The Synergistic Effect of Combined Treatment with Systemic Ketamine and Morphine on Experimentally Induced Windup-Like Pain in Humans

Influence of ketamine versus fentanyl on pain relief for pediatric orthopedic emergencies: A meta-analysis of randomized controlled studies

INTRODUCTION

The comparison of ketamine with fentanyl for pain control of pediatric orthopedic emergencies remains controversial. We conduct a systematic review and meta-analysis to explore the influence of ketamine versus fentanyl on pain management among pediatric orthopedic emergencies.

METHODS

We have searched PubMed, EMbase, Web of science, EBSCO, and Cochrane library databases through September 2020 for randomized controlled trials assessing the effect of ketamine versus fentanyl on pain management for pediatric orthopedic emergencies.

RESULTS

Five randomized controlled trials are included in the meta-analysis. Overall, compared with fentanyl for pediatric orthopedic emergencies, ketamine led to similar change in pain scores at 15 to 20 minutes (standard mean difference = -0.05; 95% confidence interval [CI] = -0.38 to 0.28; P = .77) and 30 minutes (standard mean difference = 0.11; 95% CI = -0.20 to 0.42; P = .49), as well as rescue analgesia (RR = 0.90; 95% CI = 0.54 to 1.51; P = .69), but revealed the increase in nausea/vomiting (RR = 2.65; 95% CI = 1.13 to 6.18; P = .02) and dizziness (RR = 3.83; 95% CI = 1.38 to 10.60; P = .01).

CONCLUSIONS

Ketamine may be similar to fentanyl in terms of the analgesic efficacy for pediatric orthopedic emergencies.

Methodology and applicability of the human contact burn injury model: A systematic review

The contact burn injury model is an experimental contact thermode-based physiological pain model primarily applied in research of drug efficacy in humans. The employment of the contact burn injury model across studies has been inconsistent regarding essential methodological variables, challenging the validity of the model. This systematic review analyzes methodologies, outcomes, and research applications of the contact burn injury model. Based on these results, we propose an improved contact burn injury testing paradigm. A literature search was conducted (15-JUL-2020) using PubMed, EMBASE, Web of Science, and Google Scholar. Sixty-four studies were included. The contact burn injury model induced consistent levels of primary and secondary hyperalgesia. However, the analyses revealed variations in the methodology of the contact burn injury heating paradigm and the post-burn application of test stimuli. The contact burn injury model had limited testing sensitivity in demonstrating analgesic efficacy. There was a weak correlation between experimental and clinical pain intensity variables. The data analysis was limited by the methodological heterogenicity of the different studies and a high risk of bias across the studies. In conclusion, although the contact burn injury model provides robust hyperalgesia, it has limited efficacy in testing analgesic drug response. Recommendations for future use of the model are being provided, but further research is needed to improve the sensitivity of the contact burn injury method. The protocol for this review has been published in PROSPERO (ID: CRD42019133734).

Fixed-Dose Combination of NSAIDs and Spasmolytic Agents in the Treatment of Different Types of Pain-A Practical Review

This review presents the most common disease entities in which combinations of NSAIDs and spasmolytic drugs are used to reduce pain. The benefits of fixed-dose combination products (FDCs) are that they improve the response in people with insufficient monotherapy. Using the synergy or additive effect of drugs, it is possible to obtain a significant therapeutic effect and faster action with the use of smaller doses of individual drugs. In addition, one active ingredient may counteract adverse reactions from the other. Another essential aspect of the use of FDCs is the improvement of medical adherence due to the reduction in the pill burden on patients. It is also possible to develop a fixed-dosed combination product de novo to address a new therapeutic claim and be protected by patents so that the manufacturer can obtain exclusive rights to sell a particular FDC or a formulation thereof. The proposed fixed-dose combinations should always be based on valid therapeutic principles and consider the combined safety profile of all active substances included in the medicinal product. This review aims to identify which combinations of NSAIDs and spasmolytics have been developed and tested and which combinations are still under development.

An Update on the Basic and Clinical Science of Ketamine Analgesia

OBJECTIVE

In the context of the current opioid epidemic, there has been a renewed interest in the use of ketamine as an analgesic agent.

METHODS

We reviewed ketamine analgesia.

RESULTS

Ketamine is well-known as an antagonist for N-methyl-D-aspartate receptors. In addition, it can regulate the function of opioid receptors and sodium channels. Ketamine also increases signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. These myriad of molecular and cellular mechanisms are responsible for a number of pharmacological functions including pain relief and mood regulation. Clinically, a number of studies have investigated the role of ketamine in the setting of acute and chronic pain, and there is evidence that ketamine can provide analgesia in a variety of pain syndromes.

DISCUSSION

In this review, we examined basic mechanisms of ketamine and its current clinical use and potential novel use in pain management.

