LY 300164, a novel antagonist of AMPA/kainate receptors, potentiates the anticonvulsive activity of antiepileptic drugs

21st century excitatory amino acid research: A Q & A with Jeff Watkins and Dick Evans

In 1981 Jeff Watkins and Dick Evans wrote what was to become a seminal review on excitatory amino acids (EAAs) and their receptors (Watkins and Evans, 1981). Bringing together various lines of evidence dating back over several decades on: the distribution in the nervous system of putative amino acid neurotransmitters; enzymes involved in their production and metabolism; the uptake and release of amino acids; binding of EAAs to membranes; the pharmacological action of endogenous excitatory amino acids and their synthetic analogues, and notably the actions of antagonists for the excitations caused by both nerve stimulation and exogenous agonists, often using pharmacological tools developed by Jeff and his colleagues, they provided a compelling account for EAAs, especially l-glutamate, as a bona fide neurotransmitter in the nervous system. The rest, as they say, is history, but far from being consigned to history, EAA research is in rude health well into the 21st Century as this series of Special Issues of Neuropharmacology exemplifies. With EAAs and their receptors flourishing across a wide range of disciplines and clinical conditions, we enter into a dialogue with two of the most prominent and influential figures in the early days of EAA research: Jeff Watkins and Dick Evans.

Targeting α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors in epilepsy

INTRODUCTION

Despite epilepsies being between the oldest and most studied neurological diseases, new treatment remains an unmet need of scientific research due to the high percentage of refractory patients. Several studies have identified new suitable anti-seizure targets. Glutamate activation of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) have long ago been identified as suitable targets for the development of anti seizure drugs.

AREAS COVERED

Here, we describe: i) AMPARs' structure and their involvement and role during seizures and in epilepsy and ii) the efficacy of AMPAR antagonists in preclinical models of seizures and epilepsy.

EXPERT OPINION

The physiological and pathological role of AMPAR in the CNS and the development of AMPAR antagonists have recently gained attention considering their recent involvement in status epilepticus and the marketing of perampanel. The need for new anti-seizure drugs represents a major challenge in both preclinical and clinical epilepsy. The introduction into the market of perampanel for the treatment of epilepsy will shed new light on the real potential of AMPAR antagonism in clinical settings outside the limited world of clinical trials. While research will go on in this area, fundamental will be the post-marketing evaluation of perampanel efficacy and tolerability and a better definition of the role of this receptor in the epileptic brain.

Synergistic combinations of anticonvulsant agents: what is the evidence from animal experiments?

PURPOSE

Combination therapy is often used in the treatment of seizures refractory to monotherapy. At the same time, the pharmacodynamic mechanisms that determine the combined efficacy of antiepileptic drugs (AEDs) are unknown, and this prevents a rational use of these drug combinations. We critically evaluate the existing evidence for pharmacodynamic synergism between AEDs from preclinical studies in animal models of epilepsy to identify useful combinations of mechanisms and to determine whether study outcome depends on the various research methods that are in use.

METHODS

Published articles were included if the studies were placebo-controlled, in vivo, or ex vivo animal studies investigating marketed or experimental AEDs. The animal models that were used in these studies, the primary molecular targets of the tested drugs, and the methods of interpretation were recorded. The potential association of these factors with the study outcome (synergism: yes or no) was assessed through logistic regression analysis.

RESULTS

In total, 107 studies were identified, in which 536 interaction experiments were conducted. In 54% of these experiments, the possibility of a pharmacokinetic interaction was not investigated. The majority of studies were conducted in the maximal electroshock model, and other established models were the pentylenetetrazole model, amygdala kindling, and the DBA/2 model. By far the most widely used method for interpretation of the results was evaluation of the effect of a threshold dose of one agent on the median effective dose (ED50) of another agent. Experiments relying on this method found synergism significantly more often compared with experiments relying on other methods (p<0.001). Furthermore, experiments including antagonists of the AMPA receptor were more likely to find synergism in comparison with all other experiments (p<0.001).

CONCLUSIONS

Intensive preclinical research into the effects of AED combinations has not led to an understanding of the pharmacodynamic properties of AED combinations. Specifically, the majority of the preclinical studies are not adequately designed to distinguish between additive, synergistic, and antagonistic interactions. Quantitative pharmacokinetic-pharmacodynamic studies of selectively acting AEDs in a battery of animal models are necessary for the development of truly synergistic drug combinations.

