A randomized study of the effect of oral lamotrigine and hydromorphone on pain and hyperalgesia following heat/capsaicin sensitization

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Human experimental pain models

Pain is an unpleasant sensory experience, associated with existing or potential tissue damage. It has also strong cognitive and emotional components. Stimuli that causes pain goes through process of nociception, which includes transduction, transmission, modulation and perception of said stimuli. Depending on the type of stimuli, we can classify human experimental pain models into mechanical, electrical, thermal and chemical. Information about pain mechanisms can be obtained from the following: 1) in vitro studies, 2) animal experiments, 3) human experimental pain studies and 4) clinical studies. Chosing the appropriate method for pain evaluation is a key step in the design of pain studies. Combining it with different electro-physiological and imaging methods, it can provide better objectivity and quantification of pain mechanisms. Focus in experimental pain studies is slowly shifting from static parameters of pain, such as pain threshold and maximum tolerance, to dynamic parameters, which can give us valuable insight in function of endogenous analgesic systems. This can be done using conditioned pain modulation. Using experimental pain on healthy voulenteers is key step in switching from animal models to clinical studies, foremost for validization of data from animals, making them important in translational research. Results from experimental pain studies can help us in understanding nociceptive mechanisms of acute and chronic pain, alongside development of new therapeutic modalities.

Does Electroacupuncture Have Different Effects on Peripheral and Central Sensitization in Humans: A Randomized Controlled Study

Background

Acupuncture is used to reduce chronic musculoskeletal pain. The common mechanism underlying these types of pain are peripheral and/or central sensitization. In the clinical setting, it is difficult to separate the peripheral from the central component of pain. Heat/capsaicin 45°C/0.075%-induced hyperalgesia provides a stable, human central sensitization model in which the peripheral component is also assessed.

Aim

This randomized, sham-controlled study aimed to investigate the effect of electroacupuncture (EA) on the severity of heat (peripheral sensitization) and mechanical hyperalgesia (central sensitization) in a heat/capsaicin pain model in humans.

Methods

Twenty-six healthy young participants (24 ± 3.9 years) were recruited. After baseline assessment, heat/capsaicin 45°C/0.075% was applied to the non-dominant forearm to induce hyperalgesia. The primary outcome measures were the size of the area of mechanical hyperalgesia, intensity of pain to heat stimulation and heat pain thresholds. The intensity of pain was recorded using modified 10-cm visual analogues scales (VAS). Participants were assessed at 70 min after the initial application of capsaicin then randomly allocated to receive either real electroacupuncture (REA, n = 14) or sham non-invasive EA (SEA, n = 12) for 30 min. The main outcome measures were assessed again immediately and then 90 min following EA. Credibility of blinding was assessed. Data were analyzed with t-tests or analysis of variance (ANOVA) where appropriate.

Results

After the model was established, the area of mechanical hyperalgesia was formed (55.64 cm²), as was heat hyperalgesia, as the rating to heat stimulation, increased from 2/10 to 6/10. The REA and SEA groups were comparable. Immediately after the allocated acupuncture treatment, the rating to heat stimulation was statistically significantly lower in the REA group (2.94 ± 1.64) than in the SEA group (4.62 ± 2.26) (p < 0.05). The area of mechanical hyperalgesia reduced significantly without any group difference. No group difference was detected in heat pain threshold. Blinding of the participants was successful.

Conclusion

Peripheral and central sensitization in the heat/capsaicin 45°C/0.075% model responded to EA differently, suggesting that acupuncture analgesia could vary, depending on the types of pain. This observation may explain some inconsistent findings from clinical trials of acupuncture.

Effect of Intravenous Alfentanil on Nonpainful Thermally Induced Hyperalgesia in Healthy Volunteers

Experimental interventions that activate specific components of clinical pain are necessary for characterization of underlying mechanisms and pharmacology. Cutaneous hyperalgesia has been described that uses nonpainful heat to induce secondary hyperalgesia. This study evaluated the effect of intravenous alfentanil on experimental cutaneous hyperalgesia created using this method. Eighteen subjects participated in a randomized, double-blinded, placebo-controlled crossover study consisting of 2 sessions, 1 with alfentanil and 1 with placebo. Using a computer-controlled infusion pump, alfentanil or matching placebo was maintained at a constant plasma level of 75 ng/mL for 1 hour followed by the application of a 40°C heat stimulus to the right thenar eminence for 15 minutes. The temperature was raised by 1°C every 15 minutes until the subject reported pain or 45°C was reached. After the end point was reached, the temperature was maintained, and repeat testing was performed. The nonpainful heat created an area of secondary cutaneous hyperalgesia and significant decrease in mechanical pain threshold on heat-treated right vs untreated left during placebo administration. Alfentanil prevented the hypersensitivity when compared to placebo (P < .05) but failed to reduce the area of secondary hyperalgesia created by nonpainful heat when compared to placebo (P = .06). Neither alfentanil nor the heat lamp treatment showed any significant effect on other neurosensory measures. This study demonstrated a reliable production of cutaneous hyperalgesia using a nonpainful stimulus that is affected by the systemic delivery of alfentanil. This model for human cutaneous experimental pain may be a useful method for scientific characterization of analgesics.

A literature review on the pharmacological sensitivity of human evoked hyperalgesia pain models

AIMS

Human evoked pain models can be used to determine the efficacy of new and existing analgesics and to aid in the identification of new targets. Aspects of neuropathic pain can be simulated by inducing hyperalgesia resulting from provoked sensitization. The present literature review aimed to provide insight into the sensitivity of different hyperalgesia and allodynia models of pharmacological treatment.

METHODS

A literature search was performed to identify randomized, double-blind, placebo-controlled studies that included human hyperalgesia pain models and investigated the pharmacodynamic effects of different classes of drugs.

