Challenges in the development of novel treatment strategies for neuropathic pain

Peripheral neuropathies associated with DNA repair disorders

Repair of genomic DNA is a fundamental housekeeping process that quietly maintains the health of our genomes. The consequences of a genetic defect affecting a component of this delicate mechanism are quite harmful, characterized by a cascade of premature aging that injures a variety of organs, including the nervous system. One part of the nervous system that is impaired in certain DNA repair disorders is the peripheral nerve. Chronic motor, sensory, and sensorimotor polyneuropathies have all been observed in affected individuals, with specific physiologies associated with different categories of DNA repair disorders. Cockayne syndrome has classically been linked to demyelinating polyneuropathies, whereas xeroderma pigmentosum has long been associated with axonal polyneuropathies. Three additional recessive DNA repair disorders are associated with neuropathies, including trichothiodystrophy, Werner syndrome, and ataxia-telangiectasia. Although plausible biological explanations exist for why the peripheral nerves are specifically vulnerable to impairments of DNA repair, specific mechanisms such as oxidative stress remain largely unexplored in this context, and bear further study. It is also unclear why different DNA repair disorders manifest with different types of neuropathy, and why neuropathy is not universally present in those diseases. Longitudinal physiological monitoring of these neuropathies with serial electrodiagnostic studies may provide valuable noninvasive outcome data in the context of future natural history studies, and thus the responses of these neuropathies may become sentinel outcome measures for future clinical trials of treatments currently in development such as adeno-associated virus gene replacement therapies.

Neuro-Immunity and Gut Dysbiosis Drive Parkinson's Disease-Induced Pain

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1-2% of the population aged 65 and over. Additionally, non-motor symptoms such as pain and gastrointestinal dysregulation are also common in PD. These impairments might stem from a dysregulation within the gut-brain axis that alters immunity and the inflammatory state and subsequently drives neurodegeneration. There is increasing evidence linking gut dysbiosis to the severity of PD's motor symptoms as well as to somatosensory hypersensitivities. Altogether, these interdependent features highlight the urgency of reviewing the links between the onset of PD's non-motor symptoms and gut immunity and whether such interplays drive the progression of PD. This review will shed light on maladaptive neuro-immune crosstalk in the context of gut dysbiosis and will posit that such deleterious interplays lead to PD-induced pain hypersensitivity.

Development of a Phantom Limb Pain Model in Rats: Behavioral and Histochemical Evaluation

Therapeutic strategies targeting phantom limb pain (PLP) provide inadequate pain relief; therefore, a robust and clinically relevant animal model is necessary. Animal models of PLP are based on a deafferentation injury followed by autotomy behavior. Clinical studies have shown that the presence of pre-amputation pain increases the risk of developing PLP. In the current study, we used Sprague-Dawley male rats with formalin injections or constriction nerve injury at different sites or time points prior to axotomy to mimic clinical scenarios of pre-amputation inflammatory and neuropathic pain. Animals were scored daily for PLP autotomy behaviors, and several pain-related biomarkers were evaluated to discover possible underlying pathological changes. Majority displayed some degree of autotomy behavior following axotomy. Injury prior to axotomy led to more severe PLP behavior compared to animals without preceding injury. Autotomy behaviors were more directed toward the pretreatment insult origin, suggestive of pain memory. Increased levels of IL-1β in cerebrospinal fluid and enhanced microglial responses and the expression of NaV1.7 were observed in animals displaying more severe PLP outcomes. Decreased expression of GAD65/67 was consistent with greater PLP behavior. This study provides a preclinical basis for future understanding and treatment development in the management of PLP.

Histamine, histamine receptors, and neuropathic pain relief

Histamine, acting via distinct histamine H₁, H₂, H₃, and H₄ receptors, regulates various physiological and pathological processes, including pain. In the last two decades, there has been a particular increase in evidence to support the involvement of H₃ receptor and H₄ receptor in the modulation of neuropathic pain, which remains challenging in terms of management. However, recent data show contrasting effects on neuropathic pain due to multiple factors that determine the pharmacological responses of histamine receptors and their underlying signal transduction properties (e.g., localization on either the presynaptic or postsynaptic neuronal membranes). This review summarizes the most recent findings on the role of histamine and the effects mediated by the four histamine receptors in response to the various stimuli associated with and promoting neuropathic pain. We particularly focus on mechanisms underlying histamine‐mediated analgesia, as we aim to clarify the analgesic potential of histamine receptor ligands in neuropathic pain.

Linked Articles

This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc

Fulranumab in Patients With Pain Associated With Postherpetic Neuralgia and Postraumatic Neuropathy: Efficacy, Safety, and Tolerability Results From a Randomized, Double-blind, Placebo-controlled, Phase-2 Study

Supplemental Digital Content is available in the text.

Objective:

Fulranumab is an antibody that specifically neutralizes the biological activity of human nerve growth factor. This multicenter, phase-2, randomized, double-blind (DB), placebo-controlled study evaluated the analgesic efficacy and safety of fulranumab in postherpetic neuralgia (PHN) and posttraumatic neuropathy (PTN) patients.

