Fixed Dose Flavonoid Isoquercetin on Thrombo-Inflammatory Biomarkers in Subjects With Stable Sickle Cell Disease

Background: Sickle cell disease (SCD) is an inherited hemoglobin disorder. People with SCD have an increased chance for getting blood clots. Researchers want to see if a dietary supplement called isoquercetin can decrease levels of inflammation and blood clotting in people with SCD. Objective: To see how isoquercetin works in people with SCD. Eligibility: Adults age 18-70 years old who have SCD and are in a steady-state (have not experienced a pain crisis in the last 60 days and, if taking hydroxyurea, have not had a dose change in the past 90 days). Design: Participants will be screened with a physical exam, medical history, medicine review, and blood tests. Participants will be put in 1 of 2 treatment groups. They will take 4 capsules of isoquercetin or placebo all at once, by mouth, every day for 4 weeks. They will get a pill dispenser and keep a medicine diary. Participants may have an optional near infrared spectroscopy (NIRS) test to measure how treatment affects blood flow. In this test, probes will be placed on the skin to measure tissue oxygen level and blood flow. A blood pressure cuff placed on the arm will be filled with air briefly to restrict the blood flow in the arm (for up to 5 minutes) and then released. Participants may also be asked to breathe at a certain rate or hold their breath for as long as they can during measurements. Participants will take folic acid once a day. Participants will have an end-of-study drug visit. They will discuss any side effects and repeat some of the screening tests. They may have an additional optional NIRS test. About a month after the end of study drug visit, participants will be contacted by phone to see if they have any side effects. Those who do may have a follow-up visit. At this visit, they may have additional blood tests performed. Participation will last from 8 to 12 weeks.