Novel small molecule α9α10 nicotinic receptor antagonist prevents and reverses chemotherapy-evoked neuropathic pain in rats

Centralizing the Knowledge and Interpretation of Pain in Chemotherapy-Induced Peripheral Neuropathy: A Paradigm Shift towards Brain-Centric Approaches

Chemotherapy-induced peripheral neuropathy (CIPN) is a side effect of cancer treatment, often linked with pain complaints. Patients report mechanical and thermal hypersensitivity that may emerge during chemotherapy treatment and may persist after cancer remission. Whereas the latter situation disturbs the quality of life, life itself may be endangered by the appearance of CIPN during cancer treatment. The causes of CIPN have almost entirely been ascribed to the neurotoxicity of chemotherapeutic drugs in the peripheral nervous system. However, the central consequences of peripheral neuropathy are starting to be unraveled, namely in the supraspinal pain modulatory system. Based on our interests and experience in the field, we undertook a review of the brain-centered alterations that may underpin pain in CIPN. The changes in the descending pain modulation in CIPN models along with the functional and connectivity abnormalities in the brain of CIPN patients are analyzed. A translational analysis of preclinical findings about descending pain regulation during CIPN is reviewed considering the main neurochemical systems (serotoninergic and noradrenergic) targeted in CIPN management in patients, namely by antidepressants. In conclusion, this review highlights the importance of studying supraspinal areas involved in descending pain modulation to understand the pathophysiology of CIPN, which will probably allow a more personalized and effective CIPN treatment in the future.

αO-Conotoxin GeXIVA[1,2] Reduced Neuropathic Pain and Changed Gene Expression in Chronic Oxaliplatin-Induced Neuropathy Mice Model

Chemotherapy-induced peripheral neuropathy (CIPN) is a dose-limiting painful neuropathy that occurs commonly during cancer management, which often leads to the discontinuation of medication. Previous studies suggest that the α9α10 nicotinic acetylcholine receptor (nAChR)-specific antagonist αO-conotoxin GeXIVA[1,2] is effective in CIPN models; however, the related mechanisms remain unclear. Here, we analyzed the preventive effect of GeXIVA[1,2] on neuropathic pain in the long-term oxaliplatin injection-induced CIPN model. At the end of treatment, lumbar (L4-L6) spinal cord was extracted, and RNA sequencing and bioinformatic analysis were performed to investigate the potential genes and pathways related to CIPN and GeXIVA[1,2]. GeXIVA[1,2] inhibited the development of mechanical allodynia induced by chronic oxaliplatin treatment. Repeated injections of GeXIVA[1,2] for 3 weeks had no effect on the mice's normal pain threshold or locomotor activity and anxiety-like behavior, as evaluated in the open field test (OFT) and elevated plus maze (EPM). Our RNA sequencing results identified 209 differentially expressed genes (DEGs) in the CIPN model, and simultaneously injecting GeXIVA[1,2] with oxaliplatin altered 53 of the identified DEGs. These reverted genes were significantly enriched in immune-related pathways represented by the cytokine-cytokine receptor interaction pathway. Our findings suggest that GeXIVA[1,2] could be a potential therapeutic compound for chronic oxaliplatin-induced CIPN management.

Exploring Cholinergic Compounds for Peripheral Neuropathic Pain Management: A Comprehensive Scoping Review of Rodent Model Studies

Neuropathic pain affects about 7-8% of the population, and its management still poses challenges with unmet needs. Over the past decades, researchers have explored the cholinergic system (muscarinic and nicotinic acetylcholine receptors: mAChR and nAChR) and compounds targeting these receptors as potential analgesics for neuropathic pain management. This scoping review aims to provide an overview of studies on peripheral neuropathic pain (PNP) in rodent models, exploring compounds targeting cholinergic neurotransmission. The inclusion criteria were original articles on PNP in rodent models that explored the use of compounds directly targeting cholinergic neurotransmission and reported results of nociceptive behavioral assays. The literature search was performed in the PubMed and Web of Science databases (1 January 2000-22 April 2023). The selection process yielded 82 publications, encompassing 62 compounds. The most studied compounds were agonists of α4β2 nAChR and α7 nAChR, and antagonists of α9/α10 nAChR, along with those increasing acetylcholine and targeting mAChRs. Studies mainly reported antinociceptive effects in traumatic PNP models, and to a lesser extent, chemotherapy-induced neuropathy or diabetic models. These preclinical studies underscore the considerable potential of cholinergic compounds in the management of PNP, warranting the initiation of clinical trials.

Nicotinic acetylcholine receptors: Therapeutic targets for novel ligands to treat pain and inflammation

Nicotinic acetylcholine receptors (nAChRs) have been historically defined as ligand-gated ion channels and function as such in the central and peripheral nervous systems. Recently, however, non-ionic signaling mechanisms via nAChRs have been demonstrated in immune cells. Furthermore, the signaling pathways where nAChRs are expressed can be activated by endogenous ligands other than the canonical agonists acetylcholine and choline. In this review, we discuss the involvement of a subset of nAChRs containing α7, α9, and/or α10 subunits in the modulation of pain and inflammation via the cholinergic anti-inflammatory pathway. Additionally, we review the most recent advances in the development of novel ligands and their potential as therapeutics.

Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

Stimulation of cholinergic efferent neurons innervating the inner ear has profound, well-characterized effects on vestibular and auditory physiology, after activating distinct ACh receptors (AChRs) on afferents and hair cells in peripheral endorgans. Efferent-mediated fast and slow excitation of vestibular afferents are mediated by α4β2*-containing nicotinic AChRs (nAChRs) and muscarinic AChRs (mAChRs), respectively. On the auditory side, efferent-mediated suppression of distortion product otoacoustic emissions (DPOAEs) is mediated by α9α10nAChRs. Previous characterization of these synaptic mechanisms utilized cholinergic drugs, that when systemically administered, also reach the CNS, which may limit their utility in probing efferent function without also considering central effects. Use of peripherally-acting cholinergic drugs with local application strategies may be useful, but this approach has remained relatively unexplored. Using multiple administration routes, we performed a combination of vestibular afferent and DPOAE recordings during efferent stimulation in mouse and turtle to determine whether charged mAChR or α9α10nAChR antagonists, with little CNS entry, can still engage efferent synaptic targets in the inner ear. The charged mAChR antagonists glycopyrrolate and methscopolamine blocked efferent-mediated slow excitation of mouse vestibular afferents following intraperitoneal, middle ear, or direct perilymphatic administration. Both mAChR antagonists were effective when delivered to the middle ear, contralateral to the side of afferent recordings, suggesting they gain vascular access after first entering the perilymphatic compartment. In contrast, charged α9α10nAChR antagonists blocked efferent-mediated suppression of DPOAEs only upon direct perilymphatic application, but failed to reach efferent synapses when systemically administered. These data show that efferent mechanisms are viable targets for further characterizing drug access in the inner ear.

The α9α10 nicotinic acetylcholine receptors antagonist α-conotoxin RgIA reverses colitis signs in murine dextran sodium sulfate model

Nicotinic acetylcholine receptors can regulate inflammation primarily through the vagus nerve via the cholinergic anti-inflammatory pathway. α9α10 nicotinic receptors (nAChRs) are a new promising target for chronic pain and inflammation. Recently, α9α10 selective α-conotoxin antagonists were shown to have antinociception effect in neuropathic and tonic inflammatory pain animal models. However, limited data available on the role of α9α10 nAChRs in experimental colitis. In this study, we report for the first time, the role of α9α10 nAChRs in the dextran sodium sulfate (DSS) experimental animal colitis model. We determined the effect of the α9α10 nAChRs antagonist, α-conotoxin RgIA (α-RgIA) in DSS-induced colitis model in adult male and female C57BL/6 J mice. DSS solution was freely given in the drinking water for seven consecutive days, and tap water was given on the 8th day. We then sacrificed mice on day 8 to examine the entire colon. Disease severity, colon tissue histology, and tumor necrosis factor-α (TNF-α) were evaluated. The lower doses (0.02 and 0.1 nmol/mouse, s.c.) of α-RgIA treatment in DSS-treated mice were inactive, whereas the higher dose (0.2 nmol/mouse, s.c.) reversed the disease activity index (DAI) score, loss of body weight, total histological damage score, as well as the colonic level of TNF-α compared to the DSS-control group. Moreover, the highest dose of α-RgIA (0.2 nmol/mouse, s.c.) significantly rescued the colon length shortening in DSS-treated mice compared to the DSS-control mice. The availability of α9*-selective conotoxins has opened new avenues in pharmacology research and potential targets in inflammatory disorders.

Behavioral and Molecular Basis of Cholinergic Modulation of Pain: Focus on Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) have emerged as a novel therapeutic strategy for pain and inflammatory disorders. In particular, α4β2∗, α7, and α9α10 nAChR subtypes have been investigated as potential targets to treat pain. The nAChRs are distributed on the pain transmission pathways, including central and peripheral nervous systems and immune cells as well. Several agonists for α4β2∗ nAChR subtypes have been investigated in multiple animal pain models with promising results. However, studies in human indicated a narrow therapeutic window for α4β2∗ agonists. Furthermore, animal studies suggest that using agonists for α7 nAChR subtype and antagonists for α9α10 nAChR subtypes are potential novel therapies for chronic pain management, including inflammatory and neuropathic pain. More recently, alternative nAChRs ligands such as positive allosteric modulators and silent agonists have shown potential to develop into new treatments for chronic pain.

RgIA4 Accelerates Recovery from Paclitaxel-Induced Neuropathic Pain in Rats

Chemotherapeutic drugs are widely utilized in the treatment of human cancers. Painful chemotherapy-induced neuropathy is a common, debilitating, and dose-limiting side effect for which there is currently no effective treatment. Previous studies have demonstrated the potential utility of peptides from the marine snail from the genus Conus for the treatment of neuropathic pain. α-Conotoxin RgIA and a potent analog, RgIA4, have previously been shown to prevent the development of neuropathy resulting from the administration of oxaliplatin, a platinum-based antineoplastic drug. Here, we have examined its efficacy against paclitaxel, a chemotherapeutic drug that works by a mechanism of action distinct from that of oxaliplatin. Paclitaxel was administered at 2 mg/kg (intraperitoneally (IP)) every other day for a total of 8 mg/kg. Sprague Dawley rats that were co-administered RgIA4 at 80 µg/kg (subcutaneously (SC)) once daily, five times per week, for three weeks showed significant recovery from mechanical allodynia by day 31. Notably, the therapeutic effects reached significance 12 days after the last administration of RgIA4, which is suggestive of a rescue mechanism. These findings support the effects of RgIA4 in multiple chemotherapeutic models and the investigation of α9α10 nicotinic acetylcholine receptors (nAChRs) as a non-opioid target in the treatment of chronic pain.

