Abstracts

Rationale: Dravet Syndrome is a condition marked by diminished expression of Na_V1.1 in inhibitory neurons, resulting in heightened neuronal excitability due to impaired inhibitory activity. This, in turn, leads to epilepsy and various associated issues such as significant cognitive and motor impairments. The non-seizure symptoms typically appear early in life and worsen with time, imposing a substantial burden on patients. Thus, there is an urgent need for treatments targeting these developmental endpoints.

To meet this clinical demand, we have focused on developing a precision medicine therapy that enhances Na_V1.1 activity exclusively, without interfering with other neuronal functions or proteins. In particular, we're developing potent, isoform-selective small molecule compounds capable of crossing the blood-brain barrier to amplify Na_V1.1 channel currents. Our aim is to have the compounds possess favorable characteristics for central nervous system (CNS) drugs, including allowing for oral administration in vivo. By directly potentiating the remaining Na_V1.1 channels in Dravet Syndrome, these compounds represent a promising avenue for disease-modifying pharmacotherapy.


Methods: Automated patch clamp electrophysiology was used to screen and evaluate the potency, and selectivity of compounds. Compounds were also evaluated in brain slices from Scn1a heterozygous null mice (Scn1a^+/-) to assess the effects on interneuron excitability and synaptic inhibitory and excitatory balance. Efficacy was assessed after oral dosing in Scn1a^+/- mice in a 6Hz electrically induced seizure assay and in a rotorod motor performance assay.


Results: A selected Xenon developed Na_V1.1 compound, XPC-A, exhibited robust enhancement of Na_V1.1 channels, demonstrating more than a 100-fold preference over Na_V1.2, 1.6, and 1.5 variants. Through biophysical analysis, we established that XPC-A disrupts the inactivated state of Na_V1.1 channels. In brain slices obtained from Scn1a^+/- mice, XPC-A increased the firing activity of fast-spiking cortical PV+ interneurons and reinstated the equilibrium between spontaneous excitatory and inhibitory synaptic input to pyramidal neurons. In vivo experiments further show that XPC-A effectively mitigates 6Hz electrically induced seizures in Scn1a^+/- mice and improves their motor performance deficits as evaluated by the rotarod assay.


Conclusions: Potent and selective enhancers of Na_V1.1 can augment the excitability of fast-spiking inhibitory neurons and rebalance excitation in brain slices from Scn1a^+/- mice. Notably, XPC-A significantly enhances motor performance in Scn1a^+/- mice during the rotarod test, supporting the potential to mitigate the non-seizure symptoms linked with Dravet Syndrome. The effectiveness observed in the 6Hz seizure model for XPC-A validated target engagement and restoration of inhibitory activity in the brain via Na_V1.1 potentiation. From these studies, XPC-A emerges as a pioneering mechanism for enhancing voltage-gated sodium channels, offering promise as a therapeutic approach that could modify the course of Dravet Syndrome.

Funding: Xenon Pharmaceuticals