Potent and selective antagonists of the voltage-gated sodium channel Na1.7 represent a promising avenue for the development of new chronic pain therapies. We generated a small molecule atropisomer quinolone sulfonamide antagonist AMG8379 and a less active enantiomer AMG8380. Here we show that AMG8379 potently blocks human Na1.7 channels with an IC of 8.5 nM and endogenous tetrodotoxin (TTX)-sensitive sodium channels in dorsal root ganglion (DRG) neurons with an IC of 3.1 nM in whole-cell patch clamp electrophysiology assays using a voltage protocol that interrogates channels in a partially inactivated state. AMG8379 was 100- to 1000-fold selective over other Na family members, including Na1.4 expressed in muscle and Na1.5 expressed in the heart, as well as TTX-resistant Na channels in DRG neurons. Using an ex vivo mouse skin-nerve preparation, AMG8379 blocked mechanically induced action potential firing in C-fibers in both a time-dependent and dose-dependent manner. AMG8379 similarly reduced the frequency of thermally induced C-fiber spiking, whereas AMG8380 affected neither mechanical nor thermal responses. In vivo target engagement of AMG8379 in mice was evaluated in multiple Na1.7-dependent behavioral endpoints. AMG8379 dose-dependently inhibited intradermal histamine-induced scratching and intraplantar capsaicin-induced licking, and reversed UVB radiation skin burn-induced thermal hyperalgesia; notably, behavioral effects were not observed with AMG8380 at similar plasma exposure levels. AMG8379 is a potent and selective Na1.7 inhibitor that blocks sodium current in heterologous cells as well as DRG neurons, inhibits action potential firing in peripheral nerve fibers, and exhibits pharmacodynamic effects in translatable models of both itch and pain.

Because of its strong genetic validation, Na1.7 has attracted significant interest as a target for the treatment of pain. We have previously reported on a number of structurally distinct bicyclic heteroarylsulfonamides as Na1.7 inhibitors that demonstrate high levels of selectivity over other Na isoforms. Herein, we report the discovery and optimization of a series of atropisomeric quinolinone sulfonamide inhibitors [ Bicyclic sulfonamide compounds as sodium channel inhibitors and their preparation . WO 2014201206, 2014 ] of Na1.7, which demonstrate nanomolar inhibition of Na1.7 and exhibit high levels of selectivity over other sodium channel isoforms. After optimization of metabolic and pharmacokinetic properties, including PXR activation, CYP2C9 inhibition, and CYP3A4 TDI, several compounds were advanced into in vivo target engagement and efficacy models. When tested in mice, compound 39 (AM-0466) demonstrated robust pharmacodynamic activity in a Na1.7-dependent model of histamine-induced pruritus (itch) and additionally in a capsaicin-induced nociception model of pain without any confounding effect in open-field activity.

Human genetic evidence has identified the voltage-gated sodium channel Na V 1.7 as an attractive target for the treatment of pain. We initially identified naphthalene sulfonamide 3 as a potent and selective inhibitor of Na V 1.7. Optimization to reduce biliary clearance by balancing hydrophilicity and hydrophobicity (Log D) while maintaining Na V 1.7 potency led to the identification of quinazoline 16 (AM-2099). Compound 16 demonstrated a favorable pharmacokinetic profile in rat and dog and demonstrated dose-dependent reduction of histamine-induced scratching bouts in a mouse behavioral model following oral dosing. KEYWORDS: Sodium channel, Na V 1.7, Na V 1.5, pain, histamine scratching model H uman genetics has implicated the voltage-gated sodium channel Na V 1.7, which is expressed in nociceptive sensory neurons in dorsal root ganglia (DRG), 1 as a compelling target for pain.2−4 The primary challenge associated with the development of Na V 1.7 inhibitors has historically been achieving selectivity over the other eight Na V isoforms. These isoforms are differentially expressed throughout the body, but inhibition of Na V 1.5, which is expressed in cardiac tissue, is of particular concern as it has been shown to prolong the cardiac QRS wave in humans. 5,6 Previous efforts, including our own, have met with limited success. 4 Here we report the characterization, structure−activity relationship (SAR) and optimization of a series of sulfonamide-derived Na V 1.7 inhibitors. These efforts delivered an isoform-selective compound that was effective in a histamine-induced scratching model that is representative of Na V 1.7 target engagement. Recently Pfizer and Icagen described a series of heteroarylsulfonamide Na V 1.7 inhibitors with high levels of selectivity over Na V 1.5. 7−9 These results were reproduced by our group and are exemplified by compound 1 (Figure 1A). The lack of Na V 1.5 activity was noteworthy, and we envisioned this as a good starting point for our own lead optimization efforts, which would initially be aimed at addressing some of the liabilities and shortcomings associated with this class of compounds. Namely, this series suffered from low passive permeability and high clearance in rodents. We believed constraining the linker within a bicyclic core such as indole 2 or naphthalene 3 would afford a similar conformation and potentially help address the pharmacokinetic liabilities of this class of compounds. An overlay of global minima conformations of compounds 1, 2, and 3 supported this hypothesis ( Figure 1B), 10 and we were pleased to find that 2 and 3 were potent Na V 1.7 inhibitors and showed greater than 200-fold selectivity over Na V 1.5 ( Figure 1C). 11 In general, analogues in the naphthalene series demonstrated superior Na V 1.7 inhibition compared to the corresponding indole analogues, thus the naphthalene scaffold was chosen for further optimization. Additional profiling showed 3 also suffered from high clearance; however, we believed that 3 represented a promising starting poin...

Inhibitors of the voltage-gated sodium channel Na V 1.7 are being investigated as pain therapeutics due to compelling human genetics. We previously identified Na V 1.7-inhibitory peptides GpTx-1 and JzTx-V from tarantula venom screens. Potency and selectivity were modulated through attribute-based positional scans of native residues via chemical synthesis. Herein, we report JzTx-V lead optimization to identify a pharmacodynamically active peptide variant. Molecular docking of peptide ensembles from NMR into a homology model-derived Na V 1.7 structure supported prioritization of key residues clustered on a hydrophobic face of the disulfide-rich folded peptide for derivatization. Replacing Trp24 with 5-Br-Trp24 identified lead peptides with activity in electrophysiology assays in engineered and neuronal cells. 5-Br-Trp24 containing peptide AM-6120 was characterized in X-ray crystallography and pharmacokinetic studies and blocked histamine-induced pruritis in mice after subcutaneous administration, demonstrating systemic Na V 1.7-dependent pharmacodynamics. Our data suggests a need for high target coverage based on plasma exposure for impacting in vivo end points with selectivity-optimized peptidic Na V 1.7 inhibitors.

Several reports have recently emerged regarding the identification of heteroarylsulfonamides as Na1.7 inhibitors that demonstrate high levels of selectivity over other Na isoforms. The optimization of a series of internal Na1.7 leads that address a number of metabolic liabilities including bioactivation, PXR activation, as well as CYP3A4 induction and inhibition led to the identification of potent and selective inhibitors that demonstrated favorable pharmacokinetic profiles and were devoid of the aforementioned liabilities. The key to achieving this within a series prone to transporter-mediated clearance was the identification of a small range of optimal cLogD values and the discovery of subtle PXR SAR that was not lipophilicity dependent. This enabled the identification of compound 20, which was advanced into a target engagement pharmacodynamic model where it exhibited robust reversal of histamine-induced scratching bouts in mice.