Fifteen years of Na V 1.7 channels as an analgesic target: Why has excellent in vitro pharmacology not translated into in vivo analgesic efficacy?

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.

Frontiers in Acute Pain Management: Emerging Concepts in Pain Pathways and the Role of VX-548 as a Novel NaV1.8 Inhibitor: A Narrative Review

PURPOSE OF REVIEW

Despite ongoing research into alternative postsurgical pain treatments, opioids remain widely used analgesics regardless of associated adverse effects, including dependence and overdose, as demonstrated throughout the current opioid crisis. This is likely related to a failure in proving the efficacy of alternative analgesics in clinical trials, despite strong evidence supporting the potential for effective analgesia through in vitro studies. While NaV1.7 and NaV1.8 channels have shown to be key components of pain perception, studies regarding pharmacological agents utilizing these channels as targets have largely failed to demonstrate the efficacy of these proposed analgesics when compared to current multimodal pain treatment regimens.

RECENT FINDINGS

However, the novel NaV1.8 channel inhibitor, VX-548 has surpassed previously studied NaV1.8 inhibitors in clinical trials and continues to hold promise of a novel efficacious analgesic to potentially be utilized in multimodal pain treatment on postsurgical patients. Additionally, NaV1.8 is encoded by the SCN10A, which has been shown to be minimally expressed in the brain, suggesting a lower likelihood of adverse effects in the CNS, including dependence and abuse. Novel pharmacologic analgesics that are efficacious without the significant side effects associated with opioids have lacked meaningful development. However, recent clinical trials have shown promising results in the safety and efficacy of the pharmacological agent VX-548. Still, more clinical trials directly comparing the efficacy of VX-548 to standard of care post-surgical drugs, including opioids like morphine and hydromorphone are needed to demonstrate the long-term viability of the agent replacing current opioids with an unfavorable side effect profile.

Carbenoid-involved reactions integrated with scaffold-based screening generates a Nav1.7 inhibitor

The discovery of selective Nav1.7 inhibitors is a promising approach for developing anti-nociceptive drugs. In this study, we present a novel oxindole-based readily accessible library (OREAL), which is characterized by readily accessibility, unique chemical space, ideal drug-like properties, and structural diversity. We used a scaffold-based approach to screen the OREAL and discovered compound C4 as a potent Nav1.7 inhibitor. The bioactivity characterization of C4 reveals that it is a selective Nav1.7 inhibitor and effectively reverses Paclitaxel-induced neuropathic pain (PINP) in rodent models. Preliminary toxicology study shows C4 is negative to hERG. The consistent results of molecular docking and molecular simulations further support the reasonability of the in-silico screening and show the insight of the binding mode of C4. Our discovery of C4 paves the way for pushing the Nav1.7-based anti-nociceptive drugs forward to the clinic.

The pain target NaV1.7 is expressed late during human iPS cell differentiation into sensory neurons as determined in high-resolution imaging

Human-induced pluripotent stem cells (iPS cells) are efficiently differentiated into sensory neurons. These cells express the voltage-gated sodium channel NaV1.7, which is a validated pain target. NaV1.7 deficiency leads to pain insensitivity, whereas NaV1.7 gain-of-function mutants are associated with chronic pain. During differentiation, the sensory neurons start spontaneous action potential firing around day 22, with increasing firing rate until day 40. Here, we used CRISPR/Cas9 genome editing to generate a HA-tag NaV1.7 to follow its expression during differentiation. We used two protocols to generate sensory neurons: the classical small molecule approach and a directed differentiation methodology and assessed surface NaV1.7 expression by Airyscan high-resolution microscopy. Our results show that maturation of at least 49 days is necessary to observe robust NaV1.7 surface expression in both protocols. Electric activity of the sensory neurons precedes NaV1.7 surface expression. A clinically effective NaV1.7 blocker is still missing, and we expect this iPS cell model system to be useful for drug discovery and disease modeling.

