LAUSR

Background: Approximately half of women with estrogen receptor (ER)-positive breast cancer who undergo adjuvant endocrine therapy (ET) develop disease progression and metastasis. Studies have revealed that up to 40% of metastatic patients develop tumors bearing a mutation in ESR1. ESR1 mutant (ESR1m) tumors are associated with a more aggressive metastatic disease course. Studies have shown that cancers can undergo “kinome reprogramming” in response to therapy and acquired genetic alterations within the tumor. Thus, we hypothesize that expression of ESR1 mutations induces kinome reprogramming, which presents novel potential therapeutic targets to selectively treat ESR1m metastatic breast cancer. Methods: We performed Kinobeads Precipitation (KiP) using broad specificity kinase inhibitors to capture and quantify by MS the level of active kinases in MCF-7 ESR1 wild-type and ESR1 Y537S mutant cells. We performed targeted siRNA screens and immunoblots to validate the hyperactivity of several of these kinases. Specific kinase inhibitors are being used to test their effects on proliferation and invasion in vitro, and in ex vivo organoid growth assays. Xenograft models are being used to evaluate the effects of the inhibitors on primary tumor growth and distant metastasis. Results: Hyperactivation of 169 kinases in ESR1m cells was revealed by KiP analysis. Hyperactivation of several growth factor receptors, the PI3K/Akt/mTOR pathway, and the JAK/STAT pathway was demonstrated in Y537S mutant cells. We tested specific inhibitors of these pathways, and have shown that the mutant cells have reduced proliferation when these pathways are targeted. We developed organoid growth assays using primary and metastatic tumors from MCF-7 and T47D Y537S ESR1 xenograft tumors, and the Y537S ESR1 WHIM20 PDX model, and found that ET in combination with these selected targeted kinase inhibitors decreased the growth of organoids. We are currently testing inhibitors in combination with ET in vivo, and preliminary results demonstrate a significant inhibition of distant metastatic frequency. These results suggest that selective inhibition of hyperactivated “reprogrammed” kinase pathways can restore ET sensitivity in ESR1m tumors, and may show promise for use in ER-positive metastatic breast cancer patients. Citation Format: Derek Dustin, Guowei Gu, Doug Chan, Amanda Beyer, David Edwards, Arnoldo Corona-Rodriguez, Matthew Ellis, Suzanne Fuqua. Novel clinical targets revealed by kinome reprogramming in mutant ESR1 metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 921.