Combination therapy for neuropathic pain: a review of current evidence

Towards Multitargeted Ligands as Pain Therapeutics: Dual Ligands of the Cavα2δ-1 Subunit of Voltage-Gated Calcium Channel and the μ-Opioid Receptor

The synthesis and pharmacological activity of a new series of dual ligands combining activities towards the α2δ-1 subunit of voltage-gated calcium channels (Cavα2δ-1) and the μ-opioid receptor (MOR) as novel pain therapeutics are reported. A careful exploration of the pharmacophores related to both targets, which in principle had few common characteristics, led to the design of novel compounds exhibiting both activities. The construction of the dual ligands started from published Cavα2δ-1 ligands, onto which MOR ligand pharmacophoric elements were added. This exercise led to new amino-acidic substances with good affinities on both targets as well as good metabolic and physicochemical profiles and low potential for drug-drug interactions. A representative compound, (2S,4S)-4-(4-chloro-3-(((cis)-4-(dimethylamino)-4-phenylcyclohexyl)methyl)-5-fluorophenoxy)pyrrolidine-2-carboxylic acid, displayed promising analgesic activities in several in vivo pain models as well as a reduced side-effect profile in relation to morphine.

RTA-408 Regulates p-NF-κB/TSLP/STAT5 Signaling to Ameliorate Nociceptive Hypersensitivity in Chronic Constriction Injury Rats

Neuropathic pain following nerve injury is a complex condition, which often puts a negative impact on life and remains a sustained problem. To make pain management better is of great significance and unmet need. RTA 408 (Omaveloxone) is a traditional Asian medicine with a valid anti-inflammatory property. Thus, we aim to investigate the therapeutic effect of RTA-408 on mechanical allodynia in chronic constriction injury (CCI) rats as well as the underlying mechanisms. Neuropathic pain was induced by using CCI of the rats' sciatic nerve (SN) and the behavior testing was measured by calibrated forceps testing. Activation of Nrf-2, the phosphorylation of nuclear factor-κB (NF-κB), and the inflammatory response were assessed by western blots. The number of apoptotic neurons and degree of glial cell reaction were examined by immunofluorescence assay. RTA-408 exerts an analgesic effect on CCI rats. RTA-408 reduces neuronal apoptosis and glial cell activation by increasing Nrf-2 expression and decreasing the inflammatory response (TNF-α/ p-NF-κB/ TSLP/ STAT5). These data suggest that RTA-408 is a candidate with potential to reduce nociceptive hypersensitivity after CCI by targeting TSLP/STAT5 signaling.

RALY participates in nerve trauma-induced nociceptive hypersensitivity through triggering Eif4g2 gene expression in primary sensory neurons

BACKGROUND AND PURPOSE

Peripheral nerve trauma-induced dysregulation of pain-associated genes in the primary sensory neurons of dorsal root ganglion (DRG) contributes to neuropathic pain genesis. RNA-binding proteins participate in gene transcription. We hypothesized that RALY, an RNA-binding protein, participated in nerve trauma-induced dysregulation of DRG pain-associated genes and nociceptive hypersensitivity.

METHODS AND RESULTS

Immunohistochemistry staining showed that RALY was expressed exclusively in the nuclei of DRG neurons. Peripheral nerve trauma caused by chronic constriction injury (CCI) of unilateral sciatic nerve produced time-dependent increases in the levels of Raly mRNA and RALY protein in injured DRG. Blocking this increase through DRG microinjection of adeno-associated virus 5 (AAV5)-expressing Raly shRNA reduced the CCI-induced elevation in the amount of eukaryotic initiation factor 4 gamma 2 (Eif4g2) mRNA and Eif4g2 protein in injured DRG and mitigated the development and maintenance of CCI-induced nociceptive hypersensitivity, without altering basal (acute) response to noxious stimuli and locomotor activity. Mimicking DRG increased RALY through DRG microinjection of AAV5 expressing Raly mRNA up-regulated the expression of Eif4g2 mRNA and Eif4g2 protein in the DRG and led to hypersensitive responses to noxious stimuli in the absence of nerve trauma. Mechanistically, CCI promoted the binding of RALY to the promoter of Eif4g2 gene and triggered its transcriptional activity.

CONCLUSION AND IMPLICATIONS

Our findings indicate that RALY participates in nerve trauma-induced nociceptive hypersensitivity likely through transcriptionally triggering Eif4g2 expression in the DRG. RALY may be a potential target in neuropathic pain management.

Transcription factor EBF1 mitigates neuropathic pain by rescuing Kv1.2 expression in primary sensory neurons

Nerve injury-induced alternations of gene expression in primary sensory neurons of the dorsal root ganglion (DRG) are molecular basis of neuropathic pain genesis. Transcription factors regulate gene expression. In this study, we examined whether early B cell factor 1 (EBF1), a transcription factor, in the DRG, participated in neuropathic pain caused by chronic constriction injury (CCI) of the sciatic nerve. EBF1 was distributed exclusively in the neuronal nucleus and coexpressed with cytoplasmic/membrane Kv1.2 in individual DRG neurons. The expression of Ebf1 mRNA and protein was time-dependently downregulated in the ipsilateral lumbar (L) 3/4 DRGs after unilateral CCI. Rescuing this downregulation through microinjection of the adeno-associated virus 5 expressing full-length Ebf1 mRNA into the ipsilateral L3/4 DRGs reversed the CCI-induced decrease of DRG Kv1.2 expression and alleviated the development and maintenance of mechanical, heat and cold hypersensitivities. Conversely, mimicking the downregulation of DRG EBF1 through microinjection of AAV5-expressing Ebf1 shRNA into unilateral L3/4 DRGs produced a reduction of Kv1.2 expression in the ipsilateral L3/4 DRGs, spontaneous pain, and the enhanced responses to mechanical, heat and cold stimuli in naive mice. Mechanistically, EBF1 not only bound to the Kcna2 gene (encoding Kv1.2) promoter but also directly activated its activity. CCI decreased the EBF1 binding to the Kcna2 promoter in the ipsilateral L3/4 DRGs. Our findings suggest that DRG EBF1 downregulation contributes to neuropathic pain likely by losing its binding to Kcna2 promoter and subsequently silencing Kv1.2 expression in primary sensory neurons. Exogenous EBF1 administration may mitigate neuropathic pain by rescuing DRG Kv1.2 expression.

4-Cyanamido-substituted benzenesulfonamides act as dual carbonic anhydrase and cathepsin inhibitors

A set of novel N-cyano-N-substituted 4-aminobenzenesulfonamide derivatives were synthesized and investigated for their inhibitory activity against four cytosolic carbonic anhydrase (CA, EC 4.2.1.1) isoforms (hCA I, II, VII and XIII) and two cathepsins (S and B). N-alkyl/benzyl-substituted derivatives were revealed to be very potent inhibitors against brain-associated hCA VII, but inactive against both cathepsins. On the other hand, N-acyl-substituted derivatives displayed significant inhibitory activities against cathepsin S, but only moderate to poor inhibitory potency against hCA VII. Both hCA VII and cathepsin S have recently been validated as therapeutic targets in neuropathic pain. This study provided an excellent starting point for further structural optimization of this class of bifunctional compounds to enhance their inhibitory activity and selectivity against hCA VII and cathepsin S and to achieve new compounds with an attractive dual mechanism of action as anti-neuropathic agents.

