Synthesis and Pharmacological Evaluation of Fluorinated Quinoxaline-Based κ-Opioid Receptor (KOR) Agonists Designed for PET Studies

Giovanni Tangherlini, Frederik Börgel, Dirk Schepmann, Samuel Slocum, Tao Che, Stefan Wagner, Katrin Schwegmann, Sven Hermann, Nadine Mykicki, Karin Loser, Bernhard Wünsch

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5 Scopus citations


κ-Opioid receptors (KORs) play a predominant role in pain alleviation, itching skin diseases, depression and neurodegenerative disorders such as multiple sclerosis. Therefore, imaging of KOR by a fluorinated PET tracer was envisaged. Two strategies were followed to introduce a F atom into the very potent class of cis,trans-configured perhydroquinoxalines. Whereas the synthesis of fluoroethyltriazole 2 has already been reported, fluoropyrrolidines 14 (1-[2-(3,4-dichlorophenyl)acetyl]-8-[(R)-3-fluoropyrrolidin-1-yl]-perhydroquinoxalines) were prepared by SN2 substitution of a cyclic sulfuric acid derivative with hydroxypyrrolidine and subsequent transformation of the OH moiety into a F substituent. Fluoropyrrolidines 14 showed similar low-nanomolar KOR affinity and selectivity to the corresponding pyrrolidines, but the corresponding alcohols were slightly less active. In the cAMP and β-arrestin assay, 14b (proton at the 4-position) exhibited similar KOR agonistic activity as U-50,488. The fluoro derivatives 14b and 14c (CO2CH3 at the 4-position) revealed KOR-mediated anti-inflammatory activity as CD11c and the IFN-γ production were reduced significantly in mouse and human dendritic cells. Compounds 14b and 14-c also displayed anti-inflammatory and immunomodulatory activity in mouse and human T cells. The PET tracer [18F]-2 was prepared by 1,3-dipolar cycloaddition. In vivo, [18F]-2 did not label KOR due to very fast elimination kinetics. Nucleophilic substitution of a mesylate precursor provided [18F]-14c. Unfortunately, defluorination of [18F]-14c occurred in vivo, which was analyzed in detail by in vitro studies.

Original languageEnglish
Pages (from-to)1834-1853
Number of pages20
Issue number19
StatePublished - Oct 5 2020


  • PET tracers
  • anti-inflammatory activity
  • effector cells
  • fluorine
  • opioid receptor agonists
  • perhydroquinoxaline


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