour) or a metabolic (e.g., hypoxia or hypocalcemia due to primary hypoparathyroidism) cause underlies chronic seizure disorder. • An AED is advised if the patient has had two or more seizures. A patient who gives history of a single seizure is treated with medication if there are one or more risk factors for recurrence of seizures such as abnormal neurological finding, the presence of structural lesions, abnormal EEG, partial seizures or a family history of seizures; otherwise the patient is only observed and not prescribed drugs. The initial choice of the drug depends upon the type of epileptic seizure, and not on whether it is idiopathic or secondary. A seizure diary should be maintained. • The drug therapy should be simple. Commonly used drugs are listed in Table 9.5. Treatment should be started with the effective, least toxic and convenient to take single antiepileptic drug appropriate for the particular seizure.
Table 9.5 Drugs commonly employed in the treatment of epilepsy
Plasma valproate levels are not a useful index of therapeutic efficacy. *
In adults
**
Hepatic microsomal enzyme inducers.
†
Hepatic microsomal enzyme inhibitor.
• Drug therapy should always be started with a single drug (monotherapy) in small dose targeting lower portion of therapeutic range, increasing it gradually till the maximum benefit is obtained without an increase in adverse effects. Repeated EEG evaluation and determination of the plasma level of the drug (phenytoin and carbamazepine) may help in difficult cases. • Mild toxicity can be managed by reducing the dose by 25-30% and waiting for tolerance to develop. Most antiepileptics may give rise to skin rash during the first few weeks of therapy. The drug need not be stopped for that reason. • The patient, or in case of children, the parents, should be counselled regarding the duration of treatment and the need for compliance, to keep a seizure record and to attend the follow-up clinic regularly. • Changes in therapy should be made after careful weighing of pros and cons and not every time a new drug appears in the market. Adequate trial (for 2-3 months) should be given to an antiepileptic drug before rejecting it in favour of another one. • Epileptic seizures that initially respond to drug therapy sometimes escape from control. In the case of phenobarbitone, primidone, phenytoin and carbamazepine, the escape may be due to hepatic enzyme induction. • Monotherapy with standard drugs results in satisfactory control of seizures, in almost 50% of patients. If adequate control is not achieved by a single drug in maximum tolerated doses and if compliance and absence of precipitating factors (as sleep deprivation, febrile illness, use of concomitant drugs) are confirmed, another drug is substituted. If this fails, combined therapy should be considered. • While changing from one drug to another, the first drug must be tapered off slowly (unless serious ADR demand abrupt stoppage) while the second one is introduced in gradually increasing doses. • Patients with focal epilepsy due to an underlying structural lesion, and those with multiple seizure types mostly require multi-drug therapy (resistant epilepsy). The drugs acting through different mechanisms should be combined considering their ADR and potential drug interactions. In them, the initial combination should be from among the older drugs such as phenytoin, carbamazepine, phenobarbitone and valproate. If
resistance persists, then a newer drug such as gabapentine, lamotrigine or topiramate is added. • When a drug combination is used, each drug should be prescribed separately and fixeddose drug combinations should be avoided. • It has been customary to prescribe the AED 2-3 times a day. Patients often tend to forget one or more of the doses, leading to poor seizure control. With phenobarbitone, phenytoin, primidone and ethosuximide, sustained therapeutic plasma levels can be achieved by giving the entire daily dose once a day. Sodium valproate is best prescribed on a twice a day basis and carbamazepine, 2-3 times a day. Extended release formulations are now available. Young children require frequent doses as they metabolise AED more rapidly than adults. • When experience shows that in a given patient the frequency or likelihood of attacks increases under stressful circumstances e.g. examination or social events, it is advisable to increase the dose, often to the limit of tolerance, well in advance of the event, and to reduce it gradually afterwards. Trauma, including that of surgery, also increases the drug requirement. • Sudden withdrawal of an antiepileptic drug can precipitate status epilepticus. The patient should be warned about this. • Most patients will need treatment for their lifetime, except in petit mal. However, an attempt may be made to discontinue the drug in those individuals with idiopathic grand mal epilepsy who have remained seizure free for 2 years. The drug should be tapered off slowly over weeks or months. The risk of recurrence is about 25% in patients without risk factors such as abnormal EEG, structural lesions and resistant seizures. About 80% recurrences occur within 4 months of discontinuation of drugs. Patients with focal seizures are more likely to have a recurrence of seizures. • Routine periodic determination of the blood level of an AED (Therapeutic Drug Monitoring, TDM) is not necessary. Clinical monitoring by recording the seizure frequency and ADR is more important than TDM. TDM is not useful with sodium valproate, and rarely required with phenobarbitone and ethosuximide. With other drugs, it has been usefully employed as a help: (a) in adjusting the dose of a drug during the initial days of its use; (b) in detecting non-compliance; and (c) to attribute toxicity to a particular drug and adjust its dose. • The patient should be advised to avoid using OTC formulations and drugs from alternative medicine as some of them may contain drugs which lower seizure threshold and precipitate seizures. Finally, it must be remembered that epilepsy syndromes are often associated with psychiatric, cognitive and social complications, even in cases considered as uncomplicated. Further age related brain atrophy leads to increased vulnerability to seizure induced cognitive defects. In addition, certain antiepileptic drugs can cause cognitive side effects. Phenobarbitone, phenytoin and valproate have negative effects on motor and cognitive speed and memory. On the other hand, carbamazepine and valproate may have positive effects on mood. Hence it is advisable to screen for psychiatric and cognitive co-morbidity in all patients before starting antiepileptic drugs. Table 9.6 lists some drugs used for other indications and can induce seizures as ADR.
Table 9.6 Drug induced seizures
Epilepsy and Pregnancy Sudden cessation of AED is liable to precipitate status epilepticus and consequently abortion. Hence, AED should not be stopped abruptly during pregnancy. Their dose should be reduced to a minimum. Women on AED should receive folic acid supplements 5 mg/day starting before the conception, and the drug continued throughout the pregnancy. Well over 90% of women taking AED give birth to healthy babies, and pregnancy need not be terminated in well controlled epileptics. However, women with epilepsy have almost twice the rate of complications such as toxemia, intrapartum hemorrhage and premature labour, regardless of the drug used. The major fetal malformations such as cardiac defects, cleft palate, neural tube defects and spina bifida occur in almost 2% of pregnancies in epileptics; they are probably drug-related. The use of two or more AED increases the frequency to 10%. Minor malformations like nail hypoplasia, low set ears and prominent lips also occur with higher than normal frequency in infants born of mothers on antiepileptics. The newborn of mothers who have received an AED during pregnancy should be examined for congenital abnormalities. Most AED promote hemorrhagic diathesis in the neonate. Hence, they should receive vitamin K at birth, in order to prevent bleeding due to deficiency of vitamin K dependent clotting factors. Further, phytomenadione (not menadione) 20 mg/day should be given to the mother in the last month of pregnancy. AED can be used safely during breast feeding. Sometimes, the babies can get sedated.
Drug Therapy of Epilepsy Drugs used in the treatment of epilepsy may be clinically classified into: • Those that are effective in petit mal: Ethosuximide, Sodium Valproate, Clonazepam and Acetazolamide. • Those that are effective in all other varieties: Phenobarbitone, Diphenylhydantoin, Primidone, Carbamazepine, Valproate and Oxcarbazepine; and • Newer add-on drugs Table 9.7 lists the factors which determine the selection of the antiepileptic drug. Table 9.7 Factors determining the selection of the AED
I PETIT MAL: It is essential to confirm the diagnosis of petit mal by an EEG as the specific drugs are effective only in patients with typical EEG changes. It is also essential to inquire about concomitant grand mal attacks as anti-petit mal drugs are liable to aggravate grand mal. Ethosuximide and sodium valproate are equally effective in this condition. Ethosuximide is started in the dose of 250 mg tid increasing upto 1500 mg per day, if necessary. Sodium valproate as a single drug is the drug of choice if tonic-clonic seizures are also co-exist or emerge during therapy with ethosuximide. Alternatively, phenobarbitone may be added to ethosuximide; this combination is preferred in children below three years in whom valproate is known to cause a higher incidence of fatal hepatotoxicity. Lamotrigine can also be used in newly diagnosed petit mal. Drug treatment of petit mal can be withdrawn 3-4 years after cessation of attacks. It is rare for this variety of epilepsy to recur. II GRAND MAL: During an epileptic attack: If a known epileptic person is under close observation, it may be possible to recognise an attack early enough to avert a fall. More often, by the time a fit is noticed, tonic or clonic phase has already started. When flaccidity of the muscle supervenes after the clonic phase is over, the patient can be choked by his own saliva and by his tongue falling back into the pharynx. This can be prevented by turning the patient into a semiprone, head-low position and by inserting a pharyngeal airway once the seizure is over. The patient is then watched till he recovers consciousness. A child, unless it is known to be an epileptic, should be admitted to a hospital at this stage, as meningitis is a common cause of seizures in childhood. Prevention of attacks: An epileptic should be advised to use hard pillow to prevent being smothered, if an attack occurs during sleep. Therapy with sodium valproate is initiated with 600 mg daily, divided into 2 doses. It can be increased every 3rd day by 200 mg. The usual maintenance dose is 1-2 g/day. The other suitable alternative, carbamazepine, should be started in the dose of 200 mg
twice or thrice a day and the dose should be increased gradually, until seizures are controlled or a total daily dose of 1200 mg is reached. If seizures continue, change over to phenytoin sodium. Because of its long shelf life, inexpensive phenobarbitone monotherapy at night is still a useful first line drug in adults especially in developing countries. The fewer side effects as compared with phenytoin is an advantage. It is, however, not easily available. Higher doses of phenobarbitone cause a high incidence of sedation and cognitive impairment. Lamotrigine appears to be effective in epilepsy syndromes with mixed, generalised seizure types and is currently considered as monotherapy as well as adjunctive therapy. When phenytoin is used in the maintenance dose (300-400 mg/day) from the outset, therapeutically effective plasma levels are achieved only after 7-10 days. When the need to control seizures is more urgent, therapeutic plasma levels can be reached within 12-24 hours by giving a loading dose of 1g followed by the daily maintenance dose. The maximum tolerated maintenance dose for an adult is 600 mg per day. Because of saturation kinetics, even a small increase in the maintenance dose of phenytoin may cause toxicity. This is its disadvantage. Phenytoin can occasionally cause cosmetic adverse effects, which makes carbamazepine the drug of choice particularly in young women. There is considerable variation in the bioavailability of phenytoin sodium from the many marketed formulations. This might account for the sudden appearance of toxicity or of loss of seizure control on changing the proprietary preparation without changing the dose. Once seizures are under control, the drug or drug combination must be continued for a least 3-4 years after the last seizure episode. Patients must be warned against sudden cessation of drug treatment. Drug treatment is not effective in preventing mental deterioration. This is, however, rare in well controlled epileptics. Epilepsy during drowsiness is a condition where fits occur when the patient is drowsy but not asleep. Anticonvulsants like phenobarbitone which produce drowsiness increase the frequency of attacks in this condition and must be avoided. Phenobarbitone is the drug of choice for grand mal epilepsy in children under the age of 5 years, as they do not seem to tolerate phenytoin, as well as older children and adults do. However, phenobarbitone is known to cause behavioral disturbances in children. III ATONIC SEIZURES: These are resistant to almost all drugs; however, valproate, benzodiazepines and lamotrigine have been used with some success. A ketogenic diet may be helpful. IV MYOCLONIC SEIZURES: These are often refractory to treatment. Sodium valproate seems to be the drug of choice; other alternatives are lamotrigine and topiramate. V INFANTILE SPASMS: The treatment is disappointing. The drugs used are benzodiazepines, glucocorticoids, corticotropin and vigabatrine. Though they may control the seizures, they do not improve the mental retardation. A ketogenic diet may be helpful. Lennox-Gastaut syndrome is a severe form of childhood epilepsy with multiple types of seizures, developmental delays and impaired intellectural function. Lamotrigine, topiramate valproic acid and rufinamide have been found to be effective. VI FOCAL SEIZURES: The drug treatment of this condition is similar to that of grand mal epilepsy. The seizures are often refractory to drug therapy. Carbamazepine and
phenytoin are the preferred drugs in this condition. Table 9.8 summarises the choice of antiepileptic drugs for above-mentioned types of seizures. Table 9.8 Choice of Antiepileptic drugs
*
In developing countries, because of low cost.
**
For patients resistant to established drugs.
VII POST-TRAUMATIC SEIZURES: Head injury predisposes to the development of seizures. Clinical evidence suggests that prophylactic treatment with phenobarbitone, carbamazepine or phenytoin may help such patients. VIII FEBRILE SEIZURES: Febrile convulsions are not associated with, nor do they cause, mental retardation, affect IQ, poor scholastic performance or behavioral problems; hence, routine prophylactic administration of antiepileptic drugs is not warranted. Brief febrile seizures need only treatment of the fever (tepid sponging and paracetamol) in an otherwise normal child. If a febrile convulsion lasts longer than 15 minutes, rectal administration of diazepam in solution (10 mg per 2 ml) in the same dose as in status epilepticus is rapidly effective. Chronic prophylactic therapy should be started in a child with a febrile convulsion if: • The first seizure occurs before the age of 18 months. • The convulsion was focal (one sided) or prolonged (longer than 15 minutes). • The patient has any neurological abnormality or developmental delay. • Any sibling or either parent of the patient has epilepsy; or • The febrile convulsions are recurrent. Above-mentioned factors suggest risk of developing epilepsy in approximately 10% of patients. When indicated, therapy is initiated with phenobarbitone, 3-4 mg/kg/day. Carbamazepine is another alternative. Phenytoin is not well tolerated by small children and valproate is more toxic at this age. The child should be treated for 2 years or for 1 year after the last seizure, whichever is longer. IX ECLAMPTIC SEIZURES: See Chapter 30. X STATUS EPILEPTICUS (SE): This term is used to indicate either a convulsive seizure
lasting for more than 15 minutes; or rapidly recurring seizures without return of full consciousness between seizures. Such patients need emergency IV medication if death or permanent brain damage, especially hippocampal sclerosis (which can occur in 30-60 minutes in untreated patients), is to be prevented. Table 9.9 summarises the principles of management of grand mal status epilepticus. SE occurs in epileptics but may also occur in patients with other disorders such as brain tumours or meningitis as well in alcoholics. Table 9.9 Management of tonic-clonic SE
The IV administration of anticonvulsant drugs requires the availability of cardio-pulmonary resuscitative support equipment.
Lorazepam IV is considered as the treatment of choice for controlling seizures. Diazepam, though has a longer t½, it has a higher volume of distribution and more lipid solubility. Hence, its therapeutic effects are of shorter duration than those of lorazepam (28 hours). The dose of lorazepam is 4-8 mg, given over 1-2 min. It can be repeated if there is no response within 5 min. In patients taking valproate, start IV sodium valproate, 25 mg/kg. Lorazepam has also been administered by squirting it intranasally, using an atomising pump. It reaches the brain rapidly along the perineural pathways of olfactory and trigeminal nerves bypassing the BBB, and acts within minutes. In case lorazepam is not available, IV diazepam can be administered. It may be given in the dose of 10 mg IV, slowly (over 5 minutes) in adults, 5 mg for children over 7 years of age and 2.5 mg for those between 1-7 years of age. The dose may be repeated twice more at 15 minute intervals. The dose may be further repeated every 2 to 4 hours, if necessary. Hypotension and respiratory depression should be watched for, especially in patients who have received barbiturates earlier. Rarely, cardiac arrest has been reported after IV diazepam. If IV administration is not possible, diazepam may be given rectally: 10 mg in adults and children over 3 years, and 5 mg in children 1-3 years and in elderly patients; it may be repeated, if necessary, after 5 minutes. Midazolam (buccal or IV, 10 mg) is an alternative to diazepam. Lorazepam/diazepam should be immediately followed by IV phenytoin sodium (15-20 mg/kg) at not more than 50 mg/min. (not more than 25 mg/min in elderly patients). Phenytoin should not be diluted in glucose solution as it precipitates. Fosphenytoin, which is freely soluble can be given IV, 20 mg/kg at a rate of 150 mg/min. It is potent and considered safer. If seizures persist, intubate the patient and institute IV sodium valproate (25 mg/kg). If seizures still persist, IV general anaesthesia is administered with a short acting barbiturate in consultation with an anaesthesiologist. Propofol or midazolam can also be
used. Lignocaine IV can be used in resistant cases but it must be remembered that lignocaine itself can induce seizures; it should be used only after adequate amounts of anti-epileptic drugs (phenytoin and/or phenobarbital) have been injected. Phenobarbitone (10 mg/kg) IV at not more than 50 mg/minute may be given. Where facilities for intubation/resuscitation are not available, inject phenobarbitone IV at a similar rate to a total of 800 mg. Phenobarbitone (200 mg IM, repeated as necessary) or paraldehyde deep IM (5 ml into each buttock, in an adult) may be useful. Alternatively, paraldehyde may be given rectally as follows: 0.2 ml/kg mixed with a threefold volume of a vegetable oil (maximum single dose 10 ml) every 4-6 hours upto 48 hours. Further management for the next 24 to 48 hours is that of an unconscious patient. Patients in SE are liable to develop hyperpyrexia. It should be looked for and treated. Between convulsions, a soft object, such as a folded napkin, should be inserted between jaws to prevent tongue biting during convulsions. For patients with absence seizures, sodium valproate IV may be required. Immediately on recovery, the patient should be put on the oral antiepileptic drug therapy. Table 9.10 lists ‘The Points to Remember ’ in the treatment of status epilepticus. Table 9.10 Points to remember while treating SE
Emergency management of convulsions due to other disorders such as withdrawal of sedatives (including barbiturates), tetanus, eclampsia, cerebral hemorrhage, poisoning from convulsive agents and during administration of local anaesthetics is similar to that of SE. Diazepam or lorazepam is the drug of choice; paraldehyde IM may also be used. Phenytoin is not useful. The same is true of convulsions occurring during the withdrawal of CNS depressant agents in addicts. Surgical treatment of refractory epilepsy: Surgery can be effective in reducing seizure frequency and providing complete seizure control in patients with refractory epilepsy. Depending on the localisation of the epileptogenic focus, the procedure is selected. Clinically significant complications of surgery are less than 5%. Post-operatively, though antiepileptic drugs are needed to be continued, marked reduction of seizures help to improve quality of life. Other modalities include neurostimulator devices. These divices are programmable, battery operated and deliver electrical impulses to prevent seizures. These devices either stimulate vagus nerve or directly the seizure foci in the brain. AED in non-epileptic disorders: These drugs have been used in many non-epileptic disorders with variable results. The examples are: carbamazepine in trigeminal neuralgia and bipolar disorder; gabapentin in neuropathic pain syndromes, migraine/tension headache, spasticity, and social phobia; lamotrigine in neuropathic pain syndromes; primidone in essential tremor; topiramate in migraine/tension headache, essential tremor
and binge disorder; vigabatrin in spasticity; valproate in migraine/tension headache and bipolar disorder; phenytoin in digoxin-induced ventricular tachycardia; and phenobarbitone in neonatal hyperbilirubinemia.
10
Opioid Analgesics and Antagonists Pain is an unpleasant sensory and emotional experience, associated with actual or potential tissue damage or described in terms of such damage. It is a subjective experience which cannot be objectively defined or quantified satisfactorily. Pain acts as a warning signal against disturbances in the body and thus has a protective function. However, on many occasions pain seems pointless, only contributing to the discomfort in the subject. As a symptom, pain demands instant relief and in practice its dramatic relief highly impresses a layman. Pain receptors are distributed throughout the body. Clinically, pain can be considered as: • Superficial or cutaneous pain • Deep non-visceral pain from muscles, joints, ligaments and bones • Visceral pain • Referred pain • Deafferentiation or neuropathic pain; and • Psychogenic or functional pain Pain arising from the skin and from the deep non-visceral structures like muscles, bones and joints is also termed as somatic pain. It is usually well defined and is generally caused by an inflammatory reaction in the tissues; it may be accompanied by contraction of the surrounding skeletal muscles as in patients with rheumatoid arthritis. However, other causes such as direct irritation of a nerve as in trigeminal neuralgia, herpes zoster, increased pulsation of the intracranial arteries as in migraine, or vascular insufficiency as in thromboangitis obliterans are also incriminated in the genesis of somatic pain. Pain arising from the skin and superficial mucous membrance or nerves is felt as pricking, if brief, and stinging, smarting or burning if prolonged. Deep nonvisceral (skeletomuscular) pain usually has a dull character and it may be accompanied by a sickening sensation due to an autonomic response. Sometimes, it spreads to other areas and may even occur as referred pain. BP and pulse, however, are not much affected, unless the pain is acute and severe. Visceral pain, unlike the somatic pain, is diffuse, less easily localised and often ‘referred’. It is dull-aching or colicky in character and is often accompanied by sweating, nausea, fall in BP and even shock. In addition, muscle rigidity and hyperaesthesia are common accompaniments. In practice, visceral pain may be due to spasm (renal or biliary colic), ischemia (myocardial infarction), inflammation (appendicitis, pancreatitis) or stimulation of the sensory nerve endings (peptic ulcer). Deep pain, whether visceral or somatic in origin, may be misinterpreted as coming from some part of the body other than the actual site of stimulation. This is called referred pain. Thus, cardiac pain is commonly referred to the left arm and diaphragmatic pain to the shoulder. Usually, the pain is referred to a cutaneous area which receives its nerve supply from the same spinal segment as the affected viscus. Although various theories have been proposed to explain the pain mechanism none can explain all its aspects. The assimilation of sensory pain at the level of consciousness depends on various factors such as the nature of sensory receptors, the intensity of the impulses transmitted to the CNS, their integration and finally their modulation by other
sensory information. The conscious appreciation of pain appears to depend upon the widespread activity of the entire cortex; and individuals differ widely in their reactions to similar painful experiences. Deafferentiation pain is caused by partial damage to axons and nerve membranes, which become very sensitive to mechanical and chemical stimuli. Such pain may either be of burning, superficial (dysasthetic) type; or of stabbing (lancinating) character. It has a peculiarly unpleasant quality about it and may not respond to opioids nor to NSAID (see later). Psychogenic or functional pain is usually a vague pain which follows no definite anatomical pattern of distribution. Such pain is usually continuous from day to day and involves more than one part of the body. It however, does not disturb sleep. Psychogenic pain is often preceded by a phase of exhaustion while organic pain brings about exhaustion. However, psychic element is present in all types of pain. Pain pathways: Painful stimuli may primarily be physical stimuli such as pressure or heat, or they may be chemical stimuli from the products of inflammation. A variety of naturally occurring compounds can elicit pain response in experimental animals, e.g. histamine, acetylcholine, bradykinin, PGs, 5-HT and substance P. These substances are present in venoms and products of inflammation. Nociception is a physiological process by which pain is perceived. The specialised peripheral neurons responsible for this are called nociceptors. Their cell bodies are located in posterior horns of the spinal cord. It appears that tactile sensation is transmitted by large diameter (L), fast conducting nerve fibres, and pain via small diameter (S), slow conducting (C) nerve fibres (nociceptors). Impulses from the nociceptors, on reaching the spinal cord, activate the first transmission cell and also the collateral cells in the substantia gelatinosa (SG). Anatomically, these nerve fibres are carried in the dorsal nerve roots and end in the SG at the apex of the dorsal gray horn. The SG cells inhibit the passage of signals and thus decrease the output reaching the higher centres. If, however, pain stimulus is more intense, then the SG cells are inhibited, releasing the dorsal horn cell from inhibition, resulting in higher output reaching the higher centres, leading to perception of pain. This gate control mechanism allows the sensory input to be decreased or augmented depending on the relative activity of L fibres and S fibres. Activation of nociceptor causes release of various neurotransmitters leading to activation of secondary axons. The secondary axons arising from the dorsal horn travel through the opposite spinothalamic tract, which terminate in the thalamus that projects to the post central gyrus which is mainly responsible for localisation of pain. Although the thalamus is responsible for perception of pain, the cerebral cortex is essential for its discriminative, exact and meaningful interpretation and for some of its emotional components. The other intermingled fibres which form an ascending multisynaptic pathway terminate in the thalamus and from there project to frontal and limbic systems, and the hypothalamus. This system is concerned with the emotional concomitants of pain. Higher centres, through their central inhibitory and facilitatory mechanisms, exert modulating influence on the gating mechanism. Thus, clinically the sensation of pain has several components including the emotional, psychic reaction. Analgesics are the drugs which relieve pain without causing loss of consciousness.
Experimental evaluation of analgesics: Analgesics can be evaluated in various ways: (a) Prevention or relief of artificially induced pain in experimental animals. (b) Relief of experimental pain in human volunteers, induced by radiant heat, ischemia induced with sphygmomanometer cuff or intraperitoneal bradykinin and (c) Relief of pathological or incisional pain, post-puerperal pain, post-operative pain and pain due to malignancy. Evaluation of analgesics against pathological pain is preferred to that against experimentally induced pain. It is usually desirable to compare the effects of several doses of several drugs in the same patient, before drawing conclusions. Because of the subjective component of pain, use of a cross over double blind technique using a placebo or a standard drug is essential in evaluating analgesics in man. Classification: Analgesics are classified into: I Opioid II Non-opioid. I Opioid analgesics: The word opiates refers to the products obtained from the opium poppy. The term opioid (opiate-like) is used to denote all naturally occurring, semi-synthetic and synthetic drugs which have a morphine like action viz relief from pain and depression of the CNS, both reversed by naloxone. These drugs were formerly called ‘narcotic’ analgesics because some of them (such as morphine) induce sleep. The term ‘narcotic’ is no longer applied to opioids but is restricted in the legal sense to drugs capable of producing dependance. The opioids are further subclassified as: (a) Agonists such as morphine and compounds which resemble it in most of their actions, viz, derivatives of morphine; codeine and its derivatives; synthetic compounds such as pethidine, methadone, propoxyphene, levorphanol and tramadol. (b) Partial agonists e.g. buprenorphine and meptazinol. They have partial agonist action only on the mu receptors (see later). (c) Mixed agonist-antagonists which act as agonists at one type of opioid receptors and as competitive antagonists at another type of receptors, e.g., nalbuphine, pentazocine, and butorphanol. Patients who have received repeated doses of a morphine-like drug to the point of physical dependence may experience an opioid withdrawal reaction when given a mixed agonist-antagonist. Endogenous opioid peptides: Peptides with strong opiate-like analgesic and mu receptor binding activity are present in the CNS and other tissues. They act as: (a) Endogenous analgesics, (b) Neuro-transmitters and (c) Behaviour modulators. They are: • Beta-endorphin, a potent analgesic, is derived in the pituitary from a larger, parent molecule, pro-opio-melanocortin (POMC). It predominantly binds to mu receptors and acts as a pain modulator in the CNS. • Enkephalins, derived from pro-enkephalin, are more widespread; they are found in the pituitary, brain, GI tract, spinal cord, pancreas and adrenal medulla. They predominantly bind to the delta receptors. • Dynorphins A and B, derivatives of pro-dynorphin, have been found to be widely distributed in the CNS. They predominantly bind to k receptors.
• Nociceptin/orphanin FQ is a peptide with behavioural and pain modulating effects that are complex. It is also implicated in learning, cough reflex and Parkinson’s disease. • Endomorphins 1 and 2, tetrapeptides with high, selective affinity for mu receptors. Endorphins, enkephalins and endomorphins are released during stressful conditions like pain or in anticipation of pain and serve as natural pain modulators. Milk and milk products contain opioid peptides such as beta-casomorphins, which are released from casein in the intestine during digestion of milk and may modulate GI function. Opioid antagonists, by themselves, produce few effects unless an opioid agonist has been administered previously e.g. naloxone. However, when endogenous opioids are activated as in shock or stress, an opioid antagonist does produce visible effects. II Non-opioid analgesics do not interact with opioid receptors and relieve pain without CNS depression e.g. NSAID (Chapter 11). Mechanism of action of opioids : The opioid drugs produce their effects by binding to opioid receptors (Table 10.1) which are widely distributed in the CNS and other tissues. In the CNS, they are localised to: Table 10.1 Pharmacological effects associated with opioid receptor types
(1) The periaqueductal grey matter of the brain stem, and the thalamus. (2) The area postrema which has the CTZ and the solitary nuclei which receive visceral sensory fibres from the vagus and glossopharyngeal nerves. (3) The amygdalae which may be responsible for the influences of the opioids on the emotional reactions; and (4) The spinal cord substantia gelatinosa, the first site in the CNS which integrates the sensory information. Opioid receptors are a part of family of G-protein coupled receptors. They have been classified into mu (mu1, mu2), delta (delta1 delta2), kappa (k1, k2, k3) and nociceptin (orphanin) types. When activated they: (i) Open K+ channels to inhibit post-synaptic neurons and (ii) Close Ca++ channels on the presynaptic neurons to inhibit release of the neurotransmitters from nociceptive nerve terminals. These actions reduce neuronal excitability. The above-mentioned actions are modulated by inhibitory descending pathways which communicate with nociceptor neurons in the dorsal horn of spinal cord and thalamus. Opioid receptors are also present in the peripheral nerves where they respond to
peripherally applied opioids and locally released endogenous peptides during inflammation. The pharmacological effects associated with these receptor subtypes and selectivity of the various opioid drugs for these receptors are summarised in Tables 10.1 and 10.2. Table 10.2 Selectivity of common opioid analgesic drugs for different receptors
+ = agonist, (+) = partial agonist, – = antagonist, 0 = no action MOR = Mu opioid receptors KOR = Kappa opioid receptors *
Antagonist of Mu receptors in high doses.
The vast majority of opioid drugs used as analgesics are agonists at mu receptors. Similarly the opioid antagonists naloxone and naltrexone, show a high selectivity for mu receptors. Drugs with mixed agonist-antagonist properties bind to more than one receptor class at the usual clinical doses.
Opium Alkaloids Opium is the milky exudate obtained by incising the unripe seed capsule of the poppy plant Papaver somniferum. The poppy seeds, however, are devoid of pharmacological activity and are in fact used in food preparations. On drying, the exudate forms a gummy, brownish mass. The pharmacologically active alkaloids of opium (Table 10.3) are divided chemically into: Table 10.3 Opium alkaloids
• Phenanthrene group; and • Benzyl isoquinoline group (Fig 10.1). The benzyl isoquinoline alkaloids are devoid of analgesic activity but act as smooth muscle relaxants. They are described later.
FIG. 10.1 Phenanthrene Benzylisoquinoline
MORPHINE is the most important alkaloid of opium and is used as sulfate or hydrochloride; both salts are soluble in water. Pharmacological actions: Morphine acts predominantly on mu receptors situated both in the CNS and other tissues such as GI tract. The exact mechanism of pain relief is not known. However, opioids and endogenous opioid-like peptides decrease the release of glutamate from nociceptive nerve terminals, and also acetylcholine, NA, 5HT and substance P. Central nervous system: • Analgesia: Morphine, by acting on µ1 receptors, produces relief of pain in a dose that usually does not cause motor incoordination. Other modalities such as touch, vibration and hearing are not obtunded.
In subanaesthetic doses, morphine and its analogues have little effect on pinprick sensation and the withdrawal reflex, though pain arising from the tissues is well suppressed. In moderate doses, it is effective in relieving continuous, dull pain. Larger doses, relieve sharp intermittent pain caused by trauma and by visceral pathology. Morphine, pethidine, methadone and the other agonists do not have a ceiling dose as far as their analgesic effect is concerned; the dose can be increased to a level just short of causing toxicity (as in the treatment of terminal cancer). Morphine raises the pain threshold, reducing the perception of pain and also causes the feeling of well-being (euphoria). It modifies the emotional reaction to pain. Thus, it may not completely abolish pain perception but the latter is no longer a source of concern to the patient. • Euphoria, sedation and hypnosis: With therapeutic doses, morphine produces a sense of emotional well-being termed euphoria (µ1 receptors). Euphoria eliminates the normal fear, panic, withdrawal and flight response to pain and aids the analgesic action of morphine. The ability to produce euphoria even in the absence of pain makes morphine one of the worst drugs of abuse. Rarely, it may produce a sense of anxiety or fear termed dysphoria, particularly in pain-free individuals. Sedation induced by morphine is characterised by drowsiness, difficulty in concentration and mental apathy. Thoughts may lack a logical sequence and imagination becomes extravagant, producing vivid and colourful daydreams. Larger doses induce sleep. The normal NREM and REM cycle is disrupted. Morphine depresses the respiratory and cough centers and stimulates the vagal and oculomotor centers and the CTZ. • Respiratory depression: Morphine in therapeutic doses depresses all phases of respiratory activity. It acts by: (i) Direct depressant action on the respiratory centre (µ2 receptors); and (ii) Reducing its sensitivity to increased plasma CO2 concentration. Retention of CO2 brought about by initial respiratory depression by morphine increases the rate and depth of respiration to the pre-drug value. At a later stage, the hypoxic drive tends to maintain the minute volume despite diminished sensitivity of the respiratory centre to accumulated CO2. The respiratory rate and minute volume, therefore, are not adequate monitors of respiratory depression caused by morphine. Bronchoconstriction as a result of histamine released by morphine and indifference to breathing as a result of psychological action of morphine, further enhance respiratory difficulties. With toxic doses, breathing is entirely maintained by the ‘hypoxic drive’ mediated through the carotid and the aortic body chemoreceptors, and this may result in CheyneStokes respiration. Inhalation of pure oxygen at this stage abolishes the hypoxic drive and produces apnoea. Hence, in such cases controlled assisted ventilation and moderate concentrations of oxygen are indicated. • Cough suppression: Morphine depresses the cough reflex as a result of direct depression of the cough centre. • Pupillary constriction: Morphine produces characteristic pin-point pupils. The miosis is due to an action on the Edinger-Westphal nucleus of the oculomotor nerve.
• Nausea and emesis: Morphine produces vomiting by stimulation of the CTZ in the area postrema of the medulla. Morphine-induced vomiting is abolished by nalorphine and by prochlorperazine (5-10 mg 4-8 hourly), metoclopramide (10 mg 4-8 hourly) and haloperidol (1-2 mg daily) but not by antihistaminics. In large doses, it depresses the vomiting centre. Thus in case of morphine poisoning, vomiting is absent and emetics are ineffective. • Vagus stimulation: Morphine stimulates the medullary vagal nucleus and may cause bradycardia. • Spinal cord: Morphine increases the reflex excitability of the spinal cord. This action is, however, usually masked by the depression of the higher centres in the CNS. Therapeutic doses of morphine may produce a significant increase in the CSF pressure. • Miscellaneous: Morphine in large doses lowers the body temperature by central action. Gastrointestinal tract: Morphine induces spasm of the smooth muscle of the gut, the ileocolic and the anal sphincters, and reduces the propulsive peristaltic movements (µ2 receptors). Spasmogenic action of morphine is particularly evident in the duodenum and the large intestine. There is a reduction in the secretion of saliva, gastric acid and the intestinal secretions. Desiccation of the faeces, abolition of the peristaltic movements, spasm of the sphincters, particularly the anal sphincter, and inattention to normal sensory stimuli from a loaded rectum as a result of the psychological effect of morphine, all lead to constipation. Atropine partially antagonises the spasmogenic action of morphine on the colon. Patients on long-term use of morphine should receive a high fibre diet and a laxative such as senna regularly. Lubiprostone (Chapter 41) may also be helpful in these patients. Morphine increases the intrabiliary pressure by producing a spasm of the sphincter of Oddi. Hence, although it may relieve the biliary colic because of its analgesic action, the underlying disease is exacerbated. Atropine partially antagonises this action. In mice, morphine produces a severe spasm of the anal sphincter resulting in erection of the tail. This test, termed Straub’s test, was formerly employed to detect morphine in biological fluids. Other smooth muscles: Morphine produces: • An increase in the tone of the ureters and the detrusor muscle of the bladder. The vesical sphincter is contracted. These effects are augmented by inattention to stimuli arising from the bladder and cause urinary retention. • An increase in the tone of the bronchi and the bronchioles. Except in large doses, it has no significant effect on the normal human uterus at full term. Cardiovascular system: Therapeutic doses of morphine have negligible effect on the myocardium, the BP or the heart rate. Morphine produces dilatation of the peripheral blood vessels, particularly the cutaneous blood vessels. This may reduce the pre-load on the heart (see later). Pruritus, sweating and flushing often accompany the cutaneous capillary dilatation. Large doses may produce hypotension. Neuroendocrine system: Morphine inhibits the release of GnRH and CRH from the hypothalamus. It thus decreases the plasma concentration of FSH, LH and ACTH. The plasma prolactin increases. Morphine produces a release of ADH with resultant decrease in the urinary output. Administration of morphine reduces the efficacy of diuretics in patients with CHF.
Immune system: Opioids suppress the various immune functions and increase the susceptibility to infection in experimental animals, both by a direct action and an indirect action mediated by the CNS. Absorption, fate and excretion: Given orally, morphine is adequately absorbed. It is extensively metabolised during first pass through the liver; hence its oral bioavailability is about 20-40%. Sustained-release preparations have a longer duration of action. The drug can also be given rectally. Given subcutaneously, it produces analgesic effect within 15 to 20 minutes with peak effect at 60 to 90 minutes; it persists for 3 to 5 hours. Given IV, it produces an immediate effect. Morphine circulates in the plasma partly protein-bound and partly in the free form. It enters the brain rapidly. It crosses the placental barrier readily and is also secreted in milk. It is metabolised by the liver and the kidney. Morphine is conjugated with glucuronic acid to form both active and inactive products; morphine -6- glucuronide, the active metabolite, is more potent than morphine. In adults the plasma t½ of morphine is about 2-3 hours, whereas that of the glucuronide is longer. Approximately 90% of the administered dose is eliminated in urine within 24 hours, mainly in conjugated form. Biliary excretion of this form accounts for approximately 7 to 10% of the dose. Preparations and dosage: (i) Controlled release tablets (10, 30 and 60 mg) of morphine sulfate for prolonged action. (ii) Morphine solution (2-20 mg/ml) for oral use. Dose in adults is 10-30 mg. Larger doses (up to 200 mg) are needed in patients with terminal cancer. Orally, it is only about one sixth as effective as parenteral administration. (iii) Morphine hydrochloride or sulfate injection. Dose: 10-20 mg SC or IM; 2.5-5 mg IV over 5 minutes. It has also been used by a continuous, low dose, IV infusion and by epidural and intrathecal administration. If given by IV infusion, 10 mg are infused over the first 1 hour and 10 mg over the next 4 hours. Adverse reactions: • Intolerance: These reactions include allergic skin rashes, pruritus and contact dermatitis. It is a histamine liberator. Anaphylactoid reaction with fall in BP has been reported after morphine injection. • CNS and respiratory depression: It causes nausea, vomiting, dysphoria, mental clouding, vertigo, headache, fatigue and paraesthesiae. Morphine may occasionally produce tremors and delirium. Respiratory depression occurs even in small doses (see earlier). • Constipation following morphine may be dangerous in old people as it may precipitate intestinal obstruction. It may also cause abdominal distension and increased biliary pressure. • Hypotension: Morphine occasionally produces hypotension as a result of peripheral vasodilatation, more so in patients with reduced blood volume. • Urinary retention: Morphine may cause urinary retention post-operatively and in old people with prostatic hyperplasia. • On the foetus: Morphine administered to the mother during labour can depress fetal respiration. This asphyxia can be reversed by naloxone 10 mcg/kg given IV, IM or SC. • Tolerance: Repeated administration of morphine at short intervals results in loss of its effectiveness (tolerance). With intermittent use, however, tolerance to analgesic and
sedative effects does not develop. Tolerance also develops to the respiratory depressant, emetic, hypotensive and euphoriant effects of morphine as well as to urinary retention, but the pupils and the GIT do not share this tolerance. A morphine addict thus has characteristically pinpoint pupils and is habitually constipated. Tolerance to morphine is attributed primarily to the ability of the cells of the central nervous system to withstand large doses of the drug. Persons tolerant to morphine exhibit cross tolerance to other opioid analgesics and even to compounds like barbiturates and alcohol. Tolerance does not develop to antagonistic actions of pure or mixed agonist antagonist. • Drug dependence: This is a major drawback of morphine therapy. Opium has been in use as a drug of abuse for several centuries and has precipitated wars. Dependence on morphine and morphine-like drugs results mainly from their euphoriant effects. In addition, these drugs produce a variety of sensations such as a ‘turning in the stomach’, a feeling of warmth in the epigastrium and other parts of the body due to flushing; and sensations in the lower abdomen described by addicts as akin to sexual orgasm, and known as “kick” or “thrill”. Morphine addicts are usually malnourished and debilitated. Even though they do not suffer from motor incoordination and are capable of performing complex motor and intellectual tasks, their productivity and utility to the society usually suffer. As the drug is commonly self injected, the incidence of injection abscesses, tetanus, AIDS and serum hepatitis is high among the addicts. Manifestations of opioid withdrawal syndrome are summarised in Table 10.4. Table 10.4 Manifestations of opiate withdrawal Abstinence period Manifestations 6–12 hours Intense c raving for the drug, lethargy and weakness. 12 hours Yawning, lac rimation, perspiration, rhinorrhoea, tremors and anorexia. 48 hours Peak of the withdrawal syndrome. Fever, rise in BP, inc rease in heart rate, dilatation of the previously c onstric ted pupils and intestinal c ramps. 7–10 days S ymptoms c lear up but the patient may c omplain of restlessness, insomnia, weakness, and bac k and leg pains for several weeks.
In order to prevent morphine dependence, morphine should not be prescribed readily for chronic pain except in cases of terminal cancer pain. Such patients rarely develop psychological dependence on morphine. The mechanism of opiate tolerance and withdrawal syndrome is not known clearly but the involvement of NA and NMDA receptor complex has been demonstrated. The treatment of morphine dependence: In principle, it is similar to that of alcohol or barbiturate dependence. The results, however, are unsatisfactory because of the severity of withdrawal syndrome and the high relapse rate. Its gradual withdrawal with substitution of another opioid analgesic to decrease the severity of withdrawal syndrome is usually advocated. Methadone orally is often used for replacement as it has a longer duration of action than morphine. One milligram of methadone will substitute for 4 mg of morphine. Once the patient is stabilized on methadone, its dose is gradually reduced by 10-20% daily and the drug can be completely stopped from 6th to 10th day. Acute opiate withdrawal symptoms and signs can be controlled to a certain extent by drugs like chlorpromazine, propranolol and clonidine which counter the noradrenergic
overactivity. • Acute morphine poisoning: Acute morphine poisoning may occur from clinical overdosage, accidental overingestion by an addict or from suicidal or homicidal intention. It is difficult to define the lethal dose of morphine. A dose of 60 mg is usually toxic but rarely fatal in a normal adult who is not in pain. Doses of 250 mg are usually fatal. Larger doses are generally required to produce toxicity in individuals with pain, whereas in addicts, the toxic as well as the fatal doses are much higher. Morphine poisoning is characterised by respiratory depression, pinpoint pupils, cyanosis, reduced body temperature, decreased urinary output, hypotension, shock and coma. Convulsions may occur in infants. Death is usually due to respiratory depression, or shock and pulmonary edema. If a toxic dose of morphine has been ingested, even late gastric lavage is justified as the spasmogenic action of morphine frequently delays its absorption. Naloxone and nalorphine are the specific morphine-antagonists. They produce dramatic reversal of morphine-induced respiratory depression. Naloxone is usually preferred because of its specific antagonistic and negligible agonistic action (see later). They should be administered with caution in treating acute morphine poisoning in addicts as they may produce a severe withdrawal syndrome. The duration of action of opioid antagonists is shorter than that of opioids and the patient has to be carefully monitored to prevent relapse into coma. Drug interactions: CNS depressants, phenothiazines, monoamine oxidase inhibitors and tricyclic antidepressants enhance the sedative effects of morphine and increase the respiratory depression. Therapeutic uses: • For relief of pain: Morphine is one of the most potent analgesics, employed to alleviate severe pain in conditions such as acute myocardial infarction, fractures of long bones, burns, terminal stage of malignancy, pulmonary embolism, acute pericarditis, and spontaneous pneumothorax. For relief of sudden excruciating pain, morphine is usually administered IV; prompt relief of pain minimises shock. Morphine SC is not advocated in the presence of shock, as its absorption is hampered. Repeated SC administration of morphine under these conditions may result in a sudden absorption of toxic quantities into the systemic circulation after the correction of hypotension. Morphine can be used for relief of pain in renal and biliary colic. However, for this purpose it is always combined with atropine which produces smooth muscle relaxation and thus helps to relieve spasm. Parenteral morphine has been used to reduce post-operative pain; thoughtless use for this purpose should be avoided as it can produce respiratory depression, urinary retention and constipation; it reduces coughing and may mask the signs of recovery and of complications. Since opiate receptors are located within the spinal cord, intrathecal and epidural morphine produces long lasting analgesia. Such analgesia is essentially segmental in distribution, the pain relief being remarkable and it was assumed to be selective without any interference with motor function or autonomic changes. However, respiratory depression, nausea, vomiting and pruritus may occur. Because of greater safety and ease of administration, most investigators prefer the epidural route to intrathecal route. It has been used following thoracic and upper abdominal surgery and in the treatment of cancer
pain. Not all types of pain respond to opioids. For example, the deafferentiation pain is relatively resistant (Chapter 11). • In acute left ventricular failure: Morphine is valuable in the treatment of acute left ventricular failure and pulmonary edema. It acts by reducing apprehension and the effort of breathing. Morphine-induced peripheral vasodilatation results in shunting of the blood from the pulmonary arteries to the dilated peripheral vasculature and this in turn reduces preload. Thus, it reduces the cardiac work load, and relieves dyspnea, provided oxygenation is maintained. • Sedation and sleep: Morphine is a valuable sedative in the presence of severe pain. As it does not affect the uterine motility, it has been used as a sedative in threatened abortion. Although morphine has been used routinely for sedating patients with internal bleeding such as haematemesis, a tranquillizing drug like diazepam is safer for this purpose. • As preanaesthetic medication: See Chapter 7. • To control diarrhoea: Tincture opium (0.5 to 1 ml) and paregoric are now rarely used for symptomatic relief of severe diarrhoea. • As an anaesthetic: Morphine IV, it has been used, alone or in combination with other drugs, to produce general anaesthesia especially in subjects who are considered as bad anaesthetic risks. Precautions with morphine therapy: • COPD: Morphine should be administered with caution to persons with diminished respiratory reserve e.g., individuals with emphysema, kyphoscoliosis and chronic obstructive pulmonary disease (COPD). Such patients are already on the verge of hypoxia which they avert by increasing their respiratory rate. Opioids decrease ciliary activity, depress cough reflex, cause bronchospasm and depress respiration, all of which can precipitate respiratory failure in such individuals. Deaths have been reported in patients with COPD following therapeutic doses of morphine. • Myxoedema: The lower BMR and reduced rate of metabolic disposal of drugs make such patients more sensitive to opioids and sedatives, and frank coma may be precipitated by even their conventional therapeutic doses. Patients with hypopituitarism and Addison’s disease are also more sensitive. • Old people and infants are more prone to develop respiratory depression with morphine. • Head injuries: Morphine should be avoided in such cases as it produces an increase in the CSF pressure, stimulates the spinal cord and produces respiratory depression, vomiting and miosis. Miosis and mental clouding may interfere with the diagnosis. • Acute abdomen: In this condition, morphine relieves pain symptomatically without modifying the underlying pathological process. It does not alter the physical signs such as abdominal rigidity. It may, therefore, facilitate clinical evaluation and permit diagnostic procedures. It may be helpful provided the pain is not forgotten. However, morphine-induced vomiting and its spasmogenic action on the GI and biliary tracts are its drawbacks. • Shock: Morphine IV may produce hypotension if administered during hypovolemic shock. Restoration of blood volume is more important in that condition. • Severe impairment of liver and/or kidney function: Cumulative toxicity can occur.
Other phenanthrene alkaloids of opium: CODEINE: Codeine, by itself, is a much less potent analgesic than morphine (weak agonist). It does not produce significant depression of respiration and has a low dependence liability. In toxic doses, it may produce excitement and convul sions. It enhances the analgesic effect of aspirin and is often combined with it. Unlike morphine, it is much better absorbed when given orally, with the bioavailability of about 50%. About 10% codeine is converted to morphine in the liver by CYP2D6, and which is responsible for its analgesic property. Extensive metabolizers are more susceptible to its toxicity. It is often used as an antitussive. Codeine phosphate is available for oral and IM use. Its main disadvantage is constipation. Therapeutic uses: • As an analgesic, 30-60 mg t.i.d. • As an antitussive, for suppression of dry cough (Chapter 26); and • As an antidiarrhoeal agent. (Chapter 41). Hydrocodone and oxycodone are other opioids used orally for treating cancer pain and also as antitussives. Bioavailability of oxycodone is better than that of morphine. Oxycodone is also used as an alternative to morphine in both acute and chronic pain. Sustained release formulation of oxycodone is given bid. Both hydrocodone and oxycodone can be combined with paracetamol or ibuprofen for synergistic effect. Currently hydrocodone and oxycodone are misused as drug of abuse. TRAMADOL: This synthetic codeine derivative is a weak agonist of the µ receptors and also exerts a part of its analgesic action by inhibiting NA and 5-HT uptake. It is rapidly absorbed and is metabolised in the liver to an active compound with analgesic action. Its t½ is 6 hours and 30% is excreted unchanged in urine. It is as effective as pethidine in mild to moderate pain and causes less respiratory depression and constipation. It has a low addiction potential. It can be used in labour pains. The drug causes dizziness, sedation, and nausea and rarely seizures. It should not be used concurrently with agents that enhance monamine activity or lower the seizure threshold e.g. MAO inhibitors and SSRI. The dose is 25-100 mg per day. Tapentadol: This structural analog of tramadol has similar activity and ADR profile as tramadol but its major pathway of metabolism is glucuronidation with subsequent excretion in urine. Benzylisoquinoline alkaloids of opium: PAPAVERINE: Papaverine is devoid of opioid and analgesic activities. It is a smooth muscle relaxant and causes vasodilation. Intracavernosal injection of papaverine causes penile erection and was used for symptomatic treatment of erectile dysfunction (Chapter 69). NOSCAPINE: This alkaloid has significant antitussive action in therapeutic doses, without disadvantages of morphine. It is a potent releaser of histamine and large doses can cause bronchospasm and hypotension. Its use in the therapy of cough is described in Chapter 26.
Semisynthetic Derivatives of Natural Opium Alkaloids Codeine derivatives are mainly antitussive (see above). Only the derivatives of morphine are described below. HEROIN (Diacetylmorphine): It is a more potent analgesic than morphine, produces greater euphoria and consequently has a higher dependence liability. It is now rarely employed therapeutically because of this drawback but is extensively used (as brown sugar) as a drug of abuse. The newborn children of mothers who are heroin addicts have been known to develop a ‘withdrawal syndrome’ a few hours after birth. The treatment of heroin dependence is similar to that of morphine dependence; one mg. of methadone can substitute for 2 mg of heroin. Hydromorphone, oxymorphone and methyldihydromorphinone in the doses of 1.5 mg, 1.5 mg, and 3.5 mg respectively are effective analgesics with 4-5 hours of duration of action. Their toxicity is similar to that of morphine. Sustained released hydromorphone is administered once daily for treating cancer pain. APOMORPHINE: This drug is obtained by the acid-catalysed rearrangement of morphine. The drug acts on both pre-and post-synaptic DA receptors and thus produces a variety of behavioural, neuro-pharmacological and endocrine effects. It stimulates CTZ and acts as potent emetic due to activation of dopaminergic (DA) receptors which in turn stimulate the emetic centre. The effect is blocked by neuroleptics like chlorpromazine but not by antihistaminic agents. It also produces, in threshold doses, an increase in exploratory activity and discontinuous sniffing in rats. Larger doses cause purposeless behaviour characterised by continuous sniffing, grooming, biting and licking, described as stereotyped behaviour syndrome. In cats and dogs, it causes side to side head movements and incessant running around the cage. These effects due to direct stimulation of DA receptors located in the neostriatum are also blocked by neuroleptics. Apomorphine is of great pharmacological interest and is used to evaluate the action of psychotropic drugs in experimental animals. In man, a dose of 0.1 mg/kg SC ordinarily causes vomiting within a few minutes. Adverse reactions include nausea, severe vomiting, dizziness, hypotension and bradycardia.
Synthetic Morphine Substitutes These are: I Pethidine and its congeners II Methadone and its congeners III Morphinan compounds and congeners, e.g., Levorphanol and Butorphanol IV Benzomorphan derivatives, e.g., Pentazocine. V Miscellaneous: Nalbuphine, Buprenorphine. PETHIDINE (Meperidine): As the pharmacological actions of this synthetic opioid (Fig 10.2) closely resemble those of morphine, only the salient differences between these two compounds will be pointed out. Thus:
FIG. 10.2 Pethidine
• On weight basis, it is about 1/10th as potent as morphine as an analgesic when given IV but it has a rapid onset and shorter duration of action (t½ 4 hour). • In equianalgesic doses, pethidine produces as much sedation, euphoria and respiratory depression as morphine. However, unlike morphine, it reduces the tidal volume without significantly affecting the respiratory rate. Therefore, respiratory depression may be missed if respiratory rate alone is watched. • The incidence of nausea and vomiting is higher than with morphine. • Pethidine is devoid of antitussive activity. • Pethidine exerts an autimuscarinic effect. Compared to morphine, it is less spasmogenic and causes less constipation. It may, however, raise intrabiliary pressure by producing spasm of the sphincter of Oddi. • Pethidine occasionally produces hypotension and syncope due to peripheral vasodilatation. Unlike morphine, however, IV pethidine may produce tachycardia and dryness of the mouth due to its vagolytic effect. Absorption, fate and excretion: Oral bioavailability of pethidine is about 50%; the analgesic effect appears within 10 to 15 minutes. On parenteral administration the action lasts for 2 to 4 hours as compared to 3 to 5 hours with parenteral morphine. It crosses the placental barrier and is also secreted in milk. It is mainly metabolised by the liver; the metabolic product norpethidine possesses significant excitatory action on the CNS. Norpethidine may accumulate during the chronic use. Only a small portion of pethidine is
excreted unchanged in the urine. The urinary excretion of the drug is enhanced when the urine is acidic. Preparations and dosage: (i) Pethidine hydrochloride tablets. Dose: 25 to 100 mg. (ii) Pethidine hydrochloride injection 50 mg per ml. Dose: IM/SC 25 to 100 mg IV: 25 to 50 mg to be repeated, if neces- sary, after 4 hours. Adverse reactions: • The adverse effects, apart from local irritation on parenteral administration, include sweating, euphoria, dizziness, dry mouth, vomiting, dysphoria, visual disturbances, weakness and palpitation. Anaphylactoid shock following pethidine has been reported. Constipation and urinary retention are less common. • Pethidine administered to mothers at term produces significant depression of foetal respiration. • It can produce bronchospasm and drying of secretions. As it also produces respiratory depression, it is not a suitable drug in patients with bronchial asthma. • Pethidine overdose causes respiratory depression and coma, or convulsions. Naloxone can antagonise the respiratory depression and coma but fails to modify the convulsant action of pethidine which sometimes is due to norpethidine. Acute pethidine poisoning should be treated on similar lines as acute morphine poisoning. Tolerance and dependence: Tolerance to analgesic and emetic effects develop on prolonged administration. The pethidine addict often shows dilated pupils, tremors, mental confusion, twitchings and occasionally convulsions. Dependence on pethidine is fairly common. The withdrawal syndrome usually develops within 3 hours after the last dose, reaches a peak by 8 to 12 hours and declines by 4 to 5 days. There is little nausea, vomiting or diarrhoea but the patient may show more excitement than during morphine withdrawal. The treatment of pethidine addiction is similar to that of morphine addiction. Methadone is employed initially as a substitute. One mg of methadone can substitute for 20 mg of pethidine. Drug interactions: Phenytoin increases the biodegradation of pethidine. Cimetidine (but not ranitidine) reduces the clearance of pethidine (but not morphine). Thus, morphine is a safer drug than pethidine to use in patients who are on cimetidine. Its administration to patients receiving imipramine or an MAOI may result in confusion, cerebral excitement and collapse. Contraindications to the use of pethidine are similar to those for morphine. Therapeutic uses: • Analgesia: It is particularly useful when short duration of action is required, as in gastroscopy, cystoscopy or ascending pyelography. Pethidine serves as a morphine substitute for relief of acute visceral pain e.g. in myocardial infarction, particularly that associated with bradycardia, and in burns. The precautions to be observed with morphine for the treatment of shock also apply to pethidine. Because of unacceptable pharmacological profile it is not the drug of first choice in severe/prolonged pain. • Preanaesthetic medication: Chapter 7. • Obstetrical analgesia: As small doses of pethidine do not interfere with uterine contractility, it was used in minor procedures like dilatation and curettage. • Epidural and intrathecal analgesia (see Chapter 7).
PETHIDINE CONGENERS: These examples are fentanyl (Chapter 7), alfentanil and remifentanil. They are used mainly as anaesthetic adjuncts. Fentanyl is the most widely used agent. Buccal transmucosal route can be used for fentanyl lozenges and “Lollipops”. It is also available as nasal spray. Alphaprodine has a shorter duration of action (one half to two hours) on SC administration and causes emesis less frequently than other opioid analgesics. It is administered orally or parenterally in the dose of 40 to 60 mg. It has been used for relief of pain in the first stage of labour. Diphenoxylate and its metabolite difenoxin are used in the treatment of diarrhoea in therapeutic doses, they do not produce morphine-like subjective effects; large doses, however, cause typical opioid symptoms. Loperamide, a piperidine derivative, is used for its selective GI mu receptor antidiarrhoeal effect (Chapter 41). It is also used for neuropathic pain. Loperamide 5% ointment is used to relieve pain in diabetic neuropathy METHADONE: Methadone has analgesic potency almost equivalent to that of morphine. Pharmacological actions: These are more or less similar to those of morphine but have a longer duration of action. A single therapeutic dose, however, exerts much less hypnotic activity than an equianalgesic dose of morphine. The drug depresses respiration to the same degree as morphine and has a marked antitussive effect. The actions of the drug on the GIT and the cardiovascular system are similar to those of morphine. In addition, methadone inhibits the re-uptake of noradrenaline and 5-HT; further, it also blocks the action of NMDA receptors, known modulators of neuropathic pain. Absorption, fate and excretion: Unlike morphine, methadone has bioavailability of about 80%. The analgesic effect occurs within 10 to 15 minutes following parenteral and 20 to 30 minutes after oral medication. It is highly bound to plasma proteins and also to the tissue proteins, including those in the brain. Plasma t½ of single oral dose is 15 hours which increases upto 24-36 hours following repeated administration. It crosses the placental barrier. Methadone is slowly metabolised in the liver, which may explain its longer duration of action. Less than 10% of the drug is excreted unchanged by the kidneys. Preparations and dosage: (i) Methadone hydrochloride, 5 mg tablets. Dose: 5 to 10 mg. (ii) Methadone hydrochloride injection, 5 mg per ml. Dose 5 to 10 mg by IM/SC injection. Adverse reactions: They are similar to those of morphine. Methadone may produce irritation on parenteral injection and its repeated administration can result in cumulative toxicity. Methadone shares the respiratory depressant action of morphine. It has been reported to cause dose dependent lengthening of QTc interval, which may precipitate cardiac arrhythmia. Acute methadone intoxication responds to naloxone. Tolerance and the withdrawal syndrome develop more slowly and are less intense. The symptoms, however, persist much longer, approximately 10 to 15 days. Codeine is often used as a substitute during treatment of methadone addiction. Therapeutic uses: It is used for the management of chronic pain. It can be used as a substitute for morphine and pethidine for relief of severe visceral pain. Its longer duration of action, analgesic potency and lack of marked hypnosis make it an useful drug in the treatment of opioid abstinence syndrome; it reduces severity of withdrawal symptoms.
As an antitussive, codeine is preferred to methadone owing to the higher addiction liability of the latter. METHADONE CONGENERS: These are levomethadyl acetate and propoxyphene. Levomethadyl acetate exhibits slow onset and prolonged duration of action, contributed partly by its active metabolite. A single dose every 72 hours is used in the long-term management of heroin addicts to prevent protracted withdrawal syndrome. d-Propoxyphene has been claimed to produce less depression of respiration, and fewer GI side effects. As an analgesic, it is only 1/25th-1/50th as potent as morphine and is half as potent as codeine. It is administered orally in the dose of 32.5-65 mg 3-4 times a day. It can cause respiratory depression and has abuse potential. It enhances the anticoagulant effect of warfarin. The drug has little advantage over codeine. MORPHINAN COMPOUNDS: The compound, Levorphanol is a more potent analgesic than morphine, is better absorbed orally and produces less constipation. It can cause drug dependence. It is administered orally or IM/SC in the dose of 2 to 3 mg. BUTORPHANOL, a morphinan congener, is a mixed agonist antagonist. It is a competitive antagonist at mu and exerts agonistic action on kappa receptors. Its agonist activity (on weight basis) is 4-7 times that of morphine and it is 20 times as potent as pentazocine. It is administered IM or IV in the dose of 2 mg, as well as intranasally. PENTAZOCINE: This benzomorphan derivative is a mixed agonist-antagonist. It has potent analgesic (agonist) action at the kappa receptors in the spinal cord and a weak opioid antagonist activity at the mu receptors. Compared to morphine: • It is half as effective as an analgesic and has a shorter duration of action • It does not cause euphoria • It has lower dependence liability • Constipation is uncommon • It causes less respiratory depression • It raises the systemic and pulmonary arterial BP with resulting increase in cardiac load; hence it is not recommended in MI. Absorption, fate and excretion: Pentazocine is given orally, rectally, SC, IM and IV. Although it is well absorbed orally, only 20% is bioavailable due to first pass metabolism. It is extensively metabolised by the liver and is excreted as glucuronide. Smokers metabolise 40% more pentazocine than non-smokers. It is available as 25 mg tablets and as 30 mg (lactate) per ml injection. The oral dose is 25100 mg every 3-4 hours. The parenteral dose (SC, IM, IV) is 30-60 mg every 3-4 hours, as necessary. Adverse reactions: These include: • CNS: Sedation, sweating, dizziness and nausea. • Psychomimetic reactions, hallucinations and unpleasant dreams. This is an important limitation to its use. • Precipitation of acute withdrawal syndrome in a morphine addict because of its antagonist action at µ receptor. • Tolerance and physical dependence have been reported, though the incidence is low. Nalorphine is valueless as an antidote to pentazocine but naloxone is useful. NALBUPHINE: This synthetic compound is chemically related to oxymorphone and the opioid antagonist naloxone. It is a mixed agonist antagonist (kappa agonist and mu
antagonist). As an agonist, it is 3-4 times more potent than pentazocine while its antagonistic property is about 10 times more than that of pentazocine. The adverse effects are similar to those of pentazocine. It probably causes fewer psychotomimetic effects and its adverse hemodynamic effects are less than those of pentazocine. It is administered SC, IM, or IV in the dose of 10-20 mg every 3-6 hours. MEPTAZINOL: This partial agonist opioid is given orally or by injection. It is 1/10th as potent as morphine as an analgesic and has a shorter duration of action. It does not cause euphoria and causes less respiratory depression than morphine. BUPRENORPHINE: Buprenorphine, a highly lipophilic semisynthetic derivative of thebaine, has mainly partial mu agonist properties. It is a weak antagonist at the kappa receptors and does not precipitate acute withdrawal symptoms in morphine addicts. Pharmacological actions: As an analgesic, it is: • More potent than morphine on a weight basis and has a longer duration of action (6 hours). • It causes less respiratory depression. • It has similar cardiovascular actions as morphine and can be used in MI. • It has much less abuse potential than morphine; • Naloxone does not precipitate withdrawal syndrome. When it is given following induction of anaesthesia with nitrous oxide and fentanyl, it reverses the anaesthetic and respiratory depressant effects of fentanyl but prolongs the analgesia. Absorption, fate and excretion: It is inactive orally because of the first pass effect and hence given sublingually, IM or IV. The drug is highly protein bound and is excreted largely unchanged in the faeces, and in smaller amount in the urine. Adverse reactions: It can cause respiratory depression similar to morphine at equianalgesic doses. However, unlike morphine, the action is not readily reversed by naloxone. Doxapram, a respiratory stimulant, may be useful. Other adverse effects are drowsiness, nausea, vomiting, constipation, miosis, bradycardia and hypotension. Preparations and dosage: Buprenorphine available as tablets 0.2 mg, as injections 0.3 mg/ml and as transdermal patches. The dose is 0.2-0.4 mg sublingually every 8 hours and 0.3-0.6 mg IM or slow IV every 6-8 hours. All partial agonists and mixed agonist/antagonists have a ceiling on their analgesic effect roughly equivalent to that of moderate doses of morphine. Non-analgesic uses of opioids: • Anti-diarrhoeal: Diphenoxylate, loperamide (Chapter 41). • Central cough suppressant: Codeine dextromethorphan (Chapter 26). • Emetic: Apomorphine (Chapter 41); and • In acute left ventricular failure: Morphine (Chapter 31).
