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 (2×1012 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.)

Lessons learned from lung and liver in-vivo gene therapy: implications for the future

“…There are examples of promoters used in non-clinical and clinical experiments that utilize enhancer/promoter regions from proteins that are exclusively expressed in hepatocytes, such as apolipoprotein A, AAT, transthyretin and thyroxine-binding globulin [53,59,69]. One chimeric enhancer/promoter, termed APoE/hAAT which contains a portion of the hepatic control region of the apolipoprotein A enhancer and a region of the human AAT promoter, has been used in clinical trials successfully [59,70]. These promoters owe their specificity to the incorporation of transcription factor binding sites that correspond to transcription factors expressed exclusively in the liver.…”

Development of Optimized AAV Serotype Vectors for High-Efficiency Transduction at Further Reduced Doses

“…Furthermore, the single-stranded nature of the AAV genome, which is transcriptionally inactive, also renders the first generation of ssAAV vectors less than optimal, although it is quite remarkable that clinical efficacy has been achieved with the first-generation ssAAV vectors in at least five human clinical trials. [1][2][3][4][5][6][7][8][9] The elegant work of Samulski and colleagues has overcome this barrier with the development of double-stranded self-complementary AAV (scAAV) vectors, but the packaging capacity of these vectors is compromised in that expression cassettes exceeding *2.5 kb are excluded. 28,29 Although the successful use of scAAV8-F.IX vectors in the gene therapy of hemophilia B trial is a clear testament to this approach, 6 it is unlikely to be applicable to hemophilia A, the incidence of which is significantly higher, as the F.VIII therapeutic gene far exceeds the packaging capacity of scAAV vectors.…”

“…[1][2][3][4][5][6][7][8][9] However, in some cases, the use of large vector doses to achieve therapeutic benefits has also been shown to trigger the host immune response to the capsid proteins. 10 In the first clinical for the potential gene therapy of hemophilia B, AAV2 serotype vectors failed to express therapeutic levels of human factor IX (hF.IX) at a dose of 10 13 vg in a patient.…”

“…[11][12][13] In a subsequent clinical trial with AAV8 serotype vectors, clinical efficacy was observed, but once again, at the highest dose, and AAV8 vectors also induced an immune response in two patients. 5,6 Thus, the vector-induced toxicity appears to correlate directly with the vector dose. It is, therefore, desirable to develop strategies to achieve clinical efficacy with AAV vectors at significantly reduced doses.…”

Preclinical and translational evaluation of coagulation factor IXa as a novel therapeutic target

“…The relative safety of such strategy is reinforced by the fact that severe and spontaneous bleeding appear to be associated with <1% residual activity, an observation recently confirmed in a gene therapy study in severe hemophilia B patients whose bleeding episodes were inhibited by expressing 1–6% of the normal level of fIX (Nathwani et al. 2014). Lower bleeding risks by targeting fIXa are expected from the phenotype of fIX knockout mice in comparison with fX‐deficient mice as deficiency of fX causes partial embryonic and fatal neonatal bleeding (Dewerchin et al.…”