G protein-coupled receptors (GPCRs) are a diverse group of membrane-spanning proteins that serve as conduits for external molecular signals to influence intracellular signalling pathways. While they are central to numerous physiological functions, GPCRs are also implicated in various pathophysiological conditions including metabolic and psychiatric disorders, cardiovascular diseases, and cancer. Among them, the complement C3a receptor (C3aR) holds particular significance due to its pivotal role in orchestrating immune responses. It operates within the complement system – a cornerstone of innate immune defence that also aids adaptive immunity. The recent development of selective C3aR modulators has emerged as a promising avenue for novel treatments of inflammatory diseases caused by dysregulated C3aR signalling.

This study first established the selectivity of three small molecule agonists (BR103, TR16, C061) for activating human C3aR. Their selectivity was compared against > 300 druggable human GPCRs using the PRESTO-Tango β-arrestin2 recruitment assay and they were shown to be highly selective for C3aR. G protein specificity of these C3aR-selective ligands also revealed that all three agonists couple to the same Gα proteins as C3a (Gα_i1 ≈ Gα_i3 > Gα_oB > Gα_oA > Gα_i2 > Gα_z) as transducers for signalling propagation, indicating a conserved mechanism of action. These findings support the use of these small molecule agonists as substitutes for C3a, which is highly unstable, to interrogate the roles of C3aR in disease.