Evaluation of Therapies for Peripheral and Neuraxial Opioid-induced Pruritus based on Molecular and Cellular Discoveries

Prophylatic use of IV nalmefene to prevent epidural opioid-induced pruritus: A multicenter, randomized clinical trial

STUDY OBJECTIVE

The incidence of pruritus from neuraxial opioids is about 60%. Pruritus causes discomfort and decreases the quality of recovery. This randomized double-blinded clinical trial was aimed to evaluate the prophylactic effects of a single dose IV nalmefene on the incidence and severity of epidural opioid-induced pruritus within 24 h after surgeries.

DESIGN

A two-center, randomized, double blinded, controlled clinical trial.

SETTING

The study was conducted from March 2022 to February 2023 at two tertiary care hospitals in China.

PATIENTS

Patients aged between 18 and 80 years-old who underwent elective surgeries and received epidural analgesia intra- and post-operatively were screened for study enrollment. A total of 306 patients were enrolled, 302 patients underwent randomization and 296 patients were included in the final analysis.

INTERVENTIONS

The nalmefene group was prophylactically given 0.5 μg/kg nalmefene intravenously while the control group was given the same volume of saline.

MEASUREMENTS

The primary endpoint was the incidence of pruritus within 24 h after surgeries. The secondary endpoints included time of the first patient-reported pruritus, severity of pruritus after surgeries, severity of acute pain scores after surgeries and other anesthesia/analgesia related side effects.

MAIN RESULTS

Pruritus occurred in 51 of the 147 (34.69%) patients in the control group and 35 of the 149 (23.49%) patients in the nalmefene group (odds ratio, 0.58; 95% CI, 0.35 to 0.96; P = 0.034) within 24 h postoperatively. Nalmefene group demonstrated delayed onset of pruritus, reduced severity of pruritus and decreased vomiting within 24 h after surgery. There were no significant differences in postoperative analgesia and the incidence of other anesthesia/analgesia associated side effects.

CONCLUSIONS

A single dose of 0.5 μg/kg nalmefene intravenously significantly reduced the incidence and severity of epidural-opioid induced pruritus within 24 h after surgery without affecting the efficacy of epidural analgesia.

TRIAL REGISTRATION

Chinese Clinical Trial Registry (www.chictr.org.cn) and the registration number is ChiCTR2100050463. Registered on August 27th, 2021.

Ondansetron for the Prevention of Pruritus in Women Undergoing Cesarean Delivery With Intrathecal Opioid: A Systematic Review and Meta-Analysis

BACKGROUND

Pruritus is a frequently reported and unpleasant side effect following intrathecal opioid use with frequency further increased among parturients. We have performed a systematic review to assess the overall efficacy of ondansetron for the prevention of pruritus in patients receiving intrathecal opioid as part of spinal anesthesia for cesarean delivery.

METHODS

A literature search of MEDLINE, Embase, Cochrane, and Web of Science databases was conducted from date of inception to September 2022. Studies that included patients undergoing cesarean delivery with spinal anesthesia using intrathecal opioid were included. The primary outcome was the presence of pruritus, and the secondary outcome was time to onset of pruritus. Data from included studies were pooled for analysis using an appropriately determined random-effects model. Outcomes were presented using forest plots and 95% confidence intervals. Additional sensitivity and subgroup analysis were performed. Trial sequential analysis was conducted for the primary outcome.

RESULTS

Twenty-three randomized controlled trials with a total of 2586 patients were included: 1219 received ondansetron, 1030 received a placebo, and a further 337 received a different study drug and were excluded from analysis. Opioids used in the included studies were morphine, fentanyl, and sufentanil. Patients who received ondansetron showed a significant reduction in the incidence of pruritus compared to the control group (RR, 0.81; 95% confidence interval [CI], 0.71-0.92; I 2 = 64%). There was no significant difference in pruritus onset between the groups (mean difference [MD], 17.54 minutes; 95% CI, -2.18 to 37.26; I 2 = 83%). The overall Grading of Recommendations Assessment, Development, and Evaluation (GRADE) assessment of quality of evidence was low.

