Abstract B33: Intratumoral conversion of 5-fluorocytosine to 5-fluorouracil using a novel prodrug activator gene transfer technology in an immune-competent model of glioblastoma

Abstract

Abstract Background: Patients with the most common and aggressive form of brain cancer, glioblastoma multiforme (GBM), have poor prognosis and current treatments remain only palliative. Previous gene therapy approaches using replication-deficient viral vectors have demonstrated limited efficacy likely due to poor gene delivery to the tumor. Methods: We used a novel replication-competent retroviral vector (Toca 511) designed and engineered to efficiently deliver a modified cytosine deaminase (CD) prodrug activating gene to glioma cells. Previous studies showed that such a replicating vector can preferentially infect intracranial human xenografts in nude mice (Tai et al. Mol Ther. 2005;12:842) and lead to increased survival in treatment groups compared to controls. In preparation for clinical trials we have improved the CD gene activity and further stabilized the vector structure to create Toca 511. We investigated the functionality of Toca 511 using stereotactic, intratumoral delivery in a syngeneic, orthotopic glioma model in immune-competent B6C3F1 mice. Freshly excised tumors were analyzed by quantitative PCR, Western, and HPLC. Results: Brain tumors isolated from mice treated with Toca 511 and fluorocytosine (5FC) had observable expression of CD protein and vector copy numbers per genome that demonstrate efficient tumor infection. Extensive tumor specific viral spread with almost no spread of vector from the site of injection was observed in the first 24 days. Highly efficient intratumoral conversion of the 5FC prodrug into the anticancer drug 5-fluorouracil (5FU) was demonstrated after intraperitoneal or oral dosing with 5FC. Studies with Toca 511 at high and mid dose levels in combination with 5FC resulted in prolonged survival compared to controls (> 5 months, p<0.0036 and <0.0354 respectively) in this syngeneic glioma model. The highest Toca 511 dose level tested in combination with 5-FC yielded no safety related findings in the mice. Conclusions: Toca 511 efficiently delivers a functional CD gene for expression in glioma cells in a syngeneic, orthotopic model in immune-competent mice. Toca 511 increases survival through the delivery of CD gene and subsequent efficient conversion of 5FC into 5FU within the brain tumor with no safety related findings. A phase 1 ascending dose trial investigating the safety and tolerability of Toca 511 in combination with 5FC in patients with recurrent GBM is about to initiate. Citation Information: Clin Cancer Res 2010;16(14 Suppl):B33.