The toxicity of metabolites of sodium valproate in cultured hepatocytes

Analytical Method Development for Sodium Valproate through Chemical Derivatization

Background

Sodium valproate has anticonvulsant activity and is structurally different to conventional antiepileptic drugs. The problem with valproic acid is the lack of a chromophore, which means that gas chromatography is the sole assay methodology. The introduction of benzoyl and phenyl groups to the molecule is a useful derivatisation, which enables the creation of detectable chromophores for HPLC analysis for pharmaceutical dosages as well as biological systems. Methodology. Sodium valproate was derivatised by the addition of a chromophore to its structure by introducing a methyl benzoyl or a phenyl group. Trichlorophenol and 2-hydroxyacetophenone were used to introduce phenyl and benzoyl groups to valproic acid, respectively. The reaction used was estrification reaction using coupling agents. An analytical method was then developed and validated using reverse-phase HPLC. The method was validated for parameters like linearity, range, accuracy precision, and robustness.

Results

The developed method was easy and feasible and can be applied to both routine analysis and bioanalysis. The method was very sensitive and could quantify valproic acid at a very low concentration of 0.75 × 10⁻⁵ mg/ml. The developed method was found to be linear (R² = 0.997), accurate, precise, and robust.

Conclusion

The proposed chemical derivatisation and the developed analytical method are novel. The developed analytical procedure is the first of its kind; it is easy and feasible and can be used to quantify and detect sodium valproate at very low concentrations compared to other available methods in the literature.

Strategies, models and biomarkers in experimental non-alcoholic fatty liver disease research

Non-alcoholic fatty liver disease encompasses a spectrum of liver diseases, including simple steatosis, steatohepatitis, liver fibrosis and cirrhosis and hepatocellular carcinoma. Non-alcoholic fatty liver disease is currently the most dominant chronic liver disease in Western countries due to the fact that hepatic steatosis is associated with insulin resistance, type 2 diabetes mellitus, obesity, metabolic syndrome and drug-induced injury. A variety of chemicals, mainly drugs, and diets is known to cause hepatic steatosis in humans and rodents. Experimental non-alcoholic fatty liver disease models rely on the application of a diet or the administration of drugs to laboratory animals or the exposure of hepatic cell lines to these drugs. More recently, genetically modified rodents or zebrafish have been introduced as non-alcoholic fatty liver disease models. Considerable interest now lies in the discovery and development of novel non-invasive biomarkers of non-alcoholic fatty liver disease, with specific focus on hepatic steatosis. Experimental diagnostic biomarkers of non-alcoholic fatty liver disease, such as (epi)genetic parameters and '-omics'-based read-outs are still in their infancy, but show great promise. In this paper, the array of tools and models for the study of liver steatosis is discussed. Furthermore, the current state-of-art regarding experimental biomarkers such as epigenetic, genetic, transcriptomic, proteomic and metabonomic biomarkers will be reviewed.

Effect of saffron (Crocus sativus L.) on sodium valporate induced cytogenetic and testicular alterations in albino rats

The present study investigated the cytogenetic and testicular damage induced by the antiepileptic drug, sodium valporate (SVP) in albino rats and the effect of saffron aqueous extracts. Treating rats with SVP caused a significant increase in the chromosomal aberrations either structural or numerical and decreased the mitotic index. Besides, animals administered SVP showed DNA damage appeared in the single strand breaks (comet assay). Testis of SVP-treated rats showed many histopathological changes. A significant decrease in seminiferous tubules and their epithelial heights diameters and inhibition of spermatogenesis was recorded. In addition, the number of sperm head abnormalities was increased. Biochemical results revealed an increase in malondialdhyde (MDA) which is lipid peroxidation marker and a significant decrease in the level of serum antioxidant enzyme, catalase (CAT) and reducing antioxidant power (RAP). Animals given SVP and saffron showed an improvement in chromosomal aberrations, mitotic index, DNA damage and testicular alterations caused by SVP. Moreover, MDA decreased and CAT and RAP increased. It is concluded from the present results that the ameliorative effects of saffron extract against SVP-induced cytogenetic and testicular damage in albino rats may be due to the presence of one or more antioxidant components of saffron.

Toxicity of valproic acid in liver slices from sprague-dawley rats and domestic pigs

The reason for sensitivity to valproic acid (VPA) hepatotoxicity in humans is not known and requires further investigation. We investigated two in vitro animal models that might represent the unpredictably sensitive and the predictably non-sensitive populations of patients. VPA-induced hepatotoxicity was evaluated in vitro using precision-cut liver slices prepared from adult Sprague-Dawley rats and 4-wk-old domestic pigs. Protein synthesis, K(+) retention and cytosolic lactate dehydrogenase leakage in the slices were used as parameters of viability, with protein synthesis being the most sensitive indicator of viability. Exposure to 300 or 500 mug/ml produced damage in the rat liver slices after 24 hr. However, these VPA concentrations produced damage after 12 hr in slices from rats in which hepatic metabolism had been induced by administering phenobarbital. Damage to liver slices from domestic pigs was more severe than in those from rats. Slices from non-induced pigs showed damage 8 hr after culturing in the presence of 100, 300 or 500 mug VPA/ml. These data suggest that these two animal models may illustrate the different profiles of VPA-induced hepatotoxicity that are seen in the human population.

Determination of drug toxicity using 3D spheroids constructed from an immortal human hepatocyte cell line

Numerous publications have documented that the immortal cells grown in three-dimensional (3D) cultures possess physiological behavior, which is more reminiscent of their parental organ than when the same cells are cultivated using classical two-dimensional (2D) culture techniques. The goal of this study was to investigate whether this observation could be extended to the determination of LD₅₀ values and whether 3D data could be correlated to in vivo observations. We developed a noninvasive means to estimate the amount of protein present in a 3D spheroid from it is planar area (± 21%) so that a precise dose can be provided in a manner similar to in vivo studies. This avoided correction of the actual dose given based on a protein determination after treatment (when some cells may have lysed). Conversion of published in vitro LC₅₀ data (mM) for six common drugs (acetaminophen, amiodarone, diclofenac, metformin, phenformin, and valproic acid) to LD₅₀ data (mg compound/mg cellular protein) showed that the variation in LD₅₀ values was generally less than that suggested by the original LC₅₀ data. Toxicological analysis of these six compounds in 3D spheroid culture (either published or presented here) demonstrated similar LD₅₀ values. Although in vitro 2D HepG2 data showed a poor correlation, the primary hepatocyte and 3D spheroid data resulted in a much higher degree of correlation with in vivo lethal blood plasma levels. These results corroborate that 3D hepatocyte cultures are significantly different from 2D cultures and are more representative of the liver in vivo.

