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.