EGF ligand fused to truncated Pseudomonas aeruginosa exotoxin A specifically targets and inhibits EGFR­positive cancer cells.

Cancer cells have been known to overexpress the epidermal growth factor receptor (EGFR) and hence relevant multiple­targeted therapies have been developed, with a recent clinical application of the antibody­mediated inhibition of the EGFR. However, this strategy is not useful in cancer cells with mutations in KRAS; a GTPase downstream of EGFR which constitutively activates the pathway without EGF stimulation. Furthermore, mutations in EGFR also reduce the binding of monoclonal antibodies and thereby render them ineffective. In the present study, we designed a chimeric EGF protein fused to the truncated N­terminal domain fragment of Pseudomonas aeruginosa exotoxin A (EGF­ETA), which has ADP­ribosylation activity and induces apoptosis. The EGF­ETA protein was expressed in E. coli as a His­tagged fusion. Our results showed that EGF­ETA significantly inhibited the proliferation of EGFR­positive A431 epidermoid carcinoma (IC50 27 ng/ml) and HN5 head and neck squamous cell carcinoma (IC50 36 ng/ml) cells. However, its effect on cancer cells with little or no EGFR expression was limited (A549­IC50 1,000 ng/ml; MCF­7­IC50 >10,000 ng/ml). Compared to cetuximab, EGF­ETA was highly potent in its killing capacity of HN5 cancer cells at 1,000 ng/ml, while cetuximab had little effect at 1,000 ng/ml. Furthermore, EGF­ETA was just as potent in HCT116 (KRAS G13D) and SW480 (KRAS G12V) colon cancer cell lines harbouring KRAS hyperactivating mutations when compared to KRAS wild­type HT29 colon cancer cells. Finally, co­incubation of EGF­ETA with an anti­EGF antibody abrogated its effect on the EGFR­positive A431 cells. Our results show that the chimeric EGF­ETA toxin is extremely effective against EGFR­positive cancers and raises the potential to further develop this chimera for use in targeting EGFR­positive tumours resistant to monoclonal antibodies.