Selective Biotransformation of Taxol to 6a-Hydroxytaxol by Human Cytochrome P4502C81

The prIncipal taxol biotransformation reaction of humans and of hu man hepatic in vitro preparations is 6a-hydroxylation of the taxane ring, but a separate, minor hydroxylation pathway (metabolite B formation) also exlst& Taxol metabolism was studied using membrane fractions from Hep G2 cells Infected with recombinant vacdnla viruses that contain complementary DNAS encoding several human cytochrome P450 en zymes. Only P450 2C8 formed detectable 6a-hydroxytaxol. Metaboilte B formation was catalyzed by complementary DNA-expressed 3A3 and 3A4, but not by 3A5. Each P450 3A preparation catalyzed felodipine oxidation. The apparent Km *fld V@ values for taxed 6a-hydroxylation were 5.4 ± 1.0 @1Mand 30 ± 1.5 mnol/inIn/nmol P450, respectively, for corn plernentary DNA-expressed P450 2C8; the values were 4.0 ±1.0 @iMand 0.87±0.06nmol/min/mgprotein,respectively, for humanhepaticmicro senses. The Inhibition of 6a-hydroxytaxol formation by quercetin was competitive with an apparent K, of 1.3 or 1.1 p@swith 2C8 or hepatic microsomes, res@veiy; retinoic acid was Inhibitory, showing an appar ent K1of 27 @tMwith hepatic microsomes; inhibition by tolbutamide was complex, weak, and unlikely to be clinically relevant. The correlation between hepatic 2C8 protein content and 6a-hydroxytaxol formation was high (r@ = 0,82), while the correlation with 2C9 content was low