The Kappa Opioid Receptor: A Promising Therapeutic Target for Multiple Pathologies

Neuronal and therapeutic perspectives on empathic pain: A rational insight

Empathy is the capacity to experience and understand the feelings of others, thereby playing a key role in a person's mental well-being essentially by promoting kindness and a sense of belongingness to the group. However, too much empathy may result in psychological problems such as empathic distress, compassion fatigue, and burnout, collectively termed empathic pain. Several brain regions are implicated in processing empathic pain perception. Neuroimaging investigations bring in the context of brain structures involved in this emotional exchange, pointing toward the anterior insula (AI) and anterior cingulate cortex (ACC), indicating an overlap between the neural representation of direct and simulative pain. To discern such overlaps, therapeutic techniques for managing empathic pain require understanding different brain regions and their respective neural networks. At the moment, empathic pain is being treated using various methods, including pharmacological treatments such as antidepressants and psychological treatments such as mindfulness or meditation. For instance, researchers have been exploring the modulatory effects of neurotransmitters like serotonin, norepinephrine, and oxytocin on individuals' responses to empathic pain experience. Importantly, this review focuses on the specific brain parts and their unique roles in neurobiological pathways associated with emphatic pain and how shared neural networks play into available treatment options, suggesting possible future health benefits. Such an understanding of empathy can lead to more efficient management of types of care, focusing on enhancing social connections and mental well-being.

Novel tertiary diarylethylamines as functionally selective agonists of the kappa opioid receptor

Novel kappa opioid receptor (KOR) agonists that preferentially activate G-protein signaling versus β-arrestin-2 recruitment are described. Starting from a literature-reported phenol-containing diphenethylamine KOR agonist, structure-activity relationship (SAR) studies revealed replacement of the phenol with various non-hydroxylated bicyclic heteroaromatics led to tertiary diarylethylamines which retained KOR agonist activity and improved metabolic stability in human liver microsomes. Further optimizations produced compound 39, a potent activator of G-protein signaling (GTPγS EC50 = 14 nM, 83 % Emax) that did not elicit a β-arrestin-2 recruitment functional response (Emax < 10 %). Compound 39 demonstrated moderate to high intrinsic clearance in human hepatocytes and low potential for Pgp-mediated efflux when evaluated in the MDR1-MDCK permeability assay. Compound 39 exhibited 60- and 810-fold selectivities versus the related mu (MOR) and delta (DOR) opioid receptors in recombinant radioligand binding (Ki) assays. These findings highlight compound 39 and related structures as potential leads toward safe and tolerable therapeutics that target central nervous system (CNS) disorders for which KOR agonism could provide benefit.

Opioid Analgesics: Rise and Fall of Ligand Biased Signaling and Future Perspectives in the Quest for the Holy Grail

Opioid analgesics have been used for more than 5000 years and remain the main pain medications prescribed today. Although morphine is considered the gold standard of pain relief, this selective µ-opioid receptor (MOP) agonist provides only moderate relief for many chronic pain conditions and produces a number of unwanted effects that can affect the patient's quality of life, prevent adherence to treatment or lead to addiction. In addition to the lack of progress in developing better analgesics, there have been no significant breakthroughs to date in combating the above-mentioned side effects. Fortunately, a better understanding of opioid pharmacology has given renewed hope for the development of better and safer pain medications. In this review, we describe how clinically approved opioids were initially characterized as biased ligands and what impact this approach might have on clinical practice. We also look at the preclinical and clinical development of biased MOP agonists, focusing on the history of oliceridine, the first specifically designed biased analgesic. In addition, we explore the discrepancies between ligands with low intrinsic efficacy and those with biased properties. Finally, we examine the rationale behind the development of biased ligands during the opioid crisis.

Targeting the kappa opioid receptor for analgesia and antitumour effects

Kappa opioid (KOP) receptor agonists have analgesic actions in a range of models, but central side-effects can limit their utility. In addition, non-analgesic actions of opioid receptors are receiving significant research interest. A recent article reports that high KOP receptor expression in glioma tissue correlates with improved survival and that KOP receptor agonism further promotes apoptosis of glioma cells through regulation of the p38 MAPK oncogenic pathway. This suggests that KOP receptor ligands could be developed as chemotherapeutic adjuncts in addition to their use as analgesics.

Xylazine as an emerging new psychoactive substance; focuses on both 5-HT7 and κ-opioid receptors' molecular interactions and isosteric replacement

Xylazine, traditionally used as a veterinary sedative, has recently emerged as a new psychoactive substance, being typically ingested in combination with fentanyl derivatives and hence raising significant public health concerns. Despite its increasing prevalence, little is known about its molecular interactions with human neuroreceptors, specifically the serotonin 7 (5-HT7R) and kappa-opioid (KOR) receptors, which play critical roles in mood regulation, consciousness and nociception. Hence, the binding affinity and molecular interactions of xylazine with both 5-HT7R and KOR through docking simulations and molecular dynamics calculations were investigated. These computational approaches revealed critical insights into receptor binding motifs and highlighted structural modifications that could enhance receptor affinity. The isosteric replacements within the xylazine structure to improve its binding efficacy were assessed, demonstrating that minimal structural modifications can potentiate its interaction with 5-HT7R and KOR. These findings may well advance our understanding of xylazine's mechanism of action, possibly contributing to identifying suitable treatment/management approaches in treating xylazine-related overdoses.

Kappa opioid receptor internalisation-induced p38 nuclear translocation suppresses glioma progression

BACKGROUND

Recent studies have implicated a role for perioperative medications in determining patient outcomes after surgery for malignant tumours, including relapse and metastasis.

METHODS

A combined approach spanned molecular, cellular, and organismal levels, including bioinformatics, immunohistochemical staining of clinical and animal samples, RNA sequencing of glioblastoma multiforme (GBM) cells with Ingenuity Pathway Analysis, lentiviral-mediated gene expression modulation, in vitro cell experiments, and in vivo orthotopic tumour transplantation.

RESULTS

We observed a significant correlation between increased kappa opioid receptor (KOP receptor) expression and better prognosis in patients with glioma. Exogenous KOP receptor overexpression in GBM cells in vitro induced cell cycle arrest, suppressed cell growth, and promoted apoptosis. Conversely, reducing KOP receptor expression in GBM cells reduced the proportion of cells in S and G2/M phases, accelerating cell growth. KOP receptor overexpression inhibited glioma cell growth and prolonged survival in mice in vivo, while KOP receptor knockdown had the opposite effect. Mechanistically, internalised KOP receptors were found to bind cytoplasmic p38, facilitating its nuclear translocation and phosphorylation, which influences downstream gene expression. The selective KOP receptor agonist TRK-820 triggered KOP receptor internalisation, activated the p38 pathway, and diminished glioma cell viability in vitro.

CONCLUSIONS

This combined molecular, cellular, and in vivo approach supports use of KOP receptor agonists as potential adjuvant therapeutics for glioma.

