| Despite recent advances in cancer immunotherapy, two important challenges limit the utility of vaccine therapies: the relatively low efficiency of gene transfer afforded by available vectors to tumor cells in vivo, and the lack of selectivity of these vectors to specifically target cancer cells. Recombinant adenoviruses mediate a high degree of gene transfer to a variety of cell types. Nevertheless, certain cancers remain refractory to adenoviral infection, due to a lack of cell-surface expression of CAR, the primary adenoviral receptor. Moreover, the widespread distribution of CAR on normal human tissues results in non-tumor-specific cellular toxicity. Following intravenous administration of adenoviruses, the majority appear to target the liver based on high levels of hepatic CAR expression, as well as a secondary site of viral attachment mediated by heparan sulfate proteoglycans. Therefore, the liver becomes a reservoir to trap viral particles, thus decreasing the circulating pool of available vectors that can infect target tissues. This might also account for the hepatotoxicity noted with certain adenoviral therapies.; Recent interest has focused on the tumor neovasculature as a specific target for cancer therapies. Studies have revealed that the integrins αvβ3 and αvβ5 are expressed preferentially on the surface of the neovasculature at levels significant enough to ensure the selective homing of phage, as well as labeled antibodies and drugs coupled to an RGD-4C peptide. Therefore, we generated a series of adenoviral vectors in which we ablated native binding to CAR and/or heparan sulfate. In a murine model of angiogenesis, as well as a murine xenograft model of human head and neck cancer, we found that by ablating CAR binding, liver infectivity could be inhibited by nearly ninety percent. Additionally ablating heparan sulfate binding further attenuated liver uptake. By then introducing an RGD motif into the HI loop of the fiber protein of these recombinant adenoviruses, we achieved selective targeting of these viruses to the neovasculature. These adenoviral constructs provide a unique set of tools with which to deliver target genes specifically to angiogenic vessels within a tumor, while limiting the dose-related toxicity that accompanies traditional viral vector therapies. |
