Which term describes compounds that can activate a receptor but are unable to elicit a maximal response?

Which term describes compounds that can activate a receptor but are unable to elicit a maximal response?

• A. Compounds that can activate a receptor but are unable to elicit a maximal response.
• B. Compounds that completely activate receptor.
• C. Compounds with no affinity.
• D. Compounds that act as antagonists.

Answer: (A)

This question hinges on the idea of partial agonism and efficacy. A compound that can activate a receptor but cannot produce the full effect has lower intrinsic activity, or efficacy, than a full agonist. Even when it binds and activates the receptor, the maximal response it can generate is submaximal. Because of this, at a given receptor occupancy it yields a smaller effect than a full agonist, and in the presence of a full agonist it can compete for receptors and dampen the overall response, acting like an antagonist in that context. This distinguishes it from full agonists (which produce the maximum possible response) and from antagonists (which bind but don’t activate). Non-binding compounds have no affinity and produce no effect.

This question hinges on the idea of partial agonism and efficacy. A compound that can activate a receptor but cannot produce the full effect has lower intrinsic activity, or efficacy, than a full agonist. Even when it binds and activates the receptor, the maximal response it can generate is submaximal. Because of this, at a given receptor occupancy it yields a smaller effect than a full agonist, and in the presence of a full agonist it can compete for receptors and dampen the overall response, acting like an antagonist in that context. This distinguishes it from full agonists (which produce the maximum possible response) and from antagonists (which bind but don’t activate). Non-binding compounds have no affinity and produce no effect.