Investigating the potential of GalR2 as a drug target for neuropathic pain

Neuropathic pain is a chronic and debilitating condition characterised by episodes of hyperalgesia and allodynia. It occurs following nerve damage from disease, inflammation or injury and currently impacts up to 17% of the UK population. Existing therapies lack efficacy and have deleterious side effects that can be severely limiting. Galanin receptor 2 (GalR2) is a G-protein coupled receptor (GPCR) implicated in the control and processing of painful stimuli. Within the nervous system it is expressed in key tissues involved in these actions such as dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. Stimulation of GalR2 is widely reported to have a role in the attenuation of inflammatory and neuropathic pain. Several studies have indicated GalR2 as a possible drug target, highlighting the potential of specific GalR2 agonists to both provide efficacy and to address the side-effect profiles of current pain therapies in clinical use. A strong biological target for drug discovery will be well validated with regards to its role in the relevant disease pathology. Ideally there will be good translational models, sensitive probes, selective and appropriate molecular tools, translational biomarkers, a clearly defined patient population and strong opportunities for commercialisation. Before GalR2 can be considered as a drug target suitable for investment, key questions need to be asked regarding its expression profile, receptor signalling and ligand interactions. This article aims to critically review the available literature and determine the current strength of hypothesis of GalR2 as a target for the treatment of neuropathic pain.

FUS Contributes to Nerve Injury-Induced Nociceptive Hypersensitivity by Activating NF-κB Pathway in Primary Sensory Neurons

Dysregulation of pain-associated genes in the dorsal root ganglion (DRG) is considered to be a molecular basis of neuropathic pain genesis. Fused in sarcoma (FUS), a DNA/RNA-binding protein, is a critical regulator of gene expression. However, whether it contributes to neuropathic pain is unknown. This study showed that peripheral nerve injury caused by the fourth lumbar (L4) spinal nerve ligation (SNL) or chronic constriction injury (CCI) of the sciatic nerve produced a marked increase in the expression of FUS protein in injured DRG neurons. Blocking this increase through microinjection of the adeno-associated virus (AAV) 5-expressing Fus shRNA into the ipsilateral L4 DRG mitigated the SNL-induced nociceptive hypersensitivities in both male and female mice. This microinjection also alleviated the SNL-induced increases in the levels of phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2) and glial fibrillary acidic protein (GFAP) in the ipsilateral L4 dorsal horn. Furthermore, mimicking this increase through microinjection of AAV5 expressing full-length Fus mRNA into unilateral L3/4 DRGs produced the elevations in the levels of p-ERK1/2 and GFAP in the dorsal horn, enhanced responses to mechanical, heat and cold stimuli, and induced the spontaneous pain on the ipsilateral side of both male and female mice in the absence of SNL. Mechanistically, the increased FUS activated the NF-κB signaling pathway by promoting the translocation of p65 into the nucleus and phosphorylation of p65 in the nucleus from injured DRG neurons. Our results indicate that DRG FUS contributes to neuropathic pain likely through the activation of NF-κB in primary sensory neurons.SIGNIFICANCE STATEMENT In the present study, we reported that fused in sarcoma (FUS), a DNA/RNA-binding protein, is upregulated in injured dorsal root ganglion (DRG) following peripheral nerve injury. This upregulation is responsible for nerve injury-induced translocation of p65 into the nucleus and phosphorylation of p65 in the nucleus from injured DRG neurons. Because blocking this upregulation alleviates nerve injury-induced nociceptive hypersensitivity, DRG FUS participates in neuropathic pain likely through the activation of NF-κB in primary sensory neurons. FUS may be a potential target for neuropathic pain management.

E74-like factor 1 contributes to nerve trauma-induced nociceptive hypersensitivity through transcriptionally activating matrix metalloprotein-9 in dorsal root ganglion neurons

ABSTRACT

Nerve trauma-induced alternations of gene expression in the neurons of dorsal root ganglion (DRG) participate in nerve trauma-caused nociceptive hypersensitivity. Transcription factors regulate gene expression. Whether the transcription factor E74-like factor 1 (ELF1) in the DRG contributes to neuropathic pain is unknown. We report here that peripheral nerve trauma caused by chronic constriction injury (CCI) of unilateral sciatic nerve or unilateral fourth lumbar spinal nerve ligation led to the time-dependent increases in the levels of Elf1 mRNA and ELF1 protein in injured DRG, but not in the spinal cord. Preventing this increase through DRG microinjection of adeno-associated virus 5 expressing Elf1 shRNA attenuated the CCI-induced upregulation of matrix metallopeptidase 9 (MMP9) in injured DRG and induction and maintenance of nociceptive hypersensitivities, without changing locomotor functions and basal responses to acute mechanical, heat, and cold stimuli. Mimicking this increase through DRG microinjection of AAV5 expressing full-length Elf1 upregulated DRG MMP9 and produced enhanced responses to mechanical, heat, and cold stimuli in naive mice. Mechanistically, more ELF1 directly bond to and activated Mmp9 promoter in injured DRG neurons after CCI. Our data indicate that ELF1 participates in nerve trauma-caused nociceptive hypersensitivity likely through upregulating MMP9 in injured DRG. E74-like factor 1 may be a new target for management of neuropathic pain.

Efficacy of Combination Therapy with Pregabalin in Neuropathic Pain: A Preclinical Study in the Rat L5 Spinal Nerve Ligation Model

PURPOSE

Neuropathic pain is sometimes difficult to manage because of limited efficacy of analgesic monotherapy even at high doses. Combination therapy may help address this issue, but there is little evidence for its effectiveness. Therefore, we evaluated the efficacy of combination therapy with pregabalin, an anchor drug for treating neuropathic pain, using the rat L5 spinal nerve ligation model.

METHODS

Experiments were performed on four-week-old L5 spinal nerve ligated male Sprague-Dawley rats. Mechanical allodynia was assessed using the von Frey test, where the 50% withdrawal threshold was evaluated for five drugs: pregabalin, duloxetine, venlafaxine, tramadol, and celecoxib. The single-drug experiment included 112 rats, where each drug was tested independently. Median effective doses (ED₅₀s) were determined. Combinations of pregabalin with each of the other four drugs were tested (n=84). The 50% withdrawal threshold in the von Frey test was evaluated. The ED₅₀ of each combination was determined experimentally. Isobolographic analyses were conducted to assess the synergistic potential of the drug combinations, excluding pregabalin-celecoxib, since the ED₅₀ of celecoxib could not be determined.

