TAMOXIFEN METABOLIC-ACTIVATION - COMPARISON OF DNA-ADDUCTS FORMED BY MICROSOMAL AND CHEMICAL ACTIVATION OF TAMOXIFEN AND 4-HYDROXYTAMOXIFENWITH DNA-ADDUCTS FORMED IN-VIVO

One of our laboratories recently showed by P-32-postlabeling that admi nistration of tamoxifen to mice induces two groups of hepatic DNA addu cts comprising two major spots, nos. 3 and 5, respectively. 4-Hydroxyt amoxifen and alpha-hydroxytamoxifen appear to be the proximate metabol ites of groups I and TT adducts, respectively. The relative significan ce 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 P-32-postlab eling with adducts generated by microsomal or chemical activation in v itro. Microsomal activation of 4-hydroxytamoxifen and tamoxifen, respe ctively, in the presence of DNA and cumene hydroperoxide, induced two adducts, which mapped similarly to the corresponding in vivo adduct sp ots 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 (Q1), while tamoxifen itself did not react. Rechromatographic analyses revealed that in vitro fractions 3 and Q1 (from 4-hydroxytamoxifen) matched the major in vivo group I adduct fra ction 3, consistent with the hypothesis that 4-hydroxytamoxifen is a p recursor for adduct fraction 3 in vivo. The in vitro adduct fraction 5 (from tamoxifen) was identical to that formed in vivo, indicating tha t 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 t o form DNA adducts, which are detected in vivo after treatment with ta moxifen or 4-hydroxytamoxifen.