Opioid use and abuse in adolescents and young adults; dealing with science, laws and ethics: Charming the COBRAS

The subject of substance use disorders in the pediatric population remains a disturbing conundrum for clinicians, researchers and society in general. Many of our youth are at risk of being damaged and even killed by drug addictions that result from the collision of rapidly developing as well as vulnerable central nervous systems encountering the current global drug addiction crisis. A major motif of this chemical calamity is opioid use disorder in adolescents and young adults that was stimulated by the 19th century identification of such highly addictive drugs as morphine, heroin and a non-opiate, cocaine. This analysis focuses on the pervasive presence of opioid drugs such as heroin and fentanyl that has become a major tragedy in the 21st century arising from an overall substance use and misuse phenomenon rampant in global society. Themes covered in this article include the history of addictive drugs in humans, diagnostic terms in use, the role of neurobiology in drug addiction, and current psychopharmacologic approaches to opioid overdose as well as addiction. Our youth are continuously confronted by dangers of high-risk behaviors including death and injury from opioid use disorders due to their central nervous system neuroplasticity as well as the widespread availability of these harmful chemicals. Healthcare professionals should actively assist our youth who unknowingly and even innocently encounter this deadly menace in the 21st century.

Deletion of β-arrestin 2 in mice affects kappa opioid receptor-mediated behaviors depending on sex, ovariectomy status, and behavioral endpoints

We previously demonstrated that in a mouse line expressing a kappa opioid receptor (KOR) mutant with all the four phosphorylation sites mutated to alanines (K4A) the selective KOR agonist U50,488H (U50)-induced anti-scratching tolerance was attenuated in males and conditioned place aversion (CPA) was reduced in females, without affecting acute U50-induced anti-scratching effect and hypo-locomotion (Huang et al, 2022, Neuropharmacology). KOR phosphorylation deficiency in K4A mice would lead to little recruitment of β-arrestin2 (arrb2) and hence greatly reduced arrb2-mediated KOR regulation, downstream signaling and behaviors. Herein we examined effects of arrb2 deletion in mice on KOR-mediated behaviors in arrb2 knockout (arrb2(-/-)) mice vs wildtype (WT) mice. We found that arrb2 deletion enhanced anti-scratching effects produced by acute U50 in males, but not in females. Intriguingly, in ovariectomized (OVX) but not sham-operated females, arrb2 deletion increased U50-induced anti-scratching effect, similar to males. Furthermore, OVX enhanced U50-induced anti-scratching effects specifically in arrb2(-/-) females, but not in WT females. Thus, ovarian hormones-related modulations may obscure the phenotype associated with arrb2(-/-) to promote the KOR-mediated anti-scratching signaling in females, while OVX unmasked it. In contrast, arrb2 deletion did not affect U50-induced CPA and had no effects on anti-scratching tolerance to repeated U50 in either male or female mice. The findings in arrb2(-/-) mice revealed both similarities and differences compared to our previous results in K4A mice. Overall, the effects of arrb2 deletion on KOR-mediated behaviors depended on specific behavioral endpoints, sex, and OVX status.

Positive allosteric modulation of µ-opioid receptor - A new possible approach in the pain management?

The antinociceptive effect of the opioid drugs is achieved through activation of the µ-opioid receptor (MOP). The orthosteric and allosteric sites of opioid receptors may be modulated, orthosteric site by endogenous i.e.β-endorphin and exogenous opioids (morphine, oxycodone, fentanyl); whereas BMS-986121, BMS-986122, Comp5, MS1, Ignavine or even oxytocin act on the allosteric site of the MOP. Opioid therapy is associated with numerous side effects, such as: respiratory depression, sedation, constipation, and importantly, prolonged therapy can influence the development of tolerance, overdose, and addiction. Opioid tolerance is a result of MOP internalization and desensitization, preceded by MOP phosphorylation, performed by protein kinases such as: PKA, PKC, GRKs or CaMKII. In vitro and in vivo data suggest that positive allosteric modulators may enhance antinociception triggered by orthosteric ligands and reduce side effects, which would allow the dose of opioids to be reduced and thus provide a more effective therapy. In this review, we present that positive modulation of the allosteric sites of MOP may constitute a new strategy for pain therapy.

Effects of Biased Analogues of the Kappa Opioid Receptor Agonist, U50,488, in Preclinical Models of Pain and Side Effects

Kappa opioid receptor (KOR) agonists have well-established antinociceptive effects. However, many KOR agonists have negative side effects, which limit their therapeutic potential. Some researchers have suggested that the development of biased agonists that preferentially stimulate KOR G-protein pathways over β-arrestin pathways may yield drugs with fewer adverse side effects. This was investigated in the current study. We describe the synthesis and characterization of three U50,488 analogues, 1, 2, and 3. We evaluated the acute and chronic antinociceptive effects of these compounds in mice using the warm-water tail flick assay and in a paclitaxel-induced neuropathic pain model. Side effects were investigated using open-field, passive wire hang, rotarod, elevated zero maze, conditioned place aversion, and whole-body plethysmography, with some tests being conducted in KOR or β-arrestin2 knock out mice. All compounds were highly potent, full agonists of the KOR, with varying signaling biases in vitro. In the warm-water tail withdrawal assay, these agonists were ~10 times more potent than U50,488, but not more efficacious. All KOR agonists reversed paclitaxel-induced neuropathic pain, without tolerance. Compound 3 showed no significant side effects on any test. Signaling bias did not correlate with the antinociceptive or side effects of any compounds and knockout of β-arrestin2 had no effect on U50,488-induced sedation or motor incoordination. These findings highlight the therapeutic potential of 3, with its lack of side effects typically associated with KOR agonists, and also suggest that G-protein signaling bias is a poor predictor of KOR agonist-induced side effects.

Opioidergic tuning of social attachment: reciprocal relationship between social deprivation and opioid abuse

Individuals misusing opioids often report heightened feelings of loneliness and decreased ability to maintain social connections. This disruption in social functioning further promotes addiction, creating a cycle in which increasing isolation drives drug use. Social factors also appear to impact susceptibility and progression of opioid dependence. In particular, increasing evidence suggests that poor early social bond formation and social environments may increase the risk of opioid abuse later in life. The brain opioid theory of social attachment suggests that endogenous opioids are key to forming and sustaining social bonds. Growing literature describes the opioid system as a powerful modulator of social separation distress and attachment formation in rodents and primates. In this framework, disruptions in opioidergic signaling due to opioid abuse may mediate social reward processing and behavior. While changes in endogenous opioid peptides and receptors have been reported in these early-life adversity models, the underlying mechanisms remain poorly understood. This review addresses the apparent bidirectional causal relationship between social deprivation and opioid addiction susceptibility, investigating the role of opioid transmission in attachment bond formation and prosocial behavior. We propose that early social deprivation disrupts the neurobiological substrates associated with opioid transmission, leading to deficits in social attachment and reinforcing addictive behaviors. By examining the literature, we discuss potential overlapping neural pathways between social isolation and opioid addiction, focusing on major reward-aversion substrates known to respond to opioids.

A Comprehensive Analysis of Fibromyalgia and the Role of the Endogenous Opioid System

Fibromyalgia represents a chronic pain disorder characterized by musculoskeletal pain, fatigue, and cognitive impairments. The exact mechanisms underlying fibromyalgia remain undefined; as a result, diagnosis and treatment present considerable challenges. On the other hand, the endogenous opioid system is believed to regulate pain intensity and emotional responses; hence, it might be expected to play a key role in the enhanced sensitivity experienced by fibromyalgia patients. One explanation for the emergence of disrupted pain modulation in individuals with fibromyalgia is a significant reduction in opioid receptor activity or an imbalance in the levels of endogenous opioid peptides. Further research is essential to clarify the complex details of the mechanisms underlying this abnormality. This complexity arises from the notion that an improved understanding could contribute to the development of innovative therapeutic strategies aimed at targeting the endogenous opioid system in the context of fibromyalgia. Although progress is being made, a complete understanding of these complexities remains a significant challenge. This paradigm has the potential to revolutionize the complex management of fibromyalgia, although its implementation may experience challenges. The effectiveness of this approach depends on multiple factors, but the implications could be profound. Despite the challenges involved in this transformation, the potential for improving patient care is considerable, as this condition has long been inadequately treated.

