Tumor-specific oncolytic adenoviruses expressing granulocyte macrophage colony-stimulating factor or anti-CTLA4 antibody for the treatment of cancers

The purpose of this study was to examine the tumor specificity, cytotoxicity and the antitumor activity of two conditionally replicating oncolytic adenoviruses, SKL001 and SKL002, which expressed granulocyte macrophage colony-stimulating factor (GM-CSF) or anti-cytotoxic T lymphocyte-associated antigen-4 (CTLA4) antibody, respectively, and determine their antitumor efficacy in A549 lung tumor model, B16F10 mouse melanoma tumor model and CMT-64 mouse small lung carcinoma tumor model. Virus yield and cytotoxicity were used to determine tumor specificity and virus replication-mediated cytotoxicity of SKL001 and SKL002 in a panel of human tumor cell lines and primary cells in vitro. Two subcutaneous (s.c.) tumor nexograft tumor models were used to assess their antitumor activity. Under the control of the E2F promoter, the expression of E1a genes appeared only in tumor cells, whereas the wild-type Ad5 expressed its E1a genes in both tumor cells and normal cells. GM-CSF and anti-CTLA4 production were significantly higher in tumor cells than normal cells. SKL001 and SKL002 replicated in Rb-defective cell lines as efficiently as wild-type adenovirus but produced 100-fold less virus in normal human cells. SKL001 and SKL002 was up to 1000-fold more cytotoxic in Rb pathway-defective human tumor cells in comparison with normal human cells. Antitumor activity of SKL001 and SKL002 following intravenous administration was shown in a human lung A549 s.c. xenograft tumor model and mouse B16F10 melanoma tumor model when compared with phosphate-buffered saline treatment. In immune-competent mice, the addition of GM-CSF produced a stronger antitumor activity and induced a higher number of mature dendritic cells and macrophages, whereas additive antitumor activity was observed in the group when SKL001 and SKL002 were combined. In vitro and in vivo studies showed the selective replication, cytotoxicity, gene production and antitumor efficacy of SKL001 and SKL002 in human tumor model, suggesting a potential utility of this oncolytic agent for the treatment of human cancer. Further studies are warranted to show the role of human GM-CSF and anti-CTLA4 antibody in the antitumor efficacy of these two oncolytic viruses.