Studies on the metabolic fate of valproic acid in the rat using stable isotope techniques

1. The metabolic fate of two specifically deuterated analogues of valproic acid (VPA), [2-2H1]VPA and [3,3-2H2]VPA, was studied in the rat following i.p. injection. 2. A total of 11 urinary metabolites of each labelled substrate were detected by g.l.c.-mass spectrometry. Those metabolites which resulted from oxidation of the drug at C-4 and/or C-5 retained the deuterium label(s), whereas products of oxidation at C-2 and/or C-3 exhibited varying degrees of deuterium loss. 3. The deuterium content of 3-hydroxy-VPA indicated that this metabolite has a dual origin, and arises in part by β-oxidation of VPA and in part by direct hydroxylation at C-3. An apparent intramolecular isotope effect (kH/kD) of ca. 8 was associated with the latter process. 3-Oxo-VPA appeared to be formed mainly by oxidation of Δ2-VPA, rather than by oxidation of 3-hydroxy-VPA. 4. Evidence was obtained that Δ3-VPA is formed reversibly from Δ2-VPA, and that further desaturation of Δ3-VPA gives rise to a metabolite believed to have a 2,3′-diene structure. 5. The stable isotope method employed in this investigation represents a powerful technique for studies on the origin of drug metabolites and for the elucidation of complex metabolic inter-relationships in vivo.