Mechanism of valproate-phenobarbital interaction in epileptic patients.

Valproate effects on phenobarbital biodisposition were examined in a search for the mechanisms of the valproate-induced elevation of plasma phenobarbital during antiepileptic therapy. The study involved patients who were treated with phenobarbital alone and phenobarbital plus valproate. Several kinetic parameters were determined after a pulse dose of stable isotope-labeled phenobarbital, with plasma phenobarbital levels at a steady state. Plasma elimination of labeled phenobarbital was studied by selected ion monitoring. The addition of valproate to the phenobarbital regimen resulted in elevation of plasma phenobarbital and increase in urinary output of unchanged phenobarbital. There was no effect on urinary pH. The rise in plasma phenobarbital was paralleled by lengthening of phenobarbital elimination half-life while the decrease of plasma phenobarbital clearance paralleled the decrease in phenobarbital elimination rate constant. These findings suggest that inhibition of phenobarbital metabolism by valproate is the mechanism for this clinically important drug-drug interaction.

Isoniazid as an inhibitor of primidone metabolism.

Isoniazid inhibited the metabolism of primidone in a patient with focal seizures. The steady-state serum level of primidone rose when the patient received both drugs simultaneoulsy. The serum levels of the primidone metabolites, phenobarbital and phenylethylmalonamide, fell and the rate of metabolism of primidone decreased. The results are similar to those observed when isoniazid is adminstered with diphenylhydantoin.

Serum clonazepam concentrations in children with absence seizures.

Clonazepam, a chlorinated derivative of nitrazepam, was administered to 10 children with absence seizures. Serum concentrations were measured after 8 weeks of treatment, at steady state. Seizure frequency reports and the 12-hour telemetered electroencephalogram were studied before and after 8 weeks of treatment to determine the frequency and duration of generalized spike-wave paroxysms. The clonazepam dosage ranged from 0.028 to 0.111 mg per kilogram and was reflected in serum levels ranging from 13 to 72 ng per milliliter, with an excellent correlation between dose and serum level. Eight of the 10 patients showed a significant decrease in seizure frequency, with three experiencing no seizures at all. Six patients had side effects, predominantly drowsiness and ataxia. This preliminary study shows clonazepam to be useful in the treatment of absence seizures in children and to merit further study.

Sodium valproate: pharmacokinetics and effectivensss in treating intractable seizures.

Sodium valproate (VPA) was first marketed in the United States in 1978. In this pilot study of pharmacokinetics and toxicity, VPA was added to the treatment regimens of 20 patients (10 adults and 10 children) with intractable seizures. The drug was absorbed and excreted rapidly; the mean half-life was 9.6 hours. Drowsiness and gastrointestinal symptoms were the most common side effects, but they were usually minor and transient. An increase in some plasma phenobarbital levels and a decrease in some plasma phenytoin levels were attributed to drug interaction. Control of absence attacks was assessed by 12-hour telemetered electroencephalograms. Sodium valproate was most efficacious in generalized seizure disorders, particularly absence seizures.

Plasma concentrations of phensuximide, methsuximide, and their metabolites in relation to clinical efficacy.

The clinical efficacy of phensuximide and methsuximide was studied in relation to plasma concentrations of these compounds and their desmethyl metabolites. Single- and chronic-dose studies of each drug were carried out in five patients with intractable seizures. Patients were evaluated before and during treatment by 6-hour simultaneous video and telemetered electroencephalographic recordings to characterize the seizure type and by daily determinations of plasma antiepileptic drug concentrations. Phensuximide had a mean half-life of 7.8 hours and accumulated to an average fasting level of only 5.7 micrograms per milliliter. Desmethylphensuximide averaged only 1.7 micrograms per milliliter with a similar half-life. Methsuximide had an even shorter half-life, averaging 1.4 hours, but its desmethyl metabolite had a mean half-life of 38 hours and therefore accumulated to levels in excess of 40 micrograms per milliliter. The addition of phensuximide to their regimens benefited none of the patients, but two had an excellent response to methsuximide. The failure of phensuximide and its desmethyl metabolite to accumulate to reasonable levels is the likely explanation for the relatively weak antiepileptic effect of phensuximide as compared with methsuximide.

