A Study to Investigate the Effects of AZD0901 Monotherapy in Adult Participants With 2L+ Advanced or Metastatic Gastric or GEJ Adenocarcinoma Expressing CLDN18.2

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

Inclusion Criteria: * Histologically confirmed unresectable, locally advanced or metastatic adenocarcinoma of gastric, GEJ, or distal esophagus, with positive CLDN18.2 expression. * Disease progression on or after at least one prior regimen for advanced or metastatic disease, which included a fluoropyrimidine and a platinum, for advanced or metastatic disease. * Must have at least one measurable lesion assessed by the Investigator based on RECIST 1.1. * ECOG performance status of 0 or 1. * Minimum life expectancy of ≥12 weeks. * Adequate organ and bone marrow function. * Minimum body weight of 40 kg. * Sex and Contraceptive Requirements. Exclusion Criteria: * Participants with known HER2 positive status as defined as IHC 3+ or IHC 2+/ISH +. * Participant has significant or unstable gastric bleeding and/or untreated gastric ulcers. * CNS metastases or CNS pathology. * Participant has known clinically significant corneal disease (eg, active keratitis or corneal ulcerations). * Persistent toxicities (CTCAE Grade ≥2) caused by previous anticancer therapy. * History of thromboembolic events.