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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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14 Targeted Drug Delivery to the Eye Enabled by Microneedles
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Fig. 14.9 Graph showing the effect of pressure on fluid delivery into anterior, medial, and posterior
regions of human cadaver sclera. Individual microneedles were inserted into the sclera with an
infusion pressure of 5, 10, 15, 20, or 25 psi and the volume infused was recorded. Data are mean
values (n ³ 3) with standard deviation bars. Adapted from Jiang et al. (2009) with permission from
Springer Science
challenges in drug delivery to the back of the eye: targeting and extended release.
Biodegradable particles can provide extended release while targeting the delivery
intrasclerally by localizing the delivery near the choroid and retina tissues.
Furthermore, if a single hollow microneedle can be simply inserted into the sclera
and inject a particle suspension into the sclera, it may be possible to perform the
procedure in a minimally invasive way.
Administration of nanoparticle suspensions using a hollow microneedle within
the sclera in a minimally invasive way appears to be possible. Nanoparticles of
280 nm in diameter at concentrations up to 10 wt.% suspension were injected into
the sclera with an applied pressure of 15 psi. Figure 14.10 shows the delivery of
nanoparticles within the sclera tissue. Injections were performed in different
regions of the sclera tissue to determine if a hollow microneedle was capable of
injecting nanoparticles into all regions. Hollow microneedles were capable of
injecting into all regions of the sclera, suggesting that a hollow microneedle can
inject intrasclerally to any site that can be accessible on the eye. This showed that
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Fig. 14.10 Representative histological images of intrascleral infusion of fluorescent nanoparticles in
human cadaver sclera using a hollow microneedle as a function of nanoparticle concentration and
scleral position. Microneedles were inserted into anterior, medial, and posterior regions of the sclera.
A 20 mL suspension with a solids content of 0.5, 1, 5, or 10 wt.% was infused in each attempt into the
tissue at a constant pressure of 15 psi. Dotted lines in each image indicate the upper and lower edges
of the scleral tissue. Adapted from Jiang et
al. (2009) with permission from Springer Science
nanoparticle suspensions could be injected intrasclerally similarly to the way fluids
were injected into the sclera (Jiang et al. 2009).
However, administration of microparticle suspensions intrasclerally was not as
straightforward. Suspensions of microparticles did not flow through the sclera tissue.
This is in large part attributed to the spacing of the sclera collagen fibers rather than
a limitation on the microneedle capability. The collagen fiber spacing is on the order
of several hundred nanometers, and as a result microparticles may not easily flow
within this medium (Edwards and Prausnitz 1998). To test this hypothesis, two
approaches were employed to aid the movement of particles in the dense collagen
matrix. One was the use of collagenase to break up the collagen structure and provide
larger pathways for microparticles to flow through the sclera. The sclera tissue was
either soaked in collagenase prior to injection or the collagenase was co-injected
with the microparticle suspension. Both of these steps allowed infusion of the
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microparticle suspension, demonstrating that disruption of the collagen structure
allowed microparticle infusion within the sclera. The second approach involved
co-injecting hyaluronidase, a dispersive agent known to help injectable formulations
flow through densely packed tissues such as the skin, for which it has FDA approval.
Microparticles were also successfully administered using this approach. As a result,
incorporation of hyaluronidase into a suspension may be a feasible way to deliver
controlled-release microparticle formulations within the sclera for drug delivery to
the back of the eye (Jiang et al. 2009).
14.3.3 Suprachoroidal Delivery Using Hollow Microneedles
For treating diseases of the back of the eye that involves targets such as the choroid
and retina, it would be beneficial to deliver the drug as close as possible to these tissues. In addition, it would be beneficial to localize the drug to these regions to maintain high levels of drug over time without exposing other eye tissues to the drug. The
complication with this approach is that the choroidal and retinal tissues cover a large
region of the back of the eye and much of it is inaccessible directly. This is especially
true for treating the macula in cases of neovascular AMD. If a method of administration could allow direct injection of a formulation in close proximity to the retinochoroidal tissues from a site that is easily accessible, it would provide a much needed
advantage over currently practiced methods. If a formulation can be injected in a circumferential manner so that it flows from an anterior location in the eye to the posterior near the macula while bathing the retinochoroidal tissue, it would also allow large
doses to be delivered, as well as cover a large portion of the back of the eye.
