Nav1.7 is essential for nociceptor action potentials in the mouse in a manner independent of endogenous opioids

Loss-of-function mutations in Nav1.7, a voltage-gated sodium channel, cause congenital insensitivity to pain (CIP) in humans, demonstrating that Nav1.7 is essential for the perception of pain. However, the mechanism by which loss of Nav1.7 results in insensitivity to pain is not entirely clear. It has been suggested that loss of Nav1.7 induces overexpression of enkephalin, an endogenous opioid receptor agonist, leading to opioid-dependent analgesia. Using behavioral pharmacology and single-cell RNA-seq analysis, we find that overexpression of enkephalin occurs only in cLTMR neurons, a subclass of sensory neurons involved in low-threshold touch detection, and that this overexpression does not play a role in the analgesia observed following genetic removal of Nav1.7. Furthermore, we demonstrate using laser speckle contrast imaging (LSCI) and in vivo electrophysiology that Nav1.7 function is required for the initiation of C-fiber action potentials (APs), which explains the observed insensitivity to pain following genetic removal or inhibition of Nav1.7.

New aryl and acylsulfonamides as state-dependent inhibitors of Nav1.3 voltage-gated sodium channel

Voltage-gated sodium channels (Na_vs) play an essential role in neurotransmission, and their dysfunction is often a cause of various neurological disorders. The Na_v1.3 isoform is found in the CNS and upregulated after injury in the periphery, but its role in human physiology has not yet been fully elucidated. Reports suggest that selective Na_v1.3 inhibitors could be used as novel therapeutics to treat pain or neurodevelopmental disorders. Few selective inhibitors of this channel are known in the literature. In this work, we report the discovery of a new series of aryl and acylsulfonamides as state-dependent inhibitors of Na_v1.3 channels. Using a ligand-based 3D similarity search and subsequent hit optimization, we identified and prepared a series of 47 novel compounds and tested them on Na_v1.3, Na_v1.5, and a selected subset also on Na_v1.7 channels in a QPatch patch-clamp electrophysiology assay. Eight compounds had an IC₅₀ value of less than 1 μM against the Na_v1.3 channel inactivated state, with one compound displaying an IC₅₀ value of 20 nM, whereas activity against the inactivated state of the Na_v1.5 channel and Na_v1.7 channel was approximately 20-fold weaker. None of the compounds showed use-dependent inhibition of the cardiac isoform Na_v1.5 at a concentration of 30 μM. Further selectivity testing of the most promising hits was measured using the two-electrode voltage-clamp method against the closed state of the Na_v1.1-Na_v1.8 channels, and compound 15b displayed small, yet selective, effects against the Na_v1.3 channel, with no activity against the other isoforms. Additional selectivity testing of promising hits against the inactivated state of the Na_v1.3, Na_v1.7, and Na_v1.8 channels revealed several compounds with robust and selective activity against the inactivated state of the Na_v1.3 channel among the three isoforms tested. Moreover, the compounds were not cytotoxic at a concentration of 50 μM, as demonstrated by the assay in human HepG2 cells (hepatocellular carcinoma cells). The novel state-dependent inhibitors of Na_v1.3 discovered in this work provide a valuable tool to better evaluate this channel as a potential drug target.

NaV1.7 Channel Blocker [Ala5, Phe6, Leu26, Arg28]GpTx-1 Attenuates CFA-induced Inflammatory Hypersensitivity in Rats via Endogenous Enkephalin Mechanism

Venom-derived Na_V1.7 channel blockers have promising prospects in pain management. The 34-residue tarantula peptide GpTx-1 is a potent Na_V1.7 channel blocker. Its powerful analog [Ala⁵, Phe⁶, Leu²⁶, Arg²⁸]GpTx-1 (GpTx-1-71) displayed excellent Na_V1.7 selectivity and analgesic properties in mice. The current study aimed to elucidate the anti-hyperalgesic activities of GpTx-1-71 in inflammatory pain and reveal the underlying mechanisms. Our results demonstrated that intrathecal and intraplantar injections of GpTx-1-71 dose-dependently attenuated CFA-induced inflammatory hypersensitivity in rats. Moreover, GpTx-1-71-induced anti-hyperalgesia was significantly reduced by opioid receptor antagonists and the enkephalin antibody and diminished in proenkephalin (Penk) gene knockout animals. Consistently, GpTx-1-71 treatment increased the enkephalin level in the spinal dorsal horn and promoted the Penk transcription and enkephalin release in primary dorsal root ganglion (DRG) neurons, wherein sodium played a crucial role in these processes. Mass spectrometry analysis revealed that GpTx-1-71 mainly promoted the secretion of Met-enkephalin but not Leu-enkephalin from DRG neurons. In addition, the combination of subtherapeutic Met-enkephalin and GpTx-1-71 produced synergistic anti-hyperalgesia in CFA-induced inflammatory hypersensitivity. These findings suggest that the endogenous enkephalin pathway is essential for GpTx-1-71-induced spinal and peripheral analgesia in inflammatory pain. PERSPECTIVE: This article presents a possible pharmacological mechanism underlying Na_V1.7 blocker-induced analgesia in inflammatory pain, which helps us to better understand and develop venom-based painkillers for incurable pain.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of Na_V1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of Na_V1.7 has been recently achieved, multiple Na_V1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and Na_V1.7 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.

