Respiratory Effects of the Nociceptin/Orphanin FQ Peptide and Opioid Receptor Agonist, Cebranopadol, in Healthy Human Volunteers

Cebranopadol, a novel long-acting opioid agonist with low abuse liability, to treat opioid use disorder: Preclinical evidence of efficacy

Maintenance therapy with buprenorphine and methadone is the gold standard pharmacological treatment for opioid use disorder (OUD). Despite these compounds demonstrating substantial efficacy, a significant number of patients do not show optimal therapeutic responses. The abuse liability of these medications is also a concern. Here we used rats to explore the therapeutic potential of the new long-acting pan-opioid agonist Cebranopadol in OUD. We tested the effect of cebranopadol on heroin self-administration and yohimbine-induced reinstatement of heroin seeking. In addition, we evaluated the abuse liability potential of cebranopadol in comparison to that of heroin under fixed ratio 1 (FR1) and progressive ratio (PR) operant self-administration contingencies. Oral administration of cebranopadol (0, 25, 50μg/kg) significantly attenuated drug self-administration independent of heroin dose (1, 7, 20, 60μg/inf). Cebranopadol also reduced the break point for heroin (20 μg/inf). Finally, pretreatment with cebranopadol significantly attenuated yohimbine-induced reinstatement of drug seeking. In abuse liability experiments under FR1 contingency, rats maintained responding for heroin (1, 7, 20, 60μg/inf) to a larger extent than cebranopadol (0.03, 0.1, 0.3, 1.0, 6.0μg/inf). Under PR contingency, heroin maintained responding at high levels at all except the lowest dose, while the break point (BP) for cebranopadol did not differ from that of saline. Together, these data indicate that cebranopadol is highly efficacious in attenuating opioid self-administration and stress-induced reinstatement, while having limited abuse liability properties. Overall, the data suggest clinical potential of this compound for OUD treatment.

Novel Opioids in the Setting of Acute Postoperative Pain: A Narrative Review

Although traditional opioids such as morphine and oxycodone are commonly used in the management of acute postoperative pain, novel opioids may play a role as alternatives that provide potent pain relief while minimizing adverse effects. In this review, we discuss the mechanisms of action, findings from preclinical studies and clinical trials, and potential advantages of several novel opioids. The more established include oliceridine (biased ligand activity to activate analgesia and downregulate opioid-related adverse events), tapentadol (mu-opioid agonist and norepinephrine reuptake inhibitor), and cebranopadol (mu-opioid agonist with nociceptin opioid peptide activity)-all of which have demonstrated success in the clinical setting when compared to traditional opioids. On the other hand, dinalbuphine sebacate (DNS; semi-synthetic mu partial antagonist and kappa agonist), dual enkephalinase inhibitors (STR-324, PL37, and PL265), and endomorphin-1 analog (CYT-1010) have shown good efficacy in preclinical studies with future plans for clinical trials. Rather than relying solely on mu-opioid receptor agonism to relieve pain and risk opioid-related adverse events (ORAEs), novel opioids make use of alternative mechanisms of action to treat pain while maintaining a safer side-effect profile, such as lower incidence of nausea, vomiting, sedation, and respiratory depression as well as reduced abuse potential.

IUPHAR Review - Bivalent and bifunctional opioid receptor ligands as novel analgesics

Though efficacious in managing chronic, severe pain, opioid analgesics are accompanied by significant adverse effects including constipation, tolerance, dependence, and respiratory depression. The life-threatening risks associated with µ opioid receptor agonist-based analgesics challenges their use in clinic. A rational approach to combatting these adverse effects is to develop agents that incorporate activity at a second pharmacologic target in addition to µ opioid receptor activation. The promise of such bivalent or bifunctional ligands is the development of an analgesic with an improved side effect profile. In this review, we highlight ongoing efforts in the development of bivalent and bifunctional analgesics that combine µ agonism with efficacy at κ and δ opioid receptors, the nociceptin opioid peptide (NOP) receptor, σ receptors, and cannabinoid receptors. Several examples of bifunctional analgesics in preclinical and clinical development are highlighted, as are strategies being employed toward the rational design of novel agents.

Cebranopadol for the Treatment of Chronic Pain

PURPOSE OF REVIEW

Regardless of the etiology, if pain persists chronically, it can detrimentally impact multiple aspects of a patient's well-being. Both physical and psychological effects are significant in many chronic pain patients. In this regard, psychological consequences can alter a patient's quality of life, functionality, and social functioning. Opioids have been the long-established gold standard for acute pain treatment in settings such as the postoperative period. An alternative to opioids in pain management has been highly sought after. Through a non-selective mechanism, cebranopadol is a first-in-class oral drug which combines agonism of the mu and nociceptin opioid peptide (NOP) receptors to provide improved analgesia, while reducing the occurrence of many typically opioid side effects. This manuscript is a narrative review of the possible use of cebranopadol in pain management.

RECENT FINDINGS

In pre-clinical studies, cebranopadol was similar to morphine in its pain control efficacy. In a phase IIa trial, cebranopadol was superior to placebo in reducing pain. In a randomized clinical trial, cebranopadol was superior to morphine. Another study concluded that cebranopadol had a lower misuse potential when compared to hydromorphone. In summary, cebranopadol offers new opportunities in treating chronic moderate to severe pain, while also countering risks of addiction. Additional studies are warranted to further evaluate the safety and efficacy of cebranopadol. In this regard, cebranopadol could prove to be a promising alternative to current pain treatment options.

Cebranopadol, a novel long-acting opioid agonist with low abuse liability, to treat opioid use disorder: Preclinical evidence of efficacy

The gold standard pharmacological treatment for opioid use disorder (OUD) consists of maintenance therapy with long-acting opioid agonists such as buprenorphine and methadone. Despite these compounds having demonstrated substantial efficacy, a significant number of patients do not show optimal therapeutic responses. Moreover, the abuse liability of these medications remains a major concern. Cebranopadol, is a new, long-acting pan-opioid agonist that also activates the nociception/orphanin FQ NOP receptor. Here we used rats to explore the therapeutic potential of this agent in OUD. First, in operant intravenous self-administration experiments we compared the potential abuse liability of cebranopadol with the prototypical opioid heroin. Under a fixed ratio 1 (FR1) contingency, rats maintained responding for heroin (1, 7, 20, 60 μg/inf) to a larger extent than cebranopadol (0.03, 0.1, 0.3, 1.0, 6.0 μg/inf). When the contingency was switched to a progressive ratio (PR) reinforcement schedule, heroin maintained responding at high levels at all except the lowest dose. Conversely, in the cebranopadol groups responding decreased drastically and the break point (BP) did not differ from saline controls. Next, we demonstrated that oral administration of cebranopadol (0, 25, 50 μg/kg) significantly attenuated drug self-administration independent of heroin dose (1, 7, 20, 60 μg/inf). Cebranopadol also reduced the break point for heroin (20 μg/inf). Furthermore, in a heroin self-administration training extinction/reinstatement paradigm, pretreatment with cebranopadol significantly attenuated yohimbine stress-induced reinstatement of drug seeking. Together, these data indicate that cebranopadol has limited abuse liability compared to heroin and is highly efficacious in attenuating opioid self-administration and stress-induced reinstatement, suggesting clinical potential of this compound for OUD treatment.

