Morphology and Composition of the Inner Limiting Membrane: Species-Specific Variations and Relevance toward Drug Delivery Research

Müller cells trophism and pathology as the next therapeutic targets for retinal diseases

Müller cells are a crucial retinal cell type involved in multiple regulatory processes and functions that are essential for retinal health and functionality. Acting as structural and functional support for retinal neurons and photoreceptors, Müller cells produce growth factors, regulate ion and fluid homeostasis, and facilitate neuronal signaling. They play a pivotal role in retinal morphogenesis and cell differentiation, significantly contributing to macular development. Due to their radial morphology and unique cytoskeletal organization, Müller cells act as optical fibers, efficiently channeling photons directly to the photoreceptors. In response to retinal damage, Müller cells undergo specific gene expression and functional changes that serve as a first line of defense for neurons, but can also lead to unwarranted cell dysfunction, contributing to cell death and neurodegeneration. In some species, Müller cells can reactivate their developmental program, promoting retinal regeneration and plasticity-a remarkable ability that holds promising therapeutic potential if harnessed in mammals. The crucial and multifaceted roles of Müller cells-that we propose to collectively call "Müller cells trophism"-highlight the necessity of maintaining their functionality. Dysfunction of Müller cells, termed "Müller cells pathology," has been associated with a plethora of retinal diseases, including age-related macular degeneration, diabetic retinopathy, vitreomacular disorders, macular telangiectasia, and inherited retinal dystrophies. In this review, we outline how even subtle disruptions in Müller cells trophism can drive the pathological cascade of Müller cells pathology, emphasizing the need for targeted therapies to preserve retinal health and prevent disease progression.

Intravitreal AAV2 gene delivery to feline retinal ganglion cells

Effective strategies for the neuroprotection and preservation of retinal ganglion cells (RGCs) remain elusive in the management of glaucoma. A spontaneous genetic model of glaucoma has been identified in cats and extensively characterized as a viable translational model, with eye size and anatomy similar to humans. In this study we sought to establish initial proof of concept for gene delivery to feline RGCs via intravitreal injection of AAV2 in normal cats. Pre-retinal, posterior vitreal injection of AAV2/2-CMV-GFP, was performed overlying the area centralis in 5 adult cats. Immunosuppressive oral prednisolone was administered perioperatively and gradually tapered over 6-10wks post-injection. Ophthalmic examination was performed pre- and post-injection. The GFP reporter expression and morphological effects of viral transduction on the retina were monitored in vivo using confocal scanning laser ophthalmoscopy (cSLO) and optical coherence tomography (OCT), respectively (Spectralis OCT-HRA, Heidelberg), at 1-2wk intervals over 6-10wks. Full-field electroretinograms (ERG) and visual evoked potentials (VEP) were recorded at baseline and post-injection. Retinas were examined by histology and immunolabeling for the RGC marker RBPMS and Müller cell and astrocyte marker SOX9, and GFP expression was examined in the retina, optic nerve (ON), optic tract and lateral geniculate nucleus (LGN). GFP+ retinal cells and RGC axons were visualized by cSLO at 1-2 weeks post-injection. No retinal morphological changes were observed by OCT in vivo but 3/5 eyes exhibited mild retinal inflammation on histology. Retinal and ON function were preserved in injected eyes compared to baseline and untreated eyes. GFP expression was predominantly identified in RBPMS+ RGC cells as well as SOX9+ Müller cells. GFP fluorescence was observed throughout RGC nerve fiber tract in the central visual pathway. Peak transduction in RGCs (up to ∼ 20 %) was observed in the regions with high GFP expression, but < 1 % of RGCs expressed GFP across the whole retina. Our data provide proof of concept that pre-retinal injection of AAV2/2 may represent a feasible platform for gene delivery to feline RGCs in vivo but highlight a need for further refinement to improve RGC transduction efficiency and control low-grade retinal inflammation.

Proliferative vitreoretinopathy: a revised concept of retinal injury and response

Previous concepts for the pathogenesis of proliferative vitreoretinopathy (PVR) have focused on the central role of retinal pigment epithelium cells only, potentially contributing to the lack of clinical advances. More recent studies have demonstrated the essential role of retinal glial cells in the PVR healing response but failed to identify a consistent triggering mechanism.We propose a revised concept for the pathogenesis of PVR based on retinal injury and response. A posterior vitreous detachment (PVD) is invariably present in patients with rhegmatogenous retinal detachment and PVR. There is evidence to suggest that the shearing forces of acute PVD can cause mechanical injury to the inner retina and trigger a subsequent intraretinal glial healing response. That response is characterised by subclinical glial cell activation and proliferation that may then be amplified into full-blown PVR by coexisting pathology such as retinal breaks and detachment.Whether a PVD causes interface pathology depends on the plane of separation of the posterior vitreous and areas of increased vitreoretinal adhesions. If the vitreous separates in a plane or location that damages the inner retina then glial cell activation and proliferation are likely to develop. The severity of the subclinical inner retinal damage may then represent one of the missing links in our understanding of the pathogenesis of PVR and would explain many of the findings we encounter in clinical practice. Controlling the process of acute PVD and subsequent intraretinal response may be essential in the prevention and management of PVR.

