Characterization of the blood-brain barrier choline transporter using the in situ rat brain perfusion technique

MFSD7c functions as a transporter of choline at the blood-brain barrier

Mutations in the orphan transporter MFSD7c (also known as Flvcr2), are linked to Fowler syndrome. Here, we used Mfsd7c knockout (Mfsd7c-/-) mice and cell-based assays to reveal that MFSD7c is a choline transporter at the blood-brain barrier (BBB). We performed comprehensive metabolomics analysis and detected differential changes of metabolites in the brains and livers of Mfsd7c-/-embryos. Particularly, we found that choline-related metabolites were altered in the brains but not in the livers of Mfsd7c-/- embryos. Thus, we hypothesized that MFSD7c regulates the level of choline in the brain. Indeed, expression of human MFSD7c in cells significantly increased choline uptake. Interestingly, we showed that choline uptake by MFSD7c is greatly increased by choline-metabolizing enzymes, leading us to demonstrate that MFSD7c is a facilitative transporter of choline. Furthermore, single-cell patch clamp analysis showed that the import of choline by MFSD7c is electrogenic. Choline transport function of MFSD7c was shown to be conserved in vertebrates, but not in yeasts. We demonstrated that human MFSD7c is a functional ortholog of HNM1, the yeast choline importer. We also showed that several missense mutations identified in patients exhibiting Fowler syndrome had abolished or reduced choline transport activity. Mice lacking Mfsd7c in endothelial cells of the central nervous system suppressed the import of exogenous choline from blood but unexpectedly had increased choline levels in the brain. Stable-isotope tracing study revealed that MFSD7c was required for exporting choline derived from lysophosphatidylcholine in the brain. Collectively, our work identifies MFSD7c as a choline exporter at the BBB and provides a foundation for future work to reveal the disease mechanisms of Fowler syndrome.

Presynaptic Congenital Myasthenic Syndromes: Understanding Clinical Phenotypes through In vivo Models

Presynaptic congenital myasthenic syndromes (CMS) are a group of genetic disorders affecting the presynaptic side of the neuromuscular junctions (NMJ). They can result from a dysfunction in acetylcholine (ACh) synthesis or recycling, in its packaging into synaptic vesicles, or its subsequent release into the synaptic cleft. Other proteins involved in presynaptic endplate development and maintenance can also be impaired.Presynaptic CMS usually presents during the prenatal or neonatal period, with a severe phenotype including congenital arthrogryposis, developmental delay, and apnoeic crisis. However, milder phenotypes with proximal muscle weakness and good response to treatment have been described. Finally, many presynaptic genes are expressed in the brain, justifying the presence of additional central nervous system symptoms.Several animal models have been developed to study CMS, providing the opportunity to identify disease mechanisms and test treatment options. In this review, we describe presynaptic CMS phenotypes with a focus on in vivo models, to better understand CMS pathophysiology and define new causative genes.

A merged molecular representation deep learning method for blood-brain barrier permeability prediction

The ability of a compound to permeate across the blood-brain barrier (BBB) is a significant factor for central nervous system drug development. Thus, for speeding up the drug discovery process, it is crucial to perform high-throughput screenings to predict the BBB permeability of the candidate compounds. Although experimental methods are capable of determining BBB permeability, they are still cost-ineffective and time-consuming. To complement the shortcomings of existing methods, we present a deep learning-based multi-model framework model, called Deep-B3, to predict the BBB permeability of candidate compounds. In Deep-B3, the samples are encoded in three kinds of features, namely molecular descriptors and fingerprints, molecular graph and simplified molecular input line entry system (SMILES) text notation. The pre-trained models were built to extract latent features from the molecular graph and SMILES. These features depicted the compounds in terms of tabular data, image and text, respectively. The validation results yielded from the independent dataset demonstrated that the performance of Deep-B3 is superior to that of the state-of-the-art models. Hence, Deep-B3 holds the potential to become a useful tool for drug development. A freely available online web-server for Deep-B3 was established at http://cbcb.cdutcm.edu.cn/deepb3/, and the source code and dataset of Deep-B3 are available at https://github.com/GreatChenLab/Deep-B3.

A Historical Review of Brain Drug Delivery

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

Smart Nanomedicine to Enable Crossing Blood-Brain Barrier Delivery of Checkpoint Blockade Antibody for Immunotherapy of Glioma

Immune checkpoint blockade (ICB) therapy has shown tremendous promises in the treatment of various types of tumors. However, ICB therapy with antibodies appears to be less effective for glioma, partly owing to the existence of the blood-brain barrier (BBB) that impedes the entrance of therapeutics including most proteins to the central nervous system (CNS). Herein, considering the widely existing nicotinic acetylcholine receptors (nAChRs) and choline transporters (ChTs) on the surface of BBB, a choline analogue 2-methacryloyloxyethyl phosphorylcholine (MPC) is employed to fabricate the BBB-crossing copolymer via free-radical polymerization, followed by conjugation with antiprogrammed death-ligand 1 (anti-PD-L1) via a pH-sensitive traceless linker. The obtained nanoparticles exhibit significantly improved BBB-crossing capability owing to the receptor-mediated transportation after intravenous injection in an orthotopic glioma tumor model. Within the acidic glioma microenvironment, anti-PD-L1 would be released from such pH-responsive nanoparticles, further triggering highly effective ICB therapy of glioma to significantly prolong animal survival. This work thus realizes glioma microenvironment responsive BBB-crossing delivery of ICB antibodies, promising for the next generation immunotherapy of glioma.

