This is a Phase 1, open-label, first-in-human (FIH) study designed to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD) and antineoplastic activity of BLU-285, administered orally (PO), in adult patients with unresectable GIST or other relapsed or refractory solid tumors. The study consists of 2 parts, a dose-escalation part (Part 1) and an expansion part (Part 2).