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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5636_Библиотеки_им_академика_М_И_Перельмана
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increased, the size of the micelles increased. All micelles fabricated with a molar
ratio of e-CL to MePEG of 50, 75 and 100 exhibited ~30% cumulative release at
day 14, micelles with a ratio of 25 exhibited 15% release at day 6 and plain indomethacin had nearly 100% cumulative release at day 2. Interestingly, micelles were
capable of prolonging the release of their contents by a significant factor.
Very few researchers have tested the ability of micelles to improve drug pharmacokinetics in ocular tissues. Gupta et al. in 2000 are among the first to investigate
the permeability of ketorolac-loaded micelles made with N-isopropylacrylamide
(NIPAAM) copolymer, vinyl pyrrolidone, and acrylic acid (AA) crosslinked with
N¢,N¢-methylene bis-acrylamide across the cornea of excised rabbit cornea. The
micelles were of
dependent. Slowest release (approximately 40% in 8 h) was seen for acidic pH (pH 5)
while pH 7.2 and 10 exhibited faster release. At pH 7.2, about 50% drug was released
at 6 h and at pH 10, about 75% drug was released in 6 h (Gupta et al. 2000). The
micellar formulation of ketorolac and the aqueous suspension of ketorolac had ~ 7%
and 4% cumulative amount of keterolac that permeated the cornea, respectively,
after 60 min. The study also found that the micellar formulations were able to
prevent ocular inflammation more quickly than plain ketorolac as denoted by lid
closure induced by prostaglandin E2 in a rabbit eye. Lid closure was rated as: 0 – fully
open, 2 2/3 – open, and 3 – fully closed. At 30 min, the lid closure rating for the
micellar formulation and plain ketorolac was ~0.5 and 1.7, respectively. The lid
closure rating for the micelle-treated rabbits was consistent up to 3 h after which the
lid closure rating was 0 for 4 h and 5 h time points. The lid closure rating for
the plain ketorolac did not drop to ~0.5 until 3 h and decreased to 0 at 5 h. The micellar
formulation of ketorolac showed increased residence time of the drug in ocular tissues
as well as sustained release of the drug from the formulation.
< 50 nm in size and the release of drug from the micelles was pH-
11.2.4 Protein Nanoparticles
Nanoparticles or nanosystems can be prepared using a variety of naturally occurring
or synthetic proteins. While naturally occurring proteins are likely safer, all proteins
suffer from the potential for immunogenicity, especially when administered in forms
that are altered when compared to their endogenous forms in the human body. While
immunogenicity remains a challenge, protein-based delivery systems are still viable,
given the success of AbraxaneTM, an albumin-based, intravenously administered
paclitaxel nanoparticle for cancer therapy. Eye, being relatively immuno-privileged,
might tolerate protein-based nanosystems better than some other parts of the body.
Albumin is a commonly assessed protein for drug delivery due to ease of synthesis
(Zimmer et al. 1994) and knowledge regarding its biocompatibility, ability to bind
various drug molecules, and its nontoxic nature. In fact, one study has evaluated the
ocular disposition and tolerance of ganciclovir-loaded albumin nanoparticles after
intravitreal injections in rats (Merodio et al. 2002). Albumin nanoparticles were
detected in the vitreous up to 2 weeks after injection, and the histopathology of the
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retina, ciliary muscle, neuronal interplay area, outer and inner nuclear layers, and
the vitreous cavity showed no signs of inflammation after 2 weeks. Further, the
cytoarchitecture of the retina showed no signs of alteration in photoreceptors or
neuronal layers. In addition, the mechanism of degradation of albumin nanoparticles is known to involve phagocytosis by the RES (Schafer et al. 1994). Not only are
protein nanoparticles safe and biocompatible, they have a unique inherent ability to
bind drugs with various physiochemical properties due to the wide-range of charged,
lipophilic, and hydrophilic amino acids.
