Date Published: December 27, 2007
Publisher: Public Library of Science
Author(s): David H Kirn, Yaohe Wang, Fabrice Le Boeuf, John Bell, Steve H Thorne, Grant McFadden
Abstract: BackgroundOncolytic viruses hold much promise for clinical treatment of many cancers, but a lack of systemic delivery and insufficient tumor cell killing have limited their usefulness. We have previously demonstrated that vaccinia virus strains are capable of systemic delivery to tumors in mouse models, but infection of normal tissues remains an issue. We hypothesized that interferon-beta (IFN-β) expression from an oncolytic vaccinia strain incapable of responding to this cytokine would have dual benefits as a cancer therapeutic: increased anticancer effects and enhanced virus inactivation in normal tissues. We report the construction and preclinical testing of this virus.Methods and FindingsIn vitro screening of viral strains by cytotoxicity and replication assay was coupled to cellular characterization by phospho-flow cytometry in order to select a novel oncolytic vaccinia virus. This virus was then examined in vivo in mouse models by non-invasive imaging techniques. A vaccinia B18R deletion mutant was selected as the backbone for IFN-β expression, because the B18R gene product neutralizes secreted type-I IFNs. The oncolytic B18R deletion mutant demonstrated IFN-dependent cancer selectivity and efficacy in vitro, and tumor targeting and efficacy in mouse models in vivo. Both tumor cells and tumor-associated vascular endothelial cells were targeted. Complete tumor responses in preclinical models were accompanied by immune-mediated protection against tumor rechallenge. Cancer selectivity was also demonstrated in primary human tumor explant tissues and adjacent normal tissues. The IFN-β gene was then cloned into the thymidine kinase (TK) region of this virus to create JX-795 (TK−/B18R−/IFN-β+). JX-795 had superior tumor selectivity and systemic intravenous efficacy when compared with the TK−/B18R− control or wild-type vaccinia in preclinical models.ConclusionsBy combining IFN-dependent cancer selectivity with IFN-β expression to optimize both anticancer effects and normal tissue antiviral effects, we were able to achieve, to our knowledge for the first time, tumor-specific replication, IFN-β gene expression, and efficacy following systemic delivery in preclinical models.
Partial Text: Oncolytic viruses have promise as cancer therapeutics due to their targeted nature and ability to destroy cancer cells through novel mechanisms-of-action (oncolysis and/or necrosis) [1–3]. Selective intratumoral replication of the virus leads to viral multiplication and spread to adjacent cancer cells and subsequent lysis of all infected cancer cells. One targeting strategy that has proven successful in a variety of oncolytic strains involves deletions of viral anti–type-I interferon (IFN) gene products. Cancer selectivity results through IFN-mediated inhibition of replication in normal tissues, whereas replication and oncolysis proceeds unhindered in tumor cells with defects in IFN responses [4–7]. Examples include mutations of the M-protein genes in vesicular stomatitis virus (VSV), the gamma-34.5 genes in herpes simplex virus (HSV), and viral-associated (VA) RNA in adenovirus. In addition, a variety of unmodified small RNA viruses have demonstrated natural tumor tropism mediated by their inherent sensitivity to the interferon-mediated antiviral state in normal cells [4,8,9].
We report here that IFN-β expression from a B18R-deleted vaccinia results in a systemically effective, highly selective oncolytic virus. To our knowledge, this is the first time that such a rationally designed combination of attenuating viral gene deletion and transgene expression has been incorporated into a systemically deliverable vector, and the first successful report of systemic IFN-β gene delivery to a tumor.