9 Hepcidin mimetics in PV
Comprehensive Landscape Review — updated for SANRECO Phase 2 topline (10 Aug 2026)
About this update. This revision incorporates the Silence Therapeutics SANRECO Phase 2 topline results (investor presentation and press release, 10 August 2026) as the primary new source. It also brings the rusfertide regulatory section current (NDA submitted January 2026; FDA acceptance with Priority Review 2 March 2026; PDUFA in Q3 2026), adds a full section on sapablursen based on the IMPRSSION Phase 2a results presented at ASH 2025 (Blood 2025;146(Suppl 1):82), and updates DISC-3405 status from the Disc Medicine Q2 2026 business update. Earlier content derives from the Protagonist post-ASCO VERIFY IR deck (2 Jun 2025), the VERIFY Week-52 ASH 2025 abstract (abs25-4119), the Silence corporate presentation (Nov 2025), the SANRECO EHA 2025 abstract (S224), and the DISC-3405-201 ASH 2025 trials-in-progress poster. Tables are sortable and searchable; charts are interactive. Both R and Python were used to build them.
9.1 What changed in this update
9.2 Background
Polycythemia vera (PV) is a myeloproliferative neoplasm characterized by excessive production of red blood cells. Elevated hematocrit (HCT) is the hallmark, and the primary treatment goal is to maintain HCT <45% to reduce cardiovascular and major thrombotic events. PV affects approximately 150,000 people in the US and ~3.5 million worldwide, is diagnosed most commonly at age 50–70, and carries a median survival of roughly 20 years.
The randomized evidence anchoring the HCT <45% target is CYTO-PV (Marchioli, N Engl J Med 2013;368:22–33): 365 patients randomized to HCT <45% versus 45–50%, with death from cardiovascular causes or major thrombosis in 2.7% versus 9.8% over a median 31 months (HR 3.91, 95% CI 1.45–10.53).
Key unmet needs include inconsistent HCT control, iron deficiency exacerbated by phlebotomy and by iron-restrictive treatment, and a substantial symptom burden dominated by fatigue. Roughly 30–40% of patients on cytoreductive therapy have a suboptimal response or tolerability problems.
Therapeutic strategies targeting iron-mediated erythropoiesis in PV now span four agents across three mechanistic classes:
- Hepcidin mimetic peptide — rusfertide (Protagonist / Takeda)
- TMPRSS6-silencing siRNA — divesiran / SLN124 (Silence Therapeutics)
- TMPRSS6 antisense oligonucleotide — sapablursen (Ionis / Ono / Deciphera)
- Anti-TMPRSS6 monoclonal antibody — DISC-3405 (Disc Medicine)
9.3 Rusfertide
Rusfertide is a first-in-class hepcidin mimetic that reduces iron availability for erythropoiesis via weekly subcutaneous self-injection. Under the Protagonist–Takeda partnership, Takeda led the NDA submission. Estimated peak revenue potential is $1–2 billion.
9.3.1 REVIVE Study (Phase 2)
REVIVE was a Phase 2, open-label study (n=70; N Engl J Med 2024;390:723–35) that established proof of concept in PV patients requiring frequent phlebotomy, showing meaningful reductions in phlebotomy requirement and improved HCT control. It supported the pivotal Phase 3 VERIFY trial. The THRIVE long-term extension (n=46) and PACIFIC study (elevated HCT >48%, n=20) are additional program studies. Four-year REVIVE/THRIVE efficacy and safety data formed part of the NDA package.
9.3.2 VERIFY Study (Phase 3) — NCT05210790
VERIFY is the pivotal Phase 3 randomized, double-blind, placebo-controlled trial of rusfertide added to current standard-of-care (CSC) therapy. Part 1a (Weeks 0–32) primary results were presented at the ASCO 2025 Plenary Session (LBA3) on 1 June 2025 by Andrew Kuykendall, MD (Moffitt). Part 1b (Weeks 32–52) durability results were presented at ASH 2025 (abs25-4119).
9.3.2.1 Study Design & Baseline Characteristics
Concurrent cytoreductive therapy comprised hydroxyurea (39.2%), interferons (13.3%), and ruxolitinib (2.7%). Source: ASCO 2025 Plenary (LBA3); data cutoff 7 January 2025.
