Adsorptive-mediated Transcytosis Research Articles

Preserved blood-brain barrier and neurovascular coupling in female 5xFAD model of Alzheimer's disease.

Dysfunction of the cerebral vasculature is considered one of the key components of Alzheimer's disease (AD), but the mechanisms affecting individual brain vessels are poorly understood. Here, using in vivo two-photon microscopy in superficial cortical layers and ex vivo imaging across brain regions, we characterized blood-brain barrier (BBB) function and neurovascular coupling (NVC) at the level of individual brain vessels in adult female 5xFAD mice, an aggressive amyloid-β (Aβ) model of AD. We report a lack of abnormal increase in adsorptive-mediated transcytosis of albumin and preserved paracellular barrier for fibrinogen and small molecules despite an extensive load of Aβ. Likewise, the NVC responses to somatosensory stimulation were preserved at all regulatory segments of the microvasculature: penetrating arterioles, precapillary sphincters, and capillaries. Lastly, the Aβ plaques did not affect the density of capillary pericytes. Our findings provide direct evidence of preserved microvascular function in the 5xFAD mice and highlight the critical dependence of the experimental outcomes on the choice of preclinical models of AD. We propose that the presence of parenchymal Aβ does not warrant BBB and NVC dysfunction and that the generalized view that microvascular impairment is inherent to Aβ aggregation may need to be revised.

Shedding Light on the Blood-Brain Barrier Transport with Two-Photon Microscopy In Vivo.

Treatment of brain disorders relies on efficient delivery of therapeutics to the brain, which is hindered by the blood-brain barrier (BBB). The work of Prof. Margareta Hammarlund-Udenaes was instrumental in understanding the principles of drug delivery to the brain and developing new tools to study it. Here, we show how some of the concepts developed in her research can be translated to in vivo 2-photon microscopy (2PM) studies of the BBB. We primarily focus on the methods developed in our laboratory to characterize the paracellular diffusion, adsorptive-mediated transcytosis, and receptor-mediated transcytosis of drug nanocarriers at the microscale, illustrating how 2PM can deepen our understanding of the mechanisms of drug delivery to the brain.

Transportation of Single-Domain Antibodies through the Blood-Brain Barrier.

Single-domain antibodies derive from the heavy-chain-only antibodies of Camelidae (camel, dromedary, llama, alpaca, vicuñas, and guananos; i.e., nanobodies) and cartilaginous fishes (i.e., VNARs). Their small size, antigen specificity, plasticity, and potential to recognize unique conformational epitopes represent a diagnostic and therapeutic opportunity for many central nervous system (CNS) pathologies. However, the blood–brain barrier (BBB) poses a challenge for their delivery into the brain parenchyma. Nevertheless, numerous neurological diseases and brain pathologies, including cancer, result in BBB leakiness favoring single-domain antibodies uptake into the CNS. Some single-domain antibodies have been reported to naturally cross the BBB. In addition, different strategies and methods to deliver both nanobodies and VNARs into the brain parenchyma can be exploited when the BBB is intact. These include device-based and physicochemical disruption of the BBB, receptor and adsorptive-mediated transcytosis, somatic gene transfer, and the use of carriers/shuttles such as cell-penetrating peptides, liposomes, extracellular vesicles, and nanoparticles. Approaches based on single-domain antibodies are reaching the clinic for other diseases. Several tailoring methods can be followed to favor the transport of nanobodies and VNARs to the CNS, avoiding the limitations imposed by the BBB to fulfill their therapeutic, diagnostic, and theragnostic promises for the benefit of patients suffering from CNS pathologies.

Enhancement of Therapies for Glioblastoma (GBM) Using Nanoparticle-based Delivery Systems.

