TUMOR CELL-SPECIFIC TRANSGENE EXPRESSION PREVENTS LIVER TOXICITY OF THE ADENO-HSVTK GCV APPROACH/

Treatment of colorectal liver metastases with the HSVtk/GCV approach a nd adenoviral Vectors is highly toxic. We present a nontoxic alternati ve using the cell type-specific CEA promoter instead of the widely use d hCMV immediate-early promoter to drive tk gene expression in the con text of a recombinant adenovirus. Analysis of CEA promoter-dependent t k gene expression showed significant activity of this promoter in seve ral human and rat tumor-derived cell lines but not in rat primary hepa tocytes arid in mouse liver, whereas the CMV promoter was highly activ e in all cell types and tissues investigated. CEA promoter-dependent t k gene expression was sufficient to kill 100% of cancer cells in vitro , even if less than 10% were infected by the adenoviral vector, indica ting a significant bystander effect. Moreover, treatment of subcutaneo us tumors in SCID mice with Ad.CEA-tk led to a several-fold reduction of tumor growth, and tail vein injection of a high dose of Ad.CEA-tk c aused no side-effects in the liver. The CMV promoter was more potent t han the CEA promoter in mediating GCV sensitivity to cancer cells in v itro and in vivo, but even a 20-fold reduction of the dose of Ad.CMV-t k did not prevent its liver cell toxicity after systemic application t o mice and still resulted in the death of all animals within 4 days af ter the start of GCV treatment. These results indicate that restrictio n of tk gene expression to tumor cells in the liver prevents systemic toxicity. Moreover, the CEA promoter is a safe and efficient tool for tumor cell-specific expression of suicide genes in the liver.