Small molecular drug transfer across the blood-brain barrier via carrier-mediated transport systems

Alpha7 nicotinic acetylcholine receptor is a target in pharmacology and toxicology

Alpha7 nicotinic acetylcholine receptor (α7 nAChR) is an important part of the cholinergic nerve system in the brain. Moreover, it is associated with a cholinergic anti-inflammatory pathway in the termination of the parasympathetic nervous system. Antagonists of α7 nAChR are a wide group represented by conotoxin and bungarotoxin. Even Alzheimer's disease drug memantine acting as an antagonist in its side pathway belongs in this group. Agonists of α7 nAChR are suitable for treatment of multiple cognitive dysfunctions such as Alzheimer's disease or schizophrenia. Inflammation or even sepsis can be ameliorated by the agonistic acting compounds. Preparations RG3487, SEN34625/WYE-103914, SEN12333, ABT-107, Clozapine, GTS-21, CNI-1493, and AR-R17779 are representative examples of the novel compounds with affinity toward the α7 nAChR. Pharmacological, toxicological, and medicinal significance of α7 nAChR are discussed throughout this paper.

Nanostructure-based drug delivery systems for brain targeting

CONTEXT

It is well-known fact that blood brain barrier (BBB) hinders the penetrance and access of many pharmacotherapeutic agents to central nervous system (CNS). Many diseases of the CNS remain undertreated and the inability to treat most CNS disorders is not due to the lack of effective CNS drug discovery, rather, it is due to the ineffective CNS delivery. Therefore, a number of nanostructured drug delivery carriers have been developed and explored over the past couple of years to transport the drugs to brain.

OBJECTIVE

The present review will give comprehensive details of extensive research being done in field of nanostructured carriers to transport the drugs through the BBB in a safe and effective manner.

METHODS

The method includes both the polymeric- and lipid-based nanocarriers with emphasis on their utility, methodology, advantages, and the drugs which have been worked on using a particular approach to provide a noninvasive method to improve the drug transport through BBB.

RESULTS

Polymeric- and lipid-based nanocarriers enter brain capillaries before reaching the surface of the brain microvascular endothelial cells without the disruption of BBB. These systems are further modified with specific ligands vectors and pegylation aiming to target and enhance their binding with surface receptors of the specific tissues inside brain and increase long circulatory time which favors interaction and penetration into brain endothelial cells.

CONCLUSION

This review would give an insight to the researchers working on neurodegenerative and non-neurodegenerative diseases of the CNS including brain tumor.

Cerebral microvascular endothelium and the pathogenesis of neurodegenerative diseases

Diseases of the central nervous system (CNS) pose a significant health challenge, but despite their diversity, they share many common features and mechanisms. For example, endothelial dysfunction has been implicated as a crucial event in the development of several CNS disorders, such as Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, multiple sclerosis, human immunodeficiency virus (HIV)-1-associated neurocognitive disorder and traumatic brain injury. Breakdown of the blood–brain barrier (BBB) as a result of disruption of tight junctions and transporters, leads to increased leukocyte transmigration and is an early event in the pathology of these disorders. The brain endothelium is highly reactive because it serves as both a source of, and a target for, inflammatory proteins and reactive oxygen species. BBB breakdown thus leads to neuroinflammation and oxidative stress, which are implicated in the pathogenesis of CNS disease. Furthermore, the physiology and pathophysiology of endothelial cells are closely linked to the functioning of their mitochondria, and mitochondrial dysfunction is another important mediator of disease pathology in the brain. The high concentration of mitochondria in cerebrovascular endothelial cells might account for the sensitivity of the BBB to oxidant stressors. Here, we discuss how greater understanding of the role of BBB function could lead to new therapeutic approaches for diseases of the CNS that target the dynamic properties of brain endothelial cells.

Functional characterization of rat plasma membrane monoamine transporter in the blood-brain and blood-cerebrospinal fluid barriers

