STAT3 activation confers trastuzumab-emtansine (T-DM1) resistance in HER2-positive breast cancer.

Trastuzumab-emtansine (T-DM1) is an antibody-drug conjugate that has been approved for the treatment of human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer. Despite the remarkable efficacy of T-DM1 in many patients, resistance to this therapeutic has emerged as a significant clinical problem. In the current study, we used BT-474/KR cells, a T-DM1-resistant cell line established from HER2-positive BT-474 breast cancer cells, as a model to investigate mechanisms of T-DM1 resistance and explore effective therapeutic regimens. We show here for the first time that activation of signal transducer and activator of transcription 3 (STAT3) mediated by leukemia inhibitory factor receptor (LIFR) overexpression confers resistance to T-DM1. Moreover, secreted factors induced by activated STAT3 in resistant cells limit the responsiveness of cells that were originally sensitive to T-DM1. Importantly, STAT3 inhibition sensitizes resistant cells to T-DM1, both in vitro and in vivo, suggesting that the combination T-DM1 with STAT3-targeted therapy is a potential treatment for T-DM1-refractory patients.