Talampanel

Talampanel is a novel anticonvulsant that acts as an allosteric inhibitor of the AMPA receptor. Talampanel has a broad spectrum of action in animal models of epilepsy and neuroprotection. Clinical experience to date has been able to show effectiveness in reduction of seizures in patients with refractory partial seizures.

Simultaneous determination of YM928, a novel noncompetitive AMPA receptor antagonist, and its demethylated metabolite in rat, dog and monkey plasma by high-performance liquid chromatography with ultraviolet detection

A specific HPLC method for the simultaneous determination of YM928, a novel noncompetitive AMPA receptor antagonist, and its demethylated metabolite (YM-58875) in rat, dog and monkey plasma was developed and validated. The method utilized multiple-step liquid-liquid extraction followed by a reversed-phase HPLC with UV detection at 275 nm. No interfering peaks were observed at the retention times of YM928, YM-58875 or internal standard. The validated quantitation range was 5-5000 ng/mL for both YM928 and YM-58875 when 1 mL of the plasma sample was used. The intra- and inter-day precision was less than 5.3 and 2.5% for YM928, and 3.7 and 2.3% for YM-58875, respectively. The intra- and inter-day accuracies were -8.7-5.3% and 0.7-1.9% for YM928, and -10.0-6.1% and 1.3-3.4% for YM-58875, respectively. The mean recoveries in the extraction process were 52.7-62.8%. The utility of this analytical method was demonstrated by the investigation of the pharmacokinetics of the unchanged drug and its metabolite in preclinical studies.

The AMPA-antagonist talampanel is neuroprotective in rodent models of focal cerebral ischemia

Cerebroprotection after administration of glutamate receptor antagonists has been well documented. The present study is intended to determine whether the non-competitive alpha-amino-3-hydroxy-methyl-4-isoxazolyl-propionic acid (AMPA) receptor antagonist talampanel, known as antiepileptic drug, has neuroprotective effects in stroke models in rodents. The infarct size was measured in three models of stroke by 2,3,5-triphenyltetrazolium chloride staining. Therapeutic time window was also examined in rats subjected to 1h middle cerebral artery occlusion. The degree of neuroprotection was tested in mice, using 1.5, 2 h or permanent middle cerebral artery occlusions. Effect on photochemically induced thrombosis was investigated in rats applying 30 min time window after brain irradiation. Talampanel reduced the infarct size by 47.3% (p<0.01) after a 30 min delay and 48.5% (p<0.01) after 2 h delay following middle cerebral artery occlusion in rats. In mice, talampanel reduced the extension of the infarcted tissue at the levels of striatum and hippocampus by 44.5% (p<0.05) and 39.3% (p<0.01) after 1.5 h transient ischemia and still caused 37.0% (p<0.05) and 37.0% (p<0.05) inhibitions when 2 h occlusion was applied. In photothrombosis talampanel showed a 40.1% (p<0.05) inhibition. Protective actions of talampanel in various stroke models, in rats and mice, suggest a possible therapeutic role of the compound in stroke patients.

Progress report on new antiepileptic drugs: a summary of the Seventh Eilat Conference (EILAT VII)

The Seventh Eilat Conference on New Antiepileptic Drugs (AEDs) (EILAT VII) took place in Villasimius, Sardinia, Italy from the 9th to 13th May 2004. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference,whose main themes included advances in pathophysiology of drug resistance, new AEDs in pediatric epilepsy syndromes, modes of AED action and spectrum of adverse effects and a re-appraisal of comparative responses to AED combinations. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on second-generation AEDs. This article summarizes the information presented on drugs in development, including atipamezole, BIA-2-093, fluorofelbamate, NPS 1776, pregabalin, retigabine, safinamide, SPM 927, stiripentol, talampanel,ucb 34714 and valrocemide (TV 1901). Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine,topiramate, vigabatrin, zonisamide, new oral and parenteral formulations of valproic acid and SPM 927 and the antiepileptic vagal stimulator device are also presented.

