Effects of drug interactions on biotransformation and antiplatelet effect of clopidogrel in vitro

The conversion of clopidogrel to its active metabolite, R-130964, is a two-step cytochrome P450 (CYP)-dependent process. The current investigations were performed to characterize in vitro the effects of different CYP inhibitors on the biotransformation and on the antiplatelet effect of clopidogrel.; Clopidogrel biotransformation was studied using human liver microsomes (HLM) or specific CYPs and platelet aggregation using human platelets activated with ADP.; Experiments using HLM or specific CYPs (3A4, 2C19) revealed that at clopidogrel concentrations <10 microM, CYP3A4 was primarily responsible for clopidogrel biotransformation. At a clopidogrel concentration of 40 microM, ketoconazole showed the strongest inhibitory effect on clopidogrel biotransformation and clopidogrel-associated inhibition of platelet aggregation with IC(50) values of 0.03 +/- 0.07 microM and 0.55 +/- 0.06 microM respectively. Clarithromycin, another CYP3A4 inhibitor, impaired clopidogrel biotransformation and antiplatelet activity almost as effectively as ketoconazole. The CYP3A4 substrates atorvastatin and simvastatin both inhibited clopidogrel biotransformation and antiplatelet activity, less potently than ketoconazole. In contrast, pravastatin showed no inhibitory effect. As clopidogrel itself inhibited CYP2C19 at concentrations <10 microM, the CYP2C19 inhibitor lansozprazole affected clopidogrel biotransformation only at clopidogrel concentrations > or =10 microM. The carboxylate metabolite of clopidogrel was not a CYP substrate and did not affect platelet aggregation.; At clopidogrel concentrations <10 microM, CYP3A4 is mainly responsible for clopidogrel biotransformation, whereas CYP2C19 contributes only at clopidogrel concentrations > or =10 microM. CYP2C19 inhibition by clopidogrel at concentrations <10 microM may explain the conflicting results between in vitro and in vivo investigations regarding drug interactions with clopidogrel.