Brain delivery of biotin bound to a conjugate of neutral avidin and cationized human albumin

Small molecules as cancer targeting ligands: Shifting the paradigm

Conventional chemotherapeutics exploration is hampered due to their nonspecific distribution leading to unintended serious toxicity. Toxicity is so severe that deciding to go for chemotherapy becomes a question of concern for many terminally ill cancer patients. However, with evolving times nanotechnology assisted in reducing the haywire distribution and channelizing the movement of drug-enclosing drug delivery systems to cancer cells to a greater extent, yet toxicity issues still could not be obliterated. Thus, active targeting appeared as a refuge, where ligands actively or specifically deliver linked chemotherapeutics and carriers to cancer cells. For a very long time, large molecule weight/macromolecular ligands (peptides and big polymers) were considered the first choice for ligand-directed active cancer targeting, due to their specificity towards overexpressed native cancer receptors. However, complex characterization, instability, and the expensive nature demanded to reconnoitre better alternatives for macromolecule ligands. The concept of small molecules as ligands emerged from the idea that few chemical molecules including chemotherapeutics have a higher affinity for cancer receptors, which are overexpressed on cell membranes, and may have the ability to assist in drug cellular uptake through endocytosis. But now the question is, can they assist the conjugated macro cargos to enter the cell or not? This present review will provide a holistic overview of the small molecule ligands explored till now.

Brain Disposition of Antibody-Based Therapeutics: Dogma, Approaches and Perspectives

Due to their high specificity, monoclonal antibodies have been widely investigated for their application in drug delivery to the central nervous system (CNS) for the treatment of neurological diseases such as stroke, Alzheimer’s, and Parkinson’s disease. Research in the past few decades has revealed that one of the biggest challenges in the development of antibodies for drug delivery to the CNS is the presence of blood–brain barrier (BBB), which acts to restrict drug delivery and contributes to the limited uptake (0.1–0.2% of injected dose) of circulating antibodies into the brain. This article reviews the various methods currently used for antibody delivery to the CNS at the preclinical stage of development and the underlying mechanisms of BBB penetration. It also describes efforts to improve or modulate the physicochemical and biochemical properties of antibodies (e.g., charge, Fc receptor binding affinity, and target affinity), to adapt their pharmacokinetics (PK), and to influence their distribution and disposition into the brain. Finally, a distinction is made between approaches that seek to modify BBB permeability and those that use a physiological approach or antibody engineering to increase uptake in the CNS. Although there are currently inherent difficulties in developing safe and efficacious antibodies that will cross the BBB, the future prospects of brain-targeted delivery of antibody-based agents are believed to be excellent.

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

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

Designing biotin-human serum albumin nanoparticles to enhance the targeting ability of binuclear ruthenium(III) compound

On the one hand, to obtain a novel next-generation anticancer metal agent; on the other hand, to improve the targeting ability and decrease side effects of metal agent, we proposed to design active-targeting human serum albumin (HSA) nanoparticles (NPs) to achieve the end. Thus, we not only designed and synthesized two ruthenium (Ru) thiosemicarbazone compounds (C1 and C2) but also succeeded in constructing active Biotin-HSA NPs for Ru(III) compounds. Importantly, Biotin-HSA-C2 NPs not only possessed a stronger capacity for killing MCF-7 cells and inhibiting their migration versusC2 alone but also increased accumulation compared to non-malignant WI-38 cells. Additionally, C2 and Biotin-HSA-C2 NPs act against MCF-7 cells by the following potential mechanism: 1) arresting the cell cycle in the S phase by regulating cyclin and cyclin-dependent kinases; 2) inducing apoptosis by releasing cytochrome c to activate caspase-9/3; 3) inhibiting the expression of p-EGFR and regulating its neighboring cellular pathways, followed by the inactivation of PI3K/Akt and activation of p38 MAPK signaling pathways.