Regulating neuronal excitability: The role of S-palmitoylation in NaV1.7 activity and voltage sensitivity

S-palmitoylation, a reversible lipid post-translational modification, regulates the functions of numerous proteins. Voltage-gated sodium channels (NaVs), pivotal in action potential generation and propagation within cardiac cells and sensory neurons, can be directly or indirectly modulated by S-palmitoylation, impacting channel trafficking and function. However, the role of S-palmitoylation in modulating NaV1.7, a significant contributor to pain pathophysiology, has remained unexplored. Here, we addressed this knowledge gap by investigating if S-palmitoylation influences NaV1.7 channel function. Acyl-biotin exchange assays demonstrated that heterologously expressed NaV1.7 channels are modified by S-palmitoylation. Blocking S-palmitoylation with 2-bromopalmitate resulted in reduced NaV1.7 current density and hyperpolarized steady-state inactivation. We identified two S-palmitoylation sites within NaV1.7, both located in the second intracellular loop, which regulated different properties of the channel. Specifically, S-palmitoylation of cysteine 1126 enhanced NaV1.7 current density, while S-palmitoylation of cysteine 1152 modulated voltage-dependent inactivation. Blocking S-palmitoylation altered excitability of rat dorsal root ganglion neurons. Lastly, in human sensory neurons, NaV1.7 undergoes S-palmitoylation, and the attenuation of this post-translational modification results in alterations in the voltage-dependence of activation, leading to decreased neuronal excitability. Our data show, for the first time, that S-palmitoylation affects NaV1.7 channels, exerting regulatory control over their activity and, consequently, impacting rodent and human sensory neuron excitability. These findings provide a foundation for future pharmacological studies, potentially uncovering novel therapeutic avenues in the modulation of S-palmitoylation for NaV1.7 channels.

Using automated patch clamp electrophysiology platforms in ion channel drug discovery: an industry perspective

INTRODUCTION

Automated patch clamp (APC) is now well established as a mature technology for ion channel drug discovery in academia, biotech and pharma companies, and in contract research organizations (CRO), for a variety of applications including channelopathy research, compound screening, target validation and cardiac safety testing.

AREAS COVERED

Ion channels are an important class of drugged and approved drug targets. The authors present a review of the current state of ion channel drug discovery along with new and exciting developments in ion channel research involving APC. This includes topics such as native and iPSC-derived cells in ion channel drug discovery, channelopathy research, organellar and biologics in ion channel drug discovery.

EXPERT OPINION

It is our belief that APC will continue to play a critical role in ion channel drug discovery, not only in 'classical' hit screening, target validation and cardiac safety testing, but extending these applications to include high throughput organellar recordings and optogenetics. In this way, with advancements in APC capabilities and applications, together with high resolution cryo-EM structures, ion channel drug discovery will be re-invigorated, leading to a growing list of ion channel ligands in clinical development.

From genome to proteome: Comprehensive identification of venom toxins from the Chinese funnel-web spider (Macrothelidae: Macrothele yani)

Macrothelidae is a family of mygalomorph spiders containing the extant genera Macrothele and Vacrothele. China is an important center of diversity for Macrothele with 65 % of the known species occurring there. Previous work on Macrothele was able to uncover several important toxin compounds including Raventoxin which may have applications in biomedicine and agricultural chemistry. Despite the importance of Macrothele spiders, high-quality reference genomes are still lacking, which hinders our understanding and application of the toxin compounds. In this study, we assembled the genome of the Macrothele yani to help fill gaps in our understanding of toxin biology in this lineage of spiders to encourage the future study and applications of these compounds. The final assembled genome was 6.79 Gb in total length, had a contig N50 of 21.44 Mb, and scaffold N50 of 156.16 Mb. Hi-C scaffolding assigned 98.19 % of the genome to 46 pseudo-chromosomes with a BUSCO score of 95.7 % for the core eukaryotic gene set. The assembled genome was found to contain 75.62 % repetitive DNA and a total of 39,687 protein-coding genes were annotated making it the spider genome with highest number of genes. Through integrated analysis of venom gland transcriptomics and venom proteomics, a total of 194 venom toxins were identified, including 38 disulfide-rich peptide neurotoxins, among which 12 were ICK knottin peptides. In summary, we present the first high-quality genome assembly at the chromosomal level for any Macrothelidae spider, filling an important gap in our knowledge of these spiders. Such high-quality genomic data will be invaluable as a reference in resolving Araneae spider phylogenies and in screening different spider species for novel compounds applicable to numerous medical and agricultural applications.

