An EGFRvIII-specific IHC IUO test for patient selection in AMG 595 phase I trial.

Abstract

2071 Background: EGFRvIII is a mutant version of EGF receptor resulting from the genomic deletion of exons 2 through 7 and is expressed only in certain tumors. AMG 595 is an experimental therapeutic specifically targeting EGFRvIII and consists of an EGFRvIII-specific antibody conjugated to the maytansinoid antimicrotubule agent DM-1. Due to its specificity, mechanism of action, and pre-clinical activity, AMG 595 is expected to have clinical effect only in tumors expressing EGFRvIII. Since the reported prevalence of EGFRvIII in glioblastoma multiforme (GBM) is ~30%, prospective selection of patients with EGFRvIII positive tumors was desired for clinical development. An immunohistochemical (IHC) assay developed with Dako using a novel EGFRvIII antibody is currently being employed for patient selection for the AMG 595 phase I study in recurrent GBM (NCT01475006). Methods: An appropriate IHC reagent for human tissue was created using the variable region of a novel EGFRvIII-specific antibody developed using Xenomouse technology. Staining conditions were optimized using Dako pharmDx reagents, the Dako Link 48 Autostainer, and FFPE tissue sections. Confirmatory transcript analyses of adjacent sections were conducted using the NanoString platform. Results: Robust and reproducible staining for EGFRvIII was observed using archived GBM resections. Percentage of stained cells correlated with levels of EGFRvIII transcripts, and tumors without staining did not express EGFRvIII transcripts. Although some tumors exhibited homogenous staining, most were heterogeneous with varying distribution and percentages of stained tumor cells similar to literature reports of IHC analysis utilizing other EGFRvIII-specific antibodies. Tumor samples from patients entering into the AMG 595 Phase 1 study analyzed with this IHC test have displayed the predicted staining prevalence. Conclusions: The developed EGFRvIII IHC assay, approved under an Investigational Device Exemption, is currently being successfully employed to prospectively select patients in an ongoing phase I trial. Use of this well characterized IHC test will enable correlation of clinical outcome and staining characteristics to inform subsequent studies.