Population pharmacokinetic modeling and exposure-response analysis of anrikefon: insights and implications in clinical analgesia

BACKGROUND

Anrikefon (HSK21542), a potent and selective peripheral kappa opioid receptor (KOR) agonist developed by Haisco, effectively blocks pain and itch signals.

AIM

To develop a population pharmacokinetic (PK) model for anrikefon and conduct exposure-response (E-R) analysis for safety and efficacy in postoperative pain patients.

METHOD

The Population PK analysis uses NONMEM software with data from six trials. E-R relationships were assessed using safety and efficacy data from three trials. Covariate screening and Bayesian post-hoc simulations identified relevant factors and compared exposure metrics. The fixed dosing regimen was evaluated by simulation. Safety and efficacy endpoints were evaluated using logistic regression and Emax models.

RESULTS

A three-compartment model with linear elimination accurately described anrikefon's PK, incorporating weight through allometric scaling. Significant covariates affecting clearance included creatinine clearance, total bilirubin, albumin, aspartate transaminase, and age. Fixed and weight-based dosing showed similar exposures. No apparent E-R trend was observed for safety endpoints. The Emax model indicated that most of subjects achieved over 90% of the maximum effect for SPID0-24 h at 1.0 μg/kg. Safety analysis confirmed this dose was well tolerated with no safety issues.

CONCLUSION

This study provides valuable insights into dose selection, PK variability, and safety and efficacy endpoints.

Discovery of 6α-Thiazolylcarboxamidonaltrexamine Derivative (NTZ) as a Potent and Central Nervous System Penetrant Opioid Receptor Modulator with Drug-like Properties for Potential Treatment of Opioid Use Disorder

The development of highly potent and selective μ opioid receptor (MOR) modulators with favorable drug-like properties has always been a focus in the opioid domain. Our previous efforts led to the discovery of a lead compound designated as NAT, a potent centrally acting MOR modulator. However, the fact that NAT precipitated considerable withdrawal effects at higher doses largely impaired its further development. In the light of the concept of activity cliff and CNS multiparameter optimization algorithm, a nitrogen atom was incorporated into the thiophene ring of NAT, aiming to preserve desirable pharmacological activities and CNS permeability while alleviating withdrawal symptoms. Among all 16 new analogs, compound 6 (NTZ) exhibited improved opioid receptor selectivity, enhanced in vivo antagonistic effect, and overall fewer withdrawal symptoms compared to NAT. Further assessment of several key drug-like properties suggested a favorable ADMET profile of NTZ. Taken together, NTZ shows promise as a potential lead to treat opioid use disorder.

Discovery of Potent Kappa Opioid Receptor Agonists Derived from Akuammicine

Akuammicine (1), an alkaloid isolated from Picralima nitida, is an agonist of the kappa opioid receptor (κOR). To establish structure-activity relationships (SARs) for this structurally unique κOR ligand, a collection of semisynthetic derivatives was synthesized. Evaluating these derivatives for their ability to activate the κOR and mu opioid receptor (μOR) revealed key SAR trends and identified derivatives with enhanced κOR potency. Most notably, substitutions to the C10 position of the aryl ring led to a > 200-fold improvement in κOR potency and nearly complete selectivity for the κOR. A selection of the most potent ligands was shown to possess differing abilities recruitment of β-Arrestin-2 to the κOR, indicating they have distinct signaling properties from each other and existing κOR ligands. The discovery of these κOR agonists underscores the potential of using natural products to identify new classes of potent and selective ligands and provides new tools to probe the κOR.

Emerging Psychotropic Drug for the Treatment of Trigeminal Pain: Salvinorin A

Trigeminal neuralgia (TN) is chronic pain caused by damage to the somatosensorial system on the trigeminal nerve or its branches, which involves peripheral and central dysfunction pain pathways. Trigeminal pain triggers disruptive pain in regions of the face, including within and around the mouth. Besides clinical experiences, translating the language of suffering into scientific terminology presents substantial challenges. Due to the complex and multifactorial pathophysiology underlying trigeminal pain, elucidating its social impact presents significant difficulties. Carbamazepine and oxcarbazepine are first-line treatments for TN, achieving approximately 50% pain reduction in 60-70% of treated patients. However, their efficacy is often limited by common side effects, such as dizziness, vertigo, nausea, seizures, and cognitive symptoms. In some cases, patients experience severe side effects, including myelosuppression, hyponatremia, hormonal imbalances, liver toxicity, suicidal ideation, teratogenicity, and other adverse reactions. Given these clinical limitations, the search for new painkiller candidates continues. Hence, we focused this review on salvinorin A (SalA), a natural agonist of κ-opioid receptors (KORs), which demonstrated anti-nociceptive, anti-inflammatory, and anti-neuropathic properties in various experimental models of the spinal sensory system. Furthermore, preclinical evidence indicates that SalA does not induce dependence and demonstrates a favorable toxicological and safety profile in comparison with currently marketed opioid drugs. We propose Salvinorin A as a promising candidate for treating trigeminal neuralgia, offering the potential for reduced adverse effects.

The μ-opioid receptor differentiates two distinct human nociceptive populations relevant to clinical pain

The shortfall in new analgesic agents is a major impediment to reducing reliance on opioid medications for control of severe pain. In both animals and man, attenuating nociceptive transmission from primary afferent neurons with a μ-opioid receptor agonist yields highly effective analgesia. Consequently, deeper molecular characterization of human nociceptive afferents expressing OPRM1, the μ-opioid receptor gene, is a key component for advancing analgesic drug discovery and understanding clinical pain control. A co-expression matrix for the μ-opioid receptor and a variety of nociceptive channels as well as δ- and κ-opioid receptors is established by multiplex in situ hybridization. Our results indicate an OPRM1-positive population with strong molecular resemblance to rodent peptidergic C-nociceptors associated with tissue damage pain and an OPRM1-negative population sharing molecular characteristics of murine non-peptidergic C-nociceptors. The empirical identification of two distinct human nociceptive populations that differ profoundly in their presumed responsiveness to opioids provides an actionable translational framework for human pain control.

Identification of c[D-Trp-Phe-β-Ala-β-Ala], the First κ-Opioid Receptor-Specific Negative Allosteric Modulator

Recently, the fungus secondary metabolite cyclotetrapetide c[Trp-Phe-D-Pro-Phe] (CJ-15,208) and its derivatives deserved some attention for their unusual structure and distinctive in vitro and in vivo activity. These tryptophan-containing noncationic opioid peptides can be truly regarded as versatile picklocks capable of activating all opioid receptors. Intriguingly, minimal modification of the potent κ-opioid receptor (KOR) agonist c[D-Trp-Phe-Gly-β-Ala] (3) yielded c[D-Trp-Phe-β-Ala-β-Ala] (11), the first KOR-specific negative allosteric modulator (NAM) reported to-date. KOR exerts control over numerous functions in the central nervous system, including pain, depression, stress, mood, and reward. Hence, this KOR-selective NAM looks promising for modulating the KOR in addiction and neuropsychiatric disorders.

