Nanoparticle-based technologies for retinal gene therapy

High-Efficiency Ocular Delivery of Brain-Derived Neurotrophic Factor and Oligomycin for Neuroprotection in Glaucoma

Glaucoma is a retinal neurodegenerative disease characterized by progressive apoptosis of retinal ganglion cells (RGCs) and irreversible visual impairment. Current therapies rarely offer direct protection for RGCs, highlighting the need for new neuroprotective approaches. Although viral delivery of brain-derived neurotrophic factor (BDNF) has shown potential, concerns about retinal inflammation and limited applicability persist. Meanwhile, non-viral vectors remain inefficient for in vivo ocular gene delivery. Here, a highly biocompatible nanoplatform-PBAE-PLGA-Oligomycin-pBDNF nanoparticles (PPOB NPs) is reported-that co-delivers oligomycin (an ATP inhibitor) and a BDNF plasmid to Müller cells in vivo. This nanoplatform attains an unprecedented transfection efficiency of 64.26% in Müller cells, thereby overcoming the limitations of monotherapeutic neurotrophic approaches that fail to inhibit ATP overproduction and attendant inflammatory responses. In a chronic ocular hypertension rat model, oligomycin effectively mitigated RGC damage by suppressing Müller cell hyperactivation and excessive ATP production under elevated intraocular pressure. Concurrently, it synergistically enhanced BDNF expression in Müller cells, achieving robust protection of RGCs and preservation of optic nerve function. These findings underscore the promise of PPOB NPs as a dual-functional platform, featuring high biocompatibility and efficient gene delivery, for multifaceted therapies against glaucoma and other ocular diseases.

Advancing ocular gene therapy: a machine learning approach to enhance delivery, uptake and gene expression

Ocular gene therapy offers a promising approach for treating various eye diseases, centered on the process of transfection, including delivery, cellular uptake and gene expression. This study addresses anatomical and physiological barriers, such as the eyelids, tear film, conjunctiva, cornea, sclera, choroid and retina, affecting therapeutic success. A three-step machine-learning approach is proposed. The first step predicts gene delivery efficacy by integrating molecular characteristics of the ocular gene therapy product, ocular barrier properties and patient demographics. The second step predicts cellular uptake rates, analyzing product penetration and cellular interactions. The final step forecasts gene expression levels, considering factors like nucleic acid type and endosomal escape. An artificial neural network model is recommended to capture complex, nonlinear relationships, enhancing our understanding of therapeutic and biological interactions.

The Contribution of Nanomedicine in Ocular Oncology

Nanomedicine is a novel and emerging field that has noted significant progress in both the fields of ophthalmology and cancer treatment. Expanding into ocular oncology, it holds the potential to overcome the limitations of conventional therapies, such as poor drug penetration due to anatomical and physiological ocular barriers and insufficient targeting, which can lead to collateral damage to healthy tissues. By reviewing a series of clinical and preclinical studies, we aim to outline the recent advancements, current trends and future perspectives in nanomedicine for ocular cancer treatment. Beyond improving the existing therapies, like chemotherapy, phototherapies and brachytherapy, nanomedicine enables multimodal applications by combining multiple treatments or integrating imaging for theranostic approaches. Additionally, it paves the way for experimental therapies, such as gene therapy, offering new possibilities for more effective and less invasive treatment strategies in ocular oncology.

Use of Quantum Dots as Nanotheranostic Agents: Emerging Applications in Rare Genetic Diseases

Rare genetic diseases (RGDs) affect a small percentage of the global population but collectively have a substantial impact due to their diverse manifestations. Although the precise reasons behind these diseases remain unclear, roughly 80% of cases are genetically linked. Recent efforts focus on understanding pathology and developing new diagnostic and therapeutic approaches for RGDs. However, there persists a gap between fundamental research and clinical therapeutic approaches, where advancements in nanotechnology offer promising improvements. In this context, nanosized light-emitting quantum dots (QDs), ranging from 2-10 nm, are promising materials for diverse applications. Their size-tunable light emission, high quantum yield, and photostability allow for precise tracking of cargo. Additionally, QDs can be functionalized with therapeutic agents, antibodies, or peptides to target specific cellular pathways, enhancing treatment efficacy while minimizing side effects. By combining diagnostic and therapeutic capabilities in a single platform, QDs thus offer a versatile and powerful approach to tackle rare genetic disorders. Despite several reviews on various therapeutic applications of QDs, their utilization in the specific domain of RGDs is not well documented. This review highlight QDs' potential in diagnosing and treating certain RGDs and addresses the challenges limiting their application.

Progressing nanotechnology to improve diagnosis and targeted therapy of Diabetic Retinopathy

The inherent limitations of traditional treatments for Diabetic Retinopathy (DR) have spurred the development of various nanotechnologies, offering a safer and more efficient approach to managing the disease. Nanomedicine platforms present promising advancements in the diagnosis and treatment of DR by enhancing imaging capabilities, enabling targeted and controlled drug delivery. These innovations ultimately lead to more effective and personalized treatments with fewer side effects. This review highlights the progress, challenges, and opportunities in developing effective diagnostics and therapeutics for DR. Additionally, it explores innovative engineering techniques that leverage our growing understanding of nano-bio interactions to create more potent nanotherapeutics for patients.

Nanotechnology in retinal diseases: From disease diagnosis to therapeutic applications

Nanotechnology has demonstrated tremendous promise in the realm of ocular illnesses, with applications for disease detection and therapeutic interventions. The nanoscale features of nanoparticles enable their precise interactions with retinal tissues, allowing for more efficient and effective treatments. Because biological organs are compatible with diverse nanomaterials, such as nanoparticles, nanowires, nanoscaffolds, and hybrid nanostructures, their usage in biomedical applications, particularly in retinal illnesses, has increased. The use of nanotechnology in medicine is advancing rapidly, and recent advances in nanomedicine-based diagnosis and therapy techniques may provide considerable benefits in addressing the primary causes of blindness related to retinal illnesses. The current state, prospects, and challenges of nanotechnology in monitoring nanostructures or cells in the eye and their application to regenerative ophthalmology have been discussed and thoroughly reviewed. In this review, we build on our previously published review article in 2021, where we discussed the impact of nano-biomaterials in retinal regeneration. However, in this review, we extended our focus to incorporate and discuss the application of nano-biomaterials on all retinal diseases, with a highlight on nanomedicine-based diagnostic and therapeutic research studies.

The impact of NF-κB on inflammatory and angiogenic processes in age-related macular degeneration

Age-related macular degeneration (AMD) is a prominent cause of vision loss, characterized by two different types, dry (atrophic) and wet (neovascular). Dry AMD is distinguished by the progressive deterioration of retinal cells, which ultimately causes a decline in vision. In contrast, wet AMD is defined by the abnormal development of blood vessels underneath the retina, leading to a sudden and severe vision impairment. The course of AMD is primarily driven by chronic inflammation and pathological angiogenesis, in which the NF-κB signaling pathway plays a crucial role. The activation of NF-κB results in the generation of pro-inflammatory cytokines, chemokines, and angiogenic factors like VEGF, which contribute to inflammation and the formation of new blood vessels in AMD. This review analyzes the intricate relationship between NF-κB signaling, inflammation, and angiogenesis in AMD and assesses the possibility of using NF-κB as a target for therapy. The evaluation involves a comprehensive examination of preclinical and clinical evidence that substantiates the effectiveness of NF-κB inhibitors in treating AMD by diminishing inflammation and pathological angiogenesis.

