The kappa-opioid receptor agonist, triazole 1.1, reduces oxycodone self-administration and enhances oxycodone-induced thermal antinociception in male rats

Structure-activity relationship analysis of meta-substituted N-cyclopropylmethyl-nornepenthones with mixed KOR/MOR activities

Substance Use Disorder (SUD) remains a significant global challenge, with current treatment options offering limited efficacy. Agonists targeting the kappa opioid receptor (KOR), especially those with additional mu opioid receptor (MOR) antagonistic activity, have shown promise in addressing SUD. In this study, a series of meta-substituted N-cyclopropylmethyl-nornepenthone derivatives were designed and synthesized, and their biological activities were assessed, leading to the identification of a KOR/MOR dual modulator, compound 10a. Unlike its para-positional isomer SLL-1062, where KOR activity is completely abolished, compound 10a displayed a single-digit nanomolar affinity for KOR, while its binding profiles for MOR and delta opioid receptor (DOR) were comparable to those of SLL-1062. Functional assays in vitro confirmed that compound 10a exhibited agonistic activity at KOR and antagonistic activity at MOR. The molecular basis for the introduction of a KOR component into compound 10a was further elucidated. Although compound 10a did not produce apparent antinociception in vivo, it effectively blocked morphine-induced antinociception and intestinal motility inhibition in rodent models. This study provides valuable insights into the development of MOR/KOR dual modulators and presents new lead compounds for potential treatments for SUD.

Morphine-Induced Antinociception Is Potentiated and Dopamine Elevations Are Inhibited by the Biased Kappa Opioid Receptor Agonist Triazole 1.1

Traditional analgesic opioid compounds, which act through μ opioid receptors (MORs), engender a high risk for misuse and dependence. κ opioid receptor (KOR) activation, a potential target for pain treatment, produces antinociception without euphoric side effects but results in dysphoria and aversion. Triazole 1.1 is a KOR agonist biased toward G-protein coupled signaling, potentially promoting antinociception without dysphoria. We tested whether triazole 1.1 could provide antinociception and its effects in combination with morphine. We employed a lactic acid abdominal pain model, which induced acute pain behaviors, decreased basal dopamine levels in the nucleus accumbens (NAc), and increased KOR function. We administered several interventions including triazole 1.1 (30 mg/kg) and morphine (12 or 24 mg/kg), individually and in combination. Triazole 1.1 alone reduced the pain behavioral response and changes to KOR function but did not prevent the reduction in basal dopamine levels. Morphine not only dose-dependently prevented behavioral pain responses but also elevated NAc dopamine and did not prevent the pain-induced increase in KOR function. However, combining low-dose morphine with triazole 1.1 prevents behavioral pain responses, changes to NAc dopamine levels, and changes to KOR function. Therefore, we present triazole 1.1 as a dose-sparing pain treatment to be used in combination with a lower dose of morphine, thus reducing the potential for opioid misuse.

Triazole 187 is a biased KOR agonist that suppresses itch without sedation and induces anxiolytic-like behaviors in mice

Kappa opioid receptor agonists are clinically used to treat pruritis and have therapeutic potential for the treatment of pain and neuropsychiatric disorders. We have previously shown that triazole 1.1 is a G protein signaling-biased KOR agonist, that can suppress itch without producing signs of sedation in mice. This profile was recapitulated in rats and non-human primates however, triazole 1.1 had limited potency as an antipruritic. Here we describe a more potent, G protein signaling-biased agonist, triazole 187. Triazole 187 is a potent antipruritic agent and does not decrease spontaneous locomotor activity; interestingly, it produces anxiolytic-like behaviors in mice, an effect not observed for triazole 1.1. In addition to curbing sedation, triazole 187 produces only mild diuresis, resulting in 30% of urine output induced by U50,488H at dose that is 188-fold the antipruritic potency dose. Compounds like triazole 187 may present a means to treat anxiety accompanied by persistent chronic itch while avoiding sedation and diuresis accompanied by typical KOR agonists.

