Voltage-gated sodium channels: the search for subtype-selective analgesics

A phase I, randomized, double-blind, placebo-controlled, single- and multiple dose escalation study evaluating the safety, pharmacokinetics and pharmacodynamics of VX-128, a highly selective Nav 1.8 inhibitor, in healthy adults

Selective inhibition of certain voltage-gated sodium channels (Nav s), such as Nav 1.8, is of primary interest for pharmacological pain research and widely studied as a pharmacological target due to its contribution to repetitive firing, neuronal excitability, and pain chronification. VX-128 is a highly potent and selective Nav 1.8 inhibitor that was being developed as a treatment for pain. We evaluated the safety, tolerability, and pharmacokinetics of VX-128 in healthy subjects in a single- and multiple-ascending dose (MAD) first-in-human study. Pharmacodynamics were evaluated in the MAD part using a battery of evoked pain tests. Overall, single doses of VX-128 up to 300 mg were well-tolerated, although adverse effect (AE) incidence was higher in subjects receiving VX-128 (41.7%) compared with placebo (25.0%). After multiple dosing of up to 10 days, skin rash events were observed at all dose levels (up to 100 mg once daily [q.d.]), in five of 26 (19.2%) subjects, including one subject receiving VX-128 (100 mg q.d.) who had a serious AE of angioedema. A trend in pain tolerance were observed for cold pressor- and pressure pain, which was dose-dependent for the latter. VX-128 was rapidly absorbed (median time to maximum plasma concentration between 1 and 2 h) with a half-life of ~80 h at 10 mg q.d., and approximately two-fold accumulation ratio after 10 and 30 mg q.d. Although VX-128, when given in a multiple dose fashion, resulted in early study termination due to tolerability issues, effects were observed on multiple pain tests that may support further investigation of Nav 1.8 inhibitors as pain treatments.

A Phase 1, Randomized, Double-Blind, Placebo-Controlled, Crossover Study to Evaluate the Pharmacodynamic Effects of VX-150, a Highly Selective NaV1.8 Inhibitor, in Healthy Male Adults

OBJECTIVE

To evaluate the analgesic potential, safety, tolerability, and pharmacokinetics of VX-150, a pro-drug of a highly selective NaV1.8 inhibitor, in healthy subjects.

DESIGN

This was a randomized, double-blind, placebo-controlled, crossover study in healthy subjects.

SUBJECTS

Twenty healthy male subjects with an age of 18-55 years, inclusive, were enrolled. Eligibility was based on general fitness, absence of current or previous medical conditions that could compromise subject safety, and a training assessment of pain tolerance across pain tests to exclude highly tolerant individuals whose tolerance could compromise the ability to detect analgesic responses. All dosed subjects completed the study.

METHODS

Subjects were randomized 1:1 to one of two sequences receiving a single VX-150 dose and subsequently placebo, or vice versa, with at least 7 days between dosing. A battery of pain tests (pressure, electrical stair, [capsaicin-induced] heat, and cold pressor) was administered before dosing and repetitively up to 10 h after dosing, with blood sampling up to 24 h after dosing. Safety was monitored throughout the study. Data were analyzed with a repeated-measures mixed-effects model.

RESULTS

VX-150 induced analgesia in a variety of evoked pain tests, without affecting subject safety. Significant effects were reported for the cold pressor and heat pain thresholds. Maximum median concentration for the active moiety was 4.30 µg/mL at 4 h after dosing.

CONCLUSION

Results of this proof-of-mechanism study are supportive of the potential of VX-150, a highly selective NaV1.8 channel inhibitor, to treat various pain indications.

The Hypotensive Role of Acupuncture in Hypertension: Clinical Study and Mechanistic Study

As a component of traditional Chinese medicine (TCM), acupuncture has the potential to lower blood pressure (BP) in patients with hypertension. Emerging evidence indicates that the acupuncture-induced inhibition of high BP occurs through the activation of the pathway in the afferent, central, and efferent pathways. An increasing number of studies have demonstrated that acupuncture not only activates distinct brain regions under conditions of hypertension caused by an imbalance between the sympathetic and parasympathetic systems but also modulates neurotransmitters in related brain regions to alleviate the autonomic response. The activity of these pathways can be assessed by injecting agonists or inhibitors or by performing neurotomy. This review focuses on the clinical and mechanistic studies of acupuncture in modulating BP, which might provide a neurobiological foundation for the effects of acupuncture. Although many mechanisms underlying the effects of acupuncture on cardiovascular function have been identified, further investigation is warranted.

