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

Physiological Sensor Modality Sensitivity Test for Pain Intensity Classification in Quantitative Sensory Testing

Chronic pain is prevalent and disproportionately impacts adults with a lower quality of life. Although subjective self-reporting is the "gold standard" for pain assessment, tools are needed to objectively monitor and account for inter-individual differences. This study introduced a novel framework to objectively classify pain intensity levels using physiological signals during Quantitative Sensory Testing sessions. Twenty-four participants participated in the study wearing physiological sensors (blood volume pulse (BVP), galvanic skin response (GSR), electromyography (EMG), respiration rate (RR), skin temperature (ST), and pupillometry). This study employed two analysis plans. Plan 1 utilized a grid search methodology with a 10-fold cross-validation framework to optimize time windows (1-5 s) and machine learning hyperparameters for pain classification tasks. The optimal time windows were identified as 3 s for the pressure session, 2 s for the pinprick session, and 1 s for the cuff session. Analysis Plan 2 implemented a leave-one-out design to evaluate the individual contribution of each sensor modality. By systematically excluding one sensor's features at a time, the performance of these sensor sets was compared to the full model using Wilcoxon signed-rank tests. BVP emerged as a critical sensor, significantly influencing performance in both pinprick and cuff sessions. Conversely, GSR, RR, and pupillometry demonstrated stimulus-specific sensitivity, significantly contributing to the cuff session but with limited influence in other sessions. EMG and ST showed minimal impact across all sessions, suggesting they are non-critical and suitable for reducing sensor redundancy. These findings advance the design of sensor configurations for personalized pain management. Future research will focus on refining sensor integration and addressing stimulus-specific physiological responses.

Development and validation of a home quantitative sensory testing tool-kit to assess changes in sensory and pain processing: a study in healthy young adults

ABSTRACT

Quantitative sensory testing (QST) is a set of methods for quantifying somatosensory functioning. Limitations of laboratory-based QST (LQST) include high cost, complexity in training, lack of portability, and time requirements for testing. Translating QST to a home setting could facilitate future research and clinical care. The objective of this study was to develop a home QST (HQST) tool-kit that is cost-effective, easy to use, and detects changes in sensory and pain processing. Thirty-two young healthy adults underwent sensory testing on their nondominant forearm using standard in-person LQST, followed by "simulated HQST" using video guidance in a separate room from the investigator before and after application of either a lidocaine or capsaicin cream. We observed good agreement between HQST and LQST scores, with significant correlations observed between the pinprick, pressure, cold and heat measures (|ρ| range = 0.36-0.54). The participants rated the HQST protocol as highly acceptable and safe but can be improved in future implementations. Home QST was able to detect hypoesthesia to vibration after lidocaine cream application ( P = 0.024, d = 0.502) and could detect hypoalgesia and hyperalgesia to pressure and heat pain sensitivity tests after application of lidocaine and capsaicin creams, respectively ( P -value range = <0.001-0.036, d -value range = 0.563-0.901). Despite limitations, HQST tool-kits may become a cost-effective, convenient, and scalable approach for improving sensory profiling in clinical care and clinical research.

Topical capsaicin modulates the two-point discrimination threshold-Modulation depends on stimulation modality and intensity

BACKGROUND

Spatial acuity concerns the ability to localize and discriminate sensory input and is often tested using the two-point discrimination threshold (2PDT). Sensitization of the pain system can affect the spatial acuity, but it is unclear how 2PDTs of different testing modalities are affected. The aim was to investigate if the 2PDTs for mechanical and heat stimulation at different intensities were modulated by topical capsaicin sensitization.

METHODS

30 healthy subjects were divided into either a capsaicin or a placebo group. The 2PDT was tested using two different modalities, mechanical and thermal (laser) delivered at innocuous and noxious intensities. The 2PDT were determined at baseline and re-assessed 48 h later. In the follow-up session, the subjects either had a capsaicin patch (8%) or placebo patch placed in the testing area for 30 min before re-testing the 2PDT.

RESULTS

The 2PDT was highly dependent on stimulation modality and intensity. The lowest 2PDT was found for innocuous mechanical stimuli (40.0 mm, 95% CI 38.1-41.9 mm), and the highest 2PDT was found for innocuous thermal stimuli (81.7 mm, 95% CI 73.9-89.5 mm). Topical capsaicin generally increased the 2PDT, but this was only significant for innocuous mechanical stimuli. The perceived intensity of the stimuli was increased following capsaicin and was generally higher for noxious stimuli than for innocuous stimuli (ANOVA, p < 0.001).

CONCLUSIONS

This study showed that capsaicin provoked pain sensitization increased the 2PDT. The 2PDT tested using innocuous mechanical stimuli showed less variable results indicating that this test is most suitable to detect this aspect of spatial acuity.

SIGNIFICANCE STATEMENT

This study investigated how the two-point discrimination threshold (2PDT) can be modulated by topical capsaicin. The 2PDT was assessed for two different modalities (thermal and mechanical) and for two different intensities (innocuous and noxious) before and after capsaicin. The results showed that the 2PDT was generally impaired following capsaicin, but this was only significant for mechanical innocuous stimuli. Furthermore, it was shown that mechanical innocuous stimuli assessed the 2PDT with lower variability than other combinations.

Analgesia of noninvasive electrical stimulation of the dorsolateral prefrontal cortex: A systematic review and meta-analysis

OBJECTIVE

The dorsolateral prefrontal cortex (DLPFC) is implicated in pain modulation, suggesting its potential as a therapeutic target for pain relief. However, studies on transcranial electrical stimulation (tES) over the DLPFC yielded diverse results, likely due to differences in stimulation protocols or pain assessment methods. This study aims to evaluate the analgesic effects of DLPFC-tES using a meta-analytical approach.

METHODS

A meta-analysis of 29 studies involving 785 participants was conducted. The effects of genuine and sham DLPFC-tES on pain perception were examined in healthy individuals and patients with clinical pain. Subgroup analyses explored the impact of stimulation parameters and pain modalities.

RESULTS

DLPFC-tES did not significantly affect pain outcomes in healthy populations but showed promise in reducing pain-intensity ratings in patients with clinical pain (Hedges' g = -0.78, 95% CI = [-1.33, -0.24], p = 0.005). Electrode placement significantly influenced the analgesic effect, with better results observed when the anode was at F3 and the cathode at F4.

CONCLUSIONS

DLPFC-tES holds potential as a cost-effective pain management option, particularly for clinical populations. Optimizing electrode placement, especially with an symmetrical configuration, may enhance therapeutic efficacy. These findings underscore the promise of DLPFC-tES for alleviating perceived pain intensity in clinical settings, emphasizing the importance of electrode placement optimization.

Randomized, placebo-controlled study on the effects of intravenous GSK3858279 (anti-CCL17) on a battery of evoked pain tests in healthy participants

C-C Motif Chemokine Ligand 17 (CCL17) is a chemokine that binds and signals through the G-protein coupled CC-chemokine receptor 4 and has been implicated in the development of inflammatory and arthritic pain. GSK3858279 is a high-affinity, first-in-class, monoclonal antibody, binding specifically to CCL17 and inhibiting downstream signaling. In this phase I, randomized, single-center, double-blind, placebo-controlled, three-period, incomplete-block crossover study (NCT04114656), the analgesic effects and safety of intravenous GSK3858279 were assessed in a battery of evoked acute pain assessments on healthy, adult (aged ≥18 years), male participants. Participants were randomized 1:1 to receive either one placebo (0.9% w/v NaCl) dose followed by two GSK3858279 doses (PAA treatment sequence), or one GSK3858279 dose followed by two placebo doses (APP treatment sequence). The co-primary end points were ultraviolet B heat pain detection threshold (°C), cold pressor time to pain tolerance threshold (PTT, sec), and electrical PTT (mA, single stimulus). Twenty-one participants were enrolled (PAA = 11; APP = 10). Mean age (standard deviation) was 29.3 (7.9) years for PAA, 31.1 (7.7) years for APP. No significant differences were observed in the analgesic effect between GSK3858279 and placebo for any end point. Exposure to GSK3858279 was similar between Period 1 (APP sequence), and Periods 2 and 3 (PAA sequence), with some GSK3858279 carry-over. Changes in serum CCL17 levels were consistent with the expected GSK3858279 activity. All drug-related adverse events were mild in intensity and caused no discontinuations. The absence of an efficacy signal in this acute pain model does not preclude efficacy in chronic pain states.

Exercise-induced pain within endurance exercise settings: Definitions, measurement, mechanisms and potential interventions

Pain can be defined as an unpleasant sensory and emotional experience associated with or resembling that associated with actual or potential tissue damage. Though consistent with this definition, different types of pain result in different behavioural and psychophysiological responses. For example, the transient, non-threatening, acute muscle pain element of exercise-induced pain (EIP) is entirely different from other pain types like delayed onset muscle soreness, muscular injury or chronic pain. However, studies often conflate the definitions or assume parity between distinct pain types. Consequently, the mechanisms through which pain might impact exercise behaviour across different pain subcategories may be incorrectly assumed, which could lead to interventions or recommendations that are inappropriate. Therefore, this review aims to distinguish EIP from other subcategories of pain according to their aetiologies and characteristics, thereby providing an updated conceptual and operational definition of EIP. Secondly, the review will discuss the experimental pain models currently used across several research domains and their relevance to EIP with a focus on the neuro-psychophysiological mechanisms of EIP and its effect on exercise behaviour and performance. Finally, the review will examine potential interventions to cope with the impact of EIP and support wider exercise benefits. HIGHLIGHTS: What is the topic of this review? Considerations for future research focusing on exercise-induced pain within endurance exercise settings. What advances does it highlight? An updated appraisal and guide of research concerning exercise-induced pain and its impact on endurance task behaviour, particularly with reference to the aetiology, measurement, and manipulation of exercise-induced pain.