RESULTS

Three hyperalgesia models [ultraviolet B (UVB) irradiation, capsaicin and thermode burn] have been used extensively. Assessment of hyperalgesia/allodynia and pharmacological effect are measured using challenge tests, which generally comprise thermal (heat/cold) or mechanical stimulation (pin-prick, stroking or impact). The UVB model was sensitive to the antihyperalgesic effects of nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids. The capsaicin model was partially sensitive to opioids. The burn model did not detect any antihyperalgesic effects when NSAIDs or local anaesthetics were administered but responded to the effects of N-methyl D-aspartate (NMDA) receptor antagonists by moderately reducing mechanical hyperalgesia.

CONCLUSIONS

Based on pharmacological sensitivity, the UVB model adequately reflects inflammatory pain and was sensitive to NSAIDs and opioids. Findings from the capsaicin and burn models raised questions about the translatability of these models to the treatment of neuropathic pain. There is a need for a reproducible and predictive model of neuropathic pain, either in healthy subjects or in patients.

Demarcation of secondary hyperalgesia zones: Punctate stimulation pressure matters

BACKGROUND

Secondary hyperalgesia is increased sensitivity in normal tissue near an injury, and it is a measure of central sensitization reflecting injury-related effects on the CNS. Secondary hyperalgesia areas (SHAs), usually assessed by polyamide monofilaments, are important outcomes in studies of analgesic drug effects in humans. However, since the methods applied in demarcating the secondary hyperalgesia zone seem inconsistent across studies, we examined the effect of a standardized approach upon the measurement of SHA following a first degree burn injury (BI).

NEW METHOD

The study was a two-observer, test-retest study with the two sessions separated by 6wk. An observer-blinded design adjusted to examine day-to-day and observer-to-observer variability in SHA was used. In 23 healthy volunteers (12 females/11 males) a BI was induced by a contact thermode (47.0°C, 420s, 2.5×5.0cm(2)). The SHA, demarcated by polyamide monofilaments (bending force: 0.2, 69 and 2569mN) and a "weighted-pin" stimulator (512mN), were assessed 45 to 75min after each BI.

RESULTS

A random effect, linear mixed model demonstrated a logarithmic correlation between elicited skin pressures (mN/mm(2)) and the SHAs (P<0.0001). No day-to-day or observer-to-observer differences in SHAs were observed. Intraclass correlation coefficients, in the range of 0.51 to 0.84, indicated a moderate to almost perfect reliability between observers.

COMPARISON WITH EXISTING METHODS

No standardized approach in SHA-assessment has hitherto been presented.

CONCLUSIONS

This is the first study to demonstrate that demarcation of secondary hyperalgesia zones depends on the developed pressure of the punctate stimulator used.

Skin Matters: A Review of Topical Treatments for Chronic Pain. Part One: Skin Physiology and Delivery Systems

Chronic pain is a complex disorder with multiple etiologies for which the pathologic mechanisms are still largely unknown, making effective treatment a difficult clinical task. Achieving pain relief along with improved function and quality of life is the primary goal of pain clinicians; however, most patients and healthcare professionals consider 30% pain improvement to be clinically significant—a success level that would be unacceptable in other areas of medicine. Furthermore, patients with chronic pain frequently have multiple comorbidities, including depression and sleep apnea, and most have seen several physicians prior to being seen by a pain specialist, have more than three specific pain generators, and are taking multiple medications. The addition of further oral medications to control pain increases the risk of drug–drug interactions and side effects. However, topical analgesics have the advantage of local application with limited systemic levels of drug. Topical therapies benefit from reduced side effects, lower risk of drug–drug interactions, better patient acceptability/compliance, and improved tolerability. This two-part paper is a review of topical analgesics and their potential role in the treatment of chronic pain.

A randomized, double-blind, crossover study to evaluate the depth response relationship of intradermal capsaicin-induced pain and hyperalgesia in healthy adult volunteers

OBJECTIVE

The purpose of this study was to evaluate pain and hyperalgesia in response to different depths of intradermal (ID) capsaicin injections in healthy volunteers.

DESIGN

Double-blind, cross-over study.

SETTING

Clinical Research Laboratory.

SUBJECTS

Fifteen healthy male subjects received ID capsaicin injections into the volar aspect of each forearm at depths of 1 mm, 3 mm, 5 mm, and 7 mm. After injection, spontaneous pain, elicited pain, flare response, heat thresholds, and area of hyperalgesia were measured at various time points.

OUTCOMES MEASURE

Spontaneous pain, elicited pain (pinprick, stroking, and hot pain), hyperalgesia area, and allodynia area.

RESULTS

No significant difference was found between any depths in spontaneous pain, elicited pain (pinprick, stroking, hot pain), hyperalgesia area, or allodynia area. A significant difference was found in the change in heat threshold between 5 mm and 1 mm, 7 mm and 1 mm, 5 mm and 3 mm, 7 mm and 3 mm depths. A significant difference was found in flare area between 5 mm and 3 mm depths. A significant difference was found in systolic blood pressure area under the curve (AUC) between 7 mm and 1 mm depths, and for both systolic and diastolic pressures for 5 mm and 1 mm depths, and 5 mm and 3 mm depths. A significant difference was found in pulse AUC between 5 mm and 1 mm depths and 5 mm and 3 mm depths.

CONCLUSIONS

Injection of capsaicin at different depths in the skin had different effects on heart rate and blood pressure but no effect on pain. These results may have implications on the pharmacology and analgesic predictive value of the model of ID capsaicin.

Lamotrigine for chronic neuropathic pain and fibromyalgia in adults

BACKGROUND

This is an update of the original Cochrane review entitled Lamotrigine for acute and chronic pain published in Issue 2, 2007, and updated in Issue 2, 2011. Some antiepileptic medicines have a place in the treatment of neuropathic pain (pain due to nerve damage). This updated review adds no new additional studies looking at evidence for lamotrigine as an effective treatment for chronic neuropathic pain or fibromyalgia. The update uses higher standards of evidence than previously.

OBJECTIVES

To assess the analgesic efficacy of lamotrigine in the treatment of chronic neuropathic pain and fibromyalgia, and to evaluate adverse effects reported in the studies.

