To Assess the Safety, Tolerability and Efficacy of Itacitinib Immediate Release Tablets in Participants With Primary or Secondary Myelofibrosis Who Have Received Prior Ruxolitinib and/or Fedratinib Monotherapy. (LIMBER-213)

Inclusion Criteria:

• Diagnosis of primary MF meeting the 2016 WHO criteria for overt PMF or secondary MF (PPV-MF or PET-MF) meeting the 2008 IWG-MRT criteria.
• At least Intermediate 1 risk MF according to the DIPSS.
• Prior treatment with ruxolitinib and/or fedratinib monotherapy
• Currently receiving ruxolitinib or fedratinib monotherapy for PMF or secondary MF.
• Splenomegaly defined as palpable spleen at least 5 cm below the left costal margin orvolume ≥ 450 cm3 on imaging assessed during screening.
• Allogeneic stem cell transplant not planned.
• Platelet is greater than or equal to 50 × 109/L at screening.
• Ability to comprehend and willingness to sign a written ICF for the study.
• Willingness to avoid pregnancy or fathering children.

Exclusion Criteria:

• Prior treatment with a JAK inhibitor other than ruxolitinib or fedratinib
• Record of ≥ 10% myeloid blasts in the peripheral blood (on peripheral blood smear) orbone marrow prior to or at the time of screening
• For participants on ruxolitinib or fedratinib, unable to be tapered from thattreatment over the course of 14 days without corticosteroids, hydroxyurea, or otheragents
• Treatment with ruxolitinib, fedratinib or other MF-directed therapy (approved orinvestigational) within 2 weeks of Day 1
• Prior splenectomy or splenic irradiation within 6 months before receiving the firstdose of itacitinib
• Unable or unwilling to undergo serial MRI or CT scans for spleen volume measurement
• Unable or unwilling to complete MFSAF v4.0 diary on a daily basis during the study
• ECOG performance status ≥ 3
• Life expectancy less than 24 weeks
• Not willing to receive RBC or platelet transfusions
• Participants with laboratory values at screening outside of protocol defined ranges
• Significant concurrent, uncontrolled medical condition
• Participants with impaired cardiac function or clinically significant cardiac diseaseunless approved by medical monitor/sponsor
• History or presence of an abnormal ECG that, in the investigator's opinion, isclinically meaningful
• Chronic or current active infectious disease requiring systemic antibiotics,antifungal, or antiviral treatment.
• Evidence of HBV or HCV infection or risk of reactivation
• Known HIV infection.