TAMOXIFEN - EVIDENCE BY P-32 POSTLABELING AND USE OF METABOLIC-INHIBITORS FOR 2 DISTINCT PATHWAYS LEADING TO MOUSE HEPATIC DNA ADDUCT FORMATION AND IDENTIFICATION OF 4-HYDROXYTAMOXIFEN AS A PROXIMATE METABOLITE

Exposure to pentachlorophenol (PCP) strongly intensifies the formation of mouse hepatic DNA adducts elicited by oral administration of tamox ifen (TAM), as previously shown by P-32-postlabeling. To explain this effect, PCP was proposed to interfere with the detoxication by sulfate conjugation of an as yet unidentified hydroxylated proximate TAM meta bolite. A comparison of the present and earlier results shows that the hepatic TAM adduct pattern in female ICR mice depended on the route o f administration of TAM (120 mu mol/kg), with oral administration prim arily eliciting formation of more polar adducts (termed group I adduct s), while after i.p. administration less polar adducts (group II) pred ominated over group I adducts by a factor of 17.5. All these adducts w ere also formed in female Sprague-Dawley rats after i.p, dosing with T AM, but total adduct levels were 3.5- to 5-fold higher than in mice. A fter four daily i.p. treatments, TAM adducts accumulated in mouse live r DNA in a non-linear fashion. Adduct levels were 30-50 times lower in mouse kidney and lung than in liver. The phenolic metabolite 4-hydrox y TAM (120 mu mol/kg) exclusively led to formation of polar (group I) hepatic adducts, and this process was stimulated 8-fold by coadministr ation of PCP (75 mu mol/kg). Co-administration of PCP with the parent compound led to an 11-fold enhancement of group I adduct formation; si multaneously, levels of group II adducts were suppressed 6-fold. Anoth er inhibitor of sulfate conjugation, 2,6-dichloro-4-nitrophenol, unlik e PCP, had no effect on group I adducts, but it reduced group II adduc t formation 2.2-fold. The PCP metabolite 2,3,5,6-tetrachlorohydroquino ne (75 mu mol/kg) did not significantly affect any major TAM adduct, s uggesting that PCP itself was the active compound, Similar to group II TAM adducts, the formation of hepatic safrole-DNA adducts was inhibit ed in female ICR mice by both sulfotransferase inhibitors, consistent with the proposal that metabolic alpha-hydroxylation of the ethyl grou p of TAM followed by sulfate conjugation represented a mechanism of TA M activation. On the other hand, the strong intensification of group I adducts by PCP and the lack of this effect by 2,6-dichloro-4-nitrophe nol suggested that inhibition of sulfate conjugation may not have been the primary mechanism underlying the intensification of group I adduc ts formed from TAM or 4-hydroxy TAM. The results presented herein demo nstrate conclusively that TAM was activated to DNA-reactive compounds along two distinct pathways which contrasted in their responses to met abolic inhibitors.