Advances in ophthalmic therapeutic delivery: A comprehensive overview of present and future directions

Ophthalmic treatment demands precision and consistency in delivering therapeutic agents over extended periods to address many conditions, from common eye disorders to complex diseases. This diversity necessitates a range of delivery strategies, each tailored to specific needs. We delve into various delivery cargos that are pivotal in ophthalmic care. These cargos encompass biodegradable implants that gradually release medication, nonbiodegradable implants for sustained drug delivery, refillable tools allowing flexibility in treatment, hydrogels capable of retaining substances while maintaining ocular comfort, and advanced nanotechnology devices that precisely target eye tissues. Within each cargo category, we explore cutting-edge research-level approaches and FDA-approved methods, providing a thorough overview of the current state of ophthalmic drug delivery. In particular, our focus on nanotechnology reveals the promising potential for gene delivery, cell therapy administration, and the implantation of active devices directly into the retina. These advancements hold the key to more effective, personalized, and minimally- invasive ophthalmic treatments, revolutionizing the field of eye care.

Lipid nanoparticle technology-mediated therapeutic gene manipulation in the eyes

Millions of people worldwide have hereditary genetic disorders, trauma, infectious diseases, or cancer of the eyes, and many of these eye diseases lead to irreversible blindness, which is a major public health burden. The eye is a relatively small and immune-privileged organ. The use of nucleic acid-based drugs to manipulate malfunctioning genes that target the root of ocular diseases is regarded as a therapeutic approach with great promise. However, there are still some challenges for utilizing nucleic acid therapeutics in vivo because of certain unfavorable characteristics, such as instability, biological carrier-dependent cellular uptake, short pharmacokinetic profiles in vivo (RNA), and on-target and off-target side effects (DNA). The development of lipid nanoparticles (LNPs) as gene vehicles is revolutionary progress that has contributed the clinical application of nucleic acid therapeutics. LNPs have the capability to entrap and transport various genetic materials such as small interfering RNA, mRNA, DNA, and gene editing complexes. This opens up avenues for addressing ocular diseases through the suppression of pathogenic genes, the expression of therapeutic proteins, or the correction of genetic defects. Here, we delve into the cutting-edge LNP technology for ocular gene therapy, encompassing formulation designs, preclinical development, and clinical translation.

Intravitreal therapeutic nanoparticles for age-related macular degeneration: Design principles, progress and opportunities

Age-related macular degeneration (AMD) is a leading cause of vision loss in the elderly. The current standard treatment for AMD involves frequent intravitreal administrations of therapeutic agents. While effective, this approach presents challenges, including patient discomfort, inconvenience, and the risk of adverse complications. Nanoparticle-based intravitreal drug delivery platforms offer a promising solution to overcome these limitations. These platforms are engineered to target the retina specifically and control drug release, which enhances drug retention, improves drug concentration and bioavailability at the retinal site, and reduces the frequency of injections. This review aims to uncover the design principles guiding the development of highly effective nanoparticle-based intravitreal drug delivery platforms for AMD treatment. By gaining a deeper understanding of the physiology of ocular barriers and the physicochemical properties of nanoparticles, we establish a basis for designing intravitreal nanoparticles to optimize drug delivery and drug retention in the retina. Furthermore, we review recent nanoparticle-based intravitreal therapeutic strategies to highlight their potential in improving AMD treatment efficiency. Lastly, we address the challenges and opportunities in this field, providing insights into the future of nanoparticle-based drug delivery to improve therapeutic outcomes for AMD patients.

Lipid nanoparticles for gene therapy in ocular diseases

OBJECTIVES

Lipid nanoparticles, as a nucleic acid delivery system, have been used as an alternative to treat ocular diseases, since they can cross the ocular barrier and efficiently transfecting nucleic acids to various cells of the eye. The size influences the transfection of genes, biological distribution, diffusion, and cellular uptake. It is therefore important to establish a relationship between size, formulation, and encapsulation percentage.

EVIDENCE ACQUISITION

In this review, we used a search strategy to compare studies of nanomedicine systems aimed at eye diseases where the size of the nanoparticles and the efficiency of encapsulation of genetic material are reported based on the criteria of Preferred Reporting Items for Systematic Reviews (PRISMA ScR 2020 guidelines).

RESULTS

Out of the initial 5932, 169 studies met the inclusion criteria and were included to form the basis of the analysis. Nanoparticles reported are composed mainly of PEG-modified lipids, cholesterol, and cationic lipids, that in combination with messenger or interference RNA, allow the formulation of a nanoparticle with an encapsulation efficiency greater than 95%. The diseases treated mainly focus on conditions related to the retina and cornea. Certain characteristics of nanoparticles increase encapsulation efficiency, such as the size of the nanoparticle and the charge of the outer layer of the nanoparticle.

CONCLUSION

It is still unknown what characteristics lipid nanoparticles should have to successfully treat human eye illnesses. The in vitro and in vivo investigations covered in this review, however, present encouraging results. To improve encapsulation effectiveness and disease gene silencing, nanoparticle formulation is essential. The most stable nanoparticles are those made mostly of cationic lipids, PEG lipids, and cholesterol, which also effectively encapsulate RNA. The encapsulation efficiency is not only influenced by size, but also by other factors such as methods of preparation.

Bruch's Membrane: A Key Consideration with Complement-Based Therapies for Age-Related Macular Degeneration

The complement system is crucial for immune surveillance, providing the body's first line of defence against pathogens. However, an imbalance in its regulators can lead to inappropriate overactivation, resulting in diseases such as age-related macular degeneration (AMD), a leading cause of irreversible blindness globally affecting around 200 million people. Complement activation in AMD is believed to begin in the choriocapillaris, but it also plays a critical role in the subretinal and retinal pigment epithelium (RPE) spaces. Bruch's membrane (BrM) acts as a barrier between the retina/RPE and choroid, hindering complement protein diffusion. This impediment increases with age and AMD, leading to compartmentalisation of complement activation. In this review, we comprehensively examine the structure and function of BrM, including its age-related changes visible through in vivo imaging, and the consequences of complement dysfunction on AMD pathogenesis. We also explore the potential and limitations of various delivery routes (systemic, intravitreal, subretinal, and suprachoroidal) for safe and effective delivery of conventional and gene therapy-based complement inhibitors to treat AMD. Further research is needed to understand the diffusion of complement proteins across BrM and optimise therapeutic delivery to the retina.

