2612 Background: Activated MAPK and PI3K pathway signaling are associated with poor prognosis in triple negative breast cancer (TNBC). Although some TNBC cell models are sensitive to MEK inhibition, feedback activation of the PI3K pathway mediates resistance. Thus, suppression of both arms of the MAPK/PI3K/mTOR network is a rational approach to targeting TNBC. Here we explore the anti-tumor efficacy of combinations of MEK inhibitor with PI3K, AKT, or mTOR inhibitors with a focus on biomarker development. Methods: Combinations of the MEK inhibitor PD-0325901 with the PI3K inhibitor GDC-0941, AKT inhibitor MK-2206, dual mTORC 1/2 inhibitor Torin 1, or the rapalog temsirolimus were evaluated in TNBC cell lines. Synergy was assessed using the combination index method of Chou and Talalay. We utilized reverse-phase protein array to map the signaling architecture of the treated lines to verify target suppression and identify pharmacodynamic biomarkers. Results: All combinations demonstrated synergy that was mediated by both suppression of proliferation and cell death in a dose-dependent manner. Cell death was delayed, peaking at least 96 hours post-dosing, and was associated with sustained suppression of target proteins in both pathways, including pERKT202/Y204, pS6rpS235/236, p4EBP-1S65, and pPRAS40T246. However, suppression of pAKT (at T308 or S473) was variable and not consistently required for cell death. Pathway mapping identified a protein network ‘signature’ specific to all combination therapies that emerged at 72 hours and was associated with cell death. Thus, all combinations appear to share common downstream effectors. All combinations showed promising efficacy and will be evaluated in a human-in-mouse model of TNBC. Conclusions: These data support therapeutic strategies for TNBC that simultaneously inhibit both arms of the MAPK/PI3K/mTOR signaling network. For continued biomarker development, we stress the importance of studying the delayed effects of combination therapy. This strategy coupled with a protein network based approach uncovered a unique functional signaling ‘signature’.
A novel RAF/MEK inhibitor CH5126766 in phase I clinical trial has an effectiveness in the combination with eribulin for the treatment of triple negative breast cancer