RESULTS

In the single-drug experiment, all drugs except celecoxib resulted in a dose-dependent increase in the 50% withdrawal threshold 2 h after administration, with a maximum possible effect ranging from 4.4% to 79.6%. Similarly, all pregabalin combinations demonstrated a dose-dependent increase in the 50% withdrawal threshold, with pregabalin-tramadol showing the greatest increment. Isobolographic analysis of this combination revealed synergistic effects. Specifically, the combination index was γ=0.4 (<1). Combinations of pregabalin with duloxetine and venlafaxine demonstrated additive (γ=0.9) and antagonistic effects (γ=2.0), respectively.

CONCLUSION

This study demonstrated that combination of pregabalin with tramadol has synergistic antiallodynic effects, while that with duloxetine has additive effects. Moreover, pregabalin combined with venlafaxine was potentially antagonistic. Pregabalin combined with tramadol may serve as a promising drug combination for the effective management of neuropathic pain.

Curcumin and metformin synergistically modulate peripheral and central immune mechanisms of pain

Metformin is a well-tolerated antidiabetic drug and has recently been repurposed for numerous diseases, including pain. However, a higher dose of metformin is required for effective analgesia, which can potentiate its dose-dependent gastrointestinal side effects. Curcumin is a natural polyphenol and has beneficial therapeutic effects on pain. Curcumin has been used as an analgesic adjuvant with several analgesic drugs, allowing synergistic antinociceptive effects. Nevertheless, whether curcumin can exert synergistic analgesia with metformin is still unknown. In the present study, the nature of curcumin-metformin anti-inflammatory interaction was evaluated in in vitro using lipopolysaccharide-induced RAW 264.7 macrophage and BV-2 microglia cells. In both macrophage and microglia, curcumin effectively potentiates the anti-inflammatory effects of metformin, indicating potential synergistic effects in both peripheral and central pathways of pain. The nature of the interaction between curcumin and metformin was further recapitulated using a mouse model of formalin-induced pain. Coadministration of curcumin and metformin at a 1:1 fixed ratio of their ED₅₀ doses significantly reduced the dose required to produce a 50% effect compared to the theoretically required dose in phase II of the formalin test with a combination index value of 0.24. Besides, the synergistic interaction does not appear to involve severe CNS side effects indicated by no motor alterations, no alterations in short-term and long-term locomotive behaviors, and the general well-being of mice. Our findings suggest that curcumin exerts synergistic anti-inflammation with metformin with no potential CNS adverse effects.

Recent advances in clinical trials targeting the kynurenine pathway

The kynurenine pathway (KP) is the major catabolic pathway for the essential amino acid tryptophan leading to the production of nicotinamide adenine dinucleotide. In inflammatory conditions, the activation of the KP leads to the production of several bioactive metabolites including kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, kynurenic acid and quinolinic acid. These metabolites can have redox and immune suppressive activity, be neurotoxic or neuroprotective. While the activity of the pathway is tightly regulated under normal physiological condition, it can be upregulated by immunological activation and inflammation. The dysregulation of the KP has been implicated in wide range of neurological diseases and psychiatric disorders. In this review, we discuss the mechanisms involved in KP-mediated neurotoxicity and immune suppression, and its role in diseases of our expertise including cancer, chronic pain and multiple sclerosis. We also provide updates on the clinical trials evaluating the efficacy of KP inhibitors and/or analogues in each respective disease.

ZNF382 controls mouse neuropathic pain via silencer-based epigenetic inhibition of Cxcl13 in DRG neurons

Nerve injury-induced changes of gene expression in dorsal root ganglion (DRG) are critical for neuropathic pain genesis. However, how these changes occur remains elusive. Here we report the down-regulation of zinc finger protein 382 (ZNF382) in injured DRG neurons after nerve injury. Rescuing this down-regulation attenuates nociceptive hypersensitivity. Conversely, mimicking this down-regulation produces neuropathic pain symptoms, which are alleviated by C-X-C motif chemokine 13 (CXCL13) knockdown or its receptor CXCR5 knockout. Mechanistically, an identified cis-acting silencer at distal upstream of the Cxcl13 promoter suppresses Cxcl13 transcription via binding to ZNF382. Blocking this binding or genetically deleting this silencer abolishes the ZNF382 suppression on Cxcl13 transcription and impairs ZNF382-induced antinociception. Moreover, ZNF382 down-regulation disrupts the repressive epigenetic complex containing histone deacetylase 1 and SET domain bifurcated 1 at the silencer-promoter loop, resulting in Cxcl13 transcriptional activation. Thus, ZNF382 down-regulation is required for neuropathic pain likely through silencer-based epigenetic disinhibition of CXCL13, a key neuropathic pain player, in DRG neurons.

Topical Capsaicin for the Treatment of Neuropathic Pain

BACKGROUND

Neuropathic pain (NP) is an egregious problem worldwide. Due to the side-effects of oral drugs, drugs delivered directly to the affected area of pain are preferred.

OBJECTIVE

Capsaicin, a chemical compound isolated from chili peppers, is used as an analgesic in topical ointments and dermal patches to alleviate pain. Objective of the study is to review the application and functionality of topical capsaicin in treatment of neuropathic pain.

DATA SOURCES

To systematically review capsaicin's functions on NP, we retrieved articles from the PubMed database published in the last ten years.

STUDY ELIGIBILITY CRITERIA

The inclusion criteria were capsaicin and the use of capsaicin for the treatment of NP; on the other hand, articles were excluded according to the mentioned criteria such as abstracts, articles written in any language other than English, incomplete articles, and conference papers.

PARTICIPANTS AND INTERVENTIONS

Out of 265 articles, 108 articles were selected after filtering through the inclusion and exclusion criteria. The data and knowledge currently existing for capsaicin treatment in NP are summarized.

RESULTS

This review indicates that capsaicin effectively improves NP treatment without affecting the motor and large nerve fibres involved in sensory function. Transient receptor potential channel vanilloid type 1 (TRPV1) is the capsaicin receptor expressed in central and peripheral terminals of a sensitive primary nerve cell. Conclusions and implications of key findings: Topical capsaicin has a sensible safety profile and is effective in reducing NP. Therefore, studies over the last decade suggest that capsaicin might be a potential drug for NP treatment.

Antinociceptive Effects of Sinomenine Combined With Ligustrazine or Paracetamol in Animal Models of Incisional and Inflammatory Pain

The management of postoperative and inflammatory pain has been a pressing challenge in clinical settings. Sinomenine (SN) is a morphinan derived alkaloid with remarkable analgesic properties in various kinds of pain models. The aim of the current study is to investigate if SN can enhance the effect of ligustrazine hydrochloride (LGZ) or paracetamol (PCM) in animal models of postoperative and inflammatory pain. And to determine if the combined therapeutic efficacies can be explained by pharmacokinetics changes. Pharmacological studies were performed using a rat model of incisional pain, and a mouse model of carrageenan induced inflammatory pain. Pharmacokinetic studies were performed using a microdialysis sampling and HPLC-MS/MS assay method to quantify SN, LGZ, and PCM levels in blood and extracellular fluid in brain. We found that SN plus LGZ or SN plus PCM produced marked synergistic analgesic effects. However, such synergy was subjected to pain modalities, and differed among pain models. Pharmacological discoveries could be partially linked to pharmacokinetic alterations in SN combinations. Though further evaluation is needed, our findings advocate the potential benefits of SN plus LGZ for postoperative pain management, and SN plus PCM for controlling inflammatory pain.

