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.