Design and Synthesis of a Novel and Selective Kappa Opioid Receptor (KOR) Antagonist (BTRX-335140)

Navacaprant, a novel and selective kappa opioid receptor antagonist, has no agonist properties implicated in opioid-related abuse

Kappa opioid receptors (KORs) are implicated in the pathophysiology of various psychiatric and neurological disorders creating interest in targeting the KOR system for therapeutic purposes. Accordingly, navacaprant (NMRA-140) is a potent, selective KOR antagonist being evaluated as a treatment for major depressive disorder. In the present report, we have extended the pharmacological characterization of navacaprant by further demonstrating its selective KOR antagonist properties and confirming its lack of agonist activity at KORs and related targets involved in opioid-related abuse. Using CHO-K1 cells expressing human KOR, mu (MOR), or delta (DOR) opioid receptors, navacaprant demonstrated selective antagonist properties at KOR (IC50 = 0.029 μM) versus MOR (IC50 = 3.3 μM) and DOR (IC50 > 10 μM) in vitro. In vivo, navacaprant (10-30 mg/kg, i.p.) dose-dependently abolished KOR-agonist induced analgesia in the mouse tail-flick assay. Additionally, navacaprant (10, 30 mg/kg, p.o.) significantly reduced KOR agonist-stimulated prolactin release in mice and rats, confirming KOR antagonism in vivo. Navacaprant showed no agonist activity at any opioid receptor subtype (EC50 > 10 μM) in vitro and exhibited no analgesic effect in the tail-flick assays at doses ≤100 mg/kg, p.o. thereby confirming a lack of opioid receptor agonist activity in vivo. Importantly, navacaprant did not alter extracellular dopamine concentrations in the nucleus accumbens shell of freely-moving rats following doses ≤100 mg/kg, p.o., whereas morphine (10, 20 mg/kg, i.p.) significantly increased dopamine levels. These results demonstrate that navacaprant is a KOR-selective antagonist with no pharmacological properties implicated in opioid-related abuse.

Aticaprant: (a κ-opioid receptor antagonist) for major depressive disorder

INTRODUCTION

Major depression is a common, disabling mental health condition associated with the highest disease burden for any neuropsychiatric disorder worldwide, according to the WHO. Due to the imperfect efficacy and tolerability profiles of existing treatments, investigational compounds in novel treatment classes are needed. Opioid-receptor antagonists are a potential new class of treatments currently under investigation.

AREAS COVERED

Major depressive disorder is first overviewed. Existing treatments, both their mechanisms of action and their place within the antidepressant space, are discussed herein. Then, the profile of Aticaprant and the wider context of kappa-opioid antagonism for depression are discussed in focus.

EXPERT OPINION

Early evidence indicates that Aticaprant may possess desirable pharmacodynamic and pharmacokinetic properties. A lack of convincing efficacy data at the time of writing precludes any definitive statement on its potential as an antidepressant.

Opposing Effects on Descending Control of Nociception by µ and κ Opioid Receptors in the Anterior Cingulate Cortex

BACKGROUND

The efficiency of descending pain modulation, commonly assessed with the conditioned pain modulation procedure, is diminished in patients with chronic pain. The authors hypothesized that the efficiency of pain modulation is controlled by cortical opioid circuits.

METHODS

This study evaluated the effects of µ opioid receptor activation in the anterior cingulate cortex on descending control of nociception, a preclinical correlate of conditioned pain modulation, in male Sprague-Dawley rats with spinal nerve ligation-induced chronic pain or in sham-operated controls. Additionally, the study explored the consequences of respective activation or inhibition of κ opioid receptor in the anterior cingulate cortex of naive rats or animals with neuropathic pain. Descending control of nociception was measured as the hind paw withdrawal response to noxious pressure (test stimulus) in the absence or presence of capsaicin injection in the forepaw (conditioning stimulus).

RESULTS

Descending control of nociception was diminished in the ipsilateral, but not contralateral, hind paw of rats with spinal nerve ligation. Bilateral administration of morphine in the anterior cingulate cortex had no effect in shams but restored diminished descending control of nociception without altering hypersensitivity in rats with neuropathic pain. Bilateral anterior cingulate cortex microinjection of κ opioid receptor antagonists, including nor-binaltorphimine and navacaprant, also re-established descending control of nociception in rats with neuropathic pain without altering hypersensitivity and with no effect in shams. Conversely, bilateral injection of a κ opioid receptor agonist, U69,593, in the anterior cingulate cortex of naive rats inhibited descending control of nociception without altering withdrawal thresholds.