Innovative Use of Nanomaterials in Treating Retinopathy of Prematurity

BACKGROUND/OBJECTIVES

Retinopathy of prematurity (ROP) is a severe condition primarily affecting premature infants with a gestational age (GA) of 30 weeks or less and a birth weight (BW) of 1500 g or less. The objective of this review is to examine the risk factors, pathogenesis, and current treatments for ROP, such as cryotherapy, laser photocoagulation, and anti-VEGF therapy, while exploring the limitations of these approaches. Additionally, this review evaluates emerging nanotherapeutic strategies to address these challenges, aiming to improve ROP management.

METHODS

A comprehensive literature review was conducted to gather data on the pathogenesis, traditional treatment methods, and novel nanotherapeutic approaches for ROP. This included assessing the efficacy and safety profiles of cryotherapy, laser treatment, anti-VEGF therapy, and nanotherapies currently under investigation.

RESULTS

Traditional treatments, while effective in reducing disease progression, exhibit limitations, including long-term complications, tissue damage, and systemic side effects. Nanotherapeutic approaches, on the other hand, have shown potential in offering targeted drug delivery with reduced systemic toxicity, improved ocular drug penetration, and sustained release, which could decrease the frequency of treatments and enhance therapeutic outcomes.

CONCLUSIONS

Nanotherapies represent a promising advancement in ROP treatment, offering safer and more effective management strategies. These innovations could address the limitations of traditional therapies, reducing complications and improving outcomes for premature infants affected by ROP. Further research is needed to confirm their efficacy and safety in clinical practice.

Utilizing extracellular vesicles as a drug delivery system in glaucoma and RGC degeneration

Retinal diseases are the leading cause of blindness, resulting in irreversible degeneration and death of retinal neurons. One such cell type, the retinal ganglion cell (RGC), is responsible for connecting the retina to the rest of the brain through its axons that make up the optic nerve and is the primary cell lost in glaucoma and traumatic optic neuropathy. To date, different therapeutic strategies have been investigated to protect RGCs from death and preserve vision, yet currently available strategies are restricted to treating neuron loss by reducing intraocular pressure. A major barrier identified by these studies is drug delivery to RGCs, which is in large part due to drug stability, short duration time at target, low delivery efficiency, and undesired off-target effects. Therefore, a delivery system to deal with these problems is needed to ensure maximum benefit from the candidate therapeutic material. Extracellular vesicles (EV), nanocarriers released by all cells, are lipid membranes encapsulating RNAs, proteins, and lipids. As they naturally shuttle these encapsulated compounds between cells for communicative purposes, they may be exploitable and offer opportunities to overcome hurdles in retinal drug delivery, including drug stability, drug molecular weight, barriers in the retina, and drug adverse effects. Here, we summarize the potential of an EV drug delivery system, discussing their superiorities and potential application to target RGCs.

Natural products for the treatment of age-related macular degeneration

BACKGROUND

Age-related macular degeneration (AMD) is a chronic retinal disease that significantly influences the vision of the elderly.

PURPOSE

There is no effective treatment and prevention method. The pathogenic process behind AMD is complex, including oxidative stress, inflammation, and neovascularization. It has been demonstrated that several natural products can be used to manage AMD, but systematic summaries are lacking.

STUDY DESIGN AND METHODS

PubMed, Web of Science, and ClinicalTrials.gov were searched using the keywords "Biological Products" AND "Macular Degeneration" for studies published within the last decade until May 2023 to summarize the latest findings on the prevention and treatment of age-related macular degeneration through the herbal medicines and functional foods.

RESULTS

The eligible studies were screened, and the relevant information about the therapeutic action and mechanism of natural products used to treat AMD was extracted. Our findings demonstrate that natural substances, including retinol, phenols, and other natural products, prevent the development of new blood vessels and protect the retina from oxidative stress in cells and animal models. However, they have barely been examined in clinical studies.

CONCLUSION

Natural products could be highly prospective candidate drugs used to treat AMD, and further preclinical and clinical research is required to validate it to control the disease.

Hollow microneedles for ocular drug delivery

Microneedles (MNs) are micron-sized needles, typically <2 mm in length, arranged either as an array or as single needle. These MNs offer a minimally invasive approach to ocular drug delivery due to their micron size (reducing tissue damage compared to that of hypodermic needles) and overcoming significant barriers in drug administration. While various types of MNs have been extensively researched, significant progress has been made in the use of hollow MNs (HMNs) for ocular drug delivery, specifically through suprachoroidal injections. The suprachoroidal space, situated between the sclera and choroid, has been targeted using optical coherence tomography-guided injections of HMNs for the treatment of uveitis. Unlike other MNs, HMNs can deliver larger volumes of formulations to the eye. This review primarily focuses on the use of HMNs in ocular drug delivery and explores their ocular anatomy and the distribution of formulations following potential HMN administration routes. Additionally, this review focuses on the influence of formulation characteristics (e.g., solution viscosity, particle size), HMN properties (e.g., bore or lumen diameter, MN length), and routes of administration (e.g., periocular transscleral, suprachoroidal, intravitreal) on the ocular distribution of drugs. Overall, this paper highlights the distinctive properties of HMNs, which make them a promising technology for improving drug delivery efficiency, precision, and patient outcomes in the treatment of ocular diseases.

