title: UDP-glucuronosyltransferase-mediated protection against in vitro DNA oxidation and micronucleus formation initiated by phenytoin and its embryotoxic metabolite 5-(p-hydroxyphenyl)-5-phenylhydantoin.
abstract: UDP-Glucuronosyltransferases (UGTs) are important in the elimination of most xenobiotics, including 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), the major, reputedly nontoxic, metabolite of the anticonvulsant drug phenytoin. However, HPPH alternatively may be bioactivated by peroxidases, such as prostaglandin H synthase, to a reactive intermediate that initiates DNA oxidation (reflected by 8-hydroxy-2'-deoxyguanosine), genotoxicity (reflected by micronuclei) and embryopathy. This hypothesis was evaluated in skin fibroblasts cultured from heterozygous (+/j) and homozygous (j/j) UGT-deficient Gunn rats and in mouse embryo culture, with confirmation of direct NG-glucuronidation of phenytoin in Gunn rats in vivo. HPPH (80 microM) increased micronuclei by 2.0-, 4.8- and 4.6-fold in +/+ UGT-normal cells (P = .03) and +/j and j/j UGT-deficient cells (P = .0001), respectively. HPPH-initiated micronucleus formation was increased 3.0- and 3.4-fold in +/j (P = .02) and j/j (P = .04) UGT-deficient cells, respectively, vs. +/+ UGT-normal cells. Micronuclei were not initiated by 10 microM HPPH in +/+ UGT-normal cells but were increased by 4- and 3.8-fold in +/j and j/j UGT-deficient cells (P = .0001), respectively, and were increased 2.7- and 3.0-fold in +/j (P = .007) and j/j (P = .0002) UGT-deficient cells, respectively, vs. +/+ UGT-normal cells. 8-Hydroxy-2'-deoxyguanosine was increased in j/j UGT-deficient but not +/+ UGT-normal cells treated with 80 microM HPPH (P < .05). The embryopathic potency of 80 microM HPPH in embryo culture, reflected by decreases in anterior neuropore closure, turning, yolk sac diameter and crown-rump length (P < .05), was equivalent to that reported for phenytoin. Phenytoin (80 microM) enhanced micronucleus formation 1.7-, 4.4- and 3.8-fold in +/+ cells (P = .03) and +/j and j/j UGT-deficient cells (P = .0001), respectively. Phenytoin-initiated micronucleus formation was increased about 4-fold in both +/j (P = .006) and j/j (P = .009) UGT-deficient cells vs. +/+ UGT-normal cells, providing the first evidence that the bioactivation and oxidative toxicity of phenytoin itself may be avoided by direct N-glucuronidation, which was confirmed by tandem mass spectrometry. These results further indicate that, with UGT deficiencies, HPPH potentially is a potent mediator of phenytoin-initiated genotoxicity and embryopathy, which may be relevant to teratogenesis and other adverse effects of phenytoin.
authors.affiliation: Faculty of Pharmacy, University of Toronto, Ontario, Canada.
authors.forename: P M
authors.initials: PM
authors.lastname: Kim
authors.forename: L M
authors.initials: LM
authors.lastname: Winn
authors.forename: T
authors.initials: T
authors.lastname: Parman
authors.forename: P G
authors.initials: PG
authors.lastname: Wells
mesh_terms: D000818:Animals; D000927:Anticonvulsants / Q000633:toxicity*; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D004247:DNA / Q000378:metabolism*; D004622:Embryo, Mammalian / Q000187:drug effects*; D005260:Female; D014453:Glucuronosyltransferase / Q000502:physiology*; D008297:Male; D051379:Mice; D048629:Micronuclei, Chromosome-Defective / Q000187:drug effects*; D009924:Organ Culture Techniques; D010084:Oxidation-Reduction; D010672:Phenytoin / Q000031:analogs & derivatives* / Q000633:toxicity*; D051381:Rats; D011911:Rats, Gunn; D017208:Rats, Wistar
chemical_list: D000927:Anticonvulsants; C027364:5-(3-hydroxyphenyl)-5-phenylhydantoin; D010672:Phenytoin; D004247:DNA; D014453:Glucuronosyltransferase