Oxidative Stress as a Mechanism of Valproic Acid-Associated Hepatotoxicity

Valproic acid (2-n-propylpentanoic acid; VPA) has several therapeutic indications, but it is used primarily as an anticonvulsant. VPA is a relatively safe drug, but its use is associated with idiosyncratic hepatotoxicity, which in some cases may lead to fatality. The underlying mechanism responsible for the hepatotoxicity is still not well understood, but various hypotheses have been proposed, including oxidative stress. This article discusses the experimental evidence on the effect of VPA on the various indices of oxidative stress and on the potential role of oxidative stress in VPA-associated hepatotoxicity.

Effect of Ginkgo biloba extract on oxidative metabolism of valproic acid in hepatic microsomes from donors with the CYP2C9*1/*1 genotypeThis article is one of a selection of papers published in this special issue (part 1 of 2) on the Safety and Efficacy of Natural Health Products

We investigated the effect of Ginkgo biloba extracts and some of its individual constituents on the oxidative metabolism of valproic acid (VPA) in hepatic microsomes from donors with the CYP2C9*1/*1 genotype. G. biloba extract decreased 4-ene-VPA, 3-OH-VPA, 4-OH-VPA, and 5-OH-VPA formation with mean (± SE) IC50 values of 340 ± 40 μg/mL, 370 ± 100 μg/mL, 180 ± 30 μg/mL, and 210 ± 20 μg/mL, respectively. This was associated with inhibition of not only CYP2C9*1, but also CYP2A6 and CYP2B6. Bilobalide, ginkgolide A, ginkgolide B, ginkgolide C, ginkgolide J, quercetin-3-O-rutinoside, kaempferol-3-O-rutinoside, and isorhamnetin-3-O-rutinoside were not responsible for the inhibition of VPA metabolism by the extract. When analyzed as the sum of the aglycone and total glycosides present in the extract, quercetin decreased 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA formation by 76%, 51%, and 70%, respectively, kaempferol decreased 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA formation by 65%, 46%, and 49%, respectively, and isorhamnetin decreased 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA formation by 29%, 26%, and 31%, respectively. The 3 aglycones did not affect 3-OH-VPA formation. In summary, G. biloba extract decreased hepatic microsomal formation of 4-ene-VPA, 4-OH-VPA, 5-OH-VPA, and 3-OH-VPA, but the effect was not due to the terpene trilactones or flavonol glycosides investigated in our study.

Teratogenicity of antiepileptic drugs: role of drug metabolism and pharmacogenomics

The approach to clinical decision-making pertaining to the use of antiepileptic drugs (AEDs) during pregnancy has relied on previous accumulated experience and, since the 1990s, on data from pregnancy registries. The limitations of this process are that no information regarding the chemical attributes of the AED under consideration, nor the role of a number of enzyme systems that are known to interact with foreign compounds to modify their potential for harm, are included. The role of the hepatic mixed function oxidase system may be especially important in conferring teratogenic risk. However, systems such as epoxide hydrolase, glutathione reductase, superoxide dismutase and other toxin-scavenging systems may be important modifiers that lower the risk. Knowledge is also accumulating on the interactions of AEDs with molecular targets such as histone deacetylase and peroxisome proliferator-activated receptors that may play important roles in teratogenesis. While our knowledge of these factors are incomplete, progress can be achieved by beginning to include these concepts in our discussion on the topic and by promoting research that may improve our ability to individualize the analysis of risk for a specific patient with regards to specific AEDs.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Valproic acid II: effects on oxidative stress, mitochondrial membrane potential, and cytotoxicity in glutathione-depleted rat hepatocytes

Oxidative stress has been associated with valproic acid (VPA) treatment, and mitochondrial dysfunction has been implicated in the pathogenesis of VPA-idiosyncratic hepatotoxicity. The present study investigated the effect of VPA and the role of GSH on oxidative stress, mitochondrial membrane potential, and toxicity in freshly isolated rat hepatocytes. Hepatocytes were isolated from Sprague-Dawley rats, and total levels of glutathione (GSH) reduced by pretreatment with a combination of L-buthionine sulfoximine (2 mM) and diethylmaleate (0.5 mM) prior to VPA (0-1000 microg/ml) treatment. Oxidative stress was determined by measuring the levels of 15-F(2t)-isoprostane (15-F(2t)-IsoP) and 2',7'-dichlorofluorescein (DCF). Mitochondrial membrane potential (Deltapsi(m)) was determined by using the dual-fluorescent dye JC-1, and cell viability was evaluated by the water-soluble tetrazolium salt WST-1 assay. Exposure of rat hepatocytes to VPA (0-1000 mug/ml) resulted in a time- and dose-dependent increase in 15-F(2t)-IsoP and DCF fluorescence, and these levels were further elevated in GSH-reduced hepatocytes. In control hepatocytes, VPA had no effect on cell viability; however, significant cytotoxicity was observed in the glutathione-depleted hepatocytes treated with 1000 mug/ml VPA. The Deltapsi(m) was only reduced in glutathione-reduced hepatocytes at 500 and 1000 microg/ml VPA. Our novel findings indicate that acute treatment of freshly isolated rat hepatocytes with VPA resulted in oxidative stress, which occurred in the absence of cytotoxicity, and that glutathione confers protection to hepatocytes against mitochondrial damage by VPA.

