We investigated the role of the MEK/MAPK pathway in the sensitivity/resistance of breast carcinoma cells to the EGFR tyrosine kinase inhibitor gefitinib (IRESSA). We assessed the effects of gefitinib on the growth of three breast cancer cell lines that showed high (SK-Br-3; IC50 4 microM), intermediate (MDA-MB-361; IC50 5.3 microM), and low (MDA-MB-468; IC50 6.8 microM) 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 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 resistance to gefitinib, and provide the rationale for novel therapeutic approaches based on combinations of signal transduction inhibitors.