Oral Administration of Injectable Ketamine During Burn Wound Dressing Changes

Providing adequate analgesia during burn wound care is essential to patient-centered care. Both oral and intravenous (IV) ketamine are often used for analgesia and sedation. Ketamine may improve analgesia and decrease opioid requirements for burn wound care. Oral ketamine wafers and tablets have been used as a safe alternative internationally but are unavailable in the United States. The purpose of this study was to compare opioid usage and patient satisfaction scores in patients with and without the use of oral injectable ketamine for burn wound care, with each patient serving as their own control. Ketamine, opioid, and benzodiazepine dosages recorded during dressing changes were compared to dressing changes without ketamine use that occurred before and after ketamine-associated sessions in each patient. Fourteen patients received oral ketamine at a median (interquartile range [IQR]) dose of 2.5 (2.2-2.7) mg/kg. Ketamine use significantly decreased opioid requirements when compared to wound care sessions that did not use ketamine both before (50 [IQR: 30-75] mg vs 75 [IQR: 46-91] mg median IV morphine equivalents, P = .0097) and after (50 [IQR: 30-75] mg vs 63 [IQR: 50-96] mg median IV morphine equivalents, P = .0042) the ketamine-associated sessions. One patient experienced hallucinations, and no adverse events were observed. Hence, oral administration of injectable ketamine was associated with a decrease in opioid requirements during dressing changes. Additionally, ketamine use improved patient satisfaction (P = .0034). Preliminary data suggest this promising analgesia method is safe and effective for burn wound care.

Burn Pain: A Systematic and Critical Review of Epidemiology, Pathophysiology, and Treatment

Objective

This review aims to examine the available literature on the epidemiology, pathophysiology, and treatment of burn-induced pain.

Methods

A search was conducted on the epidemiology of burn injury and treatment of burn pain utilizing the database Medline, and all relevant articles were systemically reviewed. In addition, a critical review was performed on the pathophysiology of burn pain and animal models of burn pain.

Results

The search on the epidemiology of burn injury yielded a total of 163 publications of interest, 72 of which fit the inclusion/exclusion criteria, with no publications providing epidemiological data on burn injury pain management outcomes. The search on the treatment of burn pain yielded a total of 213 publications, 14 of which fit the inclusion/exclusion criteria, highlighting the limited amount of evidence available on the treatment of burn-induced pain.

Conclusions

The pathophysiology of burn pain is poorly understood, with limited clinical trials available to assess the effectiveness of analgesics in burn patients. Further studies are needed to identify new pharmacological targets and treatments for the effective management of burn injury pain.

Randomized Controlled Feasibility Trial of Intranasal Ketamine Compared to Intranasal Fentanyl for Analgesia in Children with Suspected Extremity Fractures

OBJECTIVES

We compared the tolerability and efficacy of intranasal subdissociative ketamine to intranasal fentanyl for analgesia of children with acute traumatic pain and investigated the feasibility of a larger noninferiority trial that could investigate the potential opioid-sparing effects of intranasal ketamine.

METHODS

This randomized controlled trial compared 1 mg/kg intranasal ketamine to 1.5 μg/kg intranasal fentanyl in children 4 to 17 years old with acute pain from suspected isolated extremity fractures presenting to an urban Level II pediatric trauma center from December 2015 to November 2016. Patients, parents, treating physicians, and outcome assessors were blinded to group allocation. The primary outcome, a tolerability measure, was the frequency of cumulative side effects and adverse events within 60 minutes of drug administration. The secondary outcomes included the difference in mean pain score reduction at 20 minutes, the proportion of patients achieving a clinically significant reduction in pain in 20 minutes, total dose of opioid pain medication in morphine equivalents/kg/hour (excluding study drug) required during the emergency department (ED) stay, and the feasibility of enrolling children presenting to the ED in acute pain into a randomized trial conducted under U.S. regulations. All patients were monitored until 6 hours after their last dose of study drug or until admission to the hospital ward or operating room.

RESULTS

Of 629 patients screened, 87 received the study drug and 82 had complete data for the primary outcome (41 patients in each group). The median (interquartile range) age was 8 (6-11) years and 62% were male. Baseline pain scores were similar among patients randomized to receive ketamine (73 ± 26) and fentanyl (69 ± 26; mean difference [95% CI] = 4 [-7 to 15]). The cumulative number of side effects was 2.2 times higher in the ketamine group, but there were no serious adverse events and no patients in either group required intervention. The most common side effects of ketamine were bad taste in the mouth (37; 90.2%), dizziness (30; 73.2%), and sleepiness (19; 46.3%). The most common side effects of fentanyl were sleepiness (15; 36.6%), bad taste in the mouth (9; 22%), and itchy nose (9; 22%). No patients experienced respiratory side effects. At 20 minutes, the mean pain scale score reduction was 44 ± 36 for ketamine and 35 ± 29 for fentanyl (mean difference = 9 [95% CI = -4 to 23]). Procedural sedation with ketamine occurred in 28 ketamine patients (65%) and 25 fentanyl patients (57%) prior to completing the study.

