An agonist produces 50% maximal response at a concentration of 10 nM (EC50). A competitive antagonist at 100 nM shifts the dose-response curve to the right, and the EC50 for the agonist increases to 100 nM. The apparent affinity (Ki) of the antagonist is closest to:

• A 100 nM
• B 11.1 nM ✓
• C 50 nM
• D 10 nM

Explanation

Using the Cheng-Prusoff equation for competitive antagonism: DR (dose ratio) = EC50(antagonist)/EC50(control) = 100/10 = 10. By the Schild equation: DR = 1 + [B]/Ki, so 10 = 1 + 100/Ki, giving Ki = 100/9 ≈ 11.1 nM. The Ki represents the antagonist's equilibrium inhibition constant; a lower Ki indicates higher affinity. This calculation is commonly tested in advanced pharmacodynamics questions involving receptor-binding analysis.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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