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Recent Trends in Brain Targeted Drug Delivery Systems:An Overview

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Tejas B Patel

The brain is a delicate organ, and evolution built very efficient ways to protect it. Unfortunately, the same mechanisms that protect it against intrusive chemicals can also frustrate therapeutic interventions. Many existing pharmaceuticals are rendered ineffective in the treatment of cerebral diseases due to our inability to effectively deliver and sustain them within the brain.

General methods that can enhance drug delivery to the brain are, therefore, of great interest. Despite aggressive research, patients suffering from fatal and/or debilitating central nervous system (CNS) diseases, such as brain tumors, HIV encephalopathy, epilepsy, cerebrovascular diseases and neurodegenerative disorders, far outnumber those dying of all types of systemic cancer or heart disease. The clinical failure of much potentially effective therapeutics is often not due to a lack of drug potency but rather to shortcomings in the method by which the drug is delivered. In response to the insufficiency in conventional delivery mechanisms, aggressive research efforts have recently focused on the development of new strategies to more effectively deliver drug molecules to the CNS. This review intends to detail the recent advances in the field of brain-targeting, rational drug design approach and drug delivery to brain. To illustrate the complexity of the problems that have to be overcome for successful brain targeting, a brief intercellular characterization of the blood–brain barrier (BBB) is also included.

Introduction

The delivery of drugs to central nervous system (CNS) is a challenge in the treatment of neurological disorders. Drugs may be administered directly into the CNS or administered systematically (e.g., by intravenous injection) for targeted action in the CNS. The major challenge to CNS drug delivery is the blood-brain barrier (BBB), which limits the access of drugs to the brain substance. Advances in understanding of the cell biology of the BBB have opened new avenues and possibilities for improved drug delivery to the CNS. Several carrier or transport systems, enzymes, and receptors that control the penetration of molecules have been identified in the BBB endothelium. Receptor-mediated transcytosis can transport peptides and proteins across the BBB. Methods are available to assess the BBB permeability of drugs at the discovery stage to avoid development of drugs that fail to reach their target site of action in the CNS. Various strategies that have been used for manipulating the blood-brain barrier for drug delivery to the brain include osmotic and chemical opening of the blood-brain barrier as well as the use of transport/carrier systems. Other strategies for drug delivery to the brain involve bypassing the BBB. Various pharmacological agents have been used to open the BBB and direct invasive methods can introduce therapeutic agents into the brain substance. It is important to consider not only the net delivery of the agent to the CNS, but also the ability of the agent to access the relevant target site within the CNS. Various routes of administration as well as conjugations of drugs, e.g., with liposomes and nanoparticles, are considered. Some routes of direct administration to the brain are non-invasive such as transnasal route whereas others involve entry into the CNS by devices and needles such as in case of intrathecal and intracerebroventricular delivery 1 . Systemic therapy by oral and parenteral routes is considered along with sustained and controlled release to optimize the CNS action of drugs.

Among the three main approaches to drug delivery to the CNS - systemic administration, injection into CSF pathways, and direct injection into the brain - the greatest developments is anticipated to occur in the area of targeted delivery by systemic administration. Many of the new developments in the treatment of neurological disorders will be biological therapies and these will require innovative methods for delivery. Cell, gene and antisense therapies are not only innovative treatments for CNS disorders but also involve sophisticated delivery methods. RNA interference (RNAi) as a form of antisense therapy is also described. The role of drug delivery is depicted in the background of various therapies for neurological diseases including drugs in development and the role of special delivery preparations. Pain is included as it is considered to be a neurological disorder. Cell and gene therapies will play an important role in the treatment of neurological disorders in the future. The method of delivery of a drug to the CNS has an impact on the drug's commercial potential. The market for CNS drug delivery technologies is directly linked to the CNS drug market. Values are calculated for the total CNS market and the share of drug delivery technologies. Estimates are made for the year 2005 based on current markets and projections are made to the year 2015. The markets values are tabulated according to therapeutic areas, technologies and geographical areas. Unmet needs for further development in CNS drug delivery technologies are identified according to the important methods of delivery of therapeutic substances to the CNS. Finally suggestions are made for strategies to expand CNS delivery markets. Besides development of new products, these include application of innovative methods of delivery to older drugs to improve their action and extend their patent life 2 .

