Heterodimerization of Dibenzodiazepinone-Type Muscarinic Acetylcholine Receptor Ligands Leads to Increased M(2)R Affinity and Selectivity

[Image: see text] In search for selective ligands for the muscarinic acetylcholine receptor (MR) subtype M(2), the dimeric ligand approach, that is combining two pharmacophores in one and the same molecule, was pursued. Different types (agonists, antagonists, orthosteric, and allosteric) of monomeric MR ligands were combined by various linkers with a dibenzodiazepinone-type MR antagonist, affording five types of heterodimeric compounds (“DIBA-xanomeline,” “DIBA-TBPB,” “DIBA-77-LH-28-1,” “DIBA-propantheline,” and “DIBA-4-DAMP”), which showed high M(2)R affinities (pK(i) > 8.3). The heterodimeric ligand UR-SK75 (46) exhibited the highest M(2)R affinity and selectivity [pK(i) (M(1)R–M(5)R): 8.84, 10.14, 7.88, 8.59, and 7.47]. Two tritium-labeled dimeric derivatives (“DIBA-xanomeline”-type: [(3)H]UR-SK71 ([(3)H]44) and “DIBA-TBPB”-type: [(3)H]UR-SK59 ([(3)H]64)) were prepared to investigate their binding modes at hM(2)R. Saturation-binding experiments showed that these compounds address the orthosteric binding site of the M(2)R. The investigation of the effect of various allosteric MR modulators [gallamine (13), W84 (14), and LY2119620 (15)] on the equilibrium (13–15) or saturation (14) binding of [(3)H]64 suggested a competitive mechanism between [(3)H]64 and the investigated allosteric ligands, and consequently a dualsteric binding mode of 64 at the M(2)R.