Synthesis and evaluation of xanomeline analogs-Probing the wash-resistant phenomenon at the M₁ muscarinic acetylcholine receptor

A series of xanomeline analogs were synthesized and evaluated for binding at the M₁ muscarinic acetylcholine receptor (M₁ receptor). Specifically, compounds that substitute the O-hexyl chain of xanomeline with polar, ionizable, or conformationally restricted moieties were assessed for their ability to bind to the M₁ receptor in a wash-resistant manner (persistent binding). From our screen, several novel ligands that persistently bind to the M₁ receptor with greater affinity than xanomeline were discovered. Results indicate that persistent binding may arise not only from hydrophobic interactions but also from ionic interactions with a secondary M₁ receptor binding site. Herein, a qualitative model that accounts for both binding scenarios is proposed and applied to understand the structural basis to wash-resistant binding and long-acting effects of xanomeline-based compounds.