Cancer Therapy: Preclinical Dual Blockade of HER2 inHER2-Overexpressing TumorCells Does Not Completely Eliminate HER3 Function

Purpose: Dual blockade of HER2 with trastuzumab and lapatinib or with pertuzumab is a superior treatment approach compared with single-agent HER2 inhibitors. However, many HER2-overexpressing breast cancers still escape from this combinatorial approach. Inhibition of HER2 and downstream phosphoinositide 3-kinase (PI3K)/AKT causes a transcriptional and posttranslational upregulation of HER3 which, in turn, counteracts the antitumor action of the HER2-directed therapies. We hypothesized that suppression of HER3 would synergize with dual blockade of HER2 in breast cancer cells sensitive and refractory to HER2 antagonists. Experimental Design: Inhibition of HER2/HER3 in HER2þ breast cancer cell lines was evaluated by Western blotting. We analyzed drug-induced apoptosis and twoand three-dimensional growth in vitro. Growth inhibition of PI3K was examined in vivo in xenografts treated with combinations of trastuzumab, lapatinib, and the HER3-neutralizing monoclonal antibody U3-1287. Results: Treatment with U3-1287 blocked the upregulation of total and phosphorylated HER3 that followed treatment with lapatinib and trastuzumab and, in turn, enhanced the antitumor action of the combination against trastuzumab-sensitive and -resistant cells.Mice bearingHER2þ xenografts treatedwith lapatinib, trastuzumab, andU3-1287 exhibited fewer recurrences and better survival thanmice treated with lapatinib and trastuzumab. Conclusions: Dual blockade of HER2 with trastuzumab and lapatinib does not eliminate the compensatory upregulation of HER3. Therapeutic inhibitors of HER3 should be considered as part of multidrug combinations aimed at completely and rapidly disabling the HER2 network in HER2-overexpressing breast cancers. Clin Cancer Res; 1–10. 2012 AACR.