{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,13]],"date-time":"2026-02-13T08:52:09Z","timestamp":1770972729490,"version":"3.50.1"},"reference-count":48,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2020,2,12]],"date-time":"2020-02-12T00:00:00Z","timestamp":1581465600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Biomolecules"],"abstract":"<jats:p>Kynurenine (kyn) and kynurenic acid (kyna) are well-defined metabolites of tryptophan catabolism collectively known as \u201ckynurenines\u201d, which exert regulatory functions in host-microbiome signaling, immune cell response, and neuronal excitability. Kynurenine containing peptides endowed with opioid receptor activity have been isolated from natural organisms; thus, in this work, novel opioid peptide analogs incorporating L-kynurenine (L-kyn) and kynurenic acid (kyna) in place of native amino acids have been designed and synthesized with the aim to investigate the biological effect of these modifications. The kyna-containing peptide (KA1) binds selectively the \u03bc-opioid receptor with a Ki = 1.08 \u00b1 0.26 (selectivity ratio \u03bc\/\u03b4\/\u03ba = 1:514:10,000), while the L-kyn-containing peptide (K6) shows a mixed binding affinity for \u03bc, \u03b4, and \u03ba-opioid receptors, with efficacy and potency (Emax = 209.7 + 3.4%; LogEC50 = \u22125.984 + 0.054) higher than those of the reference compound DAMGO. This novel oligopeptide exhibits a strong antinociceptive effect after i.c.v. and s.c. administrations in in vivo tests, according to good stability in human plasma (t1\/2 = 47 min).<\/jats:p>","DOI":"10.3390\/biom10020284","type":"journal-article","created":{"date-parts":[[2020,2,21]],"date-time":"2020-02-21T10:49:16Z","timestamp":1582282156000},"page":"284","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":11,"title":["Discovery of Kynurenines Containing Oligopeptides as Potent Opioid Receptor Agonists"],"prefix":"10.3390","volume":"10","author":[{"given":"Edina","family":"Sz\u0171cs","sequence":"first","affiliation":[{"name":"Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, Temesv\u00e1ri krt. 62., H-6726 Szeged, Hungary"},{"name":"Doctoral School of Theoretical Medicine, Faculty of Medicine, University of Szeged, D\u00f3mt\u00e9r 10, H-6720 Szeged, Hungary"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Azzurra","family":"Stefanucci","sequence":"additional","affiliation":[{"name":"Department of Pharmacy, University of Chieti-Pescara \u201cG. d\u2019Annunzio\u201d, Via dei Vestini 31, 66100 Chieti, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Marilisa Pia","family":"Dimmito","sequence":"additional","affiliation":[{"name":"Department of Pharmacy, University of Chieti-Pescara \u201cG. d\u2019Annunzio\u201d, Via dei Vestini 31, 66100 Chieti, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ferenc","family":"Z\u00e1dor","sequence":"additional","affiliation":[{"name":"Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, Temesv\u00e1ri krt. 62., H-6726 Szeged, Hungary"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5926-6194","authenticated-orcid":false,"given":"Stefano","family":"Pieretti","sequence":"additional","affiliation":[{"name":"National Center for Drug Research and Evaluation, Istituto Superiore di Sanit\u00e0, Viale Regina Elena 299, 00161 Rome, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6548-7823","authenticated-orcid":false,"given":"Gokhan","family":"Zengin","sequence":"additional","affiliation":[{"name":"Department of Biology, Science Faculty, Selcuk University, 42250 Konya, Turkey"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"L\u00e1szl\u00f3","family":"V\u00e9csei","sequence":"additional","affiliation":[{"name":"MTA-SZTE Neuroscience Research Group, Department of Neurology, Interdisciplinary Excellence Centre, Faculty of Medicine, University of Szeged, H-6725 Szeged, Hungary"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"S\u00e1ndor","family":"Benyhe","sequence":"additional","affiliation":[{"name":"Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, Temesv\u00e1ri krt. 62., H-6726 Szeged, Hungary"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Marianna","family":"Nalli","sequence":"additional","affiliation":[{"name":"Laboratory affiliated with the Institute Pasteur Italy-Cenci Bolognetti Foundation, Department of Drug Chemistry and Technologies, Sapienza University of Rome, Piazzale Aldo Moro 5, I-00185 Roma, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7242-4860","authenticated-orcid":false,"given":"Adriano","family":"Mollica","sequence":"additional","affiliation":[{"name":"Department of Pharmacy, University of Chieti-Pescara \u201cG. d\u2019Annunzio\u201d, Via dei Vestini 31, 66100 Chieti, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2020,2,12]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"9794","DOI":"10.1126\/science.aaf9794","article-title":"Kynurenines: Tryptophan\u2019s metabolites in exercise, inflammation, and mental health","volume":"357","author":"Cervenka","year":"2017","journal-title":"Science"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"8609218","DOI":"10.1155\/2019\/8609218","article-title":"Multifunctional pharmaceutical effects of the antibiotic daptomycin","volume":"2019","author":"Ye","year":"2019","journal-title":"BioMed Res. Int."