Heterogeneity and potentiation of AMPA type of glutamate receptors in rat cultured microglia

alpha-amino-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor in rat cultured microglia were analyzed precisely using flop- and flip-preferring allosteric modulators of AMPA receptors, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA) and cyclothiazide (CTZ), respectively. Glutamate (Glu)- or kainite (KA)-induced currents were completely inhibited by a specific blocker of AMPA receptor, LY300164, indicating that functional Glu-receptors in cultured microglia are mostly AMPA receptor but not KA receptor in many cells. Glu- and KA-induced currents were potentiated by PEPA and CTZ in a concentration-dependent manner. The ratio of the potentiation by PEPA to the potentiation by cyclothiazide varied with cells between 0.1 and 0.9, suggesting cell-to-cell heterogeneity of AMPA receptor subunits expressed in microglia. Quantitative RT-PCR revealed that GluR1-3 mainly occurred in the flip forms, which agreed with the stronger potentiation of receptor currents by CTZ vs. PEPA. Finally, the potentiation of microglial AMPA receptors by PEPA and CTZ inhibited the Glu-induced release of tumor necrosis factor-alpha (TNF-alpha) unpredictably. The increase in TNF-alpha release by Glu or KA required extracellular Na+ and Ca2+ ions but not mitogen-activated protein kinase (MAPK), suggesting the effects of PEPA and CTZ were not due to the inhibition of MAPK. These results suggest that potentiation of microglial AMPA receptors serves as a negative feedback mechanism for the regulation of TNF-alpha release and may contribute to the ameliorating effects of allosteric modulators of AMPA receptors.

Effects of 2-[N-(4-Chlorophenyl)-N-methylamino]-4H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), an Orally Active α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor Antagonist, in Models of Generalized Epileptic Seizure in Mice and Rats

The anticonvulsant activity of 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3.2-e]-1,3-thiazin-4-one (YM928), a novel alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, was studied in animal models of generalized seizure. YM928 exerted significant anticonvulsant effects in the maximal electroshock (MES) seizure test (ED50 = 7.4 mg/kg p.o.), pentylenetetrazol (PTZ)-induced seizure test (ED50 = 9.6 mg/kg p.o.), AMPA-induced seizure test (ED50 = 5.5 mg/kg p.o.), and strychnine-induced seizure test (ED50 = 14.0 mg/kg p.o.) in mice. Effects in rats were detected in the MES seizure test (ED50 = 4.0 mg/kg p.o.) and PTZ-induced seizure test (ED50 = 6.2 mg/kg p.o.). The profile of YM928 was compared with that of established antiepileptics. Valproate showed beneficial effects in all tests used. In contrast, carbamazepine, phenytoin, lamotrigine, phenobarbital, diazepam, ethosuximide, and gabapentin were not active against seizures induced by at least one stimulant. In the rotarod test, YM928 impaired motor coordination (TD50 = 22.5 mg/kg p.o.). The protective index (TD50 value of the rotarod test/ED50 value of MES seizure) was 3.0, suggesting that YM928 can exert antiepileptic effects with only minor motor disturbances. YM928 at doses of 2, 4, and 8 mg/kg p.o. did not significantly affect the threshold of electroshock seizure in rats after 16 days of repeated administration. These data indicate that YM928 does not induce tolerance after subchronic administration. These results indicate that YM928 is a broad-spectrum anticonvulsant that would prove useful for the treatment of generalized seizure in human epileptic patients.

Effect of topiramate on the anticonvulsant activity of conventional antiepileptic drugs in two models of experimental epilepsy

PURPOSE

The objective of this study was to evaluate the interaction of the novel antiepileptic drug (AED), topiramate (TPM), with conventional AEDs against amygdala-kindled seizures in rats and pentylenetetrazol-induced convulsions in mice.

METHODS

Experiments were performed on mice and fully kindled rats. In pentylenetetrazol test, the chemoconvulsant was used at its CD97 dose of 105 mg/kg, producing clonic seizures in 97% of mice. Adverse effects were evaluated with the chimney test and passive avoidance task. Plasma levels of AEDs were measured with immunofluorescence.

