Abstract 2349: Evaluation of the combination of the prodrug-mediated gene therapy vector AdV-tk and immune checkpoint inhibitor for glioblastoma treatment in a syngeneic mouse model

There has been a recent surge of interest in cancer immunotherapies due to newly FDA approved immunologic treatments based on targeting immune checkpoint inhibition pathways. Despite the dramatic responses in animal models and early clinical human trials, particularly in melanoma, durable clinical responses are observed only in approximately 30% of patients, therefore combinatorial immune therapeutic approaches may be needed to improve outcomes further. While the brain has traditionally been considered to be an immune-privileged site, evidence supporting the use of immunotherapeutics in brain tumors has been rapidly accumulating. Given that virus-based cancer therapies can be immunostimulatory and immune checkpoint inhibitors block the body9s natural checkpoint (ICI) response, the combination of these two approaches offers a potentially advantageous interaction. AdV-tk is an immunostimulatory virus-based approach that involves the intra-tumoral delivery of a non-replicating adenoviral vector carrying the Herpes virus thymidine kinase gene (AdV-tk) followed by administration of an anti-herpetic prodrug (ganciclovir - GCV). This approach has shown benefit in clinical trials of glioblastoma among other tumor types. The immunological component results from the delivery vehicle being a virus, the mode of cell death, through both necrosis and apoptosis, and the pro-immunogenic properties of the thymidine kinase protein (TK). This approach has consistently demonstrated anti-tumor immune stimulation and an increased intra-tumoral CD8+ T-cell infiltrate. Not surprisingly, however, this immune stimulation also leads to increased expression of immune checkpoint inhibitory ligands on tumor cells, including PD-L1. Our initial in vitro studies showed that the immune checkpoint ligand PD-L1 is expressed across a panel of human and mouse conventional and stem-like glioblastoma-derived cells (GSCs). After AdV-tk infection and GCV treatment PD-L1 is consistently up-regulated both at protein and mRNA levels, while no changes were detected after AdV-tk alone. In vivo experiments in immunocompetent mouse models with GL261 glioma cell lines demonstrate a significant improvement in survival when we combine AdV-tk+GCV and anti-PD-1 antibodies. This data suggests that combination of immune checkpoint blockade with AdV-tk treatment should be explored in glioblastoma clinical trials. Citation Format: Maria Carmela Speranza, Kazue Kasai, Johanna Kaufmann, Estuardo Aguilar-Cordova, Brian W. Guzik, E. Antonio Chiocca, Sean Lawler. Evaluation of the combination of the prodrug-mediated gene therapy vector AdV-tk and immune checkpoint inhibitor for glioblastoma treatment in a syngeneic mouse model. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2349.