Enzyme/prodrug-based tumor vaccination: all politics (and immunity) are local.

A major focus of gene therapy for cancer has been the effort to introduce into cancer cells a number of foreign genes that encode enzymes that will selectively convert nontoxic prodrugs into toxic compounds, producing high local concentrations that result in tumor cell killing—so-called “suicide” gene therapy. A number of enzyme/prodrug systems have been described [reviewed in (1)], including herpes simplex virus-thymidine kinase (HSV-tk)/ganciclovir (GCV) and Escherichia coli cytosine deaminase (CD)/5-fluorocytosine. A surprising early observation in many of these systems was that not every cell in a tumor need express the transgene to achieve meaningful cell killing and tumor regression (2,3). This phenomenon, the bystander effect, is defined as the ability of the genetically modified cells, in the presence of the prodrug, to cause cytotoxic effects in cells that lack the suicide gene. The result is that the fraction of cells killed is in excess of the fraction in the tumor that actually expresses the suicide gene (4). The bystander effect is a powerful enhancement of many suicide gene/prodrug systems that compensates for the inability of current vector systems to transduce all but a small fraction cells in a given tumor (5). The degree and mechanism of the bystander effect differ with various cell lines and the enzyme/prodrug system studied. At the cellular level, several explanations have been advanced for the bystander effect; however, current evidence supports direct transfer of activated prodrug from the transgene-expressing cells to untransduced wild-type cells (6–8). In the HSV-tk/GCV system, in which the activated prodrug is highly ionized and is unable to diffuse across cell membranes, the bystander effect appears to be mediated by transfer of phoshorylated GCV through cellular gap junctions (6,8,9). Nonadherent cells and those with few gap junctions do not exhibit a significant HSVtk/GCV bystander effect. 5-Fluorouracil (5-FU), the activated prodrug of the CD/5-fluorocytosine system, is a small nonpolar molecule that passively diffuses across the cell membrane down its concentration gradient, from CD+ cells to untransduced cells (7,10). The clinical importance of the bystander effect is illus-