Recent advances and strategies for nanocarrier-mediated topical therapy and theranostic for posterior eye disease

Posterior eye disorders, such as age-related macular degeneration, diabetic retinopathy, and glaucoma, have a significant impact on human quality of life and are the primary cause of age-related retinal diseases among adults. There is a pressing need for innovative topical approaches to treat posterior eye disorders, as current methods often rely on invasive procedures with inherent risks. Limited success was attained in the realm of topical ophthalmic delivery through non-invasive means. Additionally, there exists a dearth of literature that delves into the potential of this approach for drug delivery and theranostic purposes, or that offers comprehensive design strategies for nanocarrier developers to surmount the significant physiological ocular barriers. This review offers a thorough and up-to-date state-of-the-art overview of 40 studies on therapeutic loaded nanocarriers and theranostic devices that, to the best of our knowledge, represent all successful works that reached posterior eye segments through a topical non-invasive administration. Most importantly, based on the successful literature studies, this review provides a comprehensive summary of the potential design strategies that can be implemented during nanocarrier development to overcome each ocular barrier.

Ophthalmologists' assessment of the handling characteristics of the novel Finesse Reflex Handle in comparison to those of a conventional handle

Internal limiting membrane (ILM) peeling requires a delicate handling technique. It is also important that ophthalmologists can use the ILM forceps handle of their preference. This study objectively and subjectively evaluated the handling of the novel Finesse Reflex Handle (Reflex) in comparison with that of a conventional handle. The force required to close the forceps tips, evaluated using a digital force gauge, was significantly lesser for Reflex than for the conventional handle (3.14 ± 0.09 N vs. 3.84 ± 0.06 N, P < 0.001). Twenty-one ophthalmologists with various levels of experience answered a questionnaire after using both handles, and the total questionnaire score for Reflex was higher than that for the conventional handle (35.0 ± 3.7 vs. 30.0 ± 6.9, P = 0.01). Furthermore, the duration of experience as an ophthalmologist was negatively correlated with the vertical motion, assessed by video analysis, for the conventional handle (P = 0.02, r =  - 0.50) but not for Reflex (P = 0.26). In conclusion, objective and subjective analyses revealed that compared with the conventional handle, the novel Reflex handle had more favourable handling characteristics. Most ophthalmologists preferred the handling of Reflex. Reflex may compensate for a lack of surgical experience.

Next generation therapeutics for retinal neurodegenerative diseases

Neurodegenerative diseases affecting the visual system encompass glaucoma, macular degeneration, retinopathies, and inherited genetic disorders such as retinitis pigmentosa. These ocular pathologies pose a serious burden of visual impairment and blindness worldwide. Current treatment modalities include small molecule drugs, biologics, or gene therapies, most of which are administered topically as eye drops or as injectables. However, the topical route of administration faces challenges in effectively reaching the posterior segment and achieving desired concentrations at the target site, while injections and implants risk severe complications, such as retinal detachment and endophthalmitis. This necessitates the development of innovative therapeutic strategies that can prolong drug release, deliver effective concentrations to the back of the eye with minimal systemic exposure, and improve patient compliance and safety. In this review, we introduce retinal degenerative diseases, followed by a discussion of the existing clinical standard of care. We then delve into detail about drug and gene delivery systems currently in preclinical and clinical development, including formulation and delivery advantages/drawbacks, with a special emphasis on potential for clinical translation.

Permanent transduction of retinal ganglion cells by rAAV2-retro

Adeno-associated virus (AAV) is widely used as a vector for delivery of gene therapy. Long term therapeutic benefit depends on perpetual expression of the wild-type gene after transduction of host cells by AAV. To address this issue in a mass population of identified single cells, 4 rats received an injection of a 1:1 mixture of rAAV2-retro-hSyn-EGFP and rAAV2-retro-hSyn-mCherry into each superior colliculus. After the virus was transported retrogradely to both retinas, serial fundus imaging was performed at days 14, 45, 211, and 375 to visualize individual fluorescent ganglion cells. The location of each cell was plotted to compare labeling at each time point. In 12/16 comparisons, 97% or more of the cells identified in the initial baseline fundus image were still labeled at day 375. In 4 cases the percentage was lower, but in these cases the apparent reduction in the number of labeled cells at day 375 was attributable to the lower quality of follow-up fundus images, rather than true loss of transgene expression. These data indicate that retinal ganglion cells transduced by rAAV2-retro are transduced permanently.

Less is more: Self-amplifying mRNA becomes self-killing upon dose escalation in immune-competent retinal cells

In the last few years, mRNA therapeutics experienced a new wave of interest as therapy for retinal diseases. Nevertheless, despite the widespread use of mRNA vaccines in the COVID-19 pandemic, mRNA delivery to the eye is still in its infancy. Recently, our research group has demonstrated that after subretinal and intravitreal delivery of modified mRNA, the number of transfected retinal cells and protein expression per cell remains limited. In this study, we aimed to tackle this limitation by using self-amplifying mRNA (saRNA), which in theory will increase the duration and level of protein expression when only a few mRNA molecules reach their target cells. A one-on-one comparison between modified mRNA and saRNA in two immune-competent human retinal cell types, including Müller cells and retinal pigment epithelial cells, and in immune-deficient BHK-21 cells revealed that saRNA delivery induced an innate immune response blocking its own translation above a certain dose threshold. Removal of double-stranded (ds)RNA byproducts by cellulose-based purification and addition of the innate immune inhibitor B18R remarkably improved translation from saRNA through a reduction in innate immune response. Taken together, when saRNA is applied for retinal disease, the dose should be controlled and measures should be taken to limit immunogenicity.

