An Approach to Discovering Novel Muscarinic M₁ Receptor Positive Allosteric Modulators with Potent Cognitive...

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 drug-screening 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 μM and their α-value, a PAM parameter associated with the binding cooperativity between acetylcholine and M₁ PAM. M₁ PAMs with lower α-value showed lower impact on EFS-induced ileum contraction. Next, we characterized in vivo profiles of two M₁ 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 M₁ PAMs yielding potent cognitive improvement with a lower risk of GI effects.