Opioids and opioid receptors; understanding pharmacological mechanisms as a key to therapeutic advances and mitigation of the misuse crisis

Opioids are a mainstay in acute pain management and produce their effects and side effects (e.g., tolerance, opioid-use disorder and immune suppression) by interaction with opioid receptors. I will discuss opioid pharmacology in some controversial areas of enquiry of anaesthetic relevance. The main opioid target is the µ (mu,MOP) receptor but other members of the opioid receptor family, δ (delta; DOP) and κ (kappa; KOP) opioid receptors also produce analgesic actions. These are naloxone-sensitive. There is important clinical development relating to the Nociceptin/Orphanin FQ (NOP) receptor, an opioid receptor that is not naloxone-sensitive. Better understanding of the drivers for opioid effects and side effects may facilitate separation of side effects and production of safer drugs. Opioids bind to the receptor orthosteric site to produce their effects and can engage monomer or homo-, heterodimer receptors. Some ligands can drive one intracellular pathway over another. This is the basis of biased agonism (or functional selectivity). Opioid actions at the orthosteric site can be modulated allosterically and positive allosteric modulators that enhance opioid action are in development. As well as targeting ligand-receptor interaction and transduction, modulating receptor expression and hence function is also tractable. There is evidence for epigenetic associations with different types of pain and also substance misuse. As long as the opioid narrative is defined by the 'opioid crisis' the drive to remove them could gather pace. This will deny use where they are effective, and access to morphine for pain relief in low income countries.

Nociceptin Receptor-Related Agonists as Safe and Non-addictive Analgesics

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.

Strategies towards safer opioid analgesics-A review of old and upcoming targets

Opioids continue to be of use for the treatment of pain. Most clinically used analgesics target the μ opioid receptor whose activation results in adverse effects like respiratory depression, addiction and abuse liability. Various approaches have been used by the field to separate receptor-mediated analgesic actions from adverse effects. These include biased agonism, opioids targeting multiple receptors, allosteric modulators, heteromers and splice variants of the μ receptor. This review will focus on the current status of the field and some upcoming targets of interest that may lead to a safer next generation of analgesics. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Multitargeted Opioid Ligand Discovery as a Strategy to Retain Analgesia and Reduce Opioid-Related Adverse Effects

The global "opioid crisis" has placed enormous pressure on the opioid ligand discovery community to produce novel opioid analgesics with superior opioid-related adverse-effect profiles compared with morphine. In this Perspective, the multitargeted opioid ligand strategy for the discovery of opioid analgesics with superior preclinical therapeutic indices relative to morphine is reviewed and discussed. Dual-targeted μ-opioid (MOP)/δ-opioid (DOP) ligands in which the in vitro DOP antagonist potency at least equals that of the MOP agonist activity, and are devoid of DOP or κ-opioid (KOP) agonist activity, are sufficiently promising candidates to warrant further investigation. Dual-targeted MOP/NOP partial agonists have superior preclinical therapeutic indices to morphine and/or fentanyl in nonhuman primates and are also considered promising. Based on the poor preclinical and clinical therapeutic indices of cebranopadol, which is a full agonist at MOP, DOP, and NOP receptors and a partial agonist at the KOP receptor, this pharmacologic template should be avoided.

Oral Oxycodone-Induced Respiratory Depression during Normocapnia and Hypercapnia: A Pharmacokinetic-Pharmacodynamic Modeling Study

The widely prescribed opioid oxycodone may cause lethal respiratory depression. We compared the effects of oxycodone on breathing and antinociception in healthy young volunteers. After pharmacokinetic/pharmacodynamic (PK/PD) modeling, we constructed utility functions to combine the wanted and unwanted end points into a single function. We hypothesized that the function would be predominantly negative over the tested oxycodone concentration range. Twenty-four male and female volunteers received 20 (n = 12) or 40 (n = 12) mg oral oxycodone immediate-release tablets. Hypercapnic ventilatory responses (visit 1) or responses to 3 nociceptive assays (pain pressure, electrical, and thermal tests; visit 2) were measured at regular intervals for 7 hours. the PK/PD analyses, that included carbon dioxide kinetics, stood at the basis of the utility function: probability of antinociception minus probability of respiratory depression. Oxycodone had rapid onset/offset times (30-40 minutes) with potency values (effect-site concentration causing 50% of effect) ranging from 0.05 to 0.13 ng/mL for respiratory variables obtained at hypercapnia and antinociceptive responses. Ventilation at an extrapolated end-tidal carbon dioxide partial pressure of 55 mmHg, was used for creation of 3 utility functions, one for each of the nociceptive tests. Contrary to expectation, the utility functions were close to zero or positive over the clinical oxycodone concentration range. The similar or better likelihood for antinociception relative to respiratory depression may be related to oxycodone's receptor activation profile or to is high likeability that possibly alters the modulation of nociceptive input. Oxycodone differs from other μ-opioids, such as fentanyl, that have a consistent negative utility.

