The Selective Nav1.7 Inhibitor, PF-05089771, Interacts Equivalently with Fast and Slow Inactivated Nav1.7 Channels

Sodium channels as a new target for pain treatment

Voltage-gated sodium channels, especially the Nav1.7, Nav1.8, and Nav1.9 subtypes, play a crucial role in the transmission of pain signals. Nav1.7 is considered a threshold channel that regulates the generation of action potentials and is closely associated with various hereditary pain disorders. Nav1.8 primarily participates in inflammatory and neuropathic pain within the peripheral nervous system. Its characteristic of not involving the central nervous system makes it a potential target for non-addictive analgesics. Nav1.9 has shown significant involvement in cold pain sensing and small fiber neuropathy, although its mechanism of action is still under further investigation. Currently, despite promising results from preclinical studies, sodium channel inhibitors have not fully met expectations in clinical trials due to issues such as drug selectivity, dosing, and safety. The development of Nav1.7 and Nav1.8 inhibitors faces challenges such as drug intolerance, insufficient target occupancy, and off-target side effects. Future research may promote the development of non-opioid analgesics through combined inhibition strategies targeting multiple Nav subtypes, as well as improving drug selectivity and bioavailability. Overall, sodium channel inhibitors remain a key area of research in pain management, but their clinical application prospects still require further exploration.

Unique electrophysiological property of a novel Nav1.7, Nav1.8, and Nav1.9 sodium channel blocker, ANP-230

Voltage-gated sodium channel subtypes, Nav1.7, Nav1.8, and Nav1.9 are predominantly expressed in peripheral sensory neurons. Recent genetic studies have revealed that they are involved in pathological pain processing and that the blockade of Nav1.7, Nav1.8, or Nav1.9 will become a promising pharmacotherapy especially for neuropathic pain. A growing number of drug discovery programs have targeted either of the subtypes to obtain a selective inhibitor which can provide pain relief without affecting the cardiovascular and central nervous systems, though none of them has been approved yet. Here we describe the in vitro characteristics of ANP-230, a novel sodium channel blocker under clinical development. Surprisingly, ANP-230 was shown to block three pain-related subtypes, human Nav1.7, Nav1.8, and Nav1.9 with similar potency, but had only low inhibitory activity to human cardiac Nav1.5 channel and rat central Nav channels. The voltage clamp experiments using different step pulse protocols revealed that ANP-230 had a "tonic block" mode of action without state- and use-dependency. In addition, ANP-230 caused a depolarizing shift of the activation curve and decelerated gating kinetics in human Nav1.7-stably expressing cells. The depolarizing shift of activation curve was commonly observed in human Nav1.8-stably expressing cells as well as rat dorsal root ganglion neurons. These data suggested a quite unique mechanism of Nav channel inhibition by ANP-230. Finally, ANP-230 reduced excitability of rat dorsal root ganglion neurons in a concentration dependent manner. Collectively, these promising results indicate that ANP-230 could be a potent drug for neuropathic pain.

Nanomedicine and voltage-gated sodium channel blockers in pain management: a game changer or a lost cause?

Pain, a complex and debilitating condition affecting millions globally, is a significant concern, especially in the context of post-operative recovery. This comprehensive review explores the complexity of pain and its global impact, emphasizing the modulation of voltage-gated sodium channels (VGSC or NaV channels) as a promising avenue for pain management with the aim of reducing reliance on opioids. The article delves into the role of specific NaV isoforms, particularly NaV 1.7, NaV 1.8, and NaV 1.9, in pain process and discusses the development of sodium channel blockers to target these isoforms precisely. Traditional local anesthetics and selective NaV isoform inhibitors, despite showing varying efficacy in pain management, face challenges in systemic distribution and potential side effects. The review highlights the potential of nanomedicine in improving the delivery of local anesthetics, toxins and selective NaV isoform inhibitors for a targeted and sustained release at the site of pain. This innovative strategy seeks to improve drug bioavailability, minimize systemic exposure, and optimize therapeutic outcomes, holding significant promise for secure pain management and enhancing the quality of life for individuals recovering from surgical procedures or suffering from chronic pain.

Similar excitability through different sodium channels and implications for the analgesic efficacy of selective drugs

Nociceptive sensory neurons convey pain-related signals to the CNS using action potentials. Loss-of-function mutations in the voltage-gated sodium channel NaV1.7 cause insensitivity to pain (presumably by reducing nociceptor excitability) but clinical trials seeking to treat pain by inhibiting NaV1.7 pharmacologically have struggled. This may reflect the variable contribution of NaV1.7 to nociceptor excitability. Contrary to claims that NaV1.7 is necessary for nociceptors to initiate action potentials, we show that nociceptors can achieve similar excitability using different combinations of NaV1.3, NaV1.7, and NaV1.8. Selectively blocking one of those NaV subtypes reduces nociceptor excitability only if the other subtypes are weakly expressed. For example, excitability relies on NaV1.8 in acutely dissociated nociceptors but responsibility shifts to NaV1.7 and NaV1.3 by the fourth day in culture. A similar shift in NaV dependence occurs in vivo after inflammation, impacting ability of the NaV1.7-selective inhibitor PF-05089771 to reduce pain in behavioral tests. Flexible use of different NaV subtypes exemplifies degeneracy - achieving similar function using different components - and compromises reliable modulation of nociceptor excitability by subtype-selective inhibitors. Identifying the dominant NaV subtype to predict drug efficacy is not trivial. Degeneracy at the cellular level must be considered when choosing drug targets at the molecular level.

