Effects of pre-existing anti-adenovirus antibodies on transgene expression levels and therapeutic efficacies of arming oncolytic adenovirus

Oncolytic adenoviruses (OAds), most of which are based on species C human adenovirus serotype 5 (Ad5) (OAd5), have recently received much attention as potential anticancer agents. High seroprevalence of anti-Ad5 neutralizing antibodies is a major hurdle for Ad5-based gene therapy. However, the impacts of anti-Ad5 neutralizing antibodies on OAd5-mediated transgene expression in the tumor and antitumor effects remain to be fully elucidated. In this study, we examined the impact of anti-Ad5 neutralizing antibodies on the OAd5-mediated antitumor effects and OAd5-mediated transgene expression. The luciferase expression of OAd-tAIB-Luc, which contains the cytomegalovirus promoter-driven luciferase gene, was inhibited in human cultured cells in the presence of human serum. Although the inhibitory effects of human serum possessing the low anti-Ad5 neutralizing antibody titers were overcome by long-term infection, the in vitro tumor cell lysis activities of OAd-tAIB-Luc were entirely attenuated by human serum containing the high titers of anti-Ad5 neutralizing antibodies. OAd-tAIB-Luc-mediated luciferase expression in the subcutaneous tumors 3 days after administration and tumor growth suppression levels following intratumoral administration were significantly lower in mice possessing the high titers of anti-Ad5 neutralizing antibodies, compared to those in control mice. These results suggested that pre-existing anti-Ad5 antibodies attenuated both transgene expression and potential antitumor effects of OAd5 following intratumoral administration.