This study investigated the expression and functional roles of rat plasma membrane monoamine transporter (rPMAT) in the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier by using in vitro brain barrier model cells (TR-BBB13 and TR-CSFB3 cells) and multiple in vivo experimental techniques. Quantitative reverse transcription-polymerase chain reaction analysis showed relatively high expression of rPMAT mRNA in TR-BBB13 and TR-CSFB3 cells. 1-Methyl-4-phenylpyridinium (MPP(+) ) was transported into rPMAT-expressing cells in a sodium-independent manner. [(3) H]MPP(+) was taken up concentration dependently by TR-BBB13 and TR-CSFB3 cells with K(m) values similar to that of rPMAT-expressing cells. [(3) H]MPP(+) transports into these cells were markedly inhibited by serotonin, dopamine, and cationic drugs. rPMAT small interfering RNA (siRNA) significantly suppressed the [(3) H]MPP(+) uptake by TR-BBB13 cells. Intracerebrally injected [(3) H]MPP(+) was eliminated from the brain parenchymal region, whereas brain [(3) H]MPP(+) uptake did not increase with time during in situ brain perfusion, suggesting that the brain-to-blood transport across the BBB predominates over the blood-to-brain transport. Brain microdialysis studies revealed that the elimination across the BBB was significantly decreased by coperfusion of unlabelled MPP(+) , serotonin, or dopamine. [(3) H]MPP(+) was also eliminated from the CSF. These findings suggest that PMAT in brain barriers functions as the brain-to-blood transporter to regulate brain concentrations of organic cations including monoamines and cationic neurotoxins.

Transport characteristics of a novel peptide platform for CNS therapeutics

New and effective therapeutics that cross the blood-brain barrier (BBB) are critically needed for treatment of many brain diseases. We characterize here a novel drug development platform that is broadly applicable for the development of new therapeutics with increased brain penetration. The platform is based on the Angiopep-2 peptide, a sequence derived from ligands that bind to low-density lipoprotein receptor-related protein-1 (LRP-1), a receptor expressed on the BBB. Fluorescent imaging studies of a Cy5.5Angiopep-2 conjugate and immunohistochemical studies of injected Angiopep-2 in mice demonstrated efficient transport across the BBB into brain parenchyma and subsequent co-localization with the neuronal nuclei-selective marker NeuN and the glial marker glial fibrillary acidic protein (GFAP). Uptake of [(¹²⁵I]-Angiopep-2 into brain endothelial cells occurred by a saturable mechanism involving LRP-1. The primary sequence and charge of Angiopep-2 were crucial for its passage across the BBB. Overall, the results demonstrate the significant potential of this platform for the development of novel neurotherapeutics.

In silico design of a peptidomimetic carrier for levodopa

Ab initio molecular orbital calculations at the Hartree Fock level utilizing 6-31G basis set have been performed on small cyclic peptides and peptidomimetic compounds to explore their utility as carriers of levodopa (L-3,4-dihydroxyphenylalanine) to enhance its brain bioavailability. A cyclic peptidomimetic compound with hydrophobic CH2NH backbone is suggested as possible carrier. This carrier is predicted to efficiently carry Levodopa held by non covalent interactions encompassed in its cyclic backbone without chances of expulsion before delivery inside brain. Carrier is expected to undergo passive diffusion alongwith the drug held inside. Once inside the brain, drug may be delivered enzymatically or non enzymatically.

MR-based molecular imaging of the brain: the next frontier

In the foreseeable future, the MI field could greatly assist neuroradiologists. Reporter molecules provide information on specific molecular or cellular events that could not only aid diagnosis but potentially differentiate stages of disorders and treatments. To accomplish this, reporter molecules literally need to pass a barrier, the BBB, which is designed to repel nonessential molecules from the brain. Although this is not a trivial task, several transport systems could be tricked into guiding molecules into the brain. The noninvasive nature in conjunction with a wide availability makes MR imaging particularly suitable for longitudinal neurologic imaging studies. This review explains the principles of MR imaging contrast, delineates different types of reporter molecules, and describes strategies to transport reporters into the brain. It also discusses recent advances in MR imaging hardware, pulse sequences, the development of targeted reporter probes, and future directions of the MR neuroimaging field.

Involvement of an influx transporter in the blood-brain barrier transport of naloxone

Naloxone, a potent and specific opioid antagonist, has been shown in previous studies to have an influx clearance across the rat blood-brain barrier (BBB) two times greater than the efflux clearance. The purpose of the present study was to characterize the influx transport of naloxone across the rat BBB using the brain uptake index (BUI) method. The initial uptake rate of [(3)H]naloxone exhibited saturability in a concentration-dependent manner (concentration range 0.5 microM to 15 mM) in the presence of unlabeled naloxone. These results indicate that both passive diffusion and a carrier-mediated transport mechanism are operating. The in vivo kinetic parameters were estimated as follows: the Michaelis constant, K(t), was 2.99+/-0.71 mM; the maximum uptake rate, J(max), was 0.477+/-0.083 micromol/min/g brain; and the nonsaturable first-order rate constant, K(d), was 0.160+/-0.044 ml/min/g brain. The uptake of [(3)H]naloxone by the rat brain increased as the pH of the injected solution was increased from 5.5 to 8.5 and was strongly inhibited by cationic H(1)-antagonists such as pyrilamine and diphenhydramine and cationic drugs such as lidocaine and propranolol. In contrast, the BBB transport of [(3)H]naloxone was not affected by any typical substrates for organic cation transport systems such as tetraethylammonium, ergothioneine or L-carnitine or substrates for organic anion transport systems such as p-aminohippuric acid, benzylpenicillin or pravastatin. The present results suggest that a pH-dependent and saturable influx transport system that is a selective transporter for cationic H(1)-antagonists is involved in the BBB transport of naloxone in the rat.

