Treatment schedule is of importance when gefitinib is combined with irradiation of glioma and endothelial cells in vitro.

Amplified epidermal growth factor receptor (EGFR) signaling is supposed to contribute to clinical radiation resistance of glioblastoma multiforme (GBM). Therefore, inhibition of EGFR signaling pathways by the selective EGFR tyrosine kinase inhibitor, gefitinib (ZD1839, Iressa), may increase the therapeutic effects of radiotherapy. The effects of different schedules for administration of gefitinib on sensitivity to irradiation of the human glioma cell lines (251MG and SF-767), a rat glioma cell line (BT4C), and an immortalized rat brain endothelial cell line (RBE4) is reported. Differences in effects of the combined treatment on cell toxicity were determined by a fluorometric cytotoxicity assay, and nuclear DNA fragmentation was used for quantification of apoptosis. Pre-administration with gefitinib for 30 min prior to irradiation followed by continuous incubation with gefitinib significantly increased the cytotoxicity of SF-767, BT4C, and RBE4 cells. However, the human glioma cell line 251MG was protected against radiation-induced damage by this treatment schedule, at lower concentrations of gefitinib. Pre-administration with gefitinib for 24 h prior to irradiation without following incubation with gefitinib increased the cytotoxicity of SF-767 and BT4C cells. Post-irradiation treatment with gefitinib significantly increased the cytotoxicity in all cell lines except for 251MG. We demonstrated heterogeneity in the cytotoxic effects of gefitinib between cell lines. Response to gefitinib might be due to other mechanisms than through the EGF receptor as some of the cell lines showed sensitivity to gefitinib despite no or low expression of EGFR. This study also demonstrates the importance of timing of gefitinib administration when this agent is combined with irradiation.