Non-viral ocular gene therapy: potential ocular therapeutic avenues

Recent Advancements and Applications of Ophthalmic Gene Therapy Strategies: A Breakthrough in Ocular Therapeutics

Over the past twenty years, ocular gene therapy has primarily focused on addressing diseases linked to various genetic factors. The eye is an ideal candidate for gene therapy due to its unique characteristics, such as easy accessibility and the ability to target both corneal and retinal conditions, including retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), age-related macular degeneration (AMD), and Stargardt disease. Currently, literature documents 33 clinical trials in this field, with the most promising results emerging from trials focused on LCA. These successes have catalyzed further research into other ocular conditions such as glaucoma, AMD, RP, and choroideremia. The effectiveness of gene therapy relies on the efficient delivery of genetic material to specific cells, ensuring sustained and optimal gene expression over time. Viral vectors have been widely used for this purpose, although concerns about potential risks such as immune reactions and genetic mutations have led to the development of non-viral vector systems. Preliminary laboratory research and clinical investigations have shown a connection between vector dosage and the intensity of immune response and inflammation in the eye. The method of administration significantly influences these reactions, with subretinal delivery resulting in a milder humoral response compared to the intravitreal route. This review discusses various ophthalmic diseases, including both corneal and retinal conditions, and their underlying mechanisms, highlighting recent advances and applications in ocular gene therapies.

Intravitreal Delivery of PEGylated-ECO Plasmid DNA Nanoparticles for Gene Therapy of Stargardt Disease

OBJECTIVE

Current gene therapy of inherited retinal diseases is achieved mainly by subretinal injection, which is invasive with severe adverse effects. Intravitreal injection is a minimally invasive alternative for gene therapy of inherited retinal diseases. This work explores the efficacy of intravitreal delivery of PEGylated ECO (a multifunctional pH-sensitive amphiphilic amino lipid) plasmid DNA (pGRK1-ABCA4-S/MAR) nanoparticles (PEG-ELNP) for gene therapy of Stargardt disease.

METHODS

Pigmented Abca4-/- knockout mice received 1 µL of PEG-ELNP solution (200 ng/uL, pDNA concentration) by intravitreal injections at an interval of 1.5 months. The expression of ABCA4 in the retina was determined by RT-PCR and immunohistochemistry at 6 months after the second injection. A2E levels in the treated eyes and untreated controls were determined by HPLC. The safety of treatment was monitored by scanning laser ophthalmoscopy and electroretinogram (ERG).

RESULTS

PEG-ELNP resulted in significant ABCA4 expression at both mRNA level and protein level at]6 months after 2 intravitreal injections, and a 40% A2E accumulation reduction compared with non-treated controls. The PEG-ELNP also demonstrated excellent safety as shown by scanning laser ophthalmoscopy, and the eye function evaluation from electroretinogram.

CONCLUSIONS

Intravitreal delivery of the PEG-ELNP of pGRK1-ABCA4-S/MAR is a promising approach for gene therapy of Stargardt Disease, which can also be a delivery platform for gene therapy of other inherited retinal diseases.

Ocular therapies with biomacromolecules: From local injection to eyedrop and emerging noninvasive delivery strategies

The last two decades have witnessed a continuously increasing number of biomacromolecules approved for the treatment of ocular diseases. The eye possesses multiple protective mechanisms to resist the invasion of exogenous substances, but meanwhile these physiological defense systems also act as strong barriers, impeding absorption of most biomacromolecules into the eye. As a result, local injections play predominant roles for posterior ocular delivery of biomacromolecules in clinical practice. To achieve safe and convenient application of biomacromolecules, alternative strategies to realize noninvasive intraocular delivery are necessary. Various nanocarriers, novel penetration enhancers and physical strategies have been explored to facilitate delivery of biomacromolecules to both anterior and posterior ocular segments but still suffered difficulties in clinical translation. This review compares the anatomical and physiological characteristics of the eyes from those frequently adopted experimental species and profiles the well-established animal models of ocular diseases. We also summarize the ophthalmic biomacromolecules launched on the market and put emphasis on emerging noninvasive intraocular delivery strategies of peptides, proteins and genes.

Current trends in gene therapy for retinal diseases (Review)

The eye is considered an effective target for genetic therapy, as it has a privileged immune status, it is easily accessed for medication delivery and it is affected by a number of inherited disorders. In particular, the retina is considered for gene therapy due to the fact that it can be visualized with ease, it does not have lymphatic vessels, nor a direct blood network for the outer layers and its cells do not divide after birth, and thus transgene expression is not affected. As gene therapy is currently on a continuously progressive development trend, this emerging field of gene manipulation techniques has yielded promising results. This involves the development of treatments for a number of debilitating and blinding diseases, which were to date considered intractable. However, numerous unanswered questions remain as regards the long-term efficacy and safety profile of these treatments. The present review article discusses the current research status regarding genetic manipulation techniques aimed at addressing visual impairment related to retinal disorders, both inherited and degenerative.

