A Phase 1/2 Open Label, Basket Study to Assess the Safety, Tolerability and Anti-Tumor Activity of Afamitresgene Autoleucel in Pediatric Subjects With MAGE-A4 Positive Tumors

Inclusion Criteria: * Subject has histologically confirmed diagnosis of any one of the following cancers: (A) Synovial Sarcoma (SS), (B) MPNST, (C) Neuroblastoma, or (D) Osteosarcoma (OS). * Age: (A) Synovial Sarcoma: 2 to 17 years (B) MPNST, Neuroblastoma and Osteosarcoma: 2 to 21 years * Body weight ≥ 10 kg * Must have previously received a systemic chemotherapy * Measurable disease prior to lymphodepletion according to RECIST v1.1 (or INCR, 2017 Neuroblastoma only). * HLA-A\*02 positive * Tumor shows MAGE-A4 expression confirmed by central laboratory. * Performance Status: (A) Subjects ≥16: Eastern Cooperative Oncology Group (ECOG) 0 or 1 (B) Subjects 2 to 16: Lansky score ≥ 80 • Subject has anticipated life expectancy of greater than 3 months in the opinion of the investigator. Exclusion Criteria: * Positive for HLA-A\*02:05 in either allele; or any A\*02 having same protein sequence as HLA-A\*02:05 * History of allergic reactions attributed to compounds of similar chemical or biologic composition to fludarabine, cyclophosphamide. * History of autoimmune or immune mediated disease * Known central nervous system (CNS) metastases. * Other prior malignancy that is not considered by the Investigator to be in complete remission * Clinically significant cardiovascular disease * Active infection with human immunodeficiency virus, hepatitis B virus, hepatitis C virus, or human T cell leukemia virus * Pregnant or breastfeeding * Experiencing ongoing rapid disease progression that in the opinion of the Investigator significantly increases the subjects risk associated with treatment.