Recent advances in Bio-conjugated nanocarriers for crossing the Blood-Brain Barrier in (pre-)clinical studies with an emphasis on vesicles

The brain is one of the most challenging organs for drug delivery. It is preserved by the blood-brain barrier (BBB), which controls the transport of components into and out of the brain. Although various approaches have been utilized to cross the BBB, the drug delivery into the brain is still less successful than the other human body parts. Comprehensive knowledge about the mechanisms and functionalities of brain delivery for different components is required to overcome this complex barrier. In this review, we have discussed the BBB structure, its characteristics, and the BBB's mechanisms for transporting components between the brain and blood. Furthermore, the barriers in front of drug delivery systems, the strategies which should be employed for overcoming these barriers, and different pathways for brain targeting are discussed. Then, the drug delivery systems utilized for crossing the BBB, especially nanocarriers and novel approaches, are considered. Finally, the bioconjugated vesicles as versatile nanocarriers that combine the bioconjugation approach and vesicles are reviewed. This review focuses on the biomolecules used in nano vesicular systems for overcoming the BBB. Various biomolecules such as amino acids, peptides, proteins, antibodies, and carbohydrates could be used for bioconjugation. Bioconjugated vesicles like liposomes and niosomes utilize the related receptor or transporter to cross the BBB. These drug delivery systems have shown enhanced drug loading into the brain, which is discussed.