A Possible Mechanism for the Differences in Efficiency and Variability of Active Metabolite Formation from Thienopyridine Antiplatelet Agents, Prasugrel and Clopidogrel

The efficiency and interindividual variability in bioactivation of prasugrel and clopidogrel were quantitatively compared and the mechanisms involved were elucidated using 20 individual human liver microsomes. Prasugrel and clopidogrel are converted to their thiol-containing active metabolites through corresponding thiolactone metabolites. The formation rate of clopidogrel active metabolite was much lower and more variable [0.164 +/- 0.196 mu l/min/mg protein, coefficient of variation (CV) = 120%] compared with the formation of prasugrel active metabolite (8.68 +/- 6.64 mu l/min/mg protein, CV = 76%). This result was most likely attributable to the less efficient and less consistent formation of clopidogrel thiolactone metabolite (2.24 +/- 1.00 mu l/min/mg protein, CV = 45%) compared with the formation of prasugrel thiolactone metabolite (55.2 +/- 15.4 mu l/min/mg protein, CV = 28%). These differences may be attributed to the following factors. Clopidogrel was largely hydrolyzed to an inactive acid metabolite (approximately 90% of total metabolites analyzed), and the clopidogrel concentrations consumed were correlated to human carboxylesterase 1 activity in each source of liver microsomes. In addition, 48% of the clopidogrel thiolactone metabolite formed was converted to an inactive thiolactone acid metabolite. The oxidation of clopidogrel to its thiolactone metabolite correlated with variable activities of CYP1A2, CYP2B6, and CYP2C19. In conclusion, the active metabolite of clopidogrel was formed with less efficiency and higher variability than that of prasugrel. This difference in thiolactone formation was attributed to hydrolysis of clopidogrel and its thiolactone metabolite to inactive acid metabolites and to variability in cytochrome P450-mediated oxidation of clopidogrel to its thiolactone metabolite, which may contribute to the poorer and more variable active metabolite formation for clopidogrel than prasugrel.