Disposition of mephenytoin and its metabolite, nirvanol, in epileptic patients.

We investigated the conversion of mephenytoin to nirvanol in five patients with uncontrolled complex partial seizures. After a 50-mg single oral dose, mean peak mephenytoin level was 0.48 microgram/ml and nirvanol 0.37 microgram/ml. After 400 mg, peak mephenytoin level was 3.9 micrograms/ml and nirvanol 2.5 micrograms/ml. On 400 mg daily, mephenytoin reached a mean steady-state level of 1.5 micrograms/ml. Nirvanol mean steady-state level was 18 micrograms/ml. Mean plasma half-life was 17 hours for mephenytoin and 114 hours for nirvanol. Two patients had reduced seizures during mephenytoin therapy and one a transient increase during drug withdrawal. No toxicity was seen, but mephenytoin was not more effective than phenytoin.

Effects of pharmacological manipulations on basal and newly synthesized levels of GABA, glutamate, aspartate and glutamine in mouse brain cortex.

Concentrations of basal and newly synthesized inhibitory (gamma-aminobutyric acid, GABA) and excitatory (glutamate and aspartate) neurotransmitter amino acids and glutamine were determined in mouse brain cortex. Isotopic enrichment following an intravenous infusion of a stable-labeled precursor, [13C6]D-glucose, was used to estimate the newly synthesized amino acid content. Effects of various pharmacological agents (valproate, aminooxyacetic acid, 3-mercaptopropionic acid, N-methyl-D-aspartate, and 2-amino-7-phosphonohepatanoic acid) were evaluated. The effects of 3-mercaptopropionic acid (an inhibitor of glutamate decarboxylase, a GABA-synthesizing enzyme) were restricted to the GABAergic system. On the other hand, N-methyl-D-aspartate (an agonist of a glutamate receptor subtype) was selective for the glutamate-glutamine system, and its effects were prevented by its selective antagonist, 2-amino-7-phosphonoheptanoic acid. In some cases, divergent effects were observed on basal and new amino acids. This suggested that basal and new amino acids may represent different compartments. The anticonvulsant drug valproate caused an increase in basal but a decrease in newly synthesized GABA. Aminooxyacetic acid caused a dramatic increase in basal GABA without affecting the newly synthesized GABA. This approach may be useful in studying compartmentation and fluxes of neurotransmitters.

A system for testing the development and reversal of anticonvulsant tolerance to benzodiazepines in mice.

Tolerance to the anticonvulsant effects of benzodiazepines limits their use in epilepsy treatment. Animal models producing tolerance have been developed, but they require repetitive injections over several days or use silastic capsules which must be made for each drug and do not provide a constant infusion rate. Alzet 2001 osmotic pumps deliver at a constant rate (1 microliter/h) and dosage can be easily adjusted. Various solvents, PEG 400, propylene glycol, 2% Tween, 50% DMSO, saline, Molecusol, and 0.5% methyl cellulose, were tried and found unsuitable because benzodiazepines were not maintained in solution or proconvulsant activity was seen. Tetraglycol was chosen as it did not demonstrate these shortcomings. Anticonvulsant activity was evaluated by PTZ i.v. tail infusion using forelimb clonus as the endpoint. This study describes a simple method for testing the development of tolerance and its reversal with flumazenil or ZK 93426. At 72 h of pump infusion with diazepam or flunitrazepam, tolerance to anticonvulsant activity was evident. Acute treatment with flumazenil or ZK 93426 reversed this tolerance. When flumazenil or ZK 93426 was given to diazepam tolerant mice, this reversal was complete. In flunitrazepam tolerant mice reversal with flumazenil was partial, but significant. When flumazenil was chronically coinfused with diazepam or flunitrazepam, anticonvulsant activity was antagonized. Similarly, when ZK 93426 was coinfused with diazepam, anticonvulsant activity was antagonized. The method described is suitable for screening putative anticonvulsant drugs for development of tolerance and the reversal of tolerance by other compounds.