Increase of 5-HT levels is induced both in mouse brain and HEK-293 cells following their exposure to a non-viral tryptophan hydroxylase construct

Tryptophan hydroxylase type 2 (Tph2) is the rate-limiting enzyme for serotonin (5-HT) biosynthesis in the brain. Dysfunctional Tph2 alters 5-HT biosynthesis, leading to a deficiency of 5-HT, which could have repercussions on human behavior. In the last decade, several studies have associated polymorphisms of the TPH2 gene with suicidal behavior. Additionally, a 5-HT deficiency has been implicated in various psychiatric pathologies, including alcoholism, impulsive behavior, anxiety, and depression. Therefore, the TPH2 gene could be an ideal target for analyzing the effects of a 5-HT deficiency on brain function. The aim of this study was to use the construct pIRES-hrGFP-1a-Tph2-FLAG to treat CD1-male mice and to transfect HEK-293-cells and then to evaluate whether this treatment increases 5-HT production. 5-HT levels were enhanced 48 h post-transfection, in HEK-293 cells. Three days after the ocular administration of pIRES-hrGFP-1a-Tph2-FLAG to mice, putative 5-HT production was significantly higher than in the control in both hypothalamus and amygdala, but not in the brainstem. Further research will be needed on the possible application of this treatment for psychiatric diseases involving a Tph2 dysfunction or serotonin deficiency.

Neuroprotective Effect of siRNA Entrapped in Hyaluronic Acid-Coated Lipoplexes by Intravitreal Administration

Since the possibility of silencing specific genes linked to retinal degeneration has become a reality with the use of small interfering RNAs (siRNAs), this technology has been widely studied to promote the treatment of several ocular diseases. Despite recent advances, the clinical success of gene silencing in the retina is significantly reduced by inherent anatomical and physiological ocular barriers, and new strategies are required to achieve intraocular therapeutic effectiveness. In this study, we developed lipoplexes, prepared with sodium alginate as an adjuvant and strategically coated with hyaluronic acid (HA-LIP), and investigated the potential neuroprotective effect of these systems in a retinal light damage model. Successful functionalization of the lipoplexes with hyaluronic acid was indicated in the dynamic light scattering and transmission electron microscopy results. Moreover, these HA-LIP nanoparticles were able to protect and deliver siRNA molecules targeting caspase-3 into the retina. After retinal degeneration induced by high light exposure, in vitro and in vivo quantitative reverse transcription-PCR (RT-qPCR) assays demonstrated significant inhibition of caspase-3 expression by HA-LIP. Furthermore, these systems were shown to be safe, as no evidence of retinal toxicity was observed by electroretinography, clinical evaluation or histology.

Minicircles for Investigating and Treating Arthritic Diseases

Gene delivery systems have become an essential component of research and the development of therapeutics for various diseases. Minicircles are non-viral vectors with promising characteristics for application in a variety of fields. With their minimal size, minicircles exhibit relatively high safety and efficient delivery of genes of interest into cells. Cartilage tissue lacks the natural ability to heal, making it difficult to treat osteoarthritis (OA) and rheumatoid arthritis (RA), which are the two main types of joint-related disease. Although both OA and RA affect the joint, RA is an autoimmune disease, while OA is a degenerative joint condition. Gene transfer using minicircles has also been used in many studies regarding cartilage and its diseased conditions. In this review, we summarize the cartilage-, OA-, and RA-based studies that have used minicircles as the gene delivery system.

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.

Ophthalmic Administration of a DNA Plasmid Harboring the Murine Tph2 Gene: Evidence of Recombinant Tph2-FLAG in Brain Structures

Tryptophan hydroxylase-type 2 (Tph2) is the first rate-limiting step in the biosynthesis of serotonin (5-HT) in the brain. The ophthalmic administration (Op-Ad) is a non-invasive method that allows delivering genetic vehicles through the eye and reaches the brain. Here, the murine Tph2 gene was cloned in a non-viral vector (pIRES-hrGFP-1a), generating pIRES-hrGFP-1a-Tph2, plus the FLAG-tag. Recombinant Tph2-FLAG was detected and tested in vitro and in vivo, where 25 μg of pIRES-hrGFP-1a-Tph2-FLAG was Op-Ad to mice. The construct was capable of expressing and producing the recombinant Tph2-FLAG in vitro and in vivo. The in vivo assays showed that the construct efficiently crossed the Hemato-Ocular Barrier and the Blood-Brain Barrier, reached brain cells, passed the optical nerves, and transcribed mRNA-Tph2-FLAG in different brain areas. The recombinant Tph2-FLAG was observed in amygdala and brainstem, mainly in raphe dorsal and medial. Relative Tph2 expression of threefold over basal level was recorded three days after Op-Ad. These results demonstrated that pIRES-hrGFP-Tph2-FLAG, administrated through the eyes was capable of reaching the brain, transcribing, and translating Tph2. In conclusion, this study showed the feasibility of delivering therapeutic genes, such as the Tph2, the first enzyme, rate-limiting step in the 5-HT biosynthesis.

[Anti-TNF-α in the treatment of non-infectious uveitis]

Non-infectious uveitis is a heterogenous group of potentially blinding ocular autoimmune diseases that may represent a manifestation of a systemic condition or may affect the eyes only. A systemically administered anti-TNF has recently been approved for the treatment of non-infectious uveitis, broadening the therapeutic arsenal available to control intraocular inflammation and reduce uveitis complications that can lead to vision loss. When uveitis affects only the eyes, a local anti-TNF-α administration strategy could optimize the ocular therapeutic effect and reduce undesirable systemic side-effects. A new ocular method of non-viral gene therapy, currently in development, may broaden the indications for ocular anti-TNF-α agents, not only for uveitis but also for other diseases in which TNF-α-mediated neuro-inflammation has been demonstrated.

