Phase 1-2 Clinical Trial of a Recombinant AAV5 Vector Containing the Human FIX Gene in Patients with Severe or Moderately Severe Haemophilia B

Gene therapy has been successfully used in a research setting in a limited number of patients with severe haemophilia B, primarily using vectors based on adeno-associated virus (AAV) serotype 2 and 8. Several research groups and companies are exploring new developments to improve current treatment strategies to eventually make gene therapy available for a larger number of patients. Pre-existing immunity to AAV capsid proteins may limit the availability of such therapies to small sub-groups of patients. A low prevalence of natural neutralizing antibodies against AAV5 compared with other serotypes has been demonstrated (Calcedo et al., Clin Vaccine Immunol 2009, 18:1586-1588). We use a recombinant AAV5 containing the codon optimized human factor IX gene (AAV5-hFIX), using a baculovirus production process that allows commercial scale manufacturing of the AAV5-hFIX drug product. Human FIX expression levels in macaques treated i.v. with AAV5-hFIX at a dose of 5 × 10^12 gc/kg were high enough to expect therapeutic effect on the haemophiliac phenotype in haemophilia B patients. Such levels were achieved in humans during AAV8-hFIXco clinical study (Nathwani et al., NEJM 2011, 365:2357-2365). hFIX levels in dose-escalating GLP toxicity studies in cynomolgus monkeys and mice showed linear dose responses using doses up to 1 × 10˄14 and 2.3 × 10^14 cg/kg, respectively, and no specific safety concerns. In human studies with AAV5 containing the human porbilinogen deaminase (PBGD) gene, administered in doses up to 2 x 10^13 gc/kg to patients with acute intermittent porphyria, no safety concerns were raised (D’Avola et al., ASGCT, Washington DC, 2014). Importantly, in this study no liver enzyme perturbations were observed following administration of AAV5.

The primary objective of the proposed study is to investigate the safety of systemic administration of AAV5-hFIX to adult patients with severe or moderately severe haemophilia B. The multicentre trial has an open-label, uncontrolled, single-dose, dose-ascending design and consists of two cohorts. The study population consists of male patients, aged ≥18 years, with severe or moderately severe haemophilia B and a severe bleeding phenotype [i.e. known FIX deficiency with plasma FIX activity level <1% (severe), or plasma FIX activity level ≥1% and ≤2% (moderately severe) and currently on prophylactic FIX replacement therapy for a history of bleeding, or currently on on-demand therapy with a current or past history of frequent bleeding defined as four or more bleeding episodes in the last 12 months, or chronic haemophilic arthropathy. Patients should have had more than 150 previous exposure days of treatment with FIX protein. Subjects are allocated to one of two cohorts with the following planned dose levels: Cohort 1 (5 subjects) with AAV5-hFIX 5.0 × 10^12 gc/kg and Cohort 2 (5 subjects) with AAV5-hFIX 2.0 × 10^13 gc/kg. Key efficacy assessments include factor IX plasma levels, the need for FIX replacement therapy, incidence of spontaneous bleedings and health-related quality of life measurements.

In conclusion, AAV5-hFIX produced in commercial scale represents a novel approach to gene therapy of haemophilia B.