Will peripherally restricted kappa-opioid receptor agonists (pKORAs) relieve pain with less opioid adverse effects and abuse potential?

The efficacy and safety of difelikefalin for pruritus in hemodialysis patients: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND

Chronic kidney disease-associated pruritus (CKD-ap) is a common complication that negatively affects the quality of life. Difelikefalin has emerged as a novel FDA-approved drug to manage CKD-ap. This systematic review and meta-analysis will assess the efficacy and safety of Difelikefalin versus placebo to manage CKD-ap.

METHODS

PubMed, Scopus, WOS, Central, and Embase were systematically searched until November 2023. RevMan was used to perform meta-analysis. Quality assessment was conducted using the Cochrane RoB 2.0 tool. Results were reported as risk ratio (RR) and mean difference (MD) with a 95% confidence interval (CI). PROSPERO ID: (CRD42023485979).

RESULTS

Five RCTs with a total of 896 participants were included. Difelikefalin significantly decreased the weekly mean WI-NRS score (MD: -0.99 [-1.22, -0.75], p ˂ .00001), 5-D itch scale total score (MD: -1.51 [-2.26, -0.76], p > .0001), and Skindex-10 total score (MD: -7.39 [-12.51, -2.28], p = .005), but showed significantly higher adverse events (RR: 1.26 [1.03, 1.55], p = .03), versus placebo. However, there was no significant difference between both groups in serious adverse events (RR: 1.42 [0.78, 2.57], p = .25) or death (RR: 0.81 [0.19, 3.34], p = .77).

CONCLUSION

Difelikefalin appears to be a promising agent for the management of CKD-induced pruritus in patients with end-stage renal disease. However, evidence is still underpowered due to the paucity of the current data; therefore, more robust RCTs are required to confirm the benefit of Difelikefalin.

Intrathecal administration of MCRT produced potent antinociception in chronic inflammatory pain models via μ-δ heterodimer with limited side effects

An important goal in the opioid field is to discover effective analgesic drugs with minimal side effects. MCRT demonstrated potent antinociceptive effects with limited side effects, making it a promising candidate. However, its pharmacological properties and how it minimizes side effects remain unknown. Various mouse pain and opioid side effect models were used to evaluate the antinociceptive properties and safety at the spinal level. The targets of MCRT were identified through cAMP measurement, isolated tissue assays, and pharmacological experiments. Immunofluorescence was employed to visualize protein expression. MCRT displayed distinct antinociceptive effects between acute and chronic inflammatory pain models due to its multifunctional properties at the μ opioid receptor (MOR), µ-δ heterodimer (MDOR), and neuropeptide FF receptor 2 (NPFFR2). Activation of NPFFR2 reduced MOR-mediated antinociception, leading to bell-shaped response curves in acute pain models. However, activation of MDOR produced more effective antinociception in chronic inflammatory pain models. MCRT showed limited tolerance and opioid-induced hyperalgesia in both acute and chronic pain models and did not develop cross-tolerance to morphine. Additionally, MCRT did not exhibit addictive properties, gastrointestinal inhibition, and effects on motor coordination. Mechanistically, peripheral chronic inflammation or repeated administration of morphine and MCRT induced an increase in MDOR in the spinal cord. Chronic administration of MCRT had no apparent effect on microglial activation in the spinal cord. These findings suggest that MCRT is a versatile compound that provides potent antinociception with minimal opioid-related side effects. MDOR could be a promising target for managing chronic inflammatory pain and addressing the opioid crisis.

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

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

Strategies for developing μ opioid receptor agonists with reduced adverse effects

Opioids are currently the most effective and widely used painkillers in the world. Unfortunately, the clinical use of opioid analgesics is limited by serious adverse effects. Many researchers have been working on designing and optimizing structures in search of novel μ opioid receptor(MOR) agonists with improved analgesic activity and reduced incidence of adverse effects. There are many strategies to develop MOR drugs, mainly focusing on new low efficacy agonists (potentially G protein biased agonists), MOR agonists acting on different Gα subtype, targeting opioid receptors in the periphery, acting on multiple opioid receptor, and targeting allosteric sites of opioid receptors, and others. This review summarizes the design methods, clinical applications, and structure-activity relationships of small-molecule agonists for MOR based on these different design strategies, providing ideas for the development of safer novel opioid ligands with therapeutic potential.

Blocking Aδ- and C-fiber neural transmission by sub-kilohertz peripheral nerve stimulation

Introduction

We recently showed that sub-kilohertz electrical stimulation of the afferent somata in the dorsal root ganglia (DRG) reversibly blocks afferent transmission. Here, we further investigated whether similar conduction block can be achieved by stimulating the nerve trunk with electrical peripheral nerve stimulation (ePNS).

Methods

We explored the mechanisms and parameters of conduction block by ePNS via ex vivo single-fiber recordings from two somatic (sciatic and saphenous) and one autonomic (vagal) nerves harvested from mice. Action potentials were evoked on one end of the nerve and recorded on the other end from teased nerve filaments, i.e., single-fiber recordings. ePNS was delivered in the middle of the nerve trunk using a glass suction electrode at frequencies of 5, 10, 50, 100, 500, and 1000 Hz.

Results

Suprathreshold ePNS reversibly blocks axonal neural transmission of both thinly myelinated Aδ-fiber axons and unmyelinated C-fiber axons. ePNS leads to a progressive decrease in conduction velocity (CV) until transmission blockage, suggesting activity-dependent conduction slowing. The blocking efficiency is dependent on the axonal conduction velocity, with Aδ-fibers efficiently blocked by 50-1000 Hz stimulation and C-fibers blocked by 10-50 Hz. The corresponding NEURON simulation of action potential transmission indicates that the disrupted transmembrane sodium and potassium concentration gradients underly the transmission block by the ePNS.

