Recent Advancements in Nanotechnology-based Systems and Specific Nanoparticles Used for Different Purposes in Ophthalmology

Abstract

Nowadays, reaching to more efficient and convenient ocular drug delivery methods is a high regarded accomplishment. Nanotechnology via nanoparticles as effective and feasible carriers, raised as an interesting treatment method to transport ocular drugs to specific target cells. Due to presence of several barriers and unique anatomy of the eye the straight drug access to the particular sites is a great challenge. The ocular surface epithelium, tear turnover, presence of blood aqueous and blood retina barrier has conspicuous impediment impact on ocular drug administration. Furthermore, conventional methods of ocular drug treatment are hindered by low bioavailability and severe adverse ocular effects. Nanocarriers and nanotechnology can enormously be useful in treating ocular disease and drug delivery to the targeted regions. In this review, we present the recent advancements in nanotechnology-based systems and specific nanoparticles used for different purposes in ophthalmology.

Introduction

Eye diseases directly could be negatively influential on human vision and his life quality. Based on a previous research, 285 million people had visual difficulties globally, with the proportion of 39 million blind, which 82% of them are over their fifties. Because of the burden of world population growth, it has been estimated that overall cost of visual impairment will increase significantly by 2020.Regarding this rising trend and critical role of ocular dysfunctions in individuals’ lives, reaching to more affordable and beneficial diagnostic and therapeutic pathways has been investigated. There have been significant discoveries in ophthalmic pathology mechanism and ocular disease treatments so far. However, effective ocular drug and gene therapy methods are still an attention demanding challenge.

There are some particular features of our eyes that favor investigating of therapeutic methods. These days there is access to noninvasive ways for direct ocular visualization and locally treatment. Moreover, the possibility of simultaneous observation of other eye is another benefit in this matter. On the other hand, due to ocular anatomical barriers local and systemic ocular drug administrations are suffering from lack of specificity and low efficiency. So, overcoming on these obstacles is a major challenging. Furthermore, most of available and current treatments are infrequent to restore ocular diseases or vision loss.

Nowadays, developing of both nanotechnology and nanomedicine application make tremendous progress in medicine. Nanotechnology is a part of science related to designing functional structures based on nanoscales. Nanomedicine is the application of nanotechnology for medical interventions including diagnosing, treatment and prevention of human disease based on molecular studies of human bodie. In Nanomedicine system, there are different nanocarriers for ocular delivery purposes, such as: nanoparticles, liposome, emulsion, nanotubes, nanosuspensions and dendrimers. Nanoparticles (NPs) are microscopic materials measured based on nanoscale, which act as a whole unit. They could be used in two categories of Nano capsules and nanospheres and predominantly consist of lipid, protein and polymers.

Based on recent findings regarding outstanding impacts of NPs in ophthalmology, development of smart diagnostics and therapeutics solutions is highly beneficial. In this study we discuss the benefits of nanomedicine and nanoparticles application in the field of ophthalmology. First, we introduce the ocular anatomic features, barriers and delivery routes. Then, samples of nanoparticles used for various ocular purposes are reviewed. Eventually, we reach to a conclusion besides future perspectives in this domain. This review aims to provide better know-how about NPs eye-catching roles in different sections of ophthalmology in the present time and future.

Ocular structure, barriers and delivery systems

The eye with its spherical shape is consisted of three main layers. Eye globe is divided to anterior (cornea, conjunctiva, aqueous humor, iris, ciliary body and lens) and posterior segment (retina, choroid, sclera, vitreous humor and optic nerve).

The primary barrier of the eye is cornea including five layers. Corneal epithelial layer via its cellular tight junctions has a considerable impeding role in drug penetration. Also, there is a constant tear turnover on ocular surface between 2-20 minutes, which significantly declines the bioavailability and absorption of topical administered drugs. Although drug permeability is apparently more in conjunctiva compared to cornea, it is not properly efficient as a result of conjunctival capillaries and lymphatic existence. Blood ocular barriers consisting of blood- aqueous (BAB) and blood-retina barrier (BRB) limit systemic drug diffusion. BAB plays significant preventative part in anterior segment of the eye. BAB created by endothelium of iris capillaries, non-pigment ciliary and iris epithelium. Blood-retina barrier (BRB) is in association with back of the eye, including inner and outer parts. Retinal vascular endothelium, pericytes and astrocytes build up inner part and retinal pigment epithelium cells (RPE) is related to outer section of BRB.