One approach to accomplish this circumferential delivery would be to inject a formulation into the suprachoroidal space. Suprachoroidal delivery refers to a relatively
new route of administration to deliver drugs to the back of the eye. Unlike many
approaches, this approach attempts to target delivery not within tissues or media of the
eye, but to deliver a drug formulation between two tissue layers. The suprachoroidal
space refers to a space in the eye that is created when there is fluid buildup between the
sclera and choroid layers of the eye (Emi et
Figure 14.11a shows an idealized cartoon of what delivery into this region would look
like. This region is particularly attractive because a drug in the suprachoroidal space is
in direct contact with the choroid, which is adjacent to the retina (Patel et al. 2010).
These two tissues are the targets for many diseases such as AMD, uveitis, and diabetic
macular edema, which can lead to blindness. A drug delivery method that can reliably
deliver into this region could provide a more targeted approach to treat these diseases.
Recently, researchers have shown that there are several ways to take advantage of
this region and access the space. These methods, however, are invasive and may not
be suitable for long-term clinical therapy of chronic back of the eye diseases. They
involve the use of catheters or implants that are surgically placed in the eye to access
the suprachoroidal space (Einmahl et al. 2002; Gilger et al. 2006; Olsen et al. 2006;
Kim et al. 2007). A hollow microneedle is an attractive alternative to inject formulations
al. 1989; Krohn and Bertelsen 1997).
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Fig. 14.11 Suprachoroidal delivery. (a) An idealized image of the anatomy of the periocular tissues
near the insertion site before and after suprachoroidal injection. Image of the eye was adapted from
National Eye Institute, National Institutes of Health, with permission. (b, c) Brightfield images of a
cross-section of a frozen pig eye showing (b) normal ocular tissue and (c) showing the delivery of
sulforhodamine B (pink) between the sclera and choroid (i.e., in the suprachoroidal space). Scale bar:
500
mm. Reproduced from Patel et al. (2010) with permission from Springer Science
into the suprachoroidal space, because it offers a minimally invasive route. If a hollow
microneedle can access the suprachoroidal space, it may provide micron-scale targeting of the sclera and choroid interface in a minimally invasive procedure.
Hollow microneedles have been shown to target the suprachoroidal space and
deliver fluids and particles within the suprachoroidal space of rabbit, pig, and
human eyes. Hollow microneedles inserted ex vivo into whole pig eyes showed
that a sulforhodamine solution could be injected into the suprachoroidal space.
The space could be selectively targeted, causing the sclera–choroid interface to
expand and fill with fluid (Fig. 14.11b, c). Volumes up to 35 mL could be injected
into this space ex vivo and the delivery of the solution looks to be well targeted to
the suprachoroidal space. Additional experiments revealed that particles up to
1 mm in diameter could be delivered into the suprachoroidal space of rabbit, pig,
and also human eyes. Figure 14.12 shows the delivery of particle suspensions in
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Fig. 14.12 Infusion of particles into the suprachoroidal space. Image of a cryosection of a pig
eye with no injection into the suprachoroidal space (a). The following layers of the eye are
shown: (1) sclera, (2) choroid, and (3) retina. Fluorescence microscopy images of tissue cryosections show the delivery of (b) 500 nm particles into a rabbit eye, (c) 500 nm particles into a
pig eye, and (d) 1,000 nm particles into a human eye, all ex vivo. Each image also displays an
inset with a magnified view of the microneedle insertion site. The images show targeted delivery of particles into the suprachoroidal space and indicate that the microneedle did not penetrate into the choroid or retina. Scale bar: 500 mm. Reproduced from Patel et al. (2010) with
permission from Springer Science
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Fig. 14.13 A graph showing the effect of infusion pressure and microneedle length on the success
rate of suprachoroidal delivery for (a) 20 nm, (b) 100 nm, (c) 500 nm, and (d) 1,000 nm particles
in porcine eyes. A total of five infusions were attempted at each condition. Overall, increasing
microneedle length and increasing infusion pressure increased the delivery success rate for all
particle sizes. Reproduced from Patel et al. (2010) with permission from Springer Science
these different species (Patel et al. 2010). This shows that a hollow microneedle is
versatile enough to deliver fluids and particles into the suprachoroidal space of
eyes in three different species.
