Enhanced antitumor efficacy of adriamycin in combination with interferon-gamma: effects on macrophages

The combination of the immunomodulator interferon-gamma (IFN-gamma) with the chemotherapeutic drug adriamycin (ADM) was assessed in vitro and in vivo in murine tumor models. When tested in vivo against the murine Lewis lung carcinoma, significantly greater reduction of spontaneous pulmonary metastases was obtained by combination treatment with IFN-gamma, followed 1 day later by ADM. Intraperitoneal ADM treatment also resulted in an increased recruitment of peritoneal mononuclear cells. It is noteworthy that, although the antitumor efficacy was significantly increased by the IFN-gamma/ADM combination treatment, gross toxicity of ADM was not increased. Thus, a net increase in the therapeutic index of ADM was achieved. In vitro, the effects of ADM on the ability of murine peritoneal macrophages, with or without the addition of immunological macrophage activators, to kill tumor cells was studied. Resident macrophages were able to sequester ADM (when present at 10 micrograms/ml) from the medium, and could subsequently mediate killing of target tumor cells. However, incubation of macrophages with low (ineffective by themselves) doses of ADM (1 microgram/ml) prevented their simultaneous or subsequent activation to the tumoricidal state after incubation with the normal macrophage-activating mixture of IFN-gamma plus a muramyl dipeptide (MDP) analog. When the order of addition of reagents was reversed such that the macrophages were preincubated for 24 hr with IFN-gamma (100 U/ml) plus the MDP analog (0.1-10 micrograms/ml), no antagonism of tumoricidal activity was obtained upon subsequent incubation with ADM. There were no interactions between IFN-gamma and ADM on the direct proliferation of tumor cells. Taken together, these results suggest that the enhanced antitumor efficacy of IFN-gamma/ADM combinations in vivo was not due to direct antiproliferative effects on the tumor cells, but rather may be mediated by direct cytotoxicity of ADM on tumor cells enhanced by phagocytic mononuclear cells.