Synergistic Effects of Oncolytic Reovirus and Cisplatin Chemotherapy in Murine Malignant Melanoma

Purpose: To test combination treatment schedules of reovirus and cisplatin chemotherapy in human and murine melanoma cell lines and murine models of melanoma and to investigate the possible mechanisms of synergistic antitumor effects. Experimental Design: The effects of reovirus +/- chemotherapy on in vitro cytotoxicity and viral replication were assessed using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay and plaque assay. Interactions between agents were assessed by combination index analysis. Mode of cell death was assessed by Annexin V/propidium iodide fluorescence-activated cell sorting-based assays; gene expression profiling of single versus combination treatments was completed using the Agilent microarray system. Single agent and combination therapy effects were tested in vivo in two immunocompetent models of murine melanoma. Results: Variable degrees of synergistic cytotoxicity between live reovirus and several chemotherapy agents were observed in B16.F10 mouse melanoma cells, most significantly with cisplatin (combination index of 0.42 +/- 0.03 at ED(50)). Combination of cisplatin and reovirus exposure led to increased late apoptotic/necrotic cell populations. Cisplatin almost completely abrogated the inflammatory cytokine gene up-regulation induced by reovirus. Combination therapy led to significantly delayed tumor growth and improved survival in vivo (P < 0.0001 and P = 0.0003, respectively). Cisplatin had no effect on the humoral response to reovirus in mice. However, cisplatin treatment suppressed the cytokine and chemokine response to reovirus in vitro and in vivo. Conclusion: The combination of reovirus and several chemotherapeutic agents synergistically enhanced cytotoxicity in human and murine melanoma cell lines in vitro and murine tumors in vivo. The data support the current reovirus/chemotherapy combination phase I clinical studies currently ongoing in the clinic. (Clin Cancer Res 2009; 15(19):6158-66)