Identification of metabolites of valproic acid in serum of humans, dog, rat, and mouse

Successful LC-MS/MS assay development and validation for determination of valproic acid and its metabolites supporting proactive pharmacovigilance

Valproic acid (VPA) is a well-documented contributor to liver injury, which is likely caused by the formation of its toxic metabolites. Monitoring VPA and its metabolites is very meaningful for the pharmacovigilance, but the availability of a powerful assay is a prerequisite. In this study, for the first time, a sensitive and specific LC-MS/MS method was developed and validated to simultaneously quantify the concentrations of VPA and its six pestering isomer metabolites (3-OH-VPA, 4-OH-VPA, 5-OH-VPA, 2-PGA, VPA-G, and 2-ene-VPA) in human plasma, using 5-OH-VPA-d7 and VPA-d6 as the internal standards (ISs). We also figured out another tricky problem that the concentrations of the parent drug and the metabolites vary widely. Of note, after protein precipitation and dilution with acetonitrile (ACN) and 50% ACN successively, the analytes and the ISs were successfully separated on a Kinetex C18 column. Intriguingly, sacrificing its signal intensity by elevated collision energy of VPA finally achieved the simultaneous determination. As expected, the method showed great linearity (r > 0.998) over the concentration ranges for all analytes. The inter-day and intra-day accuracy and precision were both acceptable. The method was successfully applied in 127 children with epilepsy. This novel assay will support the VPA-associated pharmacovigilance in the future.

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

Efficacy of anticonvulsant substances in the 6Hz seizure test: Comparison of two rodent species

Usually performed in the mouse, the 6Hz seizure test is used for screening potential new anticonvulsant substances against complex partial seizures. Nevertheless, advanced models of temporal lobe epilepsy (TLE) are more often performed in rats, so that possible species-related differences may complicate the development of anticonvulsant substances. The aim of the present study was to evaluate the feasibility of adapting the 6Hz test in the rat. We first compared the effects of increasing current intensities for inducing seizures in the mouse and in the rat. This step was followed by the evaluation of the activity of anticonvulsant substances. Animals received an electrical stimulation with a constant current via corneal electrodes. The seizure was characterized by the presence of forelimb clonus immediately after stimulation. Spontaneous locomotion was evaluated following the 6Hz test. In the rat, the forelimb seizure score was intensity-dependently increased and seizures were observed in all animals tested at 44mA. In the mouse, the seizures were of lower magnitude and they were not observed in all mice stimulated at 44mA. In both species, levetiracetam (LEV) clearly decreased the forelimb seizure score over the dose-range 100-300mg/kg without affecting locomotion. Valproate (VPA) displayed anticonvulsant activity at 200mg/kg and fully protected both species at 300mg/kg, a dose producing sedative effects in the mouse. Phenytoin (PHT) showed slight to moderate anticonvulsant activity at 100mg/kg in the mouse and at 60 and 100mg/kg in the rat without modifying locomotor activity. Lamotrigine (LTG) partially antagonized forelimb seizure at 60mg/kg in the mouse and at 30-60mg/kg in the rat, but it induced clear motor impairments at high dose in both species. Our data suggest that in the 6Hz test, the magnitude and the nature of seizures differed between the mouse and the rat for a given current intensity. Nevertheless, the pharmacological profile of anticonvulsant substances was similar in both species for the 4 substances tested. Dose-dependent efficacy of LEV and VPA was observed and LTG and PHT also showed anticonvulsant activity, even though the magnitude of the effects remained moderate for these two last substances. The 6Hz test in the rat therefore appears as a useful model which may be performed prior to follow-up models of partial seizures performed in the same species.

Valproic acid metabolism and its effects on mitochondrial fatty acid oxidation: a review

Valproic acid (VPA; 2-n-propylpentanoic acid) is widely used as a major drug in the treatment of epilepsy and in the control of several types of seizures. Being a simple fatty acid, VPA is a substrate for the fatty acid beta-oxidation (FAO) pathway, which takes place primarily in mitochondria. The toxicity of valproate has long been considered to be due primarily to its interference with mitochondrial beta-oxidation. The metabolism of the drug, its effects on enzymes of FAO and their cofactors such as CoA and/or carnitine will be reviewed. The cumulative consequences of VPA therapy in inborn errors of metabolism (IEMs) and the importance of recognizing an underlying IEM in cases of VPA-induced steatosis and acute liver toxicity are two different concepts that will be emphasized.

