Therapeutic Potential of Kappa Opioid Agonists

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

Conorphin-66 produces peripherally restricted antinociception via the kappa-opioid receptor with limited side effects

With the current unmet demand for effective pain relief, analgesics without major central adverse effects are highly appealing, such as peripherally restricted kappa-opioid receptor (KOR) agonists. In this study, Conorphin-66, an analog of the selective KOR peptide agonist Conorphin T, was pharmacologically characterized in a series of experiments, with CR845 serving as the reference compound. Firstly, in vitro functional assay indicated that Conorphin-66 selectively activates KOR and exhibits weak β-arrestin2 signaling bias (-1.54 versus -4.35 for CR845). Additionally, subcutaneous Conorphin-66 produced potent antinociception in mouse pain models with ED50 values ranged from 0.02 to 3.28 μmol/kg, including tail-flick test, post-operative pain, formalin pain, and acetic acid-induced visceral pain. Similarly, CR845 exert potent antinociception in mouse pain models ranged from 0.15 to 1.47 μmol/kg. Notably, antagonism studies revealed that the analgesic effects of Conorphin-66 were mainly mediated by the peripheral KOR. Furthermore, Conorphin-66 produced non-tolerance-forming antinociception over 8 days. Unlike CR845, subcutaneous Conorphin-66 did not promote the sedation, anxiogenic effects, depressive-like effects, but did exhibit diuretic activity. Further study showed that Conorphin-66 does not have apparent antipruritic effects in an acute itch model. Overall, Conorphin-66 emerges as a novel peripherally restricted KOR agonist that produced potent antinociception with reduced side effects.

Blocking potential metabolic sites on NAT to improve its safety profile while retaining the pharmacological profile

The number of opioid-related overdose deaths and individuals that have suffered from opioid use disorders have significantly increased over the last 30 years. FDA approved maintenance therapies to treat opioid use disorder may successfully curb drug craving and prevent relapse but harbor adverse effects that reduce patient compliance. This has created a need for new chemical entities with improved patient experience. Previously our group reported a novel lead compound, NAT, a mu-opioid receptor antagonist that potently antagonized the antinociception of morphine and showed significant blood-brain barrier permeability. However, NAT belongs to thiophene containing compounds which are known structural alerts for potential oxidative metabolism. To overcome this, 15 NAT derivatives with various substituents at the 5'-position of the thiophene ring were designed and their structure-activity relationships were studied. These derivatives were characterized for their binding affinity, selectivity, and functional activity at the mu opioid receptor and assessed for their ability to antagonize the antinociceptive effects of morphine in vivo. Compound 12 showed retention of the basic pharmacological attributes of NAT while improving the withdrawal effects that were experienced in opioid-dependent mice. Further studies will be conducted to fully characterize compound 12 to examine whether it would serve as a new lead for opioid use disorder treatment and management.

Biased signalling in analgesic research and development

Ligand bias offers a novel means to improve the therapeutic profile of drugs. With regard to G protein-coupled receptors involved in analgesia, it could be advantageous to develop such drugs if the analgesic effect is mediated by a different cellular signalling pathway than the adverse effects associated with the drug. Whilst this has been explored over a number of years for the μ receptor, it remains unclear whether this approach offers significant benefit for the treatment of pain. Nevertheless, the development of biased ligands at other G protein-coupled receptors in the CNS does offer some promise for the development of novel analgesic drugs in the future. Here we summarise and discuss the recent evidence to support this.

Discovery of an Ortho-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaine Derivative as a Selective and Potent Kappa Opioid Receptor Agonist with Subsided Sedative Effect

Research into kappa opioid receptor (KOR) agonists with attenuated central-nervous-system side effects is a critical focus for developing productive and safe analgesics. Herein, a series of ortho-substituted N-cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaines were designed, synthesized, and subjected to bioassays. Compound 7a exhibited high subtype selectivity and potent agonistic activity toward KOR (KOR, Ki = 3.9 nM, MOR/KOR = 270, DOR/KOR = 1075; [35S]GTPγS binding, EC50 = 3.4 nM). Additionally, this compound exhibited robust and persistent antinociceptive effects in rodent models with different animal strains (hot plate test, ED50 = 0.20-0.30 mg/kg, i.p.; abdominal constriction test, ED50 = 0.20-0.60 mg/kg, i.p.), with its KOR-mediated mechanism for antinociception firmly established. Notably, compound 7a, unlike conventional KOR agonists, displayed minimal sedation and aversion at the antinociceptive ED50 dose. This feature addresses a crucial limitation in existing KOR agonists, positioning compound 7a as a promising novel therapeutic agent.

Contingent administration of typical and biased kappa opioid agonists reduces cocaine and oxycodone choice in a drug vs. food choice procedure in male rhesus monkeys

RATIONALE

Combinations of mu and kappa-opioid receptor (KOR) agonists have been proposed as analgesic formulations with reduced abuse potential. The feasibility of this approach has been increased by the development of KOR agonists with biased signaling profiles that produce KOR-typical antinociception with fewer KOR-typical side effects.

OBJECTIVE

The present study determined if the biased KOR agonists, nalfurafine and triazole 1.1, could reduce choice for oxycodone in rhesus monkeys as effectively as the typical KOR agonist, salvinorin A.

METHODS

Adult male rhesus monkeys (N = 5) responded under a concurrent schedule of food delivery and intravenous cocaine injections (0.018 mg/kg/injection). Once trained, cocaine (0.018 mg/kg/injection) or oxycodone (0.0056 mg/kg/injection) was tested alone or in combination with contingent injections of salvinorin A (0.1-3.2 µg/kg/injection), nalfurafine (0.0032-0.1 µg/kg/injection), triazole 1.1 (3.2-100.0 µg/kg/injection), or vehicle. In each condition, the cocaine or oxycodone dose, as well as the food amount, was held constant across choice components, while the dose of the KOR agonist was increased across choice components.

RESULTS

Cocaine and oxycodone were chosen over food on more than 80% of trials when administered alone or contingently with vehicle. When KOR agonists were administered contingently with either cocaine or oxycodone, drug choice decreased in a dose-dependent manner. Salvinorin A and triazole 1.1 decreased drug-reinforcer choice without altering total trials completed (i.e., choice allocation shifted to food), while nalfurafine dose dependently decreased total trials completed.

CONCLUSIONS

These results demonstrate that salvinorin A and triazole 1.1, but not nalfurafine, selectively reduce cocaine and oxycodone self-administration independent of nonspecific effects on behavior, suggesting that G-protein bias does not appear to be a moderating factor in this outcome. Triazole 1.1 represents an important prototypical compound for developing novel KOR agonists as deterrents for prescription opioid abuse.

Psychedelic therapies reconsidered: compounds, clinical indications, and cautious optimism

The clinical investigation of psychedelic medicines has blossomed over the last 5 years. Data from a Phase 3 industry trial and a multicenter Phase 2 industry trial, in addition to multiple early phase investigator-initiated and industry trials, have now been published in peer-reviewed journals. This narrative review summarizes both the recent data and the current clinical trials that are being conducted with various classes of "psyche-manifesting" substances, which may prove beneficial in the treatment of a broad range of conditions. Methodological considerations, unique challenges, and next steps for research are discussed in keeping with the uniquely "experiential" nature of these therapies.

