The blood-brain barrier choline transporter as a brain drug delivery vector

The Microbiome in Child and Adolescent Psychiatry

Recent research has increasingly emphasized the function of the microbiome in human health. The gut microbiome is essential for digesting food and seems to play a vital role in mental health as well. This review briefly overviews the gut microbiome and its interplay with the central nervous system. We then summarize some of the latest findings on the possible role of the microbiome in psychiatric disorders in children and adolescents. In particular, we focus on autism spectrum disorder, attention-deficit/hyperactivity disorder, anorexia nervosa, bipolar disorder, and major depressive disorder. Although the role of microbiota in mental development and health still needs to be researched intensively, it has become increasingly apparent that the impact of microbiota must be considered to better understand psychiatric disorders.

Drug-Loaded Zwitterion-Based Nanomotors for the Treatment of Spinal Cord Injury

Spinal cord injury (SCI) treatment requires a nanosystem for drug delivery that can effectively penetrate the blood-spinal cord barrier (BSCB). Herein, we designed poly(2-methacryloyloxyethyl phosphorylgallylcholine) (PMPC)/l-arginine (PMPC/A)-based nanomotors that can release nitric oxide (NO). The nanomotors were loaded with the inducible NO synthase inhibitor 1400W and nerve growth factor (NGF). PMPC with a zwitterionic structure not only provided good biocompatibility for the nanomotors but also facilitated their passage through the BSCB owing to the assistance of a large number of choline transporters on the BSCB. Additionally, the l-arginine loaded on the nanomotors was able to react with reactive oxygen species in the microenvironment of the injured nerve to produce NO, thereby conferring the ability of autonomic movement to the nanomotors, which facilitated the uptake of drugs by cells in damaged areas and penetration in pathological tissues. Moreover, in vivo animal experiments indicated that the PMPC/A/1400W/NGF nanomotors could effectively pass through the BSCB and restore the motion function of a rat SCI model by regulating its internal environment as well as the release of therapeutic drugs. Thus, the drug delivery system based on nanomotor technology offers a promising strategy for treating central nervous system diseases.

Lipid nanoparticle-mediated drug delivery to the brain

Lipid nanoparticles (LNPs) have revolutionized the field of drug delivery through their applications in siRNA delivery to the liver (Onpattro) and their use in the Pfizer-BioNTech and Moderna COVID-19 mRNA vaccines. While LNPs have been extensively studied for the delivery of RNA drugs to muscle and liver targets, their potential to deliver drugs to challenging tissue targets such as the brain remains underexplored. Multiple brain disorders currently lack safe and effective therapies and therefore repurposing LNPs could potentially be a game changer for improving drug delivery to cellular targets both at and across the blood-brain barrier (BBB). In this review, we will discuss (1) the rationale and factors involved in optimizing LNPs for brain delivery, (2) ionic liquid-coated LNPs as a potential approach for increasing LNP accumulation in the brain tissue and (3) considerations, open questions and potential opportunities in the development of LNPs for delivery to the brain.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

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.

Cysteine Donor-Based Brain-Targeting Prodrug: Opportunities and Challenges

Overcoming blood-brain barrier (BBB) to improve brain bioavailability of therapeutic drug remains an ongoing concern. Prodrug is one of the most reliable approaches for delivering agents with low-level BBB permeability into the brain. The well-known antioxidant capacities of cysteine (Cys) and its vital role in glutathione (GSH) synthesis indicate that Cys-based prodrug could potentiate therapeutic drugs against oxidative stress-related neurodegenerative disorders. Moreover, prodrug with Cys moiety could be recognized by the excitatory amino acid transporter 3 (EAAT3) that is highly expressed at the BBB and transports drug into the brain. In this review, we summarized the strategies of crossing BBB, properties of EAAT3 and its natural substrates, Cys and its donors, and Cys donor-based brain-targeting prodrugs by referring to recent investigations. Moreover, the challenges that we are faced with and future research orientations were also addressed and proposed. It is hoped that present review will provide evidence for the pursuit of novel Cys donor-based brain-targeting prodrug.

