Isobolographic characterization of interactions of retigabine with carbamazepine, lamotrigine, and valproate in the mouse maximal electroshock-induced seizure model

The aim of this study was to characterize the pharmacodynamic, pharmacokinetic, and adverse-effect profiles of retigabine (RTG) in combination with carbamazepine (CBZ), lamotrigine (LTG), and valproate (VPA). The isobolographic analysis for parallel and nonparallel dose–response effects was used in the mouse maximal electroshock seizure (MES) model for evaluation of pharmacodynamic interaction. Potential adverse-effect profiles of interactions of RTG with CBZ, LTG, and VPA at the fixed ratio of 1:1 in the MES test were evaluated in the chimney (motor performance), passive avoidance (long-term memory), and grip strength (muscular strength) tests. Free plasma and total brain concentrations of CBZ, LTG, and VPA were determined by immunofluorescence and chromatography to assess pharmacokinetic interaction. In the MES model, RTG administered singly had its dose–response relationship curve (DRRC) parallel to that for VPA and nonparallel to that for CBZ and LTG. With isobolography for parallel DRRCs, the combination of RTG with VPA at fixed ratios of 1:3, 1:1, and 3:1 exerted supraadditive (synergistic) interaction. Isobolography for nonparallel DRRCs revealed that the combinations of RTG with CBZ and LTG at the fixed ratio of 1:1 produced additive interaction. In all combinations, neither motor coordination, long-term memory, nor muscular strength were affected. Only the combination of RTG with VPA at the fixed ratio of 3:1 was complicated by a pharmacokinetic increase in both free plasma and total brain VPA concentrations. All remaining combinations of RTG with VPA, CBZ, and LTG were pharmacodynamic in nature. RTG synergistically interacted with VPA and exerted additive interaction with CBZ and LTG in the mouse MES model.