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– Amount of drug that interactions with receptors is negligible compared to the amount of drug to which the receptors are exposed; effective drug concentration does not change during drug-receptor interaction
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Quantitative aspects of drug-receptor interactions•C = concentration of drug (=concentration of the drug which is essentially equal to that of unbound drug)•Y = % of total number of receptors occupied by a drug•(100-Y) = % unoccupied, free receptors•k1: constant specific for the given reaction of the drug/receptor combination•k2: constant specific for the reverse reaction of the given drug/receptor combination•The rate at which a drug-receptor complex is formed is proportional to the product of the concentration of the drug (C) and the amount of free receptors (100-Y)–Rate of drug-receptor combination = k1C(100-Y)•The rate of dissociation of a drug-receptor complex is proportion to the drug-receptor complex–Rate of dissociation of the drug-receptor combination = k2Y•In equilibrium, rate of combination = rate of dissociationk1C(100-Y) = k2Y or C=k2Y k1(100-Y)•Ratios of 2 constants is a constant, substitute Kd (dissociation constant) for k2/k1C=Y Kd(100-Y)k1Drug + free receptor Drug-receptor complexC (100-Y) k2 Y