The α9α10 Nicotinic Acetylcholine Receptor Antagonist αO-Conotoxin GeXIVA[1,2] Alleviates and Reverses Chemotherapy-Induced Neuropathic Pain

Oxaliplatin is a third-generation platinum drug and is widely used as a first-line therapy for the treatment of colorectal cancer (CRC). However, a large number of patients receiving oxaliplatin develop dose-limiting painful neuropathy. Here, we report that αO-conotoxin GeXIVA[1,2], a highly potent and selective antagonist of the α9α10 nicotinic acetylcholine receptor (nAChR) subtype, can relieve and reverse oxaliplatin-induced mechanical and cold allodynia after single and repeated intramuscular (IM) injections in rats. Treatments were started at 4 days post oxaliplatin injection when neuropathic pain emerged and continued for 8 and 16 days. Cold score and mechanical paw withdrawal threshold (PWT) were detected by the acetone test and von Frey test respectively. GeXIVA[1,2] significantly relieved mechanical and cold allodynia in oxaliplatin-treated rats after a single injection. After repeated treatments, GeXIVA[1,2] produced a cumulative analgesic effect without tolerance and promoted recovery from neuropathic pain. Moreover, the long lasting analgesic effect of GeXIVA[1,2] on mechanical allodynia continued until day 10 after the termination of the 16-day repeated treatment procedure. On the contrary, GeXIVA[1,2] did not affect acute mechanical and thermal pain behaviors in normal rats after repeated injections detected by the von Frey test and tail flick test. GeXIVA[1,2] had no influence on rat hind limb grip strength and body weight after repeated treatments. These results indicate that αO-conotoxin GeXIVA[1,2] could provide a novel strategy to treat chemotherapy-induced neuropathic pain.

Structure-Activity Studies Reveal the Molecular Basis for GABAB-Receptor Mediated Inhibition of High Voltage-Activated Calcium Channels by α-Conotoxin Vc1.1

α-Conotoxins are disulfide-bonded peptides from cone snail venoms and are characterized by their affinity for nicotinic acetylcholine receptors (nAChR). Several α-conotoxins with distinct selectivity for nAChR subtypes have been identified as potent analgesics in animal models of chronic pain. However, a number of α-conotoxins have been shown to inhibit N-type calcium channel currents in rodent dissociated dorsal root ganglion (DRG) neurons via activation of G protein-coupled GABA_B receptors (GABA_BR). Therefore, it is unclear whether activation of GABA_BR or inhibition of α9α10 nAChRs is the analgesic mechanism. To investigate the mechanisms by which α-conotoxins provide analgesia, we synthesized a suite of Vc1.1 analogues where all residues, except the conserved cysteines, in Vc1.1 were individually replaced by alanine (A), lysine (K), and aspartic acid (D). Our results show that the amino acids in the first loop play an important role in binding of the peptide to the receptor, whereas those in the second loop play an important role for the selectivity of the peptide for the GABA_BR over α9α10 nAChRs. We designed a cVc1.1 analogue that is >8000-fold selective for GABA_BR-mediated inhibition of high voltage-activated (HVA) calcium channels over α9α10 nAChRs and show that it is analgesic in a mouse model of chronic visceral hypersensitivity (CVH). cVc1.1[D11A,E14A] caused dose-dependent inhibition of colonic nociceptors with greater efficacy in ex vivo CVH colonic nociceptors relative to healthy colonic nociceptors. These findings suggest that selectively targeting GABA_BR-mediated HVA calcium channel inhibition by α-conotoxins could be effective for the treatment of chronic visceral pain.

Pain therapeutics from cone snail venoms: From Ziconotide to novel non-opioid pathways

There have been numerous attempts to develop non-opioid drugs for severe pain, but the vast majority of these efforts have failed. A notable exception is Ziconotide (Prialt®), approved by the FDA in 2004. In this review, we summarize the present status of Ziconotide as a therapeutic drug and introduce a wider framework: the potential of venom peptides from cone snails as a resource providing a continuous pipeline for the discovery of non-opioid pain therapeutics. An auxiliary theme that we hope to develop is that these venoms, already a validated starting point for non-opioid drug leads, should also provide an opportunity for identifying novel molecular targets for future pain drugs. This review comprises several sections: the first focuses on Ziconotide as a therapeutic (including a historical retrospective and a clinical perspective); followed by sections on other promising Conus venom peptides that are either in clinical or pre-clinical development. We conclude with a discussion on why the outlook for discovery appears exceptionally promising. The combination of new technologies in diverse fields, including the development of novel high-content assays and revolutionary advancements in transcriptomics and proteomics, puts us at the cusp of providing a continuous pipeline of non-opioid drug innovations for pain. SIGNIFICANCE: The current opioid epidemic is the deadliest drug crisis in American history. Thus, this review on the discovery of non-opioid pain therapeutics and pathways from cone snail venoms is significant and timely.