Current and Future Therapeutic Options in Pain Management: Multi-mechanistic Opioids Involving Both MOR and NOP Receptor Activation

Opioids are widely used in chronic pain management, despite major concerns about their risk of adverse events, particularly abuse, misuse, and respiratory depression from overdose. Multi-mechanistic opioids, such as tapentadol and buprenorphine, have been widely studied as a valid alternative to traditional opioids for their safer profile. Special interest was focused on the role of the nociceptin opioid peptide (NOP) receptor in terms of analgesia and improved tolerability. Nociceptin opioid peptide receptor agonists were shown to reinforce the antinociceptive effect of mu opioid receptor (MOR) agonists and modulate some of their adverse effects. Therefore, multi-mechanistic opioids involving both MOR and NOP receptor activation became a major field of pharmaceutical and clinical investigations. Buprenorphine was re-discovered in a new perspective, as an atypical analgesic and as a substitution therapy for opioid use disorders; and buprenorphine derivatives have been tested in animal models of nociceptive and neuropathic pain. Similarly, cebranopadol, a full MOR/NOP receptor agonist, has been clinically evaluated for its potent analgesic efficacy and better tolerability profile, compared with traditional opioids. This review overviews pharmacological mechanisms of the NOP receptor system, including its role in pain management and in the development of opioid tolerance. Clinical data on buprenorphine suggest its role as a safer alternative to traditional opioids, particularly in patients with non-cancer pain; while data on cebranopadol still require phase III study results to approve its introduction on the market. Other bifunctional MOR/NOP receptor ligands, such as BU08028, BU10038, and AT-121, are currently under pharmacological investigations and could represent promising analgesic agents for the future.

Advances in Reversal Strategies of Opioid-induced Respiratory Toxicity

Opioids may produce life-threatening respiratory depression and death from their actions at the opioid receptors within the brainstem respiratory neuronal network. Since there is an increasing number of conditions where the administration of the opioid receptor antagonist naloxone is inadequate or undesired, there is an increased interest in the development of novel reversal and prevention strategies aimed at providing efficacy close to that of the opioid receptor antagonist naloxone but with fewer of its drawbacks such as its short duration of action and lesser ability to reverse high-affinity opioids, such as carfentanil, or drug combinations. To give an overview of this highly relevant topic, the authors systematically discuss predominantly experimental pharmacotherapies, published in the last 5 yr, aimed at reversal of opioid-induced respiratory depression as alternatives to naloxone. The respiratory stimulants are discussed based on their characteristics and mechanism of action: nonopioid controlled substances (e.g., amphetamine, cannabinoids, ketamine), hormones (thyrotropin releasing hormone, oxytocin), nicotinic acetylcholine receptor agonists, ampakines, serotonin receptor agonists, antioxidants, miscellaneous peptides, potassium channel blockers acting at the carotid bodies (doxapram, ENA001), sequestration techniques (scrubber molecules, immunopharmacotherapy), and opioids (partial agonists/antagonists). The authors argue that none of these often still experimental therapies are sufficiently tested with respect to efficacy and safety, and many of the agents presented have a lesser efficacy at deeper levels of respiratory depression, i.e., inability to overcome apnea, or have ample side effects. The authors suggest development of reversal strategies that combine respiratory stimulants with naloxone. Furthermore, they encourage collaborations between research groups to expedite development of viable reversal strategies of potent synthetic opioid-induced respiratory depression.

Non-Opioid Peptides Targeting Opioid Effects

Opioids are the most potent widely used analgesics, primarily, but not exclusively, in palliative care. However, they are associated with numerous side effects, such as tolerance, addiction, respiratory depression, and cardiovascular events. This, in turn, can result in their overuse in cases of addiction, the need for dose escalation in cases of developing tolerance, and the emergence of dose-related opioid toxicity, resulting in respiratory depression or cardiovascular problems that can even lead to unintentional death. Therefore, a very important challenge for researchers is to look for ways to counteract the side effects of opioids. The use of peptides and their related compounds, which have been shown to modulate the effects of opioids, may provide such an opportunity. This short review is a compendium of knowledge about the most important and recent findings regarding selected peptides and their modulatory effects on various opioid actions, including cardiovascular and respiratory responses. In addition to the peptides more commonly reported in the literature in the context of their pro- and/or anti-opioid activity-such as neuropeptide FF (NPFF), cholecystokinin (CCK), and melanocyte inhibiting factor (MIF)-we also included in the review nociceptin/orphanin (N/OFQ), ghrelin, oxytocin, endothelin, and venom peptides.

Development of Multifunctional and Orally Active Cyclic Peptide Agonists of Opioid/Neuropeptide FF Receptors that Produce Potent, Long-Lasting, and Peripherally Restricted Antinociception with Diminished Side Effects

We previously reported that a multifunctional opioid/neuropeptide FF receptor agonist, DN-9, achieved peripherally restricted analgesia with reduced side effects. To develop stable and orally bioavailable analogues of DN-9, eight lactam-bridged cyclic analogues of DN-9 between positions 2 and 5 were designed, synthesized, and biologically evaluated. In vitro cAMP assays revealed that these analogues, except 7, were multifunctional ligands that activated opioid and neuropeptide FF receptors. Analogue 1 exhibited improved potency for κ-opioid and NPFF₂ receptors. All analogues exhibited potent, long-lasting, and peripherally restricted antinociception in the tail-flick test without tolerance development after subcutaneous administration and produced oral analgesia. Oral administration of the optimized compound analogue 1 exhibited powerful, peripherally restricted antinociceptive effects in mouse models of acute, inflammatory, and neuropathic pain. Remarkably, orally administered analogue 1 had no significant side effects, such as tolerance, dependence, constipation, or respiratory depression, at effective analgesic doses.

Functional Profile of Systemic and Intrathecal Cebranopadol in Nonhuman Primates

BACKGROUND

Cebranopadol, a mixed nociceptin/opioid receptor full agonist, can effectively relieve pain in rodents and humans. However, it is unclear to what degree different opioid receptor subtypes contribute to its antinociception and whether cebranopadol lacks acute opioid-associated side effects in primates. The authors hypothesized that coactivation of nociceptin receptors and μ receptors produces analgesia with reduced side effects in nonhuman primates.

METHODS

The antinociceptive, reinforcing, respiratory-depressant, and pruritic effects of cebranopadol in adult rhesus monkeys (n = 22) were compared with μ receptor agonists fentanyl and morphine using assays, including acute thermal nociception, IV drug self-administration, telemetric measurement of respiratory function, and itch-scratching responses.