The effects of D-23129, a new experimental anticonvulsant drug, on neurotransmitter amino acids in the rat hippocampus in vitro.

D-23129 [N-(2-amino-4-(4-fluorobenzylamino)phenyl)carbamic acid ethyl ester] and D-20443 (dihydrochloride of D-23129) are promising anticonvulsant compounds with a broad spectrum activity in animal models of epilepsy. Their effects on de novo synthesis of excitatory (glutamate and aspartate) and inhibitory (GABA) amino acids were studied in rat hippocampal slices. Like phenytoin, carbamazepine, lamotrigine, losigamone, U54494A, and flupirtine, D-23129 and D-20443 were effective in preventing the effects of a chemoconvulsant, 4-aminopyridine, on de novo synthesis of the three amino acids. However, unlike the other compounds, D-23129 and D-20443 also preferentially increased the concentrations of newly synthesized GABA. Their effect on the neosynthesis of GABA was unique, dose dependent, and not tetrodotoxin sensitive. A total of 15 compounds (including standard, new and candidate anticonvulsants) either had no effect on new GABA or decreased it. Therefore, D-23129 and D-20443 exhibited two different effects on de novo synthesis of neurotransmitter amino acids, both of which could potentially be anticonvulsant in nature.

Antagonism between intracerebroventricularly administered N-methyl-D-aspartate and bicuculline methiodide in induction of clonic seizures in mice.

N-Methyl-D-aspartate and bicuculline were administered alone or as a combination by intracerebroventricular injection to mice, and their convulsant activity was monitored. Both of these compounds elicited clonic seizures, though by different mechanisms. However, their simultaneous administration resulted in less than additive induction of clonic activity.

The effects of anticonvulsant compounds on 4-aminopyridine-induced de novo synthesis of neurotransmitter amino acids in rat hippocampus in vitro.

4-Aminopyridine, a voltage-dependent potassium channel blocker, causes tonic-clonic and electrographic seizures in vivo and evokes epileptiform activity and release of glutamate, aspartate and GABA in vitro. This study examined the effects of 4-aminopyridine (4AP) on de novo synthesis of neuroactive amino acids and a subsequent response to various anticonvulsant compounds (phenytoin, carbamazepine, phenobarbital, valproate, ethosuximide, diazepam, lamotrigine, felbamate, losigamone, U54494A, CPP, MK801 and CNQX) using a hippocampal slice preparation. 4-Aminopyridine had a minimal effect on total tissue concentrations of glutamate, aspartate, and GABA, but caused a significant increase in their de novo synthesis. Phenytoin, carbamazepine, lamotrigine, losigamone and U54494A were the only compounds which were effective in blocking the 4AP-induced increase in all newly synthesized amino acids. It appears that these compounds inhibit 4AP effects in this paradigm by blocking depolarization, probably at use-dependent voltage-sensitive sodium channels. Therefore, this paradigm may be useful in selectively identifying anticonvulsants which act by blocking depolarization.

The effects of anticonvulsant agents on 4-aminopyridine induced epileptiform activity in rat hippocampus in vitro.

Six anticonvulsant drugs, phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), U-54494A, losigamone (LOS), and D-20443, were studied using rat hippocampal slices and standard electrophysiological techniques. The K+ channel blocker, 4-aminopyridine (4-AP), was used as neuronal stimulant. The extracellular parameters evaluated in areas CA3 and CA1 were: (1) interictal-type bursting, (2) evoked population spike (PS) amplitude, (3) latency to PS onset, and (4) duration of the excitatory postsynaptic potential (EPSP). VPA was ineffective in altering any of the parameters. PHT and CBZ partially reversed the increase in EPSP duration produced by 4-AP in area CA3, while the spontaneous bursting was not affected. The experimental drugs, U-54494A, LOS, and D-20443 (dihydrochloride salt of D-23129 from Asta Medica), tended to reverse to varying degrees the 4-AP effects, especially the increase in the EPSP duration. U-54494A tended to depress responses even under control conditions. LOS partially reversed the 4-AP excitation, but abolished bursting in only one of five slices. D-20443 abolished bursting in all slices. It also partially reversed the 4-AP induced increase in the EPSP duration without depressing the normal evoked potential. The results show that 4-AP induced changes in vitro can help differentiate drugs with similar in vivo spectrums of anticonvulsant activity. While the drug induced changes may not truly define the mechanisms of action of these promising new agents, these experimental anticonvulsants can be differentiated from standard agents using the experimental paradigm in this study.