Brain microdialysis study of meropenem in two patients with acute brain injury

Concentrations of unbound meropenem in the cerebral extracellular fluid (ECF) of two patients with acute brain injury were assessed by microdialysis. Brain ECF unbound-meropenem concentrations were lower than serum unbound-meropenem concentrations, with brain-to-serum area under the concentration-time curve ratios of 0.73 and 0.14. A pharmacokinetic model was developed to fit the experimental data adequately.

Compact energy metabolism model: brain controlled energy supply

The regulation of the energy metabolism is crucial to ensure the functionality of the entire organism. Deregulations may lead to severe pathologies such as obesity and type 2 diabetes mellitus. The decisive role of the brain as the active controller and heavy consumer in the complex whole body energy metabolism is the matter of recent research. Latest studies suggest that the brain's energy supply has the highest priority while all organs in the organism compete for the available energy resources. In our novel mathematical model, we address these new findings. We integrate energy fluxes and their control signals such as glucose fluxes, insulin signals as well as the ingestion momentum in our new dynamical system. As a novel characteristic, the hormone insulin is regarded as central feedback signal of the brain. Hereby, our model particularly contains the competition for energy between brain and body periphery. The analytical investigation of the presented dynamical system shows a stable long-term behavior of the entire energy metabolism while short time observations demonstrate the typical oscillating blood glucose variations as a consequence of food intake. Our simulation results demonstrate a realistic behavior even in situations like exercise or exhaustion, and key elements like the brain's preeminence are reflected. The presented dynamical system is a step towards a systemic understanding of the human energy metabolism and thus may shed light to defects causing diseases based on deregulations in the energy metabolism.

Multifunctional Nanocarriers for diagnostics, drug delivery and targeted treatment across blood-brain barrier: perspectives on tracking and neuroimaging

Nanotechnology has brought a variety of new possibilities into biological discovery and clinical practice. In particular, nano-scaled carriers have revolutionalized drug delivery, allowing for therapeutic agents to be selectively targeted on an organ, tissue and cell specific level, also minimizing exposure of healthy tissue to drugs. In this review we discuss and analyze three issues, which are considered to be at the core of nano-scaled drug delivery systems, namely functionalization of nanocarriers, delivery to target organs and in vivo imaging. The latest developments on highly specific conjugation strategies that are used to attach biomolecules to the surface of nanoparticles (NP) are first reviewed. Besides drug carrying capabilities, the functionalization of nanocarriers also facilitate their transport to primary target organs. We highlight the leading advantage of nanocarriers, i.e. their ability to cross the blood-brain barrier (BBB), a tightly packed layer of endothelial cells surrounding the brain that prevents high-molecular weight molecules from entering the brain. The BBB has several transport molecules such as growth factors, insulin and transferrin that can potentially increase the efficiency and kinetics of brain-targeting nanocarriers. Potential treatments for common neurological disorders, such as stroke, tumours and Alzheimer's, are therefore a much sought-after application of nanomedicine. Likewise any other drug delivery system, a number of parameters need to be registered once functionalized NPs are administered, for instance their efficiency in organ-selective targeting, bioaccumulation and excretion. Finally, direct in vivo imaging of nanomaterials is an exciting recent field that can provide real-time tracking of those nanocarriers. We review a range of systems suitable for in vivo imaging and monitoring of drug delivery, with an emphasis on most recently introduced molecular imaging modalities based on optical and hybrid contrast, such as fluorescent protein tomography and multispectral optoacoustic tomography. Overall, great potential is foreseen for nanocarriers in medical diagnostics, therapeutics and molecular targeting. A proposed roadmap for ongoing and future research directions is therefore discussed in detail with emphasis on the development of novel approaches for functionalization, targeting and imaging of nano-based drug delivery systems, a cutting-edge technology poised to change the ways medicine is administered.

