Double suicide gene therapy augments the antitumor activity of a replication-competent lytic adenovirus through enhanced cytotoxicity and radiosensitization

Replication-competent adenoviruses may provide a highly efficient means of delivering therapeutic genes to tumors. Previously, we evaluated in vitro a replication-competent adenovirus (AdS-CD/TKrep) containing a cytosine deam inase (CD)/herpes simplex type 1 thymidine kinase (HSV-1 TK) fusion gene th at allows lytic viral therapy to be combined with double suicide gene thera py. Both the CD/5-FC and HSV-1 TK/GCV enzyme/prodrug systems enhanced the t umor cell-specific cytopathic effects of the AdS-CD/TKrep virus in vitro an d sensitized cells to radiation. To extend these in vitro findings in vivo, we evaluated the antitumor activity of the Ad5-CD/TKrep virus in combinati on with double prodrug therapy and radiation therapy. The Ad5-CD/TKrep viru s independently demonstrated significant antitumor activity against C33A ce rvical carcinoma xenografts. Therapeutic outcome was dramatically improved with systemic administration of double, but not single, prodrug (5-FC + GCV ) therapy. When used in a neoadjuvant setting, Ad5-CD/TKrep-mediated double suicide gene therapy dramatically potentiated the effectiveness of radiati on therapy. The trimodal approach of Ad5-CD/TKrep viral, double suicide gen e, and radiotherapies produced significant tumor regression and ultimately 100% tumor cure. The results demonstrate the high therapeutic potential of the trimodal approach and provide a solid foundation for future clinical tr ials.