Freshly isolated human peripheral blood monocytes from healthy volunteers are not cytotoxic to allogeneic A375 melanoma cells, but they were rendered tumoricidal by incubation in vitro with either liposomes containing 5 micrograms/mumol phospholipid of muramyl tripeptide phosphatidylethanolamine (liposome-MTP-PE; optimal dose, 500 nmol/ml) or recombinant human interferon gamma (rIFN-gamma; optimal dose, 100 U/ml). A combination of sub-threshold concentrations of liposome-MTP-PE (50 nmol/ml) and rIFN-gamma (1 or 10 U/ml) also induced significant tumor-cell killing, indicating that the effects of rIFN-gamma and liposome-MTP-PE in monocyte activation are synergistic. In contrast to rIFN-gamma, recombinant IFN-alpha and IFN-beta had additive effects with liposome-MTP-PE in human monocyte activation. Since recombinant human IFN-gamma has a synergistic effect with liposome-MTP-PE in monocyte activation, unlike IFN-alpha or IFN-beta, and liposome-MTP-PE as well as rIFN-gamma is available at standardized concentrations, this combination could be of clinical value in the treatment of disseminated malignant disease.

Studies were performed on the activation of human blood monocytes to the antitumor state by a dried preparation of multilamellar vesicle (MLV) liposomes in which synthetic muramyl tripeptide phosphatidylethanolamine (MTP-PE) was inserted directly into the liposome membrane. Dried liposomes composed of synthetic phospholipids [phosphatidylcholine (PC) and phosphatidylserine (PS) in a molar ratio of 7:3] were prepared by lyophilization. Dried liposome-MTP-PE was found to be superior in several ways to free desmethyl muramyl dipeptide (norMDP) or conventional liposome-MTP-PE, prepared immediately before use. First, dried liposome-MTP-PE was stable and strongly activated monocytes when stored for over 3 months in a freezer at -20 degrees C or even in suspension at 4 degrees C. Second, human monocytes in suspension, as well as in the adherent form, were activated to the tumoricidal state by interaction for at least 4 h with the dried preparation of liposome-MTP-PE. Third, monocytes activated with the dried liposome-MTP-PE or conventionally prepared liposome-MTP-PE maintained their tumoricidal activity for a longer period (4 days) than those activated with free norMDP. These results indicate that the dried preparation of liposome-MTP-PE can be stored for a long time, has a reproducible effect that can be standardized and should be valuable for in situ activation of human monocytes to the tumoricidal state, which is associated with eradication of cancer metastases.

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The effects of lung cancer on the abilities of blood monocytes to produce interleukin-1 and to mediate antitumor activity were examined. The functional integrity of blood monocytes was determined by their capacity to respond in vitro to a variety of activating agents and become tumoricidal, as assessed by a radioactive release assay and ability to produce interleukin-1 in vitro. The results show that the presence of lung cancer significantly increased the number of harvested blood monocytes and that the spontaneous tumoricidal activity of these monocytes was slightly high as compared to monocytes obtained from healthy donors. The production of interleukin-1 by monocytes of healthy donors and lung cancer patients was similar. Blood monocytes obtained from lung cancer patients were less cytotoxic against allogeneic A375 melanoma cells as compared with those of healthy donors subsequent to incubation with a soluble muramyl dipeptide analog or lipopolysaccharide, but were as tumoricidal as those from healthy donors when activated with lipophilic muramyl tripeptide (MTP-PE) entrapped in multilamellar liposomes. The finding that monocytes of patients with lung cancer can respond to MTP-PE encapsulated in liposomes, recommends the use of these liposomes in therapy of human lung cancer.

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