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

Human Experimental Pain Models for Assessing the Therapeutic Efficacy of Analgesic Drugs

Opioids and abnormal pain perception: New evidence from a study of chronic opioid addicts and healthy subjects.

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

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2125.2009.03456.x

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

BACKGROUND: Oxycodone is a mu-opioid receptor agonist that is metabolized mainly in the liver by cytochrome P450 3A and 2D6 enzymes. Rifampin is a strong inducer of several drug-metabolizing enzymes. The authors studied the interaction of rifampin with oxycodone. Their hypothesis was that rifampin enhances the CYP3A-mediated metabolism of oxycodone and attenuates its pharmacologic effect. METHODS: The protocol was a four-session, paired crossover. Twelve volunteers were given 600 mg oral rifampin or placebo once daily for 7 days. Oxycodone was given on day 6. In the first part of the study, 0.1 mg/kg oxycodone hydrochloride was given intravenously. In the second part of the study, 15 mg oxycodone hydrochloride was given orally. Concentrations of oxycodone and its metabolites noroxycodone, oxymorphone, and noroxymorphone were determined for 48 h. Psychomotor effects were characterized for 12 h by several visual analog scales. Analgesic effects were characterized by measuring the heat pain threshold and cold pain sensitivity. RESULTS: Rifampin decreased the area under the oxycodone concentration-time curve of intravenous and oral oxycodone by 53% and 86%, respectively (P < 0.001). Oral bioavailability of oxycodone was decreased from 69% to 21% (P < 0.001). Rifampin greatly increased the plasma metabolite-to-parent drug ratios for noroxycodone and noroxymorphone (P < 0.001). Pharmacologic effects of oral oxycodone were attenuated. CONCLUSIONS: Induction of cytochrome P450 3A by rifampin reduced the area under the oxycodone concentration-time curve of intravenous and oral oxycodone. The pharmacologic effects of oxycodone were modestly attenuated. To maintain adequate analgesia, dose adjustment of oxycodone may be necessary, when used concomitantly with rifampin.

Rifampin greatly reduces the plasma concentrations of intravenous and oral oxycodone.

Endogenous opiates and behavior: 2004

Aims

The coadministration of subantinociceptive doses of oxycodone with morphine has recently been shown to result in a synergistic antinociceptive effect in rats. The present study was aimed to investigate the possibility that coadministration of morphine and oxycodone can produce a similar synergistic effect in humans exposed to an experimental model of cold pressor test (CPT).



Methods

The enriched enrolment design was used to exclude ‘stoic’ and ‘placebo responders’ in a single-blind fashion. ‘Nonstoic’, placebo ‘nonresponder’ female volunteers (n = 30) were randomly assigned to receive 0.5 mg kg−1 oral morphine sulphate, 0.5 mg kg−1 oral oxycodone hydrochloride, and the combination of 0.25 mg kg−1 morphine sulphate with 0.25 mg kg−1 oxycodone hydrochloride, 1 week apart from each other, in a double-blind crossover design. Latency to pain onset (threshold), pain intensity (VAS), and pain tolerance (time until removal of the hand from the water) were measured six times over a 3-h period, subsequent to the administration of each medication, and were used to assess their antinociceptive effect.



Results

The combination produced a significantly higher effect on latency to pain onset than that of morphine alone [difference in mean postbaseline value 2.2; 95% confidence interval (CI) 0.48, 3.9; P = 0.01] but the effect was nonsignificantly smaller that that of oxycodone alone. Similarly, the effect of the combination on pain tolerance was significantly larger than that of morphine alone (combination difference 8.4; 95% CI 2.5, 14.3; P = 0.007), whereas oxycodone alone caused a nonsignificantly larger effect than that of the combination treatment. Comparisons of pain magnitude failed to show any significant differences between the three treatments.



Conclusions

These results indicate that at the doses tested, morphine and oxycodone do not produce synergistic antinociceptive effects in healthy humans exposed to the CPT.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2125.2004.02141.x

Can coadministration of oxycodone and morphine produce analgesic synergy in humans? An experimental cold pain study.

The present study examined the impact of intraoperative acupuncture on anesthesia-related parameters in patients undergoing gynecological oncology surgery.Participants underwent preoperative integrative oncology (IO) touch/relaxation treatments, followed by intraoperative acupuncture (Group A); preoperative IO treatments without acupuncture (Group B); or standard care only (Group C). Mean arterial pressure (MAP), heart rate (HR), MAP variability (mean of MAP standard deviation), bispectral index (BIS), and calculated blood pressure Average Real Variability (ARV) were measured intraoperatively.A total of 91 patients participated: Group A, 41; Group B, 24; Group C, 26. Among patients undergoing open laparotomy, Group A showed lower and more stable MAP and HR compared to Group B, (MAP, p = 0.026; HR, p = 0.029) and Group C (MAP, p = 0.025). Mean BIS, from incision to suture closing, was lower in Group A (vs. controls, p = 0.024). In patients undergoing laparoscopic surgery, MAP was elevated within Group A (p = 0.026) throughout surgery, with MAP variability significantly higher in Group A (P = 0.023) and Group B (P = 0.013) 10 min post-incision (vs. pre-incision). All groups showed similar intraoperative and post-anesthesia use of analgesic medication.Intraoperative acupuncture was shown to reduce and stabilize MAP and HR, and reduce BIS in gynecology oncology patients undergoing laparotomy, with no impact on perioperative analgesic medication use. In the laparoscopic setting, intraoperative acupuncture was associated with elevated MAP. Further research is needed to explore the hemodynamic and BIS-associated benefits and risks of intraoperative acupuncture, and the impact on the use of analgesic drugs in response to these changes. 

/pdf/a-randomized-controlled-trial-assessing-the-effect-of-2sffe1mx.pdf

A randomized-controlled trial assessing the effect of intraoperative acupuncture on anesthesia-related parameters during gynecological oncology surgery

Michael Grach

Papers

Can coadministration of oxycodone and morphine produce analgesic synergy in humans? An experimental cold pain study.

A randomized-controlled trial assessing the effect of intraoperative acupuncture on anesthesia-related parameters during gynecological oncology surgery