A Study of Itacitinib in Combination With Low-Dose Ruxolitinib or Itacitinib Alone Following Ruxolitinib in Participants With Myelofibrosis

Inclusion Criteria

• Inclusion Criteria: Cohort A only •Receiving ruxolitinib dose of less than 20 mg daily with no dose increase or no dose modification in the last 8 weeks before screening visit. Cohort B only •Must have had initial reduction in spleen on ruxolitinib treatment: - Followed by documented evidence of progression in spleen length or volume OR - Discontinued ruxolitinib for hematologic toxicities, after the initial reduction in spleen length or volume. All participants - Confirmed diagnosis of primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythemia myelofibrosis according to revised World Health Organization 2016 criteria. - Must have palpable spleen of greater than or equal to (≥) 5 centimeter (cm) below the left subcostal margin on physical examination at the screening visit. - Eastern Cooperative Oncology Group performance status of 0, 1, or 2. - Screening bone marrow biopsy specimen available or willingness to undergo a bone marrow biopsy at screening/baseline; willingness to undergo bone marrow biopsy at Week 24. - Life expectancy of at least 24 weeks. - Willingness to avoid pregnancy or fathering children Exclusion Criteria: - Lack of recovery from all toxicities from previous therapy (except ruxolitinib) to Grade 1 or better. - Previous treatment with itacitinib or Janus kinase (JAK1) inhibitors (JAK1/JAK2 inhibitor ruxolitinib is permitted). - Inability to swallow food or any condition of the upper gastrointestinal tract that precludes administration of oral medications. - Recent history of inadequate bone marrow reserve as demonstrated by protocol-defined criteria. - Inadequate liver function at screening and baseline visits as demonstrated by protocol-defined criteria. - Inadequate renal function at screening and baseline visits as demonstrated by protocol-defined criteria. - Active bacterial, fungal, parasitic, or viral infection that requires therapy. - Evidence of hepatitis B virus (HBV) or hepatitis C virus (HCV) infection or risk of reactivation: HBV deoxyribonucleic acid (DNA) and HCV ribonucleic acid (RNA) must be undetectable. Participants cannot be positive for hepatitis B surface antigen or anti-hepatitis B core antibodies. Participants who have positive anti-HBs as the only evidence of prior exposure may participate in the study provided that there is both 1) no known history of HBV infection and 2) verified receipt of hepatitis B vaccine. - Known human immunodeficiency virus infection. - Clinically significant or uncontrolled cardiac disease. - Active invasive malignancy over the previous 2 years except treated basal or squamous carcinomas of the skin, completely resected intraepithelial carcinoma of the cervix, and completely resected papillary thyroid and follicular thyroid cancers. Participants with malignancies with indolent behavior such as prostate cancer treated with radiation or surgery may be enrolled as long as they have a reasonable expectation to have been cured with the treatment modality received. - Splenic irradiation within 6 months before receiving the first dose of itacitinib. - Use of any prohibited concomitant medications. - Active alcohol or drug addiction that would interfere with their ability to comply with the study requirements. - Use of any potent/strong cytochrome P450 3A4 inhibitors within 14 days or 5 half-lives (whichever is longer) before the first dose of itacitinib or anticipated during the study. - Use of concomitant treatment of fluconazole at a dose > 200 mg (for ruxolitinib participants treated in Cohort A only). - Inadequate recovery from toxicity and/or complications from a major surgery before starting therapy. - Currently breastfeeding or pregnant.