Inhibition of murine hepatic tumor growth by liposomes containing a lipophilic muramyl dipeptide

Abstract

We have investigated the ability of liposomes containing a lipophilic muramyl dipeptide, N-acetylmuramyl-l-alanyl-d-isoglutamine glycerol dipalmitate (MDP-GDP) to activate Kupffer cell tumoricidal activity in situ and to inhibit the growth of experimental hepatic micrometastases of tumor cell line H-59, a liver-homing variant of the Lewis lung carcinoma. Liposomes prepared from distearoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DSPC/DMPG) and containing MDP-GDP (1 μmol and 2 μg, respectively) were efficiently taken up by the liver after i.v. administration. A single i.v. injection of DSPC/DMPG liposomes containing MDP-GDP was capable of inducing Kupffer cell tumoricidal activity against H-59 tumor cells as measured in vitro. Control liposomes or 100 μg free MDP were ineffective in inducing Kupffer cell tumoricidal activity in situ. Two treatment regimens were evaluated in vivo: firstly, C57BL/6 mice were injected with tumor cell line H-59 and subsequently treated with multiple injections of liposomal MDP-GDP. Secondly, treatment with liposomal MDP-GDP was initiated prior to tumor cell injection and continued after tumor cell injection. The ability of liposomes containing MDP-GDP to reduce the number of hepatic micrometastases using the first protocol was related to the tumor cell inoculum, significant inhibition being observed at lower liver tumor burdens (<25 tumor nodules). Pretreatment of the mice prior to tumor cell challenge followed by treatment afterwards greatly enhanced the efficacy of liposomal MDP-GDP and brought about a highly significant inhibition of the growth of experimental metastases even at high liver tumor burdens (>50 nodules).