In the European Union, the high-concentration capsaicin patch is licensed for the management of neuropathic pain conditions in nondiabetic patients, including postherpetic neuralgia (PHN) and HIV-associated distal sensory polyneuropathy (HIV-DSP). However, in the USA, the Food and Drug Administration approved its use only in PHN patients. Capsaicin is a transient receptor potential vanilloid-1 agonist, which increases the intracellular calcium ion concentration. This triggers calcium-dependent protease enzymes causing cytoskeletal breakdown and leads to the loss of cellular integrity and ‘defunctionalization’ of nociceptor fibres. Efficacy and therapeutic effect has been shown in several clinical studies of PHN and HIV-DSP. The high-concentration capsaicin patch and its practical application are different from low-concentration creams; one application can help for up to 3 months. The process of setting up of a service to use the capsaicin 8% patch is also discussed.
Capsaicin; high concentration patch; neuropathic pain; post herpetic neauralgia; HIV; efficacy
In ancient times, physicians had a limited number of therapies to provide pain relief. Not surprisingly, plant extracts applied topically often served as the primary analgesic plan. With the discovery of the capsaicin receptor (TRPV1), the search for ‘new’ analgesics has returned to compounds used by physicians thousands of years ago. One such compound, capsaicin, couples the paradoxical action of nociceptor activation (burning pain) with subsequent analgesia following repeat or high-dose application. Investigating this ‘paradoxical’ action of capsaicin has revealed several overlapping and complementary mechanisms to achieve analgesia including receptor desensitization, nociceptor dysfunction, neuropeptide depletion and nerve terminal destruction. Moreover, the realization that TRPV1 is both sensitized and activated by endogenous products of inflammation including bradykinin, H+, ATP, fatty acid derivatives, NGF and trypsins, has renewed interest in TRPV1 as an important site of analgesia. Building on this foundation, a new series of preclinical and clinical studies targeting TRPV1 have been reported. These include trials using brief exposure to high-dose topical capsaicin in conjunction with prior application of a local anesthetic. Clinical use of resiniferatoxin (RTX), another ancient but potent TRPV1 agonist, is also being explored as a therapy for refractory pain. The development of orally-administered high affinity TRPV1 antagonists hold promise for pioneering a new generation of analgesics capable of blocking painful sensations at the site of inflammation and tissue injury. With the isolation of other members of the TRP channel family such as TRPA1, additional opportunities are emerging in the development of safe and effective analgesics.
Analgesics; Non-Narcotic; Pain; Sensory Receptors; transient receptor potential cation channel; TRP channels; TRPV1; capsaicin receptor
The treatment of neuropathic pain is difficult. Oral pharmaceuticals have significant side effects, and treatment efficacy tends to be modest. The use of topical analgesics reduces the potential for systemic side effects and allows direct application of medications to the area of pain. The natural spicy substance, capsaicin, has historically been known for its topical use. Capsaicin, once applied to the skin, causes a brief initial sensitization followed by a prolonged desensitization of the local pain nerves. This occurs through stimulation of the transient receptor potential vanilloid-1 (TRPV1) expressing pain nerve fibers. While low-dose capsaicin has not resulted in good efficacy, the larger dose 8% topical capsaicin has had some of the best data currently available in the treatment of post-herpetic neuralgia (PHN) and other neuropathic conditions. This paper discusses the data currently existing for capsaicin 8% in the treatment of PHN. It further reviews data for the low-dose capsaicin products and the current status in the development of other capsaicinoids, e.g. resiniferotoxin, and high-concentration liquid capsaicin.
capsaicin; capsaicinoids; neuropathic pain; TRPV1
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.
Treatment of chronic pain is associated with high variability in the response to pharmacological interventions. A mathematical pharmacodynamic model was developed to quantify the magnitude and onset/offset times of effect of a single capsaicin 8% patch application in the treatment of painful diabetic peripheral neuropathy in 91 patients. In addition, a mixture model was applied to objectively match patterns in pain-associated behavior. The model identified four distinct subgroups that responded differently to treatment: 3.3% of patients (subgroup 1) showed worsening of pain; 31% (subgroup 2) showed no change; 32% (subgroup 3) showed a quick reduction in pain that reached a nadir in week 3, followed by a slow return towards baseline (16% ± 6% pain reduction in week 12); 34% (subgroup 4) showed a quick reduction in pain that persisted (70% ± 5% reduction in week 12). The estimate of the response-onset rate constant, obtained for subgroups 1, 3, and 4, was 0.76 ± 0.12 week−1 (median ± SE), indicating that every 0.91 weeks the pain score reduces or increases by 50% relative to the score of the previous week (= t½). The response-offset rate constant could be determined for subgroup 3 only and was 0.09 ± 0.04 week−1 (t½ 7.8 weeks). The analysis allowed separation of a heterogeneous neuropathic pain population into four homogenous subgroups with distinct behaviors in response to treatment with capsaicin. It is argued that this model-based approach may have added value in analyzing longitudinal chronic pain data and allows optimization of treatment algorithms for patients suffering from chronic pain conditions.
