Combination therapy for neuropathic pain: a review of current evidence

Menin regulates spinal glutamate-GABA balance through GAD65 contributing to neuropathic pain

BACKGROUND

Our previous work found that tumor suppressor menin potentiates spinal synaptic plasticity in the context of peripheral nerve injury-induced neuropathic hypersensitivity, but the underlying molecular mechanisms are not clear. We hereby assessed the role of menin in regulating the spinal balance between glutamate and GABA and its contribution to the pathological condition of nerve injury-induced hypersensitivity.

METHODS

In spared nerve injury induced C57BL/6 mice, mechanical withdrawal threshold was measured with von Frey filaments after intrathecal administration of small interfering RNA (siRNA) of MEN1 or/and subcutaneous histone deacetylase (HDAC) inhibitors to control the level of glutamic acid decarboxylase 65 (GAD65). Immunoblotting and high-performance liquid chromatography were used to detect the level of protein expression and spinal glutamate and GABA, respectively.

RESULTS

Genetic knockdown of spinal menin alleviated nerve injury evoked mechanical hypersensitivity, which was strongly associated with the alteration of the spinal level of GAD65 that resulted in an imbalance of glutamate/GABA ratio from the baseline ratio of 5.8 ± 0.9 (×10(-4)) to the peak value of 58.6 ± 11.8 (×10(-4)) at the day 14 after SNI (p < 0.001), which was reversed by MEN1 siRNA to 14.7 ± 2.1 (×10(-4)) at the day 14 after nerve injury (p < 0.01). In further, selective inhibitors of HDACs considerably reversed the ratio of spinal glutamate and GABA, and also alleviated the mechanical withdrawal threshold markedly.

CONCLUSION

Our findings provide mechanistic insight into the contribution of the upregulated spinal menin to peripheral nerve injury induced neuropathic hypersensitivity by regulating glutamate-GABA balance through deactivating GAD65.

Semi-mechanistic modelling of the analgesic effect of gabapentin in the formalin-induced rat model of experimental pain

PURPOSE

The formalin-induced rat model of nociception involves moderate continuous pain. Formalin-induced pain results in a typical repetitive flinching behaviour, which displays a biphasic pattern characterised by peaks of pain. Here we described the time course of pain response and the analgesic effect of gabapentin using a semi-mechanistic modelling approach.

METHODS

Male Sprague-Dawley rats received gabapentin (10-100 mg/kg) or placebo 1 h prior to the formalin injection, as per standard protocol. A reduction in the frequency of the second peak of flinching was used as a behavioural measure of gabapentin-mediated anti-nociception. The flinching response was modelled using a mono-exponential function to characterise the first peak and an indirect response model with a time variant synthesis rate for the second. PKPD modelling was performed using a population approach in NONMEM v.7.1.2.

RESULTS

The time course of the biphasic response was adequately described by the proposed model, which included separate expressions for each phase. Gabapentin was found to reversibly decrease, but not suppress the flinching frequency of the second response peak only. The mean IC50 estimate was 7,510 ng/ml, with relative standard error (RSE%) of 40%.

CONCLUSIONS

A compartmental, semi-mechanistic model provides the basis for further understanding of the formalin-induced flinching response and consequently to better characterisation of the properties of gabapentin, such as the potency in individual animals. Moreover, despite high exposure levels, model predictions show that gabapentin does not completely suppress behavioural response in the formalin-induced pain model.

Interaction of local anesthetics with biomembranes consisting of phospholipids and cholesterol: mechanistic and clinical implications for anesthetic and cardiotoxic effects