A Review of Mathematics Determining Solute Uptake at the Blood-Brain Barrier in Normal and Pathological Conditions

The blood-brain barrier (BBB) limits movement of solutes from the lumen of the brain microvascular capillary system into the parenchyma. The unidirectional transfer constant, K_in, is the rate at which transport across the BBB occurs for individual molecules. Single and multiple uptake experiments are available for the determination of K_in for new drug candidates using both intravenous and in situ protocols. Additionally, the single uptake method can be used to determine K_in in heterogeneous pathophysiological conditions such as stroke, brain cancers, and Alzheimer's disease. In this review, we briefly cover the anatomy and physiology of the BBB, discuss the impact of efflux transporters on solute uptake, and provide an overview of the single-timepoint method for determination of K_in values. Lastly, we compare preclinical K_in experimental results with human parallels.

A Bioinspired Platform for Effective Delivery of Protein Therapeutics to the Central Nervous System

Central nervous system (CNS) diseases are the leading cause of morbidity and mortality; their treatment, however, remains constrained by the blood-brain barrier (BBB) that impedes the access of most therapeutics to the brain. A CNS delivery platform for protein therapeutics, which is achieved by encapsulating the proteins within nanocapsules that contain choline and acetylcholine analogues, is reported herein. Mediated by nicotinic acetylcholine receptors and choline transporters, such nanocapsules can effectively penetrate the BBB and deliver the therapeutics to the CNS, as demonstrated in mice and non-human primates. This universal platform, in general, enables the delivery of any protein therapeutics of interest to the brain, opening a new avenue for the treatment of CNS diseases.

Choline Rescues Behavioural Deficits in a Mouse Model of Rett Syndrome by Modulating Neuronal Plasticity

Rett syndrome (RTT) is a postnatal neurodevelopmental disorder that primarily affects girls, with 95% of RTT cases resulting from mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Choline, a dietary micronutrient found in most foods, has been shown to be important for brain development and function. However, the exact effects and mechanisms are still unknown. We found that 13 mg/day (1.7 × required daily intake) of postnatal choline treatment to Mecp2-conditional knockout mice rescued not only deficits in motor coordination, but also their anxiety-like behaviour and reduced social preference. Cortical neurons in the brains of Mecp2-conditional knockout mice supplemented with choline showed enhanced neuronal morphology and increased density of dendritic spines. Modelling RTT in vitro by knocking down the expression of the MeCP2 protein with shRNA, we found that choline supplementation to MeCP2-knockdown neurons increased their soma sizes and the complexity of their dendritic arbors. Rescue of the morphological defects could lead to enhanced neurotransmission, as suggested by an observed trend of increased expression of synaptic proteins and restored miniature excitatory postsynaptic current frequency in choline-supplemented MeCP2-knockdown neurons. Through the use of specific inhibitors targeting each of the known physiological pathways of choline, synthesis of phosphatidylcholine from choline was found to be essential in bringing about the changes seen in the choline-supplemented MeCP2-knockdown neurons. Taken together, these data reveal a role of choline in modulating neuronal plasticity, possibly leading to behavioural changes, and hence, a potential for using choline to treat RTT.

In vitro models and systems for evaluating the dynamics of drug delivery to the healthy and diseased brain

The blood-brain barrier (BBB) plays a crucial role in maintaining brain homeostasis and transport of drugs to the brain. The conventional animal and Transwell BBB models along with emerging microfluidic-based BBB-on-chip systems have provided fundamental functionalities of the BBB and facilitated the testing of drug delivery to the brain tissue. However, developing biomimetic and predictive BBB models capable of reasonably mimicking essential characteristics of the BBB functions is still a challenge. In addition, detailed analysis of the dynamics of drug delivery to the healthy or diseased brain requires not only biomimetic BBB tissue models but also new systems capable of monitoring the BBB microenvironment and dynamics of barrier function and delivery mechanisms. This review provides a comprehensive overview of recent advances in microengineering of BBB models with different functional complexity and mimicking capability of healthy and diseased states. It also discusses new technologies that can make the next generation of biomimetic human BBBs containing integrated biosensors for real-time monitoring the tissue microenvironment and barrier function and correlating it with the dynamics of drug delivery. Such integrated system addresses important brain drug delivery questions related to the treatment of brain diseases. We further discuss how the combination of in vitro BBB systems, computational models and nanotechnology supports for characterization of the dynamics of drug delivery to the brain.

Expression of the High-affinity Choline Transporter CHT1 in Rat and Human Arteries

The arterial vascular wall contains a non-neuronal intrinsic cholinergic system. The rate-limiting step in acetylcholine (ACh) synthesis is choline uptake. A high-affinity choline transporter, CHT1, has recently been cloned from neural tissue and has been identified in epithelial cholinergic cells. Here we investigated its presence in rat and human arteries and in primary cell cultures of rat vascular cells (endothelial cells, smooth muscle cells, fibroblasts). CHT1-mRNA was detected in the arterial wall and in all isolated cell types by RT-PCR using five different CHT1-specific primer pairs. Antisera raised against amino acids 29-40 of the rat sequence labeled a single band (50 kD) in Western blots of rat aorta, and an additional higher molecular weight band appeared in the hippocampus. Immunohistochemistry demonstrated CHT1 immunoreactivity in endothelial and smooth muscle cells in situ and in all cultured cell types. A high-affinity [3H]-choline uptake mechanism sharing characteristics with neuronal high-affinity choline uptake, i.e., sensitivity to hemicholinium-3 and dependence on sodium, was demonstrated in rat thoracic aortic segments by microimager autoradiography. Expression of the high-affinity choline transporter CHT1 is a novel component of the intrinsic non-neuronal cholinergic system of the arterial vascular wall, predominantly in the intimal and medial layers.