Nav1.7 Modulator Bearing a 3-Hydroxyindole Backbone Holds the Potential to Reverse Neuropathic Pain

Chronic pain is a growing global health problem affecting at least 10% of the world's population. However, current chronic pain treatments are inadequate. Voltage-gated sodium channels (Navs) play a pivotal role in regulating neuronal excitability and pain signal transmission and thus are main targets for nonopioid painkiller development, especially those preferentially expressed in dorsal root ganglial (DRG) neurons, such as Nav1.6, Nav1.7, and Nav1.8. In this study, we screened in virtual hits from dihydrobenzofuran and 3-hydroxyoxindole hybrid molecules against Navs via a veratridine (VTD)-based calcium imaging method. The results showed that one of the molecules, 3g, could inhibit VTD-induced neuronal activity significantly. Voltage clamp recordings demonstrated that 3g inhibited the total Na+ currents of DRG neurons in a concentration-dependent manner. Biophysical analysis revealed that 3g slowed the activation, meanwhile enhancing the inactivation of the Navs. Additionally, 3g use-dependently blocked Na+ currents. By combining with selective Nav inhibitors and a heterozygous expression system, we demonstrated that 3g preferentially inhibited the TTX-S Na+ currents, specifically the Nav1.7 current, other than the TTX-R Na+ currents. Molecular docking experiments implicated that 3g binds to a known allosteric site at the voltage-sensing domain IV(VSDIV) of Nav1.7. Finally, intrathecal injection of 3g significantly relieved mechanical pain behavior in the spared nerve injury (SNI) rat model, suggesting that 3g is a promising candidate for treating chronic pain.

Structural mapping of Nav1.7 antagonists

Voltage-gated sodium (Na_v) channels are targeted by a number of widely used and investigational drugs for the treatment of epilepsy, arrhythmia, pain, and other disorders. Despite recent advances in structural elucidation of Na_v channels, the binding mode of most Na_v-targeting drugs remains unknown. Here we report high-resolution cryo-EM structures of human Na_v1.7 treated with drugs and lead compounds with representative chemical backbones at resolutions of 2.6-3.2 Å. A binding site beneath the intracellular gate (site BIG) accommodates carbamazepine, bupivacaine, and lacosamide. Unexpectedly, a second molecule of lacosamide plugs into the selectivity filter from the central cavity. Fenestrations are popular sites for various state-dependent drugs. We show that vinpocetine, a synthetic derivative of a vinca alkaloid, and hardwickiic acid, a natural product with antinociceptive effect, bind to the III-IV fenestration, while vixotrigine, an analgesic candidate, penetrates the IV-I fenestration of the pore domain. Our results permit building a 3D structural map for known drug-binding sites on Na_v channels summarized from the present and previous structures.

Somatic and terminal CB1 receptors are differentially coupled to voltage-gated sodium channels in neocortical neurons

Endogenous cannabinoid signaling is vital for important brain functions, and the same pathways can be modified pharmacologically to treat pain, epilepsy, and posttraumatic stress disorder. Endocannabinoid-mediated changes to excitability are predominantly attributed to 2-arachidonoylglycerol (2-AG) acting presynaptically via the canonical cannabinoid receptor, CB1. Here, we identify a mechanism in the neocortex by which anandamide (AEA), another major endocannabinoid, but not 2-AG, powerfully inhibits somatically recorded voltage-gated sodium channel (VGSC) currents in the majority of neurons. This pathway involves intracellular CB1 that, when activated by anandamide, decreases the likelihood of recurrent action potential generation. WIN 55,212-2 similarly activates CB1 and inhibits VGSC currents, indicating that this pathway is also positioned to mediate the actions of exogenous cannabinoids on neuronal excitability. The coupling between CB1 and VGSCs is absent at nerve terminals, and 2-AG does not block somatic VGSC currents, indicating functional compartmentalization of the actions of two endocannabinoids.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of NaV1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of NaV1.7 has been recently achieved, multiple NaV1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and NaV1.7 coverage among three types of voltage-gated sodium channel (VGSC) inhibitors, namely conventional VGSC inhibitors, sulphonamides and acyl sulphonamides, is discussed. By demonstrating the PK/PD discrepancy of preclinical studies versus in vivo models and clinical results, the potential reasons behind the disconnect between preclinical results and clinical outcomes are discussed together with strategies for developing ideal analgesic agents.