Prodrug approaches for enhancing the bioavailability of drugs with low solubility

Low water solubility and low bioavailability are frequent problems in drug development, particularly in the area of central nervous system (CNS) drugs. This short review describes selected prodrug approaches which have been developed to enhance the bioavailability of drugs, especially that of poorly soluble drugs. Some of the most successful drugs on the market are prodrugs. With a better understanding of active-transport processes at cell membranes in the gut as well as at the blood-brain barrier, the importance of prodrug approaches will further increase in the future. Prodrug approaches will already be considered in the early phase of drug discovery.

Targeting anti-HIV drugs to the CNS

The development of antiretroviral drugs over the past couple of decades has been commendable owing to the identification of several new targets within the overall HIV replication cycle. However, complete control over HIV/AIDS is yet to be achieved. This is because the current anti-HIV drugs, although effective in reducing plasma viral levels, cannot eradicate the virus completely from the body. This occurs because most anti-HIV drugs do not accumulate in certain cellular and anatomical reservoirs including the CNS. Insufficient delivery of anti-HIV drugs to the CNS is attributed to their low permeability across the BBB. Hence, low and sustained viral replication within the CNS continues even during prolonged antiretroviral drug therapy. Therefore, developing novel approaches that are targeted at enhancing the CNS delivery of anti-HIV drugs are required. In this review, we discuss the potential of nanocarriers and the role of cell-penetrating peptides in enhancing drug delivery to the CNS. Such drug delivery approaches could also lead to higher drug delivery to other cellular and anatomical reservoirs where the virus harbors than with conventional treatment, thus providing an effective therapy to eliminate the virus completely from the body.

Nature and consequences of mammalian brain and CSF efflux transporters: four decades of progress

In the last 40 years, especially with the application of new neurochemical and molecular biological techniques, there has been explosive progress in understanding how certain ligands and drugs are transported across the blood-brain barrier and choroid plexus out of brain and CSF. In the CNS, there are several separate efflux transporters with very broad specificity that are responsible for much of the efflux transport. This review focuses on three such transporters: organic acid transporter-3, peptide transporter-2 and P-glycoprotein for which there is substantial new information including 'knockout' models in mice and, in one case, dogs. Moreover, the structural biology and transport mechanism of P-glycoprotein at 3.8 angstroms is described. The overall objective is to show how this new knowledge provides a more thorough understanding (e.g., of molecular mechanisms) of efflux transport and in several cases leads to clinically relevant information that allows better treatment of certain CNS disorders (e.g., meningitis and brain cancer).

Dual-targeting daunorubicin liposomes improve the therapeutic efficacy of brain glioma in animals

Chemotherapy for brain glioma has been of limited value due to the inability of transport of drug across the blood-brain barrier (BBB) and poor penetration of drug into the tumor. For overcoming these hurdles, the dual-targeting daunorubicin liposomes were developed by conjugating with p-aminophenyl-alpha-D-manno-pyranoside (MAN) and transferrin (TF) for transporting drug across the BBB and then targeting brain glioma. The dual-targeting effects were evaluated on the BBB model in vitro, C6 glioma cells in vitro, avascular C6 glioma tumor spheroids in vitro, and C6 glioma-bearing rats in vivo, respectively. After applying dual-targeting daunorubicin liposomes, the transport ratio across the BBB model was significantly increased up to 24.9%. The most significant uptake by C6 glioma was evidenced by flow cytometry and confocal microscope. The C6 glioma spheroid volume ratio was significantly lowered to 54.7%. The inhibitory rate to C6 glioma cells after crossing the BBB was significantly enhanced up to 64.0%. The median survival time of tumor bearing rats after administering dual-targeting daunorubicin liposomes (22 days) was significantly longer than that after giving free daunorubicin (17 days, P=0.001) or other controls. In conclusion, the dual-targeting daunorubicin liposomes are able to improve the therapeutic efficacy of brain glioma in vitro and in animals.

[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.

[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.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

Prodrugs of thyrotropin-releasing hormone and related peptides as central nervous system agents

Prodrug design for brain delivery of small- and medium-sized neuropeptides was reviewed, focusing on thyrotropin-releasing hormone and structurally related peptides as examples. We have summarized our most important advances in methodology, as well as assessed the benefits and limitations of bioreversible chemical manipulation techniques to achieve targeting of the parent molecules into the central nervous system. The value of prodrug-amenable analogues as potential drug-like central nervous systems agents was highlighted.