Abnormal alterations in the metabolic patterns of patients on valproate therapy

Four cases of abnormal metabolic patterns which were obtained from three infantile patients and one adult on valproate (valproic acid; 2-n-propyl-pentanoic acid) therapy are reported. Serum levels of valproate and 15 metabolites were measured by gas chromatography/mass spectrometry. A mentally retarded, 11-month-old boy developed an extremely altered metabolic profile after having been treated with valproate polytherapy for 3 months. The altered pattern included strongly elevated serum levels of the 4-ene as well as of the omega-/omega1-metabolites, with the beta-metabolites (2-ene; 2,3'-diene) being diminished. Two samples obtained previously had shown a common pattern. The infant died 3 weeks after the last sample had been taken. Two boys of the same age showed similar but less intense deviations in their metabolic profiles at the onset of valproate therapy. Within a few weeks they approached, in a step-wise fashion, the average pattern common for children under 3 years of age. The striking alterations were paralleled by the metabolic profiles of an adult patient who suffered from intrahepatic metastasis and renal insufficiency. From the close resemblance of the abnormal metabolic patterns it was concluded that liver dysfunction results in alteration of the whole metabolic system. Regular inspection of the entire profile of an individual might help to recognize conspicuous alterations in time to avoid severe side effects.

CYP2E1-mediated modulation of valproic acid-induced hepatocytotoxicity

OBJECTIVES

To determine the cytotoxicity of valproic acid (VPA) and its metabolite, 4-ene-valproic acid (4-ene-VPA) in human hepatoblastoma cells (Hep G2), and to study the modulatory effect of cytochrome P450 2E1 induction in this model.

METHODS

Cells were exposed to VPA or 4-ene-VPA in the presence of either ethanol (EtOH), or EtOH combined with disulphiram (DS). Some cells were exposed to alpha-fluoro-VPA or to alpha-fluoro-4-ene-VPA in the absence of CYP2E1 inducers. Apoptosis and necrosis were measured by analyzing 6000 cells per sample using transmission electron microscopy, while cytokine release and apoptosis were quantitated by ELISA.

RESULTS

VPA + EtOH increased VPA cytotoxicity. 4-ene-VPA + EtOH significantly increased toxicity, while DS + EtOH significantly reduced this toxicity. Alpha-fluorinated analogues reduced cytotoxicity compared to the corresponding VPA compounds. Neither VPA nor alpha-fluorinated VPA increased the release of IL-6 or TNF-alpha in media. A significant increase in the release of TNF-alpha was observed in cells exposed to 4-ene-VPA that further increased with EtOH exposure.

CONCLUSIONS

Cells exposed to 4-ene-VPA experience greater cytotoxicity than those treated with VPA. Cytochrome P450 2E1 inducers enhance toxicity in VPA-exposed cells, while alpha-fluorination of VPA diminishes cytotoxicity by directly interfering with the beta-oxidation of the 4-ene-VPA metabolite.

Establishing individual metabolite patterns for patients on valproate therapy

The aim of this study was to establish individual metabolic profiles of patients receiving valproate (VPA) mono- or polytherapy in order to estimate inter- and intraindividual variability under normal conditions. Serum levels of VPA and 15 metabolites were measured by gas chromotography/mass spectrometry (GC/MS) with selected ion monitoring (SIM). Because of a huge inter-subject variability, calculating means for large epileptic populations resulted in broad and vague ranges for serum levels of VPA and its metabolites. It therefore remained difficult to recognize any significant alteration in the individual metabolic profile. Over long term periods, within-patient changes appeared to be much less intense than inherent interindividual differences. In epileptics consecutively receiving various forms of polytherapy, alterations in the metabolic profiles occurred. Therefore, integrating different kinds of co-medication into a single polytherapy group seemed to be inadequate. An adult patient on VPA monotherapy, suffering form intrahepatic metastasis and renal insufficiency, showed an extremely altered metabolic pattern, with the 4-ene and the omega-/omega1-metabolites being strongly elevated and the major beta-metabolites (E)-2-ene and (E,E)-2,3'-diene being significantly diminished. We suggest determining the individual metabolic profile, consisting of accessible major and minor metabolites, for every patient when VPA therapy has been started or been modified. The moment any clinical complications arise, the previously obtained specific pattern of the individual can be taken as reference in order to assess the possible presance of significant alterations which might indicate or even cause any severe side effects. There seems to be no need of monitoring metabolite levels of the average patient continuously except for the high risk group (e.g. infants under 3 years age receiving polytherapy) which exhibited the highest between-subject as well as within-patient variability.

Valproate: a reappraisal of its pharmacodynamic properties and mechanisms of action

Valproate is currently one of the major antiepileptic drugs with efficacy for the treatment of both generalized and partial seizures in adults and children. Furthermore, the drug is increasingly used for therapy of bipolar and schizoaffective disorders, neuropathic pain and for prophylactic treatment of migraine. These various therapeutic effects are reflected in preclinical models, including a variety of animal models of seizures or epilepsy. The incidence of toxicity associated with the clinical use of valproate is low, but two rare toxic effects, idiosyncratic fatal hepatotoxicity and teratogenicity, necessitate precautions in risk patient populations. Studies from animal models on structure-relationships indicate that the mechanisms leading to hepatotoxicity and teratogenicity are distinct and also differ from the mechanisms of anticonvulsant action of valproate. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA synthesis and release and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to attenuate neuronal excitation induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate exerts direct effects on excitable membranes, although the importance of this action is equivocal. Microdialysis data suggest that valproate alters dopaminergic and serotonergic functions. Valproate is metabolized to several pharmacologically active metabolites, but because of the low plasma and brain concentrations of these compounds it is not likely that they contribute significantly to the anticonvulsant and toxic effects of treatment with the parent drug. By the experimental observations summarized in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels of valproate's mechanisms of action.

A simplified model of hypoxic injury in primary cultured rat hepatocytes

The Anaeropack system for cell culture, which was originally designed for the growth of anaerobic bacteria, was used to produce a hypoxic atmosphere for cultured hepatocytes. We measured changes in the oxygen and carbon dioxide concentrations and the atmospheric temperature in an airtight jar. We also measured changes in the pH of the medium during hypoxia to assess the accuracy of this system. Moreover, we used three durations (2, 3, and 4 h) of hypoxia and 8 h of reoxygenation in cultured rat hepatocytes, and then measured the lactate dehydrogenase (LDH), ketone body concentration (acetoacetate + beta-hydroxybutyrate), and the ketone body ratio (KBR: acetoacetate/beta-hydroxybutyrate) in the medium in order to assess the suitability of this system as a model for reperfusion following liver ischemia. The oxygen concentration dropped to 1% or less within 1 h. The concentration of carbon dioxide rose to about 5% at 30 min after the induction of the hypoxic conditions, and was maintained at this level for 5 h. No effect of the reaction heat produced by the oxygen absorbent in the jar was recognized. The extent of cell injury produced by changing the hypoxic parameters was satisfactorily reflected by the KBR, the ketone body concentration, and the LDH activity released into the medium. Because this model can duplicate the conditions of the hepatocytes during revascularization following ischemic liver, and the Anaeropack system for cell culture is easy to manipulate, it seems suitable for the experimental study of hypoxic injury and revascularization in vitro.

