Deletion of apoptosis inhibitor F1L in vaccinia virus increases safety and oncolysis for cancer therapy

Abstract Vaccinia virus (VACV) possesses a great safety record as a smallpox vaccine and has been intensively used as oncolytic virus against various type of cancer over the past decade. Different strategies were developed to make vaccinia virus safe and selective to cancer cells. Leading clinical candidates, such as Pexa-Vec, are attenuated through deletion of the viral thymidine kinase (TK) gene, which limits virus growth to replicate in cancer tissue. However, tumors are not the only tissues whose metabolic activity can overcome the lack of viral TK. In this study we sought to further increase the tumor specific replication and oncolytic potential of Copenhagen strain VACV ΔTK. We show that deletion of the anti-apoptosis viral gene F1L not only increases the safety of the Copenhagen ΔTK virus, but also improves its oncolytic activity in an aggressive glioblastoma model. The additional loss of F1L does not affect vaccinia virus replication capacity, yet its ability to induce cancer cell death is significantly increased. Our results also indicate that cell death induced by the Copenhagen ΔTK/F1L mutant releases more immunogenic signals, as indicated by increased levels of IL-1B production. A cytotoxicity screen in NCI-60 panel shows that the ΔTK/F1L virus induces faster tumor cell death in different cancer types. Most importantly, we show that compared to the TK-deleted virus, the ΔTK/F1L virus is attenuated in human normal cells and causes fewer pox lesions in murine models. Collectively, our findings describe a new oncolytic vaccinia deletion strain that improves safety and increases tumor cell killing.