Caveolae-Mediated Transport at the Injured Blood-Brain Barrier as an Underexplored Pathway for Central Nervous System Drug Delivery

Drug delivery to the central nervous system (CNS) is generally hindered by the selectivity of the blood-brain barrier (BBB). However, there is strong evidence that the integrity of the BBB is compromised under certain pathological conditions, potentially providing a window to deliver drugs to injured brain regions. Recent studies suggest that caveolae-mediated transcytosis, a transport pathway suppressed in the healthy BBB, becomes elevated as an immediate response to ischemic stroke and at early stages of aging, where it may precede irreversible neurological damage. This article reviews early-stage caveolar transcytosis as a novel and promising drug delivery opportunity. We propose that albumin-binding and nanoparticle approaches have the potential to leverage this window of transcellular BBB disruption for trafficking therapeutic agents into the CNS.

Physical insights into the blood-brain barrier translocation mechanisms

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood-brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Recent advancements in liposomes targeting strategies to cross blood-brain barrier (BBB) for the treatment of Alzheimer's disease

In this modern era, with the help of various advanced technologies, medical science has overcome most of the health-related issues successfully. Though, some diseases still remain unresolved due to various physiological barriers. One such condition is Alzheimer; a neurodegenerative disorder characterized by progressive memory impairment, behavioral abnormalities, mood swing and disturbed routine activities of the person suffering from. It is well known to all that the brain is entirely covered by a protective layer commonly known as blood brain barrier (BBB) which is responsible to maintain the homeostasis of brain by restricting the entry of toxic substances, drug molecules, various proteins and peptides, small hydrophilic molecules, large lipophilic substances and so many other peripheral components to protect the brain from any harmful stimuli. This functionally essential structure creates a major hurdle for delivery of any drug into the brain. Still, there are some provisions on BBB which facilitate the entry of useful substances in the brain via specific mechanisms like passive diffusion, receptor-mediated transcytosis, carrier-mediated transcytosis etc. Another important factor for drug transport is the selection of a suitable drug delivery systems like, liposome, which is a novel drug carrier system offering a potential approach to resolving this problem. Its unique phospholipid bilayer structure (similar to physiological membrane) had made it more compatible with the lipoidal layer of BBB and helps the drug to enter the brain. The present review work focused on various surface modifications with functional ligand (like lactoferrin, transferrin etc.) and carrier molecules (such as glutathione, glucose etc.) on the liposomal structure to enhance its brain targeting ability towards the successful treatment of Alzheimer disease.

Targeting brain cells with glutathione-modulated nanoliposomes: in vitro and in vivo study

Background

The blood–brain barrier prevents many drug moieties from reaching the central nervous system. Therefore, glutathione-modulated nanoliposomes have been engineered to enhance the targeting of flucytosine to the brain.

Methods

Glutathione-modulated nanoliposomes were prepared by thin-film hydration technique and evaluated in the primary brain cells of rats. Lecithin, cholesterol, and span 65 were mixed at 1:1:1 molar ratio. The molar percentage of PEGylated glutathione varied from 0 mol% to 0.75 mol%. The cellular binding and the uptake of the targeted liposomes were both monitored by epifluorescent microscope and flow cytometry techniques. A biodistribution and a pharmacokinetic study of flucytosine and flucytosine-loaded glutathione–modulated liposomes was carried out to evaluate the in vivo brain-targeting efficiency.

Results

The size of glutathione-modulated nanoliposomes was <100 nm and the zeta potential was more than −65 mV. The cumulative release reached 70% for certain formulations. The cellular uptake increased as molar percent of glutathione increased to reach the maximum at 0.75 mol%. The uptake of the targeted liposomes by brain cells of the rats was three times greater than that of the nontargeted liposomes. An in vivo study showed that the relative efficiency was 2.632±0.089 and the concentration efficiency was 1.590±0.049, and also, the drug-targeting index was 3.670±0.824.

Conclusion

Overall, these results revealed that glutathione-PEGylated nanoliposomes enhance the effective delivery of flucytosine to brain and could become a promising new therapeutic option for the treatment of the brain infections.

