Metabolism of 7-benzyloxy-4-trifluoromethyl-coumarin by human hepatic cytochrome P450 isoforms

1. The metabolism of 7-benzyiosy-1-trifluoromethylcoumarin (BFC) to 7-hydro xy-4-trifluoromethylcoumarin (HFC) was studied in human liver microsomal pr eparations and in cDNA-expressed human cytochrome P450 (CYP) isoforms. 2. Kinetic analysis of the NADPH-dependent metabolism of BFC to HFC in four preparations of pooled human liver microsomes revealed mean (+/- SEM) K-m and V-max of 8.3+/-1.3 muM and 454+/-98 pmol/min/mg protein respectively. 3. The metabolism of BFC to HFC was determined in a characterized bank of 2 4 individual human liver microsomal preparations employing BFC substrate co ncentrations of 20 and 50 muM (i.e. about two and six times K-m respectivel y). With 20 muM BFC the highest correlations were observed between BFC meta bolism and markers of CYP1A2 (r(2) = 0.784-0.797) and then with CYP3A (r(2) = 0.434-0.547) isoforms, whereas with 50 muM BFC the highest correlations were observed between BFC metabolism and markers of CYP3A (r(2) = 0.679-0.8 37) and then with CYP1A2 (r(2) = 0.421-0.427) isoforms. At both BFC substra te concentrations, lower correlations were observed between BFC metabolism and enzymatic markers for CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 a nd CYP4A9/11. 4. Using human beta -lymphoblastoid cell microsomes containing cDNA-express ed CYP isoforms, 20 muM BFC was metabolized by CYP1A2 and CYP3A4, with lowe r rates of metabolism being observed with CYP2C9 and CYP2C19. Kinetic studi es with the CYP1A2 and CYP3A4 preparations demonstrated a lower K-m with th e CYP1A2 preparation, but a higher V-max with the CYP3A4 preparation. 5. The metabolism of 20 muM BFC in human liver microsomes was inhibited to 37-48% of control by 5-100 muM of the mechanism-based CYP1A2 inhibitor fura fylline and to 64-69% of control by 5-100 muM of the mechanism-based CYP3A4 inhibitor troleandomycin. While some inhibition of BFC metabolism was obse rved in the presence of 100 and 200 muM diethyldithiocarbamate, the additio n of 2-50 muM sulphaphenazole, 50-500 muM S-mephenytoin and 2-50 muM quinid ine had little effect. 6. The metabolism of 20 muM BFC to HFC in human liver microsomes was also i nhibited by an antibody to CYP3A4, whereas antibodies to CYP2C8/9 and CYP2D 6 had no effect. 7. In summary, by correlation analysis, use of cDNA-expressed CYP isoforms, chemical inhibition and inhibitory antibodies, BFC appears metabolized by a number of CYP isoforms in human liver. BFC metabolism appears to be prima rily catalysed by CYP1A2 and CYP3A4, with possibly some contribution by CYP 2C9, CYP2C19 and perhaps other CYP isoforms. 8. The results also demonstrate the importance of the selection of an appro priate substrate concentration when conducting reaction phenotyping studies with human hepatic CYP isoforms.