Long-term safety and efficacy of factor IX gene therapy in hemophilia B

Abstract

Background: In patients with severe hemophilia B, gene therapy that is mediated by a novel self-complementary adeno-associated virus serotype 8 (AAV8) vector has been shown to raise factor IX levels for periods of up to 16 months. We wanted to determine the durability of transgene expression, the vector dose-response relationship, and the level of persistent or late toxicity.

Methods: We evaluated the stability of transgene expression and long-term safety in 10 patients with severe hemophilia B: 6 patients who had been enrolled in an initial phase 1 dose-escalation trial, with 2 patients each receiving a low, intermediate, or high dose, and 4 additional patients who received the high dose (2×10(12) vector genomes per kilogram of body weight). The patients subsequently underwent extensive clinical and laboratory monitoring.

Results: A single intravenous infusion of vector in all 10 patients with severe hemophilia B resulted in a dose-dependent increase in circulating factor IX to a level that was 1 to 6% of the normal value over a median period of 3.2 years, with observation ongoing. In the high-dose group, a consistent increase in the factor IX level to a mean (±SD) of 5.1±1.7% was observed in all 6 patients, which resulted in a reduction of more than 90% in both bleeding episodes and the use of prophylactic factor IX concentrate. A transient increase in the mean alanine aminotransferase level to 86 IU per liter (range, 36 to 202) occurred between week 7 and week 10 in 4 of the 6 patients in the high-dose group but resolved over a median of 5 days (range, 2 to 35) after prednisolone treatment.

Conclusions: In 10 patients with severe hemophilia B, the infusion of a single dose of AAV8 vector resulted in long-term therapeutic factor IX expression associated with clinical improvement. With a follow-up period of up to 3 years, no late toxic effects from the therapy were reported. (Funded by the National Heart, Lung, and Blood Institute and others; ClinicalTrials.gov number, NCT00979238.).

Figure 1. Summary of Factor IX Activity and Liver Function after Vector Infusion in 10 Patients with Hemophilia B

The mean (±SD) levels of factor IX coagulation activity were determined at the indicated time points with the use of a one-stage clotting assay after the infusion of a low dose of vector (in Patients 1 and 2), an intermediate dose (in Patients 3 and 4), or a high dose (in Patients 5 through 10), as shown on the left y axis. The arrows at the top of the graphs show the time points for treatment with factor IX concentrate. The decline in plasma factor IX activity that was observed in some patients (i.e., Patients 1, 5, 6, 7, and 8) in the initial phase after gene transfer probably represents a washout of the factor IX concentrate from the circulation. Levels of serum alanine aminotransferase (ALT) over time are shown on the right y axis. The duration of treatment with prednisolone (P), starting at a dose of 60 mg per day that was followed by a gradual tapering of the dose, is shown in purple at the top of the graphs for Patients 5, 6, 7, and 10. The left y axis has been adjusted for the patients in the high-dose group to reflect higher factor IX levels in these patients. The x axis has been adjusted to show with greater detail the kinetics of elevations in ALT levels in these patients.

Figure 2. Effect of Gene Transfer on the Administration of Factor IX Concentrate and the Number of Bleeding Episodes before and after Gene Transfer

Shown are box plots comparing the year before gene transfer and the year after gene transfer with respect to the mean amount of factor IX concentrate that was administered (Panel A) and the number of bleeding episodes (Panel B) in the 10 study patients. The top and bottom of each box represent the interquartile range, the horizontal line within the box represents the median (with the absence of a horizontal line indicating a median >10 times the 99th percentile), and the I bars represent the minimum and maximum values.