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1365","DOI":"10.1021\/np8001282","article-title":"Cyclopeptides with Anti-inflammatory Activity from Seeds of Annona Montana","volume":"71","author":"Chuang","year":"2008","journal-title":"J. Nat. Prod."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"1735","DOI":"10.1002\/rcm.5041","article-title":"An unusual kynurenine-containig opioid tetrapeptide from the skin gland secretion of the Australian red tree frog Litoria rubella. Sequence determination by electrospray mass spectrometry","volume":"25","author":"Scanlon","year":"2011","journal-title":"Rapid Commun. Mass Spectrom."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"842","DOI":"10.1046\/j.1471-4159.2001.00498.x","article-title":"Kynurenine pathway metabolism in human astrocytes: A paradox for neuronal protection","volume":"78","author":"Guillemin","year":"2001","journal-title":"J. Neurochem."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"465","DOI":"10.1038\/nrn3257","article-title":"Kynurenines in the mammalian brain: When physiology meets pathology","volume":"13","author":"Schwarcz","year":"2012","journal-title":"Nat. Rev. Neurosci."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"64","DOI":"10.1038\/nrd3793","article-title":"Kynurenines in the CNS: Recent advances and new questions","volume":"12","author":"Toldi","year":"2013","journal-title":"Nat. Rev. Drug. Discov."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"607","DOI":"10.1007\/s40261-019-00793-6","article-title":"Melatonin and multiple sclerosis: From plausible neuropharmacological mechanisms of action to experimental and clinical evidence","volume":"39","author":"Mollica","year":"2019","journal-title":"Clin. Drug Investig."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"313","DOI":"10.1016\/0014-2999(88)90814-X","article-title":"Kynurenate and FG9041 have both competitive and non-competitive antagonist actions at excitatory amino acid receptors","volume":"151","author":"Birch","year":"1988","journal-title":"Eur. J. Pharmacol."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"570","DOI":"10.1016\/0014-4886(85)90072-X","article-title":"Actions of kynurenic acid and quinolinic acid in the rat hippocampus in vivo","volume":"88","author":"Perkins","year":"1985","journal-title":"Exp. Neurol."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"22021","DOI":"10.1074\/jbc.M603503200","article-title":"Kynurenic acid as a Ligand for Orphan G Protein-coupled Receptor GPR35","volume":"281","author":"Wang","year":"2006","journal-title":"J. Biol. Chem."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"69","DOI":"10.3389\/fphar.2015.00069","article-title":"The therapeutic potential of orphan GPCRs, GPR35 and GPR55","volume":"6","author":"Shore","year":"2015","journal-title":"Front. Pharmacol."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"79","DOI":"10.1016\/S0378-5173(00)00421-X","article-title":"Synthesis, pharmacokinetics and anticonvulsant activity of 7-chlorokynurenic acid prodrugs","volume":"202","author":"Bonina","year":"2000","journal-title":"Int. J. Pharm."},{"key":"ref_14","first-page":"8394","article-title":"Biosynthesis of l-4-Chlorokynurenine, an antidepressant prodrug and a non-proteinogenic amino acid found in lipopeptide","volume":"58","author":"Luhavaya","year":"2019","journal-title":"Antibiotics"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"559","DOI":"10.1021\/jm015556r","article-title":"Design, synthesis and activity of ascorbic acid prodrugs of nipecotic, kynurenic and diclophenamic acids, liable to increase neurotropic activity","volume":"45","author":"Manfredini","year":"2002","journal-title":"J. Med. Chem."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"21","DOI":"10.1016\/j.jns.2009.02.326","article-title":"The role of kynurenines in disorders of the central nervous system: Possibilities for neuroprotection","volume":"283","author":"Vamos","year":"2009","journal-title":"J. Neurol. Sci."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"417","DOI":"10.1007\/s00702-006-0545-z","article-title":"Kynurenine in combination with probenecid mitigates the stimulation-induced increase of c-fos immunoreactivity of the rat caudal trigeminal nucleus in an experimental migraine model","volume":"114","author":"Toldi","year":"2007","journal-title":"J. Neural. Transm."},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"G\u00e1bor, N.