AZD0901 in Participants With Advanced Solid Tumours Expressing Claudin18.2

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.