The high inter-individual variability in the inhibition of platelet function by clopidogrel is mostly explained by high variability in its transformation to an active metabolite (CAM).Objective:
We investigated the relations between pharmacokinetics and pharmacodynamics of CAM by comparing two methods of platelet function.Methods:
We enrolled 14 patients undergoing percutaneous coronary interventions for non-ST-segment elevation acute coronary syndrome or inducible myocardial ischemia. Plasma concentrations of clopidogrel and CAM, phosphorylation of vasodilator-stimulated phosphoprotein (VASP-P), expressed as a platelet reactivity index (PRI) and whole-blood platelet aggregation (multiple electrode aggregometer, MEA) were measured before and after a 600-mg clopidogrel loading dose (nine time-points) and before and after 75-mg maintenance doses on days 2, 7 and 30.Results:
Plasma concentrations of clopidogrel and CAM were highly variable. CAM reached maximal concentration (Cmax) (median, 110.8 nm; range, 41.9–484.8) 0.5–2 h after the loading dose. A sigmoid dose-response curve defined the relations between CAMCmax and PRI after 3 to 24 h (IC50, 459.6 nm; 95% confidence interval, 453.4–465.7; R2 = 0.82). PRI was unchanged from baseline in patients with the lowest CAMCmax (< 83 nm, n = 7), indicating low sensitivity of VASP-P. PRI values were also predicted by CAMCmax at days 2, 7 and 30. Platelet aggregation measured by MEA did not show significant relations with either PRI or with CAM pharmacokinetics at any time-point.Conclusions:
After 600 mg clopidogrel, VASP-P, but not whole-blood platelet aggregation measured by MEA, is almost entirely predicted by CAMCmax. VASP-P could be useful in studies aimed at investigating relations between CAM bioavailability and clinical events.