The Promise of Nanomedicine for Ocular Disease

This editorial aims to demonstrate that the eye is an excellent organ to consider for nanomedicine. It discusses two major causes of blindness and describes ocular nanomedicine studies to date that illustrate the potential for ocular nanomedicine.

As a target organ the eye has many advantages when considering developing a novel clinical treatment. First, we are blessed with two eyes so for clinical trials we have a built-in control in which to demonstrate, in an ethical manner, that our intervention is better than the best current clinical practice. Second, the eye is transparent, making it easy to observe the effects of the intervention. Third, the eye is a small organ that offers easy accessibility for application of a nano-treatment, either as a topical drop or as an injection into the anterior or posterior chamber of the eye. A much smaller amount of clinical grade reagent will need to be manufactured for eye treatment compared with, say, a treatment for liver disease or a muscular disease. Fourth, treatments that may have a systemic toxicity can be given intraocularly and thus avoid systemic complications. Finally, losing sight is devastating. Patients would do anything to avoid blindness and, as such, there is a multi-billion dollar market in drug development for ophthalmology. Therefore, pharmaceutical industries will support novel interventions and clinical trials of these in the eye.

For many of the above reasons, several of the most innovative new medical treatments are being assessed first in the eye. For example, Phase I and II clinical trials assessing siRNA treatments are being tested in the eye (for macular degeneration). In addition, ocular gene therapy for inherited retinal degeneration is showing benefit in early clinical trials [1–3].

Common eye diseases that may be considered for nano-solutions include ‘wet’ age-related macular degeneration (AMD) and glaucoma. AMD is the most common cause of blindness in the western world. By 75 years of age, one in three people will show some signs of this disease and this figure rises to one in two by 85 years of age. Therefore, any nano-treatment would have a large market of patients. In this condition, vision rapidly deteriorates (possibly overnight) with the complication of choroidal neovascularization. Happily, treatment has improved recently with the introduction of aptamers that inhibit VEGF [4,5]. However, these aptamers must be given by monthly intravitreal injection; sometimes repeatedly for 2 years or longer. What is now needed is less frequent intravitreal injections and biofeedback to detect upregulation of VEGF. In the other form of the disease, ‘dry’ AMD, the pathology begins with lipofuscin accumulation within retinal pigment epithelial cells. They and their associated photoreceptors then die and patients become progressively blind. Currently, the only treatment is vitamin supplementation. Possible ways nanotechnology could assist the treatment of this prevalent cause of blindness include: prolonged drug delivery, introduction of a nanoparticle (NP) detoxification enzyme to metabolize lipofuscin intracellularly and biofeedback to detect elevated intravitreal VEGF levels, which would permit further treatment to suppress this growth factor and prevent recurrence of choroidal neovascularization.

Primary open angle glaucoma affects 1–2% of the population over the age of 40. It is a progressive optic neuropathy. Currently, the only partially effective treatment is reduction of intraocular pressure. This is achieved by the application of daily topical eye drops or surgery (which can result in complications such as infection). Compliance with using daily intraocular drops for life is a serious issue. Therefore, for both AMD and primary open angle glaucoma, prolonged drug delivery would improve treatment efficacy. Nanocarriers such as NPs, dendrimers and liposomes may be used to enhance ocular drug delivery [6,7]. NPs are structures smaller than 100 nm. They are designed as drug carriers, whereby the active ingredient can be dissolved, entrapped/encapsulated or absorbed [8]. Different NPs afford different drug-release kinetics, capacities and stability [9]. NPs represent promising drug carriers for ophthalmic applications. Sustained intravitreal therapeutic drug concentrations can be achieved, and the ocular bioavailability of many drugs is significantly enhanced in comparison to normal aqueous eye drop solutions. These particulate delivery systems can improve patient compliance and may reduce systemic side effects. NPs have been engineered from various synthetic and natural biocompatible polymers. NPs derived from natural materials such as albumin can serve as an efficient drug-delivery system as they are biodegradable, nontoxic and non-antigenic. Albumin also has a high content of charged amino acids, which allows for attachment of positively or negatively charged drugs or oligonucleotides [7]. The most commonly used synthetic polymers are polylactide (PLA) and poly(lactide-co-glycolide) (PLGA), which degrade in vivo to form natural metabolites. Their degradation rate can be tailored via changes in polymer composition and molecular weight, such that they can provide controlled drug release from a few days up to a few years [10]. Such sustained drug delivery would be vastly preferable to the current situation of elderly patients requiring monthly intravitreal injections. Furthermore, importantly, specific single-procedure, extended-release drug delivery would reduce both the economic burden of treatment and the risks of repeated intraocular operations.

