Sustained Clinical Benefit of AAV Gene Therapy in Severe Hemophilia B

Abstract

Background: Adeno-associated virus (AAV)-mediated gene therapy has emerged as a promising treatment for hemophilia B. Data on safety and durability from 13 years of follow-up in a cohort of patients who had been successfully treated with scAAV2/8-LP1-hFIXco gene therapy are now available.

Methods: Ten men with severe hemophilia B received a single intravenous infusion of the scAAV2/8-LP1-hFIXco vector in one of three dose groups (low-dose: 2×10¹¹ vector genomes [vg] per kilogram of body weight [in two participants]; intermediate-dose: 6×10¹¹ vg per kilogram [in two]; or high-dose: 2×10¹² vg per kilogram [in six]). Efficacy outcomes included factor IX activity, the annualized bleeding rate, and factor IX concentrate use. Safety assessments included clinical events, liver function, and imaging.

Results: Participants were followed for a median of 13.0 years (range, 11.1 to 13.8). Factor IX activity remained stable across the dose cohorts, with mean factor IX levels of 1.7 IU per deciliter in the low-dose group, 2.3 IU per deciliter in the intermediate-dose group, and 4.8 IU per deciliter in the high-dose group. Seven of the 10 participants did not receive prophylaxis. The median annualized bleeding rate decreased from 14.0 episodes (interquartile range, 12.0 to 21.5) to 1.5 episodes (interquartile range, 0.7 to 2.2), which represented a reduction by a factor of 9.7. Use of factor IX concentrate decreased by a factor of 12.4 (interquartile range, 2.2 to 27.1). A total of 15 vector-related adverse events occurred, primarily transient elevations in aminotransferase levels. Factor IX inhibitor, thrombosis, or chronic liver injury did not develop in any participant. Two cancers were identified but were deemed by the investigators, together with an expert multidisciplinary team, as being unrelated to the vector. A liver biopsy that was conducted in 1 participant 10 years after the infusion revealed transcriptionally active transgene expression in hepatocytes without fibrosis or dysplasia. Levels of neutralizing antibodies to AAV8 remained high throughout follow-up, thus indicating potential barriers to readministration of the vector.

Conclusions: A single administration of scAAV2/8-LP1-hFIXco gene therapy resulted in durable factor IX expression, sustained clinical benefit, and no late-onset safety concerns over a period of 13 years. These data support the long-term efficacy and safety of AAV gene therapy for severe hemophilia B. (Funded by the U.K. Medical Research Council and others; ClinicalTrials.gov number, NCT00979238; EudraCT number, 2005-005711-17.).

Figure 1. Factor IX activity levels following peripheral vein administration of scAAV2/8-LP1-hFIXco using factor IX levels uninfluenced by prophylaxis use or treatment with factor IX concentrate.

Annual and steady-state FIX:C (median, interquartile range) was determined at the indicated time points using a one-stage clotting assay following administration of the high vector dose (2x10¹² vg per kilogram of body weight, dotted blue line, N=6) compared with levels in the low (2x10¹¹ vg per kilogram) and intermediate (6x10¹¹ vg per kilogram) vector doses combined (N=4, solid yellow line). Annual factor IX activity (A) was determined for each group using the mean factor IX activity collected during the indicated time point. Steady-state FIX:C (B) for each group was the average of all accumulated factor IX activity levels from five months post-gene therapy up to the indicated time point. Only factor IX levels measured at least 10 days after factor IX use were included.

Figure 2. Longitudinal analysis of anti-AAV capsid antibody level following peripheral vein administration of scAAV2/8-LP1-hFIXco.

Total IgG antibody levels by ELISA (A) or neutralizing antibody levels by transduction inhibition assay (B) against AAV8, AAV5 and AAV3b in serum samples collected over time after gene transfer in individual subjects treated at the low and intermediate vector dose levels combined (Lower doses) or the high dose. Shown are antibody levels over time for individual subjects (grey lines) as well as the median and interquartile range (25^th percentile=bottom bar, 75^th percentile=top bar) for the dose level (solid yellow line and "I" bars for low and intermediate dose, solid blue line and "I" bars and for high dose). For reference, the maximum (dashed black line) and median (dotted red line) AAV antibody levels observed in 38 samples from individuals presumed to have been infected with wild-type AAV are shown.