The Efficacy of PD-1 Monoclonal Antibody Combined With Chemotherapy in Metastatic Gastric Cancer Based on Gut Microbiota

This study is designed as a single-center, open-label, phase II exploratory trial. Patients with advanced or locally advanced gastric cancer who have not received chemotherapy or immunotherapy before are eligible for inclusion. They will receive a first-line two-drug combination chemotherapy regimen (FOLFOX, XELOX, or SOX, determined by the attending physician) in combination with a PD-1 monoclonal antibody (nivolumab, sintilimab, tislelizumab, or pembrolizumab, determined by the attending physician). Fruquintinib can be added on this basis (only for patients enrolled in the HMPL-013-SH-GC103 study). The aim is to evaluate whether the gut microbiota has a predictive role in the efficacy of immunotherapy combined with chemotherapy for metastatic gastric cancer. A total of 100 patients will be enrolled in the study. Gut microbiota will be measured at the following three time points: 1. Within 3 days before the first chemotherapy (designated as time 0) 2. Within 3 days before the third chemotherapy (designated as time 1) 3. After the sixth chemotherapy cycle, or within 1 week after disease progression if it occurs within the sixth cycle (designated as time 2)