Synthesis and investigation of α-hydroxy-N,N-didesmethyltamoxifen as a proximate carcinogen in the metabolic activation of tamoxifen

Tamoxifen is an adjuvant chemotherapeutic agent for the treatment of breast cancer and a chemoprotective agent for breast cancer prevention. Despite being beneficial in regard to breast cancer, tamoxifen is known to increase the risk of endometrial cancer and thromboembolic events in women; in addition, it induces liver tumors in rats and endometrial tumors in rats and mice. Tamoxifen and its metabolite, N-desmethyltamoxifen, are metabolically activated to DNA binding electrophiles through alpha-hydroxylation, followed by O-esterification, primarily via sulfation. In the present study, we have investigated whether a second desmethylated metabolite of tamoxifen, N,N-didesmethyltamoxifen, is also involved in the metabolic activation of this antiestrogen to a genotoxic species. alpha-Hydroxy-N,N-didesmethyltamoxifen was synthesized, further activated by sulfation, and then reacted with DNA. After enzymatic hydrolysis to deoxynucleosides, HPLC analysis indicated the formation of one major DNA adduct, which was characterized as (E)-alpha-(deoxyguanosin-N-2-yl)-N,N-didesmethyltamoxifen. Using P-32-postlabeling, in combination with HPLC, the same adduct was detected in liver DNA from rats treated intraperitoneally with alpha-hydroxy-N,N-didesmethyltamoxifen. In contrast, only a low extent of adduct formation could be found in rats administered N,N-didesmethyltamoxifen. These data indicate that although alpha-hydroxy-N,N-didesmethyltamoxifen can be converted to a genotoxin in rat liver, this pathway is a minor one in the metabolic activation of tamoxifen.