Glioblastoma multiforme (GBM) is the most aggressive type of malignant brain tumor. Current FDA-approved treatments include surgical resection, radiation, and chemotherapy, while hyperthermia, immunotherapy, and most relevantly, nanoparticle (NP)-mediated delivery systems or combinations thereof have shown promise in preclinical studies. Drug-carrying NPs are a promising approach to brain delivery as a result of their potential to facilitate the crossing of the blood-brain barrier (BBB) via two main types of transcytosis mechanisms: adsorptive-mediated transcytosis (AMT) and receptor-mediated transcytosis (RMT). Their ability to accumulate in the brain can thus provide local sustained release of tumoricidal drugsat or near the site of GBM tumors. NP-based drug delivery has the potential to significantly reduce drug-related toxicity, increase specificity, and consequently improve the lifespan and quality of life of patients with GBM. Due to significant advances in the understanding of the molecular etiology and pathology of GBM, the efficacy of drugs loaded into vectors targeting this disease has increased in both preclinical and clinical settings. Multitargeting NPs, such as those incorporating multiple specific targeting ligands, are an innovative technology that can lead to decreased off-target effects while simultaneously having increased accumulation and action specifically at the tumor site. Targeting ligands can include antibodies, or fragments thereof, and peptides or small molecules, which can result in a more controlled drug delivery system compared to conventional drug treatments. This review focuses on GBM treatment strategies, summarizing current options and providing a detailed account of preclinical findings with prospective NP-based approaches aimed at improving tumor targeting and enhancing therapeutic outcomes for GBM patients.

Overcoming the Blood-Brain Barrier: Functionalised Chitosan Nanocarriers.

The major impediment to the delivery of therapeutics to the brain is the presence of the blood-brain barrier (BBB). The BBB allows for the entrance of essential nutrients while excluding harmful substances, including most therapeutic agents; hence, brain disorders, especially tumours, are very difficult to treat. Chitosan is a well-researched polymer that offers advantageous biological and chemical properties, such as mucoadhesion and the ease of functionalisation. Chitosan-based nanocarriers (CsNCs) establish ionic interactions with the endothelial cells, facilitating the crossing of drugs through the BBB by adsorptive mediated transcytosis. This process is further enhanced by modifications of the structure of chitosan, owing to the presence of reactive amino and hydroxyl groups. Finally, by permanently binding ligands or molecules, such as antibodies or lipids, CsNCs have showed a boosted passage through the BBB, in both in vivo and in vitro studies which will be discussed in this review.

Nanoparticle-Based Technology Approaches to the Management of Neurological Disorders.

Neurological disorders are the most devastating and challenging diseases associated with the central nervous system (CNS). The blood-brain barrier (BBB) maintains homeostasis of the brain and contributes towards the maintenance of a very delicate microenvironment, impairing the transport of many therapeutics into the CNS and making the management of common neurological disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), cerebrovascular diseases (CVDs) and traumatic brain injury (TBI), exceptionally complicated. Nanoparticle (NP) technology offers a platform for the design of tissue-specific drug carrying systems owing to its versatile and modifiable nature. The prospect of being able to design NPs capable of successfully crossing the BBB, and maintaining a high drug bioavailability in neural parenchyma, has spurred much interest in the field of nanomedicine. NPs, which also come in an array of forms including polymeric NPs, solid lipid nanoparticles (SLNs), quantum dots and liposomes, have the flexibility of being conjugated with various macromolecules, such as surfactants to confer the physical or chemical property desired. These nanodelivery strategies represent potential novel and minimally invasive approaches to the treatment and diagnosis of these neurological disorders. Most of the strategies revolve around the ability of the NPs to cross the BBB via various influx mechanisms, such as adsorptive-mediated transcytosis (AMT) and receptor-mediated transcytosis (RMT), targeting specific biomarkers or lesions unique to that pathological condition, thereby ensuring high tissue-specific targeting and minimizing off-target side effects. In this article, insights into common neurological disorders and challenges of delivering CNS drugs due to the presence of BBB is provided, before an in-depth review of nanoparticle-based theranostic strategies.

A Peptide-Based Nanocarrier for an Enhanced Delivery and Targeting of Flurbiprofen into the Brain for the Treatment of Alzheimer's Disease: An In Vitro Study.

