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Ophthalmic drug discovery:
novel targets and mechanisms for
retinal diseases and glaucoma
Kang Zhang1,2,3, Liangfang Zhang3,4 and Robert N. Weinreb2
Abstract | Blindness affects 60 million people worldwide. The leading causes of irreversible
blindness include age-related macular degeneration, retinal vascular diseases and glaucoma.
The unique features of the eye provide both benefits and challenges for drug discovery and
delivery. During the past decade, the landscape for ocular drug therapy has substantially
changed and our knowledge of the pathogenesis of ophthalmic diseases has grown
considerably. Anti-angiogenic drugs have emerged as the most effective form of therapy
for age-related macular degeneration and retinal vascular diseases. Lowering intraocular
pressure is still the mainstay for glaucoma treatment but neuroprotective drugs represent
a promising next-generation therapy. This Review discusses the current state of ocular
drug therapy and highlights future therapeutic opportunities.
Age-related macular
degeneration
(AMD). A disease process
that is characterized by the
degeneration of photoreceptor
cells in the macula, leading to
loss of central vision.
Department of
Ophthalmology and
Molecular Medicine Research
Center, State Key Laboratory
of Biotherapy, West China
Hospital, Sichuan University,
Chengdu 610041, China.
2
Department of
Ophthalmology and Shiley
Eye Center, University of
California San Diego, La Jolla,
California 92093, USA.
3
Institute for Genomic
Medicine, University of
California San Diego, La Jolla,
California 92093, USA.
4
Department of
Nanoengineering,
University of California
San Diego, La Jolla,
California 92093, USA.
Correspondence to K.Z. e‑mail:
[email protected]
doi:10.1038/nrd3745
Published online 15 June 2012
Corrected online 29 June 2012
1
Visual impairment is one of the most feared disabilities.
The vast majority of information we acquire from the
environment is dependent on our ability to see. Loss of
sight affects activities of daily living and substantially
reduces quality of life. The leading causes of visual
impairment are age-related, and cause damage to the
retina and optic nerve. These causes include age-related
macular degeneration (AMD), diabetic retinopathy and
glaucoma (FIGS 1,2; TABLE 1).
Blindness and visual impairment represent a substantial burden to the health-care system. The total annual
economic impact of major visual disorders among
Americans aged 40 years or older has been estimated at
US$35.4 billion1. As the lifespan of our population continues to increase and advances in health care improve
longevity, there will be an increasing number of people
who are at risk of developing visual impairment, and the
economic impact of visual impairment will continue
to grow.
The unique features of the eye provide both benefits
and challenges for drug discovery and delivery (FIG. 1;
FIG. 2b). In contrast to other parts of the central nervous system, the eye is clinically accessible because it is
located outside the cranium. With new non-invasive
diagnostic technologies, such as optical coherence
tomography, the retina and optic nerve can be clearly
visualized and their macro- and microstructures evaluated in situ (FIG. 2a). The accessibility of these tissues also
facilitates drug delivery using methods such as eye drops
or local injections. Local delivery and exposure to a drug
minimizes systemic toxic effects and therefore increases
the therapeutic index.
In addition, there are many modalities available to
test visual function, thus allowing easy assessment of
drug effectiveness. However, despite these many bene­
fits there are also challenges related to ocular barriers.
For example, ocular barriers impede drug transport and
lower the efficacy of many drugs. Such barriers include
the blood–ocular barriers, blood flow, lymphatic clearance and tear dilution2.
During the past decade, there have been considerable
advances in the understanding of the pathogenesis and
genetics of ophthalmic diseases. Although there were no
effective treatments for wet AMD and retinal vascular
diseases 10 years ago, effective therapies such as antiangiogenic agents now exist. By contrast, latanoprost was
launched a decade ago for the treatment of glaucoma
(and soon became the first-line treatment) but no new
classes of glaucoma drugs have emerged since then.
Various drugs that act by inhibiting complement
activation and angiogenic pathways are currently under
development for the treatment of AMD, and neuroprotective drugs are being investigated for both AMD and
glaucoma3–6. However, the effective treatment of ocular
diseases remains an unmet medical need, and a vigorous effort to develop new drugs and therapies is now
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Retina (DR, RD)
Choroid (blood
vessel layer
under the retina)
Pupil
Light
Optic nerve:
goes to the
brain (glaucoma)
Cornea
(keratitis)
Lens
(cataract)
Macula (AMD, DME)
Figure 1 | Human eye structure and diseases. Schematic diagram of the anatomy of
the eye and associated diseases. AMD, age-related macular degeneration; DME, diabetic
macular oedema; DR, diabetic retinopathy; RD, retinal degeneration.
Nature Reviews | Drug Discovery
underway. In this Review we discuss the current state of
ocular drug therapy, highlight innovations within drug
discovery and delivery, and discuss potential concepts
for the future treatment of retinal diseases and glaucoma.
Diabetic retinopathy
A microvascular complication
of type 2 diabetes that is
characterized by increased
vascular permeability in the
initial stages of disease.
Glaucoma
A progressive disease that is
characterized by optic disc
cupping and peripheral visual
field loss.
Retinal pigment epithelium
(RPE). A monolayer of
pigmented cells in the
retina, located between
the photoreceptor cells
and choroid.
Macula
A highly pigmented area near
the centre of the retina that
is responsible for detailed
central vision.
Geographic atrophy
An advanced form of
age-related macular
degeneration that is
characterized by the atrophy
of retinal pigment epithelium
and photoreceptor cells.
Choroidal
neovascularization
(CNV). An advanced form
of age-related macular
degeneration that is
characterized by the growth
of abnormal blood vessels
into the subretinal space.
Ocular diseases
Age-related macular degeneration. AMD is defined as
an abnormality of the retinal pigment epithelium (RPE)
that leads to degeneration of the overlying photoreceptor in the macula and consequent loss of central vision7,8.
AMD usually occurs in individuals over 50 years of age,
and is the most common cause of vision loss in elderly
individuals throughout the developed world9–15. AMD
represents a major public health burden, with both economic and social consequences. It is estimated that over
9 million people in the United States suffer from intermediate or advanced forms of AMD16.
The aetiology and pathophysiology of AMD are
poorly understood. Histologically, the disease is characterized by the accumulation of membranous debris
underneath the RPE basement membrane, which forms
drusen. Drusen are small, yellowish extracellular deposits
of lipid, cellular debris and protein, including complement components, anaphylatoxins, modulators12,17–20,21–26
and high-temperature requirement A serine peptidase 1
(HTRA1)27. The formation of drusen can be caused by
RPE dysfunction or by a change in the composition or
permeability (to nutrients) of Bruch’s membrane.
Therefore, early AMD is characterized by drusen
(FIG. 2c) and hyper- or hypopigmentation of the RPE
without loss of vision. Advanced AMD leads to loss of
vision and can be classified into two categories: geographic
atrophy, which is characterized by atrophy of the RPE
(FIG. 2d) ; and choroidal neovascularization (CNV; also
known as wet AMD) (FIG. 2e), which is due to the abnormal growth of blood vessels.
It has been suggested that one or more of the following key processes is involved in AMD (FIG. 3): oxidative
damage; lipofuscin accumulation and impaired activity
or function of the RPE; increased apoptosis; abnormal
immune system activation28; senescent loss of homeostatic control29; or abnormalities in Bruch’s membrane.
Recent evidence suggests that there may be crosstalk
and interaction among these pathways30. For example,
oxidative stress, abnormal complement activation or
inflammatory pathways may lead to RPE apoptosis or
abnormal angiogenesis26,30,31.
Vascular endothelial growth factor A (VEGFA) has
been implicated in CNV and increased vascular permea­
bility that results in loss of vision32. VEGFA is a 46 kDa
homodimeric glycoprotein that was first identified in
vascularized tumours32–34. It is produced by many ocular
cell types in response to hypoxia and has multiple functions in the eye. It stimulates endothelial cell growth, promotes vascular permeability and induces dissociation of
tight junction components.
Animal models of AMD have contributed to the understanding of disease pathogenesis and the development of
treatments. Current models with features of dry AMD
include monkeys with naturally occurring drusen, mice
that have been genetically manipulated or have had high
exposure to environmental risk factors for AMD (for
example, a high-fat diet, exposure to cigarette smoke,
and so on), and mice that have been immunized with
carboxyethylpyrrole-modified mouse serum albumin35.
Animal models of wet AMD include primates and
rodents with laser-induced CNV, animals with mechanically induced CNV, rats that have been transfected with
angiogenic factors as well as transgenic and knockout
mice. Genetically modified mice are the only animal
models developed to date that recapitulate the features
of both dry and wet AMD35–40.
Ranibizumab (Lucentis; Genentech/Roche), bevacizumab (Avastin; Genentech/Roche) and aflibercept
(Eylea; Regeneron Pharmaceuticals) — all anti-VEGF
agents — are currently the most common therapies for
neovascular AMD. Other less commonly used treatments include photodynamic therapy, pegaptanib
(Macugen; Eyetech) and retinal laser photocoagulation.
