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.


This work was supported by Grant 108666 from the NIH (to J. C. G.) and by a grant from GenVec, Inc., Rockville, MD.

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