Opioid Antagonists Drugs that antagonise the effects of morphine and other opioid analgesics act mainly by competitive antagonism. In addition, some of them also exert other actions not related to morphine receptors. They are classified as: I Pure antagonists such as Naloxone, Naltrexone. II Partial agonists of Nalorphine-type e.g. Nalorphine, Levallorphan and Cyclazocine; and III Partial agonists of the Morphine-type e.g. Propiram and Profadol. Drugs from Group III produce similar agonistic actions as morphine, which are antagonised by nalorphine or naloxone. However, they precipitate withdrawal symptoms in subjects maintained on very potent agonists such as morphine and heroin. NALOXONE: This drug, N-allyl analogue of oxymorphone (Fig. 10.3), a pure antagonist, selectively antagonises the respiratory depressant action of morphine and other opioids. By itself, it is not a respiratory depressant, analgesic or euphoriant. It is not effective orally because of its first pass metabolism in the liver. One mg of naloxone given IV completely blocks the effects of 25 mg of heroin. Its duration of action is 3-4 hours. It is almost completely metabolised in the liver.
FIG. 10.3 Morphine Naloxone
It is available in 1 ml vials containing 0.4 mg/ml and is the antagonist of choice in the treatment of opioid poisoning. It is administered as an IV bolus in the dose of 0.8 - 2.0 mg every 2-3 minutes to a total maximum dose of 10 mg. It can be administered IM or SC. The IV dose in children is 10 mcg/kg (bolus); if there is no response, inject a 100 mcg/kg (bolus). It is also used to reverse the residual respiratory depressant effects of an opioid analgesic at the end of an operative procedure. Endogenous opioid peptides, released by stress, may be responsible for hypotension observed in shock. Naloxone, IV, has been reported to correct the hypotension in septicaemic shock, though the effect is short lived. Low doses of naloxone are used to treat ADR associated with epidural opioids. Adverse reactions: Hypersensitivity reaction may occur. The others are related to withdrawal syndrome precipitation in opioid-dependent patients and include nausea, vomiting, sweating, tachycardia, hypertension, tremulousness and pain. Acute pulmonary edema has been reported in patients with heart failure. It lowers seizure threshold in patients with seizure disorder. In neonates, shrill cry or failure to take feed can be observed. NALTREXONE: This is an orally administered, long acting, pure opioid antagonist. Naloxone is too short acting and is ineffective by mouth. Naltrexone is well tolerated and has no euphoric effect, does not cause physical dependence and consistently blocks the
effects of heroin and other addictive opiates for upto four days. Naltrexone is available as 50 mg scored tablets. For treating heroin addiction, small doses (25 mg/day) are used initially, followed by 50 mg/day. For better compliance, the total weekly dose (350 mg) may be given on three days of the week (e.g.100 mg on Monday and Wednesday, and 150 mg on Friday). Since naltrexone blocks the euphoric action of opioid agonists, it is given to former addicts to prevent re-addiction. For its use in alcoholism, see Chapter 6. Naltrexone in much smaller dose (nearly in 1/10th of the dose used in addiction) is being used (not yet approved) in patients with multiple sclerosis and other degenerative diseases. It is claimed to arrest their progression. This needs confirmation. Adverse reactions: The drug can cause GI disturbances, nervousness, sleeping difficulty and muscular pain. Rarely, thrombocytopenia and liver function abnormalities may occur. Methylnaltrexone bromide is a peripherally acting mu receptor antagonist. In a dose of 8-12 mg SC, it is used to treat opioid induced constipation in patients receiving palliative care, when laxative therapy fails to produce adequate response. Nalmefene is a pure µ receptor antagonist, more potent than naltrexone. NALORPHINE (N-allyl normorphine): Nalorphine, a semi-synthetic congener of morphine. It is used mainly in the treatment of acute morphine poisoning. By itself it acts as a partial agonist, and produces analgesia and respiratory depression. When administered after morphine, it acts as competitive antagonist, reversing the effects of morphine (Fig. 10.4).
FIG. 10.4 Effect of nalorphine (NA) on morphine (M) induced respiratory depression in anaesthetised cat.
Pharmacological actions: • When administered alone, nalorphine exerts significant analgesic effect comparable to that of morphine. Many patients experience dysphoric symptoms such as anxiety, confusion, and visual hallucinations. Nalorphine shares many other pharmacological actions of morphine. Thus, it has spasmogenic activity, antitussive effect, and miosis. Large doses also induce respiratory depression. However, it does not produce drug dependence. • Administered after morphine, nalorphine promptly abolishes the effects produced by morphine. Nalorphine is also a potent antagonist of codeine, heroin and synthetic morphine substitutes. However, it is much less effective against pethidine than against the other opioid analgesics.
• Administered to a morphine addict in the dose of 1 to 3 mg., nalorphine precipitates the typical morphine withdrawal syndrome within 3 to 15 minutes, reaching a peak by 45 minutes and lasting for 2 hours. Another test to diagnose an addict is to observe the pupils. In normal individuals nalorphine produces miosis but in an addict, it either dilates the pupils or fails to produce any demonstrable effect. Nalorphine precipitates withdrawal syndrome in patients addicted to heroin and methadone. Absorption, fate and excretion: Given orally, it is absorbed poorly, but more rapidly than morphine on SC administration. The drug is metabolised in the liver by conjugation. Preparations and dosage: Nalorphine injection 10 mg per ml, administered SC or IV in the dose of 3 to 10 mg. The dose may be repeated to a total of 40 mg. Therapeutic uses: • Acute poisoning due to morphine and related compounds. • Diagnosis of morphine addiction. • Nalorphine has been administered to morphine addicts along with morphine. Withdrawal of the mixture produces a milder withdrawal syndrome than that observed after withdrawal of morphine alone. Levallorphan: This opioid antagonist is similar to but more potent than nalorphine. It is given IV in the dose of 0.2 mg. It generally fails to reverse pethidine induced respiratory depression. ALVIMOPAN: This drug, a selective opioid-mu receptor antagonist, is given orally. It is not much absorbed and blocks the GI effects of opioids by binding to GI opioid-mu receptors, without blocking the central analgesic action of opioids. It accelerates return of GI motility and reduces risk of paralytic ileus. It has been used for treating post-operative ileus after bowel resection. It is given 30 min–5 hours before surgery and continued for 7 days. It is not likely to be useful, unless started before paralytic ileus develops. It also helps in achieving early recovery of opioid induced bowel dysfunction. It is usually well tolerated.
11
Analgesic-Antipyretics and Nonsteroidal Anti-inflammatory Drugs (NSAIDs) In contrast to the opioid analgesics, the non-opioid analgesics as a group : • Relieve pain without interacting with opioid receptors. • Reduce elevated body temperature (antipyretic effect). • Possess anti-inflammatory property and are known as Non Steroidal Anti-inflammatory Drugs (NSAID). • Have antiplatelet activity to varying degrees. • Do not cause sedation and sleep; and • Are non-addicting. In the last chapter, pain pathophysiology has been outlined. The temperature regulation and inflammatory responses are described below. Temperature regulation: The hypothalamus controls the body temperature by two mechanisms: (i) Cutaneous vasodilatation; and (ii) Increase in sweat gland activity through sympathetic cholinergic fibres. Normally, 60% of the body heat loss occurs by radiation, 20% by evaporation of water and the rest by convection and conduction. The commonest manifestation of a change in the core temperature is ‘fever ’. Although the body surface temperature is ordinarily measured in clinical practice, it is the body core temperature which is physiologically important. The rectal temperature (which reflects core temperature closely) is about 0.60 C (100 F) higher than oral temperature and about 1.40 C (2.50 F) higher than axillary temperature. The generally accepted normal limits of rectal temperature in adults are 36.10 C and 37.80 C (970 F and 1000 F); the body temperature is higher in infants. If the core temperature rises by more than a few degrees in man, mental changes occur. It is well known that an individual with high fever is often confused and delirious. The working of many tissue enzymes is adversely affected and hyperpyrexia (core temperature 410 C or 1060 F) may result in death. However, core temperature below 40.50 C (1050 F) is generally well tolerated by most individuals. The increase in temperature is brought about by the hypothalamus which reduces heat loss by peripheral vasoconstriction and increases heat production by inducing shivering. It then regulates the temperature around the new setting. Bacterial liposaccharides (LPS) activate the mononuclear phagocytes to release interleukin-1 (IL-1) and tumour necrosis factor (TNF-α). These then act on the vascular endothelial cells in the hypothalamus and stimulate the local synthesis of prostaglandins, PGE2 which causes fever and anorexia. The PG inhibitors reduce fever by inhibiting the PG synthesis. Thus, IL-1 and TNF-α are now accepted as the endogenous pyrogens. IL-1 and TNF-α along with other cytokines, also play a major role in the manifestations of inflammation. The role of fever in the defence reaction is not clear, though increased destruction of T. pallidum which causes syphilis, at high temperature, has been reported. In practice, as with pain, relief from fever with drugs adds to the comfort of the patient. It also impresses the
patient and the relatives favourably about the therapeutic capability of the doctor! Finally fever (as caused by infections, inflammatory disorders, malignancies, tissue infarction and trauma) must be distinguished from heat illnesses (as due to malignant hyperthermia, heat stroke, atropine overdose and the use of street drugs). Fever is due to an upward resetting of the hypothalamic thermostat; the core temperature rarely exceeds 410 C (1060 F); and the drugs described in this chapter are effective in lowering the body temperature. On the other hand, heat illness is due to failure of the thermoregulatory mechanisms; the body temperature rises to 1060 F or higher (hyperpyrexia) and does not respond to the antipyretic drugs. Heat illness could be fatal and needs urgent lowering of the body temperature with external cooling including bath with cool water and use of fans. Inflammation: It is a complex process which acts as a body defense against invading foreign agents. It helps to protect, repair and remodel tissues and involves innate immune components with multiple effectors such as leucocytes, mast cells, macrophages and locally produced cytokines. It comprises systemic response (involving nervous and hormonal adjustments, and proliferation of the lymphoreticular system); and local response (pain, redness, warmth and swelling). These inflammatory responses are usually beneficial but often they cause functional impairment requiring drug therapy to prevent/suppress tissue damage and chronicity. The three important aspects of inflammation that can be readily measured are: (i) Erythema (local vasodilatation), (ii) Edema (increased capillary permeability) (iii) Formation of granulation tissue. Compounds claimed to possess anti-inflammatory activity can be evaluated by their ability to reduce these phenomena in experimentally induced inflammation or by testing their anti-inflammatory activity in experimental arthritis. Experimental evaluation of anti-inflammatory activity: The commonly employed methods are: • Erythema assays: Irradiation of the shaven back skin of a guinea pig with ultra violet light causes erythema. Erythema can also be produced in human beings with certain specific irritants like tetrahydrofuryl nicotinate. The anti-inflammatory property of a new agent is assayed by comparing its ability to reduce the erythema with that of a known anti-inflammatory drug. • Edema assays: Anti-inflammatory activity of a drug can also be measured by noting the reduction in edema produced by the local injection of substances like formaldehyde, carrageenan, histamine, dextran and ovalbumin. A modification involves the measurement of leakage of a protein bound marker (Evans blue, 131I) from the circulation into the tissues. • Granuloma assays: The ‘Cotton wool pellet’ and the ‘granuloma pouch’ are the most commonly used methods. The former involves SC implantation of weighed cotton wool pellets, impregnated with a ‘foreign’ material like carrageenin, in rat. This causes localised inflammation. The animals are sacrificed after the drug treatment; the cotton pellets, now encapsulated and heavily infiltrated with connective tissue are removed, dried, weighed and compared with those in control animals not given the drug. In the granuloma pouch assay, an irritant like croton oil diluted with cotton seed oil or air is injected SC in the rat, usually on its back. After drug treatment the animal is
sacrificed, the pouch is dissected, its exudate content is measured and compared with that in control animals. • Experimental arthritis assays: Poly-arthritis induced in rats by injection of dead tubercle bacilli suspended in liquid paraffin (Freund’s mycobacterial adjuvant) is a frequently used method for measurement of anti-inflammatory activity. Kaolin and talc have also been injected directly into the joints of rats and pigeons to induce arthritis. • Miscellaneous: Localised inflammatory reaction can be produced in rats by intrapleural injection of turpentine or by intraperitoneal injection of acetic acid. Ability of the new agent to suppress acute inflammatory reaction to albumin or horse serum in animals previously sensitised to these antigens (Arthus reaction) can also be studied. The inflamed paw technique and the adjuvant arthritis model (both in rats) are the most successful methods of predicting anti-inflammatory activity in man. Paw inflammation and edema are produced by intra-plantar injection of napthoylheparamine or carrageenan. Classification of NSAID: I Non-selective COX inhibitors: • Salicylates and their congeners. • Para-aminophenol derivatives e.g., Paracetamol. • Pyrazolone derivatives e.g., Phenylbutazone and Oxyphenbutazone. • Indoles and related drugs: Indomethacin, Sulindac. • Heterocyclic arylacetic acid derivatives: Diclofenac, Tolmetin, Ketorolac. • Propionic acid derivatives: Ibuprofen, Fenoprofen, Naproxen, Ketoprofen and Pirprofen. • Fenamates, e.g., Flufenamic acid and Mefenamic acid. • Oxicams, e.g., Piroxicam. II Preferential COX-2 inhibitors: Nimesulide, Nabumetone, Etodolac, Meloxicam. III Selective COX-2 inhibitor: Celecoxib General properties of NSAID are given in Table 11.1. Table 11.1 General properties of NSAID
Mechanism of analgesic-antipyretic action: Though these drugs have different chemical structures, they produce qualitatively similar analgesic, antipyretic and anti-inflammatory effects. During inflammation, pain or fever, arachidonic acid (AA) is liberated from phospholipid fraction of the cell membrane by phospholipase A2. Arachidonic acid is then converted by cyclo-oxygenase (COX-1 and 2) to prostaglandins (PGs). The steps are: (i) Oxidation of AA to the endoperoxide PGG2; (ii) Its subsequent reduction to the hydroxy-endoperoxide PGH2; and (iii) Transformation of PGH2 into the primary prostanoids PGE2, PGF2, PGD2, PGI 2 and TXA2 (Chapter 25). PGs sensitise blood vessels to the effects of other inflammatory mediators that increase permeability. PGs particularly PGE2 and PGI 2 produce hyperalgesia associated with
inflammation. They sensitise the chemical receptors of the afferent pain endings to other mediators such as bradykinin and histamine. Further, release of PGs in the CNS may lower the threshold of the central pain circuits. Intravenous infusion of PGs causes headache and vascular pain; PGs are also involved in the pyretic response in man. COX-1 and COX-2 both, use the same endogenous substrate AA and form the same products by the same catalytic mechanism. Their major difference lies in the pathophysiological functions: • COX-1 activity is constitutively present in nearly all cell types at a constant level and is involved in tissue homeostasis; whereas • COX-2 activity is normally absent from cells (except those of kidneys and brain) but is inducible by bacterial liposaccharides, IL-1 and TNF-α in activated leucocytes and other inflammatory cells. Thus, COX-1 is physiological while COX-2 is usually (but not always) pathological. Aspirin and aspirin like NSAIDs act by inhibiting COX-1 and 2 and thus blocking synthesis of PGs. A similar mechanism also explains some of their adverse effects e.g. nephrotoxicity (see later). They are effective as analgesics only in pathological states where PGs are synthesised locally. They are not effective in sharp ‘stabbing’ pain caused by direct stimulation of sensory nerves. The quantitative differences in the actions of different PG inhibitors and their propensity to cause various adverse reactions may be explained by the differences in the sensitivities of COX in different tissues to the various NSAID. Thus, piroxicam and indomethacin are 10-40 fold selective for COX-1 whereas nabumetone is 15 fold selective for COX-2. Propionic acid derivatives (e.g. ibuprofen), fenamates, and aspirin inhibit both COX-1 and COX-2 equally. In addition to its conversion to PGs via the cyclo-oxygenase pathway, arachidonic acid is converted via lipo-oxygenase pathway to leukotrienes (Chapter 25). Most NSAID do not inhibit the production of leukotrienes; in fact, by blocking the synthesis of PGs, they may make more AA available for synthesis of leukotrienes. This might explain the symptoms of bronchospasm in some subjects following the ingestion of aspirin and other NSAID. Although inhibition of PG biosynthesis can explain many effects of NSAID, other mechanisms may also play an important role. Thus, indomethacin inhibits phosphodiesterase and increases the intra-cellular concentration of cAMP. Cyclic AMP has been shown to stabilize membranes, including lysosomal membranes in polymorphs. This prevents the release of enzymes important in the inflammatory response. Further, the antiinflammatory drugs which are weak PG inhibitors inhibit the activation of T-lymphocytes which, are abundant in the inflamed tissues and release pro-inflammatory cytokines (Chapter 25). Aspirin has both the properties: (i) PG synthesis inhibition and (ii) inhibition of T-lymphocyte activation. Diclofenac and indomethacin also inhibit the lipooxygenase pathway, thus decreasing the production of leukotrienes by the leucocytes and the synovial cells. NSAID may also unmask T cell suppressing activity, thereby suppressing the production of rheumatoid factors.
Salicylates Salicylates are esters of salicylic acid e.g. methyl salicylate and sodium salicylate; or alternatively, also occur as salicylate esters of organic acids such as acetyl salicylic acid (aspirin), the most commonly used salicylate (Fig. 11.1) Birth of aspirin stems from the original observation by Rev Edmund Stone regarding the antipyretic and analgesic properties of the bark of the willow tree. This subsequently led to the isolation of its active principle salicin and later salicylic acid. Aspirin was synthesised by Felix Hoffman in 1897 and is still being used in therapeutics after more than a 100 years!
FIG. 11.1 Aspirin Sodium Salicylate
Pharmacological actions of salicylates: Local actions: Salicylic acid and methyl salicylate are irritants; salicylic acid also has keratolytic, antiseptic and fungistatic actions (Chapters 62 and 71). The salts of salicylic acid do not irritate the unbroken skin but when ingested, may release free salicylic acid in the stomach causing local irritation. Central nervous system: • Analgesia: Salicylates, unlike the opioid analgesics, produce relief of pain without hypnosis or impairment of mental activity. Their analgesic action is mainly peripheral and only partly central. They do not affect the emotional reaction to pain. Therapeutically it may be rational to combine aspirin with opioid analgesic like codeine for a synergistic analgesic effect. Aspirin inhibits the biosynthesis of PGs by irreversible acetylation and inactivation of COX in contrast to other NSAID which cause its reversible inhibition. They are mainly useful for relieving (1) dull aching, throbbing pain of low intensity arising from integumental structures such as muscles and joints; (2) dysmenorrhoea; and (3) toothache. They are not useful in either deafferentiation pain or in visceral pain. In smaller doses, salicylates exert mainly analgesic action. With larger doses, they exert anti-inflammatory activity as well and relieve vascular congestion and edema. Toxic doses produce stimulation of the CNS followed by depression (see later). • Antipyretic action: Salicylates act centrally and reset thermostatic mechanism to the normal level and thereby reduce the temperature; they do not lower the body temperature in a normal individual. Salicylates and other NSAID act by inhibiting brain PG synthesis and release. They do not reduce heat production but increase dissipation of heat mainly by producing cutaneous vasodilatation. Accompanying sweating assists the reduction of body temperature. They, however, do not affect the pathological process responsible for fever. • Respiratory stimulation: Salicylates stimulate respiration as a result of direct and indirect actions. Therapeutic doses of salicylates:
(i) Increase the consumption of oxygen primarily by the skeletal muscles by acting on the mitochondria (see later); the resultant increased production of carbon dioxide directly stimulates the respiratory centre. (ii) Stimulate the medullary respiratory centre directly. (iii) Stimulate the chemoreceptors. This causes hyperventilation, and washing out of the plasma carbon dioxide, resulting in respiratory alkalosis. A plasma level of 35 mg % of salicylates is usually associated with hyperventilation and severe dyspnoea occurs when the level approaches 50 mg %. Acid-base disturbances and hypokalemia: The mild respiratory alkalosis produced by therapeutic doses of salicylates is countered by excretion of alkaline urine containing bicarbonate along with sodium and potassium. This is the stage of compensated respiratory alkalosis. Hypokalemia as a result of urinary loss of potassium is accompanied by water loss through lungs due to hyperventilation, through skin via augmented sweating and through urine as a result of alkalosis. This may lead to dehydration and hypernatremia. With acute toxic doses of salicylates, hypokalemia is aggravated, the respiratory centre is depressed and metabolic acidosis develops (see later). Gastrointestinal system: Depending on the dose, salicylates may produce • Dyspepsia, nausea and vomiting as a result of gastric irritation; and • Gastrointestinal erosions and bleeding, leading to hematemesis or melena. The acid pH of the stomach favours the existence of salicylate in non-ionised form. The non-ionised form is, however, relatively water insoluble; hence, it tends to adhere to the gastric mucosa thereby producing irritation. Further, local absorption into the mucous cell causes inhibition of local COX-1 leading to decreased PGE2 and PGI 2 synthesis, thus, causing a loss of the protective effect of PGE on the stomach. Salicylates also reduce the gastric motility and prolong the gastric emptying time. These effects increase the period of contact of salicylate with the gastric mucosa. Alkalies induce ionisation of salicylates and thereby reduce their gastric absorption and local irritant effect. As the ionised salicylate is more water soluble, it leaves the stomach more quickly. Thus, to avoid gastric irritation, salicylates are administered: • With plenty of water, after food • With milk • As soluble or buffered aspirin • With an alkali such as sodium bicarbonate; or • With misoprostol, a PGE1 analogue (Chapter 43). Anti-inflammatory and anti-rheumatic effect: Salicylates suppress the clinical signs and improve the clinical picture in acute rheumatic fever and rheumatoid arthritis. Salicylates and other NSAID reduce the ‘inflammatory component’ of these diseases by: • Inhibiting PG synthesis in the peripheral tissues. • Reducing the capillary permeability, thereby minimising the exudation of fluid and development of inflammatory edema. • Inhibition of neutrophil aggregation and activation. During inflammation neutrophils release proteases, leukotrienes and cytokines (Chapter 25) and injure the tissues. Prostaglandins present in the inflammatory exudate are potent vasodilators and can cause edema, erythema and pain. Aspirin-like drugs, by inhibiting the synthesis of PG,
prevent sensitisation of the pain receptors to agents such as histamine, 5-HT and bradykinin, the known chemical mediators of pain and inflammation. The kinins (e.g. bradykinin) are formed from kininogen by the action of kallikrein. Aspirin inhibits the formation of activated kallikrein from inactive plasma and leucocytic kallikrein. The acidic mucopolysaccharides such as hyaluronic acid, chondroitin and mucoitin sulfuric acid constitute the ground substances of the extracellular matrix. Salicylates and other NSAID inhibit the mucopolysaccharide biosynthesis and may thereby, reduce edema and tissue swelling. Immunological actions: Salicylates suppress a variety of antigen-antibody reactions in vivo including systemic anaphylaxis induced by egg-white challenge in rabbits, allergic encephalomyelitis in guinea pigs and serum sickness in man. They prevent the release of histamine as a result of antigen-antibody reaction in vitro. Further, they may inhibit the CMI. However, the amounts of NSAID required to produce such effects are large; and their contributions to the clinical antirheumatic activity is not clear. Antiplatelet activity: Aspirin inhibits platelet aggregation. It is unique in that it irreversibly inhibits COX by acetylation. Platelets play an important role in thrombus formation. Aspirin, by inhibiting COX suppresses the synthesis of thromboxane A2 (TXA2) in the platelets. Platelets, being non-nucleated are unable to regenerate the enzyme, which explains the prolonged action of aspirin. Thus, daily doses of 75-150 mg almost completely suppress the synthesis of TXA2 for 7-10 days (Chapter 33). Other NSAIDs inhibit the enzyme reversibly so that the platelet function is restored when the drug is eliminated. Salicylates do not affect the normal leucocyte count. However, they reduce the leucocytosis and lower the high ESR observed in acute rheumatic fever. The latter effect is due to a reduction in the plasma fibrinogen content. Hepatic and renal effects: Salicylates in therapeutic doses do not modify hepatic and renal functions significantly. They can, however, affect renal function in compromised kidneys by inhibiting COX-1. Salicylates increase the secretion of bile by stimulation of the hepatic parenchyma (choleretic action) but reduce the total concentration of cholates. Large doses, particularly in children, can cause hepatic damage and even necrosis. Urate present in the glomerular filtrate is reabsorbed by the proximal tubules of the kidney and the main excretion of urate in urine occurs due to its secretion by the distal tubule. Aspirin exerts biphasic action on the excretion of urate. • In small doses (1-2 g per day), aspirin interferes with urate secretion by the distal tubule, thereby elevating the plasma urate level, and block the action of other uricosuric drugs such as probenecid. • Large doses (over 5 g per day) inhibit the reabsorption of urate by the proximal tubule, which can cause uricosuria. Such doses, however, invariably result in adverse effects. Cardiovascular system: Therapeutic doses of aspirin do not produce any deleterious effects on the CVS. However, NSAID users may show some rise in BP due to retention of sodium and water. Endocrine effects: Salicylates interfere with the binding of thyroid hormones to their binding proteins, especially thyroxine binding albumin. This comes in the way of interpretation of serum thyroxine and tri-iodothyronine values. Metabolic effects: Salicylates uncouple oxidative phosphorylation (Chapter 64). The energy derived from oxidation is converted into heat. Toxic doses of salicylates may, thus,
lead to hyperpyrexia, increased protein catabolism, aminoaciduria and a negative nitrogen balance. In certain diabetic individuals, salicylates may reduce the blood sugar level and glycosuria, probably due to an enhanced peripheral utilisation of glucose and inhibition of neoglucogenesis. However, large doses cause hyperglycemia and glycosuria in normal individuals. Salicylates reduce lipogenesis and at the same time inhibit adrenaline induced lipolysis in the fat cells. Toxic doses lead to formation of ketone bodies. Absorption, fate and excretion: Both salicylic acid and methyl salicylate are absorbed from intact skin, especially when applied in alcohol, petrolatum, lard or lanolin base and systemic poisoning in children has been reported following such local applications. Salicylates are absorbed from the stomach and largely from the upper small intestine. Factors such as particle size, pH of the GI tract, solubility of the salicylate preparation and presence of food in the stomach modify the absorption. Sodium salicylate in a single therapeutic dose is absorbed within 30 minutes, peak plasma level is achieved within 2 hours; approximately 50% of the dose is eliminated in urine within 24 hours. After absorption, approximately 80% of the salicylate (but only 50% of aspirin) is bound to plasma proteins, mainly albumin. It is rapidly distributed in most of the tissues and achieves a significant concentration in the saliva, milk, spinal, synovial and peritoneal fluids and in the erythrocytes. High salicylate concentrations are observed in the liver, heart, muscle and smaller amounts in the brain. Aspirin, even though absorbed as such, is subject to rapid metabolism (50-60%) to salicylate by deacetylation during first pass and is further hydrolysed in the blood and tissues to salicylic acid (t½ of 15 minutes). Like phenytoin, aspirin exhibits dose dependent pharmacokinetics: • At lower dose levels (300-600 mg individual doses), the plasma level increases in proportion to the dose (first order kinetics). • At higher dose levels (1-2 g individual doses), the increase in the plasma level is disproportionate and severe toxicity can occur (zero order kinetics). Salicylates are mainly excreted in urine in the form of conjugates with glycine and glucuronic acid. A small portion is oxidised to gentisic acid and excreted in the urine. An alkaline urine, by ionizing the salicylate to a water soluble and indiffusible form, prevents salicylate back-diffusion in the distal tubule and enhances its excretion. Its excretion is reduced by probenecid, oliguria and kidney failure and is augmented by diuresis and alkaline pH. Adverse reactions: • Allergic or pseudoallergic reactions: These include skin rashes, urticaria, pruritus, angioneurotic edema, bronchospasm, anaphylaxis-like shock or thrombocytopenic purpura. Angioedema and anaphylactoid reaction respond to adrenaline. Aspirin can induce idiosyncratic, mild haemolysis in individuals with G6PD deficiency. Salicylic acid and its derivatives are ingredients of a large number of substances including fruits like apples, grapes, oranges, peaches and plums, soaps containing oil of wintergreen, perfumes, beverages (especially birch beer), tooth pastes, gum and lozenges. Individuals with idiosyncrasy to salicylates should also be warned against taking proprietary drug mixtures, which often contain salicylates.
• Gastrointestinal tract: The commonest ADR of all NSAID are dyspepsia, nausea, vomiting, heartburn and ulceration. In subjects taking 3-4 g of aspirin daily, the blood loss may be about 3-6 ml per day. Occasionally, aspirin can cause haematemesis. Prolonged therapy even with low dose aspirin can also lead to anemia. The incidence of major GI bleeding due to aspirin is estimated at 15 per 100,000 chronic aspirin users per year. Gastric bleeding is due to (a) local mucosal action; (b) inhibition of platelets; and (c) hypoprothrombinemia. The mucosal damage occurs in stomach (gastropathy) as well as in small intestine (enteropathy). NSAID induced enteropathy is mostly subclinical or may cause vague symptoms. PPI which block the gastric acid secretion prevent the gastropathy but are not useful in enteropathy. There is no drug that can prevent NSAID induced enteropathy; use of probiotics could be helpful. • Haemopoietic system: Salicylates but not the newer NSAID, in large doses, reduce the plasma prothrombin level by interfering with action of vitamin K in the liver. Salicylateinduced hypoprothrombinemia can be reversed by administration of vitamin K. Salicylates-exert a synergistic effect with the coumarin anticoagulants and hence, these drugs should be avoided in patients receiving oral anticoagulants. Cautious use of salicylates is also indicated in hepatic damage. • Kidneys: See later. • Reye’s syndrome: This serious and often fatal complication occurs a few days after a viral infection, especially influenza, in children below the age 12 years. There occurs anicteric liver dysfunction due to hepatic mitochondrial injury and a consequent metabolic encephalopathy. There is epidemiological evidence to associate administration of aspirin during the initial viral infection and the subsequent occurrence of this serious disease. Aspirin should, therefore, be avoided in infants and in children < 12 years, unless specifically indicated e.g. for juvenile rheumatoid arthritis. • Pregnant women and infants: Taken at term, aspirin by inhibiting PG synthesis in the uterus may delay the onset of labour and cause greater blood loss at delivery. The administration of aspirin or indomethacin at term has been reported to cause premature closure of the ductus arteriosus with resultant serious pulmonary hypertension in the newborn. Salicylates readily cross the placental barrier, and may prove toxic to the newborn. The toxic manifestations in newborn include hyperpnoea and haemorrhages. Hypoglycemia after prolonged salicylate therapy has also been reported in infants. Hence repeated use of NSAID in pregnant women should be avoided. Low dose aspirin is probably safe. • Salicylism: High doses of salicylates may produce a condition of mild salicylate intoxication termed salicylism. The syndrome usually develops when the plasma salicylate level exceeds 25 mg% and is characterised by headache, dizziness, vertigo, tinnitus, difficulty in hearing and dimness of vision; drowsiness, lethargy and mental confusion, nausea, vomiting and diarrhoea may also occur. These symptoms may be associated with tachypnoea and respiratory alkalosis. It is reversible on cessation of therapy. • Acute salicylate intoxication: Acute salicylate intoxication may be due to overzealous therapy in infants or following an accidental ingestion. A serum salicylate level of 50 mg% indicates mild toxicity; levels above 75 mg% are potentially fatal. The characteristic features of acute intoxication are acid-base and electrolyte disturbances, hyperglycemia,
dehydration, hyperpyrexia, GI irritation and occasional haemorrhages. The stimulation of CNS causes restlessness, vertigo, tremor, apprehension, hallucinations and convulsions. With toxic doses, the respiratory centre is depressed resulting in CO2 accumulation and true metabolic acidosis. Salicylates induce derangement of the carbohydrate metabolism and cause accumulation of pyruvic, lactic and acetoacetic acids. Table 11.2 gives the principles of management of salicylate poisoning. Correction of acidosis and urinary alkalinisation should be carried out cautiously using normal saline containing 2% dextrose and 2% sodium bicarbonate, at the rate of 2 litres/hr, with frequent determination of blood pH and plasma bicarbonate, to prevent metabolic alkalosis. Table 11.2 Principles of management of salicylate poisoning
Sedatives like barbiturates are dangerous when the patients show excitement with salicylate intoxication. These agents, by producing respiratory depression, may aggravate metabolic acidosis and precipitate coma. Preparations and dosage: (i) Acetyl salicylic acid (Aspirin 300 mg tablets). Dose: 0.3 to 0.6 g 4-6 hourly (maximum 4 g/day) orally, for relief of integumental pain; in the treatment of acute rheumatic fever, 4 to 8 g daily in divided doses. (ii) Soluble aspirin tablet contains aspirin (300 mg), citric acid (30 mg) with calcium carbonate (100 mg) and saccharin sodium (3 mg). When mixed with water, citric acid reacts with calcium carbonate to form calcium citrate solution and this dissolves aspirin forming calcium acetyl salicylate. (iii) Buffered aspirin tablets contain aspirin and an antacid like magnesium hydroxide, aluminium hydroxide or aluminum glycinate. (iv) Sodium salicylate has a characteristic sweetish, saline, unpleasant taste and is soluble in water. It is administered in a mixture form with alkali. It is available as 500 mg tab (Succisalyl forte containing sodium salicylate 500 mg and sodium succinate 300 mg per tab). Dose: for integumental pain 0.5 to 2 g, for acute rheumatic fever, 5 to 10g, daily in divided doses. (v) Methyl salicylate (oil of Wintergreen): liniment 25% v/v in peanut oil and, methyl salicylate ointment 50% in white bees wax and hydrous wool fat. (vi) Salicylic acid: ointment contains 2-6% salicylic acid w/w. Whitfield’s ointment contains 6% benzoic acid and 3% salicylic acid. (vii) Lysine acetyl salicylate: for IV infusion. Therapeutic uses: • Local application: Salicylic acid is used for its keratolytic, fungistatic and mild antiseptic activity (Chapter 71). Methyl salicylate is used as a counter irritant. For local use of 5-
aminosalicylic acid in the treatment of inflammatory bowel disease, see Chapter 45. • As analgesic-antipyretic: Salicylates are beneficial in a variety of conditions such as arthralgias, myalgias, neuralgias, toothache, headache, backache and dysmenorrhoea. For analgesia, combination of aspirin with an opioid analgesic like codeine. is synergistic. In single doses of 300-1200 mg, aspirin shows graded responses. Doses higher than 1200 mg however, simply increase the risk of toxicity and should be avoided. Aspirin and other NSAIDs are valuable in primary dysmenorrhoea. Treatment is started on the first day of the menstrual period and is continued for 2-3 days. They are also very useful as antipyretics in the symptomatic treatment of fever. • As anti-inflammatory: NSAID are used to treat inflammatory conditions such as arthritis and fibromyositis; they diminish but does not arrest the inflammatory response. • As antirheumatic: Salicylates, in a sufficiently large dose, produce within 24 to 48 hours dramatic relief of pain and inflammation in acute rheumatic fever. There is a significant reduction in swelling, immobility, heat and redness of the joints involved. The fever is reduced, the pulse rate slows down and further joint involvement is prevented. Salicylates, however, cannot prevent or reverse the cardiac complications, chorea, subcutaneous nodules or encephalopathy and fail to shorten the duration of the disease. In SLE, aspirin may ameliorate arthritis and serositis but the vasculitic component caused by immune complex deposition is not affected. Aspirin, being a better analgesic, is preferred to sodium salicylate. The adult dose is 4 to 8 g daily given at intervals, in 1 g dose. For children the recommended daily dose is 120 mg per kg of body weight per day (with a maximum of 8 gm) given in 4-6 divided doses. A plasma salicylate level of 25 to 40 mg% usually achieves adequate control. Full doses are continued for at least 2 weeks after disappearance of symptoms and signs of inflammation, and the drug is then gradually discontinued over a period of 7 to 10 days. Sudden salicylate discontinuation may produce a relapse. Large doses of salicylates in patients with rheumatic carditis may increase the plasma volume, cardiac output and metabolic rate, which can precipitate heart failure. Naproxen can also be administered in the dose of 10-20 mg/kg/day. Glucocorticoids (prednisolone 1-2 mg/kg/day) are useful in cases not responding to salicylates and appear to be more effective in the severely ill patients with high fever, rheumatic pericarditis and concomitant congestive cardiac failure or cardiac arrhythmias. The incidence of relapse after stoppage of corticosteroid therapy is relatively high. Some authorities recommend combined aspirin and glucocorticoid therapy. Concurrent penicillin therapy is recommended for eradication of streptococcal infection. This is discussed in Chapter 46. For use of aspirin in RA, see Chapter 75. • As antiplatelet agent: Aspirin is used to prevent platelet aggregation. It blocks COX activity, inhibiting the platelets for 8-10 days after a single dose of 75-100 mg (Chapter 33). • Miscellaneous: (a) PGs have been implicated in the maintenance of patency of ductus arteriosus. Indomethacin has been used for the closure of persistent patent ductus in neonates. (b) Bartter syndrome: This rare condition associated with hypokalemia and increased
plasma renin and aldosterone levels can be treated with aspirin successfully. (c) Prophylactic doses of aspirin (600-900 mg), indomethacin or ibuprofen have been shown to prevent symptoms of food intolerance in patients who showed acute GI symptoms after eating specific foodstuffs; it may exert the beneficial effect in radiation induced and other diarrhoeas. These symptoms are probably mediated through PG release. (d) NSAID have been used locally in the treatment of ocular inflammation (see later). (e) PGD2 release from the mast cells in the tissues causes vasodilatation and hypotension in patients with mastocytosis. Addition of an NSAID to antihistaminic for the treatment of this condition gives better results. (f) For prolongation of gestation, see Chapter 44. An inverse correlation has recently been reported between the chronic use of aspirin/NSAID and colorectal cancer. This is probably due to inhibition of COX-2 which is upregulated in this condition. Diflunisal is a non-acetylated difluorinated salicylate which has analgesic and antiinflam-matory properties with a weak antipyretic and antiplatelet activity. It has better tolerability and a longer duration of action than aspirin.
Para-Aminophenol Derivatives The commonly used drug is paracetamol (Fig. 11.2).
FIG 11.2 Paracetamol
PARACETAMOL: This compound exerts analgesic and antipyretic effects like salicylates. It has weak activity on COX in the inflamed peripheral tissues which have high concentration of peroxides; however, it equals the blocking effect of aspirin on this enzyme in the brain. Therefore, paracetamol is a potent antipyretic and is equianalgesic with aspirin in therapeutic doses but devoid of significant anti-inflammatory effect. Compared to salicylates, it does not produce GI irritation, acid-base imbalance nor does it affect platelet activity. Absorption, fate and excretion: It is rapidly absorbed orally and peak plasma levels are reached within ½ to 1 hour. It is metabolised in the liver and excreted in urine as conjugation products of glucuronic and sulfuric acids. The ability of infant liver for glucuronidation of paracetamol is poor and this may enhance its toxicity. Adverse reactions: At recommended therapeutic doses (500-1000 mg) in healthy subjects, paracetamol is generally well tolerated and causes minimal adverse effects. • Hepatic and renal toxicity: Large doses (>6g) of paracetamol as in acute poisoning produce extensive hepatocellular damage and renal tubular necrosis, and death. The liver and renal toxicity is due to the metabolite N-acetyl-P-benzoquinoneimiene which is normally turned harmless by conjugation with glutathione. Depletion of hepatic and renal glutathione potentiates its toxicity whereas treatment with sulfhydryl compounds such as cysteamine, l-methionine and N-acetyl cysteine (NAC) is beneficial. In acute poisoning, NAC is administered by infusion initially, in the dose of 150 mg/kg in 15 minutes, and then 50-100 mg/kg slowly to total maximum of 300 mg/kg in 20 hours. It can be combined with methionine. NAC has also been used orally. Prior liver damage as in chronic alcoholics may increase the liability to hepatotoxicity. • Paracetamol may cause skin reactions, fever, neutropenia, thrombocytopenia and nephropathy. • It may produce anemia as a result of haemolysis in individuals with G6PD deficiency. Preparation and dosage: (1) Paracetamol 500 mg tablets. Dose: 250 to 500 mg. The total daily dose should not exceed 4 g in adults. It can be used in a liquid dosage form in children. (2) Injection paracetamol IM and IV. Therapeutic uses: It is preferred to aspirin for mild pain, for fever in children and for treating osteoarthritic pain in elderly. Pyrazolone Derivatives These are: • Phenylbutazone and oxyphenbutazone
• Other drugs like metamizole sodium or dipyrone. PHENYLBUTAZONE: It is a potent antiinflammatory drug. Its anti-inflammatory activity exceeds that of salicylates. However, the drug is poorly tolerated by patients and causes various GI, hepatic, renal and fatal hematologic adverse effects. It gives rise to various drug interactions. Hence it is now rarely used. OXYPHENBUTAZONE: This metabolic degradation product of phenylbutazone, is claimed to cause less gastric irritation than phenylbutazone. It shares all the toxic effects of phenylbutazone. It is sometimes used in the symptomatic treatment of ankylosing spondylitis. Metamizole/Dipyrone: This potent analgesic and antipyretic injectable NSAID can cause fatal agranulocytosis and hence is banned.
Indoles and Related Drugs INDOMETHACIN: This indole acetic acid derivative (Fig. 11.3) is a potent analgesic, antipyretic and anti-inflammatory agent.
FIG. 11.3 Indomethacin
Pharmacological actions: In patients with RA with swollen joints, it brings about a quick relief of pain and reduction in the joint swellings. However, it is not superior to aspirin. Indomethacin is particularly useful in the treatment of acute attacks of gout, where it relieves pain within 2 hours. It also acts as an analgesic even in the absence of clinically obvious inflammation e.g., ankylosing spondylitis. Absorption, fate and excretion: Given orally, it is absorbed rapidly and almost completely, with a peak plasma concentration within 1 to 2 hours. It is mainly metabolised by the liver, and is rapidly eliminated by the kidneys as glucuronide. Nearly 50-90% of a single dose is excreted in urine within 24 hours. Its action is more prolonged than is suggested by its t½ (2 hours). Adverse reactions: The reported incidence of ADR has ranged from 15-20%, even with low doses. Headache is most common, followed by giddiness, mental confusion, blurring of vision, depression and psychotic disturbances. Some of these effects would make it dangerous for the patient to drive a vehicle. Such neuropsychiatric adverse reactions are more frequent with indomethacin than with other NSAIDs. Less common adverse effects are nausea, vomiting, dyspepsia, diarrhoea, skin rashes and rarely blood dyscrasias. Peptic ulceration associated with bleeding and liver damage have been reported. It may cause sodium retention, edema and nephrotoxicity. It can cause reduction in renal clearance of lithium and a rise in serum lithium level in patients on lithium therapy. Preparation and dosage: Indomethacin 25 mg capsules. Total daily dose recommended is 50-150 mg in divided doses, after food. Indomethacin suppository is also available. Therapeutic uses: It may be preferred in the treatment of acute gouty attacks and in ankylosing spondylitis for short term therapy. SULINDAC is a fluorinated derivative of indomethacin. It is a prodrug and has a longer duration of action. Its active metabolite is claimed to be much less nephrotoxic. It is given orally in the dose of 100-200 mg twice a day.
Heterocyclic Arylacetic Acid Derivatives DICLOFENAC (Voveran): This drug (Fig 11.4) probably has substantially greater activity than indomethacin, naproxen and other NSAIDs because of its higher COX-2 inhibiting property. Fifty per cent of the sodium salt is metabolized during the first pass through the liver. It is extensively bound to plasma proteins, with a t½ of 1-2 hours. It accumulates in the synovial fluid, which probably is responsible for its longer duration of action than its plasma t½ suggests. Its relative selectivity for COX-2 also explains the increased cardiovascular risk associated with this drug.
FIG. 11.4 Diclofenac
Adverse reactions: The incidence of ADR is about 20%. Commonly, it causes adverse effects similar to indomethacin. GI symptoms including bleeding and elevation of liver enzymes can occur. Hence, liver enzymes should be evaluated in the first few weeks of long term therapy. Other adverse effects include CNS effects and fluid retention. The drug may cause severe oliguria due to marked reduction in renal blood flow and GFR and cause renal damage. Its use should be avoided in children, pregnant women and nursing mothers, and in patients with suspected renal disease, such as diabetic nephropathy. Its long term use should be avoided. Diclofenac is commonly used in veterinary practice and has been found to be highly toxic to vultures which consume caracass of animals previously treated with diclofenac. Therapeutic uses: (a) As an anti-inflammatory agent in rheumatoid arthritis, severe osteoarthritis and in ankylosing spondylitis. Its dose is 75 - 100 mg orally, daily, in 2-3 divided doses after food; and 75 mg by deep IM injection once or twice a day. (b) As eye drops 0.1% for the inhibition of intraoperative miosis (but it does not possess intrinsic mydriatic activity) and to prevent postoperative inflammation in cataract surgery. (c) For postoperative analgesia, the drug is used rectally as suppositories (Voltarol) in the dose of 75-100 mg per day in divided doses. Studies confirm that dipyrone is a major cause of agranulocytosis and that phenylbutazone, indomethacin and diclofenac can cause aplastic anemia. They may be used only when other NSAID are ineffective and that too for a short term. KETOROLAC: Ketorolac IM, 20-30 mg (single dose), is a moderately effective analgesic in patients with moderate to severe postoperative pain. Ketorolac IV has been reported to be as effective as, and have fewer side effects than, morphine in surgical and chronic cancer pain. It has a longer duration of action (t½ of 5 hours). Parenteral administration can cause as much gastric mucosal injury as oral administration. The initial dose of 20-30 mg IM may be followed by 10-15 mg by the same route 6-8 hourly, to a maximum of 80 - 120
mg daily, for 2 days; IV doses are similar to IM doses. Ketorolac promethamine is available as nasal spray. It can be administered as the spray in each nostril (15.75 mg) 6-8 hrly (not more than 4 doses per day) for 5 days. Tolmetin: This drug, a pyrrole acetic acid derivative, has a t½ of 1-2 hours. It resembles ibuprofen in its actions and its toxicity. It is, however, less potent than indomethacin.
Propionic Acid Derivatives These compounds like ibuprofen, naproxen (Fig. 11.5), fenoprofen, flurbiprofen and ketoprofen have analgesic-antipyretic and antiinflammatory properties similar to aspirin but are better tolerated orally. The incidence of adverse reactions is lower than that after high doses of aspirin and indomethacin.
FIG. 11.5 Naproxen
Ketoprofen inhibits both COX and lipooxygenase, whereas flurbiprofen also inhibits TNFα and nitric oxide synthesis. These drugs are highly bound to plasma albumin (92-99%) and, like aspirin, can displace drugs such as hydantoins, sulfonylureas and warfarin. They, however, differ in their pharmacokinetics and hence, in their duration of action. Adverse reactions: They may cause GI disturbances such as epigastric pain, nausea, sensation of fullness in the stomach and heart-burn. Occult blood loss is less common. Less frequently, they may cause CNS symptoms such as headache, dizziness, blurred vision and tinnitus. In a few cases, fluid retention and edema may occur. Hepatitis, impairment of renal function and thrombocytopenia can occur. Any patient who is intolerant to aspirin may also suffer a severe reaction following administration of propionic acid derivaties. Therapeutic uses: They are particularly useful in patients with RA, osteoarthritis and ankylosing spondylitis. The pharmacodynamic profiles of various propionic acid derivatives do not differ significantly, and the choice depends upon the relative cost and convenience. In general, ibuprofen is the better tolerated drug among the propionic acid derivatives, is cost effective and is a good substitute for aspirin as an antiinflammatory agent. Flurbiprofen is also available as eye drops for eye inflammation. The use of ibuprofen simultaneously with aspirin reduces its anti-inflammatory effect of the latter. Further, ibuprofen and other NSAIDs interfere with the antiplatelet action and hence reduce the cardioprotective effect of low dose aspirin.
Fenamates MEFENAMIC ACID: This is an anthranilic acid derivative useful in chronic and dull aching pains. Fenamates have shown no clear advantages over other NSAIDs and frequently cause adverse effects such as diarrhoea. Mefenamic acid is a weaker analgesic than aspirin. Adverse reactions include gastric upset, diarrhoea, dizziness, headache, skin rashes and hemolytic anemia. The dose is 500 mg 2-3 times a day. Therapeutic uses: • Dysmenorrhoea (Chapter 67) • Menstrual bleeding in menorrhagic women may diminish by upto 50% with the use of PG inhibitors such as ibuprofen and mefenamic acid when used during menses. Flufenamic acid has similar properties.
Oxicams PIROXICAM: This NSAID is structurally different from other agents (Fig. 11.6). Given orally, it is well absorbed and has a long half life of (38-45 hrs). Hence, it can be administered once a day. Doses between 10 and 20 mg produce analgesic-antipyretic effect whereas larger doses (20-40 mg) are needed for the anti-inflammatory effect. It commonly causes GI upset, peptic ulceration and CNS disturbances. It has been used to treat RA, ankylosing spondylitis, osteoarthritis and acute gout. It has no advantage except a longer duration of action.
FIG. 11.6 Piroxicam
Tenoxicam, meloxicam and lornoxicam are the other oxicams. The NSAIDs commonly used as antiinflammatory agents are listed in Table 11.3. Table 11.3 Anti-inflammatory doses of non-selective NSAID Name Ibuprofen Fenoprofen Ketoprofen Naproxen Flurbiprofen Diclofenac Indomethacin Sulindac P iroxicam
Available as 200, 400 mg tab 300 mg tablets 50 mg c apsules 250 mg tablets 100 mg tablets 50 mg tablets 25 mg c apsules 100 mg tablets 10, 20 mg c apsules
Dose/Frequency 400 – 600 mg tid 300–600 mg tid 50 mg tid 250–500 mg bid 100–150 mg bid 50 mg bid 25–50 mg tid 200 mg bid 10–20 mg od
Preferential and Selective COX-2 Inhibitors Drugs belonging to this group selectively block COX-2 activity more than COX-1 activity, thus interfering less with the protective action of COX-1 in the stomach, blood vessels and kidneys. The group includes nimesulide, meloxicam, nabumetone and celecoxib (Table 11.4) Celecoxib is a highly selective COX-2 inhibitor. Other highly selective COX-2 inhibitors include etoricoxib, paracoxib, lumiracoxib. Table 11.4 Preferential and selective COX-2 inhibitors Drug Nabumetone Nimesulide Meloxicam Celecoxib
Dose (mg) Frequency 500–1000 od 100 tid 7.5–15 od 100 od or bid
Given orally, their absorption is complete. They are as effective as the nonselective analgesic-antiinflammatory NSAIDs in the treatment of osteoarthritis and rheumatoid arthritis. Their major advantage is that they cause fewer gastric ulcers (‘stomach-friendly’) and do not inhibit platelet aggregation. Adverse reactions: The most common adverse reactions are nausea, vomiting, dyspepsia, abdominal pain, diarrhoea, skin reactions and the renal adverse effects such as decrease in renal blood flow, edema and dose-related worsening of hypertension. Nimesulide has been reported to cause nephrotoxicity and hepatotoxicity. The drug was not licensed for use in some developed countries, and it has been banned/withdrawn from others (e.g. paediatric formulation in India). The use of nimesulide should be avoided in old persons. Studies in animals suggest that inhibiting COX-2 may interfere with wound (ulcer) healing, bone remodeling, ovulation and prenatal renal development. Their use is not recommended in children and women of child bearing age, and during lactation, as they are excreted in breast milk. Celecoxib is contraindicated in patients allergic to sulfonamides. Theoretically, selective inhibition of endothelial COX-2 would decrease the synthesis of PGI 2, which is a vasodilator and inhibitor of platelet aggregation; the continual production of COX-1, however, would produce TXA2. This may increase the risk of thrombosis (prothrombotic effect). It is now established that selective COX-2 inhibitors, rofecoxib and valdecoxib although stomach-friendly, confer dose-related increased risk of heart attack and stroke. Hence, they have been withdrawn by the manufacturers. Currently, all the selective COX-2 inhibitors are under suspicion regarding their cardiovascular toxicity. They have been described as drugs with “marginal efficacy, heightened risk and excessive cost as compared with traditional NSAIDs.” Table 11.5 summarises the effects of commonly used analgesic–antipyretic agents.
Table 11.5 Effects of commonly used analgesic-antipyretic agents
Low Dose = 75–300 mg/day; Intermediate dose = 500 mg – 3 g/day; High dose = more than 3 g/day. *
Other NSAIDs are indomethacin, ibuprofen, naproxen, diclofenac, piroxicam.
Nefopam: It is a nonopioid centrally acting analgesic with structural similarity to benzoxazocaine. Its mechanism of action is not known but blockade of voltage gated sodium channel and inhibition of serotonin, dopamine and NA reuptake may contribute to its effects. At high doses it causes sweating, dizziness and nausea. It is used as an alternative to or an adjunct with opioid analgesics. It is also advocated for post-operative shivering. Curcumin: It is an active constituent of Indian spice, turmeric roots (Curcuma longa), which is advocated by Ayurveda to treat inflammation. Curcumin has been demonstrated to possess antioxidant and anti-inflammatory activities. It probably acts inhibiting inflammatory cytokines. However, it has low solubility and poor bioavailablity.
Pharmacotherapy of Pain Phenomenon of pain is complex and the boundaries between normal discomfort and pathologic pain are often obscure. The intensity of pain suffered differs enormously with the personality, intelligence and culture of the individual. Tribal people often display a stoic disregard for pain. As a generalisation, pain is complained of more vehemently by people belonging to the more affluent and elite sections of the society. Emotional stress and anxiety adversely affect the pain response, while other factors which enhance its severity are debility and fatigue. Pain often becomes worse during the night when the distractions of daytime are absent and the patient has time to ruminate his ailment. Protracted severe pain can become so dominant a factor in a patient’s life that it can eventually lead to both physical and psychological exhaustion. The management of pain is always multidisciplinary and involves pharmacotherapy, physiotherapy and cognitive behavioral therapy. The choice of therapy depends upon: • The type of pain and underlying cause • The mechanism of the pain • Associated conditions • Physical and psychological condition of the patient; and • The risk of toxicity. An attempt should always be made to find out the probable cause, and if possible, treat it. Thus, pain due to an abscess can be relieved by appropriate chemotherapy and surgery or that of duodenal ulcer by antacids and anti-secretary drugs. In patients in whom, for some reason, the cause cannot be treated, immediate relief of pain can be obtained by modifying the mechanism by which pain is produced e.g. use of nitrates in angina pectoris, miotics in glaucoma and muscle relaxants in certain musculoskeletal disorders. Mechanism of production of abdominal pain is often obscure and a demonstrable cause is absent. Individuals with “functional dyspepsia”, are known to be benefited by ‘carminatives’ which have been used for ages. It is a common experience that a few seeds of cardamom, fennel or a little ginger can make the stomach comfortable after a sumptuous meal. These agents form the traditional ingredients of many stomach ache powders and gripe waters sold in the market. Genuine intestinal or biliary colic, however, needs administration of an anti-muscarinic drug like atropine or its substitutes. For the symptomatic relief of acute pain, opioid and non-opioid analgesics (NSAIDs) are the most commonly employed agents. It is important to administer analgesics at the very onset of pain. The longer the pain is allowed to continue untreated, the less effective the analgesics become. This is seen especially in such conditions as migraine. In the treatment of severe pain, morphine is not only more potent but more efficacious than aspirin. Starting equianalgesic doses of opioids are given in Table 11.6; the optimum dose for each patient is determined by titration.
Table 11.6 Equianalgesic doses of opioids Drug IM Dose mg Oral Dose mg Morphine 10 60 P ethidine 75 300 Methadone 10 60 Codeine − 120 P entazocine 60 180 Buprenorphine 0.4 0.8 * *
Sublingual, not oral
Severe pain of sudden onset, particularly the visceral pain (e.g. MI), can produce shock. In such cases, opioids are indispensable and should be administered immediately e.g., in acute MI, fractures and pneumothorax. The opioids also induce a state of tranquillity, thus creating an indifference towards residual discomfort. When used with skill and discrimination, the adverse effects are not very bothersome. Opioids should be administered in full doses, if necessary IV as patients with severe pain are remarkably tolerant of full therapeutic doses of morphine. When morphine is to be administered IV, it is injected slowly, the dose being 2.5-5 mg over 5 minutes. Pethidine has a shorter duration of action than morphine; this can lead to unsuspected undertreatment. Conditions in which morphine or pethidine is recommended and the frequency of doses suggested make the possibility of drug addiction very remote. However, patients with acute pain treated with opioids for more than 5-7 days are likely to develop tolerance to the analgesic effects, so that if they need relief from pain after additional surgery, the usual doses may not give relief. As the onset of pain relief by NSAID is much slower than that by the opioids, the latter may be combined with NSAID with synergistic effect. Pain is not simply a perception. It is a complex syndrome, one component of which is the sensation described by the subject as pain. The other component, the emotional and psychological one, contributes considerably to the “suffering”, an affective reaction. In fact, the affective reaction can often be of overriding importance. In such cases, analgesics alone may not be adequate and adjuvant drugs as well as non-drug therapy may be more important. The concurrent use of a phenothiazine, an anxiolytic or an antidepressant (Chapters 13 and 14), helps to relieve the pain by modifying the affective component of the pain. Used in combination with analgesics, these drugs reduce the doses of the latter as well as relieve the suffering, and improve the quality of life. For the symptomatic relief of dull aching, chronic pain, non-opioid, non-addicting analgesics like NSAID are preferred; and aspirin deserves the widespread popularity it enjoys for this purpose. Clinical studies show no substantial difference in the therapeutic benefits of various NSAID, although the tolerability and individual preference may vary. Further, the duration of action (as in early morning stiffness of RA) and the availability in injectable form may determine the choice of NSAID. Table 11.7 gives the plasma half lives of NSAID. In most instances, the selection of NSAID for an individual patient remains more of an art than science.
Table 11.7 Plasma half-lives of NSAID Less than 5 hours Aspirin, Dic lofenac , Ketoprofen, Flurbiprofen, Indomethac in, Tolmetin, Flufenamic ac id. 10–30 hours Diflunisal, Fenoprofen, Naproxen, S ulindac , Nabumetone. More than 50 hours Phenylbutazone, Piroxic am, Tenoxic am.
