A Study of Ziftomenib in Combination With Imatinib in Patients With Advanced Gastrointestinal Stromal Tumors (GIST)

Key Inclusion Criteria: * Documented diagnosis of advanced/metastatic KIT-mutant GIST * Documented disease progression on imatinib as current or prior therapy or is treatment naïve, as follows: * DES: Participant is progressing or progressed on imatinib in any prior line of therapy. * EXP Cohort A: Participant is progressing or progressed on imatinib and has not received additional intervening therapy before study participation. * EXP Cohort B: Participant has progressed on imatinib and sunitinib only and has not received additional intervening therapy before study participation. * EXP Cohort C: Participant has newly diagnosed advanced/metastatic GIST and is treatment naïve in the advanced/metastatic setting. * Eastern Cooperative Oncology Group (ECOG) performance status of ≤2 at screening. * At least 1 measurable lesion per RECIST v1.1 modified for GIST. * Negative pregnancy test for participants of childbearing potential. * Adequate organ function per protocol requirements. * Resolution of all clinically significant toxicities from prior therapy to \<Grade 1 (or participant baseline) within 1 week before the first dose of study intervention. * Participant, or legally authorized representative, must be able to understand and provide written informed consent before the first screening procedure. Key Exclusion Criteria: * Diagnosis of GIST without a KIT mutation or with a T670X KIT mutation. * History of prior or current cancer that has potential to interfere with obtaining study results. * Received a prohibited medication, including investigational therapy, less than 14 days or within 5 drug half-lives before the first dose of study intervention. * Active central nervous system metastases. * Uncontrolled intercurrent illness, including, but not limited to protocol defined cardiac disease. * Mean corrected QT interval (QTcF) greater than 470ms. * Left ventricular ejection fraction (LVEF) \<50%. * Major surgery within 2 weeks before the first dose of study intervention. * Is pregnant or breastfeeding. * Gastrointestinal abnormalities that may impact taking study intervention by mouth. * Actively bleeding, excluding hemorrhoidal or gum bleeding.