Selective Kappa-opioid Receptor Agonist Research Articles

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.

Effect of U50488, a selective kappa opioid receptor agonist and levetiracetam against lithium-pilocarpine-induced status epilepticus, spontaneous convulsive seizures and related cognitive impairment

PurposeKappa opioid receptor (KOR) agonists have anticonvulsant effect but their antiepileptogenic effect is unknown. U50488, a selective KOR agonist is used to determine its effect on status epilepticus (SE), spontaneous convulsive seizures (SS) and cognitive impairment in rat lithium-pilocarpine SE model. Effect of an antiepileptic drug levetiracetam is also studied. MethodMale Wistar rats with SE were divided into three groups namely, LiP, LiP + U50488 (10 mg/kg, i.p.) and LiP + levetiracetam (400 mg/kg, i.p.) group. SE was terminated after 90 min of its onset with diazepam (15 mg/kg, i.p.) and phenobarbitone (25 mg/kg, i.p.). Drug treatment was started after 15 min of onset of SE and repeated once after 4 h. Rats were video monitored 12 h daily (9 AM to 9 PM) to determine severity of SE using modified Racine scale and onset and frequency of SS from day 0 to day 21. Morris water maze (MWM) test was done at baseline i.e. day −1 (before lithium administration) and day 22, to assess cognitive impairment. ResultsAs compared to LiP, U50488 decreased the severity of SE (1.98 ± 0.13 vs 2.95 ± 0.12; p-value < 0.0001) but not levetiracetam (2.62 ± 0.09; p-value = 0.3112). Survival increased with both U50488 (90%, n = 10) and levetiracetam (81.8%, n = 11) as compared to NS (56.2%, n = 16). No effect on onset and frequency of SS was found in U50488/levetiracetam group. U50488 improved seizures-induced cognitive impairment. Levetiracetam group showed thigmotactic (wall hugging) behaviour in MWM in 8 out of 9 rats. ConclusionAcute treatment with U50488, a kappa opioid receptor agonist has a beneficial effect on SE, SE-related mortality and memory impairment. The dual protective effect of U50488 on seizures and related cognitive impairment is advantageous over currently used antiseizure drugs which are known to cause cognitive impairment.

Assessment of the physical dependence potential of difelikefalin: randomized placebo-controlled study in patients receiving hemodialysis.

Difelikefalin is a selective kappa opioid receptor agonist approved for treating moderate-to-severe pruritus in adults undergoing hemodialysis (HD). Difelikefalin is not a controlled substance under the Controlled Substances Act. This study assessed the potential for developing physical dependence on difelikefalin in patients undergoing HD. Eligible patients received open-label difelikefalin after each dialysis session for 3 weeks before entering a 2-week double-blind phase, when they were randomized to either continue difelikefalin or to switch to receiving placebo. Signs of physical withdrawal were assessed using the Clinical Opiate Withdrawal Scale (COWS), several patient-reported scales, and physiological measures. The primary endpoint was the between-group difference in mean maximum COWS total scores during the double-blind phase; the mean difference (placebo - difelikefalin) was compared against a pre-defined noninferiority limit (+4). Thirty-five patients (57.1% male; 91.4% Black or African American; median [range] age 58 [28-77] years) were included, of which 30 were randomized (placebo, n=14; difelikefalin, n=16). The least square mean difference in maximum COWS total scores was 0.52 (95% confidence interval [CI]: -0.56, 1.59). The upper CI limit (1.59) was below +4, indicating that patients who discontinued difelikefalin (placebo group) had similar withdrawal scores to patients who continued difelikefalin. Additional assessments supported the COWS results, showing no meaningful differences between groups in physiological measures or in patient-reported measures of sleep or physical withdrawal. These results demonstrate that abruptly discontinuing chronic difelikefalin treatment in patients undergoing HD does not produce signs or symptoms of physical withdrawal.

