Experimental Drugs for Neuropathic Pain

Guanidine Derivative ADS1017, a Potent Histamine H3 Receptor Antagonist with Promising Analgesic Activity and Satisfactory Safety Profile

In this study, we selected 12 guanidine derivatives from the previously described ligand library and determined their affinity at histamine H3 and H4 receptors (H3R and H4R, respectively). Moreover, we also checked their intrinsic activity toward H3R and muscarinic M1, M2, and M4 receptors (M1R, M2R, and M4R, respectively). Since ADS1017 has been proved to be the most selective and highly potent H3 antagonist in our series, we chose it as the lead structure for further biological evaluation. To extend the study of its in vivo efficacy, we proposed an alternative synthetic route that resulted in an increased yield. Interestingly, ADS1017 showed a broad spectrum of analgesic activity in both nociceptive and neuropathic pain models. Finally, as a result of comprehensive analysis of its off-target activity and ADMETox parameters, we confirmed the moderate selectivity of ADS1017 and its promising drug-like properties.

The Antinociceptive Role of Nrf2 in Neuropathic Pain: From Mechanisms to Clinical Perspectives

Neuropathic pain, a chronic condition resulting from nerve injury or dysfunction, presents significant therapeutic challenges and is closely associated with oxidative stress and inflammation, both of which can lead to mitochondrial dysfunction. The nuclear factor erythroid 2-related factor 2 (Nrf2) pathway, a critical cellular defense mechanism against oxidative stress, has emerged as a promising target for neuropathic pain management. Nrf2 modulators enhance the expression of antioxidant and cytoprotective genes, thereby reducing oxidative damage, inflammation, and mitochondrial impairment. This review explores the antinociceptive effects of Nrf2, highlighting how pharmacological agents and natural compounds may be used as potential therapeutic strategies against neuropathic pain. Although preclinical studies demonstrate significant pain reduction and improved nerve function through Nrf2 activation, several clinical challenges need to be addressed. However, emerging clinical evidence suggests potential benefits of Nrf2 modulators in several conditions, such as diabetic neuropathy and multiple sclerosis. Future research should focus on further elucidating the molecular role of Nrf2 in neuropathic pain to optimize its modulation efficacy and maximize clinical utility.

Novel Drug Targets and Emerging Pharmacotherapies in Neuropathic Pain

Neuropathic pain is a debilitating condition characterized by abnormal signaling within the nervous system, resulting in persistent and often intense sensations of pain. It can arise from various causes, including traumatic nerve injury, neuropathy, and certain diseases. We present an overview of current and emerging pharmacotherapies for neuropathic pain, focusing on novel drug targets and potential therapeutic agents. Current pharmacotherapies, including tricyclic antidepressants, gabapentinoids, and serotonin norepinephrine re-uptake inhibitors, are discussed, as are emerging treatments, such as ambroxol, cannabidiol, and N-acetyl-L-cysteine. Additionally, the article highlights the need for further research in this field to identify new targets and develop more effective and targeted therapies for neuropathic pain management.

Ambroxol for neuropathic pain: hiding in plain sight?

ABSTRACT

Ambroxol is a multifaceted drug with primarily mucoactive and secretolytic actions, along with anti-inflammatory, antioxidant, and local anaesthetic properties. It has a long history of use in the treatment of respiratory tract diseases and has shown to be efficacious in relieving sore throat. In more recent years, ambroxol has gained interest for its potential usefulness in treating neuropathic pain. Research into this area has been slow, despite clear preclinical evidence to support its primary analgesic mechanism of action-blockade of voltage-gated sodium (Na v ) channels in sensory neurons. Ambroxol is a commercially available inhibitor of Na v 1.8, a crucial player in the pathophysiology of neuropathic pain, and Na v 1.7, a particularly exciting target for the treatment of chronic pain. In this review, we discuss the analgesic mechanisms of action of ambroxol, as well as proposed synergistic properties, followed by the preclinical and clinical results of its use in the treatment of persistent pain and neuropathic pain symptoms, including trigeminal neuralgia, fibromyalgia, and complex regional pain syndrome. With its well-established safety profile, extensive preclinical and clinical drug data, and early evidence of clinical effectiveness, ambroxol is an old drug worthy of further investigation for repurposing. As a patent-expired drug, a push is needed to progress the drug to clinical trials for neuropathic pain. We encourage the pharmaceutical industry to look at patented drug formulations and take an active role in bringing an optimized version for neuropathic pain to market.