A nonneglectable stereochemical factor in drug development: Atropisomerism

Chirality is one of the key factors affecting the medicinal efficacy of compounds. In addition to central chirality, sterically hindered chiral axes commonly appear in drugs and the resulting chirality is known as atropisomerism. With developments in medicinal chemistry, atropisomerism has attracted increasing attention. This review discusses the classification, biological activity, pharmacokinetics, toxicity and side effects of atropisomers, and can serve as a reference in the research and development of potential chiral drugs.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

Examination of the contribution of Nav1.7 to axonal propagation in nociceptors

ABSTRACT

Nav1.7 is a promising drug target for the treatment of pain. However, there is a mismatch between the analgesia produced by Nav1.7 loss-of-function and the peripherally restricted Nav1.7 inhibitors, which may reflect a lack of understanding of the function of Nav1.7 in the transmission of nociceptive information. In the periphery, the role of Nav1.7 in transduction at nociceptive peripheral terminals has been comprehensively examined, but its role in axonal propagation in these neurons is less clearly defined. In this study, we examined the contribution of Nav1.7 to axonal propagation in nociceptors using sodium channel blockers in in vivo electrophysiological and calcium imaging recordings in mice. Using the sodium channel blocker tetrodotoxin (TTX) (1-10 µM) to inhibit Nav1.7 and other tetrodotoxin-sensitive sodium channels along the sciatic nerve, we first showed that around two-thirds of nociceptive L4 dorsal root ganglion neurons innervating the skin, but a lower proportion innervating the muscle (45%), are blocked by TTX. By contrast, nearly all large-sized cutaneous afferents (95%-100%) were blocked by axonal TTX. Many cutaneous nociceptors resistant to TTX were polymodal (57%) and capsaicin sensitive (57%). Next, we applied PF-05198007 (300 nM-1 µM) to the sciatic nerve between stimulating and recording sites to selectively block axonal Nav1.7 channels. One hundred to three hundred nanomolar PF-05198007 blocked propagation in 63% of C-fiber sensory neurons, whereas similar concentrations produced minimal block (5%) in rapidly conducting A-fiber neurons. We conclude that Nav1.7 is essential for axonal propagation in around two-thirds of nociceptive cutaneous C-fiber neurons and a lower proportion (≤45%) of nociceptive neurons innervating muscle.

Towards Structure-Guided Development of Pain Therapeutics Targeting Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels are critical molecular determinants of action potential generation and propagation in excitable cells. Normal Na_V channel function disruption can affect physiological neuronal signaling and lead to increased sensitivity to pain, congenital indifference to pain, uncoordinated movement, seizures, or paralysis. Human genetic studies have identified human Na_V1.7 (hNa_V1.7), hNa_V1.8, and hNa_V1.9 channel subtypes as crucial players in pain signaling. The premise that subtype selective Na_V inhibitors can reduce pain has been reinforced through intensive target validation and therapeutic development efforts. However, an ideal therapeutic has yet to emerge. This review is focused on recent progress, current challenges, and future opportunities to develop Na_V channel targeting small molecules and peptides as non-addictive therapeutics to treat pain.