Phase 2 Trial of Difelikefalin in Notalgia Paresthetica

BackgroundNotalgia paresthetica is a neuropathic disorder characterized by pruritus in a circumscribed region of the upper back. Difelikefalin, a selective kappa opioid receptor agonist, has shown efficacy in other chronic pruritic conditions and is being investigated for the treatment of notalgia paresthetica.MethodsIn this phase 2, double-blind, placebo-controlled trial, we randomly assigned, in a 1:1 ratio, patients with moderate-to-severe pruritus caused by notalgia paresthetica to receive 2 mg of oral difelikefalin or placebo twice daily for 8 weeks. The primary outcome was the change from baseline at week 8 in the weekly mean score on the daily Worst Itch Numeric Rating Scale (WI-NRS; scores range from 0 [no itch] to 10 [worst itch imaginable]). The secondary clinical outcomes were itch-related quality-of-life and itch-related sleep measures.ResultsA total of 126 patients were enrolled; 62 patients were assigned to receive difelikefalin, and 63 were assigned to receive placebo. One patient who had been assigned to receive difelikefalin withdrew consent before the first dose and is not included in the main analyses. The mean baseline WI-NRS score was 7.6 (indicating severe itch) in each group. The change from baseline in the weekly mean WI-NRS score at week 8 was −4.0 points in the difelikefalin group and −2.4 points in the placebo group (difference in change, −1.6 points; 95% confidence interval, −2.6 to −0.6; P=0.001). The results for the secondary outcomes generally did not support those of the primary analysis. Headache, dizziness, constipation, and increased urine output occurred more frequently in the difelikefalin group than in the placebo group.ConclusionsAmong patients with notalgia paresthetica, oral treatment with difelikefalin resulted in modestly greater reductions in itch intensity scores than placebo over a period of 8 weeks but was associated with adverse events. Larger and longer trials are needed to assess the efficacy and safety of difelikefalin treatment in this disorder. (Funded by Cara Therapeutics; KOMFORT ClinicalTrials.gov number, NCT04706975.)

Effects of a New Kappa Agonist (Fluorophenyl Derivative of Imidazo[1,2-&lt;i&gt;а&lt;/i&gt;]benzimidazole) on the Rat Genome

Selective kappa-opioid receptor (KOR) agonists are considered a promising group of substances for developing opioid analgesics characterised with an original mechanism of action without the risk of respiratory depression and drug addiction. Previous studies identified a fluorophenyl derivative of imidazo[1,2-a]benzimidazole (RU-1205) with a KOR-based mechanism of analgesic action established in in vitro and in vivo experiments.The aim of the study was to assess the effect of 9-(2-morpholinoethyl)-2-(4-fluorophenyl)imidazo[1,2-a]benzimidazole dihydrochloride on the level of DNA damage in rats after a single subcutaneous injection.Materials and methods. The study was conducted in adult white outbred laboratory rats of both sexes. DNA damage was estimated using the comet assay. The study involved a single subcutaneous injection of an aqueous solution of RU-1205 in three doses: 1, 10, and 100 mg/kg. The authors used intraperitoneal methyl methanesulfonate (40 mg per kg of animal body weight) as a positive control and 0.9% NaCl (100 μL per 100 g of animal body weight) as a negative control.Results. A single subcutaneous injection of RU-1205 to rats did not produce a significant dose-dependent increase in % tail DNA when compared with the state of the corresponding organ/tissue cell genome in negative control animals after normal saline administration at the same time points. In the negative control groups, % tail DNA in cells of various organs/tissues ranged from 1.83% to 3.82% (median values [25–75%]). On the contrary, the administration of 40 mg/kg of genotoxic methyl methanesulfonate led to an increase in damaged DNA in all studied organs and tissues when compared with negative control animals.Conclusions. The study of 9-(2-morpholinoethyl)-2-(4-fluorophenyl)imidazo[1,2-a] benzimidazole dihydrochloride genotoxicity demonstrated that a single subcutaneous injection of 1, 10, or 100 mg/kg of RU-1205 to rats did not damage the cell genome of the studied organs.

