Defective Cyclin B1 Induction in Trastuzumab-emtansine (T-DM1) Acquired Resistance in HER2-positive Breast Cancer

Abstract

<b>Purpose:</b> Trastuzumab-emtansine (T-DM1) is a standard treatment in advanced HER2-positive breast cancer. However, resistance inevitably occurs. We aimed to identify mechanisms of acquired T-DM1 resistance.<b>Experimental Design:</b> HER2-positive breast cancer cells (HCC1954, HCC1419, SKBR3, and BT474) were treated in a pulse-fashion with T-DM1 to induce a resistant phenotype. Cellular and molecular effects of T-DM1 in parental versus resistant cells were compared. CDK1 kinase activity and cyclin B1 expression were assayed under various conditions. Genetic modifications to up- or downregulate <i>cyclin B1</i> were conducted. Effects of T-DM1 on cyclin B1 levels, proliferation, and apoptosis were assayed in human <i>HER2</i>-positive breast cancer explants.<b>Results:</b> We obtained three cell lines with different levels of acquired T-DM1 resistance (HCC1954/TDR, HCC1419/TDR, and SKBR3/TDR cells). HER2 remained amplified in the resistant cells. Binding to HER2 and intracellular uptake of T-DM1 were maintained in resistant cells. T-DM1 induced cyclin B1 accumulation in sensitive but not resistant cells. <i>Cyclin B1</i> knockdown by siRNA in parental cells induced T-DM1 resistance, while increased levels of cyclin B1 by silencing <i>cdc20</i> partially sensitized resistant cells. In a series of 18 HER2-positive breast cancer fresh explants, T-DM1 effects on proliferation and apoptosis paralleled cyclin B1 accumulation.<b>Conclusions:</b> Defective cyclin B1 induction by T-DM1 mediates acquired resistance in HER2-positive breast cancer cells. These results support the testing of cyclin B1 induction upon T-DM1 treatment as a pharmacodynamic predictor in HER2-positive breast cancer. <i>Clin Cancer Res; 23(22); 7006-19. ©2017 AACR</i>.