Regulators of G-Protein Signaling (RGS) in Sporadic and Colitis-Associated Colorectal Cancer

Colorectal cancer (CRC) is one of the most common neoplasms worldwide. Among the risk factors of CRC, inflammatory bowel disease (IBD) is one of the most important ones leading to the development of colitis-associated CRC (CAC). G-protein coupled receptors (GPCR) are transmembrane receptors that orchestrate a multitude of signaling cascades in response to external stimuli. Because of their functionality, they are promising targets in research on new strategies for CRC diagnostics and treatment. Recently, regulators of G-proteins (RGS) have been attracting attention in the field of oncology. Typically, they serve as negative regulators of GPCR responses to both physiological stimuli and medications. RGS activity can lead to both beneficial and harmful effects depending on the nature of the stimulus. However, the atypical RGS-AXIN uses its RGS domain to antagonize key signaling pathways in CRC development through the stabilization of the β-catenin destruction complex. Since AXIN does not limit the efficiency of medications, it seems to be an even more promising pharmacological target in CRC treatment. In this review, we discuss the current state of knowledge on RGS significance in sporadic CRC and CAC with particular emphasis on the regulation of GPCR involved in IBD-related inflammation comprising opioid, cannabinoid and serotonin receptors.

Historical perspectives and recent advances in small molecule ligands of selective/biased/multi-targeted μ/δ/κ opioid receptor (2019-2022)

The opioids have been used for more than a thousand years and are not only the most widely prescribed drugs for moderate to severe pain and acute pain, but also the preferred drugs. However, their non-analgesic effects, especially respiratory depression and potential addiction, are important factors that plague the safety of clinical use and are an urgent problem for pharmacological researchers to address. Current research on analgesic drugs has evolved into different directions: de-opioidization; application of pharmacogenomics to individualize the use of opioids; development of new opioids with less adverse effects. The development of new opioid drugs remains a hot research topic, and with the in-depth study of opioid receptors and intracellular signal transduction mechanisms, new research ideas have been provided for the development of new opioid analgesics with less side effects and stronger analgesic effects. The development of novel opioid drugs in turn includes selective opioid receptor ligands, biased opioid receptor ligands, and multi-target opioid receptor ligands and positive allosteric modulators (PAMs) or antagonists and the single compound as multi-targeted agnoists/antagonists for different receptors. PAMs strategies are also getting newer and are the current research hotspots, including the BMS series of compounds and others, which are extensive and beyond the scope of this review. This review mainly focuses on the selective/biased/multi-targeted MOR/DOR/KOR (mu opioid receptor/delta opioid receptor/kappa opioid receptor) small molecule ligands and involves some cryo-electron microscopy (cryoEM) and structure-based approaches as well as the single compound as multi-targeted agnoists/antagonists for different receptors from 2019 to 2022, including discovery history, activities in vitro and vivo, and clinical studies, in an attempt to provide ideas for the development of novel opioid analgesics with fewer side effects.

Kappa opioid agonists in the treatment of itch: just scratching the surface?

Chronic pruritus is a debilitating condition affecting 23-44 million Americans. Recently, kappa opioid agonists (KOAs) have emerged as a novel class of potent antipruritic agents. In 2021, the Food and Drug Administration approved difelikefalin (Korsuva) for the treatment of moderate-to-severe pruritus associated with chronic kidney disease in adults undergoing hemodialysis. Difelikefalin is a potent, peripherally restricted KOA that is intravenously available. Although promising, difelikefalin is currently available as an intravenous composition only, limiting the scope of use. Oral formulations of difelikefalin did not meet the primary endpoint criteria in recent phase 2 clinical trials; however, additional clinical studies are ongoing. The future for KOAs in the treatment of pruritus is encouraging. Orally active pathway-biased KOAs, such as triazole 1.1, may serve as viable alternatives with broader applications. Extended-release compositions, such as the TP-2021 ProNeura subdermal implant, may circumvent the pharmacokinetic issues associated with peptide-based KOAs. Lastly, dual-acting kappa opioid receptor agonist/mu opioid receptor antagonists are orally bioavailable and may be useful in the treatment of various forms of chronic itch. In this review, we summarize the results of KOAs in clinical and preclinical trials and discuss future directions of drug development.

Pharmacologic hyperreactivity of kappa opioid receptors in periaqueductal gray matter during alcohol withdrawal syndrome in rats

BACKGROUND

Periaqueductal gray matter (PAG) is a brain region rich in kappa-opioid receptors (KOR). KOR in PAG mediates behavioral responses related to pain integration, and panic response, among others. Its participation in the addiction phenomena has been poorly studied. Hence, this preliminary study explored the pharmacological effects of KOR stimulation/blockade in dorsal-PAG (D-PAG) during alcohol withdrawal on anxiety-type behaviors and alcohol intake/preference.

METHODS

Juvenile male Wistar rats were unexposed (A-naïve group) or exposed to alcohol for 5 weeks and then restricted (A-withdrawal group). Posteriorly, animals received intra D-PAG injections of vehicle (10% DMSO), salvinorin A (SAL-A; a selective KOR agonist), or 2-Methyl-N-((2'-(pyrrolidin-1-ylsulfonyl)biphenyl-4-yl)methyl)propan-1-amine (PF-04455242; a highly selective KOR-antagonist). Subsequently, the defensive burying behavior (DBB) and alcohol intake/preference paradigms were evaluated.

RESULTS

SAL-A markedly increased burying time, the height of bedding, and alcohol consumption/preference in A-withdrawal, while slightly increased the height of bedding in A-näive rats. PF-04455242 decreased both burying and immobility duration, whereas increases latency to burying, frequency of rearing, and the number of stretches attempts with no action on alcohol intake/preference in A-withdrawal rats.

CONCLUSIONS

In general, stimulation/blockade of KOR in A-withdrawal animals exert higher responses compared to A-naïve ones. SAL-A produced anxiety-like behaviors and increased alcohol consumption/preference, especially/solely in the alcohol-withdrawal condition, while PF-04455242 augmented exploration with no effects on alcohol intake/preference. Our findings suggest a possible pharmacologic hyperreactivity of the KOR in PAG during alcohol withdrawal.

Within subject, double blind, examination of opioid sensitivity in participant-reported, observed, physiologic, and analgesic outcomes

Background

Inter-individual differences in opioid sensitivity may underlie different opioid risk profiles but have often been researched in persons who have current or past opioid use disorder or physical dependence. This study examined how opioid sensitivity manifests across various assessments of opioid effects in a primarily opioid-naïve population.

Procedures

Data were harmonized from two within-subject, double-blind trials wherein healthy participants (N = 123) received placebo and 4 mg oral hydromorphone. Demographics, self-report ratings, observer ratings, physiological, and cold pressor measures were collected. Participants were categorized as being responsive or nonresponsive to the opioid dose tested and compared using mixed-models, Pearson product correlations, and paired t-tests.

Findings

Participants were 49.6% female, mean 33.0 (SD=9.3) years old, and 44.7% Black/African American and 41.5% White, with 89.4% reporting no prior exposure to opioids. Within-subject sensitivity to opioids varied depending on the measure. One in five participants did not respond subjectively to the 4 mg hydromorphone dose based on their "Drug Effects" rating. Persons who were responsive showed more evidence of drug-dependent effects than did persons who were not responsive on ratings of Bad Effects (p= .03), feeling High (p= .01), Nausea (p= .03), pupil diameter (p< 0.01), and on the circular lights task (p< 0.001).