Agonist-Promoted Phosphorylation and Internalization of the Kappa Opioid Receptor in Mouse Brains: Lack of Connection With Conditioned Place Aversion.

Selective kappa opioid receptor (KOR) agonists are promising antipruritic agents and analgesics. However, clinical development of KOR agonists has been limited by side effects, including psychotomimetic effects, dysphoria, and sedation, except for nalfurafine, and recently. CR845 (difelikefalin). Activation of KOR elicits G protein- and β-arrestin-mediated signaling. KOR-induced analgesic and antipruritic effects are mediated by G protein signaling. However, different results have been reported as to whether conditioned place aversion (CPA) induced by KOR agonists is mediated by β-arrestin signaling. In this study, we examined in male mice if there was a connection between agonist-promoted CPA and KOR phosphorylation and internalization, proxies for β-arrestin recruitment in vivo using four KOR agonists. Herein, we demonstrated that at doses producing maximal effective analgesic and antiscratch effects, U50,488H, MOM-SalB, and 42B, but not nalfurafine, promoted KOR phosphorylation at T363 and S369 in mouse brains, as detected by immunoblotting with phospho-KOR-specific antibodies. In addition, at doses producing maximal effective analgesic and antiscratch effects, U50,488H, MOM-SalB, and 42B, but not nalfurafine, caused KOR internalization in the ventral tegmental area of a mutant mouse line expressing a fusion protein of KOR conjugated at the C-terminus with tdTomato (KtdT). We have reported previously that the KOR agonists U50,488H and methoxymethyl salvinorin B (MOM-SalB) cause CPA, whereas nalfurafine and 42B do not, at doses effective for analgesic and antiscratch effects. Taken together, these data reveal a lack of connection between agonist-promoted KOR-mediated CPA with agonist-induced KOR phosphorylation and internalization in male mice.

MO424: Relationship Between Itch Intensity and Patient-Reported Disease Improvement in Patients With Chronic Kidney Disease-Associated Pruritus Treated With Difelikefalin

Abstract BACKGROUND AND AIMS Chronic kidney disease-associated pruritus (CKD-aP) is a common condition in patients with CKD undergoing haemodialysis (HD), leading to poor sleep quality, reduced quality of life and depression. Difelikefalin is a selective kappa opioid receptor agonist approved in the United States for the treatment of moderate-to-severe pruritus in adults undergoing HD, which significantly reduced itch intensity in HD patients with CKD-aP in the Phase 3 KALM-1 and KALM-2 multicentre, placebo-controlled trials. The KALM trials used the Worst-Itching Numerical Rating Scale (WI-NRS) to assess itch-intensity [range from 0 (no itching) to 10 (worst itching imaginable)] and the Patient Global Impression of Change (PGIC) to assess the patient's overall perception of change in itch (range from ‘very much improved’ to ‘very much worse’) relative to the start of the study. The aim of this post hoc analysis was to establish how the WI-NRS score relates to the PGIC and to assess a clinically relevant change in WI-NRS. METHOD A post hoc analysis of data from KALM-1 and KALM-2 was performed. Studies enrolled maintenance HD patients with mean baseline 24-h WI-NRS score &amp;gt; 4 in KALM-1 or ≥ 5 in KALM-2. Patients were randomized 1:1 to intravenous difelikefalin 0.5 mcg/kg or placebo three times/week for 12 weeks. Patients completed the PGIC at the end of the 12-week, double-blind treatment period, as well as completing the WI-NRS daily. Cumulative distribution function (CDF) analyses were performed at Week 12. Results are reported for 50% CDF at week 12. RESULTS For the overall population, ‘very much improved’ corresponded to a 5-to-6-point WI-NRS reduction; ‘much improved’ corresponded to a 3-to-4-point WI-NRS reduction; and ‘minimally improved’ corresponded to a 1-to-2-point WI-NRS reduction (Figure 1). CONCLUSION There is a clear relationship between the PGIC and the WI-NRS, with greater reductions in itch severity corresponding to perceptions of better overall improvement in itch following treatment with difelikefalin. A 3-point reduction in WI-NRS can be considered clinically relevant, since it was perceived by patients as equivalent to their itch being ‘much improved’. This analysis further corroborates similar clinically meaningful changes in pruritus, previously reported in patients with CKD-aP, as ≥ 3-points [1, 2].

