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 or 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 that treatment over the course of 14 days without corticosteroids, hydroxyurea, or other agents
Treatment with ruxolitinib, fedratinib or other MF-directed therapy (approved or investigational) within 2 weeks of Day 1
Prior splenectomy or splenic irradiation within 6 months before receiving the first dose 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 disease unless approved by medical monitor/sponsor
History or presence of an abnormal ECG that, in the investigator's opinion, is clinically 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.