Pioneer research completed by Merodio and colleagues has focused on the ocular
use of albumin nanoparticles in sustaining the release of ganciclovir in the treatment
of cytomegalovirus retinitis (Merodio et
al. 2000). Drug release from albumin
nanoparticles followed a biphasic model whereby an initial rapid release of drug
was followed by a period of slow release. The nature of the concentration vs. time
curve for ganciclovir directly depended on the method of synthesis and the addition
of excipients. Three different methods of synthesis were used: Model A, B and C.
For Model A nanoparticles, ethanol was added dropwise to a 2% (w/v) albumin
solution while continuously stirring. Glutaraldehyde was then added to harden the
coacervates. The nanoparticles were then purified by centrifugation to remove unreacted gluteraldehyde and albumin. The pelleted albumin nanoparticles were suspended with a ganciclovir solution and allowed to incubate up to 4 h. Unencapsulated
drug was removed by centrifugation. Model B nanoparticles were made by adding
ganciclovir directly to a 2% (w/v) albumin solution up to 4 h and afterwards the pH
was adjusted to the isoelectric point of albumin (pI 5.5). The coacervates were dissolved with ethanol and then hardened with glutaraldehyde for 2 h. Finally, centrifugation was completed to remove unreacted glutaraldehyde, albumin, and ganciclovir.
Model C nanoparticles were made by adding ganciclovir to a 2% (w/v) albumin
solution containing a crosslinking agent and incubated up to 4
h. The pH of the solution was then adjusted to 5.5 (the pI of albumin) and afterwards ethanol was added.
Again, centrifugation was used lastly as a purification step to remove unreacted
compounds.
Addition of ganciclovir to albumin nanoparticles formed 4
h prior to the addition
of ganciclovir (Model A nanoparticles) resulted in release of 60% of encapsulated
drug within 1 h; however, addition of ganciclovir directly to the albumin solution in
the initial step (Model B nanoparticles) decreased the amount of drug released to
40% and only 20% of drug was released from Model C nanoparticles. However,
for all formulations of ganciclovir-loaded albumin nanoparticles, percent cumulative release of drug after 1 h remained constant over 5 days. The mechanism of
drug release was found to be directly dependent on pH whereby increased ganciclovir release was observed under extremely basic and acidic conditions, but minimal
release was observed near pH 7. Thus, sustained release properties in the order of a
few days can easily be obtained using albumin nanoparticles by optimizing formulation pH and excipients.
Albumin nanoparticles also demonstrated superiority over lipofectamine in gene
therapy. Human serum albumin nanoparticles loaded with the Cu, Zn superoxide
dismutase (SOD1) gene were prepared (Fig. 11.4) and tested for their safety,
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Fig. 11.4 Scheme depicting the methods for preparing human serum albumin (HSA) nanoparticles
loaded with the plasmid capable of expressing superoxide dismutase 1 (pSOD1). Degree of crosslinking controls particle size (Mo et al. 2007)
release profiles, and efficacy (gene expression) by Mo et al. (2007). The albumin
nanoparticles loaded with the SOD1 gene were synthesized using a modified
desolvation-crosslinking method: a 2% (w/v) albumin solution was mixed with
pSOD1 (plasmid encoding the SOD1 gene) in a Tris-EDTA solution at pH 8.0 for
5 min at room temperature. The solution was then added dropwise to an ethanol
solution while stirring and the nanoparticles were crosslinked by adding 1% glutaraldehyde and stirring for 12 h. Excess glutaraldehyde was removed by addition of
ethanol and centrifuging. The SOD1-loaded albumin nanoparticles were ~120 nm
with 20 mL glutaraldehyde and ~160 nm if only 1 mL of glutaraldehyde was added
as a crosslinker agent. The larger nanoparticles (160
nm) exhibited a biphasic release
profile with release of 65% of the DNA in the first 6 h followed by sustained release
for the next 44 h. The smaller nanoparticles (120 nm) had a slightly less drastic burst
effect by which only 23% of the DNA was released in 6 h, followed by sustained
release for 6 days. The nanoparticles were shown to be protective against DNAse
I-induced degradation of the plasmid and were noncytotoxic to retinal pigment epithelial (ARPE-19) cells over 96 h at nanoparticle concentrations up to 5 mg/mL.
