Transport Mechanisms at the Blood-Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

The role of oxidative stress in blood-brain barrier disruption during ischemic stroke: Antioxidants in clinical trials

Ischemic stroke is one of the leading causes of death and disability. Occlusion and reperfusion of cerebral blood vessels (i.e., ischemia/reperfusion (I/R) injury) generates reactive oxygen species (ROS) that contribute to brain cell death and dysfunction of the blood-brain barrier (BBB) via oxidative stress. BBB disruption influences the pathogenesis of ischemic stroke by contributing to cerebral edema, hemorrhagic transformation, and extravasation of circulating neurotoxic proteins. An improved understanding of mechanisms for ROS-associated alterations in BBB function during ischemia/reperfusion (I/R) injury can lead to improved treatment paradigms for ischemic stroke. Unfortunately, progress in developing ROS targeted therapeutics that are effective for stroke treatment has been slow. Here, we review how ROS are produced in response to I/R injury, their effects on BBB integrity (i.e., tight junction protein complexes, transporters), and the utilization of antioxidant treatments in ischemic stroke clinical trials. Overall, knowledge in this area provides a strong translational framework for discovery of novel drugs for stroke and/or improved strategies to mitigate I/R injury in stroke patients.

The Multifaceted Role of L-Type Amino Acid Transporter 1 at the Blood-Brain Barrier: Structural Implications and Therapeutic Potential

L-type amino acid transporter 1 (LAT1) is integral to the transport of large neutral amino acids across the blood-brain barrier (BBB), playing a crucial role in brain homeostasis and the delivery of therapeutic agents. This review explores the multifaceted role of LAT1 in neurological disorders, including its structural and functional aspects at the BBB. Studies using advanced BBB models, such as induced pluripotent stem cell (iPSC)-derived systems and quantitative proteomic analyses, have demonstrated LAT1's significant impact on drug permeability and transport efficiency. In Alzheimer's disease, LAT1-mediated delivery of anti-inflammatory and neuroprotective agents shows promise in overcoming BBB limitations. In Parkinson's disease, LAT1's role in transporting L-DOPA and other therapeutic agents highlights its potential in enhancing treatment efficacy. In phenylketonuria, studies have revealed polymorphisms and genetic variations of LAT1, which could be correlated to disease severity. Prodrugs of valproic acid, pregabalin, and gabapentin help use LAT1-mediated transport to increase the therapeutic activity and bioavailability of the prodrug in the brain. LAT1 has also been studied in neurodevelopment disorders like autism spectrum disorders and Rett syndrome, along with neuropsychiatric implications in depression. Its implications in neuro-oncology, especially in transporting therapeutic agents into cancer cells, show immense future potential. Phenotypes of LAT1 have also shown variations in the general population affecting their ability to respond to painkillers and anti-inflammatory drugs. Furthermore, LAT1-targeted approaches, such as functionalized nanoparticles and prodrugs, show promise in overcoming chemoresistance and enhancing drug delivery to the brain. The ongoing exploration of LAT1's structural characteristics and therapeutic applications reiterates its critical role in advancing treatments for neurological disorders.

Blood brain barrier dysfunction in healthy aging and dementia: Why, how, what for?

The blood brain barrier (BBB) is an indispensable structure that maintains the central nervous system (CNS) microenvironment for a correct neuronal function. It is composed by highly specialized microvessels, surrounded by astrocytes, pericytes, neurons and microglia cells, which tightly control the influx and efflux of substances to the brain parenchyma. During aging, the BBB becomes impaired, and it may contribute to the development of neurodegenerative and neurological disorders including Alzheimer's disease and other dementias. Restoring the BBB can be a strategy to prevent disease onset and development, reducing the symptoms of these conditions. This work critically reviews the major mechanisms underlying BBB breakdown in healthy and unhealthy aging, as well as biomarkers and methodologies that accurately assess its impairment. Complementarily, potential therapeutic targets are discussed as new strategies to restore the normal function of the BBB in aging.

