Functional Expression of P-glycoprotein and Organic Anion Transporting Polypeptides at the Blood-Brain Barrier: Understanding Transport Mechanisms for Improved CNS Drug Delivery?

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.

Molecular Mechanisms of Organic Anion Transporting Polypeptide-Mediated Organic Anion Clearance at the Blood-Cerebrospinal Fluid Barrier

The blood-cerebrospinal fluid barrier (BCSFB), formed by the choroid plexus epithelial (CPE) cells, plays an active role in removing drugs and metabolic wastes from the brain. Recent functional studies in isolated mouse choroid plexus (CP) tissues suggested the presence of organic anion transporting polypeptides (OATPs, encoded by SLCOs) at the apical membrane of BCSFB, which may clear large organic anions from the cerebrospinal fluid (CSF). However, the specific OATP isoform involved is unclear. Using quantitative fluorescence imaging, we showed that the fluorescent anions sulforhodamine 101 (SR101), fluorescein methotrexate (FL-MTX), and 8-fluorescein-cAMP (fluo-cAMP) are actively transported from the CSF to the subepithelial space in CP tissues isolated from wild-type mice. In contrast, transepithelial transport of these compounds across the CPE cells was abolished in Oatp1a/1b-/- mice due to impaired apical uptake. Using transporter-expressing cell lines, SR101, FL-MTX, and fluo-cAMP were additionally shown to be transported by mouse OATP1A5 and its human counterpart OATP1A2. Kinetic analysis showed that estrone-3-sulfate and SR101 are transported by OATP1A2 and OATP1A5 with similar Michaelis-Menten constants (Km). Immunofluorescence staining further revealed the presence of OATP1A2 protein in human CP tissues. Together, our results suggest that large organic anions in the CSF are actively transported into CPE cells by apical OATP1A2 (OATP1A5 in mice), then subsequently effluxed into the blood by basolateral multidrug resistance-associated proteins (MRPs). As OATP1A2 transports a wide array of endogenous compounds and xenobiotics, the presence of this transporter at the BCSFB may imply a novel clearance route for drugs and neurohormones from the CSF. SIGNIFICANCE STATEMENT: Drug transporters at the blood-cerebrospinal fluid (CSF) barrier play an important but understudied role in brain drug disposition. This study revealed a functional contribution of rodent organic anion transporting polypeptide (OATP) 1A5 towards the CSF clearance of organic anions and suggested a similar role for OATP1A2 in humans. Delineating the molecular mechanisms governing CSF organic anion clearance may help to improve the prediction of central nervous system (CNS) pharmacokinetics and identify drug candidates with favorable CNS pharmacokinetic properties.

New strategy for rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments

High-altitude hypoxic environments have critical implications on cardiovascular system function as well as blood pressure regulation. Such environments place patients with hypertension at risk by activating the sympathetic nervous system, which leads to an increase in blood pressure. In addition, the high-altitude hypoxic environment alters the in vivo metabolism and antihypertensive effects of antihypertensive drugs, which changes the activity and expression of drug-metabolizing enzymes and drug transporters. The present study reviewed the pharmacodynamics and pharmacokinetics of antihypertensive drugs and its effects on patients with hypertension in a high-altitude hypoxic environment. It also proposes a new strategy for the rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments. The increase in blood pressure on exposure to a high-altitude hypoxic environment was mainly dependent on increased sympathetic nervous system activity. Blood pressure also increased proportionally to altitude, whilst ambulatory blood pressure increased more than conventional blood pressure, especially at night. High-altitude hypoxia can reduce the activities and expression of drug-metabolizing enzymes, such as CYP1A1, CYP1A2, CYP3A1, and CYP2E1, while increasing those of CYP2D1, CYP2D6, and CYP3A6. Drug transporter changes were related to tissue type, hypoxic degree, and hypoxic exposure time. Furthermore, the effects of high-altitude hypoxia on drug-metabolism enzymes and transporters altered drug pharmacokinetics, causing changes in pharmacodynamic responses. These findings suggest that high-altitude hypoxic environments affect the blood pressure, pharmacokinetics, and pharmacodynamics of antihypertensive drugs. The optimal hypertension treatment plan and safe and effective medication strategy should be formulated considering high-altitude hypoxic environments.

Characterization of the Stereoselective Disposition of Bupropion and Its Metabolites in Rat Plasma and Brain

BACKGROUND AND OBJECTIVES

Bupropion is an atypical antidepressant and smoking cessation aid; its use is associated with wide intersubject variability in efficacy and safety. Knowledge of the brain pharmacokinetics of bupropion and its pharmacologically active metabolites is considered important for understanding the cause-effect relationships driving this variability.

METHODS

Brain concentrations from rats administered a 10 mg/kg subcutaneous dose of racemic bupropion were analyzed using a stereoselective LC/MS-MS method. A 2 mg/kg dose of (S,S)-hydroxybupropion, which has comparable pharmacologic potency to bupropion, was administered to a separate group of rats. Plasma exposure and unbound concentrations in both matrices from companion equilibrium dialysis experiments were determined to assess potential carrier-mediated transport at the blood-brain barrier.

RESULTS

Exposures to unbound forms of bupropion enantiomers were similar in plasma; this was also true in brain. This trend held for reductive diastereomer metabolite pairs in the two matrices. Unbound (R,R)-hydroxybupropion exposure was 1.5-fold higher than (S,S)-hydroxybupropion exposure in plasma and brain following bupropion administration. Unbound concentration ratios (K_p,uu) of a given molecular form decreased over time: between 4 and 6 h, these were < 1 for the two bupropion enantiomers, and they were ~ 1 for metabolites that formed. Administration of preformed (S,S)-hydroxybupropion also demonstrated a declining K_p,uu.

CONCLUSIONS

The temporal shift in K_p,uu among the different molecular forms provides evidence regarding the operation of carrier-mediated transport and/or within-brain metabolism of bupropion, and, thereby, fresh insight regarding the causes of intersubject variability in the safety and efficacy of bupropion therapy.

Relationship between blood-brain barrier changes and drug metabolism under high-altitude hypoxia: obstacle or opportunity for drug transport?

The blood-brain barrier is essential for maintaining the stability of the central nervous system and is also crucial for regulating drug metabolism, changes of blood-brain barrier's structure and function can influence how drugs are delivered to the brain. In high-altitude hypoxia, the central nervous system's function is drastically altered, which can cause disease and modify the metabolism of drugs in vivo. Changes in the structure and function of the blood-brain barrier and the transport of the drug across the blood-brain barrier under high-altitude hypoxia, are regulated by changes in brain microvascular endothelial cells, astrocytes, and pericytes, either regulated by drug metabolism factors such as drug transporters and drug-metabolizing enzymes. This article aims to review the effects of high-altitude hypoxia on the structure and function of the blood-brain barrier as well as the effects of changes in the blood-brain barrier on drug metabolism. We also hypothesized and explore the regulation and potential mechanisms of the blood-brain barrier and associated pathways, such as transcription factors, inflammatory factors, and nuclear receptors, in regulating drug transport under high-altitude hypoxia.

