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Clinical studies of the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) gefitinib (IRESSA) in breast cancer resulted in few clinical responses and in a disease control rate of approximately 10%. This observation indicates that intrinsic resistance to gefitinib, and more generally to anti-EGFR agents, is a common phenomenon in breast cancer. We investigated the role of the MEK/MAPK pathway in the sensitivity/resistance of human breast carcinoma cells to the EGFR TKI gefitinib. For this purpose, we assessed the effects of gefitinib on growth, survival and activation of signal transduction pathways in three breast cancer cell lines with relatively high (SK-Br-3; IC50 4 μM), intermediate (MDA-MB-361; IC50 5.3 μM) and low (MDA-MB-468; IC50 6.8 μM) sensitivity to the drug. Although treatment with gefitinib inhibited EGFR activation in the three cell lines in a similar fashion, significant reduction of both p42/p44-MAPK and AKT phosphorylation was observed in SK-Br-3 and MDA-MB-361, but not in MDA-MB-468 cells. The latter cell line carries a mutated PTEN tumor suppressor gene and has constitutively high levels of activation of the PI3K/AKT pathway. However, the growth of MDA-MB-468 cells was significantly inhibited by treatment with either the PI3K-inhibitor LY294002 or the MEK-inhibitor PD98059. In agreement with these findings, treatment of MDA-MB-468 cells with a combination of PD98059 and gefitinib produced a synergistic anti-tumor effect, whereas this combination was only additive in SK-Br-3 and MDA-MB-361 cells. The combination of gefitinib and PD98059 also produced a significant increase in the levels of apoptosis in MDA-MB-468 cells as compared with treatment with a single agent. This phenomenon was associated with a profound decrease in MAPK activation, reduction of BAD(ser112) phosphorylation and a paradoxical increase in the levels of AKT activation. Finally, overexpression of a constitutively activated form of p42-MAPK in MCF-10A non-transformed human mammary epithelial cells resulted in a two- to three-fold increase in the IC50 to gefitinib. Taken together, these data strongly support the role of the MEK/MAPK pathway in the intrinsic resistance of breast cancer cells to gefitinib, and provide the rationale for novel therapeutic approaches based on combinations of signal transduction inhibitors.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]