RESULTS

Subcutaneous cebranopadol (ED50, 2.9 [95% CI, 1.8 to 4.6] μg/kg) potently produced antinociception compared to fentanyl (15.8 [14.6 to 17.1] μg/kg). Pretreatment with antagonists selective for nociceptin and μ receptors, but not δ and κ receptor antagonists, caused rightward shifts of the antinociceptive dose-response curve of cebranopadol with dose ratios of 2 and 9, respectively. Cebranopadol produced reinforcing effects comparable to fentanyl, but with decreased reinforcing strength, i.e., cebranopadol (mean ± SD, 7 ± 3 injections) versus fentanyl (12 ± 3 injections) determined by a progressive-ratio schedule of reinforcement. Unlike fentanyl (8 ± 2 breaths/min), systemic cebranopadol at higher doses did not decrease the respiratory rate (17 ± 2 breaths/min). Intrathecal cebranopadol (1 μg) exerted full antinociception with minimal scratching responses (231 ± 137 scratches) in contrast to intrathecal morphine (30 μg; 3,009 ± 1,474 scratches).

CONCLUSIONS

In nonhuman primates, the μ receptor mainly contributed to cebranopadol-induced antinociception. Similar to nociceptin/μ receptor partial agonists, cebranopadol displayed reduced side effects, such as a lack of respiratory depression and pruritus. Although cebranopadol showed reduced reinforcing strength, its detectable reinforcing effects and strength warrant caution, which is critical for the development and clinical use of cebranopadol.

EDITOR’S PERSPECTIVE

Cebranopadol, a novel first-in-class drug candidate: Method validation and first exploratory pharmacokinetic study in rabbits

Cebranopadol is a novel, centrally acting, potent, first-in-class analgesic drug candidate with a unique mode of action that combines nociceptin/orphanin FQ peptide receptor and opioid peptide receptor agonism. The present study aimed to develop and validate a novel UHPLC-MS/MS method to quantify cebranopadol in rabbit plasma and to assess its pharmacokinetics in rabbits after subcutaneous (s.c.) administration. Twelve adult females were administered with 200 µg/kg s.c. injection. Blood samples were withdrawn at 15, 30 and 45 min and 1, 1.5, 2, 4, 6, 8, 10 and 24 hr after administration. The plasma samples were extracted with a liquid/liquid extraction. The new analytical method complied with the EMA requirements for the bioanalytical method validation. The method was selective, repeatable, accurate, precise and robust with a lower limit of quantification of 0.1 ng/ml. In all the rabbits, cebranopadol was quantifiable from 0.25 to 10 hr. Mean Cmax and Tmax were 871 ng/ml and 0.25 hr, respectively. Further studies including the i.v. administration are necessary to fully evaluate the pharmacokinetic features of this novel active compound.

Translational value of non-human primates in opioid research

Preclinical opioid research using animal models not only provides mechanistic insights into the modulation of opioid analgesia and its associated side effects, but also validates drug candidates for improved treatment options for opioid use disorder. Non-human primates (NHPs) have served as a surrogate species for humans in opioid research for more than five decades. The translational value of NHP models is supported by the documented species differences between rodents and primates regarding their behavioral and physiological responses to opioid-related ligands and that NHP studies have provided more concordant results with human studies. This review highlights the utilization of NHP models in five aspects of opioid research, i.e., analgesia, abuse liability, respiratory depression, physical dependence, and pruritus. Recent NHP studies have found that (1) mixed mu opioid and nociceptin/orphanin FQ peptide receptor partial agonists appear to be safe, non-addictive analgesics and (2) mu opioid receptor- and mixed opioid receptor subtype-based medications remain the only two classes of drugs that are effective in alleviating opioid-induced adverse effects. Given the recent advances in pharmaceutical sciences and discoveries of novel targets, NHP studies are posed to identify the translational gap and validate therapeutic targets for the treatment of opioid use disorder. Pharmacological studies using NHPs along with multiple outcome measures (e.g., behavior, physiologic function, and neuroimaging) will continue to facilitate the research and development of improved medications to curb the opioid epidemic.

Molecular Biology of Opioid Analgesia and Its Clinical Considerations

Understanding the molecular biology of opioid analgesia is essential for its proper implementation and mechanistic approach to its modulation in order to maximize analgesia and minimize undesired effects. By appreciating the molecular mechanisms intrinsic to opioid analgesia, one can manipulate a molecular target to augment or diminish a specific effect using adjuvant drugs, select an appropriate opioid for opioid rotation or define a molecular target for new opioid drug development. In this review, we present the cellular and molecular mechanisms of opioid analgesia and that of the associated phenomena of tolerance, dependence, and hyperalgesia. The specific mechanisms highlighted are those that presently can be clinically addressed.

Strategies for Developing κ Opioid Receptor Agonists for the Treatment of Pain with Fewer Side Effects

There is significant need to find effective, nonaddictive pain medications. κ Opioid receptor (KOPr) agonists have been studied for decades but have recently received increased attention because of their analgesic effects and lack of abuse potential. However, a range of side effects have limited the clinical development of these drugs. There are several strategies currently used to develop safer and more effective KOPr agonists. These strategies include identifying G-protein-biased agonists, developing peripherally restricted KOPr agonists without centrally mediated side effects, and developing mixed opioid agonists, which target multiple receptors at specific ratios to balance side-effect profiles and reduce tolerance. Here, we review the latest developments in research related to KOPr agonists for the treatment of pain. SIGNIFICANCE STATEMENT: This review discusses strategies for developing safer κ opioid receptor (KOPr) agonists with therapeutic potential for the treatment of pain. Although one strategy is to modify selective KOPr agonists to create peripherally restricted or G-protein-biased structures, another approach is to combine KOPr agonists with μ, δ, or nociceptin opioid receptor activation to obtain mixed opioid receptor agonists, therefore negating the adverse effects and retaining the therapeutic effect.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Analgesic and Respiratory Depressant Effects of R-dihydroetorphine

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

There is an ongoing need for potent opioids with less adverse effects than commonly used opioids. R-dihydroetorphine is a full opioid receptor agonist with relatively high affinity at the μ-, δ- and κ-opioid receptors and low affinity at the nociception/orphanin FQ receptor. The authors quantified its antinociceptive and respiratory effects in healthy volunteers. The authors hypothesized that given its receptor profile, R-dihydroetorphine will exhibit an apparent plateau in respiratory depression, but not in antinociception.

Methods

The authors performed a population pharmacokinetic–pharmacodynamic study (Eudract registration No. 2009-010880-17). Four intravenous R-dihydroetorphine doses were studied: 12.5, 75, 125, and 150 ng/kg (infused more than 10 min) in 4 of 4, 6 of 6, 6 of 6, and 4 of 4 male subjects in pain and respiratory studies, respectively. The authors measured isohypercapnic ventilation, pain threshold, and tolerance responses to electrical noxious stimulation and arterial blood samples for pharmacokinetic analysis.