Prophylactic treatments include oral vitamins and antioxidants (comprising vitamin C, vitamin E, β-carotene,
zinc and copper), and are used in patients who are at a
high risk of developing advanced AMD41.
Diabetic retinopathy and retinal vein-occlusive diseases.
Diabetic retinopathy is the leading cause of blindness
and visual impairment in working-age individuals. In
the United States, 4.1 million individuals have diabetic
retinopathy and, of these, approximately 900,000 have
sight-threatening retinopathy 42. These rates are expected
to double by 2025. Retinal vein-occlusive diseases
include central retinal vein occlusions, hemiretinal vein
occlusions and branch retinal vein occlusions. Retinal
vein-occlusive diseases affect approximately 100,000
individuals in the United States.
Diabetic retinopathy and retinal vein-occlusive diseases result in loss of vision through the following major
mechanisms: retinal vascular leakage and exudation,
which results in macular oedema; and retinal ischaemia
and secondary neovascularization, which can lead to
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vitreous haemorrhage and retinal detachment. VEGFA
has been implicated in increasing retinal vascular permeability and leakage, which leads to macular oedema
and ocular neovascularization34,43. Current treatments
for diabetic macular oedema or macular oedema arising
from retinal vein-occlusive diseases include anti-VEGF
therapies, focal laser therapy and steroids. Laser panretinal photocoagulation is the main therapy for neovascularization in proliferative diabetic retinopathy and
retinal vein-occlusive diseases.
Glaucoma. Glaucomas are a group of progressive optic
neuropathies that are characterized by a slow and progressive degeneration of retinal ganglion cells (RGCs)
and their axons, resulting in a distinct appearance of the
optic disc (FIG. 2f) and a concomitant pattern of vision
loss6. Characteristic changes to the optic nerve in glaucoma include the enlargement and elongation of the
optic nerve cup, the thinning and eventual notching of
the neuroretinal rim, asymmetry in cup size between the
two eyes and disc haemorrhages. Changes in the visual
field with perimetric testing include scotomas that correspond to local and/or diffuse loss of nerve fibres within the
retina. These characteristic changes consist of arcuate scotomas, nasal steps and paracentral scotomas. The disease
involves the entire visual pathway, including the brainstem
and the visual cortex — not just the eye44.
It has been estimated that glaucoma will affect more
than 80 million individuals worldwide by 2020, with
at least 6–8 million individuals becoming bilaterally
blind45. Glaucoma is also the leading global cause of irreversible blindness46 and is perhaps the most prevalent of
all neurodegenerative diseases.
The biological basis of glaucoma is not fully understood, and the factors contributing to its progression
are currently not well characterized. Intraocular pressure
(IOP) is the only proven treatable risk factor, and lowering IOP is the main approach for reducing disease progression. Impaired ocular blood flow is also thought to
be a risk factor 47 but the evidence for this is conflicting,
perhaps largely owing to the absence of an accurate and
reproducible test of ocular blood flow or the optic nerve
microcirculation. With or without treatment, glaucoma
can progress through a continuum48 that is initiated
by an acceleration of RGC apoptosis, with the disease
initially being asymptomatic but eventually resulting in
blindness in some individuals, particularly those who are
inadequately treated6.
Intraocular pressure
(IOP). The fluid pressure inside
the eye, which is determined
by the production and
drainage of aqueous humour.
Aqueous humour
A clear, watery fluid produced
by the ciliary epithelium that
fills the anterior and posterior
chambers of the eye.
Current status of ocular drugs
Anti-VEGF therapy. Over the past decade, considerable
progress has been made in the treatment of ocular neovascular diseases owing to an increased understanding
of the mechanisms of ocular angiogenesis32,33 (FIG. 4).
Anti-angiogenic therapies targeting VEGFA have proven
to be highly effective in treating neovascular AMD and
have become the mainstay of treatment. Anti-VEGF
treatment is the only therapy to date that improves vision
in patients with neovascular ocular diseases, as demonstrated by several randomized, controlled Phase III
studies49–51.
Ranibizumab, a recombinant humanized antibody
fragment that binds all known isoforms of human
VEGFA, is formulated for ocular administration (via an
intravitreal injection) and approved in the United States
for the treatment of neovascular AMD. Bevacizumab,
which was initially approved for the treatment of certain types of cancers, has also been used (off-label) to
treat wet AMD52 but it is not formulated for intraocular
administration. A recent study compared the safety
and efficacy of these two drugs in a multicentre, randomized controlled trial51. The primary results of the 1‑year
follow-up study showed that monthly administration of
bevacizumab is non-inferior to monthly administration
of ranibizumab. However, there were some safety concerns in the trial; a statistically higher number of serious
adverse events was observed in the bevacizumab group51
(mainly hospitalizations, perhaps owing to longer systemic
retention of bevacizumab compared to ranibizumab).
Another therapeutic approach is the use of aflibercept,
which is a fusion protein that incorporates portions of
extracellular domains of the human VEGF receptor 1
(VEGFR1) and VEGFR2 that are fused to the constant
region of human immunoglobulin G53,54. Aflibercept
binds to all isoforms of VEGFA with a higher affinity
than ranibizumab and bevacizumab. It also binds placental growth factor 1 and placental growth factor 2 (REF. 55).
Two Phase III trials using aflibercept (as an intravitreal
injection) to treat wet AMD have been completed and
showed equivalent efficacy to ranibizumab. The drug
was well tolerated and approved by the US Food and
Drug Administration (FDA) for the treatment of wet
AMD in 2011. The application of anti-VEGF agents
in the treatment of retinal vascular diseases, including
diabetic retinopathy and retinal vein occlusions, has
been very successful56–58 but ranibizumab has only been
approved by the FDA for treating retinal vein-occlusive
diseases.
IOP-lowering therapies. There are two major forms of
glaucoma: open-angle glaucoma and closed-angle glaucoma. Both forms are initially treated by lowering IOP,
typically with eye drops, but rapid advancement to surgical treatment is required for some types of closed-angle
glaucoma.
IOP is determined by the balance between the aqueous
humour secreted into the eye by the ciliary body and the
amount that is drained from the eye via the two major
outflow pathways: the trabecular meshwork and the
uveo­scleral pathway (consisting of the iris root, longitudinal ciliary muscle, anterior choroid and sclera). IOP is
in a steady state when the rate of aqueous inflow is the
same as the rate of aqueous outflow.
Pharmacological treatment of glaucoma lowers IOP
by decreasing the rate of aqueous inflow and/or increasing the rate of aqueous outflow (FIG. 5a). There are five
major classes of drugs (administered as eye drops) that
are approved for lowering IOP in glaucoma (TABLE 2).
Cholinergic agents have been used for more than
100 years to lower IOP. They act on muscarinic receptors located on the ciliary muscle to increase outflow
through the trabecular meshwork59. Although the most
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a
RPE
Bipolar cell
Choroid
Retina
Cone Rod
Choroid
Ganglion cell
Horizontal cell
Vitreous
Vitreous
Amacrine cell
Bruch’s membrane
b
Retina
RPE
Choroid
Fovea
ON
Fovea
Fovea
Retina
RPE
Choroid
Macula
c
d
e
f
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Figure 2 | Retinal anatomy and structure in health and diseases. Imaging of the
structures of an eye by optical coherence tomography. a | A schematic diagram of
the retina is shown at the top of the panel. The inset shows the choroid and layers
of the retina, including the retinal pigment epithelium (RPE). Optical coherence
tomography images are also shown, of a normal retina (top) and a retina with pigmented
epithelium detachment due to choroidal neovascularization (CNV) (indicated by the
white arrows) in wet age-related macular degeneration (AMD) (bottom). b | The image
shown is of a normal retina and macula. c | The image depicts a macula with confluent
soft drusen (indicated by the white arrows) — a hallmark of early AMD. d | The image
shows a macula of geographic atrophy (within the dashed circle). e | A macula of CNV
with haemorrhage (indicated by the white arrows) is shown. f | The image illustrates an
optic nerve with glaucomatous excavation and loss of neuroretinal rim between the
disc margin (white dashed lines) and the border of the optic cup (black dashed lines).
Images courtesy of K.Z. ON, optic nerve.
Complement system
A part of the innate immune
system that consists of
approximately 25 proteins.
Three pathways activate
the complement system: the
classical complement pathway,
the alternative complement
pathway and the
mannose-binding pathway.
widely used drug in this class — pilocarpine — is inexpensive and has few systemic side effects, its widespread
use is largely limited by ocular side effects (such as miosis, myopia, brow ache and dimming of vision) and the
inconvenience of requiring dosing four times daily 59,60.
Owing to their convenient once-daily dosing, prostaglandin F receptor (PTGFR) analogues are generally
administered as first-line agents and increase aqueous
outflow by altering the composition of the extracell­
ular matrix in the ciliary muscle and trabecular meshwork60,61. These agents are the most effective of the
IOP-lowering agents, and a single daily dose is effective at reducing IOP throughout the day and during the
night 62. PTGFR analogues cause lengthening and thickening of eyelashes and can also cause hazel and blue
irides with brown patches to become diffusely brown.