G-Protein Coupled Receptors Targeted by Analgesic Venom Peptides

Chronic pain is a complex and debilitating condition associated with a large personal and socioeconomic burden. Current pharmacological approaches to treating chronic pain such as opioids, antidepressants and anticonvulsants exhibit limited efficacy in many patients and are associated with dose-limiting side effects that hinder their clinical use. Therefore, improved strategies for the pharmacological treatment of pathological pain are urgently needed. G-protein coupled receptors (GPCRs) are ubiquitously expressed on the surface of cells and act to transduce extracellular signals and regulate physiological processes. In the context of pain, numerous and diverse families of GPCRs expressed in pain pathways regulate most aspects of physiological and pathological pain and are thus implicated as potential targets for therapy of chronic pain. In the search for novel compounds that produce analgesia via GPCR modulation, animal venoms offer an enormous and virtually untapped source of potent and selective peptide molecules. While many venom peptides target voltage-gated and ligand-gated ion channels to inhibit neuronal excitability and blunt synaptic transmission of pain signals, only a small proportion are known to interact with GPCRs. Of these, only a few have shown analgesic potential in vivo. Here we review the current state of knowledge regarding venom peptides that target GPCRs to produce analgesia, and their development as therapeutic compounds.

Nicotinic acetylcholine receptors in neuropathic and inflammatory pain

Nicotinic acetylcholine receptors (nAChRs) are actively being investigated as therapeutic targets for the treatment of pain and inflammation, but despite more than 30 years of research, there are currently no FDA-approved analgesics that are specific for these receptors. Much of the initial research effort focused on the α4β2 nAChR subtype, but more recently, additional subtypes have been identified as promising new leads and include α6β4, α7, and α9-containing nAChRs. This Review will focus on the distribution of these nAChRs in the cell types involved in neuropathic pain and inflammation and the activity of currently available nicotinic ligands.

A novel endpoint for the assessment of chemotherapy-induced peripheral neuropathy in rodents: biomechanical properties of peripheral nerve

Chemotherapy-induced peripheral neuropathy (CiPN) is a frequent adverse effect in patients and a leading safety consideration in oncology drug development. Although behavioral assessment and microscopic examination of the nerves and dorsal root ganglia can be incorporated into toxicity studies to assess CiPN risk, more sensitive and less labor-intensive endpoints are often lacking. In this study, rats and mice administered vincristine (75 μg kg^-1  day^-1 , i.p., for 10 days in rats and 100 μg kg^-1  day^-1 , i.p., for 11 days in mice, respectively) were employed as the CiPN models. Behavioral changes were assessed during the dosing phase. At necropsy, the sural or sciatic nerve was harvested from the rats and mice, respectively, and assessed for mechanical and histopathological endpoints. It was found that the maximal load and the load/extension ratio were significantly decreased in the nerves collected from the animals dosed with vincristine compared with the vehicle-treated animals (P < 0.05). Additionally, the gait analysis revealed that the paw print areas were significantly increased in mice (P < 0.01), but not in rats following vincristine administration. Light microscopic histopathology of the nerves and dorsal root ganglia were unaffected by vincristine administration. We concluded that ex vivo mechanical properties of the nerves is a sensitive endpoint, providing a new method to predict CiPN in rodent. Gait analysis may also be a useful tool in these pre-clinical animal models.

α9-containing nicotinic acetylcholine receptors and the modulation of pain

Neuropathic pain is a complex and debilitating syndrome for which there are few effective pharmacological treatments. Opioid-based medications are initially effective for acute pain, but tolerance to their analgesic effects quickly develops, and long-term use often leads to physical dependence and addiction. Furthermore, neuropathic pain is generally resistant to non-steroidal anti-inflammatory drugs. Other classes of medications including antidepressants, antiepileptics and voltage-gated calcium channel inhibitors are only partially effective in most patients, may be associated with significant side effects and have few disease-modifying effects on the underlying pathology. Medications that act through new mechanisms of action, and particularly ones that have disease-modifying properties, would be highly desirable. In the last decade, a potential new target for the treatment of neuropathic pain has emerged: the α9-containing nicotinic acetylcholine receptor (nAChR). Recent studies indicate that antagonists of α9-containing nAChRs are analgesic in animal models of neuropathic pain. These nerve injury models include chronic constriction injury, partial sciatic nerve ligation, streptozotocin-induced diabetic neuropathy and chemotherapeutic-induced neuropathy. This review details the history and state of the field regarding the role that α9-containing nAChRs may play in neuropathic pain. An alternative hypothesis that α-conotoxins exert their therapeutic effect through blocking N-type calcium channels via activation of GABA_B receptors is also reviewed. Understanding how antagonists of α9-containing nAChRs exert their therapeutic effects may ultimately result in the development of medications that not only treat but also prevent the development of neuropathic pain states.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

RgIA4 Potently Blocks Mouse α9α10 nAChRs and Provides Long Lasting Protection against Oxaliplatin-Induced Cold Allodynia

