Clopidogrel is one of the most common antiplatelet drugs used to treat and prevent atherothrombotic events. The effects of clopidogrel are mediated by its active thiol metabolite, and clopidogrel is biotransformed into thiol metabolite by the hepatic cytochrome P450 isoforms including CYP2C19, CYP2B6, CYP3A4, and CYP3A5.[2, 3] Recently, PON1 genotype was issued as another genetic biomarker associated with formation of thiol metabolite with serious controversy.[3-7] Additionally, the efflux transporter P-glycoprotein affects clopidogrel bioavailability and active metabolite formation. Thus, polymorphisms leading to functional alterations in genes modulating clopidogrel pharmacokinetics are expected to reduce active metabolite formation and antiplatelet effect leading to poor clinical outcomes.[9, 10] Indeed, carriers of a CYP2C19 loss-of-function (LOF) alleles (*2 and *3 alleles) have a higher rate of subsequent cardiovascular events than noncarriers.
Despite its robust association with poor cardiovascular outcomes of clopidogrel therapy, several questions regarding clinical use of CYP2C19 genotype as a predictive biomarker for personalized antiplatelet therapy remain to be answered. The rapid onset of clopidogrel is generally required to improve early clinical outcomes, particularly during emergencies requiring percutaneous coronary intervention (PCI) for acute myocardial infarction. The onset time for maximal inhibition of platelet aggregation (IPA) is dependent on the dose of clopidogrel. Therefore, CYP2C19 genotypes that influence the plasma concentration of clopidogrel active thiol metabolite are expected to be associated with differential time to maximal IPA after clopidogrel administration. However, there are few reports describing the interaction between CYP2C19 genotypes and time course of antiplatelet aggregation response of clopidogrel.
To address these questions, we evaluated the effects of CYP2C19 genotypes on the time course of platelet aggregation inhibition after clopidogrel administration. In addition, we explored possible associations between clopidogrel pharmacokinetics or pharmacodynamics and genetic variants of PON1, CYP2B6, CYP3A4/5, ABCB1, and P2Y12, which are involved in the formation or action of active clopidogrel metabolites.
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We investigated the effects of CYP2C19 genotype on the time course of platelet aggregation after of clopidogrel administration in 92 Korean healthy subjects. In this study, we confirmed that CYP2C19 LOF alleles reduce the active metabolite formation and antiplatelet effects of clopidogrel.[11, 19] And, we found the new finding that CYP2C19 PM and IM genotypes exhibited delayed time to MPA after a clopidogrel loading dose, and the antiplatelet effects of clopidogrel decreased gradually in those with the CYP2C19 PM genotype. These results add to our mechanistic understanding of the effects of CYP2C19 genotype on pharmacokinetics and pharmacodynamics of clopidogrel and its thiol metabolites. In this study, subjects with CYP2C19 PM and IM genotypes had delayed time to MPA after a clopidogrel loading dose, platelet aggregation decreased over time in subjects the CYP2C19 PM genotype compared to those with CYP2C19 EM.
It is well known that the 600 mg loading dose of clopidogrel achieves maximal platelet inhibition at 2 hours while 300 mg dose achieves at 6 hours. Common clinical practice guideline recommend pretreatment with a loading dose of clopidogrel as early as possible before or at the time of PCI. Thus, delayed time to maximum IPA as well as decreased antiplatelet effects in the carriers of CYP2C19 PM and IM (9 and 12 hours of median Tmax in this study) may influence the increased risk of the acute cardiac events that require rapid onset of clopidogrel. Because Tmax to reach maximal response is dose-dependent, higher loading dose (600 or 900 mg) may overcome this delayed Tmax. It remained to be evaluated.
