Modulation of DNA repair in vitro after treatment with chemotherapeutic agents by the epidermal growth factor receptor inhibitor gefitinib (ZD1839).

PURPOSE The epidermal growth factor receptor (EGFR) is commonly expressed in human tumors and provides a target for therapy. Gefitinib (Iressa, ZD1839) is a quinazoline derivative that inhibits EGFR tyrosine kinase activity. Gefitinib demonstrated anticancer efficacy in vivo, and although experiments in vitro have suggested that inhibition of EGFR modulates the activity of chemotherapeutic agents, the mechanism of this interaction is unclear. We investigated mechanisms for this modulation. EXPERIMENTAL DESIGN The antiproliferative effect of gefitinib alone or combined with cisplatin, melphalan, and etoposide was determined in a human breast (MCF-7) cancer cell line. Using the alkaline single-cell gel electrophoresis (comet) assay, we investigated kinetics of DNA damage and repair after treatment with the chemotherapeutic drugs combined with gefitinib. To investigate whether the phosphatidylinositol 3'-kinase pathway was contributing to repair-inhibition produced by gefitinib, cells were exposed to chemotherapy in combination with the phosphatidylinositol 3'-kinase inhibitor LY294002. RESULTS A superadditive (synergistic) increase in growth inhibition for combined treatment with gefitinib was found for cisplatin and etoposide, but not with melphalan. There was delayed repair of DNA strand breaks after treatment with etoposide combined with gefitinib, and repair of DNA interstrand cross-links produced by cisplatin is delayed in combination with gefitinib. Inhibition of cell proliferation and DNA repair was identical in cells treated with LY294002. Immunoprecipitation of cell extracts demonstrated that after exposure to gefitinib, there was an association between EGFR and DNA-PK(CS). CONCLUSION Gefitinib acts through inhibition of repair of cisplatin and etoposide-induced DNA damage; this effect is mimicked by inhibitors of the phosphatidylinositol 3'-kinase suggesting similar mechanisms of action.