An Agonist Radioligand for the Proinflammatory Lipid-Activated G Protein-Coupled Receptor GPR84 Providing Structural Insights

The orphan G protein-coupled receptor (GPCR) GPR84 is
expressed
on immune cells mediating proinflammatory and immunostimulatory effects.
In this study, we prepared the fully efficacious, nonbiased GPR84
agonist 6-hexylamino-2,4­(1H,3H)-pyrimidinedione
(6) in tritium-labeled form ([3H]­PSB-1584)
by hydrogenation of a hexenyl-substituted precursor with tritium gas.
The radioligand was characterized by kinetic, saturation, and competition
assays using membranes of Chinese hamster ovary cells recombinantly
expressing the human GPR84. [3H]6 reversibly
labeled the receptor with high affinity (KD 2.08 nM). Structurally diverse orthosteric and allosteric ligands,
including newly designed and synthesized compounds, were studied in
competition binding assays. A homology model of GPR84 was generated
to perform docking studies rationalizing the experimental data. The
radioligand was additionally used for labeling GPR84 in native cells
and tissues. [3H]6 constitutes the first GPR84
agonist radioligand representing a powerful tool for this poorly investigated
GPCR, which has potential as a future drug target.