Control of the blood-brain barrier function in cancer cell metastasis

Cerebral metastases are the most common brain neoplasms seen clinically in the adults and comprise more than half of all brain tumours. Actual treatment options for brain metastases that include surgical resection, radiotherapy and chemotherapy are rarely curative, although palliative treatment improves survival and life quality of patients carrying brain-metastatic tumours. Chemotherapy in particular has also shown limited or no activity in brain metastasis of most tumour types. Many chemotherapeutic agents used systemically do not cross the blood-brain barrier (BBB), whereas others may transiently weaken the BBB and allow extravasation of tumour cells from the circulation into the brain parenchyma. Increasing evidence points out that the interaction between the BBB and tumour cells plays a key role for implantation and growth of brain metastases in the central nervous system. The BBB, as the tightest endothelial barrier, prevents both early detection and treatment by creating a privileged microenvironment. Therefore, as observed in several in vivo studies, precise targetting the BBB by a specific transient opening of the structure making it permeable for therapeutic compounds, might potentially help to overcome this difficult clinical problem. Moreover, a better understanding of the molecular features of the BBB, its interrelation with metastatic tumour cells and the elucidation of cellular mechanisms responsible for establishing cerebral metastasis must be clearly outlined in order to promote treatment modalities that particularly involve chemotherapy. This in turn would substantially expand the survival and quality of life of patients with brain metastasis, and potentially increase the remission rate. Therefore, the focus of this review is to summarise the current knowledge on the role and function of the BBB in cancer metastasis.

The blood-brain barrier: structure, function and therapeutic approaches to cross it

The blood-brain barrier (BBB) is constituted by a specialized vascular endothelium that interacts directly with astrocytes, neurons and pericytes. It protects the brain from the molecules of the systemic circulation but it has to be overcome for the proper treatment of brain cancer, psychiatric disorders or neurodegenerative diseases, which are dramatically increasing as the population ages. In the present work we have revised the current knowledge on the cellular structure of the BBB and the different procedures utilized currently and those proposed to cross it. Chemical modifications of the drugs, such as increasing their lipophilicity, turn them more prone to be internalized in the brain. Other mechanisms are the use of molecular tools to bind the drugs such as small immunoglobulins, liposomes or nanoparticles that will act as Trojan Horses favoring the drug delivery in brain. This fusion of the classical pharmacology with nanotechnology has opened a wide field to many different approaches with promising results to hypothesize that BBB will not be a major problem for the new generation of neuroactive drugs. The present review provides an overview of all state-of-the-art of the BBB structure and function, as well as of the classic strategies and these appeared in recent years to deliver drugs into the brain for the treatment of Central Nervous System (CNS) diseases.

Interactive association of drugs binding to human serum albumin

Human serum albumin (HSA) is an abundant plasma protein, which attracts great interest in the pharmaceutical industry since it can bind a remarkable variety of drugs impacting their delivery and efficacy and ultimately altering the drug’s pharmacokinetic and pharmacodynamic properties. Additionally, HSA is widely used in clinical settings as a drug delivery system due to its potential for improving targeting while decreasing the side effects of drugs. It is thus of great importance from the viewpoint of pharmaceutical sciences to clarify the structure, function, and properties of HSA–drug complexes. This review will succinctly outline the properties of binding site of drugs in IIA subdomain within the structure of HSA. We will also give an overview on the binding characterization of interactive association of drugs to human serum albumin that may potentially lead to significant clinical applications.

Characterization of cellular uptake and toxicity of aminosilane-coated iron oxide nanoparticles with different charges in central nervous system-relevant cell culture models

Background

Aminosilane-coated iron oxide nanoparticles (AmS-IONPs) have been widely used in constructing complex and multifunctional drug delivery systems. However, the biocompatibility and uptake characteristics of AmS-IONPs in central nervous system (CNS)-relevant cells are unknown. The purpose of this study was to determine the effect of surface charge and magnetic field on toxicity and uptake of AmS-IONPs in CNS-relevant cell types.

