Tamoxifen metabolic activation: comparison of DNA adducts formed by microsomal and chemical activation of tamoxifen and 4-hydroxytamoxifen with DNA adducts formed in vivo.
One of our laboratories recently showed by 32P-postlabeling that administration of tamoxifen to mice induces two groups of hepatic DNA adducts comprising two major spots, nos. 3 and 5, respectively. 4-Hydroxytamoxifen and alpha-hydroxytamoxifen appear to be the proximate metabolites of groups I and II adducts, respectively. The relative significance 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-postlabeling with adducts generated by microsomal or chemical activation in vitro. Microsomal activation of 4-hydroxytamoxifen and tamoxifen, respectively, 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 (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 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.
Moorthy
B
B
Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77030, USA.
Sriram
P
P
Pathak
D N
DN
Bodell
W J
WJ
Randerath
K
K
D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology; D001555:Benzene Derivatives / Q000494:pharmacology; D004247:DNA / Q000187:drug effects / Q000378:metabolism*; D018736:DNA Adducts / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D051379:Mice; D008813:Mice, Inbred ICR; D008862:Microsomes, Liver / Q000378:metabolism*; D051381:Rats; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology
D018931:Antineoplastic Agents, Hormonal; D001555:Benzene Derivatives; D018736:DNA Adducts; D013629:Tamoxifen; C016601:afimoxifene; D004247:DNA; C007164:cumene hydroperoxide