RESULTS

TPM at 20 mg/kg exerted a significant anticonvulsant effect as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats, being ineffective at lower doses. Coadministration of TPM (10 mg/kg) with valproate (VPA; at a subtherapeutic dose of 50 mg/kg) resulted in essential reductions of seizure and afterdischarge durations. TPM (10 mg/kg) combined with carbamazepine (CBZ; at a subtherapeutic dose of 15 mg/kg) significantly increased afterdischarge threshold, simultaneously decreasing the remaining seizure parameters (duration or severity of seizures and afterdischarge duration). TPM (10 mg/kg) given with phenobarbital (PB; 15 mg/kg) markedly shortened seizure severity and seizure and afterdischarge durations. Combinations of TPM with diphenylhydantoin (PHT) were ineffective against kindled seizures in rats. TPM combined with VPA and PB did not alter their plasma levels, but its combination with CBZ resulted in an increased free plasma CBZ concentration. TPM (10 and 20 mg/kg) alone and its combinations with conventional AEDs affected neither motor coordination nor long-term memory, evaluated in the chimney and passive avoidance tests, respectively, in rats. In pentylenetetrazol-evoked convulsions in mice, TPM (175 and 200 mg/kg) showed anticonvulsant effects per se. Moreover, TPM (at its subtherapeutic dose of 150 mg/kg), significantly potentiated the anticonvulsant action of ethosuximide (ESM), but not that of VPA, PB, or clonazepam (CZP) against pentylenetetrazol-induced seizures. Either TPM alone (150 mg/kg) or its combination with ESM did not result in significant undesired effects.

CONCLUSIONS

The experimental data indicate that except for PHT, the combinations of TPM with conventional AEDs are beneficial against amygdala-kindled seizures in rats. In the pentylenetetrazol test, this novel AED potentiated only the protection offered by ESM.

Talampanel, a new antiepileptic drug: single- and multiple-dose pharmacokinetics and initial 1-week experience in patients with chronic intractable epilepsy

PURPOSE

Talampanel (LY300164), a potent and selective alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-receptor antagonist, is a potential new antiepileptic drug (AED). This study examines the single- and multiple-dose pharmacokinetics, safety, and tolerability of talampanel in patients with intractable epilepsy and assesses the potential for pharmacokinetic interaction.

METHODS

Eleven of 14 patients entered into the study completed. Fourteen patients were evaluated for safety, 13 patients were used in the single-dose, and 11 patients in the multiple-dose pharmacokinetic analysis. Each patient initially received a single 35-mg dose of talampanel followed by the measurement of pharmacokinetic profiles. A 21-day t.i.d. dosing regimen was then determined for each patient based on his or her initial pharmacokinetic profile. Adverse events were recorded by patients or their carers.

RESULTS

After oral ingestion, talampanel was rapidly absorbed, with maximal plasma concentrations achieved within 1-3 h. Talampanel concentrations in patients taking enzyme-inducing AEDs were 50% lower than those seen in healthy volunteers. Mean talampanel t1/2 values were 3.0 h compared with 4.2 h in healthy volunteers. After multiple-dose and steady-state, talampanel t1/2 values were increased to 5.6 h Talampanel and valproic acid (VPA) appear to inhibit each other's metabolism mutually. Talampanel had no effect on plasma concentrations of other AEDs. Multiple-dose talampanel administration was associated with nonlinear pharmacokinetics. No serious adverse events were reported; the most frequently reported being dizziness, ataxia, drowsiness, and headaches

CONCLUSIONS

Talampanel dosing strategies may be reliant on concomitant AED medication, as enzyme-inducing AEDs enhance, whereas VPA inhibits its metabolism. Talampanel was well tolerated, although adverse events occurred at lower doses compared with those in healthy subjects, probably because of the additive effect of concomitant AEDs.

Polytherapy in epilepsy: the experimental evidence

Monotherapy is recommended preferentially among newly diagnosed epileptic patients. In monotherapy-resistant patients polytherapy may be necessary. Two antiepileptic drugs may produce antagonistic, additive, and supra-additive (synergistic) anticonvulsant effects. The drug combination providing the supra-additive effect seems of clinical significance. However, when the supra-additive anticonvulsant efficacy is also associated by a distinct increase in toxicity, the protective index may be not affected or even lowered. Synergistic interactions have been shown for the combinations of valproate-phenytoin/ethosuximide, topiramate-carbamazepine/phenobarbital and felbamate-all major conventional antiepileptics. In contrast, the protective action of conventional antiepileptics has not been affected by felbamate at subprotective doses against maximal electroshock in mice. This is indicative that synergism is evident at only some drug ratios. Potential antiepileptic drugs, excitatory amino acid antagonists and calcium channel inhibitors, generally enhanced the protection offered by antiepileptic drugs. The experimental data may be helpful for predicting which drug combinations may prove effective in epileptic patients.