Progress report on new antiepileptic drugs: a summary of the Fifth Eilat Conference (EILAT V).

The Fifth Eilat Conference on New Antiepileptic Drugs (AEDs) took place at the Dan Hotel, Eilat, Israel, 25-29 June 2000. Basic scientists, clinical pharmacologists and neurologists from 20 countries attended the conference, whose main themes included recognition of unexpected adverse effects, new indications of AEDs, and patient-tailored AED therapy. According to tradition, the central part of the conference was devoted to a review of AEDs in development, as well to updates on AEDs that have been marketed in recent years. This article summarizes the information presented on drugs in preclinical and clinical development, including AWD 131-138, DP-valproate, harkoseride, LY300164, NPS 1776, NW 1015, pregabalin, remacemide, retigabine, rufinamide and valrocemide. The potential value of an innovative strategy, porcine embryonic GABAergic cell transplants, is also discussed. Finally, updates on felbamate, fosphenytoin, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide, and the antiepileptic vagal stimulator device are presented.

Progress report on new antiepileptic drugs: a summary of the Sixth Eilat Conference (EILAT VI).

The Sixth Eilat Conference on New Antiepileptic Drugs (AEDs) took place in Taormina, Sicily, Italy from 7th to 11th April, 2002. Basic scientists, clinical pharmacologists and neurologists from 27 countries attended the conference, whose main themes included dose-response relationships with conventional and recent AEDs, teratogenic effects of conventional and recent AEDs, update on clinical implications of AED metabolism, prevention of epileptogesis, and seizure aggravation by AEDs. According to tradition, the central part of the conference was devoted to a review of AEDs in development, as well to updates on AEDs, which have been marketed in recent years. This article summarizes the information presented on drugs in preclinical and clinical development, including carabersat (SB-204269), CGX-1007 (Conantokin-G), pregabalin, retigabine (D-23129), safinamide, SPD421 (DP-VPA), SPM 927, talampanel and valrocemide (TV 1901). Updates on fosphenytoin, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide, new formulations of valproic acid, and the antiepileptic vagal stimulator device are also presented.

Progress report on new antiepileptic drugs: a summary of the Third Eilat Conference.

The Third Eilat Conference on New Antiepileptic Drugs was held at the Royal Beach Hotel from May 27 to May 30, 1996. Epileptologists and scientists from 20 countries attended the conference, which was held to discuss critical issues in drug development, new antiepileptic drugs (AEDs) in development, progress reports and recent findings of newly marketed AEDs, the use of AEDs in special populations and their utilization in non-epileptic disorders. Over the last seven years, six new AEDs have been introduced worldwide and new information on their safety and efficacy has become available. These include felbamate, gabapentin, lamotrigine, oxcarbazepine, topiramate and vigabatrin. Drugs in development include those at an advanced stage, such as remacemide and tiagabine, as well as those just entering clinical trials, such as rufinamide (CGP 331010) and levetiracetam (ucb LO59). The following is a summary of the presentations for drugs in development and recent findings on newly marketed drugs.

Progress report on new antiepileptic drugs: a summary of the fourth Eilat conference (EILAT IV).

