For Patients with Polycythemia Vera, Fewer Phlebotomies Could Mean More Time Back

The book had to be funny, something to lighten the start of the trip. She read it on the hour-and-a-half train from her farmhouse in the South Downs of England into London, and it carried her to the hospital doors, where the rest of the routine was waiting.

The blood draw came first. If her hematocrit (the share of blood made up of red blood cells) was high, she would go up to the day unit for a venesection, where blood would be removed to lower her red blood cell counts to thin her blood as a result of her polycythemia vera (PV) diagnosis.

Since 2004, that routine has been how Nona Baker controlled her PV, a rare blood cancer in which the bone marrow makes too many red blood cells. The excess cells thicken the blood and raise the risk of blood clots. Removing blood brings the count back down.

For decades, drawing blood has been one of the most reliable ways to control PV. A drug now in clinical trials is testing a different approach that slows the production of red blood cells instead.

In the phase 2 SANRECO trial, which met its primary end point, demonstrated that, among patients treated with divesiran, 88% needed no phlebotomy and kept their hematocrit below 45% between weeks 18 and 36, compared with 19% of patients who received placebo alongside standard treatment, which could include hydroxyurea, interferon and/or Jakafi (ruxolitinib).

The research had progressed since 1991, when a hematologist told Baker she had an incurable blood disorder and 15 years to live. She was 40, raising three children and working as a psychotherapist. Thirty-five years later, Baker is still here.

Venesection, the term used in the United Kingdom for therapeutic phlebotomy, has long been a standard treatment for PV. The procedure removes blood to help keep red blood cell levels under control, but for patients, it can take up a significant part of the day.

"Patients with polycythemia vera are typically people who are working, who have families and who do all of the normal activities," said Dr. Marina Kremyanskaya, lead investigator of the SANRECO trial and associate professor of medicine, hematology and medical oncology at the Icahn School of Medicine at Mount Sinai in New York. "So, it takes a significant chunk of their life to do this."

An appointment can run anywhere from an hour to more than three, and patients may need to travel to a hematology practice, have labs drawn, wait for the results and then undergo the procedure.

Polycythemia vera treatment over 35 years

Over the years, Baker's treatment options had narrowed, reopened and narrowed again.

One early medication was a chemotherapy drug, something she discovered by reading the patient information leaflet.

"I remember going into my [general practitioner] and saying, 'Nobody's told me I've got cancer. Why am I on this drug?'" she said.

Her doctor explained that she had a proliferation of cells and that the medication reduced them.

A second drug worked well for years, until the skin damage accumulated. She developed widespread actinic keratosis, rough, scaly patches of damaged skin that can become cancerous, as well as basal cell carcinomas that required removal.

Interferons came next, but her liver did not tolerate one and her gut did not tolerate the other. She is now back on the earlier medication at a reduced dose, with more venesections needed to keep her counts controlled.

"It's a chicken and egg situation," Baker said.

How does divesiran control hematocrit in polycythemia vera?

Kremyanskaya and her colleagues studied whether controlling how iron reaches the bone marrow could reduce the need for phlebotomy. Bone marrow needs iron to make red blood cells, and normally, when iron levels fall, red blood cell production slows, and a person may become anemic.

"Even though the body is iron deficient, the bone marrow [in PV] still takes all the iron that is available and still makes the red blood cells," Kremyanskaya said. "The body becomes really low on iron, because all of the iron is being funneled into the bone marrow."

Phlebotomy removes blood to bring hematocrit down, and over repeated draws it depletes the body's iron, which limits how many new red blood cells the marrow can make.

Divesiran is designed to limit that iron supply before another blood draw is needed. The drug increases levels of hepcidin, a hormone that controls how iron moves through the body, and by limiting how much iron reaches the bone marrow, it is designed to slow the production of excess red blood cells.

"What we are doing is turning off that flow of iron into the bone marrow," Kremyanskaya explained. "That gives a little bit of iron back to the rest of the body, which does not need that much for normal cell function."

In the SANRECO trial, researchers measured whether patients could maintain a hematocrit below 45% without phlebotomy between weeks 18 and 36.

"What it suggests is that with a drug like divesiran, we can potentially keep hematocrit under control without the need for therapeutic phlebotomies in the majority of patients," Kremyanskaya said. "That is a pretty exciting response rate."

The study also compared two dosing intervals, and Kremyanskaya said dosing every 12 weeks would be easier on patients than a six-week or shorter schedule, though close monitoring continues either way while the drug is still in trials.

Divesiran is given as an injection under the skin and can cause temporary redness or itching at the injection site. Two patients developed grade 1 (mild) anemia, a low red blood cell count, which Kremyanskaya said was expected based on how the drug works. SANRECO is a phase 2 study, and a phase 3 trial is being planned.

What could divesiran change about living with polycythemia vera?

Baker was not part of the SANRECO trial, and her routine has not changed. Phlebotomy still does what she needs it to do. But after decades of appointments, she has started thinking about what a lighter schedule would give back.

"Our health service here is incredibly stretched," Baker said. "I am taking up clinic time because I have to go up there to have my blood count done to see if I need a phlebotomy."

Fewer visits would mean less time planning around blood tests, train rides and hours at the hospital, and an appointment slot freed for someone else.

"I'm just so hopeful that one day something will happen that I won't have to think about having to have these phlebotomies," she said.

References

1. News release. "Silence Therapeutics Announces Positive Topline Results from Phase 2 SANRECO Trial of Divesiran in Polycythemia Vera, Supporting its Potential Best-in-Class Profile." Silence Therapeutics plc. Posted: August 10, 2026.

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