Comparison of the steady-state pharmacokinetics of topiramate and valproate in patients with epilepsy during monotherapy and concomitant therapy

PURPOSE

The steady-state pharmacokinetics of valproate (VPA) and topiramate (TPM) were compared during VPA monotherapy, concomitant VPA and TPM therapy, and TPM monotherapy to evaluate pharmacokinetic interactions.

METHODS

After a 3-week baseline period, 12 patients receiving VPA monotherapy (500 to 2,250 mg every 12 h) received TPM at three escalating doses (from 100 to 200 to 400 mg every 12 h), each for 2 weeks. Thereafter, the VPA dose was tapered by 25% weekly. Blood and urine samples were collected over 12-h intervals during VPA monotherapy and at the end of each stage of TPM dose escalation and TPM monotherapy.

RESULTS

All patients reached TPM monotherapy, and nine achieved satisfactory seizure control for > or = 2 weeks without VPA. TPM plasma peak concentration (C(max)) and area under the concentration-versus-time curve during a 12-h dosing interval (AUC(0-12)) were slightly higher (17%; n = 8) during TPM monotherapy than during concomitant VPA therapy. TPM oral and renal clearances (n = 8) were 25.9 +/- 4.6 and 11.6 +/- 3.2 ml/min during TPM monotherapy and were 29.8 +/- 4.2 and 12.4 +/- 2.7 ml/min during VPA concomitant therapy. VPA AUC(0-12) decreased (11.3%; n = 10) with the addition of TPM 400 mg every 12 h. VPA oral clearance was 12.8 +/- 4.1 ml/min during monotherapy and was 13.8 +/- 4.0, 14.1 +/- 3.9, and 14.5 +/- 5.2 ml/min during coadministration of TPM 100, 200, and 400 mg every 12 h, respectively. Cognitive dysfunction, observed in some patients receiving high doses of VPA with TPM, reversed or improved with VPA dose reduction and discontinuation. The lower-than-normal prestudy platelet count measured in one patient increased to normal levels when VPA was discontinued.

CONCLUSIONS

Because changes in TPM and VPA pharmacokinetics were small, it is unlikely that their concomitant use will have a significant impact on the clinical condition of the patient.

Bidirectional interaction of valproate and lamotrigine in healthy subjects

OBJECTIVE

To evaluate the steady-state pharmacokinetics of lamotrigine and valproate at three dosing levels of lamotrigine in normal volunteers receiving steady-state therapeutic doses of valproate.

METHODS

This was an open-label, randomized, three-way crossover study of 18 normal male volunteers. Subjects received oral valproate (500 mg Depakote twice a day) throughout the study. Each subject subsequently received three oral dosage regimens of lamotrigine (50, 100, or 150 mg/day) for 1 week each, with a 2-week washout period between lamotrigine treatment periods. Valproate and lamotrigine trough plasma samples were determined by a capillary gas chromatography method and immunofluorometric assay, respectively. Urine samples were assayed for 11 valproate metabolites by gas chromatography/mass spectrometry.

RESULTS

When compared to other studies in which lamotrigine was administered with no concurrent antiepileptic drug, concomitant valproate markedly increased the half-life of lamotrigine and decreased lamotrigine clearance, without substantial alteration in the linear kinetics of the drug. The addition of lamotrigine was associated with a small but significant 25% decrease in steady-state valproate plasma concentration. Oral clearance of valproate was increased (from 7.2 +/- 1.1 ml/hr/kg before lamotrigine treatment to 9.0 +/- 2.0 ml/hr/kg on day 28; p < 0.05). The formation clearance of the hepatotoxic valproate metabolites, 2-n-propyl-4-pentenoic acid (4-ene-valproate) and 2-propyl-2,4-pentadienoic acid [2(E),4-diene-valproate], was unaffected by lamotrigine administration.

CONCLUSIONS

As a consequence of the interaction between lamotrigine and sodium valproate, a dosage reduction of lamotrigine should be considered in patients taking a combination of valproate and lamotrigine.

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.

CYP enzymes catalyze the formation of a terminal olefin from 2-ethylhexanoic acid in rat and human liver

1. The metabolism of 2-ethylhexanoic acid (2-EHA) was studied in rat, mouse and human liver microsomes in vitro. The metabolites of 2-EHA were identified as methylated derivatives by gas chromatography-mass spectrometry. 2. 2-Ethyl-1,6-hexanedioic acid was the main metabolite produced in rat, mouse and human liver microsomes. Unsaturated 2-ethyl-5-hexenoic acid, a terminal olefin, was produced only in human liver microsomes and phenobarbital-induced rat liver microsomes. The cytochrome P450 (CYP) inhibitors metyrapone, SKF 525A, triacetyloleandomycin (TAO), quinidine and the cytochrome P450 reductase antibody abolished its formation both in rat and human microsomes. 3. The metabolites were analyzed also in vivo in urine of 2-EHA-exposed rats and in urine of sawmill workers exposed occupationally to 2-EHA. Both rat and human urine contained 2-ethyl-1,6-hexanedioic acid as the main metabolite and also 2-ethyl-5-hexenoic acid. Metyrapone, SKF 525A and TAO all decreased drastically the formation of 2-ethyl-5-hexenoic acid in the rat. 4. The data indicate that (1) several CYP families (CYP2A, CYP2B, CYP2D and CYP3A) could be responsible for the hepatic metabolism of 2-EHA, (2) the same metabolites were formed in rats and man and (3) an unsaturated terminal olefin, 2-ethyl-5-hexenoic acid is formed in the liver.