Effect of epiretinal electrical stimulation on the glial cells in a rabbit retinal eyecup model

Introduction

We examined how pulse train electrical stimulation of the inner surface of the rabbit retina effected the resident glial cells. We used a rabbit retinal eyecup preparation model, transparent stimulus electrodes, and optical coherence tomography (OCT). The endfeet of Müller glia processes line the inner limiting membrane (ILM).

Methods

To examine how epiretinal electrode stimulation affected the Müller glia, we labeled them post stimulation using antibodies against soluble glutamine synthetase (GS). After 5 min 50 Hz pulse train stimulation 30 μm from the surface, the retina was fixed, immunostained for Müller glia, and examined using confocal microscopic reconstruction. Stimulus pulse charge densities between 133-749 μC/cm2/ph were examined.

Results

High charge density stimulation (442-749 μC/cm2/ph) caused significant losses in the GS immunofluorescence of the Müller glia endfeet under the electrode. This loss of immunofluorescence was correlated with stimuli causing ILM detachment when measured using OCT. Müller cells show potassium conductances at rest that are blocked by barium ions. Using 30 msec 20 μA stimulus current pulses across the eyecup, the change in transretinal resistance was examined by adding barium to the Ringer. Barium caused little change in the transretinal resistance, suggesting under low charge density stimulus pulse conditions, the Müller cell radial conductance pathway for these stimulus currents was small. To examine how epiretinal electrode stimulation affected the microglia, we used lectin staining 0-4 h post stimulation. After stimulation at high charge densities 749 μC/cm2/ph, the microglia under the electrode appeared rounded, while the local microglia outside the electrode responded to the stimulated retina by process orientation inwards in a ring by 30 min post stimulation.

Discussion

Our study of glial cells in a rabbit eyecup model using transparent electrode imaging suggests that epiretinal electrical stimulation at high pulse charge densities, can injure the Müller and microglia cells lining the inner retinal surface in addition to ganglion cells.

The Road towards Gene Therapy for X-Linked Juvenile Retinoschisis: A Systematic Review of Preclinical Gene Therapy in Cell-Based and Rodent Models of XLRS

X-linked juvenile retinoschisis (XLRS) is an early-onset progressive inherited retinopathy affecting males. It is characterized by abnormalities in the macula, with formation of cystoid retinal cavities, frequently accompanied by splitting of the retinal layers, impaired synaptic transmission of visual signals, and associated loss of visual acuity. XLRS is caused by loss-of-function mutations in the retinoschisin gene located on the X chromosome (RS1, MIM 30083). While proof-of-concept studies for gene augmentation therapy have been promising in in vitro and rodent models, clinical trials in XLRS patients have not been successful thus far. We performed a systematic literature investigation using search strings related to XLRS and gene therapy in in vivo and in vitro models. Three rounds of screening (title/abstract, full text and qualitative) were performed by two independent reviewers until consensus was reached. Characteristics related to study design and intervention were extracted from all studies. Results were divided into studies using (1) viral and (2) non-viral therapies. All in vivo rodent studies that used viral vectors were assessed for quality and risk of bias using the SYRCLE's risk-of-bias tool. Studies using alternative and non-viral delivery techniques, either in vivo or in vitro, were extracted and reviewed qualitatively, given the diverse and dispersed nature of the information. For in-depth analysis of in vivo studies using viral vectors, outcome data for optical coherence tomography (OCT), immunohistopathology and electroretinography (ERG) were extracted. Meta-analyses were performed on the effect of recombinant adeno-associated viral vector (AAV)-mediated gene augmentation therapies on a- and b-wave amplitude as well as the ratio between b- and a-wave amplitudes (b/a-ratio) extracted from ERG data. Subgroup analyses and meta-regression were performed for model, dose, age at injection, follow-up time point and delivery method. Between-study heterogeneity was assessed with a Chi-square test of homogeneity (I2). We identified 25 studies that target RS1 and met our search string. A total of 19 of these studies reported rodent viral methods in vivo. Six of the 25 studies used non-viral or alternative delivery methods, either in vitro or in vivo. Of these, five studies described non-viral methods and one study described an alternative delivery method. The 19 aforementioned in vivo studies were assessed for risk of bias and quality assessments and showed inconsistency in reporting. This resulted in an unclear risk of bias in most included studies. All 19 studies used AAVs to deliver intact human or murine RS1 in rodent models for XLRS. Meta-analyses of a-wave amplitude, b-wave amplitude, and b/a-ratio showed that, overall, AAV-mediated gene augmentation therapy significantly ameliorated the disease phenotype on these parameters. Subgroup analyses and meta-regression showed significant correlations between b-wave amplitude effect size and dose, although between-study heterogeneity was high. This systematic review reiterates the high potential for gene therapy in XLRS, while highlighting the importance of careful preclinical study design and reporting. The establishment of a systematic approach in these studies is essential to effectively translate this knowledge into novel and improved treatment alternatives.