CONCLUSIONS

Anterior cingulate cortex κ opioid receptor activation therefore diminishes descending control of nociception both in naive animals and as an adaptive response to chronic pain, likely by enhancing net descending facilitation. Descending control of nociception can be restored by activation of μ opioid receptors in the anterior cingulate cortex, but also by κ opioid receptor antagonists, providing a nonaddictive alternative to opioid analgesics. Navacaprant is now in advanced clinical trials.

EDITOR’S PERSPECTIVE

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.

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

Antinociceptive effect of cyclic and linear diterpenoids as new atypical agonists of κ-opioid receptors obtained from four species of the Baccharis genus, and vehiculated in nanometric niosomes

New natural analgesic compounds that act in KORs are important alternatives for potential therapeutical use in medicine. In this work, we report and compare here the antinociceptive activity displayed by cyclic and linear diterpenes, obtained from the genus Baccharis. The antinociceptive activities determined were relatively strong, in comparison whit morphine. The antinociceptive mechanism of action was made through naloxone administration (a non-selective antagonist of opioid receptors). The more active compounds were vehiculized successfully in niosomes at nanometric scale. The observed antinociceptive activity for Bartemidiolide oxide (BARTO), obtain from Baccharis artemisioides, was greater than Flabeloic acid dimer (DACD), the first compound isolated from Baccharis flabellata that was reported possessing antinociceptive effects. We also conducted docking calculations and molecular dynamics simulations, which suggested that the newly identified diterpenes might share the molecular action mechanism reported for Salvinorin A (SalA). Molecular simulations have allowed us to appreciate some subtle differences between molecular interactions of these ligands stabilizing their respective complexes; such information might be useful for designing and searching for new inhibitors of KORs.

An Analysis of Successful Hit-to-Clinical Candidate Pairs

An analysis of 156 published clinical candidates from the Journal of Medicinal Chemistry between 2018 and 2021 was conducted to identify lead generation strategies most frequently employed leading to drug candidates. As in a previous publication, the most frequent lead generation strategies resulting in clinical candidates were from known compounds (59%) followed by random screening approaches (21%). The remainder of the approaches included directed screening, fragment screening, DNA-encoded library screening (DEL), and virtual screening. An analysis of similarity was also conducted based on Tanimoto-MCS and revealed most clinical candidates were distant from their original hits; however, most shared a key pharmacophore that translated from hit-to-clinical candidate. An examination of frequency of oxygen, nitrogen, fluorine, chlorine, and sulfur incorporation in clinical candidates was also conducted. The three most similar and least similar hit-to-clinical pairs from random screening were examined to provide perspective on changes that occur that lead to successful clinical candidates.

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.

Chronic pain recruits hypothalamic dynorphin/kappa opioid receptor signalling to promote wakefulness and vigilance

Increased vigilance in settings of potential threats or in states of vulnerability related to pain is important for survival. Pain disrupts sleep and conversely, sleep disruption enhances pain, but the underlying mechanisms remain unknown. Chronic pain engages brain stress circuits and increases secretion of dynorphin, an endogenous ligand of the kappa opioid receptor (KOR). We therefore hypothesized that hypothalamic dynorphin/KOR signalling may be a previously unknown mechanism that is recruited in pathological conditions requiring increased vigilance. We investigated the role of KOR in wakefulness, non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep in freely moving naïve mice and in mice with neuropathic pain induced by partial sciatic nerve ligation using EEG/EMG recordings. Systemic continuous administration of U69,593, a KOR agonist, over 5 days through an osmotic minipump decreased the amount of NREM and REM sleep and increased sleep fragmentation in naïve mice throughout the light-dark sleep cycle. We used KORcre mice to selectively express a Gi-coupled designer receptor activated by designer drugs (Gi-DREADD) in KORcre neurons of the hypothalamic paraventricular nucleus, a key node of the hypothalamic-pituitary-adrenal stress response. Sustained activation of Gi-DREADD with clozapine-N-oxide delivered in drinking water over 4 days, disrupted sleep in these mice in a similar way as systemic U69,593. Mice with chronic neuropathic pain also showed disrupted NREM and total sleep that was normalized by systemic administration of two structurally different KOR antagonists, norbinaltorphimine and NMRA-140, currently in phase II clinical development, or by CRISPR/Cas9 editing of paraventricular nucleus KOR, consistent with endogenous KOR activation disrupting sleep in chronic pain. Unexpectedly, REM sleep was diminished by either systemic KOR antagonist or by CRISPR/Cas9 editing of paraventricular nucleus KOR in sham-operated mice. Our findings reveal previously unknown physiological and pathophysiological roles of dynorphin/KOR in eliciting arousal. Physiologically, dynorphin/KOR signalling affects transitions between sleep stages that promote REM sleep. Furthermore, while KOR antagonists do not promote somnolence in the absence of pain, they normalized disrupted sleep in chronic pain, revealing a pathophysiological role of KOR signalling that is selectively recruited to promote vigilance, increasing chances of survival. Notably, while this mechanism is likely beneficial in the short-term, disruption of the homeostatic need for sleep over longer periods may become maladaptive resulting in sustained pain chronicity. A novel approach for treatment of chronic pain may thus result from normalization of chronic pain-related sleep disruption by KOR antagonism.