MO467: Time to Improvement of Itch Intensity in Patients With Chronic Kidney Disease-Associated Pruritus Treated With Difelikefalin

Abstract BACKGROUND AND AIMS Chronic kidney disease-associated pruritus (CKD-aP) is frequently reported by patients with CKD undergoing haemodialysis (HD), impacting sleep quality, quality of life and depression. Difelikefalin is a selective kappa-opioid receptor agonist approved in the United States for the treatment of moderate-to-severe pruritus in adults undergoing HD. In the Phase 3 KALM-1 and KALM-2 multicentre, placebo-controlled trials, difelikefalin significantly reduced itch intensity in HD patients with CKD-aP. The KALM trials used the Worst-Itching Numerical Rating Scale (WI-NRS) to assess itch intensity [range from 0 (no itching) to 10 (worst itching imaginable)], for which a ≥ 3-point improvement has been validated as a clinically relevant reduction in itch in this patient population. The aim of this post hoc analysis was to determine the time to clinically relevant improvement in WI-NRS score in patients with CKD-aP treated with difelikefalin. METHOD The KALM-1 and KALM-2 studies enrolled maintenance HD patients with moderate-to-severe CKD-aP [mean baseline 24-h WI-NRS score &amp;gt; 4 (KALM-1) or ≥ 5 (KALM-2)]. Patients were randomized 1:1 to intravenous difelikefalin 0.5 mcg/kg or placebo 3 times/week for 12 weeks. Patients completed the WI-NRS questionnaire daily. A post hoc analysis was performed to assess the cumulative proportion of patients achieving a ≥ 3-point improvement in WI-NRS score each week. RESULTS Two-thirds of patients treated with difelikefalin who reported a response did so within 4 weeks. A further 4 weeks of treatment resulted in over 90% of difelikefalin responders reporting a clinically relevant improvement (Figure 1). Fewer patients receiving placebo reported a reduction in itch at any time during treatment (44% versus 57% of difelikefalin-treated patients) and any improvements in pruritus were reported later than for difelikefalin-treated patients. CONCLUSION A clinically relevant response to difelikefalin is likely to be reported by most patients within 4 weeks and almost all patients benefit from improvement after a further 4 weeks of treatment. Compared with patients receiving a placebo, those treated with difelikefalin are more likely to experience clinically-relevant reductions in itch, as well as an earlier improvement in pruritus.

ZYKR1, a novel, potent, and peripherally selective kappa opioid receptor agonist reduces visceral pain and pruritus in animal models

Opioid receptor agonists are effective analgesic agents. Central activation of the mu and/or kappa opioid receptors (KOR) is associated with CNS side effects, which limits their effectiveness. Recent studies indicated that peripherally restricted, selective KOR agonists were potent analgesics and devoid of CNS-related side effects. To confirm this hypothesis, we designed a novel, potent, and peripherally restricted KOR-selective agonist, ZYKR1. The analgesic efficacy, brain penetration and safety of ZYKR1 were assessed in pre-clinical models. ZYKR1 showed KOR agonistic activity in the cAMP assay, with an EC50 of 0.061 nM and more than 105-fold selectivity over the mu and delta opioid receptors (EC50 > 10 μM). ZYKR1 was not found to bind mu, delta opioid, and NOP receptors in radioligand binding assays. ZYKR1 produced concentration-dependent inhibition of electrically evoked contractions in isolated mouse vas deferens with an IC50 of 1.6 nM ZYKR1 showed peripheral restriction and potent analgesic efficacy in various in-vivo animal models (acetic acid induced visceral pain mouse model, ED50: 0.025 mg/kg, IV; ovariohysterectomy induced postoperative pain rat model, ED50: 0.023 mg/kg, IV; and C48/80 induced pruritus mouse model, ED50: 0.063 mg/kg, IV). In addition, ZYKR1 was devoid of motor coordination, physical dependence, dysphoria, and respiratory depression at 30, 400, 10 and 10-fold of efficacy dose, respectively. In conclusion, ZYKR1 has potent antinociceptive action in visceral pain and pruritus with limited CNS side effects in preclinical models owing to its peripheral restriction.

