The role of drug transporters at the blood-brain barrier

Developing therapeutic antibodies for neurodegenerative disease

The central nervous system has been considered off-limits to antibody therapeutics. However, recent advances in preclinical and clinical drug development suggest that antibodies can cross the blood-brain barrier in limited quantities and act centrally to mediate their effects. In particular, immunotherapy for Alzheimer's disease has shown that targeting beta amyloid with antibodies can reduce pathology in both mouse models and the human brain, with strong evidence supporting a central mechanism of action. These findings have fueled substantial efforts to raise antibodies against other central nervous system targets, particularly neurodegenerative targets, such as tau, beta-secretase, and alpha-synuclein. Nevertheless, it is also apparent that antibody penetration across the blood-brain barrier is limited, with an estimated 0.1-0.2 % of circulating antibodies found in brain at steady-state concentrations. Thus, technologies designed to improve antibody uptake in brain are receiving increased attention and are likely going to represent the future of antibody therapy for neurologic diseases, if proven safe and effective. Herein we review briefly the progress and limitations of traditional antibody drug development for neurodegenerative diseases, with a focus on passive immunotherapy. We also take a more in-depth look at new technologies for improved delivery of antibodies to the brain.

Functional evidence for physiological mechanisms to circumvent neurotoxicity of cardenolides in an adapted and a non-adapted hawk-moth species

Because cardenolides specifically inhibit the Na(+)K(+)-ATPase, insects feeding on cardenolide-containing plants need to circumvent this toxic effect. Some insects such as the monarch butterfly rely on target site insensitivity, yet other cardenolide-adapted lepidopterans such as the oleander hawk-moth, Daphnis nerii, possess highly sensitive Na(+)K(+)-ATPases. Nevertheless, larvae of this species and the related Manduca sexta are insensitive to injected cardenolides. By radioactive-binding assays with nerve cords of both species, we demonstrate that the perineurium surrounding the nervous tissue functions as a diffusion barrier for a polar cardenolide (ouabain). By contrast, for non-polar cardenolides such as digoxin an active efflux carrier limits the access to the nerve cord. This barrier can be abolished by metabolic inhibitors and by verapamil, a specific inhibitor of P-glycoproteins (PGPs). This supports that a PGP-like transporter is involved in the active cardenolide-barrier of the perineurium. Tissue specific RT-PCR demonstrated expression of three PGP-like genes in hornworm nerve cords, and immunohistochemistry further corroborated PGP expression in the perineurium. Our results thus suggest that the lepidopteran perineurium serves as a diffusion barrier for polar cardenolides and provides an active barrier for non-polar cardenolides. This may explain the high in vivo resistance to cardenolides observed in some lepidopteran larvae, despite their highly sensitive Na(+)K(+)-ATPases.

A pregnancy physiologically based pharmacokinetic (p-PBPK) model for disposition of drugs metabolized by CYP1A2, CYP2D6 and CYP3A4

AIMS

Pregnant women are usually not part of the traditional drug development programme. Pregnancy is associated with major biological and physiological changes that alter the pharmacokinetics (PK) of drugs. Prediction of the changes to drug exposure in this group of patients may help to prevent under- or overtreatment. We have used a pregnancy physiologically based pharmacokinetic (p-PBPK) model to assess the likely impact of pregnancy on three model compounds, namely caffeine, metoprolol and midazolam, based on the knowledge of their disposition in nonpregnant women and information from in vitro studies.

METHODS

A perfusion-limited form of a 13-compartment full-PBPK model (Simcyp® Simulator) was used for the nonpregnant women, and this was extended to the pregnant state by applying known changes to all model components (including the gestational related activity of specific cytochrome P450 enzymes) and through the addition of an extra compartment to represent the fetoplacental unit. The uterus and the mammary glands were grouped into the muscle compartment. The model was implemented in Matlab Simulink and validated using clinical observations.

RESULTS

The p-PBPK model predicted the PK changes of three model compounds (namely caffeine, metoprolol and midazolam) for CYP1A2, CYP2D6 and CYP3A4 during pregnancy within twofold of observed values. The changes during the third trimester were predicted to be a 100% increase, a 30% decrease and a 35% decrease in the exposure of caffeine, metoprolol and midazolam, respectively, compared with the nonpregnant women.

CONCLUSIONS

In the absence of clinical data, the in silico prediction of PK behaviour during pregnancy can provide a valuable aid to dose adjustment in pregnant women. The performance of the model for drugs metabolized by a single enzyme to different degrees (high and low extraction) and for drugs that are eliminated by several different routes warrants further study.

