HSV vector cytotoxicity is inversely correlated with effective TK/GCV suicide gene therapy of rat gliosarcoma

Herpes simplex virus (HSV)-mediated delivery of the HSV thymidine kinase (t k) gene to tumor cells in combination with ganciclovir (GCV) administration may provide an effective suicide gene therapy for destruction of malignant glioblastomas. However, because HSV is a highly cytotoxic agent, gene expr ession from the virus is short-lived which may limit the effectiveness of H SVtk/GCV therapy. Using different replication-defective HSVtk gene vectors, we compared Hsv vector backgrounds for their cytotoxic activity on infecti on of 9L gliosarcoma cells in culture and brain tumors in rats and evaluate d the impact of vector toxicity on the effectiveness of tk/GCV-mediated sui cide gene therapy. As reported previously for other cell lines, a vector de leted for both copies of the immediate-early (IE) gene ICP4 (SOZ.1) was hig hly toxic for 9L cells in culture while a vector deleted in addition for th e ICP22 and lCP27 IE genes (T.1) reduced or arrested 9L cell proliferation with more limited cell killing. Nevertheless, both vectors supported widesp read killing of uninfected cells in the presence of GCV following low multi plicity infections, indicating that vector cytotoxicity did not preempt the production of vector-encoded TK enzyme necessary for the killing of uninfe cted cells by the HSV-tk/GcV bystander effect. Although an SOZ.1-related ve ctor (SHZ.2) caused tumor cell necrosis in vivo, injection of SHZ.2 at mult iple coordinates thoughout the tumor followed by GCV administration failed to prolong markedly the survival of tumor-bearing rats. in contrast, a sing le injection of T.1 produced a life-extending response to GCV. These result s indicate that vector cytotoxicity can limit the efficacy of HSV-tk/GCV tr eatment in vivo, which may be due to premature termination of tk gene expre ssion with attendant abortion of the bystander effect.