Interaction of polyethyleneglycol-phospholipid conjugates with cholesterol-phosphatidylcholine mixtures: sterically stabilized liposome formulations.

Purpose: The purpose of this study was to investigate polyethyleneglycol (PEG)-phosphatidylethanolamine (PE) conjugate interaction with cholesterol-phospholipid mixtures in an attempt to explain the effect of cholesterol on liposome circulation time.
Methods: Differential scanning calorimetry, NMR, electron microscopy, dynamic light scattering and fluorescence spectroscopy were the major methods used.
Results: Studies performed in the absence of cholesterol indicated the formation of three distinct physical states depending on the chain length of PEG in PEG-PE. Mixed micelle formation was observed at concentrations of PEG(1,000)-DPPE above 7 mol-% of lipid. Phase separated lamellae were observed at all concentrations of PEG( 12,000)-DPPE (Bedu-Addo et al. Pharm. Res. 13:710-717 (1996)). Upon incorporation of high concentrations of cholesterol >30 mol% into the lipid bilayer, the formation of phase separated lamellae was completely inhibited and the formation of mixed micelles significantly reduced. At high concentrations of PEG(1,000)-PE, solubilization of the bilayer occurred with preferential solubilization of cholesterol over phosphatidylcholine. Maximum steric stabilization (surface protection) was observed with low concentrations of short chain PEG-PE and high concentrations of cholesterol.
Conclusions: The study provides a physical mechanism for the following observations: the blood circulation time is significantly increased or decreased with liposomes highly enriched with cholesterol or PEG-PE respectively.