Stereoselective synthesis of the diunsaturated metabolites of valproic acid

Metabolic profiling of valproic acid by cDNA-expressed human cytochrome P450 enzymes using negative-ion chemical ionization gas chromatography-mass spectrometry

A sensitive negative ion chemical ionization (NCI) gas chromatographic-mass spectrometric (GC-MS) method was modified for the quantitation of valproic acid (VPA) metabolites generated from in vitro cDNA-expressed human microsomal cytochrome P450 incubations. The use of the inherent soft ionization nature of electron-capture NCI to achieve high sensitivity enabled us to conduct kinetic studies using small amounts of recombinant human P450 enzymes. The assay is based on the selective ion monitoring of the intense [M-181] fragments of pentafluorobenzyl (PFB) esters in the NCI mode, and has the following features: (1) a micro-extraction procedure to isolate VPA metabolites from small incubation volumes (100 microl); (2) a second step derivatization with tert.-butyldimethylsilylating reagents to enhance sensitivity for hydroxylated metabolites; (3) a short run-time (<30 min) while maintaining full separation of 15 VPA metabolites by using a narrow-bore non-polar DB-1 column plus a new temperature gradient; and (4) good reproducibility and accuracy (intra- and inter-assay RSDs <15%, bias <15%) by using seven deuterated derivatives of analytes as internal standards. The derivatives of mono-and diunsaturated metabolites, like the parent drug, produced abundant [M-181](-) ions while the hydroxylated metabolites gave an ion at m/z of 273, corresponding to the [M-181](-) ion of the tert.-butyldimethylsilyl ethers. In conclusion, the GC-NCI-MS analysis of valproate metabolites provided us with a high resolution and sensitivity necessary to conduct metabolic and kinetic studies of valproic acid in small volume samples typical of the in vitro cDNA-expressed micro-incubation enzymatic systems.

Cytotoxicity of unsaturated metabolites of valproic acid and protection by vitamins C and E in glutathione-depleted rat hepatocytes

Valproic acid (VPA) and the unsaturated metabolites, 2-ene VPA and (E)-2,(Z)-3'-diene VPA, demonstrated dose-dependent cytotoxicity in primary cultures of rat hepatocytes, as evaluated by lactate dehydrogenase (LDH) leakage. Cellular glutathione (GSH) was depleted by adding buthionine sulfoximine (BSO) to the culture medium. Induction of cytochrome P450 by pretreatment of rats with phenobarbital or pregnenolone-16 alpha-carbonitrile enhanced the cytotoxicity of parent VPA in BSO-treated hepatocytes. The cytotoxicity of 4-ene VPA was apparent in BSO-treated hepatocytes with detectable loss of cell viability at 1 microM of added 4-ene VPA. Depletion of cellular GSH also increased the cytotoxicities of 2-ene VPA and (E)-2,(Z)-3'-diene VPA. The cytotoxicity of 2-ene VPA was comparable to or higher than that of VPA, producing loss of viability at concentrations > or = 5 mM. Time-course evaluation of hepatocyte response to 4-ene VPA in the GSH-depleted state revealed a delayed cytotoxicity with no effect during the first 12 h of exposure followed by a pronounced toxicity between 12 and 14 h. Two major GSH conjugates of 4-ene VPA metabolites, namely 5-GS-4-hydroxy VPA lactone and 5-GS-3-ene VPA, were detected in 4-ene VPA treated hepatocytes. Consistent with this finding, a 50% decrease in cellular GSH levels was observed following 4-ene VPA treatment. Under similar conditions, neither toxicity nor the GSH conjugated metabolite were detected in cells treated with the alpha-fluorinated 4-ene VPA analogue (alpha-F-4-ene VPA). The antioxidants, vitamin C and vitamin E, demonstrated a cytoprotective effect against 4-ene VPA-induced injury in GSH-depleted hepatocytes. These results are in support of hepatocellular bioactivation of VPA via 4-ene VPA to highly reactive species, which are detoxified by GSH. The susceptibility of hepatocytes to VPA metabolite-mediated cytotoxicity depends on cellular GSH homeostasis.

