Abstract P2-03-11: Genetic background determines the algorithm of effectiveness of targeted drugs of RAS and PI3K pathways in TNBC: Testing a combination of MEK 1/2 inhibitor with mTOR kinase inhibitor or AKT inhibitor
Authors: Carlson JH , De P , Dey N , Leyland-Jones B 
Published: May 2019

Introduction: Most TNBC patients with single-agent targeted therapy in multiple clinical trials develop resistance, leading to disease progression, posing a major challenge in clinical management of mBRCA. In TNBC patients there remains a lack of obvious predictive biomarkers to guide targeted therapy. Although signaling mechanisms of resistance may be either intrinsic or acquired, the role of driver pathways are known be associated with oncogenic evolution of tumor cells to a resistant state. As the PI3K-AKT-mTOR1/2 (PAM) pathway is activated in TNBC (due to alterations in EGFR, 50%; PTEN, 40%; and PIK3CA, 5-10%), the PAM pathway is a candidate for potential molecular targeting of anti-cancer therapeutics. Despite the involvement of PI3K pathway activation in TNBC tumors, single agent PI3K inhibitors show modest clinical activity. Literature references suggest extensive cross-talk exists between the (PAM) and the RAS-RAF-MEK-ERK (MAPK) pathways. Here we tested the efficacy of a combination of MEK 1/2i (AZD6244, GDC0973) with mTOR kinasei (AZD2014, TAK228) or AKTi (AKT5363) in TNBC models. Methods: TNBC cell lines with alterations in KRAS/RAF (MDA-MB231), BRCA-deficient PTEN-loss (SUM149), PTEN-loss (MDA-MB468) and PIK3CA mutation (BT20) were used. Proliferation, apoptosis, cell cycle and 3D clonogenic growth were tested following drug treatment alone or in combination. Signaling events in the respective pathways following drug treatment were interrogated by WB. Results: MEKi led to a brief proliferation inhibition in MDA-MB231 cells. A combination of MEKi and mTORi was additive in decreasing short term proliferation and 10 day 3D growth. AKTi alone had limited effect on 3D growth but in combination with MEKi was profoundly inhibitory. A limited effect of MEKi was observed in BT20 cells; blocking 2D and 3D growth, while a profound effect of MEKi was observed in combination with mTORi (to a lesser extent with AKTi). In MDA-MB468 cells, MEKi did not have an initial appreciable effect (proliferation and G1 arrest), though 3D growth was significantly reduced at 10 days following drug alone or in combination with AKTi. SUM149 cells had G1 arrest with no appreciable change in 3D growth following MEKi. In contrast, single agent mTORi and AKTi reduced 3D growth. MEKi and mTORi doublet slowed proliferation, increased apoptosis and disrupted 3D growth. No single agent treatment with AKTi or mTORi disrupted colony formation however either agent in combination with MEKi blocked 3D growth. Single agents treatments were mostly cytostatic with no increase in apoptosis while a doublet of MEKi plus mTORi induced significant apoptosis.A combination of MEKi with PI3K pathway specific inhibitor significantly blocked phosphrylation of downstream effector molecules. Upregulation of ERK signals following PAM pathway inhibitors were abrogated by prior addition of MEKi. Summary: Combined blockade of the PAM and MAPK pathways (AKTi or mTORi) plus MEK1/2 inhibitor were more effective in attenuating molecular signals. This combination showed enhanced efficacy in TNBC cell models with specific PAM and or MAPK pathway alterations.

Citation Format: Carlson JH, De P, Dey N, Leyland-Jones B. Genetic background determines the algorithm of effectiveness of targeted drugs of RAS and PI3K pathways in TNBC: Testing a combination of MEK 1/2 inhibitor with mTOR kinase inhibitor or AKT inhibitor [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-03-11.