The mastermind approach to CNS drug therapy: translational prediction of human brain distribution, target site kinetics, and therapeutic effects

Despite enormous advances in CNS research, CNS disorders remain the world's leading cause of disability. This accounts for more hospitalizations and prolonged care than almost all other diseases combined, and indicates a high unmet need for good CNS drugs and drug therapies.Following dosing, not only the chemical properties of the drug and blood-brain barrier (BBB) transport, but also many other processes will ultimately determine brain target site kinetics and consequently the CNS effects. The rate and extent of all these processes are regulated dynamically, and thus condition dependent. Therefore, heterogenious conditions such as species, gender, genetic background, tissue, age, diet, disease, drug treatment etc., result in considerable inter-individual and intra-individual variation, often encountered in CNS drug therapy.For effective therapy, drugs should access the CNS "at the right place, at the right time, and at the right concentration". To improve CNS therapies and drug development, details of inter-species and inter-condition variations are needed to enable target site pharmacokinetics and associated CNS effects to be translated between species and between disease states. Specifically, such studies need to include information about unbound drug concentrations which drive the effects. To date the only technique that can obtain unbound drug concentrations in brain is microdialysis. This (minimally) invasive technique cannot be readily applied to humans, and we need to rely on translational approaches to predict human brain distribution, target site kinetics, and therapeutic effects of CNS drugs.In this review the term "Mastermind approach" is introduced, for strategic and systematic CNS drug research using advanced preclinical experimental designs and mathematical modeling. In this way, knowledge can be obtained about the contributions and variability of individual processes on the causal path between drug dosing and CNS effect in animals that can be translated to the human situation. On the basis of a few advanced preclinical microdialysis based investigations it will be shown that the "Mastermind approach" has a high potential for the prediction of human CNS drug effects.

Functional inhibitors of acid sphingomyelinase (FIASMAs)

Sphingolipids are not only structural components of biological membranes, but also play an important role in cellular signalling and, thus, are involved in cell proliferation and differentiation but also stress and cell death. It is therefore of great clinical relevance to define inhibitors of the enzymes involved in sphingolipid metabolism. Here, we describe the state of the art of functional inhibitors of the acid sphingomyelinase. The acid sphingomyelinase converts sphingomyelin to ceramide, a compound often involved in cell stress. We describe the structural and physicochemical properties, the distribution, the pharmacokinetics, the pharmocodynamics and the clinical use of direct and functional inhibitors of the acid sphingomyelinase.

Role of ABC transporters in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common form of age-related dementia that begins with memory loss and progresses to include severe cognitive impairment. A major pathological hallmark of AD is the accumulation of beta amyloid peptide (Aβ) in senile plaques in the brain of AD patients. The exact mechanism by which AD takes place remains unknown. However, an increasing number of studies suggests that ATP-binding cassette (ABC) transporters, which are localized on the surface of brain endothelial cells of the blood-brain barrier (BBB) and brain parenchyma, may contribute to the pathogenesis of AD. Recent studies have unraveled important roles of ABC transporters including ABCB1 (P-glycoprotein, P-gp), ABCG2 (breast cancer resistant protein, BCRP), ABCC1 (multidrug resistance protein 1, MRP1), and the cholesterol transporter ABCA1 in the pathogenesis of AD and Aβ peptides deposition inside the brain. Therefore, understanding the mechanisms by which these transporters contribute to Aβ deposition in the brain is important for the development of new therapeutic strategies against AD. This review summarizes and highlights the accumulating evidence in the literature which describe the role of altered function of various ABC transporters in the pathogenesis and progression of AD and the implications of modulating their functions for the treatment of AD.

Drug penetration across the blood-brain barrier: an overview

The brain is one of the most protected organs in the body. There are two key barriers that control the access of endogenous substances and xenobiotics (drugs or toxins) to the CNS. These physiological structures are the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier. The BBB represents the main determinant of the effective delivery of drugs to the CNS. Good access through the BBB is essential if the target site is located within the CNS or, in contrast, can be a disadvantage if adverse reactions occur at central level. The development of new drugs targeted to the CNS requires a better knowledge of the factors affecting BBB permeation as well as in vitro and in silico predictive tools to optimize screening, and to reduce the attrition rate at later stages of drug development. This review discusses the particular characteristics of the biology and physiology of the BBB with respect to the permeation and distribution of drugs into the brain. The factors affecting rate, extent and distribution into the brain are discussed and a brief description of the in silico, in vitro, in situ and in vivo methods used to measure BBB transport are presented. Finally, the lastest proposals and strategies to enhance transport across the BBB of new CNS drugs are summarized.

Nanotechnological advances for the delivery of CNS therapeutics

Effective non-invasive treatment of neurological diseases is often limited by the poor access of therapeutic agents into the central nervous system (CNS). The majority of drugs and biotechnological agents do not readily permeate into brain parenchyma due to the presence of two anatomical and biochemical dynamic barriers: the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB). Therefore, one of the most significant challenges facing CNS drug development is the availability of effective brain targeting technology. Recent advances in nanotechnology have provided promising solutions to this challenge. Several nanocarriers ranging from the more established systems, e.g. polymeric nanoparticles, solid lipid nanoparticles, liposomes, micelles to the newer systems, e.g. dendrimers, nanogels, nanoemulsions and nanosuspensions have been studied for the delivery of CNS therapeutics. Many of these nanomedicines can be effectively transported across various in vitro and in vivo BBB models by endocytosis and/or transcytosis, and demonstrated early preclinical success for the management of CNS conditions such as brain tumors, HIV encephalopathy, Alzheimer's disease and acute ischemic stroke. Future development of CNS nanomedicines need to focus on increasing their drug-trafficking performance and specificity for brain tissue using novel targeting moieties, improving their BBB permeability and reducing their neurotoxicity.

