Current Drug Delivery, 2010, 7, 000-000
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Current Approaches for Drug Delivery to Central Nervous System Sharif Hossaina, Toshihiro Akaikea and Ezharul Hoque Chowdhurya,b,* a
Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan; bSchool of Medicine and Health Sciences, International Medical University (IMU), No. 126, Jalan 19/155B, Bukit Jalil 57000, Kuala Lumpur, Malaysia Abstract: Brain, the center of the nervous system in all vertebrate, plays the most vital role in every function of human body. However, many neurodegenerative diseases, cancer and infections of the brain become more prevalent as populations become older. In spite of the major advances in neuroscience, many potential therapeutics are still unable to reach the central nervous system (CNS) due to the blood–brain barrier (BBB) which is formed by the tight junctions within the capillary endothelium of the vertebrate brain .This results in the capillary wall behaving as a continuous lipid bilayer and preventing the passage of polar and lipid insoluble substances. Several approaches for delivering drugs to the CNS have been developed to enhance the capacity of therapeutic molecules to cross the BBB by modifying the drug itself, or by coupling it to a vector for receptor-mediated, carrier mediated or adsorption-mediated transcytosis. The current challenge is to develop drug delivery systems that ensure the safe and effective passage of drugs across the BBB. This review focuses on the strategies and approaches developed to enhance drug delivery to the CNS.
Keywords: Central nervous system, blood brain barrier, drug delivery system, drug targeting, Nanoparticle. 1. INTRODUCTION The central nervous system (CNS) being enclosed in the meninges of vertebrates, is the part of the nervous system that functions to coordinate the activities of all parts of the body. Drug delivery to the CNS is made difficult by the esistence of the blood–brain barrier (BBB), which is formed by the complex tight junctions between the endothelial cells of the brain capillaries and the low endocytic activity of the vertebrate brain [1]. The BBB is a metabolic or cellular structure in the CNS that restricts the passage of various chemical substances and microscopic objects like bacteria between the bloodstream and the neural tissue itself, while still allowing the passage of substances essential for metabolic function. The tight junctions of BBB eliminate the normal porous transcellular or paracellular pathways for solute diffusion from plasma to organ interstitial space. Circulating drugs, with the exception of lipid-soluble small molecules with a molecular mass under a 400–600 threshold [2, 3], have restricted passage through the BBB, and do not enter the CNS in pharmacologically significant amounts from the bloodstream. The concept of BBB has been illustrated in Fig. (1). 2. BARRIER SYSTEMS OF THE BRAIN There are other barrier systems within the CNS in addition to the brain microvascular endothelial barrier (BBB), including the arachnoid epithelial membrane which covers the surface of the brain and the choroid plexus epithelium which forms the blood-cerebrospinal fluid (CSF) barrier. There are approximately 400 miles of capillaries perfusing *Address correspondence to this author at the School of Medicine and Health Sciences, International Medical University (IMU), No. 126, Jalan 19/155B, Bukit Jalil 57000, Kuala Lumpur, Malaysia; Tel: (45)-924-5879; Fax: (45)-924-5879; E-mail:
[email protected] 1567-2018/10 $55.00+.00
the brain in humans, and the surface area of the brain microvascular endothelium is approximately 20 m2 [4], which is 1000-fold greater than the surface area of either the bloodCSF barrier or the arachnoid membrane [5]. Therefore, the quantitatively important barrier system within the brain is the BBB at the capillary endothelium. Despite the vast surface area of the human BBB, the thickness of the BBB is very thin, and the total intracellular volume of the brain capillary endothelium is only 5 ml in the entire human brain and 1 μl in the rat brain. The thickness of the brain capillary endothelial cell is about 200–300 nm. This very thin cellular barrier has some of the most restrictive permeability properties of any biological membrane [6]. Multiple mechanisms, including a physical endothelial barrier, an enzymatic BBB and an efflux barrier restrict the drug entry from blood into the brain. This multifunctionality of the BBB arises from the multicellularity of the brain microvasculature being formed by the triad of brain capillary endothelial cells, capillary pericytes, and perivascular astrocyte foot processes [4]. The endothelium and pericyte share a common microvascular basement membrane and 99% of the brain surface of the capillary basement membrane is invested by the end-feet of processes extending from astrocyte cell bodies originating within brain parenchyma. The main barriers include endothelial tight junctions, enzymatic BBB and active efflux barrier. 3. EXISTING WAYS THROUGH BBB
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Owing to these unique properties of BBB, circulating molecules in the blood gain access to brain interstitial space via either lipid-mediated free diffusion (for small molecules) or catalyzed transport (for small or large molecules). Through the lipid-mediated transport, certain small molecules can traverse the BBB nonspecifically. Molecules that © 2010 Bentham Science Publishers Ltd.
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Fig. (1). Schematic diagram of the neurovascular unit or cell association forming the BBB. The blood brain barrier is created by the tight apposition of endothelial cells lining blood vessels in the brain preventing easy passage of large macromolecules and pathogens between the circulation and the brain. Pericytes are distributed discontinuously along the length of the cerebral capillaries and partially surround the endothelium. Foot processes from astrocytes form a network fully surrounding the capillaries. Microglia (perivascular macrophages) are the resident immunocompetent cells of the brain and are derived from systemic circulating monocytes and macrophages.
