Activation of alveolar macrophage tumoricidal activity and eradication of experimental metastases by freeze-dried liposomes containing a new lipophilic muramyl dipeptide derivative.

The ability of a member of a new class of lipophilic muramyl dipeptide (MDP) derivative, muramyl dipeptide-glyceryldipalmitate (MDP-GDP), to induce alveolar macrophage cytotoxic activity in vitro towards B16 melanoma cells when incorporated into two types of liposome was studied. MDP-GDP incorporated into conventionally prepared liposomes formulated from distearoylphosphatidylcholine and phosphatidylserine (7:3 molar ratio) was 10-fold more effective than liposomes containing MDP, and 7000-fold more effective than free MDP in inducing macrophage cytotoxic activity. MDP-GDP incorporated into freeze-dried liposomes was 50,000- to 100,000-fold more effective than free MDP in inducing such activity. Freeze-dried liposomes containing MDP-GDP were efficiently localized in the lungs of normal mice, and induced cytotoxic activity in the alveolar macrophages. Such liposomes were able to significantly reduce the pulmonary metastatic burden of mice carrying the B16 melanoma. These data provide evidence that this class of lipophilic MDP derivative, when incorporated into freeze-dried liposomes, is a potent inducer of macrophage cytotoxic activity in vitro and in situ, and has antitumor activity in vivo. In addition, the use of a freeze-drying procedure allows the preparation and long-term storage of reproducible liposome formulations.