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Taxotere, a promising anticancer agent, is metabolized almost exclusively in liver and excreted from bile in all species. To determine which cytochrome P450 is involved in taxotere biotransformation, 11 cDNA-expressed human cytochrome P450s were examined for their activity in the metabolism of taxotere and its derivatives. Of all P450s, cytochrome P450 3A4 and 3A5 were the most active for the oxidation of taxotere to the primary metabolite RPR 1049 5 2 and for subsequent oxidation of RPR104952 to RPR111059 and RPR111026. RP70617, an epimer of taxotere was also metabolized by both P450 3A enzymes to form metabolite XII. The activity of 3A4/5 enzymes for these substrates was 4-50-fold greater than the other P450s examined. The Kms of 3A4 and 3A5 for taxotere were 0.91 and 9.28 μM, and Vmax for the formation of RPR104952 were 1.17 and 1.36 m-1, respectively. The contribution of the 3 A enzyme complex to the metabolism of taxotere in human livers from 21 individuals was assessed with the inhibitory monoclonal antibody and ranged from 64-93%. The primary oxidative metabolism of taxotere by human liver microsomes was well correlated with 3A4-dependent reactions for testosterone 6β -hydroxyIation (r2=0.84), taxol aromatic hydroxylation (r2=0.67) and aflatoxin B, 3α -hydroxylation (r2=0.63); whereas a poor correlation was found for reactions specifically catalysed by other P450s (all r2 ≤ 0.17). The extent of taxotere metabolism also closely correlated with levels of 3A4 enzyme in human livers quantified with immunoblot monoclonal antibody (r2=0.61). These results demonstrate that the P450 3A4 and 3A5 enzymes are major determinants in taxotere oxidation and suggest that care must be taken when administering this drug with other drugs that are also Substrates for these enzymes.