Abstract 337: In vitro evaluation of dual pan-PI3-kinase/mTOR inhibitor in HER2 overexpressing breast cancer cells

The HER/EGFR is a target of significant interest in human cancer due to the high frequency and broad spectrum aberrations in the pathway observed in human tumors, including breast tumors. Following ligand induced homodimeraziation/ heterodimeraziation HER2 transmits its signal mainly via PI3Kinase-AKT-mTOR and RAS-RAF-ERK1/2 pathways that promote proliferation, survival and angiogenesis. PI3Kinase-AKT-mTOR is the most frequently activated oncopathway in human cancer (De P and Leyland -Jones B, JCO Editorial in Press; Holbro T 2003 PNAS 100: 8933). Tumor cells with hyperactive PI3Kinase-AKT-mTOR pathway are not only resistant to external stresses that can induce cell death and also to chemotherapy. Literature suggests that hyyperactivation of PI3Kinase-AKT-mTOR pathway is one the major causes for the development of de novo or acquired resistance to trastuzumab (Berns K et al 2007 Cancer Cell 12: 395-402; Junttila TT et al 2009 Cancer Cell 15: 429-440). The importance of PI3kinase pathway in Her2 amplified and/or trastuzumab resistance breast cancer implies that interruption of this pathway is important in order for HER2-directed therapies to exert an anticancer effect. The present study was carried out to investigate the preclinical efficacy of a dual pan-PI3K plus mTOR inhibitor, BEZ235 alone or in combination with trastuzumab in HER2 overexpressing trastuzumab-sensitive and trastuzumab-resistant breast cancer cell lines. Here we utilized HER2 overexpressing SKBR3, BT474 and trastuzumab -resistant BT474 cells as preclinical models. Our results show 1) BEZ235 time dependently and dose dependently suppressed the pathway downstream of PI-3Kinase including p-AKT, p-mTOR and p-P70S6Kinase in both trastuzumab-sentsitive and trastuzumab-resistance breast cancer cell lines at 50nM, 100nM and 200nM concentrations and 100% inhibition was observed at higher concentration 2) inhibition of AKT and its downstream effectors were more pronounced when trastuzumab was treated along with BEZ235 even at lower concentrations, 3) the well described AKT activation due to mTOR inhibition was prevented by higher doses of BEZ235 (at 200 nm concentration) treatment, and 4) unlike other pan-PI3Kinase inhibitor (e.g. LY294002, SF1126), BEZ235 has no effect on hypoxia-mediated stabilization of hypoxia-inducible factor −1α (HIF1α) however, heregulin-induced HIF1α synthesis which is an important angiogenic modulator in breast cancer cells, was significantly decreased (> 70%) following the treatment of BEZ235. Our in vitro data provide evidence that clinically pertinent dual pan-PI3K plus mTOR inhibitor can reverse trastuzumab resistance in breast cancer cells, and further support to carry out in vivo efficacy studies (tumor growth and tumor-induced angiogenesis) with BEZ 235 alone or in combination with trastuzumab. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 337.