A Randomized, Controlled, Open-Label Study of the Long-Term Effects of NGX-4010, a High-Concentration Capsaicin Patch, on Epidermal Nerve Fiber Density and Sensory Function in Healthy Volunteers

Mechanism- and experience-based strategies to optimize treatment response to the capsaicin 8% cutaneous patch in patients with localized neuropathic pain

The capsaicin 8% cutaneous patch is an emergent new treatment option for patients with peripheral neuropathic pain. In randomized controlled clinical studies relevant pain relief for 12 weeks was achieved in about one third of patients following a single application. The first part of this paper is a review of the pathophysiology, pharmacology, and published clinical trials with the capsaicin 8% cutaneous patch. The second part reports on outcomes of an interdisciplinary expert workshop, where new treatment results of three major German pain centers were presented and reviewed with the objectives of obtaining responder rates for different pain syndromes, assessing maintenance of effect under real-life conditions, and giving recommendations for practical care. The 12 week responder rates with pain relief of ≥ 30% were comparable in patients with mononeuropathies (37.9%) and postherpetic neuralgia (38.8%). Similar responder rates were seen in a subgroup of patients with cervical spine radiculopathy and back pain (46.7%). In HIV-associated neuropathy the responder rates were high (47.8%) but lower in patients with other polyneuropathies (17.6%). Response rates were nearly identical after 1 week (46.6%) and 4 weeks (43.3) and dropped only slightly at 12 weeks (37.4%). In a subgroup of 54 patients who underwent a second treatment, efficacy was maintained. Response rates in patients with or without lidocaine pretreatment were comparable. Treatment with the capsaicin 8% cutaneous patch was generally safe and well tolerated. The workshop panel recommended further investigation of opportunities to improve the application procedure and to perform studies on the skin penetration and distribution of capsaicin. A modified quantitative sensory testing (QST) should be developed for clinical practice in order to better understand the correlation of sensory profiles and response to capsaicin treatment.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Transient receptor potential vanilloid 1 as a therapeutic target in analgesia

INTRODUCTION

The selective excitatory action of capsaicin followed by long-term chemoanalgesia due to an action on the 'capsaicin receptor' of C-polymodal nociceptors, cloned 15 years ago, opened up fascinating perspectives for a class of nociceptor blocking analgesics.

AREAS COVERED

The TRPV1/capsaicin receptor is an integrative, chemoceptive, noxious heat-gated cation channel also gated by several endogenous ligands and sensitized by phosphorylation through intracellular cascades triggered from receptors of bradykinin, prostanoids, NGF and interactions with TRPA1. In this review, types of sensory receptors and unique mechanisms for blocking nociceptor action, e.g., 'pore dilation' intracellular acidosis and the long-term function-related mitochondrial swelling at the nerve terminals and sensory neurons are summarized. In humans the 8% capsaicin dermal patch is already in usage for nondiabetic neuropathic pain and two topical preparations of civamide have also been approved recently for cluster headache and osteoarthritis. Evidence for epidermal nerve terminal loss in humans after topical applications and misleading results on sensory neuron death evoked by TRPV1 agonism in animals are discussed.

EXPERT OPINION

The unique 'multisteric' gating of TRPV1 channel which is opened and modulated in various conformational changes to natural stimuli differs from the operation of canonical ligand-gated channels and makes it suitable to initiate development of second generation of TRPV1 antagonists without on-target side effects of hyperthermia and risk of burn injury.

NGX-4010, a capsaicin 8% patch, for the treatment of painful HIV-associated distal sensory polyneuropathy: integrated analysis of two phase III, randomized, controlled trials

Background

HIV-associated distal sensory polyneuropathy (HIV-DSP) is the most frequently reported neurologic complication associated with HIV infection. NGX-4010 is a capsaicin 8% dermal patch with demonstrated efficacy in the treatment of HIV-DSP. Data from two phase III, double-blind studies were integrated to further analyze the efficacy and safety of NGX-4010 and explore the effect of demographic and baseline factors on NGX-4010 treatment in HIV-DSP.