Synthesis and evaluation of 3,4,5-trisubstituted triazoles as G protein-biased kappa opioid receptor agonists

Kappa opioid receptor (KOR) agonists represent promising therapeutics for pain relief due to their analgesic properties along with lower abuse potential than opioids that act at the mu opioid receptor. However, typical KOR agonists produce sedation and dysphoria. Previous studies have shown that G protein signaling-biased KOR agonists may present a means to untangle the desired analgesic properties from undesired side effects. In this paper, we report a new series of G protein signaling-biased KOR agonists entailing -S- → -CH2- replacement in a previously reported KOR agonist, triazole 1.1. With an optimized carbon linker in hand, further development of the scaffold was undertaken to investigate the appendages of the triazole core. The structure-activity relationship study of this series is described, including several analogues that display enhanced potency while maintaining G protein-signaling bias compared to triazole 1.1.

Toxicological Evaluation of Kaempferol and Linearolactone as Treatments for Amoebic Liver Abscess Development in Mesocricetus auratus

Several studies with kaempferol (KP) and linearolactone (LL) have demonstrated their antiparasitic activity. However, the toxicity of these treatments is unknown. Therefore, this study aimed to evaluate the possible toxicological effects of intraperitoneal (i.p.) administration of KP or LL on the amoebic liver abscess model (ALA) in Mesocricetus auratus. An ALA was induced in male hamsters with 1.5 × 105Entamoeba histolytica (E. histolytica) trophozoites inoculated in the left hepatic lobe. The lesion evolved for 4 days, and then KP (5 mg/kg body weight/day) or LL (10 mg/kg body weight/day) was administered for 4 consecutive days. Then, magnetic resonance imaging (MRI), paraclinical analyses, and necropsy for histopathological evaluation were performed. There was similar ALA inhibition by KP (19.42%), LL (28.16%), and metronidazole, the antiamoebic control (20.87%) (p ≤ 0.05, analysis of variance [ANOVA]). There were hepatic and renal biochemical alterations in all treatment groups, mainly for KP (aspartate aminotransferase: 347.5 ± 37.5 U/L; blood urea nitrogen: 19.4 ± 1.9 g/dL; p ≤ 0.05, ANOVA). Lesions found in the organs were directly linked to the pathology. In conclusion, KP and LL decreased ALA development and exerted fewer toxicological effects compared with metronidazole. Therefore, both compounds exhibit therapeutic potential as an alternative treatment of amoebiasis caused by E. histolytica. However, additional clinical studies in different contexts are required to reaffirm this assertion.

Preclinical and clinical efficacy of kappa opioid receptor antagonists for depression: A systematic review

BACKGROUND

Approximately 30 % of persons with Major Depressive Disorder (MDD) inadequately respond to conventional antidepressants. Kappa opioid receptor (KOR) antagonists, aticaprant and navacaprant, are in development as treatments for MDD. Herein, we aim to comprehensively evaluate the safety, efficacy and pharmacology of aticaprant and navacaprant for MDD.

METHODS

We performed a systematic review of primary research investigating aticaprant and navacaprant on PubMed, OVID, and Scopus databases from inception to April 2024. Studies that reported on the pharmacological profile and/or safety and efficacy of aticaprant and navacaprant were included.

RESULTS

Navacaprant monotherapy and aticaprant adjunctive therapy are in development for MDD. Navacaprant exhibits 300-fold selectivity for the KOR compared to the mu-opioid receptor, while aticaprant exhibits 30-fold selectivity. At clinically-relevant doses, navacaprant and aticaprant occupy 87-95 % and 73-94 % of KORs, respectively. Clinical trials of the foregoing agents (navacaprant as monotherapy and actiprant as adjunctive therapy) reported significant improvement in depressive symptoms and may clinically benefit measures of anhedonia. Both agents appear well-tolerated, with most adverse events mild and no known safety concerns.

LIMITATIONS

Aticaprant and navacaprant treatment for MDD are in early stages of clinical trials and results from Phase 3 pivotal trials are not yet available.

CONCLUSIONS

Kappa opioid receptor antagonists may serve as mechanistically-novel treatments for MDD and persons who inadequately respond to index conventional antidepressants. Anhedonia is debilitating and insufficiently treated with conventional antidepressants. Future research vistas should establish the efficacy and safety of KORAs in phase 3 studies in both acute and maintenance paradigms.

Nalfurafine promotes myelination in vitro and facilitates recovery from cuprizone + rapamycin-induced demyelination in mice

The kappa opioid receptor has been identified as a promising therapeutic target for promoting remyelination. In the current study, we evaluated the ability of nalfurafine to promote oligodendrocyte progenitor cell (OPC) differentiation and myelination in vitro, and its efficacy in an extended, cuprizone-induced demyelination model. Primary mouse (C57BL/6J) OPC-containing cultures were treated with nalfurafine (0.6-200 nM), clemastine (0.01-100 μM), T3 (30 ng/mL), or vehicle for 5 days. Using immunocytochemistry and confocal microscopy, we found that nalfurafine treatment increased OPC differentiation, oligodendrocyte (OL) morphological complexity, and myelination of nanofibers in vitro. Adult male mice (C57BL/6J) were given a diet containing 0.2% cuprizone and administered rapamycin (10 mg/kg) once daily for 12 weeks followed by 6 weeks of treatment with nalfurafine (0.01 or 0.1 mg/kg), clemastine (10 mg/kg), or vehicle. We quantified the number of OLs using immunofluorescence, gross myelination using black gold staining, and myelin thickness using electron microscopy. Cuprizone + rapamycin treatment produced extensive demyelination and was accompanied by a loss of mature OLs, which was partially reversed by therapeutic administration of nalfurafine. We also assessed these mice for functional behavioral changes in open-field, horizontal bar, and mouse motor skill sequence tests (complex wheel running). Cuprizone + rapamycin treatment resulted in hyperlocomotion, poorer horizontal bar scores, and less distance traveled on the running wheels. Partial recovery was observed on both the horizontal bar and complex running wheel tests over time, which was facilitated by nalfurafine treatment. Taken together, these data highlight the potential of nalfurafine as a remyelination-promoting therapeutic.

Sex chromosomes and hormones independently influence healthy brain development but act similarly after cranial radiation

The course of normal development and response to pathology are strongly influenced by biological sex. For instance, female childhood cancer survivors who have undergone cranial radiation therapy (CRT) tend to display more pronounced cognitive deficits than their male counterparts. Sex effects can be the result of sex chromosome complement (XX vs. XY) and/or gonadal hormone influence. The contributions of each can be separated using the four-core genotype mouse model (FCG), where sex chromosome complement and gonadal sex are decoupled. While studies of FCG mice have evaluated brain differences in adulthood, it is still unclear how sex chromosome and sex hormone effects emerge through development in both healthy and pathological contexts. Our study utilizes longitudinal MRI with the FCG model to investigate sex effects in healthy development and after CRT in wildtype and immune-modified Ccl2-knockout mice. Our findings in normally developing mice reveal a relatively prominent chromosome effect prepubertally, compared to sex hormone effects which largely emerge later. Spatially, sex chromosome and hormone influences were independent of one another. After CRT in Ccl2-knockout mice, both male chromosomes and male hormones similarly improved brain outcomes but did so more separately than in combination. Our findings highlight the crucial role of sex chromosomes in early development and identify roles for sex chromosomes and hormones after CRT-induced inflammation, highlighting the influences of biological sex in both normal brain development and pathology.