9.3.2.2 Study Timeline
9.3.2.3 Patient Disposition
Of 293 patients randomized 1:1 to rusfertide + CSC (n=147) or placebo + CSC (n=146), the safety analysis set was 291 (145 rusfertide, 146 placebo; two rusfertide patients not dosed). Screening enrolled 399, with 106 screen failures. 274 patients (93.5%) began open-label Part 1b, 267 enrolled in Part 2, and at the ASH 2025 data cut 254 (86.7%) remained on study receiving open-label rusfertide. Median rusfertide exposure was 61 weeks (range 2–133).
9.3.2.4 Endpoint Definitions
9.3.2.5 Primary & Key Secondary Endpoint Results (Part 1a, Weeks 0–32)
Rusfertide remains the only PV therapy to prospectively demonstrate a statistically significant improvement in patient-reported outcomes (PROMIS Fatigue and MFSAF) against placebo in a randomized Phase 3. pp = percentage points; CSC = current standard of care; LS = least squares.
9.3.2.6 Efficacy Visualizations — built in Python (Plotly)
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Among patients initially randomized to rusfertide, 61.9% (91/147) had a durable response, defined as absence of phlebotomy eligibility across Weeks 0–52. Patients originally on placebo who crossed over at Week 32 achieved absence of phlebotomy eligibility in 78.0% (110/141) during Part 1b (Weeks 40–52), versus 32.9% on placebo in Part 1a. Mean HCT stayed <43% throughout Part 1b in both continuing and crossover patients.
9.3.2.8 Safety Profile
Through Week 52 (rusfertide-treated, n=285): injection site reactions 47.4%, anemia 25.6%, fatigue 19.6%, most grade 1/2; serious AEs 8.1%; 2 thromboembolic events total (1 each in Parts 1a and 1b). Note the 6.9% versus 0% thrombocytosis signal, which is the class-level iron-restriction effect discussed in the platelet section below. Source: ASCO 2025 (LBA3) and ASH 2025 (abs25-4119).
9.3.2.9 Regulatory Status (current as of 10 August 2026)
The submission is based primarily on VERIFY, supported by four-year efficacy and safety data from REVIVE/THRIVE. Rusfertide will therefore almost certainly be the first approved agent in this class, and every subsequent entrant will be positioned against it.
9.4 TMPRSS6-Targeting Agents
TMPRSS6 (matriptase-2) is a serine protease expressed almost exclusively in the liver that negatively regulates hepcidin. Loss-of-function TMPRSS6 mutations cause iron-refractory iron-deficiency anemia (IRIDA), with elevated circulating hepcidin, providing human genetic validation that blocking TMPRSS6 raises hepcidin and restricts iron availability for erythropoiesis. Three modalities are now in clinical development against this target: an siRNA (divesiran), an antisense oligonucleotide (sapablursen), and a monoclonal antibody (DISC-3405).
9.4.1 Divesiran (SLN124) — Silence Therapeutics
Divesiran is a first-in-class GalNAc-conjugated siRNA targeting TMPRSS6 mRNA in hepatocytes, built on the mRNAi GOLD™ platform. Silencing TMPRSS6 produces sustained increases in endogenous hepcidin, restricting iron delivery to the marrow. Divesiran holds FDA Fast Track and Orphan Drug designations for PV.
9.4.1.1 SANRECO Phase 1 (NCT05499013)
An open-label, dose-finding study in 21 PV patients, completed February 2025; updated data presented at EHA 2025 (abstract S224; data cutoff 24 April 2025).
9.4.1.2 Phase 1 phlebotomy reduction (R / Plotly)
9.4.1.3 Phase 1 dose-dependent hemoglobin reduction (Python / Plotly)
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hemoglobin reduction at Day 169 (SANRECO Phase 1)"},"yaxis":{"title":{"text":"Mean change in hemoglobin (g\u002fdL)"},"gridcolor":"#eceff3"},"font":{"family":"Helvetica, Arial, sans-serif","size":13},"margin":{"t":60,"b":40},"plot_bgcolor":"white","paper_bgcolor":"white"}, {"responsive": true} ) }; </script> </div>Phase 1 safety: TEAEs in 19/21 (91%), majority grade 1; injection site reactions in 13/21; no dose-limiting toxicities, no treatment-related SAEs, no discontinuations. Data cutoff 24 April 2025.
