Which statement best describes a partial agonist at a receptor?
- A A partial agonist has lower affinity than a full agonist but causes the same maximal response
- B A partial agonist has intrinsic efficacy between 0 and 1 — it activates the receptor but produces a submaximal response even at full receptor occupancy; it can act as an antagonist in the presence of a full agonist ✓
- C A partial agonist binds irreversibly and activates only a fraction of receptors before becoming permanently bound
- D A partial agonist activates only a subset of receptor subtypes, behaving as a full agonist on that subset
Correct answer: B. A partial agonist has intrinsic efficacy between 0 and 1 — it activates the receptor but produces a submaximal response even at full receptor occupancy; it can act as an antagonist in the presence of a full agonist
Explanation
A partial agonist has an intrinsic efficacy (alpha) of greater than 0 but less than 1. Even when all available receptors are occupied, it cannot produce the same maximum response as a full agonist. In the presence of a full agonist, a partial agonist competes for receptor binding; at high concentrations it can displace the full agonist and reduce the overall response, acting functionally as an antagonist. Examples include buprenorphine (partial mu agonist), buspirone (partial 5-HT1A agonist), and pindolol (partial beta-1 agonist).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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