Overview of PET Tracers for Brain Tumor Imaging

This article provides an overview of the key considerations for the development and application of molecular imaging agents for brain tumors and the major classes of PET tracers that have been used for imaging brain tumors in humans. The mechanisms of uptake, biological implications, primary applications, and limitations of PET tracers in neuro-oncology are reviewed. The available data indicate that several of these classes of tracers, including radiolabeled amino acids, have imaging properties superior to those of (18)F-fluorodeoxyglucose, and can complement contrast-enhanced magnetic resonance imaging in the evaluation of brain tumors.

Molecular determinants of blood-brain barrier permeation

The blood-brain barrier (BBB) is a microvascular unit which selectively regulates the permeability of drugs to the brain. With the rise in CNS drug targets and diseases, there is a need to be able to accurately predict a priori which compounds in a company database should be pursued for favorable properties. In this review, we will explore the different computational tools available today, as well as underpin these to the experimental methods used to determine BBB permeability. These include in vitro models and the in vivo models that yield the dataset we use to generate predictive models. Understanding of how these models were experimentally derived determines our accurate and predicted use for determining a balance between activity and BBB distribution.

In vitro porcine blood-brain barrier model for permeability studies: pCEL-X software pKa(FLUX) method for aqueous boundary layer correction and detailed data analysis

In vitro blood-brain barrier (BBB) models from primary brain endothelial cells can closely resemble the in vivo BBB, offering valuable models to assay BBB functions and to screen potential central nervous system drugs. We have recently developed an in vitro BBB model using primary porcine brain endothelial cells. The model shows expression of tight junction proteins and high transendothelial electrical resistance, evidence for a restrictive paracellular pathway. Validation studies using small drug-like compounds demonstrated functional uptake and efflux transporters, showing the suitability of the model to assay drug permeability. However, one limitation of in vitro model permeability measurement is the presence of the aqueous boundary layer (ABL) resulting from inefficient stirring during the permeability assay. The ABL can be a rate-limiting step in permeation, particularly for lipophilic compounds, causing underestimation of the permeability. If the ABL effect is ignored, the permeability measured in vitro will not reflect the permeability in vivo. To address the issue, we explored the combination of in vitro permeability measurement using our porcine model with the pKa(FLUX) method in pCEL-X software to correct for the ABL effect and allow a detailed analysis of in vitro (transendothelial) permeability data, Papp. Published Papp using porcine models generated by our group and other groups are also analyzed. From the Papp, intrinsic transcellular permeability (P0) is derived by simultaneous refinement using a weighted nonlinear regression, taking into account permeability through the ABL, paracellular permeability and filter restrictions on permeation. The in vitro P0 derived for 22 compounds (35 measurements) showed good correlation with P0 derived from in situ brain perfusion data (r(2)=0.61). The analysis also gave evidence for carrier-mediated uptake of naloxone, propranolol and vinblastine. The combination of the in vitro porcine model and the software analysis provides a useful tool to better predict BBB permeability in vivo and gain better mechanistic information about BBB permeation.

In silico predictive model to determine vector-mediated transport properties for the blood-brain barrier choline transporter

The blood–brain barrier choline transporter (BBB-ChT) may have utility as a drug delivery vector to the central nervous system (CNS). We therefore initiated molecular docking studies with the AutoDock and AutoDock Vina (ADVina) algorithms to develop predictive models for compound screening and to identify structural features important for binding to this transporter. The binding energy predictions were highly correlated with r²=0.88, F=692.4, standard error of estimate =0.775, and P-value<0.0001 for selected BBB-ChT-active/inactive compounds (n=93). Both programs were able to cluster active (Gibbs free energy of binding <−6.0 kcal*mol⁻¹) and inactive (Gibbs free energy of binding >−6.0 kcal*mol⁻¹) molecules and dock them significantly better than at random with an area under the curve value of 0.86 and 0.84, respectively. In ranking smaller molecules with few torsional bonds, a size-related bias in scoring producing false-negative outcomes was detected. Finally, important blood–brain barrier parameters, such as the logBB_passive and logBB_active values, were assessed to predict compound transport to the CNS accurately. Knowledge gained from this study is useful to better understand the binding requirements in BBB-ChT, and until such time as its crystal structure becomes available, it may have significant utility in developing a highly predictive model for the rational design of drug-like compounds targeted to the brain.

Hyperpolarized choline as an MR imaging molecular probe: feasibility of in vivo imaging in a rat model

PURPOSE

To assess the feasibility of choline MRI using a new choline molecular probe for dynamic nuclear polarization (DNP) hyperpolarized MRI.

MATERIALS AND METHODS

Male Sprague-Dawley rats with an average weight of 400 ± 20 g (n = 5), were anesthetized and injection tubing was placed in the tail vein. [1,1,2,2-D4 , 1-(13) C]choline chloride (CMP1) was hyperpolarized by DNP and injected into rats at doses ranging from 12.6 to 50.0 mg/kg. Coronal projection (13) C imaging was performed on a 3 Tesla clinical MRI scanner (bore size 60 cm) using a variable flip angle gradient echo sequence. Images were acquired 15 to 45 s after the start of bolus injection. Signal intensities in regions of interest were determined at each time point and compared.

RESULTS

(13) C MRI images of hyperpolarized CMP1 at a 50 mg/kg dose showed time-dependent organ distribution patterns. At 15 s, high intensities were observed in the inferior vena cava, heart, aorta, and kidneys. At 30 s, most of the signal intensity was localized to the kidneys. These distribution patterns were reproduced using 12.6 and 25 mg/kg doses. At 45 s, only signal in the kidneys was detected.

CONCLUSION

Hyperpolarized choline imaging with MRI is feasible using a stable-isotope labeled choline analog (CMP1). Nonradioactive imaging of choline accumulation may provide a new investigatory dimension for kidney physiology. J. Magn. Reson. Imaging 2015;41:917-923. © 2014 Wiley Periodicals, Inc.