9.4.2 SANRECO Phase 2 — topline results (reported 10 August 2026)
The Phase 2 portion is a three-part, global, randomized, double-blind, placebo-controlled study in 48 phlebotomy-dependent PV patients across 9 countries on 4 continents (US, Canada, Germany, Poland, Bulgaria, Spain, Italy, Malaysia, Australia). Patients were randomized 1:1:1 to divesiran 6 mg/kg Q6W, divesiran 6 mg/kg Q12W, or placebo for a 36-week double-blind period, followed by a 144-week extension: placebo patients are re-randomized 1:1 to the two divesiran regimens in a double-blind extension, then all participants transition to a preferred dose level in an open-label extension. All 48 patients have completed the placebo-controlled portion.
ImportantTwo design features that matter for cross-trial interpretation
- Patients had to be at HCT <45% before the first dose. The trial enrolled patients with historically uncontrolled hematocrit who were phlebotomy-dependent despite standard of care, but required phlebotomy down to HCT <45% prior to dosing. The primary endpoint therefore measures maintenance of control, not induction of control.
- The primary endpoint window runs Weeks 18–36, giving an 18-week lead-in before assessment begins. This is a longer run-in than VERIFY’s 20-week titration but a longer assessment window than VERIFY’s 12 weeks (Weeks 20–32).
9.4.2.1 Primary efficacy endpoint
Response defined as HCT remaining below 45% and no phlebotomies received during Weeks 18–36; p<0.0001 for the pooled comparison. Company-reported placebo-adjusted response rate = 69%. Wilson score intervals for proportions and Newcombe intervals for differences are computed here to make the precision explicit; they were not reported in the topline materials.
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Phase 2 primary endpoint, Weeks 18\u201336 (95% Wilson intervals)"},"yaxis":{"title":{"text":"% maintaining HCT \u003c45% without phlebotomy"},"range":[0,110],"gridcolor":"#eceff3"},"font":{"family":"Helvetica, Arial, sans-serif","size":13},"margin":{"t":70,"b":40},"plot_bgcolor":"white","paper_bgcolor":"white","showlegend":false,"annotations":[{"align":"left","font":{"color":"grey","size":11},"showarrow":false,"text":"n=16 per arm: intervals are wide even where point estimates are striking","x":0,"xref":"paper","y":1.06,"yref":"paper"}]}, {"responsive": true} ) }; </script> </div>9.4.2.2 Sensitivity analysis in the more phlebotomy-dependent subset
Restricting to the 40 patients who met the stricter Phase 1 entry criterion (a minimum of 3 phlebotomies in the previous 3 months or 5 in the previous 12 months), the separation widened rather than narrowed.
The direction of this sensitivity analysis is reassuring: the drug effect does not depend on including patients with lighter phlebotomy burden, and the placebo response falls to 1/13 in the more dependent subset. The caveat is that with 13 patients in the placebo comparator, that 7.7% carries an interval running from 1.4% to 33.3%.
9.4.2.3 Key secondary endpoint — phlebotomy burden
Mean phlebotomies per patient across Weeks 0–36 were 0.2 for divesiran versus 2.1 for placebo (p<0.0001). The proportion receiving no phlebotomy at all from baseline through Week 36 was 90.6% (29/32) versus 12.5% (2/16).
9.4.2.4 Other secondary endpoints and safety
9.4.2.5 Next steps
Silence is carrying the Q12W regimen into Phase 3, which maximizes the convenience differentiation but also means the pivotal programme inherits the arm with the lower point estimate (81.3%) and the wider interval (57.0–93.4%).
9.4.3 Sapablursen — Ionis / Ono / Deciphera
Sapablursen (ISIS 702843, IONIS-TMPRSS6-LRx) is a liver-directed, ligand-conjugated antisense oligonucleotide suppressing TMPRSS6. Ionis discovered the compound and ran IMPRSSION; Ono acquired exclusive global rights in March 2025, and Deciphera is running the pivotal programme. It received FDA Breakthrough Therapy designation in May 2025.
