Structural Features of Iperoxo–BQCA Muscarinic Acetylcholine Receptor Hybrid Ligands Determining Subtype Selectivity and Efficacy

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Selective agonists for the human M1 and M4 muscarinic acetylcholine receptors (mAChRs) are attractive candidates for the treatment of cognitive disorders, such as Alzheimer's disease and schizophrenia. Past efforts to optimize a ligand for selective agonism at any one of the M1-M5 mAChR subtypes has proven to be a significant challenge. Recently, research efforts have demonstrated that hybrid ligands may offer a potential solution to the lack of selectivity at mAChRs. In an attempt to design M1 mAChR selective agonists by hybridizing an M1 mAChR selective positive allosteric modulator [1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid] and a potent agonist [(4-[(4,5-dihydro-3-isoxazolyl)oxy]-N,N,N-trimethyl-2-butyn-1-aminium iodide) (iperoxo)], we unexpectedly discovered that these ligands possessed noticeable M2/M4 mAChR selectivity. Evaluation of truncated derivatives of the hybrid ligands at the M1-M5 mAChR subtypes suggests that the allosteric pharmacophore of iperoxo-based mAChR hybrid ligands likely sterically disrupts the allosteric site of the mAChRs, attenuating the efficacy of M1/M3/M5 mAChR responses compared to M2/M4 mAChRs, resulting in a preference for the M2/M4 mAChRs. However, at certain intermediate linker lengths, the effects of this apparent disruption of the allosteric site are diminished, restoring nonselective agonism and suggesting a possible allosteric interaction which is favorable to efficacy at all M1-M5 mAChRs.