Activation of macrophage in vitro to acquire antitumor activity by a muramyl dipeptide derivative encapsulated in microspheres composed of lactide copolymer

Abstract

An immunopotentiator, lipophilic muramyi dipeptide derivative (MDP-B30), was encapsulated in microspheres composed of biodegradable polymers from l- lactic acid and glycolic acid copolymers. Tumor growth inhibitory activity of mouse peritoneal macrophages (PM) was investigated by the use of these microspheres. PM were activated by in vitro phagocytosis of the microspheres to suppress the growth of syngeneic, allogeneic, and xenogeneic tumor cells, but left normal (nontumorigenic) cells unharmed. When encapsulated in microspheres, MDP-B30 was far more efficient than free MDP-B30 in enhancing the tumor growth inhibitory activity of PM. The necessary minimum dose of MDP-B30 in the microspheres per unit volume of culture medium for PM activation was approximately one thousand times lower than that of free MDP-B30. A significant degree of activation was observed after incubation of PM with the microsphere-encapsulated MDP-B30 only for 3 h. The activated state of PM which phagocytosed microspheres maintained for a longer period than that of free MDP-B30. The growth inhibition of PM activated by the microspheres was found to increase with an enhancement in the phagocytosis which could be regulated by changing the microsphere size. Moreover, the maintenance of the in vitro activity could be readily controlled by changing the molecular weight, and/or monomer composition of polylactides and was in good accord with the release profiles of MDP-B30 from the microspheres. The growth inhibitory activity of PM was also observed when the microspheres were phagocytosed in situ. Mice bearing Meth-A tumor cells could survive when injected with the microsphereencapsulated MDP-B30, whereas injection with the free, unencapsulated MDP-B30 had little effect on the survival.