Different Drug Delivery System Targeting to Brain

Various strategies have been studied to circumvent the multitude of barriers inhibiting brain penetration by therapeutic agents. 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.

5.1.   Lipophilic Analogs (Drug Manipulations)

Drug penetration through the BBB is favored by lipophilicity. Because a drug's lipophilicity correlates so strongly with cerebrovascular permeability, hydrophobic analogs of small hydrophilic drugs ought to more readily penetrate the BBB. However, the drug molecule should have an optimum octanol-water partition coefficient with Log P value of approximately 1.5 to 2.5 to be efficacious when delivered via the circulatory system (Madrid et al., 1991).

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 (Siegal and Zylber-Katz, 2002). 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 often 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.

Table - 02: Strategies for Linking Drugs to Transport Vectors

Class	Target Amino Acid	Agent	Linkage	Cleavability
Chemical	Lysine	m-maleimidobenzoyl N-hrdroxy succinimide ester Traut’s	Thio-ether (-S-)	No
	Lysine
	Lysine	N-Succinimidyl-3-2-pyridyldithio propionate Traut’s	Disulfide (-SS)	yes
	Lysine
Avidin-biotin	Lysine	N-Hydroxy Succinimide-SS-biotin	Disulfide	yes
	Lysine	N-Hydroxy Succinimide-PEG-biotin	Extended Amide	No
	Aspartic Acid, Glutamic Acid	Hydrazine-PEG-biotin	Extended Hydrazide	No
Genetic Engineering	Fusion GeneElement	Fusion Gene Element	Recombinant Vector	No
		Fusion Gene Element	Recombinant Avidin	Flexible

However, certain receptor specific monoclonal antibodies (mAbs) undergo recep to r-mediated transcytosis through the BBB, and mAb-gold conjugates are transcytosed through the BBB in vivo. Therefore, the present studies were designed to achieve the following goals. First, PEG-conjugated immune liposomes were synthesized using thiolated mAb and a bi-functional 2000-Da PEG (PEG 2000 ) that contains a lipid at one end and a maleimide at the other end (Cole et al., 1992) . Second, the pharmacokinetics and brain uptake of (3H) daunomycin was examined following intravenous administration of the drug in free form, as a conventional liposome, as a PEG-conjugated liposome, and as a PEGconjugated Immunoliposomes. The mAb used in these studies is the OX26 mAb to the rat transferrin receptor, which is abundant on brain micro vascular endothelium (Taylor, 2002).

5.2.   Prodrugs

Brain uptake of drugs can be improved via prodrug formation (Bodor et al., 1987). A prodrug consists of a drug covalently attached to an unrelated chemical moiety that improves the drug's pharmacokinetic properties. The prodrug itself is inactive but becomes active when the attached moiety is cleaved in vivo by enzymatic or hydrolytic processes. A prodrug approach to delivery to the CNS involves the administration of the drug in a form that is inactive, or weakly active, but is readily able to penetrate the BBB. Ideally, the prodrug should be fairly lipid soluble so that it penetrates the BBB with ease and is converted into the active form solely within the CNS. Ideally, the active form of the molecule should be more polar than the prodrug so that it effectively becomes locked into the CNS with the consequence that brain levels of the active drug can remain high in the CNS when peripheral levels of the prodrug have declined markedly. An excellent example of this is illustrated by the series of compounds morphine, codeine, and heroin. Their brain uptakes determined by the brain uptake index (BUI) technique (Oldendorf, 1970) are illustrated in Fig. 5. Morphine, although an effective analgesic, does not enter the CNS readily and its brain entry is at the limit of quantification with this technique. Substituting one of the hydroxyl groups in the morphine molecule, thus forming codeine, increases lipid solubility and significantly increases brain uptake. The further substitution of 2 acetyl groups to form acetyl morphine produces a very substantial increase in brain penetration. Thus far, this is an excellent example of increasing brain penetration by chemical lipidization. However, once within the brain, the diacetyl morphine is rapidly metabolized to 6- acetyl morphine and then back to morphine, and it is in this form that it interacts with opioid receptors within the brain. Thus, heroin is acting as a prodrug for morphine within the CNS. It rapidly penetrates into the brain, produces the rush that the heroin addict craves, but is effectively acting as morphine. Once converted back into morphine, it is again polar and effectively becomes locked into the CNS, as it cannot easily back diffuse across the BBB; thus, morphine delivered in this way maintains significant CNS levels after the plasma levels of heroin and its metabolites have fallen. Clearly, several variants of the prodrug approach can be applied. The prodrug may simply have a higher lipid solubility favoring entry or its half-life or stability in plasma may be extended, thus enhancing and maintaining the diffusion gradient of active drug into brain over an extended period. The lipid moiety may consist of a vector attached to the drug by a linker such as an ester or disulfide bond, which is subsequently enzymatically cleaved within the brain; this type of linkage has been termed a chemical delivery system (Bodor & Buchwald, 2003; Bodor & Brewster, 1991). It is estimated that some 5% of listed drugs act as prodrugs (Bodor & Buchwald, 2003), which are subsequently converted into an active metabolite at their site of action. They almost certainly have not been deliberately designed as prodrugs and this particular mode of action, involving a crucial metabolic conversion in the target tissue, has just been serendipitously chanced upon.

