in Vivo and 4-Hydroxytamoxifen with DNA Adducts Formed Formed by Microsomal and Chemical Activation of Tamoxifen Tamoxifen Metabolic Activation : Comparison of DNA Adducts

One of our laboratories recently showed by 32P.postlabeling that ad ministration of tamoxifen to mice induces two groups of hepatic DNA adducts comprising two major spots, nos. 3 and 5, respectively. 4-Hy droxytamoxifen and a-hydroxytamoxifen appear to be the proximate metabolites of groups I and II adducts, respectively. The relative signifi cance of these two adduct groups for tamoxifen carcinogenicity remains to be established. To determine the activation mechanism(s) of tamoxifen and 4-hydroxytamoxifen, in vivo adducts were compared by 32P-postla beling with adducts generated by microsomal or chemical activation in vitro. Microsomal activation of 4-hydroxytamoxifen and tamoxifen, re spectively, in the presence of DNA and cumene hydroperoxide, induced two adducts, which mapped similarly to the corresponding in vivo adduct spots 3 and 5. Chemical oxidation of 4-hydroxytamoxifen with silver(II) oxide, followed by incubation of the product(s) with DNA, elicited the formation of a major spot (Qi), whlle tamoxifen itself did not react. Rechromatographic analyses revealed that in vitro fractions 3 and Qi (from 4-hydroxytamoxifen) matched the major in vivo group I adduct fraction 3, consistent with the hypothesis that 4-hydroxytamoxifen is a precursor for adduct fraction 3 in vivo. The in vitro adduct fraction 5 (from tamoxifen) was identical to that formed in vivo, indicating that the metabolic pathway for the formation of group II adducts did not involve 4-hydroxytamoxifen. In conclusion, the results support a model where primary metabolites of tamoxifen undergo secondary metabolism to form DNA adducts, which are detected in vivo after treatment with tamoxifen or 4-hydroxytamoxifen.