POETIC Plerixafor as a Chemosensitizing Agent for Relapsed Acute Leukemia and MDS in Pediatric Patients

Inclusion Criteria: * \>= 3 years of age and \<30 years old at study entry * diagnosis of relapsed/refractory AML, ALL, secondary AML/MDS, or acute leukemia of ambiguous lineage and meet the following criteria: * AML/MDS or leukemia with ambiguous lineage must have \>5% blast in bone marrow * ALL must have an M3 marrow * ALL and AML must not have CNS disease * patients must have fully recovered from the acute toxic effects of all prior chemotherapy, immunotherapy or radiotherapy prior to entering study * Karnofsky score \>50% for patients \>16 years of age and Lansky \>50% for patients \<= 16 years of age * adequate renal and hepatic function as defined in protocol * adequate cardiac function as defined in protocol Exclusion Criteria: * ALL and AML patients with CNS disease * Absolute blast count greater than 50,000/mcl * Systemic fungal, bacterial, viral or other infection without improvement despite appropriate antibiotics or other treatment * Significant concurrent disease, illness, psychiatric disorder or social issue that would compromise patient safety or compliance * Patients who have second cancer, not including secondary AML * Patients who are pregnant