Mechanisms of topical analgesics in relieving pain in an animal model of muscular inflammation

Making Sense of Topical Pain Relief Options: Comparing Topical Analgesics in Efficacy and Safety

CONTEXT

In patients with musculoskeletal (MSK) conditions, pain is the leading contributor to disability and significantly limits mobility and dexterity. This narrative review describes the efficacy and safety of topical analgesics in common use today.

EVIDENCE ACQUISITION

Secondary literature gained via a literature search using PubMed.gov and the Cochrane library were used.

STUDY DESIGN

Recent literature (2000-2023) on several major classes of topical analgesics and topical delivery systems were reviewed to provide strength of recommendation taxonomy (SORT) levels. A total of 86 articles were reviewed.

LEVEL OF EVIDENCE

Level 2.

RESULTS

Topical nonsteroidal anti-inflammatory drugs (NSAIDs) and cabbage leaf wraps (CLW) appear to be best suited for multiple types of acute MSK pain, and topical nitroglycerin is helpful when used specifically for rotator cuff pain in patients seeking relief while performing activities of daily living and willing to treat for long periods of time. For compounded topical formulations, it may be better to offer single agent creams based on patient preferences. Little data support the use of cryotherapy. Traumeel could be a promising natural analgesic that compares with diclofenac. Topical lidocaine appears best suited for postherpetic neuropathic pain. O24 is a reasonable alternative with a low risk profile to treat pain in patients with fibromyalgia syndrome.

CONCLUSION

Choice of topical agents should be guided by current evidence accounting for type of pain, medication side effects, patient comorbidities, as well as patient preference, convenience, and cost.

STRENGTH-OF-RECOMMENDATION TAXONOMY (SORT)

Of the topical analgesics and modalities reviewed, SORT level A evidence was found for topical NSAID use in decreasing MSK pain, topical lidocaine for postherpetic neuralgia, and nitroglycerin patches for treating rotator cuff pain if used for prolonged periods of time. Alternative treatments such as CLW and Traumeel show promising results (SORT level B).

Traditional Chinese Medicine for Topical Treatment of Skeletal Muscle Injury

Muscle injuries are common musculoskeletal problems, but the pharmaceutical agent for muscle repair and healing is insufficient. Traditional Chinese Medicine (TCM) frequently uses topical treatments to treat muscle injuries, although scientific evidence supporting their efficacy is scarce. In this study, an in vitro assay was used to test the cytotoxicity of a topical TCM formula containing Carthami Flos, Dipsaci Radix, and Rhei Rhizoma (CDR). Then, a muscle contusion rat model was developed to investigate the in vivo effect and basic mechanisms underlying CDR on muscle regeneration. The in vitro assay illustrated that CDR was non-cytotoxic to immortalized rat myoblast culture and increased cell viability. Histological results demonstrated that the CDR treatment facilitated muscle repair by increasing the number of new muscle fibers and promoting muscle integrity. The CDR treatment also upregulated the expression of Pax7, MyoD and myogenin, as evidenced by an immunohistochemical study. A gene expression analysis indicated that the CDR treatment accelerated the regeneration and remodeling phases during muscle repair. This study demonstrated that topical CDR treatment was effective at facilitating muscle injury repair.

Tibetan Herbal Pain-Relieving Plaster for Chronic Musculoskeletal Pain Among Cancer Survivors: Study Protocol of a Randomized, Double-Blind, Placebo-Controlled Trial

Chronic pain is common and debilitating in cancer survivors. Tibetan herbal pain-relieving plaster is used as an external analgesic to treat musculoskeletal pain in China; however, its safety and efficacy have not been evaluated via clinical trials in cancer survivors. We designed this Phase II randomized, double-blind, placebo-controlled trial (ClinicalTrials.gov Identifier: NCT04916249) to assess the efficacy and safety of the pain-relieving plaster for temporary pain relief among cancer survivors with chronic musculoskeletal pain. Under ethical approval from the Institutional Review Board at the Memorial Sloan Kettering Cancer Center, we will enroll eligible cancer survivors who have a clinical diagnosis of moderate to severe chronic musculoskeletal pain in this study. We use a central randomization system to allocate the eligible participants to either the treatment or the control group in a 1:1 ratio, with stratification by baseline opioid use. We will instruct the participants to apply the herbal patch (Tibetree Pain-Relieving Plaster, Tibet Cheezheng Tibetan Medicine Co. Ltd., Tibet, China) or placebo patch daily at the focal area with worst pain for 14 consecutive days. Study physician, participant, outcome assessor, and biostatistician are blinded to the group allocation. The primary outcome is pain severity measured by the Brief Pain Inventory on Days 2–7. Secondary outcomes include changes in insomnia, anxiety, depression, fatigue, pressure pain threshold, pain medication use, and global impression of change. We will also monitor the adverse events throughout the study period. Statistical analysis will follow the intention-to-treat principle and linear mixed modeling will be used. With rigorous design and implementation, this randomized, placebo-controlled trial will provide the initial evidence on the efficacy and safety of the pain-relieving plaster for pain relief among cancer survivors with chronic musculoskeletal pain.

