Biphasic action of vigabatrin on cortical epileptic after-discharges in rats

Molecular mechanisms of antiseizure drug activity at GABAA receptors

The GABAA receptor (GABAAR) is a major target of antiseizure drugs (ASDs). A variety of agents that act at GABAARs s are used to terminate or prevent seizures. Many act at distinct receptor sites determined by the subunit composition of the holoreceptor. For the benzodiazepines, barbiturates, and loreclezole, actions at the GABAAR are the primary or only known mechanism of antiseizure action. For topiramate, felbamate, retigabine, losigamone and stiripentol, GABAAR modulation is one of several possible antiseizure mechanisms. Allopregnanolone, a progesterone metabolite that enhances GABAAR function, led to the development of ganaxolone. Other agents modulate GABAergic "tone" by regulating the synthesis, transport or breakdown of GABA. GABAAR efficacy is also affected by the transmembrane chloride gradient, which changes during development and in chronic epilepsy. This may provide an additional target for "GABAergic" ASDs. GABAAR subunit changes occur both acutely during status epilepticus and in chronic epilepsy, which alter both intrinsic GABAAR function and the response to GABAAR-acting ASDs. Manipulation of subunit expression patterns or novel ASDs targeting the altered receptors may provide a novel approach for seizure prevention.

New approach to predicting proconvulsant activity with the use of Support Vector Regression

Antiepileptic drugs are commonly used for many therapeutic indications, including epilepsy, neuropathic pain, bipolar disorder and anxiety. Accumulating data suggests that many of them may lower the seizure threshold in men. In the present paper we deal with the possibility of using Support Vector Regression (SVR) to forecast the proconvulsant activity of compounds exerting anticonvulsant activity in the electroconvulsive threshold test in mice. A new approach to forecast this drug-related toxic effect by means of the support vector machine (SVM) in the regression mode is discussed below. The efficacy of this mathematical method is compared to the results obtained in vivo. Since SVR investigates the anticonvulsant activity of the compounds more thoroughly than it is possible using animal models, this method seems to be a very helpful tool for predicting additional dose ranges at which maximum anticonvulsant activity without toxic effects is observed. Good generalizing properties of SVR allow to assess the therapeutic dose range and toxicity threshold. Noteworthy, this method is very interesting for ethical reasons as this mathematical model enables to limit the use of living animals during the anticonvulsant screening process.

GABAergic drugs in the treatment of epilepsy: modern or outmoded?

Antiepileptic drugs have a number of mechanisms of action that target brain excitability systems. The potentiation of GABAergic inhibitory neurotransmission represents a classic and well-known antiseizure effect. Currently available GABAergic antiepileptic drugs mainly target GABA(A) receptor-associated complexes, GABA reuptake or GABA catabolism. All these compounds, although generally effective, are limited by their deleterious effects on cognition and behavior. The challenge will be to find GABAergic drugs that exhibit the beneficial effects, without the adverse ones.

Isobolographic characterization of interactions between vigabatrin and tiagabine in two experimental models of epilepsy

To characterize the type of interactions between vigabatrin (VGB) and tiagabine (TGB) -- two newer antiepileptic drugs influencing GABA-ergic neurotransmitter system, the isobolographic analysis was used in two experimental models of epilepsy: the maximal electroshock seizure threshold (MEST) test and pentylenetetrazole (PTZ)-induced seizures in mice. Results indicated that VGB and TGB administered separately (i.p.) increased the electroconvulsive threshold in mice, which allowed the calculation of their TID(20) values (threshold increasing doses by 20% over the threshold of control animals) in the MEST test. The TID(20) for VGB was 226.2 mg/kg and that for TGB was 4.4 mg/kg. With isobolography, the combinations of VGB with TGB (at fixed-ratios of 1:3, 1:1 and 3:1) exerted additive interactions in the MEST test in mice. Similarly, VGB and TGB injected separately (i.p.) suppressed the PTZ-induced seizures, and their ED(50) values (median effective doses, protecting 50% of the animals tested against clonic convulsions) for VGB and TGB were 622.5 mg/kg and 0.8 mg/kg, respectively. Isobolographic analysis of interactions revealed that the combinations of VGB with TGB at the fixed-ratios of 1:3 and 1:1 produced supra-additive (synergistic) interactions against PTZ-induced seizures. Only the combination of VGB with TGB at the fixed-ratio of 3:1 was additive in the PTZ test. The evaluation of acute adverse-effect potential for all fixed-ratio combinations of VGB with TGB (administered at their TID(20) and ED(50) values from the MEST and PTZ tests) revealed that none of the examined combinations affected motor coordination in the chimney test and altered neuromuscular tone in the grip-strength test in mice. In contrast, VGB in combinations with TGB produced the antinociceptive effects with respect to suppression of acute thermal pain in animals subjected to the hot-plate test. Based on this preclinical study, one can ascertain that the combination of VGB with TGB would provide an adequate seizure control in epileptic patients.

Characterization of the Anticonvulsant, Behavioral and Pharmacokinetic Interaction Profiles of Stiripentol in Combination with Clonazepam, Ethosuximide, Phenobarbital, and Valproate Using Isobolographic Analysis

PURPOSE

Isobolographic analysis was used to characterize the interactions between stiripentol (STP) and clonazepam (CZP), ethosuximide (ETS), phenobarbital (PB), and valproate (VPA) in suppressing pentylenetetrazole (PTZ)-induced clonic seizures in mice.

