[Two complex action principles in a single structure. Mixed opioid agonists-antagonists and partial agonists]

Enantiomerically Pure Quinoline-Based κ-Opioid Receptor Agonists: Chemoenzymatic Synthesis and Pharmacological Evaluation

Racemic K -opioid receptor (KOR) agonist 2-(3,4-dichlorophenyl)-1-[(4aRS,8SR,8aSR)-8-(pyrrolidin-1-yl)-3,4,4a,5,6,7,8,8a-octahydroquinolin-1(2H)-yl]ethan-1-one ((±)-4) was prepared in a diastereoselective synthesis. The first key step of the synthesis was the diastereoselective hydrogenation of the silyl ether of 1,2,3,4-tetrahydroquinoin-8-ol ((±)-9) to afford cis,cis-configured perhydroquinoline derivative (±)-10. Removal of the TBDMS protecting group led to a β-aminoalcohol that reacted with SO2 Cl2 to form an oxathiazolidine. Nucleophilic substitution with pyrrolidine resulted in the desired cis,trans-configured perhydroquinoline upon inversion of the configuration. In order to obtain enantiomerically pure KOR agonists 4 (99.8 % ee) and ent-4 (99.0 % ee), 1,2,3,4-tetrahydroquinolin-8-ols (R)-8 (99.1 % ee) and (S)-8 (98.4 % ee) were resolved by an enantioselective acetylation catalyzed by Amano lipase PS-IM. The absolute configuration was determined by CD spectroscopy. The 4aR,8S,8aS-configured enantiomer 4 showed sub-nanomolar KOR affinity (Ki =0.81 nM), which is more than 200 times higher than the KOR affinity of its enantiomer ent-4. In the cAMP assay and the Tango β-arrestin-2 recruitment assay, 4 behaved as a KOR agonist. Upon incubation of human macrophages, human dendritic cells, and mouse myeloid immune cells with 4, the number of cells expressing co-stimulatory receptor CD86 and proinflammatory cytokines interleukin 6 and tumor necrosis factor α was significantly reduced; this indicates the strong anti-inflammatory activity of 4. The anti-inflammatory effects correlated well with the KOR affinity: (4aR,8S,8aS)-4 was slightly more potent than the racemic mixture (±)-4, and the distomer ent-4 was almost inactive.

Stereoselective synthesis of conformationally restricted KOR agonists based on the 2,5-diazabicyclo[2.2.2]octane scaffold

It has been postulated that the KOR affinity depends on the dihedral angle of the ethylenediamine pharmacophore. Herein, 2,5-diazabicyclooctanes bearing a pyrrolidino moiety in the 7-position were envisaged to study KOR agonists with a conformationally rigid ethylenediamine pharmacophore and thus a defined N(pyrrolidine)-C7-C1-N2 dihedral angle. The first approach with an intramolecular addition at the chiral sulfinylimines 9 failed to give bicyclic products. The key step in the second approach was a Dieckmann analogous cyclization providing mixed methyl silyl ketals 11a-e as key intermediates. The highest KOR affinity was found for the 2,5-dibenzyl substituted derivatives (S,R,S)-16a (K_i = 31 nM) and (R,S,R)-16a (K_i = 74 nM) with the pyrrolidine ring oriented towards N-5. The high KOR affinity of (S,R,S)-16a is unexpected, since the KOR pharmacophoric ethylenediamine system adopts a dihedral angle of about 160°, which is quite different from the angle of the energetically most favored conformer of the flexible and potent KOR agonist 2. (S,R,S)-16a represents a KOR agonist with moderate selectivity over MOR (8-fold) and DOR (5-fold), but high selectivity over both σ receptor subtypes. In the [³⁵S]GTPγS assay (S,R,S)-16a reacted as a full KOR agonist with an EC₅₀ value of 240 nM.