are lipid soluble and have a molecular weight 98% of all small molecule drug candidates do not cross the BBB, and no BBB drug targeting technology is used by the pharmaceutical industry[4]. Therefore, CNS drug delivery is highly expected using different new approaches and strategies. 4. STRATEGIES AND APPROACHES OF CNS DRUG DELIVERY CNS drug delivery and targeting strategies are needed for at least two reasons. First, with the exception of lipid-soluble molecules having a molecular weight under a 400–600d
threshold, virtually all drugs that originate from either biotechnology or classical small molecule pharmacology rarely transports through the BBB. Second, disorders of the brain are surprisingly common, and in the United States there are >80 million individuals suffering from some form of CNS disorders, including alcohol abuse, anxiety/phobia, sleep disorders, depression/mania, drug abuse, obsessivecompulsive disorder, Alzheimer's disease, schizophrenia, stroke, epilepsy, cerebral acquired immunodeficiency syndrome, and Parkinson's disease [8]. There are both chemical and biological approaches for developing BBB drug-targeting strategies [9]. The biologybased approaches rely on the endogenous transport systems within the brain capillary endothelium, which forms the BBB in vivo. The chemistry-based strategies are the conventional approaches that rely on nanoparticle and lipid-mediated drug transport across the BBB. 4.1. Biological Approach The biology based strategies for brain drug delivery are founded on the principle that there are numerous endogenous
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transport systems within the BBB. The endogenous BBB transport systems may be broadly classified as carriermediated transport (CMT), active efflux transport (AET), and receptor-mediated transport (RMT). These BBB transport systems are situated on the luminal and abluminal membranes of the brain capillary endothelium [10]. Drug delivery to the brain through the many endogenous transport systems within the BBB requires reformulation of the drug so that the drug can access the BBB transport system and enter the brain. The biology-based approaches to solving the BBB drug-delivery problem require advanced knowledge of the endogenous transporters. Researchers within brain drugdiscovery and brain drug-targeting could work closely together in the drug development process to ensure that a viable reformulation of the drug is accomplished. Thus, the dual goals of brain drug formulation are to enable BBB transport and retain the biological activity of the pharmaceu-
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tical. The potential routes for transport across the BBB has been described in Fig. (2). 4.1.1. Carrier Mediated Transport (CMT) CMT is a way of transporting through BBB. Small watersoluble nutrients and vitamins traverse the BBB rapidly via CMT. The CMT systems generally mediate the blood-tobrain transport of nutrients and include the GLUT1 glucose transporter, the LAT1 large neutral amino acid transporter, the MCT1 monocarboxylic acid transporter, the CNT2 nucleoside transporter, and many other small molecule transporters [4]. Noninvasive drug delivery has been studied using CMT systems by conjugating therapeutics to the natural substrates. Since CMT systems are typically small, stereospecific pores, they are not particularly suitable to the transport of large-molecule therapeutics [11].
Fig. (2). A schematic diagram of the potential routes for transport across the BBB. (A) Leukocytes may cross the BBB adjacent to, or by modifying, the tight junctions but normally, the tight junctions severely restrict penetration of water-soluble compounds, including polar drugs. (B) However, the large surface area of the lipid membranes of the endothelium offers an effective diffusive route for lipid-soluble agents and some solutes. (C) Carrier-mediated transport, which may be passive or secondarily active, can transport many essential polar molecules such as glucose, amino acids, and nucleosides into the CNS. (D) Active efflux carriers may intercept some of these passively penetrating solutes and pump them out of the endothelial cell. Some transporters are energy-dependent (for example, P-glycoprotein) and act as efflux transporters. AZT, azidothymidine (E) Receptor Mediated Transport (RMT) can transport macromolecules such as peptides and proteins across the cerebral endothelium. Certain proteins, such as insulin and transferrin, are taken up by specific receptor-mediated endocytosis and transcytosis. (F) Native plasma proteins such as albumin are poorly transported, but cationization can increase their uptake by adsorptive-mediated endocytosis and transcytosis.
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4.1.2. Receptor Mediated Transport (RMT) In addition to the CMT systems, certain large molecules (peptides or plasma proteins) are selectively transported across the BBB via RMT systems, including the insulin receptor, the transferrin receptor (TfR), or the leptin receptor. RMT of circulating peptides is comprised of three sequential steps: (1) receptor-mediated endocytosis of the circulating peptide at the luminal membrane of the capillary endothelium, (2) movement through the 200–300 nm of endothelial cytoplasm, and (3) exocytosis of the peptide into the brain interstitial fluid at the abluminal membrane of the capillary endothelium [4]. In order to exploit endogenous RMT systems for drug delivery, the therapeutic or the carrier of the therapeutic must be conjugated to a molecule which might be a natural ligand or an artificial ligand (antibody or peptide) with the capability of targeting an RMT system [11]. In contrast to CMT, there is less restriction for the sizes of therapeutic cargo for targeting an RMT system since it employs vesicle-based transport rather than a stereoselective carrier. 4.1.3. Absorptive-Mediated Transport (AMT) While RMT systems are selective in that they require the initial binding of a ligand to something on or in the plasma membrane of the endothelial cells, AMT relies on nonspecific charge-based interactions. AMT can be initiated by polycationic molecules binding to negative charges on the plasma membrane. However, this method due to the lack of specific targeting may lead to widespread absorption and in deed, protein transduction domains such as the HIV TAT peptide lacking the targeting moieties have been shown to have a broad biodistribution, thus necessitating the prohibitive doses [12]. 4.1.4. Active Efflux Transport (AET) AET system takes the advantages for transporting certain materials across BBB. P-glycoprotein (Pgp) as well as many other AETs function at the BBB to cause the selective export of metabolites from brain back to blood. Pgp is expressed at both the capillary endothelium and at astrocyte processes in primate and human brains [13]. The GLUT1 glucose transporter is expressed at both the luminal and abluminal endothelial membranes in rat brain [14], and this transporter comigrates with Pgp in fractionated plasma membranes from rat brain endothelia [15]. 4.2. Chemical Approach CNS-targeted chemical delivery systems are inactive chemical derivatives of a drug, obtained by one or more chemical modifications, which provide a site-specific or siteenhanced delivery of the drug through multistep enzymatic or chemical transformations [16]. Chemical delivery systems, in addition to providing access by increasing the lipophilicity, exploit the specific bidirectional properties of the BBB to lock inactive drug precursors in the brain on arrival, preventing exit back across the BBB. Chemical delivery systems can be used not only to deliver compounds that otherwise have no access to the brain but also to retain lipophilic compounds within the brain, as has been achieved with a variety of steroid hormones. The following part will focus on nanobiotechnology-based methods to facilitate drug delivery across the BBB.