Lacosamide Inhibition of NaV1.7 Channels Depends on its Interaction With the Voltage Sensor Domain and the Channel Pore

Lacosamide, developed as an anti-epileptic drug, has been used for the treatment of pain. Unlike typical anticonvulsants and local anesthetics which enhance fast-inactivation and bind within the pore of sodium channels, lacosamide enhances slow-inactivation of these channels, suggesting different binding mechanisms and mode of action. It has been reported that lacosamide's effect on Na_V1.5 is sensitive to a mutation in the local anesthetic binding site, and that it binds with slow kinetics to the fast-inactivated state of Na_V1.7. We recently showed that the Na_V1.7-W1538R mutation in the voltage-sensing domain 4 completely abolishes Na_V1.7 inhibition by clinically-achievable concentration of lacosamide. Our molecular docking analysis suggests a role for W1538 and pore residues as high affinity binding sites for lacosamide. Aryl sulfonamide sodium channel blockers are also sensitive to substitutions of the W1538 residue but not of pore residues. To elucidate the mechanism by which lacosamide exerts its effects, we used voltage-clamp recordings and show that lacosamide requires an intact local anesthetic binding site to inhibit Na_V1.7 channels. Additionally, the W1538R mutation does not abrogate local anesthetic lidocaine-induced blockade. We also show that the naturally occurring arginine in Na_V1.3 (Na_V1.3-R1560), which corresponds to Na_V1.7-W1538R, is not sufficient to explain the resistance of Na_V1.3 to clinically-relevant concentrations of lacosamide. However, the Na_V1.7-W1538R mutation conferred sensitivity to the Na_V1.3-selective aryl-sulfonamide blocker ICA-121431. Together, the W1538 residue and an intact local anesthetic site are required for lacosamide's block of Na_V1.7 at a clinically-achievable concentration. Moreover, the contribution of W1538 to lacosamide inhibitory effects appears to be isoform-specific.

Cinacalcet inhibition of neuronal action potentials preferentially targets the fast inactivated state of voltage-gated sodium channels

Voltage-gated sodium channel (VGSC) activation is essential for action potential generation in the brain. Allosteric calcium-sensing receptor (CaSR) agonist, cinacalcet, strongly and ubiquitously inhibits VGSC currents in neocortical neurons via an unidentified, G-protein-dependent inhibitory molecule. Here, using whole-cell patch VGSC clamp methods, we investigated the voltage-dependence of cinacalcet-mediated inhibition of VGSCs and the channel state preference of cinacalcet. The rate of inhibition of VGSC currents was accelerated at more depolarized holding potentials. Cinacalcet shifted the voltage-dependence of both fast and slow inactivation of VGSC currents in the hyperpolarizing direction. Utilizing a simple model, the voltage-dependence of VGSC current inhibition may be explained if the affinity of the inhibitory molecule to the channel states follows the sequence: fast-inactivated > slow-inactivated > resting. The state dependence of VGSC current inhibition contributes to the non-linearity of action potential block by cinacalcet. This dynamic and abundant signaling pathway by which cinacalcet regulates VGSC currents provides an important voltage-dependent mechanism for modulating central neuronal excitability.

Role of NaV 1.7 in action potential conduction along human bronchial vagal afferent C-fibres

BACKGROUND AND PURPOSE

The purpose of this study was to determine the role of NaV 1.7 in action potential conduction in C-fibres in the bronchial branches of the human vagus nerve.

EXPERIMENTAL APPROACH

Bronchial branches of the vagus nerve were dissected from human donor tissue. The C-wave of the electrically evoked compound action potential was quantified in the absence and presence of increasing concentrations of the selective NaV 1.7 blocking drugs, PF-05089771 and ST-2262, as well as the NaV 1.1, 1.2, and 1.3 blocking drug ICA121-431. The efficacy and potency of these inhibitors were compared to the standard NaV 1 blocker, tetrodotoxin. We then compared the relative potencies of the NaV 1 blockers in inhibiting the C-wave of the compound action potential, with their ability to inhibit parasympathetic cholinergic contraction of human isolated bronchi, a response previously shown to be strictly dependent on NaV 1.7 channels.

KEY RESULTS

The selective NaV 1.7 blockers inhibited the C-wave of the compound action potential with potencies similar to that observed in the NaV 1.7 bronchial contractions assay. Using rt-PCR, we noted that NaV 1.7 mRNA was strongly expressed and transported down the vagus nerve bundles.

CONCLUSIONS AND IMPLICATIONS

NaV 1.7 blockers can prevent action potential conduction in the majority of vagal C-fibres arising from human bronchi. Blockers of NaV 1.7 channels may therefore have value in inhibiting the responses to excessive airway C-fibre activation in inflammatory airway disease, responses that include coughing as well as reflex bronchoconstriction and secretions.

