Current Landscape of Immunotherapy in Breast Cancer: A Review

Importance: There is tremendous interest in using immunotherapy to treat breast cancer, as evidenced by the more than 290 clinical trials ongoing at the time of this narrative review. The objective of this review is to describe the current status of immunotherapy in breast cancer, highlighting its potential in both early-stage and metastatic disease. Observations: After searching ClinicalTrials.gov on April 24, 2018, and PubMed up to June 30, 2018, to identify breast cancer immunotherapy trials, we found that immune checkpoint blockade (ICB) is the most investigated form of immunotherapy in breast cancer. Use of ICB as monotherapy has achieved objective responses in patients with breast cancer, with higher rates seen when administered in earlier lines of therapy. For responding patients, those responses are durable. More recent data suggest clinical efficacy when ICB is given in combination with chemotherapy. Ongoing studies are evaluating combination strategies pairing ICB with additional chemotherapeutic agents, targeted therapy, vaccines, and local ablative therapies to enhance response. To date, robust predictive biomarkers for response to ICB have not been established. Conclusions and Relevance: It is anticipated that combination therapy strategies will be the way forward for immunotherapy in breast cancer, with an improved understanding of tumor, microenvironment, and host factors informing treatment combination decisions. Thoughtful study design incorporating appropriate end points and correlative studies will be critical in identifying optimal strategies for enhancing the immune response against breast tumors.