The purpose of this Phase 1/2 master protocol study is to evaluate if DCC-3009 is safe, tolerable and works effectively in the treatment of GIST. The study will use a modular approach with each module being defined according to therapy: DCC-3009 alone or DCC-3009 in combination with other anticancer therapies. Each module will be conducted in 2 parts: Part 1 (Dose Escalation) and Part 2 (Dose Expansion). Participants will be treated in 28-day treatment cycles with an estimated duration of up to 2 years.

Inclusion Criteria:

Module A Part 1 (Escalation):

• Any participant with histologically or cytologically confirmed advanced/unresectable or metastatic GIST with documented KIT or platelet-derived growth factor receptor alpha (PDGFRA) mutation, who has progressed on or was intolerant to at least 1 approved tyrosine kinase inhibitor (TKI) regimen in the advanced/metastatic setting
• Have at least 1 measurable lesion as defined by mRECIST, v1.1
• Have Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) of 0 or 1
• Adequate organ function, bone marrow function, and electrolytes
• All participants agree to comply with the contraception requirements
• Have a life expectancy of more than 3 months

Exclusion Criteria:

• Received systemic anticancer therapy or radiotherapy within 14 days prior to first dose of study drug
• Prior or concurrent malignancy that requires treatment or is expected to require treatment for active cancer
• Has known active central nervous system (CNS) metastases or an active primary CNS cancer
• History or presence of clinically relevant cardiovascular abnormalities
• Major surgery within 28 days of the first dose of study drug
• Had systemic arterial thrombotic or embolic events within 6 months prior to the first dose of study drug
• Had venous thrombotic events (e.g., deep vein thrombosis) or venous thrombotic embolic events (e.g., pulmonary embolism) within 1 month prior to the first dose of study drug
• Known allergy or hypersensitivity to any component of the study drug
• Malabsorption syndrome or other illness that could affect oral absorption
• Any other clinically significant comorbidities