On the whole in developed countries, posterior segment eye disease like age related maculopathy and diabetic retinopathy are more common compared to anterior part disfunctions. But, anterior segment drugs (e.g. antibiotics, anti-glaucoma drugs,…) reflect more dominancy notably in the form of eye drops. There are several routs regarding ocular drug applications. Depending on the ocular target site, modalities for ocular drug therapy are: topical administration, peri ocular, trans septal, retro-orbital, intracameral, sub-retinal and intravitreal injections. Anterior segment oculopathies (cornea, conjunctiva, sclera and anterior uvea) commonly are treated by topical ocular eye drops. Unfortunately, as a result of rapid eye tear turnover, blinking and nasolacrimal system drainage, bioavailability of eye drops are less than 5%. It is obvious that topical therapy methods could not be properly effective on posterior segment (retina, vitreous, choroid) disorders. Therefore, the systemic drug delivery (intravenous or intravitreal) is more popular regarding posterior segment therapies. However, distinct complications of frequent and high dosage intra-ocular injections (retinal hemorrhage, retinal detachment, cataract, endophthalmitis,) make concern. Subsequently, there have been outstanding investigations to accomplish new alternative methods with more beneficial outcomes in this regard. For instance, by injection of nanoparticles via hollow microneedles through sclera, drugs showed faster with longstanding efficacy.

NPs used in ophthalmology

NPs as novel carriers have numerous benefits according to their good targeting potential and constant drug release. Also, various studies have exhibited long-term bioavailability, decent biodegradability and biocompatibility descriptions of NPs, which make them as promising tools in ophthalmology. Nanoparticles for molecular therapy could be categorized in 4 groups: (1) lipid based-NPs (lipid protamine DNA(LPD)), (2) metal-based NPs, (3) polymer-based NPs and gelatin based. The NPs-based delivery mechanisms include enrolling the cell, avoiding endosomal degradation and entering DNA for delivery of therapeutic agent. NPs are made of various materials, such as chitosan, poly lactic glycolic acid, hyaluronic acid (HA), cerium oxide, silver, gold and silica.

Specific applications of NPs in Ophthalmology

As it mentioned, nanotechnology is the science of synthesis and characterization of nanomaterials, which could functionalize those materials with other different molecules for specific purposes. So far, the combination of nanotechnology and biomedicine has produced interesting results in many medical fields. The advent of nanotechnology is promising and can substantially accelerate the ophthalmic therapeutic and diagnosis approaches via using specific NPs by improved penetration, sustained and controlled drug delivery.

NPs for ocular disease diagnosis

There are various methods used for ocular disease diagnosis including fluorescein and indocyanine green angiography, electroretinography, ultrasonography, ocular coherence tomography (OCT), computed tomography (CT) and magnetic resonance imaging (MRI). Although these methods have influenced on recovery process of ocular disease remarkably, each of them suffers from restrictions in disease diagnosis and monitoring. For the sake of managing these limitations, it seems that nanotechnology have provided several routes. Application of GNPs in ocular imaging is one of its myriad roles in ophthalmology (e.g. photothermal therapy, gene delivery, drug delivery). It has been reported that GNPs could be used as a good contrast agent for OCT. Anderson et al. prepared an emulsion of Gold-perfluorocarbon NPs conjugated with anti-monoclonal integrin (àvß3) antibody DM101. They indicated that after injection of targeted agent, the average signal intensity of MRI was raised to 25% in rabbit in vivo model. Zagaynova et al. demonstrated that silica-gold NPs enhanced intensity of OCT signal and brightness of related parts of OCT image. Similarly in a murine model study, there was almost 50-fold rise in OCT contrast after injection of GNR in anterior chamber [109]. Noteworthy, biosensing papers made of GNPs could be used for diagnosis of infectious disease like keratoconjunctivitis. likewise, utilization of GNPs with Raman spectroscopy remarkably enhanced detecting signals, which analyze human tears. This method not merely revealed discrimination between normal and contagious ocular tissue, but also determined the type of infectious eye (viral, bacterial and allergic). Putting together, GNPs have potential impacts on improving early detection of ocular disease.

Though application of nanotechnology methods for tumor diagnosis and treatment showed a growing improvement recently, there are few studies related to their usage in ophthalmology. But, approaches used for other disease could be a feasible guidance regarding diagnosis and treatment of ocular disease. In one study, merging hydrogel Nano system with tumor targeting, induced drug delivery and photo to heat transformation. In this report, ligand peptide-based with phage particles, heat sensitive liposomes (HSL) or mesoporous silica (MSNPs) were assembled in a hydrogel for tumor targeted drug delivery. Taking together, authors reached to multimodal imaging and monitored delivery of therapeutic agent in human tumor xenograft.