Barriers to Drug Delivery in brain

The Blood-Brain Barrier

The blood-brain barrier (BBB) is a very specialized barrier system of endothelial cells that separates the blood from the underlying brain cells, providing protection to brain cells and preserving brain homeostasis (stability). The brain endothelium has a complex arrangement of tight junctions between the cells that restrict the passage of molecules. This physical barrier is further enhanced by interactions with glial processes that form end feet and surround the brain microvessels (seeFigure1). The BBB is permeable to small and lipophilic (fat-loving) molecules (up to 800 atomic mass units), but larger molecules are not transported across unless there is an active transport system available 2 . Thus this is one of the stumbling blocks for drug delivery. An additional problem is the very effective drug efflux systems (P-gly-coprotein – P-gp – see later), which pump the drug back out of cells.

Morphology of the Blood-Brain Barrier

Figure 1: Morphology of the Blood-Brain Barrier.

The blood cerebrospinal fluid

The brain has four major fluid compartments: the blood that flows through entire brain structures; the interstitial fluid (ISF) bathing neurons and neuroglia; the cerebrospinal fluid (CSF), which circulates around brain ventricles, and spinal cord; and the intracellular fluid within brain cells. There is no major diffusional barrier between the ISF and CSF 3 . Thus, materials present in either of these two fluid compartments are free to exchange and reach their destination cells 3 . However, for a bloodborne substance to enter the ISF or CSF, it must first pass across the cell layers that safeguard the milieu of the central nervous system (CNS). Such cell layers possess unique cell types that join tightly together through intercellular connections, forming two major barrier systems designed to protect the brain microenvironment from fluctuations in concentrations of ions, metabolites, nutrients, and unwanted materials in the blood. The barrier located in the cerebral endothelia that separates the systemic circulation from the ISF compartment is defined as the blood-brain barrier, and the barrier in the choroidal epithelia that separates the systemic circulation from the CSF compartment is known as the blood-CSF barrier. While the largest interface between blood and brain is the BBB, this is also smaller less direct interface between blood and cerebrospinal fluid (CSF). The choroid plexus and the arachnoids membrane act together at the barriers between the blood and CSF 4 . On the external surface of the brain the ependymal cells fold over onto them to form a double layered structure, which lies between the dura and pia, this is called the arachnoids membrane. Within the double layer is the subarachnoid space, which participates in CSF drainage. Passage of substances from the blood through the arachnoids membrane is prevented by tight junctions. The arachnoids membrane is generally impermeable to hydrophilic substances, and its role is forming the Blood-CSF barrier is largely passive. The choroid plexus forms the CSF and actively regulates the concentration of molecules in the CSF 5 . The choroid plexus consist of highly vascularized, "cauliflower-like" masses of pia mater tissue that dip into pockets formed by ependymal cells. The preponderance of choroid plexus is distributed throughout the fourth ventricle near the base of the brain and in the lateral ventricles inside the right and left cerebral hemispheres. The cells of the choroidal epithelium are modified and have epithelial characteristics. These ependymal cells have microvilli on the CSF side, basolateral interdigitations, and abundant mitochondria. The ependymal cells, which line the ventricles, form a continuous sheet around the choroid plexus 6 . While the capillaries of the choroid plexus are fenestrated, non-continuous and have gaps between the capillary endothelial cells allowing the free-movement of small molecules, the adjacent choroidal epithelial cells form tight junctions preventing most macromolecules from effectively passing into the CSF from the blood 7 . However, these epithelial-like cells have shown a low resistance as compared the cerebral endothelial cells, approximately 200 W ·cm 2 , between blood and CSF ( see Figure 2 ).

Morphology of Blood Cerebrospinal Fluid Barrier

Figure 2: Morphology of Blood Cerebrospinal Fluid Barrier

Physiological Factors Influencing Brain Uptake

Brain penetration, brain uptake and ability to cross the BBB need to be defined exactly to understand concepts involved in brain uptake. Following are the general parameters which are used for measurement of brain uptake 8 :

The log Po/w probably still represents the most informative physicochemical parameter used in medicinal chemistry and countless examples where it proved as useful descriptors are available in the literature. On the other hand, increasing lipophilicity with the intent to improve membrane permeability might not only make chemical handling difficult, but also increase the volume of distribution in particular plasma protein binding and tends to affect all other pharmacokinetic parameters . Furthermore, increasing lipophilicity tends to increase the rate of oxidative metabolism by cytochromes P450 and other enzymes. Hence, to improve bioavailability, the effects of lipophilicity on membrane permeability and first pass metabolism have to be balanced 9 .

