Gene Therapy · eu
Keeping Clotting Factor in the Blood Longer: Hemophilia B Gene Therapy Shows Greater Potency in Mice
A research team combined a highly active factor IX with engineered albumin, producing higher and more sustained clotting function in mice; the design could reduce the viral vector dose required in the future, but its safety and efficacy in humans still await clinical validation.
Hemophilia B gene therapy can already enable some patients to produce factor IX (FIX) themselves after a single administration, but the benefits are not equally durable for everyone, and liver inflammation and immune responses associated with viral vector dose also limit the therapeutic scope. A team at the Université libre de Bruxelles in Belgium has now approached the issue from another direction: not only delivering the gene into cells, but also making the clotting factor produced by those cells more durable and efficient.
Published in *Blood Advances*, the study was based on the highly active FIX-Padua variant FIX-R338L, which was then fused with engineered human albumin containing three amino acid substitutions to form FIX-R338L-ALB(QMP). Albumin naturally remains in the blood for long periods; through the fusion design, the researchers sought to extend FIX circulation time, while QMP refers to three albumin modifications: E505Q, T527M, and K573P.
The team packaged this gene construct into AAV8 or AAV8/DJ vectors and tested them in normal mice and mice with hemophilia B, respectively. Compared with control vectors expressing unfused FIX-R338L, the new construct produced sustained FIX antigen levels approximately four times as high and clotting activity approximately three times as high; in the hemophilia model, the bleeding phenotype was also corrected over the long term. The comparator here was an experimental unfused construct, so the findings cannot be directly interpreted as showing fourfold superiority over currently approved therapies.
Preliminary safety results revealed no additional warning signals. During the study, no anti-FIX antibodies, elevated alanine aminotransferase, or infiltration of immune cells into the liver were detected, and D-dimer levels did not increase, indicating that under the conditions and observation periods used in these animals, there was no evidence of increased hepatotoxicity or thrombotic risk. Most treated mice with hemophilia B developed immune tolerance involving regulatory T cells, even when they were later actively immunized with FIX protein and an adjuvant.
The practical significance of greater potency may extend beyond generating more clotting activity. If the same efficacy can be achieved with less AAV vector, it could theoretically reduce the likelihood of vector-induced immune responses and liver burden and might also widen the therapeutic window for gene therapy. However, the study has not directly demonstrated in humans that a lower dose is sufficient to maintain efficacy; this remains a development direction inferred from the mouse findings.
Animal models also cannot fully reproduce human immune and clotting responses to AAV, engineered albumin, or highly active FIX. The next steps must include determining the minimum effective vector dose and assessing whether long-term expression remains stable, whether pre-existing AAV antibodies in different patients affect treatment, and whether thrombotic risk can remain controlled in larger studies. The findings therefore represent a specific candidate design for next-generation hemophilia B gene therapy rather than human evidence that can already replace current treatments.