Results

R-dihydroetorphine displayed a dose-dependent increase in peak plasma concentrations at the end of the infusion. Concentration-effect relationships differed significantly between endpoints. R-dihydroetorphine produced respiratory depression best described by a sigmoid EMAX-model. A 50% reduction in ventilation in between baseline and minimum ventilation was observed at an R-dihydroetorphine concentration of 17 ± 4 pg/ml (median ± standard error of the estimate). The maximum reduction in ventilation observed was at 33% of baseline. In contrast, over the dose range studied, R-dihydroetorphine produced dose-dependent analgesia best described by a linear model. A 50% increase in stimulus intensity was observed at 34 ± 11 pg/ml.

Conclusions

Over the dose range studied, R-dihydroetorphine exhibited a plateau in respiratory depression, but not in analgesia. Whether these experimental advantages extrapolate to the clinical setting and whether analgesia has no plateau at higher concentrations than investigated requires further studies.

Cebranopadol, a Novel First-in-Class Analgesic Drug Candidate: First Experience With Cancer-Related Pain for up to 26 Weeks

CONTEXT

Pain is one of the most prevalent symptoms associated with cancer. Strong opioids are commonly used in the analgesic management of the disease, but carry the risk of severe side effects. Cebranopadol is a first-in-class drug candidate, combining nociceptin/orphanin FQ peptide and opioid peptide receptor agonism. For cancer patients, frequently experiencing multimorbidities and often exposed to polypharmacy, cebranopadol is easy to handle given its once-daily dosing, the small tablet size that enables swallowing, and the option to flexibly titrate to an effective dose.

OBJECTIVES

We assessed the safety and tolerability of prolonged treatment with oral cebranopadol for up to 26 weeks in patients suffering from chronic moderate-to-severe cancer-related pain.

METHODS

This was a non-randomized, multi-site, open-label, single-arm clinical trial with patients who had completed a double-blind trial comparing morphine prolonged release with cebranopadol. In this extension trial, patients were treated with oral cebranopadol for up to 26 weeks.

RESULTS

Cebranopadol was safe and well tolerated in patients with chronic moderate-to-severe pain related to cancer in the dose range tested (200-1000 μg once daily). The median and mean pain levels remained in the range of mild pain during the treatment period.

CONCLUSION

Our data suggest that cebranopadol was safe and well tolerated when administered for up to 26 weeks in patients with chronic cancer-related pain who were previously treated with cebranopadol or morphine prolonged release.

Averting Opioid-induced Respiratory Depression without Affecting Analgesia

The ventilatory control system is highly vulnerable to exogenous administered opioid analgesics. Particularly respiratory depression is a potentially lethal complication that may occur when opioids are overdosed or consumed in combination with other depressants such as sleep medication or alcohol. Fatalities occur in acute and chronic pain patients on opioid therapy and individuals that abuse prescription or illicit opioids for their hedonistic pleasure. One important strategy to mitigate opioid-induced respiratory depression is cotreatment with nonopioid respiratory stimulants. Effective stimulants prevent respiratory depression without affecting the analgesic opioid response. Several pharmaceutical classes of nonopioid respiratory stimulants are currently under investigation. The majority acts at sites within the brainstem respiratory network including drugs that act at α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (ampakines), 5-hydroxytryptamine receptor agonists, phospodiesterase-4 inhibitors, D1-dopamine receptor agonists, the endogenous peptide glycyl-glutamine, and thyrotropin-releasing hormone. Others act peripherally at potassium channels expressed on oxygen-sensing cells of the carotid bodies, such as doxapram and GAL021 (Galleon Pharmaceuticals Corp., USA). In this review we critically appraise the efficacy of these agents. We conclude that none of the experimental drugs are adequate for therapeutic use in opioid-induced respiratory depression and all need further study of efficacy and toxicity. All discussed drugs, however, do highlight potential mechanisms of action and possible templates for further study and development.

BPR1M97, a dual mu opioid receptor/nociceptin-orphanin FQ peptide receptor agonist, produces potent antinociceptive effects with safer properties than morphine

There is unmet need to design an analgesic with fewer side effects for severe pain management. Although traditional opioids are the most effective painkillers, they are accompanied by severe adverse responses, such as respiratory depression, constipation symptoms, tolerance, withdrawal, and addiction. We indicated BPR1M97 as a dual mu opioid receptor (MOP)/nociceptin-orphanin FQ peptide (NOP) receptor full agonist and investigated the pharmacology of BPR1M97 in multiple animal models. In vitro studies on BPR1M97 were assessed using cyclic-adenosine monophosphate production, β-arrestin, internalization, and membrane potential assays. In vivo studies were characterized using the tail-flick, tail-clip, lung functional, heart functional, acetone drop, von Frey hair, charcoal meal, glass bead, locomotor activity, conditioned place preference (CPP) and naloxone precipitation tests. BPR1M97 elicited full agonist properties for all cell-based assays tested in MOP-expressing cells. However, it acted as a G protein-biased agonist for NOP. BPR1M97 initiated faster antinociceptive effects at 10 min after subcutaneous injection and elicited better analgesia in cancer-induced pain than morphine. Unlike morphine, BPR1M97 caused less respiratory, cardiovascular, and gastrointestinal dysfunction. In addition, BPR1M97 decreased global activity and induced less withdrawal jumping precipitated by naloxone. Thus, BPR1M97 could serve as a novel small molecule dual receptor agonist for antinociception with fewer side effects than morphine. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Agonist-selective NOP receptor phosphorylation correlates in vitro and in vivo and reveals differential post-activation signaling by chemically diverse agonists

Agonists of the nociceptin/orphanin FQ opioid peptide (NOP) receptor, a member of the opioid receptor family, are under active investigation as novel analgesics, but their modes of signaling are less well characterized than those of other members of the opioid receptor family. Therefore, we investigated whether different NOP receptor ligands showed differential signaling or functional selectivity at the NOP receptor. Using newly developed phosphosite-specific antibodies to the NOP receptor, we found that agonist-induced NOP receptor phosphorylation occurred primarily at four carboxyl-terminal serine (Ser) and threonine (Thr) residues, namely, Ser³⁴⁶, Ser³⁵¹, Thr³⁶², and Ser³⁶³, and proceeded with a temporal hierarchy, with Ser³⁴⁶ as the first site of phosphorylation. G protein-coupled receptor kinases 2 and 3 (GRK2/3) cooperated during agonist-induced phosphorylation, which, in turn, facilitated NOP receptor desensitization and internalization. A comparison of structurally distinct NOP receptor agonists revealed dissociation in functional efficacies between G protein-dependent signaling and receptor phosphorylation. Furthermore, in NOP-eGFP and NOP-eYFP mice, NOP receptor agonists induced multisite phosphorylation and internalization in a dose-dependent and agonist-selective manner that could be blocked by specific antagonists. Our study provides new tools to study ligand-activated NOP receptor signaling in vitro and in vivo. Differential agonist-selective NOP receptor phosphorylation by chemically diverse NOP receptor agonists suggests that differential signaling by NOP receptor agonists may play a role in NOP receptor ligand pharmacology.

