Characterization of human cytochrome P450 isoenzymes involved in the metabolism of vinorelbine.

Vinorelbine (VRL) (IV Navelbine) is a semi-synthetic vinca alkaloid, used in therapeutics for the treatment of non-small-cell lung cancer and advanced breast cancer. The aim of this study was to characterize the cytochrome P450 (CYP) isoenzymes involved in VRL metabolism. VRL was incubated at 1.28 x 10(-5) m for 90 min with human hepatic microsomes prepared from 14 donors (one woman and 13 men aged from 27 to 76 years old) and characterized for CYP1A2, CYP2D6, CYP2E1 and CYP3A4 activities. A four-combined-approach study was performed, including correlation between CYP activities and VRL metabolism among the 14 batches of microsomes, inhibition of VRL biotransformation by isoform-selective substrates and by specific inhibitory antibodies, and incubation with supersomes. Analysis of unchanged VRL and its metabolites was performed using an HPLC method coupled with both radioactive and UV detections. No correlation between CYP1A2 or CYP2E1 and VRL metabolism was observed in the 14 batches of microsomes used. A correlation was shown between VRL metabolism and CYP3A4 activity as determined with the dextromethorphan N-demethylase and nifedipine oxidase activities (r(2)=0.80 and 0.59, respectively). These results were strengthened by a correlation between the metabolism extent of VRL and CYP3A4 protein content determined by immunoblotting (r(2)=0.75). Furthermore, VRL biotransformation was inhibited by troleandomycine, the CYP3A4-specific inhibitor substrate (80% of inhibition) and by anti-CYP3A antibodies (36% of inhibition). On the contrary, a low correlation with CYP2D6 activity as determined by dextrometorphan O-demethylation (r(2)=0.31) was established. CYP2D6 supersomes did not metabolize the drug whereas 63.4% of VRL were metabolized by microsomes overexpressing CYP3A4 isoform. These data indicated that CYP3A4 is the main enzyme involved in the hepatic metabolism of VRL in human, whereas CYP2D6 is not involved.

Effects of paclitaxel, cyclophosphamide, ifosfamide, tamoxifen and cyclosporine on the metabolism of methoxymorpholinodoxorubicin in human liver microsomes.

The effects of paclitaxel, cyclosporine, cyclophosphamide, ifosfamide and tamoxifen on the metabolism of methoxymorpholinodoxorubicin (MMDx), a novel anticancer agent, were investigated using human liver microsomes. Paclitaxel, tamoxifen and cyclosporine dramatically inhibited MMDx metabolism, whereas ifosfamide had only a slight effect at high concentrations (200-300 microM) and cyclophosphamide had no effect. The inhibition was dependent on the concentrations of both MMDx and the coincubated drug. Thus, with 1 microM MMDx, paclitaxel (5 microM), tamoxifen (1 microM) and cyclosporine (1 microM) decreased the metabolic rate of MMDx by 36%, 53% and 62%, respectively. At higher concentrations (10, 5 and 5 microM, respectively, with paclitaxel, tamoxifen and cyclosporine) the inhibition was 52%, 91% and 91%, respectively. These three drugs preferentially inhibited the formation of three metabolites (M2, M3 and M6) among eight metabolites produced in liver microsomes. The inhibitory concentrations of paclitaxel, tamoxifen and cyclosporine on MMDx metabolism were in the range of those observed in patients upon administration of these drugs, which are known to be CYP3A4 substrates. These findings suggest that CYP3A4 drug substrates and MMDx in combination must be used with caution, particularly in view of the fact that MMDx is considered as a prodrug whose activation is entirely dependent upon metabolic transformation by CYP3A4.