Conclusions

This study provides initial evidence that the experience of opioids may be domain specific. Data suggest potentially clinically meaningful differences exist regarding opioid response patterns, evident following one dose among opioid inexperienced individuals.

The Utility of Peripherally Restricted Kappa-Opioid Receptor Agonists for Inhibiting Below-Level Pain After Spinal Cord Injury in Mice

Pain after spinal cord injury (SCI) can be difficult to treat. Drugs that target the opioid receptor (OR) outside the central nervous system (CNS) have gained increasing interest in pain control owing to their low risk of central side effects. Asimadoline and ICI-204448 are believed to be peripherally restricted KOR agonists withlimited access to the CNS. This study examined whether they can attenuate pain hypersensitivity in mice subjected to a contusive T10 SCI. Subcutaneous (s.c.) injection of asimadoline (5, 20 mg/kg) and ICI-204448 (1, 10 mg/kg) inhibited heat hypersensitivity at both doses, but only attenuated mechanical hypersensitivity at the high dose. However, the high-dose asimadoline adversely affected animals' exploratory performance in SCI mice and caused aversion, suggesting CNS drug penetration. In contrast, high-dose ICI-204448 did not impair exploration and remained effective in reducing both mechanical and heat hypersensitivities after SCI. Accordingly, we chose to examine the potential peripheral neuronal mechanism for ICI-204448-induced pain inhibition by conducting in vivo calcium imaging of dorsal root ganglion (DRG) in Pirt-GCaMP6s+/- mice. High-dose ICI-204448 (10 mg/kg, s.c.) attenuated the increased fluorescence intensity of lumbar DRG neurons activated by a noxious pinch (400 g) stimulation in SCI mice. In conclusion, systemic administration of ICI-204448 achieved SCI pain inhibition at doses that did not induce notable side effects and attenuated DRG neuronal excitability which may partly contribute to its pain inhibition. These findings suggest that peripherally restricted KOR agonists may be useful for treating SCI pain, but the therapeutic window must be carefully examined.

Spirocyclic Iodonium Ylides for Fluorine-18 Radiolabeling of Non-Activated Arenes: From Concept to Clinical Research

Positron emission tomography (PET) is a powerful imaging tool for drug discovery, clinical diagnosis, and monitoring of disease progression. Fluorine-18 is the most common radionuclide used for PET, but advances in radiotracer development have been limited by the historical lack of methodologies and precursors amenable to radiolabeling with fluorine-18. Radiolabeling of electron-rich (hetero)aromatic rings remains a long-standing challenge in the production of PET radiopharmaceuticals. In this personal account, we discuss the history of spirocyclic iodonium ylide precursors, from inception to applications in clinical research, for the incorporation of fluorine-18 into complex non-activated (hetero)aromatic rings.

Kappa opioid receptor agonists produce sexually dimorphic and prolactin-dependent hyperalgesic priming

ABSTRACT

Repeated stress produces hyperalgesic priming in preclinical models, but underlying mechanisms remain uncertain. As stress engages kappa opioid receptors (KORs), we hypothesized that repeated administration of KOR agonists might mimic, in part, stress-induced hyperalgesic priming. The potential contribution of circulating prolactin (PRL) and dysregulation of the expression of PRL receptor (PRLR) isoforms in sensory neurons after KOR agonist administration was also investigated. Mice received 3 daily doses of U-69593 or nalfurafine as a "first-hit" stimulus followed by assessment of periorbital tactile allodynia. Sixteen days after the first KOR agonist administration, animals received a subthreshold dose of inhalational umbellulone, a TRPA1 agonist, as the second-hit stimulus and periorbital allodynia was assessed. Cabergoline, a dopamine D2 receptor agonist, was used to inhibit circulating PRL in additional cohorts. Prolactin receptor isoforms were quantified in the V1 region of the trigeminal ganglion after repeated doses of U-69593. In both sexes, KOR agonists increased circulating PRL and produced allodynia that resolved within 14 days. Hyperalgesic priming, revealed by umbellulone-induced allodynia in animals previously treated with the KOR agonists, also occurred in both sexes. However, repeated U-69593 downregulated the PRLR long isoform in trigeminal neurons only in female mice. Umbellulone-induced allodynia was prevented by cabergoline co-treatment during priming with KOR agonists in female, but not male, mice. Hyperalgesic priming therefore occurs in both sexes after either biased or nonbiased KOR agonists. However, a PRL/PRLR-dependence is observed only in female nociceptors possibly contributing to pain in stress-related pain disorders in females.

Ligand and G-protein selectivity in the κ-opioid receptor

The κ-opioid receptor (KOR) represents a highly desirable therapeutic target for treating not only pain but also addiction and affective disorders1. However, the development of KOR analgesics has been hindered by the associated hallucinogenic side effects2. The initiation of KOR signalling requires the Gi/o-family proteins including the conventional (Gi1, Gi2, Gi3, GoA and GoB) and nonconventional (Gz and Gg) subtypes. How hallucinogens exert their actions through KOR and how KOR determines G-protein subtype selectivity are not well understood. Here we determined the active-state structures of KOR in a complex with multiple G-protein heterotrimers-Gi1, GoA, Gz and Gg-using cryo-electron microscopy. The KOR-G-protein complexes are bound to hallucinogenic salvinorins or highly selective KOR agonists. Comparisons of these structures reveal molecular determinants critical for KOR-G-protein interactions as well as key elements governing Gi/o-family subtype selectivity and KOR ligand selectivity. Furthermore, the four G-protein subtypes display an intrinsically different binding affinity and allosteric activity on agonist binding at KOR. These results provide insights into the actions of opioids and G-protein-coupling specificity at KOR and establish a foundation to examine the therapeutic potential of pathway-selective agonists of KOR.

Difelikefalin: A New κ-Opioid Receptor Agonist for the Treatment of Hemodialysis-Dependent Chronic Kidney Disease-Associated Pruritus

OBJECTIVE

To review data for difelikefalin (Korsuva) intravenous solution for management of moderate-to-severe pruritus in hemodialysis (HD) patients.

DATA SOURCES

Literature search of PubMed (January 1946-May 2022) and SCOPUS (January 1946-May 2022) was performed using the terms: Korsuva, CR845, and difelikefalin. Additional information sources include ClinicalTrials.gov, prescribing information, meeting posters, and references of identified articles.

STUDY SELECTION AND DATA EXTRACTION

Clinical trials and articles evaluating difelikefalin for chronic kidney disease-associated pruritis (CKD-aP) in HD patients.

DATA SYNTHESIS

Difelikefalin is a peripherally acting κ-opioid receptor agonist with antipruritic effects for HD patients with moderate-to-severe CKD-aP. A phase 3 study showed significant improvement of patient itch intensity and itch-related quality of life (QOL) when compared with placebo. More patients had decreased pruritus on the 24-hour Worst Itch Intensity Numerical Rating Scale with difelikefalin (49.1%) compared with placebo (27.9%, P < 0.001). A positive effect was seen with or without use of additional antipruritic agents. Common adverse events include diarrhea, dizziness, and vomiting; there were no signs of physical dependence or centrally acting opioid effects (euphoria, hallucinations).

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Difelikefalin reduced itch intensity and improved QOL for patients with CKD-aP. Whether the benefit is continued long-term as well as how it compares with other effective agents is currently unknown.

CONCLUSION

Difelikefalin is the only Food and Drug Administration-approved treatment for moderate-to-severe CKD-aP with additional research into its benefit in this and other types of pruritus ongoing.

