Immunotoxins that target an oncogenic mutant epidermal growth factor receptor expressed in human tumors

Human cancers arise from a series of mutations, many of which direct the expression of mutant proteins with altered functions. These aberrant proteins are attractive targets for new therapeutic agents. One such protein is a mutant epidermal growth factor receptor (EGFRvIII) that has an in-frame deletion near the NH2 terminus of its extracellular domain. This protein was first identified in human gliomas, but has also been shown to be present in lung and breast carcinomas. The deletion results in a receptor with constitutive tyrosine kinase activity that enhances the tumorigenicity of glioblastomas in vivo. The deletion also creates a tumor-specific cell-surface sequence at the deletion junction. Three specific anti-EGFRvIII mAbs have been isolated following immunization with a mixture of a deletion junction synthetic peptide and EGFRvIII as present on cell membranes. We have constructed immunotoxins by conjugating a modified version of Pseudomonas exotoxin A to these mAbs. Immunotoxins were tested on cells that had been transfected with cDNA for the EGFRvIII receptor and expressed receptor protein at 5 x 10(5) receptors/cell. All three immunotoxins were cytotoxic to these cells, with 50% inhibition of protein synthesis occurring in a 15-50 pM range. The immunotoxins specifically targeted EGFRvIII, as their cytotoxicity could be blocked by their respective free antibody. They showed little or no cytotoxicity to cells expressing high levels of normal epidermal growth factor receptors, demonstrating that they are able to discriminate between cells expressing the mutant receptor and those expressing the wild-type receptor. Immunotoxins targeted to mutant epidermal growth factor receptors are promising candidates for further development as tumor cell-specific therapeutic agents.