Targeting MAO‐A With Ultrafast Kinetics: Affinity Probes for Precision Imaging of Brain Pathologies and Depression Biomarker Validation

ABSTRACT Major depressive disorder (MDD) lacks objective biomarkers, hindering diagnosis. Monoamine oxidase A (MAO‐A)—upregulated in depressed brains and degrading monoamine neurotransmitters—is a key pathological biomarker. To address limitations of current MAO‐A probes (reliance on enzymatic reactions, > 1 h), we designed JBD s: near‐infrared ( λ _em >700 nm) probes with large Stokes shifts (> 200 nm), using affinity‐based recognition (independent of enzymatic reactions), a purine core for superior MAO‐A affinity, and showing long conjugated system and negatively charged tails do not enhance affinity. Among them, JBD stands out with extraordinary performance: < 5 s response (2–3 orders faster than reported) and exceptional specificity for MAO‐A. It enables precise targeted MAO‐A imaging in overexpressing live cells and visualizes MAO‐A in orthotopic glioma mice. Critically, JBD achieves non‐invasive imaging of MAO‐A upregulation in depressive mouse brains, directly bridging the gap between MAO‐A detection and MDD‐associated biomarker visualization. RNA‐seq further confirms MAO‐A elevation aligns with MDD pathways, supporting JBD ’s role in detecting and dissecting MAO‐A‐mediated depressive mechanisms.