Abstract 5647: Receptor tyrosine kinase plasticity as a mechanism of acquired resistance to cetuximab in vitro: potential for co-targeting with antibody mixtures.

Abstract

Abstract Cetuximab is an EGFR-blocking antibody that belongs to the first generation of monoclonal antibodies approved for treatment of head and neck squamous cell carcinoma and metastatic colorectal cancer. Although anti-EGFR therapy is clinically successful, development of cetuximab resistance is an increasing problem in the clinic. Thus, a better understanding of the mechanisms responsible for development of cetuximab resistance is valuable for development of the next generation of antibody therapeutics. Here we report the establishment of a number of cetuximab-resistant clones derived from the cetuximab-sensitive head and neck squamous carcinoma cell line HN5. Initial characterization of the cetuximab-resistant cells shows that the total level of EGFR is lower compared to the parental cells, but that the cetuximab binding is unaltered. However, cetuximab resistant clones remain dependent on EGFR for growth and proliferation, as a mixture of antibodies targeting non-overlapping epitopes on EGFR is able to partially overcome the cetuximab-induced resistance. Inititial results indicate that the antibody mixture overcomes cetuximab resistances by inducing efficient EGFR internalization followed by lysosomal degradation of the receptor. Removal of EGFR from the cell surface blocks interactions of EGFR with other receptor tyrosine kinases (RTKs) and intracellular proteins as compensatory response to cetuximab inhibition. The observed level of inhibition of the resistant clones by the anti-EGFR antibody mixture did not, however, induce as efficient growth inhibition as in the parental cells, indicating additional resistance mechanisms. Indeed we were able to show that adding antibodies to other RTKs to the anti-EGFR antibody mixture restored sensitivity of the resistant cells to a level similar to parental cells. In conclusion, our results demonstrate that cetuximab resistant clones remain partially dependent on EGFR for growth and proliferation. Additional RTKs are activated as a compensatory response to the EGFR inhibition, demonstrating the plasticity of the RTK family. Our results provide a rationale for targeting multiple RTKs to overcome acquired resistance to currently approved mAbs or to delay the emerge of resistance if used upfront. An update will be presented. Citation Format: Ida Kjaer, Michael Kragh, Ivan D. Horak, Mikkel Wandahl Pedersen. Receptor tyrosine kinase plasticity as a mechanism of acquired resistance to cetuximab in vitro: potential for co-targeting with antibody mixtures. [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 5647. doi:10.1158/1538-7445.AM2013-5647