Methods:

Patients (18 to 80 y) with inadequately controlled moderate-to-severe pain received study medication (subcutaneous injection) every 4 weeks. PHN patients were randomized (3:2:2:3) to receive either placebo or one of 3 doses of fulranumab: 1 mg (1 mgQ4 wk), 3 mg (3 mgQ4 wk), or 10 mg (10 mgQ4 wk). PTN patients were randomized (1:1) to receive either placebo or fulranumab 10 mgQ4 wk.

Results:

The US Food and Drug Administration placed a clinical hold (December 23, 2010) on all trials of antinerve growth factor drugs, including fulranumab, due to identified risks of osteonecrosis or rapidly progressing osteoarthritis; therefore, only 49 (of 150 planned) PHN patients and 34 (of 50 planned) PTN patients completed the DB efficacy evaluation. There was no significant difference (P>0.05, fulranumab vs. placebo) for change in 7-day average of daily pain intensity scores from DB baseline to end of 12-week DB efficacy phase in PHN or PTN patients (primary endpoint). No significant difference was found with fulranumab versus placebo (P>0.05) in other efficacy measures in either PHN or PTN patients. The most common treatment-emergent adverse events (>10% incidence) in PTN patients were sinusitis, carpal tunnel syndrome, and headache, whereas in PHN patients it was arthralgia.

Discussion:

Fulranumab did not demonstrate efficacy in either PHN or PTN patients, but was generally well-tolerated in this small underpowered and abbreviated study.

Peripherally Selective Cannabinoid 1 Receptor (CB1R) Agonists for the Treatment of Neuropathic Pain

Alleviation of neuropathic pain by cannabinoids is limited by their central nervous system (CNS) side effects. Indole and indene compounds were engineered for high hCB1R affinity, peripheral selectivity, metabolic stability, and in vivo efficacy. An epithelial cell line assay identified candidates with <1% blood-brain barrier penetration for testing in a rat neuropathy induced by unilateral sciatic nerve entrapment (SNE). The SNE-induced mechanical allodynia was reversibly suppressed, partially or completely, after intraperitoneal or oral administration of several indenes. At doses that relieve neuropathy symptoms, the indenes completely lacked, while the brain-permeant CB1R agonist HU-210 (1) exhibited strong CNS side effects, in catalepsy, hypothermia, and motor incoordination assays. Pharmacokinetic findings of ∼0.001 cerebrospinal fluid:plasma ratio further supported limited CNS penetration. Pretreatment with selective CB1R or CB2R blockers suggested mainly CB1R contribution to an indene's antiallodynic effects. Therefore, this class of CB1R agonists holds promise as a viable treatment for neuropathic pain.

PARP inhibition attenuates neuroinflammation and oxidative stress in chronic constriction injury induced peripheral neuropathy

AIM

Peripheral nerve degeneration after nerve injury is accompanied with oxidative stress that may activate poly ADP-ribose polymerase (PARP, DNA repair enzyme). PARP overactivation amplifies the neuronal damage either due to energy crisis or through inflammatory process by facilitating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Hence investigated the role of PARP inhibitors, 3-Aminobenzamide (3-AB) and 1,5-isoquinolinediol (ISO) in the attenuation of chronic constriction injury (CCI) induced peripheral neuropathy in rats.

METHODS

3-AB and ISO (at doses 30 and 3mg/kg i.p., respectively) were tested in rats subjected to standard tests for evaluating hyperalgesia and allodynia. Sciatic functional index (SFI) was assessed by performing walking track analysis. Oxidative stress and inflammation induced biochemical alterations were estimated after 14 days in sciatic nerve and lumbar spinal cord. Molecular changes were explored by immunohistochemistry and DNA fragmentation by TUNEL assay.

KEY FINDINGS

Treatment significantly improved sensorimotor responses (p<0.001), SFI (p<0.001) and foot posture. PARP inhibition significantly (p<0.01 and p<0.001) reduced the elevated levels of nitrite, inflammatory markers and also normalized the depleted NAD(total) levels. The protein expression of poly (ADP-ribose) (PAR), NF-κB, cyclooxygenase-2 (COX-2) and nitrotyrosine were significantly (p<0.01 and p<0.001) decreased in both sciatic nerve and lumbar spinal cord, evident through immunohistochemistry.

SIGNIFICANCE

Present study outcomes fortify the pathological role of PARP overactivation in CCI induced neuropathy and PARP inhibition ameliorated oxidative stress and neuroinflammation associated with CCI induced nerve injury. Therefore, the current study suggests the PARP inhibitors can further be evaluated for designing futuristic strategies for the management of trauma induced neuropathy.