Disposition of valproic acid in self-poisoned adults

Acute intoxication with valproic acid is increasingly being observed in clinical practice. In Poland, such intoxication frequently occurs as a result of mixing different drugs or alcohol. We studied the pharmacokinetics of valproic acid in five intoxicated patients. Apart from valproic acid, barbiturates, chlorprotixene, tricyclic antidepressants, tetrahydrocannabinols and alcohol were detected and measured. The absorption of the drug was rapid and the maximum concentration was observed after the period of 3.5-5.6 hr. The lowering of the valproic acid level in plasma was biphasic, with terminal half-life ranging between 8.8-30.9 hr. The calculated apparent volume of distribution was 0.17-0.72 l/kg and could be affected by varied levels of doses as well as time of drug intake (data from interviews of patients) used for calculation and reduction in plasma protein binding at higher concentration of valproic acid. Frequent multiple drug poisonings oblige toxicological laboratories not only to monitor valproic acid concentration in serum, but also to perform the toxicological screenings.

Valproic acid toxicity: overview and management

Acute valproic acid intoxication is an increasing problem, accounting for more than 5000 calls to the American Association of Poison Control Centers in 2000. The purpose of this paper is to review the pharmacology and toxicology of valproic acid toxicity. Unlike earlier antiepileptic agents, valproic acid appears to function neither through sodium channel inhibition nor through direct gamma-aminobutyric acid agonism, but through an indirect increase in regional brain gamma-aminobutyric acid levels. Manifestations of acute valproic acid toxicity are myriad, and reflect both exaggerated therapeutic effect and impaired intermediary metabolism. Central nervous system depression is the most common finding noted in overdose, and may progress to coma and respiratory depression. Cerebral edema has also been observed. Although hepatotoxicity is rare in the acute overdose setting, pancreatitis and hyperammonemia have been reported. Metabolic and hematologic derangements have also been described. Management of acute valproic acid ingestion requires supportive care and close attention to the airway. The use of controversial adjunctive therapies, including extracorporeal drug elimination and L-carnitine supplementation, will be discussed.

Urine 4-heptanone: a beta-oxidation product of 2-ethylhexanoic acid from plasticisers

4-Heptanone is a common volatile constituent of human urine and is of unknown origin. We hypothesised that it arises from in vivo beta-oxidation of 2-ethylhexanoic acid (EHA) from plasticisers, similar to formation of 3-heptanone from valproic acid. We investigated urine from individuals with normal and increased plasticiser exposure. Using GC/MS, solvent-extracted organic acids were analysed as trimethylsilyl (TMS) derivatives and heptanone with headspace solid-phase microextraction. We identified 3-oxo-2-ethylhexanoic acid, the beta-oxidation product of EHA, as an enol in all samples. This is the first report of its TMS mass spectrum. We also found 2-ethyl-1,6-hexanedioic acid and 5-hydroxyEHA, omega- and omega-1-oxidation products of EHA, respectively, and 2-ethylhexanoylglucuronide, but only in trace amounts in some plasticiser samples. These compounds have not been reported in human urine, nor has the TMS mass spectrum of 5-hydroxyEHA. The median concentrations of 3-oxoethylhexanoic acid and total 4-heptanone of seven plasticiser samples were around 30--175-fold higher than normal samples. 4-Heptanone was barely detectable and 3-oxoethylhexanoic acid was not increased in an eighth plasticiser sample, from a baby with deficiency of 2-methylbranched-chain acyl-CoA dehydrogenase. beta-Oxidation is a major catabolic pathway of EHA in man, and might be involved in the metabolism of other branched-chain drugs and environmental pollutants.

Differentiation between valproate-induced anticonvulsant effect, teratogenicity and hepatotoxicity. Aspects of species variation, pharmacokinetics, metabolism and implications of structural specificity for the development of alternative antiepileptic agents such as delta 2-valproate

Valproate is metabolized into a large number of compounds via various metabolic routes. Metabolic profiles depend on species and age. Hepatotoxicity may be correlated with abnormal metabolism, especially in young age. Teratogenicity is associated with specific structural requirements: a free carboxyl atom connected to a carbon atom which also carries a hydrogen, and two carbon chains. This provides a clue for the development of alternative antiepileptic agents.

