Vol. 28, Issue 12, 1457-1463, December 2000

Roles of Cytochromes P450 1A2, 2A6, and 2C8 in 5-Fluorouracil Formation from Tegafur, an Anticancer Prodrug, in Human Liver Microsomes

Tomoko Komatsu, Hiroshi Yamazaki, Noriaki Shimada, Miki Nakajima, and Tsuyoshi Yokoi

Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa, Japan (T.K., H.Y., M.N., T.Y.); and Daiichi Pure Chemicals, Ibaraki, Japan (N.S.)

Tegafur, an anticancer prodrug, is bioactivated to 5-fluorouracil (5-FU) mainly by cytochrome P450 (P450) enzymes. The conversion from tegafur into 5-FU catalyzed by human liver microsomal P450 enzymes was investigated. In fourteen cDNA-expressed human P450 enzymes having measurable activities, CYP1A2, CYP2A6, CYP2E1, and CYP3A5 were highly active in catalyzing 5-FU formation at a tegafur concentration of 100 µM. Kinetic analysis revealed that CYP1A2 had the highest V_max/K_m value and that the V_max value of CYP2A6 was high in 5-FU formation. In human liver microsomes, the activities of 5-FU formation from 10 µM, 100 µM, and 1 mM tegafur were significantly correlated with both coumarin 7-hydroxylation (r = 0.83, 0.86, and 0.74) and paclitaxel 6-hydroxylation (r = 0.77, 0.62, and 0.85) activities, respectively. Coumarin efficiently inhibited the 5-FU formation activities from 100 µM and 1 mM tegafur catalyzed by human liver microsomes that had high coumarin 7-hydroxylation activity. On the other hand, furafylline, fluvoxamine, and quercetin, as well as coumarin, showed inhibitory effects in liver microsomes that had high catalytic activities of 5-FU formation. The other P450 inhibitors examined showed weak or no inhibition in human liver microsomes. Polyclonal anti-CYP1A2 antibody, monoclonal anti-CYP2A6, and anti-CYP2C8 antibodies inhibited 5-FU formation activities to different extents in those two microsomal samples. These results suggest that CYP1A2, CYP2A6, and CYP2C8 have important roles in human liver microsomal 5-FU formation and that the involvement of these three P450 forms differs among individual humans.