PH 2 Pemigatinib in SDH-deficient GIST

Inclusion Criteria: * Participant must have histologically confirmed SDH-deficient GIST. Participants must have locally advanced or metastatic disease that is not amenable to surgery. * Participants must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as ≥20 mm (≥2 cm) by chest x-ray or as ≥10 mm (≥1 cm) with CT scan, MRI, or calipers by clinical exam. See Section 12 (Measurement of Effect) for the evaluation of measurable disease. * Participants must have radiographically documented progressive disease prior to study enrollment as per investigator assessment. * Age ≥18 years * ECOG performance status ≤2 * Participants must have adequate organ and marrow function as defined below: * Hemoglobin \> 8 g/dL. Patients with a hemoglobin of 7.5-8.0 g/dL may be eligible if the value is chronic, there is no clinical evidence of active bleeding, and eligibility is confirmed upon review and approval of the Sponsor-Investigator * Absolute neutrophil count ≥1,000/mcL * platelets ≥100,000/mcL * total bilirubin ≤1.5 × institutional upper limit of normal (ULN) * AST(SGOT)/ALT(SGPT) ≤3.0 × institutional ULN (unless liver metastases are present in which case it must be ≤ 5 × ULN) * glomerular filtration rate (GFR) ≥60 mL/min/1.73 m2 (using the CDK-EPI formula see Appendix B) * Human immunodeficiency virus (HIV)-infected partici…