Phase 3 Study of DCC-2618 vs Placebo in Advanced GIST Patients Who Have Been Treated With Prior Anticancer Therapies

Inclusion Criteria: 1. Histologic diagnosis of GIST 2. Patients must have progressed on imatinib, sunitinib, and regorafenib or have documented intolerance to any of these treatments. 3. ECOG PS of 0 to 2 at screening. 4. Able to provide an archival tumor tissue sample if no anticancer therapy was administered since the sample was collected; otherwise, a fresh tumor tissue sample is required prior to the first dose of study drug. 5. Female patients of childbearing potential must have a negative serum beta-human chorionic gonadotrophin (β-hCG) pregnancy test at screening and negative urine pregnancy test at Cycle 1 Day 1 prior to the first dose of study drug. 6. Patients of reproductive potential must agree to follow the contraception requirements. 7. The patient is capable of understanding and complying with the protocol and has signed the informed consent document. A signed informed consent form must be obtained before any study-specific procedures are performed. 8. At least 1 measurable lesion according to modified RECIST Version 1.1 (non-nodal lesions must be ≥1.0 cm in the long axis or ≥double the slide thickness in the long axis) within 21 days prior to the first dose of study drug. 9. Adequate organ function and bone marrow reserve as indicated by the following laboratory assessments performed at screening. * Absolute neutrophil count ≥1000/uL * Hemoglobin ≥8 g/dL * Platelet count ≥75,000/uL * Total bilirubin ≤1.5 x the upper limit of normal (ULN) * Aspartate transaminase or alanine transaminase ≤3 x ULN (≤5x ULN in the presence of hepatic metastases) * Serum creatinine ≤1.5 x ULN or creatinine clearance ≥50 mL/min based on either urine collection or Cockcroft Gault estimation. * Prothrombin time (PT) or international normalized ratio (INR) or partial thromboplastin time ≤1.5 x ULN. Patients on a stable, maintenance regimen of anticoagulant therapy for at least 30 days prior to study drug administration may have PT/INR measurements \>1.5 x ULN if, in the opinion of the Investigator, the patient is suitable for the study. An adequate rationale must be provided to the Sponsor prior to randomization. 10. Resolution of all toxicities from prior therapy to ≤Grade 1 (or baseline) within 1 week prior to the first dose of study drug (excluding alopecia and ≤Grade 3 clinically asymptomatic lipase, amylase, and creatine phosphokinase laboratory abnormalities). Exclusion Criteria: 1. Treatment with anticancer therapy, including investigational therapy, or investigational procedures within 14 days or 5 x the half-life (whichever is longer) prior to the first dose of study drug. For prior biological therapies, eg, monoclonal antibodies with a half-life longer than 3 days, the interval must be at least 28 days prior to the first dose of study drug. 2. Prior treatment with DCC-2618 3. Prior or concurrent malignancy whose natural history or treatment have the potential to interfere with the safety or efficacy assessment of DCC-2618. Patients receiving adjuvant cancer treatment are not eligible if those medications are potentially active against GIST or excluded per protocol. 4. Patient has known active central nervous system metastases. 5. New York Heart Association class II - IV heart disease, active ischemia or any other uncontrolled cardiac condition such as angina pectoris, clinically significant cardiac arrhythmia requiring therapy, uncontrolled hypertension or congestive heart failure. 6. Arterial thrombotic or embolic events such as cerebrovascular accident (including ischemic attacks) or hemoptysis within 6 months before the first dose of study drug. 7. Venous thrombotic events (eg, deep vein thrombosis) or pulmonary arterial events (eg, pulmonary embolism) within 3 months before the first dose of study drug. Patients with venous thrombotic events ≥3 months before the first dose of study drug on stable anticoagulation therapy are eligible. 8. 12-lead electrocardiogram (ECG) demonstrating QT interval corrected by Fridericia's formula \>450 ms in males or \>470 ms in females at screening or history of long QT interval corrected syndrome. 9. Left ventricular ejection fraction (LVEF) \<50% at screening. 10. Use of proton-pump inhibitors within 4 days prior to the first dose of study drug. Other medications that increase gastric pH, ie, histamine H2 receptor antagonists and antacids may be taken provided they are not administered within 2 hours before or after administration of study drug. 11. Use of strong or moderate inhibitors and inducers of cytochrome P450 (CYP) 3A4, including certain herbal medications (eg, St. John's Wort) and consumption of grapefruit or grapefruit juice within 14 days or 5 x the half-life (whichever is longer) prior to the first dose of study drug. 12. Use of known substrates or inhibitors of breast cancer resistance protein (BCRP) transporters within 14 days or 5 x the half-life (whichever is longer) prior to the first dose of study drug. 13. Major surgeries (eg, abdominal laparotomy) within 4 weeks of the first dose of study drug. Following major surgeries, \>4 weeks prior to the first dose of study drug, all surgical wounds must be healed and free of infection or dehiscence. 14. Any other clinically significant comorbidities, such as uncontrolled pulmonary disease, active infection, or any other condition, which in the judgment of the Investigator, could compromise compliance with the protocol, interfere with interpretation of the study results, or predispose the patient to safety risks. 15. Known human immunodeficiency virus or hepatitis C infection only if the patient is taking medications that are excluded per protocol, active hepatitis B, or active hepatitis C infection. 16. If female, the patient is pregnant or lactating. 17. Known allergy or hypersensitivity to any component of the investigational drug product. Patients with a history of Stevens-Johnson syndrome on a prior TKI are excluded. 18. Gastrointestinal abnormalities including but not limited to: * inability to take oral medication * malabsorption syndromes * requirement for intravenous alimentation 19. Any active bleeding excluding hemorrhoidal or gum bleeding.