Intensified regression of colon cancer liver metastases in mice treated with irinotecan and the immunomodulator JBT 3002.

The authors recently reported that tumoricidal activation of macrophages by a new synthetic bacterial lipopeptide, JBT 3002, can augment chemotherapy-mediated tumor-cell killing. The aim of this study was to identify the mechanism responsible for the destruction of metastatic cells. Three daily oral doses of JBT 3002 before once-weekly intraperitoneal injections of 100 mg/kg irinotecan for 3 weeks significantly increased the eradication of established CT-26 murine colon cancer liver metastases compared with treatment with irinotecan alone. Immunohistochemical analyses revealed that the hepatic metastases in mice given combination therapy contained infiltrating CD8+ lymphocytes and a dense infiltrate of macrophages expressing both inducible nitric oxide synthase (iNOS) and interleukin-15. In vitro treatment of peritoneal macrophages with JBT 3002 plus interferon-gamma induced the expression of iNOS and the production of nitric oxide. In the presence of a low (subtoxic) dose of irinotecan, these activated macrophages produced significant lysis of CT-26 cells. The high level of cytotoxicity was inhibited by the specific inducible nitric oxide synthase inhibitor, NG-methyl-L-arginine. In contrast, irinotecan-mediated lysis of normal intestinal epithelial IEC-6 cells was not increased by activated macrophages. Scanning electron microscopy revealed that activated macrophages bound to CT-26 tumor cells but not to normal IEC-6 cells, confirming that nitric oxide-mediated cytotoxicity is specific for tumor cells. Collectively, the results suggest that augmentation of the therapeutic efficacy of irinotecan against colon cancer liver metastases by immunomodulation with JBT 3002 may be associated with elevated inducible nitric oxide synthase and endogenous interleukin-15 in tumor-infiltrating macrophages.