Recently, there have been promising steps in nanotechnology to early disease detection by engineering monitoring tools like biosensors, especially for chronic eye disease like glaucoma, retinal degenerative disease. Lin et al. demonstrated a novel technique based on light targeting NPs for treating pigmented cells in ophthalmology. Nanoindentation system is another attractive example in this regard, which determines efficient hydraulic conductivity and modulus of human ocular surface. By this method, it is possible to reach ideal drug delivery routes via NPs from ocular surface. As a consequence of theses developing methods, there will be a notable rise in therapeutic intervention efficacy and remarkable decline in patient expenses.

NPs for ocular disease therapy

As it declared, the ocular anatomical barriers restrict drugs penetration from anterior to the posterior part. Also, drugs infiltrate in the differing direction of liquid distribution in the eye. So, pointed obstacles impressively disturb intraocular drug delivery efficacy. According to rising prevalence of chronic ocular disease in elderlies, numerous methods of drug-delivery system have been investigated. Eye-catching achievements in gene delivery, stem cell and protein therapy accentuated the necessity for enhancing stability and bioavailability of new therapeutic units [119]. NPs-based delivery methods diminish ocular injections frequency, and enhance efficiency, causing less side effects beside patients convenient.

Efficacy of drug delivery based on gene or protein is hampered by their chemical and physical instability. Thus, designing sustainable drug/gene therapy methods (e.g. encapsulation or nanotechnology-based methods) could be a great approach for tackling with nominated obstacles and side effects of frequent ocular injections.

Nanoparticulate drug/gene/protein-release efficacy depends on their charge, polarity, size and shape and structure. Diverse formulations made of basic salts, surfactant, … have been tried for enhancing molecular steadiness during intraocular administration. Here, there are explanations of major applications of NPs for ocular therapeutic purposes.

NPs for ocular gene therapy

Successful gene therapy is determined by two basic factors: (1) safe and effective gene delivery to target cells in vivo and in vitro. (2) effectual monitoring of modifying agents or modified cells via non-invasive imaging methods, which allows gene delivery tracking. There are two types of vectors for gene therapy: viral and non-viral ones. Although, viral vectors are more popular regarding their higher gene expression efficacy, their significant restrictions and side effects were motivative for investigating other alternative vectors. Usage of NPs with less proved side effects, long term gene expression, larger capacity, better bioavailability and biodegradability are getting more and more attention. There have been considerable usage of NPs loading gene transcription factors, which facilitate cell programming via in vivo studies.

Ocular anterior segment gene therapy

Cornea is simply accessible, transparent and somewhat separated from blood circulation. So, these properties make cornea as a noble target for gene therapy or delivery. The foremost purpose of corneal gene therapy is gene transferring to the cornea or ocular tissues close to this. Latest studies productively utilized corneal gene therapy for avoiding corneal disorders like: corneal neovascularization, corneal rejection and herpetic stromal keratitis. In Sharma et al. 15 μmole of conjugated polyethyleneimine (PEI) to GNPs containing (GFP) gene was topically administered on rabbit cornea. Therapeutic dosage of GNPs, which accumulated in corneal keratocytes and extra cellular matrix did not lead to cytotoxicity. Moreover, after topical usage of GNPs encapsulating BMP7 (bone morphogenic protein 7) gene, there was a remarkable reduction of surgery induced fibrosis in rabbit cornea. Vicente et al. accomplished magnificent results via solid lipid NPs gene delivery by transfecting human corneal epithelial cell lines. In another study, we notice about worthy outcomes of topical usage of PLGA (poly lactic-co-glycolic acid) NPs caring plasmid of anti VEGF RNA expression cassette on cornea. Finally, there have been conspicuous decrease of corneal neovascularization 4 weeks after the formulated materials administration. Similar case was seen in subconjunctival application of PLGA NPs for Flt23k (anti VEGF intraceptor), which reduced corneal neovascularization and graft rejection fraction. In Iriyama et al. there was a successful subconjunctival delivery of PEG-b-P (ASP(DET)) Nano micelles caring plasmid expressing VEGF1 on corneal neovascularization regression. Interestingly, it is showed that direct application of dendritic polymers rapidly fostered corneal wound repair compared to sutures. In another study chitosan (CS) and thiolated chitosan (TCS) NPs were used on mouse corneal and cultured human corneal fibroblast cells. CS and TCS NPs enhanced anti-angiogenic and anti-fibrotic therapies by downregulation of TGFβ1 and PDGF expression.