Methods

The toxicity and uptake profile of positively charged AmS-IONPs and negatively charged COOH-AmS-IONPs of similar size were examined using a mouse brain microvessel endothelial cell line (bEnd.3) and primary cultured mouse astrocytes and neurons. Cell accumulation of IONPs was examined using the ferrozine assay, and cytotoxicity was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

Results

No toxicity was observed in bEnd.3 cells at concentrations up to 200 μg/mL for either AmS-IONPs or COOH-AmS-IONPs. AmS-IONPs at concentrations above 200 μg/mL reduced neuron viability by 50% in the presence or absence of a magnetic field, while only 20% reductions in viability were observed with COOH-AmS-IONPs. Similar concentrations of AmS-IONPs in astrocyte cultures reduced viability to 75% but only in the presence of a magnetic field, while exposure to COOH-AmS-IONPs reduced viability to 65% and 35% in the absence and presence of a magnetic field, respectively. Cellular accumulation of AmS-IONPs was greater in all cell types examined compared to COOH-AmS-IONPs. Rank order of cellular uptake for AmS-IONPs was astrocytes > bEnd.3 > neurons. Accumulation of COOH-AmS-IONPs was minimal and similar in magnitude in different cell types. Magnetic field exposure enhanced cellular accumulation of both AmS- and COOH-AmS-IONPs.

Conclusion

Both IONP compositions were nontoxic at concentrations below 100 μg/mL in all cell types examined. At doses above 100 μg/mL, neurons were more sensitive to AmS-IONPs, whereas astrocytes were more vulnerable toward COOH-AmS-IONPs. Toxicity appears to be dependent on the surface coating as opposed to the amount of iron-oxide present in the cell.

Brain microvascular endothelial cell association and distribution of a 5 nm ceria engineered nanomaterial

Purpose:

Ceria engineered nanomaterials (ENMs) have current commercial applications and both neuroprotective and toxic effects. Our hypothesis is that ceria ENMs can associate with brain capillary cells and/or cross the blood–brain barrier.

Methods:

An aqueous dispersion of ∼5 nm ceria ENM was synthesized and characterized in house. Its uptake space in the Sprague Dawley rat brain was determined using the in situ brain perfusion technique at 15 and 20 mL/minute flow rates; 30, 100, and 500 μg/mL ceria perfused for 120 seconds at 20 mL/minute; and 30 μg/mL perfused for 20, 60, and 120 seconds at 20 mL/minute. The capillary depletion method and light and electron microscopy were used to determine its capillary cell and brain parenchymal association and localization.

Results:

The vascular space was not significantly affected by brain perfusion flow rate or ENM, demonstrating that this ceria ENM did not influence blood–brain barrier integrity. Cerium concentrations, determined by inductively coupled plasma mass spectrometry, were significantly higher in the choroid plexus than in eight brain regions in the 100 and 500 μg/mL ceria perfusion groups. Ceria uptake into the eight brain regions was similar after 120-second perfusion of 30, 100, and 500 μg ceria/mL. Ceria uptake space significantly increased in the eight brain regions and choroid plexus after 60 versus 20 seconds, and it was similar after 60 and 120 seconds. The capillary depletion method showed 99.4% ± 1.1% of the ceria ENM associated with the capillary fraction. Electron microscopy showed the ceria ENM located on the endothelial cell luminal surface.

Conclusion:

Ceria ENM association with brain capillary endothelial cells saturated between 20 and 60 seconds and ceria ENM brain uptake was not diffusion-mediated. During the 120-second ceria ENM perfusion, ceria ENM predominately associated with the surface of the brain capillary cells, providing the opportunity for its cell uptake or redistribution back into circulating blood.

In vitro and in vivo investigation of dexibuprofen derivatives for CNS delivery

AIM

Dexibuprofen, the S(+)-isomer of ibuprofen, is an effective therapeutic agent for the treatment of neurodegenerative disorders. However, its clinical use is hampered by a limited brain distribution. The aim of this study was to design and synthesize brain-targeting dexibuprofen prodrugs and to evaluate their brain-targeting efficiency using biodistribution and pharmacokinetic analysis.

METHODS

In vitro stability, biodistribution and pharmacokinetic studies were performed on male Sprague-Dawley rats. The concentrations of dexibuprofen in biosamples, including the plasma, brain, heart, liver, spleen, lung, and kidney, were measured using high pressure lipid chromatography (HPLC). The pharmacokinetic parameters of the drug in the plasma and tissues were calculated using obtained data and statistics.