Baicalein isolated from Oroxylum indicum acts as a potent µ- and κ-opioid receptor antagonist agent via the reversal of agonist-mediated cAMP inhibition

The opioid receptors play a pivotal role in the treatment of several neuropsychiatric and neurological disorders. Oroxylum indicum (L.) Kurtz is a very important medicinal plant with several therapeutic applications. It is a main constituent of the Ayurvedic formulation 'Dashmool' used for multifaceted disorders by the Indians. However, the constituents of this plant in neurological conditions have not been well studied. Here, we performed activity-guided isolation of compounds for opioid receptor modulator activity. In the study, we found that the isolated compound baicalein (3) has shown the most potent and competitive antagonistic activity at 20 mg/kg dose in vivo experiments. The acute dose of 3 (20 mg/kg) and pan opioid receptor antagonist naloxone (20 mg/kg) block the morphine-induced antinociception and the paw withdrawal latency decreases up to 8.3 s and 9.6 s, respectively. The in silico studies also support our in vitro data that compound 3 binds with MOR and KOR.

Discovery of Novel, Selective, and Nonbasic Agonists for the Kappa-Opioid Receptor Determined by Salvinorin A-Based Virtual Screening

Modulating the kappa-opioid receptor (KOR) is a promising strategy for treating various human diseases. KOR agonists show potential for treating pain, pruritus, and epilepsy, while KOR antagonists show potential for treating depression, anxiety, and addiction. The diterpenoid Salvinorin A (SalA), a secondary metabolite of Salvia divinorum, is a potent and selective KOR agonist. Unlike typical opioids, SalA lacks a basic nitrogen, which encouraged us to search for nonbasic KOR ligands. Through structure-based virtual screening using 3D pharmacophore models based on the binding mode of SalA, we identified novel, nonbasic, potent, and selective KOR agonists. In vitro studies confirmed two virtual hits, SalA-VS-07 and SalA-VS-08, as highly selective for the KOR and showing G protein-biased KOR agonist activity. Both KOR ligands share a novel spiro-moiety and a nonbasic scaffold. Our findings provide novel starting points for developing therapeutics aimed at treating pain and other conditions in which KOR is a central player.

Safety, tolerability, pharmacokinetics, and pharmacodynamics of a novel kappa opioid receptor agonist ZYKR1: a randomized double-blind placebo-control phase 1 study in healthy adult human participants

To perform first-in-human single-dose escalation trial of ZYKR1, which is a potent, selective, and peripherally-restricted kappa opioid receptor agonist, is the purpose of this study. This randomized, double-blind, placebo-controlled single ascending dose study conducted at Zydus Research Centre, Ahmedabad, India included healthy male participants aged 18-55 years and weighing > 50 kg. The primary objective was to evaluate the safety and tolerability of ZYKR1. The secondary objective was to evaluate the pharmacokinetics and pharmacodynamics (PD) of ZYKR1. Participants received ZYKR1 (0.5 - 6 mcg/kg) or placebo infused intravenously in 15 ± 1 min. Of total five dose groups (0.5 - 6 mcg/kg), each group included eight participants with six and two randomized to ZYKR1 and placebo, respectively. Three participants experienced six treatment-emergent adverse events (TEAEs); two were gastrointestinal disorders (nausea and vomiting at 2 mcg/kg); and four were related to the nervous system (headache (at 2 mcg/kg) and facial tingling, facial numbness and paresthesia (at 6 mcg/kg)); all TEAEs were mild and resolved without sequelae. The Cmax of ZYKR1 was achieved after 15 - 20 min of start of infusion. The mean exposures (Cmax and AUC0 - t) increased in a dose-proportional manner. The mean t1/2 ranged from 2.20 to 2.98 h across the dose range. Increase in the mean prolactin level was significantly higher in treatment groups compared with that in the placebo group. Intravenous ZYKR1 at doses up to 6 mcg/kg showed acceptable safety and tolerability and demonstrated a short half-life with principal route of excretion as renal. ZYKR1 displayed a potent PD effect reflected by increased prolactin levels, supporting further study in patients. Trial registration Clinical Trial Registry of India: CTRI/2018/07/014927. Date of registration: 18/07/2018.

Activation of kappa opioid receptor suppresses post-traumatic osteoarthritis via sequestering STAT3 on the plasma membrane

OBJECTIVE

Kappa opioid receptor (KOR) signaling is involved in joint development and inflammation in Osteoarthritis (OA), while the biochemical mechanism remains unclarified. This study aims to investigate downstream molecular events of KOR activation, to provide novel perspectives in OA pathology.

METHODS

U50,488H, a selective KOR agonist, was intra-articularly injected in mice upon destabilization of the medial meniscus (DMM) as OA models, with PBS injection as control. The behavioral and histological evaluation was assessed by hot plate test and red solid green staining, respectively. Alterations in mRNA and protein expression were assessed by RNA-seq, RT-qPCR, immunohistochemistry and western blotting (WB) in chondrocytes treated with TNF-α or TNF-α + U50,488H. Proteins interacted with KOR were explored using proximity labeling followed by mass spectrometry and then testified by co-immunoprecipitation (Co-IP) assay and immunofluorescence (IF).

RESULTS

OA-induced pain was reduced and cartilage degeneration was alleviated upon KOR activation in DMM mice. In chondrocytes, activation of KOR reversed the upregulation of MMPs, IL-6, IL-1β and phosphorylated(p-) STAT3, stimulated by TNF-α, while the expression of NF-κB, MAPKs and AKT signaling weren't reversed. RNA-seq and IF results presented that KOR activation evidently reduced STAT3 nuclear translocation in chondrocytes upon TNF-α stimuli. The reduction may be resulted from the binding of KOR and STAT3 in the plasma membrane, revealed by proximity labeling and Co-IP results.

CONCLUSIONS

KOR activation protects cartilage from OA, and this protective effect is mainly exerted via sequestering STAT3 on the plasma membrane, resulting in inactivation of STAT3-dependent immune responses which otherwise contributes to OA.

Systematic Structure-Activity Relationship Study of Nalfurafine Analogues toward Development of Potentially Nonaddictive Pain Management Treatments

Despite the availability of numerous pain medications, the current array of Food and Drug Administration-approved options falls short in adequately addressing pain states for numerous patients and consequently worsens the opioid crisis. Thus, it is imperative for basic research to develop novel and nonaddictive pain medications. Toward addressing this clinical goal, nalfurafine (NLF) was chosen as a lead and its structure-activity relationship (SAR) systematically studied through design, syntheses, and in vivo characterization of 24 analogues. Two analogues, 21 and 23, showed longer durations of action than NLF in a warm-water tail immersion assay, produced in vivo effects primarily mediated by KOR and DOR, penetrated the blood-brain barrier, and did not function as reinforcers. Additionally, 21 produced fewer sedative effects than NLF. Taken together, these results aid the understanding of NLF SAR and provide insights for future endeavors in developing novel nonaddictive therapeutics to treat pain.

Impact of butorphanol versus sufentanil on postoperative cognition and inflammation in elderly: a pilot study

Background

This randomized controlled trial aimed to compare the effects of butorphanol and sufentanil on early post-operative cognitive dysfunction (POCD) and systemic inflammation in older surgical patients.

Methods

Patients (aged 65 years or above) undergoing surgeries with general anesthesia were randomized to either the butorphanol group (40 μg/kg during anesthesia induction) or the sufentanil group (0.4 μg/kg). Cognitive function changes during the perioperative period were assessed using the Mini-Mental State Examination (MMSE) and the Montreal Cognitive Assessment (MoCA) scale up to 3 days after surgery. POCD was defined as a Z-score or composite Z-score greater than 1.96 for both MMSE and MoCA scores. Circulating inflammatory factors, including tumor necrosis factor-alpha (TNF-α), interleukin 1 beta (IL-1β), and interleukin 10 (IL-10), were measured using enzyme-linked immunosorbent assay.

