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The synthesis of 2-((E)-1'-propenyl)-(E)-2-pentenoic acid and its metabolism and pharmacokinetics in rats Lee, Ronald Duane


The anticonvulsant drug valproic acid (VPA) is extensively metabolized with 16 metabolites identified in man. Of interest are the unsaturated metabolites which appear to be responsible for the observed secondary antiepileptic activity and/or idiosyncratic hepatotoxicity. A study by Abbott et al . (1986) has shown 2-((E)-1'-propenyl)-(E)-2-pentenoic acid ((E,E)-2,3'-diene VPA) to be a major unsaturated metabolite of VPA. Acheampong (1985) demonstrated that an isomeric mixture of 2,3'-diene VPA prevented pentylenetetrazole-induced seizures in mice and had 60% the potency of VPA. Research on (E,E)-2,3'-diene VPA is limited even though the diene appears to be a contributor to the secondary antiepileptic effect. Available synthesis of 2,3'-diene VPA result in two or more isomers with very low yields as shown by Acheampong and Abbott (1985). The object of this work was, therefore, to synthesize (E,E)-2,3'-diene VPA in sufficient quantity and isomeric purity for metabolic and pharmacokinetic studies in rats. Synthesis of (E,E)-2,3'-diene VPA was achieved by the alkylation of ethyl (Z)-2-pentenoate to afford ethyl 2-(l'-hydroxypropyl)-(E)-2-pentenoate. Dehydration using methanesulfonyl chloride and potassium hydride gave the ethyl ester of (E,E)-2,3'-diene VPA. Hydrolysis of the ester in 1 N NaOH afforded an 81.2% pure sample of (E,E)- 2,3'-diene VPA as determined by GCMS and NMR. A second method was used to unequivocally synthesize (E,E)-2,3' - diene VPA whereby an O-trimethylsilylketene acetal was oxidized via a hydride abstractor to yield two isomers of 2,3'-diene VPA plus the starting material (E)-3-ene VPA. The identity of the products were determined by GCMS with the major isomer being (E,E)-2,3'-diene VPA. In the metabolism studies, Wistar rats were given 100 mg/kg i.p. of (E,E)-2,3'-diene VPA and bile and urine collected for 24 hours. GCMS analysis revealed three metabolites present in both bile and urine. These were reduction products of (E,E)-2,3'-diene VPA metabolism and consisted of the monounsatured (E)-3-ene VPA and (E)-2-ene VPA plus the fully saturated VPA. These results suggest that trace levels of (E)-3-ene VPA observed after VPA dosing may not be a direct metabolic product of VPA itself but rather a reduced metabolite of (E,E)-2,3'-diene VPA. All polar metabolites are yet to be identified. For the pharmacokinetic studies, two doses, 20 and 100 mg/kg of (E,E)-2,3'-diene VPA were administered i.v. to Wistar rats and the plasma concentration vs time decline curve determined using GCMS analysis of the plasma samples. The bile duct of these animals was then cannulated and the study repeated. A comparison between the elimination rate constant (KE), clearance (C1), and volume of distribution (Vd) between the bile duct intact and cannulated rats for both dose groups revealed no significant differences (p>0.1). A comparison of the KE, Vd, and C1 between dosage groups of both bile duct intact and cannulated rats revealed no significant difference (p>0.08-0.7). Therefore, extensive enterohepatic recirculation was not apparent and the elimination of (E,E)-2,3'-diene VPA appeared to be dose-independent. The diene seems to follow a simple one-compartment model with a half-life of 35.9±8.9 (S.D.) minutes and a large volume of distribution of 0.95±0.22 (S.D.) L/kg. In vitro protein binding studies revealed that (E,E)-2,3'-diene VPA saturates plasma proteins between concentrations of 30 - 600 ug/mL. The percent of (E,E)-2,3'-diene VPA bound decreases from 92.1% to 28.7% as the concentration of diene increases suggesting concentration-dependent protein binding. The synthesis of (E,E)-2,3'-diene VPA in substantial quantities has allowed metabolic and pharmacokinetic studies to be performed in rats. Preliminary studies showed that (E,E)-2,3' -diene VPA was metabolically reduced to monounsaturated and saturated products. Pharmacokinetic data appear to indicate a potential for the diene to accumulate in the central nervous system. Further studies are required to determine the contribution of (E,E)-2,3'-diene VPA to the secondary antiepileptic activity of VPA.

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