CNSMolGen: A Bidirectional Recurrent Neural Network-Based Generative Model for De Novo Central Nervous System Drug Design

Central nervous system (CNS) drugs have had a significant impact on treating a wide range of neurodegenerative and psychiatric disorders. In recent years, deep learning-based generative models have shown great potential for accelerating drug discovery and improving efficacy. However, specific applications of these techniques in CNS drug discovery have not been widely reported. In this study, we developed the CNSMolGen model, which uses a framework of bidirectional recurrent neural networks (Bi-RNNs) for de novo molecular design of CNS drugs. Results showed that the pretrained model was able to generate more than 90% of completely new molecular structures, which possessed the properties of CNS drug molecules and were synthesizable. In addition, transfer learning was performed on small data sets with specific biological activities to evaluate the potential application of the model for CNS drug optimization. Here, we used drugs against the classical CNS disease target serotonin transporter (SERT) as a fine-tuned data set and generated a focused database against the target protein. The potential biological activities of the generated molecules were verified by using the physics-based induced-fit docking study. The success of this model demonstrates its potential in CNS drug design and optimization, which provides a new impetus for future CNS drug development.

CNS Viral Infections-What to Consider for Improving Drug Treatment: A Plea for Using Mathematical Modeling Approaches

Neurotropic viruses may cause meningitis, myelitis, encephalitis, or meningoencephalitis. These inflammatory conditions of the central nervous system (CNS) may have serious and devastating consequences if not treated adequately. In this review, we first summarize how neurotropic viruses can enter the CNS by (1) crossing the blood-brain barrier or blood-cerebrospinal fluid barrier; (2) invading the nose via the olfactory route; or (3) invading the peripheral nervous system. Neurotropic viruses may then enter the intracellular space of brain cells via endocytosis and/or membrane fusion. Antiviral drugs are currently used for different viral CNS infections, even though their use and dosing regimens within the CNS, with the exception of acyclovir, are minimally supported by clinical evidence. We therefore provide considerations to optimize drug treatment(s) for these neurotropic viruses. Antiviral drugs should cross the blood-brain barrier/blood cerebrospinal fluid barrier and pass the brain cellular membrane to inhibit these viruses inside the brain cells. Some antiviral drugs may also require intracellular conversion into their active metabolite(s). This illustrates the need to better understand these mechanisms because these processes dictate drug exposure within the CNS that ultimately determine the success of antiviral drugs for CNS infections. Finally, we discuss mathematical model-based approaches for optimizing antiviral treatments. Thereby emphasizing the potential of CNS physiologically based pharmacokinetic models because direct measurement of brain intracellular exposure in living humans faces ethical restrictions. Existing physiologically based pharmacokinetic models combined with in vitro pharmacokinetic/pharmacodynamic information can be used to predict drug exposure and evaluate efficacy of antiviral drugs within the CNS, to ultimately optimize the treatments of CNS viral infections.

Blood-brain barrier transporters: a translational consideration for CNS delivery of neurotherapeutics

INTRODUCTION

Successful neuropharmacology requires optimization of CNS drug delivery and, by extension, free drug concentrations at brain molecular targets. Detailed assessment of blood-brain barrier (BBB) physiological characteristics is necessary to achieve this goal. The 'next frontier' in CNS drug delivery is targeting BBB uptake transporters, an approach that requires evaluation of brain endothelial cell transport processes so that effective drug accumulation and improved therapeutic efficacy can occur.

AREAS COVERED

BBB permeability of drugs is governed by tight junction protein complexes (i.e., physical barrier) and transporters/enzymes (i.e., biochemical barrier). For most therapeutics, a component of blood-to-brain transport involves passive transcellular diffusion. Small molecule drugs that do not possess acceptable physicochemical characteristics for passive permeability may utilize putative membrane transporters for CNS uptake. While both uptake and efflux transport mechanisms are expressed at the brain microvascular endothelium, uptake transporters can be targeted for optimization of brain drug delivery and improved treatment of neurological disease states.

EXPERT OPINION

Uptake transporters represent a unique opportunity to optimize brain drug delivery by leveraging the endogenous biology of the BBB. A rigorous understanding of these transporters is required to improve translation from the bench to clinical trials and stimulate the development of new treatment paradigms for neurological diseases.

The Role of Immune Dysfunction in Parkinson's Disease Development

Recent research has unveiled intriguing insights suggesting that the body's immune system may be implicated in Parkinson's disease (PD) development. Studies have observed disparities in pro-inflammatory and anti-inflammatory markers between PD patients and healthy individuals. This finding underscores the potential influence of immune system dysfunction in the genesis of this condition. A dysfunctional immune system can serve as a primary catalyst for systemic inflammation in the body, which may contribute to the emergence of various brain disorders. The identification of several genes associated with PD, as well as their connection to neuroinflammation, raises the likelihood of disease susceptibility. Moreover, advancing age and mitochondrial dysfunction can weaken the immune system, potentially implicating them in the onset of the disease, particularly among older individuals. Compromised integrity of the blood-brain barrier could facilitate the immune system's access to brain tissue. This exposure may lead to encounters with native antigens or infections, potentially triggering an autoimmune response. Furthermore, there is mounting evidence supporting the notion that gut dysbiosis might represent an initial trigger for brain inflammation, ultimately promoting neurodegeneration. In this comprehensive review, we will delve into the numerous hypotheses surrounding the role of both innate and adaptive immunity in PD.