Claudin-1-Targeted Nanoparticles for Delivery to Aging-Induced Alterations in the Blood-Brain Barrier

Aging-induced alterations to the blood-brain barrier (BBB) are increasingly being seen as a primary event in chronic progressive neurological disorders that lead to cognitive decline. With the goal of increasing delivery into the brain in hopes of effectively treating these diseases, a large focus has been placed on developing BBB permeable materials. However, these strategies have suffered from a lack of specificity toward regions of disease progression. Here, we report on the development of a nanoparticle (C1C2-NP) that targets regions of increased claudin-1 expression that reduces BBB integrity. Using dynamic contrast enhanced magnetic resonance imaging, we find that C1C2-NP accumulation and retention is significantly increased in brains from 12 month-old mice as compared to nontargeted NPs and brains from 2 month-old mice. Furthermore, we find C1C2-NP accumulation in brain endothelial cells with high claudin-1 expression, suggesting target-specific binding of the NPs, which was validated through fluorescence imaging, in vitro testing, and biophysical analyses. Our results further suggest a role of claudin-1 in reducing BBB integrity during aging and show altered expression of claudin-1 can be actively targeted with NPs. These findings could help develop strategies for longitudinal monitoring of tight junction protein expression changes during aging as well as be used as a delivery strategy for site-specific delivery of therapeutics at these early stages of disease development.

Normalization of Cerebral Blood Flow, Neurochemicals, and White Matter Integrity after Kidney Transplantation

BACKGROUND

CKD is associated with abnormalities in cerebral blood flow, cerebral neurochemical concentrations, and white matter integrity. Each of these is associated with adverse clinical consequences in the non-CKD population, which may explain the high prevalence of dementia and stroke in ESKD. Because cognition improves after kidney transplantation, comparing these brain abnormalities before and after kidney transplantation may identify potential reversibility in ESKD-associated brain abnormalities.

METHODS

In this study of patients with ESKD and age-matched healthy controls, we used arterial spin labeling to assess the effects of kidney transplantation on cerebral blood flow and magnetic resonance spectroscopic imaging to measure cerebral neurochemical concentrations (N-acetylaspartate, choline, glutamate, glutamine, myo-inositol, and total creatine). We also assessed white matter integrity measured by fractional anisotropy (FA) and mean diffusivity (MD) with diffusion tensor imaging. We used a linear mixed model analysis to compare longitudinal, repeated brain magnetic resonance imaging measurements before, 3 months after, and 12 months after transplantation and compared these findings with those of healthy controls.

RESULTS

Study participants included 29 patients with ESKD and 19 controls; 22 patients completed post-transplant magnetic resonance imaging. Cerebral blood flow, which was higher in patients pretransplant compared with controls (P=0.003), decreased post-transplant (P<0.001) to values in controls. Concentrations of neurochemicals choline and myo-inositol that were higher pretransplant compared with controls (P=0.001 and P<0.001, respectively) also normalized post-transplant (P<0.001 and P<0.001, respectively). FA increased (P=0.001) and MD decreased (P<0.001) post-transplant.

CONCLUSIONS

Certain brain abnormalities in CKD are reversible and normalize with kidney transplantation. Further studies are needed to understand the mechanisms underlying these brain abnormalities and to explore interventions to mitigate them even in patients who cannot be transplanted.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Cognitive Impairment and Imaging Correlates in End Stage Renal Disease, NCT01883349.

Ligand Conjugated Targeted Nanotherapeutics for Treatment of Neurological Disorders

BACKGROUND

Human brain is amongst the most complex organs in human body, and delivery of therapeutic agents across the brain is a tedious task. Existence of blood brain barrier (BBB) protects the brain from invasion of undesirable substances; therefore it hinders the transport of various drugs used for the treatment of different neurological diseases including glioma, Parkinson's disease, Alzheimer's disease, etc. To surmount this barrier, various approaches have been used such as the use of carrier mediated drug delivery; use of intranasal route, to avoid first pass metabolism; and use of ligands (lactoferrin, apolipoprotein) to transport the drug across the BBB. Ligands bind with proteins present on the cell and facilitate the transport of drug across the cell membrane via. receptor mediated, transporter mediated or adsorptive mediated transcytosis.

OBJECTIVE

The main focus of this review article is to illustrate various studies performed using ligands for delivering drug across BBB; it also describes the procedure used by various researchers for conjugating the ligands to the formulation to achieve targeted action.

METHODS

Research articles that focused on the used of ligand conjugation for brain delivery and compared the outcome with unconjugated formulation were collected from various search engines like PubMed, Science Direct and Google Scholar, using keywords like ligands, neurological disorders, conjugation, etc. Results and Conclusion: Ligands have shown great potential in delivering drug across BBB for treatment of various diseases, yet extensive research is required so that the ligands can be used clinically for treating neurological diseases.

