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 −2663delTG, 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. These findings may also be relevant to the clinical use of other PXR, CAR, and Nrf2 activators.
Footnotes
This work was supported by the National Institutes of Health National Institute of General Medical Sciences [Grants GM60346, GM61393]; the National Institutes of Health National Cancer Institute [Cancer Center Support Grant P30 CA21765]; the National Institutes of Health Neurological Disorders and Stroke [Grant P50 NS16308]; the National Institutes of Health National Institute of Diabetes and Digestive and Kidney Diseases [Contract N01-DK-9-2310]; the Welcome Trust [Grant 084730]; the UK Medical Research Council [Grant G0400126]; a Postdoctoral Research Fellowship from the American Epilepsy Society; the National Society for Epilepsy; the American Lebanese Syrian Associated Charities; the Research Grants Council of the Hong Kong Special Administrative Region, China [Project CUHK4519/06M]; the University College London Hospital Comprehensive Biomedical Research Centre [Grant F136]; and, in part, the United Kingdom Department of Health National Institute for Health Research Biomedical Research Centre.
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
doi:10.1124/jpet.109.161026
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The online version of this article (available at http://jpet.aspetjournals.org) contains supplemental material.-
ABBREVIATIONS:
- PHT
- phenytoin, 5,5-diphenylhydantoin, 5,5-diphenyl-2,4-imidazolinedione
- PXR
- pregnane X receptor
- CAR
- constitutive androstane receptor
- YY1
- Yin Yang 1 transcription factor
- LD
- linkage disequilibrium
- p-HPPH
- 5-(4′-hydroxyphenyl)-5-phenylhydantoin
- AED
- antiepileptic drug
- VAR
- variant
- WT
- wild type
- HAP
- haplotype
- rPM
- regulatory polymorphisms
- PCR
- polymerase chain reaction
- SNP
- single-nucleotide polymorphism
- HNF
- hepatic nuclear factor
- EMSA
- electrophoretic mobility shift assay
- ARE
- antioxidant response elements
- 5′-RACE
- 5′ rapid amplification of cDNA ends.
- Received August 26, 2009.
- Accepted October 19, 2009.
- Copyright © 2010 by The American Society for Pharmacology and Experimental Therapeutics
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