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Enhanced In vitro and In vivo Cytotoxicity of Combined Reovirus and Radiotherapy

Authors' Affiliations: ¹ The Institute of Cancer Research, Chester Beatty Laboratories, London, United Kingdom; ² Drug Development Unit, Royal Marsden Hospital, Sutton, United Kingdom; ³ Oncolytics Biotech, Inc., Calgary, Alberta, Canada; ⁴ Molecular Medicine Program, Mayo Clinic, Rochester, Minnesota; ⁵ University of Surrey, Guildford, United Kingdom; and ⁶ St. James's University Hospital, Leeds, United Kingdom

Requests for reprints: Kevin J. Harrington, The Institute of Cancer Research, Targeted Therapy Laboratory, Cancer Research UK Centre for Cell and Molecular Biology, Chester Beatty Laboratories, 237 Fulham Road, London SW3 6JB, United Kingdom. Phone: 44-20-7153-5157; Fax: 44-20-7808-2235; E-mail: kevinh{at}icr.ac.uk.

Purpose: To test combination treatment schedules of reovirus and radiation in human and murine tumor cells in vitro and in vivo.

Experimental Design: In vitro cytotoxicity and cell cycle effects of reovirus given alone and combined with radiotherapy were assessed by colorimetric, tissue culture infectious dose 50, and fluorescence-activated cell sorting–based assays. Interactions between the agents were evaluated using combination index analysis. The effect of different schedules of reovirus and radiotherapy on viral replication and cytotoxicity was tested in vitro and the combination was assessed in three tumor models in vivo.

Results: Characterization of reovirus cytotoxicity in a panel of cell lines yielded a range of sensitivities. Combined reovirus and radiotherapy yielded statistically significantly increased cytotoxicity, particularly in cell lines with moderate susceptibility to reovirus alone. The enhanced cytotoxicity of the combination occurred independently of treatment sequence or schedule. Radiation did not affect viral replication and only reduced reoviral cytotoxicity after clinically irrelevant single doses (>50 Gy). Combination index analysis revealed synergy between radiation (3-10 Gy) and reovirus at multiplicities of infection between 0.001 and 1. Combination treatment significantly increased apoptosis in tumor cells relative to either single-agent treatment. In vivo studies using xenograft and syngeneic tumors showed enhanced activity of the combination relative to reovirus or radiation alone (P < 0.001).

Conclusions: Combining reovirus and radiotherapy synergistically enhances cytotoxicity in a variety of tumor cells in vitro and in vivo. These results offer strong support for translational clinical trials of reovirus plus radiotherapy that have been initiated in the clinic.

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