A drug shows a shift in its concentration-response curve to the right with no change in maximum effect (Emax) in the presence of a second compound. Which type of antagonism is demonstrated?
- A Non-competitive antagonism — the antagonist binds an allosteric site, reducing agonist efficacy and lowering Emax
- B Irreversible antagonism — covalent binding at the orthosteric site permanently reduces receptor availability, compressing the curve rightward and reducing Emax
- C Functional (physiological) antagonism — two drugs oppose each other's effects at different receptors via separate signal transduction pathways
- D Competitive (reversible) antagonism — the antagonist competes for the same binding site, displaceably shifting the EC50 rightward while Emax is preserved ✓
Explanation
A parallel rightward shift of the concentration-response curve with preserved Emax (maximum effect) is the hallmark of competitive (surmountable) reversible antagonism. The antagonist competes with the agonist for the same orthosteric binding site; at any given antagonist concentration, increasing the agonist concentration can displace the antagonist and ultimately restore the full maximum response. This shifts the EC50 (concentration producing 50% of Emax) proportionally to the antagonist concentration in a mathematically predictable manner described by the Schild equation. Non-competitive antagonism (allosteric binding to a site that reduces efficacy coupling) and irreversible competitive antagonism (covalent binding, like phenoxybenzamine) both reduce Emax and are insurmountable. Functional antagonism involves two drugs acting at different receptors producing opposing biological effects.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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