Oral Difelikefalin Reduces Atopic Dermatitis–Associated Pruritus

Many patients with atopic dermatitis (AD) exhibit itch that is disproportionate to rash. Limited therapies specifically target itch in AD. Difelikefalin (DFK), a selective kappa-opioid receptor agonist, is being developed for chronic pruritic conditions.

Antinociceptive and Antipruritic Effects of HSK21542, a Peripherally-Restricted Kappa Opioid Receptor Agonist, in Animal Models of Pain and Itch.

Kappa opioid receptor (KOR) agonists have been promising therapeutic candidates, owing to their potential for relieving pain and treating intractable pruritus. Although lacking morphine-like central nervous system (CNS) effects, KOR agonists do elicit sedation, dysphoria and diuresis which seriously impede their development. Peripherally-restricted KOR agonists have a poor ability to penetrate into the CNS system, so that CNS-related adverse effects can be ameliorated or even abolished. However, the only approved peripherally-restricted KOR agonist CR845 remains some frequent CNS adverse events. In the present study, we aim to address pharmacological profiles of HSK21542, with an expectation to provide a safe and effective alternative for patients who are suffering from pain and pruritus. The in vitro experimental results showed that HSK21542 was a selective and potent KOR agonist with higher potency than CR845, and had a brain/plasma concentration ratio of 0.001, indicating its peripheral selectivity. In animal models of pain, HSK21542 significantly inhibited acetic acid-, hindpaw incision- or chronic constriction injury-induced pain-related behaviors, and the efficacy was comparable to CR845 at 15 min post-dosing. HSK21542 had a long-lasting analgesic potency with a median effective dose of 1.48 mg/kg at 24 h post-drug in writhing test. Meanwhile, the antinociceptive activity of HSK21542 was effectively reversed by a KOR antagonist nor-binaltorphimine. In addition, HSK21542 had powerful antipruritic activities in compound 48/80-induced itch model. On the other hand, HSK21542 had a weak ability to produce central antinociceptive effects in a hot-plate test and fewer effects on the locomotor activity of mice. HSK21542 didn’t affect the respiratory rate of mice. Therefore, HSK21542 might be a safe and effective KOR agonist and promising candidate for treating pain and pruritus.

Evaluation of the abuse potential of difelikefalin, a selective kappa-opioid receptor agonist, in recreational polydrug users.

Difelikefalin, a selective kappa‐opioid receptor agonist with limited central nervous system penetration, is being developed for the treatment of chronic pruritic conditions. This randomized, double‐blind, active‐ and placebo‐controlled, four‐way crossover study was designed to evaluate the abuse potential of difelikefalin in healthy recreational polydrug users. Using a 4 × 4 Williams design, nondependent adult users of opioids and hallucinogens (N = 44) were randomized to receive single intravenous (i.v.) injections of difelikefalin at supratherapeutic doses (5 and 15 mcg/kg); pentazocine (0.5 mg/kg), a schedule IV mu‐opioid partial agonist and kappa‐opioid receptor agonist; and placebo. The abuse potential of difelikefalin was compared with pentazocine and placebo using the maximal score (maximum effect [Emax]) of the Drug Liking visual analog scale (VAS; primary end point), along with multiple secondary end points of subject‐rated measures and pupillometry. Difelikefalin produced significantly lower Drug Liking VAS Emax, and lower peak positive, sedative, and perceptual effects compared with pentazocine. These effects of difelikefalin were small, brief, and not dose‐dependent, although marginally greater than those observed with placebo. Neither dose of difelikefalin elicited significant negative or hallucinogenic effects. On end‐of‐session measures of overall drug liking and willingness to take the drug again, difelikefalin did not differ from placebo, indicating subjects neither liked nor disliked the effects overall and did not feel motivated to take the drug again. Consistent with its lack of mu agonist activity, difelikefalin did not induce miosis compared with pentazocine. All treatments were generally well‐tolerated. This study indicates that difelikefalin presents a low potential for abuse.

