The potential of therapeutic targeting of tumor cell surface epidermal growth factor receptors (EGFRs) by modified ligands or specific antibodies has been limited by the normal tissue distribution of the receptor. The identification and characterization of a variant of this receptor, EGFRvIII, which is not expressed in normal tissues but has been described in gliomas, non-small cell lung carcinomas, and breast carcinomas, has provided a highly specific, internalizing target for antibody-mediated approaches. To determine the feasibility of immunotargeting EGFRvIII, we have assessed the qualitative distribution and quantitative expression at both the population and cellular levels of EGFRvIII in 21 biopsy samples of human gliomas by indirect analytical and quantitative flow cytometry and by immunohistochemical assay of frozen and formalinfixed tissue. Consistent with previous reports, 50% of gliomas tested (1 of 2 anaplastic astrocytomas, 7 of 12 glioblastoma multiforme, and 2 of 6 oligodendrogliomas) expressed EGFRvIII, as determined by a minimum of 2 separate assays. Minimum estimates of the proportion of positive tumor cells in these populations ranged from 37–86%; in four of five cases in which quantitation of the EGFRvIII density/cell was performed, vaiues of 2.7–6.8 × 105 were obtained with monoclonal antibody (mAb) L8A4 (EGFRvIII specific), levels consistent with successful in vivo immunotargeting. Confocal microscopic analysis confirmed that the subcellular localization of EGFRvIII was identical to that described for EGFR: predominant cell membrane expression, with some perinuclear distribution suggestive of localization to the Golgi region. Neither EGFR nor EGFRvIII was found within the nucleus. This study establishes for the first time that approximately 50% of human glioma biopsies contain cell populations expressing a sufficient number of membrane-expressed EGFRvIIIs to mediate specific anti-EGFRvIII mAb localization. Coupled with previous demonstrations of the rapid internalization of specific mAb-EGFRvIII complexes and the susceptibility of the targeted cells to isotope or toxinmediated cytotoxicity, this study establishes the validity of targeting EGFRvIII for therapy of mutant receptor-positive gliomas, breast carcinomas, and non-small cell lung carcinomas.
Supported in part by NIH Grants CA 11898 and NS20023.