The blood-brain barrier in brain homeostasis and neurological diseases

Brain endothelial cells are unique among endothelial cells in that they express apical junctional complexes, including tight junctions, which quite resemble epithelial tight junctions both structurally and functionally. They form the blood-brain-barrier (BBB) which strictly controls the exchanges between the blood and the brain compartments by limiting passive diffusion of blood-borne solutes while actively transporting nutrients to the brain. Accumulating experimental and clinical evidence indicate that BBB dysfunctions are associated with a number of serious CNS diseases with important social impacts, such as multiple sclerosis, stroke, brain tumors, epilepsy or Alzheimer's disease. This review will focus on the implication of brain endothelial tight junctions in BBB architecture and physiology, will discuss the consequences of BBB dysfunction in these CNS diseases and will present some therapeutic strategies for drug delivery to the brain across the BBB.

Involvement of the pyrilamine transporter, a putative organic cation transporter, in blood-brain barrier transport of oxycodone

The purpose of this study was to characterize blood-brain barrier (BBB) transport of oxycodone, a cationic opioid agonist, via the pyrilamine transporter, a putative organic cation transporter, using conditionally immortalized rat brain capillary endothelial cells (TR-BBB13). Oxycodone and [3H]pyrilamine were both transported into TR-BBB13 cells in a temperature- and concentration-dependent manner with Km values of 89 and 28 microM, respectively. The initial uptake of oxycodone was significantly enhanced by preloading with pyrilamine and vice versa. Furthermore, mutual uptake inhibition by oxycodone and pyrilamine suggests that a common mechanism is involved in their transport. Transport of both substrates was inhibited by type II cations (quinidine, verapamil, and amantadine), but not by classic organic cation transporter (OCT) substrates and/or inhibitors (tetraethylammonium, 1-methyl-4-phenylpyridinium, and corticosterone), substrates of OCTN1 (ergothioneine) and OCTN2 (L-carnitine), or organic anions. The transport was inhibited by metabolic inhibitors (rotenone and sodium azide) but was insensitive to extracellular sodium and membrane potential for both substrates. Furthermore, the transport of both substrates was increased at alkaline extracellular pH and decreased in the presence of a protonophore (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone). Intracellular acidification induced with ammonium chloride enhanced the uptakes, suggesting that the transport is driven by an oppositely directed proton gradient. The brain uptake of oxycodone measured by in situ rat brain perfusion was increased in alkaline perfusate and was significantly inhibited by pyrilamine. These results suggest that blood-brain barrier transport of oxycodone is at least partly mediated by a common transporter with pyrilamine, and this transporter is an energy-dependent, proton-coupled antiporter.

A unique carrier for delivery of therapeutic compounds beyond the blood-brain barrier

Background

Therapeutic intervention in many neurological diseases is thwarted by the physical obstacle formed by the blood-brain barrier (BBB) that excludes most drugs from entering the brain from the blood. Thus, identifying efficacious modes of drug delivery to the brain remains a “holy grail” in molecular medicine and nanobiotechnology. Brain capillaries, that comprise the BBB, possess an endogenous receptor that ferries an iron-transport protein, termed p97 (melanotransferrin), across the BBB. Here, we explored the hypothesis that therapeutic drugs “piggybacked” as conjugates of p97 can be shuttled across the BBB for treatment of otherwise inoperable brain tumors.

Approach

Human p97 was covalently linked with the chemotherapeutic agents paclitaxel (PTAX) or adriamycin (ADR) and following intravenous injection, measured their penetration into brain tissue and other organs using radiolabeled and fluorescent derivatives of the drugs. In order to establish efficacy of the conjugates, we used nude mouse models to assess p97-drug conjugate activity towards glioma and mammary tumors growing subcutaneously compared to those growing intracranially.

Principal Findings

Bolus-injected p97-drug conjugates and unconjugated p97 traversed brain capillary endothelium within a few minutes and accumulated to 1–2% of the injected by 24 hours. Brain delivery with p97-drug conjugates was quantitatively 10 fold higher than with free drug controls. Furthermore, both free-ADR and p97-ADR conjugates equally inhibited the subcutaneous growth of gliomas growing outside the brain. Evocatively, only p97-ADR conjugates significantly prolonged the survival of animals bearing intracranial gliomas or mammary tumors when compared to similar cumulated doses of free-ADR.

Significance

This study provides the initial proof of concept for p97 as a carrier capable of shuttling therapeutic levels of drugs from the blood to the brain for the treatment of neurological disorders, including classes of resident and metastatic brain tumors. It may be prudent, therefore, to consider implementation of this novel delivery platform in various clinical settings for therapeutic intervention in acute and chronic neurological diseases.

