Biopharmaceutical drug targeting to the brain

Strategies for delivery of therapeutics into the central nervous system for treatment of lysosomal storage disorders

Lysosomal storage disorders (LSDs) are a group of about fifty life-threatening conditions caused by genetic defects affecting lysosomal components. The underscoring molecular deficiency leads to widespread cellular dysfunction through most tissues in the body, including peripheral organs and the central nervous system (CNS). Efforts during the last few decades have rendered a remarkable advance regarding our knowledge, medical awareness, and early detection of these genetic defects, as well as development of several treatment modalities. Clinical and experimental strategies encompassing enzyme replacement, gene and cell therapies, substrate reduction, and chemical chaperones are showing considerable potential in attenuating the peripheral pathology. However, a major drawback has been encountered regarding the suboptimal impact of these approaches on the CNS pathology. Particular anatomical and biochemical constraints of this tissue pose a major obstacle to the delivery of therapeutics into the CNS. Approaches to overcome these obstacles include modalities of local administration, strategies to enhance the blood-CNS permeability, intranasal delivery, use of exosomes, and those exploiting targeting of transporters and transcytosis pathways in the endothelial lining. The later two approaches are being pursued at the time by coupling therapeutic agents to affinity moieties and drug delivery systems capable of targeting these natural transport routes. This approach is particularly promising, as using paths naturally active at this interface may render safe and effective delivery of LSD therapies into the CNS.

Pharmacokinetic characteristics, pharmacodynamic effect and in vivo antiviral efficacy of liver-targeted interferon alpha

Interferon alpha (IFNα) is used for the treatment of hepatitis B virus infection, and whilst efficacious, it is associated with multiple adverse events caused by systemic exposure to interferon. We therefore hypothesise that targeting IFN directly to the intended site of action in the liver would reduce exposure in blood and peripheral tissue and hence improve the safety and tolerability of IFNα therapy. Furthermore we investigated whether directing IFN to the reservoir of infection in the liver may improve antiviral efficacy by increasing local concentration in target organs and tissues. Our previous results show that the mIFNα2 fused to an ASGPR specific liver targeting antibody, DOM26h-196-61, results in a fusion protein which retains the activity of both fusion partners when measured in vitro. In vivo targeting of the liver by mIFNα2-DOM26h-196-61, hereafter referred to as targeted mIFNα2, was observed in microSPECT imaging studies in mice. In this study we show by pharmacokinetic analysis that antibody mediated liver-targeting results in increased uptake and exposure of targeted mIFNα2 in target tissues, and correspondingly reduced uptake and exposure in systemic circulation, clearance organs and non-target tissues. We also show that cytokine activity and antiviral activity of liver-targeted IFN is observed in vivo, but that, contrary to expectations, liver-targeting of mIFNα2 using ASGPR specific dAbs actually leads to a reduced pharmacodynamic effect in target organs and lower antiviral activity in vivo when compared to non-targeted mIFNα2-dAb fusions.

Smuggling Drugs into the Brain: An Overview of Ligands Targeting Transcytosis for Drug Delivery across the Blood-Brain Barrier

The blood-brain barrier acts as a physical barrier that prevents free entry of blood-derived substances, including those intended for therapeutic applications. The development of molecular Trojan horses is a promising drug targeting technology that allows for non-invasive delivery of therapeutics into the brain. This concept relies on the application of natural or genetically engineered proteins or small peptides, capable of specifically ferrying a drug-payload that is either directly coupled or encapsulated in an appropriate nanocarrier, across the blood-brain barrier via receptor-mediated transcytosis. Specifically, in this process the nanocarrier-drug system ("Trojan horse complex") is transported transcellularly across the brain endothelium, from the blood to the brain interface, essentially trailed by a native receptor. Naturally, only certain properties would favor a receptor to serve as a transporter for nanocarriers, coated with appropriate ligands. Here we briefly discuss brain microvascular endothelial receptors that have been explored until now, highlighting molecular features that govern the efficiency of nanocarrier-mediated drug delivery into the brain.

Open challenges in magnetic drug targeting

The principle of magnetic drug targeting, wherein therapy is attached to magnetically responsive carriers and magnetic fields are used to direct that therapy to disease locations, has been around for nearly two decades. Yet our ability to safely and effectively direct therapy to where it needs to go, for instance to deep tissue targets, remains limited. To date, magnetic targeting methods have not yet passed regulatory approval or reached clinical use. Below we outline key challenges to magnetic targeting, which include designing and selecting magnetic carriers for specific clinical indications, safely and effectively reaching targets behind tissue and anatomical barriers, real-time carrier imaging, and magnet design and control for deep and precise targeting. Addressing these challenges will require interactions across disciplines. Nanofabricators and chemists should work with biologists, mathematicians, and engineers to better understand how carriers move through live tissues and how to optimize carrier and magnet designs to better direct therapy to disease targets. Clinicians should be involved early on and throughout the whole process to ensure the methods that are being developed meet a compelling clinical need and will be practical in a clinical setting. Our hope is that highlighting these challenges will help researchers translate magnetic drug targeting from a novel concept to a clinically available treatment that can put therapy where it needs to go in human patients.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

Shuttle-cargo fusion molecules of transport peptides and the hD2/3 receptor antagonist fallypride: a feasible approach to preserve ligand-receptor binding?

To determine if the conjugation of a small receptor ligand to a peptidic carrier to potentially facilitate transport across the blood-brain barrier (BBB) by "molecular Trojan horse" transcytosis is feasible, we synthesized several transport peptide-fallypride fusion molecules as model systems and determined their binding affinities to the hD2 receptor. Although they were affected by conjugation, the binding affinities were found to be still in the nanomolar range (between 1.5 and 64.2 nM). In addition, homology modeling of the receptor and docking studies for the most potent compounds were performed, elucidating the binding modes of the fusion molecules and the structure elements contributing to the observed high receptor binding. Furthermore, no interaction between the hybrid compounds and P-gp, the main excretory transporter of the BBB, was found. From these results, it can be inferred that the approach to deliver small neuroreceptor ligands across the BBB by transport peptide carriers is feasible.