Regenerative medicine for neuropathic pain: physiology, ultrasound and therapies with a focus on alpha-2-macroglobulin

The currently available drugs to treat neuropathic pain do not provide adequate pain management. As such, other treatments including stem cells, platelet-rich plasma and plasma-derived molecules such as alpha-2 macroglobulin (A2M) are being explored because they show promising potential for neuropathic pain. The various mechanisms and immunomodulatory effects could be a desirable approach in targeting neuropathic pain. This review indicates that A2M can be highly efficacious due to its conformational change during activation and specificity of action on various cytokines. Its ability to reduce neuropathic pain can further the future of neuropathic intervention. However, there is a lack of robust clinical studies and thus further research is needed to verify and expand the understanding of its therapeutic effects.

Herpes zoster: A Review of Clinical Manifestations and Management

The Varicella-zoster virus (VZV) or human herpes virus 3 is a neurotropic human alpha herpes virus responsible for chickenpox/varicella and shingles/Herpes zoster (HZ). This review will focus on HZ. Since HZ is secondary to varicella, its incidence increases with age. In children and youngsters, HZ is rare and associated to metabolic and neoplastic disorders. In adults, advanced age, distress, other infections (such as AIDS or COVID-19), and immunosuppression are the most common risk factors. HZ reactivation has recently been observed after COVID-19 vaccination. The disease shows different clinical stages of variable clinical manifestations. Some of the manifestations bear a higher risk of complications. Among the possible complications, postherpetic neuralgia, a chronic pain disease, is one of the most frequent. HZ vasculitis is associated with morbidity and mortality. Renal and gastrointestinal complications have been reported. The cornerstone of treatment is early intervention with acyclovir or brivudine. Second-line treatments are available. Pain management is essential. For (secondary) prophylaxis, currently two HZV vaccines are available for healthy older adults, a live attenuated VZV vaccine and a recombinant adjuvanted VZV glycoprotein E subunit vaccine. The latter allows vaccination also in severely immunosuppressed patients. This review focuses on manifestations of HZ and its management. Although several articles have been published on HZ, the literature continues to evolve, especially in regard to patients with comorbidities and immunocompromised patients. VZV reactivation has also emerged as an important point of discussion during the COVID-19 pandemic, especially after vaccination. The objective of this review is to discuss current updates related to clinical presentations, complications, and management of HZ.

SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner

The serum and glucocorticoid-regulated kinase 1 (SGK1) is a widely expressed protein in the Central Nervous System (CNS), involved in regulating the activity of a wide variety of ion channels and transporters and physiological functions, such as neuronal excitability. SGK1.1 is a neuronal splice isoform of SGK1, expressed exclusively in the CNS, distributed in brain and cerebellum, that decreases neuronal excitability via up-regulation of M-current, linked to Kv7.2/3 potassium channels. Strategies to maintain increased SGK1.1 activity could be helpful in decreasing neuronal hyperexcitability, as occurs in neuropathic pain. Transgenic mice overexpressing SGK1.1 (B6.Tg.sgk1) offer a particularly relevant opportunity to assess the physiological involvement of this protein in nociception. Behavior and physiological nociception were evaluated in male and female B6.Tg.sgk1 and wild-type mice (B6.WT), characterizing nociceptive thresholds to different nociceptive stimuli (thermal, chemical and mechanical), as well as the electrophysiological properties of cutaneous sensory Aδ-fibres isolated from the saphenous nerve. The acute antinociceptive effect of morphine was also evaluated. Compared with B6.WT animals, male and female B6.Tg.sgk1 mice showed increased spontaneous locomotor activity. Regarding nociception, there were no differences between transgenic and wild-type mice in heat, chemical and mechanical thresholds, but interestingly, male B6.Tg.sgk1 mice were less sensitive to cold stimulus; B6.Tg.sgk1 animals showed lower sensitivity to morphine. Electrophysiological properties of cutaneous primary afferent fibres were maintained. This is the first demonstration that the SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner. B6.Tg.sgk1 mice offer a particularly relevant opportunity to further analyze the involvement of this protein in nociception, and studies in models of chronic, neuropathic pain are warranted.

