Suicide gene therapy for brain tumor by means of adeno-associated virus (AAV) vector

Clinical trials are currently being performed at several institutions in United States and European countries to evaluate suicide gene therapy using retrovirus vectors to deliver the Herpes Simplex thymidine kinase (HS-tk) gene. However, since retroviral vectors generally are ineffective when administered directly into the body, these protocols involve the delivery of mouse cells that produce the retroviral vectors into the brains of patients. At present, the results is not satisfactory from both efficacy and safety points of view. In contrast, AAV vectors are derived from a non-pathogenic human virus and they offer several potential advantages including efficient delivery of genes to both dividing and non-diving target cells and absence of viral genes that may be responsible for causing an undesirable immune response. We investigated AAV vector-based suicide gene therapy in collaboration with Avigen Company, and demonstrated that it offers the potential for increased efficacy and decreased toxicity. Using 1 × 106 particles of AAV-LacZ, nearly 100% of human glioma (U251-SP) cells were expressed the LacZ gene in vitro without any cytopathic effect and transduction of AAV-tk vector rendered them sensitive to the cytocidal effect of ganciclovir (GCV). Stereotactic injection of 3 × 1010 particles of AAV-LacZ into transplanted human glioma resulted in 30-40% transduction into tumor cells at injection site. Intratumoral injection of 6 × 1010 particles of AAV-tk followed by intraperitoneal administration of GCV for 6 days, resulted in a 34.7 fold reduction in the mean volume of tumors at the 17 days after the treatment as compared to controls. Furthermore, transplanted human gliomas were noted to be completely regressed by three courses of the AAV-tk/GCV treatment.