Hemophilia B is caused by a single missing protein — clotting factor IX — and for decades the only answer was to keep replacing it, infusion after infusion, for a lifetime. Gene therapy proposes something different and audacious: deliver a working copy of the factor IX gene to the patient's own liver cells once, and let the body manufacture the missing protein itself. CSL Behring's pivotal Phase 3 study of AMT-061 (NCT03569891), the program widely known by its HOPE-B identity and now marketed as etranacogene dezaparvovec, is the trial that turned that proposition into posted clinical evidence. The registry record, complete with results, is now public.
The therapy itself is a piece of engineering worth pausing on. AMT-061 is a recombinant adeno-associated virus of serotype 5 (AAV5) carrying a codon-optimized human factor IX gene — specifically the “Padua” variant, a naturally occurring hyperactive form of factor IX that produces several times the clotting activity of the standard protein per molecule. The gene sits under a liver-specific promoter so it expresses where clotting factors are normally made, and the whole construct is delivered as a single intravenous infusion at a dose of 2 × 1013 genome copies per kilogram. The use of the Padua variant is the clever part: it lets a modest amount of gene expression translate into a clinically meaningful amount of clotting activity.
How the trial was built
This was an open-label, single-dose, multicenter, multinational Phase 3 study enrolling 67 adult men with severe or moderately severe hemophilia B; 54 went on to receive the gene therapy. Its design solved a hard methodological problem elegantly. Because you cannot ethically randomize patients to forgo treatment for a one-time gene therapy, the trial used each patient as his own control. Participants first went through a lead-in period of at least 26 weeks on their standard prophylactic factor IX replacement therapy, recording every infusion and every bleed in an electronic diary. Then they received the single AMT-061 infusion and were followed for months afterward, with the post-treatment period compared directly against that personal baseline. The primary endpoint was the annualized bleeding rate, comparing the lead-in window against months 7 through 18 after dosing.
What the evidence shows
The most telling numbers in the registry are the factor IX activity levels, because they reveal whether the transferred gene actually did its job. At baseline these men sat at a mean factor IX activity of about 1.19% of normal — the near-total deficiency that defines severe disease. After the infusion, mean activity rose into the high-thirties, recorded at roughly 38.95%, 41.48%, and 36.90% across post-treatment timepoints. Moving a patient from roughly 1% to roughly 39% of normal factor IX activity is the difference between severe hemophilia and the mild end of the spectrum, where spontaneous bleeds become uncommon.
The downstream consequence shows up in the consumption data. During the lead-in period on standard prophylaxis, patients used a mean of about 257,339 international units of factor IX per year. After gene therapy, that fell to under 13,000 IU per year and trended lower still at later timepoints — roughly an order-of-magnitude reduction, with most of the residual use concentrated in the early post-infusion window. The adjusted annualized infusion rate told the same story, dropping from about 72 infusions per year to roughly 2.5. For a disease whose entire treatment burden is the relentless cadence of infusions, that collapse in usage is arguably the most patient-relevant result the trial produced.
There is a subtlety in those activity numbers worth dwelling on, because it explains the design philosophy. The Padua variant produces roughly five to eight times the clotting activity of standard factor IX per unit of protein, so the “39%” figure reflects clotting function, not the raw amount of factor IX protein circulating. That is the entire reason the trial used Padua: a liver dosed with the gene only needs to express a relatively modest quantity of this hyperactive variant to reach a clotting capacity that would otherwise demand much higher expression of the ordinary protein. It is a way of buying clinical efficacy without pushing the gene-delivery system harder than it can safely go.
The safety story is real and specific
Gene therapy is not a free lunch, and the posted adverse-event tables make that plain: 20 of the 54 treated participants recorded a serious adverse event over the study. The known risks of liver-directed AAV gene therapy are well characterized — transient elevations in liver enzymes that often require a course of corticosteroids, infusion-related reactions, and the immune-mediated loss of expression that can occur if the body reacts against the AAV capsid. The single-dose, single-arm nature of the trial means there is no concurrent control for these events, which is why each patient's own lead-in data is doing the heavy lifting on efficacy while the safety profile must be read against what is known about the modality as a whole.
Why this trial matters
The AMT-061 Phase 3 record is one of the clearest demonstrations of the gene-therapy bargain for hemophilia: a single infusion in exchange for years of near-normal factor activity and a near-elimination of routine prophylaxis. The within-patient design is the right answer to an unavoidable ethical constraint, and it produces numbers — 1% to 39% activity, 257,000 to under 13,000 IU — that are hard to misread. Whether the expression endures over a full lifetime is the question that only long-term follow-up can settle, and the registry's later timepoints are where that durability story will be written. The complete activity-level and consumption tables, along with the full adverse-event listings, remain on the ClinicalTrials.gov study page.