Background: Olaratumab (O), a PDGFRα antagonist, is a targeted human IgG1 monoclonal antibody that specifically binds PDGFRα, blocking PDGF-AA, -BB, and -CC binding and receptor activation, and has improved survival outcomes in adults with advanced sarcoma. Methods: This ongoing Phase 1, multicenter, dose-escalation study (NCT02677116) enrolled patients (pts) aged < 18 years, with a diagnosis of relapsed or refractory solid tumors, to 2 dose levels (Parts A and B) of O combined with fixed doses of standard chemotherapy with doxorubicin (D), vincristine/irinotecan (VI), or high-dose ifosfamide (I). Pts in Part A received 1 cycle (21 days) of O monotherapy at 15mg/kg IV on Days 1 and 8 followed by O + (D, VI, or I) for subsequent 21-day cycles. Each combination arm was complete when 6 pts received 2 full cycles. The primary objective of Part A was to determine the safety and tolerability of O 15mg/kg + (D, VI, or I) based on any dose-limiting toxicity (DLT) and O serum exposure matching between adult and pediatric pts. Results: Pts enrolled by O + chemotherapy regimen (n) were D (11), VI (10), and I (9). One pt (3.3%) experienced a DLT during the DLT period (Cycles 1 and 2), which consisted of grade (G) 4 elevated ALT while on O monotherapy. No ≥G3 infusion- or cardiac-related treatment emergent adverse events (AEs) occurred. Treatment-related AEs (TRAEs) ≥G3 reported in ≥2 pts are presented (Table). Serum concentrations of O in pediatric pts were within expected ranges based on adult exposure. Conclusions: Based on Part A results,O 15mg/kg as monotherapy or in combination with D, VI, or I is tolerable in pediatric pts with relapsed or refractory solid tumors. Part B of this trial is currently enrolling. Clinical trial information: NCT02677116