Voltage gated sodium channels as drug discovery targets

Voltage-gated sodium (Na_V) channels are a family of transmembrane ion channel proteins. They function by forming a gated, water-filled pore to help establish and control cell membrane potential via control of the flow of ions between the intracellular and the extracellular environments. Blockade of Na_Vs has been successfully accomplished in the clinic to enable control of pathological firing patterns that occur in a diverse range of conditions such as chronic pain, epilepsy, and cardiac arrhythmias. First generation sodium channel modulator drugs, despite low inherent subtype selectivity, preferentially act on over-excited cells which reduces undesirable side effects in the clinic. However, the limited therapeutic indices observed with the first generation demanded a new generation of sodium channel inhibitors. The structure, function and the state of the art in sodium channel modulator drug discovery are discussed in this chapter.

Novel sodium channel antagonists in the treatment of neuropathic pain

INTRODUCTION

Effective and safe drugs for the treatment of neuropathic pain are still an unmet clinical need. Neuropathic pain, caused by a lesion or disease that affects the somatosensory system, is a debilitating and hampering condition that has a great economic cost and, above all, a tremendous impact on the quality of life. Sodium channels are one of the major players in generating and propagating action potentials. They represent an appealing target for researchers involved in the development of new and safer drugs useful in the treatment of neuropathic pain. The actual goal for researchers is to target sodium channels selectively to stop the abnormal signaling that characterizes neuropathic pain while leaving normal somatosensory functions intact.

AREAS COVERED

This review covers the most recent publications regarding sodium channel blockers and their development as new treatments for neuropathic pain. The main areas discussed are the natural sources of new blockers, such as venom extracts and the recent efforts from many pharmaceutical companies in the field.

EXPERT OPINION

There have been serious efforts by both the pharmaceutical industry and academia to develop new and safer therapeutic options for neuropathic pain. A number of different strategies have been undertaken; the main efforts directed towards the identification of selective blockers starting from both natural products or screening chemical libraries. At this time, researchers have identified and characterized selective compounds against NaV1.7 or NaV1.8 voltage-gated sodium channels but only time will tell if they reach the market.

SDF1-CXCR4 signaling contributes to persistent pain and hypersensitivity via regulating excitability of primary nociceptive neurons: involvement of ERK-dependent Nav1.8 up-regulation

BACKGROUND

Pain is one critical hallmark of inflammatory responses. A large number of studies have demonstrated that stromal cell-derived factor 1 (SDF1, also named as CXCL12) and its cognate receptor C-X-C chemokine receptor type 4 (CXCR4) play an important role in immune reaction and inflammatory processes. However, whether and how SDF1-CXCR4 signaling is involved in inflammatory pain remains unclear.

METHODS

Under the intraplantar (i.pl.) bee venom (BV) injection-induced persistent inflammatory pain state, the changes of SDF1 and CXCR4 expression and cellular localization in the rat dorsal root ganglion (DRG) were detected by immunofluorescent staining. The role of SDF1 and CXCR4 in the hyperexcitability of primary nociceptor neurons was assessed by electrophysiological recording. Western blot analysis was used to quantify the DRG Nav1.8 and phosphorylation of ERK (pERK) expression. Behavioral tests were conducted to evaluate the roles of CXCR4 as well as extracellular signal-regulated kinase (ERK) and Nav1.8 in the BV-induced persistent pain and hypersensitivity.

RESULTS

We showed that both SDF1 and CXCR4 were dramatically up-regulated in the DRG in i.pl. BV-induced inflammatory pain model. Double immunofluorescent staining showed that CXCR4 was localized in all sizes (large, medium, and small) of DRG neuronal soma, while SDF1 was exclusively expressed in satellite glial cells (SGCs). Electrophysiological recording showed that bath application with AMD3100, a potent and selective CXCR4 inhibitor, could reverse the hyperexcitability of medium- and small-sized DRG neurons harvested from rats following i.pl. BV injection. Furthermore, we demonstrated that the BV-induced ERK activation and Nav1.8 up-regulation in the DRG could be blocked by pre-antagonism against CXCR4 in the periphery with AMD3100 as well as by blockade of ERK activation by intrathecal (i.t.) or intraplantar (i.pl.) U0126. At behavioral level, the BV-induced persistent spontaneous pain as well as primary mechanical and thermal hypersensitivity could also be significantly suppressed by blocking CXCR4 and Nav1.8 in the periphery as well as by inhibition of ERK activation at the DRG level.