diabetic neuropathic pain; capsaicin 8%; modeling; mixture model
Capsaicin, the pungent ingredient in hot pepper, activates nociceptors to produce pain and inflammation. However, prolonged exposures of capsaicin will cause desensitization to nociceptive stimuli. Hyperpolarization-activated cation currents (Ih) contribute to the maintenance of the resting membrane potential and excitability of neurons. In the cultured dorsal root ganglion (DRG) neurons, we investigated mechanisms underlying capsaicin-mediated modulation of Ih using patch clamp recordings. Capsaicin (1 µM) inhibited Ih only in the capsaicin-sensitive neurons. The capsaicin-induced inhibition of Ih was prevented by preexposing the TRPV1 antagonist, capsazepine (CPZ). Capsaicin-induced inhibition of Ih was dose dependent (IC50= 0.68 µM) and partially abolished by intracellular BAPTA and cyclosporin A, specific calcineurin inhibitor. In summary, the inhibitory effects of capsaicin on Ih are mediated by activation of TRPV1 and Ca2+-triggered cellular responses. Analgesic effects of capsaicin have been thought to be related to desensitization of nociceptive neurons due to depletion of pain-related substances. In addition, capsaicin-induced inhibition of Ih is likely to be important in understanding the analgesic mechanism of capsaicin.
capsaicin; DRG neuron; hyperpolarization-activated cation current; rat
Neuropathic pain is difficult to treat and can have a severe effect on quality of life. The capsaicin 8% patch is a novel treatment option that directly targets the source of peripheral neuropathic pain. It can provide pain relief for up to 12 weeks in patients with peripheral neuropathic pain. Treatment with the capsaicin 8% patch follows a clearly defined procedure, and patch application must be carried out by a physician or a health care professional under the supervision of a physician. Nonetheless, in our experience, nurses often take the lead role in capsaicin 8% patch application without the involvement of a physician. We believe that the nurse’s key role is of benefit to the patients, as he or she may be better placed, because of time constraints and patient relationships, to support the patient through the application procedure than a physician. Moreover, a number of frequently prescribed drugs, including botulinum toxin and infliximab, can be administered by health care professionals without the requirement for physician supervision. Here we argue that current guidance should be amended to remove the requirement for physician supervision during application of the capsaicin 8% patch.
capsaicin; neuropathic pain; topical; health care professional; physician; nurse
Dilute capsaicin produces a differential effect on incision-related pain behaviors depending upon the test; it reduces heat hyperalgesia and guarding pain but not mechanical hyperalgesia. This suggests that common mechanisms for heat hyperalgesia and guarding pain occur, and distinct mechanisms exist for mechanical hyperalgesia. The purpose of the present study was to evaluate the effect of capsaicin treatment on the activity of cutaneous nociceptors sensitized by incision to understand the mechanisms for the selective action of dilute capsaicin on incisional pain. We compared the effect of 0.05% capsaicin versus vehicle treatment on pain behaviors after incision and on the activity of nociceptors from these same rats using the in vitro glabrous skin nerve preparation. Immunohistochemical expression of protein gene product 9.5 (PGP9.5), neurofilament 200, calcitonin gene related peptide (CGRP) and isolectin B4 (IB4) in skin was also evaluated 1 week after 0.05% capsaicin infiltration. Infiltration of 0.05% capsaicin decreased CGRP and IB4/PGP9.5-immunoreactivity of nociceptors in skin. The same dose of capsaicin that inhibited heat hyperalgesia and guarding behavior interfered with chemo- and heat sensitivity of C-fibers. Neither mechanical hyperalgesia nor mechanosensitivity of nociceptors was affected by capsaicin, suggesting that the concentration of capsaicin used in this study did not cause fiber degeneration. These results demonstrate that nociceptors desensitized by capsaicin contribute to heat hyperalgesia and guarding pain after plantar incision. These putative TRPV1-expressing C-fibers are sensitized to heat and acid after incision, and the transduction of heat and chemical stimuli after plantar incision is impaired by dilute capsaicin.