Small molecule targeting NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces and prevents pain chronification in a mouse model of oxaliplatin-induced neuropathic pain

Treatment with anti-neoplastic agents can lead to the development of chemotherapy induced peripheral neuropathy (CIPN), which is long lasting and often refractory to treatment. This neuropathic pain develops along dermatomes innervated by peripheral nerves with cell bodies located in the dorsal root ganglia (DRG). The voltage-gated sodium channel NaV1.7 is expressed at high levels in peripheral nerve tissues and has been implicated in the development of CIPN. Efforts to develop novel analgesics directly inhibiting NaV1.7 have been unsuccessful, and our group has pioneered an alternative approach based on indirect modulation of channel trafficking by the accessory protein collapsin response mediator protein 2 (CRMP2). We have recently reported a small molecule, compound 194, that inhibits CRMP2 SUMOylation by the E2 SUMO-conjugating enzyme Ubc9 (Cai et al. , Sci. Transl. Med. 2021 13(6 1 9):eabh1314). Compound 194 is a potent and selective inhibitor of NaV1.7 currents in DRG neurons and reverses mechanical allodynia in models of surgical, inflammatory, and neuropathic pain, including spared nerve injury and paclitaxelinduced peripheral neuropathy. Here we report that, in addition to its reported effects in rats, 194 also reduces mechanical allodynia in male CD-1 mice treated with platinumcomplex agent oxaliplatin. Importantly, treatment with 194 prevented the development of mechanical allodynia when co-administered with oxaliplatin. No effects were observed on the body weight of animals treated with oxaliplatin or 194 throughout the study period. These findings support the notion that 194 is a robust inhibitor of CIPN that reduces established neuropathic pain and prevents the emergence of neuropathic pain during treatment with multiple anti-neoplastic agents in both mice and rats.

Improving NKCC1 Function Increases the Excitability of DRG Neurons Exacerbating Pain Induced After TRPV1 Activation of Primary Sensory Neurons

Background Our aim was to investigate the effects of the protein expression and the function of sodium, potassium, and chloride co-transporter (NKCC1) in the dorsal root ganglion (DRG) after activation of transient receptor potential vanilloid 1 receptor (TRPV1) in capsaicin-induced acute inflammatory pain and the possible mechanism of action.

Methods Male Sprague–Dawley rats were randomly divided into control, capsaicin, and inhibitor groups. The expression and distribution of TRPV1 and NKCC1 in rat DRG were observed by immunofluorescence. Thermal radiation and acetone test were used to detect the pain threshold of heat and cold noxious stimulation in each group. The expressions of NKCC1 mRNA, NKCC1 protein, and p-NKCC1 in the DRG were detected by PCR and western blotting (WB). Patch clamp and chloride fluorescent probe were used to observe the changes of GABA activation current and intracellular chloride concentration. After intrathecal injection of protein kinase C (PKC) inhibitor (GF109203X) or MEK/extracellular signal-regulated kinase (ERK) inhibitor (U0126), the behavioral changes and the expression of NKCC1 and p-ERK protein in L_4–6 DRG were observed.

Result: TRPV1 and NKCC1 were co-expressed in the DRG. Compared with the control group, the immunofluorescence intensity of NKCC1 and p-NKCC1 in the capsaicin group was significantly higher, and the expression of NKCC1 in the nuclear membrane was significantly higher than that in the control group. The expression of NKCC1 mRNA and protein of NKCC1 and p-NKCC1 in the capsaicin group were higher than those in the control group. After capsaicin injection, GF109203X inhibited the protein expression of NKCC1 and p-ERK, while U0126 inhibited the protein expression of NKCC1. In the capsaicin group, paw withdrawal thermal latency (WTL) was decreased, while cold withdrawal latency (CWL) was prolonged. Bumetanide, GF109203X, or U0126 could reverse the effect. GABA activation current significantly increased in the DRG cells of the capsaicin group, which could be reversed by bumetanide. The concentration of chloride in the DRG cells of the capsaicin group increased, but decreased after bumetanide, GF109203X, and U0126 were administered.

Conclusion Activation of TRPV1 by exogenous agonists can increase the expression and function of NKCC1 protein in DRG, which is mediated by activation of PKC/p-ERK signaling pathway. These results suggest that DRG NKCC1 may participate in the inflammatory pain induced by TRPV1.

Characterization of Vixotrigine, a Broad-Spectrum Voltage-Gated Sodium Channel Blocker

Voltage-gated sodium channels (Navs) are promising targets for analgesic and antiepileptic therapies. Although specificity between Nav subtypes may be desirable to target specific neural types, such as nociceptors in pain, many broadly acting Nav inhibitors are clinically beneficial in neuropathic pain and epilepsy. Here, we present the first systematic characterization of vixotrigine, a Nav blocker. Using recombinant systems, we find that vixotrigine potency is enhanced in a voltage- and use-dependent manner, consistent with a state-dependent block of Navs. Furthermore, we find that vixotrigine potently inhibits sodium currents produced by both peripheral and central nervous system Nav subtypes, with use-dependent IC50 values between 1.76 and 5.12 μM. Compared with carbamazepine, vixotrigine shows higher potency and more profound state-dependent inhibition but a similar broad spectrum of action distinct from Nav1.7- and Nav1.8-specific blockers. We find that vixotrigine rapidly inhibits Navs and prolongs recovery from the fast-inactivated state. In native rodent dorsal root ganglion sodium channels, we find that vixotrigine shifts steady-state inactivation curves. Based on these results, we conclude that vixotrigine is a broad-spectrum, state-dependent Nav blocker. SIGNIFICANCE STATEMENT: Vixotrigine blocks both peripheral and central voltage-gated sodium channel subtypes. Neurophysiological approaches in recombinant systems and sensory neurons suggest this block is state-dependent.

