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Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

Opioid Therapy in the 21st Century, Second Edition provides clinicians with up-to-date, evidence-based information about appropriate opioid use, including the potential benefits and adverse affects of the therapy.

In a double-blind, placebo-controlled, phase 3 trial, patients undergoing hemodialysis who had moderate-to-severe pruritus were randomly assigned to receive the selective kappa opioid receptor agonist difelikefalin or placebo for 12 weeks. Difelikefalin significantly reduced itch intensity and improved quality of life as compared with placebo.

To perform first-in-human single-dose escalation trial of ZYKR1, which is a potent, selective, and peripherally-restricted kappa opioid receptor agonist, is the purpose of this study. This randomized, double-blind, placebo-controlled single ascending dose study conducted at Zydus Research Centre, Ahmedabad, India included healthy male participants aged 18–55 years and weighing > 50 kg. The primary objective was to evaluate the safety and tolerability of ZYKR1. The secondary objective was to evaluate the pharmacokinetics and pharmacodynamics (PD) of ZYKR1. Participants received ZYKR1 (0.5 − 6 mcg/kg) or placebo infused intravenously in 15 ± 1 min. Of total five dose groups (0.5 − 6 mcg/kg), each group included eight participants with six and two randomized to ZYKR1 and placebo, respectively. Three participants experienced six treatment-emergent adverse events (TEAEs); two were gastrointestinal disorders (nausea and vomiting at 2 mcg/kg); and four were related to the nervous system (headache (at 2 mcg/kg) and facial tingling, facial numbness and paresthesia (at 6 mcg/kg)); all TEAEs were mild and resolved without sequelae. The C max of ZYKR1 was achieved after 15 − 20 min of start of infusion. The mean exposures ( C max and AUC 0 − t ) increased in a dose-proportional manner. The mean t 1/2 ranged from 2.20 to 2.98 h across the dose range. Increase in the mean prolactin level was significantly higher in treatment groups compared with that in the placebo group. Intravenous ZYKR1 at doses up to 6 mcg/kg showed acceptable safety and tolerability and demonstrated a short half-life with principal route of excretion as renal. ZYKR1 displayed a potent PD effect reflected by increased prolactin levels, supporting further study in patients. Trial registration Clinical Trial Registry of India: CTRI/2018/07/014927. Date of registration: 18/07/2018.

Rationale and objectivesThe present study characterized the behavioral pharmacology of a novel, mixed-action delta-selective (78:1) opioid receptor agonist, BBI-11008. This glycopeptide drug candidate was tested in assays assessing antinociception (acute, inflammatory, and neuropathic pain-like conditions) and side-effect endpoints (respiratory depression and drug self-administration).ResultsBBI-11008 had a 78-fold greater affinity for the delta opioid receptor than the mu receptor, and there was no binding to the kappa opioid receptor. BBI-11008 (3.2–100; 10–32 mg kg−1, i.v.) and morphine (1–10; 1–3.2 mg kg−1, i.v.) produced antinociceptive and anti-allodynic effects in assays of acute thermal nociception and complete Freund’s adjuvant (CFA)-induced inflammatory pain, with BBI-11008 being less potent than morphine in both assays. BBI-11008 (1–18 mg kg−1, i.v.) had similar efficacy to gabapentin (10–56 mg kg−1, i.v.) in a spinal nerve ligation (SNL) model of neuropathic pain. In the respiration assay, with increasing %CO2 exposure, BBI-11008 produced an initial increase (32 mg kg−1, s.c.) and then decrease (56 mg kg−1, s.c.) in minute volume (MV) whereas morphine (3.2–32 mg kg−1, s.c.) produced dose-dependent decreases in MV. In the drug self-administration procedure, BBI-11008 did not maintain self-administration at any dose tested.ConclusionsThese results suggest that the glycopeptide drug candidate possesses broad-spectrum antinociceptive and anti-allodynic activity across a range of pain-like conditions. Relative to morphine or fentanyl, the profile for BBI-11008 in the respiration and drug self-administration assays suggests that BBI-11008 may have less pronounced deleterious side effects. Continued assessment of this compound is warranted.

Breakthrough pain (BTP) is episodic pain that emerges through the treatment of otherwise well-managed chronic background pain. Often called pain flare or transient pain, BTP negatively affects the function and quality of life of the patient and often results in a number of other physical, psychological and social problems. Breakthrough pain is a common occurrence affecting approximately two-thirds of the estimated 50 to 100 million chronic pain sufferers in the US. It can have multiple causes with various pathophysiologies, and can present with numerous clinical features and complications. The clinical features vary from individual to individual, and may vary within an individual over time. The successful management of breakthrough pain depends on proper assessment, treatment, and reassessment. Inadequate assessment can lead to ineffective or inappropriate treatment. Similarly, inadequate reassessment may lead to continuance of ineffective or even harmful treatment. In recent years, the need to educate physicians about pain management has been garnering increased attention from prominent medical associations and the media. Despite ongoing efforts to improve pain treatment, however, the need persists for evidence-based educational materials for physicians in the area of pain diagnosis and management. Part of the Oxford American Pain Library, this highly practical guide covers current approaches and new developments in the assessment and management of breakthrough pain, including both cancer-related pain and non-cancer chronic pain. It addresses the roles of opioid and non-opioid pharmacotherapy and presents non-pharmacologic interventions as well.

