Discovery of Kynurenines Containing Oligopeptides as Potent Opioid Receptor Agonists

N-formylkynurenine but not kynurenine enters a nucleophile-scavenging branch of the immune-regulatory kynurenine pathway

Tryptophan catabolism along the kynurenine pathway (KP) mediates key physiological functions ranging from immune tolerance to lens UV protection, but the contributory roles and chemical fates of individual KP metabolites are incompletely understood. This particularly concerns the first KP metabolite, N-formylkynurenine (NFK), canonically viewed as a transient precursor to the downstream kynurenine (KYN). Here, we challenge that canon and show that hydrolytic enzymes act as a rheostat switching NFK's fate between the canonical KP and a novel non-enzymatic branch of tryptophan catabolism. In the physiological environment (37 °C, pH 7.4), NFK deaminated into electrophilic NFK-carboxyketoalkene (NFK-CKA), which rapidly (<2 min) formed adducts with nucleophiles such as cysteine and glutathione, the key intracellular antioxidants. Serum hydrolases suppressed NFK deamination as they hydrolysed NFK to KYN ∼3 times faster than NFK deaminates. Whilst KYN did not deaminate, its deaminated product (KYN-CKA) rapidly reacted with cysteine but not glutathione. The new NFK transformations of a yet to be discovered function highlight NFK's significance beyond hydrolysis to KYN and suggests the dominance of its chemical transformations over those of KYN. Enzyme compartmentalisation and abundance offer insights into the regulation of non-enzymatic KP metabolite transformations such as KYN involved in immune regulation, protein modification, lens aging or neuropathology.

Lipid-Based Nanocarriers for Delivery of Neuroprotective Kynurenic Acid: Preparation, Characterization, and BBB Transport

Encapsulation possibilities of an extensively investigated neuroprotective drug (kynurenic acid, KYNA) are studied via lipid-based nanocarriers to increase the blood-brain barrier (BBB) specific permeability. The outcomes of various preparation conditions such as stirring and sonication time, concentration of the lipid carriers and the drug, and the drug-to-lipid ratio are examined. Considering the experimentally determined encapsulation efficiency, hydrodynamic diameter, and ζ-potential values, the initial lipid and drug concentration as well as the stirring and sonication time of the preparation were optimized. The average hydrodynamic diameter of the prepared asolectin-(LIP) and water-soluble lipopolymer (WSLP)-based liposomes was found to be ca. 25 and 60 nm under physiological conditions. The physicochemical characterization of the colloidal carriers proves that the preparation of the drug-loaded liposomes was a successful process, and secondary interactions were indicated between the drug molecule and the polymer residues around the WSLP membrane. Dissolution profiles of the active molecule under physiological conditions were registered, and the release of the unformulated and encapsulated drug is very similar. In addition to this outcome, the in vitro polar brain lipid extract (porcine)-based permeability test proved the achievement of two- or fourfold higher BBB specific penetration and lipid membrane retention for KYNA in the liposomal carriers relative to the unformatted drug.

Analog of kynurenic acid decreases tau pathology by modulating astrogliosis in rat model for tauopathy

Kynurenines have immunomodulatory and neuroactive properties and can influence the central nervous system. Previous studies showed the involvement of the kynurenines in the pathogenesis and progression of neurodegenerative disease. In neurodegenerative disorders, including tauopathies, the tryptophan metabolism is shifted toward neurotoxic agents and the reduction of neuroprotectant products. Astrocyte-derived kynurenic acid serves as a neuroprotectant. However, systemic administration of kynurenic acid is not effective because of low permeability across the blood-brain barrier (BBB). We used a kynurenic acid analog with similar biological activity but higher brain permeability to overcome BBB limitations. In the present study, we used amide derivate of kynurenic acid N-(2-N, N-dimethylaminoethyl)- 4-oxo-1 H-quinoline-2-carboxamid (KYNA-1). We administered KYNA-1 for three months to tau transgenic rats SHR-24 and analyzed the effect on tau pathology and activation of glial cells. Primary glial cell cultures were applied to identify the mechanism of the KYNA-1 effect. KYNA-1 was not toxic to rats after chronic three-month administration. When chronically administered, KYNA-1 reduced hyperphosphorylation of insoluble tau in the brain of transgenic rats. Noteworthily, the plasma total tau was also reduced. We determined that the effect of KYNA-1 on tau pathology was induced through the modulation of glial activation. KYNA-1 inhibited LPS induced activation of astrocytes and induced transformation of microglia to M2 phenotype. We identified that the administration of KYNA-1 reduced tau hyperphosphorylation and neuroinflammation. KYNA-1 may serve as a promising treatment for tauopathies.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

The Kynurenine Pathway as a Potential Target for Neuropathic Pain Therapy Design: From Basic Research to Clinical Perspectives

Accumulating evidence suggests the key role of the kynurenine pathway (KP) of the tryptophan metabolism in the pathogenesis of several diseases. Despite extensive research aimed at clarifying the mechanisms underlying the development and maintenance of neuropathic pain, the roles of KP metabolites in this process are still not fully known. Although the function of the peripheral KP has been known for several years, it has only recently been acknowledged that its metabolites within the central nervous system have remarkable consequences related to physiology and behavior. Both the products and metabolites of the KP are involved in the pathogenesis of pain conditions. Apart from the neuroactive properties of kynurenines, the KP regulates several neurotransmitter systems in direct or indirect ways. Some neuroactive metabolites are known to have neuroprotective properties (kynurenic acid, nicotinamide adenine dinucleotide cofactor), while others are toxic (3-hydroxykynurenine, quinolinic acid). Numerous animal models show that modulation of the KP may turn out to be a viable target for the treatment of diseases. Importantly, some compounds that affect KP enzymes are currently described to possess analgesic properties. Additionally, kynurenine metabolites may be useful for assessing response to therapy or as biomarkers in therapeutic monitoring. The following review describes the molecular site of action and changes in the levels of metabolites of the kynurenine pathway in the pathogenesis of various conditions, with a particular emphasis on their involvement in neuropathy. Moreover, the potential clinical implications of KP modulation in chronic pain therapy as well as the directions of new research initiatives are discussed.

