424. AAV Re-Administration Following Pediatric Application: Effect of Age at First Administration

Gene therapy delivery in pediatric patients may prevent or reduce the deleterious impact of disease long-term. However, due to the cellular turn-over and growth of some target organs such as the liver re-administration of AAV vectors will have to be considered. The goal of our study is to determine the time frame after birth in which AAV can be delivered without the development of an immune response against capsid, but with a proper transgene expression, thereby permitting re-administration and a continuous transgene expression. Mice were administered with AAV1-hAAT-hSEAP intraperitoneally at 2, 7, 14, 21 days and adult mice were used as controls. Two weeks after administration SEAP concentration was measured in plasma as a measure for transgene activity. Initial transgene expression was higher in mice injected at two and seven days of age than in mice injected at 14 or 21 days; As expected, transgene expression decreased over time in all mice. At week 8 SEAP concentrations in plasma of all mice were similar. AAV1 neutralizing antibody (NAB) responses were assessed at 9 weeks following primary administration. Mice administered at 21 days of age had a NAB titer slightly above PBS level; while the other age groups of 2, 7, and 14 days were similar to PBS. Mice of all groups were re-administrated 9 weeks after primary administration with AAV1-LP1-hFIX in order to assess the effect of NAB induction on the expression of a novel transgene. Mice primed at 21 days of age had 67% less FIX than adult control mice. In the other groups that were primed at 2, 7 or 14 days of age the second administration generated the same FIX plasma levels as measured in adult controls. With this study, we show that AAV1 can be effectively used for pediatric application up to 14 days after birth without inducing an immune response. This strategy prevents the formation of neutralizing antibodies, and the same serotype can be used for re-administrated later in life.