Choline attenuates inflammatory hyperalgesia activating nitric oxide/cGMP/ATP-sensitive potassium channels pathway

New findings on neural regulation of immunity are allowing the design of novel pharmacological strategies to control inflammation and nociception. Herein, we report that choline, a 7-nicotinic acetylcholine receptor (α7nAChRs) agonist, prevents carrageenan-induced hyperalgesia without affecting inflammatory parameters (neutrophil migration or cytokine/chemokines production) or inducing sedation or even motor impairment. Choline also attenuates prostaglandin-E₂ (PGE₂)-induced hyperalgesia via α7nAChR activation and this antinociceptive effect was abrogated by administration of LNMMA (a nitric oxide synthase inhibitor), ODQ (an inhibitor of soluble guanylate cyclase; cGMP), andglibenclamide(an inhibitor of ATP-sensitive potassium channels). Furthermore, choline attenuates long-lasting Complete Freund's Adjuvant and incision-induced hyperalgesia suggesting its therapeutic potential to treat pain in rheumatoid arthritis or post-operative recovery, respectively. Our results suggest that choline modulates inflammatory hyperalgesia by activating the nitric oxide/cGMP/ATP-sensitive potassium channels without interfering in inflammatory events, and could be used in persistent pain conditions.

Efficient Delivery of Nerve Growth Factors to the Central Nervous System for Neural Regeneration

The central nervous system (CNS) plays a central role in the control of sensory and motor functions, and the disruption of its barriers can result in severe and debilitating neurological disorders. Neurotrophins are promising therapeutic agents for neural regeneration in the damaged CNS. However, their penetration across the blood-brain barrier remains a formidable challenge, representing a bottleneck for brain and spinal cord therapy. Herein, a nanocapsule-based delivery system is reported that enables intravenously injected nerve growth factor (NGF) to enter the CNS in healthy mice and nonhuman primates. Under pathological conditions, the delivery of NGF enables neural regeneration, tissue remodeling, and functional recovery in mice with spinal cord injury. This technology can be utilized to deliver other neurotrophins and growth factors to the CNS, opening a new avenue for tissue engineering and the treatment of CNS disorders and neurodegenerative diseases.

Emerging transporter-targeted nanoparticulate drug delivery systems

Transporter-targeted nanoparticulate drug delivery systems (nano-DDS) have emerged as promising nanoplatforms for efficient drug delivery. Recently, great progress in transporter-targeted strategies has been made, especially with the rapid developments in nanotherapeutics. In this review, we outline the recent advances in transporter-targeted nano-DDS. First, the emerging transporter-targeted nano-DDS developed to facilitate oral drug delivery are reviewed. These include improvements in the oral absorption of protein and peptide drugs, facilitating the intravenous-to-oral switch in cancer chemotherapy. Secondly, the recent advances in transporter-assisted brain-targeting nano-DDS are discussed, focusing on the specific transporter-based targeting strategies. Recent developments in transporter-mediated tumor-targeting drug delivery are also discussed. Finally, the possible transport mechanisms involved in transporter-mediated endocytosis are highlighted, with special attention to the latest findings of the interactions between membrane transporters and nano-DDS.

Identification of Novel Agents for the Treatment of Brain Metastases of Breast Cancer

BACKGROUND

Brain cancer from metastasized breast cancer has a high mortality rate in women. The treatment of lesions is hampered in large part by the blood-brain barrier (BBB), which prevents adequate distribution of anti-cancer compounds to brain metastases.

METHOD

In this study we used a novel screening method to identify candidate molecules that are well-suited to utilizing the BBB choline transporter for distribution into the brain parenchyma.

RESULTS

From our screen we identified two compounds, Ch-1 and Ch-2 that were able to reduce the brain tumor burden in a murine mouse model of brain metastasis of breast cancer. These compounds also significantly increased the survival of mice by more than 10 days. Mechanistic studies indicated that Ch-1 is able to prevent the activation of the pro-survival mitogen-activated kinases (MAPKs) by osteoactivin (OA; Glycoprotein nonmetastatic melanoma protein B GPNMB).

CONCLUSION

The results from this study show that nutrient transporter virtual screening is a viable novel alternative to traditional drug screening programs to identify anti-cancer compounds for the treatment of brain cancers.