The brain uptake index is a more rigorous measure of brain uptake in which there is a relative measure of brain uptake by intra-carotid injection of a mixture of 14Clabeled compound and 3H-labeled water (i.e. a saline solution in 3H-labeled water). The radioactivity in brain tissue is recorded 15 seconds after administration, and a brain uptake index (BUI) is defined in equation:

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Where the BUI for water is 100. Although, the BUI is useful as a rank order index of brain uptake, is not easily amenable to analysis by physicochemical methods. A more well-defined measure of rapid brain uptake is the permeability 11 , expressed either as a permeability-surface area product (PS) or as a permeability coefficient (PC), obtained by intravenous injection and measurement of the drug profile in arterial blood. Both the PS product and PC are quantitative measures of the rate of transport obtained by in-situ vascular perfusion technique and so are amenable to analysis through standard physicochemical procedures. An advantage of the perfusion technique as a measure of brain uptake is that the time scale for determination of PS products is very short, so that back transport and biological degradation are minimized. Although there are numerous physicochemical studies on brain perfusion, it is not possible to reach any general conclusions.

Strategies for enhanced brain drug delivery :

To circumvent the multitude of barriers inhibiting CNS penetration by potential therapeutic agents, numerous drug delivery strategies have been developed. These strategies generally fall into one or more of the following three categories: manipulating drugs, disrupting the BBB and finding alternative routes for drug delivery 12, 13, 14 .

Drug manipulating

Lipophilic analogues

CNS penetration is favored by low molecular weight, lack of ionization at physiological pH, and lipophilicity. Delivery of poorly lipid-soluble compounds to the brain requires some way of getting past the BBB. There are several possible strategies, such as transient osmotic opening of the BBB, exploiting natural chemical transporters, high dose chemotherapy, or even biodegradable implants. But all of these methods have major limitations: they are invasive procedures, have toxic side effects and low efficiency, and are not sufficiently safe. A possible strategy is to smuggle compounds across as their lipophilic precursors. Because drug’s lipophilicity correlates so strongly with cerebro-vascular permeability, hydrophobic analogues of small hydrophilic drugs ought to more readily penetrate the BBB 15 . This strategy has been frequently employed, but the results have often been disappointing Immunoliposomes (antibody-directed liposome) have been recognized as a promising to ol for the site-specific delivery of drug s and diagnostic agents. However, the in vivo use of classical Immunoliposomes is hampered by the very rapid clearance of immunoliposomes from the circulation by the reticuloendothelial system. Avoidance of this obstacle is possible if gangliosides or PEG-derivatized lipids are inserted within the bilayer of conventional liposomes, as these modifications prolong considerably the liposome half-life in the circulation. Liposomes coated with the inert and biocompatible polymer PEG are widely used and are of ten referred to as "sterically stabilized" or "stealth liposomes". PEG coating is believed to prevent recognition of liposomes by macrophages due to reduced binding of plasma proteins. Unfortunately, it has been difficult to combine steric stabilization of liposomes with efficient immunotargeting. PEG coating of liposomes can create steric hindrances for antibody-target interaction. It has therefore been proposed to attach a cell-specific ligand to the distal end of a few lipid-conjugated PEG molecules rather than conjugate the ligand to a lipid head group on the surface of a PEG-conjugated liposome. This has been done recently with folic acid and monoclonal antibodies to target liposome to cells in tissue culture and organs in vivo . The application of PEG-conjugated immunoliposomes to in vivo brain targeting of drug s has not been attempted thus far. Conventional liposomes are not delivered to brain in vivo , because these agents are not transported through the brain capillary endothelial wall, which makes up the blood- brain barrier (BBB) in vivo . However, certain recep to r specific monoclonal antibodies (mAbs) undergo recep to r-mediated transcy to sis through the BBB, and mAb-gold conjugates are transcy to sed through the BBB in vivo . Therefore, the present studies were designed to achieve the following goals. First, PEG-conjugated immunoliposomes were synthesized using thiolated mAb and a bifunctional 2000-Da PEG (PEG 2000 ) that contains a lipid at one end and a maleimide at the other end 16 . Second, the pharmacokinetics and brain uptake of [ 3 H] daunomycin was examined following intravenous administration of the drug in free form, as a conventional liposome, as a PEG-conjugated liposome, and as a PEG-conjugated Immunoliposomes. The mAb used in these studies is the OX26 mAb to the rat transferrin recep to r, which is abundant on brain micro vascular endothelium 17 .

