Dual Kinase Inhibition of EGFR and HER2 Overcomes Resistance to Cetuximab in a Novel In Vivo Model of Acquired Cetuximab Resistance

Purpose: Acquired resistance to cetuximab, a chimeric epidermal growth factor receptor (EGFR)targeting monoclonal antibody, is a widespread problem in the treatment of solid tumors. The paucity of preclinical models has limited investigations to determine the mechanism of acquired therapeutic resistance, thereby limiting the development of effective treatments. The purpose of this study was to generate cetuximab-resistant tumors in vivo to characterize mechanisms of acquired resistance. Experimental Design: We generated cetuximab-resistant clones from a cetuximab-sensitive bladder cancer cell line in vivo by exposing cetuximab-sensitive xenografts to increasing concentrations of cetuximab, followed by validation of the resistant phenotype in vivo and in vitro using invasion assays. A candidate-based approach was used to examine the role of HER2 on mediating cetuximab resistance both in vitro and in vivo. Results: We generated a novel model of cetuximab resistance, and, for the first time in the context of EGFR-inhibitor resistance, we identified increased phosphorylation of a C-terminal fragment of HER2 (611-CTF) in cetuximab- resistant cells. Afatinib (BIBW-2992), an irreversible kinase inhibitor targeting EGFR and HER2, successfully inhibited growth of the cetuximab- resistant cells in vitro. When afatinib was combined with cetuximab in vivo, we observed an additive growth inhibitory effect in cetuximab- resistant xenografts. Conclusions: These data suggest that the use of dual EGFR-HER2 kinase inhibitors can enhance responses to cetuximab, perhaps in part due to downregulation of 611-CTF. This study conducted in a novel in vivo model provides a mechanistic rationale for ongoing phase I clinical trials using this combination treatment modality. Clin Cancer Res; 17(18); 5935-44. (C) 2011 AACR.