Small extracellular vesicle loading systems in cancer therapy: Current status and the way forward

Abstract

Systemic chemotherapy is a conventional and important strategy for inhibition of cancer progression, but it is usually accompanied by various adverse effects. Targeting drug delivery systems, effective tools to avoid the adverse effects of chemotherapy, have been intensively studied and developed. Recently, the emerging application of exosomes and exosome-mimics (small extracellular vesicles [sEVs]) in targeted drug delivery and therapeutics has been widely appreciated. The sEVs-based delivery system comprises three basic components: vesicles, cargoes and surface decorations. In this article, we review the current status, existing challenges and future directions in this field from the following aspects: selection and production of vesicles; cargoes and methods to load them into vesicles; modifications to the surfaces of vesicles; as well as ways to prolong the half-life of sEVs in the circulation. Existing and emerging data indicate that sEVs are promising nanocarriers for clinical use, but additional efforts are needed to translate research findings into therapeutic products.

Introduction

Cancer has long been a global burden and is a leading cause of death worldwide [1,2]. Over the past decades, several breakthrough therapies have been developed, including targeted and immune therapies [3,4]. Currently, comprehensive treatment, which consists of a combination of chemotherapy, surgery, radiotherapy as well as the burgeoning targeted therapy or immune therapy, is used for advanced cancers. Although systemic chemotherapy exists as the most conventional strategy to inhibit cancer progression [5], most chemotherapeutic agents are highly toxic to both cancerous and healthy cells, and the low specificity of chemotherapeutic agents results in unwanted toxicity and side effects [6,7]. To address these problems, targeting drug delivery systems (DDSs) have been intensively studied and developed, and the most promising DDSs to have emerged in the past two decades are nanocarriers [8].

Typically, synthesized nanoparticles (NPs) range from 100 to 1000 nm in size and have several advantages, such as high drug-loading capacity (due to their high surface area to volume ratio), adjustable physiochemical properties and modifiable flexibiliy [8,9]. NPs represent a large family, which includes the eight most representative nanocarriers and their ramifications: liposomes, micelles, dendrimers, meso-porous silica nanoparticles, gold nanoparticles, superparamagnetic iron oxide nanoparticles (SPIONs), carbon nanotubes and quantum dots [8,10]. However, a major concern regarding NP-based DDSs is that the fate of the artificial nanocarriers in a physiological environment depends on its inherent characteristics, such as chemical composition, size, shape, specific surface area, surface charge, as well as surface modifications. Consequently, while an ideal nanocarrier should have a good balance of all these factors, this is impossible to achieve in reality [8], [9], [10], [11]. For example, abundant blood proteins usually interact with NPs to form a dynamic “nanoparticle-protein corona,” which results in some unwanted and uncertain biological effects [9]. In addition, most of the conventional nanocarriers tend to accumulate in different vital organs such as the lungs, spleen, kidneys, liver and heart, and, therefore, these organs may be vulnerable to greater toxicity [8], [9], [10], [11], [12]. As a result, it is difficult to construct a widely applicable DDS based on conventional NPs in the foreseeable future, and, as such, the discovery and design of more biocompatible tools are urgently needed.

Recently, the emerging application of exosomes and exosome-mimics in targeted drug and therapeutics delivery is being widely recognized. Exosomes, with a diameter of 30–150 nm, are a specific subtype of extracellular vesicles (EVs) originating from the late endosomal compartment [13]. They are secreted by all cell types and have been found in numerous body fluids, including blood, amniotic fluid, urine, malignant ascites, cerebrospinal fluid, breast milk, saliva, lymph, bile and pancreatic juice under both healthy and disease conditions [13,14]. As shown in Figure 1A, exosomes contain lipids, proteins, messenger RNAs (mRNAs) and micro RNAs (miRNAs)[16,17]. However, the mechanisms of exosome biogenesis and cargo sorting remain to be understood. The exosomes released by donor cells may be selectively taken up by recipient cells, so that exosomes serve as mediators in intercellular communications [13,14,18]. As far as we know, receptor-ligand interactions, direct membrane fusion and endocytosis/phagocytosis are three ways by which exosomes interact with recipient cells [19]. Their nanoscale size, potential to carry biomolecules, specificity of organtropism, easy accessibility, ability to pass through the blood-brain barrier and innate biocompatibility make exosomes suitable therapeutics carriers as compared with artificial nanocarriers in cancer therapy [20,21].

Although exosomes derived from malignant cells may promote cancer by altering the function of normal cells, exosomes can also be engineered or naturalized to deliver therapeutics to treat cancers. A consensus has not been reached on specific markers of exosomes, making purification of one specific subpopulation of extracellular vesicles (e.g., exosomes) difficult to achieve. Consequently, “exosomes” used in many research studies may be potentially contaminated with other heterogeneous populations of extracellular vesicle subtypes [22,23]. According to the latest Minimal Information for Studies of Extracellular Vesicles 2018 proposed by the International Society for Extracellular Vesicles, “small extracellular vesicles (<200 nm, sEVs)” might be a more appropriate term. Therefore, we have adopted the term “sEVs” in this review to refer to nanovesicles, including exosomes and exosome-mimics. The sEV-based delivery systems comprise three basic components: vesicles, cargoes and decorations. Therefore, we review studies in the field from these aspects, and discuss the approaches of cargo loading and surface modification.