CONCLUSIONS

Intranasal ketamine was associated with more minor side effects than intranasal fentanyl. Pain relief at 20 minutes was similar between groups. Our data support the feasibility of a larger, noninferiority trial to more rigorously evaluate the safety, efficacy, and potential opioid-sparing benefits of intranasal ketamine analgesia for children with acute pain.

Anaesthetic and cardiopulmonary effects of intramuscular morphine, medetomidine and ketamine administered to telemetered cats

The quality and duration of anaesthesia, cardiorespiratory effects and recovery characteristics of a morphine, medetomidine, ketamine (MMK) drug combination were determined in cats. Six healthy, adult female cats were administered 0.2 mg/kg morphine sulphate, 60 μg/kg medetomidine hydrochloride, and 5 mg/kg ketamine hydrochloride intramuscularly. Atipamezole was administered intramuscularly at 120 min after MMK administration. Time to lateral recumbency, intubation, extubation and sternal recumbency were recorded. Cardiorespiratory variables and response to a noxious stimulus were recorded before and at 3 min and 10 min increments after drug administration until sternal recumbency. The time to lateral recumbency and intubation were 1.9±1.2 and 4.3±1.2 min, respectively. Body temperature and haemoglobin saturation with oxygen remained unchanged compared to baseline values throughout anaesthesia. Respiratory rate, tidal volume, minute volume, heart rate, and blood pressure were significantly decreased during anaesthesia compared to baseline values. One cat met criteria for hypotension (systolic blood pressure <90 mmHg). End tidal carbon dioxide increased during anaesthesia compared to baseline values. All but one cat remained non-responsive to noxious stimuli from 3 to 120 min. Time to extubation and sternal recumbency following atipamezole were 2.9±1.1 and 4.7±1.0 min, respectively. MMK drug combination produced excellent short-term anaesthesia and analgesia with minimal cardiopulmonary depression. Anaesthesia lasted for at least 120 min in all but one cat and was effectively reversed by atipamezole.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Pain Management in Elderly Patients

In elderly patients, persistent pain negatively impacts quality of life. An interdisciplinary approach to pain management and emphasis on quality improvement will help to achieve better therapeutic outcomes. Managing pain in the geriatric population is challenging because of age-related changes in pain perception, cognition, pharmacokinetics, and drug effects. Improvement and maintenance of physical and emotional function is the goal. Pharmacotherapy should be initiated conservatively and titrated to effective doses with minimal adverse effects. Milder pain should be treated with non-opioid analgesics with a progression toward opioids and/or adjuvant medications as the pain intensifies. Acetaminophen, nonsteroidal anti-inflammatory drugs (NSAIDs), opioids, and adjuvant medications represent most of the analgesic agents used in pain management. Knowing the underlying mechanism of pain will help guide pharmacologic therapy. The patient should be monitored initially, with every dose change, and periodically to assess efficacy and severity of adverse effects.

Application of fast-track surgery principles to evaluate effects of atipamezole on recovery and analgesia following ovariohysterectomy in cats anesthetized with dexmedetomidine-ketamine-hydromorphone

OBJECTIVE

To evaluate the effects of atipamezole hydrochloride on recovery and analgesia following ovariohysterectomy in cats anesthetized with a dexmedetomidine hydrochloride, ketamine hydrochloride, and hydromorphone hydrochloride combination, in accordance with fast-track surgery principles.

DESIGN

Prospective, randomized, clinical trial.

ANIMALS

44 cats.

PROCEDURES

Cats were anesthetized with a combination of dexmedetomidine (15 μg/kg [6.8 μg/lb]), ketamine (5 mg/kg [2.3 mg/lb]), and hydromorphone (0.05 mg/kg [0.023 mg/lb]), IM, supplemented with isoflurane in oxygen. Immediately after ovariohysterectomy, cats received meloxicam (0.2 mg/kg [0.09 mg/lb]) SC and either atipamezole (75 μg/kg [34.1 μg/lb]) or an equivalent volume of saline (0.9% NaCl) solution IM. Pain and sedation were scored at baseline (prior to surgery) and at predetermined intervals after surgery. Time to sternal recumbency was recorded.

RESULTS

The atipamezole group recovered to sternal recumbency faster (median, 15 minutes; range, 5 to 60 minutes) than the saline solution group (median, 60 minutes; range, 15 to 90 minutes]). Pain scores did not differ between groups or at any time, compared with baseline, and were below the intervention threshold for most cats. Sedation scores were significantly greater in the saline solution group (median, 0; range, 0 to 2) at 2 hours after surgery, compared with the atipamezole group (median, 0; range, 0 to 0).

