Bronchiolitis Obliterans Syndrome

The goal of this research study is to test the efficacy of a novel immunosuppressive agent, belumosudil, in allogeneic hematopoietic stem cell transplant (HSCT) recipients who have been newly diagnosed or have developing (early stage) bronchiolitis obliterans syndrome (BOS). The name of the study drugs involved in this study are: * Belumosudil (an immunotherapy) * Fluticasone (an intranasal corticosteroid) * Azithromycin (an antibiotic) * Montelukast (a leukotriene receptor antagonist) * Prednisone (a corticosteroid)

The goal of this clinical trial is to learn about the safety and effectiveness of LAM-001 in patients who have developed bronchiolitis obliterans syndrome (BOS), a form of chronic rejection, after lung transplantation. The main questions it aims to answer are: * Is LAM-001 safe in these patients? * Is LAM-001 effective in slowing BOS progression? Participants will: * Be randomly assigned to inhale either LAM-001 or placebo (a look-alike substance that contains no active drug) daily for 48 weeks * Attend 10 study visits (mixture of in-person and telehealth) over the 48 week period * Undergo pulmonary function testing, bronchoscopy, lab testing, and physical examination * Submit weekly home spirometry monitoring Researchers will compare participants assigned to LAM-001 versus placebo to see if LAM-001 is safely tolerated and to assess the effectiveness of LAM-001 on slowing BOS progression.

In this study, we will study 129Xe-MRI and LCI as tools for diagnosis of BOS in pediatric patients who have received Hematopoietic Stem Cell Transplantation (HSCT) and have been identified as eligible for this study. Participants will be required to have vital signs collected, complete breathing tests and complete an MRI. The MRI will require participants to perform breath holds in the MRI scanner with xenon gas while being coached by a research assistant.