Purified herpes simplex thymidine kinase retroviral particles. II. Influence of clinical parameters and bystander killing mechanisms

High-titer, purified herpes simplex virus thymidine kinase (HSV-TK) retrovi ral particles, followed with intraperitoneal ganciclovir (GCV) were tested in Fischer rats bearing 1-week established 9L gliosarcomas. 9L cells were i nfused intracranially through a cannula on day 0, given intracranial infusi ons of HSV-TK retroviral particles on days 7-12, and given 5 or 10 daily do ses of intraperitoneal GCV starling at day 14. Tumor volumetric studies per formed on rat brains at day 26 after tumor infusion revealed significant di fferences in experimental groups receiving HSV-TK retroviral particles plus 10-day GCV or the 100% transduced 9L-TK group receiving GCV versus control groups treated with either buffer, HSV-TK vector, or either 5- or 10-day r egimens of GCV alone. The duration of GCV administration and the volume of tumor burden influenced efficacy. Adjuvant dexamethasone did not significan tly affect efficacy. Experiments in which 9L rechallenge of animals treated with 9L-TK/GCV or 9L tumors treated with HSV-TK vector/GCV indicated that a host immune response was involved in rejecting the rechallenge tumor. Out come was dependent upon the site and size of the rechallenge inoculum. In v itro, bystander effects were significant but were especially marked when ce ll-to-cell contact was maintained. Cumulatively, the data indicate that bot h the bystander effect and the host immune response are responsible for the efficacy observed in the suicide gene therapy paradigm using purified HSV- TK retroviral particles and GCV to treat smaller tumor burden in rats with 9L gliosarcoma.