Inclusion Criteria:

   - Has undergone 1 allo-HSCT from any donor (related or unrelated with any degree of HLA
   matching) and any donor source (bone marrow, peripheral blood stem cells, or cord
   blood) for a hematologic malignancy or disorder. Recipients of myeloablative and
   reduced-intensity conditioning regimens are eligible.

   - Clinically suspected Grade II to IV aGVHD as per MAGIC criteria, occurring after
   allo-HSCT and any GVHD prophylaxis regimen.

   - Evidence of myeloid engraftment. Use of growth factor supplementation is allowed.

   - Serum creatinine ≤ 2.0 mg/dL or creatinine clearance ≥ 40 mL/min measured or
   calculated by Cockroft Gault equation.

   - Willing to avoid pregnancy or fathering children.

   - Able to give written informed consent and comply with all study visits and procedures.

   - Able to swallow and retain oral medication.

Exclusion Criteria:

   - Has received more than 1 allo-HSCT.

   - Has received more than 2 days of systemic corticosteroids for aGVHD.

   - Presence of GVHD overlap syndrome.

   - Presence of an active uncontrolled infection.

   - Known human immunodeficiency virus infection.

   - Active hepatitis B virus (HBV) or hepatitis C virus infection that requires treatment
   or at risk for HBV reactivation.

   - Participants with evidence of relapsed primary disease, or participants who have been
   treated for relapse after the allo-HSCT was performed.

   - Any corticosteroid therapy for indications other than GVHD at doses > 1 mg/kg per day
   methylprednisolone (or prednisone equivalent) within 7 days of randomization.

   - Severe organ dysfunction unrelated to underlying GVHD, including:

      - Cholestatic disorders or unresolved veno-occlusive disease of the liver.

      - Clinically significant or uncontrolled cardiac disease.

      - Clinically significant respiratory disease that requires mechanical ventilation
      support or 50% oxygen.

   - Currently breast feeding.

   - Received JAK inhibitor therapy after allo-HSCT for any indication. Treatment with a
   JAK inhibitor before allo-HSCT is permitted.

   - Treatment with any other investigational agent, device, or procedure within 21 days
   (or 5 half-lives, whichever is greater) of enrollment.

   - Any medical complications or conditions that would, in the investigator's judgment,
   interfere with full participation in the study, including administration of study drug
   and attending required study visits; pose a significant risk to the participant; or
   interfere with interpretation of study data.

   - Known allergies, hypersensitivity, or intolerance to any of the study medications,
   excipients, or similar compounds.