Strategic nanocarriers to control neurodegenerative disorders: Concept, challenges, and future perspective

Various neurodegenerative diseases (parkinson, huntington, alzheimer, and amyotrophic lateral sclerosis) are becoming serious global health challenges. Despite various treatment options, successful delivery and effective outcomes have been challenged with several physiological-anatomical barriers, formulation related issues, post-administration hurdles, regulatory constraints, physical hurdles, environmental issues, and safety concern. In the present review, we addressed a brief understanding of pathological and normal condition of blood brain barrier (BBB), rational for brain delivery using nanocarriers, major challenges, advantages of nanomedicine, critical aspects of nanomedicine to translate from bed to clinics, and strategic approaches for improved delivery across BBB. The review addressed various mechanistic perspective for delivery of drug loaded nanocarriers across BBB. Moreover, several reports have been published wherein phytomedicine, exosomes, magnetic nanopartilces, functionalized nanocarriers, cationic nanopartilces, and nano-phytomedicine were investigated for remarkable improvement in neurological disorders. These findings are informative for healthcare professionals, researchers, and scientists working in the domains. The successful application and convincing outcomes of nanomedicines were envisaged with clinical trials conducted on various drugs intended to control neurological disorders (NDs). Conclusively, the review addressed comprehensive findings on various aspects of drug loaded nanocarrier delivery across BBB, considerable risks, potential therapeutic benefits, clinical trial based outcomes, and recent advances followed by future perspectives.

Every nano-step counts: a critical reflection on do's and don'ts in researching nanomedicines for retinal gene therapy

INTRODUCTION

Retinal disease affects millions of people worldwide, generating a massive social and economic burden. Current clinical trials for retinal diseases are dominated by gene augmentation therapies delivered with recombinant viruses as key players. As an alternative, nanoparticles hold great promise for the delivery of nucleic acid therapeutics as well. Nevertheless, despite numerous attempts, 'nano' is in practice not as successful as aspired and major breakthroughs in retinal gene therapy applying nanomaterials are yet to be seen.

AREAS COVERED

In this review, we summarize the advantages of nanomaterials and give an overview of nanoparticles designed for retinal nucleic acid delivery up to now. We furthermore critically reflect on the predominant issues that currently limit nano to progress to the clinic, where faulty study design and the absence of representative models play key roles.

EXPERT OPINION

Since the current approach of in vitro - in vivo experimentation is highly inefficient and creates misinformation, we advocate for a more prominent role for ex vivo testing early on in nanoparticle research. In addition, we elaborate on several concepts, including systematic studies and open science, which could aid in pushing the field of nanomedicine beyond the preclinical stage.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

A Trojan horse biomimetic delivery system using mesenchymal stem cells for HIF-1α siRNA-loaded nanoparticles on retinal pigment epithelial cells under hypoxia environment

AIM

To demonstrate the feasibility of mesenchymal stem cell (MSC)-mediated nano drug delivery, which was characterized by the "Trojan horse"-like transport of hypoxia-inducible factor-1α small interfering RNA (HIF-1α siRNA) between MSCs and retinal pigment epithelial cells (RPE) under hypoxia environment.

METHODS

Plasmid and lentivirus targeting the human HIF-1α gene were designed and constructed. HIF-1α siRNA was encapsulated into poly(lactic-co-glycolic acid) nanoparticles (PLGA-NPs) through the water-in-oil-in-water (w/o/w) multiple emulsion technique. The effect of PLGA-NPs uptake on the expression of HIF-1α mRNA was tested in RPE cells by real-time quantitative polymerase chain reaction (qPCR) and additional transfected conditions were used as control, including lentivirus group, nude plasmid group and blank PLGA group. MSCs were transfected with the NPs and the transfection efficacy was evaluated by flow cytometry. Transwell co-culture system of transfected MSCs and RPE cells was constructed under hypoxia environment. The effects of MSC-loaded HIF-1α siRNA PLGA-NPs on proliferation, apoptosis, and migration of RPE cells were then evaluated. The effect of transfected MSCs on HIF-1α expression of RPE cells was analyzed by using qPCR at the time points 24h, 3d, and 7d.

RESULTS

The average diameter of PLGA-NPs loaded with HIF siRNA was 314.1 nm and the zeta potential was -0.36 mV. The transfection efficiency of PLGA-NPs was 67.3%±5.2% into MSCs by using flow cytometry. Compared with the lentivirus group, the PLGA-NPs loaded with HIF-1α siRNA can effectively reduce the expression of HIF-1α mRNA up to 7d in RPE (0.63±0.05 at 7d, P<0.001). In the Transwell co-culture system of transfected MSCs and RPE, the abilities of proliferation (2.34±0.17, 2.40±0.28, 2.47±0.24 at 48h, F=0.23, P=0.80), apoptosis (14.83%±2.43%, 12.94%±2.19%, 12.39%±3.21%; F=0.70, P=0.53) and migration (124.5±7.78, 119.5±5.32, 130±9.89, F=1.33, P=0.33) of the RPE cells had no differences between MSC-loaded HIF-1α siRNA PLGA-NPs and other groups. The inhibition of PLGA on the HIF-1α mRNA expression in RPE cells could continue until the 7^th day, the level of HIF-1α mRNA was lower than that of other groups (F=171.98, P<0.001).

CONCLUSION

The delivery of PLGA-NPs loaded with HIF-1α siRNA carried by MSCs is found to be beneficial temporally for HIF-1α mRNA inhibition in RPE cells under hypoxia environment. The MSC-based bio-mimetic delivery of HIF-1α siRNA nanoparticles is a potential method for therapy against choroidal neovascularization.

Optogenetic Prosthetization of Retinal Bipolar Cells

Abstract

Although the experience of optogenetic retinal prosthetics in animal models dates back to more than 16 years, the first results obtained on humans have only been reported in the last year. Over this period, the main challenges of prosthetics became clear and the approaches to their solution were proposed. In this review, we aim to present the achievements in the field of optogenetic prosthetization of retinal bipolar cells with a focus mainly on relatively recent publications. The review addresses the advantages and disadvantages of bipolar cell prosthetics as compared to the alternative target, retinal ganglion cells, and provides a comparative analysis of the effectiveness of ionotropic light-sensitive proteins (channelrhodopsins) or metabotropic receptors (rhodopsins) as prosthetic tools.

Advancing precision medicines for ocular disorders: Diagnostic genomics to tailored therapies

Successful sequencing of the human genome and evolving functional knowledge of gene products has taken genomic medicine to the forefront, soon combining broadly with traditional diagnostics, therapeutics, and prognostics in patients. Recent years have witnessed an extraordinary leap in our understanding of ocular diseases and their respective genetic underpinnings. As we are entering the age of genomic medicine, rapid advances in genome sequencing, gene delivery, genome surgery, and computational genomics enable an ever-increasing capacity to provide a precise and robust diagnosis of diseases and the development of targeted treatment strategies. Inherited retinal diseases are a major source of blindness around the world where a large number of causative genes have been identified, paving the way for personalized diagnostics in the clinic. Developments in functional genetics and gene transfer techniques has also led to the first FDA approval of gene therapy for LCA, a childhood blindness. Many such retinal diseases are the focus of various clinical trials, making clinical diagnoses of retinal diseases, their underlying genetics and the studies of natural history important. Here, we review methodologies for identifying new genes and variants associated with various ocular disorders and the complexities associated with them. Thereafter we discuss briefly, various retinal diseases and the application of genomic technologies in their diagnosis. We also discuss the strategies, challenges, and potential of gene therapy for the treatment of inherited and acquired retinal diseases. Additionally, we discuss the translational aspects of gene therapy, the important vector types and considerations for human trials that may help advance personalized therapeutics in ophthalmology. Retinal disease research has led the application of precision diagnostics and precision therapies; therefore, this review provides a general understanding of the current status of precision medicine in ophthalmology.