Regulation of P-Glycoprotein in the Brain

Maintenance of the tightly regulated homeostatic environment of the brain is facilitated by the blood-brain barrier (BBB). P-glycoprotein (P-gp), an ATP-binding cassette transporter, is expressed on the luminal surface of the endothelial cells in the BBB, and actively exports a wide variety of substrates to limit exposure of the vulnerable brain environment to waste buildup and neurotoxic compounds. Downregulation of P-gp expression and activity at the BBB have been reported with ageing and in neurodegenerative diseases. Upregulation of P-gp at the BBB contributes to poor therapeutic outcomes due to altered pharmacokinetics of CNS-acting drugs. The regulation of P-gp is highly complex, but unravelling the mechanisms involved may help the development of novel and nuanced strategies to modulate P-gp expression for therapeutic benefit. This review summarises the current understanding of P-gp regulation in the brain, encompassing the transcriptional, post-transcriptional and post-translational mechanisms that have been identified to affect P-gp expression and transport activity.

Delivering the Promise of Gene Therapy with Nanomedicines in Treating Central Nervous System Diseases

Central Nervous System (CNS) diseases, such as Alzheimer's diseases (AD), Parkinson's Diseases (PD), brain tumors, Huntington's disease (HD), and stroke, still remain difficult to treat by the conventional molecular drugs. In recent years, various gene therapies have come into the spotlight as versatile therapeutics providing the potential to prevent and treat these diseases. Despite the significant progress that has undoubtedly been achieved in terms of the design and modification of genetic modulators with desired potency and minimized unwanted immune responses, the efficient and safe in vivo delivery of gene therapies still poses major translational challenges. Various non-viral nanomedicines have been recently explored to circumvent this limitation. In this review, an overview of gene therapies for CNS diseases is provided and describes recent advances in the development of nanomedicines, including their unique characteristics, chemical modifications, bioconjugations, and the specific applications that those nanomedicines are harnessed to deliver gene therapies.

Need for a Paradigm Shift in the Treatment of Ischemic Stroke: The Blood-Brain Barrier

Blood-brain barrier (BBB) integrity is essential to maintaining brain health. Aging-related alterations could lead to chronic progressive leakiness of the BBB, which is directly correlated with cerebrovascular diseases. Indeed, the BBB breakdown during acute ischemic stroke is critical. It remains unclear, however, whether BBB dysfunction is one of the first events that leads to brain disease or a down-stream consequence. This review will focus on the BBB dysfunction associated with cerebrovascular disease. An added difficulty is its association with the deleterious or reparative effect, which depends on the stroke phase. We will first outline the BBB structure and function. Then, we will focus on the spatiotemporal chronic, slow, and progressive BBB alteration related to ischemic stroke. Finally, we will propose a new perspective on preventive therapeutic strategies associated with brain aging based on targeting specific components of the BBB. Understanding BBB age-evolutions will be beneficial for new drug development and the identification of the best performance window times. This could have a direct impact on clinical translation and personalised medicine.

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

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood–brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a “gateway” that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

Dysregulation of BMP, Wnt, and Insulin Signaling in Fragile X Syndrome

Drosophila models of neurological disease contribute tremendously to research progress due to the high conservation of human disease genes, the powerful and sophisticated genetic toolkit, and the rapid generation time. Fragile X syndrome (FXS) is the most prevalent heritable cause of intellectual disability and autism spectrum disorders, and the Drosophila FXS disease model has been critical for the genetic screening discovery of new intercellular secretion mechanisms. Here, we focus on the roles of three major signaling pathways: BMP, Wnt, and insulin-like peptides. We present Drosophila FXS model defects compared to mouse models in stem cells/embryos, the glutamatergic neuromuscular junction (NMJ) synapse model, and the developing adult brain. All three of these secreted signaling pathways are strikingly altered in FXS disease models, giving new mechanistic insights into impaired cellular outcomes and neurological phenotypes. Drosophila provides a powerful genetic screening platform to expand understanding of these secretory mechanisms and to test cellular roles in both peripheral and central nervous systems. The studies demonstrate the importance of exploring broad genetic interactions and unexpected regulatory mechanisms. We discuss a number of research avenues to pursue BMP, Wnt, and insulin signaling in future FXS investigations and the development of potential therapeutics.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Blood-Brain Barrier Transporters: Opportunities for Therapeutic Development in Ischemic Stroke

Globally, stroke is a leading cause of death and long-term disability. Over the past decades, several efforts have attempted to discover new drugs or repurpose existing therapeutics to promote post-stroke neurological recovery. Preclinical stroke studies have reported successes in identifying novel neuroprotective agents; however, none of these compounds have advanced beyond a phase III clinical trial. One reason for these failures is the lack of consideration of blood-brain barrier (BBB) transport mechanisms that can enable these drugs to achieve efficacious concentrations in ischemic brain tissue. Despite the knowledge that drugs with neuroprotective properties (i.e., statins, memantine, metformin) are substrates for endogenous BBB transporters, preclinical stroke research has not extensively studied the role of transporters in central nervous system (CNS) drug delivery. Here, we review current knowledge on specific BBB uptake transporters (i.e., organic anion transporting polypeptides (OATPs in humans; Oatps in rodents); organic cation transporters (OCTs in humans; Octs in rodents) that can be targeted for improved neuroprotective drug delivery. Additionally, we provide state-of-the-art perspectives on how transporter pharmacology can be integrated into preclinical stroke research. Specifically, we discuss the utility of in vivo stroke models to transporter studies and considerations (i.e., species selection, co-morbid conditions) that will optimize the translational success of stroke pharmacotherapeutic experiments.

Nanotechnology-Abetted Astaxanthin Formulations in Multimodel Therapeutic and Biomedical Applications

Astaxanthin (AXT) is one of the most important fat-soluble carotenoids that have abundant and diverse therapeutic applications namely in liver disease, cardiovascular disease, cancer treatment, protection of the nervous system, protection of the skin and eyes against UV radiation, and boosting the immune system. However, due to its intrinsic reactivity, it is chemically unstable, and therefore, the design and production processes for this compound need to be precisely formulated. Nanoencapsulation is widely applied to protect AXT against degradation during digestion and storage, thus improving its physicochemical properties and therapeutic effects. Nanocarriers are delivery systems with many advantages─ease of surface modification, biocompatibility, and targeted drug delivery and release. This review discusses the technological advancement in nanocarriers for the delivery of AXT through the brain, eyes, and skin, with emphasis on the benefits, limitations, and efficiency in practice.