Results

The study included 114 patients (median age: 71 years, 57.7% male). Compared to sufentanil, butorphanol significantly reduced the incidence of POCD on the first (11.5% versus 32.7%, p = 0.017) and third day (3.8% versus 15.4%, p = 0.046) after surgery. Additionally, patients receiving butorphanol had significantly lower circulating levels of TNF-α and IL-1β at the time of discharge from the post-anesthesia care unit and on the first and third day after surgery (p < 0.05 for all comparisons). Furthermore, circulating IL-10 levels were significantly higher in patients receiving butorphanol (p < 0.05 for all comparisons).

Conclusion

Administration of butorphanol during anesthesia induction, as opposed to sufentanil, was associated with a significant reduction in the early incidence of POCD in older surgical patients, possibly attributed to its impact on systemic inflammation.Clinical trial registration: The present study was registered in the China Clinical Trial Center (ChiCTR2300070805, 24/04/2023).

ERNEST COST action overview on the (patho)physiology of GPCRs and orphan GPCRs in the nervous system

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a critical role in nervous system function by transmitting signals between cells and their environment. They are involved in many, if not all, nervous system processes, and their dysfunction has been linked to various neurological disorders representing important drug targets. This overview emphasises the GPCRs of the nervous system, which are the research focus of the members of ERNEST COST action (CA18133) working group 'Biological roles of signal transduction'. First, the (patho)physiological role of the nervous system GPCRs in the modulation of synapse function is discussed. We then debate the (patho)physiology and pharmacology of opioid, acetylcholine, chemokine, melatonin and adhesion GPCRs in the nervous system. Finally, we address the orphan GPCRs, their implication in the nervous system function and disease, and the challenges that need to be addressed to deorphanize them.

Targeting of KOR by famotidine promotes OPC maturation differentiation and CNS remyelination via STAT3 signaling pathway

This study aims to identify FDA-approved drugs that can target the kappa-opioid receptor (KOR) for the treatment of demyelinating diseases. Demyelinating diseases are characterized by myelin sheath destruction or formation that results in severe neurological dysfunction. Remission of this disease is largely dependent on the differentiation of oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes (OLGs) in demyelinating lesions. KOR is an important regulatory protein and drug target for the treatment of demyelinating diseases. However, no drug targeting KOR has been developed due to the long clinical trials for drug discovery. Here, a structure-based virtual screening was applied to identify drugs targeting KOR among 1843 drugs of FDA-approved drug libraries, and famotidine was screen out by its high affinity cooperation with KOR as well as the clinical safety. We discovered that famotidine directly promoted OPC maturation and remyelination using the complementary in vitro and in vivo models. Administration of famotidine was not only effectively enhanced CNS myelinogenesis, but also promoted remyelination. Mechanically speaking, famotidine promoted myelinogenesis or remyelination through KOR/STAT3 signaling pathway. In general, our study provided evidence of new clinical applicability of famotidine for the treatment of demyelinating diseases for which there is currently no effective therapy.

Neuroimaging of opioid effects in humans across conditions of acute administration, chronic pain therapy, and opioid use disorder

Evidence of central nervous system (CNS) exogenous opioid effects in humans has been primarily gained through neuroimaging of three participant populations: individuals after acute opioid administration, those with opioid use disorder (OUD), and those with chronic pain receiving opioid therapy. In both the brain and spinal cord, opioids alter processes of pain, cognition, and reward. Opioid-related CNS effects may persist and accumulate with longer opioid use duration. Meanwhile, opioid-induced benefits versus risks to brain health remain unclear. This review article highlights recent accumulating evidence for how exogenous opioids impact the CNS in humans. While investigation of CNS opioid effects has remained largely disparate across contexts of opioid acute administration, OUD, and chronic pain opioid therapy, integration across these contexts may enable advancement toward effective interventions.

Therapeutic stapled peptides: Efficacy and molecular targets

Peptide stapling, by employing a stable, preformed alpha-helical conformation, results in the production of peptides with improved membrane permeability and enhanced proteolytic stability, compared to the original peptides, and provides an effective solution to accelerate the rapid development of peptide drugs. Various reviews present peptide stapling chemistries, anchoring residues and one- or two-component cyclization, however, therapeutic stapled peptides have not been systematically summarized, especially focusing on various disease-related targets. This review highlights the latest advances in therapeutic peptide drug development facilitated by the application of stapling technology, including different stapling techniques, synthetic accessibility, applicability to biological targets, potential for solving biological problems, as well as the current status of development. Stapled peptides as therapeutic drug candidates have been classified and analysed mainly by receptor- and ligand-based stapled peptide design against various diseases, including cancer, infectious diseases, inflammation, and diabetes. This review is expected to provide a comprehensive reference for the rational design of stapled peptides for different diseases and targets to facilitate the development of therapeutic peptides with enhanced pharmacokinetic and biological properties.

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.

The impact of multifunctional enkephalin analogs and morphine on the protein changes in crude membrane fractions isolated from the rat brain cortex and hippocampus

Endogenous opioid peptides serve as potent analgesics through the opioid receptor (OR) activation. However, they often suffer from poor metabolic stability, low lipophilicity, and low blood-brain barrier permeability. Researchers have developed many strategies to overcome the drawbacks of current pain medications and unwanted biological effects produced by the interaction with opioid receptors. Here, we tested multifunctional enkephalin analogs LYS739 (MOR/DOR agonist and KOR partial antagonist) and LYS744 (MOR/DOR agonist and KOR full antagonist) under in vivo conditions in comparison with MOR agonist, morphine. We applied 2D electrophoretic resolution to investigate differences in proteome profiles of crude membrane (CM) fractions isolated from the rat brain cortex and hippocampus exposed to the drugs (10 mg/kg, seven days). Our results have shown that treatment with analog LYS739 induced the most protein changes in cortical and hippocampal samples. The identified proteins were mainly associated with energy metabolism, cell shape and movement, apoptosis, protein folding, regulation of redox homeostasis, and signal transduction. Among these, the isoform of mitochondrial ATP synthase subunit beta (ATP5F1B) was the only protein upregulation in the hippocampus but not in the brain cortex. Contrarily, the administration of analog LYS744 caused a small number of protein alterations in both brain parts. Our results indicate that the KOR full antagonism, together with MOR/DOR agonism of multifunctional opioid ligands, can be beneficial in treating chronic pain states by reducing changes in protein expression levels but retaining analgesic efficacy.

Identification and In Vivo Evaluation of Myelination Agent PIPE-3297, a Selective Kappa Opioid Receptor Agonist Devoid of β-Arrestin-2 Recruitment Efficacy

Structure-activity relationship studies led to the discovery of PIPE-3297, a fully efficacious and selective kappa opioid receptor (KOR) agonist. PIPE-3297, a potent activator of G-protein signaling (GTPγS EC50 = 1.1 nM, 91% Emax), did not elicit a β-arrestin-2 recruitment functional response (Emax < 10%). Receptor occupancy experiments performed with the novel KOR radiotracer [3H]-PIPE-3113 revealed that subcutaneous (s.c.) administration of PIPE-3297 at 30 mg/kg in mice achieved 90% occupancy of the KOR in the CNS 1 h post dose. A single subcutaneous dose of PIPE-3297 in healthy mice produced a statistically significant increase of mature oligodendrocytes (P < 0.0001) in the KOR-enriched striatum, an effect that was not observed in animals predosed with the selective KOR antagonist norbinaltorphimine. An equivalent dose given to mice in an open-field activity-monitoring system revealed a small KOR-independent decrease in total locomotor activity versus vehicle measured between 60 and 75 min post dose. Daily doses of PIPE-3297 at both 3 and 30 mg/kg s.c. reduced the disease score in the mouse experimental autoimmune encephalomyelitis (EAE) model. Visually evoked potential (VEP) N1 latencies were also significantly improved versus vehicle in both dose groups, and latencies matched those of untreated animals. Taken together, these findings highlight the potential therapeutic value of functionally selective G-protein KOR agonists in demyelinating disease, which may avoid the sedating side effects typically associated with classical nonbiased KOR agonists.