The use of the antimicrobial peptide piscidin (PCD)-1 as a novel anti-nociceptive agent

The antimicrobial peptide piscidin (PCD)-1 has been reported to have antibacterial and immunomodulatory functions. Here, we investigated the anti-neuropathic properties of PCD-1, in order to determine its potential as a compound to alleviate pain. Treatment with PCD-1 suppressed the inflammatory proteins COX-2 and iNOS in murine macrophage (RAW264.7) and microglial (BV2) cell lines stimulated by lipopolysaccharide (LPS). For studies of the effect of PCD-1 in vivo, mononeuropathy in rats was induced by chronic constriction injury (CCI), and the resulting anti-nociceptive behaviors were compared between CCI controls and CCI rats given intrathecal injections of PCD-1. Much like gabapentin, PCD-1 exerts anti-nociceptive effects against thermal hyperalgesia, with a median effective dose (ED50) of 9.5 μg in CCI rats. In CCI rats, PCD-1 exerted effects against mechanical and cold allodynia, thermal hyperalgesia, and weight-bearing deficits. Furthermore, CCI-mediated activation of microglia and astrocytes in the dorsal horn of the lumbar spinal cord were decreased by PCD-1. In addition, PCD-1 suppressed up-regulation of interleukin-1β (IL-1β) and phosphorylated mammalian target of rapamycin (phospho-mTOR) in CCI rats. Finally, CCI-induced down-regulation of transforming growth factor-β1 (TGF-β1) in rats was attenuated by injection of PCD-1. Taken together, the present findings demonstrate that the marine antimicrobial peptide PCD-1 has anti-nociceptive effects, and thus may have potential for development as an alternative pain-alleviating agent.

Antinociceptive effect of hydantoin 3-phenyl-5-(4-ethylphenyl)-imidazolidine-2,4-dione in mice

Imidazolidine derivatives, or hydantoins, are synthetic compounds with different therapeutic applications. Many imidazolidine derivatives have psychopharmacological properties, such as phenytoin, famous for its anticonvulsant efficacy, but also effective in the treatment of neuropathic pain. The hydantoin, 3-phenyl-5-(4-ethylphenyl)-imidazolidine-2,4-dione (IM-3), synthesized from the amino acid, glycine, was selected for psychopharmacological studies in mice on the basis of its chemical and structural similarity with phenytoin. The first step of this study was to define the LD₅₀, which determined the doses of 50, 100 and 200 mg/kg for subsequent tests. The results obtained from the behavioral screening indicated that IM-3 produces decreased ambulation and analgesia in mice. Motor coordination and anxiety behavior were not affected by treatment with IM-3, as observed in the rotarod and elevated plus-maze tests, respectively. Regarding its antinociceptive properties, IM-3 showed efficacy in the acetic acid-induced writhing test by increasing the latency of the first writhe and reducing the number of writhes, as well as reducing the paw licking time in the second phase of the formalin test. The behavior of treated animals exposed to the hot plate test, however, did not differ from that of the control group. These data suggest that IM-3 has antinociceptive effects in mice, which is probably mediated by anti-inflammatory mechanisms.

The perception and endogenous modulation of pain

Pain is often perceived an unpleasant experience that includes sensory and emotional/motivational responses. Accordingly, pain serves as a powerful teaching signal enabling an organism to avoid injury, and is critical to survival. However, maladaptive pain, such as neuropathic or idiopathic pain, serves no survival function. Genomic studies of individuals with congenital insensitivity to pain or paroxysmal pain syndromes considerable increased our understanding of the function of peripheral nociceptors, and especially of the roles of voltage-gated sodium channels and of nerve growth factor (NGF)/TrkA receptors in nociceptive transduction and transmission. Brain imaging studies revealed a "pain matrix," consisting of cortical and subcortical regions that respond to noxious inputs and can positively or negatively modulate pain through activation of descending pain modulatory systems. Projections from the periaqueductal grey (PAG) and the rostroventromedial medulla (RVM) to the trigeminal and spinal dorsal horns can inhibit or promote further nociceptive inputs. The "pain matrix" can explain such varied phenomena as stress-induced analgesia, placebo effect and the role of expectation on pain perception. Disruptions in these systems may account for the existence idiopathic pan states such as fibromyalgia. Increased understanding of pain modulatory systems will lead to development of more effective therapeutics for chronic pain.

Growth factors and neuropathic pain

A treatment for neuropathic pain is an important unmet medical need because this pain often is refractory to many medical interventions. An important element in the development of neuropathic pain is a dysfunction in the activity of peripheral nerves. Because neurotrophic factors affect nerve development and maintenance, modulating the activity of these factors can alter neuronal pathophysiology and produce a disease-modifying effect. Blocking the activity of nerve growth factor or enhancing the activity of either glial-derived neurotrophic factor or artemin has shown potential for normalizing neuronal activity and attenuating signs of neuropathic pain in animal models and clinical studies. This article discusses the role of these factors in neuropathic pain and the implications for the development of novel therapeutics.

Engagement of descending inhibition from the rostral ventromedial medulla protects against chronic neuropathic pain

