Discovery of Potent Agonists for the Predominant Variant of the Orphan MAS-Related G Protein-Coupled Receptor X4 (MRGPRX4).

The MAS-related Gq protein-coupled receptor X4 (MRGPRX4) is poorly investigated. MRGPRX4 has been proposed to be involved in pain transmission, itch, inflammation, wound healing, and cancer. However, so far only a few moderately potent, nonselective MRGPRX4 agonists have been described, most of which appear to preferably activate the minor receptor variant MRGPRX4-83L but not the main variant 83S. In the present study, we discovered a xanthine derivative bearing a phosphate substituent that activates the main variant of MRGPRX4. Optimization resulted in analogs with high potency and metabolic stability. The best compounds of the present series include 8-(m-methoxyphenethyl)-1-propargylxanthine substituted with a butyl linker in the 3-position containing a terminal phosphonate (30d, PSB-22034, EC50 Ca2+ assay/ß-arrestin assay, 11.2 nM/32.0 nM) and its N7-methyl derivative 31d (PSB-22040, EC50, 19.2/30.0 nM) showing high selectivity versus all other MRGPRX subtypes. They present promising tool compounds for exploring the potential of MRGPRX4 as a future drug target.

MAS-related G protein-coupled receptors X (MRGPRX): Orphan GPCRs with potential as targets for future drugs.

MAS-related G protein-coupled receptors (GPCRs) of subfamily X, designated MRGPRX, are primate-specific orphan receptors that belong to the δ-branch of rhodopsin-like, class A GPCRs. Four distinct subtypes exist, MRGPRX1, -2, -3, and -4, MRGPRX2 having the lowest degree of similarity with the others. Due to their expression on sensory neurons and immune cells, and their roles in pain perception and transmission, itch, inflammation, immune defense, pseudo-allergic reactions, wound healing, and possibly cancer, they have recently attracted much attention as novel drug targets. In particular MRGPRX2 was identified as an important mast cell receptor, responsible for anaphylactoid drug reactions and involved in skin and mucosal diseases, e.g. urticaria, atopic dermatitis, rosacea, and allergic rhinitis. A major hurdle has been the lack of animal models for studying these primate-specific receptors. However, recently humanized mice have been created. Moreover, a mouse ortholog of MRGPRX2, MRGPRB2, was identified, both receptors having a certain degree of similarity. MRGPRX1 and -2 can be activated by various peptides and small (partly peptidomimetic) molecules. MRGPRX2 is additionally activated by a very broad range of basic molecules, positively charged at physiologic pH value of 7.4, including many drugs. MRGPRX4 is activated by small acidic molecules including bile acids. For MRGPRX3, no ligands have been reported yet. Antagonists with reasonable potency and selectivity have been described for MRGPRX1, and few antagonists also for MRGPRX2, but not for the other subtypes. The recent elucidation of cryogenic electron microscopy structures of MRGPRX2 and -4 is expected to facilitate and advance drug development for these receptors. Currently, research on MRGPRX is still in its infancy, and exciting discoveries can be awaited. These receptors have great potential as future drug targets.