LB-20 Background: HER3 is a member of the Human Epidermal Growth Factor Receptor (HER) family. Although HER3 lacks intrinsic kinase activity, it contains six docking sites for the p85 regulatory subunit of PI3K, and serves as a scaffold for PI3K/AKT signaling for the HER family via heterodimeric interactions. Recent reports have demonstrated that HER3 activation may be associated with a resistance mechanism to therapeutic anti-EGFR inhibitors. We report the generation of the first anti-HER3 fully human mAbs and the in vitro and in vivo functional anti-tumoractivities of these antibodies.
Methods: Fully human Anti-HER3 mAbs were generated using Xenomouse® technology. To determine the inhibition of ligand-induced phosphorylation of HER2 and HER3 and downstream signaling targets ERK1/2 and AKT, T47D and MCF7 cells, respectively were treated with control or anti-HER3 monoclonal antibodies (between 10-3 and 102 nM ) 1 hour prior to heregulin-beta (HRG) stimulation. To determine the activity of the anti-HER3 mAbs on anchorage-independent growth, BxPC3 pancreatic cancer cells were treated with10 μg/ml anti-HER3 or control mAbs in serum containing medium. Tumor cell colonies formed in the absence of exogenous ligand for 10-14 days, were stained with MTT overnight and quantified using a Scanalyzer HTS camera imaging system. To determine in vivo efficacy, mice bearing ~200mm3 established BxPC-3 pancreatic, Calu-3 NSCLC, or HT-29 colorectal carcinoma xenografts were treated twice per week with anti-HER3 mAbs at 25 mpk. BxPC-3 xenograft tumors were further analyzed for pHER3 by western blotting.
Results: Treatment of both T47D and MCF7 tumor cells with anti-HER3 mAbs resulted in a significant inhibition of ligand-induced phosphorylation of HER2 and HER3, and the downstream effectors ERK1/2 and AKT, respectively. In the anchorage-independent growth assay, treatment of BxPC3 pancreatic cancer cells with anti-HER3 mAbs resulted in > 50% inhibition of colony formation (p