Selective MOR activity of DAPEA and Endomorphin-2 analogues containing a (R)-γ-Freidinger lactam in position two

The use of α-amino-γ lactam of Freidinger (Agl) may serve as an impressive method to increase the biological stability of peptides and an appropriate tool to elucidate their structure-activity relationships. The endomorphin-2 (EM-2) and [D-Ala², des-Leu⁵] enkephalin amide (DAPEA) are two linear opioid tetrapeptides agonists of MOR and MOR/DOR respectively. Herein, we investigated the influence of the incorporation of (R/S)-Agl in position 2 and 3 on the biological profile of the aforementioned products in vitro and in vivo. Receptor radiolabeled displacement and functional assays were used to measure in vitro the binding affinity and receptors activation of the novel analogues. The mouse tail flick and formalin tests allowed to observe their antinociceptive effect in vivo. Data revealed that peptide A2D was able to selectively bind and activate MOR with a potent antinociceptive effect after intracerebroventricular (i.c.v.) administration, performing better than the parent compounds EM-2 and DAPEA. Molecular docking calculations helped us to understand the key role exerted by the Freidinger Agl moiety in A2D for the interaction with the MOR binding pocket.

Novel Natural-based Biomolecules Discovery for Tackling Chronic Diseases

In the last decade, natural-derived/-based biomolecules have continuously played an important role in novel drug discovery (as a prototype drug template) for potential chronic disease treatment [...]

Development of Diphenethylamines as Selective Kappa Opioid Receptor Ligands and Their Pharmacological Activities

Among the opioid receptors, the kappa opioid receptor (KOR) has been gaining substantial attention as a promising molecular target for the treatment of numerous human disorders, including pain, pruritus, affective disorders (i.e., depression and anxiety), drug addiction, and neurological diseases (i.e., epilepsy). Particularly, the knowledge that activation of the KOR, opposite to the mu opioid receptor (MOR), does not produce euphoria or leads to respiratory depression or overdose, has stimulated the interest in discovering ligands targeting the KOR as novel pharmacotherapeutics. However, the KOR mediates the negative side effects of dysphoria/aversion, sedation, and psychotomimesis, with the therapeutic promise of biased agonism (i.e., selective activation of beneficial over deleterious signaling pathways) for designing safer KOR therapeutics without the liabilities of conventional KOR agonists. In this review, the development of new KOR ligands from the class of diphenethylamines is presented. Specifically, we describe the design strategies, synthesis, and pharmacological activities of differently substituted diphenethylamines, where structure-activity relationships have been extensively studied. Ligands with distinct profiles as potent and selective agonists, G protein-biased agonists, and selective antagonists, and their potential use as therapeutic agents (i.e., pain treatment) and research tools are described.

Long-term systemic administration of kynurenic acid brain region specifically elevates the abundance of functional CB1 receptors in rats

Kynurenic acid (KYNA) is one of the most significant metabolite of the kynurenine pathway both in terms of functional and potential therapeutic value. It is an N-methyl-D-aspartate (NMDA) receptor antagonist, but it can also activate the G-protein coupled receptor 35 (GPR35), which shares several structural and functional properties with cannabinoid receptors. Previously our group demonstrated that systemic chronic KYNA treatment altered opioid receptor G-protein activity. Opioid receptors also overlap in many features with cannabinoid receptors. Thus, our aim was to examine the direct in vitro and systemic, chronic in vivo effect of KYNA on type 1 cannabinoid receptor (CB₁R) binding and G-protein activity. Based on competition and [³⁵S]GTPγS G-protein binding assays in rat brain, KYNA alone did not show significant binding towards the CB₁R, nor did it alter CB₁R ligand binding and agonist activity in vitro. When rats were chronically treated with KYNA (single daily, i.p., 128 mg/kg for 9 days), the KYNA plasma and cerebrospinal fluid levels significantly increased compared to vehicle treated group. Furthermore, in G-protein binding assays, in the whole brain the amount of G-proteins in basal and in maximum activity coupled to the CB₁R also increased due to the treatment. At the same time, the overall stimulatory properties of the receptor remained unaltered in vehicle and KYNA treated samples. Similar observations were made in rat hippocampus, but not in the cortex and brainstem. In saturation binding assays the density of CB₁Rs in rat whole brain and hippocampus were also significantly enhanced after the same treatment, without significantly affecting ligand binding affinity. Thus, KYNA indirectly and brain region specifically increases the abundance of functional CB₁Rs, without modifying the overall binding and activity of the receptor. Supposedly, this can be a compensatory mechanism on the part of the endocannabinoid system induced by the long-term KYNA exposure.