Identification of the main metabolites of 2-ethylhexanoic acid in rat urine using gas chromatography-mass spectrometry

The metabolites of 2-ethylhexanoic acid, an industrial chemical and the active ingredient in wood preservatives, were investigated in rat urine. Male Wistar rats were given 2-ethylhexanoic acid (2-EHA) in drinking water (600 mg/kg daily) for nine weeks, and then urine specimens were collected and analysed. The compounds were identified by gas chromatography-mass spectrometry in both electron-impact mode and chemical ionization mode. In addition to 2-EHA, ten different 2-EHA-related metabolites were found in the urine of 2-EHA-treated rats. The main metabolite was 2-ethyl-1,6-hexanedioic acid. Urine also contained 2-ethyl-6-hydroxyhexanoic acid and five other hydroxylated metabolites and two lactones, the detailed structures of which have not yet been elucidated. The unsaturated 5,6-dehydro-EHA was also identified; this is the metabolite corresponding to 2-n-propyl-4-pentenoic acid, the hepatotoxic metabolite of valproic acid. At least part of the 2-EHA is present in urine as a glucuronide conjugate.

The enzymatic basis for the metabolism and inhibitory effects of valproic acid: dehydrogenation of valproyl-CoA by 2-methyl-branched-chain acyl-CoA dehydrogenase

Five distinct acyl-CoA dehydrogenases are currently known. These are short, medium, long and 2-methyl-branched-chain acyl-CoA dehydrogenases, and isovaleryl-CoA dehydrogenase. We tested these five acyl-CoA dehydrogenases for their ability to dehydrogenate valproyl-CoA using pure enzyme preparations isolated from rat liver mitochondria. The activities of the pure human short-chain, medium-chain and isovaleryl enzymes purified from post-mortem livers, and a long-chain acyl-CoA dehydrogenase preparation partially purified from placental mitochondria, were also tested. Valproyl-CoA was dehydrogenated at a significant rate (0.167 mumol/min per mg protein) only by rat 2-methyl-branched-chain acyl-CoA dehydrogenase. Human 2-methyl-branched-chain acyl-CoA dehydrogenase has not been purified; therefore, it could not be tested. Since four other human acyl-CoA dehydrogenases did not dehydrogenate isobutyryl-CoA, 2-methylbutyryl-CoA (obligatory intermediates from valine and isoleucine, respectively) nor valproyl-CoA, it is reasonable to assume that valproyl-CoA is dehydrogenated by 2-methyl-branch-chain acyl-CoA dehydrogenase in man as well. We identified 2-propyl-2-pentenoyl-CoA as the reaction product from valproyl-CoA by mass spectral analysis of the acyl moiety. Valproyl-CoA, at 0.3 mM, moderately inhibited human acyl-CoA dehydrogenases with the exception of the long-chain enzyme. 5 mM free valproic acid inhibited the activities of various acyl-CoA dehydrogenases only very weakly.

Beta-oxidation of valproate. II. Effects of fasting, glucose, and clofibrate in rats

The effects of glucose infusion, fasting, and clofibrate pretreatment on valproate (VPA) disposition were investigated in rats to determine the role of endogenous fatty acid beta-oxidation in the metabolic formation of 2-en-VPA. Rats undergoing each treatment received a continuous steady-state infusion of VPA and a single intravenous (i.v.) bolus of 2-en-VPA. Elimination clearance of VPA was significantly higher (median 31%, p = 0.002) with glucose infusion as compared with fasting but was unchanged by clofibrate pretreatment as compared with control. Formation clearance of 2-en-VPA was significantly higher with glucose infusion as compared with fasting (median 147%, p = 0.001) and with clofibrate pretreatment as compared with control (median 73%, p = 0.041). Fractional metabolism of VPA by this route averaged 6% in fasted and control rats and 10% in glucose-infused and clofibrate-pretreated rats. Thus, VPA elimination clearance was not greatly influenced by effects on this route in rats. Elimination clearance of 2-en-VPA was also higher with glucose infusion as compared with fasting (median 149%, p = 0.002), and with clofibrate pretreatment as compared with control (median 167%, p less than 0.001). These observations are consistent with glucose-sparing release of endogenous fatty acids (FAs) to compete with VPA for beta-oxidation, and increased beta-oxidative activity after clofibrate treatment. The results of this study provide strong in vivo evidence for involvement of beta-oxidation in metabolism of VPA.