Recent studies investigating the pharmacological management of neuropathic pain support the efficacy of certain antidepressants, anticonvulsants, and opioids. Novel directions in drug applications include topical applications of patches with either lidocaine or capsaicin and intradermal injections of botulinum toxin. In cases of partial pain relief, drug combinations may be considered.
There has been an increasing focus on development of new routes of drug administration to provide tailored treatments for patients, without decreasing efficacy of analgesia, in proportion to the progression of the knowledge of pain mechanisms. While acute pain acts as an alarm, chronic pain is a syndrome requiring meticulous selection of analgesic drugs of high bioavailability for long-term use. Such criteria are challenges that topical medications aim to overcome, allowing progressive delivery of active component, maintaining stable plasma levels, with a good safety profile. This review presents recent findings regarding topical formulations of the most widely used drugs for pain treatment, such as nonsteroidal anti-inflammatory agents, anesthetics, and capsaicin, and the role of physical agents as delivery enhancers (phonophoresis and iontophoresis). Although the number of topical agents is limited for use in peripheral conditions, increasing evidence supports the efficacy of these preparations in blocking nociceptive and neuropathic pain. Patient adherence to medical treatment is also a challenge, especially in chronic painful conditions. It is known that reduction of treatment complexity and pill burden are good strategies to increase patient compliance, as discussed here. However, the role of topical presentations, when compared to traditional routes, has not yet been fully explored and thus remains unclear.
medication adherence; administration; topical; patient compliance; pain; therapeutics
Neuropathic pain is common and difficult to treat. Recently a technique was developed to selectively inhibit nociceptive inputs by simultaneously applying two drugs: capsaicin, a transient receptor potential vanilloid receptor 1 channel activator and QX-314, a lidocaine derivative that intracellularly blocks sodium channels. We used this technique to investigate whether transient receptor potential vanilloid receptor 1-expressing nociceptors contribute to neuropathic pain.
The rat chronic constriction injury model was used to induce neuropathic pain in order to test the analgesic effects of both peripheral (perisciatic) and central (intrathecal) administration of the QX-314/capsaicin combination. The Hargreaves and von Frey tests were used to monitor evoked pain-like behaviors and visual observations were used to rank spontaneous pain-like behaviors.
Perisciatic injections of the QX-314/capsaicin combination transiently increased the withdrawal thresholds by ~3 fold for mechanical and thermal stimuli in rats (n = 6/group) with nerve injuries suggesting that peripheral transient receptor potential vanilloid receptor 1-expressing nociceptors contribute to neuropathic pain. In contrast, intrathecal administration of the QX-314/capsaicin combination did not alleviate pain-like behaviors (n = 5/group). Surprisingly, intrathecal QX-314 alone (n = 9) or in combination with capsaicin (n = 8) evoked spontaneous pain-like behaviors.
Data from the perisciatic injections suggested that a component of neuropathic pain was mediated by peripheral nociceptive inputs. The role of central nociceptive terminals could not be determined because of the severe side effects of the intrathecal drug combination. We concluded that only peripheral blockade of transient receptor potential vanilloid receptor 1-expressing nociceptive afferents by the QX-314/capsaicin combination was effective at reducing neuropathic allodynia and hyperalgesia.
Itch evoked by cowhage or histamine is reduced or blocked by capsaicin desensitization, suggesting that pruriceptive neurons are capsaicin-sensitive. Topical capsaicin can evoke both nociceptive sensations and itch, whereas intradermal injection of capsaicin evokes only burning pain. To dissociate the pruritic and nociceptive sensory effects caused by the chemical activation of sensory neurons, chemicals were applied in a punctiform manner to the skin of the forearm using individual, heat-inactivated cowhage spicules treated with various concentrations of capsaicin (1–200 mg/ml) or histamine (0.01–100 mg/ml). Perceived intensities of itch, pricking/stinging and burning were obtained every 30s using the general version of the Labeled Magnitude Scale and compared with ratings evoked by individual native cowhage spicules. Similar to cowhage, capsaicin and histamine spicules reliably evoked sensations of itch in a dose-dependent manner that were most often accompanied by pricking/stinging and to a lesser extent burning. Spicules containing 200 mg/ml capsaicin or 10 mg/ml histamine yielded peak magnitudes and durations of sensations comparable to those elicited by cowhage. Each type of spicule also produced comparable areas of dysesthesias (enhanced mechanically evoked itch or pain) and/or skin reactions (wheal and/or flare) in surrounding skin, though inconsistently. The incidence of flare was greater in response to histamine than to capsaicin or cowhage. These results suggest the possibility that capsaicin, histamine and cowhage activate common peripheral or central neural mechanisms that mediate pruritic sensations and associated dysesthesias.
