Nanotechnology involves the development of structures and chemicals on a molecular scale, with wide-reaching applications in multiple disciplines, including medicine. Nanoparticles are structures that are typically between 10-100 nanometers, which is in a range of 50 times to 500 times smaller than a typical red blood cell.¹ The development of technology on this scale is well suited for aiding in the treatment of ocular diseases, because of the structural complexity of the eye on the microscopic scale. Nanotechnology allows us to potentially treat at the cellular level rather than the tissue level as with current available treatments.

Delivery of therapy to the posterior segment has unique challenges, given the inability of topical formulations to penetrate deep enough into the eye for effect. Intravitreal treatments have been effective; however, each repeat treatment carries the risks associated with injection, notably endophthalmitis. Therefore, therapies that can enhance drug delivery, increase durability with controlled release, prevent drug degradation, and prolong half-life could decrease treatment burden. This article discusses novel and emerging therapies that have been developed in the nanotechnology field that have important implications for vitreoretinal disease management.

GENE THERAPY

Ocular gene therapy, consisting of gene replacement or gene regulation, may be an effective way of treating retinal disorders due to ocular immune privilege and monogenetic inheritance of various retinal diseases. Vectors are used to deliver the gene therapy to the affected cell; however, this can be difficult due to issues with immunogenicity and determining the amount of gene necessary to yield expression.

Viral Vectors

Studies using adenovirus-associated vector (AAV) date back to as early as 2001, where it was first successful in dog studies with RPE65 mutations; this led to further trials in patients with Leber congenital amaurosis. Adenovirus-associated vector is a nonpathogenic human parvovirus with demonstrated safety that can infect cells without inducing insertional mutagenesis. The virus cannot replicated without co-infection of an additional virus, and approximately 90% of adults are seropositive for AAV2. However, the size of the gene to be transfected is limited to approximately 5 kb.²

Nonviral Vectors

Complexes of plasmid DNA formed due to cationic charges have allowed for controlled delivery of gene therapy with reduced immunogenicity compared with viral vectors. Additional nanoparticles using polyethylene glycol complexes have been delivered into the subretinal space with success in retinal degeneration slow mouse models of retinitis pigmentosa. Mice in these studies showed improvement and stabilization of the photoreceptor layers on electron microscopy and increased amplitudes on scotopic and photopic electroretinogram.^3-7 Coating of nanoparticle vectors with biodegradable polymers results in longer gene expression by preventing protein adsorption to the particles, which would promote faster clearance. An additional advantage of nonviral vectors is the ability to carry plasmids up to 20 kb.²

Drug Delivery

Sustained intraocular drug delivery remains the goal of therapeutic innovation in the posterior segment. The development of novel therapies is currently focused on reducing the treatment burden of frequent intravitreal injections for various pathologies. Nanoparticles not only provide controlled and steady drug delivery to the posterior segment but also can target certain cells with alterations of their chemical properties. They are able to gain intracellular access via endocytic mechanisms and can further target specific areas within the cell using targeted peptides. In addition, the nanoparticles can release drugs when exposed to certain light wavelengths, temperature, and pH, which can alter the formulation to selectively target diseased cells. Despite their advantages, there are still some concerns regarding immunogenicity and inflammation, as well as uniformity and stability of the nanoparticles during production and storage.⁸

Liposomes

Liposomes are stable phospholipid bilayers of varying size analogous to a cell membrane that can incorporate both aqueous and lipid phase drugs. Several studies have documented the extended half-life of medications with good safety profile. A liposomal formulation of ganciclovir, a medication used in the treatment of cytomegaloviral retinitis, was tested in rabbits and showed greater durability of the medication with less required treatments compared with nonliposomal formulation.¹ Liposomal-encapsulated tacrolimus has been used in rat models on experimental autoimmune uveoretinitis with promising results as well.⁹

Nanospheres

Natural and synthetic polymers have been used to encapsulate drugs and provide controlled delivery of medication to the posterior segment over longer periods of time compared to nonencapsulated formulations of the same medication. A commercially available formulation of ganciclovir (Vitrasert; Bausch + Lomb) has been FDA approved for the treatment of cytomegaloviral retinitis. In rabbit studies, smaller nanospheres were able to directly target the retina, while larger nanospheres remained in the vitreous cavity; this demonstrates that varying size of nanoparticles can have a significant effect on targeted drug delivery.¹

