A mixed Ca2+ channel blocker, A-1264087, utilizes peripheral and spinal mechanisms to inhibit spinal nociceptive transmission in a rat model of neuropathic pain

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.

Characterization and pharmacological modulation of noci-responsive deep dorsal horn neurons across diverse rat models of pathological pain

This overview compares the activity of wide dynamic range (WDR) and nociceptive specific (NS) neurons located in the deep dorsal horn across different rat models of pathological pain and following modulation by diverse pharmacology. The data were collected by our group under the same experimental conditions over numerous studies to facilitate comparison. Spontaneous firing of WDR neurons was significantly elevated (>3.7 Hz) in models of neuropathic, inflammation, and osteoarthritic pain compared with naive animals (1.9 Hz) but was very low (<0.5 Hz) and remained unchanged in NS neurons. WDR responses to low-intensity mechanical stimulation were elevated in neuropathic and inflammation models. WDR responses to high-intensity stimuli were enhanced in inflammatory (heat) and osteoarthritis (mechanical) models. NS responses to high-intensity stimulation did not change relative to control in any model examined. Several therapeutic agents reduced both evoked and spontaneous firing of WDR neurons (e.g., TRPV1, TRPV3, Nav1.7, Nav1.8, P2X7, P2X3, H3), other targets affected neither evoked nor spontaneous firing of WDR neurons (e.g., H4, TRPM8, KCNQ2/3), and some only modulated evoked (e.g, ASIC1a, Cav3.2) whereas others decreased evoked but affected spontaneous activity only in specific models (e.g., TRPA1, CB2). Spontaneous firing of WDR neurons was not altered by any peripherally restricted compound or by direct administration of compounds to peripheral sites, although the same compounds decreased evoked activity. Compounds acting centrally were effective against this endpoint. The diversity of incoming/modulating inputs to the deep dorsal horn positions this group of neurons as an important intersection within the pain system to validate novel therapeutics.

NEW & NOTEWORTHY Data from multiple individual experiments were combined to show firing properties of wide dynamic range and nociceptive specific spinal dorsal horn neurons across varied pathological pain models. This high-powered analysis describes the sensitization following different forms of injury. Effects of diverse pharmacology on these neurons is also summarized from published and unpublished data all recorded under the same conditions to facilitate comparison. This comprehensive overview describes the function and utility of these neurons.

Calcium channel modulation as a target in chronic pain control

Neuropathic pain remains poorly treated for large numbers of patients, and little progress has been made in developing novel classes of analgesics. To redress this issue, ziconotide (Prialt™) was developed and approved as a first-in-class synthetic version of ω-conotoxin MVIIA, a peptide blocker of Cav 2.2 channels. Unfortunately, the impracticalities of intrathecal delivery, low therapeutic index and severe neurological side effects associated with ziconotide have restricted its use to exceptional circumstances. Ziconotide exhibits no state or use-dependent block of Cav 2.2 channels; activation state-dependent blockers were hypothesized to circumvent the side effects of state-independent blockers by selectively targeting high-frequency firing of nociceptive neurones in chronic pain states, thus alleviating aberrant pain but not affecting normal sensory transduction. Unfortunately, numerous drugs, including state-dependent calcium channel blockers, have displayed efficacy in preclinical models but have subsequently been disappointing in clinical trials. In recent years, it has become more widely acknowledged that trans-aetiological sensory profiles exist amongst chronic pain patients and may indicate similar underlying mechanisms and drug sensitivities. Heterogeneity amongst patients, a reliance on stimulus-evoked endpoints in preclinical studies and a failure to utilize translatable endpoints, all are likely to have contributed to negative clinical trial results. We provide an overview of how electrophysiological and operant-based assays provide insight into sensory and affective aspects of pain in animal models and how these may relate to chronic pain patients in order to improve the bench-to-bedside translation of calcium channel modulators.

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.

Breaking barriers to novel analgesic drug development

Acute and chronic pain complaints, although common, are generally poorly served by existing therapies. This unmet clinical need reflects a failure to develop novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms, and the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities for developing novel therapeutic strategies and revisiting existing targets, including modulating ion channels, enzymes and G-protein-coupled receptors.

