Rusfertide and the hepcidin mimetics: a peptide for polycythemia vera

Polycythemia vera and why blood-letting has been the mainstay

Polycythemia vera is a blood cancer, a myeloproliferative neoplasm, in which the bone marrow makes too many red cells. Thicker blood raises the risk of clots, so treatment aims to keep the haematocrit, the share of blood made up of red cells, below 45%. For decades the main tool has been phlebotomy: removing blood, like a blood donation, sometimes many times a year, often alongside drugs that suppress the marrow. A 2026 review notes that although the disease is driven by constant JAK2 signalling, red-cell overproduction still depends on iron availability, which is the rationale for targeting iron (review) ¹.

Phlebotomy works partly by draining iron, but it is a blunt instrument: repeated procedures, fatigue and swings in haematocrit between sessions. Treatment choices are built around each patient's thrombosis risk, and phlebotomy is recommended for patients in every risk group ².

How a hepcidin mimetic works

Hepcidin is the body's master iron hormone, made by the liver. It works by blocking ferroportin, the channel that lets iron out of gut cells and stores into the blood. Rusfertide is a synthetic peptide that copies this action. Its label explains that it blocks ferroportin, and that this reduces the iron available for making red cells, lowering haematocrit (drug label) ³. In healthy volunteers, serum iron fell most 24 to 48 hours after a dose, with higher doses keeping it down for up to 96 hours ³.

The effect is a controlled iron restriction aimed at red-cell production, maintained by weekly dosing and adjusted to each patient's haematocrit ³. Interestingly, iron stores rebound rather than fall: in the phase 3 trial, average ferritin rose from 21 mcg/L at baseline to 124 mcg/L at week 32 ³, because iron is no longer being removed by phlebotomy and is held in stores instead.

Iron deficiency: the hidden cost of phlebotomy

There is a further reason iron-directed treatment interests haematologists. A 2024 review notes that people with polycythemia vera often already have iron deficiency at diagnosis, because their marrow uses so much iron, and that repeated phlebotomy can worsen it, producing fatigue, lethargy and poor concentration; it describes rusfertide as controlling haematocrit without requiring phlebotomy (review) ². By restricting iron's use for red cells rather than removing iron from the body, a hepcidin mimetic lets stores recover, which the ferritin rise in VERIFY reflects ³. Whether that translates into less fatigue for patients is one of the questions the trial's patient-reported measures were designed to explore ³.

The approval, as of October 3, 2026

Takeda announced FDA approval of Mimrylo on August 28, 2026, for erythrocytosis in adults with polycythemia vera; the application had received priority review ⁴. Drugs@FDA lists it as NDA 220605 ⁵. The label sets a starting dose of 19 mg by injection under the skin once a week, adjusted for efficacy or safety within a range of 9.5 to 108 mg weekly; its half-life is about 28.6 hours (drug label) ³. Rusfertide was discovered by Protagonist Therapeutics and licensed to Takeda ⁴.

What the VERIFY trial showed

VERIFY was a randomised, double-blind, placebo-controlled phase 3 trial in 293 adults with polycythemia vera who needed frequent phlebotomy, at least three in 28 weeks or five in a year, despite standard care, which included phlebotomy alone or with marrow-suppressing drugs. Patients took rusfertide or placebo on top of their existing treatment for 32 weeks. A responder was someone who did not become eligible for phlebotomy between weeks 20 and 32 (drug label) ³.

Responders made up 76.9% of the rusfertide group against 32.9% on placebo, and the mean number of phlebotomies from baseline to week 32 fell to 0.53 against 1.82 (randomised trial, as reported in the label) ³. Earlier, in the phase 2 REVIVE trial, patients who had averaged 8.7 phlebotomies a year before treatment needed 0.6 a year during rusfertide dose-finding, and in the randomised withdrawal part 60% of those kept on rusfertide responded against 17% switched to placebo (randomised trial) ⁶.