Figure – 9: The BUI of the series: morphine, codeine, and heroin; each of which exhibits a progressive increase in lipid solubility

5.3.   Mucoadhesive formulation

The incorporation of mucoadhesive polymers into nasal formulation can increase the mucosal contact time and prolong the residence time of the dosage forms in the nasal cavity. The pharmacokinetic profiles of apomorphine after nasal administration were improved with mucoadhesive polymers of polyacrylic acid, Carbopol, and carboxymethylcellulose (Ugwoke et al., 1999; 2000). Hyaluronan is another example of mucoadhesive polymer used in a nasal formulation. It has demonstrated its ability to improve the brain penetration of a hydrophilic peptide via the nasal route (Horvát et al., 2009).

Chitosan was also extensively studied by formulators due to the non-toxic nature and its absorption enhancing and mucoadhesive properties (Charlton et al., 2006). Chitosan enhanced the brain bioavailability of intranasally administered nerve growth factor by a 14-fold increase comparing with a preparation without chitosan (Vaka et al., 2009). Chitosan hydrochloride in combination with hydroxypropyl beta-cyclodextrin was used as mucoadhesive formulation in brain targeting studies on buspirone hydrochloride with a high drug targeting index (Khan et al., 2009). Chitosan and hydroxylpropylmethyl cellulose can be formulated as mucoadhesive temperature-mediated in situ gel to enhance intranasal delivery of ropinirole, the dopamine D2 agonist, to the brain (Khan et al., 2010).

5.4.   Penetration enhancers

Penetration enhancers are used to improve the permeability and bioavailability of the drug upon contacting the nasal mucosa. The bioavailability of nerve growth factor in the brain could be enhanced by intranasal administration of peppermint oil (Vaka and Murthy, 2010). Intranasal administration of hexarelin, a growth hormone releasing neuropeptide for nose-to-brain targeting, was also enhanced by N-tridecyl-beta-D-maltoside as a permeation enhancer. Markedly greater hexarelin concentrations in olfactory bulb and olfactory tract on the treated side of brain tissues were observed (Yu and Kim, 2009)

5.5.   Vasoconstrictor

Phenylephrine hydrochloride, a short-acting vasoconstrictor showed remarkably reduced blood concentrations and increased CNS concentrations of hypocretin-1, a peptide involved in appetite and sleep regulation, and dipeptide L-Tyr-D-Arg, a morphine-like analgesic (Dhuria et al., 2009a). In this case, vasoconstrictor was used to enhance intranasal drug targeting to the CNS by limiting absorption into the systemic circulation and increasing the amount of neuropeptide available for direct transport into the CNS along olfactory pathways.

5.6.   The olfactory route

A route into the CNS via the olfactory epithelium and nerves is a viable and interesting possibility for the delivery of some types of drug to the brain (Okuyama, 1997; Illum, 2003).

