To Evaluate the Efficacy and Safety of Naxitamab in Patients With Refractory Ewing's Sarcoma (Butterfly)

Inclusion Criteria:

1. Histologically proven Ewing sarcoma of the bone or soft tissues.
2. Subject's archival tumour sample (formalin-fixed, paraffin-embedded; FFPE) available for evaluation of GD2 expression.
3. Documented disease progression (during or after completion of at least one line treatment) or any subsequent recurrence.
4. GD2 positive tumor assessed by IHC.
5. Age ≥ 2 years and ≤ 21 years.
6. Life expectancy of at least 12 weeks from the time informed consent was signed.
7. Previous systemic anticancer treatment completed ≥ 3 weeks, major surgery ≥ 2 weeks, and radiation therapy ≥ 4 weeks prior to study enrollment.
8. Recovered from adverse effects of prior surgery, radiotherapy, or Clinical trial protocol BUTTERFLY version 1.0 of 30.09.2022 r.anti-neoplastic therapy at the discretion of the investigator.
9. Signing of informed consent for trial participation (including for naxitamab treatment) according with current legal regulations.
10. Consent to the use of effective contraception throughout the period of the study and a minimum of 1 year after discontinuation of study treatment in patients at puberty and sexual maturity

Exclusion Criteria:

1. Failure to meet any of the inclusion criteria.
2. Not eligible to IT.
3. Previous treatment with an anti-GD2 antibody.
4. Hypersensitivity to the study drugs or any of their ingredients (covers IT and naxitamab).
5. Simultaneous treatment with other drugs which might interact with naxitamab or IT regimen.
6. Persistent toxicity related to prior therapy, making it impossible to treat with naxitamab.
7. Significant cardiac conduction abnormalities, including known familial prolonged QT syndrome, or screening corrected QT interval (QTc) \>480 msec.
8. Symptoms of congestive heart failure or left ventricular ejection fraction \<50%.
9. Inadequate pulmonary function defined as evidence of dyspnea at rest, exercise intolerance, and/or chronic oxygen requirement. In addition, room air pulse oximetry \< 94% and/or abnormal pulmonary function tests if these assessments are clinically indicated.
10. Requirement, or likely requirement, for corticosteroids at doses \>10 mg prednisolone (or equivalent) per day or other immunosuppressive agents.
11. Diagnosis of other malignancies before study inclusion.
12. Planning to become pregnant (while being treated with IT or naxitamab), pregnancy or breastfeeding.
13. Other acute or persistent disorders, behaviors or abnormal laboratory test results, which might increase the risk related to the participation in this clinical trial or to taking the study drug, or which might influence the interpretation of the study results, or which, in the investigator's opinion, disqualify a patient from participating in the tri