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 the formation of mouse hepatic DNA adducts elicited by oral administration of tamoxifen (TAM), as previously shown by 32P-postlabeling. To explain this effect, PCP was proposed to interfere with the detoxication by sulfate conjugation of an as yet unidentified hydroxylated proximate TAM metabolite. A comparison of the present and earlier results shows that the hepatic TAM adduct pattern in female ICR mice depended on the route of administration of TAM (120 mumol/kg), with oral administration primarily eliciting 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 these adducts were also formed in female Sprague-Dawley rats after 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 and lung than in liver. The phenolic metabolite 4-hydroxy TAM (120 mumol/kg) exclusively led to formation of polar (group I) hepatic adducts, and this process was stimulated 8-fold by co-administration of PCP (75 mumol/kg). Co-administration of PCP with the parent compound led to an 11-fold enhancement of group I adduct formation; simultaneously, levels of group II adducts were suppressed 6-fold. Another inhibitor of sulfate conjugation, 2,6-dichloro-4-nitrophenol, unlike PCP, had no effect on group I adducts, but it reduced group II adduct formation 2.2-fold. The PCP metabolite 2,3,5,6-tetrachlorohydroquinone (75 mumol/kg) did not significantly affect any major TAM adduct, suggesting that PCP itself was the active compound. Similar to group II TAM adducts, the formation of hepatic safrole-DNA adducts was inhibited in female ICR mice by both sulfotransferase inhibitors, consistent with the proposal that metabolic alpha-hydroxylation of the ethyl group of TAM followed by sulfate conjugation represented a mechanism of TAM 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-nitrophenol suggested that inhibition of sulfate conjugation may not have been the primary mechanism underlying the intensification of group I adducts formed from TAM or 4-hydroxy TAM. The results presented herein demonstrate conclusively that TAM was activated to DNA-reactive compounds along two distinct pathways which contrasted in their responses to metabolic inhibitors.
Department of Pharmacology, Baylor College of Medicine, Houston, TX 77030.
K
K
Randerath
B
B
Moorthy
N
N
Mabon
P
P
Sriram
D000818:Animals; D004247:DNA / Q000187:drug effects* / Q000378:metabolism*; D018736:DNA Adducts / Q000096:biosynthesis*; D005260:Female; D006873:Hydroquinones / Q000494:pharmacology; D007553:Isotope Labeling; D007668:Kidney / Q000187:drug effects / Q000378:metabolism; D008099:Liver / Q000187:drug effects* / Q000378:metabolism*; D008168:Lung / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D008813:Mice, Inbred ICR; D010416:Pentachlorophenol / Q000494:pharmacology*; D010761:Phosphorus Radioisotopes; D051381:Rats; D017207:Rats, Sprague-Dawley; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000493:pharmacokinetics / Q000633:toxicity*
D018736:DNA Adducts; D006873:Hydroquinones; D010761:Phosphorus Radioisotopes; D013629:Tamoxifen; C016601:afimoxifene; C044578:2,3,5,6-tetrachlorohydroquinone; D004247:DNA; C069879:2,5-dichlorohydroquinone; D010416:Pentachlorophenol