This study is designed to evaluate the safety and effectiveness of CART123 cells either alone or when combined with ruxolitinib in pediatric and young adult subjects with relapsed or refractory AML. Subjects will be enrolled into one of two treatment cohorts: subjects who will receive CART123 alone (Cohort A) or subjects who will receive CART123 in combination with ruxolitinib (Cohort B).

This trial will be conducted at the Children's Hospital of Philadelphia with CART123 infusions occurring in an outpatient setting with close follow-up. Approximately 18 subjects will be treated on Cohort A and 12 patients treated on Cohort B.

Cohort A will consist of a dose escalation of CART123 cells administered intravenously on Day 0 after lymphodepleting chemotherapy. Subjects enrolled on Cohort A will receive a standard regimen of fludarabine (30 mg/m2/day x 4 days) and cyclophosphamide (500 mg/m2/day x 2 days). There is the most experience with the use of this regimen in facilitating adoptive immunotherapy in completed and ongoing pediatric CART trials.

Cohort B will consist of a fixed dose of CART123 cells (2x10^6 CART123 cells/kg) to be administered intravenously on Day 0 in combination with age and BSA-based dosing of ruxolitinib given orally from the start of lymphodepleting chemotherapy until Day -2 and again from Day+7 to Day+13. Subjects enrolled on Cohort B will receive the regimen fludarabine (30 mg/m2/day x 4 days) and cyclophosphamide (1000 mg/m2/day x 3 days). This regimen of enhanced lymphodepletion has been employed with other investigational CAR T-cell therapies with the goal of improving CAR T-cell expansion. There is no dose escalation of CART123 cells or ruxolitinib on this cohort, but a dose de-escalation of ruxolitinib is planned in the event of unacceptable toxicity defined by dose de-escalation rules.

Inclusion Criteria:

• 1. Age at time of consent: Cohort A: 0-29 years. Cohort B: 1-29 years (Note: the first subject at each dose level of Cohort B must be ≥12 years old)

• 2. Subjects with AML in second or greater relapse, post-transplant relapse, or with chemotherapy-refractory disease. Specifically:

  1. Second or greater relapse defined as bone marrow flow cytometric confirmation of myeloid leukemia of at least 0.1% or development of extramedullary disease by imaging or biopsy after second documented complete remission; OR
  2. Any detectable disease post-allogeneic transplant with bone marrow flow cytometric confirmation of myeloid leukemia of at least 0.1% or development of extramedullary disease by imaging or biopsy; OR
  3. Refractory disease, defined as: Persistent bone marrow involvement with ≥0.1% disease by flow cytometry or persistent extramedullary disease by imaging or biopsy after two courses of induction chemotherapy for patients at initial presentation, ≥ 0.1% bone marrow disease by flow cytometry or persistent extramedullary disease by imaging or biopsy after one course of induction chemotherapy for patients who have relapsed after previously achieving a CR , and ≥ 0.1% bone marrow disease by flow cytometry or persistent extramedullary disease by imaging or biopsy after one course of AML-directed chemotherapy for those with myeloid lineage switch.

• 3. Subjects must have an identified stem cell donor with the ability to proceed rapidly to transplant following CART123 treatment if indicated.

• 4. Adequate organ function defined as:

  1. Serum creatinine based on age/gender.
  2. Adequate liver function: ALT ≤ 500 U/L, Bilirubin ≤3x the upper limit of normal, and ALT and/or bilirubin results that exceed this range are acceptable if, in the opinion of the physician-investigator (or as confirmed by liver biopsy), the abnormalities are directly related to AML infiltration of the liver.
  3. Must have a minimum level of pulmonary reserve defined as ≤Grade 1 dyspnea and \