The presence of endogenous opioid peptides in different testicular cell types has been extensively characterized and provides evidence for the participation of the opioid system in the regulation of testicular function. However, the exact role of the opioid system during the spermatogenesis has remained controversial since the presence of the mu-, delta- and kappa-opioid receptors in spermatogenic cells was yet to be demonstrated. Through a combination of quantitative real-time PCR, immunofluorescence, immunohistochemistry and flow cytometry approaches, we report for the first time the presence of active mu-, delta- and kappa-opioid receptors in mouse male germ cells. They show an exposition time-dependent response to opioid agonist, hence suggesting their active involvement in spermatogenesis. Our results contribute to understanding the role of the opioid receptors in the spermatogenesis and could help to develop new strategies to employ the opioid system as a biochemical tool for the diagnosis and treatment of male infertility.

Biphalin, one of the opioid agonists, is a dimeric analog of enkephalin with a high affinity for opioid receptors. Opioid receptors are widespread in the central nervous system and in peripheral neuronal and non-neuronal tissues. Hence, these receptors and their agonists, which play an important role in pain blocking, may also be involved in the regulation of other physiological functions. Biphalin was designed and synthesized in 1982 by Lipkowski as an analgesic peptide. Extensive further research in various laboratories on the antinociceptive effects of biphalin has shown its excellent properties. It has been demonstrated that biphalin exhibits an analgesic effect in acute, neuropathic, and chronic animal pain models, and is 1000 times more potent than morphine when administered intrathecally. In the course of the broad conducted research devoted primarily to the antinociceptive effect of this compound, it has been found that biphalin may also potentially participate in the regulation of other opioid system-dependent functions. Nearly 40 years of research on the properties of biphalin have shown that it may play a beneficial role as an antiviral, antiproliferative, anti-inflammatory, and neuroprotective agent, and may also affect many physiological functions. This integral review analyzes the literature on the multidirectional biological effects of biphalin and its potential in the treatment of many opioid system-dependent pathophysiological diseases.

To evaluate the effects of low-dose butorphanol on hyperalgesia induced by high-dose remifetanil in patients undergoing laparoscopic cholecystectomy. Randomized double-blind clinical trial. Intraoperative. Seventy-five patients scheduled for laparoscopic cholecystectomy were enrolled. Randomly allocated into 3 groups, low dose of remifentanil (LR) group and high dose of remifentanil (HR) group received low (0.1μg kg−1 min−1) or high (0.3μg kg−1 min−1) doses of remifentanil, respectively, and butorphanol combined with remifentanil (BR) group received remifentanil (0.3μg kg−1 min−1) and butorphanol (0.2μg/kg). The visual analog scale scores and cumulative consumption of fentanyl were recorded. Visual analog scale scores were significantly higher in the HR group than in the LR and BR groups (P<.001). The dose of intravenously given fentanyl was significantly higher in the HR group than in the LR and BR groups (P<.001). In addition, the HR group showed a significantly higher cumulative consumption of fentanyl during 5 to 8 hours after the operation (P<.001). A high dose of remifentanil induces postoperative hyperalgesia, which could be prevented by a continuous intravenous administration of a low dose of butorphanol. •Remifentanil enhances postoperative pain through the activation of mu-opioid receptor.•High dose of remifentanil during laparoscopic cholecystectomy induced postoperative hyperalgesia.•Low dose of butorphanol prevented remifentanil-induced postoperative hyperalgesia.

As clinical use of currently available opioid analgesics is often impeded by dose-limiting adverse effects, such as abuse liability and respiratory depression, new approaches have been pursued to develop safe, effective, and non-addictive pain medications. After the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor more than 25 years ago, NOP receptor-related agonists have emerged as a promising target for developing novel and effective opioids that modulate the analgesic and addictive properties of mu-opioid peptide (MOP) receptor agonists. In this review, we highlight the effects of the NOP receptor-related agonists compared with those of MOP receptor agonists in experimental rodent and more translational non-human primate (NHP) models and the development status of key NOP receptor-related agonists as potential safe and non-addictive analgesics. Several lines of evidence demonstrated that peptidic and non-peptidic NOP receptor agonists produce potent analgesic effects by intrathecal delivery in NHPs. Moreover, mixed NOP/MOP receptor partial agonists (e.g., BU08028, BU10038, and AT-121) display potent analgesic effects when administered intrathecally or systemically, without eliciting adverse effects, such as respiratory depression, itch behavior, and signs of abuse liability. More importantly, cebranopadol, a mixed NOP/opioid receptor agonist with full efficacy at NOP and MOP receptors, produces robust analgesic efficacy with reduced adverse effects, conferring promising outcomes in clinical studies. A balanced coactivation of NOP and MOP receptors is a strategy that warrants further exploration and refinement for the development of novel analgesics with a safer and effective profile. Graphical Abstract