Because of variation in the response of individuals to different NSAIDs, several NSAID may be tried before one finds the drug which suits a patient most. If used as an analgesic, the NSAID should be changed if no response is obtained within a week. If used as an antiinflammatory agent, the NSAID should be changed if no response is obtained within 3 weeks. Aspirin is effective in many kinds of pain, not merely those related to the musculoskeletal disorders. It is also anti-inflammatory and anti-pyretic. It can be used over prolonged periods without fear of addiction. Aspirin and other NSAID are also useful in pain associated with injury to soft tissues e.g., sprains and postoperative pain. The combination of aspirin tbuprofen with codeine oxycodone/hydroxycodone is synergistic as these drugs alleviate pain by different mechanisms. Indomethacin, diclofenac and related compounds are potent NSAID but are more likely to cause GI bleeding, increase CVS risk and damage kidneys. Use of these drugs in any dosage in treatment of mild to moderate pain is unjustifiables. In chronic pain, the abnormal activity of the pain-mediating afferent system may continue irrespective of the original cause, and blocking of the neural pathways may not be helpful. This is because there are several mediators of chronic pain (e.g. cytokines, bradykinin, substance P etc). Countering their effects is useful. The cause of chronic headache is often obscure in practice and needs detailed investigations. Migraine can be treated with sumatriptan (Chapter 24). Many headaches are, however, caused by anxiety, tension, fatigue or depression; and use of proper psychotherapeutic drugs like benzodiazepines or antidepressants can give dramatic results. Realisation of this psychic aspect of pain will prevent other unnecessary therapy including extensive and irrelevant sinus operations. In such cases some minor adjustments in patient’s life can be more therapeutic than even drugs. Acute headaches due to common cold, influenza or other fevers respond to administration of paracetamol or aspirin. Headaches due to sinusitis and eyestrain need specific treatment. Backache presents similar diagnostic problems as headache. It can be due to many causes. The commonest causes are faulty posture and lack of exercise. Mental depression and nervous tension can often produce backache. Failure to recognise this in women may lead to unnecessary correction of normal retroverted uterus or even its removal for no sensible reason. Mild backache of spinal osteoporosis may be relieved by oral calcium, vitamin D, bisphosphonates and physiotherapy. However, severe pain of ankylosing spondylitis needs treatment with indomethacin/diclofenac that of an acute vertebral fracture in osteoporosis needs a short period of bed rest followed by physiotherapy, in addition to an NSAID. Unremitting pain due to spinal osteoporosis may respond to calcitonin (Chapter 70). In inflammatory conditions such as RA and gout, the NSAID relieve pain without affecting the basic disorder (Chapter 75). In single doses, NSAID have analgesic activity comparable to that of paracetamol. They may, therefore, be used on demand to treat mild or intermittent pain or to supplement
regular treatment. In regular full doses, NSAID also have an anti-inflammatory effect and are effective in treating continuous or regular pain associated with inflammatory arthritides such as RA and ankylosing spondylitis. NSAID are also useful in the pain of advanced osteoarthritis when paracetamol fails to work; even in such cases much smaller doses of a mild drug such as ibuprofen are needed to control pain than those required in RA. Some drugs are relatively specific in that they act in a specific, inflammatory, painful condition viz. colchicine in acute gout (Chapter 75). Chronic cancer pain: Pain is a major problem in cancer patients and 70% of patients with advanced cancer have it. It can be caused by the cancer itself or by other associated conditions such as osteoarthritis, bedsores or surgery. It may be related to bones, nerve compression, metastases in soft tissues; further, psychological reactions to the illness including depression and a sense of helplessness (distress) may worsen the pain. Unremitting pain itself can cause secondary symptoms such as anorexia, disturbed sleep, irritability and impaired concentration. It is, therefore, necessary to assess the causes of pain, as well as its effects, at the very outset in a patient with cancer; further, periodic reassessment of both is necessary so as to offer the patient appropriate treatment. The therapy consists of drug treatment and other methods like local radiation to a painful bony metastasis. Irradiation of the metastases by beta rays from injected 89SrCl alleviates the bone pain. The drugs used in the treatment of cancer pain can be classified as: I Analgesics: (i) Non-opioids (paracetamol and NSAID); (ii) Weak opioids (codeine and dextropropoxyphene); and (iii) Strong opioids (morphine, buprenorphine, pethidine and methadone); and II Adjuvant analgesics: These are the drugs used primarily for indications other than pain. They are particularly useful in the treatment of chronic cancer or non-malignant pain as “add- on” drugs. Sometimes they are used as “first line” therapy e.g. carbamazepine in trigeminal neuralgia and serve as ‘opioid-sparing’drugs. Several such compounds are available: (i) Antidepressants; (ii) Anticonvulsants; (iii) α2 agonists-Clonidine, Tizanidine; (iv) Glucocorticoids; (v) Local anaesthetics-Transdermal ligno caine, Mexiletine; (vi) Topical agents-Capsaicin; (vii) NMDA antagonists- Dextromethorphan, Methadone, Amantadine, Ketamine; (viii) Cannabinoid-Δ-9-THC; (ix) Bisphosphonates and Calcitonin; (x) GABA agonists-Baclofen; (xi) Neuroimmunomodulatory agents-Thalidomide and its newer analogues. They are described elsewhere. Usually, in chronic cancer pain analgesics are used in a step ladder fashion, commencing with NSAID, and changing to weak and finally strong opioids as the drugs from the earlier used group cease to be effective. Ibuprofen and other NSAID may be useful in relieving pain of bony metastases and that due to mechanical compression of tissues other than
nerves. The opioids are added sequentially when the need arises. Glucocorticoids are very often prescribed in the palliative treatment of cancer pain. They have anti-inflammatory action, reduce edema around damaged nerves, reduce nausea, and improve appetite and the quality of life in general. Dexamethasone may be used 1-2 mg bid. In severe cancer pain, morphine is used by the clock and one does not wait till the pain returns and the patient demands relief. The oral route is the preferred one; a commonly used dose is 5-30 mg every 4 hours; but much larger doses may be needed. It has no ceiling dose but dosage may be limited by ADR. Nausea may be common at the beginning of treatment with morphine and is treated with a drug such as prochlorpromazine (5-10 mg 48 hourly), metoclopramide (10 mg 4-8 hourly) or haloperidol (1-2 mg daily). Constipation may be troublesome; physical dependance and tolerance may develop during treatment with morphine; but psychological dependence rarely, if ever, occurs in cancer patients receiving opioids. Metastases in the liver are not a contraindication to opioids; but care should be exercised in patients with concomitant hepatic dysfunction. In cancer patients, the use of potent analgesics is dictated by the intensity of pain and not by the brevity of prognosis. The starting doses of other strong opioids are: buprenorphine (0.2-0.4 mg sublingually); methadone (5-10 mg orally) and pethidine (50-100 mg orally), on a 4 hourly basis. Being a mixed agonist-antagonist, buprenorphine can reverse the analgesia caused by other strong opioids, and therefore should not be combined with drugs such as morphine. Extended release oral preparations of hydromorphone are available for treating severe cancer pain. Its effect lasts for 24 hrs. The tablet must not be crushed or chewed but swallowed. A self-adhesive skin patch which releases the opioid fentanyl transdermally can also be used. Its effects last for 72 hours. Patient-controlled analgesia: This is an approach where the patient himself injects a programmed dose of an opioid analgesic. This has been shown to deliver better pain control. Careful monitoring is however needed. Ziconotide, a synthetic peptide, non-opioid, neuronal calcium channel blocker, by intrathecal infusion, has been reported to help some patients with chronic, severe, refractory pain. Chronic neuropathic (deafferentiation) pain occurs in diabetic neuropathy as well as following herpes zoster. It is due to altered neuronal excitability resulting in abnormal spontaneous discharge. The shooting or stabbing pain occurring in these conditions responds to antiepileptics such as phenytoin, carbamazepine, gabapentin, pregabalin (Chapter 9) and to duloxetine, a SNRI (Chapter 14). The deep, dull aching pain in these situations may respond to antidepressants like amitriptyline. Pain resistant to amitriptyline may respond to capsaicin derived from chillies (hot peppers). Capsaicin is applied to the concerned skin area as a 0.025 to 0.075% cream. It is believed to act by interfering with pain transmission in the peripheral neurons by depleting and inhibiting the reuptake of substance P in these neurons. It may also cause local burning sensation. Lignocaine ointment 5%, applied locally, may also sometimes relieve pain due to herpetic neuralgia. Miscellaneous pains: Obstetric analgesia is achieved by the use of nitrous oxide (Chapter 7) or pethidine (Chapter 10). Analgesics are often used prior to surgery (pre-emptive analgesia) as they are known to
reduce postoperative pain. Relief of postoperative pain not only increases the patient’s comfort but can also remove an impediment to adequate ventilation in major upper abdominal surgery. If an NSAID such as paracetamol, ketorolac or diclofenac does not give adequate relief, buprenorphine (0.2-0.4 mg sublingually every 6-8 hours) may be used. The use of phenazopyridine to relieve lower urinary tract pain is discussed in Chapter 52. Local treatment of pain: Counterirritants such as methyl salicylate applied as a liniment relieve dull-aching, localised musculo-skeletal pain. Further, counterirritants do not mask underlying internal disease. Several NSAIDs in the form of gels/creams/ointments for topical use, are available as OTC products for treatment of sprains, arthritis and post-extraction dental pain. However, they can induce photosensitivity reaction and should not be applied to broken or inflamed skin. Eye drops containing diclofenac 0.1%, flurbiprofen 0.03% and suprofen 1% are useful in patients undergoing cataract surgery. When started before surgery, they act synergistically with local mydriatics to prevent miosis (Chapter 72).
NSAID and Renal Damage The relation between analgesic use and renal damage has now been confirmed. Phenacetin was the first drug proven to cause ‘analgesic nephropathy’. But, all NSAID can cause acute or chronic renal damage following repeated use, sometimes as short as two weeks. Drugs with a long half life (e.g. naproxen, diclofenac and piroxicam) are more likely to cause renal damage than those with shorter half life (e.g. ibuprofen). Clinically, the renal injury can present itself in several forms: • Acute renal failure. • Mild asymptomatic renal impairment. • Chronic renal impairment due to papillary necrosis or interstitial fibrosis; and • Serious hyperkalemia. The first three are due to NSAID induced inhibition of intra-renal PG synthesis. Locally produced PGE2 acts as intra-renal vasodilators to counteract the vasoconstricting effect of angiotensin II and noradrenaline (as in shock); PGE2 also influences the tubular transport of ions and water. The renal perfusion is less dependent on the locally produced PGE2 in healthy, young persons than in old people and individuals with diseases such as diabetic nephropathy. Inhibition of synthesis of PGs within the kidney has several adverse effects: (a) The protective intrarenal vasodilator effect is lost. (b) Renal blood flow and GFR are reduced (in pre-existing renal impairment). (c) The natriuretic effect of PGE2 on the renal medulla is lost with consequent sodium retention This may cause edema and congestive heart failure. Hyperkalemia is due to diminished aldosterone synthesis secondary to inhibition of renin synthesis by NSAID (hyporeninimic hypoaldosteronism). Hyperkalemia is particularly likely to occur in patients with diabetes mellitus, renal disease and in those on potassium sparing diuretics. Apart from nephropathy, NSAIDs can cause allergic type of interstitial nephritis and urate nephropathy. Finally, NSAID enhance the effects of vasopressin on the kidneys and can diminish excretion of free water. Renal toxic effects are often overlooked because their onset is insidious and the patients may forget to inform the doctor that they are taking an NSAID. The risk of developing renal damage due to NSAID is increased by the concurrent presence of: old age; hypertension; congestive heart failure; cirrhosis of liver; diabetic nephropathy; gout; renal or renovascular disease; and salt/volume depletion. Studies indicate that aspirin, in less than full anti-inflammatory doses, is perhaps safer as its chronic use is less associated with analgesic nephropathy. Slow release, long acting pain relieving NSAID formulation are not ‘superior ’ to individual active agents. Their only advantage is that they have longer duration of action. In fact, often individual agent is superior to ‘slow releasing’ one for prompt relief of pain. Such formulations are always expensive. Moreover, the drug remains in the body for longer time. The patients may ‘self administer ’ such formulation repeatedly for chronic pain. This is dangerous, when we know that NSAID can cause ‘silent and irreparable’ kidney damage. Remember that all NSAID can cause renal toxicity in the elderly. The possibility of chronic
ingestion of analgesics, especially OTC preparations, should always be borne in mind when dealing with unexplained chronic renal damage.
12
Central Nervous System Stimulants The stimulants of the CNS are therapeutically, in general, not as useful as the CNS depressants because they lack selectivity of action. Further, excessive stimulation of CNS is followed by its depression. Some of the CNS stimulants are mainly used as analeptics. Analepsis is a Greek word which can be loosely translated as ‘picking up those who have been cast down’. Analeptics stimulate the CNS particularly the respiratory centre and, in large doses, they cause generalised convulsions. Classification: The CNS stimulants are: I Those acting directly on the CNS. • Predominantly cortical stimulants like Xanthine alkaloids, Amphetamine, Methyl amphetamine, Methylphenidate and Pipradrol. • Predominantly medullary stimulants, e.g. Picrotoxin, Pentylenetetrazol, Nikethamide, Amiphenazole, Camphor and Carbon dioxide. • Predominantly spinal stimulants, e.g. Strychnine. II Those which stimulate the CNS reflexly, e.g., Lobeline, Ammonia, Veratrum and Nicotine. The above classification is arbitrary and a CNS stimulant can stimulate the entire CNS.
Stimulants of the Cerebral Cortex XANTHINE ALKALOIDS: The three naturally occurring methyl xanthine alkaloids, caffeine, theophylline and theobromine, are purine bases (Fig. 12.1) which occur in several plants all over the world. These alkaloids leave behind a yellow residue when heated with nitric acid and hence, the term xanthine derived from the Greek word ‘xanthos’ meaning yellow. Coffee prepared by grinding the seeds of Coffea arabica contains caffeine; tea from leaves of Thea sinensis contains caffeine and small amounts of theobromine and theophylline; while cocoa obtained by grinding seeds of Theobroma cacao contains caffeine and theobromine. The cola-flavoured soft drinks also contain caffeine.
FIG. 12.1 Xanthine
Mechanism of action: Methyl xanthines act in many ways: • By inhibiting phosphodiesterase (PDE), thus preventing the conversion of cAMP to inactive 5’ AMP and thereby increasing tissue concentration of cAMP. This is its main action. The catecholamines, which also increase the concentration of cAMP by a different mechanism, act synergistically with methylxanthines. • By bringing about changes in distribution of calcium at the intracellular sites; and • By blocking adenosine receptors which modulate adenylyl cyclase activity. The relative contributions of these mechanisms in producing different pharmacological actions is not established. Pharmacological actions: Central nervous system: Of the three xanthine alkaloids, caffeine possesses the most significant action on CNS, followed by theophylline and theobromine. Caffeine mainly acts on the cerebral cortex; larger amounts stimulate the medullary centres and toxic doses may cause convulsions. • Cerebral cortex: Caffeine is a recreational drink. In small doses it produces a more rapid and clearer flow of thoughts, increases mental alertness, and delays fatigue and drowsiness. It stimulates mental activity when it is below normal following fatigue or boredom; it does not raise it above normal. Caffeine also reduces reaction time, improves motor activity and augments conditioned responses. Thus, it improves physical performance. The cortical effects may be produced by ingestion of 1 or 2 cups of coffee, one cup containing 100-150 mg of caffeine. Larger doses of caffeine (exceeding 300 to 500 mg) produce irritability, nervousness, confusion of thought, insomnia, headache and tremors. Recently acquired motor skills calling for delicate muscular coordination and accurate timing may be affected adversely as a result of nervousness and tremors. Toxic doses cause focal and generalised convulsions in animals.
• Medulla: Larger doses of caffeine stimulate the respiratory, vasomotor and vagal centres. Caffeine-induced respiratory stimulation is more marked in individuals breathing 3-5% CO2 than in normal individuals; this suggests that the drug probably increases the sensitivity of the respiratory centre to carbon dioxide. The stimulation of vasomotor and vagal centres tends to raise the BP and induces bradycardia, respectively. • Spinal Cord: Very large doses of caffeine increase the reflex excitability of the spinal cord, and may produce clonic convulsions and death in animals. In man, no fatalities after caffeine administration have been reported. Cardiovascular system: Theophylline has the most prominent action on the CVS. • Heart: Xanthines directly stimulate the myocardium and increase the heart rate, the force of contraction and the myocardial oxygen consumption. The positive chronotropic action on the myocardium is antagonized by central vagal stimulation, particularly with caffeine. Thus, therapeutic doses of caffeine produce a variable effect on the heart rate. Large doses of caffeine, however usually cause palpitation, tachycardia and rarely cardiac arrhythmias. Increase in cardiac output with xanthines may occur even in the absence of tachycardia. The increased force of contraction assures a better emptying of the heart and reduces the central venous pressure. In healthy individuals, the lowering of the venous pressure may outlast the cardiac stimulant effect, resulting in a fall in the cardiac output following an initial rise, but in individuals with congestive heart failure, the lowered venous pressure produces an increase in the cardiac output. • Blood vessels: Xanthines tend to produce peripheral vasodilatation by a direct action on vascular smooth muscle, and cause a decrease in cardiac preload. Coronaries: The coronary arterial blood vessels are dilated and the coronary blood flow is increased. Cerebral blood vessels: Xanthines produce a marked increase in the cerebral vascular resistance and reduce the cerebral blood flow and the CSF pressure. This constriction may be responsible for the relief of migraine headache. Pulmonary blood vessels: Xanthines produce relaxation of the pulmonary arterioles and reduce the pulmonary artery pressure. Blood pressure: The direct cardiac stimulant action of xanthines tends to raise the BP. This action is aided by the stimulant action on the vasomotor centre, and is antagonised by the central vagal stimulation and vasodilation. Changes in BP are, therefore, unpredictable. The combination of vasodilatation and increased cardiac output, however, raises the pulse pressure and the velocity of blood flow and helps to improve circulation. Aminophylline IV may produce a fall in BP. Kidneys: The xanthines reduce tubular reabsorption of sodium and cause moderate diuresis. Theophylline is the most potent compound in this respect, followed by theobromine and caffeine (Chapter 39). Smooth muscle: Xanthines also relax other smooth muscles, particularly the bronchial smooth muscle. Theophylline abolishes bronchospasm produced by histamine, pilocarpine and anaphylactic shock. Voluntary muscles: Xanthines, particularly caffeine, strengthen the contraction, increase the metabolism, and postpone fatigue of skeletal muscles by both central and peripheral actions. Improved contractility of the diaphragm contributes to the therapeutic efficacy of
aminophylline in bronchial asthma. Miscellaneous actions: The xanthines increase the gastric acid secretion. Decaffeinated coffee also has similar effect. The basal metabolic rate is slightly increased by caffeine, probably as a result of increased metabolism of the skeletal muscles. Theophylline also elevates plasma renin activity in man. Absorption, fate and excretion: The xanthines are readily absorbed on oral, rectal or parenteral administration. After absorption, about 17% of caffeine, 20% of theophylline and 3% of theobromine are bound to plasma proteins. They are metabolised in the liver, mainly by demethylation and oxidation by mixed function oxidase enzymes and xanthine oxidase. Caffeine metabolism varies widely among individuals; its t½ varies from 2 to 12 hours. None of the xanthines is converted into uric acid and hence, beverages containing xanthines are not contraindicated in gout. Adverse reactions: In the usual doses, caffeine does not cause serious toxicity. • CNS: Excessive, prolonged use of the drug may produce confusion, tremors, insomnia and excitement which may progress to mild delirium. The individual may complain of ringing in the ears, headache, and may develop tachypnoea, tachycardia, emesis, fever and occasionally extrasystoles or cardiac arrhythmias. Hence, inquiry into anxiety symptoms, especially in a subject with recurrent headache, should include questions about excessive tea or coffee drinking. The symptoms can be treated with sedatives. • GI tract: The xanthine alkaloids and beverages should be administered with caution in patients with peptic ulcer. Theophylline, in such patients, may produce hyperacidity, nausea, vomiting and epigastric pain even when it is given parenterally. GI irritation can be reduced by the administration of theophylline with food. • Miscellaneous: Aminophylline, on parenteral administration, may produce dizziness, hypotension, severe precordial pain, and even ventricular fibrillation. Fatalities have been reported following IV use. In children, aminophylline intoxication is characterised by vomiting, severe thirst, dehydration, delirium, convulsions and shock. • Tolerance: Tolerance develops after prolonged use of xanthines, mainly to their cortical stimulant, diuretic and peripheral vasodilator effects. It is usually abolished after abstinence from xanthine beverages. • Habituation: Habituation to xanthine beverages such as tea, coffee and cola is extremely common. However, it does not seem to be harmful. Tea, coffee and cola drinks are better avoided in small children as they are thought to be less tolerant of the stimulant effects of caffeine. Preparations and dosage: (i) Caffeine citrate Dose: 120 to 600 mg. (ii) Caffeine and sodium benzoate ampoules 250 mg per ml Dose: 250 to 500 mg (iii) Aminophylline: Chapter 27. (iv) Deriphylline (Chapter 27). Therapeutic uses: • As a CNS stimulant : Caffeine in the form of coffee or tea is often employed for relief from fatigue. Theophylline and caffeine are used to treat neonatal apnoea. • Migraine: Because of its action on the cerebral blood vessels, caffeine is used along with ergotamine tartrate for relief of migraine (Chapter 24). • Acute left ventricular failure: Aminophylline is an adjuvant in the treatment of
paroxysmal nocturnal dyspnoea due to LVF. Aminophylline increases the cardiac output, reduces the pulmonary artery pressure and the cardiac preload, induces bronchodilatation, stimulates the respiratory centre and causes diuresis. When the distinction between dyspnoea due to acute LVF (cardiac asthma) and bronchial asthma is not clear, it is safer to administer IV aminophylline rather than adrenaline (which is dangerous in the former condition) or morphine (which is dangerous in the latter condition). It is administered slowly IV in the dose of 500 mg along with other treatment such as morphine, oxygen and phlebotomy. Its use has now declined because of the availability of more specific preload reducing drugs (Chapter 31). • In bronchial asthma (Chapter 27). • As a diuretic (Chapter 39). • Strong coffee is used in orthostatic hypotension of autonomic failure (Chapter 30). AMPHETAMINE: Amphetamine and methylamphetamine are sympathomimetic amines. Their central actions are similar to xanthines but peripherally they produce adrenaline-like actions (Chapter 18). PIPRADROL AND METHYLPHENIDATE are mild psychomotor stimulants (Chapter 14).
Stimulants of the Brain Stem and Medullary Centres Most drugs belonging to this group (picrotoxin, pentylenetetrazole, nikethamide and camphor) are no longer used as respiratory stimulants (analeptics) because of lack of specificity, toxicity and unproven efficacy. In large doses these drugs produce clonic convulsions followed by tonic convulsions.
FIG. 12.2 Effect of pentylenetetrazole on respiration in dog under barbiturate anaesthesia.
DOXAPRAM: This is a non-specific analeptic used mainly as a respiratory stimulant in the post-anaesthetic period and in patients with hypoventilation. It has a reasonable margin of safety. It is administered by IV drip in the total dose of 0.5-1.5 mg per kg at the rate of 5 mg per minute. It may be repeated after 1 hour. A single IV injection (0.7 mg per kg) produces peak action in 1 minute lasting for 5-10 minutes. It is used to treat opioid induced postoperative respiratory depression. Doxapram has also been used as a temporary measure to correct acute respiratory insufficiency in patients with COPD (Chapter 27). Adverse reactions include vomiting, hypertension, tachycardia, arrhythmias, muscle twitchings, tremors and convulsions. MEDROXYPROGESTERONE ACETATE: Like progesterone, medroxyprogesterone acetate has respiratory stimulant action. It is effective orally. It has been used with some success in patients with chronic ventilatory failure due to pathological obesity (Pickwickian syndrome). CARBON DIOXIDE: Chapter 77. Therapeutic uses of analeptics: Analeptics are of limited use in practice. The uses are : • Opioid-induced postoperative respiratory depression: Naloxone (Chapter 10) can be used if the depression is caused by opioid analgesics. Unless the dose is carefully titrated, it can reverse analgesia as well. Doxapram does not reverse opioid induced analgesia. • Ventilatory failure in patients with COPD (Chapter 27): When such a patient develops hypercapneic respiratory failure and becomes drowsy or comatose and in patients in whom mechanical ventilatory support is contraindicated, doxapram may be useful to tide over the crisis; it is used by IV infusion or by slow IV injection.
• Primary apnoea of the newborn: Caffeine and theophylline are effective in the treatment of this condition. • Pickwickian syndrome: See above.
Stimulants of the Spinal Cord STRYCHNINE: Strychnine is an alkaloid obtained from the button-shaped seeds of the plant Strychnos nux vomica. Given orally or parenterally in animals, it produces convulsions characterised by tonic extension of the body and opisthotonus. Death may occur as a result of asphyxia after seizures. Strychnine acts mainly on the spinal cord but it stimulates the entire neuraxis in large doses. It is a competitive antagonist of the inhibitory transmitter glycine at the post-synaptic inhibitory sites. Excessive stimulation is followed by depression and death. Strychnine has no place in therapeutics. In the treatment of strychnine poisoning, the most urgent need is the control of convulsions with IV diazepam (10 mg in adults, repeated as necessary). All forms of sensory stimulation must be avoided. Tracheal intubation and assisted ventilation are indicated if adequate ventilation is not restored. Only after controlling the convulsions should a gastric lavage be performed. The universal antidote, if administered without delay, would adsorb the alkaloid and prevent its systemic absorption. Alternatively, oxidising solutions like 1 : 1000 potassium permanganate or 2% tannic acid (as strong tea) may be employed.
Reflex Stimulants of the Central Nervous System Lobeline: Lobeline is an alkaloid obtained from the leaves of Lobelia inflata. Lobeline stimulates the CNS through the chemoreceptors of the carotid sinus. In addition it stimulates autonomic ganglia, and the axon reflex which induces sweating. It is now rarely used. Other reflex stimulants of the central nervous system such as nicotine, veratrum and apomorphine are discussed elsewhere. Liquor ammonia and smelling salts (ammonium carbonate) inhalation in syncope is a common household procedure which stimulates the respiratory and vasomotor centres reflexly.
13
Psychopharmacology - 1: Introduction, Antipsychotic Drugs and Pharmacotherapy of Major Psychotic Disorders The term ‘tranquillisation’ or ‘ataraxia’ is considered more or less synonymous with ‘peace of mind’. Obviously, such a state can be produced by many drugs, depending upon the cause of disturbed ‘peace of mind’. The term ‘tranquilliser ’ was used originally to describe the calming effect of reserpine and chlorpromazine which have the ability to calm without affecting wakefulness. Regardless of terminology, the objective of drug therapy in psychiatry is to induce an improved mental state in mentally disturbed patients. Drugs which selectively modify the behavioural pattern are known as psychotropic or psychoactive drugs. It is extremely difficult to define what constitutes psyche or mind which is supposed to carry out three functions : • Cognition, the reception of environmental stimuli. • Affect, analysing the information received and formation of a reaction pattern; and • Conation, the behavioural response. Little is known about the neurophysiological and biochemical differences between normal individuals and mentally ill patients. The mind cannot be separated from the physical body. Many physical illnesses cause associated psychic problems, whereas mental illnesses can produce somatic symptoms. The etiology of psychic illnesses is complex, and includes both psychological and physiological/biochemical factors. In some of them, psychological problems predominate, whereas in others, disturbances of endogenous neurochemicals clearly exist. For pure psychological problems (affective disorders) psychotherapy is a good alternative to drugs, whereas drugs are most effective in treating biologically (neurochemically) based psychotic illnesses such as manic depressive psychosis. However, in all patients both psychotherapy and drugs are necessary for the best results. Clinically, the term psychotic disorders refers to the major mental illnesses like schizophrenia and manic depression in which (a) insight is said to be lost; and (b) the patient’s experience e.g. hallucinations, is outside the normal range of human experience. In contrast, the term neurotic disorders implies the rest of recognised psychiatric conditions in which (1) insight is preserved; and (2) the patient’s experience, although unpleasant and extreme, is within the range of normal human experience. A major affective disorder of mind is the syndrome of mental depression. Evaluation of psychotropic drugs in animals: Models of psychic disturbances analogous to those seen in humans cannot be produced in animals. Further, the intellectual superiority of man over the highest primate is so great that it is difficult to predict usefulness of drugs in the treatment of human mental illnesses from the behavioural studies in animals. It is not surprising, therefore, that therapeutic application of many compounds in humans originated from accidental observations in patients getting such drugs for other purposes. Pre-clinical screening, however, does give useful information. Some of the experimental methods employed are: • Natural behavioural patterns: Quietening property of a drug can be demonstrated by its
taming effect on an aggressive monkey or on a cat in the presence of a mouse. Similarly, the modification of natural activities of various animals by drugs is helpful in evaluating their general stimulant or depressant properties. • Spontaneous motor activity: This is usually studied in rats and mice. The animal is kept in a cage through which a beam of light passes. An electronic device records the number of times the beam is interrupted by movements of the animal. Spontaneous activity is also measured by using a jiggle cage. It is a cage suspended on springs and hence, produces oscillatory movements every time the animal moves; these can be recorded. • Motor co-ordination and muscle tone: Screening for behavioural pattern usually reveals drug-induced ataxia. This can be quantified by rota rod test, using a horizontally mounted rod with a diameter of 2-3 cm. Normal mice maintain their position on the rod for at least five minutes while ataxic mice fall off earlier. • Drug induced behavioural patterns: Administration of reserpine to animals induces a condition resembling retarded depression in man, where there is general reduction in activity, slowing of movements, reduced responsiveness to stimuli and neglect of activities such as feeding and sexual behaviour. Such ‘model illness’ has been used for testing the antidepressant properties of drugs. Toxicity of amphetamine is considerably higher when tested on mice kept together in the same cage than on animals kept individually in separate cages. Drugs like phenothiazines reduce this group toxicity. ‘Behavioural models’ also include forced swim test (FST) and tail suspension test (TST) in rodents. When forced to swim or suspended in a restricted space from which there is no possibility of an escape, the animals eventually cease to struggle, and surrender themselves (despair or helplessness) and enter in a readily identifiable immobile state. This immobility can be reduced by drugs which are clinically effective in human depression. Such tests are widely used for screening of antidepressants. • Learning and discrimination behaviour: The maze has provided the psychological setting for most of these studies. Thus, drug effect on maze learning or on perfected maze habit in animals, using the time required for performance as a criterion, can give some idea about the influence of a drug on learning and discrimination behaviour. • Emotional behaviour and conditioned neurosis: Experimental neurosis is known since the work of Pavlov. The drug effect has been studied on a variety of induced ‘neurotic’ reactions (phobias, compulsions etc.) in animals. Drugs expected to inhibit selectively certain abnormal reactions or behaviour patterns such as fear and/or anxiety, without impairing the innate behaviour, are studied in ‘conditioned animals’. In conditioned avoidance the animal first learns a response like running, pressing a bar or climbing up a pole in order to escape from an electric shock. Then it learns to avoid the shock by responding promptly to a danger signal such as a buzzer sound which precedes the shock. Antipsychotics like chlorpromazine selectively block such conditioned responses but not the unconditioned ones, where the animal still escapes once the shock is applied. Barbiturates, on the contrary, abolish both conditioned and unconditioned avoidance responses. • Effects of drugs on behaviour are also studied by implanting or injecting them directly into various parts of the brain.
The psychotropic drugs that reduce apomorphine-induced stereotype and amphetamine-induced hyperactivity, and inhibit conditioned avoidance responses, are likely to be useful as antipsychotic drugs. The animal pharmacologic test that correlates best with antipsychotic activity is the prevention of apomorphine (a dopamine agonist) induced vomiting in dogs. These effects are mediated by the mesolimbic dopamine receptors. The drugs which reduce aggressiveness but increase the exploratory activity in a maze without causing ataxia are likely to be useful as antianxiety drugs (anxiolytics). Drugs with possible application as antidepressants usually potentiate the actions of amphetamine and increase the spontaneous motor activity, but antagonize reserpine and apomorphine-induced hypothermia. They reduce the immobile states in FST and TST. For the evaluation of psychotropic drugs in man a large number of rating scales have been designed to obtain an overall assessment of the mental state and to quantify the drug-induced modification of parameters such as anxiety, depression and the adverse effects. Rating scales can be used to assess both objective and subjective features of the condition. Classification of Psychoactive drugs: I Antipsychotics, used mainly in major psychoses like schizophrenia and manic depressive psychosis (MDP). For details see below. These drugs are called neuroleptics because they reduce the agitation and disturbed behaviour often associated with delusions and hallucinations in schizophrenia. II Anti-anxiety agents (Anxiolytics), mainly useful in anxiety states and neurosis, e.g., Benzodiazepines and Buspirone. They have a calming effect in anxiety states associated with neurotic personality, situational crisis or physical disease. III Anti-depressants also called mood elevators or psychic energizers. For detailed classification see later. IV Mood stabilizers, e.g., Lithium carbonate. V Psychomotor stimulants: Methyl phenidate, Dextroamphetamine and Pemoline. VI Psychotogenic drugs which induce behavioural abnormalities resembling psychosis, e.g., LSD, Cannabis and hallucinogens.
Antipsychotic Drugs Introduction of antipsychotics revolutionised the treatment of schizophrenia, the most common of the serious mental illnesses. They are classified as: I Conventional/typical antipsychotics • Phenothiazines, e.g., Chlorpromazine, Trifluoperazine, Fluphenazine • Butyrophenones, e.g., Haloperidol, Trifluperidol. • Diphenylbutylpiperidines, e.g., Pimozide, Penfluridol and Fluspirilene. • Thioxanthenes, e.g., Chlorprothixene, Flupentixol and Zuclopenthixol. • Indolic derivatives, e.g., Molindone, Oxypertine. II Atypical antipsychotics • Dibenzodiazepines, e.g., Clozapine. • Substituted benzamides, e.g., Sulpiride, Risperidone, Paliparidone • Miscellaneous, e.g., Olanzapine, Quetiapine, Ziprasidone, Aripiprazole. Phenothiazine Compounds: The first clinically useful phenothiazine compound chlorpromazine, synthesized in 1950 as an antihistaminic, was shown to possess an amazingly large number of actions. Phenothiazine has a three ring structure in which two benzene rings are linked by sulphur and nitrogen atoms (Fig. 13.1). According to the chemical structure, phenothiazines could be predominantly antipsychotic, anticholinergic or antihistaminic (Table 13.1). Table 13.1 Some commonly used phenothiazine derivatives Compound and side chain Group I (P ropyldimethylamino) Chlorpromazine Promazine Triflupromazine Group II (P iperidine) Thioridazine Group III (P iperazine) Fluphenazine Trifluoperazine Perphenazine Proc hlorperazine Thiethylperazine maleate Group IV (Ethyldiethylamino) Diethazine Ethopropazine Promethazine
P harmacological properties Marked sedative and autonomic effec ts Marked autonomic , moderate EPR and antiemetic effec ts Marked EPR, high antiemetic and low autonomic effec ts
Daily dose mg* 100 to 1000 100 to 800 5 to 50
Moderate sedative and autonomic , less EPR but more c ardiotoxic 50 to 300 Moderate sedative, marked EPR and less autonomic effec ts. Less sedative, less autonomic and marked EPR effec ts Moderate sedative, marked EPR and less autonomic effec ts Antiemetic
2.5 to 10 5 to 10 12 to 24 10 to 30 10 to 30
Antiparkinsonism
Chapter 15
Antihistaminic (S ee Chapter 23)
Chapter 23
EPR = extrapyramidal reactions Note: Gp I, II and III are used as antipsychotics and sometimes as antiemetics. *
In divided dose
FIG. 13.1 Phenothiazine nucleus
Mechanism of action of antipsychotics: Antipsychotic drugs produce beneficial effects probably by affecting three of the major integrating systems in the brain, viz. (i) Mesolimbic system. (ii) Mesocortical system; and (iii) The hypothalamus. The drugs : • Block mainly postsynaptic dopaminergic D2 receptors (D2-antagonists) and to a smaller extent 5-HT receptors. • Modify the function of the mesolimbic system; and • Reduce the incoming sensory stimuli by acting on the brainstem reticular formation. Their therapeutic efficacy is mostly related to their ability to bind and to block the dopaminergic (D2) receptors in the mesolimbic system. At least 5 subtypes of dopamine receptors (D1-D5) have been described (Chapter 18). They are distributed in the limbic region, the frontal cortex, the basal ganglia, the midbrain and the medulla. Although several of these subtypes need to be blocked simultaneously for the maximum benefit, the predominant action appears to be at the D2. Blockade of dopamine action in the corpus striatum is responsible for the extrapyramidal reactions (EPR) often associated with these drugs. In addition to the D2 receptor blockade, most of the atypical antipsychotics have potent 5-HT antagonist action. Antipsychotics like risperidone and clozapine also block α2 adrenoreceptors. This may explain their usefulness in improving negative symptoms. Drugs that block muscarinic receptors cause less EPR e.g. chlorpromazine, clozapine. Thus, the action profile of these drugs and their adverse reactions can be explained to some extent on the basis of their affinity to multiple receptors. High dose (i.e. less potent) neuroleptics such as chlorpromazine tend to cause sedation and autonomic nervous system (ANS) adverse effects but have a lower propensity to cause EPR. Low dose (i.e. potent) neuroleptics such as haloperidol, fluphenazine and trifluoperazine are more selective in binding to the D2 receptors. Hence they cause EPR more often. There is, however, less sedation. CHLORPROMAZINE: Introduction of chlorpromazine into psychiatric practice by Delay and Deniker in 1952 marked the beginning of modern psychopharmacology. Since chlorpromazine is the most extensively studied antipsychotic phenothiazine, it is discussed below as a prototype. Other antipsychotic phenothiazine drugs differ from chlorpromazine mainly in potency and to a certain extent in their profile of actions (Table 13.1). Pharmacological actions: Behavioural and CNS actions: When chlorpromazine is administered to a normal monkey, the animal: • Loses its aggressiveness and its interest in the surroundings (quietening effect)
• Shows indifference to happenings around • Develops complete lack of initiative • Does not attack spontaneously; instead, it sits motionless. There is no change in the state of wakefulness and consciousness even with high doses. Control over the muscles and withdrawal from noxious stimuli remain unaffected. Chlorpromazine effectively blocks the conditioned avoidance responses, so that the animal forgets what it has learnt but, unlike after barbiturates, still escapes to safety as soon as a shock is felt. In patients with major psychosis with agitation, chlorpromazine produces psychomotor slowing, emotional quietening, diminution of initiative and anxiety, without affecting wakefulness (Neurolepsis). The subject sits in silence and shows indifference to the events around him, responding minimally to external stimuli. Although tolerance develops rapidly to its sedative action, the antipsychotic effect continues. Unlike with barbiturates, there is minimal ataxia and incoordination. Central nervous system: It causes: • Diminution of spontaneous motor activity. It produces a state of catalepsy where the body and limbs are moulded into various postures and remain immobile for prolonged periods. Catalepsy resembles but is not the same as catatonia seen in some schizophrenics; the latter is relieved by phenothiazines. • Induction of sleep with characteristic slow wave pattern on the EEG, and normalisation of sleep in schizophrenics. • Improvement of cognitive and intellectual functions but impairment of vigilance and motor response required in a variety of tests. • Antiemetic action: Chlorpromazine depresses the chemoreceptor trigger zone (CTZ) and thus acts as a potent antiemetic. It counters the effects of apomorphine (a dopamine agonist) on the CTZ in the medulla. It, however, is not effective in vomiting due to vestibular stimulation or that caused by local GI irritation. • Potentiation of the action of opioid analgesic drugs. • Prolongation of pentobarbitone sleep in animals. • Phenothiazines with an aliphatic side chain (Group I, Table 13.1) increase strychnine toxicity in animals and can precipitate seizures in epileptic patients. Autonomic nervous system: It acts as an autonomic suppressant. Because of its alpha adrenergic blocking action, it blocks certain actions of adrenaline and NA. It also has moderate anti-muscarinic and anti-5HT actions. It has a central depressant action on the hypothalamic centre controlling sympathetic activity. Cardiovascular system: Chlorpromazine may produce orthostatic hypotension due to inhibition of centrally mediated pressor reflexes along with peripheral adrenergic blocking action. It also dilates the blood vessels directly. It is a myocardial depressant and may cause defects in intra-ventricular conduction, prolongation of QT interval and blunting of T waves in the ECG. Hypothalamic pituitary-gonadal axis: As it blocks the dopamine receptors in the hypothalamus and the pituitary, it inhibits ovulation and produces amenorrhoea and galactorrhoea due to elevation of serum prolactin. It diminishes the libido in men. It also blocks the release of growth hormone. Miscellaneous effects: It has a potent local anaesthetic action. It prevents the shivering
response to cold and thus favours the development of hypothermia. Tolerance: In practice, patients develop tolerance to the sedative effect of phenothiazines. However, tolerance to the antipsychotic effect has not been observed. After sudden cessation of treatment with phenothiazines, withdrawal nausea and vomiting may develop in as many as 30% of subjects. Muscular discomfort, exacerbation of the psychotic state and insomnia may also occur. Although some degree of dependence on phenothiazines has thus been accepted, real drug dependence has not been demonstrated. The characteristic craving is absent and the withdrawal symptoms are essentially mild. Absorption, fate and excretion: Phenothiazines are well absorbed orally and parenterally. After absorption, phenothiazines are distributed in all the body tissues. Brain concentrations are much higher than the plasma concentrations. An active enterohepatic circulation prolongs the biological half life of chlorpromazine and its duration of action. Thus, chlorpromazine or its metabolites can be detected in urine even 6 to 12 months after discontinuation of therapy. Phenothiazines are metabolised in the liver by hydroxylation and subsequent glucuronide conjugation, sulfoxidation and demethylation. Adverse reactions: The phenothiazines are divided into three major groups (Table 13.1) based on their adverse effects (Table 13.2). Table 13.2 Phenothiazines grouped according to adverse effects
*
Refer Table 13.1
Neuroleptics from other chemical groups tend to resemble Group III. They include the butyrophenones (droperidol, haloperidol and trifluperidol); diphenylbutylpiperidines (fluspirilene and pimozide); thioxanthenes (flupentixol), oxypertine; and loxapine. Apart from common effects such as nasal stuffiness, dryness of mouth and palpitation, the adverse reactions include: • Intolerance: Skin eruptions of various types, photosensitivity and contact dermatitis are common. Rarely, yellowish brown or purple discoloration of the exposed skin may develop on prolonged therapy. The colour is due to melanin or a melanin-like substance formed by the phototoxic action of sunlight acting on the phenothiazine in the skin. The pigment can also occur in the brain, liver, kidneys, retina and the cornea. Visual impairment due to pigmentary retinopathy is known. • Extrapyramidal reactions (EPR): Many patients receiving phenothiazines show EP symptoms of parkinsonism, viz, tremor, muscular rigidity, excessive salivation and akinesia. These are due to blocking of dopamine receptors in the basal ganglia and can be countered by anticholinergic drugs such as benzhexol but not by levodopa or amantidine (Chapter 15). Motor restlessness, akathisia (inability to sit still), acute dystonic reactions and dyskinesias can also occur. Tardive dyskinesia is a late-appearing neurological
syndrome in patients on long term therapy. It is characterized by repetitive involuntary movements of lips and tongue with/without choreoathetosis. • Behavioural reactions: These include drowsiness, impaired psychomotor function, restlessness, excitement, psychotic reactions and toxic confusional states. However such reactions are rarely serious. Endogenous depression may develop after the patient has been on therapy for many weeks and the patient may commit suicide. Such depression should be watched for and should be treated. • CNS: The phenothiazines and other neuroleptics in large doses may occasionally produce epileptic seizures (particularly in individuals with history of seizures), and disturbances in temperature regulation. The effects however, are reversible. • Autonomic nervous system: The phenothiazines, by virtue of their antimuscarinic activity, may produce dryness of mouth blurring of vision, tachycardia, constipation or even paralytic ileus, difficulty in micturition and sometimes inhibition of ejaculation. Suppression of sympathetic system can cause postural hypotension and prolongation of QTC interval. Rarely hypotensive crisis on parenteral administration has been observed particularly in the elderly. Chlorpromazine should not be given IV, as fatalities due to a sudden fall in BP have been reported. Concomitant use of alcohol predisposes to this effect. Cardiotoxicity is more evident with thioridazine. • Haemopoietic system and the liver: These drugs may rarely produce agranulocytosis, thrombocytopenia and aplastic anemia. Reversible intrahepatic obstructive (cholestatic) jaundice occurs in about 0.5 to 2% of the patients receiving chlorpromazine. It is probably allergic in origin. Usually, it appears within first 6 weeks of therapy. Administration of phenothiazines during pregnancy has been associated with increased incidence of neonatal jaundice. • Endocrine and metabolic disturbances: Long term therapy may occasionally produce gynaecomastia and impotence, galactorrhea and menstrual irregularities due to increased prolactin. Aggravation of diabetes mellitus and weight gain as a result of increased food intake have been reported with most classical antipsychotics. • Neuroleptic malignant syndrome: This is a rare but potentially fatal reaction to the neuroleptic drugs. The manifestations include hyperthermia, fluctuating level of consciousness, muscular rigidity, and autonomic dysfunction with tachycardia, sweating, urinary incontinence and labile BP. The offending drug should be discontinued immediately as there is no proven effective treatment, although bromocriptine and dantrolene have been used. The syndrome may last for 5-15 days after the drug is discontinued. Preparations and dosage: The phenothiazines, divided according to the side chain attached to the nitrogen atom, are listed in Table 13.1. The commonly used preparations are: (i) Chlorpromazine hydrochloride tablets 10, 25, 50 and 100 mg; syrup 25 mg/(for adults) and 5 mg/ml (pediatric), and suppositories. Dose 25 mg to 1000 mg. (ii) Chlorpromazine injection 25 mg per ml. It is usually administered IM in the dose of 25 to 50 mg. The patient should be confined to bed for 30 minutes following IM injection in order to avoid postural hypotension.
(iii) Depot phenothiazines: Esters (such as decanoate, enanthate and palmitate) of fluphenazine, perphenazine, flupentixol and oxyprothepine, given IM or SC, release the active drug slowly. Satisfactory therapeutic response in schizophrenia can be obtained by injecting fluphenazine decanoate in the dose of 12.5-50 mg every 2 to 4 weeks. Drug interactions: See Table 13.3. Table 13.3 Interactions of phenothiazines with other drugs
Therapeutic uses: • Schizophrenia: This is discussed later. • Manic depressive psychosis (MDP): Chlorpromazine and haloperidol are both effective in the treatment of mania (see later). • Senile psychosis: The phenothiazines are sometimes useful in senile psychosis for controlling delusions and hallucinations. Care should be taken to use small doses as they can cause postural hypotension and falls. • Other neuropsychiatric disorders e.g., Huntington’s disease, where haloperidol is a preferred drug. • Drug dependence: They are useful in the management of psychosis associated with chronic alcoholism (alcoholic hallucinations) but are contraindicated in acute withdrawal syndromes (alcohol, opiates and other sedatives) for fear of precipitating seizures. • Behavioural disorders in children: Phenothiazines are sometimes used to control the excessively aggressive and destructive behaviour in children, and exert a quietening effect. In such cases, other causes of aggressiveness such as temporal lobe epilepsy, schizophrenia, hypoglycemia and drug (amphetamine) abuse should be ruled out. • Antiemetic and antihiccup: Chlorpromazine in small, nonsedating doses, is useful to control vomiting due to uraemia, radiation sickness and certain drugs. It can also be employed to treat nausea and vomiting of pregnancy; but the other phenothiazines (e.g. prochlorperazine) are preferred. It is not effective in motion sickness. Chlorpromazine is sometimes effective in the treatment of intractable hiccup. • Miscellaneous: Certain phenothiazines are used as preanaesthetic medication (Chapter 7). HALOPERIDOL: This butyrophenone is a very potent antipsychotic with similar clinical effects as piperazine phenothiazines. It is more effective in highly agitated or manic patients and has less prominent sedative and autonomic effects. It is given orally in the dose of 1.5 to 7.5 mg bid. It can also be given IM in the dose of 210 mg, repeated every hour up to a total of 30 mg, in highly agitated and violent patients. Depot injectable preparations of haloperidol are also available. The incidence of EPR with this drug is high. Irreversible toxic encephalopathy has been reported in patients on lithium, given high doses of haloperidol. The other drugs of this series are trifluperidol (Triperidol) and droperidol which are used in combination with
fentanyl for neuroleptanalgesia (Chapter 7). DIPHENYL BUTYL PIPERIDINES: Pimozide 2-10 mg once daily may have the advantage in that the prescribed dosage can be administered under supervision. Penfluridol, structurally related to haloperidol, has a long duration of action. It is given orally in the dose of 20-100 mg once a week. Such oral preparations are useful in practice to eliminate failure of patient compliance. Other antipsychotics such as chlorprothixene, clopenthixol, centbutandol, molindone and prothipendyl mainly differ from the phenothiazines in their pharmacokinetic properties and in their sedative, autonomic and extra-pyramidal effects. Unlike other neuroleptics which react with various dopamine receptors, sulpiride is a more specific antagonist at post-synaptic D2 receptors.
Atypical Antipsychotics (Second Generation) The classical antipsychotic drugs described above can cause EPR even in therapeutic doses. Further, they are less effective against negative symptoms. ‘Atypical’ antipsychotics in general: • Have lower propensity to cause EPR and tardive dyskinesia than phenothiazines and haloperidol. • Help to improve negative symptoms • Are less likely (except risperidone) to cause hyperprolactinemia; • Have greater affinity for other neuroreceptors such as 5-HT, α adrenergic, histaminergic and muscarinic, than the classical antipsychotic drugs (Table 13.4); Table 13.4 Actions of typical and atypical antipsychotic drugs on various CNS receptors
D = Dopamine, 5-HT = 5-Hydroxytryptamine H = Histamine, M = Muscarinic
• Cause dose dependent ADR which include sedation (more with clozapine, olanzapine), anticholinergic effects and postural hypotension, and • Have been found useful as an adjuntive in the treatment of major depressive disorders (MDD). CLOZAPINE: This antipsychotic drug, related to tricyclic compounds such as imipramine, was synthesised in 1960. As it was found to cause agranulocytosis, its use was abandoned. It has now staged a comeback for a specific indication viz. in the treatment of schizophrenia resistant to classical anti-psychotics. It has selective effects in the limbic, dopaminergic systems wherein it blocks D1, D2, D3 and D4 receptors. However, it has more potent action in blocking the 5HT2 receptors than D2 receptors. Its other actions include antiadrenergic and anticholinergic actions. It differs from phenothiazines in that it causes fewer EPRs and does not cause hyperprolactinemia. Given orally, it produces antipsychotic effects similar to those of haloperidol. Its major advantage is that the drug improves not only the positive symptoms but also the negative symptoms such as emotional withdrawal, blunted affect, retardation and social withdrawal. It is started in the dose of 12.5 mg once daily and gradually increased to 200-450 mg/day in
divided doses. Adverse reactions: These include nausea, vomiting, sedation, postural hypotension, marked tachycardia, ileus, sialorrhoea, confusion and delirium. Its main drawback is the relatively high incidence of grand mal seizures and agranulocytosis. Because of its toxicity, the drug should be used only in patients resistant to standard therapy (see above) and that too under supervision and regular blood counts. OLANZAPINE: This dibenzothiazepine causes greater 5-HT2 than D2 receptor blockade. Further, it does not antagonize α2 receptor function; and hence, it causes less EPR and cardiovascular toxicity. Orally, it is absorbed well but about 40% is metabolised during first pass through the intestinal wall. In dose of 5-25 mg, once daily, the drug is as effective as haloperidol in reducing psychotic symptoms and also acts against negative symptoms. The incidence of hyperprolactinemia and sexual dysfunction is also lower than with risperidone. The drug may also be useful in children with developmental CNS disorders and in patients with Tourette syndrome. Adverse reactions: These include dry mouth, sedation, nausea, postural hypotension and constipation. It has a propensity to cause weight gain, and increase in glycosylated hemoglobin, total cholesterol and triglycerides. It should be avoided in elderly patients, especially hypertensives, because of increased risk of cerebrovascular accidents. QUETIAPINE: It has effects similar to those of olanzapine; but it is less likely to cause weight gain. It has a short t½ and is administered bid. RISPERIDONE: This atypical antipsychotic though less effective has a profile similar to that of clozapine in respect of negative symptoms. It has action on D2, 5-HT, alpha adrenergic and histaminergic receptors. It, however, causes dyskinesias such as akathisia and hyperprolactinemia. Other adverse reactions include postural hypotension, dizziness, insomnia and constipation. It is as effective as haloperidol. It is less likely to precipitate epileptic seizures. It does not cause blood dyscrasias. The starting dose is 2 mg/day in divided doses increased gradually to 4-6 mg/day. A depot preparation is also available. The drug also appears to be effective and well tolerated for the treatment of tantrums, aggression and self-injurious behaviour of children with autistic disorders. Risperidone and olanzapine, although reported to be beneficial for calming agitated or aggressive patients with dementia, their use in elderly patients is not recommended. Paliperidone is the primary active metabolite of risperidone. It is well absorbed orally. It is not extensively metabolised by CYP450 enzymes; hence it is less likely to have drug interactions. Iloperidone, another analogue of risperidone can prolong QTc interval. Ziprasidone and aripiprazole are the other, second generation antipsychotics. Ziprasidone is also claimed to possess anxiolytic and antidepressant properties because of its affinity for 5-HT receptors. Table 13.5 summarises the efficacy and ADR of the second generation antipsychotics.
Table 13.5 Relative efficacy and toxicity of Second Generation antipsychotics
*
Can cause agranulocytosis, myonecrosis; Posthypo=Postural hypotension; Hyperprolact=Hyperprolactinaemia
Carbamazepine, an antiepileptic, is useful as an adjunct in the treatment of schizophrenic patients with aggressive or violent behaviour or agitation, who show resistance to the usual antipsychotic drugs (Chapter 9). RESERPINE: This plant-derived drug is no longer used as an atispychotic agent but is described here for its historical value. The plant Rauwolfia serpentina (Benth) is a climbing shrub indigenous to India. It was so named in honour of Dr. Leonard Rauwolf, a 16th century botanist. It is called serpentina (sarpagandha) because of the resemblance of its root to a snake. The crude preparation was used regularly in Indian traditional medicine to quiten babies, to treat insomnia and even insanity. From amongst the various alkaloides of this plant (reserpine, serpentine and ajmaline), reserpine is well studied. It is interesting to note that Dr. RA Hakim from Bombay (Mumbai) received a gold medal at the regional conference (1953) for his presentation, “Indegious drugs in the treatment of mental diseases” reporting the results of its use in schizophrenics. Mechanism of actions: Reserpine is of great pharmacological interest because it depletes endogenous catecholamines and 5-HT from the brain and peripheral sites by interfering with amine storage. Such depletion can last for days or weeks. A single dose of 5 mg/kg. body weight in animals is sufficient to cause 90% reduction in brain NA and 5-HT over a period of 10 days. The depletion of cerebral monoamines is responsible for its central tranquilising actions. Pharmacological actions of reserpine: • Central nervous system: It has antipsychotic action resembling that of chlorpromazine; however, it has no antihistaminic, anticholinergic or direct antiadrenergic action. Like chlorpromazine, it produces a calming effect as well as EPR in man, but without clouding the consciousness. Reserpine is less effective than chlorpromazine in the treatment of schizophrenia. It may also cause mental depression precipitating suicidal tendencies; hence, it is not used as an antipsychotic. • Cardiovascular system: Reserpine lowers BP and is used as an antihypertensive drug (Chapter 29).
Management of Schizophrenia Schizophrenia is a serious mental disorder characterised by persistent disturbance in the perception and evaluation of reality, leading to characteristic changes in the perception, thinking, affective responses and behaviour. The word ‘schizophrenia’ was coined from a Greek word meaning ‘split mind’ to describe a mental syndrome where an individual is dominated by one set of ideas or a complex to the exclusion of others. Thus, the harmonious working of the mind is split. The schizophrenic patient, therefore, lives in his own world, dissociated from reality. He is a victim of illusions (perception falsified, e.g., mistaking a rope for a snake), hallucinations (perception without a stimulus, e.g., hearing God talking) and delusions (false beliefs, not based on cultural mores, that cannot be corrected by logic and reasoning); and believes that only his behaviour and actions are rational, without realising that he is ill (lack of insight). His mental functioning is sufficiently impaired to interfere grossly with his capacity to meet the ordinary demands of life. The disease is common in young people between the ages of 18 and 28, exhibits a hereditary tendency, and generally is a recurring illness. It may exist in several varieties; and the paranoid form (delusional disorder), in which the individual becomes suspicious of and belligerent towards the entire society, is perhaps the most dangerous. The etiology of schizophrenia is complex and still unknown. It is believed to be due to disturbances in cortico-striatal-thalamic circuit. Several structural brain abnormalities have been described in schizophrenia. Although increased activity of the brain dopamine pathways is important, the other brain neuro transmitters are also involved in pathogenesis. The symptoms of schizophrenia can be both positive and negative. The positive symptoms include hallucinations, delusions, agitation, repetitive behaviour and thought disorder. The negative symptoms include psychomotor slowing, marked social withdrawal, anhedonia (inability to experience pleasure), paucity of speech, apathy, as well as lack of energy and motivation. Antipsychotics do not cure schizophrenia but they alleviate disturbing symptoms. Thus, they reduce hallucinations, aggression, agitation and anxiety and make the patient more co-operative and acceptable. Disturbed thinking, paranoid symptoms, delusions and personal neglect improve. Improvement usually commences during the first 7-21 days, but may be delayed by as much as 5-6 weeks. About 70% of patients with first episode show favourable response. It is not possible to predict which cases will respond promptly. A proportion of patients who do not respond to antipsychotics alone, may respond to antipsychotics and electroconvulsive therapy (ECT) combination. Patients in catatonic excitement are better controlled initially with ECT, and drug therapy is started later. Unless there are physical contraindications, ECT and drugs can be combined if indicated. Antipsychotics have made it possible for the patients, who otherwise would have needed prolonged hospitalisation in mental ‘asylums’, to stay at home and engage in productive activity. The drugs are started in the smallest possible dose and increased gradually as needed. All antipsychotics are effective in controlling the core symptoms of schizophrenia and schizophrenia like illness. The initial choice is guided by symptom profile and possible ADR of the drug. Chlorpromazine (medium potency) or haloperidol (high potency) may be
preferred. The oral dose of chlorpromazine varies widely from 100 to 1000 mg per day. The initial dose usually is 25 mg three times a day in adults, increased gradually to 300 mg a day. Highly agitated, rowdy and violent patients need larger doses, sometimes given parenterally. Haloperidol is administered orally in the dose of 2.5-7.5 mg/day and increased gradually upto 20 mg/day. High doses of neuroleptics offer little advantage over smaller doses, at least in the majority of acute psychotic episodes. Although relapse rates may be lower in patients maintained on what are regarded as standard doses, those maintained on lower doses may have the advantage of improved social and vocational functioning. Chlorpromazine may cause more drowsiness and depression while haloperidol may cause more EPR. Some authorities routinely combine phenothiazines with benzhexol for preventing EPR. Most neuroleptics, if consumed in very high doses, do not cause life-threatening coma, and the lethal dose is very high. Patients who cannot be relied upon to take the drug regularly can be treated with weekly to biweekly injections of a depot phenothiazine. Schizophrenia is a relapsing disease; hence, after the therapeutic response is obtained, the drug should be continued in smaller maintenance doses for a long time (sometimes lifelong) even after the first episode of illness. Daytime drowsiness may interfere with the patient’s ability to work, but this could be reduced if he takes most of his daily dose at night. The withdrawal should be slow (6-12 months) as reappearance of symptoms following withdrawal is not uncommon. A longer maintenance period is recommended, particularly in patients with a history of relapse. Table 13.6 gives the equipotent oral doses of various neuroleptics. Table 13.6 Equipotent oral doses of neuroleptics
Acute schizophrenic reaction needs initiation of treatment with the equivalent of 400 mg of chlorpromazine per day given in four divided doses. On subsequent days, the daily dose is increased by the equivalent of 200 mg of chlorpromazine till the acute reaction is controlled. The maintenance dose for preventing recurrence in chronic cases is the equivalent of 50-200 mg of chlorpromazine per day. Several randomised controlled studies have reported no significant differences in overall
effects between typical and atypical agents. The possible advantage of an atypical antipsychotic is that they may cause fewer EPR and may show more activity against negative symptoms. However, they may cause weight gain, hyperglycemia, hyper-prolactinaemia and disturbed lipid metabolism. Some of them are associated with increased cardiovascular risk (prolongation of QTc). Atypical antipsychotics also are much more expensive. Hence, an atypical antipsychotic is reserved for some selected patients. Clozapine is considered the most effective atypical antipsychotic but is probably more toxic. Compared with placebo, an increased risk of stroke and transient ischaemic attacks has been reported in elderly patients with dementia receiving atypical antipsychotics for the treatment of behavioural disorders. The causes of non-response to treatment are listed in Table 13.7. Irrespective of the drug used, many chronic schizophrenics discontinue the medication probably because of lack of efficacy, adverse reactions or lack of supervision; in this respect, cooperation of patient’s relatives is vital. Table 13.7 Causes of non-response to neuroleptic therapy
The neuroleptics that stimulate the secretion of prolactin are better avoided in patients with established breast cancer. Both typical and atypical antipsychotics may provoke seizures in susceptible subjects e.g. epileptic patients.
Manic Depressive Psychosis – Management Manic depressive psychosis (MDP), a bipolar disorder, is a highly recurrent and heterogenous illness. It has a strong genetic propensity; about 50% of the patients have a positive family history. The central features of MDP consist of unpredictable swings in mood as Mania followed by Depressive episodes, with near normal behaviour in between. Mania is characterised by elevation of mood and overactivity. A manic patient is energetic, cheerful and optimistic. Sleep is reduced; speech is often rapid and copious. Appetite and sexual desire may increase. The patient believes that his ideas are brilliant and that his work is of outstanding quality. This may be accompanied by grandiose delusions and occasionally hallucinations. It may also be presented as hypomania, which is less dramatic but with similar features as above. Subject may be over talkative and mildly reckless. This is followed by a brief episode of depression, characterised by low mood, poor appetite and insomnia. Many patients, however, can exert some control over their symptoms, at least for a short term, and often remain undiagnosed. Depressive and maniac symptoms sometimes occur at the same time. Patients who are overactive and talkative may be having profoundly depressive thoughts. Some manic patients may become intensely depressed for a few hours and then return quickly to manic state. The main objectives of treatment are: (a) Immediate control of acute mania or depression; and (b) Long term prevention of recurrences. Pharmacotherapy of bipolar depression is summarised in Table 13.8. Table 13.8 Pharmacotherapy of bipolar depression (MDP)
SSRI = Selective serotonin (5-HT) reuptake inhibitors ECT= Electroconvulsive therapy Levothyroxine is used only as an adjunctive drug along with other definitive drugs
The immediate treatment of acute mania includes either lithium, sodium valproate or carbamazepine. However, all the 3 drugs have delayed onset of effects. Hence, the treatment is started usually with anti-psychotic drugs, and haloperidol is usually preferred; chlorpromazine can also be used though it causes more sedation. They control hyperactivity and psychotic features of the severe mania. After initial large doses (IM, if necessary), the patient is maintained on smaller oral doses depending upon the degree of overactivity. Lorazepam (1-2 mg every 4 hrly) may be used in severe cases to control over
activity. Once the patient is able to cooperate, kidney and thyroid function should be tested and lithium treatment started. Atypical antipsychotic drugs like olanzapine, ziprasidone, and aripiprazole given IM also induce rapid control. Adjunct therapy with BDZ also helps to calm the patient and induce sleep. Lithium salts remain the treatment of choice in acute manic state, and are generally effective as monotherapy in mild to moderate severity. They should be used only in patients with normal sodium intake. Lithium carbonate is usually given initially in the loading dose of 600 mg followed by 300 mg bid or tid. The dose is increased by 300 mg every 2-3 days till plasma levels are 1-1.5 mEq/L. It takes 2-3 weeks for full therapeutic effect. The maintenance dose recommended is 300-400 mg twice a day. Doses are adjusted to maintain a plasma level of 0.5 to 1.0 mEq/L 12 hours after the preceding dose. In patients resistant to lithium, other drugs may be added (For details see Chapter 14). SODIUM VALPROATE, (Chapter 9), acts faster and produces beneficial effects in 3-5 days. It is preferred in patients who get frequent attacks (4 or more per year). The dose is 250 mg tid to achieve plasma levels of 90-120 mcg/mL. CARBAMAZEPINE: Response rate to carbamazepine is lower than to lithium or sodium valproate but it is used as an alternative to lithium in patients with acute mania who do not tolerate or do not respond to lithium. It does not show rebound effect as seen following early withdrawal of lithium. It is generally used in the total daily dose of 400-600 mg, given in divided doses. Oxcarbazepine can also be used in a dose of 150mg bid. Patients presenting with bipolar depression are treated with lithium and antidepressant agents (Table 13.8). The atypical antipsychotics and supraphysiological doses of levothyroxine have been used as add on drugs in severe and refractory bipolar depression. However, in such cases ECT is preferred because of its proven, rapid anti-depressant effect. For long term prophylaxis of MDP, atypical antipsychotic drugs are used with moodstabilisers. Combination therapy is given for 2-4 months after control of mania, followed by mood-stabiliser alone. Lithium is used in the dose of 600-1000 mg daily in two divided doses, 12 hours apart. Prophylactic treatment with lithium reduces risk of suicide and is needed for more than 2 years. Early withdrawal can cause recurrence. Sodium valproate, lamotrigine or gabapentin are also used for maintenance. Verapamil, a CCB, is also claimed to be useful.