Nociception level index variations in ICU: curarized vs non-curarized patients - a pilot study

PURPOSE

Pain is a major physiological stressor that can worsen critical medical conditions in many ways. Currently, there is no reliable monitoring tool which is available for pain monitoring in the deeply sedated ± curarized critically ill patients. This study aims to assess the effectiveness of the multiparameter nociception index (NOL®) in the critical care setting. We compared NOL with traditionally used neurovegetative signs and examined its correlation with sedation depth measured by bispectral index (BIS®) electroencephalographic (EEG) monitoring.

METHODS

This retrospective monocentric cohort study was conducted in a general intensive care unit, including patients who required moderate-to-deep levels of sedation with or without continuous neuromuscular blockade. The performance of NOL was evaluated both in the entire studied population, as well as in two subgroups: curarized and non-curarized patients.

RESULTS

NOL demonstrated greater accuracy than all other indicators in pain detection in the overall population. In the non-curare subgroup, all indices correctly recognized painful stimulation, while in the patients subjected to neuromuscular blocking agent's infusion, only NOL properly identified nociception. In the former group, EEG's relation to nociception was on the border of statistical significance, whereas in the latter BIS showed no correlation with NOL.

CONCLUSION

NOL emerges as a promising device for pain assessment in the critical care setting and exhibits its best performance precisely in the clinical context where reliable pain assessment methods are most lacking. Furthermore, our research confirms the distinction between sedation and analgesia, highlighting the necessity for distinct monitoring instruments to accurately assess them.

A matter of personality and point of view: How the interplay of reinforcement sensitivity and general attitudes towards pain impacts the responsivity to acute pain

BACKGROUND

The relatively stable individual differences reflected in Grey's revised reinforcement sensitivity theory (rRST), with foundations in neurophysiology and learning theory, appear particularly applicable to the study of pain. However, remarkably little research has been conducted in this area. In acute pain, activation of the behavioural approach system (BAS), the behavioural inhibition system (BIS) and the fight, flight, freezing system might depend on an individual's evaluation of pain. It was thus hypothesised that higher-order interactions of rRST traits and pain attitudes affect pain responsiveness.

METHODS

To investigate relationships between rRST traits and pain attitudes and to identify patterns between clusters of participants, we conducted a study in which N = 275 healthy participants filled in Reuter and Montag's rRST-Q and the General Attitudes Towards Pain Inventory (GATPI). Experimental (pressure, electrical, thermal) and self-report data of pain responsiveness were collected in a subsample (N = 113).

RESULTS

We found significant correlations between rRST-Q and GATPI subscales of up to r = 0.34. Two clusters were identified, significantly differing in rRST-Q and GATPI scores. Pain avoiders, characterized by high BIS, flight, freezing and negative pain attitudes, were found to be significantly more pain sensitive than pain approachers, characterized by high BAS, fight and positive pain attitudes. Moderate effects (d = 0.56 to d = 0.70) were demonstrated in subjective and objective measures and across pain modalities.

CONCLUSIONS

The present study expands the scientific knowledge on factors influencing pain responsiveness. Future research is needed to confirm implications for chronic pain prevention and therapy, particularly concerning pain avoiders.

SIGNIFICANCE

We have identified two clusters of participants, pain avoiders and pain approachers, that not only present differential patterns of revised reinforcement theory traits and general attitudes towards pain but also differ in their pain responsiveness. Pain avoiders appeared more pain sensitive compared to pain approachers, both in objective and subjective measurements, with implications for the improvement of chronic pain prevention and therapy.

Systematic review and co-ordinate based meta-analysis to summarize the utilization of functional brain imaging in conjunction with human models of peripheral and central sensitization

BACKGROUND AND OBJECTIVE

Functional magnetic resonance imaging, in conjunction with models of peripheral and/or central sensitization, has been used to assess analgesic efficacy in healthy humans. This review aims to summarize the use of these techniques to characterize brain mechanisms of hyperalgesia/allodynia and to evaluate the efficacy of analgesics.

DATABASES AND DATA TREATMENT

Searches were performed (PubMed-Medline, Cochrane, Web of Science and Clinicaltrials.gov) to identify and review studies. A co-ordinate based meta-analysis (CBMA) was conducted to quantify neural activity that was reported across multiple independent studies in the hyperalgesic condition compared to control, using GingerALE software.

RESULTS

Of 217 publications, 30 studies met the inclusion criteria. They studied nine different models of hyperalgesia/allodynia assessed in the primary (14) or secondary hyperalgesia zone (16). Twenty-three studies focused on neural correlates of hyperalgesic conditions and showed consistent changes in the somatosensory cortex, prefrontal cortices, insular cortex, anterior cingulate cortex, thalamus and brainstem. The CBMA on 12 studies that reported activation coordinates for a contrast comparing the hyperalgesic state to control produced six activation clusters (significant at false discovery rate of 0.05) with more peaks for secondary (17.7) than primary zones (7.3). Seven studies showed modulation of brain activity by analgesics in five of the clusters but also in four additional regions.

CONCLUSIONS

This meta-analysis revealed substantial but incomplete overlap between brain areas related to neural mechanisms of hyperalgesia and those reflecting the efficacy of analgesic drugs. Studies testing in the secondary zone were more sensitive to evaluate analgesic efficacy on central sensitization at brainstem or thalamocortical levels.

SIGNIFICANCE

Experimental pain models that provide a surrogate for features of pathological pain conditions in healthy humans and functional imaging techniques are both highly valuable research tools. This review shows that when used together, they provide a wealth of information about brain activity during pain states and analgesia. These tools are promising candidates to help bridge the gap between animal and human studies, to improve translatability and provide opportunities for identification of new targets for back-translation to animal studies.

How to make calibration less painful-A proposition for an automatic, reliable and time-efficient procedure

In behavioral and neurophysiological pain studies, multiple types of calibration methods are used to quantify the individual pain sensation stimuli. Often, studies lack a detailed calibration procedure description, data linearity, and quality quantification and omit required control for sex pain differences. This hampers study repetition and interexperimental comparisons. Moreover, typical calibration procedures require a high number of stimulations, which may cause discomfort and stimuli habituation among participants. To overcome those shortcomings, we present an automatic calibration procedure with a novel stimuli estimation method for intraepidermal stimulation. We provide an in-depth data analysis of the collected self-reports from 70 healthy volunteers (37 males) and propose a method based on a dynamic truncated linear regression model (tLRM). We compare its estimates for the sensation (t) and pain (T) thresholds and mid-pain stimulation (MP), with those calculated using traditional estimation methods and standard linear regression models. Compared to the other methods, tLRM exhibits higher R2 and requires 36% fewer stimuli applications and has significantly higher t intensity and lower T and MP intensities. Regarding sex differences, t and T were found to be lower for females compared to males, regardless of the estimation method. The proposed tLRM method quantifies the calibration procedure quality, minimizes its duration and invasiveness, and provides validation of linearity between stimuli intensity and subjective scores, making it an enabling technique for further studies. Moreover, our results highlight the importance of control for sex in pain studies.

Insights into pain distraction and the impact of pain catastrophizing on pain perception during different types of distraction tasks

Introduction

Distraction is commonly used to reduce pain, but the effectiveness of distractions remains inconclusive. Studies have shown that pain catastrophizing could modulate the effectiveness of distraction strategies. The present study aimed to compare various distraction tasks, then control for pain catastrophizing, and examine how this relationship varies with pain intensity and unpleasantness across different distraction tasks.

Methods

Forty-one pain-free participants (aged 27.00 ± 5.41) were recruited for a cross-sectional study. Four types of distraction (cognitive, sensory, emotional, and social) were presented, while moderate pain intensity was induced by electrical stimulation. Before starting the experiment, moderate pain intensity was individually calibrated as six on the Numerical Pain Rating Scale (NRS) to control individual differences in pain sensitivity. Each participant performed all four distraction tasks in a random order. NRS measured pain assessment. Pain catastrophizing was measured by the Pain Catastrophizing Scale (PCS). A repeated measure ANCOVA was conducted to examine the effects of pain dimensions during distraction tasks as a within-subject and pain catastrophizing as a covariate factor.

Results

A significant difference was observed in the pain intensity and unpleasantness during cognitive distraction. After controlling for PCS, there were diverse associations between PCS and pain intensity across distinct distraction tasks: social vs. sensory, and cognitive vs. sensory distraction. A consistent pattern in pain unpleasantness emerged with minor variations. This interaction underscored notable distinctions between social vs. sensory and emotional distractions, as well as between cognitive vs. sensory and emotional distractions. However, only the correlation in social distraction remained significant in both pain dimensions.

Discussion

Our findings reveal that the link between PCS and pain dimensions varies across different distraction tasks, suggesting diverse interactions. Particularly, social distraction, characterized by both emotional and cognitive states, proves beneficial with lower PCS scores; however, this advantage diminishes as PCS scores increase.

