Discovery of a selective, state-independent inhibitor of Na1.7 by modification of guanidinium toxins.

The voltage-gated sodium channel isoform Na1.7 is highly expressed in dorsal root ganglion neurons and is obligatory for nociceptive signal transmission. Genetic gain-of-function and loss-of-function Na1.7 mutations have been identified in select individuals, and are associated with episodic extreme pain disorders and insensitivity to pain, respectively. These findings implicate Na1.7 as a key pharmacotherapeutic target for the treatment of pain. While several small molecules targeting Na1.7 have been advanced to clinical development, no Na1.7-selective compound has shown convincing efficacy in clinical pain applications. Here we describe the discovery and characterization of ST-2262, a Na1.7 inhibitor that blocks the extracellular vestibule of the channel with an IC of 72 nM and greater than 200-fold selectivity over off-target sodium channel isoforms, Na1.1-1.6 and Na1.8. In contrast to other Na1.7 inhibitors that preferentially inhibit the inactivated state of the channel, ST-2262 is equipotent in a protocol that favors the resting state of the channel, a protocol that favors the inactivated state, and a high frequency protocol. In a non-human primate study, animals treated with ST-2262 exhibited reduced sensitivity to noxious heat. These findings establish the extracellular vestibule of the sodium channel as a viable receptor site for the design of selective ligands targeting Na1.7.