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N Engl J Med. 2017 Dec 7;377(23):2215-2227. doi: 10.1056/NEJMoa1708538.

Hemophilia B Gene Therapy with a High-Specific-Activity Factor IX Variant.

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From the Division of Hematology (L.A.G., B.J.S.-J., A.W., V.R.A.) and the Raymond G. Perelman Center for Cellular and Molecular Therapeutics (L.A.G., B.J.S.-J., A.W., V.R.A., J.C.M.L., O.Z.), Children's Hospital of Philadelphia, the Departments of Pediatrics (L.A.G., B.J.S.-J., V.R.A.) and Medicine (A.C.), Perelman School of Medicine at the University of Pennsylvania, and Spark Therapeutics (A.Y.L., D.H., J.F.W., Y.C., Y.L., K.W., D.T., M.E.C., L.B.C., X.M.A., K.A.H.) - all in Philadelphia; the Department of Pediatrics, Mississippi Center for Advanced Medicine, Madison (S.K.S.), and the Departments of Pathology (L.M.S.) and Pediatrics (S.M.), University of Mississippi Medical School, Jackson; the Departments of Medicine (A.G.) and Pediatrics (J.D.), University of California-Davis Medical School, Sacramento; the Department of Medicine, Sydney Medical School, and the Gene and Stem Cell Therapy Program, Centenary Institute (J.E.J.R.), University of Sydney, and Cell and Molecular Therapies, Royal Prince Alfred Hospital (J.E.J.R.) - both in Camperdown, NSW, Australia; the Department of Medicine, University of Toronto Faculty of Medicine and St. Michael's Hospital, Toronto (J.T.); the Department of Pediatrics, Weill Cornell Medical College, New York (C.E.M.); the Department of Medicine, University of Pittsburgh, Pittsburgh (M.V.R.); and Colorado Coagulation, Laboratory Corporation of America Holdings, Englewood, CO (S.T.).



The prevention of bleeding with adequately sustained levels of clotting factor, after a single therapeutic intervention and without the need for further medical intervention, represents an important goal in the treatment of hemophilia.


We infused a single-stranded adeno-associated viral (AAV) vector consisting of a bioengineered capsid, liver-specific promoter and factor IX Padua (factor IX-R338L) transgene at a dose of 5×1011 vector genomes per kilogram of body weight in 10 men with hemophilia B who had factor IX coagulant activity of 2% or less of the normal value. Laboratory values, bleeding frequency, and consumption of factor IX concentrate were prospectively evaluated after vector infusion and were compared with baseline values.


No serious adverse events occurred during or after vector infusion. Vector-derived factor IX coagulant activity was sustained in all the participants, with a mean (±SD) steady-state factor IX coagulant activity of 33.7±18.5% (range, 14 to 81). On cumulative follow-up of 492 weeks among all the participants (range of follow-up in individual participants, 28 to 78 weeks), the annualized bleeding rate was significantly reduced (mean rate, 11.1 events per year [range, 0 to 48] before vector administration vs. 0.4 events per year [range, 0 to 4] after administration; P=0.02), as was factor use (mean dose, 2908 IU per kilogram [range, 0 to 8090] before vector administration vs. 49.3 IU per kilogram [range, 0 to 376] after administration; P=0.004). A total of 8 of 10 participants did not use factor, and 9 of 10 did not have bleeds after vector administration. An asymptomatic increase in liver-enzyme levels developed in 2 participants and resolved with short-term prednisone treatment. One participant, who had substantial, advanced arthropathy at baseline, administered factor for bleeding but overall used 91% less factor than before vector infusion.


We found sustained therapeutic expression of factor IX coagulant activity after gene transfer in 10 participants with hemophilia who received the same vector dose. Transgene-derived factor IX coagulant activity enabled the termination of baseline prophylaxis and the near elimination of bleeding and factor use. (Funded by Spark Therapeutics and Pfizer; number, NCT02484092 .).

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