Study of Pharmacokinetics, Activity and Safety of Ruxolitinib in Pediatric Patients With Grade II-IV Acute Graft vs. Host Disease

Inclusion Criteria: * Male or female patients age ≥28 days and \<18 years at the time of informed consent. * Patients who 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 myeloablative or reduced intensity conditioning are eligible. * Patients with a clinically confirmed diagnosis of grades II-IV aGvHD within 48 hours prior to study treatment start. Patients may have either: Treatment-naïve aGvHD (criteria per Harris et al. 2016) OR Steroid refractory aGvHD as per institutional criteria, or per physician decision in case institutional criteria are not available, and the patient is currently receiving systemic corticosteroids. * Evident myeloid engraftment with ANC \> 1,000/µl and platelet count \>20,000/µl. (Use of growth factor supplementation and transfusion support is allowed.) Exclusion Criteria: * Has received the following systemic therapy for aGvHD: a) Treatment-naïve aGvHD patients have received any prior systemic treatment of aGvHD except for a maximum 72h of prior systemic corticosteroid therapy of methylprednisolone or equivalent after the onset of acute GvHD. Patients are allowed to have received prior GvHD prophylaxis which is not counted as systemic treatment (as long as the prophylaxis was started prior to the diagnosis of aGvHD); OR b) SR-aGvHD patients have received two or more prior systemic treatments for aGvHD in addition to corti…