Inhibiting endocytosis in CGRP+ nociceptors attenuates inflammatory pain-like behavior

The advantage of locally applied anesthetics is that they are not associated with the many adverse effects, including addiction liability, of systemically administered analgesics. This therapeutic approach has two inherent pitfalls: specificity and a short duration of action. Here, we identified nociceptor endocytosis as a promising target for local, specific, and long-lasting treatment of inflammatory pain. We observed preferential expression of AP2α2, an α-subunit isoform of the AP2 complex, within CGRP⁺/IB4^- nociceptors in rodents and in CGRP⁺ dorsal root ganglion neurons from a human donor. We utilized genetic and pharmacological approaches to inhibit nociceptor endocytosis demonstrating its role in the development and maintenance of acute and chronic inflammatory pain. One-time injection of an AP2 inhibitor peptide significantly reduced acute and chronic pain-like behaviors and provided prolonged analgesia. We evidenced sexually dimorphic recovery responses to this pharmacological approach highlighting the importance of sex differences in pain development and response to analgesics.

The authors show the endocytotic adaptor subunit called AP2A2 is differentially expressed in CGRP⁺ nociceptors. Locally inhibiting nociceptor endocytosis with a lipidated AP2 inhibitor peptide reduces acute and chronic pain-like behaviour in mice and rats, indicating prolonged analgesia.

A Systematic Review of CheeZheng Pain Relieving Plaster for Musculoskeletal Pain: Implications for Oncology Research and Practice

BACKGROUND

Musculoskeletal pain is a common and debilitating condition for cancer patients. Existing therapies for these pain conditions have substantial limitations. To identify an integrative approach to pain management, we conducted a systematic review to evaluate the safety and efficacy of a Tibetan herbal-based topical agent, CheeZheng Pain Relieving Plaster (PRP), for the treatment of osteoarthritis (OA) pain.

METHODS

We extracted citations from PubMed and Chinese databases (CNKI, WanFang Data, and CQVIP). We included randomized clinical trials evaluating the effectiveness and safety of CheeZheng PRP compared to conventional OA pain treatments.

RESULTS

Twenty-two randomized clinical trials (n = 2556 participants) compared CheeZheng PRP against nonsteroidal anti-inflammatory drugs (11), glucosamine (2), intraarticular corticosteroid (2), hyaluronic acid injections (6), and acetaminophen (1). Ten studies found a statistically greater effectiveness (assessed by ≥30% reduction in symptom severity) of CheeZheng PRP in improving OA pain (measured by the Visual Analogue Scale), stiffness, and function compared to control. Ten studies reported that 4.8% of participants experienced application site skin irritation that resolved after discontinuing the plaster. Randomization was not sufficiently described in most studies, and no placebo-controlled trials were identified.

CONCLUSIONS

There is promising evidence for the safety and clinical effectiveness of CheeZheng PRP to treat OA; however, lack of placebo control and unclear descriptions of randomization increase the potential risk for bias. Future randomized, placebo-controlled trials are needed to establish the safety and efficacy of CheeZheng PRP for pain management in oncology settings.

Electroacupuncture and Moxibustion-Like Stimulation Relieves Inflammatory Muscle Pain by Activating Local Distinct Layer Somatosensory Afferent Fibers

Recent studies have shown that both superficial and deep acupuncture produced clinically relevant and persistent effect on chronic pain, and several subtypes of somatic primary afferents played critical roles in acupuncture and moxibustion analgesia. However, which kind of primary afferents in the superficial and deep tissue of the acupoint is activated by acupuncture or moxibustion to relieve pain persistently remains unclear. The aim of this study is to investigate the roles of distinct peripheral afferents in different layers of the tissue (muscle or skin) in the acupoint for pain relief. Muscular A-fibers activated by deep electroacupuncture (dEA) with lower intensity (approximately 1 mA) persistently alleviated inflammatory muscle pain. Meanwhile, cutaneous C-nociceptors excited by noxious moxibustion-like stimulation (MS) and topical application of capsaicin (CAP) on local acupoint area produced durable analgesic effect. Additionally, spontaneous activity of C-fibers caused by muscular inflammation was also inhibited by dEA and CAP. Furthermore, decreases in pain behavior induced by dEA disappeared after deep A-fibers were demyelinated by cobra venom, whereas CAP failed to relieve pain following cutaneous denervation. Collectively, these results indicate that dEA and MS ameliorate inflammatory muscle pain through distinct primary afferents in different layers of somatic tissue; the former is achieved by activating muscular A-fibers, while the latter is mediated by activating cutaneous C-fibers.