RESULTS

Five dexibuprofen prodrugs that were modified to contain ethanolamine-related structures were designed and synthesized. Their chemical structures were confirmed using (1)H NMR, (13)C NMR, IR, and HRMS. In the biodistribution study, 10 min after intravenous administration of dexibuprofen (11.70 mg/kg) and its prodrugs (the dose of each compound was equivalent to 11.70 mg/kg of dexibuprofen) in male Sprague-Dawley rats, the dexibuprofen concentrations in the brain and plasma were measured. The C(brain)/C(plasma) ratios of prodrugs 1, 2, 3, 4, and 5 were 17.0-, 15.7-, 7.88-, 9.31-, and 3.42-fold higher than that of dexibuprofen, respectively (P<0.01). Thus, each of the prodrugs exhibited a significantly enhanced brain distribution when compared with dexibuprofen. In the pharmacokinetic study, prodrug 1 exhibited a brain-targeting index of 11.19 {DTI=(AUC(brain)/AUC(plasma))(1)/(AUC(brain)/AUC(plasma))(dexibuprofen)}.

CONCLUSION

The ethanolamine-related structures may play an important role in transport across the brain blood barrier.

A prediction model for blood-brain barrier permeation and analysis on its parameter biologically

The objective of this paper is to build a reliable model based on the artificial neural network (ANN) for predicting the blood-brain barrier (BBB) permeability and reveal the effects of the molecular descriptor on the BBB permeability. Eight descriptors including high-affinity P-gp substrate probability and plasma protein binding ratio are selected to develop the model. The three layers feedforward neural network (8-5-1) is employed for the prediction of logBB. By analyzing the experimental results, polar surface area (PSA) seems to be the most important factor for BBB permeability. Different from traditional view, the Abraham's hydrogen-bond basicity (HBB) can make a positive contribution to logBB in rational range. The experimental results show that the ANN based model with eight selected descriptors as inputs can achieve good performance for logBB prediction, and the results of sensitivity analysis can be confirmed by the present biological and chemical research.

Getting into the brain: approaches to enhance brain drug delivery

Being the most delicate organ of the body, the brain is protected against potentially toxic substances by the blood-brain barrier (BBB), which restricts the entry of most pharmaceuticals into the brain. The developmental process for new drugs for the treatment of CNS disorders has not kept pace with progress in molecular neurosciences because most of the new drugs discovered are unable to cross the BBB. The clinical failure of CNS drug delivery may be attributed largely to a lack of appropriate drug delivery systems. Localized and controlled delivery of drugs at their desired site of action is preferred because it reduces toxicity and increases treatment efficiency. The present review provides an insight into some of the recent advances made in the field of brain drug delivery.The various strategies that have been explored to increase drug delivery into the brain include (i) chemical delivery systems, such as lipid-mediated transport, the prodrug approach and the lock-in system; (ii) biological delivery systems, in which pharmaceuticals are re-engineered to cross the BBB via specific endogenous transporters localized within the brain capillary endothelium; (iii) disruption of the BBB, for example by modification of tight junctions, which causes a controlled and transient increase in the permeability of brain capillaries; (iv) the use of molecular Trojan horses, such as peptidomimetic monoclonal antibodies to transport large molecules (e.g. antibodies, recombinant proteins, nonviral gene medicines or RNA interference drugs) across the BBB; and (v) particulate drug carrier systems. Receptor-mediated transport systems exist for certain endogenous peptides, such as insulin and transferrin, enabling these molecules to cross the BBB in vivo.The use of polymers for local drug delivery has greatly expanded the spectrum of drugs available for the treatment of brain diseases, such as malignant tumours and Alzheimer's disease. In addition, various drug delivery systems (e.g. liposomes, microspheres, nanoparticles, nanogels and bionanocapsules) have been used to enhance drug delivery to the brain. Recently, microchips and biodegradable polymers have become important in brain tumour therapy.The intense search for alternative routes of drug delivery (e.g. intranasal drug delivery, convection-enhanced diffusion and intrathecal/intraventricular drug delivery systems) has been driven by the need to overcome the physiological barriers of the brain and to achieve high drug concentrations within the brain. For more than 30 years, considerable efforts have been made to enhance the delivery of therapeutic molecules across the vascular barriers of the CNS. The current challenge is to develop drug delivery strategies that will allow the passage of drug molecules through the BBB in a safe and effective manner.