Clinical pharmacokinetics of valproic acid--1988

Sodium valproate (valproic acid) has been widely used in the last decade and is now considered a relatively safe and effective anticonvulsant agent. Recently, several investigators have proposed its use in the treatment of anxiety, alcoholism and mood disorders, although these indications require further clinical studies. Valproic acid is available in different oral formulations such as solutions, tablets, enteric-coated capsules and slow-release preparations. For most of these formulations bio-availability approaches 100%, while the absorption half-life varies from less than 30 minutes to 3 or 4 hours depending on the type of preparation used. Once absorbed, valproic acid is largely bound to plasma proteins and has a relatively small volume of distribution (0.1 to 0.4 L/kg). Its concentration in CSF is approximately one-tenth that in plasma and is directly correlated with the concentration found in tears. At therapeutic doses, valproic acid half-life varies from 10 to 20 hours in adults, while it is significantly shorter (6 to 9 hours) in children. Valproic acid undergoes extensive liver metabolism. Numerous metabolites have been positively identified and there is reasonable evidence that several of them contribute to its pharmacological and toxic actions. In fact, several valproic acid metabolites have anti-convulsant properties, while many of the side effects it may cause (e.g. those related to hyperammonaemia or liver damage) are most often observed in patients previously treated with phenobarbitone. This could indicate that induction of liver enzymes is responsible for the formation of toxic valproic acid metabolites.

Valproate-associated hepatotoxicity and its biochemical mechanisms

Intake of the anticonvulsant drug valproic acid, or its sodium salt, has been associated with occasional instances of severe and sometimes fatal hepatotoxicity. Probably at least 80 cases have occurred worldwide. The syndrome affects perhaps 1 in 10,000 persons taking the drug, and usually develops in the early weeks or months of therapy. Most instances have involved children, usually those receiving more than 1 anticonvulsant. Multiple cases have occurred in 2 families. The typical presentation is of worsening epilepsy, increasing depression of consciousness, and progressive clinical and biochemical evidence of liver failure. The liver has sometimes shown hepatocyte necrosis, and on other occasions widespread microvesicular steatosis, while cholestatic changes have also occurred. The appearances are interpreted as consistent with a drug toxicity reaction. During the hepatotoxicity increased amounts of unsaturated metabolites of valproate, notably 4-en-valproate, have been found in blood and urine. In 4 cases there has been evidence of impaired beta-oxidation of valproate with, in 1 case, accumulation of isomers of valproate glucuronide caused by intramolecular rearrangement of the conjugate. There are molecular structural similarities between 4-en-valproate and 2 known hepatotoxins (4-en-pentanoate and methylenecyclopropylacetic acid, the latter being responsible for hypoglycin poisoning). There are also clinical and histopathological similarities between valproate hepatotoxicity and both hypoglycin poisoning and certain spontaneous disorders of isoleucine metabolism (one pathway of valproate metabolism is analogous to oxidative degradation of isoleucine). Unsaturated metabolites of valproate, in particular 4-en-valproate, may contribute to the hepatotoxicity of the drug. However, since the hepatotoxicity appears to involve an element of idiosyncrasy, the primary defect in some cases may be an inherited or acquired deficiency in the drug's beta-oxidation. This defect may divert valproate metabolism towards omega-oxidation, with increased formation of the toxin 4-en-valproate, but may also allow increased formation of a toxic metabolite derived from isoleucine, since beta-oxidation of isoleucine derivatives will also be impaired.

Oxidation and glucuronidation of valproic acid in male rats--influence of phenobarbital, 3-methylcholanthrene, beta-naphthoflavone and clofibrate