A systematic review on the kappa opioid receptor and its ligands: New directions for the treatment of pain, anxiety, depression, and drug abuse

Kappa opioid receptor (KOR) is a member of the opioid receptor system, the G protein-coupled receptors that are expressed throughout the peripheral and central nervous systems and play crucial roles in the modulation of antinociception and a variety of behavioral states like anxiety, depression, and drug abuse. KOR agonists are known to produce potent analgesic effects and have been used clinically for the treatment of pain, while KOR antagonists have shown efficacy in the treatment of anxiety and depression. This review summarizes the history, design strategy, discovery, and development of KOR ligands. KOR agonists are classified as non-biased, G protein-biased, and β-arrestin recruitment-biased, according to their degrees of bias. The mechanisms and associated effects of the G protein signaling pathway and β-arrestin recruitment signaling pathway are also discussed. Meanwhile, KOR antagonists are classified as long-acting and short-acting, based on their half-lives. In addition, we have special sections for mixed KOR agonists and selective peripheral KOR agonists. The mechanisms of action and pharmacokinetic, pharmacodynamic, and behavioral studies for each of these categories are also discussed in this review.

In Vitro, In Vivo and In Silico Characterization of a Novel Kappa-Opioid Receptor Antagonist

Kappa-opioid receptor (KOR) antagonists are promising innovative therapeutics for the treatment of the central nervous system (CNS) disorders. The new scaffold opioid ligand, Compound A, was originally found as a mu-opioid receptor (MOR) antagonist but its binding/selectivity and activation profile at the KOR and delta-opioid receptor (DOR) remain elusive. In this study, we present an in vitro, in vivo and in silico characterization of Compound A by revealing this ligand as a KOR antagonist in vitro and in vivo. In the radioligand competitive binding assay, Compound A bound at the human KOR, albeit with moderate affinity, but with increased affinity than to the human MOR and without specific binding at the human DOR, thus displaying a preferential KOR selectivity profile. Following subcutaneous administration in mice, Compound A effectively reverse the antinociceptive effects of the prototypical KOR agonist, U50,488. In silico investigations were carried out to assess the structural determinants responsible for opioid receptor subtype selectivity of Compound A. Molecular docking, molecular dynamics simulations and dynamic pharmacophore (dynophore) generation revealed differences in the stabilization of the chlorophenyl moiety of Compound A within the opioid receptor binding pockets, rationalizing the experimentally determined binding affinity values. This new chemotype bears the potential for favorable ADMET properties and holds promise for chemical optimization toward the development of potential therapeutics.

Rational Design, Chemical Syntheses, and Biological Evaluations of Peripherally Selective Mu Opioid Receptor Ligands as Potential Opioid Induced Constipation Treatment

Opioid-induced constipation (OIC) is a common adverse effect of opioid analgesics. Peripherally acting μ opioid receptor antagonists (PAMORAs) can be applied in the treatment of OIC without compromising the analgesic effects. NAP, a 6β-N-4-pyridyl-substituted naltrexamine derivative, was previously identified as a potent and selective MOR antagonist mainly acting peripherally but with some CNS effects. Herein, we introduced a highly polar aromatic moiety, for example, a pyrazolyl or imidazolyl ring to decrease CNS MPO scores in order to reduce passive BBB permeability. Four compounds 2, 5, 17, and 19, when administered orally, were able to increase intestinal motility during morphine-induced constipation in the carmine red dye assays. Among them, compound 19 (p.o.) improved GI tract motility by 75% while orally administered NAP and methylnaltrexone showed no significant effects at the same dose. Thus, this compound seemed a promising agent to be further developed as an oral treatment for OIC.

