METABOLISM OF THE ANTIMAMMARY CANCER ANTIESTROGENIC AGENT TAMOXIFEN .2. FLAVIN-CONTAINING MONOOXYGENASE-MEDIATED N-OXIDATION

Earlier studies demonstrated that the major metabolites of tamoxifen g enerated by mammalian liver microsomes are the corresponding N-oxide, N-desmethyl, and 4-hydroxy derivatives. This study examines the enzyma tic activity catalyzing the formation of tamoxifen N-oxide by mammalia n liver microsomes. Incubations of tamoxifen with liver microsomes fro m the various species, supplemented with NADPH, yielded the N-oxide, N -desmethyl, and 4-hydroxy derivatives. Inhibition of N-oxide accumulat ion by mild heat and low concentrations of methimazole in rat liver mi crosomes indicated that this reaction is catalyzed by the flavin-conta ining monooxygenase (FMO). Antibodies to NADPH-P-450 reductase inhibit ed N-demethylation and 4-hydroxylation, but not N-oxidation, supportin g the aforementioned conclusion. Purified mouse liver microsomal FMO c onverted tamoxifen solely into the N-oxide, providing direct evidence for FMO involvement. Human liver microsomes formed the same tamoxifen metabolites, albeit at a much lower rate. Inhibitors of FMO diminished the accumulation of N-oxide by human liver microsomes, indicating inv olvement of FMO. Tamoxifen-N-oxide was found to be readily reduced to tamoxifen by rat or human liver microsomes, in the presence of NADPH; the extent of reduction was dramatically increased when incubations we re supplemented with methimazole. The facile reduction of tamoxifen N- oxide back to tamoxifen suggests that the N-oxide may serve as a stora ge form for tamoxifen in vivo.