To Evaluate Efficacy and Safety of Parsaclisib and Ruxolitinib in Participants With Myelofibrosis Who Have Suboptimal Response to Ruxolitinib (LIMBER-304)

Inclusion Criteria: * Diagnosis of PMF, PPV-MF, or PET-MF. * DIPSS risk category of intermediate-1, intermediate-2, or high. * Treated with ruxolitinib for ≥ 3 months with a stable dose for at least the last 8 weeks prior to Day 1 * Palpable spleen of ≥ 5 cm below the left costal margin on physical examination at the screening visit. * Active symptoms of MF at the screening visit, as demonstrated by the presence of a TSS of ≥ 10 using the Screening Symptom Form. * Participants with an ECOG performance status score of 0, 1, or 2. * Screening bone marrow biopsy specimen and pathology report(s) available that was obtained within the prior 2 months or willingness to undergo a bone marrow biopsy at screening/baseline; willingness to undergo bone marrow biopsy at Week 24 and every 24 weeks there after. Screening/baseline biopsy specimen must show diagnosis of MF. * Life expectancy of at least 24 weeks. * Willingness to avoid pregnancy or fathering children. Exclusion Criteria: * Prior therapy with any drug that inhibits PI3K (examples of drugs targeting this pathway include but are not limited to INCB040093, idelalisib, duvelisib, buparlisib, copanlisib, and umbralisib). * Use of experimental drug therapy for MF or any other standard drug used for MF (whether for treatment of MF or another indication) with the exception of ruxolitinib, within 3 months of starting study drug, and/or lack of recovery from all toxicities from previous therapy (except ruxolitinib) to Grade 1 or bett…