A Study of Pimitespib in Combination With Imatinib in Patients With GIST (CHAPTER-GIST-101)

Inclusion Criteria: * Provided written informed consent * Histologically confirmed GIST * Has radiographic progression based on RECIST 1.1 during or within 6 months of the last imatinib administration at enrollment. If surgery/radiotherapy has been performed, radiographic progression based on RECIST 1.1 with imatinib must have been observed after the last surgery /radiotherapy * Has at least one measurable lesion based on the RECIST version 1.1, except lymph nodes (not dependent on size), which should be chosen as nontarget lesions; * Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 Exclusion Criteria: * Corrected visual acuity \< 0.5 (using the International Visual Acuity Measurement Standard) for both eyes * Received treatment with any other line of therapy besides imatinib for advanced GIST * History of total gastrectomy and/or whole resection of the small intestine * A serious illness or medical condition * Previous or concurrent cancer that is distinct in primary disease or histology from cancer that is being evaluated in this study. However, any previous cancer curatively treated \> 5 years before the enrollment can be eligible * Pregnancy or lactation (including lactation interruption)