Tamoxifen metabolism in rat liver microsomes: Identification of a dimeric metabolite derived from free radical intermediates by liquid chromatographymass spectrometry

Tamoxifen has been shown to be a potent liver carcinogen in rats, and gener ates covalent DNA adducts, Online high performance liquid chromatography/el ectrospray ionisation mass spectrometry (HPLC/ESI-MS) has been used to furt her study the metabolites of tamoxifen formed by rat liver microsomes in th e presence of NADPH with a view to identifying potential reactive metabolit es which may be responsible for the formation of DNA adducts, and liver car cinogenesis. A metabolite has been detected with a protonated molecule at m /z 773. The mass of this compound is consistent with a dimer of hydroxylate d tamoxifen (m/z 388). Analysis of 4-hydroxytamoxifen incubated with a rat liver microsomal preparation showed the formation of a similar metabolite w ith an apparent MH+ ion at m/z 773, believed to be a dimer of 4-hydroxytamo xifen formed by a free radical reaction. The retention time for this metabo lite from 4-hydroxytamoxifen is identical to that of the tamoxifen metaboli te, suggesting that these two compounds are the same. The levels of the dim er were higher when 4-hydroxytamoxifen was used as substrate and, in additi on, two isomers were detected. It is proposed that tamoxifen was first conv erted to arene oxides which react with DNA or to 4-hydroxytamoxifen, either directly or via 3,4-epoxytamoxifen, which then undergoes activation via a free radical reaction to give reactive intermediates which can then react w ith DNA and protein, or with themselves, to give the dimers (m/z 773), Copy right (C) 1999 John Wiley & Sons, Ltd.