Abstract S5-7: Combined Blockade of PI3K/AKT and EGFR/HER3 Enhances Anti-Tumor Activity in Triple Negative Breast Cancer
Authors: Tao J , Ip P , Auricchio N , Juric D , Yu M , Shyamala M , Kim P , Singh S , Hazra S , Haber D , Scaltriti M , Baselga J 
Published: August 2014

Abstract
Background: Up to 60% of triple negative breast cancers (TNBCs) express high levels of EGFR (HER1). 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 HER family receptors and the PI3K/AKT pathway will result in superior antitumor activity compared to single agent blockade in TNBC.
Materials and Methods: TNBC cell lines MDA-MB-231, MDA-MB-468, HCC70, HCC1143 and HCC1937 were treated with MEHD7945A, a dual-action antibody that targets both EGFR and HER3, AKT inhibitor GDC0068, and pan-PI3K inhibitor GDC0941. Cell viability was measured by CellTiter-Glo and Crystal Violet. Both cell line- and patient-derived xenograft models of TNBC were grown subcutaneously in Nu/Nu mice and then treated with MEHD7945A, GDC0068, GDC0941, or the combination of MEHD7945A with either GDC0068 or GDC0941. Tumor size and histology were examined. Circulating tumor cells (CTCs) were isolated from murine peripheral blood by Herring Bone CTC chip. Protein expression was measured by Western blot, Reverse Phase Protein Analysis (RPPA), and immunohistochemistry.
Results: GDC0068 and GDC0941 treatment resulted in variable inhibition of cell viability, with IC50s ranging from 170 nM to &gt;1 µM across all TNBC cell lines. Under full-serum (10% FBS) conditions, MEHD7945A led to a very mild decrease in cell viability in vitro. Combination treatments were modestly more effective than monotherapy in vitro. In vivo, MEHD7945, GDC0941 and GDC0068 showed variable delay in tumor growth whereas combination of MEHD7945A with either GDC0068 or GDC0941 was superior to single agent treatment. While untreated tumors increased over 14 fold in size, tumor growth of MEHD7945, GDC0941 and GDC0068 treated xenografts reached 6, 4 and 2.5 fold increase respectively. However, both combination arms prevented tumor growth, with complete responses achieved in 1/9 mice in each of the combination 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, as well as decreased expression of total EGFR and HER3. Circulating tumor cells are being used as surrogate markers of pathway inhibition.
Conclusions: Combined therapy with MEHD7945A and either GDC0068 or GDC0941 was superior to monotherapy in preclinical models of TNBC. We observed significantly greater effect with the combination treatment in vivo compared to in vitro. We are currently exploring whether expression/activation of the HER receptors following PI3K/Akt blockade are accentuated in vivo. Additionally, this may be the first mouse model enabling CTC collection from subcutaneous xenografts, allowing for real-time pharmacodynamic investigation. These findings provide the rationale for combined targeting of PI3K/AKT and EGFR/HER3 in triple negative breast cancers.
Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S5-7.