Electrostatics of PEGylated Micelles and Liposomes Containing Charged and Neutral Lipopolymers

Abstract

The electrostatics of large unilamellar vesicles (LUVs) of various lipid compositions were determined and correlated with steric stabilization. The compositional variables studied include (a) degree of saturation, comparing the unsaturated egg phosphatidylcholine (EPC) and the fully hydrogenated soy phosphatidylcholine (HSPC) as liposome-forming lipids; (b) the effect of 40 mol % cholesterol; (c) the effect of mole % of three methyl poly(ethylene glycol) (mPEG)-lipids (the negatively charged mPEG-distearoyl phosphoethanolamine (DSPE) and two uncharged lipopolymers, mPEG-distearoyl glycerol (DSG) and mPEG-oxycarbonyl-3-amino-1,2-propanediol distearoyl ester (DS)); and (d) the negatively charged phosphatidyl glycerol (PG). The lipid phases were as follows: liquid disordered (LD) for the EPC-containing LUV, solid ordered (SO) for the HSPC-containing LUV, and liquid ordered (LO) for either of those LUV with the addition of 40 mol % cholesterol. The LUV zeta potential and electrical surface potential (psi(0)) were determined. It was found that progressive addition of mPEG(2k)-DSPE to liposomes increases negative surface potential and reduces surface pH to a similar extent as the addition of PG. However, due to the "hidden charge effect", zeta potential was more negative for liposomes containing PG than for those containing mPEG(2k)-DSPE. Replacing mPEG-DSPE with mPEG(2k)-DS or mPEG-DSG had no effect on surface pH and surface potential, and zeta potential was approximately zero. Addition of low concentrations of cationic peptides (protamine sulfate and melittin) to PG- or mPEG-DSPE-containing liposomes neutralized the liposome negative surface potential to a similar extent. However, only in liposomes containing PG, did liposome aggregation occur. Replacing the negatively charged lipopolymer mPEG-DSPE with the neutral lipopolymers mPEG-DS or mPEG-DSG eliminates or reduces such interactions. The relevance of this effect to the liposome performance in vivo is discussed.

Intratracheal Versus Intravenous Liposomal Delivery of siRNA, Antisense Oligonucleotides and Anticancer Drug

“…The finding that the zeta potential of lipid assemblies containing PEG-DSPE is negative is in agreement with our previously reported data (31). It also supports the "hidden charge effect" that was suggested for liposomes sterically stabilized through grafting of mPEG-DSPE with PEG moiety ≥0.75 kDa (34,35). Similar to their conventional counterpart, these liposomes can be considered "neutral."…”

Characterization of phosphatidylcholine/polyethylene glycol‐lipid aggregates and their use as coatings and carriers in capillary electrophoresis

“…Comparison of the effective electrophoretic mobilities (m ep = m tot 2 m eo ) of atenolol with the different coatings and carriers showed that there were no strong interactions of the positively charged analyte with the PEG-lipid membranes under these conditions (HEPES, pH 7.4). A recent study of the interaction between spermine (cationic, molar mass about 200 g/mol) and DSPEG2000-containing liposomes by fluorescence spectroscopy, showed low (or no) affinity of spermine to the liposome surface [30]. Our results agree with this.…”

“…The higher the molar mass of PEG (i.e., the longer the PEG chain), the lower was the surface charge of the vesicles. This can be explained in terms of shielding of the negatively charged vesicle surface by the exposed PEG chains, which decreased the zeta potential of the vesicle [29,30]. However, the zeta potential of the POPC/DMPEG3000 aggregates was about 213 mV, which shows that the shorter C14 chains of the DMPEG-lipid clearly affect the packing of the lipids in the liposome.…”

siRNA-Lipoplex-Mediated Bcl-2 and Bcl-xL Gene Silencing Induces Apoptosis in MCF-7 Human Breast Carcinoma Cells

“…An aliquot of 100 L of liposomes or lipoplexes was diluted in 600 L of 20 mM Hepes (pH 7.4). The principle of zeta potential measurement and equation for calculation are described elsewhere [37].…”