Human glioblastoma xenografts overexpressing a tumor-specific mutant epidermal growth factor receptor sensitized to cisplatin by the AG1478 tyrosine kinase inhibitor

Object. Activation of signaling by the epidermal growth factor receptor (EG FR) through gene amplification or rearrangement is common in human malignan cy, especially in a large fraction of de novo glioblastomas multiforme, (GB Ms). The most common mutant EGFR, (Delta EGFR, also known as de2-7 EGFR and EGFRvIII) lacks a portion of the extracellular domain, enhances tumorigeni city in vivo, and causes resistance to the chemotherapeutic drug cisplatin (CDDP). This resistance is due to the suppression of CDDP-induced apoptosis by the constitutively active tyrosine kinase activity of the receptor. The authors have investigated whether inhibition of Delta EGFR signaling by th e tyrosine kinase inhibitor, tyrphostin AG1478, could sensitize tumor xenog rafts to CDDP and, thereby, enhance its therapeutic efficacy in animals. Methods. Nude mice were inoculated either subcutaneously or intracerebrally with human GBM cells expressing Delta EGFR and were then systemically trea ted with CDDP and/or AG1478. Tumor volumes were monitored and tumor section s were analyzed by using terminal deoxynucleotidyl transferase-mediated deo xyuridine triphosphate nick-end labeling (TUNEL) assays or MIB-1 staining. Expression of Delta EGFR, but not wild-type EGFR, conferred CDDP resistance to the cells in vivo. Inhibition of receptor signaling by the EGFR-specifi c tyrosine, kinase inhibitor, AG1478, sensitized the xenografts to the cyto toxic effects of CDDP. This combined CDDP/AG1478 treatment significantly su ppressed growth Of subcutaneous xenografts in nude mice in a synergistic ma nner (p < 0.01 compared with vehicle control) without causing generalized t oxicity, whereas treatments with CDDP or AG1478 alone were ineffective. The synergistic growth suppression by the CDDP/AG1478 combination was not obse rved in xenografts overexpressing wild-type EGFR or kinase-deficient <Delta >EGFR. The combined CDDP/ AG1478 treatment induced tumor growth suppression , which correlated with increased apoptosis and reduced proliferation. This treatment also extended the life span of mice bearing intracerebral xenogr afts (p < 0.01 compared with controls). Conclusions. The results of this study may provide the basis for the develo pment of a novel and safe therapeutic strategy for the very aggressive <Del ta>EGFR-expressing GBM.