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.

Readiness of nociceptor cell bodies to generate spontaneous activity results from background activity of diverse ion channels and high input resistance

ABSTRACT

Nociceptor cell bodies generate "spontaneous" discharge that can promote ongoing pain in persistent pain conditions. Little is known about the underlying mechanisms. Recordings from nociceptor cell bodies (somata) dissociated from rodent and human dorsal root ganglia have shown that previous pain in vivo is associated with low-frequency discharge controlled by irregular depolarizing spontaneous fluctuations of membrane potential (DSFs), likely produced by transient inward currents across the somal input resistance. Using mouse nociceptors, we show that DSFs are associated with high somal input resistance over a wide range of membrane potentials, including depolarized levels where DSFs approach action potential (AP) threshold. Input resistance and both the amplitude and frequency of DSFs were increased in neurons exhibiting spontaneous activity. Ion substitution experiments indicated that the depolarizing phase of DSFs is generated by spontaneous opening of channels permeable to Na + or Ca 2+ and that Ca 2+ -permeable channels are especially important for larger DSFs. Partial reduction of the amplitude or frequency of DSFs by perfusion of pharmacological inhibitors indicated small but significant contributions from Nav1.7, Nav1.8, TRPV1, TRPA1, TRPM4, and N-type Ca 2+ channels. Less specific blockers suggested a contribution from NALCN channels, and global knockout suggested a role for Nav1.9. The combination of high somal input resistance plus background activity of diverse ion channels permeable to Na + or Ca 2+ produces DSFs that are poised to reach AP threshold if resting membrane potential depolarizes, AP threshold decreases, or DSFs become enhanced-all of which can occur under painful neuropathic and inflammatory conditions.

Differential expression of slow and fast-repriming tetrodotoxin-sensitive sodium currents in dorsal root ganglion neurons

Sodium channel Nav1.7 triggers the generation of nociceptive action potentials and is important in sending pain signals under physiological and pathological conditions. However, studying endogenous Nav1.7 currents has been confounded by co-expression of multiple sodium channel isoforms in dorsal root ganglion (DRG) neurons. In the current study, slow-repriming (SR) and fast-repriming (FR) tetrodotoxin-sensitive (TTX-S) currents were dissected electrophysiologically in small DRG neurons of both rats and mice. Three subgroups of small DRG neurons were identified based on the expression pattern of SR and FR TTX-S currents. A majority of rat neurons only expressed SR TTX-S currents, while a majority of mouse neurons expressed additional FR TTX-S currents. ProTx-II inhibited SR TTX-S currents with variable efficacy among DRG neurons. The expression of both types of TTX-S currents was higher in Isolectin B4-negative (IB4-) compared to Isolectin B4-positive (IB4+) neurons. Paclitaxel selectively increased SR TTX-S currents in IB4- neurons. In simulation experiments, the Nav1.7-expressing small DRG neuron displayed lower rheobase and higher frequency of action potentials upon threshold current injections compared to Nav1.6. The results suggested a successful dissection of endogenous Nav1.7 currents through electrophysiological manipulation that may provide a useful way to study the functional expression and pharmacology of endogenous Nav1.7 channels in DRG neurons.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

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.

