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  • Title: Evidence for lipoxygenase-catalyzed bioactivation of phenytoin to a teratogenic reactive intermediate: in vitro studies using linoleic acid-dependent soybean lipoxygenase, and in vivo studies using pregnant CD-1 mice.
    Author: Yu WK, Wells PG.
    Journal: Toxicol Appl Pharmacol; 1995 Mar; 131(1):1-12. PubMed ID: 7878664.
    Abstract:
    Teratogenicity of the anticonvulsant drug phenytoin and structurally related compounds has been postulated to result in part from its cooxidative bioactivation to an embryotoxic reactive intermediate by the hydroperoxidase component of arachidonic acid-dependent prostaglandin H synthase (PHS). However, lipoxygenases (LPOs) could constitute a competing, arachidonate-dependent bioactivating system. The teratologic importance of LPO alone and together with PHS was evaluated in vitro and in vivo. To evaluate directly the potential role of LPO in phenytoin bioactivation, in vitro incubation conditions measuring the linoleic acid-dependent, soybean LPO-catalyzed covalent binding of phenytoin to protein, believed to constitute teratologic initiation, were optimized for substrate concentration, enzyme activity, incubation time, inhibitor vehicle, and inhibitor concentration. Under optimal conditions, 4.24 nM [3H]phenytoin, 100,000 units of soybean LPO (Type 1B, Sigma Chemical Co.), and 5 mg bovine serum albumin, with or without 5 mM linoleic acid, were adjusted with 100 mM Tris-HCl buffer (pH 9) to a 1-ml final volume and incubated at 37 degrees C for 60 min. Addition of linoleic acid substrate to the incubation medium enhanced the covalent binding of phenytoin by over threefold (p < 0.05). Linoleic acid-dependent covalent binding of phenytoin was inhibited in a concentration-dependent fashion by the selective and nonselective LPO/PHS inhibitors indomethacin, nordihydroguaiaretic acid, quercetin, BW755C, and 5,8,11,14-eicosatetraynoic acid (ETYA) and by the hydroperoxidase inhibitor methimazole (p < 0.05). In vivo, on Gestational Days 12 and 13, pregnant CD-1 mice were treated with the irreversible dual PHS/LPO inhibitor ETYA (0, 0.5, 5, 50, 100, and 125 mg/kg ip) or its vehicle, 2 hr before a teratogenic dose of phenytoin (65 mg/kg ip). Dams were killed on Day 19 and the fetuses were delivered by Caesarian section. ETYA pretreatment produced a dose-related decrease in phenytoin teratogenicity, with maximal reductions by 50 mg/kg of ETYA in the incidence of phenytoin-induced fetal cleft palates (1.8 +/- 1.8% vs 26.8 +/- 3%) (% +/- SE; p < 0.00001) and resorptions (22.5 +/- 5% vs 60 +/- 2%) (p < 0.05). Higher ETYA doses were embryotoxic. The reduction in phenytoin teratogenicity by the dual PHS/LPO inhibitor ETYA was considerably greater than that previously reported for acetylsalicylic acid, which inhibits only PHS.(ABSTRACT TRUNCATED AT 250 WORDS)
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