Discussion

The current study provides direct evidence of reversible Aδ- and C-fiber transmission blockage by low-frequency (<100 Hz) electrical stimulation of the nerve trunk, a previously overlooked mechanism that can be harnessed to enhance the therapeutic effect of ePNS in treating neurological disorders.

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

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

Model informed drug development: HSK21542 PBPK model supporting dose decisions in specific populations

HKS21542, a highly selective activator of peripheral kappa opioid receptor agonists, plays a critical role in antinociception and itch inhibition during clinical development. Due to its indication population and elimination characteristics, it is imperative to evaluate the potential HSK21542 systemic exposure in individuals with renal impairment, hepatic impairment, the elderly, and the geriatric population. Here, a physiologically-based pharmacokinetic (PBPK) model for HSK21542 was developed based on in vitro metabolism and transport characteristics and in vivo elimination mechanism. Meanwhile, the potential systemic exposure of HSK21542 in specific populations was evaluated. The predicted results indicated increased systemic exposure in patients with renal impairment, hepatic impairment and in the elderly. Compared to the healthy volunteers aged 20-60 years, the AUC0-24h increased by 52 %-71 % in population with moderate to severe renal impairment, by 46 %-77 % in those with mild to severe hepatic impairment, and by 45 %-85 % in the elderly population aged 65-95-years. Conversely, the pediatric population demonstrated a potential decrease in systemic exposure, ranging from 20 % to 37 % in patients aged 0-17 years due to the physiological characteristics. Combined with the predicted results and the exposure-response relationship observed for HSK21542 and its analog (CR845), dosage regimens were designed for the target population with renal and hepatic impairment, supporting the successfully conducted trials (CTR20201702 and CTR20211940). Moreover, the observed exposure of HSK21542 in the elderly closely matched the predicted results within the same age group. Additionally, based on the predicted results, potential reductions in systemic exposure in pediatric patients should be carefully considered to avoid potential treatment failure in future clinical trials.

In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

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

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

Comparison of Effectiveness and Safety of Oxycodone Hydrochloride and Fentanyl for Post-operative Pain Following Total Hip Arthroplasty: A Randomized Triple-Blind Trial

Background

Total hip replacement (THR) is frequently associated with intense post-surgical pain. Effective pain management is of crucial importance to improving patient's condition and increasing his/her satisfaction in the post-operative time.

Objectives

This study aimed to compare the analgesic effect and safety of oxycodone and fentanyl after THR.

Methods

Seventy-two cases scheduled for elective THR were included in this randomized, triple-blind trial. The patients were equally randomized into 2 groups: Fentanyl group (50 ug of fentanyl) and oxycodone group (oxycodone 4 mg). Drugs were received 20 min prior to the end of the operation.

Results

Post-operative visual analog scale (VAS) measurements at rest and movement at the post-anesthesia care unit (PACU) and in the ward, 2 h, 4 h, and 8 h post-operatively exhibited a significantly reduced value in the oxycodone group compared to the fentanyl group (P-value < 0.05). Time to first rescue for analgesia was delayed significantly in the oxycodone compared to the fentanyl group (P-value < 0.001). Fentanyl consumption (ug) in the 1st post-operative 12 h, 24 h, and 48 h decreased significantly in the oxycodone group compared to the fentanyl group (P-value < 0.001). Post-operative nausea, vomiting, headache, and pruritus were matched between the 2 groups (P > 0.05).

Conclusions

A bolus dose of 4 mg of oxycodone provided superior analgesic efficacy than 50 ug fentanyl as evidenced by significantly lower pain score, delayed onset to first request for analgesia, and the smaller amount of fentanyl consumption at 12, 24, and 48 h post-total hip arthroplasty compared to fentanyl. The incidence of adverse events was comparable between the 2 groups.

Structure-Activity Relationships of a Novel Cyclic Hexapeptide That Exhibits Multifunctional Opioid Agonism and Produces Potent Antinociceptive Activity

The cyclic peptide c[d-Lys2, Asp5]-DN-9 has recently been identified as a multifunctional opioid/neuropeptide FF receptor agonist, displaying potent analgesic activity with reduced side effects. This study utilized Tyr-c[d-Lys-Gly-Phe-Asp]-d-Pro-NH2 (0), a cyclic hexapeptide derived from the opioid pharmacophore of c[d-Lys2, Asp5]-DN-9, as a chemical template. We designed, synthesized, and characterized 22 analogs of 0 with a single amino acid substitution to investigate its structure-activity relationship. Most of these cyclic hexapeptide analogs exhibited multifunctional activity at μ and δ opioid receptors (MOR and DOR, respectively) and produced antinociceptive effects following subcutaneous administration. The lead compound analog 15 showed potent agonistic activities at the MOR, κ opioid receptor (KOR), and DOR in vitro and produced a strong and long-lasting analgesic effect through peripheral MOR and KOR in the tail-flick test. Further biological evaluation identified that analog 15 did not cause significant side effects such as tolerance, withdrawal, or reward liability.