Transporter-Guided Delivery of Nanoparticles to Improve Drug Permeation across Cellular Barriers and Drug Exposure to Selective Cell Types

Targeted nano-drug delivery systems conjugated with specific ligands to target selective cell-surface receptors or transporters could enhance the efficacy of drug delivery and therapy. Transporters are expressed differentially on the cell-surface of different cell types, and also specific transporters are expressed at higher than normal levels in selective cell types under pathological conditions. They also play a key role in intestinal absorption, delivery via non-oral routes (e.g., pulmonary route and nasal route), and transfer across biological barriers (e.g., blood–brain barrier and blood–retinal barrier. As such, the cell-surface transporters represent ideal targets for nano-drug delivery systems to facilitate drug delivery to selective cell types under normal or pathological conditions and also to avoid off-target adverse side effects of the drugs. There is increasing evidence in recent years supporting the utility of cell-surface transporters in the field of nano-drug delivery to increase oral bioavailability, to improve transfer across the blood–brain barrier, and to enhance delivery of therapeutics in a cell-type selective manner in disease states. Here we provide a comprehensive review of recent advancements in this interesting and important area. We also highlight certain key aspects that need to be taken into account for optimal development of transporter-assisted nano-drug delivery systems.

Chirality sensing of choline derivatives by a triple anion helicate cage through induced circular dichroism

A racemic A₂L₃ triple anion helicate cage is able to sense chiral choline derivatives by induced circular dichroism.

Nanocarriers for brain specific delivery of anti-retro viral drugs: challenges and achievements

HIV/AIDS is a global pandemic and the deleterious effects of human immunodeficiency virus in the brain cannot be overlooked. Though the current anti-retro viral therapy is able to reduce the virus load in the peripheral tissues of the body, the inability of the anti-retro viral drugs to cross the blood brain barrier, as such, limits its therapeutic effect in the brain. The development of newer, successful nanoparticulate drug delivery systems to enhance the feasibility of the anti-retro viral drugs to the brain, offers a novel strategy to treat the AIDS-related neuronal degradation. This review summarised the neuropathogenesis of neuroAIDS, the challenges and achievements made in the delivery of therapeutics across the BBB and the use of nanocarriers as a safe and effective way for delivering anti-retro viral drugs to the brain.

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.

Molecular properties of the high-affinity choline transporter CHT1

This article summarizes molecular properties of the high-affinity choline transporter (CHT1) with reference to the historical background focusing studies performed in laboratories of the author. CHT1 is present on the presynaptic terminal of cholinergic neurons, and takes up choline which is the precursor of acetylcholine. The Na(+)-dependent uptake of choline by CHT1 is the rate-limiting step for synthesis of acetylcholine. CHT1 is the integral membrane protein with 13 transmembrane segments, belongs to the Na(+)/glucose co-transporter family (SLC5), and has 20-25% homology with members of this family. A single nucleotide polymorphism (SNP) for human CHT1 has been identified, which has a replacement from isoleucine to valine in the third transmembrane segment and shows the choline uptake activity of 50-60% as much as that of wild-type CHT1. The proportion of this SNP is high among Asians. Possible importance of choline diet for those with this SNP was discussed.

A choline derivate-modified nanoprobe for glioma diagnosis using MRI

Gadolinium (Gd) chelate contrast-enhanced magnetic resonance imaging (MRI) is a preferred method of glioma detection and preoperative localisation because it offers high spatial resolution and non-invasive deep tissue penetration. Gd-based contrast agents, such as Gd-diethyltriaminepentaacetic acid (DTPA-Gd, Magnevist), are widely used clinically for tumor diagnosis. However, the Gd-based MRI approach is limited for patients with glioma who have an uncompromised blood-brain barrier (BBB). Moreover, the rapid renal clearance and non-specificity of such contrast agents further hinders their prevalence. We present a choline derivate (CD)-modified nanoprobe with BBB permeability, glioma specificity and a long blood half-life. Specific accumulation of the nanoprobe in gliomas and subsequent MRI contrast enhancement are demonstrated in vitro in U87 MG cells and in vivo in a xenograft nude model. BBB and glioma dual targeting by this nanoprobe may facilitate precise detection of gliomas with an uncompromised BBB and may offer better preoperative and intraoperative tumor localization.

