Wanted and unwanted properties of surface PEGylated nucleic acid nanoparticles in ocular gene transfer

Controlling the Biological Fate of Liposomal Spherical Nucleic Acids Using Tunable Polyethylene Glycol Shells

Liposomal spherical nucleic acids (LSNAs) modified with polyethylene glycol (PEG) units are studied in an attempt to understand how the circulation time and biodistribution of the constructs can be manipulated. Specifically, the effect of (1) PEG molecular weight, (2) PEG shell stability, and (3) PEG modification method (PEG in both the core and shell versus PEG in the shell only) on LSNA blood circulation, biodistribution, and in vivo cell internalization in a syngeneic, orthotopic triple-negative breast cancer mouse model is studied. Generally, high PEG molecular weight extends blood circulation lifetime, and a more lipophilic anchor stabilizes the PEG shell and improves circulation and tumor accumulation but at the cost of cell uptake efficiency. The PEGylation strategy has a minor effect on in vitro properties of LSNAs but significantly alters in vivo cell uptake. For example, surface-only PEG in one design contributed to higher in vivo cell internalization than its counterpart with PEG both in the shell and core. Taken together, this work provides guidelines for designing LSNAs that exhibit maximal in vivo cancer cell uptake characteristics in the context of a breast cancer model.

Antisense microRNA185 loaded liposome for efficient inhibition of the hepatic endogenous microRNA185 level

MicroRNA185 (miR185), an endogenous noncoding RNA with 23 nucleotides, is one of key posttranscriptional modulators of cholesterol metabolism in hepatic cells. The antisense inhibitor of miR185 (miR185i) could decrease cholesterol level in vivo, providing a promising agent for anti-atherosclerosis strategy. In this work, a novel LipomiR185i was constructed by thin film hydration method and post-PEGylation as DOPE: DOTAP: Chol: DSPE-PEG₂₀₀₀ at the molar ratio of 1:1:1:0.1 with a nitrogen-to-phosphate ratio of 3, through the optimization of three cationic lipids (DOTAP, DODMA and DLin-MC3-DMA), six helper lipids (PC-98T, HSPC, DOPE, DMPC, DPPC and DSPC), different amounts and incorporation approaches of DSPE-PEG₂₀₀₀ and nitrogen-to-phosphate ratio. LipomiR185i was characterized with a particle size of 174 ± 11 nm, a zeta potential of 7.0 ± 3.3 mV, high encapsulation efficiency and transfection activity. It could protect miR185i from the rapid degradation by nucleases in serum, enhance cellular uptake and promote lysosomal escape in HepG2 cells. LipomiR185i could accumulate in the liver and remain for at least two weeks. More importantly, LipomiR185i significantly down-regulated the hepatic endogenous miR185 level in vitro and in vivo without significant tissue damage at 14 mg⋅kg^-1. The construction of LipomiR185i provides a potential anti-atherosclerotic nanodrug as well as a platform for delivering small RNAs to the liver efficiently and safely.

Small interfering RNAs (siRNAs) based gene silencing strategies for the treatment of glaucoma: Recent advancements and future perspectives

RNA-interference-based mechanisms, especially the use of small interfering RNAs (siRNAs), have been under investigation for the treatment of several ailments and have shown promising results for ocular diseases including glaucoma. The eye, being a confined compartment, serves as a good target for the delivery of siRNAs. This review focuses on siRNA-based strategies for gene silencing to treat glaucoma. We have discussed the ocular structures and barriers to gene therapy (tear film, corneal, conjunctival, vitreous, and blood ocular barriers), methods of administration for ocular gene delivery (topical instillation, periocular, intracameral, intravitreal, subretinal, and suprachoroidal routes) and various viral and non-viral vectors in siRNA-based therapy for glaucoma. The components and mechanism of siRNA-based gene silencing have been mentioned briefly followed by the basic strategies and challenges faced during siRNA therapeutics development. We have emphasized different therapeutic targets for glaucoma which have been under research by scientists and the current siRNA-based drugs used in glaucoma treatment. We also mention briefly strategies for siRNA-based treatment after glaucoma surgery.