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that administration of atipamezole, compared with saline solution, allowed for a faster recovery from anesthesia with dexmedetomidine-ketamine-hydromorphone in cats following ovariohysterectomy without compromising analgesia. These findings have implications for the provision of appropriate postoperative analgesia following ovariohysterectomy in cats.

Can Gradual Dose Titration of Ketamine for Management of Neuropathic Pain Prevent Psychotomimetic Effects in Patients With Advanced Cancer?

Background:

Ketamine is often used to manage neuropathic pain in patients with cancer. However, it occasionally causes psychotomimetic effects such as vivid dreams, nightmares, illusions, hallucinations, and altered body image. Objective: To examine whether gradual dose titration of ketamine for management of neuropathic pain prevents psychotomimetic effects in patients with advanced cancer.

Methods:

This was a retrospective chart review. We administered ketamine when neuropathic pain in patients with advanced cancer became refractory to opioids and oral adjuvant analgesics. The starting dose of ketamine was 10 mg/d by continuous intravenous infusion. The dose was gradually increased by 10 mg/d every 4 to 6 hours to 50 mg/d or until the pain was relieved. It was subsequently increased by 25 mg/d every 12 to 24 hours until the pain was relieved.

Results:

For this study, we enrolled 46 patients with advanced cancer. The mean age was 52.2 ± 16.9 years. The mean dose at onset of action and maximum dose of ketamine were 56 ± 58 and 272 ± 214 mg/d, respectively. The mean pain intensity (numerical rating scale) decreased significantly from 7.3 ± 2.0 to 3.5 ± 2.2 after the administration of ketamine ( P < .01). The effectiveness was 69.5%. No psychotomimetic effect of less than 300 mg/d was observed during the introduction phase even though psychotropic drugs were not prescribed. Mild sedation was observed in 3 patients (7%) as the only adverse effect during the introduction phase.

Conclusion:

Gradual dose titration of ketamine for management of neuropathic pain can prevent psychotomimetic effects in patients with advanced cancer.

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

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

Reduction of human experimental muscle pain by alfentanil and morphine

Musculoskeletal pain is a major clinical problem. By using various experimental models in humans, the understanding of the basic mechanisms behind muscle pain can increase, thereby giving hope for new and optimized treatment. Opioids are increasingly often used to treat muscle pain. There are, however, a limited number of previous studies on opioids and muscle pain, most of them using a relative low, single dose. Therefore, we wanted to further study the effect of two rather high doses of alfentanil (25 and 75ng/ml) and morphine (0.14 and 0.28mg/kg) in human volunteers. The study consisted of two parallel studies with morphine and alfentanil, respectively, and was conducted as randomized, double-blinded, placebo-controlled, 3-way cross-over. We used intramuscular infusion of hypertonic saline and intramuscular electrical stimulation to induce experimental pain. Visual analog scale (VAS)-score, intramuscular electrical pain thresholds and pain area (local and referred) were measured. Both alfentanil and morphine at their highest doses induced a 6 to 7-fold increase in pain thresholds to single and repetitive (5 stimulations, 2Hz) electrical stimulation. Alfentanil and morphine also reduced VAS score about 4 to 5-fold during suprathreshold electric stimulation and during infusion of hypertonic saline. None of the drugs decreased referred pain. There were no apparent differences between the drugs, in terms of effect or adverse reactions. In conclusion, this is the first study to compare two high doses of alfentanil and morphine on experimental muscle pain in humans. Both alfentanil and morphine reduced experimental muscle pain. There were no indications of any true pharmacodynamic differences between the two drugs.

A systematic review of ketamine as an analgesic agent in adult burn injuries

OBJECTIVE

To assess the current literature regarding the effectiveness and side-effect profile of intravenous ketamine as a means of pain relief when compared with placebo or as an adjunct to opioid analgesia in patients exposed to burn injury.

DESIGN

Electronic searches of MEDLINE, CINAHL, Embase, and The Cochrane Library databases from 1966 onward were used to identify clinical trials comparing ketamine with placebo in the adult burn population.

OUTCOMES MEASURED

Effectiveness and side-effect profile of ketamine as an analgesic agent in burn injuries.

RESULTS

Four experimental trials involving 67 patients were identified. Due to heterogeneity of studies, pooling of the results and meta-analysis were not possible. Intravenous ketamine showed some efficacy as an analgesic for burn injuries, with a reduction in secondary hyperalgesia when compared with opioid analgesia alone. Combination therapy of ketamine and morphine resulted in the abolishment of windup pain phenomena. The side-effect profile did not result in the withdrawal of any participants included in the studies' results.

CONCLUSIONS

Further well-designed randomized controlled trials conducted in burn-specific populations are warranted, thus enabling the development of a relevant evidence base to support its clinical use.