In vivo pharmacological interactions between a type II positive allosteric modulator of α7 nicotinic ACh receptors and nicotinic agonists in a murine tonic pain model

BACKGROUND AND PURPOSE

The α7 nicotinic ACh receptor subtype is abundantly expressed in the CNS and in the periphery. Recent evidence suggests that α7 nicotinic ACh receptor (nAChR) subtypes, which can be activated by an endogenous cholinergic tone comprising ACh and the α7 agonist choline, play an important role in chronic pain and inflammation. In this study, we evaluated whether type II α7 positive allosteric modulator PNU-120596 induces antinociception on its own and in combination with choline in the formalin pain model.

EXPERIMENTAL APPROACH

We assessed the effects of PNU-120596 and choline and the nature of their interactions in the formalin test using an isobolographic analysis. In addition, we evaluated the interaction of PNU-120596 with PHA-54613, an exogenous selective α7 nAChR agonist, in the formalin test. Finally, we assessed the interaction between PNU-120596 and nicotine using acute thermal pain, locomotor activity, body temperature and convulsing activity tests in mice.

KEY RESULTS

We found that PNU-120596 dose-dependently attenuated nociceptive behaviour in the formalin test after systemic administration in mice. In addition, mixtures of PNU-120596 and choline synergistically reduced formalin-induced pain. PNU-120596 enhanced the effects of nicotine and α7 agonist PHA-543613 in the same test. In contrast, PNU-120596 failed to enhance nicotine-induced convulsions, hypomotility and antinociception in acute pain models. Surprisingly, it enhanced nicotine-induced hypothermia via activation of α7 nAChRs.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that type II α7 positive allosteric modulators produce antinociceptive effects in the formalin test through a synergistic interaction with the endogenous α7 agonist choline.

In vivo SELEX for Identification of Brain-penetrating Aptamers

The physiological barriers of the brain impair drug delivery for treatment of many neurological disorders. One delivery approach that has not been investigated for their ability to penetrate the brain is RNA-based aptamers. These molecules can impart delivery to peripheral tissues and circulating immune cells, where they act as ligand mimics or can be modified to carry payloads. We developed a library of aptamers and an in vivo evolution protocol to determine whether specific aptamers could be identified that would home to the brain after injection into the peripheral vasculature. Unlike biopanning with recombinant bacteriophage libraries, we found that the aptamer library employed here required more than 15 rounds of in vivo selection for convergence to specific sequences. The aptamer species identified through this approach bound to brain capillary endothelia and penetrated into the parenchyma. The methods described may find general utility for targeting various payloads to the brain.Molecular Therapy - Nucleic Acids (2013) 2, e67; doi:10.1038/mtna.2012.59; published online 8 January 2013.

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.

Gene expression of zebrafish embryos exposed to titanium dioxide nanoparticles and hydroxylated fullerenes

Increased release of engineered nanoparticles to the environment suggests a rising need for the monitoring and evaluation of potential toxicity. Zebrafish frequently have been used as a model species in human and aquatic toxicology studies. In this study, zebrafish embryos were microinjected in the otic vesicle with a sublethal dose of engineered nanoparticles (titanium dioxide/TiO(2) and hydroxylated fullerenes/C(60)(OH)(24)). A gene microarray analysis was performed on injected and control embryos to determine the potential for nanoparticles to change the expression of genes involved in cross talk of the nervous and immune systems. The exposure to TiO(2) and hydroxylated fullerenes caused shifts in gene regulation response patterns that were similar for downregulated genes but different for upregulated genes. Significant effects on gene regulation were observed on genes involved in circadian rhythm, kinase activity, vesicular transport and immune response. This is the first report of circadian rhythm gene deregulation by nanoparticles in aquatic animals, indicating the potential for broad physiological and behavioral effects controlled by the circadian system.

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.

Intrathecal baclofen in patients with persistent vegetative state: 2 hypotheses

Sporadic cases of recovery from persistent vegetative state (PVS) after administration of intrathecal baclofen (ITB) have been reported without giving any possible explanation for its paradoxical effect. We summarize our recent findings on 5 patients with PVS treated with ITB and make some speculations on the mechanisms responsible for the observed clinical improvement. The patients developed spasticity and were judged eligible for ITB therapy. Two weeks after pump implantation, patients began to show a clinical improvement that, at the end of the 6 months' follow-up, was stable in all but 1 patient, ranging from a mere increased alertness to a full recovery of consciousness, as revealed by changes of the Coma Recovery Scale-Revised (CRS-R) score. Our findings suggest that ITB might favor a variable degree of clinical improvement. A proposal for a pharmacodynamic explanation of this effect has not been formally put forward. We hypothesize 2 possible mechanisms: first, a modulation confined to spinal cord segmental activities and to neuronal centripetal outputs reaching the cortex; and second, a modulation of sleep-wake cycles that, although present, may be dysregulated and interfere with alertness and awareness. Although our research is confined to a few subjects, it provides follow-up information by means of the CRS-R that is a validated standardized neurobehavioral instrument expressly designed for use in patients with PVS. Our observations indicate that further systematic investigation of the mechanisms and the putative clinical applications of ITB should be undertaken.