SEARCH METHODS

We identified randomised controlled trials (RCTs) of lamotrigine for chronic neuropathic pain and fibromyalgia (including cancer pain) from MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials (CENTRAL). We ran searches for the original review in 2006, in 2011 for the first update, and subsequent searches in August 2013 for this update. We sought additional studies from the reference lists of the retrieved papers. The original review and first update included acute pain, but no acute pain studies were identified.

SELECTION CRITERIA

RCTs investigating the use of lamotrigine (any dose, by any route, and for any study duration) for the treatment of chronic neuropathic pain or fibromyalgia. Assessment of pain intensity or pain relief, or both, using validated scales. Participants were adults aged 18 and over. We included only full journal publication articles.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted efficacy and adverse event data, and examined issues of study quality. We performed analysis using three tiers of evidence. The first tier used data where studies reported the outcome of at least 50% pain reduction from baseline, lasted at least eight weeks, had a parallel group design, included 200 or more participants in the comparison, and reported an intention-to-treat analysis. First-tier studies did not use last observation carried forward (LOCF) or other imputational methods for dropouts. The second tier used data that failed to meet this standard and second-tier results were therefore subject to potential bias.

MAIN RESULTS

Twelve included studies in 11 publications (1511 participants), all with chronic neuropathic pain: central post-stroke pain (1), chemotherapy-induced neuropathic pain (1), diabetic neuropathy (4), HIV-related neuropathy (2), mixed neuropathic pain (2), spinal cord injury-related pain (1), and trigeminal neuralgia (1). We did not identify any additional studies. Participants were aged between 26 and 77 years. Study duration was two weeks in one study and at least six weeks in the remainder; eight were of eight-week duration or longer.No study provided first-tier evidence for an efficacy outcome. There was no convincing evidence that lamotrigine is effective in treating neuropathic pain and fibromyalgia at doses of 200 mg to 400 mg daily. Almost 10% of participants taking lamotrigine reported a skin rash.

AUTHORS' CONCLUSIONS

Large, high-quality, long-duration studies reporting clinically useful levels of pain relief for individual participants provided no convincing evidence that lamotrigine is effective in treating neuropathic pain and fibromyalgia at doses of about 200 to 400 mg daily. Given the availability of more effective treatments including antiepileptics and antidepressant medicines, lamotrigine does not have a significant place in therapy based on the available evidence. The adverse effect profile of lamotrigine is also of concern.

Reproducibility of the heat/capsaicin skin sensitization model in healthy volunteers

Introduction

Heat/capsaicin skin sensitization is a well-characterized human experimental model to induce hyperalgesia and allodynia. Using this model, gabapentin, among other drugs, was shown to significantly reduce cutaneous hyperalgesia compared to placebo. Since the larger thermal probes used in the original studies to produce heat sensitization are now commercially unavailable, we decided to assess whether previous findings could be replicated with a currently available smaller probe (heated area 9 cm² versus 12.5–15.7 cm²).

Study design and methods

After Institutional Review Board approval, 15 adult healthy volunteers participated in two study sessions, scheduled 1 week apart (Part A). In both sessions, subjects were exposed to the heat/capsaicin cutaneous sensitization model. Areas of hypersensitivity to brush stroke and von Frey (VF) filament stimulation were measured at baseline and after rekindling of skin sensitization. Another group of 15 volunteers was exposed to an identical schedule and set of sensitization procedures, but, in each session, received either gabapentin or placebo (Part B).

Results

Unlike previous reports, a similar reduction of areas of hyperalgesia was observed in all groups/sessions. Fading of areas of hyperalgesia over time was observed in Part A. In Part B, there was no difference in area reduction after gabapentin compared to placebo.

Conclusion

When using smaller thermal probes than originally proposed, modifications of other parameters of sensitization and/or rekindling process may be needed to allow the heat/capsaicin sensitization protocol to be used as initially intended. Standardization and validation of experimental pain models is critical to the advancement of translational pain research.

Does naloxone reinstate secondary hyperalgesia in humans after resolution of a burn injury? A placebo-controlled, double-blind, randomized, cross-over study

INTRODUCTION

Development of secondary hyperalgesia following a cutaneous injury is a centrally mediated, robust phenomenon. The pathophysiological role of endogenous opioid signalling to the development of hyperalgesia is unclear. Recent animal studies, carried out after the resolution of inflammatory pain, have demonstrated reinstatement of tactile hypersensitivity following administration of μ-opioid-receptor-antagonists. In the present study in humans, we analyzed the effect of naloxone when given after the resolution of secondary hyperalgesia following a first-degree burn injury.

METHODS

Twenty-two healthy volunteers were included in this placebo-controlled, randomized, double-blind, cross-over study. Following baseline assessment of thermal and mechanical thresholds, a first-degree burn injury (BI; 47°C, 7 minutes, thermode area 12.5 cm(2)) was induced on the lower leg. Secondary hyperalgesia areas around the BI-area, and separately produced by brief thermal sensitization on the contralateral thigh (BTS; 45°C, 3 minutes, area 12.5 cm(2)), were assessed using a polyamide monofilament at pre-BI and 1, 2, and 3 hours post-BI. At 72 hrs, BI and BTS secondary hyperalgesia areas were assessed prior to start of a 30 minutes intravenous infusion of naloxone (total dose 21 microg/kg) or placebo. Fifteen minutes after start of the infusion, BI and BTS secondary hyperalgesia areas were reassessed, along with mechanical and thermal thresholds.

RESULTS

Secondary hyperalgesia areas were demonstrable in all volunteers 1-3 hrs post-BI, but were not demonstrable at 72 hrs post-burn in 73-86% of the subjects. Neither magnitude of secondary hyperalgesia areas nor the mechanical and thermal thresholds were associated with naloxone-treated compared to placebo-treated subjects.

CONCLUSION

Naloxone (21 microg/kg) did not reinstate secondary hyperalgesia when administered 72 hours after a first-degree burn injury and did not increase BTS-generated hyperalgesia. The negative results may be due to the low dose of naloxone or insufficient tissue injury to generate latent sensitization.

