The effects of analogues of valproic acid on seizures induced by pentylenetetrazol and GABA content in brain of mice

Anticonvulsant Profile of Selected Medium-Chain Fatty Acids (MCFAs) Co-Administered with Metformin in Mice in Acute and Chronic Treatment

In contrast to the other components of the medium-chain triglycerides ketogenic diet (MCT KD), i.e., caprylic acid (CA8), a comprehensive evaluation of caproic (CA6) and lauric acids' (CA12) properties in standard chemical and electrical seizure tests in mice has not yet been performed. We investigated their effects in maximal electroshock seizure threshold (MEST), 6 Hz seizure threshold and intravenous (i.v.) pentylenetetrazole (PTZ) seizure tests. Since ketone body production can be regulated by the activation of 5'AMP-activated protein kinase (AMPK), we hypothesized that metformin (an AMPK activator) enhance ketogenesis and would act synergistically with the fatty acids to inhibit convulsions. We assessed the effects of acute and chronic co-treatment with metformin and CA6/CA8 on seizures. CA6 and CA12 (p.o.) increased seizure threshold in the 6 Hz seizure test. CA6 at the highest tested dose (30 mmol/kg) developed toxicity in several mice, impaired motor performance and induced ketoacidosis. Acute and chronic co-treatment with metformin and CA6/CA8 did not affect seizure thresholds. Moreover, we observed the pro-convulsive effect of the acute co-administration of CA8 (5 mmol/kg) and metformin (100 mg/kg). Since this co-treatment was pro-convulsive, the safety profile and risk/benefit ratio of MCT KD and metformin concomitant therapy in epileptic patients should be further evaluated.

Effect of valproate on sleep patterns disturbed by epilepsy

Nocturnal epilepsy is a neurological disease that has a significant effect on sleep. Various treatments have been implemented to help mitigate these effects and improve patients' quality of life. The use of experimental animal models for epilepsy has facilitated efficacy assessment and the development of different medications to treat the symptoms of this disease. The objective of this study was to evaluate the effect of valproate on sleep patterns altered by epilepsy. Chronically implanted Wistar rats were used to study sleep patterns over three consecutive days under different experimental conditions. The animals were separated into two groups. The first day was considered the control recording; on the second day, one group received pentylenetetrazol (PTZ) alone, and the other group received valproate prior to induction of convulsive seizures with PTZ administration. The results show that in addition to its antiepileptic effect, valproate has hypnotic properties. It is considered to facilitate the action of GABAergic mechanisms to mitigate the effect of convulsive seizures and increase the occurrence of sleep.

Insights into Structural Modifications of Valproic Acid and Their Pharmacological Profile

Valproic acid (VPA) is a well-established anticonvulsant drug discovered serendipitously and marketed for the treatment of epilepsy, migraine, bipolar disorder and neuropathic pain. Apart from this, VPA has potential therapeutic applications in other central nervous system (CNS) disorders and in various cancer types. Since the discovery of its anticonvulsant activity, substantial efforts have been made to develop structural analogues and derivatives in an attempt to increase potency and decrease adverse side effects, the most significant being teratogenicity and hepatotoxicity. Most of these compounds have shown reduced toxicity with improved potency. The simple structure of VPA offers a great advantage to its modification. This review briefly discusses the pharmacology and molecular targets of VPA. The article then elaborates on the structural modifications in VPA including amide-derivatives, acid and cyclic analogues, urea derivatives and pro-drugs, and compares their pharmacological profile with that of the parent molecule. The current challenges for the clinical use of these derivatives are also discussed. The review is expected to provide necessary knowledgebase for the further development of VPA-derived compounds.

Bioassay-guided isolation of anti-seizure principles from Semen Pharbitidis using a zebrafish pentylenetetrazol seizure model

ETHNOPHARMACOLOGICAL RELEVANCE

Semen Pharbitidis, the seeds of Pharbitis nil (Linn.) Choisy (Convolvulaceae) is a well-known traditional Chinese medicinal plant used for treating helminthiasis and epilepsy in China.

AIM OF THE STUDY

This study aims to identify the anti-seizure components from Semen Pharbitidis.

METHODS

A bioassay-guided isolation of anti-seizure compounds from Semen Pharbitidis was performed using a zebrafish pentylenetetrazol seizure model. The structures of active compounds were elucidated by high resolution mass spectrometry. The fragments of active compounds were tested for anti-seizure activity as well.

RESULTS

The bioassay-guided isolation of ethanol extract of Semen Pharbitidis led to a group of resin glucosides, namely pharbitin. One of the fragments of pharbitin, 2-methylbutyric acid, also showed anti-seizure activity.