Inner blood-retinal barrier transporters: role of retinal drug delivery

The inner blood-retinal barrier (inner BRB) forms complex tight junctions of retinal capillary endothelial cells to prevent the free diffusion of substances between the circulating blood and the neural retina. Thus, understanding of the inner BRB transport mechanisms could provide a basis for the development of strategies for drug delivery to the retina. Recent progress in inner BRB research has revealed that retinal endothelial cells express a variety of unique transporters which play a role in the influx transport of essential molecules and the efflux transport of xenobiotics. In this review we focus on the transport mechanism at the inner BRB in relation to its importance in influencing the inner BRB permeability of drugs.

Nanotechnology for delivery of drugs to the brain for epilepsy

Epilepsy results from aberrant electrical activity that can affect either a focal area or the entire brain. In treating epilepsy with drugs, the aim is to decrease seizure frequency and severity while minimizing toxicity to the brain and other tissues. Antiepileptic drugs (AEDs) are usually administered by oral and intravenous routes, but these drug treatments are not always effective. Drug access to the brain is severely limited by a number of biological factors, particularly the blood-brain barrier, which impedes the ability of AEDs to enter and remain in the brain. To improve the efficacy of AEDs, new drug delivery strategies are being developed; these methods fall into the three main categories: drug modification, blood-brain barrier modification, and direct drug delivery. Recently, all three methods have been improved through the use of drug-loaded nanoparticles.

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.

The possible role of ammonia toxicity on the exposure, deposition, retention, and the bioavailability of nicotine during smoking

A complete and rigorous review is presented of the possible effect(s) of ammonia on the exposure, deposition and retention of nicotine during smoking and the bioavailability of nicotine to the smoker. There are no toxicological data in humans regarding ammonia exposure within the context of tobacco smoke. Extrapolation from occupational exposure of ammonia to smoking in humans suggests minimal, non-toxicological effects, if any. No direct study has examined the effect of the ammonia on the total rate or amount of nicotine reaching the arterial bloodstream or brains of smokers. Machine-smoking methods have been reported which accurately quantify >99% of the nicotine in mainstream (MS) smoke for a wide variety of commercial and test cigarettes, including a series of experimental cigarettes having a range in MS smoke ammonia yields using the US Federal Trade Commission (FTC) protocol. However, the actual exposure of nicotine to smokers depends on their own smoking behavior. The nicotine ring system is relatively thermally stable. Protonated nicotine forms nicotine which evaporates before the nicotine ring system decomposes. The experimental data indicate that neither nicotine transfer from tobacco to MS smoke nor nicotine bioavailability to the smoker increases with an increase in any of the following properties: tobacco soluble ammonia, MS smoke ammonia, "tobacco pH" or "smoke pH" at levels found in commercial cigarettes. Gas phase nicotine deposits primarily in the mouth and upper respiratory tract. To the extent that ammonia increases the deposition of nicotine in the buccal cavity and upper respiratory tract during smoking, the total rate and amount of nicotine into the arterial bloodstream and to the central nervous system will decrease. Charged nicotine analogues are actively transported in a number of tissues. This active transport system appears to be insensitive to pH and the form of nicotine in the biological milieu, suggesting that protonated nicotine may be a substrate for active transport. Neither "smoke pH" of commercial cigarettes nor "smoke pHeff" nor the fraction of non-protonated nicotine in tobacco smoke particulate matter are useful, practical smoke parameters for providing understanding or predictability of nicotine bioavailability to smokers. Greater than 95% of both ammonia and nicotine are in the gas phase of environmental tobacco, and both are likely to deposit in the buccal cavity and upper respiratory tract following exposure.

Acetylcholine receptors in dementia and mild cognitive impairment

PURPOSE

To clarify whether changes in the cholinergic transmission occur early in the course of Alzheimer's disease (AD), we carried out positron emission tomography (PET) with the radioligand 2-[(18)F]F-A-85380, which is supposed to be specific for alpha4beta2 nicotinic acetylcholine receptors (nAChRs).

