A Stratified Sickle Event Randomized Trial (ASSERT)

Inclusion Criteria: * 16 to 65 years of age (inclusive) * Male or female (not capable of becoming pregnant or using appropriate birth control) * Medical history of sickle cell disease * Have a history of at least two or more acute sickle-related painful crises requiring a visit to a medical facility within the preceding 12 months Exclusion Criteria: * Hemoglobin \<4 or \>11 g/dL * On a chronic transfusion program * Has significant active and poorly controlled (unstable) cardiovascular, neurologic, endocrine, hepatic, or renal disorders clearly unrelated to sickle cell disease