Abstracts

Rationale: Mature CNS neurons predominantly express three voltage gated sodium channels, NaV1.1 (SCN1A), NaV1.2 (SCN2A), and NaV1.6 (SCN8A).  NaV1.1 is the dominant isoform in inhibitory interneurons, and block of NaV1.1 is expected to be proconvulsant since patients with heterozygous null mutations in the SCN1A gene encoding NaV1.1 have Early Infantile onset Epileptic Encephalopathy 6 (EIEE6, a.k.a. Dravet Syndrome).  NaV1.2 and NaV1.6 are both highly expressed in axon initial segment of excitatory neurons, and gain of function mutations in both genes have been linked to epilepsy syndromes (EIEE11 and EIEE13, respectively). Recombinant mice with a heterozygous patient identified SCN8A mutation have a gain of function variant NaV1.6 channel (N1768D+/-) and display symptoms of epilepsy (Wagnon et al., 2015. H. Mol. Genet. 24(2):506-15).  Mice have reduced seizure thresholds, spontaneous seizures, and sudden death. Methods:   We evaluated the ability of NaV1.6 inhibitors to prevent seizures in a 6 Hz psychomotor seizure assay (12 mA stimulating current) in N1768D+/- mice. We also tested the same compounds in wild type CF-1 mice using the maximal electroshock assay (MES).   Results: Drugs with little NaV selectivity, novel compounds that block NaV1.6 and NaV1.2, but spare NaV1.1, and novel NaV1.6 selective compounds all exhibited robust (70%) protection of seizures in N1768D+/- mice when the brain levels of the test compound reached levels near the IC50 for in vitro inhibition of NaV1.6.  This could be expected since the seizure phenotype is caused by excess NaV1.6 current. The effective brain concentration in wild type mice was closely aligned to that efficacious in N1768D+/- mice for all tested NaV inhibitors, regardless of whether the inhibitor was NaV1.6 selective, NaV1.6 + NaV1.2 selective, or non-selective.    Conclusions: This suggests that inhibition of NaV1.6 is the primary driver of efficacy in both wild type and N1768D+/- models, and that inhibition of additional channels, such as NaV1.2 and NaV1.1 does not greatly impact compound efficacy.  Increased selectivity for NaV1.6 inhibition is predicted to reduce the likelihood of adverse events due to the block of other ion channel subtypes. Our data indicate that improved NaV1.6 selectivity does not compromise the efficacy of seen with pan-NaV channel inhibitors.  Funding: This work was entirely funded by Xenon Pharmaceuticals, Inc. All authors are employed by Xenon Pharmaceuticals, Inc.