Abstract 726: Erlotinib overcomes acquired resistance to cetuximab
Authors: Brand Toni M. , Dunn Emily F. , Iida Mari , Myers Rebecca , Li Chunrong , Wheeler Deric L. 
Published: June 2012

Abstract
The epidermal growth factor receptor (EGFR) is a ubiquitously expressed receptor tyrosine kinase (RTK) and is recognized as a key mediator of proliferation and progression in many human epithelial tumors. Following more than 20 years of preclinical development, five EGFR inhibitors, two monoclonal antibodies and three small molecule tyrosine kinase inhibitors (TKIs), have gained FDA approval in oncology (panitumumab, cetuximab, erlotinib, gefitinib and lapatinib). Both strategies of EGFR inhibition have demonstrated clinical successes. However, many tumors do not show response to EGFR inhibition and some of the responders eventually manifest resistance to the treatment. The underlying mechanisms of intrinsic and acquired resistance to EGFR inhibitors remain an intense area of scientific and clinical investigation.
To explore potential molecular mechanisms of acquired resistance to cetuximab we established a series of cetuximab-resistant clones by chronically exposing the NCI-H226 NSCLC cell line to cetuximab. Cetuximab-resistant clones exhibited a dramatic increase in the steady-state expression of EGFR, due to alterations in trafficking and endocytosis as compared to parental controls. This increase in steady-state expression of the EGFR led to constitutive activation of HER2 and HER3 and downstream signals mediated by MAPK and AKT. Cells with acquired resistance to cetuximab remained dependent on EGFR signaling as shown by RNAi studies. This finding prompted investigation on whether or not cells with acquired resistance to cetuximab would be sensitive to erlotinib, an EGFR directed TKI. In vitro, erlotinib was able to decrease AKT and MAPK activation and subsequent cell proliferation measured by BrdU incorporation. Additionally, erlotinib treatment induced apoptosis in cetuximab resistant cells as measured by Annexin V analysis. To determine if erlotinib could have therapeutic benefit in vivo, we treated cetuximab sensitive parental lines, H226, with cetuximab in mouse xenograft studies. Mice harboring tumors were treated for approximately 70-150 days before resistant tumors manifested. At this time a series of mice with resistant tumors were treated with erlotinib for 15-30 days. Mice treated with erlotinib exhibited either a regression or tumor growth delay as compared to controls maintained on cetuximab. Analysis of the erlotinib treated tumors demonstrated a decrease in cell proliferation as measured by Ki67 and higher rates of apoptosis, as measured by TUNEL assays when compared to tumors maintained on cetuximab treatment. The work presented herein suggest that 1) cells with acquired resistance to cetuximab maintain their dependence on EGFR signaling via the MAPK and AKT pathways and 2) tumors developing resistance to cetuximab can be treated with erlotinib. This work may provide data strengthening the rationale for the use of erlotinib to treat patients with cetuximab resistant tumors.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 726. doi:10.1158/1538-7445.AM2011-726