Assessing the efficacy of a novel gene therapy approach for treating ovarian cancer: Combination RRV-mediated prodrug activated suicide gene therapy

Introduction/Aim: Retroviral replicating vectors (RRVs) afford a unique strategy for gene therapy targeting cancer cells. By “activating” RRVs with prodrug activator suicide genes, these RRVs become strong gene delivery vehicles that replicate selectively in cancer cells, and stably persist through proviral genomic integration, leading to inducible cell death upon prodrug administration. This strategy has shown highly promising clinical results in early phase trials (published), and is currently being evaluated in an international Phase III clinical trial for patients with recurrent high-grade glioma. Recently, we have also applied RRV-mediated gene therapy to preclinical models of ovarian cancer. The aim of this current study was to evaluate feasibility and efficacy of combination suicide gene therapy using two RRVs encoated (‘pseudotyped9) with different envelope proteins and delivering different prodrug activators for the treatment of ovarian cancer. Methods: Viral spread was monitored in established and primary patient-derived ovarian cancer cell lines (SKOV3, OCI-P5X, OCI-C5X) over the course of ≥27 days using amphotropic murine leukemia virus (AMLV) envelope- and gibbon ape leukemia virus (GALV) envelope-pseudotyped RRVs delivering either fluorescent protein reporter genes (GFP, mStrawberry) or prodrug activator genes (yeast cytosine deaminase, E. coli nitroreductase), either individually or in combination, at various multiplicities of infection (MOI). Flow cytometry for GFP, mStrawberry, and Gag viral protein was used to determine transduction levels and transgene expression levels of RRVs at serial time points. Each therapeutic RRV was tested by MTS assay for its ability to induce cytotoxicity upon exposure to different concentrations prodrug. To determine viral integration and vector stability, PCR of genomic DNA extracted from infected cells was performed using RRV specific primers, and qRT-PCR was performed to determine vector copy number per cell. Results: All ovarian cancer cell lines tested could be u003e80% transduced by AMLV- and GALV-pseudotyped RRVs, both individually and in combination, within 6-24 days even after initial inoculation at MOI 0.001, with replication kinetics varying between different cell lines. Both RRVs could stably integrate into the target cell genome without evidence of transgene deletion. MTS assays demonstrated that prodrug activated cytotoxicity led to efficient killing of RRV-transduced ovarian cancer cells in vitro, with synergistic cytotoxicity observed at lower prodrug concentrations after combined treatment. Conclusion: RRV-mediated prodrug activator gene therapy may be an effective approach for efficiently and selectively inducing cytotoxicity in ovarian cancer cells. These data support further evaluation of this strategy through translational studies in vivo. Citation Format: James Grosso, Sara Collins, Aki Inagaki, Brian Slomovitz, Noriyuki Kasahara. Assessing the efficacy of a novel gene therapy approach for treating ovarian cancer: Combination RRV-mediated prodrug activated suicide gene therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5927.