The purpose of this study is to assess the safety, tolerability, pharmacokinetic, and pharmacodynamic effects of GBT440 compared with placebo in healthy subjects and subjects with sickle cell disease (SCD).

Inclusion Criteria:

• Healthy male or female of non-child bearing potential; 18 to 55 years old; are non-smokers and have not used nicotine products within 3 months prior to screening.
• Male or female, 18 to 60 years old, with sickle cell disease (hemoglobin SS, HbS/β0thalassemia, HbS/β+thalassemia, or HbSC) not requiring chronic blood transfusion therapy; without hospitalization in 30 days before screening or receiving blood transfusion within 30 days before screening; subjects are allowed concomitant use of hydroxyurea if the dose has been stable for the 3 months prior to screening.

Exclusion Criteria:

• 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.
• Subjects who consume more than 14 (female subjects) or 21 (male subjects) units of alcohol a week.
• Subjects who have used any investigational product in any clinical trial within 30 days of screening
• Subjects with sickle cell disease who smoke >10 cigarettes per day; have hemoglobin level <6 g/dL or >10.4 g/dL (> ULN (appropriately corrected for gender) for Cohort 15) at screening; have aspartate aminotransferase (AST) >4x upper limit of normal or alanine aminotransferase (ALT), or alkaline phosphatase (ALK) >3x upper limit of normal reference range (ULN) at screening; have moderate or severe renal dysfunction