CONCLUSIONS

The present results suggest that peripheral inflammatory pain state can trigger over release of SDF1 from the activated SGCs in the DRG by which SGC-neuronal cross-talk is mediated by SDF1-CXCR4 coupling that result in subsequent ERK-dependent Nav1.8 up-regulation, leading to hyperexcitability of tonic type of the primary nociceptor cells and development and maintenance of persistent spontaneous pain and hypersensitivity.

Structure and function of μ-conotoxins, peptide-based sodium channel blockers with analgesic activity

μ-Conotoxins block voltage-gated sodium channels (VGSCs) and compete with tetrodotoxin for binding to the sodium conductance pore. Early efforts identified µ-conotoxins that preferentially blocked the skeletal muscle subtype (NaV1.4). However, the last decade witnessed a significant increase in the number of µ-conotoxins and the range of VGSC subtypes inhibited (NaV1.2, NaV1.3 or NaV1.7). Twenty µ-conotoxin sequences have been identified to date and structure-activity relationship studies of several of these identified key residues responsible for interactions with VGSC subtypes. Efforts to engineer-in subtype specificity are driven by in vivo analgesic and neuromuscular blocking activities. This review summarizes structural and pharmacological studies of µ-conotoxins, which show promise for development of selective blockers of NaV1.2, and perhaps also NaV1.1,1.3 or 1.7.

Discriminative sensory characteristics of the lateral femoral cutaneous nerve after mepivacaine-induced block

Background and objectives

Unmyelinated C-fibres comprise the largest group of somatic afferents and have demonstrated a crucial role not only in the perception of high-threshold mechanically, thermally or chemically induced pain, but also in non-harmful low-threshold mechanical stimuli [1,2]. The objective of our study was to characterize differential sensitivity changes of C-fibre related subclasses of high-threshold and low-threshold polymodal nociceptors and low-threshold mechanoreceptors to the local anaesthetic (LA) mepivacaine during nerve block of the purely sensory lateral femoral cutaneous nerve (LFCN) in human. We assumed a diverse response of different classes of afferents to the two different concentrations of the LA mepivacaine (Scandicaine).

Methods

In a double-blind randomized experimental setting, an ultrasound-guided nerve block of the LFCN was performed in 10 healthy male subjects, each with two different concentrations of mepivacaine (0.5 and 1%). Responsiveness of afferent nerve fibres to different noxious and non-noxious stimuli was tested by Quantitative Sensory Testing (QST) 30, 180, and 300 min after nerve block. Both LA concentrations of mepivacaine were compared for time course of the areas of anaesthesia for the tested sensory modalities.

Results

Initial extension of anaesthetic areas at 30 min did not differ between both LA concentrations. At 180 min only the anaesthetic areas to nociceptive stimuli were reduced at the site of lower mepivacaine injection (260mN: 204mm2 (18; 244; median difference and 95% confidence interval; p < 0.05), heat: 276mm2 (3; 305)). In contrast, no significant differences were found between the two concentration when non-nociceptive stimuli were used (100mN: 187mm2 (4; 240), p >0.05, brush: 159mm2 (–59; 202)).

Conclusion

Equal initial sizes of anaesthesia areas for all sensory modalities can be explained by supramaximal perineural LA molecule concentration in both administered mepivacaine dosages. Upon washout of the LA nociceptive function is restored faster as compared to non-nociceptive sensation and higher concentration of the LA are required to maintain the analgesia. Quantitative sensory testing is able to detect different susceptibility of low threshold mechanosensors and subtypes of nociceptive C-fibres to mepivacaine. Using painful mechanical, heat and electrical stimulation different classes of nociceptors will be activated. The analgesic areas to electrical stimulation were particularly small; one might therefore hypothesize that the proposed protocol allows to also differentiate mechano-insensitive (“silent”) and mechanosensitive (“polymodal”) nociceptors.

Implications

QST is a non-invasive method to functionally examine sensory modalities and their pharmacological modulation in humans. The method is sufficiently sensitive to differentiate the analgesic properties of mepivacaine at 0.5 and 1% and might also be adequate to different classes of nociceptors. Further development of nociceptive stimuli including supra-threshold encoding characteristics will enable to investigate peripheral analgesic effects more specifically and thus might help to design new analgesics with preferential effect on high frequency discharge of nociceptors.