Methods

Data from two similarly designed studies in which patients with HIV-DSP received NGX-4010 or a low-concentration control patch (capsaicin 0.04% w/w) for 30 or 60 minutes were integrated. Efficacy assessments included the mean percent change from baseline in Numeric Pain Rating Scale (NPRS) scores to Weeks 2–12. Safety and tolerability assessments included adverse events (AEs) and pain during and after treatment.

Results

Patients (n = 239) treated with NGX-4010 for 30 minutes demonstrated significantly (p = 0.0026) greater pain relief compared with controls (n = 100); the mean percent change in NPRS scores from baseline to Weeks 2–12 was −27.0% versus −15.7%, respectively. Patients who received a 60-minute application of NGX-4010 (n = 243) showed comparable pain reductions (−27.5%) to patients treated for 30 minutes, but this was not statistically superior to controls (n = 115). NGX-4010 was effective regardless of gender, baseline pain score, duration of HIV-DSP, or use of concomitant neuropathic pain medication, although NGX-4010 efficacy was greater in patients not receiving concomitant neuropathic pain medications. NGX-4010 was well tolerated; the most common AEs were application-site pain and erythema, and most AEs were mild to moderate. The transient increase in pain associated with NGX-4010 treatment decreased the day after treatment and returned to baseline by Day 2.

Conclusions

A single 30-minute application of NGX-4010 provides significant pain relief for at least 12 weeks in patients with HIV-DSP and is well tolerated.

Trial registration

C107 = NCT00064623; C119 = NCT00321672

Changes in hind paw epidermal thickness, peripheral nerve distribution and mechanical sensitivity after immobilization in rats

This study was designed to investigate histological changes in skin tissue accompanying immobilization-induced hypersensitivity. Changes in mechanical sensitivity, epidermal thickness, and peripheral nerve profiles in the upper dermis were examined in glabrous skin of rat hind paw after 1, 2, and 4 weeks of ankle joint immobilization by plaster casts. Induction of mechanical hypersensitivity was confirmed after 2 and 4 weeks of joint immobilization. Epidermal thinning and increase in peripheral nerve profiles were observed in skin tissues in immobilized rats. The time course of epidermal thinning and increase in peripheral nerve profiles were similar closely to that of hypersensitivity, with significant differences between the immobilized and control rats after 2 weeks of immobilization, which became even more remarkable at 4 weeks of immobilization. These findings suggest that joint immobilization by cast induces epidermal thinning and increases peripheral nerve profiles in the upper dermis and that these changes might be partly responsible for immobilization-induced hypersensitivity.

Capsaicin for neuropathic pain: linking traditional medicine and molecular biology

Capsaicin has long been used as a traditional medicine to treat pain and, recently, its mechanism of analgesic action has been discovered. This review article documents the clinical development of capsaicin to demonstrate that pharmacognosy still has a profound influence on modern-day drug development programs. Capsaicin is a highly selective agonist for the transient receptor potential channel vanilloid-receptor type 1 (TRPV1), which is expressed on central and peripheral terminals of nociceptive primary sensory neurons. Knockout studies have revealed the importance of TRPV1 as a molecular pain integrator and target for novel analgesic agents. Topical application of capsaicin at the peripheral terminal of TRPV1-expressing neurons superficially denervates the epidermis in humans in a highly selective manner and results in hypoalgesia. In three recent randomized controlled trials, a patch containing high-concentration capsaicin demonstrated meaningful efficacy and tolerability relative to a low-concentration capsaicin control patch in patients with peripheral neuropathic pain. Data from clinical practice will determine if the high-concentration capsaicin patch is effective in real-world settings.

