The blood-brain barrier (BBB) is considered an important protective barrier in the central nervous system (CNS). The barrier is mainly formed by endothelial cells (ECs) interconnected by various junctions such as tight junctions (TJs), gap junctions, and adherent junctions. They collectively constitute an intensive barrier to the transit of different substances into the brain, selectively permitting small molecules to pass through by passive movement but holding off large ones such as peptides and proteins to cross the brain. Hence some molecules selectively transfer across the BBB by active routes via transcytosis. The BBB also forms a barrier against neurotoxins as well as pathogenic agents. Although various CNS disorders like Alzheimer's disease (AD), and Parkinson's disease (PD) could hamper the integrity of the border. Nevertheless, the BBB acts as a barrier for CNS disorders treatment because it prevents the drugs from reaching their target in the CNS. In recent years, different strategies, including osmotic disruption of BBB or chemical modification of drugs, have been used to transfer the chemotherapeutic agents into brain substances. Nowadays, nanoparticles (NPs) have been used as an effective and non-invasive tool for drug delivery and diagnosis of CNS disorders. In this review, we discuss the structural characteristic of BBB, safe passageways to cross the BBB, and the relation of barrier lesions with different CNS disorders. In the end, we explore various progresses in drug delivery, diagnosis, imaging, and treatment of CNS disorders using nanoparticles.

: Quantum dots (QDs) are nanoparticles (NPs) with electronic and optical properties such as emitting bright light and fluorescence. They also carry specific characters such as photostability, high quantum yield, high emission, and size-turnable. Nowadays, a great interest is given to the extensive use of theranostic-NPs for sensing and imaging, as well as drug delivery. Moreover, QDs may yield great potential for the diagnosis and treatment of various central nervous system (CNS) diseases (e.g., Parkinson’s disease, Alzheimer’s disease, and multiple sclerosis). The blood-brain barrier (BBB) protects the brain tissue. Only certain small molecules like water and gases can cross BBB, whereas larger molecules enter via receptors, but many drugs are incapable of passing the barrier. A series of great advances have been achieved concerning using different NPs (e.g., QDs) to deliver drugs to the brain and CNS imaging. In this review, we discussed a wide variety of QDs along with their production, passive or active delivery of therapeutic agents for neurodegenerative diseases, and different image production.

One of the main obstacles to most medication administrations (such as the vaccine constructs) is the cellular membrane’s inadequate permeability, which reduces their efficiency. Cell-penetrating peptides (CPPs) or protein transduction domains (PTDs) are well-known as potent biological nanocarriers to overcome this natural barrier, and to deliver membrane-impermeable substances into cells. The physicochemical properties of CPPs, the attached cargo, concentration, and cell type substantially influence the internalization mechanism. Although the exact mechanism of cellular uptake and the following processing of CPPs are still uncertain; but however, they can facilitate intracellular transfer through both endocytic and non-endocytic pathways. Improved endosomal escape efficiency, selective cell targeting, and improved uptake, processing, and presentation of antigen by antigen-presenting cells (APCs) have been reported by CPPs. Different in vitro and in vivo investigations using CPP conjugates show their potential as therapeutic agents in various medical areas such as infectious and non-infectious disorders. Effective treatments for a variety of diseases may be provided by vaccines that can cooperatively stimulate T cell-mediated immunity (T helper cell activity or cytotoxic T cell function), and immunologic memory. Delivery of antigen epitopes to APCs, and generation of a potent immune response is essential for an efficacious vaccine that can be facilitated by CPPs. The current review describes the delivery of numerous vaccine components by various CPPs and their immunostimulatory properties.