The Role of Proteasome Inhibitors and the Trail Apoptotic Pathway in the Treatment of Chronic Lymphocytic Leukemia.

TRAIL (tumour necrosis factor-related apoptosis-inducing ligand) triggers the TRAIL apoptotic pathway selectively in tumour cells by binding to two death receptors, DR4 and DR5, that are present on the surface of many cancer cells. It is effective at inducing apoptosis in a variety of haematological malignancies, such as multiple myeloma. However, chronic lymphocytic leukemia (CLL) cells are relatively resistant to TRAIL-induced apoptosis. We have previously shown that the chemotherapeutic drugs, fludarabine and chlorambucil, increase the cell surface expressions of DR4 and DR5, and give synergistic apoptotic responses when combined with TRAIL. Proteasome inhibitors, that are used in the treatment of multiple myeloma, also upregulate TRAIL and its death receptors in CLL cells but not in normal B cells. Herein we have determined that proteasome inhibitors are effective at inducing apoptosis in CLL cells, and that the activation of the TRAIL apoptotic pathway contributes significantly to proteasome inhibitor cytotoxicity. Combining proteasome inhibitors with TRAIL enhanced apoptosis in CLL cells by approximately 15% over treatment with the proteasome inhibitor alone. Another novel approach to trigger the TRAIL apoptotic pathway is to use activating monoclonal antibodies directed against DR4 and DR5. Similar to TRAIL, we demonstrated that when used alone, the monoclonal antibodies were minimally cytotoxic against CLL cells. Proteasome inhibitors in combination with activating monoclonal antibodies against DR4 or DR5 increased the amount of apoptosis in CLL cells. Cell death was enhanced by 23% and 17% when proteasome inhibitors were combined with monoclonal antibodies against DR4 and DR5, respectively. While proteasome inhibitors may have a potential role in CLL treatment as single therapy, they are also cytotoxic to normal B cells. However, when activating monoclonal antibodies against TRAIL death receptors are given in combination with proteasome inhibitors, that upregulate DR4 and DR5 expression, the anti-tumor specificity and cytotoxicity of these agents may be increased in CLL.