Central pain syndromes

The management of pain in persons with neurological injuries is challenging and complex. A holistic view and clinical approach are necessary when addressing pain in patients with neurological impairment because interpreting signs and symptoms and deciphering sources of pain is never a straightforward process. This problem is further magnified with the management of central pain syndromes. The best approach is to have a good understanding of the clinical characteristics commonly found in this patient population, in particular for patients with stroke, multiple sclerosis (MS), or spinal cord injury (SCI), as central pain manifests differently between these groups. This paper will focus on the history, clinical presentation, pathophysiology, assessment, and treatment of central pain in patients with these types of neurological conditions. In addition to being at risk for a decline in quality of life, patients with pain syndromes are also prone to adverse responses to treatments (e.g., opioid addiction). It is therefore important to methodically analyze the similarities and differences between patients with different pain syndromes.

Lack of analgesic effects of transcranial pulsed electromagnetic field stimulation in neuropathic pain patients: A randomized double-blind crossover trial

BACKGROUND

Neuromodulation is nowadays investigated as a promising method for pain relief. Research indicates that a single 30-minute stimulation with transcranial pulsed electromagnetic fields (tPEMF) can induce analgesic effects. However, it is unknown whether tPEMF can induce analgesia in neuropathic pain patients.

OBJECTIVE

To evaluate the effect of tPEMF on spontaneous pain and heat pain in neuropathic pain patients.

METHODS

This study had a randomized double-blind crossover design. Twenty neuropathic pain patients received 30-minutes of tPEMF and 30-minutes sham stimulation. Primary outcomes were pain intensity, pain aversion and heat pain. Secondary outcomes included affect, cognition, and motor function, to investigate safety, tolerability and putative working mechanisms of tPEMF. Outcomes were assessed before, during and after stimulation.

RESULTS

No differences in analgesic effects between tPEMF and sham stimulation were found for pain intensity, pain aversion or heat pain. No differences between tPEMF and sham stimulation were observed for affect, motor, and cognitive outcomes.

CONCLUSION

A single 30-minute tPEMF stimulation did not induce analgesic effects in neuropathic pain patients, compared to sham. Further study is needed to determine whether prolonged stimulation is necessary for analgesic effects.

Fn14 Participates in Neuropathic Pain Through NF-κB Pathway in Primary Sensory Neurons

Fibroblast growth factor-inducible-14 (Fn14), a receptor for tumor necrosis-like weak inducer of apoptosis, is expressed in the neurons of dorsal root ganglion (DRG). Its mRNA is increased in the injured DRG following peripheral nerve injury. Whether this increase contributes to neuropathic pain is unknown. We reported here that peripheral nerve injury caused by spinal nerve ligation (SNL) increased the expression of Fn14 at both protein and mRNA levels in the injured DRG. Blocking this increase attenuated the development of SNL-induced mechanical, thermal, and cold pain hypersensitivities. Conversely, mimicking this increase produced the increases in the levels of phosphorylated extracellular signal-regulated kinase ½ and glial fibrillary acidic protein in ipsilateral dorsal horn and the enhanced responses to mechanical, thermal, and cold stimuli in the absence of SNL. Mechanistically, the increased Fn14 activated the NF-κB pathway through promoting the translocation of p65 into the nucleus of the injured DRG neurons. Our findings suggest that Fn14 may be a potential target for the therapeutic treatment of peripheral neuropathic pain.

Sinomenine facilitates the efficacy of gabapentin or ligustrazine hydrochloride in animal models of neuropathic pain

Management of chronic pain is restricted by the lack of effective tools. This study evaluated the efficacies of sinomenine combined gabapentin or ligustrazine hydrochloride in treating peripheral and central chronic neuropathic pain. The study was conducted in mice with photochemically induced sciatic nerve injury, and in rats with photochemically induced spinal cord injury. For assessing the effectiveness of combined therapy, sinomenine, gabapentin or ligustrazine hydrochloride was injected intraperitoneally (i.p.), and pain behavioral tests were performed. At sub-effective dosages, pre-administration of sinomenine (for 60 min) plus gabapentin or ligustrazine hydrochloride, generated significant anti-allodynic effects in mice or rats with peripheral or central neuropathic pain. However, these effects were abolished when gabapentin or ligustrazine hydrochloride was pre-administered, and then sinomenine was given 60 min later. The combined efficacies of sinomenine and gabapentin or ligustrazine hydrochloride, cannot be blocked or reversed by the naloxone, suggesting the underlying mechanism is not mediated by opioid receptors. Moreover, following repeated treatments, sinomenine and gabapentin combination increased the baseline mechanical threshold, while generating prolonged analgesia, without introducing notable side effects. Sinomenine can enhance the efficacy of gabapentin or ligustrazine hydrochloride in rodent models of peripheral or central neuropathic pain, without introducing tolerance or other notable side effects. Findings of current study suggest that combing sinomenine and gabapentin or ligustrazine hydrochloride could be highly beneficial in neuropathic pain therapies.

Spinal α2 -adrenoceptors and neuropathic pain modulation; therapeutic target

Neuropathic pain can arise from disease or damage to the nervous system. The most common symptoms of neuropathic pain include spontaneous pain, allodynia, and hyperalgesia. There is still limited knowledge about the factors that initiate and maintain neuropathic pain. However, ample evidence has proved the antinociceptive role of spinal α-adrenoceptors following nerve injury. It is well-documented that noradrenergic descending pathways from supraspinal loci exert an inhibitory influence on the spinal cord nociceptive neurons, mostly through the activation of spinal α₂ -adrenoceptors. This, in turn, suppresses transmission of pain input and the hyperexcitability of spinal dorsal horn neurons. There is considerable evidence demonstrating that spinal application of α₂ -adrenoceptor agonists leads to analgesic effects in animal models of neuropathic pain. Today, despite the recent rapid development of neuroscience and drug discovery, effective drugs with clear basic mechanisms have remained a mystery. Here, we give an overview of the cellular mechanisms through which brainstem adrenergic descending inhibitory processing can alter spinal pain transmission to the higher centres, and how these pathways change in neuropathic pain conditions focusing on the role of spinal α₂ -adrenoceptors in the spinal dorsal horn. We then suggest that α₂ -adrenoceptor agonist may be useful to treat neuropathic pain. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

The importance of chemokines in neuropathic pain development and opioid analgesic potency

The treatment of neuropathic pain resulting from nervous system malfunction remains a challenging problem for doctors and scientists. The lower effectiveness of conventionally used analgesics in neuropathic pain is associated with complex and not fully understood mechanisms of its development. Undoubtedly, interactions between immune and nervous system are crucial for maintenance of painful neuropathy. Nerve injury induces glial cell activation and thus enhances the production of numerous pronociceptive factors by these cells, including interleukins and chemokines. Increased release of those factors reduces the analgesic efficacy of opioids, which is significantly lower in neuropathic pain than in other painful conditions. This review discusses the role of chemokines from all four subfamilies as essential mediators of neuron-glia interactions occurring under neuropathic pain conditions. Based on available data, we analyse the influence of chemokines on opioid properties. Finally, we identify new direct and indirect pharmacological targets whose modulation may result in effective therapy of neuropathic pain, possibly in combination with opioids.

