Voltage-gated sodium channels, transmembrane proteins that initiate action potentials in nerve, muscle and other electrically excitable cells, are a necessary component of normal sensation, emotions, thoughts and movements (Catterall, W. A., Nature (2001), Vol. 409, pp. 988-990). These channels consist of a highly processed alpha subunit that is associated with auxiliary beta subunits. The pore-forming alpha subunit is sufficient for channel function, but the kinetics and voltage dependence of channel gating are in part modified by the beta subunits (Goldin et al., Neuron (2000), Vol. 28, pp. 365-368). Each alpha-subunit contains four homologous domains, I to IV, each with six predicted transmembrane segments. The alpha-subunit of the sodium channel, forming the ion-conducting pore and containing the voltage sensors regulating sodium ion conduction has a relative molecular mass of 260,000. Electrophysiological recording, biochemical purification, and molecular cloning have identified nine different sodium channel alpha subunits and four beta subunits (Yu, F. H., et al., Sci. STKE (2004), 253; and Yu, F. H., et al., Neurosci. (2003), 20:7577-85).
The hallmarks of sodium channels include rapid activation and inactivation when the voltage across the plasma membrane of an excitable cell is depolarized (voltage-dependent gating), and efficient and selective conduction of sodium ions through conducting pores intrinsic to the structure of the protein (Sato, C., et al., Nature (2001), 409:1047-1051). At negative or hyperpolarized membrane potentials, sodium channels are closed. Following membrane depolarization, sodium channels open rapidly and then inactivate. Channels only conduct currents in the open state and, once inactivated, have to return to the resting state, favored by membrane hyperpolarization, before they can reopen. Different sodium channel subtypes vary in the voltage range over which they activate and inactivate as well as their activation and inactivation kinetics. Alterations in the gating mechanism of sodium channels can contribute to disease. A loss of the ability to inactivate results in a sustained influx of sodium into cells. This process is termed a persistent sodium current. Depending on the amplitude of this current changes in cell excitability or triggered cell death can occur.
The sodium channel family of proteins has been extensively studied and shown to be involved in a number of vital body functions. Research in this area has identified variants of the alpha subunits that result in major changes in channel function and activities, which can ultimately lead to major pathophysiological conditions. Implicit with function, this family of proteins is considered a prime point of therapeutic intervention. Nav1.1 and Nav1.2 are highly expressed in the brain (Raymond, C. K., et al., J. Biol. Chem. (2004), 279 (44):46234-41) and retina, and are vital to normal brain function. In humans, mutations in Nav1.1 and Nav1.2 result in severe epileptic states and in some cases mental decline (Rhodes, T. H., et al., Proc. Natl. Acad. Sci. USA (2004), 101(30):11147-52; Kamiya, K., et al., J. Biol. Chem. (2004), 24(11):2690-8; Pereira, S., et al., Neurology (2004), 63(1):191-2). As such both channels have been considered as validated targets for the treatment of epilepsy (see PCT Published Patent Publication No. WO 01/38564).
Voltage-gated sodium channels comprise a family of proteins designated from Nav1.3 through Nav1.9. The sodium channel isoforms show differential expression throughout the central and peripheral nervous system. (Catterall, W. A., Nature (2001), Vol. 409, pp. 988-990; Goldin et al., Neuron (2000), Vol. 28, pp. 365-368; Ehring, George R., et al. “Diversity of Expression of Voltage-Gated Sodium Channels in the Rat Retina.” ARVO Meeting Abstracts 53.6 (2012): 5337; O'Brien, B. J., et al. “Tetrodotoxin-resistant voltage-gated sodium channels Na(v)1.8 and Na(v)1.9 are expressed in the retina.” J Comp Neurol 508.6 (2008): 940-51).
U.S. Pat. No. 7,309,716 (assigned to Vertex Pharmaceuticals) discloses benzimidazole compounds that are useful as inhibitors of voltage-gated sodium channels.
U.S. Pat. Nos. 7,125,908, 7,763,651, 7,767,718, and 8,153,645 (all assigned to Allergan) disclose methods of treating chronic pain in a mammal by administering to the mammal an effective amount of a selective persistent sodium channel antagonist that has at least 20-fold selectivity for persistent sodium current relative to transient sodium current.
U.S. Pat. No. 7,754,440 (assigned to Allergan) discloses a method for identifying a selective persistent Na+ channel blocker by measuring the ability of the blocker to reduce or inhibit a persistent Na+ current to a greater degree than a transient Na+ current. Aspects of the present method provide Na+ depletion/repletion methods for identifying a selective blocker of a persistent Na+ channel, hyperpolarization methods for identifying a blocker of a persistent Na+ channel, and Na/K ATPase pump inhibitor methods for identifying a selective blocker of a persistent Na+ channel.