Antitussive effects of NaV 1.7 blockade in Guinea pigs

Our previous studies implicated the voltage-gated sodium channel subtype NaV 1.7 in the transmission of action potentials by the vagal afferent nerves regulating cough and thus identified this channel as a rational therapeutic target for antitussive therapy. But it is presently unclear whether a systemically administered small molecule inhibitor of NaV 1.7 conductance can achieve therapeutic benefit in the absence of side effects on cardiovascular function, gastrointestinal motility or respiration. To this end, we have evaluated the antitussive effects of the NaV 1.7 selective blocker Compound 801 administered systemically in awake guinea pigs or administered topically in anesthetized guinea pigs. We also evaluated the antitussive effects of ambroxol, a low affinity NaV blocker modestly selective for tetrodotoxin resistant NaV subtypes. Both Compound 801 and ambroxol dose-dependently inhibited action potential conduction in guinea pig vagus nerves (assessed by compound potential), with ambroxol nearly 100-fold less potent than the NaV 1.7 selective Compound 801 in this and other NaV 1.7-dependent guinea pig and human tissue-based assays. Both drugs also inhibited citric acid evoked coughing in awake or anesthetized guinea pigs, with potencies supportive of an NaV 1.7-dependent mechanism. Notably, however, the antitussive effects of systemically administered Compound 801 were accompanied by hypotension and respiratory depression. Given the antitussive effects of topically administered Compound 801, we speculate that the likely insurmountable side effects on blood pressure and respiratory drive associated with systemic dosing make topical formulations a viable and perhaps unavoidable therapeutic strategy for targeting NaV 1.7 in cough.

Tetrodotoxin: A New Strategy to Treat Visceral Pain?

Visceral pain is one of the most common symptoms associated with functional gastrointestinal (GI) disorders. Although the origin of these symptoms has not been clearly defined, the implication of both the central and peripheral nervous systems in visceral hypersensitivity is well established. The role of several pathways in visceral nociception has been explored, as well as the influence of specific receptors on afferent neurons, such as voltage-gated sodium channels (VGSCs). VGSCs initiate action potentials and dysfunction of these channels has recently been associated with painful GI conditions. Current treatments for visceral pain generally involve opioid based drugs, which are associated with important side-effects and a loss of effectiveness or tolerance. Hence, efforts have been intensified to find new, more effective and longer-lasting therapies. The implication of VGSCs in visceral hypersensitivity has drawn attention to tetrodotoxin (TTX), a relatively selective sodium channel blocker, as a possible and promising molecule to treat visceral pain and related diseases. As such, here we will review the latest information regarding this toxin that is relevant to the treatment of visceral pain and the possible advantages that it may offer relative to other treatments, alone or in combination.

Discovery of Arylsulfonamide Nav1.7 Inhibitors: IVIVC, MPO Methods, and Optimization of Selectivity Profile

The voltage-gated sodium channel Nav1.7 continues to be a high-profile target for the treatment of various pain afflictions due to its strong human genetic validation. While isoform selective molecules have been discovered and advanced into the clinic, to date, this target has yet to bear fruit in the form of marketed therapeutics for the treatment of pain. Lead optimization efforts over the past decade have focused on selectivity over Nav1.5 due to its link to cardiac side effects as well as the translation of preclinical efficacy to man. Inhibition of Nav1.6 was recently reported to yield potential respiratory side effects preclinically, and this finding necessitated a modified target selectivity profile. Herein, we report the continued optimization of a novel series of arylsulfonamide Nav1.7 inhibitors to afford improved selectivity over Nav1.6 while maintaining rodent oral bioavailability through the use of a novel multiparameter optimization (MPO) paradigm. We also report in vitro-in vivo correlations from Nav1.7 electrophysiology protocols to preclinical models of efficacy to assist in projecting clinical doses. These efforts produced inhibitors such as compound 19 with potency against Nav1.7, selectivity over Nav1.5 and Nav1.6, and efficacy in behavioral models of pain in rodents as well as inhibition of rhesus olfactory response indicative of target modulation.

A Solid Supported Membrane-Based Technology for Electrophysical Screening of B0AT1-Modulating Compounds

Classical high-throughput screening (HTS) technologies for the analysis of ionic currents across biological membranes can be performed using fluorescence-based, radioactive, and mass spectrometry (MS)-based uptake assays. These assays provide rapid results for pharmacological HTS, but the underlying, indirect analytical character of these assays can be linked to high false-positive hit rates. Thus, orthogonal and secondary assays using more biological target-based technologies are indispensable for further compound validation and optimization. Direct assay technologies for transporter proteins are electrophysiology-based, but are also complex, time-consuming, and not well applicable for automated profiling purposes. In contrast to conventional patch clamp systems, solid supported membrane (SSM)-based electrophysiology is a sensitive, membrane-based method for transporter analysis, and current technical developments target the demand for automated, accelerated, and sensitive assays for transporter-directed compound screening. In this study, the suitability of the SSM-based technique for pharmacological compound identification and optimization was evaluated performing cell-free SSM-based measurements with the electrogenic amino acid transporter B⁰AT1 (SLC6A19). Electrophysiological characterization of leucine-induced currents demonstrated that the observed signals were specific to B⁰AT1. Moreover, B⁰AT1-dependent responses were successfully inhibited using an established in-house tool compound. Evaluation of current stability and data reproducibility verified the robustness and reliability of the applied assay. Active compounds from primary screens of large compound libraries were validated, and false-positive hits were identified. These results clearly demonstrate the suitability of the SSM-based technique as a direct electrophysiological method for rapid and automated identification of small molecules that can inhibit B⁰AT1 activity.