Difelikefalin for Hemodialysis Patients with Pruritus in Japan

BACKGROUND: Difelikefalin, a peripherally specific selective agonist of kappa opioid receptors, has been approved for the treatment of pruritus in hemodialysis patients in the United States. However, there is limited evidence for postdialysis intravenous use in non-U.S. populations. METHODS: In this double-blind, placebo-controlled, phase 3 trial in Japan, patients with moderate to severe pruritus were randomly allocated 1:1 to receive either placebo or 0.5 μg/kg of difelikefalin three times per week intravenously for 6 weeks. The primary end point was change from baseline in the Worst Itching Intensity Numerical Rating Scale (NRS; 0 to 10; higher scores indicate more severe itching) score at week 4. RESULTS: A total of 230 patients were screened, of whom 178 were randomly assigned to receive placebo (n=89) or difelikefalin (n=89). The change from baseline in the weekly adjusted mean NRS score (95% confidence interval) at week 4 in the placebo and difelikefalin groups was −1.09 (−1.47 to −0.70) and −2.06 (−2.45 to −1.66), respectively. The difference between the groups was −0.97 (−1.52 to −0.42), demonstrating that difelikefalin was superior to placebo (P<0.001). Prespecified secondary quality-of-life end points showed consistent improvement associated with difelikefalin. The incidence of treatment-related adverse events in the placebo and difelikefalin groups was 3 of 89 patients (3%) and 13 of 89 patients (15%), respectively, of which the majority in the difelikefalin group were gastrointestinal (e.g., constipation and abdominal discomfort). CONCLUSIONS: Intravenous difelikefalin reduced itching and improved quality of life in patients with moderate to severe pruritus who were undergoing maintenance hemodialysis. (Funded by Kissei Pharmaceutical Co., Ltd.; ClinicalTrials.gov number, NCT04711603.)

Safety and effectiveness of HSK21542 for hemodialysis patients: a multiple ascending dose study

Background: HSK21542, a novel selective peripherally-restricted κ-opioid receptor agonist has been proven to be a safe and effective analgesic and antipruritic drug in both in vitro and in vivo studies. We aimed to evaluate its safety, pharmacokinetics and efficacy in hemodialysis patients over a 1-week treatment period, and to establish the optimal dosage for a further 12-week stage 2 trial. Methods: In this multiple ascending dose study, hemodialysis patients were randomly assigned to receive HSK21542 (0.05-0.80 μg/kg), or a placebo three times within 2.5 h at the end of each dialysis session for 1 week. Safety evaluations included reports of treatment-emergent adverse events (TEAEs); pharmacokinetics and efficacy outcomes were also assessed. Results: Among the 44 screened patients, 41 were enrolled and completed the trial. The overall incidence of TEAEs was higher in the HSK21542 group compared to the placebo group, with an incidence of 75.0%, 50.0%, 75.0%, and 88.9% in the range of 0.05-0.80 μg/kg. All TEAEs were grade 1 or 2 in severity. HSK21542 exhibited linear pharmacokinetics characteristics within the dose range 0.05-0.80 μg/kg, without drug accumulation after multiple-doses. Compared to the placebo, a significant decrease of the weekly mean Worst Itching Intensity Numerical Rating Scale was found in the HSK21542-0.30 μg/kg group (p = 0.046), but without significant improvement in the Skindex-16 score. Conclusion: HSK21542 was well tolerated in the dose range 0.05-0.80 μg/kg in hemodialysis patients. HSK21542-0.3 μg/kg exhibited promising efficacy in patients with moderate to severe pruritus and warrants a further Stage 2 trial. Clinical Trial Registration: https://clinicaltrials.gov/, identifier NCT04470154.

Difelikefalin suppresses itch and reduces scratching independent of inflammation in a murine model of atopic dermatitis

BACKGROUND

Therapies specifically targeting nonhistaminergic pruritus are largely lacking. Difelikefalin (DFK) has been found to reduce itch in various chronic pruritic conditions, including atopic dermatitis (AD).

OBJECTIVE

We sought to investigate the ability of DFK to impact scratching behavior, inflammatory mediators, and neuronal signaling in a murine model of AD.

METHODS

The ears of C57BL/6 mice were topically treated with MC903 for 12 consecutive days to induce AD-like inflammation and itch. Before MC903 treatment, mice were treated with either DFK (0.5 mg/kg, intraperitoneal injection twice daily) or vehicle (saline). Skin ear thickness, histological analysis, flow cytometry, RNA-sequencing, and differential gene expression analyses of mouse ear skin were used to examine the effect of DFK on skin inflammation. Scratching behavior was quantified to measure itch behavior in mice that were topically treated with MC903 for 6 consecutive days; then, mice received a single injection of either DFK (1.0 mg/kg, intraperitoneal injection) or saline. Calcium imaging and single-cell RNA-sequencing were used in mouse dorsal root ganglia neurons to determine the size of the neurons activated with DFK treatment. Statistical significance was determined by Mann-Whitney test, unless otherwise noted.

RESULTS

DFK rapidly suppressed itch without altering AD-like skin inflammation in MC903 (calcipotriol)-treated mice. In vitro Ca2+ influx trace of dorsal root ganglia suggested that a major target for DFK is the larger-diameter mechanoreceptors (eg, Aꞵ-fibers), rather than small-diameter pruriceptive C-fibers.

CONCLUSIONS

These studies support a potential neuromodulatory role of DFK for reducing itch associated with AD in mice.

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

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

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

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

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 end point 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 predefined 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 squares 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.

Pharmacokinetics, Metabolism, and Excretion of Intravenous [14C]Difelikefalin in Healthy Subjects and Subjects on Hemodialysis

BACKGROUND AND OBJECTIVE

Difelikefalin, a selective kappa-opioid receptor agonist, is the first approved treatment for moderate-to-severe pruritus in patients with end-stage renal disease (ESRD) on hemodialysis (HD) in the USA and Europe. The purpose of this open-label study was to investigate the pharmacokinetics and disposition of [14C]difelikefalin following a single intravenous dose in subjects with normal renal function and subjects on HD.

