Impact of the haplotype CYP3A4*16B harboring the Thr185Ser substitution on paclitaxel metabolism in Japanese patients with cancer

Objective: Paclitaxel is one of the most important anticancer drugs for the treatment of various tumors such as non-small cell lung cancer. We investigated the association between CYP3A4 haplotypes and pharmacokinetic parameters of paclitaxel metabolism. Methods: This study enrolled 235 Japanese patients with cancer who were receiving paclitaxel. These patients were screened for CYP3A4 gene polymorphisms by either direct sequencing or pyrosequencing. Plasma concentrations of paclitaxel and its 3 metabolites were determined by HPLC in 229 patients. Results: Median values of paclitaxel clearance, normalized for body surface area, were lower in the high-dose group (>= 175 mg/m(2), n = 199) than in the low-dose group (:5100 mg/m(2), n = 30), suggesting nonlinearity of pharmacokinetics caused by Cremophor EL. Therefore we mainly used the patients in the high-dose group for further analysis. The median value of the area under the curve (AUC) ratio of 3'-p-hydroxypaclitaxel to paclitaxel was 20% greater (95% confidence interval, 2.4% to 41.7%) in the women than in the men (P = .01). Although no significant difference in paclitaxel clearance was observed among the CYP3A4 haplotypes, *16B-bearing (*1/*16B) patients (n = 8) showed a 20% lower (95% confidence interval, -58.5 to -11.5%) median AUC ratio of 3'-p-hydroxypaclitaxel to paclitaxel (P = .04) and a 2.4-fold higher median AUC ratio of 6 alpha-hydroxypaclitaxel to paclitaxel (P < .001) compaed with those in the wild-type (*1/*1) patients (n = 180). In contrast, no significant differences were seen between *1/*1 and *18B/*1 (n = 10) patients for the pharmacokinetic parameters examined. Conclusions: Our results suggest that CYP3A4*16B is associated with both reduced 3'-p-hydroxylation of paclitaxel and probably increased levels of 6et-hydroxypaclitaxel.