Novel anticonvulsant medications in development

Epilepsy is currently the most prevalent neurological disorder worldwide. Pharmacological therapy remains the cornerstone of epilepsy treatment, however, refractory epilepsy is still a significant clinical problem despite the release of the second generation of anticonvulsants. Anticonvulsant treatment failures may result from lack of efficacy and presence of significant side effects. One rationale for incomplete effectiveness of the currently available anticonvulsants is that they were identified using the same classical models and therefore work largely by the same actions. These mechanisms fail to consider variations in the pathophysiological process that results in epilepsy, nor have they been shown to prevent the process of developing epilepsy (epileptogenesis). The next generation of anticonvulsants has taken into account the shortcomings of existing agents and attempted to improve on the currently available treatments using rationale drug design. This group of investigational anticonvulsants may be broadly classified as possessing one or more of the following: 1) increased tolerability through improvement in drug chemical structure or better delivery to the site of action, 2) new mechanisms (or combinations of mechanisms) of action, 3) improved pharmacokinetic properties. This article will discuss the next generation of anticonvulsants (carabersat, CGX-1007, fluorofelbamate, harkoseride, losigamone, pregabalin, retigabine, safinamide, SPD-421, talampanel, valrocemide) and the possible populations in which they would be clinically useful.

Effect of gabapentin on the anticonvulsant activity of antiepileptic drugs against electroconvulsions in mice: an isobolographic analysis

PURPOSE

The objective of this study was the isobolographic evaluation of the interactions between the novel antiepileptic drug (AED) gabapentin (GBP) and a number of other AEDs against electroconvulsion-induced convulsions in mice.

METHODS

Electroconvulsions were produced by means of an alternating current (ear-clip electrodes, 0.2-s stimulus duration, tonic hindlimb extension taken as the end point). Adverse effects were evaluated with the chimney test (motor performance) and passive-avoidance task (long-term memory). Plasma levels of AEDs were measured by immunofluorescence or high-pressure liquid chromatography.

RESULTS

GBP (< or =50 mg/kg) remained ineffective on the electroconvulsive threshold. According to the isobolographic analysis, GBP appears to act synergistically with carbamazepine, valproate, phenytoin, phenobarbital (PB), lamotrigine (LTG), and LY 300164. The pharmacokinetic events may be responsible for the interactions of GBP/PB and GBP/LTG, because only PB and LTG significantly elevated the plasma concentration of this AED. Conversely, GBP did not affect the plasma levels of other AEDs used in this study. No adverse effects were induced by combinations of GBP with these AEDs.

CONCLUSIONS

The isobolographic analysis revealed that combinations of GBP with other AEDs generally results in synergistic (supraadditive) interactions.

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.

Determination of antiepileptic drugs in biological material

Current analytical methodologies applied to the determination of antiepileptic drugs in biological material are reviewed. The role of chromatographic techniques is emphasized. Special attention is focused on new chemical entities as well as current trends such as high-speed liquid chromatographic techniques, hyphenated techniques and electrochromatography techniques. A review with 542 references.

AMPA and GABA(B) receptor antagonists and their interaction in rats with a genetic form of absence epilepsy

The effects of combined and single administration of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 7,8-methylenedioxy-1-(4-aminophenyl)-4-methyl-3-acetyl-4,5-dihydro-2,3-benzodiazepine (LY 300164), and of the GABA(B) receptor antagonist gamma-aminopropyl-n-butyl-phosphinic acid (CGP 36742), on spontaneously occurring spike-wave discharges were investigated in WAG/Rij rats. LY 300164 had minor effects; only the highest dose (16 mg/kg) reduced the number of spike-wave discharges in a short time window. CGP 36742 was more effective as it significantly reduced the number of spike-wave discharges and shortened their duration at the doses of 25 and 100 mg/kg. The ED(50) values for the inhibition of spike-wave discharges by LY 300164 and CGP 36742 in a time window 30-60 min after injection were 15.5 and 16.6 mg/kg, respectively. The ED(50) of CGP 36742 was reduced to 8.0 mg/kg when this antagonist was administered in combination with LY 300164 (6 mg/kg). The interaction between the two antagonists appeared to be additive according to isobolographic analysis. Importantly, CGP 36742 and LY 300164 administered either alone or in combination had no apparent effects on behavior. These results may provide information for a rational approach to polytherapy for the treatment of generalized absence epilepsy.