Recent progress in generating intracellular functional antibody fragments to target and trace cellular components in living cells

In biomedical research there is an ongoing demand for new technologies, which help to elucidate disease mechanisms and provide the basis to develop novel therapeutics. In this context a comprehensive understanding of cellular processes and their pathophysiology based on reliable information on abundance, localization, posttranslational modifications and dynamic interactions of cellular components is indispensable. Besides their significant impact as therapeutic molecules, antibodies are arguably the most powerful research tools to study endogenous proteins and other cellular components. However, for cellular diagnostics their use is restricted to endpoint assays using fixed and permeabilized cells. Alternatively, live cell imaging using fluorescent protein-tagged reporters is widely used to study protein localization and dynamics in living cells. However, only artificially introduced chimeric proteins are visualized, whereas the endogenous proteins, their posttranslational modifications as well as non-protein components of the cell remain invisible and cannot be analyzed. To overcome these limitations, traceable intracellular binding molecules provide new opportunities to perform cellular diagnostics in real time. In this review we summarize recent progress in the generation of intracellular and cell penetrating antibodies and their application to target and trace cellular components in living cells. We highlight recent advances in the structural formulation of recombinant antibody formats, reliable screening protocols and sophisticated cellular targeting technologies and propose that such intrabodies will become versatile research tools for real time cell-based diagnostics including target validation and live cell imaging. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Specific binding, uptake, and transport of ICAM-1-targeted nanocarriers across endothelial and subendothelial cell components of the blood-brain barrier

PURPOSE

The blood-brain barrier (BBB) represents a target for therapeutic intervention and an obstacle for brain drug delivery. Targeting endocytic receptors on brain endothelial cells (ECs) helps transport drugs and carriers into and across this barrier. While most receptors tested are associated with clathrin-mediated pathways, clathrin-independent routes are rather unexplored. We have examined the potential for one of these pathways, cell adhesion molecule (CAM)-mediated endocytosis induced by targeting intercellular adhesion molecule -1 (ICAM-1), to transport drug carriers into and across BBB models.

METHODS

Model polymer nanocarriers (NCs) coated with control IgG or antibodies against ICAM-1 (IgG NCs vs. anti-ICAM NCs; ~250-nm) were incubated with human brain ECs, astrocytes (ACs), or pericytes (PCs) grown as monocultures or bilayered (endothelial+subendothelial) co-cultures.

RESULTS

ICAM-1 was present and overexpressed in disease-like conditions on ECs and, at a lesser extent, on ACs and PCs which are BBB subendothelial components. Specific targeting and CAM-mediated uptake of anti-ICAM NCs occurred in these cells, although this was greater for ECs. Anti-ICAM NCs were transported across endothelial monolayers and endothelial+subendothelial co-cultures modeling the BBB.

CONCLUSIONS

CAM-mediated transport induced by ICAM-1 targeting operates in endothelial and subendothelial cellular components of the BBB, which may provide an avenue to overcome this barrier.

Conjugation of a brain-penetrant peptide with neurotensin provides antinociceptive properties

Neurotensin (NT) has emerged as an important modulator of nociceptive transmission and exerts its biological effects through interactions with 2 distinct GPCRs, NTS1 and NTS2. NT provides strong analgesia when administered directly into the brain; however, the blood-brain barrier (BBB) is a major obstacle for effective delivery of potential analgesics to the brain. To overcome this challenge, we synthesized chemical conjugates that are transported across the BBB via receptor-mediated transcytosis using the brain-penetrant peptide Angiopep-2 (An2), which targets LDL receptor-related protein-1 (LRP1). Using in situ brain perfusion in mice, we found that the compound ANG2002, a conjugate of An2 and NT, was transported at least 10 times more efficiently across the BBB than native NT. In vitro, ANG2002 bound NTS1 and NTS2 receptors and maintained NT-associated biological activity. In rats, i.v. ANG2002 induced a dose-dependent analgesia in the formalin model of persistent pain. At a dose of 0.05 mg/kg, ANG2002 effectively reversed pain behaviors induced by the development of neuropathic and bone cancer pain in animal models. The analgesic properties of ANG2002 demonstrated in this study suggest that this compound is effective for clinical management of persistent and chronic pain and establish the benefits of this technology for the development of neurotherapeutics.

Enhanced blood-brain barrier transmigration using a novel transferrin embedded fluorescent magneto-liposome nanoformulation

The blood-brain barrier (BBB) is considered as the primary impediment barrier for most drugs. Delivering therapeutic agents to the brain is still a big challenge to date. In our study, a dual mechanism, receptor mediation combined with external non-invasive magnetic force, was incorporated into ferrous magnet-based liposomes for BBB transmigration enhancement. The homogenous magnetic nanoparticles (MNPs), with a size of ∼10 nm, were synthesized and confirmed by TEM and XRD respectively. The classical magnetism assay showed the presence of the characteristic superparamagnetic property. These MNPs encapsulated in PEGylated fluorescent liposomes as magneto-liposomes (MLs) showed mono-dispersion, ∼130 ± 10 nm diameter, by dynamic laser scattering (DLS) using the lipid-extrusion technique. Remarkably, a magnetite encapsulation efficiency of nearly 60% was achieved. Moreover, the luminescence and hydrodynamic size of the MLs was stable for over two months at 4 ° C. Additionally, the integrity of the ML structure remained unaffected through 120 rounds of circulation mimicking human blood fluid. After biocompatibility confirmation by cytotoxicity evaluation, these fluorescent MLs were further embedded with transferrin and applied to an in vitro BBB transmigration study in the presence or absence of external magnetic force. Comparing with magnetic force- or transferrin receptor-mediated transportation alone, their synergy resulted in 50-100% increased transmigration without affecting the BBB integrity. Consequently, confocal microscopy and iron concentration in BBB-composed cells further confirmed the higher cellular uptake of ML particles due to the synergic effect. Thus, our multifunctional liposomal magnetic nanocarriers possess great potential in particle transmigration across the BBB and may have a bright future in drug delivery to the brain.