Acute in vivo effect of valproic acid on the GABAergic system in rat brain: A [11C]Ro15-4513 microPET study

γ-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the nervous system acting mainly through GABA_A receptors. In the presence of high levels of GABA, an allosteric shift in the GABA_A receptors can change the affinity of benzodiazepine (BZD) ligands. Valproic acid (VPA) is an anticonvulsant that enhances the level of endogenous GABA in the brain. The BZD ligand, Ro15-4513 has a high affinity for GABA_A receptors containing the α₅ subunit and can be used to investigate the GABA shift in the brains of living rats after VPA exposure. Seven Wistar rats were scanned using a Mediso NanoScan PET/MRI. A baseline 90-min dynamic [¹¹C]Ro15-4513 PET scan was acquired prior to an intravenous injection of 50 mg/kg VPA, and was followed by a second [¹¹C]Ro15-4513 PET scan. Standardized uptake values were obtained for regions of high GABA binding, including the hippocampus and amygdala, and low GABA binding such as the cerebellum. We showed a significant increase in [¹¹C]Ro15-4513 uptake in hippocampus and amygdala, but no significant differences in cerebellar uptake, after acute VPA exposure. In contrast to several in vitro studies, we demonstrated a positive allosteric change in the GABA_A receptors after pharmacologically enhanced GABA levels resulting in enhanced Ro15-4513 uptake. Knowledge of how subtypes of the GABA_A receptors react will provide us with information useful to fine-tune pharmacological interventions and design receptor subtype specific drugs.

Role of MicroRNA-143 in Nerve Injury-Induced Upregulation of Dnmt3a Expression in Primary Sensory Neurons

Peripheral nerve injury increased the expression of the DNA methyltransferase 3A (Dnmt3a) mRNA and its encoding Dnmt3a protein in injured dorsal root ganglia (DRG). This increase is considered as an endogenous instigator in neuropathic pain genesis through epigenetic silencing of pain-associated genes (such as Oprm1) in injured DRG. However, how DRG DNMT3a is increased following peripheral nerve injury is still elusive. We reported here that peripheral nerve injury caused by the fifth spinal nerve ligation (SNL) downregulated microRNA (miR)-143 expression in injured DRG. This downregulation was required for SNL-induced DRG Dnmt3a increase as rescuing miR-143 downregulation through microinjection of miR-143 mimics into injured DRG blocked the SNL-induced increase in Dnmt3a and restored the SNL-induced decreases in Oprm1 mRNA and its encoding mu opioid receptor (MOR) in injured DRG, impaired spinal cord central sensitization and neuropathic pain, and improved morphine analgesic effects following SNL. Mimicking SNL-induced DRG miR-143 downregulation through DRG microinjection of miR143 inhibitors in naive rats increased the expression of Dnmt3a and reduced the expression of Oprm1 mRNA and MOR in injected DRG and produced neuropathic pain-like symptoms. These findings suggest that miR-143 is a negative regulator in Dnmt3a expression in the DRG under neuropathic pain conditions and may be a potential target for therapeutic management of neuropathic pain.

Nortriptyline Enhances Morphine-Conditioned Place Preference in Neuropathic Rats: Role of the Central Noradrenergic System

BACKGROUND

Combination drug therapy is commonly used to treat chronic pain conditions such as neuropathic pain, and antidepressant is often used together with opioid analgesics. While rewarding is an intrinsic property of opioid analgesics, it is unknown whether the use of an antidepressant would influence opioid reward, which may contribute to opioid addiction. In this study, we examined whether nortriptyline (a tricyclic antidepressant and a first-line medication for neuropathic pain) would enhance the morphine rewarding property in both naive and chronic constriction sciatic nerve injury (CCI) rats.

METHODS

The rewarding effect of these drugs was assessed using conditioned place preference (CPP). The real-time polymerase chain reaction, western blot, and enzyme-linked immunosorbent assay analysis were used to investigate the function of central noradrenergic system.

RESULTS

In naive rats, coadministration of nortriptyline with morphine did not change the acquisition of morphine-induced CPP. However, nortriptyline enhanced the acquisition, delayed the extinction, and augmented the reinstatement of morphine-induced CPP in CCI rats. In CCI rats treated with both nortriptyline and morphine, the expression of α2A-adrenergic receptors, norepinephrine transporter, and tyrosine hydroxylase was markedly decreased in the locus coeruleus, whereas the norepinephrine concentration in the nucleus accumbens was remarkably increased.

CONCLUSIONS

These results demonstrate that nortriptyline enhanced morphine reward when both drugs were used to treat neuropathic pain in rats and that this behavioral phenotype is likely to be mediated by upregulation of the central noradrenergic system. These findings may have implications in opioid therapy commonly used for chronic pain management.

The transcription factor C/EBPβ in the dorsal root ganglion contributes to peripheral nerve trauma-induced nociceptive hypersensitivity

Changes in gene transcription in the dorsal root ganglion (DRG) after nerve trauma contribute to the genesis of neuropathic pain. We report that peripheral nerve trauma caused by chronic constriction injury (CCI) increased the abundance of the transcription factor C/EBPβ (CCAAT/enhancer binding protein β) in the DRG. Blocking this increase mitigated the development and maintenance of CCI-induced mechanical, thermal, and cold pain hypersensitivities without affecting basal responses to acute pain and locomotor activity. Conversely, mimicking this increase produced hypersensitivity to mechanical, thermal, or cold pain. In the ipsilateral DRG, C/EBPβ promoted a decrease in the abundance of the voltage-gated potassium channel subunit Kv1.2 and μ opioid receptor (MOR) at the mRNA and protein levels, which would be predicted to increase excitability in the ipsilateral DRG neurons and reduce the efficacy of morphine analgesia. These effects required C/EPBβ-mediated transcriptional activation of Ehmt2 (euchromatic histone-lysine N-methyltransferase 2), which encodes G9a, an epigenetic silencer of the genes encoding Kv1.2 and MOR. Blocking the increase in C/EBPβ in the DRG improved morphine analgesia after CCI. These results suggest that C/EBPβ is an endogenous initiator of neuropathic pain and could be a potential target for the prevention and treatment of this disorder.

