Antiplatelet Effect of Clopidogrel Can Be Reduced by Calcium-Channel Blockers

Purpose: Clopidogrel is metabolized by the hepatic cytochrome P450 (CYP) system into its active thiol metabolite. CYP3A4 is involved in the metabolism of both clopidogrel and dihydropyridine calcium channel blockers (CCBs). A few reports have suggested an inhibitory interaction between CCBs and clopidogrel. Accordingly, the aim of this study was to determine the effect of CCBs on the antiplatelet activity of clopidogrel by serial P2Y12 reaction unit (PRU) measurements. Materials and Methods: We assessed changes in antiplatelet activity in patients receiving both clopidogrel and CCBs for at least 2 months prior to enrollment in the study. The antiplatelet activity of clopidogrel was measured by VerifyNow P2Y12 assay in the same patient while medicated with CCBs and at 8 weeks after discontinuation of CCBs. After discontinuation of the CCBs, angiotensin receptor blockers were newly administered to the patients or dosed up for control of blood pressure. Results: Thirty patients finished this study. PRU significantly decreased after discontinuation of CCBs (238.1 +/- 74.1 vs. 215.0 +/- 69.3; p=0.001). Of the 11 patients with high post-treatment platelet reactivity to clopidogrel (PRU >= 275), PRU decreased in nine patients, decreasing below the cut-off value in seven of these nine patients after 8 weeks. Decrease in PRU was not related to CYP2C19 genotype. Conclusion: CCBs inhibit the antiplatelet activity of clopidogrel.