ERBB2 and breast cancer: The two agents showed consistent strong synergistic interactions against HER‐2‐overexpressing established human breast cancer cell lines, with drug concentrations used between 0.039 and 5.0 Amol/L for lapatinib and 0.31 and 4.0 Ag/mL for trastuzumab.5 Another mechanism was found that Lapatinib‐induced accumulation of HER2 and trastuzumab‐mediated downregulation of HER2 triggered enhanced immune‐mediated trastuzumab‐dependent cytotoxicity in SKBR3 and MCF7‐HER2 cells.6