Lack of Analgesic Synergy of the Cholecystokinin Receptor Antagonist Proglumide and Spinal Cord Stimulation for the Treatment of Neuropathic Pain in Rats

OBJECTIVE

Neuropathic pain is difficult to manage and treat. Spinal cord stimulation (SCS) has become an established procedure for treating chronic neuropathic pain that is refractory to pharmacological therapy. In order to achieve better analgesia, a number of studies have evaluated the effectiveness of combining drug therapy with SCS. Cholecystokinin antagonists, such as proglumide, enhance the analgesic efficacy of endogenous opioids in animal models of pain. We previously reported that both systemic and spinal administration of proglumide enhances analgesia produced by both low- and high-frequency transcutaneous electrical nerve stimulation (TENS). Since SCS produces analgesia through endogenous opioids, we hypothesized that the analgesic effect of SCS would be enhanced through co-administration with proglumide in animals with neuropathic pain.

MATERIALS AND METHODS

Male Sprague-Dawley rats (n = 40) with spared nerve injury were given proglumide (20 mg/kg, i.p.) or saline prior to treatment with SCS (sham, 4 Hz, and 60 Hz). Mechanical withdrawal thresholds of the paw were measured before and after induction of nerve injury, and after SCS. Physical activity levels were measured after SCS.

RESULTS

Both proglumide and SCS when given independently significantly increased withdrawal thresholds two weeks after nerve injury. However, there was no additional effect of combining proglumide and SCS on mechanical withdrawal thresholds or activity levels in animals with nerve injury.

DISCUSSION AND CONCLUSIONS

Proglumide may be a candidate for achieving analgesia for patients with refractory neuropathic pain conditions, but does not enhance analgesia produced by SCS.

G9a participates in nerve injury-induced Kcna2 downregulation in primary sensory neurons

Nerve injury-induced downregulation of voltage-gated potassium channel subunit Kcna2 in the dorsal root ganglion (DRG) is critical for DRG neuronal excitability and neuropathic pain genesis. However, how nerve injury causes this downregulation is still elusive. Euchromatic histone-lysine N-methyltransferase 2, also known as G9a, methylates histone H3 on lysine residue 9 to predominantly produce a dynamic histone dimethylation, resulting in condensed chromatin and gene transcriptional repression. We showed here that blocking nerve injury-induced increase in G9a rescued Kcna2 mRNA and protein expression in the axotomized DRG and attenuated the development of nerve injury-induced pain hypersensitivity. Mimicking this increase decreased Kcna2 mRNA and protein expression, reduced Kv current, and increased excitability in the DRG neurons and led to spinal cord central sensitization and neuropathic pain-like symptoms. G9a mRNA is co-localized with Kcna2 mRNA in the DRG neurons. These findings indicate that G9a contributes to neuropathic pain development through epigenetic silencing of Kcna2 in the axotomized DRG.

Gabapentin for post-operative pain management - a systematic review with meta-analyses and trial sequential analyses

BACKGROUND

Perioperative pain treatment often consist of combinations of non-opioid and opioid analgesics, 'multimodal analgesia', in which gabapentin is currently used. The aim was to document beneficial and harmful effects of perioperative gabapentin treatment.

METHODS

Randomized clinical trials comparing gabapentin vs. placebo or active placebo in adult surgical patients receiving gabapentin perioperatively were included. This review was conducted using Cochrane standards, trial sequential analysis (TSA), and Grading of Recommendations Assessment, Development, and Evaluation (GRADE). The primary outcomes were 24-h opioid consumption and incidence of serious adverse events (SAE).

RESULTS

One hundred and thirty-two trials with 9498 patients were included. Thirteen trials with low risk of bias reported a reduction in 24-h opioid consumption of 3.1 mg [0.5, 5.6] [corrected]. In the analysis of gabapentin as add-on analgesic to another non-opioid analgesic regimen found a mean reduction in 24-h morphine consumption of 1.2 mg [-0.3, 2.6; TSA-adjusted CI: -0.3, 2.6] in trials with low risk of bias. [corrected]. Nine trials with low risk of bias reported a risk ratio of SAEs of 1.61 [0.91; 2.86; TSA-adjusted CI: 0.57, 4.57].

CONCLUSION

Based on GRADE assessment of the primary outcomes in trials with low risk of bias, the results are low or very low quality of evidence due to imprecision, inconsistency, and in some outcomes indirectness. Firm evidence for use of gabapentin is lacking as clinically relevant beneficial effect of gabapentin may be absent and harm is imminent, especially when added to multimodal analgesia.

Transdermal Buprenorphine Relieves Neuropathic Pain: A Randomized, Double-Blind, Parallel-Group, Placebo-Controlled Trial in Diabetic Peripheral Neuropathic Pain

OBJECTIVE

To evaluate the efficacy and safety of transdermal buprenorphine in patients with diabetic peripheral neuropathic pain (DPNP).

RESEARCH DESIGN AND METHODS

This multicenter, randomized, double-blind, placebo-controlled, parallel-group trial enrolled patients with type 1 or type 2 diabetes and stable glycemic control who had been experiencing moderate to severe DPNP for at least 6 months on maximal tolerated conventional therapy. Patients were randomly assigned to receive buprenorphine (5 μg/h) or placebo patches. The dose was titrated to effect to a maximum of 40 μg/h. Paracetamol was available as rescue analgesia. The severity of pain and other symptoms of DPNP were assessed daily in a patient diary and at clinic visits.

RESULTS

One hundred eight-six patients were enrolled, with 93 randomized to either buprenorphine or placebo. A high proportion of patients did not complete the study (buprenorphine 37 of 93, placebo 24 of 93). The main reason for premature withdrawal in the buprenorphine group was adverse events commonly due to untreated nausea and/or vomiting. Among the per-protocol population, more patients in the buprenorphine group (86.3%) experienced a 30% reduction in average versus baseline pain at week 12 than those in the placebo group (56.6%, P < 0.001). A nonsignificant trend favored the buprenorphine group within the intention-to-treat analysis of the same end point (51.7% vs. 41.3%, P = 0.175).

CONCLUSIONS

Transdermal buprenorphine, when tolerated, is an effective therapy for DPNP and provides another option to manage this challenging painful condition. Nausea and constipation need to be managed proactively to optimize treatment outcomes.

Opioid analgesics increase incidence of somnolence and dizziness as adverse effects of pregabalin: a retrospective study

BACKGROUND

Pregabalin, a gabapentinoid, is an adjuvant analgesic for treatment of neuropathic pain, but it has serious adverse effects such as somnolence and dizziness, particularly in elderly patients. Although decreased renal function is considered to the contributing factor for high frequency of these adverse effects in elder patients, only a few systematic clinical investigations, especially for hospitalized patients, have been performed on factors that might affect the incidence of its adverse effects. In this study, we performed a retrospective study on the effect of concomitant drugs on induction of somnolence and dizziness as adverse effects of pregabalin in hospitalized patients.