Estimation of ZYKR1 in human urine and plasma utilizing LC-MS/MS positive electrospray ionization; a kappa opioid receptor (KOR) agonist

ZYKR1, a short chain novel peptide with selective kappa opioid receptor agonist activity used as analgesics for the treatment of pain management. A sensitive and selective LC-MS/MS assay was developed and validated for estimation of ZYKR1 in human urine and plasma. ZY17258, an analogue compound was used as an internal standard. ZYKR1 was quantified using a selective reaction monitoring in electrospray ionization positive mode. The chromatographic separation was performed using mobile phase consisted of 0.05% v/v formic acid in water and methanol in gradient elution by analytical column Kinetex C8, 100 A°, 5 µm, 100 × 4.6 mm with 8.0 min analytical run time. Solid Phase extraction technique was used for purification of ZYKR1 and IS from human urine and plasma. The calibration curves were linear over range of 0.300 ng/mL to 300 ng/mL and 0.500 ng/mL to 500 ng/mL for human urine and plasma, respectively. No matrix effect and no significant carryover were observed. The extraction recovery was consistent and ranged from about 85% to 93% in human urine and in plasma respectively. Inter-day and intra-day accuracy (bias, %) and precision (CV, %) was −11.11 to 5.91 % and −2.25 to 6.65 % in human urine and −2.74 to 7.17 % and 2.24 to 15.18 % in plasma respectively were well within the acceptance criteria. Both the assays were devoid of endogenous matrix interference and commonly used concomitant drug interference. The validated assays were used for estimation of ZYKR1 from clinical pharmacokinetic study sample bioanalysis in healthy human subjects.

Intranasal Salvinorin A Improves Long-term Neurological Function via Immunomodulation in a Mouse Ischemic Stroke Model

Salvinorin A (SA), a highly selective kappa opioid receptor agonist, has been shown to reduce brain infarct volume and improve neurological function after ischemic stroke. However, the underlying mechanisms have not been fully understood yet. Therefore, we explored whether SA provides neuroprotective effects by regulating the immune response after ischemic stroke both in the central nervous system (CNS) and peripheral circulation. In this study, adult male mice were subjected to transient Middle Cerebral Artery Occlusion (tMCAO) and then were treated intranasally with SA (50 μg/kg) or with the vehicle dimethyl sulfoxide (DMSO). Multiple behavioral tests were used to evaluate neurofunction. Flow cytometry and immunofluorescence staining were used to evaluate the infiltration of peripheral immune cells into the brain. The tracer cadaverine and endogenous immunoglobulin G (IgG) extravasation were used to detect blood brain barrier leakage. We observed that SA intranasal administration after ischemic stroke decreased the expression of pro-inflammatory factors in the brain. SA promoted the polarization of microglia/macrophages into a transitional phenotype and decreased the pro-inflammatory phenotype in the brain after tMCAO. Interestingly, SA treatment scarcely altered the number of peripheral immune cells but decreased the macrophage and neutrophil infiltration into the brain at 24 h after tMCAO. Furthermore, SA treatment also preserved BBB integrity, reduced long-term brain atrophy and white matter injury, as well as improved the long-term neurofunctional outcome in mice. In this study, intranasal administration of SA improved long-term neurological function via immuno-modulation and by preserving blood–brain barrier integrity in a mouse ischemic stroke model, suggesting that SA could potentially serve as an alternative treatment strategy for ischemic stroke.Graphic

Discovery of an M-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethanotetrahydronorthebaine as a Selective, Potent, and Orally Active κ-Opioid Receptor Agonist with an Improved Central Nervous System Safety Profile.