Dysregulation of serum prolactin links the hypothalamus with female nociceptors to promote migraine

Migraine headache results from activation of meningeal nociceptors, however, the hypothalamus is activated many hours before the emergence of pain. How hypothalamic neural mechanisms may influence trigeminal nociceptor function remains unknown. Stress is a common migraine trigger that engages hypothalamic dynorphin/kappa opioid receptor (KOR) signalling and increases circulating prolactin. Prolactin acts at both long and short prolactin receptor isoforms that are expressed in trigeminal afferents. Following downregulation of the prolactin receptor long isoform, prolactin signalling at the prolactin receptor short isoform sensitizes nociceptors selectively in females. We hypothesized that stress may activate the kappa opioid receptor on tuberoinfundibular dopaminergic neurons to increase circulating prolactin leading to female-selective sensitization of trigeminal nociceptors through dysregulation of prolactin receptor isoforms. A mouse two-hit hyperalgesic priming model of migraine was used. Repeated restraint stress promoted vulnerability (i.e. first-hit priming) to a subsequent subthreshold (i.e. second-hit) stimulus from inhalational umbellulone, a TRPA1 agonist. Periorbital cutaneous allodynia served as a surrogate of migraine-like pain. Female and male KORCre; R26lsl-Sun1-GFP mice showed a high percentage of KORCre labelled neurons co-localized in tyrosine hydroxylase-positive cells in the hypothalamic arcuate nucleus. Restraint stress increased circulating prolactin to a greater degree in females. Stress-primed, but not control, mice of both sexes developed periorbital allodynia following inhalational umbellulone. Gi-DREADD activation (i.e. inhibition through Gi-coupled signalling) in KORCre neurons in the arcuate nucleus also increased circulating prolactin and repeated chemogenetic manipulation of these neurons primed mice of both sexes to umbellulone. Clustered regularly interspaced short palindromic repeats-Cas9 deletion of the arcuate nucleus KOR prevented restraint stress-induced prolactin release in female mice and priming from repeated stress episodes in both sexes. Inhibition of circulating prolactin occurred with systemic cabergoline, a dopamine D2 receptor agonist, blocked priming selectively in females. Repeated restraint stress downregulated the prolactin receptor long isoform in the trigeminal ganglia of female mice. Deletion of prolactin receptor in trigeminal ganglia by nasal clustered regularly interspaced short palindromic repeats-Cas9 targeting both prolactin receptor isoforms prevented stress-induced priming in female mice. Stress-induced activation of hypothalamic KOR increases circulating prolactin resulting in trigeminal downregulation of prolactin receptor long and pain responses to a normally innocuous TRPA1 stimulus. These are the first data that provide a mechanistic link between stress-induced hypothalamic activation and the trigeminal nociceptor effectors that produce trigeminal sensitization and migraine-like pain. This sexually dimorphic mechanism may help to explain female prevalence of migraine. KOR antagonists, currently in phase II clinical trials, may be useful as migraine preventives in both sexes, while dopamine agonists and prolactin/ prolactin receptor antibodies may improve therapy for migraine, and other stress-related neurological disorders, in females.

A new paradigm shift in antidepressant therapy? From dual-action to multitarget-directed ligands

Major Depressive Disorder is a chronic, recurring, and potentially fatal disease affecting up to 20% of the global population. Since the monoamine hypothesis was proposed more than 60 years ago, only a few relevant advances have been achieved, with very little disease course changing, from a pharmacological perspective. Moreover, since negative efficacy studies with novel molecules are frequent, many pharmaceutical companies have put new studies on hold. Fortunately, relevant clinical studies are currently being performed, and extensive striving is being developed by universities, research centers, and other public and private institutions. Depression is no longer considered a simple disease but a multifactorial one. New research fields are emerging in what could be a paradigm shift: the multitarget approach beyond monoamines. In this review, we summarize the present and the past of antidepressant drug discovery, with the aim to shed some light on the current state of the art in clinical and preclinical advances to face this increasingly devastating disease.

Contributions of the International Narcotics Research Conference to Opioid Research Over the Past 50 years

The International Narcotics Research Conference (INRC), founded in 1969, has been a successful forum for research into the actions of opiates, with an annual conference since 1971. Every year, scientists from around the world have congregated to present the latest data on novel opiates, opiate receptors and endogenous ligands, mechanisms of analgesic activity and unwanted side effects, etc. All the important discoveries in the opiate field were discussed, often first, at the annual INRC meeting. With an apology to important events and participants not discussed, this review presents a short history of INRC with a discussion of groundbreaking discoveries in the opiate field and the researchers who presented from the first meeting up to the present.