The physiological function of different voltage-gated sodium channels in pain

Evidence from human genetic pain disorders shows that voltage-gated sodium channel α-subtypes Nav1.7, Nav1.8 and Nav1.9 are important in the peripheral signalling of pain. Nav1.7 is of particular interest because individuals with Nav1.7 loss-of-function mutations are congenitally insensitive to acute and chronic pain, and there is considerable hope that phenocopying these effects with a pharmacological antagonist will produce a new class of analgesic drug. However, studies in these rare individuals do not reveal how and where voltage-gated sodium channels contribute to pain signalling, which is of critical importance for drug development. More than a decade of research utilizing rodent genetic models and pharmacological tools to study voltage-gated sodium channels in pain has begun to unravel the role of different subtypes. Here, we review the contribution of individual channel subtypes in three key physiological processes necessary for transmission of sensory information to the CNS: transduction of stimuli at peripheral nerve terminals, axonal transmission of action potentials and neurotransmitter release from central terminals. These data suggest that drugs seeking to recapitulate the analgesic effects of loss of function of Nav1.7 will need to be brain-penetrant - which most of those developed to date are not.

Antinociceptive properties of an isoform-selective inhibitor of Nav1.7 derived from saxitoxin in mouse models of pain

ABSTRACT

The voltage-gated sodium channel Nav1.7 is highly expressed in nociceptive afferents and is critically involved in pain signal transmission. Nav1.7 is a genetically validated pain target in humans because loss-of-function mutations cause congenital insensitivity to pain and gain-of-function mutations cause severe pain syndromes. Consequently, pharmacological inhibition has been investigated as an analgesic therapeutic strategy. We describe a small molecule Nav1.7 inhibitor, ST-2530, that is an analog of the naturally occurring sodium channel blocker saxitoxin. When evaluated against human Nav1.7 by patch-clamp electrophysiology using a protocol that favors the resting state, the Kd of ST-2530 was 25 ± 7 nM. ST-2530 exhibited greater than 500-fold selectivity over human voltage-gated sodium channel isoforms Nav1.1-Nav1.6 and Nav1.8. Although ST-2530 had lower affinity against mouse Nav1.7 (Kd = 250 ± 40 nM), potency was sufficient to assess analgesic efficacy in mouse pain models. A 3-mg/kg dose administered subcutaneously was broadly analgesic in acute pain models using noxious thermal, mechanical, and chemical stimuli. ST-2530 also reversed thermal hypersensitivity after a surgical incision on the plantar surface of the hind paw. In the spared nerve injury model of neuropathic pain, ST-2530 transiently reversed mechanical allodynia. These analgesic effects were demonstrated at doses that did not affect locomotion, motor coordination, or olfaction. Collectively, results from this study indicate that pharmacological inhibition of Nav1.7 by a small molecule agent with affinity for the resting state of the channel is sufficient to produce analgesia in a range of preclinical pain models.

In silico development of potential therapeutic for the pain treatment by inhibiting voltage-gated sodium channel 1.7

The voltage-gated sodium channel Nav1.7 can be considered as a promising target for the treatment of pain. This research presents conformational-independent and 3D field-based QSAR modeling for a series of aryl sulfonamide acting as Nav1.7 inhibitors. As descriptors used for building conformation-independent QSAR models, SMILES notation and local invariants of the molecular graph were used with the Monte Carlo optimization method as a model developer. Different statistical methods, including the index of ideality of correlation, were used to test the quality of the developed models, robustness and predictability and obtained results were good. Obtained results indicate that there is a very good correlation between 3D QSAR and conformation-independent models. Molecular fragments that account for the increase/decrease of a studied activity were defined and used for the computer-aided design of new compounds as potential analgesics. The final evaluation of the developed QSAR models and designed inhibitors were carried out using molecular docking studies, bringing to light an excellent correlation with the QSAR modeling results.