The Microglial Activation Inhibitor Minocycline, Used Alone and in Combination with Duloxetine, Attenuates Pain Caused by Oxaliplatin in Mice

The antitumor drug, oxaliplatin, induces neuropathic pain, which is resistant to available analgesics, and novel mechanism-based therapies are being evaluated for this debilitating condition. Since activated microglia, impaired serotonergic and noradrenergic neurotransmission and overexpressed sodium channels are implicated in oxaliplatin-induced pain, this in vivo study assessed the effect of minocycline, a microglial activation inhibitor used alone or in combination with ambroxol, a sodium channel blocker, or duloxetine, a serotonin and noradrenaline reuptake inhibitor, on oxaliplatin-induced tactile allodynia and cold hyperalgesia. To induce neuropathic pain, a single dose (10 mg/kg) of intraperitoneal oxaliplatin was used. The mechanical and cold pain thresholds were assessed using mouse von Frey and cold plate tests, respectively. On the day of oxaliplatin administration, only duloxetine (30 mg/kg) and minocycline (100 mg/kg) used alone attenuated both tactile allodynia and cold hyperalgesia 1 h and 6 h after administration. Minocycline (50 mg/kg), duloxetine (10 mg/kg) and combined minocycline + duloxetine influenced only tactile allodynia. Seven days after oxaliplatin, tactile allodynia (but not cold hyperalgesia) was attenuated by minocycline (100 mg/kg), duloxetine (30 mg/kg) and combined minocycline and duloxetine. These results indicate a potential usefulness of minocycline used alone or combination with duloxetine in the treatment of oxaliplatin-induced pain.

T Cells as Guardians of Pain Resolution

Despite successful research efforts aimed at understanding pain mechanisms, there is still no adequate treatment for many patients suffering from chronic pain. The contribution of neuroinflammation to chronic pain is widely acknowledged. Here, we summarize findings indicating that T cells play a key role in the suppression of pain. An active contribution of the immune system to resolution of pain may explain why immunosuppressive drugs are often not sufficient to control pain. This would also imply that dysregulation of certain immune functions promote transition to chronic pain. Conversely, stimulating the endogenous immune-mediated resolution pathways may provide a potent approach to treat chronic pain.

Combination of Botulinum Toxin and minocycline Ameliorates Neuropathic Pain Through Antioxidant Stress and Anti-Inflammation via Promoting SIRT1 Pathway

Neuropathic pain (NP) is one of the intractable complications of spinal cord injury (SCI), with poor prognosis and seriously affects the quality of life of patients. This study aims to determine the treatment effect and mechanism of multimodal therapies in a rat model of SCI-induced NP by combining treatment with the anti-inflammatory agent minocycline (MC) and botulinum toxin (BoNT). The combined utilization alleviated SCI-induced NP and reduced apoptosis, inflammation, and oxidative stress of SCI by activating SIRT1 and dampening pAKT, P53, and p-NF-KB. BoNT with a concentration of 0.1 nm and MC with a concentration of 20 uM were selected for the experiment in the primary microglia and astrocytes treated with LPS. It was found that the combination of BoNT and MC obviously inhibits the inflammatory response and oxidative stress of glial cells, and notably activates SIRT1 and restrains pAKT, P53, and p-NF-KB. Therefore, in the treatment of SCI-induced NP, the combination of BoNT and MC markedly improves the therapeutic effect of NP by promoting the SIRT1 expression, thereby inactivating NF-KB, P53, and PI3K/AKT signaling pathway, inhibiting inflammation and oxidative stress as well as relieving SCI-induced NP.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Synthesis, anticonvulsant, and antinociceptive activity of new 3-(3-methyl-2,5-dioxo-3-phenylpyrrolidin-1-yl)propanamides and 3-phenyl-butanamides

A focused library of new 3-(3-methyl-2,5-dioxo-3-phenylpyrrolidin-1-yl)propanamides and their nonimide analogs were synthesized and tested for anticonvulsant activity. These compounds were obtained through the coupling reaction of the starting carboxylic acids with appropriate amines. The initial anticonvulsant screening was performed in mice (intraperitoneal administration) using the maximal electroshock seizure (MES) and the subcutaneous pentylenetetrazole (scPTZ) seizure models. The most promising compound 6 showed more potent protection in the MES and scPTZ tests than valproic acid, which is still recognized as one of the most relevant first-line anticonvulsants. The structure-activity relationship analysis revealed that the presence of the pyrrolidine-2,5-dione ring is important but not indispensable to retain anticonvulsant activity. Additionally, compound 6 showed potent antinociceptive properties in the oxaliplatin-induced neuropathic pain model in mice. The most plausible mechanism of action for compound 6 may result from its influence on the neuronal sodium channel (Site 2) and the high-voltage-activated L-type calcium channel.

