• 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.