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

Abstract

Background: 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.

Methods: 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.

Results: 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.

Conclusions: 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; ClinicalTrials.gov number, NCT02484092 .).

Figure 1. Factor IX Activity after One Peripheral Infusion of SPK-9001 in the Eight Participants Who Did Not Have an Adeno-Associated Viral Capsid-Directed Immune Response

The vector SPK-9001 was administered at a dose of 5×10¹¹ vector genomes per kilogram of body weight.

Figure 2. Laboratory Values in Two Participants Who Had an Immune Response to the Adeno-Associated Viral (AAV) Vector

Evidence of an immune response was determined by monitoring each participant’s factor IX activity and alanine aminotransferase and aspartate aminotransferase levels and by evaluating the results of the interferon-γ enzyme-linked immunosorbent spot (ELISPOT) assay regarding the reaction of the participant’s peripheral-blood mono nuclear cells (PBMCs) to the AAV capsid peptides and the transgene product (factor IX–R338L), as compared with a media control. The dotted lines in the top graph in each panel indicate the upper limit of the normal range for the alanine aminotransferase (black) and aspartate aminotransferase (gray) levels in each participant, according to the local laboratory. The arrow after day 56 in Panel A (showing values for Participant 7) indicates the receipt of a single dose of prednisone. The duration of a full course prednisone treatment (started a few days later) is shown, with the slope indicating tapering. The levels of alanine aminotransferase and aspartate aminotransferase returned to baseline levels in these two participants after prednisone initiation, and transgene expression was maintained in the two participants after prednisone was tapered and stopped. Results of the ELISPOT assay are shown as the number of spot-forming units (SFU) per 1 million PBMCs; values that are more than 50 SFU or that are above the media control (dotted line) by a factor of three are considered to be positive.

Figure 3. Comparison of Annualized Bleeding Rate, Number of Exogenous Factor IX Infusions, and Use of Factor IX Concentrate in the 52 Weeks before Screening and after Vector Infusion

Panel A shows the annualized bleeding rate for each participant, Panel B the number of factor IX infusions, and Panel C the total number of international units (IU) of factor IX concentrate that were infused. The left side of each graph shows the values before vector administration (dashed line), and the right side the values after infusion. Values were normalized for each participant’s follow-up period. Significant reductions in the annualized bleeding rate (P = 0.02, W = 28), in the number of infusions (P = 0.004, W = 45), and in the amount of factor used (P = 0.004, W = 45) were observed in each participant after vector infusion.