Amino Acid Conjugates: Metabolites of 2-Propylpentanoic Acid (Valproic Acid) in Epileptic Patients

Abstract

ABSTRACT:In this study, spectroscopic and chromatographic evidence is presented for the identification and characterization of the metabolites, valproyl glutamate (2-propylpentanoyl glutamate, VPA-GLU) and valproyl glutamine (2-propylpentanoyl glutamine, VPA-GLN) in the urine, serum, and cerebrospinal fluid (CSF) of patients on valproic acid (VPA) therapy. Moreover, the identification of valproyl glycine (2-propylpentanoyl glycine, VPA-GLY) in the serum and urine of patients on VPA, albeit in trace concentrations, is also reported here. The three amino acid conjugates excreted in urine accounted for about 1% of the VPA dose in four patients who were on VPA therapy chronically and had reached steady state. VPA-GLU was quantitatively the most prominent metabolite (0.66-13.1 g/mg creatinine) compared with VPA-GLN (0.78-9.93 g/mg creatinine) and VPA-GLY (trace-1.0 g/mg creatinine) in overnight urine samples of all patients studied (n ‫؍‬ 29). The relatively low serum concentrations of the three amino acid conjugates of VPA in six patients suggest that the metabolites are readily excreted once formed. In contrast, whereas VPA GLY was absent in the CSF of one patient on VPA, the concentrations of VPA-GLU and VPA-GLN in this CSF sample were 9 and 5 times, respectively, their corresponding serum concentrations.The antiepileptic drug, valproic acid (VPA 3 ), is primarily used for the management of generalized and absence seizures in children. Concerns over its teratogenicity and hepatotoxicity side effect have encouraged the need to develop analogs of VPA devoid of the adverse effects. However, this task is complicated by the fact that the mechanism of action of the drug itself remains to be understood.A large body of evidence has emerged to link the hepatotoxicity of VPA to its metabolism and has been reviewed over the years (Eadie et al., 1988;Abbott and Anari, 1999;Radatz and Nau, 1999). Moreover, a review of mass-balance studies indicated that the total recovery of a VPA dose cannot be completely accounted for in humans (Baillie and Sheffels, 1995), suggesting the existence of yet unknown metabolites. For all these reasons, despite the extensive work that has been conducted on the metabolism of VPA, the subject remains an area that warrants further investigation.

High sensitivity determination of valproic acid in mouse plasma using semi‐automated sample preparation and liquid chromatography with tandem mass spectrometric detection

“…Sample pretreatment is a crucial step for the analysis of VPA because the analyte is highly volatile and thus sample losses can occur during the pretreatment. Most extractive procedures developed for the analysis of VPA in plasma samples have been based on LLE16, 20, 23, 27, 28 and SPE17, 22, 24, 29 procedures, which are time‐consuming and often lead to sample loss. A few authors report the use of acetonitrile for the deproteinization of serum samples,7, 30 but interference from the matrix was found in the analysis of VPA 30…”

“…Capillary electrophoretic methods using UV detection have been described,12–14 and satisfactory results in terms of sensitivity have been obtained using gas chromatography with tandem mass spectrometry (GC/MS/MS) 15–21. However, GC/MS methods require derivatization, and the analysis of VPA using GC/MS has been conducted with selected ion monitoring of the trimethylsilylate,20 tert ‐butyldimethylsilyl,19, 21 glutamate,22 and pentafluorobenzyl ester derivatives,15 in one case with isotope dilution 18. An LC/atmospheric pressure chemical ionization (APCI)‐MS method for quantification of VPA in plasma has been described,23 which does not require chemical modification of VPA and yields linearity in the 5–1000 µg/mL concentration range.…”

The glycine deportation system and its pharmacological consequences

“…Interestingly, valproate itself undergoes very little conjugation with GLY, the valproyl-CoA intermediate being used almost exclusively for β-oxidation. Three amino acid conjugates of valproic acid have been reported, with glutamate, glutamine, and GLY, and these three urinary metabolites accounted for only 1% of the dose (Gopaul et al, 2003). However, valproate has very subtle effects on the GDS.…”

Biotransformation Reactions and their Enzymes

“…In addition, intermediate metabolites of β-oxidation may be seen, as illustrated in Figure 24.10 with valproic acid, whose acyl-CoA intermediate (19) is a substrate for some first steps of β-oxidation [76]. In addition, intermediate metabolites of β-oxidation may be seen, as illustrated in Figure 24.10 with valproic acid, whose acyl-CoA intermediate (19) is a substrate for some first steps of β-oxidation [76].…”