Ocular posterior segment gene therapy

Choroidal neovascularization (CNV) is a major cause of blindness, which its treatment depends on the underlying intentions. For CNV therapy, in one study PLGA NPs loading anti VEGF plasmid were functionalized with transferrin and surface peptide. As a results, rats studied by this method had smaller area of CNV compared to the group treated with unfunctionalized NPs.

Moreover, CNV suppression reached by intravitreal gene therapy of PLGA/chitosan NPs wrapping plasmid of proteolytic plasminogen kringle 5 (K5). Likewise, PLG NPs loading anti-VEGF intraceptor tagged with RGD peptide have been studied successfully for gene expression in laser induced CNV model via intravenous (IV) injection.

Retinal gene therapy has reached to terrific outcomes particularly in animal models with retinal dystrophic disease. It is known that retina is immunologically privileged and separated of other parts of body by BRB. Noteworthy, surgical access to retina is convenient. That is why it is the fascinating part of the eye for gene delivery methods. But, one of basic requirements in this regard is selecting effectual vectors like useful and specific nanoparticles pondering the retinal high sensitivity. Both viral and non-viral vectors like NPs have demonstrated worthwhile success in retinal gene therapy. However, considering advantages of NPs against viral vectors, their contribution in gene delivery is acquiring more attention. Application of PEI NPs via oligonucleotide (ODN) encapsulating anti TGF-ß2 plasmid was assessed for retinal gene delivery. This in vivo study exhibited appropriate spreading of conjugated NPs in retinal muller ganglionic (RMG) cells after intravitreal injection. Also, the accumulation of formulated materials in RMG cells inhibited cell growth procedures. Retinal pigment epithelial (RPE) cells have prominent roles in visual system. RPE cells not merely the outer part of BRB, but also extensively involved in the ophthalmic pathologic conditions, such as AMD, retinitis pigmentosa (RP). RPE cells are able to uptake various types of NPs. Bejjani et al. evaluated delivery of GFP plasmid internalized via PLA and PLGA NPs to human ARPE-19 cell line, with conspicuous results without toxic effects on RPE cells. Bourges et al. concluded the effective and prolonged accumulation of PLA NPs (containing Rh-6G and Nile red Fluorochromes (RNFP)) in RPE cells by IV injection. There have been successive studies of novel methods for rescue photoreceptors function in retinal degenerative conditions. Also, subretinal injection of DNA NPs with CK30-PEG encapsulating rhodopsin gDNA promoted gene expression and therapeutic efficiency for rhodopsin knocking out in mouse model of RP. By this route, there was remarkable gene expression besides rescue of photoreceptors function.

It worth to know that another merit of PLA/PLGA NPs utilization in gene delivery is that FDA has already approved their application in this field [78, 150]. Naash et al. has evaluated effects of CK30-PEG driving pZEO-GFP incorporating CMV promoter. After subretinal injection on PI- 2 wild mice, GFP expression was seen in retinal ganglionic cells, inner and outer retina. Moreover, there was GFP expression in lens and cornea, in the P5 mice eye. This could be good reason for usage of cell-/gene specific promoters for reducing possibility of ectopic expression. In another study, CK30-PEG DNA NPs caring human ABCA4 cDNA and the mouse opsin (MOP-ABCA4) was formulated for gene delivery to mice with Stargardt dystrophy. During subretinal injection of Abca4(-/-) mice at P30, there was a constant ABCA4 expression for more than 8 months beside remarkable improvement of structural and functional Stargardt phenotype, like enhanced of dark adaption and depleted lipofuscin granules. Han et. al. demonstrated DNA NPs as a promising substitute for viral vectors, particularly, regarding too large genes. Intriguingly, subretinal injection of DNA NPs loading plasmid with S/MAR (scaffold matrix attachment region) led to outstanding RPE65 expression in rpe65(-/-) mouse model with LCA. Similarly, some studies reported the effective ocular gene therapy via subretinal delivery of compacted DNA NPs for nucleic acid expression in the ocular disorders, like retinal degenerative disease. In another study, there was a side-by-side comparison of subretinal delivery of wild type rds gene loaded via compacted DNA NPs and naked DNA. Gene delivery by NPs promoted gene expression to four times more and it continued for up to four months compared to control group. There are other reports regarding good efficiency of lipid NPS (e.g. LPD) via promoters for sustained protein (like recoverin) and gene expression in animal models with visual impairment, such as blindness. Additionally, intravitreal gene delivery by solid lipid NPs (SLN) via special mouse rhodopsin promotor (mOPS) facilitated structural improvement in mouse model of retinoschisis (Rs1h-deficient). Sun et al. reported effective and safe subretinal therapy of Leber’s congenital amaurosis by delivery of ECO/pDNA NPs in to mouse RPE cells.