14
Psychopharmacology - 2: Anxiolytics, Antidepressants and Mood Modifying Agents Anxiety disorders are perhaps the most common psychiatric illness encountered in general practice. Anxiolytics are the drugs used for the treatment of anxiety disorders. Their CNS depressant effect is dose dependent: (a) In smaller doses, they relieve anxiety (b) In larger single doses they induce sleep and can, therefore, be grouped together with sedative hypnotics. (c) Because of their depressant effect on the motor cortex, many of them also act as muscle relaxants and anticonvulsants (Chapter 9). BENZODIAZEPINES (BDZ): They are the most commonly prescribed anti-anxiety agents. All anxiolytic BDZ have similar properties; however, they differ in their pharmacokinetic profiles (Chapter 8). Diazepam is the most commonly used drug. Mechanism of action: The exact mechanism of antianxiety action is not known though they act at many levels of the neuraxis. Experimentally, they have been shown to act on the limbic system, the hypothalamus and the brain stem reticular system. They bind to BDZ binding sites on GABA receptors and facilitate the action of GABA (Chapter 8). BDZ also reduce the turnover of brain 5-HT and NA. Pharmacological actions: In both animals and humans, BDZ produce sedation, reduce aggressiveness and thus have a calming (taming) effect. Unlike chlorpromazine, they block conditioned as well as unconditioned responses. Clinically, they produce beneficial effects in anxious, neurotic patients. Such benefits are, however, difficult to assess. BDZ, however, are capable of causing memory impairment (anterograde amnesia) and other cognitive impairment; this is their major drawback (Chapter 8). CHLORMETHIAZOLE: Chlormethiazole ethane-di-sulphonate is a thiazol derivative with sedative, hypnotic and anti-convulsant actions. Given orally, it is absorbed rapidly but undergoes extensive first pass metabolism. It has been used orally or by injection in delirium tremens to induce sedation. The adverse effects include tingling sensation particularly in nose, and a moderate fall of BP on IV administration. Failure to produce marked respiratory depression even in excessive doses is an advantage. BUSPIRONE: This azaspiro decanedione (azapirone) is an anxiolytic not related to BDZ and lacks the sedative-hypnotic, muscle relaxant and anticonvulsant properties of BDZ. It acts as a partial agonist of inhibitory, presynaptic 5-HT1A receptors and inhibits autoreceptors; this reduces the release of 5-HT which probably explains its anxiolytic action. It also has a weak D2 receptor antagonistic action. The drug does not potentiate the CNS depressant effect of the commonly used depressant medications and, therefore, may be particularly useful in anxious, elderly patients. It causes less cognitive and psychomotor impairment than diazepam. It has a short t½ (2-5 hours) and it is prescribed in the dose of 30 mg/day in divided doses. It does not cause tolerance/dependence nor interacts with alcohol. It has a wide
margin of safety. Its important drawback is that its onset of action is slow, requires thrice a day dosing and may take as much as two weeks for its anxiolytic effect. This effect is weaker than that of benzodiazepines. Further, it is not useful in severe anxiety with panic disorder and in alcohol withdrawal syndrome. The adverse reactions include GI disturbances, nervousness, dizziness, confusion and tachycardia. Patients on MAOI can develop hypertension when given buspirone. Gepirone, ipsapirone and trospirone are the newer analogues of buspirone. Other non-barbiturate sedative-hypnotics like hydroxyzine hydrochloride, diphenhydramine and buclizine hydrochloride are sedative antihistaminics and are promoted as OTC drugs for treating insomnia. More sedative tricyclic anti-depressants such as amitriptyline and doxepin are also promoted for the treatment of anxiety state. They do not cause muscle relaxation and are likely to cause adverse effects such as dryness of mouth, palpitation, daytime sedation and confusion. None is superior to BDZ either as anxiolytic or hypnotic; however, they are not addictive.
Treatment of Anxiety Disorders Some amount of anxiety (fear of the known or unknown) is a normal physiological response that assists the individual in solving various problems in life. Amygdala modulates fear and anxiety. Pathological anxiety is that which has no apparent external cause, and exhibits heightened amygdala responses resulting in high intensity of symptoms which persist over time, and lead to the development of harmful behavioral strategies (avoidance, compulsions etc.) that impair function. The latter are associated with reduced activation thresholds of prefrontal cortex and limbic system. Anxiety may be secondary to stressful situations, bodily disease or the use or withdrawal of drugs/substances of abuse. It is a cardinal symptom of many psychiatric disorders. Primary anxiety disorders are those in which no cause is discernible. Depending on the manifestations, these disorders are given such names as : stress related adjustment disorders, phobias, panic disorders, obsessive compulsive disorder and generalised anxiety disorder (GAD). Anxiety-associated disorder is a common condition in which anxiety is the most prominent symptom. The patient is generally aware of his/her symptoms and the probable cause. These reactions have been considered as the maladaptive results of conflict between unfulfilled desires and repressive tendencies. Anxiety has two components: (1) A psychological component (dread, unpleasant anticipation or a feeling of impending doom); and (2) A physical component (autonomic arousal). The reactions and emotions of a patient suffering from anxiety disorder are usually an exaggeration of those experienced by normal persons in day to day life. Subjects generally complain of headache, tension, feeling of a tight band round the head, palpitation, tremulousness, dryness of mouth, hyperhidrosis, coldness of extremities, spasm of back muscle giving rise to vague bodyaches, and insomnia. Such patients may also suffer from bowel disturbances and phobias such as fear of dying, insanity and heart disease. The appetite and libido, however, are not much affected, and thoughts of committing suicide are absent unless there is underlying severe depression or major psychosis. Currently, BDZ and SSRI are the most commonly used drugs to treat anxiety disorders. In general, there is little difference among the various BDZ as anti-anxiety agents. However, a short course of fast-acting, high-potency BDZ such as alprazolam, clonazepam or lorazepam is preferred in severe anxiety states with marked autonomic overactivity. Oxazepam in small, divided doses may be preferred in the elderly and in those with hepatic dysfunction, because of its short duration of action. Anxiolytics can reduce the somatic and autonomic disturbances, abolishing physical malaise, bodyache and anxiety. None of these agents, however, produce permanent benefit which can come only from realisation by the patient of the nature of his problems and his adjustment to them. Counselling and psychotherapy are more effective. Drugs should be used only for a short term to lessen the patient’s distress. Anti-anxiety agents are particularly useful in treating acute stress reactions. Since insomnia is a common complaint of these patients and as most anti-anxiety drugs (not buspirone) induce sleep, the major dose should be given at bedtime. BDZ are considered safer when
suicidal tendencies are suspected. As they are quick acting, the initial treatment always should be with BDZ. However, they can cause dependence and interaction with alcohol. Hence, they are not recommended for chronic disorders. SSRI do not cause sedation, but they have slow onset of action and can delay orgasm. Once started these drugs must not be stopped suddenly, but withdrawn slowly over 4-8 weeks, if needed. Since anxiety is often episodic and varying in intensity, drugs should be used to treat each episode and not be given continuously for prolonged periods. It must be emphasised that in anxious patients, placebo responses are frequent. In some patients with severe anxiety, somatic symptoms such as palpitation, trembling and giddiness dominate the clinical picture. In such cases, the beta-adrenergic blocking agent, propranolol may be useful. In practice, many patients complain of ‘tension’ with vague symptoms without any obvious signs of illness. These are due to minor maladjustments in day to day life and do not really need drug therapy. However, in the highly technical age that we live in, one always seeks a technical solution to every problem. Unfortunately, such ideas are encouraged by drug firms and further enhanced by media by publishing confusing reports. Drugs can temporarily modify the patient’s emotional response to environmental factors. They are not expected to influence the environment or socioeconomic situations. As pointed out, “It is required that we cope actively and almost constantly with an outer environment and that we learn how to do this without disintegrating in new anxiety. To live is to be under tension, to be dissatisfied; to be anxious, sometimes unbearably so; to be angry, sometimes potently and impotently; to be everlastingly hungry, to some degree, for things that may be consciously well defined or very vague; to become depressed and discouraged; to become physically and psychosomatically ill; to worry obsessively and to become hysterically emotional. The range of normal functioning of mind is wide and flexible.” Certainly, anxiolytics should not be used to modify such normal cyclic behavioural changes without assessing the disability produced. Indiscriminate use of anti-anxiety agents for prolonged periods may kill all the initiative in the individual and may cause drug dependence. Effective treatment of anxiety neurosis needs patient co-operation. The majority can be helped more by an empathetic approach of the doctor and relatives than by drugs. Severe anxiety, however, may be extremely disabling and often may be the presenting symptom of a more serious psychiatric disorder such as schizophrenia or depression. In such cases appropriate treatment of the underlying disorder is important, and BDZ, in general, can be added to antidepressants or antipsychotics. Phobic anxiety is observed in specific situation. Its intensity increases as the person approaches the feared situation such as air or ship travel, public speaking, interviews etc. and it remains as long as the exposure to that situation lasts. The person will try to avoid the situation if possible. Patients with phobic anxiety may suffer from panic attacks when suddenly or overwhelmingly exposed to the feared situation. The term panic disorder is used to describe a condition in which panic attacks appear to occur spontaneously and repeatedly. Because of marked somatic symptoms, its diagnosis is likely to be missed. Panic disorder has distinct symptoms. The patient abruptly develops a feeling of intense fear, impending disaster or death accompanied by various physical symptoms such as palpitation, sweating, trembling, feeling of choking, abdominal distress,
chills or hot flushes etc. Panic attacks are many times associated with agoraphobia, a morbid fear, and avoidance of being alone or being in a public place, resulting in a marked restriction on travel. Medical conditions that are commonly noted in subjects with panic attacks include episodic high blood pressure, acute dyspepsia, cardiac arrhythmias, and mitral valve prolapse. In fact, such patients may often receive treatment primarily for such secondary disorders and not the anxiety state. Hyperventilation (over-breathing) with resulting symptoms such as carpopedal spasms is an important diagnostic feature of panic attacks. Not all panic attacks constitute panic disorder. Often, a similar picture may be observed in specific phobias e.g. at heights or on seeing a snake; or in social phobias such as facing a stranger or an examination (where a person is likely to be scrutinised). However, patients with panic disorder may not be aware of the source of their fearfulness. Further, panic disorder is often associated with underlying major depression. Panic-like attacks may also occur in hyper-thyroidism, pheochromecytoma and drug abuse. The treatment of choice for panic disorder is: • A selective serotonin reuptake inhibitor (SSRI) such as fluoxetine, paroxetine or sertraline is effective in inducing response and remission. They are considered as first line therapy. However, they take some time to act (see later). In addition, they help to control the co-morbid depression. Fluoxetine appears to be the most efficacious but sertraline is better tolerated. Escitalopram is less likely to cause hepatic enzyme interactions and may be appropriate for patients receiving other medications for associated illnesses (see later). • A tricyclic antidepressant such as imipramine or amitriptyline are as effective as the SSRI and are less expensive. However, TCA with prominent anticholinergic effects such as amitriptyline may not be tolerated by the elderly. • A high potency benzodiazepine, alprazolam, in the starting dose of 0.25 mg 2-4 times a day orally, works faster (in days, unlike antidepressants which take weeks). If needed, it can be given IV. It causes sedation. Other BDZ are equally effective, and the long acting ones such as clonazepam (t½ 18-50 hours) are easier to withdraw and are favoured. The β-adrenergic blocker, propranolol, is an adjunct for controlling tachycardia, palpitation and tremors during social phobia. By itself, it does not counter the basic anxiety. BDZ as anxiolytics are not useful in depression, phobic or obsessional states and chronic psychosis. However, they may be used initially, concurrently with SSRI/TCA for a speedier response. They are tapered over 4-12 weeks while SSRI is continued. In bereavement, psychological adjustment may be inhibited by BDZ. In children, anxiolytics should only be used to relieve acute anxiety caused by fear e.g. before surgery. Drug therapy should be combined with cognitive-behavioural therapy (CBT). CBT alone may not be as helpful; but it may certainly enhance the long term well-being of the patient. The term general anxiety disorder (GAD) is used when excessive anxiety (tension) and persistent worry are present on most days of the week for at least six months. The symptoms are restlessness, difficulty in concentrating, easy fatiguability, irritability, muscle tension and disturbed sleep, along with symptoms such as palpitation, dry mouth and sweating. Such symptoms are gradual in onset and do not create life-threatening fear but relapses are common. Major depression is the most commonly coexisting psychiatric disorder in
patients with GAD, occurring in almost 60% of the patients, whereas panic disorder occurs in 25% of the cases. The links between GAD and the personality trait in neuroticism are very strong and are controlled by genetic factors; hence, the prevention of GAD is very difficult. The current therapy of GAD includes a combination of BDZ, SSRI/TCA and cognitive behavioral therapy. Some patients will need maintenance drug therapy almost lifelong. The initial therapy should be with a combination of BDZ and SSRI; the dose of BDZ is tapered after 2-3 weeks when SSRI become effective. Severe, intractable GAD may need MAO inhibitors. Emphasis should be on counselling, exercise, mental relaxation/meditation and behavioural therapy. Excessive intake of stimulants such as caffeine and cola in any form, and diseases such as thyrotoxicosis should be excluded. Many patients with mild anxiety symptoms are able to function well, sometimes even better than normal subjects, in daily life. Hence, drugs should be used only if symptoms interfere with normal social functioning. Even in these, many patients are likely to respond to the minimum therapy, and one should not rush into treatment approaches that involve long term risk and expense.
Antidepressant Drugs The syndrome of depression is a major affective disorder, common in the general population, and is many times underdiagnosed. Although biochemically, it is associated with depletion of brain monoamines, 5-HT and NA, its causes are complex and not well understood. Successful treatment of depression with drugs is one of the major advances in psychopharmacology in recent years. Several drugs are now available as ‘antidepressants’, sometimes also called as ‘psychoanaleptics’ or ‘mood elevators’. They act by increasing the intrasynaptic availability of the monoamines (NA, 5-HT) in the brain. This is achieved by: (1) Inhibiting the neuronal reuptake of such amines, (2) Receptor blockade or (3) Inhibiting amine metabolism by enzyme inhibitors such as MAOI. Thus, drugs can be classified into: I Monoamine oxidase inhibitors (MAOI) • Irreversible: (a) Hydrazine MAOI, e.g., Isocarboxazid, Iproniazid and Phenelzine. (b) Nonhydrazine MAOI, e.g., Tranylcypromine. • Reversible: e.g. Moclobemide II Serotonin-noradrenaline reuptake inhibitors • Tricyclic antidepressants (TCA) mainly act by inhibiting reuptake of NA. Their action on reuptake of serotonin is variable. They can be classified as: (a) Predominantly NA-reuptake inhibitors e.g Desimipramine, Amitriptyline, Protriptyline etc. (b) Predominantly 5-HT-reuptake inhibitors e.g. Clomipramine. These agents are termed “nonselective” as they also interact with H1, α1 and muscarinic receptors to variable extent. • Selective 5-HT-NA reuptake inhibitors (SNRI) Venlafaxine, Duloxetine, Milnacipran. III Selective serotonin (5-HT) reuptake inhibitors(SSRI): Fluoxetine, Paroxetine, Fluvoxamine, Sertraline, Citalopram, Escitalopram. IV Selective NA reuptake inhibitor (NARI): Reboxetine. V 5-HT 2 receptor antagonists: Trazodone, Nefazodone. VI Miscellaneous: • Unicyclic: Bupropion • Tetracyclic: Amoxapine, Mirtazapine, Maprotiline
Monoamine Oxidase Inhibitors (MAOI) This heterogeneous group of drugs acts by blocking the oxidative deamination of naturally occurring amines such as NA, 5-HT and DA. Mechanism of action: Relatively large amounts of 5-HT and NA are present in the hypothalamus and in other subcortical regions of the brain. These amines are stored in granules in the neurons and are released following neuronal stimuli. The active amines thus liberated act on the postsynaptic receptors but do not accumulate as they are immediately metabolised by the enzyme MAO. It is present intracellularly in most of the tissues, particularly the CNS, gut and liver. The two types, MAO-A and MAO-B, differentially affect the metabolism of neurotransmitters in humans. • Inhibition of the MAO-A decreases the deamination of NA and 5-HT. This causes increase in local NA and 5-HT which is associated with both antidepressant action and hypertensive interactions with foods containing tyramine and with sympathomimetic drugs. In animal experiments, accumulation of these amines is associated with excitement and enhanced motor activity. • Selective inhibition of MAO-B, which preferentially decreases the deamination of dopamine, is not associated with antidepressant action or hypertensive interactions, but is useful in treating parkinsonism (Chapter 15). Given orally, these drugs exert a considerable effect on liver MAO enzymes because of their high concentration in portal circulation. Pharmacological actions: • Behavioural effects: These drugs elevate the mood of depressed individuals. Subjects feel more energetic, less sleepy and more fresh. Tendency for suicidal rumination diminishes. In some cases agitation, talkativeness and restlessness may occur. The action is seen after a latent period of a few days to 3-4 weeks. • Cardiovascular effects: There is no specific action on heart or the coronary flow. Some MAOI may, however, cause hypotension. • Reserpine reversal: Normally, animals treated with reserpine are inert, apathetic and do not take interest in the surroundings. In animals pretreated with MAOI, administration of reserpine produces agitation and excitement due to accumulation of amines. This is known as ‘reserpine reversal’. • Potentiation of action of sympatho-mimetic amines: MAOI potentiate the sympathomimetic actions of other amines like amphetamine and ephedrine. • Miscellaneous: As these drugs also inhibit MAO and other enzymes present in the liver, they prevent the metabolism of many drugs and prolong their actions. This may precipitate toxicity. They are potent REM sleep inhibitors. Absorption, fate and excretion: All compounds are well absorbed orally. Information about their metabolism in man is inadequate, but the effect of MAOI continues for 10 to 14 days after the drug is withdrawn. This is due to their irreversible action on the MAO enzyme, which may take over 2 weeks to return to normal level. Adverse reactions: • Behavioural effects: These include headache, excitement, agitation, hallucinations and disturbed sleep. These drugs may activate latent psychosis, hence their use alone in cases of schizophrenia is contraindicated.
• CNS effects: They may cause insomnia and CNS stimulation as demonstrated by tremors, twitching, ataxia, hyperreflexia, hyperthermia and even convulsions. Iproniazid and isocarboxazid sometimes cause peripheral neuritis which responds to pyridoxine. • Hypertensive crisis: This can be precipitated by concurrent administration of sympathomimetic pressor amines like amphetamine and ephedrine. Sudden rise in BP may even cause subarachnoid haemorrhage. Hypertensive crisis can also occur in patients taking MAOI, if they consume cheese or red wine which contain tyramine. Normally, tyramine is metabolised in the liver by MAO enzymes. MAOI, by inhibiting its metabolism, lead to tyramine accumulation which releases NA from binding sites causing a marked rise in BP. Eating of broad beans can also produce similar complication due to their content of DOPA. Yeast extract contains both tyramine and histamine. Some other foodstuffs which are incompatible with MAOI include yoghurt, buttermilk, meat extracts, soyabeans, chocolates, ripe bananas and figs. A hypertensive crisis should be treated with phentolamine (Regitine) 5 mg IV slowly or sodium nitroprusside infusion. (Chapter 30) • Autonomic effects: Hydrazine compounds can cause antimuscarinic effects such as constipation, dry mouth, blurring of vision, impotence, difficulty in micturition and orthostatic hypotension. • Miscellaneous effects: They induce weight gain. Hydrazine compounds, particularly iproniazid, may cause hepatocellular jaundice. • Acute toxic effects following overdosage include agitation, hallucinations, hyperpyrexia and convulsions. Blood pressure may be low or high. The treatment is mainly symptomatic. Drugs like vasopressor agents and barbiturates should be administered cautiously. Drug interactions: By blocking drug degradation, MAOI potentiate the action of several drugs (Table 14.1). Thus, the normal dose of pethidine in a patient receiving MAOI can cause shock, collapse, respiratory. depression and death. The effects of adrenaline, noradrenaline and isoprenaline, however, are not enhanced as they are inactivated by catechol-omethyl transferase, another enzyme present in the liver and blood. Table 14.1 Some drugs whose action is potentiated by MAOI
Preparations and dosages: (i) Isocarboxazid 10 mg tablet, usual daily dose 10-30 mg increased upto 50 mg (ii) Phenelzine sulfate 15 mg tablet. Usual daily dose 45-60 mg. Maximum daily dose recommended 75 mg. (iii) Tranylcypromine 10 mg tablet, 10-20 mg daily increased upto 30 mg. With all these drugs, smaller doses should be used for maintenance therapy and in old people.
MOCLOBEMIDE: This drug acts by selective, reversible inhibition of MAO-A enzyme, and hence is termed as reversible inhibitor of MAO (RIMA). It causes less potentiation of pressor amines and lower incidence of drug interactions than irreversible MAOI. It does not need strict dietary restriction as the intestinal MAO is mainly MAO-B.
Tricyclic Antidepressants (TCA) IMIPRAMINE, a dibenzazepine derivative, is the well studied tricyclic antidepressant (TCA) and is discussed as a prototype. Structurally, it differs from phenothiazines in that the sulphur is replaced by an ethylene linkage (Fig. 14.1).
FIG. 14.1
Mechanism of action: All drugs which modify depression or mania have distinct effects on reuptake of 5-HT, NA and/or DA. Normally, a large proportion of 5-HT/NA liberated at the nerve endings is inactivated by reabsorption into its storage sites. Tricyclic antidepressants (TCA): • Inhibit neuronal NA and to a variable extent, 5-HT reuptake in the brain by binding to their transporters. This causes a localised increase in NA/5-HT in the synaptic gap. • Cause variable blockade of α1 and to a lesser extent, presynaptic α2 adrenoreceptors; and • Possess central anti-muscarinic properties. In addition, the different TCA also block other neurotransmitters such as histamine ACh, and dopamine to differing degrees. This may explain the differences in the action profiles of various TCA. Long term treatment with TCA causes adaptive changes in presynaptic α2 adrenergic receptors to increase synaptic availability of NA. It also alters sensitivity of several other receptors such as muscarinic receptors and GABA-B receptors. The importance of such complex phenomena in various actions of the antidepressants is not clear. Pharmacological actions: Behavioural effects: Imipramine produces similar anti-depressant effect as MAOI, but the mechanism of action is different. The drug can reverse the depressant action of reserpine without restoring brain monoamines. It also has some anxiolytic action. Central nervous system: A single dose of 100 mg in normal subjects causes drowsiness and a feeling of light headedness. It produces some degree of sedation, enhances sleep and disrupts obsessive rumination. The drug suppresses REM sleep but increases stage 4 sleep. Repeated administration may produce difficulty in concentration and thinking. The
drug has no euphoriant effect, and drug dependence is rare. It lowers the seizure threshold in animals and hence, should be prescribed with caution in patients with history of seizures. Autonomic nervous system: It exerts antimuscarinic effects (See below). Cardiovascular system: See later. Absorption, fate and excretion: Given orally, TCA are well absorbed. They are highly lipophilic, widely distributed and get strongly bound to proteins in the various tissues. Antidepressants in general are metabolised by hepatic CYP3A4 and CYP2D6 and some of them are converted to an active metabolite having longer duration of action (Table 14.2). Imipramine is converted in the liver to its active metabolite desmethylimipramine. Some patients are poor metabolisers of TCA and may not tolerate standard doses. It is important to note that there is a marked variation in steady state plasma levels between individuals following similar doses. Further, Asians, Americans and African Americans are known to require much lower doses than Caucasian Americans. Table 14.2 Half-lives of antidepressants and their metabolites Drug Drug t½, hours Metabolite, t½, hours Tricyclics Imipramine 12 14–62 Amitriptyline 31 20–92 Doxepin 16 30 Protriptyline 80 — Clomipramine 32 54–77 SSRI Fluoxetine 50 180 Paroxetine 20 — Fluvoxamine 15 14–16 S ertraline 22 62–104 Citalopram 32 —
Most tricyclics are completely eliminated within 7-10 days. Adverse reactions: Some of these can be explained on the basis of blocking of various neurotransmitter receptors in the brain (Table 14.3). Blocking of D2 receptor is responsible for the endocrine adverse effects e.g. galactorrhoea following clomipramine, amoxapine, trimipramine, etc. Usually TCA are well tolerated.
Table 14.3 Antidepressant drugs
0 = Negligible + = Minimal ++ = Moderate +++ = Marked *
Injections 100 mg/10 ml for IM use. H1 = Histaminic; M = Muscarinic; α1 = Alpha1 adrenergic.
• Allergic reactions like urticaria, skin rashes, pruritus and photosensitivity. • Antimuscarinic effects: These are common and the most troublesome adverse effects of TCA. These include dryness of mouth, difficulty in accommodation, tachycardia, constipation, difficulty in micturition, impotence, delayed ejaculation, and rarely hyperpyrexia. The drug should be used cautiously in patients with glaucoma or enlarged prostate. Rarely, it can cause paralytic ileus. Central antimuscarinic action may cause
confusion, disorientation or psychosis. • Central nervous system: Feeling of tiredness, lethargy, headache and weight gain may be observed. Amitriptyline, trimipramine, doxepin, trazodone and mirtazepine are potent sedatives and can cause sleep. Like MAOI, these drugs can cause tremors, muscle jerking, ataxia and hyper-reflexia. They are best avoided in epileptics. • Cardiovascular system: Postural hypotension, cardiomyopathy and heart failure have been reported following long term therapy. Both imipramine and amitriptyline may rarely cause inverted or flattened T wave, prolongation of QT interval and depressed ST segment in the ECG. With overdosage, they can precipitate cardiac arrhythmias. • Miscellaneous: Like chlorpromazine, imipramine can cause cholestatic jaundice, agranulocytosis and edema. Priapism can occur with trazodone, a heterocyclic agent. Tricyclic antidepressants cross the placental barrier and can cause jitteriness, suckling problem, hyperexcitability and rarely cardiac arrhythmias in the neonate. Sudden discontinuation of TCA may rarely lead to cholinergic crisis and a flu-like syndrome. • Acute poisoning with tricyclics produces hyperpyrexia, hypertension or hypotension, convulsions and coma. Cardiac arrhythmias may be present. It may also cause metabolic acidosis. The treatment is symptomatic. Physostigmine salicylate, given parenterally, in the dose of 1-4 mg every 1 hour, is used to treat anticholinergic CNS manifestations. Drug interactions: See Table 14.4. Table 14.4 Drug interactions of imipramine
Preparations and dosage: See Table 14.3. Therapeutic uses: • Major depression, discussed later. • Acute panic attacks, anxiety disorders, social phobias, (see earlier). • Obsessive compulsive neurosis: This disorder is characterised by obsessional pattern of thinking and ritualistic compulsive behaviour as a defence against anxiety. If such thoughts or action is prevented or interrupted, the patient becomes anxious. The treatment is either with a TCA or with a SSRI. • Nocturnal enuresis: Enuresis is defined as bedwetting that occurs after the age at which bladder control should have been achieved, usually between the ages 2-3 years. The disorder exists in two forms : (a) primary (persistent), and (b) secondary (acquired or regressional). Primary enuresis is the commoner of the two types. The main treatment consists of bladder training and correction of psychopathologic factors. Imipramine, in a single bedtime dose of 10 to 75 mg, has been used with variable success. The exact mechanism of action is not known. The other drugs used are amitriptyline and desmopressin (Chapter 39).
• Bulimia nervosa: This is a behavioural disorder characterised by episodes of overeating, usually followed by self-induced vomiting, cathartic or diuretic abuse or fasting to undo the threat of weight gain. Antidepressants such as imipramine and SSRI are the favoured drugs treatment for this condition. • Migraine, see Chapter 24. • Deafferentiation pain, see Chapter 11. • Attention deficit hyperactivity disorder (ADHD), (see later). • Chronic fatigue syndrome (CFS). • Nonspecific fibromyalgias. • Pruritus: Doxepin is used because of its antihistaminic effects. • Erectile dysfunction (Trazodone; see latter) Imipramine and amitriptyline are still the most cost-effective TCA for general use.
Selective Serotonin (5-HT) Reuptake Inhibitors (SSRI) Drugs belonging to this group are listed in Tables 14.2 and 14.3. Mechanism of action: As the name suggests, the selective serotonin (5-HT) reuptake inhibitors (SSRI) act mainly by inhibiting the reuptake of serotonin by the tryptaminergic neurons. They bind to the serotonin transporter (SERT) at a site other than the binding site of 5-HT and inhibit the transporter. Pharmacological action: They are as effective as TCA in moderate depression but may be less effective in the severely depressed patients. Because of their selective receptor action, they cause: • Less marked antimuscarinic effects • Less antihistaminic effects, with less sedation. • Fewer cardiovascular effects (bradycardia, hypotension, conduction disturbances); and they • Are generally safer than the TCA in the elderly. As with TCA, repeated administration leads to gradual downregulation and desensitisation of autoreceptor mechanisms over several weeks. Absorption, fate and excretion: SSRI are well absorbed orally and have long half lives, which permits their once-a-day administration. In general, like TCA many of these are metabolised in the liver. Some of the metabolites are also active, with half lives longer than those of the parent drugs (Table 14.3). All SSRI except citalopram and escitalopram inhibit one or more CYP450 e.g. fluoxetine and paroxetine inhibit CYP2D6 and may cause increased toxicity of TCA, tramadol, methadone, type 1C antiarrhythmics, alcohol and theophylline while sertraline inhibits CYP3A4 and alters blood levels of digoxin. On the other hand, antiepileptics e.g. carbamazepine which induces CYP 450 can cause decreased efficacy of SSRI. Adverse reactions: These include: • Gastrointestinal: Anorexia, nausea, abdominal pain. • CNS: Anxiety, agitation, akathisia, headache and transient insomnia. • Anorgasmia: They decrease sexual thoughts and desire, decrease libido, and may delay orgasm in both sexes (anorgasmia). This side effect however, makes them useful in the therapy of paraphilias and sexual obsession associated with increased suicidal tendencies. Derangement in sexual function can be treated by reducing the dose, giving weekend drug holidays or using amantadine, bethanechol, buspirone or bupropion. • Serotonergic syndrome: This may be due to hyperstimulation of 5-HT1A receptors in brain stem. It is necessary to monitor patients closely if taking serotonergic drugs in combination. It comprises of : (a) Cognitive-behavioural symptoms: mainly agitation, insomnia, anxiety hypomania and seizures. (b) Autonomic symptoms: nausea, salivation, diaphoresis, diarrhoea, abdominal cramps, flushed skin, hypertension and hyperthermia; and (c) Neuromuscular symptoms: hyperreflexia, shivering, twitching, rigidity. Rhabdomyolysis, secondary to severe rigidity and hyperthermia, may occur. The
reaction may be rarely fatal. The treatment is symptomatic. The administration of an MAOI concurrently with or immediately prior to one of these drugs can cause a serotonergic syndrome. Other drugs which may also cause such syndrome are listed in Table 14.5. Table 14.5 Some drugs which may induce serotonergic syndrome
Like other antidepressants, the SSRI may cause mania when used in patients with undiagnosed bipolar depression. Sudden stoppage of any of the SSRIs can produce withdrawal symptoms such as anxiety, agitation, confusion, insomnia, sweating, tremor, vomiting and shock like syndrome. Fluoxetine with a longer t½ is less likely to cause these symptoms. Preparations and dosage: See Table 14.3 Fluoxetine is used initially in the dose of 20 mg once a day, in the morning, increased by 20 mg once in several weeks, to a maximum of 80 mg daily (less in the elderly). Doses higher than 20 mg should be given in two divided doses in the morning and at noon. Fluvoxamine and sertraline are used initially in the dose of 50 mg once a day, increased by 50 mg once a week, to a maximum of 200 mg daily (less in the elderly). Therapeutic uses: They should be given in the morning hours as they can disturb sleep. They are not the first line drugs in all depressed patients. They may be preferred in patients who: • Cannot tolerate the TCA • Have a high risk of suicide • Are accident prone, as the older antidepressants have sedative and autonomic adverse effects; or • Have an obsessive compulsive disorder: A major role for serotonin system in mediation of obsessive compulsive neurosis has been suggested. SSRI are clinically effective and also have better safety and tolerability profile than tricyclics. These drugs are also preferred in the depressed geriatric patients as the TCA can cause dizziness, postural hypotension, constipation and difficulty in micturition. If used concurrently, they may increase the TCA, lithium and carbamazepine plasma levels. SSRI should not be used: • In the manic phase of bipolar depression • Together with MAOI; and • Concurrently with ECT
Choice of SSRI: All SSRIs are almost equally effective in the treatment of depression. Patient who do not respond to one drug may respond to another. Because of the differences in their hepatic metabolism, they differ in their potential for drug interactions. Fluoxetine is currently considered the drug of choice for routine use. Paroxetine exerts more antimuscarinic effects, may cause more weight gain as well as pose a high risk of mood changes and withdrawal syndrome. Sertraline has relatively lower risk of drug interaction than the former drugs while citalopram and escitalopram carry no such risk. Newer antidepressants: These drugs (Table 14.6) have diverse structures, mechanisms of action and ADR profile but they have efficacy similar to SSRI. Because of their selective profile, they may be useful in matching individual patient’s needs in terms of efficacy and tolerability. For example, sexual dysfunction is less common with mirtazapine while weight gain is not a problem with venlafaxine and reboxitine. The latter also relieves depressionassociated sleep disturbances like insomnia, fatigue and anergia. They should not be used with MAO inhibitors. Because of suspected effect on mood fluctuations and possible suicidal thoughts, paroxetine, mirtazapine, venlafaxine and nefazodone should be avoided in children. A few drugs like trazodone, nefazodone, venlafaxine and bupropion require frequent dosing.
Table 14.6 Newer antidepressants
For tetracyclic compounds amoxapine and maprotiline, see Table 14.3.
Agomelatine: This melatonin analogue which acts as a selective agonist of melatonin, is also an antagonist of 5HT2B and 5HT2C receptors. Many patients with depression may have disturbed/desynchronised circadian system resulting into difficulty in getting sleep, frequent arousals and awakenings during the night with the resultant hypersomnia, daytime fatigue or napping. In these patients use of SSRI, hypnotics or anxiolytics though useful, may not restore normal circadian function. In such cases melatonin or its analogues, because of their chronobiotic effects may be useful. Although, they may improve the quality of sleep, they are not effective antidepressants. Agomelatine, in addition to its chronobiotic effects, has been claimed to exert some antidepressant and anxiolytic properties; this may offer some advantages over SSRI/TCA in the treatment of depression associated with sleep disturbances. It needs further evaluation. St. John’s Wort: This herbal product is derived from the plant Hypericum pertforetum.
Many phytoconstituents probably contribute to its action. Given orally, it has variable beneficial effects in mild to moderate depression. The ADR include GI disturbances, dry mouth, dizziness, sedation and confusion. It is an inducer of hepatic CYP450 and hence can reduce plasma levels of several drugs e.g. warfarin, anticonvulsants, antipsychotics and COC pills.
Treatment of Major Depression Depression is a major disorder of mood (affect) prevalent in a large percentage of the population and has a strong familial predisposition. It is a serious condition as it can disrupt the normal social life and may drive the individual to commit suicide. Hence, it is essential that it is diagnosed early as antidepressant drugs can alleviate the majority of depressive illnesses. Furthermore, such effective treatment can now be given at home by the family doctor. Although there is no unanimity about the clinical classification of depression, it can be broadly divided into two main groups: • Unipolar depressive disorders which involve major depression. • Bipolar disorders (Manic Depressive Psychosis, MDP): Patients identified as having bipolar disorders have different pathologies; their management has been discussed in Chapter 13. Unipolar depression can be thought of as of two types. (i) Reactive, neurotic or psychological depression, is an exaggerated reaction to adversity, manifested as gloom, unhappiness and tearfulness. It is precipitated by such factors as sudden monetary loss, failure in examinations or an accident. The individual blames the situation rather than himself. It responds favourably to moral, spiritual and social support from friends and relatives, who can share the problems and discuss the possibilities of solution. In such situational crises, anxiolytics like BDZ help to tide over the crisis. The patient improves with the change of situation. Death of a close person normally results in a grief reaction, which resolves in course of time. Short term use of an anxiolytic can be helpful immediately after such loss; but their prolonged use may actually hinder the resolution of the grief reaction. If severe, the grief reaction may be complicated by depression, physical ill health and even suicide. The use of antidepressants in such a situation helps the depressive symptoms but may not help in the resolution of the grief reaction. (ii) Melancholic, earlier known as endogenous, depression produces a varied picture. The dictionary meaning of melancholia is “a mental state characterised by dejection and misery”. It usually occurs in the middle or later years of life. In its classical form, an individual shows retardation of thoughts, movement and speech; he remains withdrawn from the usual activities. It is associated with early-morning waking, occasional nightmares, anxiety, feeling of guilt, and unworthiness. There is a tendency for self blaming. The subjects generally complain of various aches and pains, tiredness, loss of appetite, loss of libido, lack of concentration and loss of weight. There is a greater tendency to commit suicide. Prompt use of antidepressants and/or ECT can produce dramatic results in such cases. Pathophysiology of major depression is complex although it may have a biochemical basis which is related to decreased synthesis and turnover of brain 5-HT, NA and DA as well as an increased accumulation of ACh. Studies in depressed persons indicate decreased metabolic activity in the caudate nuclei and frontal lobes and altered NA activity in various parts of the brain. Patients who have attempted suicide have significantly lower CSF levels of the 5-HT metabolite, 5-Hydroxyindolacetic acid (HIAA) than those who have not. However, there appears to be no definite biological marker for depression. Other systems such as hypothalmo-pituitary-adrenal axis may also be involved in its genesis.
Hence, treatment is guided mainly by patient’s symptom profile. The aims of therapy are to: (1) Reduce the symptoms and prevent suicidal tendency. (2) Prevent relapse/recurrence of symptoms. (3) Improve cognitive and functional state; (4) Help in rehabilitation. For the successful treatment of depression, doctor-patient relationship is important. One must try to create confidence in such patients. Nothing should be done to increase their guilt feeling. It is worth searching for possible external factors contributing to the depressive illness and to try to reduce their impact by modifying the environment or by psychotherapy. Cognitive behavioral and interpersonal therapy are effective and counseling or patient education alone may be sufficient in mild to moderate depression. From the various antidepressants available, it is better to be familiar with a few rather than go for the unfamiliar ‘latest’ ones in the market. None of the newer antidepressants appears to be consistently therapeutically superior to the TCA-imipramine and amitriptyline except in a few selected subgroups of patients. The choice of tricyclics should be determined by the side effects one wishes to avoid (orthostatic hypotension and antimuscarinic effects) or produce such as sedation. All TCA are considered equally effective in uncomplicated, nondelusional depression. They are relatively cheap. They are, however, contraindicated in patients having serious cardiovascular risk factors. Considering their secondary psychotropic effects, amitriptyline, doxepin and dothiepin are more sedative than imipramine whereas nortriptyline, desipramine and protriptyline have negligible sedating action. Patients with agitation or anxiety are best treated with a sedative antidepressant. Treatment with amitriptyline is often associated with substantial weight gain. Imipramine/desipramine is perhaps the most suitable general purpose anti-depressant, particularly in young patients. To begin with, imipramine is generally administered in a dose of 25 mg bid and then increased to 50 mg 2-3 times daily during second and third week respectively depending upon the response. Smaller doses are employed in elderly people. Because of pharmacokinetic considerations, the entire daily dose of imipramine or amitriptyline may be given at bedtime; their sedative effect may eliminate the need for an additional hypnotic. No response occurs below a threshold drug concentration, and wide variations are known to occur in the serum concentration in different people on the same dose. Asians generally require lower doses than Caucasians (see earlier). The elderly metabolise TCA more slowly and may achieve therapeutic plasma concentrations with doses as low as 25-50 mg daily. Once the therapeutic dose has been established, the entire daily dose may be given at night. In majority of cases, improvement occurs within 8 weeks, but the maintenance treatment should be continued for 6-9 months to prevent relapse. Some cases, however, may need lifelong maintenance treatment as risk of relapse after cessation of treatment is very high. Although some tolerance to the sedative and autonomic effects may develop, the drugs remain clinically effective for long time. Although the list of ADR following TCA is formidable, their incidence is not high and they usually occur during the first few days of treatment. Anticholinergic adverse effects are the most common and annoying, particularly in the elderly. If improvement does not occur after 3-4 weeks of therapy with recommended doses, further increase of dosage is
unlikely to bring about further improvement. In such case, the use of an SSRI is indicated. The advantages of SSRI are that they are given in a single daily dose and they cause less sedation and no antimuscarinic or cardiovascular effects; hence, compliance may be better. Impairment of sexual function is, however, common. They may be preferred in the elderly. Commonly used drugs are fluoxetine, sertraline and citalopram. Sometimes patients on SSRI show waning of response over time. These are benefitted by addition of buspirone or small dose of TCA. An antidepressant must be given a trial for at least 4 weeks before it is judged ineffective; in that case, a drug from a different class is prescribed either alone or in combination with earlier agent. In general, combined treatment approach is beneficial than the single medication in such patients. Patients requiring long term antidepressant therapy are maintained on lower doses, determined by trial and error. A less common adverse effect of all antidepressants is hyponatremia. It should be suspected in patients who develop drowsiness, confusion or convulsions while on antidepressants. Subjects with erectile dysfunction may benefit from trazodone or citalopram. Withdrawal of TCA and SSRI should be gradual as sudden stoppage can cause withdrawal symptoms (discontinuation syndrome; see earlier). This is more often seen with antidepressant drugs with short t½ In general, TCA appear to be safe during pregnancy. They are, however, secreted in the breast milk. MAOI, though faster acting and highly effective in atypical depression, are potentially more hazardous. Although it is claimed that younger patients do better with MAOI, these compounds cannot be recommended as drugs of choice. Combination of TCA or SSRI with MAOI is hazardous and can cause agitation, convulsions, coma and death. (serotonergic reaction). A stable personality before the illness, psychomotor retardation and intermediate severity of depression with melancholia predict a good therapeutic response. Anti-anxiety agents like BDZ (eg alprazolam) may be combined to lessen anxiety in early stages while a sedative like diazepam and flurazepam may be given at bedtime in those who complain of early waking. Phenothiazines can be combined with TCA in bipolar depression with accompanying agitation or psychotic symptoms. In severe cases with delusions, suicidal ruminations, marked retardation or severe agitation, ECT is preferred to drug therapy, as the beneficial results can be obtained more quickly. A history of suicidal attempt or strong suicidal thoughts points to the need for immediate hospitalisation. Finally, safety of self administration of large doses with suicidal intention must be a major determinant in the choice of an antidepressant in these patients. Studies indicate that some atypical antipsychotics (risperidone, olanzapine, aripiprazole) are useful as adjunctive drugs in patients with major depressive disorders (MDD) who do not respond adequately to monotherapy even after 6-8 weeks. Usually some patients not responding to SSRI may respond to another SSRI or SNRI like extended release venlafaxine. Although the atypical antipsychotics may augment the response to standard antidepressants, they may cause unwanted effects such as weight gain, akathesia,
hyperglycemia and hyperprolactinaemia. The other drugs used for augmenting the response are thyroxine and triiodothyronine. The relationship between depression and anxiety is complex. Depression can be precipitated as a reaction to severe anxiety. Both can coexist in a patient with neurotic illness, schizophrenia or an organic syndrome. Although the drugs and counselling can achieve remarkable therapeutic results, the kindness from relatives and friends could be of immense benefit and the patient should be made to feel that he is useful to the family and community and not an unwanted, useless burden. It must be realised that mild depression is a normal manifestation of cyclic variations in mood. It is self-limited. Antidepressant drugs, which are potentially toxic, should not be used indiscriminately as euphoriants in such subjects. The important points to remember about depression are listed in Table 14.7. Table 14.7 Important points to remember about depression and its therapy
Mood Stabilisers LITHIUM CARBONATE: The use of lithium carbonate in mental illness was described by Cade in 1949. Lithium is useful as a mood stabiliser in manic depressive psychosis (MDP). As compared to chlorpromazine, lithium causes less drowsiness while controlling the marked psychomotor overactivity in about 75% of the patients. The drug, however, does not exhibit positive activity in psycho-pharmacological screening in animals. Mechanism of action: Its exact mechanism of action is not known. It causes: (a) Inhibition of phospholipase C synthesis with resultant decrease in brain inositol triphosphate and diacylglycerol concentration; this reduces the sensitivity of some neurons to the action of various neurotransmitters. (b) Modification of GABA concentration in the brain and modulation of synaptic glutarnate availability. (c) Decrease in the synthesis of DA and NA in the brain, and facilitation of their neuronal re-uptake; and (d) Decrease in the function of brain protein kinases, leading to alterations in the release of neurotransmitters and hormones. Absorption, fate and excretion: Given orally, it is well absorbed and gets distributed throughout the total body water. Being a metallic ion, it is not metabolised nor gets protein-bound, but is mostly excreted in the urine, the renal clearance being proportional to its plasma concentration. Lithium decreases the sodium reabsorption by the renal tubules leading to sodium depletion. Patients on lithium treatment, therefore, should maintain adequate salt and water intake. Adverse reactions: Lithium toxicity is closely related to its serum level and the therapeutic window is narrow. Hence, the drug must be administered under supervision, with facilities for estimating serum lithium levels. Blood levels exceeding 2.0 mEq/1 are associated with dangerous toxic effects. Salt depletion from any cause, including a diuretic, increases the renal tubular reabsorption of lithium and its plasma level, thus precipitating toxicity. • Mild toxicity includes GI disturbances, drowsiness, muscular weakness and alopesia. It can also cause allergic reactions, blurred vision, glycosuria, polyuria and weight gain. • Large doses (level >1.5 mmol/L) cause sodium depletion, cerebellar ataxia, tremors, cardiac arrythmias seizures, hypotension and coma. • Chronic administration may give rise to goitre formation, hypothyroidism (rare) and ECG changes. The drug should be administered cautiously in the presence of cardiovascular, renal or brain damage. • Embryotoxicity : Lithium is embryotoxic and increases the risk of Ebstein’s anomaly. Because of toxicity, lithium should be prescribed only by a specialist who will also be responsible for monitoring it. Therapeutic uses: The principal uses are: • To prevent the recurrence of mania and of depressive episodes in bipolar disorder. Lithium salts are the first choice for long term prevention of MDP (see Chapter 13). • To treat acute episodes of mania it is combined with an anti-psychotic like haloperidol. • To treat alcohol dependence (Chapter 6). Psychotropic drug combinations: It is rational to combine various psychotropic drugs
when psychiatric illnesses are associated with a comorbid condition which needs to be treated. Thus, schizophrenia is often associated with depression which needs combined therapy with an antipsychotic and antidepressant. Combinations are also needed in the treatment of MDP and BDZ are often combined with antidepressants in patients with associated insomnia. However, such combinations must be chosen with specific goals in mind. The dangers of combinations are: • The opposing effects may attenuate the desired therapeutic effect; and • The summation of the anticholinergic effects of phenothiazines, the tricyclics and antiparkinsonian agents may produce bladder or bowel paralysis, precipitate an attack of acute glaucoma or cause mental symptoms. “The fact that many patients improve mentally when all such drugs are withdrawn may indicate that they were suffering from mental confusion associated with overdoses of centrally acting anticholinergic drugs.” Several psychotropic drugs and other drugs are substrates for CYP2D6. e.g: citalopram, desipramine, chlorpromazine, risperidone etc. or to CYP3A4. e.g.: alprazolam, diazepam, buspirone, carbamazepine, amlodipine, simvastatin etc. or to both. e.g.: fluoxetine, sertraline, amitriptyline, imipramine, haloperidol etc. Further, drugs like fluoxetine, cimetidine, erythromycin and ketoconazole inhibit CYP3A4 or CYP2D6. When multiple drugs that are substrates for the same hepatic enzyme are prescribed together, they may compete for the same enzyme and thus may inhibit the metabolism of other drugs leading to increased plasma level and toxicity.
Psychomotor Stimulants They are central stimulants with nervousness, insomnia and anorexia as the common adverse effects (Chapter 12). CAFFEINE and AMPHETAMINE: Amphetamine evokes release of brain NA and dopamine and blocks reuptake of these amines. (Chapters 12 and 17). Although these drugs stimulate the CNS and can act as “psychic energizers”, they are not true antidepressants. They do not correct the depressive state nor prevent suicidal tendencies. They may increase physical activity, alertness and confidence. Dextroamphetamine is twice as active as levo isomer. PIPERIDYL DERIVATIVES: The drugs pipradrol and methylphenidate act by inhibiting reuptake of central NA and DA. Their actions are similar to those of amphetamine. Thus, they reduce fatigue and produce a feeling of well-being and alertness. They have no place in the treatment of depression. In large doses, they cause restlessness, insomnia, nervousness, tremors, palpitation, ataxia and even convulsions. Possibility of drug dependence is a drawback. Therapeutic uses: • Narcolepsy is a heritable neurologic disorder with varied manifestations which begin to appear in late teens to twenties. The manifestations are: (a) Daytime Excessive Sleepiness (DES) and poor or disturbed sleep at night. (b) Sudden, sleep attacks during any activity during day. (c) Cataplexy: sudden onset of flaccid paralysis precipitated by anticipation, anger or surprise. (d) Hypnogogic hallucinations : frightening hallucinations at the onset of sleep; and (e) Sleep paralysis: Muscle paralysis on awakening. The underlying biochemical abnormalities are believed to be: (1) A widespread underrelease of dopamine; and (2) A brainstem specific hyper-response to acetylcholine. Current evidence suggests that the hypothalamic neuropeptide hypocortin (orexin) is involved in its pathogenesis. The treatment is symptomatic; the drug of choice is modafinil (see below) given as single daily dose. Methylphenidate or dextroamphetamine are used as alternative to modafinil. Cataplexy, hypnogoic hallucinations and sleep paralysis are related to REM sleep and are treated with REM suppressing antidepressants such as TCA (protriptyline) or SSRI (fluoxetine). DES, sleep attacks and cataplexy (see below) also may respond to gamma-hydroxybutyric acid taken at bedtime and again during the night. It is available as oral solution. The drug causes headache, nausea, dizziness, confusion and sleep walking. Its abuse has been associated with ‘date-rape.’ MODAFINIL is a non-amphetamine drug which reduces DES and improves daytime performance. It promotes wakefulness in normal people and may help to work for longer time. Its mechanism of action is not known. Its main side effects are headache, nervousness, dizziness and insomnia. It is a re-inforcing drug. It should not be used in persons with severe hepatic impairment. The dose is 200 mg (single dose) in the morning. • Attention deficit hyperactivity disorder (ADHD): This pediatric condition is characterised by inattentiveness and impulsiveness with or without hyperactivity, impaired learning and emotional lability. These children are easily distracted and
accident prone. Psychomotor stimulants like d-amphetamine, methylphenidate and pemoline cause beneficial effects when given for 1-3 months. Methylphenidate has properties similar to amphetamine and is preferred. ADR include decreased appetite, weight loss, insomnia, tachycardia and abdominal pain. It also shares abuse potential of amphetamine. It is contraindicated in CV disorders, hypertension, hyperthyroidism and glaucoma. Pemoline has lower abuse potential. It is, however, a relatively weak drug and may cause liver damage. Family counselling and psychotherapy are perhaps important. Other drugs used are non-stimulant anti-hypertensive drugs clonidine and guanfacine (Chapter 30); and a NA reuptake inhibitor atomoxetine which probably acts by increasing NA and DA in frontal cortex.
Psychotogenic Drugs These drugs, when consumed, produce psychosis – “a state characterised by maladaptive behaviour in which an individual reacts inappropriately to his environment.” They produce depersonalisation, changes in mood and a variety of effects on memory and learned behaviour. As some of these effects resemble those observed in psychosis such as schizophrenia, these drugs are also called psychotomimetic; and because of their ability to produce hallucinations they are sometimes designated as hallucinogens/psychodysleptics. Toxic psychosis is known following toxic doses of many pharmacological agents; but these are associated with neurological disturbances. Psychotogenic drugs, however, can produce psychotic states selectively, without delirium and neurological disturbances. The important psychotogenic drugs can be classified as: • Indolic such as Lysergic acid diethylamide (LSD) and Psilocybine • Non-indolic such as Mescaline, Phencyclidine, Tenamphetamine and Cannabis. LYSERGIC ACID DIETHYLAMIDE: LSD is an amine alkaloid, synthesized from ergot by Stoll and Hoffmann in 1938. It has some resemblance to ergometrine and possesses oxytocic action. Its central actions were recognised accidentally by Hoffmann in 1943. Mechanism of action: The exact mechanism of the central action is not known. But, LSD is a potent agonist at central DA receptors, central presynaptic 5-HT1A autoreceptors and 5HT2C receptors; its hallucinogenic action is probably related to its 5-HT2 receptor action. The drug also blocks peripheral 5-HT2 receptors. Pharmacological actions: The drug is rapidly absorbed on oral administration and produces its actions in doses as low as 20-25 mcg. Generally, LSD has a disintegrating effect on both inborn and learned behaviour patterns. Animal behaviour under LSD is known to be disorganised. Thus, the garden spider produces a defective web and a fish becomes disoriented. Individuals under LSD effect exhibit marked changes in mood with emotional outbursts; they may laugh or cry on slightest provocation. Motivation is impaired. The subject may experience an uninterrupted stream of fantastic images of extraordinary plasticity and vividness, accompanied by an intense kaleidoscopic like play of colours. The visual hallucinations take the person to “dream world”, a world appearing more real and better than the one he lives in. There is a cognitive distortion of time and space. If the dose is not large, the consciousness and memory are retained. Many subjects experience a fear of disintegration of the self. The syndrome clears up after about 12 hours. Associated with the behavioural changes, it also produces sympathomimetic actions such as dilatation of pupil, tachycardia, tremor, piloerection and hyperglycemia. It may cause nausea and frequency of micturition. Tachyphylaxis to behavioural effects of LSD is known, and a cross tolerance exists between LSD, mescaline and psilocybine. The drug causes psychic but not physical dependence. Adverse reactions: These vary markedly from species to species. In man, the margin of safety between effective dose and lethal dose appears to be wide. Sometimes, it produces suicidal tendencies. Although the psychotic changes are generally reversible, the drug can cause permanent psychosis and personality changes. The drug is also suspected to produce chromosomal damage. Phenothiazines such as chlorpromazine can antagonize many acute effects of LSD.
Other agents chemically related to LSD such as psilocybine, 5-hydroxy-dimethyl tryptamine (bufotenine) and harmine produce similar psychotogenic actions. MESCALINE: The alkaloid mescaline was isolated in 1846 from the cactus Lophophora williamsii. The Red Indian tribes in Mexico and in North America were using this cactus as an intoxicant to produce ecstatic states on special religious occasions. Given orally, the drug produces anxiety, tremors, sympatho-mimetic effects and hallucinations. The subjects get visual hallucinations of fantastic and brilliantly coloured figures, animals and people. They may get the feeling of floating in space with ever increasing feeling of dissolution. Beside such colourful hallucinations, the drug also produces delusions, depersonalisation and disturbances of thought. The effects of a single dose persist for about 12 hours. Uses: Both LSD and mescaline have been employed as an experimental tool to produce model psychosis. Tenamphetamine: is related to mescaline and amphetamine. It acts as central and peripheral adrenoreceptor stimulant. It is often misused as ‘dance’ drug at rave parties. CANNABIS (MARIJUANA): Cannabis is one of the oldest herbal remedies, known since 4000 BC for its therapeutic and recreational properties. It is obtained from the hemp plants, Cannabis sativa and Cannabis indica. The active ingredients are present in the resinous exudate of the tops of the female plant. The resin is known as Hashish or Charas. Bhang is prepared from the dried leaves and the flowering shoots while ganja is the resinous mass obtained from the small leaves and brackets of inflorescence. The term Marijuana is used to describe any plant part or extract containing the active principle. The psychoactive principle of Cannabis is known as Δ-9-tetrahydrocannabinol (THC). Its synthetic analogous are also available. Mechanism of action: Two cannabinoid receptor types, CB1 and CB2, have been identified. CB1 is widely distributed in the mammalian tissues, with the highest concentration found in the brain neurons, particularly in hypothalamus, the limbic system, cerebellum and the basal ganglia, and in the GI tract and adipocytes. CB2 receptors are found in the cells of the immune system. Pharmacological actions: • Acute effects: These have been studied in man following the administration of synthetic THC. When smoked, THC is rapidly absorbed and effects appear within minutes and last for about 2-3 hours. Given orally, the onset of action is delayed upto 30 min–2 hours. The pulse rate increases, conjunctiva becomes red and BP may fall slightly; at higher doses, orthostatic hypotension occurs. Muscle strength is decreased; appetite may be increased, leading to increase in food intake. CNS and behavioural effects vary according to the dose, the route of administration and the individual personality. There is initial euphoria or “high”, which is followed by drowsiness. It produces a dreamy state, feeling of well being, excitement and inner joyousness. An individual under its influence may become garrulous and hilarious, exhibiting uncontrollable laughter even with minimal stimuli. Violent or aggressive behaviour, however, is rare. Time sense is altered and hearing becomes less discriminating. Vision becomes apparently sharper with many visual distortions. The drug causes difficulty in concentrating and thinking. Often, it causes nausea, vomiting, increased urinary frequency and dryness of mouth.
• Chronic effects: The effects of chronic use are less certain. Some degree of tolerance is known to develop rapidly and a mild withdrawal reaction may occur; this is, however, not associated with craving or physical dependence. Some of the acute effects may be reversible; thus a decrease in heart rate is observed instead of tachycardia as seen following acute use. Heavy chronic cannabis users can develop an amotivated syndrome, with apathy and loss of academic performance in students. Such an effect is expected since cannabis is concentrated in the limbic system, the motivational centre of the brain, and also interferes with memory, cognition and psychomotor performance. Even social doses impair car driving ability because of distortions of time and space estimations, reduced vigilance and coordination. Effects persist for many hours because the drug is eliminated slowly. Interestingly, the drug lowers intraocular pressure in some individuals. Absorption, fate and excretion: The crude oily resinous extracts contain many ingredients, the THC being most active. The THC content in various preparations varies widely depending upon the source. It is well absorbed following inhalation or ingestion. Cannabinoids are extensively metabolised to various active metabolites. They are highly lipid soluble and are stored in body fat. Their slow release prolongs their action. Acute administration of THC inhibits hepatic metabolising enzymes while chronic intake may induce them. Adverse reactions: Large doses of cannabis may cause an acute panic reaction, a toxic delirium, an acute paranoid state or acute mania. • Acute panic state is perhaps the most common psychic reaction and is characterised by anxiety, confusion and other unpleasant experiences. Occasionally it may cause a dissociative reaction, and depersonalisation which may be long lasting. Very large doses of cannabis may cause toxic delirium characterised by marked memory impairment, confusion and disorientation. Similar reactions are seen with many other drugs e.g. Dhatura strammonium. A self-limiting hypomania-schizophrenia-like psychosis following marijuana can occur. Cannabis can unmask latent psychiatric disorders and can aggravate schizophrenia. Sometimes, the drug may produce “flashback” reactions in which events associated with drug use are suddenly thrust into consciousness in the non-drugged state. This phenomenon is common with LSD and other hallucinogens, and may occur many months after the last use of such drugs. In case of cannabis, the reaction is mild. • Chronic cannabis users may have decreased sperm production. Women may have anovulatory menstrual cycles associated with decreased LH. The drug may cause deterioration of glucose tolerance and aggravation of diabetes mellitus. ‘T’ cell function may be inhibited. Chronic cannabis smokers, like tobacco smokers often develop significant airway obstruction, bronchitis, cough and precancerous mucosal changes. Subjects can develop tolerance but withdrawal symptoms are usually mild. Cross tolerance with alcohol is known. Cannabinoids are teratogenic in animals. Therapeutic uses: Although cannabis possesses antiemetic, mild analgesic, muscle relaxant, anticonvulsant and sedative-hypnotic actions, it cannot be recommended for these purposes because of its adverse effects. Small doses of oral THC can stimulate
appetite without causing serious psychotropic effects and have been used to treat chronic wasting in AIDS. Synthetic cannabinoids, nabilone and dronabinol are useful as antiemetics in patients receiving cancer chemotherapy (Chapter 41). Endocannabinoids (EC): The first endogenous cannabinoid ligand was discovered in 1992 and named Anandamide. This name is derived from the Indian Sanskrit word Anand, meaning bliss, joy or tranquility. The other EC discovered was 2-arachidonoylglycerol (2AG). Both are derivatives of the long chain polyunsaturated fatty acid, arachidonic acid, in the cell membrane (Chapter 24). Like Δ-9-THC, they bind to the G-protein coupled CB1 and CB2 receptors and exert almost identical actions as the classical cannabinoid CB agonists. The EC system appears to be a natural, physiological system, activation of which is believed to affect the accumulation of body fat, especially the intra-abdominal fat. The system, particularly in the brain, is believed to be relatively silent (turned off) in normal conditions, and is activated in special circumstances. Anandamide and 2-AG are released from the cell membrane, when needed, and act immediately. CB1 receptors in the brain control appetite and modulate the hypothalamic neuropeptides to control the size of meals, and through the adipocytes, regulate lipid metabolism. Their stimulation by EC increases the food consumption. By acting at the hypothalamus, EC promote anabolic processes and inhibit catabolic processes. It is suggested that overweight and obesity in humans may be related with hyperactive EC system. Rimonabant is a selective CB1 antagonist, once promoted as antiobesity drug. It has been, withdrawn from the market due to severe depression and suicidal tendencies in patients.
Drug Induced Psychiatric Syndromes Psychiatric disturbances are often attributed to concomitantly administered drugs; yet, it is generally difficult to establish the causal relationship between the two. Such a druginduced reaction should, however, be suspected whenever an unexpected psychiatric disturbance arises suddenly in a person of good previous personality, after a new drug has been consumed. The psychiatric reactions to drugs can be broadly categorised into: • Delirium (acute brain syndrome, toxic confusional state): This is characterised by a fluctuating clouding of consciousness, restlessness, emotional changes usually fear and perplexity and, in severe cases, paranoid delusions or visual hallucinations. The elderly are particularly susceptible. They may follow overdose or drug withdrawal, or may be due to intolerance to a normal therapeutic dose. Although many drugs can cause such states, CNS depressants (including alcohol), anticholinergics and cimetidine are the ones implicated most frequently. • Psychotic states: Hallucinogens such as LSD can induce a psychotic state with clear consciousness, paranoid delusions and visual hallucinations. States closely resembling schizophrenia with auditory hallucinations, thought disorder, aggressive behaviour and occasionally violence and suicide are seen with the CNS stimulants (cocaine and amphetamine), sympathomimetic nasal sprays, anorexiants and beta adrenergic agonists. Other drugs which can cause psychotic states are beta-adrenergic blockers, anticholinergics, opioids, dopamine agonists, glucocorticoids and rarely NSAID. • Manic states are sometimes observed with antidepressants, anticholinergics, high doses of corticosteroids, isoniazid, levodopa, dexamphetamine and clonidine. • Depression can occur with antihypertensive drugs (reserpine, methyldopa, clonidine, propranolol and pindolol), levodopa, chloroquine, anti-convulsants, OC pills and cimetidine. • Behavioural disorders reported are withdrawal syndrome after cessation of BDZ and akathisia during treatment with neuroleptic drugs. Pathological gambling and other compulsive behaviours such as compulsive eating and drinking have been reported in patients on non-ergoline dopamine agonists, particularly pramipexole (Chapter 15).
