673. Therapeutic Efficacy of Retroviral Replicating Vector (RRV) -Mediated Prodrug Activator Gene Therapy for Pancreatic Cancer

Abstract

Pancreatic ductal adenocarcinoma is one of the most lethal cancers, thus new therapeutic strategies for this disease are urgently needed. Retroviral replicating vectors (RRV)-mediated prodrug activator gene therapy with Toca 511 (vocimagene amiretrorepvec), an optimized RRV encoding yeast cytosine deaminase (yCD) which converts the prodrug 5-fluorocytosine (5-FC) to the anticancer drug 5-fluorouracil (5-FU), is showing promising clinical activity in patients with recurrent high grade glioma, and is now being evaluated in a multi-center Phase II/III clinical trial. In the present study, we evaluated the therapeutic efficacy of RRV-mediated prodrug activator gene therapy in preclinical models of pancreatic cancer. We first examined the replication kinetics of RRV expressing the GFP reporter gene (RRV-GFP) in murine (Pan02) and human (MIAPaCa-2, BxPC-3, PANC-1 and SUIT-2) pancreatic cancer cell lines by flow-cytometric analysis and quantitative PCR. In all of these pancreatic cancer lines, RRV-GFP inoculated at MOI=0.05 (~5% initial transduction levels) showed rapid viral replication subsequently resulting in high levels of transduction, with the majority of pancreatic cancer lines reaching >90% GFP-positive cells over time. Next, we tested in vitro cytotoxicity by MTS assay after prodrug treatment of pancreatic cancer cells (Pan02 and MIAPaCa-2) transduced with Toca 511. In RRV-transduced pancreatic cancer cells, significant (~90%) cytotoxicity was induced by exposure to 0.1-1.0 mM 5-FC prodrug for 4 days, compared to untransduced and RRV-GFP transduced controls. We then evaluated in vivo therapeutic efficacy of Toca 511/5-FC prodrug activator gene therapy in Pan02 pancreatic tumor models established subcutaneously in immunocompetent syngeneic C57BL/6 mice. While 5-FC treatment alone showed no obvious inhibition of tumor growth as well as the nontreated control group, the majority (n=7/8) of Toca 511-transduced tumors showed complete regression after 5-FC treatment. Notably, systemic biodistribution studies showed no detectable RRV signals in genomic DNA from normal tissues of treated mice. In orthotopic models using luciferase-marked MIAPaCa-2 tumors established in the pancreas in nude mice, significant inhibition of bioluminescent signals was observed by optical imaging after 5-FC administration in mice with Toca 511-infected tumors, as compared to untreated control tumors. Thus, RRVs are highly efficient vehicles for delivering prodrug activator genes such as yCD to pancreatic cancer cells, thereby achieving significant cell killing upon pro-drug administration. Further preclinical studies are ongoing toward translating this novel therapeutic strategy into clinical trials for patients with pancreatic cancer.