Botulinum Neurotoxin for the Treatment of Neuropathic Pain

Botulinum neurotoxin is widely used for the treatment of central and peripherical neurological conditions. Initially used to treat strabismus, over the years its use has been expanded also to spasticity and other neurological disorders. This review summarizes the evidence from the published literature regarding its effect on neuropathic pain. Almost all investigations were performed using onabotulinum toxin type A (BoNT/A). Most studies provided positive results, even though toxin formulation, dose, dilution, injection techniques, and sites are heterogeneous across studies. Future larger, high-quality, specifically designed clinical trials are warranted to confirm botulinum neurotoxin efficacy in neuropathic pain.

Mechanisms Involved in Superiority of Angiotensin Receptor Blockade over ACE Inhibition in Attenuating Neuropathic Pain Induced in Rats

Although previous reports described the beneficial role of angiotensin-converting enzyme inhibitors (ACE-Is) or AT1 receptor blockers (ARBs) in attenuating neuropathic pain (NP), no study has yet explored the exact underlying mechanisms, as well as the superiority of using centrally versus peripherally acting renin-angiotensin-aldosterone system (RAAS) drugs in NP. We investigated the effects of 14 days of treatment with centrally (telmisartan and ramipril) or peripherally (losartan and enalapril) acting ARBs and ACE-Is, respectively, in attenuating peripheral NP induced by sciatic nerve chronic constriction injury (CCI) in rats. We also compared these with the effects of pregabalin, the standard treatment for NP. Behavioral changes, inflammatory markers (NFкB, TNF-α, COX-2, PGE2, and bradykinin), oxidative stress markers (NADPH oxidase and catalase), STAT3 activation, levels of phosphorylated P38-MAPK, ACE, AT1 receptor (AT1R), and AT2 receptor (AT2R), as well as histopathological features, were assessed in the brainstem and sciatic nerve. CCI resulted in clear pain-related behavior along with increased levels of inflammatory and oxidative stress markers, and STAT3 activity, as well as increased levels of phosphorylated P38-MAPK, ACE, AT1R, and AT2R, along with worsened histopathological findings in both the brainstem and sciatic nerve. ARBs improved both animal behavior and all measured parameters in CCI rats and were more effective than ACE-Is. At the tested doses, centrally acting ARBs or ACE-Is were not superior to the peripherally acting drugs of the same category. These findings suggest that ARBs (centrally or peripherally acting) are an effective treatment modality for NP.

Statins and Neuropathic Pain: A Narrative Review

The frequently prescribed drug class of statins have pleiotropic effects and have been implicated in neuropathic pain syndromes. This narrative review examines studies of statin-induced neuropathic pain which to date have been conducted only in animal models. However, the pathophysiology of diabetic neuropathy in humans may shed some light on the etiology of neuropathic pain. Statins have exhibited a paradoxical effect in that statins appear to reduce neuropathic pain in animals but have been associated with neuropathic pain in humans. While there are certain postulated mechanisms offering elucidation as to how statins might be associated with neuropathic pain, there is, as the American Heart Association stated, to date no definitive association between statins and neuropathic pain. Statins are important drugs that reduce cardiovascular risk factors and should be prescribed to appropriate patients with these risk factors but some of this population is also at elevated risk for neuropathic pain from other causes.

