Silent agonism mediated by the α7 nicotinic acetylcholine receptor: the role of trifluoromethyl group in the NS6740 molecular skeleton

The hypothesis that states a relationship between non-ionotropic activity of the α7 nicotinic acetylcholine receptor (nAChR) and its desensitized states is an emerging research topic. The ability of a new class of compounds, defined as “silent agonists”, to stabilize the desensitized states of the α7 nicotinic acetylcholine receptor engendering anti-inflammatory responses, most likely via a metabotropic mechanism, would seem to confirm that hypothesis (1). In this study, we dissected the exemplary compound NS6740 (2) ((1,4-Diazabicyclo[3.2.2]-non-4-yl[5-[3-(trifluoromethyl)phenyl]-2-furanyl]methanone)), the most potent desensitizing agent for the α7 nAChR, characterized by both profound desensitization and relatively long term binding to the receptor. NS6740 shows promising anti-inflammatory activity, both in vitro and in vivo (3,4). In particular, we explored the role of the meta trifluoromethyl substituent of the phenyl ring in inducing the silent agonist binding mode. Compounds MCP5, MCP6, MCP7, MCP8 were prepared by introducing halogen atoms, i.e. fluorine, chlorine, bromine, iodine with increasing size in the meta position of the phenyl ring; MCP18, instead, showed the original trifluromethyl group moved on para position. (Figure 1). Figure 1. Structures of NS6740 and the newly synthesized derivatives. Two-electrode voltage clamping was employed to assess the electrophysiological profile of the newly synthesized compounds. Each experiment was conducted with 10 μM of drug, 60 μM ACh pre- and post-control, 10 μM type-II PAM PNU-120596 (5). Among the new compounds, we observed for MCP5 the highest residual potentiated response in the presence of type-II PAM PNU-120596. Taken together, our data suggest the meta trifluoromethyl group has a crucial role in minimizing the partial agonist behavior. Moreover, we found that the ability to stabilize the desensitized states of the α7 nAChR is preserved when trifluoromethyl is replaced by halogen atoms. When the CF3 group is moved on para position, the desensitizing activity is compromised, suggesting the meta substitution is strictly required. (1) Horenstein, N.A. and Papke, R.L., ACS Med. Chem. Lett. 2017, 8, 989-991 (2) Peters, D. et al., Novel diazabicyclic aryl derivatives, WO 2004/076453 (3) Thomsen, M. S. et al., Journal of Neuroimmunology 2012, 251, 65-72 (4) Papke, R.L. et al., Neuropharmacology 2015, 91, 34-42 (5) Papke, R.L. et al., Methods, 2010, 51, 121-133