TAXOL METABOLISM BY HUMAN LIVER-MICROSOMES - IDENTIFICATION OF CYTOCHROME-P450 ISOZYMES INVOLVED IN ITS BIOTRANSFORMATION

The biotransformation of taxol by human liver was investigated in vitr o with microsomes isolated from adult and developing human tissues. In vitro, no metabolism was detected with kidney microsomes, whereas two metabolites were generated by liver microsomes. The most prominent me tabolite, termed M5, corresponded to an hydroxylation at the C6 positi on on the taxane ring, while the other metabolite, termed M4, correspo nded to an hydroxylation at the para-position on the phenyl ring at th e C3'-position of the C13 side chain. These two taxol derivatives have been shown to be the major metabolites recovered in bile from a patie nt infused with taxol. Several approaches have been used to identify t he cytochrome P450 (CYP) isozymes involved in these reactions. No posi tive correlation was observed between the in vitro synthesis of these two metabolites, suggesting that two cytochrome P450 isozymes could be involved, although they could not be distinguished by their apparent affinities (K(m) almost-equal-to 15 muM). The formation of metabolite M4 was substantially reduced both by antibody directed against CYP3A a nd by the addition of CYP3A substrates such as orphenadrine, erythromy cin, troleandomycin, and testosterone. Conversely, the formation of me tabolite M5 remained unaffected by antibodies against CYP3A and by CYP 3A substrates but was sensitive to diazepam inhibition, a preferential substrate of CYP2C. Correlation between CYP2C content or diazepam dem ethylation and the synthesis of metabolite M5 was highly positive. The formation of metabolite M4 developed during the early postnatal perio d. tn contrast, the synthesis of metabolite M5 rose only after 3 month s of age. These data clearly implicate CYP3A in the formation of metab olite M4 and CYP2C in the synthesis of metabolite M5. Microsomes from patients treated with barbiturates and benzodiazepines increased the f ormation of metabolite M4 to the level of metabolite M5, demonstrating that drug interactions could modify the human metabolism of taxol.