METHODS

Twelve adult males (n = 6 healthy subjects; n = 6 subjects on HD) received single intravenous doses of [14C]difelikefalin containing 100 µCi (total doses of 1.7-3.0 μg/kg difelikefalin). Blood, urine, feces, and dialysate samples (when applicable) were collected after dosing.

RESULTS

The median time to maximum concentration was similar for HD and healthy subjects, occurring at 5 min post-dose. The mean area under the concentration-time curve (AUC) was approximately 11-fold higher in HD versus healthy subjects; mean plasma half-life was 38.0 h and 2.6 h, respectively. In healthy subjects, 80.5% of the dose was recovered in urine, and 11.3% was recovered in feces. In subjects on HD, 58.8% of the dose was recovered in feces, and 19.5% was recovered in dialysate [for subjects on HD with residual kidney function (n = 3), 11.2% was recovered in urine]. Based on plasma AUClast, parent [14C]difelikefalin was the most abundant analyte in systemic circulation (> 99% of total exposure) for both cohorts. Metabolite profiles in urine and feces suggested minimal metabolism of the parent compound.

CONCLUSION

In subjects on HD, difelikefalin total exposure was higher and plasma half-life was longer compared with subjects with intact renal function. Metabolism was low in both healthy subjects and subjects on HD, with unchanged drug representing > 99% of systemic circulation; however, the route of excretion was primarily into urine versus feces in healthy subjects, and feces versus dialysate in subjects on HD.

REGISTRATION

ClinicalTrials.gov NCT03947970.

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

Synergistic interaction between DAMGO-NH2 and NOP01 in peripherally acting antinociception in two mouse models of formalin pain

The most commonly used opioid analgesics are limited by their severe side-effects in the clinical treatment of pain. Preliminary reports indicate that the combination of classical opioids and N/OFQ receptor (NOP) ligands may be an effective strategy to reduce unwanted side-effects and improve antinociception. But the interaction of these two receptor ligands in pain regulation at the peripheral level remains unclear. In this study, the antinociception of a designed amide analogue of the mu opioid receptor (MOP) peptide agonist DAMGO, DAMGO-NH₂, and its antinociceptive interaction with the peripherally limited NOP peptide agonist NOP01 was investigated in two mouse models of formalin pain. Our results showed that DAMGO-NH₂ acted as a MOP agonist in in vitro functional assays. Moreover, local subcutaneous or intraplantar injection of DAMGO-NH₂ exerted dose-related antinociception in both phases of the formalin orofacial and intraplantar pain, which could be mediated by the classical opioid receptor. Peripheral but not central pretreatment with the peripherally restricted opioid antagonist naloxone methiodide inhibited local DAMGO-NH₂-induced antinociception, supporting the involvement of the peripheral opioid receptor in local DAMGO-NH₂-induced antinociception. Furthermore, co-administration of the inactive doses of DAMGO-NH₂ and NOP01 produced effective antinociception. More importantly, isobolographic analysis indicates that the combination of DAMGO-NH₂ and NOP01 elicited supra-additive antinociception in these two models of formalin pain. In addition, the combination of DAMGO-NH₂ and NOP01 did not change motor function of mice in rotarod test. In conclusion, these data suggest that peripheral DAMGO-NH₂ and particularly its combination therapy with NOP01 may be effective for pain management.

Solving an Old Puzzle: Elucidation and Evaluation of the Binding Mode of Salvinorin A at the Kappa Opioid Receptor

The natural product Salvinorin A (SalA) was the first nitrogen-lacking agonist discovered for the opioid receptors and exhibits high selectivity for the kappa opioid receptor (KOR) turning SalA into a promising analgesic to overcome the current opioid crisis. Since SalA's suffers from poor pharmacokinetic properties, particularly the absence of gastrointestinal bioavailability, fast metabolic inactivation, and subsequent short duration of action, the rational design of new tailored analogs with improved clinical usability is highly desired. Despite being known for decades, the binding mode of SalA within the KOR remains elusive as several conflicting binding modes of SalA were proposed hindering the rational design of new analgesics. In this study, we rationally determined the binding mode of SalA to the active state KOR by in silico experiments (docking, molecular dynamics simulations, dynophores) in the context of all available mutagenesis studies and structure-activity relationship (SAR) data. To the best of our knowledge, this is the first comprehensive evaluation of SalA's binding mode since the determination of the active state KOR crystal structure. SalA binds above the morphinan binding site with its furan pointing toward the intracellular core while the C2-acetoxy group is oriented toward the extracellular loop 2 (ECL2). SalA is solely stabilized within the binding pocket by hydrogen bonds (C210ECL2, Y3127.35, Y3137.36) and hydrophobic contacts (V1182.63, I1393.33, I2946.55, I3167.39). With the disruption of this interaction pattern or the establishment of additional interactions within the binding site, we were able to rationalize the experimental data for selected analogs. We surmise the C2-substituent interactions as important for SalA and its analogs to be experimentally active, albeit with moderate frequency within MD simulations of SalA. We further identified the non-conserved residues 2.63, 7.35, and 7.36 responsible for the KOR subtype selectivity of SalA. We are confident that the elucidation of the SalA binding mode will promote the understanding of KOR activation and facilitate the development of novel analgesics that are urgently needed.

Perception of Spanish nephrologists on an old unsolved problem: Pruritus associated with chronic kidney disease (CKD-aP)

INTRODUCTION

Pruritus associated with chronic kidney disease is defined as the sensation of itching, in people with chronic kidney disease, in a one area or all over the body that causes the need to scratch, after having ruled out other dermatological or systemic causes. It is an old and known problem whose prevalence has been able to decrease with the improvement of dialytic techniques but which still persists and is underdiagnosed.

