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Epidermal growth factor receptor (EGFR) is a transmembrane receptor tyrosine kinase overexpressed in non-small cell lung cancer (NSCLC) and many other solid tumors. EGFR is activated by specific ligands and various cell stresses, such as oxidative stress and UV irradiation. The present study investigates the effect of ligand-independent EGFR activation on gefitinib mediated cytotoxicity using the NSCLC cell line, PC-9. The induction of EGFR autophosphorylation by non-cytotoxic levels of hydrogen peroxide (H2O2) and cisplatin (CDDP) is completely inhibited by 100 nM gefitinib. Pretreatment of cells with both H2O2 and CDDP enhances gefitinib cytotoxicity in vitro. The growth inhibitory effect of gefitinib was examined in vivo using the xenografted severe combined immunodeficiency (SCID) mouse model. PC-9 cells were pretreated with / without a low dose of CDDP (1 ƒÊM) for 1 h and injected subcutaneously into the right flank of SCID mice. Following the appearance of measurable tumors mice were treated by subcutaneous injection into the left flank with / without 10 mg / kg gefitinib for 4 days. Pretreatment with CDDP enhanced tumor growth by 20-30% compared to the control. Subsequent treatment with gefitinib resulted in disappearance of the tumor mass by day 10 in the CDDP-pretreated group and by day 16 in the control group. There was no reappearance of tumors in the CDDP-pretreated group. By comparison, tumors reappeared in the non-pretreated group by day 20 in 4 / 5 animals. These results suggest that chemotherapy may enhance tumor growth due to ligand-independent EGFR activation and that combination chemotherapy may result in enhanced sensitivity of tumors to the sequential administration of gefitinib.

Gefitinib Cytotoxicity in Non-small Cell Lung Cancer Cells is Enhanced by Low Dose Cisplatin Due to Ligand-independent EGFR Autophosphorylation