THE EFFECT OF PROMOTER STRENGTH IN ADENOVIRAL VECTORS CONTAINING HERPES-SIMPLEX VIRUS THYMIDINE KINASE ON CANCER GENE-THERAPY IN-VITRO AND IN-VIVO

The use of adenoviral vectors to deliver the herpes simplex virus thym idine kinase (HSVtk) gene followed by treatment with the prodrug ganci clovir (GCV) has promise for a variety of applications where excess ce ll proliferation is detrimental such as treatment of tumors and vascul ar restenosis. Optimizing this system is thus an important goal. The p urpose of this study was to determine if the induction of higher level s of HSVtk expression would augment the sensitivity to GCV. This was a ccomplished by generating adenoviral vectors that expressed HSVtk from promoters of different efficiencies (the CMV versus RSV promoters). D espite higher levels of HSVtk expression per cell with the CMV promote r, there was no significant enhancement of antitumor effects between R SV- and CMV-driven adenovirus vectors in in vitro and in vivo studies indicating that simply increasing HSVtk enzyme levels per cell above a minimal threshold level will not be effective in augmenting the HSVtk /GCV system. These results suggest that other strategies, e.g., the us e of higher doses of GCV, augmentation of the ''bystander effect,'' th e generation of mutant HSVtk genes with higher substrate affinities, t he discovery of improved vectors with increased transduction efficienc ies, or the development of new prodrugs with higher affinities for HSV tk will therefore be needed to enhance therapeutic responses.