424. EFFECT OF GUT MICROBIOTA ON THE RESPONSE TO NEOADJUVANT IMMUNOTHERAPY COMBINED WITH CHEMOTHERAPY IN PATIENTS WITH ESOPHAGEAL SQUAMOUS CELL CARCINOMA

Background To explore the impact of gut microbiota diversity on the efficacy of immunotherapy combined with chemotherapy in esophageal squamous cell carcinoma (ESCC) patients, aiming to identify microbial markers for predicting treatment outcomes. Methods In this study, we prospectively enrolled patients diagnosed with locally advanced esophageal squamous cell carcinoma who were scheduled to undergo neoadjuvant immunotherapy combined with chemotherapy; stool specimens were collected before the first cycle, the second cycle of immunotherapy, and at the time of preoperative disease evaluation. To examine the gut microbiota's composition and abundance, we employed the 16S rDNA sequencing technique. The sequencing data were preprocessed with FLASH and Uparse software, and species annotation was performed using the SSUrRNA database. Microbiota analysis was conducted using the R language, and group differences were assessed using appropriate statistical methods. Results In this study, 43 patients (totally 122 stool samples) with neoadjuvant immunotherapy combined with chemotherapy for esophageal squamous cell carcinoma were enrolled from December 2020 to January 2022. 16S rDNA sequencing results suggested that patients in the sensitive group had higher total fecal microbiota diversity than those in the drug-resistant group, and a notable variation in the structure of the microbiota was detected between the two groups. During treatment, the results did not show any significant statistical difference in the number of species (alpha diversity) in the sensitive group compared to the resistant group, although there was a trend towards a higher number of species in the pre-treatment sensitive group. As the treatment progressed, starting from the second cycle of treatment, the sensitive group had a higher alpha diversity than the resistant group. And we found that Bray-Curtis distance assessment of beta diversity showed that the difference between the sensitive and resistant groups also became more pronounced as treatment progressed. At the phylum level, the gut flora composition of the sensitive group remained relatively stable as treatment progressed, while the microbial diversity of the resistant group appeared to decrease. At the genus level, Bifidobacterium, Faecalibacterium, Akkermansia and Ruminococcus were more abundant in the immunotherapy-sensitive group; whereas Ligilactobacillus and Escherichia-Shigella was more rich in the immunotherapy-resistant group. The prediction model based on the baseline differences of gut microbiota in the two groups using a random forest method can be applied to predict the efficacy of immunotherapy for esophageal squamous cell carcinoma. Conclusion There are differences in species diversity and microbiota structure between the sensitive and resistant groups of esophageal squamous cell carcinoma patients receiving immunotherapy combined with chemotherapy, and the predictive model, which is based on the microbiota differences between the two groups before treatment, could be clinically relevant in predicting the efficacy of immunotherapy combined with chemotherapy for esophageal squamous cell carcinoma.