Prodrugs

Brain uptake of drugs can be improved via prodrug formation. Prodrugs are pharmacologically inactive compounds that result from transient chemical modifications of biologically active species. The chemical change is usually designed to improve some deficient physicochemical property, such as membrane permeability or water solubility. After administration, the prodrug, by virtue of its improved characteristics, is brought closer to the receptor site and is maintained there for longer periods of time. Here it gets converted to the active form, usually via a single activating step 18 . Unfortunately, simple prodrugs suffer from several important limitations. Going to extremes on the lipophilic precursor scale, a possible choice for CNS prodrugs is coupling the drug to a lipid moiety, such as fatty acid, glyceride or phospholipids. Such prodrug approaches were explored for a variety of acid-containing drugs, like levodopa, GABA, Niflumic acid, valproate or vigabatrin are coupled to diglycerides or modified diglycerides 19 . While increased lipophilicity may improve movement across the BBB, it also tends to increase uptake into other tissues, causing an increased tissue burden.

Chemical Drug Delivery

Chemical drug delivery systems (CDDS) represent novel and systematic ways of targeting active biological molecules to specific target sites or organs based on predictable enzymatic activation. They are inactive chemical derivatives of a drug obtained by one or more chemical modifications so that the newly attached moieties are monomolecular units (generally comparable in size to the original molecule) and provide a site-specific or site enhanced delivery of the drug through multi-step enzymatic and/or chemical transformations 20, 21, 22, 23 . During the chemical manipulations, two types of bio removable moieties are introduced to convert the drug into an inactive precursor form. A targetor (T) moiety is responsible for targeting, site-specificity, and lock-in, while modifier functions (F1...Fn) serve as lipophilizers, protect certain functions, or fine-tune the necessary molecular properties to prevent premature, unwanted metabolic conversions. The CDDS is designed to undergo sequential metabolic conversions, disengaging the modifier functions and finally the targetor, after this moiety fulfils its site- or organ-targeting role.

Carrier Mediated Drug Delivery

Carrier-mediated transport (CMT) and receptor-mediated transport (RMT) pathways are available for certain circulating nutrients or peptides. The availability of these endogenous CMT or RMT pathways means that portals of entry to the brain for circulating drugs are potentially available. In the braincapillary endothelial cells, which make up the BBB, there are several transport systems for nutrients and endogenous compounds. They are (a) the hexose transport system for glucose and mannose, (b) the neutral amino acid transport system for phenylalanine, leucine and other neutral amino acids, (c) the acidic amino acid transport system for glutamate and aspartate, (d) the basic amino acid transport system for arginine and lysine, (e) the b-amino acid transport system for b-alanine and taurine, (f) the monocarboxylic acid transport system for lactate and short-chain fatty acids such as acetate and propionate, (g) the choline transport system for choline and thiamine, (h) the amine transport system for mepyramine, (i) the nucleoside transport system for purine bases such as adenine and guanine, but not pyrimidine bases, and (j) the peptide transport system for small peptides such as enkephalins, thyrotropin-releasing hormone, argininevasopressin etc 24,25 . Utilization of differences in the affinity and the maximal transport activity among these transportsystems expressed at the BBB is an attractive strategy for controlling the delivery and retention of drugs into the brain.

Receptor/Vector Mediated Drug Delivery

Receptor-mediated drug delivery to the brain employs chimeric peptide technology, wherein a non-transportable drug is conjugated to a BBB transport vector. The latter is a modified protein or receptor-specific monoclonal antibody that undergoes receptor-mediated transcytosis through the BBB in-vivo. Conjugation of drug to transport vector is facilitated with chemical linkers, avidin–biotin technology, polyethylene glycol linkers, or liposomes. Multiple classes of therapeutics have been delivered to the brain with the chimeric peptide technology, including peptide- based pharmaceuticals, such as a vasoactive peptide analog or neurotrophins such as brain-derived neurotrophic factor, anti-sense therapeutics including peptide nucleic acids (PNAs), and small molecules incorporated within liposomes. The attachment of the drug that normally does not undergo transport through the BBB to a BBB transport vector such as the MAb, results in the formation of a chimeric peptide, provided the bifunctionality of the conjugate is retained. That is, the chimeric peptide must have not only a BBB transport function, but also a pharmaceutical function derived from the attached drug. Certain drugs may not be pharmacologically active\ following attachment to a BBB transport vector. In this case, it may be desirable to attach the drug to the transport vector via a cleavable disulfide linkage that ensures the drug is still pharmacologically active following release from the transport vector owing to cleavage of the disulfide bond. Depending on the chemistry of the disulfide linker, a molecular adduct will remain attached to the drug following disulfide cleavage, and the molecular adduct must not interfere with drug binding to the drug receptor 26, 27, 28, 29 . A second consideration with respect to the use of a disulfide linker is that virtually all of the cell disulfide reducing activity may be contained within the cytosol Therefore, the chimeric peptide must undergo endosomal release following receptor-mediated endocytosis into the target brain cell, in order to distribute to the reductase compartment. Figure 3 Illustrate the multiplicity of approaches for linking drugs to transport vectors, and the availability of these multiple approaches allows for designing transport linkers to suit the specific functional needs of the therapeutic under consideration.