Design of κ-Opioid Receptor Agonists for the Development of Potential Treatments of Pain with Reduced Side Effects

The κ-opioid receptor (KOR) has recently emerged as an alternative therapeutic target for the development of pain medications, without deleterious side effects associated with the μ-opioid receptor (MOR). However, modulation of KOR is currently under investigation for the treatment of depression, mood disorders, psychiatric comorbidity, and specific drug addictions. However, KOR agonists also trigger adverse effects including sedation, dysphoria, and hallucinations. In this respect, there is currently much debate on alternative paradigms. Recent effort has been devoted in search of biased ligands capable of selectively activating favorable signaling over signaling associated with unwanted side effects. On the other hand, the use of partial agonists is expected to allow the analgesia to be produced at dosages lower than those required to produce the adverse effects. More empirically, the unwanted central effects can be also avoided by using peripherally restricted agonists. In this review, we discuss the more recent trends in the design of KOR-selective, biased or partial, and finally, peripherally acting agonists. Special emphasis is given on the discussion of the most recent approaches for controlling functional selectivity of KOR-specific ligands.

Discovery of κ Opioid Receptor (KOR)-Selective d-Tetrapeptides with Improved In Vivo Antinociceptive Effect after Peripheral Administration

Peripherally active tetrapeptides as selective κ opioid receptor (KOR) agonists have been prepared in good overall yields and high purity following solid-phase peptide synthesis via Fmoc protection strategy. Structural modifications at the first and second position of the lead compound FF(d-Nle)R-NH2 (FE200041) were contemplated with aromatic side chains containing d-amino acids, such as (d)-pF-Phe, (d)-mF-Phe, (d)-oF-Phe, which led to highly selective and efficacious KOR agonists endowed with strong antinociceptive activity in vivo following intravenous (i.v.) and subcutaneous (s.c.) administration in the tail flick and formalin tests. These results suggest potential clinical applications in the treatment of neuropathic and inflammatory pain.

Exploring the neurobiology of the premonitory phase of migraine preclinically - a role for hypothalamic kappa opioid receptors?

Background

The migraine premonitory phase is characterized in part by increased thirst, urination and yawning. Imaging studies show that the hypothalamus is activated in the premonitory phase. Stress is a well know migraine initiation factor which was demonstrated to engage dynorphin/kappa opioid receptors (KOR) signaling in several brain regions, including the hypothalamus. This study proposes the exploration of the possible link between hypothalamic KOR and migraine premonitory symptoms in rodent models.

Methods

Rats were treated systemically with the KOR agonist U-69,593 followed by yawning and urination monitoring. Apomorphine, a dopamine D1/2 agonist, was used as a positive control for yawning behaviors. Urination and water consumption following systemic administration of U-69,593 was also assessed. To examine if KOR activation specifically in the hypothalamus can promote premonitory symptoms, AAV8-hSyn-DIO-hM4Di (Gi-DREADD)-mCherry viral vector was microinjected into the right arcuate nucleus (ARC) of female and male KOR^CRE or KOR^WT mice. Four weeks after the injection, clozapine N-oxide (CNO) was administered systemically followed by the assessment of urination, water consumption and tactile sensory response.

Results

Systemic administration of U-69,593 increased urination but did not produce yawning in rats. Systemic KOR agonist also increased urination in mice as well as water consumption. Cell specific Gi-DREADD activation (i.e., inhibition through Gi-coupled signaling) of KOR^CRE neurons in the ARC also increased water consumption and the total volume of urine in mice but did not affect tactile sensory responses.

Conclusion

Our studies in rodents identified the KOR in a hypothalamic region as a mechanism that promotes behaviors consistent with clinically-observed premonitory symptoms of migraine, including increased thirst and urination but not yawning. Importantly, these behaviors occurred in the absence of pain responses, consistent with the emergence of the premonitory phase before the headache phase. Early intervention for preventive treatment even before the headache phase may be achievable by targeting the hypothalamic KOR.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01497-7.

Designer Benzodiazepines' Activity on Opioid Receptors: A Docking Study

BACKGROUND

Previous studies have reported that benzodiazepines (BZDs) seem to enhance euphoric and reinforcing properties of opioids in opioid users so that a direct effect on opioid receptors has been postulated, together with a possible synergistic induction of severe side effects due to co use of BDZs and opioids. This is particularly worrisome given the appearance on the market of designer benzodiazepines (DBZDs), whose activity/toxicity profiles are scarcely known.

OBJECTIVES

This study aimed to evaluate, through computational studies, the binding affinity (or lack thereof) of 101 DBZDs identified online on the kappa, mu, and delta opioid receptors (K, M, DOR); and to assess whether their mechanism of action could include activation of the latter.

METHODS

MOE® was used for the computational studies. Pharmacophore mapping based on strong opioids agonist binders' 3D chemical features was used to filter the DBZDs. Resultant DBZDs were docked into the crystallised 3D active conformation of KOR (PDB6B73), DOR (PDB6PT3) and MOR (PDB5C1M). Co-crystallised ligands and four strong agonists were used as reference compounds. A score (S, Kcal/mol) representative of the predicted binding affinity, and a description of ligand interactions were obtained from MOE®.

RESULTS

The docking results, filtered for S < -8.0 and the interaction with the Asp residue, identified five DBZDs as putative binders of the three ORs : ciclotizolam, fluloprazolam, JQ1, Ro 48-6791, and Ro 48-8684.

CONCLUSION

It may be inferred that at least some DBZDs may have the potential to activate opioid receptors. This could mediate/increase their anxiolytic, analgesic, and addiction potentials, as well as worsen the side effects associated with opioid co-use.

The role of kappa opioid receptors in immune system - An overview

Opioids are one of the most effective anti-nociceptive agents used in patients with cancer pain or after serious surgery in most countries. The endogenous opioid system participates in pain perception, but recently its role in inflammation was determined. κ-opioid receptors (KOP receptors), a member of the opioid receptor family, are expressed in the central and peripheral nervous system as well as on the surface of different types of immune cells, e.g. T cells, B cells and monocytes. In this review, we focused on the involvement of KOP receptors in the inflammatory process and described their function in a number of conditions in which the immune system plays a key role (e.g. inflammatory bowel disease, arthritis, subarachnoid hemorrhage, vascular dysfunction) and inflammatory pain. We summed up the application of known KOP ligands in pathophysiology and we aimed to shed new light on KOP receptors as important elements during inflammation.

Is a Highly Selective and Potent κ Opioid Receptor (KOR) Agonist with an Unexpected Nonreduction in Locomotor Activity

Undue central nervous system (CNS) side effects including dysphoria and sedation remain to be a challenge for the development of κ opioid receptor (KOR) agonists as effective and safe analgesics. On the basis of our previous work on morphinan-based KOR agonists, a series of 7α-methyl-7β-substituted northebaine derivatives were designed, synthesized, and biologically assayed. Among others, compound 4a (SLL-627) has been identified as a highly selective and potent KOR agonist both in vitro and in vivo, and its molecular basis was also examined and discussed. Besides low liability to conditioned place aversion (CPA) test, treatment of SLL-627 was associated with a nonreduction in locomotor activity, compared to most of the other arylacetamide- or morphinan-based KOR agonists which generally exhibited apparently sedative effects. This unexpected finding provides new insights to dissociate analgesia from sedation for future discovery of innovative KOR agonists.