The effects of PEGylation on LNP based mRNA delivery to the eye

Gene therapy is now an effective approach to treat many forms of retinal degeneration. Delivery agents that are cell-specific, allow for multiple dosing regimens, and have low immunogenicity are needed to expand the utility of gene therapy for the retina. We generated eight novel lipid nanoparticles (LNPs) ranging in size from 50 nm to 150 nm by changing the PEG content from 5% to 0.5%, respectively. Subretinal injections of LNP-mRNA encoding luciferase revealed that 0.5% PEG content within nanoparticles elicits the highest expression. Similar injections of LNP delivered cre mRNA into Ai9 mice revealed cell-specific protein expression in the retinal pigment epithelium (RPE), confirmed by fundus photography and immunohistochemistry of whole globe cross-sections. To investigate mechanisms of LNP delivery to the eye, we injected mCherry mRNA using the subretinal approach in apoE-/- and Mertk-/- mice. RPE transfection was observed in both mouse models suggesting that LNP intracellular delivery is not solely dependent on apolipoprotein adsorption or phagocytosis. To investigate LNP penetration, particles were delivered to the vitreous chamber via an intravitreal injection. The 0.5% PEG particles mediated the highest luciferase activity and expression was observed in the Müller glia, the optic nerve head and the trabecular meshwork, but failed to reach the RPE. Overall, particles containing less PEG (~150 nm in size) mediated the highest expression in the eye. Thus far, these particles successfully transfect RPE, Müller cells, the optic nerve head and the trabecular meshwork based on route of administration which can expand the utility of LNP-mediated gene therapies for the eye.

PEGylated microemulsion for dexamethasone delivery to posterior segment of eye

Dexamethasone (Dex) is one of the most commonly used anti-vascular endothelial growth factor (anti-VEGF) drugs being used in ocular diseases whether it is associated with anterior segment or posterior segment. For diseases of posterior segment of eye, Dex is delivered as intravitreal implant but the route used for the same is very invasive and poses several hazards on long term use. Thus, topical formulation with ability to outreach retina from ocular surface was intended. Thus, polyethylene glycolylated (PEGylated) microemulsion (ME) was attempted as it can cross the membranous barrier of eye (cornea, conjunctiva, and sclera) and remain afloat in fluidic barrier (aqueous humor, choroid, etc.) as well. Present investigation involved development of Dex-loaded PEGylated ME which was stable, non-toxic to ocular surface, capable to cross cornea and enhanced residence as well as availability of loaded drug in retina. The developed PEGylated ME had physicochemical properties like size (15.98 ± 3.05 nm), polydispersity index (0.25 ± 0.04), zeta potential (-0.04 ± 0.47 mV), percentage transmittance (99.84 ± 1.17%), and drug content (99.32 ± 3.21%). It showed sustained Dex release in in vitro conditions. It also displayed efficiency in enhancing retention of drugs in retina in in vivo pharmacokinetic study on Sprague-Dawley rats. PEGylated ME can retain the drug in retina of rats longer than simple eye drop solution via topical ocular route.

Optimization, stabilization, and characterization of amphotericin B loaded nanostructured lipid carriers for ocular drug delivery

