Overcoming Endosomal Entrapment in Drug Delivery

Abstract

Intracellular delivery of biological agents such as peptides, proteins, and nucleic acids generally rely on the endocytic pathway as the major uptake mechanism, resulting in their entrapment inside the endosome and lysosome. The recent discovery of cell-penetrating molecules of exceptionally high endosomal escape and cytosolic delivery efficiencies and elucidation of their mechanism of action represent major breakthroughs in this field. In this Topical Review, we provide an overview of the recent progress in understanding and enhancing the endosomal escape process and the new opportunities opened up by these recent findings.

Figure 1.

Endocytic uptake and intracellular trafficking of biological cargoes by mammalian cells. Biological cargoes internalized by different endocytic mechanisms are initially all transported to the early endosome, where the luminal pH is slightly acidic (~6.3). Maturation of the early endosome into the late endosome is accompanied by further acidification (to pH~5.5). Finally, the late endosome fuses with lysosome (pH~4.7) and the cargoes are degraded by hydrolytic enzymes. A fraction of the cargoes may recycle back to the cell surface via the recycling endosome. For most of the non-viral delivery systems, cargo escape from the endo-/lysosomal pathway into the cytosol is inefficient to nonexistent.

Figure 2.

Proposed mechanisms of endosomal escape. a) Proton sponge effect and osmotic lysis of the endosome/lysosome; b) Membrane fusion mechanism for liposome-based delivery systems; c) Barrel-stave pore formation; d) Toroidal pore formation; e) Membrane destabilization mechanism for polymer-based delivery systems; and e) Vesicle budding and collapse mechanism.

Figure 3.

Conformationally constrained CPPs and CPMs. a) Scheme showing the structure of miniature protein 5.3. b) Structure of cyclic CPP12. c) Structure of CPM2.