The in vitro data clearly demonstrates that albumin pSOD1-loaded nanoparticles
have higher SOD1 activity due to gene expression than pSOD1 + lipofectamine.
Intravitreal injection of albumin pSOD1-loaded nanoparticles into mice had high
protein levels of SOD1 compared to intravitreal injection of pSOD1 only.
Most recently, the effects of surface charge on albumin nanoparticle disposition
within the vitreous and retina of rat eyes were determined (Kim et al. 2009b).
Anionic nanoparticles were found to penetrate the retina after intravitreal injection;
however, these nanoparticles could not penetrate the blood–retinal barrier. Cationic
albumin nanoparticles were not able to efficiently penetrate the retina as only few
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particles made it to the retina after 5 h due to the aggregates formed in the vitreous.
Neither albumin nanoparticle formulations were found in the choroid or Bruch’s
membrane. Depending on the target for the disease to be treated, albumin nanoparticles may not be an appropriate delivery device. For example, for the treatment of
wet age-related macular degeneration (AMD), the drug must be able to reach the
choroid and therefore, albumin nanoparticles may not be desirable if they are unable
to reach the choroid. Further studies are needed to investigate the ability for albumin
nanoparticles to deliver drug over extended periods of time and to compare albumin
nanoparticles to current treatment regimes. This field of ocular drug delivery is relatively new and will likely expand within the next decade as the necessity for ocular
drug delivery vehicles increases.
11.2.5 Carbohydrate Nanoparticles
Chitosan, a polysaccharide, in particular has been investigated extensively over the
past three decades as a material for making drug delivery devices (Paolicelli et al.
2009). Chitosan is an acetylated form of chitin, which is found in lobster, crab and
shrimp shells as well as in other insects and fungi. In addition, degraded forms of
chitin are also found in plant soil to help plants defend against bacterium and other
pests including the pine beetle. The chemical structure of chitosan consists of randomly oriented units of b-(1→4)-d-glucosamine and N-acetyl-d-glucosamine.
The method of biodegradation of chitin within the body is relatively well understood and involves both deacetylation and lysosomal degradation (Pangburn et al.
1982). The rate of degradation and cellular toxicity is highly dependent on the per-
centage of N-acetylation of chitosan (Freier et al. 2005). With 30–70% acetylation,
50% of the chitosan mass was lost by lysosomal degradation over 4 weeks. Samples
with extremely high or low percentages of acetylation showed minimal weight loss
over 4 weeks. Chitosan with an extremely low percentage of acetylation (0.5%) had
the highest cell viability compared to chitosan with more acetylation. Both toxicity
and rate of degradation can be controlled by synthesizing chitosan with specific
amounts of N-acetylation.
In 2001, it was proposed by De Campos that chitosan nanoparticles may be
effective ocular drug delivery vehicles (De Campos et al. 2001). Cyclosporin A
(CyA)-loaded chitosan nanoparticles were shown to have at least twofold higher
corneal concentrations than CyA in solution at all time points assessed up to 48 h
(De Campos et al. 2001). In the conjunctiva, chitosan CyA nanoparticles had
~4,000 ng/g at 2 h compared with only ~900 ng/g at 2 h for the aqueous solution of
CyA. At 6 and 24 h, chitosan CyA nanoparticles had twofold higher concentrations
than the aqueous solution. The amount of drug in blood, iris/ciliary body, and aqueous humor were nearly indifferent. The same research group reported in 2006 that
the cell viability in the presence and absence of chitosan nanoparticles was the
same, with no signs of inflammation after cell uptake of nanoparticles (Salamanca
et al. 2006). Chitosan nanoparticles are emerging as a new class of drug delivery
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vehicles in the field of ocular therapeutics due to their ability to enhance drug levels
within ocular tissues. However, safety of chitosan nanoparticles after repeated
administrations should be ensured since chitosan is known to disrupt epithelial tight
junctions (Schipper et al. 1997).