Medicinal chemistry based approaches and nanotechnology-based systems to improve CNS drug targeting and delivery

The central nervous system (CNS) is protected by various barriers, which regulate nervous tissue homeostasis and control the selective and specific uptake, efflux, and metabolism of endogenous and exogenous molecules. Among these barriers is the blood-brain barrier (BBB), a physical and physiological barrier that filters very efficiently and selectively the entry of compounds from the blood to the brain and protects nervous tissue from harmful substances and infectious agents present in the bloodstream. The BBB also prevents the entry of potential drugs. As a result, various drug targeting and delivery strategies are currently being developed to enhance the transport of drugs from the blood to the brain. Following a general introduction, we briefly overview in this review article the fundamental physiological properties of the BBB. Then, we describe current strategies to bypass the BBB (i.e., invasive methods, alternative approaches, and temporary opening) and to cross it (i.e., noninvasive approaches). This section is followed by a chapter addressing the chemical and technological solutions developed to cross the BBB. A special emphasis is given to prodrug-targeting approaches and targeted nanotechnology-based systems, two promising strategies for BBB targeting and delivery of drugs to the brain.

Morphine metabolism, transport and brain disposition

The chemical structures of morphine and its metabolites are closely related to the clinical effects of drugs (analgesia and side-effects) and to their capability to cross the Blood Brain Barrier (BBB). Morphine-6-glucuronide (M6G) and Morphine-3-glucuronide (M3G) are both highly hydrophilic, but only M6G can penetrate the BBB; accordingly, M6G is considered a more attractive analgesic than the parent drug and the M3G. Several hypotheses have been made to explain these differences. In this review we will discuss recent advances in the field, considering brain disposition of M6G, UDP-glucoronosyltransferases (UGT) involved in morphine metabolism, UGT interindividual variability and transport proteins.

Qualitative prediction of blood-brain barrier permeability on a large and refined dataset

The prediction of blood-brain barrier permeation is vitally important for the optimization of drugs targeting the central nervous system as well as for avoiding side effects of peripheral drugs. Following a previously proposed model on blood-brain barrier penetration, we calculated the cross-sectional area perpendicular to the amphiphilic axis. We obtained a high correlation between calculated and experimental cross-sectional area (r = 0.898, n = 32). Based on these results, we examined a correlation of the calculated cross-sectional area with blood-brain barrier penetration given by logBB values. We combined various literature data sets to form a large-scale logBB dataset with 362 experimental logBB values. Quantitative models were calculated using bootstrap validated multiple linear regression. Qualitative models were built by a bootstrapped random forest algorithm. Both methods found similar descriptors such as polar surface area, pKa, logP, charges and number of positive ionisable groups to be predictive for logBB. In contrast to our initial assumption, we were not able to obtain models with the cross-sectional area chosen as relevant parameter for both approaches. Comparing those two different techniques, qualitative random forest models are better suited for blood-brain barrier permeability prediction, especially when reducing the number of descriptors and using a large dataset. A random forest prediction system (n(trees) = 5) based on only four descriptors yields a validated accuracy of 88%.

Preparation and characterization of liposomal formulations of neurotensin-degrading enzyme inhibitors

Neurotensin-degrading enzyme (NTDE) inhibitors hold great potential for treating psychotic disorders. However, brain uptake of such compounds in vivo is generally low due to the presence of the blood-brain barrier. In this study, liposomal formulations of two NTDE inhibitors, named compound 1 (C1) and compound 2 (C2) were prepared. Association of these compounds with the liposomal bilayer, subsequent liposomal stability, and compound release in the presence of albumin was studied. Entrapment of the compounds in the liposomal bilayer showed the solubilizing properties of the liposomes. Size and polydispersity index of the compound-entrapped liposomes did not change over 1 month, showing colloidal stability of the liposomal drug formulations. The amount of compounds associated with the liposomes decreased within one day. After this, the association remained stable at 4°C. For C1, association remained stable at 37°C in HEPES buffered saline, and the compound was gradually released in the presence of bovine serum albumin. For C2, the release was rapid in both HBS and BSA at 37°C. In conclusion, the formulation of NTDE inhibitors C1 and C2 in liposomes has been demonstrated and holds promise to deliver NTDE inhibitors in vivo.

Primary and secondary thyroid hormone transporters

Thyroid hormones (TH) are essential for the development of the human brain, growth and cellular metabolism. Investigation of TH transporters became one of the emerging fields in thyroid research after the discovery of inactivating mutations in the Monocarboxylate transporter 8 (MCT8), which was found to be highly specific for TH transport. However, additional transmembrane transporters are also very important for TH uptake and efflux in different cell types. They transport TH as secondary substrates and include the aromatic amino acid transporting MCT10, the organic anion transporting polypeptides (e.g. OATP1C1, OATP1A2, OPTP1A4) and the large neutral amino acid transporters (LAT1 and LAT2). These TH transporters characteristically possess 12 transmembrane spanners but due to the strong differing sequences between the three transporter families we assume an identical conformation is not very likely. In contrast to the others, the LAT family members form a heterodimer with the escort protein 4F2hc/CD98. A comparison of sequence proportions, locations and types of functional sensitive features for TH transport discovered by mutations, revealed that transport sensitive charged residues occur as conserved amino acids only within each family of the transporter types but not in all putative TH transporters. Based on the lack of highly conserved sensitive charged residues throughout the three transporter families as a common counterpart for the amino acid moiety of the substrates, we conclude that the molecular transport mechanism is likely organized either a) by different molecular determinants in the divergent transporter types or b) the counterparts for the substrates` amino acid moiety at the transporter are not any charged side chains but other proton acceptors or donators. However, positions of transport sensitive residues coincide at transmembrane helix 8 in the TH transporter MCT8, OATP1C1 and another amino acid transporter, the L-cystine and L-glutamate exchanger xCT, which is highly homologous to LAT1 and LAT2. Here we review the data available and compare similarities and differences between these primary and secondary TH transporters regarding sequences, topology, potential structures, trafficking to the plasma membrane, molecular features and locations of transport sensitive functionalities. Thereby, we focus on TH transporters occurring in the blood-brain barrier.

