O-alkyl dioleoylphosphatidylcholinium compounds: The effect of varying alkyl chain length on their physical properties and in vitro DNA transfection activity

1,2-Dioleoyl-sn-3-ethylphosphocholine (EDOPC) has been previously shown be a highly effective DNA transfection reagent in vitro. To assess the effect of alkyl chain length on transfection efficiency, the O-methyl, O-propyl, O -hexyl, O-decyl, and O-octadecyl derivatives have been prepared from dioleo ylphosphatidylcholine using the corresponding alkyl trifluoromethylsulfonat e. The methyl, ethyl, and propyl derivatives formed liposomes which were ve ry large and unilamellar. The ethyl and propyl derivatives were equally eff icient at mediating transfection (even in the presence of serum) of BHK cel ls, but the chemically labile methyl derivative was a much weaker transfect ion agent. The O-decyl and O-octadecyl compounds, which assume the inverted hexagonal phase in excess water las determined by X-ray diffraction), were almost inactive after manual agitation in both water and in saline; howeve r, after sonication, these compounds exhibited good transfection activity. The O-hexyl derivative displayed novel behavior, assuming the lamellar phas e at low and a cubic phase at high ionic strength. All compounds, whether l amellar or not, formed lamellar structures when complexed with DNA. In wate r, where the hexyl compound dispersed well, sonication diminished transfect ion activity, whereas at physiological ionic strength, which led to poor ma nual dispersion, sonication was essential for good transfection. These resu lts emphasize the importance of optimal dispersion of a cationic lipid: too little, and interaction with DNA is handicapped, too much, and the resulta nt particle transfects poorly. Lipid dispersibility is thus an important va riable in assessing lipid transfection agents, and caution is advised in at tributing too much significance to chemical structure until interaction wit h DNA has been optimized.