The olfactory neurons penetrate the cribiform plate and are surrounded by a sleeve of arachnoid membrane, which contains subarachnoid CSF between the nerve and the membrane. This sleeve then terminates in an open-ended manner as the olfactory sensory endings, which penetrate through the olfactory mucosa (Mathison et al., 1998; Fig. 10). The CSF contained in these arachnoid sleeves appears to move outward into the lamina propria and to drain into the local lymphatic system (Bradbury et al., 1981). However, a significant fraction of this fluid appears to recirculate back into the subarachnoid CSF and may carry drug applied to the olfactory mucosa back into the subarachnoid space of the CNS (Begley, 2003; Begley & Brightman, 2003). An alternative hypothesis is that nasally administered drug is taken up by the olfactory nerves themselves and transported by retrograde axonal cytoplasmic flow back into the CNS (Begley, 2003; Illum, 2003). However, a cellular mechanism involving cytoplasmic flow as the major route of transport would probably be much slower than is actually observed, certainly with the drugs that have thus far been investigated, all of which appear in CSF within a few minutes of introduction into the nasal cavity (Sakane et al., 1991a, 1991b, 1995; Illum, 2003). Several drugs have been successfully delivered to the CNS by the nasal route including several sulfonamides (Sakane et al., 1991a), cephalexin (Sakane et al., 1991b), progesterone (Anand Kumar et al., 1982), zidovudine (Seki et al., 1994), and several peptides including the hormone insulin and hyaluronidase (Okuyama, 1997; Fehm et al., 2000). Transport to the brain via the nasal route is enhanced by an increasing lipophilicity of the transported molecule, which suggests that transmembrane movement may be one of the steps in the process of drug transport (Sakane et al., 1991a). Experiments with fluorescently labeled dextrans have also shown that there is an apparent molecular weight cut-off for these tracers of between 20 and 40 kDa (Sakane et al., 1995). However, in spite of these caveats, some relatively large peptides and even viruses and bacteria can enter the brain via the nasal route, which may form an important route for the introduction of CNS infective agents such as meningococcus.

Figure - 10: The olfactory route into the CNS. Diagram of the olfactory neurons (a) penetrating the cribiform plate (C). The arachnoid membrane (j) forms a sleeve (k), which encloses the olfactory neurons as they pass through the cribiform plate and the lamina propria (B). This sleeve is open ended at the base of the olfactory mucosa (A). Schwann cells, which have no barrier function, also surrounds the olfactory neurons. Thus, the subarachnoid space (D) is continuous with the extracellular space of the olfactory mucosa.

5.7.   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 siteenhanced delivery of the drug through multi-step enzymatic and/or chemical transformations. 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...F_n) serve as lipophilizers, protect certain functions, or fine-tune the necessary molecular properties to prevent premature, unwanted metabolic conversions. The chemical drug delivery system (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. Undoubtedly, the concept evolved from the prodrug concept, but became essentially different by the introduction of multi-step activation and targetor moieties. Within the present formalism, one can say that prodrugs contain one or more F moieties for protected or enhanced overall delivery, but they do not contain T moieties. Brain-targeting chemical delivery systems represent just one class of chemical drug delivery system (CDDS); however, this is the most developed class. Using the general chemical drug delivery system (CDDS) concept, successful deliveries have been achieved to the brain, to the eye, and to the lung (Bodor and Buchwald, 1997). These chemical drug delivery system (CDDS) are based on the idea that, if a lipophilic compound that enters the brain is converted there into a lipid-insoluble molecule, it will no longer be able to come out, i.e. it will become ‘locked- in’. If the same conversion also takes place in the rest of the body, it accelerates peripheral elimination and improves targeting. In principle, many targetor moieties are possible for a general system of this kind, but the one based on the 1,4-dihydrotrigonelline ´trigonelline (coffearine) system, where the lipophilic 1,4-dihydro form (T) is converted in-vivo to the hydrophilic quaternary form (T*), proved the most useful. This conversion takes place easily everywhere in the body since it is closely related to that of the ubiquitous NAD(P)H´NAD(P)+ coenzyme system associated with numerous oxidoreductases and cellular respiration. Since, oxidation takes place with direct hydride transfer and without generating highly active or reactive radical intermediates; it provides a nontoxic targetor system. Furthermore, since for small quarternary pyridinium ions rapid elimination from the brain, probably due to involvement of an active transport mechanism that eliminates small organic ions, has been shown (Palomino et al., 1989), the T+ moiety formed during the final release of the active drug D from the charged T –D form will not accumulate within the brain. Meanwhile, the charged T –D form is locked behind the BBB into the brain, but is easily eliminated from the body due to the acquired positive charge, which enhances water solubility. After a relatively short time, the delivered drug D (as the inactive, locked-in T+ –D) is present essentially only in the brain, providing sustained and brain-specific release of the active drug. It has to be emphasized that the system not only achieves delivery to the brain, but it provides preferential delivery, which means brain targeting. Ultimately, this should allow smaller doses and reduce peripheral side effects.

