Physicochemical and morphological properties of complexes made of cationic liposomes and oligonucleotides

Abstract

Cationic liposomes are a potential delivery system for antisense oligonucleotides, but physical chemistry of this system is still poorly understood. We studied physicochemical properties (size distribution, lipid mixing reactions) and the morphology of oligonucleotide–cationic lipid complexes in buffer and in cell culture medium using quasi-elastic light scattering (QELS), resonance energy transfer (RET) and freeze-fracture electron microscopy. In addition, interaction of the complexes with model membranes was studied at extracellular (pH 7.4), intracellular (pH 7.1) and endosomal (pH 5 and 6) conditions. Phosphorothioate oligonucleotides (ODN, 15 mer) were complexed with cationic liposomes composed of DOTAP, or DOTAP with DOPE at 1/1 and 1/2 molar ratios and investigated at different charge ratios (−/+, ODN–cationic lipid). The size of the complexes formed in water increased prominently in buffer and DMEM and it was maximal at charge ratio (−/+) of 1.2–1.5. In the case of sole DOTAP liposomes the complex sizes were smaller at −/+ ratios <0.5 and >1.7 in all media. In the presence of DMEM the DOPE-containing complexes gave average diameter in μm range irrespect of the charge ratio. Lipid mixing, in general, increased with increasing −/+ ratio and with DOPE in the presence of buffer or DMEM. Freeze-fracture electron micrographs showed cationic liposomes undergoing aggregation and fusion during interaction with ODNs. DOPE in cationic liposomes induced hexagonal lipid tubule formation that was most pronounced in cell culture medium. Upon incubation with endosomal model liposomes ODN was partly released from the complexes. The release was more pronounced when the liposomes contained DOPE. It appears that DOPE as a helper lipid affects the behaviour of ODN/lipid complexes at several stages.