Absence of bioactivity of lipid derivatives of serotonin

BACKGROUND

Serotonin (5-HT) and its receptors are present in central, the brain stem, and peripheral, the carotid body, tissues controlling the ventilatory responses to hypoxia. The exact action of serotonin and its nature are, however, unsettled. We hypothesized that the discrepant results on the ventilatory action of serotonin could be caused by the inability of serotonin to penetrate into the brain or the plasma membrane lipid bilayers, the target site of signal transduction cascades, after its exogenous administration.

OBJECTIVE

To study the penetrability of novel lipid derivatives of serotonin of varying fatty acid chain length and number of saturated/unsaturated bonds, the oleic, caprylic, and caprolic amides of 5-HT, into the brain, and their functional effects on the hypoxic ventilatory response in awake rats after systemic administration.

MATERIAL AND METHODS

Adult Wistar rats were used for the experiments. In the biochemical part of the study, the presence and stability of the compounds tested, after i.p. injection, was assessed in brain extracts using spectrophotometry and thin-layered chromatography. In the functional part, the ventilatory responses to 8 and 12% hypoxia were compared before and 1 h after the compound administration using a whole body plethysmography.

RESULTS

The "lipidized" serotonin compounds turned out to be stable in brain extracts in vitro for up to 3 h of the test. However, we could not substantiate the presence of any of the compounds in the brain, with either method used, after i.p. administration. Likewise, none of the compounds had any appreciable effect on the profile of the stimulatory hypoxic ventilatory response.

CONCLUSIONS

Synthetically attaching lipophilic groups to the serotonin molecule does not make it penetrate into the brain. The lack of serotonin penetrability likely depends on the planarity of its molecule, as it does not seem to depend on the size, number of carbons or bond saturation of the "lipidized" molecules. Such molecules do not directly interfere with the carotid chemoreceptor-mediated hypoxic ventilatory response. The study failed to substantiate the bioactive potential of the lipid derivatives of serotonin.

Connecting vascular and nervous system development: angiogenesis and the blood-brain barrier

The vascular and nervous systems share a common necessity of circuit formation to coordinate nutrient and information transfer, respectively. Shared developmental principles have evolved to orchestrate the formation of both the vascular and the nervous systems. This evolution is highlighted by the identification of specific guidance cues that direct both systems to their target tissues. In addition to sharing cellular and molecular signaling events during development, the vascular and nervous systems also form an intricate interface within the central nervous system called the neurovascular unit. Understanding how the neurovascular unit develops and functions, and more specifically how the blood-brain barrier within this unit is established, is of utmost importance. We explore the history, recent discoveries, and unanswered questions surrounding the relationship between the vascular and nervous systems with a focus on developmental signaling cues that guide network formation and establish the interface between these two systems.

In vivo methods to study uptake of nanoparticles into the brain

Several in vivo techniques have been developed to study and measure the uptake of CNS compounds into the brain. With these techniques, various parameters can be determined after drug administration, including the blood-to-brain influx constant (K_in), the permeability-surface area (PS) product, and the brain uptake index (BUI). These techniques have been mostly used for drugs that are expected to enter the brain via transmembrane diffusion or by carrier-mediated transcytosis. Drugs that have limitations in entering the brain via such pathways have been encapsulated in nanoparticles (based on lipids or synthetic polymers) to enhance brain uptake. Nanoparticles are different from CNS compounds in size, composition and uptake mechanisms. This has led to different methods and approaches to study brain uptake in vivo. Here we discuss the techniques generally used to measure nanoparticle uptake in addition to the techniques used for CNS compounds. Techniques include visualization methods, behavioral tests, and quantitative methods.

Synthesis and biological evaluation of novel peripherally active morphiceptin analogs

Morphiceptin (Tyr-Pro-Phe-Pro-NH(2)), a tetrapeptide present in the enzymatic digest of bovine beta-casein, is a selective ligand of the mu-opioid receptor. In the present study, we describe the synthesis of a series of novel morphiceptin analogs modified in positions 1-3. Two of the obtained analogs, [Dmt(1), D-Ala(2), D-1-Nal(3)]morphiceptin and [Dmt(1), D-NMeAla(2), D-1-Nal(3)]morphiceptin (Dmt-2',6'-dimethyltyrosine and d-1-Nal-3-(1-naphthyl)-D-alanine)) displayed very high mu-receptor affinity, resistance to enzymatic degradation, and remarkable supraspinally mediated analgesia, as shown in the hot-plate test after intracerebroventricular but not intravenous administration, which indicated that they could not cross the blood-brain barrier. Therefore, these two analogs were further tested in vitro and in vivo towards their possible peripheral analgesic activity and inhibitory effect on gastrointestinal (GI) motility. We report that both peptides showed strong antinociceptive effect in the writhing test after intraperitoneal administration, inhibited smooth muscle contractility in vitro and GI motility in vivo. Taken together, these findings indicate that the novel morphiceptin analogs which induce peripheral, but not central antinociception, inhibit GI transit, and possess exceptional metabolic stability, may provide an interesting approach to the development of peripherally restricted agents for the treatment of GI motility disorders, such as diarrhea or diarrhea-predominant irritable bowel syndrome.