Furthermore, since the ‘lock-in’ mechanism works against the concentration gradient, it provides more prolonged effects. Consequently, chemical drug delivery systems (CDDSs) 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 indeed been achieved, for example, with a variety of steroid hormones. During the last decade, the system has been explored with a wide variety of drug classes. In a recent addition to the drug-targeting arsenal, targeted drug delivery to the brain via phosphonate derivatives was also explored, and so-called anionic chemical delivery systems (aCDDS) were designed, synthesized, and evaluated for testosterone and zidovudine (Boder et al., 1992). Here, an (acyloxy) alkyl phosphonate-type targetor moiety is used, and formation of an anionic 2 intermediate (T- –D) is expected to provide the ‘lock-in’. In addition, molecular packaging, an extension of the CDDS approach, achieved the first documented noninvasive brain delivery of neuropeptides in pharmacologically significant amounts. In this approach the peptide unit is part of a bulky molecule, dominated by lipophilic modifying groups that direct BBB penetration and prevent recognition by peptidases (Boder et al., 1992; Bodor, and Prokai, 1995; Chen et al., 1998; and Wu et al., 2002). Such a brain targeted packaged peptide delivery system contains the following major components: the redox targetor (T); a spacer function (S), consisting of strategically used amino acids to ensure timely removal of the charged targetor from the peptide; the peptide itself (P); and abulky lipophilic moiety (L) attached through an ester bond or sometimes through a C- terminal adjuster (A) at the carboxyl terminal to enhance lipid solubility and to disguise the peptide nature of the molecule. To achieve delivery and sustained activity with such complex systems, it is very important that the designated enzymatic reactions take place in a specific sequence. Upon delivery, the first step must be the conversion of the targetor to allow for ‘lockin’.This must be followed by removal of the L function to form a direct precursor of the peptide that is still attached to the charged targetor. Subsequent cleavage of the targetor– spacer moiety finally leads to the active peptide.

 Another method called redox chemical delivery systems involves linking a drug to the lipophilic dihydropyridine carrier, creating a complex that after systemic administration readily transverses the BBB because of its lipophilicity. Once inside the brain parenchyma, the dihydropyridine moiety is enzymatically oxidized to the ionic pyridinium salt. The acquisition of charge has the dual effect of accelerating the rate of systemic elimination by the kidney and bile and trapping the drug-pyridinium salt complex inside the brain. Subsequent cleavage of the drug from the pyridinium carrier leads to sustained drug delivery in the brain parenchyma (Bodor et al., 1981). This methodology increases intracranial concentrations of a variety of compounds, including neurotransmitters, antibiotics, and antineoplastic agents. This methodology has been extended to deliver neuroactive peptides such as enkephalin to the brain and has demonstrated promise in laboratory models, and evaluation of clinical efficacy in neurological patients is awaited with interest (Bodor et al., 1992). These approaches should be useful in medicinal chemistry and research on drug delivery to the brain.

5.8.   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 (Sakane et al., 1991; Sakane et al., 1995; Illum, 2003). 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 - 11 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.

 

Figure - 11: 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 (Seki at al., 1994). The mechanisms responsible for BBBD with some of these techniques are not well understood.