Effective transvascular delivery of nanoparticles across the blood-brain tumor barrier into malignant glioma cells

BACKGROUND

Effective transvascular delivery of nanoparticle-based chemotherapeutics across the blood-brain tumor barrier of malignant gliomas remains a challenge. This is due to our limited understanding of nanoparticle properties in relation to the physiologic size of pores within the blood-brain tumor barrier. Polyamidoamine dendrimers are particularly small multigenerational nanoparticles with uniform sizes within each generation. Dendrimer sizes increase by only 1 to 2 nm with each successive generation. Using functionalized polyamidoamine dendrimer generations 1 through 8, we investigated how nanoparticle size influences particle accumulation within malignant glioma cells.

METHODS

Magnetic resonance and fluorescence imaging probes were conjugated to the dendrimer terminal amines. Functionalized dendrimers were administered intravenously to rodents with orthotopically grown malignant gliomas. Transvascular transport and accumulation of the nanoparticles in brain tumor tissue was measured in vivo with dynamic contrast-enhanced magnetic resonance imaging. Localization of the nanoparticles within glioma cells was confirmed ex vivo with fluorescence imaging.

RESULTS

We found that the intravenously administered functionalized dendrimers less than approximately 11.7 to 11.9 nm in diameter were able to traverse pores of the blood-brain tumor barrier of RG-2 malignant gliomas, while larger ones could not. Of the permeable functionalized dendrimer generations, those that possessed long blood half-lives could accumulate within glioma cells.

CONCLUSION

The therapeutically relevant upper limit of blood-brain tumor barrier pore size is approximately 11.7 to 11.9 nm. Therefore, effective transvascular drug delivery into malignant glioma cells can be accomplished by using nanoparticles that are smaller than 11.7 to 11.9 nm in diameter and possess long blood half-lives.

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.

New systems for delivery of drugs to the brain in neurological disease

The restricted or regulated entry of most blood-borne substances into the brain has been recognised for more than a century. The blood-brain barrier (BBB)-shielding function provided by endothelial cells is important in the treatment of neurological diseases because this exclusion of foreign substances also restricts entry of many potentially therapeutic agents into the brain. The recent identification of several neuroactive proteins of potential therapeutic value has highlighted the crucial need for effective and safe transcapillary delivery methods to the brain. One promising method is delivery through brain capillaries by augmentation of pinocytotic vesicles delivery systems that use this cellular mechanism are in development. Recent investigations in animal models show that large molecules of neurotherapeutic potential can be conjugated to peptidomimetic ligands, which bind to selected peptide receptors, and are then internalised and transported in small vesicles across the cytoplasmic brain capillary barrier. These conjugates have been shown to remain functionally active and effective in animal models of neurological disease.

Vector-mediated drug delivery to the brain

As a consequence of the growing ageing population, many neurodegenerative diseases, cancer and infections of the brain will become more prevalent. Despite major advances in neuroscience, many potential therapeutic agents are denied access to the central nervous system (CNS) because of the existence of the blood-brain barrier (BBB). This barrier is formed by the endothelial cells of the brain capillaries and its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. The BBB behaves as a continuous lipid bilayer and prevents the passage of polar and lipid-insoluble substances. The BBB is, therefore, the major obstacle to drugs that are potentially useful for combating diseases affecting the CNS. Extensive efforts have been made to develop CNS drug delivery strategies in order to enhance delivery of therapeutic molecules across the BBB. The current challenge is to develop drug-delivery strategies that will allow the passage of therapeutic drugs through the BBB in a safe and effective manner. This review focuses specifically on the strategies developed to enhance drug delivery across the BBB with an emphasis on the vector-mediated strategy.

Brain drug delivery technologies: novel approaches for transporting therapeutics

The blood-brain barrier (BBB) denies many therapeutic agents access to brain tumours and other diseases of the central nervous system (CNS). Despite remarkable advances in our understanding of the mechanisms involved in the development of the brain diseases and the actions of neuroactive agents, drug delivery to the brain remains a challenge. For more than 20 years, extensive efforts have been made to enhance delivery of therapeutic molecules across vascular barriers of the CNS. The current challenge is to develop drug-delivery strategies that will allow the passage of drug molecules through the BBB in a safe and effective manner, and this review will provide an insight into some of the strategies developed to enhance drug delivery across the BBB.