Biological functionalization of drug delivery carriers to bypass size restrictions of receptor-mediated endocytosis independently from receptor targeting

Targeting of drug carriers to cell-surface receptors involved in endocytosis is commonly used for intracellular drug delivery. However, most endocytic receptors mediate uptake via clathrin or caveolar pathways associated with ≤200-nm vesicles, restricting carrier design. We recently showed that endocytosis mediated by intercellular adhesion molecule 1 (ICAM-1), which differs from clathrin- and caveolae-mediated pathways, allows uptake of nano- and microcarriers in cell culture and in vivo due to recruitment of cellular sphingomyelinases to the plasmalemma. This leads to ceramide generation at carrier binding sites and formation of actin stress-fibers, enabling engulfment and uptake of a wide size-range of carriers. Here we adapted this paradigm to enhance uptake of drug carriers targeted to receptors associated with size-restricted pathways. We coated sphingomyelinase onto model (polystyrene) submicro- and microcarriers targeted to clathrin-associated mannose-6-phosphate receptor. In endothelial cells, this provided ceramide enrichment at the cell surface and actin stress-fiber formation, modifying the uptake pathway and enhancing carrier endocytosis without affecting targeting, endosomal transport, cell-associated degradation, or cell viability. This improvement depended on the carrier size and enzyme dose, and similar results were observed for other receptors (transferrin receptor) and cell types (epithelial cells). This phenomenon also enhanced tissue accumulation of carriers after intravenous injection in mice. Hence, it is possible to maintain targeting toward a selected receptor while bypassing natural size restrictions of its associated endocytic route by functionalization of drug carriers with biological elements mimicking the ICAM-1 pathway. This strategy holds considerable promise to enhance flexibility of design of targeted drug delivery systems.

68Ga-labeled gold glyconanoparticles for exploring blood-brain barrier permeability: preparation, biodistribution studies, and improved brain uptake via neuropeptide conjugation

New tools and techniques to improve brain visualization and assess drug permeability across the blood-brain barrier (BBB) are critically needed. Positron emission tomography (PET) is a highly sensitive, noninvasive technique that allows the evaluation of the BBB permeability under normal and disease-state conditions. In this work, we have developed the synthesis of novel water-soluble and biocompatible glucose-coated gold nanoparticles (GNPs) carrying BBB-permeable neuropeptides and a chelator of the positron emitter (68)Ga as a PET reporter for in vivo tracking biodistribution. The small GNPs (2 nm) are stabilized and solubilized by a glucose conjugate. A NOTA ligand is the chelating agent for the (68)Ga, and two related opioid peptides are used as targeting ligands for improving BBB crossing. The radioactive labeling of the GNPs is completed in 30 min at 70 °C followed by purification via centrifugal filtration. As a proof of principle, a biodistribution study in rats is performed for the different (68)Ga-GNPs. The accumulation of radioactivity in different organs after intravenous administration is measured by whole body PET imaging and gamma counter measurements of selected organs. The biodistribution of the (68)Ga-GNPs varies depending on the ligands, as GNPs with the same gold core size show different distribution profiles. One of the targeted (68)Ga-GNPs improves BBB crossing near 3-fold (0.020 ± 0.0050% ID/g) compared to nontargeted GNPs (0.0073 ± 0.0024% ID/g) as measured by dissection and tissue counting.

Rapid and reversible enhancement of blood-brain barrier permeability using lysophosphatidic acid

The present study characterizes the effects of lysophosphatidic acid (LPA) on blood-brain barrier (BBB) permeability focusing specifically on the time of onset, duration, and magnitude of LPA-induced changes in cerebrovascular permeability in the mouse using both magnetic resonance imaging (MRI) and near infrared fluorescence imaging (NIFR). Furthermore, potential application of LPA for enhanced drug delivery to the brain was also examined by measuring the brain accumulation of radiolabeled methotrexate. Exposure of primary cultured brain microvessel endothelial cells (BMECs) to LPA produced concentration-dependent increases in permeability that were completely abolished by clostridium toxin B. Administration of LPA disrupted BBB integrity and enhanced the permeability of small molecular weight marker gadolinium diethylenetriaminepentaacetate (Gd-DTPA) contrast agent, the large molecular weight permeability marker, IRdye800cwPEG, and the P-glycoprotein efflux transporter probe, Rhodamine 800 (R800). The increase in BBB permeability occurred within 3 minutes after LPA injection and barrier integrity was restored within 20 minutes. A decreased response to LPA on large macromolecule BBB permeability was observed after repeated administration. The administration of LPA also resulted in 20-fold enhancement of radiolabeled methotrexate in the brain. These studies indicate that administration of LPA in combination with therapeutic agents may increase drug delivery to the brain.

Image-guided drug delivery to the brain using nanotechnology

Targeting across the blood-brain barrier (BBB) for treatment of central nervous system (CNS) diseases represents the most challenging aspect of, as well as one of the largest growing fields in, neuropharmaceutics. Combining nanotechnology with multiple imaging techniques has a unique role in the diagnosis and treatment (theranostics) of CNS disease. Such imaging techniques include anatomical imaging modalities, such as magnetic resonance imaging (MRI), ultrasound (US), X-ray computed tomography (CT), positron emission tomography (PET), single-photon emission computed tomography (SPECT), electron microscopy, autoradiography and optical imaging as well as thermal images. In this review, we summarize and discuss recent advances in formulations, current challenges and possible hypotheses concerning the use of such theranostics across the BBB.

