This study is for adults who have had an allogeneic hematopoietic stem cell transplant (HSCT) and now have high-risk acute GvHD (aGvHD). GvHD is a condition where the new stem cells attack the patient's body. The study compares a new treatment called CYP-001 combined with corticosteroids (medicines that reduce inflammation) against a placebo (a substance with no therapeutic effect) combined with corticosteroids. Participants will be randomly assigned to receive either the new treatment or the placebo through an IV (intravenous) on Days 0 and 4, along with ongoing corticosteroid therapy. The study will monitor participants' GvHD severity using specific guidelines called MAGIC.

• Duration: Primary Evaluation up to Day 100, Follow-Up up to 24 months.
• Visits: Regular visits required up to 24 months.
• Eligibility: Must have undergone HSCT and be diagnosed with high-risk aGvHD.