New analgesic: Focus on botulinum toxin

In 2010, Kissin concluded pessimistically that of the 59 new drugs introduced in the fifty-year period between 1960 and 2009 and still in use, only seven had new molecular targets. Of these, only one, sumatriptan, was effective enough to lead to the introduction of multiple drugs targeting the same target molecules (triptans) (Kissin, 2010). Morphine and acetylsalicylic acid (aspirin), introduced for the treatment of pain more than a century ago, continue to dominate biomedical publications despite their limited effectiveness in many areas (e.g., neuropathic pain) and serious adverse effects. Today, are we really closer to ideal analgesics that would work hard enough, long enough, and did not have unwanted side effects? The purpose of the present article is to analyze where we are now. Several drugs, like long-acting opioids or botulinum toxins open some hope. Advantage of botulinum toxin A is unique duration of action (months). New discoveries showed that after peripheral application botulinum toxin by axonal transport reaches the CNS. Major analgesic mechanism of action seems to be of central origin. Will botulinum toxin in the CNS bring new indications and or/adverse effects? Much more basic and clinical research should be in front of us. Although relatively safe as a drug, botulinum toxin is not without adverse effect. Policy makers, clinicians and all those applying botulinum toxin should be aware of that. Unfortunately the life without the pain is still not possible.

Sustained relief of trigeminal neuropathic pain by a blood-brain barrier penetrable PPAR gamma agonist

The blood–brain barrier (BBB) and blood–nerve barrier ensure protection of the nervous system but pose a challenge for the treatment of pain since it restricts passage of many therapeutic drugs. Although it is unknown which blood–neural barrier is more relevant, or whether permeabilities are the same for different barriers, we proposed that the inefficiency of thiazolidinedione-type agonists for peroxisome proliferator-activated receptor gamma (PPARɣ) is due to their difficulty in passage through the BBB. We developed a new highly BBB penetrable PPARɣ agonist for the treatment of neuropathic pain, assuming BBB permeability is a rule of thumb to estimate the overall permeability of relevant blood–neural barriers.  As an index of brain penetration, the brain–plasma ratio (Kp) of ELB00824 is 5.13, suggesting very high brain bioavailability, which is 58-fold that of pioglitazone. The series of studies presented here indicate that ELB00824 may be the most potent PPARɣ agonist currently known for acute reduction of neuropathic pain in trigeminal nerve in rat and mouse models. Low-dose PPARɣ agonist, ELB00824 (10 mg/kg), effectively decreased neuropathic hypersensitivity in mice and rats at both acute and chronic time points, a dose 100-fold lower than the effective dose (1000 mg/kg, i.p.) of pioglitazone. Comparisons of ELB00824 alone or in combination with gabapentin or carbamazepine are provided. While PPARɣ agonists used to treat Type 2 diabetes produce several adverse side effects, sub-chronic oral toxicity study provided promising results that ELB00824 does not produce any significant short-term toxicity. The study animals of either sex remained alive and healthy with no significant alteration of body weight long term. Toxicity study results obtained were satisfactory, with no significant alterations in any serum biochemistry parameters.

The antihyperalgesic effect of docosahexaenoic acid in streptozotocin-induced neuropathic pain in the rat involves the opioidergic system

Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid that has shown an antinociceptive effect in multiple pain models, such as inflammatory and neuropathic pain by chronic constriction injury in rats; however, its mechanism of action is still not well-understood. Reports suggest that DHA activates opioid signaling, but there is no information on this from a model of neuropathic pain. As a result, the aims of this study were (1) to determine the antihyperalgesic and antiallodynic effect of peripheral DHA administration, and (2) to evaluate the participation of the opioid receptors in the antihyperalgesic effect of DHA on streptozotocin-induced neuropathic pain in the rat. Female Wistar rats were injected with streptozotocin (50 mg/kg, i.p.) to induce hyperglycemia. The formalin, Hargreaves, and von Frey filaments tests were used to assess the nociceptive activity. Intraplantar administration of DHA (100-1000 μg/paw) or gabapentin (562-1778 μg/paw) decreased formalin-evoked hyperalgesia in diabetic rats, in a dose-dependent manner. Furthermore, DHA (562 μg/paw) and gabapentin (1000 μg/paw) reduced thermal hyperalgesia and allodynia. Local peripheral administration of naloxone (non-selective opioid receptor antagonist; 100 μg/paw), naltrindole (selective δ receptor antagonist; 1 μg/paw), and CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, μ receptor antagonist; 20 μg/paw) prevented formalin-evoked hyperalgesia in diabetic rats but not by GNTI (guanidinonaltrindole, κ receptor antagonist;1 µg/paw). It is suggested that peripheral DHA shows an antihyperalgesic effect in neuropathic pain in the rat. Furthermore, δ and μ receptors are involved in the antihyperalgesic peripheral effect of DHA in diabetic rats.