423P Gut Microbiome Predicts a Response after Preoperative Chemoradiation in Rectal Cancer Patients

There are ongoing investigations to find promising biomarkers for predicting a complete response (CR) after concurrent chemoradiation (CCRT) in rectal cancer. We aimed to find the predictive value in the gut microbiome in terms of response following preoperative CCRT. We collected a total of 45 fecal samples from rectal cancer patients before CCRT. Tumor response following CCRT was assessed according to the AJCC Tumor Regression Grading System. Analysis of LEfSe (Linear discriminant analysis Effect Size) and MetaCyc pathway abundance predictions were performed, to compare composition and metabolic function of microbiome between CR and non-CR patients. We also established a Bayesian network model to identify microbial networks and species that can modulate the response of preoperative CCRT. Seven patients (15.6%) demonstrated a pathologically CR and 38 patients (84.4%) showed non-CR after preoperative CCRT. Between CR and non-CR patients, there was a significant difference in terms of β-diversity (P=0.028), but no difference in α-diversity was found. Bacteroidales (Bacteroidaceae, Rikenellaceae, Bacteroides) were relatively more abundant in non-CR than CR patients. Pathways related to anabolic function predominated in CR patients. According to in silico Bayesian network analysis, intervention of commensal Duodenibacillus massiliensis, a bacteria classified as a member of the Sutterella family in the Proteobacteria phylum, could potentially elevate the CR rate in rectal cancer patients. From the fecal microbiome using samples obtained before preoperative CCRT, differences in microbial community composition and functions were observed between CR and non-CR patients in rectal cancer. In silico analysis identified a specific microbial taxa that could potentially modulate the response following anti-cancer treatment, however, this finding needs to be verified in a prospective setting.