Currently, in vivo studies using drugs entrapped in NPs for intraocular delivery are relatively limited. Of note, NPs of different sizes and electrical charges can migrate through the retinal layers and tend to accumulate and remain in the retinal pigment epithelium (RPE) when injected into the vitreous of rabbits [11,12]. Recently, Kim et al. demonstrated that intravitreally injected anionic human serum albumin NPs diffused readily through the vitreous, and trans-retinal movement of NPs was more pronounced following experimental injury. This is promising for drug carriage to the RPE and choroid [13]. The ability of drugs that are injected intravitreally to be able to reach the RPE and choroid is advantageous as intravitreal injections are technically less challenging than subretinal injections.

Nanoparticles afford a number of advantages for controlled ocular drug release. They are relatively simple to prepare, are injectable and sterilizable with a long shelf-life after lypholization. Drug solubility issues may be circumvented, and encapsulation of drugs offers protection against immediate dilution and degradation. NPs also afford improved residence time, while the ability to target drugs to the sight of action leads to a decrease in the dosage required [14,15]. NPs act at a cellular level and can enter into cells either via endocytosis or phagocytosis, leading to internalization of any encapsulated material, which may include proteins, DNA, lipids and organic/inorganic substances [16]. In the future, transporter/receptor-targeted NPs may play a crucial role in delivering drugs and antibodies efficiently to the posterior eye segment [6,17].

Nanoparticles are also promising vehicles for gene therapy and delivery of oligonucleotides [18,19]. Delivery of full genes (gene replacement) or siRNA (gene suppression), such as knockout of VEGF and its receptor (VEGFR), are particularly advantageous for ophthalmologists [20,21]. Ocular diseases are good candidates for gene therapy as the eye is easily accessible and disease-causing defects are beginning to be understood. Gene therapy relies on delivery of genetic material into cells safely, efficiently and specifically. Viral vectors have been used with success in animal models of retinal disease [22] and are now progressing to clinical trials [1–3,23]. However, there are a number of drawbacks associated with viral vectors: their efficacy is limited by the amount of genetic material that they can carry, there is also the possibility of host immunity following repeated exposure, preference to infect certain cell types as well as the possibility of viral reversion to wild-type. Clinical use of NPs for gene delivery may overcome some of these significant issues [24].

Of note, NPs appear to be a more efficient transduction system than other nonviral vectors [25]. There are a number of studies in which NPs have been used for ocular gene delivery [21,26,27]. AMD has pathogenic features in RPE cells and studies have demonstrated that RPE cells readily take up NPs [26], with transgene expression being maintained for several weeks [28]. NPs injected into the vitreous can migrate through retinal layers and accumulate in the RPE [11]. Thus, trans-retinal movement of NPs may facilitate gene delivery to the RPE and choroid. VEGF antisense oligonucelotides encapsulated by NPs have been successfully delivered to RPE cells with inhibition of VEGF secretion and mRNA expression [26]. Currently, several of the most advanced clinical trials for RNAi focus on the treatment of AMD. For example, naked siRNA targeted to genes for VEGF and VEGFR have shown therapeutic potential [20]. Phase I and II trials demonstrated that siRNA is well tolerated, lacks side effects and has led to visual improvement in some patients [29]. If these trials prove to be effective following Phase III studies, siRNA delivery in tandem with nanotechnology may become a widespread treatment option for AMD patients. Cell adhesion molecules or antibodies may be attached or conjugated onto NPs, which holds promise to further improve their selectivity and transduction efficiency for gene delivery applications [21].

Nanoparticles are also being successfully used for intraocular delivery of trophic factors. Intravitreal administration of peptide trophic factors in a sustained low level formulation can both prevent photoreceptor degeneration in the Royal College of Surgeons (RCS) rat model of retinitis pigmentosa and also retinal ganglion cell death associated with glaucoma [30–32]. Intravitreal ciliary neurotrophic factor delivery using encapsulated cell transplants has completed Phase I clinical trials [33] and has advanced to Phase II and III trials. In addition, PLGA nanospheres encapsulating PEDF have been shown to be neuroprotective in experimentally induced retinal ischemia [34].

Nanostructures also show promise in ocular tissue replacement therapies, for example, as artificial corneas, or for retinal or optic nerve repair. The nanostructure of scaffold materials may play a role in creating niches to promote cell proliferation, functionality and integration. Nanoscale physical characteristics of the extracellular matrix, including surface topography, compliance and geometry, can also influence the rate and direction of cell migration. Lithography used to pattern silicon wafers with nano- and microscale pitches transformed human corneal cells alignment and migratory behavior [35]. Self-assembling peptide nanofibers have been shown to aid axon regeneration of the visual pathways of the brain [36], which may have significant implications for retinal repair in the future.

