Study on New Treatments Before Surgery for Locally Advanced Gastroesophageal Cancer

Inclusion Criteria

• Have a confirmed diagnosis of gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma that can be surgically removed.
• For Sub-study 1, participants must be CLDN18.2-positive and HER2-negative; for Sub-study 2, participants must be HER2-positive; no specific requirements for Sub-study 3.
• Be in good overall health with an ECOG performance status of 0 or 1, meaning fully active or restricted in physically strenuous activity but ambulatory.
• Have adequate organ and bone marrow function as determined by specific medical tests.
• Weigh more than 35 kg (about 77 pounds).

Exclusion Criteria

• Have received any prior treatment or surgery for the current gastric, GEJ, or esophageal cancer.
• Have active or previously documented autoimmune or inflammatory disorders requiring treatment with steroids or other immunosuppressive drugs.
• Have any central nervous system (CNS) disorders.
• Have uncontrolled infections.
• Have a history of lung diseases like interstitial lung disease (ILD) or pneumonitis, or currently have or are suspected to have these conditions.
• Have a history of another primary cancer.
• Have any known or suspected cancer spread to distant parts of the body.
• Have uncontrolled hepatitis B or chronic or active hepatitis B.
• Have used immunosuppressive medication within 14 days before starting the study treatment.