The Oncolytic Activity of Myxoma Virus against Soft Tissue Sarcoma Is Mediated by the Overexpression of Ribonucleotide Reductase

Background: Myxoma virus (MYXV) is an oncolytic poxvirus that lacks the gene for 1 of the subunits of ribonucleotide reductase (RR), a crucial DNA synthesis and repair enzyme. The overexpression of RR has been implicated in the invasiveness of several cancers, including soft tissue sarcomas (STS). The purpose of the study was to investigate the oncolytic efficacy of MYXV in STS with different levels of RR expression. Methods: The oncolytic effect of recombinant MYXV was evaluated in 4 human STS cell lines, LS141 (a dedifferentiated liposarcoma), DDLS8817 (a dedifferentiated liposarcoma), RDD2213 (recurrent dedifferentiated liposarcoma), and HSSYII (a synovial sarcoma) using infectivity and cytotoxicity assays. Following the overexpression of RRM2 by cDNA transfection and silencing of RRM2 by siRRM2 in these STS cell lines, the RRM2 expression levels were analyzed by Western blot. Results: We observed a direct correlation between viral oncolysis and RRM2 mRNA levels ( R = 0.96) in STS. Higher RRM2 expression was associated with a more robust cell kill. Silencing the RRM2 gene led to significantly greater cell survival (80%) compared with the control group ( P = .003), whereas overexpression of the RRM2 increased viral oncolysis by 33% ( P < .001). Conclusions: Our results show that the oncolytic effects of MYXV correlate directly with RR expression levels and are enhanced in STS cell lines with naturally occurring or artificially induced high expression levels of RR. Myxoma virus holds promise in the treatment of advanced soft tissue cancer, especially in tumors overexpressing RR.

THER-07. INHIBITION OF THE RAS SIGNALING ENHANCES VIRAL ONCOLYSIS IN MALIGNANT GLIOMAS

Pediatric malignant glioma indicates rapid proliferation, widely infiltrative properties and resistance to various therapies, and carries a very poor prognosis. There are methods of using virus within novel therapies under development against malignant neoplasms, which have been studied for many years already. We examined the treatment with sunitinib or GW5074 to our experimental model of vaccinia virus therapy for malignant glioma, and then evaluated changes in the tumoricidal activity, the viral infectivity, and the impact on the Ras signaling pathway. Glioma cells (U251MG, LN229, LN18, rat C6) infected with vaccinia virus was fatal, in its course of death, apoptosis and autophagy were induced. The activity of Ras signaling in vaccinia-infected cells heightened in the early stage and declined in the late stage Inhibition of the Ras signaling pathway at the early stage of viral infection prevented vaccinia virus replication, while viral oncolysis was not inhibited when the pathway was blocked after sufficient viral spread. Glioma cells infected with vaccinia virus are led to cell death. Vaccinia virus regulates Ras or other survival signaling pathways in the infected cells. It enhances the signaling in the early stage (viral replicative period), however suppresses in the later stage (virus-releasing stage). Inhibition of the Ras signaling pathway at the early stage of viral infection prevents vaccinia virus from replicating, while viral oncolysis appears to be accelerated when the pathway was blocked after sufficient viral reproduction.

Viral Oncolytic Therapy

Viral oncolysis broadly refers to the use of modified viruses to infect and subsequently lyse tumor cells. This concept arises from the observation that viral replication is itself effective in destroying tumor cells. This effect is then amplified by reinfection of adjacent tumor cells by the progeny virion released from lysed tumor cells. Herpes simplex virus 1 (HSV-1) has been the primary focus of current efforts in viral oncolysis. It is a double-stranded DNA virus that is a ubiquitous pathogen transmitted by direct mucosal contact. HSV-1 possesses several features well suited to viral oncolytic therapy. It does not integrate into the cellular genome, has a large transgene capacity of up to 50 kb, and is already highly prevalent in the general population. In addition, effective antiherpetic agents are available to stop unwanted viral replication. HSV-1 mutants that preferentially replicate in neoplastic cells rather than normal cells have been characterized, and several variants of replication deficient HSV-1 mutants have been created and studied. They follow a common theme in that their replication is significantly attenuated in normal cells, while activated in cancer cells. Studies have been performed in various strains including those known as G207, NV1020, talimogene laherparepvec, and rRp450, and are reviewed here. Viral oncolysis is an exciting area of research with applications to tumors throughout the body. It holds promise as a new treatment for primary and metastatic liver cancer and may soon become a relevant therapy in interventional oncology.