Voltage-gated sodium ion channel subtype 1.7 (NaV1.7) is a high interest target for the discovery of non-opioid analgesics. Compelling evidence from human genetic data, particularly the finding that persons lacking functional NaV1.7 are insensitive to pain, has spurred considerable effort to develop selective inhibitors of this Na+ ion channel target as analgesic medicines. Recent clinical setbacks and disappointing performance of preclinical compounds in animal pain models, however, have led to skepticism around the potential of selective NaV1.7 inhibitors as human therapeutics. In this Perspective, we discuss the attributes and limitations of recently disclosed investigational drugs targeting NaV1.7 and review evidence that, by better understanding the requirements for selectivity and target engagement, the opportunity to deliver effective analgesic medicines targeting NaV1.7 endures.

The waters of the "red tide" are awash with noxious agents, the most infamous of which are the paralytic shellfish poisons (PSPs). 1 Small molecule, bis-guanidinium structuressaxitoxin, neosaxitoxin, and the gonyautoxins -unique in both their form and function, represent the principle constituents of PSPs. 2 These highly polar, heteroatom-rich compounds are exquisitely designed corks that act to stopper ion flux through voltage-gated Na + channels (Na V ), thus inhibiting electrical conduction in cells. 3 The intricate molecular shape common to these toxins coupled with their importance as pharmacological tools for ion channel study have inspired efforts aimed at their de novo assembly. Three prior works have described preparations of saxitoxin (STX) and one a decarbamoyloxy form. 4,5 The first synthetic path to any member of the more than 20 known sulfated poisons, gonyautoxin 3 (GTX 3), is outlined in this report (Figure 1). 6,7The five-membered cyclic guanidine in GTX 3 became the focal point of our synthetic analysis following our recent disclosure of an oxidative method for 2-aminoimidazoline formation. 8 This transformation is thought to proceed through the intermediacy of a Rh-bound guanidine nitrene, a reactive species capable of modifying both C-H and π-bonds. For the purpose of crafting GTX 3, amination of a pyrrole nucleus by the guanidine nitrenoid presented a novel application of this technology (Figure 1). Such a reaction could occur through either a strained aziridine 3 or dipolar species 4, attack of which by a nucleophile at either C10 or C12 would generate the desired tricyclic core. 9 This regiochemical issue notwithstanding, such a strategy simplifies the GTX problem to a rather unassuming bicyclic intermediate 1. Pursuant to this approach, a route to bis-guanidine 1 was formulated that would exploit an intramolecular addition of a pyrrole to an activated imine. Although limited in precedent, this type of PictetSpengler reaction could be quickly evaluated, as the necessary precursor 2 is easily accessed from serine.The synthesis of GTX 3 commences with a three-step sequence that transforms L-serine methyl ester to aldehyde 5 (Scheme 1). 10 Condensation of this aldehyde with allylamine is followed by treatment with BF 3 · OEt 2 , which effects the desired ring closure to furnish the transsubstituted urea 6 with >20:1 diastereoselectivity. 11 Assuming the C5/C6 stereochemistry (GTX numbering) in this product is established under kinetic control, a conformation that minimizes allylic strain between the substituents on C6 and N7 could account for the observed sense of induction. Forwarding 6 to the requisite amination precursor 7 was efficiently achieved through a sequence of four transformations; of note is the development of a single step process for sequential allyl deprotection and isothiourea formation (cf., step e, 6→7, Tces = SO 3 CH 2 CCl 3 ). Successful application of the Rh-catalyzed amination reaction with guanidine 7 assembles the tricyclic frame of GTX 3 in a singular, defining event...

Determining permeability of a given compound through human skin is a principal challenge owing to the highly complex nature of dermal tissue. We describe the application of an ambient mass spectrometry imaging method for visualizing skin penetration of sodium channel modulators, including novel synthetic analogs of natural neurotoxic alkaloids, topically applied ex vivo to human skin. Our simple and label-free approach enables successful mapping of the transverse and lateral diffusion of small molecules having different physicochemical properties without the need for extensive sample preparation.

The voltage-gated sodium channel isoform NaV1.7 is highly expressed in dorsal root ganglion neurons and is obligatory for nociceptive signal transmission. Genetic gain-of-function and loss-of-function NaV1.7 mutations have been identified in select individuals, and are associated with episodic extreme pain disorders and insensitivity to pain, respectively. These findings implicate NaV1.7 as a key pharmacotherapeutic target for the treatment of pain. While several small molecules targeting NaV1.7 have been advanced to clinical development, no NaV1.7-selective compound has shown convincing efficacy in clinical pain applications. Here we describe the discovery and characterization of ST-2262, a NaV1.7 inhibitor that blocks the extracellular vestibule of the channel with an IC50 of 72 nM and greater than 200-fold selectivity over off-target sodium channel isoforms, NaV1.1–1.6 and NaV1.8. In contrast to other NaV1.7 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 NaV1.7.

The paralytic shellfish poisons are a collection of guanidine-containing natural products that are biosynthesized by prokaryote and eukaryote marine organisms. These compounds bind and inhibit isoforms of the mammalian voltage-gated Na(+) ion channel at concentrations ranging from 10(-11) to 10(-5) M. Here, we describe the de novo synthesis of three paralytic shellfish poisons, gonyautoxin 2, gonyautoxin 3, and 11,11-dihydroxysaxitoxin. Key steps include a diastereoselective Pictet-Spengler reaction and an intramolecular amination of an N-guanidyl pyrrole by a sulfonyl guanidine. The IC50's of GTX 2, GTX 3, and 11,11-dhSTX have been measured against rat NaV1.4, and are found to be 22 nM, 15 nM, and 2.2 μM, respectively.