Doing more with less in hemophilia B gene therapy

In this issue of Blood Advances, De Wolf et al¹ have investigated a transgene-engineering strategy that yields more functional factor IX from each dose of adeno-associated virus (AAV) gene therapy. By fusing the hyperactive factor IX–Padua variant to an engineered, recycling-optimized human albumin, they obtained a sustained approximately fourfold increase in circulating factor IX antigen and approximately threefold in activity over a matched, nonfused control vector given at the same dose in hemophilia B mice, with durable correction of the bleeding phenotype and no rise in anti–factor IX antibodies, liver enzymes, or markers of thrombosis. Because this benefit comes from the transgene rather than a higher vector load, it is achieved without increasing the dose, which would otherwise raise the immune and hepatotoxic risk.

More factor IX from the same dose addresses a central unsolved problem in hemophilia B gene therapy: how to ensure that more patients reach a durably protective factor IX level.

Three AAV products for hemophilia B have been approved, and all 3 express the same hyperactive factor IX–Padua (R338L) variant, which already raises factor IX potency roughly 8- to 12-fold.² What differs is the capsid and dose, summarized with the long-term outcomes in the table.3, 4, 5 This Padua strategy is specific to hemophilia B. The approved hemophilia product valoctocogene roxaparvovec, which itself has been withdrawn in 2026 for commercial reasons, delivers a B-domain–deleted factor VIII; that deletion solves a packaging problem rather than boosting potency, and no comparable gain-of-function factor VIII variant is in routine clinical use.⁶

Approved factor IX–Padua AAV gene therapies for hemophilia B

Product (trial)	Capsid	n; follow-up	Not normalized (FIX <40%)	Resumed prophylaxis/extra factor needed
Etranacogene dezaparvovec (Hemgenix; HOPE-B)3; approved and marketed	AAV5	N = 54; 5 y	Mean, ∼36 IU/dL; approximately two-thirds below 40%; 2 nonresponders	3/54 (∼6%) on prophylaxis; others needed factor for bleeds and surgery
Fidanacogene elaparvovec (Beqvez; BENEGENE-2 and phase 1/2a)4; withdrawn 2025	AAVRh74var	Pivotal n = 45; follow-up (n = 15), 6 y	Mostly mild range (∼7-44 IU/dL); slow decline over time	Pivotal, 6/45 (∼13%) resumed prophylaxis; follow-up: none resumed; bleeds at FIX <25% treated with factor
Dalnacogene ponparvovec (BBM-H901)5; approved in China (2025)	Engineered liver tropic	Phase 3, N = 26; 52 wk	Mean, ∼42 IU/dL at 52 wk; 81% 0 bleeds; short follow-up	None resumed; longer follow-up needed

All 3 products express the factor IX–Padua (R338L) variant. Across different capsids, a consistent minority of recipients stay below normal and still need treatment. Normal factor IX activity is above ∼40%; the mild hemophilia range is ∼5% to 40%. Figures are approximate and drawn from long-term cohorts.

FIX, factor IX.

Across these trials, with follow-up now extending from ∼1 year to 5 to 6 years, mean factor IX activity has settled into the mild-to-normal range (see table). But that number hides how widely individual patients differ, and it is the lower end that defines the unmet need: most recipients are not fully normalized. In HOPE-B, only approximately one-third reached factor IX >40% (the normal range) at 2 years, and the mean held near 36% through year 5; therefore, most remained below normal throughout follow-up.³ These participants can still bleed, still need factor concentrate for breakthrough bleeds and surgery, and, in the most affected cases, return to prophylaxis. They are precisely the patients who would benefit from higher and more uniform expression, yet they have spent their one opportunity with this capsid because anti-AAV antibodies make redosing impractical. For two-thirds of participants who remain below the normal range, raising expression rather than the average is among the gaps recently identified in the field,⁷ and it is what this study targets. Simply increasing the dose is not an answer because larger vector loads drive transaminase elevations and capsid-directed immune responses that can compromise transgene expression and, in other AAV indications, have proved fatal.⁸

De Wolf et al approached the problem from the transgene side rather than the dose side. An albumin-fused factor IX already exists as an infused replacement protein with an extended half-life: albutrepenonacog alfa (Idelvion), which links recombinant factor IX to recombinant albumin.⁹ The difference here is that the same albumin-fusion concept is delivered by gene therapy and combined with the hyperactive Padua variant.

That distinction matters: an earlier effort to express albumin-fused factor IX from AAV encountered expression limits and offered little advantage over nonfused factor IX¹⁰; therefore, the benefit seen here rests on the specific protein engineering rather than on the fusion concept alone. The engineered albumin carries 3 substitutions (E505Q, T527M, and K573P) that tighten binding to the neonatal Fc receptor, which recycles albumin and immunoglobulin G, thereby prolonging their time in the circulation. A cleavable linker is the main design element: the albumin remains attached during circulation but is released when factor IX is activated; hence, clotting function is preserved. The codon-optimized cassette was driven by a liver-specific promoter and packaged into 2 liver-targeting capsids.

The advantage held across a full dose response in normal mice and at the lowest clinically relevant dose in hemophilia B mice. It was also not an artifact of greater vector uptake: when factor IX was normalized to liver vector-copy number and transcript level, the fusion still produced more circulating protein, so each delivered genome yields more functional factor IX. A western blot of liver tissue confirmed the fusion protein was produced intact rather than cleaved prematurely. The treated mice stayed immunologically tolerant to the factor IX they produced, with most (5 of the 6) raising no antibody even when deliberately challenged with an albumin-fused factor IX and adjuvant, and that tolerance depended on regulatory T cells.¹ Toxicity appeared only at a dose >2 orders of magnitude above the therapeutic range, which defines the safety ceiling. A transgene that combines both extended half-life and superactivity improves the therapeutic index, raising a practical question for trial design: could the dose be reduced? Reaching a protective range at a lower dose would sit further from the zone in which capsid-driven hepatotoxicity and immune responses cluster.

Two cautions are warranted; the first was flagged by the authors. Mice poorly predict the human AAV-capsid immune response; the transaminitis and T-cell responses seen in patients have been hard to reproduce in rodents and only emerged in the first human studies. Therefore, the favorable immune profile here cannot be assumed to carry over, and the authors call for confirmation in nonhuman primates or in patients. Second, supraphysiologic factor IX activity raises a theoretical thrombotic concern that a stable murine D-dimer cannot fully exclude; dose calibration will matter because the goal is robust hemostasis not the highest level.

These results also matter because the clinical field is contracting. Uptake of these 1-time therapies has been slow, and in 2025, fidanacogene elaparvovec was withdrawn worldwide barely a year after approval, with no patient treated commercially, leaving etranacogene dezaparvovec (Hemgenix) as the only hemophilia B gene therapy available in Western countries. A therapy that did more with less (higher, more uniform expression at a lower dose and cost) would address several reasons uptake has lagged. By gaining expression without a measurable safety penalty, the study points to the transgene itself as an underused source of therapeutic index for hemophilia B and potentially for other disorders needing durable expression of a secreted protein.

Conflict-of-interest disclosure: W.M. reports support from Bayer, Biomarin, Biotest, CSL Behring, Chugai, LFB, Grifols, Novo Nordisk, Octapharma, Pfizer, Regeneron, Roche, Sanofi, Sobi, and Takeda/Shire.