FDA Approves First-of-Its-Kind Drug for Rare Blood Disorder Polycythemia Vera

For people living with polycythemia vera, the routine has often looked the same for years: regular trips to a clinic for therapeutic phlebotomy, a procedure that drains blood to keep dangerously thick, cell-packed blood in check. It works, but it treats the symptom, not the underlying problem. Now there's a new option that takes a different approach entirely.

The FDA has approved Mimrylo, known during development as rusfertide, for treating erythrocytosis, the excessive red blood cell buildup that defines polycythemia vera, in adults. The once-weekly injection comes from Takeda Pharmaceutical, though it was originally developed by Protagonist Therapeutics. Regulators announced the decision Friday after markets closed.

Polycythemia vera belongs to a family of rare blood cancers called myeloproliferative neoplasms. These conditions start with acquired mutations that push bone marrow into overproducing blood cells. For patients, that translates into headaches, dizziness, and a fatigue that doesn't lift with rest. More seriously, the thickened blood raises the odds of heart attack and stroke, turning a chronic condition into a persistent cardiovascular threat.

Here's where Mimrylo's design gets interesting. Instead of simply removing excess blood after the fact, the drug works upstream, closer to how a healthy body actually regulates red blood cell production. Mimrylo is an engineered version of hepcidin, a hormone that normally limits how much iron is available for bone marrow to build new red blood cells. Less available iron means fewer red blood cells produced in the first place.

Natural hepcidin, though, makes for a poor medicine on its own. "It's unstable, not soluble, and not very potent," Protagonist CEO Dinesh Patel said in an interview ahead of the approval. Protagonist's engineering platform reworked the peptide into something stable enough, and potent enough, to function as a weekly injection.

Trial Results and a Crowded but Limited Field

The Phase 3 data behind the approval are striking. At 32 weeks, 76.9% of patients receiving weekly Mimrylo injections avoided needing therapeutic phlebotomy. In the placebo group, only 32.9% managed the same. One-year results, presented last December at the American Society of Hematology annual meeting, reinforced those findings. The most common side effects were injection site reactions and anemia, manageable trade-offs for many patients weighing their options.

Dr. Andrew Kuykendall, the study's lead investigator and an associate member in the hematology department at Moffitt Cancer Center, didn't mince words about what this could mean for everyday care. "It's replacing therapeutic phlebotomy, which is archaic," he said at the ASH meeting. Since phlebotomy remains, in his words, "the mainstay of treatment for everyone at some point in time during their [polycythemia vera] course," a drug that reduces reliance on it could reshape standard practice.

Polycythemia vera patients haven't been without options before now. Incyte's Jakafi, a JAK inhibitor first approved for myelofibrosis in 2011, picked up a polycythemia vera indication in 2014, but only as a second-line treatment for patients who fail hydroxyurea, an older cancer drug. PharmaEssentia's Besremi, an engineered interferon alpha, has served as a first-line option since 2021.

Besremi comes with real limitations, though. Kuykendall noted that interferon-based therapies like it can't be used by patients with mood disorders, since the drug class tends to worsen depression symptoms. Patients with autoimmune conditions face similar restrictions, because interferon therapies can overstimulate an already overactive immune system. Those risks appear as a black box warning on Besremi's label, the FDA's strongest safety alert. That warning, Kuykendall said, leaves a real gap for patients who either can't tolerate existing therapies or need better disease control than what's currently available.

Mimrylo's label sidesteps those restrictions almost entirely. There's no requirement tying the drug to a specific line of therapy, and though the pivotal trial enrolled patients with uncontrolled high red blood cell counts who relied on phlebotomy, the FDA approval itself doesn't require patients to be phlebotomy-dependent to qualify. That's a meaningfully broader door than what Jakafi or Besremi offer.

The business side of this story matters too, if only because it shapes what happens next for both companies. Takeda's original 2024 deal with Protagonist put $300 million upfront toward the collaboration, with Protagonist handling the pivotal trial and the two companies agreeing to share U.S. profits. But Protagonist had an opt-out clause, and in April, the company took it. That decision brought an immediate $200 million payment, with another $200 million tied to approval, plus a $75 million milestone payment triggered by the FDA's decision. Protagonist remains eligible for up to $975 million more in future milestones, along with royalties on Takeda's sales.

Patel framed the choice as a strategic bet on independence rather than a retreat. "It takes us on a path of financial independence, relatively speaking, funding our R&D pipeline in a non-hesitant manner, not a careless manner, but in a confident manner, and creating value for shareholders without diluting them," he said. Takeda now holds exclusive global commercialization rights and has projected Mimrylo could reach $2 billion in peak revenue, a meaningful boost as the company's current top seller, the inflammatory bowel disease drug Entyvio, faces looming patent expirations. Leerink Partners had separately modeled $2 billion in sales by 2035, which would work out to roughly $470 million in royalties flowing back to Protagonist.

For patients, the practical numbers matter most. A Takeda spokesperson confirmed Mimrylo's list price at $4,200 per vial. Assuming one vial per weekly dose, that adds up to about $218,400 a year, a steep cost that will raise questions about insurance coverage and access even as the clinical case for the drug looks strong. Mimrylo is available now, and an open-label extension of the pivotal study continues, with Takeda planning to share further data at upcoming medical conferences.

In related news, PharmaEssentia's Besremi just picked up a separate FDA approval, this time for essential thrombocythemia, another myeloproliferative neoplasm marked by excessive platelet production rather than red blood cells. Patients with this condition face elevated risks of blood clots, abnormal bleeding, and enlarged spleens. Standard first-line treatment remains hydroxyurea, with anagrelide typically used second-line. Besremi's Phase 3 study showed durable responses and fewer blood clot events compared with anagrelide over twelve months. Dr. Ruben Mesa, the study's principal investigator and president of Advocate Health's Cancer National Service Line, called it a treatment that "works at the source of the disease rather than solely managing symptoms."

Why It Matters

Rare blood disorders like these don't make daily headlines, but the people living with them face real, compounding risks: strokes, heart attacks, and a quality of life eroded by fatigue and repeated medical procedures. New treatment options that work with the body's own regulatory systems, rather than against them, offer something beyond incremental improvement. They offer patients and physicians more paths toward disease control that fit individual circumstances, whether that means avoiding phlebotomy, sidestepping interferon's psychiatric risks, or simply having a first-line choice that wasn't available before. Cost will remain a real barrier for many, but the clinical direction here, treating root causes rather than just symptoms, is one worth watching closely as more data emerges.