Overview of processed excipients in ocular drug delivery: Opportunities so far and bottlenecks

Ocular drug delivery presents a unique set of challenges owing to the complex anatomy and physiology of the eye. Processed excipients have emerged as crucial components in overcoming these challenges and improving the efficacy and safety of ocular drug delivery systems. This comprehensive overview examines the opportunities that processed excipients offer in enhancing drug delivery to the eye. By analyzing the current landscape, this review highlights the successful applications of processed excipients, such as micro- and nano-formulations, sustained-release systems, and targeted delivery strategies. Furthermore, this article delves into the bottlenecks that have impeded the widespread adoption of these excipients, including formulation stability, biocompatibility, regulatory constraints, and cost-effectiveness. Through a critical evaluation of existing research and industry practices, this review aims to provide insights into the potential avenues for innovation and development in ocular drug delivery, with a focus on addressing the existing challenges associated with processed excipients. This synthesis contributes to a deeper understanding of the promising role of processed excipients in improving ocular drug delivery systems and encourages further research and development in this rapidly evolving field.

Adeno-associated virus vectors for retinal gene therapy in basic research and clinical studies

Retinal degenerative diseases, including glaucoma, age-related macular degeneration, diabetic retinopathy, and a broad range of inherited retinal diseases, are leading causes of irreversible vision loss and blindness. Gene therapy is a promising and fast-growing strategy to treat both monogenic and multifactorial retinal disorders. Vectors for gene delivery are crucial for efficient and specific transfer of therapeutic gene(s) into target cells. AAV vectors are ideal for retinal gene therapy due to their inherent advantages in safety, gene expression stability, and amenability for directional engineering. The eye is a highly compartmentalized organ composed of multiple disease-related cell types. To determine a suitable AAV vector for a specific cell type, the route of administration and choice of AAV variant must be considered together. Here, we provide a brief overview of AAV vectors for gene transfer into important ocular cell types, including retinal pigment epithelium cells, photoreceptors, retinal ganglion cells, Müller glial cells, ciliary epithelial cells, trabecular meshwork cells, vascular endothelial cells, and pericytes, via distinct injection methods. By listing suitable AAV vectors in basic research and (pre)clinical studies, we aim to highlight the progress and unmet needs of AAV vectors in retinal gene therapy.

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

Gene therapy for inherited retinal diseases: exploiting new tools in genome editing and nanotechnology

Inherited retinal diseases (IRDs) encompass a diverse group of genetic disorders that lead to progressive visual impairment and blindness. Over the years, considerable strides have been made in understanding the underlying molecular mechanisms of IRDs, laying the foundation for novel therapeutic interventions. Gene therapy has emerged as a compelling approach for treating IRDs, with notable advancements achieved through targeted gene augmentation. However, several setbacks and limitations persist, hindering the widespread clinical success of gene therapy for IRDs. One promising avenue of research is the development of new genome editing tools. Cutting-edge technologies such as CRISPR-Cas9 nucleases, base editing and prime editing provide unprecedented precision and efficiency in targeted gene manipulation, offering the potential to overcome existing challenges in gene therapy for IRDs. Furthermore, traditional gene therapy encounters a significant challenge due to immune responses to viral vectors, which remain crucial obstacles in achieving long-lasting therapeutic effects. Nanotechnology has emerged as a valuable ally in the quest to optimize gene therapy outcomes for ocular diseases. Nanoparticles engineered with nanoscale precision offer improved gene delivery to specific retinal cells, allowing for enhanced targeting and reduced immunogenicity. In this review, we discuss recent advancements in gene therapy for IRDs and explore the setbacks that have been encountered in clinical trials. We highlight the technological advances in genome editing for the treatment of IRDs and how integrating nanotechnology into gene delivery strategies could enhance the safety and efficacy of gene therapy, ultimately offering hope for patients with IRDs and potentially paving the way for similar advancements in other ocular disorders.

Vitrectomy and ILM peeling in rhesus macaque: pitfalls and tips for success

BACKGROUND

The non-human primate (NHP) model is ideal for pre-clinical testing of novel therapies for human retinal diseases due to its similarity to the human visual system. However, intra-ocular delivery of gene therapy or cell transplantation to the retina gets hampered by the sticky vitreous body and poorly permeable inner limiting membrane (ILM) in primates. Although vitrectomy and ILM peeling are commonly performed in patients, many pitfalls exist in carrying out these procedures in the rhesus macaque, which have not been reported previously.

METHODS

We summarised common surgical pitfalls after performing vitrectomy and ILM peeling in four eyes of two rhesus macaques (one male and one female). We provided corresponding hands-on technical tips based on our surgical experience and literature search. Orbital CT scans were compared between adult rhesus macaques and humans. High-resolution surgical videos were recorded to demonstrate each critical surgical step.

RESULTS

Due to size difference, poor post-operative compliance, and high-standard requirements of a controlled experiment, there were eleven common surgical pitfalls during vitrectomy and ILM peeling in rhesus macaque. Falling into these pitfalls may produce discomfort, add fatigue, cause surgical complications, or even lead to the exclusion of the NHP from an experimental group.

CONCLUSION

Recognition and circumvention of these pitfalls during vitrectomy and ILM peeling in NHP are essential. By focusing on these surgical pitfalls, we can better carry out preclinical tests of novel therapies for retinal diseases in the NHP model.