CONCLUSIONS

We provided further experimental scientific evidence to support the traditional use of Semen Pharbitidis for the treatment of epilepsy. Pharbitin was identified to be the main anti-seizure component in Semen Pharbitidis.

Design and Comparative Evaluation of the Anticonvulsant Profile, Carbonic-Anhydrate Inhibition and Teratogenicity of Novel Carbamate Derivatives of Branched Aliphatic Carboxylic Acids with 4-Aminobenzensulfonamide

Epilepsy is one of the most common neurological diseases, with between 34 and 76 per 100,000 people developing epilepsy annually. Epilepsy therapy for the past 100⁺ years is based on the use of antiepileptic drugs (AEDs). Despite the availability of more than twenty old and new AEDs, approximately 30% of patients with epilepsy are not seizure-free with the existing medications. In addition, the clinical use of the existing AEDs is restricted by their side-effects, including the teratogenicity associated with valproic acid that restricts its use in women of child-bearing age. Thus, there is an unmet clinical need to develop new, effective AEDs. In the present study, a novel class of carbamates incorporating phenethyl or branched aliphatic chains with 6-9 carbons in their side-chain, and 4-benzenesulfonamide-carbamate moieties were synthesized and evaluated for their anticonvulsant activity, teratogenicity and carbonic anhydrase (CA) inhibition. Three of the ten newly synthesized carbamates showed anticonvulsant activity in the maximal-electroshock (MES) and 6 Hz tests in rodents. In mice, 3-methyl-2-propylpentyl(4-sulfamoylphenyl)carbamate(1), 3-methyl-pentan-2-yl-(4-sulfamoylphenyl)carbamate (9) and 3-methylpentyl, (4-sulfamoylphenyl)carbamate (10) had ED₅₀ values of 136, 31 and 14 mg/kg (MES) and 74, 53, and 80 mg/kg (6 Hz), respectively. Compound (10) had rat-MES-ED₅₀ = 13 mg/kg and ED₅₀ of 59 mg/kg at the mouse-corneal-kindling test. These potent carbamates (1,9,10) induced neural tube defects only at doses markedly exceeding their anticonvuslnat-ED₅₀ values. None of these compounds were potent inhibitors of CA IV, but inhibited CA isoforms I, II and VII. The anticonvulsant properties of these compounds and particularly compound 10 make them potential candidates for further evaluation and development as new AEDs.

Seizure control by ketogenic diet-associated medium chain fatty acids

The medium chain triglyceride (MCT) ketogenic diet is used extensively for treating refractory childhood epilepsy. This diet increases the plasma levels of medium straight chain fatty acids. A role for these and related fatty acids in seizure control has not been established. We compared the potency of an established epilepsy treatment, Valproate (VPA), with a range of MCT diet-associated fatty acids (and related branched compounds), using in vitro seizure and in vivo epilepsy models, and assessed side effect potential in vitro for one aspect of teratogenicity, for liver toxicology and in vivo for sedation, and for a neuroprotective effect. We identify specific medium chain fatty acids (both prescribed in the MCT diet, and related compounds branched on the fourth carbon) that provide significantly enhanced in vitro seizure control compared to VPA. The activity of these compounds on seizure control is independent of histone deacetylase inhibitory activity (associated with the teratogenicity of VPA), and does not correlate with liver cell toxicity. In vivo, these compounds were more potent in epilepsy control (perforant pathway stimulation induced status epilepticus), showed less sedation and enhanced neuroprotection compared to VPA. Our data therefore implicates medium chain fatty acids in the mechanism of the MCT ketogenic diet, and highlights a related new family of compounds that are more potent than VPA in seizure control with a reduced potential for side effects.

This article is part of the Special Issue entitled ‘New Targets and Approaches to the Treatment of Epilepsy’.

Chemical properties of antiepileptic drugs (AEDs)

Between 1990 and 2011 the following fifteen new antiepileptic drugs (AEDs) were approved: eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, retigabine, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin, and zonisamide. These AEDs (except felbamate) offer appreciable advantages in terms of their favorable pharmacokinetics, improved tolerability and lower potential for drug interactions. All AEDs introduced after 1990 that are not second generation drugs (with the exception of vigabatrin and tiagabine) were developed empirically (sometimes serendipitously) utilizing mechanism-unbiased anticonvulsant animal models. The empirical nature of the discovery of new AEDs in the last three decades coupled with their multiple mechanisms of action explains their diverse chemical structures. The availability of old and new AEDs with various activity spectra and different tolerability profiles enables clinicians to better tailor drug choice to the characteristics of individual patients. With fifteen new AEDs having entered the market in the past 20years the antiepileptic market is crowded. Consequently, epilepsy alone is not attractive in 2011 to the pharmaceutical industry even though the clinical need of refractory epilepsy remains unmet. Due to this situation, future design of new AEDs must also have a potential in non-epileptic CNS disorders such as neuropathic pain, migraine prophylaxis and bipolar disorder or fibromyalgia as demonstrated by the sales revenues of pregabalin, topiramate and valproic acid. This review analyzes the effect that the emerging knowledge on the chemical properties of the old AEDs starting from phenobarbital (1912) has had on the design of subsequent AEDs and new therapeutics as well as the current approach to AED discovery.