Trigeminal postherpetic neuralgia responsive to treatment with capsaicin 8 % topical patch: a case report

Postherpetic neuralgia has been variably defined but is generally understood to be pain that persists for longer than a few months after an attack of herpes zoster. Pain persists for years in approximately 10 % of those afflicted with acute herpes zoster. The likelihood of postherpetic neuralgia increases with older age, severity of the zoster, trigeminal location, and other factors. Postherpetic neuralgia is a neuropathic pain and treatment usually involves sequential trials of topical and systemic drugs; a variety of other therapies may be considered in refractory cases. A new topical capsaicin 8 % patch has been approved for this indication based on the positive studies in patients with non-trigeminal postherpetic neuralgia. Experience with the use of the capsaicin 8 % patch for trigeminal distribution neuralgia is lacking. We report a case of trigeminal postherpetic neuralgia which was safely and effectively treated with capsaicin 8 % patch.

Efficacy of a 0.1% capsaicin hydrogel patch for myofascial neck pain: a double-blinded randomized trial

OBJECTIVE

The objective of this study was to evaluate the efficacy of a hydrogel patch containing capsaicin 0.1% compared with a placebo hydrogel patch without capsaicin to treat chronic myofascial neck pain.

DESIGN

The study was designed as a double-blinded randomized controlled trial. Setting.  The study was set at Kyung-hee University Hospital at Gangdong, Korea.

SUBJECTS

Sixty-one participants between 18 and 65 years with at least 3 months duration of neck pain and a clinical presentation of myofascial pain syndrome were enrolled in the study from September 1 to November 20, 2010. Interventions.  Participants received capsaicin 0.1% hydrogel patches or control hydrogel patches without capsaicin according to the randomization scheme. All participants were instructed to apply one patch to each side of the neck and shoulder girdle overlying the point of maximal pain for 12 hours daily during the duration of the 4-week study.

OUTCOME MEASURES

Each participant completed five surveys at baseline, at 2 weeks after the start of treatments, and at the conclusion of the 4-week study. The primary outcome measure was visual analog scale (VAS). Other outcome measures included the Neck Disability Index (NDI), Beck's Depression inventory (BDI), Short Form 36 Korean version, and Euroqol 5-D.

RESULTS

Fifty-seven patients completed the study. The mean VAS, NDI, and BDI scores were significantly decreased at 2 and 4 weeks after the start of the intervention in both groups. There was no significant difference between the two groups in any of the outcome measures.

CONCLUSIONS

  Future research may help to discern specific effects of capsaicin, trigger point stimulation by application of the patch, and the placebo effect.

Interventional management of cancer pain

PURPOSE OF REVIEW

Interventional techniques were the mainstay for cancer pain management before the WHO ladder and opioids were freely available. The three-step WHO ladder has its limitations, and cancer pain is often under treated. Advances in treatment options mean that cancer patients are living longer and pain interventions may have a role to play even early in the cancer diagnosis for better quality of analgesia. The role of high doses of opioids in pain management is also currently under scrutiny.

RECENT FINDINGS

Recent advances in intrathecal analgesia, radiofrequency techniques, both in tumour ablation and neurotomies, are being widely used for palliation. Vertebroplasty techniques have been used not only for pain relief, but also for stabilization. Improved imaging and thoracoscopic techniques have made coeliac plexus and splanchnic blockade safer and more efficacious. There has been recent interest in percutaneous cordotomy with newer techniques using computed tomography/MRI and endoscopy guidance. Percutaneous electrical nerve stimulation and 8% capsaicin patches have been successfully used for managing neuropathic pain in cancer.

SUMMARY

Interventions form an integral part in providing pain relief in complex cancer pains. Oncologists and palliative care physicians are to be educated on the usefulness and timing of interventions in the management of complex cancer pain.

NGX-4010, a capsaicin 8% dermal patch, administered alone or in combination with systemic neuropathic pain medications, reduces pain in patients with postherpetic neuralgia

OBJECTIVES

Analyses of integrated data from 4 controlled postherpetic neuralgia studies evaluated the effect of NGX-4010, a capsaicin 8% patch, administered alone or together with systemic neuropathic pain medications.

