Silencing the Blood: What Divesiran's Phase 2 Win Means for Polycythemia Vera and the siRNA Era

On August 10, 2026, Silence Therapeutics announced that its Phase 2 SANRECO trial of divesiran in polycythemia vera had met its primary endpoint and all secondary endpoints. The headline number was striking: 88 percent of patients treated with divesiran achieved a clinical response during weeks 18 through 36, compared with 19 percent in the placebo arm. The p-value was less than 0.0001. In a disease where the standard of care has not meaningfully changed in decades, that is not a modest result.

Polycythemia vera is a rare myeloproliferative neoplasm in which the bone marrow produces too many red blood cells. The excess drives hematocrit above 45 percent, a threshold associated with a fourfold higher rate of death from cardiovascular and thrombotic events. The disease is chronic, progressive, and burdensome. Patients require repeated phlebotomies to reduce hematocrit, sometimes every few weeks, and many eventually need cytoreductive agents that carry their own toxicity profiles. There are currently no approved therapies that specifically target the underlying mechanism driving red blood cell overproduction. Divesiran is designed to change that.

A Different Kind of Silence

Divesiran is a short interfering RNA, or siRNA, that silences TMPRSS6, a gene expressed almost exclusively in the liver. TMPRSS6 is a negative regulator of hepcidin, the body's master regulator of iron metabolism. By silencing TMPRSS6, divesiran increases hepcidin production, which in turn restricts iron availability to the bone marrow and reduces the excessive red blood cell production that defines polycythemia vera. The mechanism is upstream, precise, and durable in a way that conventional small molecules rarely achieve.

The SANRECO Phase 2 trial enrolled 48 phlebotomy-dependent patients with polycythemia vera in a randomized, double-blind, placebo-controlled design. Patients received divesiran at 6 mg/kg subcutaneously either every six weeks or every twelve weeks. The primary endpoint was the proportion of patients achieving a response during weeks 18 through 36, defined as maintaining hematocrit below 45 percent without requiring a phlebotomy. Both dosing regimens met the primary endpoint, and the quarterly dosing arm performed comparably to the six-week arm. That finding matters considerably for the drug's commercial and patient-experience profile.

The Dosing Story Is the Real Story

The efficacy numbers are compelling, but the dosing frequency is what separates divesiran from the existing treatment landscape in a meaningful way. Polycythemia vera is a lifelong condition. Patients who require phlebotomy every few weeks are managing a disease that intrudes on their daily lives in a concrete, recurring way. A therapy that can be administered once every twelve weeks, with durable hematocrit control between doses, represents a qualitative shift in what disease management looks like for this population.

The SANRECO data showed that the mean number of phlebotomies in the divesiran arm was 0.2 over the 36-week study period, compared with 2.1 in the placebo arm. Patient-reported outcomes using the MPN-SAF Total Symptom Score also improved in the divesiran groups, capturing the fatigue, cognitive disturbance, and pruritus that make polycythemia vera more than a laboratory abnormality. The safety profile was consistent with prior divesiran trials: injection site reactions were infrequent and self-limiting, and no treatment-related serious adverse events or discontinuations were reported.

Silence Therapeutics has FDA Fast Track and Orphan Drug designations for divesiran in polycythemia vera. The company plans to submit the full SANRECO Phase 2 results abstract to the ASH Congress and expects an end-of-Phase 2 meeting with the FDA by year-end, with Phase 3 initiation targeted for the first half of 2027.

Where siRNA Fits in the Rare Blood Disease Landscape

The SANRECO results arrive at a moment when RNA interference is establishing itself as a durable therapeutic modality rather than a scientific curiosity. Inclisiran, the siRNA cholesterol-lowering drug approved in 2021, demonstrated that twice-yearly dosing could achieve sustained LDL reductions in a large cardiovascular population. Givosiran and lumasiran have shown that siRNA can address rare metabolic diseases with infrequent dosing and favorable safety profiles. The platform has proven itself across multiple disease areas, and divesiran extends that track record into myeloproliferative neoplasms.

What makes the polycythemia vera application particularly interesting is the specificity of the target. TMPRSS6 is expressed almost exclusively in the liver, which means the off-target risk profile is narrow. Hepcidin regulation is the primary biological consequence of TMPRSS6 silencing, and the downstream effect on iron availability to the bone marrow is the intended therapeutic mechanism. This is not a broad immunosuppressant or a cytotoxic agent. It is a precisely engineered intervention in a single regulatory pathway, and the Phase 2 data suggest that precision is translating into clinical benefit.

The Competitive Context and What Comes Next

Polycythemia vera is not a crowded therapeutic space, but it is not empty either. Ruxolitinib, a JAK1/2 inhibitor, is approved for patients who are inadequately controlled on or intolerant of hydroxyurea, and it has become a meaningful treatment option for higher-risk patients. Ropeginterferon alfa-2b, which received FDA approval for polycythemia vera in 2021, offers a disease-modifying approach with durable responses in some patients. Neither of these agents specifically targets the iron-hepcidin axis, and neither offers the dosing convenience that divesiran's quarterly regimen could provide.

The end-of-Phase 2 meeting with the FDA will be the critical near-term milestone. The agency's feedback on the Phase 3 design, including the primary endpoint, patient population, and comparator arm, will determine how quickly divesiran can move toward a registration trial and ultimately toward approval. The company has indicated that Phase 3 initiation is targeted for the first half of 2027, which would put a potential approval on a timeline that is meaningful for patients who are currently managing their disease with phlebotomy schedules and cytoreductive agents that have not changed substantially in a generation.

The SANRECO Phase 2 results are not a Phase 3 readout, and the history of rare disease drug development counsels appropriate caution about extrapolating from 48 patients to a registration trial. But the signal is clean, the mechanism is well-characterized, the dosing profile is differentiated, and the unmet need is real. For a disease that has been waiting for a therapy that addresses its root cause rather than its consequences, divesiran's Phase 2 win is the kind of result that earns serious attention.