Abstract 2255: Enhancing immunogenicity in immunologically cold ER+ breast cancer using estrogen receptor blockade and radiation therapy

Metastatic, hormone therapy-resistant breast cancers expressing estrogen receptor (ER+) account for the majority of breast cancer deaths. ER+ breast cancers are immunologically “cold” with low tumor mutation burden and few tumor-infiltrating lymphocytes (TILs). Consequently, application of immune checkpoint inhibitors (ICIs) to ER+ breast cancers has not proven effective. Radiation therapy (RT) can increase immune susceptibility of “cold” tumors, but can also increase recruitment of immunosuppressive myeloid-derived suppressor cells (MDSCs) to the radiated tumor microenvironment (TME). Recent studies demonstrate that ER inhibition may antagonize the trafficking and activation of 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 ICIs in a syngeneic murine model of ER+ breast cancer. Using a murine breast cancer model of ER+ breast cancer developed (TC11), we examined effects in vitro and in vivo. In vitro, TC11 cells were treated with 8Gy RT and/or fulvestrant (1uM) to evaluate cell autonomous effects. In vivo, 25,000 TC11 cells were transplanted to caudal mammary fat pads of female FVB/N mice. When tumors reached ~200mm3, mice were randomized and treated. We initially evaluated the time course of immune responses to 8Gy RT in the TC11 TME, euthanizing mice every 5 days up to 20 days after RT. We subsequently treated mice with vehicle, fulvestrant (250mg/kg sc, weekly), 8Gy RT, and fulvestrant+8Gy RT. Flow cytometry was performed on day 5 and day 10 after RT to examine the innate and adaptive immune cell infiltrates, respectively. A therapeutic intervention study was performed using combinations of fulvestrant, 8Gy RT, and/or anti-PDL1 (200µg, ip, at days 3, 6, and 9 after RT). 8Gy RT elicited a type-1 interferon response in TC11 cells in vitro and in vivo; 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 in TC11 tumors. RT increased MDSCs (Ly6C+Ly6G+) in the TC11 TME compared to vehicle or fulvestrant alone in tumors taken at day 5, but MDSCs were reduced in tumors treated with fulvestrant and RT. Together, fulvestrant and RT increased TILs (CD45+) and CD8+ T cells at day 10 compared to vehicle control and single treatments. Preliminary data indicate RT and fulvestrant together primed a greater response to anti-PDL1 ICI compared to single or dual combinations of these treatments. 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 study is warranted to determine whether this combination may elicit a more effective in situ vaccine effect compared to RT alone. Citation Format: Amber M. Bates, Kathleen A. O9Leary, Sarah Emma, Erin Nystuen, Elizabeth G. Sumiec, Linda A. Schuler, Zachary S. Morris. Enhancing immunogenicity in immunologically cold ER+ breast cancer using estrogen receptor blockade and radiation therapy [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 2255.