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)
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...Fn) 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).