Biodistribution and radiation dosimetry of deuterium-substituted 18F-fluoromethyl-[1, 2-2H4]choline in healthy volunteers

(11)C-choline and (18)F-fluoromethylcholine ((18)F-FCH) have been used in patients to study tumor metabolic activity in vivo; however, both radiotracers are readily oxidized to respective betaine analogs, with metabolites detectable in plasma soon after injection of the radiotracer. A more metabolically stable FCH analog, (18)F-fluoromethyl-[1,2-(2)H4]choline ((18)F-D4-FCH), based on the deuterium isotope effect, has been developed. We report the safety, biodistribution, and internal radiation dosimetry profiles of (18)F-D4-FCH in 8 healthy human volunteers.

METHODS

(18)F-D4-FCH was intravenously administered as a bolus injection (mean ± SD, 161 ± 2.17 MBq; range, 156-163 MBq) to 8 healthy volunteers (4 men, 4 women). Whole-body (vertex to mid thigh) PET/CT scans were acquired at 6 time points, up to 4 h after tracer injection. Serial whole-blood, plasma, and urine samples were collected for radioactivity measurement and plasma radiotracer metabolites. Tissue (18)F radioactivities were determined from quantitative analysis of the images, and time-activity curves were generated. The total numbers of disintegrations in each organ normalized to injected activity (residence times) were calculated as the area under the curve of the time-activity curve normalized to injected activities and standard organ volumes. Dosimetry calculations were performed using OLINDA/EXM 1.1.

RESULTS

The injection of (18)F-D4-FCH was well tolerated in all subjects, with no radiotracer-related serious adverse event reported. The mean effective dose averaged over both men and women (± SD) was estimated to be 0.025 ± 0.004 (men, 0.022 ± 0.002; women, 0.027 ± 0.002) mSv/MBq. The 5 organs receiving the highest absorbed dose (mGy/MBq) were the kidneys (0.106 ± 0.03), liver (0.094 ± 0.03), pancreas (0.066 ± 0.01), urinary bladder wall (0.047 ± 0.02), and adrenals (0.046 ± 0.01). Elimination was through the renal and hepatic systems.

CONCLUSION

(18)F-D4-FCH is a safe PET radiotracer with a dosimetry profile comparable to other common (18)F PET tracers. These data support the further development of (18)F-D4-FCH for clinical imaging of choline metabolism.

Quantitative fluorescence microscopy provides high resolution imaging of passive diffusion and P-gp mediated efflux at the in vivo blood-brain barrier

Quantitative fluorescent microscopy is an emerging technology that has provided significant insight into cellular dye accumulation, organelle function, and tissue physiology. However, historically dyes have only been used to qualitatively or semi-quantitatively (fold change) determine changes in blood-brain barrier (BBB) integrity. Herein, we present a novel method to calculate the blood to brain transfer rates of the dyes rhodamine 123 and Texas red across the in situ BBB. We observed that rhodamine 123 is subject to p-glycoprotein mediated efflux at the rat BBB and can be increased nearly 20-fold with p-glycoprotein inhibition. However, Texas Red appears to not be subject to MRP2 mediated efflux at the rat BBB, agreeing with literature reports suggesting MRP2 may lack functionality at the normal rat BBB. Lastly, we present data demonstrating that once dyes have crossed the BBB, diffusion of the dye molecule is not as instantaneous as has been previously suggested. We propose that future work can now be completed to (1) match BBB transfer coefficients to interstitial diffusion constants and (2) use dyes with specific affinities to cellular organelles or that have specific properties (e.g., subject to efflux transporters) to more fully understand BBB physiology.

Brain efflux index to investigate the influence of active efflux on brain distribution of pemetrexed and methotrexate

Antifolates, in particular methotrexate (MTX), have been widely used in the treatment of primary and secondary tumors of the central nervous system (CNS). Pemetrexed (PMX) is a novel antifolate that also exhibits potent antitumor activity against CNS malignancies. Studies have shown that brain distribution of both antifolates is significantly restricted, possible due to active efflux transport at the blood-brain barrier (BBB). This study characterizes the brain-to-blood transport of PMX and MTX and examines the role of several efflux transporters in brain distribution of the antifolates by use of the intracerebral microinjection technique (brain efflux index). The results from this study show that both PMX and MTX undergo saturable efflux transport across the BBB, with elimination half-lives of approximately 39 minutes and 29 minutes, respectively. Of the various efflux transporters this study investigated, multidrug resistance-associated protein 2 (Mrp2) does not play an important role in the brain distribution of the two antifolate drugs. Interestingly, breast-cancer resistance protein (Bcrp) makes a significant contribution to the brain elimination of MTX but not PMX. In addition, the brain-to-blood transport of both antifolates was inhibited by probenecid and benzylpenicillin, suggesting the involvement of organic anion transporters in the efflux of these compounds from the brain, with organic anion transporter 3 (Oat3) being a possibility. Our results suggest that one of the underlying mechanisms behind the limited brain distribution of PMX and MTX is active efflux transport processes at the BBB, including a benzylpenicillin-sensitive transport system and/or the active transporter Bcrp.

Exploiting nutrient transporters at the blood-brain barrier to improve brain distribution of small molecules

The blood-brain barrier (BBB) is a major physiological barrier for drugs that target CNS receptors or enzymes. Several methods exist by which permeability to the CNS can be increased, one of which is using native nutrient transporters to carry these drugs through the endothelial cells of the BBB. In this review, we focus on work that characterizes the use of nutrient transporters of the BBB in delivering drugs to the CNS.