15
Drug Therapy of Parkinsonism and Other Neurodegenerative Disorders Parkinsonism as a clinical entity was first described by James Parkinson in 1817 (Parkinson’s disease; PD; paralysis agitans). It is a syndrome of varied etiology and its important features are bradykinesia, muscular rigidity, postural instability, loss of associated movements and tremor. Excessive salivation, seborrhoea, depression and liver damage may occur. Besides the idiopathic PD, arteriosclerotic and post-encephalitic forms, the syndrome is seen in hepatolenticular degeneration of Wilson’s disease and can be induced by drugs like reserpine, haloperidol, triperidol, chlorpromazine and other halogenated phenothiazines. Point mutations in genes on several chromosomes have been reported in some patients. Pathophysiology: The basal ganglia consist of the corpus striatum (the caudate nucleus and the putamen), globus pallidus, and substantia nigra. They modulate the extrapyramidal (EP) control of motor activity. The substantia nigra pars compacta (SNpc, which is rich in dopaminergic neuronal cell bodies), projects to the corpus striatum where dopamine is released. The latter, in turn, projects back via the globus pallidus and substantia nigra pars reticulata (SNpr) to the thalamus and finally to the cerebral, motor cortex, and regulates their involvement in voluntary movement. The nigro-striatal neurons make efferent connections with the striatum where they make contact with two types of striatal neurons: (i) those bearing excitatory D1 receptors and (ii) those bearing inhibitory D2 receptors. The neurons which bear D1 receptors relay impulses via a direct excitatory pathway (medial globus pallidus → thalamus) to the cerebral motor cortex and uses GABA, the inhibitory NT. The final outcome is enhanced stimulation by the latter of the spinal motor neurons. On the other hand, the neurons which bear D2 receptors relay impulses via an indirect inhibitory pathway (lateral globus pallidus → subthalalmic nucleus → medial globus pallidus and SNpr → thalamus) to the same cerebral motor cortical neurons; the indirect pathway has two GABAergic links and one glumatergic link. It finally decreases stimulation by them of the same spinal motor neurons. In health, the direct pathway (excitatory) predominates as the dopamine released in the neostriatum enhances the activity of the concerned neurons. In PD, deficiency of dopamine (DA), due to the degeneration of nigrostriatal dopaminergic neurons, leads to dominance of the indirect pathway (inhibitory). This accounts for the major symptoms and signs of PD. The dopamine agonist bromocriptine (see later) helps to correct this imbalance and relieves many, but not all, symptoms and signs of parkinsonism. Other defects may account for the unrelieved symptoms and signs e.g. degeneration of the noradrenergic locus coeruleus may contribute to autonomic symptoms and depression; and degeneration of the cholinergic nucleus basalis may account for the dementia. Thus, the clinical features of PD can be explained by a combination of: • Predominantly dopamine deficiency • Relative cholinergic preponderance
• Increased activity of GABAergic neurons in basal ganglia, and • NA deficiency Initially, the dopamine deficiency is compensated for by an increased sensitivity of the denervated striatal neurons to DA. However, as the disease progresses, more and more nigrostriatal neurons fall out. As no drug can halt the progressive loss of the nigro-striatal neurons, the disease is progressive and incurable. In drug-induced parkinsonism, the DA receptors in the striatum are blocked; there is no deficiency of DA. Hence, it is reversible following omission of the offending drug. Experimental administration of 1-methyl - 4–phenyl, 1,2,3,6 tetrahydropyridine (MPTP) in mice and monkeys results in selective destruction of dopaminergic neurons of the nigrostriatal pathway. This effect of MPTP is due to its conversion to a neurotoxic metabolite methylphenyl-pyridium (MPP) by MAO-B. The MPTP-treated primates represent the best animal model of PD. It is postulated that PD in humans may be caused by chronic exposure to MPTP-like substances in the environment, combined with effects of ageing and oxidative stress. The aims of therapy in PD are outlined in Table 15.1. Table 15.1 Aims of therapy of parkinsonism
• Relief of rigidity, tremors and bradykinesia: Most of the drugs reduce rigidity more than tremor and bradykinesia. Tremor, in fact, may be aggravated after reduction in rigidity. Levodopa ameliorates all the three. Tremor is best relieved by anticholinergic drugs. Reduction in rigidity and tremor allows the patient more free and easy movements, increases the mobility and boosts his morale. Physiotherapy acts as a valuable adjuvant in such cases. • Correction of mood changes: Most parkinsonian patients have a mild intellectual disability as a result of frontal lobe dysfunction. Depression is often a marked feature of arteriosclerotic parkinsonism. If the primary drug employed fails to correct it, a tricyclic antidepressant may be helpful. A substantial minority of patients with progressive dementia are, however, difficult to treat, and drug therapy itself may lead to hallucinations. Optimism is infectious and hence, the physician’s attitude must be one of hope and cheerfulness. • Treatment of other symptoms such as excessive salivation, seborrhoea and of complications like oculogyric or sweating crisis. • Treatment of cause, if possible: Parkinsonism following drugs is completely reversible after stopping the drugs. Reduction in high tissue copper levels associated with Wilson’s disease also results in some relief. Successful management of parkinsonism depends on a multidisciplinary approach, and demands compassionate care of the patient. The various forms of treatment available are: (i) Drug therapy; (ii) Non-pharmacologic therapy comprising education, support,
physiotherapy, nutrition; and (iii) Surgery.
Drug Therapy The drugs used in the treatment of parkinsonism can be classified as: I Those that increase the dopaminergic activity: • Precursors of DA, e.g. Levodopa. • Drugs that inhibit DA metabolism (a) MAO-B inhibitors, e.g. Selegiline. (b) COMT inhibitors, e.g. Tolcapone, Entacapone • Drugs that release DA, e.g. Amantadine • DA receptor agonists, e.g., (a) Ergot derived: Bromocriptine (b) Non-ergot: Pramipexole, Ropinirole II Those that suppress the cholinergic activity: Atropine, and atropine substitutes such as Benzhexol, Procyclidine; and Antihistaminics with anticholinergic properties. Anticholinergic drugs help to diminish the cholinergic preponderance; further, some of them (eg. benztropine) inhibit active DA re-uptake in the striatum and increase the local DA concentration. In general, they are less effective but safer than levodopa. LEVODOPA: Levodopa, 3-4, dihydroxy-phenylalanine (Fig. 15.1), is a ‘universal antiparkinsonian drug’.
FIG 15.1 Levodopa
Mechanism of action: Levodopa, a prodrug, is a metabolic precursor of DA (Chapter 18). It crosses the BBB by an active process mediated by a carrier of aromatic amino acids. This process may be competitively inhibited by a protein-rich diet. It is taken up by the dopaminergic neurons and is decarboxylated to DA. Levodopa can thus, be looked upon as a ‘replacement therapy’ of sorts. Dopamine itself does not cross the BBB and hence is ineffective. In advanced PD, as there are few dopaminergic neurons remaining, levodopa becomes less and less effective. In the peripheral tissues, such as the liver and the kidneys, it is converted to DA which accounts for its peripheral effects. Pharmacological actions: CNS actions: Levodopa improves all the major manifestations of PD. Bradykinesia responds first, followed by rigidity and tremor. Tremor may, however, be initially aggravated in some patients. Other manifestations such as seborrhoea, sialorrhoea and aphonia may also improve. The drug improves mood, memory and makes the patients more alert and interested in themselves and in their surroundings. The subjective improvement and the improvement in the general motor performance far surpass the more modest improvement in the conventional physical signs of parkinsonism. About 30% of the patients show ‘impressive’ improvement whereas another 30% show ‘worthwhile’ improvement. In general, younger patients with milder symptoms derive greater benefit
than elderly, debilitated patients who may not tolerate full doses of levodopa. Newly diagnosed patients who do not respond to 1.2 g of levodopa daily over a period of three months probably do not have PD. Improvement of parkinsonian symptoms arising from manganese poisoning has also been reported. Levodopa has also been found to ameliorate idiopathic dystonia in children and adolescents. Cardiovascular actions: It produces its cardiovascular effects by being converted peripherally to DA which, • Acts on specific dopaminergic receptors to cause renal and mesenteric vasodilatation which causes fall in BP following small doses. • Stimulates beta-adrenergic receptors in the heart (positive inotropic action); this effect can be blocked by beta blockers; and • Stimulates alpha-adrenergic receptors in blood vessels to produce vasoconstriction. Thus, large doses of levodopa may cause rise in BP which can be countered by alphaadrenergic blocking agents such as prazosin. These effects can be prevented following prior administration of a decarboxylase (DC) inhibitor such as carbidopa, which does not cross the BBB. Endocrine actions: Levodopa inhibits prolactin secretion and suppresses lactation by acting on the D2 receptors on the pituitary lactotropes. Absorption, fate and excretion: The drug is rapidly absorbed from the small intestine by active transport with peak plasma levels at ½- 2 hours, and its plasma t½ is 1-3 hours. As more than 95% of orally taken levodopa is rapidly decarboxylated to DA in the lumen of the GI tract, liver (first pass effect) and other tissues, less than 1% is left to enter the CNS. Hence, large doses of levodopa are needed to permit enough to penetrate into the brain to raise its_DA_content. Pyridoxine accelerates its peripheral decarboxylation as the decarboxylase (DC) is pyridoxine dependent. The blood levels of levodopa can be increased by inhibiting the DC by using DC inhibitors (see later). Since the DC inhibitors do not penetrate the BBB, conversion of levodopa to DA in the brain is not affected. As high protein content of a meal interferes with the absorption of l-dopa, the drug is taken 1 hour before or 1 hour after a meal. Only in patients difficult to control, who need frequent doses and who have a fluctuating motor response, protein intake should be restricted. Levodopa competes with ‘large neutral amino acids’ for passage through the gut wall as well as into the brain. It is metabolised by both MAO and COMT. The drug is excreted in the urine partly unchanged and partly as DA and mainly as its metabolite, homovanillic acid. Some of the drug is also converted to NA. The dose of l-dopa generally needs no adjustment in patients with renal or hepatic disease. Adverse reactions: Almost every patient may show some adverse effect. GI and CVS reactions occur early in the course of therapy. Most patients develop tolerance to it. Behavioral and CNS effects occur during prolonged treatment, and as tolerance does not develop to these symptoms, they prove dose-limiting. (A) Early ADR: • GI: Nausea, vomiting and anorexia are common. They occur because of its action on CTZ and are minimised by taking levodopa with food, by increasing the dose slowly
and by domperidone 10 mg tid. Phenothiazine antiemetics are not used as they worsen parkinsonism. Carbidopa, in addition to that present in the levodopa-carbidopa combination, in the dose of 25 mg three times a day may help in resistant cases. • Cardiovascular: It causes: (a) Postural hypotension, generally asymptomatic, by its central action; this is not prevented by DC inhibitors; instead it can be helped by fludrocortisone. (b) Peripherally, DA causes palpitation, sinus tachycardia, increased A-V conduction and ventricular arrhythmias; these can be countered by a DC inhibitor and by adrenergic beta blocker such as atenolol. Tolerance develops over weeks to postural hypotension as well as to the cardiac effects. The drug should be used cautiously in patients with history of MI and with ECG evidence of ectopic activity. (B) Late ADR: • Behavioural: This includes agitation, confusion, restlessness, hypomania, hallucinations, delusions and depression with attempted suicide. The drug exacerbates latent or active psychotic states, making it necessary to abandon treatment. The use of conventional antipsychotics may worsen PD. Patients with history of psychiatric disturbances should not be treated with levodopa. • CNS: On prolonged therapy, abnormal movements (choreoathetosis) involving head, neck and sometimes even the extremities may occur. They can be quite disturbing and even incapacitating. Unfortunately, they coincide with optimum therapeutic effect and correlate with the duration of therapy and the dosage. They are not prevented by a DC inhibitor. Parkinsonian patients suffer from insomnia but do not complain of it because their daytime symptoms are more disabling. An improvement in their daytime symptoms can make them more aware of their insomnia which, in fact, neither improves nor worsens on levodopa. A syndrome similar to the neuroleptic malignant syndrome (Chapter 13) can occur rarely after rapid reduction in dosage or abrupt discontinuation of levodopa. (C) Miscellaneous: Some patients may show positive Coomb’s test, though hemolytic anemia has not been reported. Blood urea nitrogen and serum SGOT may show a transient rise. Rise in plasma cholesterol levels and a decrease in carbohydrate tolerance can occur. The urine is red coloured when passed and becomes dark on exposure to air or alkali. It gives a false positive test for ketone bodies with the dip-stick test. Drug interactions: They are many and are listed in Table 15.2. Table 15.2 Drug interactions of levodopa
Anticholinergics, benzodiazepines, tricylic antidepressants, diuretics, oral hypoglycemic agents,
antibiotics, glyceryl trinitrate, digoxin, propranolol and anti-arrhythmic agents may, however, be safely used along with levodopa. Preparation and dosage: Levodopa is available as 100, 250 and 500 mg tablets. The initial dose is 50 mg tid in combination with carbidopa. The total daily dose is increased progressively (see later). As the drug has a short plasma t½, frequent dosage is necessary to maintain an even therapeutic effect and to minimize ADR. Concurrent administration of a DC inhibitor permits a 75% reduction in the daily dose of levodopa. Hence combination is preferred. Decarboxylase inhibitors (DCI): By themselves, these drugs are inactive. They do not enter the brain. The concurrent administration of all DCI decreases the peripheral decarboxylation of levodopa to DA, thus increasing l-dopa plasma level. This endows certain advantages (Table 15.3). Table 15.3 Advantages of adding DCI to levodopa
Postural hypotension, abnormal involuntary movements and psychiatric disturbances, all of which are central in origin, are not prevented or eliminated by concurrent use of DC inhibitors. Carbidopa (methyl dopa hydrazine) and Benserazide hydrochloride are the two DC inhibitors available in combination with levodopa. They are available as: (a) Carbidopa and levodopa in the ratio of 1 : 10. Approximately 75-100 mg of carbidopa is needed to totally block the effect of dopa-decarboxylase. Sustained released preparations and disintegrating tablets are also available. (b) Benserazide and levodopa in the ratio of 1:4. The dose, expressed as levodopa, is initially 50-125 mg 3-4 times a day; the maintenance dose is 0.75 to 1.0 g per day. The plant Mucuna pruriens Bak has been shown to contain levodopa and the crude seed powder has been used to treat parkinsonism. MAO-B inhibitors: SELEGILINE: The two types of monoamine oxidases (MAO) are: • Type A which causes oxidative deamination of tyramine, NA and 5-HT; and • Type B which acts on DA in human platelets and brain. Selegiline is a relatively selective, irreversible inhibitor of MAO-B. It prevents DA from degradation, thus increasing the concentration and storage of DA within the striatum. Hence, it prolongs the duration of improvement brought about by levodopa. Further, it diminishes the diurnal fluctuations in physical strength and reduces the end-of-dose ‘wearing-off ’ effects. It has no antidepressant action. Adverse reactions: They are due to an increase in the incidence of central dopaminergic effects e.g. dyskinesia, nausea and hallucinations. As selegiline is a MAO inhibitor, the drug should be discontinued for some time before starting an antidepressant. Its use with pethidine or an SSRI can precipitate serotonergic reaction. Selegiline is
generally well tolerated in mild PD. However, in advanced disease it may accentuate the adverse motor and cognitive effects of levodopa. Metabolites of selegiline include amphetamine and methamphetamine, which may cause anxiety and insomnia. The daily dose is 10 mg, administered as 5 mg at breakfast and at lunch. Administration later in the day can cause insomnia. Higher doses can cause inhibition of MAO-A as well, and should be avoided. Rasagiline is another MAO-B inhibitor, claimed to be more potent and selective. Unlike seligiline, it does not have an active metabolite. Inhibitors of CYP1A2 such as fluoxetine and ciprofloxacin increase the blood levels of rasagiline. Clinical studies have indicated that the mortality due to hypertensive crises may be higher in patients treated with levodopa-carbidopa-selegiline combination than in those with levodopa-carbidopa combination. Hence, newly diagnosed patients should not be treated with such triple drug regimens. COMT inhibitors: TOLCAPONE: By inhibiting COMT, this drug reduces the central and peripheral, metabolic degradation of levodopa and prolongs its plasma t½. Given alone, it is not useful. Adverse effects reported are nausea, orthostatic hypotension, dyskinesia, diarrhoea, induction of hallucinations due to increase in the serum level of levodopa, and serious hepatotoxicity. It is used as an adjunct to l-dopa. Entacapone, an analogue, is claimed to be less hepatotoxic than tolcapone. Its action is primarily peripheral and is of shorter duration as compared to tolcapone. The main indication for this drug is to treat early ‘end-of-dose’ deterioration as it does not cause dyskinesia. Dopamine releasers: AMANTADINE: This drug, developed originally as an antiviral agent, has been found to ameliorate bradykinesia, rigidity and tremor in parkinsonism. It acts by liberating DA from the residual intact nerve terminals. It also inhibits the activity of NMDA receptors and has some antimuscarinic action. Its therapeutic efficacy in this respect is 15-20% that of levodopa but slightly higher than that of anticholinergics. But, it produces a more rapid response (2-5 days) than levodopa and is less toxic. The drug is well absorbed orally and is excreted unchanged in urine. Adverse reactions: These are similar to those of anticholinergic drugs whose adverse effects it potentiates. In general, the drug is well tolerated. In toxic doses, it causes convulsions and mania. (Chapter 59). The dose is 100 mg per day, increased to 100 mg twice a day after 7-10 days. As restlessness is one of its major adverse effects, the second dose should not be taken late in the day. Addition of amantadine in patients receiving near maximum benefit from levodopa causes little further improvement. The best way of using this drug may be to give it for 2-4 weeks at a time, as an adjunct to l-dopa; tolerance to it develops quickly. Dopamine (D2) agonists: used in the treatment of PD are summarised in Table 15.4. In general:
Table 15.4 Dopamine agonists used in parkinsonism Drug Ergot derived (Ergolines): Bromoc riptine Non-ergot derived: Ropinirole Pramipexole Rotigotine Apomorphine
Dose 20–30 mg/day Upto 24 mg/day Upto 4.5 mg/day 4–6 mg/day, transdermal patc h. Parenteral dose as needed, after priming with an antiemetic suc h as domperidone
All drugs except rotigotine and apomorphine are given orally.
(a) They do not require functional nigrostriatal neuron; (b) They do not need conversion to active metabolire; (c) They have substantially longer duration of action than levodopa; (d) They are better tolerated; and (e) Their dose in parkinsonism is higher than in hyperprolactinemia. They are: BROMOCRIPTINE: This synthetic ergoline acts as a specific dopamine D2 receptor agonist (Chapter 67). It crosses the BBB. It is slower acting but less toxic than levodopa. It is used in the dose of 2.5 mg bid to begin with, slowly increased to a maintenance dose of 20-30 mg per day. It causes nausea by its dopaminergic action on the medullary CTZ. This can be treated by domperidone (Chapter 41). ROPINIROLE: This synthetic nonergoline, a selective D2 agonist, is well absorbed and its actions are similar to those of bromocriptine. However, it is effective within a week or less. The adverse reactions include nausea (40%), somnolence, vomiting, dizziness and fatigue. It may cause sudden sleep attacks, postural hypotension, and rarely hallucinations. Pathological gambling and other compulsive behaviours have been reported. The drug causes embryonic loss in animals. Pramipexole has properties similar to those of ropinirole. It particularly induces pathological compulsive behaviours. It has been used to treat restless leg syndrome (Chapter 14). Rotigotine, an non-ergot DA agonist, is available as transdermal patch. For apomorphine, see chapter 10. All the dopamine agonists in large doses can cause severe neuro-psychiatric adverse effects. All ergolines can produce pleural as well as retroperitoneal fibrosis and digital spasms. Further, thickening and dysfunction of cardiac valves has been reported during therapy with pergolide and cabergoline; they are not recommended in parkinsonism. Pergolide has now been withdrawn from the market. Anticholinergics: By their antimuscarinic action, both atropine and hyoscine can relieve, to some extent, rigidity, tremor, hyperhidrosis, seborrhoea and sialorrhoea. Atropine substitutes are moderately effective and have been used for many years in the treatment of parkinsonism; they are often the only drugs patients can afford for prolonged periods. Benzhexol is discussed below in detail as a prototype. BENZHEXOL HYDROCHLORIDE (Trihexyphenidyl): Benzhexol is an effective and the most commonly used drug from this group. It has weaker peripheral anticholinergic actions and is well tolerated.
Pharmacological actions: The drug is useful in controlling muscular rigidity, tremor, sialorrhoea and seborrhoea. It improves the mood. In patients with excessive muscular rigidity; however, it may increase tremor while reducing rigidity. It does not improve bradykinesia and loss of postural reflexes significantly. Larger doses cause cerebral stimulation. Absorption, fate and excretion: Benzhexol is well absorbed orally. It rapidly disappears from the tissues but its fate is unknown. Adverse reactions: It is free from serious adverse reactions; however, atropine-like side effects may develop even in therapeutic doses in 10 to 20% of patients. These include xerostomia, blurred vision, nausea, dizziness, restlessness and urinary retention in the presence of prostatic enlargement. Overdosage causes confusion, hallucinations and delirium. Preparations and dosage: It is available as 2 and 5 mg tablets and 5 mg sustained release capsules. The initial dose is 1 to 2 mg, increased gradually upto 10 to 15 mg per day. A single dose of the sustained release capsule maintains the effect for 15 hours and may be employed after initial stabilisation. The central stimulation produced by the drug can be countered by combining it with diphenhydramine. Therapeutic use: Before the advent of levodopa, benzhexol was called a ‘universal’ antiparkinsonian drug as it controls, to some extent, all the signs. It can be used even in the presence of hypertension and cardiac disease. It is effective in drug-induced parkinsonism and is helpful in postencephalitic parkinsonism. It is also useful in certain forms of dystonias. The other drugs used are listed in Table 15.5. Table 15.5 Drugs with atropine like action used in parkinsonism
*
A potent blocker of re-uptake of dopamine. Has a prolonged action and a sedative effect
**
Useful IV, in rapidly controlling drug-induced acute dystonic reactions. Other doses are oral.
Miscellaneous: Certain antihistaminics (Table 15.5) used to treat parkinsonism also have anticholinergic action. Diphenhydramine (50-100 mg/day) is well tolerated, relieves rigidity but not tremor or sialorrhoea; it can cause drowsiness and giddiness. Promethazine (25 mg IV) is useful in rapidly controlling acute drug induced dystonic reactions (Chapter 23).
Management of Parkinsonism Levodopa still remains the most effective drug in the treatment of Parkinson’s disease. However, its adverse effects make continued close supervision mandatory and its high cost puts serious limitations on its routine use in some subjects. Levodopa is commonly used in combination with a DC inhibitor, carbidopa. Usual starting dose is 50 mg of 1-dopa with 12.5 mg of carbidopa given 3 times a day. Dose is then gradually increased till maximum benefit is achieved without serious toxicity; this may be 500-1000 mg daily in 3-4 divided doses. If no benefit is observed even with a dose of 1000 mg, the clinical diagnosis of PD should be reviewed. Although slow, constant-release preparations of 1-dopa are available, they generally do not achieve desired prolongation of plasma level or help to avoid pulsatile dopaminergic stimulation. Its withdrawal, if necessary, must be gradual as sudden withdrawal may precipitate symptoms resembling neuroleptic malignant syndrome. In advanced cases, levodopa can be combined with amantadine, dopaminergic agonists or COMT inhibitors. Many patients derive substantial benefit from l-dopa over the entire course of their illness. Levodopa increases life expectancy in patients with PD, and survival is significantly reduced if the administration is delayed until greater disability develops. Hence, some authorities advocate early treatment with l-dopa or dopamine agonist such as ropinirole to provide improved quality of life. However, they are expensive. Levodopa with a DC inhibitor is the only drug that promotes active life, and should be prescribed for patients who are incapacitated due to severe disease. The drug is preferred in the elderly patients (more than 70 years) who tolerate the anticholinergics poorly. The major drawbacks of long term levodopa treatment are: (1) Dyskinesias involving involuntary choreoform movements, and rapid fluctuations in motor strength. New symptoms develop that may be resistant to levodopa. Younger patients develop fluctuations and dyskinesia to a more severe degree and sooner than the older patients. (2) Motor complications that include (a) Predictable ‘off’ periods of immobility or greater severity of parkinsonian signs, when the medication effect ‘wears off ’. This has a relation to the timing of the antiparkinsonian medication, and can be helped by a COMT inhibitor (entacapone) and (b) Unpredictable ‘on-off’ fluctuations which are of sudden occurrence (lasting seconds) of shifts between on and off periods that are apparently not related to the timing of medication. With the passage of time, 60% of initial responders start experiencing ‘end-of-dose’ wearing effect (advanced disease). (3) Psychiatric complications due to the stimulant action of levodopa on dopamine receptors in the mesolimbic and mesocortical dopamine systems. An atypical neuroleptic such as clozapine may be helpful. Parkinsonism due to generalised degenerative brain disease and postencephalitic parkinsonism responds less well than PD. Dopamine agonists like bromocriptine though useful, have limited efficacy. About 1/3rd of the patients have a good response to these drugs and may not need l-dopa for 3-5 years. Their advantage is that these patients do not have fluctuations or dyskinesias till l-dopa is added. However, with prolonged use, their effect diminishes. Non-ergot dopamine agonists may be preferred to levodopa as they are both better tolerated and less toxic.
The synthetic atropine substitutes are still important in the treatment of parkinsonism, benzhexol being the most effective. They, in combination with physiotherapy, may be sufficient and cost effective in the relatively inactive patients with minimal disease and no functional impairment. They are particularly well tolerated by and hence preferred in the ‘younger ’ patients (less than 50 years of age), in whom cognitive impairment is rare. However early levodopa-carbidopa therapy (see below) is indicated even in these younger patients if rapid response is essential for some reason. Treatment with benzhexol should be started with a small dose and increased gradually. As maximally tolerated doses do not give much more benefit than slightly smaller and better tolerated doses, no attempt should be made to push the dose to the limit of tolerance. Some patients do not improve adequately on this drug or the initial improvement is lost due to the development of tolerance. In such cases, another synthetic atropine substitute should be tried or a drug from the other groups (antihistaminics or levodopa) may be added to benzhexol. The change over to a new drug should be gradual and overlapping. Anticholinergics can be combined with either amantadine or levodopa. However, it is better to avoid such combination if there is history of psychosis or dementia. Diphenhydramine, an antihistaminic is particularly useful in elderly patients with mild disease, who cannot tolerate the anticholinergic drugs. Further, they help to counter the insomnia in patients on levodopa or anticholinergics. Amantadine is a useful alternative to anti-cholinergic drugs in patients with mild PD. It can also be used as an adjunct in patients who are unable to tolerate levodopa. It may help in dyskinesias. Disease-slowing benefit observed following selegiline given initially is now thought to be due to the amelioration of symptoms. The initial effect following selegiline was not sustained and the drug did not delay the development of dyskinesias. The treatment strategies in PD are outlined in Table 15.6. Acute infections and surgery tend to cause rapid deterioration in the patient’s health. Hence, surgery should be avoided unless it is absolutely necessary. Further, levodopa should be continued post-operatively and during an infectious illness. Table 15.6 Treatment strategies in PD
Symptomatic treatment: Regular treatment with the standard drug therapy can prevent oculogyric crises. Addition of dexamphetamine in the dose of 5 -10 mg once or twice daily offers additional protection against oculogyric crises; but, dexamphetamine must not be used along with levodopa. Orphenadrine and trihexyphenidyl are effective in preventing and treating the rarer sweating crises. Depression is common in PD and can occur especially in patients on levodopa. It can be treated with an antidepressant. If a patient develops an organic, confusional state or
psychosis, the drug therapy should be discontinued in the following order: anticholinergics, selegiline, amantidine, and dopa agonists. Finally, the dose of levodopa should be gradually tapered. If psychosis persists, the use of clozapine or other antipsychotics is indicated. Sialorrhoea is usually controlled by atropine substitutes. If dryness of the mouth is bothersome, it can be relieved by the use of hard candy. Limitations of drug therapy: • Drugs do not cure the disease nor retard its progression. All that can be expected is 20 to 70% symptomatic improvement in 60-80% of the patients. Tremor is not much helped by drugs except 1-dopa. • Tolerance develops to most of the drugs. • Drugs with prominent anti-cholinergic actions must be used cautiously in patients with glaucoma and prostatic enlargement. • Levodopa is not recommended during pregnancy, lactation or in children below 18 years. • Levodopa therapy is expensive, has many adverse effects and needs regular supervision. Education of the patient and his family is important to ensure compliance and emotional and social support to the patient. Exercise, especially stretching and strengthening exercises, can prevent or alleviate secondary orthopedic effects of rigidity and flexed posture such as shoulder, hip, and back pain, and may improve function in some motor tasks. The other useful exercises are brisk walking, swimming and aquatic aerobic exercises. Nutrition: High fibre diet and adequate hydration can prevent constipation. Large, high fat meals which slow gastric emptying and interfere with drug absorption should be avoided. Dietary protein restriction becomes necessary in some patients with advanced disease and motor fluctuations in whom competition with amino acids interferes with l-dopa absorption. Surgery: Thalamotomy, pallidotomy and deep-brain stimulation with implanted electrodes may benefit patients under 50 who suffer from severe symptoms unresponsive to drugs. Attempts are being made to transplant stem cells into the substantia nigra.
Drug Therapy of Other Extrapyramidal Syndromes • Tremor: Propranolol in the total daily dose of 30-120 mg is helpful in alleviating essential tremor and that due to anxiety and thyrotoxicosis; cardioselective β blockers are less effective. Primidone (in the total daily dose of 125-500 mg) is also helpful in relieving essential tremor. It may be combined with propranolol in resistant cases. • Chorea: Levodopa is palliative for short periods in juvenile onset Huntington’s chorea; it aggravates the adult-onset disease; the latter benefits from the use of D2 receptor blocking antipsychotic drugs such as haloperidol. • Dystonias: A variety of drugs (trihexyphenidyl, carbamazepine, diazepam) have been tried with variable results. For the use of botulinum toxin type A, in muscle spasticity see Chapter 22. • Myoclonus: Valproic acid, clonazepam and a combination of l-5-hydroxytryptophan with carbidopa have been reported to alleviate myoclonus.
Drug-induced Extrapyramidal Reactions (EPR) Certain drugs used in therapeutics (phenothiazines, butyrophenones, thioxanthenes, metoclopramide, reserpine, methyldopa and levodopa) cause a variety of EPR. These reactions can be broadly grouped into four syndromes: (1) Parkinsonism: It is almost indistinguishable from idiopathic parkinsonism but tremor is an infrequent feature. It has been reported with all drugs mentioned above except levodopa. Drug-induced parkinsonism is treated by adding a synthetic atropine substitute and withdrawing the offending agent. Anticholinergics, however, impair cognitive function and have peripheral antimuscarinic effects. They also exacerbate tardive dyskinesia. Hence amantadine is recommended as treatment. It reduces parkinsonian symptoms without increasing psychotic symptoms. (2) Akathisia (not to sit): In this syndrome, the patient exhibits a compulsive motor restlessness, is constantly on the move and is apprehensive. It needs to be distinguished from psychotic agitation which it resembles, because unlike the latter it is aggravated by an increase in the dose of the antipsychotic drugs. It has been reported with the antipsychotic phenothiazines and with metoclopramide. The anti-cholinergic benzotropine and small doses of propranolol or clonazepam may be helpful. (3) Acute dystonic reactions: These are characterised by painless, spasmodic contraction of one or more muscle groups resulting in trismus, torticollis, opisthotonus or oculogyric crisis. They are seen mainly with phenothiazines, butyrophenones, metoclopramide and prochlorperazine. Acute dystonic reactions respond well to an IV injection of an antihistaminic such as diphenhydramine 10 mg or promethazine 25 mg, followed by 25 mg of the same drug orally. (4) Tardive dyskinesia: This syndrome, reported to occur late during phenothiazine therapy, develops gradually and consists of involuntary movements such as repetitive sucking, smacking of lips, grimacing and movements of the tongue and extremities. Old age, prior brain damage, schizophrenia and cerebral hypoxia seem to predispose to it. It may persist indefinitely even after stopping the drug and is presumed to be related to DA receptor supersensitivity. There is no satisfactory treatment. Atypical antipsychotics may be used as replacement. Episodes of EPR can be prevented by: (i) Prescribing the antipsychotic drugs in the minimal, effective doses; (ii) Concurrent administration of low doses of benzhexol with antipsychotic drugs; and (iii) Using less toxic drugs such as benzodiazepines for sedation or cyclizine for vomiting. Anti-cholinergics, however, fail to prevent the development of tardive dyskinesia. Keeping a watch for the ADR during therapy is mandatory. At the first sign of EPR, the dose of the offending drug should be reduced or if possible, replaced by a less toxic drug.
Motor Neuron Disease (MND) Drug Therapy Amyotrophic Lateral Sclerosis or Motor Neuron Disease (MND) is one of the most severe degenerative neurological disorders. Both central and peripheral neurons are affected. The course of MND is rapidly progressive and the 50% survival probability after the first signs of MND is just about 3 years. The etiology of the disease is unknown. The currently favoured hypothesis centres around glutamate. Glutamate is the principle excitatory amino acid (EAA) neurotransmitter in the brain and the spinal cord. Excessive stimulation of glutamate receptors (excitotoxicity) may cause neuronal injury or death in various neuropathological conditions. Experimentally, selective inhibition of glutamate in the culture medium results in slowing of degeneration of motor neurons. It is suggested, therefore, that a drug that reduces the effect of glutamate may help in MND. RILUZOLE: This 2-amino-6 trifluoro-methoxy benzothiazole, given orally, is rapidly absorbed and crosses the BBB. It is largely metabolised in the liver. It blocks the release of glutamate from the neuronal cells as well as blocks the postsynaptic NMDA receptors and thus protects against the excitotoxicity of glutamate. Riluzole has anticonvulsant properties in animals. Further, it protects against memory impairment and degeneration of neurons in animal models of acute cerebral ischemia. Riluzole retards the progression of MND and increases the duration of survival by a few months. The adverse effects include nausea, diarrhoea and elevation of liver enzymes. Baclofen, tizanidine and dantrolene (Chapter 22) have been tried for symptomatic treatment of spasticity in MND. Neurotrophic factors: These naturally occurring proteins have been shown to keep the neurons alive and healthy during embryonic development and subsequently. They are under evaluation for the treatment of MND.
Drugs and Memory In recent years, attempts have been made to develop drugs for improving memory in disorders of cognitive deterioration such as occurs in Alzheimer ’s disease and other dementias. Alzheimer’s disease is a chronic, progressive, degenerative, nonpsychiatric disorder of the brain. There is damage to cortical and subcortical areas of the brain, resulting into disturbances of multiple higher functions such as thinking, memory, judgment and orientation. It is characterised by cognitive deficit and debility but consciousness is not affected. Its etiology is not known. Currently it is believed that deposition of amyloid β (AB) inside neuronal cells and extracellularlly causes synaptic dysfunction and neuronal cell death. Toxic concentration of AB target tau, microtubule-associated protein, a major constituent of neurofibrillary protein. The disease is associated with degeneration of cholinergic neurons. There is also a deficit of choline acetyl transferase, the enzyme responsible for formation of ACh. This results in decreased central cholinergic transmission. There is a genetic predisposition and there is no cure. Alzheimer ’s disease must be differentiated from other types of dementia such as due to depression, vitamin deficiency and hypothyroidism, which are treatable. Before starting treatment, other causes of cognitive impairment such as intracranial lesions and cardiovascular disease have to be ruled out. Symptomatic drug treatment includes use of: (i) Cholinesterase inhibitors e.g. Donepezil, Rivastigmine, Galantamine (ii) NMDA receptor antagonist: Memantine (iii) Treatment of neuropsychiatric symptoms by antipsychotics and antidepressants TACRINE (Tetrahydroaminoacridine): This anti-ChE drug acts by preventing the degradation of ACh. It readily crosses the BBB and is retained in the CNS for long time. Tacrine improves cognitive performance in 15-30% of the patients. The most important and frequent adverse effect is hepatotoxicity. Other adverse effects are similar to those of other anti-ChEs. (Chapter 19). The selective cholinesterase inhibitors used to treat Alzheimer ’s disease include: (1) Donepezil (non-competitive inhibitor with t½ 70 hours; 5 mg -10 mg), (2) Galantamine (competitive inhibitor; t½ 8 hours; 4 mg -12 mg bid) and (3) Rivastigmine (non-competitive inhibitor; t½ 1 hour; 1.5 mg – 6mg bid). These drugs are less hepatotoxic, but can give rise to GI adverse reactions infrequently. They may be preferred to tacrine. Rivastigmine is also available as transdermal patches. Memantine is a non-competitive, NMDA receptor antagonist. It is believed to protect neurons from glutamate-induced excitotoxicity. Its usefulness needs confirmation. Anticholinesterases produce moderate improvement in cognition and mood. They can be combined with memantine. Antipsychotics are commonly used to treat agitation, aggression and psychosis in patients with dementia. Various vasodilators, metabolic enhancers (Nootropics), e.g., DHE, piracetam, estrogen, statins the extract of Ginkgo biloba and antioxidants such as vitamin E have been tried with doubtful results. The overall benefit of drug therapy is disappointing. Drug therapy is combined with nonpharmacological treatment such as social interaction and person centered care training.
Multiple Sclerosis (MS) Multiple sclerosis (MS) is a complex inflammatory disease of the brain and spinal cord, characterised by focal lymphocytic infiltration, leading to damage of the oligodendrocytes (which synthesise myelin) and axons. Initially the inflammation is reversible and remyelination can occur but later episodes cause permanent damage. MRI shows focal or confluent abnormalities in the white matter of the brain and spinal cord. Its etiology probably involves both environmental exposure and genetic susceptibility. Inflammation is driven by IL-17 secreting T-lymphocytic-subtype which allows penetration of BBB by Thelper 17 (Th-17) cells into the brain, where they attack the neurons. This results in inflammation, demyelination, oligodendrocyte depletion, and finally neuronal degeneration. Specific antibodies to myelin-base proteins are found to be associated with MS. In the initial phase, relapsing-remitting pattern of the disease is observed. The symptoms include visual impairment due to optic neuritis, spasticity, unstable blader, increased mechanical sensitivity, and electric sensation felt in the spine or the limbs on neck flexion. The symptoms worsen on hot bath or exercise. The diagnosis in clinical but MRI is helpful. The disease causes much disability, and leads to secondary progressive MS with shortened life expectancy. There is no cure, the therapy includes: (a) Rest, physiotherapy family/social support; (b) Symptomatic treatment e.g carbamazepine and gabapentin to treat paroxysmal symptoms; and (c) Treatment with disease modifying drugs in, • In Acute attacks: Methylprednisolone in pulse doses is useful (Chapter 66) is useful. • Relapsing form: Immuno-modulators (Chapter 74) such as interferon beta (Chapter 25) and glatiramer acetate/copolymer are considered as first line drugs to reduce the frequency of relapses (Chapter 74). Their usefulness for delaying or preventing long term disability is, however, unpredictable. Second line drugs are usually reserved for severe disease. They are: (i) Natalizumab, a humanised anti α-4, β-1 integrin antibody acting against the α-4, β-1 integrin on the surface of lymphocytes, has been shown to decrease relapse rate by 65%. Rarely it may cause fatal encephalopathy due to activation of JC virus. (ii) Alemtuzumab, a humanized monoclonal antibody which targets CD52 on the surface of T cell population, B cells and monocytes has been reported to be highly effective against multiple sclerosis. It induces a pronounced and long lasting depletion of T cells and is claimed to be superior to Interferon β-1a. The ADR reported are opportunistic infections such as herpes and UTI and increased chances of secondary autoimmunity especially thyroid disorders and thrombocytopenic purpura. (iii) Fingolimod, a sphingosin 1- phosphate receptor modulator, interferes with T cell migration, and thus helps to sequester circulating T Iymphocytes to lymph nodes. It is the first orally active agent for MS and has been shown to reduce relapse by 50%. (iv) Mitoxantrone, an anticancer agent. However, it is cardiotoxic and its long term use may increase risk of acute myeloid leukemia. (v) Teriflunomide, is a new FDA approved orally active drug approved for relapsing form of MS. It is an active metabolite of leflunomide (Chapter 75). It inhibits mitochondrial
enzyme dihydroorotate dehydrogenase and blocks pyrimidine synthesis, which result in reduction in T and B cell activation, proliferation and function. Adverse effect include diarrhea, nausea, alopecia, neutropenia, peripheral, neuropathy, hyperkalemia, hypophosphatemia, hypertension, hepatic failure and acute renal failure. Monitoring of LFT is needed before starting the drug and every 6 monthly during therapy. It inhibits CYP2C8 but induces CYP1A2 and therefore can alter the serum concentrations of the concomitant drugs which are metabolised by these enzymes. Other oral drugs which are undergoing evaluation for relapsing MS are laquinimod and dimethyl fumarate.
S E C T IO N III
Local Anaesthetics OUT LINE Chapter 16: Cocaine, Procaine and Other Synthetic Local Anaesthetics
16
Cocaine, Procaine and Other Synthetic Local Anaesthetics Local anaesthetics are drugs which, when applied directly to peripheral nerves, block nerve conduction and abolish all sensations in the part supplied by the nerve. They are applied to somatic nerves and act on axons, cell body, dendrites and synapses. Local anaesthetics are classified as: I Natural: Cocaine. II Synthetic nitrogenous compounds: • Derivatives of para-aminobenzoic acid. (i) Freely soluble: Procaine, Amethocaine. (ii) Poorly soluble: Benzocaine, Orthocaine. • Derivatives of acetanilide, e.g., Lignocaine (Lidocaine). • Quinoline derivatives, e.g., Cinchocaine (Nupercaine). • Acridine derivatives, e.g., Bucricaine III Synthetic non-nitrogenous compounds: Benzyl alcohol, Propanediol. IV Miscellaneous drugs with local action: e.g. Clove oil, Phenol, Chlorpromazine, antihistaminics like Diphenhydramine. These are discussed elsewhere. Local anaesthesia can also be produced by physical methods such as refrigeration, application of ice and ethyl chloride spray. General properties of local anaesthetics: Synthetic local anaesthetic drugs have many properties in common. They possess varying degrees of water and lipoidal solubility; since the nervous tissue is rich in lipid, lipoidal solubility helps the drug to move into the neuronal fibre, while water solubility helps to get the drug to the site of action from the site of injection or application. Thus, the local anaesthetic with high lipid but low water solubility will not be much useful because of difficulty in transportation through the aqueous phase surrounding the neuronal fibre. Chemically, the useful local anaesthetics consist of three parts (Fig. 16.1):
FIG. 16.1 Structure of local anaesthetics
(1) A hydrophilic amino group. (2) An intermediate chain; and (3) A lipophilic aromatic group. Due to presence of amino nitrogen, these drugs are bases and form water soluble salt with acids (pH 4.6) and are generally dispensed as hydrochlorides. In the tissues where the pH is alkaline (pH 7.4), the free base is liberated and produces its pharmacological action. Majority of the clinically useful local anaesthetics are nitrogenous compounds, either: (a) Esters e.g. procaine, tetracaine; or (b) Amides e.g. lignocaine, prilocaine bupivacaine and ropivacaine. Their generic name ends with the suffix ‘caine’. Non-nitrogenous compounds with local anaesthetic properties like benzyl alcohol have their generic names ending with suffix ‘ol’, e.g. propanediol, cyclohexanol. Mechanism of action: Local anaesthetics block both the generation and the conduction of the nerve impulse. They bind to receptors near the intracellular end of the voltage gated sodium channels. This prevents the increase in permeability of the cell membrane to Na + ion, the first event in depolarisation (Sodium Channel Block). Thus, an action potential is not generated. This action affecting the process of depolarisation, leading to failure of impulse propagation without affecting the resting potential, is known as membrane stabilising effect. Increased extracellular calcium antagonises the action of local anaesthetics on nerves and muscles, while potassium facilitates the same. A smaller nerve fibre presents a greater surface area per unit volume for the action of an anaesthetic than a larger fibre. Smaller fibres are, therefore, blocked first. Thus, the various fibres are blocked in the following order: • Autonomic fibres
• Sensory fibres conducting temperature and pain • Sensory fibres carrying touch, pressure and vibration sensations; and • Motor fibres Recovery of function occurs in the reverse order. Local anaesthetics are less effective when injected into an inflamed area. The exact cause for this phenomenon is not known. Pharmacological actions: Besides the local anaesthetic properties, cocaine and the other nitrogenous synthetic substitutes have important actions on other systems. • Central nervous system: They stimulate the CNS and cause restlessness, tremors and in toxic doses, convulsions. Central stimulation is followed by depression; death is usually due to respiratory depression. Cocaine in smaller doses acts more prominently on the higher centres causing euphoria, mental alertness and hallucinations. • Cardiovascular system: These drugs are myocardiac depressants. They decrease heart rate and the amplitude of contraction; increase the excitability threshold and the refractory period while slowing down conduction (membrane stabilising effect). Higher concentrations may lead to cardiac arrhythmias and cardiac arrest. Procainamide (related to procaine) and lignocaine are used therapeutically for their cardiac depressant properties (Chapter 28). Except cocaine, which is not used therapeutically, all the other drugs produce hypotension, by a direct action on the vessel wall; this is related to their neuron blocking potency. Cocaine is a vasoconstrictor. • Other actions: Synthetic nitrogenous local anaesthetics have a direct spasmolytic action on smooth muscle and in large doses, they can produce neuromuscular blockade. Absorption, fate and excretion: Local anaesthetics are not absorbed from unbroken skin. Applied to the mucous membrane, the absorption varies with the mucous surface. Thus, absorption is more rapid from the trachea than from the pharynx while it is poor through the urinary bladder. A large amount of drug can be absorbed from a raw granulating surface and can precipitate toxicity. When used by infiltration, the absorption can be retarded by combining it with a vasoconstrictor agent like adrenaline. It must be emphasised that a latent period of several minutes elapses between drug application and therapeutic effect. Failure to allow sufficient time for establishment of block may convey the erroneous impression that the dose employed is inadequate; this may lead to use of unnecessarily large toxic doses. Many of the common local anaesthetics are esters and are metabolised by hydrolysis in both the liver and plasma. The amide-like local anaesthetics such as lignocaine are dealkylated by the liver. There is a species variation; thus, cocaine is largely detoxified in rabbits while it is excreted unchanged in human urine. Slowly and incompletely detoxified drugs, if absorbed, would obviously produce greater systemic toxicity. Anticholinesterases increase the duration of action of procaine by inhibiting its degradation by plasma pseudocholinesterase. Adverse reactions: • Allergic reaction: This may manifest as a mild allergic dermatitis, a typical asthmatic attack or a severe fatal anaphylactoid reaction. It is generally seen with the local anaesthetics of the ester type and may show cross sensitivity with chemically related
compounds. Intradermal sensitivity test is recommended before using these drugs. A negative response, however, does not rule out drug sensitivity. • CVS: Fall of BP and cardiac arrest can occur sometimes. Hypotension should be treated with vasoconstrictor drugs such as noradrenaline and ephedrine. If the heart stops, external cardiac massage is indicated. During emergency, mouth to mouth breathing keeps the airway patent (Chapter 18). • CNS: This toxicity can be countered to a certain extent by preanaesthetic administration with diazepam. If convulsions occur, IV diazepam or IV thiopentone is used. Oxygen is given to prevent hypoxia. Factors which determine the toxicity are: • The rate of absorption, diffusion and inactivation of the drug • Individual susceptibility • Inherent toxicity of the drug Prevention of toxicity: See Table 16.1. Table 16.1 Prevention of toxicity of local anaesthetics
Therapeutic uses: The choice of the local anaesthetic mainly depends upon its desired duration of action. Procaine is short acting, lignocaine and mepivacaine are intermediate acting, whereas tetracaine and bupivacaine are long acting. • Surface anaesthesia: Amethocaine is used as a surface anaesthetic for the eye, throat, urethra, rectum and skin. Similarly, benzocaine and lidocaine hydrochloride are used as all purpose surface anaesthetics except for the eye. Dibucaine is used for the ear, rectum and skin. Proparacaine and tetracaine are used exclusively for the eye. • Infiltration anaesthesia: In this procedure, the nerve endings are anaesthetised by their direct exposure to the drug. The drug is infiltrated subcutaneously. Procaine 2% and lignocaine 2% are most commonly used. They are mixed with adrenaline (1:200,000) to prolong the action. Lignocaine acts longer than procaine, but procaine is cheaper and more easily available. Adrenaline should be avoided when local anaesthetics are used to produce ring block to anaesthetize the digits or penis, in order to avoid local ischaemia and in patients with known myocardial disease. • Nerve block or conduction block where the drug is injected very close to the nerve e.g. brachial plexus. Choice of the anaesthetic is determined by the duration of anaesthesia needed. • Spinal anaesthesia: In this procedure the drug is injected into the subarachnoid space. Its level in the space is adjusted by using solutions with higher (hyperbaric) or lower (hypobaric) specific gravity than that of CSF, as vehicles. Usually, the injection is made
‘heavy’ by adding dextrose or ‘light’ (approximately isotonic) by adding saline. The position of the patient is important in limiting the block to the desired level. Lignocaine and bupivacaine are the most commonly used drugs. When the anaesthetic is injected outside the dura, the technique is known as epidural anaesthesia. In that case, the spread of the anaesthetic is restricted to a specific region and hence, causes fewer complications. Due to sympathetic blockade these drugs produce arteriolar dilatation, decreased venous tone, post-arteriolar pooling of blood and diminished venous return to the heart. The cardiac output and the BP are reduced. Thus, hypotension is one of the most important complications of spinal anaesthesia. It is treated by: (a) Elevation of the legs or wrapping the legs in elastic bandages to increase venous return. (b) Rapid intravenous infusion of fluids for filling the dilated vascular bed; and/or (c) Use of vasopressor drugs e.g. ephedrine or methoxamine to restore arteriolar and venous tone. When used as spinal anaesthetics, lignocaine, tetracaine and other related compounds give good muscle relaxation and allow the use of cautery and electrical appliances during surgery. These drugs, however, are not suitable for surgery above the diaphragm and in apprehensive and mentally disturbed patients. Failure to block the vagus may precipitate hypotension and hiccough due to reflex stimulation during abdominal surgery. Headache, which is commonly observed following spinal anaesthesia, is probably due to leakage of CSF from site of puncture and it responds to analgesic drugs. Other complications include post-operative urinary retention and intestinal atony. Treatment of these is discussed in Chapter 19. Systemic uses: • In the treatment of cardiac arrhythmias (Chapter 28). • As IV analgesics in the treatment of severe pruritus and pain due to malignancy; they are of limited use. COCAINE is an alkaloid from the leaves of the coca tree (Erythroxylon coca) and other species. It is the methylbenzoyl ester of ecgonine which is chemically closely related to atropine. Cocaine is not used as a local anaesthetic; but it is an important drug of abuse for its psychotropic effects. Pharmacological actions: • Local anaesthetic action: It acts on peripheral nerves as a membrane stabiliser (see earlier) and hence as a local anaesthetic. The concentration used to produce local anaesthesia is poisonous to many structures like leucocytes and tissue cells. • Central Nervous System: It is a central stimulant. It blocks the reuptake of dopamine and causes activation of the dopaminergic system, leading to sense of euphoria (mesolimbic and mesocortical pathways), strongly reinforcing the addicting property of the drug. The euphoric effect of cocaine consumed by smoking lasts for 20 minutes whereas that following intranasal administration may last for 1-1½ hours. Later, depletion of dopamine from the nerve endings gives rise to dysphoria so characteristic of cocaine withdrawal. It impairs the homeostasis of 5-HT by blocking the uptake of tryptophan and 5-HT itself.
This may account for the striking alteration of sleep-wake cycle and may enhance the central excitatory effect of dopamine. • Cardiovascular system: It impairs the reuptake of adrenaline and NA by presynaptic nerve endings, which causes accumulation of these neurotransmitters at the synapses and thus activates the adrenergic system resulting in hypertension, tachycardia and peripheral vasospasm. Adverse reactions: These are: • Acute (a) Allergic reactions. (b) Activation of sympathetic nervous system produces vasoconstriction, an acute rise in BP, tachycardia and a predisposition to acute myocardial infarction, ventricular arrhythmias and convulsions. It may also result in mydriasis, hyperglycemia and hyperthermia. (c) Sudden death after administration by any route. Since it is metabolised by plasma and liver cholinesterases, people with deficiency of these enzymes (liver disease), infants, pregnant women and old persons are at greater risk of cocaine toxicity. Most deaths are due to convulsions, respiratory failure and cardiac arrhythmias. • Chronic (a) Although at low dose levels cocaine delays ejaculation and orgasm and causes heightened sensory awareness, sexual dysfunction and sexual disinterest are seen in long term. It is not a true aphrodisiac. (b) Anorexia, emaciation, tremors, disturbances of sensation and emotion, hallucinations and insanity are observed in cocaine addicts. (c) Cocaine when used by pregnant women may cause prematurity, intrauterine growth retardation and microcephaly. It has teratogenic effects on brain development. It can cause neurological symptoms including sudden death in the newborn. Drug dependence: Persons dependent on cocaine show paranoid and suicidal tendencies. Cocaine is one of the major drugs of abuse and its withdrawal can cause severe CNS depression. PROCAINE: It is the diethyl aminoethyl ester of para aminobenzoic acid. It is nonirritant and as effective as cocaine as a local anaesthetic, but is much less toxic. It is a vasodilator. Its disadvantages are that it is poorly absorbed from the mucous membranes and, therefore, has no topical use. Procaine is rapidly hydrolysed by esterases in the plasma and liver and is partly excreted in the urine, conjugated with glucuronic acid and glycine. LIGNOCAINE (Lidocaine): This is the most commonly employed local anaesthetic. • It is stable, can be stored for a long time at room temperature and can be autoclaved. • It has a quick onset of action and a high degree of penetration. • Its toxicity is similar to that of other local anaesthetics in equipotent doses. • It is also an excellent surface anaesthetic. Following infiltration of 0.25-0.5% solution, the duration of action varies between 30 and 60 minutes. Addition of adrenaline (1 in 200,000) prolongs the action for about 2 hours. Analgesia is complete within a few minutes and recovery occurs quickly, within 2-3 hours
after spinal anaesthesia. The drug is recommended for topical use, nerve blocks, infiltration and epidural injection and for dental analgesia. It may cause drowsiness but has no vasoconstricting action. It can be used in subjects allergic to procaine and other ester-type local anaesthetics. Its use in cardiac arrhythmias is discussed in Chapter 28. Prilocaine has similar actions as lignocaine. It does not require adrenaline. CNS toxicity is less and is used for IV regional blocks. It is used in dentistry. Lignocaine and prilocaine are solid bases but the combination of equal quantities (by weight) of the two agents results in an eutectic mixture. This means that the mixture has lower melting point than of either solid ingredient alone. The lignocaine/prilocaine mixture (either 2 or 7% of each) exists as oil with the melting point as 1800C. It can be emulsified with water to form a cream that can penetrate intact skin. This also allows higher concentrations of anaesthetic agent in the cream. Applied topically under occlusive dressing 30- 60 mins prior to any procedure, it serves as an alternative to infiltration anaesthesia for procedures such as venipuncture, cannulation, skin graft harvesting or minor dermatological procedures. The common ADR include mild skin blanching and erythema. However, application to abraded skin and mucous membranes results in rapid absorption with systemic toxicity. BUPIVACAINE: This local anaesthetic is about four times as potent as lignocaine and has more prolonged action (up to 8 hours). Its toxicity is similar to that of lignocaine but it is more cardiotoxic. It is used for spinal anaesthesia and epidural analgesia. Mepivacaine has N-methyl substituent in the place of the butyl group of bupivacaine. AMETHOCAINE (Tetracaine) is a potent long acting local anaesthetic. It is effective topically; its absorption from the vascular mucous membranes is very rapid and deaths have ocurred following its use in urethra and respiratory tract. It should never be used on inflamed, injured or very vascular surfaces. Ropivacaine: This amide local anaesthetic agent, though less potent than bupivacaine, has been claimed to be less cardiotoxic. It is used for epidural and regional anaesthesia and is more motor sparing. Duration of action is 2-4 hr. Cinchocaine (Dibucaine, Nupercaine) is a potent but toxic local anaesthetic. It can be used locally in the form of 1% ointment for anorectal conditions. Benzocaine is poorly water soluble and is used topically as ointment, gel or liquid spray. BUCRICAINE: This acridine derivative, is used locally in ophthalmic, dental and general surgical procedures in much the same way as lignocaine. Bucricaine has a longer duration of action than lignocaine. Further, the drug has some inherent vasopressor activity and, therefore, does not require the addition of a vasopressor for infiltration anaesthesia. Its CNS and cardiac toxicity appears to be less than that of lignocaine, and it can be used in patients allergic to lignocaine. The concentrations in which various local anaesthetics are used are given in Table 16.2.
Table 16.2 Some injectable local anaesthetics
Spinal opioid analgesia: Small amounts of morphine or fentanyl administered intrathecally or epidurally, produce analgesia without sensory loss. This technique is sometimes used for relief of intractable pain such as that of cancer. The effect last for 12-15 hours. (Chapter 10).
S E C T IO N IV
Autonomic Nervous System OUT LINE Chapter Chapter Chapter Chapter Chapter Chapter
17: 18: 19: 20: 21: 22:
General Considerations Adrenergic Agonists and Antagonists Cholinergic Drugs Muscarinic Receptor Blocking Drugs; Pharmacotherapy of Bladder Dysfunction Ganglion Stimulating and Blocking Drugs Skeletal Muscle Relaxants
17
General Considerations Autonomic nervous system (ANS) was so named by Langley (1898), because of the fact that unlike the somatic nervous system of the skeletal muscles, it is independent of volitional control and thus enjoys some degree of autonomy. ANS innervates the heart, the smooth muscles, the glands and the viscera. Unlike the somatic structures, the structures receiving the autonomic nerve supply possess an inherent physiological activity and the nervous influences only augment or reduce the initial functional level. Interference with autonomic nerve supply, therefore, does not completely abolish the vegetative functions. This is in contrast to skeletal muscles which develop complete paralysis and atrophy following interruption of their motor supply. The presence of this inherent physiological activity appears to be a built-in protective mechanism. The ANS comprises the parasympathetic (cholinergic), the sympathetic (adrenergic) and the enteric nervous systems. The parasympathetic system mainly participates in tissue building reactions while the sympathetic system enables the individual to respond to stress and prepares the body for ‘flight or fight’. An animal can survive complete elimination of sympathetic but not of parasympathetic nervous system. Usually, these systems are in a state of dynamic equilibrium. The control of autonomic functions is represented at all the levels of the CNS. The reason for this appears to be phylogenetic. Thus, an animal or a man with the absence of entire neuraxis except the spinal cord is still capable of maintaining BP and other vegetative functions except respiration. The autonomic functions are regulated through the reticular formation and its constituents, along with the cranial nerve nuclei. In the hypothalamus, the posterior and the lateral nuclei are regarded as being associated with sympathetic activity while the parasympathetic function is modulated by the midline nuclei. The thalamus, the centre and relay station for sensory perception, can modify the autonomic activity. The limbic system is postulated to co-ordinate the autonomic reactions with emotions but the ultimate synchronisation of the somatic and vegetative functions is undoubtedly achieved in the cortex. The autonomic innervation consists of a myelinated preganglionic fibre which forms a synapse with the cell body of a non-myelinated, second neuron, termed the postganglionic neuron. The postganglionic fibre in turn terminates in a synapse with the receptors of the organ supplied by it. The synapse may thus be defined conceptually as a structure that is formed by the close apposition of a neuron either with another neuron or with effector cell. The synapse merely transmits impulses from one neuron to another; the effect (excitation or inhibition) on the second neuron depends upon the type of neurotransmitter released and the types of receptor on which it acts. The synapse between the preganglionic and postganglionic fibres is termed as a ganglion while that between the postganglionic fibre and the receptors is termed the neuroeffector junction. It must be emphasised that the synapse is a physiological and not an anatomical entity. Passage of an impulse across a synapse is carried out by the process of transmission while it is carried along the preganglionic or postganglionic fibres by the process of conduction.
Distribution of Parasympathetic Nervous System The parasympathetic nervous system serves two important functions: • It carries from the viscera the afferent impulses (visceral afferents) which reflexly modify the autonomic functions; and • It supplies motor fibres to smooth muscle, glands, heart and viscera through its craniosacral outflow (Fig. 17.1).
FIG. 17.1 Schematic representation of the sites of release of neurohumoral transmitters acetylcholine (ACh) and noradrenaline. ACh is released at all the ganglia (G), postganglionic cholinergic nerve endings (PG-ACh), myoneural junctions (MN) and the adrenal medulla (AD). Noradrenaline is released at postganglionic adrenergic nerve endings (PG-Ad). Adrenaline is released from the adrenal medulla. PR-G = Preganglionic.
Visceral afferents: The visceral afferent fibres are non-myelinated. They: (1) Mediate visceral sensations except pain,
(2) Regulate vasomotor, respiratory and viscerosomatic reflexes and (3) Co-ordinate the autonomic activity in general. The important afferents are: • Afferents from the carotid sinus and carotid body carried through the glossopharyngeal nerves: Stimulation of these afferent fibres occurs as a result of local elevation of BP or decrease in blood pH respectively; this results in a fall in BP and bradycardia (carotid sinus reflex) and stimulation of respiration (carotid body reflex) respectively. Hypotension is due to a reduction of sympathetic outflow and bradycardia occurs through increased vagal tone. Respiration is stimulated by increased activity of the medullary respiratory centre. • Afferents from the aortic arch, carried through the vagus nerve: Their stimulation also produces hypotension by reducing peripheral sympathetic outflow. • Afferent fibres from the lungs, heart and the GI tract carried through the vagus: These afferents mediate visceral sensations. The reflex responses vary from hypotension (Bezold Jarisch reflex) to vomiting (afferents from stomach). Craniosacral outflow: The craniosacral outflow, mainly efferent in nature, consists of • Midbrain or tectal outflow through the Edingar Westphal nucleus of the oculomotor (III) nerve which terminates in the ciliary ganglion in the orbit. The postganglionic fibres supply the ciliary muscle and the circular fibres of sphincter pupillae (Fig. 17.1). • Medullary outflow comprising parasympathetic components of the facial (VII), glossopharyngeal (IX) and vagus (X) nerves. (i) The facial nerve supplies secretomotor and vasodilator fibres to the submaxillary and sublingual salivary glands and probably also to the lacrimal glands. (ii) The glossopharyngeal nerve carries the parasympathetic supply of the parotid glands via the otic ganglia while, (iii) The vagus provides secretomotor and vasodilator fibres for the thoracic and the abdominal viscera with the exception of the lower third of the GI tract. • Sacral outflow consists of axons arising from the second, third and fourth sacral segments of the spinal cord and forms the pelvic nerves (nervi erigentis) which synapse near or within the bladder, the lower third of the GI tract including the rectum, and the sexual organs, and supplies secretomotor and vasodilator fibres. The distribution of the parasympathetic system is much more limited than that of the sympathetic system. Usually, a single preganglionic parasympathetic fibre synapses with a single postganglionic cell body of the same system. An exception to this rule is the vagus nerve, the preganglionic fibres of which synapse with approximately 8000 ganglion cells in the Auerbach’s plexus of small intestine.
