The concept of 'therapeutic window' relates to the relationship between the minimum effective concentration (MEC) and minimum toxic concentration (MTC). Which pharmacodynamic concept is defined as the ratio of MTC to MEC?

• A Efficacy
• B Potency
• C Receptor reserve
• D Therapeutic index (TI) ✓

Explanation

The therapeutic index (TI) is classically defined as the ratio of the median toxic dose (TD50) to the median effective dose (ED50) in animals, or clinically approximated as the ratio of MTC to MEC. A wide TI (e.g., penicillins, TI>100) allows substantial dosing latitude, while a narrow TI (e.g., digoxin, theophylline, warfarin, lithium, TI<2–3) requires careful monitoring. Efficacy is the maximal drug effect; potency is the dose required for 50% of maximal effect; receptor reserve is the fraction of receptors that can be occupied without producing a maximal response.

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

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