The purpose of this study is to measure the efficacy and safety of AZD0901 compared to Investigator's choice of therapy as 2L+ treatment for participants with advanced or metastatic gastric or GEJ adenocarcinoma expressing CLDN18.2.

This is a Phase III, multi-center, open-label, sponsor-blinded, randomized, global study to assess the efficacy and safety of AZD0901 compared to Investigator's choice of therapy as the 2L+ treatment for participants with advanced or metastatic gastric or GEJ adenocarcinoma expressing CLDN18.2, and the clinical performance of the investigational IVD. As part of this combined approach, the efficacy analyses from this study will also provide the basis to evaluate the clinical performance of Ventana CLDN18.2 assay as an IVD device for the identification of patients with advanced or metastatic gastric or GEJ adenocarcinoma expressing CLDN18.2 who may benefit from AZD0901.

Inclusion Criteria:

1. Capable of giving signed informed consent prior to any study procedure.

2. Participant must be at least 18 years or the legal age of consent in the jurisdiction in which the study is taking place at the time of signing the ICF.

3. Histologically confirmed unresectable, locally advanced or metastatic adenocarcinoma of gastric, GEJ, or distal esophagus (distal third of the esophagus) and the following requirement:

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

4. 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, for advanced or metastatic disease.

5. Must have at least one measurable or evaluable lesion assessed by the Investigator based on RECIST 1.1.

6. ECOG performance status of 0 or 1 with no deterioration over the previous 2 weeks prior to baseline or day of first dosing.

7. Predicted life expectancy of ≥ 12 weeks.

8. Adequate organ and bone marrow function

9. Body weight of ≥ 35 kg.

10. Sex and Contraceptive Requirements

Exclusion Criteria:

1. 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). Participants must undergo local (or have had) HER2 testing by IHC/ISH, and the most recent result of HER2 status will be used to determine the eligibility.

2. Participant has significant or unstable gastric bleeding and/or untreated gastric ulcers.

3. CNS metastases or CNS pathology including: epilepsy, seizures or aphasia within 3 months prior to consent, severe brain injury, dementia, Parkinson's disease, neurodegenerative diseases, cerebellar disease, severe uncontrolled mental illness, psychosis, CNS involvement of autoimmune diseases.

4. Participant has known clinically significant corneal disease (eg, active keratitis or corneal ulcerations).

5. Persistent toxicities (CTCAE Grade ≥ 2) caused by previous anticancer therapy, excluding alopecia. Participants with irreversible toxicity that is not reasonably expected to be exacerbated by study intervention may be included (eg, hearing loss).

6. Prior exposure to any ADC with MMAE payload or 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).

7. History of thromboembolic events:

   1. Participants with venous thromboembolism within the past 6 months prior to randomization: participants with venous port or catheter thrombosis or superficial venous thrombus that do not require treatment or are stable on treatment with anticoagulants are excepted
   2. History of arterial thromboembolism within the past 12 months prior to randomization

8. As judged by the Investigator, any evidence of diseases which in the Investigator's opinion, makes it undesirable for the participant to participate in the study or that would jeopardize compliance with the protocol.

Qualified researchers can request access to anonymized individual patient-level data from AstraZeneca group of companies sponsored clinical trials via the request portal Vivli.org. All requests will be evaluated as per the AZ disclosure commitment: https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure

"Yes", indicates that AZ are accepting requests for IPD, but this does not mean all requests will be approved.

AstraZeneca will meet or exceed data availability as per the commitments made to the EFPIA/PhRMA Data-Sharing Principles. For details of our timelines, please refer to our disclosure commitment at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure

When a request has been approved AstraZeneca will provide access to the anonymized individual patient-level data via secure research environment Vivli.org. A Signed Data Usage Agreement (non-negotiable contract for data accessors) must be in place before accessing requested information.

This is an open-label study; however, it will be conducted 'sponsor-blind' and the specific treatment to be taken by a participant will be assigned using an IRT/RTSM. To maintain the integrity of the study, sponsor access to treatment records will be restricted, and, in particular, under no circumstances will the sponsor undertake any efficacy analysis by treatment arm during the study. A Study Integrity Plan will be generated in which nominated individuals who will be granted access to any treatment-revealing data will be pre-specified, with their reason for requiring access detailed.