Multi-time point metabolomics reveals key metabolic features from the ultra-early stage of intracerebral hemorrhage in mice

Despite decades of intensive research, there are still very limited options for the effective treatment of intracerebral hemorrhage (ICH). Recently, mounting evidence has indicated that the ultra-early stage (<3 h), serving as the primary phase of ICH, plays a pivotal role and may even surpass other stages in terms of its significance. Therefore, uncovering the metabolic alterations induced by ICH in the ultra-early stage is of crucial importance. To investigate this, the collagenase ICH mouse model was employed in this study. ICH or sham-operated mice were euthanized at the ultra-early stage of 3 h and the acute stage of 24 h and 72 h after the operation. Then, the metabolic changes in the perihematomal tissues were detected by liquid chromatography coupled with tandem mass spectrometry. In total, alterations in the levels of 465 metabolites were detected. A total of 136 metabolites were significantly changed at 3 h. At 24 h and 72 h, the amounts were 132 and 126, respectively. Additionally, the key corresponding metabolic pathways for these time points were analyzed through KEGG. To gather additional information, quantitative real-time transcription polymerase chain reaction, enzyme-linked immunosorbent assay and Western blots were performed to validate the metabolic changes. Overall, ICH significantly alters important physiological functions such as cysteine metabolism, purine metabolism, synaptic alterations, the synaptic vesicle cycle, and the ATP-binding cassette transporter system. These might be the key pathologic mechanisms of the ultra-early stage induced by ICH.

Imaging the impact of blood-brain barrier disruption induced by focused ultrasound on P-glycoprotein function

The P-glycoprotein (P-gp/ABCB1) is a major efflux transporter which impedes the brain delivery of many drugs across the blood-brain barrier (BBB). Focused ultrasound with microbubbles (FUS) enables BBB disruption, which immediate and delayed impact on P-gp function remains unclear. Positron emission tomography (PET) imaging using the radiolabeled substrate [11C]metoclopramide provides a sensitive and translational method to study P-gp function at the living BBB. A FUS protocol was devised in rats to induce a substantial and targeted disruption of the BBB in the left hemisphere. BBB disruption was confirmed by the Evan's Blue extravasation test or the minimally-invasive contrast-enhanced MRI. The expression of P-gp was measured 24 h or 48 h after FUS using immunostaining and fluorescence microscopy. The brain kinetics of [11C]metoclopramide was studied by PET at baseline, and both immediately or 24 h after FUS, with or without half-maximum P-gp inhibition (tariquidar 1 mg/kg). In each condition (n = 4-5 rats per group), brain exposure of [11C]metoclopramide was estimated as the area-under-the-curve (AUC) in regions corresponding to the sonicated volume in the left hemisphere, and the contralateral volume. Kinetic modeling was performed to estimate the uptake clearance ratio (R1) of [11C]metoclopramide in the sonicated volume relative to the contralateral volume. In the absence of FUS, half-maximum P-gp inhibition increased brain exposure (+135.0 ± 12.9%, p < 0.05) but did not impact R1 (p > 0.05). Immediately after FUS, BBB integrity was selectively disrupted in the left hemisphere without any detectable impact on the brain kinetics of [11C]metoclopramide compared with the baseline group (p > 0.05) or the contralateral volume (p > 0.05). 24 h after FUS, BBB integrity was fully restored while P-gp expression was maximally down-regulated (-45.0 ± 4.5%, p < 0.001) in the sonicated volume. This neither impacted AUC nor R1 in the FUS + 24 h group (p > 0.05). Only when P-gp was inhibited with tariquidar were the brain exposure (+130 ± 70%) and R1(+29.1 ± 15.4%) significantly increased in the FUS + 24 h/tariquidar group, relative to the baseline group (p < 0.001). We conclude that the brain kinetics of [11C]metoclopramide specifically depends on P-gp function rather than BBB integrity. Delayed FUS-induced down-regulation of P-gp function can be detected. Our results suggest that almost complete down-regulation is required to substantially enhance the brain delivery of P-gp substrates.

