Inclusion Criteria:

- Diagnosis of SCD or sickle beta thalassemia (excluding Hemoglobin C)

- Between 12 and 60 years of age, inclusive

- At least one episode of a SCD-related crisis or complication (e.g., vaso-occlusive
crisis, acute chest syndrome, priapism) per year for an average of 3 years or one
episode of acute chest syndrome over the prior 5 years

- Screening (untransfused) HbF level >/= 2% as analyzed by a central laboratory

- If receiving hydroxyurea therapy, must be receiving a stable dose for at least 6
months

- Able and willing to give informed consent

- If female, must have a negative serum pregnancy test within 7 days of dosing

- If female, must not be of childbearing potential defined as post-menopausal by at
least 2 years or surgically sterile, or must agree to use a medically accepted form
of contraception throughout the study

- If the sexual partner of a male subject is a WCBP, she must agree to use a medically
accepted form of birth control for themselves or their partner throughout the study

- In the view of the Investigator, able to comply with necessary study procedures

Exclusion Criteria:

- Red blood cell (RBC) transfusion within 3 months prior to beginning study medication

- Participation in a regular blood transfusion program

- More than 4 hospitalizations for acute sickle cell-related events in the previous 12
months

- An acute vaso-occlusive event within 3 weeks prior to receiving first dose of study
medication

- Pulmonary hypertension requiring oxygen

- QTc > 450 msec on screening

- Alanine transaminase (ALT) > 3X upper limit of normal (ULN)

- Creatinine phosphokinase (CPK) > 20% above the ULN

- Serum creatinine >1.2 mg/dL

- An acute illness (e.g., febrile, gastrointestinal [GI], respiratory) within 72 hours
prior to receiving first dose of study medication

- History of syncope, clinically significant dysrhythmias or resuscitation from sudden
death

- Chronic opiate use which, in the view of the Investigator, could confound evaluation
of an investigational drug

- Current abuse of alcohol or drugs

- Received another investigational agent within 4 weeks, or 5 half-lives, whichever is
longer, prior to administration of study medication

- Currently pregnant or breast feeding a child

- Known infection with HIV-1

- Infection with hepatitis B or hepatitis C such that patients are currently on therapy
or will be placed on therapy during the trial