Deuteration of a molecular probe for DNP hyperpolarization--a new approach and validation for choline chloride

The promising dynamic nuclear polarization (DNP) for hyperpolarized (13)C-MRI/MRS of real-time metabolism in vivo is challenged by the limited number of agents with the required physical and biological properties. The physical requirement of a liquid-state T(1) of tens of seconds is mostly found for (13)C-carbons in small molecules that have no direct protons attached, i.e. carbonyl, carboxyl and certain quaternary carbons. Unfortunately, such carbon positions do not exist in a large number of metabolic agents, and chemical shift dispersion often limits detection of their chemical evolution. We have previously shown that direct deuteration of protonated carbon positions significantly prolongs the (13)C T(1) in the liquid state and provides potential (13)C-labeled agents with differential chemical shift with respect to metabolism. The Choline Molecular Probe [1,1,2,2-D(4), 2-(13)C]choline chloride (CMP2) has recently been introduced as a means of studying choline metabolism in a hyperpolarized state. Here, the biophysical properties of CMP2 were characterized and compared with those of [1-(13)C]pyruvate to evaluate the impact of molecular probe deuteration. The CMP2 solid-state polarization build-up time constant (30 min) and polarization level (24%) were comparable to those of [1-(13)C]pyruvate. Both compounds' liquid state T(1) increased with temperature. The high-field T(1) of CMP2 compared favorably with [1-(13)C]pyruvate. Thus, a deuterated agent demonstrated physical properties comparable to a hyperpolarized compound of already proven value, whereas both showed chemical shift dispersion that allowed monitoring of their metabolism. It is expected that the use of deuterated carbon-13 positions as reporting hyperpolarized nuclei will substantially expand the library of agents for DNP-MR.

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.

How well can in vitro brain microcapillary endothelial cell models predict rodent in vivo blood-brain barrier permeability?

The object of the study was to improve the blood-brain barrier (BBB) permeability in vitro-invivo correlations (IVIVC) between in vitro brain microcapillary endothelial cell (BMEC) models and the well-tested rodent in situ brain perfusion technique. Porcine, bovine, rat, mouse, and human in vitro BMEC apparent permeability values, P(e), (14 studies from several laboratories: 229 P(e), 60 compounds) were analyzed by a novel biophysical model encoded in a weighted nonlinear regression procedure to determine the aqueous boundary layer (ABL) thickness and the paracellular parameters: porosity-pathlength (dual-pore model), pore radius, and water channel electrostatic potential. The refined parameters were then used to transform the P(e) values into the transendothelial permeability (P(c)) values. Porcine BMEC mono-culture models showed tight junctions comparable to those reported in several Caco-2 studies. Bovine cultures were somewhat leakier. In the human primary cultured cell and the hCMEC/D3 cell line data, IVIVC based on P(e) values has r(2) = 0.14. With transformed permeability values, r(2) = 0.58. Comparable improvements were found in the other species data. By using the in vitro transendothelial P(c) values in place of the apparent P(e) values, IVIVC can be dramatically improved.

A hyperpolarized choline molecular probe for monitoring acetylcholine synthesis

Choline as a reporter molecule has been investigated by in vivo magnetic resonance for almost three decades. Accumulation of choline metabolites (mainly the phosphorylated forms) had been observed in malignancy in preclinical models, ex-vivo, in vivo and in patients. The combined choline metabolite signal appears in (1) H-MRS of the brain and its relative intensity had been used as a diagnostic factor in various conditions. The advent of spin hyperpolarization methods for in vivo use has raised interest in the ability to follow the physiological metabolism of choline into acetylcholine in the brain. Here we present a stable-isotope labeled choline analog, [1,1,2,2-D(4) ,2-(13) C]choline chloride, that is suitable for this purpose. In this analog, the (13) C position showed 24% polarization in the liquid state, following DNP hyperpolarization. This nucleus also showed a long T(1) (35 s) at 11.8 T and 25 °C, which is a prerequisite for hyperpolarized studies. The chemical shift of this (13) C position differentiates choline and acetylcholine from each other and from the other water-soluble choline metabolites, namely phosphocholine and betaine. Enzymatic studies using an acetyltransferase enzyme showed the synthesis of the deuterated-acetylcholine form at thermal equilibrium conditions and in a hyperpolarized state. Analysis using a comprehensive model showed that the T(1) of the formed hyperpolarized [1,1,2,2-D(4) ,2-(13) C]acetylcholine was 34 s at 14.1 T and 37 °C. We conclude that [1,1,2,2-D(4) ,2-(13) C]choline chloride is a promising new molecular probe for hyperpolarized metabolic studies and discuss the factors related to its possible use in vivo.

Nicotine and cotinine increases the brain penetration of saquinavir in rat

Endothelial tight junctions and efflux transporters of the blood-brain barrier (BBB) significantly limit brain accumulation of many drugs, including protease inhibitors such as saquinavir. The cholinergic agonist nicotine is one of the most commonly used drugs in the world and the incidence is even higher in the human immune deficiency virus population (∼ 70%). We examined the ability of nicotine and its primary metabolite cotinine to modify brain uptake of saquinavir in rats. Both nicotine and cotinine at pharmacological concentrations matching those in smokers, increased brain saquinavir uptake by two fold. Co-perfusion with nicotinic receptor antagonists and passive permeability markers showed that the effect was not caused by receptor activation or BBB permeability disruption. Transport inhibition studies demonstrated that brain saquinavir uptake is limited by multiple efflux transporters, P-glycoprotein (P-gp), breast cancer resistance protein and multidrug resistance-associated protein. In situ perfusion and in vitro experiments using a classical P-gp substrate rhodamine 123 linked the effect of nicotine to inhibition of BBB P-gp transport. The effect was confirmed in vivo in chronic 14 day nicotine administration animals. These data suggest nicotine increases antiretroviral drug exposure to brain and may represent a significant in vivo drug-drug interaction at the BBB. Although this may slightly benefit CNS antiretroviral efficacy, it may also expose the brain to potential serious neurotoxicity.