Electroacupuncture-Induced Muscular Inflammatory Pain Relief Was Associated With Activation of Low-Threshold Mechanoreceptor Neurons and Inhibition of Wide Dynamic Range Neurons in Spinal Dorsal Horn

Acupuncture is an effective alternative therapy for pain management. Evidence suggests that acupuncture relieves pain by exciting somatic afferent nerve fibers. However, the mechanism underlying the interaction between neurons in different layers of the spinal dorsal horn induced by electroacupuncture (EA) remains unclear. The aim of this study was to explore the mechanism of EA relieving inflammatory muscle pain, which was associated with activation of the spontaneous firing of low-threshold mechanoreceptor (LTM) neurons and inhibition of wide dynamic range (WDR) neuronal activities in the spinal dorsal horn of rats. Inflammatory muscle pain was induced by injecting complete Freund’s adjuvant into the right biceps femoris muscle. EA with intensity of threshold of A fibers (Ta) in Liangqiu (ST34) muscle considerably inhibited the abnormal spontaneous activities of electromyography (EMG) due to muscle inflammation. While EA with intensity of C-fiber threshold (Tc) increased the abnormal activities of EMG. EA with Ta also ameliorated the imbalance of weight-bearing behavior. A microelectrode array with 750-μm depth covering 32 channels was used to record the neuronal activities of WDR and LTM in different layers of the spinal dorsal horn. The spontaneous firing of LTM neurons was enhanced by EA-Ta, while the spontaneous firing of WDR neurons was inhibited. Moreover, EA-Ta led to a significant inverse correlation between changes in the frequency of WDR and LTM neurons (r = −0.64, p < 0.05). In conclusion, the results indicated that EA could alleviate inflammatory muscle pain, which was associated with facilitation of the spontaneous firing of LTM neurons and inhibition of WDR neuronal activities. This provides a promising evidence that EA-Ta could be applied to relieve muscular inflammatory pain in clinical practice.

Herbal Topical Analgesic for Pain Management: Perspectives from Cancer Patients

OBJECTIVE

Herbs and natural products are increasingly used by cancer patients for pain management, but few studies have examined their integration within conventional cancer care. This study describes the characteristics, experiences, and perspectives of cancer patients who were prescribed an herbal topical analgesic for pain management.

DESIGN AND SETTING

Program evaluation of a pilot herbal dispensary at a National Cancer Institute-designated comprehensive cancer center.

SUBJECTS

Cancer patients who were prescribed the Tibetree Pain-Relieving Plaster (PRP) by an integrative medicine physician.

METHODS

Sociodemographic and clinical characteristics of patients were abstracted from medical records. Semistructured phone interviews were conducted 1-2 weeks after PRP prescription to evaluate patient experiences with using PRP for pain. Interviews were analyzed through thematic content analysis.

RESULTS

From February 2019 to February 2020, 50 patients were prescribed PRP. Median age (range) was 63 years (21-86), 37 patients (74%) were female, 14 (28%) were non-White, and 38 (76%) were using oral analgesics. During interviews, the majority of patients reported that the PRP improved pain and health-related outcomes, was convenient to use, and addressed pain management needs that were not fulfilled by oral analgesics. However, a few patients described adverse experiences with PRP, including skin irritation.

CONCLUSIONS

Understanding patient experiences and perspectives is a critical step toward evidence-based integration of herbs and natural products into cancer pain management. Findings from this program evaluation will inform the design of a randomized clinical trial on the efficacy and safety of PRP for pain in patients with cancer.

Tibetan herbal pain-relieving plaster for low back pain: A systematic review and meta-analysis

ETHNOPHARMACOLOGICAL RELEVANCE

Tibetan traditional medicine CheeZheng Pain-Relieving Plaster (CZPRP) is frequently used as an over-the-counter external analgesic for musculoskeletal pain; however, its evidence for low back pain (LBP) has not been evaluated.

AIM OF THE STUDY

This study aims to assess the efficacy and safety of CZPRP for both acute, subacute and chronic LBP through a systematic review and meta-analysis of clinical trials.

MATERIALS AND METHODS

PubMed, CENTRAL, CNKI, CQVIP, and Wanfang databases were searched through April 20, 2020 for randomized controlled trials of CZPRP for LBP. Eligible comparators were placebo, active treatment, or usual care. Clinical outcomes included pain severity, lower back function score, pain-free rate, and adverse events (AEs). Qualitative evaluations were conducted using the Cochrane risk of bias assessment tools. Quantitative analyses were conducted using a random-effects model.