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

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

Management of pain in children with burns

Burn injuries are common in children under 10 years of age. Thermal injury is the most common mechanism of injury and scalds account for >60% of such injuries. All children with burns will experience pain, regardless of the cause, size, or burn depth. Undertreated pain can result in noncompliance with treatment and, consequently, prolonged healing. It is acknowledged that the monitoring and reporting of pain in children with burns has generally been poor. Due to the adverse physiological and emotional effects secondary to pain, adequate pain control is an integral and requisite component in the management of children with burns. A multidisciplinary approach is frequently necessary to achieve a robust pain relief. Key to successful treatment is the continuous and accurate assessment of pain and the response to therapy. This clinical review article discusses the essential aspects of the pathophysiology of burns in children provides an overview of pain assessment, the salient principles in managing pain, and the essential pharmacodynamics of commonly used drugs in children with burn injuries. Both pharmacological and nonpharmacological treatment options are discussed, although a detailed review of the latter is beyond the scope and remit of this article.

Selective enhancement of fentanyl-induced antinociception by the delta agonist SNC162 but not by ketamine in rhesus monkeys: Further evidence supportive of delta agonists as candidate adjuncts to mu opioid analgesics

Mu-opioid receptor agonists such as fentanyl are effective analgesics, but their clinical use is limited by untoward effects. Adjunct medications may improve the effectiveness and/or safety of opioid analgesics. This study compared interactions between fentanyl and either the noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist ketamine or the delta-opioid receptor agonist SNC162 [(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-phenyl)methyl]-N,N-diethylbenzamide] in two behavioral assays in rhesus monkeys. An assay of thermal nociception evaluated tail-withdrawal latencies from water heated to 50 and 54°C. An assay of schedule-controlled responding evaluated response rates maintained under a fixed-ratio 30 schedule of food presentation. Effects of each drug alone and of three mixtures of ketamine+fentanyl (22:1, 65:1, 195:1 ketamine/fentanyl) or SNC162+fentanyl (59:1, 176:1, 528:1 SNC162/fentanyl) were evaluated in each assay. All drugs and mixtures dose-dependently decreased rates of food-maintained responding, and drug proportions in the mixtures were based on relative potencies in this assay. Ketamine and SNC162 were inactive in the assay of thermal antinociception, but fentanyl and all mixtures produced dose-dependent antinociception. Drug interactions were evaluated using dose-addition and dose-ratio analysis. Dose-addition analysis revealed that interactions for all ketamine/fentanyl mixtures were additive in both assays. SNC162/fentanyl interactions were usually additive, but one mixture (176:1) produced synergistic antinociception at 50°C. Dose-ratio analysis indicated that ketamine failed to improve the relative potency of fentanyl to produce antinociception vs. rate suppression, whereas two SNC162/fentanyl mixtures (59:1 and 176:1) increased the relative potency of fentanyl to produce antinociception. These results suggest that delta agonists may produce more selective enhancement than ketamine of mu agonist-induced antinociception.

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.

Clarithromycin, a potent inhibitor of CYP3A, greatly increases exposure to oral S-ketamine

BACKGROUND

Oral ketamine is used as an adjuvant in the treatment of refractory neuropathic and cancer-related pain. Drug interactions may alter the analgesic or other effects of ketamine.

AIM AND METHODS

The aim of the study was to investigate the effect of cytochrome P450 3A enzyme inhibition with clarithromycin on the pharmacokinetics and pharmacodynamics of oral S-ketamine in a randomized controlled cross-over study with two phases. Ten healthy subjects were pre-treated with oral clarithromycin or placebo for 4 days. On day 4, they ingested an oral dose of 0.2mg/kg of S-ketamine syrup. Plasma concentrations of ketamine and norketamine were measured for 24h. Analgesic effects were evaluated in a cold pressor test and psychomotor effects were followed for 12h.

RESULTS

Clarithromycin increased the mean C(max) of ketamine by 3.6-fold (p<0.001) and the mean AUC(0-infinity) of ketamine by 2.6-fold (p=0.001). The relative amount of the CYP3A dependent metabolite norketamine was decreased by 54% by clarithromycin (p=0.004). Self-rated drug effect of S-ketamine was enhanced by clarithromycin (p<0.05) but other behavioral effects or cold pain scores were not affected.

CONCLUSIONS

Clarithromycin strongly increases plasma concentrations of oral S-ketamine probably by inhibiting its CYP3A-mediated N-demethylation. This increase is reflected as modest changes in behavioral effects of oral S-ketamine.

