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

Inclusion Criteria:

• 18 years or older at the time of signing the ICF.

• Body weight > 35 kg.

• Previously untreated for unresectable or metastatic gastric or gastroesophagealjunction 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 tumourcollected within past 24 months or from a fresh biopsy when Substudy 3, Substudy 4 is open for recruitment.

Exclusion Criteria:

• Participants with HER2-positive (3+ by IHC, or 2+ by IHC and positive by insituhybridisation) or indeterminate gastric or GEJ carcinoma.

• Untreated or progressive CNS metastatic disease, any leptomeningeal disease, or cordcompression.

• Participants with ascites which cannot be controlled with appropriate interventions.

• Active infectious diseases, including tuberculosis, HIV infection, or hepatitis B/C.

• Uncontrolled intercurrent illness.

• Active or prior documented autoimmune or inflammatory disorders requiring systemictreatment 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 MMAEexposure (when Substudy 3, Substudy 4 is open for recruitment).