β-adrenergic receptor blockers have been widely
used for more than 30 years and have various strengths,
including their ocular tolerability and IOP-lowering efficacy (which is surpassed only during the day by PTGFR
analogues). They reduce aqueous humour secretion by
inhibiting the activity of the predominant β-adrenergic
receptor in the ciliary epithelium63. However, they are
only effective during the day and not during the night 62,
and can reduce ocular perfusion pressure. They have
minimal efficacy in combination with PTGFR analogues, and for this reason the use of these agents in
a combination product has not been approved in the
United States.
Another approach is the use of carbonic anhydrase
inhibitors to inhibit the activity of carbonic anhydrase 2
in the non-pigmented ciliary epithelium, thus reducing aqueous humour formation. These drugs lower
Table 1 | Leading causes of irreversible visual impairment
Disease
Incidence
Cataract
8.4–29.7% of patients over 43 years of age
Refs
183
Age-related macular
degeneration
6–22% of patients over 70 years of age
184
Glaucoma
1–4% of patients over 45 years of age
6
Diabetic retinopathy
74.9–92.3% of diabetic patients over
30 years of age
185,186
Retinitis pigmentosa
1 in 4,000
187
IOP throughout the day and during the night, and are
widely used as second-line agents64. α-adrenergic receptor agonists lower IOP primarily through stimulation of
α2‑adrenergic receptors in the eye. Like the beta blockers, they effectively lower IOP during the day but not
during the night 65. They appear to decrease aqueous
inflow and increase uveoscleral outflow66. In experimental animal models they have a neuroprotective effect on
RGCs67 but this has not been demonstrated convincingly
in patients with glaucoma.
Ongoing clinical trials
Complement pathway inhibitors. Recent studies have
shown that AMD is associated with certain genotypic
variants of complement pathways, including complement factor H (CFH)21,68–70, complement component 2
(C2)71, complement factor B71,72, complement component 3 (C3)72,73 and complement factor I74. Owing to
multiple gene associations in the complement activation pathway, several complement inhibitors are now in
clinical trials. Various strategies have been exploited to
treat AMD by modulating the complement system, such
as the replacement of defective complement components
as well as inhibition of the complement pathway (FIG. 6).
Inhibition of the complement pathway disables downstream complement cascade activation, thus hindering
inflammatory responses. However, the complement
system is an important component of the immune system, and local or systemic complement inhibition may
increase the risk of infections such as endophthalmitis.
POT‑4 (also known as AL‑78898A) is a synthetic
peptide C3 inhibitor that binds reversibly to C3. An
appealing feature of this intravitreal drug is its gel-like
preparation, which can be deposited in the vitreous
at high concentrations and can last for 6 months as a
sustained-delivery vesicle. A Phase I study of POT‑4 in
patients with wet AMD was completed successfully without any safety concerns74. A Phase II study of AL‑78898A
in conjunction with ranibizumab is currently underway.
ARC1905 is a complement component 5 (C5) inhibitor that is administered by intravitreal injection. As a
small-molecule aptamer-based drug it exhibits several
advantages, including low activation of the immune
response and the ability to formulate it in a sustainedrelease form. A Phase I trial investigating the efficacy of
ARC1905 in combination with ranibizumab for treating
wet AMD was performed and the agent was shown to be
safe and well tolerated75. Another Phase I study investigating the efficacy of ARC1905 in dry AMD is underway.
Eculizumab is a monoclonal antibody derived from a
murine anti-human C5‑specific antibody. Like ARC1905,
it is also intravenously administered and inhibits the cleavage of C5 to C5a and C5b. Eculizumab is already approved
by the FDA for the treatment of paroxysmal nocturnal
haemoglobinuria and is currently in Phase II trials known
as COMPLETE (‘complement inhibition with eculizumab
for the treatment of non-exudative age-related macular
degeneration’) for the treatment of dry AMD.
FCFD4514S is a recombinant, humanized monoclonal antibody Fab fragment directed against complement
factor D, currently in a Phase I/II study to evaluate its
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REVIEWS
Chronic oxidative stress
Antioxidants
Visual cycle
inhibitors
Lipofuscin
+
Photoreceptor–RPE–Bruch’s
choriocapillaris complex
Anti-inflammatory
agents
+
Chronic inflammation
+
+
Abnormal complement activity
Accumulation of extracellular and intracellular debris
Drusen formation, damage to photoreceptors and RPE
Geographic atrophy
Neovascularization
Neuroprotectants
Anti-angiogenic agents
Figure 3 | Proposed pathophysiology of AMD, and locations in the pathway in
which different therapeutic interventions might be effective. One or more of the
Nature macular
Reviews degeneration
| Drug Discovery
following key processes is likely to have a role in age-related
(AMD): oxidative damage; lipofuscin accumulation and impaired function of retinal
pigment epithelium (RPE); increased cell death; abnormal immune system activation;
senescent loss of homeostatic control; and abnormalities in Bruch’s membrane.
VEGF, vascular endothelial growth factor. Potential strategies for the treatment of
AMD are shown in orange boxes.
safety, tolerability, pharmacokinetics and immunogenicity when intravitreally injected in the treatment of
geographic atrophy.
Small interfering RNA
(siRNA). Small,
double-stranded RNA
molecules that interfere with
gene expression by binding
to and promoting the
degradation of mRNA.
Visual cycle inhibitors. A prominent feature of AMD is
the aberrant accumulation of cellular debris or lipofuscin
within the RPE. Lipofuscin is a product of autofluorescent material composed of lipids, proteins and vitamin A
derivatives76–78. The rationale for the use of visual cycle
modulators is to reduce the accumulation of fluorophores (for example, A2E and lipofuscin) in RPE cells
(FIG. 3). Retinol is essential for vision, and its metabolism
is mediated through the visual cycle (see the Webvision
website). Retinol binding protein (RBP) binds to all-transretinol with a high affinity. The retinol–RBP complex
serves as a substrate for the uptake of retinol by the RPE
within the eye. Two visual cycle inhibitors are currently
in clinical trials.
N‑(4‑hydroxyphenyl)-retinamide (Fenretinide; Sirion
Therapeutics) is an oral synthetic retinoid derivative that
binds to RBP in the circulation and blocks its association
with retinol, thereby preventing the transport of retinol
to the RPE and lowering its availability in the visual
cycle. Fenretinide reduces lipofuscin and A2E accumulation in the RPE of mice in which ATP-binding cassette subfamily A member 4 (ABCA4) is knocked out
(ABCA4‑knockout mice) and causes modest delays in
dark adaptation79. A Phase II clinical trial of this oral
agent has been completed, and the drug has been shown
to slow the progression of geographic atrophy and reduce
the incidence of CNV. Potential side effects include night
blindness and dry eye.
ACU‑4429 is another orally administered visual cycle
modulator. It inhibits the conversion of all-trans-retinyl
ester to 11‑cis-retinol via inhibition of the isomerase
retinoid isomerohydrolase (RPE65). It was shown to
slow retinal degeneration in preclinical models. However,
side effects of this drug include nyctalopia, dyschroma­
topsia and delayed dark adaptation. By modulating
isomerization, ACU‑4429 slows the visual cycle in rod
photoreceptors and decreases the accumulation of A2E.
A Phase II study of ACU‑4429 is currently underway
for the treatment of patients with AMD who have
geographic atrophy.
Anti-angiogenic agents. Because ocular angiogenesis is a
multistep process (FIG. 4), it can be intercepted or inhibited at many steps using various approaches, and several
agents are now under investigation (TABLE 3). The various
strategies and related agents are discussed below.
One strategy is to inhibit mammalian target of rapamycin (mTOR), which is a tyrosine kinase that has a
key role in regulating cell growth and proliferation. Its
activation leads to the production of hypoxia-inducible
factors (for example, HIF1α), which in turn can induce
the expression of the angiogenesis stimulator VEGF80.
Examples of mTOR inhibitors include rapamycin (also
known as sirolimus), everolimus and Palomid 529.
Sirolimus is currently in Phase II clinical trials for the
treatment of patients with diabetic macular oedema,
and everolimus in combination with ranibizumab is in
Phase II trials in patients with wet AMD.
Small interfering RNA (siRNA) is another approach that
is used to inhibit VEGF expression. PF‑655 is a synthetic
siRNA that inhibits the expression of DNA damageinducible transcript 4 protein (DDIT4; also known
as RTP801), which has been shown to be involved
in pathological retinal neovascularization in animal
models81. It is currently in Phase II trials for diabetic
macular oedema and wet AMD. Bevasiranib is a naked,
21‑nucleotide-long siRNA that specifically targets VEGF.
