An improved helper-dependent adenoviral vector allows persistent gene expression after intramuscular delivery and overcomes preexisting immunity to adenovirus

Helper-dependent adenoviral vectors deleted of all viral coding sequences h ave shown an excellent gene expression profile in a variety of animal model s, as well as a reduced toxicity after systemic delivery. What is still unc lear is whether long-term expression and therapeutic dosages of these vecto rs can be obtained also in the presence of a preexisting immunity to adenov irus, a condition found in a high proportion of the adult human population. In this study we performed intramuscular delivery of helper-dependent vect ors carrying mouse erythropoietin as a marker transgene. We found that low doses of helper-dependent adenoviral vectors can direct long-lasting gene e xpression in the muscles of fully immunocompetent mice. The best performanc e-i.e., 100% of treated animals showing sustained expression after 4 months -was achieved with the latest generation helper-dependent backbones, which replicate and package at high efficiency during vector propagation. Moreove r, efficient and prolonged transgene expression after intramuscular injecti on was observed with limited vector load also in animals previously immuniz ed against the same adenovirus serotype. These data suggest that human gene therapy by intramuscular delivery of helper-dependent adenoviral vectors i s feasible.