Analytical characterization and comparison of the blood-brain barrier permeability of eight opioid peptides

Opioid drugs, including the newly developed peptides, should penetrate the blood-brain barrier (BBB) for pain management activity. Although BBB transport is fragmentarily described for some mu-opioid peptides, a complete and comparative overview is currently lacking. In this study, the BBB transport of eight opioid peptides (EM-1, EM-2, CTAP, CTOP, DAMGO, dermorphin, TAPP and TAPS) is described and compared. In addition, the metabolic stability in plasma and brain was evaluated. The highest influx rate was obtained for dermorphin (K(in)=2.18 microl/(g x min)), followed by smaller rates for EM-1, EM-2 and TAPP (K(in)=1.06-1.14 microl/(g x min)). Negligible influx was observed for DAMGO, CTOP and TAPS (K(in)=0.18-0.40 microl/(g x min)) and no influx for CTAP. Capillary depletion revealed that all peptides reached brain parenchyma for over 75%. Efflux was shown for TAPP (t(1/2)=2.82 min) and to a lesser extent for EM-1, EM-2 and DAMGO (t(1/2)=10.66-21.98 min), while no significant efflux was observed for the other peptides. All peptides were stable in mouse plasma and brain, with generally higher stability in brain, except for EM-1 and EM-2 which showed plasma half-life stabilities of a few minutes only.

ABC transporters and drug efflux at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic physical and biological barrier between blood circulation and the central nervous system (CNS). This unique feature of the BBB lies in the structure of the neurovascular unit and its cerebral micro-vascular endothelial cells. The BBB restricts the passage of blood-borne drugs, neurotoxic substances and peripheral immune cells from entering the brain, while selectively facilitating the transport of nutrients across the BBB into the brain. Thus, the integrity and proper function of the BBB is crucial to homeostasis and physiological function of the CNS. A number of transport and carrier systems are expressed and polarized on the luminal or abluminal surface of the BBB to realize these discrete functions. Among these systems, ABC transporters play a critical role in keeping drugs and neurotoxic substances from entering the brain and in transporting toxic metabolites out of the brain. A number of studies have demonstrated that ABCB1 and ABCG2 are critical to drug efflux at the BBB and that ABCC1 is essential for the blood-cerebral spinal fluid (CSF) barrier. The presence of these efflux ABC transporters also creates a major obstacle for drug delivery into the brain. We have comprehensively reviewed the literature on ABC transporters and drug efflux at the BBB. Understanding the molecular mechanisms of these transporters is important in the development of new drugs and new strategies for drug delivery into the brain.

Role of human placental apical membrane transporters in the efflux of glyburide, rosiglitazone, and metformin

OBJECTIVE

Substrates of placental efflux transporters could compete for a single transporter, which could result in an increase in the transfer of each substrate to the fetal circulation. Our aim was to determine the role of placental transporters in the biodisposition of oral hypoglycemic drugs that could be used as monotherapy or in combination therapy for gestational diabetes.

STUDY DESIGN

Inside-out brush border membrane vesicles from term placentas were used to determine the efflux of glyburide, rosiglitazone, and metformin by P-glycoprotein, breast cancer resistance protein, and multidrug resistance protein.

RESULTS

Glyburide was transported by multidrug resistance protein (43 +/- 4%); breast cancer resistance protein (25 +/- 5%); and P-glycoprotein (9 +/- 5%). Rosiglitazone was transported predominantly by P-glycoprotein (71 +/- 26%). Metformin was transported by P-glycoprotein (58 +/- 20%) and breast cancer resistance protein (25 +/- 14%).

CONCLUSION

Multiple placental transporters contribute to efflux of glyburide, rosiglitazone, and metformin. Administration of drug combinations could lead to their competition for efflux transporters.

Mechanisms of the penetration of blood-borne substances into the brain

The blood-brain barrier (BBB) impedes the influx of intravascular compounds from the blood to the brain. Few blood-borne macromolecules are transferred into the brain because vesicular transcytosis in the endothelial cells is considerably limited and the tight junction is located between the endothelial cells. At the first line of the BBB, the endothelial glycocalyx which is a negatively charged, surface coat of proteoglycans, and adsorbed plasma proteins, contributes to the vasculoprotective effects of the vessels wall and are involved in maintaining vascular permeability. In the endothelial cytoplasm of cerebral capillaries, there is an asymmetrical array of metabolic enzymes such as alkaline phosphatase, acid phosphatase, 5'-nucleotidase, adenosine triphosphatase, and nucleoside diphosphatase and these enzymes contribute to inactivation of substrates. In addition, there are several types of influx or efflux transporters at the BBB, such as P-glycoprotein (P-gp), multidrug resistance associated protein, breast cancer resistance protein, organic anion transporters, organic cation transporters, organic cation transporter novel type transporters, and monocarboxylic acid transporters. P-gp, energy-dependent efflux transporter protein, is instrumental to the barrier function. Several findings recently reported indicate that endothelial P-gp contributes to efflux of undesirable substances such as beta-amyloid protein from the brain or periarterial interstitial fluid, while P-gp likely plays a crucial role in the genesis of multiple vascular abnormalities that accompany hypertension. In this review, influx and efflux mechanisms of drugs at the BBB are also reviewed and how medicines pass the BBB to reach the brain parenchyma is discussed.

