Inclusion Criteria:  Age ≥ 12 years at the time of informed consent Bodyweight ≥ 40 kg Histologically documented relapsed or metastatic osteosarcoma. Must have measurable disease according to RECIST Guideline version 1.1 criteria. Adequate hematologic and organ function. Female subjects of childbearing potential and male subjects must agree to use an effective method of contraception per institutional standard prior to the first dose and for 6 months after study treatment discontinuation. Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.  Exclusion Criteria:  Unresolved toxicity of Grade >1 attributed to prior therapies (excluding: Grade ≤2 neuropathy, alopecia, or skin pigmentation) Prior therapy with a WEE1 inhibitor A serious illness or medical condition(s). Pregnant or lactating females. Females of childbearing potential with a positive serum pregnancy test <14 days to Day 1. Subjects with active (uncontrolled, metastatic) second malignancies or requiring therapy. 12-lead ECG demonstrating a corrected QT interval using Fridericia's formula (QTcF) of >470 ms, except for subjects with atrioventricular pacemakers or other conditions (e.g., right bundle branch block) that render the QT measurement invalid. History or current evidence of congenital or family history of long QT syndrome or Torsades de Pointes (TdP). Taking medications with a known risk of TdP. Administration of strong and moderate CYP3A4 inhibitors/inducers and strong and moderate P-gp inhibitors.