Gene-Based Therapeutics for Inherited Retinal Diseases

Inherited retinal diseases (IRDs) are a heterogenous group of orphan eye diseases that typically result from monogenic mutations and are considered attractive targets for gene-based therapeutics. Following the approval of an IRD gene replacement therapy for Leber's congenital amaurosis due to RPE65 mutations, there has been an intensive international research effort to identify the optimal gene therapy approaches for a range of IRDs and many are now undergoing clinical trials. In this review we explore therapeutic challenges posed by IRDs and review current and future approaches that may be applicable to different subsets of IRD mutations. Emphasis is placed on five distinct approaches to gene-based therapy that have potential to treat the full spectrum of IRDs: 1) gene replacement using adeno-associated virus (AAV) and nonviral delivery vectors, 2) genome editing via the CRISPR/Cas9 system, 3) RNA editing by endogenous and exogenous ADAR, 4) mRNA targeting with antisense oligonucleotides for gene knockdown and splicing modification, and 5) optogenetic approaches that aim to replace the function of native retinal photoreceptors by engineering other retinal cell types to become capable of phototransduction.

The Next Generation of Molecular and Cellular Therapeutics for Inherited Retinal Disease

Inherited retinal degenerations (IRDs) are a diverse group of conditions that are often characterized by the loss of photoreceptors and blindness. Recent innovations in molecular biology and genomics have allowed us to identify the causative defects behind these dystrophies and to design therapeutics that target specific mechanisms of retinal disease. Recently, the FDA approved the first in vivo gene therapy for one of these hereditary blinding conditions. Current clinical trials are exploring new therapies that could provide treatment for a growing number of retinal dystrophies. While the field has had early success with gene augmentation strategies for treating retinal disease based on loss-of-function mutations, many novel approaches hold the promise of offering therapies that span the full spectrum of causative mutations and mechanisms. Here, we provide a comprehensive review of the approaches currently in development including a discussion of retinal neuroprotection, gene therapies (gene augmentation, gene editing, RNA modification, optogenetics), and regenerative stem or precursor cell-based therapies. Our review focuses on technologies that are being developed for clinical translation or are in active clinical trials and discusses the advantages and limitations for each approach.

Molecular Therapy for Choroideremia: Pre-clinical and Clinical Progress to Date

Choroideremia is an inherited retinal disease characterised by a degeneration of the light-sensing photoreceptors, supporting retinal pigment epithelium and underlying choroid. Patients present with the same symptoms as those with classic rod-cone dystrophy: (1) night blindness early in life; (2) progressive peripheral visual field loss, and (3) central vision decline with a slow progression to legal blindness. Choroideremia is monogenic and caused by mutations in CHM. Eight clinical trials (three phase 1/2, four phase 2, and one phase 3) have started (four of which are already finished) to evaluate the therapeutic efficacy of gene supplementation mediated by subretinal delivery of an adeno-associated virus serotype 2 (AAV2/2) vector expressing CHM. Furthermore, one phase 1 clinical trial has been initiated to evaluate the efficiency of a novel AAV variant to deliver CHM to the outer retina following intravitreal delivery. Lastly, a non-viral-mediated CHM replacement strategy is currently under development, which could lead to a future clinical trial. Here, we summarise the rationale behind these various studies, as well as any results published to date. The diversity of these trials currently places choroideremia at the forefront of the retinal gene therapy field. As a consequence, the trial outcomes, regardless of the results, have the potential to change the landscape of gene supplementation for inherited retinal diseases.

A core-shell nanoplatform as a nonviral vector for targeted delivery of genes to the retina

Retinal diseases, including age-related macular degeneration (AMD), are a major cause of blindness. Efficient delivery of therapeutic genes to retinal cells to treat retinal disease is a formidable challenge. In this study, we developed a core-shell nanoplatform composed of a core and two external layers for targeted delivery of the gene to the retina. The inner core was composed of amino acid-functionalized dendrimers and a nuclear localization signal (NLS) for DNA complexation, nuclear transport and efficient transfection. The inner core was coated in a lipid bilayer that comprised pH-sensitive lipids as the inner shell layer. Hyaluronic acid (HA)-1,2-dioleoylphosphatidylethanolamine (DOPE) as the outermost shell layer was used for retinal cell targeting. This core-shell nanoplatform was developed so that the mobility in the vitreous body of these negatively charged carriers would not be affected by their surface charge, allowing diffusion into the retina, uptake into the retinal cells via CD44-mediated internalization, and finally transport into the nucleus by the NLS. The designed nanoparticles showed safety both in vitro and in vivo and inhibited the expression of VEGF under hypoxia-mimicking conditions. In vitro angiogenesis assays exhibited significant inhibitory effects on cell migration and tube formation. The in vivo assays indicated that this nanoplatform could be delivered to the retina. Taken together, this nanoplatform has the potential to transfer gene material into the retina for the treatment of retinal diseases, including AMD. STATEMENT OF SIGNIFICANCE: It remains a challenge to develop an efficient nonviral vector for gene therapy, especially retinal gene therapy. Various barriers exist in gene delivery and the unique ocular environment, making gene delivery to the retina difficult. In this study, we designed a negatively charged core-shell nanoplatform (HD-NP_PND) for the targeted delivery of gene to the retina. The developed nanoplatform possessed excellent transfection efficiency and safety both in vitro and in vivo. It efficiently delivered a gene to the retina. The results of this study suggested that this core-shell nanoplatform has the potential to deliver genes to the retina to treat retinal diseases, including age-related macular degeneration (AMD).

Gene therapy for inherited retinal diseases

Inherited retinal diseases (IRDs) are a genetically variable collection of devastating disorders that lead to significant visual impairment. Advances in genetic characterization over the past two decades have allowed identification of over 260 causative mutations associated with inherited retinal disorders. Thought to be incurable, gene supplementation therapy offers great promise in treating various forms of these blinding conditions. In gene replacement therapy, a disease-causing gene is replaced with a functional copy of the gene. These therapies are designed to slow disease progression and hopefully restore visual function. Gene therapies are typically delivered to target retinal cells by subretinal (SR) or intravitreal (IVT) injection. The historic Food and Drug Administration (FDA) approval of voretigene neparvovec for RPE65-associated Leber's congenital amaurosis (LCA) spurred tremendous optimism surrounding retinal gene therapy for various other monogenic IRDs. Novel disease-causing mutations continue to be discovered annually, and targeted genetic therapy is now under development in clinical and preclinical models for many IRDs. Numerous clinical trials for other IRDs are ongoing or have recently completed. Disorders being targeted for genetic therapy include retinitis pigmentosa (RP), choroideremia (CHM), achromatopsia (ACHM), Leber's hereditary optic neuropathy, usher syndrome (USH), X-linked retinoschisis, and Stargardt disease. Here, we provide an update of completed, ongoing, and planned clinical trials using gene supplementation strategies for retinal degenerative disorders.

