Abstract P6-03-08: Androgen receptor (AR): A novel target for radiosensitization and treatment in triple-negative breast cancers (TNBC)

Abstract

Abstract Background: Increased rates of locoregional recurrence have been observed in TNBC despite the use of chemotherapy and radiation (RT). Thus, approaches that result in radiosensitizaton of TNBC are critically needed. We have previously characterized the radiation response of 21 breast cancer cell (BCC) lines using clonogenic survival assays. We now pair this data with high-throughput drug screen data available through cancer cell line encyclopedia studies to identify AR as a top target for radiosensitization and assess AR inhibition as a radiosensitization strategy for TNBC. Methods: Clonogenic survival assays were performed to determine the intrinsic RT sensitivity of 21 BCC lines (0-8 Gy RT). IC50 values were determined for 130 clinically available compounds and correlation coefficients were calculated using IC50 values (for drug sensitivity) and SF-2Gy (for radiation sensitivity). Gene expression was measured using Affymetrix microarrays and protein expression was measured using reverse-phase protein lysate arrays (RPPA) of human tumor samples (n=2,061) and BCC lines (n=51). AR function was assessed using siRNA knockdown or inhibition with MDV3100 (enzalutamide). Kaplan-Meier analysis was performed to determine the clinical impact of AR expression on local control and survival. A Cox proportional hazards model was constructed to identify potential factors of survival, and multivariate analysis was used to determine variables most significantly associated with LRF survival. Results: Our radiosensitizer screen nominated bicalutamide as one of the most effective drugs in treating radioresistant BCC lines (R2= 0.46, p-value <0.001). Recognizing that a subgroup of TNBC includes AR expressing tumors, we interrogated the expression of AR in >2000 human breast tumor samples and found signifi[not]cant heterogeneity in AR expression with an increase in TNBC (35% of tumors) compared to non-TNBC (28% of tumors). This same heterogeneity was also identified in human BCC lines. There was a strong correlation between AR RNA expression and protein expression (R2= 0.72, p <0.0001). Inhibition of AR using both siRNA and MDV3100 induced radiation sensitivity in vitro with an enhancement ratio (ER) of 1.35-1.42 in AR-positive TNBC lines. No such radiosensitization was seen in AR-negative TNBC or ER-positive, AR-negative BCC lines. Radiosensitization was at least partially dependent on impaired dsDNA break repair mediated by DNAPKcs. In vivo assessment of tumor growth inhibition with RT and anti-AR strategies are currently underway. Clinically, analyses of patients with TNBC showed that patients whose tumors had high expression of AR had markedly higher rates of LR after RT than patients with low expression of AR (HR for LR 2.9-3.2, p-value <0.01, 2 independent datasets). There was no difference in LR in TNBC patients not treated with RT when stratified by AR expression status. In multivariate analysis, AR expression was the variable most significantly associated with worse LRF survival after RT with a HR of 3.58 (p-value < 0.01). Conclusion: Our results implicate AR as a mediator of radioresistance in breast cancer and support the rationale for developing clinical strategies to inhibit AR as a novel radiosensitizing target in TNBC. Citation Format: Corey Speers, Shuang G Zhao, Meilan Liu, Joseph Evans, Prasanna Alluri, Daniel F Hayes, Felix Y Feng, Lori J Pierce. Androgen receptor (AR): A novel target for radiosensitization and treatment in triple-negative breast cancers (TNBC) [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-03-08.