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 muM) and cyclophosphamide had no effect. The inhibition was dependent on the concentrations of both MMDx and the coincubated drug. Thus, with 1 muM MMDx, paclitaxel (5 muM), tarnoxifen (1 muM) and cyclosporine (1 muM) decreased the metabolic rate of MMDx by 36%, 53% and 62%, respectively. At higher concentrations (10, 5 and 5 muM, 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.