Pain, negative affective states and opioid-based analgesics: Safer pain therapies to dampen addiction

Across centuries and civilizations opioids have been used to relieve pain. In our modern societies, opioid-based analgesics remain one of the most efficient treatments for acute pain. However, the long-term use of opioids can lead to the development of analgesic tolerance, opioid-induced hyperalgesia, opioid use disorders, and overdose, which can ultimately produce respiratory depressant effects with fatal consequences. In addition to the nociceptive sensory component of pain, negative affective states arising from persistent pain represent a risk factor for developing an opioid use disorder. Several studies have indicated that the increase in prescribed opioid analgesics since the 1990s represents the root of our current opioid epidemic. In this review, we will present our current knowledge on the endogenous opioid system within the pain neuroaxis and the plastic changes occurring in this system that may underlie the occurrence of pain-induced negative affect leading to misuse and abuse of opioid medications. Dissecting the allostatic neuronal changes occurring during pain is the most promising avenue to uncover novel targets for the development of safer pain medications. We will discuss this along with current and potential approaches to treat pain-induced negative affective states that lead to drug misuse. Moreover, this chapter will provide a discussion on potential avenues to reduce the abuse potential of new analgesic drugs and highlight a basis for future research and drug development based on recent advances in this field.

Peptide Kappa Opioid Receptor Ligands and Their Potential for Drug Development

Ligands for kappa opioid receptors (KOR) have potential uses as non-addictive analgesics and for the treatment of pruritus, mood disorders, and substance abuse. These areas continue to have major unmet medical needs. Significant advances have been made in recent years in the preclinical development of novel opioid peptides, notably ones with structural features that inherently impart stability to proteases. Following a brief discussion of the potential therapeutic applications of KOR agonists and antagonists, this review focuses on two series of novel opioid peptides, all-D-amino acid tetrapeptides as peripherally selective KOR agonists for the treatment of pain and pruritus without centrally mediated side effects, and macrocyclic tetrapeptides based on CJ-15,208 that can exhibit different opioid profiles with potential applications such as analgesics and treatments for substance abuse.

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.

Rapid-Onset Anti-Stress Effects of a Kappa-Opioid Receptor Antagonist, LY2795050, Against Immobility in an Open Space Swim Paradigm in Male and Female Mice

The kappa-opioid receptor (KOR) / dynorphin system is implicated with behavioral and neurobiological effects of stress exposure (including heavy exposure to drugs of abuse) in translational animal models. Thus some KOR-antagonists can decrease the aversive, depressant-like and anxiety-like effects caused by stress exposure. The first generation of selective KOR-antagonists have slow onsets (hours) and extremely long durations of action (days-weeks), in vivo. A new generation of KOR antagonists with rapid onset and shorter duration of action can potentially decrease the effects of stress exposure in translational models, and may be of interest for medication development. This study examined the rapid onset anti-stress effects of one of the shorter acting novel KOR-antagonists (LY2795050, (3-chloro-4-(4-(((2S)-2-pyridin-3-ylpyrrolidin-1-yl)methyl) phenoxy)benzamide)) in a single-session open space swim (OSS) stress paradigm (15 min duration), in adult male and female C57BL/6 J mice. LY2795050 (0.32 mg/kg, i.p.) had rapid onset (within 15 min) and short duration (<3 h) of KOR-antagonist effects, based on its blockade of the locomotor depressant effects of the KOR-agonist U50,488 (10 mg/kg). LY2795050 (0.32 mg/kg), when administered only 1 min prior to the OSS stress paradigm, decreased immobility in males, but not females. With a slightly longer pretreatment time (15 min), this dose of LY2795050 decreased immobility in both males and females. A 10-fold smaller dose of LY2795050 (0.032 mg/kg) was inactive in the OSS, showing dose-dependence of this anti-stress effect. Overall, these studies show that a novel KOR-antagonist can produce very rapid onset anti-immobility effects in this model of acute stress exposure.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Profile of a short-acting κ-antagonist, LY2795050, on self-grooming behaviors, forced swim test and locomotor activity: sex comparison in mice

BACKGROUND

Novel short-acting κ(kappa)-opioid receptor selective antagonists are translational tools to examine the impact of the κ-receptor/dynorphin system in assays related to central nervous system dysfunction (e.g., substance use disorders, anhedonia and depression). The effects of such compounds have been compared in males and females under very limited conditions.

AIMS

The goal of this study was to examine potential sex differences in the effects of a κ-agonist and a short-acting κ-antagonist in an ethologically relevant test of anhedonia, the "splash test" of self-grooming, and also in the forced swim test and in locomotor activity.

METHODS

We examined the dose-dependence of grooming deficits caused by the κ-agonist U50,488 (0.1-3.2 mg/kg intraperitoneal (i.p.)) in gonadally intact adult male and female C57BL/6J mice. We then compared the effects of the short-acting κ-antagonist LY2795050 ((3-chloro-4-(4-(((2S)-2-pyridin-3-ylpyrrolidin-1-yl)methyl) phenoxy)benzamide)); 0.032-0.1 mg/kg i.p.) in blocking grooming deficits caused by U50,488 (3.2 mg/kg). The effects of LY2795050 were also studied in the forced swim test (FST). The effects of LY2795050 in blocking the locomotor depressant effects of U50,488 (10 mg/kg) were also studied.

