Topical amitriptyline and ketamine for post-herpetic neuralgia and other forms of neuropathic pain

Amitriptyline protects afferent synapses in the cochlea against excitotoxic trauma in vitro

Afferent synapses between inner hair cells (IHCs) and the type I spiral ganglion neurons (SGNs) in the cochlea provide over 95% of sensory signals for auditory perception in the brain. However, these afferent synapses are particularly vulnerable to damage, for example from excitotoxicity, and exposure to noise in the environment which often leads to noise-induced cochlear synaptopathy (NICS). In this study, we simulated excitotoxic trauma by incubating kainic acid, a non-desensitizing agonist for AMPA type glutamate receptors on cultured cochleae. The possible protective effects of amitriptyline against NICS were examined. We found that, in IHCs, amitriptyline reversed the decrease of Ca2+ current and exocytosis caused by excitotoxic trauma. In SGNs, amitriptyline promoted the recovery of neurite loss caused by excitotoxic trauma. Furthermore, we found that the protective effects of amitriptyline are likely mediated by suppressing apoptosis factors that were upregulated during excitotoxic trauma. In conclusion, our results suggest that amitriptyline could protect afferent synapses in the cochlea from NICS, making it a potential drug candidate for hearing protection.

Investigational Drugs for the Treatment of Postherpetic Neuralgia: Systematic Review of Randomized Controlled Trials

The pharmacological treatment of postherpetic neuralgia (PHN) is unsatisfactory, and there is a clinical need for new approaches. Several drugs under advanced clinical development are addressed in this review. A systematic literature search was conducted in three electronic databases (Medline, Web of Science, Scopus) and in the ClinicalTrials.gov register from 1 January 2016 to 1 June 2023 to identify Phase II, III and IV clinical trials evaluating drugs for the treatment of PHN. A total of 18 clinical trials were selected evaluating 15 molecules with pharmacological actions on nine different molecular targets: Angiotensin Type 2 Receptor (AT2R) antagonism (olodanrigan), Voltage-Gated Calcium Channel (VGCC) α2δ subunit inhibition (crisugabalin, mirogabalin and pregabalin), Voltage-Gated Sodium Channel (VGSC) blockade (funapide and lidocaine), Cyclooxygenase-1 (COX-1) inhibition (TRK-700), Adaptor-Associated Kinase 1 (AAK1) inhibition (LX9211), Lanthionine Synthetase C-Like Protein (LANCL) activation (LAT8881), N-Methyl-D-Aspartate (NMDA) receptor antagonism (esketamine), mu opioid receptor agonism (tramadol, oxycodone and hydromorphone) and Nerve Growth Factor (NGF) inhibition (fulranumab). In brief, there are several drugs in advanced clinical development for treating PHN with some of them reporting promising results. AT2R antagonism, AAK1 inhibition, LANCL activation and NGF inhibition are considered first-in-class analgesics. Hopefully, these trials will result in a better clinical management of PHN.

Recent Progress in Gels for Neuropathic Pain

Neuropathic pain is a complex and debilitating condition that affects millions of people worldwide. While several treatment options are available, they often have limited efficacy and are associated with adverse effects. In recent years, gels have emerged as a promising option for the treatment of neuropathic pain. Inclusion of various nanocarriers, such as cubosomes and niosomes, into gels results in pharmaceutical forms with higher drug stability and increased drug penetration into tissues compared to products currently marketed for the treatment of neuropathic pain. Furthermore, these compounds usually provide sustained drug release and are biocompatible and biodegradable, which makes them a safe option for drug delivery. The purpose of this narrative review was to provide a comprehensive analysis of the current state of the field and identify potential directions for future research in the development of effective and safe gels for the treatment of neuropathic pain, ultimately improving the quality of life for patients suffering from neuropathic pain.

A randomised sham-controlled study evaluating rTMS analgesic efficacy for postherpetic neuralgia

Context

Postherpetic neuralgia (PHN) is a refractory neuropathic pain condition in which new treatment options are being developed. Repetitive transcranial magnetic stimulation (rTMS) may have the potential to reduce pain sensations in patients with postherpetic neuralgia.

Objectives

This study investigated the efficacy on postherpetic neuralgia by stimulating two potential targets, the motor cortex (M1) and the dorsolateral prefrontal cortex (DLPFC).

Methods

This is a double-blind, randomised, sham-controlled study. Potential participants were recruited from Hangzhou First People's Hospital. Patients were randomly assigned to either the M1, DLPFC or Sham group. Patients received ten daily sessions of 10-Hz rTMS in 2 consecutive weeks. The primary outcome measure was visual analogue scale (VAS) assessed at baseline, first week of treatment (week 1), post-treatment (week 2), 1-week (week 4), 1-month (week 6) and 3-month (week 14) follow-up.