Despite a long history in medical and dental application, the molecular mechanism and precise site of action are still arguable for local anesthetics. Their effects are considered to be induced by acting on functional proteins, on membrane lipids, or on both. Local anesthetics primarily interact with sodium channels embedded in cell membranes to reduce the excitability of nerve cells and cardiomyocytes or produce a malfunction of the cardiovascular system. However, the membrane protein-interacting theory cannot explain all of the pharmacological and toxicological features of local anesthetics. The administered drug molecules must diffuse through the lipid barriers of nerve sheaths and penetrate into or across the lipid bilayers of cell membranes to reach the acting site on transmembrane proteins. Amphiphilic local anesthetics interact hydrophobically and electrostatically with lipid bilayers and modify their physicochemical property, with the direct inhibition of membrane functions, and with the resultant alteration of the membrane lipid environments surrounding transmembrane proteins and the subsequent protein conformational change, leading to the inhibition of channel functions. We review recent studies on the interaction of local anesthetics with biomembranes consisting of phospholipids and cholesterol. Understanding the membrane interactivity of local anesthetics would provide novel insights into their anesthetic and cardiotoxic effects.

Mechanisms and management of diabetic painful distal symmetrical polyneuropathy

Although a number of the diabetic neuropathies may result in painful symptomatology, this review focuses on the most common: chronic sensorimotor distal symmetrical polyneuropathy (DSPN). It is estimated that 15-20% of diabetic patients may have painful DSPN, but not all of these will require therapy. In practice, the diagnosis of DSPN is a clinical one, whereas for longitudinal studies and clinical trials, quantitative sensory testing and electrophysiological assessment are usually necessary. A number of simple numeric rating scales are available to assess the frequency and severity of neuropathic pain. Although the exact pathophysiological processes that result in diabetic neuropathic pain remain enigmatic, both peripheral and central mechanisms have been implicated, and extend from altered channel function in peripheral nerve through enhanced spinal processing and changes in many higher centers. A number of pharmacological agents have proven efficacy in painful DSPN, but all are prone to side effects, and none impact the underlying pathophysiological abnormalities because they are only symptomatic therapy. The two first-line therapies approved by regulatory authorities for painful neuropathy are duloxetine and pregabalin. α-Lipoic acid, an antioxidant and pathogenic therapy, has evidence of efficacy but is not licensed in the U.S. and several European countries. All patients with DSPN are at increased risk of foot ulceration and require foot care, education, and if possible, regular podiatry assessment.

Evaluation of analgesic, antioxidant, cytotoxic and metabolic effects of pregabalin for the use in neuropathic pain

OBJECTIVE

The aim of this research was to evaluate analgesic, antioxidant, metabolic, and cytotoxic effects of pregabalin (PGB), which is widely applied for the treatment of neuropathic pain syndromes in diabetic patients.

METHODS

We used the streptozotocin (STZ) model of painful diabetic neuropathy (PDN) in mice and we measured the effect of intraperitoneally administered PGB on tactile and thermal nociceptive thresholds in the von Frey and hot plate assays, respectively. The influence of PGB on the motor coordination of diabetic animals was investigated in the rotarod test. In vitro in HepG2 and 3T3-L1 cell lines cytotoxicity of PGB, its influence on glucose utilization, and lipid accumulation were assessed. The antioxidant capacity of PGB was evaluated spectrophotometrically using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical method.

RESULTS

Pregabalin was a very efficacious antiallodynic and analgesic drug capable of increasing the pain thresholds for tactile allodynia and thermal hyperalgesia in diabetic mice. In the von Frey test at a dose of 30 mg/kg it elevated the pain threshold for 168% versus diabetic control and in the hot plate test this dose prolonged the latency time to pain reaction for 130% versus control value of diabetic mice. No motor deficits were observed in PGB-treated diabetic animals. In vitro PGB did not influence glucose utilization or lipid accumulation. No antioxidant or cytotoxic effects of PGB were observed at concentrations 1-100 μM.

DISCUSSION AND CONCLUSION

Our experiments demonstrated significant antiallodynic and analgesic properties of PGB in mice. In vitro studies showed that this drug is metabolically neutral. It did not cause motor coordination impairments in diabetic animals either. These effects might be of great importance for diabetic patients.