The current study sought to formulate, optimize, and stabilize amphotericin B (AmB) loaded PEGylated nanostructured lipid carriers (NLC) and to study its ocular biodistribution following topical instillation. AmB loaded PEGylated NLC (AmB-PEG-NLC) were fabricated by hot-melt emulsification followed by high-pressure homogenization (HPH) technique. 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)] (mPEG-2K-DSPE) was used for surface PEGylation. mPEG-DSPE with different PEG molecular weight, 1 K, 2 K, 5 K, 10 K, and 20 K, were screened for formulation stability. Furthermore, the AmB loaded PEGylated (2K) NLC (AmB-PEG2K-NLC) was optimized using Box-Behnken design with respect to the amount of AmB, castor oil, mPEG-2K-DSPE, and number of high-pressure homogenization cycles as the factors; particle size, zeta potential, PDI, entrapment efficiency, and loading efficiency as responses. Stability of the optimized AmB-PEG2K-NLC was assessed over 4 weeks, at 4 °C as well as 25 °C and effect of autoclaving was also evaluated. AmB-PEG2K-NLC were tested for their in vitro antifungal activity against Candida albicans (ATCC 90028), AmB resistant Candida albicans (ATCC 200955) and Aspergillus fumigatus (ATCC 204305). Cytotoxicity of AmB-PEG2K-NLC was studied in human retinal pigmented epithelium cells. In vivo ocular biodistribution of AmB was evaluated in rabbits, following topical application of PEGylated NLCs or marketed AmB preparations. PEGylation with mPEG-2K-DSPE prevented leaching of AmB and increased the drug load significantly. The optimized formulation was prepared with a particle size of 218 ± 5 nm; 0.3 ± 0.02 PDI, 4.6 ± 0.1% w/w drug loading, and 92.7 ± 2.5% w/w entrapment efficiency. The optimized colloidal dispersions were stable for over a month, at both 4 °C and 25 °C. AmB-PEG2K-NLCs showed significantly (p < 0.05) better antifungal activity in both wild-type and AmB resistant Candida strains and, was comparable to, or better than, commercially available parenteral AmB formulations like Fungizone™ and AmBisome®. AmB-PEG2K-NLC did not show any toxicity up to a highest concentration of 1% (v/v) (percent formulation in medium). Following topical instillation, AmB was detected in all the ocular tissues tested and statistically significant (p > 0.05) difference was not observed between the formulations tested. An optimized autoclavable and effective AmB-PEG2K-NLC ophthalmic formulation with at least one-month stability, in the reconstituted state, has been developed.

The obstacle course to the inner retina: Hyaluronic acid-coated lipoplexes cross the vitreous but fail to overcome the inner limiting membrane

Considerable research over the last few years has revealed dysregulation of growth factors in various retinal diseases, such as glaucoma, diabetic retinopathy and photoreceptor degenerations. The use of messengerRNA (mRNA) to transiently overexpress a specific factor could compensate for this imbalance. However, a critical challenge of this approach lies in the ability to efficiently deliver mRNA molecules to the retinal target cells. In this study we found that intravitreal (IVT) injection is an attractive approach to deliver mRNA to the retina, providing two critical barriers can be overcome: the vitreous and the inner limiting membrane (ILM). We demonstrated that the vitreous is indeed a major hurdle in the delivery of the cationic mRNA-complexes to retinal cells, both in terms of vitreal mobility and cellular uptake. To improve their intravitreal mobility and avoid unwanted extracellular interactions, we evaluated the use of hyaluronic acid (HA) as an electrostatic coating strategy. This HA-coating provided the complexes with a negative surface charge, markedly enhancing their mobility in the vitreous humor, without reducing their cellular internalization and transfection efficiency. However, although this coating strategy allows the mRNA-complexes to successfully overcome the vitreal barrier, the majority of the particles accumulated at the ILM. This study therefore underscores the crucial barrier function of the ILM toward non-viral retinal gene delivery and the need to smartly design mRNA-carriers able to surmount the vitreous as well as the ILM.

In vitro and ex vivo models to study drug delivery barriers in the posterior segment of the eye

Many ocular disorders leading to blindness could benefit from efficient delivery of therapeutics to the retina. However, despite extensive research into drug delivery vehicles and administration techniques, efficacy remains limited because of the many static and dynamic barriers present in the eye. Comprehension of the various barriers and especially how to overcome them can improve our ability to estimate the potential of existent drug delivery vectors and support the design of new ones. To this end, this review gives an overview of the most important ocular barriers for each administration route to the back of the eye. For each barrier, its biological composition and its role as an obstacle towards macromolecules, nanoparticles and viral vectors will be discussed; special attention will be paid to the influence of size, charge and lipophilicity of drug(s) (carrier) on their ability to overcome each barrier. Finally, the most significant available in vitro and ex vivo methods and models to test the potential of a therapeutic to cross each barrier are listed.

Beyond PEGylation: Alternative surface-modification of nanoparticles with mucus-inert biomaterials

Mucus is a highly hydrated viscoelastic gel present on various moist surfaces in our body including the eyes, nasal cavity, mouth, gastrointestinal, respiratory and reproductive tracts. It serves as a very efficient barrier that prevents harmful particles, viruses and bacteria from entering the human body. However, the protective function of the mucus also hampers the diffusion of drugs and nanomedicines, which dramatically reduces their efficiency. Functionalisation of nanoparticles with low molecular weight poly(ethylene glycol) (PEGylation) is one of the strategies to enhance their penetration through mucus. Recently a number of other polymers were explored as alternatives to PEGylation. These alternatives include poly(2-alkyl-2-oxazolines), polysarcosine, poly(vinyl alcohol), other hydroxyl-containing non-ionic water-soluble polymers, zwitterionic polymers (polybetaines) and mucolytic enzymes. This review discusses the studies reporting the use of these polymers or potential application to facilitate mucus permeation of nanoparticles.

