The human liver metabolism of paclitaxel (Taxol), an anticancer drug, leads to three metabolites: 6alpha-hydroxypaclitaxel, 3'-p-hydroxypaclitaxel and 6alpha,3'-p-dihydroxypaclitaxel. The inter-individual variability of paclitaxel metabolism was investigated first in vitro using 22 human liver microsomes. Three metabolites have been detected by HPLC. This preliminary work revealed marked inter-individual differences in paclitaxel metabolism. The amount of major metabolite 6alpha-hydroxypaclitaxel formed varied 16-fold (0.7 to 11.5 nmol/mg/h). We next studied the effect of 29 compounds (antineoplastics, antiemetics, histamine-2 receptor antagonist, antalgics, antifungals, antivirals, psychotropics, antibiotic, corticoid, antiarrhythmic, calcium channel blocker) on paclitaxel metabolism in human liver microsomes. Among the compounds studied, quercetin, antifungal drugs such as ketoconazole and miconazole, and the antineoplastic drug doxorubicin inhibited formation of 6alpha-hydroxypaclitaxel. Dixon plots indicated that quercetin and doxorubicin inhibited 6alpha-hydroxypaclitaxel formation through a competitive mechanism with a Ki of 10.1 microM and 64.8 microM, respectively. The inhibition of this metabolite by ketoconazole was through a noncompetitive mechanism with a Ki of 11.8 microM. Our data thus suggest that special attention should be paid when these drugs are combined in clinical practice.