Therapeutic applications of extracellular vesicles: clinical promise and open questions

Abstract

This review provides an updated perspective on rapidly proliferating efforts to harness extracellular vesicles (EVs) for therapeutic applications. We summarize current knowledge, emerging strategies, and open questions pertaining to clinical potential and translation. Potentially useful EVs comprise diverse products of various cell types and species. EV components may also be combined with liposomes and nanoparticles to facilitate manufacturing as well as product safety and evaluation. Potential therapeutic cargoes include RNA, proteins, and drugs. Strategic issues considered herein include choice of therapeutic agent, means of loading cargoes into EVs, promotion of EV stability, tissue targeting, and functional delivery of cargo to recipient cells. Some applications may harness natural EV properties, such as immune modulation, regeneration promotion, and pathogen suppression. These properties can be enhanced or customized to enable a wide range of therapeutic applications, including vaccination, improvement of pregnancy outcome, and treatment of autoimmune disease, cancer, and tissue injury.

Keywords: drug delivery; exosomes; extracellular RNA; gene medicine; gene therapy; liposomes; mesenchymal stem cells; microvesicles; nanoparticles.

Figure 1

Strategies for incorporating therapeutic agents into extracellular vesicles (EVs). EVs can carry DNA, RNA, proteins, lipids, or drugs, which are incorporated via various mechanisms. ❶ Directing highly oligomeric proteins to EVs using plasma membrane anchors (100). ❷ Including zipcode-like sequences in the 3′ untranslated region to facilitate mRNA loading into EVs (105). ❸ Overexpressing mRNA to drive incorporation into EVs by passive loading. ❹ Delivering an adeno-associated virus expression vector termed a vexosome (114). ❺ Loading EVs with cargo by physical methods, including mixing (e.g., for curcumin) (71) and electroporation for drugs (74) or RNA (76).

Figure 2

Bioengineering extracellular vesicles (EVs) for therapeutic delivery. Targeting EVs to specific recipient cells may be achieved by expressing proteins in EV-producing cells, including natural proteins such as EBV gp350 [selectively binds to B cells (121)], ICAM-1 [binds to T cells (119)], or sialic acid residues [promote uptake by macrophages (117)] (blue text) and engineered proteins such as EV transmembrane proteins that display peptides on the EV surface [e.g., GE11 (73), iRGD (74), and RVG (76)] (red text). Expression of other exogenous proteins may facilitate other steps of EV-mediated delivery, including fusion of EVs and donor cells [e.g., VSV-G (127)] (purple text). Other abbreviations: EBV, Epstein Barr virus; EGFR, epidermal growth factor receptor; ICAM-1, intercellular adhesion molecule-1; iRGD, internalizing arginine-glycine-asparagine; LFA-1, lymphocyte function–associated antigen-1; nACh, nicotinic acetylcholine; PDGFR TM domain, platelet-derived growth factor receptor, transmembrane domain; RVG, rabies virus glycoprotein; VSV-G, vesicular stomatitis virus glycoprotein.