On the Molecular Mechanism of Steric Stabilization of Liposomes in Biological Fluids

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Abstract
Liposomes with specific surface modification overcome rapid in vivo uptake by cells of the mononuclear phagocytic system (MPS) resulting in prolonged circulation in the blood. The structure-function relationship of this effect has been examined by measurements both in vitro and in vivo. The results are reviewed and compared with those from liposomes without surface modification. For example, in the best cases with polyethylene glycol-derivatized phosphatidylethanolamine (PEG-PE) up to 35% of the injected dose remains in the blood and less than 10% is taken up by the two major organs of the MPS, liver and spleen, after 24 hr. This compares with less than 1% in the blood and up to 40% uptake for liposomes without PEG-PE. Steric stabilization has been proposed as a theoretical basis for these results, and some initial results testing this basis have been reported. Here, we discuss steric stabilization in terms of the physico-chemical properties of the liposomes.