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Delivering drugs across the BBB is one of the most promising applications of nanotechnology in clinical neuroscience. The primary advantage of nanoparticle carrier technology is that nanoparticles mask the BBB-limiting characteristics of the therapeutic drug molecule [17]. Furthermore, this system may slow drug release in the brain, decreasing peripheral toxicity. Various factors that influence the transport include the type of polymer or surfactant used, nanoparticle size and surface property, and the drug molecule (described elaborately in section 5.3). Drugs that have successfully been transported into the brain using this carrier include the hexapeptide dalargin, the dipeptide kyotorphin, loperamide, tubocurarine and doxorubicin. Nanotemplate engineering technology (NanoMed Pharmaceuticals) is being used to manufacture nanoparticles that mask the BBB-limiting characteristics of a drug, enabling targeted delivery via BBB transporters. NanoMed is developing paclitaxel nanoparticle (an approved chemotherapeutic agent) to treat primary and secondary brain tumors. At standard therapeutic doses, paclitaxel is limited in its access to the brain by a P-glycoprotein efflux pump. By effectively overcoming the BBB, paclitaxel nanoparticle enables a lower and safer drug dose that still maintains efficacy. Several types of nanoparticle are used for drug delivery, such as liposomes, immunoliposome, polymeric nanoparticles and so on. 4.2.1. Liposomes The attachment of water soluble drugs to lipid soluble carriers has been proposed as a strategy for targeting drugs through the BBB. Lipid carriers that have been used in the past include dihydropyridine [18], adamantine [19], and fatty acyl carriers [20], such as N-docosahexaenoyl [21]. The attachment of a lipid carrier to a drug may increase the lipid solubility of the compound. Due to the characteristics advantages, liposome is highly promising not only for delivery of neurotherapeutics to CNS, but also for CNS drug discovery. Liposomes are stable nanosize vesicles (20–100 nm) formed by phospholipids and similar amphipathic lipids. Lipid bilayers of liposomes are similar in structure to those found in living cell membranes and can carry lipophilic substances such as drugs within these layers in the same way as cell membranes. Liposomal formulations can transport drugs across the BBB and also release some of their content within the BBB. Nerve growth factor has been encapsulated into liposomes (size 100 nm) in order to protect it from the enzyme degradation in vivo and promote its permeability across the BBB [22]. Liposomes loaded with proteins or peptide drugs can be targeted to the brain capillary endothelial cells and more specifically to the lysosomes, which is an advantage for the treatment of lysosomal storage disease [23]. Antibody-conjugated liposomes or immunoliposomes are nanoparticulate drug carriers that can be used to direct encapsulated drug molecules to diseased tissues or organs. 4.2.2. Immunoliposome Small molecules may be delivered thorugh the BBB by conjugation to a delivery vector, such as monoclonal antibody (Mab). However, the number of small molecules that can be individually conjugated to MAb vectors is limited. The carrying capacity of the vector could be greatly increased by attaching the vector to the liposome which can
Current Approaches for Drug Delivery to Central Nervous System
entrap up to 10,000 small molecules. Liposomes are not normally transported across the BBB because they are too large to undergo lipid-mediated transport across the endothelial membrane [24]. While small unilamellar vesicles (40 to 80 nm) do not accumulate in brain, multilamellar vesicles (0.3 to 2 m) accumulate in brain because they cause embolism of brain capillaries [25]. If liposomes are directly injected into the brain, they dissolve in the lipid membranes of brain cells. Modification of the surface could enable the liposome or nanoparticle to be targeted to the CNS via specific BBB mechanisms. Pegylated immunoliposomes was employed to transfect -galactosidase and luciferase genes into the brain [26, 27]. The gene was incorporated into the center of the liposome while the surface of the liposome was coated with polyethylene glycol (PEG) to prolong the blood circulation time of the liposomes. In addition, 2% of the PEG strands have a mAb atatched to them for targeting the transferrin receptor in the liver and the brain. Fig. (3) illustrates a typical immunoliposome described by Pardridge [27]. If a tissueor organ-specific promoter is selected for a gene, the protein expression becomes confined to that particular tissue or organ. Using this approach, both -galactosidase and luciferase was targeted to the brain [26]. Although the mechanism by which the immunoliposome carries the gene across the BBB to the brain cells is unknown, an initial step might be the endocytosis of the immunoliposome after binding to the transferrin receptor [28]. 4.2.3. Chimeric Peptides A chimeric peptide is formed by fusing or conjugating a small or large molecule drug that is normally not transported
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across the BBB, to a BBB transport vector such as an endogenous peptide, modified protein, or peptidomimetic monoclonal antibody (MAb) that undergoes RMT through the BBB on endogenous endothelial receptor systems [4]. A peptidomimetic MAb transport vector binds an exofacial epitope on the BBB receptor. The MAb acts as a transport vector and can deliver any attached drug or gene to the brain. A panel of species-specific peptidomimetic MAbs were developed to allow for transport of drugs or genes into the brain of either animal models or humans [4]. 4.2.4. Polymeric Nanoparticles Drugs can be delivered to the brain with the aid of poly(butylcyanoacrylate) (PBCA) nanoparticles coated with polysorbate 80 [29]. These carriers can penetrate the BBB and deliver drugs of various structures, including peptides, hydrophilic compounds and lipophilic compounds that are usually eliminated from the brain by P-glycoprotein. Apolipoprotein E attached to the surface of nanoparticles facilitates the transport of drugs across the BBB by interaction with lipoprotein receptors on the brain capillary endothelial cell membranes [30]. Conjugations between a biodegradable copolymer, poly ( D , L -lactide-co-glycolide), and five short peptides were made by means of amidic linkages and a fluorescent probe was included to render the resulting nanoparticles fluorescent for allowing their localization after administration [31]. Fluorescent and confocal microscopy studies showed that while poly( D , L -lactide-co-glycolide) nanoparticles were unable to cross the BBB, surface modification with the peptides allowed them to do so. These polymer nanoparticles could be used to deliver imaging agents and anticancer drugs to brain tumors.
