OSU-03012 and lapatinib act to kill ER-positive and ER-negative breast cancer cells in combination. A, D-E: ER-positive BT474 and ER negative MDA-MB-231 cell lines were treated with the indicated concentrations of OSU-03012 and lapatinib (A) or 2 μM lapatinib OSU-03012 and 2 μM lapatinib (D-E) for 48 h. Cells were then singly plated onto 6-well plates to assay for clonogenic capacity. B-C: BT474 and MDA-MB-231 cells were treated with 2 μM OSU-03012 (OSU) or 2 μM lapatinib (Lap) or the combination, incubated for 48 h, then assayed using Annexin V/PI for cell death. All measurements are ± SEM. * indicates a p < 0.05 when compared to the vehicle-treated condition.