RES-loaded pegylated CS NPs: for efficient ocular delivery

The objective of this study is to develop resveratrol (RES) loaded polyethylene glycols (PEGs) modified chitosan (CS) nanoparticles (NPs) by ionic gelation method for the treatment of glaucoma. While increasing the concentration of PEG, the particle size and polydispersity index of the formulations increased. Entrapment efficiency and RES loading (RL) of NPs decreased while increasing PEG concentration. The in vitro release of NPs showed an initial burst release of RES (45%) followed by controlled release. Osmolality of formulations revealed that the prepared NPs were iso-osmolar with the tear. Ocular tolerance of the NPs was evaluated using hen's egg test on the chorioallantoic membrane and it showed that the NPs were non-irritant. RES-loaded PEG-modified CS NPs shows an improved corneal permeation compared with RES dispersion. Fluorescein isothiocyanate loaded CS NPs accumulated on the surface of the cornea but the PEG-modified CS NPs crossed the cornea and reached retinal choroid. RES-loaded PEG-modified CS NPs reduced the intra-ocular pressure (IOP) by 4.3 ± 0.5 mmHg up to 8 h in normotensive rabbits. These results indicate that the developed NPs have efficient delivery of RES to the ocular tissues and reduce the IOP for the treatment of glaucoma.

PEGylation as a strategy for improving nanoparticle-based drug and gene delivery

Coating the surface of nanoparticles with polyethylene glycol (PEG), or "PEGylation", is a commonly used approach for improving the efficiency of drug and gene delivery to target cells and tissues. Building from the success of PEGylating proteins to improve systemic circulation time and decrease immunogenicity, the impact of PEG coatings on the fate of systemically administered nanoparticle formulations has, and continues to be, widely studied. PEG coatings on nanoparticles shield the surface from aggregation, opsonization, and phagocytosis, prolonging systemic circulation time. Here, we briefly describe the history of the development of PEGylated nanoparticle formulations for systemic administration, including how factors such as PEG molecular weight, PEG surface density, nanoparticle core properties, and repeated administration impact circulation time. A less frequently discussed topic, we then describe how PEG coatings on nanoparticles have also been utilized for overcoming various biological barriers to efficient drug and gene delivery associated with other modes of administration, ranging from gastrointestinal to ocular. Finally, we describe both methods for PEGylating nanoparticles and methods for characterizing PEG surface density, a key factor in the effectiveness of the PEG surface coating for improving drug and gene delivery.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Recombinant adeno-associated virus-, polyethylenimine/plasmid- and lipofectamine/carboxyfluorescein-labeled small interfering RNA-based transfection in retinal pigment epithelial cells with ultrasound and/or SonoVue

The present study was conducted to investigate the efficacy and safety of ultrasound (US)‑mediated transfection of the type 2 recombinant adeno‑associated virus (AAV) vectors encoding the enhanced green fluorescent protein (EGFP) gene (rAAV), polyethylenimine (PEI)/plasmid EGFP‑N1 (pDNA) or lipofectamine (L)/carboxyfluorescein (FAM)‑labeled small interfering RNA (siRNA) in the human ARPE‑19 retinal pigment epithelial (RPE) cell line, with or without the addition of SonoVue. Cultured RPE cells were exposed to US, with or without SonoVue under different conditions, including variation in the intensity and duration of treatment, and the dose of microbubbles. The effects of ultrasound‑targeted microbubble destruction (UTMD) on the structure of pDNA and the transfection ability of rAAV, PEI/pDNA and L/siRNA were also evaluated. Furthermore, the effect of UTMD on RPE cells was evaluated at 0 and 24 h following UTMD. US‑mediated transfection (USMT) significantly increased L/siRNA transfection efficiency, as measured by the transgene expression per cell and the percentage of transfected cells. UTMD significantly increased rAAV and PEI/pDNA transfer to RPE cells. UTMD‑mediated rAAV or PEI/pDNA delivery was more effective than USMT‑mediated delivery of siRNA. Evaluating cell viability at 24 h post‑UTMD provided more valuable information than immediate evaluation following UTMD. Furthermore, there was minimal cytotoxicity and minimal change to the structure of pDNA under the optimal parameters. UTMD/US may be of use in enhancing rAAV, PEI/pDNA and L/siRNA transgene expression of ARPE‑19 cells in vitro. Studies on the transfection of different nucleotides (such as pDNA and siRNA) and different types of vectors (chemical and biological) mediated by UTMD may provide useful information to guide future in vivo and transfection studies.