Discovery of a selective, state-independent inhibitor of NaV1.7 by modification of guanidinium toxins

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.

Differential effect of lacosamide on Nav1.7 variants from responsive and non-responsive patients with small fibre neuropathy

Small fibre neuropathy is a common pain disorder, which in many cases fails to respond to treatment with existing medications. Gain-of-function mutations of voltage-gated sodium channel Na_v1.7 underlie dorsal root ganglion neuronal hyperexcitability and pain in a subset of patients with small fibre neuropathy. Recent clinical studies have demonstrated that lacosamide, which blocks sodium channels in a use-dependent manner, attenuates pain in some patients with Na_v1.7 mutations; however, only a subgroup of these patients responded to the drug. Here, we used voltage-clamp recordings to evaluate the effects of lacosamide on five Na_v1.7 variants from patients who were responsive or non-responsive to treatment. We show that, at the clinically achievable concentration of 30 μM, lacosamide acts as a potent sodium channel inhibitor of Na_v1.7 variants carried by responsive patients, via a hyperpolarizing shift of voltage-dependence of both fast and slow inactivation and enhancement of use-dependent inhibition. By contrast, the effects of lacosamide on slow inactivation and use-dependence in Na_v1.7 variants from non-responsive patients were less robust. Importantly, we found that lacosamide selectively enhances fast inactivation only in variants from responders. Taken together, these findings begin to unravel biophysical underpinnings that contribute to responsiveness to lacosamide in patients with small fibre neuropathy carrying select Na_v1.7 variants.

Rat NaV1.7 loss-of-function genetic model: Deficient nociceptive and neuropathic pain behavior with retained olfactory function and intra-epidermal nerve fibers

Recapitulating human disease pathophysiology using genetic animal models is a powerful approach to enable mechanistic understanding of genotype–phenotype relationships for drug development. Na_V1.7 is a sodium channel expressed in the peripheral nervous system with strong human genetic validation as a pain target. Efforts to identify novel analgesics that are nonaddictive resulted in industry exploration of a class of sulfonamide compounds that bind to the fourth voltage-sensor domain of Na_V1.7. Due to sequence differences in this region, sulfonamide blockers generally are potent on human but not rat Na_V1.7 channels. To test sulfonamide-based chemical matter in rat models of pain, we generated a humanized Na_V1.7 rat expressing a chimeric Na_V1.7 protein containing the sulfonamide-binding site of the human gene sequence as a replacement for the equivalent rat sequence. Unexpectedly, upon transcription, the human insert was spliced out, resulting in a premature stop codon. Using a validated antibody, Na_V1.7 protein was confirmed to be lost in the brainstem, dorsal root ganglia, sciatic nerve, and gastrointestinal tissue but not in nasal turbinates or olfactory bulb in rats homozygous for the knock-in allele (HOM-KI). HOM-KI rats exhibited normal intraepidermal nerve fiber density with reduced tetrodotoxin-sensitive current density and action potential firing in small diameter dorsal root ganglia neurons. HOM-KI rats did not exhibit nociceptive pain responses in hot plate or capsaicin-induced flinching assays and did not exhibit neuropathic pain responses following spinal nerve ligation. Consistent with expression of chimeric Na_V1.7 in olfactory tissue, HOM-KI rats retained olfactory function. This new genetic model highlights the necessity of Na_V1.7 for pain behavior in rats and indicates that sufficient inhibition of Na_V1.7 in humans may reduce pain in neuropathic conditions. Due to preserved olfactory function, this rat model represents an alternative to global Na_V1.7 knockout mice that require time-intensive hand feeding during early postnatal development.

Lidocaine Binding Enhances Inhibition of Nav1.7 Channels by the Sulfonamide PF-05089771

PF-05089771 is an aryl sulfonamide Nav1.7 channel blocker that binds to the inactivated state of Nav1.7 channels with high affinity but binds only weakly to channels in the resting state. Such aryl sulfonamide Nav1.7 channel blockers bind to the extracellular surface of the S1-S4 voltage-sensor segment of homologous Domain 4, whose movement is associated with inactivation. This binding site is different from that of classic sodium channel inhibitors like lidocaine, which also bind with higher affinity to the inactivated state than the resting state but bind at a site within the pore of the channel. The common dependence on gating state with distinct binding sites raises the possibility that inhibition by aryl sulfonamides and by classic local anesthetics might show an interaction mediated by their mutual state dependence. We tested this possibility by examining the state-dependent inhibition by PF-05089771 and lidocaine of human Nav1.7 channels expressed in human embryonic kidney 293 cells. At -80 mV, where a small fraction of channels are in an inactivated state under drug-free conditions, inhibition by PF-05089771 was both enhanced and speeded in the presence of lidocaine. The results suggest that lidocaine binding to the channel enhances PF-05089771 inhibition by altering the equilibrium between resting states (with D4S4 in the inner position) and inactivated states (with D4S4 in the outer position). The gating state-mediated interaction between the compounds illustrates a principle applicable to many state-dependent agents. SIGNIFICANCE STATEMENT: The results show that lidocaine enhances the degree and rate of inhibition of Nav1.7 channels by the aryl sulfonamide compound PF-05089771, consistent with state-dependent binding by lidocaine increasing the fraction of channels presenting a high-affinity binding site for PF-05089771 and suggesting that combinations of agents targeted to the pore-region binding site of lidocaine and the external binding site of aryl sulfonamides may have synergistic actions.