A puzzling observation is why peripheral nerve injury results in chronic pain in some, but not all, patients. We explored potential mechanisms that may prevent the expression of chronic pain. Sprague Dawley (SD) or Holtzman (HZ) rats showed no differences in baseline sensory thresholds or responses to inflammatory stimuli. However, spinal nerve ligation (SNL)-induced tactile allodynia occurred in approximately 85% of SD and 50% of HZ rats, respectively. No apparent differences were observed in a survey of dorsal root ganglion or spinal neuropathic markers after SNL regardless of allodynic phenotype. SNL-induced allodynia was reversed by administration of lidocaine within the rostral ventromedial medulla (RVM), a site that integrates descending pain modulation via pain inhibitory (ie, OFF) and excitatory (ie, ON) cells. However, in SD or HZ rats with SNL but without allodynia, RVM lidocaine precipitated allodynia. Additionally, RVM lidocaine produced conditioned place preference in allodynic SD or HZ rats but conditioned place aversion in nonallodynic HZ rats. Similarly, RVM U69,593 (kappa opioid agonist) or blockade of spinal α(2) adrenergic receptors precipitated allodynia in previously nonallodynic HZ rats with SNL. All rats showed an equivalent first-phase formalin responses. However, HZ rats had reduced second-phase formalin behaviors along with fewer RVM OFF cell pauses and RVM ON cell bursts. Thus, expression of nerve injury-induced pain may ultimately depend on descending modulation. Engagement of descending inhibition protects in the transition from acute to chronic pain. These unexpected findings might provide a mechanistic explanation for medications that engage descending inhibition or mimic its consequences.

Ambroxol: a multifaceted molecule with additional therapeutic potentials in respiratory disorders of childhood

INTRODUCTION

Mucoactive drugs are currently used to cleanse the respiratory tract following disturbance of the normal mucociliary clearance due to mucous hyperproduction and/or modification of its physicochemical characteristics. However, in addition to possessing the ability to perform specific actions on airway secretion, these compounds have the capability to modulate the mechanisms involved in abnormal secretions. Indeed, over the years, in the postmarketing phase, a large number of studies have been published showing interesting pharmacological activities in addition to their secretagogue activity.

AREAS COVERED

This article collates available data on ambroxol (2-amino-3,5-dibromo-N-[trans-4-hydroxycyclohexyl]benzylamine), a metabolite of bromhexine, used as a secretagogue in the treatment of childhood airway diseases. This article goes beyond the mucoactive aspects of the drug covering its multiple pharmacological properties.

EXPERT OPINION

The non-mucoactive functions exhibited by the compound may provide beneficial effects on airway structure and function in health and disease. Beyond the mucokinetic and secretagogue effects, ambroxol showed great antioxidant, anti-inflammatory, local anesthetic and surfactant synthesis stimulatory activities. Moreover, some antiviral and antibacterial activities were shown. These findings may better explain the clinical results observed in a variety of airway disorders and suggest additional therapeutic potential. Further studies are needed to better define the clinical relevance of these non-mucolytic activities.

The multifunctional and multi-system influence of Ghrelin in the treatment of diabetic and spinal cord injury induced Neuropathy

Ghrelin (Gh), a small peptide, which was originally discovered as a gastrointestinal (GI) tropic hormone, has shown to have a presence and function within multiple tissue systems. Recently, Gh has shown to exhibit anti-inflammatory and regenerative abilities in response to both chemical and mechanical stressors within neural tissues. By continuing to elucidate the potential applications of Gh on pathological neural states, the viability of this peptide hormone for therapeutic uses can be explored for future clinical application.

Novel peptide ligands with dual acting pharmacophores designed for the pathophysiology of neuropathic pain

The conventional design of high affinity drugs targeted to a single molecule has not resulted in clinically useful therapies for pain relief. Recent reviews have suggested that newly designed analgesic drugs should incorporate multiple targets. The distributions of cholecystokinin (CCK) and CCK receptors in the central nervous system (CNS) overlap significantly with endogenous opioid systems and can be dually targeted. CCK has been shown to act as an endogenous "anti-analgesic" peptide and neuropathic pain conditions promote endogenous CCK release in CNS regions of pain modulation. Administration of CCK into nuclei of the rostral ventromedial medulla induces pronociceptive behaviors in rats. RSA 504 and RSA 601 are novel bifunctional compounds developed to target neuropathic pain by simultaneously acting as agonists at two distinct opioid receptors and antagonizing CCK receptors in the CNS. RSA 504 and RSA 601 demonstrate agonist activity in vitro and antihypersensitivity to mechanical and thermal stimuli in vivo using the spinal nerve ligation model of neuropathic pain. Intrathecal administration of RSA 504 and RSA 601 did not demonstrate antinociceptive tolerance over 7 days of administration and did not display motor impairment or sedation using a rotarod. These are the first behavioral studies that demonstrate how multi-targeted molecule design can address the pathology of neuropathic pain. These compounds with δ and μ opioid agonist activity and CCK antagonist activity within one molecule offer a novel approach with efficacy for neuropathic pain while lacking the side effects typically caused by conventional opioid therapies.

Functional role of cannabinoid receptors in urinary bladder

Cannabinoids, the active components of Cannabis sativa (maijuana), and their derivatives produce a wide spectrum of central and peripheral effects, some of which may have clinical applications. The discovery of specific cannabinoid receptors and a family of endogenous ligands of those receptors has attracted much attention to the general cannabinoid pharmacology. In recent years, studies on the functional role of cannabinoid receptors in bladder have been motivated by the therapeutic effects of cannabinoids on voiding dysfunction in multiple sclerosis patients. In this review, we shall summarize the literature on the expression of cannabinoid receptors in urinary bladder and the peripheral influence of locally and systemically administered cannabinoids in the bladder. The ongoing search for cannabinoid-based therapeutic strategies devoid of psychotropic effects can be complemented with local delivery into bladder by the intravesical route. A greater understanding of the role of the peripheral CB₁ and CB₂ receptor system in lower urinary tract is necessary to allow the development of new treatment for pelvic disorders.