The Fourth Eilat Conference on New Antiepileptic Drugs (AEDs) was held at the Royal Beach Hotel, Eilat, Israel, from 6th to 10th September 1998. Epileptologists and scientists from 20 countries attended the conference, which was held to discuss a number of issues in drug development, including outcome assessment in epilepsy (long-term efficacy, quality of life, safety), cost-effectiveness, an update on drugs in development, a progress report on recently marketed AEDs, and controversies in strategies for drug development. This review focuses on drugs in development and recently marketed AEDs. Drugs in development include ADCI, AWD 131-138, DP16, ganaxolone (CCD 1042), levetiracetam (ucb L059), losigamone, pregabalin (isobutyl GABA [CI-1008]), remacemide hydrochloride, retigabine (D-23129), rufinamide (CGP 33101), soretolide (D2916), TV1901, and 534U87. New information on the safety and efficacy of recently marketed drugs (felbamate, fosphenytoin, gabapentin, lamotrigine, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide) and of a new antiepileptic device, the neurocybernetic prosthesis (NCP), has become available. This paper summarizes the presentations made at the conference.

Progress report on new antiepileptic drugs. A summary of the Second Eilat Conference.

The Second Eilat Conference on New Antiepileptic Drugs was held at the King Solomon's Palace Hotel from October 31 to November 3, 1994. Epileptologists and scientists from 20 countries attended the conference, which was held to discuss new trial designs, drug approval, early use of new antiepileptic drugs, and new drugs in development. Over the last six years, several novel antiepileptic drugs have been introduced worldwide, and new information on their safety and efficacy has become available. These include felbamate, gabapentin, lamotrigine, oxcarbazepine, and vigabatrin. Drugs in development include those at an advanced stage, such as topiramate and tiagabine, as well as those just entering clinical trials, such as remacemide and levetiracetam. The following is a summary of the presentations for drugs in development and newly marketed drugs. The meeting concluded with a presentation, 'Still Searching for the Magic Bullet'.

Effect of GABA agonists on the neurotoxicity and anticonvulsant activity of benzodiazepines.

Progabide (50 mg/kg, i.p.), a GABA receptor agonist, significantly decreases the median minimal neurotoxic dose (TD50) of clobazam, chlordiazepoxide, and diazepam; the receptor binding of these substances is highly enhanced by muscimol. Progabide has no significant effect on the TD50 of clonazepam and triazolam; the receptor bindings of these substances is either only slightly enhanced or not altered by muscimol. Progabide also significantly decreases the median antimaximal electroshock dose (MES ED50) of all the benzodiazepines tested. However, progabide has no effect on the median antipentylenetetrazol dose (PTZ ED50) of the benzodiazepines. Likewise, THIP (2.5 mg/kg, i.p.) significantly decreases the TD50 of chlordiazepoxide but not that of triazolam. THIP significantly decreases the MES ED50 of chlordiazepoxide and triazolam but has no effect on the PTZ ED50 of these two substances. The above data suggest that benzodiazepine receptors linked to GABA receptors contribute to the minimal neurotoxicity and anti-MES activity but not to the anti-PTZ activity of benzodiazepines.

The National Institutes of Health Anticonvulsant Drug Development Program: screening for efficacy.

The procedures employed by the ASP provide detailed information pertaining to the anticonvulsant profile of new candidate substances. In addition, the results obtained from tolerance and liver microsomal studies furnish critical information for predicting whether tolerance and/or serious drug-drug interactions are likely to develop following long-term administration of a candidate substance. Finally, in vitro mechanism-of-action studies supply preliminary information regarding the site of action of promising new anticonvulsant drugs. It is anticipated that the testing protocol outlined above will identify safer and mechanistically novel substances to enhance significantly the quality of life of those epilepsy patients still suffering from uncontrolled seizure disorders and/or experiencing significant adverse drug effects.

GC and GC-mass spectrometric determination of p-hydroxyphenobarbital extracted from plasma, urine, and hepatic microsomes.

Analytical methodology was developed for the quantitation of p-hydroxyphenobarbital extracted from plasma, urine, and hepatic microsomes. p-Hydroxyphenobarbital was derivatized with an appropriate n-alkyl iodide in the presence of a methanolic base in aprotic solvent medium. The peralkylated derivatives were stable indefinitely and were quantitated by the sensitive and selective method of GC nitrogen-selective detection and/or selected ion monitoring. The accuracy, precision, and cross verification of all methods were good. The analysis was subsequently used to study the effects of other drugs on phenobarbital biodisposition.