NOP agonist action of cebranopadol counteracts its liability to promote physical dependence

Cebranopadol is a mixed NOP/opioid receptor agonist currently under development as innovative analgesic. In this study the liability of cebranopadol to produce opioid-type physical dependence has been evaluated in comparison with morphine in wild type mice and in mice knockout for the NOP receptor gene (NOP(-/-)). Mice were treated twice a day for 5 days with increasing doses of cebranopadol or morphine (cumulative doses 10.2 and 255 mg/kg, respectively) and the number of jumping in response to naloxone 10 mg/kg were measured after 2 h from the last injection. In wild type mice naloxone evoked a similar withdrawal jumping behavior in animal pretreated with morphine or cebranopadol. In NOP(-/-) mice morphine treatment produced the same signs of withdrawal as in NOP(+/+) animals, while cebranopadol treatment elicited a stronger withdrawal syndrome in NOP(-/-) than of NOP(+/+) mice. These results demonstrated that the activation of the NOP receptor reduces the liability of cebranopadol to produce opioid-like physical dependence. Thus, the simultaneous activation of NOP and opioid receptors can be an effective pharmacological strategy to counteract physical dependence to opioid drugs.

NOP-Targeted Nonpeptide Ligands

The development of nonpeptide systemically active small-molecule NOP-targeted ligands has contributed tremendously to validating the NOP receptor as a promising target for therapeutics. Although a NOP-targeted compound is not yet approved for clinical use, a few NOP ligands are in clinical trials for various indications. Both successful and failed human clinical trials with NOP ligands provide opportunities for rational development of new and improved NOP-targeted compounds. A few years after the discovery of the NOP receptor in 1994, and its de-orphanization upon discovery of the endogenous peptide nociceptin/orphanin FQ (N/OFQ) in 1995, there was a significant effort in the pharmaceutical industry to discover nonpeptide NOP ligands from hits obtained from high-throughput screening campaigns of compound libraries. Depending on the therapeutic indication to be pursued, NOP agonists and antagonists were discovered, and some were optimized as clinical candidates. Advances such as G protein-coupled receptor (GPCR) structure elucidation, functional selectivity in ligand-driven GPCR activation, and multi-targeted ligands provide new scope for the rational design of novel NOP ligands fine-tuned for successful clinical translation. This article reviews the field of nonpeptide NOP ligand drug design in the context of these exciting developments and highlights new optimized nonpeptide NOP ligands possessing interesting functional profiles, which are particularly attractive for several unmet clinical applications involving NOP receptor pharmacomodulation.

Assessment of the Abuse Potential of Cebranopadol in Nondependent Recreational Opioid Users: A Phase 1 Randomized Controlled Study

BACKGROUND

Cebranopadol is a nociceptin/orphanin FQ peptide/opioid receptor agonist with central antinociceptive activity. We hypothesize that this novel mechanism of action may lead to a lower risk of abuse compared with pure μ-opioid peptide receptor agonists.

METHODS

We conducted a single-dose, nested-randomized, double-blind crossover study in nondependent recreational opioid users to evaluate the abuse potential of single doses of cebranopadol relative to hydromorphone immediate release and placebo. The study consisted of a qualification phase and a 7-period treatment phase (cebranopadol 200, 400, and 800 μg; hydromorphone 8 and 16 mg; and 2 placebos). The primary end point was the peak effect of drug liking at this moment, measured by visual analog scale (VAS). Various secondary end points (eg, VAS rating for good drug effects, high, bad drug effects, take drug again, drug similarity, and pupillometry) were also investigated.

RESULTS

Forty-two subjects completed the study. Cebranopadol 200 and 400 μg did not differentiate from placebo on the abuse potential assessments and generated smaller responses than hydromorphone. Responses observed with cebranopadol 800 μg were similar to hydromorphone 8 mg and smaller than hydromorphone 16 mg. The maximum effect for VAS drug liking at this moment was delayed compared with hydromorphone (3 and 1.5 hours, respectively). Cebranopadol administration was safe; no serious adverse events or study discontinuation due to treatment-emergent adverse events occurred.

CONCLUSIONS

These results confirm our hypothesis that cebranopadol, a nociceptin/orphanin FQ peptide/opioid receptor agonist, has lower abuse potential than hydromorphone immediate release, a pure μ-opioid peptide agonist.

NOP-Related Mechanisms in Substance Use Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995 and has been widely studied since. The role of the N/OFQ system in drug abuse has attracted researchers' attention since its initial discovery. The first two scientific papers describing the effect of intracranial injection of N/OFQ appeared 20 years ago and reported efficacy of the peptide in attenuating alcohol intake, whereas heroin self-administration was insensitive. Since then more than 100 scientific articles investigating the role of the N/OFQ and N/OFQ receptor (NOP) system in drug abuse have been published. The present article provides an historical overview of the advances in the field with focus on three major elements. First, the most robust data supportive of the efficacy of NOP agonists in treating drug abuse come from studies in the field of alcohol research, followed by psychostimulant and opioid research. In contrast, activation of NOP appears to facilitate nicotine consumption. Second, emerging data challenge the assumption that activation of NOP is the most appropriate strategy to attenuate consumption of substances of abuse. This assumption is based first on the observation that animals carrying an overexpression of NOP system components are more prone to consume substances of abuse, whereas NOP knockout rats are less motivated to self-administer heroin, alcohol, and cocaine. Third, administration of NOP antagonists also reduces alcohol consumption. In addition, NOP blockade reduces nicotine self-administration. Hypothetical mechanisms explaining this apparent paradox are discussed. Finally, we focus on the possibility that co-activation of NOP and mu opioid (MOP) receptors is an alternative strategy, readily testable in the clinic, to reduce the consumption of psychostimulants, opiates, and, possibly, alcohol.

