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 t he expression of mutant proteins with altered functions. These aberran t 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 do main. 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 irt vivo. The deletion al so creates a tumor-specific cell-surface sequence at the deletion junc tion. Three specific anti-EGFRvIII mAbs have been isolated following i mmunization with a mixture of a deletion junction synthetic peptide an d EGFRvIII as present on cell membranes, We have constructed immunotox ins by conjugating a modified version of Pseudomonas exotoxin A to the se mAbs, Immunotoxins were tested on cells that had been transfected w ith cDNA for the EGFRvIII receptor and expressed receptor protein at 5 x 10(5) receptors/cell. All three immunotoxins were cytotoxic to thes e cells, with 50% inhibition of protein synthesis occurring in a 15-50 pM range. The immunotoxins specifically targeted EGFRvIII, as their c ytotoxicity could be blocked by their respective free antibody. They s howed little or no cytotoxicity to cells expressing high levels of nor mal epidermal growth factor receptors, demonstrating that they are abl e to discriminate between cells expressing the mutant receptor and tho se 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.