Fig. (3). Structure of a model of an immunoliposome. A gene of interest is packaged into the center of a nano size liposome (Diameter85nm). ~2000 strands of PEG are used to coat the surface of the liposome which reduces uptake by the RER. Between 1% and 2% of these strands are conjugated to the transferrin receptor targeting mAb. Concept was taken from Pardridge (2002).
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5. DELIVERY OF POTENTIAL NEUROTHERAPEUTICS Both large molecules and small molecule drugs are delivered using different strategies of drug targeting described above. Large molecule neurotherapeutics include peptides, recombinant proteins, antisense agents, and gene medicines, on the other hand low molecular weight drugs represent as small molecules. It is widely believed that the BBB transport of large molecule drugs is not possible, and large molecule drug development programs are frequently terminated in favor of small molecule drug discovery. However, recombinant proteins, antisense drugs, and non-viral gene medicines can be delivered to the brain with brain drug targeting technology. 5.1. Antisense Drugs RNA interference (RNAi) is a new strategy that knocks down gene expression post-transcriptionally [32]. RNAi mechanisms may involve either the degradation of target RNA as in the case of a short interfering RNA (siRNA), or cause translation arrest of the target RNA as in the case of micro RNA (miRNA). There are two types of RNAi-based therapeutics: one is DNA-based RNAi in which plasmid DNA encodes for a short hairpin RNA (shRNA) and another is RNA-based RNAi in which a siRNA duplex is chemically synthesized without a DNA intermediate. Similar to the delivery of non-viral gene therapies, plasmid DNA encoding for short hairpin RNA (shRNA) may be delivered to the brain following intravenous administration with pegylated immunoliposomes (PILs). Weekly, intravenous RNAi with PILs enabled 90% knockdown of the human epidermal growth factor receptor, resulting in 90% increase in survival time in mice with intra-cranial brain cancer [33]. Similar to the delivery of antisense agents, siRNA duplexes could be delivered with the combined use of targeting MAb's and avidin–biotin technology, where siRNA was monobiotinylated [34] in parallel with the production of a conjugate of the targeting MAb and streptavidin. Moreover oligodeoxynucleotides (ODN) and antisense peptide nucleic acids (PNA) were also successfully delivered to the brain in vivo [35]. 5.2. Gene Therapeutics Neurodegenerative diseases can be treated with different gene medicines. Widespread distribution of a therapeutic gene to the brain can only be achieved by delivery via the transvascular route. It is possible to deliver non-viral plasmid DNA throughout the brain with gene targeting technology that uses pegylated immunoliposomes (PIL), which are able to access endogenous RMT systems within the BBB [36]. Uptake of the liposome by the reticulo-endothelial system (RES) is minimized by conjugating several thousand strands of 2000 Da polyethyleneglycol (PEG) to the surface of the liposome. The PIL is targeted across the BBB and across brain cell membranes by attaching receptor-specific MAbs to the tips of 1%–2% of the PEG strands. With this approach to brain gene delivery, intravenous antisense gene therapy led to a 100% increase in survival time in animals with experimental human brain cancer [36].
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5.3. Synthetic Drugs Synthetic drugs are very much promising therapeutic to treat neurodegenerative disease like brain tumors. Many drugs have been or are being evaluated for the treatment of brain tumors [37]. Among them are doxorubicin, paclitaxel, cisplatin, iirinotecan, methotrexate, temozolomide, carmustine and so on. However, the BBB permits transport across the brain vascular system of a very limited number of small hydrophobic molecules. Many anti-cancer agents, however, are large hydrophobic molecules unable to freely cross the BBB and are also substrates for the MDR efflux pumps, expressed by both the BBB vasculature and the tumor cells. Of those mentioned above, only irinotecan (a topoisomerase I inhibitor), melphalan, and temozolomide are capable of being transported across the BBB. Most of the approaches attempted have evaluated drug conjugation to a blood-to-brain transporter. To be successful, this approach requires the drug to mimic the endogenous ligand, since most transporters, such as the glucose transporter, are highly selective. The anti-cancer agent melphalan resembles the amino acid phenylalanine and can be transported by the LAT1 carrier [38]. Very recently, the small hydrophilic drug ketoprofen, an anti-inflammatory agent that is not a substrate for LAT1, was chemically bound via an ester linkage to the phenolic hydroxyl group of the amino acid tyrosine, a LAT1 substrate, and was recognized by the LAT1 transporter [39], opening new possibilities for small anti-cancer drugs. Melphalan has also been conjugated to L-glutamate, with some success [40]. However, for transport by amino acid transporters, the amino and carboxyl groups of the amino acid must be free, limiting chemical possibilities. The glucose transporter GLUT1 was shown to be able to transport the anti-cancer pro-drug chlorambucilglucose across the BBB [41].The most extensively studied has been the Tf-receptor (TfR) that is expressed on highly proliferating cells, such as cancer cells and at much higher levels on