Bioactivation of a toxic metabolite of valproic acid, (E)-2-propyl-2,4-pentadienoic acid, via glucuronidation. LC/MS/MS characterization of the GSH-glucuronide diconjugates

The hepatotoxicity of the anticonvulsant drug valproic acid may be associated with the formation of potentially reactive metabolites, one of which is (E)-2-propyl-2,4-pentadienoic acid ((E)-2,4-diene VPA). This report describes the characterization of new GSH-related conjugates of this diene. Bile samples collected from male Sprague-Dawley rats dosed ip with (E)-2,4-diene VPA (100 mg/kg) were analyzed by LC/MS/MS. Initial Q1 parent in scanning indicated that the daughter ions m/z 162 and 123 could be derived from the ions at m/z 624 and 480, respectively. Subsequent collision-induced dissociation (CID) of these parent ions revealed a common neutral loss of 176 Da which is diagnostic for glucuronides. A similar neutral loss of 176 Da was observed in daughter ion spectra of the biliary metabolites arising from [2H7]-4-ene VPA dosed ip to rats, where the ion fragments containing the VPA portion were 7 amu higher than those derived from the unlabeled drug. CID of the ion at m/z 624 also gave fragments characteristics for GSH conjugates such as the loss of glycine and glutamate moieties. Based on the MS data, the metabolites were assigned the diconjugate structures 1-O-(2-propyl-5-(glutathion-S-yl)-3-pentenoyl)-beta-D-glucur onide (5-GS-3-ene VPA-glucuronide I, MH+, 624) and the corresponding 5-NAC-3-ene VPA-glucuronide (MH+, 480). Further proof of structural identity was obtained from 1H NMR of HPLC-purified metabolites. The amount of biliary 5-GS-3-ene VPA-glucuronide I was 7-fold greater than the corresponding 5-GS-3-ene VPA, the sum of the two metabolites accounting for 6.6% of the dose. Incubation of 1-O-(2-propyl-2,4-pentadienoyl)-beta-D-glucuronide (2,4-diene VPA-glucuronide) with GSH in the presence or absence of GST enzyme led to the formation of 5-GS-3-ene VPA-glucuronide I which was readily detected by LC/MS/MS, suggesting that in vivo the diconjugate may arise from the reaction of GSH with 2,4-diene VPA-glucuronide. To our knowledge, this is the first recorded instance in which glucuronide formation activates a drug to further conjugate with GSH via a Michael addition reaction.

Effects of the anticonvulsant, valproate, on the expression of components of the cytochrome-P-450-mediated monooxygenase system and glutathione S-transferases

It has been shown previously that the anticonvulsant agent, sodium valproate, induces certain cytochrome P-450 monooxygenase activities and decreases glutathione S-transferase activity. We have used Western blotting, RNase protection assays and Northern blot hybridization to determine the effects of valproate on the abundance of individual components of the cytochrome P-450 monooxygenase and of glutathione S-transferase subunits. Due to the short half-life of the drug in rats we have used an in vitro experimental system comprised of rat hepatocytes co-cultured with rat primitive biliary epithelial cells. Valproate was shown to be a potent inducer of two members of the cytochrome P-450 (CYP)2B subfamily, CYP2B1 and 2B2. The induction of the proteins was mediated at the level of the mRNAs, with the mRNA for CYP2B1 being more highly induced than that for CYP2B2. The drug also induced, but to a much lesser extent, two important components of the cytochrome-P-450-mediated monooxygenase system, NADPH-dependent cytochrome P-450 reductase and cytochrome b5, and their corresponding mRNAs. Thus, the effects of valproate on cytochromes P-450 and other components of the cytochrome-P450-mediated monooxygenase system mimic those of another, structurally diverse, antiepileptic drug, phenobarbital. However, in contrast to phenobarbital, which induces glutathione S-transferase subunits 1, 2, 3, 4 and 7, valproate selectively decreases the abundance of subunits 3 and/or 4. It has been shown previously that CYP2B1 is involved in the production of metabolites of valproate implicated in hepatotoxicity. The induction of this protein by valproate would thus contribute substantially to the hepatotoxic effects associated with the drug.

CYP4 isozyme specificity and the relationship between omega-hydroxylation and terminal desaturation of valproic acid

The cytochrome P450-dependent terminal desaturation of valproic acid (VPA) is of both toxicological and mechanistic interest because the product, 4-ene-VPA, is a more potent hepatotoxin than the parent compound and its generation represents a rather novel metabolic reaction for the cytochrome P450 system. In the present study, lung microsomes from rabbits were identified as a rich source of VPA desaturase activity. Monospecific polyclonal antibodies directed against CYP4B1 (anti-4B) inhibited 82% of 4-ene-VPA formation, whereas monospecific polyclonal antibodies directed against CYP2B4 (anti-2B) inhibited only 15% of 4-ene-VPA formation. Anti-4B also inhibited 95% of the 5-hydroxy-VPA formation, but only 42% of 4-hydroxy-VPA formation. These data suggest that CYP4B1 accounts for more than 80% of the 4-ene- and 5-hydroxy-VPA metabolites generated by rabbit lung microsomes. CYP4B1 expressed in HepG2 cells metabolized VPA with a turnover number of 35 min-1 and formed the 5-hydroxy-, 4-hydroxy-, and 4-ene-VPA metabolites in a ratio of 110:2:1, respectively. In contrast, the lauric acid omega-hydroxylases, CYP4A1 and CYP4A3, did not give rise to detectable levels of any of these VPA metabolites. Therefore, these studies demonstrate a new functional role for CYP4B1 in the terminal desaturation and omega-hydroxylation of this short, branched-chain fatty acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of glycine on valproate toxicity in rat hepatocytes

The interaction between the amino acid glycine and valproate (VPA), an antiepileptic drug (AED) that occasionally causes hepatotoxicity, was studied in rat hepatocytes in monolayer culture. Valproate caused a dose-dependent increase in leakage of lactic acid dehydrogenase (LDH), and glycine prevented this toxic response. L-Carnitine, L-alanine, and L-cysteine did not protect hepatocytes from VPA. Glycine also partially antagonized inhibition of fatty acid beta-oxidation by VPA, as estimated by the generation of acid-soluble products from [14C]palmitic acid. These results are consistent with the hypothesis that glycine prevents VPA toxicity by removing acyl-CoA esters, which accumulate during VPA exposure and interfere with fatty acid beta-oxidation. Glycine, however, also antagonized the toxic effects of acetaminophen on hepatocytes, although at higher concentrations than required to protect hepatocytes from VPA. Because the mechanism of toxicity of acetaminophen probably is different from that of VPA, a nonspecific cytoprotective effect may contribute to glycine antagonism of valproate toxicity. Our results emphasize the importance of glycine in protecting hepatocytes from noxious insult in general as well as from VPA in particular.