Design, synthesis, and biological evaluation of acyl sulfonamide derivatives with spiro cycles as NaV1.7 inhibitors for antinociception

Chronic pain, as an unmet medical need, severely impacts the quality of life. The voltage-gated sodium channel Na_V1.7 preferentially expressed in sensory neurons of dorsal root ganglia (DRG) serves a promising target for pain therapy. Here, we report the design, synthesis, and evaluation of a series of acyl sulfonamide derivatives targeting Nav1.7 for their antinociceptive activities. Among the derivatives tested, the compound 36c was identified as a selective and potent Na_V1.7 inhibitor in vitro and exhibited antinociceptive effects in vivo. The identification of 36c not only provides a new insight into the discovery of selective Na_V1.7 inhibitors, but also may hold premise for pain therapy.

Discovery of selective NaV1.8 inhibitors based on 5-chloro-2-(4,4-difluoroazepan-1-yl)-6-methyl nicotinamide scaffold for the treatment of pain

The Na_V1.8 channel is a genetically validated target for pain and it is mostly expressed in the peripheral nervous system. Based on the disclosed structures of Na_V1.8-selective inhibitors, we designed and synthesized a series of compounds by introducing bicyclic aromatic fragments based on the nicotinamide scaffold. In this research, a systematic structure-activity relationship study was carried out. While compound 2c possessed moderate inhibitory activity (IC₅₀ = 50.18 ± 0.04 nM) in HEK293 cells stably expressing human Na_V1.8 channels, it showed potent inhibitory activity in DRG neurons and isoform selectivity (>200-fold against human Na_V1.1, Na_V1.5 and Na_V1.7 channels). Moreover, the analgesic potency of compound 2c was identified in a post-surgical mouse model. These data demonstrate that compound 2c can be further evaluated as a non-addictive analgesic agent with reduced cardiac liabilities.

NaV1.7 mRNA and protein expression in putative projection neurons of the human spinal dorsal horn

NaV1.7, a membrane-bound voltage-gated sodium channel, is preferentially expressed along primary sensory neurons, including their peripheral & central nerve endings, axons, and soma within the dorsal root ganglia and plays an integral role in amplifying membrane depolarization and pain neurotransmission. Loss- and gain-of-function mutations in the gene encoding NaV1.7, SCN9A, are associated with a complete loss of pain sensation or exacerbated pain in humans, respectively. As an enticing pain target supported by human genetic validation, many compounds have been developed to inhibit NaV1.7 but have disappointed in clinical trials. The underlying reasons are still unclear, but recent reports suggest that inhibiting NaV1.7 in central terminals of nociceptor afferents is critical for achieving pain relief by pharmacological inhibition of NaV1.7. We report for the first time that NaV1.7 mRNA is expressed in putative projection neurons (NK1R+) in the human spinal dorsal horn, predominantly in lamina 1 and 2, as well as in deep dorsal horn neurons and motor neurons in the ventral horn. NaV1.7 protein was found in the central axons of sensory neurons terminating in lamina 1-2, but also was detected in the axon initial segment of resident spinal dorsal horn neurons and in axons entering the anterior commissure. Given that projection neurons are critical for conveying nociceptive information from the dorsal horn to the brain, these data support that dorsal horn NaV1.7 expression may play an unappreciated role in pain phenotypes observed in humans with genetic SCN9A mutations, and in achieving analgesic efficacy in clinical trials.

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.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Small molecule targeting NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces pain in chronic constriction injury (CCI) rats

The voltage-gated sodium channel isoform NaV1.7 is a critical player in the transmission of nociceptive information. This channel has been heavily implicated in human genetic pain disorders and is a validated pain target. However, targeting this channel directly has failed, and an indirect approach – disruption of interactions with accessory protein partners – has emerged as a viable alternative strategy. We recently reported that a small-molecule inhibitor of CRMP2 SUMOylation, compound 194, selectively reduces NaV1.7 currents in DRG neurons across species from mouse to human. This compound also reversed mechanical allodynia in a spared nerve injury and chemotherapy-induced model of neuropathic pain. Here, we show that oral administration of 194 reverses mechanical allodynia in a chronic constriction injury (CCI) model of neuropathic pain. Furthermore, we show that orally administered 194 reverses the increased latency to cross an aversive barrier in a mechanical conflict-avoidance task following CCI. These two findings, in the context of our previous report, support the conclusion that 194 is a robust inhibitor of NaV1.7 function with the ultimate effect of profoundly ameliorating mechanical allodynia associated with nerve injury. The fact that this was observed using both traditional, evoked measures of pain behavior as well as the more recently developed operator-independent mechanical conflict-avoidance assay increases confidence in the efficacy of 194-induced anti-nociception.