METHODS

Patients recorded their “average pain for the past 24 hours” daily for 12 weeks using an 11-point Numeric Pain Rating Scale (NPRS). Efficacy assessment included the percentage NPRS score reduction from baseline during weeks 2 to 8 and 2 to 12, the proportion of patients responding during weeks 2 to 8 and 2 to 12 and the Patient Global Impression of Change (PGIC) at weeks 8 and 12.

RESULTS

During the studies, 302 NGX-4010 and 250 control (capsaicin, 0.04% wt/wt) patients were using at least 1 systemic neuropathic pain medication; 295 NGX-4010 and 280 control patients were not. During weeks 2 to 8, NGX-4010 patients reported greater reductions in NPRS scores compared with control both in patients using systemic neuropathic pain medications (26.1% vs. 18.1%, P=0.0011) and in patients not using these medications (36.5% vs. 26.2%, P=0.0002). Patients not using systemic neuropathic pain medications reported a greater reduction in pain compared with patients using these medications in both, NGX-4010 and control groups, resulting in comparable treatment differences between NGX-4010 and control regardless of systemic neuropathic pain medication use. Similar results were seen during weeks 2 to 12, for the responder and PGIC analyses. Transient, capsaicin-related application site reactions were the most common adverse events and not affected by systemic neuropathic pain medication use.

CONCLUSION

A single 60-minute NGX-4010 treatment reduces PHN for up to 12 weeks regardless of concomitant systemic neuropathic pain medication use.

Skin innervation at different depths correlates with small fibre function but not with pain in neuropathic pain patients

BACKGROUND

Neuropathy can lead not only to impaired function but also to sensory sensitization. We aimed to link reduced skin nerve fibre density in different levels to layer-specific functional impairment in neuropathic pain patients and tried to identify pain-specific functional and structural markers.

METHODS

In 12 healthy controls and 36 patients with neuropathic pain, we assessed clinical characteristics, thermal thresholds (quantitative sensory testing) and electrically induced pain and axon reflex erythema. At the most painful sites and at intra-individual control sites, skin biopsies were taken and innervation densities in the different skin layers were assessed. Moreover, neuronal calcitonin gene-related peptide staining was quantified.

RESULTS

Perception of warm, cold and heat pain and nerve fibre density were reduced in the painful areas compared with the control sites and with healthy controls. Warm and cold detection thresholds correlated best with epidermal innervation density, whereas heat and cold pain thresholds and axon reflex flare correlated best with dermal innervation density. Clinical pain ratings correlated only with epidermal nerve fibre density (r = 0.38, p < 0.05) and better preserved cold detection thresholds (r = 0.39, p < 0.05), but not with other assessed functional and structural parameters.

CONCLUSIONS

Thermal thresholds, axon reflex measurements and assessment of skin innervation density are valuable tools to characterize and quantify peripheral neuropathy and link neuronal function to different layers of the skin. The severity of small fibre neuropathy, however, did not correspond to clinical pain intensity and a specific parameter or pattern that would predict pain intensity in peripheral neuropathy could not be identified.

Translational investigation and treatment of neuropathic pain

Neuropathic pain develops from a lesion or disease affecting the somatosensory system. Translational investigations of neuropathic pain by using different animal models reveal that peripheral sensitization, spinal and cortical plasticity may play critical roles in neuropathic pain. Furthermore, descending facilitatory or excitatory modulation may also act to enhance chronic pain. Current clinical therapy for neuropathic pain includes the use of pharmacological and nonpharmacological (psychological, physical, and surgical treatment) methods. However, there is substantial need to better medicine for treating neuropathic pain. Future translational researchers and clinicians will greatly facilitate the development of novel drugs for treating chronic pain including neuropathic pain.

Tolerability of NGX-4010, a capsaicin 8% patch, in conjunction with three topical anesthetic formulations for the treatment of neuropathic pain

BACKGROUND

The objective of this study was to assess the safety, tolerability, and preliminary efficacy of NGX-4010, a capsaicin 8% patch, following pretreatment with three different topical anesthetics in patients with peripheral neuropathic pain.

