This clinical trial is focused on studying a condition known as chronic lung allograft dysfunction (CLAD), which can occur after a person has received a lung transplant. The study is testing a treatment called belumosudil, which is taken as a tablet. The purpose of the study is to evaluate how effective belumosudil is in improving lung function in adults who have developed CLAD after a lung transplant. Participants in the study will be randomly assigned to receive either belumosudil or a placebo, which is a tablet that looks like the treatment but does not contain the active medicine.

During the study, participants will take the tablets by mouth. The study will be conducted in two parts: an initial phase where participants will not know if they are receiving belumosudil or the placebo, followed by an open-label phase where all participants will receive belumosudil. The study will last for several months, and participants will have regular check-ups to monitor their lung function and overall health. Lung function will be measured using a test called forced expiratory volume in 1 second (FEV1), which assesses how much air a person can exhale in one second.

Participants in the study will have already been treated with another medication called azithromycin for at least eight weeks before starting the trial. The study aims to see if belumosudil can help improve lung function and slow down the progression of CLAD. Throughout the study, researchers will also monitor for any side effects or adverse events to ensure the safety of the participants. The results of this study could provide valuable information about the potential benefits of belumosudil for people with CLAD following a lung transplant.