On the rate and extent of drug delivery to the brain

To define and differentiate relevant aspects of blood–brain barrier transport and distribution in order to aid research methodology in brain drug delivery. Pharmacokinetic parameters relative to the rate and extent of brain drug delivery are described and illustrated with relevant data, with special emphasis on the unbound, pharmacologically active drug molecule. Drug delivery to the brain can be comprehensively described using three parameters: K_p,uu (concentration ratio of unbound drug in brain to blood), CL_in (permeability clearance into the brain), and V_u,brain (intra-brain distribution). The permeability of the blood–brain barrier is less relevant to drug action within the CNS than the extent of drug delivery, as most drugs are administered on a continuous (repeated) basis. K_p,uu can differ between CNS-active drugs by a factor of up to 150-fold. This range is much smaller than that for log BB ratios (K_p), which can differ by up to at least 2,000-fold, or for BBB permeabilities, which span an even larger range (up to at least 20,000-fold difference). Methods that measure the three parameters K_p,uu, CL_in, and V_u,brain can give clinically valuable estimates of brain drug delivery in early drug discovery programmes.

Identification of six putative human transporters with structural similarity to the drug transporter SLC22 family

The solute carrier family 22 (SLC22) is a large family of organic cation and anion transporters. These are transmembrane proteins expressed predominantly in kidneys and liver and mediate the uptake and excretion of environmental toxins, endogenous substances, and drugs from the body. Through a comprehensive database search we identified six human proteins not yet cloned or annotated in the reference sequence databases. Five of these belong to the SLC22 family, SLC22A20, SLC22A23, SLC22A24, SLC22A25, and SPNS3, and the sixth gene, SVOPL, is a paralog to the synaptic vesicle protein SVOP. We identified the orthologs for these genes in mouse and rat and additional homologous proteins and performed the first phylogenetic analysis on the entire SLC22 family in human, mouse, and rat. In addition, we performed a phylogenetic analysis which showed that SVOP and SV2A-C are, in a comparison with all vertebrate proteins, most similar to the SLC22 family. Finally, we performed a tissue localization study on 15 genes on a panel of 30 rat tissues using quantitative real-time polymerase chain reaction.

Active targeting of brain tumors using nanocarriers

The delivery of drugs to brain tumors is limited by the presence of the blood-brain barrier (BBB) separating the blood from the cerebral parenchyma. An understanding of the specific mechanisms of the brain capillary endothelium has led to the development of various strategies to enhance the penetration of drugs into the brain tissue. Active targeting is a non-invasive approach, which consists in transporting drugs to target organs using site-specific ligands. Drug-loaded nanocarriers capable of recognizing brain capillary endothelial cells and cerebral tumoral cells have shown promising potential in oncology. Endogenous and chimeric ligands binding to carriers or receptors of the BBB have been directly or indirectly conjugated to nanocarriers. This review indexes the main targeted colloidal systems used for drug delivery to the brain. Their pharmacological behavior and their therapeutic effect are discussed.

Emerging treatments and gene expression profiling in high-risk medulloblastoma

The past decades have seen an increase in the survival rates of patients with standard-risk medulloblastoma. Efforts have, therefore, been focused on obtaining better results in the treatment of patients with high-risk tumors. In addition to consolidated therapies, novel approaches such as small molecules, monoclonal antibodies, and antiangiogenic therapies that aim to improve outcomes and quality of life are now available through new breakthroughs in the molecular biology of medulloblastoma. The advent of innovative anticancer drugs tested in brain tumors has important consequences for personalized therapy. Gene expression profiling of medulloblastoma can be used to identify the genes and signaling transduction pathways that are crucial for the tumorigenesis process, thereby revealing both new targets for therapy and sensitive/resistance phenotypes. The interpretation of microarray data for new treatments of patients with high-risk medulloblastoma, as well as other poor prognosis tumors, should be developed through a consensus multidisciplinary approach involving oncologists, neurosurgeons, radiotherapists, biotechnologists, bioinformaticists, and other professionals.