The search for selective kappa opioid receptor (κOR) agonists with an improved safety profile is an area of interest in opioid research. In this work, a series of m-substituted analogs were designed, synthesized, and assayed, resulting in the identification of compound 6c (SLL-1206) as a κOR agonist with single-digit nanomolar activities. The subtype selectivity of compound 6c appeared to be a consequence of an enormous decrease in the affinity for μOR and δOR, rather than a significant increase in the affinity for κOR, which was not the case for SLL-039, another selective and potent κOR agonist identified in our previous work. Besides reduced central nervous system effects, SLL-1206 exhibited substantially improved physicochemical and pharmacokinetic properties compared with SLL-039, with increases of over 20-fold in aqueous solubility and approximately 40-fold in oral bioavailability in rats.

Effect of difelikefalin, a selective kappa opioid receptor agonist, on respiratory depression: A randomized, double-blind, placebo-controlled trial.

Difelikefalin, a selective kappa opioid receptor agonist designed to limit central nervous system (CNS) penetration, is under development for the treatment of pruritus. Its hydrophilic, small‐peptidic structure limits CNS entry, minimizing potential CNS‐mediated adverse events (AEs). This study assessed the effect of difelikefalin on key relevant measures of respiratory depression in healthy volunteers. This single‐center, randomized, double‐blind, placebo‐controlled, three‐way crossover study enrolled healthy, nonsmoking volunteers. Subjects were randomized to 1 of 3 treatment sequences of difelikefalin (1.0 or 5.0 mcg/kg i.v.) or placebo on sequential days with an intervening 24 (±2) h washout period. The primary end points included incidence of increased end‐tidal carbon dioxide (ETCO2) greater than or equal to 10 mm Hg versus baseline or a level greater than 50 mm Hg sustained greater than or equal to 30 seconds, and incidence of reduction in saturation of peripheral oxygen (SpO2) to less than 92% sustained greater than or equal to 30 seconds. Secondary end points included incidence of reduced respiratory rate and other safety assessments. Fifteen subjects were randomized and completed the study. No subject on placebo or difelikefalin met the increased ETCO2 or reduced SpO2 primary end point criteria for respiratory depression. All respiratory measures in each group remained near baseline values during 4‐h postdose observations. No subject met the reduced respiratory rate criterion or experienced clinically significant changes in ETCO2, SpO2, or respiratory rate. The most commonly reported treatment‐emergent AEs (TEAEs; ≥20% of subjects) were paresthesia, hypoesthesia, and somnolence in the difelikefalin arms. All TEAEs were mild and resolved without intervention. Difelikefalin 1.0 and 5.0 mcg/kg i.v. did not produce respiratory depression.

FC 022AN OPEN-LABEL, MULTICENTER STUDY TO EVALUATE THE SAFETY AND EFFECTIVENESS OF INTRAVENOUS DIFELIKEFALIN IN PATIENTS WITH MODERATE-TO-SEVERE CKD–ASSOCIATED PRURITUS UNDERGOING HEMODIALYSIS