Progress on ocular siRNA gene-silencing therapy and drug delivery systems

Age-related macular degeneration (AMD) and glaucoma are global ocular diseases with high blindness rate. RNA interference (RNAi) is being increasingly used in the treatment of these disorders with siRNA drugs, bevasiranib, AGN211745 and PF-04523655 for AMD, and SYL040012 and QPI-1007 for glaucoma. Administration routes and vectors of gene drugs affect their therapeutic effect. Compared with the non-viral vectors, viral vectors have limited payload capacity and potential immunogenicity. This review summarizes the progress of the ocular siRNA gene-silencing therapy by focusing on siRNA drugs for AMD and glaucoma already used in clinical research, the main routes of drug delivery and the non-viral vectors for siRNA drugs.

Pharmaceutical Development of AAV-Based Gene Therapy Products for the Eye

A resurgence of interest and investment in the field of gene therapy, driven in large part by advances in viral vector technology, has recently culminated in United States Food and Drug Administration approval of the first gene therapy product targeting a disease caused by mutations in a single gene. This product, LUXTURNA™ (voretigene neparvovec-rzyl; Spark Therapeutics, Inc., Philadelphia, PA), delivers a normal copy of the RPE65 gene to retinal cells for the treatment of biallelic RPE65 mutation–associated retinal dystrophy, a blinding disease. Many additional gene therapy programs targeting both inherited retinal diseases and other ocular diseases are in development, owing to an improved understanding of the genetic basis of ocular disease and the unique properties of the ocular compartment that make it amenable to local gene therapy. Here we review the growing body of literature that describes both the design and development of ocular gene therapy products, with a particular emphasis on target and vector selection, and chemistry, manufacturing, and controls.

Non-viral ocular gene therapy, pEYS606, for the treatment of non-infectious uveitis: Preclinical evaluation of the medicinal product

Non-infectious uveitis (NIU) is the first cause of blindness that can be cured if optimal anti-inflammatory therapy can be achieved. Systemic anti-TNF (Tumor Necrosis Factor) agents have been recently approved for NIU but no local delivery of anti-TNF is available. For sustained production of secreted therapeutic proteins into the eye, non-viral gene therapy using plasmid electrotransfer in the ciliary muscle has been proposed. In this paper, we report the development steps of pEYS606, a clinical-grade plasmid DNA, devoid of antiobiotic selection gene, encoding a fusion protein consisting of the extracellular domain of the soluble p55 TNF-α receptor linked to the human IgG1 Fc domain (hTNFR-Is/hIgG1 or Protein 6), with high affinity for human TNF-α, for non-viral gene transfer into the ocular ciliary muscle. Electrotransfer of pEYS606 in the ciliary muscle significantly reduced ocular inflammation in two well-established rat models of uveitis, the endotoxin-induced uveitis (EIU) and the experimental autoimmune uveitis (EAU). In addition, in EAU, a significant protection of photoreceptors was demonstrated after pEYS606 treatment. The improved pharmacokinetic profile of intraocularly-secreted protein as compared to direct intravitreous injection of recombinant protein allowed to demonstrate Protein 6 efficacy at very low concentrations. Based on these results, a phase I/II clinical trial is conducted [ClinicalTrials.gov Identifier: NCT03308045].

Principles of pharmacology in the eye

The eye is a highly specialized organ that is subject to a huge range of pathology. Both local and systemic disease may affect different anatomical regions of the eye. The least invasive routes for ocular drug administration are topical (e.g. eye drops) and systemic (e.g. tablets) formulations. Barriers that subserve as protection against pathogen entry also restrict drug permeation. Topically administered drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiological properties of the cornea, the limited volume that can be accommodated by the cul-de-sac, the lacrimal drainage system and reflex tearing. The tissue layers of the cornea and conjunctiva are further key factors that act to restrict drug delivery. Using carriers that enhance viscosity or bind to the ocular surface increases bioavailability. Matching the pH and polarity of drug molecules to the tissue layers allows greater penetration. Drug delivery to the posterior segment is a greater challenge and, currently, the standard route is via intravitreal injection, notwithstanding the risks of endophthalmitis and retinal detachment with frequent injections. Intraocular implants that allow sustained drug release are at different stages of development. Novel exciting therapeutic approaches include methods for promoting transscleral delivery, sustained release devices, nanotechnology and gene therapy.

Noninvasive delivery of oligonucleotide by penetratin-modified polyplexes to inhibit protein expression of intraocular tumor

Our present study aimed to develop an antisense oligonucleotide (ASO) delivery system to achieve gene silencing in intraocular tumor via topical instillation. ASO specific for luciferase was chosen as model drug, polyamidoamine (PG5) was employed to condense ASO, and penetratin (Pene) was used to enhance cellular uptake. Nanoscale PG5/ASO/Pene polyplex was stabilized via noncovalent bonding. In vitro evaluations indicated that PG5/ASO/Pene exhibited improved cell-penetrating and gene silencing ability compared with naked ASO and PG5/ASO. Subcutaneous and orthotopic tumor models expressing luciferase were established in nude mice. After treated by PG5/ASO/Pene, immunohistochemical results of subcutaneous tumors showed significant inhibition of luciferase expression via peritumoral injection, and bioluminescence from orthotopic tumor was obviously weakened via topical instillation. To date, few works were successful in noninvasive treatment of intraocular diseases using antisense strategy, this penetratin-modified polyplex could be a promising vector to inhibit protein expression by effectively delivering ASOs into the eye.