The antiepileptic drug valproic acid and other medium-chain fatty acids acutely reduce phosphoinositide levels independently of inositol in Dictyostelium

Valproic acid (VPA) is the most widely prescribed epilepsy treatment worldwide, but its mechanism of action remains unclear. Our previous work identified a previously unknown effect of VPA in reducing phosphoinositide production in the simple model Dictyostelium followed by the transfer of data to a mammalian synaptic release model. In our current study, we show that the reduction in phosphoinositide [PtdInsP (also known as PIP) and PtdInsP(2) (also known as PIP(2))] production caused by VPA is acute and dose dependent, and that this effect occurs independently of phosphatidylinositol 3-kinase (PI3K) activity, inositol recycling and inositol synthesis. In characterising the structural requirements for this effect, we also identify a family of medium-chain fatty acids that show increased efficacy compared with VPA. Within the group of active compounds is a little-studied group previously associated with seizure control, and analysis of two of these compounds (nonanoic acid and 4-methyloctanoic acid) shows around a threefold enhanced potency compared with VPA for protection in an in vitro acute rat seizure model. Together, our data show that VPA and a newly identified group of medium-chain fatty acids reduce phosphoinositide levels independently of inositol regulation, and suggest the reinvestigation of these compounds as treatments for epilepsy.

Anticonvulsant profile of caprylic acid, a main constituent of the medium-chain triglyceride (MCT) ketogenic diet, in mice

The purpose of the present study was to evaluate the acute anticonvulsant effects of caprylic acid (CA), the main constituent of the medium-chain triglyceride ketogenic diet (MCT KD), in seizure tests typically used in screening for potential antiepileptic drugs in mice. Pharmacodynamic and pharmacokinetic interactions between CA and valproate (VPA) were also investigated. CA (p.o.) and VPA (i.p.) were administered 30 min before testing. Acute effects on motor coordination were assessed in the chimney test. Total plasma and brain concentrations of CA and VPA, when administered alone or in combination, were determined by high performance liquid chromatography. CA (10-30 mmol/kg) increased the threshold for i.v. pentylenetetrazole-induced myoclonic and clonic convulsions, but not tonic convulsions. CA (5-30 mmol/kg) increased the threshold for 6-Hz psychomotor seizures but was ineffective in the maximal electroshock seizure threshold test. CA (10-60 mmol/kg p.o.) impaired motor performance in the chimney test (TD(50) value, 58.4 mmol/kg). Increasing doses of CA (5-30 mmol/kg) produced proportional increases in plasma and brain exposure with constant brain/plasma partitioning. CA increased anticonvulsant potency of VPA in the maximal electroshock seizure and 6-Hz seizure tests. Co-administration of CA and VPA had no effect on brain and plasma concentrations of either compound. In summary, CA exerts acute anticonvulsant effects and potentiates the anticonvulsant effect of VPA at doses that result in plasma exposures comparable to those reported in epileptic patients on the MCT KD. Thus, this acute anticonvulsant property of CA may benefit and add to the overall clinical efficacy of the MCT KD.

Inhibition of long-term potentiation by valproic acid through modulation of cyclic AMP

PURPOSE

Valproic acid (VPA) is widely used clinically in epilepsy, bipolar disorder, and migraine. In experimental models, it has also been shown to have neuroprotective and antiepileptogenic effects. Its mechanisms of action in these diverse conditions are, however, unclear, but there is some evidence indicating an effect of VPA upon protein kinase A (PKA) activity. We, therefore, asked whether VPA modulates cyclic adenosine monophosphate (cAMP)/PKA-dependent synaptic plasticity and whether this mode of action could explain its anticonvulsant effect.

METHODS

We first tested the effects of VPA on PKA-dependent synaptic plasticity at mossy fiber to CA3 synapses in rat hippocampus slices following very high-frequency stimulation or application of the adenylyl cyclase activator forskolin. Using biochemical assays, we then tested whether VPA had a direct effect on PKA activity or an indirect effect through modulating cAMP production. Lastly, VPA and inhibitors of adenylyl cyclase (SQ22536) and PKA (H89) were tested in in vitro models of epileptiform activity induced in hippocampal-entorhinal cortex slices using either pentylenetetrazol (2 mM) or low magnesium.