Polymeric Nanogels

Aqueous-soluble drugs can be loaded onto hydrophilic polymers to provide more sustained release of medication. An amino-acid-based polymer, such as polylactic glycolic acid, is currently used in dexamethasone intravitreal implants (Ozurdex; Allergan) to allow for controlled delivery of steroid to the posterior segment in inflammation-mediated conditions over an average of 3 months.⁴ In addition, light-activated crosslinking of polycaprolactone and hydroxyethyl methacrylate has been demonstrated to produce hydrogels that can provide sustained release of bevacizumab in rabbits with choroidal neovascularization; however, light-related toxicity remains a concern.¹⁰

THERAPEUTIC MODALITIES

Photodynamic Therapy

Used for select cases of choroidal neovascularization and central serous chorioretinopathy, photodynamic therapy (PDT) is an early example of nanotechnology in retina. Light-activated photosensitizer called verteporfin (Visudyne; Bausch + Lomb) is injected intravenously and a 689-nm laser is applied to the treatment area. The photosensitizer selectively targets neovascular endothelial cells and generates reactive oxidative species that result in vascular occlusion via endothelial cell death.¹⁰

PROSTHESES

The development of prostheses on the microscale for intraocular implantation in retinal diseases serves to either bypass or enhance the function of diseased retina. The goal of nanotechnology prosthetics is to improve functional vision for patients with severe retinal disease.

Argus II Retinal Prosthesis

The Argus II retinal prosthesis (Second Sight Medical) bypasses the diseased peripheral retina in retinitis pigmentosa with an electrode array that is placed directly over the macula. The array is connected to a radiofrequency transmitter that communicates information from the patient’s glasses, which are connected to a video processing unit. Brightness values on the video input are converted into current amplitudes through each of the 60 electrodes in the array; in turn, this stimulates retinal neuronal action potentials that can be perceived as light patterns. Five-year safety data and outcomes show improved functional vision in a majority of patients with 60% of patients experiencing no serious adverse events.¹¹

Retinal Pigment Epithelial Stem Cell Grafts

Polymer scaffolds facilitate the orientation and organization of progenitor and retinal pigment epithelial (RPE) cells for subretinal implantation. Microfilms using various materials such as polymethylmethacrylate and polycaprolactone have been developed as biodegradable porous scaffolds with increased cell adherence.^12-14 Kashani et al describe a polarized monolayer of human embryonic stem-cell derived RPE on a synthetic parylene scaffold that mimic Bruch’s membrane. In a phase 1/2a study, 3 out of 4 of the eyes that successfully underwent subretinal implantation of the stem-cell scaffold experienced visual improvement in terms of fixation, with one patient gaining 17 letters.¹⁵

Biomimetic Films

Bareket et al developed a nanosystem for wire-free retinal photostimulation using semiconductor nanorods and carbon nanotubes, which was tested in a chick model. In the study, successful photostimulation was achieved in the otherwise light-insensitive chick retina. This technology has promising implications for artificial retina in the future.¹⁶

A polypyrrole/polydimethylsiloxane electronic skin matrix with photodetecting and signal transmission processing capability has been developed by Dai et al in China. The matrix does not require battery power and is driven by blinking. It can map out single and multiple illumination‐stimuli via a multichannel data acquisition method.17 Therefore, the advantage of this type of nanostructure over previously developed systems is the opportunity for improved resolution and the lack of need for an external power source.⁸

THE FUTURE OF NANOTECHNOLOGY IN VITREORETINAL DISEASE

The application of nanotechnology to vitreoretinal diseases has augmented the available treatments for a variety of pathologies. Current advancements in nanotherapy in retina have increased durability, stability, and targeted delivery of treatments and have restored functional vision in patients who otherwise did not have any available treatments. While most therapies have been tested in vitro or in animal models only at this point, continued developments in this field will alter the treatment paradigm of retinal conditions in the future. RP

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