Synthesis and pharmacological investigation of allosteric and bitopic ligands for the M₁ muscarinic acetylcholine receptor

Abstract

This thesis describes the development of allosteric and bitopic molecular tools for the M₁ muscarinic acetylcholine receptor (M₁ mAChR), a potential therapeutic target for the treatment of cognitive deficits experienced in central nervous system (CNS) disorders. Chapter 1 presents a comprehensive review article that provides a rationale for targeting the M₁ mAChR and an account of the M₁ mAChR-selective allosteric and bitopic ligands that have emerged to date. The perspective at the end of Chapter 1 highlights the ongoing complexities encountered in the advancement of allosteric and bitopic ligand drug discovery. This perspective serves as a basis for the research undertaken in this thesis, as we contend that the development of molecular tools that are able to serve as structural and functional probes may contribute to the resolution of these complexities. Chapters 2 and 3 describe the synthesis and pharmacological evaluation of a panel of putative irreversible allosteric ligands based on the structure of benzyl quinolone carboxylic acid (BQCA), the first reported orally-bioavailable M₁ mAChR allosteric ligand exhibiting absolute subtype selectivity. From this research, an electrophilic irreversible allosteric ligand MIPS1262 (11, Chapter 2) and a photoactivatable irreversible allosteric ligand MIPS1455 (4, Chapter 3), were developed, affording novel molecular tools for further probing allosteric interactions at the M1 mAChR. Chapter 4 details the development of novel putative bitopic ligands for the M1 mAChR, including the selection of the constituent ligands (iperoxo and BQCA), the design and synthetic incorporation of linkers into their respective structures, and the synthesis and preliminary pharmacology of two putative bitopic ligands (33a and 44, Chapter 4). Such ligands may prove useful in investigating specific structural and functional hypotheses at the M1 mAChR, in addition to presenting as a prospective therapeutic approach for the treatment of cognitive deficits. In Chapter 5, known BQCA analogues exhibiting improved properties compared to the parent molecule were synthesized and subjected to more detailed pharmacological techniques and analysis than have been previously reported. Briefly, we demonstrate that it may be possible to introduce structural modifications into allosteric ligands to specifically ‘fine-tune’ individual pharmacological properties and use such molecules as in vitro and in vivo tools to delineate the requirements for optimal therapeutic effect without side effects. A diverse toolbox of ligands were synthesized and pharmacologically validated for use in future studies of the M₁ mAChR. A discussion of potential directions for future research is provided in Chapter 6.