Hepcidin mimetic · polycythemia vera
A synthetic peptide that mimics hepcidin, the main hormone that controls how much iron the body makes available. It is approved in the US as Mimrylo to control excess red blood cell production in adults with polycythemia vera, a chronic bone-marrow neoplasm, and is injected under the skin. Protagonist Therapeutics developed it under the code PTG-300.
In polycythemia vera, the bone marrow makes too many red blood cells, which raises the hematocrit (the share of blood made up of red cells). Patients have traditionally needed repeated phlebotomy, or blood removal, to keep it down3. Making red cells requires iron. Hepcidin is the hormone that limits how much iron is available, and rusfertide copies that action by blocking the iron transporter ferroportin1,2. With less iron reaching the bone marrow, fewer red cells are made and the hematocrit falls1. Because iron is held back rather than used, ferritin, a marker of stored iron, rose during treatment in the phase 3 trial1. In a smaller study, markers of iron deficiency such as ferritin improved on treatment4.
The key trials enrolled patients who needed frequent phlebotomy. In the phase 2 REVIVE trial of 70 patients, mean hematocrit stayed below 45% during a 28-week dose-finding period. In a 12-week randomized withdrawal phase, 60% of those kept on rusfertide responded versus 17% switched to placebo2. The pivotal phase 3 trial randomized 293 patients, on top of their usual care, to rusfertide or placebo for 32 weeks. A response, meaning no longer meeting the criteria for phlebotomy during weeks 20 to 32, was seen in 77% versus 33%. Phlebotomies fell (mean 0.5 vs 1.8), and fatigue and symptom scores improved3.
In an open-label phase 2 study of 20 patients with high starting hematocrit, 85% were below 45% by week 8 without phlebotomy4.
FDA-approved in August 2026 as Mimrylo (Takeda) for erythrocytosis, or too many red blood cells, in adults with polycythemia vera1. It was approved as a new molecular entity.
Per the FDA label, the most common side effects in the phase 3 trial were injection-site reactions (56% vs 33% on placebo) and anemia (16% vs 4%); thrombocytosis and shortness of breath were also more frequent than with placebo1. Platelet counts rose by about 31% on average within four weeks, so blood counts are monitored. Animal studies showed harm to the fetus, so effective contraception is advised, and breastfeeding is not recommended. The trial report also listed fatigue among the most common events3.
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