Delivery Systems in Ocular Retinopathies: The Promising Future of Intravitreal Hydrogels as Sustained-Release Scaffolds

Slow-release delivery systems are needed to ensure long-term sustained treatments for retinal diseases such as age-related macular degeneration and diabetic retinopathy, which are currently treated with anti-angiogenic agents that require frequent intraocular injections. These can cause serious co-morbidities for the patients and are far from providing the adequate drug/protein release rates and required pharmacokinetics to sustain prolonged efficacy. This review focuses on the use of hydrogels, particularly on temperature-responsive hydrogels as delivery vehicles for the intravitreal injection of retinal therapies, their advantages and disadvantages for intraocular administration, and the current advances in their use to treat retinal diseases.

Co-delivery of microRNA-150 and quercetin by lipid nanoparticles (LNPs) for the targeted treatment of age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is characterized by choroidal neovascularization (CNV), which leads to severe vision loss in middle-aged and elderly patients. Current treatments for CNV show weak, transient efficacy, and they can cause several adverse effects. A potential new treatment is to use microRNA-150 (mR150), which regulates physiological and pathological angiogenesis by modulating the expression of CXCR4 at the post-transcriptional level. Here, we developed solid lipid nanoparticles that we modified with an Asp-Gly-Arg peptide to target endothelial cells during abnormal angiogenesis, then we co-loaded them with mR150 and the anti-angiogenic drug quercetin. The resulting nanoparticles had an average size around 200 nm and showed strong ability to target the fundus and inhibit CNV for up to two weeks in a mouse model without causing retinal toxicity. They significantly enhanced the uptake of mR150 in vitro compared to free mR150 or nanoparticles without peptide. Our study suggests that co-administration of mR150 and quercetin has potential for treating age-related macular degeneration and that nanoparticles modified with Asp-Gly-Arg peptide are an effective platform for the co-delivery of small-molecule and nucleic acid drugs via intravitreal injection.

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.

Ocular Delivery of Therapeutic Proteins: A Review

Therapeutic proteins, including monoclonal antibodies, single chain variable fragment (ScFv), crystallizable fragment (Fc), and fragment antigen binding (Fab), have accounted for one-third of all drugs on the world market. In particular, these medicines have been widely used in ocular therapies in the treatment of various diseases, such as age-related macular degeneration, corneal neovascularization, diabetic retinopathy, and retinal vein occlusion. However, the formulation of these biomacromolecules is challenging due to their high molecular weight, complex structure, instability, short half-life, enzymatic degradation, and immunogenicity, which leads to the failure of therapies. Various efforts have been made to overcome the ocular barriers, providing effective delivery of therapeutic proteins, such as altering the protein structure or including it in new delivery systems. These strategies are not only cost-effective and beneficial to patients but have also been shown to allow for fewer drug side effects. In this review, we discuss several factors that affect the design of formulations and the delivery of therapeutic proteins to ocular tissues, such as the use of injectable micro/nanocarriers, hydrogels, implants, iontophoresis, cell-based therapy, and combination techniques. In addition, other approaches are briefly discussed, related to the structural modification of these proteins, improving their bioavailability in the posterior segments of the eye without affecting their stability. Future research should be conducted toward the development of more effective, stable, noninvasive, and cost-effective formulations for the ocular delivery of therapeutic proteins. In addition, more insights into preclinical to clinical translation are needed.

Formulation and Evaluation of Polymer-Based Nanoparticles for Intravitreal Gene-Delivery Applications

The advent of the first-ever retinal gene therapy product, involving subretinal administration of a virus-based gene delivery platform, has garnered hope that this state-of-the-art therapeutic modality may benefit a broad spectrum of patients with diverse retinal disorders. On the other hand, clinical studies have revealed limitations of the applied delivery strategy that may restrict its universal use. To this end, intravitreal administration of synthetic gene-delivery platforms, such as polymer-based nanoparticles (PNPs), has emerged as an attractive alternative to the current mainstay. To achieve success, however, it is imperative that synthetic platforms overcome key biological barriers in human eyes encountered following intravitreal administration, including the vitreous gel and inner limiting membrane (ILM). Here, we introduce a series of experiments, from the fabrication of PNPs to a comprehensive evaluation in relevant experimental models, to determine whether PNPs overcome these barriers and efficiently deliver therapeutic gene payloads to retinal cells. We conclude the article by discussing a few important considerations for successful implementation of the strategy. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Preparation and characterization of PNPs Basic Protocol 2: Evaluation of in vitro transfection efficacy Basic Protocol 3: Evaluation of PNP diffusion in vitreous gel Basic Protocol 4: Ex vivo assessment of PNP penetration within vitreoretinal explant culture Basic Protocol 5: Assessment of in vivo transgene expression mediated by intravitreally administered PNPs.

Nanoparticles for the treatment of glaucoma-associated neuroinflammation

Recently, a considerable amount of literature has emerged around the theme of neuroinflammation linked to neurodegeneration. Glaucoma is a neurodegenerative disease characterized by visual impairment. Understanding the complex neuroinflammatory processes underlying retinal ganglion cell loss has the potential to improve conventional therapeutic approaches in glaucoma. Due to the presence of multiple barriers that a systemically administered drug has to cross to reach the intraocular space, ocular drug delivery has always been a challenge. Nowadays, studies are focused on improving the current therapies for glaucoma by utilizing nanoparticles as the modes of drug transport across the ocular anatomical and physiological barriers. This review offers some important insights on the therapeutic advancements made in this direction, focusing on the use of nanoparticles loaded with anti-inflammatory and neuroprotective agents in the treatment of glaucoma. The prospect of these novel therapies is discussed in relation to the current therapies to alleviate inflammation in glaucoma, which are being reviewed as well, along with the detailed molecular and cellular mechanisms governing the onset and the progression of the disease.

