Oral pharmacotherapeutics for the management of peripheral neuropathic pain conditions - a review of clinical trials

Interventions for treating pain and disability in adults with complex regional pain syndrome- an overview of systematic reviews

BACKGROUND

Complex regional pain syndrome (CRPS) is a chronic pain condition that usually occurs in a limb following trauma or surgery. It is characterised by persisting pain that is disproportionate in magnitude or duration to the typical course of pain after similar injury. There is currently no consensus regarding the optimal management of CRPS, although a broad range of interventions have been described and are commonly used. This is the first update of the original Cochrane review published in Issue 4, 2013.

OBJECTIVES

To summarise the evidence from Cochrane and non-Cochrane systematic reviews of the efficacy, effectiveness, and safety of any intervention used to reduce pain, disability, or both, in adults with CRPS.

METHODS

We identified Cochrane reviews and non-Cochrane reviews through a systematic search of Ovid MEDLINE, Ovid Embase, Cochrane Database of Systematic Reviews, CINAHL, PEDro, LILACS and Epistemonikos from inception to October 2022, with no language restrictions. We included systematic reviews of randomised controlled trials that included adults (≥18 years) diagnosed with CRPS, using any diagnostic criteria.  Two overview authors independently assessed eligibility, extracted data, and assessed the quality of the reviews and certainty of the evidence using the AMSTAR 2 and GRADE tools respectively. We extracted data for the primary outcomes pain, disability and adverse events, and the secondary outcomes quality of life, emotional well-being, and participants' ratings of satisfaction or improvement with treatment.  MAIN RESULTS: We included six Cochrane and 13 non-Cochrane systematic reviews in the previous version of this overview and five Cochrane and 12 non-Cochrane reviews in the current version. Using the AMSTAR 2 tool, we judged Cochrane reviews to have higher methodological quality than non-Cochrane reviews. The studies in the included reviews were typically small and mostly at high risk of bias or of low methodological quality. We found no high-certainty evidence for any comparison.  There was low-certainty evidence that bisphosphonates may reduce pain intensity post-intervention (standardised mean difference (SMD) -2.6, 95% confidence interval (CI) -1.8 to -3.4, P = 0.001; I2 = 81%; 4 trials, n = 181) and moderate-certainty evidence that they are probably associated with increased adverse events of any nature (risk ratio (RR) 2.10, 95% CI 1.27 to 3.47; number needed to treat for an additional harmful outcome (NNTH) 4.6, 95% CI 2.4 to 168.0; 4 trials, n = 181).  There was moderate-certainty evidence that lidocaine local anaesthetic sympathetic blockade probably does not reduce pain intensity compared with placebo, and low-certainty evidence that it may not reduce pain intensity compared with ultrasound of the stellate ganglion. No effect size was reported for either comparison. There was low-certainty evidence that topical dimethyl sulfoxide may not reduce pain intensity compared with oral N-acetylcysteine, but no effect size was reported. There was low-certainty evidence that continuous bupivacaine brachial plexus block may reduce pain intensity compared with continuous bupivacaine stellate ganglion block, but no effect size was reported. For a wide range of other commonly used interventions, the certainty in the evidence was very low and provides insufficient evidence to either support or refute their use. Comparisons with low- and very low-certainty evidence should be treated with substantial caution. We did not identify any RCT evidence for routinely used pharmacological interventions for CRPS such as tricyclic antidepressants or opioids.

AUTHORS' CONCLUSIONS

Despite a considerable increase in included evidence compared with the previous version of this overview, we identified no high-certainty evidence for the effectiveness of any therapy for CRPS. Until larger, high-quality trials are undertaken, formulating an evidence-based approach to managing CRPS will remain difficult. Current non-Cochrane systematic reviews of interventions for CRPS are of low methodological quality and should not be relied upon to provide an accurate and comprehensive summary of the evidence.

