Tamoxifen metabolism in rat liver microsomes: identification of a dimeric metabolite derived from free radical intermediates by liquid chromatography/mass spectrometry.
Tamoxifen has been shown to be a potent liver carcinogen in rats, and generates covalent DNA adducts. On-line high performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS) has been used to further study the metabolites of tamoxifen formed by rat liver microsomes in the presence of NADPH with a view to identifying potential reactive metabolites which may be responsible for the formation of DNA adducts, and liver carcinogenesis. A metabolite has been detected with a protonated molecule at m/z 773. The mass of this compound is consistent with a dimer of hydroxylated tamoxifen (m/z 388). Analysis of 4-hydroxytamoxifen incubated with a rat liver microsomal preparation showed the formation of a similar metabolite with an apparent MH+ ion at m/z 773, believed to be a dimer of 4-hydroxytamoxifen formed by a free radical reaction. The retention time for this metabolite from 4-hydroxytamoxifen is identical to that of the tamoxifen metabolite, suggesting that these two compounds are the same. The levels of the dimer were higher when 4-hydroxytamoxifen was used as substrate and, in addition, two isomers were detected. It is proposed that tamoxifen was first converted 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 with DNA and protein, or with themselves, to give the dimers (m/z 773).
Jones
R M
RM
MRC Toxicology Unit, University of Leicester, UK.
Yuan
Z X
ZX
Lim
C K
CK
D000818:Animals; D002853:Chromatography, Liquid / Q000379:methods*; D019281:Dimerization; D004965:Estrogen Antagonists / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*; D005609:Free Radicals; D013058:Mass Spectrometry / Q000379:methods*; D008862:Microsomes, Liver / Q000378:metabolism*; D009249:NADP / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D013629:Tamoxifen / Q000031:analogs & derivatives / Q000032:analysis / Q000737:chemistry / Q000378:metabolism*
D004965:Estrogen Antagonists; D005609:Free Radicals; D013629:Tamoxifen; C016601:afimoxifene; D009249:NADP