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K Kassahun, K Farrell and F Abbott
Faculty of Pharmaceutical Science, University of British Columbia, Vancouver, Canada.
The severe hepatotoxicity of valproic acid (VPA) is believed to be mediated through reactive metabolites. The formation of glutathione (GSH) and N-acetylcysteine (NAC) adducts of reactive intermediates derived from VPA and two of its metabolites, 2-propyl-4-pentenoic acid (4-ene-) and 2-propyl-2,4-pentadienoic acid [(E)-2,4-diene VPA], was investigated in the rat. Rats were dosed ip with 100 mg/kg of VPA, 4- ene-, or 2,4-diene-VPA, and methylated bile and urine extracts were analyzed by LC/MS/MS and GC/MS, respectively. The GSH conjugate of (E)- 2,4-diene VPA was detected in the bile of rats treated with 4-ene- and (E)-2,4-diene VPA. The NAC conjugate was a major urinary metabolite of rats given (E)-2,4-diene VPA and was a prominent urinary metabolite of those animals given 4-ene VPA. The NAC conjugate was also found to be a metabolite of VPA in patients. Both the GSH and NAC adducts were chemically synthesized and their structures established to be 5- (glutathion-S-yl)3-ene VPA and 5-(N-acetylcystein-S-yl)3-ene VPA by NMR and mass spectrometry. In contrast to the very slow reaction of the free acid of (E)-2,4-diene VPA with GSH, the methyl ester reacted rapidly with GSH to yield the adduct. In vivo it appears the diene forms an intermediate with enhanced electrophilic reactivity to GSH as indicated by the facile reaction of the diene with GSH in vivo [about 40% of the (E)-2,4-diene VPA administered to rats was excreted as the NAC conjugate in 24 hr]. The characterization of the GSH and NAC (in humans and rats) conjugates of (E)-2,4-diene VPA suggests that VPA is metabolized to a chemically reactive intermediate that may contribute to the hepatotoxicity of the drug.
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