Evaluation of a polymer-coated nanoparticle cream formulation of resiniferatoxin for the treatment of painful diabetic peripheral neuropathy

ABSTRACT

Painful diabetic peripheral neuropathy (PDPN) is one of the major complications of diabetes. Currently, centrally acting drugs and topical analgesics are used for treating PDPN. These drugs have adverse effects; some are ineffective, and treatment with opioids is associated with use dependence and addiction. Recent research indicates that transient receptor potential vanilloid 1 (TRPV1) expressed in the peripheral sensory nerve terminals is an emerging target to treat pain associated with PDPN. Block of TRPV1 ion channel with specific antagonists, although effective as an analgesic, induced hyperthermia in clinical trials. However, TRPV1 agonists are useful to treat pain by virtue of their ability to cause Ca 2+ influx and subsequently leading to nerve terminal desensitization. Here, we report the effectiveness of an ultrapotent TRPV1 agonist, resiniferatoxin (RTX) nanoparticle, in a topical formulation (RTX-cream; RESINIZIN) that alleviates pain associated with DPN in animal models of diabetes. Resiniferatoxin causes nerve terminal depolarization block in the short term, which prevents pain during application and leading to nerve terminal desensitization/depletion in the long term resulting in long-lasting pain relief. Application of RTX cream to the hind limbs suppresses thermal hyperalgesia in streptozotocin-induced diabetic rats and mini pigs without any adverse effects as compared with capsaicin at therapeutic doses, which induces intense pain during application. Resiniferatoxin cream also decreases the expression of TRPV1 in the peripheral nerve endings and suppresses TRPV1-mediated calcitonin gene-related peptide release in the skin samples of diabetic rats and mini pigs. Our preclinical data confirm that RTX topical formulation is an effective treatment option for PDPN.

Overcoming clinical challenges of refractory neuropathic pain

INTRODUCTION

: Refractory neuropathic pain (ReNP), and its definition, is widely disputed amongst clinicians due in part to unclear categorical diagnosing guidelines, overall time duration of neuropathic pain, and the exhaustiveness of treatment options. Usually ReNP is defined as chronic, intractable, and unresponsive neuropathic pain that have otherwise been untreatable.

AREAS COVERED

: In this narrative review, we discuss and summarize the effectiveness of prospective ReNP research conducted over the past 10 years. This research looks at pharmacological and interventional therapies in clinical trial settings. The pharmacological therapies discussed include the use of adjuvant treatments to improve the safety and efficacy of conventional approaches. Different modalities of administration, such as injection therapy and intrathecal drug delivery systems, provide targeted drug delivery. Interventional therapies such as neuromodulation, pulse radiofrequency, and nerve lesioning are more invasive, however, they are increasingly utilized in the field, as reflected in ongoing clinical trials.

EXPERT OPINION

: Based on the current data from RCTs and systematic reviews, it is clear that single drug therapy cannot be effective and has significant limitations. Transitioning to interventional modalities that showed more promising results sooner rather than later may be even more cost-efficient than attempting different conservative treatments with a high failure rate.

Cilostazol Ameliorates Peripheral Neuropathic Pain in Streptozotocin-Induced Type I Diabetic Rats

Background: Cilostazol is an antiplatelet agent with vasodilating, endothelial function restoration, and anti-inflammatory effects. This study aims to investigate the efficacy of oral cilostazol for preventing the development of diabetic peripheral neuropathy (DPN).

Materials and Methods: Ninety adult male Sprague-Dawley rats were divided into five groups: 1) naïve (control); 2) diabetic (DM); 3) DM receiving 10 mg/kg cilostazol (cilo-10); 4) DM receiving 30 mg/kg cilostazol (cilo-30); and 5) DM receiving 100 mg/kg cilostazol (cilo-100). Hindpaw responses to thermal and mechanical stimuli were measured. Activation of microglia and astrocytes in the spinal dorsal horn (SDH) and expression of NaVs in the dorsal root ganglia (DRG) were examined with Western blots and immunofluorescence.

