Additional Study Information: The main purpose of this research study is to determine the effect of the study drug compared to placebo in increasing red blood cell counts and reducing the severity of symptoms of Sickle Cell Disease (SCD). A placebo is a dummy drug: it looks like the study drug but contains no medicine. Subjects being asked to take part in this research study have SCD. SCD is an inherited blood disorder. People with SCD have abnormal hemoglobin (the part of your red blood cells that carries oxygen), called hemoglobin S or sickle hemoglobin in their red blood cells. Red blood cells that contain normal hemoglobin are disc shaped (like a doughnut without a hole). This shape allows the cells to be flexible so that they can move through large and small blood vessels to deliver oxygen. Sickle hemoglobin is not like normal hemoglobin. It can form stiff rods within the red cell, changing it into a crescent or sickle shape. Sickle-shaped cells are not flexible and can stick to vessel walls, causing a blockage that slows or stops the flow of blood. When this happens, oxygen cant reach nearby tissues.