Cathepsin Inhibitor3 (Compound 53k) is a nonradioactive intermediate in the synthesis of [18F]2k, showing selectivity for cathepsin S. This compound is capable of undergoing radiolabeling with fluorine-18, making it useful for related studies. As a click chemistry reagent, Cathepsin Inhibitor3 contains an alkyne group, enabling it to participate in copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions with azide-containing molecules.