Therapeutic potential of matrix metalloprotease inhibitors in neuropathic pain

IMPORTANCE OF THE FIELD

Millions of people suffer from neuropathic pain (NP), but the treatment is empirical and results in transient relief in only a few patients. This is primarily because of the poor understanding of the molecular mechanism underlying NP. Following nerve injury, there is a differential and temporal pattern of MMPs expression that coincides with changes in levels of pro-inflammatory cytokines, suggesting that MMPs not only act as mediators for neuroinflammation but might also be directly involved in pain associated with nerve damage.

AREAS COVERED IN THIS REVIEW

The present review describes the different mechanisms of NP. The main focus of the review is to highlight the importance of MMPs in NP and their inhibition as a novel approach for treating NP.

WHAT THE READER WILL GAIN

A comprehensive overview of the role of MMPs in the pathogenesis of NP and the potential of MMP inhibition as a therapeutic intervention for NP.

TAKE HOME MESSAGE

Targeted therapy using specific MMP inhibitors, siRNAs, peptide inhibitors and monoclonal antibodies can provide a better way of treatment by blocking a single MMP and can reduce the side effects of broad-spectrum MMP inhibitors.

Rat bone marrow stromal cells and oligonucleotides in pain research

In the last years, significant progress has been made in the medical treatment of pain. However, pathological pains, such us neuropathic pain, remain refractory to the currently available analgesics. Therefore, new therapeutic strategies are being evaluated. We have recently shown that both bone marrow stromal cells (MSCs) and the oligonucleotide IMT504 can prevent the development of mechanical and thermal allodynia when they are administered to rats subjected to a sciatic nerve crush. This chapter summarizes the laboratory techniques used to isolate and culture MSCs, administer both MSCs and IMT504, perform the nerve injury and determine mechanical and thermal sensitivities.

Desvenlafaxine succinate: a newer antidepressant for the treatment of depression and somatic symptoms

Desvenlafaxine succinate (DVS) is one of several serotonin-norepinephrine reuptake inhibitors (SNRIs). Others are venlafaxine hydrochloride, milnacipran, and duloxetine. Desvenlafaxine has been approved by the US Food and Drug Administration (FDA) for the treatment of major depressive disorder (MDD) based on a number of randomized, placebo-controlled clinical trials. Clinical studies have investigated the efficacy of DVS in doses ranging from 50 to 400 mg/day for the treatment of MDD in adult outpatients. The effects of DVS 50 mg/day have been clearly distinguished from placebo in the reduction of MDD symptoms in such clinical trials. No additional therapeutic benefits were found at doses > 50 mg/day. The recommended dose of DVS ranges from 50 to 100 mg. Desvenlafaxine is currently the third SNRI approved by the FDA for this indication. Preliminary evidence also suggests the clinical usefulness of DVS in the treatment of vasomotor symptoms of menopause, anxiety symptoms, and painful physical symptoms. The modified pharmacokinetic and pharmacodynamic profiles of DVS differentiate this drug from the original product, venlafaxine. Significant points of difference, compared with venlafaxine, are once-daily dosing and the achievement of steady-state plasma concentrations within 4 to 5 days. To summarize, current evidence indicates that DVS has proven efficacy, acceptable safety and tolerability profiles, convenient dosing, and minimal impact on the cytochrome P450 enzyme system. A reduced risk for pharmacokinetic drug interactions is a potential advantage over other selective serotonin noradrenaline reuptake inhibitors. Desvenlafaxine succinate has demonstrated its efficacy for treating MDD but its variable efficacy, as shown in individual studies, limited long-term data, and its different risk-to-benefit ratio compared with earlier antidepressants, means that further investigation of this drug is necessary.

Block of sensory neuronal Na+ channels by the secreolytic ambroxol is associated with an interaction with local anesthetic binding sites

Voltage-gated Na(+) channels (Na(v)) regulate the excitability of sensory neurons and are potential targets for novel analgesics. The secreolytic ambroxol reduces pain-related behavior in rodents and alleviates pain in humans. With properties resembling those of local anesthetics, ambroxol has been reported to block Na(+) currents in sensory neurons with a preference for tetrodotoxin-resistant (TTXr) Na(+) currents encoded by Na(v)1.8. However, the molecular determinants for ambroxol-induced block of Na(+) channels and a preferential block of Na(v)1.8 opposed to tetrodotoxin-sensitive (TTXs) Na(v) alpha-subunits have not been studied in detail. By means of whole-cell voltage clamp recordings, we studied the effects of ambroxol and local anesthetics on the recombinant TTXr subunit Na(v)1.8, on TTXs Na(v) alpha-subunits and on mutants of Na(v)1.4 that are insensitive to local anesthetics. Tonic and use-dependent block by ambroxol was strongly alleviated in local anesthetic-insensitive Na(v)1.4 mutants. Use-dependent block, but not tonic block was significantly stronger on Na(v)1.8 than on TTXs channels. The TTXs subunit Na(v)1.3 displayed the least degree of use-dependent block by ambroxol. The local anesthetics mepivacaine and S(-)-bupivacaine also blocked Na(v)1.8 and TTXs channels differentially. While mepivacaine displayed a preferential use-dependent block of Na(v)1.8, S(-)-bupivacaine displayed a preference for TTXs Na(+) channels. Our data show that ambroxol acts as a typical local anesthetic on Na(+) channels interacting with specific residues in the S6 segments. This property probably meditates the analgesic effect of ambroxol. Ambroxol preferentially blocks Na(v)1.8, however shares this property with established local anesthetics like mepivacaine.