The future of retinal gene therapy: evolving from subretinal to intravitreal vector delivery

Inherited retinal degenerations are a leading and untreatbale cause of blindness, and as such they are targets for gene therapy. Numerous gene therapy treatments have progressed from laboratory research to clinical trails, and a pioneering gene therapy received the first ever FDA approval for treating patients. However, currently retinal gene therapy mostly involves subretinal injection of the therapeutic agent, which treats a limited area, entails retinal detachment and other potential complications, and requires general anesthesia with consequent risks, costs and prolonged recovery. Therefore there is great impetus to develop safer, less invasive and cheapter methods of gene delivery. A promising method is intravitreal injection, that does not cause retinal detachment, can lead to pan-retinal transduction and can be performed under local anesthesia in out-patient clinics. Intravitreally-injected vectors face several obstacles. First, the vector is diluted by the vitreous and has to overcome a long diffusion distance to the target cells. Second, the vector is exposed to the host's immune response, risking neutralization by pre-existing antibodies and triggering a stronger immune response to the injection. Third, the vector has to cross the inner limiting membrane which is both a physical and a biological barrier as it contains binding sites that could cause the vector's sequestration. Finally, in the target cell the vector is prone to proteasome degradation before delivering the transgene to the nucleus. Strategies to overcome these obstacles include modifications of the viral capsid, through rational design or directed evolution, which allow resistance to the immune system, enhancement of penetration through the inner limiting membrane or reduced degradation by intracellular proteasomes. Furthermore, physical and chemical manipulations of the inner limiting membrane and vitreous aim to improve vector penetration. Finally, compact non-viral vectors that can overcome the immunological, physical and anatomical and barriers have been developed. This paper reviews ongoing efforts to develop novel, safe and efficacious methods for intravitreal delivery of therapeutic genes for inherited retinal degenerations. To date, the most promising results are achieved in rodents with robust, pan-retinal transduction following intravitreal delivery. Trials in larger animal models demonstrate transduction mostly of inner retinal layers. Despite ongoing efforts, currently no intravitreally-injected vector has demonstrated outer retinal transduction efficacy comparable to that of subretinal delivery. Further work is warranted to test promising new viral and non-viral vectors on large animal models of inherited retinal degenerations. Positive results will pave the way to development of the next generation of treatments for inherited retinal degeneration.

Preparation of Targeted Mitochondrion Nanoscale-Release Peptides and Their Efficiency on Eukaryotic Cells

We established a self-decomposable SiO₂ encapsulated mitochondrial targeting short peptide SS31 drug loading system (SiO₂@SS31) to determine its nano-sustained release characteristics in eukaryotic cells. We explored the protection of SiO₂@SS31 on the 661W cells after oxidative injury by H₂O₂. After the drug loading, we detected the morphology of SiO₂@SS31 by transmission electron microscopy (TEM). Moreover, high-pressure liquid chromatography (HPLC) was used to determine the drug capacity and encapsulation efficiency of the nanoparticles. Then, the release curve in vitro was drawn. The 661W cells were cultured in vitro to allow the detection of cytotoxicity by the MTT assay. The SS31loaded nanoscale microspheres labeled with fluorescein isothiocyanate (SiO₂@FITC-SS31) were prepared, and their sustained release effect was detected with intracellular endocytosis, using confocal microscopy and flow cytometry. Within 15 days, the SiO2@SS31 nanoparticles were completely decomposed and simultaneously released the SS31 peptide in deionized water and normal saline. Nonetheless, the process was faster in simulated body fluid and serum. The MTT assay suggested that SiO₂@SS31 has sustained protection compared with SS31 in the 661W cells at 48 h. Flow cytometry proved SiO₂@FITC-SS31 could maintain a high level and last longer after 24 h. The SS31 peptide, which has excellent medical application prospects, can be slowly and continuously released from self-decomposable SiO₂ and targeted to concentrate on mitochondria.

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.

Early and late stage gene therapy interventions for inherited retinal degenerations

Inherited and age-related retinal degeneration is the hallmark of a large group of heterogeneous diseases and is the main cause of untreatable blindness today. Genetic factors play a major pathogenic role in retinal degenerations for both monogenic diseases (such as retinitis pigmentosa) and complex diseases with established genetic risk factors (such as age-related macular degeneration). Progress in genotyping techniques and back of the eye imaging are completing our understanding of these diseases and their manifestations in patient populations suffering from retinal degenerations. It is clear that whatever the genetic cause, the majority of vision loss in retinal diseases results from the loss of photoreceptor function. The timing and circumstances surrounding the loss of photoreceptor function determine the adequate therapeutic approach to use for each patient. Among such approaches, gene therapy is rapidly becoming a therapeutic reality applicable in the clinic. This massive move from laboratory work towards clinical application has been propelled by the advances in our understanding of disease genetics and mechanisms, gene delivery vectors, gene editing systems, and compensatory strategies for loss of photoreceptor function. Here, we provide an overview of existing modalities of retinal gene therapy and their relevance based on the needs of patient populations suffering from inherited retinal degenerations.

Nonviral Locally Injected Magnetic Vectors for In Vivo Gene Delivery: A Review of Studies on Magnetofection

Magnetic nanoparticles have been widely used in nanobiomedicine for diagnostics and the treatment of diseases, and as carriers for various drugs. The unique magnetic properties of "magnetic" drugs allow their delivery in a targeted tumor or tissue upon application of a magnetic field. The approach of combining magnetic drug targeting and gene delivery is called magnetofection, and it is very promising. This method is simple and efficient for the delivery of genetic material to cells using magnetic nanoparticles controlled by an external magnetic field. However, magnetofection in vivo has been studied insufficiently both for local and systemic routes of magnetic vector injection, and the relevant data available in the literature are often merely descriptive and contradictory. In this review, we collected and systematized the data on the efficiency of the local injections of magnetic nanoparticles that carry genetic information upon application of external magnetic fields. We also investigated the efficiency of magnetofection in vivo, depending on the structure and coverage of magnetic vectors. The perspectives of the development of the method were also considered.