Transcripts for α9 and α10 nicotinic acetylcholine receptor (nAChR) subunits are found in diverse tissues. The function of α9α10 nAChRs is best known in mechanosensory cochlear hair cells, but elsewhere their roles are less well-understood. α9α10 nAChRs have been implicated as analgesic targets and α-conotoxins that block α9α10 nAChRs produce analgesia. However, some of these peptides show large potency differences between species. Additionally several studies have indicated that these conotoxins may also activate GABA_B receptors (GABA_BRs). To further address these issues, we cloned the cDNAs of mouse α9 and α10 nAChR subunits. When heterologously expressed in Xenopus oocytes, the resulting α9α10 nAChRs had the expected pharmacology of being activated by acetylcholine and choline but not by nicotine. A conotoxin analog, RgIA4, potently, and selectively blocked mouse α9α10 nAChRs with low nanomolar affinity indicating that RgIA4 may be effectively used to study murine α9α10 nAChR function. Previous reports indicated that RgIA4 attenuates chemotherapy-induced cold allodynia. Here we demonstrate that RgIA4 analgesic effects following oxaliplatin treatment are sustained for 21 days after last RgIA4 administration indicating that RgIA4 may provide enduring protection against nerve damage. RgIA4 lacks activity at GABA_B receptors; a bioluminescence resonance energy transfer assay was used to demonstrate that two other analgesic α-conotoxins, Vc1.1 and AuIB, also do not activate GABA_BRs expressed in HEK cells. Together these findings further support the targeting of α9α10 nAChRs in the treatment of pain.

Crystal structure of 4,4'-bis-[3-(piperidin-1-yl)prop-1-yn-1-yl]-1,1'-biphen-yl

The title compound, C28H32N2, (I), is one of a second generation of compounds designed and synthesized based on a very potent and selective α9α10 nicotinic acetyl-choline receptor antagonist ZZ161C {1,1'-[[1,1'-biphen-yl]-4,4'-diylbis(prop-2-yne-3,1-di-yl)]bis-(3,4-di-methyl-pyridin-1-ium) bromide}, which has shown analgesic effects in a chemotherapy-induced neuropathy animal model. Compound (I) was synthesized by the reaction of 4,4'-bis-(3-bromo-prop-1-yn-1-yl)-1,1'-biphenyl with piperidine at room temperature in aceto-nitrile. The single-crystal used for X-ray analysis was obtained by dissolving (I) in a mixture of di-chloro-methane and methanol, followed by slow evaporation of the solvent. In the crystal of (I), the biphenyl moiety has a twisted conformation, with a dihedral angle of 25.93 (4)° between the benzene rings. Both piperidine head groups in (I) are in the chair conformation and are oriented so that the N-atom lone pairs of each piperidine group point away from the central biphenyl moiety.

Inhibition of α9α10 nicotinic acetylcholine receptors prevents chemotherapy-induced neuropathic pain

Opioids are first-line drugs for moderate to severe acute pain and cancer pain. However, these medications are associated with severe side effects, and whether they are efficacious in treatment of chronic nonmalignant pain remains controversial. Medications that act through alternative molecular mechanisms are critically needed. Antagonists of α9α10 nicotinic acetylcholine receptors (nAChRs) have been proposed as an important nonopioid mechanism based on studies demonstrating prevention of neuropathology after trauma-induced nerve injury. However, the key α9α10 ligands characterized to date are at least two orders of magnitude less potent on human vs. rodent nAChRs, limiting their translational application. Furthermore, an alternative proposal that these ligands achieve their beneficial effects by acting as agonists of GABA_B receptors has caused confusion over whether blockade of α9α10 nAChRs is the fundamental underlying mechanism. To address these issues definitively, we developed RgIA4, a peptide that exhibits high potency for both human and rodent α9α10 nAChRs, and was at least 1,000-fold more selective for α9α10 nAChRs vs. all other molecular targets tested, including opioid and GABA_B receptors. A daily s.c. dose of RgIA4 prevented chemotherapy-induced neuropathic pain in rats. In wild-type mice, oxaliplatin treatment produced cold allodynia that could be prevented by RgIA4. Additionally, in α9 KO mice, chemotherapy-induced development of cold allodynia was attenuated and the milder, temporary cold allodynia was not relieved by RgIA4. These findings establish blockade of α9-containing nAChRs as the basis for the efficacy of RgIA4, and that α9-containing nAChRs are a critical target for prevention of chronic cancer chemotherapy-induced neuropathic pain.

The α9α10 nicotinic receptor antagonist α-conotoxin RgIA prevents neuropathic pain induced by oxaliplatin treatment

Oxaliplatin, a third-generation diaminocyclohexane platinum drug, is widely used alone or in combination with 5-fluorouracil and leucovorin to treat metastatic colorectal, ovarian, and pancreatic cancers. Oxaliplatin long-term treatment is associated with the development of a dose-limiting painful neuropathy that dramatically impairs the patient's quality of life and therapy possibility. To study novel strategies to treat oxaliplatin-induced neuropathy, we evaluated α-conotoxin RgIA, a peptide that potently blocks the α9α10 nicotinic acetylcholine receptor (nAChR) subtype in a rat model of oxaliplatin-dependent neurotoxicity (2.4mgkg(-1) oxaliplatin intraperitoneally daily for 21days). The administration of RgIA (2 and 10nmol injected intramuscularly once a day concomitantly with oxaliplatin treatment), reduced the oxaliplatin-dependent hypersensitivity to mechanical and thermal noxious and non-noxious stimuli. Moreover, morphological modifications of L4-L5 dorsal root ganglia were significantly prevented. In the spinal cord the numerical increase of astrocyte cell density present in oxaliplatin-treated rats is partially prevented by RgIA treatment. Nevertheless, the administration of the α-conotoxin is able per se to elicit a numerical increase and a morphological activation of microglia and astrocytes in specific brain areas.

