AZD0901 Compared With Investigator's Choice of Therapy in Participants With Second- or Later-line Advanced or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma Expressing Claudin18.2

Required: CLDN18 positive expression

Participants with positive CLDN18.2 expression from archival tumor collected within past 24 months or from a fresh biopsy

Required: HER2 (ERBB2) overexpression (IHC 3+ or IHC 2+/ISH+ (HER2:CEP17 ratio ≥ 2 or average HER2 copy number ≥ 6.0 signals/cell))

Participants with known HER2 positive status as defined as IHC 3+ or IHC 2+/ISH + (Cases with HER2: CEP17 ratio ≥ 2 or an average HER2 copy number ≥ 6.0 signals/cell are considered positive by ISH) [excluded]

Must have received: fluoropyrimidine — advanced or metastatic

Disease progression on or after at least one prior line of treatment (LoT) for advanced or metastatic disease, which included a fluoropyrimidine and a platinum

Must have received: platinum-based chemotherapy — advanced or metastatic

Disease progression on or after at least one prior line of treatment (LoT) for advanced or metastatic disease, which included a fluoropyrimidine and a platinum

Cannot have received: antibody-drug conjugate

Exception: ADC with MMAE payload

Prior exposure to any ADC with MMAE payload

Cannot have received: CLDN18.2 targeting treatment

Exception: naked monoclonal antibody

Prior exposure to any CLDN18.2 targeting treatment other than naked monoclonal antibody (eg, CLDN18.2 targeting CAR-T cell therapy, multi-specific antibody including targeting CLDN18.2, etc)