The purpose of this study is to assess the safety, feasibility, and effectiveness of a consolidative B7-H3 CAR T cell therapy in patients with newly diagnosed high-risk osteosarcoma who have undergone upfront standard chemotherapy.

Primary Objectives:

- To evaluate 1-year RFS from the time of SJCARB7H3_41BBL infusion for patients with newly diagnosed metastatic osteosarcoma who received standard chemotherapy.

Secondary Objectives:

• To evaluate the OS from time of SJCARB7H3_41BBL infusion for patients with newly diagnosed metastatic osteosarcoma who received standard chemotherapy.
• To evaluate the feasibility of delivering SJCARB7H3_41BBL at the end of standard therapy in patients with newly diagnosed metastatic osteosarcoma.
• To describe the safety of autologous SJCARB7H3_41BBL therapy when delivered at the end of standard therapy in patients with newly diagnosed metastatic osteosarcoma.

This is a phase 2 study of SJCARB7H3_41BBL for participants with newly diagnosed high-risk metastatic osteosarcoma who received standard chemotherapy.

All participants will receive standard chemotherapy (for example methotrexate, anthracycline, platinum), local control surgery, and pulmonary metastasectomy if applicable, and this is not considered part of protocol therapy. Participants will undergo apheresis prior to standard local control surgery (about 12 weeks after diagnosis) for SJCARB7H3_41BBL manufacture and then resume standard consolidation therapy. A safety run will initiate with Regimen A. For Regimen A, eligible participants with available SJCARB7H3_41BBL product will receive lymphodepletion chemotherapy 14-28 days after the completion of standard chemotherapy (31 weeks after diagnosis), followed by SJCARB7H3_41BBL infusion. If Regimen A is not cleared, then Regimen B will be evaluated, where lymphodepletion and SJCARB7H3_41BBL infusion occur post pulmonary metastasectomy. Following successful clearance of either regimen A or, if necessary, regimen B, then the efficacy cohort will be initiated. Participants will be followed with serial disease evaluations for 2 years from SJCARB7H3_41BBL infusion prior to transfer to our institutional long-term follow-up (LTFU) protocol. The total duration from completion of standard therapy, including experimental therapy and follow-up on 3CAR4OS, is approximately 2 years.

Inclusion Criteria:

1. Participant and/or legally authorized representative has signed the Informed Consent Form for this study

2. Prior cancer therapy:

   • Regimen A only: Completed all planned cycles of consolidation therapy between 14-28 days prior.
   • Regimen B only: has completed all planned cycles of consolidation chemotherapy at least 14 days prior and if clinically indicated, participant has undergone pulmonary metastasectomy. They must have recovered from any surgical complications with no ongoing sequelae of category 2 or higher by the Clavien-Dindo classification system and less than 6 weeks must have passed from time of pulmonary metastasectomy.

3. No evidence of progressive disease since enrolled on study

4. Lansky performance status score of ≥ 50 for participants <16 years of age or Karnofsky score ≥ 50 for participants ≥ 16 years. Participants who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for purposes of assessing performance status

5. Adequate organ function as indicated by:

   • Renal: Serum creatinine ≤ 1.5 X the upper limit of normal (ULN) based on enrollment eligibility table.
   • Hepatic: Total bilirubin ≤ 3 times ULN for age OR conjugated bilirubin ≤ 2 mg/dL AND ALT (SGPT) ≤ 5 times ULN
   • Cardiac: Shortening fraction ≥ 28% OR ejection fraction ≥ 50% as measured by echocardiogram
   • Respiratory: Oxygen saturation ≥ 90% on room air without supplemental oxygen or mechanical ventilation

6. Laboratory values meet the following criteria:

   • Absolute Neutrophil Count (ANC) ≥ 750 cells/uL
   • Platelet Count of ≥ 75,000 (can be transfused)
   • Hemoglobin ≥ 7 g/dL (can be transfused)

7. Participant is ≥ 7 days from receiving supra-physiologic dosing of systemic (IV or PO) corticosteroids. Glucocorticosteroid physiologic replacement therapy for management of adrenal insufficiency is allowed.

8. Participant and/or legally authorized representative has signed the Informed Consent Form for the treatment phase of this study.

Exclusion Criteria:

1. Major surgical adverse event related to the primary tumor local control defined as Clavien-Dindo category 3 requiring ongoing wound care.

2. Evidence of clinically significant encephalopathy/new focal neurologic deficits.

3. Presence of active severe infection, defined as:

   • positive blood culture within 48 hours of enrollment, OR
   • fever above 38.2° C, AND clinical signs of infection within 48 hours of enrollment

4. Participant has received prior disease-directed therapy other than 1st line therapy with methotrexate, an anthracycline, and a platinum and local control surgery

   • Regimen B only - okay to have undergone initial pulmonary metastasectomy

5. Pregnant or breastfeeding

6. Presence of any condition that, in the opinion of the investigator, would prohibit the participant from undergoing treatment under this protocol

Individual participant de-identified datasets containing the variables analyzed in the published article will be made available (related to the study primary or secondary objectives contained in the publication). Supporting documents such as the protocol, statistical analyses plan, and informed consent are available through the CTG website for the specific study. Data used to generate the published article will be made available at the time of article publication. Investigators who seek access to individual level de-identified data will contact the computing team in the Department of Biostatistics (ClinTrialDataRequest@stjude.org) who will respond to the data request.

Data will be made available at the time of article publication.

Data will be provided to researchers following a formal request with the following information: full name of requestor, affiliation, data set requested, and timing of when data is needed. As an informational point, the lead statistician and study principal investigator will be informed that primary results datasets have been requested.