Analysis of DNA adducts formed by tamoxifen and related antiestrogenic and estrogenic compounds in rats

Abstract: The work presented herein has focused on DNA adduct formation by the non-steroidal antiestrogen tamoxifen and its analogues. Tamoxifen, [(Z)-1-{4-[2-(dimethylamino)-ethoxy]phenyl}-1,2-diphenyl-1-butene], is widely used in the treatment of breast cancer and its recognised side effect is endometrial cancer. In rats tamoxifen is a potent liver carcinogen and cancer development is associated with the formation of high levels of DNA adducts in this tissue. The characterisation of these adducts and comparison of DNA binding properties between structural analogues, provide means by which to elucidate the pathways of metabolic activation of tamoxifen and, ultimately, to better understand the mechanism how tamoxifen is inducing cancer. A large part of the work presented in this thesis has focused on the development of specific and sensitive 32P-postlabelling methods and HPLC separations of the adducts, which would allow the further characterisation of these adducts. The main observations are summarised as follows: ' Only tamoxifen and toremifene were found to induce DNA adducts when the DNA binding of four triphenylethylene derivatives: tamoxifen, toremifene, clomiphene and triparanol, and a stilbene derivative diethylstilbestrol as well as a benzothiophene derivative raloxifene, was studied in rat liver. Equimolar dose of toremifene caused ~300-fold lower adduct levels than tamoxifen and the adducts were chromatographically different. ' An unknown, main tamoxifen adduct in rat liver was identified by mass spectrometry to be the N-desmethyltamoxifen-deoxyguanosine adduct in which the [alpha]-position of the metabolite N-desmethyltamoxifen was linked covalently to the amino group of deoxyguanosine. The other main adduct was confirmed to be [alpha]-(N2-deoxyguanosinyl)tamoxifen. The minor adducts in rat liver sample were shown to be identical to other diastereomers of these main adducts. The identified adducts accounted for at least 95% of the total adducts detected, implicating that in rat liver [alpha]-hydroxylation of the ethyl group followed by conjugation by phase H enzymes is the major activation pathway for formation of tamoxifen-DNA adducts. ' Only tamoxifen was shown to induce DNA adducts in the uterus of the neonatal rats when the reproductive tract toxicity was studied by comparing tamoxifen, toremifene, diethylstilbestrol and ethinylestradiol. Altogether, our results showed that [alpha]-hydroxylated tamoxifen and N-desmethyltarnoxifen undergo subsequent phase II activation at the [alpha]-position in rat liver. The structurally related compounds, with blocked cc-positions, were forming either low levels or no adducts which is consistent with the non-carcinogenicity of these compounds. Additionally, we showed that tamoxifen can be activated to DNA reactive species in neonatal rat uterus. Since the adducts were identical to those seen in the liver, similar activation pathway is suggested in both organs. These data imply that a genotoxic mechanism may be involved also in the induction of endometrial cancer in the rat.