Ticagrelor is preferred over clopidogrel in acute coronary syndrome per current guidelines. Which unique pharmacological property distinguishes ticagrelor from clopidogrel in terms of receptor interaction?

• A Ticagrelor is a prodrug requiring less hepatic metabolism than clopidogrel, explaining faster onset
• B Ticagrelor selectively inhibits P2Y1 receptors while clopidogrel inhibits P2Y12, targeting different platelet ADP receptors
• C Ticagrelor is a reversible, direct-acting P2Y12 ADP receptor antagonist that binds an allosteric site outside the ADP-binding domain, whereas clopidogrel (via active metabolite) irreversibly alkylates the ADP binding site ✓
• D Ticagrelor directly inhibits thromboxane A2 receptors in addition to P2Y12, providing dual antiplatelet action

Explanation

Clopidogrel is a thienopyridine prodrug requiring hepatic CYP2C19 bioactivation to an active metabolite that irreversibly binds and alkylates the P2Y12 ADP receptor at the cysteine residue in the ADP-binding domain, causing permanent receptor blockade for the platelet's lifetime (~7-10 days). Ticagrelor is a cyclopentyltriazolopyrimidine that is NOT a prodrug and binds REVERSIBLY to a distinct allosteric site on the P2Y12 receptor outside the ADP binding domain — it is an allosteric antagonist that prevents ADP-induced conformational changes. Because it does not bind the ADP site and is reversible, platelet function recovers within 3-5 days of stopping ticagrelor. Ticagrelor's advantages include: no prodrug conversion needed (no CYP2C19 pharmacogenomic variability), faster onset, and more consistent platelet inhibition.

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

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