Peptide-Mediated Release of Folate-Targeted Liposome Contents from Endosomal Compartments1

An egg phosphatidylcholine-derived liposome preparation has been developed that achieves low pH-dependent cargo unloading in the absence of a pH-sensitive lipid component by use of the 30 amino acid residue peptide, EALA. This peptide adopts an amphiphilic α-helical conformation in a mildly acidic environment, and when encapsulated into neutral liposomes or covalently attached to one of its lipid components, the peptide can facilitate the release of liposome contents. Our liposomes have also been designed to be tumor cell specific by incorporation of a folate−PEG−PE lipid conjugate which allows the liposomes to enter receptor-bearing cells through the pathway of folate receptor-mediated endocytosis. In combination, these features allow for the cytoplasmic release of 20−25% of the encapsulated cargo of internalized liposomes within several hours of administration. To quantitate the extent of the cytoplasmic unloading of liposomal contents, a new methodology was developed that exploits the dramatic increase in quantum yield of propidium iodide upon binding DNA. As the internalized liposomes interact with the endosomal membranes and release their dye into the cytoplasm, a rapid increase in fluorescence is measured to yield quantitative data on the kinetics of intracellular unloading.