κ-Opioid receptor activation attenuates osteoarthritis synovitis by regulating macrophage polarization through the NF-κB pathway

Osteoarthritis (OA) is a prevalent and chronic joint disease that affects the aging population, causing pain and disability. Macrophages in synovium are important mediators of synovial inflammatory activity and pathological joint pain. Previous studies have demonstrated the significant involvement of κ-opioid receptor (KOR) in the regulation of pain and inflammation. Our study reveals a significant reduction in synovial KOR expression among patients and mice with OA. Here, we find that KOR activation effectively inhibits the expressions of the LPS-induced-inflammatory cytokines TNF-α and IL-6 by inhibiting macrophage M1 phenotype. Mechanistically, KOR activation effectively suppresses the proinflammatory factor secretion of macrophages by inhibiting the translocation of NF-κB into the nucleus. Our animal experiments reveal that activation of KOR effectively alleviates knee pain and prevents synovitis progression in OA mice. Consistently, KOR administration suppresses the expressions of M1 macrophage markers and the NF-κB pathway in the synovium of the knee. Collectively, our study suggests that targeting KOR may be a viable strategy for treating OA by inhibiting synovitis and improving joint pain in affected patients.

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.

Design and structural validation of peptide-drug conjugate ligands of the kappa-opioid receptor

Despite the increasing number of GPCR structures and recent advances in peptide design, the development of efficient technologies allowing rational design of high-affinity peptide ligands for single GPCRs remains an unmet challenge. Here, we develop a computational approach for designing conjugates of lariat-shaped macrocyclized peptides and a small molecule opioid ligand. We demonstrate its feasibility by discovering chemical scaffolds for the kappa-opioid receptor (KOR) with desired pharmacological activities. The designed De Novo Cyclic Peptide (DNCP)-β-naloxamine (NalA) exhibit in vitro potent mixed KOR agonism/mu-opioid receptor (MOR) antagonism, nanomolar binding affinity, selectivity, and efficacy bias at KOR. Proof-of-concept in vivo efficacy studies demonstrate that DNCP-β-NalA(1) induces a potent KOR-mediated antinociception in male mice. The high-resolution cryo-EM structure (2.6 Å) of the DNCP-β-NalA-KOR-Gi1 complex and molecular dynamics simulations are harnessed to validate the computational design model. This reveals a network of residues in ECL2/3 and TM6/7 controlling the intrinsic efficacy of KOR. In general, our computational de novo platform overcomes extensive lead optimization encountered in ultra-large library docking and virtual small molecule screening campaigns and offers innovation for GPCR ligand discovery. This may drive the development of next-generation therapeutics for medical applications such as pain conditions.

Opioids and the Kidney: A Compendium

Opioids are a class of medications used in pain management. Unfortunately, long-term use, overprescription, and illicit opioid use have led to one of the greatest threats to mankind: the opioid crisis. Accompanying the classical analgesic properties of opioids, opioids produce a myriad of effects including euphoria, immunosuppression, respiratory depression, and organ damage. It is essential to ascertain the physiological role of the opioid/opioid receptor axis to gain an in-depth understanding of the effects of opioid use. This knowledge will aid in the development of novel therapeutic interventions to combat the increasing mortality rate because of opioid misuse. This review describes the current knowledge of opioids, including the opioid epidemic and opioid/opioid receptor physiology. Furthermore, this review intricately relates opioid use to kidney damage, navigates kidney structure and physiology, and proposes potential ways to prevent opioid-induced kidney damage.

Oral difelikefalin reduces moderate to severe pruritus and expression of pruritic and inflammatory biomarkers in subjects with atopic dermatitis

BACKGROUND

Pruritus is the most common and burdensome symptom of atopic dermatitis (AD). Pruritus-targeted treatments in AD are lacking, particularly for patients with milder skin disease.

OBJECTIVE

We sought to evaluate the impact of the selective κ-opioid receptor agonist difelikefalin (DFK) on pruritus intensity and pruritus- and immune-related biomarkers in subjects with moderate to severe AD-related pruritus.

METHODS

A phase 2 clinical trial investigated the efficacy and safety of oral DFK 0.25, 0.5, and 1.0 mg in subjects with moderate to severe AD-related pruritus. A biomarker substudy evaluated the effects of DFK on the expression of pruritus, TH2-associated genes, and skin barrier-related genes.

RESULTS

In the clinical trial (N = 401), all DFK doses reduced itch versus placebo; however, the results were not statistically significant at week 12. In a subgroup of subjects in the trial with mild to moderate skin inflammation and moderate to severe itch (itch-dominant AD phenotype), DFK reduced itch at week 12 versus placebo. In the biomarker substudy, DFK downregulated the expression of key pruritus-related genes (eg, IL-31 and TRPV1) and the AD phenotype (eg, CCL17). Gene set variation analysis confirmed that DFK, but not placebo, downregulated pruritus-related genes and TH2 pathways. DFK improved skin barrier integrity markers and upregulated the expression of claudins and lipid metabolism-associated genes (eg, SEC14L6, ELOVL3, CYP1A2, and AKR1D1).

CONCLUSIONS

DFK treatment reduced itch in subjects with moderate to severe AD-related pruritus, particularly those with an "itch-dominant" AD phenotype, and had an impact on the expression of pruritus, TH2-associated genes, and skin barrier-related genes. DFK is a promising therapy for AD-related pruritus; further clinical studies are warranted.

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.

Development of a Selective Peptide κ-Opioid Receptor Antagonist by Late-Stage Functionalization with Cysteine Staples

The κ-opioid receptor (KOR) is an attractive target for the development of novel drugs. KOR agonists are potentially safer pain medications, whereas KOR antagonists are promising drug candidates for the treatment of neuropsychiatric disorders. Hitherto, the vast majority of selective drug leads that have been developed for KOR are small molecules. In this study, novel peptide probes were designed by using an endogenous dynorphin A1-13 sequence as a template for peptide stapling via late-stage cysteine functionalization. Leveraging this strategy, we developed a stable and potent KOR antagonist, CSD-CH2(1,8)-NH2, with approximately 1000-fold improved selectivity for KOR over μ- and δ-opioid receptors. Its potent competitive KOR antagonism was verified in KOR-expressing cells, peripheral dorsal root ganglion neurons, and using the tail-flick and rotarod tests in mice. This work highlights the value of cysteine stapling to develop selective peptide probes to modulate central KOR function, as innovative peptide drug candidates for the treatment of KOR-related illnesses.

