Liposomes and Extracellular Vesicles as Drug Delivery Systems: A Comparison of Composition, Pharmacokinetics, and Functionalization

Abstract

Over the past decades, lipid-based nanoparticle drug delivery systems (DDS) have caught the attention of researchers worldwide, encouraging the field to rapidly develop improved ways for effective drug delivery. One of the most prominent examples is liposomes, which are spherical shaped artificial vesicles composed of lipid bilayers and able to encapsulate both hydrophilic and hydrophobic materials. At the same time, biological nanoparticles naturally secreted by cells, called extracellular vesicles (EVs), have emerged as promising more complex biocompatible DDS. In this review paper, the differences and similarities in the composition of both vesicles are evaluated, and critical mediators that affect their pharmacokinetics are elucidate. Different strategies that have been assessed to tweak the pharmacokinetics of both liposomes and EVs are explored, detailing the effects on circulation time, targeting capacity, and cytoplasmic delivery of therapeutic cargo. Finally, whether a hybrid system, consisting of a combination of only the critical constituents of both vesicles, could offer the best of both worlds is discussed. Through these topics, novel leads for further research are provided and, more importantly, gain insight in what the liposome field and the EV field can learn from each other.

Keywords: biodistribution; cellular uptake; targeting moiety incorporation; therapeutic cargo delivery; vesicle functionalization.

Figure 1

Overview of key compositional parameters of liposomes and EVs. Liposomes typically contain increasing levels of 1) cholesterol to improve vesicle stability and also in EVs high levels of cholesterol have been reported. Liposomes can be made from various lipids, with different 2) head groups, 3) chain lengths, and 4) chain saturation. This also holds true for EVs, which have shown highly complex lipid compositions. Together with cholesterol content, the characteristics of the lipids present are key parameters for pharmacokinetic properties. 5) Lamellarity and 6) size depend on the method that is used for the production of liposomes. 7) Fluidity and mechanics of the membrane are dependent on the mixture of the different lipids and cholesterol that is being used. In EVs, only limited control over (5–7) is present. 8–10) The net charge of the vesicles is dependent on the mixture of the different lipids and cholesterol that is being used. EVs typically carry a net negative charge, whereas for liposomes this parameter can be controlled. 11) EVs potentially have an asymmetry in their lipid bilayers. Incorporation of this feature into liposomes is part of ongoing research.

Figure 2

Schematic overview of pharmacokinetics of liposomes and EVs. Intravenous (i.v.) injection will deliver vesicles into the systemic circulation. Unmodified vesicles most likely are taken up by the RES/MPS, the main site being the liver, followed by the spleen. Resident macrophages in these tissues take up vesicles for clearance. Parameters can be modified by the properties of the vesicles and the administration route by which uptake can be driven to alternative target tissues (see text for further discussion).

Figure 3

Naturally occurring EV‐surface proteins that alter circulation time, intrinsic targeting, and cellular uptake. The presence of 1) CD47 or 2) CD55/CD59 on the membrane of EVs can prolong the blood circulation time by evading phagocytic clearance. EVs show intrinsic targeting capacity that is associated with the incorporation of 3) integrins, 4) tetraspanins, and 5) other proteins, such as, fibronectin and Wnt4, on the surface of EVs. The 6) tetraspanin, 7) integrin, and 8) proteoglycan compositions of EVs affect their uptake into recipient cells.

Figure 4

Overview of proteins used to tweak pharmacokinetics of liposomes and EVs. The blood circulation time of liposomes and EVs can be prolonged by the incorporation of 1) dysopsonic proteins, such as, albumin, and 2) phagocytic clearance evading proteins, such as, CD47. Liposomes and EVs have been actively targeted toward tumors and diseased tissues by the incorporation of 3) antibodies, 4) targeting peptides, 5) antibody‐fragments, and 6) targeting proteins. 7) Cellular uptake and subsequent cytoplasmic delivery of therapeutic cargo has been enhanced by decorating the surface of liposomes and EVs with virus‐inspired peptides, such as, GALA‐peptide. Moreover, cellular delivery has been enhanced by incorporating 8) albumin, 9) gap‐junction proteins such as, CxC43, and 10) other fusogenic peptides.

Figure 5

Overview of non‐protein related strategies to actively target liposomes and EVs. The surface of liposomes and EVs has been decorated with various small molecules, such as, 1) glutathione and 2) folic acid, to actively target diseased tissue. In addition, targeting of vesicles has been demonstrated by incorporating 3) DNA and 4) RNA aptamers on the vesicle surface. 5) Moreover, the acidic environment of tumor tissue has been exploited to target liposomes and EVs using pH responsive polymers. 6,7) Finally, liposomes and EVs have been modified with iron‐based nanoparticles to obtain magnetic‐responsive vesicles, which can be targeted by applying an external magnetic field.

Figure 6

Overview of next‐generation vesicle formulations for DDS. Comparing EVs and liposomes on various aspects (no. 1–5), shows that they have inherent advantages and disadvantages. The generation of EV‐inspired liposomes or EV/liposome hybrid vesicles could be key to improving lipid vesicles as a DDS as they could incorporate the beneficial aspects of both EVs and liposomes. Another alternative for the generation of vesicles of biological origin, is to use the cellular membranes in a cell shearing approach.