RecruitingPhase 1Drug trialStudy to Evaluate the Safety and Tolerability of Escalating Doses of Fostamatinib in Subjects With Stable Sickle Cell DiseaseBackground: Sickle cell disease (SCD) is a genetic disease that causes the body to produce abnormal ( sickled ) red blood cells. SCD can cause anemia and life-threatening complications in the lungs, heart, kidney, and nerves. People with SCD are also at increased risk of forming blood clots in the veins and lungs, but the standard treatments for these clots can cause increased bleeding in people with SCD. Better treatments are needed. Objective: To test a drug (fostamatinib) in people with SCD. Eligibility: People aged 18 to 65 with SCD. Design: Participants will have 6 clinic visits over 12 weeks. Each visit will be 2 to 3 hours. Participants will be screened. They will have a physical exam with blood tests. They will tell the researchers about the medications they take. Fostamatinib is a tablet taken by mouth. Participants will take the drug at home, twice a day, for up to 6 weeks. Participants will have a clinic visit every 2 weeks while they are taking the drug. At each visit they will have a physical exam with blood tests. They will talk about any side effects the drug may be causing. If they are tolerating the drug well after the first 2 weeks, they may begin taking a higher dose. Participants will have a final visit 4 weeks after they stop taking the drug. They will have a physical exam and blood tests; they will be checked for any side effects of the drug.