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.