An approach to discovering novel muscarinic M1 receptor positive allosteric modulators with potent cognitive improvement and minimized gastrointestinal dysfunctions

Abstract

Activation of muscarinic M1 receptor (M1R) is a promising approach for improving cognitive impairment in Alzheimer's disease. However, an M1R-selective positive allosteric modulator (PAM), benzyl quinolone carboxylic acid (BQCA), at 30 mg/kg, induced diarrhea in wild-type mice, but not in M1R 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 drugscreening strategy to discover novel M1 PAMs producing potent cognitive improvement with minimized gastrointestinal (GI) dysfunction. We assessed PAM parameters of various M1 PAMs with $100-fold selectivity over other muscarinic receptor subtypes by using in vitro binding and functional analysis. Evaluation of these M1 PAMs in the Magnus assay revealed a significant correlation between percentage of ileum contractions at 1 mM and their a-value, a PAM parameter associated with the binding cooperativity between acetylcholine and M1 PAM. M1 PAMs with lower a-value showed lower impact on EFS-induced ileum contraction. Next, we characterized in vivo profiles of twoM1 PAMs: compound A (log a51.18) and compound B (log a 5 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 a-values could be a key to discoveringM1 PAMs yielding potent cognitive improvement with a lower risk of GI effects.