Co-chromatography of a tamoxifen epoxide-deoxyguanylic acid adduct with a major DNA adduct formed in the livers of tamoxifen-treated rats

Tamoxifen is a potent liver carcinogen in rats and has beenshown to form covalent DNA adducts in the livers of severalspecies of rodent. We have shown previously by ³²P-postlabelling(Carcinogenesis, 13, 2197–2203) that >85% of the totaladducts detected and resolved by multi-directional TLC migrateas a single spot. In the present study, this material was furtheranalysed by reverse-phase HPLC and resolved into two approximatelyequal components. Tamoxifen 1,2-epoxide, a postulated metaboliteof tamoxifen, was reacted with DNA and polydeoxyribonucleotidesand the products analysed. ³²P-Postlabelling revealed threemajor adduct spots on TLC which comigrated with the three majoradduct spots seen with DNA from livers of tamoxifen-treatedrats. Moreover, the major epoxide adduct, which contained guanineas the modified base, eluted on HPLC as a single major peakcoincident with one of the major peaks derived from the liverDNA of tamoxifen-treated rats. These results demonstrate that{small tilde}40% of the tamoxifen-DNA adducts formed in vivoare chromatographically indistinguishable with the major productof the reaction of tamoxifen epoxide with guanine residues inDNA and provide important clues to the mechanism of activationof tamoxifen to a genotoxic carcinogen.