Correlation of Optical and Automated Patch Clamp Electrophysiology for Identification of NaV1.7 Inhibitors

The voltage-gated sodium channel Nav1.7 is a genetically validated target for pain; pharmacological blockers are promising as a new class of nonaddictive therapeutics. The search for Nav1.7 subtype selective inhibitors requires a reliable, scalable, and sensitive assay. Previously, we developed an all-optical electrophysiology (Optopatch) Spiking HEK platform to study activity-dependent modulation of Nav1.7 in a format compatible with high-throughput screening. In this study, we benchmarked the Optopatch Spiking HEK assay with an existing validated automated electrophysiology assay on the IonWorks Barracuda (IWB) platform. In a pilot screen of 3520 compounds, which included compound plates from a random library as well as compound plates enriched for Nav1.7 inhibitors, the Optopatch Spiking HEK assay identified 174 hits, of which 143 were confirmed by IWB. The Optopatch Spiking HEK assay maintained the high reliability afforded by traditional fluorescent assays and further demonstrated comparable sensitivity to IWB measurements. We speculate that the Optopatch assay could provide an affordable high-throughput screening platform to identify novel Nav1.7 subtype selective inhibitors with diverse mechanisms of action, if coupled with a multiwell parallel optogenetic recording instrument.

Discovery of aryl sulfonamide-selective Nav1.7 inhibitors with a highly hydrophobic ethanoanthracene core

Nav1.7 channels are mainly distributed in the peripheral nervous system. Blockade of Nav1.7 channels with small-molecule inhibitors in humans might provide pain relief without affecting the central nervous system. Based on the facts that many reported Nav1.7-selective inhibitors contain aryl sulfonamide fragments, as well as a tricyclic antidepressant, maprotiline, has been found to inhibit Nav1.7 channels, we designed and synthesized a series of compounds with ethanoanthracene and aryl sulfonamide moieties. Their inhibitory activity on sodium channels were detected with electrophysiological techniques. We found that compound 10o potently inhibited Nav1.7 channels stably expressed in HEK293 cells (IC₅₀ = 0.64 ± 0.30 nmol/L) and displayed a high Nav1.7/Nav1.5 selectivity. In mouse small-sized dorsal root ganglion neurons, compound 10o (10, 100 nmol/L) dose-dependently decreased the sodium currents and dramatically suppressed depolarizing current-elicited neuronal discharge. Preliminary in vivo experiments showed that compound 10o possessed good analgesic activity: in a mouse visceral pain model, administration of compound 10o (30−100 mg/kg, i.p.) effectively and dose-dependently suppressed acetic acid-induced writhing.

Complementary roles of murine NaV1.7, NaV1.8 and NaV1.9 in acute itch signalling

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (Na_V) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, Na_V1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing Na_V-specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, β-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in Na_V channel-mediated itch signalling. Na_V1.7^−/− showed substantial scratch reduction mainly towards strong pruritogens. Na_V1.8^−/− impaired histamine and 5-HT-induced scratching while Na_V1.9 was involved in itch signalling towards 5-HT, C48/80 and SLIGRL. Furthermore, similar microfluorimetric calcium responses of sensory neurons and expression of itch-related TRP channels suggest no change in sensory transduction but in action potential transformation and conduction. The cumulative sum of scratching over all pruritogens confirmed a leading role of Na_V1.7 and indicated an overall contribution of Na_V1.9. Beside the proposed general role of Na_V1.7 and 1.9 in itch signalling, scrutiny of time courses suggested Na_V1.8 to sustain prolonged itching. Therefore, Na_V1.7 and 1.9 may represent targets in pruritus therapy.

Novel charged sodium and calcium channel inhibitor active against neurogenic inflammation

Voltage-dependent sodium and calcium channels in pain-initiating nociceptor neurons are attractive targets for new analgesics. We made a permanently charged cationic derivative of an N-type calcium channel-inhibitor. Unlike cationic derivatives of local anesthetic sodium channel blockers like QX-314, this cationic compound inhibited N-type calcium channels more effectively with extracellular than intracellular application. Surprisingly, the compound is also a highly effective sodium channel inhibitor when applied extracellularly, producing more potent inhibition than lidocaine or bupivacaine. The charged inhibitor produced potent and long-lasting analgesia in mouse models of incisional wound and inflammatory pain, inhibited release of the neuropeptide calcitonin gene-related peptide (CGRP) from dorsal root ganglion neurons, and reduced inflammation in a mouse model of allergic asthma, which has a strong neurogenic component. The results show that some cationic molecules applied extracellularly can powerfully inhibit both sodium channels and calcium channels, thereby blocking both nociceptor excitability and pro-inflammatory peptide release.