Non-viral strategies for ocular gene delivery

The success of gene therapy relies on efficient gene transfer and stable transgene expression. The in vivo efficiency is determined by the delivery vector, route of administration, therapeutic gene, and target cells. While some requirements are common to several strategies, others depend on the target disease and transgene product. Consequently, it is unlikely that a single system is suitable for all applications. This review examines current gene therapy strategies, focusing on non-viral approaches and the use of natural polymers with the eye, and particularly the retina, as their gene delivery target.

In vivo genome editing as a potential treatment strategy for inherited retinal dystrophies

In vivo genome editing represents an emerging field in the treatment of monogenic disorders, as it may constitute a solution to the current hurdles in classic gene addition therapy, which are the low levels and limited duration of transgene expression. Following the introduction of a double strand break (DSB) at the mutational site by highly specific endonucleases, such as TALENs (transcription activator like effector nucleases) or RNA based nucleases (clustered regulatory interspaced short palindromic repeats - CRISPR-Cas), the cell's own DNA repair machinery restores integrity to the DNA strand and corrects the mutant sequence, thus allowing the cell to produce protein levels as needed. The DNA repair happens either through the error prone non-homologous end-joining (NHEJ) pathway or with high fidelity through homology directed repair (HDR) in the presence of a DNA donor template. A third pathway called microhomology mediated endjoining (MMEJ) has been recently discovered. In this review, the authors focus on the different DNA repair mechanisms, the current state of the art tools for genome editing and the particularities of the retina and photoreceptors with regard to in vivo therapeutic approaches. Finally, current attempts in the field of retinal in vivo genome editing are discussed and future directions of research identified.

Span poly-L-arginine nanoparticles are efficient non-viral vectors for PRPF31 gene delivery: An approach of gene therapy to treat retinitis pigmentosa

Retinitis pigmentosa (RP) is the most common cause of inherited blindness in adults. Mutations in the PRPF31 gene produce autosomal dominant RP (adRP). To date there are no effective treatments for this disease. The purpose of this study was to design an efficient non-viral vector for human PRPF31 gene delivery as an approach to treat this form of adRP. Span based nanoparticles were developed to mediate gene transfer in the subretinal space of a mouse model of adRP carrying a point mutation (A216P) in the Prpf31 gene. Funduscopic examination, electroretinogram, optomotor test and optical coherence tomography were conducted to further in vivo evaluate the safety and efficacy of the nanosystems developed. Span-polyarginine (SP-PA) nanoparticles were able to efficiently transfect the GFP and PRPF31 plasmid in mice retinas. Statistically significant improvement in visual acuity and retinal thickness were found in Prpf31^A216P/+ mice treated with the SP-PA-PRPF31 nanomedicine.

Application of lipid nanoparticles to ocular drug delivery

INTRODUCTION

Although eye drops are widely used as drug delivery systems for the anterior segment of the eye, they are also associated with poor drug bioavailability due to transient contact time and rapid washout by tearing. Moreover, effective drug delivery to the posterior segment of the eye is challenging, and alternative routes of administration (periocular and intravitreal) are generally needed, the blood-retinal barrier being the major obstacle to systemic drug delivery. Areas covered: Nanotechnology, and especially lipid nanoparticles, can improve the therapeutic efficiency, compliance and safety of ocular drugs, administered via different routes, to both the anterior and posterior segment of the eye. This review highlights the main ocular barriers to drug delivery, as well as the most common eye diseases suitable for pharmacological treatment in which lipid nanoparticles have proved efficacious as alternative delivery systems. Expert opinion: Lipid-based nanocarriers are among the most biocompatible and versatile means for ocular delivery. Mucoadhesion with consequent increase in pre-corneal retention time, and enhanced permeation due to cellular uptake by corneal epithelial cells, are the essential goals for topical lipid nanoparticle delivery. Gene delivery to the retina has shown very promising results after intravitreal administration of lipid nanoparticles as non-viral vectors.

The influence of the polar head-group of synthetic cationic lipids on the transfection efficiency mediated by niosomes in rat retina and brain

The development of novel non-viral delivery vehicles is essential in the search of more efficient strategies for retina and brain diseases. Herein, optimized niosome formulations prepared by oil-in water (o/w) and film-hydration techniques were characterized in terms of size, PDI, zeta potential, morphology and stability. Three ionizable glycerol-based cationic lipids containing a primary amine group (lipid 1), a triglycine group (lipid 2) and a dimethylamino ethyl pendent group (lipid 3) as polar head-groups were part of such niosomes. Upon the addition of pCMS-EGFP plasmid, nioplexes were obtained at different cationic lipid/DNA ratios (w/w). The resultant nioplexes were further physicochemically characterized and evaluated to condense, release and protect the DNA against enzymatic digestion. In vitro experiments were performed to evaluate transfection efficiency and cell viability in HEK-293, ARPE-19 and PECC cells. Interestingly, niosome formulations based on lipid 3 showed better transfection efficiencies in ARPE-19 and PECC cells than the rest of cationic lipids showed in this study. In vivo experiments in rat retina after intravitreal and subretinal injections together with in rat brain after cerebral cortex administration showed promising transfection efficiencies when niosome formulations based on lipid 3 were used. These results provide new insights for the development of non-viral vectors based on cationic lipids and their applications for efficient delivery of genetic material to the retina and brain.

