Control of Breast Tumor Cell Growth Using a Targeted Cysteine Protease Inhibitor¹ Free

The purpose of this study was to determine whether inhibition of lysosomal proteolysis could be used to selectively inhibit proliferation of tumor cells. The lysosomal cysteine protease inhibitor 9-fluorenylmethy-loxycarbonyl-tyrosylalanyl-diazomethane was found to inhibit growth of the breast cancer cell lines SK-Br-3 and MCF-7. A humanized monoclonal antibody (huMAb 4D5) directed against the extracellular domain of p185^HER2 specifically inhibited growth of the SK-Br-3 cells, which overexpress this antigen. The antibody and inhibitor together showed enhanced inhibition of growth of the SK-Br-3 cells only. When the protease inhibitor was radiolabeled and conjugated to the antibody (huMAb 4D5-¹²⁵I-Tyr-Ala-CHN₂) it was selectively bound to and taken up by the SK-Br-3 cell line. The conjugated inhibitor was delivered and targeted to cathepsin B and an unidentified protein of M_r 39,000 in the SK-Br-3 cells. Internalization of huMAb 4D5-¹²⁵I-Tyr-Ala-CHN₂ and inhibitor labeling of the proteins were temperature-dependent processes. huMAb 4D5-Tyr-Ala-CHN₂ was significantly more effective in inhibiting proliferation of SK-Br-3 cells than the inhibitor-free analogue but was ineffective against MCF-7 cells. The results in this report show that targeting of cysteine protease inhibitors can selectively control tumor cell growth and that targeted cysteine protease inhibitors could prove valuable in the development of novel anticancer immunotherapies.