Fludarabine is a chemotherapy drug used extensively in bone marrow transplantation. The goal of this study is to determine what causes some children to have different drug concentrations of fludarabine in their bodies and if drug levels are related to whether or not a child experiences severe side-effects during their bone marrow transplant. The hypothesis is that clinical and genetic factors cause changes in fludarabine drug levels in pediatric bone marrow transplant patients and that high levels may cause severe side-effects.

Fludarabine is a nucleoside analog with potent antitumor and immunosuppressive properties used in conditioning regimens of pediatric allogeneic hematopoietic cell transplantation (alloHCT) to promote stem cell engraftment.

This is a single-center, pharmacokinetic-pharmacodynamic (PK-PD) study investigating the clinical pharmacology of fludarabine in 45 children undergoing alloHCT at University of California, San Francisco Benioff Children's Hospital.

Patients would receive fludarabine regardless of whether or not they decide to consent to PK sampling.

Fludarabine doses will not be adjusted based on PK data.

We will apply the combination of a D-optimality-based limited sampling strategy and population PK methodologies to determine specific factors influencing fludarabine exposure in pediatric alloHCT recipients and identify exposure-response relationships.

Subjects will undergo PK sampling of plasma (f-ara-a) and intracellular (f-ara-ATP) drug concentrations over the duration of fludarabine therapy (3 to 5 days).

To evaluate sources of variability impacting fludarabine exposure clinical data will be obtained from the patient's medical chart on each day of PK sampling.

A single blood draw for the collection of DNA and genotyping of single nucleotide polymorphisms of genes involved in fludarabine activation, transport or elimination will occur in all patients.

To assess exposure-response relationships neutrophil engraftment, treatment-related toxicity, and survival data will be collected through day 100 post-transplant.

Inclusion Criteria:

• Children 0-17 years of age
• Undergoing alloHCT for the treatment of malignant or nonmalignant disorder
• Receiving fludarabine-based preparative regimen

Exclusion Criteria:

• Any child 7-17 years of age unwilling to provide assent
• Parent or guardian unwilling to provide written consent

The target population for the proposed study includes children 0-17 years of age undergoing alloHCT for the treatment of malignant and nonmalignant disorders.

Patients receiving fludarabine over 3 to 5 days are eligible to participate.

All patients enrolled in this study will undergo PK sampling on the inpatient pediatric BMT unit at UCSF Benioff Children's Hospital. The proposed research will not study any patients receiving fludarabine in a clinic or any other out-patient setting.