Current and emerging therapies of HER2-positive metastatic breast cancer.

The HER2 receptor as measured by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) is overexpressed in 15-20% of all breast cancers and traditionally represents adverse biology and a guarded prognosis, particularly in HER2 positive metastatic breast cancer (MBC). Trastuzumab and newer anti-HER2 targeting agents have significantly improved the clinical outcomes of patients with HER2 positive MBC. The development of new techniques has led to discovery of promising biomarkers that can lead to more precise selection of patients for anti-HER2 therapies. This paper summarizes these new biomarkers, useful in selecting patients for treatment with new and emerging therapies for HER2 positive MBC. Emerging next generation sequencing techniques have truly changed the landscape of HER2 positive MBC. Deployment of multiple anti-HER2 therapies in combination is a strategy which has yielded additive or even synergistic effects and has led to markedly improved patient outcomes in HER2+ MBC. In the future, in order to further improve the treatment of these patients and to reduce toxicities, we need to improve our understanding of HER2-dependent pathways and their function, and to develop further treatment combinations while optimizing selection of patients by identifying new biomarkers. The results of prospective studies using CTCs, cDNA and other promising new biomarkers are awaited with great interest.