The Rare Marine Bioactive Compounds in Neurological Disorders and Diseases: Is the Blood-Brain Barrier an Obstacle or a Target?

The blood-brain barrier (BBB) is a dynamic barrier separating neurocytes and brain tissues from blood that is extremely sealed and strictly regulated by transporters such as aquaporin-4 (AQP-4), glucose transporter (GLUT), and specialized tight junctional complexes (TJCs) including tight junctions (TJs), adherens junctions (AJs), and Zonulae occludens (ZOs). With specifically selective transcellular and paracellular permeability, the BBB maintains a homeostatic microenvironment to protect the central nervous system (CNS). In recent years, increasing attention has been paied to the importance of BBB disruption and dysfunction in the pathology of neurological disorders and diseases, such as Alzheimer's diseases (AD), Parkinson diseases (PD), stroke and cerebral edema. However, the further research on how the integral structure and function of BBB are altered under the physiological or pathological conditions is still needed. Focusing on the ultrastructural features of the BBB and combining the latest research on associated proteins and transporters, physiological regulation and pathological change of the BBB were elucidated. By summarizing the protective effects of known bioactive compounds derived from marine life on the BBB, this review aims to highlight the BBB as a key to the treatment of several major neurological diseases instead of a normally described obstacle to drug absorption and transport. Overall, the BBB's morphological characteristics and physiological function and their regulation provide the theoretical basis for the study on the BBB and inspire the diagnosis of and therapy for neurological diseases.

Towards Novel Biomimetic In Vitro Models of the Blood-Brain Barrier for Drug Permeability Evaluation

Current available animal and in vitro cell-based models for studying brain-related pathologies and drug evaluation face several limitations since they are unable to reproduce the unique architecture and physiology of the human blood-brain barrier. Because of that, promising preclinical drug candidates often fail in clinical trials due to their inability to penetrate the blood-brain barrier (BBB). Therefore, novel models that allow us to successfully predict drug permeability through the BBB would accelerate the implementation of much-needed therapies for glioblastoma, Alzheimer's disease, and further disorders. In line with this, organ-on-chip models of the BBB are an interesting alternative to traditional models. These microfluidic models provide the necessary support to recreate the architecture of the BBB and mimic the fluidic conditions of the cerebral microvasculature. Herein, the most recent advances in organ-on-chip models for the BBB are reviewed, focusing on their potential to provide robust and reliable data regarding drug candidate ability to reach the brain parenchyma. We point out recent achievements and challenges to overcome in order to advance in more biomimetic in vitro experimental models based on OOO technology. The minimum requirements that should be met to be considered biomimetic (cellular types, fluid flow, and tissular architecture), and consequently, a solid alternative to in vitro traditional models or animals.

Advanced Blood-Brain Barrier Drug Delivery

This Special Issue of Pharmaceutics, "Advanced Blood-Brain Barrier Drug Delivery," comprises 16 articles or reviews, which cover a cross-section of brain drug delivery for either small-molecule or large-molecule therapeutics [...].

New Strategies for Stroke Therapy: Nanoencapsulated Neuroglobin

Stroke is a global health and socio-economic problem. However, no efficient preventive and/or palliative treatments have yet been found. Neuroglobin (Ngb) is an endogen neuroprotective protein, but it only exerts its beneficial action against stroke after increasing its basal levels. Therefore, its systemic administration appears to be an efficient therapy applicable to stroke and other neurodegenerative pathologies. Unfortunately, Ngb cannot cross the blood-brain barrier (BBB), making its direct pharmacological use unfeasible. Thus, the association of Ngb with a drug delivery system (DDS), such as nanoparticles (NPs), appears to be a good strategy for overcoming this handicap. NPs are a type of DDS which efficiently transport Ngb and increase its bioavailability in the infarcted area. Hence, we previously built hyaluronate NPS linked to Ngb (Ngb-NPs) as a therapeutic tool against stroke. This nanoformulation induced an improvement of the cerebral infarct prognosis. However, this innovative therapy is still in development, and a more in-depth study focusing on its long-lasting neuroprotectant and neuroregenerative capabilities is needed. In short, this review aims to update the state-of-the-art of stroke therapies based on Ngb, paying special attention to the use of nanotechnological drug-delivering tools.