Inclusion Criteria:

1. Age 18 years or older at the time of signing the informed consent (ICF). 2. For Phase 1b: Dynamic International Prognostic Scoring System (DIPSS) score of 3 to 4 (intermediate-2 risk) or ≥ 5 (high-risk) primary MF, post-PV MF, and/or post-ET MF, as confirmed in the most recent local bone marrow biopsy report, according to World Health Organization (WHO) 2016 criteria. 3. Washout of at least 28 days prior to Screening of the following treatments: androgens, erythropoietin, cladribine, immunomodulators (lenalidomide, thalidomide), interferon alpha-2a or any other MF-directed therapy. Systemic corticosteroids are permitted for non-hematological conditions if stable or decreasing dose for ≥ 28 days prior to Screening and receiving an equivalent to ≤ 10 mg prednisone for the 28 days immediately prior to Screening. 4. Anemia: For Phase 1b: Hemoglobin (Hgb) < 10 g/dL on ≥ 3 assessments over 84 days prior to Screening, without RBC transfusion, or Hgb < 10 g/dL and receiving RBC transfusions periodically but not meeting criteria for TD participant as defined for the TD cohort. The baseline Hgb value for these participants is the lowest Hgb level during the 84 days prior to Screening, or RBC transfusion dependence, defined as an RBC transfusion frequency of ≥ 6 units packed RBCs (PRBC) over the 84 days immediately prior to Screening. There must not be any consecutive 42-day period without an RBC transfusion in the 84-day period, and the last transfusion must be within 28 days prior to Screening. For Phase 2a: RBC transfusion dependence, defined as an RBC transfusion frequency of ≥ 6 units PRBC over the 84 days immediately prior to Screening. There must not be any consecutive 42-day period without an RBC transfusion in the 84-day period, and the last transfusion must be within 28 days prior to Screening. 5. Stable dose of JAK inhibitor (except momelotinib) and/or hydroxyurea, or, if taking any other treatment for MF, stable for at least 28 days prior to Screening. Momelotinib use requires 12 weeks of stable dosing prior to Screening. If subject discontinues JAK inhibitor (including momelotinib) and/or hydroxyurea prior to Screening, a 28-day washout period is required. 6. Eastern Cooperative Oncology Group (ECOG) performance score ≤ 2. 7. Infusion of hematopoietic stem cell transplant not anticipated within 8 months after Screening. 8. Transferrin saturation <75% (local lab acceptable). 9. Liver iron concentration by MRI < 7 mg/g dry weight within 3 months of eligibility confirmation. 10. Serum ferritin ≥ 30 μg/L at Screening. 11. Platelet count ≥ 25,000/μL and < 1,000,000/μL; neutrophils ≥ 1,000/μL; and total white blood cell (WBC) count < 50,000/μL at Screening. 12. Estimated glomerular filtration rate (eGFR) ≥ 30 mL/min/1.73m2 by the Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI) formula. 13. Aspartate aminotransferase (AST) and alanine transaminase (ALT) < 3.0 x upper limit of normal (ULN) at Screening. 14. Direct bilirubin < 2x ULN at Screening. Higher levels are acceptable if these can be attributed by the Investigator to ineffective erythropoiesis.

Exclusion Criteria:

Medical History: 1. Hereditary hemochromatosis. 2. Hemoglobinopathy or intrinsic RBC defect associated with anemia. 3. Total splenectomy. 4. Hematopoietic cell transplant within the past 10 years. 5. Current anemia from iron deficiency, vitamin B12 or folate deficiency, infection, or bleeding. 6. Active immune-mediated hemolytic anemia. 7. Symptomatic bleeding, unrelated to surgery, in a critical area or organ and/or bleeding causing a decrease in Hgb of ≥ 2 g/dL or leading to transfusion of ≥ 2 units of RBCs in the 6 months prior to Screening. 8. Major surgery within 8 weeks prior to Screening or incomplete recovery from any previous surgery. 9. Malignancy within the past 3 years, other than primary MF, post-ET, or post-PV MF. The following history or concurrent conditions are allowed: 1. basal or squamous cell carcinoma. 2. carcinoma in situ of the cervix or the breast. 3. histologic finding of prostate cancer (T1a or T1b using the tumor, nodes, metastasis [TNM] clinical staging system) A history of completed treatment (medical or surgical) of stage 1-2 cancers may be permitted with prior Sponsor agreement. 10. Stroke, deep vein thrombosis, or pulmonary or arterial embolism within 6 months prior to Screening. 11. Known allergic reaction to any study drug excipient, or anaphylaxis to any food or drug. 12. A history of anti-drug antibody formation. 13. Inadequately controlled heart disease (New York Heart Association Classification 3 or 4) and/or known to have left ventricular ejection fraction < 35% 14. Active Hepatitis B or C, or human immunodeficiency virus (HIV) with detectable viral load. 15. Uncontrolled fungal, bacterial, or viral infection (ongoing signs/symptoms related to the infection, without improvement despite appropriate treatment) Treatment History: 16. Iron chelation therapy in the 28 days prior to Screening. 17. Change in anticoagulant therapy regimen within 8 weeks prior to Screening. Laboratory Exclusions: 18. Peripheral blood myeloblasts ≥ 10% of WBC differential at most recent evaluation prior to Screening. 19. Positive direct antiglobulin test in conjunction with a reactive RBC eluate at Screening

Arms

Experimental: Phase 1b: Dose Escalation

In the Phase 1b (dose-escalation) portion of the study, DISC-0974 will be administered subcutaneously every 4 weeks.

Experimental: Phase 2a: Expansion

In the Phase 2a (expansion) portion of the study, DISC-0974 will be administered subcutaneously every 4 weeks.

Interventions

Drug: - DISC-0974

DISC-0974 is administered subcutaneously.