Objective: To determine whether the gene dosage of CYP2C19 affects the metabolism of diazepam and desmethyldiazepam in healthy Chinese subjects.
Subjects and methods: Eighteen unrelated adult men were recruited for the study from a total of 101 healthy Chinese volunteers who had been screened for CYP2C19 phenotype and genotype. All subjects received a single oral dose (5 mg) of diazepam, and the pharmacokinetics of diazepam and desmethyldiazepam were compared in six ml homozygotes (ml/ml), six ml heterozygotes (wt/ml), and six wild-type homozygotes (wt/wt).
Results: The plasma elimination half-life values of diazepam (84.0 +/- 13.7 hours) and desmethyldiazepam (176.0 +/- 28.9 hours) in subjects of ml/ml were significantly longer than those (62.9 +/- 9.8 hours for diazepam; 132.1 +/- 24.9 hours for desmethyldiazepam; both P < .01) in subjects of wt/ml or those (20.0 +/- 10.8 hours for diazepam; 99.2.+/- 21.7 hours for desmethyldiazepam; both P < .01) in subjects of wt/wt. A significant difference in the corresponding half-life values existed between the wt/ml and wt/wt subjects (P < .01). As expected, the slowest mean clearance of diazepam was observed in the ml/ml subjects (2.8 +/- 0.9 mL/min) and the fastest in the wt/wt subjects (19.5 +/- 9.8 mL/min), with the wt/ml heterozygotes having an intermediate value (7.2 +/- 2.6 mL/min).
Conclusion: The presence of a single-nucleotide polymorphism (G681A) of the CYP2C19 gene cosegregates with the impaired metabolism of diazepam and desmethyldiazepam among Chinese subjects in a gene-dosage effect manner.