Alzheimer’s disease (AD) is an age-related disease caused by abnormal accumulation of amyloid-β in the brain leading to progressive tissue degeneration. Flurbiprofen (FP), a drug used to mitigate the disease progression, has low efficacy due to its limited permeability across the blood–brain barrier (BBB). In a previous work, FP was coupled at the uppermost branching of an ε-lysine-based branched carrier, its root presenting a phenylalanine moiety able to increase the hydrophobicity of the complex and enhance the transport across the BBB by adsorptive-mediated transcytosis (AMT). The present study explores a different molecular design of the FP-peptide delivery system, whereby its root presents an ApoE-mimicking peptide, a targeting ligand that could enhance transport across the BBB by receptor-mediated transcytosis (RMT). The functionalised complex was synthesised using a solid-phase peptide synthesis and characterised by mass spectrometry and FTIR. Cytotoxicity and permeability of this complex across an in vitro BBB model were analysed. Moreover, its activity and degradation to release the drug were investigated. The results revealed successful synthesis and grafting of FP molecules at the uppermost molecular branches of the lysine terminal without observed cytotoxicity. When covalently linked to the nanocarrier, FP was still active on target cells, albeit with a reduced activity, and was released as a free drug upon hydrolysis in a lysosome-mimicking medium. Noticeably, this work shows the high efficiency of RMT-driven FP delivery over delivery systems relying on AMT.

Transcellular brain drug delivery: A review on recent advancements.

The blood-brain barrier (BBB) has a pivotal role in maintaining brain homeostasis. It robustly protects the brain parenchyma against the invasion of irrelevant substances, which may interrupt its critical function. From a pharmaceutical point of view, such a barrier may cause central nervous system (CNS) disorders refractory by restricting the therapeutics from accessing to their target sites in cerebral parenchyma. On the other side, the increasing rate of CNS disorders demands novel strategies to be developed for effective transferring the drugs through the BBB. Transcellular pathways seem to be more promising in ferrying across the BBB than paracellular route due to using the regular biological routes and retaining the BBB integrity, as well. The transcellular pathway contains several mechanisms for the transportation of therapeutic molecules, which are alternately applicable based on the physicochemical characteristics of the crossing molecule. In the present article, the most considerable transcellular routes, including the adsorptive mediated transcytosis (AMT), receptor-mediated transcytosis (RMT), cell-mediated transcytosis (CMT), and the efflux pumps-mediated drug delivery approaches were reviewed. Exosome, as a new drug carrier, utilizable in various transcellular routes, was also discussed.

Ligand Conjugated Targeted Nanotherapeutics for Treatment of Neurological Disorders.

Human brain is amongst the most complex organs in human body, and delivery of therapeutic agents across the brain is a tedious task. Existence of blood brain barrier (BBB) protects the brain from invasion of undesirable substances; therefore it hinders the transport of various drugs used for the treatment of different neurological diseases including glioma, Parkinson's disease, Alzheimer's disease, etc. To surmount this barrier, various approaches have been used such as the use of carrier mediated drug delivery; use of intranasal route, to avoid first pass metabolism; and use of ligands (lactoferrin, apolipoprotein) to transport the drug across the BBB. Ligands bind with proteins present on the cell and facilitate the transport of drug across the cell membrane via. receptor mediated, transporter mediated or adsorptive mediated transcytosis. The main focus of this review article is to illustrate various studies performed using ligands for delivering drug across BBB; it also describes the procedure used by various researchers for conjugating the ligands to the formulation to achieve targeted action. Research articles that focused on the used of ligand conjugation for brain delivery and compared the outcome with unconjugated formulation were collected from various search engines like PubMed, Science Direct and Google Scholar, using keywords like ligands, neurological disorders, conjugation, etc. Results and Conclusion: Ligands have shown great potential in delivering drug across BBB for treatment of various diseases, yet extensive research is required so that the ligands can be used clinically for treating neurological diseases.

Antibodies for the Treatment of Brain Metastases, a Dream or a Reality?

The incidence of brain metastases (BM) in cancer patients is increasing. After diagnosis, overall survival (OS) is poor, elicited by the lack of an effective treatment. Monoclonal antibody (mAb)-based therapy has achieved remarkable success in treating both hematologic and non-central-nervous system (CNS) tumors due to their inherent targeting specificity. However, the use of mAbs in the treatment of CNS tumors is restricted by the blood–brain barrier (BBB) that hinders the delivery of either small-molecules drugs (sMDs) or therapeutic proteins (TPs). To overcome this limitation, active research is focused on the development of strategies to deliver TPs and increase their concentration in the brain. Yet, their molecular weight and hydrophilic nature turn this task into a challenge. The use of BBB peptide shuttles is an elegant strategy. They explore either receptor-mediated transcytosis (RMT) or adsorptive-mediated transcytosis (AMT) to cross the BBB. The latter is preferable since it avoids enzymatic degradation, receptor saturation, and competition with natural receptor substrates, which reduces adverse events. Therefore, the combination of mAbs properties (e.g., selectivity and long half-life) with BBB peptide shuttles (e.g., BBB translocation and delivery into the brain) turns the therapeutic conjugate in a valid approach to safely overcome the BBB and efficiently eliminate metastatic brain cells.