Identification of peptide ligands for targeting to the blood-brain barrier

Purpose

Transport of drugs to the brain is limited by the blood-brain barrier. New, specific brain endothelium ligands can facilitate brain-specific delivery of drugs.

Methods

We used phage display in an in situ brain perfusion model to screen for new brain endothelium peptide ligands.

Results

Two phage clones, displaying 15 amino acid-peptides (GLA and GYR) that were selected for brain binding in the mouse model, showed significant binding to human brain endothelium (hCMEC/D3), compared to a random control phage. This binding was not seen for other human endothelial cells (HUVEC). Binding to hCMEC/D3 cells was dose dependent. When phage GLA and GYR were individually perfused through the murine brain, their ability to bind to the brain was 6-fold (GLA) and 5-fold (GYR) higher than the control phage. When compared to lung perfusion, phage showed an 8.5-fold (GYR) and 48-fold (GLA) preference for brain over lung compared to the control.

Conclusions

These results indicate that two new peptide ligands have been identified that may be used for specific targeting of drugs to the blood-brain barrier.

Comparison of two blood-brain barrier in vitro systems: cytotoxicity and transfer assessments of malathion/oxon and lead acetate

Toxicity and integrity disruption in response to transport through the blood-brain barrier (BBB) of the organophosphates malathion and malaoxon and heavy metal lead acetate were assessed in two in vitro barrier systems. One system was constructed using bovine brain microvascular endothelial cells (BMEC), while the other system was constructed with rat brain microvascular endothelial cells (RBE4); both were cocultured with rat astrocytes. We hypothesized that these models would respond differently to neurotoxic compounds. Concentrations of malathion, malaoxon, and lead acetate between 0.01 microM and 1 mM were assessed for their capacity to cause cytotoxicity to the astrocytes and endothelial cells utilized to construct the BBB systems, with the least cytotoxic concentrations chosen for transfer assessments of neurotoxicants through the barrier systems. Concentrations of malathion at 10 microM, malaoxon at 1 microM, and lead acetate at 1 and 10 microM were selected. Lead concentrations were measured in media of the abluminal and luminal sides of both systems using graphite furnace atomic absorption at the beginning of the treatment (T0) and 14 h later (T14). Passage of organophosphate compounds was determined utilizing inhibition of acetylcholinesterase enzyme in a neuroblastoma cell line (SH-SY5Y) localized below the barrier system. Transendothelial electrical resistance was assessed as a measurement of integrity of the barrier systems, with baseline values higher with the RBE4-astrocyte system than with the BMEC-astrocyte system. Metabolic capability, as measured by esterase activity, was higher in BMECs, which were more likely to retain lead than RBE4 cells. Results suggest that differences in endothelial cell source can affect the outcome of studies on toxicant transfer through in vitro BBB systems.

Role of membrane transporters in the safety profile of drugs

It has increasingly been recognized that few molecules move across the cell membrane without the assistance of transporter proteins. Large superfamilies of transporter proteins have been identified in every living cell, including microorganisms and mitochondria. This report reviews the role of transporters in physiology and pharmacology, and identifies where this may have an impact on drug efficacy and toxicity. This new understanding will require a fresh appreciation of pharmacokinetics and drug effects, as the current paradigms are based largely on the assumption that drug molecules have a reasonable unrestricted permeability across membranes. Rather than just focusing on clearance changes and central compartment pharmacokinetics, it will become increasingly necessary to examine the peripheral tissue distribution of drugs to more accurately predict drug efficacy and toxicity.

Drug metabolism and pharmacokinetics

In this article, aspects of absorption, distribution, metabolism, and excretion have been described bearing in mind the pathogenesis of allergic diseases and their possible therapeutic opportunities. The importance of the routes of administration of the different therapeutic groups has been emphasized. The classical aspects of drug metabolism and disposition related to oral administration have been reviewed, but special emphasis has been given to intranasal, cutaneous, transdermal, and ocular administration as well as to the absorption and the subsequent bioavailability of drugs. Drug-metabolizing enzymes and transporters present in extrahepatic tissues, such as nasal mucosa and the respiratory tract, have been particularly discussed. As marketed antiallergic drugs include both racemates and enantiomers, aspects of stereoselective absorption, distribution, metabolism, and excretion have been discussed. Finally, a new and promising methodology, microdosing, has been presented, although it has not yet been applied to drugs used in the treatment of allergic diseases.