-G., Dvor\u00e1csk\u00f3, S., Boh\u00e1r, Z., Benyhe, S., T\u00f6mb\u00f6ly, C., P\u00e1rdutz, A., and V\u00e9csei, L. (2017). Interactions between the kynurenine and the endocannabinoid system with special emphasis on migraine. Int. J. Mol. Sci., 18.","DOI":"10.3390\/ijms18081617"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1386","DOI":"10.1111\/j.1742-4658.2012.08487.x","article-title":"Kynurenine pathway inhibition as a therapeutic strategy for neuroprotection","volume":"279","author":"Stone","year":"2012","journal-title":"FEBS J."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"2667","DOI":"10.1021\/acschemneuro.7b00229","article-title":"Multifunctional analogs of kynurenic acid for the treatment of Alzheimer\u2019s disease: Synthesis, pharmacology, and molecular modeling studies","volume":"8","author":"Deora","year":"2017","journal-title":"ACS Chem. Neurosci."},{"key":"ref_21","first-page":"865","article-title":"Neuroprotective effects of a novel kynurenic acid analogue in a transgenic mouse model of Huntington\u2019s disease","volume":"18","author":"Nyiri","year":"2011","journal-title":"J. Neurol. Transm."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"1928","DOI":"10.1021\/bc4003635","article-title":"Synthesis of multifunctional PAMAM-Aminoglycoside conjugates with enhanced transfection efficiency","volume":"24","author":"Ghilardi","year":"2013","journal-title":"Bioconjug Chem."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"22","DOI":"10.1111\/j.1751-1097.2010.00841.x","article-title":"Deactivation of excited states of kynurenine covalently linked to nitroxides","volume":"87","author":"Tsentalovich","year":"2011","journal-title":"Photochem. Photobiol."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"449","DOI":"10.1021\/acsmedchemlett.7b00044","article-title":"Opioid receptor activity and analgesic potency of DPDPE peptide analogues containing a xylene bridge","volume":"8","author":"Stefanucci","year":"2017","journal-title":"ACS Med. Chem. Lett."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"10302","DOI":"10.1021\/jm301258w","article-title":"Discovery and pharmacological evaluation of a diphenethylamine derivative (HS665), a highly potent and selectivek-opioid receptor agonist","volume":"55","author":"Spetea","year":"2012","journal-title":"J. Med. Chem."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"1763","DOI":"10.1016\/0024-3205(91)90214-V","article-title":"Opioid receptor labeling with the chloromethyl ketone derivative of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) II: Covalent labeling of mu opioid binding site by 3H-Tyr-D-Ala-Gly-(Me)Phe chloromethyl ketone","volume":"48","author":"Oktem","year":"1991","journal-title":"Life Sci."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"456","DOI":"10.1021\/cn5002792","article-title":"Synthesis and pharmacological evaluation of [(3)H]HS665, a novel, highly selective radioligand for the kappa opioid receptor","volume":"6","author":"Guerrieri","year":"2015","journal-title":"ACS Chem. Neurosci."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"587","DOI":"10.1016\/j.neuro.2006.12.007","article-title":"Decreased N-methyl-d-aspartic acid (NMDA) receptor levels are associated with mercury exposure in wild and captive mink","volume":"28","author":"Basu","year":"2007","journal-title":"Neurotoxicology"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"249","DOI":"10.1002\/(SICI)1097-4547(19970501)48:3<249::AID-JNR7>3.0.CO;2-F","article-title":"Met5-enkephalin-Arg6-Phe7, an endogenous neuropeptide, binds to multiple opioid and nonopioid sites in rat brain","volume":"48","author":"Benyhe","year":"1997","journal-title":"J. Neurosci. Res."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"14","DOI":"10.1016\/j.neuint.2013.12.005","article-title":"Micromolar concentrations of rimonabant directly inhibits delta opioid receptor specific ligand binding and agonist-induced G-protein activity","volume":"67","author":"Kocsis","year":"2014","journal-title":"Neurochem. Int."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"248","DOI":"10.1016\/0003-2697(76)90527-3","article-title":"A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding","volume":"72","author":"Bradford","year":"1976","journal-title":"Anal. Biochem."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"7242","DOI":"10.1073\/pnas.92.16.7242","article-title":"In vitro autoradiography of receptor-activated G proteins in rat brain by agonist-stimulated guanylyl 5\u2019-[gamma-[35S]thio]-triphosphate binding","volume":"92","author":"Sim","year":"1995","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_33","first-page":"848","article-title":"Modulation by mu-opioid agonists of guanosine-5\u2019-O-(3-[35S]thio)triphosphate binding to membranes from human neuroblastoma SH-SY5Y cells","volume":"47","author":"Traynor","year":"1995","journal-title":"Mol. Pharmacol."