Homing of negatively charged albumins to the lymphatic system: general implications for drug targeting to peripheral tissues and viral reservoirs

The present study shows the lymphatic distribution of the negatively charged anti-HIV-1 agents succinylated or aconytilated human serum albumins (HSAs) in rats. Quantitation of blood and lymphatic concentrations of these proteins was performed through fluorescence detection of the fluorescein isothiocyanate (FITC)-labeled proteins. At several time points after i.v. injection, samples were taken from the cannulated thoracic duct and the carotid artery. Distribution of the negatively charged albumins (NCAs) to lymph was much more rapid than that of albumin itself and was dependent on the total net negative charge added to the protein: the half-life times of lymphatic equilibration were 15, 30, and 120 min for FITC-labeled aconytilated HSA, FITC-labeled succinylated HSA, and FITC-labeled HSA, respectively. Lymph to blood concentration ratios of the studied compounds obtained at steady state approached unity. In addition, the fluorescence in both body fluids was shown to represent unchanged labeled proteins. It was therefore inferred that the NCAs efficiently passed the endothelial barrier from blood to the interstitial compartment. Subsequently, we studied whether a specialized process was involved in the endothelial passage of the NCAs to the lymph. The following observations supported such a mechanism: a) preinjection of the scavenger receptor blockers polyinosinic- and formaldehyde-treated HSA reduced the transport from blood to the lymphatic compartment of FITC-labeled aconytilated HSA by more than 90%; b) the rate of lymphatic distribution was largely reduced when the body temperature of the rat was lowered to 28 degrees; and c) pre-administration of chloroquine resulted in a significant reduction in the lymphatic distribution of the NCAs. These data collectively indicate that a scavenger receptor-mediated process is involved in the transendothelial transport of NCAs. In situ localization in lymph nodes of the rat showed that FITC-labeled aconytilated and succinylated HSA are mainly present in the germinal center and parafollicular zones. The efficient distribution of these anionized proteins to the lymphatic system is of particular interest for HIV therapy, taking into account that replication of HIV mainly takes place in the lymphoid system. The observation that macromolecules, through charge modification, can extravasate through a receptor-mediated transcytotic process is potentially of major importance for the delivery of drugs with macromolecular carriers to cells not directly in contact with the blood.

Blood-brain barrier permeability to small and large molecules

The objective of this article is to provide the reader with an update of some of the BBB research highlights which have occurred in recent times, and to review the impact and contributions of immunogold electron microscopic studies on our understanding of the brain capillary endothelium. Glucose and monocarboxylic acids are two small molecules which this review will focus upon; and advances in immunogold characterization of the GLUT1 glucose transporter and the MCT1 and MCT2 monocarboxylic acid nutrient transporters will be discussed. Human serum albumin is chosen as a representative large molecule, and it has recently been shown that immunogold identification of this protein can serve as an indicator of compromised BBB function in a variety of pathophysiological conditions.

Drug transfer across the blood-brain barrier and improvement of brain delivery

The blood-brain barrier is formed by the endothelial cells of the brain capillaries. Its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. Essential nutrients are delivered to the brain by selective transport mechanisms, such as the glucose transporter and a variety of amino acid transporters. Although most drugs enter the brain by passive diffusion through the endothelial cells depending on their lipophilicity, degree of ionization, molecular weight, relative brain tissue and plasma bindings, some others can use specific endogenous transporters. In such cases, binding competition on the transporter with endogenous products or nutrients can occur and limits drug transfer. The blood-brain barrier can be a major impediment for the treatment of diseases of the central nervous system, since many drugs are unable to reach this organ at therapeutic concentrations. Various attempts have been made to overcome the limiting access of drugs to the brain, e.g. chemical modification, development of more hydrophobic analogs or linking an active compound to a specific carrier. Transient opening of the blood-brain barrier in humans has been achieved by intracarotid infusion of hypertonic mannitol solutions or of bradykinin analogs. Another way to increase or decrease brain delivery of drugs is to modulate the P-glycoprotein (P-gp) whose substrates are actively pumped out the cell into the capillary lumen. Many P-gp inhibitors or inducers are available to enhance the therapeutic effects of centrally acting drugs or to decrease central adverse effects of peripherally active drugs.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