Radiosynthesis and in Vivo and ex Vivo Evaluation of Isomeric [11C]methoxy Analogs of Nimesulide as Brain Cyclooxygenase-2-Targeted Imaging Agents

Our previous studies identified that nimesulide analogs which bear a methoxy substituent at the para-position of the phenyl ring could be potential radiotracer candidates for detecting disorders related to cyclooxygenase-2 (COX-2) expression and activity in vivo using positron emission tomography (PET) in the brain. The present study was conducted to evaluate the in vivo characteristics of ¹¹C-labeled para-methoxy nimesulide ([¹¹C]1d) as a brain COX-2-targeted imaging agent compared to other isomeric methoxy analogs of nimesulide ([¹¹C]1b and [¹¹C]1c). [¹¹C]1b-d were synthesized with reasonable yield and purity by the methylation of the O-desmethyl precursor with [¹¹C]methyl triflate in the presence of NaOH at room temperature. We performed in vivo biodistribution analysis, brain PET imaging, ex vivo autoradiography, and metabolite analysis in mice. The uptake of [¹¹C]1b-d was lower in the brain than in other tissues, including in the blood, and both [¹¹C]1c and [¹¹C]1d were rapidly metabolized. However, [¹¹C]1d showed a small, but significant, specific signal and heterogeneous distribution in the brain. In vivo evaluation suggested that [¹¹C]1d might correlate with COX-2 expression in the brain. Given its instability in vivo, [¹¹C]1d seems unsuitable as a brain-COX-2 radioimaging agent. Further structural refinement of these radiotracers is necessary to enhance their uptake in the brain and to achieve sufficient metabolic stability.

Blood-Brain Barrier Mechanisms in Stroke and Trauma

The brain microenvironment is tightly regulated. The blood-brain barrier (BBB), which is composed of cerebral endothelial cells, astrocytes, and pericytes, plays an important role in maintaining the brain homeostasis by regulating the transport of both beneficial and detrimental substances between circulating blood and brain parenchyma. After brain injury and disease, BBB tightness becomes dysregulated, thus leading to inflammation and secondary brain damage. In this chapter, we overview the fundamental mechanisms of BBB damage and repair after stroke and traumatic brain injury (TBI). Understanding these mechanisms may lead to therapeutic opportunities for brain injury.

Tailoring of P-glycoprotein for effective transportation of actives across blood-brain-barrier

P-Glycoprotein serves as a gatekeeper in the blood-brain-barrier wherein, it shows a vital part in the elimination of xenobiotics, drugs, foreign molecules etc. and guards the central nervous system from infections and external toxic molecules by functioning as an efflux transporter. It plays an essential role in various brain-related conditions like Parkinsonism, Alzheimer's disease, depression, cancer, etc. and terminates the entry of therapeutic agents across blood-brain-barrier which remains a significant challenge serving as major hindrance in pharmacotherapy of disease. The physiological structure and topology of P-glycoprotein and its relation with blood-brain-barrier and central nervous system gives an idea for targeting nanocarriers across the barrier into brain. This review article provides an overview of current understanding of the nanoformulations-based P-gp trafficking strategies like nanocarriers, stem cell therapy, drugs, substrates, polymeric materials, chemical compounds as well as naturally occurring active constituents for improving drug transport in brain across blood-brain-barrier and contributing in effective nanotherapeutic development for treatment of CNS disorders.

Disease-Induced Modulation of Drug Transporters at the Blood-Brain Barrier Level

The blood–brain barrier (BBB) is a highly selective and restrictive semipermeable network of cells and blood vessel constituents. All components of the neurovascular unit give to the BBB its crucial and protective function, i.e., to regulate homeostasis in the central nervous system (CNS) by removing substances from the endothelial compartment and supplying the brain with nutrients and other endogenous compounds. Many transporters have been identified that play a role in maintaining BBB integrity and homeostasis. As such, the restrictive nature of the BBB provides an obstacle for drug delivery to the CNS. Nevertheless, according to their physicochemical or pharmacological properties, drugs may reach the CNS by passive diffusion or be subjected to putative influx and/or efflux through BBB membrane transporters, allowing or limiting their distribution to the CNS. Drug transporters functionally expressed on various compartments of the BBB involve numerous proteins from either the ATP-binding cassette (ABC) or the solute carrier (SLC) superfamilies. Pathophysiological stressors, age, and age-associated disorders may alter the expression level and functionality of transporter protein elements that modulate drug distribution and accumulation into the brain, namely, drug efficacy and toxicity. This review focuses and sheds light on the influence of inflammatory conditions and diseases such as Alzheimer’s disease, epilepsy, and stroke on the expression and functionality of the BBB drug transporters, the consequential modulation of drug distribution to the brain, and their impact on drug efficacy and toxicity.

Optimization of dose and route of administration of the P-glycoprotein inhibitor, valspodar (PSC-833) and the P-glycoprotein and breast cancer resistance protein dual-inhibitor, elacridar (GF120918) as dual infusion in rats

Transporters can play a key role in the absorption, distribution, metabolism, and excretion of drugs. Understanding these contributions early in drug discovery allows for more accurate projection of the clinical pharmacokinetics. One method to assess the impact of transporters in vivo involves co‐dosing specific inhibitors. The objective of the present study was to optimize the dose and route of administration of a P‐glycoprotein (P‐gp) inhibitor, valspodar (PSC833), and a dual P‐gp/breast cancer resistance protein (BCRP) inhibitor, elacridar (GF120918), by assessing the transporters’ impact on brain penetration and absorption. A dual‐infusion strategy was implemented to allow for flexibility with dose formulation. The chemical inhibitor was dosed intravenously via the femoral artery, and a cassette of known substrates was infused via the jugular vein. Valspodar or elacridar was administered as 4.5‐hour constant infusions over a range of doses. To assess the degree of inhibition, the resulting ratios of brain and plasma concentrations, Kp's, of the known substrates were compared to the vehicle control. These data demonstrated that doses greater than 0.9 mg/hr/kg valspodar and 8.9 mg/hr/kg elacridar were sufficient to inhibit P‐gp‐ and BCRP‐mediated efflux at the blood‐brain barrier in rats without any tolerability issues. Confirmation of BBB restriction by efflux transporters in preclinical species allows for subsequent prediction in humans based upon the proteomic expression at rodent and human BBB. Overall, the approach can also be applied to inhibition of efflux at other tissues (gut absorption, liver clearance) or can be extended to other transporters of interest using alternate inhibitors.

Pharmacokinetics profiles of polyphyllin II and polyphyllin VII in rats by liquid chromatography with tandem mass spectrometry

Polyphyllin II (PII) and polyphyllin VII (PVII) are the main active ingredients in Paris Polyphylla with an excellent antitumor effect in vitro and in vivo. In this study, a rapid and precise LC-MS/MS method was developed and validated for the separation and simultaneous determination of PII and PVII in rat plasma, tissues, feces and urine using ginsenoside Rg3 as the internal standard. Positive linearity ranged from 1 to 1,000 ng/ml in samples. At the same time, intra- and inter-day precisions were in range of 1.8-12.0%. The accuracy ranged from 95.9 to 100.8%. Mean extraction recoveries of PII and PVII ranged from 86.6 to 96.4%. The analytical method has been successfully applied to the pharmacokinetic studies of PII and PVII in rats after their i.v. administration. After entering systemic circulation, PII and PVII were rapidly distributed in organs, mainly including liver, lung and spleen. Their elimination rate was slow. All of these data provided a theoretical basis for the application of PII and PVII in the treatment of liver- and lung-related diseases.

