Abstract PO-044: Exploiting a radiotherapy induced immunologic double bind in advanced prostate cancer
Authors: Luddy Kimberly , West Jeffrey , Robertson-Tessi Mark , Anderson Alexander , Marignol Laure , Gatenby Robert , O'Farrelly Cliona 
Published: April 2021

Abstract
An evolutionary double bind occurs in cancer when resistance to a commonly used therapy increases the sensitivity to a second therapy. Typically, the second therapy is most effective in the recurrent setting and less effective when given up front. There is a tremendous need for novel second-line therapies able to exploit these new sensitivities. Prostate cancer remains one of the most treatable cancers, however, advanced disease is typically metastatic and currently incurable. Radiation therapy (RT) is often used in the resistant setting. Given locally, RT induces double-strand DNA breaks in tumor cells. This results in cellular stress responses that either trigger apoptosis and cell death, or DNA repair and cell survival. Radiosensitivity is related to the DNA repair capabilities of a tumor. Activation of DNA damage response pathways, which are mediated by ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related protein (ATR), induces changes in surviving cancer cells. Most notably, it has been shown that RT, as well as other DNA damaging agents, can increase natural killer cell (NK) ligand expression on tumor cells. Here we utilize an isogenic cell line model of radioresistant prostate cancer. Our pre-clinical model shows that alterations in DNA repair pathways required to maintain resistance to RT correlate with changes in NK-cell ligands, including: major histocompatibility complex class I chain-related protein A/B (MICA/B), Nectin-2 (CD112), and poliovirus receptor (PVR). This results in a 2-fold increase in sensitivity to NK-cell mediated killing in the radiation resistant cell line. An “evolutionary game assay” was performed by preparing mixtures of parental and radiation-resistant cell lines at 4 different ratios: 100:0, 90:10, 10:90 and 0:100. This was repeated for six treatment scenarios: untreated, 6Gy radiation, 1:1 NK cells, 5:1 NK cells, 6Gy with 1:1 NK cells, and 6Gy with 5:1 NK cells. Comparison of monotypic growth rates to coculture mixtures of parental and radiation-resistant cell lines enable quantification of competitive interactions through the use of evolutionary math models. This assay quantifies effective fitness (growth rate) to experimentally determine the frequency-dependent gradient of selection. Preliminary results indicate the emergence of three qualitatively unique evolutionary scenarios: 1) dominance of resistance (radiation therapy), 2) dominance of sensitive (5:1 NK), and 3) coexistence (untreated; 1:1 NK). These results suggest the existence of an evolutionary double-bind when NK-cell administration follows radiation. This was experimentally validated and confirmed by repeating the evolutionary assay with sequential radiation and NK therapy. Importantly, there is a minimum threshold of NK cells required to achieve the double-bind. Direct measurement of the fitness landscape underlying radiation and NK treatment enabled the discovery and validation of an evolutionary strategy to re-sensitize tumors after radiation treatment.
Citation Format: Kimberly Luddy, Jeffrey West, Mark Robertson-Tessi, Alexander Anderson, Laure Marignol, Robert Gatenby, Cliona O'Farrelly. Exploiting a radiotherapy induced immunologic double bind in advanced prostate cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-044.