The Emax model of pharmacodynamics describes the relationship between drug concentration and effect. At what drug concentration is 50% of the maximum effect (Emax) achieved?

• A Cmax/2
• B Therapeutic index midpoint
• C MEC (minimum effective concentration)
• D EC50 ✓

Explanation

In the Emax (Emax/EC50 sigmoidal Hill equation) model, EC50 is the drug concentration that produces 50% of the maximum possible effect (Emax/2). EC50 is a measure of drug potency: a lower EC50 means greater potency (smaller concentration needed to produce a half-maximal effect). This is mathematically derived from the Hill equation: E = Emax × C^n / (EC50^n + C^n). EC50 is distinct from Cmax (peak plasma concentration during dosing), the therapeutic index (ratio of toxic to effective dose), and MEC.

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

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