Uptake and transfection efficiency of PEGylated cationic liposome-DNA complexes with and without RGD-tagging

Steric stabilization of cationic liposome-DNA (CL-DNA) complexes is required for in vivo applications such as gene therapy. PEGylation (PEG: poly(ethylene glycol)) of CL-DNA complexes by addition of PEG2000-lipids yields sterically stabilized nanoparticles but strongly reduces their gene delivery efficacy. PEGylation-induced weakening of the electrostatic binding of CL-DNA nanoparticles to cells (leading to reduced uptake) has been considered as a possible cause, but experimental results have been ambiguous. Using quantitative live-cell imaging in vitro, we have investigated cell attachment and uptake of PEGylated CL-DNA nanoparticles with and without a custom synthesized RGD-peptide grafted to the distal ends of PEG2000-lipids. The RGD-tagged nanoparticles exhibit strongly increased cellular attachment as well as uptake compared to nanoparticles without grafted peptide. Transfection efficiency of RGD-tagged PEGylated CL-DNA NPs increases by about an order of magnitude between NPs with low and high membrane charge density (σM; the average charge per unit area of the membrane; controlled by the molar ratio of cationic to neutral lipid), even though imaging data show that uptake of RGD-tagged particles is only slightly enhanced by high σM. This suggests that endosomal escape and, as a result, transfection efficiency of RGD-tagged NPs is facilitated by high σM. We present a model describing the interactions between PEGylated CL-DNA nanoparticles and the anionic cell membrane which shows how the PEG grafting density and membrane charge density affect adhesion of nanoparticles to the cell surface.

On the role of specific interactions in the diffusion of nanoparticles in aqueous polymer solutions

Understanding nanoparticle diffusion within non-Newtonian biological and synthetic fluids is essential in designing novel formulations (e.g., nanomedicines for drug delivery, shampoos, lotions, coatings, paints, etc.), but is presently poorly defined. This study reports the diffusion of thiolated and PEGylated silica nanoparticles, characterized by small-angle neutron scattering, in solutions of various water-soluble polymers such as poly(acrylic acid) (PAA), poly(N-vinylpyrrolidone) (PVP), poly(ethylene oxide) (PEO), and hydroxyethylcellulose (HEC) probed using NanoSight nanoparticle tracking analysis. Results show that the diffusivity of nanoparticles is affected by their dimensions, medium viscosity, and, in particular, the specific interactions between nanoparticles and the macromolecules in solution; strong attractive interactions such as hydrogen bonding hamper diffusion. The water-soluble polymers retarded the diffusion of thiolated particles in the order PEO > PVP > PAA > HEC whereas for PEGylated silica particles retardation followed the order PAA > PVP = HEC > PEO. In the absence of specific interactions with the medium, PEGylated nanoparticles exhibit enhanced mobility compared to their thiolated counterparts despite some increase in their dimensions.

Nanotechnology approaches for ocular drug delivery

Blindness is a major health concern worldwide that has a powerful impact on afflicted individuals and their families, and is associated with enormous socio-economical consequences. The Middle East is heavily impacted by blindness, and the problem there is augmented by an increasing incidence of diabetes in the population. An appropriate drug/gene delivery system that can sustain and deliver therapeutics to the target tissues and cells is a key need for ocular therapies. The application of nanotechnology in medicine is undergoing rapid progress, and the recent developments in nanomedicine-based therapeutic approaches may bring significant benefits to address the leading causes of blindness associated with cataract, glaucoma, diabetic retinopathy and retinal degeneration. In this brief review, we highlight some promising nanomedicine-based therapeutic approaches for drug and gene delivery to the anterior and posterior segments.

