Bystander effect of purine nucleoside analogues in HSV-1tk suicide gene therapy is superior to that of pyrimidine nucleoside analogues

Introduction of the herpes simplex virus type I thymidine kinase gene into tumor cells, followed by the administration of the antiherpes nucleoside an alogue ganciclovir has been demonstrated to be effective in eliminating sol id tumors in animals. The success of this combination treatment largely dep ends on the bystander effect, ie the killing of nontransfected tumor cells by activated drug carried over from the nearby herpes thymidine kinase (fk) gene-transfected cells. We evaluated the in vitro bystander effect of seve ral antiherpes purine and pyrimidine nucleoside analogues, using a colorime tric assay. All pyrimidine nucleoside analogues, including (E)-5-(2-bromovi nyl)-2'-deoxyuridine (BVDU), showed low, if any, bystander killing effect. In contrast, purine nucleoside analogues, such as ganciclovir, were endowed with a pronounced bystander killer effect. Lobucavir (Cyclobut-G), a ganci clovir analogue, displayed a two- to three-fold more pronounced bystander k iller effect than ganciclovir, eliminating, at a concentration of 10 mu M, 75% and 90% of a cell population that contained 5% and 10% tk gene-transfec ted cells, respectively. These findings were corroborated by autoradiograph ic analysis that showed that 2'-H-3-BVDU metabolites formed in the herpes t k gene-transfected tumor cells were much less efficiently incorporated in t he DNA of bystander cells than 8-H-3-GCV. This indicates that, under the sa me experimental conditions, BVDU metabolites are less prone to pass the gap junctions than GCV metabolites.