Combined effects of lapatinib and bortezomib in human epidermal receptor 2 (HER2)‐overexpressing breast cancer cells and activity of bortezomib against lapatinib‐resistant breast cancer cells

Lapatinib and bortezomib are highly active against breast cancer cells. Breast cancer patients who initially respond to lapatinib may eventually manifest acquired resistance to this treatment. Thus, the identification of novel agents that may prevent or delay the development of acquired resistance to lapatinib is critical. In the current study, we show that the combination of lapatinib and bortezomib results in a synergistic growth inhibition in human epidermal receptor 2 (HER2)‐overexpressing breast cancer cells and that the combination enhances apoptosis of SK‐BR‐3 cells. Importantly, we found that the combination of lapatinib plus bortezomib more effectively blocked activation of the HER2 pathway in SK‐BR‐3 cells, compared with monotherapy. In addition, we established a model of acquired resistance to lapatinib by chronically challenging SK‐BR‐3 breast cancer cells with increasing concentrations of lapatinib. Here, we showed that bortezomib notably induced apoptosis of lapatinib‐resistant SK‐BR‐3 pools and further inhibited HER2 signaling in the resistant cells. Taken together, the current data indicate a synergistic interaction between lapatinib and bortezomib in HER2‐overexpressing breast cancer cells and provide the rationale for the clinical evaluation of these two noncross‐resistant targeted therapies. The combination of lapatinib and bortezomib may be a potentially novel approach to prevent or delay the onset of acquired resistance to lapatinib in HER2‐overxpressing/estrogen receptor (ER)‐negative breast cancers. (Cancer Sci 2010); 00: 000–000