Abstract P1-09-05: Radiosensitization of androgen receptor (AR)-positive triple-negative breast cancer (TNBC) cells using seviteronel (INO-464), a selective CYP17 lyase and AR inhibitor
Authors: Speers CW , Chandler B , Olsen E , Wilder-Romans K , Moubadder L , Nyati S , Rae J , Hayes DF , Spratt DE , Wahl DR , Eisner J , Feng FY , Pierce LJ 
Published: February 2018

Background: Increased rates of locoregional recurrence (LR) have been observed in TNBC despite chemotherapy and radiation (RT). A novel radiosensitizer screen nominated the AR as a promising target for treatment of radioresistant breast cancer, including TNBC. We assessed the activity of seviteronel (Sevi), a selective CYP17 lyase and AR inhibitor in Phase 2 clinical development for advanced breast and prostate cancer, as a potential radiosensitizer in AR+ TNBC model.

Methods: Clonogenic survival assays were used to determine the intrinsic RT sensitivity of 21 breast cancer cell (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 or qRT-PCR and protein expression was measured using RPPA arrays. AR function was assessed using functional inhibition with Sevi in MDA-MB-453, ACC-422, ACC-460, SUM-185 (all four AR+ TNBC), MDA-MB-231 (AR- TNBC), and T47D (AR- ER+) BCC lines. Double-stranded DNA (dsDNA) break repair was assessed with γH2AX foci counting. In vivo tumor growth was measured 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 multi-variate analysis (MVA) were used to determine variables associated with LRF survival.

Results: Our novel radiosensitizer screen identified the activity of anti-androgen therapy as a potentially effective strategy for radiosensitization in RT-resistant BCC lines (R2 =0.46, p-value &lt; 0.01) (Speers et al, J Clin Oncol 35, 2017 (suppl; abstr e12102). Heterogeneity in AR expression was identified in human BCC lines and TNBC samples from patients (N=2098). There was a strong correlation between AR RNA expression and protein expression across all BC intrinsic subtypes. AR inhibition using Sevi induced radiation sensitivity in vitro with an enhancement ratio (ER) of 1.24-1.69 in four different AR+ TNBC lines. No such radiosensitization was seen in AR(-) TNBC or ER+, AR(-) BCC lines. Radiosensitization was at least partially dependent on impaired dsDNA break repair with significant delays in dsDNA break repair at 16 and 24 hours in all AR+ TNBC lines examined (p-value &lt; 0.01). AR inhibition with Sevi significantly radiosensitized AR+ TNBC xenografts in mouse models and markedly delayed tumor-volume tripling time (TTT) and tumor growth (MDA-MB-453: median TTT 16.1 days for RT alone vs. not reached after 45 days for Sevi+RT, p-value &lt;0.001). Similar delays were seen in tumor growth, weight, and tumor doubling. 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 &lt;0.01, 2 independent datasets). In MVA, high AR expression was the variable most significantly associated with worse LR survival after RT in TNBC patients, outperforming all other variables (HR of 3.42; p-value &lt; 0.01).

Conclusions: Our results implicate the AR as a mediator of radioresistance in breast cancer and support the rationale for developing Sevi as a novel radiosensitizing agent in AR+ TNBC.

Citation Format: Speers CW, Chandler B, Olsen E, Wilder-Romans K, Moubadder L, Nyati S, Rae J, Hayes DF, Spratt DE, Wahl DR, Eisner J, Feng FY, Pierce LJ. Radiosensitization of androgen receptor (AR)-positive triple-negative breast cancer (TNBC) cells using seviteronel (INO-464), a selective CYP17 lyase and AR inhibitor [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-09-05.