Metabolic Barrier against Bisphenol A in Rat Uterine Endometrium

Exposure to environmental chemicals with estrogenic activity during the early stage of pregnancy can seriously affect embryonic development and the maintenance of pregnancy. To estimate the metabolism and pharmacodynamics of a xenoestrogen, bisphenol A, in a reproductive organ, the metabolite of bisphenol A was analyzed after incubating a rat uterine sac in buffer solutions containing the chemical. When the inner or the outer side of the uterine sac was exposed to bisphenol A, the concentration of the parent chemical was decreased in buffer solution and then, only one metabolite, bisphenol A-glucuronide, was observed only in the outer, that is, the maternal, side. A small amount of the parent chemical could pass through the uterine sac without being modified. Uridine diphosphate (UDP)-glucuronosyltransferase (UGT) was shown by immunohistochemical staining analysis to be distributed in epithelial cells of the endometrium, oviduct, and uterine glands. Based on measurements of enzyme activity and on Western blot analysis, UGT activity toward bisphenol A and UGT protein were identified in the microsomal fractions prepared from rat uterus. UGT isoforms, such as UGT1A1, 1A2, 1A5, 1A6, and 1A7, were expressed, and MRP-1 (multidrug resistance-associated protein) and MRP-3, which are well-known to be transporters of various drug-glucuronides, were detected in the rat uterus by reverse transcription-PCR. These results elucidate the rat uterine barrier system by showing that most bisphenol A perfused into the uterus was glucuronidated in the epithelium, resulting in transport of glucuronides to the maternal side.

Blood-brain barrier transport of pramipexole, a dopamine D2 agonist

The blood-brain barrier (BBB) transport of pramipexole, a potent dopamine receptor agonist with high efficacy for Parkinson's disease, was mainly characterized using immortalized rat brain capillary endothelial cells (RBEC)1 as an in vitro BBB model. [(14)C]Pramipexole uptake by RBEC1 was dependent on temperature and pH, but not sodium ion concentration or membrane potential. The uptake was inhibited by several organic cations including pyrilamine. Mutual inhibition was observed between pramipexole and pyrilamine. In addition, [(14)C]pramipexole uptake was stimulated by preloading unlabeled pramipexole. RT-PCR analysis for organic cation transporters (rOCT1-3, rOCTN1-2) in RBEC1 was performed. The mRNA level of rOCTN2 was the highest, followed by rOCTN1, while expression of rOCT1, rOCT2 and rOCT3 was negligible. The brain uptake of [(14)C]pramipexole, which was measured by the in situ rat brain perfusion technique, was significantly inhibited by unlabeled pramipexole. These results suggest that pramipexole is, at least in part, transported across the BBB by an organic cation-sensitive transporter. The pramipexole transport in RBEC1 was pH-dependent, but sodium- and membrane potential-independent.

Antiepileptic Drug Formulation and Treatment in the Elderly: Biopharmaceutical Considerations

The pharmacokinetics of antiepileptic drugs (AEDs) determine their effectiveness in the treatment of patients with epilepsy. Given the likelihood of comorbid medical conditions that require polytherapy, as well as the normal physiological changes associated with aging, an understanding of AED pharmacokinetics and pharmacodynamics in the elderly patient is critical. There is a relative sparsity of data regarding changes in the oral absorption patterns of AEDs that may accompany aging. Therefore, the objective of this chapter is to discuss fundamental principles related to oral drug absorption, and to discuss their potential impact on AED treatment in the older patient. Although most drugs are absorbed via the diffusion process, active transport also plays a role in absorption. While the gastrointestinal tract shows remarkable resilience during aging, physiological changes that influence oral and esophageal function, gastric pH, gastric emptying rates, and intestinal transit times do occur. Oral administration of AEDs may be affected by changes associated with aging, including altered oral protective reflexes, xerostomia, thickening of the esophageal smooth muscle layer, reduced contraction velocity and duration, altered esophageal emptying rates, and enteric plexus neuron reduction. Gastric acid secretion is similar between older and younger patients, but older patients require more time to return to baseline gastric pH values and have prolonged gastric emptying rates compared to younger patients. Elderly patients may similarly have reduced numbers of myenteric neurons, decreased postprandial contractions, reduced frequency of migrating motor complex, and diminished rectal compliance as well as reduced sphincter tones. All of these effects observed in the aging patient, in turn, produce numerous opportunities for changes in AED absorption, particularly for those agents demonstrating poor water solubility or variable absorption patterns.

Paraoxon has only a minimal effect on pralidoxime brain concentration in rats

Clinical experience with oximes, cholinesterase reactivators used in organophosphorus poisoning, has been disappointing. Their major anatomic site of therapeutic action and their ability to pass the blood-brain barrier (BBB) are controversial. Although their physico-chemical properties do not favour BBB penetration, access of oximes to the brain may be facilitated by organophosphates. The effect of the organophosphate paraoxon (POX) on pralidoxime (2-PAM) brain entry was therefore determined. Rats either received 50 micromol 2-PAM only (G(1)) or additionally 1 micromol POX ( approximately LD(75)) (G(2)). Three animals each were killed after 5, 15, 30, 60, 90, 120, 180, 240, 360, 480 min, and 2-PAM concentrations in the brain and plasma were measured using HPLC. Moreover, the effect of brain perfusion with isotonic saline on subsequent 2-PAM measurements was assessed. The maximal 2-PAM concentration (C(max)) in G(1) brain was 6% of plasma C(max), while in G(2) brains it was 8%. Similarly, the ratio of the area under the curve (AUC) brain to plasma was 8% in G(1) and 12% in G(2). Brain t(max) (15 min) was slightly higher than plasma t(max) (5 min). The AUC of plasma 2-PAM did not differ between G(1) and G(2). However, in G(1), AUC brain was significantly lower than in G(2), the differences probably being clinically irrelevant. In perfused brains, 2-PAM concentrations were very close to those of non-perfused brains. The results indicate that brain penetration of 2-PAM is poor and that organophosphates only have a modest effect on 2-PAM BBB penetration. Brain perfusion does not significantly alter 2-PAM measurements and is therefore considered unnecessary.

