pemetrexed on EGFR-TKI-sensitive and EGFR-TKI-resistant human non-small cell lung cancer (NSCLC) cell lines. The antiproliferative effects of gefitinib and pemetrexed, alone and in combination, on the growth of NSCLC cell lines, were assessed using an MTT assay. The cytotoxic interaction

Currently, chemotherapy and targeted therapies share the principal limitation of the emergence of drug resistance, which prevents these strategies from having lasting clinical benefits. The combination of epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) with concurrent chemotherapy has been proposed as one strategy to overcome acquired resistance to EGFR-TKIs. The purpose of the present study was to investigate the combined effects of gefitinib and pemetrexed on EGFR-TKI-sensitive and EGFR-TKI-resistant human non-small cell lung cancer (NSCLC) cell lines. The antiproliferative effects of gefitinib and pemetrexed, alone and in combination, on the growth of NSCLC cell lines, were assessed using an MTT assay. The cytotoxic interaction between the two drugs was evaluated in vitro using the combination index (CI) method. Cell cycle distribution and apoptosis were analyzed by flow cytometry and alterations in signaling pathways were determined by western blot analysis. In the present study, it was identified that when cells were concurrently exposed to pemetrexed and gefitinib, cytotoxic synergism was present in the gefitinib-resistant PC9/GR human NSCLC cell line and antagonistic interactions were observed in the gefitinib-sensitive PC9 cell line. Synergism was associated with a combination of cell cycle effects of the different agents. In addition, the combination of pemetrexed and gefitinib decreased the levels of phosphorylated AKT, phosphorylated extracellular-signal-regulated kinase and B-cell lymphoma 2 as compared with those in the control. By contrast, antagonism was associated with gefitinib-induced G0/G1-phase blockade of gefitinib-sensitive cells, which interfered with the cell cycle-specific cytotoxicity of chemotherapy. The combination of pemetrexed and gefitinib generated synergistic effects in gefitinib-acquired resistant cells and antagonistic effects in gefitinib-sensitive cells, suggesting that EGFR-TKIs combined with pemetrexed may be a beneficial treatment strategy for NSCLC patients with acquired resistance to EGFR-TKIs.