Urothelial carcinoma (UC) remains a common cancer with significant morbidity and mortality worldwide. Immune checkpoint inhibitors (ICIs) have helped revolutionize the treatment of UC, and there is growing evidence suggesting the crucial role of the gut microbiome in immune system function influences immunotherapy outcomes in this disease. Herein, we review the preclinical basis for how manipulation of the gut microbiome may alter the efficacy of immunotherapy for patients with cancer, highlight interventions optimizing gut microbiome diversity currently in use, review recent and ongoing clinical trials supporting the role of the gut microbiome in improving immunotherapy outcomes, and discuss clinical implications to improve outcomes for UC patients with immunotherapy in the real world. There is growing evidence that suggests that specific gut microbiome compositions significantly modulate the host immune system and response to ICIs. Early studies have shown that certain microbial taxa enhance antitumor immunity by influencing T cell priming, dendritic cell activation, and cytokine production. Fecal microbiota transplantation (FMT), probiotic supplementation, and dietary modulation have emerged as promising methods to alter microbiomes to improve immunotherapy outcomes. Taxa from positive immunotherapy responders across a variety of cancers demonstrate beneficial effects when transplanted into both treatment-naive or prior nonresponders. Increasing evidence suggests that the gut microbiome plays a crucial role in cancer care, particularly when patients are treated with immunotherapy. Future studies are needed to better understand the underlying mechanisms. While some studies are currently underway to explore gut manipulation for patients with UC, more studies are needed to investigate the potential to convert nonresponders into responders through microbiome manipulation.
Reddy et al. (2026) studied this question.