Metastatic triple negative breast cancer: Optimizing treatment options, new and emerging targeted therapies

Triple negative breast cancer (TNBC) is a heterogeneous disease, not only on the molecular level, but also on the pathologic and clinical levels. It also has a distinct epidemiology. TNBCs are frequently of high histologic grade, typically more aggressive and difficult to treat than hormone receptor-positive tumors, and they are associated with a higher risk of early relapse with visceral metastasis after surgery, chemotherapy and/or radiotherapy. The lack of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2 expression precludes the use of targeted therapies in advanced stages, and the only approved systemic treatment option is chemotherapy with or without bevacizumab. In patients with advanced TNBC, responses to chemotherapy occur, but are often of short duration and it is associated with poor prognosis. The median overall survival for patients with metastatic TNBC is about 9-12 months with conventional cytotoxic agents. Given the suboptimal outcomes with chemotherapy, new targeted therapies for TNBC are urgently needed. This review summarizes the clinical efficacy, perspectives and future challenges of using new treatment options for metastatic TNBC, such as poly-ADP-ribose-polymerase inhibitors, antiandrogen therapies and immune checkpoint inhibitors (antiprogrammed death receptor-1/PD-L1 monoclonal antibodies).