title: Influence of poly(ethylene glycol) grafting density and polymer length on liposomes: relating plasma circulation lifetimes to protein binding.
abstract: The incorporation of poly(ethylene glycol) (PEG)-conjugated lipids in lipid-based carriers substantially prolongs the circulation lifetime of liposomes. However, the mechanism(s) by which PEG-lipids achieve this have not been fully elucidated. It is believed that PEG-lipids mediate steric stabilization, ultimately reducing surface-surface interactions including the aggregation of liposomes and/or adsorption of plasma proteins. The purpose of the studies described here was to compare the effects of PEG-lipid incorporation in liposomes on protein binding, liposome-liposome aggregation and pharmacokinetics in mice. Cholesterol-free liposomes were chosen because of their increasing importance as liposomal delivery systems and their marked sensitivity to protein binding and aggregation. Specifically, liposomes containing various molecular weight PEG-lipids at a variety of molar proportions were analyzed for in vivo clearance, aggregation state (size exclusion chromatography, quasi-elastic light scattering, cryo-transmission and freeze fracture electron microscopy) as well as in vitro and in vivo protein binding. The results indicated that as little as 0.5 mol% of 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine (DSPE) modified with PEG having a mean molecular weight of 2000 (DSPE-PEG(2000)) substantially increased plasma circulation longevity of liposomes prepared of 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC). Optimal plasma circulation lifetimes could be achieved with 2 mol% DSPE-PEG(2000). At this proportion of DSPE-PEG(2000), the aggregation of DSPC-based liposomes was completely precluded. However, the total protein adsorption and the protein profile was not influenced by the level of DSPE-PEG(2000) in the membrane. These studies suggest that PEG-lipids reduce the in vivo clearance of cholesterol-free liposomal formulations primarily by inhibition of surface interactions, particularly liposome-liposome aggregation.
authors.affiliation: Department of Advanced Therapeutics, British Columbia Cancer Agency, 675 West 10th Avenue, Vancouver, BC, Canada.
authors.forename: Nancy
authors.initials: N
authors.lastname: Dos Santos
authors.forename: Christine
authors.initials: C
authors.lastname: Allen
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Doppen
authors.forename: Malathi
authors.initials: M
authors.lastname: Anantha
authors.forename: Kelly A K
authors.initials: KA
authors.lastname: Cox
authors.forename: Ryan C
authors.initials: RC
authors.lastname: Gallagher
authors.forename: Goran
authors.initials: G
authors.lastname: Karlsson
authors.forename: Katarina
authors.initials: K
authors.lastname: Edwards
authors.forename: Gail
authors.initials: G
authors.lastname: Kenner
authors.forename: Lacey
authors.initials: L
authors.lastname: Samuels
authors.forename: Murray S
authors.initials: MS
authors.lastname: Webb
authors.forename: Marcel B
authors.initials: MB
authors.lastname: Bally
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002850:Chromatography, Gel; D016503:Drug Delivery Systems / Q000379:methods*; D004591:Electrophoresis, Polyacrylamide Gel; D005614:Freeze Fracturing; D008055:Lipids / Q000493:pharmacokinetics; D008081:Liposomes / Q000097:blood* / Q000378:metabolism*; D051379:Mice; D008807:Mice, Inbred BALB C; D046529:Microscopy, Electron, Transmission; D010714:Phosphatidylethanolamines / Q000378:metabolism*; D011092:Polyethylene Glycols / Q000378:metabolism*; D011485:Protein Binding
chemical_list: D008055:Lipids; D008081:Liposomes; D010714:Phosphatidylethanolamines; C038089:1,2-distearoylphosphatidylethanolamine; D011092:Polyethylene Glycols