Design of sterile muramyldipeptide-containing oligolamellar liposomes suitable for i.v. injection. Effect of liposome size and lipid composition on their ability to render peritoneal macrophages antitumoral

Abstract

The water-soluble immunomodulator muramyldipeptide has been encapsulated in oligolamellar liposomes (mean diameter 200 nm) of various lipid compositions. These liposomes were sterilized by Millipore filtration without loss of entrapped solute and are therefore suitable for i.v. administration. The amount of encapsulated MDP increased when the volume of aqueous phase (containing a given MDP concentration) used for hydrating the phospholipid film increased and this did not vary for at least 15 days when the liposomes were kept at 4°C. The presence of cholesterol did not modify the encapsulation results but prevented the leakage of encapsulated water-soluble solute in serum-containing media. The internalization of various types of liposomes by murine peritoneal macrophages has been studied in order to define the best conditions for activating macrophages. For a given lipid composition, sterile oligolamellar vesicles (suitable for i.v. injection) were more efficient than multilamellar vesicles in rendering macrophages capable of inhibiting the growth of a syngeneic tumor, particularly when these vesicles contained muramyldipeptide in their aqueous space and cholesterol in their lipidic phase.