9.4.3.1 IMPRSSION Phase 2a (NCT05143957) — ASH 2025
Source: Palmer J, How J, Bose P, et al. Blood 2025;146(Suppl 1):82. The Weeks 17–37 “HCT <45% without phlebotomy” endpoint is operationally almost identical to the SANRECO Phase 2 primary endpoint, which makes it the single most useful external anchor for interpreting the divesiran result. The critical asymmetry is that IMPRSSION had no placebo arm and enrolled an older, higher-risk population.
9.4.4 DISC-3405 — Disc Medicine
DISC-3405 is a humanized IgG1 anti-TMPRSS6 monoclonal antibody (in-licensed from Mabwell; formerly MWTX-003), the only antibody in this class. In healthy volunteers (DISC-3405-101, NCT06050915), subcutaneous doses of 37.5–300 mg were well tolerated with no injection site reactions, produced dose-dependent PK, significantly raised hepcidin with corresponding falls in serum iron, and reduced hematocrit.
Disc is also running a Phase 1b in sickle cell disease (initial data expected Q4 2026). Sources: DISC-3405-201 ASH 2025 trials-in-progress poster (Gangat et al., Blood 2025;146(Suppl 1):2053); Disc Medicine Q2 2026 business update, 30 July 2026.
10 Cross-Trial Comparison
10.1 Why the headline numbers are not directly comparable
The four programmes use different response definitions, different assessment windows, different entry hematocrit requirements, and in two cases no control arm. The table below aligns them so that the comparison can be made with the caveats visible rather than buried.
10.2 Responder rates on a common axis
The metric plotted below is “proportion maintaining HCT <45% without needing phlebotomy” or the closest available equivalent from each trial. VERIFY’s primary endpoint is shown separately because its definition permits a hematocrit rise that the other trials would score as failure.
*VERIFY primary uses “absence of phlebotomy eligibility”, a more permissive definition than HCT <45% without phlebotomy. †VERIFY’s HCT <45% key secondary spans Weeks 0–32 including the 20-week titration period, permits a single excursion ≥45%, and does not require zero phlebotomy.
10.3 Head-to-head landscape
11 Clinical Interpretation and Q&A
The sections below work through the questions raised on and after the SANRECO Phase 2 call. Where a question asks for a share of a clinical panel, the answer is a judgement calibrated to a typical academic MPN practice and should be adjusted for local case mix.
11.1 1. How does the efficacy stack up against rusfertide?
The 87.5% versus 76.9% comparison is not an apples-to-apples read, and the honest answer is that divesiran looks at least as good and possibly better, but the trial cannot establish superiority.
Three points carry the argument.
The placebo-adjusted delta favours divesiran, but the intervals overlap. SANRECO’s pooled difference is 68.8 pp (95% CI 39.9–83.1); VERIFY’s primary difference is 44.0 pp (95% CI 33.1–53.3). The Q12W arm, the one going into Phase 3, gives 62.5 pp (95% CI 28.2–79.7). That lower bound of 28 pp sits well below VERIFY’s point estimate of 44 pp. With 16 patients per arm, the data are compatible with divesiran being clearly better, roughly equivalent, or somewhat worse than rusfertide.
SANRECO’s endpoint is stricter on one axis and easier on another. Stricter: it requires HCT below 45% throughout with no permitted excursion, whereas VERIFY’s primary endpoint scores a patient as a responder unless hematocrit reaches 45% and rises 3% above baseline, or reaches 48%. A divesiran patient drifting to 46% fails; a rusfertide patient doing the same may still count as a responder. Easier: SANRECO required phlebotomy to HCT <45% before the first dose, so every patient entered the study already controlled. VERIFY enrolled phlebotomy-eligible patients and used a 20-week titration to get there. The strict definition and the controlled starting point pull in opposite directions and roughly cancel.
The placebo arms differ more than the active arms do. SANRECO’s placebo rate was 18.8% against VERIFY’s 32.9%. Some of that gap is the endpoint definition, some is the smaller and more phlebotomy-dependent SANRECO population, and some is noise around 3 events in 16 patients. A good deal of divesiran’s apparent margin comes from a lower placebo rate rather than a higher drug rate.