METHODS

The subjects were all pregabalin-administered patients in Shiga University of Medical Science Hospital from September 2010 to September 2012, and the subject number was 195. Multivariate logistic regression analysis was performed to determine predictors of the adverse effects, creatinine clearance, duration of pregabalin therapy, initial and maintenance doses of pregabalin, and concomitant drugs, including hypoglycemic drugs, anti-hypertensive ones, non-steroidal anti-inflammatory ones, opioids and central nervous system depressants, being used as independent variables.

RESULTS

The median initial doses of pregabalin in each renal function group were the same with the case of the defined dose. Although renal function is a well-known factor for prediction of development of adverse effects of pregabalin, we did not detect significant contribution of it. Alternatively, it was demonstrated that concomitant administration of opioids was the significant factor of the incidence of somnolence and dizziness. The first onset date of the adverse effects was frequently detected in the early days of the pregabalin therapy.

CONCLUSIONS

The fine tuning of pregabalin dosage schedule based on the renal function appeared to be critical for prevention of development of its adverse effects. Adverse effects tended to develop in the initial phase of pregabalin therapy. Concomitant administration of opioids with pregabalin has the potential to increase the incidence of adverse effects, and thus much more careful attention has to be paid especially in those situations.

Effect of combination therapy consisting of enalapril, α-lipoic acid, and menhaden oil on diabetic neuropathy in a high fat/low dose streptozotocin treated rat

We have previously demonstrated that treating diabetic rats with enalapril, an angiotensin converting enzyme (ACE) inhibitor, α-lipoic acid, an antioxidant, or menhaden oil, a natural source of omega-3 fatty acids can partially improve diabetic peripheral neuropathy. In this study we sought to determine the efficacy of combining these three treatments on vascular and neural complications in a high fat fed low dose streptozotocin treated rat, a model of type 2 diabetes. Rats were fed a high fat diet for 8 weeks followed by a 30 mg/kg dose of streptozotocin. Eight weeks after the onset of hyperglycemia diabetic rats were treated with a combination of enalapril, α-lipoic acid and menhaden oil. Diabetic rats not receiving treatment were continued on the high fat diet. Glucose clearance was impaired in diabetic rats and significantly improved with treatment. Diabetes caused steatosis, elevated serum lipid levels, slowing of motor and sensory nerve conduction, thermal hypoalgesia, reduction in intraepidermal nerve fiber profiles, decrease in cornea sub-basal nerve fiber length and corneal sensitivity and impairment in vascular relaxation to acetylcholine and calcitonin gene-related peptide in epineurial arterioles of the sciatic nerve. Treating diabetic rats with the combination of enalapril, α-lipoic acid and menhaden oil reversed all these deficits to near control levels except for motor nerve conduction velocity which was also significantly improved compared to diabetic rats but remained significantly decreased compared to control rats. These studies suggest that a combination therapeutic approach may be most effective for treating vascular and neural complications of type 2 diabetes.

Correspondence between neurophysiological and clinical measurements of chemotherapy-induced peripheral neuropathy: secondary analysis of data from the CI-PeriNomS study

Chemotherapy-induced peripheral neuropathy (CIPN) lacks standardized clinical measurement. The objective of the current secondary analysis was to examine data from the CIPN Outcomes Standardization (CI-PeriNomS) study for associations between clinical examinations and neurophysiological abnormalities. Logistic regression estimated the strength of associations of vibration, pin, and monofilament examinations with lower limb sensory and motor amplitudes. Examinations were classified as normal (0), moderately abnormal (1), or severely abnormal (2). Among 218 participants, those with class 1 upper extremity (UE) and classes 1 or 2 lower extremity (LE) monofilament abnormality were 2.79 (95% confidence interval [CI]: 1.28-6.07), 3.49 (95%CI: 1.61-7.55), and 4.42 (95%CI: 1.35-14.46) times more likely to have abnormal sural nerve amplitudes, respectively, compared to individuals with normal examinations. Likewise, those with class 2 UE and classes 1 or 2 LE vibration abnormality were 8.65 (95%CI: 1.81-41.42), 2.54 (95%CI: 1.19-5.41), and 7.47 (95%CI: 2.49-22.40) times more likely to have abnormal sural nerve amplitudes, respectively, compared to participants with normal examinations. Abnormalities in vibration and monofilament examinations are associated with abnormal sural nerve amplitudes and are useful in identifying CIPN.

Dorsal root ganglion myeloid zinc finger protein 1 contributes to neuropathic pain after peripheral nerve trauma

Peripheral nerve injury-induced changes in gene transcription and translation in primary sensory neurons of the dorsal root ganglion (DRG) are considered to contribute to neuropathic pain genesis. Transcription factors control gene expression. Peripheral nerve injury increases the expression of myeloid zinc finger protein 1 (MZF1), a transcription factor, and promotes its binding to the voltage-gated potassium 1.2 (Kv1.2) antisense (AS) RNA gene in the injured DRG. However, whether DRG MZF1 participates in neuropathic pain is still unknown. Here, we report that blocking the nerve injury-induced increase of DRG MZF1 through microinjection of MZF1 siRNA into the injured DRG attenuated the initiation and maintenance of mechanical, cold, and thermal pain hypersensitivities in rats with chronic constriction injury (CCI) of the sciatic nerve, without affecting locomotor functions and basal responses to acute mechanical, heat, and cold stimuli. Mimicking the nerve injury-induced increase of DRG MZF1 through microinjection of recombinant adeno-associated virus 5 expressing full-length MZF1 into the DRG produced significant mechanical, cold, and thermal pain hypersensitivities in naive rats. Mechanistically, MZF1 participated in CCI-induced reductions in Kv1.2 mRNA and protein and total Kv current and the CCI-induced increase in neuronal excitability through MZF1-triggered Kv1.2 AS RNA expression in the injured DRG neurons. MZF1 is likely an endogenous trigger of neuropathic pain and might serve as a potential target for preventing and treating this disorder.

Suppression of network activity in dorsal horn by gabapentin permeation of TRPV1 channels: implications for drug access to cytoplasmic targets

The effectiveness of gabapentin (GBP) in the treatment of neuropathic pain depends on access to the α2δ-1 accessory subunit of voltage-gated Ca(2+) channels. Access may be limited by its rate of entry via the neuronal system L-neutral amino acid transporter. The open pore of capsaicin-activated TRPV1 channel admits organic molecules such as local anesthetics and we calculated that GBP entry via this route would be 500× more rapid than via the transporter. Capsaicin should therefore increase GBP effectiveness. We used a quaternary GBP derivative (Q-GBP) as sole charge carrier in whole-cell recording experiments on rat dorsal root ganglion (DRG) neurons. Under these conditions, capsaicin produced a capsazepine-sensitive inward current thereby confirming Q-GBP permeation of TRPV1 channels. We have previously established that 5-6 days exposure to 100 μM GBP decreases excitability of dorsal horn neurons whereas 10 μM is ineffective. Excitability was monitored using confocal Ca(2+) imaging of rat spinal cord slices in organotypic culture. GBP effectiveness was augmented by transient exposures of cultures to capsaicin and robust suppression of excitability was seen with 10 μM GBP. Experiments with an inhibitor of the neutral amino acid transporter, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH 300 μM), showed the actions of GBP seen in the presence of capsaicin were independent of its entry by this route. Capsaicin potentiation of GBP depression of dorsal horn activity may therefore reflect drug permeation of TRPV1 channels. Agonist activation of TRP channels may provide a means for improving drug access to cytoplasmic targets in selective neuronal populations defined on the basis of type of TRP channel expressed.

