Abstract 1940: Dual inhibition of mTORC1/C2 and HER2 results in maximal antitumor efficacy in preclinical model of HER2+ breast cancer resistant to trastuzumab therapy
Authors: De Pradip Kr , Sun Yuliang , Carlson Jennifer H. , Lin Xiaoqian , Dey Nandini , Leyland-Jones Brian 
Published: August 2015

Abstract
Background: Approximately 30% of HER2+ breast cancers (BC) harbor activating mutations (helical or kinase domain) in PIK3CA (Ellis and Perou Cancer Discovery 2013 3:27) and PIK3CA mutation is associated with lower rates of pathologic complete response to anti-HER2 therapies in primary HER2+ BC (Loibl S et al JCO 2014 55:7876). The hypothesis is that the suppression of one of the nodal points of this pathway results in sensitization to anti-HER2 agents. However, the combination of trastuzumab (T) and mTORC1/C2 inhibitor has not been comprehensively tested in T-sensitive and HER2+/PIK3CA mutated models. Methodology: Here we investigate the preclinical efficacy of a dual mTORC1/C2 inhibitor (MLN-0128, formally called INK128) alone or in combination with T in HER2+/T-sensitive/PIK3CA wild type (neither helical nor kinase domain mutation) (BT474), and HER2+/PIK3CA mutated (HCC1954, UACC893, MDA-MB453) cell lines. We analyzed in vitro cell viability and induction of apoptosis in all HER2+ cell lines. PI3K signaling was examined by immunoblot for phosphoproteins in HER2-PI3K-AKT-mTOR signaling cascade. Tumor growth inhibition was evaluated after the treatment with T, MLN-0128 or the combination in HER2 amplified models with wild type or mutant PIK3CA. Results: Our data showed that 1) PIK3CA mutation in HER2+ cells were responsible for resistance to the HER2-specific antibody T, 2) T-resistance conferred by this activating PIK3CA mutation was overcome by blockade of mTORC1/C2 with MLN-0128, 3) inhibition of phosphorylation of AKT(S473), P70S6K, S6RP, and 4EBP1(T37/46, T70) was observed following MLN-0128 treatment and the combination of MLN-0128 and T more effectively blocked the mTORC1/C2 signaling pathway, 4) MLN-0128 induced apoptosis of both BT474 and HCC1954 cells as indicated by Annexin V staining, 5) in vivo, both cell line-based xenografts showed exquisite sensitivity to the antitumor activity of the combination of T and MLN-0128, which resulted in durable tumor shrinkage and exhibited no signs of toxicity in these models and 6) immununo-staining of tumor tissues from MLN-0128 treated mice showed growth inhibition (Ki67), tumor-induced angiogenesis (CD31) and a decreased in associated phospho-proteins (p-S6RP, p-4EBP1). Conclusion: These results suggest that the combination of MLN-0128 plus T may be a potential strategy for the treatment of T-resistant breast cancer mediated by activating mutation of PIK3CA and this combination also provides benefit to HER2+/PIK3CA wild type tumors.
Citation Format: Pradip Kr De, Yuliang Sun, Jennifer H. Carlson, Xiaoqian Lin, Nandini Dey, Brian Leyland-Jones. Dual inhibition of mTORC1/C2 and HER2 results in maximal antitumor efficacy in preclinical model of HER2+ breast cancer resistant to trastuzumab therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1940. doi:10.1158/1538-7445.AM2015-1940