Where divesiran does look distinctly strong is phlebotomy elimination: 90.6% received no phlebotomy at all across Weeks 0–36, against 72.8% freedom from phlebotomy over Weeks 0–32 in VERIFY. That is a cleaner comparison because the outcome definition is nearly identical.
Against the other TMPRSS6 agents the comparison is more lopsided. Sapablursen’s IMPRSSION endpoint is operationally the same construct over the same window length, and Cohort A reached 53.1% (17/32; 95% CI 36.4–69.1). Divesiran at 87.5% (28/32; 95% CI 71.9–95.0) sits well above that, and even the Q12W lower bound of 57% exceeds the sapablursen point estimate. IMPRSSION enrolled an older population with 59% high-risk patients and had no control arm, so the gap is not proof, but it is the most direct within-class read available and it favours divesiran substantially.
11.2 2. How much should the Phase 3 result be expected to degrade?
Expect the Q12W response rate to land somewhere around 72–82%, the placebo rate to rise to 20–33%, and the placebo-adjusted difference to compress from 62.5 pp to roughly 40–55 pp. That is still a decisive result. The reasoning:
The single largest technical risk is not average efficacy but end-of-interval escape on the Q12W schedule. Phase 1 showed hepcidin elevation persisting through 16 weeks after the last dose, which is encouraging, but the Phase 2 topline gives no hematocrit trajectory data. A programme that looks excellent on a period-summary responder endpoint can still show a sawtooth in individual patients, with hematocrit creeping toward 45% at Weeks 10–12 before the next dose. The full dataset should settle this.
11.3 3. Hematocrit versus platelet count as drivers of thrombosis in PV
This question deserves a direct answer because the arithmetic is lopsided.
Hematocrit carries essentially all of the randomized evidence. CYTO-PV assigned 365 PV patients to HCT <45% or 45–50% and found cardiovascular death or major thrombosis in 2.7% versus 9.8% over a median 31 months, roughly 1.0 versus 3.8 events per 100 patient-years. That is the only randomized demonstration that manipulating a hematologic parameter in PV changes thrombotic outcome. It is the reason the <45% threshold exists.
Platelet count has repeatedly failed to show an independent association with thrombosis in PV. In ECLAP (n=1,638) and subsequent series, platelet count was not an independent predictor of arterial or venous events. Leukocytosis above 15 × 10⁹/L showed a more consistent signal than platelets, particularly for myocardial infarction. Where extreme thrombocytosis does matter in the MPNs, above roughly 1,000–1,500 × 10⁹/L, the associated risk is bleeding through acquired von Willebrand syndrome, not clotting. The ELN complete hematologic response criteria include platelets ≤400 × 10⁹/L, but achieving that composite has never been shown to reduce thrombosis, and MAJIC-PV suggested molecular response rather than count normalization tracks with outcome.
The strategy has already been tested with its platelet effect included. Iron restriction raises platelets. Chronic phlebotomy is itself an iron-restrictive strategy, and CYTO-PV’s low-hematocrit arm achieved control largely through more phlebotomy and hydroxyurea. Whatever reactive thrombocytosis that produced was baked into the 4-fold benefit that was actually observed. A hepcidin-pathway agent reproduces the same physiology by a different route.
11.3.1 So is a platelet rise from ~460 to ~610 with hematocrit held below 45% a net gain?
Net risk reduction, and not a close call. Moving a patient from intermittent hematocrit above 45% to sustained control below it removes on the order of 2 to 3 major events per 100 patient-years, using the CYTO-PV event rates. For a platelet increment of roughly 150 × 10⁹/L within the 400–700 range to offset that, platelets would need a per-unit hazard that no PV dataset supports and that several datasets specifically fail to find. Both values also sit well below the range where acquired von Willebrand syndrome and bleeding become the operative concern.
Three qualifications are worth keeping in view rather than dismissing.