Delivery of particles into the suprachoroidal space offers the potential for controlled
or sustained delivery to the chorioretinal surface. If the parameters necessary for particle administration into this space using microneedles can be determined, then a minimally invasive delivery method and device can be designed. Detailed experiments were
performed on pig eyes ex vivo to determine the necessary parameters for delivering
particles of 20, 100, 500, and 1,000 nm in diameter into the suprachoroidal space.
These studies showed that as the particle size increased, the applied pressure and
microneedle length were critical parameters for achieving realiable suprachoroidal
delivery into pig eyes ex vivo (Fig. 14.13). The hypothesis for this is that suprachoroidal
administration using a hollow microneedle is performed by inserting the microneedle
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Fig. 14.14 Images showing the effect of particle size on particle distribution in the eye.
Fluorescence microscopy images of tissue cryosections show the delivery of (a) 20 nm particles
and (b) 1,000 nm particles into the suprachoroidal space of porcine eyes ex vivo. The images show
that 20 nm particles can spread in the suprachoroidal space and within the sclera. However,
1,000 nm particles are primarily in the suprachoroidal space. The insets show a magnified view of
the insertion sites, which are indicated by arrows. Scale bar: 500
(2010) with permission from Springer Science
mm. Reproduced from Patel et al.
to the base of the sclera as opposed to directly inserting all the way into the suprachoroidal space. As a result, the initial barrier that must be overcome is movement of particles from the base of the sclera into the suprachoroidal space or choroid. This is
governed by the anatomy of the sclera and the issues associated with intrascleral delivery also apply here as well. It was significantly easier to deliver particles less than
500 nm vs. larger than 500 nm in diameter (Patel et al. 2010).
This hypothesis was further confirmed by imaging the delivery of different-sized
particles within the ocular tissues. The effect of collagen fiber spacing in the sclera
discussed above suggests that particles of 20 and 100 nm should be able to spread
within the sclera as well as the suprachoroidal space, whereas particles of 500 and
1,000 nm should localize exclusively in the suprachoroidal space. Figure 14.14
shows the spread of 20 nm particles and 1,000 nm particles under identical injection
conditions within the layers of the eye. As expected, the smaller particles spread
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significantly in the sclera as well as the suprachoroidal space. In contrast, the larger
particles are confined primarily to the suprachoroidal space and are excluded from
spreading within the sclera. Although scleral collagen fiber spacing may vary
between species, this segregation effect should be consistent since the hollow
microneedle delivery mechanism involves flow of the formulation through the
sclera. Particles on the same order of magnitude or smaller in size than the collagen
spacing of the species’ sclera should spread in the sclera and those larger will be
limited mainly to the suprachoroidal space (Patel et al. 2010).
The intraocular pressure (IOP) may also have an important role in successful suprachoroidal delivery of large microparticles since it is important to reach the base of the
sclera, as close as possible to the suprachoroidal space. An increase in IOP should allow
more efficient insertion of the microneedle into the sclera since internal pressure within
the eye provides a back pressure that keeps the eye inflated. The infusion pressures of
150–300
of approximately 2 kPa (i.e., 15–16 mmHg) (Klein et al. 1992). As a result, a doubling
or tripling of this IOP should contribute insignificant back pressure to counter the infusion pressure. Instead, the main effect of elevated IOP would be to make the sclera surface firmer, and reduce deflection of the tissue surface during microneedle insertion and
thereby increase the depth of microneedle penetration into sclera.
