NBI-921352 (formerly XEN901) is a novel sodium channel inhibitor designed to specifically target Na _V 1.6 channels. Such a molecule provides a precision-medicine approach to target SCN8A -related epilepsy syndromes ( SCN8A -RES), where gain-of-function (GoF) mutations lead to excess Na _V 1.6 sodium current, or other indications where Na _V 1.6 mediated hyper-excitability contributes to disease (Gardella and Moller, 2019; Johannesen et al., 2019; Veeramah et al., 2012). NBI-921352 is a potent inhibitor of Na _V 1.6 (IC ₅₀ 0.051 µM), with exquisite selectivity over other sodium channel isoforms (selectivity ratios of 756X for Na _V 1.1, 134X for Na _V 1.2, 276X for Na _V 1.7, and >583X for Na _V 1.3, Na _V 1.4, and Na _V 1.5). NBI-921352 is a state-dependent inhibitor, preferentially inhibiting activated (inactivated or open) channels. The state dependence leads to potent stabilization of inactivation, inhibiting Na _V 1.6 currents, including resurgent and persistent Na _V 1.6 currents, while sparing the closed/rested channels. The isoform-selective profile of NBI-921352 led to a robust inhibition of action-potential firing in glutamatergic excitatory pyramidal neurons, while sparing fast-spiking inhibitory interneurons, where Na _V 1.1 predominates. Oral administration of NBI-921352 prevented electrically induced seizures in a Scn8a GoF mouse, as well as in wild-type mouse and rat seizure models. NBI-921352 was effective in preventing seizures at lower brain and plasma concentrations than commonly prescribed sodium channel inhibitor antiseizure medicines (ASMs) carbamazepine, phenytoin, and lacosamide. NBI-921352 was well tolerated at higher multiples of the effective plasma and brain concentrations than those ASMs. NBI-921352 is entering phase II proof-of-concept trials for the treatment of SCN8A- developmental epileptic encephalopathy ( SCN8A -DEE) and adult focal-onset seizures.

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