Cathepsin Inhibitor 3 (Compound 53k) is a non-radioactive intermediate in compound [ 18 F] 2k radioactive synthesis, which has selectivity for cathepsin S. Cathepsin Inhibitor 3 can undergo radioactive fluorination and can be used for fluorine-18 labeling research. Cathepsin Inhibitor 3 is a click chemistry reagent, it contains an Alkyne group and can undergo copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules containing Azide groups.