Table 4 Strengths, weaknesses, applications, ang alternatives of vesicle-vector system (VVS) for targeted drug delivery

●Protection from degradation: Lipid-based vesicles (e.g. liposomes and extracellular vesicles) are superior vectors for drug delivery because they protect the cargo from degradation and increase its bioavailability

●Biocompatibility: The use of safe and biocompatible extracellular vesicles (EVs) for drug delivery promote cellular uptake of target cells

●Target specificity: Conjugation of bioactive probing molecules on the surface of lipid-based vesicles help target specific cells and promote targeted drug delivery

●Multi-targeted delivery: Multi-targeted delivery can be achieved by conjugating a combination of bioactive probes with different targets or by using molecules with multiple targeting ability (e.g. bispecific antibodies)

●Multiple approaches in designing VVS:

  → Conjugation of target-specific bioactive probing materials like proteins, peptides, antibodies, and aptamers with vesicles enhances targeted delivery

  →Hybridization of two vesicle types (e.g. liposome and exosome) is a promising approach in creating modified vesicles with enhanced targeted drug delivery characteristics. It takes advantage of each vesicle’s characteristics that are helpful for targeted delivery

  →Genetic engineering of EV’s parent cells a practical approach in displaying bioactive probing materials on EV surface to be used for targeted delivery. This can be achieved through a variety of surface display methods largely divided into two types: direct and indirect surface display

●Scale up production of EVs can be a challenge due to complexity of the isolation process and requirement of sophisticated equipment. Currently, the most widely used method for EV extraction is ultracentrifugation, but there are problems with protein contamination and low product yield

●Drug loading: In the encapsulation of drugs, current drug loading methods on EVs are still low efficiency. Additionally, the stability and long-term preservation of drug cargoes remain to be an issue

●Bioavailability: Liposomes, in general, have a low bioavailability because they are easily cleared by the immune system. Even when using PEG to evade immune response, repeated administration of PEGylated liposomes has been reported to lead to the accelerated blood clearance (ABC) phenomenon

●Storage and stability: Cell culture derived EVs cannot be kept in storage for an extended period of time. Negative 80 °C frozen storage is now the most effective all-around storage method

●Immunogenicity: Protein markers or other properties that are inherently present in EVs may be toxic and induce immune response from hosts. For instance, Lipopolysaccharide (LPS), one of the main components of OMV membranes, but it is highly immunogenic to human cells and triggers host immune response

●Multi-targeted delivery: multi-targeted ligands, such as bispecific antibodies can bind to or inhibit two or more targets at once, increasing the compound's therapeutic potential

●Vaccine/gene delivery VVS can be used in vaccine delivery applications. The type of vesicle and surface displayed probing material can be altered to fit the requirements for a successful vaccine/gene delivery

●Tissue regeneration: EVs are involved in the maintenance of tissue homeostasis, and they contribute to tissue repair and regeneration. For example, exosomes were reported to promote cartilage regeneration. Using an exosome-based VVS can further enhance this ability which can be applied to the regeneration of other types of tissues as well

●Immunological regulation: EVs play discrete roles in the immune regulatory functions, such as antigen presentation, and activation or suppression of immune cells. Likewise, EV-based VVS can be applied in immune regulation applications, such as for the treatment of autoimmune diseases

●Inorganic nanoparticles: Silver NP, gold NP, zinc oxide NP, and magnetic nanoparticles are used in various biomedical applications (including targeted delivery) due to their good biocompatibility, small size, low toxicity, easy surface modification, and controlled drug release

●Robotic nanomaterials: Micro/nanorobots can be designed to perform any task, including the effective delivery of drugs to body tissues. Besides targeted delivery in cancer treatment, they are also predicted to carry out other small-scale tasks, such as microsurgery of cells, assisted fertilization, and tissue engineering. Despite very promising advantages, much more research is necessary regarding micro/nanorobots on clinical applications

●Carbon nanotubes imaging probe: Carbon nanotubes have versatile applications. Its strong absorption of near-infrared regions enable their application in photothermal therapy. Carbon nanotubes can also transform the laser energy to acoustic signals and exhibit great resonant Raman scattering and photoluminescence in near infrared region, which are all beneficial to their utilization in cancer imaging