Characterization of Novel Small Molecule Potentiators of Oncolytic Virotherapy

Abstract

The use of oncolytic viruses (OVs) to selectively destroy cancer cells is poised to make a major impact in the clinic and potentially revolutionize cancer therapy. Pre-clinical and clinical studies have shown that OV therapy is safe, well-tolerated and effective in a broad range of cancers. Still, resistance due to tumour heterogeneity highlights areas for improvement in OV based therapeutics. Combining OVs and small molecules is a promising strategy to selectively enhance OV-mediated anti-tumour effects. To this end, we have previously identified the synthetic compound Viral Sensitizer 1 (VSe1) that enhances the spread of oncolytic vesicular stomatitis virus (VSVΔ51) in resistant cancer cell lines up to 1000-fold, resulting in synergistic cell killing and improved efficacy in vitro and in vivo. The electrophilic nature of VSe1 prompted us to investigate the scaffold to identify active analogs with more favourable physiochemical properties and explore structure-activity relationships (SAR). In vitro assays and a rational approach in the design of VSe1 analogs allowed us to identify functional groups that can be modified without hampering activity. Lead compounds created in this study based on a pyrrole scaffold increase OV growth up to 2000-fold in vitro and demonstrate remarkable selectivity for cancer cells over normal tissue ex vivo and in vivo. Compared to the parental VSe1, these small molecules also possess enhanced stability with reduced electrophilicity and are well-tolerated in animals, leading to reduced tumour burden and prolonged survival in vivo when used in combination with VSVΔ51. It was known from previous studies that VSe1 suppresses the type I interferon response generated by cancer cells to defend against viral infection. In this study, further investigation revealed that VSe1 and its analogs inhibit the nuclear translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB), resulting in dampened transcriptional expression and secretion of IFN-β and interferon stimulated genes, thereby increasing viral replication and spread. While these findings further elucidated the effect these compounds have on the innate antiviral response, the molecular mechanisms leading to NFκB inhibition remained unclear. We used the newly generated VSe1 analogs to perform ligand-based affinity capture studies leading to the identification of glutathione-s-transferases as interacting proteins, catalytically inhibited by VSe1 and to a lesser extent by its pyrrole analogs. Further inquiry revealed that VSe1 and its analogs cause an imbalance in cellular glutathione homeostasis and increase oxidative stress, which is associated with inhibition of the nuclear translocation of NFκB. However, further studies are required to assess whether these phenomena are directly or indirectly linked. Overall, this study highlights a novel approach to improving OV therapy by using a previously uncharacterized class of compounds that ultimately alter the innate cellular antiviral response through inhibition of NFκB.