Preparation and In Vitro/In Vivo Evaluation of Luteinizing Hormone Releasing Hormone (LHRH)-Loaded Polyhedral and Spherical/Tubular Niosomes

Abstract

Niosomes are vesicles formed by the self‐assembly of nonionic surfactants in aqueous dispersions. They can entrap drugs and have been used experimentally as sustained drug delivery systems. Apart from conventional spherical niosomes, various types of vesicle ultrastructures can be formed by varying the composition of the vesicle membrane. Hexadecyl diglycerol ether (C16G2), cholesterol, and poly‐24‐oxyethylene cholesteryl ether (Solulan C24) in the ratio 91:0:9 gave polyhedral niosomes, whereas spherical and tubular niosomes are produced at a composition ratio of 49:49:2. The mean size of both polyhedral and spherical/tubular niosomes were within the range of 6 to 9 µm. Both types of vesicle were visualized by cryo‐scanning electron microscopy. The properties of the two forms of niosomes were studied using luteinizing hormone releasing hormone (LHRH) as a model peptide. Analysis by high‐performance liquid chromatography demonstrated high entrapment of LHRH acetate in polyhedral niosomes when prepared by remote loading methods using pH or (NH4)2SO4 gradients; in contrast, only low entrapment was achieved by passive loading methods (direct hydration at pH 7.4 or pH 3.0, dehydration–rehydration, and reversed‐phase evaporation). In vitro studies demonstrated that both polyhedral and spherical/tubular niosomes were more stable in 5% rat skeletal muscle homogenate than in rat plasma. Also, polyhedral niosomes released more radiolabeled LHRH ([125|]LHRH) than spherical/tubular niosomes in both muscle homogenate and plasma. In clearance experiments in the rat, following intramuscular injection, both polyhedral and spherical/ tubular niosomes gradually released [125|]LHRH into the blood, but some radioactivity remained at the injection site for 25 and 49 h, respectively. In contrast, [125|]LHRH in phosphate buffered saline was completely cleared from the injection site at 2h. The release of drug is sustained by both niosome formulations, but spherical/tubular niosomes possess more stable membranes than polyhedral niosomes due to the presence of cholesterol.