Targeting GPR84: a receptor involved in regulating inflammation

GPR84 is a G protein-coupled receptor that is activated by medium-chain fatty acids and is highly expressed in macrophages, suggesting a role for dietary fatty acids in the regulation of inflammatory responses. The validation of GPR84 as a therapeutic target has been hindered by the structural homogeneity and poor selectivity of available chemical tools, and the consequent poor understanding of GPR84 physiology. The aim of this thesis was to develop structurally novel GPR84 agonists that may reveal new aspects of GPR84 biology. A ligand-based virtual screening approach was used to identify a compound active in recombinant cell cAMP assays that represents a new scaffold of GPR84 agonist. The compound was chemically optimised and extensively characterised for potency, stability, solubility, and selectivity, resulting in DL-175 which shows properties appropriate for an in vitro chemical probe. Furthermore, unlike previously reported GPR84 ligands, DL-175 is inactive in beta-arrestin recruitment assays, showing a marked bias for G protein signalling pathways. In primary murine bone marrow-derived macrophages (BMDMs), DL-175 induced dosedependent responses in impedance signalling assays that were absent in GPR84-/- BMDMs. Moreover, the BMDM response to DL-175 was significantly more sustained than that induced by range of publishedGPR84 agonists, indicative of biased signalling in disease-relevant primary cells. The functional consequences of biased agonism at GPR84 in immune cells were investigated by a direct comparison between DL-175 and widely used GPR84 agonist 6-OAU. Both agonists enhance U937macrophage phagocytosis, but have strikingly different abilities to promote chemotaxis in the same cells. This work demonstrates that biased agonism enables the selective activation of functional responses in immune cells and delivers a high-quality chemical probe for further investigation.