Discovery, Optimization, and Characterization of Novel Subtype-Selective M5 Muscarinic Acetylcholine Receptor Ligands

Abstract

The muscarinic acetylcholine receptors (mAChRs) are a family of G protein-coupled receptors comprised of five subtypes (M1-5) expressed throughout the central nervous system (CNS). These receptors, together with their endogenous agonist, acetylcholine (ACh), play a vital role in regulating a wide range of physiological functions. Recent advances in the discovery of highly subtype-specific ligands for M1 and M4 have enabled pharmacologists to begin elucidating the discrete functions of these subtypes. However, discovery of M5-selective ligands has remained challenging due to the highly conserved nature of the orthosteric pocket across the mAChR subtypes. Phenotypic studies of M5-knockout mice have suggested that activation of M5 may be therapeutically relevant for the treatment of cerebrovascular diseases, acute ischemic stroke, and Alzheimer’s disease. Furthermore, M5’s localization in dopaminergic neurons of the ventral tegmental area suggests that inhibition of M5 signaling may provide novel therapies for the treatment of addictive behavior.

Here we use multidimensional iterative parallel synthesis and in vitro molecular pharmacology techniques to identify, optimize, and pharmacologically characterize highly selective M5 positive or negative allosteric modulators (PAMs or NAMs, respectively) for use as probes in the investigation of M5’s role in the CNS. The optimization and characterization of four chemical series possessing sub-micromolar potencies are described: an isatin-based M5-preferring PAM, VU0467903 (ML326; hM5 EC50 = 0.41 μM); a non-isatin M5-preferring PAM, VU0481443 (ML380; hM5 EC50 = 0.19 μM); the first highly M5-selective NAM, VU0483253 (ML375; hM5 IC50 = 0.30 μM); and a highly M5-selective orthosteric antagonist, VU0488130 (ML381; hM5 IC50 = 0.45 μM).