Gut Microbiome and Breast Cancer in the Era of Cancer Immunotherapy

Purpose of Review Immune checkpoint inhibitors (ICIs) are an emerging therapy in breast cancer, but not all patients will have benefit with these medications. It has been proposed that certain gut microbes may play a role in protecting the host against inappropriate inflammation and modulating the immune response. Here, we review the current evidence on the association of the gut microbiome, antitumor immunity, and response to immunotherapy and discuss open questions, ongoing trials, and future directions for modulating the gut microbiome as part of breast cancer treatment. Recent Findings Several groups have showed that the composition of gut microbiota modulates responses to ICI in preclinical cancer models, and the composition of gut microbiota can predict which patients with solid tumors are more likely to respond to ICI. In addition, it was also showed that fecal microbiota transplant was able to make non-responder animals into responders when they received feces from patients who had benefited to ICI. Recent studies suggest that ICIs can be active in breast cancer but identifying the patients who are most likely to benefit remains a challenge. In other tumor types, the gut microbiome differs between responders and non-responders, suggesting that it can be used as a predictive biomarker of response. In addition, future investigations will determine whether manipulating the gut microbiota can improve responses to ICIs in breast cancer.