Basic physiology of the blood-brain barrier in health and disease: a brief overview

The blood-brain barrier (BBB), a dynamic interface between blood and brain constituted mainly by endothelial cells of brain microvessels, robustly restricts the entry of potentially harmful blood-sourced substances and cells into the brain, however, many therapeutically active agents concurrently cannot gain access into the brain at effective doses in the presence of an intact barrier. On the other hand, breakdown of BBB integrity may involve in the pathogenesis of various neurodegenerative diseases. Besides, certain diseases/disorders such as Alzheimer's disease, hypertension, and epilepsy are associated with varying degrees of BBB disruption. In this review, we aim to highlight the current knowledge on the cellular and molecular composition of the BBB with special emphasis on the major transport pathways across the barrier type endothelial cells. We further provide a discussion on the innovative brain drug delivery strategies in which the obstacle formed by BBB interferes with effective pharmacological treatment of neurodegenerative diseases/disorders.

Organic Cation Transporter (OCT/OCTN) Expression at Brain Barrier Sites: Focus on CNS Drug Delivery

Therapeutic delivery to the central nervous system (CNS) continues to be a considerable challenge in the pharmacological treatment and management of neurological disorders. This is primarily due to the physiological and biochemical characteristics of brain barrier sites (i.e., blood-brain barrier (BBB), blood-cerebrospinal fluid barrier (BCSFB)). Drug uptake into brain tissue is highly restricted by expression of tight junction protein complexes and adherens junctions between brain microvascular endothelial cells and choroid plexus epithelial cells. Additionally, efflux transport proteins expressed at the plasma membrane of these same endothelial and epithelial cells act to limit CNS concentrations of centrally acting drugs. In contrast, facilitated diffusion via transporter proteins allows for substrate-specific flux of molecules across the plasma membrane, directing drug uptake into the CNS. Organic Cation Transporters (OCTs) and Novel Organic Cation Transporters (OCTNs) are two subfamilies of the solute carrier 22 (SLC22) family of proteins that have significant potential to mediate delivery of positively charged, zwitterionic, and uncharged therapeutics. While expression of these transporters has been well characterized in peripheral tissues, the functional expression of OCT and OCTN transporters at CNS barrier sites and their role in delivery of therapeutic drugs to molecular targets in the brain require more detailed analysis. In this chapter, we will review current knowledge on localization, function, and regulation of OCT and OCTN isoforms at the BBB and BCSFB with a particular emphasis on how these transporters can be utilized for CNS delivery of therapeutic agents.

Peripheral inflammation and blood-brain barrier disruption: effects and mechanisms

The blood-brain barrier (BBB) is an important physiological barrier that separates the central nervous system (CNS) from the peripheral circulation, which contains inflammatory mediators and immune cells. The BBB regulates cellular and molecular exchange between the blood vessels and brain parenchyma. Normal functioning of the BBB is crucial for the homeostasis and proper function of the brain. It has been demonstrated that peripheral inflammation can disrupt the BBB by various pathways, resulting in different CNS diseases. Recently, clinical research also showed CNS complications following SARS-CoV-2 infection and chimeric antigen receptor (CAR)-T cell therapy, which both lead to a cytokine storm in the circulation. Therefore, elucidation of the mechanisms underlying the BBB disruption induced by peripheral inflammation will provide an important basis for protecting the CNS in the context of exacerbated peripheral inflammatory diseases. In the present review, we first summarize the physiological properties of the BBB that makes the CNS an immune-privileged organ. We then discuss the relevance of peripheral inflammation-induced BBB disruption to various CNS diseases. Finally, we elaborate various factors and mechanisms of peripheral inflammation that disrupt the BBB.

Circadian regulation of transporter expression and implications for drug disposition

Introduction: Solute Carrier (SLC) and ATP-binding cassette (ABC) transporters expressed in the intestine, liver, and kidney determine the absorption, distribution, and excretion of drugs. In addition, most molecular and cellular processes show circadian rhythmicity controlled by circadian clocks that leads to diurnal variations in the pharmacokinetics and pharmacodynamics of many drugs and affects their therapeutic efficacy and toxicity.Area covered: This review provides an overview of the current knowledge on the circadian rhythmicity of drug transporters and the molecular mechanisms of their circadian control. Evidence for coupling drug transporters to circadian oscillators and the plausible candidates conveying circadian clock signals to target drug transporters, particularly transcription factors operating as the output of clock genes, is discussed.Expert opinion: The circadian machinery has been demonstrated to interact with the uptake and efflux of various drug transporters. The evidence supports the concept that diurnal changes that affect drug transporters may influence the pharmacokinetics of the drugs. However, more systematic studies are required to better define the timing of pharmacologically important drug transporter regulation and determine tissue- and sex-dependent differences. Finally, the transfer of knowledge based on the results and conclusions obtained primarily from animal models will require careful validation before it is applied to humans.

Regulation of blood-brain barrier integrity by microglia in health and disease: A therapeutic opportunity

The blood-brain barrier (BBB) is a critical regulator of CNS homeostasis. It possesses physical and biochemical characteristics (i.e. tight junction protein complexes, transporters) that are necessary for the BBB to perform this physiological role. Microvascular endothelial cells require support from astrocytes, pericytes, microglia, neurons, and constituents of the extracellular matrix. This intricate relationship implies the existence of a neurovascular unit (NVU). NVU cellular components can be activated in disease and contribute to dynamic remodeling of the BBB. This is especially true of microglia, the resident immune cells of the brain, which polarize into distinct proinflammatory (M1) or anti-inflammatory (M2) phenotypes. Current data indicate that M1 pro-inflammatory microglia contribute to BBB dysfunction and vascular "leak", while M2 anti-inflammatory microglia play a protective role at the BBB. Understanding biological mechanisms involved in microglia activation provides a unique opportunity to develop novel treatment approaches for neurological diseases. In this review, we highlight characteristics of M1 proinflammatory and M2 anti-inflammatory microglia and describe how these distinct phenotypes modulate BBB physiology. Additionally, we outline the role of other NVU cell types in regulating microglial activation and highlight how microglia can be targeted for treatment of disease with a focus on ischemic stroke and Alzheimer's disease.