METHODS

This open-label, multicenter study enrolled 117 patients with post-herpetic neuralgia, HIV-associated distal sensory polyneuropathy, or painful diabetic neuropathy. Patients received pretreatment with one of three lidocaine 4%-based topical anesthetics (L.M.X.4(®) [Ferndale Laboratories Inc, Ferndale, MI], Topicaine(®) Gel [Estela Basso, Jupiter, FL], or Betacaine Enhanced Gel 4 [Tiberius Inc, Tampa, FL]) for 60 minutes followed by a single 60- or 90-minute NGX-4010 application, and were followed for 12 weeks. Tolerability and safety measures included "pain now" Numeric Pain Rating Scale (NPRS) scores, dermal assessments, medication use for treatment-related pain, adverse events (AEs), clinical laboratory parameters, physical examinations, and vital signs. The primary efficacy variable was the percentage change in mean NPRS scores for "average pain for the past 24 hours" from baseline to weeks 2 through 12.

RESULTS

Treatment with NGX-4010 following pretreatment with any of the three topical anesthetics was generally safe and well tolerated. Nearly all patients completed ≥90% of the planned NGX-4010 application duration. The most common treatment-related AEs, application-site burning and application-site pain, were transient, mostly mild or moderate, and could be adequately managed by local cooling or short-acting oral opioid analgesics. Although slightly more patients used medication for treatment-related discomfort following pretreatment with Topicaine compared with L.M.X.4 or Betacaine, there were no statistical differences between the topical anesthetics. Neuropathic pain reduction from baseline to weeks 2 through 12 was approximately 30% and was similar among the topical anesthetics; the proportion of responders ranged from 45% to 50%.

CONCLUSION

Treatment with NGX-4010 following pretreatment with any of the three topical anesthetics was generally safe and well tolerated; no significant differences in the parameters measured were noted between the pretreatment groups.

New developments in the treatment algorithm for peripheral neuropathic pain

Neuropathic pain often imposes a substantial and unrelenting burden on those individuals who have it; single-agent analgesics typically only reduce pain at best. Worldwide, five sets of treatment recommendations offer insight into managing neuropathic pain, including two European guidelines, one Canadian, one Latin American, and another constructed under the auspices of the International Association for the Study of Pain (IASP). The analgesics common to these guidelines are topical lidocaine, secondary amine tricyclic antidepressants, serotonin and norepinephrine dual reuptake inhibitors, calcium channel α(2)-δ ligands, tramadol, and opioid antagonists. Still, significant knowledge gaps in the treatment of neuropathic pain conditions have hampered the development of algorithms and multimodal approaches. As the evidence base expands, the addition of new comparative trial data will further refine the development of new guidance for clinical management of neuropathic pain. New alternatives for managing neuropathic pain, such as the high-concentration capsaicin patch, will enlarge the treatment armamentarium and potentially impact therapeutic guidelines.

Tolerability of NGX-4010, a capsaicin 8% patch for peripheral neuropathic pain

BACKGROUND/PURPOSE

NGX-4010 (QUTENZA(™); NeurogesX Inc, San Mateo, CA), a capsaicin 8% dermal patch, is licensed in the European Union for the treatment of peripheral neuropathic pain (PNP) in nondiabetic adults and in the United States for the treatment of neuropathic pain associated with postherpetic neuralgia (PHN). While NGX-4010 treatment is associated with a low risk of systemic adverse events, patch application-related pain is common and may be managed with local cooling and/or oral analgesics. This article characterizes the tolerability of NGX-4010 and will help to guide any pain management.

METHODS

This integrated analysis of tolerability data collected from the NGX-4010 clinical study program included 1696 patients with PNP. Patch application-related pain on the treatment day was captured as Numeric Pain Rating Scale (NPRS) "pain now" scores while "average pain for the past 24 hours" NPRS scores were analyzed for 7 days following treatment. Other tolerability assessments included the percentage of patients completing ≥90% of the intended treatment duration and patients using medication for patch application-related pain.

