title: Enantioselectivity in the cytochrome P450-dependent conversion of tegafur to 5-fluorouracil in human liver microsomes.
abstract: Tegafur (FT) is a prodrug of 5-fluorouracil (5-FU) used in cancer chemotherapy, and the bioactivation of FT to 5-FU is mainly catalyzed by cytochrome P450 (CYP) in hepatic microsomes. FT has a chiral center and is a racemate consisting of the enantiomers, R- and S-FT. In the present study, we clarified the enantioselectivity in the conversion of FT to 5-FU and identified human CYP isoforms involved in the metabolism of its enantiomers using human hepatic preparations and recombinant CYP isoforms. Although 5-FU was generated from both FT enantiomers, R-FT was a preferred substrate than S-FT, because of the considerably higher intrinsic clearance for 5-FU formation from R-FT in liver. Eadie-Hofstee plots in microsomes showed that the conversions of R- and S-FT to 5-FU followed biphasic and monophasic kinetics, respectively. Based on the evaluation using cDNA-expressed enzymes, CYP2A6 showed the highest activity for 5-FU formation from R-FT with the K m value similar to that of the high-affinity component in microsomes. Also, CYP2A6 was the most effective catalyst for S-FT. Inhibition studies using CYP-selective inhibitors and anti-CYP antibodies demonstrated that CYP2A6 mainly contributed to the enantioselective metabolism of FT, and were almost in accordance with the relative percentage contribution of each CYP isoform to the metabolism of FT estimated using relative activity factor methods. These results suggest that the enantioselectivity in the bioactivation of FT to 5-FU in humans is mainly due to the large difference of the catalytic activity of CYP2A6 between R- and S-FT.
authors.affiliation: Pharmacokinetics Research Laboratories, Taiho Pharmaceutical Co. Ltd. Tsukuba, Japan ; Graduate School of Pharmaceutical Sciences, Kyushu University Fukuoka, Japan.
authors.forename: Ikuo
authors.initials: I
authors.lastname: Yamamiya
authors.affiliation: Pharmacokinetics Research Laboratories, Taiho Pharmaceutical Co. Ltd. Tsukuba, Japan.
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Yoshisue
authors.affiliation: Graduate School of Pharmaceutical Sciences, Kyushu University Fukuoka, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Ishii
authors.affiliation: Graduate School of Pharmaceutical Sciences, Kyushu University Fukuoka, Japan.
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Yamada
authors.affiliation: Pharmacokinetics Research Laboratories, Taiho Pharmaceutical Co. Ltd. Tsukuba, Japan.
authors.forename: Ken-Ichiro
authors.initials: K
authors.lastname: Yoshida
keywords: 5-fluorouracil; cytochrome P450 2A6; enantioselectivity; microsomes; tegafur; thymidine phosphorylase