Mechanisms of acquired afatinib resistance in HER2-positive breast cancer cells.

620 Background: Afatinib is a potent irreversible ErbB family tyrosine kinase inhibitor. We have previously shown that afatinib has anti-proliferative activity in a panel of 8 HER2 positive breast cancer cell lines (IC50values 5-80 nM). The aim of this study was to investigate potential mechanisms by which HER2 positive breast cancer cells may develop acquired resistance to afatinib. Methods: SKBR3 cells were continuously exposed to 150 nM afatinib for 6 months. Proliferation assays were performed to determine the IC50values for the parental (SKBR3-P) and the afatinib conditioned cells (SKBR3-A), and to assess sensitivity to lapatinib and neratinib. The molecular profiles of the resistant cells were examined by Reverse Phase Protein Array (RPPA). Results: The SKBR3-A cells were more resistant to afatinib than the SKBR3-P cells (IC50 > 500 nM vs IC50 = 10.9 ± 3.4 nM). Furthermore, the resistant cells were cross-resistant to lapatinib (IC50 = 1.0 µM ± 44.0 nM vs IC50 = 25.9 ± 3.0 nM)and neratinib (IC50 = 28...