HER2 overexpression is determined by immunohistochemistry of the biopsy sample, and HER2 amplification is analyzed by fluorescence in situ hybridization. The status of these receptors is critical in the management of breast cancer. Breast cancer cells are evaluated for biomarkers at the time of initial biopsy, including the estrogen receptor status, progesterone receptor status, and human epidermal growth factor receptor 2 (HER2). Keywords: T-DM1, trastuzumab emtansine, Kadcyla ®, breast cancer, HER2īreast cancer is divided into different types based upon pathology and biomarkers. This review article will summarize the evidence from the preclinical studies, summarize evidence from the clinical trials, discuss current clinical trials, discuss current approval of trastuzumab emtansine, and discuss future directions of research. Trastuzumab is used to target this potent chemotherapy agent directly to the HER2-expressing cancer cells. Emtansine is a potent antimicrotubule agent. Trastuzumab emtansine is one of these antibody–drug conjugates and was the first Food and Drug Administration approved drug for a solid tumor. Antibody–drug conjugates are a new class of powerful drugs designed to target high-dose chemotherapy directly to the cancer cells. In the adjuvant setting, women often progress despite therapy that includes trastuzumab. The original studies of trastuzumab showed improved survival in metastatic breast cancer however, resistance often develops. These cancers are referred to as HER2-positive breast cancer. Trastuzumab targets the human epidermal growth factor receptor 2 (HER2) receptor on breast cancer cells that express this tyrosine kinase receptor. University of Minnesota, Minneapolis, MN, USA 2Division of Medical Oncology, The James Cancer Hospital, The Ohio State University Comprehensive Cancer Center, Columbus, OH, USAĪbstract: Trastuzumab is a monoclonal antibody that is used in the treatment of breast cancer. 1Division of Hematology, Oncology, and Transplant.