Learning pain from others: a systematic review and meta-analysis of studies on placebo hypoalgesia and nocebo hyperalgesia induced by observational learning

ABSTRACT

Observing someone experience pain relief or exacerbation after an intervention may induce placebo hypoalgesia or nocebo hyperalgesia. Understanding the factors that contribute to these effects could help in the development of strategies for optimizing treatment of chronic pain conditions. We systematically reviewed and meta-analyzed the literature on placebo hypoalgesia and nocebo hyperalgesia induced by observational learning (OL). A systematic literature search was conducted in the databases PubMed, PsycINFO, Web of Science, ScienceDirect, PsycARTICLES, Scopus, and Academic Search Ultimate. Twenty-one studies were included in the systematic review, 17 of which were suitable for meta-analysis (18 experiments; n = 764 healthy individuals). The primary end point was the standardized mean difference (SMD) for pain following placebo cues associated during OL with low vs high pain. Observational learning had a small-to-medium effect on pain ratings (SMD 0.44; 95% confidence interval [CI] 0.21-0.68; P < 0.01) and a large effect on pain expectancy (SMD 1.11; 95% CI 0.49-2.04; P < 0.01). The type of observation (in-person vs videotaped) modulated the magnitude of placebo hypoalgesia/nocebo hyperalgesia ( P < 0.01), whereas placebo type did not ( P = 0.23). Finally, OL was more effective when observers' empathic concern (but no other empathy-related factors) was higher ( r = 0.14; 95% CI 0.01-0.27; P = 0.03). Overall, the meta-analysis demonstrates that OL can shape placebo hypoalgesia and nocebo hyperalgesia. More research is needed to identify predictors of these effects and to study them in clinical populations. In the future, OL could be an important tool to help maximize placebo hypoalgesia in clinical settings.

Intra- and interindividual reliability of muscle pain induced by an intramuscular injection of hypertonic saline injection into the quadriceps

BACKGROUND

Intramuscular injections of hypertonic saline are commonly used to induce experimental muscle pain, but reliability data on this technique are lacking. This study investigated the intra- and interindividual reliability of pain measures from a hypertonic saline injection into the vastus lateralis.

METHODS

Fourteen healthy participants (6 female) attended three laboratory visits where they received an intramuscular injection of 1 mL hypertonic saline into the vastus lateralis. Changes in pain intensity were recorded on an electronic visual analogue scale, and pain quality was assessed after pain had resolved. Reliability was assessed with the coefficient of variation (CV), minimum detectable change (MDC) and intraclass correlation coefficient (ICC) with 95% CIs.

RESULTS

Mean pain intensity displayed high levels of intraindividual variability (CV = 16.3 [10.5-22.0]%) and 'poor' to 'very good' relative reliability (ICC = 0.71 [0.45-0.88]) but had a MDC of 11 [8-16] au (out of 100). Peak pain intensity exhibited high levels of intraindividual variability (CV = 14.8 [8.8-20.8]%) with 'moderate' to 'excellent' levels of relative reliability (ICC = 0.81 [0.62-0.92]), whereas the MDC was 18 [14-26] au. Measures of pain quality exhibited good reliability. Interindividual variability in pain measures was high (CV > 37%).

CONCLUSIONS

Intramuscular injections of 1 mL of hypertonic saline into the vastus lateralis display substantial levels of interindividual variability, but MDC is below the clinically important changes in pain. This model of experimental pain is suitable for studies involving repeated exposures.

SIGNIFICANCE

Many pain research studies have performed intramuscular injections of hypertonic saline to investigate responses to muscle pain. However, the reliability of this technique is not well established. We examined the pain response over three repeated sessions of a hypertonic saline injection. The pain induced by hypertonic saline has considerable interindividual variability but has largely acceptable intraindividual reliability. Therefore, the injections of hypertonic saline to induce muscle pain are a reliable model of experimental muscle pain.

Effects of oral morphine on experimentally evoked itch and pain: a randomized, double-blind, placebo-controlled trial

OBJECTIVES

Pain and itch share similar neuronal networks; hence, it is difficult to explain why opioids can relieve pain but provoke itching. The present human volunteer study aimed to investigate the similarities and differences in responses to experimentally provoked pain and itching to explore the underlying fundamental mechanisms.

METHODS

Twenty-four healthy volunteers were enrolled in this single-center, randomized, double-blind, placebo-controlled, parallel-group trial. Three volar forearms and two mandibular areas were marked, and participants randomly received morphine (20 mg) or identical placebo tablets. Heat, cold, and pressure pain thresholds, and vasomotor responses were assessed at baseline and after oral morphine administration. Itch provocations were induced by intradermal application of 1 % histamine or a topical cowhage (non-histaminergic itch) to a marked area of the skin. The participants were subsequently asked to rate their itching and pain intensities. The assessments were repeated for all marked areas.

RESULTS

Morphine caused analgesia, as assessed by the significant modulation of cold and pressure pain thresholds (p<0.05). There were no significant differences in histaminergic or non-histaminergic itch or pain intensity between the morphine and placebo groups. Superficial blood perfusion (vasomotor response) following histamine provocation was significantly increased by morphine (p<0.05) in both areas. No correlation was found between the provoked itch intensity and analgesic efficacy in any area or group.

CONCLUSIONS

Oral administration of morphine caused analgesia without modulating itch intensities but increased neurogenic inflammation in response to histamine, suggesting that different opioid mechanisms in histaminergic and non-histaminergic neurons evoke neurogenic inflammation.

Morphine and Hydromorphone Effects, Side Effects, and Variability: A Crossover Study in Human Volunteers

BACKGROUND

Balancing between opioid analgesia and respiratory depression continues to challenge clinicians in perioperative, emergency department, and other acute care settings. Morphine and hydromorphone are postoperative analgesic standards. Nevertheless, their comparative effects and side effects, timing, and respective variabilities remain poorly understood. This study tested the hypothesis that IV morphine and hydromorphone differ in onset, magnitude, duration, and variability of analgesic and ventilatory effects.

METHODS

The authors conducted a randomized crossover study in healthy volunteers. Forty-two subjects received a 2-h IV infusion of hydromorphone (0.05 mg/kg) or morphine (0.2 mg/kg) 1 to 2 weeks apart. The authors measured arterial opioid concentrations, analgesia in response to heat pain (maximally tolerated temperature, and verbal analog pain scores at discrete preset temperatures to determine half-maximum temperature effect), dark-adapted pupil diameter and miosis, end-expired carbon dioxide, and respiratory rate for 12 h after dosing.

RESULTS

For morphine and hydromorphone, respectively, maximum miosis was less (3.9 [3.4 to 4.2] vs. 4.6 mm [4.0 to 5.0], P < 0.001; median and 25 to 75% quantiles) and occurred later (3.1 ± 0.9 vs. 2.3 ± 0.7 h after infusion start, P < 0.001; mean ± SD); maximum tolerated temperature was less (49 ± 2 vs. 50 ± 2°C, P < 0.001); verbal pain scores at end-infusion at the most informative stimulus (48.2°C) were 82 ± 4 and 59 ± 3 (P < 0.001); maximum end-expired CO2 was 47 (45 to 50) and 48 mmHg (46 to 51; P = 0.007) and occurred later (5.5 ± 2.8 vs. 3.0 ± 1.5 h after infusion start, P < 0.001); and respiratory nadir was 9 ± 1 and 11 ± 2 breaths/min (P < 0.001), and occurred at similar times. The area under the temperature tolerance-time curve was less for morphine (1.8 [0.0 to 4.4]) than hydromorphone (5.4°C-h [1.6 to 12.1] P < 0.001). Interindividual variability in clinical effects did not differ between opioids.

CONCLUSIONS

For morphine compared to hydromorphone, analgesia and analgesia relative to respiratory depression were less, onset of miosis and respiratory depression was later, and duration of respiratory depression was longer. For each opioid, timing of the various clinical effects was not coincident. Results may enable more rational opioid selection, and suggest hydromorphone may have a better clinical profile.

EDITOR’S PERSPECTIVE

Neuromuscular adaptations to experimentally induced pain in the lumbar region: systematic review and meta-analysis

ABSTRACT

Experimental pain models are frequently used to understand the influence of pain on the control of human movement. In this systematic review, we assessed the effects of experimentally induced pain in the lumbar region of healthy individuals on trunk muscle activity and spine kinematics. Databases were searched from inception up to January 31, 2022. In total, 26 studies using either hypertonic saline injection (n = 19), heat thermal stimulation (n = 3), nociceptive electrical stimulation (n = 3), or capsaicin (n = 1) were included. The identified adaptations were task dependent, and their heterogeneity was partially explained by the experimental pain model adopted. Meta-analyses revealed an increase of erector spinae activity (standardized mean difference = 0.71, 95% confidence interval [CI] = 0.22-1.19) during full trunk flexion and delayed onset of transversus abdominis to postural perturbation tasks (mean difference = 25.2 ms, 95% CI = 4.09-46.30) in the presence of pain. Low quality of evidence supported an increase in the activity of the superficial lumbar muscles during locomotion and during voluntary trunk movements during painful conditions. By contrast, activity of erector spinae, deep multifidus, and transversus abdominis was reduced during postural perturbation tasks. Reduced range of motion of the lumbar spine in the presence of pain was supported by low quality of evidence. Given the agreement between our findings and the adaptations observed in clinical populations, the use of experimental pain models may help to better understand the mechanisms underlying motor adaptations to low back pain.

Are catechol-O-methyltransferase gene polymorphisms genetic markers for pain sensitivity after all? - A review and meta-analysis

The catechol-O-methyltransferase (COMT) gene has arguably been the designated pain sensitivity gene for nearly two decades. However, the literature provides inconsistent evidence. We performed several meta-analyses including k = 31 samples and n = 4631 participants thereby revealing small effects of rs4680 on pain thresholds in fibromyalgia, headache and across chronic pain conditions. Moreover, rs4680 effects were found across pain patients when affected, but not unaffected, body sites were assessed. No effect was detected for any other SNP investigated. Importantly, our results corroborate earlier findings in that we found a small effect of COMT haplotypes on pain sensitivity. Our review and meta-analysis contribute to the understanding of COMT-dependent effects on pain perception, provide insights into research issues and offer future directions. The results support the theory that rs4680 might only impact behavioural measures of pain when descending pain modulatory pathways are sufficiently challenged. After all, COMT polymorphisms are genetic markers of pain sensitivity, albeit with some limitations which are discussed with respect to their implications for research and clinical significance.