Who was in the trial

VERIFY's participants had a mean age of 57, ranging from 27 to 86; 73% were men and 89% were White. At randomisation, 44.7% were on phlebotomy alone, 38.9% on phlebotomy plus hydroxyurea, 13.3% on phlebotomy plus interferon and 2.4% on phlebotomy plus ruxolitinib. About half, 46.8%, were classed as high risk, aged 60 or over or with a past clot (drug label) ³. Patients who completed 32 weeks could continue on open-label rusfertide to week 52 and into a long-term extension to week 156 ³. That breadth of background treatment matters: rusfertide was added to existing care, not used in place of it, so the results describe its effect on top of standard therapy.

How it is given

The label describes weekly injections under the skin, starting at 19 mg and titrated to keep haematocrit below 45%, within a 9.5 to 108 mg weekly range; doses above 54 mg need two injections and doses above 82 mg are given on separate days (drug label) ³. Rusfertide is broken down into smaller peptides by two pathways, and two active metabolites account for a meaningful share of exposure at steady state ³. A thorough QT study in healthy adults, given 90 mg alongside placebo and moxifloxacin controls, found no clinically relevant effect on heart rate or cardiac conduction, with changes in the corrected QT interval closely following placebo (randomised crossover study) ⁷.

Side effects and warnings

The label lists three warnings. Platelets can rise: within four weeks of starting, platelet counts rose by an average of 31%, 36% of patients exceeded 600 x10^9/L and 6% exceeded 1,000 x10^9/L, usually plateauing by week 8, and 1% stopped because of high platelets; the label advises complete blood counts every 2 to 4 weeks after starting and during dose changes. Injection-site reactions are common, and the label suggests ice, topical corticosteroids, antihistamines or painkillers as needed. Based on animal data, it can harm a fetus, so effective contraception is advised (drug label) ³. The most common adverse reactions in the trial were injection-site reactions in 56% and anaemia in 16% ³.

The platelet rise matters in a disease where clotting is the main danger, which is why monitoring is built into the label.

What it does not yet prove

VERIFY's endpoints were haematocrit control and phlebotomy, which are surrogate markers. The 2026 review points out that current evidence rests largely on surrogate haematological endpoints, and that whether iron-directed treatment reduces thrombotic risk or changes the course of the disease remains undefined (review) ¹. In other words, rusfertide is proven to keep haematocrit down without frequent blood-letting; it is not yet proven to prevent strokes or heart attacks, or to slow the cancer itself, which drugs aimed at the malignant clone attempt.

Where it fits among polycythemia vera treatments

The treatments already used in polycythemia vera, such as hydroxyurea, interferon and ruxolitinib, act on the overactive marrow or its signalling; VERIFY enrolled patients taking each of them ³. Rusfertide works differently, by controlling the iron supply rather than the malignant cells. The 2026 review frames targeting the hepcidin-ferroportin axis as a complementary approach to therapies aimed at the disease clone, not a replacement for them (review) ¹. In practice that means a haematologist will weigh it as an addition for patients who still need frequent phlebotomy, which is exactly who VERIFY studied, rather than as a stand-alone answer to the disease.

The trial also measured fatigue with a standard patient questionnaire, PROMIS Fatigue Short Form 8a, because fatigue is a major complaint in the disease ³; patients and doctors will want to see how those results hold up over longer follow-up.

Beyond polycythemia vera

Because it controls iron, rusfertide has been studied in other conditions. In an open-label phase 2 trial in hereditary haemochromatosis, adults on maintenance phlebotomy received weekly rusfertide with the aim of keeping transferrin saturation under 40% (open-label trial) ⁸. In a study of 20 patients with poorly controlled polycythemia vera, 85% had a haematocrit below 45% by week 8 and none needed phlebotomy during treatment (open-label study) ⁹. These are not approved uses. Protagonist has also announced an oral hepcidin-mimetic candidate, though it is early-stage ¹⁰.

A designed peptide, not a research peptide

Rusfertide is a good example of what a modern peptide drug looks like: a molecule designed to copy a natural hormone, tested in placebo-controlled trials, and approved with a label that spells out monitoring. Protagonist's other peptide to reach approval, icotrokinra (Icotyde, marketed by Janssen for plaque psoriasis), followed a similar path ^{5 10}. That contrasts with research peptides sold online, which skip every one of those steps; our guide to 'research use only' explains the difference, and our guide to how peptides are made covers quality.

