The ability of liposomes containing a lipophilic muramyl dipeptide, N-acetylmuramyl-L-alanyl-D-isoglu- tamine-glycerol dipalmitate, to induce Kupffer cell tumoricidal activity has been investigated. Liposomal Nacetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate was 16-fold more potent than liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine and 2,400-fold more potent than N-acetylmuramyl-L-alanyl-D-isoglu- tamine in inducing Kupffer cell tumoricidal activity in uitro. A single i.v. injection of liposomes containing Nacetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate was capable of inducing Kupffer cell tumoricidal activity as measured against B 16-melanoma cells after Kupffer cell isolation. Maximal cytotoxic activity was obtained with 1 pg muramyl dipeptide-glycerol dipalmitate encapsulated within liposomes: doses of 10 or 100 pg inhibited tumoricidal activity. Kupffer cells from mice treated with liposomes containing N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate remained cytotoxic for at least 6 days after injection. Liposomal N-acetylmuramyl-L-alanyl-D-isoghtamine was significantly less potent than liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate in inducing Kupffer cell tumoricidal activity in situ. N- Acetylmuramyl-L-alanyl-D-isoglutamine was capable of inducing Kupffer cell tumoricidal activity in uitro: its failure to induce tumoricidal activity in situ at doses of 1,000 pg demonstrates the utility of liposomal carriers for the in viuo activation of Kupffer cells by muramyl dipeptides.
The fixed macrophage of the liver, the Kupffer cell, is an attractive target for the nonspecific immunotherapy of primary and metastatic liver cancer. It is present in large numbers throughout the liver and is in close proximity to growing tumors. The continual influx of monocytes (1) and the local proliferation of Kupffer cells in response to noxious stimuli (2,3) ensure sufficient numbers for effector activity. The exposure of macrophages to a synthetic immunomodulator, N-acetylmuramyl-Lalanyl-D-isoglutamine (MDP) (4), results in the induc-