Fusion protein linkers: property, design and functionality

As an indispensable component of recombinant fusion proteins, linkers have shown increasing importance in the construction of stable, bioactive fusion proteins. This review covers the current knowledge of fusion protein linkers and summarizes examples for their design and application. The general properties of linkers derived from naturally-occurring multi-domain proteins can be considered as the foundation in linker design. Empirical linkers designed by researchers are generally classified into 3 categories according to their structures: flexible linkers, rigid linkers, and in vivo cleavable linkers. Besides the basic role in linking the functional domains together (as in flexible and rigid linkers) or releasing the free functional domain in vivo (as in in vivo cleavable linkers), linkers may offer many other advantages for the production of fusion proteins, such as improving biological activity, increasing expression yield, and achieving desirable pharmacokinetic profiles.

The blood-brain barrier: an engineering perspective

It has been more than 100 years since Paul Ehrlich reported that various water-soluble dyes injected into the circulation did not enter the brain. Since Ehrlich's first experiments, only a small number of molecules, such as alcohol and caffeine have been found to cross the blood-brain barrier, and this selective permeability remains the major roadblock to treatment of many central nervous system diseases. At the same time, many central nervous system diseases are associated with disruption of the blood-brain barrier that can lead to changes in permeability, modulation of immune cell transport, and trafficking of pathogens into the brain. Therefore, advances in our understanding of the structure and function of the blood-brain barrier are key to developing effective treatments for a wide range of central nervous system diseases. Over the past 10 years it has become recognized that the blood-brain barrier is a complex, dynamic system that involves biomechanical and biochemical signaling between the vascular system and the brain. Here we reconstruct the structure, function, and transport properties of the blood-brain barrier from an engineering perspective. New insight into the physics of the blood-brain barrier could ultimately lead to clinical advances in the treatment of central nervous system diseases.

Bioengineering heterodimeric cytokines: turning promiscuous proteins into therapeutic agents

The interleukin 12 (IL-12) family comprises a group of heterodimeric cytokines that can cope with a great variety of immune conditions as the microenvironment demands. By sharing cytokine and receptor subunits, IL-12 (comprised of p40/p35 subunits), IL-23 (p40/p19), IL-27 (p28/EBI3), and IL-35 (p35/EBI3) represent, as a whole, a highly versatile system participating in controlling the continuum from inflammation to tolerance. Promiscuity, a peculiar feature of those cytokines, is a powerful and economic means of producing individual factors with distinct activities via different combinations of a single set of subunits. Whereas IL-12 and IL-23 have a clearly dominant immunostimulatory functional profile and IL-35 is a potent immunosuppressive agent, IL-27 can exert both adjuvant and regulatory effects, depending on the cytokine milieu. Promiscuity itself, however, may significantly hamper the therapeutic use of heterodimeric cytokines. The subunits of a recombinant cytokine, when administered in its native form, will rapidly dissociate in vivo and reassociate with alternative partners, thus generating different heterodimeric or even homodimeric molecules (i.e., p40/p40) with unwanted effects. As in other areas, bioengineering has provided a formidable tool to overcome the constraints associated with the potential use of IL-12 family cytokines. The generation of several gene constructs expressing IL-12, IL-23, IL-27, IL-35, or even the homodimer p40/p40, in their monomerized, single-chain form has allowed us to unveil the efficacy of those molecules in several experimental settings, including neoplasia, viral infection, chronic inflammation, allergy and autoimmunity. Although work is still needed to obtain an overall picture of therapeutic vs. adverse effects of individual molecules before any use in humans, the new frontiers of bioengineering are now driving the production of completely new combinations of cytokine subunits that may further extend the potential clinical use of such eclectic proteins.

Peptide pills for brain diseases? Reality and future perspectives

The peptide therapeutic market is one of the fastest growth areas of the pharmaceutical industry. Although few orally administered peptides are marketed and many are in different phases of clinical development, there is no marketed oral peptide therapeutic used for CNS disorders. The major challenges involved in orally delivering peptides to the brain relate to their enzymatic instability and inability to permeate across physiological barriers. The paucity of therapies for the treatment of brain diseases and the presence of the blood-brain barrier excluding 98% of therapeutic molecules necessitates parenteral administration. Various approaches have been applied to enhance oral peptide bioavailability, but only nanoparticulate strategies were able to deliver orally therapeutic peptides to the brain. Although industry may be reluctant to invest in developing oral peptide nanomedicines, the increasingly unmet clinical need and economic burden associated with brain diseases will fuel the development of the first marketed oral-to-brain peptide therapy.

Physiology of the intrathecal bolus: the leptomeningeal route for macromolecule and particle delivery to CNS

Presently, there are no effective treatments for several diseases involving the CNS, which is protected by the blood-brain, blood-CSF, and blood-arachnoid barriers. Traversing any of these barriers is difficult, especially for macromolecular drugs and particulates. However, there is significant experimental evidence that large molecules can be delivered to the CNS through the cerebrospinal fluid (CSF). The flux of the interstitial fluid in the CNS parenchyma, as well as the macro flux of CSF in the leptomeningeal space, are believed to be generally opposite to the desirable direction of CNS-targeted drug delivery. On the other hand, the available data suggest that the layer of pia mater lining the CNS surface is not continuous, and the continuity of the leptomeningeal space (LMS) with the perivascular spaces penetrating into the parenchyma provides an unexplored avenue for drug transport deep into the brain via CSF. The published data generally do not support the view that macromolecule transport from the LMS to CNS is hindered by the interstitial and CSF fluxes. The data strongly suggest that leptomeningeal transport depends on the location and volume of the administered bolus and consists of four processes: (i) pulsation-assisted convectional transport of the solutes with CSF, (ii) active "pumping" of CSF into the periarterial spaces, (iii) solute transport from the latter to and within the parenchyma, and (iv) neuronal uptake and axonal transport. The final outcome will depend on the drug molecule behavior in each of these processes, which have not been studied systematically. The data available to date suggest that many macromolecules and nanoparticles can be delivered to CNS in biologically significant amounts (>1% of the administered dose); mechanistic investigation of macromolecule and particle behavior in CSF may result in a significantly more efficient leptomeningeal drug delivery than previously thought.