Questions to ask a haematologist

• How often do I currently need phlebotomy, and would rusfertide reduce that?
• How will my platelets and blood counts be monitored, and what would change the plan?
• Should I continue my current marrow-suppressing treatment alongside it?
• What do we know about clot risk on rusfertide compared with phlebotomy?
• What contraception is needed, and for how long after stopping?

The bottom line

Rusfertide, approved as Mimrylo in August 2026, gives polycythemia vera patients a way to control red-cell counts by restricting iron, mimicking the body's own hepcidin. In its phase 3 trial it greatly reduced the need for blood-letting. It raises platelets in many patients and needs regular blood counts, and it has not yet been shown to prevent clots or change the disease itself. For a condition long managed by draining blood, it is a genuinely new tool.

Frequently asked questions

Is rusfertide FDA-approved?

Yes. FDA approved it as Mimrylo on August 28, 2026, for the treatment of erythrocytosis in adults with polycythemia vera.

How does rusfertide work?

It mimics hepcidin, the iron-control hormone, by blocking the iron transporter ferroportin. That reduces the iron available for making red blood cells, lowering haematocrit.

How well did rusfertide work in trials?

In the phase 3 VERIFY trial, 76.9% of patients on rusfertide avoided phlebotomy eligibility in weeks 20 to 32, against 32.9% on placebo.

What are rusfertide's side effects?

The most common were injection-site reactions (56%) and anaemia (16%). Platelet counts rose in many patients, so blood counts are checked every 2 to 4 weeks after starting.

Does rusfertide prevent blood clots?

Not shown. Its trials measured haematocrit control and phlebotomy; a 2026 review notes that effects on thrombotic risk remain undefined.

Related on Grey Peptides

• Rusfertide
• Icotrokinra
• Wire: FDA approves rusfertide

Sources

1. Costa, A., et al. (2026). Targeting the hepcidin-ferroportin axis in polycythemia vera: a complementary approach to clone-directed therapies. Blood Rev, , 101420. PMID: 42660705
2. Edahiro, Y., et al. (2025). Iron deficiency and phlebotomy in patients with polycythemia vera. Int J Hematol, 121(1), 39-44. PMID: 39528901
3. Takeda Pharmaceuticals America. MIMRYLO (rusfertide) US prescribing information: Indications and Usage, Dosage and Administration, Warnings and Precautions 5.1-5.3, Adverse Reactions, Clinical Pharmacology 12.1-12.3, Clinical Studies (VERIFY, NCT05210790). DailyMed version 2, effective August 28, 2026; read October 3, 2026.
4. Takeda. FDA Approves Mimrylo (rusfertide) for the Treatment of Polycythemia Vera. Press release, August 28, 2026; and Takeda and Protagonist, NDA acceptance and priority review, March 2, 2026. Read October 3, 2026.
5. Grey Peptides encyclopedia entries for rusfertide (Drugs@FDA NDA 220605) and icotrokinra (approved as Icotyde). Read October 3, 2026.
6. Kremyanskaya, M., et al. (2024). Rusfertide, a Hepcidin Mimetic, for Control of Erythrocytosis in Polycythemia Vera. N Engl J Med, 390(8), 723-735. PMID: 38381675
7. Modi, N. B., et al. (2025). Evaluation of Rusfertide, a Hepcidin Mimetic, on Cardiac Repolarization: A Randomized, Placebo- and Positive-Controlled Crossover Thorough QT Study in Healthy Participants. Clin Ther, 47(11), 1043-1052. PMID: 41033871
8. Kowdley, K. V., et al. (2023). Rusfertide for the treatment of iron overload in HFE-related haemochromatosis: an open-label, multicentre, proof-of-concept phase 2 trial. Lancet Gastroenterol Hepatol, 8(12), 1118-1128. PMID: 37863080
9. Chew, L. P., et al. (2025). Rusfertide rapidly decreases hematocrit in patients with suboptimally controlled polycythemia vera. Leuk Res, 159, 108132. PMID: 41175501
10. Protagonist Therapeutics. Fourth quarter and full year 2025 results (Form 8-K exhibit, February 25, 2026), announcing PN-8047, an oral hepcidin functional mimetic. Read October 3, 2026.

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