Minor products of reaction of DNA with alpha-acetoxytamoxifen

The drug tamoxifen shows evidence of genotoxicity and induces liver tumoursin rats. Covalent DNA adducts have been detected in the liver of ratstreated with tamoxifen and these arise, at least in part, from itsmetabolite alpha-hydroxytamoxifen. This probably undergoes conjugation inthe liver tissue to give an ester, which alkylates DNA. We have preparedalpha-acetoxytamoxifen as a model for this reactive intermediate andstudied its reaction with DNA in vitro. The products of this reaction werechromatographically identical to DNA adducts found in the liver of ratstreated with tamoxifen. We have isolated three of these products as thenucleosides TG1, TG2 and TA1 and identified them by ultraviolet, mass andproton magnetic resonance spectroscopy. TG1 and TG2 weretamoxifen-deoxyguanosine adducts in which the alpha-position of tamoxifenwas linked to the amino group of guanine; TG1,(E)-4-[4-[2-(dimethylamino)ethoxy]phenyl]-3,4-diphenyl-2- (9beta-deoxyribofuranosyl-6-oxopurin-2-ylamino)-3-butene; TG2, (Z) isomer of TG1. InTG2, the tamoxifen group had undergone trans-cis isomerization. The minorproduct TA1 was a tamoxifen-deoxyadenosine adduct, where linkage wasthrough the amino group of adenine: (E)-4-[4- [2-(dimethylamino)ethoxy]phenyl]-3,4-diphenyl-2-(9beta- deoxyribofuranosylpurin-6-ylamino)-3-butene. These three adducts accounted for >90% of thereaction products (approximately 67% TG1, 18% TG2 and 7% TA1); traceproducts included other stereoisomers of these and dinucleotide adductswhich resisted enzymatic digestion.