Multiplicity of approaches for linking drugs to transport vectors

Figure 3: Multiplicity of approaches for linking drugs to transport vectors

Disrupting the Blood Brain Barrier

The second invasive strategy for enhanced CNS drug delivery involves the systemic administration of drugs in conjunction with transient BBB disruption (BBBD). Theoretically, with the BBB weakened, systemically administered drugs can undergo enhanced extravasations rates in the cerebral endothelium, leading to increased parenchymal drug concentrations. A variety of techniques that transiently disrupt the BBB have been investigated; however, albeit physiologically interesting, many are unacceptably toxic and therefore not clinically useful. These include the infusion of solvents such as dimethyl sulfoxide or ethanol and metals such as aluminum; X-irradiation; and the induction of pathological conditions including hypertension, hypercapnia, hypoxia or ischemia 30, 31 . The mechanisms responsible for BBBD with some of these techniques are not well understood.

Osmotic blood brain barrier disruption

Osmotic opening of the BBB was developed. Intracarotid injection of an inert hypertonic solution such as mannitol or arabinose has been employed to initiate endothelial cell shrinkage and opening of BBB tight junctions for a period of a few hours, and this permits delivery of antineoplastic agents to the brain. Though this treatment is still investigational, the fact that some patients who fail systemic chemotherapy have responded to similar or lower doses of intracarotid drugs is an often-cited argument in favor of the method. One reason for the unfavorable toxic/therapeutic ratio often observed with hyperosmotic BBBD is that this methodology results in only a 25% increase in the permeability of the tumor microvasculature, in contrast to a 10-fold increase in the permeability of normal brain endothelium 32 . Osmotic disruption of the BBB has also been suggested as a delivery strategy for recombinant adenoviral vectors for gene transfer to intracerebral tumors, and for magnetic resonance imaging agents for diagnosis of brain metastases using iron oxide conjugates, but there are problems which must be overcome before the routine clinical use of this technique can be realized. Osmotic disruption seems to be most successful in treating primary non-AIDS CNS lymphoma. The risk factors include the passage of plasma proteins, the altered glucose uptake, and the expression of heat shock proteins, microembolism or abnormal neuronal function.

Biochemical Blood-Brain Barrier Disruption

Recently, new and potentially safer biochemical techniques have been developed to disrupt the BBB. Selective opening of brain tumor capillaries (the blood–tumor barrier), by the intracarotid infusion of leukotriene C4 was achieved without concomitant alteration of the adjacent BBB. In contrast to osmotic disruption methods, biochemical opening utilizes the novel observation that normal brain capillaries appear to be unaffected when vasoactive leukotriene treatments are used to increase their permeability. However, brain tumor capillaries or injured brain capillaries appear to be sensitive to treatment with vasoactive leukotrienes, and the permeation is dependent on molecular size 33 .

Alternative Routes to CNS Drug Delivery

Despite advances in rational CNS drug design and BBBD, many potentially efficacious drug molecules still cannot penetrate into the brain parenchyma at therapeutic concentrations. A third class of strategies aimed at enhancing CNS penetration of drug molecules is composed of delivery methodologies that do not rely on the cardiovascular system 33, 34, 35, 36 . These alternative routes for controlled CNS drug delivery obviate the need for drug manipulation to enhance BBB permeability and/or BBBD by circumventing the BBB altogether. Since, most aforementioned techniques aim to enhance the CNS penetration of drugs delivered via the circulatory system; the result is higher drug penetration throughout the entire body and frequently unwanted systemic side effects. Additionally, systemically administered agents must penetrate the BBB to enter the brain, which is a formidable task.