Protamine/DNA/Niosome Ternary Nonviral Vectors for Gene Delivery to the Retina: The Role of Protamine

The present study aimed to evaluate the incorporation of protamine into niosome/DNA vectors to analyze the potential application of this novel ternary formulation to deliver the pCMS-EGFP plasmid into the rat retina. Binary vectors based on niosome/DNA and ternary vectors based on protamine/DNA/niosomes were prepared and physicochemically characterized. In vitro experiments were performed in ARPE-19 cells. At 1:1:5 protamine/DNA/niosome mass ratio, the resulted ternary vectors had 150 nm size, positive charge, spherical morphology, and condensed, released, and protected the DNA against enzymatic digestion. The presence of protamine in the ternary vectors improved transfection efficiency, cell viability, and DNA condensation. After ocular administration, the EGFP expression was detected in different cell layers of the retina depending on the administration route without any sign of toxicity associated with the formulations. While subretinal administration transfected mainly photoreceptors and retinal pigment epithelial cells at the site of injection, intravitreal administration produced a more uniform distribution of the protein expression through the inner layers of the retina. The protein expression in the retina persisted for at least one month after both administrations. Our study highlights the flattering properties of protamine/DNA/niosome ternary vectors for efficient and safe gene delivery to the rat retina.

Gene therapy for ocular diseases meditated by ultrasound and microbubbles (Review)

The eye is an ideal target organ for gene therapy as it is easily accessible and immune‑privileged. With the increasing insight into the underlying molecular mechanisms of ocular diseases, gene therapy has been proposed as an effective approach. Successful gene therapy depends on efficient gene transfer to targeted cells to prove stable and prolonged gene expression with minimal toxicity. At present, the main hindrance regarding the clinical application of gene therapy is not the lack of an ideal gene, but rather the lack of a safe and efficient method to selectively deliver genes to target cells and tissues. Ultrasound‑targeted microbubble destruction (UTMD), with the advantages of high safety, repetitive applicability and tissue targeting, has become a potential strategy for gene‑ and drug delivery. When gene‑loaded microbubbles are injected, UTMD is able to enhance the transport of the gene to the targeted cells. High‑amplitude oscillations of microbubbles act as cavitation nuclei which can effectively focus ultrasound energy, produce oscillations and disruptions that increase the permeability of the cell membrane and create transient pores in the cell membrane. Thereby, the efficiency of gene therapy can be significantly improved. The UTMD‑mediated gene delivery system has been widely used in pre‑clinical studies to enhance gene expression in a site‑specific manner in a variety of organs. With reasonable application, the effects of sonoporation can be spatially and temporally controlled to improve localized tissue deposition of gene complexes for ocular gene therapy applications. In addition, appropriately powered, focused ultrasound combined with microbubbles can induce a reversible disruption of the blood‑retinal barrier with no significant side effects. The present review discusses the current status of gene therapy of ocular diseases as well as studies on gene therapy of ocular diseases meditated by UTMD.

Inorganic nanovectors for nucleic acid delivery

Nucleic acids show immense potential to treat cancer, acquired immune deficiency syndrome, neurological diseases and other incurable human diseases. Upon systemic administration, they encounter a series of barriers and hence barely reach the site of action, the cell. Intracellular delivery of nucleic acids is facilitated by nanovectors, both viral and non-viral. A major advantage of non-viral vectors over viral vectors is safety. Nanovectors evaluated specifically for nucleic acid delivery include polyplexes, lipoplexes and other cationic carrier-based vectors. However, more recently there is an increased interest in inorganic nanovectors for nucleic acid delivery. Nevertheless, there is no comprehensive review on the subject. The present review would cover in detail specific properties and types of inorganic nanovectors, their preparation techniques and various biomedical applications as therapeutics, diagnostics and theranostics. Future prospects are also suggested.

Magnetic nanoparticle and magnetic field assisted siRNA delivery in vitro

This chapter describes how to design and conduct experiments to deliver siRNA to adherent cell cultures in vitro by magnetic force-assisted transfection using self-assembled complexes of small interfering RNA (siRNA) and cationic lipids or polymers that are associated with magnetic nanoparticles (MNPs). These magnetic complexes are targeted to the cell surface by the application of a gradient magnetic field. A further development of the magnetic drug-targeting concept is combining it with an ultrasound-triggered delivery using magnetic microbubbles as a carrier for gene or drug delivery. For this purpose, selected MNPs, phospholipids, and siRNAs are assembled in the presence of perfluorocarbon gas into flexible formulations of magnetic lipospheres (microbubbles). Methods are described how to accomplish the synthesis of magnetic nanoparticles for magnetofection and how to test the association of siRNA with the magnetic components of the transfection vector. A simple method is described to evaluate magnetic responsiveness of the magnetic siRNA transfection complexes and estimate the complex loading with magnetic nanoparticles. Procedures are provided for the preparation of magnetic lipoplexes and polyplexes of siRNA as well as magnetic microbubbles for magnetofection and downregulation of the target gene expression analysis with account for the toxicity determined using an MTT-based respiration activity test. A modification of the magnetic transfection triplexes with INF-7, fusogenic peptide, is described resulting in reporter gene silencing improvement in HeLa, Caco-2, and ARPE-19 cells. The methods described can also be useful for screening vector compositions and novel magnetic nanoparticle preparations for optimized siRNA transfection by magnetofection in any cell type.

