Amount of drug that interactions with receptors is

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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 dissociation k1C(100-Y) = k2Y or C= k2Y k1(100-Y) Ratios of 2 constants is a constant, substitute Kd (dissociation constant) for k2/k1 C= Y Kd(100-Y) k1 Drug + free receptor Drug-receptor complex C (100-Y) k2 Y
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