Engineering of highly potent and selective HNTX-III mutant against hNav1.7 sodium channel for treatment of pain

Human voltage-gated sodium channel Na_v1.7 (hNa_v1.7) is involved in the generation and conduction of neuropathic and nociceptive pain signals. Compelling genetic and preclinical studies have validated that hNa_v1.7 is a therapeutic target for the treatment of pain; however, there is a dearth of currently available compounds capable of targeting hNav1.7 with high potency and specificity. Hainantoxin-III (HNTX-III) is a 33-residue polypeptide from the venom of the spider Ornithoctonus hainana. It is a selective antagonist of neuronal tetrodotoxin-sensitive voltage-gated sodium channels. Here, we report the engineering of improved potency and Na_v selectivity of hNa_v1.7 inhibition peptides derived from the HNTX-III scaffold. Alanine scanning mutagenesis showed key residues for HNTX-III interacting with hNa_v1.7. Site-directed mutagenesis analysis indicated key residues on hNa_v1.7 interacting with HNTX-III. Molecular docking was conducted to clarify the binding interface between HNTX-III and Nav1.7 and guide the molecular engineering process. Ultimately, we obtained H4 [K0G1-P18K-A21L-V] based on molecular docking of HNTX-III and hNa_v1.7 with a 30-fold improved potency (IC₅₀ 0.007 ± 0.001 μM) and >1000-fold selectivity against Na_v1.4 and Na_v1.5. H4 also showed robust analgesia in the acute and chronic inflammatory pain model and neuropathic pain model. Thus, our results provide further insight into peptide toxins that may prove useful in guiding the development of inhibitors with improved potency and selectivity for Na_v subtypes with robust analgesia.

FGF13 Is Required for Histamine-Induced Itch Sensation by Interaction with NaV1.7

Itch can be induced by activation of small-diameter DRG neurons, which express abundant intracellular fibroblast growth factor 13 (FGF13). Although FGF13 is revealed to be essential for heat nociception, its role in mediating itch remains to be investigated. Here, we reported that loss of FGF13 in mouse DRG neurons impaired the histamine-induced scratching behavior. Calcium imaging showed that the percentage of histamine-responsive DRG neurons was largely decreased in FGF13-deficient mice; and consistently, electrophysiological recording exhibited that histamine failed to evoke action potential firing in most DRG neurons from these mice. Given that the reduced histamine-evoked neuronal response was caused by knockdown of FGF13 but not by FGF13A deficiency, FGF13B was supposed to mediate this process. Furthermore, overexpression of histamine Type 1 receptor H1R, but not H2R, H3R, nor H4R, increased the percentage of histamine-responsive DRG neurons, and the scratching behavior in FGF13-deficient mice was highly reduced by selective activation of H1R, suggesting that H1R is mainly required for FGF13-mediated neuronal response and scratching behavior induced by histamine. However, overexpression of H1R failed to rescue the histamine-evoked neuronal response in FGF13-deficient mice. Histamine enhanced the FGF13 interaction with Na_V1.7. Disruption of this interaction by a membrane-permeable competitive peptide, GST-Flag-Na_V1.7CT-TAT, reduced the percentage of histamine-responsive DRG neurons, and impaired the histamine-induced scratching, indicating that the FGF13/Na_V1.7 interaction is a key molecular determinant in the histamine-induced itch sensation. Therefore, our study reveals a novel role of FGF13 in mediating itch sensation via the interaction of Na_V1.7 in the peripheral nervous system.SIGNIFICANCE STATEMENT Scratching induced by itch brings serious tissue damage in chronic itchy diseases, and targeting itch-sensing molecules is crucial for its therapeutic intervention. Here, we reveal that FGF13 is required for the neuronal excitation and scratching behavior induced by histamine. We further provide the evidence that the histamine-evoked neuronal response is mainly mediated by histamine Type 1 receptor H1R, and is largely attenuated in FGF13-deficent mice. Importantly, we identify that histamine enhances the FGF13/Na_V1.7 interaction, and disruption of this interaction reduces histamine-evoked neuronal excitation and highly impairs histamine-induced scratching behavior. Additionally, we also find that FGF13 is involved in 5-hydroxytryptamine-induced scratching behavior and hapten 1-fluoro-2,4-dinitrobenzene-induced chronic itch.

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.

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.

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.

Molecular determinants for ligand binding at Nav1.4 and Nav1.7 channels: Experimental affinity results analyzed by molecular modeling

Here, we focused on exploring the selectivity mechanism against Nav1.7 over Nav1.4 due to different binding modes of two selected inhibitors. By the superposition of Nav1.7 and Nav1.4 proteins, we selected the most homologous chain of Nav1.7 with Nav1.4, defining the active site of Nav1.4-VSD4 based on the aryl sulfonamide binding site of Nav1.7-VSD4. Comparison of the conformations exhibited by Tyr1386 (Nav1.4) and Tyr1537 (Nav1.7) suggested that the steric hindrance caused by Tyr1386 owned primary influence on inhibition selectivity, which was further verified through molecular docking and MD simulation of two representative inhibitors. Our finding would be helpful for discovery of selective Nav1.7 inhibitors over Nav1.4.