A multicomponent nanoparticle comprised of PLGA and chitosan has recently
been investigated for the delivery of a plasmid encoding the plasminogen kringle 5
(PK5) protein, an angiogenic inhibitor for the treatment of diabetic retinopathy
(Park et al. 2009). The PLGA–chitosan nanoparticles were prepared by addition of
ethyl acetate to PLGA and addition of chitosan chloride to a polyvinyl acetate (1%
w/v) solution. The K5 plasmid was then added to the chitosan solution for complexation and DNA condensation. The chitosan–plasmid solution was then mixed
with the PLGA solution for 4
allowed to stir for 3 h. Centrifugation was then completed to purify the nanoparticles and lastly, lyophillization was conducted to obtain a dry powder. The PLGA–
chitosan nanoparticles (~260 nm) were injected intravitreally into a rat eye and PK5
gene expression was detected up to 4 weeks after injection. The nanoparticle formulation was also capable of decreasing cell viability of bovine retinal capillary
endothelial cells, but had no effect on ARPE-19 cells. In vivo, the nanoparticles
diminished the neovascular area and preretinal vascular cells when intravitreally
injected. PLGA–chitosan nanoparticles provide for effective means to treat diabetic
retinopathy by inhibiting neovascularization.
min and then water was added to the mixture and
11.2.6 Dendrimers
Dendrimers are organic chemical structures with macromorphology consisting of
branched tree-like structures. Dendrimer morphology comprises a core, which
serves as the initiation site of branching. The core is then branched out to create the
linkage between the core and the branches. The ends of each branch typically carry
surface functional groups, which can serve as binding locations for drug molecules,
targeting molecules, or imaging agents. Various organic compounds can be used in
the synthesis of dendrimers depending on the functionalization and physiochemical
properties desired. Some commonly synthesized dendrimers are based on polyamido amine (PAMAM) and poly-propylene imine (PPI). PAMAM dendrimers are
especially desirable in designing drug delivery systems since the amine functional
group can be easily tethered to a drug. Dendrimer morphology and size can vary
depending on the number and type of building blocks used.
The use of dendrimers for ocular delivery was proposed at least 5–10 years ago
(Robinson and Mlynek
tors are still discovering the benefits of dendrimers. Similar to other drug delivery
devices, dendrimers can be designed to have desirable properties such as controlled
release and enhanced bioavailability or tissue penetration. Dendrimers have been
designed to cross cellular barriers such as epithelial cells in the gastrointestinal tract
(Wiwattanapatapee et al. 2000). Enhanced permeation was reported across canine
1995; Vandamme and Brobeck 2005) and several investiga-
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kidney cells (Tajarobia et al. 2001) as well as Caco-2 monolayers (Jevprasesphant
et al. 2004). In addition, dendrimers have been shown to improve the cell permeabil-
ity of ibuprofen by reducing the time it takes the cell to uptake ibuprofen from 3 to
1 h (Kolhea et al. 2003). In 2005, Vandamme and Brobeck determined which physiochemical parameters (molecular weight, size, number of amines, carboxyl, and
hydroxyl groups) contribute to the controlled release properties of the PAMAM
dendrimer in rabbit eye (Vandamme and Brobeck 2005). Fluorescently labeled
PAMAM dendrimer preparations (25 mL each) was instilled into the center of the
rabbit cornea and concentrations of the dendrimer at specific time points up to 24
h
were determined. PAMAM dendrimers with an amine group functionalization at
generation 2 had lowest corneal mean residence time (MRT) of 100 min and the
presence of a carboxyl group or a hydroxyl group at generation 2 had the highest
corneal MRT of 300 min. The corneal MRT of the amine-functionalized PAMAM
dendrimer was increased to 203 min if the amine was placed at generation 4. The
corneal MRT is a function of the molecular weight and functional group since
enhanced MRT was observed with an increase in molecular weight and with
hydroxyl or carboxyl functionalization.