New frontiers in assessing pain and analgesia in laboratory animals

BACKGROUND

Translating promising analgesic compounds into reliable pain therapeutics in humans is made particularly challenging by the difficulty in measuring the pain quantitatively. This problem is manifest not only in clinical settings in which patient pain assessments involve mostly subjective measures but also in preclinical settings wherein laboratory animals, most commonly rodents, are typically evaluated in stimulus-evoked response tests.

OBJECTIVE

Given the limitations of traditional pain tests, we sought out new approaches to measure pain, and analgesia, in laboratory animals.

METHODS

We reviewed the peer reviewed literature to identify pain tests that could be utilized in preclinical settings to understand the effects of new and established analgesics.

RESULTS/CONCLUSIONS

The tests identified include weight bearing differential, suppression of feeding, reduction in locomotor activity, gait analysis, conditioning models and functional MRI. Although the pharmacology of known and new analgesics has not been broadly established in these models, they hold the promise of better predictive utility for the discovery of pain relievers.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

Lamotrigine effectively blocks synaptic transmission between nociceptive primary afferents and secondary sensory neurons in the rat superficial spinal dorsal horn

It has been demonstrated that in the superficial spinal dorsal horn, Lamotrigine, which is known to block voltage-sensitive Na+ and Ntype Ca2+ channels, depresses neural activities evoked by sustained activation of nociceptive primary afferent fibres. In the present experiments, we study how Lamotrigine exerts its inhibitory effect on spinal nociceptive information-processing mechanisms. We show that Lamotrigine in an in vitro slice preparation effectively blocks synaptic transmission between primary afferents and secondary sensory neurons. Together with the robust increase in the failure rate and reduction in the amplitude of excitatory post-synaptic potentials (EPSPs) evoked by stimulation of nociceptive primary afferents, Lamotrigine causes a marked decrease in the number and amplitude of spontaneous EPSPs and a gradual shift of the resting membrane potential towards hyperpolarization. In addition, Lamotrigine treatment also changes the intrinsic firing pattern of superficial dorsal horn neurons. The results suggest that the effect of Lamotrigine on spinal nociceptive information-processing mechanisms is multiple: it depresses synaptic inputs from nociceptive primary afferents to secondary spinal sensory neurons and also weakens the intrinsic activities of nociceptive spinal neural circuits in the superficial spinal dorsal horn.

Lamotrigine for acute and chronic pain

BACKGROUND

This is an update of the original Cochrane review published in Issue 2, 2007. Some antiepileptic medicines have a place in the treatment of neuropathic pain (pain due to nerve damage). This updated review adds five new additional studies looking at evidence for Lamotrigine as an effective treatment for acute and chronic pain.

OBJECTIVES

To assess analgesic efficacy and adverse effects of the antiepileptic drug lamotrigine in acute and chronic pain.

SEARCH STRATEGY

Randomised controlled trials (RCTs) of lamotrigine in acute, and chronic pain (including cancer pain) were identified from MEDLINE, EMBASE and CENTRAL up to January 2011. Additional studies were sought from the reference list of the retrieved papers.

SELECTION CRITERIA

RCTs investigating the use of lamotrigine (any dose, by any route, and for any study duration) for the treatment of acute or chronic pain. Assessment of pain intensity or pain relief, or both, using validated scales. Participants were adults aged 18 and over. Only full journal publication articles were included.

DATA COLLECTION AND ANALYSIS

Dichotomous data (ideally for the outcome of at least 50% pain relief) were used to calculate relative risk with 95% confidence intervals. Meta-analysis was undertaken using a fixed-effect model. Numbers needed to treat to benefit (NNTs) were calculated as the reciprocal of the absolute risk reduction. For unwanted effects, the NNT becomes the number needed to harm (NNH) and was calculated.

MAIN RESULTS

Twelve included studies in 11 publications (1511 participants), all with chronic neuropathic pain: central post stroke pain (1), chemotherapy induced neuropathic pain (1), diabetic neuropathy (4), HIV related neuropathy (2), mixed neuropathic pain (2), spinal cord injury related pain (1), and trigeminal neuralgia (1); none investigated lamotrigine in acute pain. The update had five additional studies (1111 additional participants). Participants were aged between 26 and 77 years. Study duration was 2 weeks in one study and at least 6 weeks in the remainder; eight were of eight week duration or longer. There is no convincing evidence that lamotrigine is effective in treating acute or chronic pain at doses of about 200-400 mg daily. Almost 10% of participants taking lamotrigine reported a skin rash.

AUTHORS' CONCLUSIONS

The additional studies tripled participant numbers providing data for analysis, and new, more stringent criteria for outcomes and analysis were used; conclusions about lamotrigine's lack of efficacy in chronic pain did not change. Given availability of more effective treatments including antiepileptics and antidepressant medicines, lamotrigine does not have a significant place in therapy based on available evidence.

Assessing efficacy of non-opioid analgesics in experimental pain models in healthy volunteers: an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of analgesics and target the clinical indications for their use. This review, the second in a series of two, addresses how the efficacy of non-opioid analgesics have been assessed in human volunteers using experimental pain models.

METHODS

A literature search was completed for randomized controlled studies that included human experimental pain models, healthy volunteers and non-opioid analgesics.

RESULTS

Nonsteroidal anti-inflammatory drugs worked against various types of acute pain as well as in hyperalgesia. Analgesia from paracetamol was difficult to detect in experimental pain and the pain needed to be assessed with very sensitive methods like evoked brain potentials. The N-methyl-D-aspartate antagonists exemplified by ketamine generally needed strong, long-lasting or repeated pain in the skin for detectable analgesia, whereas pain in muscle and viscera generally was more easily attenuated. Gabapentin worked well in several models, particularly those inducing hyperalgesia, whereas lamotrigine was weak in modulation of experimental pain. Imipramine attenuated pain in most experimental models, whereas amitriptyline had weaker effects. Delta-9-tetrahydrocannabinol attenuated pain in only a few models.

CONCLUSIONS

Pain induction and assessment are very important for the sensitivity of the pain models. Generally, experimental pain models need to be designed with careful consideration of the pharmacological mechanisms and pharmacokinetics of analgesics. The drawback with the different study designs is also discussed. This knowledge can aid the decisions that need to be taken when designing experimental pain studies for compounds entering Phase I and II trials.