Outer retinal transduction by AAV2-7m8 following intravitreal injection in a sheep model of CNGA3 achromatopsia

Sheep carrying a mutated CNGA3 gene exhibit diminished cone function and provide a naturally occurring large animal model of achromatopsia. Subretinal injection of a vector carrying the CNGA3 transgene resulted in long-term recovery of cone function and photopic vision in these sheep. Research is underway to develop efficacious vectors that would enable safer transgene delivery, while avoiding potential drawbacks of subretinal injections. The current study evaluated two modified vectors, adeno-associated virus 2-7m8 (AAV2-7m8) and AAV9-7m8. Intravitreal injection of AAV2-7m8 carrying enhanced green fluorescent protein under a cone-specific promoter resulted in moderate photoreceptor transduction in wild-type sheep, whereas peripheral subretinal delivery of AAV9-7m8 resulted in the radial spread of the vector beyond the point of deposition. Intravitreal injection of AAV2-7m8 carrying human CNGA3 in mutant sheep resulted in mild photoreceptor transduction, but did not lead to the clinical rescue of photopic vision, while day-blind sheep treated with a subretinal injection exhibited functional recovery of photopic vision. Transgene messenger RNA levels in retinas of intravitreally treated eyes amounted to 4-23% of the endogenous CNGA3 levels, indicating that expression levels >23% are needed to achieve clinical rescue. Overall, our results indicate intravitreal injections of AAV2.7m8 transduce ovine photoreceptors, but not with sufficient efficacy to achieve clinical rescue in CNGA3 mutant sheep.

Photodisruption of the Inner Limiting Membrane: Exploring ICG Loaded Nanoparticles as Photosensitizers

The inner limiting membrane (ILM) represents a major bottleneck hampering efficient drug delivery to the retina after intravitreal injection. To overcome this barrier, we intend to perforate the ILM by use of a light-based approach which relies on the creation of vapor nanobubbles (VNBs) when irradiating photosensitizers with high intensity laser pulses. Upon collapse of these VNBs, mechanical effects can disrupt biological structures. As a photosensitizer, we explore indocyanine green (ICG) loaded nanoparticles (NPs) specifically designed for our application. In light of this, ICG liposomes and PLGA ICG NPs were characterized in terms of physicochemical properties, ICG incorporation and VNB formation. ICG liposomes were found to encapsulate significantly higher amounts of ICG compared to PLGA ICG NPs which is reflected in their VNB creating capacity. Since only ICG liposomes were able to induce VNB generation, this class of NPs was further investigated on retinal explants. Here, application of ICG liposomes followed by laser treatment resulted in subtle disruption effects at the ILM where zones of fully ablated ILM were alternated by intact regions. As the interaction between the ICG liposomes and ILM might be insufficient, active targeting strategies or other NP designs might improve the concept to a further extent.

ICG-mediated photodisruption of the inner limiting membrane enhances retinal drug delivery

Many groundbreaking therapies for the treatment of blindness require delivery of biologics or cells to the inner retina by intravitreal injection. Unfortunately, the advancement of these therapies is greatly hampered by delivery difficulties where obstruction of the therapeutics at the inner limiting membrane (ILM) represents the dominant bottleneck. In this proof-of-principle study, we explore an innovative light-based approach to locally ablate the ILM in a minimally invasive and highly controlled manner, thus making the ILM more permeable for therapeutics. More specifically, we demonstrate that pulsed laser irradiation of ILM-bound indocyanine green (ICG), a clinically applied ILM dye, results in the formation of vapor nanobubbles which can disrupt the bovine ILM as well as the extraordinary thick human ILM. We have observed that this photodisruption allows for highly successful retinal delivery of model nanoparticles which are otherwise blocked by the intact ILM. Strikingly, this treatment is furthermore able of enhancing the efficacy of mRNA-loaded lipid nanoparticles within the bovine retina by a factor of 5. In conclusion, this study provides evidence for a light-based approach to overcome the ILM which has the potential to improve the efficacy of all retinal therapies hampered by this delivery barrier.

Targeted drug delivery to the retinal pigment epithelium: Untapped therapeutic potential for retinal diseases

The retinal pigment epithelium (RPE) plays a crucial part in sight-threatening diseases. In this review, we shed light on the pivotal implication of the RPE in age-related macular degeneration, diabetic retinopathy and retinopathy of prematurity; and explain why a paradigm shift toward targeted RPE therapy is needed to efficiently fight these retinal diseases. We provide guidance for the development of RPE-specific nanotherapeutics by giving a comprehensive overview of the possibilities and challenges of drug delivery to the RPE and highlight successful nanotherapeutic approaches targeting the RPE.

