Joshua I. Siner

AAV liver expression of FIX-Padua prevents and eradicates FIX inhibitor without increasing thrombogenicity in hemophilia B dogs and mice

Emerging successful clinical data on gene therapy using adeno-associated viral (AAV) vector for hemophilia B (HB) showed that the risk of cellular immune response to vector capsid is clearly dose dependent. To decrease the vector dose, we explored AAV-8 (1-3 × 10(12) vg/kg) encoding a hyperfunctional factor IX (FIX-Padua, arginine 338 to leucine) in FIX inhibitor-prone HB dogs. Two naïve HB dogs showed sustained expression of FIX-Padua with an 8- to 12-fold increased specific activity reaching 25% to 40% activity without antibody formation to FIX. A third dog with preexisting FIX inhibitors exhibited a transient anamnestic response (5 Bethesda units) at 2 weeks after vector delivery following by spontaneous eradication of the antibody to FIX by day 70. In this dog, sustained FIX expression reached ∼200% and 30% of activity and antigen levels, respectively. Immune tolerance was confirmed in all dogs after challenges with plasma-derived FIX concentrate. Shortening of the clotting times and lack of bleeding episodes support the phenotypic correction of the severe phenotype, with no clinical or laboratory evidence of risk of thrombosis. Provocative studies in mice showed that FIX-Padua exhibits similar immunogenicity and thrombogenicity compared with FIX wild type. Collectively, these data support the potential translation of gene-based strategies using FIX-Padua for HB.

Minimal modification in the factor VIII B-domain sequence ameliorates the murine hemophilia A phenotype

Recombinant canine B-domain deleted (BDD) factor VIII (FVIII) is predominantly expressed as a single-chain protein and exhibits greater stability after activation compared with human FVIII-BDD. We generated a novel BDD-FVIII variant (FVIII-RH) with an amino acid change at the furin cleavage site within the B domain (position R1645H) that mimics the canine sequence (HHQR vs human RHQR). Compared with human FVIII-BDD, expression of FVIII-RH protein revealed a 2.5-fold increase in the single-chain form. Notably, FVIII-RH exhibited a twofold increase in biological activity compared with FVIII-BDD, likely due to its slower dissociation of the A2-domain upon thrombin activation. Injection of FVIII-RH protein in hemophilia A (HA) mice resulted in more efficacious hemostasis following vascular injury in both the macro- and microcirculation. These findings were successfully translated to adeno-associated viral (AAV)-based liver gene transfer in HA mice. Expression of circulating FVIII-RH was approximately twofold higher compared with AAV-FVIII-BDD-injected mice. Moreover, FVIII-RH exhibits superior procoagulant effects compared with FVIII-BDD following a series of hemostatic challenges. Notably, the immunogenicity of FVIII-RH did not differ from FVIII-BDD. Thus, FVIII-RH is an attractive bioengineered molecule for improving efficacy without increased immunogenicity and may be suitable for both protein- and gene-based strategies for HA.

Safety of Liver Gene Transfer Following Peripheral Intravascular Delivery of Adeno-Associated Virus (AAV)-5 and AAV-6 in a Large Animal Model

Intravascular delivery of adeno-associated virus (AAV) vector is commonly used for liver-directed gene therapy. In humans, the high prevalence of neutralizing antibodies to AAV-2 capsid and the wide cross-reactivity with other serotypes hamper vector transduction efficacy. Moreover, the safety of gene-based approaches depends on vector biodistribution, vector dose, and route of administration. Here we sought to characterize the safety of AAV-5 and AAV-6 for liver-mediated human factor IX (hFIX) expression in rabbits at doses of 1 × 10(12) or 1 × 10(13) viral genomes/kg. Circulating therapeutic levels of FIX were observed in both cohorts of AAV-6-hFIX, whereas for AAV-5-hFIX only the high dose was effective. Long-lasting inhibitory antibodies to hFIX were detected in three of the 10 AAV-6-injected animals but were absent in the AAV-5 group. Overall, vector shedding in the semen was transient and vector dose-dependent. However, the kinetics of clearance were remarkably faster for AAV-5 (3-5 weeks) compared with AAV-6 (10-13 weeks). AAV-6 vector sequences outside the liver were minimal at 20-30 weeks post-injection. In contrast, AAV-5 exhibited relatively high amounts of vector DNA in tissues other than the liver. Together these data are useful to further define the safety and potential for clinical translation of these AAV vectors.

Novel factor VIII variants with a modified furin cleavage site improve the efficacy of gene therapy for hemophilia A

Essentials Factor (F) VIII is an inefficiently expressed protein. Furin deletion FVIII variants were purified and characterized using in vitro and in vivo assays. These minimally modified novel FVIII variants have enhanced function. These variants provide a strategy for increasing FVIII expression in hemophilia A gene therapy.

Circumventing furin enhances factor VIII biological activity and ameliorates bleeding phenotypes in hemophilia models

Processing by the proprotein convertase furin is believed to be critical for the biological activity of multiple proteins involved in hemostasis, including coagulation factor VIII (FVIII). This belief prompted the retention of the furin recognition motif (amino acids 1645-1648) in the design of B-domain-deleted FVIII (FVIII-BDD) products in current clinical use and in the drug development pipeline, as well as in experimental FVIII gene therapy strategies. Here, we report that processing by furin is in fact deleterious to FVIII-BDD secretion and procoagulant activity. Inhibition of furin increases the secretion and decreases the intracellular retention of FVIII-BDD protein in mammalian cells. Our new variant (FVIII-ΔF), in which this recognition motif is removed, efficiently circumvents furin. FVIII-ΔF demonstrates increased recombinant protein yields, enhanced clotting activity, and higher circulating FVIII levels after adeno-associated viral vector-based liver gene therapy in a murine model of severe hemophilia A (HA) compared with FVIII-BDD. Moreover, we observed an amelioration of the bleeding phenotype in severe HA dogs with sustained therapeutic FVIII levels after FVIII-ΔF gene therapy at a lower vector dose than previously employed in this model. The immunogenicity of FVIII-ΔF did not differ from that of FVIII-BDD as a protein or a gene therapeutic. Thus, contrary to previous suppositions, FVIII variants that can avoid furin processing are likely to have enhanced translational potential for HA therapy.