GPR18 and GPR55-related Ligands Serving as Antagonists or Agonists: Current Situation, Challenges and Perspectives

GPCR superfamily, the largest known family of membrane receptors, consists of six classes from A to F. GPR18 and GPR55, δ-branch of A class, had been reported to have no confirmed endogenous ligand and were named as "orphan receptors". Previous studies suggest that both GPR18 and GPR55 are possibly related to the migration and proliferation of cancer cells, macrophages and other inflammation-associated immune cells. Thus, they may be potential targets for inflammation, cancer and analgesia therapy. In this paper, we aimed to summarize the chemical structures and bioactivities of the agonists and antagonists of GPR18 and GPR55; moreover, we have briefly discussed the challenges and future perspectives in this field. This review will be beneficial for further design and synthesis of efficient agonists and antagonists towards GPR18 and GPR55- related disease treatment.