Human experimental pain models 3: heat/capsaicin sensitization and intradermal capsaicin models

The heat/capsaicin sensitization and intradermal capsaicin injection models are safe and noninvasive paradigms to generate stable, long-lasting, and reproducible injury capable of producing an area of both primary and secondary hyperalgesia. Risk of skin injury is substantially reduced since lower levels of thermal and chemical irritation produce long-lasting cutaneous hyperalgesia. Rekindling sustains central sensitization by providing peripheral nociceptive input. The intradermal capsaicin model has been widely used to test analgesic efficacy for a wide range of analgesics. Unlike the heat/capsaicin sensitization model, intradermal capsaicin results in a brief painful stimulus followed by a long lasting area of secondary hyperalgesia. The intradermal injection of capsaicin results in a transient, intense stinging sensation at the site of injection (e.g. heat allodynia) followed by a persistent area of secondary tactile allodynia.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers - an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of action of analgesics and target the clinical indications for their use. This review addresses how the efficacy of opioids can be assessed in human volunteers using experimental pain models. The drawback with the different study designs is also discussed.

METHOD

A literature search was completed for randomized controlled studies which included human experimental pain models, healthy volunteers and opioids.

RESULTS

Opioids with a strong affinity for the micro-opioid receptor decreased the sensation in a variety of experimental pain modalities, but strong tonic pain was attenuated more than short lasting pain and non-painful sensations. The effects of opioids with weaker affinity for the micro-opioid receptor were detected by a more narrow range of pain models, and the assessment methods needed to be more sensitive.

CONCLUSION

The way the pain is induced, assessed and summarized is very important for the sensitivity of the pain models. This review gives an overview of how different opioids perform in experimental pain models. Generally experimental pain models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. This knowledge can aid the decisions needed to be taken when designing experimental pain studies for compounds entering phase 1 clinical trials.

Changes in morphine analgesia and side effects during daily subcutaneous administration in healthy volunteers

Tolerance to the anti-nociceptive effects of opioids develops rapidly in animals. In contrast, humans with chronic pain show little or no loss of pain relief in prospective opioid trials of 4-8 weeks duration. Employing the Brief Thermal Sensitization model to induce transient cutaneous secondary hyperalgesia, we tested the hypothesis that opioid analgesic tolerance would develop rapidly. In this outpatient randomized placebo-controlled study, subjects in the MMMMP group received two injections of subcutaneous morphine 6 mg (150 min apart) on Monday-Thursday (total 48 mg over 4 days) and matching saline placebo on Friday. Subjects in the PPPPM group received placebo on Monday-Thursday and morphine (total 12 mg) on Friday. Sixty-one healthy volunteers were enrolled; morphine side effects accounted for all nine non-completions. Compared to the first placebo day, the reduction in the area of secondary hyperalgesia on the first morphine day was significant and robust in both groups. Morphine suppression of the painfulness of skin heating and elevation of the heat pain detection threshold were also significant. During 4 days of twice-daily injections, the decline in anti-hyperalgesic effects of morphine did not reach statistical significance (p=0.06) compared to placebo. Morphine side effects did not correlate with anti-hyperalgesic effects and withdrawal symptoms did not emerge. As 4 days is the threshold for demonstrating analgesic tolerance to twice-daily morphine in animal models, a longer period of opioid exposure in healthy volunteers might be needed to detect analgesic tolerance.

The role of pacemaker currents in neuropathic pain

Hyperpolarization-activated cation nonselective cyclic nucleotide-gated (HCN) channels mediate pacemaker currents that control basic rhythmic processes including heartbeat. Alterations in HCN channel expression or function have been described in both epilepsy and cardiac arrhythmias. Recent evidence suggests that pacemaker currents may also play an important role in ectopic neuronal activity that manifests as neuropathic pain. Pacemaker currents are subject to endogenous regulation by cyclic nucleotides, pH and perhaps phosphorylation. In addition, a number of neuromodulators with known roles in pain affect current density and kinetics. The pharmacology of a number of drugs that are commonly used to treat neuropathic pain includes effects on pacemaker currents. Altered pacemaker currents in injured tissues may be an important mechanism underlying neuropathic pain, and drugs that modulate these currents may offer new therapeutic options.

Lamotrigine for acute and chronic pain

BACKGROUND

Anticonvulsant medicines have a place in the treatment of neuropathic pain (pain due to nerve damage). This review looks at the evidence for the pain relieving properties of lamotrigine.

OBJECTIVES

To assess the analgesic efficacy and adverse effects of the anticonvulsant lamotrigine for acute and chronic pain.

SEARCH STRATEGY

Randomised Controlled Trials (RCTs) of lamotrigine (and key brand names Lamictal, Lamictin, Neurium) in acute, chronic or cancer pain were identified from MEDLINE (1966 to August 2006), EMBASE 1994 to August 2006 and the CENTRAL register on The Cochrane Library (Issue 3, 2006). Additional reports were sought from the reference list of the retrieved papers.

SELECTION CRITERIA

RCTs investigating the use of lamotrigine (any dose and by any route) for treatment of acute or chronic pain. Assessment of pain intensity or pain relief, or both, using validated scales. Participants were adults aged 18 and over. Only full journal publication articles were included.

DATA COLLECTION AND ANALYSIS

Dichotomous data were used to calculate relative risk with 95% confidence intervals using a fixed effects model unless significant statistical heterogeneity was found. Continuous data was also reported where available. Meta-analysis was undertaken using a fixed effect model unless significant heterogeneity was present (I(2) >50%) in which case a random effects model was used. Numbers-needed-to-treat (NNTs) were calculated as the reciprocal of the absolute risk reduction. For unwanted effects, the NNT becomes the number-needed-to-harm (NNH) and was calculated.

