Sirolimus (Rapamune ) for Relapse Prevention in People With Severe Aplastic Anemia Responsive to Immunosuppressive Therapy

Background: People with severe aplastic anemia (SAA) do not make enough red and white blood cells, and/or platelets. Their body's immune system stops the bone marrow from making these cells. Immunosuppression (IST) with anti-thymocyte globulin (ATG) and cyclosporine (CSA) leads to better blood counts. But when cyclosporine is stopped, the disease may return in 1 in 3 people. The drug sirolimus (SRL) may help by suppressing the immune system to prevent the return of aplastic anemia. Objective: To evaluate and compare the usefulness of SRL in preventing aplastic anemia from returning after cyclosporine is stopped, compared with stopping cyclosporine alone. Eligibility: 1. People ages 2 and older with SAA who: 2. Have responded to IST and continue to take cyclosporine 3. Are not taking drugs with hematologic effects Study Design: Participants will be screened with: • Medical history, physical exam, blood and urine tests, bone marrow biopsy and aspirate. Participants will be randomly assigned to a group. All will stop cyclosporine. Group 1 will take sirolimus by mouth at the same time each day for 12 weeks with close monitoring. Group 2 will not receive the study drug but will be monitored closely (Observation arm). Participants will have clinical tests for the first 3 months: * Weekly blood test Monthly fasting blood test * For group 1, measurements of sirolimus in the blood every 1-2 weeks Participants will have clinic visits at 3 months, 12 months, and annually for 5 years after the study starts. They may have another visit if their SAA returns.