Additional synbiotics to combination therapy with immune checkpoint inhibitors for unresectable or recurrent gastric and esophageal cancer

INTERVENTION: LBG‐P or placebo is administered orally 3 times daily after each meal, starting at least 5 days before the start of chemotherapy and continuing for 6 months after the start of chemotherapy or until the end of first‐line therapy, whichever is shorter. CONDITION: Gastric cancer , esophageal cancer PRIMARY OUTCOME: Progression free survaival SECONDARY OUTCOME: (1) Progression free survival for gastric cancer; (2) Progression free survival for esophageal cancer; (3) Objective resoponce rate of 1st line chemotherapy; (4) Adverse events during 1st line chemotherapy; (5) Duration of response; (6) Time to treatment failure; (7) Relative Dose Intensity; (8) Intestinal environmental index; (9) iPFS; (10) Overall survival; (11) Best response according to iRECIST INCLUSION CRITERIA: (1) Gastric cancer histologically diagnosed as adenocarcinoma (papillary adenocarcinoma, tubular adenocarcinoma, poorly differentiated adenocarcinoma), signet ring cell carcinoma, mucinous carcinoma, or hepatoid adenocarcinoma. Esophageal cancer histologically diagnosed as squamous cell carcinoma, adenosquamous cell carcinoma, adenocarcinoma, or basaloid carcinoma. (2) Subjects with unresectable advanced gastric cancer/esophageal cancer or recurrent gastric cancer/esophageal cancer who undergo ICI+chemotherapy or ICI combination therapy, which is the first‐line treatment, as usual clinical practice. [1] In the case of unresectable advanced gastric cancer, those who satisfy both i) and ii) below. i) Diagnosed as cStage IVB or pStage IV based on clinical or surgical findings. However, it does not include the case of stage IV only by peritoneal washing cytology (CY1). Also, it does not include localized peritoneal dissemination or cases in which macroscopic