Trial Testing Safety of IL-21 NK Cells for Induction of R/R AML

Inclusion Criteria: Patient Inclusion Criteria for Induction Phase * Primary Relapsed AML including * Relapsed AML after allogeneic stem cells * Isolated CNS or extramedullary disease (Note: a response monitoring plan must be developed a priori for subjects with extramedullary disease) * 1-3 prior lines of therapy which includes chemotherapy, hypomethylating agents, venetoclax or targeted therapy. * Patient weight ≥ 42 kg * Performance status: Karnofsky or Lansky Performance Scale (PS) greater or equal to 70, or, ECOG score 0-2. * Renal function: Serum creatinine ≤ 2 mg/dl and/or creatinine clearance greater or equal than 40 cc/min. * Pulmonary function: FEV1, FVC and DLCO ≥ 50% of expected, corrected for hemoglobin. * Liver function: Total bilirubin ≤ 2 mg/dl or ≤ 2.5 x ULN for age (unless Gilbert's syndrome) and SGPT (ALT) ≤ 2.5 x ULN for age. * Cardiac function: left ventricular ejection fraction ≥ 40%. * CNS: Patients with seizure disorder are eligible if seizures well controlled. * Negative serum test to rule out pregnancy within 2 weeks prior to enrollment in females of childbearing potential (non-childbearing potential defined as premenarchal, greater than one year post-menopausal, or surgically sterilized). * Sexually active males and females of childbearing potential must agree to use a form of contraception considered effective and medically acceptable by the Investigator. * Ability to understand and willingness to sign the written informed consent document. * Negative serology for human immunodeficiency virus (HIV). * Patients on hydrocortisone for adrenal insufficiency or on inhaled or topical steroids are eligible. Maintenance Phase: Patients that complete induction therapy and who achieve a CR/CRi/PR within the designated follow-up period and who are ineligible, unable or unwilling to undergo HSCT; these patients will not receive fludarabine or cytarabine. Exclusion Criteria for Induction Phase: * Investigational therapies in the 3 weeks prior to beginning treatment on this protocol. * Patients receiving any concurrent therapy including but not limited to chemotherapy, targeted therapy, radiation therapy, or immunotherapy for R/R AML. * Any comorbidities that in the opinion of the investigator will preclude receiving fludarabine or cytarabine. * Uncontrolled infection, defined as an infection which has not resolved spontaneously or does not show evidence of significant resolution after initiating appropriate therapy. Asymptomatic viremia such as CMV, HPV, BK virus, HCV, HBV etc. is NOT considered as an exclusion criteria. * Uncontrolled arrhythmias or uncontrolled symptomatic cardiac disease. * Active GVHD * Prednisone dose is \> 20 mg/day or \>0.25mg/kg, whichever is higher will be excluded. * Patients with donor-specific antibodies with MFI \> 5000 will be ineligible * Maintenance Phase: Patients must continue to meet exclusion criteria as defined in Section 4.4.