RESULTS

VPA (1 mm) inhibited PKA-dependent long-term potentiation of mossy fiber to CA3 pyramidal cell transmission. However, VPA did not directly modulate PKA activity but rather inhibited the accumulation of cAMP. In acute in vitro seizure models, the anticonvulsant activity of VPA is not mediated through modulation of adenylyl cyclase or PKA.

CONCLUSIONS

  These results indicate that VPA through an action on cAMP accumulation can inhibit synaptic plasticity, but this cannot fully explain its anticonvulsant effect.

Structure-function studies for the panacea, valproic acid

The anticonvulsant properties of VPA (valproic acid), a branched short-chain fatty acid, were serendipitously discovered in 1963. Since then, therapeutic roles of VPA have increased to include bipolar disorder and migraine prophylaxis, and have more recently been proposed in cancer, Alzheimer's disease and HIV treatment. These numerous therapeutic roles elevate VPA to near 'panacea' level. Surprisingly, the mechanisms of action of VPA in the treatment of many of these disorders remain unclear, although it has been shown to alter a wide variety of signalling pathways and a small number of direct targets. To analyse the mechanism of action of VPA, a number of studies have defined the structural characteristics of VPA-related compounds giving rise to distinct therapeutic and cellular effects, including adverse effects such as teratogenicity and hepatotoxicity. These studies raise the possibility of identifying target-specific novel compounds, providing better therapeutic action or reduced side effects. This short review will describe potential therapeutic pathways targeted by VPA, and highlight studies showing structural constraints necessary for these effects.

Anticonvulsant efficacy of valproate-like carboxylic acids: a potential target for anti-bipolar therapy

BACKGROUND

Bipolar disorder (BPD) is a severe and chronic illness, with a lifetime prevalence of approximately 1.5%. Despite the availability of some mood stabilizing drugs including lithium, valproate (valproic acid), lamotrigine and carbamazepine, BPD is characterized by high rates of recurrence, as treatment with these and other drugs is ineffective for and not well-tolerated by a significant percentage of patients. Most drugs currently used for the maintenance treatment of BPD are anticonvulsants (e.g., valproate, carbamazepine and lamotrigine).

OBJECTIVES

The aim of this paper is to review the studies characterizing the anticonvulsant efficacy of valproate-like carboxylic acids and related compounds, some of which may have potential for the treatment of manic-depressive illness.

RESULTS

The data reviewed herein demonstrate clearly that some dietary fatty acids and other valproate-like carboxylic acids exhibit potent anticonvulsant activity, and may thus be candidates for mood stabilizing treatment options for BPD.

Examining the correlations between GSK-3 inhibitory properties and anti-convulsant efficacy of valproate and valproate-related compounds

A family of compounds based upon the chemical structure of valproate were synthesized and assayed for their ability to inhibit glycogen synthase kinase (GSK)-3 alpha and beta activity in vitro. This data is correlated to the known anti-convulsant properties of these compounds in order to determine the potential role of GSK-3 inhibition in the therapeutic efficacy of these drugs.

Characterization of the anticonvulsant profile of valpromide derivatives

The antiepileptic activity of nine derivatives of valpromide is discussed. They comply with a pharmacophore model that establishes the essential structural and electronic features responsible for the protection against the MES test. The model results from the comparison of 17 structures, using density functional methodologies combined with an active analog approach. The derivatives of valpromide have been tested for anticonvulsant activity in mice. These compounds displayed a phenytoin-like profile, being active in the MES test and inactive in the PTZ test. 4-(Valproylamido)benzenesulfonamide is the most active compound, with an ED(50) of 53 micromol/kg and no neurotoxicity at doses up to 1000 micromol/kg. The pharmacological behavior of the drugs points to a sodium channel blocking effect as one of the associated mechanisms. This mechanism was tested positive for N-ethylvalpromide through its competition with the binding of [(3)H]batrachotoxin-A-20 alpha-benzoate to the voltage-dependent sodium channels from rat brain synaptosomes.

Continuum solvent model studies of the interactions of an anticonvulsant drug with a lipid bilayer

Valproic acid (VPA) is a short, branched fatty acid with broad-spectrum anticonvulsant activity. It has been suggested that VPA acts directly on the plasma membrane. We calculated the free energy of interaction of VPA with a model lipid bilayer using simulated annealing and the continuum solvent model. Our calculations indicate that VPA is likely to partition into the bilayer both in its neutral and charged forms, as expected from such an amphipathic molecule. The calculations also show that VPA may migrate (flip-flop) across the membrane; according to our (theoretical) study, the most likely flip-flop path at neutral pH involves protonation of VPA pending its insertion into the lipid bilayer and deprotonation upon departure from the other side of the bilayer. Recently, the flip-flop of long fatty acids across lipid bilayers was studied using fluorescence and NMR spectroscopies. However, the measured value of the flip-flop rate appears to depend on the method used in these studies. Our calculated value of the flip-flop rate constant, 20/s, agrees with some of these studies. The limitations of the model and the implications of the study for VPA and other fatty acids are discussed.

