What determines the concentration of drug required to form a significant number of drug-receptor complexes or magnitude of the response?

• A. Lipophilicity
• B. Binding affinity
• C. Drug solubility
• D. Molecular weight

Answer: (B)

The key idea is receptor occupancy. The concentration of drug needed to form a significant number of drug-receptor complexes—and thus produce a measurable response—depends on how tightly the drug binds to the receptor, i.e., its binding affinity. A higher affinity (lower dissociation constant, Kd) means the drug binds more tightly and at lower concentrations, you achieve substantial receptor occupancy. The relationship can be described simply: fractional occupancy ≈ [drug] / ([drug] + Kd). As the drug concentration rises above the Kd, occupancy—and the response—approach their maximum.

Pharmacokinetic properties like lipophilicity, solubility, and molecular weight influence how much drug actually reaches the receptor in the body, but they don’t change the intrinsic ability of the drug to bind. They affect concentration at the site of action, not the affinity itself, which is why affinity is the primary determinant of the concentration needed for a significant response.