Prospective, interventional, open, randomized, national, multicenter, non-commercial trial

The study includes:

1. Biology screening: to estimate expression on GD2 on Ewing sarcoma cells from tumor tissue from archival material. Availability of tumor tissue is required for pre-screening testing to determine GD2 expression. To be screened for potential enrollment into the study patients or their legal representatives must have signed the pre-screening informed consent form (ICF) to consent to using their archival tumor sample to test the expression of GD2 in their tumor. The expression level of GD2 will be characterized in tumor tissue by immunohistochemistry (IHC) at a local and a central diagnostic testing laboratory.
2. Standard stratifying diagnostic tests (laboratory assessment: morphology, blood chemistry including ALT, AST, eGFR, creatinine, sodium, potassium, coagulation, urine analysis including pH, blood, protein, leukocytes, glucose, urobilinogen, bilirubin, ketones, nitrites, specific gravity, vital signs (body temperature, systolic and diastolic blood pressure, and pulse rate), ECG, cardiac function test, imaging test: CT/MRI scan).
3. Patients with GD2 expression will be randomized in proportions (2:1) to the experimental (D) and control groups (S). The cohort D will consist of 16 subjects, the cohort S 8 subjects. The exploratory cohort D will receive the experimental regimen in 3-week cycles consisting of irinotecan given intravenously (iv) 50 mg/m2 after oral temozolomide 100 mg/m2 on days 1-5 and naxitamab administered iv 2.25 mg/kg/day over 30 - 60 minutes, days 2, 4, 8 and 10 (up to 150 mg/day; total 9 mg/kg per cycle), and GM-CSF 250 mg/m2/day subcutaneously, days 6-10. Patients randomized to arm S will receive IT alone. Treatment cycles will be repeated every 21 days summary to 6 cycles or until disease progression, or subsequent relapsed, or occurrence of intolerable toxicity, or any event making impossible treatment continuation, or investigator's judgment, or withdrawal of consent. All activities are presented in Schedule of Assessments (SoA) at the end of the study synopsis.
4. Patients will be recalculated according to the intent to treat (ITT) rule.
5. The study will be conducted in accordance to GCP and after EC approval of the protocol.

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