Recently it was shown that recombinant vaccinia virus GLV-1h68 is a promising tool for treating different type of cancers in animal models. The goal of the present study was to enhance the oncolytic potential of GLV-1h68 without decreasing its safety. A derivative of GLV-1h68 containing the gene for a Walker A motif mutant of the essential cell cycle protein Cdc6, GLV-1h237, was engineered. The characteristics of GLV-1h237 and its efficiency in treating human breast cancer GI-101A cells were compared with that of GLV-1h236 (carrying the wild-type gene for Cdc6), GLV-1h71 (a derivative of GLV-1h68) and GLV-1h68, respectively. RT-PCR and immunoblot analyses revealed that Cdc6 is efficiently overexpressed in GLV-1h237-infected GI-101A cells. GLV-1h237 was found to have higher replication efficiency and enhanced cytotoxity than GLV-1h68 in cell culture. In the GI-101A tumor xenograft animal model, GLV-1h237 turned out to be the most potent oncolytic virus strain investigated. A single i.v. injection of GLV-1h237 resulted in enhanced anti-tumor activity compared to GLV-1h68 concomitant with a high tumor selectivity and a comparable safety profile. Thus, the strategy to combine oncolytic virotherapy with agents that interfere with host cell DNA synthesis is a promising approach for effective cancer therapy.

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