NPs could be utilized effectively for gene delivery to stem cells assisting diagnosis and treatment of ocular disease. For instance, subretinal application of compacted DNA NPs loading mouse opsin promotor and wild type Rds gene was promising in mouse models of retinitis pigmentosa. Yanai et al. reported superparamagnetic iron oxide NPs (SPIONs) internalizing rat mesenchymal stem cells for retinal targeted delivery. There was a remarkable therapeutic outcome regarding this monitored intravitreal delivery beside concentration of anti-inflammatory agents in retina. Mitra et al. evaluated efficacy of CK30PEG10K loading miR200-p plasmid (an antiangiogenic factor). There was remarkable decrease of VEGFR-2 protein expression after intravitreal injection of materials in mouse model with diabetic retinopathy.

Retinoblastoma (RB) as an infrequent pediatric ocular tumor have benefited of NPs application, as well. There are several Nano-based drug and gene delivery systems illustrating great impacts for RB therapy. In Mitra et al. they provided polyethyleneimine (PEI) loaded gold NPS in conjugation with epithelial cell adhesion molecule (EpCAM) antibody and siRNA molecules. There was a remarkable decrease in EpCAM expression in RB Y79 cells after application of combined GNPs via EpCAM antibody. Authors considered this method as a novel gene delivery, which noticeably was internalized for leading cytotoxicity in cultured RB cells.

There is a developing trend in gene therapy for several ocular disease, like primary open angel glaucoma, Stargardt disease, Leber’s congenital amaurosis, AMD, retinitis pigmentosa and red-green ocular blindness.In spite of these eye-catching accomplishments in this domain, there are still so much to do for effectual drug and gene delivery to achieve explicit cures [166, 167]. By pondering these delivery methods, as a potential role in gene delivery efficacy, nanoparticulate delivery systems could accomplish promising outcomes. Moreover, there have been remarkable efforts for overcoming the problem of transient expression by non-viral gene delivery system.

NPs for drug therapy purposes

As it mentioned ahead, emerging of nanomedicine with nanotechnology have provided precious consequences in ophthalmic pharmacotherapy. A part of these accomplishments refer to NPs capability to penetrate from vitreous and orbital layers. Here we have summarized some distinguished applications of NPs in ocular disease therapy.

Anterior segment drug therapy

Commonly, anterior oculopathies (e.g. corneal wounds, cataract, keratitis) are treated by eye drops. But this therapeutic method is not effectively potential because of corneal and pre-corneal barriers. Moreover, frequent and lasting applications of them induce side effects like corneal surface deficiency, corneal and conjunctival inflammation. Instead, substantial endeavor is being focused on enhancing drug maintenance and infiltration from ocular superficial parts.

As it proposed formerly, NPs could be highly beneficial as new carriers of therapeutic agent for pointed tissues. NPs could enhance precorneal residence time and drug efficacy, avoid from enzymatic degradation and enable sustainable release of ocular drugs. Formulated NPs with mucoadhesive properties enhance their precorneal retention time for topical administration. For instance, hyaluronic acid, Carbopol and PEG have been used for improving NPs retention time in cul-de-sac.

Extensively usage of poly lactic acid (PLA) and PLGA NPs could be as respectable instances in this regard. Moreover, poly lactic glycolic acid (PLGA) NPs attracted attention regarding their ability of reducing dosing frequency for ocular disease therapy . They could improve corneal penetration of some drugs like levofloxacin, dexamethasone and sparfloxacin. for instance, PLGA NPs improved the anti-inflammatory impacts of flurbiprofen on in vitro model of rabbit cornea. Additionally, this method reached to identical suppressing roles on cell mitotic process compared to common eye drops. Such advancement specifies the remarkable effects of NPs on some ocular drugs bioavailability like flurbiprofen. Calvo et al. reported 3-fold higher penetration of indomethacin loaded by polymer NPs in rabbit cornea.