RESULTS

The mean maximum change in "pain now" NPRS scores from pretreatment levels during and after patch application was 2.6 for all patients. This pain was transient and resolved following patch removal. Mean "average pain for the past 24 hours" NPRS scores returned to baseline by the evening of the treatment day for patients with PHN, and the evening of day 2 for patients with human immunodeficiency virus-associated distal sensory polyneuropathy or painful diabetic neuropathy. Repeated NGX-4010 applications did not affect the intensity of patch application-related pain. Almost all patients (≥98%) completed ≥90% of the full treatment duration, regardless of the number of treatments received.

CONCLUSION

Transient patch application-related pain with NGX-4010 can be managed with local cooling and/or oral analgesics in nearly all cases. Patient adherence to the full intended treatment duration indicated that patch application-related pain was not a barrier to NGX-4010 use.

Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch

Topical capsaicin formulations are used for pain management. Safety and modest efficacy of low-concentration capsaicin formulations, which require repeated daily self-administration, are supported by meta-analyses of numerous studies. A high-concentration capsaicin 8% patch (Qutenza™) was recently approved in the EU and USA. A single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks. Advantages of the high-concentration capsaicin patch include longer duration of effect, patient compliance, and low risk for systemic effects or drug-drug interactions. The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as 'defunctionalization' of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals. Peripheral neuropathic hypersensitivity is mediated by diverse mechanisms, including altered expression of the capsaicin receptor TRPV1 or other key ion channels in affected or intact adjacent peripheral nociceptive nerve fibres, aberrant re-innervation, and collateral sprouting, all of which are defunctionalized by topical capsaicin. Evidence suggests that the utility of topical capsaicin may extend beyond painful peripheral neuropathies.

Efficacy, safety, and tolerability of NGX-4010, capsaicin 8% patch, in an open-label study of patients with peripheral neuropathic pain

AIMS

To assess efficacy, safety, and tolerability of NGX-4010, capsaicin 8% patch, in patients with peripheral neuropathic pain.

METHODS

This open-label, uncontrolled, 12-week study enrolled 25 patients with postherpetic neuralgia (PHN), one with HIV-distal sensory polyneuropathy, and 91 with painful diabetic neuropathy (PDN). Patients received pre-treatment with one of three 4% lidocaine topical anesthetics (L.M.X.4¹, Topicaine Gel², or Betacaine Enhanced Gel 4³) followed by a single 60- or 90-min NGX-4010 application. The primary efficacy variable was the percentage change in Numeric Pain Rating Scale scores from baseline to Weeks 2-12. Adverse events (AEs), laboratory parameters, vital signs, neurosensory examinations, dermal assessments, treatment-related pain scores, and medication use for treatment-related pain were collected.

RESULTS

PDN and PHN patients achieved a 31% and 28% mean pain decrease from baseline during Weeks 2-12, respectively, and 47% and 44%, respectively, were responders (≥30% pain decrease). Mild or moderate treatment-site-related burning and pain were the most common AEs and there was no evidence of impaired neurosensory function.

CONCLUSIONS

NGX-4010 in conjunction with any of the three topical anesthetics tested was generally safe and well tolerated and reduced pain over a 12-week period in patients with PDN and PHN.

Capsaicin 8% topical patch (Qutenza)--a review of the evidence

Qutenza is a high-potency capsaicin (8%) topical patch, labeled for treating pain associated with postherpetic neuralgia (PHN). Qutenza decreases pain sensation by reducing transient receptor potential vanilloid 1 (TRPV1) expression and decreasing the density of epidermal nerve fibers in the application area. Systemic absorption from Qutenza is low. Qutenza has not been directly compared to any other medications for the treating PHN. Two pivotal clinical trials compared Qutenza to a control patch (0.04% capsaicin) in PHN. The primary endpoint of both trials was the reduction in numeric pain rating scale (NPRS) score. Qutenza reduced pain from baseline to weeks 2 to 8 (29.6% and 32% reductions) compared to control (19.9% and 24.4% reductions; P ≤ .01). The improvement in NPRS scores persisted, with score reductions greater with Qutenza (29.9% and 32.3% reductions) compared to control (20.4% and 25% reductions; P ≤ .03) for the period 2 to 12 weeks. Safety and efficacy of capsaicin 8% has been demonstrated in open-label trials for up to 48 weeks. The most common adverse drug reactions occurring with capsaicin 8% are application site erythema (63%) and application site pain (42%). Some patients experienced transient increases in blood pressure during Qutenza application. Qutenza must be administered by a physician or under the close supervision of a physician. Prior to application, pretreat the affected area with a topical anesthetic to reduce application site pain. Some patients may require systemic analgesics during and after treatment for treatment-associated pain. Applications of Qutenza can be repeated no sooner than once every 3 months, as needed.

