Identification and characterization of selective inhibitors of Nav1.9 voltage dependent sodium channels

Voltage-gated sodium channels play fundamental roles in physiological and pathophysiological electrical excitability of the nervous and muscular systems. This involves several sodium channel subtypes, including Nav1.9, which has been genetically linked to altered pain sensation in humans. This association makes Nav1.9 a promising target for new pain therapies. However, progress has been limited by the historical challenges of expressing functional recombinant Nav1.9 as well as the absence of selective pharmacological tools. The current study expands on previous success of: (1) functionally expressing recombinant Nav1.9; (2) developing electrophysiological assays which enabled a more detailed assessment of the sensitivity of Nav1.9 to existing sodium channel modulators and; (3) screening for, identifying and characterizing novel potent Nav1.9 subtype selective sodium channel inhibitors. This study shows that previously described sodium channels inhibitors exhibit similar sensitivity and gating dependence at Nav1.9 as to other Nav1.x family members. This study also describes the identification of several novel Nav1.9 subtype selective inhibitors including ICA604025, which inhibits human Nav1.9 in a state dependent manner, with EC50s of 9 nM and 1.1 μM for the activated/inactivated state versus resting closed state. ICA604025 exhibits a similar potency for inhibition of cynomolgus monkey Nav1.9 but ∼50-fold lower potency for inhibition of rodent Nav1.9. It is a 300- to 1800-fold more potent inhibitor of human Nav1.9 compared with other human Nav1.x family members. In conclusion, this work demonstrates that subtype selective inhibition of Nav1.9 is achievable and offers valuable tools for further exploration of its physiological role and therapeutic potential. SIGNIFICANCE STATEMENT: Nav1.9 is a voltage-gated sodium channel genetically associated with human pain disorders and thus is a key target for new analgesics. Several novel potent and highly subtype selective Nav1.9 inhibitors have been identified that may enable further advances in pain research and therapeutic development.