A Study of Azenosertib (ZN-c3) in Combination With Gemcitabine in Subjects With Osteosarcoma

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 …