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Background: Peritoneal carcinomatosis from pancreatic cancer has a poor prognosis with a median survival of 3.1 months. This is mainly due to lack of effective treatment. Interleukin 1 2 (IL12) is a proinflammatory cytokine that has a potent antitumoral effect by stimulating innate and adoptive immunity. Aim: To examine the antitumoral effect and toxicity of intraperitoneal delivery of IL12 using an ex vivo gene therapy approach in a murine model of pancreatic peritoneal carcinomatosis. Methods: Peritoneal carcinomatosis was generated by direct intraperitoneal inoculation of the pancreatic cancer cell line Capon-1 in athymic mice. Syngenic fibroblasts were genetically modified in vitro to secrete IL12 using a polycistronic TFG murine IL12 retroviral vector coding for both p35 and p40 murine IL12 subunits. Ex vivo gene therapy involved injection of the genetically modified fibroblasts intraperitoneally twice a week for 4 weeks. Results: Treatment of pre-established peritoneal carcinomatosis with fibroblasts genetically modified to express IL12 induced a marked inhibition of tumour growth as measured by comparison of the weights of the intraperitoneal tumour nodules in the treated and control animals (3.52 (SD 0.47) v 0.93 (SD 0.21) g, p<0.05) and improved survival. This effect was associated with infiltration of the peritoneal tumour nodules with macrophages. Peritoneal lavage confirmed enhancement of the innate peritoneal inflammatory activity, with an increased number of activated macrophages and natural killer cells. Moreover, macrophages harvested from animals with peritoneal carcinomatosis and treated with IL12-expressing fibroblasts expressed an activated proinflammatory antitumoral Ml phenotype that included strongly enhanced reactive oxygen species and nitric oxide production. There was no treatment-related toxicity. Conclusion: Multiple injections of genetically modified fibroblasts to express IL12 is an effective and well- tolerated treatment for experimental murine pancreatic peritoneal corcinomatosis via activated innate immunity and in particular activated Ml macrophages. [ABSTRACT FROM AUTHOR]