Iloprost-Containing Liposomes for Aerosol Application in Pulmonary Arterial Hypertension: Formulation Aspects and Stability

Abstract

Pulmonary arterial hypertension (PAH) is a severe and progressive disease. The prostacyclin analogue iloprost is effective against PAH, but requires six to nine inhalations per day. The feasibility of liposomes to provide a sustained release formulation to reduce inhalation frequency is evaluated from a technological point of view. Liposomal formulations consisting of di-palmitoyl-phosphatidyl-choline (DPPC), cholesterol (CH) and polyethyleneglycol-di-palmitoyl-phosphatidyl-ethanolamine (DPPE-PEG) were prepared. Their physico-chemical properties were investigated using dynamic light scattering, atomic force microscopy and differential scanning calorimetry. Stability of liposomes during aerosolization using three different nebulizers (air-jet, ultrasonic and vibrating mesh) was investigated with respect to drug loading and liposome size, pre- and post-nebulization. The phospholipid composition affected the diameters of liposomes only slightly in the range of 200-400 nm. The highest iloprost loading (12 microg/ml) and sufficient liposome stability (70% drug encapsulation post-nebulization) was observed for the DPPC/CH (70:30 molar ratio) liposomes. The formulation's stability was confirmed by the relatively high phase transition temperature (53 degrees C) and unchanged particle sizes. The incorporation of DPPE-PEG in the liposomes (DPPC/CH/DPPE-PEG, 50:45:5 molar ratio) resulted in decreased stability (20-50% drug encapsulation post-nebulization) and a phase transition temperature of 35 degrees C. The vibrating mesh nebulizer offered a number of significant advantages over the other nebulizers, including the production of small aerosol droplets, high output, and the lowest deleterious physical influence upon all investigated liposomes. Iloprost-loaded liposomes containing DPPC and CH components yield formulations which are well suited to aerosolization by the vibrating mesh nebulizer. The investigation of sustained release effects for the treatment of PAH in ex vivo and in vivo models is under way.