A Pediatric Study of a Plerixafor Containing Regimen In Second Allogeneic Stem Cell Transplantation

This study will determine the following objectives: 1. To determine the dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of plerixafor in combination with fludarabine, thiotepa, and melphalan as a conditioning regimen for children and young adults undergoing a second allogeneic stem cell transplant (SCT) procedure. 2. The study determines the secondary objectives: * To describe the pharmacokinetic properties of plerixafor in this study population * To estimate the cumulative incidence of relapse and overall survival in study participants at one year after this second transplant procedure 3. Other exploratory objectives include: * To study the correlation between the pharmacokinetic properties of plerixafor and key regulatory and effector T cell populations in blood before and after exposure to plerixafor. * To evaluate the content of leukemia cells in bone marrow and in blood before and after exposure to plerixafor * To evaluate key regulatory and effector T cell populations in bone marrow and in blood before and after exposure to plerixafor