Abstract 4471: Combined blockade of PI3K/AKT and EGFR/HER3 enhances antitumor activity in triple negative breast cancer.
Authors: Scaltriti Maurizio , Tao Jessica , Juric Dejan , Auricchio Neil , Castel Pau , Morse Natasha , Kim Phillip , Singh Sharat , Hazra Saswati , Hembrough Todd , Burrows Jon , Baselga Jose 
Published: August 2014

Abstract
Background: Up to 60% of triple negative breast cancers (TNBCs) express high levels of EGFR. Moreover, TNBCs are associated with increased frequency of phosphatase and tension homologue (PTEN) loss of function, leading to hyperactivation of the phosphoinositide 3-kinase (PI3K)/AKT pathway. This provides the rationale for using PI3K/AKT inhibitors in this subset of patients. However, compensatory expression of receptor tyrosine kinases (RTKs) such as HER3 may limit efficacy of PI3K/AKT inhibitors. Therefore, we hypothesize that combined targeting of both EGFR and HER3 and the PI3K/AKT pathway will result in superior antitumor activity compared to single agent in TNBC. Materials and Methods: Several TNBC cell lines were treated with MEHD7945A, a dual-action antibody that targets both EGFR and HER3, AKT inhibitor GDC-0068, and pan-PI3K inhibitor GDC-0941. Cell viability was measured by CellTiter-Glo and Crystal Violet. Both cell line- and patient-derived xenograft models of TNBC were treated with MEHD7945A, GDC-0068, GDC-0941, or the combination of MEHD7945A with either GDC-0068 or GDC-0941. Tumor size and histology were examined. Protein expression was measured by Western blot, Mass Spectometry, CEER and immunohistochemistry. Results: GDC-0068 and GDC-0941 treatment resulted in variable inhibition of cell viability, with IC50s ranging from 170 nM to &gt;1 μM across all TNBC cell lines. In cells stimulated with either EGF or Heregulin MEHD7945A prevented EGFR/HER3 receptors phosphorylation and improved the antiproliferative activity of the PI3K/AKT inhibitors. To test the activity of these compounds in vivo we used three different models of TNBC, two cell line (MDA-MB-468 and HCC70)-based and a patient-derived xenograft. MEHD7945, GDC-0941 and GDC-0068 showed variable delay in tumor growth whereas combination of MEHD7945A with either GDC-0068 or GDC-0941 was always superior to single agent treatment. Both combinations either prevented tumor growth or led to tumor shrinkage with complete responses achieved in 1/2 mice in each cohorts. Of note, all the treatments (up to 9 weeks of therapeutic exposure) were well tolerated. Analysis of treated tumors reveals potent inhibition of the PI3K/AKT pathway, with decreased levels phospho-PRAS40 and phospho-S6. Moreover, we confirmed that MEHD7945A effectively prevented EGFR and HER3 phosphorylation consequent to PI3K inhibition. Conclusions: Combined therapy with MEHD7945A and either GDC-0068 or GDC-0941 was superior to monotherapy in preclinical models of TNBC. We are currently exploring the levels of expression/activation of EGFR and HER3 following PI3K/Akt blockade in samples from patients treated with GDC-0068. These findings provide the rationale for combined targeting of PI3K/AKT and EGFR/HER3 in triple negative breast cancers.
Citation Format: Maurizio Scaltriti, Jessica Tao, Dejan Juric, Neil Auricchio, Pau Castel, Natasha Morse, Phillip Kim, Sharat Singh, Saswati Hazra, Todd Hembrough, Jon Burrows, Jose Baselga. Combined blockade of PI3K/AKT and EGFR/HER3 enhances antitumor activity in triple negative breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4471. doi:10.1158/1538-7445.AM2013-4471