✓ Retroviral-mediated transfer of the herpes simplex virus thymidine kinase (HSVtk) gene into malignant tumors confers drug susceptibility to the antiviral drug ganciclovir. The authors have recently shown that in situ transduction of the rat 9L brain tumor following HSVtk-producer cell implantation led to tumor regression after ganciclovir administration in treated rats. A wide spectrum of potential adverse effects may, however, be associated with the application of this approach to treat brain tumors, including dissemination of the retroviral vector to nontumoral tissues within or outside the central nervous system, proliferation of the injected murine vector-producer cells at the injection site, immune-mediated responses to the implantation of xenogeneic cells, and damage to the brain from toxic by-products of the HSVtk-ganciclovir interaction. These possibilities were investigated using intracerebral and systemic injections of retroviral vector-producer cells carrying the HSVtk or the lacZ gene in mice, rats, and nonhuman primates.
Using the lacZ gene as a reporter gene, no evidence of β-galactosidase activity consistent with vector transduction was detected in any major body organ in the treated mice or rats. Similarly, the HSVtk gene transfer did not result in toxicity, with or without ganciclovir administration. In studies using rat and monkey models, no proliferation of the vector-producer cells occurred after intracerebral injection. Vector-producer cell survival was limited to 7 to 14 days. High-dose steroid therapy did not appear to extend the survival of these xenogeneic cells in rats. No significant inflammatory response was observed in the meninges or brain parenchyma. Endothelial cells were occasionally transduced in brain capillaries adjacent to the injected site of the vector-producer cells. Injection of producer cells into brain tissue elicited mild edema and reactive gliosis surrounding the injection site, which were probably the cause of a transient toxic response arising 4 to 5 days following injection of the producer cells; short-term administration of dexamethasone eliminated that response. No neurological deficits were observed in the rats or primates treated with the HSVtk vector-producer cells, with or without ganciclovir. In primates injected with producer cells, magnetic resonance imaging before, during, and after ganciclovir administration showed minimal and localized breakdown of the blood-brain barrier without significant edema or mass effect. Similarly, histological examination of the monkeys' brains showed no damage to neurons, astroglia, or myelin. Long-term clinical (> 9 months) and radiological (3 months) assessment of the primates has revealed no evidence of toxicity. The results of these studies indicate that intratumoral implantation of HSVtk-producer cells can be attempted for the treatment of brain tumors, without anticipating significant adverse toxicity to normal brain or remote proliferating tissues.