Boosting Tumor Immunogenicity to Enhance Response to Immune Checkpoint Inhibitors in Soft Tissue Sarcomas

Breelyn Wilky, MD,  University of Colorado Cancer Center
Recipient of the: $50,000 Race to Cure Sarcoma Research Award

Most soft tissue sarcomas (STS) remain incurable in the metastatic setting with traditional chemotherapy. Immune checkpoint inhibitors (ICIs), including antibodies to PD-1/PD-L1 and CTLA-4, act by reversing tumor-mediated suppression of T cells. While these drugs have dramatically improved outcomes in many solid tumors, ICIs are effective in less than 20% of STS patients. Recent studies have shown that sarcoma patients who do not achieve benefit with ICIs have few T cells infiltrating the tumor tissue, and low expression of immune-related genes. One potential explanation is that due to the low mutational burden in many STS, antigen-presenting cells do not recognize the sarcoma cells as a threat, leading to a poor immune response. The inability of sarcoma cells to trigger an immune response, termed poor tumor immunogenicity, may lead to a lack of tumor-specific T cells, and accordingly, failure of ICIs. Therapies that increase tumor immunogenicity and promote downstream T cell generation and activation may improve the response to ICIs and immune-mediated tumor kill. Doxorubicin has been shown to improve sarcoma cell immunogenicity, by releasing new tumor antigens from necrosing cells and stimulating “danger signals” that attract antigen-presenting cells to the tumor bed. In mouse models of fibrosarcoma, we showed that the combination of doxorubicin with PD1/CTLA-4 inhibition suppressed tumor growth more than either doxorubicin or ICIs alone. However, no tumor regressions were observed in mice, and a recent clinical trial of doxorubicin with anti-PD1 therapy did not improve tumor responses in patients. In this research, we propose using a library of over 2500 FDA approved drugs to treat sarcoma cell lines and determine which drugs act by increasing tumor immunogenicity. We have developed a multicolor flow cytometry panel to survey treated tumor cells for “danger factors” and suppressive checkpoint proteins. Additionally, we engineered a human antigen-presenting cell line to express luminescent signals upon activation of NF-kB and type 1 interferon, two key pathways that induce antigen-presenting cell activation. In a high-throughput fashion, we can rapidly identify the most promising drugs that increase tumor immunogenicity superiorly to doxorubicin. The best drugs will then be tested in two mouse sarcoma models in our laboratory, MCA-205, which is immunogenic, and KP, which is highly resistant to the immune system. Mice bearing these two sarcoma types will be treated with the candidate drug alone and in combination with PD1/CTLA-4 antibodies to determine tumor kill, and the effects on the immune system in the tumors, blood, and related immune organs. This study will establish key preclinical data to support the next combination clinical trial with ICIs, and offer more patients with incurable sarcoma the chance to benefit from the durable responses and generally favorable side effect profile of immunotherapy.