EVALUATING HUMAN MICROBIOME SIGNATURES ASSOCIATED WITH DEVELOPMENT, OUTCOME, AND TREATMENT RESPONSE IN GLIOMA AND BRAIN METASTASIS: A SCOPING REVIEW

The human microbiome is made up of over 500 species of bacteria in the body. Microbiome dysbiosis has been implicated in cancer development and treatment response, including in both primary brain tumors and brain metastases, by acting through the gut-brain axis. We conducted a scoping review of the current evidence surrounding the human microbiome and brain tumors. METHODS: A systematic search of relevant studies and abstracts from 5 electronic databases from 1946-2023 was conducted based on a search strategy developed by an information specialist. Eligible studies included human, in vivo, or in vitro studies that focused on the relationship between microbiome and glioma and brain metastasis development, response to therapies, and outcomes. RESULTS: Of 383 citations, 31 studies met inclusion criteria, including 15 articles and 16 conference abstracts. There were 19 human studies and 18 mouse studies, of which 24 studies focused on glioma and 7 studies focused on brain metastases. They either characterized the microbiome in patients with brain tumors (n=19) or its correlation to systemic therapies for glioma (n=12). Changes in the Firmicutes/Bacteroidetes ratio, which is a marker of dysbiosis in patients with brain tumors was observed in 7 studies; however, the direction of change varied. Microbiome dysbiosis seemed to occur in response to both immunotherapy (5 studies) and temozolomide (3 studies), with variable impacts on treatment response. CONCLUSION: Evidence surrounding the impact of the gut-brain axis on brain tumors remains in its infancy with a large amount of heterogeneity.