Targeting PI3K/mTOR overcomes resistance to HER2-targeted therapy independent of feedback activation of AKT

Clin Cancer Res. 2014 Jul 1;20(13):3507-20. doi: 10.1158/1078-0432.CCR-13-2769. Epub 2014 May 30.


Purpose: Altered PI3K/mTOR signaling is implicated in the pathogenesis of a number of breast cancers, including those resistant to hormonal and HER2-targeted therapies.

Experimental design: The 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.

Results: Each 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.

Conclusion: These 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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Antibodies, Monoclonal, Humanized / pharmacology
  • Antineoplastic Agents / pharmacology*
  • Apoptosis / drug effects
  • Breast Neoplasms / drug therapy
  • Breast Neoplasms / metabolism
  • Breast Neoplasms / mortality
  • Cell Cycle Checkpoints / drug effects
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Disease Models, Animal
  • Drug Resistance, Neoplasm*
  • Female
  • Humans
  • Mice
  • Molecular Targeted Therapy
  • Mutation
  • Phosphatidylinositol 3-Kinases / genetics
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoinositide-3 Kinase Inhibitors*
  • Protein Kinase Inhibitors / pharmacology
  • Proto-Oncogene Proteins c-akt / metabolism*
  • Receptor, ErbB-2 / antagonists & inhibitors*
  • Signal Transduction / drug effects
  • TOR Serine-Threonine Kinases / antagonists & inhibitors*
  • Trastuzumab
  • Xenograft Model Antitumor Assays


  • Antibodies, Monoclonal, Humanized
  • Antineoplastic Agents
  • Phosphoinositide-3 Kinase Inhibitors
  • Protein Kinase Inhibitors
  • TOR Serine-Threonine Kinases
  • Receptor, ErbB-2
  • Proto-Oncogene Proteins c-akt
  • Trastuzumab