The Role of Orthosteric Building Blocks of Bitopic Ligands for Muscarinic M1 Receptors

The muscarinic M-1 acetylcholine receptor is an important drug target for the treatment of various neurological disorders. Designing M-1 receptor-selective drugs has proven challenging, mainly due to the high conservation of the acetylcholine binding site among muscarinic receptor subtypes. Therefore, less conserved and topographically distinct allosteric binding sites have been explored to increase M-1 receptor selectivity. In this line, bitopic ligands, which target orthosteric and allosteric binding sites simultaneously, may provide a promising strategy. Here, we explore the allosteric, M-1-selective BQCAd scaffold derived from BQCA as a starting point for the design, synthesis, and pharmacological evaluation of a series of novel bitopic ligands in which the orthosteric moieties and linker lengths are systematically varied. Since beta-arrestin recruitment seems to be favorable to therapeutic implication, all the compounds were investigated by G protein and beta-arrestin assays. Some bitopic ligands are partial to full agonists for G protein activation, some activate beta-arrestin recruitment, and the degree of beta-arrestin recruitment varies according to the respective modification. The allosteric BQCAd scaffold controls the positioning of the orthosteric ammonium group of all ligands, suggesting that this interaction is essential for stimulating G protein activation. However, beta-arrestin recruitment is not affected. The novel set of bitopic ligands may constitute a toolbox to study the requirements of beta-arrestin recruitment during ligand design for therapeutic usage.