Affinity Mass Spectrometry-Based Fragment Screening Identified a New Negative Allosteric Modulator of the Adenosine A Receptor Targeting the Sodium Ion Pocket.

Allosteric ligands provide new opportunities to modulate G protein-coupled receptor (GPCR) function and present therapeutic benefits over orthosteric molecules. Negative allosteric modulators (NAMs) can inhibit the activation of a receptor and downstream signal transduction. Screening NAMs for a GPCR target is particularly challenging because of the difficulty in distinguishing NAMs from antagonists bound to the orthosteric site as they both show inhibitory effects in receptor signaling assays. Here we report an affinity mass spectrometry (MS)-based approach tailored to screening potential NAMs of a GPCR target especially from fragment libraries. Compared to regular surface plasmon resonance or NMR-based methods for fragment screening, our approach features a reduction of the protein and compound consumption by 2-4 orders of magnitude and an increase in the data acquisition speed by 2-3 orders of magnitude. Our affinity MS-based fragment screening led to the identification of a new NAM of the adenosine A receptor (AAR) bearing an unprecedented azetidine moiety predicted to occupy the allosteric sodium binding site. Molecular dynamics simulations, ligand structure-activity relationship (SAR) studies, and in-solution NMR analyses further revealed the unique binding mode and antagonistic property of this compound that differs considerably from HMA (5-(,-hexamethylene)amiloride), a well-characterized NAM of AAR. Taken together, our work would facilitate fragment-based screening of allosteric modulators, as well as guide the design of novel NAMs acting at the sodium ion pocket of class A GPCRs.