Alpha7 nicotinic acetylcholine receptor silent agonists: exploration of the SAR for NS6740

Silent agonists are a new class of nicotinic acetylcholine receptor (nAChR) ligands, able to desensitize the receptor with little if any channel activation. A number of silent agonists, including NS6740 ((1,4-Diazabicyclo[3.2.2]non-4-yl[5-[3-(trifluoromethyl)phenyl]-2-furanyl]methanone)) show promising anti-inflammatory activity, presumably via non-ionotropic mechanisms. Here we explore the importance of nitrogen quaternization and the role of the meta trifluoromethyl substituent. The synthesis of compounds MCP4 (1,4-diazabicyclo[3.2.2]nonan-4-yl(5-phenylfuran-2-yl)methanone)), MCP9 (1-methyl-4-(5-(3-(trifluoromethyl)phenyl)furan-2-carbonyl)-1,4-diazabicyclo[3.2.2]nonan-1-ium iodide) , MCP12 (1,4-Diazabicyclo[3.2.2]non-4-yl[5-[3-(methyl)phenyl]-2-furanyl]methanone) were synthesized in ~ 50 overall yields in four steps featuring Suzuki-Miyaura couplings of methyl-5-bromo-2-furoate with differently substituted aryl boronic moieties. The new compounds were assayed employing the human α7 nAChRs expressed in Xenopus laevis oocytes and two electrode voltage clamping. The tests were conducted with 60μM ACh as pre- and post-application controls, with 10μM drug and 10 μM of type II PAM PNU-120596. Applications of PAM render desensitized states conductive, and thus detectable by this method. MCP4 and MCP12 were weak partial agonists (40% of ACh response), with EC50 ≈ 30-50 μM and an intermediate desensitizing ability. MCP9 appeared to be a more potent partial agonist, characterized by an EC50 ≈ 3-5 µM, with an efficacy about 60% of ACh. Intermediate desensitizing properties were observed for this quaternary ammonium salt as well. The trifluoromethyl group appears critical for effective desensitization. The N-methyl compound (MCP9) lost silent agonism, possibly due to a steric effect in the receptor binding site. These results suggest the charged ammonium and trifluoromethyl groups are critical components of the NS6740 silent agonism pharmacophore.