Abstract B161: U3-1287 (AMG 888), a fully human anti-HER3 mAb, demonstrates preclinical efficacy in HER2+ and HER2− breast cancer models
Authors: Hettman Thore , Schneider Matthias , Blum Sabine , Hartmann Susanne , Hendrich Marco , Moor Robert , Ogbagabriel Selam , Radinsky Robert , Freeman Dan 
Published: June 2012

Abstract
Background: HER3 is required for proliferation in HER2 amplified (HER2+) breast cancer cell lines, but may also be important in the HER2 negative (HER2−) setting. Though HER3 lacks kinase activity, it is a scaffold for PI3K signaling for the HER family and other receptor kinases via heterodimeric interactions. In preclinical models, activation of HER3 is a resistance mechanism to current HER family inhibitors. We report the efficacy of U3-1287 (AMG 888) in HER2+ and HER2− preclinical breast cancer models. We also evaluate human breast tumor samples for detectable levels of pHER3 and pAKT.
Methods: Levels of pHER3, pERK and pAKT were determined by western blotting. To determine the inhibition of HER3 oncogenic signaling, HER2+ (SKBR3, MDA-MB-453 and HCC1569) and HER2− (MDA-MB-175 VII) breast cancer cells were treated with 10 µg/ml of U3-1287 (AMG 888), cetuximab, c2C4, trastuzumab, lapatinib (250 to 1500 nM) or controls prior to heregulin (HRG) stimulation. To determine the activity on cell proliferation in the presence of 0.4% FBS, breast cancer cell lines were incubated with 10 µg/ml of U3-1287 (AMG 888), cetuximab, c2C4, trastuzumab, lapatinib (50 to 1500 nM) or control for 1 hour prior to HRG stimulation. After 4 days, the growth of treated cells was measured with alamarBlue™. To determine the activity on anchorage-independent growth, breast cancer cells were treated with 2 to 5 g/ml U3-1287 (AMG 888), anti-HER antibodies, 500 nM lapatinib, or control. Tumor cell colonies formed in the absence or presence of HRG for 6 to 10 days and were stained with MTT for 4 to 6 hours and quantified.
Results: Treatment of breast cancer cell lines with U3-1287 (AMG 888) resulted in an inhibition of pHER3 and pAKT. In cell proliferation assays, U3- 1287 (AMG 888) reduced heregulin-stimulated SkBR-3 proliferation up to 40% (p&lt;0.05) as a single agent, and up to 80% (p&lt;0.05) when combined with other HER inhibitors. Additive activity was observed in MDA-MB 175 VII cells when U3-1287 (AMG 888) was combined with standard of care chemotherapeutics (gemcitabine and cisplatin) (p&lt;0.05 vs either single agent alone). U3-1287 (AMG 888) did not inhibit colony growth in the ZR- 75-30 model, but inhibits colony growth by 50%, 25%, 85% in the SkBr-3, MDA-MB453 and MDA-MB175VII breast cancer models, respectively, as a single agent (p&lt;0.05) and up to 95% when combined with other HER inhibitors (p&lt;0.05). 40%, 35% of the HER2+ and 10%, 35% of the HER2− human tumor samples had detectable pHER3 and pAKT, respectively. Conclusions: U3-1287 (AMG 888) inhibits proximal and distal HER3 oncogenic signaling in breast cell lines in vitro. Breast cancer cells are sensitive to U3-1287 (AMG 888) treatment as single agent and in combination with anti-HER agents. The preclinical data together with detectable levels of pHER3 in patient samples provide evidence for the potential clinical application of U3-1287 (AMG 888) in HER2+ and HER2− breast cancer.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B161.