Measuring the intravitreal mobility of nanomedicines with single-particle tracking microscopy

Aim: To develop a robust assay to evaluate and compare the intravitreal mobility of nanoparticles in the intact vitreous body. Materials & methods: Excised bovine eyes were prepared to preserve the fragile structure of the vitreous humor, while permitting high-resolution fluorescence microscopy and single-particle tracking analysis of intravitreally injected nanoparticles. This assay was validated by analyzing polystyrene beads and further employed to evaluate gene nanomedicines composed of poly(amido amine)s and plasmid DNA. Results: The assay was able to distinguish immobilized cationic nanoparticles from mobile PEGylated nanoparticles. PEGylation of the polyplexes resulted in a drastic improvement of their mobility. Conclusion: An ex vivo eye model is presented for studying nanoparticle mobility in intact vitreous humor by single-particle tracking microscopy. These results give important guidelines for developing gene- and drug-delivery nanomedicines that are compatible with intravitreal administration.

Original submitted 20 April 2012; Revised submitted 22 November 2012

Topical delivery of ocular therapeutics: carrier systems and physical methods

OBJECTIVE

The basic concepts, major mechanisms, technological developments and advantages of the topical application of lipid-based systems (microemulsions, nanoemulsions, liposomes and solid lipid nanoparticles), polymeric systems (hydrogels, contact lenses, polymeric nanoparticles and dendrimers) and physical methods (iontophoresis and sonophoresis) will be reviewed.

KEY FINDINGS

Although very convenient for patients, topical administration of conventional drug formulations for the treatment of eye diseases requires high drug doses, frequent administration and rarely provides high drug bioavailability. Thus, strategies to improve the efficacy of topical treatments have been extensively investigated. In general, the majority of the successful delivery systems are present on the ocular surface over an extended period of time, and these systems typically improve drug bioavailability in the anterior chamber whereas the physical methods facilitate drug penetration over a very short period of time through ocular barriers, such as the cornea and sclera.

SUMMARY

Although in the early stages, the combination of these delivery systems with physical methods would appear to be a promising tool to decrease the dose and frequency of administration; thereby, patient compliance and treatment efficacy will be improved.

Strategies on the nuclear-targeted delivery of genes

To improve the nuclear-targeted delivery of non-viral vectors, extensive effort has been carried out on the development of smart vectors which could overcome multiple barriers. The nuclear envelope presents a major barrier to transgene delivery. Viruses are capable of crossing the nuclear envelope to efficiently deliver their genome into the nucleus through the specialized protein components. However, non-viral vectors are preferred over viral ones because of the safety concerns associated with the latter. Non-viral delivery systems have been designed to include various types of components to enable nuclear translocation at the periphery of the nucleus. This review summarizes the progress of research regarding nuclear transport mechanisms. "Smart" non-viral vectors that have been modified by peptides and other small molecules are able to facilitate the nuclear translocation and enhance the efficacy of gene expression. The resulting technology may also enhance delivery of other macromolecules to the nucleus.

Dendrimers for gene delivery – a potential approach for ocular therapy?

Objectives

A vast number of blinding diseases have genetic aetiologies and may be treated by molecular based therapies such as antisense oligonucleotides or short interfering RNA. However, treatment success of ocular gene therapy is highly dependent on efficient delivery of such molecules.

Key findings

The majority of clinical studies for ocular gene therapy utilize viral vectors. While these have proven highly efficient, they show limited loading capacity and pose significant safety risks owing to their oncogenic and immunogenic effects. Non-viral gene carriers have emerged as a promising alternative with dendrimers providing great potential for gene therapy because of their size, shape and high density of modifiable surface groups. However, while dendrimers have been used extensively for drug and gene delivery to other organs, only a few studies have been reported on the eye.

Summary

This review focuses on the development of dendrimers for gene delivery with special emphasis on ocular gene therapy. Different synthesis approaches and types of dendrimers are discussed. Ocular gene therapy targets are highlighted with an overview of current clinical studies. The use of dendrimers in ocular gene delivery in comparison to liposomes and nanoparticles is also discussed. Finally, future prospects of tailored multifunctional dendrimers for ocular gene therapy are highlighted.

