Lurbinectedin in FET-Fused Tumors

The purpose of this study is to find out if a drug called lurbinectedin (the "study drug") is safe and effective at treating people with recurrent or relapsed solid tumors, including Ewing sarcoma.

Inclusion Criteria: 1. Age ≥ 10 years. 2. Phase 1: Histological confirmed diagnosis of recurrent or relapsed solid tumor failing primary therapy. Patients must have a known FET fusion (fusion that contains EWSR1, FUS, or TAF15) as documented by next generation sequencing, polymerase chain reaction (PCR) or Fluorescence in situ hybridization (FISH). Patients with a histological diagnosis of Ewing sarcoma with EWS-FLI1 are eligible for dose escalation but not for the exploratory cohort. Please note patients with Ewing sarcoma and alternative FET-ETS fusions (including but not limited to EWS-ERG, EWS-ETV1, EWS-ETV4, EWS-FEV, FUS-ERG, FUS-FEV) are eligible for the exploratory cohort. 3. Phase 2: Histologically confirmed diagnosis of recurrent or relapsed Ewing sarcoma failing primary therapy with confirmation of EWS-FLI1 fusion and breakpoint by Next generation sequencing or PCR or EWSR1 rearrangement confirmed by FISH and available tissue for central confirmation of EWS-FLI1 fusion and breakpoint. 4. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 (age \>16 years) or Lansky of at least 70 (age \