Isobolographic and subthreshold analysis of interactions among felbamate and four conventional antiepileptic drugs in pentylenetetrazole-induced seizures in mice

PURPOSE: Despite possibility of idiosyncratic reaction development, felbamate (FBM) is recommended in Lennox-Gastaut syndrome and partial refractory epilepsy. The aim of this study was to evaluate the profile of interactions between FBM and four conventional antiepileptic drugs (AEDs): clonazepam (CZP), ethosuximide (ESM), phenobarbital (PB), and valproate (VPA), in pentylenetetrazole (PTZ)-induced convulsions in mice, a model of myoclonic seizures in humans. METHODS: Data obtained from PTZ-evoked seizures were compared by use of two basic procedures, the subthreshold method and isobolographic analysis. Results of the chimney test (evaluating motor coordination) also were elaborated isobolographically. Thus it was possible to determine both median toxic dose (TD50) and protective index (PI) for each drug combination. RESULTS: FBM reduced the clonic seizure activity [with an ED50 of 9.7 mg/kg; TD50, 439.1 mg/kg; and PI, 45.3]. FBM at the dose of 10 mg/kg, but not 7.5 mg/kg, significantly reduced PTZ-induced convulsions in mice. In the subthreshold method, FBM (7.5 mg/kg) did not affect the protective activity of conventional AEDs used in the study. However, when applied at 10 mg/kg, it enhanced the protective activity of PB and ESM, but not that of VPA or CZP. The nature of these interactions could not be precisely estimated with this method. The exact profile of drug interactions was determined with the use of isobolography. In terms of seizure inhibition, antagonism was found between FBM and VPA applied at the fixed-dose ratio of 3:1. Synergy was detected between FBM and PB (1:3). Combinations of FBM with VPA (1:3, 1:1), PB (1:1, 3:1), and ESM or CZP (1:3, 1:1, 3:1) led to additive interactions. As regards motor impairment, the combinations of FBM with VPA (1:3) or CZP (1:1, 3:1) were synergistic. Remaining combinations exhibited pure additivity. Pharmacokinetic events may influence FBM/ESM and FBM/CZP interactions, because FBM lowered the brain concentration of ESM and increased that of CZP. CONCLUSIONS: The profitable benefit index was found only for the combination of FBM with PB (1:3). Conversely, the combinations of FBM with either VPA (1:3) or CZP (1:1, 3:1) do not seem promising for the therapy of refractory myoclonic convulsions. Isobolographic analysis provides more reliable clues to be considered by the clinicians willing to introduce AED combinations for the therapy of epilepsy.