An Extension Study to Further Evaluate the Safety, Tolerability of GBT440 in Patients With Sickle Cell Disease Who Participated in the Study GBT440-001

Inclusion Criteria: 1. Male or female subjects with SCD aged 18 to 60 years inclusive and \>50 kg who have participated in the GBT440-001 study. 2. Subjects, who if female and of child bearing potential, agree to continue to use highly effective methods of contraception prior to enrollment in this study and for 3 months after the last dose of study drug. 3. Subjects, who if male are willing to continue to use barrier methods of contraception, prior to enrollment in this study to 3 months after the last dose of study drug. Exclusion Criteria: 1. Subjects requiring chronic transfusion therapy. 2. Subjects receiving a blood transfusion within 30 days of enrollment in this study. 3. Female subjects who are pregnant, trying to become pregnant or lactating. 4. Subjects who have a clinically relevant history or presence of respiratory, gastrointestinal, renal, hepatic, haematological, lymphatic, neurological, cardiovascular, psychiatric, musculoskeletal, genitourinary, immunological, dermatological, connective tissue diseases or disorders, or additional risk factors for torsades de pointe (e.g., heart failure, hypokalemia, personal or family history of long QTc interval). 5. Subjects who have a significant infection or known inflammatory process on admission to this study. 6. Subjects who have acute gastrointestinal symptoms at the time of admission (e.g. nausea, vomiting, diarrhoea, heartburn).