CGT9486 and Sunitinib for Gastrointestinal Stromal Tumors

Key Inclusion Criteria: 1. Histologically confirmed locally advanced, metastatic, and/or unresectable GIST. Molecular pathology report must be available for Part 2; if molecular pathology report is unavailable or inadequate, an archival or fresh tumor tissue sample will be required to evaluate mutational status prior to randomization. (GIST 1L Substudy: must have documented mutation in KIT Exon 9 with an available molecular pathology report; archival or fresh tumor tissue sample will be required) 2. Documented disease progression on or intolerance to imatinib (Part 1a, Part 1b, Part 2, DDI Substudy) 3. Subjects must have received the following treatment: * DDI Substudy/Part 1a: Treatment with ≥1 prior lines of therapy for GIST * Part 1b: Treatment with ≥2 prior TKI for GISTs * Part 2: Prior treatment with imatinib only * GIST 1L Substudy: No prior systemic therapy for GIST including adjuvant therapy. Exception: up to 10 subjects with ongoing imatinib therapy of ≤4 weeks 4. Have at least 1 measurable lesion according to mRECIST v1.1 (Part1a, Part 1b, Part 2, GIST 1L Substudy) 5. Eastern Cooperative Oncology Group (ECOG) Status * 0 to 2 (Part 1a, Part 1b, Part 2, DDI Substudy) * 0 to 1 (GIST 1L Substudy) 6. Have clinically acceptable local laboratory screening results (clinical chemistry and hematology) within certain limits Key Exclusion Criteria: 1. Known Platelet-Derived Growth Factor Receptor (PDGFR) driving mutations or known succinate dehydrogenase deficiency (Part 1a, Part 1b, Part 2, DDI Substudy) 2. Clinically significant cardiac disease 3. Major surgeries (eg, abdominal laparotomy) within 4 weeks of the first dose of study drug (Part 1a, Part 1b, Part 2, DDI Substudy) 4. Gastrointestinal abnormalities including, but not limited to, significant nausea and vomiting, malabsorption, external biliary shunt, or significant bowel resection that would preclude adequate absorption 5. Any active bleeding excluding hemorrhoidal or gum bleeding 6. Seropositive for HIV 1 or 2, or positive for hepatitis B surface antigen or hepatitis C virus (HCV) antibody. 7. Active, uncontrolled, systemic bacterial, fungal, or viral infections at Screening 8. Received strong CYP3A4 inhibitors or inducers (Part 1a, Part 1b, Part 2, DDI Substudy) 9. Received sunitinib within 3 weeks (Part 1a, Part 1b, DDI Substudy)