Evaluating Efficacy of B7-H3-CAR T Cells Administered at the End of Upfront Map Chemotherapy in Patients With Newly Diagnosed High-Risk Osteosarcoma

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