CYP3A activity in European American and Japanese men using midazolam as an in vivo probe.

OBJECTIVE: To investigate putative differences in CYP3A activity between European American and Japanese subjects using midazolam as an in vivo probe. METHODS: Midazolam was administered orally (2 mg) to 22 young healthy Japanese men and, on a separate occasion, to 19 of these by the intravenous route (1 mg). The disposition of the drug and its 1'-hydroxy metabolite were determined and compared with data collected in a similar fashion in 20 young healthy European American men. RESULTS: Plasma concentrations of midazolam, especially those attained soon after drug administration, were higher after intravenous injection in Japanese subjects than those in European American men. This observation was associated with smaller initial (2.5-fold) and steady-state (1.8-fold) volumes of distribution for the drug; normalization for body weight only modestly reduced these differences. The systemic clearance value of midazolam was 25% lower (P < .03) in Japanese subjects, but this difference was not apparent after accounting for the smaller body weights of that group. No statistical differences were noted in the elimination half-life (t 1/2) of midazolam between European American and Japanese subjects. Much greater interindividual variability was observed after oral administration compared with intravenous administration, but significant differences were not found between the 2 groups with respect to the maximum midazolam plasma level or its oral clearance. Absolute oral bioavailability and its associated gastrointestinal and hepatic extraction ratios also showed no statistically significant interracial differences. CONCLUSIONS: On average, hepatic CYP3A, as measured by the metabolism of midazolam, is lower in young healthy Japanese men compared with similar European Americans. However, there is considerable interindividual variability, and body size appears to be an important determinant. After oral administration, even greater variability in the plasma level-time profile of midazolam is present, and no statistically significant or clinically important interracial/ethnic difference is present. Possibly because of smaller body mass and differences in body composition, midazolam has a smaller distribution volume(s) in Japanese men than in European American men that might be an important factor when drugs are administered intravenously.

Structural characterization of a new variant of the CYP2A6 gene (CYP2A6*1B) apparently diagnosed as heterozygotes of CYP2A6*1A and CYP2A6*4C.

During the course of investigating the frequency of a CYP2A6 whole deletion-type polymorphism (CYP2A6*4C) in Japanese, an unexpectedly large population of heterozygotes for CYP2A6*4C and the wild-type (CYP2A6*1A) was found. Cloning of a cDNA encoding CYP2A6 from the liver of individuals judged as heterozygotes for CYP2A6*4C and the CYP2A6*1A was carried out to identify the causal allele(s) responsible for a possible overestimation. A clone isolated from the liver cDNA library possessed 58 bp sequences in the 3'-untranslated region, which was replaced with the corresponding region of the CYP2A7 gene. The same gene conversion existed in the genomic DNA, indicating that the replacement was not a cloning artifact. Based on the gene structure of the allele (CYP2A6*1B), this variant was thought to be one of the causal alleles responsible for overestimation of heterozygotes for CYP2A6*4C and CYP2A6* A. To investigate this further, we developed a genotyping method which could distinguish the CYP2A6*A, CYP2A6*1B and CYP2A6*4C alleles from each other. The results clearly showed that CYP2A6*1B was the sole allele responsible for the overestimation. We conclude that the new genotyping method allows determination of six genotypes of the CYP2A6 gene, simultaneously and precisely, in both Oriental and Caucasian populations.

Analysis of the CYP2D6 gene in relation to dextromethorphan O-demethylation capacity in a Japanese population.

OBJECTIVE: To analyze the CYP2D6 allele frequencies in a Japanese population and to evaluate the effects of CYP2D6 variants on in vivo CYP2D6 activity as measured by the dextromethorphan metabolic ratio (MR). METHODS: Ninety-eight unrelated, healthy Japanese men were phenotyped with dextromethorphan and genotyped by the polymerase chain reaction amplification method for 7 CYP2D6 alleles. RESULTS: The CYP2D6*1, CYP2D6*10, CYP2D6*2, CYP2D6*5, CYP2D6*4, and CYP2D6*21 allele frequencies in our Japanese subjects were 0.423, 0.408, 0.092, 0.061, 0.020, and 0.010, respectively. Thirty-three subjects (33.7%) were heterozygous for *10/*1, and 18 (18.4%) and 17 (17.3%) subjects were homozygous for *1 and *10, respectively. Subjects who were homozygous for *10 showed the highest dextromethorphan MR among these 3 genotypes. Eighteen subjects (18.3%) were heterozygous for *2, but their dextromethorphan MR values were not greater than the MR values of subjects who were homozygous for *1. One subject was a poor metabolizer phenotypically, and he was homozygous for *5. CONCLUSIONS: The CYP2D6 allele frequencies in our Japanese subjects differed from those determined in previous studies of white subjects or mainland Chinese subjects. Individuals homozygous for *10 who have relatively low in vivo CYP2D6 activity represent almost 20% of the Japanese population. In addition, we did not identify any subjects with amplified *2 among our 98 Japanese men.