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Trabectedin and low-dose irinotecan to target EWS::FLI1 in Ewing sarcoma: a phase 1/2 trial. Ewing sarcoma (ES) is a bone and soft tissue sarcoma that is absolutely dependent on the EWS::FLI1 transcription factor for cell survival. No compound has been shown to reverse EWS::FLI1 activity in patients, and outcomes for relapsed patients remain poor. Trabectedin above a threshold concentration reverses the activity of EWS::FLI1 and is potentiated by low-dose irinotecan in vivo. This open-label phase 1/2 trial of trabectedin with irinotecan (SARC037) enrolled 37 relapsed/refractory patients with ES. The primary objectives were to determine the safety, tolerability, recommended phase 2 dose (RP2D; phase 1) and objective response rate (ORR; phase 2) of trabectedin administered as a 1-hour infusion in combination with low-dose irinotecan in patients with ES. The secondary objectives were to determine the progression-free survival (PFS), 6-month PFS, duration of response and 18F-fluorothymidine positron emission tomography (18F-FLT PET) avidity of ES tumors. The RP2D was trabectedin 1.0 mg m-2 over 1 hour (day 1) and irinotecan 25 mg m-2 (days 2 and 4) of a 21-day cycle. Toxicities were manageable with grade 3 or higher toxicities (>15%) of myelosuppression and alanine aminotransferase elevations at RP2D. The phase 2 ORR was 33% (39%, including RP2D phase 1 patients), and 6-month PFS was 48%. Transcriptional profiling demonstrated reversal of the EWS::FLI1 transcriptome in tumors from a subset of patients. Additional correlative objectives captured molecular profiling, circulating tumor DNA levels, pharmacokinetics and 18F-FLT PET avidity. Here we provide the basis for further development of trabectedin/irinotecan for patients with ES by the international cooperative groups. ClinicalTrials.gov: NCT04067115 .