Gene Therapy to the Retina and the Cochlea

Vision and hearing disorders comprise the most common sensory disorders found in people. Many forms of vision and hearing loss are inherited and current treatments only provide patients with temporary or partial relief. As a result, developing genetic therapies for any of the several hundred known causative genes underlying inherited retinal and cochlear disorders has been of great interest. Recent exciting advances in gene therapy have shown promise for the clinical treatment of inherited retinal diseases, and while clinical gene therapies for cochlear disease are not yet available, research in the last several years has resulted in significant advancement in preclinical development for gene delivery to the cochlea. Furthermore, the development of somatic targeted genome editing using CRISPR/Cas9 has brought new possibilities for the treatment of dominant or gain-of-function disease. Here we discuss the current state of gene therapy for inherited diseases of the retina and cochlea with an eye toward areas that still need additional development.

Nanocarriers, Progenitor Cells, Combinational Approaches, and New Insights on the Retinal Therapy

Progenitor cells derived from the retinal pigment epithelium (RPECs) have shown promise as therapeutic approaches to degenerative retinal disorders including diabetic retinopathy, age-related macular degeneration and Stargardt disease. However, the degeneration of Bruch's membrane (BM), the natural substrate for the RPE, has been identified as one of the major limitations for utilizing RPECs. This degeneration leads to decreased support, survival and integration of the transplanted RPECs. It has been proposed that the generation of organized structures of nanofibers, in an attempt to mimic the natural retinal extracellular matrix (ECM) and its unique characteristics, could be utilized to overcome these limitations. Furthermore, nanoparticles could be incorporated to provide a platform for improved drug delivery and sustained release of molecules over several months to years. In addition, the incorporation of tissue-specific genes and stem cells into the nanostructures increased the stability and enhanced transfection efficiency of gene/drug to the posterior segment of the eye. This review discusses available drug delivery systems and combination therapies together with challenges associated with each approach. As the last step, we discuss the application of nanofibrous scaffolds for the implantation of RPE progenitor cells with the aim to enhance cell adhesion and support a functionally polarized RPE monolayer.

Suprachoroidally Delivered DNA Nanoparticles Transfect Retina and Retinal Pigment Epithelium/Choroid in Rabbits

Purpose

This study evaluated ocular tolerability and transfectability of nonviral DNA nanoparticles (DNPs) after microneedle-based suprachoroidal (SC) administration, in comparison to subretinal (SR) administration.

Methods

The DNPs consisted of a single copy of plasmid DNA with a polyubiquitin C/luciferase transcriptional cassette compacted with 10 kDa PEG-substituted lysine 30-mer peptides (CK30PEG10k). New Zealand White rabbits (n = 4 per group) received a unilateral SC injection (0.1 mL via a microneedle technique) of ellipsoid-shaped DNPs, rod-shaped DNPs, or saline (negative control). A cohort of rabbits (n = 4) also received a single unilateral SR injection (0.05 mL via a transvitreal approach) of rod-shaped DNPs. At day 7, luciferase activity was measured in the retina and retinal pigment epithelium (RPE)–choroid via bioluminescence assay. A cohort of rabbits received a SC injection of analogous DNPs to assess spread of DNP injectate in the suprachoroidal space (SCS) via optical coherent tomography and histology.

Results

Suprachoroidal injection of DNPs resulted in reversible opening of the SCS circumferentially and posteriorly and was generally well tolerated, with no significant ocular examination score changes, intraocular pressure abnormalities, or changes in electroretinography amplitudes on day 7 compared to the baseline. High luciferase activity was observed in the retina and RPE-choroid of eyes that received SC DNPs (rod and ellipsoid shape) and SR DNPs (rod shape) compared to controls. The mean luciferase activity in RPE-choroid and retina was comparable between SC and SR administrations. Transfection in the RPE-choroid was approximately 10-fold higher than in the retina after either SC or SR administration of DNPs.

Conclusions

Suprachoroidal and SR administration of DNPs resulted in comparable transfection of retina and RPE-choroid.

Translational Relevance

Suprachoroidal delivery of DNPs offers the potential to precisely target chorioretinal tissues while avoiding surgical risks associated with SR injection, and it may offer an office-based nonsurgical gene therapy option for the treatment of retinal diseases.

Emerging approaches for restoration of hearing and vision

Impairments of vision and hearing are highly prevalent conditions limiting the quality of life and presenting a major socioeconomic burden. For long, retinal and cochlear disorders have remained intractable for causal therapies, with sensory rehabilitation limited to glasses, hearing aids, and electrical cochlear or retinal implants. Recently, the application of gene therapy and optogenetics to eye and ear has generated hope for a fundamental improvement of vision and hearing restoration. To date, one gene therapy for the restoration of vision has been approved and undergoing clinical trials will broaden its application including gene replacement, genome editing, and regenerative approaches. Moreover, optogenetics, i.e. controlling the activity of cells by light, offers a more general alternative strategy. Over little more than a decade, optogenetic approaches have been developed and applied to better understand the function of biological systems, while protein engineers have identified and designed new opsin variants with desired physiological features. Considering potential clinical applications of optogenetics, the spotlight is on the sensory systems. Multiple efforts have been undertaken to restore lost or hampered function in eye and ear. Optogenetic stimulation promises to overcome fundamental shortcomings of electrical stimulation, namely poor spatial resolution and cellular specificity, and accordingly to deliver more detailed sensory information. This review aims at providing a comprehensive reference on current gene therapeutic and optogenetic research relevant to the restoration of hearing and vision. We will introduce gene-therapeutic approaches and discuss the biotechnological and optoelectronic aspects of optogenetic hearing and vision restoration.

Noble Metals and Soft Bio-Inspired Nanoparticles in Retinal Diseases Treatment: A Perspective

We are witnessing an exponential increase in the use of different nanomaterials in a plethora of biomedical fields. We are all aware of how nanoparticles (NPs) have influenced and revolutionized the way we supply drugs or how to use them as therapeutic agents thanks to their tunable physico-chemical properties. However, there is still a niche of applications where NP have not yet been widely explored. This is the field of ocular delivery and NP-based therapy, which characterizes the topic of the current review. In particular, many efforts are being made to develop nanosystems capable of reaching deeper sections of the eye such as the retina. Particular attention will be given here to noble metal (gold and silver), and to polymeric nanoparticles, systems consisting of lipid bilayers such as liposomes or vesicles based on nonionic surfactant. We will report here the most relevant literature on the use of different types of NPs for an efficient delivery of drugs and bio-macromolecules to the eyes or as active therapeutic tools.

Nanoscale systems for local drug delivery

Many diseases and conditions affect a relatively localized area of the body. They can be treated either by direct deposition of drug in the target area, or by giving the drug systemically. Here we review nanoparticle-based approaches to achieving both. We highlight advantages and disadvantages that nanoscale solutions have for locally administered therapies, with emphasis on the former. We discuss strategies to enable systemically delivered nanoparticles to deliver their payloads at specific locations in the body, including triggering (local and remote) and targeting.

