A Study to Find the Highest Dose of Imetelstat in Combination With Fludarabine and Cytarabine for Patients With AML, MDS or JMML That Has Come Back or Does Not Respond to Therapy

PRIMARY OBJECTIVE: I. To estimate the maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D) of imetelstat administered in combination with fludarabine and cytarabine to children with second or greater relapse of acute myeloid leukemia or first or greater relapse of myelodysplastic syndrome (MDS) or juvenile myelomonocytic leukemia (JMML). SECONDARY OBJECTIVES: I. To define and describe the toxicities of imetelstat administered on this schedule in combination with fludarabine and cytarabine in patients with refractory and/or relapsed AML, MDS, or JMML. II. To characterize the pharmacokinetics of imetelstat in combination with fludarabine and cytarabine in patients with refractory and/or relapsed AML, MDS, or JMML. III. To describe the antileukemic activity of imetelstat (CR \[complete remission\]/PR \[partial response\]/CRi \[complete remission with incomplete blood count recovery\] and rates of minimal residual disease \[MRD\] negative response after up to two cycles of therapy) in combination with fludarabine and cytarabine within the limits of a phase 1 study. IV. To estimate the overall survival (OS) of children with second or greater relapse of AML or first or greater relapse of MDS or JMML treated with imetelstat administered in combination with fludarabine and cytarabine. EXPLORATORY OBJECTIVES; I. To conduct pharmacodynamics studies of imetelstat in combination with fludarabine and cytarabine in patients with refractory and/or relapsed AML, MDS, or JMML. II. To analyze telomerase activity in peripheral blood mononuclear cells. III. To evaluate baseline and change of cytogenetic abnormalities after treatment with imetelstat in combination with fludarabine and cytarabine. IV. To evaluate baseline mutational status and change of mutational status after treatment with imetelstat in combination with fludarabine and cytarabine. OUTLINE: This is a dose-escalation study of imetelstat. Patients receive imetelstat intravenously (IV) over 2 hours on days 1 and 8, fludarabine IV over 1 hour on days 2-6, and cytarabine IV over 1-3 hours on days 2-6 of each cycle. Patients also receive cytarabine intrathecally (IT) alone or with methotrexate IT, and hydrocortisone IT at the provider's discretion. Patients may also receive leucovorin calcium IV or orally (PO) 24 and 30 hours after each IT triples dose. Treatment repeats every 28 days for up to 2 cycles in the absence of disease progression or unacceptable toxicity. Patients undergo echocardiography (ECHO), bone marrow biopsy and/or aspiration, blood sample collection, and lumbar puncture for cerebrospinal fluid (CSF) sample collection during screening and on the trial. After completion of study treatment, patients are followed up for 5 years.