Abstract 5839: Androgen receptor (AR): A novel target for radiosensitization in triple-negative breast cancers (TNBC)

Abstract

Abstract Purpose: Increased rates of local recurrence (LR) have been observed in TNBC despite chemotherapy and radiation (RT). Thus, approaches that result in radiosensitization in TNBC are critically needed. We characterized the RT response of 21 breast cancer cell (BCC) lines using clonogenic survival assays and paired this with high-throughput drug screen data, identifying AR as a top target for radiosensitization. We demonstrate that AR inhibition confers radiosensitization in vitro and in vivo, identified a biomarker of response, and characterize the mechanism of AR-mediated radiosensitization in TNBC. Materials/Methods: Clonogenic survival assays determined the intrinsic RT sensitivity of 21 BCC lines. IC50 values were determined for 130 clinical compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Gene and protein expression was measured using RNA Seq and RPPA arrays, respectively, in tumor samples (n=2,061) and BCC lines (n=51). AR function was assessed using gene knockdown (KD) or functional inhibition with anti-androgen drugs. 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. A Cox proportional hazards model and MVA were used to determine variables associated with LRF survival. Results: Our unbiased drug radiosensitizer screen nominated bicalutamide as an 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 51 BCC lines and found heterogeneity in AR expression with strongly correlated expression of protein and RNA levels in TNBC (R2=0.89, p-value <0.001). Inhibition of AR, using both KD and drug (MDV3100) induced RT sensitivity with an enhancement ratio (ER) of 1.35-1.42 in AR+ TNBC lines with no effect on controls. Radiosensitization was at least partially dependent on impaired dsDNA break repair mediated by DNAPKcs. AR inhibition with MDV3100 significantly radiosensitized TNBC xenografts in mouse models and markedly delayed tumor tripling time (TTT) and tumor growth (median TTT 17.4 days for RT alone vs. not reached after 50 days for MDV3100+RT, p-value <0.001). Activated DNAPK was identified as a biomarker of response. Clinically, TNBC patients whose tumors had higher than median expression of AR had higher rates of LR after RT (HR for LR ~3, p-value <0.01, 2 independent datasets). In MVA, high AR expression was the variable most significantly associated with worse LRF survival after RT in TNBC patients, outperforming all other variables (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: Benjamin C. Chandler, Corey W. Speers, Shuang G. Zhao, Meilan Liu, Kari Wilder-Romans, Eric Olsen, Shyam Nyati, Daniel Spratt, Daniel Wahl, Daniel Hayes, Felix Y. Feng, Lori J. Pierce. Androgen receptor (AR): A novel target for radiosensitization in triple-negative breast cancers (TNBC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5839. doi:10.1158/1538-7445.AM2017-5839