[Nanoparticles as drug delivery systems in ophthalmology]

Nanoparticles are perfectly suited as drug delivery systems due to their size and the diversity of materials used. They are able to penetrate biological barriers, can directly deliver drugs to the target site and provide a sustained release profile. Having long been established in oncology, in the last decade research has started to take a closer look at the potential of nanoparticles for ocular drug delivery. Obstacles, such as poor delivery of drugs via eye drops and the side effects of invasive methods, such as placing implants as drug depots could be overcome. Among the most relevant investigated structures are polymeric nanoparticles, micelles, liposomes, solid lipid nanoparticles, dendrimers and cyclodextrins. Besides the composition of the nanoparticle itself, its efficacy and stability can be optimized through coatings; however, long-term stability, standardization of production and toxicity remain the major challenges. The preclinical and partly clinical results obtained so far will hopefully give impulse to the idea of applying nanoparticles for optimized ocular drug delivery in the near future.

Non-viral delivery of chemically modified mRNA to the retina: Subretinal versus intravitreal administration

mRNA therapeutics have recently experienced a new wave of interest, mainly due to the discovery that chemical modifications to mRNA's molecular structure could drastically reduce its inherent immunogenicity and perceived instability. On this basis, we aimed to explore the potential of chemically stabilized mRNA for ocular applications. More specifically, we investigated the behavior of mRNA-loaded lipid-based carriers in human retinal cells (in vitro), in bovine retinal explants (ex vivo) and in mouse retinas (in vivo). We demonstrate a clear superiority of mRNA over pDNA to induce protein expression in different retinal cell types, which was further enhanced by chemical modification of the mRNA, providing up to ~1800-fold higher reporter gene expression compared to pDNA. Moreover, transgene expression could be detected for at least 20 days after a single administration of chemically modified mRNA in vitro. We furthermore determined the localization and extent of mRNA expression depending on the administration route. After subretinal (SR) administration, mRNA expression was observed in vivo and ex vivo. By contrast, intravitreal (IVT) administration resulted in limited expression in vivo. Using ex vivo bovine explants with an intact vitreoretinal (VR) interface we could attribute this to the inner limiting membrane (ILM), which presents a large barrier for non-viral delivery of mRNA, trapping mRNA complexes at the vitreal side. When the vitreous was removed, which compromises the ILM, mRNA expression was apparent and seemed to colocalize with Müller cells or photoreceptors after respectively IVT or SR administration. Taken together, this study represents a first step towards mRNA-mediated therapy for retinal diseases.

Ocular gene therapies in clinical practice: viral vectors and nonviral alternatives

Ocular gene therapy has entered into clinical practice. Although viral vectors are currently the best option to replace and/or correct genes, the optimal method to deliver these treatments to the retinal pigment epithelial (RPE) cells and/or photoreceptor cells remains to be improved to increase transduction efficacy and reduce iatrogenic risks. Beyond viral-mediated gene replacement therapies, nonviral gene delivery approaches offer the promise of sustained fine-tuned expression of secreted therapeutic proteins that can be adapted to the evolving stage of the disease course and can address more common nongenetic retinal diseases, such as age-related macular degeneration (AMD). Here, we review current gene therapy strategies for ocular diseases, with a focus on clinical stage products.

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.

Lipid nanoparticles for delivery of messenger RNA to the back of the eye

Retinal gene therapy has had unprecedented success in generating treatments that can halt vision loss. However, immunogenic response and long-term toxicity with the use of viral vectors remain a concern. Non-viral vectors are relatively non-immunogenic, scalable platforms that have had limited success with DNA delivery to the eye. Messenger RNA (mRNA) therapeutics has expanded the ability to achieve high gene expression while eliminating unintended genomic integration or the need to cross the restrictive nuclear barrier. Lipid-based nanoparticles (LNPs) remain at the forefront of potent delivery vectors for nucleic acids. Herein, we tested eleven different LNP variants for their ability to deliver mRNA to the back of the eye. LNPs that contained ionizable lipids with low pKa and unsaturated hydrocarbon chains showed the highest amount of reporter gene transfection in the retina. The kinetics of gene expression showed a rapid onset (within 4 h) that persisted for 96 h. The gene delivery was cell-type specific with majority of the expression in the retinal pigmented epithelium (RPE) and limited expression in the Müller glia. LNP-delivered mRNA can be used to treat monogenic retinal degenerative disorders of the RPE. The transient nature of mRNA-based therapeutics makes it desirable for applications that are directed towards retinal reprogramming or genome editing. Overall, non-viral delivery of RNA therapeutics to diverse cell types within the retina can provide transformative new approaches to prevent blindness.

In Vitro Gene Delivery in Retinal Pigment Epithelium Cells by Plasmid DNA-Wrapped Gold Nanoparticles

Many rare diseases course with affectation of neurosensory organs. Among them, the neuroepithelial retina is very vulnerable due to constant light/oxidative stress, but it is also the most accessible and amenable to gene manipulation. Currently, gene addition therapies targeting retinal tissue (either photoreceptors or the retinal pigment epithelium), as a therapy for inherited retinal dystrophies, use adeno-associated virus (AAV)-based approaches. However, efficiency and safety of therapeutic strategies are relevant issues that are not always resolved in virus-based gene delivery and alternative methodologies should be explored. Based on our experience, we are currently assessing the novel physical properties at the nanoscale of inorganic gold nanoparticles for delivering genes to the retinal pigment epithelium (RPE) as a safe and efficient alternative approach. In this work, we present our preliminary results using DNA-wrapped gold nanoparticles (DNA-gold NPs) for successful in vitro gene delivery on human retinal pigment epithelium cell cultures, as a proof-of-principle to assess its feasibility for retina in vivo gene delivery. Our results show faster expression of a reporter gene in cells transfected with DNA-gold NPs compared to DNA-liposome complexes. Furthermore, we show that the DNA-gold NPs follow different uptake, internalization and intracellular vesicle trafficking routes compared to pristine NPs.

Gene Therapy for Inherited Retinal Degeneration

Inherited retinal degeneration (IRD), a group of rare retinal diseases that primarily lead to the progressive loss of retinal photoreceptor cells, can be inherited in all modes of inheritance: autosomal dominant (AD), autosomal recessive (AR), X-linked (XL), and mitochondrial. Based on the pattern of inheritance of the dystrophy, retinal gene therapy has 2 main strategies. AR, XL, and AD IRDs with haploinsufficiency can be treated by inserting a functional copy of the gene using either viral or nonviral vectors (gene augmentation). Different types of viral vectors and nonviral vectors are used to transfer plasmid DNA both in vitro and in vivo. AD IRDs with gain-of-function mutations or dominant-negative mutations can be treated by disrupting the mutant allele with (and occasionally without) gene augmentation. This review article aims to provide an overview of ocular gene therapy for treating IRDs using gene augmentation with viral or nonviral vectors or gene disruption through different gene-editing tools, especially with the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system.

Viral Delivery Systems for CRISPR

The frontiers of precision medicine have been revolutionized by the development of Clustered Regularly-Interspaced Short Palindromic Repeats (CRISPR)/Cas9 as an editing tool. CRISPR/Cas9 has been used to develop animal models, understand disease mechanisms, and validate treatment targets. In addition, it is regarded as an effective tool for genome surgery when combined with viral delivery vectors. In this article, we will explore the various viral mechanisms for delivering CRISPR/Cas9 into tissues and cells, as well as the benefits and drawbacks of each method. We will also review the history and recent development of CRISPR and viral vectors and discuss their applications as a powerful tool in furthering our exploration of disease mechanisms and therapies.