However, clinical trials of bevasiranib were discontinued
because they did not meet their end point (stabilization of vision), which was defined as a loss of less than
three lines of vision on the ETDRS (Early Treatment for
Diabetic Retinopathy Study) vision chart. However, in
hindsight, the failure of these trials is not surprising as
siRNAs need to be formulated to achieve cell permeation in order to cause bona fide RNA interference. They
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• Sirolimus
• RAD001
• Palomid 529
• PF4523665
Smooth
muscle cell
Angiogenic
factor
production
Release
PEDF
• Aflibercept
• Ranibizumab
• Bevacizumab
Sirna-027
Binding to
endothelial
cell receptor
Sonepcizumab
Tube
formation
Directional
migration
• JSM6427
• Volociximab
Endothelial cell proliferation
Endothelial
cell
Blood vessel
Fosbretabulin
α5β1 integrin
Intracellular
signalling
E10030
Pazopanib
Vascular
stabilization
Figure 4 | Mechanism of action of ocular angiogenesis inhibitors in clinical care or development. Schematic
diagram of new capillary blood vessel growth. Several classes of angiogenic factors, including
vascular
endothelial
growth
Nature
Reviews
| Drug Discovery
factor (VEGF), platelet-derived growth factor (PDGF) and pigment epithelium-derived factor (PEDF), bind to specific
receptors on vascular endothelial cells to induce specific signalling pathways that activate the cells. Angiogenesis can be
inhibited by blocking these signalling pathways as well as through various other approaches. Tyrosine kinase inhibitors
(sirolimus, RAD001, Palomid 529 and PF4523665) prevent the production of angiogenic factors. Bevacizumab, ranibizumab
and aflibercept bind to and sequester VEGF, thereby inhibiting its interaction with VEGF receptors. Sirna‑027 inhibits the
synthesis of VEGF receptor 1. JSM6427 and volociximab both bind to α5β1 integrin and block the migration of endothelial
cells. The active metabolite of fosbretabulin (combretastatin A4) inhibits microtubule polymerization in proliferating
endothelial cells, inducing vessel regression.
also need to be modified to avoid off-target effects and
recognition of their nucleotide structure by the innate
immune system82–84.
In contrast to VEGF, pigment epithelium-derived
factor (PEDF) has an antagonistic effect on endothelial cell proliferation and migration, and its levels are
decreased in AMD85–87. A Phase I clinical trial has been
initiated using an adenoviral vector (AdGVPEDF.11D)
to deliver PEDF via intravitreal injections. Many agents,
such as ranibizumab or bevacizumab, have now been
developed to inhibit the binding of VEGF to its receptor.
Aflibercept is another example of an agent that prevents
receptor binding by trapping VEGF in the extracellular space. AAV2‑sFLT01 is an adeno-associated virus
(AAV)88 that produces a soluble VEGFR1 that can bind
to free VEGF89,90 to interrupt VEGF signalling.
VEGFA binds primarily to VEGFR1 and VEGFR2, both
of which are tyrosine kinases91. A siRNA (AGN211745)
designed to inhibit VEGFR1 synthesis has been tested in a
Phase II trial, but this was terminated presumably because
AGN211745 was not formulated for cell permeation.
Another approach that is currently undergoing testing
involves the blockade of chemokine CC motif receptor 3, which mediates angiogenic signalling initiated by
eotaxins92.
VEGF binds and activates its receptors VEGFR1,
VEGFR2 and VEGFR3. Phosphorylation of the tyrosine
residue of the kinase domain of VEGFRs is crucial for
receptor activation. Small-molecule tyrosine kinase inhibitors therefore have considerable potential in targeting
the kinase domain and blocking the action of VEGFR1,
VEGFR2 and VEGFR3. Such inhibitors are usually also
active against platelet-derived growth factor receptor
(PDGFR), KIT and fibroblast growth factor receptor 1
(FGFR1). Some non-specific tyrosine kinase inhibition
and cross-inhibition may be beneficial. As these growth
factors are also involved in ocular angiogenesis, they can
potentially provide synergistic effects on CNV inhibition.
Pazopanib is a tyrosine kinase inhibitor that inhibits
VEGFR1, VEGFR2, VEGFR3, PDGFR, KIT and FGFR1.
Pazopanib is effective in preclinical models of CNV93.
It is administered topically and has shown promising
results in a Phase II trial for CNV94. Vatalanib is an oral
tyrosine kinase inhibitor that has been tested previously
as a treatment for CNV in Phase I and Phase II clinical
studies. AL39324 is an intravitreally administered tyro­
sine kinase inhibitor that is in a Phase II clinical trial in
combination with ranibizumab.
TG100801 is a tyrosine kinase inhibitor that binds
and inhibits VEGFR and PDGFR. It is administered as
an eye drop and has been found to suppress CNV and
retinal oedema95. A Phase I trial was completed successfully but a Phase II trial was discontinued because of
corneal toxicity.
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Lens
Anterior chamber
Cornea
Iris
Inflow
• β-adrenergic receptor
blockers
• α-adrenergic receptor
agonists
• Carbonic anhydrase
inhibitors
Trabecular meshwork outflow
• Cholinergic agents
• Latrunculins
• ROCK inhibitors
• siRNA β-adrenergic
receptor antagonist
Uveoscleral outflow
• Prostaglandin analogues
• α-adrenergic receptor agonists
• Prostaglandin EP2 agonists
• Nitric oxide-donating
prostaglandin F2α analogue
• Drug-eluting punctal plug
with latanoprost
Ciliary processes
Ciliary muscle
Sclera
Current treatments
Investigational drugs
Figure 5 | Agents that lower intraocular pressure. Current and investigational drugs
for lowering intraocular pressure, decreasing aqueous inflow
and/or
increasing
aqueous
Nature
Reviews
| Drug Discovery
outflow are indicated on the figure. Aqueous humour is secreted into the posterior
chamber by the ciliary processes and eventually circulates into the anterior chamber
angle, where it drains from the eye via one or both outflow pathways: the trabecular
meshwork or the uveoscleral outflow route. ROCK, RHO-associated protein kinase.
An alternative approach to inhibiting the action of
VEGF involves preventing the activation of the nicotinic
acetylcholine receptor (nAChR) pathway in the vasculature, which inhibits VEGF-induced angiogenesis in
endothelial cells96. ATG‑3 is an eye drop formulation of
mecamylamine, an antagonist of the nAChR pathway 97.
It is the first anti-angiogenic eye drop that has been studied in humans. ATG‑3 has successfully completed two
Phase II clinical trials, one in patients with diabetic macular oedema98 and the other in patients with wet AMD
who were receiving anti-VEGF treatments.
Sphingosine-1‑phosphate (S1P) is the extracellular ligand for S1P receptor 1 (also known as EDG1), a
G protein-coupled lysophospholipid receptor, and it is a
pro-angiogenic and profibrotic mediator 99,100. Intra­
vitreous injection of an anti‑S1P monoclonal antibody
inhibited CNV formation and subretinal collagen deposition in a preclinical model of laser-induced CNV and in a
preclinical model of diabetic retinopathy 101,102. A humanized S1P monoclonal antibody (iSONEP) has completed
a Phase I clinical trial in patients with wet AMD. iSONEP
was well tolerated, and no drug-related serious adverse
events were observed in any of the patients. iSONEP
also exhibited positive biological effects (including lesion
regression, reduction of retinal thickness and resolution
of pigment epithelium detachment). The mechanisms
responsible for the positive effects exerted by S1P appear
to be independent to those of anti-VEGF agents, indicating the potential of S1P antibodies to serve as a monotherapy (or an adjunct therapy to anti-VEGF agents) for
the treatment of neovascular AMD.
Endothelial cell migration involves interactions
between integrins (transmembrane heterodimeric proteins) and extracellular matrix ligands. For example,
α5β1 integrin is expressed on the surface of vascular
endothelial cells and mediates cell migration103. JSM6427
and volociximab both bind to α5β1 integrin and block
the migration of endothelial cells. Both agents have
been shown to inhibit CNV formation in preclinical
models104,105.
Fosbretabulin is a novel vascular disrupting agent,
and its active metabolite is combretastatin A4 (CA4)106.
CA4 binds to tubulin and prevents microtubule poly­
merization in proliferating endothelial cells, thus inducing vessel regression107. In animal models, CA4 has been
demonstrated to inhibit retinal neovascularization and
suppress the development of CNV108,109. Phase II trials
are underway to evaluate the safety and efficacy of fosbretabulin in patients with CNV and vasculopathy.
Pericyte recruitment is a crucial step in vascular
maturation and stabilization. PDGF has a key role in this
process110,111. With this in mind, E10030 — a pegylated
aptamer that inhibits PDGF-β — was developed. Inhibition
of PDGF results in the stripping of pericytes from endothelial cells, which increases the sensitivity of mature CNV
to VEGF inhibition. PDGF inhibition may therefore synergize with VEGF inhibition in CNV treatment. Indeed,
in a Phase I dose-escalating study, some CNV regression
was observed in over 90% of patients receiving the combination therapy of E10030 and ranibizumab, compared
to about 10% of patients receiving ranibizumab alone112.
A Phase II study is currently underway.
Pharmacologic vitreolysis. Pharmacological interventions for retinal diseases, such as macular oedema and
macular holes, that are associated with the vitreoretinal
interface (VRI) may soon include one for pharmacologic
vitreolysis. The goals of an intravitreal pharmacologic vitreolysis agent are to cleanly cleave the VRI at the internal
limiting membrane, removing the potential for fibro­
cellular traction, and to liquefy the vitreous by altering
its molecular organization and structure203.
