Background: Colorectal cancer is the second most frequent malignant disease in Europe. Thanks to advances in therapeutic options, rectal cancer detected at an early stage is now highly treatable, with combined modalities (including systemic therapy, radiotherapy, and surgery). Neoadjuvant chemoradiotherapy (nCRT) followed by surgery has become the standard treatment for locally advanced rectal cancer and significantly improves survival outcomes. However, it is frequently associated with significant adverse effects. Therefore, identifying reliable biomarkers to predict treatment response is highly important. Microbiome changes during nCRT have been proposed as promising biomarkers for predicting the response to treatment and adverse events in rectal cancer.
Objectives: This study aims to characterize microbiota and microbiome changes during neoadjuvant chemoradiation therapy (nCRT) for rectal cancer patients. Despite advancements in radiotherapy planning, pelvic radiation can inadvertently damage normal tissues and disrupt gut microbiota, leading to severe side effects. Among key microbial components, short-chain fatty acid (SCFA)-producing bacteria play a pivotal role in preserving gut homeostasis by maintaining epithelial integrity and producing anti-inflammatory metabolites. This study seeks to identify specific SCFA-producing bacterial taxa critical for gut health and evaluate their potential as biomarkers for predicting treatment response and adverse events.
Methods: Thirteen patients with histologically confirmed rectal adenocarcimona undergoing nCRT per protocol, were included in the study. Fecal samples were collected at three time points: pre-treatment, mid-treatment, and post-treatment. Adverse events were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0, and treatment responses were categorized according to the Tumor Regression Grade (TRG) system. A sex- and age-matched voluntary healthy control group was used for comparison. DNA extraction from 0.25 g fecal samples was performed using the DNeasy PowerSoil Pro Kit. Shotgun metagenomic sequencing was conducted on an Illumina NovaSeq 6000 (150-bp paired-end run, Novogene Bioinformatics Technology). Microbial analysis was carried out using the SqueezeMeta pipeline (v1.6.3), with taxonomic assignment via DIAMOND v2.19 against the GenBank nr database. Antibiotic resistome prediction was performed using the Resistance Gene Identifier (RGI) software with the Comprehensive Antibiotic Resistance Database (CARD).
Results: Among the 13 patients, nine exhibited stable microbiota composition throughout treatment, whereas four showed significant microbial alterations. Five patients experienced mild to severe diarrhea, and four of these cases coincided with notable shifts in gut microbiota composition. Analysis of TRG grading revealed a correlation between SCFA-producing taxa, therapeutic response, and severe adverse events. Patients with pronounced SCFA-producing bacterial shifts exhibited stronger associations with both treatment outcomes and toxicity profiles.
Conclusions: Gut microbiota alterations, particularly changes in SCFA-producing bacterial taxa, are closely linked to therapeutic response and treatmentrelated toxicity in nCRT for rectal cancer. These findings suggest that monitoring SCFA-producing microbiota could aid in developing postbiotic interventions to mitigate adverse events and enhance treatment efficacy. Future research should explore microbiome-targeted strategies to optimize patient outcomes and establish predictive biomarkers for radiation-induced toxicity.