MAIN RESULTS

Sixteen studies were identified. Nine studies were excluded. No studies for acute pain were identified. The seven included studies involved 502 participants, all for neuropathic pain. The studies covered the following conditions: central post stroke pain (1), diabetic neuropathy (1), HIV related neuropathy (2), intractable neuropathic pain (1), spinal cord injury related pain (1) and trigeminal neuralgia (1). The studies included participants in the age range of 26 to 77 years. Only one study for HIV related neuropathy had a statistically significant result for a sub group of patients on anti-retroviral therapy; this result is unlikely to be clinically significant NNT 4.3 (95% CI 2.3 to 37). Approximately 7% of participants taking lamotrigine reported a skin rash.

AUTHORS' CONCLUSIONS

Given the availability of more effective treatments including anticonvulsants and antidepressant medicines, lamotrigine does not have a significant place in therapy at present. The limited evidence currently available suggests that lamotrigine is unlikely to be of benefit for the treatment of neuropathic pain.

Butyl 2-(4-[1.1′-biphenyl]-4-yl-1H-imidazol-2-yl)ethylcarbamate, a potent sodium channel blocker for the treatment of neuropathic pain

A series of 4-arylimidazole carbamates was synthesized and their binding affinities to the site-2 sodium (Na+) channel were determined. SAR studies led to the identification of compound 10, a potent Na+ channel blocker which was efficacious in pain models in vivo.

Anti-convulsants and anti-depressants

Damage to a nerve should only lead to sensory loss. While this is common, the incidence of spontaneous pain, allodynia and hyperalgesia indicate marked changes in the nervous system that are possible compensations for the loss of normal function that arises from the sensory loss. Neuropathic pain arises from changes in the damaged nerve which then alter function in the spinal cord and the brain and lead to plasticity in areas adjacent to those directly influenced by the neuropathy. The peripheral changes drive central compensations so that the mechanisms involved are multiple and located at a number of sites. Nerve damage increases the excitability of both the damaged and undamaged nerve fibres, neuromas and the cell bodies in the dorsal root ganglion. These peripheral changes are substrates for the ongoing pain and the efficacy of excitability blockers such as carbamazepine, lamotrigine and mexiletine, all anti-convulsants. A better understanding of ion channels at the sites of injury has shown important roles of particular sodium, potassium and calcium channels in the genesis of neuropathic pain. Within the spinal cord, increases in the activity of calcium channels and the receptors for glutamate, especially the N-methyl-D-aspartate (NMDA) receptor, trigger wind-up and central hyperexcitability. Increases in transmitter release, neuronal excitability and receptive field size result from the damage to the peripheral nerves. Ketamine and gabapentin/pregabalin, again with anti-convulsant activity, may interact with these mechanisms. Ketamine acts on central spinal mechanisms of excitability whereas gabapentin acts on a subunit of calcium channels that is responsible for the release of pain transmitters into the spinal cord. In addition to these spinal mechanisms of hyperexcitability, spinal cells participate in a spinal-supraspinal loop that involves parts of the brain involved in affective responses to pain but also engages descending excitatory and inhibitory systems that use the monoamines. These pathways become more active after nerve injury and are the site of action of anti-depressants. This chapter reviews the evidence and mechanisms of drugs, both anti-depressants and anti-convulsants, that are believed to be effective in pain control, with a major emphasis on the neuropathic state.

Placebo-Controlled Comparison of a Morphine/Dextromethorphan Combination With Morphine on Experimental Pain and Hyperalgesia in Healthy Volunteers

In this double-blind, placebo-controlled, crossover study we compared the analgesic effect of a single oral dose of 30-mg dextromethorphan and 30-mg morphine combination (MS/DM) to 30 mg morphine (MS) alone and either placebo or 30 mg dextromethorphan (DM) on cutaneous sensitization induced by heat/capsaicin (topical) sensitization on the forearm and the brief thermal sensitization model on the thigh in 22 healthy volunteers. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation in both sensitization models and the painfulness of acute thermal noxious stimulation on the upper arm. Compared with placebo, both MS/DM and morphine had some effect on the secondary hyperalgesia and reduced the painfulness of a noxious thermal stimulus. The analgesic effect of MS/DM was not superior to that of morphine on any outcome measure. These results differ from preclinical studies with animal experimental pain models in which DM markedly potentiated the analgesic effects of opioids, but they are in accordance with recent clinical trials for chronic pain.

PERSPECTIVE

Adding dextromethorphan to morphine (1:1 ratio) did not enhance analgesia on measures of experimental cutaneous sensitization and acute noxious thermal stimulation in healthy volunteers. The results differ from preclinical studies but agree with clinical trials. Human experimental models of pain and neuronal sensitization, which are responsive to oral opioids, allow efficient study of opioid combination analgesics and simplify the process for determining the optimal dose and/or dose ratio.

Differential sensitivity of three experimental pain models in detecting the analgesic effects of transdermal fentanyl and buprenorphine

This is the first randomized controlled trial that tests the analgesic efficacy of transdermally delivered opioids in healthy volunteers and that assesses the sensitivity of different experimental pain tests to detect analgesia in this setting. Transdermal application of the full agonist fentanyl (TDF: 12.5 or 25 microg/h) and the partial agonist buprenorphine (TDB: 35 microg/h) was compared in three experimental models of acute pain (heat pain, painful electrical stimulation, cold pressor) in a double-blind, randomized, placebo-controlled, 4-arm crossover study with 20 healthy subjects (15 men, 5 women). Patches were administered for 72 h and pain levels measured at baseline and 24 and 72 h, with an 11-day wash-out. The cold pressor test was most sensitive to analgesic effects, with significant reductions in area under the pain intensity curve for all active compounds at 24 h (average reductions: 14% TDF 12.5 microg/h, 35% TDF 25 microg/h, 43% TDB 35 microg/h). There were significant increases in heat pain threshold for TDF 25 microg/h and TDB 35 microg/h. Painful electrical stimulation failed to demonstrate an analgesic effect. The magnitude of analgesia in the cold pressor model showed some correlation with TDF dosage and comparable effects for the full agonist fentanyl and the partial agonist buprenorphine. We conclude that the cold pressor test was most sensitive to analgesic effects in healthy subjects and that a transdermal dose of 12.5 microg/h fentanyl achieved significant pain reduction compared with placebo. Subjects experienced opioid-typical AEs including dizziness, nausea and vomiting. No serious AEs occurred.