OBJECTIVES

The objective of this study was to analyse the current perception of nephrologists about this problem that influences the quality of life of people with chronic kidney disease through a survey.

RESULTS

135 nephrologists, most of them engaged in haemodialysis, participated. 86% considered that pruritus associated with chronic kidney disease is still a problem today that affects the quality of life. Most nephrologists believe that the main pathophysiological cause is uremic toxins (60%) and only 16% believe that it is due to the dysregulation of the opioid system/endorphins-dynorphins. Only 16% comment that the prevalence of pruritus in their centre is greater than 20%. 40% believe that the diagnosis is made because it is manifested by the patient and only 27% because it is asked by the doctor. Moreover, it is not usual to use scales to measure it or the codification in the medical records. The main treatment used is antihistamines (96%), followed by moisturizers/anaesthetics (93%) and modification of the dialysis regimen (70%).

CONCLUSIONS

Pruritus associated with chronic kidney disease is still a current problem, it is underdiagnosed, not codified and with a lack of indicated, effective and safe treatments. Nephrologists do not know its real prevalence and the different pathophysiological mechanisms involved in its development. Many therapeutic options are used with very variable results, ignoring their efficacy and applicability at the present time. The new emerging kappa-opioid-receptor agonist agents offer us an opportunity to reevaluate this age-old problem and improve the quality of life for our patients with chronic kidney disease.

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

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

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.

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Pruritus in chronic kidney disease

PURPOSE OF REVIEW

Among the many difficult symptoms that patients with kidney disease experience, pruritus is one of the most frequent and troubling. Because a substantial amount of new information has accumulated, we seek here to review the subject.

RECENT FINDINGS

Pruritus is not only a common problematic symptom among patients with kidney disease, but its considerably more frequent than nephrologists recognize. The result for patients is not just uncomfortable itch but degraded quality of life as well. The pathogenesis is increasingly understood, but many aspects remain to be fully resolved. Importantly, research is progressing on treatment, leading to the first approved medication in the United States, difelikefalin, in August, 2021.

SUMMARY

As nephrology is progressing to a greater focus on patient symptoms, recognition of the importance of pruritus has led to increased interest and improved diagnosis and treatment options.

Efficacy of Difelikefalin for the Treatment of Moderate to Severe Pruritus in Hemodialysis Patients: Pooled Analysis of KALM-1 and KALM-2 Phase 3 Studies

Rationale & Objective

Chronic kidney disease-associated pruritus (CKD-aP) in patients treated by hemodialysis (HD) impairs quality of life (QoL). Difelikefalin, a selective κ-opioid receptor agonist, decreased the intensity of CKD-aP in patients undergoing HD. This pooled analysis evaluated difelikefalin's efficacy and the itch-related QoL overall and in subgroups defined by demographics or disease characteristics.

Study Design

In KALM-1 and KALM-2, participants were randomized (1:1) to receive intravenous difelikefalin or placebo 3 times/wk for 12 weeks, followed by a 52-week open-label extension.

Setting & Participants

Adults with moderate to severe CKD-aP treated by HD in North America, Europe, and the Asia-Pacific region.

Intervention

Intravenous difelikefalin at 0.5 mcg/kg or placebo.

Outcomes

Itch intensity (Worst Itching Intensity Numerical Rating Scale [WI-NRS]) and itch-related QoL (Skindex-10 and 5-D Itch questionnaires).

Results

851 participants were randomized (difelikefalin, n = 426; placebo, n = 425). This pooled analysis demonstrated early     (week 1), sustained difelikefalin efficacy, with significantly greater achievement of ≥3-point WI-NRS reduction with difelikefalin (51.1%) versus placebo (35.2%; P < 0.001). Achievement of a ≥4-point WI-NRS reduction was significantly greater with difelikefalin (38.7%) versus placebo (23.4%; P < 0.001). Difelikefalin reduced itch intensity in subgroups based on age, sex, anti-itch medication use, the presence of specific medical conditions, and gabapentin or pregabalin use. More participants receiving difelikefalin versus placebo achieved clinically meaningful decreases of ≥15 points on the Skindex-10 scale (55.5% vs 40.5%, respectively; P < 0.001) and ≥5 points on the 5-D Itch scale (52.1% vs 42.3%, respectively; P = 0.01), with sustained 5-D Itch effects up to 64 weeks.

Limitations

Subgroup samples were small. The WI-NRS, Skindex-10, and 5-D Itch are not used in routine clinical care of dialysis patients; therefore, findings may not reflect the real-world effectiveness of difelikefalin.

Conclusions

Difelikefalin demonstrated rapid, sustained efficacy, with consistent results in diverse populations of patients treated by HD.

Funding

Cara Therapeutics, Inc.

Trial Registration

The KALM-1 trial is registered as NCT03422653 and the KALM-2 trial is registered as NCT03636269.

Safety and Tolerability of Difelikefalin for the Treatment of Moderate to Severe Pruritus in Hemodialysis Patients: Pooled Analysis From the Phase 3 Clinical Trial Program

Rationale & Objective

We report a pooled safety analysis of intravenous difelikefalin in participants with moderate to severe chronic kidney disease–associated pruritus (CKD-aP) treated by hemodialysis in 4 phase 3 clinical studies.

Study Design

KALM-1 and KALM-2 were randomized, double-blind, placebo-controlled, pivotal phase 3 studies; CLIN3101 (52 weeks) and CLIN3105 (12 weeks) were open-label studies.

Setting & Participants

Adults with moderate to severe CKD-aP treated by hemodialysis in North America, Europe, and the Asia-Pacific region.

Intervention

At least 1 intravenous placebo or difelikefalin dose of 0.5 mcg/kg for up to 64 weeks.