endothelial cells of the BBB than on endothelial cells at other locations within the body. Diphtheria toxin-Tf conjugates [42] improved outcome in patients with chemotherapy- refractory brain tumors. The active efflux transporters [43, 44], which are responsible for drug resistance and efflux of therapeutic agents into the blood flow as soon as internalized by cells of the BBB, require that drugs should be coupled or co-injected with inhibitors of these systems to pass the BBB. A major concern with approaches using large molecules is the potential risk of immunological responses associated with longterm treatment. Drug-carrier nanoparticles [45-46] are defined as submicroscopic colloidal systems that may act as drug vehicles, either as nanospheres (matrix system in which the drug is dispersed) or nanocapsules (reservoirs in which the drug is confined in a hydrophobic or hydrophilic core surrounded by a single polymeric membrane). The design of nanoparticles as multifunctional platforms, their preparations and uses have been recently reviewed [45, 46]. Some examples of nanoparticles able to ferry drugs across the BBB to CNS tumors have been reported and some of the necessary characteristics of such tools have been defined. Chemical derivatization or encapsulation into polymeric particles [45-49] has been evaluated as a possibility for enhancing drug selectivity. Nanoparticles are generally internalized into cells via
Current Approaches for Drug Delivery to Central Nervous System
fluid phase endocytosis, receptor-mediated endocytosis, or phagocytosis. Nanoparticle surface manipulations may be performed to increase cell uptake and the potential delivery of the nanoparticles in different cell compartments [50-52]. Nanocontainers have been decorated with ligands for BBB transporters. Anti-cancer agents have been loaded in Tfcoated nanoparticles, for example 5-fluorouracyl [53]. The size of even a small protein, such as Tf or an antibody, however, is of the same order of magnitude as that of the container, which will prevent significant numbers of molecules to decorate the nanocontainer, thus limiting a favorable binding equilibrium. Adsorptive-mediated endocytosis, involving electrostatic interaction between a positively charged ligand and the negatively charged membrane of cells at the BBB [54], mainly sialic acid, may also be of interest. Cationized albumin was efficiently transported across the BBB [55]. The immunogenic properties of proteins may, however, be issues in long-term treatment. Attempts using carriermediated systems to transport nanoparticles included the GLUT1, which showed that a-mannose, but not b-mannose derivatives incorporated on the surface of liposomes [56] induced transport across the BBB. Nanoparticles coated with a choline derivative were transported across brain-derived endothelial cells faster by the cation transporter than uncoated nanoparticles [57] whereas coating with thiamine derivatives were not effective for enhanced transport [58]. Another interesting system is the folic acid receptor, specifically expressed at the BBB and able to transport doxorubicin-loaded folic acid-decorated nanoparticles [59]. Some hydrophilic surfactants, in particular polysorbates, interact with the surface of the BBB [60]. Polysorbate-coated doxorubicin nanoparticles [61], but not PEG-coated nanoparticles [62], may be promising for nanoparticulate drug delivery to the brain. Toxicity issues, non-biocompatibility of the surfactant, increased permeability, and tight junction disruption of the BBB by these surfactants may, however, be important issues. Finally, as for direct pro-drug conjugates, the active brain-to-blood transport efflux transporters [43, 44], which are mainly responsible of drug resistance, require additional coupling or co-injection of inhibitors of these systems to pass the BBB. Depending on the idea from the above mentioned nanoparticles, the structure of a model BBB drug nanoparticle delivery system has been illustrated in Fig. (4). A few strategies exist to enhance transport of anti-cancer agents across the BBB for the treatment of high-grade brain tumors: (i) passive permeation of lipidated drugs, however, this strategy is possible only for small molecules; (ii) the development of prodrugs hijacking the transport mechanisms at the BBB, however, the high selectivity of these transport mechanisms limits this approach; (iii) the development of drug-loaded nanocarriers able to take advantage of any disruption of the BBB at tumor sites. The most promising tools to deliver therapeutic drugs to tumors of the brain may be nanoparticles [63]. 5.4. Protein Drugs Though recombinant proteins do not cross the BBB in pharmacologically significant amounts, neurotrophins could be used for a wide variety of brain diseases. Consequently, virtually all current neurotrophin CNS drug development programs are focused on the discovery of small molecule
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Fig. (4). Diagram of a model BBB drug nanoparticle delivery system. The optimal nanocarrier will contain the anti-cancer agent in the core of a polymeric sheet, whose surface has been decorated with a BBB targeting and transport-enhancing molecule, and has enough positive charges to enhance uptake by brain tumor. In order to be efficient and selective, nanocarriers able to ferry anti-cancer agents across the BBB to treat primary and secondary sites of aggressive brain cancers, must be very complex entities.
