131I-apamistamab-based Conditioning for Hematopoietic Stem Cell Transplant (HSCT) in Advanced Sickle Cell Disease (SCD)

The purpose of this study is to find the smallest amount of the 131 I-apamistamab needed for preparing patients with severe sickle cell disease (SCD) for a bone marrow transplant. This is the first time 131 I-apamistamab is being used for advanced Sickle Cell Disease (SCD) in the setting of allogeneic stem cell transplant. 131 I-apamistamab is an investigational product. This means that 131 I-apamistamab has not been approved by the Food and Drug Administration (FDA) for medical use in patients. The study treatment that is given before the transplant is called the conditioning regimen. In this study, the investigators are adding a drug called 131 I-apamistamab instead of the conditioning regimen typically given before a stem cell transplant.

Inclusion Criteria: * Availability of an HLA-matched sibling donor * Patients with sickle cell anemia (Hb SS, Sβ0 thalassemia or severe SC) who are 12 - 50 years of age inclusive AND who have 1 or more of the following: 1. Clinically significant neurologic event (stroke) or any neurological deficit lasting at least 24 hours. Stroke will be defined as a clinically significant neurologic event that is accompanied by an infarct on cerebral MRI or cerebral arteriopathy requiring chronic transfusion therapy. 2. History of two or more episodes of ACS in the 2-year period preceding enrollment despite supportive care measures (i.e. asthma therapy and/or hydroxyurea). 3. History of three or more severe vaso-occlusive pain crises per year in the 2-year period preceding enrollment despite the institution of supportive care measures (i.e. a pain management plan and/or treatment with hydroxyurea). 4. Administration of regular RBC transfusion therapy, defined as receiving 8 or more transfusions per year for 1 year or more to prevent vaso-occlusive clinical complications (i.e. pain, stroke, and ACS) 5. An echocardiographic finding of tricuspid valve regurgitant jet (TRJ) velocity \> or equal to 2.7 m/sec or pulmonary hypertension diagnosed by right heart catherization. 6. Sickle hepatopathy defined as EITHER ferritin \>1000mcg/L OR direct bilirubin \>0.4mg/dl but \