This pilot trial compares drug exposure levels using a new method for dosing vincristine in infants and young children compared to the standard dosing method based on body surface area (BSA) in older children. Vincristine is an anticancer drug used to a variety of childhood cancers. The doses anticancer drugs in children must be adjusted based on the size of the child because children vary significantly in size (height, weight, and BSA) and ability to metabolize drugs from infancy to adolescence. The dose of most anticancer drugs is adjusted to BSA, which is calculated from a patients weight and height. However, infants and young children have more severe side effects if the BSA is used to calculate their dose, so new dosing models have to be made to safely give anticancer drugs to the youngest patients. This new method uses a BSA-banded approach to determine the dose. Collecting blood samples before and after a dose of the drug will help researchers determine whether this new vincristine dosing method results in equivalent drug levels in the blood over time in infants and young children compared to older children.

Severe Aplastic Anemia (SAA) is a rare condition in which the body stops producing enough new
blood cells. SAA can be cured with immune suppressive therapy or a bone marrow transplant.
Regular treatment for patients with aplastic anemia who have a matched sibling (brother or
sister), or family donor is a bone marrow transplant. Patients without a matched family donor
normally are treated with immune suppressive therapy (IST). Match unrelated donor (URD) bone
marrow transplant (BMT) is used as a secondary treatment in patients who did not get better
with IST, had their disease come back, or a new worse disease replaced it (like leukemia).

This trial will compare time from randomization to failure of treatment or death from any
cause of IST versus URD BMT when used as initial therapy to treat SAA.

The trial will also assess whether health-related quality of life and early markers of
fertility differ between those randomized to URD BMT or IST, as well as assess the presence
of marrow failure-related genes and presence of gene mutations associated with MDS or
leukemia and the change in gene signatures after treatment in both study arms.

This study treatment does not include any investigational drugs. The medicines and procedures
in this study are standard for treatment of SAA.