Acute application of topical capsaicin produces spontaneous burning and stinging pain similar to that seen in some neuropathic states, with local hyperalgesia. Use of capsaicin applied topically or injected intradermally has been described as a model for neuropathic pain, with patterns of activation in brain regions assessed using functional magnetic resonance imaging (fMRI) and positron emission tomography. The Contact Heat Evoked Potential Stimulator (CHEPS) is a noninvasive clinically practical method of stimulating cutaneous A-delta nociceptors. In this study, topical capsaicin (1%) was applied to the left volar forearm for 15 minutes of twelve adult healthy human volunteers. fMRI scans and a visual analog pain score were recorded during CHEPS stimulation precapsaicin and postcapsaicin application. Following capsaicin application there was a significant increase in visual analog scale (mean ± standard error of the mean; precapsaicin 26.4 ± 5.3; postcapsaicin 48.9 ± 6.0; P < 0.0001). fMRI demonstrated an overall increase in areas of activation, with a significant increase in the contralateral insular signal (mean ± standard error of the mean; precapsaicin 0.434 ± 0.03; postcapsaicin 0.561 ± 0.07; P = 0.047). The authors of this paper recently published a study in which CHEPS-evoked A-delta cerebral potential amplitudes were found to be decreased postcapsaicin application. In patients with neuropathic pain, evoked pain and fMRI brain responses are typically increased, while A-delta evoked potential amplitudes are decreased. The protocol of recording fMRI following CHEPS stimulation after topical application of capsaicin could be combined with recording of evoked potentials to provide a simple, rapid, and robust volunteer model to develop novel drugs for neuropathic pain.
capsaicin; fMRI; contact heat evoked potentials; neuropathy; pain model
The aim of this study was to differentiate the processing of nociceptive information, matched for pain intensity, from capsaicin-induced hyperalgesic vs. control skin at multiple levels in the trigeminal nociceptive pathway. Using an event-related fMRI approach, 12 male subjects underwent three functional scans beginning 1 hour after topical application of capsaicin to a defined location on the maxillary skin, when pain from capsaicin application had completely subsided. Brush and two levels of painful heat (low - Thermal-1 and high - Thermal-2) were applied to the site of capsaicin application and to the mirror image region on the opposite side. Temperatures for each side were set to evoke perceptually-matched pain (mean temperatures [capsaicin/control]: Thermal-1=38.4/42.8°C; Thermal-2=44.9/47.8°C). We found differences in activation patterns following stimuli to treated and untreated sides in sensory circuits across all stimulus conditions. Across the trigeminal nociceptive pathway, Thermal-2 stimulation of hyperalgesic skin evoked greater activation in trigeminal ganglion and nucleus, thalamus, and somatosensory cortex than the control side. Thus, trigeminal nociceptive regions showed increased activation in the context of perceptually equal pain levels. Beyond these regions, contrast analyses of capsaicin vs. control skin stimulation indicated significant changes in bilateral dorsolateral prefrontal cortex and amygdala. The involvement of these emotion-related regions suggests that they may be highly sensitive to context, such as prior experience (application of capsaicin) and the specific pain mechanism (hyperalgesic vs. normal skin).