Traumatic Stress-Induced Vulnerability to Addiction: Critical Role of the Dynorphin/Kappa Opioid Receptor System

Substance use disorders (SUD) may emerge from an individual’s attempt to limit negative affective states and symptoms linked to stress. Indeed, SUD is highly comorbid with chronic stress, traumatic stress, or post-traumatic stress disorder (PTSD), and treatments approved for each pathology individually often failed to have a therapeutic efficiency in such comorbid patients. The kappa-opioid receptor (KOR) and its endogenous ligand dynorphin (DYN), seem to play a key role in the occurrence of this comorbidity. The DYN/KOR function is increased either in traumatic stress or during drug use, dependence acquisition and DYN is released during stress. The behavioural effects of stress related to the DYN/KOR system include anxiety, dissociative and depressive symptoms, as well as increased conditioned fear response. Furthermore, the DYN/KOR system is implicated in negative reinforcement after the euphoric effects of a drug of abuse ends. During chronic drug consumption DYN/KOR functions increase and facilitate tolerance and dependence. The drug-seeking behaviour induced by KOR activation can be retrieved either during the development of an addictive behaviour, or during relapse after withdrawal. DYN is known to be one of the most powerful negative modulators of dopamine signalling, notably in brain structures implicated in both reward and fear circuitries. KOR are also acting as inhibitory heteroreceptors on serotonin neurons. Moreover, the DYN/KOR system cross-regulate with corticotropin-releasing factor in the brain. The sexual dimorphism of the DYN/KOR system could be the cause of the gender differences observed in patients with SUD or/and traumatic stress-related pathologies. This review underlies experimental and clinical results emphasizing the DYN/KOR system as common mechanisms shared by SUD or/and traumatic stress-related pathologies, and suggests KOR antagonist as a new pharmacological strategy to treat this comorbidity.

Sex Differences in Kappa Opioid Receptor Agonist Mediated Attenuation of Chemotherapy-Induced Neuropathic Pain in Mice

Chemotherapy-induced neuropathic pain is a common side effect for cancer patients which has limited effective treatment options. Kappa opioid receptor (KOR) agonists are a promising alternative to currently available opioid drugs due to their low abuse potential. In the current study, we have investigated the effects of Salvinorin A (SalA) analogues, 16-Ethynyl SalA, 16-Bromo SalA and ethyoxymethyl ether (EOM) SalB, and in a preclinical model of paclitaxel-induced neuropathic pain in male and female C57BL/6J mice. Using an acute dose-response procedure, we showed that compared to morphine, 16-Ethynyl SalA was more potent at reducing mechanical allodynia; and SalA, 16-Ethynyl SalA, and EOM SalB were more potent at reducing cold allodynia. In the mechanical allodynia testing, U50,488 was more potent in males and SalA was more potent in females. There were no sex differences in the acute cold allodynia testing. In the chronic administration model, treatment with U50,488 (10 mg/kg) reduced the mechanical and cold allodynia responses to healthy levels over 23 days of treatment. Overall, we have shown that KOR agonists are effective in a model of chemotherapy-induced neuropathic pain, indicating that KOR agonists could be further developed to treat this debilitating condition.

Mechanistic Characterization of the Pharmacological Profile of HS-731, a Peripherally Acting Opioid Analgesic, at the µ-, δ-, κ-Opioid and Nociceptin Receptors

Accumulated preclinical and clinical data show that peripheral restricted opioids provide pain relief with reduced side effects. The peripherally acting opioid analgesic HS-731 is a potent dual μ-/δ-opioid receptor (MOR/DOR) full agonist, and a weak, partial agonist at the κ-opioid receptor (KOR). However, its binding mode at the opioid receptors remains elusive. Here, we present a comprehensive in silico evaluation of HS-731 binding at all opioid receptors. We provide insights into dynamic interaction patterns explaining the different binding and activity of HS-731 on the opioid receptors. For this purpose, we conducted docking, performed molecular dynamics (MD) simulations and generated dynamic pharmacophores (dynophores). Our results highlight two residues important for HS-731 recognition at the classical opioid receptors (MOR, DOR and KOR), particular the conserved residue 5.39 (K) and the non-conserved residue 6.58 (MOR: K, DOR: W and KOR: E). Furthermore, we assume a salt bridge between the transmembrane helices (TM) 5 and 6 via K2275.39 and E2976.58 to be responsible for the partial agonism of HS-731 at the KOR. Additionally, we experimentally demonstrated the absence of affinity of HS-731 to the nociceptin/orphanin FQ peptide (NOP) receptor. We consider the morphinan phenol Y1303.33 responsible for this affinity lack. Y1303.33 points deep into the NOP receptor binding pocket preventing HS-731 binding to the orthosteric binding pocket. These findings provide significant structural insights into HS-731 interaction pattern with the opioid receptors that are important for understanding the pharmacology of this peripheral opioid analgesic.

Pleiotropic Effects of Kappa Opioid Receptor-Related Ligands in Non-human Primates

The kappa opioid receptor (KOR)-related ligands have been demonstrated in preclinical studies for several therapeutic potentials. This chapter highlights (1) how non-human primates (NHP) studies facilitate the research and development of ligands targeting the KOR, (2) effects of the endogenous opioid peptide, dynorphin A-(1-17), and its analogs in NHP, and (3) pleiotropic effects and therapeutic applications of KOR-related ligands. In particular, synthetic ligands targeting the KOR have been extensively studied in NHP in three therapeutic areas, i.e., the treatment for itch, pain, and substance use disorders. As the KORs are widely expressed in the peripheral and central nervous systems, pleiotropic effects of KOR-related ligands, such as discriminative stimulus effects, neuroendocrine effects (e.g., prolactin release and stimulation of hypothalamic-pituitary-adrenal axis), and diuresis, in NHP are discussed. Centrally acting KOR agonists are known to produce adverse effects including dysphoria, hallucination, and sedation. Nonetheless, with strategic advances in medicinal chemistry, three classes of KOR-related agonists, i.e., peripherally restricted KOR agonists, mixed KOR/mu opioid receptor partial agonists, and G protein-biased KOR agonists, warrant additional NHP studies to improve our understanding of their functional efficacy, selectivity, and tolerability. Pharmacological studies in NHP which carry high translational significance will facilitate future development of KOR-based medications.

MedXercise: a promising strategy to promote remyelination

Multiple sclerosis is an inflammatory and demyelinating disease of the central nervous system. While remyelination facilitates functional recovery in animal models, it is limited in people with multiple sclerosis. Thus, multiple strategies have been put forth to promote remyelination, including exercise and medication. Exercise promotes the release of growth factors and induces protein-level changes, while remyelinating medications act through a variety of mechanisms to promote oligodendrocyte maturation within the lesion. In animal models, the combination of medication and exercise (Medication + eXercise = MedXercise) has an additive effect on remyelination and other pathological features of multiple sclerosis. In this review, we highlight the existing literature on the effects of exercise and medication on remyelination both independently and in combination.

Postoperative pain: a review of emerging therapeutic options

INTRODUCTION

Postoperative pain is often managed by opioid medications, even though they carry a risk of adverse effects such as vomiting, constipation, sedation, respiratory depression and physical dependence. Furthermore, opioid use in the healthcare setting has likely contributed to the epidemic. However, the mismanagement of postoperative pain can result in delayed recovery time, impaired physical function, increased risk of morbidity and mortality, chronic pain, and higher healthcare costs.

