Metabolism of the antimammary cancer antiestrogenic agent tamoxifen. II. Flavin-containing monooxygenase-mediated N-oxidation.
Earlier studies demonstrated that the major metabolites of tamoxifen generated by mammalian liver microsomes are the corresponding N-oxide, N-desmethyl, and 4-hydroxy derivatives. This study examines the enzymatic activity catalyzing the formation of tamoxifen N-oxide by mammalian liver microsomes. Incubations of tamoxifen with liver microsomes from the various species, supplemented with NADPH, yielded the N-oxide, N-desmethyl, and 4-hydroxy derivatives. Inhibition of N-oxide accumulation by mild heat and low concentrations of methimazole in rat liver microsomes indicated that this reaction is catalyzed by the flavin-containing monooxygenase (FMO). Antibodies to NADPH-P-450 reductase inhibited N-demethylation and 4-hydroxylation, but not N-oxidation, supporting the aforementioned conclusion. Purified mouse liver microsomal FMO converted 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 involvement 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 were supplemented with methimazole. The facile reduction of tamoxifen N-oxide back to tamoxifen suggests that the N-oxide may serve as a storage form for tamoxifen in vivo.
Mani
C
C
Worcester Foundation for Experimental Biology, Shrewsbury, MA 01545.
Hodgson
E
E
Kupfer
D
D
D000818:Animals; D000970:Antineoplastic Agents / Q000378:metabolism* / Q000627:therapeutic use; D006224:Cricetinae; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D008325:Mammary Neoplasms, Experimental / Q000188:drug therapy*; D008647:Mesocricetus; D051379:Mice; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D010084:Oxidation-Reduction; D010105:Oxygenases / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013629:Tamoxifen / Q000378:metabolism* / Q000627:therapeutic use
D000970:Antineoplastic Agents; D013629:Tamoxifen; D010105:Oxygenases; C020066:dimethylaniline monooxygenase (N-oxide forming)