Human Brain Organoids in Migraine Research: Pathogenesis and Drug Development

Human organoids are small, self-organized, three-dimensional (3D) tissue cultures that have started to revolutionize medical science in terms of understanding disease, testing pharmacologically active compounds, and offering novel ways to treat disease. Organoids of the liver, kidney, intestine, lung, and brain have been developed in recent years. Human brain organoids are used for understanding pathogenesis and investigating therapeutic options for neurodevelopmental, neuropsychiatric, neurodegenerative, and neurological disorders. Theoretically, several brain disorders can be modeled with the aid of human brain organoids, and hence the potential exists for understanding migraine pathogenesis and its treatment with the aid of brain organoids. Migraine is considered a brain disorder with neurological and non-neurological abnormalities and symptoms. Both genetic and environmental factors play essential roles in migraine pathogenesis and its clinical manifestations. Several types of migraines are classified, for example, migraines with and without aura, and human brain organoids can be developed from patients with these types of migraines to study genetic factors (e.g., channelopathy in calcium channels) and environmental stressors (e.g., chemical and mechanical). In these models, drug candidates for therapeutic purposes can also be tested. Here, the potential and limitations of human brain organoids for studying migraine pathogenesis and its treatment are communicated to generate motivation and stimulate curiosity for further research. This must, however, be considered alongside the complexity of the concept of brain organoids and the neuroethical aspects of the topic. Interested researchers are invited to join the network for protocol development and testing the hypothesis presented here.

Personal Pain Sensitivity Prediction from Ultra-short-term Resting Heart Rate Variability

Pain is a subjective experience with interpersonal perception sensitivity differences. Pain sensitivity is of scientific and clinical interest, as it is a risk factor for several pain conditions. Resting heart rate variability (HRV) is a potential pain sensitivity measure reflecting the parasympathetic tone and baroreflex function, but it remains unclear how well the prediction can achieve. This work investigated the relationship between different ultra-short-term HRV features and various pain sensitivity representations from heat and electrical pain tests. From leave-subject-out cross-validated results, we found that HRV can better predict a composite pain sensitivity score built from different tests and measures than a single measure in terms of the agreement between predictions and observations. Heat pain sensitivity was more possibly predicted than electrical pain. SDNN, RMSSD and LF better predicted the composite pain sensitivity score than other feature combinations, consis-tent with pain's physical and emotional attributes. It should be emphasized that the validity is probably limited within HRV at the resting state rather than an arbitrary measurement. This work implies a potential pain sensitivity prediction possibility that may be worth further validation.

Synergistic effect of chronic pain and nonsuicidal self-harm on pain sensitivity

Abnormalities in pain processing have been observed in patients with chronic pain conditions and in individuals who engage in self-harm, specifically nonsuicidal self-injurious behaviors (NSSI). Both increased and decreased pain sensitivity have been described in chronic pain patients, while decreased pain sensitivity is consistently observed in individuals with NSSI. The objective of the study was to identify the differential effects of chronic pain and NSSI on experimental pain sensitivity, specifically pressure pain threshold, in depressed patients. Moreover, the role that hopelessness may play between depression severity and pain sensitivity was also examined. Depressed patients with and without chronic pain, and with and without lifetime self-harm behaviors were analyzed into four groups. Group 1 (N = 42) included depressed patients with both Chronic pain ( +) and Self-harm ( +), Group 2 (N = 53) included depressed patients with Chronic pain ( +) but no Self-harm (-), Group 3 (N = 64) included depressed patients without Chronic pain (-), but Self-harm ( +), and Group 4 (N = 81) included depressed patients with neither Chronic pain (-) nor Self-harm (-). Healthy controls (N = 45) were also recruited from the community. Depressed patients with both Chronic pain ( +) and Self-harm ( +) reported higher pressure pain threshold measures when compared with the other groups. Mediation analysis indicated that hopelessness mediates the relationship between depression severity and pressure pain threshold. Our findings suggest that a multiprong approach including adequate mental health services and pain control for depressed patients with comorbid chronic pain and nonsuicidal self-harm is needed to yield effective outcomes.

Combining Electrodermal Activity With the Peak-Pain Time to Quantify Three Temporal Regions of Pain Experience

Background

Self-reported pain levels, while easily measured, are often not reliable for quantifying pain. More objective methods are needed that supplement self-report without adding undue burden or cost to a study. Methods that integrate multiple measures, such as combining self-report with physiology in a structured and specific-to-pain protocol may improve measures.

Method

We propose and study a novel measure that combines the timing of the peak pain measured by an electronic visual-analog-scale (eVAS) with continuously-measured changes in electrodermal activity (EDA), a physiological measure quantifying sympathetic nervous system activity that is easily recorded with a skin-surface sensor. The new pain measure isolates and specifically quantifies three temporal regions of dynamic pain experience: I. Anticipation preceding the onset of a pain stimulus, II. Response rising to the level of peak pain, and III. Recovery from the peak pain level. We evaluate the measure across two pain models (cold pressor, capsaicin), and four types of treatments (none, A=pregabalin, B=oxycodone, C=placebo). Each of 24 patients made four visits within 8 weeks, for 96 visits total: A training visit (TV), followed by three visits double-blind presenting A, B, or C (randomized order). Within each visit, a participant experienced the cold pressor, followed by an hour of rest during which one of the four treatments was provided, followed by a repeat of the cold pressor, followed by capsaicin.

Results

The novel method successfully discriminates the pain reduction effects of the four treatments across both pain models, confirming maximal pain for no-treatment, mild pain reduction for placebo, and the most pain reduction with analgesics. The new measure maintains significant discrimination across the test conditions both within a single-day's visit (for relative pain relief within a visit) and across repeated visits spanning weeks, reducing different-day-physiology affects, and providing better discriminability than using self-reported eVAS.

Conclusion

The new method combines the subjectively-identified time of peak pain with capturing continuous physiological data to quantify the sympathetic nervous system response during a dynamic pain experience. The method accurately discriminates, for both pain models, the reduction of pain with clinically effective analgesics.

Coolsense® versus EMLA® for peripheral venous cannulation in adult volunteers: A randomised crossover trial

Peripheral venous cannulation (PVC) is a commonly performed invasive medical procedure. Topical treatments such as the eutectic mixture of local anaesthetics (EMLA®, Aspen Pharmacare Australia Pty Ltd, St Leonards, NSW) attenuate the associated pain, but are limited by requiring up to one hour of application before becoming effective. The Coolsense® (Coolsense Medical Ltd., Tel Aviv, Israel) pain numbing applicator is a new device using a cryoanalgesic means to anaesthetise skin within seconds. Coolsense is being increasingly used for cannulation, but comparative studies are lacking. We recruited 64 healthy adult volunteers to this open-label two sequence, two period randomised crossover trial. Participants had two 20 gauge venous cannulae inserted, one on the dorsum of each hand. Each cannulation attempt was preceded by treatment with Coolsense or an EMLA patch containing 2.5% lidocaine and 2.5% prilocaine. The primary outcome was participant pain using the 0-10 numerical pain rating scale. Secondary outcomes were participant satisfaction scores on a 0-10 scale, treatment preference, and failed cannulation attempts. Participants were randomly assigned to either the Coolsense EMLA (n = 32) or EMLA Coolsense (n = 32) sequence. All participants completed the trial. The pooled mean paired difference of the numerical pain rating scale was -1.84 (95% confidence intervals -1.28 to -2.41; P < 0.001) in favour of EMLA. The pooled mean paired difference for satisfaction score was 2.26 (95% confidence intervals 1.46 to 3.07; P < 0.001) higher with EMLA. Most participants preferred EMLA over Coolsense (P < 0.001). There was no significant difference regarding failed cannulation between the two treatments (P = 0.14). Among healthy individuals undergoing elective PVC, EMLA was associated with reduced pain, increased satisfaction, and was the preferred treatment compared to Coolsense.

Elevated customary alcohol consumption attenuates opioid effects

BACKGROUND

Regular alcohol consumption is on the rise among older adults and has the potential of altering the subjective experience of pain and response to pain medications. This study examined the cognitive, analgesic and side effect response to oxycodone in middle age and older adults with elevated levels of customary alcohol consumption in a human laboratory setting.

METHODS

After refraining from alcohol for one day, eligible participants underwent baseline assessment cognition and side effects by means of questionnaires that were repeated at three time points (90 min, 5 and 8 h) following administration of a 10 mg oral dose of oxycodone. Response to pain stimulus (Cold Pressor Test (CPT)), pupil size, and plasma oxycodone were also measured.

RESULTS

One hundred twenty-eight adults (age 35-85) completed the study day. Compared to those with lower customary alcohol consumption, participants with elevated alcohol consumption showed attenuated opioid-induced pupil constriction and cognitive decline on objective measures of working memory, sustained attention, inhibitory control, coordination on a simulated driving task, and subjective dysphoric effects with enhanced subjective euphoric effects. Oxycodone pharmacokinetics, pain tolerance to CPT, and Berg balance were impacted comparably between alcohol consumption groups. Women endorsed greater negative drug effects, whereas men endorsed positive drug effects.

CONCLUSION

Independent of subject's age, elevated customary alcohol consumption attenuates opioid central effects (i.e., pupil miosis, impaired cognition) and gender influences subjective drug effects. Clinicians should consider alcohol consumption and gender when prescribing opioid medications.

Opioid-induced analgesia among persons with opioid use disorder receiving methadone or buprenorphine: A systematic review of experimental pain studies

BACKGROUND

Treating acute pain among persons with opioid use disorder (OUD) on opioid agonist therapy (OAT) is complex, and the therapeutic benefits of opioids remain unclear when weighted against their abuse potential and respiratory depressant effects.