PAMAM dendrimers, although smaller than 10 nm in their molecular form, can
form lose aggregates in buffers at physiological pH that can be separated by filtration. In the periocular region of the eye, while 20 nm particles disappear rapidly
within a few hours, 200 nm particles remain almost completely at the site of administration for at least 2 months (Amrite and Kompella 2005). Thus, administration of
larger nanoparticles will retain the drug better at the site of administration in the
periocular space. Using this concept Kang et al. prepared PAMAM dendrimers of
carboplatin with a particle size of approximately 260 nm and administered them in
the subconjunctival space of a murine model for retinoblastoma (Kang et al. 2009).
With this approach, nanoparticle formulation was shown to be much superior to
equivalent carboplatin solution at the end of 22 days following a single dose.
For efficient cellular response by a particular drug that is associated with a drug
delivery device, it is typically desired for the drug delivery vehicle enter cells by
crossing biological membranes. One possible mechanism to evade this hurdle is to
synthesize dendrimers that have specific functional groups that enhance cell uptake.
A polyguanidilyated dendrimer termed dendritic guanidilyated translocator (DPT)
by Durairaj and Kompella in 2009 was recently shown to increase gatifloxacin
(GFX) solubility, activity, permeability, and tissue retention (Durairaj and Kompella
2009; Durairaj et al. 2010). DPT enhanced solubility for GFX in a dose-dependent
manner. Within 5 min, preservative-free DPT–GFX rapidly entered human corneal
epithelium cells (HCE). DPT–GFX formulation increased the sclera–choroid–RPE
(SCRPE) transport of GFX by 40%. Further, DPT–GFX formulation was shown to
be as efficient or superior to GFX alone in antibacterial activity. In rabbit single and
multiple dosing studies, DPT–GFX (1.2% w/v GFX) was well tolerated and resulted
in about 13- and 2-fold greater tissue exposure of the drug compared to preservative
containing commercial formulation of GFX (0.3%). Further, drug levels persisted
longer in various tissues with DPT formulation. Thus, DPT formulations may reduce
the frequency of dosing of GFX.
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Dendrimers are generally synthesized by a stepwise addition of finite chemical
units. One well-known approach is the orthogonal coupling-strategy approach
(Zeng and Zimmerman 1996). This method starts with mixing a compound with
two repeating units such as dimethyl 5-hydroxyisophthalate with harsh chemicals
such as MeOH, H2SO4, LiAlH4, and Et2O to generate the dendritic core. Once the
core has been synthesized, the next unit can be covalently linked to the core to
branch out. Stepwise addition of polymer generations can be repeated until the
desired amount of generations is completed.
DPTs can be synthesized by a two-step process: the first step is to synthesize the
core, which contains three guanidine groups attached to tris-(hydroxymethyl) aminomethane (HMAM) (Durairaj and Kompella
of 3,5-diethoyoxycarbonylbenzoic acid to create as many generations as desired.
Lastly, units of guanidine can be added to react with the amine group of (HMAM).
Dendrimers are extremely desirable and useful in ocular drug delivery because
their composition and function can be readily controlled. Unlike other methods of
nanoparticle synthesis, dendrimer synthesis can be highly controlled and regulated.
The functional groups on the surface of the dendrimer may be optimized to allow
for enhanced cell permeability, targeting, or drug retention. However, the sustained
release from dendrimeric systems may be of a shorter duration compared to solid
nanoparticles.
2009). The second step is to add units
11.2.7 Combination Nanosystems
Controlled release or release at a particular site and/or for a particular duration is
employed to enhance drug efficacy while minimizing the risk for toxicity. Hoare
et al. developed a nanosystem comprised of a liposome with hydrogels embedded in
the membrane that act as a pore when Iron(III)oxide particles are magnetically
induced (Hoare et
off” switch that controls the magnetic induction and therefore the opening and closing
of the hydrogels embedded in the membrane. Another possible mechanism may
involve light irradiation. This was alluded to in Sect. “Polymer Nanoparticles.” Gold
nanoshells which undergo surface plasmon resonance upon laser irradiation and
create a local heating effect can also be used as actuators in a drug delivery device
(Prevo et al. 2008). Other mechanisms including thermosensitive and enzymatic
release may be possible as well.
al. 2009). Drug release from the liposome is controlled by an “on,
11.3 Using Nanotechnology to Improve Ocular Therapeutics
An introduction to the usefulness of drug delivery systems in ocular therapeutics
was discussed in the previous section for polymer, liposomal, protein, carbohydrate,
dendrimer nanoparticles as well as drug delivery systems with multiple components.
