HER2-overexpressing cell lines exhibit distinct responses when treated with potent anti-HER2 therapy. (A) A panel of HER2-overexpressing breast cancer cell lines was treated with lapatinib (1 μM) plus trastuzumab (10 μg/ml) for 48 h and whole-cell extracts were analyzed by immunoblotting with indicated antibodies. (B) Combination therapy (trastuzumab plus lapatinib) growth response in the HER2-overexpressing breast cancer cell line panel. Growth inhibition was determined by methylene blue assay. Shown are the percent inhibitions of cells treated for six days normalized to non-treated cells. The experiment was performed in quadruplicate. Error bars on plots represent +/- standard error (SE). (C) Growth curves of de novo resistant MDA-MB-361 cells treated with different target therapies/regimens for nine days: trastuzumab (T) (10 μg/ml), lapatinib (L) (1 μM), trastuzumab plus lapatinib (L + T), or endocrine therapy, fulvestrant (F) (10-7 M), untreated (C). Cell numbers were quantified by absorbance at 655 nm after staining with methylene blue. Conditions were repeated in quadruplicate. Significance between groups was determined by multiple comparisons using the Sidak method (*P < 0.0001, F versus C, T, L, or L + T).