Factors influencing the oligonucleotides release from O-W submicron cationic emulsions

We recently described a positively charged O-W emulsion as a delivery system for oligonucleotides (ON) [Teixeira et al., Pharm. Res. 16 (1999) 30-36]. The present paper investigates the role of the main formulation parameters that may have an influence on the release-rate of a model ON in a protein-containing medium, i.e. the nature of the oily core, the presence of pegylated lipids, the lipid phase transition temperature, and the cationic lipid structure. The use of cationic lipids bearing diacyl chains (and especially polycations) appeared as the only efficient strategy to reduce the ON release rate. In order to have a better insight on the nature of the interactions between the ON and the interfacial lipids, adsorption isotherms at the air-water interface, fluorescence resonance energy transfer and zeta-potential measurements have been performed. Electrostatic interactions were found to play a crucial role. In contrast, the incorporation of PEG-phospholipids acted as a barrier and maintained the ON molecules distant from the interface, leading to a more rapid release. Finally, ON integrity was assessed by a competitive hybridization assay. The results suggest the existence of a transient ion-pair (ON-cationic lipids) protecting ON against nuclease degradation even after its release from the emulsions.