An Approach to Discovering Novel Muscarinic M1 Receptor Positive Allosteric Modulators with Potent Cognitive Improvement and Minimized Gastrointestinal Dysfunction

Activation of muscarinic M 1 receptor (M 1 R) is a promising approach for improving cognitive impairment in Alzheimer’s disease. However, an M 1 R-selective positive allosteric modulator (PAM), benzyl quinolone carboxylic acid (BQCA), at 30 mg/kg, induced diarrhea in wild-type mice, but not in M 1 R knockout mice. Moreover, BQCA (0.1–1000 nM) augmented electric field stimulation (EFS)-induced ileum contraction in an in vitro Magnus assay. Thus, we decided to establish a drug-screening strategy to discover novel M 1 PAMs producing potent cognitive improvement with minimized gastrointestinal (GI) dysfunction. We assessed PAM parameters of various M 1 PAMs with ≥100-fold selectivity over other muscarinic receptor subtypes by using in vitro binding and functional analysis. Evaluation of these M 1 PAMs in the Magnus assay revealed a significant correlation between percentage of ileum contractions at 1 μ M and their α -value, a PAM parameter associated with the binding cooperativity between acetylcholine and M 1 PAM. M 1 PAMs with lower α -value showed lower impact on EFS-induced ileum contraction. Next, we characterized in vivo profiles of two M 1 PAMs: compound A (log α = 1.18) and compound B (log α = 3.30). Compound A, at 30 mg/kg, significantly improved scopolamine-induced cognitive deficits without prominent signs of diarrhea at up to 1000 mg/kg in mice. In contrast, compound B, at 10 mg/kg, showed both significant improvement of scopolamine-induced cognitive deficits and severe diarrhea. Thus, fine adjustment of the α -values could be a key to discovering M 1 PAMs yielding potent cognitive improvement with a lower risk of GI effects.