AREAS COVERED

This review explores emerging therapeutic options and strategies in the management of acute postoperative pain and focuses on opioid-sparing, multimodal analgesia. This includes regional anesthetic techniques, non opioid pharmacotherapy, novel opioids and non-pharmacologic therapy. We have also discussed examples of novel analgesics and formulations which have potential benefits in reducing postoperative pain and opioid use after surgery.

EXPERT OPINION

The development of novel regional anesthesia techniques allows for opioid minimization in increasing number of surgical procedures. This synergizes with the availability of novel non-opioid analgesic adjucts. In addition, several novel opioid drugs have been developed which may be pathway selective and associated with less adverse effect than conventional opioids.

κ‑opioid receptor agonist, U50488H, inhibits pyroptosis through NLRP3 via the Ca2+/CaMKII/CREB signaling pathway and improves synaptic plasticity in APP/PS1 mice

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder with slow onset in most cases. Clinically, dementia associated with AD is characterized by memory disorders, aphasia, executive dysfunction and personality and behavior changes. Currently, treatment strategies attempt to reduce certain symptoms, however there is no cure for AD. The aim of the present study was to identify a novel treatment strategy for AD. Thus, the protective effects of a κ‑opioid receptor (KOR) agonist, U50488H on neural damage in AD mice were investigated. The underlying mechanism of the Ca2+/calcium/calmodulin‑dependent protein kinase II/cyclic adenosine monophosphate‑response element binding protein (Ca2+/CaMKII/CREB) signaling pathway was evaluated. Amyloid precursor protein (APP)/presenilin‑1 (PS1) mice were treated subcutaneously with a KOR agonist for 28 days. The learning and memory abilities of the APP/PS1 mice were evaluated using the Morris water maze test. Damage to hippocampal neurons was assessed using hematoxylin and eosin staining. Inflammatory factors and brain injury markers were detected using ELISA. Neurons were examined using immunofluorescence and dendritic spines were observed using Golgi‑Cox staining. Western blotting was used to detect NOD‑, LRR‑ and pyrin domain‑containing protein 3, microglial ptosis and the Ca2+/CaMKII/CREB‑related protein pathway. The KOR agonist significantly improved the brain injury observed in APP/PS1 mice, inhibited microglia pyroptosis and improved the synaptic plasticity of APP/PS1 mice, which was reversed by a KOR antagonist. Thus, the KOR agonist improved the symptoms of APP/PS1 mice by inhibiting the Ca2+/CaMKII/CREB signaling pathway.

"I'll Be Back": The Resurrection of Dezocine

Beginning with opium itself, natural and synthetic opioids have been used as analgesics for over 8000 years and were likely abused as drugs of recreation for that long as well. However, the "opioid crisis" resulted in attempts to avoid or limit opioid analgesics in favor of other therapies and methods. Mu opioid agonists can be effective analgesics but suffer from addiction, tolerance, and dangerous, sometimes fatal, side effects. One exception to this generalization is dezocine (Dalgan), a mixed mu/kappa opioid partial agonist. Dezocine is at least as effective as morphine in reducing acute pain in animal models and clinical applications such as postoperative pain. And while dezocine was discontinued in western markets in 2011, it has become the favored opioid analgesic in China, capturing over 40% of the market. Additionally, dezocine possesses norepinephrine uptake inhibitory activity, which may synergize with mu agonism in the case of acute pain treatment and possibly endow the drug with antinociceptive activity in neuropathic pain conditions. This Innovations article summarizes the history and properties of dezocine and presents evidence and rationale for why dezocine has undergone a resurrection.

Effect of Dezocine on the Ratio of Th1/Th2 Cytokines in Patients Receiving Postoperative Analgesia Following Laparoscopic Radical Gastrectomy: A Prospective Randomised Study

Purpose

To evaluate the effect of dezocine on the postoperative ratio of Th1/Th2 cytokines in patients undergoing laparoscopic radical gastrectomy.

Patients and Methods

Sixty patients undergoing laparoscopic radical gastrectomy were randomly divided into two groups (n=30): dezocine group (Group D) and sufentanil group (Group S). They received patient-controlled intravenous analgesia (PCIA) after the operation with either dezocine 0.8 mg/kg (Group D) or sufentanil 2 µg/kg (Group S). Both groups also received ondansetron 8 mg diluted to 100 mL with saline. The primary outcome was the Th1/Th2 cytokines ratio at predetermined intervals, 30 min before the induction of general anaesthesia and 0, 12, 24 and 48 h after surgery. The secondary endpoints were patients’ pain scores, measured on a visual analogue scale (VAS) at predetermined intervals (0, 12, 24 and 48 h after surgery), and side effects at follow-up 48 h after surgery.

Results

The Th1/Th2 cytokines ratio in Group D was significantly higher than Group S (P<0.05) 12, 24 and 48 h after the operation. There were no significant differences in VAS pain scores between groups at 0, 12, 24 and 48 h after surgery (P>0.05). Compared to Group S, the incidence of postoperative nausea, vomiting and lethargy was significantly lower in Group D (P<0.05).

Conclusion

Dezocine increases the ratio of Th1/Th2 cytokines, relieves postoperative pain and causes fewer side effects in patients undergoing laparoscopic radical gastrectomy.

Addressing opioid tolerance and opioid-induced hypersensitivity: Recent developments and future therapeutic strategies

Opioids are a commonly prescribed and efficacious medication for the treatment of chronic pain but major side effects such as addiction, respiratory depression, analgesic tolerance, and paradoxical pain hypersensitivity make them inadequate and unsafe for patients requiring long-term pain management. This review summarizes recent advances in our understanding of the outcomes of chronic opioid administration to lay the foundation for the development of novel pharmacological strategies that attenuate opioid tolerance and hypersensitivity; the two main physiological mechanisms underlying the inadequacies of current therapeutic strategies. We also explore mechanistic similarities between the development of neuropathic pain states, opioid tolerance, and hypersensitivity which may explain opioids' lack of efficacy in certain patients. The findings challenge the current direction of analgesic research in developing non-opioid alternatives and we suggest that improving opioids, rather than replacing them, will be a fruitful avenue for future research.

Topical administration of the kappa opioid receptor agonist nalfurafine suppresses corneal neovascularization and inflammation

Corneal neovascularization (CNV) causes higher-order aberrations, corneal edema, ocular inflammation, and corneal transplant rejection, thereby decreasing visual acuity. In this study, we investigated the effects of topical administration of the kappa opioid receptor agonist nalfurafine (TRK-820) on CNV. To induce CNV, intrastromal corneal sutures were placed on the corneal stroma of BALB/c mice for 2 weeks. Nalfurafine (0.1 µg/2 μL/eye) was topically administered to the cornea once or twice daily after CNV induction. The CNV score, immune cell infiltration, and mRNA levels of angiogenic and pro-inflammatory factors in neovascularized corneas were evaluated using slit-lamp microscopy, immunohistochemistry, flow cytometry, and polymerase chain reaction. The mRNA expression of the kappa opioid receptor gene Oprk1 was significantly upregulated following CNV induction. Topical administration of nalfurafine twice daily significantly suppressed CNV and lymphangiogenesis, as well as reduced the mRNA levels of angiogenic and pro-inflammatory factors in the neovascularized corneas. Moreover, nalfurafine administration twice daily reduced the numbers of infiltrating leukocytes, neutrophils, macrophages, and interferon-γ-producing CD4⁺ T cells in the neovascularized corneas. In this study, we demonstrated that topical administration of nalfurafine suppressed local CNV in a mouse model along with the activation of KOR, suggesting that nalfurafine may prevent and control CNV in humans.

