Abstract A35: Proteomic and transcriptional profiling reveal differential responses to combined MEK and PI3K-mTOR network inhibition in basal-like and mesenchymal subtypes of triple negative breast cancer

Background: Activated MAPK and PI3K/AKT/mTOR pathway signaling are associated with poor prognosis in triple negative Breast cancer (TNBC). Therefore, suppression of both arms of the MAPK/PI3K/AKT/mTOR network is a promising strategy for targeting TNBC. Here we explore the anti-tumor effects of combinations of MEK inhibitor with PI3K, AKT, or mTOR inhibitors with a focus on cell fate and biomarker development in two major subtypes of TNBC, basal-like and mesenchymal. Methods: Combinations of a MEK inhibitor with a PI3K inhibitor, AKT inhibitor, dual mTORC 1/2 inhibitor, or rapalog were evaluated in TNBC cell lines and an orthotopically implanted patient-derived xenograft (PDX) model of TNBC. We utilized reverse-phase protein array (RPPA) and quantitative real-time PCR to interrogate the signaling architecture and transcriptional activity of the treated cell lines. Results: All combinations synergistically suppress the growth of basal-like and mesenchymal TNBC models. RPPA confirms that all combinations suppress common network targets including pERK Th202/T204, pPRAS40 T246, pS6rp S235/236, and p4E-BP1 S65 in all cell lines. Notably, however, comparable repression of MAPK/PI3K/AKT/mTOR signaling produces distinct fates in basal-like and mesenchymal subtypes. Basal-like cell lines preferentially undergo delayed cell death with surviving cells displaying profound growth arrest. In contrast, mesenchymal cell lines respond with uniform quiescence exhibiting little or no cell death. Transcriptional analysis corroborates these phenotypic effects demonstrating differential modulation of genes regulating apoptosis and proliferation in basal-like versus mesenchymal cell lines. Drug treated mesenchymal cells exhibit transcriptional ‘reprogramming’ of epithelial-mesenchymal status as evidenced by reduced mesenchymal gene expression inferring potential effects on invasion and metastasis. In a PDX model of basal-like TNBC that is paclitaxel resistant, early and sustained suppression of tumor growth was achieved by treating once daily with the MEK inhibitor, GDC-0973, in combination with either the PI3K inhibitor, GDC-0941, or the rapalog, Temsirolimus. Concurrent dosing on this intense schedule was associated with some toxicity that could likely be diminished with dose modifications. Conclusions: These data highlight the therapeutic potential of combined MEK and PI3K/AKT/mTOR inhibition in chemo-resistant TNBC. Importantly, they demonstrate innate differences in the response of basal-like and mesenchymal subtypes of TNBC to these combinations, supporting the concept that molecular subtype will be an important predictor of response both in this setting and for other targeted therapies. The combination of GDC-0973 and GDC-0941 performed particularly well in a taxane resistant PDX model and deserves further evaluation in clinical trials for the treatment of TNBC. Citation Format: Sarah J. Schweber, Alicia Rodriguez-Gabin, Jinghang Zhang, Valerie Calvert, Huiping Liu, Emanuel M. Petricoin, III, Eleni Andreopoulou, Susan Band Horwitz, Hayley M. McDaid. Proteomic and transcriptional profiling reveal differential responses to combined MEK and PI3K-mTOR network inhibition in basal-like and mesenchymal subtypes of triple negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr A35.