M4 mAChR Modulator-1 (compound 23i) is a M4 mAChR positive allosteric modulator (PAM). M4 mAChR Modulator-1 exhibits significantly greater cooperativity with ACh in β-arrestin recruitment over G protein activation. M4 mAChR Modulator-1 displays weak PAM effect in G protein-mediated responses, but strong PAM effect in β-arrestin recruitment.