Targeting Of Nav1.6 And Nav1.2 To Inhibit Excitatory Vs Inhibitory Neural Circuits

Sodium channel inhibitors are widely used to treat a variety of neurological disorders of hyperexcitability. Human epilepsy patients with channelopathies such as Dravet Syndrome and GEFS+ reveal that Nav1.1 is the dominant channel in inhibitory circuits while Nav1.6 and Nav1.2 are the dominant channels in excitatory pyramidal neurons. All available Nav targeting anti-epileptic drugs are non-selective and thus should inhibit both excitatory and inhibitory neurons, potentially limiting efficacy and therapeutic safety margins. This non-selectivity is a function of these compounds’ binding-site in the inner pore where key residues in S6 are highly conserved across the orthologues. Xenon has developed a new class of compounds with an unprecedented selectivity profile targeting inhibition of Nav1.6 alone (XEN224), or equipotent inhibition of Nav1.6 and Nav1.2 (XEN462), both with >100 fold selectivity against Nav1.1 and >1000 fold against Nav1.5. Here, we describe the comparative pharmacology of our compounds alongside non-selective Nav inhibitors. Using heterologously expressed Nav's, we show that both XEN compounds produce high affinity inhibition of the target channel with pseudo-first order kinetics, consistent with 1:1 binding. We demonstrate that these compounds are molecularly selective, interacting with a site in Voltage-Sensing Domain-IV that controls inactivation, similar to previously reported selective inhibitors of Nav1.7. The compounds slow recovery from inactivated states in a voltage dependent manner by stabilizing the inactivated-state of the channel. The selective compounds have significantly longer residency times than pore targeting compounds that contribute to higher potency. Finally, we have established that in adult mouse brain slices, the selective targeting of Nav1.6 and Nav1.2 inhibits action potential firing in excitatory neurons, but not in inhibitory interneurons, providing a unique approach for treatment of epilepsy that selectively down regulates excitatory and spares inhibitory networks.