Abstract 6650: Targeted radiation in combination with immunotherapy agents in murine breast cancer models
Authors: Urs Sumithra , Draper David , Franklin Maryland R. 
Published: September 2020

Abstract
Clinically, most breast cancers are refractory to checkpoint inhibitor (CPI) monotherapy and are regarded as immunologically “cold” due to minimal tumor CD8+ T cell infiltrate and low immunogenicity. These characteristics have encouraged the evaluation of potential synergy of immunotherapy with other treatment modalities such as targeted radiotherapy (RT). While RT by itself can have many immunostimulatory effects, it is insufficient to induce a curative anti-tumor immune response especially against metastatic breast cancers, highlighting the need for combination strategies that boost the immune system. Here, we evaluate three syngeneic triple negative breast cancer models, 4T1, E0771 and EMT6 and their responses to immune modulatory agents and focal radiation. Single agent CPI and costimulatory antibodies show anti-tumor activity with tumor growth delay (TGD) (varying from 1-24 days) and tumor free survivors (TFS) in the EMT6 model, whereas E0771 and 4T1 show little/no response to CPI treatment. All three models are sensitive to focal beam radiation (5-20Gy) delivered using the Small Animal Radiation Research Platform (SARRP; Xstrahl). RT monotherapy results in TGD in all three models. Combination of immune modulators and RT were evaluated in these models. The EMT6 model is the most responsive with RT in combination with anti-mPD-1 resulting in 9 days TGD (anti-mPD-1 monotherapy = 1.2 TGD) and 33 days TGD in combination with anti-mCD137 (anti-mCD137 monotherapy = 24 TGD) along with 50% TFS. RT in combination with anti-mCTLA-4 in the 4T1 model improves overall treatment response. The baseline tumor immune profile for the three models was established to understand their potential immunogenicity. The T and B cell populations (relative to CD45+) was comparable between models. In the myeloid compartment, 4T1 and EMT6 models are similar with respect to G-MDSCs (Granulocytic Myeloid Derived Suppressor Cells) being the predominant subset. On the other hand, E0771 tumors have a predominating M-MDSC (Monocytic-MDSC) population and a near absence of G-MDSCs. MDSCs are well known for contributing to immunosuppression and can play a role in promoting tumor metastasis by participating in the formation of angiogenesis and invasion. We determine that in the metastatic 4T1 model, combination treatment of anti-mCTLA-4 with RT reduces overall metastatic tumor burden in the lung and axillary lymph nodes which was concomitant with reduced MDSCs in the primary tumor. We have luciferase-enabled these cell lines to further evaluate combination therapy on metastatic spread. Combination of radiotherapy with immunotherapy offers benefits of targeting the tumor, activating immune cells and enhancing the local and potentially abscopal effects of immunotherapy. These data can be useful in providing a guide to choose between three murine breast cancer models for rational design of combination strategies.
Citation Format: Sumithra Urs, David Draper, Maryland R. Franklin. Targeted radiation in combination with immunotherapy agents in murine breast cancer models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6650.