ABSTRACT:The CYP3A subfamily represents the most abundant cytochrome P450 in the human liver and gastrointestinal tract and plays very important role in xenobiotic metabolism. CYP3A5 is expressed in a relatively small population of whites and Orientals. We recruited 42 Chinese volunteers to determine the genotypes of CYP3A5 by polymerase chain reaction-restriction fragment length polymorphism. Genotype analyses revealed that CYP3A5*3 allele existed in 39 of 42 volunteers. CYP3A5*4 and CYP3A5*5 alleles were found in one volunteer each; and CYP3A5*2 and CYP3A5*6 alleles were not found. The most frequent CYP3A5*3 allele is known not to express CYP3A5. We excluded other genotypes of CYP3A5 to study the significance of CYP3A5*3 in midazolam pharmacokinetics. In this study, each volunteer was given a midazolam tablet (7.5 mg) orally.Blood samples were collected to analyze the time-dependent concentrations of midazolam and 1-hydroxymidazolam by high-performance liquid chromatography. The average area under plasma concentration curve (AUC, 0-8 h) of midazolam was 9237 ؎ 1050 ng-min/ml (mean ؎ S.E.M.) in homozygous CYP3A5*3 (n ‫؍‬ 14) subjects and 7934 ؎ 768 ng-min/ml in heterozygous CYP3A5*1/*3 (n ‫؍‬ 12) subjects, respectively. The average AUC (0-8 h) of 1-hydroxymidazolam was 3748 ؎ 427 ng-min/ml in homozygous CYP3A5*3 subjects and 3920 ؎ 402 ng-min/ml in heterozygous CYP3A5*1/*3 subjects. The results indicated that the pharmacokinetics of midazolam and 1-hydroxymidazolam was independent of CYP3A5 expression. Although the genetic polymorphism of CYP3A5 is well known, the results of this study suggested that the clinical consequence might be insignificant.