Abstract P3-14-14: Activation of SRC and MAPK Mediates Resistance to Lapatinib in ER-/ERBB2+ Breast Cancer

Abstract Background Resistance to lapatinib represents a significant problem in the treatment of ERBB2+ breast cancer, especially after failure of prior trastuzumab therapy. A number of resistance mechanisms have been identified in recent years that limit the anti-tumor effects of lapatinib. These mechanisms include the activation of ERα signaling, and sustained activation of AKT either through coexpression of receptor tyrosine kinases (IGF-IR, AXL, MET, ERBB3), loss of PTEN or activating mutations in PIK3CA. However, these mechanisms have been shown to confer resistance primarily in ER+/ERBB2+ breast cancer cells; the mechanisms of resistance to lapatinib in ER-/ERBB2+ breast cancer (∼ 50% of ERBB2+ breast cancers) remain poorly understood. We hypothesized that resistance to lapatinib in ER-/ERBB2+ breast cancer cells can be mediated by activation of alternative signaling pathways which bypass the PI3K/AKT signaling axis. Materials and Methods To investigate the mechanisms of acquired resistance to lapatinib we generated ER-/ERBB2+ and ER+/ERBB2+ breast cancer cells resistant to lapatinib at the clinically relevant concentration of 2.6 uM (SK-lapR and BT-lapR). Resistance to lapatinib was achieved by exposure of SKBR3 and BT474 cells to increasing concentrations of the drug, and resistant cells were selected through serial passage. The activation status of multiple signaling pathways in lapatinib sensitive and resistant cells was determined by western blot analysis, and the role of individual signaling pathways in promoting resistance to lapatinib was investigated in cell proliferation assays using selective small molecule inhibitors. Results and Discussion Consistent with previous studies, we show that inhibition of ERα signaling using 100 nM fulvestrant in combination with lapatinib restored lapatinib sensitivity and effectively leads to growth arrest and cell death in ER+/ERBB2+ BT-lapR cells (P<0.001). However, none of the previously proposed mechanisms of resistance to lapatinib were apparent in our SK-lapR cells (ER-/ERBB2+). Strikingly, these cells which are stably resistant to lapatinib show no activation of AKT even in the absence of lapatinib. Rather, they exhibit increased phosphorylation of SRC kinase (3.9 fold, analyzed with ImageJ) and no inhibition of ERK1/2 after treatment with lapatinib compared to sensitive SKBR3 cells. Inhibition of SRC activity with 1uM AZD0530 in combination with lapatinib significantly reduced growth of SK-lapR cells compared to treatment with lapatinib alone (P<0.05). These data reveal potential novel mechanisms for lapatinib resistance in ER-/ERBB2+ breast cancer, and may indicate therapeutic strategies to prevent or overcome lapatinib resistance. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P3-14-14.