Distribution of Sympathetic Nervous System The sympathetic division consists of the thoracolumbar outflow. The cells of the preganglionic sympathetic fibres are situated in the intermediolateral column of the spinal cord and extend from the 8th cervical to the 2nd or 3rd lumbar segments. The sympathetic ganglia are of five types : • Paravertebral • Prevertebral • Terminal • Intermediate; and • The adrenal medulla Paravertebral ganglia consists of 22 pairs of ganglia that form a lateral chain on either side of the vertebral column. The preganglionic sympathetic fibres emerge from the vertebral column along with the anterior spinal roots and end in the paravertebral ganglia as white rami communicantes. The ganglia give rise to gray rami communicantes which carry secretomotor fibres along the anterior spinal roots to sweat glands, pilomotor muscles, blood vessels of skeletal muscles and of the skin. The first three pairs of the paravertebral ganglia are superior, middle and inferior cervical ganglia which mainly innervate the radial muscle fibres of the sphincter pupillae, sublingual and submaxillary salivary glands and supply vasodilator and pilomotor fibres to the facial skin and neck. The fourth pair is called the stellate ganglia. The prevertebral ganglia lie in the abdomen and the pelvis. They are the coeliac, superior and inferior mesentric and aortico-renal ganglia. The postganglionic fibres from these supply the abdominal viscera, the urinary bladder and the external genitalia. The terminal ganglia are few and are distributed in close proximity to the viscera such as urinary bladder and the rectum. The intermediate ganglia are closely associated with the anterior spinal roots and lie outside the paravertebral ganglia. A preganglionic adrenergic fibre may end in any of these ganglia. Thus, many preganglionic fibres arising from the 5th to the 12th thoracic segment form the splanchnic nerves which synapse into the coeliac ganglion. The postganglionic sympathetic fibres from the upper thoracic ganglia (1st to 4th) form cardiac, oesophageal and pulmonary plexuses and end as arborizations in these organs. The adrenal medulla is anatomically, embryologically and functionally a sympathetic ganglion. However, it does not have a postganglionic continuation and serves a secretory function. It secretes mainly adrenaline and small amounts of noradrenaline. Enteric Nervous System (ENS) The ENS, the third division of the ANS, consists of collections of highly organised neurons situated in the wall of the GI tract. It includes the myenteric plexus (Auerbach’s plexus) and the submucosal plexus (Meissner’s plexus). This network receives preganglionic fibres from the parasympathetic system and from the postganglionic sympathetic neurons (Fig. 17.2).
FIG. 17.2 Enteric Nervous System Ach: Acetylcholine; VIP: Vasoactive Intestinal Peptide; NO: Nitric Oxide; SP: Substance P; PACAP: Pituitary adenylatecyclase activating polypeptide. Diagram modified from: Tally NJ. Serotoninergic neuroenteric modulators. The Lancet, 2001; 358:2061-8.
ENS controls GI motility, secretions and the mucosal blood flow. Stimulation of the ENS causes release of other putative transmitters, leading to relaxation or stimulation of smooth muscles. Some of the neurons have been identified as containing peptides (VIP), nucleotides (ATP), and nitric oxide (NO) which, cause inhibition. On the other hand, noncholinergic, excitatory transmitters such as substance P, released locally, have also been identified in the enteric plexus. They play a modulatory role in controlling ENS functions. Some enteric neurons acts as mechanoreceptors or chemoreceptors providing local reflex pathways that can control the secretory and motor GI function without external inputs. Although the adrenergic and cholinergic systems are traditionally believed to act antagonistically, their actions on specific tissues may be either discrete and independent or integrated and interdependent. Thus, they act as complementary in an integrated fashion on the male sexual organs to promote sexual function.
Neurohumoral Transmission Transmission of an impulse across the synapse occurs mainly as a result of release of a neurohumoral transmitter into the synaptic cleft. Electrical transmission of impulses has, however, been demonstrated in lower organisms like crayfish and annelids. Neurohumoral transmission: In 1905 Langley postulated the presence of excitatory and inhibitory ‘receptor substances’ in the effector cell; and Dixon (1906) proposed that parasympathetic nerve impulses acted by liberating a muscarine-like substance. The pioneering investigations of Otto Loewi (1921) and Loewi and Navratil (1926) showed that the vagus inhibited the heart by means of a chemical transmitter acetylcholine. Loewi allowed perfusion fluid from a frog heart (donor) to come into contact with a second frog heart (recipient). Stimulation of the vagosympathetic trunk of the donor frog produced cardiac arrest of both the donor and the recipient heart. As no anatomical communication existed between the donor and the recipient hearts, Loewi proposed that the arrest of the recipient heart was brought about by a substance released into the perfusion fluid from the donor heart on vagosympathetic stimulation. This substance was initially termed vagusstoff. Loewi presented evidence which established it as acetylcholine (ACh). He also noted that if the vagosympathetic trunk of the donor heart was stimulated after its initial atropinisation, both the donor and the recipient heart accelerated. This led him to postulate another substance, released from the atropinised donor heart following vagosympathetic stimulation. He named this substance as acceleranstoff. The accelerator neurohumoral transmitter was established as noradrenaline (NA) by Von Euler (1946). The work of Dale, Gaddum, Feldberg and others led to the extension of the chemical transmitter hypothesis to the autonomic ganglia and myoneural junctions where ACh was identified as the transmitter. Barger and Dale (1910), while describing the pharmacological actions of adrenaline and related substances, employed the term sympathomimetic as these actions resembled those seen following sympathetic stimulation; similarly the actions of pilocarpine, muscarine and related substances were described by them as parasympathomimetic. Dale in his later work (1914) used the term Nicotinic action to describe the ganglionic and neuromuscular actions of acetylcholine and Muscarinic action to describe the actions at the postganglionic parasympathetic nerve endings, because of their resemblance to those observed following the alkaloids, nicotine and muscarine respectively. Since the terms sympathetic and parasympathetic do not give any idea about the chemical transmitter at the nerve endings, Dale classified autonomic nerves as: (a) Adrenergic, which release NA; and (b) Cholinergic, which release ACh. A neuron can receive chemical messages at various active sites called receptors (Chapter 5), three groups of which are considered important: • Soma-dendritic receptors, located on the cell body and dendrites, when acted upon, primarily modify the functions of the soma-dendritic region such as generation of action potential or protein synthesis. • Presynaptic receptors located in or near the axon terminals, when activated, primarily modify the function of the terminal region, such as facilitation or inhibition of
transmitter synthesis and release (Chapter 5). These receptors are of two types: (1) Autoreceptors; and (2) Heteroreceptors. Autoreceptors respond to the neuron’s own transmitter, and are involved in synaptic feed back mechanism. They are usually inhibitory to further release of the transmitter. However, somatic motor fibres of the cholinergic system have excitatory presynaptic receptors. The nerve terminals also have regulatory receptors that are activated by bloodborne agents or neurotransmitters (NT) from the neighbouring cells. They are termed heteroreceptors e.g. angiotensin II type 2 (AT2) receptors on adrenergic cells. • Postsynaptic receptors are associated with the target organs/tissues (Fig. 17.1).
Neurohumoral Transmitters ACh, NA, dopamine (DA), gamma-amino-butyric acid (GABA) and 5-hydroxytryptamine (5-HT) are considered as the classical neurotransmitters (NT). Steps involved in the synthesis and the storage of the neurohumor, its metabolism and its interaction with the receptors are potential points where a drug can act. The latter can thus mimic or antagonize the action of the corresponding neurohumor. Acetylcholine: an ester of choline, acts as the neurohumoral transmitter at sites shown in Table 17.1. Table 17.1 Sites of action of ACh
ACh is synthesised inside the nerve fibre by combination of choline taken up from the ECF with an acetyl group. The acetyl group is obtained from acetylcoenzyme A, a product of the intermediary metabolism. The axon terminals contain a large number of mitochondria where acetylcoenzyme-A is synthesised. The coupling of choline with the acetyl group is catalysed by the enzyme choline acetyl transferase to form ACh, which is stored in the synaptic vesicles. It is released by exocytosis through the pre-junctional membrane. ACh is hydrolysed into choline and acetic acid by the enzymes choline esterases. Two main types of cholinesterase have been identified: • Acetylcholinesterase (AChE) or true cholinesterase, present in neurons, ganglia and at myoneural junctions, which rapidly hydrolyzes acetylcholine and another choline ester, acetyl beta methylcholine (methacholine) but not benzoylcholine. • Butyrocholinesterase (BuChE) or pseudocholinesterase, present mainly in the plasma, RBCs, liver, ganglia and other organs, which hydrolyzes ACh slowly, but not methacholine. The activity of the cholinesterases can be inhibited by the anticholinesterase drugs. Noradrenaline and Dopamine: NA and DA are monoamines and act as neurohumoral transmitters at the post-ganglionic sympathetic nerve endings and certain regions within the brain. These amines are present in the highest concentration in the terminal axonal processes of specific neurons, where they are synthesised and stored in the vesicles within the terminals. The enzymes participating in the formation of NA are synthesised in the cell bodies of the adre-neregic neurons and are transported to the axon terminals. These enzymes are not completely specific as they also play a part in the synthesis of 5-HT. The three neurotransmitters viz. dopamine, noradrenaline and adrenaline, are sequentially synthesised from the amino acid phenylalanine (Fig. 17.3).
FIG. 17.3 Biosynthesis of catecholamines
The hydroxylation of tyrosine by tyrosine hydroxylase to DOPA and decarboxylation of DOPA to DA occur in the cytoplasm. About 50% of DA is then actively transported into the vesicles containing dopamine-beta-hydroxylase enzyme, where it is converted to NA, which is stored as granules. Tyrosine hydroxylase disappears from the tissues if the sympathetic nerves degenerate. Alpha-methyl tyrosine and 3 iodotyrosine are inhibitors of this enzyme. Unlike ACh, NA released into the synaptic cleft is only partially degraded and a substantial part is taken up (reuptake) by the sympathetic neurons (Fig. 17.4). The mechanism of NA metabolism is discussed in Chapter 18. The end-products of NA and adrenaline metabolism are excreted in urine in a free form and as conjugates of glucuronic and sulfuric acids.
FIG. 17.4 Synthesis and metabolism of NA at adrenergic neuronal endings
DA is degraded predominantly by MAO-B in the brain and by MAO-A outside the CNS, and partly by COMT. Adrenaline is formed in the adrenal medulla by methylation of NA and is stored in the chromaffin granules. The adrenaline and NA forming cells in the adrenal medulla are two distinct cell types. Because of their unique blood supply, the adrenal chromaffin cells are exposed to high concentrations of cortisol in the venous drainage from the adrenal cortex. Glucocorticoids cause induction of the enzyme noradrenaline-N methyl-transferase and thus control the rate of synthesis of adrenaline. It is released into the blood stream on stimulation of the adrenal medulla. The hypothalamus plays an important role in the regulation of catecholamine secretion by adrenal medulla. Stimulation of the splanchnic nerves results in the release of ACh from the nerve endings which, by increasing the permeability of the chromaffin cells to calcium ions, increases the intracellular calcium and causes the secretion of catecholamines. Calcium is thus, important both for the release of ACh from nerve endings and for the secretion of catecholamines by chromaffin cells. Stress stimulates the release of cortisol from the adrenal cortex (via ACTH) and of adrenaline from the adrenal medulla (via neuronal impulses). In addition, adrenaline secretion increases in response to various circulating substances such as glucagon, histamine, angiotensin II and bradykinin. The adrenal medulla is innervated by preganglionic sympathetic neurons whose cell bodies are located in the spinal cord segments T-3 to L-3. People who have spinal cord transection at the level of T-3 or higher have reduced plasma levels of adrenaline. Mechanisms of neurohumoral transmission: The space between the pre- and postganglionic fibres or that between the nerve ending and the receptor is termed the synaptic cleft. The terminal portions of the pre- and post-ganglionic cholinergic axons contain vesicles, known as the synaptic vesicles. Acetylcholine stored in the synaptic vesicles is termed as depot acetylcholine. Approximately 25% of this is released into the synaptic cleft as a result of nerve impulse. This is releasable ACh. Acetylcholine which is not releasable serves the function of replenishing the stores of releasable ACh. Small quantities of Ach are released continually into the synaptic cleft and are responsible for the postjunctional miniature end plate potentials (MEPP) recorded intracellularly. ACh released as a result of nerve impulse produces a change in permeability of the effector cell, leading to its depolarisation by ionic fluxes. Thus, with inward flux of sodium and outward flux of potassium, the negativity of the intra-axonal voltage diminishes and this produces a nerve action potential (NAP). The NAP leads to either conduction of the nerve impulse along the axon or activation of the effector organ resulting in a secretory or a motor response. Acetylcholine released into the synaptic cleft is rapidly degraded by the true cholinesterase. This reverses the ionic changes and enables the postsynaptic membrane or the receptor site to get repolarised. Two distinctly separate but related systems exist at the level of adrenergic neuron: (i) one is concerned with the intraneuronal amine concentrating-storage mechanism. The vesicles containing the granules have a vesicular monoamine transporter (VMAT) located in their wall; it has high affinity for NA; and
(ii) the other is responsible for the reuptake (Uptake1 process) of NA following its prior release from the nerve terminals using neuronal membrane amine pump (NET; norepinephrine transporter). The maximum concentration of NA is found in the adrenergic nerve terminals in the brain as well as the peripheral adrenergic neurons. NA in the adrenergic nerve terminals exists in several pools, the major portion, over 60%, being present in protein bound form as granules. In the granules it exists with calcium and ATP. An influx of Ca++ into the axonal terminals results in fusion of the vesicles with the plasma membrane and exocytosis of NA. Its movement from the extracellular space back to the cytoplasm, however, involves active transport mechanisms. (Uptake1 in Fig. 17.4). Thus, NA released into the synaptic cleft following a nerve impulse binds to the receptors on the effector cells. Only a part of the stored NA is released into the synaptic cleft as a result of nerve impulse. This portion is termed the mobile or functional pool of NA and is in equilibrium with a fixed or nonfunctional pool which replenishes it on depletion. Presynaptic NA release is regulated through a negative feedback mechanism mediated by adrenergic presynaptic α-2 receptors (Fig. 17.4) and a positive feedback mechanism mediated by presynaptic β-receptors. According to this hypothesis, beta-receptors are more sensitive to agonists so that during the initiation of release, low concentrations of NA in the synaptic cleft accelerate the release process. When the concentration of NA reaches high levels, the presynaptic α2 receptors are stimulated and the secretion is terminated by a negative feedback mechanism. Predominantly α1 receptor antagonists such as phenoxybenzamine, on the other hand, enhance NA release. The combined effects of the positive and negative feedback mechanism may thus control the ‘need oriented’ release of the transmitter. It appears that a presynaptic regulating mechanism similar to that described in the periphery operates in the CNS as well. Drugs could produce actions by altering the release of these neurotransmitters centrally or peripherally, by modifying the presynaptic regulatory mechanisms (Chapter 5). A part of released NA is metabolised outside the cell by the enzyme catechol-O- methyltransferase (COMT) but a large part (75-80%) is taken back into the cell by an active process and re-stored mostly in mobile pool. Only a small portion is metabolised intracellularly by mono-amine oxidase (MAO). Rebinding of NA with the granules represents a way by which it is immobilised but can be used again. In fact, physiologically, uptake and restorage are the major routes of NA inactivation and enzymatic destruction plays only a minor role. The enzymes MAO and COMT are widely distributed throughout the body including the brain, with the highest concentrations in the liver and the kidneys. It is also present in the intestinal mucosa. There are two types of MAO. (i) ‘MAO-A’, which oxidises mainly NA and 5-HT, can be selectively inhibited by very low concentration of inhibitors, clorgyline and moclobemide; and (ii) ‘MAO-B’, which oxidizes DA in the brain and can be selectively inhibited by selegiline (Chapter 15). Tyramine and DA are substrates for both forms of the enzyme. The liver contains both forms in equal amounts while the brain MAO is predominantly type B. For details about adrenergic, DA and cholinergic receptors, see Chapters 18 and 19.
Neurotransmitter Uptake Mechanisms and Drugs Many studies have defined the properties of the catecholamine ‘uptake’ mechanisms involved and their modification by drugs. (1) Uptake1 is the picking up of catecholamines from the extracellular space by the axoplasm of the adrenergic neurons and by other extraneuronal cells. This process demonstrates a greater affinity for NA than for adrenaline. Catecholamines taken up by ‘Uptake1’ are then transferred to the storage vesicles in the adrenergic neurons by a separate process; both these processes are carrier-mediated transport system. (2) Uptake2 is the picking up of catecholamines by an organic cation transporter (OCT3) located on the effector cells in the peripheral tissues such as the vascular smooth muscle, the heart and the exocrine glands. Such uptake is followed by rapid degradation of the catecholamines. In contrast to ‘Uptake1‘, ‘Uptake2’ demonstrates a higher affinity for adrenaline and isoprenaline than for NA. ‘Uptake1’ may be looked upon as ‘uptake with retention’; by contrast, ‘Uptake2’ is an ‘uptake followed by metabolism’. Noradrenergic Uptake1 and Uptake2 transport systems can be blocked selectively by a number of drugs (Table 17.2). Table 17.2 Inhibitors of NA Uptake1 and Uptake2
Many sympathomimetic amines are also taken up by “Uptake1” process and hence, can act as competitive substrates, thus inhibiting the NA uptake. Other important drugs which are known to inhibit the “Uptake1“ mechanism are listed in Table 17.2. Drugs that inhibit “Uptake1“ potentiate and prolong the responses of sympathetically innervated organs to nerve stimulation. The drug 6-hydroxydopamine accumulates selectively through ‘Uptake1’ process and produces selective destruction of adrenergic neurones (chemical sympathectomy) in both peripheral and central nervous system. Various drugs known to act on the adrenergic neuron produce their effects by modifying the synthesis, storage and ‘Uptake1, mechanisms (Fig. 17.5) by:
FIG. 17.5 Schematic diagram showing the effect of some drugs on noradrenaline (NA) uptake and release mechanisms. Drugs may act by inhibiting its Uptake1 process (imipramine, cocaine), by interfering with the synthesis of transmitter, e.g. methyltyrosine, by depletion of NA from storage site (reserpine), by blocking the release from the storage site (guanethidine), by producing a false transmitter (methyl-dopa), by promoting the NA release (tyramine), by inhibiting the destruction of NA by MAO and by blocking both Uptake1 and Uptake2 mechanisms (phenoxybenzamine).
• Supplying an amine precursor e.g., Levodopa used in Parkinson’s disease is a precursor of dopamine. • Blocking the Uptake1 of NA by inhibiting NET e.g. TCA like imipramine used in the treatment of mental depression; Cocaine. • Interfering with the synthesis of NA, e.g., α-Methyltyrosine. • Inhibiting the transport into the vesicles, thus interfere with the storage of NA, leading to its depletion from the sites, e.g., antihypertensive Reserpine. • Blocking the release of NA from the binding stores in the terminals, e.g., Guanethidine, a drug used in the treatment of hypertension. • Promoting a synthesis of a false transmitter which displaces NA, e.g. Alpha methyldopa. • Promoting the release of NA from the storage sites, e.g., Tyramine. • Inhibiting the intraneuronal degradation of NA, e.g., MAO inhibitors used as antidepressants; and • Blocking postsynaptic receptors, e.g., Adrenergic receptor blocking drugs. These mechanisms are discussed elsewhere. Specialised uptake mechanisms, similar to that described for NA neurons, are also known to exist in DA and 5-HTergic neurons in CNS and in cholinergic neurons in the periphery and CNS. In cholinergic neurons, however, the transport mechanism is for transmitter precursor choline rather than for ACh. Various agents known to act on the cholinergic system produce their effects by: • Blocking synthesis of ACh, e.g. Hemicholinium which blocks the uptake of precursor, choline.
• Blocking the uptake of ACh into synaptic vesicles, e.g. Vesamicol • Inhibiting the release of ACh, e.g. Botulinum toxin • Increasing the release of ACh, Black widow spider toxin • Preventing the destruction of ACh by cholinesterase, e.g. Anti-cholinesterases • Interacting with post-synaptic receptors, (a) Muscarinic receptors: e.g. Muscarine (as an agonist) and Atropine (as an antagonist) (b) Nicotinic receptors on ganglia (Nn): e.g. DMPP (as an agonist) and Hexamethonium (as an antagonist) (c) Nicotinic receptors on N-M junction (Nm): e.g. Nicotine (as agonist) and dTubocurarine (as an antagonist) Both ACh and NA are also termed as local hormones because they act at the site of their synthesis. Although ACh and NA are the main classical neurotransmitters at the autonomic nerve endings, the neurons may also possess other chemical messengers such as VIP, neuropeptide Y etc., which function as primary neurotransmitters, co-transmitters or neuro-modulators. Thus, autonomic transmission may, therefore, be mediated by the release of multiple neurochemicals. Supersensitivity: Interruption of the nerve supply of an effector organ (denervation) makes it more sensitive to the neurohumor of the system supplying it. Thus, a skeletal muscle, after sectioning the motor nerve, becomes highly sensitive to ACh and the nictitating membrane becomes highly sensitive to NA after sectioning its postganglionic sympathetic supply. This phenomenon is termed denervation supersensitivity. The exact mechanism is not known. It may be related to the elimination of the neuronal uptake mechanism. It could also partly be due to degeneration of the nerve terminals after sectioning, leading to disappearance of associated enzymes that normally inactivate the transmitter. It may also be due to an increase in receptor number (up-regulation) induced by the fall in the catecholamine concentration within the synaptic cleft (also Chapter 2). Supersensitivity to transmitter substances has also been observed after prolonged administration of blocking agents.
18
Adrenergic Agonists and Antagonists The sympathomimetic or adrenergic drugs mimic the responses obtained after stimulation of the sympathetic or adrenergic nerves. Majority of these substances contain an intact or a partially substituted amino (-NH2) group and hence, are also called as sympathomimetic amines. From the therapeutic point of view these drugs can be classified as: I Adrenergic drugs used for raising blood pressure, e.g., Noradrenaline, Metaraminol and Phenylephrine. II Those used for their inotropic actions on the heart, e.g., Dopamine, Dobutamine and Isoprenaline. III Those used as central stimulants, e.g., Amphetamine, Dextroamphetamine and Methylphenidate. IV Those used as smooth muscle relaxants e.g. (a) nonselective beta stimu lants such as Adrenaline, Isoprenaline,; and (b) selective β2 stimulants, e.g., Isoxsuprine Salbutamol and Terbutaline. V Those used in allergic reactions, e.g., Adrenaline, Ephedrine. VI Those used for local vasoconstrictor effect, e.g., Adrenaline. VII Those used for nasal decongestion e.g. Naphazoline, Phenylephrine, Xylometazoline. VIII Those used for suppressing the appetite (anorectic), e.g., Fenfluramine, Phenteramine. These drugs can also be structurally classified as: Catecholamines, which are compounds containing a catechol nucleus (i.e. a benzene ring with two adjacent OH groups) and an amine-containing side-chain (Fig. 18.1); and
FIG. 18.1 Catecholamines
• Non-catecholamines which lack the hydroxyl groups.
Catecholamines The catecholamines include the sympathetic, neurohumoral transmitters noradrenaline (NA) and dopamine (DA); the main hormone of the adrenal medulla adrenaline; and the synthetic compound isoprenaline (isoproterenol, isopropylarterenol). Catecholamine content of adrenal medulla normally is 85% adrenaline and 15% NA. Dopamine not only serves as a precursor of NA but also acts as a neurohumoral sympathetic transmitter (Chapter 17). The word ‘nor ’ in noradrenaline was originally coined to indicate nitrogen (N) without (O-Ohne) a radical (R), in this case a methyl (CH3) group. Mechanism of action of catecholamines: The catecholamines produce their action by direct interaction with receptors located on the cell membrane. This drug-receptor combination leads to either an increase (excitation) or a decrease (inhibition) in the tissue activity. In order to explain these differences in responses by different tissues, the concept of two different receptors, alpha and beta, was proposed by Ahlquist (1948). Thus, the α receptor stimulation is mainly responsible for the excitatory effects while the β receptor stimulation usually produces inhibitory effects. Noradrenaline specifically acts on the α and β1 receptors while adrenaline acts nonspecifically on both α and β receptors; isoprenaline acts only on β receptors. A given tissue may contain either α or β or both types of receptors. Even though α receptors are generally excitatory and β receptors inhibitory in character, there are certain exceptions. Thus, β receptors, predominantly present in the heart, are excitatory in character; their stimulation increases the rate and force of cardiac contraction. Similarly, both the α and β receptors of the GI tract are inhibitory and their stimulation produces smooth muscle relaxation. Adrenergic receptors are further subclassified according to their selective sensitivity to agonists and antagonists. At the molecular level, the adrenergic receptor-effector mechanism involves three interacting proteins: (i) the receptor, (ii) the G-protein and (iii) the effector enzyme-ion channel. Three distinct beta receptor subtypes have been distinguished: • Beta1 receptors responsible for myocardial stimulation and renin release. • Beta2 receptors responsible for bronchial muscle relaxation, skeletal muscle vasodilation and uterine relaxation; and • Beta3 receptors expressed primarily in brown and white adipose tissue. They regulate NA-induced changes in energy metabolism and thermogenesis. Beta3 agonists stimulate metabolic rate and induce weight loss without reducing food intake in animals. It is relatively resistant to blockade by beta antagonists. Generally, beta-adrenergic responses appear to result from binding of the catecholamine to beta receptors (β1 and β2) which via the mediation of a stimulatory G-protein (GS) stimulate a plasma membrane enzyme, adenylyl cyclase. This results in a rise of the intracellular cyclic AMP (see Fig. 2.2 in Chapter 2). The cyclic AMP alters the cellular function by stimulation of a protein kinase, which causes phosphorylation of certain enzyme proteins, resulting in activation of some and inactivation of others. This explains
the stimulatory and inhibitory actions of drugs acting on the β receptors. Phosphodiesterase, another enzyme, promotes the breakdown of cyclic AMP. Drugs like caffeine, theophylline and other methyl xanthines which inhibit this enzyme potentiate the beta receptor stimulant action of adrenaline. Presynaptic β receptors facilitate the release of NA at the adrenergic nerve terminals. (Chapter 17). Alpha adrenergic receptors are of two types: alpha1 and alpha2, with three subtypes each (α1A, and α1B, α1C, and α2A, α2B, α2C). • Alpha1 adrenergic receptors (postsynaptic) are predominantly in the vascular smooth muscles. Activation of these receptors which are excitatory in nature: (i) Increases the intracellular concentration of calcium by activation of phospholipase C in the cell membrane via stimulatory G-protein (GS). (ii) The phospholipase C then hydrolyses membrane bound phosphoinositides with the generation of two second messengers, diacylglycerol and inositol triphosphate. (iii) This results in an increase in the intracellular Ca++ which accounts for the vasoconstrictor effect. • Alpha2 adrenergic receptors are found both in effector tissues (postsynaptic) and on the neuronal endings (presynaptic) where they are autoreceptors. Activation of the presynaptic α2 receptors by agents acting through the inhibitory G protein (Gi) inhibits adenylyl cyclase and reduces the intracellular concentration of cyclic AMP. These receptors activate G-protein gated K+ channels. This inhibits NA release from adrenergic nerves. The activation of the post-synaptic vascular α2 receptors however, causes release of ‘endothelium derived relaxing factor ’ (EDRF, NO) which brings about vasodilatation. Activation of venous α2 receptors, on the other hand, causes venoconstriction. Activation of post-synaptic α2 receptors in the GI tract causes inhibition of voltage sensitive calcium channels leading to relaxation. Alpha2 adrenergic receptors are also present at the post-junctional or nonjunctional sites in several tissues such as the brain. Activation of post-junctional α2 receptors in the brain by clonidine causes the antihypertensive effect. The presence of α receptors has been also demonstrated on human leukocytes and platelets. The predominant receptors in various organs and the usual responses to their stimulation are given in the Table 18.1; however, such responses in isolated tissues may differ from those in the whole animal owing to the presence of compensatory reflex activity in the latter. Further, the initial condition of the tissue may also determine the resultant responses. Table 18.2 lists the agonists and antagonists of the adrenergic receptor subtypes.
Table 18.1 Distribution and responses of adrenergic receptors Tissue Response P redominantly alpha receptors (a) Medulla oblongata (α2) Reduc tion of BP and heart rate (b) Blood vessels: (α1) S kin and muc osa Constric tion Cerebral Constric tion (slight) (c ) S kin: Pilomotor musc le (α1) Contrac tion Apoc rine sweat glands S ec retion inc reases (d) Radial musc le of iris (α1) Contrac tion (mydriasis) (e) S alivary glands, exc ept parotids Thic k, visc ous sec retion (f) S ex organ, male (α) Emission P redominantly beta receptors (a) Heart: (β 1, α1) S -A node – β 1 Inc reased heart rate (positive c hronotropic ac tion) Atria – β 1 Inc reased c ontrac tion (positive inotropic ac tion) A- V node – β 1 Faster c onduc tion Ventric les – β 1 Inc reased c ontrac tility and c onduc tivity, inc reased automatic ity (positive dromotropic ac tion) (b) Bronc hial musc le – β 2 Relaxation (c ) S keletal musc le c hanges – β 2 Changes in c ontrac tility (d) S keletal musc le blood vessels – β 2 Dilatation (e) Kidney: JG apparatus – β 1 Renin sec retion Both alpha and beta receptors (a) G.I. trac t: Motility and tone (α2 β 2) Dec reased S phinc ters (α1) Contrac tion Panc reas: Alpha2 Inhibiting insulin release Beta2 S timulation of insulin release (b) Urinary bladder: Trigone – α1A Contrac tion Detrusor – β 2 Relaxation (c ) Blood vessels: Coronary – α, β 2 Constric tion; dilatation Pulmonary – α, β 2 Constric tion; dilatation Abdominal visc era – α, β 2 Constric tion (mainly); dilatation Renal – α, β 2 Constric tion; dilatation S keletal musc le – α, β 2 Constric tion; dilatation (d) Adipoc yte – α2 Inhibit lipolysis β 3 Lipolysis Liver – α1, β 2 Glyc ogenolysis, neogluc ogenesis, inhibition of glyc ogen synthetase (e) Leukoc yte (human) – β 2 Inhibits c hemotaxis and lysosomal enzyme release Platelet (human) – α2 Platelet aggregation (f) Uterus: α1 Contrac tion β 2 Relaxation
Table 18.2 Drugs acting on adrenergic receptor subtypes
Pharmacological actions of adrenaline and noradrenaline: Adrenaline acts as a nonselective α and β receptor agonist, whereas NA acts more selectively as α and β1 agonist. Cardiovascular system: • Heart: Adrenaline stimulates the β1 receptors in the heart and increases the rate, the force of contraction and the conduction velocity. This is associated with increased metabolism of the myocardium and increased oxygen consumption. It also increases cardiac output by stimulating venoconstriction, thus increasing the venous return, and increasing the force of atrial contraction which augments the ventricular diastolic volume. Very high cardiac rate, however, prevents proper diastolic filling and may produce fall in the cardiac output. Adrenaline enhances conduction across the A-V node and may cause ventricular arrhythmias. Noradrenaline, though β1 stimulant does not usually increase the heart rate in an intact animal but tends to produce bradycardia due to compensatory vascular reflex activity. It should be noted, however, that NA is the physiological transmitter in the heart and its capacity to stimulate cardiac β1 receptors is of vital importance. The myocardial effects of adrenaline and noradrenaline can be blocked by the beta receptor blocking agents like propranolol. • Blood vessels and blood pressure (Fig. 18.2): Adrenaline and NA constrict the blood vessels of the skin and mucous membranes. Adrenaline dilates the blood vessels of the skeletal muscles on account of the preponderance of β2 receptors; and decreases the total peripheral resistance. Hence, although adrenaline raises the systolic BP mainly by its cardiac actions, it lowers the diastolic pressure by its peripheral actions; and therefore, it is not suitable for routine use in hypovolemic shock.
FIG. 18.2 Effect of Adrenaline (Adr), Noradrenaline (N.Adr), Ephedrine (Eph), and Isoprenaline (Isop) on blood pressure in anaesthetised dog.
As the rise in systolic BP with adrenaline is only of moderate magnitude, compensatory reflexes do not antagonise its cardiac actions and the rise in systolic BP is accompanied by tachycardia, increased cardiac output and increased stroke volume. The rise in systolic BP produced by moderate doses of adrenaline is often followed by a fall. By stimulating the α receptors, it produces a rise in BP. However, its action on beta receptors is more persistent and hence, when the action on α receptors wears off, the action on β receptors is unmasked producing a fall of BP. This response to moderate doses of adrenaline is termed biphasic response. Sir Henry Dale noted that this biphasic response was converted to a depressor response by prior administration of ergot extract which blocks the α receptor, leading to stimulation of peripheral β2 receptors by adrenaline, resulting in a fall in BP. This phenomenon is termed as Dale’s vasomotor reversal (Fig. 18.3). Such reversal is not seen with NA.
FIG. 18.3 Dale’s Vasomotor Reversal. Note the fall in BP with Adrenaline after injection of Dihydroergotamine (DHE).
Noradrenaline produces a rise in both systolic and diastolic BP; the pulse pressure usually remains unaltered. As compared to adrenaline, its β2 receptor actions are very feeble. The rise of BP is associated with reflex bradycardia. Renal blood flow is reduced by both, adrenaline and NA, even in doses that have no significant effect on BP. The urine output and urinary excretion of sodium, potassium and chloride decreases. Renin secretion is increased by direct β1 receptor effect, independent of vascular changes in the kidney; whereas α2 receptors are responsible for inhibition of renin release from the kidney.
Both adrenaline and NA constrict the hepatic and mesenteric blood vessels and raise the portal venous pressure. The pulmonary arterial and venous pressures are raised by both, more by adrenaline. Constriction of the musculature of the great systemic veins tends to push the blood from the periphery into the pulmonary circulation and this may occasionally result in pulmonary edema following adrenaline administration. Adrenaline in moderate doses increases the coronary blood flow, cerebral blood flow and oxygen consumption. Smooth muscle: (Table 18.1). • Bronchi: Adrenaline by its β2 receptor agonist action causes relaxation of the bronchial smooth muscle and act as a bronchodilator. It antagonises the bronchospasm produced by vagal stimulation, choline esters, histamine or an antigen-antibody reaction, bradykinin, leukotrienes or prostaglandin F2α. • Uterus: The response of the uterus to the catecholamines varies according to species, the phase of oestrous cycle, presence or absence of gestation, period of gestation and the dose administered. The rat uterus is relaxed irrespective of all these factors. The human non-pregnant uterine strip is stimulated to contract by adrenaline. In the last month of pregnancy adrenaline inhibits uterine contraction and causes relaxation. • Gastrointestinal tract: Adrenaline and NA relax the smooth muscle of the gut and reduce its motility; the sphincters are constricted. • Other smooth muscles: Adrenaline contracts the pilomotor muscle of the hair follicle. It also produces contraction of the vesical sphincter and the trigone (α receptor), while relaxing the detrusor muscle (β receptor). Adrenaline and NA produce contraction of the splenic capsule producing a release of erythrocytes into the circulation. This probably serves as a protective mechanism during stress such as hypoxia and haemorrhage. Eye: Sympathetic stimulation causes mydriasis due to contraction of the radial muscle fibres of the iris, and exophthalmos due to contraction of the orbital muscles. Nictitating membrane, present in lower mammals, contracts with adrenaline. Adrenaline, on topical application, does not readily penetrate the eyeball. However, it produces a moderate reduction in IOT. Respiration : Adrenaline is a weak stimulant of respiration. Given IV, both adrenaline and NA may induce apnoea partly by stimulating the baroreceptors and mainly by a direct central action. Adrenaline, particularly in aerosol form, constricts the pulmonary vessels and relieves bronchial congestion. Metabolic effects: Adrenaline increases: • Blood sugar level by enhancing hepatic glycogenolysis (β2 effect) and by decreasing peripheral glucose uptake. • Blood lactate by enhancing the breakdown of glycogen to lactate in the skeletal muscles. • Plasma free fatty acid concentration by increasing lipolysis in adipose tissue. • Serum K+ level transiently, followed by a more sustained hypokalemia. Both adrenaline and NA promote cellular uptake of K+. Activation of pancreatic α2 adrenergic receptors by adrenaline and by severe stress (via activation of adrenergic nervous system) inhibits insulin release; β2 adrenergic receptor agonists and vagal nerve stimulation enhance it. Central nervous system: The catecholamines do not cross the BBB satisfactorily and
hence, their central actions are limited. Adrenaline may produce excitement, tremor stupor, vomiting and restlessness. Noradrenaline and dopamine are important neurotransmitters in the CNS (Chapter 5). Skeletal muscle: Catecholamines influence skeletal muscle contractions by acting on both sides of the neuromuscular junction. The α effect on the motor nerve endings increases the amount of ACh released and is probably the main factor in the improvement of neuromuscular transmission by adrenaline. The β action on the muscle fibre itself probably contributes to the improvement of muscle contractions and tremor, sometimes observed following these drugs. Antiallergic action : Adrenaline and similar compounds prevent the release of mediators of allergy such as histamine from the tissue mast cells (Chapter 27). Miscellaneous: Adrenaline produces thick viscid secretion from salivary glands, leucocytosis and eosinopenia, and accelerates blood coagulation. It also stimulates platelet aggregation through α2 receptors. The important pharmacological actions of the three catecholamines in man are summarised in Table 18.3. Also see Chapter 32.
Table 18.3 Comparison of the pharmacological actions of catecholamines in man
+ = Increase, 0 = No change, – = Decrease.
Absorption, fate and excretion: Catecholamines are not effective orally because they are rapidly inactivated by MAO in the gut and the liver. On inhalation as aerosol, small quantities may be absorbed into circulation. Adrenaline and NA are metabolised by: • Catechol-O-methyl transferase (COMT) located extracellularly; and • Monoamine oxidase (MAO), located inside the mitochondria of the adrenergic neurons. These enzymes are also present in the liver and kidney. They act sequentially as shown in Fig. 18.4 and convert them into the final urinary excretory products: VMA, metanephrine (metadrenaline) and normetanephrine (normetadrenaline). A small quantity of catecholamines is excreted unchanged.
FIG. 18.4 Metabolism of Adrenaline and Noradrenaline
The normal 24 hour urinary excretion of VMA and free catecholamines is 4-8 mg and 50100 mcg respectively. A significant increase in these values is considered diagnostic of pheochromocytoma, an adrenal medullary tumor, producing excessive catecholamines. Adverse reactions: These are mostly cardiovascular and are due to extension of the pharmacological actions. Adrenaline, given SC, may produce palpitation, throbbing headache and tremors. Noradrenaline, employed in the form of IV drip may cause anxiety, pallor and headache. • Both adrenaline and NA, injected rapidly, IV, may cause a sudden, marked increase in BP, precipitating subarachnoid haemorrhage and occasionally a stroke. • They can cause ventricular arrhythmias including fatal ventricular fibrillation. In individuals with cardiac decompensation; adrenaline may precipitate acute pulmonary edema. Noradrenaline infusion has to be carefully titrated, and BP has to be checked at least every 15 minutes to prevent the above mentioned complications. The infusion must never be left unattended. Noradrenaline infusion, if stopped suddenly, may result in alarming hypotension. • Noradrenaline infusion, if extravasated, produces local vasospasm and sloughing. • Adrenaline and NA (particularly the former) may precipitate anginal pain in persons with ischemic heart disease. Thyrotoxic or hypertensive individuals are more sensitive to the pressor effects of these agents. Preparations and dosage: (i) Adrenaline injection: 0.5 or 1 ml ampoules containing 1:1000 adrenaline (1 mg/ml) in water. Dose : 0.2 to 0.5 ml SC or IM. Rarely, the drug is administered IV, in the dose of 0.25 mg, further diluted with saline and given slowly, under monitoring and supervision. (ii) Adrenaline inhalation; is a nonsterile, 1:100, aqueous solution of adrenaline tartrate. (iii) Noradrenaline injection; 0.2% solution of noradrenaline bitartrate in 2 ml ampoules. It is administered as an IV infusion. For this purpose, 2 ml of noradrenaline bitartrate (equivalent to 4 mg of the salt and 2 mg of the base) is added to 500 ml of 5% dextrose solution (which is generally acidic) resulting in a concentration of 4 mcg of the base per ml. After judging the cardiovascular response with a test dose of 2 to 3 ml, the drug is administered at the rate of 0.5 to 1 ml per minute. The dose is controlled according to the
blood pressure response. Noradrenaline is unstable at the neutral pH of normal saline and vitamin C (500-1000 mg) should be added to the infusion, if noradrenaline needs to be infused in normal saline. Therapeutic uses of adrenaline: • Anaphylaxis and angioneurotic edema: Adrenaline is the drug of choice in the treatment of anaphylactic shock. (Chapter 23). It is life-saving in angioneurotic edema of the larynx. • Bronchial asthma: Adrenaline, given SC is a potent bronchodilator. (Chapter 27). Cardiopulmonary resuscitation (CPR): Cardiac arrest is diagnosed if the patient collapses suddenly and becomes unconscious, is not breathing, and the carotid pulse absent. Table 18.4 summarises the general guidelines for cardiopulmonary resuscitation, which comprises attention to: A (airway), B (breathing) and C (circulation). Table 18.4 Steps of cardiopulmonary resuscitation
The therapy of cardiac arrest comprises (1) Basic life support, to be administered at the site of the occurrence; and (2) Advanced life support, to be provided in the intensive care facility of a hospital. The most important component of basic life support is external cardiac massage, administered by precordial chest wall compressions (C) started immediately and continued uninterruptedly till hospitalisation. This is supported by very rapid clearing of the upper airway (A); and attention to breathing (B) by mouth to mouth ventilation of the patient. (A) and (B) should be attempted only if more than one person is available for administering emergency treatment. If only one person is available, he should stick to (C) only. The American Heart Association’s Guidelines 2010 for CPR in adults, teens and children recommend Compressions > Airway > Breathing (C > A > B) in that order. The first cycle should include at least 30 compressions before (A) and (B) are attempted. Recommendations for neonates are still based on the older A>B>C protocol. When cardiac arrest is diagnosed outside an intensive care facility, a bystander, even if untrained, should first shout for help, and immediately initiate precordial chest wall compressions at the rate of at least 100/minute. The compressions should not be interrupted for any reason till the patient reaches an intensive care facility. Use of atropine for pulseless asystole, and airway suctioning for all newborns (except for those with obvious obstruction) is no more recommended. Advanced life support in the ICCU is within the purview of experts, and is not described here. However, in patients with ventricular fibrillation or pulseless ventricular tachycardia, IV adrenaline in a dose of 1 mg (1 ml of 1:1000 dilution) is administered if two attempts of defibrillation fail to restore the rhythm. With subsequent attempts of defibrillation, the
adrenaline may be given after intervals of 3–5 min. Cardiac arrest due to bradyarrhythmias or asystole is treated first with basic life support and treatment of identifiable causes (like hypoxia, electrolyte imbalance, acidosis, pulmonary embolism, myocardial infarction or drug overdose). Subsequent non-specific therapy includes IV or intracardiac 1mg of adrenaline and/or atropine. • Control of haemorrhage: Adrenaline in the concentration of 1 : 1000 to 1 : 20,000 is sometimes used topically for controlling bleeding eg. epistaxis and bleeding after tooth extraction. • Other local uses: Adrenaline, because of its vasoconstrictor effect, is used in the concentration of 1 : 50,000 to 1 : 100,000 along with local anaesthetics. It reduces the systemic absorption of the local anaesthetic, thus prolonging its action and minimising its toxicity. Although adrenaline hydrochloride 0.5-2% can be used to reduce the production of aqueous humour in glaucoma, selective α2 agonists apraclonidine and brimonidine are preferred (Chapter 72). Therapeutic uses of noradrenaline: • Noradrenaline is mainly used for elevating the BP in shock (Chapter 32) due to peripheral vasodilatation. Routinely, alpha-receptor blocking drugs are used for the control of the BP before and during operative removal of a pheochromocytoma. This antagonism of alpha receptor usually does not lead to irrecoverable hypotension after removal of the tumor. However, if there is a fall in BP even after correction of hypovolemia, administration of NA may be useful. ISOPRENALINE (Isoproterenol, Isopropyl arterenol): Isoprenaline is the most potent, synthetic, non-selective beta agonist. Pharmacological actions: Its main actions are on the heart, skeletal muscle vasculature and smooth muscles. • It stimulates the heart and causes tachycardia. • It lowers the peripheral vascular resistance in skeletal, renal and mesenteric vascular beds and produces a fall mainly in the diastolic pressure. • It relaxes the smooth muscles particularly those of bronchi and GI tract. • Its calorigenic and FFA releasing actions are similar to those of adrenaline. Absorption, fate and excretion: Isoprenaline is inconsistently absorbed sublingually and orally. Absorption is quicker after IM injection and by inhalation. It is less effective orally. It is rapidly inactivated by uptake into tissue and metabolised by COMT. Adverse reactions: It can cause palpitation, tachycardia, arrhythmias, anginal pain, headache and flushing. Combined isoprenaline-adrenaline administration in bronchial asthma may prove fatal. Preparations and dosage: (i) Isoprenaline sulfate, 10 mg. Dose: 5 to 10 mg sublingually. (ii) Isoprenaline injection (Isoprin), 2 mg/ml to be diluted for IV infusion. Therapeutic uses: • Stokes-Adams syndrome: Isoprenaline, because of its lesser liability to produce cardiac arrhythmias, is preferred to adrenaline in this condition. It may be administered either
IV or orally in the dose 30 to 120 mg 6 to 8 hourly. • To counter cardiotoxicity due to beta blocker overuse. Precautions and contraindications for catecholamines: Catecholamines should be administered cautiously in the presence of hypertension, hyperthyroidism, angina pectoris, acute left ventricular failure and hypotension during halothane anaesthesia. DOPAMINE (DA): This naturally occurring precursor of NA acts on dopaminergic and other adrenergic receptors. Two classes of postsynaptic DA receptors have been described: D1-like (D1 and D5) and D2-like (D2, D3, D4). Table 18.5 summarises the distribution and properties of D1 and D2 receptors. Presynaptic receptors or autoreceptors for DA are present in the brain. Dopamine is also a weak α and β adrenergic receptor agonist. It is metabolised by MAO and COMT. It has extremely short half life. Its effects can be blocked, by the use of alpha blocker phentolamine. Dopamine does not cross the BBB. Table 18.5 Distribution and properties of D1 and D2 receptors Location Type of D receptors Limbic system D 1 and D 2 Corpus striatum D 1 and D 2 Hypothalamus Mainly D 1 P ituitary Mainly D 2 Blood vessels Mainly D 1 Heart : muscle Mainly D 2 Heart : sympathetic fibres Mainly D 2
Functions Mood and emotional stability; stereotypy Motor c ontrol Autonomic c ontrol Inhibition of prolac tin sec retion Dilatation S timulation of inotropic func tion (Chapter 31) Inhibition of NA release
Mechanism of action: D1 Increases cAMP synthesis. D2 Decreases cAMP synthesis.
Table 18.6 summarises the pharmacological effects of DA at different dose levels. Table 18.6 Pharmacological effects of dopamine Dose (by infusion) (mcg/kg/min) 2–5 Dopaminergic range 5–10 Dopaminergic + β1range 11–20 β1 range More than 20 α 1 range
Effects Renal, mesenteric and c erebral va sodila tion by a ction on dopa mine (D 1) receptors. S ame as above. Plus inc rease in myoc ardial c ontrac tility with inc rease in heart rate and c ardiac output, by ac tion on dopamine and beta1 rec eptors. This is of advantage in the treatment of shoc k. The peripheral resistanc e is lowered or unc hanged. Predomina ntly ca rdia c a ction. Va soconstriction, renal blood flow and urine dec rease in output, and sometimes aggravation of heart failure, by ac tion on α1 adrenergic rec eptors.
Adverse reactions: These include nausea, vomiting, palpitation, ectopic beats and anginal pain. A sudden rise in BP may occur. Small doses occasionally precipitate a fall in BP. Infusion of large doses for long time may cause ischemia and gangrene of limbs. Reduction in urine output, tachycardia and development of arrhythmias indicate toxicity. Therapeutic uses: • Shock: See Table in Chapter 32. • Severe CHF in patients with oliguria (Chapter 31). • Threatened acute renal failure: In low dose (less than 5 mcg/kg/min), it preferentially
dilates the renal vessels and may produce diuresis. If the patient does not respond in 1012 hours, the drug should be discontinued. FENOLDOPAM: This D1 receptor agonist acts as a peripheral vasodilator and is used in hypertensive crises (Chapter 30). For D2 receptor agonists (e.g. bromcriptine) see Chapter 67. DOBUTAMINE, though structurally related to DA, has a selective β1 receptor agonist action with predominant effect on cardiac contractility. DA receptors are not involved in its action. It has negligible chronotropic and peripheral vascular actions. In patients with low-output cardiac failure, it increases the cardiac output without increasing the heart rate. Unlike DA, it does not cause renal vasodilatation. It has a short duration of action and is given by slow IV infusion in 5% dextrose, at the rate of 2.5 - 15 mcg/kg/minute. Its toxicity is similar to that of DA. Like other catecholamines, dobutamine loses its effect in an alkaline medium. It is particularly useful in refractory, chronic, congestive heart failure, unresponsive to digoxin. It may also have beneficial hemodynamic effects in patients with bacteremic shock. Dopexamine is another inotropic agent used in heart failure associated with cardiac surgery.
Noncatecholamines The sympathomimetic amines devoid of the catechol nucleus comprise compounds like ephedrine, amphetamine and other vasopressors as well as smooth muscle relaxing compounds. Mechanism of action: These drugs: • Release NA and/or DA from the sympathetic neurons. Noncatecholamines are taken up in the neuron by NET and are concentrated in the vesicles by VMAT. They displace NA, which is subsequently released into the synaptic cleft by reverse transport via norepinephrine transporter (NET). This indirect action does not involve the usual exocytic process secondary to Ca++ influx, and produces mainly α effects resembling those of NA. • Amphetamine and ephedrine, in particular, also inhibit the reuptake of neurotransmitters DA, NA and 5-HT by membrane transporters. • Act as partial agonists of NA and are capable of directly stimulating the adrenergic α and/or β receptors. This explains the relaxation of the bronchial smooth muscle by ephedrine, of uterine smooth muscle by isoxsuprine, and of vascular smooth muscle by nylidrin, and stimulation of the myocardium by mephenteramine. Compared to catecholamines, they (1) Are effective orally; (2) Are relatively resistant to the action of MAO; (3) Have longer duration of action; and (4) Cross the BBB and therefore have significant CNS effects. EPHEDRINE is an alkaloid (Fig 18.5) obtained from plants of the genus ephedra. The herb containing ephedrine, ma huang, has been employed in Chinese indigenous medicine for over 5000 years. Surprisingly, the drug has not been studied extensively in humans. Plants of this genus are commonly encountered in northern India and China.
FIG. 18.5 Ephedrine Amphetamine
Pharmacological actions: Ephedrine mainly acts indirectly by releasing NA from sympathetic nerve endings. It also directly stimulates both the adrenergic receptors. • Cardiovascular actions: Ephedrine increases the force of myocardial contraction, cardiac output and may cause tachycardia. The rise in BP is due to both peripheral vasoconstriction and increase in the cardiac output. Repeated administration at short intervals fails to elicit the same pressor response, (tachyphylaxis). Qualitatively, its actions on various blood vessels are similar to those of adrenaline but it is a less potent pressor agent. • Smooth muscles: Ephedrine relaxes the bronchial smooth muscle. The relaxation is less prompt than with adrenaline but persists for a longer time. Ephedrine also relaxes the
uterine smooth muscle but enhances the tone of trigone and the sphincter of the bladder. • CNS: Ephedrine stimulates the CNS probably by acting on the reticular activating system. Therapeutic doses, often cause increases in mental activity, restlessness, insomnia, anxiety and tremors. It enhances the monosynaptic and polysynaptic reflexes of the spinal cord and increases the depth and rate of respiration. • Eye: Ephedrine produces mydriasis on local as well as systemic administration. • Metabolic effects: Ephedrine increases the metabolic rate and oxygen consumption. It is less effective than adrenaline in raising the blood sugar level. Absorption, fate and excretion: Ephedrine is well absorbed orally. It is relatively resistant to MAO. Hence, it has a longer duration of action than adrenaline. It is deaminated to some extent in the liver, but largely (60-75%) is eliminated unchanged in urine. Preparations and dosage: (i) Ephedrine hydrochloride tablet, 30 mg. Dose: 15 to 30 mg tid. (ii) Ephedrine hydrochloride elixir, 15 mg per 5 ml. Dose: 5 to 10 ml. Ephedrine pediatric syrup NF 8 mg per 5 ml. Dose : 5 ml per year of age to a maximum of 20 ml per dose 4 to 6 hourly. (iii) Ephedrine hydrochloride injection 30 mg per ml. Dose: 15 to 45 mg SC or IM. (iv) Ephedrine hydrochloride 1% nasal drops. Pseudoephedrine hydrochloride, a stereoisomer of ephedrine, 30 and 60 mg tablets, and syrup 30 mg in 5 ml used as a nasal decongestant. It is less liable to produce tachycardia, increase in BP and CNS stimulation. Adverse reactions: The adverse reactions are similar to those encountered with catecholamines. However, in therapeutic doses it is usually well tolerated. GI upset, difficulty in micturition, insomnia, tremors and rarely psychotic symptoms have been reported. Precautions similar to those with catecholamines should be exercised during its administration. Therapeutic uses: • Bronchial asthma: Ephedrine is useful in treating chronic, persistent, moderate asthma and in preventing acute attacks (Chapter 27). • Nasal decongestion: Ephedrine drops are used for nasal decongestion. However, it may produce tachyphylaxis and after-congestion. • Hypotension: Ephedrine may be employed IM to prevent or to treat hypotension during spinal anaesthesia. • Stokes-Adams syndrome: Ephedrine 10-30 mg 3 to 4 times daily has been used to prevent ventricular asystole. However, isoprenaline is preferred for this purpose. • As a mydriatic: Ephedrine eye drops 3 to 5% are employed to produce mydriasis without cycloplegia. • In narcolepsy: (Chapter 14) • Miscellaneous: Ephedrine has been used with varying success in urinary incontinence and nocturnal enuresis. It is of some value in relieving paroxysms of whooping cough, allergic bronchospasm and myasthenia gravis. AMPHETAMINE, which is structurally related to ephedrine (Fig. 18.5), is available in racemic and dextro forms. The d-isomer is approximately 3 to 4 times as potent as the levo
form in its central effects. Pharmacological actions: • CNS stimulation: It is a potent CNS stimulant and exerts its effects indirectly by releasing NA and DA from their storage sites in the central neurons. In therapeutic doses (10-30 mg orally), amphetamine produces euphoria, wakefulness and insomnia. It also decreases and postpones fatigue and improves the physical performance. The psychic effects of amphetamine are determined by the personality of the individual and the dose. Larger doses cause tremor, restlessness, confusion, agitation and headache. Repeated and excessive stimulation by amphetamine is followed by fatigue and depression. The psychic effects of amphetamine are attributed to cortical stimulation while stimulation of the reticular activating system probably accounts for its analeptic effect. • Cardiovascular actions: These are similar to those of ephedrine. It increases the systolic and diastolic BP but does not elevate the cardiac output significantly. Tachyphylaxis to the hypertensive effect can occur. • Smooth muscle relaxation: It contracts the sphincter of the bladder and relaxes the bronchial smooth muscle only in large doses. • Appetite suppression: See Chapter 40. Absorption, fate and excretion: Amphetamine is well absorbed on oral and parenteral administration and readily enters CNS. Like ephedrine, it is relatively resistant to inactivation by MAO. Approximately 40% of the dose is excreted unchanged in urine. Adverse reactions: • CVS and CNS toxicity: Excessive sympathetic stimulation generally causes palpitation, restlessness, headache, tremors and agitation. CNS stimulation causes marked anxiety, confusion, erratic behaviour, paranoid psychosis and visual hallucinations. CNS stimulation and the danger of afterdepression and dependence should discourage its use by students during examination season. • Acute intoxication causes (a) CNS effects such as restlessness, dizziness, tremors, hyperactive reflexes, irritability, and insomnia, followed by fatigue and depression. (b) Behavioral symptoms such as delirium, confusion, acute neurotic or psychotic episodes, and suicidal/homicidal tendencies. (c) ANS instability resulting in angina pectoris, cardiac arrhythmias, headache, chilliness, flushing, pallor, hypertension, excessive sweating and circulatory collapse. (d) GI symptoms including anorexia, nausea, vomiting, abdominal cramps and diarrhoea; and (e) It increases peripheral oxygen consumption and causes hyperpyrexia. These symptoms are characteristic of serotonergic syndrome (Chapter 14). Death is usually due to convulsions and coma. Treatment of acute poisoning is symptomatic. Sedation with diazepam is indicated to control the central stimulation. Alpha adrenergic receptor blocking agents such as phentolamine are employed to control hypertension. The urine should be acidified to promote its excretion. Peritoneal dialysis may prove useful. Individuals on therapy with MAOI may develop an alarming rise in BP with therapeutic doses of amphetamine-like drugs. • Long term consumption results in irritability, aggressive and stereotyped behaviour, and
paranoid psychosis. Marked weight loss is observed. • Dependence and withdrawal symptoms: Amphetamine causes habituation, tolerance and psychic dependence. It is a drug of abuse. Withdrawal symptoms are listed in Table 18.7. Table 18.7 Amphetamine withdrawal symptoms
Preparations and dosage: Amphetamine sulfate, 5 mg. Dose: 5 to 10 mg in the morning and at midday. Dextroamphetamine sulfate as 5 mg tablets. Therapeutic uses: It is no more used for its peripheral effects; but is used for treating Attention Deficit Hyperactivity Disorder (ADHD) and narcolepsy. • ADHD: Dextroamphetamine has been used in children with ADHD. The diagnosis of ADHD must be established properly as many other conditions in children which superficially resemble ADHD do not improve on dexamphetamine. Linear growth may be hampered during long term treatment, probably because of its anorectic action. (see Chapter 14). • Narcolepsy: Amphetamine prevents attacks of sleep (Chapter 14). Methamphetamine has more central than peripheral actions and is a major drug of abuse.
Noncatecholamines Mainly Used as Vasopressor Agents The vasopressor agents increase the BP either by increasing the peripheral resistance or by increasing cardiac output or by a combination of both. They can be classified as: I Nonselective, acting on both, α and cardiac β receptors e.g. Noradrenaline; and II Selective, acting only on α1 adrenergic receptors e.g. Mephentermine, Metaraminol, Phenylephrine and Methoxamine. These agents are routinely administered by parenteral route: • To correct hypotension due to cardiogenic shock and to prevent and treat hypotension due to neurogenic shock, e.g. during or after spinal anaesthesia. They are, however, of little value in treating late haemorrhagic or endotoxic shock. • To correct cardiac arrhythmias associated with hypotension. • In the treatment of paroxysmal atrial tachycardia, as some of these agents increase the vagal tone reflexly. METARAMINOL: Metaraminol, α receptor agonist, is less potent than NA. It has a gradual onset and longer duration of action. Rise in BP is usually accompanied by reflex bradycardia. A part of its effect is mediated through NA release. Metaraminol increases the force of ventricular contraction and the cardiac output. It increases the coronary blood flow, probably as a result of increased BP, and reduces the cerebral, splanchnic, renal and limb blood flow. The pressor effect lasts for about an hour after a 5 mg IM dose. It is used to treat hypotension in the dose of 2 to 10 mg IM. The IV dose is 0.5 to 10 mg. It can also be given as a slow infusion. It is also used as a nasal decongestant. MEPHENTERMINE acts directly, as well as indirectly by releasing NA. Its actions resemble those of ephedrine. It causes CNS stimulation. PHENYLEPHRINE: This vasopressor agent has a potent selective α1 receptor stimulant action. The pressor response is accompanied by reflex bradycardia. It has minimal action on the CNS. It is used as a nasal decongestant (0.25-0.5%) and as a mydriatic (1-2%). METHOXAMINE: Methoxamine, an α adrenergic agonist, raises BP purely by peripheral vasoconstriction accompanied by reflex bradycardia. Its haemodynamic actions are similar to those of phenylephrine. A 0.25% solution is used as a nasal decongestant. When using vasopressor agents, the BP should be raised only moderately above the critical level (100/70 mm Hg) necessary for adequate tissue perfusion. Excessive vasoconstriction jeopardises tissue perfusion and defeats the very purpose of vasopressor therapy. Tachyphylaxis to certain vasopressors may develop, rarely necessitating a changeover to another drug; however, other causes of refractoriness to vasopressors like hypovolemia and metabolic acidosis should be ruled out. MIDODRINE: This prodrug acts through its active metabolite, desglymidodrine, on α1 receptors. It is used to treat postural hypotension resulting from impaired autonomic function (Chapter 30). As it causes supine hypertension, it is administered to a patient in upright position.
Nasal Decongestants In addition to the vasopressor agents mentioned above, other sympathomimetic amines are available for use topically as nasal decongestants. Also, see Chapter 27. An ideal nasal decongestant should produce a prompt, prolonged and reliable effect and should be free from tachyphylaxis, local irritation and damaging effect on nasal cilia. It should not produce after-congestion and systemic adverse effects. Only a few drugs in very dilute solution are safe; majority of them can produce temporary or even permanent damage to ciliated respiratory epithelium after repeated use, with the possibility of atrophic changes. It is important to remember that they may blunt effects of anti-hypertensive drugs. For nasal decongestion, often a simple decongestant like ephedrine is all that is required. Ephedrine hydrochloride 0.5% in normal saline is satisfactory and the safest symptomatic treatment. It gives relief for several hours. Xylometazoline hydrochloride 0.1%, tuamino heptane sulfate 1% and oxymetazoline 0.05% produce similar effects, at a much higher cost; only the duration of action may differ. Propylhexedrine by inhalation has local action similar to that of ephedrine. Pseudoephedrine, phenylephrine and phenylpropanolamine (norephedrine) are common constituents of various oral preparations promoted for the relief of nasal congestion. Phenylpropanolamine resembles ephedrine in its CVS actions. It can cause CNS stimulation and can suppress appetite. The drug has been reported to cause pulmonary valve abnormality and increase in the incidence of hemorrhagic stroke in young women. Selective α2 Receptor Stimulants Clonidine and methyldopa, α2 adrenergic receptor agonists, are used for treatment of hypertension; they act by stimulating central α2 adrenergic receptors (Chapter 30). Guanfacine is more α2 selective than clonidine. Apart from use as anti-hypertensive, its sustained release formulation is used to treat ADHD. Apraclonidine and brimonidine are used to decrease IOP in glaucoma (Chapter 72). Tizanidine is used as central muscle relaxant. Selective β2 Receptor Stimulants Isoprenaline stimulate both β1 and β2 receptors and, therefore when used as a bronchodilator causes adverse cardiac effects. Drugs with predominant action on β2 receptors are available. These are orciprenaline (metaproterenol), salbutamol (albuterol), salmeterol, fenoterol, terbutaline, ritodrine and isoetharine. Given by inhalation, they act as promptly as isoprenaline but have a longer duration of action (2-12 hours) with less stimulant action on the heart (Chapter 27). They are used as: • Bronchodilators (Chapter 27). • Uterine relaxants (tocolytics) (Chapter 44).
Anorectic Sympathomimetic Drugs The use of these agents in the treatment of obesity is discussed in Chapter 40.
Miscellaneous Compounds ISOXSUPRINE HYDROCHLORIDE: This has β2 receptor actions as well as direct action on vascular and uterine smooth muscle. On this basis, the drug is promoted in the treatment of threatened abortion, premature labour and peripheral vascular disease. It is available as 10 mg tablets and as IM/IV injection (Chapter 44).
