Transferrin-modified liposomes equipped with a pH-sensitive fusogenic peptide: an artificial viral-like delivery system.

Liposomes are one of the most promising systems for selective cellular targeting via introduction of specific ligands for cell-surface receptors. After being taken up by the cells, these liposomes usually follow intracellular pathways of receptor-mediated endocytosis. Control of intracellular trafficking is required for optimized drug delivery. In this study, we elucidated the intracellular fate of transferrin-modified liposomes and succeeded in altering it by introducing the pH-sensitive fusogenic peptide, GALA (WEAALAEALAEALAEHLAEALAEALEALAA). Transferrins that are chemically attached to a liposomal surface (Tf-L) were internalized via receptor-mediated endocytosis more slowly than unmodified transferrins. In contrast to the recyclable nature of transferrin, liposome-attached transferrins together with encapsulated rhodamines were retained in vesicular compartments. When GALA was introduced into liposomal membranes using a cholesteryl moiety for anchoring (Chol-GALA), rhodamines were efficiently released and diffused into the cytosol. The addition of GALA to the Tf-L-containing medium or the encapsulation of GALA in Tf-L did not induce similar effects. These results clearly indicate that GALA must be present on the surface of liposomes to exert its function. In vitro energy transfer and dynamic light scattering experiments suggested that the endosomal escape of the encapsulates in Tf-L equipped with Chol-GALA can be attributed to pH-dependent membrane fusion. With GALA present on the surface, intracellular trafficking of liposomes after receptor-mediated endocytosis could be successfully controlled.