7-Nitroindazole potentiates the anticonvulsant action of some second-generation antiepileptic drugs in the mouse maximal electroshock-induced seizure model

Summary. The effects of 7-nitroindazole (7NI, a preferential neuronal nitric oxide synthase inhibitor) on the anticonvulsant activity of four second-generation antiepileptic drugs (AEDs: felbamate [FBM], lamotrigine [LTG], oxcarbazepine [OXC] and topiramate [TPM]) were studied in the mouse maximal electroshock-induced seizure (MES) model. Moreover, the influence of 7NI on the acute neurotoxic (adverse-effect) profiles of the studied AEDs, with regard to motor coordination, was determined in the chimney test in mice. Results indicate that 7NI (50 mg/kg; i.p.) significantly potentiated the anticonvulsant activity of OXC, but not that of FBM, LTG and TPM against MES-induced seizures and, simultaneously, it enhanced the acute neurotoxic effects of TPM, but not those of FBM, LTG and OXC in the chimney test in mice. 7NI at the lower dose of 25 mg/kg had no effect on the antiseizure activity and acute neurotoxic profiles of all investigated AEDs. Pharmacokinetic evaluation of interactions between 7NI and LTG, OXC and TPM against MES-induced seizures revealed no significant changes in free (non-protein bound) plasma AED concentrations following 7NI administration. Moreover, none of the examined combinations of 7NI with AEDs from the MES test were associated with long-term memory impairment in mice subjected to the step-through passive avoidance task. Based on our preclinical study, it can be concluded that only the combination of 7NI with OXC was beneficial, when considering its both anticonvulsant and acute neurotoxic effects. Moreover, the lack of impairment of long-term memory and no pharmacokinetic interactions in plasma of experimental animals make the combination of 7NI with OXC worthy of consideration for the treatment of patients with refractory epilepsy. The other combinations tested between 7NI and LTG, FBM and TPM were neutral, when considering their both anticonvulsant effects and acute neurotoxic profiles, therefore, no useful recommendation can be made for their clinical application.     

Update on the Mechanism of Action of Antiepileptic Drugs

Summary: Novel antiepileptic drugs (AEDs) are thought to act on voltage-sensitive ion channels, on inhibitory neurotransmission or on excitatory neurotransmission. Two successful examples of rational AED design that po tentiate GABA-mediated inhibition are vigabatrin (VGB) by irreversible inhibition of GABA-transaminase, and ti-agabine (TGB) by blocking GAB A uptake. Lamotrigine (LTG) prolongs inactivation of voltage-dependent sodium channels. The anticonvulsant action of remacemide (RCM) is probably largely due to blockade of NMDA receptors and prolonged inactivation of sodium channels induced by its desglycinated metabolite. Felbamate (FBM) apparently blocks NMDA receptors, potentiates GABA-mediated responses, blocks L-type calcium channels, and possibly also prolongs sodium channel inactivation. Similarly, to piramate (TPM) has multiple probable sites of action, including sodium channels, GABA receptors, and glutamate (AMPA) receptors. Gabapentin (GBP) apparently has a completely novel type of action, probably involving potentiation of GABA-mediated inhibition and possibly also inactivation of sodium channels. The therapeutic advantages of the novel AEDs are as yet only partially explained by our present understanding of their. Mechanisms of action.     

Clinical Efficacy of New Antiepileptic Drugs in Refractory Partial Epilepsy: Experience in the United States With Three Novel Drugs

Summary: A number of new antiepileptic drugs (AEDs), including topiramate (TPM), felbamate (FBM), and gabapentin (GBP), are approved or believed to be close to approval for marketing in the United States. Key efficacy findings for these AEDs in refractory partial epilepsy were reviewed. Large and significant drug-placebo differences were observed with TPM in two large dose-finding trials conducted in the United States. The minimal effective dose of TPM in the population studied was determined to be approximately 200 mg/day, and doses above 600 mg/day produced good efficacy but little incremental benefit versus the lower dosages for the overall study population. FBM is active in partial epilepsy, although seizure reduction is less marked and drug interactions complicate the findings. GBP is also active in this population, but only the 1,800 mg/day dosage was significantly better than placebo with respect to percent re-sponders. It may be useful to explore higher dosage ranges for both FBM and GBP if they can be well tolerated.     

