536. Dual-Vector Prodrug Activator Gene Therapy Using Retroviral Replicating Vectors Derived from Amphotropic Murine Leukemia Virus and Gibbon Ape Leukemia Virus

Abstract

Retroviral replicating vectors (RRVs) have been shown to achieve efficient tumor transduction and enhanced therapeutic benefit in a wide variety of cancer models. An amphotropic murine leukemia virus (AMLV)-based RRV encoding the yeast cytosine deaminase (CD) prodrug activator gene, designated Toca 511 (vocimagene amiretrorepvec), is currently being investigated in combination with Toca FC (extended-release 5-FC) in multi-center clinical trials in the United States for patients with recurrent high-grade glioma (http://www.clinicaltrials.gov NCT01156584, NCT01985256, NCT01470794). This treatment has been safe and well tolerated and median overall survival exceed historical benchmarks. RRV-mediated prodrug activator gene therapy represents the ultimate form of ‘intracellular’ chemotherapy, generated selectively and directly from within the infected cancer cells themselves, without incurring systemic toxicity. Moreover, preclinical data support subsequent activation of the immune system selectively against the cancer. However, some patients may not respond to this treatment, therefore, combination with additional therapeutic agents may be desirable to optimize treatment outcomes. Here we evaluated two different RRVs derived from AMLV and gibbon ape leukemia virus (GALV), encoding two different prodrug activator genes, CD and herpes simplex virus thymidine kinase (TK) in Hep3B human hepatocellular carcinoma cells.Both RRVs expressing the green fluorescent protein gene (AMLV-GFP and GALV-GFP) efficiently replicated in Hep3B cells and spread in culture. Additionally, AMLV-GFP can spread in GALV-mCherry pretransduced Hep3B cells but not in AMLV-mCherry pretransduced cells. Similarly, GALV-GFP can spread in AMLV-mCherry pretransduced cells but not in GALV-mCherry pretransduced cells. This mutually exclusive infection pattern is likely due to receptor interference resulting in superinfection resistance when the same viral strain is used. Notably, however, replication and spread of either RRV in culture was not affected by pretransduction with the counterpart RRV encoated with the other envelope.In order to investigate the effect of combined prodrug-dependent cell killing vs. multiple vector copy transduction in vitro, Hep3B cells were transduced with the AMLV-CD (Ca), GALV-CD (Cg), AMLV-TK (Ta), GALV-TK (Tg), respectively or in combination (Ca/Cg, Ta/Tg, Ca/Tg, Cg/Ta). The resultant cells were used to evaluate the cytotoxic effect of RRV-mediated prodrug activator gene therapy with CD and TK in the presence of their respective prodrugs, 5-fluorocytosine (5-FC) and Ganciclovir (GCV). In vitro cytocidal effects obtained by combining different prodrug activator genes (Ca/Tg and Cg/Ta) were significantly greater than when the same prodrug activator genes (Ca/Cg and Ta/Tg) were delivered with two different vectors. These data indicate the potential utility of dual-vector prodrug activator gene therapy using two different RRVs carrying different prodrug activator genes.