Recent Advances in the Treatment of Opioid Use Disorder

PURPOSE OF REVIEW

Opioid use disorder (OUD) remains a national epidemic with an immense consequence to the United States' healthcare system. Current therapeutic options are limited by adverse effects and limited efficacy.

RECENT FINDINGS

Recent advances in therapeutic options for OUD have shown promise in the fight against this ongoing health crisis. Modifications to approved medication-assisted treatment (MAT) include office-based methadone maintenance, implantable and monthly injectable buprenorphine, and an extended-release injectable naltrexone. Therapies under investigation include various strategies such as heroin vaccines, gene-targeted therapy, and biased agonism at the G protein-coupled receptor (GPCR), but several pharmacologic, clinical, and practical barriers limit these treatments' market viability. This manuscript provides a comprehensive review of the current literature regarding recent innovations in OUD treatment.

Diversity oriented Deep Reinforcement Learning for targeted molecule generation

In this work, we explore the potential of deep learning to streamline the process of identifying new potential drugs through the computational generation of molecules with interesting biological properties. Two deep neural networks compose our targeted generation framework: the Generator, which is trained to learn the building rules of valid molecules employing SMILES strings notation, and the Predictor which evaluates the newly generated compounds by predicting their affinity for the desired target. Then, the Generator is optimized through Reinforcement Learning to produce molecules with bespoken properties. The innovation of this approach is the exploratory strategy applied during the reinforcement training process that seeks to add novelty to the generated compounds. This training strategy employs two Generators interchangeably to sample new SMILES: the initially trained model that will remain fixed and a copy of the previous one that will be updated during the training to uncover the most promising molecules. The evolution of the reward assigned by the Predictor determines how often each one is employed to select the next token of the molecule. This strategy establishes a compromise between the need to acquire more information about the chemical space and the need to sample new molecules, with the experience gained so far. To demonstrate the effectiveness of the method, the Generator is trained to design molecules with an optimized coefficient of partition and also high inhibitory power against the Adenosine [Formula: see text] and [Formula: see text] opioid receptors. The results reveal that the model can effectively adjust the newly generated molecules towards the wanted direction. More importantly, it was possible to find promising sets of unique and diverse molecules, which was the main purpose of the newly implemented strategy.

The Role of the Kappa Opioid System in Comorbid Pain and Psychiatric Disorders: Function and Implications

Both pain and psychiatric disorders, such as anxiety and depression, significantly impact quality of life for the sufferer. The two also share a strong pathological link: chronic pain-induced negative affect drives vulnerability to psychiatric disorders, while patients with comorbid psychiatric disorders tend to experience exacerbated pain. However, the mechanisms responsible for the comorbidity of pain and psychiatric disorders remain unclear. It is well established that the kappa opioid system contributes to depressive and dysphoric states. Emerging studies of chronic pain have revealed the role and mechanisms of the kappa opioid system in pain processing and, in particular, in the associated pathological alteration of affection. Here, we discuss the key findings and summarize compounds acting on the kappa opioid system that are potential candidates for therapeutic strategies against comorbid pain and psychiatric disorders.

Evaluation of Antinociceptive Effects of Chitosan-Coated Liposomes Entrapping the Selective Kappa Opioid Receptor Agonist U50,488 in Mice

Background and Objectives: The selective kappa opioid receptor agonist U50,488 was reported to have analgesic, cough suppressant, diuretic and other beneficial properties. The aim of our study was to analyze the effects of some original chitosan-coated liposomes entrapping U50,488 in somatic and visceral nociceptive sensitivity in mice. Materials and Methods: The influence on the somatic pain was assessed using a tail flick test by counting the tail reactivity to thermal noxious stimulation. The nociceptive visceral estimation was performed using the writhing test in order to evaluate the behavioral manifestations occurring as a reaction to the chemical noxious peritoneal irritation with 0.6% acetic acid (10 mL/kbw). The animals were treated orally, at the same time, with a single dose of: distilled water 0.1 mL/10 gbw; 50 mg/kbw U50,488; 50 mg/kbw U50,488 entrapped in chitosan-coated liposomes, according to the group they were randomly assigned. Results: The use of chitosan-coated liposomesas carriers for U50,488 induced antinociceptive effects that began to manifest after 2 h, andwere prolonged but with a lower intensity than those caused by the free selective kappa opioid in both tests. Conclusion: In this experimental model, the oral administration of nanovesicles containing the selective kappa opioid agonist U50,488 determined a prolonged analgesic outcome in the tail flick test, as well as in the writhing test.

Oliceridine for the Management of Acute Postoperative Pain

OBJECTIVE

To review the pharmacological characteristics, clinical evidence, and place in the management of acute postoperative pain severe enough to require an intravenous opioid.

DATA SOURCES

A comprehensive literature search was conducted in PubMed (January 2000 to December 1, 2020). Key search terms included oliceridine or acute postoperative pain. Other sources were derived from product labeling and ClinicalTrials.gov.

STUDY SELECTION AND DATA EXTRACTION

All English-language articles identified from the data sources were reviewed and evaluated. Phase I, II, and III clinical trials were included.

DATA SYNTHESIS

Oliceridine is a novel selective µ-receptor G-protein pathway modulator. It has the property of activating G-protein signaling while causing low β-arrestin recruitment to the µ-receptor. Intravenous oliceridine showed statistically superior analgesia than placebo in patients with moderate or severe pain after surgery, with a favorable safety and tolerability profile regarding respiratory and gastrointestinal adverse effects, compared with morphine.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

The analgesic capacity of oliceridine is at least comparable to that of morphine at clinically relevant dosages, with a rapid onset of action. Also, it may be associated with a lower incidence of adverse events at dosing regimens associated with comparable analgesia. These data suggest that oliceridine may provide an important new treatment option for the management of moderate to severe postoperative pain where an intravenous opioid is warranted.

CONCLUSION

Oliceridine has obvious analgesic effects in patients with moderate or severe pain after surgery; additionally, it has a favorable safety and tolerability profile.

Kappa Opioid Receptor Ligands and Pharmacology: Diphenethylamines, a Class of Structurally Distinct, Selective Kappa Opioid Ligands

The kappa opioid receptor (KOR), a G protein-coupled receptor, and its endogenous ligands, the dynorphins, are prominent members of the opioid neuromodulatory system. The endogenous kappa opioid system is expressed in the central and peripheral nervous systems, and has a key role in modulating pain in central and peripheral neuronal circuits and a wide array of physiological functions and neuropsychiatric behaviors (e.g., stress, reward, emotion, motivation, cognition, epileptic seizures, itch, and diuresis). We review the latest advances in pharmacology of the KOR, chemical developments on KOR ligands with advances and challenges, and therapeutic and potential applications of KOR ligands. Diverse discovery strategies of KOR ligands targeting natural, naturally derived, and synthetic compounds with different scaffolds, as small molecules or peptides, with short or long-acting pharmacokinetics, and central or peripheral site of action, are discussed. These research efforts led to ligands with distinct pharmacological properties, as agonists, partial agonists, biased agonists, and antagonists. Differential modulation of KOR signaling represents a promising strategy for developing pharmacotherapies for several human diseases, either by activating (treatment of pain, pruritus, and epilepsy) or blocking (treatment of depression, anxiety, and addiction) the receptor. We focus on the recent chemical and pharmacological advances on diphenethylamines, a new class of structurally distinct, selective KOR ligands. Design strategies and investigations to define structure-activity relationships together with in vivo pharmacology of diphenethylamines as agonists, biased agonists, and antagonists and their potential use as therapeutics are discussed.

