Abstract
BACKGROUND: Ontuxizumab is a humanized IgG monoclonal antibody that targets the cell-surface glycoprotein endosialin (tumor endothelial marker-1[TEM-1]/CD248) found on activated mesenchymal cells and certain tumors. Ontuxizumab binding to endosialin may interfere with platelet-derived growth factor signaling, prevent tumor stroma organization, and prevent new vessel formation. METHODS: Ontuxizumab was administered intravenously on days 1, 8, 15, and 22 of a 28-day cycle at three dose levels (4, 8, and 12 mg/kg). Further dose escalation to 16 mg/kg was planned if the maximum tolerated dose (MTD) was not reached and the ontuxizumab systemic clearance was ≥30% higher in children compared to adults. Following determination of the MTD/recommended phase 2 dose, an additional cohort of six patients (<12 years) was enrolled for further pharmacokinetics (PK) evaluation. RESULTS: Twenty-seven eligible patients (17 male, median age 15 years, range 3-21 years) were enrolled. Twenty-two patients (neuroblastoma [5], Ewing sarcoma [4], rhabdomyosarcoma [4], and other tumors [9]) were fully evaluable for toxicity. Five patients did not complete cycle 1 due to tumor progression. Two of 10 patients experienced dose-limiting toxicity of bacteremia (n = 1) and hyponatremia (n = 1) at 12 mg/kg. Grade ≤2 fever or infusion-related reactions occurred in 10 patients. Clearance was dose dependent and within 30% of adult value at 12 mg/kg. CONCLUSION: Ontuxizumab administered weekly at 12 mg/kg appears to be well tolerated in children with relapsed or refractory solid tumors. The PK of ontuxizumab does not appear to be significantly different in children compared to adults.