Results

Of sixty patients enrolled, 51 received treatment and completed all outcome assessments. M1 stimulation resulted in a larger analgesia during and after treatment compared to the Sham (week 2 - week 14, p < 0.005), as well as to the DLPFC stimulation (week 1 - week 14, p < 0.05). In addition to pain, sleep disturbance was significantly improved and relieved by targeting either the M1 or the DLPFC (M1: week 4 - week 14, p < 0.01; DLPFC: week 4 - week 14, p < 0.01). Moreover, pain sensations following M1 stimulation uniquely predicted improvement in sleep quality.

Conclusion

M1 rTMS is superior to DLPFC stimulation in treating PHN with excellent pain response and long-term analgesia. Meanwhile, M1 and DLPFC stimulation were equally effective in improving sleep quality in PHN.

Clinical trial registration

https://www.chictr.org.cn/, identifier ChiCTR2100051963.

A Review on Autophagy in Orofacial Neuropathic Pain

Orofacial neuropathic pain indicates pain caused by a lesion or diseases of the somatosensory nervous system. It is challenging for the clinician to diagnose and manage orofacial neuropathic pain conditions due to the considerable variability between individual clinical presentations and a lack of understanding of the mechanisms underlying the etiology and pathogenesis. In the last few decades, researchers have developed diagnostic criteria, questionnaires, and clinical assessment methods for the diagnosis of orofacial neuropathic pain. Recently, researchers have observed the role of autophagy in neuronal dysfunction as well as in the modulation of neuropathic pain. On this basis, in the present review, we highlight the characteristics, classification, and clinical assessment of orofacial neuropathic pain. Additionally, we introduce autophagy and its potential role in the modulation of orofacial neuropathic pain, along with a brief overview of the pathogenesis, which in future may reveal new possible targets for treating this condition.

Trigeminal extracranial thermocoagulation along with patient-controlled analgesia with esketamine for refractory postherpetic neuralgia after herpes zoster ophthalmicus: A case report

BACKGROUND

Primary trigeminal neuralgia can achieve satisfactory results through clinical treatment and intervention. The pathogenesis of neuralgia caused by varicella-zoster virus infection of the trigeminal nerve is more complex, and it is still difficult to relieve the pain in some patients simply by drug treatment or surgical intervention.

CASE SUMMARY

A 66-year-old woman was hospitalized with herpetic neuralgia after herpes zoster ophthalmicus (varicella-zoster virus infects the ophthalmic branch of the trigeminal nerve). On admission, the patient showed spontaneous, electric shock-like and acupuncture-like severe pain in the left frontal parietal region, and pain could be induced by touching the herpes area. The numerical rating scale (NRS) was 9. There was no significant pain relief after pulsed radiofrequency and thermocoagulation of the ophthalmic branch of the trigeminal nerve. Combined with patient-controlled intravenous analgesia (PCIA) with esketamine, neuralgia was significantly improved. The patient had no spontaneous pain or allodynia at discharge, and the NRS score decreased to 2 points. The results of follow-up 2 mo after discharge showed that the NRS score was ≤ 3, and the Pittsburgh Sleep Quality Index score was 5 points. There were no adverse reactions.

CONCLUSION

Trigeminal extracranial thermocoagulation combined with esketamine PCIA may be a feasible method for the treatment of refractory herpetic neuralgia after herpes zoster ophthalmicus.

Orofacial Neuropathic Pain-Basic Research and Their Clinical Relevancies

Trigeminal nerve injury is known to cause severe persistent pain in the orofacial region. This pain is difficult to diagnose and treat. Recently, many animal studies have reported that rewiring of the peripheral and central nervous systems, non-neuronal cell activation, and up- and down-regulation of various molecules in non-neuronal cells are involved in the development of this pain following trigeminal nerve injury. However, there are many unknown mechanisms underlying the persistent orofacial pain associated with trigeminal nerve injury. In this review, we address recent animal data regarding the involvement of various molecules in the communication of neuronal and non-neuronal cells and examine the possible involvement of ascending pathways in processing pathological orofacial pain. We also address the clinical observations of persistent orofacial pain associated with trigeminal nerve injury and clinical approaches to their diagnosis and treatment.

