Interactions between tiagabine and conventional antiepileptic drugs in the rat model of complex partial seizures.

This study evaluated the interactions between tiagabine (TGB) and three conventional antiepileptic drugs (AEDs): valproate (VPA), carbamazepine (CBZ), and phenobarbital (PB) in amygdala-kindled rats, a reliable model of complex partial seizures in humans. Isobolographic analysis of interactions revealed that TGB interacted additively with all tested conventional AEDs for the fixed-ratio combinations of 1:3, 1:1, and 3:1. Evaluation of pharmacokinetic interactions between AEDs revealed that the observed additivity was pharmacodynamic in nature. TGB did not affect the plasma and brain concentrations of VPA, CBZ and PB. Similarly, none of the studied conventional AEDs changed the plasma or brain levels of TGB. Also, TGB, VPA, CBZ, and PB administered alone (at their median effective doses) and in combinations at the fixed-ratio of 1:1 did not impair motor performance evaluated in the chimney test. In conclusion, additivity between TGB and conventional AEDs in amygdala-kindled rats and lack of bidirectional pharmacokinetic interactions suggest that TGB appears to be a valuable drug for an add-on therapy of refractory complex partial seizures in humans.

Effect of MPEP, a selective mGluR5 antagonist, on the antielectroshock activity of conventional antiepileptic drugs.

MPEP, a selective non-competitive antagonist of group I metabotropic glutamate receptor subtype 5 (mGluR5), administered at doses ranging from 0.75 to 1 mg/kg, failed to influence the electroconvulsive threshold in mice. However, when administered at higher doses (1.25 and 1.5 mg/kg), it significantly increased the threshold. Moreover, MPEP (applied at its highest subprotective dose of 1 mg/kg) did not affect the protective action of valproate, carbamazepine, diphenylhydantoin and phenobarbital against maximal electroshock-induced seizures in mice. The presented results indicate that mGluR5 antagonists should not be considered as good candidates for add-on therapy of generalized seizures.

Low-affinity kainate receptor-mediated events reduce the protective activity of phenobarbital and diphenylhydantoin against maximal electroshock in mice.

(2S,2R)-4-Methylglutamic acid (SYM 2081), a potent selective agonist of GluR5 and GluR6 kainate receptor subtypes, applied at the dose of 15.5 mg/kg, equal to its CD(16) value (i.e., a dose required to induce convulsions in 16% of mice), significantly decreased the electroconvulsive threshold from 7.0 to 5.8 mA. When administered at the dose of 11.5 mg/kg, equal to 75% of its CD(16), it markedly attenuated the protective activity of phenobarbital and diphenylhydantoin, but not that of valproate, carbamazepine, or diazepam against maximal electroshock-induced seizures in mice. The respective ED(50) values were increased from 18.5 to 23.8 mg/kg for phenobarbital, and from 11.7 to 14.7 mg/kg for diphenylhydantoin. Since the free plasma levels of both antiepileptic drugs were not influenced by SYM 2081, the pharmacokinetic interaction does not seem to be involved in the observed results. In conclusion, low-affinity kainate receptor-mediated events might be a factor reducing the protective efficacy of some antiepileptic drugs. Furthermore, the activation of GluR5 and GluR6 kainate receptor subtypes by endogenous glutamate during seizures may be associated with the drug-resistance phenomenon.