Administration of helper-dependent adenoviral vectors and sequential delivery of different vector serotype for long-term liver-directed gene transferin baboons

The efficiency of first-generation adenoviral vectors as gene delivery tool s is often limited by the short duration of transgene expression, which can be related to immune responses and to toxic effects of viral proteins. In addition, readministration is usually ineffective unless the animals are im munocompromised or a different adenovirus serotype is used. Recently, adeno viral vectors devoid of all viral coding sequences (helper-dependent or gut less vectors) have been developed to avoid expression of viral proteins. In mice, liver-directed gene transfer with AdSTK109, a helper-dependent adeno viral (Ad) vector containing the human alpha(1)-antitrypsin (hAAT) gene, re sulted in sustained expression for longer than 10 months with negligible to xicity to the liver. In the present report we have examined the duration of expression of AdSTK109 in the liver of baboons and compared it to first-ge neration vectors expressing hAAT. Transgene expression was limited to appro ximately 3-5 months with the first-generation vectors. In contrast administ ration of AdSTK109 resulted in transgene expression for longer than a year in two of three baboons. We have also investigated the feasibility of circu mventing the humoral response to the virus by sequential administration of vectors of different serotypes. We found that the ineffectiveness of readmi nistration due to the humoral response to an Ad5 first-generation vector wa s overcome by use of an Ad2-based vector expressing hAAT. These data sugges t that long-term expression of transgenes should be possible by combining t he reduced immunogenicity and toxicity of helper-dependent vectors with seq uential delivery of Vectors of different serotypes.