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

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.

Design and Synthesis of Enantiomerically Pure Decahydroquinoxalines as Potent and Selective κ-Opioid Receptor Agonists with Anti-Inflammatory Activity in Vivo

In order to develop novel κ agonists restricted to the periphery, a diastereo- and enantioselective synthesis of (4aR,5S,8aS)-configured decahydroquinoxalines 5-8 was developed. Physicochemical and pharmacological properties were fine-tuned by structural modifications in the arylacetamide and amine part of the pharmacophore as well as in the amine part outside the pharmacophore. The decahydroquinoxalines 5-8 show single-digit nanomolar to subnanomolar κ-opioid receptor affinity, full κ agonistic activity in the [³⁵S]GTPγS assay, and high selectivity over μ, δ, σ₁, and σ₂ receptors as well as the PCP binding site of the NMDA receptor. Several analogues were selective for the periphery. The anti-inflammatory activity of 5-8 after topical application was investigated in two mouse models of dermatitis. The methanesulfonamide 8a containing the (S)-configured hydroxypyrrolidine ring was identified as a potent (K_i = 0.63 nM) and highly selective κ agonist (EC₅₀ = 1.8 nM) selective for the periphery with dose-dependent anti-inflammatory activity in acute and chronic skin inflammation.