Sympathetic Blocking Drugs The actions produced by stimulation of the sympathetic nervous system can be blocked by peripherally acting drugs: I Drugs that induce depletion of catecholamines from the various body tissues e.g. Reserpine and Tetrabenazine (Chapter 30). II Drugs that interfere with synthesis of the adrenergic transmitter either in the adrenergic neuron or in the adrenal medulla e.g. Alpha methyl paratyrosine (Chapter 30). III Drugs that interfere with transmission of impulses across the postganglionic adrenergic neurons e.g. adrenergic neuron blocking agents like Guanethidine (Chapter 30). IV Drugs which block the adrenergic receptors e.g, α and β adrenergic receptor blocking agents. They are described below.
Adrenergic Receptor Blockers Adrenergic receptor blocking agents prevent the response of effector organs to endogenous as well as exogenous adrenaline and NA. These drugs block either α or β receptors. ALPHA RECEPTOR BLOCKERS: These drugs are more effective in antagonising the alpha receptor effects of exogenously administered adrenaline and NA than direct adrenergic stimulation. They are: I Nonselective alpha adrenergic blockers: (a) Beta haloalkylamines e.g. Dibenamine and Phenoxybenzamine. (b) Natural and hydrogenated Ergot alkaloids. (c) Imidazoline derivatives, e.g., Tolazoline and Phentolamine. II Selective α1 adrenergic blockers: Quinazolines e.g. Prazosin, Terazosin, Doxazosin and Indoramin III Selective α2 adrenergic blockers, e.g., Yohimbine. PHENOXYBENZAMINE: It binds covalently to α receptors and causes prolonged and stable non-equilibrium (non-competitive) type of blockade, loosely termed irreversible blockade. It also blocks the uptake1 mechanism. Pharmacological actions: Phenoxybenzamine is a prodrug with an active metabolite. It nonselectively blocks both postsynaptic α1 and presynaptic α2 receptors. The blocking effect is established only after 1-2 hours, even on IV administration and persists for 3 to 4 days. The usual doses cause only slight lowering of diastolic BP in normals but significant lowering of BP in patients with pheochromocytoma. The compound evokes Dale’s vasomotor reversal. It can prevent cardiac arrhythmias induced by adrenaline and other sympathomimetic amines. In clinically effective doses, phenoxybenzamine causes orthostatic hypotension due to α1 blockade in the blood vessels. Accumulated NA at the adrenergic neuronal endings would normally be reduced by action of presynaptic α2 receptors (autoreceptors); but as they too are blocked, the released NA stimulates cardiac β receptors, causing tachycardia. It also antagonises the actions of ACh, histamine and 5-HT. Slow IV infusion often produces sedation and drowsiness, probably by its antihistaminic activity. Absorption, fate and excretion: Oral absorption of phenoxybenzamine is irregular. The drug may produce irritation on SC or IM administration and hence, it is given orally or by IV infusion. Because of its high lipid solubility, it tends to accumulate in the body fat. Approximately 50% of the IV dose is excreted within 12 hours and 80% within 24 hours. Adverse reactions: Phenoxybenzamine may cause palpitation, giddiness and postural hypotension. Other ADR include miosis, dryness of mouth, nasal stuffiness and inhibition of ejaculation, Large doses produce nausea, vomiting, increased excitability and even convulsions. It can cause cumulative toxicity. Preparations and dosage: Phenoxybenzamine 10 mg capsules. The usual daily maintenance dose is 20-60 mg. It is used orally to prepare a patient with a pheochromocytoma for surgery and in its long term treatment. (Chapter 30). NATURAL AND DIHYDROGENATED ERGOT ALKALOIDS: The chemistry of the
ergot alkaloids is discussed in Chapter 43. The natural amino acid ergot alkaloids, ergotamine, ergosine, ergocornine, ergocristine and ergocryptine (the last three collectively termed formerly as “ergotoxine”) and their dihydrogenated derivatives: (a) Block the α adrenergic receptors and (b) Directly stimulate the vascular and uterine muscle. The stimulant activity on the vascular smooth muscle decreases from the natural amino acid alkaloids to their dihydrogenated derivatives with a corresponding increase in the α adrenergic blockade. Thus, the natural amino acid alkaloids usually raise the BP and may constrict coronary vessels. The dihydrogenated alkaloids, on the other hand, have a minimal constricting effect on coronaries and usually reduce BP, by a combination of α adrenergic blockade and depression of the vasomotor centre. The exception to this is dihydroergotamine (DHE) which retains the vasoconstrictor effect in addition to adrenergic blocking activity. Ergot alkaloids also depress the vasomotor centre, induce bradycardia by central vagal stimulation and direct myocardial depression and may cause vomiting by stimulation of the CTZ. They are also antagonists of 5-HT3 (Chapter 41). The amine ergot alkaloid, ergometrine, is devoid of adrenergic blocking activity. Absorption, fate and excretion: The amino acid alkaloids and their dihydrogenated derivatives are poorly absorbed on oral administration. A part of the total parenteral dose is degraded in the body. Adverse reactions: These compounds often produce nausea, vomiting, miosis and postural hypotension. Anginal pain may occur particularly with natural amino acid alkaloids due to coronary constriction. The natural alkaloids, on prolonged administration, may also induce paraesthesiae, tingling, numbness and occasionally frank gangrene due to peripheral vasospasm. Headache, diarrhoea, confusion, depression and drowsiness have been reported with ergotamine (Chapter 44). Preparations and dosage: (i) Ergotamine tartrate tablet 1 mg. Dose: 1 to 2 mg as a single dose orally, 3 to 4 mg sublingually. (ii) Ergotamine tartrate injection 0.5 mg in one ml. Dose: 0.25 to 0.5 mg SC or IM. (iii) Dihydroergotamine injection (DHE): Dose: 1 to 1.5 mg SC or IM. (iv) DHE tablets : Up to 20 mg/day, in divided doses, to treat orthostatic hypotension due to autonomic neuropathy. IMIDAZOLINE DERIVATIVES: The compounds tolazoline and phentolamine, cause competitive non-selective α adrenergic blockade. They also increase the force of myocardial contraction and cause tachycardia. They dilatate the peripheral blood vessels, particularly the arterioles and capillaries of the skin, and increase the gut motility and the gastric secretion. The salivary, lacrimal, respiratory and pancreatic secretions are augmented. The compounds also exert a mild anti-5-HT activity. Tolazoline is a partial α1 agonist and because of its predominant cardiac effect, often evokes a mild rise in BP while phentolamine usually produces a moderate fall due to peripheral vasodilatation. Absorption, fate and excretion: Tolazoline is well absorbed on oral and parenteral administration and eliminated mainly unchanged in urine. Phentolamine is poorly absorbed on oral administration; approximately 1/10th of its total parenteral dose is
eliminated in urine while the rest is probably metabolised. Adverse reactions: These include palpitation, flushing and apprehension. Other disturbances are a sensation of coldness, postural hypotension, piloerection, nausea, vomiting, epigastric distress and diarrhoea. Excessive doses of tolazoline may induce profuse sweating. Preparations and dosage: Phentolamine mesylate injection 5 mg per ml. Dose: 1-10 mg IV, repeated if necessary. Quinazolines: PRAZOSIN: It (Fig. 18.6) acts selectively by blocking postsynaptic α1 receptors.
FIG. 18.6 Prazosin
Thus, it: • Dilates the arteries (lowering blood pressure) and the veins (reducing the venous return and cardiac output); • Reduces the tone of the internal sphincter of the bladder thus decreasing the resistance to urinary outflow in patients with benign prostatic hyperplasia (Chapter 69). • It does not cause tachycardia as it does not block presynaptic α2 receptors. For details, see Chapter 30. • It has a short duration of action. TERAZOSIN: This selective α1 adrenergic blocker is used in the treatment of benign prostatic hyperplasia (BPH). The drug is long acting and is used once a day in the dose of 1 mg (at bed time), increased at weekly intervals to 2-10 mg (single dose). Doxazosin, alfuzosin, bunazosin and tamsulosin are the other α1 adrenergic blockers. Tamsulosin acts more selectively on the prostatic α1A and α1D adrenergic receptors (Chapter 69). Silodosin is α1A selective and used for BPH. Miscellaneous: INDORAMIN: This selective α1 blocker can be used to treat BPH in the dose of 20 mg b.i.d. The daily dose may be increased by 20 mg every 2 weeks to a maximum of 100 mg. In the elderly 20 mg once at night may be adequate. For its use in hypertension, see Chapter 29. URAPIDIL, an α1 antagonist, has weak α2 agonist and 5-HT1A agonist actions. It is also a weak β1 antagonist. It is used as an antihypertensive and for BPH. YOHIMBINE, an alkaloid from the West African tree Pausinystalia chimbe, is a competitive α2 receptor antagonist with short duration of action. Stimulation of the presynaptic α2 receptors inhibits the release of NA from the peripheral adrenergic nerve endings. Given orally, it increases the sympathetic outflow and potentiates the release of
NA. The drug had been promoted for erectile dysfunction (aphrodisiac). However, it is no longer used. Therapeutic uses of alpha adrenergic blocking agents: These drugs have limited therapeutic applications, mainly owing to their adverse effects like tachycardia and postural hypotension. The important uses are: • Hypertension and pheochromocytoma: see Chapter 30. • Peripheral vascular disease: see Chapter 28. Phentolamine infiltrated in the dose of 2.5 to 5 mg prevents cutaneous necrosis due to extravasation of NA. • Benign prostatic hyperplasia: Prazosin is given initially in the dose of 0.5 mg b.i.d; the first dose is given at bedtime to avoid vascular collapse (first dose effect). After 3-7 days, the dose is adjusted to the usual maintenance dose of upto 2 mg bid (Chapter 69). • Scorpion sting (Chapter 73). Ketanserin, a 5-HT receptor antagonist, also blocks α1 adrenergic receptors. Alpha adrenergic blocking action of chlorpromazine and haloperidol is discussed in Chapter 13. BETA ADRENERGIC BLOCKING AGENTS: These drugs block the actions of catecholamines by acting selectively and competitively on the beta-adrenoreceptors. They inhibit the activity of the cell membrane enzyme adenylyl cyclase and decrease the production of cyclic AMP. They can be classified as: I Cardioselective β1 blockers e.g., Acebutolol, Atenolol, Metoprolol, Bisoprolol, and Esmolol. Acebutolol has intrinsic sympathomimetic activity. II Nonselective ßj and β2 blockers: (a) With membrane stabilising activity e.g., Propranolol (Fig 18.7).
FIG. 18.7
Propranolol
(b) With intrinsic sympathetic action e.g. Carteolol. (c) With membrane stabilising activity and intrinsic sympathomimetic activity e.g., Oxprenolol, Pindolol, and; (d) Without membrane stabilising action e.g. Timolol, Nadolol and Sotalol. III Beta blockers with additional properties (a) Nonselective with α blocker activity: Labetalol, Bucindolol, Carvedilol. (b) Cardioselective: Betoxolol, Nebivolol, Celiprolol Pharmacological Actions: • Cardiac effects: These drugs do not produce any marked effect on the normal heart in the subject at rest. In the presence of increased sympathetic tone, the cardiac betablockade: (a) Reduces the automaticity and prevents the rise in heart rate (Fig. 18.8);
Effects of alpha adrenergic blocking agent phenoxybenzamine (PHB) and beta adrenergic blocking agent propranolol (PROP) on the actions of adrenaline and noradrenaline on isolated rabbit auricle. Note the reduction in the rate and the amplitude of auricular contraction with PROP in contrast to phenoxybenzamine W=wash.
FIG. 18.8
(b) Reduces the myocardial contractility, cardiac output and stroke work; (c) Slows A-V conduction; and (d) Reduces myocardial oxygen requirement and improves exercise tolerance. The cardiac response to exercise and to other situations in which sympathetic tone is increased, is attenuated. Certain beta blockers are more cardioselective (β1) in action than others (Table 18.8). Such selectivity is relative.
Table 18.8 Properties of some beta-adrenergic blockers
*
Alpha receptor blocking also
**
IV infusion only, in emergency management of unstable angina, threatened infarct, cardiac arrhythmias and thyrotoxic crisis; + = Present. 0 = Absent. ***
Local anaesthetic action.
• Blood pressure: They reduce BP probably by their action on the heart and reduction in cardiac output. They reduce renal renin release and lower the plasma renin activity (PRA), but cause an increase in natriuretic peptide secretion (Chapter 30). They also reduce central sympathetic outflow and have a central hypotensive action. During their chronic use, they decrease the peripheral resistance by blocking the presynaptic β2 receptors, resulting in reduction in the release of NA from the adrenergic nerve endings. They do not cause postural or exercise induced hypotension as the α1-adrenergic receptors are not blocked. • Bronchi: The blockade of β2 receptor sites in bronchi and bronchioles causes increase in airway resistance which could be harmful in patients with asthma. • Membrane-stabilising action: Propranolol and some other beta-blockers (celiprolol) have a variable direct depressant effect on the heart similar to that of lignocaine. It occurs only with high concentrations of the drug and is probably not relevant to their clinical use. • Intrinsic sympathomimetic action: Some of the beta-blockers possess beta-receptor stimulating activity (partial agonist property). It has been suggested but not proved, that beta-blockers with additional intrinsic sympathomimetic activity may cause less cardiac depression and are thus less likely to precipitate congestive heart failure in the presence of damaged heart.
It should be noted that the pharmacological actions of beta-receptor antagonism are always present at lower concentrations of the drug than either membrane stabilising or paradoxical agonist effect. The primary beta blocking action of all these drugs is in fact mostly responsible for their beneficial as well as adverse effects. • Effects on the CNS: Lipid soluble propranolol, metoprolol and labetalol can readily cross the BBB. Propranolol alters mood and has been used in anxiety states. Atenolol, with the lowest lipid solubility, has fewer central side effects. • Metabolic effects: Beta blockers are capable of modifying carbohydrate and lipid metabolism. Nonselective BB, by blocking β2 receptors, prevent the perception of hypoglycemia by preventing indicative symptoms such as palpitation and tremor. In addition, they prevent catecholamine-induced glycogenolysis and mobilisation of glucose during hypoglycemia, and delay recovery. For this reason, selective β1 blockers are preferred in diabetes mellitus. Further, they decrease the release of FFA from the adipose tissue. Long term, it increases LDL cholesterol and triglycerides, and decreases HDL cholesterol. However, selective β1 blockers such as celiprolol, carvedilol and carteolol may improve the lipid profile. • Intraocular pressure: β-blockers, used topically or orally, cause a reduction in intraocular pressure due to a reduction in the secretion of aqueous humour (Chapter 72). Some beta blockers have additional actions e.g. through NO production (celiprolol, carteolol), calcium channel blocking (carvedilol), and potassium channel opening (tilisolol). Their contribution to the therapeutic effects, however, is not clear. Absorption, fate and excretion: The effective oral dose ranges of the β-blocking drugs are wide. Plasma concentrations vary markedly between individuals receiving the same dose. This is because although most of these compounds are completely and rapidly absorbed, some of them like propranolol and metoprolol are rapidly metabolised by the liver (first pass metabolism (Table 18.8). Thus, for propranolol, the relative oral bioavailability is low and variations in plasma levels are marked (almost upto 20 fold). The relative oral bioavailability of pidolol and sotalol is better and variations in plasma levels are less marked. Those which are largely excreted by the kidney i.e. atenolol, nadolol tend to accumulate in the presence of kidney damage. Pindolol, acebutolol, atenolol and timolol are eliminated to variable extents by both routes. The plasma half life of the beta blockers that are mostly metabolised by the liver is short (2-3 hours), whereas that of drugs excreted unchanged is longer (8-12 hours). Duration of effect is modified by liver and kidney diseases (Table 18.8) and by the active metabolite(s), e.g. the active metabolite of propranolol, 4 hydroxy-propranolol, makes twice a day administration of propranolol possible. The plasma t½, however, does not correlate well with the duration of their therapeutic effects. This is because the plasma level declines exponentially, thus following first order kinetics while the effect decreases linearly, following zero order kinetics. Hence, most agents can be given orally at much longer intervals than is suggested by their plasma half-lives. Adverse reactions: • Cardiovascular effects: These are mostly due to the extended cardiac actions. Thus, propranolol may cause hypotension and pronounced bradycardia. The ventricular function depends on increased contractility due to sympathetic activity, and betablockade prevents this homeostatic response leading to clinical heart failure in patients
with cardiac damage. Some of these drugs can aggravate A-V conduction defects. However, this action may be beneficial in patients with atrial fibrillation. Excessive bradycardia due to overdose can be countered by IV atropine 0.6-2.4 mg in divided doses (Chapter 32). Hypotension refractory to atropine may require IV glucagon (50-150 mcg/kg in 5% glucose). In severe cases, IV isoprenaline 4 mcg/min is administered increasing slowly till heart rate is restored to normal (50-70 per min) and maintained. If beta blocker therapy is to be discontinued in angina, the dosage should be reduced gradually, as myocardial infarction may be precipitated following its abrupt withdrawal. Cold extremities and absent pulses may sometimes be observed after beta-blockade. Raynaud’s phenomenon may occasionally be troublesome and intermittent claudication may be aggravated. • Metabolic effects: These drugs prevent the correction of hypoglycemia by adrenergic body mechanisms (hypoglycemia unresponsiveness) and may aggravate neurogylcopenic symptoms of hypoglycemia. However, in clinical practice cardioselective beta-blockers may be used safely in most patients with T2DM (Chapter 65). • Bronchospasm: This may occur particularly in patients with bronchial asthma. Cardioselective beta-blockers in low doses may be preferred but none is absolutely safe. • CNS effects: Prolonged use of propranolol can cause fatigue, muscle cramps, lethargy and rarely mental depression. • Miscellaneous: These include allergic reactions, thrombocytopenia and sexual dysfunction. In patient with severe renal failure, beta blockers can reduce the GFR further; hence their dose needs to be adjusted. Beta blockers can be used during pregnancy. However, they can cross placental barrier and may cause hypoglycemia and bradycardia in neonates. Drugs such as NSAID that cause sodium retention attenuate the antihypertensive effect of beta blockers. By reducing the hepatic blood flow by almost 30%, beta blockers can slow down the hepatic drug metabolism. A combination of beta blockers with verapamil and amiodarone may be dangerous. Beta blockers may increase the severity and possibly the frequency of anaphylactic reactions to drugs, biologicals, insect stings and foods, and the reaction may prove resistant to treatment with adrenaline. Table 18.9 shows the dosage schedules of the commonly used beta blockers.
Table 18.9 Dosage range of some beta adrenergic blocking drugs
Therapeutic uses: Clinically, in terms of beta blocking activity, no drug is superior to others. The choice is guided by safety, pharmacological properties, ease of administration and cost. In the presence of airway obstruction or peripheral vascular disease, a cardioselective beta-blocker such as atenolol may be preferred, keeping in mind that cardioselectivity is not absolute. I Cardiovascular uses: • Angina pectoris: Beta blockers are the mainstay of the chronic, prophylactic treatment of patients with angina of effort (Chapter 29). • Myocardial infarction: Chapter 29. • Cardiac arrhythmias: Beta blockers can be used successfully in the treatment of tachyarrhythmias precipitated by sympathetic overactivity as during exercise, emotion and anaesthesia. (Chapter 28). • Hypertension: (Chapter 30). • Heart failure and depressed EF < 40%: Chapter 31. • Hypertrophic obstructive cardiomyopathy (Idiopathic hypertrophic subaortic stenosis) is characterised by a marked hypertrophy of the ventricular musculature, commonly of the left ventricle, leading to palpitation, angina, dyspnoea or syncope. Propranolol, in the dose of 60 to 400 mg per day, causes symptomatic improvement. • Pheochromocytoma: Alpha adrenergic blocking agents are routinely used prior to surgery in patients with pheochromocytoma. In some patients, however, α blockade leads to a severe tachycardia particularly when atropine is used for preanaesthetic medication. Propranolol, in the dose of 1-5 mg IV prevents this complication. It should be pointed out, however, that beta blockade without simultaneous α blockade may lead to severe hypertension in pheochromocytoma. II Non-cardiovascular uses: • Chronic open-angle glaucoma: Timolol maleate 0.25-0.5% is used as eye-drops in lowering IOP. It has an advantage in that it does not induce spasm of accommodation nor does it affect the pupil. Further, it is convenient to administer (once or twice daily). In general, they are well tolerated and reasonably safe. Timolol eye drops may be absorbed and can affect the heart and bronchi adversely (Chapter 72). • Thyrotoxicosis: Beta blockers are valuable adjuncts to antithyroid drugs in the treatment of thyrotoxicosis, where they produce rapid symptomatic relief (Chapter 64). • Portal hypertension: Propranolol and nadolol are used orally to prevent variceal bleeding
in this condition; they act by lowering the pressure in the portal circulation. • To control the withdrawal symptoms in alcohol addicts. (Chapter 6). • Miscellaneous: Increased adrenergic activity is prominent in anxiety states and propranolol can reduce the associated symptoms such as palpitation, tachycardia and sweating Beta blockers may be used to decrease cardiac symptoms prior to important meetings and public speaking engagements in susceptible individuals. Propranolol is also useful in the prevention of migraine and treatment of essential tremor as it crosses the BBB. Contraindications to beta-blockers: are listed in Table 18.10. Table 18.10 Contraindications to beta blockers
ALPHA AND BETA-ADRENERGIC BLOCKING DRUGS: LABETALOL: Labetolol is a non-selective beta blocker consisting of 24 diastereomers, each exhibiting different relative affinity for the receptors accounting for the complex drug effects: (i) Blockade of the β1 and β2 receptors. while (ii) Partial agonistic activity at β2 receptors. (iii) Selective blockade of α1 receptors; and (iv) Inhibition of neuronal uptake of NA. These actions prevent the vasoconstriction observed following non-selective β blocker. Given orally, it is less effective than atenolol. The drug undergoes extensive first pass metabolism with t½ of 4h. It is given bid. Adverse reactions: Labetalol may cause GI disturbances, dryness of mouth and fluid retention. Cardiac effects are similar to those of other beta blockers. Postural hypotension may occur. Other reactions include nervousness, sexual dysfunction, muscle cramps and depression. The drug accumulates in tissues with high melanin content such as the choroid, and periodic eye examination is recommended. Therapeutic uses: It is mainly used by infusion (1mg/min titrated at ½ hrly interval) to treat hypertensive emergencies, as the effect is rapid due to concurrent α1 blockade (Chapter 30). Carvedilol: Like labetalol, this drug is a non-selective β blocker with selective α1 adrenergic receptor blocking action. Bisoprolol and Nebivolol are cardioselective β blockers (Table 18.8) with NO-mediated vasodilator properties. They are effective in hypertension as well as CHF and improve left ventricular function. They have t½ of 10-11 hours (Chapter 31).
19
Cholinergic Drugs Cholinergic or parasympathomimetic agents are drugs which stimulate the effector cells innervated by postganglionic parasympathetic cholinergic nerves. In general, their actions are similar to those seen following the stimulation of the parasympathetic nervous system. These drugs are classified as: I Esters of choline e.g. Acetylcholine, Methacholine, Carbachol, Bethanechol. II Cholinomimetic alkaloids, e.g., Pilocarpine, Muscarine and Arecoline. III Cholinesterase inhibitors (Anticholinesterases), e.g., Physostigmine, Neostigmine, Organophosphorus compounds.
Esters of Choline Acetylcholine (ACh) is an ester of choline with acetic acid, while carbachol and bethanechol are esters of choline and betamethylcholine respectively with carbamic acid. The base choline also possesses properties similar to acetylcholine. Esterification of choline augments the cholinergic activity. Chemically, ACh and related substances are quaternary ammonium compounds and their unique specific action is attributed to the trimethylammonium [R-N+ (CH3)3] groupings. ACETYLCHOLINE is available in powder form as chloride or bromide; it is extremely hygroscopic (Fig 19.1). Although it is not useful in therapeutics it is important to know its actions for understanding other cholinergic drugs. There is no circulating ACh in the blood.
FIG. 19.1 Acetylcholine
Mechanism of action: ACh acts on two subtypes of cholinergic receptors: (a) Muscarinic (M), located on tissues supplied by postganglionic parasympathetic nerves, CNS and non-innervated receptor on vascular endothelium; and (b) Nicotinic (N), situated in all sympathetic ganglia, adrenals, CNS and skeletal neuromuscular junction. • Muscarinic actions: The actions produced as a result of ACh released from the postganglionic parasympathetic nerve endings in various tissues are termed as muscarinic actions, e.g., secretory glands, smooth muscles and the heart. They are blocked by atropine. The designation muscarinic action comes from the fact that these actions are similar to those produced by the poisonous mushroom alkaloid muscarine. Currently, five subtypes of the muscarinic receptors M1 M2, M3, M4 and M5 have been identified. There is a great deal of homology among M1, M3 and M5 receptors and between M2 and M4 receptors: (i) M1 receptors are found in the CNS and ganglia; (ii) M2 exist in the heart and presynaptic CNS neurons; (iii) M3 in the smooth muscle of the GI tract and detrusor muscles of bladder; (iv) M4 expression is rich in CNS but they serve as autoreceptors inhibiting transmitter release and, (v) M5 are predominant in substantia nigra and ventral tegmentum area. Mj, M3 and M5 receptors act through Gs protein. Which stimulates phospholipase C. This leads to the hydrolysis of phosphati-dylinositol polyphosphate (PIP) to form inositol 1-4-5 triphosphate and diacylglycerol. The former causes release of intracellular calcium from endoplasmic reticulum while the latter activates protein kinase C. The M2 and M4 receptors interact with G proteins (Gi) with resultant inhibition of
adenylyl cyclase and activation of K+ channels, particularly in the heart, and modulation of the activity of calcium channels in certain cell types. These effects account for both the negative chronotropic and the inotropic effects of ACh. • Nicotinic actions: The actions of ACh at the nicotinic receptors are termed nicotinic actions as they resemble those produced by the tobacco alkaloid nicotine. To demonstrate these actions, atropine has to be administered to block the muscarinic actions of ACh. In such atropinised animal preparations, injection of large doses (1-5 mg) of ACh produces certain responses (Fig. 19.2) due to an initial stimulation and subsequent blockade of the nicotinic receptors in autonomic ganglia and the skeletal myoneural junctions.
FIG. 19.2 Blood pressure and spleen volume (upper tracing) in anaesthetised dog. Note the fall of B.P. following a low dose (L) of acetylcholine (ACh) which is blocked by atropine (Muscarinic action). High dose (H) of ACh in this atropinised animal produced rise in B.P. and contraction of the spleen (Nicotinic action), resembling the action of adrenaline (Adr.).
Nicotinic receptors are ligand-gated ion channels, and their activation causes a rapid increase in the cell permeability to Na+ and K+ ions with resultant depolarisation and excitation. Pharmacological actions: Given orally, ACh is rapidly destroyed in the GI tract and hence, it has to be administered by IV infusion to elicit its actions. Even large IV, bolus doses of ACh have no appreciable action in man. This is because it is rapidly metabolised in the plasma by the enzyme pseudocholinesterase, and at the site of action by the specific true cholinesterase. The only important actions seen after IV injection of ACh are flushing and transient fall in BP. Many of its actions can, however, be demonstrated in in vitro experiments. Cardiovascular system: • Heart: In mammals, the effect of ACh on the heart is similar to that obtained by stimulation of the vagus which is a cholinergic nerve. Thus, it: (1) Depresses the SA node, causes bradycardia (negative chronotropic action), and may cause cardiac arrest; (2) Decreases the contractility (negative inotropic action); (3) May cause A-V block; and
(4) Increases the conduction velocity in the atria. These changes are transient and can be blocked by atropine. Acetylcholine reduces the cardiac rate in isolated heart preparation. However, in the presence of atropine, a large dose of ACh stimulates the heart, causing ventricular arrhythmias. • Blood vessels: ACh dilates the blood vessels mainly of the skin and the mucous membranes by acting on the M3 receptors located on the endothelial cells of the vessel wall. Their stimulation causes release of nitric oxide (NO) which results in vascular relaxation. If the endothelium is damaged, ACh causes vasoconstriction. It also dilates the coronary arteries and has a doubtful vasodilator effect on the cerebral and pulmonary vessels. ACh given IV in man results in transient flushing, a sense of warmth in the skin and throbbing headache. The BP falls owing to a decrease in the peripheral resistance and in the cardiac output (vagal effect) in anaesthetised animals. Smooth muscles: Acetylcholine • Increases the tone and the rhythmic activity of the smooth muscle of the GI tract and enhances peristalsis. The sphincters are, however, relaxed resulting in a rapid forward propulsion of the intestinal contents. • Contracts the smooth muscle of the gall bladder. • Contracts the detrusor muscle of the urinary bladder while the smooth muscle of the trigonal sphincter is relaxed. • Constricts the bronchial smooth muscle and causes bronchospasm. • Usually contracts the smooth muscle of the ureter while that of the uterus shows inconsistent response. Secretions: Cholinergic stimulation increases the gastric, intestinal and pancreatic secretions; the bronchial, salivary, lacrimal and nasopharyngeal secretions are also augmented. The increased bronchial secretions, accompanied by bronchospasm, may result in cough and dyspnoea. The salivary secretion is profuse and watery. As the postganglionic sympathetic fibres supplying the sweat glands are cholinergic, ACh enhances sweating. Eye: Instillation of ACh in the eye is without any effect as it is not absorbed. However, intra-carotid injection after sectioning of the postganglionic fibres from superior cervical ganglion (removal of sympathetic tone) produces: • Constriction of the pupil (miosis) by contracting the circular fibres of sphincter pupillae, resulting in reduction in intraocular tension by increasing the drainage of ocular fluid through the canal of Schlemm. • Contraction of the ciliary muscle which results in relaxation of the suspensory ligament (zonule) of the lens. This reduces the tension on the lens and allows the lens to bulge into the anterior chamber thereby increasing its thickness and reducing the focal length. Vision is, therefore, fixed for a short distance. This is termed spasm of accommodation. Autonomic ganglia: Acetylcholine-induced ganglionic stimulation results in an increased output of ACh and NA from the postganglionic parasympathetic and sympathetic nerve endings respectively. The released NA causes rise in the BP. In addition, large doses of ACh also stimulate the adrenal medulla to increase the secretion of adrenaline which further augments and sustains the rise in blood pressure (Fig. 19.2). Myoneural junction: Acetylcholine released as a result of stimulation of the somatic
nerves induces contraction of the skeletal muscle. However, a very high concentration of ACh at the myoneural junction can produce paralysis of the skeletal muscles by keeping the muscle in a persistently depolarised state in which it is refractory to further stimuli. Intra-arterial injection of ACh into the brachial artery of human volunteers produces fasciculations and twitching of the skeletal muscle followed by a prolonged weakness. The action of ACh on the autonomic ganglia is blocked by ganglion blocking agents like hexamethonium whereas the action on the myoneural junction is antagonised by tubocurarine. (Chapter 22). Miscellaneous actions: ACh is a neurohumoral transmitter in the CNS. The recurrent collaterals of the motor neurons which synapse with the Renshaw cells of the spinal cord are cholinergic and the receptors are predominantly nicotinic in nature. In contrast, most of the cholinergic neurons at the cortical and subcortical levels of the CNS have predominately muscarinic receptors. Cholinergic system is essential to normal behaviour and cognition. Patients with Huntington’s disease, characterised by involuntary choreiform movements and dementia, have been found to have severe degeneration of cholinergicneurons within the basal ganglia. Patients suffering from Alzheimer ’s dementia have significant losses of choline acetyltransferase in the basal ganglia, frontal cortex and hippocampus. Being a quarternary ammonium compound, ACh does not cross the BBB and hence, it exerts no significant central actions. Therapeutic uses: Owing to its extremely transient action, ACh cannot be used in clinical practice. Hence, its substitutes have been synthesised which are: • Effective orally; • More selective in their actions; and • Act directly as ACh receptor agonists. METHACHOLINE: Methacholine differs from ACh in being effective orally though its oral absorption is poor. Susceptibility of methacholine to true cholinesterase is approximately ⅓rd that of ACh. It is totally resistant to pseudocholinesterase. Hence, it has a longer duration of action. It has no nicotinic actions. The drug is now rarely used. CARBACHOL: Carbachol is resistant to both true and pseudocholinesterase. Its absorption from GI tract is incomplete. Hence, it is not useful orally. Pharmacological actions: It has a relatively selective muscarinic effect on the smooth muscle of the GI tract and the urinary bladder; it also stimulates autonomic ganglia and skeletal muscles and has a more sustained miotic effect on topical application. Administered parenterally, it produces flushing of the face, sweating, salivation and lacrimation. The muscarinic effects of carbachol are not adequately antagonised by atropine. Dose: For ophthalmic use, 3% eye drops. BETHANECHOL: Like carbachol, this choline ester is resistant to hydrolysis by both true and pseudocholinesterase. It has predominantly muscarinic actions and hence has negligible cardiovascular effects. Its muscarinic effects are well antagonised by atropine and hence it is preferred to carbachol in clinical practice. Bethanechol should never be administered IV. Preparations and dosage: Bethanechol 5 or 10 mg tablets. Dose : 10 to 30 mg 3-4 times daily. It is also available as injection. Adverse reactions to choline esters: These are mostly an extension of the
pharmacological actions. Besides the minor side effects like flushing, salivation, sweating and bradycardia, the serious reactions include hypotension, syncope, bronchial spasm, and occasionally cardiac arrhythmias The increased GI and urinary tract activity may produce a desire for defaecation and micturition. Carbachol, because of its slow hydrolysis, is capable of exerting cumulative toxicity. Abdominal cramps, belching, nausea and vomiting may result. Marked hypotension, cardiac arrhythmias and death after parenteral carbachol therapy have been reported. Therapeutic uses of choline esters: • Gastrointestinal and urinary tracts: Bethanechol has been employed for the treatment of post-operative paralytic ileus and abdominal distension and for urinary retention. In chronic urinary retention 10 mg of the drug may be given 3 to 4 times a day till voluntary or automatic voiding is established.
Cholinomimetic Alkaloids PILOCARPINE: Pilocarpine, an alkaloid obtained from the South American shrubs Pilocarpus microphyllus and Pilocarpus jaborandi, directly stimulates cholinergic receptors and produces both muscarinic and nicotinic actions of ACh. When applied topically to the eye, pilocarpine produces miosis, followed by a sustained fall in intraocular tension and a spasm of accommodation. Adverse reactions: It has all the side effects of choline esters. Because of the prominent secretory response, pulmonary edema is a major hazard of systemic pilocarpine therapy. Preparations and dosage: Pilocarpine nitrate eye drops; 1-4% (Chapter 72). Therapeutic uses: Pilocarpine alone (0.5-4% aqueous solution) or in combination with 1% physostigmine is used to reduce intraocular tension in acute congestive glaucoma. Pilocarpine-induced miosis persists for 3 to 24 hours but the spasm of accommodation disappears in about 2 hours. Pilocarpine is often used alternately with mydriatics like homatropine (2 to 5%) to break adhesions between the iris and the lens (Chapter 72). Pilocarpine ocusert is a drug delivery unit specially designed to deliver pilocarpine slowly, over a period of 7 days. However, it is far more expensive than drops. The drug is too toxic for systemic use. MUSCARINE AND ARECOLINE: Muscarine is an alkaloid from the poisonous mushroom Amanita muscaria. Acute mushroom poisoning is characterised by diarrhoea, dyspnoea, abdominal pain, lacrimation, salivation, weakness, confusion, convulsions and coma. These effects are due to muscarine and can be antagonised by large doses of atropine. Delayed poisoning which develops within 6 to 15 hours after ingestion of another mushroom Amanita phalloides is characterised by nausea, vomiting, diarrhoea, jaundice and vasomotor collapse; this is attributed to other toxins and does not respond specifically to atropine. Muscarine has no therapeutic application. Arecoline, the alkaloid of betel nut Areca catechu, has cholinergic actions. It has no therapeutic application in humans. Betel nut is commonly used by chewing and may have mild euphoriant effect. Cevimeline, a quinuclidine derivative, is an M3 receptor agonist which increases salivary and lacrimal secretions on oral administration. It is long acting and is claimed to be less toxic than pilocarpine. It can be used in the treatment of xerostomia (dry mouth).
Cholinesterase Inhibitors Anticholinesterase (anti ChE) drugs inhibit the enzymes, true and pseudocholinesterase and prevent inactivation of ACh. Their pharmacological effects resemble those of stimulation of cholinergic nervous system. In addition, some antiChE have independent pharmacological actions. They are classified as: I Reversible: These produce reversible inhibition of cholinesterase; e.g. Physostigmine, Neostigmine etc. II Irreversible: These induce almost irreversible inhibition of cholinesterase e.g. Diisopropylflurophosphate (DFP), Octa methyl pyrophosphotetramide (OMPA) and other organophosphorus compounds and carbamates. Mechanism of action: Acetylcholine is inactivated by combination with two sites on the enzyme cholinesterase : an anionic site bearing a negative charge which attracts the quaternary nitrogen atom (N+) of ACh; and an esteratic site which attracts the carboxylic acid group (COOH) of the ACh molecule (Fig. 19.3). As a result of the union of ACh with cholinesterase, the esteratic site of the enzyme is acetylated and this results in splitting off of choline. The acetyl group combined with the esteratic site is, however, immediately removed as a result of hydrolysis, forming acetic acid. This sets the esteratic site of the enzyme free for inactivation of another molecule of ACh.
FIG. 19.3 The inactivation of acetylcholine by cholinesterase and the mechanism of action of reversible anticholinesterase drug, Neostigmine, and irreversible anticholinesterase drug, DFP. G-H : a protonated acidic group of the esteratic site.
The reversible anticholinesterases bear a structural resemblance to ACh and combine with the anionic and esteratic sites of cholinesterase as well as with ACh receptor. However, the complex which they form with the esteratic site of cholinesterase is much less readily hydrolysed than the acetyl-esteratic site complex formed with ACh. This produces a temporary inhibition of the enzyme. In contrast to other reversible anticholinesterases, edrophonium forms reversible complex only with the anionic site and hence, has a shorter duration of action. The irreversible anticholinesterases, organophosphorus compounds, combine only with esteratic site of cholinesterase and consequently the esteratic site is phosphorylated. The hydrolysis of the phosphorylated site, however, is slow and in certain cases does not occur at all. This produces an almost irreversible inhibition of cholinesterase. In contrast to other organophosphorus compounds, echothiophate forms complexes with both anionic and esteratic sites and hence, is much more potent than other compounds. I Reversible anticholinesterases can be further subdivided into: • Naturally occurring, e.g., Physostigmine. • Synthetic: Neostigmine, Pyridostigmine, Ambenonium, Demecarium, Edrophonium, Tacrine and Rivastigmine. PHYSOSTIGMINE: This is an alkaloid obtained from the calabar bean, the dried ripe seed of an African woody climber Physostigma venenosum. Pharmacological actions: The pharmacological effects of physostigmine are similar to those of other cholinergic agents.
Topical instillation into the eye produces miosis, spasm of accommodation and a fall in intraocular tension. It has a short duration of action. It is rapidly absorbed on oral or parenteral administration, crosses the BBB and exerts central cholinergic actions. Preparations and dosage: Physostigmine salicylate is used in eye as 0.25 - 0.50% aqueous solution or as an ointment. On storing, the solution becomes pink owing to decomposition. Physostigmine salicylate inj. 2 mg/2 ml. Therapeutic uses: (1) As miotic, (2) In the treatment of anticholinergic drug (atropine) intoxication, and (3) In poisoning with phenothiazines and tricyclic antidepressants. It is particularly valuable in patients with CNS symptoms such as delirium. It is used in doses of 2 mg SC or IV and repeated after 10-20 min. If effective, 2-4 mg may be given every 2-4 hours. NEOSTIGMINE: It is a synthetic, quaternary ammonium compound (Fig. 19.4) that inhibits both true and pseudocholinesterases.
FIG. 19.4 Neostigmine
Pharmacological actions: In addition to its anticholinesterase activity, neostigmine also directly stimulates certain organs having cholinergic receptors. Its actions are: Gastrointestinal tract: Neostigmine increases the tone and motility of the small and large bowel, enhances the production of gastric juice; and by augmenting the intestinal motor activity, it promotes the propulsion of intestinal contents. Atropine reduces but does not abolish the intestinal effects. Skeletal muscles: Neostigmine produces a striking increase in the strength of skeletal muscles in myasthenia gravis. In contrast to physostigmine, neostigmine IV can stimulate chronically denervated muscle or a muscle in which all the cholinesterase has been inactivated. It is postulated to improve muscle strength by: • Its anti-ChE activity causing greater accumulation of ACh at the motor end plates. • Increasing the amount of ACh released during each nerve impulse; and • Directly stimulating the cholinoceptive receptor sites on the motor end plate by virtue of its structural similarity with ACh. It thus acts as a partial agonist. Administration of neostigmine to normal subjects may cause twitchings and fasciculations of skeletal muscles but these are not encountered in myasthenic patients. It reverses the neuromuscular blockade produced by d-tubocurarine, but is not a satisfactory antagonist for benzoquinonium, and it actually enhances the paralysis of skeletal muscles caused by persistent depolarisers like succinylcholine. Cardiovascular system: The drug causes bradycardia and fall in BP by peripheral vasodilatation. Other actions: It increases bronchial secretions and causes bronchospasm. Absorption, fate and excretion: Being a quaternary ammonium compound, it is
absorbed incompletely orally. Hence it is administered SC or IM. The effect of IM injection starts within 10 minutes and persists for 3 to 4 hours. It is partly destroyed by the cholinesterase and partly eliminated unchanged in urine. Preparations and dosage: (i) Neostigmine bromide tablet 15 mg. Dose: 15 to 30 mg. (ii) Neostigmine methyl-sulphate inj. 0.5 mg per ml. Dose: 0.5 to 2 mg. Distigmine: is a longer acting neostigmine analogue and can be used in the dose of 5-20 mg once daily before breakfast. PYRIDOSTIGMINE: This compound resembles neostigmine structurally and in its actions. The drug has a slightly longer duration of action but is less potent. It is claimed to have fewer visceral effects. Preparations and dosage: Pyridostigmine bromide tablet, 60 mg. Dose: 60 to 240 mg. Pyridostigmine slow release tablets have a duration of action of 4 to 10 hours. Pyridostigmine inj. 1 mg/ml. Dose: 1 to 5 mg SC or IM. AMBENONIUM: This quaternary ammonium compound which is a more potent truecholinesterase-inhibitor than neostigmine and has a more marked direct stimulant effect on the skeletal muscle. The action is more sustained with a lower incidence of GI side effects. It is available as 10 mg tablets. Dose: 5-20 mg 3-4 times a day as required. DEMECARIUM: Structurally, demecarium is made up of two neostigmine molecules joined through 10 methoxy groups. It is a potent miotic. This effect may last for 3 to 10 days. It is accompanied by a spasm of accommodation. It is used as 0.1 to 0.25% eyedrops once or twice weekly for treating glaucoma. EDROPHONIUM: It is structurally related to neostigmine. It has a weak anticholinesterase activity as compared to neostigmine. But it enhances neuromuscular transmission with a dose that is too low to affect the smooth muscles, the myocardium and the glands. It has a quicker onset of action than neostigmine, and the effects of a single IV dose persists only for 10 minutes. The muscarinic side effects are mild. Preparations and dosage: Edrophonium chloride 10 ml vials containing 10 mg per ml. Adverse reactions to reversible anti-cholinesterases: The reversible anticholinesterases produce effects attributable mainly to their muscarinic actions. These are: (a) Gastrointestinal: Epigastric distress, nausea, abdominal cramps and diarrhoea (b) Secretions: Increased salivation, sweating, lacrimation (c) Neurological: Paraesthesia, fasciculations particularly around the mouth and superior extremities, tremors (d) Miscellaneous: hypotension Demecarium and physostigmine eyedrops, may pass into the nose through the nasolacrimal duct and produce muscarinic side effects. When used in the treatment of myasthenia gravis, an overdose of these compounds may produce skeletal muscle paralysis by persistent depolarisation. This phenomenon, termed cholinergic crisis, has to be differentiated from the sudden exacerbation of muscular weakness in myasthenia often associated with severe upper respiratory infection, termed myasthenic crisis. In myasthenic crisis, the requirement of the antiChE agent is increased and occasionally the patient becomes resistant to anticholinesterase medication. Edrophonium 2 mg IV (ameliorative test) brings about a prompt improvement of muscle strength in myasthenic crisis, while it exacerbates the weakness of cholinergic crisis.
However, this effect is not dangerous because of the extremely short duration of action of edrophonium. If the initial dose of 2 mg fails to produce an improvement in muscle strength within 30 seconds, a dose of 8 mg may be administered. The cholinergic crisis is treated by administration of large doses of atropine, oximes and artificial respiration. Therapeutic uses of reversible anti-cholinesterases: • Glaucoma: Chapter 72. • For decurarisation following the use of neuromuscular blockers like d-tubocurarine in anaesthetic procedures. • Myasthenia gravis: Myasthenia gravis is a disease characterised by easy fatiguability and progressive weakness of striated muscles and with intermittent periods of exacerbation. Pregnancy usually leads to an improvement or even temporary remission of this condition. Myasthenia gravis is an autoimmune disease caused by a deficiency of the postsynaptic neuromuscular ACh receptor complex. Thus the receptors in myasthenic muscle are degraded and cleared much faster than normally. The number of available ACh receptors in the involved muscles is reduced by as much as 70-90%. Circulating antibodies to ACh receptors have been demonstrated in 70-90% of patients with myasthenia. Many patients with this disorder have thymic hyperplasia or a thymoma. Another myasthenic syndrome (Lambert-Eaton syndrome) of autoimmune nature occurs in association with small cell carcinoma of the lung. The autoantibodies are directed to the calcium channel in the nerve terminals, with resultant diminution in the release of ACh. The defect is presynaptic and does not respond to anticholiesterases but may respond to 3,4-diaminopyridine (3,4 DAP) which increases neurotransmitter release. The diagnosis of myasthenia gravis depends upon typical clinical picture and a dramatic clinical response to either neostigmine or edrophonium. Administration of 1 to 1.5 mg of neostigmine IM produces a marked improvement in muscle strength, which lasts for 3 to 4 hours. Atropine sulfate 0.6 mg IM is usually administered before or along with neostigmine to counter the muscarinic effects of neostigmine. Treatment: Neostigmine was formerly used in the treatment of myasthenia. However, because of its relatively short duration of action (< 4 hours), development of tolerance and waxing and waning of muscle strength, drugs with longer duration of action and fewer side effects, e.g. pyridostigmine (4-6 hr) and ambenonium (4-8 hr) are now preferred. These drugs may, however, be combined with neostigmine as the onset of action of neostigmine is quicker. The effect of parenteral neostigmine appears within 30 minutes while oral administration produces a response within 1 hour. The therapy is initiated with neostigmine 7.5-15 mg or pyridostigmine 30-60 mg or ambenonium 2.5 to 5 mg orally, at a time, and the dose is gradually increased until the maximal benefit is obtained. The reversible antiChE are usually administered 3 to 4 hourly, orally or parenterally to ensure a smooth and sustained effect. Parenteral medication is indicated before meals so as to enable the patient to swallow his food. Most myasthenics can be improved only partially with these drugs. Thus, 80 to 90% recovery occurs in 25% individuals. Further increase in dosage precipitates toxic actions without appreciable increase in clinical improvement. It is, therefore, wiser to accept minor disability rather than overdose the patient. Infection increases the requirements of anticholinesterases. Because of the autoimmune basis of myasthenia, prednisolone, a glucocorticoid has
been used in the dose of 25-100 mg once a day. Dose should be regulated slowly. Alternate day regimen is preferred. It causes remission or improvement in 80% of cases. Such therapy may, however, cause exacerbation of weakness in the early stages. Immunosuppresants such as azathioprine, cyclosporine as well as plasmapheresis have been used as adjunctive therapy in resistant cases. Thymectomy can help patients with thymoma. Table 19.1 lists the drugs which aggravate the symptoms in myasthenic patients. Table 19.1 Drugs which aggravate myasthenic symptoms
• Paralytic ileus and urinary retention: Neostigmine can be employed parenterally in the dose of 0.5 to 1 mg to treat postoperative paralytic ileus and urinary retention. It is now rarely used for this purpose. Benzpyrinium is also employed for similar purpose. • Snake venom poisoning: An antiChE may be beneficial in the management of neurotoxicity of Asian-cobra-venom. For this purpose, atropine sulfate (0.6 mg) is given IV, slowly, for blocking muscarinic action, and is followed immediately by edrophonium chloride 10 mg IV over two minutes. Edrophonium reverses oculomotor and glossopharyngeal paralysis as well as respiratory paralysis. The improvement can be maintained by a longer acting antiAchE such as neostigmine methylsulfate, given SC or by IV or infusion. The dose of edrophonium for children is 0.25 mg/kg; that of atropine is 50 mcg/kg. The pathophysiologic nature of paralysis after cobra bite is very similar to that of myasthenia gravis. Paralysis following krait bite, however, is not likely to benefit from edrophonium therapy as the Krait beta-bungarotoxin causes pre-synaptic blockage (Chapter 73). • Curare poisoning: Edrophonium is preferred to neostigmine to antagonise curare induced skeletal muscle paralysis because of its short latent period of action. A single dose of 10 mg may at times be inadequate and 2 or 3 doses may have to be administered. • Alzheimer’s disease (See Chapter 15). II Irreversible anticholinesterases: Organophosphorus compounds, the organic esters of phosphoric acid, are potent irreversible inhibitors of cholinesterase. Unlike the quaternary ammonium Anti-ChE, most of these compounds have high lipid solubility, and hence: • Are absorbed by practically all the routes including the GI tract, the intact unbroken skin, mucous membranes and lungs; and • Cross the BBB and affect the functions of the CNS. The pharmacological actions of these compounds are those of ACh which accumulates in the tissues due to prolonged inhibition of the true and pseudocholinesterases. The organophosphorus compounds are inactivated in the body almost entirely by oxidation and hydrolysis, and the end products are eliminated in urine.
Table 19.2 lists the important organophosphorus compounds. Table 19.2 Important organophosphorus compounds
Therapeutic uses: The organophosphorus compounds have limited therapeutic uses owing to high toxicity. They are used mainly as insecticides and are of toxicological importance. • Glaucoma: Echothiophate 0.06% reduces the intraocular tension; the action persists for 1 to 3 weeks. The drug, however, produces a marked ciliary spasm, browache, headache and blurring. See Chapter 72. Worm infestation: Though dichlorovas and trichlorophos have anthelmintic properties (Chapter 60), they are not used for this purpose.
Organophosphorus Compound (OPC) Poisoning Poisoning with organophosphorus compounds may be: • Occupational as in persons engaged in spraying insecticides. • Accidental e.g. by consumption of the agricultural products sprayed with these insecticides; or • Suicidal due to intentional ingestion of any of these compounds. Acute organophosphorus pesticide poisoning is an important cause of morbidity and mortality all over the world. Of these, 99% of the fatal poisonings occur in the developing countries, mostly among agricultural workers. Symptomatology: The effects of acute intoxication are: • Muscarinic effects: Localised exposure of the eyes produces miosis, spasm of accommodation, headache and conjunctival hyperaemia. Inhalation results in bronchospasm, cough and augmented bronchiolar secretions and a sense of ‘tightness in the chest’. On ingestion, the GI symptoms are the earliest to appear and consist of anorexia, nausea, vomiting, abdominal cramps, tenesmus and diarrhoea. The other muscarinic effects, including those on the eye and the respiratory system, appear subsequently. Severe bronchospasm and pulmonary edema may be fatal. • Nicotinic effects: These are characterised by fasciculations, twitching, generalised weakness and a depolarisation type of paralysis. There may be either tachycardia or bradycardia. • Central effects: These include giddiness, anxiety, confusion, ataxia, hypotension, respiratory depression, convulsions and coma. Death is usually due to paralysis of respiratory muscles and respiratory failure. The duration of effect is longest with DFP, and shorter with echothiophate and tetraethyl pyrophosphate (TEPP). It appears that even single episodes of clinically significant organophosphorus intoxication are associated with persistent decline in neuropsychological function. • Neurotoxic effects: In addition to the acute toxic manifestations, DFP and mipafox may produce delayed symptoms due to demyelination of the nerve tracts in the central and peripheral nervous systems such as the spinocerebellar tract, pyramidal tract and the sciatic nerve, producing permanent functional derangements. This is not related to ChE inhibition. It produces weakness, fatiguability, twitching and loss of tendon reflexes, leading to paralysis. Adulteration of edible olive oil with lubricating oil containing triorthocresyl phosphate (TOCP) caused thousands of deaths in a tragedy that occurred in North Africa. No treatment is available. Treatment of acute poisoning: Table 19.3 summarises the principles of treatment of acute organophosphorus poisoning.
Table 19.3 Principles of treatment of acute organophosphorus poisoning
As recommended by the WHO, “The treatment must be instituted rapidly in order to prevent a fatal outcome. In intoxication by mouth, rapid gastric lavage is imperative. For removal of secretions and maintenance of a patent airway, place the patient in a prone position with head down and to one side, the mandible elevated and the tongue pulled forward. Clear the mouth and pharynx with finger or suction. Use an oropharyngeal or nasopharyngeal airway or endotracheal intubation if airway obstruction persists. If the body is soiled with the insecticides or if vomiting or hypersalivation has occurred, clothes must be removed and the skin washed with soap and water for at least 10 minutes. Contamination of the eyes is treated by washing of the conjunctiva”. Mouth to mouth respiration is to be avoided when it is suspected that the patient has been intoxicated by mouth since vomited material may contain toxic amounts of substances. On signs of systemic absorption, both atropine and reactivators (see below) must be given parenterally. Persons without signs of respiratory insufficiency but with manifest peripheral symptoms should be treated with 2-4 mg of atropine sulfate and 1-2 g of a soluble salt of pralidoxime (P-2-AM) or 250 mg of obidoxime chloride (adult doses, see later) by slow IV injection. More atropine (with or without the reactivator) may be given depending upon the severity of the intoxication and the response to the first dose. After the administration of oximes, less atropine may be required. Atropine is effective in antagonising the central and peripheral muscarinic effects but does not modify the ganglionic action and the neuromuscular paralysis. Reactivators of cholinesterase should not be used before atropine administration, as given alone they may increase muscle weakness. Atropine should not be given to a cyanosed patient until the cyanosis has been overcome, since it may cause ventricular fibrillation. Convulsions are treated with diazepam. In severe intoxication, 4-6 mg of atropine sulfate should be given initially, followed by repeated doses of 2 mg or as much as required to maintain full atropinisation. The patient’s condition, including respiration, convulsions, blood pressure, pulse, and salivation should be monitored as a guide to further administration of atropine. Initially, atropine may have to be given at 5 or 10 minute intervals. Cases are described in the literature in which several hundred mg have been given during the first 24 hours. Usually however, it is not necessary to exceed 50 mg. Every 2 mg dose gives a short-lasting improvement in respiration, and reduction in cyanosis and convulsions. Tachycardia may occur and watch must be kept on salivary secretion in order to prevent over-atropinisation. The pulse rate should not be allowed to exceed 120/min. Because most intoxications occur after exposure of the skin or after ingestion, any
deterioration in the patient’s condition due to delayed absorption must be carefully watched for. Reactivators are excreted fairly rapidly if kidney function is normal (in the case of pralidoxime 80% in 2-3 hours) and repeated doses of 1 g may be needed. The IV injection of oximes should be made slowly, especially in small children. Recent studies from India have shown that in “moderately severe” poisoning, pralidoxime, started within two hours of ingestion of the poison, in larger than conventional doses i.e. with a loading dose of 2 g (base) by IV infusion over 30 minutes, followed by 1 g/hour by continuous IV infusion for 48 hours, is highly effective. This is followed by pralidoxime (base) 1g IV infusion every 4 hours till the patient is weaned from the ventilator. This regimen is well tolerated and mortality is low. As the iodide salt of pralidoxime loads the body with too large a quantity of iodide, it is preferable to use either the chloride or the methanesulfonate. The doses mentioned above are of the base, and therefore the molecular weight of the compound should be taken into account while deciding the quantity injected. However, extent of therapeutic usefullness of oximes, in general, is not yet well defined. After the resolution of the initial symptoms, some patients may develop the ‘intermediate syndrome’. It is characterised by flaccid, proximal paralysis. Later, after a gap of 2-4 weeks, a ‘delayed polyneuropathy’ with sensory and motor, impairment, usually of the lower limbs, may manifest. Unfortunately, there is no specific treatment. If possible, blood samples should be taken for cholinesterase determinations before and during the continued treatment. This may help to determine when a patient may return for work; subjects should not be allowed to do the same work involving organophophorus until the plasma cholinesterase level exceeds 70% of normal. This usually takes several weeks. OXIMES (Cholinesterase reactivators): The irreversible inhibition of cholinesterase produced by the OPC is due to phosphorylation of the esteratic site of the enzyme (Fig. 19.3). Oximes combine with the phosphoryl groups of these phosphorylated esteratic sites forming soluble complex. This results in setting free the esteratic site and a reactivation of the enzyme. The oximes are particularly effective in reversing the neuromuscular paralysis where atropine is ineffective. Their effects on the autonomic ganglia and the CNS are not significant, except probably in the case of DAM which crosses the blood brain barrier. These compounds are effective only when administered within a short time after poisoning. Late administration fails to produce the expected results as by this time the phosphoryl bound with the enzyme gets more stabilised (ageing of the enzyme). Oximes are mainly metabolised in the liver. Adverse reactions: The oximes are not free from toxicity. They may produce local irritation, drowsiness, giddiness, blurred vision, diplopia, tachycardia and hypotension. Pralidoxime has weak anti-ChE activity and hence is contraindicated in the treatment of overdosage with neostigmine or physostigmine. It also does not antagonise the effects of carbamate type (Baygon/Carbaryl) of anticholinesterases (Chapter 62). High doses of oximes can themselves cause neuromuscular blockade. Preparations and dosage: (i) Pyridine-2 aldoxime chloride (P-2-AM, Pralidoxime; Fig. 19.5) by IV infusion (see earlier details). The dose can be repeated after 1-2 hours. It can also be given IM. It is also available for oral use.
FIG. 19.5
(ii) Diacetylmonoxime (DAM; Fig. 19.5) administered by IV infusion. It crosses the blood brain barrier. (iii) Obidoxime chloride is more potent than pralidoxime. It is given IV. The dose can be repeated every 20 minutes.
20
Muscarinic Receptor Blocking Drugs; Pharmacotherapy of Bladder Dysfunction Cholinergic – muscarinic receptor blocking drugs include atropine, and related alkaloids obtained from the plants Atropa belladonna (the deadly nightshade), Atropa acuminata, Hyoscyamus niger (henbane), Scopola carniolica and Datura stramonium (Datura), and synthetic or semisynthetic atropine substitutes. They block the muscarinic actions of ACh; the ganglionic and skeletal neuromuscular actions of ACh are not affected.
Belladonna Alkaloids The name Atropa belladonna represents a paradox. For whereas Atropos is the seniormost of the Three Fates who performs the inglorious function of cutting the thread of life, the term belladonna (pretty lady) is derived from the practice by Venetian court beauties of putting the extract of these plants in the eyes, to dilate the pupils and make the eyes look bigger and more seductive. Belladonna preparations were known to ancient Hindus for many centuries. The two important alkaloids of belladonna are atropine (dl-hyoscyamine) and scopolamine (hyoscine). Atropine is an ester of an aromatic organic acid ‘tropic acid’ with a complex organic base ‘tropine’ while hyoscine is an ester of tropic acid with another base ‘scopine’ (Fig. 20.1).
FIG. 20.1A Atropine
FIG. 20.1B Scopolamine
Mechanism of action: The belladonna alkaloids block both the peripheral and central muscarinic effects of ACh. The antagonism between ACh and atropine is of competitive type which can be reversed by an increase in the concentration of ACh at the muscarinic neuroeffector site. Atropine is more effective in blocking the effects of externally administered ACh than the effects of cholinergic nerve stimulation. The dose of the drug required to produce muscarinic blockade varies from organ to organ. Thus, salivary and bronchial secretions are extremely sensitive to atropine blockade, while the heart, the smooth muscle of the GI tract, gastric acid secretion and the eye, are less affected even after relatively large doses. In the CNS, cholinergic transmission at subcortical and cortical levels is predominantly muscarinic (M1) and can be blocked by atropine. Although atropine can completely abolish the effects of choline esters on the GI tract, it
does not completely abolish the effects of vagal stimulation. This is so because it does not block noncholinergic, neurohumoral transmitters nor responses to GI peptide hormones. In large doses, atropine blocks the nicotinic action of ACh at the autonomic ganglia; it is more marked following atropine substitutes containing the quaternary ammonium (-NH4) ion. Pharmacological actions: The pharmacological actions of atropine and scopolamine (hyoscine) are qualitatively similar except that atropine is a central nervous system stimulant while scopolamine is a central depressant and can act as a sedative. Scopolamine has more prominent effects on the iris, the ciliary body and the salivary, bronchial and sweat secretions while atropine is more active on the heart, the gut and the bronchial smooth muscle. Atropine has a longer duration of action than scopolamine. Secretions: The secretions of the exocrine glands except the production of milk are reduced. • Salivary secretion: Atropine blocks the watery salivary secretion giving rise to dryness of mouth and difficulty in swallowing. • Gastric secretion: Atropine reduces the volume and the total acidity of gastric secretion. The secretion of acid without any food in the stomach (interdigestive secretion) is significantly diminished. The cephalic, gastric and intestinal phases of gastric secretion are, however, blocked partially. It also reduces the secretion of mucin and gastric enzymes. • Other secretions: It reduces the secretions in the nose, mouth, pharynx and bronchi. The bronchial secretions may become viscid. It inhibits the sweat secretion but does not produce a striking inhibition of the lacrimal secretion. Atropine has little effect on the pancreatic and intestinal secretions. Smooth muscle: • Gastrointestinal tract: Atropine reduces both the tone and the motility of the gut, and acts as an antispasmodic. It antagonises the spasmogenic action of morphine on intestine and can also completely abolish the excessive motility induced by the cholinergic agents. It is, however, only partially effective in blocking the effects of vagus nerve stimulation and it does not interfere significantly with normal peristalsis. • Biliary tract: Atropine exerts a weak antispasmodic (relaxant) action on the biliary tract and the gallbladder. • Urinary tract: Atropine produces reduction in normal as well as in drug induced ureteral peristalsis. Therapeutic doses reduce the tone of the fundus of the bladder and enhance the tone of the trigonal sphincter and hence can cause urinary retention. • Bronchi: Atropine relaxes the smooth muscles of the bronchi and bronchioles. It is particularly effective in relieving broncho-constriction produced by cholinergic agents but is much less potent than adrenaline in relieving histamine induced bronchoconstriction. Since it dries up the secretions, it is not recommended in the treatment of bronchial asthma. • Uterus: Atropine and scopolamine have no significant effect on the uterine muscle. Eye: On local instillation, atropine produces: (i) Mydriasis by blocking the cholinergic nerves supplying the smooth muscle of the sphincter of the iris. Photophobia is manifested in response to bright light because of the sphincter paralysis.
