Toxicity associated with repeated administration of first-generation adenovirus vectors does not occur with a helper-dependent vector

Background: Certain gene therapy protocols may require multiple administrat ions of vectors to achieve therapeutic benefit to the patient. This may be especially relevant for vectors such as adenoviral vectors that do not inte grate into the host chromosome. Because immunocompetent animal models used for gene transfer studies develop neutralizing antibodies to adenoviral vec tors after a single administration, little is known about how repeat admini strations of vectors might affect transgene expression and vector toxicity. Materials and Methods: We used mice deficient in the membrane spanning regi on of immunoglobulin (IgM), which do not develop antibodies, to evaluate th e effect of repeated intravenous administration of first-generation and hel per-dependent adenoviral vectors expressing human alpha(1)-antitrypsin (hAA T). The duration and levels of transgene expression were evaluated after re peated administration of vectors. Toxicity was assessed by measuring the le vel of liver enzymes in the serum and the degrees of hepatocyte hypertrophy and proliferation. Results: We found that previous administration of first-generation adenovir al vectors can alter the response to subsequent doses. These alterations in cluded an increase in transgene expression early (within 1 and 3 days), fol lowed by a rapid drop in expression by day 7. In addition, previous adminis trations of first-generation vectors led to an increase in toxicity of subs equent doses, as indicated by a rise in liver enzymes and an increase in he patocyte proliferation. In contrast to first-generation vectors, use of the helper-dependent adenovirus vector, Ad-STK109, which contained no viral co ding regions, did not lead to increased toxicity after multiple administrat ions. Conclusions: We conclude that the response of the host to adenoviral vector s can be altered after repeated administration, compared with the response after the initial vector dose. In addition, these experiments provide furth er evidence for the relative safety of helper-dependent adenoviral vectors for gene therapy, compared with first-generation vectors.