Results: DM rats displayed decreased withdrawal thresholds to mechanical stimuli (mechanical allodynia) and blunted responses to thermal stimuli. In addition, the expression of microglia increased, but astrocytes were reduced in the SDH. Upregulation of Nav −1.1, 1.2, −1.3, −1.6, and −1.7 and downregulation of Nav-1.8 were observed in the DRG. The DM rats receiving cilostazol all returned DM-induced decrease in withdrawal threshold to mechanical stimuli and attenuated neuropathic pain. Additionally, all cilostazol treatments suppressed the level of activated microglial cells and ameliorated the DM-induced decline in astrocyte expression levels in the SDH. However, only the rats treated with cilo-100 demonstrated significant improvements to the aberrant NaV expression in the DRG.

Conclusion: Oral cilostazol can blunt the responses of mechanical allodynia and has the potential to treat diabetic neuropathy by attenuating NaV and glial cell dysregulation.

Noopept Attenuates Diabetes-Mediated Neuropathic Pain and Oxidative Hippocampal Neurotoxicity via Inhibition of TRPV1 Channel in Rats

Neuropathic pain and oxidative neurotoxicity are two adverse main actions of diabetes mellitus (DM). The expression levels of calcium ion (Ca²⁺) permeable TRPV1 channels are high in the dorsal root ganglion (DRGs) and hippocampus (HIPPO). TRPV1 is activated by capsaicin and reactive free oxygen radicals (fROS) to mediate peripheral neuropathy and neurotoxicity. Noopept (NP) acted several protective antioxidant actions against oxidative neurotoxicity. As DM is known to increase the levels of fROS, the protective roles of antioxidant NP were evaluated on the DM-mediated neurotoxicity and neuropathic pain via the modulation of TRPV1 in rats. Thirty-six rats were equally divided into control, NP, DM (streptozotocin, STZ), and STZ + NP groups. A decrease on the STZ-mediated increase of neuropathic pain (via the analyses of Von Frey and hot plate) and blood glucose level was observed by the treatment of NP. A protective role of NP via downregulation of TRPV1 activity on the STZ-induced increase of apoptosis, mitochondrial fROS, lipid peroxidation, caspase -3 (CASP-3), caspase -9 (CASP-9), TRPV1 current density, glutathione (GSH), cytosolic free Zn²⁺, and Ca²⁺ concentrations in the DRGs and HIPPO was also observed. The STZ-mediated decrease of glutathione peroxidase, GSH, vitamin E, and β-carotene concentrations in the brain cortex, erythrocyte, liver, kidney, and plasma was also attenuated by the treatment of NP. The STZ-mediated increase of TRPV1, CASP-3, and CASP-9 expressions was decreased in the DRGs and HIPPO by the treatment of NP. In conclusion, the treatment of NP induced protective effects against STZ-induced adverse peripheral pain and HIPPO oxidative neurotoxicity. These effects might attribute to the potent antioxidant property of NP.

Comparative safety review of current treatment options for chronic low back pain and unmet needs: a narrative review

Introduction: The healthcare expenditures in the United States are substantial for the management of refractory, chronic low back pain (CLBP). The objective of this review is to summarize and evaluate the safety profiles of different pharmacological treatment options used in the management of CLBP.Areas covered: The authors conducted a search of randomized controlled trials (RCTs) assessing the safety profiles of different pharmacological agents used in the management of CLBP. This narrative review covered corticosteroids, opioids, antidepressants, gabapentinoids, nonsteroidal anti-inflammatory drugs, muscle relaxants, anti-nerve growth factor antibodies and topical agents, as monotherapy or in combination.Expert opinion: The risk-benefit ratio of a particular treatment is a subject driving the ongoing development of pharmaceuticals. The most commonly reported AEs across all drug classes are of gastrointestinal nature, followed by neurological and skin-related. These AEs include nausea, dizziness, constipation, arthralgia, headache, dry mouth, pruritus, etc. The majority of the AEs reported are not life-threatening, although they may lower patients' quality of life, thus, affecting their compliance. One of the biggest limitations of our review stems from the paucity of safety assessments in published RCTs. Advances in our understanding of the neurobiology of pain will promote development of new therapeutic strategies.