Systemic administration of a recombinant vaccinia virus expressing the cytosine deaminase gene and subsequent treatment with 5-fluorocytosine leads to tumor-specific gene expression and prolongation of survival in mice

Suicide gene therapy using the cytosine deaminase (CD) gene and 5-fluorocyt osine (5-FC) has shown promising results for the treatment of colon carcino ma cells ire vitro. Efficient viral infection and tumor-specific gene deliv ery is crucial for clinically measurable treatment effects. After proving e fficient gene transfer in vitro, we demonstrate here that genes can be deli vered to metastatic liver tumors in vivo in a highly selective manner using systemic delivery of a thymidine kinase-deleted (TK-) recombinant vaccinia virus (Western Reserve strain), When the vector was administered systemica lly in C57BL/6 mice or nude/athymic mice with established disseminated MC38 Liver metastases, transgene expression in tumors was usually 1,000- to 10, 000-fold higher compared with other organs (n = 160; P < 0.00001). This tum or-specific gene transfer Leads to significant tumor responses and subseque nt survival benefits after the transfer of the CD gene to liver metastases and subsequent systemic treatment with the prodrug 5-FC (P < 0.0001), We de scribe reporter gene and survival experiments both in immunocompetent and a thymic nude mice, establishing a gene expression pattern over time and char acterizing the treatment effects of the virus delivery/prodrug system. Cure rates of up to 30% in animals with established liver metastases show that suicide gem therapy using TK- vaccinia virus as a vector may be a promising system for the clinical application of tumor-directed gene therapy.