Genes and epigenetic processes as prospective pain targets

Chronic pain affects approximately one in five adults, resulting in a greatly reduced quality of life and a higher risk of developing co-morbidities such as depression. Available treatments often provide inadequate pain relief, but it is hoped that through deeper understanding of the molecular mechanisms underlying chronic pain states we can discover new and improved therapies. Although genetic research has flourished over the past decade and has identified many key genes in pain processing, the budding field of epigenetics promises to provide new insights and a more dynamic view of pain regulation. This review gives an overview of basic mechanisms and current therapies to treat pain, and discusses the clinical and preclinical evidence for the contribution of genetic and epigenetic factors, with a focus on how this knowledge can affect drug development.

The role of continuous peripheral nerve blocks

A continuous peripheral nerve block (cPNB) is provided in the hospital and ambulatory setting. The most common use of CPNBs is in the peri- and postoperative period but different indications have been described like the treatment of chronic pain such as cancer-induced pain, complex regional pain syndrome or phantom limb pain. The documented benefits strongly depend on the analgesia quality and include decreasing baseline/dynamic pain, reducing additional analgesic requirements, decrease of postoperative joint inflammation and inflammatory markers, sleep disturbances and opioid-related side effects, increase of patient satisfaction and ambulation/functioning improvement, an accelerated resumption of passive joint range-of-motion, reducing time until discharge readiness, decrease in blood loss/blood transfusions, potential reduction of the incidence of postsurgical chronic pain and reduction of costs. Evidence deriving from randomized controlled trials suggests that in some situations there are also prolonged benefits of regional anesthesia after catheter removal in addition to the immediate postoperative effects. Unfortunately, there are only few data demonstrating benefits after catheter removal and the evidence of medium- or long-term improvements in health-related quality of life measures is still lacking. This review will give an overview of the advantages and adverse effects of cPNBs.

N- and C-terminal extensions of μ-conotoxins increase potency and selectivity for neuronal sodium channels

μ-Conotoxins are peptide blockers of voltage-gated sodium channels (sodium channels), inhibiting tetrodotoxin-sensitive neuronal (Na(v) 1.2) and skeletal (Na(v) 1.4) subtypes with highest affinity. Structure-activity relationship studies of μ-conotoxins SIIIA, TIIIA, and KIIIA have shown that it is mainly the C-terminal part of the three-loop peptide that is involved in binding to the sodium channel. In this study, we characterize the effect of N- and C-terminal extensions of μ-conotoxins SIIIA, SIIIB, and TIIIA on their potency and selectivity for neuronal versus muscle sodium channels. Interestingly, extending the N- or C-terminal of the peptide by introducing neutral, positive, and/or negatively charged residues, the selectivity of the native peptide can be altered from neuronal to skeletal and the other way around. The results from this study provide further insight into the binding profile of μ-conotoxins at voltage-gated sodium channels, revealing that binding interactions outside the cysteine-stablilized loops can contribute to μ-conotoxin affinity and sodium channel selectivity.

The chemokine CCL2 increases Nav1.8 sodium channel activity in primary sensory neurons through a Gβγ-dependent mechanism

Changes in function of voltage-gated sodium channels in nociceptive primary sensory neurons participate in the development of peripheral hyperexcitability that occurs in neuropathic and inflammatory chronic pain conditions. Among them, the tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.8, primarily expressed by small- and medium-sized dorsal root ganglion (DRG) neurons, substantially contributes to the upstroke of action potential in these neurons. Compelling evidence also revealed that the chemokine CCL2 plays a critical role in chronic pain facilitation via its binding to CCR2 receptors. In this study, we therefore investigated the effects of CCL2 on the density and kinetic properties of TTX-R Na(v)1.8 currents in acutely small/medium dissociated lumbar DRG neurons from naive adult rats. Whole-cell patch-clamp recordings demonstrated that CCL2 concentration-dependently increased TTX-resistant Na(v)1.8 current densities in both small- and medium-diameter sensory neurons. Incubation with CCL2 also shifted the activation and steady-state inactivation curves of Na(v)1.8 in a hyperpolarizing direction in small sensory neurons. No change in the activation and inactivation kinetics was, however, observed in medium-sized nociceptive neurons. Our electrophysiological recordings also demonstrated that the selective CCR2 antagonist INCB3344 [N-[2-[[(3S,4S)-1-E4-(1,3-benzodioxol-5-yl)-4-hydroxycyclohexyl]-4-ethoxy-3-pyrrolidinyl]amino]-2-oxoethyl]-3-(trifluoromethyl)benzamide] blocks the potentiation of Na(v)1.8 currents by CCL2 in a concentration-dependent manner. Furthermore, the enhancement in Na(v)1.8 currents was prevented by pretreatment with pertussis toxin (PTX) or gallein (a Gβγ inhibitor), indicating the involvement of Gβγ released from PTX-sensitive G(i/o)-proteins in the cross talk between CCR2 and Na(v)1.8. Together, our data clearly demonstrate that CCL2 may excite primary sensory neurons by acting on the biophysical properties of Na(v)1.8 currents via a CCR2/Gβγ-dependent mechanism.

