Abstract 6060: Can the gut microbiota both radiosensitize tumors and spare normal tissue toxicity
Authors: Then Chee Kin , Paillas Salome , Wang Xuedan , Misheva Mariya , McCullagh James , Foster Kevin R. , Kiltie Anne E. 
Published: June 2022

Abstract
Background: New non-toxic radiosensitizers are needed in the treatment of muscle-invasive bladder cancer because elderly patients are vulnerable to chemotherapy-related toxicity of currently available radiosensitizers. Our previous study showed that high fiber diets sensitized RT112 xenografts to ionizing radiation (IR) by modifying the gut microbiome and this phenotype was positively correlated with B.acidifaciens (BA) abundance. It is noted that gut microbiota can enhance anti-tumoral responses, so we hypothesise that gut microbiota including BA may radiosensitize tumors via secretion of metabolites and immunomodulation.
Methods: UPPL1591 mouse bladder cancer cell line flank xenografts were implanted into C57BL/6 mice (an immune-competent setting) at the same time as starting low (0.2% cellulose) and high fiber (5% psyllium, 5% psyllium plus 10% resistant starch/10% inulin) diets for their lifetime (N=30). 16S rRNA sequencing, IC-MS, IHC and the Nanostring platform were used for gut microbiota, untargeted metabolomics, and local tumor immunity analyses. For normal tissue toxicity studies, mice received 10-14 Gy IR (N=84). We used an intestinal crypt assay and FACS analysis of spleen to assess acute normal effects in small intestine and systemic immunity. Cell viability, clonogenic assay and Western blotting were used to validate the cytotoxic and radiosensitizing effects of bacterial supernatants on RT112 human bladder cancer cells (N=3).
Results: Psyllium plus either resistant starch (RS, p=0.007) or inulin (p&lt;0.001) significantly decreased tumors size. Psyllium plus inulin raised inosine-producing bacteria (p&lt;0.001) and faecal inosine levels (p&lt;0.001) which has been shown to enhance immunotherapy efficacy. Increased systemic (p=0.189) and local (p=0.024) anti-tumoral T cell responses were observed in the psyllium plus inulin group. Furthermore, the two diets mitigated the radiation injury from 14 Gy in intestinal crypt assays (p=0.011) and increased systemic immunity, with higher populations of B (p=0.024), cytotoxic T (p=0.019) and helper T (p&lt;0.001) cells. Psyllium plus RS or inulin significantly increased caecal size (p&lt;0.001), implying higher fermentation levels. We previously found inulin can enhance the relative abundance of BA. The supernatants of co-cultures of BA and F.praunitzii (FP, butyrate-producer) conferred greater cytotoxity than BA alone (p&lt;0.001) or co-cultures of Bifidobacterium and FP (p&lt;0.001), and also increased histone acetylation levels. Bacterial supernatants of BA increased DNA damage with 5Gy IR (p&lt;0.05) and radiosensitivity of bladder tumour cells (p=0.008 at 8Gy).
Conclusions: B.acidifaciens is a potential radiosensitiser with enhanced efficacy in combination with butyrate-producing bacteria. Our in vivo experiments suggest a role for inosine and cytotoxic T cells generated by the gut microbiota in radiosensitisation of bladder tumors.
Citation Format: Chee Kin Then, Salome Paillas, Xuedan Wang, Mariya Misheva, James McCullagh, Kevin R. Foster, Anne E. Kiltie. Can the gut microbiota both radiosensitize tumors and spare normal tissue toxicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6060.