Pain; sensitization; dorsolateral prefrontal cortex; amygdala; S1; brush
The effects of capsaicin, the ingredient of hot pepper, on rheumatoid arthritis synoviocytes have been studied. Capsaicin was shown to have a direct action on the metabolism of synovial cells. Thus at 10(-6) mol/l and at higher doses DNA synthesis was restored to the control level. Capsaicin at both doses induced an increase in the synthesis of collagenase and at the lower concentration (10(-8) mol/l) only of prostaglandins. These results indicate that the different effects of capsaicin on cellular proliferation and on metabolic activities are dependent on dose. The responses seen in rheumatoid arthritis synoviocytes in vitro might not be mediated by tachykinins if the synovial tissue is still able to produce neuropeptides in the absence of neuronal afferents. These results suggest that capsaicin, in addition to its direct action on the afferent nervous fibres and the consequent release of tachykinins, may also have a direct action on the cells. The mechanisms by which capsaicin stimulates DNA synthesis and production of collagenase and prostaglandin E2, in a manner dependent on dose, remain to be determined.
Although certain bacterial species appear to be risk factors for pain due to odontogenic infections, comparatively little is known about the potential mechanisms mediating this effect. In this study, we tested the hypothesis that trigeminal nociceptive neurons express the TLR4 or CD14 receptors, thus enabling sensory neurons to detect and respond to tissue levels of bacterial substances such as lipopolysaccharide (LPS). Immunohistochemical analyses of human and rat trigeminal neurons demonstrated that a capsaicin-sensitive subclass of nociceptors (defined by expression of TRPV1, a capsaicin receptor) expresses both TLR4 and CD14. Moreover, human dental pulp collected from patients with caries lesions demonstrated co-localization of TLR4 and CD14, with markers of peripheral sensory neurons. Collectively, these studies indicate that the capsaicin-sensitive subclass of trigeminal nociceptors expresses TLR4 and CD14. These results indicate that pain due to bacterial infections may result, in part, from direct activation of nociceptors by bacterial products such as LPS.
pain; infection; TRPV1; TLR4; CD14; bacteria; LPS
The Neuropathic Pain Special Interest Group of the International Association for the Study of Pain recently sponsored the development of evidence-based guidelines for the pharmacological treatment of neuropathic pain. Tricyclic antidepressants, dual reuptake inhibitors of serotonin and norepinephrine, calcium channel α2-δ ligands (ie, gabapentin and pregabalin), and topical lidocaine were recommended as first-line treatment options on the basis of the results of randomized clinical trials. Opioid analgesics and tramadol were recommended as second-line treatments that can be considered for first-line use in certain clinical circumstances. Results of several recent clinical trials have become available since the development of these guidelines. These studies have examined botulinum toxin, high-concentration capsaicin patch, lacosamide, selective serotonin reuptake inhibitors, and combination therapies in various neuropathic pain conditions. The increasing number of negative clinical trials of pharmacological treatments for neuropathic pain and ambiguities in the interpretation of these negative trials must also be considered in developing treatment guidelines. The objectives of the current article are to review the Neuropathic Pain Special Interest Group guidelines for the pharmacological management of neuropathic pain and to provide a brief overview of these recent studies.
Capsaicin elicits burning pain via the activation of the vanilloid receptor (TRPV1). Intriguingly, several reports showed that capsaicin also inhibits Na+ currents but the mechanisms remain unclear. To explore this non-TRPV1 action we applied capsaicin to HEK293 cells stably expressing inactivation-deficient rat skeletal muscle Na+ mutant channels (rNav1.4-WCW). Capsaicin elicited a conspicuous time-dependent block of inactivation-deficient Na+ currents. The 50% inhibitory concentration (IC50) of capsaicin for open Na+ channels at +30 mV was measured 6.8 ± 0.6 μM (n = 5), a value that is 10–30 times lower than those for resting (218 μM) and inactivated (74 μM) wild-type Na+ channels. On-rate and off-rate constants for capsaicin open-channel block at +30 mV were estimated to be 6.37 μM−1 s−1 and 34.4 s−1, respectively, with a calculated dissociation constant (KD) of 5.4 μM. Capsaicin at 30 μM produced ~70% additional use-dependent block of remaining rNav1.4-WCW Na+ currents during repetitive pulses at 1 Hz. Site-directed mutagenesis showed that the local anesthetic receptor was not responsible for the capsaicin block of the inactivation-deficient Na+ channel. Interestingly, capsaicin elicited little time-dependent block of batrachotoxin-modified rNav1.4-WCW Na+ currents, indicating that batrachotoxin prevents capsaicin binding. Finally, neuronal open Na+ channels endogenously expressed in GH3 cells were as sensitive to capsaicin block as rNav1.4 counterparts. We conclude that capsaicin preferentially blocks persistent late Na+ currents, probably via a receptor that overlaps the batrachotoxin receptor but not the local anesthetic receptor. Drugs that target such a non-TRPV1 receptor could be beneficial for patients with neuropathic pain.