An innovative approach to the treatment of Gaucher disease and possibly other metabolic disorders of the brain

The extraordinary benefit of enzyme replacement therapy (ERT) on the systemic manifestations of Gaucher disease was demonstrated in 1991. Since that time, investigators have devoted substantial effort to improve the delivery of enzymes to the brain because many hereditary metabolic disorders are characterized by extensive central nervous system involvement. Because the required supplemental enzyme is too large to cross the blood-brain barrier (BBB), ERT for central nervous system involvement was out of the question at that time. Several innovative strategies that have been reported to overcome this impediment are discussed. Recent investigations have provided additional insight concerning the pathogenesis of enzyme deficiency disorders. For many years it was presumed that alterations of the amino acid sequence of enzymes such as glucocerebrosidase reduced the catalytic activity of the enzyme. It has recently been shown that the decrease of glucocerebrosidase activity was the result of a quantitative loss of the amount of this enzyme. Significant increases of its activity were obtained with small molecule inhibitors of histone deacetylase that cross the BBB. The effect of such materials on neuronopathic Gaucher disease and other CNS metabolic disorders is discussed.

Comparative binding, endocytosis, and biodistribution of antibodies and antibody-coated carriers for targeted delivery of lysosomal enzymes to ICAM-1 versus transferrin receptor

Targeting lysosomal enzymes to receptors involved in transport into and across cells holds promise to enhance peripheral and brain delivery of enzyme replacement therapies (ERTs) for lysosomal storage disorders. Receptors being explored include those associated with clathrin-mediated pathways, yet other pathways seem also viable. Well characterized examples are that of transferrin receptor (TfR) and intercellular adhesion molecule 1 (ICAM-1), involved in iron transport and leukocyte extravasation, respectively. TfR and ICAM-1 support ERT delivery via clathrin- vs. cell adhesion molecule-mediated mechanisms, displaying different valency and size restrictions. To comparatively assess this, we used antibodies vs. larger multivalent antibody-coated carriers and evaluated TfR vs. ICAM-1 binding and endocytosis in endothelial cells, as well as in vivo biodistribution and delivery of a model lysosomal enzyme required in peripheral organs and brain: acid sphingomyelinase (ASM), deficient in types A-B Niemann Pick disease. We found similar binding of antibodies to both receptors under control conditions, with enhanced binding to activated endothelium for ICAM-1, yet only anti-TfR induced endocytosis efficiently. Contrarily, antibody-coated carriers showed enhanced binding, engulfment, and endocytosis for ICAM-1. In mice, anti-TfR enhanced brain targeting over anti-ICAM, with an opposite outcome in the lungs, while carriers enhanced ICAM-1 targeting over TfR in both organs. Both targeted carriers enhanced ASM delivery to the brain and lungs vs. free ASM, with greater enhancement for anti-ICAM carriers. Therefore, targeting TfR or ICAM-1 improves lysosomal enzyme delivery. Yet, TfR targeting may be more efficient for smaller conjugates or fusion proteins, while ICAM-1 targeting seems superior for multivalent carrier formulations.

Cell-penetrating Peptide-mediated therapeutic molecule delivery into the central nervous system

The blood-brain barrier (BBB), a dynamic and complex barrier formed by endothelial cells, can impede the entry of unwanted substances - pathogens and therapeutic molecules alike - into the central nervous system (CNS) from the blood circulation. Taking into account the fact that CNS-related diseases are the largest and fastest growing unmet medical concern, many potential protein- and nucleic acid-based medicines have been developed for therapeutic purposes. However, due to their poor ability to cross the BBB and the plasma membrane, the above-mentioned bio-macromolecules have limited use in treating neurological diseases. Finding effective, safe, and convenient ways to deliver therapeutic molecules into the CNS is thus urgently required. In recent decades, much effort has been expended in the development of drug delivery technologies, of which cell-penetrating peptides (CPPs) have the most promising potential. The present review covers the latest advances in CPP delivery technology, and provides an update on their use in CNS-targeted drug delivery.

Liver-targeting of interferon-alpha with tissue-specific domain antibodies

Interferon alpha (IFNα) is used for the treatment of hepatitis C infection and whilst efficacious it is associated with multiple adverse events including reduced leukocyte, erythrocyte, and platelet counts, fatigue, and depression. These events are most likely caused by systemic exposure to interferon. We therefore hypothesise that targeting the therapeutic directly to the intended site of action in the liver would reduce exposure in blood and peripheral tissue and hence improve the safety and tolerability of IFNα therapy. We genetically fused IFN to a domain antibody (dAb) specific to a hepatocyte restricted antigen, asialoglycoprotein receptor (ASGPR). Our results show that the murine IFNα2 homolog (mIFNα2) fused to an ASGPR specific dAb, termed DOM26h-196-61, could be expressed in mammalian tissue culture systems and retains the desirable biophysical properties and activity of both fusion partners when measured in vitro. Furthermore a clear increase in in vivo targeting of the liver by mIFNα2-ASGPR dAb fusion protein, compared to that observed with either unfused mIFNα2 or mIFNα2 fused to an isotype control dAb V_HD2 (which does not bind ASGPR) was demonstrated using microSPECT imaging. We suggest that these findings may be applicable in the development of a liver-targeted human IFN molecule with improved safety and patient compliance in comparison to the current standard of care, which could ultimately be used as a treatment for human hepatitis virus infections.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

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.

