Abstracts

Rationale: During early development, NaV1.2 is the major voltage-gated sodium channel isoform in the axons of excitatory CNS neurons.  As animals mature, the isoform distribution rebalances to contain a higher fraction of NaV1.6 in addition to NaV1.2.  Xenon is currently developing a selective inhibitor of NaV1.6 (XEN901) for epilepsy based on the hypothesis that nonselective sodium channel inhibitors like carbamazepine and phenytoin would be better therapeutics if they inhibited action potential firing in excitatory neurons (primarily driven by NaV1.2 and NaV1.6 channels) without inhibiting firing of inhibitory interneurons (primarily NaV1.1 channels) and also avoiding potential cardiac liabilities by not inhibiting NaV1.5.  Because of the dynamic nature of sodium channel expression during development, some patients, particularly very young patients in the first months of life, may benefit more from a dual acting drug that blocks both NaV1.2 and NaV1.6 than from a purely selective inhibitor of NaV1.6.  Specifically, NaV1.2 gain-of-function patients (EIEE11) would likely benefit from such a compound. Sparing block of NaV1.1 and NaV1.3 is anticipated to be a benefit for patients as these channels drive the firing of inhibitory interneurons in adults and neonates, respectively.  Based on this theoretical framework we have created a dual NaV1.2/NaV1.6 inhibitor (XEN393) that has drug-like properties and effectively prevents electrically-induced seizures in mice and rats.  Methods: We created a novel small molecule sodium channel inhibitor, XEN393. Patch-clamp electrophysiology was used to measure the potency and selectivity of XEN393 in voltage-gated sodium channel isoforms. In vivo, we evaluated the ability of orally administered XEN393 to prevent electrically induced seizures in multiple electrically induced rodent seizure models. Wild type CF-1 mice were assessed in two assays that evoked a tonic-clonic seizure with hind-limb extension in 90% of vehicle-treated mice, one using a direct current stimulation and another using alternating current stimulation. Analogous experiments were repeated in rats.  Results: XEN393 was a potent inhibitor of both human NaV1.2 (IC50 = .005 µM) and NaV1.6 (IC50 = 0.01 µM) but spared NaV1.1 and NaV1.5 (IC50's > 100 µM). XEN393 prevented electrically induced seizures invoked in both mice and rats with both direct or alternating current. The brain levels required for protection in mice stimulated with alternating current (EC70 ~ 0.3 µM) were higher than in rats or in mice stimulated with direct current (EC70 ~ 0.1 µM).  Conclusions: XEN393 is a novel, mechanistically differentiated, selective inhibitor of NaV1.2 and NaV1.6 channels. Brain concentrations of XEN393 that block the target channels but spare off-target channels like NaV1.1 were well tolerated and prevented electrical induced seizures in mice and rats. This work enables the testing of the hypothesis that dual selective NaV1.2/NaV1.6 inhibitors, such as XEN393, that spare NaV1.1, NaV1.3, and NaV1.5 may provide a new class of more effective seizure prevention AEDs that may be better tolerated in clinical practice.  Funding: Xenon Pharmaceuticals