METHOD

We included patients with moderate to severe AD and patients with amnestic mild cognitive impairment (MCI), presumed to present preclinical AD.

RESULTS

Both patients with AD and MCI showed significant reductions in alpha4beta2 nAChRs in brain regions typically affected by AD pathology. These findings indicate that a reduction in alpha4beta2 nAChRs occurs during early symptomatic stages of AD. The alpha4beta2 nAChR availability in these regions correlated with the severity of cognitive impairment, indicating a stage sensitivity of the alpha4beta2 nAChR status.

CONCLUSION

Together, our results provide evidence for the potential of 2-[(18)]F-A-85380 nAChR PET in the diagnosis of patients at risk for AD. Because of the extraordinary long acquisition time with 2-[(18)F]F-A-85380, we developed the new alpha4beta2 nAChR-specific radioligands (+)- and (-)-[(18)F]norchloro-fluoro-homoepibatidine (NCFHEB) and evaluated them preclinically. (-)-[(18)F]NCFHEB shows twofold higher brain uptake and significantly shorter acquisition times. Therefore, (-)-[(18)F]NCFHEB should be a suitable radioligand for larger clinical investigations.

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.

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.

Postnatal dietary choline supplementation alters behavior in a mouse model of Rett syndrome

Rett syndrome (RTT), a neurodevelopmental disorder primarily affecting females, is accompanied by behavioral and neuropathological abnormalities and decreases in brain cholinergic markers. Because the cholinergic system is associated with cognitive and motor functions, cholinergic deficits in RTT may underlie some of the behavioral abnormalities. In rodents, increased choline availability during development enhances transmission at cholinergic synapses and improves behavioral performance throughout life. We examined whether choline supplementation of nursing dams would attenuate deficits in Mecp2(1lox) offspring, a mouse model of RTT. Dams were given choline in drinking water, and pups nursed from birth to weaning. Offspring were assessed on development and behavior. In Mecp2(1lox) males, choline supplementation improved motor coordination and locomotor activity, whereas in females it enhanced grip strength. Choline supplementation did not improve response to fear conditioning. Postnatal choline supplementation attenuates some behavioral deficits in Mecp2(1lox) mice and should be explored further as a therapeutic agent in RTT.

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.

Molecular modeling of blood–brain barrier nutrient transporters: In silico basis for evaluation of potential drug delivery to the central nervous system

For drugs that act in the brain, the blood-brain barrier (BBB) is a considerable physical barrier which influences the distribution of drugs to the brain. The BBB is essentially impermeable for hydrophilic and/or charged compounds. Nutrient membrane transporters have an important physiological role in the transport of essential substances across the BBB required for normal brain function. We and others have shown that these transporters may have utility as drug delivery vectors, thereby increasing brain distribution of these compounds via these systems. In this review, we evaluate molecular (in silico) models of BBB transport proteins. Few BBB membrane transporters have been crystallized, but their crystal structures have a possibility for use in homology modeling. Other techniques commonly used are 2D quantitative structure-activity relationships (QSAR), as well as 3D-QSAR techniques including comparative molecular field analysis (CoMFA) and comparative similarity index analysis (CoMSIA). Each of these models provides valuable information for ascertaining their potential basis for BBB transport and brain drug delivery.

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.

Utility of nutraceutical products marketed for cognitive and memory enhancement

This article identifies a convenience sample of 14 memory-enhancing herbal products that were found to be available commercially, examines their active ingredients, states their claims, and evaluates the available evidence to determine their efficacy. The analyses identified four problematic areas. First, a majority of the products use cognitive terminology, which leads consumers to anticipate an intended cognitive benefit. Second, some ingredients are completely homeopathic and contain components not known outside of the homeopathic field. Third, the evidence of treatment efficacy is often contradictory, because products are recommended for purposes other than cognitive or memory loss. Finally, the manufacturers of the product have usually conducted the research on individual products. Until more research is available, it is suggested that holistic nursing professionals exercise caution in recommending nutraceuticals to their patients/clients for the use of cognitive improvement or memory enhancement.