Therapeutic Benefits from Nanoparticles: The Potential Significance of Nanoscience in Retinal Degenerative Diseases

Several nanotechnology podiums have gained remarkable attention in the area of medical sciences, including diagnostics and treatment. In the past decade, engineered multifunctional nanoparticles have served as drug and gene carriers. The most important aspect of translating nanoparticles from the bench to bedside is safety. These nanoparticles should not elicit any immune response and should not be toxic to humans or the environment. Lipid-based nanoparticles have been shown to be the least toxic for in vivo applications, and significant progress has been made in gene and drug delivery employing lipid-based nanoassemblies. Several excellent reviews and reports discuss the general use and application of lipid-based nanoparticles; our review focuses on the application of lipid-based nanoparticles for the treatment of ocular diseases, and recent advances in and updates on their use.

Ocular Drug Delivery: Role of Degradable Polymeric Nanocarriers for Ophthalmic Application

Ocular drug delivery has been a major challenge for clinical pharmacologists and biomaterial scientists due to intricate and unique anatomical and physiological barriers in the eye. The critical requirement varies from anterior and posterior ocular segments from a drug delivery perspective. Recently, many new drugs with special formulations have been introduced for targeted delivery with modified methods and routes of drug administration to improve drug delivery efficacy. Current developments in nanoformulations of drug carrier systems have become a promising attribute to enhance drug retention/permeation and prolong drug release in ocular tissue. Biodegradable polymers have been explored as the base polymers to prepare nanocarriers for encasing existing drugs to enhance the therapeutic effect with better tissue adherence, prolonged drug action, improved bioavailability, decreased toxicity, and targeted delivery in eye. In this review, we summarized recent studies on sustained ocular drug/gene delivery and emphasized on the nanocarriers made by biodegradable polymers such as liposome, poly lactic-co-glycolic acid (PLGA), chitosan, and gelatin. Moreover, we discussed the bio-distribution of these nanocarriers in the ocular tissue and their therapeutic applications in various ocular diseases.

Polysorbate 20 non-ionic surfactant enhances retinal gene delivery efficiency of cationic niosomes after intravitreal and subretinal administration

The success of non-viral vectors based on cationic niosomes for retinal gene delivery applications depends on the ability to achieve persistent and high levels of transgene expression, ideally from a single administration. In this work, we studied the effect of the non-ionic surfactant component of niosomes in their transfection efficiency in rat retina. For that purpose, three niosome formulations that only differed in the non-ionic tensioactives were elaborated. Niosomes contained: cationic lipid 1,2-di-O-octadecenyl-3-trimethylammonium propane (DOTMA), helper lipid squalene and polysorbate 20, polysorbate 80 or polysorbate 85. Niosomes and corresponding nioplexes were fully characterized in terms of size, polydispersity index, zeta potential, morphology and ability to protect and release DNA. In vitro experiments were carried out to evaluate transfection efficiency, cell viability and intracellular trafficking pathways of the formulations. Nioplexes based on polysorbate 20 niosomes were the most efficient transfecting retinal cells in vitro. Moreover, subretinal and intravitreal administration of those nioplexes in vivo showed also high levels of transgene expression in rat retinas. Our results demonstrate that the incorporation of polysorbate 20 in cationic niosomes enhances retinal gene delivery. Thus, this formulation emerges as a potential non-viral candidate to efficiently transfer specific therapeutic genes into the eye for biomedical purposes.

CRISPR-Cas9 genome engineering: Treating inherited retinal degeneration

Gene correction is a valuable strategy for treating inherited retinal degenerative diseases, a major cause of irreversible blindness worldwide. Single gene defects cause the majority of these retinal dystrophies. Gene augmentation holds great promise if delivered early in the course of the disease, however, many patients carry mutations in genes too large to be packaged into adeno-associated viral vectors and some, when overexpressed via heterologous promoters, induce retinal toxicity. In addition to the aforementioned challenges, some patients have sustained significant photoreceptor cell loss at the time of diagnosis, rendering gene replacement therapy insufficient to treat the disease. These patients will require cell replacement to restore useful vision. Fortunately, the advent of induced pluripotent stem cell and CRISPR-Cas9 gene editing technologies affords researchers and clinicians a powerful means by which to develop strategies to treat patients with inherited retinal dystrophies. In this review we will discuss the current developments in CRISPR-Cas9 gene editing in vivo in animal models and in vitro in patient-derived cells to study and treat inherited retinal degenerative diseases.

Phenotypic characterization of P23H and S334ter rhodopsin transgenic rat models of inherited retinal degeneration

We produced 8 lines of transgenic (Tg) rats expressing one of two different rhodopsin mutations in albino Sprague-Dawley (SD) rats. Three lines were generated with a proline to histidine substitution at codon 23 (P23H), the most common autosomal dominant form of retinitis pigmentosa in the United States. Five lines were generated with a termination codon at position 334 (S334ter), resulting in a C-terminal truncated opsin protein lacking the last 15 amino acid residues and containing all of the phosphorylation sites involved in rhodopsin deactivation, as well as the terminal QVAPA residues important for rhodopsin deactivation and trafficking. The rates of photoreceptor (PR) degeneration in these models vary in proportion to the ratio of mutant to wild-type rhodopsin. The models have been widely studied, but many aspects of their phenotypes have not been described. Here we present a comprehensive study of the 8 Tg lines, including the time course of PR degeneration from the onset to one year of age, retinal structure by light and electron microscopy (EM), hemispheric asymmetry and gradients of rod and cone degeneration, rhodopsin content, gene dosage effect, rapid activation and invasion of the outer retina by presumptive microglia, rod outer segment disc shedding and phagocytosis by the retinal pigmented epithelium (RPE), and retinal function by the electroretinogram (ERG). The biphasic nature of PR cell death was noted, as was the lack of an injury-induced protective response in the rat models. EM analysis revealed the accumulation of submicron vesicular structures in the interphotoreceptor space during the peak period of PR outer segment degeneration in the S334ter lines. This is likely due to the elimination of the trafficking consensus domain as seen before as with other rhodopsin mutants lacking the C-terminal QVAPA. The 8 rhodopsin Tg lines have been, and will continue to be, extremely useful models for the experimental study of inherited retinal degenerations.

Nanoparticles as Delivery Vehicles for the Treatment of Retinal Degenerative Diseases

Over the last few years, huge progress has been made in the understanding of molecular mechanisms underlying the pathogenesis of retinal degenerative diseases. Such knowledge has led to the development of gene therapy approaches to treat these devastating disorders. Non-viral gene delivery has been recognized as a prospective treatment for retinal degenerative diseases. In this review, we will summarize the constituent characteristics and recent applications of three representative nanoparticles (NPs) in ocular therapy.