Multilamellar cationic liposomes are efficient vectors for in vitro gene transfer in serum

Multilamellar vesicles (MLVs) containing the cationic lipid DOTAP were used as vectors to lipofect a number of culture cell lines in the presence of serum. The lipofection efficiency of lipoplexes made of MLVs and the plasmid pSV-beta galactosidase are much less sensitive to the lipofection-inhibitory effect of serum than the conventionally used lipoplexes made of sonicated small unilamellar vesicles (SUVs). In order to determine the factors favoring the lipofection efficiency of MLVs, we measured the size, as well as the cellular association and uptake of MLV and SUV lipoplexes containing DOTAP alone or DOTAP:DOPE (1:1). Electron microscope images of these complexes were taken to confirm their structure and size. The single most important factor that correlates with transfection efficiency in serum is the size of the lipoplex. SUV lipoplexes remain smaller than 300 nm in the presence of serum, and the lipofection efficiencies are low. MLV lipoplexes are larger (>300 nm) and the lipofection efficiency, as well as cellular association and uptake, are much higher than those of SUV lipoplexes. Exceptions are those lipoplexes made of MLVs of DOTAP and DOPE (1:1) combined with DNA at higher charge ratios, which form hexagonal structures and show poor lipofection as well as cellular association and uptake, even if their lipoplex size exceeds 300 nn. This finding lends credence to our theory of the serum inhibition effect upon lipofection, and suggests ways to improve the transfection efficiency in the presence of serum, by fabricating lipoplexes of defined sizes.