Adsorptive-mediated transcytosis (AME), a mechanism of brain uptake that is related to receptor-mediated transcytosis, operates for peptides and proteins with a basic isoeletric point (cationic proteins) and for some lectins (glycoprotein-binding proteins). The initial binding to the luminal plasma membrane is mediated by electrostatic interactions with anionic sites or by specific interactions with sugar residues, respectively. In order to establish the structural specificity of AME at the BBB, uptake of several synthetic peptides having various molecular sizes, basicities and hydrophobicities, and carboxyl-terminal structures was compared by using primary cultured bovine endothelial cells. These results indicated that not the number of constituent amino acids of peptides, but rather the C-terminal structure and the basicity of the molecules, are important determinants of uptake by the AME system at the BBB (Tamai et al., 1997).

Nanoparticles have also been used as transport vectors for peptides. Nanoparticles consist of colloidal polymer particles of poly-butylcyanoacrylate with the desired peptide absorbed onto the surface and then coated with polysorbate 80. Nanoparticles have been used as a vector for delivery of hexapeptide dalargin (an enkephalin analog). Intravenous injections of the vector dalargin produce analgesia, while dalargin does not alone (Kreuter et al., 1995). Drugs that have successfully been transported across the BBB with the nanoparticles include loperamide, tubocerarine and doxorubicin (Kreuter, 2001 and Kreuter, 2002). The mechanism of nanoparticle transport has not yet been fully elucidated. The most probable transport pathway seems to be endocytosis by the blood capillary endothelial cells following adsorption of blood plasma components, most likely apolipoprotein E (apo E), after intravenous injection. These particles interact with the Low Density Lipoproteins (LDL) receptors on the endothelial cells and then get internalized. After internalization by the brain capillary endothelial cells, the drug releases in these cells by desorption or degradation of the nanoparticles and diffuses into the residual brain. Alternatively,transport may occur by transcytosis of the nanoparticles with drug across the endothelial cells (Dehouck et al., 1997). Per-coating of nanoparticles with polysorbate led to adsorption of apolipoprotein E (apo E), and possibly other plasma components, which seem to be able to interact with the LDL receptors on the brain endothelial cells, which could lead to their endocytosis (Luck, 1997). In addition to these processes, polysorbates seem to be able to inhibit the efflux pump. This inhibition could contribute to the brain transport properties of the nanoparticles (Zordan-Nudo et al., 1993). However the possibility of a general toxic effect is also a serious impediment (Olivier, 1999).

5.9.   Nanoparticles as a drug delivery tool for brain targeting drug delivery

Drugs that areeffective against diseases in the CNS and reach the brain via the blood compartment must pass the BBB usually by nanoparticles drug delivery system which is an advanced technology to deliver drug molecules into the brain. The main advantage of Nanoparticles technology is that they masquerade the blood-brain barrier restrictive features of the therapeutic drug molecule.

 These systems are attractive because the methods of preparation are generally simple and easy to scale-up. Due to their small size, nanoparticles penetrate into even small capillaries and are taken up within cells, allowing an efficient drug accumulation at the targeted sites in the body. The use of biodegradable materials for nanoparticles preparation, allows sustained drug release at the targeted site over a period of days or even weeks after injection (Kumar and Santhi, 2008).

Nanoparticles is a universal tool to deliver drugs to the brain (Schroeder, 2000; Kreuter, 1997).The evidence is consistently mounting that important diseases of the brain can be treated by a combined nanoparticle/drug approach, and brain tumor treatment, in particular, has been accomplished in animal models.The nanoparticles are the solid colloidal particulate systems with size ranging from 1 to 1000nm that are utilized as drug delivery system (Bala and Kumar, 2004). It has Certain properties like hydrophobicity, lipophilicity, surface charge needs to be altered, so uptake of nanoparticles into cells increased. This can be done by manipulating the use of polymers (Kreuter, 2004). Nanoparticles generally made up of biocompatible and biodegradable polymers which are obtained from either natural or synthetic source.

Advantages of Nanoparticles (Sai, 2010; Kopecek, 2003; Torchilin, 2001; Muller-Goymann, 2004):

                          i.            Targeting ability of drugs to particular organ or tissue.

                        ii.             Increase in bioavailability.

                      iii.            Development of new formulation, which are safer.

                      iv.            Ability to sustained release of drugs.

