Using a selective, reversible, non-covalent inhibitor of cathepsin V (compound 7), we can effectively reduce the activation of cystatin F. Using this approach, we were able to improve NK cell cytotoxic function in standard NK cell cytotoxicity assays, as well as in more complex tumor models, such as static and perfusion GSC spheroids models in microfluidic chips. Here, CST7 is linked to neoplasm.