Challenges and Opportunities for Therapeutics Targeting the Voltage-Gated Sodium Channel Isoform NaV1.7

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.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

The voltage-gated sodium channel Nav1.7 associated with endometrial cancer

Background: Endometrial cancer is the most common gynecologic malignancy in women in the developed countries. Despite recent progress in functional characterization of voltage-gated sodium channel (Na_v) in multiple cancers, very little was known about the expression of Na_v in human endometrial cancer. The present study sought to determine the role of Na_v and molecular nature of this channel in the endometrial cancer.

Methods: PCR approach was introduced to determine expression level of Na_v subunits in endometrial cancer specimens. Pharmacological agents were used to investigate Na_v function in endometrial cancer cells. Flow cytometry were used to test cancer apoptosis, and invasion assays were applied to test tumor metastasis.

Results: Transcriptional levels of the all Na_v α and β subunits were determined by real time-PCR in endometrial cancer with pair tissues of carcinoma and adjacent nonneoplastic tissue, Na_v1.7 was the most highly expressed Na_v subtype in endometrial cancer tissues. Na_v1.7 level was closely associated with tumor size, local lymph node metastasis, and 5-year and 10-year survival ratio. Inhibition of this channel by Na_v1.7 blocker PF-05089771, promoted cancer apoptosis and attenuated cancer cell invasion.

Conclusion: These results establish a relationship between voltage-gated sodium channel protein and endometrial cancer, and suggest that Na_v1.7 is a potential prognostic biomarker and could serve as a novel therapeutic target for endometrial cancer.

Tetrodotoxin-Sensitive Sodium Channels Mediate Action Potential Firing and Excitability in Menthol-Sensitive Vglut3-Lineage Sensory Neurons

Small-diameter vesicular glutamate transporter 3-lineage (Vglut3^lineage) dorsal root ganglion (DRG) neurons play an important role in mechanosensation and thermal hypersensitivity; however, little is known about their intrinsic electrical properties. We therefore set out to investigate mechanisms of excitability within this population. Calcium microfluorimetry analysis of male and female mouse DRG neurons demonstrated that the cooling compound menthol selectively activates a subset of Vglut3^lineage neurons. Whole-cell recordings showed that small-diameter Vglut3^lineage DRG neurons fire menthol-evoked action potentials and exhibited robust, transient receptor potential melastatin 8 (TRPM8)-dependent discharges at room temperature. This heightened excitability was confirmed by current-clamp and action potential phase-plot analyses, which showed menthol-sensitive Vglut3^lineage neurons to have more depolarized membrane potentials, lower firing thresholds, and higher evoked firing frequencies compared with menthol-insensitive Vglut3^lineage neurons. A biophysical analysis revealed voltage-gated sodium channel (Na_V) currents in menthol-sensitive Vglut3^lineage neurons were resistant to entry into slow inactivation compared with menthol-insensitive neurons. Multiplex in situ hybridization showed similar distributions of tetrodotoxin (TTX)-sensitive Na_V transcripts between TRPM8-positive and -negative Vglut3^lineage neurons; however, Na_V1.8 transcripts, which encode TTX-resistant channels, were more prevalent in TRPM8-negative neurons. Conversely, pharmacological analyses identified distinct functional contributions of Na_V subunits, with Na_V1.1 driving firing in menthol-sensitive neurons, whereas other small-diameter Vglut3^lineage neurons rely primarily on TTX-resistant Na_V channels. Additionally, when Na_V1.1 channels were blocked, the remaining Na_V current readily entered into slow inactivation in menthol-sensitive Vglut3^lineage neurons. Thus, these data demonstrate that TTX-sensitive Na_Vs drive action potential firing in menthol-sensitive sensory neurons and contribute to their heightened excitability.SIGNIFICANCE STATEMENT Somatosensory neurons encode various sensory modalities including thermoreception, mechanoreception, nociception, and itch. This report identifies a previously unknown requirement for tetrodotoxin-sensitive sodium channels in action potential firing in a discrete subpopulation of small-diameter sensory neurons that are activated by the cooling agent menthol. Together, our results provide a mechanistic understanding of factors that control intrinsic excitability in functionally distinct subsets of peripheral neurons. Furthermore, as menthol has been used for centuries as an analgesic and anti-pruritic, these findings support the viability of Na_V1.1 as a therapeutic target for sensory disorders.

