The epidermal growth factor receptor (EGFR) is frequently overexpressed in colorectal cancer and is therefore an attractive target for treatment. (ZEGFR:1907)2 is a newly developed dimeric affibody molecule with high affinity to the extracellular part of EGFR. In this study, we evaluated the cytotoxic effects of (ZEGFR:1907)2 in combination with external radiation and the possible inhibitory effects in the EGFR signalling pathways in the colon cancer cell lines HT-29 and HCT116. The effects were compared with an EGFR antibody (cetuximab) and the tyrosine kinase inhibitors (erlotinib and sunitinib). These cell lines are genotypically different with respect to e.g. KRAS and BRAF mutational status, recently shown to be of clinical significance for therapeutic effects. Both cell lines express approximately 100,000-150,000 EGFRs per cell but differ in the radiation response (HCT116, SF2=0.28 and HT-29, SF2=0.70). Exposure to (ZEGFR:1907)2 produced a small, but significant, reduction in survival in HCT116 but did not affect HT-29 cells. Similar results were obtained after exposure to EGF and the EGFR antibody cetuximab. The EGFR tyrosine kinase targeting inhibitor erlotinib and the multi-tyrosine kinase inhibitor sunitinib reduced survival in both cell lines. However, none of the drugs had any significant radiosensitizing effects in combination with radiation. Akt and Erk are central proteins in the EGFR downstream signalling and in the cellular response to ionizing radiation. The activation of Akt (Ser 473) and Erk (Thr202/Tyr204) by radiation was both dose- and time-dependent. However the activation of EGFR was not clearly affected by radiation. Neither (ZEGFR:1907)2 nor any of the other drugs were able to completely inactivate Akt or Erk. On the contrary, erlotinib stimulated Akt phosphorylation in both cell lines and in HCT116 cells Erk was activated. Overall the results illustrate the complexity in response to radiation and drugs in cells with differential phenotypic status.

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