Small Molecule Tools to Study Cellular Target Engagement for the Intracellular Allosteric Binding Site of GPCRs

A conserved intracellular allosteric binding site (IABS) has recently been identified at several G protein‐coupled receptors (GPCRs). Ligands targeting the IABS, so‐called intracellular allosteric antagonists, are highly promising compounds for pharmaceutical intervention and currently evaluated in several clinical trials. Beside co‐crystal structures that laid the foundation for the structure‐based development of intracellular allosteric GPCR antagonists, small molecule tools that enable an unambiguous identification and characterization of intracellular allosteric GPCR ligands are of utmost importance for drug discovery campaigns in this field. Herein, we discuss recent approaches that leverage cellular target engagement studies for the IABS and thus play a critical role in the evaluation of IABS‐targeted ligands as potential therapeutic agents.