Capsaicin; Voltage-gated sodium channel; Open-channel block; Analgesia; Use-dependent block
We evaluated the effect of infiltration of dilute solutions of capsaicin, administered before plantar incision, on three pain related behaviors: guarding pain, heat withdrawal latency and mechanical withdrawal threshold. Perineural application of capsaicin was also studied and the appearance of the wound was also evaluated. Dilute solutions of capsaicin 0.025% and 0.10% were infiltrated in the plantar region one day before incision. In another group of rats, perineural capsaicin (1%) was applied to the nerves innervating the plantar aspect of the rat hindpaw. Rats were then tested for pain related behaviors before and after plantar incision and then daily thereafter. Wound appearance was graded and histopathology was evaluated. Infiltration with capsaicin reduced guarding pain and heat hyperalgesia after plantar incision; there were minimal effects on mechanical responses. Perineural capsaicin application produced a similar result. Both capsaicin infiltration and perineural capsaicin application impaired wound apposition. Histologic evaluation also confirmed impaired wound apposition after capsaicin infiltration. In conclusion, dilute solutions of capsaicin have differential effects on pain related behaviors after plantar incision. Based on the antinociception produced by capsaicin both via infiltration and perineural injection, the effect on wound appearance was likely related to its inhibitory effects on pain behaviors and was not necessarily a local effect of the drug.
This study demonstrated that capsaicin infiltration before plantar incision produced an analgesic effect that depended upon the stimulus modality tested. When evaluating novel treatments for postoperative pain, studies using a single stimulus modality may overlook an analgesic effect by not examining a variety of stimuli.
Brief Summary Statement
Capsaicin infiltration and perineural capsaicin application decrease guarding pain and heat hyperalgesia after plantar incision.
Itch can be suppressed by painful stimuli, but the underlying neural basis is unknown. We generated conditional null mice in which VGLUT2-dependent synaptic glutamate release from mainly Nav1.8-expressing nociceptors was abolished. These mice showed deficits in pain behaviors including mechanical pain, heat pain, capsaicin-evoked pain, inflammatory pain and neuropathic pain. The pain deficits were accompanied by greatly enhanced itching, as suggested by i) sensitization of both histamine-dependent and histamine-independent itch pathways, and ii) development of spontaneous scratching and skin lesions. Strikingly, intradermal capsaicin injection promotes itch responses in these mutant mice, as opposed to pain responses in control littermates. Consequently, co-injection of capsaicin was no longer able to mask itch evoked by pruritogenic compounds. Our studies suggest that synaptic glutamate release from a group of peripheral nociceptors is required to sense pain and suppress itch. Elimination of VGLUT2 in these nociceptors creates a mouse model of chronic neurogenic itch.
Capsaicin opens the TRPV1 channel, a cation channel that depolarizes and activates nociceptive neurons. Following this initial activation, neurons become desensitized to subsequent applications of capsaicin as well as to other noxious stimuli, a phenomenon attributed primarily to the entry of Ca2+ ions through the open TRPV1 channel. This ability of capsaicin to desensitize nociceptors has led to its use as an analgesic in the treatment of a variety of chronic pain states. Because treatment with capsaicin is initially quite painful, local anesthetics are sometimes used to block axonal conduction in nociceptive neurons and thus minimize pain. However, local anesthetics might also block TRPV1 and prevent the Ca2+ entry required for capsaicin-induced desensitization. We have studied the direct effect of local anesthetics on currents induced by capsaicin (1 μM) in acutely isolated rat dorsal root ganglion neurons using the whole cell patch clamp technique. At the highest concentration tested (1 mM), bupivacaine only moderately inhibited the capsaicin-induced current to 55 ± 27% of control (mean ± S.D.; n = 12, p < 0.01). Tetracaine (1 mM), on the other hand, enhanced the capsaicin-induced current to 151 ± 34% of control (mean ± S.D.; n = 7, p < 0.01). These results show that local anesthetics can be used to prevent the initial pain induced by application of capsaicin without abolishing, and perhaps even enhancing, its desensitizing actions.
