Harnessing the gut microbiota in extra-intestinal cancers: from causal evidence to immunotherapy strategies

The gut microbiota (GM) has emerged as a key modulator of cancer development and therapeutic response beyond the gastrointestinal tract. In extra-intestinal cancers, GM composition influences oncogenesis, with specific microbial taxa and their metabolites linked to either increased or decreased cancer risk, as highlighted by Mendelian Randomization studies. Beyond cancer initiation, GM plays a critical role in shaping the efficacy and toxicity of anticancer therapies, particularly immunotherapy. We searched PubMed and ClinicalTrials.gov using the terms"gut microbiota," "immune checkpoint inhibitors," "faecal microbiota transplantation," "solid tumor" in oncology patients. Evidence indicates that SCFA-producing bacteria, Akkermansia muciniphila, and members of Lachnospiraceae and Ruminococcaceae families enhance responses to immune checkpoint inhibitors (ICIs), whereas dysbiosis and immunosuppressive bacteria are associated with poor outcomes and immune-related adverse events. Therapeutic modulation of the GM through probiotics, prebiotics, fecal microbiota transplantation, and dietary interventions shows promise in optimizing immunotherapy efficacy, yet standardized clinical protocols remain lacking. Integrating GM profiling with multi-omics and artificial intelligence approaches offers a path toward personalized microbiota-targeted interventions to improve patient outcomes. This review critically summarizes current evidence linking GM to cancer immunotherapy, discusses mechanistic insights, and outlines future perspectives for translating microbiota modulation into clinical practice.