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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5394_Библиотеки_им_академика_М_И_Перельмана
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14 Targeted Drug Delivery to the Eye Enabled by Microneedles
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nontarget tissues of the eye as well. This lack of targeting is generally a concern for
all approaches that release drugs into the vitreous, including intravitreal injections
that have become a common method to deliver drugs to the back of the eye (Peyman
et al. 2009). In addition, the large volume of the vitreous dilutes the released drug.
This implies that drug delivered intravitreally needs to be of a higher dose in order
to maintain a therapeutic level than if it were delivered in a targeted way to tissues
such as the retina and/or choroid.
These two issues are particularly problematic for the above devices because they
release steroids that come in contact with the lens and cause side effects such as
cataracts (Ozkiris and Erkilic 2005). Although these devices have helped to address
one of the key goals of drug delivery, i.e., sustained delivery, they have not targeted
that delivery particularly well to the desired target tissues. In addition, these
approaches are invasive. Implants are most often surgically placed and, if they need
to be removed, an additional surgery is required.
Targeting drug delivery to the front of the eye has similarly not received sufficient attention. As an example, many glaucoma drug therapies require a drug to
act on the ciliary body or the trabecular meshwork to decrease production or
increase outflow of aqueous humor, respectively (Lee and Higginbotham 2005).
Yet, most drugs are administered using drops on the surface of the eye. Although,
drops are a convenient method of application that is noninvasive, they expose a
large surface of the eye to the drug. In addition to delivering a fraction of the drug
to the desired region of the eye, much more of the drug is administered to other
parts of the eye and even more of it is removed from the eye to other parts of the
body through the nasolacrimal duct and absorption by subconjunctival vessels
(Jarvinen et al. 1995). Once again, this applies not just to topical drops but for any
method that administers drugs to the corneal surface. As a result, topical drug
delivery is not very effective at targeting the drug to the desired location within
the eye for glaucoma therapy.
In general, a method that is effective at targeting localizes the drug at high concentration in or near the target eye tissue while minimizing exposure of other tissues
so as to avoid side effects and complications. When examining the current approaches
for ocular drug delivery, many of them are not designed with this goal in mind.
These approaches can be roughly divided into two strategies. The first is either a
periocular or superficial strategy to place the drug on the outer surface of the eye by
administration methods such as drops, injections, or implants. In many cases, the
therapeutic target is not on the outer surface of the eye and, as a result, the drug
needs to diffuse across the cornea and/or sclera to intraocular tissues to be effective
(Ghate et al. 2007; Gaudana et al. 2010). Intravitreal strategies deliver drugs directly
into the vitreous and are thereby effective at overcoming barriers that prevent getting drug into the eye. However, the drug then spreads throughout the vitreous and
this exposes multiple nontarget tissues to the drug as it moves toward the target site
(Krohne et al. 2008; Cheng et al. 2009).
Many of the tissue targets for diseases of the eye are less than 1 mm beneath
the globe of the eye. These include the corneal stroma, the ciliary body and
trabecular meshwork for front of the eye diseases and the choroid and retina
for back of the eye diseases (Lee and Higginbotham 2005; Gaudana et al. 2010).
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A method that can deliver a drug directly to these regions or just adjacent to them
and thereby localize the delivery would be more effective at targeting than the
currently practiced strategies.
14.3 Improved Methods of Drug Delivery to the Eye
Using Microneedles
A microneedle-based delivery method may be able to deliver drugs directly to
intraocular tissues in a minimally invasive manner and thus provide targeted delivery. Microneedles, because of their small size, can be inserted directly into tissues
of the eye and, in a variety of scenarios, target the delivery of drugs to tissues such
as the corneal stroma, the sclera, and the suprachoroidal space (Fig. 14.1). We show
in this chapter that microneedles, which are microscopic needles less than 1 mm
long, can be used for minimally invasive intracorneal, intrascleral, and suprachoroidal delivery. We examine the capability of microneedles as a way to effectively
target regions of the eye in a minimally invasive manner. As a result of their micronscale size, microneedles offer a unique way to deliver drug formulations within and
between tissues of the globe. This allows the use of ocular tissues as reservoirs or
conduits for drug delivery rather than just barriers to transport.
14.3.1 Intrastromal Delivery to the Cornea
Using Coated Microneedles
The cornea is often treated as a barrier to drug delivery. This is especially true in
glaucoma therapy using drops. The drug is initially placed on the surface of the eye
but needs to be delivered to tissues further within the eye so the cornea presents
itself as a barrier for this transport. However, if drugs can be delivered across the
corneal epithelium and directly into the corneal stroma (i.e., intrastromal delivery),
the cornea can be used advantageously for drug delivery. If drugs are deposited
directly into the corneal stroma, in a minimally invasive way, the cornea can act as
a reservoir to deliver drugs to the anterior segment of the eye. In cases of corneal
infections or neovascularization, the cornea can be the actual target and thus intrastromal delivery can effectively target the site of disease within the cornea (Prakash
et
al. 2008; Tabbara and Al Balushi 2010). Furthermore, if delivery can be localized
on the micron scale, then the drug can be targeted to a specific region of the cornea
without exposing the whole anterior surface of the eye.