µ-TRTX-Ca1a: a novel neurotoxin from Cyriopagopus albostriatus with analgesic effects

Human genetic and pharmacological studies have demonstrated that voltage-gated sodium channels (VGSCs) are promising therapeutic targets for the treatment of pain. Spider venom contains many toxins that modulate the activity of VGSCs. To date, only 0.01% of such spider toxins has been explored, and thus there is a great potential for discovery of novel VGSC modulators as useful pharmacological tools or potential therapeutics. In the current study, we identified a novel peptide, µ-TRTX-Ca1a (Ca1a), in the venom of the tarantula Cyriopagopus albostriatus. This peptide consisted of 38 residues, including 6 cysteines, i.e. IFECSISCEIEKEGNGKKCKPKKCKGGWKCKFNICVKV. In HEK293T or ND7/23 cells expressing mammalian VGSCs, this peptide exhibited the strongest inhibitory activity on Na_v1.7 (IC₅₀ 378 nM), followed by Na_v1.6 (IC₅₀ 547 nM), Na_v1.2 (IC₅₀ 728 nM), Na_v1.3 (IC₅₀ 2.2 µM) and Na_v1.4 (IC₅₀ 3.2 µM), and produced negligible inhibitory effect on Na_v1.5, Na_v1.8, and Na_v1.9, even at high concentrations of up to 10 µM. Furthermore, this peptide did not significantly affect the activation and inactivation of Na_v1.7. Using site-directed mutagenesis of Na_v1.7 and Na_v1.4, we revealed that its binding site was localized to the DIIS3-S4 linker region involving the D816 and E818 residues. In three different mouse models of pain, pretreatment with Cala (100, 200, 500 µg/kg) dose-dependently suppressed the nociceptive responses induced by formalin, acetic acid or heat. These results suggest that Ca1a is a novel neurotoxin against VGSCs and has a potential to be developed as a novel analgesic.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.

Antibodies and venom peptides: new modalities for ion channels

Ion channels play fundamental roles in both excitable and non-excitable tissues and therefore constitute attractive drug targets for myriad neurological, cardiovascular and metabolic diseases as well as for cancer and immunomodulation. However, achieving selectivity for specific ion channel subtypes with small-molecule drugs has been challenging, and there currently is a growing trend to target ion channels with biologics. One approach is to improve the pharmacokinetics of existing or novel venom-derived peptides. In parallel, after initial studies with polyclonal antibodies demonstrated the technical feasibility of inhibiting channel function with antibodies, multiple preclinical programmes are now using the full spectrum of available technologies to generate conventional monoclonal and engineered antibodies or nanobodies against extracellular loops of ion channels. After a summary of the current state of ion channel drug discovery, this Review discusses recent developments using the purinergic receptor channel P2X purinoceptor 7 (P2X7), the voltage-gated potassium channel KV1.3 and the voltage-gated sodium channel NaV1.7 as examples of targeting ion channels with biologics.

Engineering NaV1.7 Inhibitory JzTx-V Peptides with a Potency and Basicity Profile Suitable for Antibody Conjugation To Enhance Pharmacokinetics

Drug discovery research on new pain targets with human genetic validation, including the voltage-gated sodium channel Na_V1.7, is being pursued to address the unmet medical need with respect to chronic pain and the rising opioid epidemic. As part of early research efforts on this front, we have previously developed Na_V1.7 inhibitory peptide-antibody conjugates with tarantula venom-derived GpTx-1 toxin peptides with an extended half-life (80 h) in rodents but only moderate in vitro activity (hNa_V1.7 IC₅₀ = 250 nM) and without in vivo activity. We identified the more potent peptide JzTx-V from our natural peptide collection and improved its selectivity against other sodium channel isoforms through positional analogueing. Here we report utilization of the JzTx-V scaffold in a peptide-antibody conjugate and architectural variations in the linker, peptide loading, and antibody attachment site. We found conjugates with 100-fold improved in vitro potency relative to those of complementary GpTx-1 analogues, but pharmacokinetic and bioimaging analyses of these JzTx-V conjugates revealed a shorter than expected plasma half-life in vivo with accumulation in the liver. In an attempt to increase circulatory serum levels, we sought the reduction of the net +6 charge of the JzTx-V scaffold while retaining a desirable Na_V in vitro activity profile. The conjugate of a JzTx-V peptide analogue with a +2 formal charge maintained Na_V1.7 potency with 18-fold improved plasma exposure in rodents. Balancing the loss of peptide and conjugate potency associated with the reduction of net charge necessary for improved target exposure resulted in a compound with moderate activity in a Na_V1.7-dependent pharmacodynamic model but requires further optimization to identify a conjugate that can fully engage Na_V1.7 in vivo.