                        v.            High carrier capacity.

                      vi.             Prolonged circulation time.

                    vii.            Stable in blood.

                  viii.            Acquiescent to small molecules, peptides, proteins, or nucleic acids.

Disadvantages:

        i.            Increase in cost of formulation, due to high manufacturing costs.

      ii.            May cause allergic reactions.

    iii.            Over use of polyvinyl alcohol as a stabilizer may have toxic reactions.

5.10.   Mechanisms of Nanoparticle Transport across the BBB:

·         Adhesion of nanoparticles to brain blood vessel walls: the adhesion of nanoparticles to the inner surface of the brain blood vessels resulting in higher radioactivity levels with polysorbate 80 coated 14C label nanoparticles.

·         Fluidization of endothelium by surfactants: The possibility that enhanced drug transport is due to the surface activity of polysorbate 80 and resulting fluidization of the endothelium. Experiment with such surfactants using tail flick test and dalargin as experimental drug showed that polysorbate 20, 40 and 60 were also able to transport dalargin to brain and produced an antinoceptive effect.

·         Opening of tight junctions of endothelium: Another possible explanation of enhanced transport of drug across BBB is opening of endothelial lining of the brain blood vessels. Hyper osmotic pressures may open these junctions for instance, enhancing drug transport in to the brain.

·         Transcytosis across the brain endothelial cells: After uptake of nanoparticles by endothelial cells, the nanoparticles and adsorbed drug may be delivered to the brain cells by transcytosis. Evidence that low density lipoproteins (LDL) particles may be transported across the BBB by receptor mediated transcytosis.

·          Blockage of the glycoprotein in the brain endothelial cells: One of the possibilities for enhancement of brain transport with nanoparticles could be the inactivation of p-glycoprotein efflux pump. This glycoprotein is present in the brain endothelial. This is responsible for multidrug resistance which represents a major obstacle to cancer chemotherapy .Surfactants including polysorbate 80 were shown to inhibit the efflux system and to reverse multidrug resistance.

·         Endocytosis by the brain vessel endothelial cells: This is the most likely mechanism for brain transport of drug uptake by endothelial cells lining the brain blood vessels. These cells are similar to reticuloendothelial cells which are able to endocytose particulate matter under certain circumstances. After endocytosis, delivery of drug occurs with or without nanoparticle degradation and the drug would enter the residual brain by diffusion.

5.11.   Different types of nanoparticles used for CNS targeted drug delivery

5.11.1.1.   Inorganic nanoparticles

Ceramic nanoparticles are typically composed of inorganic compounds such as silica, alumina, metals, metal oxides, and metal sulfides can be used. Hollow silica nanoparticles have been prepared, such as calcium phosphate-based nanoshells, with surface pores leading to a central reservoir. Inorganic nanoparticles may be designed to escape the reticuloendothelial system by varying size and surface composition. Also provide a physical encasement to protect an entrapped molecular payload from degradation or denaturization. Their lack of biodegradation and slow dissolution may not be suitable for long- term administration. (Faraji and Wipf, 2009).

5.11.1.2.   Polymeric nanoparticles (NPs)

Polymeric nanoparticles (NPs are composed of a core polymer matrix in which drugs can be embedded, with sizes usually between 60 and 200 nm. A range of materials have been employed for delivery of drugs. In particular, in recent years some polymers have been designed primarily for medical applications and have entered the arena of controlled release of bioactive agents. Many of these materials are designed to degrade within the body. Most popular ones are polylactides (PLA), polyglycolides (PGA). Poly(lactide-co-glycolides) (PLGA), polyanhydrides, polycyanoacrylates, and polycaprolactone. In spite of development of various synthetic and semi-synthetic polymers, also natural polymers such as chitosan can be utilized.

