Hemophilia B: why gene therapy became the target

Hemophilia B is a rare X-linked bleeding disorder caused by deficiency of coagulation Factor IX (FIX), and its single-gene etiology made it one of the earliest and most compelling candidates for gene therapy. The clinical burden of the disease is substantial: without prophylactic treatment, patients face recurrent spontaneous bleeds into joints and soft tissues that accumulate into chronic arthropathy and disability. Standard of care for decades has been regular intravenous infusions of recombinant or plasma-derived FIX concentrate — a regimen that is effective but imposes a heavy treatment burden, requires venous access, and carries significant long-term cost for healthcare systems.

The appeal of gene therapy for hemophilia B rests on the prospect of a single treatment replacing a lifetime of infusions. By delivering a functional FIX gene to hepatocytes via an adeno-associated virus (AAV) vector, the liver is reprogrammed to produce FIX endogenously — ideally at levels sufficient to prevent spontaneous bleeding without ongoing prophylaxis. According to WIPO, gene therapy patent filings in rare bleeding disorders have grown substantially over the past decade, reflecting the commercial and scientific momentum behind this approach. The hemophilia B field has now reached a milestone that few rare disease areas achieve: two independently developed, regulatory-approved gene therapies targeting the same molecular defect.

The existence of two approved products — etranacogene dezaparvovec (Hemgenix, CSL Behring/uniQure) and fidanacogene elaparvovec (Beqvez, Pfizer) — creates an unusual competitive dynamic in the rare disease gene therapy space. Payers, clinicians, and health technology assessment bodies now face a genuine comparative question: given the high one-time acquisition cost of gene therapies, which product offers the more favourable benefit-risk profile, and for which patient subgroups? Answering that question rigorously requires both robust clinical trial data and, increasingly, real-world evidence from post-approval registries.

Two approved therapies, one disease: how Hemgenix and Beqvez differ

Etranacogene dezaparvovec (Hemgenix) and fidanacogene elaparvovec (Beqvez) share the same therapeutic goal — durable hepatic FIX expression — but were developed by different organisations using distinct vector and transgene designs. Both programmes are AAV-based and both incorporate the FIX-Padua gain-of-function variant, but the specific AAV serotype, promoter, and manufacturing approach differ between them.

Hemgenix, developed by uniQure and commercialised by CSL Behring, uses an AAV5 serotype vector. AAV5 was selected in part for its distinct seroprevalence profile relative to more common serotypes such as AAV2 and AAV8 — a consideration that affects the proportion of patients who may be excluded from treatment due to pre-existing neutralising antibodies. Beqvez, developed by Pfizer, uses a different AAV serotype and has its own liver-specific promoter architecture. The specific engineering choices made in each programme have direct implications for the IP landscape, as capsid patents, promoter patents, and transgene patents each represent distinct layers of intellectual property protection.

A critical practical difference between the two programmes concerns pre-existing immunity to the AAV vector. Patients with high titres of neutralising antibodies against the relevant AAV serotype are typically excluded from gene therapy trials and, in many cases, from approved treatment. The seroprevalence of different AAV serotypes varies by geography and age, meaning that the proportion of eligible patients may differ between Hemgenix and Beqvez — a factor with direct commercial and health economics implications. Regulatory agencies including the European Medicines Agency have required post-approval studies to characterise long-term durability and immunogenicity profiles for both products.

Pivotal trial designs and the evidence each programme generated

The HOPE-B trial was the pivotal study supporting regulatory approval of etranacogene dezaparvovec, while the BENEGENE-2 trial served the same role for fidanacogene elaparvovec. Both trials enrolled adults with moderately severe to severe hemophilia B and used annualized bleeding rate (ABR) reduction and FIX activity levels as primary endpoints — the standard efficacy measures in hemophilia gene therapy.

“Hemophilia B is a rare X-linked bleeding disorder caused by deficiency of coagulation Factor IX, representing a high-value target for gene therapy given its single-gene etiology and the clinical burden of prophylactic infusion therapy.”

Both trials were single-arm, open-label studies — a design that is standard in rare disease gene therapy given the impracticality of placebo-controlled trials in conditions with established prophylactic treatment. The primary comparator in both cases was the patient’s own pre-treatment ABR during a lead-in period on prophylaxis. This design generates strong within-patient evidence of efficacy but makes cross-trial comparisons methodologically challenging: differences in baseline ABR, lead-in duration, patient selection criteria, and outcome measurement windows can all confound apparent differences in treatment effect between the two programmes.

