Inclusion Criteria:  Males or females of any race ≥18 years age. Histologically-confirmed diagnosis of unresectable, relapsed or metastatic GIST or another advanced solid tumor. GIST patients must have progressed on or had an intolerability to imatinib and other SoCs or refused other SoCs. Patients with an advanced solid tumor other than GIST must have relapsed or had refractory disease without an available effective therapy and harbor KIT or PDGFRa gene alteration. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. Life expectancy ≥ 12 weeks. Adequate organ and marrow function. Tumor sample collection is required.  Exclusion Criteria:  Prior anti-cancer therapy within 2 weeks or at least 5 half-lives, whichever is longer, before the first dose. Major surgery within 4 weeks of the first dose. Radiotherapy with a limited field of radiation for palliation within 1 week prior to the first dose, with the exception as defined. Patients currently receiving medications or herbal supplements known to be strong inhibitors or inducers of CYP3A4. Patients currently receiving acid-reducing agents and are unable to stop use at least 2 weeks prior to the first dose. Spinal cord compression or brain metastases. Active infection including hepatitis B, hepatitis C, and HIV. Any evidence of severe or uncontrolled systemic diseases which in the Investigator's opinion makes it undesirable for the patient to participate in the trial or which would jeopardize compliance with the protocol.