This study is testing a medicine called SIL-8301 (senicapoc) for people with sickle cell disease, a blood disorder that affects red blood cells. The study compares the effects of senicapoc to a placebo, which looks like the medicine but doesn't contain the active ingredient. Participants will continue taking hydroxyurea, a standard treatment for sickle cell disease, along with senicapoc or placebo for up to 24 weeks.

**Key Points**

• The study lasts for 24 weeks.
• Participants may need to visit a medical facility once a year for a sickle cell crisis.
• Participants must be aged 16-35 years and currently using hydroxyurea.

Eligibility includes having sickle cell disease, being 16-35 years old, having certain blood levels (Hb ≤ 9.0 g/dL), and a history of no more than one severe crisis per year in the last two years. If you have been in a study with senicapoc before, have certain health conditions, or are part of a blood transfusion program, you cannot join this study. This study aims to help manage sickle cell disease by hopefully reducing symptoms and improving quality of life.