peptidomimetics. However, most small molecule peptidomimetics will not have molecular characteristics that pass the stringent criteria discussed above for effective BBB transport. Therefore, the small molecule peptidomimetic would benefit from reformulation with a BBB drug delivery strategy to be pharmacologically active in the brain. The development of a small molecule drug that crosses the BBB can be just as difficult as the development of a large molecule drug that is transported across the BBB. Therefore, one alternative is to reformulate the large molecule drug with a BBB drug delivery strategy. This has been done with the chimeric peptide technology, wherein a non-transportable peptide is conjugated to a BBB transport vector, which functions as a molecular Trojan horse and carries the peptide across the BBB. This approach has enabled the drug development of recombinant proteins, neuropeptides, and antisense drugs, which all cross the BBB and are pharmacologically active in the brain following intravenous administration. These chimeric peptides include (1) vasoactive intestinal peptide (VIP) for cerebral blood flow enhancement, (2) brain-derived neurotrophic factor for neuroprotection in either global or brain ischemia, (3) epidermal growth factor for the early detection of brain cancer, (4) A analogs for the early detection of brain amyloid of Alzheimer's disease, and (5) peptide nucleic acid antisense agents for the in vivo imaging of brain gene expression [4]. In all of these cases, the
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protein or antisense drug was not pharmacologically active in the absence of conjugation to the BBB Trojan horse, because the unmodified molecule did not cross the BBB in pharmacologically significant amounts. So, the minimal transport of neuropeptides through the BBB is consistent with the high molecular weight and water solubility of these molecules. 6. CONCLUSION Extensive efforts have been made to develop strategies for delivering drugs to the CNS by enhancing their ability to cross the BBB. The delivery of drugs, peptides, proteins and genes to the brain depends on brain-specific vectors. The development of such vectors requires the identification of new receptor–ligand and antigen–antibody interactions that are selective for the BBB. The use of vectors employing a transporter or acting in a nonspecific manner may find an increased application especially when combined with a nanoparticle or liposome containing the drug(s) for delivery. This type of approach using particulate systems potentially allows large payloads of a drug to be delivered, which may have particular application in the delivery of cytotoxic agents, neurotrophic peptides/proteins, enzymes, gene vectors and other difficult large molecules to the brain. Knowledge of the pharmacogenomics of the BBB will undoubtedly lead to the discovery of new ways of promoting the delivery of pharmaceuticals to the CNS. Since the diseases afflicting the CNS are of diverse etiology (infectious, degenerative, autoimmune, metabolic and tumors), severity and time course, their treatment must have a corresponding range of targets and aims. The influence of the brain diseases on the physiology of the BBB must also be integrated into the development of the brain delivery strategies. While there is no probably for a single universal system for delivering drugs to the brain, the techniques described above promise to provide practical methods for the delivery of a range of therapeutic agents. ACKNOWLEDGEMENTS This work was supported by Grants-in-Aid for Scientific Research from the MEXT, Ministry of Education, Science, Sports and Culture of Japan.
Hossain et al. [8] [9] [10]
[11] [12] [13] [14]
[15]
[16]
[17] [18] [19]
[20]
[21]
[22]
[23]
REFERENCES [1]
[2] [3] [4] [5] [6] [7]
Brightman, M.W.; Reese, T.S.; Feder, N. Assessment With the Electron-Microscope Of The Permeability To Peroxidase Of Cerebral Endothelium And Epithelium In Mice And Sharks; Crone, C., Lassen, N.A., Ed.; In: Capillary Permeability: Copenhagen, 1970; pp. 468-476. Levin, V.A. Relationship of octanol–water partition coefficient and molecular weight to rat brain capillary permeability. J. Med. Chem., 1980, 23, 682-684. Pardridge, W.M. CNS drug design based on principles of blood– brain barrier transport. J. Neurochem., 1998, 70, 1781-1792. Pardridge, W.M. Brain drug targeting: the future of brain drug development. Mol. Interv., 2003, 3(2), 90-105. Dohrmann, G.J. The choroid plexus: a historical review. Brain Res., 1970, 18, 197-218. Oldendorf, W.H. Brain uptake of radiolabeled amino acids, amines, and hexoses after arterial injection. Am. J. Physiol., 1971, 221, 1629-1639. Ajay, B.G.W. ; Murcko, M.A. Designing libraries with CNS activity. J. Med. Chem., 1999, 42, 4942-4951.
[24]
[25]
[26] [27] [28] [29]
Pardridge, W.M.; Golden, P.L.; Kang, Y.S.; Bickel, U. Brain microvascular and astrocyte localization of P-glycoprotein. J. Neurochem., 1997, 68, 1278-1285. Pardridge, W.M. BBB-genomics: Creating new openings for braindrug targeting. Drug Discov. Today, 2001, 6, 381-383. Cornford, E.M.; Hyman, S.; Black, K.L.; Cornford, M.E.; Vinters, H.V.; Pardridge, W.M. High expression of the GLUT1 glucose transporter in human brain hemangioblastoma endothelium. J. Neuropathol Exp. Neurol., 1995, 54, 842-851. Pottiez. G.; Flahaut, C.; Cecchelli, R.; Karamanos, Y. Understanding the blood-brain barrier using gene and protein expression profiling technologies. Brain Res. Rev., 2009, 62(1), 83-98. Lee, H.J.; Pardridge, W.M. Pharmacokinetics and delivery of tat and tat-protein conjugates to tissues in vivo. Bioconjug. Chem.,. 