Abstract P6-13-05: Androgen receptor (AR): A novel target and mechanism for radiosensitization and treatment in triple-negative breast cancers (TNBC)

Abstract

Abstract Background: Increased rates of locoregional recurrence have been observed in TNBC despite chemotherapy and radiation (RT). Thus, approaches that result in radiosensitizaton in TNBC are critically needed. We characterized the RT response of 21 breast cancer cell (BCC) lines using clonogenic survival assays. We paired this with high-throughput drug screen data to identify AR as a top target for radiosensitization and assess AR inhibition as a radiosensitization strategy for TNBC. Methods: Clonogenic survival assays were used to determine the intrinsic RT sensitivity of 21 BCC lines. IC50 values were determined for 130 clinically available compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Gene expression was measured using RNA Seq and protein expression was measured using RPPA arrays in human tumor samples (n=2,061) and BCC lines (n=51). AR function was assessed using siRNA knockdown or functional inhibition with MDV3100 (enzalutamide). We measured in vivo tumor growth with varying control and treatment groups (16-20 tumors/group). Kaplan-Meier analysis was performed to estimate local control and survival. A Cox proportional hazards model was used to identify factors of survival, and MVA was used to determine variables associated with LRF survival. Results: Our radiosensitizer screen nominated bicalutamide as a most effective drug in treating RT-resistant BCC lines (R2= 0.46, p-value <0.01). We interrogated the expression of AR in >2000 human breast tumor samples and found significant heterogeneity in AR expression with enrichment of expression at the protein and RNA level in TNBC. 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 <0.01). Inhibition of AR using both siRNA and MDV3100 (enzalutamide) 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. AR inhibition either with MDV3100 significantly radiosensitized TNBC xenografts in mouse models and markedly delayed tumor tripling time and tumor growth (median tumor tripling time 17.4 days for RT alone vs. not reached after 50 days for MDV3100 + RT, p-value <0.001). Biomarker analysis identifies DNAPKcs as a potential biomarker of response. Clinically, analyses of patients with TNBC showed that patients whose tumors had higher than median expression of AR had markedly higher rates of LR after RT (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. In MVA, AR expression was the variable most significantly associated with worse LRF survival after RT (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, including clinical trials, to inhibit AR as a novel radiosensitizing target in TNBC. Citation Format: Speers C, Zhao SG, Liu M, Rae JM, Hayes DF, Feng FY, Pierce LJ. Androgen receptor (AR): A novel target and mechanism for radiosensitization and treatment in triple-negative breast cancers (TNBC). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P6-13-05.