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"522","DOI":"10.1002\/anie.197805221","article-title":"Simple Method for the Esterification of Carboxylic Acids","volume":"17","author":"Neises","year":"1978","journal-title":"Angew. Chem. Int. Ed. Engl."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"167","DOI":"10.1016\/j.ejmech.2013.07.044","article-title":"Hybrid peptides endomorphin-2\/DAMGO: Design, synthesis and biological evaluation","volume":"68","author":"Mollica","year":"2013","journal-title":"Eur. J. Med. Chem."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"859","DOI":"10.4155\/fmc-2016-0232","article-title":"Fluorescent-labeled bioconjugates of the opioid peptides biphalin and DPDPE incorporating fluorescein-maleimide linkers","volume":"9","author":"Stefanucci","year":"2017","journal-title":"Future Med. Chem."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"1520","DOI":"10.2174\/1871527314666141205164114","article-title":"Inhibition of opioid receptor mediated G-Protein activity after chronic administration of kynurenic acid and its derivative without direct binding to opioid receptors","volume":"13","author":"Samavati","year":"2014","journal-title":"CNS Neurol. Disord. Drug Targets"},{"key":"ref_38","first-page":"314P","article-title":"Characterization of the antagonism of excitatory amino acid receptors in rat cortex by kynurenic acid","volume":"91","author":"Kemp","year":"1987","journal-title":"Br. J. Pharmacol."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"3099","DOI":"10.1016\/0006-2952(73)90196-2","article-title":"Relationship between the inhibition constant (K1) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction","volume":"22","author":"Cheng","year":"1973","journal-title":"Biochem. Pharmacol."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"209","DOI":"10.1016\/S0014-2999(99)00610-X","article-title":"Mu-opioid receptor specific antagonist cyprodime: Characterization by in vitro radioligand and [35S]GTP\u03b3S binding assays","volume":"383","author":"Monory","year":"1999","journal-title":"Eur. J. Pharmacol."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"185","DOI":"10.1016\/0014-2999(88)90502-X","article-title":"Naltrindole, a highly selective and potent non-peptide delta opioid receptor antagonist","volume":"146","author":"Portoghese","year":"1988","journal-title":"Eur. J. Pharmacol."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"121","DOI":"10.36076\/ppj.2008\/11\/S121","article-title":"Peripherally-acting opioids","volume":"11","author":"Smith","year":"2008","journal-title":"Pain Physician"},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"13525","DOI":"10.1073\/pnas.0705924104","article-title":"TRPA1 mediates formalin-induced pain","volume":"104","author":"McNamara","year":"2007","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"433","DOI":"10.1146\/annurev-med-062613-093100","article-title":"Opioid receptors","volume":"67","author":"Stein","year":"2016","journal-title":"Annu. Rev. Med."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"307","DOI":"10.1016\/j.brainres.2011.01.083","article-title":"Opioid receptor agonists reduce brain edema in stroke","volume":"1383","author":"Yang","year":"2011","journal-title":"Brain Res."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"499","DOI":"10.1124\/jpet.111.184127","article-title":"Characterization of neuroprotective effects of biphalin, an opioid receptor agonist, in a model of focal brain ischemia","volume":"339","author":"Yang","year":"2011","journal-title":"J. Pharmacol. Exp. Ther."},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.brainres.2015.03.022","article-title":"In vitro and in vivo efficacy of a potent opioid receptor agonist, biphalin, compared to subtype-selective opioid receptor agonists for stroke treatment","volume":"1609","author":"Yang","year":"2015","journal-title":"Brain Res."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"50","DOI":"10.1016\/j.peptides.2015.04.020","article-title":"Peptides at the blood brain barrier: Knowing me knowing you","volume":"72","author":"Davis","year":"2015","journal-title":"Peptides"}],"container-title":["Biomolecules"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2218-273X\/10\/2\/284\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T08:57:13Z","timestamp":1760173033000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2218-273X\/10\/2\/284"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2020,2,12]]},"references-count":48,"journal-issue":{"issue":"2","published-online":{"date-parts":[[2020,2]]}},"alternative-id":["biom10020284"],"URL":"https:\/\/doi.org\/10.3390\/biom10020284","relation":{},"ISSN":["2218-273X"],"issn-type":[{"value":"2218-273X","type":"electronic"}],"subject":[],"published":{"date-parts":[[2020,2,12]]}}}