Targeting voltage-gated sodium channels for treatment for chronic visceral pain

Voltage-gated sodium channels (VGSCs) play a fundamental role in controlling cellular excitability, and their abnormal activity is related to several pathological processes, including cardiac arrhythmias, epilepsy, neurodegenerative diseases, spasticity and chronic pain. In particular, chronic visceral pain, the central symptom of functional gastrointestinal disorders such as irritable bowel syndrome, is a serious clinical problem that affects a high percentage of the world population. In spite of intense research efforts and after the dedicated decade of pain control and research, there are not many options to treat chronic pain conditions. However, there is a wealth of evidence emerging to give hope that a more refined approach may be achievable. By using electronic databases, available data on structural and functional properties of VGSCs in chronic pain, particularly functional gastrointestinal hypersensitivity, were reviewed. We summarize the involvement and molecular bases of action of VGSCs in the pathophysiology of several organic and functional gastrointestinal disorders. We also describe the efficacy of VGSC blockers in the treatment of these neurological diseases, and outline future developments that may extend the therapeutic use of compounds that target VGSCs. Overall, clinical and experimental data indicate that isoform-specific blockers of these channels or targeting of their modulators may provide effective and novel approaches for visceral pain therapy.

The discovery and development of analgesics: new mechanisms, new modalities

Despite intensive research into pain mechanisms and significant investment in research and development, the majority of analgesics available to prescribers and patients are based on mechanistic classes of compounds that have been known for many years. With considerable ingenuity and innovation, researchers continue to make the best of the mechanistic approaches available, with novel formulations, routes of administration, and combination products. Here we review some of the mechanisms and modalities of analgesics that have recently entered into clinical development, which, coupled with advances in the understanding of the pathophysiology of chronic pain, will hopefully bring the promise of new therapeutics that have the potential to provide improved pain relief for those many patients whose needs remain poorly met.

Isoform-selective voltage-gated Na+ channel modulators as next-generation analgesics

For many patients the current therapies for controlling chronic pain are inadequate. This has driven the search for analgesics with improved efficacy and side effect profiles. Some anticonvulsants have voltage-gated Na+ channels (VGSCs) as their molecular targets and are used to treat pain, but the efficacy seen is marginal and the drugs are generally poorly tolerated. The clinically used VGSC-modulating analgesics show no isoform selectivity, which probably limits their use. Thus, focus has fallen on VGSCs expressed selectively by primary afferent neurons and the search for isoform-selective drugs. In this review, we describe developments in our understanding of the biology of VGSCs, screening technologies and the pharmacological properties of VGSC modulators with promise as analgesics. Also highlighted are the challenges associated with targeting isoform-selective VGSCs.

Sodium channel blockers for the treatment of neuropathic pain

Drugs that block voltage-gated sodium channels are efficacious in the management of neuropathic pain. Accordingly, this class of ion channels has been a major focus of analgesic research both in academia and in the pharmaceutical/biotechnology industry. In this article, we review the history of the use of sodium channel blockers, describe the current status of sodium channel drug discovery, highlight the challenges and hurdles to attain sodium channel subtype selectivity, and review the potential usefulness of selective sodium channel blockers in neuropathic pain.

Translational pain research: achievements and challenges

The achievements in both preclinical and clinical pain research over the past 4 decades have led to significant progress in clinical pain management. However, pain research still faces enormous challenges and there remain many obstacles in the treatment of clinical pain, particularly chronic pain. Translational pain research needs to involve a number of important areas including: 1) bridging the gap between pain research and clinical pain management; 2) developing objective pain-assessment tools; 3) analyzing current theories of pain mechanisms and their relevance to clinical pain; 4) exploring new tools for both preclinical and clinical pain research; and 5) coordinating research efforts among basic scientists, clinical investigators, and pain-medicine practitioners. These issues are discussed in this article in light of the achievements and challenges of translational pain research.

PERSPECTIVE

The subjective nature of clinical pain calls for innovative research approaches. As translational pain research emerges as an important field in pain medicine, it will play a unique role in improving clinical pain management through coordinated bidirectional research approaches between bedside and bench.