Capnellene, a natural marine compound derived from soft coral, attenuates chronic constriction injury-induced neuropathic pain in rats

BACKGROUND AND PURPOSE

Natural compounds obtained from marine organisms have received considerable attention as potential sources of novel drugs for treatment of human inflammatory diseases. Capnellene, isolated from the marine soft coral Capnella imbricate, 4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalene-2,3a-diol (GB9) exhibited anti-inflammatory actions on activated macrophages in vitro. Here we have assessed the anti-neuroinflammatory properties of GB9 and its acetylated derivative, acetic acid 3a-hydroxy-4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalen-2-yl ester (GB10).

EXPERIMENTAL APPROACH

Effects of GB9 or GB10 on the expression of inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) in interferon-gamma (IFN-gamma)-stimulated mouse microglial BV2 cells were measured by Western blot. The in vivo effects of these compounds were examined in the chronic constriction injury (CCI) rat model of neuropathic pain, measuring thermal hyperalgesia, and microglial activation and COX-2 protein in lumbar spinal cord, by immunohistochemistry.

KEY RESULTS

In BV2 cells, GB9 and GB10 inhibited the expression of iNOS and COX-2, stimulated by IFN-gamma. Intrathecal administration of GB9 and GB10 inhibited CCI-induced nociceptive sensitization and thermal hyperalgesia in a dose-dependent manner. Intraperitoneal injection of GB9 inhibited CCI-induced thermal hyperalgesia and also inhibited CCI-induced activation of microglial cells and up-regulation of COX-2 in the dorsal horn of the lumbar spinal cord ipsilateral to the injury.

CONCLUSION AND IMPLICATIONS

Taken together, these data indicate that the marine-derived capnellenes, GB9 and GB10, had anti-neuroinflammatory and anti-nociceptive properties in IFN-gamma-stimulated microglial cells and in neuropathic rats respectively. Therefore, capnellene may serve as a useful lead compound in the search for new therapeutic agents for treatment of neuroinflammatory diseases.

Intrathecal ziconotide for neuropathic pain: a review

Neuropathic pain is a considerable burden that affects activities of daily living. The management of neuropathic pain can be challenging because of multiple etiologies and complex manifestations. Ziconotide is a nonopioid intrathecal (IT) analgesic option for patients with neuropathic pain refractory to conventional treatments. The objective of this article is to review the published literature on ziconotide for the treatment of neuropathic pain. Relevant publications were identified through searches of all years of 6 databases, which included PubMed, EMBASE, and CINAHL. Search terms used were ziconotide, SNX-111, MVIIA, Prialt, and neuropathic pain. Publications were included if ziconotide was intrathecally administered (either alone or in combination with other IT agents) to treat neuropathic pain of any etiology and if pain assessment was an outcome measure. Data extracted included study design, IT drug doses, pain outcome measures, and adverse events (AEs). Twenty-eight articles met the inclusion criteria: 5 were preclinical studies and 23 were clinical studies. In the preclinical studies, ziconotide demonstrated antiallodynic effects on neuropathic pain. Data from double-blind, placebo-controlled (DBPC) trials indicated that patients with neuropathic pain reported a mean percent improvement in pain score with ziconotide monotherapy that ranged from 15.7% to 31.6%. A low starting dose and slow titration of ziconotide resulted in an improved safety profile in the aforementioned trials. Common AEs associated with ziconotide include nausea and/or vomiting, dizziness, confusion, urinary retention, and somnolence. Evidence from DBPC trials, open-label studies, case series, and case studies suggests that ziconotide, as either monotherapy or in combination with other IT drugs, is a potential therapeutic option for patients with refractory neuropathic pain. Additional studies are needed to establish the long-term efficacy and safety of ziconotide for neuropathic pain.

Fibromyalgia: a complex syndrome requiring a multidisciplinary approach

Fibromyalgia is a pain syndrome which is not due to tissue damage or inflammation and is thus fundamentally different from rheumatic disorders and many other pain conditions. In addition to widespread pain it is associated with a range of other symptoms such as sleep disturbance, fatigue, cognitive disturbance, stiffness and depressive symptoms. A number of multidisciplinary therapeutic programmes involving education, exercise and cognitive therapy have been shown to be effective in bringing relief. The various medications that are currently being developed for the treatment of fibromyalgia are based on different mechanistic approaches. In particular, serotonin noradrenaline reuptake inhibitors (SNRI) such as duloxetine and milnacipran and alpha2-delta receptor ligands such as pregabalin have been shown, in a variety of placebo-controlled studies, to bring significant relief from pain and other symptoms. The complex symptomatology of fibromyalgia will, however, continue to require a multidisciplinary approach including education and exercise in addition to drug therapy to achieve the most efficient management of fibromyalgia.