Treatment of ocular disorders by gene therapy

Gene therapy to treat ocular disorders is still starting, and current therapies are primarily experimental, with most human clinical trials still in research state, although beginning to show encouraging results. Currently 33 clinical trials have been approved, are in progress, or have been completed. The most promising results have been obtained in clinical trials of ocular gene therapy for Leber Congenital Amaurosis, which have prompted the study of several ocular diseases that are good candidates to be treated with gene therapy: glaucoma, age-related macular degeneration, retinitis pigmentosa, or choroideremia. The success of gene therapy relies on the efficient delivery of the genetic material to target cells, achieving optimum long-term gene expression. Although viral vectors have been widely used, their potential risk associated mainly with immunogenicity and mutagenesis has promoted the design of non-viral vectors. In this review, the main administration routes and the most studied delivery systems, viral and non-viral, for ocular gene therapy are presented. The primary ocular disease candidates to be treated with gene therapy have been also reviewed, including the genetic basis and the most relevant preclinical and clinical studies.

Sustained gene expression in the retina by improved episomal vectors

Gene and cellular therapies are nowadays part of therapeutic strategies for the treatment of diverse pathologies. The drawbacks associated with gene therapy-low levels of transgene expression, vector loss during mitosis, and gene silencing-need to be addressed. The pEPI-1 and pEPito family of vectors was developed to overcome these limitations. It contains a scaffold/matrix attachment region, which anchors its replication to cell division in eukaryotic cells while in an extrachromosomal state and is less prone to silencing, due to a lower number of CpG motifs. Recent success showed that ocular gene therapy is an important tool for the treatment of several diseases, pending the overcome of the aforementioned limitations. To achieve sustained gene delivery in the retina, we evaluated several vectors based on pEPito and pEPI-1 for their ability to sustain transgene expression in retinal cells. These vectors stably transfected and replicated in retinal pigment epithelial (RPE) cells. Expression levels were promoter dependent with constitutive promoters cytomegalovirus immediate early promoter (CMV) and human CMV enhancer/human elongation factor 1 alpha promoter yielding the highest levels of transgene expression compared with the retina-specific RPE65 promoter. When injected in C57Bl6 mice, transgene expression was sustained for at least 32 days. Furthermore, the retina-specific RPE65 promoter showed higher efficiency in vivo compared to in vitro. In this study, we demonstrate that by combining tissue-specific promoters with a mitotic stable system, less susceptible to epigenetic silencing such as pEPito-based plasmids, we can achieve prolonged gene expression and a sustained therapeutic effect.

A novel formulation based on 2,3-di(tetradecyloxy)propan-1-amine cationic lipid combined with polysorbate 80 for efficient gene delivery to the retina

PURPOSE

The aim of the present study was to evaluate the potential application of a novel formulation based on a synthesized cationic lipid 2,3-di(tetradecyloxy)propan-1-amine, combined with polysorbate 80 to deliver the pCMS-EGFP plasmid into the rat retina.

METHODS

We elaborated lipoplexes by mixing the formulation containing the cationic lipid and the polysorbate 80 with the plasmid at different cationic lipid/DNA ratios (w/w). Resulted lipoplexes were characterized in terms of size, charge, and capacity to condense, protect and release the DNA. In vitro transfection studies were performed in HEK-293 and ARPE-19 cells. Formulations were also tested in vivo by monitoring the expression of the EGFP after intravitreal and subretinal injections in rat eyes.

RESULTS

At 2/1 cationic lipid/DNA mass ratio, the resulted lipoplexes had 200 nm of hydrodynamic diameter; were positive charged, spherical, protected DNA against enzymatic digestion and transfected efficiently HEK-293 and ARPE-19 cultured cells exhibiting lower cytotoxicity than LipofectamineTM 2000. Subretinal administrations transfected mainly photoreceptors and retinal pigment epithelial cells; whereas intravitreal injections produced a more uniform distribution of transfection through the inner part of the retina.

CONCLUSIONS

These results hold great expectations for other gene delivery formulations based on this cationic lipid for retinal gene therapy purposes.

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

In vivo electroporation of minicircle DNA as a novel method of vaccine delivery to enhance HIV-1-specific immune responses

DNA vaccines offer advantage over conventional vaccines, as they are safer to use, easier to produce, and able to induce humoral as well cellular immune responses. Unfortunately, no DNA vaccines have been licensed for human use for the difficulties in developing an efficient and safe in vivo gene delivery system. In vivo electroporation (EP)-based DNA delivery has attracted great attention for its potency to enhance cellular uptake of DNA vaccines and function as an adjuvant. Minicircle DNA (a new form of DNA containing only a gene expression cassette and lacking a backbone of bacterial plasmid DNA) is a powerful candidate of gene delivery in terms of improving the levels and the duration of transgene expression in vivo. In this study, as a novel vaccine delivery system, we combined in vivo EP and the minicircle DNA carrying a codon-optimized HIV-1 gag gene (minicircle-gag) to evaluate the immunogenicity of this system. We found that minicircle-gag conferred persistent and high levels of gag expression in vitro and in vivo. The use of EP delivery further increased minicircle-based gene expression. Moreover, when delivered by EP, minicircle-gag vaccination elicited a 2- to 3-fold increase in cellular immune response and a 1.5- to 3-fold augmentation of humoral immune responses compared with those elicited by a pVAX1-gag positive control. Increased immunogenicity of EP-assisted minicircle-gag may benefit from increasing local antigen expression, upregulating inflammatory genes, and recruiting immune cells. Collectively, in vivo EP of minicircle DNA functions as a novel vaccine platform that can enhance efficacy and immunogenicity of DNA vaccines.

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.

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.

