The potential for trastuzumab emtansine in human epidermal growth factor receptor 2 positive metastatic breast cancer: latest evidence and ongoing studies

The treatment of breast cancer that is driven by amplification and overexpression of human epidermal growth factor receptor 2 (HER2) has been drastically improved by the development of HER2-targeted therapies including trastuzumab and lapatinib. While outcomes for patients diagnosed with HER2-positive breast cancer have been greatly impacted by these therapies, treatment resistance is common and toxicity to standard regimens remains a therapeutic challenge. Trastuzumab emtansine (T-DM1) is a novel antibody drug conjugate that consists of the HER2-targeted monoclonal antibody, trastuzumab, joined via a stable linker to a derivative of maytansine, a highly potent cytotoxic chemotherapy. While other antibody drug conjugates have been developed clinically, this is the first in its class that maintains the antitumor properties of the HER2-targeted antibody, trastuzumab, and also avoids release of the chemotherapy until the molecule is taken up inside the HER2-overexpressing cancer cell. Several phase I studies have shown T-DM1 is safe, tolerable and has activity in trastuzumab and lapatinib-pretreated breast cancer. Moreover, phase II studies are now being reported that confirm its safety and clinical efficacy in both the frontline and heavily pretreated settings. Preliminary data from phase II studies evaluating its use in combination with other cytotoxics have also been reported and several large phase III trials are underway to evaluate its use in the HER2-positive metastatic breast cancer setting. This paper aims to provide a detailed review of the preclinical and clinical evidence relating to the mechanism of action, efficacy and safety of T-DM1 for the treatment of HER2-positive breast cancer.