A serum-stable mixture of lipids capable of encapsulating an active agent to form a liposome, said mixture comprising phosphatidylcholine and phosphatidylethanolamine in a ratio in the range of about 0.5 to about 8. The mixture may also include pH sensitive anionic and cationic amphiphiles, such that the mixture is amphoteric, being negatively charged or neutral at pH 7.4 and positively charged at pH 4. Amphoteric liposomes comprising such a mixture may be used for encapsulating nucleic acid therapeutics, such as oligonucleotides and DNA plasmids. The drug/lipid ratio may be adjusted to target the liposomes to particular organs or other sites in the body.