Intra-ventricular / Intra thecal Route

One strategy for bypassing the BBB that has been studied extensively both in laboratory and in clinical trials is the intralumbar injection or intreventricular infusion of drugs directly into the CSF. Drugs can be infused intraventricularly using an Ommaya reservoir, a plastic reservoir implanted subcutaneously in the scalp and connected to the ventricles within the brain via an outlet catheter. Drug solutions can be subcutaneously injected into the implanted reservoir and delivered to the ventricles by manual compression of the reservoir through the scalp 37, 38 .

When compared to vascular drug delivery, intra-CSF drug administration theoretically has several advantages. Intra- CSF administration bypasses the BCB and results in immediate high CSF drug concentrations. Since, the drug is somewhat contained within the CNS, a smaller dose can be used, potentially minimizing systemic toxicity. Furthermore, drugs in the CSF encounter minimized protein binding and decreased enzymatic activity relative to drugs in plasma, leading to longer drug half-life in the CSF. Finally, because the CSF freely exchanges molecules with the extracellular fluid of the brain parenchyma, delivering drugs into the CSF could theoretically result in therapeutic CNS drug concentrations 39 .

However, this delivery method has not lived up to its theoretical potential for several reasons. These include a slow rate of drug distribution within the CSF and increase in intracranial pressure associated with fluid injection or infusion into small ventricular volumes. It results in to high clinical incidence of hemorrhage, CSF leaks, and neurotoxicity and CNS infections. The success of this approach is limited by the CSF-brain barrier, composed of barriers to diffusion into the brain parenchyma. Because the extracellular fluid space of the brain is extremely tortuous, drug diffusion through the brain parenchyma is very slow and inversely proportional to the molecular weight of the drug. For macromolecules, such as proteins, brain parenchymal concentrations following intra-CSF administration are undetectable. For these reasons, intra-CSF chemotherapy in the treatment of intraparenchymal CNS tumors has not proven to be effective. The greatest utility of this delivery methodology has been in cases where high drug concentrations in the CSF and/or the immediately adjacent parenchyma are desired, such as in the treatment of carcinomatous meningitis or for spinal anesthesia/analgesia 40 .

Intrathecal and intracerebral drug administration differs fundamentally from systemic drug administration in terms of pharmacokinetic characteristics determining brain tissue concentration, where the available dose reaching the target organ is 100%. However, there are large gradients inside the tissue with very high local concentrations at the site of administration (the ventricular surface or tissue site of injection) and zero concentration at some distance for macromolecules. Since, they have low diffusion coefficients, the gradients will be even steeper than what has been measured for small molecular weight drugs. After intracerebroventricular (icv) injection, the rate of elimination from the CNS compartment is dominated by cerebrospinal fluid dynamics. Clinical examples of intrathecal small drug delivery are the icv administration of glycopeptide and aminoglycoside antibiotics in meningitis, the intraventricular treatment of meningeal metastasis, intrathecal injection of baclofen for treatment of spasticity and the infusion of opioids for severe chronic pain 41,42 . These examples have in common the fact that the drug targets in all instances are close to the ventricular surface. Superficial targets may also be accessible for some macromolecular drugs 43, 44 .

Olfactory Pathway

An alternative CNS drug delivery strategy that has received relatively little attention is the intranasal route. Drug delivered intranasally are transported along olfactory sensory neurons to yield significant concentrations in the CSF and olfactory bulb. In recent studies, intranasal administration of wheat germ agglutinin horseradish peroxidase resulted in a mean olfactory bulb concentration in the nanomolar range. In theory, this strategy could be effective in the delivery of therapeutic proteins such as brain-delivered neurotropic factor (BDNF) to the olfactory bulb as a treatment for Alzheimer’s disease. The nasal drug delivery to the CNS is thought to involve either an intraneuronal or extraneuronal pathway. Recent evidence of direct nose-to-brain transport and direct access to CSF of three neuropeptides bypassing the bloodstream has been shown in human trials, despite the inherent difficulties in delivery 45, 46 . The difficulties that have to be overcome include an enzymatically active, low pH nasal epithelium, the possibility of mucosal irritation or the possibility of large variability caused by nasal pathology, such as common cold. An obvious advantage of this method is that it is noninvasive relative to other strategies. In practice, however, further study is required to determine if therapeutic drug concentrations can be achieved following intranasal delivery.

Interstitial Delivery

The most direct way of circumventing the BBB is to deliver drugs directly to the brain interstitium. By directing agents uniquely to an intracranial target, interstitial drug delivery can theoretically yield high CNS drug concentrations with minimal systemic exposure and toxicity. Furthermore, with this strategy, intracranial drug concentrations can be sustained, which is crucial in treatment with many chemotherapeutic agents 47 .

