1. Confirmed diagnosis of SCD (HbSS, HbSB0 thalassemia, or HbSB+ thalassemia)
2. Hemoglobin of >5.5 and <10.5 g/dL. 3. Subjects must have had at least 1 and no more than 12 documented episodes of VOCs in the past 12 months at the time of informed consent signing and at randomization (Day 1). 4. Subjects receiving HU must have received it continuously for at least 6 months prior to signing informed consent, and must have been on a stable dose for at least 3 months prior to signing the informed consent, with no anticipated need for dose adjustments during the study including the screening period, in the opinion of the investigator. 5. Female subjects must not be pregnant or breastfeeding and be highly unlikely to become pregnant. Male subjects must be unlikely to impregnate a partner. 6. Must be willing and able to complete all study assessments and procedures, and to communicate effectively with the investigator and site staff.
1. Body mass index (BMI) <17.0 kg/m2 and a total body weight <45 kg; or a BMI >35 kg/m2.
2. Subjects with known active hepatitis A, hepatitis B, or hepatitis C, with active or
acute event of malaria, or who are known to be positive for human immunodeficiency
3. Stroke requiring medical intervention within 24 weeks prior to randomization.
4. Prior exposure to IMR-687.
5. A history of use of crizanlizumab or voxelotor within 6 months prior to signing the
6. Receipt of erythropoietin or other hematopoietic growth factor treatment within 3
months of signing the ICF or anticipated need for such agents during the study.
7. Prior gene therapy.