RESULTS

U50,488 produced dose-dependent grooming deficits in male and female mice, and LY2795050 prevented these effects. In contrast, LY2795050 decreased immobility in the FST in males at a dose of 0.1 mg/kg, but not in females, up to a dose of 0.32 mg/kg. Also, LY2795050 (0.32 mg/kg) prevented and also reversed the locomotor-depressant effects of U50,488 (10 mg/kg), in males and females.

CONCLUSIONS

This study further implicates the κ-receptor system in ethologically relevant aspects of anhedonia, and confirms sexual dimorphism in some behavioral effects of novel κ-antagonists.

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.

The Role of Dynorphin and the Kappa Opioid Receptor in Schizophrenia and Major Depressive Disorder: A Translational Approach

The kappa opioid receptor (KOR) and its endogenous ligands dynorphins (DYN) have been implicated in the development or symptomatology of a variety of neuropsychiatric disorders. This review covers a brief history of the development of KOR agonists and antagonists, their effects in healthy volunteers, and the potential role of DYN/KOR dysfunction in schizophrenia and major depressive disorder from a translational perspective. The potential role of DYN/KOR dysfunction in schizophrenia is based on several lines of evidence. Selective KOR agonists induce affective states in healthy volunteers with similarities to the symptoms of schizophrenia. Studies have shown increased DYN in patients with schizophrenia, although the data have been mixed. Finally, meta-analytic data have shown that opioid antagonists are associated with reductions in the symptoms of schizophrenia. The potential role of DYN/KOR dysfunction in major depressive disorder is also based on a combination of preclinical and clinical data. Selective KOR agonists have shown pro-depressive effects in human volunteers, while selective KOR antagonists have shown robust efficacy in several preclinical models of antidepressant activity. Small studies have shown that nonselective KOR antagonists may have efficacy in treatment-resistant depression. Additionally, recent clinical data have shown that the KOR may be an effective target for treating anhedonia, a finding relevant to both schizophrenia and depression. Finally, recommendations are provided for translating preclinical models for schizophrenia and major depressive disorder into the clinic.

Differential effects of novel kappa opioid receptor antagonists on dopamine neurons using acute brain slice electrophysiology

Activation of the kappa opioid receptor (KOR) contributes to the aversive properties of stress, and modulates key neuronal circuits underlying many neurobehavioral disorders. KOR agonists directly inhibit ventral tegmental area (VTA) dopaminergic neurons, contributing to aversive responses (Margolis et al. 2003, 2006); therefore, selective KOR antagonists represent a novel therapeutic approach to restore circuit function. We used whole cell electrophysiology in acute rat midbrain slices to evaluate pharmacological properties of four novel KOR antagonists: BTRX-335140, BTRX-395750, PF-04455242, and JNJ-67953964. Each compound concentration-dependently reduced the outward current induced by the KOR selective agonist U-69,593. BTRX-335140 and BTRX-395750 fully blocked U-69,593 currents (IC₅₀ = 1.2 ± 0.9 and 1.2 ± 1.3 nM, respectively). JNJ-67953964 showed an IC₅₀ of 3.0 ± 4.6 nM. PF-04455242 exhibited partial antagonist activity asymptoting at 55% blockade (IC₅₀ = 6.7 ± 15.1 nM). In 3/8 of neurons, 1 μM PF-04455242 generated an outward current independent of KOR activation. BTRX-335140 (10 nM) did not affect responses to saturating concentrations of the mu opioid receptor (MOR) agonist DAMGO or the delta opioid receptor (DOR) agonist DPDPE, while JNJ-67953964 (10 nM) partially blocked DAMGO and DPDPE responses. Importantly, BTRX-335140 (10 nM) rapidly washed out with complete recovery of U-69,593 responses within 10 min. Collectively, we show electrophysiological evidence of key differences amongst KOR antagonists that could impact their therapeutic potential and have not been observed using recombinant systems. The results of this study demonstrate the value of characterizing compounds in native neuronal tissue and within circuits implicated in the neurobehavioral disorders of interest.

A novel, injury-free rodent model of vulnerability for assessment of acute and preventive therapies reveals temporal contributions of CGRP-receptor activation in migraine-like pain

AIM

Development and characterization of a novel injury-free preclinical model of migraine-like pain allowing mechanistic assessment of both acute and preventive treatments.

