This is a phase 3 double-blind clinical trial arm to test Ropeginterferon alfa-2b (P1101) in adult patients with Primary Myelofibrosis (PMF) at early stage or low to medium risk.

Participants will receive the study drug/placebo bi-weekly and have an assessment visit every 4 weeks. The ratio of study drug to placebo group is 2:1.

Inclusion Criteria:

1. Male or female patients aged ≥18 years at the time of signing the informed consent form;
2. Patients with pre-fibrotic/early PMF (Pre-PMF) or overt primary myelofibrosis at low to intermediate-1 risk according to DIPSS plus, diagnosed according to WHO 2016 or 2022 classification;
3. With good liver function at screening, which is defined as total bilirubin ≤1.5 × upper limit of normal (ULN), international normalized ratio (INR) ≤1.5 × ULN, albumin >3.5 g/dL, alanine aminotransferase (ALT) ≤2.0 × ULN, and aspartate aminotransferase (AST) ≤2.0 × ULN;
4. Hgb ≥10.0 g/dL at screening;
5. Neutrophil count ≥1.0 × 10^9/L at screening;
6. Creatinine clearance rate ≥30 mL/min at screening (according to the Cockcroft-Gault formula);
7. Females of childbearing potential, as well as all women <2 years after the onset of menopause, must agree to use an acceptable form of birth control until 60 days following the last dose of the study drug, and females must agree to not breastfeed during the study;
8. Written informed consent obtained from the subject and ability for the subject to comply with the requirements of the study.

Exclusion Criteria:

1. Any known contraindications to interferon α or hypersensitivity to interferon α;
2. Patients with prior interferon therapy having poor tolerability or lack of efficacy to the previous interferon therapy per investigator's judgement;
3. Patients with an ongoing cytoreduction (e.g., HU or IFN-α) at the time of screening if, in the Investigator's opinion, randomizing them into the placebo arm will lead to immediate rebound increase of peripheral blood counts and thus may jeopardize their health status;
4. With severe or serious diseases that, in the Investigator's opinion, may affect the patient's participation in this study;
5. History of major organ transplantation;
6. Pregnant or breastfeeding women;
7. Patients with any other diseases that will affect the study results or may weaken the compliance to protocol per the Investigator's judgment;
8. Use any investigational drug <4 weeks prior to the first dose of study drug, or not recovered from effects of prior administration of any investigational drug.
9. Eligible for JAK inhibitor therapy at screening.

Double-Blind