RESULTS

This study includes 1674 LBP patients from nine clinical studies. Pooled analyses among subjects with acute LBP show 1) significant pain reductions (mean difference -0.84, 95% confidence interval[CI] -1.31, -0.37) in CZPRP plus diclofenac versus diclofenac, 2) significant improvements in lower back function (standard mean difference -1.50, 95% CI -2.16, -0.85) in CZPRP versus diclofenac, and 3) a higher pain-free rate in CZPRP alone (risk ratio 1.48, 95% CI 1.16, 1.89; I² = 61%) or CZPRP plus nonsteroidal anti-inflammatory drugs (NSAIDs) (risk ratio 1.66, 95% CI 1.14, 2.40; I² = 0%) versus NSAIDs. However, in a heterogeneous population with mixed LBP subtypes, there was no significant difference in pain outcomes between CZPRP and diclofenac. Additionally, CZPRP use did not increase AEs compared with no CZPRP (p = 0.40). All nine studies are associated with moderate to high risk of bias.

CONCLUSIONS

The use of CZPRP is associated with improved acute LBP outcomes compared to diclofenac. However, due to the moderate to high risk of bias of the studies, future rigorous randomized controlled trials are needed to evaluate the effects of CZPRP for acute and chronic LBP.

Inhibition of Muscular Nociceptive Afferents via the Activation of Cutaneous Nociceptors in a Rat Model of Inflammatory Muscle Pain

Topical irritants such as capsaicin (CAP), peppermint oil (PO), and mustard oil (MO) are effective in relieving inflammatory muscle pain. We investigated the effects of topical irritants in a rat model of inflammatory muscle pain produced by injecting complete Freund's adjuvant (CFA) into the tibialis anterior muscle. CFA-induced mechanical hypersensitivity and the spontaneous activity of muscular nociceptive afferents, and decreased weight-bearing of the hindlimb were relieved by topical application of CAP, PO, or MO on the skin overlying the inflamed muscle. The effects of topical irritants were abolished when applied to the skin on the ipsilateral plantar region or on the contralateral leg, or when the relevant cutaneous nerve or dorsal root was transected. Our results demonstrated that topical irritants may alleviate inflammatory muscle pain via activating cutaneous nociceptors and subsequently inhibiting the abnormal activity of muscular nociceptive neurons.

Traditional Tibetan medicinal plants: a highlighted resource for novel therapeutic compounds

Around 70-80% of drugs used in traditional Tibetan medicine (TTM) come from Qinghai Tibet Plateau, the majority of which are plants. The biological and medicinal culture diversity on Qinghai Tibet Plateau are amazing and constitute a less tapped resource for innovative drug research and development. Meanwhile, the problem of the exhausting Tibetan medicine resources is worrying. Here, the latest awareness, as well as the gaps of the traditional Tibetan medicinal plant issues in drug development and clinical usage of TTM compounds, was systematically reviewed and highlighted. The TTM resource studies should be enhanced within the context of deeper and more extensive investigations of molecular biology and genomics of TTM plants, phytometabolites and metabolomics and ethnopharmacology-based bioactivity, thus enabling the sustainable conservation and exploitation of Tibetan medicinal resource.

Sensitization of group III and IV muscle afferents in the mouse after ischemia and reperfusion injury

Ischemic myalgia is a unique type of muscle pain in the patient population. The role that discrete muscle afferent subpopulations play in the generation of pain during ischemic events, however, has yet to be determined. Using 2 brachial artery occlusion models to compare prolonged ischemia or transient ischemia with reperfusion of the muscles, we found that both injuries caused behavioral decrements in grip strength, as well as increased spontaneous pain behaviors. Using our ex vivo forepaw muscles, median and ulnar nerves, dorsal root ganglion, and spinal cord recording preparation, we found after both prolonged and transient ischemia that there was a significant increase in the number of afferents that responded to both noxious and non-noxious chemical (lactate, adenosine triphosphate, varying pH) stimulation of the muscles compared to uninjured controls. However, we found an increase in firing to heat stimuli specifically in muscle afferents during prolonged ischemia, but a distinct increase in afferent firing to non-noxious chemicals and decreased mechanical thresholds after transient ischemia. The unique changes in afferent function observed also corresponded with distinct patterns of gene expression in the dorsal root ganglia. Thus, the development of ischemic myalgia may be generated by unique afferent-based mechanisms during prolonged and transient ischemia.

PERSPECTIVE

This study analyzed the response properties of thinly myelinated group III and unmyelinated group IV muscle afferents during prolonged and transient ischemia in addition to pain behaviors and alterations in DRG gene expression in the mouse. Results suggest that mechanisms of pain generation during prolonged ischemia may be different from ischemia/reperfusion.