Prospective, non-interventional study on the tolerability and analgesic effectiveness over 12 weeks after a single application of capsaicin 8% cutaneous patch in 1044 patients with peripheral neuropathic pain: first results of the QUEPP study

BACKGROUND

Reversible defunctionalisation of nociceptors by the TRPV1 agonist capsaicin in high concentration is an emerging new concept for the treatment of peripheral neuropathic pain.

OBJECTIVES

The capsaicin 8% cutaneous patch with a long-lasting effect for up to 3 months after a single application is available in Germany by prescription since October 2010. The aim of this study was to monitor its usage and therapeutic performance in clinical practice.

METHODS

Patients had a single patch application with up to 4 patches and were followed up after 7-14 days, 4, 8, and 12 weeks. Average pain intensity (NPRS-11), pain attacks, neuropathy symptoms, sleep parameters, quality of life, working capacity and concomitant neuropathic pain medication were assessed during at least two visits.

RESULTS

A total of 509 females (48.8%; effectiveness population N = 1044) and 531 males (50.9%) were included; the mean age was 61.2 ± 14.4 (SD) years. Postherpetic neuralgia was the most frequent diagnosis (31.9%), followed by postsurgical neuralgia (22.8%), post-traumatic neuropathy (12.4%), polyneuropathy (14.3%), and mixed pain syndromes (16.6%). Thirty and 50% responder rates were 42.7% and 23.7%, respectively, with a mean relative reduction of pain intensity during weeks 1-12 of 24.7% (1.1 SEM) and significant improvements in pain attacks, sleep duration and sleep quality, while the consumption of opioids and antiepileptics decreased significantly. In 106 patients (10.0%; safety population n = 1063) 146 adverse drug reactions (ADRs) were reported, mainly application site reactions (erythema, pain). A total of 27 serious ADRs were documented in 17 patients (1.6%).

CONCLUSIONS

Analgesic treatment of peripheral neuropathic pain with the capsaicin 8% cutaneous patch is safe and effective.

LIMITATIONS

The study did not include a control group; therefore, a comparison of the results with that of therapeutic alternatives is not justified.

Duloxetine and pregabalin: high-dose monotherapy or their combination? The "COMBO-DN study"--a multinational, randomized, double-blind, parallel-group study in patients with diabetic peripheral neuropathic pain

This multicentre, double-blind, parallel-group study in diabetic peripheral neuropathic pain addressed whether, in patients not responding to standard doses of duloxetine or pregabalin, combining both medications is superior to increasing each drug to its maximum recommended dose. For initial 8-week therapy, either 60 mg/day duloxetine (groups 1, 2) or 300 mg/day pregabalin (groups 3, 4) was given. Thereafter, in the 8-week combination/high-dose therapy period, only nonresponders received 120 mg/day duloxetine (group 1), a combination of 60 mg/day duloxetine and 300 mg/day pregabalin (groups 2, 3), or 600 mg/day pregabalin (group 4). Primary outcome (Brief Pain Inventory Modified Short Form [BPI-MSF] 24-hour average pain change after combination/high-dose therapy) was analyzed comparing combination (groups 2, 3 pooled) with high-dose monotherapy (groups 1, 4 pooled). Secondary end points included response rates, BPI-MSF severity items, and comparison of duloxetine and pregabalin in BPI-MSF average pain. Eight hundred four patients were evaluated for initial therapy and 339 for combination/high-dose therapy. There were no significant differences between combination and high-dose monotherapy regarding BPI-MSF average pain (mean change: combination: -2.35; high-dose monotherapy: -2.16; P = 0.370) and most secondary end points, which, however, consistently favoured combination therapy. Fifty-percent response rates were 52.1% for combination and 39.3% for high-dose monotherapy (P = 0.068). In exploratory analyses of the initial 8-week therapy uncorrected for multiple comparisons, 60 mg/day duloxetine was found superior to 300 mg/day pregabalin (P < 0.001). Both drugs and their combination were well tolerated. Although not significantly superior to high-dose monotherapy, combination therapy was considered to be effective, safe, and well tolerated.

