Selinexor With Fludarabine and Cytarabine for Treatment of Refractory or Relapsed Leukemia or Myelodysplastic Syndrome

The purpose of this study is to test the safety of selinexor (KPT-330) and to find the highest dose of selinexor (KPT-330) that can be given safely when it is combined with two chemotherapy drugs (fludarabine and cytarabine). This study will be done in two parts: Phase I and Phase II. The goal of Phase I is to find the highest tolerable dose of selinexor (KPT-330) that we can give to patients with leukemia or MDS, when it is combined with fludarabine and cytarabine. The goal of the subsequent Phase II portion of the study (insert NCT ID of SELHEM-2) is to give the highest dose of selinexor (KPT-330) in combination with fludarabine/cytarabine that was found in Phase I to be safe for children with leukemia or MDS. The investigators will examine the effect of this combination treatment. PRIMARY OBJECTIVE: * Determine a tolerable combination of selinexor, fludarabine, and cytarabine in pediatric patients with relapsed or refractory hematologic malignancies included acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), mixed phenotype acute leukemia (MPAL) and myelodysplastic syndrome (MDS). SECONDARY OBJECTIVES: * To characterize the pharmacokinetics of selinexor, when administered in tablet form, after the first dose and at steady-state, as well as in combination with fludarabine and cytarabine * To estimate the overall response rate of selinexor given with fludarabine and cytarabine in patients with relapsed or refractory hematologic malignancies