Factors influencing the stability of dehydrated liposomal systems

Plant seeds, yeasts, bacterial spores, rotifers, and other organisms are capable of suspending their metabolism and entering a state of latency when dehydrated. These organisms may maintain this state for extremely long periods of time, yet upon rehydration resume normal metabolism, without evidence of severe membrane disruption. With many of these organisms, the ability to survive dehydration has been correlated to the production of large amounts of carbohydrates, including glycerol, glycogen and the disaccharide trehalose. Trehalose has been shown to protect isolated sarcoplasmic reticulum microsomes and phospholipid vesicles from dehydration damage, implying that the site of protective action of trehalose and other carbohydrates is the lipid portion of membranes. In this thesis, the effects of carbohydrate composition, vesicle size, and lipid composition on the protection of liposomes from dehydration was investigated, as was the structure of the solid lipid-trehalose complex. Electron microscopy of dried liposomes indicated that vesicles protected with trehalose remain essentially intact even when dry, while vesicles not protected by sugar are severely disrupted by drying . ³¹-P and ¹³-C NMR results suggested that the lipid of protected vesicles is in a similar phase as that of unprotected vesicles, and that this state is similar to powdered anhydrous phospholipid. Using carboxyfluorescein as a probe, it was demonstrated that trehalose, other sugars can prevent vesicle disruption upon dehydration. Different lipid compositions of the liposomes showed nearly identical behavior, with the exception of vesicles composed of dipalmitoylphosphatidylcholine and egg phosphatidylcholine, which showed greater and lower stability to dehydration respectively. Light scattering experiments indicated that a wide variety of carbohydrate and lipid vesicle combinations can withstand dehydration and maintain their original size when protected by sugars. The implications of these results in the development of liposomes as pharmaceuticals are discussed, and a hypothesis is advanced regarding the role of carbohydrates in the preservation of dry lipid membranes.Medicine, Faculty ofBiochemistry and Molecular Biology, Department ofGraduat