P-glycoprotein Expression Is Upregulated in a Pre-Clinical Model of Traumatic Brain Injury

Athletes participating in contact sports are at risk for sustaining repeat mild traumatic brain injury (rmTBI). Unfortunately, no pharmacological treatment to lessen the pathophysiology of brain injury has received U.S. Food and Drug Administration (FDA) approval. One hurdle to overcome for potential candidate agents to reach effective therapeutic concentrations in the brain is the blood-brain barrier (BBB). Adenosine triphosphate (ATP)-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp), line the luminal membrane of the brain capillary endothelium facing the vascular space. Although these transporters serve to protect the central nervous system (CNS) from damage by effluxing neurotoxicants before they can reach the brain, they may also limit the accumulation of therapeutic drugs in the brain parenchyma. Thus, increased Pgp expression following brain injury may result in reduced brain availability of therapeutic agents. We therefore questioned if repeat concussive injury increases Pgp expression in the brain. To answer this question, we used a rodent model of repeat mild closed head injury (rmCHI) and examined the messenger RNA (mRN) and protein expression of both isoforms of rodent Pgp (Abcb1a and Abcb1b). Compared with sham-operated controls (n = 5), the mRNA levels of both Abcb1a and Abcb1b were found to be increased in the hippocampus at day 1 (n = 5) and at day 5 (n = 5) post-injury. Using a validated antibody, we show increased immunolabeling for Pgp in the dorsal cortex at day 5 and in the hippocampus at day 1 (n = 5) and at day 5 (n = 5) post-injury compared with sham controls (n = 6). Taken together, these results suggest that increased expression of Pgp after rmCHI may reduce the brain accumulation of therapeutic drugs that are Pgp substrates. It is plausible that including a Pgp inhibitor with a candidate therapeutic agent may be an effective approach to treat the pathophysiology of rmCHI.

Probing the drug delivery strategies in ischemic stroke therapy

Ischemic stroke, which is caused by a sudden clot in the blood vessels, may cause severe brain tissue damage and has become a leading cause of death globally. Currently, thrombolysis is the gold standard primary treatment of ischemic stroke in clinics. However, the short therapeutic window of opportunity limits thrombolysis utility. Secondary cerebral damage caused by stroke is also an urgent problem. In this review, we discuss the present methods of treating ischemic stroke in clinics and their limitations. Various new drug delivery strategies targeting ischemic stroke lesions have also been summarized, including pharmaceutical methods, diagnostic approaches and other routes. These strategies could change the pharmacokinetic behavior, improve targeted delivery or minimize side effects. A better understanding of the novel approaches utilized to facilitate drug delivery in ischemic stroke would improve outcomes.

Transport Properties of Statins by Organic Anion Transporting Polypeptide 1A2 and Regulation by Transforming Growth Factor-β Signaling in Human Endothelial Cells

Our in vivo rodent studies have shown that organic anion transporting polypeptide (Oatp) 1a4 is critical for blood-to-brain transport of statins, drugs that are effective neuroprotectants. Additionally, transforming growth factor-β (TGF-β) signaling via the activin receptor-like kinase 1 (ALK1) receptor regulates Oatp1a4 functional expression. The human ortholog of Oatp1a4 is OATP1A2. Therefore, the translational significance of our work requires demonstration that OATP1A2 can transport statins and is regulated by TGF-β/ALK1 signaling. Cellular uptake and monolayer permeability of atorvastatin, pravastatin, and rosuvastatin were investigated in vitro using human umbilical vein endothelial cells (HUVECs). Regulation of OATP1A2 by the TGF-β/ALK1 pathway was evaluated using bone morphogenetic protein 9 (BMP-9), a selective ALK1 agonist, and LDN193189, an ALK1 antagonist. We showed that statin accumulation in HUVECs requires OATP1A2-mediated uptake but is also affected by efflux transporters (i.e., P-glycoprotein, breast cancer resistance protein). Absorptive flux (i.e., apical-to-basolateral) for all statins was higher than secretory flux (i.e., basolateral-to-apical) and was decreased by an OATP inhibitor (i.e., estrone-3-sulfate). OATP1A2 protein expression, statin uptake, and cellular monolayer permeability were increased by BMP-9 treatment. This effect was attenuated in the presence of LDN193189. Apical-to-basolateral statin transport across human endothelial cellular monolayers requires functional expression of OATP1A2, which can be controlled by therapeutically targeting TGF-β/ALK1 signaling. Taken together with our previous work, the present data show that OATP-mediated drug transport is a critical mechanism in facilitating neuroprotective drug disposition across endothelial barriers of the blood-brain barrier. SIGNIFICANCE STATEMENT: Transporter data derived from rodent models requires validation in human models. Using human umbilical vein endothelial cells, this study has shown that statin transport is mediated by OATP1A2. Additionally, we demonstrated that OATP1A2 is regulated by transforming growth factor-β/activin receptor-like kinase 1 signaling. This work emphasizes the need to consider endothelial transporter kinetics and regulation during preclinical drug development. Furthermore, our forward-thinking approach can identify effective therapeutics for diseases for which drug development has been challenging (i.e., neurological diseases).

Novel Intrinsic Mechanisms of Active Drug Extrusion at the Blood-Brain Barrier: Potential Targets for Enhancing Drug Delivery to the Brain?

The blood-brain barrier (BBB) limits the pharmacotherapy of several brain disorders. In addition to the structural and metabolic characteristics of the BBB, the ATP-driven, drug efflux transporter P-glycoprotein (Pgp) is a selective gatekeeper of the BBB; thus, it is a primary hindrance to drug delivery into the brain. Here, we review the complex regulation of Pgp expression and functional activity at the BBB with an emphasis on recent studies from our laboratory. In addition to traditional processes such as transcriptional regulation and posttranscriptional or posttranslational modification of Pgp expression and functionality, novel mechanisms such as intra- and intercellular Pgp trafficking and intracellular Pgp-mediated lysosomal sequestration in BBB endothelial cells with subsequent disposal by blood neutrophils are discussed. These intrinsic mechanisms of active drug extrusion at the BBB are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the treatment of brain diseases and enhance drug delivery to the brain.

Structure, Function, and Regulation of the Blood-Brain Barrier Tight Junction in Central Nervous System Disorders

The blood-brain barrier (BBB) allows the brain to selectively import nutrients and energy critical to neuronal function while simultaneously excluding neurotoxic substances from the peripheral circulation. In contrast to the highly permeable vasculature present in most organs that reside outside of the central nervous system (CNS), the BBB exhibits a high transendothelial electrical resistance (TEER) along with a low rate of transcytosis and greatly restricted paracellular permeability. The property of low paracellular permeability is controlled by tight junction (TJ) protein complexes that seal the paracellular route between apposing brain microvascular endothelial cells. Although tight junction protein complexes are principal contributors to physical barrier properties, they are not static in nature. Rather, tight junction protein complexes are highly dynamic structures, where expression and/or localization of individual constituent proteins can be modified in response to pathophysiological stressors. These stressors induce modifications to tight junction protein complexes that involve de novo synthesis of new protein or discrete trafficking mechanisms. Such responsiveness of BBB tight junctions to diseases indicates that these protein complexes are critical for maintenance of CNS homeostasis. In fulfillment of this vital role, BBB tight junctions are also a major obstacle to therapeutic drug delivery to the brain. There is an opportunity to overcome this substantial obstacle and optimize neuropharmacology via acquisition of a detailed understanding of BBB tight junction structure, function, and regulation. In this review, we discuss physiological characteristics of tight junction protein complexes and how these properties regulate delivery of therapeutics to the CNS for treatment of neurological diseases. Specifically, we will discuss modulation of tight junction structure, function, and regulation both in the context of disease states and in the setting of pharmacotherapy. In particular, we will highlight how these properties can be potentially manipulated at the molecular level to increase CNS drug levels via paracellular transport to the brain.

