Gastrointestinal Microbiota and Breast Cancer Chemotherapy Interactions: A Systematic Review.

Breast cancer is the most common type of cancer in women besides basal cell and squamous cell skin cancer. The current systemic therapy guidelines for this heterogeneous disease are mainly based on the molecular subtypes. However, more research is required to improve rates of therapy resistance and prevent side effects. Previous studies have shown that the human gut microbiota may have an important role in carcinogenesis as well as therapy outcomes, but this factor has not yet been integrated into therapy protocols. This systematic review aims to analyze how response rates and side effect profiles of breast cancer systemic therapies may be affected by the gastrointestinal microbiota. A literature search was performed using multiple databases and keywords related to gastrointestinal microbiota, breast cancer, and anticancer drugs. Studies were excluded if they primarily focused on diseases other than breast cancer. Abstracts, reviews, meta-analyses, and animal experiments were also excluded. After screening, nine studies met all selection criteria and included a total of 566 participants. Most studies described the impact of the gut microbiota on therapy response, but a few additionally discussed chemotherapy side effects, probiotics, or antibiotics. In general, diversity and specific microbiota were linked to chemotherapy response as well as prognosis. Microbiota diversity was also predictive of side effects such as neurological symptoms, weight gain, and constipation. The diversity and composition of gastrointestinal microbiota may serve as biomarkers and provide pathways for the optimization of chemotherapy in breast cancer patients.