Physicochemical and biopharmaceutical characterization of dipalmitoyl phosphatidylcholine liposomes sterically stabilized by copolymers bearing short blocks of lipid-mimetic units

Abstract

Amphiphilic block copolymers were used to provide steric stabilization of dipalmitoyl phosphatidylcholine (DPPC) liposomes. The copolymers are based on poly(ethylene glycol) (PEG), but unlike the commercial PEG-lipids, bear short blocks of lipid-mimetic units. The liposomes were prepared by the lipid film hydration method and extrusion which yielded liposomes in the range of 140–170 nm in diameter as revealed by dynamic light scattering. The cryogenic transmission electron microscopy study documented unilamellar, spherical, well-separated and predominantly intact liposomes, even at copolymer contents as high as 10 mol%. The leakage of 5(6)-carboxyfluorescein was typically reduced compared to the plain liposomes. Some differences in the leakage profiles and membrane permeability were discussed in terms of shapes of the macromolecules and phase propensity of the copolymers studied. The pharmacokinetic parameters showed that the liposomes modified by a copolymer bearing four lipid-mimetic anchors exhibit superior longevity in vivo and two times lower accumulation in the liver compared to the plain liposomes and liposomes stabilized by commercial PEG-lipid. The cellular interactions were evaluated by means of fluorescence microscopy. Both the plain and the sterically stabilized liposomes were found to adhere to the membrane of MGH-U1 cells thus showing potential of serving as containers for sustained release of cytostatics. By using an ammonium transmembrane gradient method, mitoxantrone was loaded into liposomes with efficiency greater than 96%. The presence of a stabilizing copolymer did not compromise the drug loading efficacy. The liposomal mitoxantrone showed pronounced cytotoxicity towards an HL-60/Dox, which is of clinical importance as far as by-passing of multi-drug resistance is concerned.