Injections, Catheters, and Pumps

Several techniques have been developed for delivering drugs directly to the brain interstitium. One such methodology is the Ommaya reservoir or implantable pump as discussed earlier under intraventricular/intrathecal route. This technique, however, does achieve truly continuous drug delivery. More recently, several implantable pumps have been developed that possess several advantages over the Ommaya reservoir. This can be implanted subcutaneously and refilled by subcutaneous injection and are capable of delivering drugs as a constant infusion over an extended period of time 48 . Furthermore, the rate of drug delivery can be varied using external handheld computer control units. Currently each of the three different pumps available for interstitial CNS drug delivery operates by a distinct mechanism. The Infusaid pump uses the vapour pressure of compressed Freon to deliver a drug solution at a constant rate; the MiniMed PIMS system uses a solenoid pumping mechanism, and the Medtronic SynchroMed system delivers drugs via a peristaltic mechanism. The distribution  of small and large drug molecules in the brain can be enhanced by maintaining a pressure gradient during interstitial drug infusion to generate bulk fluid convection through the brain interstitium or by increasing the diffusion gradient by maximizing the concentration of the infused agent as a supplement to simple diffusion. Another recent study shows that the epidural (EPI) delivery of morphine encapsulated in multivesicular liposomes (DepoFoam drug delivery system) produced a sustained clearance of morphine and a prolonged analgesia, and theresults suggest that this delivery system is without significant pathological effects at the dose of 10mg/ml morphine after repeated epidural delivery in dogs 49, 50, 51, 52 .

Biodegradable polymer Wafers, Microspheres and Nanoparticles

Though interstitial drug delivery to the CNS has had only modest clinical impact, its therapeutic potential may soon be realized using new advances in polymer technologies to modify the aforementioned techniques. Polymeric or lipidbased devices that can deliver drug molecules at defined rates for specific periods of time are now making a tremendous impact in clinical medicine. Drug delivery directly to the brain interstitium using polyanhydride wafers can circumvent the BBB and release unprecedented levels of drug directly to an intracranial target in a sustained fashion for extended periods of time 53 . The fate of drug delivered to the brain interstitium from the biodegradable  polymer wafer was predicted by a mathematical model based on (a) rates of drug transport via diffusion and fluid convection; (b) rates of elimination from the brain via degradation,metabolism and permeation through capillary networks; and (c) rates of local binding and internalization. Such models are used to predict the intracranial drug concentrations that result from BCNU-loaded pCPP:SA (1,3 bis-para-carboxyphenoxypropane: sebacic acid) wafers as well as other drug-polymer combinations, paving the way for the rational design of drugs specifically for intracranial polymeric delivery. Conjugation of a polymerically delivered chemotherapeutic agent to a water-soluble macromolecule increases drug penetration into the brain by increasing the period of drug retention in brain tissue. Hanes et al have recently developed IL-2-loaded biodegradable polymer microspheres for local cytokine delivery to improve the immunotherapeutic approach to brain tumor treatment. In theory, polymeric cytokine delivery has several advantages over delivery from transducted cells, including obviatingthe need for transfecting cytokine genes, producing longer periods of cytokine release in-vivo and yielding more reproducible cytokine release profiles and total cytokine dose 54, 55 . Microparticles can also be easily implanted by stereotaxy in discrete, precise and functional areas of the brain without damaging the surrounding tissue. This type of implantation avoids the inconvenient insertion of large implants by open surgery and can be repeated if necessary. The feasibility of polymer-mediated drug delivery by the standard chemotherapeutic agent 1,3-bis(2-chloroethyl)-1- nitrosourea (BCNU) showed that local treatment of gliomas by this method is effective in animal models of intracranial tumors. This led to clinical trials for glioma patients, and subsequent approval of GliadelTM [(3.8% BCNU): p(CPP:SA)] by the FDA and other worldwide regulatory agencies. Obviously, such an invasive approach can only be useful in a very limited number of patients, but this approach has been shown to prolong survival in patients with recurrent glioblastoma multiform brain tumors. Nevertheless, because of diffusion problems, even in this case, the therapeutic agent is likely to reach only nearby site. Polymeric nanoparticles have been proposed as interesting colloidal systems that allow the enhancement of therapeutic efficacy and reduction of toxicity of large variety of drugs. Nanoparticles were found to be helpful for the treatment of the disseminated and very aggressivebrain tumors. Intravenously injected doxorubicin-loaded polysorbate 80-coated nanoparticles were able to lead to 40% cure in rats with intracranially transplanted glioblastomas. Another Study shows that PEGylated PHDCA (n-hexadecylcyanoacrylate) nanoparticles made by PEGyalated amphiphilic copolymer penetrate into the brain to a larger extent than all the other tested nanoparticle formulations, without inducing any modification of the BBB permeability. And the result defines two important requirements to take into account in the design of adequate brain delivery systems, long-circulating properties of the carrier and appropriate surface characteristics to permit interactions with endothelial cells. Valproic acid-loaded nanoparticles showed reduced toxic side effects of valporate therapy, not by reducing the therapeutically necessary dosage but by inhibition of formation of toxic metabolites 56, 57, 58, 59 . In conclusion, the capacity of the biodegradable polymer delivery methodology to deliver drugs directly to the brain interstitium is vast.