Magnetic NPs may also be used for localization, delivery and guidance of cell growth. In the eye, RPE cells grow as a monolayer and in a proof-of-principle study it has been shown that magnetic NPs can be used to construct RPE cell sheets ex vivo[37]. This was achieved by adding magnetic cationic liposomes to ARPE-19 cells. These cells were then grown in the presence of a magnet for 24 h, in which time they formed a multilayered sheet. Once the magnet was removed, cells were transferred to a tissue culture dish where they proliferated and formed a monolayer. It is possible that a similar technique could be used to make sheets of RPE for transplantation. Magnetic NPs may also be used for tissue engineering at the subcellular level. It may be possible to extend an injured ganglion cell axon to the optic nerve to help neurons reconnect. This would be achieved through attachment of magnetic NPs to the tip of the axon and directing cell growth with the application of an external magnetic field [38]. Magnetic NPs may also enhance cell invasion into tissue-engineering scaffolds. Chitosan-coated magnetic NPs have been investigated to improve seeding density in 3D scaffolds to circumvent problems with superficial cell seeding [39].

One of the emerging goals of nanotechno logy is to functionalize inert biocompatible materials to impart or regulate precise biological functions [40]. Several hybrid organic/inorganic NPs have been described for therapeutic or diagnostic use. Interest in stimuli-responsive polymers is gaining increasing momentum especially in the fields of controlled and self-regulated drug delivery [41]. Drugs may be released in response to specific environmental stimuli such as oxidative stress or enzymatic activity. These polymers are designed to experience rapid and reversible changes in their structure in response to changes in their microenvironment [41]. Such technologies could be exploited on a nanoscale.

There are a number of ocular applications for so-called ‘smart’ materials. For example, cerium is a rare earth element. Its valence state can be dynamically switched between trivalent and quadrivalent in a redox reaction. Owing to this redox capacity cerium oxides are excellent oxygen buffers. Photoreceptor cells have the highest rate of oxygen metabolism in the body and are continuously exposed to oxidative stress [42]. An elevated intracellular concentration of reactive oxygen intermediates (ROIs) has been noted in all forms of visual loss. It has been postulated that engineered nanoceria particles could scavenge ROIs within retinal neurons [42]. Nanoceria particles prevented an elevation in intracellular ROIs in primary cell cultures in rat retina. This was borne out in vivo using albino rats with photoreceptors highly sensitive to light damage. Intravitreal injection of ceria NPs prior to, and following induction of, light damage prevented loss of functional vision in a concentration-dependent manner [42].

Cerium has also been utilized in the treatment of glaucoma. Destruction of optic nerve cells noted in glaucoma is the result of increased intraocular pressure, which is caused in part by a build-up of carbon dioxide in the eye. An enzyme that aids in the production of carbon dioxide is human carbonic anhydrase II (hCAII) [43]. Nanoceria particles have been functionalized with hCAII inhibitors and investigated as a potential ophthalmic drug-delivery tool for glaucoma. Benzenesulfonamide inhibitors of carbonic anhydrases were bound to nanoceria [44]. The study aimed to show that conjugated NPs can be used for hCAII inhibition. Results of this study were promising; furthermore, inhibitors for other pathogenic enzymes, deficient enzymes or enzymes that have detoxification properties, could be immobilized onto nanoceria particles.

Stimuli-responsive NPs may also act as biosensors in the intraocular milieu. A model of premature retinopathy has been described using an enhanced green fluorescent protein reporter gene driven by an antioxidant response element, which is a sensitive indicator of oxidative stress in the retina [45]. Biosensor DNA was tethered to magnetic NPs. Upon analysis these stimuli-responsive NPs were taken up by endothelial cells. This approach could allow cells of the retinal vasculature to prevent injury or promote repair following an oxidative insult. Thus, manipulation of phenotypes such as intraocular pressure (in glaucoma) could provide an opportunity for early therapeutic interventions.

Fabrication of nanodevices is another exciting area where nanotechnology is being exploited for ophthalmology, ranging from retinal prosthesis [46] to nanodrainage implants for glaucoma treatment [47]. Contact lenses provide a unique opportunity for integration of miniature functions to monitor the biochemical environment of the surface of the eye [101] or facilitate extended delivery of drugs [48] and are also being engineered and investigated.

In summary, the eye is a very suitable and important organ for developing novel nanomedicines. Recent advances in the treatment of macular degeneration and inherited retinal dystrophies have demonstrated that new medical technologies can be rapidly and successfully translated into clinically relevant treatments in the eye. Nanoscientists should therefore focus their efforts on nano-ophthalmology. Early studies suggest they could make a significant impact in treating common causes of blindness such as macular degeneration and glaucoma.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

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The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.