Probing the Activation Mechanisms of Agonist DPI-287 to Delta-Opioid Receptor and Novel Agonists Using Ensemble-Based Virtual Screening with Molecular Dynamics Simulations

Pain drugs targeting mu-opioid receptors face major addiction problems that have caused an epidemic. The delta-opioid receptor (DOR) has shown to not cause addictive effects when bound to an agonist. While the active conformation of the DOR in complex with agonist DPI-287 has been recently solved, there are still no FDA-approved agonists targeting it, providing the opportunity for structure-based virtual screening. In this study, the conformational plasticity of the DOR was probed using molecular dynamics (MD) simulations, identifying two representative conformations from clustering analysis. The two MD conformations as well as the crystal conformation of DOR were used to screen novel compounds from the ZINC database (17 million compounds), in which 69 drugs were picked as potential compounds based on their docking scores. Notably, 37 out of the 69 compounds were obtained from the simulated conformations. The binding stability of the 69 compounds was further investigated using MD simulations. Based on the MM-GBSA binding energy and the predicted drug properties, eight compounds were chosen as the most favorable, six of which were from the simulated conformations. Using a dynamic network model, the communication between the crystal agonist and the top eight molecules with the receptor was analyzed to confirm if these novel compounds share a similar activation mechanism to the crystal ligand. Encouragingly, docking of these eight compounds to the other two opioid receptors (kappa and mu) suggests their good selectivity toward DOR.

Association Between Opioid Use Disorder and Seizure Incidents Among Alcohol Use Disorder Patients

Many previous studies have discussed an association between alcohol use disorder (AUD) and seizure incidents. There are also case reports of seizures during opioid withdrawals. Therefore, it is possible that AUD patients may have a higher risk of seizures if they also have opioid use disorder (OUD). However, it remains unproven whether AUD patients with a dual diagnosis of OUD have higher seizure incidents, to our knowledge. This study explored seizure incidents among the patients with a dual diagnosis of AUD and OUD as well as seizures among AUD only or OUD only patients. This study utilized de-identified data from 30 777 928 hospital inpatient encounters at 948 healthcare systems over 4 years (9/1/2018-8/31/2022) from the Vizient^® Clinical Database for this study. Applying the International Classification of Diseases 10th Revision (ICD-10) diagnostic codes, AUD (1 953 575), OUD (768 982), and seizure (1 209 471) encounters were retrieved from the database to examine the effects of OUD on seizure incidence among AUD patients. This study also stratified patient encounters for demographic factors such as gender, age, and race, as well as the Vizient-categorized primary payer. Greatest gender differences were identified among AUD followed by OUD, and seizure patient groups. The mean age for seizure incidents was 57.6 years, while that of AUD was 54.7 years, and OUD 48.9 years. The greatest proportion of patients in all 3 groups were White, followed by Black, with Medicare being the most common primary payer in all 3 categories. Seizure incidents were statistically more common (P < .001, chi-square) in patients with a dual diagnosis of AUD and OUD (8.07%) compared to those with AUD only (7.55%). The patients with the dual diagnosis had a higher odd ratio than those with AUD only or OUD only. These findings across more than 900 health systems provide a greater understanding of seizure risks. Consequently, this information may help in triaging AUD and OUD patients in certain higher-risk demographic groups.

The Kappa Opioid Receptor Agonist 16-Bromo Salvinorin A Has Anti-Cocaine Effects without Significant Effects on Locomotion, Food Reward, Learning and Memory, or Anxiety and Depressive-like Behaviors

Kappa opioid receptor (KOR) agonists have preclinical antipsychostimulant effects; however, adverse side effects have limited their therapeutic development. In this preclinical study, conducted in Sprague Dawley rats, B6-SJL mice, and non-human primates (NHPs), we evaluated the G-protein-biased analogue of salvinorin A (SalA), 16-bromo salvinorin A (16-BrSalA), for its anticocaine effects, side effects, and activation of cellular signaling pathways. 16-BrSalA dose-dependently decreased the cocaine-primed reinstatement of drug-seeking behavior in a KOR-dependent manner. It also decreased cocaine-induced hyperactivity, but had no effect on responding for cocaine on a progressive ratio schedule. Compared to SalA, 16-BrSalA had an improved side effect profile, with no significant effects in the elevated plus maze, light-dark test, forced swim test, sucrose self-administration, or novel object recognition; however, it did exhibit conditioned aversive effects. 16-BrSalA increased dopamine transporter (DAT) activity in HEK-293 cells coexpressing DAT and KOR, as well as in rat nucleus accumbens and dorsal striatal tissue. 16-BrSalA also increased the early phase activation of extracellular-signal-regulated kinases 1 and 2, as well as p38 in a KOR-dependent manner. In NHPs, 16-BrSalA caused dose-dependent increases in the neuroendocrine biomarker prolactin, similar to other KOR agonists, at doses without robust sedative effects. These findings highlight that G-protein-biased structural analogues of SalA can have improved pharmacokinetic profiles and fewer side effects while maintaining their anticocaine effects.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Design of Analgesic Trivalent Peptides with Low Withdrawal Symptoms: Probing the Antinociceptive Profile of Novel Linear and Cyclic Peptides as Opioid Pan Ligands

The discovery of efficacious and safe analgesics with reduced side effects is the foremost challenge in the pain field. In this work, we report the in vitro and in vivo evaluation of linear and cyclic analogues of biphalin with the aim to complete the series of structural modifications previously applied in the development of opioid peptides incorporating a xylene bridge. Replacement of Tyr1,1' by Dmt (2,5-dimethyltyrosine) in the linear biphalin analogue AM94 and cyclic analogue MACE4 resulted in two new compounds (namely, MJ2 and MJ5) endowed with improved KOR/MOR/DOR binding affinity. Both compounds showed a strong antinociceptive profile in in vivo models of nociception, allodynia, and hyperalgesia via the tail flick, hot plate, and formalin tests after intracerebroventricular and subcutaneous administration. One of these ligands, MJ2, was also tested in tolerance and dependence studies, exhibiting very little withdrawal symptoms.

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.

Bifunctional Peptidomimetic G Protein-Biased Mu-Opioid Receptor Agonist and Neuropeptide FF Receptor Antagonist KGFF09 Shows Efficacy in Visceral Pain without Rewarding Effects after Subcutaneous Administration in Mice

There is still an unmet clinical need to develop new pharmaceuticals for effective and safe pain management. Current pharmacotherapy offers unsatisfactory solutions due to serious side effects related to the chronic use of opioid drugs. Prescription opioids produce analgesia through activation of the mu-opioid receptor (MOR) and are major contributors to the current opioid crisis. Multifunctional ligands possessing activity at more than one receptor represent a prominent therapeutic approach for the treatment of pain with fewer adverse effects. We recently reported on the design of a bifunctional MOR agonist/neuropeptide FF receptor (NPFFR) antagonist peptididomimetic, KGFF09 (H-Dmt-DArg-Aba-βAla-Bpa-Phe-NH2), and its antinociceptive effects after subcutaneous (s.c.) administration in acute and persistent pain in mice with reduced propensity for unwanted side effects. In this study, we further investigated the antinociceptive properties of KGFF09 in a mouse model of visceral pain after s.c. administration and the potential for opioid-related liabilities of rewarding and sedation/locomotor dysfunction following chronic treatment. KGFF09 produced a significant dose-dependent inhibition of the writhing behavior in the acetic acid-induced writhing assay with increased potency when compared to morphine. We also demonstrated the absence of harmful effects caused by typical MOR agonists, i.e., rewarding effects (conditioned-place preference test) and sedation/locomotor impairment (open-field test), at a dose shown to be highly effective in inhibiting pain behavior. Consequently, KGFF09 displayed a favorable benefit/side effect ratio regarding these opioid-related side effects compared to conventional opioid analgesics, such as morphine, underlining the development of dual MOR agonists/NPFFR antagonists as improved treatments for various pain conditions.