Effect of alpha-fluorination of valproic acid on valproyl-S-acyl-CoA formation in vivo in rats

Studies designed to compare valproic acid (VPA) with its alpha -fluorinated derivative (F-VPA) for their abilities to form acyl-CoA thioester derivati ves in vivo are described. Recent studies have shown that alpha -fluorinati on of a hepatotoxic metabolite of VPA (Delta (4)-VPA) resulted in a nonhepa totoxic derivative. We hypothesize that the decrease in hepatotoxicity may be related to a lack of formation of the intermediary acyl-CoA thioester. T o determine the effect of alpha -fluoro substitution on acyl-CoA formation, we synthesized F-VPA and compared it with VPA for its ability to form the acyl-CoA thioester derivative in vivo in rat liver. Thus, after dosing rats with VPA or F-VPA, animals were sacrificed (0.05-, 0.5-, 1-, 2-, and 5-h p ostadministration) for the analysis of liver tissue. High-performance liqui d chromatography (HPLC) and electrospray ionization/tandem mass spectrometr y analysis of liver extracts from VPA-dosed rats showed the presence of VPA -CoA that was maximal after 0.5 h (185 nmol/g of liver) and was still measu rable 5-h postadministration (90 nmol/g of liver). In agreement with our hy pothesis, F-VPA did not form the corresponding acyl-CoA derivative as deter mined by the absence of F-VPA-CoA upon HPLC analysis of liver extracts from F-VPA-dosed rats. Further examination of liver tissue for the presence of free acids revealed that the differences in acyl-CoA formation cannot be ex plained by differences in VPA and F-VPA free acid concentrations. From thes e observations and related studies showing the lack of toxicity due to alph a -fluoro substitution, we propose that metabolism of VPA by acyl-CoA forma tion may mediate the hepatotoxicity of the drug.