METHODS

We conducted a systematic review of experimental pain studies examining opioid-induced analgesia among persons with OUD on OAT. We searched multiple databases from inception to July 30, 2021. Study quality was assessed by previously established validity measures.

RESULTS

Nine studies were identified, with a total of 225 participants, of whom 63% were male, and 37% were female. Six studies included methadone-maintained persons with OUD; four studies included buprenorphine-maintained persons with OUD; and three studies included healthy persons as comparison groups. Either additional doses of OAT or other opioids - morphine, oxycodone, hydromorphone, or remifentanil - were administered. In seven studies, persons with OUD on OAT did not experience analgesia, despite receiving opioid doses up to 20 times greater than those clinically used to treat severe pain among the opioid naïve. Conversely, in two studies, high-potency opioids did produce analgesia, albeit with greater abuse potential. Notably, persons with OUD on OAT remained vulnerable to respiratory depression.

CONCLUSIONS

Although persons with OUD on OAT can derive analgesic effects from opioids, high-potency compounds may be required to achieve clinically significant pain relief. Further, persons with OUD on OAT may remain vulnerable to opioid-induced abuse potential and respiratory depression. Together, these finding have clinical, methodological, and mechanistic implications for the treatment of acute pain in the context of OAT.

Neuromuscular adaptations to experimentally induced pain in the lumbar region: protocol for a systematic review and meta-analysis

BACKGROUND

Numerous studies report changes in neuromuscular control in people with low back pain (LBP). However, the relationship between pain and altered neuromuscular control is challenging to unravel given the heterogeneity that exists in clinical populations. One approach commonly adopted to overcome this issue is the use of experimental pain models, but it is currently unclear if the effects of experimental pain are consistent between studies. Therefore, this planned study will systematically evaluate and summarise the effect of experimentally induced pain in the lumbar region on neuromuscular control at sites both locally and remote to the low back.

METHODS

This protocol has been developed following the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P). MEDLINE, EMBASE, CINAHL, ZETOC, Web of Science, and grey literature will be searched up to August 31, 2021. Screening processes (title/abstract and full-text), data extraction, and risk of bias assessment will be conducted by two independent reviewers. Studies investigating the effects of exogenous pain models delivered to the low back region on neuromuscular control in healthy individuals will be included. Muscle activity and body kinematics will be the outcomes of interest. The comparisons of interest will be between baseline or control conditions and the experimental pain condition, as well as between the experimental pain and post-pain conditions. Randomised crossover and non-randomised studies of interventions will be included and their risk of bias will be evaluated with the Cochrane Risk-of-Bias tool or with the Risk Of Bias In Non-randomised Studies of Interventions tool, respectively. A random-effect meta-analysis will be conducted for quantitative synthesis when clinical and methodological consistency is ensured. Quality of evidence will be evaluated using the Grading of Recommendations, Assessment, Development and Evaluation guidelines.

DISCUSSION

The current review will provide new insights to understand if and what neuromuscular adaptations are caused by pain experimentally induced in the lumbar region. Our findings will reveal which experimental pain model is able to better reproduce adaptations similar to those identified in people with low back pain, possibly contributing to improving our understanding of motor adaptation to low back pain in the long term.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42020220130.

Tapentadol and oxycodone reduce cingulate glutamate in healthy volunteers

Tapentadol and oxycodone are commonly used analgesics. Preclinical studies have shown that oxycodone modulates brain metabolites related to opioid pathways, whereas tapentadol also affects noradrenergic activity. However, knowledge about the function of the medications in the human brain is limited. The aim was to investigate effects of tapentadol and oxycodone on brain glutamate, the most important neurotransmitter in pain processing. Magnetic resonance spectroscopy was obtained in 21 healthy subjects from the anterior cingulate cortex, prefrontal cortex, and insula at baseline and after 14 days of treatment with either 50 mg tapentadol, 10 mg oxycodone (equipotent dose, both extended release) or placebo twice daily in a randomized double-blind cross-over study. Compared to baseline, decreased glutamate/creatine levels were identified in anterior cingulate cortex after tapentadol (1.26 ± 0.14 vs. 1.35 ± 0.18, P = .04) and oxycodone (1.26 ± 0.10 vs. 1.35 ± 0.12, P = .05) treatments, both with 7% reduction. This indicates that both analgesics modulate the glutamatergic system at the supraspinal level in humans.

Automated Pain Assessment in Children using Electrodermal Activity and Video Data Fusion via Machine Learning

Pain assessment in children continues to challenge clinicians and researchers, as subjective experiences of pain require inference through observable behaviors, both involuntary and deliberate. The presented approach supplements the subjective self-report-based method by fusing electrodermal activity (EDA) recordings with video facial expressions to develop an objective pain assessment metric. Such an approach is specifically important for assessing pain in children who are not capable of providing accurate self-pain reports, requiring nonverbal pain assessment. We demonstrate the performance of our approach using data recorded from children in post-operative recovery following laparoscopic appendectomy. We examined separately and combined the usefulness of EDA and video facial expression data as predictors of childrens self-reports of pain following surgery through recovery. Findings indicate that EDA and facial expression data independently provide above chance sensitivities and specificities, but their fusion for classifying clinically significant pain vs. clinically nonsignificant pain achieved substantial improvement, yielding 90.91% accuracy, with 100% sensitivity and 81.82% specificity. The multimodal measures capitalize upon different features of the complex pain response. Thus, this paper presents both evidence for the utility of a weighted maximum likelihood algorithm as a novel feature selection method for EDA and video facial expression data and an accurate and objective automated classification algorithm capable of discriminating clinically significant pain from clinically nonsignificant pain in children.

The Effect of Pain Resilience on Experimental Pain Experience Across Different Stimuli

OBJECTIVE

Pain resilience, one's ability to maintain behavioral engagement and adaptively regulate cognitions and emotions despite intense or prolonged pain, has been shown to protect against negative pain-related outcomes in experimental settings. A weakness of this research, and much of experimental pain research in general, has been the lack of rationale behind the selection of noxious stimuli, which can activate different nociceptive fibers. The present study sought to determine if the relationship between pain resilience and pain ratings differed across stimuli based on the stimulated nociceptors.

METHODS

Healthy undergraduate students (N = 100; mean [SD] age = 19.4 [1.2] years; 60% female) completed the Pain Resilience Scale and provided continuous pain ratings during exposure to three different tasks, each selected based on their ability to stimulate specific combinations of nociceptive fibers: pinprick (Aδ fibers), cold water immersion (Aδ and C fibers), and ischemic tourniquet (C fibers).

RESULTS

Participants with high pain resilience reported lower pain ratings over time during cold water immersion than did those with low pain resilience (F(1, 39) = 8.526, p = .006); however, there was no relationship between pain resilience and pain ratings during either of the pinprick or ischemic tourniquet stimuli.

CONCLUSIONS

This study provides further support for the use of multiple pain stimuli for pain assessment given their unique characteristics and concludes that outcome variables aside from pain ratings may provide additional insight into the role of resilience on pain adaptation.

Preclinical efficacy of stem cell therapy for skin flap: a systematic review and meta-analysis

Background

A skin flap is one of the most critical surgical techniques for the restoration of cutaneous defects. However, the distal necrosis of the skin flap severely restricts the clinical application of flap surgery. As there is no consensus on the treatment methods to prevent distal necrosis of skin flaps, more effective and feasible interventions to prevent skin flaps from necrosis are urgently needed. Stem therapy as a potential method to improve the survival rate of skin flaps is receiving increasing attention.

Methods

This review followed the recommendations from the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statements. Twenty studies with 500 animals were included by searching Web of Science, EMBASE, PubMed, and Cochrane Library databases, up until October 8, 2020. Moreover, the references of the included articles were searched manually to obtain other studies. All analyses were conducted using Review Manager V.5.3 software.

Results

Meta-analysis of all 20 studies demonstrated stem cell treatment has significant effects on reducing necrosis of skin flap compared with the control group (SMD: 3.20, 95% CI 2.47 to 3.93). Besides, subgroup analysis showed differences in the efficacy of stem cells in improving the survival rate of skin flaps in areas of skin flap, cell type, transplant types, and method of administration of stem cells. The meta-analysis also showed that stem cell treatment had a significant effect on increasing blood vessel density (SMD: 2.96, 95% CI 2.21 to 3.72) and increasing the expression of vascular endothelial growth factor (VEGF, SMD: 4.34, 95% CI 2.48 to 6.1).

Conclusions

The preclinical evidence of our systematic review indicate that stem cell-based therapy is effective for promoting early angiogenesis by up regulating VEGF and ultimately improving the survival rate of skin flap. In summary, small area skin flap, the administration method of intra-arterial injection, ASCs and MSCs, and xenogenic stem cells from humans showed more effective for the survival of animal skin flaps. In general, stem cell-based therapy may be a promising method to prevent skin flap necrosis.

Experimenter- and Infrared Thermography-Derived Measures of Capsaicin-Induced Neurogenic Flare Among Non-Hispanic White and Black Adults

OBJECTIVE

Capsaicin is a widely utilized experimental pain stimulus; however, few studies have reported on ethnic differences in pain responses to capsaicin. The present study used infrared thermography to 1) measure differences in capsaicin-induced neurogenic flare between non-Hispanic black (NHB) and non-Hispanic white (NHW) adults and 2) determine the association between neurogenic flare and secondary hyperalgesia.

METHODS

Fifty-four participants (NHB N = 28) underwent heat/capsaicin sensitization model procedures. Neurogenic flare was examined using experimenter (i.e., subjective) and thermography (i.e., objective) measurements. A typically nonpainful mechanical punctate probe was used to measure secondary hyperalgesia.

