S2-5: An Anti-HER3 Antibody That Stabilizes the Inactive Conformation Inhibits Both HER2 and Ligand Driven Tumor Growth.
Authors: Garner AP , Bialucha CU , Chen D , Elis W , Kunz C , Li S , Murtie J , Saxena P , Sineshchekova O , Sprague E , Ettenberg S 
Published: June 2012

Abstract
Background: HER3 (ErbB3) is a member of the ErbB family of receptor tyrosine kinases (RTK). In normal physiology, ligands (e.g. neuregulin) activate HER3 by promoting dimerization with other RTK's such as HER2 (ErbB2). Inappropriate HER2/HER3 dimerization as a result of HER2 over-expression in cancer results in HER3 mediated activation of the oncogenic PI3K pathway. The HER2− targeted antibody trastuzumab inefficiently inhibits HER2 mediated HER3 activation allowing persistent HER3 signaling that is speculated to limit clinical responses. Consequently, combination of a HER3- targeted agent with trastuzumab may be of clinical benefit. Furthermore, ectopic HER3 activation has recently been implicated in the relief of a feedback loop induced by PI3K inhibitors that may perhaps limit their efficacy in HER2 driven tumors. Since HER3 activation in HER2 driven cancers occurs in a ligand- independent manner, antibodies that primarily inhibit ligand- induced HER3 activation are largely inactive in HER2 driven xenograft models.
Results: H3F15, a high affinity (26pM) HER3- targeted fully human IgG1 antibody was selected from the Human Combinatorial Antibody Library (HuCAL) using phage display technology. In a broad range of HER2 amplified breast and gastric cell lines, H3F15 displayed potent inhibition (IC50 &lt;1nM) of HER3/AKT phosphorylation and proliferation. Interestingly, H3F15 also effectively inhibited neuregulin stimulated HER3 phosphorylation and downstream signaling in MCF7 cells indicating that H3F15 inhibits multiple modes of HER3 activation. Determining the H3F15/HER3 crystal structure revealed that H3F15 binds a novel conformational epitope that traps HER3 in the inactive conformation thus preventing its activation by either HER2 or neuregulin. In vivo testing of H3F15 using xenograft tumor models driven by either HER2 (BT474) or HER3 ligands (BxPC3) confirmed that single agent H3F15 (20mg/kg) significantly inhibited tumor growth (83 and 77% inhibition respectively). Furthermore, combinations of H3F15 with either trastuzumab or PI3K- targeted agents were synergistic in a panel of HER2 driven cell lines whilst the in vivo combination of H3F15 (20mg/kg) with trastuzumab (1mg/kg) was sufficient to induce tumor regression.
Discussion: H3F15 is a HER3- targeted antagonist IgG1 that stabilizes the inactive form of HER3. This novel mechanism of action enables H3F15 to uniquely inhibit both ligand-induced and HER2−mediated activation of HER3. Thus H3F15 is the first HER3 antibody to display single-agent efficacy in both HER2 and ligand driven xenograft models whilst also inducing tumor regressions in combination with trastuzumab. Based on preclinical data, combining H3F15 with either trastuzumab or PI3K- targeted agents fully inhibits the HER2/HER3 signaling pathway, which may lead to greater and more sustained clinical efficacy in HER2 driven cancers.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr S2-5.