[New protein vectors for physiological transfer of therapeutic agents to the central nervous system]

The central nervous system is a sanctuary protected by barriers, among which the blood-brain barrier (BBB). The BBB is formed by the specific nature of the endothelial cells of the brain capillaries, that allow brain access only to nutrients necessary for brain cell survival and function. These properties of the BBB result in the incapacity of therapeutic compounds, both small and large, to reach the brain at therapeutic concentrations. Various strategies are now being developed to enhance the amount and concentration of these compounds in the brain parenchyma. The development of new technologies such as peptide vectors will achieve the delivery of active agents in therapeutic concentration across the BBB to treat brain diseases such as cancer or neurodegenerative disorders. In this paper, design of new active peptides and the development of new protein, peptide, vectors for drug brain delivery using physiological approaches will be addressed. In addition to the Angiopep platform technology which is in development at Angiochem Inc. and is to date the most advanced in human clinical trials, the new Transcend technology using a protein melanotransferrin for the transport of biologics such as lysosomal enzymes and antibodies will be discussed.

Peptide derived from HIV-1 TAT protein destabilizes a monolayer of endothelial cells in an in vitro model of the blood-brain barrier and allows permeation of high molecular weight proteins

Most chemotherapeutic agents are blood-brain barrier (BBB) impermeants. HIV-1-derived TAT protein variants contain a transmembrane domain, which may enable them to cross the BBB and reach the brain. Here we synthesized CAYGRKKRRQRRR, a peptide containing a cysteine moiety attached to the N terminus of the transmembrane domain (C-TAT peptide), and studied its effects in an in vitro BBB model, which we found to reflect penetration by a receptor-independent pathway. Incubation of the brain capillary endothelial cell monolayer with 0.3-0.6 μmol/ml of this C-TAT peptide, for a period of 1-2 h, destabilizes brain capillary endothelial cell monolayer and introduces the ability of impermeant therapeutic agents including high molecular weight proteins to penetrate it substantially. The cysteinyl moiety at position 1 of the C-TAT peptide contributes largely to the destabilizing potency and the penetration efficacy of impermeant substances. The destabilizing effect was reversed using heparin. In summary, experimental conditions allowing a significant increase in entry of impermeant low and high molecular weight substances from the luminal (blood) to the abluminal side (brain) were found in an in vitro BBB model reflecting in vivo protein penetrability by a receptor-independent pathway.

Drug transport across the blood-brain barrier

The blood-brain barrier (BBB) prevents the brain uptake of most pharmaceuticals. This property arises from the epithelial-like tight junctions within the brain capillary endothelium. The BBB is anatomically and functionally distinct from the blood-cerebrospinal fluid barrier at the choroid plexus. Certain small molecule drugs may cross the BBB via lipid-mediated free diffusion, providing the drug has a molecular weight <400 Da and forms <8 hydrogen bonds. These chemical properties are lacking in the majority of small molecule drugs, and all large molecule drugs. Nevertheless, drugs can be reengineered for BBB transport, based on the knowledge of the endogenous transport systems within the BBB. Small molecule drugs can be synthesized that access carrier-mediated transport (CMT) systems within the BBB. Large molecule drugs can be reengineered with molecular Trojan horse delivery systems to access receptor-mediated transport (RMT) systems within the BBB. Peptide and antisense radiopharmaceuticals are made brain-penetrating with the combined use of RMT-based delivery systems and avidin-biotin technology. Knowledge on the endogenous CMT and RMT systems expressed at the BBB enable new solutions to the problem of BBB drug transport.

Peptide sequences mediating tropism to intact blood-brain barrier: an in vivo biodistribution study using phage display

Peptide motifs that demonstrate tropism for the blood brain barrier (BBB) are of real translational value in developing innovative delivery strategies for biological brain targeted therapies. In vivo peptide-phage display affords peptide selection against the full complement of biological markers within the correct cellular macro- and micro-environments. Here a stringent in vivo biopanning protocol was employed in the rat aimed at identifying cyclic 7-mer peptide motifs that mediate tropism to brain microvasculature. Five rounds of biopanning identified 349 unique peptide motifs in the brain tissue gray matter compartment (microvasculature and parenchyma). While in general no consensus was evident linking peptide physico-chemical properties and brain tropism, peptides bearing c-SxTSSTx-c or c-xxxSSTx-c motifs were found to be present in high abundance. Based on amino acid frequency distribution of the 349 unique peptides sequences a theoretical 'idealized' peptide pattern, c-PP(S/P)SSST-c, could be derived. For the most abundant experimental peptide sequence found in brain tissue, c-SYTSSTM-c, an in vivo pharmacokinetic and whole body tissue biodistribution study was performed. Based upon tissue exposure data (i.e. tissue AUC((0-infinity))) the sequence c-SYTSSTM-c efficiently retargeted phage virions to the brain providing an approximate 5-fold greater (P<0.05) accumulation in brain over control phage; in all other organs no significant (P>0.05) difference in tissue tropism between c-SYTSSTM-c and control phages were evident. This peptide and more generally the peptide motifs, -SxTSSTx- or -xxxSSTx-, warrant further investigation as agents mediating sequence-dependent tropism to brain microvasculature potentially able to deliver biologic cargo to the CNS.