Dermal delivery of amitriptyline for topical analgesia

Abstract

Amitriptyline, administered orally, is currently one of the treatment options for the management of neuropathic pain and migraine. Because of the physicochemical properties of the molecule, amitriptyline is also a promising candidate for delivery as a topical analgesic. Here we report the dermal delivery of amitriptyline from a range of simple formulations. The first stage of the work required the conversion of amitriptyline hydrochloride to the free base form as confirmed by nuclear magnetic resonance (NMR). Distribution coefficient values were measured at pH 6, 6.5, 7, and 7.4. Solubility and stability of amitriptyline were assessed prior to conducting in vitro permeation and mass balance studies. The compound demonstrated instability in phosphate-buffered saline (PBS) dependent on pH. Volatile formulations comprising of isopropyl alcohol (IPA) and isopropyl myristate (IPM) or propylene glycol (PG) were evaluated in porcine skin under finite dose conditions. Compared with neat IPM, the IPM:IPA vehicles promoted 8-fold and 5-fold increases in the amount of amitriptyline that permeated at 24 h. Formulations containing PG also appear to be promising vehicles for dermal delivery of amitriptyline, typically delivering higher amounts of amitriptyline than the IPM:IPA vehicles. The results reported here suggest that further optimization of topical amitriptyline formulations should be pursued towards development of a product for clinical investigational studies.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13346-021-00982-x.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Peripheral neuropathic pain

Neuropathic pain (NP) can have either central nervous system causes or ones from the peripheral nervous system. This article will focus on the epidemiology, classifications, pathology, non-invasive treatments and invasive treatments as a general review of NP involving the peripheral nervous system. NP has characteristic symptomatology such as burning and electrical sensations. It occurs in up to 10% of the general population. Its frequency can be attributed to its occurrence in neck and back pain, diabetes and patients receiving chemotherapy. There are a wide range of pharmacologic options to control this type of pain and when such measures fail, numerous interventional methods can be employed such as nerve blocks and implanted stimulators. NP has a cost to the patient and society in terms of emotional consequences, quality of life, lost wages and the cost of assistance from the medical system and thus deserves serious consideration for prevention, treatment and control.

Comprehensive Review of Topical Analgesics for Chronic Pain

PURPOSE OF REVIEW

Topical analgesics are a non-opioid option for the treatment of chronic pain conditions including neuropathic pain, musculoskeletal pain, and osteoarthritis. There are many topical medications available; however their efficacy is variable. This article reviews the various topical analgesics, their mechanisms of action, and their efficacy.

RECENT FINDINGS

Studies have found topical NSAIDs are useful in treating acute musculoskeletal pain syndromes (strains and sprains) and show some efficacy in treating hand and knee osteoarthritis (Derry et al. Cochrane Database Syst Rev 5:CD008609, 2017). Topical capsaicin 8% has been shown to be efficacious in the treatment of postherpetic neuralgia, painful diabetic peripheral neuropathy, and HIV-neuropathy (Derry et al. Cochrane Database Syst Rev 1:CD007393, 2017). Topical lidocaine has been widely studied and found to reduce pain in patients with postherpetic neuralgia (Knezevic et al. Pain Manag 7:537-58, 2017). Although many other topical analgesics are available, there is limited data to support the efficacy of other agents. Topical analgesics are a relatively benign treatment for chronic pain conditions including neuropathic pain, musculoskeletal, and myofascial pain. There is evidence to support the use of topical NSAIDs, high concentration topical capsaicin, and topical lidocaine for various painful conditions.

Peripheral Mechanisms of Neuropathic Pain-the Role of Neuronal and Non-Neuronal Interactions and Their Implications for Topical Treatment of Neuropathic Pain

Neuropathic pain in humans arises as a consequence of injury or disease of somatosensory nervous system at peripheral or central level. Peripheral neuropathic pain is more common than central neuropathic pain, and is supposed to result from peripheral mechanisms, following nerve injury. The animal models of neuropathic pain show extensive functional and structural changes occurring in neuronal and non-neuronal cells in response to peripheral nerve injury. These pathological changes following damage lead to peripheral sensitization development, and subsequently to central sensitization initiation with spinal and supraspinal mechanism involved. The aim of this narrative review paper is to discuss the mechanisms engaged in peripheral neuropathic pain generation and maintenance, with special focus on the role of glial, immune, and epithelial cells in peripheral nociception. Based on the preclinical and clinical studies, interactions between neuronal and non-neuronal cells have been described, pointing out at the molecular/cellular underlying mechanisms of neuropathic pain, which might be potentially targeted by topical treatments in clinical practice. The modulation of the complex neuro-immuno-cutaneous interactions in the periphery represents a strategy for the development of new topical analgesics and their utilization in clinical settings.