2001, 12, 995-999. Golden, P.L.; Pardridge, W.M. P-glycoprotein on astrocyte foot processes of unfixed isolated human brain capillaries. Brain Res., 1999, 819, 143-146. Farrell, C.L.; Pardirdge, W.M. Blood-brain barrier glucose transporter is asymmetrically distributed on brain capillary endothelial luminal and abluminal plasma membranes; an electron microscopic immunogold study. Proc. Natl. Acad. Sci. USA, 1991, 88, 57795783. Beaulieu, E.; Demeule, M.; Ghitescu, L.; Beliveau, R. Pglycoprotein is strongly expressed in the luminal membranes of the endothelium of blood vessels in the brain. Biochem. J. Vol., 1997, 326, 539-544. Sziraki, I.; Horvath, K.; Bodor, N. Comparative evaluation of Estredox, a brain-targeted estradiol delivery system versus traditional estrogen replacement therapy. Pharmazie, 2006, 61, 140143. Tiwari, SB; Amiji, MM. A review of nanocarrier- based CNS delivery systems. Curr. Drug. Deliv., 2006, 3, 219-232. Bodor, N; Farag, HH; Brewster, ME. Site-specific, sustained release of drugs to the brain. Science, 1981, 214, 1370-1372. Tsuzuki, N; Hama, T; Hibi, T; Konishi, R; Futaki, S; Kitagawa, K. Rapid communication: adamantane as a brain-directed drug carrier for poorly absorbed drug: antinociceptive effects of [DAla2]leuenkephalin derivatives conjugated with the 1-adamantane moiety. Biochem. Pharmacol., 1991, 41, R5-8. Chekhonin, V.P.; Kabanov, A.V., Zhirkov, Y.A.; Morozov, G.V. Fatty acid acylate Fab-fragments of antibodies to neurospecific proteins as carriers for neuroleptic targeted delivery in brain. FEBS Lett., 1991, 287, 149-152. Shashoua, V.E.; Hesse, G.W. N-docosahexaenoyl, 3 hydroxytyramine: a dopaminergic compound that penetrates the blood-brain barrier and suppresses appetite. Life Sci., 1996, 58, 1347-1357. Xie, Y.; Ye, L.; Zhang, X.; Cui, W.; Lou, J.; Nagai, T.; Hou, X. Transport of nerve growth factor encapsulated into liposomesacross the blood-brain barrier: in vitro and in vivo studies. J. Control. Release, 2005, 105, 106-119. Visser, C.C.; Stevanovic, S., Voorwinden, L.H.; van Bloois, L.; Gaillard, P.J.; Danhof, M.; Crommelin, D.J.; de Boer. A.G. Targeting liposomes with protein drugs to the blood-brain barrier in vitro. Eur. J. Pharm. Sci., 2005, 25, 299-305. Gennuso, R.G.; Spigelman, M.K.; Chinol. M.; Zappulla, R.A.; Nieves, J.; Vallabhajosula, S.; Paciucci, A.; Goldsmith, S.J., Holland, J.F. Effect of blood-brain barrier and blood-tumor barrier and blood-tumor barrier modification on central nervous system liposomal uptake. Cancer Invest., 1993, 11, 118-128. Schackert, G.; Fan, D.; Nayar, R.; Fidler, I.J. Arrest and retention of multilamellar liposomes in the brain of normal mice or mice bearing experimental brain metastases. Selective Cancer Ther., 1989, 5, 73-79. Shi, N.; Zhang, Y.; Zhu, C.; Boado, R.J.; Pardridge, W.M. Brainspecific expression of an exogenous gene after i.v. administration. Proc. Natl. Acad. Sci USA, 2001, 98, 12754-12759. Pardridge, W.M. Drug and gene targeting to the brain with molecular Trojan horses. Nat. Rev. Drug Discov., 2002, 1, 131-139. Huwyler, J.; Pardridge, W.M. Examination of blood-brain barrier transferrin receptor by confocal fluorescent microscopy of unfixed isolated rat brain capillaries. J. Neurochem., 1998, 70, 883-886. Aliautdin, R.N., Kreuter, J.; Kharkevich, D.A. Drug delivery to the brain with nanoparticles. Eksp. Klin. Farmakol., 2003, 66, 65-68.
Current Approaches for Drug Delivery to Central Nervous System [30]
[31] [32] [33]
[34] [35]
[36] [37]
[38]
[39]
[40]
[41]
[42] [43] [44] [45]
Michaelis, K., Hoffmann, M.M.; Dreis, S.; Herbert, E.; Alyautdin, R.N.; Michaelis, M.; Kreuter, J.; Langer, K. Covalent linkage of apolipoprotein E to albumin nanoparticles strongly enhances drug transport into the brain. J. Pharmacol. Exp. Ther., 2006, 317, 12460-1253. Costantino, L.; Gandolfi, F.; Tosi, G..; Rivasi, F.; Vandelli, M.A.; Forni, F. Peptide- derivatized biodegradable nanoparticles able to cross the blood-brain barrier. J. Control. Release, 2005, 108, 84-96. McManus, M.T.; Sharp, P.A. Gene silencing in mammals by small interfering RNAs. Nat. Rev. Genet., 2002, 3, 737-747. Zhang, Y.; Zhang, Y.F.; Bryant, J.; Charles, A.; Boado, R.J.; Pardridge, WM. Intravenous RNA interference gene therapy targeting the human epidermal growth factor receptor prolongs survival in intracranial brain cancer. Clin. Cancer Res., 2004, 10, 36673677. Chiu, Y.L.; Rana, T.M. RNAi in human cells: basic structural and functional features of small interfering RNA. Mol. Cell, 2002, 10, 549-561. Suzuki, T.; Wu, D.; Schlachetzki, F.; Li, J.Y.; Boado, R.J.; Pardridge, W.M. Imaging endogenous gene expression in brain cancer in vivo with 111In-peptide nucleic acid antisense radiopharmaceuticals and brain drug-targeting technology. J. Nucl. Med., 2004, 45, 1766-1775. Zhang, Y.; Zhu, C.; Pardridge, W.M. Antisense gene therapy of brain cancer with an artificial virus gene delivery system. Mol. Ther., 2002, 6, 67-72. Muldoon, L.L.; Soussain, C.; Jahnke, K.; Johanson, C.; Siegal, T.; Smith, Q.R.; Hall, W.A,.; Hynynen, K.; Senter, P.D.; Peereboom, D.M.; Neuwelt, E.A. Chemotherapy delivery issues in central nervous system malignancy: a reality check. J. Clin. Oncol., 2008, 25, 2295-2305. Cornford, E.M.; Young, D.; Paxton, J.W.; Finlay, G J.; Wilson, W.R.; Pardridge, W.M. Melphalan penetration of the blood–brain barrier via the neutral aminoacid transporter in tumor-bearing brain. Cancer Res., 1992, 52, 138-143. Gynther, M.; Laine, K.; Ropponen, J.; Leppanen, J.; Mannila, A.; Nevalainen, T.; Savolainen, J.; Jarvinen, T.; Rautio, J. Large neutral amino acid transporter enables brain delivery via prodrugs. J. Med. Chem., 2008, 51, 932-936. Sakaeda, T.; Siahaan, T.J.; Audus, K.L.; Stella, V.J. Enhancement of transport of D-melphalan analogue by conjugation with Lglutamate across bovine brain microvessels endothelial cell monolayers. J. Drug Target, 2000, 8, 195-204. Halmos, T.; Santarromana, M.; Antonakis, K.; Scherman, D. Synthesis of glucose-chlorambucil derivatives and their recognition by the human GLUT1 glucose transporter. Eur. J. Pharm., 1996, 318, 477-484. Laske, D.W.; Youle, R.J.; Oldfield, E.H. Tumor regression with regional distribution of the targeted toxin TF-CRM107 in patients with malignant brain tumors. Nat. Med., 1997, 3, 1362-1368. Deeken, J.F.; Loscher, W. The blood–brain barrier and cancer: transporters, treatment, and Trojan horses. Clin. Cancer Res., 2007, 13, 1663-1674. Nies. A.T. The role of membrane transporters in drug delivery to brain tumors. Cancer Lett., 2007, 254, 11-29. Beduneau, A.; Saulnier, P. ; Benoit, J.P. Active targeting of brain tumors using nanocarriers. Biomat.,, 2007, 28, 4947-4967.