Structure-activity relationships of bifunctional cyclic disulfide peptides based on overlapping pharmacophores at opioid and cholecystokinin receptors

Prolonged opioid exposure increases the expression of cholecystokinin (CCK) and its receptors in the central nervous system (CNS), where CCK may attenuate the antinociceptive effects of opioids. The complex interactions between opioid and CCK may play a role in the development of opioid tolerance. We designed and synthesized cyclic disulfide peptides and determined their agonist properties at opioid receptors and antagonist properties at CCK receptors. Compound 1 (Tyr-c[d-Cys-Gly-Trp-Cys]-Asp-Phe-NH(2)) showed potent binding and agonist activities at delta and mu opioid receptors but weak binding to CCK receptors. The NMR structure of the lead compound displayed similar conformational features of opioid and CCK ligands.

Oxycodone plus ultra-low-dose naltrexone attenuates neuropathic pain and associated mu-opioid receptor-Gs coupling

Both peripheral nerve injury and chronic opioid treatment can result in hyperalgesia associated with enhanced excitatory neurotransmission at the level of the spinal cord. Chronic opioid administration leads to a shift in mu-opioid receptor (MOR)-G protein coupling from G(i/o) to G(s) that can be prevented by cotreatment with an ultra-low-dose opioid antagonist. In this study, using lumbar spinal cord tissue from rats with L(5)/L(6) spinal nerve ligation (SNL), we demonstrated that SNL injury induces MOR linkage to G(s) in the damaged (ipsilateral) spinal dorsal horn. This MOR-G(s) coupling occurred without changing G(i/o) coupling levels and without changing the expression of MOR or Galpha proteins. Repeated administration of oxycodone alone or in combination with ultra-low-dose naltrexone (NTX) was assessed on the SNL-induced MOR-G(s) coupling as well as on neuropathic pain behavior. Repeated spinal oxycodone exacerbated the SNL-induced MOR-G(s) coupling, whereas ultra-low-dose NTX cotreatment slightly but significantly attenuated this G(s) coupling. Either spinal or oral administration of oxycodone plus ultra-low-dose NTX markedly enhanced the reductions in allodynia and thermal hyperalgesia produced by oxycodone alone and minimized tolerance to these effects. The MOR-G(s) coupling observed in response to SNL may in part contribute to the excitatory neurotransmission in spinal dorsal horn in neuropathic pain states. The antihyperalgesic and antiallodynic effects of oxycodone plus ultra-low-dose NTX (Oxytrex, Pain Therapeutics, Inc., San Mateo, CA) suggest a promising new treatment for neuropathic pain.

PERSPECTIVE

The current study investigates whether Oxytrex (oxycodone with an ultra-low dose of naltrexone) alleviates mechanical and thermal hypersensitivities in an animal model of neuropathic pain over a period of 7 days, given locally or systemically. In this report, we first describe an injury-induced shift in mu-opioid receptor coupling from G(i/o) to G(s), suggesting why a mu-opioid agonist may have reduced efficacy in the nerve-injured state. These data present a novel approach to neuropathic pain therapy.

Roles of axonal voltage-dependent ion channels in damaged peripheral nerves

Ion channels are fundamental to the correct functioning of nerves. A high density of Na(v) channels at nodes of Ranvier is crucial for generating sufficient local current to initiate a self-regenerating action potential. K(v)7 (KCNQ) channels are also functional components of nodes of Ranvier and contribute to the repolarization of the action potential. Paranodes underlying the paranodal loops have no channels whereas K(v)1.1 and K(v)1.2 channels cluster at juxtaparanodes. The disruption of the normal balance of different channels causes the inappropriate nerve activity that underlies a variety of nervous system diseases. Axonal voltage-dependent channels, nodal Na(v) and juxtaparanodal K(v) channels disperse after the nerve injury and reorganize with regeneration.

Ibudilast (AV-411)

The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.

Gene therapy applications for the treatment of neuropathic pain

Neuropathic pain is notoriously difficult to treat; currently available pharmaceutical drugs result in moderate analgesia in approximately a third of patients. As our understanding of the biological processes involved in the establishment and maintenance of neuropathic pain increases, so does the development of novel treatment options. Significant advancements have been made in the past few years in gene transfer, a very powerful potential therapy that can be used to directly target affected areas of the neuraxis or body tissues involved in neuropathic pain. Candidate gene products include directly analgesic proteins as well as proteins that interfere with pain-associated biochemical changes in nerve or other tissues underlying the disease process.

Antihyperalgesic effects of loperamide in a model of rat neuropathic pain are mediated by peripheral delta-opioid receptors

The possible antihyperalgesic and antiallodynic activity of loperamide, an opioid agonist which does not readily penetrate the blood-brain barrier, were examined in the spinal nerve ligation model of experimental neuropathic pain. Intraperitoneal (i.p.) injection of loperamide effectively reversed thermal hyperalgesia. In contrast, loperamide had minimal effects on cold allodynia and no effects on mechanical allodynia. The antihyperalgesic action of loperamide against noxious heat was antagonized by naltrindole, a delta-opioid receptor selective antagonist, but not by pretreatment with beta-funaltrexamine, a mu-opioid receptor selective antagonist, or administration of nor-binaltorphimine, a kappa-opioid receptor selective antagonist. Furthermore, i.p. injection of [d-Ala(2), Glu(4)]-deltorphin II, a delta-opioid receptor selective peptide agonist, also reversed thermal hyperalgesia. The present results suggest that thermal hyperalgesia in experimental neuropathic pain can be reduced through activation of peripheral delta-opioid receptors. The data suggest the possible application of peripherally restricted and delta-opioid receptor selective agonists in the treatment of some aspects of neuropathic pain without many of the side effects associated with centrally acting opioids and without the peripheral side effects of opioid agonists acting at mu-receptors.

