Discovery of Small Molecule Modulators targeting Secretin Receptors

Abstract

Class B G protein-coupled receptors (GPCRs) are a highly important group of therapeutic targets with strong physiologic implications in cardiovascular, metabolic, or gastrointestinal systems. Endogenously activated by moderate-length peptide hormones, class B GPCRs entertain a complex binding mechanism to induce stimulatory signaling processes. The secretin receptor (SCTR) is a prototypical member of this receptor family, having clinical relevance in multiple diseases including heart failure, digestive disorders, diabetes and obesity. [1-4] However, its endogenous ligand secretin and peptide-based analogs are the only ligands known to interact with SCTRs, likely due to challenges in class B GPCR drug discovery [5]. To find orally available therapeutic leads targeting this “undruggable” receptor, we focused on identifying positive allosteric modulators (PAMs), which act via structurally distinct binding pockets and potentiate hormonal signaling of native agonists. Thus, we developed a testing funnel tailored to discover SCTR small molecule modulators. After screening a 12,000-compound library with a unique primary assay that used a mixture of stimulators, we identified three structurally related scaffolds (A-C) enhancing peptidic SCTR activation [5]. Applying a comprehensive toolbox with binding and functional assays, we studied structure-activity relationships of a well-considered set of commercially available analogs incorporating these three scaffolds. Our investigations afforded an optimized analog B2, which showed substantial positive cooperativity to low-potent peptide agonists and significantly decelerated secretin dissociation. Due to the innate electrophilicity of scaffold set A, we recognized a structural similarity to PAMs acting on glucagon-like peptide-1 receptor (GLP-1R), a prominent member of class B GPCR family. We further demonstrated that a subset of SCTR PAMs was able to augment not only SCTR signaling but also GLP-1R functions. A recent study described that dual activation of SCTRs and GLP-1Rs was benefiting glucose homeostasis and appetite regulation while avoiding proliferative effects on pancreas mass [6]. Determining cooperativity between PAMs and selected orthosteric peptides, we established distinct selectivity and probe dependency profiles. To decipher reversible and covalent mechanisms-of-action as well as potential binding sites of novel PAMs, we conducted glutathione reactivity, cAMP-washout and receptor mutational studies. [7] To conclude, we discovered a unique class of SCTR modulators providing useful pharmacologic tools to study SCTR- or GLP-1R-specific activation pathways and explore novel allosteric binding sites with potential to instigate the development of first-in-class therapeutics for treating comorbid conditions obesity and diabetes. [7] References [1] Annu Rev Pharmacol Toxicol, 2020, 60, 89-107; [2] Trends in Biochemical Sciences, 2017, 12, 946-960; [3] Experimental Physiology, 2013, 5, 973-987; [4] Current Neuropharmacology, 2007, 3, 168-179; [5] SLAS Discovery, 2021, 26, 1-16; [6] Journal of Peptide Science, 2017, 12, 845-854; [7] submitted to Biochemical Pharmacology 2021.