Biophysical model for organic acid excretion in Ascaris suum

To develop a model for the mechanisms of organic acid excretion in nematodes, we measured the concentrations of volatile fatty acids (VFAs), pH and electrical potentials across hypodermal and muscle membranes and across the composite body wall (consisting of hypodermis, muscle and cuticle) of Ascaris suum using standard chromatographic and microelectrode recording techniques. In incubates containing one parasite in 20 ml modified Ascaris Ringer's solution, the level of combined VFAs excreted into the medium increased linearly for about 18 h, then plateaued at a concentration of 4.2 mM; the medium acidified rapidly to a plateau at about pH 5.0 within 4-6 h. Following 24 h incubations, the concentrations of VFAs in the hypodermis, muscle, and pseudocoelomic compartments were 62.4 +/- 8.1, 62.3 +/- 7.8 and 74.4 +/- 3.2 mM, respectively. The pseudocoelomic fluid was more acidic (pH 6.52 +/- 0.06) than the hypodermis (pH 6.78 +/- 0.03) or muscle (pH 6.77 +/- 0.03). These data and the electrical potentials across hypodermal (-57.9 +/- 6.3 mV) and muscle (-30.3 +/- 0.8 mV) membranes were used to determine the equilibrium concentrations for protonated (HVFA) and anionic (VFA-) forms of the acids across these membranes and across the cuticle. Under these conditions, little transmembrane or transmural excretion of HVFAs is expected to occur in A. suum. However, a 16-27 mV driving force for VFA- excretion exists across hypodermal and muscle membranes, and a larger driving force is predicted to exist for these anions across the cuticle. This driving force could provide potential energy for VFA- excretion through anion channels which exist in muscle and hypodermal membranes of this parasite, or for facilitated transport systems.

Valnoctamide, valpromide and valnoctic acid are much less teratogenic in mice than valproic acid

The teratogenic properties of valproic acid (VPA) and its analogues depend to a great extent on their chemical structure. We investigated the structure-teratogenicity relationships of VPA, its structural isomer, valnoctic acid (VCA), and their two amide analogues, valpromide (VPD) and valnoctamide (VCD), respectively. Each substance was injected (3 mmol/kg) in NMRI-mice on the morning of day 8 of gestation. Embryolethality, fetal weight and exencephaly rates were recorded on day 18 of gestation. VPA caused 53% exencephaly, VPD induced 6%, VCA and VCD produced only 1% exencephaly (control values between 0 and 1%). VPA-treated mice also had increased embryolethality rates (52%). There was no significant change of embryolethality in the other treatment groups. Pharmacokinetic studies showed that VCD was eliminated from plasma at a slower rate than VPA. Also, the residual teratogenic activity of VPD was not accounted for by the relatively small amounts of its hydrolysis product VPA. This study indicates that VPD, VCA and VCD were distinctly less teratogenic than VPA. Apparently the amidation of the free carboxylic group and/or methyl-substitution at the beta-position of the carbon chain greatly decreased the teratogenic activity of VPA.

Pharmacokinetics and pharmacodynamics of valproate analogues in rats. IV. Anticonvulsant action and neurotoxicity of octanoic acid, cyclohexanecarboxylic acid, and 1-methyl-1-cyclohexanecarboxylic acid

We examined the pharmacodynamics of valproate (VPA) and three structural analogues, octanoic acid (OA), cyclohexanecarboxylic acid (CCA), and 1-methyl-1-cyclohexanecarboxylic acid (MCCA) in rats. A pentylenetetrazol (PTZ) infusion seizure model was used to determine threshold convulsive doses of PTZ; the increase in PTZ threshold dose after administration of test compound was taken as an index of anticonvulsant activity. Each of the compounds investigated antagonized PTZ-induced seizures, with MCCA evidencing the highest potency. Both CCA and MCCA appeared to have an approximate twofold advantage relative to VPA in protective index (i.e., the ratio of concentrations that produce toxicity to concentrations that produce anticonvulsant effect), based on a rotorod assay of neurotoxicity. Examination of the time course of PTZ antagonism indicated that there was significant dissociation between pharmacokinetics and pharmacodynamics of VPA, with a marked delay in production of maximal anticonvulsant activity. In contrast, only a slight delay in production of maximal protection against PTZ-induced seizures was observed for MCCA, and no delay was evident for CCA. The data indicate that the dynamics of anticonvulsant action differ between these low-molecular-weight carboxylic acids despite their similar chemical structures.

