Purpose Phase 1 study assessing the safety and toxicity of cabozantinib in combination with topotecan and cyclophosphamide for relapsed osteosarcoma and Ewing sarcoma. Methods Oral cabozantinib (25mg/m 2) was administered daily for 21 (dose level 1) or 14 (dose level -1B) days. Topotecan (0.75mg/m 2) and cyclophosphamide (250mg/m 2) were administered IV on days 1-5. A modified 3+3 design based upon first cycle dose-limiting toxicities (DLT) was used for dose escalation. Results Twelve patients with a median age of 15 (12.9-33.2) years were enrolled (7 with Ewing sarcoma; 5 with osteosarcoma); all were evaluable for toxicity. At dose level 1, three of six patients developed first cycle DLT: grade 3 epistaxis; grade 3 transaminitis; prolonged grade 2 thrombocytopenia. Six patients were enrolled on dose level -1B (interrupted cabozantinib, given days 8-21), with one first cycle DLT (grade 3 pneumothorax) observed. Of the 10 response evaluable patients, one had partial response (Ewing sarcoma), seven had stable disease, and two had progressive disease. Conclusions The recommended phase 2 doses and schedules for this combination are topotecan 0.75mg/m 2 IV days 1-5, cyclophosphamide 250mg/m 2 IV days 1-5, and cabozantinib 25mg/m 2 days 8-21. Non-concomitant administration of cabozantinib with cytotoxic therapy in this population has acceptable toxicity while allowing for potential disease control.