A Study to Test an Oral Medicine, Belumosudil, in Combination With Corticosteroids in Participants at Least 12 Years of Age With Newly Diagnosed Chronic Graft Versus Host Disease.

Inclusion Criteria:

• Patients must be at least 12 years of age inclusive, at the time of signing theinformed consent

• Participants who have undergone allogenic HCT with newly diagnosed moderate tosevere cGVHD according to NIH consensus diagnosis and staging criteria (2014)

• Participants who require systemic treatment with corticosteroids for cGVHD

• Participants who have not received any prior systemic treatment for cGVHD (includingECP)

• If participants are receiving other immunosuppressive agents for the prophylaxis ortreatment of acute GVHD, the dose should be under the threshold pre-defined inprotocol

• For adult participants, the body weight should be ≥40 kg. For adolescentparticipants, the body weight should be ≥30 kg

• Contraceptive use by men and women should be consistent with local regulationsregarding the methods of contraception for those participating in clinical studies.

• Participants or their legally authorized representative must be capable of givingsigned informed consent

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

Medical conditions

• Any evidence (histologic, cytogenetic, molecular, hematologic, or mixed) ofprogressive or relapsed underlying disease after the most recent allogeneic HCT

• Post-transplant lymphoproliferative disease within 4 weeks prior to randomization

• Female participants who are pregnant or breastfeeding

• Unable to tolerate a prednisone equivalent dose of corticosteroids ≥ 1 mg/kg/dayPrior/concomitant therapy

• Participant has had previous exposure to belumosudil.

• Received any previous systemic treatment for cGVHD with the following exception:Corticosteroids for cGVHD received within 7 days prior to the planned administrationof IMP only if in the interest of participant.

Prior/concurrent clinical study experience

• Received any investigational agents, or any investigational device or procedure, orprohibited therapy for this study within 28 days or 5 elimination half-lives priorto randomization, whichever is longer Diagnostic assessments

• Karnofsky (if aged ≥16 years)/Lansky (if aged <16 years) Performance Score of < 60

• Platelets <25 x 109/L. Platelet transfusion is not allowed within 3 days before thescreening hematological test

• Absolute neutrophil count (ANC) <0.5 x 109/L. The use of granulocyte-colonystimulating factor (G-CSF) is not allowed to reach this level during screening

• Estimated Glomerular Filtration Rate (eGFR) <30 mL/min/1.73 m2 using the MDRD-4variable formula (if aged ≥18 years) or using the Bedside Schwartz formula (if aged <18 years)

• Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) >3 x ULNwithout liver cGVHD or>5 × ULN with liver) cGVHD

• Total bilirubin >1.5 × (ULN) (>3 × ULN if Gilbert syndrome)

• Participant has forced expiratory volume in 1 second (FEV1) of predicted ≤39% or haslung score of 3 according to NIH consensus diagnostic and staging criteria (2014)

• History or other evidence of severe illness or any other conditions that would makethe participant, in the opinion of the Investigator, unsuitable for the study (suchas malabsorption syndromes, poorly controlled psychiatric disease or coronary arterydisease)

• Known history of human immunodeficiency virus (HIV)

• Active viral disease including hepatitis B virus (HBV) or hepatitis C virus (HCV)

• Active uncontrolled cytomegalovirus (CMV) and Epstein-Barr virus (EBV) infection.Infections are considered controlled if appropriate therapy has been instituted and,at the time of screening, no signs of infection worsening are present according toInvestigator's judgement

• Diagnosed or treated for another malignancy other than the underlying diseaseallogeneic HCT was indicated for, within 3 years prior to randomization with theexception of complete resection of basal cell carcinoma or squamous cell carcinomaof the skin, an in-situ malignancy, or low risk prostate cancer after curativetherapy

• Unable to swallow tablets

• Participant not suitable for participation, whatever the reason, as judged by theInvestigator, including medical or clinical conditions, or participants potentiallyat risk of noncompliance to study procedures

• Any active, uncontrolled infections assessed to be clinically significant by theInvestigator