Prediction of antiepileptic drug efficacy: the use of intracerebral microdialysis to monitor biophase concentrations

Biophase concentrations of antiepileptic drugs can differ significantly from pharmacokinetics in plasma. A crucial determinant in the disposition of antiepileptic drugs to the brain is represented by the blood-brain barrier. There is growing evidence that this barrier can alter the availability of antiepileptic drugs at the target site. The permeability of the blood-brain barrier becomes particularly relevant in epileptic conditions and in drug refractory situations. In vivo, intracerebral microdialysis is a valuable technique to determine biophase drug concentrations as it enables investigation of antiepileptic drug transport and distribution in the brain as a function of time. The present review illustrates that intracerebral microdialysis is an indispensable tool for the assessment of the pharmacokinetics of antiepileptic drugs. In addition, we demonstrate how microdialysis data can be used in conjunction with mechanism-based pharmacokinetic/pharmacodynamic modeling for dose selection and optimization of the therapeutic regimen for novel compounds.

Plants with neurobiological activity as potential targets for drug discovery

Significant number of studies has been performed to find alternatives or treatments for diseases of the nervous forum by identifying structures with activity at the central nervous system (CNS). However most of the screenings are usually conducted on an ad hoc basis and not systematically. The initial purpose of this review was to screen plants with neurological bioactivity, in particular those that have not been fully studied and that have molecular mechanisms whose active constituents responsible for the activity remain to be identified. The second purpose was to identify potential target plants for future studies of new and alternative therapies for the treatment of neurological disorders and neurodegenerative diseases. All plants considered in this review were selected for three qualities: possible molecular requirements to act at the CNS; representative of the main classes of compounds with the referred bioactivity and the major families containing species with those compounds; and diffuse world distribution. We identified several examples of plants that have potential for further study. We have included the main families of these plants, their known molecular mechanisms involved in neurological bioactivity, and the active constituents responsible for such activity. Also included is a brief discussion about the requirements of the different compounds to reach the CNS. These requirements may be less limited than what researchers have previously thought.

[Blood-brain barrier part III: therapeutic approaches to cross the blood-brain barrier and target the brain]

Over the last few years, the blood-brain barrier has come to be considered as the main limitation for the treatment of neurological diseases caused by inflammatory, tumor or neurodegenerative disorders. In the blood-brain barrier, the close intercellular contact between cerebral endothelial cells due to tight junctions prevents the passive diffusion of hydrophilic components from the bloodstream into the brain. Several specific transport systems (via transporters expressed on cerebral endothelial cells) are implicated in the delivery of nutriments, ions and vitamins to the brain; other transporters expressed on cerebral endothelial cells extrude endogenous substances or xenobiotics, which have crossed the cerebral endothelium, out of the brain and into the bloodstream. Recently, several strategies have been proposed to target the brain, (i) by by-passing the blood-brain barrier by central drug administration, (ii) by increasing permeability of the blood-brain barrier, (iii) by modulating the expression and/or the activity of efflux transporters, (iv) by using the physiological receptor-dependent blood-brain barrier transport, and (v) by creating new viral or chemical vectors to cross the blood-brain barrier. This review focuses on the illustration of these different approaches.

Incorporating receptor theory in mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) modeling

Pharmacokinetic-Pharmacodynamic (PK-PD) modeling helps to better understand drug efficacy and safety and has, therefore, become a powerful tool in the learning-confirming cycles of drug-development. In translational drug research, mechanism-based PK-PD modeling has been recognized as a tool for bringing forward early insights in drug efficacy and safety into the clinical development. These models differ from descriptive PK-PD models in that they quantitatively characterize specific processes in the causal chain between drug administration and effect. This includes target site distribution, binding and activation, pharmacodynamic interactions, transduction and homeostatic feedback mechanisms. Compared to descriptive models mechanism-based PK-PD models that utilize receptor theory concepts for characterization of target binding and target activation processes have improved properties for extrapolation and prediction. In this respect, receptor theory constitutes the basis for 1) prediction of in vivo drug concentration-effect relationships and 2) characterization of target association-dissociation kinetics as determinants of hysteresis in the time course of the drug effect. This approach intrinsically distinguishes drug- and system specific parameters explicitly, allowing accurate extrapolation from in vitro to in vivo and across species. This review provides an overview of recent developments in incorporating receptor theory in PK-PD modeling with a specific focus on the identifiability of these models.

Differential receptor-mediated drug targeting to the diseased brain

The brain is not directly accessible for intravenously administered macro- and most small molecular drugs because of the presence of the blood-brain barrier (BBB). In this respect the BBB functions as a physical and metabolic barrier which is presented by the endothelial cells in brain capillaries. In order to overcome the BBB, therapeutic compounds have been targeted to internalizing receptors at the BBB. In this review we summarize the different approaches that have been described in current literature, including the possible difficulties for clinical application. Particularly, we focus on the possible impact of brain diseases on receptor-mediated transport to the BBB/brain and how this may affect various targeting strategies. Moreover, it is our opinion that a differential drug targeting/delivery approach should be applied to treat central nervous system (CNS) diseases that are related to the BBB alone, and for CNS diseases that are related to both the brain and the BBB.

[Biology of the blood-brain barrier: Part I]

The blood-brain barrier provides the central nervous system with a unique protection against the toxic effects of many xenobiotics. This protection results from the unique anatomic and biological structure of the endothelium of blood vessels in the brain. The main features of the blood-brain barrier are the presence of tight intercellular junctions which strictly limit the diffusion of blood-borne solutes and cells into the brain and the polarized expression of transporters which specifically control the cerebral availability of nutrients, drugs or xenobiotics. Recent findings in molecular and cellular biology improved our knowledge of blood-brain barrier permeability and its regulation. The importance of these findings has been recently highlighted by the description of dysfunctions of the blood-brain barrier which could have an impact on the pathophysiology of several neurological diseases. This review focuses on recent advances in our understanding of blood-brain barrier biology and physiology, presenting the structural organization of the blood-brain barrier and the functional regulation of solute permeability and cellular transendothelial migration.