Abstract Figure

Assessment of the antinociceptive, respiratory-depressant, and reinforcing effects of the low pKa fluorinated fentanyl analogs, FF3 and NFEPP

RATIONALE

Recent studies report that fentanyl analogs with relatively low pKa values produce antinociception in rodents without other mu opioid-typical side effects due to the restriction of their activity to injured tissue with relatively low pH values. However, it is unclear if and to what degree these compounds may produce mu opioid-typical side effects (respiratory depression, reinforcing effects) at doses higher than those required to produce antinociception.

OBJECTIVES

The present study compared the inflammatory antinociceptive, respiratory-depressant, and reinforcing effects of fentanyl and two analogs of intermediate (FF3) and low (NFEPP) pKa values in terms of potency and efficacy in male and female Sprague-Dawley rats.

METHODS

Nociception was produced by administration of Complete Freund's Adjuvant into the hind paw of subjects, and antinociception was measured using an electronic Von Frey test. Respiratory depression was measured using whole-body plethysmography. Reinforcing effects were measured in self-administration using a progressive-ratio schedule of reinforcement. The dose ranges tested for each drug encompassed no effect to maximal effects.

RESULTS

All compounds produced full effects in all measures but varied in potency. FF3 and fentanyl were equipotent in antinociception and self-administration, but FF3 was less potent than fentanyl in respiratory depression. NFEPP was less potent than fentanyl in every measure. The magnitude of potency difference between antinociception and other effects was greater for FF3 than for NFEPP or fentanyl, indicating that FF3 had the widest margin of safety when relating antinociception to respiratory-depressant and reinforcing effects.

CONCLUSIONS

Low pKa fentanyl analogs possess potential as safer analgesics, but determining the optimal degree of difference for pKa relative to fentanyl will require further study due to some differences between the current results and findings from prior work with these analogs.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Quantification of observable behaviors following oral administration of oxycodone and nalfurafine in male rhesus monkeys

BACKGROUND

Recent preclinical studies have investigated the atypical kappa-opioid receptor (KOR) agonist, nalfurafine, as a co-formulary with mu-opioid receptor (MOR) agonists as a potential deterrent for misuse. However, no study has investigated effects of nalfurafine combined with a MOR agonist using an oral route of administration. The objective of the current study was to measure behavioral effects of orally administered oxycodone and nalfurafine, alone and combined, in rhesus monkeys using a quantitative behavioral observation procedure.

METHODS

Adult male rhesus monkeys (N=5) were orally administered vehicle, oxycodone (0.56-1.8mg/kg), nalfurafine (0.001-0.0056mg/kg), or mixtures (1.0mg/kg oxycodone/0.001-0.0056mg/kg nalfurafine) in a Jell-O vehicle at multiple timepoints (10-320min). Species-typical and drug-induced behaviors were recorded by observers blinded to conditions.

RESULTS

Oxycodone alone significantly increased scratch and face-rub behaviors without affecting other behaviors. Nalfurafine decreased baseline levels of scratch without affecting other behaviors, and oxycodone-nalfurafine combinations resulted in reduced oxycodone-induced scratching at a dose (0.001mg/kg) that did not produce sedation-like effects. Oxycodone combined with larger nalfurafine doses (0.0032-0.0056mg/kg) also reduced oxycodone induced scratch that were accompanied with sedation-like effects (i.e., increased lip droop).

CONCLUSIONS

Nalfurafine was orally active in rhesus monkeys, and it reduced oxycodone-induced pruritus at a dose that did not produce sedation-like effects that are commonly observed with prototypical KOR agonists. Combinations of low doses of nalfurafine with MOR agonists such as oxycodone may be well-tolerated by humans who are prescribed MOR agonists for the treatment of pain.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Signaling underlying kappa opioid receptor-mediated behaviors in rodents

