Abstract 676: Fulvestrant and radiation modify the tumor immune microenvironment in ER+ metastatic breast cancer and cooperate to enhance response to anti-PDL1 checkpoint blockade

Abstract Metastatic, hormone therapy-resistant breast cancers expressing estrogen receptor (ER+) account for most breast cancer deaths. A lack of tumor-infiltrating lymphocytes (TILs) and low tumor mutation burden render most ER+ breast cancers immunologically “cold” and unresponsive to immune checkpoint inhibitors (ICI). Radiation therapy (RT) can augment tumor immune susceptibility, but RT may also increase immunosuppressive cell recruitment to the tumor. Recent studies demonstrate that ER inhibition may antagonize the trafficking and activation of myeloid derived suppressive cells (MDSCs) in tumors. We hypothesized that combining RT and a selective ER degrader (SERD), fulvestrant, would cooperate to relieve immunosuppression, increase tumor immune susceptibility, and facilitate response to ICI in a syngeneic murine model of ER+ breast cancer. Using a cell line (TC11) developed in the Schuler laboratory, cells were transplanted to caudal mammary fat pads of female FVB/N mice. When tumors reached ~200mm3, mice were treated with combinations of vehicle, fulvestrant (250mg/kg sc, weekly), 8Gy RT on 3 consecutive days (8Gy x 3, days -1, 0, 1), and/or anti-PDL1 (200µg, IP, at days 3, 6, and 9). Age-matched controls that received identical treatments were included. Tumors from these mice were collected at day 28, and cytokines in tumor lysates were evaluated using a multiplex immunoassay. In a similar experiment, tumors were collected at day 10 for qPCR and flow cytometry analysis. Therapeutic intervention demonstrated that fulvestrant with RT slowed tumor growth compared to either treatment alone. 8Gy RT elicited a type-1 interferon response in TC11 tumors as measured by upregulated expression of Mx1, Pdl1, Oas2, Oas3, Trex1, and IfnB mRNAs peaking at 3-5 days post RT and persisting to day 10. Cytokine analysis from tumor lysate revealed significantly increased IP-10 in tumors from mice treated with both RT and fulvestrant compared to controls, and similar trends were seen for IL6, VEGF, and MCSF. With the addition of RT and fulvestrant to anti-PDL1, IL2, IL3, IL6, IL7, IL17, and IP-10 were significantly increased compared to anti-PDL1 alone. At day 28 RT and fulvestrant together primed a greater response to anti-PDL1. Combined RT and fulvestrant may reduce immunosuppression compared to RT alone and render this immunologically cold tumor model more responsive to anti-PDL1 blockade. RT increases tumor cell immune susceptibility by activation of a type I IFN response and combining fulvestrant with RT does not affect this IFN1 response. These results demonstrate a cooperative interaction between RT and fulvestrant in favorably modulating the TME and response to ICIs in an immunologically cold, hormone-therapy resistant, murine ER+ breast cancer model. Further experiments are needed to explore the mechanisms of these immune effects. Citation Format: Erin J. Nystuen, Amber M. Bates, Kathleen A. O'Leary, Sarah E. Emma, Elizabeth G. Sumiec, Linda A. Schuler, Zachary S. Morris. Fulvestrant and radiation modify the tumor immune microenvironment in ER+ metastatic breast cancer and cooperate to enhance response to anti-PDL1 checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 676.