of IOP: 18 and 36 mmHg (Patel et al. 2010). The results indicate an increase in the
delivery success rate at shorter microneedle lengths, confirming the theory. Although
a direct measurement of microneedle insertion depth was not performed, these results
suggest that microneedle insertion at elevated IOP may cause less deflection of the
tissue and allow the microneedle to reach the base of the sclera more efficiently. This,
in turn, increases infusion success rate since the microneedle and microparticles have
more direct access to the suprachoroidal space (Patel et al. 2010).
kPa are two orders of magnitude greater than the mean IOP in a normal adult
This hypothesis was tested by injecting 1,000 nm particles at two different levels
14.4 Microneedle Types and Other Applications
The term microneedle does not represent a singular device or design of a needle, but
is instead a term used to describe a class of micrometer-scale needles that can be used
in a variety of ways to deliver drugs locally to the body. Microneedles can be made
from a variety of materials, such as metals, glass and plastics, and they can be
designed in various shapes. However, the overall purpose of all microneedles
remains similar: microneedles pierce into a tissue to create micron-scale pores or
channels through which therapeutics can be transported more effectively into the
body than without such pores or channels. We present in this section four general
approaches to using various types of microneedles to deliver molecules into tissues.
Microneedles were first envisioned for application to the skin and, as a result, many
of these strategies were developed with the skin in mind. However, all of these
approaches are applicable to the eye and may be advantageous depending on the
target tissue and necessary delivery requirements. A summary of the approaches in
a graphical format is shown in Fig. 14.15 (Arora et al. 2008).
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14 Targeted Drug Delivery to the Eye Enabled by Microneedles
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Fig. 14.15 Four different general strategies for delivering molecules into a tissue using microneedles.
(a) Poke and apply: Insert microneedles into the tissue to form pores, remove microneedles and
then apply the drug formulation to the surface of the tissue. (b) Coat and poke: Coat microneedles
with a drug formulation, insert microneedles into a tissue to dissolve off the coating formulation
within the tissue and remove microneedles to leave the deposited drug in the tissue. (c) Poke and
release: Encapsulate drug in a biodegradable microneedle, insert microneedles into the tissue and
leave them there, and as the microneedle degrades the drug is released into the surrounding tissue.
(d) Poke and flow: Insert a hollow microneedle into a tissue, apply pressure to the fluid in the
microneedle to flow the fluid into the tissue, and remove hollow microneedle after the desired
volume has been injection, leaving behind fluid in the tissue. Adapted from Arora et al. (2008) with
permission from Elsevier
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Fig. 14.16 A histological cross-section of human cadevar sclera after an uncoated microneedle
was inserted into the sclera and removed. The sclera was then stained with a blue tissue-marking
dye to identify the site of penetration (arrow). Scale bar: 250
with permission from Association for Research in Vision and Ophthalmology
mm. Adapted from Jiang et al. (2007)
14.4.1 Poke and Apply
One microneedle strategy is to simply insert solid microneedles into a tissue and
remove the microneedles, leaving behind channels or pores that make the tissue
more permeable. A therapeutic agent can then be applied to the surface of the
microneedle-treated tissue. This makes the delivery a two-step procedure: the first
step is the creation of the channels followed by a second application of the formulation to be delivered. With this approach, the microneedles serve primarily to create
a transport pathway and do not come into contact with the drug formulation.
One of the first reported studies showed that microneedles made of silicon using
microfabrication technologies borrowed from the microelectronics industry showed
that holes created by microneedles could increase the permeability of human
epidermis of calcein by four to five orders of magnitude (Henry et
Additional experiments using a similar strategy showed that pores created by
microneedle could increase the permeability to larger compounds such as insulin,
BSA, vaccines, and nanospheres up to 50 nm in radius (McAllister et al. 2003;
Martanto et al. 2006; Ding et al. 2009). Many of these early studies showed that
microneedles could create pores to enhance the delivery of many compounds that
would otherwise not be deliverable through the skin.
This strategy can be applied to the ocular tissues as well. Solid microneedles can
be inserted into tissues such as the sclera, cornea, near the limbus, or near the target
area and removed after creating pores. Solid microneedles have been shown to pierce
the sclera tissue and create pores within the sclera. Figure 14.16 shows that a microneedle
al. 1999).
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