Advances in the Treatment of Chronic Pain by Targeting GPCRs

Pain is an essential protective mechanism that the body uses to alert or prevent further damage. Pain sensation is a complex event involving perception, transmission, processing, and response. Neurons at different levels (peripheral, spinal cord, and brain) are responsible for these pro- or antinociceptive activities to ensure an appropriate response to external stimuli. The terminals of these neurons, both in the peripheral endings and in the synapses, are equipped with G protein-coupled receptors (GPCRs), voltage- and ligand-gated ion channels that sense structurally diverse stimuli and inhibitors of neuronal activity. This review will focus on the largest class of sensory proteins, the GPCRs, as they are distributed throughout ascending and descending neurons and regulate activity at each step during pain transmission. GPCR activation also directly or indirectly controls the function of co-localized ion channels. The levels and types of some GPCRs are significantly altered in different pain models, especially chronic pain states, emphasizing that these molecules could be new targets for therapeutic intervention in chronic pain.

Biased Opioid Ligands

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.

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.

Substance use disorders and chronic itch

Chronic pruritus is one dermatologic manifestation of an underlying substance use disorder. Recent literature has uncovered similarities between the general neurologic mechanisms of addiction and chronic itch, largely involving activation of the dopaminergic reward circuits within the brain and imbalances between mu and kappa opioid receptor activation. It is likely that the use of specific drugs, like central nervous system stimulants and opioids, results in further activation and imbalances within these pathways, perpetuating both addiction and pruritus simultaneously. Opioid users often present to dermatology clinics with a generalized pruritus, whereas individuals using central nervous system stimulants like cocaine and methylenedioxymethamphetamine (MDMA), as well as legally prescribed drugs like treatments for attention deficit hyperactivity disorder, frequently complain of crawling, delusional infestation-like sensations underneath the skin. Because of these overlapping mechanisms and similar clinical presentations to many other chronically itchy conditions, it is necessary for dermatologists to consider and investigate an underlying substance use disorder to effectively treat these patients.

Quantification of observable behaviors induced by typical and atypical kappa-opioid receptor agonists in male rhesus monkeys

RATIONALE

Kappa-opioid receptor (KOR) agonists are antinociceptive but have side effects that limit their therapeutic utility. New KOR agonists have been developed that are fully efficacious at the KOR but may produce fewer or reduced side effects that are typical of KOR agonists.

OBJECTIVES

We determined behavioral profiles for typical and atypical KOR agonists purported to differ in intracellular-signaling profiles as well as a mu-opioid receptor (MOR) agonist, oxycodone, using a behavioral scoring system based on Novak et al. (Am J Primatol 28:124-138, 1992, Am J Primatol 46:213-227, 1998) and modified to quantify drug-induced effects (e.g., Duke et al. J Pharmacol Exp Ther 366:145-157, 2018).

METHODS

Six adult male rhesus monkeys were administered a range of doses of the typical KOR agonists, U50-488H (0.0032-0.1 mg/kg) and salvinorin A (0.00032-0.01 mg/kg); the atypical KOR agonists, nalfurafine (0.0001-0.001 mg/kg) and triazole 1.1 (0.01-0.32 mg/kg); the MOR agonist, oxycodone (0.0032-0.32 mg/kg); and as controls, cocaine (0.032-0.32 mg/kg) and ketamine (0.32-10 mg/kg). For time-course determinations, the largest dose of each KOR agonist or MOR agonist was administered across timepoints (10-320 min). In mixture conditions, oxycodone (0.1 mg/kg) was followed by KOR-agonist administration.

RESULTS

Typical KOR agonists produced sedative-like and motor-impairing effects. Nalfurafine was similar to typical KOR agonists on most outcomes, and triazole 1.1 produced no effects on its own except for reducing scratch during time-course determinations. In the mixture, all KOR agonists reduced oxycodone-induced scratching, U50-488H and nalfurafine reduced species-typical activity, and U50-488H increased rest/sleep posture.

CONCLUSIONS

Atypical "biased" KOR agonists produce side-effect profiles that are relatively benign (triazole 1.1) or reduced (nalfurafine) compared to typical KOR agonists.

The Intriguing Effects of Substituents in the N-Phenethyl Moiety of Norhydromorphone: A Bifunctional Opioid from a Set of "Tail Wags Dog" Experiments

(−)-N-Phenethyl analogs of optically pure N-norhydromorphone were synthesized and pharmacologically evaluated in several in vitro assays (opioid receptor binding, stimulation of [³⁵S]GTPγS binding, forskolin-induced cAMP accumulation assay, and MOR-mediated β-arrestin recruitment assays). “Body” and “tail” interactions with opioid receptors (a subset of Portoghese’s message-address theory) were used for molecular modeling and simulations, where the “address” can be considered the “body” of the hydromorphone molecule and the “message” delivered by the substituent (tail) on the aromatic ring of the N-phenethyl moiety. One compound, N-p-chloro-phenethynorhydromorphone ((7aR,12bS)-3-(4-chlorophenethyl)-9-hydroxy-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 2i), was found to have nanomolar binding affinity at MOR and DOR. It was a potent partial agonist at MOR and a full potent agonist at DOR with a δ/μ potency ratio of 1.2 in the ([³⁵S]GTPγS) assay. Bifunctional opioids that interact with MOR and DOR, the latter as agonists or antagonists, have been reported to have fewer side-effects than MOR agonists. The p-chlorophenethyl compound 2i was evaluated for its effect on respiration in both mice and squirrel monkeys. Compound 2i did not depress respiration (using normal air) in mice or squirrel monkeys. However, under conditions of hypercapnia (using air mixed with 5% CO₂), respiration was depressed in squirrel monkeys.

Diuretic Activity of a Novel Peripherally-Restricted Orally-Active Kappa Opioid Receptor Agonist

Kappa-opioid agonists (KOAs) enhance cardiac performance, as well as reduce infarct size and prevent deleterious cardiac remodeling following myocardial infarction. Additionally, KOAs promote diuresis; however, there has been limited development of KOAs as a class due to the promotion of untoward central nervous system (CNS)-mediated side effects. Our laboratory has developed a peripherally-restricted, orally-active, KOA (JT09) for the treatment of pain and cardiovascular disease. Peripherally-restricted KOAs possess a limited side-effect profile and demonstrate potential in preventing heart failure. The aim of this study was to assess the diuretic activity of lead compound JT09 relative to vehicle control and Tolvaptan through single oral administration to adult male Sprague–Dawley rats. JT09-administered rats demonstrated significantly increased urine output relative to vehicle control. However, the effect persisted for 8 h, whereas Tolvaptan-administered rats demonstrated diuretic activity for 24 h. Relative to Tolvaptan, urine output was significantly reduced in JT09 administered animals at all-time points, suggesting that the overall diuretic effect of JT09 is less profound than Tolvaptan. Additionally, JT09-administered rats demonstrated alterations in clinical chemistry; reduced urine specific gravity; and increased urine pH relative to vehicle control. The following study establishes a preliminary diuretic profile for JT09.