Current designs of viruses with decreased virulence for therapeutic applications rely on an impaired ability to replicate and dependence on dividing cells. In contrast, the proposed research will develop designs based on restricting host range and impairing the virus' ability to establish latency and to cause recurrent infections. The effects of attenuated replication are reflected in the high doses of virus (>10/9) that are required to achieve even a minimal therapeutic effect. Moreover, current candidates for therapeutic applications retain a wide host range. As consequence virus delivery is largely restricted to local injection into the tumor mass with the result that cells surrounding the needle track are infected at high multiplicity whereas those a short distance away are at best exposed to relatively low multiplicities of infection. Because of restricted replication these viruses are difficult and costly to produce. The objective of this Project is to design viruses with a restricted host range; and incapable of axonal transport related to establishment of latent infections. We propose three aims; specifically, to: (i) develop a system for targeting recombinant herpes simplex viruses (HSV) to tumor cells expressing specific proteins on their surfaces. We have chosen as the first target the IL-13 receptor (IL13Ralpha2) commonly expressed on malignant glioma cells. Preliminary studies on intermediate phase viruses show increased specificity for malignant glioma cells carrying IL13Ralpha2. Other targets have also been identified and will be pursued; (ii) identify viral genes useful for enhanced replication of viruses targeted to specific cells. Once targeting of virus is achieved, the objective is to select mutants capable of enhanced replication in targeted cells; and, (iii) identify and modify viral gene products that enable anterograde and retrograde transport in neurons. The objective here is to further reduce potential virulence of therapeutic viruses by identification and ablation of genes whose products enable the virus to be transported to and from neuronal nuclei.