Manganese transport by rat brain endothelial (RBE4) cell-based transwell model in the presence of astrocyte conditioned media

Manganese (Mn), an essential nutrient, is neurotoxic at high levels and has been associated with the development of a parkinsonian syndrome termed manganism. Currently, the mechanisms responsible for transporting Mn across the blood-brain barrier (BBB) are unknown. By using rat brain endothelial 4 (RBE4) cell monolayers cultured in astrocyte-conditioned media (ACM), we examine the effects of temperature, energy, proton (pH), iron (Fe), and sodium (Na(+)) dependence on Mn transport. Our results suggest that Mn transport is temperature, energy, and pH dependent, but not Fe or Na(+) dependent. These data suggest that Mn transport across the BBB is an active process, but they also demonstrate that the presence of ACM in endothelial cell cultures decreases the permeability of these cells to Mn, reinforcing the use of ACM or astrocyte cocultures in studies examining metal transport across the BBB.

Antinociceptive effects of choline against acute and inflammatory pain

We used the hot plate test and the formalin test to evaluate the antinociception of choline after i.c.v. or i.v. administration. The analgesic mechanism of choline was also studied. The response latency of mice was significantly prolonged in the hot plate test after choline (90-120 mug/animals) i.c.v. administration in a dose-dependent manner. Pretreatment with methyllycaconitine citrate (MLA), alpha-bungarotoxin, or atropine blocked the antinociception of choline in the hot plate test. In contrast, mecamylamine and naloxone had no effect. No antinociceptive action of choline was found in the hot plate test, but it did have an effect in the late phase of the formalin test after i.v. administration. The effect of choline on anti-inflammatory pain was blocked by MLA, but not by mecamylamine, naloxone and atropine, which is indicative of the involvement of alpha7 receptors in peripheral sites. When choline (2 mg/kg) was coadministered with aspirin (9.4 mg/kg), the licking/biting times in the late phase significantly decreased, although no effects were shown when these doses of drugs were used alone. Similarly, coadministration of choline (2 mg/kg) with morphine (0.165 mg/kg) significantly increased the antinociception of morphine in the late phase, but had no effect in the early phase. These results demonstrate that activation of alpha7 nicotinic receptors by choline elicits antinociceptive effects both in an acute thermal pain model and in an inflammatory pain model. Choline holds promise for development as a non-addictive analgesic drug and in reducing the regular dose of aspirin or morphine in inflammatory pain.

3D-QSAR study of bis-azaaromatic quaternary ammonium analogs at the blood–brain barrier choline transporter

Previously, we have developed 3D-QSAR models of the blood-brain barrier (BBB) choline transporter, a transport system that may have utility as a vector for central nervous system drug delivery. In this study, we extended the model by evaluating five bis-azaaromatic quaternary ammonium compounds for their affinity for the choline binding site on the BBB-choline transporter. The compounds, and their affinities for the transporter, were then incorporated into our existing molecular model, in order to update our knowledge on the molecular recognition factors associated with interaction of ligands at the choline binding site. The current compounds are structurally related to previous substrates that we have evaluated, but offer additional three dimensional aspects compared to the series of compounds previously utilized to define the original models. The compounds showed good affinity for the BBB-choline transporter, exhibiting inhibition constants ranging from 10 to 68 microM, as determined by the in situ rat brain perfusion method. Comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) methods were used to build the new 3D QSAR models. When the new bis-azaaromatic quaternary ammonium compounds were included in the model, the best cross-validated CoMFA q2 was found to be 0.536 and the non-cross-validated r2 was 0.818. CoMSIA hydrophobic cross-validated q2 was 0.506 and the non-cross-validated r2 was 0.804. This new model was able to better predict BBB-choline transporter affinity of hemicholinium-3 (predicted 65 microM, actual 54 microM), when compared to an earlier model (predicted 316 microM).

Molecular modeling studies on the active binding site of the blood–brain barrier choline transporter

The blood-brain barrier choline transporter may have utility as a drug delivery vector to the central nervous system. Surprisingly, this transporter has as yet not been cloned and expressed. We therefore initiated a 3D-QSAR study to develop predictive models for compound binding and identify structural features important for binding to this transporter. In vivo experimental data were obtained from in situ rat brain perfusion studies. Comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) methods were used to build the models. The best cross-validated CoMFA q(2) was found to be 0.47 and the non-cross-validated r(2) was 0.95. CoMSIA hydrophobic cross-validated q(2) was 0.37 and the non-cross-validated r(2) was 0.85. These models rendered a useful approximation for binding requirements in the BBB-choline transporter and, until such time as the cloned transporter becomes available, may have significant utility in developing a predictive model for the rational design of drugs targeted to the brain.