To determine if Itacitinib in combination with corticosteroids is safe and tolerable in patients with Grade IIB-IVD acute graft-versus-host disease (GVHD).

Inclusion Criteria:

• Have undergone first allo-HSCT from any donor source (matched unrelated donor, sibling, haploidentical) using bone marrow, peripheral blood stem cells, or cord blood for hematologic malignancies. Recipients of nonmyeloablative and myeloablative transplants are eligible.
• Clinically suspected Grades IIB to IVD acute GVHD as per modified MN-CIBMTR criteria, occurring after allo-HSCT with any conditioning regimen and any anti-GVHD prophylactic program.
• Subjects may, but are not required to, have previously received corticosteroids for acute GVHD:
• Evidence of myeloid engraftment. Use of growth factor supplementation is allowed.

Exclusion Criteria:

• Has received more than 1 hematopoietic stem cell transplantation.
• Has progressed on more than 2 prior treatment regimens for acute GVHD.
• Presence of an active uncontrolled infection.
• Subjects with relapsed primary disease, or subjects who have been treated for relapse after the allogeneic hematopoietic stem-cell transplantation (allo-HSCT) was performed.
• Inadequate recovery from toxicity and/or complications from the prior allo-HSCT.
• Any corticosteroid therapy (for indications other than GVHD) at doses > 1 mg/kg per day methylprednisolone or equivalent within 7 days of randomization.
• Previously received JAK inhibitor therapy for any indication.