Radiation in combination with Reolysin for pediatric sarcomas

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With advances in the scientific knowledge of molecular mechanisms of cancer and viral replication, oncolytic viruses have emerged as an interesting potential therapeutic strategy for several human cancers. Several genetically engineered and attenuated viruses have been constructed to achieve tumor specific replication and cell lysis. Reovirus, a double stranded RNA virus, has the specific ability to enter and replicate in cells with activated ras signaling pathways. A large majority of human cancer cells have either activating ras gene mutations or other mutations in the ras signaling pathway thus making reovirus a rational therapeutic target. In preclinical studies, reovirus has shown significant anti-tumor activity in several human cancers such as gliomas, childhood sarcomas, colon and pancreatic cancer. In addition, reovirus seems to increase radiation cytotoxicity in multiple human cancer cell lines and tumor xenografts. In the current study, we assessed the effects of the combination of reovirus and radiation therapy (RT) in a panel of pediatric sarcoma cell lines and tumor xenografts. Methods: In-vitro, cytotoxicity was determined by MTT assay for the different treatment groups (control, reovirus, RT, reovirus+RT) in rhabdomyosarcoma (RMS) and Ewing’s sarcoma (ES) cell lines. In-vivo, RH30 (RMS) and SKES (ES) cells were injected in the flanks of 5-6 week old female nude mice. Once tumor volumes reached 0.2-0.7 cm3, mice were divided into 4 groups: control, reovirus only, RT only and reovirus+RT. Reovirus was administered at the dose of 5x10⁹ pfu intravenously for 3 consecutive days. RT was concurrently administered at 4 fractions of 5 Gy each, for 4 consecutive days. In the combination group, reovirus was administered 2 hours before RT. Tumor volumes were serially measured for the next 6 weeks and response measured in terms of event free survival (EFS) and mean relative tumor volumes (RTV). Tumor tissue was also harvested from each group for apoptosis assay. Results: The combination of reovirus and RT caused significant tumor regression in both RH30 and SKES xenografts. (P value for mean RTV at day 22 was 0.007 in both RH30 and SKES xenografts). The EFS was also significantly increased in the combination group as compared to the other 3 groups in both xenografts (P value 0.001 for RH30 and 0.002 for SKES). On histological analysis, increased degree of apoptosis was seen in each of the treatment groups but was markedly increased in the combination group. Conclusions: Our data suggests that the combination of reovirus and radiation therapy has enhanced efficacy than either of the individual treatments alone in pediatric sarcomas. This is especially interesting since reovirus was administered systemically instead of intratumorally suggesting the potential use of this treatment modality even in metastatic and hard to reach tumors. Our data warrants further testing of this combination in clinical trials.