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