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This section will focus on different areas of ocular therapeutic improvement and
possible solutions.
11.3.1 Improving Patient Compliance
A major concern for clinicians prescribing and administering eye injections is
patient compliance due to the lack of noninvasive treatments that can deliver adequate amounts of drug to the target site. Currently, there are no treatments available that can deliver macromolecules and small molecules to the posterior
segment of the eye efficiently without using invasive techniques (e.g., intravitreal
injection).
As the biological basis for many ocular diseases becomes more apparent, protein, peptide, and nucleic acid drugs will be used to develop new pharmaceuticals
and therefore there is a need to develop noninvasive approaches for delivering
macromolecules as well. For example, the anti-VEGF antibody formulation,
Lucentis
Many macromolecules have poor permeability across biological barriers, which
make the development of noninvasive techniques difficult. Nanotechnology
approaches may be used to improve the bioavailability of many macromolecules
by sequestering the drug from enzymatic degradation and by enhancing tissue
uptake. For instance, surface-functionalized nanoparticle technologies were
developed by Kompella et al. to enhance corneal and conjunctival uptake and
transport of nanoparticles and the associated therapeutic agents (Kompella et al.
2006). These technologies entail coating of particle surfaces with a ligand capable
of recognizing a cell surface receptor. By coating LHRH receptor and transferrin
receptor recognizing ligands, it was shown that the corneal and conjunctival
uptake as well as transport of nanoparticles can be enhanced by several fold. The
functionalized nanoparticle exposure did not alter corneal epithelial cell tight
junctions or paracellular permeability, indicating the safety of these nanoparticles. It is anticipated that functionalized nanoparticles will allow noninvasive
delivery of poorly permeable small molecules as well as macromolecules to the
back of the eye.
®
must be intravitreally injected to reach the posterior segment of the eye.
11.3.2 Increasing Drug Retention and Sustained Release
Many therapeutics designed to treat ocular diseases must be injected into the eye
and typically they are injected multiple times to prevent relapse, e.g., Lucentis®
(Valmaggia et al. 2008). It has been reported that complications related to the
injection technique can occur, resulting in infection, uvetis, endophthalmitis
(Ozkiris and Erkilic 2005), cataract progression (Cekic et al. 2005), and vitreous
hemorrhage (Ciardella et al. 2004). The risk for these complications can be
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decreased by either developing noninvasive topical modes of delivery or by
injecting less frequently. In order for treatment injections to be less frequent, the
drug must have either intrinsic sustained release properties or a controlled release
mechanism may be employed by engineering a drug carrier. Nanosystems or
nanoparticles can be designed to have sustained release properties. For example,
Bourges et al. designed polylactic acid (PLA) nanoparticles that were injected
intravitreally and were observed in RPE cells up to 4 months after injection
(Bourges et al. 2003). Compared to Macugen® and Lucentis®, which are injected
every 6 weeks and every 4 weeks, PLA nanoparticles are retained much longer.
The use of PLA nanoparticles as a drug carrier may prove to be a successful
approach to sustain the release of its contents. A reduction in dosing frequency
will increase patient compliance, which reduces the risk for many complications
associated with ocular injections.
11.3.3 Increasing Permeability and Tissue Partitioning
Many topical agents including steroids, antihistamines, prostaglandins, and topical
anesthetics have been formulated to provide for noninvasive administration, yet
these topical agents still are not able to reach the posterior segment of the eye in
sufficient quantity. In eye treatments given as eye droplets such as timolol, only 1%
or less of a topically applied dose is absorbed across the cornea to reach the anterior segment of the eye (Lee and Robinson
1998) and only about one-billionth of that reaches the vitreous (Maurice 2002).
