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

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 Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 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.

A Recent Update on Drug Delivery Systems for Pain Management

Pain remains a global health challenge affecting approximately 1.5 billion people worldwide. Pain has been an implicit variable in the equation of human life for many centuries considering different types and the magnitude of pain. Therefore, developing an efficacious drug delivery system for pain management remains an open challenge for researchers in the field of medicine. Lack of therapeutic efficacy still persists, despite high throughput studies in the field of pain management. Research scientists have been exploiting different alternatives to curb the adverse side effects of pain medications or attempting a more substantial approach to minimize the prevalence of pain. Various drug delivery systems have been developed such as nanoparticles, microparticles to curb adverse side effects of pain medications or minimize the prevalence of pain. This literature review firstly provides a brief introduction of pain as a sensation and its pharmacological interventions. Second, it highlights the most recent studies in the pharmaceutical field for pain management and serves as a strong base for future developments. Herein, we have classified drug delivery systems based on their sizes such as nano, micro, and macro systems, and for each of the reviewed systems, design, formulation strategies, and drug release performance has been discussed.

Reversal of Peripheral Neuropathic Pain by the Small-Molecule Natural Product Physalin F via Block of CaV2.3 (R-Type) and CaV2.2 (N-Type) Voltage-Gated Calcium Channels

No universally efficacious therapy exists for chronic pain, a disease affecting one-fifth of the global population. An overreliance on the prescription of opioids for chronic pain despite their poor ability to improve function has led to a national opioid crisis. In 2018, the NIH launched a Helping to End Addiction Long-term plan to spur discovery and validation of novel targets and mechanisms to develop alternative nonaddictive treatment options. Phytochemicals with medicinal properties have long been used for various treatments worldwide. The natural product physalin F, isolated from the Physalis acutifolia (family: Solanaceae) herb, demonstrated antinociceptive effects in models of inflammatory pain, consistent with earlier reports of its anti-inflammatory and immunomodulatory activities. However, the target of action of physalin F remained unknown. Here, using whole-cell and slice electrophysiology, competition binding assays, and experimental models of neuropathic pain, we uncovered a molecular target for physalin F's antinociceptive actions. We found that physalin F (i) blocks CaV2.3 (R-type) and CaV2.2 (N-type) voltage-gated calcium channels in dorsal root ganglion (DRG) neurons, (ii) does not affect CaV3 (T-type) voltage-gated calcium channels or voltage-gated sodium or potassium channels, (iii) does not bind G-protein coupled opioid receptors, (iv) inhibits the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in spinal cord slices, and (v) reverses tactile hypersensitivity in models of paclitaxel-induced peripheral neuropathy and spinal nerve ligation. Identifying CaV2.2 as a molecular target of physalin F may spur its use as a tool for mechanistic studies and position it as a structural template for future synthetic compounds.

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.

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.