The influence of voltage-gated sodium channels on human gastrointestinal nociception

BACKGROUND

Abdominal pain is a frequent and persistent problem in the most common gastrointestinal disorders, including irritable bowel syndrome and inflammatory bowel disease. Pain adversely impacts quality of life, incurs significant healthcare expenditures, and remains a challenging issue to manage with few safe therapeutic options currently available. It is imperative that new methods are developed for identifying and treating this symptom. A variety of peripherally active neuroendocrine signaling elements have the capability to influence gastrointestinal pain perception. A large and growing body of evidence suggests that voltage-gated sodium channels (VGSCs) play a critical role in the development and modulation of nociceptive signaling associated with the gut. Several VGSC isoforms demonstrate significant promise as potential targets for improved diagnosis and treatment of gut-based disorders associated with hyper- and hyposensitivity to abdominal pain.

PURPOSE

In this article, we critically review key investigations that have evaluated the potential role that VGSCs play in visceral nociception and discuss recent advances related to this topic. Specifically, we discuss the following: (a) what is known about the structure and basic function of VGSCs, (b) the role that each VGSC plays in gut nociception, particularly as it relates to human physiology, and (c) potential diagnostic and therapeutic uses of VGSCs to manage disorders associated with chronic abdominal pain.

Discovery of a Novel Nav1.7 Inhibitor From Cyriopagopus albostriatus Venom With Potent Analgesic Efficacy

Spider venoms contain a vast array of bioactive peptides targeting ion channels. A large number of peptides have high potency and selectivity toward sodium channels. Na_v1.7 contributes to action potential generation and propagation and participates in pain signaling pathway. In this study, we describe the identification of μ-TRTX-Ca2a (Ca2a), a novel 35-residue peptide from the venom of Vietnam spider Cyriopagopus albostriatus (C. albostriatus) that potently inhibits Na_v1.7 (IC₅₀ = 98.1 ± 3.3 nM) with high selectivity against skeletal muscle isoform Na_v1.4 (IC₅₀ > 10 μM) and cardiac muscle isoform Na_v1.5 (IC₅₀ > 10 μM). Ca2a did not significantly alter the voltage-dependent activation or fast inactivation of Na_v1.7, but it hyperpolarized the slow inactivation. Site-directed mutagenesis analysis indicated that Ca2a bound with Na_v1.7 at the extracellular S3–S4 linker of domain II. Meanwhile, Ca2a dose-dependently attenuated pain behaviors in rodent models of formalin-induced paw licking, hot plate test, and acetic acid-induced writhing. This study indicates that Ca2a is a potential lead molecule for drug development of novel analgesics.

In silico exploration of aryl sulfonamide analogs as voltage-gated sodium channel 1.7 inhibitors by using 3D-QSAR, molecular docking study, and molecular dynamics simulations

It has been demonstrated by human genetics that the voltage-gated sodium channel Nav1.7 is currently a promising target for the treatment of pain. In this research, we performed molecular simulation works on a series of classic aryl sulfonamide Nav1.7 inhibitors using three-dimensional quantitative structure-activity relationships (3D-QSAR), molecular docking and molecular dynamics (MD) simulations for the first time to explore the correlation between their structures and activities. The results of the relevant statistical parameters of comparative molecular field analyses (CoMFA) and comparative molecular similarity indices analyses (CoMSIA) had been verified to be reasonable, and the deep relationship between the structures and activities of these inhibitors was obtained by analyzing the contour maps. The generated 3D-QSAR model showed a good predictive ability and provided valuable clues for the rational modification of molecules. The interactions between compounds and proteins were modeled by molecular docking studies. Finally, accuracy of the docking results and stability of the complexes were verified by 100 ns MD simulations. Detailed information on the key residues at the binding site and the types of interactions they participate in involved was obtained. The van der Waals energy contributed the most in the molecular binding process according to the calculation of binding free energy. All research results provided a good basis for further research on novel and effective Nav1.7 inhibitors.

