CA-074, but not its methyl ester CA-074Me, is a selective inhibitor of cathepsin B within living cells

Studies using inhibitors that reportedly discriminate between Cathepsin B and related lysosomal cysteine proteinases have implicated the enzyme in a wide range of physiological and pathological processes. The most popular substance to selectively inhibit Cathepsin B in vivo is CA-074Me, the methyl ester of the E-64 derivative CA-074. However, we now have found that CA-074Me inactivates both Cathepsin B and Cathepsin L within murine fibroblasts. In contrast, exposure of these cells to the parental compound CA-074 leads to the selective inhibition of endogenous Cathepsin B, while intracellular Cathepsin L remains unaffected. These results indicate that CA-074 rather than CA-074Me should be used to specifically inactivate Cathepsin B within living cells.