A novel approach of targeting refractory epilepsy: Need of an hour

The refractory epilepsy adds to the global burden of epilepsy as about 25 % of all patients with epilepsy present drug-resistant epilepsy. The P-glycoprotein (P-gp) plays a vital role in the mechanism of resistance in epilepsy. The AED levels in the brain are regulated by the P-gp transport. The upregulation of P-gp results in low concentration of AEDs inside the brain parenchyma and thus leads to resistance. There are three main conditions which lead to decrease transport of AEDs in refractory epilepsy. First being AEDs as substrate of P-gp; secondly, the elevated expression of P-gp in patients with drug resistant epilepsy as compared to drug-responsive patients; thirdly, the low brain AED concentration in refractory epilepsy in comparison to drug-responsive epilepsy. Therefore, determination of P-gp substrate should be a criterion for the selection of new AED for management of refractory epilepsy. This review highlights various tools which help in identification of P-gp substrates and also illustrates a concept of using various novel non-P-gp substrates which can cross the blood brain barrier and leads to enhanced accumulation inside the brain. Hence, these non P-gp substrates can be used as an add on treatment for the management of resistant epilepsy.

Cannabinoids, Blood-Brain Barrier, and Brain Disposition

Potential therapeutic actions of the cannabinoids delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are based on their activity as analgesics, anti-emetics, anti-inflammatory agents, anti-seizure compounds. THC and CBD lipophilicity and their neurological actions makes them candidates as new medicinal approaches to treat central nervous system (CNS) diseases. However, they show differences about penetrability and disposition in the brain. The present article is an overview about THC and CBD crossing the blood-brain barrier (BBB) and their brain disposition. Several findings indicate that CBD can modify the deleterious effects on BBB caused by inflammatory cytokines and may play a pivotal role in ameliorating BBB dysfunction consequent to ischemia. Thus supporting the therapeutic potential of CBD for the treatment of ischemic and inflammatory diseases of CNS. Cannabinoids positive effects on cognitive function could be also considered through the aspect of protection of BBB cerebrovascular structure and function, indicating that they may purchase substantial benefits through the protection of BBB integrity. Delivery of these cannabinoids in the brain following different routes of administration (subcutaneous, oral, and pulmonary) is illustrated and commented. Finally, the potential role of cannabinoids in drug-resistance in the clinical management of neurological or psychiatric diseases such as epilepsy and schizophrenia is discussed on the light of their crossing the BBB.

Alteration in the Function and Expression of SLC and ABC Transporters in the Neurovascular Unit in Alzheimer's Disease and the Clinical Significance

The neurovascular unit (NVU) plays an important role in maintaining the function of the central nervous system (CNS). Emerging evidence has indicated that the NVU changes function and molecules at the early stage of Alzheimer’s disease (AD), which initiates multiple pathways of neurodegeneration. Cell types in the NVU have become attractive targets in the interventional treatment of AD. The NVU transportation system contains a variety of proteins involved in compound transport and neurotransmission. Brain transporters can be classified as members of the solute carrier (SLC) and ATP-binding cassette (ABC) families in the NVU. Moreover, the transporters can regulate both endogenous toxins, including amyloid-beta (Aβ) and xenobiotic homeostasis, in the brains of AD patients. Genome-wide association studies (GWAS) have identified some transporter gene variants as susceptibility loci for late-onset AD. Therefore, the present study summarizes changes in blood-brain barrier (BBB) permeability in AD, identifies the location of SLC and ABC transporters in the brain and focuses on major SLC and ABC transporters that contribute to AD pathology.

Transporter-Mediated Delivery of Small Molecule Drugs to the Brain: A Critical Mechanism That Can Advance Therapeutic Development for Ischemic Stroke

Ischemic stroke is the 5th leading cause of death in the United States. Despite significant improvements in reperfusion therapies, stroke patients still suffer from debilitating neurocognitive deficits. This indicates an essential need to develop novel stroke treatment paradigms. Endogenous uptake transporters expressed at the blood-brain barrier (BBB) provide an excellent opportunity to advance stroke therapy via optimization of small molecule neuroprotective drug delivery to the brain. Examples of such uptake transporters include organic anion transporting polypeptides (OATPs in humans; Oatps in rodents) and organic cation transporters (OCTs in humans; Octs in rodents). Of particular note, small molecule drugs that have neuroprotective properties are known substrates for these transporters and include 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (i.e., statins) for OATPs/Oatps and 1-amino-3,5-dimethyladamantane (i.e., memantine) for OCTs/Octs. Here, we review current knowledge on specific BBB transporters that can be targeted for improvement of ischemic stroke treatment and provide state-of-the-art perspectives on the rationale for considering BBB transport properties during discovery/development of stroke therapeutics.

Modulation of brain cation-Cl- cotransport via the SPAK kinase inhibitor ZT-1a

The SLC12A cation-Cl^- cotransporters (CCC), including NKCC1 and the KCCs, are important determinants of brain ionic homeostasis. SPAK kinase (STK39) is the CCC master regulator, which stimulates NKCC1 ionic influx and inhibits KCC-mediated efflux via phosphorylation at conserved, shared motifs. Upregulation of SPAK-dependent CCC phosphorylation has been implicated in several neurological diseases. Using a scaffold-hybrid strategy, we develop a novel potent and selective SPAK inhibitor, 5-chloro-N-(5-chloro-4-((4-chlorophenyl)(cyano)methyl)-2-methylphenyl)-2-hydroxybenzamide ("ZT-1a"). ZT-1a inhibits NKCC1 and stimulates KCCs by decreasing their SPAK-dependent phosphorylation. Intracerebroventricular delivery of ZT-1a decreases inflammation-induced CCC phosphorylation in the choroid plexus and reduces cerebrospinal fluid (CSF) hypersecretion in a model of post-hemorrhagic hydrocephalus. Systemically administered ZT-1a reduces ischemia-induced CCC phosphorylation, attenuates cerebral edema, protects against brain damage, and improves outcomes in a model of stroke. These results suggest ZT-1a or related compounds may be effective CCC modulators with therapeutic potential for brain disorders associated with impaired ionic homeostasis.

In Vitro Models of the Blood-Brain Barrier

Knowledge about the transport of active compounds across the blood-brain barrier is of essential importance for drug development. Systemically applied drugs for the central nervous system (CNS) must be able to cross the blood-brain barrier in order to reach their target sites, whereas drugs that are supposed to act in the periphery should not permeate the blood-brain barrier so that they do not trigger any adverse central adverse effects. A number of approaches have been pursued, and manifold in silico, in vitro, and in vivo animal models were developed in order to be able to make a better prediction for humans about the possible penetration of active substances into the CNS. In this particular case, however, in vitro models play a special role, since the data basis for in silico models is usually in need of improvement, and the predictive power of in vivo animal models has to be checked for possible species differences. The blood-brain barrier is a dynamic, highly selective barrier formed by brain capillary endothelial cells. One of its main tasks is the maintenance of homeostasis in the CNS. The function of the barrier is regulated by cells of the microenvironment and the shear stress mediated by the blood flow, which makes the model development most complex. In general, one could follow the credo "as easy as possible, as complex as necessary" for the usage of in vitro BBB models for drug development. In addition to the description of the classical cell culture models (transwell, hollow fiber) and guidance how to apply them, the latest developments (spheroids, microfluidic models) will be introduced in this chapter, as it is attempted to get more in vivo-like and to be applicable for high-throughput usage with these models. Moreover, details about the development of models based on stem cells derived from different sources with a special focus on human induced pluripotent stem cells are presented.

