Cathepsin L-IN-2 ((Rac) -Z-Phe-Phe-FMK) is an isomer of the Cathepsin L inhibitor Z-Phe-Phe-FMK , with an IC50 of 15 μM for cathepsin L. Z-Phe-Phe-FMK irreversibly blocks the proteolytic function of cathepsins by covalently binding to the cysteine residues in the active center of the enzyme. Cathepsin L-IN-2 and Z-Phe-Phe-FMK can be used to study neurodegenerative diseases (such as GRN-related frontotemporal dementia) and cancer invasion and metastasis.