METHODS

The anticonvulsant and acute adverse (neurotoxic) effects of STP in combination with the various conventional antiepileptic drugs (AEDs), at fixed ratios of 1:3, 1:1, and 3:1, were evaluated in the PTZ and chimney tests in mice using the isobolographic analysis. Additionally, protective indices (PI) and benefit indices (BI) were calculated to identify their pharmacological profiles so that a ranking in relation to advantageous combination could be established. Moreover, adverse-effect paradigms were determined by use of the step-through passive avoidance task (long-term memory), threshold for the first pain reaction, grip-strength test (neuromuscular tone), and the hot plate test (acute thermal pain). Brain AED concentrations were also measured so as to ascertain any pharmacokinetic contribution to the pharmacodynamic interactions.

RESULTS

All AED combinations comprising of STP and CZP, ETS, PB, and VPA (at the fixed ratios of 1:3, 1:1 and 3:1) were additive in terms of clonic seizure suppression in the PTZ test. However, these interactions were complicated by changes in brain AED concentrations consequent to pharmacokinetic interactions. Thus STP significantly increased total brain ETS and PB concentrations, and decreased VPA concentrations, but was without effect on CZP concentrations. In contrast, PB significantly decreased and VPA increased total brain STP concentrations while CZP and ETS were without effect. Furthermore, while isobolographic analysis revealed that STP and CZP in combination, at the fixed ratios of 1:1 and 3:1, were supraadditive (synergistic; p < 0.05), the combinations of STP with CZP (1:3), ETS, PB, or VPA (at all fixed ratios of 1:3, 1:1, and 3:1) were barely additivity in terms of acute neurotoxic adverse effects in the chimney test. Additionally, none of the examined combinations of STP with conventional AEDs (CZP, ETS, PB, VPA--at their median effective doses from the PTZ-test) affected long-term memory, threshold for the first pain reaction, neuromuscular tone, and acute thermal pain.

CONCLUSIONS

Based on BI values, the combination of STP with PB at the fixed ratio of 1:3 appears to be a particularly favourable combination. In contrast, STP and CZP or ETS (at the fixed ratios of 1:1 and 3:1) were unfavorable combinations. However, these conclusions are confounded by the fact that STP is associated with significant pharmacokinetic interactions. The remaining combinations of STP with PB (1:1 and 3:1), CZP (1:3), ETS (1:3), and VPA (at all fixed ratios of 1:3, 1:1, and 3:1) do not appear to be potential favorable AED combinations.

Pharmacological and behavioral characteristics of interactions between vigabatrin and conventional antiepileptic drugs in pentylenetetrazole-induced seizures in mice: an isobolographic analysis

To characterize the anticonvulsant effects and types of interactions exerted by mixtures of vigabatrin (VGB) and conventional antiepileptic drugs (valproate (VPA), ethosuximide (ESM), phenobarbital (PB), and clonazepam (CZP)) in pentylenetetrazole (PTZ)-induced seizures in mice, the isobolographic analysis for three fixed-ratio combinations of 1 : 3, 1 : 1, and 3 : 1 was used. The adverse-effect profile of the combinations tested, at the doses corresponding to their median effective doses (ED(50)) at the fixed-ratio of 1 : 1 against PTZ-induced seizures, was determined by the chimney (motor performance), step-through passive avoidance (long-term memory), pain threshold (pain sensitivity), and Y-maze (general explorative locomotor activity) tests in mice. Additionally, the observed isobolographic interactions were verified in terms of a pharmacokinetic interaction existence. VGB combined with PB or ESM exerted supra-additive (synergistic) interactions against the clonic phase of PTZ-induced seizures, which was associated with the increment of PB or ESM concentrations in the brains of examined animals. The remaining combinations tested (ie VGB+VPA and VGB+CZP) occurred additive in the PTZ test, which was associated with no significant changes in the brain concentrations of VPA and CZP. None of the examined combinations exerted motor impairment in the chimney test in mice. In the standard variant of passive avoidance task (current of 0.6 mA; 2 s of stimulus duration), the combinations of VGB+CZP and VGB+VPA significantly affected long-term memory in mice. Moreover, VGB in a dose-dependent manner lengthened the latency to the first pain reaction in the pain threshold test in mice. The modified variant of step-through passive avoidance task (current of 0.6 mA; stimulus duration based on the latency from the pain threshold test) revealed no significant changes in the long-term memory of animals for the combinations of VGB+VPA and VGB+CZP; so the observed effects in the standard variant of passive avoidance task were a result of the antinociceptive effects produced by VGB. In the Y-maze test, VGB also, in a dose-dependent manner, increased the general explorative locomotor activity of the animals tested. Similarly, the total number of arm entries in the Y-maze was significantly increased for the combinations of VGB+CZP and VGB+ESM, but not for VGB+PB and VGB+VPA. The application of VGB in combination with PB, ESM, CZP, and VPA suppressed the clonic phase of PTZ-induced seizures, having no harmful or deleterious effects on behavioral functioning of the animals tested, which might be advantageous in further clinical practice.