Affinity Selection-Mass spectrometry coupled with biophysical validation enables proof-of-concept discovery of CHI3L1...

Chitinase-3-like protein 1 (CHI3L1) is a multifunctional extracellular glycoprotein implicated in tumor progression, immune suppression, and fibrosis, making it an attractive but challenging therapeutic target. To explore its chemical tractability, we applied an affinity selection-mass spectrometry (AS-MS) workflow to screen 10,000 small molecules for CHI3L1 binding. The screen yielded 124 initial hits with a hit rate of 1.24 %, which were prioritized based on chemical suitability, and six candidates were advanced for validation using microscale thermophoresis (MST). Among these, compound A9 exhibited a clear, dose-dependent binding response in MST with a Kd of 182 ± 18 μM. Molecular docking supported these findings, revealing that A9 forms hydrophobic and hydrogen-bonding interactions within a defined pocket of the CHI3L1 structure. Although modest in affinity, A9 represents the first small molecule binder of CHI3L1 identified through AS-MS. This study provides a proof-of-concept demonstration that CHI3L1 can be chemically engaged using AS-MS, establishing a foundation for future medicinal chemistry optimization and the development of chemical probes targeting this previously undruggable extracellular protein.