Determination of valproic acid and its metabolites using gas chromatography with mass-selective detection: application to serum and urine samples from sheep

An improved method for the quantitative determination of valproic acid (VPA) and sixteen of its metabolites has been developed using gas chromatography-mass spectrometry with selected-ion monitoring. The method is applicable to serum or urine and all metabolites are measured in a single chromatographic run of 29.5 min. Ions selected for quantitative purposes were the characteristic [M-57]+ ions of the tert.-butyldimethylsilyl (tBDMS) derivatives. The method utilizes heptadeuterated VPA as well as six heptadeuterated metabolites as internal standards [i.e. 2-[2H7]propyl-2-pentenoic acid (2-ene[2H7]VPA), 2-[2H7]propyl-4-pentenoic acid (4-ene[2H7]VPA), 2-[2H7]propyl-3-oxopentanoic acid (3-keto[2H7]VPA), 2-[2H7]propyl-4-oxopentanoic acid (4-keto[2H7]VPA), 2-[2H7]propyl-3-hydroxypentanoic acid (3-OH[2H7]VPA), 2-[2H7]propyl-5-hydroxypentanoic acid (5-OH[2H7]VPA)]. The method demonstrates very good accuracy and precision over a large range of concentrations for VPA and all metabolites measured in both human and sheep biological fluids. The assay was applied to the analysis of VPA and metabolites in serum and urine samples collected from three non-pregnant ewes following intravenous bolus administration of a mixture of VPA and [13C4]VPA. Sheep were observed to produce measurable quantities of the majority of metabolites found in humans, with the notable exception of the di-unsaturated compounds (i.e. 2,3'-diene VPA and 2,4-diene VPA). The pharmacokinetics and metabolism of VPA and [13C4]VPA appear to be equivalent in the sheep model. The elimination half-life of VPA and [13C4]VPA in the ewe were estimated to be approximately 3.5 +/- 0.4 and 3.2 +/- 0.4 h, respectively.

Metabolic activation of unsaturated derivatives of valproic acid. Identification of novel glutathione adducts formed through coenzyme A-dependent and -independent processes

The ability of 2-n-propyl-4-pentenoic acid (delta 4-VPA) and 2-n-propyl-2(E)-pentenoic acid ([E]-delta 2-VPA), two unsaturated metabolites of valproic acid (VPA), to form reactive intermediates, deplete hepatic glutathione (GSH) and cause accumulation of liver triglycerides was investigated in the rat. With the aid of ionspray liquid chromatography-tandem mass spectrometry (LC-MS/MS), three GSH adducts were detected in the bile of delta 4-VPA-treated animals and were identified as 4-hydroxy-5-glutathion-S-yl-VPA-gamma-lactone, 5-glutathion-S-yl-(E)-delta 3-VPA and 3-oxo-5-glutathion-S-yl-VPA. A fourth conjugate was identified tentatively as 4-glutathion-S-yl-5-hydroxy-VPA. Quantitative analysis of the corresponding N-acetyl-cysteine (NAC) conjugates in urine indicated that metabolism of delta 4-VPA via the GSH-dependent pathways accounted for approximately 20% of an acute dose (100 mg kg-1 i.p.). In contrast, when rats were given an equivalent dose of (E)-delta 2-VPA, only one GSH adduct (5-glutathion-S-yl-(E)-delta 3-VPA) was detected at low concentrations in bile. In vitro experiments with rat liver mitochondria demonstrated that delta 4-VPA undergoes coenzyme A- and ATP-dependent metabolic activation in this organelle via the beta-oxidation pathway to intermediates which bind covalently to proteins. When liver homogenates and hepatic mitochondria from rats injected with delta 4-VPA, (E)-delta 2-VPA or VPA were analyzed for GSH content, it was found that only delta 4-VPA depleted GSH pools significantly. Treatment of rats with delta 4-VPA and (to a lesser extent) VPA led to an accumulation of liver triglycerides, whereas (E)-delta 2-VPA had no measurable effect. It is concluded that delta 4-VPA undergoes metabolic activation by both microsomal cytochrome P-450-dependent and mitochondrial coenzyme A-dependent processes, and that the resulting electrophilic intermediates, which are trapped in part by GSH, may mediate the hepatotoxic effects of this compound. In contrast, (E)-delta 2-VPA is not transformed to any appreciable extent to reactive metabolites, which thus accounts for the apparent lack of hepatotoxicity of this positional isomer in the rat.

Metabolic profiling of valproic acid in patients using negative-ion chemical ionization gas chromatography-mass spectrometry

A negative-ion chemical ionization gas chromatographic-mass spectrometric method for the determination of valproic acid (VPA) and fourteen of its metabolites in a single chromatographic run is reported. The assay features the use of four internal standards and is applicable to the analysis of small serum and urine volumes. A combination of pentafluorobenzyl and trimethylsilyl derivatization resulted in the [M - 181]- ion as the base peak for all the metabolites measured. When these ions were monitored sensitivities in the low picogram levels were achieved. The VPA metabolite profile was determined in pediatric patients on VPA monotherapy and on combined VPA therapy with either carbamazepine or clobazam. The recently characterized diene metabolite, (E,E)-2,3'-diene-VPA, was found to be a major serum metabolite of VPA. In the patient groups taking VPA in combination with carbamazepine, the induction of omega and omega-1 pathways of VPA metabolism was apparent, while the levels of the beta-oxidation products were significantly decreased.