Comparison of the crystal structures of 4,4'-bis-[3-(4-methyl-piperidin-1-yl)prop-1-yn-1-yl]-1,1'-biphenyl and 4,4'-bis-[3-(2,2,6,6-tetra-methyl-piperidin-1-yl)prop-1-yn-1-yl]-1,1'-biphen-yl

As part of a comprehensive program to discover α9α10 nicotinic acetyl-choline receptor antagonists, the title compounds C30H36N2, (I), and C36H48N2, (II), were synthesized by coupling 4,4'-bis-(3-bromo-prop-1-yn-1-yl)-1,1'-biphenyl with 4-methyl-piperidine and 2,2,6,6-tetra-methyl-piperidine, respectively, in aceto-nitrile at room temperature. In compound (I), the biphenyl system has a twisted conformation with a dihedral angle of 26.57 (6)° between the two phenyl rings of the biphenyl moiety, while in compound (II), the biphenyl moiety sits on a crystallographic inversion centre so the two phenyl rings are exactly coplanar. The terminal piperidine rings in both compound (I) and compound (II) are in the chair conformation. In compound (I), the dihedral angles about the ethynyl groups between the planes of the phenyl rings and the piperidine ring N atoms are 37.16 (16) and 14.20 (17)°. In compound (II), the corresponding dihedral angles are both 61.48 (17)°. There are no noteworthy inter-molecular inter-actions in (I), but in (II) there is a small π-overlap between inversion-related mol-ecules (1 - x, 1 - y, 1 - z), with an inter-planar spacing of 3.553 (3) Å and centroid-to-centroid separation of 3.859 (4) Å.

Comparison crystal structure conformations of two structurally related biphenyl analogues: 4,4'-bis-[3-(pyrrolidin-1-yl)prop-1-yn-1-yl]-1,1'-biphenyl and 4,4'-bis-{3-[(S)-2-methyl-pyrrolidin-1-yl]prop-1-yn-1-yl}-1,1'-biphen-yl

The title compounds, C26H28N2, (I), and C28H32N2, (II), were designed based on the structure of the potent α9α10 nicotinic acetyl-choline receptor antagonist ZZ161C {1,1'-[[1,1'-biphen-yl]-4,4'-diylbis(prop-2-yne-3,1-di-yl)]bis-(3,4-di-methyl-pyridin-1-ium) bromide}. In order to improve the druglikeness properties of ZZ161C for potential oral administration, the title compounds (I) and (II) were prepared by coupling 4,4'-bis-(3-bromo-prop-1-yn-1-yl)-1,1'-biphenyl with pyrrol-idine, (I), and (S)-2-methyl-pyrrolidine, (II), respectively, in aceto-nitrile at room temperature. The asymmetric unit of (I) contains two half mol-ecules that each sit on sites of crystallographic inversion. As a result, the biphenyl ring systems in compound (I) are coplanar. The biphenyl ring system in compound (II), however, has a dihedral angle of 28.76 (11)°. In (I), the two independent mol-ecules differ in the orientation of the pyrrolidine ring (the nitro-gen lone pair points towards the biphenyl rings in one mol-ecule, but away from the rings in the other). The torsion angles about the ethynyl groups between the planes of the phenyl rings and the pyrrolidine ring N atoms are 84.15 (10) and -152.89 (10)°. In compound (II), the corresponding torsion angles are 122.0 (3) and 167.0 (3)°, with the nitro-gen lone pairs at both ends of the mol-ecule directed away from the central biphenyl rings.

Conotoxin Interactions with α9α10-nAChRs: Is the α9α10-Nicotinic Acetylcholine Receptor an Important Therapeutic Target for Pain Management?

The α9α10-nicotinic acetylcholine receptor (nAChR) has been implicated in pain and has been proposed to be a novel target for analgesics. However, the evidence to support the involvement of the α9α10-nAChR in pain is conflicted. This receptor was first implicated in pain with the characterisation of conotoxin Vc1.1, which is highly selective for α9α10-nAChRs and is an efficacious analgesic in chronic pain models with restorative capacities and no reported side effects. Numerous other analgesic conotoxin and non-conotoxin molecules have been subsequently characterised that also inhibit α9α10-nAChRs. However, there is evidence that α9α10-nAChR inhibition is neither necessary nor sufficient for analgesia. α9α10-nAChR-inhibiting analogues of Vc1.1 have no analgesic effects. Genetically-modified α9-nAChR knockout mice have a phenotype that is markedly different from the analgesic profile of Vc1.1 and similar conotoxins, suggesting that the conotoxin effects are largely independent of α9α10-nAChRs. Furthermore, an alternative mechanism of analgesia by Vc1.1 and other similar conotoxins involving non-canonical coupling of GABAB receptors to voltage-gated calcium channels is known. Additional incongruities regarding α9α10-nAChRs in analgesia are discussed. A more comprehensive characterisation of the role of α9α10-nAChRs in pain is crucial for understanding the analgesic action of conotoxins and for improved drug design.