RESULTS

Ethnic groups did not significantly differ in age, sex, marital status, or personal income. Although experimenters rated a significantly wider area of capsaicin-related neurogenic flare among NHW compared with NHB participants (F1, 52 = 8.33, P = 0.006), thermography results showed no differences between groups in neurogenic flares (F1, 52 = 0.01, P = 0.93). Further, although NHB individuals reported greater average pain during the capsaicin procedures compared with NHW individuals (NHB = 58.57 [3.67], NHW = 46.46 [3.81]; F2, 51 = 5.19, P = 0.03), the groups did not differ in secondary hyperalgesia (F2, 51 = 0.03, P = 0.86), and ethnicity did not moderate the association between neurogenic flare and secondary hyperalgesia (F3, 50 = 0.24, P = 0.87).

CONCLUSIONS

Findings cautiously support the use of infrared thermography over subjective experimenter report when measuring neurogenic inflammation in diverse samples. However, infrared thermography should not be used as a diagnostic tool for pain, given the lack of association between these factors. Future research is warranted to replicate these findings in a larger and more diverse sample to determine accurate neurogenic inflammation measures across other ethnic minority populations.

[Establishment of an adaptable acute pain model for induction of nociceptive stimuli of defined intensity and duration using thermal stimulation]

BACKGROUND

In previous years numerous acute pain models to investigate the pathophysiological mechanisms of pain and to validate treatment procedures have been described. Due to the specific questions addressed by different trials standardized protocols are often missing. Therefore, the research results obtained are only comparable or reproducible to a limited extent. The transferability of acquired knowledge to clinical pain is limited by the mostly short test duration of already established models.

METHOD

The aim of this study was to establish a standardized protocol for an acute pain model that induces nociceptive thermal stimuli of defined intensity and variable duration using a device for quantitative sensory testing (QST). The greatest possible exclusion of factors influencing pain perception was achieved. In order to reduce the risk of thermal tissue damage a capsaicin cream was applied to the test area, which led to a significant increase in the perceived pain intensity of heat stimuli.

RESULTS

From previously performed experiments on thermal pain thresholds and temporal aspects of pain adaptation, the parameters for stimulus lengths and thermode temperatures for a cold and heat pain model could be derived. The acute pain model established here was able to induce significant heat and cold pain stimuli over variable periods of time. An average pain intensity of NRS ≥ 6 was reported by the test participants. Among the 30 subjects no tests were terminated due to intolerance.

CONCLUSION

The established acute pain model in this study is characterized by the induction of thermal pain stimuli of defined intensity and variable duration. There is no danger of significant thermal tissue damage and the pain was tolerated by all study participants. The pain model can easily be established using a device for quantitative sensory testing.

Benefit and Risk Evaluation of Biased μ-Receptor Agonist Oliceridine versus Morphine

BACKGROUND

To improve understanding of the respiratory behavior of oliceridine, a μ-opioid receptor agonist that selectively engages the G-protein-coupled signaling pathway with reduced activation of the β-arrestin pathway, the authors compared its utility function with that of morphine. It was hypothesized that at equianalgesia, oliceridine will produce less respiratory depression than morphine and that this is reflected in a superior utility.

METHODS

Data from a previous trial that compared the respiratory and analgesic effects of oliceridine and morphine in healthy male volunteers (n = 30) were reanalyzed. A population pharmacokinetic-pharmacodynamic analysis was performed and served as basis for construction of utility functions, which are objective functions of probability of analgesia, P(analgesia), and probability of respiratory depression, P(respiratory depression). The utility function = P(analgesia ≥ 0.5) - P(respiratory depression ≥ 0.25), where analgesia ≥ 0.5 is the increase in hand withdrawal latency in the cold pressor test by at least 50%, and respiratory depression ≥ 0.25 is the decrease of the hypercapnic ventilatory response by at least 25%. Values are median ± standard error of the estimate.

RESULTS

The two drugs were equianalgesic with similar potency values (oliceridine: 27.9 ± 4.9 ng/ml; morphine 34.3 ± 9.7 ng/ml; potency ratio, 0.81; 95% CI, 0.39 to 1.56). A 50% reduction of the hypercapnic ventilatory response by morphine occurred at an effect-site concentration of 33.7 ± 4.8 ng/ml, while a 25% reduction by oliceridine occurred at 27.4 ± 3.5 ng/ml (potency ratio, 2.48; 95% CI, 1.65 to 3.72; P < 0.01). Over the clinically relevant concentration range of 0 to 35 ng/ml, the oliceridine utility function was positive, indicating that the probability of analgesia exceeds the probability of respiratory depression. In contrast, the morphine function was negative, indicative of a greater probability of respiratory depression than analgesia.

CONCLUSIONS

These data indicate a favorable oliceridine safety profile over morphine when considering analgesia and respiratory depression over the clinical concentration range.

Impact of Experimental Tonic Pain on Corrective Motor Responses to Mechanical Perturbations

Movement is altered by pain, but the underlying mechanisms remain unclear. Assessing corrective muscle responses following mechanical perturbations can help clarify these underlying mechanisms, as these responses involve spinal (short-latency response, 20-50 ms), transcortical (long-latency response, 50-100 ms), and cortical (early voluntary response, 100-150 ms) mechanisms. Pairing mechanical (proprioceptive) perturbations with different conditions of visual feedback can also offer insight into how pain impacts on sensorimotor integration. The general aim of this study was to examine the impact of experimental tonic pain on corrective muscle responses evoked by mechanical and/or visual perturbations in healthy adults. Two sessions (Pain (induced with capsaicin) and No pain) were performed using a robotic exoskeleton combined with a 2D virtual environment. Participants were instructed to maintain their index in a target despite the application of perturbations under four conditions of sensory feedback: (1) proprioceptive only, (2) visuoproprioceptive congruent, (3) visuoproprioceptive incongruent, and (4) visual only. Perturbations were induced in either flexion or extension, with an amplitude of 2 or 3 Nm. Surface electromyography was recorded from Biceps and Triceps muscles. Results demonstrated no significant effect of the type of sensory feedback on corrective muscle responses, no matter whether pain was present or not. When looking at the effect of pain on corrective responses across muscles, a significant interaction was found, but for the early voluntary responses only. These results suggest that the effect of cutaneous tonic pain on motor control arises mainly at the cortical (rather than spinal) level and that proprioception dominates vision for responses to perturbations, even in the presence of pain. The observation of a muscle-specific modulation using a cutaneous pain model highlights the fact that the impacts of pain on the motor system are not only driven by the need to unload structures from which the nociceptive signal is arising.

Acute alcohol effects on conditioned pain modulation, but not temporal summation of pain

Although pain reduction after alcohol administration has repeatedly been demonstrated, alcohol effects on advanced and clinically relevant dynamic pain paradigms are still unknown. As such, temporal summation of pain (TSP) and conditioned pain modulation (CPM) indicate mechanisms of endogenous pain modulation and involve certain neurotransmitter systems crucially influenced by alcohol. Our study is the first to investigate acute alcohol effects on TSP and CPM. We investigated 39 healthy subjects in a placebo-controlled within-subject design and targeted alcohol levels of 0.06% (dose 1) and 0.08% (dose 2). Pain threshold, TSP, and CPM were evaluated before and after an alcoholic or placebo drink. Temporal summation of pain was assessed as enhanced pain response to 5 repetitive contact heat stimuli (threshold +3°C). Conditioned pain modulation was tested as pain inhibition when a conditioning stimulus (46°C hot water) was applied concurrently to a test stimulus (contact heat; threshold + 3°C). Both alcohol doses boosted CPM, with a greater effect size for the higher dose. Conditioning stimulus ratings increased after alcohol intake but were not correlated with CPM, suggesting independence of these effects. Temporal summation of pain was not affected by alcohol, and alcohol effects on pain threshold were small and limited to the higher dose. Our findings suggest that analgesic alcohol effects might be mainly driven by an enhancement of endogenous pain inhibition. The frequent use of alcohol as self-medication in chronic pain might be motivated by alcohol temporarily restoring deficient CPM, thus leading to pain relief in the short run and alcohol-related problems in the long run.

Sinus Headache: Differential Diagnosis and an Evidence-Based Approach

The diagnosis "sinus headache" has been reclassified as "headache attributed to disorder of the nose or paranasal sinuses" by the International Headache Society, but the term is still commonly used by patients and primary care doctors alike. Rhinologic symptoms and headache disorders are common, and they may coexist without a causative relationship. Patients may undergo unnecessary medical interventions because of inadequate understanding of the classifications and management of various headache disorders. Otolaryngologists frequently treat patients with these complaints, and a systematic approach to the differential diagnosis and utilization of a multidisciplinary approach are critical in providing optimal patient care.

Muscle pain induced by hypertonic saline in the knee extensors decreases single-limb isometric time to task failure

Purpose

Increased nociceptive activity and the experience of exercise-induced pain (EIP) may contribute to fatigue during endurance exercise. To investigate this, a pain model that produces pain similar to EIP and decouples its relationship to exercise intensity is required. This study (1) compared the quality of pain caused by a hypertonic saline injection into the vastus lateralis in resting and exercise conditions, and (2) investigated whether this pain contributes to changes in time to task failure.

Methods

On separate days, 18 participants completed a time to task failure at 20% maximal voluntary torque (MVT), a resting hypertonic saline intramuscular injection, and in a further three visits a time to task failure at 10% MVT following injection of isotonic saline, hypertonic saline or a control (no injection).

