Safety, PK, PD, Dosing, and Efficacy of RLS-0071for the Treatment of Hospitalized Patients With Steroid-Refractory Acute Graft-versus-Host Disease

Inclusion Criteria: * Male or female adults or adolescents (\>12 years old). * Hospitalized with steroid-refractory aGvHD (grade II-IV) after allo-HSCT * Anticipated hospital length-of-stay of at least 1 week from the time of RLS-0071 initiation. * No plans to add additional GvHD treatment medications or to add, dose-adjust, or discontinue GvHD prophylactic medications during the 7-days of RLS-0071 treatment. * Neutrophil recovery following the stem-cell transplantation, defined as blood neutrophil count \>500/mL for at least 3 consecutive measurements and not supported by growth factor supplementation * Weight ≥40 kg and ≤ 140 kg at screening. Exclusion Criteria: * Has received more than 1 allo-HSCT * Current, previous, or planned use of any systemic treatment in addition to or other than corticosteroids or ruxolitinib for aGvHD * Previous failure of ruxolitinib treatment * Uncontrolled GI infection * Endoscopic and biopsy testing (if performed) that definitively rules out lower GI aGvHD * Chronic GvHD * Participants with evidence of relapsed primary disease, or participants who have been treated for relapse after the allo-HSCT was performed. * Unresolved toxicity or complications (other than aGvHD) due to the allo-HSCT * Any corticosteroid therapy for indications other than aGvHD at doses of methylprednisolone or equivalent \>1 mg/kg per day within 7 days of enrollment. * Severe organ dysfunction unrelated to underlying aGvHD * Known hypersensitivity, allergy, or anaphylactic reaction to polyethylene glycol (PEG) * Significant liver disease that is unrelated to GvHD * Severe kidney disease * Currently breast feeding. * Known pregnancy, a positive pregnancy test at screening, or lactation for WOCBP. * Active hepatitis B virus (HBV) or hepatitis C virus infection that requires treatment or human immunodeficiency virus (HIV)-1 or HIV-2. * Active sepsis