Selective killing of AFP-positive hepatocellular carcinoma cells by adeno-associated virus transfer of the herpes simplex virus thymidine kinase gene.

The use of viral thymidine kinase (TK) gene coupled with the administration of ganciclovir to render cancer cell death has been studied extensively. Many of these experiments utilized retrovirus to transfer the TK gene under the control of a nonspecific promoter. Because nonspecific expression of the viral TK gene may cause death of proliferating cells, other than cancer cells, we explored the use of a liver-specific promoter and a tumor-specific AFP enhancer to achieve regulated viral TK gene expression for treatment of hepatocellular carcinoma. We also used the adeno-associated virus (AAV) as vector for the delivery of the TK gene because this virus is not associated with any pathological consequences in humans. Because it can infect nondividing S-phase cells, AAV can transfer genes into noncycling tumor cells. A recombinant AAV virus was constructed to include the selectable marker neoR gene and the herpes simplex virus (HSV)-TK gene driven by the human AFP enhancer and the albumin promoter. The liver-predominant expression pattern of the TK gene was observed when this construct was tested in transgenic mice. When human hepatocellular carcinoma cell lines displaying different levels of AFP and albumin and nonhepatocyte tumor cell lines were infected with the recombinant AAV virus, ganciclovir treatment caused only AFP and albumin-positive hepatocellular carcinoma cells death, but not nonhepatocyte tumor cells or AFP and albumin-negative hepatic tumor cells. Moreover, the dose required to kill the cancer cells was inversely proportional to the level of AFP expression in the cells.