How can a drug display high affinity but low efficacy?

• A. It binds weakly to the receptor but produces a strong or full response.
• B. It binds tightly to the receptor but produces a weak or no functional response, functioning as an antagonist or partial agonist.
• C. It binds irreversibly and prevents any signaling.
• D. It binds tightly and fully activates the receptor.

Answer: (B)

High affinity means the drug binds very tightly to the receptor and tends to stay bound. Efficacy is about how well that bound drug can activate the receptor and produce a response. If a compound binds strongly but doesn’t trigger much signaling, it can act as an antagonist or as a partial agonist. An antagonist occupies the receptor and blocks activation, so the response stays minimal even with strong binding. A partial agonist also binds well but can only elicit a limited response, so the maximum effect is lower than that of a full agonist. This combination—tight binding with little or no functional signal—explains how a drug can have high affinity but low efficacy. The other patterns described (weak binding with strong activation, tight binding with full activation, or irreversible binding with signaling) don’t fit this particular scenario.