(ii) The ciliary smooth muscle is paralysed by atropine. This produces a tightening of the suspensory ligament resulting in flattening of the lens with a consequent increase in its focal length. The individual, therefore, is able to see things clearly only at a distance owing to fixing of lens for far vision. This phenomenon is termed as paralysis of accommodation or cycloplegia. Atropine is thus both a mydriatic and a cycloplegic drug. Local instillation of 1% atropine drops produces maximum mydriatic response within 30 to 40 minutes and recovery occurs within 7 to 10 days. Maximum cycloplegia with atropine is seen within 1 to 3 days; it persists for 7 to 11 days. Atropine mydriasis can be distinguished from the mydriatic effect of sympathomimetic amines as the latter do not produce cycloplegia. Atropine does not alter the intraocular tension in the normal eye but in individuals with shallow anterior chamber and in those with narrow angle glaucoma, a precipitous increase in intraocular tension may occur. It is due to relaxation of the ciliary muscle and crowding of the iris in the angle of the anterior chamber interfering with the drainage of aqueous humour. Cardiovascular system: Atropine, in therapeutic doses, may initially decrease the heart rate owing to stimulation of the medullary vagal nuclei, followed by tachycardia, particularly in young individuals who have a high vagal tone and in whom the heart rate may increase by 30 to 40 beats per minute. Tachycardia is due to blocking of M2 receptors in the SA nodal pacemaker. This action is sometimes not observed in old people and in infants probably because of the lower vagal tone. Atropine abolishes the effects of cholinergic agents on the heart rate and also the bradycardia induced by manoeuvres like carotid massage and pressure on an eyeball. In therapeutic doses, atropine completely counters the vasodilatation and hypotension produced by cholinergic agents. However, by itself it has insignificant effect on the vasculature and does not modify the BP. Toxic doses cause dilatation of the cutaneous blood vessels resulting in ‘atropine flush’ and hypotension. Central nervous system: Atropine, causes a mild stimulation of the medullary vagal nuclei and higher cerebral centres. This occasionally produces bradycardia and an increase in the rate and depth of respiration. Respiratory depression produced by toxic doses of anticholinesterases can be antagonised appreciably by atropine. In moderate doses, it controls the tremors and rigidity in parkinsonism. In toxic doses, it produces marked excitation. In contrast to atropine, therapeutic doses of scopolamine depress the reticular activating system, and usually produce euphoria, drowsiness, amnesia and dreamless sleep which lasts for 1 to 2 hours. Scopolamine owes its salutary effect in motion sickness to its direct action on the vestibular function (Chapter 41). Absorption, fate and excretion: The belladonna alkaloids are satisfactorily absorbed from the GI tract, from parenteral sites and from mucous membranes. The absorption from the eye and intact skin, however, is not significant. Atropine is partly detoxified in liver and partly excreted unchanged by kidneys. It has plasma t½ of 4 hours. Approximately 50% of the parenterally administered drug appears in the urine in free form within 24 hours while 33% is excreted as metabolites. Scopolamine is mostly metabolised.
Atropine crosses the placental barrier and is secreted in milk and saliva. Animals such as rabbits having atropine esterase in their plasma and liver, and rodents detoxify atropine faster than human beings and can tolerate large doses without toxicity. Adverse reactions: Majority of these are due to extension of its pharmacological actions. Infants and young children are particularly susceptible to the CNS toxicity of muscarinic receptor blocking drugs. Mild reactions include dryness of mouth, xerostomia, flushing and constipation. Atropine can precipitate glaucoma and urinary retention, especially in the elderly. Locally, atropine can give rise to allergic reactions such as dermatitis, conjunctivitis and swelling of eyelids. • Acute atropine poisoning: Atropine has a wide margin of safety. The lethal dose of atropine is not known but is believed to be 10-20 mg in children and 80-130 mg in adults, although survival after doses over 200 mg in adults has been reported. Scopolamine is claimed to be more toxic than atropine. Poisoning may also occur following ingestion of leaves or seeds of Datura species or berries of solanaceous plants. The symptoms and signs are due to: (a) Peripheral muscarinic blockade. Dryness of mouth, difficulty in swallowing, intense thirst, tachycardia, palpitation, flushing, hyperpyrexia due to inhibition of sweating, dilatation of pupils, blurred vision and photophobia are the cardinal manifestations. Difficulty in micturition and urinary retention may occur due to spasm of the trigone. A rash may appear over the face, neck and upper part of the trunk, leading to desquamation of the skin; and (b) Central effects which are attributed to initial stimulation and subsequent depression of the CNS. The patient shows excitement, restlessness, motor incoordination, slurring of speech, disturbance of memory, confusion, hallucinations, and occasionally mania and delirium. Nausea and vomiting are infrequent. Severe poisoning depresses the vasomotor centre, leading to vasomotor collapse. Depression following initial excitement occurs more quickly with scopolamine. Belladonna poisoning may be diagnosed by adding a drop of patient’s urine into a cat’s eye, where it will produce pupillary dilatation. However, absence of such dilatation does not exclude belladonna poisoning. Dry mucous membranes, dilated nonreacting pupils, flushing, rash, fever and rapid feeble pulse seen in atropine poisoning may sometimes be mistaken for an exanthematous fever. The other drugs which cause a syndrome like acute atropine intoxication include drugs possessing muscarinic blocking action such as: tricyclic antidepressants, phenothiazine antipsychotics and the anti-histaminics promethazine and diphenhydramine. Treatment: If the poison has been ingested, prompt attempts to remove it by gastric lavage should be made. Alkaloidal inactivators like universal antidote should be administered before and after gastric lavage. The muscarinic effects can be countered by administration of slow IV physostigmine 1-4 mg (0.5-1 mg in children) or neostigmine 2 to 5 mg SC. The drugs may be repeated at intervals of 1-2 hours till satisfactory control over muscarinic blockade is established. Physostigmine is preferred in patients with CNS symptoms as it crosses the BB barrier. Restlessness and delirium may be treated with diazepam but this drug may augment the respiratory depression seen in later stages of atropine intoxication. A dark room to alleviate photophobia, catheterisation for urinary retention, tepid sponging for
pyrexia, good nursing care, oxygen and artificial ventilation when necessary, constitute the supportive treatment. • Chronic use of belladonna is manifested by dryness of mouth, skin eruptions, tremors and speech disturbances. Preparations and dosage: (i) Atropine sulfate 0.5 mg tablets. Dose: 0.25 to 2 mg. (ii) Atropine sulfate injection 0.5 mg in 1 ml. Dose: 0.25 mg to 2 mg SC or IM. (iii) Atropine eye ointment 1%. (iv) Atropine methonitrate a 0.6% alcoholic solution in the dose of 0.2-0.6 mg. (v) Hyoscyamus tincture. Dose: 2 to 4 ml. (vi) Hyoscine (Scopolamine) injection 0.4 mg in 1 ml. Dose: 0.3 to 0.6 mg by SC injection. (vii) Hyoscine hydrobromide tablets 0.3- 0.6 mg. Dose 1-2 tablets 4 times daily. (viii) Hyoscine -N-butylbromide: Tablets 10 mg; Injection 20 mg/ml. Dose: 10-20 mg 3-4 times a day orally; 20 mg IM. (ix) Hyoscine hydrobromide skin patch 1 mg (TTS) releases the drug over 72 hrs. It is to be applied behind the ear 5-6 hours before journey and repeated after 72 hours, if required. Therapeutic uses of atropine and scopolamine: • Gastrointestinal colic: Atropine is used as intestinal antispasmodic to control colicky pain. Constipation due to spastic state of the bowel may be relieved after atropine. It also controls spasticity induced by lead and morphine. • Other colics: Atropine is usually administered along with morphine in the treatment of biliary colic. Morphine tends to increase the intra-biliary pressure and this effect is countered by concomitant atropine administration. Atropine-morphine combination is also used for relief of renal colic. Atropine and its substitutes are often used to allay the frequency and urgency of micturition accompanying cystitis. They act probably by increasing the capacity of the bladder as a result of its relaxant effect on the bladder wall. Frequency of micturition associated with paraplegia is also controlled with atropine. • Ocular conditions: Atropine is used to produce mydriasis and cycloplegia. Mydriasis is necessary for a fundoscopic examination and in the treatment of acute iritis, iridocyclitis and keratitis. Atropine reduces pain in these conditions by relaxing the inflamed musculature of iris and the ciliary body. It may be instilled into the eye alternately with miotics to break the adhesions between the iris and the lens or the cornea. It also reduces the chances of adhesion formation. • As pre-anaesthetic medication: See Chapter 7. Contrary to popular belief, atropine does not directly abolish the laryngospasm during anaesthesia but prevents its development by reducing the respiratory secretions. It is administered at least 30 minutes before general anaesthesia. Use of atropine with non-irritant, volatile general anaesthetics may produce unpleasant sore throat postoperatively. Similarly, reduction in the bronchial secretion can lead to inspissation of the residual secretions and formation of thick bronchial plugs. It blocks vagal reflexes induced by surgical manipulation of viscera. • Organophosphorus poisoning: See Chapter 19. Atropine is also useful in early mushroom poisoning due to muscarine. • Parkinsonism: See Chapter 15, for atropine substitutes in parkinsonism. • Cardiovascular conditions: Atropine may be useful in abolishing A-V block due to
excessive vagal activity. It is also occasionally useful in countering the syncope and bradycardia due to hypersensitive carotid sinus. • Urinary incontinence: Various synthetic substitutes like dicyclomine, oxybutynin and propantheline have been used to reduce unstable detrusor contractions (See later). • Motion sickness: Scopolamine hydrobromide in the dose of 0.5 to 1 mg by mouth is used in the treatment of motion sickness. Dryness of mouth signifies the onset of effect and the protection conferred lasts for 4 to 6 hours. The greatest advantage of the drug is that, when given orally in above doses, it has only a slight sedative effect. Scopolamine is admirably suited to control motion sickness during short journeys. In the event of prolongation of the journey, the drug may be repeated at intervals of 2 hours in the dose of 0.1 mg. Larger doses may produce excessive sedation. Atropine is much less effective than hyoscine. Hyoscine can also be given by transdermal route (Chapter 41). • Peptic ulcer: Anticholinergics including pirenzepine are now rarely used (Chapter 43). For many of the above-mentioned conditions, atropine derivatives and substitutes are now preferred. Atropine has retained its therapeutic place only in organophosphorus compound poisoning and treatment of A-V block. Relative contraindications to atropine therapy: Atropine should be administered with caution in: (a) Patients over the age of 40, as it may precipitate an attack of acute congestive glaucoma. (b) Individuals with enlarged prostate, as retention of urine may develop. (c) Chronic lung conditions as it may reduce the secretions and produce drying, and (d) Congestive heart failure with tachycardia
Synthetic and Semisynthetic Atropine Substitutes The need for atropine substitutes arises mainly because of the lack of its selectivity in action. Thus, the dose of atropine required to produce the therapeutic effects on GI tract invariably produces many adverse effects. Drugs have been synthesised, therefore, to produce more therapeutic selectivity. Chemically, atropine substitutes are: I The quaternary ammonium compounds which are not satisfactorily absorbed orally, do not cross the BBB, and generally exhibit a greater degree of nicotinic (ganglionic) blocking action; and II The tertiary amine drugs which are better absorbed orally, cross the BBB, and act more selectively at the muscarinic receptors. These substitutes are employed mainly for their predominant actions: I As mydriatics and cycloplegics in the eye. II As antispasmodics (particularly for GI and urinary bladder muscles) III In parkinsonism. IV As bronchodilators in COPD and bronchial asthma (Chapter 27); and V As preanaesthetic medication I Mainly Used in Eye (Chapter 72). II Mainly Used as Spasmolytics: These atropine substitutes are mainly used in the treatment of colics. The quaternary ammonium atropine substitutes which do not cross the blood brain barrier are relatively free from the central effects. They are more liable to produce ganglionic blockade, resulting in impotence, postural hypotension and urinary retention. They may also block neuromuscular transmission by a curarimimetic action. ATROPINE METHONITRATE: Besides its ophthalmic use, it is administered orally in the dose of 0.2 to 0.4 mg 4 to 6 times a day to treat congenital hypertrophic pyloric stenosis. METHSCOPOLAMINE BROMIDE: This quaternary ammonium compound is devoid of the central effects of scopolamine and is used in the treatment of renal colic and frequency of micturition associated with cystitis. Dose: 2 to 5 mg orally, three times a day, or parenterally in the dose of 0.25 to 1 mg. Hyoscine-N-butyl bromide has potent smooth muscle relaxant action and similar use. METHANTHELINE: It is a synthetic quaternary ammonium compound with a high ratio of ganglion blocking to muscarinic blocking activity. The GI effects and the duration of action of this drug are greater than those of atropine. The drug may produce impotence, postural hypotension, urinary retention and neuromuscular blockade. Rarely, it can cause restlessness and acute psychosis. PROPANTHELINE: It is related to methantheline and possess more potent ganglionic and muscarinic blocking actions than methantheline. It has been used as a muscle relaxant in irritable bowel syndrome and for relieving pain of diverticulitis. It is administered orally in the dose of 15 mg tid. OXYPHENONIUM: This quaternary ammonium compound has a higher ratio of ganglion blocking to antimuscarinic activity than majority of other synthetic atropine substitutes. It is a potent antispasmodic. The usual dose is 10 mg orally. DICYCLOMINE: This tertiary amine has weak antimuscarinic effect but, it is a direct relaxant for GI smooth muscles. Dicyclomine hydrochloride is available as 10 mg tablets and as a syrup containing 10 mg per 5 ml. Dose : 10-20 mg tid. Smaller doses are used in
children. It is used in dysmenorrhoea and diarrhoea predominate irritable bowel syndrome. FLAVOXATE: This tertiary amine has a direct relaxant action on smooth muscles especially of urinary tract. It also possesses weak anticholinergic, local anaesthetic, antihistaminic and analgesic properties. It is used to treat dysuria, nocturia, suprapubic pain, and urinary urgency and frequency associated with cystitis, prostatitis and urethritis. The dose is 100-200 mg 3-4 times a day. OXYBUTYNIN: This compound has antimuscarinic (selective M3), direct antispasmodic and some local anaesthetic effects on the urinary bladder. It has been used in the treatment of urinary urgency, urge incontinence and urinary frequency except that following transurethral surgical procedures. The oral dose in adults in 5 mg tid. However, incidence of ADR are more. Hence topical gel formulations and transdermal system have been developed. TROSPIUM is a nonselective antimuscarinic with efficacy similar to those of oxybutynin. It is mainly excreted unchanged by kidney. Dose: 20 mg bid. TOLTERODINE: This tertiary amine acts as a selective M3 receptor antagonist and has been used to treat bladder detrusor instability in the treatment of urinary incontinence. It is metabolised in the liver and the metabolites possess similar actions as the parent compound. Dose: 2 mg bid. Fesoterodine is a prodrug of tolterodine. Some of the other atropine substitutes available for therapy are homatropine methylbromide, and drotavarine. Many of these quaternary antimuscarinic drugs in tolerated doses are claimed to be superior to atropine. Glycopyrrolate IV is used as preanaesthetic medication (See below). III Used in parkinsonism: See Chapter 15. IV Used in chronic bronchitis, COPD and bronchial asthma: Ipratropium bromide and Tiotropium (Chapter 27). V As preanaesthetic medication: Glycopyrrolate, a quarternary ammonium compound, is structurally unrelated to atropine but reduces GI tone and motility. Given IV, it decreases salivary secretion. Tachycardia is lesser than atropine. It is also used to abate cholinergic effects of neostigmine, when the latter is given post operatively to reverse effects of skeletal muscle relaxant.: Table 20.1 lists some drugs which may cause antimuscarinic side effects when used in therapy. Table 20.1 Some drugs with antimuscarinic side effects
Bladder Dysfunction – Pharmacology Normal urinary bladder function and the act of urination (micturition) are well coordinated, involving several neurotransmitters and neuromodulators. Normal urination involves higher centres (pons), spinal cord and peripheral autonomic, somatic and afferent sensory innervation of the lower urinary tract. Disorders of any of these can contribute to the symptoms of bladder dysfunction. Acetylcholine, by acting on the M3 receptors, causes detrusor muscle contraction while adrenergic activity causes its relaxation through the stimulation of β3 adrenergic receptors. Stimulation of dopamine D1 receptors suppresses bladder activity, whereas D2 stimulation facilitates voiding. Other neurotransmitters such as GABA, encephalin, glutamate and 5HT are also involved in the regulation of bladder function. The internal sphincter at the bladder neck is rich in α1 adrenoreceptors; their activation causes sphincter contraction; their inhibition by α1 antagonists is helpful in BPH (Chapter 69). The common functional abnormalities of micturition that call for treatment are: (1) Overactive bladder (2) Hypotonic bladder and (3) Nocturia/nocturnal enuresis. Drugs used in disturbances of bladder function are summarised in Table 20.2.
Table 20.2 Drugs used in bladder dysfunction
*
Long acting and transdermal estrogen preparations are available.
**
0.5–1.0 g/day for first 2 weeks; then twice weekly for 4 weeks.
• Overactive bladder, a symptom complex, includes urinary urgency with or without urge incontinence, urinary frequency and nocturia (awakening more than once at night to void urine). It is observed in the elderly with prevalence rate of 30-40% in subjects over 75 years of age and is often embarrassing to the subject. There is no identifiable cause nor any local abnormality. Among the types of chronic urinary incontinence, urge incontinence is the most common. It is characterised by sudden urgency with leakage of urine, and may be associated with increased frequency and nocturia. Stress incontinence; the next common type, is more common in the older females. It is associated with weakened pelvic floor muscles and consequent increased mobility of the bladder outlet and urethra. Leakage of urine occurs with increased abdominal pressure as during coughing, sneezing and laughing. The non-pharmacological treatment of these conditions includes education about bladder function in cognitively intact, motivated patients, pelvic floor exercises and bladder training. The latter involves teaching of urge suppression and regular, scheduled voiding. This is more important than drugs. Some patients can be helped by biofeedbackassisted training. Functional urinary frequency, urgency and incontinence are treated with antimuscarinic
drugs such as oxybutynin, flavoxate, tolterodine and trospium. They act by inhibiting the involuntary detrusor contractions and thus increasing the bladder capacity. They can be used for both neurogenic and non-neurogenic overactive bladder. These drugs are also useful in detrusor muscle spasm associated with cystitis due to bladder infection. However, they can cause anticholinergic side effects. Solifenacin and darifenacin are the new drugs, claimed to be relatively ‘bladder specific’ in their antimuscarinic action, (M3 antagonist) for use in overactive bladder. However, there is no convincing evidence of their superiority over the older drugs. Anticholinergics should be used cautiously in elderly patients with dementia. Further, obstruction due to enlarged prostate, carcinoma of the prostate and bladder stone must be ruled out. Mirabegron is beta-3 adrenergic agonist used for overactive bladder. It causes relaxation of detrusor smooth muscle and increases bladder capacity. It may cause GI symptoms, dizziness, tachycardia and rarely, rise in BP and urinary retention. It is moderate inhibitor of CYP2D6. Onabotulinum toxin A (Botox) is another drug used as single intradetrusor injection, to treat overactive bladder (Chapter 22). Post-synaptic α1 adrenergic blockers such as terazosin and tamsulosin are used to treat urinary frequency due to BPH (Chapter 69). • Bladder hypotonicity which sometimes causes urinary retention may be treated by cholinergic drugs such as bethanechol, neostigmine or distigmine (Chapter 19). Urinary retention due to BPH, however, will be worsened by these drugs. • Nocturia, a symptom of overactive bladder, can be due to detrusor overactivity, nocturnal polyuria or a primary sleep disorder-nocturnal eneuresis. Antidepressant imipramine has anticholinergic action, and possible central effect on voiding reflexes. It is useful in some patients with stress-urge incontinence and nocturnal eneuresis (Chapter 14). Finally, it should be remembered that the distal part of the female urethral epithelium possesses estrogen receptors. Their degeneration after menopause can cause urinary incontinence. Such patients may be helped by topical (vaginal) or oral estrogen therapy.
21
Ganglion Stimulating and Blocking Drugs The ganglion stimulating agents have hardly any place in therapeutics. The important agents belonging to this group are the alkaloids nicotine and lobeline. In addition, synthetic compounds like tetramethylammonium (TMA) and dimethylphenylpiperazinium (DMPP) are mainly used as experimental tools. Although nicotine has no therapeutic utility, it is the important constituent of tobacco. Lobeline is described in Chapter 12. NICOTINE Nicotine was isolated from leaves of the tobacco plant, Nicotiana tabacum in 1828. It is a tertiary amine consisting of a pyridine ring and a pyrolidine ring. The actions of this compound, particularly its effects on autonomic ganglia, were studied by Langley and Dickinson in their classical experiments in 1889. Mechanism of action: Nicotine acts as an agonist at cholinergic receptors present in the brain, autonomic ganglia and neuromuscular junctions, which explains its multiple effects. Nicotinic receptors are of two types: (1) Nn in the CNS and the autonomic ganglia; and (2) Nm in the skeletal neuro-muscular junction. Activation of nicotinic receptors facilitates the release of ACh, NA, DA, 5-HT and betaendorphin in the CNS. Nicotine also causes release of growth hormone, prolactin and ACTH. Pharmacological actions: The amount of nicotine absorbed during smoking is sufficient to produce measurable pharmacological and psychopharmacological effects. • Behavioural effects: These vary according to the species and the dosage used and are probably linked to DA release. High concentration of nicotinic-ACh receptors is present in the mesolimbic system, also known as the reward centre of the brain. In small doses, the effects are predominantly stimulant and may improve attention, learning, reaction time and problem solving. Smokers often report pleasure and relaxation, and reduction in anger, tension, depression and stress. However large doses cause mental depression. • Central Nervous System: Nicotine stimulates the CNS and produces tremor, while large doses may produce convulsions. Small doses reflexly stimulate respiration through aortic and carotid body chemoreceptors; while large doses directly stimulate the medullary respiratory centre. The stimulation of respiration is usually followed by depression and paralysis. Vomiting induced by nicotine is due to stimulation of the CTZ and the sensory nerve endings involved in mediation of the vomiting reflex. Nicotine, by stimulating the supraoptic nuclei of the hypothalamus, induces the release of ADH and exerts an antidiuretic effect. In sensitive individuals, this effect may become apparent after smoking 2 or 3 cigarettes. • Autonomic ganglia: Like ACh, nicotine initially stimulates the autonomic ganglia leading to stimulation of post-ganglionic nerves. However, large doses of nicotine produce persistent depolarisation of these ganglionic cell bodies resulting in ganglionic blockade. In still larger doses, nicotine, by competing with ACh for ganglionic receptor sites, also produces a competitive block. Nicotine in
large doses, therefore, paralyses the autonomic ganglia by a dual mechanism. • Adrenal medulla: This, anatomically and embryologically a sympathetic ganglion, is initially stimulated by nicotine, leading to discharge of adrenaline into the blood. Larger doses block the secretory response to splanchnic nerve stimulation. The cardiovascular actions of nicotine in an intact animal usually vary according to preponderance of sympathetic or parasympathetic stimulation. The most consistent effects of smoking in humans are an increase in the heart rate and peripheral vasoconstriction. Carotid chemoreceptors are very sensitive to low levels of nicotine. Blood pressure may rise and an increase in skeletal muscle and coronary blood flow may occur. Cardiac work and O2 consumption are increased. These effects are due to increased release of catecholamines following stimulation of the sympathetic ganglia and adrenal medulla. Tolerance to increase in the heart rate develops fairly rapidly. It increases the platelet adhesiveness. • GI tract and secretions: Nicotine increases the motility, tone and secretion of the GI tract and may cause colonic evacuation due to stimulation of the parasympathetic ganglia. Stimulation is usually followed by decrease in motility and tone, leading to constipation. Nicotine initially increases the salivary and bronchial secretions, followed by their inhibition. Salivation accompanying smoking, however, is due to irritant nature of the smoke rather than its nicotine content. • Myoneural junction: At the myoneural junction, nicotine produces a transient depolarisation of the motor end plate, resulting in a stimulation of skeletal muscles and twitchings. In large doses, this stimulant effect is followed and often overshadowed by blockade of myoneural transmission. It is usually the cause of death in nicotine poisoning. It must be pointed out, however, that nicotine has a much more prominent effect on the autonomic ganglia than on the myoneural junction. • Liver enzymes: Nicotine induces hepatic microsomal enzymes. Inhalation of cigarette smoke enhances the metabolism of several drugs including nicotine in man. • Metabolic effects: Nicotine increases the metabolic rate slightly at rest but almost doubles it during light exercise. The increase in BMR, along with appetite suppression, leads to weight loss. Nicotine, via catecholamine release, causes lipolysis and release of free fatty acids (FFA) into the circulation. Excessive release of cortisol following nicotine could affect mood and may contribute to osteoporosis. • Miscellaneous: Intradermal injection or local application of nicotine produces sweating and vasoconstriction in the area treated. This effect is attributed to cutaneous axon reflexes mediated by sympathetic nerves and serves as a basis for testing the integrity of the postganglionic sympathetic fibres. It is blocked by local anaesthetics, atropine and ganglionic blocking agents. Tolerance and dependence: One third to one half of occasional smokers develop physical dependence. Most tobacco-dependent persons never achieve lasting abstinence and one half of all smokers die prematurely of tobacco-related diseases. A regular smoker is able to withstand large amounts of nicotine in contrast to a non-smoker. The addictive effects of tobacco smoking are largely due to nicotine. Withdrawal syndrome manifests as craving, depression, nervousness, restlessness, irritability, anxiety, impaired concentration, increased appetite and weight gain. Absorption, fate and excretion: Nicotine is well absorbed from all mucous membranes
and even from intact, unbroken skin. It is concentrated in the liver, lungs and the brain. At physiologic pH, about 30% of nicotine is unionised and can cross cell membrane readily. A major portion is metabolised mainly in liver by CYP2D6 to inert cotinine which is then glucuronidated and excreted by the kidney. Its t½ is 2 hours. An acidic urine enhances the excretion of ionised nicotine. Nicotine may be secreted in the milk. Adverse reactions: Acute nicotine poisoning occurs in workers engaged in spraying nicotine as an insecticide. Nicotine poisoning may occur in children from accidental ingestion of cigarettes. Nicotine is much less toxic when swallowed in the form of tobacco than when ingested in pure form. It is, however, one of the most toxic agents and can produce death with the rapidity of cyanide. Acute nicotine poisoning is characterised by nausea, salivation, vomiting, abdominal pain and diarrhoea. Dizziness, headache, confusion and marked weakness develop. The pupils initially constrict but dilate subsequently. The initial rise in blood pressure is followed by a fall and initial bradycardia is followed by tachycardia. Cold sweat is a prominent feature. Respiration, after brief stimulation, becomes irregular. Convulsions may appear in the later stage and death occurs from respiratory paralysis. The treatment consists of gastric lavage with 1:10,000 solution of potassium permanganate. As nicotine is rapidly metabolised in the body, attempts should be made to tide over the crisis by symptomatic therapy. Paralysis of respiration should be treated by artificial ventilation with oxygen. The use of analeptics for respiratory paralysis is dangerous. Chronic nicotine toxicity and tobacco smoking: Tobacco, the dried leaf of Nicotiana tabacum, is used in various forms as snuff, as plug for chewing and for smoking. The nicotine content of tobacco varies from 0.5 to 8%. Large quantities of nicotine are absorbed from the inhaled smoke and a person who ‘drags’ on his cigar or cigarette absorbs larger quantities. A cigarette contains 6-11 mg of nicotine, of which the smoker typically absorbs 1-3 mg irrespective of the nicotine yield rating provided by the tobacco companies. The other ingredients of tobacco smoke include tar, pyridine, volatile acids, furfural, carbon monoxide and carcinogens. They also contribute to the adverse effects of tobacco smoking. Smoking and tobacco chewing are believed to be either causative or exacerbating factors in a number of conditions including cancers. Cigarette smoking is associated with increase in morbidity and a shortening of life expectancy. The latter is related to the magnitude of cigarette consumption. It is higher in older than in younger persons, is seen especially in those who start smoking early in life and in those who inhale the smoke deeply. The increase in mortality is lower in those who give up smoking than in those who continue to smoke. Most of the increase in mortality is from cancer of various organs, chronic bronchitis, emphysema and ischemic heart disease. • Carcinoma of the lung: The risk of carcinoma of the lung is 15-20 times higher in cigarette smokers than in non-smokers. This is especially so when cigarette smoking is combined with inhalation of asbestos dust, chromates, nickel, arsenic or radioactive material. Tobacco smoke contains several cancer initiators (carcinogens) and cancer promoters (co-carcinogens); all have not been identified but the best known is benzapyrene. Experimental cancer of the skin has been produced in animals by application of condensates of tobacco smoke and dogs who inhaled cigarette smoke through tracheostomies developed cancer of the lung.
• Chronic bronchitis and emphysema: The incidence of chronic bronchitis is higher in habitual smokers than in non-smokers. A smoker ’s respiratory syndrome characterised by cough, dyspnoea, wheezing, pain in chest and frequent infection of the upper respiratory tract has been described. Tobacco smoke contains a number of irritants which cause bronchoconstriction, damage to the ciliary epithelium and hypertrophy of the mucus glands. Prolonged, heavy smoking is the main cause of chronic obstructive pulmonary disease (COPD) which has two components, chronic bronchitis and emphysema. Emphysema is due to hyperinflation of the lungs coupled with destruction of the alveolar walls causing loss of elastic tissue. When younger smokers give up smoking their lung function can return to normal. However, when older persons with established chronic bronchitis and emphysema stop smoking, the benefit is less dramatic. • Ischaemic heart disease: Mortality from ischaemic heart disease and the frequency of angina pectoris are more in smokers than in non-smokers. Further, atherosclerotic changes are more extensive in smokers than in non-smokers. These effects are probably due to release of catecholamines from the adrenal medulla by nicotine. Catecholamines increase (a) platelet adhesiveness; (b) the concentration of blood lipids, and (c) the tendency to hypertension and cardiac arrhythmias. Further, smoking increases the concentration of carboxyhemoglobin in the blood. Thrombosis is an important mechanism of smoking-induced cardiac events. • Peripheral vascular disease: Nicotine causes prolonged vasoconstriction in the hands and feet. Thromboangiitis obliterans (TAO) occurs far more frequently in smokers than in non-smokers and intermittent claudication is more frequent in elderly atherosclerotic smokers. • Tobacco amblyopia: Tobacco amblyopia which usually causes a gradual, but occasionally sudden, decrease in the visual acuity, particularly in the central field, is attributed to a spasm of the retinal blood vessels caused by nicotine. Fortunately, it is rare. Smoking may increase the IOP in glaucomatous patients. • GI disturbances: Nicotine causes excessive salivation, aggravation of peptic ulcer disease and constipation. Increased incidence of cancers of the mouth, larynx and esophagus has been reported in association with smoking and tobacco chewing. • Miscellaneous: Women who smoke heavily are at risk for a premature menopause. Those who smoke during pregnancy have a higher incidence of abortions and give birth to babies with low birth weights. It is an important cause of impotence in men and infertility in women. Smoking is often claimed to have a tranquillising effect but many people feel better both physically and mentally after giving up smoking. The polycyclic hydrocarbons present in the cigarette smoke accelerate the metabolism of many drugs by inducing hepatic microsomal enzymes (Chapter 3). Prevention of tobacco use in any form is important. In this respect, education of children in the concept of positive health and in the harmful effect of smoking on health is of prime importance. This must be done as much by practice as by preaching since children tend to imitate the grown-ups around them and cannot be expected not to smoke as long as they see people, particularly family members, smoking or chewing tobacco all around them.
Doctors can help by inquiring about the smoking habits of all their patients, by educating them about its harmful effects, by strongly advising against smoking and by not smoking themselves! Pharmacotherapy of tobaco dependence may be necessary to reduce craving for nicotine during abstinence period and to inhibit reinforcing effects of nicotine during smoking. Tobacco produces both psychological and physical dependence. Abrupt cessation of heavy smoking may cause irritability and drowsiness. The dependence is likely to be mediated by mesolimbic Nn receptors and α4 β2 mediated DA release. The latter inhibits reinforcing effects of nicotine. Table 21.1 lists the drugs used in treating tobacco dependence. Psychotic and neurotic traits are found consistently more often in smokers than in non-smokers. Attention to any underlying psychological disturbances may prove rewarding. Table 21.1 Drugs used in tobacco dependence
Nicotine substitution therapy for tobacco dependence doubles or triples the rate of success of quitting smoking. Reinforcing effect of smoking is maximum with inhalation, less with oral use, and least with the slow-release, transdermal nicotine patch. Hence, the preparations used for substitution therapy are: (i) Transdermally delivered nicotine (Nicotine patches). (ii) Transmucosally delivered nicotine in form of gums and lozenges; and (iii) Oral and nasal spray These preparations, in contrast to smoking, provide slower rise in, and lower and less variable plasma nicotine concentrations, thus helping the smoker to abstain from tobacco, resulting finally in cessation of smoking. The possible mechanism by which the smoker ’s cessation efforts are strengthened by this medication are: • Nicotine substitution decreases withdrawal symptoms. • It partially decreases the craving for cigarettes by sustaining tolerance, in much the same way that methadone partially decreases the craving for morphine; and • It may provide some euphoria. Transdermal patches may occasionally cause nightmares and insomnia. VARENICLINE is a partial agonist of nicotinic Nn receptors and α4 β2 receptors. Its t½ is 14-24 hours. Oral administration of 1 mg bid, with a full glass of water, after eating, helps some patients to quit smoking. It causes nausea, headache, sleep disturbances and weight loss. Severe neuropsychiatric symptoms, agitation, depressed moods and suicidal ideation have been reported. Cardiovascular risk is also suspected.
BUPROPION an anti-depressant, helps some individuals to give up smoking. It is a selective blocker of NA and DA neuronal uptake and may help to reduce craving by acting on the mesolimbic system. Bupropion is used in the dose of 150-300 mg per day, for 7 days prior to cessation of smoking and then in the dose of 300 mg per day for 6-12 weeks. It can be used along with nicotine patches. It can cause dryness of mouth, insomnia, neuropsychiatric disorders and seizures.
GanglionBlocking Agents The ganglion blocking agents are drugs which selectively competitively block the transmission across the autonomic ganglia, both sympathetic and parasympathetic, by blocking the ACh receptors. The blockade produced by these agents, unlike that produced by nicotine, is not preceded by stimulation. They can cause adverse reactions due to: (a) Parasympathetic blockade (visual disturbances, dry mouth, urinary hesitancy, constipation and sexual impotence); and (b) Sympathetic blockade (syncope from postural hypotension). Once used to treat hypertension, these drugs are now obsolete (Chapter 30).
22
Skeletal Muscle Relaxants Skeletal muscle relaxants help to reduce unwanted spasm or spasticity without interfering with consciousness and normal voluntary movements; they have important application in various neurological or painful musculo-skeletal disorders. They are also valuable, during surgery for achieving satisfactory muscle relaxation. Spasticity is due to increase in skeletal muscle tone associated with decrease in muscle power due to damage to the corticomoto-neuronic pathways as in cerebral palsy, multiple sclerosis, CNS injury or stroke. Spasm, on the other hand, is an involuntary contraction of a muscle or group of muscles, usually accompanied by pain and limited function. Skeletal muscle relaxation without the loss of consciousness can be achieved by: I Drugs acting centrally, e.g., Benzodiazepines (Chapter 8), Baclofen and Tizanidine (Table 22.1). Table 22.1 Centrally acting muscle relaxants
Daily doses mentioned are usually given in divided doses. *
IM and IV preparations available.
II Drugs acting peripherally at neuro-muscular junction. See later. III Drugs acting directly on muscle, e.g., Dantrolene. IV Drugs effective in extrapyramidal disorders such as parkinsonism.
Centrally Acting Skeletal Muscle Relaxants Centrally acting muscle relaxants cause muscular relaxation without loss of consciousness. They act on selective areas in the CNS like cortex, brain stem and spinal cord. The exact mechanism of action is not known. However, depression of polysynaptic spinal and supraspinal reflexes, especially of the reticular system, controlling the muscle tone appears to be responsible for their effects. Inhibition of pathways in ascending reticular formation, which are involved in maintenance of wakefulness, results in sedation, a common property of most of these drugs. Sedative anxiolytic agents like diazepam also exhibit central muscle relaxant action. These drugs cause respiratory depression if combined with CNS depressants or used in combination with each other. These drugs can be classified as: I Antispastic agents: Prescribed for conditions such as cerebral palsy and multiple sclerosis: Diazepam, Baclofen, Dantrolene. II Antispasmodic agents: Prescribed for musculoskeletal conditions): Cyclobenzaprine, Tizanidine, Diazepam, Metaxalone, Methocarbamol, Orphenadrine, Carisoprodol, Chlorzoxazone. DIAZEPAM: This drug is discussed in detail in Chapter 8. It has both, antispastic and antispasmodic effects. It is useful alone, or in combination, for relieving spasticity especially in patients with lesions of the spinal cord, and occasionally in patients with cerebral palsy or multiple sclerosis. Painful spasms due to a spinal cord lesion are often reduced. It may be of benefit in the stiff-man syndrome and in localised muscle spasms due to traumatic causes. Oral dose is 2 mg twice daily, which is increased gradually to maximum of 10 mg 3-4 times daily. It has low muscle relaxant:sedation ratio and sedation limits the dose used for muscle relaxation. The drug has long elimination t½ due to an active metabolite and hence, it is better avoided in elderly patients and in patients with hepatic impairment. It is the drug of choice to control spasms in tetanus, where it can be given IM or IV. Diazepam should not be mixed with other drugs for IV use. BACLOFEN This compound, beta-4 (chlorophenyl) - gamma aminobutyric acid, is structurally related to inhibitory neurotransmitter, GABA. It is a selective GABAB receptor agonist, mainly acting, on presynaptic receptors in the spinal cord rather than on post synaptic GABAB receptors. It depresses the reflexes by reducing the calcium influx and thereby prevents release of excitatory neurotransmitters. It also appears to inhibit the release of substance P (neurokinin-1) in the spinal cord. It causes a dose-dependent ‘antinociceptive’ effect in the intact animal and may thus be useful in pain syndromes. Clinically, it produces considerable relief of painful flexor (and sometimes extensor) spasms and of increased flexor tone in patients with spinal transection. It also reduces tonic flexor dystonias of the lower extremities in patients with spinal spasticity and may improve bladder and bowel control in patients with spinal lesions. It has no action on muscle power and it does not improve muscle function. It is not useful in spasticity of cerebral origin. It may be useful as a ‘back up’ drug in the treatment of trigeminal neuralgia. It is almost completely absorbed, and 80% is excreted unchanged in the urine within 72 hours. The drug is generally well tolerated. Adverse reactions include drowsiness, lassitude, hallucinations, depression, ataxia, blurred vision and GI disturbances. Baclofen should be tapered off slowly in epileptic patients as increased seizure activity has been
reported on withdrawal. CYCLOBENZAPRINE: This drug, structurally related to TCA, probably acts on descending 5-HT pathway to reduce tonic somatic motor activity. It has a long elimination t½ Adverse effects are dose related and include sedation, dizziness and anticholinergic effects. It should be avoided in older patients and in patients with glaucoma. Seizures have been reported with concomitant use of tramadol and hence, it should be avoided in patients who are prone to seizures. It is also contraindicated in patients with arrhythmias, recent MI and CHF. TIZANIDINE This drug, an α2 adrenergic agonist, reduces muscle spasm reinforcing the presynaptic and postsynaptic inhibition of the motor neurons. It serves as antispasmodic and antispastic agent and is used for the treatment of spasticity secondary to spinal cord injury and multiple sclerosis. It inhibits nociceptive transmission in the spinal dorsal horn and exhibits intrinsic analgesic activity. It is given in the dose of 2-4 mg at bedtime but dose for optimal response varies markedly among patients. Adverse reactions include dizziness, drowsiness, asthenia, hypotension, and dry mouth. Monitoring of liver function is recommended due to hepatotoxic potential. Sudden stoppage of drug can result in rebound hypertension in patients on long term therapy. It should not be administered with CYP1A2 inhibitors like ciprofloxacin or fluvoxamine. Its efficacy as skeletal muscle relaxant is comparable to diazepam, baclofen and dantrolene. CARISOPRODOL: This drug has favourable muscle relaxant : sedative activity ratio. It has also weak analgesic, antipyretic and anticholinergic properties. It gets metabolised to meprobamate, which is a sedative-hypnotic with well documented extensive abuse. Carisoprodol also exhibits addictive potential. Adverse reactions include allergic reactions like rash, asthma and angioneurotic edema. Rarely, idiosyncratic reactions (mental status changes, transient quadriplegia, and temporary loss of vision) may occur after first dose, which may need hospitalisation. It is not recommended for children younger than 12 years. CHLORZOXAZONE: This synthetic benzoxazole is marketed as FDCs with NSAID. It inhibits degranulation of mast cells and cytokines from inflammatory cells. It is claimed to inhibit calcium and potassium influx, which causes neuronal inhibition. It causes dizziness, drowsiness, red or orange coloured urine, GI irritation and rarely GI bleeding and hepatic damage. METAXALONE: This drug is moderately effective and produces less dizziness and drowsiness compared to other skeletal muscle relaxants. It should be used with caution in patients with liver failure and is not recommended in children younger than 12 years. METHOCARBAMOL: This is a carbamate of guaifenesin, but does not produce guaifenesin as a metabolite because the carbamate bond is stable. Its abuse potential is minimal. ADR include hypersensitivity reactions, discolouration of urine, nausea, anorexia, drowsiness, dizziness, restlesness, anxiety, confusion and convulsions. Parenteral administration may cause syncope, hypotension, bradycardia and anaphylaxis. It may exacerbate myasthenia gravis by inhibiting effects of pyridostigmine and should be used with caution with anticholinesterase agents. Fetal abnormalities have been reported. It is also marketed as FDC with NSAID like ibuprofen, paracetamol and diclofenac. ORPHENADRINE: It is a centrally-acting anticholinergic agent developed to treat EPR
of antipsychotics. It is related to antihistaminic, diphenhydramine and also to the nonopioid analgesic, nefopam. It is used to treat pain of various etiologies, from headaches to radiculopathy and also to relieve muscle spasm. Orphenadrine salt used for EPR is the hydrochloride, whereas for the muscle relaxant activity is the citrate. It exerts several CNS and peripheral effects. It has long t½ Its ADR include GI irritation, hypersensitivity reaction anticholinergic effects like drowsiness, dry mouth, tachycardia, urinary retention, increased intraocular tension, and rarely aplastic anemia. It should be avoided in patients with glaucoma or myasthenia gravis. The doses of centrally acting muscle relaxants are presented in Table 22.1. Although various muscle relaxants mentioned above reduce the activity of polysynapic reflexes and resolve decerebrate rigidity in the spinal cat, these findings do no necessarily indicate their usefulness in cerebrate man. Hence though they are promoted for certain spastic neurological disorders, they are not of much use. They may be of some use in cases with localised muscle spasm and are advocated as adjuvant analgesics (Chapter 11) in conditions such as fibrositis, myalgia, myositis and spasms associated with arthritis. The marginal beneficial effects observed with these agents are probably because of their central sedative and analgesic actions, and none of these compounds except diazepam and baclofen can be recommended as effective, reliable and specific muscle relaxants. The centrally acting skeletal muscle relaxants should be used with caution in pregnant women and in the presence of renal damage. They are contraindicated in myasthenia gravis.
Peripherally Acting Skeletal Muscle Relaxants Physiology of skeletal muscle contraction: The steps in neuromuscular transmission leading to skeletal muscle contraction consist of: • Release of ACh in relatively large amounts from the synaptic vesicles of the motor nerve into the synaptic cleft as a result of a nerve impulse which is also called nerve action potential (NAP). Even in the absence of nerve impulse, minute quantities of ACh are released continually into the synaptic cleft. • Acetylcholine thus released, binds to the nicotinic receptors on the motor end plate, leading to the development of an end plate potential (EPP). Influx of sodium into the motor end plate (MEP), causes depolarisation. • When the EPP achieves sufficient magnitude, the surrounding area of the muscle fibre membrane is excited, resulting in the development of muscle action potential (MAP) which initiates contraction as a result of release of calcium into the sarcoplasm. • Acetylcholine is rapidly hydrolysed by AChE enabling the repolarisation of the MEP and the muscle fibre membrane. This is achieved by reversal of the ionic fluxes. The repolarised muscle is now capable of responding to a fresh nerve impulse. Calcium ions play a crucial role in excitation-contraction (E-C) coupling in all muscle types. However, the source of these calcium ions varies in different muscle types and under different experimental conditions. Thus, in most vertebrate and invertebrate smooth muscles, calcium ions bound to the inner surface of the cell membrane are the main source of Ca++ for coupling. Cardiac muscles use mainly extracellular calcium ions while skeletal muscle uses mainly tubular membrane bound calcium for coupling. However, in both these muscles the amount of calcium that enters the myoplasm from these sources is inadequate to complete the coupling process and is supplemented through an amplification system consisting of Ca++ release from the sarcoplasmic reticulum (SR). Drugs can modify this calcium-triggered calcium release from the SR in both skeletal and cardiac muscle. Skeletal muscle contraction can be blocked at several points as shown in Table 22.2. Table 22.2 Ways of blocking skeletal muscle contraction in response to nerve impulse
In clinical practice, paralysis of skeletal muscles is usually induced by interfering with the function of MEP. The peripherally acting skeletal muscle relaxants can be classified according to their mode of action: I Agents acting by competitive blockade of ACh at the motor end plate, e.g. dTubocurarine, Alcuronium, Atracurium, and Vecuronium. II Agents acting by persistent depolarisation of the motor end plate and the muscle fibre membrane e.g. Succinylcholine.
III Drugs which inhibit the release of ACh from the motor nerve terminals e.g. Botulinum toxin Type A. I Drugs acting by competitive blockade: d-TUBOCURARINE : This is the dextrorotatory, quaternary ammonium alkaloid obtained from the plant Chondrodendron tomentosum, indigenous to the Western Amazon region, and plants of the Strychnos species (mainly Strychnos lethalis) from Eastern Amazon region of South America. Crude curare is a dark-brown to black, gummy, resinoid mass, soluble in water. The letter ‘d’ stands for dextro, while the prefix ‘tubo’ is derived from the fact that crude curare was stored and transported by the South American Indians in bamboo tubes. It was mainly used by the tribals as an arrow poison. Although the mechanism of this action was studied by Claude Bernard in 1856, the alkaloid was isolated by King in 1935. Mechanism of action: d-Tubocurarine binds to the nicotinic receptors on the MEP and thus blocks the action of ACh by competitive blockade. The muscle paralysed by d-tubocurarine still responds to direct electrical stimulation, showing that the drug selectively acts on the MEP. If the concentration of ACh in the synaptic cleft is increased either by augmenting its release (stronger electrical stimulation of the motor nerve) or inhibiting its degradation (by an antiChE drug), dtubocurarine blockade is reversed (Fig. 22.1).
Blocking effect of d-Tubocurarine (dTc) on electrically stimulated rat phrenic nerve diaphragm preparation. Note the quick recovery following the addition of neostigmine (Neo).
FIG. 22.1
Pharmacological actions:
• Skeletal muscle: d-Tubocurarine, on parenteral administration, initially produces motor weakness followed by flaccid paralysis. Small, rapidly moving muscles of the fingers, toes, ears and eyes are affected first, making it impossible to perform delicate motor tasks and producing diplopia, slurred speech and difficulty in swallowing. The muscles of limbs, neck and trunk are affected later, followed by the intercostal muscles. Finally, the diaphragm is paralysed and death occurs from hypoxia. Consciousness and sensorium are unaffected. Recovery occurs in the reverse order, the diaphragm recovering first and the small muscles recovering last. • Autonomic ganglia: In the doses used it can produce partial blockade of both the autonomic ganglia and adrenal medulla resulting in fall of BP, after brief initial stimulation. • Histamine release: It produces histamine release from tissues by acting directly on mast cells. This may occasionally cause bronchospasm, increased salivary, tracheobronchial and gastric acid secretion and contributes to production of hypotension. Absorption, fate and excretion: d-Tubocurarine, being a quaternary ammonium compound, is not significantly absorbed from the GI tract. The drug is well absorbed on IM administration and is widely distributed in tissues. The drug owes its brief duration of action to its rapid redistribution. About 33% of the dose is eliminated unchanged in urine within 24 hours. Repeated
administration can cause cumulative toxicity. The drug does not cross the blood-brain or placental barrier. It is, therefore, devoid of CNS effects. Adverse reactions:
• Hypoxia and respiratory paralysis: Respiratory acidosis enhances neuromuscular blockade. Hypoxia should be treated with positive pressure artificial respiration with oxygen and maintenance of a patent airway till adequate recovery occurs. Neostigmine methyl sulphate, 0.5 to 2 mg, along with 0.6 to 1.2 mg of atropine should be administered cautiously. Though neostigmine counters skeletal muscle paralysis, it enhances the bronchospasm and hypotension produced by d-tubocurarine. This can be countered by atropine. However cardiac arrest, which is liable to occur with neostigmine-atropine combination must be watched for. • Hypotension: This usually responds to IV fluids. Antihistaminics are also useful to counter bronchospasm and peripheral vasodilatation produced by release of histamine. • Miscellaneous: There may be a regurgitation of gastric juice into the oesophagus and aspiration into the lungs due to paralysis of the oesophageal sphincter and the diaphragm. Drugs which synergise with d-tubocurarine are general anaesthetics (ether, halothane, isoflurane and enflurane), aminoglycosides, trimethaphan, opioids, propranolol, glucocorticoids, digitalis, chloroquine, calcium channel blockers, quinidine, diuretics and catecholamines. d-Tubocurarine is now obsolete from clinical practice and described above as a prototype. The synthetic competitive neuromuscular blockers used currently are classified in Table 22.3. Chemically, they are: Table 22.3 Competitive neuromuscular blocking agents Long acting (60–120 minutes) Doxac urium, Panc uronium, Pipec uronium and Vec uronium Intermediate acting (20–50 minutes) Atrac urium, Roc uronium Short acting (10–20 minutes) Mivac urium, Rapac uronium
(a) Benzylisoquinolones e.g., Atracurium, Doxacurium,
Mivacurium; and (b) Ammoniosteroids e.g. Pancuronium, Vecuronium, Rocuronium. They have similar properties but differ in their relative toxicities. Broadly, they differ from d-tubocurarine in that they:
• Are more selective and do not block ganglionic transmission significantly; • Have fewer cardiovascular adverse effects; and • Are less liable to release histamine; Table 22.4 summarises the properties of competitive neuromuscular blocking agents. Table 22.4 Neuromuscular blocking agents
II Drugs acting by persistent depolarisation SUCCINYLCHOLINE: Succinylcholine is a quaternary ammonium compound with a structure resembling two molecules of ACh joined together. It has a short duration of action. Mechanism of action: The drug acts like a partial agonist of acetylcholine and produces skeletal muscle depolarisation, by acting on the membrane channel. This can explain the fasciculations preceding skeletal muscle paralysis with this agent. However, in contrast to ACh, the drug is destroyed much more slowly, being susceptible only to plasma and liver pseudocholinesterase. This causes a persistent depolarisation during which the muscles are insensitive to ACh released from nerve endings and remain paralysed (Fig. 22.2).
FIG. 22.2 Blocking effect of succinylcholine (Sc) on electrically stimulated rat phrenic nerve diaphragm preparation. Note the worsening of the block following neostigmine (Neo) and recovery after wash (W).
Pharmacological actions:
• Skeletal muscle: Paralysis of skeletal muscle produced by succinylcholine is preceded by transient muscular fasciculations and twitching, seen usually in the thoracic and abdominal regions. Succinylcholine relaxes the limb and neck muscles in a dose that does not significantly affect respiratory muscles. Transient apnoea is, however, usually observed with the peak effect of succinylcholine. The skeletal muscle paralysis by succinylcholine is enhanced by antiChE neostigmine which increases the local concentration of ACh (Fig. 22.2). Succinylcholine in a concentration and time dependent manner produces a dual block, initially a depolarising block (phase 1) which later becomes non-depolarising. The latter (phase 2), is antagonised by edrophonium. The mechanism of development of nondepolarising blockade with large doses is not clear. • Hyperkalemia: Prolonged depolarisation may cause shift of intracellular potassium to the extracellular space. In patients with extensive soft tissue trauma or burns, as well as other condition which cause hyperkalemia, this can be life-threatening. • Cardiovascular system: Repeated administration leads to stimulation of the vagal nuclei and sympathetic ganglia. The former produces bradycardia, cardiac arrhythmias, and hypotension. The latter causes persistent hypertension and tachycardia. • Miscellaneous: Succinylcholine is less liable to release histamine. Large doses may cause hypotension as a result of muscarinic effect and to some extent by ganglionic blockade. Because of its muscarinic action, atropine is generally given before its use. Absorption, fate and excretion: Succinylcholine IV produces fasciculations which last for 10 to 15 seconds. Peak effect develops within 1 to 2 minutes and muscle power recovers within 5 minutes. The drug is hydrolysed by plasma and liver pseudocholinesterase to succinic acid and choline. About 10% of the dose is excreted unchanged in the urine. It crosses the placental barrier in insignificant amount and hence, can be used in obstetric cases. Adverse reactions: Apart from allergic reactions, the other adverse reactions are: • Cardiac arrest and arrhythmias: High incidence of cardiac arrhythmias with the use of succinylcholine-halothane combination has been reported. The drug may occasionally produce cardiovascular collapse. Changes in K+ distribution may cause serious arrhythmias in patients on diuretics and digitalis. • Succinylcholine apnoea: Apnea needing respiratory support longer than 15 minutes is considered abnormal. Presence of a hereditary, abnormal plasma pseudocholinesterase,
having a poor ability to hydrolyse succinylcholine, or acquired deficiency of normal pseudocholinesterase as in liver disease, predisposes to its development. Metabolic acidosis can also precipitate succinylcholine apnoea. It is treated by artificial respiration and fresh blood transfusion. No antidote is available. • Malignant hyperthermia: The drug can rarely trigger serious malignant hyperthermic crisis in patients receiving ether or halothane. • Miscellaneous: Muscle soreness is a frequent complaint following succinylcholine. Increase in intraocular tension can also occur. • Drug interactions: They are similar to those of d-tubocurarine. AntiChE drugs, however, act synergistically with depolarising blockers. Preparations and dosage: Succinylcholine chloride 50 mg per ml. Dose: 1 mg per kg given slowly IV, followed by 0.3 mg/kg as needed. Therapeutic uses of peripheral skeletal muscle relaxants:
• Adjuvant to anaesthesia: The main use of skeletal muscle relaxants is to promote skeletal muscle relaxation during abdominal surgery, orthopaedic manipulations, and various brief procedures like laryngoscopy, bronchoscopy and oesophagoscopy. When the procedure involved is of short duration, succinylcholine or mivacurium is the drug of choice. • In electroconvulsive therapy: Succinylcholine or mivacurium is often administered along with diazepam to protect the patient from injury during electroconvulsive therapy. • In spastic disorders: The curarimimetic skeletal muscle relaxants have been used to treat the severe spasms of tetanus. This use, however, involves risk and needs expert supervision. III Drugs which inhibit ACh release from motor nerve terminals. BOTULINUM TOXIN TYPE A is produced by Clostridium botulinum. It binds irreversibly to presynaptic, cholinergic sites and inhibits the release of ACh from the motor nerve terminals. Local injection of the toxin weakens the overactive muscle and decreases the hypersecretion of glands innervated by cholinergic neurons. It has been used to treat:
(1) Spastic conditions such as spasmodic torticolis, hemifacial spasm, strabismus, blepharospasm, dystonias, lower esophageal spasm and painful anal fissure; (2) Wrinkles on the face and neck; (3) Palmar hyperhidrosis. (4) Overactive bladder (Chapter 20). It appears to be safe and effective in these conditions. Its action lasts for 3-4 months. Adverse reactions include ptosis, diplopia, reduced blinking leading to dry eyes, minor bruises and lid swelling; reversible muscle atrophy can occur after repeated administration. Botulinum toxin Type B can be used in patients who have become resistant to Type A toxin.
Drugs Acting Directly on Skeletal Muscle DANTROLENE: This phenytoin analogue relaxes the skeletal muscles by binding to the ryanodine receptors (RYR) on the sarcoplasmic reticulum, blocking their opening. This prevents the release of Ca++ from the sarcoplasmic stores. Thus, the muscle contraction is weakened without muscle paralysis. It does not alter the neuromuscular transmission. It has relatively little effect on cardiac and smooth muscle. Given orally, it is incompletely (about 1/3rd) absorbed and is largely metabolised in the liver. It can also be given IV. Dose: 25 mg/d, increased weekly to a maximum of 100 mg bid or qid. Adverse reactions: These include generalised muscle weakness, dizziness, drowsiness, fatigue and diarrhoea. Rarely, it may cause serious hepatotoxicity. Therapeutic uses: • Spasticity: The drug is particularly useful for the treatment of spasticity (especially cerebral spasticity) in patients in whom nursing care is made difficult by muscle spasm. Patients in whom spastic, dystonic stiffness is useful as a sort of protective endogenous crutch should not be treated with dantrolene. • Malignant hyperthermia: Intravenous dantrolene (1 mg/kg, bolus, repeated if required to a maximum total dose of 100 mg) is life-saving in this rare, fatal and familial, genetic disorder of skeletal muscle triggered by any potent inhalation anaesthetic, depolarising muscle relaxant, curare-like neuromuscular blocking agent and even by stress. In susceptible individuals, these can cause excessive release of Ca++ from the sarcoplasmic reticulum, leading to muscle rigidity, hyperpyrexia (temperature more than 42°C), hyperkalemia, tachycardia and metabolic acidosis. It is a medical emergency. It must be noted that management of the patient with spasticity implies more than just treatment of the spasticity. For many patients the increased tone is not harmful and may even be helpful. Active measures to reduce spasticity are only justified where the reflex hyperexcitability interferes with function, making rehabilitation, physiotherapy and nursing care difficult.
S E C T IO N V
Other Biogenic Amines and Polypeptides OUT LINE Chapter 23: Histamine and Antihistaminic Drugs Chapter 24: 5-Hydroxytryptamine (Serotonin), its Agonists and Antagonists; and Treatment of Migraine Chapter 25: Angiotensin, Kinins, Leukotrienes, Prostaglandins and Cytokines
23
Histamine and Antihistaminic Drugs HISTAMINE (‘tissue amine’), a potent biogenic amine, was synthesised even before its isolation from plants and animals by Windaus and Vogt in 1907. The compound was isolated from ergot extracts by Barger and Dale in 1910 and reports regarding its pharmacological actions were published by Dale and Laidlaw in 1910-1911. The role of this extremely potent natural substance in the genesis of allergic and anaphylactic manifestations was forecast by the brilliant work of Lewis. Distribution and synthesis: Histamine, an imidazole compound, is widely distributed in plant and animal tissues, and is also present in the venom of bees and wasps. Almost all tissues in mammals synthesise histamine, from amino acid histidine by decarboxylation with the help of histidine decarboxylase (Fig 23.1). Histamine is also synthesised by the microflora in the GI tract from dietary histidine. Very little, however, reaches the circulation as most of what is absorbed is rapidly metabolised in the intestinal wall and the liver.
FIG. 23.1 Biosynthesis of histamine
Histamine is present in various biological fluids, and in platelets, leucocytes, basophils and mast cells. A major portion of histamine is stored in mast cells and circulating basophils. These cells possess histidine decarboxylase, and also contain specialised granules, wherein histamine is stored in an inactive form. In the mast cells of majority of animals, histamine is stored along with heparin, and in rodents, along with 5-HT. Tissues devoid of mast cells, e.g., human epidermis, gastric mucosa and CNS neurons also contain a significant concentration of histamine. Within the gut, the histamine concentration is highest in the stomach wall and gastric and intestinal glands. In the CNS, the area postrema, mast cells (histaminocytes) and histaminergic neurons of the pituitary stalk and the hypothalamus contain significant amounts of histamine. Mechanism of action: Four types of receptors, H1, H2, H3 and H4 have been identified and cloned. All these receptors belong to the family of GPCR (G protein coupled receptors). (Table 23.1).
Table 23.1 Histamine receptors and their functions
• Activation of H1 receptors causes pain, pruritus, bronchoconstriction, vasodilatation, increase in vascular permeability and in secretions. It is associated with increase in intracellular cyclic guanosine 3′, 5′ monophosphate (cGMP). In the tissues, histamine serves as a chemotactic agent for neutrophils and eosinophils. H1 receptor effects can be competitively blocked specifically by the conventional antihistaminics (H1 receptor blockers). • Activation of H2 receptors increases the gastric acid secretion and myocardial contractility. These effects are not blocked by the conventional antihistaminics but are specifically blocked by H2 receptor antagonists such as cimetidine. Impromidine is a potent H2 receptor agonist and behaves as a competitive antagonist at the autoinhibitory receptors. Both H1 and H2 receptors appear to be involved in vascular dilatation, hypotension and edema formation. • For H3 and H4 receptors, see Table 23.1. Pharmacological actions of histamine: Cardiovascular system: • Blood vessels: Although histamine may produce pulmonary and systemic vasoconstriction in certain herbivores like rabbits and guinea pigs, it causes marked vasodilatation in human beings. In man, the pulmonary vessels are dilated by histamine, producing a fall in pulmonary artery pressure. The cerebral blood vessels are dilated in majority of the species. It produces throbbing headache, palpable temporal pulsations, and a transient increase in the CSF pressure. The headache is attributed to stretching of sensory nerve endings around the cranial arteries. Histamine constricts large veins. It acts on the blood vessels in several ways:
(a) Activation of H1 receptors on the endothelial cells causes rapid and short-lived
vasodilatation, mainly of arterioles through the release of NO. (b) Activation of H2 receptors in the vascular smooth muscle causes slower but more prolonged vasodilatation, and (c) Direct relaxation of the smooth muscle of the arterioles and capillaries leads to their dilatation and fall in BP. In man, histamine causes marked flushing, a sense of warmth; and marked increase in capillary permeability after large doses, which causes edema and reduction in plasma volume. • Blood pressure: Hypotension, induced by moderate doses of histamine is transient due to its rapid degradation and the presence of protective reflexes. Large doses, however, produce prolonged hypotension. Histamine-induced hypotension can be prevented but only partially reversed by antihistaminic agents. However, it can be reversed by adrenaline (Physiological antagonism). Triple response: When a pointed object is drawn lightly over the skin, the stroke line becomes blanched, i.e. white reaction, owing to contraction of the precapillary sphincters. It lasts for about 15 seconds. When histamine (20 mcg) is injected ID or the skin is stroked firmly with the pointed object in man, a triple response develops at the site. It comprises sequentially:
(a) Flush or red reaction which is a red line/spot developing within 10 seconds, owing to local dilatation of capillaries and venules; then (b) Wheal which is local swelling due to edema, and mottled reddening around the injury, and lasts for about 1½ minute; it is due to increased permeability of capillaries and post capillary venules, with consequent extravasation of fluid; and finally (c) Flare in which the redness with irregular margins spreads out from the injury. The triple response (Lewis response) is a part of the normal reaction to injury. Its prevention is used to evaluate antihistaminic activity of a new drug. • Heart: Histamine increases the sinus rate (positive chronotropic action); increases the amplitude of ventricular contraction (positive inotropic action); decreases A-V conduction time and increases coronary blood flow. High concentrations induce ventricular fibrillation. Smooth muscle: Histamine stimulates the smooth muscle of various tissues directly (H1 action). However, the individual tissue responses show a marked variation. The bronchial and the uterine smooth muscle is highly sensitive to histamine. Its action on the pregnant uterus is, however, not significant. Individuals suffering from bronchial asthma and certain other pulmonary diseases develop a sharp fall in vital capacity and considerable respiratory embarrassment in respo