Tamoxifen: evidence by 32P-postlabeling and use of metabolic inhibitors for two distinct pathways leading to mouse hepatic DNA adduct formation and identification of 4-hydroxytamoxifen as a proximate metabolite

Exposure to pentachlorophenol (PCP) strongly intensifies theformation of mouse hepatic DNA adducts elicited by oral administrationof tamoxifen (TAM), as previously shown by ³²P-postlabeling.To explain this effect, PCP was proposed to interfere with thedetoxication by sulfate conjugation of an as yet unidentifiedhydroxylated proximate TAM metabolite. A comparison of the presentand earlier results shows that the hepatic TAM adduct patternin female ICR mice depended on the route of administration ofTAM (120 µmol/kg), with oral administration primarilyeliciting formation of more polar adducts (termed group I adducts),while after i.p. administration less polar adducts (group II)predominated over group I adducts by a factor of 17.5. All theseadducts were also formed in female Sprague–Dawley ratsafter i.p. dosing with TAM, but total adduct levels were 3.5-to 5-fold higher than in mice. After four daily i.p. treatments,TAM adducts accumulated in mouse liver DNA in a non-linear fashion.Adduct levels were 30–50 times lower in mouse kidney andlung than in liver. The phenolic metabolite 4-hydroxy TAM (120µimol/kg) exclusively led to formation of polar (groupI) hepatic adducts, and this process was stimulated 8-fold bycoadministration of PCP (75 µimol/kg). Co-administrationof PCP with the parent compound led to an 11-fold enhancementof group I adduct formation; simultaneously, levels of groupII adducts were suppressed 6-fold. Another inhibitor of sulfateconjugation, 2,6-dichloro-4-nitrophenol, unlike PCP, had noeffect on group I adducts, but it reduced group II adduct formation2.2-fold. The PCP metabolite 2,3,5,6-tetrachlorohydroquinone(75 µimol/kg) did not significantly affect any major TAMadduct, suggesting that PCP itself was the active compound.Similar to group II TAM adducts, the formation of hepatic safrole–DNAadducts was inhibited in female ICR mice by both sulfotransferaseinhibitors, consistent with the proposal that metabolic