Enhanced Tumor Cell Killing in the Presence of Ganciclovir by Herpes Simplex Virus Type 1 Vector-directed Coexpression of Human Tumor Necrosis Factor-α and Herpes Simplex Virus Thymidine Kinase¹

Past studies have documented the promise of herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) suicide gene therapy as a potential antitumor treatment. HSV-TK converts the pro-drug ganciclovir (GCV) into a toxic nucleotide analogue, the incorporation of which into cellular DNA blocks cell proliferation. In this report, we have examined the hypothesis that the effectiveness of HSV-TK suicide gene therapy can be enhanced by coexpression of the antitumor cytokine human tumor necrosis factor-α (TNF-α) from the same replication-defective HSV-1 vector. In vitro testing demonstrated that TNF-α expression from this vector potentiated the killing of both TNF-α-sensitive L929 tumor cells and TNF-α-resistant U-87 MG cells in the presence of GCV. Furthermore, treatment of established intradermal L929 tumors in vivo with the TNF-α/TK vector and GCV resulted in prolonged animal survival compared with treatment with parental HSV-TK vector in the presence or absence of GCV. Treatment of intracerebral U-87 MG tumors showed a clear benefit of TK therapy, but a significant further increase in survival using the TNF-α vector could not be demonstrated. We found that potentiation of cell killing in vitro required intracellular TNF-α because purified protein added to the culture medium of cells infected with HSV-TK vector failed to have the same effect. Accordingly, potentiation in vivo should depend on efficient infection, but immunohistochemical analysis indicated that virus administration by U-87 MG intratumoral injection was inadequate, resulting in an estimated <1% infection of all tumor cells. Moreover, the majority of infected tumor cells were localized at the tumor margin. Together, these results suggest that TNF-enhanced tk gene therapy should provide a useful treatment for TNF-α-sensitive tumors and perhaps also for TNF-α-resistant tumors if vector delivery can be improved to increase the percentage of transduced tumor cells.