TRPV3 modulates nociceptive signaling through peripheral and supraspinal sites in rats

TRPV3 is a nonselective cation channel activated by temperatures above 33°C and is reported to be localized in keratinocytes and nervous tissue. To investigate a role for TRPV3 in pain modulation, we conducted a series of in vivo electrophysiological studies on spinal and brain nociceptive neurons. Structurally diverse TRPV3 receptor antagonists reduced responses of spinal wide dynamic range (WDR) neurons to low-intensity mechanical stimulation in neuropathic rats, but only CNS-penetrant antagonists decreased elevated spontaneous firing. Injections of an antagonist into the neuronal receptive field, into the L₅ dorsal root ganglion, or intracerebroventricularly (ICV) attenuated the evoked firing, but only ICV injections reduced spontaneous activity. Intraspinal injections did not affect either. Spinal transection blocked the effect on spontaneous but not evoked firing after systemic delivery of a TRPV3 antagonist. Systemic administration of an antagonist to neuropathic rats also impacted the firing of On- and Off-cells in the rostral ventromedial medulla in a manner consistent with dampening nociceptive signaling. An assessment of nonevoked "pain," an EEG-measured pain-induced sleep disturbance induced by hind paw injections of CFA, was also improved with CNS-penetrant TRPV3 antagonists but not by an antagonist with poor CNS penetration. Antagonism of TRPV3 receptors modulates activity of key classes of neurons in the pain pathway in a manner consistent with limiting pathological nociceptive signaling and was mediated by receptors in the periphery and brain. Blockade of TRPV3 receptors is likely an effective means to alleviate mechanical allodynia and nonevoked pain. However, the latter will only be obtained by blocking supraspinal TRPV3 receptors.NEW & NOTEWORTHY Recent studies have linked TRPV3 to pain modulation, and much of this work has focused on its role in the skin-primary afferent interface. In this electrophysiological study, we demonstrate that receptor antagonists modulate evoked signals through peripheral mechanisms but blockade of supraspinal TRPV3 receptors contributes to dampening both evoked and nonevoked "pain" through descending modulation. Thus, the full therapeutic potential of TRPV3 antagonists may only be realized with the ability to access receptors in the brain.

3-Benzamides and 3,4,5-trimethoxyphenyl amines as calcium channel blockers

T- and N-type calcium channels have known for relating to therapy of neuropathic pain which is chronic, debilitating pain state. Neuropathic pain is caused by damage of the somatosensory system. It may be associated with abnormal sensations and pain produced by normally non-painful stimuli (allodynia). Neuropathic pain is very difficult to treat, and only some 40-60% of patients achieve partial relief. For a neuropathic pain therapy, anticonvulsant like Lamotrigine, Carbamazepine and a topical anesthetic such as Lidocaine are used. We synthesized 15 novel amine derivatives and evaluated their activities against T-type and N-type calcium channels by whole-cell patch clamp recording on HEK293 cells. Among the tested compounds, compound 10 showed good inhibitory activity for both T-type and N-type calcium channels with the IC50 value of 1.9 μM and 4.3 μM, respectively. Compound 10 also showed good analgesic activity on rat spinal cord injury model.

Inhibition of N-type calcium channels by fluorophenoxyanilide derivatives

A set of fluorophenoxyanilides, designed to be simplified analogues of previously reported ω-conotoxin GVIA mimetics, were prepared and tested for N-type calcium channel inhibition in a SH-SY5Y neuroblastoma FLIPR assay. N-type or Ca_v2.2 channel is a validated target for the treatment of refractory chronic pain. Despite being significantly less complex than the originally designed mimetics, up to a seven-fold improvement in activity was observed.

Mechanistic insights into the analgesic efficacy of A-1264087, a novel neuronal Ca(2+) channel blocker that reduces nociception in rat preclinical pain models

Voltage-gated Ca(2+) channels play an important role in nociceptive transmission. There is significant evidence supporting a role for N-, T- and P/Q-type Ca(2+) channels in chronic pain. Here, we report that A-1264087, a structurally novel state-dependent blocker, inhibits each of these human Ca(2+) channels with similar potency (IC50 = 1-2 μM). A-1264087 was also shown to inhibit the release of the pronociceptive calcitonin gene-related peptide from rat dorsal root ganglion neurons. Oral administration of A-1264087 produces robust antinociceptive efficacy in monoiodoacetate-induced osteoarthritic, complete Freund adjuvant-induced inflammatory, and chronic constrictive injury of sciatic nerve-induced, neuropathic pain models with ED50 values of 3.0, 5.7, and 7.8 mg/kg (95% confidence interval = 2.2-3.5, 3.7-10, and 5.5-12.8 mg/kg), respectively. Further analysis revealed that A-1264087 also suppressed nociceptive-induced p38 and extracellular signal-regulated kinase 1/2 phosphorylation, which are biochemical markers of engagement of pain circuitry in chronic pain states. Additionally, A-1264087 inhibited both spontaneous and evoked neuronal activity in the spinal cord dorsal horn in complete Freund adjuvant-inflamed rats, providing a neurophysiological basis for the observed antihyperalgesia. A-1264087 produced no alteration of body temperature or motor coordination and no learning impairment at therapeutic plasma concentrations.

PERSPECTIVE

The present results demonstrate that the neuronal Ca(2+) channel blocker A-1264087 exhibits broad-spectrum efficacy through engagement of nociceptive signaling pathways in preclinical pain models in the absence of effects on psychomotor and cognitive function.