EXTH-44. DEVELOPMENT OF RETROVIRAL REPLICATING VECTORS ENCODING SEQUENCE-OPTIMIZED NITROREDUCTASE FOR PRODRUG ACTIVATOR GENE THERAPY IN HUMAN GLIOMA MODELS

Abstract Early-phase clinical trials of Toca 511, a retroviral replicating vector (RRV) expressing an optimized form of yeast cytosine deaminase for prodrug activator gene therapy, showed highly promising evidence of therapeutic benefit and increased survival in recurrent high-grade glioma, but an international Phase 3 trial failed to meet primary and secondary endpoints. Nonetheless, promising results observed within predetermined subgroups, combined with the highly favorable safety profile of this approach, make it worthwhile to consider alternative prodrug-activating enzymes in order to increase therapeutic efficacy. In this study, we employed RRV to deliver modified versions of another prodrug activator gene, E.coli nitroreductase (NTR), which activates the prodrug CB1954 to generate a potent bifunctional alkylating agent. We constructed RRV encoding two different isoforms of wild-type E.coli NTR genes (NfsA, NfsB) as well as variants of both isoforms extensively modified to optimize human codon usage and vector stability. Further efforts to improve the potency of these prodrug activator enzymes included the construction of additional new vectors incorporating point mutations at the active site which have previously been demonstrated to improve prodrug conversion efficiency. NTR transgene insertion did not affect RRV replication, which resulted in increasing NTR expression over time in human glioma cells in culture. Sequence optimization resulted in enhanced genomic stability of the vectors upon serial passage and also in significantly reduced cell viability in vitro on treatment of fully transduced U87 and U87-EGFRviii glioma cells with CB1954 prodrug. In orthotopic U87-EGFRviii intracerebral tumor models, stereotactic intratumoral injection of sequence-optimized RRV-NTR and repeated cycles of prodrug treatment resulted in rapid and significant reduction of tumor burden as monitored by bioluminescence imaging, leading to prolonged survival benefit. These data indicate that improved therapeutic efficacy can be achieved with optimized NTR prodrug activator gene therapy delivered by RRV in experimental glioma models.