As described below, microneedles may be able to accomplish intrastromal
delivery of drugs to target the cornea and anterior segment of the eye. The key
advantage of using microneedles lies in the scale of the target tissue and the
dimensions of the microneedle. Although corneal thickness varies, it is on the
order of several hundred micrometers (Aghaian et al. 2004). A microneedle,
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14 Targeted Drug Delivery to the Eye Enabled by Microneedles
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Fig. 14.1 Microneedles can be inserted into intraocular tissues for targeted delivery in a variety of
scenarios. (a) Microneedles can be inserted into the cornea (5) for intrastromal delivery. (b) The
trabecular meshwork can also be targeted by inserting a microneedle near the limbus directly into or
near the trabecular meshwork. (c) Intrascleral delivery can be accomplished by inserting a microneedle into the sclera (1). (d) Microneedles inserted deeper into the sclera can target the suprachoroidal
space, the region between the sclera and choroid (2), for suprachoroidal delivery. The retina (3) is
located just below the choroid and the vitreous humor (4) just below that. Image of the eye was
adapted from National Eye Institute, National Institutes of Health, with permission
which is on the same order of magnitude in length as the corneal thickness, can be
inserted within the cornea and used to deposit the drug into the stroma. Ideally,
the drug should be delivered quickly so that the microneedle can be removed
within seconds from the eye. The microneedle strategy for intrastromal delivery
relies on inserting the microneedle into the cornea, without penetrating across the
cornea, then depositing the drug formulation within the stroma, and finally removing the microneedle as quickly as possible from the eye, thereby leaving the drug
formulation behind as a depot within the cornea.
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Fig. 14.2 Solid, coated microneedle. (a) A light micrograph of a single stainless steel microneedle
with no coating on the surface of the microneedle. (b) A magnified view of a microneedle after
coating sodium fluorescein on the surface. Sodium fluorescein is selectively coated on the
microneedle and not on the base. Scale bar: 500
permission from Association for Research in Vision and Ophthalmology
mm. Reproduced from Jiang et al. (2007) with
The first test of this proposed delivery method was designed to assess whether
microneedles could insert into, but not across, the cornea and deposit molecules within
the cornea. In order to quickly deposit the drug using a simple, inexpensive device, a
solid-coated microneedle was employed. The approach is to place a dry coating of the
drug on the surface of the solid microneedle, insert it into the wet interior of the
cornea, which allows the coating to dissolve off of the microneedle, and then remove
the microneedle, thereby leaving the dissolved coating within the cornea.
To test this, sodium fluorescein was used as a model compound and coated on
individual solid microneedles. Figure
14.2 shows a solid stainless steel microneedle
before and after coating (Jiang et al. 2007). The microneedle was tested by inserting
it into the cornea of a pig eye in vitro. Figure 14.3a shows a cross-section of the
cornea after insertion of a microneedle. The microneedle insertion site can be seen
at the break in the corneal tissue marked by the arrow. The microneedle penetrated
into the cornea without penetrating across the cornea. Figure 14.3b shows the same
image under fluorescence microscopy demonstrating delivery of the fluorescein
throughout the cornea. This shows that a microneedle can insert into the cornea and
deliver coated compounds into the corneal stroma.
As a result of microneedle administration that bypasses the corneal epithelium
and targets the corneal stroma, microneedle-based delivery should provide higher
bioavailability than topical administration. To test this hypothesis, Jiang et al. studied
the intraocular distribution of sodium fluorescein after intrastromal delivery using
coated microneedles. The work showed that microneedles can indeed deliver molecules directly within the corneal stroma in vivo in rabbits. A single solid stainless
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Fig. 14.3 A cross-section of porcine cornea after insertion of a solid stainless steel microneedle
coated with sodium fluorescein. (a) A break of the corneal surface can be seen (arrow) followed
by the path of the microneedle into the stroma of the cornea. The insertion site is confined to within
the cornea. (b) Fluorescence micrograph of the tissue shows the sodium fluorescein (in green) has
been locally delivered within the cornea near the insertion site. Scale bar: 500
mm
steel microneedle was coated with 280 ng of sodium fluorescein. The microneedle
was inserted into the cornea and the concentration in the cornea, aqueous humor and
lens was measured over time in vivo. As a comparison, experiments were also done
applying a 3 mg dose of sodium fluorescein topically to the surface of the rabbit eye
as a drop and identical measurements were made (Jiang et al. 2007).