Pharmacological characterization of potent and selective NaV1.7 inhibitors engineered from Chilobrachys jingzhao tarantula venom peptide JzTx-V

Identification of voltage-gated sodium channel NaV1.7 inhibitors for chronic pain therapeutic development is an area of vigorous pursuit. In an effort to identify more potent leads compared to our previously reported GpTx-1 peptide series, electrophysiology screening of fractionated tarantula venom discovered the NaV1.7 inhibitory peptide JzTx-V from the Chinese earth tiger tarantula Chilobrachys jingzhao. The parent peptide displayed nominal selectivity over the skeletal muscle NaV1.4 channel. Attribute-based positional scan analoging identified a key Ile28Glu mutation that improved NaV1.4 selectivity over 100-fold, and further optimization yielded the potent and selective peptide leads AM-8145 and AM-0422. NMR analyses revealed that the Ile28Glu substitution changed peptide conformation, pointing to a structural rationale for the selectivity gains. AM-8145 and AM-0422 as well as GpTx-1 and HwTx-IV competed for ProTx-II binding in HEK293 cells expressing human NaV1.7, suggesting that these NaV1.7 inhibitory peptides interact with a similar binding site. AM-8145 potently blocked native tetrodotoxin-sensitive (TTX-S) channels in mouse dorsal root ganglia (DRG) neurons, exhibited 30- to 120-fold selectivity over other human TTX-S channels and exhibited over 1,000-fold selectivity over other human tetrodotoxin-resistant (TTX-R) channels. Leveraging NaV1.7-NaV1.5 chimeras containing various voltage-sensor and pore regions, AM-8145 mapped to the second voltage-sensor domain of NaV1.7. AM-0422, but not the inactive peptide analog AM-8374, dose-dependently blocked capsaicin-induced DRG neuron action potential firing using a multi-electrode array readout and mechanically-induced C-fiber spiking in a saphenous skin-nerve preparation. Collectively, AM-8145 and AM-0422 represent potent, new engineered NaV1.7 inhibitory peptides derived from the JzTx-V scaffold with improved NaV selectivity and biological activity in blocking action potential firing in both DRG neurons and C-fibers.

Engineering Antibody Reactivity for Efficient Derivatization to Generate NaV1.7 Inhibitory GpTx-1 Peptide-Antibody Conjugates

The voltage-gated sodium channel Na_V1.7 is a genetically validated pain target under investigation for the development of analgesics. A therapeutic with a less frequent dosing regimen would be of value for treating chronic pain; however functional Na_V1.7 targeting antibodies are not known. In this report, we describe Na_V1.7 inhibitory peptide-antibody conjugates as an alternate construct for potential prolonged channel blockade through chemical derivatization of engineered antibodies. We previously identified Na_V1.7 inhibitory peptide GpTx-1 from tarantula venom and optimized its potency and selectivity. Tethering GpTx-1 peptides to antibodies bifunctionally couples FcRn-based antibody recycling attributes to the Na_V1.7 targeting function of the peptide warhead. Herein, we conjugated a GpTx-1 peptide to specific engineered cysteines in a carrier anti-2,4-dinitrophenol monoclonal antibody using polyethylene glycol linkers. The reactivity of 13 potential cysteine conjugation sites in the antibody scaffold was tuned using a model alkylating agent. Subsequent reactions with the peptide identified cysteine locations with the highest conversion to desired conjugates, which blocked Na_V1.7 currents in whole cell electrophysiology. Variations in attachment site, linker, and peptide loading established design parameters for potency optimization. Antibody conjugation led to in vivo half-life extension by 130-fold relative to a nonconjugated GpTx-1 peptide and differential biodistribution to nerve fibers in wild-type but not Na_V1.7 knockout mice. This study describes the optimization and application of antibody derivatization technology to functionally inhibit Na_V1.7 in engineered and neuronal cells.

Using automated patch clamp electrophysiology platforms in pain-related ion channel research: insights from industry and academia

Automated patch clamp (APC) technology was first developed at the turn of the millennium. The increased throughput it afforded promised a new paradigm in ion channel recordings, offering the potential to overcome the time-consuming, low-throughput bottleneck, arising from manual patch clamp investigations. This has relevance to the fast-paced development of novel therapies for chronic pain. This review highlights the advances in technology, using select examples that have facilitated APC usage in both industry and academia. It covers both first generation and the latest developments in second-generation platforms. In addition, it also provides an overview of the pain research field and how APC platforms have furthered our understanding of ion channel research and the development of pharmacological tools and therapeutics. APC platforms have much to offer to the ion channel research community, and this review highlights areas of best practice for both academia and industry. The impact of APC platforms and the prospects of ion channel research and improved therapeutics for chronic pain will be evaluated.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.