Effects of NOP-Related Ligands in Nonhuman Primates

The nociceptin/orphanin FQ peptide (NOP) receptor-related ligands have been demonstrated in preclinical studies for several therapeutic applications. This article highlights (1) how nonhuman primates (NHP) were used to facilitate the development and application of positron emission tomography tracers in humans; (2) effects of an endogenous NOP ligand, nociceptin/orphanin FQ, and its interaction with mu opioid peptide (MOP) receptor agonists; and (3) promising functional profiles of NOP-related agonists in NHP as analgesics and treatment for substance use disorders. NHP models offer the most phylogenetically appropriate evaluation of opioid and non-opioid receptor functions and drug effects. Based on preclinical and clinical data of ligands with mixed NOP/MOP receptor agonist activity, several factors including their intrinsic efficacies for activating NOP versus MOP receptors and different study endpoints in NHP could contribute to different pharmacological profiles. Ample evidence from NHP studies indicates that bifunctional NOP/MOP receptor agonists have opened an exciting avenue for developing safe, effective medications with fewer side effects for treating pain and drug addiction. In particular, bifunctional NOP/MOP partial agonists hold a great potential as (1) effective spinal analgesics without itch side effects; (2) safe, nonaddictive analgesics without opioid side effects such as respiratory depression; and (3) effective medications for substance use disorders.

Cebranopadol: A Novel First-in-Class Potent Analgesic Acting via NOP and Opioid Receptors

Cebranopadol is a novel first-in-class analgesic with highly potent agonistic activity at nociceptin/orphanin FQ peptide (NOP) and opioid receptors. It is highly potent and efficacious across a broad range of preclinical pain models. Its side effect profile is better compared to typical opioids. Mechanistic studies have shown that cebranopadol's activity at NOP receptors contributes to its anti-hyperalgesic effects while ameliorating some of its opioid-type side effects, including respiratory depression and abuse potential. Phase II of clinical development has been completed, demonstrating efficacy and good tolerability in acute and chronic pain conditions.This article focusses on reviewing data on the preclinical in vitro and in vivo pharmacology, safety, and tolerability, as well as clinical trials with cebranopadol.

Synergistic interaction between the agonism of cebranopadol at nociceptin/orphanin FQ and classical opioid receptors in the rat spinal nerve ligation model

Cebranopadol (trans-6'-fluoro-4',9'-dihydro-N,N-dimethyl-4-phenyl-spiro[cyclohexane-1,1'(3'H)-pyrano[3,4-b]indol]-4-amine) is a novel analgesic nociceptin/orphanin FQ opioid peptide (NOP) and classical opioid receptor (MOP, DOP, and KOP) agonist with highly efficacious and potent activity in a broad range of rodent models of nociceptive, inflammatory, and neuropathic pain as well as limited opioid-type side effects such as respiratory depression. This study was designed to explore contribution and interaction of NOP and classical opioid receptor agonist components to cebranopadol analgesia in the rat spinal nerve ligation (SNL) model. Assessing antihypersensitive activity in SNL rats intraperitoneal (IP) administration of cebranopadol resulted in ED 50 values of 3.3 and 3.58 μg/kg in two independent experiments. Pretreatment (IP) with J-113397 (4.64 mg/kg) a selective antagonist for the NOP receptor or naloxone (1 mg/kg), naltrindole (10 mg/kg), or nor-BNI (10 mg/kg), selective antagonists for MOP, DOP, and KOP receptors, yielded ED 50 values of 14.1, 16.9, 17.3, and 15 μg/kg, respectively. This 4-5 fold rightward shift of the dose-response curves suggested agonistic contribution of all four receptors to the analgesic activity of cebranopadol. Combined pretreatment with a mixture of the antagonists for the three classical opioid receptors resulted in an 18-fold potency shift with an ED 50 of 65.5 μg/kg. The concept of dose equivalence was used to calculate the expected additive effects of the parent compound for NOP and opioid receptor contribution and to compare them with the observed effects, respectively. This analysis revealed a statistically significant difference between the expected additive and the observed effects suggesting intrinsic synergistic analgesic interaction of the NOP and the classical opioid receptor components of cebranopadol. Together with the observation of limited respiratory depression in rats and humans the synergistic interaction of NOP and classical opioid receptor components in analgesia described in the current study may contribute to the favorable therapeutic index of cebranopadol observed in clinical trials.

Nociceptin/orphanin FQ receptor ligands and translational challenges: focus on cebranopadol as an innovative analgesic

Opioids are characterised as classical (mu, delta, and kappa) along with the non-classical nociceptin/orphanin FQ (N/OFQ) receptor or NOP. Targeting NOP has therapeutic indications in control of the cardiovascular and respiratory systems and micturition, and a profile as an antidepressant. For all of these indications, there are translational human data. Opioids such as morphine and fentanyl (activating the mu receptor) are the mainstay of pain treatment in the perioperative period, despite a challenging side-effect profile. Opioids in general have poor efficacy in neuropathic pain. Moreover, longer term use is associated with tolerance. There is good evidence interactions between opioid receptors, and receptor co-activation can reduce side-effects without compromising analgesia; this is particularly true for mu and NOP co-activation. Recent pharmaceutical development has produced a mixed opioid/NOP agonist, cebranopadol. This new chemical entity is effective in animal models of nociceptive and neuropathic pain with greater efficacy in the latter. In animal models, there is little evidence for respiratory depression, and tolerance (compared with morphine) only develops after long treatment periods. There is now early phase clinical development in diabetic neuropathy, cancer pain, and low back pain where cebranopadol displays significant efficacy. In 1996, N/OFQ was formally identified with an innovative analgesic profile. Approximately 20 yr later, cebranopadol as a clinical ligand is advancing through the human trials process.

