A Bicistronic Adenoviral Vector Carrying Cytosine Deaminase and Granulocyte-Macrophage Colony-Stimulating Factor Increases the Therapeutic Efficacy of Cancer Gene Therapy.

The combination of cytotoxic treatment modalities, including oncolytic viral gene therapies and immunotherapy, usually yields a synergistic effect. In the current study, a bicistronic adenoviral vector, Ad-CD-GMCSF, carrying the cytosine deaminase (CD) and granulocyte-macrophage colony-stimulating factor (GM-CSF) transcription units driven by a cytomegalovirus promoter was constructed, and the in vitro efficacy of the vector was tested in tumor cell lines and a syngeneic mouse model of colon cancer. The tumor cells infected with Ad-CD-GMCSF vector were found to produce a substantial amount of GM-CSF in tumor cell lines. Accordingly, the vector carrying CD and GM-CSF transcription units together induced a potent antitumor immunity with a significantly increased number of tumor-specific T cells and tumor-specific T-cell cytotoxicity (p < 0.001). The tumor growth rate of Ad-CD-GMCSF-treated mice was significantly lower when compared to the control and an adenoviral vector carrying only the CD transcription unit (Ad-CD; p < 0.05). Likewise, the median overall survival of the Ad-CD-GMCSF vector group was significantly higher than that of the control and Ad-CD groups (34.0 ± 12.8 vs. 14.0 ± 0.5 and 23.0 ± 2.8 days, respectively; p < 0.001). In conclusion, along with its cytotoxic effect, the high immunostimulatory effect of the bicistronic Ad-CD-GMCSF vector has excellent potential in the treatment of cancers.