AZD0901 in Participants With Advanced Solid Tumours Expressing Claudin18.2

The list below is a summarised eligibility criteria for the study - refer to the study protocol for full criteria. Master Inclusion Criteria applicable to all sub studies: * Participant must be ≥ 18 years or the legal age of consent at the time of signing the ICF. * Participants who are CLDN18.2 positive. * Must have at least one measurable lesion according to RECIST v1.1. * ECOG performance status of 0 to 1 with no deterioration over the previous 2 weeks prior first day of dosing. * Predicted life expectancy of ≥ 12 weeks. * Adequate organ and bone marrow function as defined by protocol. * Body weight \> 35 kg. * Participants are willing to comply with contraception requirements. Sub study 1 Specific Inclusion criteria: * Histologically confirmed adenocarcinoma of the stomach or gastroesophageal junction. * Advanced or metastatic GC/GEJC. * Maximum 2 prior lines of systemic treatment for unresectable or metastatic disease. Sub study 2 Specific Inclusion criteria: * Participants diagnosed with histologically confirmed metastatic or advanced PDAC. * Availability of an archival sample or a fresh tumour biopsy taken at screening. * No prior treatments for unresectable or metastatic disease. Prior neoadjuvant/adjuvant chemotherapy is permitted as long as participants progressed ≥ 6 months (183 days) from the last dose. Sub study 3 Specific Inclusion criteria * Histologically confirmed, unresectable advanced, or metastatic adenocarcinoma of biliary tract, including cholangiocarcinoma (intrahepatic or extrahepatic) and gallbladder carcinoma (NOTE: Ampullary cancers are not eligible). * Documented radiographic or clinical disease progression on or after at least one prior regimen and maximum 2 prior lines of systemic treatment for unresectable or metastatic disease. Master Exclusion Criteria applicable to all sub studies: * Unstable or active peptic ulcer disease or digestive tract bleeding including but not limited to clinically significant bleeding in the setting of prior CLDN18.2 directed therapy. * Participants with clinically significant ascites that require drainage. * A history of drug-induced non-infectious ILD/pneumonitis. * Central nervous system metastases or CNS pathology. * Peripheral neuropathy, sensory, or motor ≥ Grade 2 at screening. * History of another primary malignancy. * Prior exposure to any MMAE-based ADC. * Prior exposure to any CLDN18.2 targeted agents except anti-CLDN18.2 monoclonal antibody. Sub study 1 Specific Exclusion criteria: * Participants with HER2-positive (3+ by IHC, or 2+ by IHC, and positive by ISH) or indeterminate GC/GEJC unless they have failed/not tolerated/or are not eligible for standard anti-HER2 therapy, where available. * Any factors that increase the risk of QTc prolongation or risk of arrhythmic events. * The use of concomitant medications known to prolong the QT/QTc interval. Sub study 2 Specific Exclusion criteria: * Known DPD enzyme deficiency based on local testing where testing is SoC. * Use of strong inhibitor or inducer of UGT1A1. * Use of strong inhibitors or inducers of CYP3A4. * Known homozygous for the UGT1A1\*28 allele based on local testing where testing is SoC. Sub study 3 Specific Exclusion criteria • Clinically significant biliary obstruction that has not resolved before enrollment.