Double suicide gene therapy using a replication defective herpes simplex virus vector reveals reciprocal interference in a malignant glioma model

Herpes simplex virus thymidine kinase (HSV-TK) and Escherichia coli cytosine deaminase (CD) are non-mammalian enzymes capable of converting innocuous prodrugs into cytotoxic metabolites. Both enzymes have been utilized independently, as well as together in 'suicide' gene therapy protocols to eliminate tumor cells in vitro and in vivo. We have used a set of replication defective HSV vectors expressing either or both enzymes to compare the efficacies of single and double suicide gene therapies in the 9L gliosar-coma model in vitro and in vivo. In cell culture experiments at high and low multiplicities of infection, combined expression of the two genes by vector TOCD/TK along with exposure to the matching prodrugs (ganciclovir and 5-fluorocytosine) showed increased cytotoxicity compared with exposure to either prodrug alone, However, the two gene combination was inferior to single gene treatments, suggesting that HSVtk and CD are mutually counteractive in the prodrug-dependent killing of glioma cells. In animal experiments, survival was not significantly prolonged by administration of both prodrugs to TOCD/TK-treated animals, while each single gene/prodrug pair resulted in increased survival, These results indicate that single suicide gene systems employing HSVtk or CD may be preferable over combinations of the two.