Novel Combinations in Participants With Locally Advanced Unresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Eligibility Criteria

• * 18 years or older at the time of signing the ICF.
• * Body weight \> 35 kg.
• * Previously untreated for unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma.
• * Has measurable target disease assessed by the Investigator based on RECIST 1.1.
• * ECOG PS zero or one.
• * Life expectancy of at least 12 weeks.
• * Adequate organ and bone marrow function.
• * Has central lab confirmed Claudin18.2 status at screening from archival tumour collected within past 24 months or from a fresh biopsy when Substudy 3, Substudy 4 or Substudy 6 is open for recruitment.
• * Participants with HER2-positive (3+ by IHC, or 2+ by IHC and positive by in situhybridisation) or indeterminate gastric or GEJ carcinoma.
• * Untreated or progressive CNS metastatic disease, any leptomeningeal disease, or cord compression.
• * Participants with ascites which cannot be controlled with appropriate interventions.
• * Active infectious diseases, including tuberculosis, HIV infection, or hepatitis A/B/C.
• * Uncontrolled intercurrent illness.
• * Active or prior documented autoimmune or inflammatory disorders requiring systemic treatment with steroids or other immunosuppressive treatment.
• * History of another primary malignancy.
• * Previous treatment with an immune-oncology agent.
• * Previous treatment with any modalities of Claudin18.2 target therapy or MMAE exposure (when Substudy 3, Substudy 4, or Substudy 6 is open for recruitment).