Nanoparticles in the ocular drug delivery

Ocular drug transport barriers pose a challenge for drug delivery comprising the ocular surface epithelium, the tear film and internal barriers of the blood-aqueous and blood-retina barriers. Ocular drug delivery efficiency depends on the barriers and the clearance from the choroidal, conjunctival vessels and lymphatic. Traditional drug administration reduces the clinical efficacy especially for poor water soluble molecules and for the posterior segment of the eye. Nanoparticles (NPs) have been designed to overcome the barriers, increase the drug penetration at the target site and prolong the drug levels by few internals of drug administrations in lower doses without any toxicity compared to the conventional eye drops. With the aid of high specificity and multifunctionality, DNA NPs can be resulted in higher transfection efficiency for gene therapy. NPs could target at cornea, retina and choroid by surficial applications and intravitreal injection. This review is concerned with recent findings and applications of NPs drug delivery systems for the treatment of different eye diseases.

Nanoparticle diffusion in, and microrheology of, the bovine vitreous ex vivo

Intravitreal injection of biodegradable nanoparticles (NP) holds promise for gene therapy and drug delivery to the back of the eye. In some cases, including gene therapy, NP need to diffuse rapidly from the site of injection in order to reach targeted cell types in the back of the eye, whereas in other cases it may be preferred for the particles to remain at the injection site and slowly release drugs that may then diffuse to the site of action. We studied the movements of polystyrene (PS) NP of various sizes and surface chemistries in fresh bovine vitreous. PS NP as large as 510nm rapidly penetrated the vitreous gel when coated with polyethylene glycol (PEG), whereas the movements of NP 1190nm in diameter or larger were highly restricted regardless of surface chemistry owing to steric obstruction. PS NP coated with primary amine groups (NH2) possessed positively charged surfaces at the pH of bovine vitreous (pH=7.2), and were immobilized within the vitreous gel. In comparison, PS NP coated with COOH (possessing negatively charged surfaces) in the size range of 100-200nm and at particle concentrations below 0.0025% (w/v) readily diffused through the vitreous meshwork; at higher concentrations (~0.1% w/v), these nanoparticles aggregated within vitreous. Based on the mobility of different sized PEGylated PS NP (PS-PEG), we estimated the average mesh size of fresh bovine vitreous to be ~550±50nm. The bovine vitreous behaved as an impermeable elastic barrier to objects sized 1190nm and larger, but as a highly permeable viscoelastic liquid to non-adhesive objects smaller than 510nm in diameter. Guided by these studies, we next sought to examine the transport of drug- and DNA-loaded nanoparticles in bovine vitreous. Biodegradable NP with a diameter of 227nm, composed of a poly(lactic-co-glycolic acid) (PLGA)-based core coated with poly(vinyl alcohol) rapidly penetrated vitreous. Rod-shaped, highly-compacted CK30PEG10k/DNA with PEG coating (neutral surface charge; hydrodynamic diameter ~60nm) also diffused rapidly within vitreous. These findings will help guide the development of nanoparticle-based therapeutics for the treatment of vision-threatening ocular diseases.

Challenges in neuroprotective nanomedicine development: progress towards noninvasive gene therapy of glaucoma

Over the past decade the application of gene therapy of retinal diseases such as glaucoma has produced promising results. However, optic nerve regeneration and restoration of vision in patients with glaucoma is still far from reality. Neuroprotective approaches in the form of gene therapy may provide significant advantages, but are still limited by many factors both at the organ and cellular levels. In general, gene delivery systems for eye diseases range from simple eye drops and ointments to more advanced bio- and nanotechnology-based systems such as muco-adhesive systems, polymers, liposomes and ocular inserts. Most of these technologies were developed for front-of-the-eye ophthalmic therapies and are not applicable as back-of-the-eye delivery systems. Currently, only the invasive intravitreal injections are capable of successfully delivering genes to the retina. Here we review the challenges and possible strategies for the noninvasive gene therapy of glaucoma including the barriers in the eye and in neural cells, and present a cross-sectional view of gene delivery as it pertains to the prevention and treatment of glaucoma.

