Inclusion Criteria:

• Be 18 years of age or older at the time of informed consent;
• Have a diagnosis of homozygous sickle cell anemia or sickle cell-beta° thalassemia;
• Have had 2 - 10 documented pain crises in the past year (pain crises are defined as visits to a medical clinic, Emergency Department or hospital, being bedridden and requiring constant analgesia at home for at least three days, or having a three-day interruption of life's activities [i.e., school, work, planned leisure activity] because of pain);
• If female and of child bearing potential, have a negative serum or urine pregnancy test and be using an effective birth-control method with a history of reliability for the individual patient (use of mifepristone is not allowed);
• Be properly informed of the nature and risks of the clinical investigation, be willing and able to comply with all clinical investigation-related procedures and assessments, and sign an Institutional Review Board (IRB) approved Informed Consent Form prior to entering the clinical investigation.

Exclusion Criteria:

• Have a history of abnormal bleeding, stroke, moya moya vascular malformations, or any other contraindication to anticoagulation;
• Be currently taking anticoagulant or thrombolytic medication;
• Be currently taking an endothelin receptor antagonist, e.g., bosentan (Tracleer®);
• Have a known sensitivity or allergy to heparin or related drugs;
• Have a history of thrombocytopenia (platelet count < 100 x 103/mm3) induced by heparin or related drugs;
• Have had fewer than 2 documented pain crises in the past year;
• Have had a pain crisis within one month of screening or randomization;
• If currently on or recently discontinued hydroxyurea treatment, have initiated or discontinued treatment or changed regimen within the past 6 months;
• Have had a transfusion within last 120 days or have HbA% > 15% from prior transfusion;
• Creatinine levels > 1.53 mg/dL (135 umol/L);
• ALT levels ≥ 3 times normal;
• Platelet count < 100 x 103/mm3;
• INR > 2.0;
• Be unable to tolerate oral medications;
• Have unreliable venous access;
• Be noncompliant with regular care;
• Have a positive pregnancy test, be currently lactating, or be trying to become pregnant;
• Have participation in an investigational drug or medical device study within previous 30 days;
• Have any other condition or circumstance that in the opinion of the Investigator makes the patient a poor candidate for participation in the study.