Background: AR is expressed in ~60% of HER2+ and up to 50% of triple negative breast cancer (TNBC) with growing evidence that AR plays a role in BC growth and survival. Next-generation AR-targeting agents including SEVI (selective CYP17-lyase and AR inhibitor) and ENZA (AR inhibitor) have shown promise in preclinical and clinical models of prostate cancer and are currently being evaluated in BC. Approved therapeutics targeting mTOR such as everolimus (EVEROL) and HER2-targeting agents provide significant clinical benefit in some BC patients; however, de novo and acquired resistance remain critical issues that may be partially attributed to compensatory AR action.

Hypothesis: Due to cross-regulation between AR and the HER2/PI3K/mTOR pathway in BC, targeting AR in combination with approved agents for these pathways may improve responses.

Methods: Human BC cell lines HER2+ (BT474, BT474-HR20, SKBR3) and TNBC (MDAMB453, BT549) and an AR+ TNBC mouse mammary carcinoma model (Met-1) were used. Cells were treated with multiple doses of AR-targeting agents SEVI or ENZA and EVEROL, alone or in combination and assayed over time using IncuCyte instrumentation and combination indices calculated using Calcusyn. Anchorage independence was tested by growth on soft agar. Pathway component genes and proteins were measured. In vivo, NOD/SCID mice with HER2+ trastuzumab-resistant BT474-HR20 xenografts were treated with ENZA, EVEROL, or the combination. Similar in vivo experiments with SEVI are ongoing.

Results: Both SEVI and ENZA significantly inhibited proliferation of both HER2+ and TNBC cell lines. Both significantly inhibited growth on soft agar in l Met-1 cells (p=0.0005 for SEVI, p=0.032 for ENZA). Using 9 dose combinations of EVEROL with SEVI or ENZA, synergy was observed in multiple cell lines in vitro, but only in lines containing activating PIK3CA mutations. DHT induced a cell line-specific increase in pHER2 and/or pHER3 that was attenuated by AR inhibition. EVEROL alone caused an increase in total AR, pHER2, and pHER3, and these effects were also abrogated by AR inhibition. In vivo, ENZA inhibited BT474-HR20 xenografts, and combining ENZA with EVEROL decreased tumor viability more than either single agent (p<0.001, repeated measures ANOVA). Similar in vivo studies with SEVI are planned.

Conclusion: Agents targeting AR synergize with EVEROL. Combination therapies targeting AR and the mTOR and HER2/HER3 pathways may provide benefit for HER2+ and TNBC patients. SEVI was a stronger inhibitor of anchorage-independent growth than ENZA and will thus be further investigated in combination with approved HER2/PI3K/mTOR pathway-directed therapeutics in vivo.

This work was funded by DOD BCRP - Clinical Translational Award BC120183 W81XWH-13-1-0090 to JKR and 0091 to AE and R01 CA187733 to JKR

Note: This abstract was not presented at the meeting.

Citation Format: Michael A. Gordon, Nicholas D'Amato, Jessica Christenson, Beatrice Babbs, Haihua Gu, Julia Wulfkuhle, Emmanuel Petricoin, Anthoy Elias, Jennifer K. Richer. Synergy between androgen receptor (AR)-targeting agents seviteronel (SEVI) or enzalutamide (ENZA) and mTOR or HER2 pathway-targeting agents in breast cancer [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 3610. doi:10.1158/1538-7445.AM2017-3610