The solute carrier transporters and the brain: Physiological and pharmacological implications

Solute carriers (SLCs) are the largest family of transmembrane transporters that determine the exchange of various substances, including nutrients, ions, metabolites, and drugs across biological membranes. To date, the presence of about 287 SLC genes have been identified in the brain, among which mutations or the resultant dysfunctions of 71 SLC genes have been reported to be correlated with human brain disorders. Although increasing interest in SLCs have focused on drug development, SLCs are currently still under-explored as drug targets, especially in the brain. We summarize the main substrates and functions of SLCs that are expressed in the brain, with an emphasis on selected SLCs that are important physiologically, pathologically, and pharmacologically in the blood-brain barrier, astrocytes, and neurons. Evidence suggests that a fraction of SLCs are regulated along with the occurrences of brain disorders, among which epilepsy, neurodegenerative diseases, and autism are representative. Given the review of SLCs involved in the onset and procession of brain disorders, we hope these SLCs will be screened as promising drug targets to improve drug delivery to the brain.

Brain Delivery of a Potent Opioid Receptor Agonist, Biphalin during Ischemic Stroke: Role of Organic Anion Transporting Polypeptide (OATP)

Transporters (expressed) at the blood-brain barrier (BBB) can play an essential role in the treatment of brain injury by transporting neuroprotective substance to the central nervous system. The goal of this study was to understand the role of organic anion transporting polypeptide (OATP1; OATP1A2 in humans and oatp1a4 in rodents) in the transport of a potent opioid receptor agonist, biphalin, across the BBB during ischemic stroke. Brain microvascular endothelial cells (BMECs) that were differentiated from human induced pluripotent stem cells (iPSCs) were used in the present study. The effect of oxygen-glucose deprivation (OGD) and reperfusion on the OATP1 expression, uptake, and transport of biphalin was measured in induced pluripotent stem cells differentiated brain microvascular endothelial cells (iPSC–BMECs) in the presence and absence of an OATP1 substrate, estrone-3-sulfate (E3S). Biphalin brain permeability was quantified while using a highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. It was found that iPSC-BMECs expressed OATP1. In vitro studies showed that biphalin BBB uptake and transport decreased in the presence of an OATP1 specific substrate. It was also observed that OGD and reperfusion modulate the expression and function of OATP1 in BMECs. This study strongly demonstrates that OATP1 contributes to the transport of biphalin across the BBB and increased expression of OATP1 during OGD-reperfusion could provide a novel target for improving ischemic brain drug delivery of biphalin or other potential neurotherapeutics that have affinity to this BBB transporter.

Organic Anion-Transporting Polypeptide 1a4 (Oatp1a4/Slco1a4) at the Blood-Arachnoid Barrier is the Major Pathway of Sulforhodamine-101 Clearance from Cerebrospinal Fluid of Rats

The blood-arachnoid barrier (BAB), which is formed by arachnoid epithelial cells linked by tight junctions, has generally been considered impermeable to water-soluble substances. However, we recently demonstrated that organic anion transporters 1 and 3 (Oat1 and Oat3) play roles in drug clearance at the BAB. Here, we examined whether an organic anion-transporting polypeptide (Oatp) also plays a role, using the fluorescent organic anion sulforhodamine-101 (SR-101) as a model substrate. SR-101 was injected into the cisterna magna of rats in order to minimize the contribution of choroid plexus transport. The in vivo cerebrospinal fluid (CSF) elimination clearance of SR-101 after intracisternal administration was ninefold greater than that of fluorescein-labeled inulin, a bulk flow marker. In the case of pre-administration of taurocholate, a broad-spectrum inhibitor of Oatps, or digoxin, a strong substrate/inhibitor for Oatp1a4 but not for Oatp1a1, Oat1, and Oat3, the CSF elimination of SR-101 was significantly reduced, becoming similar to that of inulin, and thus indicating complete inhibition of SR-101 clearance from the CSF. The distribution of SR-101 fluorescence was restricted to the arachnoid mater in the absence of inhibitor, whereas the fluorescence was increased in the parenchyma of the spinal cord after co-injection of taurocholate or digoxin. Immunostaining confirmed the localization of Oatp1a4 in the arachnoid mater. These results indicate that Oatp1a4 at the BAB acts as an avid clearance pathway of SR-101 in the CSF to the blood. Thus, Oatp1a4 appears to play a major role in CSF detoxification by limiting the distribution of organic anions to the brain and spinal cord.

Borneol for Regulating the Permeability of the Blood-Brain Barrier in Experimental Ischemic Stroke: Preclinical Evidence and Possible Mechanism

Borneol, a natural product in the Asteraceae family, is widely used as an upper ushering drug for various brain diseases in many Chinese herbal formulae. The blood-brain barrier (BBB) plays an essential role in maintaining a stable homeostatic environment, while BBB destruction and the increasing BBB permeability are common pathological processes in many serious central nervous system (CNS) diseases, which is especially an essential pathological basis of cerebral ischemic injury. Here, we aimed to conduct a systematic review to assess preclinical evidence of borneol for experimental ischemic stroke as well as investigate in the possible neuroprotective mechanisms, which mainly focused on regulating the permeability of BBB. Seven databases were searched from their inception to July 2018. The studies of borneol for ischemic stroke in animal models were included. RevMan 5.3 was applied for data analysis. Fifteen studies investigated the effects of borneol in experimental ischemic stroke involving 308 animals were ultimately identified. The present study showed that the administration of borneol exerted a significant decrease of BBB permeability during cerebral ischemic injury according to brain Evans blue content and brain water content compared with controls (P < 0.01). In addition, borneol could improve neurological function scores (NFS) and cerebral infarction area. Thus, borneol may be a promising neuroprotective agent for cerebral ischemic injury, largely through alleviating the BBB disruption, reducing oxidative reactions, inhibiting the occurrence of inflammation, inhibiting apoptosis, and improving the activity of lactate dehydrogenase (LDH) as well as P-glycoprotein (P-GP) and NO signaling pathway.