Pathophysiology of the blood-brain barrier: animal models and methods

The specialized cerebral microvascular endothelium interacts with the cellular milieu of the brain and extracellular matrix to form a neurovascular unit, one aspect of which is a regulated interface between the blood and central nervous system (CNS). The concept of this blood-brain barrier (BBB) as a dynamically regulated system rather than a static barrier has wide-ranging implications for pathophysiology of the CNS. While in vitro models of the BBB are useful for screening drugs targeted to the CNS and indispensable for studies of cerebral endothelial cell biology, the complex interactions of the neurovascular unit make animal-based models and methods essential tools for understanding the pathophysiology of the BBB. BBB dysfunction is a complication of neurodegenerative disease and brain injury. Studies on animal models have shown that diseases of the periphery, such as diabetes and inflammatory pain, have deleterious effects on the BBB which may contribute to neurological complications associated with these conditions. Furthermore, genetic and/or epigenetic abnormalities in constituents of the BBB may be significant contributing factors in disease etiology. Research that approaches the BBB as a dynamic system integrated with both the CNS and the periphery is therefore critical to understanding and treating diseases of the CNS. Herein, we review various methodological approaches used to study BBB function in the context of disease. These include measurement of transport between blood and brain, imaging-based technologies, and genomic/proteomic approaches.

The neuroscience of affiliation: forging links between basic and clinical research on neuropeptides and social behavior

Animal studies point to the role of two neuropeptides-oxytocin and vasopressin-in the regulation of affiliative behaviors including mating, pair-bond formation, maternal/parenting behavior, and attachment. These findings may have important implications for understanding and treating clinical disorders marked by social deficits and/or disrupted attachment. This review focuses on advances made to date in the effort to forge links between basic and clinical research in the area of neuropeptides and social behavior. The literature on oxytocin and its involvement in stress response, affiliation, and prosocial behavior is reviewed, and the implications of these findings for such disorders as autism as well as other social and stress-related disorders including social phobia, post-traumatic stress disorder, and some personality disorders are considered. Finally, unresolved issues and directions for future research are discussed.

Brain uptake of nonsteroidal anti-inflammatory drugs: ibuprofen, flurbiprofen, and indomethacin

PURPOSE

To determine the roles of blood-brain barrier (BBB) transport and plasma protein binding in brain uptake of nonsteroidal anti-inflammatory drugs (NSAIDs)-ibuprofen, flurbiprofen, and indomethacin.

METHODS

Brain uptake was measured using in situ rat brain perfusion technique.

RESULTS

[14C]Ibuprofen, [3H]flurbiprofen, and [14C]indomethacin were rapidly taken up into the brain in the absence of plasma protein with BBB permeability-surface area products (PS(u)) to free drug of (2.63 +/- 0.11) x 10(-2), (1.60 +/- 0.08) x 10(-2), and (0.64 +/- 0.05) x 10(-2) mL s(-1) g(-1) (n = 9-11), respectively. BBB [14C]ibuprofen uptake was inhibited by unlabeled ibuprofen (Km = 0.85 +/- 0.02 mM, Vmax = 13.5 +/- 0.4 nmol s(-1) g(-1)) and indomethacin, but not by pyruvate, probenecid, digoxin, or valproate. No evidence was found for saturable BBB uptake of [3H]flurbiprofen or [14C]indomethacin. Initial brain uptake for all three NSAIDs was reduced by the addition of albumin to the perfusion buffer. The magnitude of the brain uptake reduction correlated with the NSAID free fraction in the perfusate.

CONCLUSIONS

Free ibuprofen, flurbiprofen, and indomethacin rapidly cross the BBB, with ibuprofen exhibiting a saturable component of transport. Plasma protein binding limits brain NSAID uptake by reducing the free fraction of NSAID in the circulation.