Tunable pH-sensitive liposomes composed of mixtures of cationic and anionic lipids

The pH-dependent fusion properties of large unilamellar vesicles (LUVs) com posed of binary mixtures of anionic and cationic lipids have been investiga ted. It is shown that stable LUVs can be prepared from the ionizable anioni c lipid cholesteryl hemisuccinate (CHEMS) and the permanently charged catio nic lipid N,N-dioleoyl-N,N-dimethylammonium chloride (DODAC) at neutral pH values and that these LUVs undergo fusion as the pH is reduced. The critica l pH at which fusion was observed (pH(f)) was dependent on the cationic lip id-to-anionic lipid ratio. LUVs prepared from DODAC/CHEMS mixtures at molar ratios of 0 to 0.85 resulted in Vesicles with pH(f) values that ranged fro m pH 4.0 to 6.7, respectively. This behavior is consistent with a model in which fusion occurs at pH values such that the DODAC/CHEMS LUV surface char ge is zero. Related behavior was observed for LUVs composed of the ionizabl e cationic lipid 3 alpha-[N-(N',N'-dimethylaminoethane)-carbamoyl] choleste rol hydrochloride (DC-Chol) and the acidic lipid dioleoylphosphatidic acid (DOPA). Freeze-fracture and P-31 NMR evidence is presented which indicates that pH-dependent fusion results from a preference of mixtures of cationic and anionic lipid far "inverted" nonbilayer lipid phases under conditions w here the surface charge is zero. It is concluded that tunable pH-sensitive LUVs composed of cationic and anionic lipids may be of utility for drug del ivery applications. It is also suggested that the ability of cationic lipid s to adopt inverted nonbilayer structures in combination with anionic lipid s may be related to the ability of cationic lipids to facilitate the intrac ellular delivery of macromolecules.