In this study, the subjects with CYP2C19 IM genotype retained the antiplatelet effects of clopidogrel at steady state unlike CYP2C19 PM genotype. The 300 mg loading dose administered to subjects with CYP2C19 IM was not sufficient to alter the pharmacodynamic effects as much as that in the CYP2C19 EM genotype groups, but 75 mg maintenance dose of clopidogrel preserved the antiplatelet effects seen with loading dose administration. In the case of CYP2C19 PM genotype, 75 mg maintenance dose did not keep even the decreased antiplatelet effects of 300 mg loading dose. Simon et al reported that 600 mg loading dose and 150 mg maintenance dose overcame diminished antiplatelet response. However, they did not report the difference in MPA decrease among different CYP2C19 genotype after administration of 75 mg clopidogrel. This discordance may be resulted from that this study has larger sample size, more frequent measurement of platelet aggregation, and conducted in homogeneous same ethnic population compared with previous study although it needs to be confirmed by replication.
The decreasing trend of IPA in subjects with CYP2C19 PM over time may be related to the delayed onset of cardiac events after clopidogrel administration in a clinical situation. This suggests that an additional clopidogrel dose may be guided differently depending on CYP2C19 IM and CYP2C19 PM genotype. Some reports based on clinical presentation not CYP2C19 genotypes have suggested that patients undergoing PCI and taking double dose of clopidogrel have significantly decreased stent thrombosis. However, recent reports based on CYP2C19 genotype such as Clinical Pharmacogenetics Implementation Consortium Guidelines and Dutch Pharmacogenetics Working Group Guideline have recommended for subjects with CYP2C19 IM and PM genotypes to use of an alternative agent (e.g., prasugrel, ticagrelor) if not contraindicated. Mega et al reported that higher dose of clopidogrel in subjects with CYP2C19 IM genotype resulted in comparable platelet inhibition compared to the standard 75 mg dose in those with CYP2C19 EM genotype. However, in CYP2C19 PM genotype, even 300 mg clopidogrel did not result in comparable degrees of platelet inhibition compared to the standard 75 mg dose in CYP2C19 EM genotype. Accordingly, the patients with the CYP2C19 IM genotype may benefit from both higher dose of clopidogrel and more potent ADP receptor blockers such as prausgrel or ticagrelor, but those with the CYP2C19 PM genotype benefit from alternative therapy rather than a higher dose of clopidogrel, particularly those with acute coronary syndromes receiving PCI.
Some studies have reported a negative correlation between clopidogrel and IPA. However, we did not observe any correlation between clopidogrel and IPA, or between clopidogrel and its thiol metabolite. Only less than 15% of Clopidogrel is biotransformed to thiol metabolite, mainly to inactive carboxyl metabolite via carboxyl esterase. Decreased formation of the thiol metabolite may not be associated with a higher plasma level of clopidogrel due to formation of carboxyl metabolite by the high capacity carboxyl esterase.
Bouman et al suggested that PON1 is involved in the formation of the active thiol metabolite of clopidogrel, and thus PON1 Q192R genotype was significantly associated with poor antiplatelet effect and clinical outcome of clopidogrel therapy. However, soon thereafter, several reports showed that PON1 genotype had no effect on the antiplatelet effects of clopidogrel.[5, 27] It has been also reported that PON1 plays a major role in formation of endo-thiol metabolite, while CYP2C19 lead to the formation of active cis-thiol metabolite. Our results confirmed that PON1 genotype was not associated with the antiplatelet effects and pharmacokinetic parameters of clopidogrel. Because the contribution of the endo-thiol metabolite concentration level to total thiol concentration level is minimal (20-fold lower than that of cis-thiol metabolite), there seemed to be no marked difference between total thiol concentration and cis-thiol metabolite concentration. Furthermore, no effects of candidate SNPs in genes involved in clopidogrel pharmacokinetic and pharmacodynamic pathways including CYP2B6, CYP2C9, CYP3A5, ABCB1, and P2Y12 on clopidogrel response were observed, which is consistent with other studies.[9, 11]
In conclusion, CYP2C19 PM and IM genotypes exhibited delayed time to MPA after a clopidogrel loading dose, and the antiplatelet effects of clopidogrel decreased gradually in those with the CYP2C19 PM genotype during maintenance dose of clopidogrel.