New approaches to transfer genetic material into tumor and normal central nervous system (CNS) tissue are being explored. The mechanisms involved in effecting antitumor activity using the suicide gene transfer approach are investigated. Normal CNS structures, tumors, normal and tumor vasculature, and choroid plexus epithelium are being targeted. New viral vectors, including adenoviruses, are being evaluated for potential therapeutic approaches. A clinical trial for treating patients with recurrent malignant brain tumors with a retroviral vector containing the gene for Herpes simplex thymidine kinase (HS Tk) and intravenous ganciclovir (GCV) was completed. The results indicate that 1) the producer-cell approach can be used successfully without toxicity in human brain tumors, 2) antitumor activity occurs in some patients, 3) limited gene transfer into tumor cells occurs with the current approach for delivery and distribution, and 4) """"""""bystander effect"""""""" probably underlies the antitumor activity. These results highlight the need for improved methods of drug delivery and distribution in solid tumors. Therefore, we are currently investigating techniques to enhance the delivery of genetic vectors to the CNS and to CNS tumors via intracarotid infusion after selectively opening the blood-tumor barrier using a short-acting nitric oxide donor. The results indicate that this approach might be an effective and safe new way of opening the blood-brain barrier in patients with brain tumors to enhance delivery and distribution of genetic vectors. In addition, we are studying the distribution of an adenoviral vector in normal brain and tumors with convection-enhanced delivery. We are also developing methods to quantify gene delivery to the brain and to tumors. Using a radiolabeled adenoviral vector, we are studying the distribution of the viral vector in normal brain tissue and tumors using quantitative autoradiography (QAR) and modern image analysis techniques. With this approach, it might be possible to develop a method to monitor the distribution of genetic material in patients with brain tumors using positron-emission tomography (PET). Improving the efficacy of gene therapy for CNS malignancies also requires the design of novel, more effective genetic vectors. In contrast to replication-deficient viral vectors whose distribution is limited to a small diameter around the injection site, replication-competent vectors are more likely to be distributed in a larger portion of the tumor. We are currently investigating the efficacy of replication-permissive adenoviral vector carrying the TK gene which preferably replicates in p53 mutant cells in a nude rat glioma model.