Transporting therapeutics across the blood-brain barrier

In 1996, we are half-way through the Decade of the Brain, yet we still have few effective treatments for major disorders of the central nervous system. These include affective disorders, epilepsy, neurodegenerative disorders, brain tumours, infections and HIV encephalopathy; sufferers far outnumber the morbidity of cancer or heart disease. Increased understanding of the pharmacology of the brain and its blood supply, and methods for rational drug design, are leading to potential new drug therapies based on highly specific actions on particular target sites, such as neurotransmitter receptors and uptake systems. These methods are capable of reducing the side effects that are common with more general treatments. However, all these treatments and potential treatments meet a formidable obstacle--the blood-brain barrier. In this article, we review the properties of this barrier that complicate drug delivery to the brain, and some of the most hopeful strategies for overcoming or bypassing the barrier in humans.

Vector-mediated delivery of a polyamide ("peptide") nucleic acid analogue through the blood-brain barrier in vivo

Polyamide ("peptide") nucleic acids (PNAs) are molecules with antigene and antisense effects that may prove to be effective neuropharmaceuticals if these molecules are enabled to undergo transport through the brain capillary endothelial wall, which makes up the blood-brain barrier in vivo. The model PNA used in the present studies is an 18-mer that is antisense to the rev gene of human immunodeficiency virus type 1 and is biotinylated at the amino terminus and iodinated at a tyrosine residue near the carboxyl terminus. The biotinylated PNA was linked to a conjugate of streptavidin (SA) and the OX26 murine monoclonal antibody to the rat transferrin receptor. The blood-brain barrier is endowed with high transferrin receptor concentrations, enabling the OX26-SA conjugate to deliver the biotinylated PNA to the brain. Although the brain uptake of the free PNA was negligible following intravenous administration, the brain uptake of the PNA was increased at least 28-fold when the PNA was bound to the OX26-SA vector. The brain uptake of the PNA bound to the OX26-SA vector was 0.1% of the injected dose per gram of brain at 60 min after an intravenous injection, approximating the brain uptake of intravenously injected morphine. The PNA bound to the OX26-SA vector retained the ability to bind to synthetic rev mRNA as shown by RNase protection assays. In summary, the present studies show that while the transport of PNAs across the blood-brain barrier is negligible, delivery of these potential neuropharmaceutical drugs to the brain may be achieved by coupling them to vector-mediated peptide-drug delivery systems.

Pharmacokinetics of [3H]biotin bound to different avidin analogues

The use of avidin-biotin technology in drug delivery facilitates the conjugation of biotinylated therapeutics to transport vectors that are enabled to undergo receptor-mediated transcytosis through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo. However, the conjugation of avidin, a cationic glycosylated protein, to transport vectors greatly increases the rate of removal of the vector from the bloodstream, owing to rapid uptake of avidin by peripheral tissues such as liver and kidney. However, modified avidins may retain high affinity biotin binding properties, but may not be rapidly removed from plasma by peripheral tissues, and such avidin analogues would provide preferred plasma pharmacokinetic profiles. Therefore, the present studies investigate the pharmacokinetics of plasma removal of [3H]biotin bound to one of six different avidin analogues: streptavidin, Neutra-lite avidin, avidin, neutral avidin, Lite-avidin, and succinylated avidin. Isoelectric focusing studies show that avidin and Lite-avidin were highly cationic proteins, whereas neutral avidin, Neutra-lite avidin, and streptavidin were neutral proteins, and succinylated avidin had an acidic isoelectric point. The avidin analogues fell into two groups with respect to rate of biotin removal from plasma. The low clearance group included streptavidin and Neutra-lite avidin, which had a mean plasma clearance of 0.41 mL/min/kg. The high clearance group consisted of succinylated avidin, neutral avidin, and Lite-avidin and had a mean plasma clearance of 17 mL/min/kg, or 40-fold faster than the low clearance avidins.(ABSTRACT TRUNCATED AT 250 WORDS)