An EGF-pseudomonas exotoxin A recombinant protein with a deletion in toxin binding domain specifically kills EGF receptor bearing cells.

We constructed two chimeric toxins; one composed of epidermal growth factor (EGF) and pseudomonas exotoxin A (PE), designated EGF-PE and the other composed of EGF and PE with a deletion of the Ia domain (cell-binding domain), designated EGF-PE (delta Ia). Both chimeric toxins reacted with anti-EGF and anti-PE antibodies. The cell-killing experiments showed that EGF-PE, but not EGF-PE (delta Ia), was cytotoxic to the murine fibroblast cell line NR6, which carried the PE receptor, but not the EGF receptor. However, after NR6 was transfected with DNA for the expression of human EGF receptor, the transfected cell line, designated NRHER5, over-expressed human EGF receptors and became sensitive to EGF-PE(delta IA). The cytotoxicity of EGF-PE(delta Ia), but not EGF-PE, to NRHER5 can be completely blocked by an excess amount of EGF. To completely reverse the cytotoxicity of EGF-PE on NRHER5, both the EGF receptor pathway and the PE receptor pathway need to be blocked. These results suggest that EGF-PE exhibits both EGF and PE binding activities, while EGF-PE(delta IA) possesses only EGF binding activity. Thus, EGF-PE(delta Ia) may be a better chimeric toxin than EGF-PE in terms of target specificity to EGF receptor bearing cells. We, therefore, examined the cytotoxicity of EGF-PE(delta Ia) to various human cancer cell lines. We find that human cancer cells containing more EGF receptors are more sensitive to EGF-PE(delta Ia).