SELECTIVE BIOTRANSFORMATION OF TAXOL TO 6-ALPHA-HYDROXYTAXOL BY HUMANCYTOCHROME-P450 2C8

The principal taxol biotransformation reaction of humans and of human hepatic in vivo preparations is 6 alpha-hydroxylation of the taxane ri ng, but a separate, minor hydroxylation pathway (metabolite B formatio n) also exists. Taxol metabolism was studied using membrane fractions from Hep G(2) cells infected with recombinant vaccinia viruses that co ntain complementary DNAs encoding several human cytochrome P450 enzyme s. Only P450 2C8 formed detectable 6 alpha-hydroxytaxol. Metabolite B formation was catalyzed by complementary DNA-expressed 3A3 and 3A4, bu t not by 3A5. Each P450 3A preparation catalyzed felodipine oxidation. The apparent K-m and V-max values for taxol 6 alpha-hydroxylation wer e 5.4 +/- 1.0 mu M and 30 +/- 1.5 nmol/min/nmol P450, respectively, fo r complementary DNA-expressed P450 2C8; the values were 4.0 +/- 1.0 mu M and 0.87 +/- 0.06 nmol/min/mg protein, respectively, for human hepa tic microsomes. The inhibition of 6 alpha-hydroxytaxol formation by qu ercetin was competitive with an apparent K-i of 1.3 or 1.1 mu M with 2 C8 or hepatic microsomes, respectively; retinoic acid was inhibitory, showing an apparent K-i of 27 mu M with hepatic microsomes; inhibition by tolbutamide was complex, weak, and unlikely to be clinically relev ant. The correlation between hepatic 2C8 protein content and 6 alpha-h ydroxytaxol formation was high (r(2) = 0.82), while the correlation wi th 2C9 content was low (r(2) = 0.38). These data show that human biotr ansformation routes of taxol result from catalysis by specific enzymes of two P450 families and that taxol 6 alpha-hydroxylation is a useful indicator of P450 2C8 activity in human hepatic microsomes.