Interactions of tiagabine with some antiepileptics in the maximal electroshock in mice

Tiagabine (TGB), a new potent gamma-aminobutyric acid (GABA) uptake inhibitor, is widely applied in adjunctive treatment of partial seizures in humans. Although, polytherapy is not an initial method of epilepsy treatment, clinicians often combine TGB with other antiepileptics as add-on therapy for assuring the anticonvulsant protection in patients with refractory seizures. To evaluate the character of pharmacological interactions between TGB and some antiepileptics, the isobolographic analysis was used as a suitable method for determining the exact types of interactions. Determination of an influence of TGB on the protective effects of diphenylhydantoin (DPH), carbamazepine (CBZ), valproate (VPA), phenobarbital (PB), lamotrigine (LTG), topiramate (TPM), and felbamate (FBM) in maximal electroshock-induced seizures was essential for this study. To exclude or confirm a pharmacokinetic character of observed interactions, the free plasma and brain concentrations of antiepileptic drugs (AEDs) studied were evaluated by using the immunofluorescence or high-pressure liquid chromatography (HPLC).TGB (up to 2.5 mg/kg) remained ineffective upon the electroconvulsive threshold, whilst the drug in doses of 5 and 10 mg/kg significantly raised the electroconvulsive threshold in mice. According to the isobolography, TGB appears to act synergistically with VPA. The remaining combinations tested exerted additive interactions. A pharmacokinetic character of interaction between TGB and VPA was evidently corroborated either in plasma or brains. Moreover, TGB significantly reduced the plasma and brain concentrations of DPH; however, pharmacokinetic events were not accompanied by any changes in anticonvulsant activity of the latter. Finally, the isobolographic analysis revealed that combinations of TGB with VPA exerted synergistic (supra-additive) interaction resulting from a pharmacokinetic interaction.