RETRACTED: Increases in inflammatory mediators in DRG implicate in the pathogenesis of painful neuropathy in Type 2 diabetes

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor-in-Chief. Image duplication has been observed within Figure 1g-h. The corresponding author has been asked to provide an acceptable explanation for this duplication but has not been able to do so neither have the original source files been supplied.

Diagnosing and managing postherpetic neuralgia

Postherpetic neuralgia (PHN) represents a potentially debilitating and often undertreated form of neuropathic pain that disproportionately affects vulnerable populations, including the elderly and the immunocompromised. Varicella zoster infection is almost universally prevalent, making prevention of acute herpes zoster (AHZ) infection and prompt diagnosis and aggressive management of PHN of critical importance. Despite the recent development of a herpes zoster vaccine, prevention of AHZ is not yet widespread or discussed in PHN treatment guidelines. Diagnosis of PHN requires consideration of recognized PHN signs and known risk factors, including advanced age, severe prodromal pain, severe rash, and AHZ location on the trigeminal dermatomes or brachial plexus. PHN pain is typically localized, unilateral and chronic, but may be constant, intermittent, spontaneous and/or evoked. PHN is likely to interfere with sleep and daily activities. First-line therapies for PHN include tricyclic antidepressants, gabapentin and pregabalin, and the lidocaine 5 % patch. Second-line therapies include strong and weak opioids and topical capsaicin cream or 8 % patch. Tricyclic antidepressants, gabapentinoids and strong opioids are effective but are also associated with systemic adverse events that may limit their use in many patients, most notably those with significant medical comorbidities or advanced age. Of the topical therapies, the topical lidocaine 5 % patch has proven more effective than capsaicin cream or 8 % patch and has a more rapid onset of action than the other first-line therapies or capsaicin. Given the low systemic drug exposure, adverse events with topical therapies are generally limited to application-site reactions, which are typically mild and transient with lidocaine 5 % patch, but may involve treatment-limiting discomfort with capsaicin cream or 8 % patch. Based on available clinical data, clinicians should consider administering the herpes zoster vaccine to all patients aged 60 years and older. Clinicians treating patients with PHN may consider a trial of lidocaine 5 % patch monotherapy before resorting to a systemic therapy, or alternatively, may consider administering the lidocaine 5 % patch in combination with a tricyclic antidepressant or a gabapentinoid to provide more rapid analgesic response and lower the dose requirement of systemic therapies.

Lack of correlation between opioid dose adjustment and pain score change in a group of chronic pain patients

Despite the increasing use of opioid analgesics for chronic pain management, it is unclear whether opioid dose escalation leads to better pain relief during chronic opioid therapy. In this study, we retrospectively analyzed clinical data collected from the Massachusetts General Hospital Center for Pain Medicine over a 7-year period. We examined 1) the impact of opioid dose adjustment (increase or decrease) on clinical pain score; 2) gender and age differences in response to opioid therapy; and 3) the influence of clinical pain conditions on the opioid analgesic efficacy. A total of 109 subjects met the criteria for data collection. We found that neither opioid dose increase, nor decrease, correlated with point changes in clinical pain score in a subset of chronic pain patients over a prolonged course of opioid therapy (an average of 704 days). This lack of correlation was consistent regardless of the type of chronic pain including neuropathic, nociceptive, or mixed pain conditions. Neither gender nor age differences showed a significant influence on the clinical response to opioid therapy in these subjects. These results suggest that dose adjustment during opioid therapy may not necessarily alter long-term clinical pain score in a group of chronic pain patients and that individualized opioid therapy based on the clinical effectiveness should be considered to optimize the treatment outcome.

PERSPECTIVE

The study reports a relationship, or lack thereof, between opioid dose change and clinical pain score in a group of chronic pain patients. The study also calls for further investigation into the effectiveness of opioid therapy in the management of chronic nonmalignant pain conditions.

Apoptosis signal-regulating kinase 1-thioredoxin complex dissociation by capsaicin causes pancreatic tumor growth suppression by inducing apoptosis

AIM

In this study, we evaluated the effect of capsaicin on the interaction of redox-sensitive thioredoxin (Trx)/apoptosis signal-regulating kinase 1 (ASK1) in pancreatic cancer cells.

