Generation and selection of targeted adenoviruses embodying optimized vector properties.

The utility of adenovirus serotype 5 (Ad5)-based vectors for gene therapy applications would be improved by cell-specific targeting. However, strategies to redirect Ad5 vectors to alternate cellular receptors via replacement of the capsid fiber protein have often resulted in structurally unstable vectors. In view of this, we hypothesized that the selection of modified adenoviruses during their rescue and propagation would be a straightforward approach that guarantees the generation of functional, targeted vectors. Based on our first generation fiber-fibritin molecule, several new chimeric fibers containing variable amounts of fibritin and the Ad5 fiber shaft were analyzed via a new scheme for Ad vector selection. Our selected chimera, composed of the entire Ad5 fiber shaft fused to the 12th coiled-coil segment of fibritin, is capable of efficient capsid incorporation and ligand display. Moreover, transduction by the resultant vector is independent of the expression of the native Ad5 receptor. The incorporation of the Fc-binding domain of Staphylococcus aureus protein A at the carboxy terminus of this chimeric fiber facilitates targeting of the vector to a variety of cellular receptors by means of coupling with monoclonal antibodies. In addition, we have concluded that Ad5 vectors incorporating individual targeting ligands require individual optimization of the fiber-fibritin chimera, which may be accomplished by selecting the optimal fiber-fibritin variant at the stage of rescue of the virus in cells of interest, as described herein.

Targeting of adenovirus via genetic modification of the viral capsid combined with a protein bridge.

A potential barrier to the development of genetically targeted adenovirus (Ad) vectors for cell-specific delivery of gene therapeutics lies in the fact that several types of targeting protein ligands require posttranslational modifications, such as the formation of disulfide bonds, which are not available to Ad capsid proteins due to their nuclear localization during assembly of the virion. To overcome this problem, we developed a new targeting strategy, which combines genetic modifications of the Ad capsid with a protein bridge approach, resulting in a vector-ligand targeting complex. The components of the complex associate by virtue of genetic modifications to both the Ad capsid and the targeting ligand. One component of this mechanism of association, the Fc-binding domain of Staphylococcus aureus protein A, is genetically incorporated into the Ad fiber protein. The ligand is comprised of a targeting component fused with the Fc domain of immunoglobulin, which serves as a docking moiety to bind to these genetically modified fibers during the formation of the Ad-ligand complex. The modular design of the ligand solves the problem of structural and biosynthetic compatibility with the Ad and thus facilitates targeting of the vector to a variety of cellular receptors. Our study shows that targeting ligands incorporating the Fc domain and either an anti-CD40 single-chain antibody or CD40L form stable complexes with protein A-modified Ad vectors, resulting in significant augmentation of gene delivery to CD40-positive target cells. Since this gene transfer is independent of the expression of the native Ad5 receptor by the target cells, this strategy results in the derivation of truly targeted Ad vectors suitable for tissue-specific gene therapy.