Phenytoin metabolic ratio: a putative marker of CYP2C9 activity in vivo

CYP2C9 mediates the oxidative metabolism of approximately 10% of drugs, som e of which are characterized by a narrow therapeutic index. We aimed to val idate genotype method and phenotype methodology, for evaluation of CYP2C9 a ctivity in vivo. Thirty-one healthy subjects (22 male) received a single 30 0 mg dose of phenytoin. Blood was drawn periodically and urine was collecte d at intervals for 96 h. Plasma phenytoin and 5-(4-hydroxyphenyl)-5-phenylh ydantoin (p-HPPH) and urine S and R enantiomers of p-HPPH were determined b y high-performance liquid chromatography. CYP2C9 genotyping was obtained by polymerase chain reaction followed by digestion with Sau961 and StyI for t he identification of CYP2C9*2 and CYP2C9*3, respectively. Eighteen subjects were CYP2C9*1 homozygous, seven were CYP2C9*2 heterozygous, four were CYP2 C9*3 heterozygous, one was CYP2C9*2 homozygous and one was compound CYP2C9* 2/CYP2C9*3 heterozygous. The allele frequencies of CYP2C9*1, CYP2C9*2 and C YP2C9*3 were 0.76 [95% confidence interval (CI) 0.73-0.79], 0.16 (95% Cl 0. 13-0.19) and 0.08 (95% Cl 0.05-0.11), respectively. The CYP2C9-mediated pro duction of (S)-p-HPPH represented the major metabolic pathway of phenytoin biotransformation as its excretion accounted for 95.6 + 0.9% of 'total' p-H PPH excretion over the 96 h collection interval. Phenytoin metabolic cleara nce to produce (S)-p-HPPH (PMC), correlated significantly with (S)-p-HPPH ( or 'total' p-HPPH) content in 0-8, 0-12 and 0-24 urine collections (r = 0.8 8, 0.85 and 0.89, respectively) and with phenytoin metabolic ratio (PMR) de fined as the ratio of urine (S)-p-HPPH (or 'total' p-HPPH) to mid-interval plasma phenytoin (r = 0.90, 0.88 and 0.94, respectively). PMC and PMR exhib ited a gene-dose effect so that the highest and lowest values were noted in homozygous subjects CYP2C9*1 and subjects carrying two defective alleles, respectively, whereas heterozygous subjects had intermediate values. CYP2C9 genotyping and several phenytoin metabolic indices are correlated with CYP 2C9 activity in vivo. The utility of phenytoin to predict the metabolism of other CYP2C9 substrates justifies further evaluation. Pharmacogenetics 11: 587-596 (C) 2001 Lippincott Williams & Wilkins.