partial agonist
Summary
A partial agonist is a drug that binds to a receptor but produces a submaximal response even at full receptor occupancy, compared to a full agonist. It can act as either an agonist or antagonist depending on the presence of a full agonist.
Detail
Partial agonists have intrinsic activity (efficacy) between 0 and 1, meaning they cannot produce the maximal response that a full agonist can, regardless of dose or concentration. This is due to differences in receptor conformational changes or signal transduction efficiency after binding. Key concept: potency (EC50) is independent of efficacy—a partial agonist can be more or less potent than a full agonist but will always have lower maximal efficacy.
Clinically important behavior: In the absence of a full agonist, a partial agonist stimulates the receptor and produces a submaximal effect (acts as an agonist). However, in the presence of a full agonist, the partial agonist competes for the same receptor and can decrease the overall response by displacing the full agonist, effectively acting as a functional antagonist (lowering the net efficacy toward its own intrinsic activity).
High-yield examples for boards: - Buprenorphine: partial agonist at mu-opioid receptors, used in opioid use disorder treatment; safer overdose profile than full agonists (ceiling effect on respiratory depression) but can precipitate withdrawal if given to a patient on a full opioid agonist. - Pindolol, acebutolol: partial agonist beta-blockers with intrinsic sympathomimetic activity (ISA), useful when bradycardia is a concern. - Aripiprazole, brexpiprazole: partial agonists at D2 dopamine receptors, used in schizophrenia—they stabilize dopamine transmission (reduce hyperactivity in mesolimbic pathway while not fully blocking dopaminergic tone in mesocortical pathway). - Nicotine replacement (varenicline): partial agonist at nicotinic acetylcholine receptors, reduces craving and withdrawal while blunting the reward from smoking.
Understanding partial agonists is essential for USMLE pharmacology questions, especially distinguishing dose-response curve shapes (partial agonists show a lower plateau/Emax on log-dose response curves) and clinical scenarios involving receptor theory (efficacy vs potency, competitive interactions).
Sources
- Katzung's Basic and Clinical Pharmacology
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- First Aid for the USMLE Step 1
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