The coated microneedle experiment showed that microneedles could be inserted
into the cornea and that sodium fluorescein dissolves off the microneedle in a matter
of seconds to create a depot within the stroma. In addition, sodium fluorescein levels in the anterior ocular tissues were higher than a topical application of an equivalent dose (Fig. 14.4). As an example, at the 3 h time point fluorescein concentrations
in the eye were about 60 times higher than a topical application of an equivalent
dose. The kinetic data also demonstrated that microneedle-based administration of
fluorescein resulted in extended residence time of fluorescein as compared to topical application. In both cases, fluorescein concentrations returned to near baseline
levels in the anterior segment with 24
h. The calculated bioavailability of coated
sodium fluorescein delivered to the eye was 69% following microneedle administration vs. only 1% for topical administration (Jiang et al. 2007). This shows that
microneedle administration effectively targeted the cornea while the topical application resulted in nearly all of the fluorescein being washed away from the eye.
These results indicate that coated microneedles should be able to deliver a therapeutically relevant molecule to the cornea and anterior segment of the eye more
effectively than topical administration. Furthermore, the targeting capability of
microneedles should allow for a high bioavailability of the drug and an enhanced
pharmacological effect. In order to test this hypothesis, pilocarpine, a drug used to
treat glaucoma, was delivered intrastromally using microneedles. Solid stainless
steel microneedles were coated with approximately 1.1 mg of pilocarpine and inserted
in the peripheral cornea of New Zealand white rabbits in vivo. A total of five
microneedles were inserted along the circumference of the cornea targeting the
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Fig. 14.4 Fluorescein concentration profiles as a function of position in the anterior chamber of
the rabbit eye in vivo after administration using microneedles (a) and topical administration at 10
times the microneedle dose (b). Adapted from Jiang et al. (2007) with permission from Association
for Research in Vision and Ophthalmology
peripheral cornea area. The microneedles were removed after 20 s, which was long
enough to deposit the pilocarpine within the corneal stroma. Since pilocarpine causes
constriction of the pupil if it reaches the ciliary muscles, the pupil size was monitored
to determine if pilocarpine had reached its intended target (Jiang et al. 2007).
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14 Targeted Drug Delivery to the Eye Enabled by Microneedles
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Fig. 14.5 Changes in rabbit
pupil diameter over time in
an untreated eye (open
circles), in eyes treated with a
topical application of 5
pilocarpine (gray circles),
microneedles coated with
5.5
mg pilocarpine (gray
square), and a topical
application of 500
pilocarpine (black circles).
Data represent the average of
at least three measurements.
Adapted from Jiang et
(2007) with permission from
Association for Research in
Vision and Ophthalmology
mg
mg
al.
Microneedle-based administration of pilocarpine caused the pupil to constrict
2.5 mm within 15 min after insertion. A similar dose of pilocarpine, 5 mg, delivered
topically cause constriction of only 1 mm. In addition, the pupil began to constrict
several minutes earlier when pilocarpine was administered using microneedles.
This indicated that the kinetics of microneedle-based delivery of pilocarpine was
faster than topical drops. When 500 mg of pilocarpine was applied topically, the
pupil constricted a total of 4 mm with kinetics similar to that of 5.5 mg administered
using coated microneedles (Fig. 14.5). These experiments demonstrate that a drug
can be administered using microneedles and can be targeted by inserting the
microneedles within the peripheral cornea. As a result, the pharmacological effect
was more effective when administered through microneedles than by a less targeted
approach, such as topical application (Jiang et
al. 2007).
14.3.2 Intrascleral Delivery Using Coated and Hollow
Microneedles
The sclera, like the cornea, is typically seen as a barrier to transport of drugs to the
back of the eye from periocular administration routes such as subconjunctival injections. Targets for posterior segment diseases such as neovascular AMD are the choroid
and retina layers, which are just below the sclera. As a result, if drugs can be delivered directly to the sclera, i.e., intrasclerally, the sclera can be converted from a
transport barrier to a reservoir for localized drug delivery to the underlying tissues
of choroid and retina. If this can be accomplished in a minimally invasive manner,
it would allow direct access to ocular tissues as natural drug delivery depots.
Microneedles, given their micron dimensions, can play an important role in accomplishing this because they can specifically target the sclera and deliver drug formulations
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intrasclerally. The thickness of the sclera tissue is on the order of hundreds of
micrometers, which means that microneedles can be inserted intrasclerally without penetrating across the tissue and deliver a drug depot within the tissue (Olsen et al. 1998).
There are two approaches to exploit the capabilities of microneedles to deliver
formulations into the sclera: solid-coated microneedles and hollow microneedles.
