Anti-Epileptic Drug Combination Efficacy in an In Vitro Seizure Model - Phenytoin and Valproate, Lamotrigine and Valproate

In this study, we investigated the relative efficacy of different classes of commonly used anti-epileptic drugs (AEDs) with different mechanisms of action, individually and in combination, to suppress epileptiform discharges in an in vitro model. Extracellular field potential were recorded in 450 mu m thick transverse hippocampal slices prepared from juvenile Wistar rats, in which "epileptiform discharges" (ED's) were produced with a high-K+ (8.5 mM) bicarbonate-buffered saline solution. Single and dual recordings in stratum pyramidale of CA1 and CA3 regions were performed with 3-5 M Omega glass microelectrodes. All drugs-lamotrigine (LTG), phenytoin (PHT) and valproate (VPA)-were applied to the slice by superfusion at a rate of 2 ml/min at 32 degrees C. Effects upon frequency of ED's were assessed for LTG, PHT and VPA applied at different concentrations, in isolation and in combination. We demonstrated that high-K+ induced ED frequency was reversibly reduced by LTG, PHT and VPA, at concentrations corresponding to human therapeutic blood plasma concentrations. With a protocol using several applications of drugs to the same slice, PHT and VPA in combination displayed additivity of effect with 50 mu M PHT and 350 mu M VPA reducing SLD frequency by 44% and 24% individually (n = 19), and together reducing SLD frequency by 66% (n = 19). 20 mu M LTG reduced SLD frequency by 32% and 350 mu M VPA by 16% (n = 18). However, in combination there was a supra-linear suppression of ED's of 64% (n = 18). In another independent set of experiments, similar results of drug combination responses were also found. In conclusion, a combination of conventional AEDs with different mechanisms of action, PHT and VPA, displayed linear additivity of effect on epileptiform activity. More intriguingly, a combination of LTG and VPA considered particularly efficacious clinically showed a supra-additive suppression of ED's. This approach may be useful as an in vitro platform for assessing drug combination efficacy.