Biodistribution of TAT or QLPVM coupled to receptor targeted liposomes for delivery of anticancer therapeutics to brain in vitro and in vivo

Combination therapy has emerged as an efficient way to deliver chemotherapeutics for treatment of glioblastoma. It provides collaborative approach of targeting cancer cells by acting via multiple mechanisms, thereby reducing drug resistance. However, the presence of impermeable blood brain barrier (BBB) restricts the delivery of chemotherapeutic drugs into the brain. To overcome this limitation, we designed a dual functionalized liposomes by modifying their surface with transferrin (Tf) and a cell penetrating peptide (CPP) for receptor and adsorptive mediated transcytosis, respectively. In this study, we used two different CPPs (based on physicochemical properties) and investigated the influence of insertion of CPP to Tf-liposomes on biocompatibility, cellular uptake, and transport across the BBB both in vitro and in vivo. The biodistribution profile of Tf-CPP liposomes showed more than 10 and 2.7 fold increase in doxorubicin and erlotinib accumulation in mice brain, respectively as compared to free drugs with no signs of toxicity.

Nanoparticle formulations in the diagnosis and therapy of Alzheimer's disease.

Alzheimer's disease (AD) is one of the most common age-related diseases that occurs because of the deposition of amyloid fibrils in a form of extracellular plaques containing β-amyloid peptide (Aβ) and tangles are found as intracellular deposit in the brain made up of twisted strands of aggregated microtubule binding protein. Scores of small molecule inhibitors have been designed for the treatment of AD. However some of these drugs cannot pass through the brain-blood-barrier (BBB). To overcome this problem, various nanoparticles (NPs) or nanomedicines (NMs) have been synthesized. These nanoparticles exploit the existing physiological mechanisms of passing through the BBB, including receptor- and adsorptive-mediated transcytosis that facilitate the transcellular transport of nanoparticle from the blood to the brain. During the last decades, varieties of nanoparticles that differ in the composition have been developed, and they have the potential application in the diagnostics and therapy of AD. The most common NP formulations that have major impact in the diagnosis and therapy of AD include polymeric NPs (PPs), gold NPs, gadolinium NPs, selenium NPs, protein-based NPs, polysaccharide-based NPs, etc. The goal of this review is to provide discussion of the application of different types of NP formulations in the diagnosis and therapy of AD.

Cationization increases brain distribution of an amyloid-beta protofibril selective F(ab’)2 fragment

Antibodies and fragments thereof are, because of high selectivity for their targets, considered as potential therapeutics and biomarkers for several neurological disorders. However, due to their large molecular size, antibodies/fragments do not easily penetrate into the brain. The aim of the present study was to improve the brain distribution via adsorptive-mediated transcytosis of an amyloid-beta (Aβ) protofibril selective F(ab’)2 fragment (F(ab’)2-h158). F(ab’)2-h158 was cationized to different extents and the specific and unspecific binding was studied in vitro. Next, cationized F(ab’)2-h158 was labelled with iodine-125 and its brain distribution and pharmacokinetics was studied in mice. Cationization did not alter the in vitro affinity to Aβ protofibrils, but increased the unspecific binding somewhat. Ex vivo experiments revealed a doubling of brain concentrations compared with unmodified F(ab’)2-h158 and in vivo imaging with single photon emission computed tomography (SPECT) showed that the cationized F(ab’)2-h158, but not the unmodified F(ab’)2-h158 could be visualized in the brain. To conclude, cationization is a means to increase brain concentrations of therapeutic antibodies or fragments and may facilitate the use of antibodies/fragments as imaging biomarkers in the brain.

Transmission of \u03b1-synuclein-containing erythrocyte-derived extracellular vesicles across the blood-brain barrier via adsorptive mediated transcytosis: another mechanism for initiation and progression of Parkinson\u2019s disease?

