Participants will be randomized 1:1 to receive either belumosudil or Best Available Therapy (BAT), with stratification based on baseline cGVHD severity as defined by the 2014◦NIH consensus criteria (moderate versus severe), use of concomitant CS and/or CNI (ie, tacrolimus or cyclosporine) at baseline (Yes versus No), and the number of prior lines of therapies (2 versus more than 2). While treatment practices for cGVHD differ across regions, ruxolitinib has been approved by the European Commission since May 2022 and is expected to be broadly accessible throughout most EU member states by study initiation. The study will target patients post-ruxolitinib treatment, except where Investigators deemed ruxolitinib treatment for cGVHD not suitable. In the BAT arm, the study doctor will select one BAT based on clinical judgement, local availability etc. prior to randomization.
Participants randomized to the BAT arm will have the option to cross-over to open-label belumosudil treatment upon meeting predefined criteria.
Study details include:
* The study duration will be defined as 3 years from LPI.
* Individual participant duration on study will consist of:
* Up to 28 days for screening.
* Treatment until clinically significant progression of cGVHD, relapse/recurrence of the underlying disease, start of a new systemic treatment for cGVHD (except change from BAT to belumosudil during the cross-over), experience of an unacceptable adverse event, request from participant or Investigator, or until the end of the study is reached, whichever comes first.
* Thirty days of post treatment safety follow-up.
* Follow-up for cGVHD status as applicable.
* Long-term follow-up until death or end of study, whichever occurs first.