Results

In a subset of eligible participants (n = 12), the hypertonic saline combined with 10% MVT produced a qualitative experience of pain (assessed by the McGill Pain Questionnaire) that felt similar to EIP. 10% MVT with hypertonic saline significantly elevated pain intensity in the first 20% of the time to task failure and caused a significantly (P < 0.05) shorter time to task failure (448 ± 240 s) compared with the isotonic saline (605 ± 285 s) and control (514 ± 197 s) conditions.

Conclusion

These findings demonstrate that hypertonic saline increases the intensity of pain during exercise, which results in a faster occurrence of exercise-induced fatigue. These results provide important evidence supporting pain as a limiting factor in endurance performance.

Investigation of topical intranasal cocaine for sinonasal procedures: a randomized, phase III clinical trial

BACKGROUND

The objective of this trial was to demonstrate the clinical efficacy and safety of topical cocaine as part of the effort to gain regulatory approval from the United States Food and Drug Administration.

METHODS

This phase III, randomized, prospective, double-blind, multicenter, single-dose, placebo- and dose-controlled, parallel-group study aimed to evaluate the safety and efficacy of topical intranasal cocaine (RX0041-002). A total of 925 subjects were screened and a total of 648 subjects were randomized: 95 to placebo; 275 to 4% RX0041-002; and 278 to 8% RX0041-002. Efficacy was assessed with subjective pain intensity scores using the visual numeric rating scale and objectively using the von Frey filament test. Adverse events (AEs), vital signs, Holter monitoring, nasal irritation on visual examination, and smell assessment were recorded. The placebo and experimental groups were compared using a two-tailed Fisher's exact test.

RESULTS

Topical 4% and 8% cocaine achieved significant subject analgesia, the primary efficacy endpoint. Both doses were safe and well-tolerated, with a safety profile similar to placebo. In the 4% and 8% groups, headache (1.5% and 2.5%, respectively), epistaxis (0.7% and 1.1%), and anxiety (0.7% and 0%) were the only AEs reported by >1 subject. No cardiovascular AEs, serious AEs, or deaths occurred. A higher percentage of subjects in the 4% and 8% groups compared with the placebo group had a modest increase in either systolic or diastolic blood pressure.

CONCLUSION

Topical 4% and 8% cocaine is an effective anesthetic that can be safely administered for nasal procedures.

Thiopental Does Not Produce Hyperalgesia: A Laboratory Study Using Two Human Experimental Pain Models

Objective

Past investigations assessing the effects of thiopental on pain are conflicting. Although several studies demonstrate hyperalgesia as a result of barbiturate administration, others show analgesia. Our objective was to assess the effects of an infusion of the GABAA agonist thiopental, compared with placebo, in healthy participants on two subjective experimental pain paradigms: noxious electrical stimulation and intradermal capsaicin.

Methods

For electrical stimulation, the milliamps required to achieve pain threshold and tolerance were recorded, and the percent change from baseline was determined for each infusion condition. In the intradermal capsaicin condition, the area of hyperalgesia was determined by von Frey technique pre- and postinfusion, and the percent change in the area of hyperalgesia was calculated.

Results

Though thiopental infusion resulted in an increase in the electrical stimulation current required to elicit pain threshold or reach pain tolerance when compared with baseline, this finding was not statistically significant. In the intradermal capsaicin condition, there was a statistically significant difference in overall pre- and postinfusion pain interpretation, as measured by the McGill Pain Questionnaire (P &lt; 0.05), but there was no significant difference in area of hyperalgesia.

Conclusions

In this human study of thiopental’s effects on two experimental pain models, our results show that thiopental does not induce hyperalgesia.

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.

The effects of kinesiology taping on experimentally-induced thermal and mechanical pain in otherwise pain-free healthy humans: A randomised controlled repeated-measures laboratory study

Background

Kinesiology taping (KT) is used to manage musculoskeletal-related pain. There is a paucity of physiological studies evaluating the effect of KT on stimulus-evoked experimental pain.

Objective

To investigate the effect of KT (applied to lumbar region) on cutaneous somatosensation to noxious and innocuous stimuli in humans with a non-sensitised normally functioning nociceptive system using quantitative sensory testing (QST).

Methods

Fifty-four participants were randomised to one of three interventions: (i) KT (ii) standard ‘rigid’ taping (ST) (iii) sham taping (ShT). QST measurements were taken at lumbar sites pre-intervention (T1), during-intervention (T2) and during-intervention (T3) in the following sequence: warm-detection-threshold (WDT), heat-pain-threshold (HTPh), heat-pain-tolerance (HPTo), mechanical-detection-threshold (MDT), mechanical-pain-threshold (MPT) and pressure-pain-threshold (PPT).

Results

Mixed ANOVA revealed statistically significant interaction between Intervention and Time on MDT (p < .0005) and MPT (p < .0005) but not on WDT (p = .09), HPTh (p = .09), HPTo (p = .51) and PPT (p = .52) datasets. There was no significant simple main effect of Intervention on MDT at T2 (p = .68) and T3 (p = .24), and MPT at T2 (p = .79) and T3 (p = .54); post-hoc tests found KT and ST groups had higher (but non-significant) MDT and MPT than the ShT group. There was a significant simple main effect of Time on MDT and MPT for KT (p < .0005) and ST (p < .0005) groups; post-hoc tests found significant increases in MDT and MPT at T3 and T2 compared with T1 in both KT and ST groups. There was no significant simple main effect of Time on MDT (p = .13) nor MPT (p = .08) for the ShT group.

Conclusion

Taping, irrespective of the elasticity, may modulate cutaneous mechanosensation. KT, ST and ShT seemed to have similar influence on cutaneous thermal and deep pressure nociception.

Pack-years of tobacco cigarette smoking as a predictor of spontaneous pain reporting and experimental pain reactivity

The pack-years formula is a widely used estimate of lifetime tobacco smoking exposure, and greater pack-years have been associated with greater risk of chronic pain development and poorer pain-related outcomes among smokers with chronic pain. The pathophysiology underlying these associations is poorly understood. Regular tobacco smoking exposure may dysregulate homeostatic pain processes, producing an allostatic state of pain facilitation. Maladaptive pain mechanisms, such as central and peripheral sensitization, are chronic pain risk factors. Yet no published research has examined the relation between lifetime-smoking exposure and dysregulated pain processing. The current study used hierarchical linear regression analyses to test pack-years of tobacco smoking as a predictor of (a) pain reporting (current pain severity, pain frequency in the last 180 days) among a sample of 228 daily smokers without chronic pain, and (b) experimental capsaicin-induced pain reactivity (pain intensity, area of flare, mechanical pain sensitivity, and area of mechanical hyperalgesia) among 101 daily smokers without chronic pain. As hypothesized, results indicated that pack-years smoking was positively and significantly associated with current pain severity, past 180-day pain frequency, experimental pain intensity, mechanical pain sensitivity ratings, and area of mechanical hyperalgesia. Pack-years smoking was not significantly associated with neurogenic flare. These findings implicate central sensitization as a factor that may underlie the association between chronic tobacco smoking and increased risk for persistent pain. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Cannabinoid modulation of opioid analgesia and subjective drug effects in healthy humans

RATIONALE

Dozens of preclinical studies have reported cannabinoid agonist potentiation of the analgesic effects of μ-opioid agonists.

OBJECTIVES

The aim of this study was to determine if a cannabinoid agonist could potentiate opioid analgesia in humans using several laboratory pain models.

METHODS

Healthy participants (n = 10) with/out current drug use/pain conditions completed this within-subject, double-blind, placebo-controlled, randomized outpatient study. Nine 8-h sessions were completed during which dronabinol (0, 2.5, 5 mg, p.o.) was administered 1 h before oxycodone (0, 5, 10 mg, p.o.) for a total of 9 test conditions. Outcomes included sensory threshold and tolerance from four experimental pain models (cold pressor, pressure algometer, hot thermode, cold hyperalgesia), along with participant- and observer-rated, performance and physiological effects.

RESULTS

Oxycodone produced miosis (p < 0.05) and analgesic responses (e.g., pressure algometer [p < 0.05]), while dronabinol did not (p > 0.05). Depending on the dose combination, dronabinol attenuated or did not alter oxycodone analgesia; for example, dronabinol (2.5 mg) decreased the analgesic effects of oxycodone (10 mg) on pressure tolerance. Conversely, dronabinol increased oxycodone subjective effects (e.g., drug liking) (p < 0.05); oxycodone (5 mg) ratings of "high" were potentiated by 5 mg dronabinol (p < 0.05; placebo = 1.1 [± 0.7]; 5 mg oxycodone = 4.7 [± 2.2]; 5 mg dronabinol = 9.9 [± 8.4]; 5 mg oxycodone + 5 mg dronabinol = 37.4 [± 11.3]).

CONCLUSIONS

This study indicates that dronabinol did not enhance the analgesic effects of oxycodone and increased abuse- and impairment-related subjective effects. These data suggest that dronabinol may not be an effective or appropriate opioid adjuvant; it could potentially increase opioid dose requirements, while increasing psychoactive opioid effects.

Notoginsenoside R1 for Organs Ischemia/Reperfusion Injury: A Preclinical Systematic Review

Notoginsenoside R1 (NGR1) exerts pharmacological actions for a variety of diseases such as myocardial infarction, ischemic stroke, acute renal injury, and intestinal injury. Here, we conducted a preclinical systematic review of NGR1 for ischemia reperfusion (I/R) injury. Eight databases were searched from their inception to February 23rd, 2019; Review Manager 5.3 was applied for data analysis. CAMARADES 10-item checklist and cell 10-item checklist were used to evaluate the methodological quality. Twenty-five studies with 304 animals and 124 cells were selected. Scores of the risk of bias in animal studies ranged from 3 to 8, and the cell studies ranged from 3 to 5. NGR1 had significant effects on decreasing myocardial infarct size in myocardial I/R injury, decreasing cerebral infarction volume and neurologic deficit score in cerebral I/R injury, decreasing serum creatinine in renal I/R injury, and decreasing Park/Chiu score in intestinal I/R injury compared with controls (all P < 0.05 or P < 0.01). The multiple organ protection of NGR1 after I/R injury is mainly through the mechanisms of antioxidant, anti-apoptosis, and anti-inflammatory, promoting angiogenesis and improving energy metabolism. The findings showed the organ protection effect of NGR1 after I/R injury, and NGR1 can potentially become a novel drug candidate for ischemic diseases. Further translation studies are needed.