The expression of P-glycoprotein is increased in vessels with blood-brain barrier impairment in a stroke-prone hypertensive model

AIMS

We previously reported that the blood-brain barrier (BBB) function was impaired in vessels in the hippocampus in 3-month-old stroke-prone spontaneously hypertensive rats (SHRSP). In this study, we examined gene and protein expressions of P-glycoprotein, a representative efflux transporter of cerebral vessels, in the BBB-damaged hippocampal vessels of SHRSP and in the vessels of Wistar Kyoto (WKY) rats as controls, to clarify roles of the efflux transporter in the BBB-damaged vessels.

METHODS

The expression of P-glycoprotein in hippocampal and cortical samples was examined by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunoelectron microscopic techniques.

RESULTS

Real-time RT-PCR and Western blotting analyses revealed that the gene and protein expressions of P-glycoprotein were increased in the hippocampal samples of 3-month-old SHRSP compared with hippocampal samples of 3-month-old WKY rats or with cortical samples of SHRSP. The gene expression of P-glycoprotein was also increased in the hippocampal samples of 4-week-old SHRSP. Immunoelectron microscopic examination revealed that immunosignals of P-glycoprotein were seen in the luminal and ab-luminal cytoplasmic membranes of endothelial cells and the basal lamina, that the labelling density of P-glycoprotein in the vessel wall was higher in the hippocampus of 3-month-old SHRSP than in other groups and that the immunosignals of P-glycoprotein were occasionally co-located with those of albumin.

CONCLUSIONS

These findings indicate that the expression of P-glycoprotein is increased in BBB-damaged hippocampal vessels in hypertensive SHRSP compared with those in WKY rats.

Abcb1a and Abcb1b expression in senescence-accelerated mouse (SAM)

It was recently reported that some strains of senescence-accelerated mouse (SAM) including SAMR1 had a spontaneous retroviral insertional mutation in the ATP-binding cassette, sub-family B, member 1A (Abcb1a) gene, while other strains including SAMP8 had not. The Abcb1 gene product, P-glycoprotein, is a representative efflux transporter of cerebral vessels. In this study, using brain samples of SAMR1, Abcb1a gene-mutant mice, and of SAMP8 without that mutation, we examined the gene expression of some representative ATP-binding cassettes, such as Abcb1a, Abcb1b, Abcc, and Abcg2, and the protein expression of P-glycoprotein by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemical techniques. The gene expression of Abcb1a was decreased in the brain samples of SAMR1 compared with those of SAMP8, while that of Abcb1b was increased in the samples of SAMR1 compared with those of SAMP8. There were no differences in the gene expression of Abcc and Abcg2 between the samples of SAMR1 and SAMP8. The protein expression of P-glycoprotein was decreased in the brain samples of SAMR1 compared with those of SAMP8. Immunosignals of P-glycoprotein were seen in vessels walls, mainly CD34-positive endothelial cells and partially astrocytic cells, in both mice. These findings indicate that SAMR1, Abcb1a-mutant mice, showed decreased expression of Abcb1a gene and P-glycoprotein and increased gene expression of Abcb1b, compared with those of SAMP8 without that mutation, suggesting no clear effect of increased gene expression of Abcb1b on decreased expression of P-glycoprotein. The combination of SAMR1 and SAMP8 may be a good tool to investigate which transporter, Abcb1a or Abcb1b, can be used in drug delivery into the brain.

MDR1A (ABCB1)-deficient CF-1 mutant mice are susceptible to cerebral malaria induced by Plasmodium berghei ANKA

Under experimental conditions, Plasmodium berghei infection causes cerebral malaria (CM) in susceptible strains of mice such as C57BL/6 and CBA/Ca, whereas BALB/c or DBA/2J strains serve as a model for CM-resistant mice. The aim of the present study was to investigate the susceptibility of the CF1 mouse strain, carrying a spontaneous mutation of the mdr1a gene, to infection with Plasmodium berghei ANKA (PbA). The mdr1a gene codes for P-glycoprotein (P-gp/ABCB1), an efflux pump that is one of the major components of the blood-brain barrier. P-gp effluxes a broad range of xenobiotics from the brain to blood, preventing accumulation and toxicity in the central nervous system. CFI mdr1a (-/-) mice are used to investigate drug transport by efflux pumps. Because many antimalarial agents are effluxed by P-gp (mefloquine, quinine), it was important to determine whether CF1 mice can develop cerebral malaria to predict drug toxicity during cerebral malaria. Our work showed that CF1 mdr1a (-/-) mice are susceptible to PbA. CF1 and C57BL/6N mice (the reference strain) infected with PbA have similar profiles with regard to clinical signs, brain histological lesions, and brain macrophagic activation observed by immunohistological methods.

Structure, function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1. 2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes. 3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs. 4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood-brain barrier. 5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins. 6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases. 7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug-drug interactions with altered drug pharmacokinetics and response. 8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.