STERICALLY STABILIZED PH-SENSITIVE LIPOSOMES - INTRACELLULAR DELIVERYOF AQUEOUS CONTENTS AND PROLONGED CIRCULATION IN-VIVO

Liposomes that destabilize at mildly acidic pH are efficient tools for delivering water-soluble drugs into the cell cytoplasm. However, thei r use in vivo is limited because of their rapid uptake from circulatio n by the reticuloendothelial system. Lipid-anchored polyethylene glyco l (PEG-PE) prolongs the circulation time of liposomes by steric stabil ization. We have found that addition of PEG-PE to the membrane of pH-s ensitive liposomes composed of cholesteryl hemisuccinate (CHEMS) and d ioleoylphosphatidylethanolamine (DOPE) confers steric stability to the se vesicles. This modification significantly decreases the pH-dependen t release of a charged water-soluble fluorophore, calcein, from liposo mes suspended in buffer or cell culture medium. However, the ability o f such liposomes to release calcein intracellularly, measured by a nov el flow cytometry technique involving dual fluorescence labeling, rema ins unaltered. As expected, the release of calcein from liposomes endo cytosed by cells is inhibited upon pretreatment of the cells with NH4C l, an inhibitor of endosome acidification. The unique properties of th ese liposomes were also demonstrated in vivo. The distribution kinetic s of In-111-containing CHEMS/DOPE/PEG-PE liposomes injected intravenou sly into rats has pharmacokinetic parameters similar to control, non-p H-sensitive, sterically stabilized CHEMS/distearoylphosphatidylcholine /PEG-PE liposomes. In contrast, regular pH-sensitive liposomes lacking the PEG-PE component are cleared rapidly. Sterically stabilized pH-se nsitive liposomes may therefore be useful, for the intracellular deliv ery in vivo of highly negatively charged molecules such as genes, anti sense oligonucleotides, and ribozymes for the treatment of various dis eases.