Anticonvulsant and neurotoxic activities of twelve analogues of valproic acid

Twelve racemic analogues of the antiepileptic drug valproic acid (VPA) were tested and compared with VPA for anticonvulsant activity by the subcutaneous pentylenetetrazol (PTZ) seizure threshold test and for neurotoxicity by the rotorod test. Four compounds produced maximal anticonvulsant activity (100% protection) in equimolar doses (1.5 mmol/kg) to VPA and two compounds showed a similar effect with lower doses (1.0 mmol/kg). Four compounds produced lower activity (38-80% protection), and two compounds showed no anticonvulsant activity at the dose used (1.5 mmol/kg). Two of the 12 compounds, (+/-)-2-n-propyl-4-hexynoic acid (11) and (+/-)-4-methyl-2-n-propyl-4-pentenoic acid (12), showed no sedation at doses that produced the maximum anticonvulsant effect. For the first time we succeeded to develop two compounds with higher protective index and safety ratios than VPA. Compound 11 had a longer duration of action and higher protective index but a lower safety ratio than 12. Comparisons of the anticonvulsant and minimal neurotoxic effects of these compounds with their calculated lipophilicity (C log P) revealed that compounds with the desired high anticonvulsant activity and minimal neurotoxicity showed C log P values between 1.84 and 2.64 and had nine carbon atoms (in contrast to eight carbon atoms for VPA).

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.

Development of aqueous parenteral formulations for carbamazepine through the use of modified cyclodextrins

The poor aqueous solubility of carbamazepine was dramatically increased via complexation with various chemically modified beta-cyclodextrins and gamma-cyclodextrins. A preparation of carbamazepine and 2-hydroxypropyl-beta-cyclodextrin was found to be stable to steam sterilization and to storage under a variety of conditions. Carbamazepine, when solubilized in this manner, was found to exert potent anticonvulsant effects in various seizure models and the formulation was tolerated in animals at high doses (100 mg/kg carbamazepine and 1200 mg/kg of the cyclodextrin excipient). The onset of anticonvulsant action was rapid and consistent with almost instantaneous in vivo complex dissociation. The low toxicity of 2-hydroxypropyl-beta-cyclodextrin, when administered via the parenteral route, and its ability to enhance the aqueous solubility of carbamazipine highly favor the use of this excipient.

Structure-pharmacokinetic relationships in a series of short fatty acid amides that possess anticonvulsant activity

Valpromide (VPD) and valnoctamide (VCD) are two isomers which are aliphatic amides derived from short fatty acids that possess anticonvulsant activity. Our previous studies with VPD, VCD, and other related compounds showed that the biotransformation of these amides to their respective homologous acids is the key issue in their possessing pharmacological activity. In this study, we explored the structure--pharmacokinetic relationships of the following five isomers or analogues of VPD: diisproprylacetamide (DID), diallylacetamide (DAD), octanamide (OAD), ethylisobutylacetamide (EID), and dimethylbutylacetamide (DBD). In addition, the anticonvulsant activity of these compounds was evaluated and compared with that of VPD and VCD. No plasma levels of OAD could be detected after its iv administration. Octanamide (OAD) was very rapidly metabolized to its homologous acid, octanoic acid (OAA). Octanamide (OAD) was different from the other four amides investigated, having a high clearance (due to metabolic processes in the blood) and possessing the least anticonvulsant activity. All of the other amides were stable in blood and showed similar pharmacokinetic parameters. Unlike the other amides, DID and VCD did not metabolize to their respective homologous acids due to the fact that they had a substituted beta position in their aliphatic side chain. Our study showed that, despite similarities in the chemical structures of the amides investigated, significant differences were observed in their pharmacokinetics and in the fraction of the amide (fm) biotransformed to its homologous acid. These differences in fm values may, therefore, account for the observed differences in the respective pharmacological activities, in general, and in the extent of the anticonvulsant activity, in particular, of the amides.(ABSTRACT TRUNCATED AT 250 WORDS)

Paradoxical role of GABA in a chronic model of petit mal (absence)-like epilepsy in the rat

Neonatal Long-Evans hooded rats were treated with AY-9944, a cholesterol biosynthesis inhibitor, every 6 days for 7 weeks to induce a permanent absence-like epileptic condition. AY-9944-treated rats averaged 50 +/- 15 generalized non-motor seizures per hour of 2-15 s duration as monitored by electrocorticography. Clinically effective anti-absence drugs were observed to reduce seizure occurrence in a dose-dependent manner. Paradoxically, GABA agonists increased seizure occurrence while GABA antagonists decreased seizure occurrence. Evaluation of the benzodiazepines, diazepam and clonazepam, in this model revealed inhibition of seizure activity by GABA-independent mechanisms. Valproic acid produced a biphasic effect suggesting a GABA-independent, antiabsence action at low doses and GABAergic augmentation of seizure occurrence at higher doses. The results of this study support the hypothesis that increased GABAergic stimulation may induce inhibitory seizures in absence epilepsy.