This polymeric coating is thought to reduce immunogenicity, and limit the phagocytosis of nanoparticles by the reticuloendothelial system, resulting in increased blood levels of drug in the brain. The US Food and Drug Administration (FDA) have approved biodegradable polymeric nanoparticles, such as PLA and PLGA, for human use. The polymer matrix prevents drug degradation and may also provide management of drug release from these nanoparticles. Changing the drug-to-polymer ratio and molecular weight and composition of the polymer can modify the extent and level of drug release can provide excellent pharmacokinetic control and are suitable for the entrapment and delivery of a wide range of therapeutic agents. Practically, large-scale production and manufacturing remains an issue with polymeric nanoparticles (Faraji and Wipf, 2009; Olivier, 2005)

Also in the case of polymeric nanoparticles (NPs), reports are available describing an enhanced drug delivery to the brain mediated by these devices. Nanoparticles (NPs)  made of PLGA (poly(lactide-co-glycolides)  embedding antituberculosis drugs (rifampicin, isoniazid, pyrazinamide, and ethambutol) for cerebral drug delivery were administered to mice, maintaining high drug levels for 5–8 days in plasma and for 9 days in the brain, much higher as compared with free drugs (Choonara et al., 2001). In Mycobacterium tuberculosis-infected mice, 5 doses of the NPs formulation (against 46 doses of conventional free drugs) resulted in undetectable bacteria in the meninges (Pandey and Khuller, 2006). In another research (Hasadsri et al., 2009), polybutyl-cyanoacrylate (PBCA) NPs were successfully utilized for delivery of functional proteins into neurons and neuronal cell lines.

5.11.1.3.   Solid lipid nanoparticles

Solid lipid nanoparticles are one of the novel potential colloidal carrier systems as alternative materials to polymers which is identical to oil in water emulsion for parenteral nutrition, but the liquid lipid of the emulsion has been replaced by a solid lipid shown on Fig. 1. They have many advantages such as good biocompatibility, low toxicity and lipophilic drugs are better delivered by solid lipid nanoparticles and the system is physically stable.

Solid lipid nanoparticles (SLNs) are considered to be the most effective lipid based colloidal carriers, introduced in early nineties. This is the one of the most popular approaches to improve the oral bioavailability of the poorly water so luble drugs. SLNs are in the submicron size range of 50-1000 nm and are composed of physiologically tolerated lipid components which are in solid state at room temperature (Houli et al., 2009; Uner & Yener, 2009).

Aims of solid lipid nanoparticles (Uner & Yener, 2009; Kaur et al., 2008)

·         Possibility of controlled drug release

·         Increased drug stability.

·         High drug pay load

·         No bio-toxicity of the carrier.

·         Avoidance of organic solvents.

·         Incorporation of lipophilic and hydrophilic drugs.

5.11.1.4.   Nanocrystals

Nanocrystals are aggregates of molecules that can be combined into a crystalline form of the drug surrounded by a thin coating of surfactant. A nanocrystalline species may be prepared from a hydrophobic compound and coated with a thin hydrophilic layer. The biological reaction to nanocrystals depends strongly on the chemical nature of this hydrophilic coating. The hydrophilic layer aids in the biological distribution and bioavailability and prevents aggregation of the crystalline drug material. These factors combine to increase the efficiency of overall drug delivery. High dosages can be achieved with this formulation. Poorly soluble drugs can be formulated to increase bioavailability via treatment with an appropriate coating layer. Both oral and parenteral deliveries are possible. The limited carrier consisting of primarily the thin coating of surfactant may reduce potential toxicity. A drawback however, is that the stability of nanocrystals is limited. Moreover, this technique requires crystallization; some therapeutic compounds may not be easily crystallized. (Faraji and Wipf, 2009)

5.11.1.5.   Carbon Nanotubes

Carbon nanotubes are used as carriers for drug or oligonucleotide delivery and represent the most investigated therapeutic strategies for intra-tumoral drug and gene therapy delivery.  They are able to carry small interfering RNA (siRNA) molecules that exert RNA interference on target gene expression. While they are potentially promising for pharmaceutical applications, human tolerance of these compounds remains unknown, and toxicity reports are conflicting. Extensive research into the biocompatibility and toxicity of nanotubes remains ongoing. (Faraji and Wipf, 2009)