Life-threatening hypothermia in a child with primary erythromelalgia

Primary erythromelalgia is a rare autosomal-dominant condition due to pathogenic variant in the SCN9A gene, characterized by childhood onset of excruciating pain, redness, and warmth of acral sites. Patients often resort to ice water baths and other cooling measures to manage the discomfort. Hypothermia is a rare complication, reported only twice previously. We report a child with primary erythromelalgia due to a confirmed pathogenic variant admitted with life-threatening hypothermia. Although the overuse of cooling mechanisms may have contributed, we postulate that the SCN9A mutation may lead to thermodysregulation and make patients with primary erythromelalgia particularly susceptible to this complication.

Paracelsus' legacy in the faunal realm: Drugs deriving from animal toxins

Given the vast number of venomous and poisonous animals, it is surprising that only relatively few animal-derived toxins have been explored and made their way into marketed drugs or are being investigated in ongoing clinical trials. In this review, we highlight marketed drugs deriving from animal toxins as well as ongoing clinical trials and preclinical investigations in the field. We emphasize that more attention should be paid to the rich supply of candidates that nature provides as valuable starting points for addressing serious unmet medical needs.

Pharmacological Inhibition of the Voltage-Gated Sodium Channel NaV1.7 Alleviates Chronic Visceral Pain in a Rodent Model of Irritable Bowel Syndrome

The human nociceptor-specific voltage-gated sodium channel 1.7 (hNa_V1.7) is critical for sensing various types of somatic pain, but it appears not to play a primary role in acute visceral pain. However, its role in chronic visceral pain remains to be determined. We used assay-guided fractionation to isolate a novel hNa_V1.7 inhibitor, Tsp1a, from tarantula venom. Tsp1a is 28-residue peptide that potently inhibits hNa_V1.7 (IC₅₀ = 10 nM), with greater than 100-fold selectivity over hNa_V1.3-hNa_V1.6, 45-fold selectivity over hNa_V1.1, and 24-fold selectivity over hNa_V1.2. Tsp1a is a gating modifier that inhibits Na_V1.7 by inducing a hyperpolarizing shift in the voltage-dependence of channel inactivation and slowing recovery from fast inactivation. NMR studies revealed that Tsp1a adopts a classical knottin fold, and like many knottin peptides, it is exceptionally stable in human serum. Remarkably, intracolonic administration of Tsp1a completely reversed chronic visceral hypersensitivity in a mouse model of irritable bowel syndrome. The ability of Tsp1a to reduce visceral hypersensitivity in a model of irritable bowel syndrome suggests that pharmacological inhibition of hNa_V1.7 at peripheral sensory nerve endings might be a viable approach for eliciting analgesia in patients suffering from chronic visceral pain.