CONCLUSIONS

This systematic review has demonstrated a significant reduction in the incidence of pruritus following the use of ondansetron. This is in contrast to previously published meta-analyses. Studies included were of varying quality and some at high risk of bias with a high degree of statistical heterogeneity. Furthermore, high-quality and well-powered studies are required to confirm these findings.

Brainstem Modulation of Nociception by Periaqueductal Gray Neurons Expressing the μ-Opioid Receptor in Mice

BACKGROUND

Pharmacologic manipulations directed at the periaqueductal gray have demonstrated the importance of the μ-opioid receptor in modulating reflexive responses to nociception. The authors hypothesized that a supraspinal pathway centered on neurons in the periaqueductal gray containing the μ-opioid receptor could modulate nociceptive and itch behaviors.

METHODS

The study used anatomical, optogenetic, and chemogenetic approaches in male and female mice to manipulate μ-opioid receptor neurons in the periaqueductal gray. Behavioral assays including von Frey, Hargreaves, cold plantar, chloroquine-induced itch, hotplate, formalin-induced injury, capsaicin-induced injury, and open field tests were used. In separate experiments, naloxone was administered in a postsurgical model of latent sensitization.

RESULTS

Activation of μ-opioid receptor neurons in the periaqueductal gray increased jumping (least-squares mean difference of -3.30 s; 95% CI, -6.17 to -0.44; P = 0.023; n = 7 or 8 mice per group), reduced itch responses (least-squares mean difference of 70 scratching bouts; 95% CI, 35 to 105; P < 0.001; n = 8 mice), and elicited modestly antinociceptive effects (least-squares mean difference of -0.7 g on mechanical and -10.24 s on thermal testing; 95% CI, -1.3 to -0.2 and 95% CI, -13.77 to -6.70, and P = 0.005 and P < 0.001, respectively; n = 8 mice). Last, the study uncovered the role of the periaqueductal gray in suppressing hyperalgesia after a postsurgical state of latent sensitization (least-squares mean difference comparing saline and naloxone of -12 jumps; 95% CI, -17 to -7; P < 0.001 for controls; and -2 jumps; 95% CI, -7 to 4; P = 0.706 after optogenetic stimulation; n = 7 to 9 mice per group).

CONCLUSIONS

μ-Opioid receptor neurons in the periaqueductal gray modulate distinct nocifensive behaviors: their activation reduced responses to mechanical and thermal testing, and attenuated scratching behaviors, but facilitated escape responses. The findings emphasize the role of the periaqueductal gray in the behavioral expression of nociception using reflexive and noxious paradigms.

EDITOR’S PERSPECTIVE

Effects of opioids administered via intravenous or epidural patient-controlled analgesia after caesarean section: A network meta-analysis of randomised controlled trials

Background

Post-caesarean section analgesia is important physiologically and psychologically for both mothers and infants. Patient-controlled analgesia is a well-established method of administering opioids for postoperative pain. However, to date, no study has systematically investigated the effects of opioids administered through intravenous patient-controlled analgesia (IVPCA) or patient-controlled epidural analgesia (PCEA) in parturients who have undergone caesarean section.

Methods

This systematic review and network meta-analysis aimed to evaluate the analgesic and adverse effects of opioids administered via IVPCA or PCEA in parturients who have undergone a caesarean section. PubMed, Embase, Scopus, Web of Science, and Cochrane Library were searched from inception through 02 10, 2022 for relevant records. Randomised controlled trials (RCTs) that compared opioids administered via IVPCA or PCEA and reported outcomes of interest were included. Studies were excluded if the solution for patient-controlled analgesia contained antiemetics and/or other analgesics in addition to opioids. The methodological quality of RCTs was assessed using the revised Cochrane Risk of Bias Tool. Summary data were extracted from each eligible study. The primary outcome was pain intensity, and the secondary outcomes were opioid-related adverse effects. Frequentist network meta-analyses were performed using a contrast-based random-effects model. This study is registered with PROSPERO, CRD42021254040.