RESULTS

Capsaicin treatment downregulated Trx and increased the phosphorylation (activation) of ASK1 at Thr845 and kinase activity in AsPC-1 and BxPC-3 cells. Capsaicin treatment also activated downstream effector molecules MKK4/7, caspase-9, and caspase-3. Antioxidants tiron or PEG-catalase blocked the activation of ASK1 cascade by capsaicin and protected the cells from apoptosis, indicating the involvement of reactive oxygen species in the activation of ASK1. Our results further revealed that Trx overexpression suppressed the effects of capsaicin, whereas ASK1 overexpression enhanced the apoptosis-inducing effects of capsaicin. β-mercaptoethanol, a reducing agent, blocked capsaicin-mediated activation of ASK1, indicating that Trx-ASK1 complex exists and requires reducing conditions in the cell. On the other hand, the Trx inhibitor (1-chloro-2-4-dinitrobenzene) increased capsaicin-induced ASK1 kinase activity, suggesting that Trx inhibition by capsaicin is essential for ASK1 activation. Oral administration of 5 mg capsaicin/kg body weight substantially suppressed the growth of tumors in xenograft and orthotopic mouse model. Tumors from capsaicin-treated mice showed reduced levels of Trx, increased phosphorylation of ASK1 at Thr845, and cleavage of caspase-3 and poly (ADP-ribose) polymerase.

INNOVATION

Our results for the first time demonstrated a new perspective that Trx-ASK1 complex can be targeted by capsaicin in pancreatic cancer.

CONCLUSION

Capsaicin reduces Trx expression and dissociates Trx-ASK1 complex resulting in the activation of ASK1 and downstream effectors leading to apoptosis in pancreatic tumor cells in vitro and in vivo.

Therapeutic utility of palmitoylethanolamide in the treatment of neuropathic pain associated with various pathological conditions: a case series

Palmitoylethanolamide (PEA), an endogenous fatty acid amide, has been demonstrated to bind to a receptor in the cell nucleus - the peroxisome proliferator-activated receptor - and performs a great variety of biological functions related to chronic and neuropathic pain and inflammation, as has been demonstrated in clinical trials. These include peripheral neuropathies such as diabetic neuropathy, chemotherapy-induced peripheral neuropathy, carpal tunnel syndrome, sciatic pain, osteoarthritis, low-back pain, failed back surgery syndrome, dental pains, neuropathic pain in stroke and multiple sclerosis, chronic pelvic pain, postherpetic neuralgia, and vaginal pains. Probably due to the fact that PEA is an endogenous modulator as well as a compound in food, such as eggs and milk, no serious side effects have been reported, nor have drug-drug interactions. This article presents a case series describing the application and potential efficacy and safety of PEA in the treatment of various syndromes associated with chronic pain that is poorly responsive to standard therapies.

Pregabalin attenuates docetaxel-induced neuropathy in rats

Chemotherapy-induced neuropathy is a serious clinical problem for patients receiving cancer treatment. The aim of this study was to investigate the potential efficacy of pregabalin in chemotherapy-induced neuropathy in rats. A total of 35 male Sprague-Dawley rats were randomly divided into 5 groups: group 1, naive control; group 2, treated with pregabalin (30 mg/kg p.o., for 8 days); group 3, docetaxel was given by single intravenous infusion at 10 mg/kg; groups 4 and 5, pregabalin at 10 mg/kg and 30 mg/kg respectively was orally administered for 8 days after the docetaxel treatment. On day 8, behavioral test was performed, and substance P and CGRP release in dorsal root ganglion (DRG) and sciatic nerve were analyzed by electron microscope. Our results showed that docetaxel induced mechanical allodynia, mechanical hyperalgesia, heat hypoalgesia, cold allodynia, and sciatic nerve impairment and substance P and CGRP release in DRG. However, oral administration of pregabalin (10 mg/kg and 30 mg/kg) for 8 consecutive days significantly attenuated docetaxel-induced neuropathy by ameliorating heat hypoalgesia, cold allodynia, impairment of sciatic nerve and reducing the release of substance P and CGRP. The findings in the present study reveal that pregabalin may be a potential treatment agent against chemotherapy-induced neuropathy.

