HOPE-PMF: Pre-fibrotic/Early Primary Myelofibrosis or Overt Primary Myelofibrosis

If you are able to take part in this research study, you will be randomly assigned to receive P1101 or a placebo (a liquid without the medication). Screening Visit (visit window: -28 days to -1 day) - tests to determine your eligibility to participate. Baseline Visit (Week 0) Dose Escalation Period (Week 2 to Week 4 visits) Study medication injection during the Core Study Drug Period: During the Baseline Visit and Dose Escalation Period, you will receive P1101 (study group) or placebo (placebo group) during your clinic visit on the following schedule: Baseline visit (Day 0) = 250 mcg; Week 2 = 350 mcg; Week 4 = 500mcg Core Study Drug Period (Week 8 to Week 76 visits) - visits will occur roughly every 4 weeks during the 76-week Core Study Drug Period End of Core Study Drug Visit (Week 80 of receiving P1101/placebo) Safety Follow-up Visit for the Core Study Phase (~28 days after the last dose of P1101/placebo) - If you participate in the Blinded Extension Phase, the Safety Follow-up Visit will also serve as the Screening Visit for the Blinded Extension Phase. On-site biweekly study procedures for visits for whom self-injection is not possible (every 2 weeks) Early Termination Visit During the study visits the following tests may be done physical examinations, echocardiogram, eye exam, vitals, blood and urine samples collected, electrocardiogram (ECG), chest x-ray, questionnaires, spleen measurements, and bone marrow samples. You are expected to be in this research study for roughly 2.5 years (80 weeks for the Core Study Phase, 4 weeks of Safety Follow-up Visit, and 60 weeks for the Blinded Extension Phase [separate consent form]) or longer, depending on disease progression. If you are found to be positive for drug antibodies at your safety follow-up (end of study), we will need to check again in 12 weeks and 24 weeks or until you test negative, whichever comes first.