Drug Delivery from Biological Tissues

Another strategy to achieve interstitial drug delivery involves releasing drugs from biological tissues. The simples  approach to this technique is to implant into the brain a tissue that naturally secretes a desired therapeutic agent. This approach has been most extensively applied to the treatment of Parkinson’s disease. Transplanted tissue often did not survive owing to a lack of neovascular innervation. Recently the enhanced vascularization and microvascular permeability in cell-suspension embryonic neural grafts relative to solid grafts has been demonstrated 60, 61 . An alternative extension of this method is to use gene therapy to develop optimized biological tissue for interstitial drug delivery. Prior to implantation, cells can be genetically modified to synthesize and release specific therapeutic agents. The therapeutic potential of this technique in the treatment of brain tumor was demonstrated. The use of nonneuronal cells for therapeutic protein delivery to the CNS has recently been reviewed. The survival of foreign tissue grafts may be improved by advancements in techniques for culturing distinct cell types. Co-grafted cells engineered to release neurotropic factors with cells engineered to release therapeutic proteins may enhance the survival and development of foreign tissue. Ideally it would be possible to perform in-vivo genetic engineering to cause specific endogenous brain tissue to express a desired protein, circumventing the ischemic and immunogenic complications encountered with the implantation of foreign tissue grafts. One such technique that has been successfully used for the treatment of CNS malignancies involves in-vivo tumor transduction with the herpes simplex thymidine kinase (HS-tk) gene followed by treatment with anti-herpes drug ganciclovir was achieved by intra-tumoral injection of retroviral vector-producing cells containing the HS-tk gene, rendering the transfected tumor cells susceptible to treatment with ganciclovir. Other vector systems used in CNS gene transfer studies include retroviruses, adenoviruses, adeno-associated viruses, encapluation of plasmid DNA into cationic liposomes and neutral and oligodendrial stem cells 62, 63, 64 .

Although this approach holds remarkable therapeutic potential in the treatment of CNS diseases, its efficacy has thus far been hindered by a number of obstacles: restricted delivery of vector systems across the BBB, inefficient transfection of host cells, and nonselective expression of the transgene and deleterious regulation of the transgene by the host 65 .

Conclusion

The treatment of CNS diseases is particularly challenging because the delivery of drug molecules to the brain is often precluded by a variety of physiological, metabolic and biochemical obstacles that collectively comprise the BBB, BCB and BTB. The present outlook for patients suffering from many types of brain diseases remains poor, but recent developments in drug delivery techniques provide reasonable hope that the formidable barriers shielding the brain may ultimately be overcome. Drug delivery directly to the brain interstitium has recently been markedly enhanced through the rational design of polymer-based drug delivery systems. Substantial progress will only come about, however, if continued vigorous research efforts to develop more therapeutic and less toxic drug molecules are paralleled by the aggressive pursuit of more effective mechanisms for delivering those drugs to brain targets.

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About Authors:

Tejas B Patel 1, Kirit A Patel , Tushar R Patel 3 , Timir B Patel 4 , Dr. Praful D Bharadia 5

1.Department of Pharmaceutical Technology, K. B. Raval College of Pharmacy, Shertha, Gujarat , India .

2. Department of Pharmacology, K. B. Raval College of Pharmacy, Shertha, Gujarat , India .

3.Department of Pharmaceutical Analysis, K. B. Raval College of Pharmacy, Shertha, Gujarat , India .

4.Department of Pharmacology, Faculty of Pharmacy, DDU, Nadiad, Gujarat , India .

5.Department of Pharmaceutics, B.S. Patel College of Pharmacy, Mehsana, Gujarat , India .

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