Chemosensory properties of the trigeminal system

The capacity of cutaneous, including trigeminal endings, to detect chemicals is known as chemesthesis or cutaneous chemosensation. This sensory function involves the activation of nociceptor and thermoreceptor endings and has a protective or defensive function, as many of these substances are irritants or poisonous. However, humans have also developed a liking for the distinct sharpness or pungency of many foods, beverages, and spices following activation of the same sensory afferents. Our understanding of the cellular and molecular mechanisms of chemosensation in the trigeminal system has experienced enormous progress in the past decade, following the cloning and functional characterization of several ion channels activated by physical and chemical stimuli. This brief review attempts to summarize our current knowledge in this field, including a functional description of various sensory channels, especially TRP channels, involved in trigeminal chemosensitivy. Finally, some of these new findings are discussed in the context of the pathophysiology of trigeminal chemosensation, including pain, pruritus, migraine, cough, airway inflammation, and ophthalmic diseases.

Capsaicin dermal patch: in non-diabetic peripheral neuropathic pain

Capsaicin dermal patch is an adhesive patch containing a high concentration (8% w/w) of synthetic capsaicin. It is indicated in the EU for the treatment of peripheral neuropathic pain in non-diabetic adults using a single 30- or 60-minute application repeated every 90 days, as required, and in the US for the treatment of neuropathic pain associated with postherpetic neuralgia (PHN). In pivotal, randomized, double-blind, multicentre trials in adults with PHN, a single 60-minute application of capsaicin dermal patch reduced the mean Numeric Pain Rating Scale (NPRS) scores from baseline to a significantly greater extent than a low-concentration (0.04% w/w capsaicin) control patch during weeks 2-8. In randomized, double-blind, multicentre trials in patients with HIV-associated neuropathy, capsaicin dermal patch reduced the mean NPRS scores from baseline significantly more than control in one study for the 30- and 90-minute, but not the 60-minute, application during weeks 2-12. In another study, the differences between capsaicin (30- and 60-minute applications) and control did not reach statistical significance. An integrated analysis of both studies showed that the 30-minute application of capsaicin dermal patch was significantly better than control for the reduction from baseline in mean NPRS scores during weeks 2-12. The efficacy of capsaicin dermal patch was maintained for up to 1 year in extension studies in which patients could receive up to three or four repeat treatments. Capsaicin dermal patch was generally well tolerated in clinical trials. The most common adverse events were transient, mostly mild to moderate, application-site reactions.

A multicenter, randomized, double-blind, controlled study of NGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia

OBJECTIVES

To confirm the efficacy, tolerability, and safety of NGX-4010, an 8% capsaicin dermal patch (capsaicin 640 µg/cm(2) ), in patients with postherpetic neuralgia (PHN). PHN is a chronic pain disorder that can be difficult to treat and for which current treatment options are often limited by poor tolerability.

DESIGN

A total of 418 patients were randomized to receive a single 60-minute application of NGX-4010 or a 0.04% capsaicin control patch (3.2 µg/cm(2) ) in a multicenter, double-blind, confirmatory, phase 3 study.

PATIENTS

Patients were 18-90 years old with a diagnosis of PHN, pain for at least 6 months, and an average baseline Numeric Pain Rating Scale (NPRS) score of 3-9.

