Assessing Physical Function in Sickle Cell Patients Taking Voxelotor

Inclusion Criteria: 1. Provision of signed and dated informed consent form 2. Stated willingness to comply with all study procedures and availability for the duration of the study 3. Male or female, 4. aged 18-65 years 5. Diagnosis of sickle cell disease with HbSS, HbSc, HbS-beta thalassemia genotypes 6. Hemoglobin level of 6 g/dL - 10.5 g/dL 7. Between 0 and 10 vaso-occlusive (VOC) episodes in previous 12 months 8. Ability to take oral medication and be willing to adhere to the voxelotor regimen 9. For females of reproductive potential, they will be educated to avoid pregnancy during study period and for one month after due to limited data surrounding pregnancy and voxelotor 10. For males of reproductive potential: use of condoms or other methods to ensure effective contraception with partner 11. Agreement to adhere to Lifestyle Considerations (see section 5.3) throughout study duration Exclusion Criteria: 1. Hemoglobin results \< 6 g/dL or \> 10.5 g/dL 2. Patients receiving hydroxyurea who have had a dose change within the prior 3 months 3. Females who are breastfeeding or pregnant 4. Patients who are receiving regularly scheduled blood (RBC) transfusion therapy (also termed chronic, prophylactic, or preventive transfusion) within 30 days of 6MWT 5. Any form of hepatic dysfunction as demonstrated by alanine aminotransferase (ALT) \>4 × ULN or Child-Pugh Class C 6. Acute kidney injury or chronic renal disease with estimated glomerular filtration rate\<30mL/min/1.73 m2 7. Current or previous diagnosis of malignancy within the last two years excluding local therapy for non-melanoma skin malignancy 8. Unstable cardiopulmonary disease within the last 6 months prior to study enrollment 9. Any conditions including behavioral or medical that in the opinion of the investigator, may negatively impact informed consent, participation and study interpretation 10. Current use of commercially available voxelotor 11. Prior hypersensitivity to voxelotor or its excipients 12. Patients with baseline gait and other physical impairments that would limit 6MWT and other functional assessment at discretion of the investigator 13. Patients who on day of assessment present with vaso-occlusive crisis or rate their pain greater than 6 on a scale of 1-10 using the Numeric Rating Scale will not be assessed that day and re-screened at the investigator's discretion.