Comparison of antinociceptive actions of standard analgesics in attenuating capsaicin and nerve-injury-induced mechanical hypersensitivity

Intradermal capsaicin injection produces immediate spontaneous pain behaviors, and a secondary mechanical hypersensitivity (SMH) that is employed in the clinic as a model potentially predictive of human neuropathic pain. Presently, we have characterized capsaicin-induced SMH in rats, and compared pharmacological actions of standard analgesics in this and two nerve injury models, the L5/L6 spinal nerve ligation (SNL) and sciatic nerve chronic constriction injury (CCI) models. Intraplantar capsaicin produced dose-related SMH (enhanced paw withdrawal response to von Frey monofilament stimulation at an area away from injection site) that lasted for over 4 h. While pretreatment with a potent selective transient receptor potential vanilloid receptor-1 (TRPV1) antagonist A-425619 (1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea) prevented development of acute nocifensive (flinching) behavior immediately following capsaicin injection (ED(50)=4.9 mg/kg), the compound failed to attenuate the SMH when administered 2 h following capsaicin (10 microg/10 microl). Additional standard analgesics were also tested 3 h following intraplantar capsaicin in the SMH model. Comparison of their potencies in attenuating mechanical hypersensitivity in capsaicin, SNL and CCI models revealed similar ED(50)s for morphine (2.3 mg/kg, 1.6 mg/kg and 3.2 mg/kg, respectively), gabapentin (33.1 mg/kg, 33.9 mg/kg and 26.3 mg/kg, respectively) and lamotrigine (9.1 mg/kg, 8.9 mg/kg and 15.5 mg/kg, respectively). Duloxetine produced 50-65% effect at the highest tested dose (50 mg/kg), whereas the highest tested doses of morphine (10 mg/kg), gabapentin (85.5 mg/kg) and lamotrigine (30 mg/kg) all produced >70% efficacy in capsaicin SMH, SNL and CCI models. In contrast, celecoxib and ibuprofen showed weak effects in all three models. All standard analgesics generally had weak efficacy in attenuating capsaicin-induced immediate acute flinching behavior when administered before capsaicin. These results provide further support to the suggestions that distinct pharmacological mechanisms underlie capsaicin-induced acute nocifensive and SMH behaviors, and certain neuronal mechanisms underlying neuropathic pain states are also contributory to capsaicin-induced SMH.

Review article: The role of anticonvulsant drugs in postoperative pain management: a bench-to-bedside perspective

PURPOSE

Anticonvulsant drugs are effective in the treatment of chronic neuropathic pain but were not, until recently, thought to be useful in more acute conditions such as postoperative pain. However, similar to nerve injury, surgical tissue injury is known to produce neuroplastic changes leading to spinal sensitization and the expression of stimulus-evoked hyperalgesia and allodynia. Pharmacological effects of anticonvulsant drugs which may be important in the modulation of these postoperative neural changes include suppression of sodium channel, calcium channel and glutamate receptor activity at peripheral, spinal and supraspinal sites. The purpose of this article is to review preclinical evidence and clinical trial data describing the efficacy and safety of anticonvulsant drugs in the setting of postoperative pain management.

SOURCE

A Medline search was performed to retrieve available literature on the basic and clinical pharmacology of anticonvulsant drugs as they pertain to postoperative pain management.

PRINCIPAL FINDINGS

Numerous laboratory studies have described analgesic effects of different anticonvulsant drugs in experimental pain models. Furthermore, several recent clinical trials have shown that anticonvulsants may reduce spontaneous and movement-evoked pain, as well as decrease opioid requirements postoperatively. Some early findings suggest further that anticonvulsant drugs may alleviate postoperative anxiety, accelerate postoperative functional recovery and reduce chronic postsurgical pain.

CONCLUSION

Given the incomplete efficacy of currently available non-opioid analgesics, and the identified benefits of opioid sparing, anticonvulsant medications may be useful adjuncts for postoperative analgesia. Further research in this field is warranted.

Gender differences in pain and secondary hyperalgesia after heat/capsaicin sensitization in healthy volunteers

In most published studies women are more sensitive to experimental pain than men. Enhanced central pain processing in women has been suggested, but psychosocial factors might also have affected the findings. Data from five completed healthy volunteer studies were analyzed to investigate gender differences in development of secondary hyperalgesia. Cutaneous hyperalgesia was induced with the heat/capsaicin sensitization model. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation after heat and capsaicin sensitization, rating of pain during heat/capsaicin sensitization, and heat pain detection thresholds. There was a trend toward smaller areas of secondary hyperalgesia in women. After adjusting for estimated gender differences in forearm surface area, areas to brush but not von Frey hair stimulation after capsaicin sensitization were larger in women. Peak pain, but not total pain, during prolonged noxious thermal stimulation was higher in women. There was no gender difference in pain ratings during capsaicin sensitization or in heat pain detection thresholds. The results provided only limited support to the hypothesis that gender differences in clinical pain syndromes can be explained by enhanced central sensitization in women.

PERSPECTIVE

Our findings suggest that gender differences in nociceptive transmission and neuronal sensitization are small and provide only limited support to the hypothesis that gender differences in acute and chronic pain syndromes can be explained by enhanced central sensitization in women.