Ultrasound-mediated blood-brain/blood-tumor barrier disruption improves outcomes with trastuzumab in a breast cancer brain metastasis model

Trastuzumab has shown positive results in many patients with metastatic HER2-positive breast cancer, but it is less effective for controlling metastases in the CNS, which remains a site of relapse. The poor prognosis for patients with brain metastases is thought to be largely due to the presence of the blood-brain barrier (BBB) that prevents delivery of most drugs to the CNS and to the heterogeneous and limited permeability of the blood-tumor barrier (BTB). Focused ultrasound (FUS) bursts combined with circulating microbubbles can temporarily permeabilize both the BBB and the BTB. This technique has been investigated as a potential noninvasive method for targeted drug delivery in the brain. Here, we investigated whether BBB/BTB permeabilization in the tumor and surrounding brain tissue induced by FUS and microbubbles can slow tumor growth and improve survival in a breast cancer brain metastases model. HER2/neu-positive human breast cancer cells (BT474) were inoculated in the brains of 41 nude (nu/nu) rats. Animals in the treatment group received six weekly treatments of BTB/BBB permeabilization under MRI guidance combined with IV administration of trastuzumab (2 mg/kg). Tumor growth and survival rates were monitored via MRI for seven weeks after sonication. Starting at week seven and continuing through the end of the study, the mean tumor volume of the FUS+trastuzumab group was significantly (P<0.05) less than those of the three control groups (no treatment, FUS alone, trastuzumab alone). Furthermore, in four out of 10 rats treated with FUS+trastuzumab, the tumor appeared to be completely resolved in MRI, an outcome which was not observed in any of the 31 rats in three control groups. Trastuzumab improved median survival by 13% compared to the no treatment group, a difference which was significant (P=0.044). Treatment with FUS+trastuzumab produced the most significant benefit compared to the no-treatment controls (P=0.0084). More than half (6/10) animals survived at the study endpoint, leading to a median survival time greater than 83 days (at least 32% longer than the untreated control group). Overall, this work suggests that BBB/BTB permeabilization induced by FUS and microbubbles can improve outcomes in breast cancer brain metastases.

Improving the transport of chemotherapeutic drugs across the blood-brain barrier

The successful treatment of brain tumors or metastases in the brain is still hampered by the very efficient blood-brain barrier, which prevents the cerebral accumulation of a pharmacologically sufficient amount of a drug. Beside the possibility of disintegrating the functionality of this effective working barrier, a nanocarrier-mediated transport is presently an interesting and promising method to increase the drug concentration in the brain. Nanocarriers are small vesicles (<200 nm) and can be prepared by polymerization, resulting in nanoparticles, or by producing superficial lipid structures to incorporate the drug. In this context, liposomes are of importance owing to their ability to adapt their properties to the pharmacological requirements. In this article, we will give an overview of current possibilities of enhancing anticancer drug transport across the blood-brain barrier, based on its structure and functionality. Special consideration will be given to recent liposomal approaches that use active targeting for receptor-mediated transport across this physiological barrier.

Drug penetration across the blood-brain barrier: an overview

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

Development of new peptide vectors for the transport of therapeutic across the blood-brain barrier

The blood-brain barrier (BBB) is formed by the special nature of the endothelial cells of the brain capillaries characterized by tight junctions between cells and a high expression of efflux pumps only allowing the brain access to nutrients necessary for cell survival and function. These properties of the BBB result in the incapacity of small and large therapeutic compounds to reach the brain at therapeutic concentrations. Various strategies are now being developed to enhance the amount and concentration of these compounds in the brain parenchyma. The development of new technologies such as peptide vectors has the potential to achieve the delivery of active agents in therapeutic concentrations across the BBB to treat brain diseases such as brain primary and metastatic cancers and neurodegenerative disorders. In this review, the design of new active peptides and development of new peptide vectors for drug brain delivery using physiological approaches will be addressed. A new chemical entity incorporating angiopep peptide in a small anticancer agent (paclitaxel) is now in clinical trials. It is the first of such designed agents to be validated for the treatment of human brain cancers and opens the door for such approaches.

Drug delivery to the brain via the blood-brain barrier: a review of the literature and some recent patent disclosures

Delivery of drugs to the brain is challenging, not only for large biopharmaceutical molecules, but also for small organics, which are effluxed from the brain capillary endothelial cells. These cells constitute, in part, the selectively permeable blood-brain barrier. Progress is being made using delivery systems comprising a vector, a linker and cargo, which are purported to enter the brain via receptors on the luminal surface of the brain capillary endothelial cells. Unfortunately, from a delivery perspective, these receptors are not expressed only on brain capillary endothelial cells; so the approaches described in this review are for enhanced delivery to the brain, not for specific brain targeting. The inventions disclosed in patents relate to technologies to screen for new blood-brain barrier receptors and to identify new vectors, or describe systems that deliver cargoes to the brain via any blood-brain barrier receptor, or define specified peptide vectors that target a specific receptor. To date, only one of the technologies has reached early clinical trials and, as always, major challenges remain to be addressed.

Pericytes and the blood-brain barrier: recent advances and implications for the delivery of CNS therapy

"Once the regulation of brain endothelial transcytosis is understood at the molecular level, it should be possible to exploit these mechanisms as targets for facilitated CNS drug delivery".

Probing cocaine-antibody interactions in buffer and human serum

Background

Despite progress in cocaine immunotherapy, the kinetic and thermodynamic properties of antibodies which bind to cocaine and its metabolites are not well understood. It is also not clear how the interactions between them differ in a complex matrix such as the serum present in the human body. In the present study, we have used microscale thermophoresis (MST), isothermal titration calorimetry (ITC), and surface plasmon resonance (SPR) we have evaluated the affinity properties of a representative mouse monoclonal (mAb08) as well as those of polyclonal antibodies purified from vaccinated mouse and human patient serum.

Results

MST analysis of fluorescently tagged mAb08 binding to cocaine reveals an approximately 15 fold decrease in its equilibrium dissociation constant in 20–50% human serum compared with that in saline buffer. A similar trend was also found using enriched polyclonal antibodies purified from vaccinated mice and patient serum, for which we have used fluorescently tagged bovine serum albumin conjugated to succinyl norcocaine (BSA-SNC). This conjugate closely mimics both cocaine and the hapten used to raise these antibodies. The ITC data also revealed that cocaine has a moderate affinity of about 2 µM to 20% human serum and very little interaction with human serum albumin or nonspecific human IgG at that concentration range. In a SPR inhibition experiment, the binding of mAb08 to immobilized BSA-SNC was inhibited by cocaine and benzoylecgonine in a highly competitive manner, whereas the purified polyclonal antibodies from vaccinated humans and mice, revealed preferential selectivity to pharmacologically active cocaine but not to the inactive metabolite benzoylecgonine. We have also developed a simple binding model to simulate the challenges associated with cocaine immunotherapy using the variable quantitative and kinetic properties of the antibodies.