Valproate (VPA) metabolites in various clinical conditions of probable VPA-associated hepatotoxicity

Of a cohort of 470 epileptic patients in whom valproate (VPA) serum metabolites had been measured, 170 subjects without symptoms or signs of hepatic side effects were chosen as a reference group to establish the usual metabolic pattern. A wide interindividual variation of VPA metabolite concentrations was noted. Infants receiving VPA monotherapy and comedication with other antiepileptic drugs (AEDs) showed lower concentrations of the potential hepatotoxin 4-ene-VPA than did older children. In 11 patients with early symptoms and signs of possible fatal VPA-associated hepatotoxicity, the following spectrum of benign clinical conditions was observed: unusually severe side effect during initiation of VPA therapy (1 patient), high VPA dosage (2 patients), reversible impairment of coagulation with bleeding manifestations in association with a slight increase in transaminase levels (1 child), and reversible liver dysfunction associated with febrile illness (7 patients). Reversible or irreversible fulminant liver failure had occurred in 5 children. Three of the 4 children with a fatal outcome had massive lactic acidosis. In all patients with probable VPA-associated hepatotoxicity, some aspects of VPA metabolism differed distinctly from that of the reference group, but the inter-individual profile of metabolites varied considerably, even in the subgroup of 4 children who died. Impairment of VPA beta-oxidation and increase of metabolites of alternative metabolic pathways (omega- and omega 1-hydroxylation, dehydrogenation reactions) were the most frequent findings. Increased values of 2-n-propyl-4-pentenoic acid metabolite of VPA (4-ene-VPA), could be detected only in 1 of the 5 patients with fulminant liver failure and in one other child with a slight hepatic dysfunction, indicating that this VPA metabolite is not the decisive hepatotoxin or indicator of hepatotoxicity. Because we cannot distinguish between benign and life-threatening hepatic adverse reactions on the basis of VPA metabolites, all identified changes are considered secondary to an as-yet-unknown primary metabolic event. The most toxic compound could be VPA itself, which may unmask an inborn or an acquired metabolic defect in the processing of fatty acids.

Pharmacokinetic analysis of the structural requirements for forming "stable" analogues of valpromide

The following valpromide (VPD) analogues were synthesized and their structure-pharmacokinetic relationships explored: 3-ethyl pentanamide (EPD), methylneopentylacetamide (MND), 1-methyl cyclohexanecarboxamide (MCD), cycloheptanecarboxamide (CHD), and t-butylacetamide (TBD). Two aliphatic (EPD and MND) and two cyclic amides (MCD and CHD) underwent complete or partial conversion to their corresponding acids. The only amide found in this study to be "stable" to the amide-acid biotransformation was TBD. It also had the lowest clearance and the longest half-life and mean residence time. Unlike the other investigated amides, TBD contained two substitutions of two methyl moieties at the beta position of its chemical structure. A "stable" valpromide analogue must have either two substitutions at the beta position, such as in the case of TBD, or a substitution in the alpha and beta positions, such as in the case of the VPD isomer valnoctamide (VCD). This paper discusses the antiepileptic potential of stable VPD analogues which may be more potent and less teratogenic than their biotransformed isomers.

Effect of valproate dose on formation of hepatotoxic metabolites

Valproate (VPA) therapy has been associated with a rare but fatal hepatotoxicity. 4-ene-VPA and 2(E),4-diene-VPA, unsaturated metabolites of VPA, are hepatotoxins several times more potent than VPA. In a previous study, a dose-dependent excretion of hepatotoxic metabolites was noted in patients receiving VPA. Our study was designed to evaluate the effect of dose on VPA metabolism under controlled conditions. Nineteen healthy volunteers sequentially received three different daily doses of VPA (250, 500, and 1,000 mg). Each dose was given twice daily for 4 days. Urine was collected for one dosage interval (12 h) at steady state for each dose and assayed for 15 VPA metabolites by gas chromatography/mass spectrometry (GCMS). Blood samples were also obtained from eight of the subjects, and VPA was assayed by GCMS. No effect of dose was noted on total plasma clearance. There was a significant dose-dependent decrease in intrinsic hepatic clearance. The intrinsic formation clearance (Clf) of the 4-ene-VPA pathway showed a statistically significant dose-dependent increase (0.22, 0.33, 0.40 ml/h/kg). The corresponding percentage of dose recovered as 4-ene-VPA and its sequential metabolites showed significant dose-dependent increases (0.15, 0.27, 0.62%). The role of VPA dose in the pathogenesis of hepatotoxicity may be more important than was previously believed.

Influence of co-medication on the metabolism of valproate

Valproate is extensively metabolized in the liver and at least six main pathways which produce about 50 metabolites have been identified in man. The enzyme-inducing antiepileptic drugs phenobarbital, primidone, phenytoin and carbamazepine increase total valproate clearance by 30-85%, whereas cimetidine and the new anticonvulsant compound striripentol display a small inhibitory effect (10-20%). Both carbamazepine and phenytoin induce a two-fold increase in the formation of delta 4-valproate and stimulate omega-oxidation and omega-1-oxidation. Acetylsalicylic acid causes a fall of 60-70% in the content in the urine of the metabolites of the beta-oxidative pathway, i.e. delta 2-valproate, 3-OH-valproate and 3-oxo-valproate, and an increase of glucuronidation (approximately 30%) and delta-dehydrogenation (approximately 20%). Stiripentol inhibits the formation clearance of delta 4-valproate by 30%. In the light of the possible therapeutic and toxic effects of some valproate metabolites, drug interactions with valproate at metabolic level may have important clinical implications.

Metabolism of valproate to hepatotoxic intermediates

A number of lines of evidence indicate that metabolites of valproate rather than the parent drug, mediate the microvesicular steatosis which characterizes valproate-associated liver injury. In this article, two mechanisms are discussed whereby valproate may cause hepatic steatosis through interference with the process of fatty acid beta-oxidation. In the first, valproate itself enters the mitochondrion where it completes for the enzymes and/or co-factors involved in the beta-oxidation of endogenous substrates, while in the second, valproate is metabolized via the hepatotoxic terminal olefin, delta 4-valproate, to a variety of chemically reactive intermediates which inhibit key enzymes in the beta-oxidation cycle.