Microprecision delivery of oligonucleotides in a 3D tissue model and its characterization using optical imaging

Despite significant potential of oligonucleotides (ONs) for therapeutic and diagnostic applications, rapid and widespread intracellular delivery of ONs in cells situated in tissues such as skin, head and neck cavity, and eye has not been achieved. This study was aimed at evaluating the synergistic combination of microneedle (MN) arrays and biochemical approaches for localized intratissue delivery of oligonucleotides in living cells in 3D tissue models. This synergistic combination was based on the ability of MNs to precisely deliver ONs into tissues to achieve widespread distribution, and the ability of biochemical agents (streptolysin O (SLO) and cholesterol conjugation to ONs) to enhance intracellular ON delivery. The results of this study demonstrate that ON probes were uniformly coated on microneedle arrays and were efficiently released from the microneedle surface upon insertion in tissue phantoms. Co-insertion of microneedles coated with ONs and SLO into 3D tissue models resulted in delivery of ONs into both the cytoplasm and nucleus of cells. Within a short incubation time (35 min), ONs were observed both laterally and along the depth of a 3D tissue up to a distance of 500 μm from the microneedle insertion point. Similar widespread intratissue distribution of ONs was achieved upon delivery of ON-cholesterol conjugates. Uniformity of ON delivery in tissues improved with longer incubation times (24 h) postinsertion. Using cholesterol-conjugated ONs, delivery of ON probes was limited to the cytoplasm of cells within a tissue. Finally, delivery of cholesterol-conjugated anti-GFP ON resulted in reduction of GFP expression in HeLa cells. In summary, the results of this study provide a novel approach for efficient intracellular delivery of ONs in tissues.

Delivery systems and local administration routes for therapeutic siRNA

With the increasing number of studies proposing new and optimal delivery strategies for the efficacious silencing of gene-related diseases by the local administration of siRNAs, the present review aims to provide a broad overview of the most important and latest developments of non-viral siRNA delivery systems for local administration. Moreover, the main disease targets for the local delivery of siRNA to specific tissues or organs, including the skin, the lung, the eye, the nervous system, the digestive system and the vagina, were explored.

Polymeric vectors for ocular gene delivery

Gene therapy holds promise for the treatment of many inherited and acquired diseases of the eye. Successful ocular gene therapy interventions depend on efficient gene transfer to targeted cells with minimal toxicity. A major challenge is to overcome both intracellular and extracellular barriers associated with ocular gene delivery. Numerous viral and nonviral vectors were explored to improve transfection efficiency. Among nonviral delivery systems, polymeric vectors have gained significant attention in recent years owing to their nontoxic and non-immunogenic nature. Polyplexes or nanoparticles can be prepared by interaction of cationic polymers with DNA, which facilitate cellular uptake, endolysosomal escape and nuclear entry through active mechanisms. Chemical modification of these polymers allows for the generation of flexible delivery vectors with desirable properties. In this article several synthetic and natural polymeric systems utilized for ocular gene delivery are discussed.

Dextran and protamine-based solid lipid nanoparticles as potential vectors for the treatment of X-linked juvenile retinoschisis

The goal of the present study was to analyze the potential application of nonviral vectors based on solid lipid nanoparticles (SLN) for the treatment of ocular diseases by gene therapy, specifically X-linked juvenile retinoschisis (XLRS). Vectors were prepared with SLN, dextran, protamine, and a plasmid (pCMS-EGFP or pCEP4-RS1). Formulations were characterized and the in vitro transfection capacity as well as the cellular uptake and the intracellular trafficking were studied in ARPE-19 cells. Formulations were also tested in vivo in Wistar rat eyes, and the efficacy was studied by monitoring the expression of enhanced green fluorescent protein (EGFP) after intravitreal, subretinal, and topical administration. The presence of dextran and protamine in the SLN improved greatly the expression of retinoschisin and EGFP in ARPE-19 cells. The nuclear localization signals of protamine, its ability to protect the DNA, and a shift in the entry mechanism from caveola-mediated to clathrin-mediated endocytosis promoted by the dextran, justify the increase in transfection. After ocular administration of the dextran-protamine-DNA-SLN complex to rat eyes, we detected the expression of EGFP in various types of cells depending on the administration route. Our vectors were also able to transfect corneal cells after topical application. We have demonstrated the potential usefulness of our nonviral vectors loaded with XLRS1 plasmid and provided evidence for their potential application for the management or treatment of degenerative retinal disorders as well as ocular surface diseases.

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.

Suprachoroidal electrotransfer: a nonviral gene delivery method to transfect the choroid and the retina without detaching the retina

Photoreceptors and retinal pigment epithelial cells (RPE) targeting remains challenging in ocular gene therapy. Viral gene transfer, the only method having reached clinical evaluation, still raises safety concerns when administered via subretinal injections. We have developed a novel transfection method in the adult rat, called suprachoroidal electrotransfer (ET), combining the administration of nonviral plasmid DNA into the suprachoroidal space with the application of an electrical field. Optimization of injection, electrical parameters and external electrodes geometry using a reporter plasmid, resulted in a large area of transfected tissues. Not only choroidal cells but also RPE, and potentially photoreceptors, were efficiently transduced for at least a month when using a cytomegalovirus (CMV) promoter. No ocular complications were recorded by angiographic, electroretinographic, and histological analyses, demonstrating that under selected conditions the procedure is devoid of side effects on the retina or the vasculature integrity. Moreover, a significant inhibition of laser induced-choroidal neovascularization (CNV) was achieved 15 days after transfection of a soluble vascular endothelial growth factor receptor-1 (sFlt-1)-encoding plasmid. This is the first nonviral gene transfer technique that is efficient for RPE targeting without inducing retinal detachment. This novel minimally invasive nonviral gene therapy method may open new prospects for human retinal therapies.

