Abstract A42: Alternate pseudotypes overcome receptor interference and enable combination suicide gene therapy with retroviral replicating vectors

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 (MLV)-based RRV encoding the yeast cytosine deaminase (CD) prodrug activator gene, designated Toca 511 (vocimagene amiretrorepvec), is now being investigated in combination with Toca FC (extended-release 5-FC) in multi-center Phase IIB / III clinical trials for patients with recurrent high-grade glioma (clinicaltrials.gov: NCT02414165). 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. We further propose that combination with additional therapies may be desirable to optimize treatment outcomes. Here we first evaluated two different RRVs derived from MLV and gibbon ape leukemia virus (GALV), and expressing either the same fluorescent reporter gene (MLV-GFP and GALV-GFP, respectively), or different reporter genes (MLV-GFP vs. GALV-RFP; MLV-RFP vs. GALV-GFP), in different types of human cancer cells. Individually, RRVs expressing the same reporter gene efficiently infected human glioma, prostate cancer, and ovarian cancer cells, and showed efficient replication and spread in culture. When marked with different fluorescent reporter genes, it was found that MLV-GFP can spread in cells that had been pre-transduced with GALV-RFP, but not in MLV-RFP pre-transduced cells. Similarly, GALV-GFP can spread in MLV-RFP pre-transduced cells, but not in GALV-RFP pre-transduced cells. This mutually exclusive infection pattern is likely due to the phenomenon of receptor interference, which occurs when vectors derived from the same viral strain compete for binding to cell surface receptors, resulting in superinfection resistance. Notably, however, replication and spread of either RRV in culture was not affected by pre-transduction with RRV derived from a different strain, indicating that this approach could overcome receptor interference. In order to investigate the effect of combined prodrug-dependent cell killing in vitro, cells were then transduced with MLV- or GALV-based RRV expressing the yeast cytosine deaminase suicide gene (MLV-CD, GALV-CD), which converts the prodrug 5-fluorocytosine (5-FC) into the active drug 5-fluorouracil (5-FU), or with RRV expressing the Herpes thymidine kinase gene (MLV-TK, GALV-TK) which converts Ganciclovir (GCV) to GCV-monophosphate, either individually or in combination. In vitro cytocidal effects obtained by combining different prodrug activator genes were significantly greater when these genes were delivered with RRV derived from two different strains. These data indicate the potential utility of using RRV pseudotyped with envelopes derived from different viral strains to overcome receptor interference leading to superinfection resistance, and achieve effective combined prodrug activator gene therapy. Citation Format: James Grosso, Sara Collins, Akihito Inagaki, Suzanne Matsuura, Brian Slomovitz, Tan Ince, Noriyuki Kasahara. Alternate pseudotypes overcome receptor interference and enable combination suicide gene therapy with retroviral replicating vectors. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A42.