USE OF TISSUE-SPECIFIC EXPRESSION OF THE HERPES-SIMPLEX VIRUS THYMIDINE KINASE GENE TO INHIBIT GROWTH OF ESTABLISHED MURINE MELANOMAS FOLLOWING DIRECT INTRATUMORAL INJECTION OF DNA

We report here the use of the 5' flanking region of the murine tyrosin ase gene to direct expression of the herpes simplex virus thymidine ki nase (tk) gene specifically to murine melanoma cells, whilst not permi tting expression in a range of other cell types. Expression of the her pes simplex virus tk gene from the tyrosinase promoter in melanoma cel ls rendered them sensitive to killing by ganciclovir (100% cell death of a tk-expressing B16 clone after 12 days in culture at 1 mu/ml ganci clovir). We also observed a substantial bystander killing effect when expressing cells were mixed with nontransfected parental B16 cells. Wh en transfected murine melanoma cells expressing tk were injected into syngeneic mice both their tumorigenicity and experimental metastatic p otential were abrogated completely when the mice were treated with gan ciclovir (27 of 28 mice treated with water developed progressively gro wing tumors versus 1 of 30 in the ganciclovir-treated group). Direct i njection of the tk gene under control of the tyrosinase promoter into established tumors in mice, followed by treatment with ganciclovir, le d to significant reductions in resultant tumor size relative to the si ze of tumor developing in mice treated with water (median tumor weight , 1.65 g versus 2.75 g). Therefore, direct transfer of recombinant gen es by injection of DNA can significantly reduce established tumor burd en in vivo.