Comment on Koltzenburg et al.: Differential sensitivity of three experimental pain models in detecting the analgesic effects of transdermal fentanyl and buprenorphine. Pain 2006;126:165-74

Midazolam as an active placebo in 3 fentanyl-validated nociceptive pain models

The use of inactive placebos in early translational trials of potentially analgesic compounds is discouraged because of the side-effect profiles of centrally acting analgesics. Therefore, benzodiazepines are used, although their use has not been validated in this context. Whether benzodiazepines confound the results of acute pain tests is unknown. Midazolam (0.06 mg/kg) as an active placebo was investigated in 3 nociceptive models that included contact heat, electrical pain, and pressure pain thresholds in 24 healthy volunteers. Fentanyl (1 μg/kg) served as an internal validator in this randomized, placebo (saline) controlled, 3-way cross-over trial. The primary outcome parameter (contact heat pain) was analyzed using a one-way, repeated measures analysis of variance and Tukey's post test. Midazolam did not reduce pain ([numeric rating scale], 0-100) in a statistically significant manner compared with placebo for the contact heat (mean difference -1.7, 95% confidence interval -10.6 to 7.3; P = 0.89) or electrical pain (4.3, -5.1 to 13.7; P = 0.51) test, nor did it raise the pressure pain thresholds (-28 kPa, -122; 64 kPa, P = 0.73). The width of the confidence intervals suggested that there were no clinically meaningful analgesic effects compared with the placebo. In contrast, the analgesic efficacy of fentanyl was effectively demonstrated in all 3 models (P < 0.01 vs midazolam and placebo). The findings of this study show that midazolam can be used as an active placebo in analgesic drug trials. Furthermore, the proposed models were simple to implement and very effective in detecting analgesia. The test battery can be used in translational trials for new compounds and comes with an active placebo and an optional active comparator.

Sensitivity of quantitative sensory models to morphine analgesia in humans

Introduction

Opioid analgesia can be explored with quantitative sensory testing, but most investigations have used models of phasic pain, and such brief stimuli may be limited in the ability to faithfully simulate natural and clinical painful experiences. Therefore, identification of appropriate experimental pain models is critical for our understanding of opioid effects with the potential to improve treatment.

Objectives

The aim was to explore and compare various pain models to morphine analgesia in healthy volunteers.

Methods

The study was a double-blind, randomized, two-way crossover study. Thirty-nine healthy participants were included and received morphine 30 mg (2 mg/mL) as oral solution or placebo. To cover both tonic and phasic stimulations, a comprehensive multi-modal, multi-tissue pain-testing program was performed.

Results

Tonic experimental pain models were sensitive to morphine analgesia compared to placebo: muscle pressure (F=4.87, P=0.03), bone pressure (F=3.98, P=0.05), rectal pressure (F=4.25, P=0.04), and the cold pressor test (F=25.3, P<0.001). Compared to placebo, morphine increased tolerance to muscle stimulation by 14.07%; bone stimulation by 9.72%; rectal mechanical stimulation by 20.40%, and reduced pain reported during the cold pressor test by 9.14%. In contrast, the more phasic experimental pain models were not sensitive to morphine analgesia: skin heat, rectal electrical stimulation, or rectal heat stimulation (all P>0.05).

Conclusion

Pain models with deep tonic stimulation including C fiber activation and and/or endogenous pain modulation were more sensitive to morphine analgesia. To avoid false negative results in future studies, we recommend inclusion of reproducible tonic pain models in deep tissues, mimicking clinical pain to a higher degree.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Social influence and pain response in women and men

The purpose of this study was to examine the effects of social influence on responses to acute pain in women and men in a randomized experimental design. Sixty-eight undergraduates (32 women; 36 men) were randomly assigned to perform a cold pressor task either alone or in the presence of a same-sex friend. Expressions of pain were assessed with the short form of the McGill Pain Questionnaire. Overall social support was measured using the Krause social support assessment scale. The presence of a same-sex friend significantly increased pain reports in women, but not in men. Persons who reported high levels of social support on the Krause scale also reported greater cold pressor pain. Results suggest that the presence of a friend can increase pain report to an acute laboratory pain stimulus in women. These findings are consistent with models of social reinforcement in chronic pain syndromes.