Nav1.7 target modulation and efficacy can be measured in nonhuman primate assays

Humans with loss-of-function mutations in the Na_v1.7 channel gene (SCN9A) show profound insensitivity to pain, whereas those with gain-of-function mutations can have inherited pain syndromes. Therefore, inhibition of the Na_v1.7 channel with a small molecule has been considered a promising approach for the treatment of various human pain conditions. To date, clinical studies conducted using selective Na_v1.7 inhibitors have not provided analgesic efficacy sufficient to warrant further investment. Clinical studies to date used multiples of in vitro IC₅₀ values derived from electrophysiological studies to calculate anticipated human doses. To increase the chance of clinical success, we developed rhesus macaque models of action potential propagation, nociception, and olfaction, to measure Na_v1.7 target modulation in vivo. The potent and selective Na_v1.7 inhibitors SSCI-1 and SSCI-2 dose-dependently blocked C-fiber nociceptor conduction in microneurography studies and inhibited withdrawal responses to noxious heat in rhesus monkeys. Pharmacological Na_v1.7 inhibition also reduced odor-induced activation of the olfactory bulb (OB), measured by functional magnetic resonance imaging (fMRI) studies consistent with the anosmia reported in Na_v1.7 loss-of-function patients. These data demonstrate that it is possible to measure Na_v1.7 target modulation in rhesus macaques and determine the plasma concentration required to produce a predetermined level of inhibition. The calculated plasma concentration for preclinical efficacy could be used to guide human efficacious exposure estimates. Given the translatable nature of the assays used, it is anticipated that they can be also used in phase 1 clinical studies to measure target modulation and aid in the interpretation of phase 1 clinical data.

Neuroscience and Neuroimmunology Solutions for Osteoarthritis Pain: Biological Drugs, Growth Factors, Peptides and Monoclonal Antibodies Targeting Peripheral Nerves

Neuroscience is a vast discipline that deals with the anatomy, biochemistry, molecular biology, physiology and pathophysiology of central and peripheral nerves. Advances made through basic, translational, and clinical research in the field of neuroscience have great potential for long-lasting and beneficial impacts on human and animal health. The emerging field of biological therapy is intersecting with the disciplines of neuroscience, orthopaedics and rheumatology, creating new horizons for interdisciplinary and applied research. Biological drugs, growth factors, therapeutic peptides and monoclonal antibodies are being developed and tested for the treatment of painful arthritic and rheumatic diseases. This concise communication focuses on the solutions provided by the fields of neuroscience and neuroimmunology for real-world clinical problems in the field of orthopaedics and rheumatology, focusing on synovial joint pain and the emerging biological treatments that specifically target pathways implicated in osteoarthritis pain in peripheral nerves.

Spider Venom Peptide Pn3a Inhibition of Primary Afferent High Voltage-Activated Calcium Channels

Despite potently inhibiting the nociceptive voltage-gated sodium (Na_v) channel, Na_v1.7, µ-theraphotoxin Pn3a is antinociceptive only upon co-administration with sub-therapeutic opioid agonists, or by itself at doses >3,000-fold greater than its Na_v1.7 IC₅₀ by a yet undefined mechanism. Na_v channels are structurally related to voltage-gated calcium (Ca_v) channels, Ca_v1 and Ca_v2. These channels mediate the high voltage-activated (HVA) calcium currents (I_Ca) that orchestrate synaptic transmission in nociceptive dorsal root ganglion (DRG) neurons and are fine-tuned by opioid receptor (OR) activity. Using whole-cell patch clamp recording, we found that Pn3a (10 µM) inhibits ∼55% of rat DRG neuron HVA-I_Ca and 60–80% of Ca_v1.2, Ca_v1.3, Ca_v2.1, and Ca_v2.2 mediated currents in HEK293 cells, with no inhibition of Ca_v2.3. As a major DRG I_Ca component, Ca_v2.2 inhibition by Pn3a (IC₅₀ = 3.71 ± 0.21 µM) arises from an 18 mV hyperpolarizing shift in the voltage dependence of inactivation. We observed that co-application of Pn3a and µ-OR agonist DAMGO results in enhanced HVA-I_Ca inhibition in DRG neurons whereas co-application of Pn3a with the OR antagonist naloxone does not, underscoring HVA channels as shared targets of Pn3a and opioids. We provide evidence that Pn3a inhibits native and recombinant HVA Ca_vs at previously reportedly antinociceptive concentrations in animal pain models. We show additive modulation of DRG HVA-I_Ca by sequential application of low Pn3a doses and sub-therapeutic opioids ligands. We propose Pn3a's antinociceptive effects result, at least in part, from direct inhibition of HVA-I_Ca at high Pn3a doses, or through additive inhibition by low Pn3a and mild OR activation.