• Neil A O'Brien, Karen McDonald, Luo Tong, Erika von Euw, Ondrej Kalous, Dylan Conklin, Sara A Hurvitz, Emmanuelle di Tomaso, Christian Schnell, Ronald Linnartz, Richard S Finn, Samit Hirawat, and Dennis J Slamon.
• Authors' Affiliations: Division of Hematology/Oncology, Department of Medicine, Geffen School of Medicine at UCLA, Los Angeles, California; nobrien@mednet.ucla.edu.
• Clin Cancer Res. 2014 Jul 1; 20 (13): 3507-20.

PurposeAltered PI3K/mTOR signaling is implicated in the pathogenesis of a number of breast cancers, including those resistant to hormonal and HER2-targeted therapies.Experimental DesignThe activity of four classes of PI3K/mTOR inhibitory molecules, including a pan-PI3K inhibitor (NVP-BKM120), a p110α isoform-specific PI3K inhibitor (NVP-BYL719), an mTORC1-specific inhibitor (NVP-RAD001), and a dual PI3K/mTORC1/2 inhibitor (NVP-BEZ235), was evaluated both in vitro and in vivo against a panel of 48 human breast cell lines.ResultsEach agent showed significant antiproliferative activity in vitro, particularly in luminal estrogen receptor-positive and/or HER2(+) cell lines harboring PI3K mutations. In addition, monotherapy with each of the four inhibitors led to significant inhibition of in vivo growth in HER2(+) breast cancer models. The PI3K/mTOR pathway inhibitors were also effective in overcoming both de novo and acquired trastuzumab resistance in vitro and in vivo. Furthermore, combined targeting of HER2 and PI3K/mTOR leads to increased apoptosis in vitro and induction of tumor regression in trastuzumab-resistant xenograft models. Finally, as previously shown, targeting mTORC1 alone with RAD001 leads to consistent feedback activation of AKT both in vitro and in vivo, whereas the dual mTOR1-2/PI3K inhibitor BEZ235 eliminates this feedback loop. However, despite these important signaling differences, both molecules are equally effective in inhibiting tumor cell proliferation both in vitro and in vivo.ConclusionThese preclinical data support the findings of the BOLERO 3 trial that shows that targeting of the PI3K/mTOR pathway in combination with trastuzumab is beneficial in trastuzumab-resistant breast cancer.©2014 American Association for Cancer Research.

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