Discovery of Pyrazolopyrimidines as Potent and Orally Bioavailable MRGPRX2 Antagonists

Abstract The Mas-related G protein-coupled receptor X2 (MRGPRX2) mediates IgE-independent mast cell activation and is implicated in pseudoallergic drug reactions, chronic urticaria, atopic dermatitis, rosacea, neurogenic inflammation, and other mast cell-mediated disorders. Herein, we report the discovery of a pyrazolopyrimidine series as potent and orally bioavailable MRGPRX2 antagonists. A scaffold hybridization strategy, supported by systematic structure–activity relationship studies, yielded antagonists with nanomolar activity in both Gq/IP1 and beta-arrestin cellular assays. Addressing solubility limitations and CYP inhibition liability, while maintaining favorable pharmacokinetic properties, led to the identification of INCB187175 (compound 23), which exhibits good cellular potency and robust oral exposure across preclinical species. Importantly, functional activity was validated in a beta-hexosaminidase release assay using primary human mast cells, demonstrating potent inhibition of MRGPRX2-mediated degranulation. This activity translated in vivo, where INCB187175 effectively suppressed mast cell activation in an Evans blue extravasation assay. These findings establish the pyrazolopyrimidine chemotype as a promising scaffold for inhibition of MRGPRX2 signaling and support INCB187175 as a compelling drug candidate for further development in mast cell-mediated diseases.