Synthesis of a Series of Structurally Diverse MB327 Derivatives and Their Affinity Characterization at the Nicotinic Acetylcholine Receptor

A novel series of 30 symmetric bispyridinium and related N‐heteroaromatic bisquaternary salts with a propane‐1,3‐diyl linker was synthesized and characterized for their binding affinity at the MB327 binding site of nicotinic acetylcholine receptor (nAChR) from Torpedo californica. Compounds targeting this binding site are of particular interest for research into new antidotes against organophosphate poisoning, as therapeutically active 4‐tert‐butyl‐substituted bispyridinium salt MB327 was previously identified as a nAChR re‐sensitizer. Efficient access to the target compounds was provided by newly developed methods enabling N‐alkylation of sterically hindered or electronically deactivated heterocycles exhibiting a wide variety of functional groups. Determination of binding affinities toward the MB327 binding site at the nAChR, using a recently developed mass spectrometry (MS)‐based Binding Assay, revealed that several compounds reached affinities similar to that of MB327 (pK_i=4.73±0.03). Notably, the newly prepared lipophilic 4‐tert‐butyl‐3‐phenyl‐substituted bispyridinium salt PTM0022 ( 3 h ) was found to have significantly higher binding affinity, with a pK_i value of 5.16±0.07, thus representing considerable progress toward the development of more potent nAChR re‐sensitizers.