PURPOSE: To determine the inhibitory potency of letrozole and its main human metabolite, 4,4'-methanol-bisbenzonitrile, on the activities of eight cytochrome P450 (CYP) enzymes. METHODS: Letrozole and its metabolite were incubated with human liver microsomes (HLMs) (or expressed CYP isoforms) and NADPH in the absence (control) and presence of the test inhibitor. RESULTS: Letrozole was a potent competitive inhibitor of CYP2A6 (K (i) 4.6 +/- 0.05 microM and 5.0 +/- 2.4 microM in HLMs and CYP2A6, respectively) and a weak inhibitor of CYP2C19 (K (i) 42.2 microM in HLMs and 33.3 microM in CYP2C19), while its metabolite showed moderate inhibition of CYP2C19 and CYP2B6. Letrozole or its metabolite had negligible effect on other CYPs. CONCLUSIONS: Based on the in vitro K (i) values, letrozole is predicted to be a weak inhibitor of CYP2A6 in vivo. Letrozole and its major human metabolite show inhibitory activity towards other CYPs, but clinically relevant drug interactions seem less likely as the K (i) values are above the therapeutic plasma concentrations of letrozole.