METHODS

A "two-hit" hyperalgesic priming strategy was used to induce vulnerability to a normally subthreshold challenge with umbellulone, a transient receptor potential ankyrin 1 (TRPA1) activator, in uninjured female and male C57BL/6 mice. Priming (i.e. the first hit) was induced by three consecutive daily episodes of restraint stress; repeated umbellulone was also evaluated for potential priming effects. Sixteen days after the first restraint stress, mice received inhalational umbellulone (i.e. the second hit) to elicit migraine-like pain. Medications currently used for acute or preventive migraine therapy including propranolol (a beta blocker) and sumatriptan (5HT1_B/D agonist), as well as olcegepant, an experimental calcitonin gene related peptide (CGRP) receptor antagonist and nor-Binaltorphimine (nor-BNI), an experimental long-acting kappa opioid receptor (KOR) antagonist, were investigated for their efficacy to block priming and prevent or reverse umbellulone-induced allodynia in primed animals. To assess migraine-like pain, cutaneous allodynia was determined by responses to periorbital or hindpaw probing with von Frey filaments.

RESULTS

Repeated restraint stress, but not umbellulone exposure, produced transient cutaneous allodynia that resolved within 16 d. Restraint stress produced long-lasting priming that persisted beyond 16 d, as demonstrated by reinstatement of cutaneous allodynia following inhalational umbellulone challenge. Pretreatment with propranolol or nor-BNI prior to restraint stress prevented both transient cutaneous allodynia and priming, demonstrated by a lack of umbellulone-induced cutaneous allodynia. Following establishment of restraint stress priming, olcegepant, but not propranolol or nor-BNI, prevented umbellulone-induced cutaneous allodynia. When administered 1 h after umbellulone, sumatriptan, but not olcegepant, reversed umbellulone-induced cutaneous allodynia in restraint stress-primed rats.

CONCLUSION

We have developed a novel injury-free model with translational relevance that can be used to study mechanisms relevant to migraine-like pain and to evaluate novel acute or preventive treatments. Restraint stress priming induced a state of vulnerability to a subthreshold stimulus that has been referred to as "latent sensitization". The development of latent sensitization could be prevented by blockade of stress pathways with propranolol or with a kappa opioid receptor antagonist. Following establishment of latent sensitization, subthreshold stimulation with umbellulone reinstated cutaneous allodynia, likely from activation of meningeal TRPA1-expressing nociceptors. Accordingly, in restraint stress-primed animals, sumatriptan reversed umbellulone-induced cutaneous allodynia, supporting peripheral sites of action, while propranolol and nor-BNI were not effective. Surprisingly, olcegepant was effective in mice with latent sensitization when given prior to, but not after, umbellulone challenge, suggesting time-dependent contributions of calcitonin gene-related peptide receptor signaling in promoting migraine-like pain in this model. Activation of the calcitonin gene-related peptide receptor participates in initiating, but has a more limited role in maintaining, pain responses, supporting the efficacy of small molecule calcitonin gene-related peptide antagonists as preventive medications. Additionally, the effectiveness of sumatriptan in reversal of established pain thus suggests modulation of additional, non-calcitonin gene-related peptide receptor-mediated nociceptive mechanisms. Kappa opioid receptor antagonists may represent a novel preventive therapy for stress-related migraine.

Regulation of Kappa Opioid Receptor Inactivation Depends on Sex and Cellular Site of Antagonist Action

The prototypical member of the receptor-inactivating kappa opioid receptor (KOR) antagonists, norbinaltorphimine (norBNI), produces prolonged receptor inactivation by a cJun kinase mechanism. These antagonists have potential therapeutic utility in the treatment of stress disorders; however, additional preclinical characterization is necessary to understand important aspects of their action. In this study, we report that norBNI does not work as effectively in female mice as in males because of estrogen regulation of G protein receptor kinase (GRK); pretreatment of ovary-intact female mice with the selective GRK2/3 inhibitor, Compound 101, made females equally sensitive to norBNI as males. Prior observations suggested that in vivo treatment with norBNI does not produce long-lasting inhibition of KOR regulation of dopamine release in the nucleus accumbens. We assessed the persistence of norBNI receptor inactivation in subcellular compartments. Fast-scan cyclic voltammetry recordings confirmed that presynaptic inhibition of dopamine release by the KOR agonist U69,593 was not blocked by in vivo pretreatment with norBNI under conditions that prevented KOR-mediated aversion and analgesia. We employed a novel in vivo proxy sensor of KOR activation, adenovirus associated double floxed inverted-HyPerRed, and demonstrated that KOR activation stimulates cJun kinase-dependent reactive oxygen species (ROS) production in somatic regions of ventral tegmental area dopamine neurons, but did not activate ROS production in dopamine terminals. The compartment selective action helps explain how dopamine somatic, but not terminally expressed, KORs are inactivated by norBNI. These results further elucidate molecular signaling mechanisms mediating receptor-inactivating KOR antagonist action and advance medication development for this novel class of stress-resilience medications. SIGNIFICANCE STATEMENT: Kappa opioid receptor (KOR) antagonists are being developed as novel proresilience therapeutics for the treatment of mood and substance use disorders. This study showed that the long-acting KOR antagonists are affected by both the sex of the animal and the subcellular compartment in which the receptor is expressed.