Anti-allodynic efficacy of botulinum neurotoxin A in a model of neuropathic pain

Neuropathic pain is typified by injuries to the peripheral and central nervous system and derives from such causes as cancer, diabetes, multiple sclerosis, post-herpetic neuralgia, physical trauma or surgery, and many others. Patients suffering neuropathic pain do not respond to conventional treatment with non-steroidal anti-inflammatory drugs and show a reduced sensitivity to opiates often associated with serious side effects. Recently, it has been demonstrated that botulinum neurotoxin serotype-A (BoNT/A) is able to induce analgesia in inflammatory pain conditions. The goal of this research was to test if BoNT/A was able to relieve also neuropathic pain symptoms. By using chronic constriction injury of the sciatic nerve, a mouse model of neuropathic pain, we observed that peripheral administration of BoNT/A strongly reduced the mechanical allodynia associated with this neuropathy. Remarkably, a single non-toxic dose of BoNT/A was sufficient to induce anti-allodynic effects, which lasted for at least 3 weeks. This result is particularly relevant since neuropathic pain is poorly treated by current drug therapies. This communication enlarges our knowledge on potentially new medical uses of BoNT/A in efforts to ameliorate human health conditions, with very important implications in the development of new pharmacotherapeutic approaches against neuropathic pain.

New paradigms and tools in drug design for pain and addiction

New modalities providing safe and effective treatment of pain, especially prolonged pathological pain, have not appeared despite much effort. In this mini-review/overview we suggest that new paradigms of drug design are required to counter the underlying changes that occur in the nervous system that may elicit chronic pain states. We illustrate this approach with the example of designing, in a single ligand, molecules that have agonist activity at mu and delta opioid receptors and antagonist activities at cholecystokinin (CCK) receptors. Our findings thus far provide evidence in support of this new approach to drug design. We also report on a new biophysical method, plasmon waveguide resonance (PWR) spectroscopy, which can provide new insights into information transduction in G-protein coupled receptors (GPCRs) as illustrated by the delta opioid receptor.

Drug therapy for chronic idiopathic axonal polyneuropathy

BACKGROUND

Chronic idiopathic axonal polyneuropathy is an insidiously progressive sensory or sensorimotor polyneuropathy that affects elderly people. Although severe disability or handicap does not occur, it reduces quality of life.

OBJECTIVES

To assess whether drug therapy for chronic idiopathic axonal polyneuropathy reduces disability, ameliorates neurological symptoms and associated impairments, and whether treatment is safe.

SEARCH STRATEGY

We searched Cochrane Library (Cochrane Neuromuscular Disease Review Group Register, Cochrane Database of Systematic Reviews, Cochrane Database of Abstracts of Reviews of Effectiveness, and the Cochrane Central Register of Controlled Trials), MEDLINE, EMBASE, ISI, and ACP Journal Club's Best Evidence, from 1981 until December 2002. We also hand searched the reference lists of relevant articles, reviews and textbooks identified electronically, and contacted authors and other experts in the field to identify additional studies.

SELECTION CRITERIA

We sought all randomised or quasi-randomised (alternate or other systematic treatment allocation), unconfounded trials that examined the effects of any drug therapy in patients with chronic idiopathic axonal polyneuropathy at least one year after the onset of treatment. Patients with chronic idiopathic axonal polyneuropathy had to fulfil the following criteria: age 40 years or older, distal sensory or sensorimotor polyneuropathy, absence of systemic or other neurological disease, chronic clinical course not reaching a nadir in less than two months, exclusion of any recognised cause of the polyneuropathy by medical history taking, clinical or laboratory investigations, electrophysiological studies in agreement with axonal polyneuropathy without evidence of demyelinating features. The primary outcome was the proportion of patients with a significant improvement in disability. Secondary outcomes were change in the mean disability score, change in the proportion of patients who make use of walking aids, change in the mean Medical Research Council sum score, degree of pain relief and/or reduction of other positive sensory symptoms, change in the proportion of patients with pain or other positive sensory symptoms, and frequency of adverse effects.

DATA COLLECTION AND ANALYSIS

Two reviewers independently reviewed and extracted details of trial methodology and outcome data of all potentially relevant trials.

MAIN RESULTS

Eighteen studies were identified and assessed for possible inclusion in the review, but all were excluded because of insufficient quality or lack of relevance.

REVIEWERS' CONCLUSIONS

Even though chronic idiopathic axonal polyneuropathy has been clearly described and delineated, no adequate randomised or quasi-randomised controlled clinical treatment trials have been performed. In their absence there is no proven efficacious drug therapy.