RESET-Myositis: An Open-Label Study to Evaluate the Safety and Efficacy of CABA-201 in Subjects With Active Idiopathic Inflammatory Myopathy or Juvenile Idiopathic Inflammatory Myopathy

Adult Cohorts Inclusion Criteria: * Age ≥18 and ≤75 * A clinical diagnosis of IIM, based on the 2017 The European League Against Rheumatism/American College of Rheumatology classification criteria * Diagnosis of DM, ASyS, or IMNM * Evidence of active disease, despite prior or current treatment with standard of care treatments, as defined by the presence of elevated creatine kinase (CK), DM rash, or active disease on muscle biopsy, magnetic resonance imaging (MRI), or electromyography * Presence of muscle weakness Other protocol-defined criteria apply. Exclusion Criteria: * Contraindication to leukapheresis * History of anaphylactic or severe systemic reaction to fludarabine, cyclophosphamide or any of their metabolites * Active infection requiring medical intervention at screening * Current symptoms of severe, progressive, or uncontrolled renal, hepatic, hematological, gastrointestinal, pulmonary, psychiatric, cardiac, neurological, or cerebral disease, including severe and uncontrolled infections, such as sepsis and opportunistic infections * Concomitant medical conditions that, in the opinion of the investigator, might place the subject at unacceptable risk for participation in this study, interfere with the assessment of the effects or safety of the investigational product or with the study procedures * Significant lung or cardiac impairment * Previous CAR T cell therapy * Prior solid organ (heart, liver, kidney, lung) transplant or hematopoietic cell transplant Other protocol-defined criteria apply. Juvenile Cohort Inclusion Criteria: * Age ≥6 and ≤17 years at enrollment * A clinical diagnosis of IIM, based on the 2017 The European League Against Rheumatism/American College of Rheumatology classification criteria * Evidence of active disease, despite prior or current treatment with standard of care treatments, as defined by the presence of elevated muscle enzymes, DM rash, or active disease on muscle biopsy, magnetic resonance imaging (MRI), or electromyography Other protocol-defined criteria apply. Exclusion Criteria: * Contraindication to leukapheresis * History of anaphylactic or severe systemic reaction to fludarabine, cyclophosphamide or any of their metabolites * Active infection requiring medical intervention at screening * Current symptoms of severe, progressive, or uncontrolled renal, hepatic, hematological, gastrointestinal, pulmonary, psychiatric, cardiac, neurological, or cerebral disease, including severe and uncontrolled infections, such as sepsis and opportunistic infections. * Concomitant medical conditions that, in the opinion of the investigator, might place the subject at unacceptable risk for participation in this study, interfere with the assessment of the effects or safety of the investigational product or with the study procedures * Significant lung or cardiac impairment * Previous CAR T cell therapy * Prior solid organ (heart, liver, kidney, lung) transplant or hematopoietic cell transplant Other protocol-defined criteria apply.