Immunotherapy for metastatic breast cancer.

The development of immune-check inhibitors has resulted in a paradigm shift for immune oncology therapeutics in the past few decades. Blocking antibodies against programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) leads to robust local tumor control and a durable response in patients with various tumors that are refractory to standard treatments. Breast cancer was traditionally thought to be poorly immunogenic and yield a relatively lower mutation burden compared to 'inflamed' tumors, including melanoma and non-small cell lung carcinoma. Accumulated results have demonstrated that higher T lymphocyte infiltration was observed in triple-negative breast cancer (TNBC) and human epidermal growth factor receptor 2 (HER2) breast cancer compared to estrogen receptor-positive, HER2-negative luminal breast cancer. Among molecular subtypes, single agent cancer immunotherapy showed the most promising results in TNBC. The development of immuno-oncology combinations is required to increase the clinical benefit of immunotherapy against breast cancer. Response to the immunotherapy depends on the dynamic interaction between tumor and immune cells in the tumor microenvironment. In the era of precise medicine, exploration of actual immune response and development of biomarkers is required to maximize the clinical benefit of cancer immunotherapy.