Is 2-Propyl-4-Pentenoic Acid, a Hepatotoxic Metabolite of Valproate, Responsible for Valproate-Induced Hyperammonemia?

To investigate the association between valproate metabolism (VPA) and VPA-induced hyperammonemia together with the contribution of VPA hepatotoxicity risk factors such as young age, polypharmacy, and high serum VPA levels to VPA-induced hyperammonemia, plasma ammonia (NH3) levels, serum levels of VPA and its metabolites, and biochemical parameters were determined in 98 patients treated with VPA (53 monopharmacy cases and 45 polypharmacy cases). In monopharmacy patients, plasma NH3 levels did not depend on age, VPA dosage or serum levels. Serum level of 2-propyl-4-pentenoic acid (4-en) showed a negative correlation with plasma NH3 level in the monopharmacy group. In polypharmacy patients, plasma NH3 levels, serum glutamic pyruvic transaminase, and gamma-glutamyl-transpeptidase were significantly higher, while level/dose VPA ratio, 2-en-VPA serum level, and bilirubin were significantly lower than those in monopharmacy patients. These results suggest that young age and relatively high VPA serum levels within the therapeutic range were unlikely to be risk factors for common hyperammonemia associated with VPA therapy and that 4-en was not causally related to this adverse effect. The decreased serum level of 2-en-VPA in polypharmacy patients may be a reflection of a certain mitochondrial dysfunction, which might be a mechanism of the increased NH3 levels. The changes in biochemical parameters in polypharmacy patients were considered results of the enzyme-inducing activity of coadministered antiepileptic drugs (AEDs).

Valproic acid hepatotoxicity in human liver slices

Precision cut human liver slices were incubated in organ culture with valproic acid (VPA) to identify patterns of sensitivity to VPA-induced hepatotoxicity. The slices were incubated in Krebs-HEPES buffer supplemented with 25mM glucose and 84 micrograms/ml gentamycin. At 2, 4, 6, 12, 18 and 24 hr slices were taken and analyzed for K+ retention, synthesis of protein and LDH leakage. All three of these viability indicators showed that certain human livers were more susceptible to VPA-induced hepatotoxicity than others. In the limited group of human livers investigated (n = 9) we found one to be particularly sensitive and two relatively insensitive to VPA toxicity. The remaining tissues were of intermediate sensitivity towards VPA. At this time there is no correlation between the human livers that were susceptible to VPA induced hepatotoxicity and age or sex. This study was designed to show that VPA does induce hepatotoxicity in vitro at therapeutically relevant concentrations. Future studies will show whether VPA hepatotoxicity correlates with VPA metabolism, nutritional status or concomitant therapy.

Preclinical toxicology of the anticonvulsant calcium valproate

The oral toxicity of the anticonvulsant calcium valproate with selected comparisons to valproic acid and sodium valproate was evaluated in mice, rats and Beagle dogs. Median lethal doses of the three forms of valproate in rodents ranged from 1100 to 3900 mg/kg. Clinical signs in acute studies and reductions in body weight or body weight gain and food consumption at high doses in rats and dogs during 2-, 13- and 52-week studies were considered to be central nervous system related. In the 13-week study in rats (calcium valproate at 200, 400, 800, 1200 and 1600 mg/kg and sodium valproate at 1200 mg/kg), reduced plasma globulin levels and low white blood cell counts due to suppressed neutrophil maturation were noted at doses of 800 mg/kg and higher. Platelet counts were reduced at 1200 and 1600 mg/kg. Testicular atrophy occurred at 1200 and 1600 mg/kg. In dogs given calcium valproate at 100, 200 and 400 mg/kg for 13 weeks, testicular atrophy was seen at 400 mg/kg and mild hepatocellular changes at all doses. In rats given calcium valproate at 125, 250 and 500 mg/kg for 1 year, reduced plasma protein and globulin levels and a dose-dependent increased incidence and severity of atrophic pancreatitis were noted at 250 and 500 mg/kg. Calcium valproate, given for 1 year to dogs at doses of 50, 100 and 200 mg/kg, was well tolerated. These studies indicated that calcium valproate has a toxicity profile similar to other forms of valproate.

Pharmacokinetics and pharmacodynamics of valproate analogues in rats. I. Spiro[4.6]undecane-2-carboxylic acid

The pharmacokinetic and pharmacodynamic properties of the spiro carboxylic acid, spiro[4.6]undecane-2-carboxylic acid (SUCA, ADD 93024), were investigated in rats and compared with those of the standard anticonvulsant carboxylic acid, valproate (VPA). The clearance of SUCA was dose-dependent, although the observed nonlinearity did not appear to be due to classical saturable elimination. The change in clearance across doses was consistent with end-product inhibition or cosubstrate depletion. The volume of distribution of the spiro compound also evidenced nonlinearity, possibly due to concentration-dependent binding to serum proteins. In contrast, the dose-dependent clearance displayed by VPA was composed of both saturable and nonsaturable components. Furthermore, the disposition of VPA was characterized by a significant enterohepatic recirculation, whereas no such recirculatory process was apparent for SUCA. Both compounds afforded significant protection from pentylenetetrazol (PTZ)-induced seizures, and the time course of anticonvulsant effect did not correspond to that of drug concentrations in serum for either anticonvulsant. The apparent dissociation between the pharmacokinetics and pharmacodynamics of VPA may be a function of the mechanism of antiepileptic action and not due to the presence of active metabolites of the drug.

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.

Markedly increased omega-oxidation of valproate in fulminant hepatic failure

Using gas chromatography-mass spectrometry, we showed that the urinary metabolite profile of valproate (VPA) in a subject receiving VPA and phenobarbital (PB) who died of fulminant hepatic failure was quite different from those of reported patients with Reye's syndrome or fatal hepatic failure. Only 2-n-propylglutarate, the end product of omega-oxidation of VPA, was excreted in markedly increased amounts, while other VPA metabolites were undetectable. Although the primary cause of fulminant hepatitis and the mechanism of enhanced VPA metabolism by the hepatic P-450 system in this patient are not clear, our findings suggest that P-450-mediated reactions become the predominant metabolic pathway of VPA in a stage of fulminant hepatic failure.