Ocriplasmin (ThromboGenics, Belgium) is currently
being investigated as a pharmacologic vitreolysis agent.
A stable, truncated form of human plasmin retaining proteolytic activity, ocriplasmin (molecular weight 27.2 kDa)
is manufactured using recombinant DNA technology204.
Preclinical studies suggest that ocriplasmin targets substrates that are important to both the vitreous structure
and the VRI, including collagen fibrils, fibronectin and
laminin204. Ocriplasmin was recently studied in two
Phase III pivotal clinical trials as a single intravitreal
injection for treatment of patients with symptomatic
vitreomacular adhesion (VMA), including macular holes.
Results from both trials indicated that treatment with
ocriplasmin met the primary end point of VMA resolution as well as secondary end points, including visual
acuity improvement and closure of full-thickness macular holes in a subset of patients with VMA when compared to placebo. Most adverse events were considered
mild and were either transient or temporary in nature,
with a majority occurring within the first 7 days after
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Table 2 | Available classes of IOP-lowering eye drops for glaucoma
Class
Drugs
Mechanisms of action
Notes
Aqueous humour outflow (conventional pathway)
Cholinergic drugs
Pilocarpine,
carbachol
•Increase trabecular
meshwork outflow through
ciliary muscle contraction59
•Highly effective but their use
is compromised by dim vision
(from small pupil) and discomfort59,60
Aqueous humour outflow (uveoscleral pathway)
Prostaglandins
PGF2α analogues :
latanoprost,
travoprost,
bimatoprost
and tafluprost
•Increase outflow by
•Highly effective and well tolerated:
increasing matrix
generally used as first-line therapy62
metalloproteinase expression
and altering the extracellular
matrix in the ciliary muscle
and trabecular meshwork61,192
Aqueous humour formation
β-adrenergic
receptor blockers
Timolol, betaxol,
carteolol and
levobunolol
•Decrease inflow by regulating •Few ocular side effects but occasional
aqueous humour formation in
systemic effects include fatigue and
the ciliary processes63
bradycardia62
α-adrenergic
receptor agonists
Apraclonidine and
brimonidine
•Decrease inflow by
inactivating adenylyl cyclase
in ciliary processes and
mediating noradrenaline
release via activation of
pre-synaptic α2-adrenergic
receptors; may also increase
uveoscleral outflow65
Carbonic anhydrase
inhibitors
Dorzolamide,
•Decrease aqueous humour
brinzolamide,
formation by inhibiting
acetazolamide* and
carbonic anhydrase and
methazolamide*
HCO3 production in the
ciliary epithelium64
•Sedation is minimized by occluding
nasolacrimal tear duct for up to
2 minutes
•These drugs have also been suggested
to have neuroprotective effects67
•Allergy is not uncommon, particularly
with the 0.2% formulation
•Eye drops are better tolerated than
orally administered drugs193
Combination of existing classes of IOP-lowering drugs
β-adrenergic
receptor blockers
plus carbonic
anhydrase inhibitors
Timolol and
brinzolamide
Decrease inflow196
–
β-adrenergic
receptor blockers
plus α-adrenergic
receptor agonists
Timolol and
brimonidine
Decrease inflow and increase
outflow197
•Combination product has less
hyperaemia than brimonidine alone
β-adrenergic
receptor blockers
plus prostaglandins
Timolol and PGF2α
analogues
Decrease inflow and increase
outflow198–202
•Not available in the United States
IOP, intraocular pressure; PGF2α, prostaglandin F2α receptor. *Available as oral medications.
treatment205. Ocriplasmin is currently under review by the
FDA. Other indications may include: vitreous liquefaction
and lysis to facilitate complex surgical cases such as paediatric vitrectomy, proliferative vitreoretinopathy and proliferative diabetic retinopathy; relief of vitreous traction on
retinal breaks in pneumatic retinopexy; and treatment of
macular oedema due to vitreomacular traction.
Novel IOP-lowering drugs. In addition to the five major
drug classes currently being used to lower IOP (FIG. 5),
several novel drugs are being evaluated in clinical trials
(FIG. 5; TABLE 4) but results of these studies have not been
published. Approved ocular hypotensive drugs with new
delivery systems, including drug-eluting punctal plugs
(for example, with a prostaglandin analogue), are also
being investigated for sustained release and to improve
patient adherence. Existing drug classes can be separated
into those that lower habitual IOP throughout the day
and during the night when administered topically (such
as prostaglandin analogues62,113 and carbonic anhydrase
inhibitors64), and those that are effective only during the
day and not during the nocturnal period (such as beta
blockers64 and α-adrenergic receptor agonists65). The
relationship of IOP lowering (throughout the day and
during the night) to glaucoma progression is unknown
and under investigation114.
The latrunculins are macrolides from marine sponges
that inhibit actin polymerization115. Studies in nonhuman primates and post-mortem analyses of eyes from
patients have demonstrated that latrunculins increase
trabecular meshwork outflow via a novel mechanism that
disrupts the actin cytoskeleton in the trabecular meshwork116. However, their performance in clinical trials has
so far been marginal and they have poor solubility. It has
been suggested that their efficacy may be improved by
changes in their delivery system117.
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Classical pathway
IgM, IgG immune
complexes or CRP
Lectin pathway
Alternative pathway
Mannose-binding
lectin complex
Endogenous or
foreign pathogens
C3 (H2O)
C1
CFB
CFH
C1 complex
+
+
C2
CFI
C3 (H2O)-B
CFI
+ C4
C3
C3 convertase (C4b2a)
C3b
C5
C3a
C6, C7, C8 and C9
C5 convertase
C5a
C5b
Terminal membrane attack complex
Figure 6 | Complement pathways, their association with AMD and drug targets.
Schematic diagram of classical, alternative and lectin complement activation pathways.
Circled proteins have corresponding gene variants that are associated with age-related
Nature
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| Drug Discovery
macular degeneration (AMD). C1, complement component
1; CFB,
complement
factor B; CFH, complement factor H; CFI, complement factor I; CRP, C-reactive protein;
IgG, immunoglobulin G.
RHO-associated protein kinase (ROCK) is a downstream effector of RHO (a small guanine triphosphatase)
in the RHO-dependent signal transduction pathway.
ROCK inhibitors induce changes in the actin cytoskeleton and the cellular motility of the outflow pathways,
and they can also lower IOP118. Several drugs in this
class have been tested in clinical trials. However, it is
still unknown whether the efficacy of these agents will
outweigh their side effects, so the use of this class of
compounds may be limited by conjunctival hyperaemia
and subconjunctival haemorrhages119,120. One strategy
for addressing these side effects is to use a pro-drug that
is converted into a more active compound once it passes
through the cornea and is present in the anterior chamber 121. Such an example is ATS907; the pro-drug form
of ATS907 is less active and highly permeable across the
cornea. After it passes through the cornea, ATS907 is
then converted into a more active ROCK inhibitor in
the aqueous humour of the anterior chamber. ATS907
is currently in Phase I/II clinical trials.
Other ROCK inhibitors that are currently in Phase II
glaucoma trials include AR‑12286 and K‑115. AR‑12286
is currently in a Phase II clinical trial to evaluate its
24‑hour efficacy in patients with open-angle glaucoma
or ocular hypertension. K‑115 has completed two
Phase II clinical trials, which demonstrated the ability
of the compound to lower IOP in patients with primary
open-angle glaucoma or hypertension122,123. Amakem
also has a ROCK inhibitor (AMA0076) in preclinical
development. It is currently unknown whether ROCK
inhibitors will be effective when used only once a day or
whether they will require administration at least twice a
day. In addition, it has not yet been determined whether
ROCK inhibitors will be effective at lowering IOP during the night or whether they will be used as primary
— rather than adjunctive — therapy.
Drugs in this class are also being experimentally
evaluated for their neuroprotective potential124. The only
ROCK inhibitor that is approved and clinically used is
fasudil (Eril; Asahi Kasei). Fasudil is used in Japan for
neuroprotection in subarachnoid haemorrhage. ROCK
inhibitors can also be useful in trabeculectomy surgery
to reduce fibrosis125. Another potential use of ROCK
inhibitors is as anti-inflammatory agents, which may
be beneficial at ameliorating ocular surface disease and
inflammation.
Other classes of drugs are also being studied for
their ability to lower IOP. Adenosine receptor agonists
increase outflow by increasing trabecular meshwork
outflow 126. Several drug companies are pursuing strategies involving modulation of the adenosine receptor
pathway by targeting different receptors. Examples of
such drugs include ATL313 (an adenosine A2A receptor
agonist), INO8875 (an adenosine A1 receptor agonist)
and OPA‑6566. However, these drugs may have limited
use owing to desensitization of their receptors.
