Which term describes the measure of how strongly a drug binds to its receptor?

• A. Intrinsic activity
• B. Binding constant
• C. Selectivity
• D. Affinity

Answer: (D)

Affinity is the measure of how strongly a drug binds to its receptor. It reflects how readily the drug associates with and stays bound to the receptor at a given concentration; a high affinity means the drug binds tightly and can occupy receptors at lower concentrations (often described by a low dissociation constant, Kd, or a high association constant, Ka). This concept is distinct from intrinsic activity, which describes the ability of the bound drug to produce a cellular response, and from selectivity, which is about preference for certain receptor types. Binding constant is a related term sometimes used in different contexts, but the classic description for “how strongly a drug binds” is affinity.