Contributions of Drug Transporters to Blood-Brain Barriers

Blood-brain interfaces comprise the cerebral microvessel endothelium forming the blood-brain barrier (BBB) and the epithelium of the choroid plexuses forming the blood-cerebrospinal fluid barrier (BCSFB). Their main functions are to impede free diffusion between brain fluids and blood; to provide transport processes for essential nutrients, ions, and metabolic waste products; and to regulate the homeostasis of central nervous system (CNS), all of which are attributed to absent fenestrations, high expression of tight junction proteins at cell-cell contacts, and expression of multiple transporters, receptors, and enzymes. Existence of BBB is an important reason that systemic drug administration is not suitable for the treatment of CNS diseases. Some diseases, such epilepsy, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and diabetes, alter BBB function via affecting tight junction proteins or altering expression and function of these transporters. This chapter will illustrate function of BBB, expression of transporters, as well as their alterations under disease status.

Functional Expression of Organic Anion Transporting Polypeptide 1a4 Is Regulated by Transforming Growth Factor-β/Activin Receptor-like Kinase 1 Signaling at the Blood-Brain Barrier

Central nervous system (CNS) drug delivery can be achieved by targeting drug uptake transporters such as Oatp1a4. In fact, many drugs that can improve neurologic outcomes in CNS diseases [3-hydroxy-3-methylglutaryl-CoA reductase inhibitors (i.e., statins)] are organic anion transporting polypeptide (OATP) transport substrates. To date, transport properties and regulatory mechanisms of Oatp1a4 at the blood-brain barrier (BBB) have not been rigorously studied. Such knowledge is critical to develop Oatp1a4 for optimization of CNS drug delivery and for improved treatment of neurological diseases. Our laboratory has demonstrated that the transforming growth factor-β (TGF-β)/activin receptor-like kinase 1 (ALK1) signaling agonist bone morphogenetic protein 9 (BMP-9) increases functional expression of Oatp1a4 in rat brain microvessels. Here, we expand on this work and show that BMP-9 treatment increases blood-to-brain transport and brain exposure of established OATP transport substrates (i.e., taurocholate, atorvastatin, and pravastatin). We also demonstrate that BMP-9 activates the TGF-β/ALK1 pathway in brain microvessels as indicated by increased nuclear translocation of specific Smad proteins associated with signaling mediated by the ALK1 receptor (i.e., pSmad1/5/8). Furthermore, we report that an activated Smad protein complex comprised of phosphorylated Smad1/5/8 and Smad4 is formed following BMP-9 treatment and binds to the promoter of the Slco1a4 gene (i.e., the gene that encodes Oatp1a4). This signaling mechanism causes increased expression of Slco1a4 mRNA. Overall, this study provides evidence that Oatp1a4 transport activity at the BBB is directly regulated by TGF-β/ALK1 signaling and indicates that this pathway can be targeted for control of CNS delivery of OATP substrate drugs.

Sex-specific differences in organic anion transporting polypeptide 1a4 (Oatp1a4) functional expression at the blood-brain barrier in Sprague-Dawley rats

Background

Targeting endogenous blood–brain barrier (BBB) transporters such as organic anion transporting polypeptide 1a4 (Oatp1a4) can facilitate drug delivery for treatment of neurological diseases. Advancement of Oatp targeting for optimization of CNS drug delivery requires characterization of sex-specific differences in BBB expression and/or activity of this transporter.

Methods

In this study, we investigated sex differences in Oatp1a4 functional expression at the BBB in adult and prepubertal (i.e., 6-week-old) Sprague–Dawley rats. We also performed castration or ovariectomy surgeries to assess the role of gonadal hormones on Oatp1a4 protein expression and transport activity at the BBB. Slco1a4 (i.e., the gene encoding Oatp1a4) mRNA expression and Oatp1a4 protein expression in brain microvessels was determined using quantitative real-time PCR and western blot analysis, respectively. Oatp transport function at the BBB was determined via in situ brain perfusion using [³H]taurocholate and [³H]atorvastatin as probe substrates. Data were expressed as mean ± SD and analyzed via one-way ANOVA followed by the post hoc Bonferroni t-test.

Results

Our results showed increased brain microvascular Slco1a4 mRNA and Oatp1a4 protein expression as well as increased brain uptake of [³H]taurocholate and [³H]atorvastatin in female rats as compared to males. Oatp1a4 expression at the BBB was enhanced in castrated male animals but was not affected by ovariectomy in female animals. In prepubertal rats, no sex-specific differences in brain microvascular Oatp1a4 expression were observed. Brain accumulation of [³H]taurocholate in male rats was increased following castration as compared to controls. In contrast, there was no difference in [³H]taurocholate brain uptake between ovariectomized and control female rats.

Conclusions

These novel data confirm sex-specific differences in BBB Oatp1a4 functional expression, findings that have profound implications for treatment of CNS diseases. Studies are ongoing to fully characterize molecular pathways that regulate sex differences in Oatp1a4 expression and activity.

Neurovascular unit transport responses to ischemia and common coexisting conditions: smoking and diabetes

Transporters at the neurovascular unit (NVU) are vital for the regulation of normal brain physiology via ion, water, and nutrients movement. In ischemic stroke, the reduction of cerebral blood flow causes several complex pathophysiological changes in the brain, one of which includes alterations of the NVU transporters, which can exacerbate stroke outcome by increased brain edema (by altering ion, water, and glutamate transporters), altered energy metabolism (by altering glucose transporters), and enhanced drug toxicity (by altering efflux transporters). Smoking and diabetes are common risk factors as well as coexisting conditions in ischemic stroke that are also reported to change the expression and function of NVU transporters. Coexistence of these conditions could cause an additive effect in terms of the alterations of brain transporters that might lead to worsened ischemic stroke prognosis and recovery. In this review, we have discussed the effects of ischemic stroke, smoking, and diabetes on some essential NVU transporters and how the simultaneous presence of these conditions can affect the clinical outcome after an ischemic episode. Further scientific investigations are required to elucidate changes in NVU transport in cerebral ischemia, which can lead to better, personalized therapeutic interventions tailor-made for these comorbid conditions.

Alterations in Expression and Function of ABC Family Transporters at Blood-Brain Barrier under Liver Failure and Their Clinical Significances

Liver failure is often associated with hepatic encephalopathy, due to dyshomeostasis of the central nervous system (CNS). Under physiological conditions, the CNS homeostasis is precisely regulated by the blood-brain barrier (BBB). The BBB consists of brain microvessel endothelial cells connected with a junctional complex by the adherens junctions and tight junctions. Its main function is to maintain brain homoeostasis via limiting the entry of drugs/toxins to brain. The brain microvessel endothelial cells are characterized by minimal pinocytotic activity, absent fenestrations, and highly expressions of ATP-binding cassette (ABC) family transporters (such as P-glycoprotein, breast cancer resistance protein and multidrug resistance-associated proteins). These ABC transporters prevent brain from toxin accumulation by pumping toxins out of brain. Accumulating evidences demonstrates that liver failure diseases altered the expression and function of ABC transporters at The BBB, indicating that the alterations subsequently affect drugs’ brain distribution and CNS activity/neurotoxicity. ABC transporters also mediate the transport of endogenous substrates across the BBB, inferring that ABC transporters are also implicated in some physiological processes and the development of hepatic encephalopathy. This paper focuses on the alteration in the BBB permeability, the expression and function of ABC transporters at the BBB under liver failure status and their clinical significances.