The angiotensin II type 1 receptor antagonist olmesartan is being tested in Japan for its ability to lower IOP by
increasing outflow 127. Cannabinoid receptor agonists128
are also being tested. The evidence for their use in glaucoma is based on the observation that smoking marijuana
transiently lowers IOP129. In a non-human primate model
of glaucoma, topical application of a cannabinoid receptor
agonist that is selective for cannabinoid receptor 1 lowered IOP by decreasing aqueous humour flow 128. A cannabinoid receptor-selective agonist that lowers IOP might
be better tolerated and have a longer ocular hypotensive
duration of action than marijuana.
SYL040012, a siRNA targeting the β2‑adrenergic
receptor, is currently in clinical trials to investigate its
effect on IOP in patients with ocular hypertension130.
PF‑04217329 is a selective agonist of prostaglandin E
receptor 2 that has completed Phase II clinical trials.
The results showed that PF‑04217329 significantly
reduces IOP in patients with primary open-angle glaucoma and ocular hyper­tension, but doses higher than
0.015% were associated with a higher incidence of photophobia and iritis131,132.
BOL‑303259‑X (latanoprostene bunod) is a nitric
oxide-donating prostaglandin F2α analogue. A Phase II
clinical trial is currently underway to compare the
safety and efficacy of BOL‑303259‑X with latanoprost
in patients with glaucoma or ocular hypertension. The
L-PPDS (Latanoprost Punctal Plug Delivery System)
study 133 by QLT recently completed a Phase II clinical
trial and latanoprost was found to be well tolerated;
adverse events observed were similar to those reported
for punctal plugs (for example, tearing). In addition, the
L‑PPDS was shown to significantly lower IOP133.
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Table 3 | Anti-angiogenic drugs in clinical trials
Drug
Company
Description
Target
Clinical
phase
ClinicalTrials.gov
identifiers
Sirolimus (also
known as
rapamycin)
MacuSight/
Santen
Tyrosine kinase
inhibitor
mTOR
Phase I/II
NCT01445548
Phase I/II
NCT00766649
Phase II
NCT00656643
RAD-001
(everolimus)
Novartis
Tyrosine kinase
inhibitor
mTOR
Phase II
NCT00304954
Palomid 529
Paloma
Tyrosine kinase
inhibitor
mTOR
Phase I
NCT01033721
Phase I
NCT01271270
PF‑655 (also
known as
PF‑04523655)
Pfizer
Synthetic siRNA
DDIT4
Phase II
NCT01445899
AdGVPEDF.11D
GenVec
Adenovirus vector
containing PEDF
Endothelial cells
Phase I
NCT00109499
(completed)
AAV2-sFLT01
Genzyme
Adeno-associated
virus (AAV) vector
carrying VEGFR
VEGF
Phase I
NCT01024998
Pazopanib
GlaxoSmithKline
(United States)
Tyrosine kinase
inhibitor
VEGFRs
Phase II
NCT01362348
Phase II
NCT01134055
PTK787
Novartis
Oral tyrosine kinase
inhibitor
VEGFRs
Phase I
NCT00138632
(completed)
AL-39324
Alcon
Tyrosine kinase
inhibitor
VEGFRs
Phase II
NCT00992563
(completed)
ATG‑3
CoMentis
Eye drop
formulation of
mecamylamine
Nicotinic
acetylcholine
receptor
Phase II
NCT00536692
(completed)
Phase II
NCT00607750
(completed)
Monoclonal
antibody
Sphingosine‑1phosphate
Phase II
NCT01414153
Phase I
NCT01334255
iSONEP
(sonepcizumab)
Lpath
JSM6427
Jerini Ophthalmic
Monoclonal
Antibody
α5β1 Integrin
Phase I
NCT00536016
Volociximab
Ophthotech
Corporation
Monoclonal
Antibody
α5β1 Integrin
Phase I
NCT00782093
Fosbretabulin
OXiGENE
Combrestatin A4
phosphate
Microtubule
assembly
Phase II
NCT01423149
(completed)
E10030
Ophthotech
Corporation
Pegylated aptamer
Platelet-derived
growth factor-β
Phase II
NCT01089517
DDIT4, DNA damage-inducible transcript 4 protein; mTOR, mammalian target of rapamycin; PEDF, pigment epithelium-derived
factor; siRNA, small interfering RNA; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor.
Neuroprotective agents. Both retinal diseases and
glaucoma are neurodegenerative disorders in which a
common end point is the apoptosis of retinal neurons.
Therefore, the use of agents that confer neuroprotection
is a very appealing therapeutic approach. Stimulation of
glutamate receptors, interleukin‑1 receptors (IL‑1Rs),
JUN receptors and tumour necrosis factor receptors
(TNFRs) triggers retinal neurons to undergo apoptosis
through a cascade of cellular signalling events134. These
signals include activation of the pro-apoptotic proteins
BCL‑2 antagonist of cell death (BAD), BH3‑interacting
domain death agonist (BID) and BCL‑2‑associated
X protein (BAX), which then promote the release of
cytochrome c and activate the caspase pathways.
The anti-apoptotic mechanisms can also be stimulated
through neurotrophin signalling pathways, including
pathways involving brain-derived neurotrophic factor
(BDNF), ciliary neurotrophic factor (CNTF) and glial
cell-derived neurotrophic factor (GDNF), resulting in
the inhibition of the pro-apoptotic cascade through
signalling molecules such as AKT, B cell lymphoma 2
(BCL‑2), BCL-XL as well as X‑linked inhibitor of apoptosis (XIAP)134.
To date, only a few clinical trials (TABLE 5) have been
carried out to investigate the efficacy of neuroprotective
agents in glaucoma (for example, memantine (Namenda;
Forest/Lundbeck) and brimonidine (Alphagan; Allergan))
and in AMD (for example, brimonidine, AL‑8309B, CNTF
and RN6G). These are discussed in more detail below.
AL‑8309B is a topical selective agonist of the 5‑hydroxy­­
tryptamine 1A receptor (5‑HT1A). 5‑HT1A agonists have
been shown to have neuroprotective effects against
NATURE REVIEWS | DRUG DISCOVERY
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REVIEWS
Table 4 | Other novel glaucoma drugs and possible mechanisms of action
Drugs
Companies
Latrunculins
Mechanisms of action
Clinical
phase
Clinical trial
identifiers
Refs
Wisconsin
Increase outflow by altering actin
Alumni Research cytoskeleton and increasing
Fund (WARF)
trabecular meshwork outflow
Preclinical
NA
116
SYL040012
Sylentis
β-adrenergic receptor antagonist;
small interfering RNA that reduces
aqueous humour inflow
Phase II
NCT01227291
130
PF‑04217329
(also known as
taprenepag)
Pfizer
Prostaglandin E receptor 2 agonist;
increases outflow
Phase II
NCT00934089
(completed);
NCT00572455
(completed)
194
BOL‑303259‑X
NicOx (with
Bausch & Lomb)
Nitric oxide-donating
prostaglandin F2α analogue;
increases outflow by increasing
uveoscleral outflow
Phase II
NCT01223378
195
Latanoprost Punctal
Plug Delivery System
(L-PPDS)
QLT
Sustained delivery of latanoprost
by the plug increases outflow
Phase II
NCT01481051
133
INO‑8875
Inotek
Pharmaceuticals
Adenosine receptor agonist;
increases outflow via trabecular
meshwork
Phase I/II
NCT01123785
126
OPA‑6566
Acucela/Otsuka
Adenosine receptor agonist;
increases outflow via trabecular
meshwork
Phase I/II
NCT01410188
126
ATL313
Santen
Adenosine receptor agonist;
increases outflow via trabecular
meshwork
Preclinical
NA
126
Angiotensin II type 1
receptor antagonists
(olmesartin,
candesartin
and losartin)
Santen
Increase outflow and may also
have neuroprotective effects on
retinal ganglion cells
Preclinical
NA
127
Cannabinoid
receptor agonist
Investigational
Decrease aqueous humour
outflow
Preclinical
NA
128
Aerie
ROCK inhibitor; alters actin
cytoskeleton and cellular motility
of outflow pathways
Phase II
NCT01330979
118
Phase II
NCT01474135
ROCK inhibitors
AR‑12286
ATS907
Altheos
ROCK inhibitor; alters actin
cytoskeleton and cellular motility
of outflow pathways
Phase I/II
NCT01520116
118
AMA0076
Amakem
ROCK inhibitor; alters actin
cytoskeleton and cellular motility
of outflow pathways
Preclinical
NA
118
K‑115
Kowa
ROCK inhibitor; alters actin
cytoskeleton and cellular motility
of outflow pathways
Phase II
JapicCTI‑101015
(completed)
118
Phase II
JapicCTI‑090708
(completed)
NA, not available; ROCK, RHO-associated protein kinase.
excitotoxic neuronal damage in animal models of central
nervous system injury 135,136. However, the mechanisms
under­lying the neuroprotective properties of 5‑HT1A agonists remain elusive. Several putative mechanisms that
have been identified include inhibition of caspase 3 and
activation of the mitogen-activated protein kinase (MAPK)
signalling pathway 137, which leads to increased expression
of anti-apoptotic proteins (for example, XIAP, BCL‑2 and
BCL-XL)138.