Cloning, synthesis, and characterization of αO-conotoxin GeXIVA, a potent α9α10 nicotinic acetylcholine receptor antagonist

We identified a previously unidentified conotoxin gene from Conus generalis whose precursor signal sequence has high similarity to the O1-gene conotoxin superfamily. The predicted mature peptide, αO-conotoxin GeXIVA (GeXIVA), has four Cys residues, and its three disulfide isomers were synthesized. Previously pharmacologically characterized O1-superfamily peptides, exemplified by the US Food and Drug Administration-approved pain medication, ziconotide, contain six Cys residues and are calcium, sodium, or potassium channel antagonists. However, GeXIVA did not inhibit calcium channels but antagonized nicotinic AChRs (nAChRs), most potently on the α9α10 nAChR subtype (IC50 = 4.6 nM). Toxin blockade was voltage-dependent, and kinetic analysis of toxin dissociation indicated that the binding site of GeXIVA does not overlap with the binding site of the competitive antagonist α-conotoxin RgIA. Surprisingly, the most active disulfide isomer of GeXIVA is the bead isomer, comprising, according to NMR analysis, two well-resolved but uncoupled disulfide-restrained loops. The ribbon isomer is almost as potent but has a more rigid structure built around a short 310-helix. In contrast to most α-conotoxins, the globular isomer is the least potent and has a flexible, multiconformational nature. GeXIVA reduced mechanical hyperalgesia in the rat chronic constriction injury model of neuropathic pain but had no effect on motor performance, warranting its further investigation as a possible therapeutic agent.

Rodent models of chemotherapy-induced peripheral neuropathy

Peripheral neuropathy is a common and dose-limiting side effect of many chemotherapeutic drugs. These include platinum compounds, taxanes, vinca alkaloids, proteasome inhibitors, and others such as thalidomide and suramin. Although many rodent models have been developed using either mice or rats, there is limited consistency in the dose or mode of delivery of the drug; the sex, age, and genetic background of the animal used in the study; and the outcome measures used in evaluation of the peripheral neuropathy. Behavioral assays are commonly used to evaluate evoked sensory responses but are unlikely to be a good representation of the spontaneous sensory paresthesias that the patients experience. Electrophysiologic tests evaluate the integrity of large myelinated populations and are useful in drugs that cause either demyelination or degeneration of large myelinated axons but are insensitive to degeneration of unmyelinated axons in early stages of neuropathy. Histopathologic tools offer an unbiased way to evaluate the degree of axonal degeneration or changes in neuronal cell body but are often time consuming and require processing of the tissue after the study is completed. Nevertheless, use of drug doses and mode of delivery that are relevant to the clinical protocols and use of outcome measures that are both sensitive and objective in evaluation of the length-dependent distal axonal degeneration seen in most chemotherapy-induced peripheral neuropathies may improve the translational utility of these rodent models.

Limited efficacy of α-conopeptides, Vc1.1 and RgIA, to inhibit sensory neuron CaV current

Chronic pain is very difficult to treat. Thus, novel analgesics are a critical area of research. Strong pre-clinical evidence supports the analgesic effects of α-conopeptides, Vc1.1 and RgIA, which block α9α10 nicotinic acetylcholine receptors (nAChRs). However, the analgesic mechanism is controversial. Some evidence supports the block of α9α10 nAChRs as an analgesic mechanism, while other evidence supports the inhibition of N-type Ca_V (Ca_V2.2) current via activation of GABA_B receptors. Here we reassess the effect of Vc1.1 and RgIA on Ca_V current in rat sensory neurons. Unlike the previous findings, we found highly variable effects among individual sensory neurons, but on average only minimal inhibition induced by Vc1.1, and no significant effect on the current by RgIA. We also investigated the potential involvement of GABA_B receptors in the Vc1.1 induced inhibition, and found no correlation between the size of Ca_V current inhibition induced by baclofen (GABA_B agonist) vs. that induced by Vc1.1. Thus, GABA_B receptors are unlikely to mediate the Vc1.1 induced Ca_V current inhibition. Based on the present findings, Ca_V current inhibition in dorsal root ganglia is unlikely to be the predominant mechanism by which either Vc1.1 or RgIA induce analgesia.

SIGNIFICANCE STATEMENT

Better analgesic drugs are desperately needed to help physicians to treat pain. While many pre-clinical studies support the analgesic effects of α-conopeptides, Vc1.1 and RgIA, the mechanism is controversial. The development of improved α-conopeptide analgesics would be greatly facilitated by a complete understanding of the analgesic mechanism. However, we show that we cannot reproduce one of the proposed analgesic mechanisms, which is an irreversible inhibition of Ca_V current in a majority of sensory neurons.

α9-nicotinic acetylcholine receptors contribute to the maintenance of chronic mechanical hyperalgesia, but not thermal or mechanical allodynia

Background

The current pharmacological treatments for chronic pain are limited. The first analgesic drug approved for clinical use in decades that has a novel molecular target is the synthetic version of a naturally occurring conotoxin. Several conotoxins that target ion channels have progressed to clinical trials for the relief of pain. Vc1.1 and RgIA are analgesic α-conotoxins that target α9-subunit-containing nicotinic acetylcholine receptors (α9-nAChR) as well as GABA_B receptor mechanisms. However, the evidence for the involvement of α9-nAChRs in pain is controversial. In the present study, the role of the α9-nAChR in pain was assessed using a battery of behavioural pain tests and pain models in α9-nAChR knockout (KO) mice.

