Effect of chemotherapy and weight change on the gut microbiome of breast cancer patients during the first year of treatment

Abstract Objective: The effects of chemotherapy and weight changes on the gut microbiome of breast cancer patients are not well understood. Methods: We conducted a 1-year follow-up study of 33 breast cancer patients and investigated gut microbiome before initiation of chemotherapy and after completion of treatment. We compared alpha diversity and mean taxa abundance at baseline and absolute changes (Δ; final-baseline) in taxa abundance by treatment (16 neoadjuvant- neoADJ, 13 adjuvant- ADJ, 4 no chemotherapy-noC) using Wilcoxon rank sum and negative binomal tests and evalauted whether these changes were affected by weight changes during follow-up. Results: Alpha diversity measures increased in the neoADJ (+16.4% in OTU p =0.03; +51.6% in Chao1 p =0.03; +7.0% in Shannon index P=0.02; +11.0% in PD whole tree p =0.09) but not in the non-neoADJ group (ADJ+noC). The difference in change in Chao1 index between groups was statistically significant ( p NEOADJ vs ADJ+noC =0.04). Wilcoxon p values of 0.03 to 0.003 were observed for five taxa: Bacteroidetes (g _Alistipes) , Firmicutes (g_Clostridium, g_Eubacterium, g_Bilophila) and Preteobacteria g_Haemophilus). In the negative binomial analysis, changes in abundance differed at Bonferroni-adjusted p values ≤0.0007 for four taxa: two Bacteroidetes taxa ( g_Alistipes, f _S247 ) and two Firmicutes taxa (g_Catenibacterium, g_Eubacterium ). The negative binomial results remained largely the same when we adjusted for weight changes. Conclusions and Relevance: This pilot longitudinal study showed changes in alpha diversity measures and abundance of select taxa that appeared to differ by chemotherapy type. Further investigations are needed to confirm these findings and to assess the impact of these microbiome changes on patient outcome.