JAK Inhibitor Before Donor Stem Cell Transplant in Treating Patients With Primary or Secondary Myelofibrosis

OUTLINE: PART 1: Patients receive ruxolitinib orally (PO) twice daily (BID) from at least 8 weeks prior to the start of conditioning through day -4 before transplantation, with a taper schedule reducing the dose every 2-3 days beginning after day -4. PART 2: Patients are assigned to 1 of 2 conditioning regimens at the discretion of the clinical provider and Clinical Coordinators Office (CCO). MYELOABLATIVE CONDITIONING: Patients receive fludarabine phosphate intravenously (IV) over 1 hour on days -8 to -6 (umbilical cord blood transplant recipients only), cyclophosphamide IV on days -7 and -6, and busulfan IV over 3 hours on days -5 to -2. REDUCED-INTENSITY CONDITIONING: Patients receive fludarabine phosphate IV over 1 hour on days -6 to -2 and melphalan IV over 15-30 minutes on days -3 and -2. Patients also undergo total-body irradiation (TBI) on day -1 (umbilical cord blood transplant recipients only). TRANSPLANT: Patients undergo allogeneic hematopoietic stem cell transplant or umbilical cord blood transplant on day 0. GRAFT-VERSUS-HOST DISEASE (GVHD) PROPHYLAXIS: Patients receive tacrolimus IV continuously (inpatients) or over 1-2 hours twice daily (BID) (outpatients) or orally (PO) BID on days -1 to +180 (patients receiving related or unrelated stem cells) or days -3 to +180 (patients receiving umbilical cord blood) with taper beginning on day +56 (related donor recipients) or +100 (unrelated donor or umbilical cord blood recipients) in the absence of GVHD. Patients also receive methotrexate IV on days +1, +3, +6, and +11 (related and unrelated donor recipients only) or mycophenolate mofetil IV or PO every 8 hours on days 0 to +40 with taper to day +96 (umbilical cord blood transplant recipients only). After completion of study treatment, patients are followed up at 6 months, 1 year, and then yearly for 4 years.