Abstract Background and Aims Chronic kidney disease–associated pruritus (CKD-aP) is a common and highly distressing condition in patients undergoing hemodialysis (HD). CKD-aP is associated with sleep disturbances, significant quality-of-life (QoL) impairment, and increased morbidity and mortality. Difelikefalin (DFK) is a selective kappa opioid receptor agonist in development for CKD-aP that has minimal central nervous system penetration. In placebo-controlled phase 3 trials of patients with moderate to severe CKD-aP undergoing HD, intravenous (IV) DFK had an acceptable safety profile and demonstrated significant reductions vs placebo in itch intensity. We report safety and effectiveness outcomes, including itch-related QoL and sleep measures, from a phase 3 open-label study of DFK in patients with moderate to severe CKD-aP. Method This multicenter, open-label study conducted in the United States and Europe enrolled patients with moderate to severe CKD-aP (mean baseline 24-hour Worst Itching Intensity Numerical Rating Scale [WI-NRS] score ≥5) undergoing HD for ≥3 months. Patients received IV DFK 0.5 mcg/kg 3 times/week at the end of each HD session for up to 12 weeks. Predefined effectiveness endpoints at week 12 included ≥3-point and ≥4-point improvement in the weekly mean of the 24-hour WI-NRS score (range from 0 [no itching] to 10 [worst itching imaginable]). Change from baseline in QoL at week 12 was assessed using the 5-D itch and Skindex-10, multidimensional itch-related questionnaires validated in CKD-aP (higher scores indicate worse QoL). The proportion of patients with no problems (score of 1) on the skin irritation and self-confidence domains of the EQ-PSO questionnaire was evaluated. Post hoc endpoints included complete resolution in WI-NRS (≥75% of week 12 scores 0 or 1) and Sleep Quality Questionnaire total score assessments (range of possible scores, 0 [did not interfere] to 10 [completely interfered]), including ≥3-point and ≥4-point improvement in weekly mean score and complete resolution (all scores of 0) at week 12. Safety assessments and adverse events (AEs) were evaluated. Data were summarized descriptively. Results Among 222 patients who received DFK, 197 (88.7%) completed the study. At baseline, mean ±SD age was 58.1 ±12.8 years and 54.5% of patients were male. Baseline mean ±SD WI-NRS score was 7.6 ±1.3, Sleep Quality score was 6.6 ±2.2, 5-D itch score was 17.1 ±3.5, and Skindex-10 score was 32.9 ±14.3. At week 12, the majority of patients achieved ≥3-point and ≥4-point improvement in WI-NRS (73.7% and 59.3%) and Sleep Quality score (66.0% and 56.7%). Complete resolution of WI-NRS and Sleep Quality score was observed in 29.4% and 19.1% of patients, respectively (Figure). DFK was associated with improvements in mean 5-D itch (−7.1 ±4.3) and Skindex-10 (−21.0 ±15.6) scores at week 12. The proportion of patients reporting no problems in the skin irritation EQ-PSO domain increased from 1.4% at baseline to 28.9% at week 12, and self-confidence EQ-PSO domain scores increased from 63.5% at baseline to 73.2% at week 12. Overall, 64.4% (143/222) of patients reported ≥1 treatment-emergent AE (TEAE). The most commonly reported TEAEs (≥4% of patients) were diarrhea (5.0% [11/222]), nausea (4.5% [10/222]), and hyperkalemia (4.1% [9/222]). Serious TEAEs were reported by 20.3% (45/222) of patients; no serious TEAEs were related to study drug. Conclusion In this phase 3 open-label study in patients with moderate to severe CKD-aP undergoing HD, DFK was generally well tolerated with an acceptable safety profile. DFK demonstrated effectiveness based on reduction of itch intensity, and improvements in sleep quality and itch-related QoL at week 12. The majority of patients reported ≥3-point or ≥4-point improvement in WI-NRS and Sleep Quality scores, with some reporting complete resolution. Findings from this open-label study provide insight into the potential real-world effectiveness of DFK in moderate to severe CKD-aP.

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

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

The pain and the itch

Itch Spinal and epidural morphine administration is used in the clinical setting as an analgesic for surgical procedures and in women during labor. However, delivery through these routes is associated with the development of itch. The molecular basis of this unwanted side effect has been unclear. Nguyen et al. found that morphine-induced itch did not involve mast cells but rather was mediated by the activity of spinal dynorphin neurons and subsequent inhibition of kappa-opioid receptor–expressing neurons in mice. Concomitant administration of morphine and nalfurafine, a selective kappa-opioid receptor agonist, inhibited itch without affecting morphine's analgesic properties in mice and monkeys, suggesting that this approach might also be effective in the clinical setting. Sci. Transl. Med. 13 , eabc3774 (2021).

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.