The application of genetically engineered herpes simplex viruses to the treatment of experimental brain tumors

Due to lack of effective therapy, primary brain tumors are the focus of intense investigation of novel experimental approaches that use vectors and recombinant viruses. Therapeutic approaches have been both indirect, whereby vectors are used, or direct to allow for direct cell killing by the introduced virus. Genetically engineered herpes simplex viruses are currently being evaluated as an experimental approach to eradicate malignant human gliomas. Initial studies with γ₁34.5 mutants, R3616 (from which both copies of the γ₁34.5 gene have been deleted) and R4009 (a construct with two stop codons inserted into the γ₁34.5 gene), have been assessed. In a syngeneic scid mouse intracranial tumor model, recombinant herpes simplex virus can be experimentally used for the treatment of brain tumors. These viruses and additional engineered viruses were subsequently tested in human glioma cells both in vitro and in vivo. Using a xenogeneic scid mouse intracranial glioma model, R4009 therapy of established tumors significantly prolonged survival. Most importantly, long-term survival was achieved, with histologic evidence that R4009 eradicated intracranial tumors in this model. Furthermore, the opportunity to evaluate γ₁34.5 mutants that have enhanced oncolytic activity, e.g., R8309 where the carboxyl terminus of the γ₁34.5 gene has been replaced by the murine homologue, MyD116, are considered.