Voltage-gated sodium channels: structures, functions, and molecular modeling

Voltage-gated sodium channels (VGSCs), formed by 24 transmembrane segments arranged into four domains, have a key role in the initiation and propagation of electrical signaling in excitable cells. VGSCs are involved in a variety of diseases, including epilepsy, cardiac arrhythmias, and neuropathic pain, and therefore have been regarded as appealing therapeutic targets for the development of anticonvulsant, antiarrhythmic, and local anesthetic drugs. In this review, we discuss recent advances in understanding the structures and biological functions of VGSCs. In addition, we systematically summarize eight pharmacologically distinct ligand-binding sites in VGSCs and representative isoform-selective VGSC modulators in clinical trials. Finally, we review studies on molecular modeling and computer-aided drug design (CADD) for VGSCs to help understanding of biological processes involving VGSCs.

Enhancing inactivation rather than reducing activation of Nav1.7 channels by a clinically effective analgesic CNV1014802

The Nav1.7 channel represents a promising target for pain relief. In the recent decades, a number of Nav1.7 channel inhibitors have been developed. According to the effects on channel kinetics, these inhibitors could be divided into two major classes: reducing activation or enhancing inactivation. To date, however, only several inhibitors have moved forward into phase 2 clinical trials and most of them display a less than ideal analgesic efficacy, thus intensifying the controversy regarding if an ideal candidate should preferentially affect the activation or inactivation state. In the present study, we investigated the action mechanisms of a recently clinically confirmed inhibitor CNV1014802 using both electrophysiology and site-directed mutagenesis. We found that CNV1014802 inhibited Nav1.7 channels through stabilizing a nonconductive inactivated state. When the cells expressing Nav1.7 channels were hold at 70 mV or 120 mV, the half maximal inhibitory concentration (IC50) values (with 95% confidence limits) were 1.77 (1.20-2.33) and 71.66 (46.85-96.48) μmol/L, respectively. This drug caused dramatic hyperpolarizing shift of channel inactivation but did not affect activation. Moreover, CNV1014802 accelerated the onset of inactivation and delayed the recovery from inactivation. Notably, application of CNV1014802 (30 μmol/L) could rescue the Nav1.7 mutations expressed in CHO cells that cause paroxysmal extreme pain disorder (PEPD), thereby restoring the impaired inactivation to those of the wild-type channel. Our study demonstrates that CNV1014802 enhances the inactivation but does not reduce the activation of Nav1.7 channels, suggesting that identifying inhibitors that preferentially affect inactivation is a promising approach for developing drugs targeting Nav1.7.

Physical basis of specificity and delayed binding of a subtype selective sodium channel inhibitor

Nerve and muscle signalling is controlled by voltage-gated sodium (Nav) channels which are the targets of local anesthetics, anti-epileptics and anti-arrythmics. Current medications do not selectively target specific types of Nav found in the body, but compounds that do so have the potential to be breakthrough treatments for chronic pain, epilepsy and other neuronal disorders. We use long computer simulations totaling more than 26 μs to show how a promising lead compound can target one Nav implicated in pain perception and specific channels found in bacteria, and accurately predict the affinity of the compound to different channel types. Most importantly, we provide two explanations for the slow kinetics of this class of compound that limits their therapeutic utility. Firstly, the negative charge on the compound is essential for high affinity binding but is also responsible for energetic barriers that slow binding. Secondly, the compound has to undergo a conformational reorientation during the binding process. This knowledge aids the design of compounds affecting specific eukaryotic and bacterial channels and suggests routes for future drug development.

Pharmacologic Characterization of AMG8379, a Potent and Selective Small Molecule Sulfonamide Antagonist of the Voltage-Gated Sodium Channel NaV1.7

Potent and selective antagonists of the voltage-gated sodium channel NaV1.7 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 NaV1.7 channels with an IC50 of 8.5 nM and endogenous tetrodotoxin (TTX)-sensitive sodium channels in dorsal root ganglion (DRG) neurons with an IC50 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 NaV family members, including NaV1.4 expressed in muscle and NaV1.5 expressed in the heart, as well as TTX-resistant NaV 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 NaV1.7-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 NaV1.7 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.

Insensitivity to pain induced by a potent selective closed-state Nav1.7 inhibitor

Pain places a devastating burden on patients and society and current pain therapeutics exhibit limitations in efficacy, unwanted side effects and the potential for drug abuse and diversion. Although genetic evidence has clearly demonstrated that the voltage-gated sodium channel, Nav1.7, is critical to pain sensation in mammals, pharmacological inhibitors of Nav1.7 have not yet fully recapitulated the dramatic analgesia observed in Nav1.7-null subjects. Using the tarantula venom-peptide ProTX-II as a scaffold, we engineered a library of over 1500 venom-derived peptides and identified JNJ63955918 as a potent, highly selective, closed-state Nav1.7 blocking peptide. Here we show that JNJ63955918 induces a pharmacological insensitivity to pain that closely recapitulates key features of the Nav1.7-null phenotype seen in mice and humans. Our findings demonstrate that a high degree of selectivity, coupled with a closed-state dependent mechanism of action is required for strong efficacy and indicate that peptides such as JNJ63955918 and other suitably optimized Nav1.7 inhibitors may represent viable non-opioid alternatives for the pharmacological treatment of severe pain.