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

AIM

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

METHOD

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

RESULTS

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

CONCLUSION

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

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Analgo-sedation of patients with burns outside the operating room

Following the initial resuscitation of burn patients, the pain experienced may be divided into a 'background' pain and a 'breakthrough' pain associated with painful procedures. While background pain may be treated with intravenous opioids via continuous infusion or patient-controlled analgesia (PCA) and/or less potent oral opioids, breakthrough pain may be treated with a variety of interventions. The aim is to reduce patient anxiety, improve analgesia and ensure immobilization when required. Untreated pain and improper sedation may result in psychological distress such as post-traumatic stress disorder, major depression or delirium. This review summarizes recent developments and current techniques in sedation and analgesia in non-intubated adult burn patients during painful procedures performed outside the operating room (e.g. staple removal, wound-dressing, bathing). Current techniques of sedation and analgesia include different approaches, from a slight increase in background pain therapy (e.g. morphine PCA) to PCA with rapid-onset opioids, to multimodal drug combinations, nitrous oxide, regional blocks, or non-pharmacological approaches such as hypnosis and virtual reality. The most reliable way to administer drugs is intravenously. Fast-acting opioids can be combined with ketamine, propofol or benzodiazepines. Adjuvant drugs such as clonidine or NSAIDs and paracetamol (acetaminophen) have also been used. Patients receiving ketamine will usually maintain spontaneous breathing. This is an important feature in patients who are continuously turned during wound dressing procedures and where analgo-sedation is often performed by practitioners who are not specialists in anaesthesiology. Drugs are given in small boluses or by patient-controlled sedation, which is titrated to effect, according to sedation and pain scales. Patient-controlled infusion with propofol has also been used. However, we must bear in mind that burn patients often show an altered pharmacokinetic and pharmacodynamic response to drugs as a result of altered haemodynamics, protein binding and/or increased extracellular fluid volume, and possible changes in glomerular filtration. Because sedation and analgesia can range from minimal sedation (anxiolysis) to general anaesthesia, sedative and analgesic agents should always be administered by designated trained practitioners and not by the person performing the procedure. At least one individual who is capable of establishing a patent airway and positive pressure ventilation, as well as someone who can call for additional assistance, should always be present whenever analgo-sedation is administered. Oxygen should be routinely delivered during sedation. Blood pressure and continuous ECG monitoring should be carried out whenever possible, even if a patient is undergoing bathing or other procedures that may limit monitoring of vital pulse-oximetry parameters.

[Systemic analgesia for postoperative pain management in the adult]

Severe postsurgical pain contributes to prolonged hospital stay and is also believed to be a risk factor for the development of chronic pain. Locoregional anesthesia, which results in faster patient recovery with fewer side effects, is favored wherever feasible, but is not applicable to every patient. Systemic analgesics are the most widely used method for providing pain relief in the postoperative period. Improvements in postoperative systemic analgesia for pain management should be applied and predictive factors for severe postoperative pain should be anticipated in order to control pain while minimizing opioid side effects. Predictive factors for severe postoperative pain include severity of preoperative pain, prior use of opiates, female gender, non-laparoscopic surgery, and surgeries involving the knee and shoulder. Pre- and intraoperative use of small doses of ketamine has a preventive effect on postoperative pain. Multimodal or balanced analgesia (the combined use of various analgesic agents) such as NSAID/morphine, NSAID/nefopam, morphine/ketamine improves analgesia with morphine-sparing effects. Nausea and vomiting, the principle side effects of morphine, can be predicted using Apfel's simplified score; patients with a high Apfel score risk should receive preemptive antiemetic agents aimed at different receptor sites, such as preoperative dexamethasone and intraoperative droperidol. Droperidol can be combined with morphine for postoperative patient-controlled anesthesia (PCA). When PCA is used, dosage parameters should be adjusted every day based on pain evaluation. Patients with presurgical opioid requirements will require preoperative administration of their daily opioid maintenance dose before induction of anesthesia: PCA offers useful options for effective postsurgical analgesia using a basal rate equivalent to the patient's hourly oral usage plus bolus doses as required.