Glutamate receptor antagonists differentially affect the protective activity of conventional antiepileptics against amygdala-kindled seizures in rats

LY 300164 (5 mg/kg), a selective non-competitive antagonist of AMPA/kainate receptors, exerted a significant anticonvulsant effect in amygdala-kindled rats, being ineffective at 2 mg/kg. LY 235959 (1--5 mg/kg), a selective competitive antagonist of NMDA receptors, failed to modify behavioral and electrographic correlates of kindled seizures. Amygdala-kindled seizures were inhibited by conventional antiepileptics, their lowest effective doses were: 20 mg/kg for carbamazepine and phenobarbital, 50 mg/kg for diphenylhydantoin, and 100 mg/kg for valproate magnesium. The combined treatment of the AMPA/kainate antagonist (2 mg/kg) with valproate at sub-effective doses (25--75 mg/kg) resulted in the reduced severity and duration of kindled seizures. Also, a clear-cut protective effect was observed when LY 235959 was co-administered with diphenylhydantoin (40 mg/kg). Any interaction at the pharmacokinetic level can be excluded because neither LY 300164 nor LY 235959 interfered with the free plasma levels of valproate or diphenylhydantoin, respectively. The combination of the AMPA/kainate receptor antagonist (2 mg/kg) with valproate (75 mg/kg) did not impair performance of rats in the rotorod test (motor co-ordination) or passive-avoidance task (long-term memory). Conversely, the NMDA receptor antagonist alone or in combination with diphenylhydantoin, produced significant mnemonic deficits. The results indicate that AMPA/kainate receptor antagonists might be of importance as adjuvant antiepileptic drugs in patients treated with valproate. A possible use of NMDA receptor antagonists may be questionable.

Interaction of topiramate with conventional antiepileptic drugs in mice

Topiramate [2,3:4, 5-bis-O-(1-methyl-ethylidene-)-beta-D-fructopyranose sulfamate], administered intraperitoneally (i.p.) up to 5 mg/kg, did not influence the threshold for electroconvulsions. In doses of 10-30 mg/kg, topiramate significantly raised the threshold. This novel antiepileptic drug, in subprotective doses, enhanced the protective activity of i.p. given valproate, carbamazepine, dihenylhydantoin and phenobarbital against maximal electroshock-induced convulsions in mice. The potentiation induced by topiramate (2.5-5 mg/kg) was most profound for carbamazepine and phenobarbital. The anticonvulsive activity of valproate and diphenylhydantoin was potentiated by topiramate only at 5 mg/kg. Topiramate (5 mg/kg) combined with valproate, phenobarbital and diphenylhydantoin did not alter their free plasma levels but its combination with carbamazepine resulted in an increased free plasma level of this antiepileptic drug. Treatment with topiramate (5 mg/kg) alone or in combination with the studied antiepileptics (providing 50% protection against maximal electroshock) resulted in no adverse effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). In contrast, valproate administered alone at its ED(50) against maximal electroshock impaired motor coordination. It is noteworthy that valproate and carbamazepine at their respective ED(50) values of 248 and 11.2 mg/kg disturbed long-term memory. The results provide an experimental basis for rational polytherapy.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Influence of LY 300164, an antagonist of AMPA/kainate receptors, on the anticonvulsant activity of clonazepam

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4, 5H)-2,3-benzodiazepine], an antagonist of AMPA/kainate receptors, at 5 mg/kg exerted a significant anticonvulsant effect, as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats. At lower doses, LY 300164 did not exert anticonvulsant activity. Clonazepam alone (0.003-0.1 mg/kg) significantly diminished seizure severity, seizure and afterdischarge durations. Coadministration of LY 300164 (2 mg/kg) with clonazepam (0.001 mg/kg) resulted in the significant anticonvulsant activity. Seizure severity score, seizure and afterdischarge durations were reduced from 5 to 4, from 32.6 s to 12.3 s, and 42.7 s to 23.2 s. LY 300164 (2 mg/kg), clonazepam (0.001-0.1 mg/kg) and the combination of clonazepam (0.001 mg/kg) with LY 300164 (2 mg/kg) did not affect long-term memory evaluated in the passive avoidance task in rats. LY 300164 (at the subprotective dose of 2 mg/kg) significantly potentiated the anticonvulsant action of clonazepam against maximal electroshock but not against pentylenetetrazol-induced convulsions in mice. The results indicate that blockade of glutamate-mediated events at AMPA/kainate receptors may differently affect the protection offered by clonazepam, which seems dependent upon the model of experimental seizures.