Mrgx2 antagonist-3 (Compound B-40) is a highly selective antagonist of the MrgX2 receptor, with an IC50 range of 0.042-2.5 nM. It blocks downstream G protein signaling and β-arrestin recruitment, inhibiting Mrgx2 receptor-mediated calcium influx and cell degranulation. Mrgx2 antagonist-3 shows potential for research in inflammation-associated diseases and pruritus, such as chronic urticaria and allergic asthma.