Abstract P3-04-03: PIK3CA mutations in androgen receptor-positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors

Abstract

Abstract Triple negative breast cancer (TNBC) is a heterogeneous collection of biologically diverse cancers, which contributes to variable clinical outcomes. Previously, we identified a TNBC subtype that has a luminal phenotype and expresses androgen receptor (AR+). TNBC cells derived from these luminal AR+ tumors have high frequency PIK3CA mutations. The purpose of this study was to determine if targeting PI3K alone or in combination with an AR antagonist is effective in AR+ TNBC. Methods: We determined the frequency of activating PIK3CA mutations in AR+ and AR- TNBC clinical cases. Using AR+ TNBC cell lines and xenograft models we evaluated the effectiveness of PI3K inhibitors, used alone or in combination with an AR antagonist, on tumor cell growth and viability. Results: PIK3CA kinase mutations were highly clonal, more frequent in AR+ vs. AR- TNBC (40% vs. 4%), and often associated with concurrent amplification of the PIK3CA locus. PI3K/mTOR inhibitors had an additive growth inhibitory effect when combined with genetic or pharmacological AR targeting in AR+ TNBC cells. We also analyzed the combination of bicalutamide +/- the pan-PI3K inhibitor GDC-0941 or the dual PI3K/mTOR inhibitor GDC-0980 in xenograft tumor studies and observed additive effects. Conclusions: While approximately one third of TNBC patients respond to neoadjuvant/adjuvant chemotherapy, recent studies have shown that patients with androgen receptor positive (AR+) TNBC are far less likely to benefit from the current standard of care chemotherapy regimens and novel targeted approaches need to be investigated. In this study, we show that activating PIK3CA mutations are enriched in AR+ TNBC; and, the growth and viability of AR+ TNBC cell line models is significantly reduced after treatment with PI3K inhibitors used in combination with an AR antagonist. These results provide rationale for pre-selection of TNBC patients with a biomarker (AR expression) to investigate the use of AR antagonists in combination with PI3K/mTOR inhibitors. Citation Format: Brian D Lehmann, Joshua A Bauer, Johanna M Schafer, Christopher S Pendleton, Luojia Tang, Kimberly C Johnson, Xi Chen, Justin M Balko, Henry Gomez, Carlos L Arteaga, Gordon B Mills, Melinda E Sanders, Jennifer A Pietenpol. PIK3CA mutations in androgen receptor-positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors [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 P3-04-03.