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 better.
* 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.
* Inadequate liver and renal function at screening.
* Active bacterial, fungal, parasitic, or viral infection that requires therapy.
* Active HBV or HCV infection that requires treatment or at risk for HBV reactivation.
* Known HIV infection.
* Uncontrolled, severe, or unstable cardiac disease that in the investigator's opinion may jeopardize the safety of the participant or compliance with the Protocol.
* Active invasive malignancy over the previous 2 years.
* Splenic irradiation within 6 months before receiving the first dose of study drug.
* Concurrent use of any prohibited medications.
* Active alcohol or drug addiction that would interfere with the ability to comply with the study requirements.
* Use of any potent CYP3A4 inhibitors or inducers within 14 days or 5 half lives(whichever is longer) before the first dose of study drug or anticipated during the study.
* Inadequate recovery from toxicity and/or complications from a major surgery before starting therapy.
* Currently breastfeeding or pregnant.
* Any condition that would, in the investigator's judgment, interfere with full participation in the study, including administration of study drug and attending required study visits; pose a significant risk to the participant; or interfere with interpretation of study data.
* History of Grade 3 or 4 irAEs from prior immunotherapy.
* Receipt of any live vaccine within 30 days of the first dose of study drug
* Unwillingness to receive RBC transfusions to treat low hemoglobin levels.
* Known hypersensitivity or severe reaction to parsaclisib or ruxolitinib or excipients of parsaclisib/matching placebo or ruxolitinib formulations.