Gene Therapy for the Treatment of Recurrent Pediatric Malignant Astrocytomas with In Vivo Tumor Transduction with the Herpes Simplex Thymidine Kinase Gene/Ganciclovir System. Childrens Hospital, Los Angeles, California

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Abstract
The possibility of transferring a “sensitivity” gene into a growing brain tumor has been investigated. The purpose is to make the tumor sensitive to a type of chemotherapy that is relatively non-toxic to the rest of the body. The gene selected is the Herpes Simplex-thymidine kinase (HS-tk) gene. Herpes Simplex is a virus that can be killed by a drug called ganciclovir (GCV). By transferring the HS-tk gene into the tumor, using a disabled mouse virus called a “vector,” the tumor will become like a herpes virus and the tumor can be killed with GCV. Experiments in rats have shown that the direct injection of mouse cells producing a HS-tk vector into a growing brain tumor can result in complete destruction of the tumor with GCV therapy. We found no evidence of spread of the virus to the normal brain tissue or to other parts of the body. Based upon these findings, an initial trial is underway at the National Institutes of Health (NIH). To date eight people have been treated without evidence of toxicity. A second trial in adults has been approved. We now propose a human clinical trial for pediatric patients who are currently ineligible for treatment in the adult trials. In this study children with recurrent malignant brain tumors will be treated by injection of HS-tk vector-producer cells (VPC) into their tumors in an attempt to induce regression of the tumor with GCV therapy. The patient population consists of children who have failed standard therapy and have recurrent primary brain tumors with an expected survival of weeks to a few months. Murine retroviral vectors can infect a wide variety of proliferating mammalian cell types (e.g., lymphocytes). Non-proliferating tissues (e.g., neurons) are not transduced by murine retroviral vectors. These findings suggest that this type of vector may be useful for the selective introduction of genes into growing tumors in the brain, since the tumor is essentially the only tissue that will integrate and express the vector genes. The possibility of direct in vivo gene transfer into growing brain tumors in animals has been investigated. Rats with a malignant brain tumor were given an intratumoral stereotaxic injection of murine retroviral vector-producer cells (VPC) that were producing vectors containing the Herpes simplex-thymidine kinase (HS-tk) gene or a control VPC line containing the β-galactosidase gene. The animals were rested 5 days to allow time for the HS-tk retroviral vectors that were produced in situ to transduce the neighboring proliferating gliosarcoma. The animals were then treated with the anti-herpes drug ganciclovir (GCV). Tumors injected with the HS-tk VPC regressed completely with GCV therapy while the tumors injected with β-galactosidase VPC developed large tumors. Staining for β-galactosidase (+) cells in control animal brain revealed transduction of 10–50% of the tumor cells without evidence of transduction of the surrounding normal brain tissue. No significant toxicity was observed in toxicity studies in mice, rats and non-human primates. Based upon these findings, we propose a human clinical trial to determine if the direct implantation of the GlTkSvNa VPC line into growing human brain tumors will induce regression with GCV therapy. The patient population primarily consists of children with recurrent malignant tumors, who are currently considered ineligible for the approved adult protocols. The RAC has previously approved two trials involving this type of therapy. In this protocol, children will undergo treatment based on the surgical accessibility of their tumor. Those with resectable tumors will undergo surgical debulking of the tumor followed by repeated treatments of HS-tk VPC into the tumor bed in an attempt to induce complete regression of the tumor with GCV therapy. Children with unresectable tumors will undergo stereotaxic injection of the tumor with VPC. These children will have failed standard therapy for their tumor and are expected to survive for several weeks to a few months.