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

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.

Development of a photoactivatable allosteric ligand for the m1 muscarinic acetylcholine receptor.

The field of G protein-coupled receptor drug discovery has benefited greatly from the structural and functional insights afforded by photoactivatable ligands. One G protein-coupled receptor subfamily for which photoactivatable ligands have been developed is the muscarinic acetylcholine receptor family, though, to date, all such ligands have been designed to target the orthosteric (endogenous ligand) binding site of these receptors. Herein we report the synthesis and pharmacological investigation of a novel photoaffinity label, MIPS1455 (4), designed to bind irreversibly to an allosteric site of the M1 muscarinic acetylcholine receptor; a target of therapeutic interest for the treatment of cognitive deficits. MIPS1455 may be a valuable molecular tool for further investigating allosteric interactions at this receptor.

Synthesis and pharmacological evaluation of analogues of benzyl quinolone carboxylic acid (BQCA) designed to bind irreversibly to an allosteric site of the M 1 muscarinic acetylcholine receptor.

Activation of the M1 muscarinic acetylcholine receptor (mAChR) is a prospective treatment for alleviating cognitive decline experienced in central nervous system (CNS) disorders. Current therapeutics indiscriminately enhance the activity of the endogenous neurotransmitter ACh, leading to side effects. BQCA is a positive allosteric modulator and allosteric agonist at the M1 mAChR that has high subtype selectivity and is a promising template from which to generate higher affinity, more pharmacokinetically viable drug candidates. However, to efficiently guide rational drug design, the binding site of BQCA needs to be conclusively elucidated. We report the synthesis and pharmacological validation of BQCA analogues designed to bind irreversibly to the M1 mAChR. One analogue in particular, 11, can serve as a useful structural probe to confirm the location of the BQCA binding site; ideally, by co-crystallization with the M1 mAChR. Furthermore, this ligand may also be used as a pharmacological tool with a range of applications.

Development of M1 mAChR allosteric and bitopic ligands: prospective therapeutics for the treatment of cognitive deficits.

Since the cholinergic hypothesis of memory dysfunction was first reported, extensive research efforts have focused on elucidating the mechanisms by which this intricate system contributes to the regulation of processes such as learning, memory, and higher executive function. Several cholinergic therapeutic targets for the treatment of cognitive deficits, psychotic symptoms, and the underlying pathophysiology of neurodegenerative disorders, such as Alzheimer's disease and schizophrenia, have since emerged. Clinically approved drugs now exist for some of these targets; however, they all may be considered suboptimal therapeutics in that they produce undesirable off-target activity leading to side effects, fail to address the wide variety of symptoms and underlying pathophysiology that characterize these disorders, and/or afford little to no therapeutic effect in subsets of patient populations. A promising target for which there are presently no approved therapies is the M1 muscarinic acetylcholine receptor (M1 mAChR). Despite avid investigation, development of agents that selectively activate this receptor via the orthosteric site has been hampered by the high sequence homology of the binding site between the five muscarinic receptor subtypes and the wide distribution of this receptor family in both the central nervous system (CNS) and the periphery. Hence, a plethora of ligands targeting less structurally conserved allosteric sites of the M1 mAChR have been investigated. This Review aims to explain the rationale behind allosterically targeting the M1 mAChR, comprehensively summarize and critically evaluate the M1 mAChR allosteric ligand literature to date, highlight the challenges inherent in allosteric ligand investigation that are impeding their clinical advancement, and discuss potential methods for resolving these issues.