This study is looking at a medicine called RLS-0071 to help people in the hospital with a condition called acute Graft-versus-Host Disease (aGvHD), which happens after a stem cell transplant when the new cells attack the body. The study is for patients who did not get better with steroids, a common treatment. The study will test how safe and helpful RLS-0071 is, and how it moves and works in the body. Participants need to be at least 13 years old and stay in the hospital for at least a week. People who have had more than one stem cell transplant, certain infections, or other severe health issues cannot join. The study will not add or change other GvHD treatments during the 7 days of RLS-0071 treatment.

• Study Length: 7 days of treatment with hospital stay.
• Eligibility: Must be 13+ years old and weigh between 40-140 kg.
• Exclusions: Includes certain infections and other severe health issues.