The first is that the supporting evidence for the platelet side is largely negative evidence from observational cohorts, and absence of a detected association in studies not designed to detect it is weaker than a demonstrated null. The second is that iron restriction produces microcytic, less deformable red cells, and whether a hematocrit of 44% composed of microcytes carries the same rheologic meaning as a hematocrit of 44% with normal indices has not been formally studied in this setting. That question applies equally to chronic phlebotomy, which has decades of use behind it, so it is a research question rather than a clinical objection. The third is that no agent in this class has been powered for thrombotic events. VERIFY recorded 2 thromboembolic events across 52 weeks in roughly 285 rusfertide-exposed patients and SANRECO Phase 1 recorded none, which is reassuring but consistent with a wide range of true rates.
The practical position: treat the platelet rise as an expected pharmacodynamic marker of iron restriction rather than a safety signal, monitor it, and reserve concern for values approaching 1,000 × 10⁹/L or for a patient with a bleeding phenotype.
11.5 5. How meaningful is the convenience advantage?
More meaningful than the usual dosing-frequency argument, for two reasons specific to this population.
Weekly self-injection for a chronic, largely asymptomatic condition in patients whose median age is late fifties has a persistence problem that trials do not capture. VERIFY reported injection site reactions in 55.9% of rusfertide patients during Part 1a and 47.4% through Week 52. Four injections a year against 52 changes both the adherence arithmetic and the cumulative injection site burden. Divesiran’s Phase 2 described injection site reactions as infrequent and self-limiting, which if it holds up compounds the advantage.
The second reason is that these patients are trading one burden for another. The alternative to drug is phlebotomy, which means clinic visits. A therapy requiring quarterly dosing and quarterly monitoring collapses into roughly the same visit schedule most PV patients already keep, whereas weekly injections add a daily-life intrusion on top of it.
The counterweight is the one already noted: infrequent dosing and durable pharmacology are the same property. The convenience is purchased with loss of control.
11.6 6. Reconciling “hemoglobin ran low in a larger number of patients” with two grade 1 anemia cases
These statements are not in conflict. They come from two separate data streams that are collected and adjudicated differently.
Laboratory hemoglobin values are captured on every patient at every visit and summarized in shift tables by CTCAE laboratory criteria. Investigator-reported adverse events are entered when the investigator judges a finding clinically meaningful. In a PV trial the whole therapeutic intent is to lower hemoglobin and hematocrit, so a value drifting below the laboratory lower limit is the drug working. Most investigators will not raise an adverse event for it unless the patient becomes symptomatic, needs a dose held, or crosses a threshold that concerns them. CTCAE grade 1 anemia spans hemoglobin from below the lower limit of normal down to 10.0 g/dL, which for a man means anything from 12.9 down to 10.0. A PV patient moving from 17 to 12 g/dL is a treatment success by protocol logic and a grade 1 laboratory abnormality by CTCAE logic.
The number that matters is not how many patients dipped below the lower limit. It is how many went low enough to require action. The reconciling data are the hemoglobin nadir distribution, the count below 10 g/dL and below 8 g/dL, any transfusions, any dose delays or omissions, and whether low values clustered in the Q6W arm, where cumulative exposure is higher, or in the Q12W arm.
Management framing this as two grade 1 events is technically accurate and rhetorically favourable. The right response is not to call it misleading but to ask for the shift table at ASH.
11.7 7. Concerns about quarterly dosing combined with low hemoglobin
Yes, and this is the most substantive clinical reservation about the profile.
The mechanism has an asymmetry that dosing interval amplifies. Hepcidin causes ferroportin internalization and degradation, so iron is trapped in enterocytes, macrophages and hepatocytes. When a patient on a hepcidin-raising agent becomes anemic, giving iron does not fix it efficiently, because the block is on iron export rather than on iron stores. The remedies are to stop the drug and wait, or to transfuse. With weekly rusfertide, stopping works within weeks. With a GalNAc-siRNA producing durable TMPRSS6 knockdown, Phase 1 showed hepcidin elevation and hematologic effect persisting through the 16-week post-dose follow-up. There is no antidote and no way to accelerate washout.
The scenarios where this matters are not exotic: a gastrointestinal bleed, elective surgery, a new malignancy requiring chemotherapy, progression to myelofibrosis with its own anemia, or simple aging into a lower baseline. None of these is common in any given quarter, but across a treated population followed for years they accumulate. Silence choosing Q12W for Phase 3 rather than Q6W maximizes the marketing advantage and simultaneously maximizes this exposure. The Phase 3 protocol should include an explicit dose-holding algorithm and pre-specified hemoglobin stopping rules, and the extension data should be scrutinized for cumulative over-suppression, since a drug given every 12 weeks reaches steady state slowly and the placebo-controlled window covered only three doses.