Novel 1,2,4-Oxadiazole Derivatives in Drug Discovery

Five-membered 1,2,4-oxadiazole heterocyclic ring has received considerable attentionbecause of its unique bioisosteric properties and an unusually wide spectrum of biological activities.Thus, it is a perfect framework for the novel drug development. After a century since the1,2,4-oxadiazole have been discovered, the uncommon potential attracted medicinal chemists'attention, leading to the discovery of a few presently accessible drugs containing 1,2,4-oxadiazoleunit. It is worth noting that the interest in a 1,2,4-oxadiazoles' biological application has been doubledin the last fifteen years. Herein, after a concise historical introduction, we present a comprehensiveoverview of the recent achievements in the synthesis of 1,2,4-oxadiazole-based compounds and themajor advances in their biological applications in the period of the last five years as well as briefremarks on prospects for further development.

Kappa Opioid Receptor Antagonists as Potential Therapeutics for Mood and Substance Use Disorders

The kappa opioid receptor (KOR) and its primary cognate ligands, the dynorphin peptides, are involved in diverse physiological processes. Disruptions to the KOR/dynorphin system have been found to likely play a role in multiple neuropsychological disorders, and hence KOR has emerged as a potential therapeutic target. Targeting KOR is complicated by close homology to the mu and delta opioid receptors (MOR and DOR), and many KOR ligands have at least moderate affinity to MOR and/or DOR. Animal models utilizing primarily very long-lasting selective KOR antagonists (>3 weeks following a single dose) have demonstrated that KOR antagonism attenuates certain anxiety-like and depression-like behaviors and blocks stress- and cue-induced reinstatement to drug seeking. Recently, relatively selective KOR antagonists with medication-like pharmacokinetic and pharmacodynamic properties and durations of action have been developed. One of these, JNJ-67953964 (also referred to as CERC-501, LY2456302, OpraKappa or Aticaprant) has been studied in humans, and shown to be safe, relatively KOR selective, and able to substantially attenuate binding of a KOR PET tracer to CNS localized KOR for greater than 24 h. While animal studies have indicated that compounds of this structural class are capable of normalizing withdrawal signs in animal models of cocaine and alcohol dependence and reducing cocaine and alcohol intake/seeking, additional studies are needed to determine the value of these second generation KOR antagonists in treating mood disorders and substance use disorders in humans.

Behavioral Pharmacology of Novel Kappa Opioid Receptor Antagonists in Rats

BACKGROUND

New treatments for stress-related disorders including depression, anxiety, and substance use disorder are greatly needed. Kappa opioid receptors are expressed in the central nervous system, including areas implicated in analgesia and affective state. Although kappa opioid receptor agonists share the antinociceptive effects of mu opioid receptor agonists, they also tend to produce negative affective states. In contrast, selective kappa opioid receptor antagonists have antidepressant- and anxiolytic-like effects, stimulating interest in their therapeutic potential. The prototypical kappa opioid receptor antagonists (e.g., norBNI, JDTic) have an exceptionally long duration of action that complicates their use in humans, particularly in tests to establish safety. This study was designed to test dose- and time-course effects of novel kappa opioid receptor antagonists with the goal of identifying short-acting lead compounds for future medication development.

METHODS

We screened 2 novel, highly selective kappa opioid receptor antagonists (CYM-52220 and CYM-52288) with oral efficacy in the warm water tail flick assay in rats to determine initial dose and time course effects. For comparison, we tested existing kappa opioid receptor antagonists JDTic and LY-2456302 (also known as CERC-501 or JNJ-67953964).

RESULTS

In the tail flick assay, the rank order of duration of action for the antagonists was LY-2456302 < CYM-52288 < CYM-52220 << JDTic. Furthermore, LY-2456302 blocked the depressive (anhedonia-producing) effects of the kappa opioid receptor agonist U50,488 in the intracranial self-stimulation paradigm, albeit at a higher dose than that needed for analgesic blockade in the tail flick assay.

CONCLUSIONS

These results suggest that structurally diverse kappa opioid receptor antagonists can have short-acting effects and that LY-2456302 reduces anhedonia as measured in the intracranial self-stimulation test.