STUDIES WITH KETAMINE AND ALFENTANIL FOLLOWING FREUND'S COMPLETE ADJUVANT-INDUCED INFLAMMATION IN RATS

1. N-Methyl-D-aspartate (NMDA) receptor antagonists suppress inflammatory hyperalgesia and the development of acute opioid tolerance. They may also enhance opioid-induced antinociception, while suppressing postopioid-induced hyperalgesia and opioid-enhanced inflammatory hyperalgesia. 2. The non-competitive NMDA receptor antagonist, ketamine, is a racemic chiral drug; its individual enantiomers have differing affinities for the NMDA receptor. The anaesthetic and antinociceptive potencies of (S)-ketamine are 1.5- and threefold higher, respectively, than those of (R)-ketamine in laboratory rodents. 3. The present study investigated the effects of racemic ketamine and enantiopure (S)-ketamine on inflammatory hyperalgesia in rats, 5 days after intraplantar injection of Freund's complete adjuvant (FCA) into one hind paw. First, racemic or (S)-ketamine was administered alone; second, racemic or (S)-ketamine was administered 30 min after initiation of i.v. infusions of the micro-opioid agonist, alfentanil. 4. Area under the curve (AUC) values for Von Frey paw withdrawal threshold (PWT) versus time curves were significantly increased (P < 0.05) for both inflamed and non-inflamed hind paws by racemic and (S)-ketamine (5 & 10 mg/kg, s.c.). Similarly, AUC values for reduction of hind paw volume versus time were significantly increased (P < 0.05) by racemic and (S)-ketamine (10 mg/kg, s.c.). 5. Alfentanil infusions significantly increased PWT in both hind paws, but neither racemic nor (S)-ketamine (5 mg/kg, s.c.) administered 30 min after initiation of alfentanil infusion produced further increases in PWT. 6. Racemic and (S)-ketamine produced antinociceptive effects in both hind paws, but an antihyperalgesic effect per se was not apparent. Additionally, there was a possible anti-inflammatory effect of both drugs in the inflamed hind paw. These findings complement previous studies in which non-competitive NMDA receptor antagonists suppressed behavioural hyperalgesia. 7. However, racemic and (S)-ketamine did not further enhance alfentanil's antinociceptive effects, although they appeared to prolong alfentanil's antinociceptive effects in the non-inflamed hind paw. These findings suggest that factors such as time-course, frequency and the mode of administration of NMDA receptor antagonists, in addition to the type of antinociceptive model (i.e. inflammatory compared with acute) and the nociceptive testing procedure (i.e. noxious mechanical compared with low threshold stimuli) may influence their effects on opioid-induced antinociception.

Effects of the NMDA-receptor antagonist ketamine on perceptual correlates of long-term potentiation within the nociceptive system

We recently reported perceptual correlates of long-term potentiation (LTP) of synaptic strength within the nociceptive system demonstrating the functional relevance of LTP for human pain sensation. LTP is generally classified as NMDA-receptor dependent or independent. Here we show that low doses of the NMDA-receptor antagonist ketamine (0.25 mg/kg) prevented the long-term increase in perceived pain to electrical test stimuli, which was induced by high-frequency electrical stimulation (HFS) of nociceptive afferents. Whereas in a control experiment HFS led to a stable increase in perceived pain by 51% for the entire observation period of 1h HFS given 4 min after i.v. ketamine was ineffective. In contrast, HFS induced a two-fold increase of pinprick-evoked pain surrounding the HFS site (secondary neurogenic hyperalgesia) in both experiments. Pain evoked by light tactile stimuli (allodynia) was also unaffected by ketamine. These data support the concept that homotopic hyperalgesia to electrical stimulation of the conditioned pathway is a perceptual correlate of NMDA-receptor sensitive homosynaptic LTP in the nociceptive system, e.g. in the spinal cord. Although secondary neurogenic hyperalgesia and allodynia are induced by the same HFS protocol, they involve additional NMDA-receptor insensitive mechanisms of heterosynaptic facilitation.

A comparison of four methods of analgesia in cats following ovariohysterectomy

OBJECTIVE

To evaluate the effectiveness of preoperative administration of oral carprofen, subcutaneous ketoprofen, and local nerve block with bupivacaine in preventing postoperative pain-associated behavior in cats after ovariohysterectomy.

ANIMALS

Fifty-two female intact cats. Materials and methods Cats received butorphanol (0.44 mg kg(-1) IM), carprofen (2.2 mg kg(-1) PO), ketoprofen (2.2 mg kg(-1) SQ), or bupivacaine infiltration block (1.1 mg kg(-1) SQ) before surgery. Cortisol and drug concentrations and visual analog scale (VAS) and interactive visual analog scale (IVAS) pain-associated behavior scores were measured 2 hours before and 0, 1, 2, 4, 8, 12, and 24 hours after ovariohysterectomy.

RESULTS

Cats receiving butorphanol had significantly increased IVAS scores 2 hours after surgery compared with baseline measurements. Cats receiving carprofen, ketoprofen, and bupivacaine had significant increases from baseline in VAS and IVAS scores 1 and 2 hours after surgery. VAS and IVAS scores for cats receiving bupivacaine were significantly greater 1 and 2 hours after surgery than for cats that received butorphanol. Cats receiving carprofen had significant increases in cortisol 1 hour after surgery and significant decreases 24 hours after surgery compared with baseline measurements.

