Synergism between lymphokines and muramyl dipeptide encapsulated in liposomes: In situ activation of macrophages and therapy of spontaneous cancer metastases

Murine alveolar macrophages (AM) can be rendered tumoricidal after the i.v. injection of multilamellar vesicles (MLV) containing muramyl dipeptide (MDP) at a minimum dose of approximately 0.6 μg or less than a 10-fold dilution of cellfree culture supernatants from mitogen-stimulated F-344 rat lymphocytes rich in macrophage-activating factor (MAF) activity. Neither MAF at dilutions exceeding 1/10 nor MDP at doses lower than approximately 0.6 μg activated AM in situ to become tumor cytotoxic. The combination of these agents at subthreshold amounts (MAF 1/20, MDP 0.3 μg), however, activated AM to significant levels of cytotoxicity. The synergism between MAF and MDP encapsulated in liposomes was also observed in therapeutic studies of mice bearing well established, spontaneous lung and lymph node metastases. Multiple i.v. injections of liposomes containing either 6.25 μg MDP or a 1/2 dilution of MAF resulted in the longterm survival (>250 days) of 30% of the treated mice. Neither liposomes containing MAF at a 1/20 dilution nor MDP at doses lower than 0.6 μg were therapeutically effective. Multiple injections of liposomes containing subthreshold amounts (MAF 1/20; and MDP 0.3 μg) of these agents, however, resulted in 50% survival in the treated group. These data allow us to conclude that the encapsulation of MAF and MDP within the same MLV is more efficient in inducing the destruction of metastases than either agent administered alone.