Pharmacological management of neuropathic pain in patients with vestibular schwannomas: Experience of the Atlantic Lateral Skull Base Clinic

Neuropathic pain is chronic pain generated by disorders of the peripheral and central nervous system, including skull base tumours. A skull base tumour can be any type of tumour that forms in the skull base, and this includes vestibular schwannomas which arise from the sheath of the inner ear vestibulocochlear nerve (eighth cranial nerve). Growth of the tumour, surgical resection, and/or stereotactic radiotherapy may result in compression and/or irritation of the fifth cranial nerve (trigeminal nerve) resulting in facial pain and/or numbness. Non-trigeminal afferent input may contribute to the wide constellation of symptoms seen in orofacial pain patients. The purpose of this report was to develop a decision tool to guide the recognition and treatment of neuropathic pain in this specialized population. Recommendations for treatment are based on evidence presented in Canadian and international neuropathic treatment guidelines. Algorithms are included for assessment and treatment of adult patients with agents that are recognized to have analgesic efficacy within the broad context of neuropathic pain.

Effectiveness and Safety of Tapentadol Prolonged Release (PR) Versus a Combination of Tapentadol PR and Pregabalin for the Management of Severe, Chronic Low Back Pain With a Neuropathic Component: A Randomized, Double-blind, Phase 3b Study

OBJECTIVE

To evaluate the effectiveness and tolerability of tapentadol PR monotherapy versus tapentadol PR/pregabalin combination therapy for severe, chronic low back pain with a neuropathic component.

METHODS

Eligible patients had painDETECT "unclear" or "positive" ratings and average pain intensity ≥ 6 (11-point NRS-3 [average 3-day pain intensity]) at baseline. Patients were titrated to tapentadol PR 300 mg/day over 3 weeks. Patients with ≥ 1-point decrease in pain intensity and average pain intensity ≥ 4 were randomized to tapentadol PR (500 mg/day) or tapentadol PR (300 mg/day)/pregabalin (300 mg/day) during an 8-week comparative period.

RESULTS

In the per-protocol population (n = 288), the effectiveness of tapentadol PR was clinically and statistically comparable to tapentadol PR/pregabalin based on the change in pain intensity from randomization to final evaluation (LOCF; LSMD [95% CI], -0.066 [-0.57, 0.43]; P < 0.0001 for noninferiority). Neuropathic pain and quality-of-life measures improved significantly in both groups. Tolerability was good in both groups, in line with prior trials in the high dose range of 500 mg/day for tapentadol PR monotherapy, and favorable compared with historical combination trials of strong opioids and anticonvulsants for combination therapy. The incidence of the composite of dizziness and/or somnolence was significantly lower with tapentadol PR (16.9%) than tapentadol PR/pregabalin (27.0%; P = 0.0302).

CONCLUSIONS

Tapentadol PR 500 mg is associated with comparable improvements in pain intensity and quality-of-life measures to tapentadol PR 300 mg/pregabalin 300 mg, with improved central nervous system tolerability, suggesting that tapentadol PR monotherapy may offer a favorable treatment option for severe low back pain with a neuropathic component.

Contributions of peripheral, spinal, and supraspinal actions to analgesia

Pain signaling involves several main compartments that can be considered as potential sites for analgesic drug actions. When drugs are given systemically, they can act at spinal, supraspinal and peripheral sites, and several methods have been developed for identifying where they act. These include (1) localized delivery of drugs to specific sites (via intracerebral, intrathecal, and intraplantar injections), (2) systemic delivery of drugs with localized delivery of antagonists for the receptor on which the drug acts or for a system recruited by the drug, (3) use of peripherally restricted analogs, and (4) use of conditional knockout technology to selectively deplete receptors on nociceptors. Delivery of drugs simultaneously to several sites (spinal/supraspinal, peripheral/spinal, and peripheral/supraspinal) reveals "self-synergy" between sites for some agents. Knowledge of peripheral contributions to drug actions is important because of the potential to develop peripherally restricted analgesics (with a diminished side effect profile due to not entering the central nervous system), the potential to deliver drugs peripherally (e.g. topically) to act on sensory nerve endings and adjacent tissue (with a diminished side effect profile due to limited systemic absorption), and the potential to use combinations of topical and oral drug regimens to obtain improved pain relief (without increasing the side effect burden). This review considers methods used for compartmental analysis, and results of such site analysis for several major classes of analgesic drugs that are in current use.

The mu-opioid receptor agonist/noradrenaline reuptake inhibition (MOR-NRI) concept in analgesia: the case of tapentadol

Tapentadol is a novel, centrally-acting analgesic drug, with an analgesic efficacy comparable to that of strong opioids such as oxycodone and morphine. Its high efficacy has been demonstrated in a range of animal models of acute and chronic, nociceptive, inflammatory, and neuropathic pain as well as in clinical studies with moderate to severe pain arising from a number of different etiologies. At the same time, a favorable gastrointestinal tolerability has been demonstrated in rodents and humans, and advantages over morphine regarding tolerance development and physical dependence were shown in animal studies. Furthermore, a low level of abuse and diversion is beginning to emerge from first post-marketing data. Tapentadol acts as a μ-opioid receptor (MOR) agonist and noradrenaline reuptake inhibitor (NRI). Both mechanisms of action have been shown to contribute to the analgesic activity of tapentadol and to produce analgesia in a synergistic manner, such that relatively moderate activity at the two target sites (MOR and noradrenaline reuptake transporter) is sufficient to produce strong analgesic effects. It has been suggested that tapentadol is the first representative of a proposed new class of analgesics, MOR-NRI. This review presents the evidence that has led to this suggestion, and outlines how the pharmacology of tapentadol can explain its broad analgesic activity profile and high analgesic potency as well as its favorable tolerability.

Update on neuropathic pain treatment: ion channel blockers and gabapentinoids

Neuropathic pain is a debilitating chronic pain condition, which remains difficult to treat. The current mainstays of treatment include physical therapy, interventional procedures and medications. Among medications, ion channel blockers and gabapentinoids are the 2 classes of drugs commonly used to treat neuropathic pain. It has been suggested that these medications may be useful to treat a variety of neuropathic pain conditions. This article provides several updates on the utility of both ion channel blockers and gabapentinoids for the treatment of neuropathic pain.