Phase 1a/1b Study of the Safety, Pharmacokinetics, and Antitumor Activity of the Oral Menin Inhibitor Ziftomenib in Combination with Imatinib in Patients with Advanced Gastrointestinal Stromal Tumors (GIST) After Imatinib Failure

Key Inclusion Criteria:
*  Documented diagnosis of advanced/metastatic KIT-mutant GIST.
*  Documented disease progression on imatinib as current or prior therapy.
*  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 \*  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.