Noninternalizing Fluorescent Probe Targeting Mas-Related G Protein Coupled Receptor X2: A Tool for Antagonist Screening and Receptor Detection in Blood Samples

MrgX2 has emerged as a critical target in allergic diseases, and the development of targeted fluorescent probes would significantly advance the study and treatment of these conditions. However, receptor internalization limits current MrgX2 probes, which are mostly agonist-based, compromising their labeling accuracy and stability. Here, we introduce TM2, a noninternalizing fluorescent probe with high specificity and favorable fluorescent properties for MrgX2. TM2 enables stable, precise receptor labeling in live cells without the induction of cellular internalization or activation. This capability facilitated the development of a competitive binding assay that screened 140 compounds, identifying C104 as a potent MrgX2 antagonist with validated antiallergic efficacy <i>in vitro</i> and <i>in vivo</i>. Moreover, TM2 quantitatively differentiated MrgX2 expression in clinical blood samples from atopic dermatitis patients versus controls, highlighting its diagnostic potential. Collectively, this work presents TM2 as a versatile tool for advancing the diagnosis and treatment of MrgX2-mediated allergic diseases.