Unique among replicating viruses being developed as oncolytic agents, replicating murine leukemia virus (RMLV) retroviruses propagate without immediate lysis of host cells and maintain viral persistence through stable integration. Due to its intrinsic inability to infect quiescent post-mitotic cells and susceptibility to innate antiviral defenses that exist in normal cells but are inactivated in many cancers, RMLV vector replication has been shown to be highly tumor-selective. We have previously demonstrated that RMLV vectors are capable of highly efficient replication in tumor cells, and we have found significantly enhanced survival benefit in a variety of cancer models in vivo when RMLV vectors are employed for delivery of prodrug activator genes such as yeast cytosine deaminase (yCD), which mediates the conversion of inactive prodrug 5-fluorocytosine (5FC) to the anti-cancer drug, 5-fluorouracil (5FU). In the present study, we tested a newly developed RMLV vector with modified virus backbone sequences and a codon-optimized yCD gene (Toca511). First, we examined in vitro cytotoxicity using human (U87) and murine (TU-2449) glioma cell lines by MTS assay. The cell viability of Toca511-transduced cells was reduced by >80% after 5 or 8 days of exposure to the prodrug, respectively. Second, we examined the therapeutic efficiency of this newly developed vector in subcutaneous U87 human glioblastoma models and TU2449 syngeneic mouse models. Toca511-mediated prodrug activator therapy strongly inhibited tumor growth in both subcutaneous models. Third, therapeutic efficacy was evaluated, and the minimum effective doses of 5FC prodrug and Toca511 vector determined in an intracranial U87 xenograft model. Systemic administration of 5FC resulted in significant inhibition of bioluminescent signals in mice whose tumors had been infected with Toca511 but not in control mice, associated with significant survival benefit even after injecting only 1×1e3 total vector units (p=0.0015). These data suggest that we have been successful in developing an improved retrovirus vector that shows strong therapeutic efficacy when used for prodrug activator gene therapy in both subcutaneous and intracranial xenograft models of human and/or murine glioma. This vector has now entered Phase I/II clinical trials for glioblastoma. Now, with the use of RMLV vectors such as Toca511, the original promise of retrovirus-mediated gene therapy strategies for cancer may finally be fulfilled.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5386. doi:10.1158/1538-7445.AM2011-5386