Conclusions

High sensitivity calorimetric determination of antibody binding to cocaine and its metabolites provide valuable information for characterization of their interactions and thermodynamic properties. In addition MST measurements of antibody affinity in the presence of biological fluids will provide a better opportunity to make reliable decisions and facilitate the design of cocaine vaccines and immunization conditions. The methods should be more widely adopted in characterization of antibody complexes.

Synthetic cell surface receptors for delivery of therapeutics and probes

Receptor-mediated endocytosis is a highly efficient mechanism for cellular uptake of membrane-impermeant ligands. Cells use this process to acquire nutrients, initiate signal transduction, promote development, regulate neurotransmission, and maintain homeostasis. Natural receptors that participate in receptor-mediated endocytosis are structurally diverse, ranging from large transmembrane proteins to small glycolipids embedded in the outer leaflet of cellular plasma membranes. Despite their vast structural differences, these receptors share common features of binding to extracellular ligands, clustering in dynamic membrane regions that pinch off to yield intracellular vesicles, and accumulation of receptor-ligand complexes in membrane-sealed endosomes. Receptors typically dissociate from ligands in endosomes and cycle back to the cell surface, whereas internalized ligands are usually delivered into lysosomes, where they are degraded, but some can escape and penetrate into the cytosol. Here, we review efforts to develop synthetic cell surface receptors, defined as nonnatural compounds, exemplified by mimics of cholesterol, that insert into plasma membranes, bind extracellular ligands including therapeutics, probes, and endogenous proteins, and engage endocytic membrane trafficking pathways. By mimicking natural mechanisms of receptor-mediated endocytosis, synthetic cell surface receptors have the potential to function as prosthetic molecules capable of seamlessly augmenting the endocytic uptake machinery of living mammalian cells.

Transcriptional profiling of human brain endothelial cells reveals key properties crucial for predictive in vitro blood-brain barrier models

Brain microvascular endothelial cells (BEC) constitute the blood-brain barrier (BBB) which forms a dynamic interface between the blood and the central nervous system (CNS). This highly specialized interface restricts paracellular diffusion of fluids and solutes including chemicals, toxins and drugs from entering the brain. In this study we compared the transcriptome profiles of the human immortalized brain endothelial cell line hCMEC/D3 and human primary BEC. We identified transcriptional differences in immune response genes which are directly related to the immortalization procedure of the hCMEC/D3 cells. Interestingly, astrocytic co-culturing reduced cell adhesion and migration molecules in both BECs, which possibly could be related to regulation of immune surveillance of the CNS controlled by astrocytic cells within the neurovascular unit. By matching the transcriptome data from these two cell lines with published transcriptional data from freshly isolated mouse BECs, we discovered striking differences that could explain some of the limitations of using cultured BECs to study BBB properties. Key protein classes such as tight junction proteins, transporters and cell surface receptors show differing expression profiles. For example, the claudin-5, occludin and JAM2 expression is dramatically reduced in the two human BEC lines, which likely explains their low transcellular electric resistance and paracellular leakiness. In addition, the human BEC lines express low levels of unique brain endothelial transporters such as Glut1 and Pgp. Cell surface receptors such as LRP1, RAGE and the insulin receptor that are involved in receptor-mediated transport are also expressed at very low levels. Taken together, these data illustrate that BECs lose their unique protein expression pattern outside of their native environment and display a more generic endothelial cell phenotype. A collection of key genes that seems to be highly regulated by the local surroundings of BEC within the neurovascular unit are presented and discussed.

In vivo real-time fluorescence visualization and brain-targeting mechanisms of lipid nanocarriers with different fatty ester:oil ratios

AIMS

The objective of the present work was to investigate the influence of the inner cores of lipid nanocarriers on the efficiency of brain targeting. Cetyl palmitate and squalene were respectively chosen as the solid lipid and liquid oil in the inner phase of the nanocarriers.

MATERIALS & METHODS

Nanoparticulate systems with different cetyl palmitate/squalene ratios were compared by evaluating the size, zeta potential, molecular environment, and mobility of lipids in the systems.

RESULTS

The particulate diameter ranged from 190 to 210 nm, with systems containing 100% cetyl palmitate in the matrix (solid lipid nanoparticles [SLN]) showing the smallest size, followed by systems with both cetyl palmitate and squalene (nanostructured lipid carriers [NLC]) and with 100% squalene (lipid emulsions [LE]). A cationic surfactant, Forestall, was used to produce a positive surface charge of 40-55 mW. The in vitro release was evaluated using various dyes located in different phases of the nanocarriers. The release of sulforhodamine B occurred in a sustained manner from the shell of the nanocarriers. The in vivo brain distribution of lipid nanosystems after an intravenous injection into rats was monitored by a real-time fluorescence imaging system. LE showed higher brain accumulation than SLN and NLC. NLC only exhibited a slightly higher brain accumulation compared with the aqueous control. Incorporation of sulforhodamine B into LE could prolong its retention in the brain from 20 to 50 min. The results were further confirmed by imaging the entire brain and brain slices. The specific association of lipid nanocarriers with rat brain endothelial cells (bEnd3) was demonstrated using fluorescence microscopy. The cellular uptake of LE and SLN was higher compared with NLC and the aqueous control. LE were observed to be internalized by cells through caveola-mediated and macropinocytotic energy-dependent endocytosis.

CONCLUSION

The experimental profiles indicated that LE with moderate additives are a promising brain-targeting nanocarrier. The composition of the lipid matrix played a significant role in delivering compounds to the brain.