• Signed informed consent must be obtained prior to participation in the study.

• Male or female Chinese participants aged 12 or older at the time of informed consent

• Able to swallow tablets.- Have undergone alloSCT from any donor source (matched unrelated donor, sibling, haplo-identical) using bone marrow, peripheral blood stem cells, or cord blood. Recipients of non-myeloablative, myeloablative, and reduced intensity conditioning are eligible.

• Evident myeloid and platelet engraftment:

‣ Absolute neutrophil count (ANC) \>1,000/mm3 AND

⁃ Platelet count ≥25,000/mm3

∙ Note: Use of growth factor supplementation and transfusion support is allowed during the trial, however, transfusion to reach a minimum platelet count for inclusion is not allowed during screening and at baseline.

• Participants with clinically diagnosed cGvHD staging of moderate to severe according to NIH Consensus Criteria (Jagasia et al 2015) prior to Cycle 1 Day 1.

‣ Moderate cGvHD: at least one organ (not lung) with a score of 2, 3 or more organs involved with a score of 1 in each organ, or lung score of 1.

⁃ Severe cGvHD: at least 1 organ with a score of 3, or lung score of 2 or 3.

• Participants currently receiving systemic corticosteroids for the treatment of cGvHD for a duration of \< 12 months prior to Cycle 1 Day 1, and have a confirmed diagnosis of corticosteroid refractory cGvHD defined per 2014 NIH consensus criteria (Martin et al 2015) irrespective of the concomitant use of a calcineurin inhibitor, as follows:

‣ A lack of response or disease progression after administration of minimum prednisone 1 mg/kg/day for at least 1 week (or equivalent) OR

⁃ Disease persistence without improvement despite continued treatment with prednisone at \>0.5 mg/kg/day or 1 mg/kg/every other day for at least 4 weeks (or equivalent) OR

⁃ Increase to prednisone dose to \>0.25 mg/kg/day after two unsuccessful attempts to taper the dose (or equivalent)

• Participants has Eastern Cooperative Oncology Group (ECOG) performance status of 0-2