Project Summary/Abstract Many pediatric and young adult patients require an allogeneic hematopoietic cell transplant (HCT) for treatment of deadly diseases besides cancer. Non-malignant disorders which are often treated with allogeneic HCT include severe inborn errors of immunity, inborn errors of metabolism, marrow failure disorders, and hematologic conditions such as thalassemia and sickle cell disease. Reduced intensity conditioning (RIC) and reduced toxicity conditioning (RTC) regimens are commonly used for patients with non-malignant disorders. RIC and RTC regimens usually contain alemtuzumab, a humanized monoclonal antibody that is directed against CD52. CD52 is expressed by the majority of lymphocytes and some other white blood cells. Alemtuzumab is included in RIC and RTC regimens for 2 main reasons. Alemtuzumab prevents graft rejection by depleting the recipient of lymphocytes including T cells which may recognize the allogeneic graft as foreign. Alemtuzumab also reduces graft versus host disease because alemtuzumab may linger at lytic levels through the administration of the hematopoietic stem cell graft and result in lymphocyte depletion of the graft. Adequate prevention of graft failure and graft versus host disease is essential to ensure successful outcomes and patient survival. We do not know the best way to dose alemtuzumab. We have previously reported that optimal peri- transplant alemtuzumab concentrations of 0.2-0.6mcg/mL on the day of graft administration (Day 0) reduce the risks of graft failure and graft versus host disease. Levels within this range also optimize early immune recovery. It is important to be able to dose alemtuzumab so that the majority of patients achieve Day 0 concentrations within this ideal target concentration window. We have performed detailed alemtuzumab pharmacokinetic (PK) studies and developed a population PK model to allow a Precision Dosing strategy to be developed. We applied this Precision Dosing strategy in a pilot feasibility study of 12 patients with good results. We are requesting funding in this current application to support a larger phase II study of Precision Alemtuzumab Dosing in pediatric and young adult patients with non-malignant disorders. We will evaluate the success of our approach in targeting patients to the ideal therapeutic concentration window of 0.2-0.6mcg/mL on Day 0 and the impact on the clinical outcomes of immune reconstitution, graft failure, and graft versus host disease.