The influence of phenobarbital, clofibrate, 3-methylcholanthrene and beta-naphthoflavone on omega- and beta-oxidation as well as on glucuronidation of valproic acid (n-dipropylacetic acid) was evaluated in male Sprague-Dawley rats by determination of urinary excretion of its metabolites by GC-MS after administration of 100 mg/kg. In controls 12% of the dose was excreted within 24 hours, primarily as glucuronides; metabolites formed by oxidation amounted to about 4%. Phenobarbital treatment led to stimulation of 4-hydroxyvalproic acid [(omega-1)-oxidation], 5-hydroxyvalproic acid and n-propylglutaric acid (omega-oxidation) excretion. Clofibrate enhanced the excretion of 4-hydroxyvalproic acid and 3-keto-valproic acid, a product of peroxisomal beta-oxidation. beta-Naphthoflavone slightly increased the excretion of 5-hydroxyvalproic acid. The most specific effect was found for 3-methylcholanthrene, which was effective in stimulating the formation of 3-hydroxyvalproic acid which might be formed by (omega-2)-oxidation. The addition of fatty acids (olive oil in which 3-methylcholanthrene and beta-naphthoflavone were suspended) led to increased excretion of 3-keto-valproic, 4-hydroxyvalproic and n-propylglutaric acid. The excretion of 3-hydroxyvalproic acid was completely suppressed by olive oil. Such specific effects were not observed for glucuronidation of valproic acid and its metabolites, although stimulation was attained after phenobarbital, clofibrate and 3-methylcholanthrene treatment, because of instability of glucuronide conjugates. Stimulation of valproic acid metabolism was also shown by increased plasma clearance after treatment with phenobarbital. In contrast, clofibrate given once 1 hr before valproic acid inhibited excretion of valproic acid, possibly by competition during renal tubular secretion. Determination of valproic acid metabolites in urine provides a useful tool for evaluation of inducer specificity of short chain fatty acid metabolism and differentiation between microsomal and peroxisomal enzyme activity.

Identification of valproic acid metabolites in human serum and urine using hexadeuterated valproic acid and gas chromatographic mass spectrometric analysis

Valproic acid, an antiepileptic drug, is extensively metabolized in humans. A hexadeuterated valproic acid was used in this study to aid the identification of metabolites and their artifacts in serum and urine extracts. A 600 mg oral dose of hexadeuterated valproic acid was administered to a human volunteer at steady state serum concentrations of unlabelled valproic acid. By using gas chromatography mass spectrometry, metabolites were characterized as their methyl, tert-butyldimethylsilyl, or trimethylsilyl derivatives. A di-unsaturated metabolite with concentrations comparable to 2-ene valproic acid was identified in serum and urine. The presence of a molecular ion doublet in the mass spectrum corresponding to the unlabelled and hexadeuterated dienes reduces the possible structures for this metabolite. A new metabolite, 2-propyl-4-ketopentanoic acid was also identified in serum and in urine. The dicarboxylic acids, 2-propylsuccinic and 2-propylmalonic were identified in the urine extract as metabolites. Identification of isolated metabolites was aided by comparison of the retention times and mass spectra to those of synthesized reference compounds. Diagnostic fragment ions and ion doublets in the mass spectra of the derivatives of newly identified metabolites were compared to those of known metabolites.

The toxicity of metabolites of sodium valproate in cultured hepatocytes

Sodium valproate is hepatotoxic in both humans and rat hepatocytes. The toxicity is dose related and frequently associated with simultaneous ingestion of drugs which induce the drug metabolizing system. For these reasons, metabolites of sodium valproate were tested for toxicity using rat hepatocyte cultures. The sodium salts of three metabolites, 2-propylpent-4-enoate, 4-hydroxyvalproate, and perhaps 5-hydroxyvalproate, were toxic in this system. In addition, 2-propylpent-4-enoate was toxic in a dose-related fashion. All are omega and omega-1 oxidation products in the microsome-mediated pathway of valproate metabolism.

Systemic interaction between valproic acid and free fatty acids in rhesus monkeys

The interaction between valproic acid (VPA) and free fatty acids (FFA) was investigated at a systemic level in eight rhesus monkeys. Intralipid emulsion (I.L.) was infused to increase plasma FFA levels. During I.L. treatment, plasma FFA levels were elevated in all cases (12 cases) with increases ranging from 38 to 525% (median, 102%). Valproate free fraction (fp) was elevated in 11 of 12 cases, with increases ranging between 7 and 100% (median, 18%). A positive correlation between the relative increase in FFA and the corresponding increase in fp of VPA was obtained. Intralipid infusion was associated with changes in the systemic clearance of VPA (an increase in five cases and a decrease in seven cases). The intrinsic clearance (Clint) of VPA was decreased in 9 of 12 cases with decreases ranging from 13 to 78% (median, 22%). Urinary excretion of valproate glucuronide was decreased in three of four monkeys during I.L. treatment. A significant positive correlation (p less than 0.05) between the relative increase in FFA levels and the decrease in Clint of VPA were also observed. The mechanism of interaction between VPA and FFA in rhesus monkey is more complex than a simple protein binding displacement and involves at least one metabolic inhibition component such as glucuronidation.