Parkinson’s disease (PD) pathophysiology develops in part from the formation, transmission, and aggregation of toxic species of the protein α-synuclein (α-syn). Recent evidence suggests that extracellular vesicles (EVs) may play a vital role in the transport of toxic α-syn between brain regions. Moreover, increasing evidence has highlighted the participation of peripheral molecules, particularly inflammatory species, which may influence or exacerbate the development of PD-related changes to the central nervous system (CNS), although detailed characterization of these species remains to be completed. Despite these findings, little attention has been devoted to erythrocytes, which contain α-syn concentrations ~1000-fold higher than the cerebrospinal fluid, as a source of potentially pathogenic α-syn. Here, we demonstrate that erythrocytes produce α-syn-rich EVs, which can cross the BBB, particularly under inflammatory conditions provoked by peripheral administration of lipopolysaccharide. This transport likely occurs via adsorptive-mediated transcytosis, with EVs that transit the BBB co-localizing with brain microglia. Examination of microglial reactivity upon exposure to α-syn-containing erythrocyte EVs in vitro and in vivo revealed that uptake provoked an increase in microglial inflammatory responses. EVs derived from the erythrocytes of PD patients elicited stronger responses than did those of control subjects, suggesting that inherent characteristics of EVs arising in the periphery might contribute to, or even initiate, CNS α-syn-related pathology. These results provide new insight into the mechanisms by which the brain and periphery communicate throughout the process of synucleinopathy pathogenesis.

Crossing the Blood-Brain Barrier: Recent Advances in Drug Delivery to the Brain.

CNS disorders are on the rise despite advancements in our understanding of their pathophysiological mechanisms. A major hurdle to the treatment of these disorders is the blood-brain barrier (BBB), which serves as an arduous janitor to protect the brain. Many drugs are being discovered for CNS disorders, which, however fail to enter the market because of their inability to cross the BBB. This is a pronounced challenge for the pharmaceutical fraternity. Hence, in addition to the discovery of novel entities and drug candidates, scientists are also developing new formulations of existing drugs for brain targeting. Several approaches have been investigated to allow therapeutics to cross the BBB. As the molecular structure of the BBB is better elucidated, several key approaches for brain targeting include physiological transport mechanisms such as adsorptive-mediated transcytosis, inhibition of active efflux pumps, receptor-mediated transport, cell-mediated endocytosis, and the use of peptide vectors. Drug-delivery approaches comprise delivery from microspheres, biodegradable wafers, and colloidal drug-carrier systems (e.g., liposomes, nanoparticles, nanogels, dendrimers, micelles, nanoemulsions, polymersomes, exosomes, and quantum dots). The current review discusses the latest advancements in these approaches, with a major focus on articles published in 2015 and 2016. In addition, we also cover the alternative delivery routes, such as intranasal and convection-enhanced diffusion methods, and disruption of the BBB for brain targeting.

Cationic albumin-conjugated chelating agent as a novel brain drug delivery system in neurodegeneration.

The critical role of metal ions and in particular iron in oxidative stress and protein aggregation offers chelation therapy as a sensible pharmaceutical strategy in oxidative stress-induced neuronal damages. In this research, we conjugated an iron-chelating agent, deferasirox, to cationized human serum albumin molecules in order to develop a novel brain delivery system for the management of neurodegenerative disorders due to the significant role of oxidative stress-induced neuronal injury in such diseases. Cationized albumin is known to be able to transport to brain tissue via adsorptive-mediated transcytosis. The developed structures were molecularly characterized, and their conjugation ratio was determined. PC12 cell line was utilized to evaluate the neuroprotective features of these newly developed molecules in the presence of hydrogen peroxide neuronal damage and to identify the mechanisms behind the observed neuronal protection including apoptotic and autophagic pathways. Furthermore, a rat model of Alzheimer's disease was utilized to evaluate the impact of conjugated structures invivo. Data analysis revealed that the conjugated species were able to hinder apoptotic cell death while enhancing autophagic process. The developed conjugated species were also able to attenuate amyloid beta-induced learning deficits when administered peripherally.