Outcomes

Safety.

Results

Safety analyses were conducted with 848 participants in the placebo-controlled cohort (424 participants each in the difelikefalin and placebo groups) and in 1,306 participants in the all-difelikefalin-exposure cohort. In the placebo-controlled cohort, the most commonly reported treatment-emergent adverse events (TEAEs), occurring in ≥2% of participants receiving difelikefalin and with a ≥1% higher incidence than placebo, were diarrhea (9.0% and 5.7%, respectively); dizziness (6.8% and 3.8%, respectively); nausea (6.6% and 4.5%, respectively); gait disturbances, including falls (6.6% and 5.4%, respectively), hyperkalemia (4.7% and 3.5%, respectively); headache (4.5% and 2.6%, respectively); somnolence (4.2% and 2.4%, respectively); and mental status changes (3.3% and 1.4%, respectively). These were mostly mild or moderate, with few leading to discontinuation. Incidence rates of TEAEs, serious TEAEs, and discontinuations because of TEAEs did not increase with long-term exposure. Three participants (0.7%) in the difelikefalin group and 5 participants (1.2%) in the placebo group died during the study.

Limitations

Pooled data from studies with different designs.

Conclusions

Intravenous difelikefalin demonstrated an acceptable safety profile, was generally well tolerated with long-term use, and may address the unmet treatment need for patients with CKD-aP treated by hemodialysis.

Funding

Cara Therapeutics, Inc.

Trial Registration

KALM-1 is registered as NCT03422653, KALM-2 as NCT03636269, CLIN3101 as NCT03281538, and CLIN3105 as NCT03998163.

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.

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.

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.

Efficacy and Safety of Difelikefalin in Japanese Patients With Moderate to Severe Pruritus Receiving Hemodialysis: A Randomized Clinical Trial

IMPORTANCE

Patients with pruritus receiving hemodialysis frequently experience oppressive physical and psychiatric symptoms that directly affect their quality of life and increase mortality. However, treatment options are limited.

OBJECTIVE

To determine the clinically recommended dose of difelikefalin, a κ-opioid receptor agonist, based on the efficacy, dose response, safety, and pharmacokinetics.

DESIGN, SETTING, AND PARTICIPANTS

This randomized, double-blind, placebo-controlled, 4-arm phase 2 trial was conducted from February 1, 2019, to October 22, 2019, at 94 sites in Japan. Patients with moderate to severe pruritus receiving hemodialysis were enrolled.

INTERVENTIONS

Difelikefalin (0.25, 0.5, and 1.0 μg/kg) and placebo were intravenously administered 3 times a week at the end of each hemodialysis session for 8 weeks.

MAIN OUTCOME AND MEASURES

The primary end point was the change from baseline in the weekly mean Worst Itching Intensity Numerical Rating Scale (NRS) score at week 8. Secondary outcomes measured changes in itch-related quality-of-life score using the Skindex-16 and 5-D itch scale. Safety was assessed according to adverse events, laboratory tests, vital signs, body weight, and 12-lead electrocardiogram.

RESULTS

A total of 247 Japanese patients (186 male [75%]; mean [SD] age, 64.5 [11.7] years) were randomized to placebo (n = 63), 0.25 μg/kg of difelikefalin (n = 61), 0.5 μg/kg of difelikefalin (n = 61), or 1.0 μg/kg of difelikefalin (n = 62). The changes from baseline in the adjusted mean (SE) of the 24-hour Worst Itching Intensity NRS score at week 8 were -2.86 (0.29) in the placebo group, -2.97 (0.29) in the 0.25 μg/kg of difelikefalin group, -3.65 (0.30) in the 0.5 μg/kg of difelikefalin group, and -3.64 (0.30) in the 1.0 μg/kg of difelikefalin group. Significant differences were found in the 0.5 μg/kg of difelikefalin group (adjusted mean difference, -0.80; 95% CI, -1.55 to -0.04; P = .04) and the 1.0 μg/kg of difelikefalin group (adjusted mean difference, -0.78; 95% CI, -1.54 to -0.03; P = .04) compared with placebo. The Skindex-16 overall score and 5-D itch scale total score indicated an improvement with treatment with 0.5 and 1.0 μg/kg of difelikefalin (adjusted weekly mean [SE] Skindex-16 overall score at week 8, -27.79 [2.05]; 95% CI, -31.83 to -23.74 for 0.5 μg/kg of difelikefalin and -22.69 [2.04]; 95% CI, -26.71 to -18.68 for 1.0 μg/kg of difelikefalin; adjusted weekly mean [SE] 5-D itch scale total score at week 8, -6.5 [0.4]; 95% CI, -7.2 to -5.8 for 0.5 μg/kg of difelikefalin and -6.8 [0.3]; 95% CI, -7.5 to -6.2 for 1.0 μg/kg of difelikefalin). The incidence of adverse events was 67% (42 of 63 patients) in the placebo group, 72% (44 of 61 patients) in the 0.25 μg/kg of difelikefalin group, 77% (47 of 61 patients) in the 0.5 μg/kg of difelikefalin group, and 85% (53 of 62 patients) in the 1.0 μg/kg of difelikefalin group. No dependency was reported.

CONCLUSIONS AND RELEVANCE

The findings of this phase 2 randomized clinical trial of difelikefalin suggest that 0.5 μg/kg of difelikefalin should be the clinically recommended dose as a new option for treating moderate to severe pruritus in patients undergoing hemodialysis because of its efficacy, acceptable tolerability, and manageable safety profile.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03802617.

