Adenovirus serotype 5 remains one of the most promising vectors for delivering genetic material to
cancer cells for imaging or
therapy, but optimization of these agents to selectively promote
tumor cell
infection is needed to further their clinical development.
Peptide sequences that bind to specific
cell surface receptors have been inserted into adenoviral
capsid proteins to improve
tumor targeting, often in the background of mutations designed to ablate normal
ligand:receptor interactions and thereby reduce off target effects and toxicities in non-target tissues. Different
tumor types also express highly variable complements of
cell surface receptors, so a customized targeting strategy using a particular
peptide in the context of specific adenoviral mutations may be needed to achieve optimal efficacy. To further investigate
peptide targeting strategies in adenoviral vectors, we used a set of
peptide motifs originally isolated using phage display technology that evince
tumor specificity in vivo. To demonstrate their abilities as targeting motifs, we genetically incorporated these
peptides into a surface loop of the fiber
capsid protein to construct targeted adenovirus vectors. We then systematically evaluated the ability of these
peptide targeted vectors to infect several
tumor cell types, both in vitro and in vivo, in a variety of mutational backgrounds designed to reduce CAR and/or HSG-mediated binding. Results from this study support previous observations that
peptide insertions in the HI loop of the fiber knob domain are generally ineffective when used in combination with HSG detargeting mutations. The evidence also suggests that this strategy can attenuate other fiber knob interactions, such as CAR-mediated binding, and reduce overall viral infectivity. The insertion of
peptides into fiber proved more effective for targeting
tumor cell types expressing low levels of CAR receptor, as this strategy can partially compensate for the very low infectivity of wild-type adenovirus in those cells. Nevertheless, the incorporation of relatively low affinity
peptide ligands into the fiber knob, while effective in vitro, has only minimal targeting efficacy in vivo and highlights the importance of high affinity
ligand:receptor interactions to achieve
tumor targeting.