Adsorptive-Mediated Brain Delivery Systems

The blood-brain barrier (BBB), which impedes drug penetration into the central nervous system, is composed of specific structures formed by brain capillary endothelial cells and sheathed by astrocytic end-feet through basement membrane. Many brain drug delivery strategies have focused on adsorptive-mediated transcytosis (AMT), which is triggered by electrostatic interaction between cationic molecules and anionic microdomains on the cytoplasm membrane of the brain capillary endothelial cells. AMT-based drug delivery to the brain can be achieved by using cationic proteins and basic oligopeptides such as cell-penetrating peptides as targetors. Large therapeutic molecules such as neuropeptides and proteins or even drug-encapsulated vectors such as liposomes and nanoparticles can be allowed to access brain parenchyma through AMT when conjugated with these cationic targetors. In this review, I briefly discuss adsorptive-mediated brain delivery systems that may provide physiologic-based strategies for enhanced delivery of therapeutic substances through the BBB.

In Vivo Biodistribution of Prion- and GM1-Targeted Polymersomes following Intravenous Administration in Mice

Due to the aging of the population, the incidence of neurodegenerative diseases, such as Parkinson's and Alzheimer's, is expected to grow and, hence, the demand for adequate treatment modalities. However, the delivery of medicines into the brain for the treatment of brain-related diseases is hampered by the presence of a tight layer of endothelial cells that forms the blood-brain barrier (BBB). Furthermore, most conventional drugs lack stability and/or bioavailability. These obstacles can be overcome by the application of nanocarriers, in which the therapeutic entity has been incorporated, provided that they are effectively targeted to the brain endothelial cell layer. Drug nanocarriers decorated with targeting ligands that bind BBB receptors may accumulate efficiently at/in brain microvascular endothelium and hence represent a promising tool for brain drug delivery. Following the accumulation of drug nanocarriers at the brain vasculature, the drug needs to be transported across the brain endothelial cells into the brain. Transport across brain endothelial cells can occur via passive diffusion, transport proteins, and the vesicular transport pathways of receptor-mediated and adsorptive-mediated transcytosis. When a small lipophilic drug is released from its carrier at the brain vasculature, it may enter the brain via passive diffusion. On the other hand, the passage of intact nanocarriers, which is necessary for the delivery of larger and more hydrophilic drugs into brain, may occur via active transport by means of transcytosis. In previous work we identified GM1 ganglioside and prion protein as potential transcytotic receptors at the BBB. GM1 is a glycosphingolipid that is ubiquitously present on the endothelial surface and capable of acting as the transcytotic receptor for cholera toxin B. Likewise, prion protein has been shown to have transcytotic capacity at brain endothelial cells. Here we determine the transcytotic potential of polymersome nanocarriers functionalized with GM1- and prion-targeting peptides (G23, P50 and P9), that were identified by phage display, in an in vitro BBB model. In addition, the biodistribution of polymersomes functionalized with either the prion-targeting peptide P50 or the GM1-targeting peptide G23 is determined following intravenous injection in mice. We show that the prion-targeting peptides do not induce efficient transcytosis of polymersomes across the BBB in vitro nor induce accumulation of polymersomes in the brain in vivo. In contrast, the G23 peptide is shown to have transcytotic capacity in brain endothelial cells in vitro, as well as a brain-targeting potential in vivo, as reflected by the accumulation of G23-polymersomes in the brain in vivo at a level comparable to that of RI7217-polymersomes, which are targeted toward the transferrin receptor. Thus the G23 peptide seems to serve both of the requirements that are needed for efficient brain drug delivery of nanocarriers. An unexpected finding was the efficient accumulation of G23-polymersomes in lung. In conclusion, because of its combined brain-targeting and transcytotic capacity, the G23 peptide could be useful in the development of targeted nanocarriers for drug delivery into the brain, but appears especially attractive for specific drug delivery to the lung.

Transport of Alzheimer Disease Amyloid-β-Binding d-Amino Acid Peptides across an In Vitro Blood–Brain Barrier Model

Previously, two D-enantiomeric amino acid peptides, D1 and D3, which specifically bind to the amyloid-beta peptide Abeta(1-42), were identified by phage display selection. To assess the diagnostic and therapeutic potentials of D1 and D3 for the diagnosis and treatment of Alzheimer disease, the blood-brain barrier transport of these D-peptides was quantitatively evaluated in vitro. Our results showed that the apical-to-basolateral transport of D3 was more efficient than that of D1. An active efflux transport mechanism seems to oppose the transport of D1, whereas D3 is likely to be transported through the blood-brain barrier via an adsorptive-mediated transcytosis mechanism.

Structure and function of the blood-brain barrier

Structure and function of the blood-brain barrier