A Study Evaluating Anvumetostat in Combination With Other Therapies in Participants With Advanced Gastrointestinal, Biliary Tract, or Pancreatic Cancers With Homozygous Methylthioadenosine Phosphorylase (MTAP)-Deletion (MTAPESTRY 103)

Subprotocol B

Inclusion:

* Age ≥ 18 years (or ≥ legal age within the country if it is older than 18 years).
* Histologically or cytologically confirmed diagnosis of metastatic and/or unresectable (locally advanced) adenocarcinoma of the pancreas.
* Tumor tissue (FFPE sample) or an archival block must be available. Participants without archived tumor tissue available may be allowed to enroll by undergoing tumor biopsy before dosing.
* Homozygous MTAP-deletion.
* Disease measurable as defined by RECIST v1.1.
* Adequate organ function as defined in the protocol.

Exclusion:

* Prior treatment with a MAT2A inhibitor or a PRMT5 inhibitor.
* Radiation therapy within 28 days of first dose.
* Major surgery within 28 days of first dose of Anvumetostat.
* Cardiovascular and pulmonary exclusion criteria as defined in the protocol.
* Gastrointestinal tract disease causing the inability to take PO medication, malabsorption syndrome, requirement for IV alimentation, gastric/jejunal tube feeds, uncontrolled inflammatory gastrointestinal disease (eg, Crohn's disease, ulcerative colitis).
* History of solid organ transplantation.

Subprotocol C

Inclusion:

* Age ≥ 18 years (or ≥ legal age within the country if it is older than 18 years).
* Histologically or cytologically confirmed diagnosis of metastatic and/or unresectable (locally advanced) adenocarcinoma of the pancreas.
* Homozygous MTAP-deletion.
* Rat Sarcoma Viral Oncogene Homolog (RAS) mutation
* Received at least 1 prior systemic therapy for advanced or metastatic PDAC.
* Disease measurable as defined by RECIST v1.1.
* Adequate organ function as defined in the protocol.

Exclusion:

* Prior treatment with aMAT2A inhibitor, a PRMT5 inhibitor, or a MAPK pathway inhibitor, including KRAS inhibitors.
* Cardiovascular and pulmonary exclusion criteria as defined in the protocol.
* Gastrointestinal tract disease causing the inability to take PO medication, malabsorption syndrome, requirement for IV alimentation, gastric/jejunal tube feeds, uncontrolled inflammatory gastrointestinal disease (eg, Crohn's disease, ulcerative colitis).
* History of solid organ transplantation.