Pharmacology of 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide: A Potent, Broad-Spectrum State-Dependent Sodium Channel Blocker for Treating Pain States

Voltage-gated Na(+) channels may play important roles in establishing pathological neuronal hyperexcitability associated with chronic pain in humans. Na(+) channel blockers, such as carbamazepine (CBZ) and lamotrigine (LTG), are efficacious in treating neuropathic pain; however, their therapeutic utility is compromised by central nervous system side effects. We reasoned that it may be possible to gain superior control over pain states and, in particular, a better therapeutic index, by designing broad-spectrum Na(+) channel blockers with higher potency, faster onset kinetics, and greater levels of state dependence than existing drugs. 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide (PPPA) is a novel structural analog of the state-dependent Na(+) channel blocker V102862 [4-(4-fluorophenoxy)benzaldehyde semicarbazone]. Tested on recombinant rat Na(v)1.2 channels and native Na(+) currents in cultured rat dorsal root ganglion neurons, PPPA was approximately 1000 times more potent, had 2000-fold faster binding kinetics, and > or =10-fold higher levels of state dependence than CBZ and LTG. Tested in rat pain models against mechanical endpoints, PPPA had minimal effective doses of 1 to 3 mg/kg p.o. in partial sciatic nerve ligation, Freund's complete adjuvant, and postincisional pain. In all cases, efficacy was similar to clinically relevant comparators. Importantly, PPPA did not produce motor deficits in the accelerating Rotarod assay of ataxia at doses up to 30 mg/kg p.o., indicating a therapeutic index >10, which was superior to CBZ and LTG. Our experiments suggest that high-potency, broad-spectrum, state-dependent Na(+) channel blockers will have clinical utility for treating neuropathic, inflammatory, and postsurgical pain. Optimizing the biophysical parameters of broad-spectrum voltage-gated Na(+) channel blockers may lead to improved pain therapeutics.

A comparative study of oxycodone and morphine in a multi-modal, tissue-differentiated experimental pain model

Visceral pain can be difficult to treat with classical mu-opioid agonists and it has been suggested to use opioids with distinct pharmacological profiles. In animal experiments, oxycodone has shown different effects compared to morphine, and clinical observations have shown that oxycodone may occasionally be superior to, e.g., morphine in the treatment of visceral pain. In the current study, we randomised 24 healthy subjects to treatment with either morphine (30 mg), oxycodone (15 mg) or placebo in a crossover study. The experimental pain model involved multi-modal (mechanical, thermal and electrical) pain tests in the skin, muscles and viscera. The pain tests were carried out at baseline and 30, 60 and 90 min after oral administration of the drugs. The model showed effect of the two opioids compared to placebo on all stimulus modalities in all three types of tissues (all P values <0.001). Both opioids attenuated the sensory response mainly to painful stimulations. Morphine and oxycodone were equipotent in pain modulation of the skin and muscles, but oxycodone had superior analgesic effect to both morphine and placebo on the mechanical (P<0.001) and thermal (P<0.001) stimulations of the oesophagus. In conclusion, the multi-modal and tissue-differentiated pain model could link findings from animal experiments to clinical findings. A different pharmacological profile of oxycodone compared to that of morphine was shown, and thus oxycodone may be a useful alternative to morphine in the treatment of visceral pain syndromes.

New Treatments for Neuropathic Pain

Existing treatments for neuropathic pain deliver inadequate pain relief, unacceptable side effects, or both. The unmet medical need for more effective treatment is driving a large volume of research to discover new drugs. Most existing treatments are drugs introduced to treat other pain conditions or other medical conditions, such as antidepressants and anticonvulsants, which were found empirically to be effective for neuropathic pain. Only recently have drug discovery efforts have become mechanistically driven, addressing targets identified by a molecular neurobiological approach to the pathophysiology of neuropathic states.

Experimental Human Pain Models: A Review of Standardised Methods for Preclinical Testing of Analgesics

Treatment of pain is one of the major challenges in clinical medicine. However, it is often difficult to evaluate the effect of a treatment, as the many symptoms of the underlying diseases often confound this assessment. Furthermore, as the pain mechanisms in many diseases are poorly understood, the limited successful trial and error approach is most often used in the selection of analgesics. Hence, there is a need for new methods in the characterization and treatment of pain. Human experimental pain models offer the possibility to explore the pain system under controlled settings. The models can also be used to screen the analgesic profiles of drugs targeted to treat pain. This review gives a brief introduction to the methods used to evoke and assess pain in the skin, muscle and viscera. New methods using multimodal stimulation and activation of central pain mechanisms can to a higher degree mimic the clinical situation, and such methods are recommended in the future screening of analgesics. Examples of the use of experimental pain models in the testing of analgesics are given. With these models the therapeutic spectrum may be defined from a differentiated knowledge on the effect of drugs on the pain system. Such information may be used in the future guidelines for trials and clinical use of analgesics.

Lack of effect of two oral sodium channel antagonists, lamotrigine and 4030W92, on intradermal capsaicin-induced hyperalgesia model

Preclinical studies have emphasized that persistent small afferent input will induce a state of central facilitation, which can be regulated by systemically administered sodium channel blockers. We have extended these preclinical studies to the human volunteers by examining the effects of lamotrigine and 4030W92, two structurally related voltage-sensitive sodium channel antagonists, on acute sensory thresholds and facilitated processing induced by intradermal capsaicin. Fifteen healthy subjects received 4030W92, lamotrigine, and placebo in a randomized order using double-blinded crossover design methodology in three sessions each separated by a 7-day washout period. In each session, baseline neurosensory testing was performed on the volar aspect of the subject's left forearm. Subjects were then dosed with either lamotrigine (300 mg), 4030W92 (100 mg), or placebo, followed 2 h later by capsaicin (100 microg) injected intradermally on the volar aspect of the left forearm. Pain scores, blood pressure, heart rate, and respiratory rate were measured at the time of injection and every 5 min for 15 min. Fifteen minutes after the capsaicin injection, the hyperalgesic area was determined by von Frey hair, stroking, and heat; the flare response was outlined; and neurosensory testing again was performed halfway between the edge of the hyperalgesic area and the capsaicin injection site. While capsaicin significantly decreased the hot pain and VF pain thresholds, oral lamotrigine and 4030W92 failed to alter this response to capsaicin, relative to placebo treatment. Similarly, oral lamotrigine or 4030W92 did not alter the pain scores reported from mechanical pain stimuli at any time postcapsaicin. This study showed a lack of effect of two structurally similar sodium channel antagonists on a human experimental pain model using intradermal capsaicin, which is consistent with other studies on the effects of sodium channel antagonists of capsaicin-induced pain and hyperalgesia. This lack of effect stands in contrast to reported effects of sodium channel antagonists on preclinical models of cutaneous hyperalgesia or effects of lamotrigine on clinical neuropathic pain.