Capsaicin; TRPV1; VR1; Ionic currents; Dorsal root ganglion neuron; Sensory neuron; Local anesthetics
Transient receptor potential channels are implicated in thermosensation both in mammals and insects. The aim of our study was to assess the effect of mammalian vanilloid receptor subtype 1 (TRPV1) agonist (capsaicin) and antagonist (capsazepine) on insect behavioral thermoregulation. We tested behavioral thermoregulation of mealworms larvae intoxicated with capsaicin and capsazepine in two concentrations (10−7 and 10−4 M) in a thermal gradient system for 3 days. Our results revealed that in low concentration, capsaicin induces seeking lower temperatures than the ones selected by the insects that were not intoxicated. After application of capsazepine in the same concentration, the mealworms prefer higher temperatures than the control group. The observed opposite effect of TRPV1 agonist and antagonist on insect behavioral thermoregulation, which is similar to the effect of these substances on thermoregulation in mammals, indicates indirectly that capsaicin may act on receptors in insects that are functionally similar to TRPV1.
Behavioral thermoregulation; Capsaicin; Capsazepine; TRPV1; Mealworms
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.
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.
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.
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.
C107 = NCT00064623; C119 = NCT00321672
NGX-4010; Capsaicin 8% patch; HIV-associated distal sensory polyneuropathy; Neuropathic pain
To investigate the response of patients with peripheral neuropathic pain (PNP) to capsaicin 8% patch treatment in a clinical setting.
The Clinic for Pain Therapy and Palliative Medicine at the Medical Centre for the region of Aachen, Germany.
Patients diagnosed with PNP who attended the clinic for capsaicin 8% patch treatment between January 13, 2010 and February 7, 2011.
Pain intensity was assessed using the Numeric Pain Rating Scale (NPRS) at baseline and following each capsaicin 8% patch treatment. Changes in prescribed concomitant neuropathic pain (NP) medications and response duration were recorded.
Overall, 68 patients with PNP conditions, including facial neuropathy (severe trigeminal neuralgia in V2), polyneuropathy, post-herpetic neuralgia, and mononeuropathies, received 96 treatments with the capsaicin 8% patch. The 53 patients with a follow-up of ≥8 weeks demonstrated a 48.4% mean reduction in NPRS score from baseline to Weeks 1–8. Among the 37 responders (those exhibiting ≥30% reduction in NPRS score from baseline to Weeks 1–8), the median time to re-treatment was 125 days. Following treatment, there was a significant (P < 0.001) 54% reduction in the mean number of prescribed concomitant NP medications taken by patients.
This analysis demonstrates that in clinical practice, the capsaicin 8% patch provides rapid and sustained pain reductions in patients with a variety of PNP conditions and a significant reduction in prescribed concomitant NP medications. The capsaicin 8% patch can be a valuable addition to the NP treatment armory for certain patients.
Capsaicin; Neuropathy; Chronic Pain; Polyneuropathy; Post-Herpetic Neuralgia; Pain Management
Objective To determine the efficacy and safety of topically applied capsaicin for chronic pain from neuropathic or musculoskeletal disorders.
Data sources Cochrane Library, Medline, Embase, PubMed, an in-house database, and contact with manufacturers of topical capsaicin.
Study selection Randomised controlled trials comparing topically applied capsaicin with placebo or another treatment in adults with chronic pain.
Data extraction Primary outcome was dichotomous information for the number of patients with about a 50% reduction in pain. Outcomes were extracted at four weeks for musculoskeletal conditions and eight weeks for neuropathic conditions. Secondary outcomes were adverse events and withdrawals due to adverse events.
Data synthesis Six double blind placebo controlled trials (656 patients) were pooled for analysis of neuropathic conditions. The relative benefit from topical capsaicin 0.075% compared with placebo was 1.4 (95% confidence interval 1.2 to 1.7) and the number needed to treat was 5.7 (4.0 to 10.0). Three double blind placebo controlled trials (368 patients) were pooled for analysis of musculoskeletal conditions. The relative benefit from topical capsaicin 0.025% or plaster compared with placebo was 1.5 (1.1 to 2.0) and the number needed to treat was 8.1 (4.6 to 34). Around one third of patients experienced local adverse events with capsaicin, which would not have been the case with placebo.
Conclusions Although topically applied capsaicin has moderate to poor efficacy in the treatment of chronic musculoskeletal or neuropathic pain, it may be useful as an adjunct or sole therapy for a small number of patients who are unresponsive to, or intolerant of, other treatments.