Frequency of cytochrome P450 CYP2C9 variants in a Turkish population and functional relevance for phenytoin

Aims The genetically polymorphic cytochrome P450 enzyme CYP2C9 metabolizes many important drugs. We studied the frequency of the amino acid variants cysteine 144 (CYP2C9*2 ) and leucine 359 (CYP2C9*3  ) in a Turkish population and the correlation between genotype and phenotype using phenytoin as probe drug.Methods CYP2C9 alleles *2 and *3 were measured in 499 unrelated Turkish subjects by PCR and restriction fragment length pattern analysis. Phenotyping was performed in a subgroup of 101 volunteers with a single oral dose of 300 mg phenytoin and concentration analysis in serum drawn 12 h after dosage.Results CYP2C9 allele frequencies in 499 unrelated Turkish subjects were 0.794 for CYP2C9*1, 0.106 for CYP2C9*2 and 0.100 for CYP2C9*3. Mean phenytoin serum concentrations at 12 h after dosage were 4.16 mg l −1 (95% CI 3.86–4.46) in carriers of the genotype CYP2C9*1/1, 5.52 mg l −1 (4.66–6.39) in CYP2C9*1/2, and 5.65 mg l −1 (4.86–6.43) in CYP2C9*1/3. These differences were significant and accounted for 31% of total variability in phenytoin trough levels. Mean 12 h concentration ratios of 5‐(para‐hydroxyphenyl)‐5‐phenylhydantoin/phenytoin (p‐HPPH/P) were 0.43 (0.39–0.47) for CYP2C9*1/1 compared with 0.26 (0.21–0.31) for CYP2C9*1/2, 0.14 (0.13–0.14) for CYP2C9*2/2, 0.21 (0.18–0.24) for CYP2C9*1/3, and 0.02 for CYP2C9*3/3; all mutant genotypes were significantly different compared with CYP2C9*1/1 .Conclusions Frequency of the two CYP2C9 variants in Turkish subjects was in a similar range as in other Caucasian populations. A significant proportion of the interindividual variability in phenytoin trough levels is explained by the genotypes. The 12 h serum concentrations after a single phenytoin dose may be used for routine phenotyping of CYP2C9 mediated metabolic clearance and the p‐HPPH/P ratios may be even more sensitive indicators of CYP2C9 activity.