Evaluation of Postsurgical Hyperalgesia and Sensitization After Open Inguinal Hernia Repair: A Useful Model for Neuropathic Pain?

Cutaneous mechanical hyperalgesia can be induced in healthy volunteers in early phase analgesic studies to model central sensitization, a key mechanism of persistent pain. However, such hyperalgesia is short-lived (a matter of hours), and is used only for assessing only single drug doses. In contrast, postsurgical peri-incisional hyperalgesia may be more persistent and hence be a more useful model for the assessment of the efficacy of new analgesics. We undertook quantitative sensory testing in 18 patients at peri-incisional and nonoperated sites before open inguinal hernia repair and up to the 24th postsurgical week. The spatial extent of punctate hyperalgesia and brush allodynia at the peri-incisional site were greatest at weeks 2 and 4, but had resolved by week 24. Heat allodynia, suggestive of local inflammation or peripheral sensitization, was not observed; instead, there were deficits in cold and heat sensory detection that persisted until week 24. The findings suggest that central sensitization contributes significantly to mechanical hyperalgesia at the peri-incisional site. The prolonged duration of hyperalgesia would be advantageous as a pain model, but there was considerable variability of mechanical hyperalgesia in the cohort; the challenges of recruitment may limit its use to small, early phase analgesic studies. PERSPECTIVE: Peri-incisional mechanical hyperalgesia persists for ≥4 weeks after open inguinal hernia repair and reflects central sensitization; this may have usefulness as a model of chronic pain to assess the potential of antineuropathic analgesics.

A Phase I, Randomized, Double-Blind, Laser-Evoked Potential Study to Evaluate the Analgesic/Antihyperalgesic Effect of ASP9226, a State-Dependent N-Type Voltage-Gated Calcium Channel Inhibitor, in Healthy Male Subjects

Objective

Evaluate the analgesic/antihyperalgesic effects of ASP9226, a state-dependent N-type voltage-gated calcium channel inhibitor, in healthy male subjects.

Design

Randomized, double-blind, double-dummy, placebo- and active comparator–controlled crossover study.

Setting

HPR Dr. Schaffler GmbH, Munich, Germany.

Subject

Healthy male subjects aged 18–55 years.

Methods

Twenty-four eligible subjects were randomly assigned to one of four treatment sequences and received single doses of ASP9226 (30 mg or 50 mg), pregabalin (150 mg), or placebo during four treatment periods. Laser-evoked potentials (LEP) and postlaser pain visual analog scales (VAS) on capsaicin-treated skin were assessed during main assessment days (the first day of each study period). Primary and secondary end points were the differences in LEP N2-P2 peak-to-peak (PtP) amplitudes and VAS score, respectively, in all subjects.

Results

Overall, treatment with pregabalin resulted in a significantly lower LEP N2-P2 PtP amplitude vs placebo (–3.30 μV, P < 0.0001). There were no clinically relevant differences in N2-P2 PtP amplitudes between placebo and either ASP9226 dose (–0.31 μV and –0.27 μV). Furthermore, subjects reported significantly lower VAS pain scores with pregabalin vs placebo (–9.90%, P < 0.0001) in contrast to ASP9226 30 mg (–2.1%) and ASP9226 50 mg (1.2%) vs placebo. Subgroup analysis of LEP and VAS pain in participants with positive prestudy capsaicin response (n = 13) were in keeping with results in all subjects.

Conclusions

ASP9226 was well tolerated; however, there was no improvement in LEP and VAS pain scores with ASP9226 at either dose vs placebo.

Reproducibility of a battery of human evoked pain models to detect pharmacological effects of analgesic drugs

BACKGROUND

Although reproducibility is considered essential for any method used in scientific research, it is investigated only rarely; thus, strikingly little has been published regarding the reproducibility of evoked pain models involving human subjects. Here, we studied the reproducibility of a battery of evoked pain models for demonstrating the analgesic effects of two analgesic compounds.

METHODS

A total of 81 healthy subjects participated in four studies involving a battery of evoked pain tests in which mechanical, thermal and electrical stimuli were used to measure pain detection and tolerance thresholds. Pharmacodynamic outcome variables were analysed using a mixed model analysis of variance, and a coefficient of variation was calculated by dividing the standard deviation by the least squares means.

RESULTS

A total of 76 subjects completed the studies. After being administered pregabalin, the subjects' pain tolerance thresholds in the cold pressor and pressure stimulation tests were significantly increased compared to the placebo group. Moreover, the heat pain detection threshold in UVB-irradiated skin was significantly increased in subjects who were administered ibuprofen compared to the placebo group. Variation among all evoked pain tests ranged from 2.2% to 30.6%.

CONCLUSIONS

Four studies using a similar design showed reproducibility with respect to the included evoked pain models. The relatively high consistency and reproducibility of two analgesics at doses known to be effective in treating clinically relevant pain supports the validity of using this pain test battery to investigate the analgesic activity and determine the active dosage of putative analgesic compounds in early clinical development.

SIGNIFICANCE

The consistency and reproducibility of measuring the profile of an analgesic at clinically relevant doses illustrates that this pain test battery is a valid tool for demonstrating the analgesic activity of a test compound and for determining the optimal active dose in early clinical drug development.

A Randomized, Double-Blind, Placebo- and Active Comparator-Controlled Phase I Study of Analgesic/Antihyperalgesic Properties of ASP8477, a Fatty Acid Amide Hydrolase Inhibitor, in Healthy Female Subjects

Objectives

To evaluate the analgesic/antihyperalgesic effect of ASP8477.

Design

Randomized, double-blind, double-dummy, cross-over, placebo- and active comparator-controlled study.

Setting

HPR Dr. Schaffler GmbH, Munich, Germany.

Subjects

Healthy female subjects aged 18–65 years.

Methods

Eligible subjects were randomly assigned to one of six treatment sequences and received multiple ascending doses of ASP8477, duloxetine, and placebo over three treatment periods (each consisting of 21-day dosing separated by 14-day washout periods). On the last day of each dose level, laser evoked potentials (LEPs) and visual analog scales (VAS pain) on capsaicin-treated skin at baseline and at multiple postdose time points were assessed. The primary end point was the difference in LEP N2-P2 peak-to-peak (PtP) amplitudes for ASP8477 100 mg vs placebo.

Results

Twenty-five subjects were randomized. In all subjects, LEP N2-P2 PtP amplitudes were numerically lower for ASP8477 100 mg vs placebo (P = 0.0721); in subjects who demonstrated positive capsaicin skin effects, a greater mean difference of –2.24 µV (P = 0.0146) was observed. Across all doses, LEP N2-P2 PtP amplitudes were lower for duloxetine compared with ASP8477 (mean difference –3.80 µV; P < 0.0001) or placebo (mean difference –5.21 µV; P < 0.0001). The effect of ASP8477 (all doses) on down-scoring the VAS pain score was significant compared with placebo (mean difference –2.55%; P < 0.0007).

Conclusions

ASP8477 was well tolerated in this study. Analysis of all subjects did not demonstrate a significant difference in LEP for ASP8477 100 mg over placebo but did in subjects who demonstrated positive capsaicin skin effects.

An experimental randomized study on the analgesic effects of pharmaceutical-grade cannabis in chronic pain patients with fibromyalgia

Supplemental Digital Content is Available in the Text.

This experimental highly controlled trial in 20 patients with fibromyalgia shows that the cannabinoid THC, but not CBD, is effective in the treatment of fibromyalgia pain.

In this experimental randomized placebo-controlled 4-way crossover trial, we explored the analgesic effects of inhaled pharmaceutical-grade cannabis in 20 chronic pain patients with fibromyalgia. We tested 4 different cannabis varieties with exact knowledge on their ∆⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD) content: Bedrocan (22.4-mg THC, <1-mg CBD; Bedrocan International BV, Veendam, the Netherlands), Bediol (13.4-mg THC, 17.8-mg CBD; Bedrocan International BV, Veendam, the Netherlands), Bedrolite (18.4-mg CBD, <1-mg THC; Bedrocan International BV, Veendam, the Netherlands), and a placebo variety without any THC or CBD. After a single vapor inhalation, THC and CBD plasma concentrations, pressure and electrical pain thresholds, spontaneous pain scores, and drug high were measured for 3 hours. None of the treatments had an effect greater than placebo on spontaneous or electrical pain responses, although more subjects receiving Bediol displayed a 30% decrease in pain scores compared to placebo (90% vs 55% of patients, P = 0.01), with spontaneous pain scores correlating with the magnitude of drug high (ρ = −0.5, P < 0.001). Cannabis varieties containing THC caused a significant increase in pressure pain threshold relative to placebo (P < 0.01). Cannabidiol inhalation increased THC plasma concentrations but diminished THC-induced analgesic effects, indicative of synergistic pharmacokinetic but antagonistic pharmacodynamic interactions of THC and CBD. This experimental trial shows the complex behavior of inhaled cannabinoids in chronic pain patients with just small analgesic responses after a single inhalation. Further studies are needed to determine long-term treatment effects on spontaneous pain scores, THC–CBD interactions, and the role of psychotropic symptoms on pain relief.