Antibodies to checkpoint inhibitors like PD-1, PD-L1 and CTLA4 activate resident Cytotoxic T-Lymphocytes to attack and kill tumor cells. The main mechanism that they employ to kill the cancer cells is through the release of cytolytic granules that contain perforin and Granzyme-A (GrA) protease. Perforin punches holes in the tumor cell membrane facilitating entry of GrA into the cytoplasm where it selectively degrades the SET oncoprotein, which is at the core of the SET complex composed of the SET oncoprotein and at least 5 different tumor suppressor proteins. Upon GrA mediated degradation of the SET oncoprotein, the tumor suppressors including nm23-H1 and TREX-1 deoxyribonucleases (DNAse's) are released into the nucleus where genomic DNA is degraded leading to cancer cells apoptosis. Importantly, we found that the SET oncoprotein is over-expressed in more than 90% of all leukemias and in 50 to 75% of all human solid tumor biopsies ranging from breast cancer to pancreatic cancer and others. As an inhibitor of tumor suppressors, SET overexpression in cancer helps to explain why tumor suppressors fail to effectively function to kill cancer cells.
We discovered that OP449, a peptide mimetic of apolipoprotein-E (apoE), specifically antagonizes SET to release tumor suppressors that then act to kill the cancer cells. Initially we found that OP449 antagonized SET in MDA-MB-231 breast cancer cells to release the nm23-H1 DNAse tumor suppressor, which accumulated in the nucleus where it actively participated in degrading genomic DNA. Using MDA-MB-231 cells, OP449 caused a dose dependent death in cultures and in orthotopic xenografts with NSG mice with severe T-cell deficiency. Interestingly, using MDA-MB-231 cells labeled with green fluorescent protein to produce xenografts, the number of metastases in brain and lung were significantly reduced in OP449 treated animals compared to controls. MDA-MB-231, MDA-MB-436 and MDA-MB-468 tumor growth was significantly impaired in xenografts treated with OP449 or with small hairpin DNA (shDNA) constructs used to reduce levels of the SET protein, but not with empty shDNA vector controls. Using a genetically engineered mouse model (GEMM) of breast cancer, tumor volumes in Myc Neu: Blg CRE (Her2/Myc) mice treated with OP449 were significantly less than their control counterparts and similar to animals treated with Lapatinib alone. In contrast, combining OP449 plus Lapatinib caused tumor regression to the limit of measurement in this GEMM model. Additional xenografts with CAPAN1 pancreatic cancer cell lines similarly showed significant reductions of tumor volumes in OP449 treated animals compared to vehicle controls, and with shDNA constructs that reduce SET levels and reduce tumor volumes. These experiments demonstrate that in the presence or absence of T-cells, that OP449 can provide effective control of tumor growth.
These results provide the groundwork for a next-generation immuno-oncology approach that directly antagonizes the SET oncoprotein to mimic proteolytic degradation of SET by Granzyme-A resulting from immune cell activation and cytolytic granule action. While clinical use of checkpoint inhibitor antibodies has proven very effective in treating deadly cancers, the rate of unwanted side effects of uncontrolled immune activation is also increasing with more widespread use of these antibodies. In contrast, SET antagonists like OP449 targeted to the cancer cells do not require the presence of activated immune cells, but do implicate the release and re-activation of endogenous tumor suppressors in the tumor tissue to kill cancer cells in vitro and in vivo.
Citation Format: Michael P. Vitek. Surpassing the emerging immune-checkpoint inhibitor market. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A092.
- ©2016 American Association for Cancer Research.