DRUG ANTAGONISM DR. SHABANA ALI

DRUG ANTAGONISM One drug or inhibits action of another drug Types of Antagonism Physical antagonism Chemical antagonism Physiological/functional antagonism Pharmacological antagonism

A) Physical Antagonism Based on physical property of drugs, e.g. charcoal (adsorb alkaloid) in alkaloidal poisoning B) Chemical Antagonism Chemical reaction between two drugs e.g., NaHCO3 HCl

C) Physiological/Functional Antagonism Opposite effects of two drugs on same function Two drugs act on two diff. types of receptors & antagonize action of each other, e.g. histamine & adrenaline (adrenaline for treatment of anaphylactic shock); Ad & insulin on blood sugar level

D) Pharmacological Antagonism Opposite effect of two drugs binding to same receptors Receptor antagonism is specific,e.g. atropine  spam of intestine by acetylcholine not by Hist.or 5-HT Types Competitive Non-competitive Equilibrium Non-equilibrium (Reversible) (Irreversible)

i) Competitive Antagonism (equilibrium or reversible) Competition between agonist & antagonist for specific site or receptor Action of agonist is blocked if conc. of antagonist is  Antagonism can be overcome (surmount) by  conc. of agonist Cont.

Agonist can produce max.response in higher conc. Competitive antagonist shifts log Dose-response curve of agonist to right EC50 of agonist  in presence of antagonist, e.g., Ach & atropine; Ad & Prop.; Morphine & naloxone

COMPETITIVE ANTAGONISM

II) Non-Equilibrium(irreversible) Antagonist Antagonist binds to receptor with covalent bond Irreversible blocking Antagonist shifts DRC to right+  max. response, e.g. adrenaline & phenoxybenzamine

iii) Non-competitive (non-surmountable Antagonist Antagonist binds to another site of receptor DRC is flattened + max. response is  e.g., verapamil (noradrenaline)

Non-competitive Antagonist