Chronic exposure to nicotine and saquinavir decreases endothelial Notch-4 expression and disrupts blood-brain barrier integrity

Since the advent of HAART, there have been substantial improvements in HIV patient survival; however, the prevalence of HIV associated dementia has increased. Importantly, HIV positive individuals who smoke progress to HIV associated neurological conditions faster than those who do not. Recent in vitro data have shown that pharmacological levels of saquinavir causes endothelial oxidative stress and significantly decreases Notch-4 expression, a primary protein involved in maintaining stability of blood-brain barrier (BBB) endothelium. This is concerning as nicotine can also generate reactive oxygen species in endothelium. It is largely unknown if pharmacological doses of these drugs can cause a similar in vivo down-regulation of Notch-4 and if there is a concurrent destabilization of the integrity of the BBB. The data herein show: (i) nicotine and protease inhibitors cause an additive oxidative stress burden in endothelium; (ii) that the integrity of the BBB is disrupted after concurrent chronic nicotine and protease inhibitor administration; and (iii) that BBB endothelial dysfunction is correlated with a decrease in Notch-4 and ZO-1 expression. Considering the high prevalence of smoking in the HIV infected population (3- to 4-fold higher than in the general population) this data must be followed up to determine if all protease inhibitors cause a similar BBB disruption or if there is a safer alternative. In addition, this data may suggest that the induced BBB disruption may allow foreign molecules to gain access to brain and be a contributing factor to the slow progression of HIV associated dementia.

Brain injury and recovery following binge ethanol: evidence from in vivo magnetic resonance spectroscopy

BACKGROUND

The binge-drinking model in rodents using intragastric injections of ethanol (EtOH) for 4 days results in argyrophilic corticolimbic tissue classically interpreted as indicating irreversible neuronal degeneration. However, recent findings suggest that acquired argyrophilia can also identify injured neurons that have the potential to recover. The current in vivo magnetic resonance (MR) imaging and spectroscopy study was conducted to test the hypothesis that binge EtOH exposure would injure but not cause the death of neurons as previously ascertained postmortem.

METHODS

After baseline MR scanning, 11 of 19 rats received a loading dose of 5 g/kg EtOH via oral gavage, then a maximum of 3 g/kg every 8 hours for 4 days, for a total average cumulative EtOH dose of 43 +/- 1.2 g/kg and average blood alcohol levels of 258 +/- 12 mg/dL. All animals were scanned after 4 days of gavage (post-gavage scan) with EtOH (EtOH group) or dextrose (control [Con] group) and again after 7 days of abstinence from EtOH (recovery scan).

RESULTS

Tissue shrinkage at the post-gavage scan was reflected by significantly increased lateral ventricular volume in the EtOH group compared with the Con group. At the post-gavage scan, the EtOH group had lower dorsal hippocampal N-acetylaspartate and total creatine and higher choline-containing compounds than the Con group. At the recovery scan, neither ventricular volume nor metabolite levels differentiated the groups.

CONCLUSIONS

Rapid recovery of ventricular volume and metabolite levels with removal of the causative agent argues for transient rather than permanent effects of a single EtOH binge episode in rats.

Bis-azaaromatic quaternary ammonium salts as ligands for the blood-brain barrier choline transporter

A series of bis-azaaromatic quaternary ammonium compounds containing flexible polymethylenic linkers as well as conformationally restricted linkers were evaluated for their affinity for the blood-brain barrier choline transporter (BBB-ChT). The preliminary structure-activity relationships obtained from this study suggest that incorporating a linear, conformationally restricted linker into the molecule improves affinity for the BBB-ChT.

Predictive screening model for potential vector-mediated transport of cationic substrates at the blood-brain barrier choline transporter

A set of semi-rigid cyclic and acyclic bis-quaternary ammonium analogs, which were part of a drug discovery program aimed at identifying antagonists at neuronal nicotinic acetylcholine receptors, were investigated to determine structural requirements for affinity at the blood-brain barrier choline transporter (BBB CHT). This transporter may have utility as a drug delivery vector for cationic molecules to access the central nervous system. In the current study, a virtual screening model was developed to aid in rational drug design/ADME of cationic nicotinic antagonists as BBB CHT ligands. Four 3D-QSAR comparative molecular field analysis (CoMFA) models were built which could predict the BBB CHT affinity for a test set with an r(2) <0.5 and cross-validated q(2) of 0.60, suggesting good predictive capability for these models. These models will allow the rapid in silico screening of binding affinity at the BBB CHT of both known nicotinic receptor antagonists and virtual compound libraries with the goal of informing the design of brain bioavailable quaternary ammonium analogs that are high affinity selective nicotinic receptor antagonists.

Nanoparticle Surface Charges Alter Blood–Brain Barrier Integrity and Permeability

PURPOSE

The blood-brain barrier (BBB) presents both a physical and electrostatic barrier to limit brain permeation of therapeutics. Previous work has demonstrated that nanoparticles (NPs) overcome the physical barrier, but there is little known regarding the effect of NP surface charge on BBB function. Therefore, this work evaluated: (1) effect of neutral, anionic and cationic charged NPs on BBB integrity and (2) NP brain permeability.

METHODS

Emulsifying wax NPs were prepared from warm oil-in-water microemulsion precursors using neutral, anionic or cationic surfactants to provide the corresponding NP surface charge. NPs were characterized by particle size and zeta potential. BBB integrity and NP brain permeability were evaluated by in situ rat brain perfusion.

RESULTS

Neutral NPs and low concentrations of anionic NPs were found to have no effect on BBB integrity, whereas, high concentrations of anionic NPs and cationic NPs disrupted the BBB. The brain uptake rates of anionic NPs at lower concentrations were superior to neutral or cationic formulations at the same concentrations.

