Abstract 6270: A radiosensitizer screen identifies a novel role for MDM2 inhibition in the radiosensitization of ER+ breast cancers in a p53 dependent manner

Background: Radiation therapy (RT) is a mainstay of treatment for most women with breast cancer (BC). Despite this treatment, response remains heterogenous for women with estrogen receptor positive (ER+) BC. Thus, approaches that result in radiosensitization of aggressive ER+ disease are critically needed. We performed a radiosensitizer screen paired with transcriptomic and proteomic data from ER+ models treated +/-RT to identify potential mediators of RT resistance. Methods: Clonogenic survival assays were used to determine RT sensitivity of 21 BCC lines as well as radiosensitization with drug treatment. IC50 values were determined for 130 clinical compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Microarray and RPPA data was used for differential gene/protein expression and pathway analysis. AlamarBlue was used to determine IC50 values of the MDM2 inhibitor AMG-232. Western blot analysis of Cleaved PARP was used to measure apoptosis and Cyclins A and E to measure cell cycle progression. Results: Our radiosensitizer screen nominated the MDM2 inhibitor (JNJ-26854165) as one of the most effective drugs in treating RT-resistant BC cell lines (R2= 0.43, p-value 10uM). Clonogenic survival assays demonstrated that MDM2 inhibition at sub-IC50 doses leads to radiosensitization in p53 WT ER+ cell lines (MCF-7 rER: 1.37-1.66;ZR751 rER: 1.30-1.65). In contrast, p53 MT ER+ cells did not demonstrate significant radiosensitization (T47D rER: 0.94-1.11). Combination of AMG-232 and RT led to an increase in apoptosis compared to RT alone in ER+ p53 WT cells but not p53 MT cells. Additionally, combination treatment led to differential cylin expression in p53 WT cells but not p53 MT cells. In vivo studies testing MDM2 inhibition w