Liposomes for intravitreal drug delivery: a state of the art

Intravitreal administration of drugs has raised a large interest during the last two decades improving the treatment of infectious diseases of the posterior segment of the eye or edematous maculopathies. This route of administration allows achieving high drug concentrations in the vitreous and avoiding adverse effects resulting from systemic administration. However, many drugs are rapidly cleared from the vitreous humor; therefore, to reach and to maintain effective therapy, repeated administrations are necessary. Unfortunately, frequent intravitreal injections increase the risk of endophthalmitis, damage to lens, retinal detachment. Moreover, some drugs provoke a local toxicity at their effective dose inducing side-effects and possible retinal lesions. This is the reason why new drug delivery systems, among which liposomes, have been developed to improve the intravitreal administration of drugs. Liposomes can reduce the toxicity and increase the residence time of several active molecules in the eye. In vivo, they can protect poorly-stable drugs such as peptides and nucleic acids from degradation. Successful reports have shown their potential for improving the treatment of retinitis induced by cytomegalovirus in human and more recently for the treatment of uveitis in rats. Moreover, recent preliminary studies about the trafficking of liposomes in ocular tissues and fluids following intravitreal injection attempted to elucidate their fate. All the data discussed in this review support the large interest raised by these colloidal carriers for intravitreal drug delivery.

Impacts of nanomedicines in ocular pharmacotherapy

INTRODUCTION

The integrity of the cells/tissues in anterior and/or posterior segments of the eye plays a crucial role in biofunctions of the vision. To maintain ocular homeostasis, selective restrictiveness of the ophthalmic membranes and barriers control must act on shuttling of biomolecules. Thus, not all attempts to apply de novo nanotechnology approaches for ocular pharmacotherapy have met with the same successes as those cited here in this review, and sometimes these novel technologies tools provoke a great deal of challenges and hurdles mainly because of functional presence of these barriers.

METHODS

Recent published articles related to applications of ocular nanomedicines were reviewed and highlighted in this review article.

RESULTS

It seems the emergence of nanomedicines have arisen great hopes for ophthalmic pharmacotherapy, in which nanostructured medicines are expected to be able to cross the restrictive barriers of the eye. Although such fast inauguration of ocular nanomedicines will literally convey new challenges in the regulatory and translational processes, it will also grant a prolific platform from which many exciting, and yet unimagined, applications of biomedical nanotechnology will emerge for pharmacotherapy of the eye.

CONCLUSION

This review provides recent advancements on ocular nanomedicines.

PEG-PLA microparticles for encapsulation and delivery of Tat-EGFP to retinal cells

The efficient and controlled delivery of genes and proteins to retinal cells remains a challenge. In this study, we evaluated polyethylene glycol-polylactic acid (PEG-PLA) microparticles for encapsulation and delivery of a Transactivator of transcription-enhanced green fluorescent protein fusion (Tat-EGFP) to retinal cells. Our main objective was to develop a microparticle system that delivers Tat-EGFP with an initial rapid release (within 24 h) followed by a sustained release. We prepared four different formulations of Tat-EGFP encapsulated PEG-PLA particles to investigate the effects of protein and polymer concentrations on particle morphology and protein release, using scanning electron microscopy (SEM) and fluorometry techniques. The optimum formulation was selected based on higher protein release, and smaller particle size. The optimum formulation was then tested in vitro for cell biocompatibility and protein internalization, and in vivo for cellular toxicity following sub-retinal injections into rat eyes. The results suggest that PEG-PLA microparticles can deliver proteins in cell culture allowing protein internalization in as little as 1 h. In vivo, protein was shown to localize within the photoreceptor layer of the retina, and persist for at least 9 weeks with no observed toxicity.

Ocular novel drug delivery: impacts of membranes and barriers

BACKGROUND

Ocular drug delivery is an extremely challenging area due to its restrictive barrier functionalities.

OBJECTIVE

Drug transport via corneal/non-corneal routes involves several intricate biological processes such as drug penetration across the ocular barriers and transfer to the anterior or posterior chambers, thus the influence of these processes on the pharmacotherapy of the eye should be fully addressed.

METHODS

To pursue the impacts of such impediments in novel drug therapy, recent publications were reviewed regarding advanced strategies such as nanomedicines.

CONCLUSION

The ocular barriers are highly specialized and selectively control the inward/outward traverse of compounds, hence a better understanding of these biological obstacles would provide a platform to advance ophthalmic drug therapy towards specified delivery/targeting with minimal adverse consequences.