CONCLUSIONS

(1) Neutral NPs and low concentration anionic NPs can be utilized as colloidal drug carriers to brain, (2) cationic NPs have an immediate toxic effect at the BBB and (3) NP surface charges must be considered for toxicity and brain distribution profiles.

bis-Pyridinium cyclophanes: novel ligands with high affinity for the blood-brain barrier choline transporter

A series of bis-pyridinium cyclophane analogs designed as conformationally restricted bis-quaternary ammonium compounds were evaluated for their affinity for the blood-brain barrier (BBB) choline transporter. All the cyclophanes investigated exhibited high affinity compared to choline. Of these compounds, N, N'-(1,10-decanediyl)3,3'-(1,9-decadiyn-1,10-diyl)-bis-pyridinium diiodide (5c) and N,N'-(1,9-nonanediyl)3,3'-(1,9-decadiyn-1,10-diyl)-bis-pyridinium dibromide (5b) exhibited highest affinity with K(i) values of 0.8 microM and 1.4 microM, respectively, and constitute some of the most potent BBB choline transporter ligands reported.

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.

Norchloro-fluoro-homoepibatidine (NCFHEB) - a promising radioligand for neuroimaging nicotinic acetylcholine receptors with PET

Cholinergic neurotransmission depends on the integrity of nicotinic acetylcholine receptors (nAChRs), and impairment of both is characteristic for various neurodegenerative diseases. Visualization of specific receptor subtypes by positron emission tomography (PET) has potential to assist with diagnosis of such neurodegenerative diseases and with design of suitable therapeutic approaches. The goal of our study was to evaluate in vivo the potential of (18)F-labelled (+)- and (-)-norchloro-fluoro-homoepibatidine ([(18)F]NCFHEB) in comparison to 2-[(18)F]F-A-85380 as PET tracers. In the brains of NMRI mice, highest levels of radioactivity were detected at 20 min post-injection of (+)-[(18)F]NCFHEB, (-)-[(18)F]NCFHEB, and 2-F-[(18)F]-A-85380 (7.45, 5.60, and 3.2% ID/g tissue, respectively). No marked pharmacological adverse effects were observed at 25 mug NCFHEB/kg. Uptake studies in RBE4 cells and in situ perfusion studies suggest an interaction of epibatidine and NCFHEB with the carrier-mediated choline transport at the blood-brain barrier. The data indicate that (+)- and (-)-[(18)F]NCFHEB have potential for further development as PET tracers.

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.

Putative proteins involved in manganese transport across the blood-brain barrier

Manganese (Mn) is an essential nutrient required for proper growth and maintenance of numerous biological systems. At high levels it is known to be neurotoxic. While focused research concerning the transport of Mn across the blood-brain barrier (BBB) is on-going, the exact identity of the transporter(s) responsible is still debated. The transferrin receptor (TfR) and the divalent metal transporter-1 (DMT-1) have long been thought to play a role in brain Mn deposition. However, evidence suggests that Mn may also be transported by other proteins. One model system of the BBB, rat brain endothelial (RBE4) cells, are known to express many proteins suspected to be involved in metal transport. This review will discuss the biological importance of Mn, and then briefly describe several proteins that may be involved in transport of this metal across the BBB. The latter section will examine the potential usefulness of RBE4 cells in characterizing various aspects of Mn transport, and basic culture techniques involved in working with these cells. It is hoped that ideas put forth in this article will stimulate further investigations into the complex nature of Mn transport, and address the importance as well as the limitation of in vitro models in answering these questions.

Discovery of a novel nicotinic receptor antagonist for the treatment of nicotine addiction: 1-(3-Picolinium)-12-triethylammonium-dodecane dibromide (TMPD)

Limitations in efficacy and high relapse rates of currently available smoking cessation agents reveal the need for more efficacious pharmacotherapies. One strategy is to develop subtype-selective nicotinic receptor (nAChR) antagonists that inhibit nicotine-evoked dopamine (DA) release, the primary neurotransmitter involved in nicotine reward. Simple alkylation of the pyridino N-atom converts nicotine from a potent agonist into a potent antagonist. The classical antagonists, hexamethonium and decamethonium, differentiate between peripheral nAChR subtypes. Using a similar approach, we interconnected varying quaternary ammonium moieties with a lipophilic linker to provide N,N'-bis-nicotinium analogs, affording a lead compound, N,N'-dodecyl-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), which inhibited nicotine-evoked DA release and decreased nicotine self-administration. The current work describes a novel compound, 1-(3-picolinium)-12-triethylammonium-dodecane dibromide (TMPD), a hybrid of bPiDDB and decamethonium. TMPD completely inhibited (IC(50)=500 nM) nicotine-evoked DA release from superfused rat striatal slices, suggesting that TMPD acts as a nAChR antagonist at more than one subtype. TMPD (1 microM) inhibited the response to acetylcholine at alpha3beta4, alpha4beta4, alpha4beta2, and alpha1beta1varepsilondelta receptors expressed in Xenopus oocytes. TMPD had a 2-fold higher affinity than choline for the blood-brain barrier choline transporter, suggesting brain bioavailability. TMPD did not inhibit hyperactivity in nicotine sensitized rats, but significantly and specifically decreased nicotine self-administration. Together, the results suggest that TMPD may have the ability to reduce the rewarding effect of nicotine with minimal side effects, a pharmacological profile indicative of potential clinical utility for the treatment of tobacco dependence.