The influence of free fatty acids on valproic acid plasma protein binding during fasting in normal humans

The effect of physiologic variations of free fatty acid levels on in vivo valproic acid plasma protein binding was studied in 6 healthy adult subjects. 14 blood samples were taken during a 12-h dosing interval at steady state while in a fed condition and also during a 27 h fast. Free fraction and total valproate concentration were determined by equilibrium dialysis and GLC, respectively. Free fatty acid levels were determined from both fresh samples and samples incubated at 37 degrees C for 12 h, the latter in order to simulate equilibrium dialysis conditions. Fasting resulted in increased serum free fatty acid levels in all subjects, ranging from 34-182% (p less than 0.01). Incubation also caused free fatty acid levels to rise, more so in fed samples (50-87%, p less than 0.01) than in fasting samples (10-50%, p less than 0.01). Fasting resulted in a 9% increase in the mean free fraction for all subjects combined (P less than 0.01). Regression analysis of 180 sets of values for free fraction, total valproate concentration and free fatty acid level suggested that valproate concentration accounts for 17% and free fatty acid level for 37% of the variation in free fraction. Mean clearance was unchanged by fasting despite an increased free fraction suggesting decreased intrinsic clearance (i.e. decreased metabolism) of valproate under these conditions.

Brain maturation following administration of phenobarbital, phenytoin, and sodium valproate to developing rats or to their dams: effects on synthesis of brain myelin and other subcellular membrane proteins

The anticonvulsant drugs phenobarbital, phenytoin, sodium valproate, and phenytoin-sodium valproate in combination were administered daily to (a) pregnant rats starting on the 5th day after conception, and continued through 17 days postpartum, or (b) to developing rats between 3 and 17 days of age. Each drug was prepared in water and administered at either a therapeutic dose (TD), three times therapeutic dose (3TD), or 9TD. Drug administration had no discernible effect on litter size or sex ratio in the offspring; however, phenobarbital administration to dams caused small but significant reductions in birth weights. Body weights of developing rats treated with anticonvulsant drugs either via dams of directly by intraperitoneal injection lagged behind controls. At 20-24 days of age the brain weights of the offspring of phenobarbital (9TD)-exposed dams lagged control weights by 5% whereas brain weights in the offspring of the other treated groups were indistinguishable from controls. In contrast, administration of phenobarbital directly to developing rats caused no significant brain weight deficits whereas significant deficits were observed with phenytoin (9TD), sodium valproate (9TD), and phenytoin-sodium valproate (9TD) in combination. AT 20-24 days of age the relative incorporation of radioactive leucine into purified myelin and crude nuclear proteins of drug-treated rats or the offspring of drug-treated dams was reduced by 10-20% in all cases. Dose-related differences were not observed however, and the effects of phenytoin and sodium valproate in combination approximated those of phenytoin administered alone.

Valproic acid and several metabolites: quantitative determination in serum, urine, breast milk and tissues by gas chromatography-mass spectrometry using selected ion monitoring

A method has been developed for the simultaneous quantitative determination of valproic acid (2-propylpentanoic acid) and its metabolites 2-propyl-2-pentenoic acid (trans), 2-propyl-3-pentenoic acid (trans), 2-propyl-4-pentenoic acid, 3-hydroxy-2-propylpentanoic acid, 4-hydroxy-2-propylpentanoic acid, 5-hydroxy-2-propylpentanoic acid, 3-oxo-2-propyl-pentanoic acid, and and 2-propylglutaric acid. All compounds were extracted at pH 5.0 with ethyl acetate. The concentrated extracts were trimethylsilylated and the resulting mixtures analyzed by a gas chromatography-mass spectrometry-computer system operated in the selected ion monitoring mode. Linear calibration curves were obtained in the concentration ranges studied (0.1-20 microgram/ml for metabolites, 0.1-150 microgram/ml for valproic acid. The recoveries of the drugs were between 92 and 97%. The relative standard deviations were between 3.9 and 8.1% (analysis of multiple 10-microliter samples of patient urine). The lower detection limits were found to be between 2.8 and 18 ng/ml using 200-microliter serum samples. The derivatized extracts were stable for at least one week. Applications of the method described include studies of placental transfer for valproic acid and metabolites in the human, the elimination of these substances by the neonate, their transfer via mother's milk, and their levels in mouse brain.

Effects of ethyl ester derivatives of valproic acid metabolites on pentylenetetrazol seizures in mice

The anticonvulsant activity of the ethyl esters of the major valproic acid metabolites was assessed against minimal pentylenetetrazol seizures in adult male ICR mice. The ethyl ester 3-hydroxy-propylpentanoic acid was found to possess significant anticonvulsant activity.