Limited potential of cebranopadol to produce opioid-type physical dependence in rodents

Cebranopadol is a novel potent analgesic agonist at the nociceptin/orphanin FQ peptide (NOP) and classical opioid receptors. As NOP receptor activation has been shown to reduce side effects related to the activation of μ-opioid peptide (MOP) receptors, the present study evaluated opioid-type physical dependence produced by cebranopadol in mice and rats. In a naloxone-precipitated withdrawal assay in mice, a regimen of seven escalating doses of cebranopadol over 2 days produced only very limited physical dependence as evidenced by very little withdrawal symptoms (jumping) even at cebranopadol doses clearly exceeding the analgesic dose range. In contrast, mice showed clear withdrawal symptoms when treated with morphine within the analgesic dose range. In the rat, spontaneous withdrawal (by cessation of drug treatment; in terms of weight loss and behavioral score) was studied after 4-week subacute administration. Naloxone-precipitated withdrawal (in terms of weight loss and behavioral score) was studied in the same groups of rats after 1-week re-administration following the spontaneous withdrawal period. In both tests, cebranopadol-treated rats showed only few signs of withdrawal, while withdrawal effects in rats treated with morphine were clearly evident. These findings demonstrate a low potential of cebranopadol to produce opioid-type physical dependence in rodents. The prospect of this promising finding into the clinical setting remains to be established.

Nociceptin/orphanin FQ opioid peptide (NOP) receptor and µ-opioid peptide (MOP) receptors both contribute to the anti-hypersensitive effect of cebranopadol in a rat model of arthritic pain

Cebranopadol is a novel, first-in-class analgesic with agonist activity at the nociceptin/orphanin FQ opioid peptide (NOP) receptor as well as the classical opioid peptide receptors. This study investigated the anti-hypersensitive effect of cebranopadol in a rat model of arthritic pain. Selective antagonists were used to probe the involvement of the NOP receptor and the µ-opioid peptide (MOP) receptors. Experimental mono-arthritis was induced by intra-articular injection of complete Freund's adjuvant into the left hind knee joint. Intravenous (i.v.) administration of cebranopadol 0.8-8.0 µg/kg to rats 5 days after induction of arthritis elicited dose-dependent increases in weight bearing on the affected limb. The quarter-maximal effective dose (ED₂₅) for this anti-hypersensitive effect of cebranopadol was 1.6 µg/kg i.v. (95% confidence interval [CI]: 0.8, 1.6). The ED₂₅ increased to 3.2 µg/kg i.v. (95% CI: 2.4, 4.0) following pretreatment with the selective NOP receptor antagonist J-113397 and to 18.3 µg/kg i.v. (95% CI: 9.6, 146.0) following pretreatment with the MOP receptor antagonist naloxone (at intraperitoneal antagonist doses of 4.64 mg/kg and 1.0 mg/kg, respectively). The MOP receptor agonist morphine and the NOP receptor agonist Ro65-6570 also elicited increases in weight bearing on the affected limb. The anti-hypersensitive effect of morphine 2.15 mg/kg i.v. was inhibited by naloxone but not by J-113397. Conversely, the anti-hypersensitive effect of Ro65-6570 0.464 mg/kg i.v. was inhibited by J-113397 but not by naloxone. In conclusion, cebranopadol evoked potent anti-hypersensitive efficacy in a rat model of arthritic pain, and this involved agonist activity at both the NOP and MOP receptors.

Benefit versus Severe Side Effects of Opioid Analgesia

Background

Previous studies integrated opioid benefit and harm into one single function—the utility function—to determine the drug toxicity (respiratory depression) in light of its wanted effect (analgesia). This study further refined the concept of the utility function using the respiratory and analgesic effects of the opioid analgesic alfentanil as example.

Methods

Data from three previous studies in 48 healthy volunteers were combined and reanalyzed using a population pharmacokinetic–pharmacodynamic analysis to create utility probability functions. Four specific conditions were defined: probability of adequate analgesia without severe respiratory depression, probability of adequate analgesia with severe respiratory depression, probability of inadequate analgesia without severe respiratory depression, and probability of inadequate analgesia with severe respiratory depression.

Results

The four conditions were successfully identified with probabilities varying depending on the opioid effect-site concentration. The optimum analgesia probability without serious respiratory depression is reached at an alfentanil effect-site concentration of 68 ng/ml, and exceeds the probability of the most unwanted effect, inadequate analgesia with severe respiratory depression (odds ratio, 4.0). At higher effect-site concentrations the probability of analgesia is reduced and exceeded by the probability of serious respiratory depression.

Conclusions

The utility function was successfully further developed, allowing assessment of specific conditions in terms of wanted and unwanted effects. This approach can be used to compare the toxic effects of drugs relative to their intended effect and may be a useful tool in the development of new compounds to assess their advantage over existing drugs.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Opioid-type Respiratory Depressant Side Effects of Cebranopadol in Rats Are Limited by Its Nociceptin/Orphanin FQ Peptide Receptor Agonist Activity

Background

Cebranopadol is a first-in-class analgesic with agonist activity at classic opioid peptide receptors and the nociceptin/orphanin FQ peptide receptor. The authors compared the antinociceptive and respiratory depressant effects of cebranopadol and the classic opioid fentanyl and used selective antagonists to provide the first mechanistic evidence of the contributions of the nociceptin/orphanin FQ peptide and μ-opioid peptide receptors to cebranopadol’s respiratory side-effect profile.

Methods

Antinociception was assessed in male Sprague–Dawley rats using the low-intensity tail-flick model (n = 10 per group). Arterial blood gas tensions (Paco2 and Pao2) were measured over time in samples from unrestrained, conscious rats after intravenous administration of cebranopadol or fentanyl (n = 6 per group).

Results

The ED50 for peak antinociceptive effect in the tail-flick model was 7.4 μg/kg for cebranopadol (95% CI, 6.6 to 8.2 μg/kg) and 10.7 μg/kg for fentanyl citrate (9 to 12.7 μg/kg). Fentanyl citrate increased Paco2 levels to 45 mmHg (upper limit of normal range) at 17.6 μg/kg (95% CI, 7.6 to 40.8 μg/kg) and to greater than 50 mmHg at doses producing maximal antinociception. In contrast, with cebranopadol, Paco2 levels remained less than 35 mmHg up to doses producing maximal antinociception. The nociceptin/orphanin FQ peptide receptor antagonist J-113397 potentiated the respiratory depressant effects of cebranopadol; these changes in Paco2 and Pao2 were fully reversible with the μ-opioid peptide receptor antagonist naloxone.

Conclusions

The therapeutic window between antinociception and respiratory depression in rats is larger for cebranopadol than that for fentanyl because the nociceptin/orphanin FQ peptide receptor agonist action of cebranopadol counteracts side effects resulting from its μ-opioid peptide receptor agonist action.