Isobolographic and subthreshold methods in the detection of interactions between oxcarbazepine and conventional antiepileptics--a comparative study.

Until now, a character of interactions among the antiepileptic drugs (AEDs), in some experimental models of epilepsy, has been determined alternatively with subthreshold and isobolographic methods. In order to elicit the precise and adequate method for evaluating two drug interactions, the comparative study was performed in the maximal electroshock-induced seizure test in mice. In this experimental model, the exact types of interactions among oxcarbazepine (OXC) and conventional AEDs (diphenylhydantoin, phenobarbital, valproate, carbamazepine, and clonazepam) were determined with both methods. Results from the subthreshold method showed a considerable reduction of ED(50) values of clonazepam, diphenylhydantoin and valproate (after administration of OXC at the highest subthreshold dose of 2.5 mg/kg), whilst ED(50)s of carbamazepine or phenobarbital were almost unchanged when OXC (2.5 mg/kg) was co-administered with these AEDs. Results from the 2-dimensional (2-D) isobolographic analysis of interactions for a 50% anticonvulsant effect, for three fixed drug dose ratio combinations of 1:2, 1:1, and 2:1, indicate antagonism between OXC and diphenylhydantoin as regards their anticonvulsant (protective) activity. Furthermore, the interactions between OXC and clonazepam occurred either antagonistic (for the fixed-ratios of 1:4 and 1:3) or synergistic (for the fixed-ratio combinations of 1:1 and 2:1) depending on the proportions of used drugs. Remaining interactions between OXC and carbamazepine, OXC and valproate, or OXC and phenobarbital (for the fixed-ratios of 1:3, 1:1, and 3:1) were isobolographically additive for a 50% anticonvulsant effect tested. The 3-dimensional (3-D) isobolographic analysis of interactions between OXC and CZP revealed that the dual character of interactions (antagonistic and synergistic) observed for a 50% anticonvulsant effect (ED(50)) was also present for additional drug-dose effects tested, i.e. ED(16) and ED(84). The 3-D isobologram for the combination of OXC with CZP clearly visualized either synergy or antagonism between the drugs in combinations.Distinct differences resulting from two experimental methods prove evidently the superiority of isobolographic analysis over the subthreshold method. The former clearly and adequately detects the exact types of interactions between two AEDs, becoming a potent and powerful paradigm for further studies evaluating the character of interactions among AEDs.