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Background & objectives: Phenytoin, a widely used anti-epileptic drug, is metabolized mainly by CYP2C9 (90%) and partly by CYP2C19 (10%) to its major metabolite 5-(para-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). The CYP2C9 and CYP2C19 genes encoding these enzymes are polymorphically expressed and most of the variants result in decreased metabolism of the respective substrates. The present study was undertaken to investigate the influence of the CYP2C9*2 and *3 as well as CYP2C19*2 and *3 variant genotypes on phenytoin hydroxylation in healthy subjects from south India.
Methods: A total of 27 healthy, unrelated, subjects were administered a single oral dose of 300 mg phenytoin. Four hours later, 5 ml of blood was collected and genotyped for CYP2C9*1, *2, *3, CYP2C19*1, *2 and *3 by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Phenytoin and the major metabolite p-HPPH were estimated by reverse phase HPLC. The metabolic ratio was calculated as concentration of phenytoin/p-HPPH.
Results: A significant correlation was observed between the CYP2C9 genotype and metabolic ratio of phenytoin/p-HPPH (r = 0.472, 95% CI 0.100 to 0.728; P = 0.01). While no association was found with CYP2C19 alone, a significant correlation was observed between the combined CYP2C9 and CYP2C19 genotypes and phenytoin metabolic ratio (r = 0.507, 95% CI 0.146 to 0.749; P< 0.01).
Interpretation & conclusion: CYP2C9*2 and *3 mutant alleles caused decreased hydroxylation of phenytoin in vivo, whereas the mutant alleles of CYP2C19 played only a minor role in the metabolism of phenytoin in subjects of our study. The results of present preliminary study needs to be confirmed with a larger sample.
Key words CYP2C9 - CYP2C19 - metabolism - phenytoin - south Indians
Phenytoin, a widely used anti-epileptic drug, is known to exhibit marked inter-individual variation in pharmacokinetics, which has, to some extent, been attributed to genetic factors such as the CYP2C9 polymorphism1,2. Phenytoin is metabolized almost extensively by the cytochrome P450 (CYP)2C9 enzyme and to a small extent by CYP2C19 to its major metabolite S-(para-hydroxyphenyl)-phenylhydantoin (p-HPPH)3. CYP2C9, the gene encoding this enzyme, is polymorphically expressed with about 20 variant alleles4. Of these, CYP2C9*2, characterized by a C430T transversion on exon 3 producing amino acid change Arg^sup 144^Cys, and CYP2C9*3, resulting from an A1075C mutation on exon 7 causing Ile^sup 359^Leu amino acid substitution are the...