Novel BET Inhibitor PLX51107 for Steroid-Refractory Acute GVHD

Inclusion Criteria: * Age \>= 18 years at the time of signing informed consent * Steroid-refractory acute GVHD as defined as progression of acute (a)GvHD within 3-5 days of therapy onset with \>= 2 mg/kg/day of prednisone equivalent OR failure to improve within 5-7 days of treatment initiation with \> 1-2 mg/kg/day of prednisone equivalent OR incomplete response after more than 28 days of immunosuppressive treatment including steroids * Recipients of ablative and reduced-intensity conditioning regimens * Recipients of human leukocyte antigen (HLA)-matched related and unrelated, 1-allele mismatched, haploidentical, or umbilical cord blood donor grafts * Prior lines of therapy for treatment of steroid-refractory acute GVHD are allowed. However, exposure to investigational therapies for the treatment of GVHD must be \> 14 days or 5 half-lives (whichever is shorter) of first administration of study drug. For patients treated with ruxolitinib for the treatment of acute GVHD, ruxolitinib must be discontinued by at least one day prior to initiation of PLX51107 * Eastern Cooperative Oncology Group (ECOG) performance status =\< 3 * Absolute neutrophil count \>= 1.0 x 10\^9/L for 3 consecutive days). Use of growth factor support is allowed * Platelet count \>= 50 x 10\^9/L without transfusion support for 2 consecutive days * Women of child-bearing potential must have a negative serum pregnancy test at Screening and must agree to use an effective form of contraception from the time of …