competitive antagonist
Summary
A competitive antagonist binds reversibly to the same receptor site as the agonist, competing for occupancy without activating the receptor. It shifts the agonist dose-response curve to the right (increases EC50) but does not change the maximal efficacy (Vmax/Emax) if enough agonist is added to overcome the antagonism.
Detail
Competitive antagonists bind reversibly to the orthosteric (active) site of a receptor, the same site the endogenous agonist or drug uses. Because binding is reversible and the antagonist competes with the agonist for the same site, increasing the agonist concentration can outcompete the antagonist and restore the maximal response (Emax unchanged), but a higher agonist concentration is required to achieve the same effect (rightward shift in the dose-response curve, increased EC50/decreased potency). This is described as a 'surmountable' antagonism.
This contrasts with: - Noncompetitive/irreversible antagonists: bind to the same site irreversibly or bind to an allosteric site, decreasing Emax without necessarily shifting EC50 (dose-response curve shows decreased maximal efficacy). - Partial agonists: bind the same receptor but produce a submaximal response even at full receptor occupancy.
Clinical/pharmacologic examples of competitive antagonism include naloxone (opioid receptor antagonist, competes with opioids), flumazenil (benzodiazepine receptor antagonist), and atropine (muscarinic receptor antagonist competing with acetylcholine). Understanding competitive vs noncompetitive antagonism is essential for predicting drug interactions, overdose reversal strategies, and receptor pharmacology on Step 1.
Key graphical representation: log dose-response curves shift rightward with increasing antagonist concentration, but the curves eventually reach the same Emax if enough agonist is present.
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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