CYP2C9*1BPromoter Polymorphisms, in Linkage withCYP2C19*2, Affect Phenytoin Autoinduction of Clearance and Maintenance Dose

Abstract

The commonly prescribed antiepileptic drug phenytoin has a narrow therapeutic range and wide interindividual variability in clearance explained in part by CYP2C9 and CYP2C19 coding variants. After finding a paradoxically low urinary phenytoin metabolite (S)/(R) ratio in subjects receiving phenytoin maintenance therapy with a CYP2C9*1/*1 and CYP2C19*1/*2 genotype, we hypothesized that CYP2C9 regulatory polymorphisms (rPMs), G-3089A and Ϫ2663delTG, in linkage disequilibrium with CYP2C19*2 were responsible. These rPMs explained as much as 10% of the variation in phenytoin maintenance dose in epileptic patients, but were not correlated with other patients' warfarin dose requirements or with phenytoin metabolite ratio in human liver microsomes. We hypothesized the rPMs affected CYP2C9 induction by phenytoin, a pregnane X receptor (PXR), and constitutive androstane receptor (CAR) activator. Transfection studies showed that CYP2C9 reporters with wild-type versus variant alleles had similar basal activity but significantly greater phenytoin induction by cotransfected PXR, CAR, and Nrf2 and less Yin Yang 1 transcription factor repression. Phenytoin induction of CYP2C9 was greater in human hepatocytes with the CYP2C9 wild type versus variant haplotype. Therefore, CYP2C9 rPMs affect phenytoin-dependent induction of CYP2C9 and phenytoin metabolism in humans, with an effect size comparable with that for CYP2C9*2 and 2C9*3.

CYP2C9 Promoter Variable Number Tandem Repeat Polymorphism Regulates mRNA Expression in Human Livers

“…The independent in vivo effects of pVNTR-S on metabolism of other substrates requires further investigation. We also confirm the previously proposed reduction of CYP2C9 inducibility caused by the minor alleles of SNP11 and SNP 12 (Kramer et al, 2008;Chaudhry et al, 2010). Together, this study provides a comprehensive analysis of regulatory variants in the CYP2C9 promoter region that can serve as a guide for assessing regulatory genetic factors in CYP2C9 with potential impact on drug therapy.…”

“…3A). SNP11 and SNP12 were shown to reduce rifampicin-or phenytoin-induced promoter activity in reporter gene assays (Kramer et al, 2008;Chaudhry et al, 2010) and were associated with the phenytoin maintenance dose (Chaudhry et al, 2010), but they did not associate with AEI status (k coefficient of 0.066 and 0.096, respectively) (Supplemental Fig. 3, B and C), indicating these two SNPs did not affect constitutive CYP2C9 expression in livers, consistent with the previous studies by Kramer et al (2008) and Chaudhry et al (2010) that these variants do not affect constitutive CYP2C9 promoter activity but only inducible activity.…”

“…One hundred sixty-eight human liver biopsy or autopsy samples were obtained from the Cooperative Human Network Midwestern and Western Division, under the approval of The Ohio State University Institutional Review Board. Because CYP2C9 promoter polymorphisms can affect CYP2C9 inducibility (Kramer et al, 2008;Chaudhry et al, 2010), we excluded livers from individuals with known usage of CYP2C9 inducers (phenytoin, phenobarbital, ethanol, carbamazepine, etc.). Four hundred thirty DNA samples from patients who were taking sulfomethoxazole [(SMX) cohort], collected for another study (Wang et al, 2011b), were also used in this study to determine the distribution of CYP2C9*2, CYP2C9*3, and the newly identified pVNTR-S.…”

“…In particular, reporter gene assays showed promoter SNP Ϫ4302CϾT and haplotypes H3A and H3B (or pattern 6, containing Ϫ981GϾA, Ϫ1537CϾT, Ϫ1885CϾG, and Ϫ1911TϾA) reduced constitutive promoter activity (Takahashi et al, 2004;Kramer et al, 2008), whereas Ϫ2663delTG and/or Ϫ3089GϾA reduced pregnane X receptor (PXR) or phenytoin-mediated induction of CYP2C9 promoter activity (Kramer et al, 2008;Chaudhry et al, 2010). However, it is unclear whether these polymorphisms or haplotypes affect CYP2C9 mRNA expression in human livers, because conclusions derived from reporter gene assays in transfected cells are not always consistent with in vivo gene expression and regulation.…”

Genetic Biomarkers in Epilepsy

“…Promising studies exist in which multivariate models, including genetics, as well as age, gender, and body weight, were used for predicting PHT and CBZ dosing in epilepsy patients [84,85].…”