Ocular barriers such as the cornea and conjunctiva also create a major hurdle for
topically applied agents (Kompella and Lee 1999; Kompella et al. 2010). Therefore,
noninvasive formulations such as eye drops are not only are being washed away by
tear drainage and blinking, but they also encounter major ocular barriers that significantly reduce the amount of drug that is able to reach the posterior segment of
the eye. Therefore, the major route of administration of ocular therapeutics for the
back of the eye is injection because it delivers the drug either directly to the site of
action or in close proximity. With the advent of nanotechnology, noninvasive routes
of administration may be finally realized for ocular treatments by overcoming the
many biological barriers and providing for increased drug retention.
Surface functionalization of nanoparticles is a common approach to enhance
the permeability and specific tissue levels of therapeutics. For instance, deslorelin,
a luteinizing releasing hormone agonist, and transferrin functionalized polystyrene
(PS) nanoparticles (approximately 100 and 85
epithelial uptake by 3- and 4.5-fold compared to unfunctionalized nanoparticles,
respectively, at 5 min when topically applied to an ex vivo model (Kompella et al.
2006). At 1 h after a single topical application of the nanoparticle solution, the
deslorelin and transferrin functionalized nanoparticles had 4.5- and 3.8-fold higher
1986; Mezei and Meisner 1993; Ding
nm, respectively) enhanced corneal
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uptake across the corneal epithelium than nonfunctionalized nanoparticles.
Functionalized nanoparticles clearly are capable of improving drug transport
across major ocular barriers.
11.3.4 Targeting Nanotherapies
Ocular treatments may also be improved by developing targeted nanotherapies that
increase drug localization in the target tissues or reduce drug delivery to nontarget
tissues associated with drug side effects. Such approaches can potentially increase
drug therapeutic index by increasing drug efficacy and/or reducing drug toxicity.
Macugen® (pegaptanib), a drug product approved for treating wet AMD, belongs to
a class of chemicals known as aptamers (a short strand of nucleotides that recognizes a specific protein sequence) that are known to bind to their targets with affinities superior to even antibodies. Potentially, such aptamers can be used to target
delivery systems following various routes of administration. Indeed, aptamers have
been designed in the field of cancer therapy to target therapeutics directly to the
cancer cells and similar approaches may be used for targeting specific cell types
within the eye. Aptamers that specifically recognize the prostate-specific membrane
antigen (PSMA) found on the surface of prostate cancer cells were ligated to PLA–
PEG nanoparticles by adding the nanoparticles to 1-(3-dimethylaminopropyl)-3ethylcarbodimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) for
15
min while stirring (Farokhzad et al. 2004). Then the NHS-activated nanoparticles
were covalently linked to the PSMA aptamer. The resulting size of the nanoparticles
was approximately 250 nm. An in vitro assay confirmed that nanoparticles with
PSMA aptamers had 77-fold higher binding to LNCaP cells (which contain the
PSMA membrane protein) than the PC3 cells (which do not contain the PSMA
membrane protein).
Further, integrin-targeting peptides with RGD (arginine, glycine, and aspartic
acid) sequence and transferrin functionalizations on nanoparticle surface are of
potential value in increasing the delivery of nanoparticles and any associated therapeutic agents to various cell types within the eye. Using intravenously administered
nanoparticles functionalized on their surface with RGD peptide or transferrin, it
was demonstrated that back of the eye delivery of anti-VEGF intraceptor plasmidloaded nanoparticles can be enhanced in a choroidal neovascularization model
(Singh et
in vascular endothelial cells, photoreceptor outer segments, and retinal pigment
epithelial cells. By encapsulating the plasmid inside the nanoparticles as opposed
to the anti-VEGF agent itself, this approach potentially minimizes the systemic
side effects of anti-VEGF antibodies such as stroke and hypertension. Further,
since the intraceptor plasmid produces an anti-VEGF protein that is selectively
retained in endoplasmic reticulum, resulting in VEGF sequestration and reduced
secretion (Singh et al. 2006), extracellular concentrations of this anti-VEGF protein
al. 2009). Further, these nanoparticles enhance gene expression efficiency
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