AL‑8309B exhibited neuroprotective effects in animal
models of excitotoxic neuronal damage and light damage,
in which it reduced the incidence of neuronal death139.
A 2‑year, multicentre, randomized, double-masked,
placebo-controlled Phase III clinical trial (the Geographic
Atrophy Treat­ment Evaluation (GATE) trial) is currently
underway to investigate the effects of topical AL‑8309B
administration (at doses of 1.0% and 1.75%) in AMD
patients with geographic atrophy; the trial is expected
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Table 5 | Neuroprotective agents in clinical trials
Drugs
Company
Description and mechanism of
action
Indication
(clinical
phase)
ClinicalTrials.
gov identifier
Refs
AL‑8309B
Alcon
Selective 5-HT1A agonist;
neuroprotective against excitotoxic
neuronal damage
Geographic
atrophy
(Phase III)
NCT00890097
(completed)
139
Brimonidine
tartrate
(intravitreal
implant)
Allergan
α-adrenergic receptor agonist;
promotes the production of
neurotrophic factors such as CNTF,
and protects photoreceptors in animal
models of retinal degeneration caused
by light damage of photoreceptor cells
Geographic
atrophy
(Phase II)
NCT00658619
140
RN6G
Pfizer
Monoclonal antibody; prevents
accumulation of amyloid-β40 and
amyloid-β42, thus preventing damage
to retina
AMD (Phase I)
NCT01003691
141,142
CNTF
Neurotech
Neurotrophic factor; protects
photoreceptors from degeneration,
slowing progression of vision loss
Geographic
atrophy
(Phase II)
NCT01408472
94,
143–150
Memantine
Allergan
NMDA glutamate receptor antagonist;
protects retinal ganglion cells and
their target (the relay neurons) in the
lateral geniculate body
Primary
open-angle
glaucoma
(Phase III)
NCT00168350
(completed)
44
5-HT1A, 5-hydroxytryptamine 1A receptor; AMD, age-related macular degeneration; CNTF, ciliary neurotrophic factor; NMDA,
N-methyl-d-aspartate.
to be completed in 2012. The primary efficacy end point
is the rate of progression to geographic atrophy.
Another promising approach is the use of brimon­
idine tartrate (delivered through an intravitreal implant),
which is an α2‑adrenergic receptor agonist that was originally designed to lower IOP in the treatment of glaucoma.
Although the mechanism of action of the compound is
largely unknown, brimonidine was shown to promote
the production of neurotrophic factors (such as CNTF)
and protect photoreceptors in animal models of retinal
degeneration induced by light damage140. Brimonidine
tartrate has been formulated as an intravitreal implant for
delivering brimonidine to the retinal tissue over a period
of 3 months. A Phase II study to evaluate the safety and
efficacy of the compound is expected to be completed in
2012. The primary end point is the change from the baseline in the surface area of geographic atrophy after 1 year.
RN6G (also known as PF‑4382923) is a humanized
monoclonal antibody that is under development for the
treatment of Alzheimer’s disease. It targets the carboxyl
termini of amyloid‑β40 and amyloid‑β42. Intravenous
treatment with RN6G is intended to prevent the accumulation of amyloid‑β40 and amyloid‑β42, and to prevent
their cytotoxic effects141. Amyloid-β is present in the eyes
of patients with AMD; systemic treatment with RN6G
decreased the amount of amyloid-β in the eyes of a mouse
model of AMD and prevented damage to the retina142.
A Phase I clinical trial has been successfully completed.
CNTF, which protects photoreceptors from apoptosis during retinal degeneration143, also has potential
as a treatment for geographic atrophy. Although the
intrinsic function of CNTF is not fully understood,
exogenous CNTF affects the survival and differentiation of various cells in the nervous system, including
retinal cells144,145. The CNTF study used an implanted
semipermeable capsule that encapsulates genetically
modified RPE cells that overexpress CNTF. RPE cells
are encapsulated within a semipermeable membrane
with small pores, which allows the release of CNTF into
the vitreous yet prevents the allogeneic rejection of RPE
cells. CNTF effectively protected photoreceptors in 12
animal models of photoreceptor degeneration94,146–149.
A multicentre, 1‑year, double-masked, sham-controlled
dose-ranging study found that CNTF delivered by the
encapsulated cell technology (ECT) implant appears to
slow the progression of vision loss in patients with geographic atrophy, especially in patients with 20/63 vision
or better vision on an eye chart at the baseline150.
Degeneration and functional loss of the glaucomatous optic nerve was once thought to be primarily due
to diffuse RGC injury; however, it has recently been
proposed that this form of neurodegeneration appears
to be compartmentalized, thus affecting neuronal processes well before the RGC body in the inner retina150.
This has important implications for the development of
neuroprotective therapies for glaucoma, as functional
changes or axonal dystrophy may be more amenable to
treatment than the replacement of lost RGC bodies or
long stretches of their axons in the optic nerve or their
projections into the brainstem151.
Excessive activation of the NMDA (N‑methyld‑aspartate) receptor signalling cascade leads to excitotoxicity, wherein intracellular calcium overloads RGCs
and other neurons, causing cell death through apoptosis.
Oral administration of memantine to non-human primates with experimentally induced glaucoma conferred
protection of RGCs and their target — the relay neurons in
the lateral geniculate body 44. A Phase III clinical trial has
been completed to investigate the efficacy of the NMDA
glutamate receptor antagonist memantine in glaucoma
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neuroprotection (protection of the structure and function
of the optic nerve to halt glaucoma progression independently of lowering IOP). However, memantine failed to
reach the primary efficacy end point of reducing visual
field loss in patients at a high risk of developing glaucoma.
Data from this study have not yet been reported.
The efficacy of topical brimonidine in preserving
visual field loss in glaucoma has also been investigated.
Brimonidine is approved by the FDA for lowering IOP in
glaucoma, and systemic administration of the drug has
been reported to protect RGCs in an experimental rat
model of ocular hypertension152. In the low-tension glaucoma treatment study, patients who were randomized to
receive topical brimonidine monotherapy had less visual
field loss than those who were randomized to receive
topical timolol (Timoptic; Merck & Co.), even though
both drugs invoked similar IOP-lowering effects153.
Although it has been suggested that this result is
consistent with the neuroprotective effects of topical
brimonidine, there are several important limitations of
the study, including the considerably small sample size,
the large number of dropouts in the brimonidine group
and the possibility that the patients who were treated
with brimonidine were unmasked as they often had conjunctival hyperaemia. Therefore, although the results are
intriguing, they do need corroboration. The brimonidine
tartrate intravitreal implant that is being investigated for
AMD might also be useful for treating glaucoma, particularly if it is used for intracameral drug delivery to
lower IOP and provide neuroprotection in glaucoma, but
such studies have not yet been undertaken.
Potential future therapeutic approaches
Target discovery through genomic research. A crucial
role of genetic variation in the occurrence and development of primary open-angle glaucoma has been affirmed
by the varying prevalence of primary open-angle glaucoma observed in different populations, family aggregation trends154,155 and in twin studies. Some populations
have a higher prevalence of primary open-angle glaucoma than others (for example, the prevalence of the
disease in individuals of African ancestry is several-fold
higher than in those of European ancestry). This suggests
that genetic background may be a primary cause of the
disease. A growing number of studies, including genomewide association studies, have identified glaucoma susceptibility loci for monogenic and common primary
open-angle glaucoma (reviewed in REFS 156,157). Genes
that are involved in cell cycle control and transforming
growth factor-β (TGFβ) pathways have emerged as the
main risk loci for primary open-angle glaucoma158,159.
However, it is not clear at present how these findings
could be translated into therapies.
There is increasing evidence that genetic susceptibilities contribute to the development of diabetic retinopathy. For instance, the frequency of diabetic retinopathy varies
among different ethnicities and its prevalence is higher in
individuals with a positive family history 160–162. Identifying
genes (such as erythropoietin)163 that are associated with
diabetic retinopathy could lead to the development of
targeted treatment strategies in the near future.
AMD is an excellent example in which novel target
discoveries have been achieved through genomic research.
More than ten genes have been identified for AMD, which
point to several new pathways that may be involved in the
pathogenesis of AMD and could be used to predict the
risk of developing the disease164. These genes encode proteins such as hepatic lipase and apolipoprotein E (which
are involved in lipid metabolism)165,166, proteases such as
HTRA1 (REFS 27,40) as well as components of the complement pathway. Owing to multiple gene associations
in each of these pathways, various drugs targeting these
pathways (for example, the alternative complement pathway) are now in clinical trials.
Gene therapy. Gene therapy has been an active area of
clinical investigation. It can provide long-term, sustained
effects with a single injection; however, reversibility
needs to be addressed as long-term continuous blockade
may not be desirable. Various ocular gene therapy trials
have been highly successful in treating retinal degeneration167–169. An AAV2‑mediated gene transfer of the MER
receptor tyrosine kinase (MERTK) gene rescued retinal
degeneration resulting from a MERTK mutation in RCS
rats170, and a Phase I clinical trial is underway in patients
with recessive MERTK mutations. TT30 is a recombinant CFH fusion protein that is under development for
replacement therapy of defective CFH (see the Taligen
Therapeutics website). Knockdown or silencing of CFHrelated 1 (CFHR1) by preventing its mRNA expression
might be useful for the treatment of AMD because deletion of CFHR1 protects against AMD independently of
CFH modulation itself 171,172.
