This study is testing a new treatment called **CYP-001** along with **corticosteroids (CS)** for adults with a condition called **high-risk acute graft-versus-host disease (HR-aGvHD)**. This happens when a person gets a **stem cell transplant** and the new cells attack their body. The study wants to see if CYP-001 works better than a **placebo** (a fake treatment) when both are used with CS. Participants will be randomly chosen to get either the CYP-001 or the placebo through an **IV (intravenous, meaning directly into the vein)** on Day 0 and Day 4. All participants will continue CS treatment. The study will have visits up to Day 100 and then check-ins up to 24 months later.

• Study Length: Visits up to 100 days, follow-ups for 24 months.
• Treatment: CYP-001 or placebo, plus ongoing corticosteroids.
• Eligibility: Must have had a stem cell transplant and specific HR-aGvHD signs.