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

Do structural properties explain the anticonvulsant activity of valproate metabolites? A QSAR analysis

PURPOSE

Differences in potency among valproate (VPA) metabolites could be explained by structural properties. Therefore, a quantitative structure-activity relation (QSAR) analysis was performed to study the relation between structural parameters and the effect of the following VPA metabolites: 4-en-VPA, 2-en-VPA, 3-en-VPA, 2,4'-dien-VPA, 4,4'-dien-VPA, 4-hydroxy-VPA, 3-ceto-VPA, 3-hydroxy-VPA, 5-hydroxy-VPA, and propylglutaric acid.

METHODS

By using the CAChe program package for biomolecules (Oxford Molecular, Ltd), we performed molecular modeling. The anticonvulsant activity determined on the threshold for maximal electroconvulsions in mice was obtained from a study of Löscher and Nau. Structural parameters were compared between metabolites with a double bond and metabolites with oxygen at either side chain (unpaired Student's t test). A single linear regression analysis between each structural parameter and the relative anticonvulsant potency was also performed.

RESULTS

Similar parameters were found between the cis and trans and R and S isomers. Biologic activity and most of the structural parameters were significantly different between metabolites with a double bond and metabolites with oxygen at either side chain. Activity was directly related to log P(oct) (r(2) = 0.77) and to reactivity parameters and was inversely related to stability parameters and to molecular weight and surface. The most potent metabolites had a log P(oct) value of higher than 2 units.

CONCLUSIONS

Similar data were identified between cis and trans, and R and S isomers of VPA metabolites. Anticonvulsant activity was mainly related to log P(oct), probably reflecting the ability of VPA metabolites to cross the blood-brain barrier.

Adolescent issues in epilepsy

Approximately one third of new cases of epilepsy have their onset before age 20 years. Many children will enter adolescence with epilepsy or have an onset of seizures during adolescence. Adolescence is a time of dramatic change in growth, hormonal, psychologic, and social situations. Seizure frequency, teenage pregnancy, driving, and alcohol and drug use often become major issues during the adolescent years. Furthermore, adolescents often have difficulty accepting the chronicity of epilepsy and complying with medications, which can result in physical injury and perceived or real obstacles to employment, thereby contributing to low morale. Both pediatricians and neurologists should be aware of adolescent issues in epilepsy.

Effects of valproate and E-2-en-valproate on functional and morphological parameters of rat liver. III. Influence of fasting

Valproate (VPA) therapy has been associated with a rare but fatal hepatotoxicity. Several possible biochemical mechanisms responsible for the hepatotoxicity have been proposed, but the matter has not been decided. There is some evidence that VPA-associated hepatotoxicity may represent the consequences of a VPA overload on a limited mitochondrial beta-oxidation capacity, causing abnormalities in metabolic pathways. If this assumption is true, fasting-induced increase of endogenous fatty acids, which compete with VPA for beta-oxidation, should enhance the hepatotoxic potential of VPA. Indeed, involuntary fasting because of anorexia, e.g., in children with febrile infections, has been discussed as one clinical risk pattern preceding VPA-associated hepatic fatalities. In the present experiments, the effects of fasting on functional and morphological parameters of the liver were studied in young male rats chronically treated with VPA. E-2-en-VPA (trans-2-en-VPA), a major active metabolite of the beta-oxidation pathway of VPA, was used for comparison. Both drugs were administered at doses of 250 mg/kg i.p. 3 times daily for 1 week. In control rats, a 40-h fasting period resulted in marked mobilization of liver lipid and glycogen stores, alterations in liver enzyme activities, and hyperammonemia. In rats treated with VPA, fasting reduced beta-oxidation of the drug, but seemed not to increase its hepatotoxic potential. Compared to experiments without fasting, alterations in liver enzymes and ammonia levels induced by VPA were less marked or absent in fasted rats, and histopathological examination of liver sections did not indicate degenerative liver lesions in response to drug treatment. Thus, compared to previous rat studies on VPA without fasting, fasting appeared to attenuate VPA's hepatotoxic potency, possibly as a result of fasting-induced increases in carnitine levels. In rats treated with E-2-en-VPA, no indication of hepatotoxicity was evident, and alterations in functional hepatic parameters were less pronounced than with VPA. The data do not indicate that fasting or poor nutrition are risk factors for VPA-associated hepatic injury.

Effects of valproate and E-2-en-valproate on functional and morphological parameters of rat liver. II. Influence of phenobarbital comedication

The effect of phenobarbital on the potential hepatotoxicity of E-2-en-valproate (E-2-en-VPA; trans-2-en-VPA) and VPA was studied in young male Sprague-Dawley rats. E-2-en-VPA and VPA were administered daily at 750 mg/kg i.p. (divided into three doses a day) for 7 consecutive days. Phenobarbital was coadministered i.p. once daily at 100 mg/kg for 2 days, followed by daily injections of 50 mg/kg for the subsequent days of the treatment period. Additional groups of rats were treated with phenobarbital alone or received once daily administration of 4-en-VPA (100 mg/kg), a potentially hepatotoxic metabolite of VPA. Clinical chemistry data were studied before and after the period of treatment. Furthermore, drug and metabolite levels were analyzed by gas chromatography-mass spectrometry. Treatment with VPA and phenobarbital resulted in deaths and histopathological liver alterations, such as marked microvesicular steatosis and degenerative lesions, whereas no death and hepatotoxicity occurred in rats treated with E-2-en-VPA and phenobarbital. Furthermore, hyperammonemia was recorded in VPA- but not E-2-en-VPA-treated rats. In comparison to treatment with VPA or E-2-en-VPA alone, combined treatment with phenobarbital markedly reduced plasma levels of the parent drugs and metabolites originating from beta-oxidation, but, in case of VPA, increased metabolites originating from omega-oxidation. Plasma levels of 4-en-VPA were increased by phenobarbital in VPA-treated rats, but 4-en-VPA was not detectable in rats treated with E-2-en-VPA. The most severe alterations in functional and morphological liver parameters were found in rats treated with 4-en-VPA. In these animals, the extent of steatosis was significantly correlated with plasma levels of 4-en-VPA, but not its major metabolite 2,4-dien-VPA. Plasma levels of 4-en-VPA or its major metabolite 2,4-dien-VPA in rats without steatosis were markedly higher than levels of these compounds in VPA-treated rats with steatosis, suggesting that 4-en-VPA and 2,4-dien-VPA are not critically involved in the hepatotoxic effects of VPA. The data substantiate that E-2-en-VPA is less hepatotoxic than VPA and may thus offer advantages for antiepileptic therapy.

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.

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.