Stem cell biology and therapy. More recently, there has
been increasing interest in the potential use of stem cell
therapy to achieve ocular tissue repair and/or regeneration173,174. The eye is a particularly attractive target organ
for stem cell therapy as it is in a contained intraocular
space and is therefore associated with minimal systemic
safety concerns and immune reactions. The effect of stem
cell therapy can also be monitored through non-invasive
diagnostic tools such as optical coherence tomography.
These stem cell advancements include our considerably
improved ability to manipulate and control stem cell fate
and function in a more precise and defined fashion, as
well as new approaches to reprogramme somatic cells
towards various tissue-specific lineages173,175. For example, RPE can be derived from patients with geographic
atrophy who have a high genetic risk genotype, and this
RPE can be subjected to an in vitro high-throughput
screen to identify small molecules that can prevent the
death of RPE cells or enhance their survival173. Ultimately,
it is possible that conventional small-molecule or biological therapeutics may be developed that could enable one’s
own cells to regenerate in vivo to alleviate damage caused
by diseases and injuries.
Advanced Cell Technology has produced human
embryonic stem cell-derived RPE cells and has received
FDA approval for their use in clinical trials. Two combined, multicentre, open-label Phase I/II trials are currently underway, using RPE cell subretinal transplantation
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REVIEWS
to treat patients with Stargardt macular dystrophy and
geographic atrophy. These trials are prospective openlabel studies to determine the safety and tolerability of
transplanted RPE cells. Preclinical studies in animals
showed that transplanted RPE cells preserved overlying
photoreceptors, slowed the progression of degeneration
and improved visual functions176. StemCells has recently
filed an investigational new drug application with the
FDA to initiate a Phase I/II multicentre study using
adult-derived neural stem cells to treat patients with
geographic atrophy.
Ocular drug delivery
Despite advances made in the discovery of novel targets
and an understanding of their mechanisms of action,
drug delivery to these targets — especially when it is to
the posterior eye — is continually challenged by anatomical and physiological constraints, particularly the blood–
ocular barriers and an array of efflux transporters. As a
result, therapeutic agents that are systemically administered are inevitably associated with adverse systemic
exposure and unsatisfying efficacy 177. Although using
intraocular or periocular injections can substantially
lower systemic exposure when delivering therapeutic
entities to the retina, poor patient compliance and the
high risk of complications (including retinal detachment, endophthalmitis and increased IOP) have limited
the frequency of administration via these routes178. For
example, anti-VEGF therapies are highly effective in
treating wet AMD, diabetic macular oedema and retinal vein-occlusive diseases, but a monthly intraocular
injection is not an ideal drug delivery platform and it is
hard to maintain patient compliance.
Overall, these challenges have hindered the use of
many novel therapeutic agents, thus emphasizing the
urgency to develop novel drug delivery strategies for
the eye. Among various approaches to optimize drug
delivery, ECT and applications of nanomedicine can
have essential roles.
Nanotechnology
The understanding and control
of matter at dimensions
between approximately
1 nm and 100 nm. It involves
imaging, measuring, modelling
and manipulating matter at
this length scale.
Nanoparticle drugs
Nanometre-scale particles that
are used to carry and transport
pharmaceutical agents to
improve therapeutic efficacy,
drug safety and patient
compliance.
Encapsulated cell technology. ECT is a cell-based delivery
system that is used to treat chronic disorders of the eye179.
In this technology, specific cells are first genetically engineered to overexpress desired therapeutic proteins, and
are then encapsulated into semipermeable capsules that
facilitate the diffusion of nutrients and proteins but prevent attack by the host immune system. For treatment,
the capsules are surgically implanted into target sites,
where the cells will survive and release proteins. As any
gene encoding a therapeutic protein can now be engineered into cells, ECT has a broad range of applications.
The implantation of cell-loaded capsules also allows for
controlled, continuous and long-term administration of
therapeutic proteins to the proximity of the eye in situ
with no systemic exposure. The implant can also be
retrieved, providing an added level of safety 180.
ECT has advanced into the clinic as a result of an
improved understanding of the mechanisms of protein
functions related to visual impairments, coupled with
advances in genetic engineering and implant technology 181. In particular, encapsulated human cells that are
genetically modified to secrete CNTF (for the treatment
of geographic atrophy associated with dry macular
degeneration and retinitis pigmentosa) have advanced
to Phase II clinical trials, with promising results150.
ECT can also be extended to other ophthalmic diseases, depending on the secreted protein. For example,
the delivery of alginate-encapsulated cells producing
glucagon-like peptide 1 (GLP1) resulted in the intraocular
production of GLP1 without causing the obvious cytotoxic effects in animal studies182. Meanwhile, a Phase I
clinical trial that is currently being planned by Neurotech
involves the implantation of encapsulated human cells
that are genetically modified to deliver a structural antagonist of VEGF for the treatment of wet AMD150.
Nanomedicine. Nanomedicine, which exploits nanotechnology to solve medical challenges, has offered numerous exciting possibilities in health care. In particular,
nanoparticle drugs have become established therapeutics
by formulating nanometre-scale carriers composed of
various therapeutic entities (including small-molecule
drugs, peptides, proteins and nucleic acids), together
with excipients such as lipids and polymers that assemble
with these therapeutic entities183,184. Although nanoparticle therapeutics were initially developed for cancer
treatment, their application has expanded into ophthalmology owing largely to their unique capability of
enhancing drug efficacy — through improved drug
encapsulation, sustained or triggered drug release, and
preferential targeting to disease sites185. Some nanoparticle therapeutics have been clinically approved for
treating ophthalmic diseases. For example, pegaptanib
— an anti-VEGF aptamer conjugated with branched
polyethylene glycol — has been approved for the treatment of AMD186.
In addition, nanomedicine has provided new opportunities for overcoming obstacles in treating ophthalmic
diseases. For example, by combining synthetic chemistry with a basic understanding of protein–polymer
interactions, nanotechnology has led to the efficient
encapsulation and controlled delivery of proteins
within biological fluid under mild conditions187. These
technologies are particularly promising as the therapeutic potential of proteins and monoclonal antibodies
is increasingly becoming realized in ophthalmology,
particularly for halting retinal angiogenesis188. In addition, nanotechnology has resulted in the formation of
sophisticated carriers that are particularly suitable for the
delivery of therapeutic agents to the cytoplasm for bio­
activity 189. Technology for intracellular delivery, if available, can be used in siRNA-based therapeutics to selectively
silence gene expression for the treatment of ophthalmic
diseases.
Moreover, nanotechnology has provided new opportunities for drug delivery, with the goal of attaining prolonged ocular drug retention. In particular, approaches
that are under active investigation include the incorporation of bioadhesive components to formulate multifunctional nanoparticle drug reservoirs that can be attached
to the corneal tissue with a high affinity 190. Furthermore,
the recent development of polymeric nanoparticles that
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are camouflaged by the red blood cell membrane allows
man-made delivery vehicles to share functionalities that
have been engineered and perfected by nature, hence
taking another step towards bridging synthetic materials
with natural components191. These biomimetic nanoparticles — that have excellent biocompatibility and
well-controlled drug release kinetics — will have many
applications in the treatment of ophthalmic diseases.
Conclusions
Currently, anti-VEGF therapies are highly effective in
treating wet AMD, diabetic macular oedema and retinal
vein-occlusive diseases, and have become the first-line
treatment for these conditions. However, there are still
several common blinding ophthalmic disorders, such
as geographic atrophy, for which no effective therapies
are currently available. Moreover, many individuals with
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Acknowledgements
We thank J. Ambati, S. Ding, I. Kozak, J. Lee, L. Zhao and
P. Shaw for their helpful comments. K.Z. is supported by
grants from the Chinese National 985 Project to Sichuan
University and West China Hospital, the National Eye Institute
and the US National Institutes of Health, VA Merit Award,
Research to Prevent Blindness, King Abdulaziz City for Science
and Technology (through the University of California San
Diego Center of Excellence in Nanomedicine centre grant)
and the BWF (Burroughs Wellcome Fund) Clinical Scientist
Award in Translational Research. L.Z. is supported by the
National Science Foundation (NSF) grants CMMI1031239
and DMR1216461; R.N.W. is supported by grants from the
National Eye Institute and the US National Institutes of
Health (EY019692) and an unrestricted grant from Research
to Prevent Blindness, New York, USA. We apologize for the
omission of references owing to page limitations.
Competing interests statement
The authors declare competing financial interests: see Web
version for details.
FURTHER INFORMATION
Kang Zhang’s homepage:
http://eyesite.ucsd.edu/retina/KZL/index.html
Taligen Therapeutics website:
http://www.taligentherapeutics.com/pipeline/index.html
Webvision website: http://webvision.med.utah.edu
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