Adenosine 2A Agonist Lexiscan in Children and Adults With Sickle Cell Disease

* In this research study we are looking for the highest dose of Lexiscan that can be given safely to patients with SCD. There are 4 stages to this study. Each stage will look for the highest dose that can be given safely in the following situations: Stage 1: Lexiscan will be given through a 12 hours infusion to adults with SCD who are not having a pain crisis. Stage 2: Lexiscan will be given through a 24 hour infusion to adults with SCD who are not having a pain crisis. Stage 2b: Lexiscan will be given through a 48 hour infusion to adults with SCD who are not having a pain crisis. Stage 3: Lexiscan will be given through a 24 hour infusion to adults with SCD who are having a pain crisis. Stage 4: Lexiscan will be given through a 24 hour infusion to children with SCD who are having a pain crisis. Stages 1-3 are now complete and closed to accrual. The study is now open to children ages 10-17 with SCD pain crisis (stage 4) only. * When participants sign the consent form, they will be told what stage they will join. * Participants in Stages 1, 2, and 2b will be given an infusion of the study drug at the time when they do not have a pain crisis. The infusion for Stage 1 participants will be 12 hours long, followed by a 6-hour observation period. The infusion for Stage 2 will be 24 hours long, followed by a 6-hour observation period. The infusion for Stage 2b will be 48 hours long, followed by a 6-hour observation period. * Participants in Stages 3 and 4 will be given one infusion of the study drug when they are admitted to the hospital for a pain crisis. The infusion will be 24 hours long, followed by a 6-hour observation period. During the infusion, they will receive standard treatment for their pain crisis. * Before the infusion the following procedures will be performed: Pulmonary function test (optional, Stage 1 only), blood test and vital signs. * During the infusion the following procedures will be performed: heart rate and amount of oxygen in the blood will be monitored continuously, blood tests and blood pressure. * During the observation period immediately following the infusion the following procedures will be performed: heart rate and amount of oxygen in the blood will be monitored continuously, blood tests, blood pressure and Pulmonary Function test (optional, Stage 1 only).