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 marginor volume ≥ 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)or bone 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 cardiacdisease unless 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.