Interaction of Calcium Channel Blockers and Excitatory Amino Acid Antagonists with Conventional Antiepileptic Drugs

Interactions between non-barbiturate injectable anesthetics and conventional antiepileptic drugs in the maximal electroshock test in mice--an isobolographic analysis

The aim of this study was the isobolographic evaluation of interactions between three non-barbiturate intravenous anesthetics and conventional antiepileptic drugs in the maximal electroshock-induced seizures in mice. Electroconvulsions were produced by means of an alternating current (ear-clip electrodes, 0.2-s stimulus duration, tonic hindlimb extension taken as the endpoint). Adverse effects were evaluated in the chimney test (motor performance) and passive avoidance task (long-term memory). Plasma levels of antiepileptic drugs were measured by immunofluorescence. Obtained results indicate that ketamine acts synergistically with valproate and carbamazepine. Also the combinations of propofol and valproate or phenobarbital led to synergistic interactions. An antagonism was found between etomidate and carbamazepine or phenobarbital. On the other hand, interactions between diphenylhydantoin and injectable anesthetics proved to be additive. The only exception was the combination of diphenylhydantoin and propofol (1:3). Pharmacokinetic phenomena do not seem to interfere with the observed interactions, since none of anesthetics influenced the free plasma concentrations of antiepileptic drugs. Referring to undesired effects, only propofol impaired long-term memory. Although propofol did not disturbed motor coordination, it enhanced motor impairment caused by carbamazepine and diphenylhydantoin. Results of the present study suggest that etomidate needs to be avoided in epileptic patients due to a possibility of negative interactions with some antiepileptic drugs and seizure precipitation.

Amlodipine enhances the activity of antiepileptic drugs against pentylenetetrazole-induced seizures

Amlodipine (AML), which belongs to the 1,4-dihydropyridine calcium channel antagonists, possesses pharmacological and pharmacokinetic profile that distinguishes it from other agents of this class. Pentylenetetrazole (PTZ)-induced clonic and tonic convulsions in mice were significantly reduced by administration of AML at 10 mg/kg. At this dose AML remained without influence upon the plasma level of PTZ. The ED50 value of AML against clonic seizures induced by PTZ was 5.4 mg/kg. This calcium channel antagonist (at 2.5 mg/kg) combined with ethosuximide (ETX), valproate magnesium (VPA) or phenobarbital (PB) significantly reduced their ED50 values against clonic phase of PTZ-induced seizures. AML administered alone or in combination with antiepileptic drugs (AEDs) worsened the motor performance of mice in the chimney test. However, these treatments remained without significant influence on the retention time in the passive avoidance test. Plasma levels of antiepileptics remained unchanged in the presence of AML. The results indicate that AML does not seem a good candidate for a combination therapy in epileptic patients because of its adverse potential.

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.

Effect of amlodipine upon the protective activity of antiepileptic drugs against maximal electroshock-induced seizures in mice

Amlodipine, a calcium channel antagonist of the dihydropyridine class, up to 10 mg kg(-1)(i.p.) did not significantly affect the threshold for electroconvulsions. However, this calcium channel antagonist (10 mg kg(-1)) enhanced the anticonvulsive activity of carbamazepine, valproate and phenobarbital against maximal electroshock-induced seizures in mice. Furthermore, amlodipine (5 mg kg(-1)) intensified the protection offered by carbamazepine. This effect was associated with the increased free plasma level of carbamazepine in the presence of amlodipine. Amlodipine did not influence the free or total plasma level of phenobarbital and valproate, so a pharmacokinetic interaction is not probable for valproate and phenobarbital. The anticonvulsive action and free plasma level of diphenylhydantoin was not modified by amlodipine. The combined treatment of the calcium channel antagonist and antiepileptics caused motor impairment (evaluated in the chimney test). Long-term memory (assessed in the passive avoidance test) in case of combinations of amlodipine with carbamazepine or diphenylhydantoin was not affected. The combination of amlodipine with valproate or phenobarbital significantly influenced the retention in this test. A possible usefulness of amlodipine as add-on therapy in epileptic patients may be limited by its considerable adverse effect revealed by behavioural tests. The pharmacokinetic interaction between carbamazepine and amlodipine might have some clinical importance for patients treated with these drugs.

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.

Influence of melatonin upon the protective action of conventional anti-epileptic drugs against maximal electroshock in mice

Melatonin (50 mg/kg; 60 min before the test) significantly raised the electroconvulsive threshold in mice. The protective action of melatonin (50 mg/kg) in the electroconvulsive threshold test was reversed by aminophylline, picrotoxin and bicuculline. Melatonin at the subconvulsive dose of 25 mg/kg potentiated the anticonvulsive activity of carbamazepine and phenobarbital (ED50 values were significantly decreased from 12.1 to 8.3 and from 18.9 to 11.8 mg/kg, respectively). No potentiation was observed in the case of valproate and diphenylhydantoin (their ED50s were changed from 253 to 249 and from 10.3 to 9.7 mg/kg, respectively). Melatonin did not influence the plasma or brain levels of anti-epileptics studied, so a pharmacokinetic interaction is not probable. Melatonin (25 mg/kg) alone and its combinations with carbamazepine or phenobarbital, providing a 50% protection against maximal electroshock, were devoid of significant motor adverse effects, but caused strong long-term memory deficit. Consequently, it does not seem to be a good candidate for the treatment of epilepsy.