Herpes Vector-Mediated Delivery of Marker Genes to Disseminated Central Nervous System Tumors

Abstract

The present study investigated the ability of a recombinant herpes simplex virus type 1 (HSV) vector to deliver genes into disseminated brain tumor foci through intrathecal injection of the vector. The animal model was designed to simulate brain tumors with cerebrospinal fluid (CSF) metastases, which are found especially in the pediatric population. 9L gliosarcoma cells were injected both into the right frontal lobe and in through the cisterna magna of adult rats. The HSV vector, hrR3, was inoculated intrathecally 5 days later. This vector is defective in the gene for ribonucleotide reductase, and, therefore, replicates preferentially in dividing cells; it retains an intact HSV-thymidine kinase gene (HSV-tk). Two days after injection of the vector, immunohistochemical staining for HSV thymidine kinase (HSV-TK) revealed expression in frontal tumors, as well as in leptomeningeal tumor foci along the entire neuroaxis. HSV-TK-immunopositive cells were most frequent in small tumors contacting the CSF pathways. Frontal lobe tumors showed the highest density of HSV-TK-immunopositive cells around their periphery with little expression in central parts. Some paraventricular neurons temporarily showed HSV-TK-immunolabeling at this early time point. The number of HSV-TK-immunopositive tumor cells markedly decreased 5 days after injection of the HSV vector. In all animals, some toxicity was observed in the first 2–4 days after virus injection with extensive leptomeningeal inflammation. In conclusion, intrathecal application of HSV vectors can mediate widespread transfer of the therapeutic HSV-tk gene into disseminated tumors throughout the brain and CSF pathways. Although there was marked toxicity associated with intrathecal injection of this vector, this mode of gene delivery offers a promising approach for treatment of CSF-metastases in conjunction with development of less toxic vectors.

Overview summary

Dissemination of brain tumors is an as yet unsolved therapeutic problem in adult and pediatric patients. Whereas in adults dissemination occurs by single-cell migration away from the main tumor mass, pediatric brain tumors often tend to disseminate via the cerebrospinal fluid (CSF) pathways. The gene therapy approach presented in this study addresses the problem of brain tumor dissemination with emphasis on CSF metastases. In a combined model of intracerebral and leptomeningeal tumor foci, a herpes simplex virus (HSV) vector with a potentially therapeutic herpes simplex virus type 1 (HSV) thymidine kinase (HSV-tk) gene was injected intrathecally. Distribution of various HSV marker proteins, including HSV thymidine kinase (HSV-TK), was investigated in the disseminated tumor foci and normal brain. The time course of HSV vector-mediated gene expression was determined and vector toxicity was evaluated.