Abstract A208: Sensitivity of syngeneic tumor models to focal radiation
Authors: Krueger Sarah , Guley Kevin , Bull Christopher , Urs Sumithra , Wong Alden , Saims Dan , Wise Scott , Franklin Maryland R. 
Published: August 2018

More than 50% of all cancer patients receive radiotherapy during the course of their illness. However, while radiation therapy is a mainstay in clinical treatment approaches, there is a paucity of drugs being developed for use with radiotherapy. Therefore, we set out to provide data on the preclinical sensitivity of mouse tumor models to radiation as a means to further define radiation doses for use in combination studies with immunotherapy. Image-guided irradiation was performed using a SARRP system (Xstrahl Inc., Suwanee, GA). Animals were imaged with an open field at 60 kV and 0.5 mA for a computed tomography (CT) image, which was used for treatment planning. The treatment was delivered at 220 kV and 13.0 mA using an appropriately sized collimator to the total indicated dose (in Gray; Gy) in 2 equally weighted beams. Subcutaneous (SC) tumor models tested were A20 (B cell lymphoma) and CT26 (colon), and orthotopic models tested were 4T1-luc (mammary) and GL261-luc (glioblastoma). For the 4T1-luc, A20, and CT26 models tumors were implanted and mice staged into study groups with a mean tumor volume of approximately 100mm3. Cohorts of mice were either untreated or dosed with a single dose of focal radiation at 5, 10, or 20Gy. Body weights were monitored over time and results indicated that in each model all doses were well tolerated. All models showed a dose response antitumor activity with 20Gy resulting in approximately 20% tumor-free survivors in both A20 and CT26 models and more than 15 days of tumor growth delay in the 4T1-luc model. In the CT26 and A20 models there was some antitumor activity at 5 and 10Gy whereas 4T1-luc showed no response at 5Gy and moderate response at 10Gy. In the intracranial GL261-luc model a single dose of 15Gy to the brain was curative whereas 10Gy was only partially responsive. We further utilized the GL261-luc model to evaluate the antitumor effects of focal irradiation in combination with anti-PD-1 antibody treatment. The GL261-luc model resulted in rapid onset of tumor growth with a median survival of approximately 20 days. We determined that monotherapy with either systemic anti-PD-1 antibody or focal irradiation improved overall survival compared to control mice. Further, the combination of 10Gy radiation with anti-PD-1 antibody resulted in 100% tumor-free survivors for the duration of the study. In a subsequent study we analyzed tumor-infiltrating lymphocytes at days 12 and 20 post GL261-luc implant. We found that the percentage of regulatory T cells increased over time in mice treated with isotype control. However, treatment with radiation decreased regulatory T cells by approximately 50% on day 20 compared to day 12. These cells further diminished upon treatment with the combination of radiation and anti-PD-1 therapy. Other immune cell marker changes will also be discussed. In mouse models, radiation treatment has been shown to increase the level of tumor antigen presentation and the variety of peptides available for cross-presentation. Current work in the field is focusing on using radiation as a tool to bridge the gap from tumor equilibrium to tumor elimination, which could improve the response rate of immuno-oncology therapeutic agents. Tools such as the SARRP greatly enhance the ability of preclinical models to provide a first-line assessment for advanced immuno-oncology therapy combinations with focal irradiation.

Citation Format: Sarah Krueger, Kevin Guley, Christopher Bull, Sumithra Urs, Alden Wong, Dan Saims, Scott Wise, Maryland R. Franklin. Sensitivity of syngeneic tumor models to focal radiation [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A208.