5.11.1.6.   Dendrimers

Dendrimers are branched polymers, reminding the structure of a tree. A dendrimer is typically symmetric around the core, and when sufficiently extended it often adopts a spheroidal three-dimensional morphology in water. A central core can be recognized in their structure with at least two identical chemical functionalities; starting from these groups, repeated units of other molecules can originate, having at least one junction of branching. The structure is therefore tightly packed in the periphery and loosely packed in the core, leaving spaces which play a key role in the drug-entrapping ability of dendrimers (Dhanikula et al., 2009). Poly (amidoamine), or PAMAM, is perhaps the most well-known molecule for synthesis of dendrimers. The core of PAMAM is a diamine (commonly ethylenediamine), which is reacted with methyl acrylate and then with another ethylenediamine to make the generation-0 PAMAM. Successive reactions create higher generations. The function of PAMAMs dendrimers has a dramatic effect on their ability to diffuse in the CNS tissue in vivo and penetrate living neurons as shown after intraparenchymal or intraventricular injections (Albertazzi et al., 2013).

The systemically administered polyamidoamine dendrimers localize in activated microglia and astrocytes in the brain of newborn rabbits with cerebral palsy, providing opportunities for clinical translation in the treatment of neuroinflammatory disorders in human (Kannan et al., 2012).

5.11.1.7.   Quantum dots (QDs)

Quantum dots (QDs) are luminescent nanocrystals with rich surface chemistry and unique optical properties that make them useful as probes or carriers for traceable targeted delivery and therapy applications. QDs can be functionalized to target specific cells or tissues by conjugating them with targeting ligands. Recent advancement in making biocompatible QD formulations has made these nanocrystals suitable for in vivo applications.

QDs are luminescent nanocrystals made of semiconductors used for imaging in biological systems. This interaction allows specific drugs such as protein, siRNA, genetic materials, and antisense oligonucleotides to penetrate targeted cancer cells in the CNS. As semiconductors are poisonous heavy metals, toxicity is a huge obstacle to clinical application of QDs for humans (Faraji and Wipf, 2009).

5.11.1.8.   Gold nanoparticles

Metallic colloidal gold nanoparticles are widely used, can be synthesized in different forms (rods, dots), are commercially available in various size ranges and can be detected at low concentrations. Cells can take up gold nanoparticles without cytotoxic effects (Connor et al., 2005; Shenoy et al., 2006). For biomedical applications, they are used as potential carriers for drug delivery, imaging molecules and even genes (Kawano et al., 2006), and for the development of novel cancer therapy products (Hirsch et al., 2003; Hainfeld et al., 2004; Loo et al., 2004; O’Neal et al., 2004; Radt et al., 2004). For gold nanorods the cytotoxicity could be attributed to the presence of the stabilizer CTAB (cetyltrimethylammonium bromide, hexadecyltrimethylammonium bromide) of which even residual presence after washing resulted in considerable cytotoxicity. PEG-modified gold nanorods with removing the excess CTAB did not show cytotoxicity (Niidome et al., 2006). In an acute oral toxicity study no signs of gross toxicity or adverse effects were noted when a nanogold suspension (nanoparticle diameter ca. 50 nm) was evaluated, the single dose for acute oral LD50 being greater than 5000 mg/kg body weight (Lai et al., 2006).

Gold solutions are also used to prepare nanoshells composed of gold and copper, or gold and silver to function as contrast agents in Magnetic Resonance Imaging (RMI) (Su et al., 2006), and gold-silica for photothermal ablation of tumor cells (Bernardi et al., 2007; Stern et al., 2007). In vitro the non targeted nanoshells did not show cytotoxicity for the tumor cells, whereas after binding to the tumor cells cell death could be obtained after laser activation (Lowery et al., 2006; Bernardi et al., 2007; Stern et al., 2007). Also in vivo positive results were obtained with photothermal ablation therapy in a mouse model for colon carcinoma after intraveneous administration of PEG coated gold nanoshells of approximately 130 nm (O’Neal et al., 2004).

5.11.1.9.   Magnetic nanoparticles

Magnetic NPs are iron oxide particles with a diameter of 10 nm. Many groups have tested these molecules as contrasting agents for MRI, through conjugation of iron oxide NPs with hydrophilic polymer coatings of dextran or polyethylene glycol (Faraji and Wipf, 2009).