HER2-positive breast cancer: Current and new therapeutic strategies.

Since the identification of the HER2 receptor amplification as an adverse prognostic factor that defined a special subtype of metastatic breast cancer, there has been a substantial improvement in survival of patients affected with this disease due to the development of anti-HER2 targeted therapies. The approval of trastuzumab and pertuzumab associated to a taxane in first line and subsequent treatment with the antibody-drug conjugate T-DM1 has certainly contributed to achieve these outcomes. The Tyrosine Kinase Inhibitor lapatinib was also approved in the basis of an improvement in progression free survival, becoming another commonly used treatment in combination with capecitabine. Inevitably, despite these therapeutic advances most patients progress on therapy due to primary or acquired resistance or because of an incorrect HER2 positivity assessment. Hence, it is crucial to correctly categorize HER2 amplified tumors and define mechanisms of resistance to design effective new treatment approaches. In addition, identifying biomarkers of response or resistance permits to tailor the therapeutic options for each patient sparing them from unnecessary toxicity as well as improving their outcomes. The aim of this review is to examine new strategies in development to treat HER2-positive metastatic breast cancer referring to the mechanisms of action of new drugs and new combinations including results reported so far.