Topical Treatment of Localized Neuropathic Pain in the Elderly

The prevalence of neuropathic pain in the older population has been reported to be very high and is most commonly localized to a circumscribed area. Treatment failure is frequent in neuropathic pain and is accompanied by central side effects with recommended oral drugs acting on the central nervous system. A number of topical pharmaceuticals are available on prescription and also sold over the counter. This review in persons aged older than 60 years shows the efficacy of lidocaine 5% and capsaicin 8% for localized neuropathic pain while results with other pharmaceuticals are rather inconsistent. Local application of drugs has a very limited systemic effect and the pharmacological advantages of local over systemic treatment are particularly interesting in older persons who often have comorbidities and take multiple medications. However, more information is needed on the efficacy and safety of lidocaine 5% and capsaicin 8% in older old persons and on the long-term effects of these pharmaceuticals. These studies should also pave the way for research and development in the field of topical analgesics with a satisfactory level of evidence-based medicine.

Topical treatments for diabetic neuropathic pain

Diabetic neuropathic pain (DNP) has a huge impact on quality of life and can be difficult to treat. Oral treatment is the most frequently used method for DNP, but its use is often limited by systemic side effects. Topical use of drugs as an alternative option for DNP treatment is currently gaining interest. In the present review, a summary is provided of the available agents for topical use in patients with DNP, including lidocaine plasters or patches, capsaicin cream, gel or patches, amitriptyline cream, clonidine gel, ketamine cream, extracts from medicinal plants including nutmeg extracts and Citrullus colocynthis extract oil, and certain compounded topical analgesics. Furthermore, the potential efficacy of these treatments is addressed according to the available clinical research literature. It has been indicated that these topical drugs have the potential to be valuable additional options for the management of DNP, with adequate safety and continuous long-term treatment efficacy. Compounded topical agents are also effective and safe for patients with DNP and could be another area worthy of further investigation based on the strategy of using low-dose, complementary therapies for DNP. The findings indicate that developing topical drugs acting on different targets in the process of DNP is a valuable area of future research.

Topical drug therapeutics for neuropathic pain

INTRODUCTION

Neuropathic pain (NP) is a particularly severe and intractable chronic condition that is not well treated by commonly recommended systemic pharmacological therapies, partly due to dose-limiting side effects or adverse events. The use of topical therapeutics for NP is growing and benefits from the reduced potential for adverse effects, as well as the ability to directly target peripheral pathological processes.

AREAS COVERED

The current review defines and describes the limitations of various commonly prescribed systemic pharmacological therapies for NP. It also provides a justification for increased research aimed at developing topical therapeutics for NP, particularly localized and peripheral NP. The review discusses the various classes of topical treatments used for NP, including agents that: block sensory inputs; activate inhibitory systems; provide mechanism-based therapeutics; are used in mucosal tissues; and include combinations that produce multimodal therapeutic effects.

EXPERT OPINION

There are arguments that the current topical therapeutics for NP rely too heavily on the use of local anesthetics and capsaicinoids, and more research is certainly needed on topical therapies that are multimodal and/or are targeted at the peripheral sources of pathology. The potential for novel topical therapeutics may be enhanced by further research on topical co-drugs, drug-drug salts, co-crystals and hydrates, and ionic liquids.

The Effect of a Combination of Diclofenac and Methadone Applied as Gel in a Human Experimental Pain Model - A Randomized, Placebo-controlled Trial

Pain involves responses in which both peripheral and central mechanisms contribute to the generation of pain. Pre-clinical laboratory data have supported that a topical formulation of combined diclofenac and methadone (Diclometh) may alleviate local pain, and potentially, the side effect profile should be low. We hypothesized that antiallodynic and antihyperalgesic effects of Diclometh could be demonstrated in a human experimental pain model and that Diclometh would be safe to administer. Thus, the aims were as follows: (i) to compare two doses of Diclometh versus placebo; and (ii) to assess the safety profile of Diclometh. The study was a crossover, randomized, double-blind, placebo-controlled comparison of two doses of Diclometh gel (0.1% and 0.2%) administered topically in healthy participants. Nerve growth factor (NGF) and capsaicin intradermal injections were used as human pain models. Pressure stimulation, contact heat stimulation, hyperalgesia (pinprick stimulation) and allodynia (brush stimulation) to mechanical stimulation were performed in the area where capsaicin and NGF were injected. Side effects were recorded on a four-point Likert scale. Twenty-one men completed the study (mean age 26.14 ± 5.3). Diclometh 0.2% reduced the capsaicin-induced dynamic mechanical allodynia compared to placebo (primary end-point, p = 0.03). No other primary or secondary end-points were found significantly different (all p > 0.05). All side effects were reported as mild with no differences between treatments (p = 0.15). Indication of antiallodynic effect of Diclometh 0.2% was found. Additionally, it was demonstrated that Diclometh was safe to use.