Nicotine Exposure Does not Alter Plasma to Brain Choline Transfer

Acute and chronic nicotine exposure in rats is associated with an increase in brain acetylcholine (ACh) transmission. The acquisition of choline for neuronal ACh synthesis occurs primarily via two pathways; first, free choline is transported from the blood across the blood-brain barrier (BBB) and/or second, from synaptic choline generated by either hydrolysis of non-bound ACh or membrane phosphatidylcholine catabolism. To determine if nicotine-induced cholinergic demand is associated with increased choline transport rates into brain, we measured BBB choline transport in naïve and S-(-) nicotine exposed rats (acute and chronic, 4.5 mg/kg/d for 1, 14, 21 and 28 d; osmotic minipumps) using the in situ rat brain perfusion technique. No significant changes in choline uptake after acute or chronic nicotine exposure were observed in whole brain or cortex. Of considerable interest was a significant decrease in regional brain choline uptake measured in the hippocampus after chronic nicotine exposure (28 d). Our data suggest that the increased ACh transmission observed after nicotine exposure does not correlate with increased blood-to-brain transfer of choline. Considering these data and previous literature reports, we propose that the additional free choline required under conditions of nicotine exposure (for ACh synthesis) is primarily recruited from membrane phospholipid metabolism.

Drug delivery across the blood–brain barrier: why is it difficult? how to measure and improve it?

The development of drugs that act in the CNS has been significantly impeded by the difficulty of delivering them across the blood-brain barrier (BBB). This article aims to provide the reader with a critical overview of important issues in the discovery and development of drugs that need to enter the brain to elicit pharmacological activity, focusing particularly on i) the role of drug transporters in brain permeation and how to manipulate them to enhance drug brain bioavailability; ii) the successful application, limitations and challenges of commonly used in vitro and in vivo methodologies for measuring drug transport across the BBB, and iii) a discussion of recently developed strategies (e.g., modulation of efflux transporters by chemical inhibitors and the employment of delivery vectors taking advantage of native transport systems at the BBB) for facilitating drug penetration into the brain.

Uptake of 18F-fluorocholine, 18F-fluoro-ethyl-L-tyrosine and 18F-fluoro-2-deoxyglucose in F98 gliomas in the rat

INTRODUCTION

The positron emission tomography (PET) tracers (18)F-fluoro-ethyl-L: -tyrosine (FET), (18)F-fluorocholine (N,N-dimethyl-N-[(18)F]fluoromethyl-2-hydroxyethylammonium (FCH]) and (18)F-fluoro-2-deoxyglucose (FDG) are used in the diagnosis of brain tumours. The aim of this study was threefold: (a) to assess the uptake of the different tracers in the F98 rat glioma, (b) to evaluate the impact of blood-brain barrier (BBB) disruption and microvessel density (MVD) on tracer uptake and (c) to compare the uptake in the tumours to that in the radiation injuries (induced by proton irradiation of healthy rats) of our previous study.

METHODS

F98 gliomas were induced in 26 rats. The uptake of FET, FCH and FDG was measured using autoradiography and correlated with histology, disruption of the BBB and MVD.

RESULTS

The mean FET, FCH and FDG standardised uptake values (SUVs) in the tumour and the contralateral normal cortex (in parentheses) were 4.19+/-0.86 (1.32+/-0.26), 2.98+/-0.58 (0.51+/-0.11) and 11.02+/-3.84 (4.76+/-1.77) respectively. MVD was significantly correlated only with FCH uptake. There was a trend towards a negative correlation between the degree of BBB disruption and FCH uptake and a trend towards a positive correlation with FET uptake. The ratio of the uptake in tumours to that in the radiation injuries was 1.97 (FCH), 2.71 (FET) and 2.37 (FDG).

CONCLUSION

MVD displayed a significant effect only on FCH uptake. The degree of BBB disruption seems to affect the accumulation of FET and FCH, but not FDG. Mean tumour uptake for all tracers was significantly higher than the accumulation in radiation injuries.

Neurochemical and oedematous changes in 1,3-dinitrobenzene-induced astroglial injury in rat brain from a 1H-nuclear magnetic resonance perspective

1,3-Dinitrobenzene (1,3-DNB), an intermediate used in the chemical industry, has toxic effects in the brain and testes. It produces focal lesions with marked astroglial necrosis in the rat brain upon repeated administration. Astrocytic death occurs in parallel with elevated local blood flow and is followed by damage to the cerebral vasculature and neurones. (1)H-nuclear magnetic resonance spectroscopic analysis before the onset of astrocytic damage, showed a global elevation of lactate, whereas choline containing compounds increased in the non-vulnerable cerebral cortex, yet decreased in the vulnerable brainstem. Similarly, glutamate increased in the cerebral cortex, cerebellum and midbrain, but decreased in the susceptible brainstem. In vivo T2-weighted NMR imaging showed high signal intensities in brain nuclei shown to develop astroglial loss by conventional neuropathology at 24 hours after completion of dosing, but not at 6-10 hours. Hence the early neurochemical changes in susceptible areas contribute to the aetiology of degeneration, and those seen elsewhere may represent adaptive responses dependent on the particular phenotype of different cell groups and underlying metabolic relationships.

Inhibition of choline uptake by N-cyclohexylcholine, a high affinity ligand for the choline transporter at the blood-brain barrier

The blood-brain barrier (BBB) choline transporter (CHT) may have utility as a drug delivery vector for drugs that act in the central nervous system. Previous studies suggested the importance of hydrophobic moieties on the cationic nitrogen of choline for improved affinity for this transporter. In a pilot study, we therefore designed five novel N-cycloalkyl derivatives of choline, one of which showed promising inhibition properties. This choline analogue had a cyclohexyl (UMBB-5) moiety substituting one of the methyl groups attached to the cationic nitrogen in choline. In situ experimental data were obtained from in situ rat brain perfusion studies. The binding affinity for the BBB-choline transporter found for UMBB-5 was K(i)=1.9 microM. Comparative molecular field analysis (CoMFA) suggested that the cyclohexyl moiety orientates towards a steric favourable area. Taken together, the results of these in situ and in silico studies provide further evidence or restrictions that occur with binding to this brain drug delivery vector.