Understanding the relationship between breast cancer, immune checkpoint inhibitors, and gut microbiota: a narrative review.

Background and Objective: The composition of gut microbiota plays an important role in predicting and influencing outcomes of cancer treated with immunotherapy. Our objective is to summarize the role of gut microbiota and immunotherapy in breast cancer. Methods: A systematic search from inception until July 2024 of key search terms including immunity, breast neoplasm, gastrointestinal microbiome/microbiota, fecal microbiota transplantation, pro- and prebiotics, antibiotics and immunotherapy using EMBASE, MEDLINE and CENTRAL was conducted. The results were screened by two reviewers independently and synthesized and presented descriptively. Key Content and Findings: Thirteen studies (5 clinical, 8 pre-clinical) met the eligibility criteria and were published from 2020-2024. Clinical studies showed that the composition and diversity of gut microbiota was associated with patient response to immunotherapy. In pre-clinical studies, dysbiotic states induced by obesity, antibiotics, and diet were associated with immunosuppression and influenced response to programmed cell death-ligand 1 (PD-L1) inhibitors. Microbiota-modulating treatments such as probiotics showed the ability to enhance response to immunotherapy, indicating their potential use as adjunct therapies in breast cancer treatment. Conclusions: The composition of gut microbiota could help predict the chance of response to immunotherapy, and modulating gut microbiota has the potential to enhance the efficacy of chemo-immunotherapy in breast cancer. However, the available data relating to breast cancer are limited. Larger prospective studies are required to further elucidate their role as a biomarker and treatment.