11.8 8. Does this increase confidence in TMPRSS6, and what should be expected from the antibody?
Confidence in the target goes up substantially. Before 10 August 2026 the TMPRSS6 hypothesis in PV rested on human genetics from IRIDA, an open-label 21-patient Phase 1, an open-label 49-patient Phase 2a with no control arm, and a healthy-volunteer study. SANRECO Phase 2 is the first randomized, double-blind, placebo-controlled demonstration that knocking down TMPRSS6 produces clinically meaningful hematocrit control and phlebotomy elimination in PV patients. The mechanism is now de-risked in a way it was not before.
What is not de-risked: the exposure-response relationship for a different modality, the width of the therapeutic window, immunogenicity for a biologic, and, for every agent in the class, whether hematocrit control by iron restriction translates into fewer thrombotic events. None of these agents alters JAK2 allele burden, controls leukocytosis, or has been powered for thrombosis.
For DISC-3405 specifically:
11.9 9. Where can other TMPRSS6 agents still differentiate?
Divesiran has set a high bar on the efficacy axis and the dosing axis. The remaining openings are mostly about control, safety and evidence quality rather than raw response rate.
11.10 10. What to look for when the full dataset is presented at ASH
NoteA methodological note on the endpoint itself
The SANRECO primary endpoint is a doubly dichotomized composite: a continuous hematocrit trajectory collapsed to a binary threshold, combined with a phlebotomy count collapsed to zero versus non-zero, summarized over a fixed window. This throws away most of the information the trial collected. With 48 patients that is an expensive choice. A model of the hematocrit trajectory itself, or a recurrent-event analysis of phlebotomies, would use all the data, estimate the treatment effect with far greater precision, and avoid the situation where a patient at 45.1% for one visit is scored identically to a patient at 52% throughout. The binary endpoint is what regulators currently accept, so the trial design is defensible, but the full ASH presentation should include the continuous analyses alongside it.
12 Summary
Four agents now target iron-mediated erythropoiesis in PV through three mechanisms: a hepcidin mimetic peptide acting directly at ferroportin, and three modalities that raise endogenous hepcidin by suppressing TMPRSS6.
Rusfertide holds the evidentiary lead with a completed Phase 3, durability through Week 52, the only placebo-controlled patient-reported outcome benefit in the class, and a PDUFA date in the current quarter. It will be first to market and it is the agent every subsequent entrant will be measured against.
Divesiran’s Phase 2 changes the competitive picture more than any single dataset since VERIFY. It provides the first randomized, double-blind, placebo-controlled evidence that TMPRSS6 knockdown controls hematocrit and eliminates phlebotomy in PV, and it does so with quarterly dosing. The efficacy point estimates are at least as good as rusfertide’s and the phlebotomy elimination is better, though with 16 patients per arm the confidence intervals cannot support a claim of superiority. The Phase 3 will use Q12W, which inherits both the stronger convenience argument and the weaker of the two point estimates.
Sapablursen validates the target from a third modality but its Phase 2a lacked a control arm and its hematocrit control rate of 53.1% sits well below divesiran’s. INTREPID corrects the design weakness. DISC-3405 reads out imminently and its differentiation is likely to rest on tolerability and on the ability to titrate, rather than on response rate.
The open questions common to all four agents are the same: none of them alters the JAK2 clone, none controls leukocytosis, none has been powered for thrombotic events, and all of them deepen functional iron restriction in a disease where iron deficiency already drives symptoms. The programme that answers the last of those questions convincingly will have the strongest claim on the low-risk population, which is where the largest untreated opportunity sits.
13 Appendix: Cross-Trial Eligibility Comparison
Where a criterion was not specified in the source materials it is marked as such rather than inferred.
HU = hydroxyurea; CRT = cytoreductive therapy; TE = thromboembolic event. Low-risk PV: age <60 and no prior thrombosis. High-risk PV: age ≥60 and/or prior thrombosis.