2021 FDA TIDES (Peptides and Oligonucleotides) Harvest

From the medical, pharmaceutical, and social perspectives, 2021 has been a year dominated by the COVID-19 pandemic. However, despite this global health crisis, the pharmaceutical industry has continued its endeavors, and 2021 could be considered an excellent year in terms of the drugs accepted by the US Food and Drug Administration (FDA). Thus, during this year, the FDA has approved 50 novel drugs, of which 36 are new chemical entities and 14 biologics. It has also authorized 10 TIDES (8 peptides, 2 oligonucleotides), in addition to 2 antibody-drug conjugates (ADCs) whose structures contain peptides. Thus, TIDES have accounted for about 24% of the approvals in the various drug categories. Importantly, this percentage has surpassed the figure in 2020 (10%), thus reflecting the remarkable success of TIDES. In this review, the approved TIDE-based drugs are analyzed on the basis of their chemical structure, medical target, mode of action, administration route, and adverse effects.

NOP01, a NOP receptor agonist, produced potent and peripherally restricted antinociception in a formalin-induced mouse orofacial pain model

Orofacial pain is one of the most common medical challenges. A preliminary report indicates that the NOP receptor may act as a therapeutic target in orofacial pain. Previous studies have shown that [(pF)Phe⁴, Aib⁷, Aib¹¹, Arg¹⁴, Lys¹⁵]N/OFQ-NH₂ (NOP01) functions as a potent NOP receptor peptide agonist. This work aims to investigate the antinociception of NOP01 and its possible action mechanisms in a formalin-induced mouse orofacial pain model at different levels. Our results demonstrated that local, intraperitoneal (i.p.) or intrathecal (i.t.) injection of NOP01 produced dose-related antinociception in both phases of the formalin pain, which could be inhibited by the NOP receptor antagonist but not the classical opioid receptor antagonist. Furthermore, the antinociception induced by systemic NOP01 was blocked by local but not spinal pretreatment with the NOP receptor antagonist, suggesting the involvement of the peripheral NOP receptor in NOP01-induced systemic antinociception. Moreover, local injection of NOP01 markedly suppressed the expression of c-Fos protein induced by formalin in ipsilateral trigeminal ganglion (TG) neurons. In conclusion, this work suggests that NOP01 exerts significant antinociception on orofacial pain at both peripheral and spinal levels via the NOP receptor. Notably, NOP01 cannot readily penetrate the blood-brain barrier. Thus, NOP01 may behave as a potential compound for developing peripherally restricted analgesics.

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

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

Analgesic Effects of Different κ-Receptor Agonists Used in Daytime Laparoscopic Cholecystectomy

Background

Comparing the effect of two different κ-receptor agonists, nalbuphine and oxycodone, and regular morphine in patients for prophylactic analgesia of acute pain after daytime laparoscopic cholecystectomy.

Methods

One hundred and twenty-four patients undergoing laparoscopic cholecystectomy were randomly allocated to receive nalbuphine (group N), oxycodone (group O), and morphine (group M). The three groups were all given intravenous injection (iv.) of 0.15 mg/kg injection before incision and 0.05 mg/kg injection at the end of pneumoperitoneum. The Visual Analogue Scale (VAS) scores (incision, visceral, and shoulder) and Ramsay sedation scores at 1, 2, 4, 8, 12, 16, 20, and 24 hours after surgery, the time of extubation, the incidence of postoperative adverse events, the satisfaction of pain treatment, and the duration of stay after surgery were all recorded.

Results

Compared with group M, the VAS scores of visceral pain at rest decreased in group N and group O at 1-8 h after surgery (P < 0.05). The VAS scores of visceral pain at movement in group N decreased longer than those in group O (P < 0.05). Compared with that of group M, the postoperative time in Ramsay sedation score of group O increased longer than that of group N (P < 0.05). Compared with group N, patients had worse sleep quality in group O, longer length of stay in group M, and lower satisfaction in both groups.

Conclusion

Compared with morphine, prophylactic use of the κ-receptor agonists, nalbuphine and oxycodone, during laparoscopic cholecystectomy can reduce postoperative visceral pain. Furthermore, the nalbuphine group had fewer adverse reactions, better analgesia, and better satisfaction.

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.

Novel approach to unpleasant symptom clusters surrounding pruritus in patients with chronic kidney disease and on dialysis therapy

PURPOSE OF REVIEW

Chronic kidney disease-associated-pruritus (CKD-aP) is a common symptom in patients with end-stage kidney disease (ESKD) undergoing dialysis. CKD-aP typically occurs alongside other debilitating symptoms and may comprise so-called 'symptom clusters' which have synergistic effects that adversely impact patient health-related quality of life (HRQoL). Importantly, symptoms in a cluster may share a common biological mechanism. Here we review the clinical impact of CKD-aP and its association with other symptoms reported by dialysis patients. The clinical benefits of treating pruritus and its potential impact on other symptoms are also addressed.

RECENT FINDINGS

Studies have shown CKD-aP significantly impairs HRQoL in patients with ESKD undergoing dialysis and is associated with adverse clinical outcomes, including increased risk of infections, hospitalizations, and mortality. Despite these negative effects, CKD-aP remains underrecognized and undertreated in clinical practice. CKD-aP is frequently associated with other symptoms, including disturbed sleep/poor sleep quality, anxiety, depression, and pain. Clinical studies of antipruritic therapies show that reduction of itch intensity may also alleviate other associated symptoms, such as poor sleep quality.

SUMMARY

CKD-aP and its associated symptoms are inadequately managed in clinical practice. Greater understanding and awareness of CKD-aP and its surrounding symptom clusters in dialysis patients may improve their overall symptom management and HRQoL.

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.

