We clarified that the single nucleotide polymorphisms in the promoter region or introns in the CYP2B6 affect the potency of cyclophosphamide activation to 4-hydroxycyclophosphamide. This information would be valuable for predicting adverse reactions and the clinical efficacy of cyclophosphamide.

Cytochrome P450 (CYP) 2A6 is a major CYP responsible for the metabolism of nicotine and coumarin in humans. We identified a novel allele, designated CYP2A6*17 , which contains A51G (exon 1), C209T (intron 1), G1779A (exon 3), C4489T (intron 6), G5065A (V365M, exon 7), G5163A (intron 7), C5717T (exon 8), and A5825G (intron 8). We developed a genotyping method by polymerase chain reaction-restriction fragment length polymorphism for the CYP2A6*17 allele, targeting the G5065A mutation. The allele frequency in black subjects (n = 96) was 9.4% (95% confidence interval [CI], 5.3%-13.5%). The allele was not found in white subjects (95% CI, 0%-0.9%; n = 163), Japanese subjects (95% CI, 0%-1.6%; n = 92), and Korean subjects (95% CI, 0%-0.7%; n = 209). To examine the effects of the amino acid change in the CYP2A6*17 allele on the enzymatic activity, we expressed a wild-type or variant (V365M) CYP2A6 together with NADPH-CYP reductase in Escherichia coli . For coumarin 7-hydroxylation, the apparent Michaelis-Menten constant value of variant CYP2A6 (1.06 +/- 0.11 micromol/L) was significantly (P < .005) higher than that of wild type (0.60 +/- 0.05 micromol/L). The maximum velocity values of the wild-type and variant CYP2A6 were 0.61 +/- 0.06 and 0.64 +/- 0.07 pmol . min -1 . pmol -1 CYP, respectively. For nicotine C -oxidation, the apparent Michaelis-Menten constant values of the wild-type or variant CYP2A6 were 31.6 +/- 2.9 micromol/L and 31.3 +/- 3.1 micromol/L, respectively. The maximum velocity value of variant CYP2A6 (0.72 +/- 0.21 pmol . min -1 . pmol -1 CYP) was significantly (P < .05) lower than that of the wild type (1.80 +/- 0.42 pmol . min -1 . pmol -1 CYP). Thus the intrinsic clearance values for coumarin 7-hydroxylation and nicotine C -oxidation by the variant were both significantly (P < .05) decreased to 40% to 60% compared with the wild type. Furthermore, cotinine/nicotine ratios after 1 piece of nicotine gum was chewed, used as an index of in vivo nicotine metabolism, were significantly (P < .05) decreased in heterozygotes of the CYP2A6*17 allele (5.4 +/- 2.7, n = 12) compared with homozygotes of the wild type (11.5 +/- 10.5, n = 37). A subject with CYP2A6*17 / CYP2A6*17 revealed the lowest cotinine/nicotine ratio (1.8). We found a novel allele in black subjects that affects the nicotine metabolism in vitro and in vivo.

This article is available online at http://dmd.aspetjournals.org ABSTRACT:CYP2C8 plays important roles in metabolizing therapeutic drugs and endogenous compounds. Although genetic polymorphisms of CYP2C8 were reported, there is little information on CYP2C8 polymorphisms in the Japanese population. In the present study, we screened for previously described polymorphisms in the coding region of this gene using polymerase chain reaction (PCR)-restriction fragment length polymorphism or allele specific-PCR analyses. Eleven polymorphisms of CYP2C8*2 (I269F), CYP2C8*3 (R139K, K399R), CYP2C8*4 (I264M), CYP2C8*5 (frameshift), T130N, E154D, N193K, K249R, L390S, P404A, and H411L have been comprehensively investigated in at least 200 Japanese individuals. A single subject was heterozygous for CYP2C8*5, and the allele frequency was calculated as 0.0025. The other single nucleotide polymorphisms (SNPs) were not found in the Japanese subjects in the present study. Thus, it appears that the frequencies of these alleles in Japanese are extremely low. In addition, concerning the SNPs of T130N, E154D, N193K, K249R, and H411L, it remains clear that these alleles exist as polymorphisms or represent sequence errors or cloning artifacts. Although several SNPs such as CYP2C8*2, CYP2C8*3, CYP2C8*4, and P404A have been reported to reduce the enzymatic activity, pharmacokinetic abnormalities of drugs metabolized by polymorphic CYP2C8 might be rare in Japanese.

Interindividual differences in the pharmacokinetics of paclitaxel and its metabolites in Japanese ovarian cancer patients were investigated in relation to genetic polymorphisms of the CYP2C8, CYP3A4, and MDR1 genes. The area under the concentration-time curve (AUC) ratios of paclitaxel/6alpha-hydroxypaclitaxel and paclitaxel/3 -p-hydroxypaclitaxel calculated as the metabolic index of CYP2C8 and CYP3A4 showed 13- and 12-fold interindividual variations, respectively. No patient had any CYP2C8 variants, while 2 patients were heterozygotes of CYP3A4*16. For the MDR1 gene, the frequencies of -129C, 1236C, 2677T, 2677A, and 3435T alleles were 2.2%, 8.7%, 56.5%, 4.4%, and 52.2%, respectively. Subjects possessing the 3435T allele had a significantly (P < .05) higher AUC of 3'- p-hydroxypaclitaxel compared to those possessing the 3435C allele. Leukocytopenia was significantly (P < .05) related to the AUC of paclitaxel. Genotyping of the CYP2C8, CYP3A4, and MDR1 genes might not be essential to predict adverse effects of paclitaxel in Japanese patients, although an allelic variant of MDR1 may functionally affect the pharmacokinetics of its metabolite.