Purpose: Malignant rhabdoid tumor (MRT) is a rare and highly aggressive neoplasm of young children. Current treatments have had only limited success. Epidermal growth factor receptor (EGFR) was found recently to be expressed on MRT cell lines. Gefitinib (trade name Iressa) is an oral and selective EGFR-tyrosine kinase inhibitor and has been demonstrated to be effective in inhibiting the proliferation of cancer cells in vivo as well as in clinical trials. This encouraged us to examine the

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... or effects of gefitinib on MRT cells in vitro and in vivo. Experimental Design: The expression of EGFR in two MRT tumors and two MRT cell lines (MP-MRT-AN and KP-MRT-NS), established from these two tumor tissues, was examined by immunohistochemistry, immunofluorescence, and immunoblot. The effect of gefitinib on EGFR phosphorylation was examined by immunoblot. The effects of gefitinib on cell growth and apoptosis were examined by cell growth assay and terminal deoxynucleotidyl transferasemediated nick end labeling assay. The in vivo effect of gefitinib was assessed in athymic mice that had been xenografted with MRT cells. Results: The expression of EGFR was detected in both tumor tissues and cell lines. Gefitinib inhibited EGFR-phosphorylation (IC 50 < 0.1 mol/L) and in vitro cell growth (IC 50 ‫؍‬ approximately 10 -12 mol/L), and a high concentration of gefitinib (20 mol/L) induced apoptosis in vitro (MP-MRT-AN, 42.9% and KP-MRT-NS, 47.2%). Furthermore, gefitinib at 150 mg/kg had a cytostatic effect on es-tablished MRT xenografts (MP-MRT-AN, P ‫؍‬ 0.039 and 0.0014; and KP-MRT-NS, P ‫؍‬ 0.048 and 0.0086). Conclusions: Our results demonstrate that gefitinib has antitumor effects in MRT cells in vitro and in vivo and, thus, has promise as a novel and therapeutic strategy for MRT.