The Gut Microbiome as a Biomarker and Therapeutic Target of Immune Checkpoint Inhibitors: A Review for Oncologists.

Immune checkpoint inhibitors (ICIs) have transformed cancer therapy, yet their benefits remain limited to a subset of patients, underscoring the need for more reliable biomarkers and novel therapeutic strategies. The gut microbiome has emerged as a critical modulator of systemic immunity and a promising determinant of ICI response. Evidence links specific microbial features, taxa, and bioactive metabolites to enhanced antitumor immunity, whereas disruptions, such as antibiotic exposure, are associated with poorer outcomes. Advances in sequencing and multi-omics technologies have provided more profound insights into microbiome-immune crosstalk, though methodological heterogeneity continues to challenge reproducibility. Translational studies demonstrate that microbiome-based intervention, including fecal microbiota transplantation (FMT), biotics supplementation, and engineered microbial strains, can enhance ICI efficacy or mitigate immune-related toxicities. Despite encouraging early clinical signals, broader implementation requires methodological rigor, standardized protocols, and innovative trial designs that account for host and environmental factors. For clinicians, the most immediate strategies involve prudent antibiotic stewardship and patient enrollment in microbiome-focused clinical trials. Overall, the gut microbiome is a promising biomarker and a therapeutic target, representing a new frontier for personalizing immunotherapy and improving patient outcomes in oncology.