Neuronal activity, amino acid concentration and amino acid release in the substantia nigra of the rat after sodium valproate

The effects of sodium valproate on extracellularly recorded spontaneous neuronal activity and striatal-evoked inhibition in the substantia nigra zona reticulata of the rat were compared with its effects on the tissue concentration of endogenous amino acids and their spontaneous release into perfusates of this region obtained with a push-pull cannula. Valproate (200 mg/kg i.p.) produced a rapid and sustained reduction in the firing rate of all reticulata neurones tested and a concomitant increase in the duration of striatal-evoked inhibition. No change in the spontaneous release of any amino acid was observed. A significant elevation of nigral gamma-aminobutyric acid concentration was seen in both anaesthetized and non-anaesthetized animals, but this occurred only after 60 minutes. Valproate produced a rapid decline in nigral aspartate in non-anaesthetized but not in anaesthetized animals. The results of this study suggest that the acute depressant effect of valproate is unrelated to its ability to alter the concentration of GABA or aspartate in brain and is most likely due to a postsynaptic action.

Improved anticonvulsant activity of phenytoin by a redox brain delivery system. III: Brain uptake and pharmacological effects

Phenytoin (DPH) was delivered to the brain by a dihydropyridine in equilibrium pyridinium salt redox system, which was evaluated for anticonvulsant activity. Following iv injection of the lipophilic delivery system of DPH (2) to rats, concentrations of DPH were lower but sustained and, after 30 min, essentially the same as the levels after equimolar administration of DPH. While 2 delivered the same levels of DPH to the brain as DPH did, it was twice as potent as DPH in rats (ED50 was 7.5 mumol/kg for 2 and 14.2 mumol/kg for DPH) and mice (2: 10.5; DPH: 23.9) against maximal electroshock seizures (MES), and seven times more potent in mice (2: 10.0, DPH: 70.6) against maximal pentylenetetrazole seizures (MPS). Moreover, 2 was active against pentylenetetrazole threshold seizures (PTS) in mice and rats (ED50 = 44.1 and 40.5 mumol/kg, respectively), while DPH was ineffective (up to a dose of 79.2 mumol/kg). After evaluation of acute neurological toxicity in rats, 2 was found to possess 1.5 times higher a protective index (for MES) than DPH. It appeared also that while DPH was 2.9 times less sensitive to MPS than to MES, 2 was equally potent to both types of convulsions. Thus, the data indicate that 2 delivered DPH more efficiently to the brain. The better anticonvulsant activity (quantitatively as well as qualitatively) of 2 can be explained on the basis of an improved distribution in the brain due to its higher lipophilicity, and by favorable regional differences in the rates of conversion of 2 to DPH at the convulsing foci.

Structure-pharmacokinetic relationships in a series of valpromide derivatives with antiepileptic activity

The following valpromide (VPD) derivatives were synthesized and their structure-pharmacokinetic relationships explored: ethylbutylacetamide (EBD), methylpentylacetamide (MPD), propylisopropylacetamide (PID), and propylallylacetamide (PAD). In addition, the anticonvulsant activity of these compounds was evaluated and compared to that of VPD, valnoctamide (VCD), and valproic acid (VPA). MPD, the least-branched compound had the largest clearance and shortest half-life of all the amides investigated and was the least active. All other amides had similar pharmacokinetic parameters. Unlike the other amides, PID and VCD did not metabolize to their respective homologous acids and were the most active compounds. Our study showed that these amides need an unsubstituted beta position in their aliphatic side chain in order to biotransform to their homologous acids. An amide which is not metabolized is more potent as an anticonvulsant than its biotransformed isomer. All amides were more active than their respective homologous acids. In this particular series of aliphatic amides, which were derived from short-branched fatty acids, the anticonvulsant activity was affected by the pharmacokinetics in general and by the biotransformation of the amide to its homologous acid in particular. This amide-acid biotransformation appeared to be dependent upon the chemical structure, especially upon the substitution at position beta of the molecule.

