Application of the Cre recombinase/loxP system further enhances antitumor effects in cell type-specific gene therapy against carcinoembryonic antigen-producing cancer

A considerable number of studies of cancer have shown that the cell type-sp ecific promoter is an effective tool for selective expression of foreign ge nes in tumor cells, However, few reports have demonstrated significant in v ivo antitumor effects using this strategy thus far, possibly because the lo w activity of such a promoter results in insufficient expression of genes i n cancer cells as well as in insignificant antitumor effects, even when the cells are infected by highly efficient gene transfer methods. To overcome this problem, we used the Cre/loxP system for the cell type-specific gene t herapy against carcinoembryonic antigen (CEA)-producing cancer. We construc ted a pair of recombinant Ads, One expresses the Cre recombinase (Cre) gene under the control of the CEA promoter (Ad.CEA-Cre). The other contains the herpes simplex virus thymidine kinase (HSV-TK) gene separated from the str ong CAG promoter by insertion of the neomycin resistance (neo) gene (Ad.lox -TK). The HSV-TK gene of the latter Ad is designed to be activated through excisional deletion of the neo gene by Cre enzyme released from the former one only when CEA-producing cells are infected simultaneously with these Ad s. Coinfection by these Ads rendered a human CEA-producing cancer cell line 8.4-fold more sensitive to ganciclovir (GCV) compared with infection by Ad .CEA-TK alone, the HSV-TK gene of which is directly regulated by the CEA pr omoter, On the other hand, coinfection with these Ads did not significantly change the GCV sensitivity of non-CEA-producing cells. Intratumoral inject ion of Ad,CEA-Cre combined with Ad,lox-TK followed by GCV treatment almost completely eradicated CEA-producing tumors established in the subcutis of a thymic mice, whereas intratumoral injection of Ad.CEA-TK with GCV administr ation at mast retarded the growth of inoculated tumors, These results sugge st distinct advantages of the Cre/loxP system applied in the conventional c ell type-specific gene therapy against cancer.