Animal model of margosa oil ingestion with Reye-like syndrome. Pathogenesis of microvesicular fatty liver

The aetiology and pathogenesis of Reye's syndrome (RS) are incompletely understood. A number of environmental toxins and biological agents, including viruses, have been postulated to cause RS, either acting alone or synergistically. Most investigations have suggested that the primary insult is in the liver mitochondria, leading to a complex biochemical catastrophe, with death from encephalopathy. Margosa oil (MO), a long-chain fatty acid compound, has been shown to cause a Reye-like syndrome with death from hepatoencephalopathy, in children in Malaysia and India. The present time-course study performed in MO-administered mice showed the development of hepatic lesions with many features of RS. MO acts rapidly, within 30 min, on the nuclei of hepatocytes inducing mitoses and binucleated cells. This is followed by mitochondrial injury, with swelling, rarefaction of matrix, loss of dense bodies, pleomorphism, and loss of ribosomes starting at 60 min. There is loss of liver glycogen, and proliferation and hypertrophy of the endoplasmic reticulum (ER), followed by the presence of lipid droplets in the hyaloplasm, and globules within dilated cisterns of the ER. Additional fatty acids from lipolysis of body adipocytes, and fat globules from intestinal MO ingestion further aggravate the liver fatty change. There is evidence of fat globule ingestion by endocytosis into hepatocytes at the level of the sinusoids. The development of microvesicular liver steatosis and glycogen depletion due to involvement of liver cell organelles occur rapidly as in RS.

Two-step activation of valproic acid (VPA) by hepatic cytochrome P-450

There is now considerable evidence that the unsaturated metabolite of VPA, delta 4-VPA, may be responsible for VPA-induced hepatic injury. It is proposed that VPA is activated to a hepatotoxic species by a two-step cytochrome P-450-mediated mechanism consisting of 1) desaturation and 2) unknown activation, possibly by epoxide formation, involving one or more P-450 isozymes with subsequent covalent binding to cellular macromolecules. In vivo and in vitro experimental protocols are indicated for testing the proposed hypothesis.

Valproic acid-induced increase in carnitine acetyltransferase in rat hepatocytes is not due to an induction of peroxisomes

Valproic acid induced a dose-dependent increase in carnitine acetyltransferase (CAT) activity in rat hepatic mitochondrial fractions isolated by differential centrifugation. An increase in CAT and carnitine palmitoyltransferase (CPT) also occurred in cultured rat hepatocytes in a concentration-and time-dependent fashion. A maximal increase of 8-fold in the activity of CAT and 2-fold in the activity of CPT was induced by 3 mM valproic acid in 72 h. Valproic acid had no effect on cytochrome P-450 levels in cultured rat hepatocytes. Electron-microscopic examination of rat hepatocytes showed that there was no increase in the number of peroxisomes but there was a marked proliferation of mitochondria in parallel with an increase in glutathione level and succinic dehydrogenase in the liver cells after incubation with valproic acid in vitro.

Cytochrome P-450--catalyzed formation of delta 4-VPA, a toxic metabolite of valproic acid

Liver damage induced by the antiepileptic drug valproic acid (VPA) is believed to be mediated by an unsaturated metabolite of the drug, delta 4-VPA. In studies of the biological origin of this hepatotoxic compound, it was found that liver microsomes from phenobarbital-treated rats catalyzed the desaturation of VPA to delta 4-VPA. Indirect evidence suggested that cytochrome P-450 was the responsible enzyme, a conclusion that was verified by studies with a purified and reconstituted form of the hemoprotein, which catalyzed the oxidation of VPA to 4- and 5-hydroxyvalproic acid and to delta 4-VPA. Desaturation of a nonactivated alkyl substituent represents a novel metabolic function of cytochrome P-450 and probably proceeds via the conversion of substrate to a transient free radical intermediate, which partitions between recombination (alcohol formation) and elimination (olefin production) pathways. These findings have broad implications with respect to the metabolic generation of olefins and may explain the increased hepatotoxic potential of VPA when it is administered in combination with potent enzyme-inducing anticonvulsants such as phenobarbital.

Lack of relationship between sodium valproate-induced adverse effects and the plasma concentration of its metabolite 2-propylpenten-4-oic acid

The concentrations of valproic acid (VPA) and of its metabolites 3-oxo-VPA and 4-en-VPA were measured in the plasma of 12 selected epileptic patients 1, 2, 3, and 4 h after administration of a loading dose of VPA. Four of the patients, all on polytherapy, had had short-term adverse effects during chronic VPA treatment, and in them there has been abnormal NH3-values after a test doese of VPA. Eight patients (4 on monotherapy and 4 on polytherapy) had been free from adverse effects. No significant difference in the VPA, 3-oxo-VPA and 4-en-VPA concentrations was found between the three groups of patients. Accumulation of 4-en-VPA is not involved in the short-term adverse effects and hyperammonaemia induced by VPA.

Accumulation and washout kinetics of valproic acid and its active metabolites

There is growing evidence that the metabolites of valproic acid (VPA) may be pharmacologically active and could contribute to both the therapeutic and toxic effects of the drug. The accumulation and washout kinetics of VPA and its oxidative metabolites were, therefore, examined in five healthy volunteers. Valproic acid (250-mg capsules) was administered bid for 15 days. Blood samples were obtained periodically during the 15 days of drug administration and for seven days following termination of treatment. Urine was also collected over the final dosing interval. Steady-state serum concentrations of VPA were achieved within three to four days of treatment. The accumulation of all metabolites in serum lagged behind that of the parent compound, with the mono-desaturated metabolites accumulating more slowly than the hydroxylated species. Furthermore, the apparent washout half-life of each metabolite was longer than the elimination half-life of VPA. In general, the unsaturated metabolites were eliminated more slowly than the hydroxylated metabolites. The serum and urinary metabolite profiles of VPA observed in the healthy volunteers were comparable with those reported for epileptic patients. The differences in the disposition kinetics of VPA and of its potentially active metabolites may explain the previously observed dissociation between the pharmacokinetics and pharmacodynamics of the drug in epileptic patients.