Temozolomide (TMZ) in Advanced Succinate Dehydrogenase (SDH)-Mutant/Deficient Gastrointestinal Stromal Tumor (GIST)

Inclusion Criteria: 1. Patient has pathologically confirmed SDH-mutant/deficient GIST. 2. Has recovered from the acute toxic effects of all prior chemotherapy, immunotherapy, or radiotherapy. 3. Patient has an ECOG Performance Status of 0-2. 4. Patient has adequate hematologic, hepatic and renal function. 5. Female patient of childbearing potential has a negative serum or urine pregnancy within 72 hours prior to receiving the first dose of study medication. 6. Female patient of childbearing potential agrees to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication. 7. Male patient with a partner of childbearing potential agrees to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy. 8. Has measurable or evaluable disease as per RECIST v1.1 (Appendix B). 9. Life expectancy of \>12 weeks. Exclusion Criteria: 1. Patients who have had major surgery within 4 weeks of initiation of study medication. 2. Patients who are receiving other concurrent anti-neoplastic therapy (e.g., chemotherapy, targeted therapy, immunotherapy, or radiotherapy) at the start of study treatment. 3. Patients with known brain metastases should be excluded from this clinical trial because of their poor prognosis and because they often develop progressive neurologic dysfunction that would confound the evaluation of neurologic and other adverse events. 4. Evidence of severe or uncontrolled systemic diseases \[e.g., unstable or uncompensated respiratory, cardiac (including life threatening arrhythmias)\]. 5. Unresolved toxicity ≥ CTCAE Grade 2 from previous anti-cancer therapy except alopecia (if applicable) unless agreed that the patient can be entered after discussion with the Medical Monitor. 6. Presence of cardiac impairment class III and IV definitions; OR history of myocardial infarction/active ischemic heart disease within one year of study entry; OR uncontrolled dysrhythmias; OR poorly controlled angina. 7. Pregnant or breast-feeding females. 8. Medical condition such as uncontrolled infection (including HIV), uncontrolled diabetes mellitus or cardiac disease which, in the opinion of the treating physician, would make this protocol unreasonably hazardous for the patient. 9. Any concurrent condition which in the investigator's opinion makes it undesirable for the subject to participate in this trial or which would jeopardize compliance with the protocol. 10. Patients who cannot swallow oral formulations of the agent