A Phase I/II Open-label Dose Escalation and Dose Expansion Study to Evaluate the Safety, Pharmacokinetics, Pharmacodynamics, and Efficacy of AZD5863, a T Cell-engaging Bispecific Antibody That Targets Claudin 18.2 (CLDN18.2) and CD3 in Adult Participants With Advanced or Metastatic Solid Tumors

Key Inclusion Criteria: * Age ≥ 18 at the time of signing the informed consent * Histologically confirmed diagnosis of adenocarcinoma of the stomach, gastro-esophageal junction, esophagus, or pancreas * Must have at least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 * Must show positive CLDN18.2 expression in tumor cells as determined by central immunohistochemistry (IHC) * Eastern Cooperative Oncology Group Performance status (ECOG PS): 0-1 at screening * Predicted life expectancy of ≥ 12 weeks * Adequate organ and bone marrow function measured within 28 days prior to first dose as defined by the protocol * Contraceptive use by men or women should be consistent with local regulations, as defined by the protocol * Must have received at least one prior line of systemic therapy in the advanced/metastatic setting Key Exclusion Criteria: * Unresolved toxicity from prior anticancer therapy of Common Terminology Criteria for Adverse Events (CTCAE) Grade ≥ 2 except for those defined by the protocol * Participant experienced unacceptable cytokine release syndrome (CRS) or Immune Effector Cell Associated Neurotoxicity (ICANS) following prior T cell engagers (TCE) or chimeric antigen receptor T (CAR-T) cell therapy * Previous history of hemophagocytic lymphohistiocytosis (HLH) / macrophage activation syndrome (MAS) * Active or prior documented autoimmune or inflammatory disorders within 3 years of start of treatment * central nervous system (CNS) metastases or CNS pathology, as defined by the protocol, within 3 months prior to consent * Infectious disease including active human immunodeficiency virus (HIV), active hepatitis B/C, uncontrolled infection with EBV, uncontrolled active systemic fungal, bacterial or other infection * Cardiac conditions as defined by the protocol * History of thromboembolic event within the past 3 months prior to the scheduled first dose of study intervention * Participant requires chronic immunosuppressive therapy * Participants on anticoagulation therapy with long-acting anticoagulants or other class of anticoagulants at therapeutic doses