Combination pharmacotherapy for the treatment of neuropathic pain in adults

BACKGROUND

Pharmacotherapy remains an important modality for the treatment of neuropathic pain. However, as monotherapy current drugs are associated with limited efficacy and dose-related side effects. Combining two or more different drugs may improve analgesic efficacy and, in some situations, reduce overall side effects (e.g. if synergistic interactions allow for dose reductions of combined drugs).

OBJECTIVES

This review evaluated the efficacy, tolerability and safety of various drug combinations for the treatment of neuropathic pain.

SEARCH METHODS

We identified randomised controlled trials (RCTs) of various drug combinations for neuropathic pain from CENTRAL, MEDLINE, EMBASE and handsearches of other reviews and trial registries. The most recent search was performed on 9 April 2012.

SELECTION CRITERIA

Double-blind, randomised studies comparing combinations of two or more drugs (systemic or topical) to placebo and/or at least one other comparator for the treatment of neuropathic pain.

DATA COLLECTION AND ANALYSIS

Data extracted from each study included: proportion of participants a) reporting ≥ 30% pain reduction from baseline OR ≥ moderate pain relief OR ≥ moderate global improvement; b) dropping out of the trial due to treatment-emergent adverse effects; c) reporting each specific adverse effect (e.g. sedation, dizziness) of ≥ moderate severity. The primary comparison of interest was between study drug(s) and one or both single-agent comparators. We combined studies if they evaluated the same drug class combination at roughly similar doses and durations of treatment. We used RevMan 5 to analyse data for binary outcomes.

MAIN RESULTS

We identified 21 eligible studies: four (578 participants) evaluated the combination of an opioid with gabapentin or pregabalin; two (77 participants) evaluated an opioid with a tricyclic antidepressant; one (56 participants) of gabapentin and nortriptyline; one (120 participants) of gabapentin and alpha-lipoic acid, three (90 participants) of fluphenazine with a tricyclic antidepressant; three (90 participants) of an N-methyl-D-aspartate (NMDA) blocker with an agent from a different drug class; five (604 participants) of various topical medications; one (313 participants) of tramadol with acetaminophen; and another one (44 participants) of a cholecystokinin blocker (L-365,260) with morphine. The majority of combinations evaluated to date involve drugs, each of which share some element of central nervous system (CNS) depression (e.g. sedation, cognitive dysfunction). This aspect of side effect overlap between the combined agents was often reflected in similar or higher dropout rates for the combination and may thus substantially limit the utility of such drug combinations. Meta-analysis was possible for only one comparison of only one combination, i.e. gabapentin + opioid versus gabapentin alone. This meta-analysis involving 386 participants from two studies demonstrated modest, yet statistically significant, superiority of a gabapentin + opioid combination over gabapentin alone. However, this combination also produced significantly more frequent side effect-related trial dropouts compared to gabapentin alone.

AUTHORS' CONCLUSIONS

Multiple, good-quality studies demonstrate superior efficacy of two-drug combinations. However, the number of available studies for any one specific combination, as well as other study factors (e.g. limited trial size and duration), preclude the recommendation of any one specific drug combination for neuropathic pain. Demonstration of combination benefits by several studies together with reports of widespread clinical polypharmacy for neuropathic pain surely provide a rationale for additional future rigorous evaluations. In order to properly identify specific drug combinations which provide superior efficacy and/or safety, we recommend that future neuropathic pain studies of two-drug combinations include comparisons with placebo and both single-agent components. Given the apparent adverse impact of combining agents with similar adverse effect profiles (e.g. CNS depression), the anticipated development and availability of non-sedating neuropathic pain agents could lead to the identification of more favourable analgesic drug combinations in which side effects are not compounded.