A Study of Belumosudil in People at Risk of Developing Graft-Versus-Host Disease After a Stem Cell Transplant

Inclusion Criteria: * Patients ≥ 18 years-old at time of consent. * Diagnosis: hematologic malignancy in morphologic remission who will be treated with RI or NMA conditioning and GVHD prophylaxis CNI-based (CNI without PTCY) plus abatacept or PTCY-based (CNI with PTCY). * Recipients of 7-8/8 related or unrelated HLA-matched or related haploidentical donor. * Peripheral blood stem cell graft * Allo-HCT day \<120 at time of consent Post-HCT inclusion criteria (within 3 weeks before start of belumosudil treatment) * Patient has received an allo-HCT transplant and is in morphologic remission (blasts \<5%, no evidence of extramedullary disease in AML or MDS). Patients with CR with incomplete count recovery (CRp or CRi) or minimal residual disease are allowed. * Patient has achieved engraftment. Engraftment is defined as ANC≥500/μL and platelets ≥ 20000/μL on 3 consecutive measurements (each occurring at least 1 day apart). The patient must not have had a platelet transfusion within 7 days before the first measurement. * Patient is ≥ 80 days and ≤ 20 days from allo-HCT infusion. * Karnofsky score ≥ 70%. * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3x upper limit of normal (ULN) * Total bilirubin ≤1.5 x ULN (unless benign congenital hyperbilirubinemia). * Glomerular filtration rate (GFR) ≥ 30 mL/min/1.73 m2 * Female subjects of childbearing potential (≤ 50 years old) have a negative serum or urine pregnancy test. Females of childbearing potential are defined as females without prior hysterectomy or who have had any evidence of menses in the past 12 months. ° Sexually active females of childbearing potential enrolled in the study must agree to consistently use two forms of accepted methods of contraception during the course of the study and for 3 months after their last dose of study drug. Effective birth control includes: \* Intrauterine device (IUD) plus one barrier method \* Stable doses of hormonal contraception for at least 3 months (eg, oral, injectable, implant, transdermal) plus one barrier method \* 2 barrier methods. Effective barrier methods are male or female condoms, diaphragms, and spermicides (creams or gel that contain a chemical to kill sperm); or \* A vasectomized partner * For male subjects who are sexually active and who are partners of females of childbearing potential: Agreement to use two forms of contraception as per above and to not donate sperm during the treatment period and for at least 3 months after the last dose of study drug. Exclusion Criteria: * Recipient of CD34+ selected or engineered stem cell graft. * Treatment with in vivo T cell depletion (e.g. anti-thymocyte globulin). * Evidence of current uncontrolled cardiovascular conditions, uncontrolled cardiac arrhythmias, symptomatic congestive heart failure, unstable angina, or myocardial infarction within the past 6 months. * Pulmonary dysfunction with DLCO \<50% corrected for hemoglobin Post-HCT exclusion criteria * Uncontrolled infection, including active hepatitis B and C. Definitive therapy for infection is required and must have no signs of progression within 7 days of the first day of study drug treatment. * Use of investigational agent within 14 days pre-HCT or anytime thereafter. * Active acute or chronic GVHD requiring systemic therapy (topical or local therapies are allowed). * Active treatment with corticosteroids at a dose of ≥ 0.25 mg/kg/day for non-GVHD indication. * Uncontrolled psychosis, active suicidal ideation, or psychiatric hospitalization within the past year * Female patient who is pregnant or breastfeeding. * Prior therapy with belumosudil. * Known allergy or sensitivity to belumosudil or any other ROCK2 inhibitor.