Capillary gas chromatographic assay for valproic acid and its 2-desaturated metabolite in brain and plasma

A capillary gas chromatographic method has been developed for quantitating the antiepileptic drug valproic acid and its pharmacologically active metabolite, E-delta 2-valproic acid, in brain and plasma. The method was designed to cover large concentration ranges of both valproic acid and E-delta 2-valproic acid for pharmacokinetic and pharmacodynamic studies in laboratory animals and human subjects. Careful optimization of the extraction and chromatographic procedures was needed to resolve the analytes from a variety of endogenous constituents and other known metabolites of valproic acid. A sensitivity limit of 0.10 micrograms/g for 300 mg of brain tissue or 0.20 micrograms/ml for 150 microliters of plasma was achieved using flame ionization detection. The within-batch coefficients of variation for both analytes were less than 8%. Reproducible calibration data were observed over a period of three to eleven months.

Quantitative structure-anticonvulsant activity relationships of valproic acid, related carboxylic acids and tetrazoles

Valproic acid and several structurally related carboxylic acids and tetrazole analogues have been shown to antagonize seizures induced by pentylenetetrazole in mice. To investigate the influence of the alkyl substituents on the anticonvulsant activity, the octanol-water partition coefficients and relative pKa values were determined. Within the series of active carboxylic acids, there was a good correlation between the anticonvulsant activity and the partition coefficient (r = 0.86). The influence of pKa on the anticonvulsant activity was small but of statistical significance. When the most active compound, 5-heptyltetrazole was added to the carboxylic acid series, a low correlation between the anticonvulsant activity and a linear combination of lipophilicity and pKa resulted. The effect of the polar moieties in alkyl-substituted anticonvulsant compounds was assessed by comparison of the regression equations with either an added pKa or dipole moment term to the term for lipophilicity. It appears that other factors, such as the nature of the alkyl substituent, influence the anticonvulsant activity. The inactivity of the cyclohexylmethyl-substituted compounds, cyclohexylacetic acid and 5-cyclohexylmethyltetrazole may be due to subtle steric effects at a critical step, either involving oxidative metabolism or an interaction at an active site.

Effect of valproate on brain GABA: comparison with various medium chain fatty acids

The antiepileptic valproic acid increases brain GABA levels, and interferes with intermediate metabolism. We have therefore evaluated the effects of a number of other medium chain fatty acids on brain GABA. Propionate, isovalerate, pent-4-enoate and diallylacetate all increased whole-brain GABA levels in mice, but only diallylacetate possessed some anticonvulsant activity. Hyperammonaemia is a common side effect observed with these acids, but artificially induced hyperammonaemia through injections of ammonium chloride did not alter brain GABA. These results suggest that high doses of valproate may increase brain GABA by interfering with intermediate metabolism, possibly at the level of medium chain acyl CoA. However alterations in brain GABA did not correlate with anticonvulsant activity.

Comparative evaluation of anticonvulsant and toxic potencies of valproic acid and 2-en-valproic acid in different animal models of epilepsy

The anticonvulsant potency of 2-propyl-2-pentenoic acid (2-en-VPA; trans isomer), a major metabolite of the antiepileptic valproic acid (VPA), was evaluated in different animal models of epilepsy and compared with the respective data for VPA. Four models were used: the maximal electroshock seizure (MES) test in mice, the pentylenetetrazol seizure test in mice, gerbils with 'major' (generalized tonic-clonic) seizures in response to specific sensory stimulation, and rats with chronically recurring, spontaneous 'petit mal' seizures. The overall anticonvulsant profile of 2-en-VPA in these models compared favourably with that of VPA. Both drugs were considerably more potent to block seizures in epileptic rats and gerbils than in the traditional MES and pentylenetetrazol mouse models. As regards toxicity, no side-effects were observed with effective doses of 2-en-VPA in rats and gerbils, whereas in the doses necessary to block MES and pentylenetetrazol seizures in mice (200-300 mg/kg i.p.) 2-en-VPA was more sedative than VPA. LD50 values determined for both drugs were comparable. A major difference between 2-en-VPA and VPA was found with respect to embryotoxicity. Single doses of VPA administered to pregnant mice gave rise to significant teratogenic effects (exencephaly, embryolethality , growth retardation), whereas 2-en-VPA was not embryotoxic, even at extremely high doses (600 mg/kg). The data suggest that 2-en-VPA may be a valuable alternative antiepileptic drug.

The quinolines PK 8165 and PK 9084 possess benzodiazepine-like activity in vitro but not in vivo

The quinolines PK 8165 and PK 9084 bind to brain benzodiazepine receptors in vitro. However, unlike diazepam, these molecules do not reduce GABA turnover, possess anxiolytic properties, or displace [3H]flunitrazepam from benzodiazepine receptors in vivo. The pharmacological properties of PK 8165 and PK 9084 in vivo are thus unrelated to the benzodiazepine receptor.