LAUSR

1‐[(3S)‐3‐[4‐Amino‐3‐[2‐(3,5‐dimethoxyphenyl)ethynyl]‐1H‐pyrazolo[3,4‐d]pyrimidin‐1‐yl]‐1‐pyrrolidinyl]‐2‐propen‐1‐one (TAS‐120) is an irreversible inhibitor of the fibroblast growth factor receptor (FGFR) family, and is currently under phase I/II clinical trials in patients with confirmed advanced metastatic solid tumours harbouring FGFR aberrations. This inhibitor specifically targets the P‐loop of the FGFR tyrosine kinase domain, forming a covalent adduct with a cysteine side chain of the protein. Our mass spectrometry experiments characterise an exceptionally fast chemical reaction in forming the covalent complex. The structural basis of this reactivity is revealed by a sequence of three X‐ray crystal structures: a free ligand structure, a reversible FGFR1 structure, and the first reported irreversible FGFR1 adduct structure. We hypothesise that the most significant reactivity feature of TAS‐120 is its inherent ability to undertake conformational sampling of the FGFR P‐loop. In designing novel covalent FGFR inhibitors, such a phenomenon presents an attractive strategy requiring appropriate positioning of an acrylamide group similarly to that of TAS‐120.