Solid-coated microneedles can be inserted into the sclera, and the coating can dissolve off the microneedle into the sclera, after which the microneedle can be
removed. This forms a local depot near the insertion site. A second approach is to
use hollow microneedles to inject a formulation directly within the sclera. In this
approach, a hollow microneedle is inserted into the sclera and a fluid is injected
within the sclera, after which the microneedle can be removed once the desired
volume is injected. A hollow microneedle functions in a way that is similar to a
standard hypodermic needle, since it allows a fluid to flow through the bore of the
microneedle. However, because a hollow microneedle has a microscopic orifice
opening and length, a microneedle can target the sclera by spreading the fluid specifically within the sclera.
Solid-coated microneedles can allow pinpoint delivery near the insertion site
within the sclera. The microneedle can be inserted into any accessible location on
the sclera to deposit the coated formulation. If the drug needs to be delivered near
the anterior segment of the eye, it can be inserted near the limbus. Multiple microneedles can be inserted either simultaneously as part of an array of microneedles or
serially, one after the other. Figure
14.6a shows that a microneedle can penetrate
into human cadaver sclera and deliver a small molecule such a sulforhodamine
locally into the tissue. In addition to a small molecule, macromolecules can also be
coated onto microneedles. Figure 14.6b shows the delivery of fluorescein-labeled
bovine serum albumin (BSA) after intrascleral administration using a coated
microneedle. The images show that microneedles can locally deliver molecules and
form a depot within the sclera (Jiang et al. 2007).
It may be advantageous to not just deliver a formulation to a specific spot in the
sclera, but to spread the formulation over a larger area of scleral tissue. This would
allow the sclera to serve as a large reservoir for subsequent drug delivery to underlying tissues. Hollow microneedles may be able to spread a fluid within the scleral
collagen matrix and accomplish intrascleral delivery of fluids. Figure 14.7 shows a
hollow glass microneedle in comparison to a standard 30-gauge needle. The first
reported study to show that a hollow microneedle was capable of intrascleral injection demonstrated delivery of a sulforhodamine solution within the sclera of human
eyes in vitro. Bare sclera was excised from human cadaver eyes and a hollow glass
microneedle was inserted into the sclera and infused with a solution. Figure 14.8
shows the delivery and spread of sulforhodamine solution within the sclera. These
images show that a hollow microneedle can inject a solution intrasclerally and target
the sclera tissue (Jiang et al. 2009).
An important parameter for determining effective delivery within the sclera is
microneedle insertion depth. This is especially important because scleral thickness
varies based on location. Scleral thickness can range from 300 mm to 1 mm within
the same eye (Olsen et al. 1998). Initial experiments revealed that in addition to the
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Fig. 14.6 Cross-sections of human cadaver sclera pierced using a single 750 mm-long microneedle
(55° tip angle). Microneedles coated with sulforhodamine (a) and fluorescein-labeled bovine serum
albumin (BSA) (b) were inserted into the sclera and deposited the coating formulation within the
sclera. The arrow indicates the site of microneedle insertion. Scale bar: 250
et al. (2007) with permission from Association for Research in Vision and Ophthalmology
microneedle insertion depth, it was also important to partially retract the microneedle
to flow a fluid within the sclera. The amount of fluid delivered did not vary significantly
with location or the insertion depth and retraction distance. The volumes delivered
were between 10 and 15 mL and all were delivered within 3 min of applied pressure.
Applied infusion pressure was also varied within the different regions of the eye,
and Fig. 14.9 shows the volume delivered with a constantly applied infusion pressure.
mm. Adapted from Jiang
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Fig. 14.7 Comparison of a hollow glass microneedle (a) to the tip of a 30-gauge hypodermic needle
(b). Scale bar: 1 mm. Adapted from Patel et al. (2010) with permission from Springer Science
Fig. 14.8 Representative images of human cadaver sclera after microneedle infusion of a sulforhodamine solution. (a) Top view image of the surface of the sclera showing the infusion of sulforhodamine over an area of several square millimeters. (b) Histological section using fluorescence
microscopy showing the site of microneedle insertion (arrows) and the distribution of injected
sulforhodamine (in red) preferentially localized within the sclera. Adapted from Jiang et
with permission from Springer Science
The data suggest that there is no direct correlation between applied pressure and
volume delivered. As a result there may be an inherent capacity of the sclera to hold
fluid and increased infusion pressure cannot overcome this limitation under the conditions tested (Jiang et al. 2009).
In addition to injecting a solution intrasclerally, injection of nano- or microparticles may be more advantageous. If designed properly, particles injected into the
sclera can release a drug into the sclera tissue and provide sustained or controlled
release of a drug. This can extend the residence time of the drug in the eye and
reduce the administration frequency. This approach would address both of the key
al. (2009)
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