OUTCOME MEASURES

The primary efficacy end point was the percentage change in NPRS score from baseline to weeks 2-8.

RESULTS

NGX-4010 recipients had a significantly greater mean reduction from baseline in pain during weeks 2-8 compared with the control group (32.0% vs 24.4%; P=0.011). A ≥ 30% reduction in mean NPRS scores was achieved in 46% of NGX-4010 recipients compared with 34% of controls (P=0.02). Pain was significantly lower in NGX-4010 recipients than controls by week 2, and greater pain reduction was maintained throughout the remaining 12-week study period. Most treatment-emergent adverse events were application site specific (notably erythema and pain), transient, and generally mild to moderate in severity.

CONCLUSIONS

In patients with PHN, a single 60-minute application of NGX-4010 produced significant reduction in pain that was maintained over a 12-week period.

The discovery and development of analgesics: new mechanisms, new modalities

Despite intensive research into pain mechanisms and significant investment in research and development, the majority of analgesics available to prescribers and patients are based on mechanistic classes of compounds that have been known for many years. With considerable ingenuity and innovation, researchers continue to make the best of the mechanistic approaches available, with novel formulations, routes of administration, and combination products. Here we review some of the mechanisms and modalities of analgesics that have recently entered into clinical development, which, coupled with advances in the understanding of the pathophysiology of chronic pain, will hopefully bring the promise of new therapeutics that have the potential to provide improved pain relief for those many patients whose needs remain poorly met.

Variation in quantitative sensory testing and epidermal nerve fiber density in repeated measurements

Quantitative sensory testing (QST) is commonly used to evaluate peripheral sensory function in neuropathic conditions. QST measures vary in repeated measurements of normal subjects but it is not known whether QST can reflect small changes in epidermal nerve fiber density (ENFd). This study evaluated QST measures (touch, mechanical pain, heat pain and innocuous cold sensations) for differences between genders and over time using ENFd as an objective-independent measure. QST was performed on the thighs of 36 healthy volunteers on four occasions between December and May. ENFd in skin biopsies was determined on three of those visits. Compared to men, women had a higher ENFd, a difference of 12.2 ENFs/mm. They also had lower tactile and innocuous cold thresholds, and detected mechanical pain (pinprick) at a higher frequency. Heat pain thresholds did not differ between genders. By the end of the 24-week study, men and women showed a small reduction (p<0.05) in the frequency of sharp mechanical pain evoked by pinprick whereas tactile and thermal thresholds showed no change. This coincided with a small decrease in ENFd, 4.18 ENFs/mm. Variation in measurements over time was large in a fraction of normal subjects. We conclude that most QST measures detect relatively large differences in epidermal innervation (12.2 ENFs/mm), but response to mechanical pain was the only sensory modality tested with the sensitivity to detect small changes in innervation (4.18 ENFs/mm). Since some individuals had large unsystematic variations, unexpected test results should therefore alert clinicians to test additional locations.

Quantitative sensory testing: a diagnostic tool for painful neuropathy

Quantitative sensory testing (QST) analyzes sensory perceptions of external stimuli of controlled intensity. Both large and small fiber function can be evaluated by applying thermal and mechanical stimuli, thus closing the diagnostic gap for the conventional electrophysiology in the examination of thin and unmyelinated nerve fibers. Detection, pain thresholds and stimulus–response curves can be determined; therefore, QST is a valuable diagnostic tool for identifying both sensory loss (i.e., hypoesthesia and hypoalgesia) and gain (i.e., hyperalgesia and allodynia) in patients with painful or painless neuropathy. Every psychophysical approach QST requires standardized stimuli administration, instructions and data evaluation to achieve valid results. QST can be used to evaluate distinct somatosensory profiles and thus give hints to the underlying mechanisms that occur with different frequencies in different pain syndromes. This might be helpful for the future establishment of mechanism-based pharmacotherapy. Since normative data are available, QST also contributes to the individual diagnosis of neuropathy. The present article gives an outline of QST application in diagnosis and its limitations for the evaluation of neuropathic pain.