Inclusion Criteria:  Male or female subjects with confirmed diagnosis of SCD, including HbS/S or HbS/βT0 genotype. Subjects who had at least 1 and no more than 6 vaso-occlusive crises (VOC) episodes reported within 12 months prior to screening. Body weight ≥50 kg and ≤100 kg at screening and baseline. Absolute reticulocyte count and percentage reticulocyte count >1.5 × ULN during screening. Subjects on concomitant hydroxyurea must be on a stable dose (mg/kg) for ≥3 months prior to screening Visit V1 Female subjects of childbearing potential, must have negative pregnancy, must have stopped breastfeeding as of first dose, and must either commit to true abstinence from heterosexual contact or must be willing to use adequate contraceptive precautions. Male subjects must practice true abstinence or agree to use a condom during sexual contact with a pregnant female or a female of childbearing potential  Exclusion Criteria:  Hb level <6.0 g/dl or >10.4 g/dl at screening Visit V1 Having received red blood cell (RBC) transfusion therapy within 4 weeks prior to screening, or ongoing or planned RBC transfusion therapy during the course of the study Low levels of Ferritin or transferrin saturation or total iron-binding capacity at screening Subjects being hospitalised for SCD-related events within 30 days before the screening visit Chronic liver disease or history of liver cirrhosis, and/or high levels of alanine aminotransferase or aspartate aminotransferase at baseline Low estimated glomerular filtration rate, and/or significant high urinary albumin/creatinine ratio at screening or on chronic dialysis. Newly diagnosed folate deficiency anaemia, which is considered clinically relevant by the Investigator at screening Any history or clinically important finding of cardiac or pulmonary disorders Known pulmonary hypertension as defined in the study protocol Any clinically relevant abnormal 12-lead electrocardiogram (ECG) finding prior to randomisation Family history of long-QT syndrome or sudden death without a preceding diagnosis of a condition that could be causative of sudden death Clinically significant bacterial, fungal, parasitic, or viral infection which requires therapy. Note: A subject meeting this criterion should delay screening and/or enrolment for a minimum of 2 weeks, or if excluded can be re-screened at maximum 2 times at a later time point. History of drug or alcohol abuse within 2 years prior to screening Pregnant or females currently breastfeeding. History or known concomitant solid tumours and/or haematological malignancies unless resolved in the ≥5 past years, except for basal or squamous cell carcinoma of the skin, carcinoma in situ of the cervix or breast, incidental histologic finding of prostate cancer Unable to take and absorb oral medications, unable to swallow Size 0 capsules. Acute peptic stomach or duodenal ulcer in the previous 6 months before screening.