Merck KGaA adds a Phase 3-ready polycythemia vera drug to its rare-disease push
Merck KGaA is buying Silence Therapeutics in a $1.1 billion all-cash transaction that brings a late-stage RNA medicine into one of hematology’s most crowded but still under-served rare blood cancer markets. The acquisition gives the German company divesiran, an investigational siRNA therapy being developed for polycythemia vera, a disease defined by excessive red blood cell production, thickened blood, and elevated cardiovascular risk. For Merck, the appeal is not just the asset itself. The deal also adds a biologically distinct platform built around gene silencing, a mechanism that could be leveraged beyond a single indication if the chemistry and delivery package continue to hold up in late-stage development.
Polycythemia vera remains anchored to a familiar clinical problem: too many red blood cells, too much viscosity, and too much thrombotic danger. Therapeutic phlebotomy is still the mainstay for many patients because it directly lowers hematocrit, but repeated blood removal is an imperfect answer to a chronic marrow disorder. The disease can progress over time to myelofibrosis or acute myeloid leukemia, so the therapeutic bar is not merely symptom control. Any agent that reduces the need for phlebotomy while keeping hematocrit below a danger threshold has the potential to change how clinicians think about disease management.
Divesiran targets iron handling through TMPRSS6 silencing
Divesiran works by silencing TMPRSS6, the gene that codes for a liver protein acting like a brake on hepcidin. Hepcidin is the hormone that helps regulate iron availability, and increasing hepcidin keeps more iron stored in the body instead of circulating to the marrow for red blood cell synthesis. In practical terms, the drug is trying to restrain the iron supply line that fuels erythrocytosis. That mechanism matters because it attacks the upstream biology of the disease rather than repeatedly chasing the downstream consequence with venesection.
Silence Therapeutics has described its approach as gene silencing through short interfering RNA, or siRNA, a platform designed to degrade messenger RNA before the target protein is made. In a disorder driven by excess red cell production, this strategy offers a clean mechanistic rationale. It also distinguishes divesiran from other emerging treatments that attempt to mimic or replace hepcidin through different molecular designs. The competitive race in this indication is no longer just about efficacy; it is also about how often a patient must be treated, whether the regimen is durable, and how tolerable the delivery schedule proves to be in real-world use.
Phase 2 data gave the drug momentum before the takeover
The transaction follows Phase 2 findings that gave divesiran substantial momentum. In August, Silence reported an 88% response rate with divesiran versus 19% in the placebo arm. Response in that study meant no phlebotomy and hematocrit below 45%, a clinically meaningful threshold because hematocrit above that level is linked with higher cardiovascular mortality. The study examined both six-week and 12-week dosing schedules for the injectable treatment.
A single result can change the tone of a development program quickly when the endpoint is tightly tied to bedside decision-making. Here, the clinical signal was not abstract biomarker movement; it was the absence of phlebotomy combined with hematocrit control. That pairing is especially relevant in polycythemia vera, where repeated procedures are burdensome and uncontrolled hematocrit is a direct path to stroke, myocardial infarction, and other vascular events. The Phase 2 outcome also gave the company a dosing story to tell, because the longer interval looked suitable for later development and potentially better suited to chronic use than more frequent injections.
Silence had already said it would move forward with every-12-week dosing in a Phase 3 study planned for the first half of 2027. That schedule could become an important competitive lever. Takeda’s newly approved Mimrylo is given weekly, making the difference between weekly and quarterly administration more than a convenience issue. In a disease that often requires long-term management, injection burden can shape adherence, patient preference, and overall uptake in hematology practice.
Competitive pressure in polycythemia vera is intensifying
The acquisition lands in a field that is getting more crowded by the month. Takeda’s Mimrylo, a peptide engineered to stand in for hepcidin, was approved in late August for polycythemia vera. PharmaEssentia’s Besremi, an engineered interferon alpha product, is given every two weeks and has held an FDA-approved position in the disease since 2021. Incyte’s Jakafi remains an established second-line option after receiving the indication in 2014. Each of these drugs addresses a different niche in treatment sequencing, but all of them reflect the same clinical need: durable control of hematocrit and disease burden without relying indefinitely on phlebotomy alone.
The TMPRSS6 space is also taking shape beyond Silence. Agios Pharmaceuticals has a TMPRSS6-silencing siRNA drug in early development, Disc Medicine is testing an antibody that blocks the protein coded by TMPRSS6, and Ono Pharmaceutical’s sapablursen, an antisense oligonucleotide targeting TMPRSS6, is in late-stage development. The field is converging on the same pathway from different molecular angles, which suggests the biology has been validated even as the best modality remains unsettled.
Merck is betting on late-stage rare disease assets
Merck KGaA has been building its pipeline with rare-disease assets that are close to commercialization or already in the clinic. Last year, the company exercised an option for global commercialization rights to pimicotinib, which is under FDA review for tenosynovial giant cell tumor. It then agreed to pay $3.9 billion to buy SpringWorks Therapeutics, a company with two FDA-approved drugs for rare tumors. The Silence acquisition extends that pattern into hematology and gives Merck a foothold in a disease area where mechanism, dosing convenience, and long-term control all matter to physicians and patients.
Merck’s healthcare chief described the fit as one based on a biologically validated mechanism of action. The company also sees the deal as a way to advance a differentiated pipeline and broaden the number of patients it can reach faster. Those corporate aims are plain enough. The harder question sits in the clinic: whether a quarterly siRNA therapy can deliver enough durable hematocrit control to compete with approved agents and emerging rivals once Phase 3 testing begins.
Financially, Merck agreed to pay $20 in cash for each American depositary share of Silence, a price representing a more than 130% premium to the biotech’s closing stock price on Friday and an 82% premium to the 30-day average. The boards of both companies have approved the deal, though it still needs shareholder and regulatory clearance. Closing is expected in the first quarter of 2027.
For now, the transaction turns a promising Phase 2 signal into a larger strategic bet. The next decisive data will come from the planned Phase 3 study, where dosing interval, durability, and tolerability will have to justify the premium Merck is paying and the clinical position Silence believes divesiran can win.