Received: May 29, 2010
Revised: June 11, 2010
Accepted: June 16, 2010
Current Drug Delivery, 2010, Vol. 7, No. 5 [46] [47] [48] [49] [50] [51] [52] [53] [54]
[55] [56] [57]
[58] [59] [60] [61]
[62]
[63]
9
Juillerat-Jeanneret, L. Critical Analysis Of Cancer Therapy Using Nanomaterials; In: Kumar, Challa S.S.R., Ed,; Nanomaterials for Cancer Therapy and Diagnosis, Wiley-VCH 2006, pp. 199-232. Allen, T.M.; Cullis, P.R. Drug delivery systems: entering the mainstream. Science, 2004, 303, 1818-1822. Duncan, R. The dawning era of polymer therapeutics. Nat. Rev. Drug Discov., 2003, 2, 347-360. Ferrari, M. Cancer nanotechnology: opportunities and challenges. Nat. Rev. Cancer, 2005, 5, 161-171. Dubowchik, G..M.; Walker, M.A. Receptor-mediated and enzymedependent targeting of cytotoxic anticancer drugs. Pharm. Ther., 1999, 83, 67–123. Panyam, J.; Labhasetwar. V. Sustained cytoplasmic delivery of drugs with intracellular receptors using biodegradable nanoparticles. Mol. Pharm., 2004, 1, 77–84. Savic, R.; Luo, L.; Eisenberg, A.; Maysinger, D. Micellar nanocontainers distribute to defined cytoplasmic organelles. Science, 2003, 300, 615–618. Soni, V.; Kohli, D.V.; Jain, S.K. Transferrin coupled liposomes as drug delivery carriers for brain targeting of 5-fluorouracil. J. Drug Target, 2005, 13, 245–250. Jallouli, Y.; Paillard, A.; Chang, J.; Sevin, E.; Betbeder D. Influence of surface charge and inner composition of porous nanoparticles to cross blood–brain barrier in vitro. Int. J. Pharm., 2007, 344, 103–109. Lu, W.; Wan, J.; She, Z.; Jiang, X. Brain delivery property and accelerated blood clearance of cationic albumin conjugated pegylated nanoparticle. J. Control. Release, 2007, 118, 38–53. Umezawa, F.; Eto, Y. Liposomes targeting to mouse brain: mannose as a recognition marker. Biochem. Biophys. Res. Commun., 1988, 153, 1038–1044. Fenart, L.; Casanova, A.; Dehouck, B.; Duhem, C.; Slupek, S.; Cecchelli, R.; Betbeder, D. Evaluation of effect of charge and lipid coating on ability of 60 nm nanoparticles to cross an in vitro model of the blood–brain barrier. J. Pharm. Exp. Ther., 1999, 29, 1017– 1022. Lockman, P.R.; Oyewumi, M.O.; Koziara, J.M.; Roder, K.E.; Mumper, R.J.; Allen, D.D. Brain uptake of thiamine-coated nanoparticles. J. Control. Release, 2003, 93, 271–282. Wu, D.; Pardridge, WM. Blood–brain barrier transport of reduced folic acid. Pharm. Res., 1999, 16, 415–419. Kreuter, J. Influence of the surface properties on nanoparticlemediated transport of drugs to the brain. J. Nanosci. Nanotechnol., 2004, 4, 484–488. Ambruosi, A.; Gelperina, S.; Khalansky, A.; Tanski, S.; Theisen, A.; Kreuter, J. Influence of surfactants, polymer and doxorubicin loading on the anti-tumour effects of poly (butyl cyanoacrylate) nanoparticles in a rat glioma model. J. Microencapsul., 2006, 23, 582–592. Brigger, I.; Morizet, J.; Laudani, L.; Aubert, G.; Appel, M.; Velasco, V.; Terrier-Lacombe, M.J.; Desmaele, D.; d'Angelo, J.; Couvreur, P.; Vassal, G. Negative preclinical results with stealth1 nanospheresencapsulateddoxorubicin in an orthopic murine brain tumor model. J. Control. Rel., 2004, 100, 29–40. Jain, K.K. Use of nanoparticles for drug delivery in glioblastoma multiforme. Exp. Rev. Neurother., 2007, 7, 363–372.
