•Timing changed for the first interim analysis to minimize treatment of patients with an ineffective dose •Randomization stratified by baseline platelet count & geographic region to ensure balance in risk factors between study arms •DNA samples collection to be used in future analyses to determine if specific mutations predict responsiveness to pacritinib • Screening visit window modified to support the investigator & site staff in the scheduling & performing of tests & assessments •Included additional changes required by the French,United Kingdom,Swedish & German regulatory bodies •Inclusion criteria changed to ensure that patients who enter the study had a documented adequate trial on ruxolitinib without demonstrated substantial efficacy •Revised text to define symptom control failure by the TSS instrument & ensure that only patients without adequate symptom control are eligible •Concomitant use of growth factor therapy prohibited •Corticosteroids to be used as supportive care for medically indicated conditions •Deleted dose re-escalation language as PAC203 is a dose-finding study •Added hemoglobin A1C & high sensitivity CRP tests to central laboratory analysis for future use as pharmacodynamics markers for pacritinib •Added modifications to specify that all prior experimental therapy requires a 28-day washout prior to randomization to ensure that drug associated AEs from prior therapy are identified & reported •Allowed patients to continue pacritinib after 24 weeks & define follow up assessments as evaluation of long-term safety of pacritinib is a secondary goal of the study •Deleted platelet count inclusion criterion to eliminate an impediment to enrollment not been shown to be associated with safety concerns •Added timing details for subsequent interim analyses to the first interim one •Deleted pacritinib dose reduction for patients requiring antiplatelet or anticoagulation agent to treat AEs •Allowed patients with platelets ≥ 100,000/μL

• Expanded sample size to approximately 150 patients (up to 50 patients/arm) • Included additional dense PK blood sampling at selected sites for approximately 6 to 8 patients per treatment group • Specified a 30-minute window for the 0-hour (predose) PK & pharmacodynamics blood sampling • Specified that samples collected for unscheduled hematology & serum chemistry tests may be analyzed locally but must also be submitted to the central laboratory for testing & entry into the EDC • Excluded patients on high-dose ruxolitinib (more than 10 mg BID or 20 mg QD) who cannot tolerate tapering off ruxolitinib prior to the first dose of pacritinib • Removed the requirement for central radiographic confirmation of disease progression prior to stopping treatment

• Revised study design to remove BPP interim futility analyses • Added text to require that if a grade 4 thrombocytopenia recurs after restarting drug, pacritinib must be discontinued per request from the French Competent Authority

• Revised study design to terminate pacritinib treatment &study assessments at and beyond the Week 24 timepoint to conclude Phase 2 dose-finding study in preparation for Phase 3 • Added provision allowing patients who are benefiting from therapy, as of study drug termination, to continue receiving pacritinib under single patient expanded access or named patient programs at investigator discretion & subject to regulatory and IEC/IRB approval • Clarified that the FAS is defined as all randomized patients who received at least one dose of study drug. Remove reference to “Per Protocol Population.”