Inhibiting Kiss1 Neurons With Kappa Opioid Receptor Agonists to Treat Polycystic Ovary Syndrome and Vasomotor Symptoms

CONTEXT

Recent evidence suggests that vasomotor symptoms (VMS) or hot flashes in the postmenopausal reproductive state and polycystic ovary syndrome (PCOS) in the premenopausal reproductive state emanate from the hyperactivity of Kiss1 neurons in the hypothalamic infundibular/arcuate nucleus (KNDy neurons).

OBJECTIVE

We demonstrate in 2 murine models simulating menopause and PCOS that a peripherally restricted kappa receptor agonist (PRKA) inhibits hyperactive KNDy neurons (accessible from outside the blood-brain barrier) and impedes their downstream effects.

DESIGN

Case/control.

SETTING

Academic medical center.

PARTICIPANTS

Mice.

INTERVENTIONS

Administration of peripherally restricted kappa receptor agonists and frequent blood sampling to determine hormone release and body temperature.

MAIN OUTCOME MEASURES

LH pulse parameters and body temperature.

RESULTS

First, chronic administration of a PRKA to bilaterally ovariectomized mice with experimentally induced hyperactivity of KNDy neurons reduces the animals' elevated body temperature, mean plasma LH level, and mean peak LH per pulse. Second, chronic administration of a PRKA to a murine model of PCOS, having elevated plasma testosterone levels and irregular ovarian cycles, suppresses circulating levels of LH and testosterone and restores normal ovarian cyclicity.

CONCLUSION

The inhibition of kisspeptin neuronal activity by activation of kappa receptors shows promise as a novel therapeutic approach to treat both VMS and PCOS in humans.

Difelikefalin: First Approval

Difelikefalin (Korsuva™) is a synthetic peptide agonist of the kappa opioid receptor being developed by Cara Therapeutics for the treatment of pruritus. In August 2021, intravenous difelikefalin was approved in the USA for the treatment of moderate-to-severe pruritus associated with chronic kidney disease (CKD) in adults undergoing haemodialysis. Intravenous difelikefalin has also been evaluated for CKD-associated pruritus in patients undergoing haemodialysis in various other countries, with Marketing Authorization Application under regulatory review in the EU and a phase III trial ongoing in Japan. Clinical studies of an oral formulation of difelikefalin have also been completed or are underway in pruritus indications, including pruritus associated with atopic dermatitis, notalgia paraesthetica or primary biliary cholangitis. This article summarizes the milestones in the development of difelikefalin leading to this first approval for CKD-associated pruritus in adults undergoing haemodialysis.

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 [E_max]) 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 E_max, 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.

The effect of opioids on gastrointestinal function in the ICU

Gastrointestinal (GI) dysfunction is common in the critical care setting and is highly associated with clinical outcomes. Opioids increase the risk for GI dysfunction and are frequently prescribed to reduce pain in critically ill patients. However, the role of opioids in GI function remains uncertain in the ICU. This review aims to describe the effect of opioids on GI motility, their potential risk of increasing infection and the treatment of GI dysmotility with opioid antagonists in the ICU setting.

Development of Multifunctional and Orally Active Cyclic Peptide Agonists of Opioid/Neuropeptide FF Receptors that Produce Potent, Long-Lasting, and Peripherally Restricted Antinociception with Diminished Side Effects

We previously reported that a multifunctional opioid/neuropeptide FF receptor agonist, DN-9, achieved peripherally restricted analgesia with reduced side effects. To develop stable and orally bioavailable analogues of DN-9, eight lactam-bridged cyclic analogues of DN-9 between positions 2 and 5 were designed, synthesized, and biologically evaluated. In vitro cAMP assays revealed that these analogues, except 7, were multifunctional ligands that activated opioid and neuropeptide FF receptors. Analogue 1 exhibited improved potency for κ-opioid and NPFF₂ receptors. All analogues exhibited potent, long-lasting, and peripherally restricted antinociception in the tail-flick test without tolerance development after subcutaneous administration and produced oral analgesia. Oral administration of the optimized compound analogue 1 exhibited powerful, peripherally restricted antinociceptive effects in mouse models of acute, inflammatory, and neuropathic pain. Remarkably, orally administered analogue 1 had no significant side effects, such as tolerance, dependence, constipation, or respiratory depression, at effective analgesic doses.

Recent Chemical and Pharmacological Developments on 14-Oxygenated-N-methylmorphinan-6-ones

Adequate pain management, particularly chronic pain, remains a major challenge associated with modern-day medicine. Current pharmacotherapy offers unsatisfactory long-term solutions due to serious side effects related to the chronic administration of analgesic drugs. Morphine and structurally related derivatives (e.g., oxycodone, oxymorphone, buprenorphine) are highly effective opioid analgesics, mediating their effects via the activation of opioid receptors, with the mu-opioid receptor subtype as the primary molecular target. However, they also cause addiction and overdose deaths, which has led to a global opioid crisis in the last decades. Therefore, research efforts are needed to overcome the limitations of present pain therapies with the aim to improve treatment efficacy and to reduce complications. This review presents recent chemical and pharmacological advances on 14-oxygenated-N-methylmorphinan-6-ones, in the search of safer pain therapeutics. We focus on drug design strategies and structure-activity relationships on specific modifications in positions 5, 6, 14 and 17 on the morphinan skeleton, with the goal of aiding the discovery of opioid analgesics with more favorable pharmacological properties, potent analgesia and fewer undesirable effects. Targeted molecular modifications on the morphinan scaffold can afford novel opioids as bi- or multifunctional ligands targeting multiple opioid receptors, as attractive alternatives to mu-opioid receptor selective analgesics.

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.

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.