Findings

Twenty-three studies with 2589 parturients were included. Compared with IVPCA morphine as a reference treatment, PCEA fentanyl had better analgesic effects at 4 h (mean difference [MD] in the visual analogue scale score, -0.75; 95% confidence interval [CI] [-1.16, -0.34]) and 8 h (MD, -0.93; 95% CI [-1.57, -0.28]) and yielded lower odds of developing nausea/vomiting (odds ratio [OR], 0.27; 95% CI [0.09, 0.80]) and sedation/drowsiness (OR, 0.22; 95% CI [0.11, 0.45]). However, PCEA fentanyl may be more likely to cause pruritus than IVPCA treatments.

Interpretation

Considering the analgesic efficacy; opioid-induced nausea, vomiting, and sedation; and the well-being of breastfed infants, PCEA fentanyl may be the treatment of choice for post-caesarean section analgesia.

Funding

The Taipei Tzu Chi Hospital, Buddhist Tzu Chi Medical Foundation (TCRD-TPE-111-27).

Role of kappa-opioid and mu-opioid receptors in pruritus: Peripheral and central itch circuits

Modern genetic approaches in animal models have unveiled novel itch-specific neural pathways, emboldening a paradigm in which drugs can be developed to selectively and potently target itch in a variety of chronic pruritic conditions. In recent years, kappa-opioid receptors (KORs) and mu-opioid receptors (MORs) have been implicated in both the suppression and promotion of itch, respectively, by acting on both the peripheral and central nervous systems. The precise mechanisms by which agents that modulate these pathways alleviate itch remains an active area of investigation. Notwithstanding this, a number of agents have demonstrated efficacy in clinical trials that influence both KOR and MOR signalling. Herein, we summarize a number of opioid receptor modulators in development and their promising efficacy across a number of chronic pruritic conditions, such as atopic dermatitis, uremic pruritus and beyond.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Autoimmune Connective Tissue Diseases-Related Pruritus: Proper Diagnosis and Possible Mechanisms

Pruritus is a well-known bothersome symptom among skin disorders, especially inflammatory skin disorders. Lately, a high prevalence of pruritus in patients with autoimmune connective tissue diseases (ACTDs) has been revealed. Patients with ACTDs may suffer from varying degrees of pruritus, which affect their quality of life. However, it is rarely recognized both by patients and physicians. Meanwhile, pruritus is not only a symptom but is also related to the disease severity of some ACTDs. The pathophysiology of ACTD related pruritus is ambiguous. This review summarizes the features and possible mechanisms of ACTD-related pruritus, which might lead to proper diagnosis and treatment.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

Pruritus, neuraxial morphine and recrudescence of oral herpes simplex and treatment: an educational review in obstetric patients

Neuraxial opioids are well known to cause itching, which may be challenging to treat. Neuraxial morphine has been demonstrated to cause recrudescent herpes simplex viruses (HSV-1), especially in women during labor and childbirth with neuraxial analgesia, and may be an occult etiology of refractory itching. This educational review summaries the clinical and epidemiological characteristics associated with recrudescent HSV-1 in patients treated with neuraxial opioids, especially morphine.

Opioidergic Signaling-A Neglected, Yet Potentially Important Player in Atopic Dermatitis

Atopic dermatitis (AD) is one of the most common skin diseases, the prevalence of which is especially high among children. Although our understanding about its pathogenesis has substantially grown in recent years, and hence, several novel therapeutic targets have been successfully exploited in the management of the disease, we still lack curative treatments for it. Thus, there is an unmet societal demand to identify further details of its pathogenesis to thereby pave the way for novel therapeutic approaches with favorable side effect profiles. It is commonly accepted that dysfunction of the complex cutaneous barrier plays a central role in the development of AD; therefore, the signaling pathways involved in the regulation of this quite complex process are likely to be involved in the pathogenesis of the disease and can provide novel, promising, yet unexplored therapeutic targets. Thus, in the current review, we aim to summarize the available potentially AD-relevant data regarding one such signaling pathway, namely cutaneous opioidergic signaling.