The absence of a direct head-to-head randomised controlled trial between the two products means that any comparative efficacy claim must be treated as an indirect comparison — with all the methodological caveats that entails. Key variables that differ between the trials include the specific FIX activity assay used, the definition of a qualifying bleed, and the duration of the primary efficacy observation window. According to data published by the U.S. Food and Drug Administration in its approval packages for both products, FIX activity levels and ABR reductions were the primary endpoints, but the specific numerical results and their confidence intervals require direct access to the primary publications and regulatory review documents for accurate citation.

What is clear from the approved labelling of both products is that neither therapy is universally applicable: patients with high neutralising antibody titres against the relevant AAV serotype are excluded, patients with active liver disease are excluded, and both products carry risks of hepatotoxicity requiring corticosteroid management. The patient selection criteria embedded in each approval therefore define distinct — though substantially overlapping — eligible populations.

IP landscape: AAV capsids, FIX-Padua, and the patent battleground

The intellectual property landscape for hemophilia B gene therapy is structured around three primary layers of patent protection: AAV capsid patents covering the vector delivery system, transgene patents covering the FIX-Padua gain-of-function variant and its codon-optimised expression cassette, and manufacturing process patents covering the production and purification of clinical-grade AAV vectors.

For Hemgenix, the foundational IP position rests substantially on uniQure’s AAV5 platform patents and the FIX-Padua transgene. The AAV5 serotype was not uniQure’s invention — it is a naturally occurring virus — but the specific engineering of an AAV5 vector carrying a codon-optimised FIX-Padua expression cassette under a liver-specific promoter represents patentable subject matter that has been pursued across multiple jurisdictions. Patent filings from uniQure and CSL Behring can be tracked across databases maintained by the European Patent Office, USPTO, and WIPO, and represent a critical intelligence resource for developers, investors, and freedom-to-operate analysts.

For Beqvez, Pfizer’s IP position reflects its own AAV vector engineering and its independently developed FIX-Padua expression system. Because both programmes use the same FIX-Padua variant, the freedom-to-operate analysis around the R338L mutation itself — and the original academic patents covering this naturally occurring variant — is a shared consideration for both programmes. The FIX-Padua variant was first described in a patient from Padua, Italy, and the academic and commercial patent rights to its therapeutic use have been the subject of licensing arrangements that underpin both programmes.

Where real-world evidence stands — and what the field still needs

Real-world evidence for both etranacogene dezaparvovec and fidanacogene elaparvovec is at an early stage, reflecting the recency of their regulatory approvals. Post-approval registry data, health economics analyses, and comparative effectiveness studies from hematology society publications represent the primary sources through which the field will build a more complete picture of long-term outcomes, durability of FIX expression, and safety signals beyond the controlled trial setting.

The key evidence gaps identified for this topic area include: (1) post-approval registry data tracking long-term FIX activity durability beyond the trial follow-up periods; (2) health economics and cost-effectiveness analyses comparing the one-time acquisition cost of each gene therapy against the lifetime cost of prophylactic FIX concentrate; (3) real-world data on the proportion of eligible patients who develop immune responses requiring corticosteroid intervention; and (4) comparative effectiveness analyses that apply consistent statistical methodology to the HOPE-B and BENEGENE-2 datasets to enable indirect treatment comparison.

Hematology society publications — including Blood, the Journal of Thrombosis and Haemostasis, and Haemophilia — are the primary venues where this evidence is expected to accumulate. The PatSnap resources library and PatSnap Eureka platform aggregate patent and literature records to support systematic drug intelligence research of exactly this kind, enabling analysts to track both the clinical evidence base and the IP landscape as they evolve in parallel.

“For research teams pursuing this question, retrieval strategies should span AAV5 FIX-Padua gene therapy patent filings, HOPE-B and BENEGENE-2 clinical trial data, and post-approval registry evidence from hematology society publications.”

For payers and health technology assessment bodies, the comparative question between Hemgenix and Beqvez will ultimately be resolved not by head-to-head trial data — which is unlikely to be generated — but by the accumulation of real-world evidence, indirect treatment comparisons, and long-term registry follow-up. The PatSnap life sciences intelligence platform enables teams to monitor this evidence as it emerges, tracking new publications, patent filings, and regulatory submissions in real time. Until that evidence base matures, the comparative clinical profile of the two approved hemophilia B gene therapies remains an open and commercially significant question.