Lyophilization and rehydration of iopromide-carrying liposomes

Abstract

Iopromide-carrying liposomes prepared by the ethanol-evaporation method which encapsulated approximately 35% X-ray contrast agent at an average vesicle size of 300 nm were stabilized by lyophilization. Freezing behaviour of the suspension was studied by differential scanning calorimetry (DSC) and resistance/temperature measurement with respect to the maximum allowable temperature during primary drying. While melting of the suspension was observed at −21°C by DSC, conductivity changes down to −40°C could be detected by simultaneous resistance/temperature measurement. Furthermore, the influence of two different shelf-temperatures (−15°C and + 5°C) and three different chamber pressures (0.03 mbar, 0.08 mbar and 0.2 mbar) during primary drying on the quality of the lyophilized and rehydrated liposomes were investigated. A chamber pressure of 0.08 mbar and a shelf-temperature of −15°C during primary drying led to optimal product quality indicated by high contrast agent encapsulation. A chamber pressure of 0.08 mbar also proved to be optimal with respect to an economic lyophilization process. The residual water content of the freeze dried material proved to be very low (about 0.1%) independent of process parameters. Rehydration of the lyophilized liposomes with mannitol solution resulted in higher encapsulation efficiency of the rehydrated vesicles compared to rehydration with water or 20 mM tromethamine buffer (pH 7.5). The encapsulation efficiency could be further improved by increasing the concentration of the mannitol solution. Rehydration with iso-osmotic solutions of sucrose and KCl proved not to be as effective as mannitol solution in increasing the encapsulation efficiency of the rehydrated vesicles. Freeze-fracture images of lyophilized drug-free liposomes in contrast to iopromide-carrying liposomes revealed the lyoprotective effect of iopromide.