CONCLUSIONS AND CLINICAL RELEVANCE

Preoperative carprofen and ketoprofen have effects on pain-associated behavior similar to butorphanol in cats undergoing ovariohysterectomy. Cats receiving bupivacaine blocks may require additional analgesics immediately after surgery.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Dose-dependent effects of morphine on experimentally induced cutaneous pain in healthy volunteers

This study examines the dose dependent analgesic effects of two doses of morphine and a single dose of alfentanil on experimentally induced cutaneous pain. In 16 healthy volunteers pain was induced by a skin burn injury and by continuous electrical skin stimulation. Mechanical pain thresholds (PT, von Frey filament), area of secondary hyperalgesia (SH) and 'wind-up like pain' upon repetitive stimulation (40-g load, 3Hz, 30s) were assessed. Analgesic effects on these pain parameters were tested at steady-state IV infusions of morphine, 50% (plasma concentration 15ng/ml) and 100% (plasma concentration 30ng/ml) of maximal tolerable dose to be given to healthy volunteers, and with an effective dose of alfentanil (plasma concentration 70ng/ml). All effects were compared to active placebo, midazolam infusion (20microg/kg for 10min). Alfentanil significantly diminished the SH area in the burn injury model as well as in the electrical pain model. Additionally, alfentanil increased PT several fold in both models. The high dose of morphine showed a similar analgesic response pattern as alfentanil even though the effects were only statistically significant in the electrical pain model. The low dose of morphine as well as placebo did not affect these pain parameters. 'Wind-up like pain' was not influenced by any of the given drugs. In conclusion, the present study clearly indicates dose dependent effects of morphine on experimentally induced cutaneous pain. The high dose of morphine (30ng/ml) was approximately equianalgesic to the administered alfentanil dose (70ng/ml).

Effects of the N-Methyl-D-Aspartate Receptor Antagonist Dextromethorphan on Temporal Summation of Pain are Similar in Fibromyalgia Patients and Normal Control Subjects

Temporal summation of second pain at least partly reflects temporal summation of dorsal horn neuronal responses, and both have been termed windup (WU), a form of nociception-dependent central sensitization. Animal and human experiments have shown that both forms of WU depend on N-methyl-D-aspartate (NMDA) and substance P receptor systems. WU of second pain (WU(SP)) in patients with fibromyalgia (FM) is enhanced compared with normal control (NC) subjects and is followed by exaggerated WU(SP) aftersensations and prolonged WU(SP) maintenance at low stimulus frequencies. Because the enhanced WU(SP) of FM patients could be related to abnormal endogenous modulation of NDMA receptors, we tested the effects of the NMDA receptor antagonist dextromethorphan (DEX) on WU(SP) in FM and NC subjects in a double-blind, placebo-controlled, crossover study. WU(SP) was elicited by trains of 0.7-second duration thermal pulses applied to the glabrous surface of the hands or by 1-second mechanical stimuli to the adductor pollicis muscle of the hands at a frequency of 0.33 Hz. In comparison to baseline and placebo conditions, single oral doses of DEX 60 and 90 mg reduced thermal and mechanical WU(SP) in NC and FM subjects, with DEX 90 mg being most effective. These effects did not differ for male and female NC subjects. FM subjects required less thermal and mechanical stimulus intensity than NC to achieve maximal WU(SP), but the extent of WU(SP) reduction by DEX did not statistically differ between NC and FM subjects for all study conditions. Thus, central pain processing of FM subjects is not different from NC in at least one important aspect, namely their NMDA receptor system responsiveness to pharmacologic inhibition by DEX.

PERSPECTIVE

Results of this study demonstrate that FM patients show abnormal WU(SP) during thermal and mechanical stimulation compared with NC. Because oral doses of the NMDA receptor antagonist DEX attenuated thermal and mechanical WU(SP) similarly in FM patients and NC, other mechanisms than WU(SP) need to be considered for the widespread pain of FM patients. These mechanisms might include tonic nociceptive input from peripheral tissues and/or enhanced descending facilitation.

Ketamina, da anestesia ao uso abusivo

Objetivo. Realizar uma revisão bibliográfica sobre a ketamina, enfocando aspectos químicos e farmacocinéticos, seus mecanismos de ação, efeitos farmacológicos, usos terapêuticos, bem como aborda o uso da ketamina como droga de abuso. Ketamina. Método. Estudo de revisão bibliográfica através das bases de dados Pubmed, SciELO, Lilacs e Medline, na qual foram selecionados estudos clássicos e recentes relevantes para a discussão do tema abordado. Resultados. Após análise dos artigos selecionados, conclui-se que a ketamina apresenta perspectivas promissoras. Alguns estudos demonstraram propriedades analgésicas em relação à dor pós-operatória e outros revelaram uma ação antidepressiva rápida. Além disto, a ketamina também apresentou efeitos neuroprotetores em vários estudos animais. Embora a ketamina seja um campo promissor, essas ações precisam ser melhor investigadas. Atualmente, observou-se que o seu uso não se restringe apenas à prática clínica ou pesquisa, sendo frequentemente utilizada como droga de abuso pelos jovens em festas como um potente alucinógeno. Conclusão. Pesquisas sobre o uso recreacional da ketamina também são necessárias, especialmente no Brasil, assim como dados que mostrem a morbidade e mortalidade pelo uso desta substância como droga de abuso.