Identification and pharmacological profile of SPP1, a potent, functionally selective and brain penetrant agonist at muscarinic M1 receptors

Background and Purpose Experimental Approach We aimed to identify and develop novel, selective muscarinic M-1 receptor agonists as potential therapeutic agents for the symptomatic treatment of Alzheimer's disease. We developed and utilized a novel M-1 receptor occupancy assay to drive a structure activity relationship in a relevant brain region while simultaneously tracking drug levels in plasma and brain to optimize for central penetration. Functional activity was tracked in relevant native in vitro assays allowing translational (rat-human) benchmarking of structure-activity relationship molecules to clinical comparators. Key Results Conclusions and Implications Using this paradigm, we identified a series of M-1 receptor selective molecules displaying desirable in vitro and in vivo properties and optimized key features, such as central penetration while maintaining selectivity and a partial agonist profile. From these compounds, we selected spiropiperidine 1 (SPP1). In vitro, SPP1 is a potent, partial agonist of cortical and hippocampal M-1 receptors with activity conserved across species. SPP1 displays high functional selectivity for M-1 receptors over native M-2 and M-3 receptor anti-targets and over a panel of other targets. Assessment of central target engagement by receptor occupancy reveals SPP1 significantly and dose-dependently occupies rodent cortical M-1 receptors. We report the discovery of SPP1, a novel, functionally selective, brain penetrant partial orthosteric agonist at M-1 receptors, identified by a novel receptor occupancy assay. SPP1 is amenable to in vitro and in vivo study and provides a valuable research tool to further probe the role of M-1 receptors in physiology and disease.