Preclinical profile of combinations of some second-generation antiepileptic drugs: an isobolographic analysis

PURPOSE: The need for an efficacious treatment of patients with intractable seizures is urgent and pressing, because approximately 30% of epilepsy patients worldwide are still inadequately medicated with current frontline antiepileptic drugs (AEDs). This study sought to determine the interactions among some newer AEDs [topiramate (TPM), felbamate (FBM), oxcarbazepine (OXC), and lamotrigine (LTG)] in the maximal electroshock-induced seizures (MES) and chimney test (motor performance) in mice, by using the isobolographic analysis. METHODS: Evaluation of the anticonvulsant and acute adverse (neurotoxic) effects in mice produced by the AEDs in combinations at the fixed ratios of 1:3, 1:1, and 3:1 allowed the assessment of their preclinical profile and the determination of benefit indices (BIs) for all individual combinations. RESULTS: Combinations of TPM+FBM at the fixed ratios of 1:3, 1:1, and 3:1 offered supraadditive (synergistic) interactions against electroconvulsions and subadditivity (antagonism) in terms of acute neurotoxic effects in the chimney test (BIs ranged between 1.90 and 2.59, the best combinations from a preclinical point of view). The examined combinations of TPM+OXC also were advantageous due to synergistic interactions in the MES, and additivity in terms of acute neurotoxic effects produced by the AEDs (BIs ranged between 1.35 and 1.71). In contrast, OXC+FBM exerted subadditive (antagonistic) interactions in the MES test and additive interactions in terms of acute motor impairment of animals (BIs ranged between 0.53 and 0.71). The worst combination was observed for OXC+LTG, at the fixed ratio of 1:1, displaying subadditivity (antagonism) against electroconvulsions and supraadditivity (synergy) with respect to neurotoxicity (BIs, 0.43). The remaining combinations of OXC+LTG tested (i.e., 1:3 and 3:1) exerted additivity in the MES test and supraadditivity in the chimney test (BIs 0.54 and 0.49, respectively). None of the studied AEDs affected the brain concentrations of other AEDs, so the existence of any pharmacokinetic interactions to be responsible for the observed effects is improbable. CONCLUSIONS: Based on the current preclinical data, the pharmacological profile of combinations of TPM+FBM and TPM+OXC evaluated with isobolography was beneficial and might be worth recommendation to further clinical practice. In contrast, utmost caution is required during the use of OXC+FBM or OXC+LTG in clinical practice, because of the high risk of neurotoxic adverse effect appearance.     

Interaction of Felbamate with Several Other Antiepileptic Drugs Against Seizures Induced by Maximal Electroshock in Mice

Summary: The anticonvulsant effects of felbamate (FBM) alone or in combination with phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), or phenobarbital (PB) were investigated against maximal electroshock (MES) seizures in mice. Nonprotective doses of the prototype antiepileptic drugs (AEDs) enhanced the protective effects of FBM against electrically induced seizures, as shown by significant reduction of FBM ED₅₀ values. Toxicity as determined by rotorod test was not significantly potentiated, however, and the protective index (PI = TD₅₀/ED₅₀) of FBM was increased by >100% for each AED interaction. The increase in anticonvulsant potency of FBM after its combination with nonprotective doses of AEDs could not be accounted for by a pharmacokinetic mechanism.     

Topiramate: Preclinical Evaluation of a Structurally Novel Anticonvulsant

Summary: Topiramate [TPM, 2,3:4,5-his-O-(1-methyl-ethylidene)-β-D-fructopyranose sulfamate] (RWJ-17021-000, formerly McN-4853) is a structurally novel antiepileptic drug (AED). The preclinical anticonvulsant profile suggests that TPM acts primarily by blocking the spread of seizures. TPM was highly effective in the maximal electroshock (MES) seizure test in rats and mice. Activity was evident 0.5. h after oral administration and lasted at least 16 h. The ED₅₀ values 4 h after oral dosing were 13.5 and 40.9 mg/kg in rats and mice, respectively. TPM blocked pentylenetetrazol (PTZ)-induced clonic seizures at high doses in mice (ED₅₀= 1,030 mg/kg orally, p.o.). With motor incoordination and loss of righting reflex used as indicators of neurologic impairment, the neuroprotective index (TD₅₀/MES ED₅₀) for TPM was equivalent or superior to that of several approved AEDs. In mice pretreated with SKF-525A (a P450 enzyme inhibitor), the anticonvulsant potency was either increased or unaffected when TPM was tested 0.5, 1, or 2 h after i.p. administration, suggesting that TPM rather than a metabolite was the active agent. In mice pretreated with reserpine or tetrabenazine, the activity of TPM in the MES test was markedly reduced. TPM was inactive in a variety of receptor binding, neurotransmitter uptake, and ion channel tests. TPM weakly inhibited erythrocyte carbonic anhydrase (CA) activity. However, the anticonvulsant activity of TPM appears to differ mechanistically from that of acetazolamide.     