Plasma levels of valproic acid and its metabolites during continued treatment in dogs

Dogs were treated with the anti-epileptic drug valproic acid (VPA) for 2 weeks in order to determine whether therapeutic plasma levels could be maintained. VPA (as the sodium salt) was administered in a dosage of 170-180 mg/kg/day divided into three equal doses in the form of enteric-coated tablets. Due to the rapid elimination of this drug in dogs, therapeutic plasma levels were reached only intermittently. However, the concentrations maintained may be sufficient for anti-epileptic therapy in dogs because the lower plasma protein binding of VPA in this species gives rise to higher central concentrations compared with man. Furthermore, some VPA metabolites previously shown to exert anticonvulsant activity (2-en-VPA and 3-keto-VPA) were found in relatively high concentrations in dog plasma. We would recommend the dosages used in this study as a basis for assessment of VPA in the treatment of canine epilepsy.

Concentration of metabolites of valproic acid in plasma of epileptic patients

With the help of synthetic reference substances, five metabolites of valproic acid (VPA) could be quantitated by gas chromatography in the plasma of 26 epileptic patients undergoing chronic therapy with sodium valproate. The products of beta-oxidation, i.e., 2-en-VPA, 3-hydroxy-VPA, and 3-keto-VPA were found to be the major metabolites of VPA in plasma, whereas the intermediates of omega-oxidation, 4-hydroxy-VPA and 5-hydroxy-VPA, were present only in markedly lower concentrations. It was thus confirmed that in addition to the excretion of VPA as the glucuronide, beta-oxidation is the preferred metabolic pathway of VPA in man. However, taking the anticonvulsant activity of the metabolites as derived from animal experiments into consideration, none of the metabolites found in human plasma seems to contribute markedly to the therapeutic effect of VPA. Thus, in most patients, VPA seems responsible for more than 90% of the antiepileptic activity during continued medication in man.

A dual role for GABA in quasi-morphine abstinence behavior induced by di-n-propylacetate involving both initiation and termination

Di-n-propylacetate (DPA) induces a behavioral syndrome in rats resembling morphine abstinence behavior. The inhibitory action of DPA on GABA degradation, resulting in an enhanced release of GABA, is probably responsible for this behavioral effect, since GABA antagonists, like bicuculline and picrotoxin, have been shown to suppress this behavior. However, the time-course of the DPA-induced behavior is much shorter than that of the DPA-induced increase of GABA concentrations. Therefore, we have studied the influence of enhanced GABA levels caused by a first injection of DPA and the behavior evoked by a second injection of DPA at different time intervals after the first injection. The results indicate that GABA fulfills a role in both the initiation and termination of DPA-induced behavior. The mechanism responsible for this dual action of GABA is ascribed to a differential sensitivity to DPA of the nerve terminal and glial metabolic compartments of GABA in the brain. The increase of GABA in the nerve terminal caused by DPA is probably responsible for the initiation of the quasi-abstinence behavior, whereas the overflow of GABA into the synaptic cleft may be responsible for the suppression of this behavior via stimulation of presynaptic autoreceptors. Another mechanism responsible for the rapid termination of the DPA-evoked behavior could be the formation of DPA metabolites which antagonize this behavior. From the results of experiments using some primary metabolites of DPA, a role for these metabolites in the termination of the DPA-induced behavior seems unlikely.

Determination of valproic acid kinetics in patients during maintenance therapy using a tetradeuterated form of the drug

A method has been developed for measuring concentrations of valproic acid and, with stable isotopes, labelled forms of VPA, in the plasma of patients undergoing maintenance therapy. One tetradeutero isomer of valproic acid-di-(2,3-dideuteropropyl) acetic acid-has been synthesized for administration to patients. A [2H14]valproic acid-diheptadeuteropropyl acetic acid-was synthesized as the internal standard. After isolation from body fluids the acids were methylated with diazomethane. The McLafferty fragments of the methyl esters were used for mass spectrometric quantification. It was demonstrated that the [2H4]valproic acid used behaves kinetically like the unlabelled drug in the human body. One dose of [2H4]valproic acid was given as a pulse dose to epileptic patients and the elimination profile was measured during continuous normal treatment. The half-life of valproic acid in patients under barbiturate or carbamazepine co-medication was reduced to at least half the value found after single dose administration to healthy volunteers.