Abstract 5646: Overcoming acquired resistance to cetuximab by dual targeting of HER family members using antibody based therapy.
Authors: Iida Mari , Brand Toni M. , Starr Megan M. , Luthar Neha , Wleklinski Matthew J. , Wheeler Deric L. 
Published: August 2014

Abstract
The EGFR is recognized as a key mediator of proliferation and progression in many human tumors and strategies to inhibit EGFR signaling have emerged as highly promising cancer therapy approaches. However, many tumors do not show response to EGFR inhibition and some of the responders eventually manifest resistance to treatment. In an effort to examine mechanisms of acquired resistance to anti-EGFR monoclonal antibody we developed a series of cetuximab-resistant clones using the NSCLC line NCI-H226. During efforts to elucidate molecular pathways that are involved in acquired resistance to cetuximab we found that 1) EGFR ubiquitination and degradation is altered in cetuximab-resistant cells resulting in increased steady-state expression of the EGFR and 2) enhanced signaling from the EGFR to HER3 and the PI(3)K/Akt pathway indicating that activation of HER3 may lead to escape from cetuximab therapy.
To investigate the role of HER3 in acquired resistance to cetuximab we used both RNAi and targeted approaches using the anit-HER3 antibody U3-1287/AMG888. siRNA directed against HER3 indicated that resistant cells had a strong dependency on HER3. Further, targeting HER3 using U3-1287/AMG888 showed a dose-dependent anti-proliferative response in resistant cells. In studies using siRNA against the EGFR and HER3 resulted in robust inhibition of cell proliferation than did silencing of either receptor alone in resistant cells. Comparable effects on cell proliferation were observed after combinatorial treatment of cells by the EGFR antibody cetuximab and the HER3 antibody U3-1287/AMG888. In addition, combined EGFR and HER3 inhibition by either siRNAs or antibodies decreased the phosphorylation of downstream signaling pathways AKT and MAPK. Annexin-V binding assays showed a significant increase in apoptosis in U3-1287/AMG888 plus cetuximab treated cells as compared to either agent alone. To determine if U3-1287/AMG888 could have therapeutic benefit as a combination treatment with cetuximab in vivo, mice harboring NCI-H226 xenografts will be treated with cetuximab for approximately 70-100 days until cetuximab-resistant tumors develop. At this time mice with resistant tumors will be divided into two groups that will be treated with both cetuximab and U3-1287/AMG888 or cetuximab alone.
The work presented herein suggests that cells with acquired resistance to cetuximab maintain their dependence on EGFR and HER3 signaling via the AKT and MAPK pathways.
Citation Format: Mari Iida, Toni M. Brand, Megan M. Starr, Neha Luthar, Matthew J. Wleklinski, Deric L. Wheeler. Overcoming acquired resistance to cetuximab by dual targeting of HER family members using antibody based therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5646. doi:10.1158/1538-7445.AM2013-5646