Amiloride enhances the anticonvulsant action of various antiepileptic drugs in the mouse maximal electroshock seizure model

Accumulating evidence indicates that amiloride (a potassium-sparing diuretic) exerts the anticonvulsant action in various in vivo and in vitro experiments. Therefore, the objective of this study was to assess the influence of amiloride on the protective action of numerous conventional and second-generation antiepileptic drugs [AEDs: carbamazepine (CBZ), lamotrigine (LTG), oxcarbazepine (OXC), phenobarbital (PB), topiramate (TPM), and valproate (VPA)] against maximal electroshock (MES)-induced seizures in mice. Results indicate that amiloride [up to 100 mg/kg, intraperitoneally (i.p.), at 30, 60, and 120 min before the test] neither altered the threshold for electroconvulsions, nor protected the animals against MES-induced seizures in mice. Moreover, amiloride (75 and 100 mg/kg, i.p., 120 min prior to the test) significantly enhanced the anticonvulsant effects of all studied AEDs, except for LTG, by reducing their ED₅₀ values in the MES test. In contrast, amiloride at 50 mg/kg (i.p.) had no significant effect on the antielectroshock action of the tested AEDs in mice. Estimation of total brain AED concentrations revealed that amiloride (75 mg/kg) significantly increased total brain concentrations of CBZ, OXC, and PB, but not those of LTG, TPM, and VPA in mice. In conclusion, one can ascertain that the potentiation of the antiseizure action of TPM and VPA by amiloride in the MES test and lack of any pharmacokinetic interactions between drugs, make the combinations of amiloride with TPM and VPA of pivotal importance for 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.     

Overview of the Safety of Newer Antiepileptic Drugs

Summary: Standard antiepileptic drugs (AEDs) are associated with a wide variety of acute and chronic adverse events and with many interactions with each other and with non-AEDs that complicate patient management. The safety and interaction profiles of the newer AEDs have also been intensively studied. Safety data are available for six of the newer AEDs, lamotrigine (LTG), vigabatrin (VGB), gabapentin (GBP), tiagabine (TGB), felbamate (FBM), and topiramate (TPM). The potential for the most recently developed AEDs for producing rare idiosyncratic reactions cannot be ascertained until additional patient exposures have been reported from careful postmarketing surveillance.     

Pharmacodynamic and pharmacokinetic interactions between common antiepileptic drugs and acetone, the chief anticonvulsant ketone body elevated in the ketogenic diet in mice

Purpose:   Acetone is the principal ketone body elevated in the ketogenic diet (KD), with demonstrated robust anticonvulsant properties across a variety of seizure tests and models of epilepsy. Because the majority of patients continue to receive antiepileptic drugs (AEDs) during KD treatment, interactions between acetone and AEDs may have important clinical implications. Therefore, we investigated whether acetone could affect the anticonvulsant activity and pharmacokinetic properties of several AEDs against maximal electroshock (MES)–induced seizures in mice.
Methods:   Effects of acetone given in subthreshold doses were tested on the anticonvulsant effects of carbamazepine (CBZ), lamotrigine (LTG), oxcarbazepine (OXC), phenobarbital (PB), phenytoin (PHT), topiramate (TPM) and valproate (VPA) against MES-induced seizures in mice. In addition, acute adverse effects of acetone–AEDs combinations were assessed in the chimney test (motor performance) and passive avoidance task (long-term memory). Pharmacokinetic interactions between acetone and AEDs were also studied in the mouse brain tissue.
Results:   Acetone (5 or 7.5 mmol/kg, intraperitoneally [i.p.]) enhanced the anticonvulsant activity of CBZ, LTG, PB, and VPA against MES-induced seizures; effects of OXC, PHT, and TPM were not changed. Acetone (7.5 mmol/kg) did not enhance the acute adverse-effect profiles of the studied AEDs. Acetone (5 or 7.5 mmol/kg, i.p.) did not affect total brain concentrations of the studied AEDs. In contrast, VPA, CBZ, LTG, OXC, and TPM significantly decreased the concentration of free acetone in the brain; PB and PHT had no effect.
Conclusions:   Acetone enhances the anticonvulsant effects of several AEDs such as VPA, CBZ, LTG, and PB without affecting their pharmacokinetic and side-effect profiles.