Results

α9-nAChR KO mice showed normal responses to acute noxious thermal and mechanical stimuli, and developed normal chronic cold and mechanical allodynia in inflammatory and nerve injury pain models. However, KO animals developed mechanical hyperalgesia to a lesser extent than their wild type (WT) counterparts in both inflammatory and neuropathic pain models. Chronic neuropathic pain is sustained in WT mice for at least 21 days post injury, while KO mice show significant recovery by 14 days post injury. KO sham mice were also resistant to the repeated-measures effect of the noxious pain test that caused a gradual onset of mild mechanical hyperalgesia in WT sham animals.

Conclusions

The α9-nAChR is not involved in acute pain perception or chronic thermal or mechanical allodynia or thermal hyperalgesia but does contribute to the intensity and duration of chronic mechanical hyperalgesia, suggesting that pain-relieving actions of antagonists that target this site may be restricted to high threshold mechanosensation. The α9-nAChR appears to be a valid target for pharmacological compounds that alleviate long-term mechanical hyperalgesia and may be of use as a prophylactic drug to prevent the development of some symptoms of chronic pain.

α-conotoxin RgIA protects against the development of nerve injury-induced chronic pain and prevents both neuronal and glial derangement

Neuropathic pain affects millions of people worldwide, causing substantial disability and greatly impairing quality of life. Commonly used analgesics or antihyperalgesic compounds are generally characterized by limited therapeutic outcomes. Thus, there is a compelling need for novel therapeutic strategies able to prevent nervous tissue alterations responsible for chronic pain. The α9α10 nicotinic acetylcholine receptor antagonist α-conotoxin RgIA (RgIA), a peptide isolated from the venom of a carnivorous cone snail, induces relief in both acute and chronic pain models. To evaluate potential disease-modifying effects of RgIA, the compound was given to rats following chronic constriction injury (CCI) of the sciatic nerve. Two or 10 nmol RgIA injected intramuscularly once a day for 14 days reduced the painful response to suprathreshold stimulation, increased pain threshold to nonnoxious stimuli, and normalized alterations in hind limb weight bearing. Histological analysis of the sciatic nerve revealed that RgIA prevented CCI-induced decreases of axonal compactness and diameter, loss of myelin sheath, and decreases in the fiber number. Moreover, RgIA significantly reduced edema and inflammatory infiltrate, including a decrease of CD86(+) macrophages. In L4-L5 dorsal root ganglia, RgIA prevented morphometric changes and reduced the inflammatory infiltrate consistent with a disease-modifying effect. In the dorsal horn of the spinal cord, RgIA prevented CCI-induced activation of microglia and astrocytes. These data suggest that RgIA-like compounds may represent a novel class of therapeutics for neuropathic pain that protects peripheral nervous tissues as well as prevents central maladaptive plasticity by inhibiting glial cell activation.

Functional expression of human α9* nicotinic acetylcholine receptors in X. laevis oocytes is dependent on the α9 subunit 5' UTR

Nicotinic acetylcholine receptors (nAChRs) containing the α9 subunit are expressed in a wide variety of non-neuronal tissues ranging from immune cells to breast carcinomas. The α9 subunit is able to assemble into a functional homomeric nAChR and also co-assemble with the α10 subunit into functional heteromeric nAChRs. Despite the increasing awareness of the important roles of this subunit in vertebrates, the study of human α9-containing nAChRs has been severely limited by difficulties in its expression in heterologous systems. In Xenopus laevis oocytes, functional expression of human α9α10 nAChRs is very low compared to that of rat α9α10 nAChRs. When oocytes were co-injected with cRNA of α9 and α10 subunits of human versus those of rat, oocytes with the rat α9 human α10 combination had an ∼-fold higher level of acetylcholine-gated currents (I_ACh) than those with the human α9 rat α10 combination, suggesting difficulties with human α9 expression. When the ratio of injected human α9 cRNA to human α10 cRNA was increased from 1∶1 to 5∶1, I_ACh increased 36-fold (from 142±23 nA to 5171±748 nA). Functional expression of human α9-containing receptors in oocytes was markedly improved by appending the 5′-untranslated region of alfalfa mosaic virus RNA4 to the 5′-leader sequence of the α9 subunit cRNA. This increased the functional expression of homomeric human α9 receptors by 70-fold (from 7±1 nA to 475±158 nA) and of human α9α10 heteromeric receptors by 80-fold (from 113±62 nA to 9192±1137 nA). These findings indicate the importance of the composition of the 5′ untranslated leader sequence for expression of α9-containing nAChRs.

Determination of the α-conotoxin Vc1.1 binding site on the α9α10 nicotinic acetylcholine receptor

α-Conotoxin Vc1.1 specifically and potently inhibits the nicotinic acetylcholine receptor subtype α9α10 (α9α10 nAChR) and is a potential novel treatment for neuropathic pain. Here, we used a combination of computational modeling and electrophysiology experiments to determine the Vc1.1 binding site on the α9α10 nAChR. Interactions of Vc1.1 with two probable binding sites, α9α10 and α10α9, were modeled. Mutational energies calculated by assuming specific interactions in the α10α9 binding site correlated better with electrophysiological recordings than those assuming interactions with the α9α10 binding site. Two novel Vc1.1 analogues, [N9F]Vc1.1 and [N9W]Vc1.1, were predicted to have large differences in affinity between the two binding sites. Data from functional studies were consistent with computational predictions that assumed preferred binding of Vc1.1 to the α10α9 pocket. Moreover, our modeling study suggested that a single hydrogen bond formed between Vc1.1 and position 59 of the α10α9 pocket confers specificity to rat versus human α9α10 nAChRs.