Kappa opioid receptor (KOR) agonists are potentially useful as analgesic and anti-pruritic agents, for prevention and treatment of substance use disorders, and for treatment of demyelinating diseases. However, side effects of KOR agonists, including psychotomimesis, dysphoria, and sedation, have caused early termination of clinical trials. Understanding the signaling mechanisms underlying the beneficial therapeutic effects and the adverse side effects may help in the development of KOR agonist compounds. In this review, we summarize the current knowledge in this regard in five sections. First, studies conducted on mutant mouse lines (GRK3-/-, p38alpha MAPK-/-, β-arrestin2-/-, phosphorylation-deficient KOR) are summarized. In addition, the abilities of four distinct KOR agonists, which have analgesic and anti-pruritic effects with different side effect profiles, to cause KOR phosphorylation are discussed. Second, investigations on the KOR agonist nalfurafine, both in vitro and in vivo are reviewed. Nalfurafine was the first KOR full agonist approved for clinical use and in the therapeutic dose range it did not produce significant side effects associated with typical KOR agonists. Third, large-scale high-throughput phosphoproteomic studies without a priori hypotheses are described. These studies have revealed that KOR-mediated side effects are associated with many signaling pathways. Fourth, several novel G protein-biased KOR agonists that have been characterized for in vitro biochemical properties and agonist biases and in vivo behavior effects are described. Lastly, possible mechanisms underlying KOR-mediated CPA, hypolocomotion and motor incoordination are discussed. Overall, it is agreed upon that the analgesic and anti-pruritic effects of KOR agonists are mediated via G protein signaling. However, there is no consensus on the mechanisms underlying their side effects. GRK3, p38 MAPK, β-arrestin2, mTOR pathway, CB1 cannabinoid receptor and protein kinase C have been implicated in one side effect or another. For drug discovery, after initial in vitro characterization, in vivo pharmacological characterizations in various behavior tests are still the most crucial steps and dose separation between beneficial therapeutic effects and adverse side effects are the critical determinant for the compounds to be moved forward for clinical development.

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.

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

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

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

Kappa-opioid receptors (KOR) are widely expressed throughout the central nervous system, where they modulate a range of physiological processes depending on their location, including stress, mood, reward, pain, inflammation, and remyelination. However, clinical use of KOR agonists is limited by adverse effects such as dysphoria, aversion, and sedation. Within the drug-development field KOR agonists have been extensively investigated for the treatment of many centrally mediated nociceptive disorders including pruritis and pain. KOR agonists are potential alternatives to mu-opioid receptor (MOR) agonists for the treatment of pain due to their anti-nociceptive effects, lack of abuse potential, and reduced respiratory depressive effects, however, dysphoric side-effects have limited their widespread clinical use. Other diseases for which KOR agonists hold promising therapeutic potential include pruritis, multiple sclerosis, Alzheimer's disease, inflammatory diseases, gastrointestinal diseases, cancer, and ischemia. This review highlights recent drug-development efforts targeting KOR, including the development of G-protein-biased ligands, mixed opioid agonists, and peripherally restricted ligands to reduce side-effects. We also highlight the current KOR agonists that are in preclinical development or undergoing clinical trials.

An updated assessment of the translational promise of G-protein-biased kappa opioid receptor agonists to treat pain and other indications without debilitating adverse effects

Kappa opioid receptor (κOR) agonists lack the abuse liability and respiratory depression effects of clinically used mu opioid receptor (μOR) analgesics and are hypothesized to be safer alternatives. However, κOR agonists have limiting adverse effects of their own, including aversion, sedation, and mood effects, that have hampered their clinical translation. Studies performed over the last 15 years have suggested that these adverse effects could result from activation of distinct intracellular signaling pathways that are dependent on β-arrestin, whereas signaling downstream of G protein activation produces antinociception. This led to the hypothesis that agonists biased away from β-arrestin signaling would have improved therapeutic windows over traditional unbiased agonists and allow for clinical development of analgesic G-protein-biased κOR agonists. Given a recent controversy regarding the benefits of G-protein-biased μOR agonists, it is timely to reassess the therapeutic promise of G-protein-biased κOR agonists. Here we review recent discoveries from preclinical κOR studies and critically evaluate the therapeutic windows of G-protein-biased κOR agonists in each of the adverse effects above. Overall, we find that G-protein-biased κOR agonists generally have improved therapeutic window relative to unbiased agonists, although frequently study design limits strong conclusions in this regard. However, a steady flow of newly developed biased κOR agonists paired with recently engineered behavioral and molecular tools puts the κOR field in a prime position to make major advances in our understanding of κOR function and fulfill the promise of translating a new generation of biased κOR agonists to the clinic.

Application of dose-addition analyses to characterize the abuse-related effects of drug mixtures

Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.