The Effect of Liposome Size on the Final Lipid/DNA Ratio of Cationic Lipoplexes

Abstract

Several studies have demonstrated that lipoplexes are two-phase systems over most mixing lipid/DNA charge ratios. Because these studies have focused on small unilamellar vesicles (SUV), they leave open the question as to whether a similar pattern is followed by other liposome types. The main purpose of this work is to examine the question further by characterizing the assembly of cationic lipoplexes prepared from 1-[2-(oleoyloxy)ethyl]-2-oleyl-3-(2-hydroxyethyl)imidazolinium chloride (DOTIM)/dioleoylphosphatidylethanolamine (DOPE) (1:1) liposomes of various types. Sedimentation in sucrose density gradients reveals that large unilamellar vesicles (LUV) and sedimented multilamellar vesicles (sMLV), as opposed to SUV, form lipoplexes that exist as a single phase over a relatively broad range of mixing (+/-) ratios. This is indicated by observing that most of the LUV and sMLV become involved in the assembly reaction up to mixing (+/-) ratios of 4 and 9, respectively, while only a small and constant fraction of SUV associates with DNA at all mixing (+/-) ratios tested. Consequently, while maximal (+/-) ratios of approximately 4.5 and 9 are found in LUV and sMLV lipoplexes, respectively, a final (+/-) ratio of only approximately 2 is determined in SUV lipoplexes. Isothermal titration calorimetry shows that this is the lowest possible charge ratio achieved when liposomes are titrated with DNA. Based on these observations and on the size differences of the liposomes used, a model of lipoplex formation is proposed.

Supramolecular Organization and siRNA Binding of Hyaluronic Acid-Coated Lipoplexes for Targeted Delivery to the CD44 Receptor

“…(36) These observations are in agreement with reports from the literature on thermodynamic studies of DNA lipoplexes containing DOPE in their composition. (37)(38)(39) They are related to the fact that DOPE amine groups (pKa = 9.5) are unprotonated before binding due to the high surface pH usually measured in cationic liposomes (around 11) whereas proton uptake occurs upon lipoplex formation. (39) Since DOPE protonation is an exothermic process, it is suggested, as shown in DOTAP/DOPE-containing lipoplex,(37) that changes in the protonation state of DOPE account for the exothermic nature of complex formation with these lipids.…”

Cholesterol as a bilayer anchor for PEGylation and targeting ligand in folate‐receptor‐targeted liposomes

“…64 Briefly, the liposomal samples were placed onto 100 mesh grids. After 3 min, excess solution was wiped off with filter paper and the grid was immersed with 2% UA.…”

Cell Biological and Biophysical Aspects of Lipid-mediated Gene Delivery

“…(A) The lipoplex size influences the transfection efficiency [43][44][45] (B) Charge ratio of the cation (from nitrogen of the cationic lipid) to anion (phosphate of the nucleic acid) in the lipoplex has strong bearing on the transfection efficiency. Charge ratios greater than one are more efficient than lipoplexes with charge ratios that are less than one [46,47].…”

“…Independent of the chemical nature of the lipid, the relation between transfection efficiency and the size of the lipoplex is bell shaped, with a particular size range showing maximum transfection [45]. Size and colloidal stability depend on the charge ratio of the lipid to DNA [20].…”