Abstracts

Rationale:
The concomitant use of multiple anti-seizure medications (ASMs) is common in the management of epilepsy. This study investigated any potential for a pharmacodynamic (PD) interaction for the ASMs cannabidiol (CBD) and sodium valproate (VPA). 

Methods:
The maximal electroshock (MES) model of acute generalized seizure in the mouse was employed to evaluate anticonvulsive properties of each ASM alone and, subsequently, any CBD:VPA PD interaction. Mice were pretreated with either CBD (10-100 mg/kg) or VPA (50-250 mg/kg) before receiving an electrical stimulus using pre-validated fixed current intensity (30 mA, 0.2 sec duration; Eur J Pharmacol 2009; 602: 298-305, Epilepsy Res 2010;90: 188-198) that reliably produced tonic hindlimb extension indicative of generalized seizures in 100% of control animals. Exposures of CBD, its active metabolite 7-OH-CBD and VPA were quantified in plasma and brain using liquid chromatography/ tandem mass spectrometry (LC-MS/MS) bioanalysis. For CBD, the overall class effect of CBD and 7-OH-CBD was estimated in brain after normalizing metabolite potency to that of parent. Using the brain exposure-response relationships derived from treatment with each ASM alone (EE50, effective brain exposures protecting 50% animals), mice were then treated with three fixed ratios of CBD and VPA (1:3, 1:1 and 3:1) calculated using Loewe’s equation of additivity (Naunyn Schmiedebergs Arch Pharmacol 2006;374:51-64) that yield a theoretically derived additive effect (EE50+ADD). An isobolographic method (Eur J Pharmacol 2009; 602:298-305, Pharmacol Rev 2006;58(3):621-81) was then used to assess additivity, antagonism or synergy between CBD and VPA at each ratio, analyzing quantitative measures of the isobologram, combination index (CI) and exposure reduction index (ERI).

Results:
CBD and VPA caused brain exposure-dependent anticonvulsive effects upon the incidence of hindlimb extension. Log(brain exposure)-probit analysis revealed that the EE50 for CBD and VPA were 6.4 and 199.4 μM, respectively. Isobolographic analysis performed on combination of CBD:VPA based on equi-effective bEE50 values ([CBD+7-OH+CBD], [VPA]) revealed PD synergism between CBD:VPA all three EE50+ADD ratios 1:3, 1:1 and 3:1, also reflected by CI values lower than one. For all ratios tested, the ERI was greater than one, indicating a reduction of total exposure required to elicit the same anticonvulsive effect. There were no notable clinical observations observed post dosing, prior to testing.

Conclusions:
CBD administered with VPA produced PD synergism in the MES mouse model of acute generalized seizures.  
Funding:
GW Research Ltd., now part of Jazz Pharmaceuticals Inc.