Subretinal delivery and electroporation in pigmented and nonpigmented adult mouse eyes

Subretinal injection offers one of the best ways to deliver many classes of drugs, reagents, cells and treatments to the photoreceptor, Müller, and retinal pigment epithelium (RPE) cells of the retina. Agents delivered to this space are placed within microns of the intended target cell, accumulating to high concentrations because there is no dilution due to transport processes or diffusion. Dilution in the interphotoreceptor space (IPS) is minimal because the IPS volume is only 10-20 μl in the human eye and less than 1 μl in the mouse eye. For gene delivery purposes, we wished to transfect the cells adjacent to the IPS in adult mouse eyes. Others transfect these cells in neonatal rats to study the development of the retina. In both neonates and adults, electroporation is found to be effective. Here we describe the optimization of electroporation conditions for RPE cells in the adult mouse eye with naked plasmids. However, both techniques, subretinal injection and electroporation, present some technical challenges that require skill on the part of the surgeon to prevent untoward damage to the eye. Here we describe methods that we have used for the past 10 years (Johnson et al. Mol Vis 14: 2211-2226, 2008).

Nano-vectors for the Ocular Delivery of Nucleic Acid-based Therapeutics

Nucleic acid-based therapeutics have gained a lot of interest for the treatment of diverse ophthalmic pathologies. The first to enter in clinic has been an oligonucleotide, Vitravene^® for the treatment of cytomegalovirus infection. More recently, research on aptamers for the treatment of age related macular degeneration has led to the development of Macugen^®. Despite intense potential, effective ocular delivery of nucleic acids is a major challenge since therapeutic targets for nucleic acid-based drugs are mainly located in the posterior eye segment, requiring repeated invasive administration. Of late, nanotechnology-based nano-vectors have been developed in order to overcome the drawbacks of viral and other non-viral vectors. The diversity of nano-vectors allows for ease of use, flexibility in application, low-cost of production, higher transfection efficiency and enhanced genomic safety. Using nano-vector strategies, nucleic acids can be delivered either encapsulated or complexed with cationic lipids, polymers or peptides forming sustained release systems, which can be tailored according to the ocular tissue being targeted. The present review focuses on developments and advances in various nano-vectors for the ocular delivery of nucleic acid-based therapeutics, the barriers that such delivery systems face and methods to overcome them.

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.

Ocular drug delivery - a look towards nanobioadhesives

IMPORTANCE OF THE FIELD

A major challenge emanating in the design of topical ophthalmic preparations is their short precorneal residence time. Retention of a drug delivery system in the front of the eye is thus desirable. One solution identified to address this concern is a retentive system that can preferably be delivered in a liquid drop form and ultimately remain attached to the corneal tissue owing to incorporation of a bioadhesive component. Forward-thinking approaches are required to achieve advancements in this approach for the attainment of an effective drug concentration at the site of action. Accordingly, several investigators have identified the benefits of nanotechnology-based drug delivery systems for ophthalmic drug delivery.

AREAS COVERED IN THIS REVIEW

A concerted effort was made to review critically all 'nanobioadhesives', that is, nanosystems designed for ocular drug delivery with the goal of attaining prolonged ocular retention, in a systematic, chronological manner, from their reported point of inception to the present.

WHAT THE READER WILL GAIN

A perspective on possible future trends in this growing field of ocular drug delivery is formulated.

TAKE HOME MESSAGE

The importance of and need for new developments in the field of ocular nanobioadhesives is emphasized.

Chitosan-based nanostructures: a delivery platform for ocular therapeutics

Nanoscience and nanotechnology has caused important breakthroughs in different therapeutic areas. In particular, the application of nanotechnology in ophthalmology has led to the development of novel strategies for the treatment of ocular disorders. Indeed, the association of an active molecule to a nanocarrier allows the molecule to intimately interact with specific ocular structures, to overcome ocular barriers and to prolong its residence in the target tissue. Over the last decade, our group has designed and developed a delivery platform based on the polysaccharide chitosan, which suits the requirements of the topical ocular route. These nanosystems have been specifically adapted for the delivery of hydrophilic and lipophilic drugs and also polynucleotides onto the eye surface. The results collected up until now suggest the potential of this delivery platform and the subsequent need of a full preclinical evaluation in order to satisfy the specific regulatory demands of this mode of administration.

Chitosan nanoparticles for drug delivery to the eye

The purpose of this review is to provide the reader with an overview of the advances made in ocular delivery of bioactive molecules by means of chitosan-based nanosystems, and their potential relevance in clinical use. The studies described clearly emphasise that chitosan-based nanostructures are versatile systems that can be tailor-made according to required compositions, surface characteristics and particle size. Such parameters, which are known to influence their in vivo performance, can be modulated by adjusting the formulation conditions of the nanotechnologies responsible for their formation, by incorporating additional materials in the preparation steps, and/or by using synthetically modified chitosan. Moreover, this review illustrates how the advances achieved in the understanding of the interaction of nanosystems with the ocular structures should result in the coming years, logically, into challenging innovations in ocular nanomedicines with significant impact on clinical practice.