The following pharmacological definition has been taken from the Pharmacology and Experimental Therapeutics Department Glossary at Boston University School of Medicine.
The equilibrium constant of the reversible reaction of a drug with a receptor to form a drug-receptor complex; the reciprocal of the dissociation constant of a drug-receptor complex. Under the most general conditions, where there is a 1:1 binding interaction, at equilibrium the number of receptors engaged by a drug at a given drug concentration is directly proportional to their affinity for each other and inversely related to the tendency of the drug-receptor complex to dissociate. Obviously, affinity depends on the chemical natures of both the drug and the receptor. (See: Ariens, E.D. et al., Pharmacol. Rev. 9: 218, 1957).
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