Synthesis and characterization of PEGylated bolaamphiphiles with enhanced retention in liposomes

Long-circulating liposomes are typically prepared with poly(ethylene glycol)- (PEG-) modified lipids, where the lipid portion is inserted in the lipid bilayers as an anchor and the hydrophilic PEG coats the surface to prevent liposome aggregation and rapid clearance in vivo. However, these steric protection effects are compromised upon systemic administration due to low retention of PEGylated lipids within liposome membranes upon dilution. Hence, a series of PEGylated bolaamphiphiles (PEG-bolas) were for the first time developed to increase retention in the lipid bilayer, presumably leading to enhanced integrity of the PEG protective layer upon dilution. We hypothesized that PEG-bolas with a sufficiently long hydrophobic domain and rigid central group could predominantly adopt a membrane-spanning configuration, taking full advantage of steric protection offered by PEG and enhanced retention in liposomes enabled by the bola geometry. In this paper, liposomes stabilized by PEG-bolas comprised of a biphenyl core and twelve-carbon alkyl chain not only exhibited similar storage and biological stability compared to conventional PEGylated lipid stabilized liposomes, but also significantly improved retention upon dilution. Our findings facilitate new designs of liposome-stabilizing agents and can be applied to improve the delivery efficiency of liposomal delivery vehicles in vivo.