Effect of PEO−PPO−PEO Triblock Copolymers on Structure and Stability of Phosphatidylcholine Liposomes

Abstract

The interactions of five poly(oxyethylene)−poly(oxypropylene)−poly(oxyethylene) (PEO−PPO−PEO), Pluronic, copolymers and phosphatidylcholine liposomes of varying composition have been studied. Structural studies were performed by means of cryo-transmission electron microscopy (c-TEM) and reveal that inclusion of low amounts (∼2 mol %) of Pluronics gives rise to significant morphological changes of the liposome preparations. Pluronics with large poly(oxyethylene) (PEO) blocks, such as F127, F108, and F87, induce the formation of bilayer disks, whereas those with comparably short PEO blocks, P105 and P85, tend to to promote a reduction in the liposome size. Inclusion of cholesterol in the liposomal preparations reduces the incorporation of copolymers in the lipid bilayer and thus reduces, and in some cases even abolishes, the morphological changes observed in the absence of cholesterol. The effect of the copolymers on liposome permeability was also investigated. All investigated copolymers were found to increase the leakage of carboxyfluorescein from preformed liposomes. This was true also in the case of cholesterol-containing liposomes despite the fact that no change in the liposome structure could be observed by means of c-TEM. The magnitude of the induced leakage was found to correlate well with the hydrophobicity, as measured by the cmc, of the respective Pluronic. By raising the temperature or decreasing the solvency of the copolymer in the solution, the effect of the copolymer on liposome leakage was found to increase significantly.

Effect of chain lengths of PEO–PPO–PEO on small unilamellar liposome morphology and stability: an AFM investigation

“…Cryo-TEM visualizes the aggregation morphology in dilute aqueous solutions. Table 3 compared morphological changes of Pluronic ® -modified EggPC systems observed by AFM in this study and by c-TEM reported by Johnsson et al [11], the results are quite different. For example, formation of bilayer disks and liposomes were observed for F87, F108, and F127, and compacted liposomes were observed for P85 and P105 in solution by cryo-TEM.…”

“…Morphological changes of liposomes after copolymer incorporation were also observed by Cryo-TEM [11]. Cryo-TEM visualizes the aggregation morphology in dilute aqueous solutions.…”

“…These are characterized by good stabilization ability, low cost, and biocompatibility [6][7][8][9][10][11][12][13][14][15][16]. PEO-PPO-PEO consists of two hydrophilic PEO chains and a hydrophobic PPO chain, and is commercially available in a number of different segment lengths.…”

“…However, significant morphological changes were observed by cryo-TEM for this system. It was found that Pluronic ® with a large PEO chain length induced the formation of small bilayer fragments, or bilayer disks at low polymer-to-lipid molar ratios [11]. Thus, it is doubtful whether PEO-PPO-PEO triblock copolymers can be incorporated into the liposome without causing serious structural changes.…”

“…In their subsequent NMR study, they concluded that the PPO was buried inside the vesicle bilayer thus following the membrane-spanning model [7][8][9]. However, Johnsson et al suggested that the preferred position of the polymers is still not fully elucidated, and they believe it is possible that both models can occur [11].…”