Long-Term Persistence of Antibodies to Adeno-Associated Viral Vectors Following Gene Therapy with scAAV8-LP1-Fixco

Background Gene therapy for hemophilia B (HB) consisting of a one-time, systemic, administration of adeno-associated viral (AAV) vectors encoding a functional Factor-IX gene is now approved and represents an important and long-awaited milestone. A rise in anti-AAV antibody titers follows systemic administration of AAV vectors. This does not have direct clinical consequences; however, persistence of neutralising anti-AAV antibodies (NAB) at high titers will preclude subsequent successful gene transfer with vector of the same serotype. This would be problematic, in the event that transgene expression should fall below therapeutic levels and repeat dosing might be needed. Furthermore, high titer NAB response to one serotype can cross-react with other serotypes thereby limiting the option of using alternative serotype AAV vectors. We report a longitudinal analysis of anti-AAV antibody response over a 10-year period in severe HB participants following systemic administration of scAAV2/8-LP1-hFIXco from the St Jude-UCL Phase I/II, gene therapy trial (AGT4HB ClinicalTrials.gov:NCT00979238). Methods Anti-AAV antibody levels were evaluated in the 10 trial participants following a bolus intravenous infusion of scAAV2/8-LP1-hFIXco at a dose of 2x10 11vg/kg (N=2), 6x10 11vg/kg (N=2), or 2x10 12vg/kg (N=6) between 2010 and 2012. Total anti-AAV antibodies (TAB) were measured by ELISA, and levels quantitated using a pooled IgG reference. NAB were quantitated by an in-vitro transduction inhibition assay (TIA) in which plasma samples were preincubated with AAV CMV NanoLuc (MOI: AAV3 1, AAV5 100 and 8 100) prior to evaluation of transduction of HEK 293T cells. Results were normalised to virus control and the 50% inhibition dilution (IC50) was calculated. For comparison, we assessed anti-AAV antibody titers in 38 normal human plasma samples using the same methodology. Results At the time of data cut-off (31 st Dec 2022) the median follow-up for the cohort was 10.7 years (range: 4-12 years) with follow-up in two participants (one mid dose and the other high dose) being limited to 4 years. A dose dependent AAV8 TAB response was observed with low dose participants having a mean of 84±101mg/ml at 1-year, mid dose 507 mg/ml and high dose 1650±603 mg/ml. This represented a >1400-fold increase from baseline, pre-AAV administration values. These TAB levels were significantly higher than levels following natural infection in 38 seropositive individuals not exposed to recombinant vect