A Clinical Study to Evaluate the Pharmacokinetics, Efficacy, and Safety of Belumosudil in Chinese Adolescents With cGVHD Who Have Had an Inadequate Response to Glucocorticoids or Other Systemic Therapies

Inclusion Criteria: * Participant must be 12 to less than 18 years of age at the time of signing the informed consent. * Participant has undergone allogeneic hematopoietic stem cell transplantation (allo-HSCT). * Has active moderate to severe cGVHD. * Has received at least one line of prior systemic therapy for cGVHD. * Participant must receive a corticosteroid therapy for cGVHD with a stable dose for at least 2 weeks prior to the first dose of the IMP. * Has a Lansky-Play performance score of ≥60. * Participants should have an expected survival of longer than 6 months. * Body weight of 30 kg and above. * Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. * The participant or their legally authorized representative (LAR) must be capable of giving signed informed consent. Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Recurrence of hematologic neoplasms (according to the corresponding criteria for recurrence of primary hematologic neoplasms) or post-transplant lymphoproliferative disease at screening. * Received investigational systemic therapy for cGVHD within 28 days prior to enrollment, unless the prior treatment had been washed out for at least 28 days or 5 half-lives prior to enrollment, whichever is shorter. * Absolute neutrophil count (ANC) \<1.0 × 10\^9/L. * Platelet count \<50 × 10\^9/L. * Alanine a…