Polymer based sustained drug delivery to the ocular posterior segment: barriers and future opportunities for the treatment of neovascular pathologies

There is an increasing momentum in research and pharmaceutical industry communities to design sustained, non-invasive delivery systems to treat chronic neovascular ocular diseases that affect the posterior segment of the eye including age-related macular degeneration and diabetic retinopathy. Current treatments include VEGF blockers, which have revolutionized the standard of care for patients, but their maximum therapeutic benefit is hampered by the need for recurrent and invasive administration procedures. Currently approved delivery systems intended to address these limitations exploit polymer technology to regulate drug release in a sustained manner. Here, we critically review sustained drug delivery approaches for the treatment of chronic neovascular diseases affecting the ocular posterior segment, with a special emphasis on novel and polymeric technologies spanning the spectrum of preclinical and clinical investigation, and those approved for treatment. The mechanism by which each formulation imparts sustained release, the impact of formulation characteristics on release and foreign body reaction, and special considerations related to the translation of these systems are discussed.

Fundus imaging of retinal ganglion cells transduced by retrograde transport of rAAV2-retro

Access of adeno-associated virus (AAV) to ganglion cells following intravitreal injection for gene therapy is impeded by the internal limiting membrane of the retina. As an alternative, one could transduce ganglion cells via retrograde transport after virus injection into a retinal target nucleus. It is unknown if recombinant AAV2-retro (rAAV2-retro), a variant of AAV2 developed specifically for retrograde transport, is capable of transducing retinal ganglion cells. To address this issue, equal volumes of rAAV2-retro-hSyn-EGFP and rAAV2-retro-hSyn-mCherry were mixed in a micropipette and injected into the rat superior colliculus. The time-course of viral transduction was tracked by performing serial in vivo fundus imaging. Cells that were labeled by the fluorophores within the first week remained consistent in distribution and relative signal strength on follow-up imaging. Most transduced cells were double-labeled, but some were labeled by only EGFP or mCherry. Fundus images were later aligned with retinal wholemounts. Ganglion cells in the wholemounts matched precisely the cells imaged by fundus photography. As seen in the fundus images, ganglion cells in wholemounts were sometimes labeled by only EGFP or mCherry. Overall, there was detectable label in 32-41% of ganglion cells. Analysis of the number of cells labeled by 0, 1, or 2 fluorophores, based on Poisson statistics, yielded an average of 0.66 virions transducing each ganglion cell. Although this represents a low number relative to the quantity of virus injected into the superior colliculus, the ganglion cells showed sustained and robust fluorescent labeling. In the primate, injection of rAAV2-retro into the lateral geniculate nucleus might provide a viable approach for the transduction of ganglion cells, bypassing the obstacles that have prevented effective gene delivery via intravitreal injection.

The MDM2 Single-Nucleotide Polymorphism T309G Is Associated With the Development of Epimacular Membranes

Purpose: To investigate the role of the mouse double minute 2 (MDM2) gene single-nucleotide polymorphism (SNP) T309G in the development of epimacular membranes (EMMs) by analyzing the genotype distribution and consistency of the polymorphism in paired membrane-blood samples.

Methods: This was a cross–sectional genetic association study of patients with proliferative vitreoretinopathy (PVR) or EMMs. PVR membranes (PVRMs), internal limiting membranes (ILMs) (PVR-ILMs) and blood samples (PVR-blood) from patients with PVR, and EMMs, EMM-ILMs and EMM-blood from patients with EMMs were collected. The genotype of all samples was determined by Sanger sequencing. Sex composition, mean age, the genotype distribution of MDM2 T309G, the allelic frequency of the MDM2 SNP309 G allele (% G) and the somatic mutation rate at the MDM2 T309G locus (% M) were analyzed and compared. The PVR and healthy Chinese donor groups were used as controls for different comparisons.

Results: The EMM group of 62 patients was older than the PVR group of 61 patients by an average of 8.87 years (p < 0.0001), but the two groups were statistically similar in the sex composition (p = 0.1754). Importantly, G allele carriers were at a higher risk of developing EMMs than non-G allele carriers (p = 0.0479; OR = 2.047). Moreover, EMM-blood exhibited a significantly higher % G than blood samples from healthy Chinese donors (EMM-blood: 56.78%, donors: 45.61%; p = 0.0256; OR = 1.567). Regarding membrane-blood consistency, % M was significantly different between PVRMs and EMMs (PVRMs: 2.63%, EMMs: 21.57%; p = 0.0097; OR = 10.18) but not between different types of ILMs (PVR-ILMs: 18.18%, EMM-ILMs: 29.17%; p = 0.6855). Furthermore, EMMs (p = 0.0053; OR = 8.250) and EMM-ILMs (p = 0.0233; OR = 14.40) from patients with preoperative macular holes were more predisposed toward somatic mutations at the MDM2 T309G locus than those from patients without preoperative macular holes.

Conclusions:MDM2 T309G is associated with the development of EMMs. Herein, the MDM2 SNP309 G allele is first reported as an associated factor of EMMs in a Chinese population. In addition, EMMs and ILMs are genetically unstable at the MDM2 T309G locus, especially when complicated with preoperative macular holes.

UV-DIB: label-free permeability determination using droplet interface bilayers

Simple diffusion of molecular entities through a phospholipid bilayer, is a phenomenon of great importance to the pharmaceutical and agricultural industries. Current model lipid systems to probe this typically only employ fluorescence as a readout, thus limiting the range of assessable chemical matter that can be studied. We report a new technology platform, the UV-DIB, which facilitates label free measurement of small molecule translocation rates. This is based upon the coupling of droplet interface bilayer technology with implemented fiber optics to facilitate analysis via ultraviolet spectroscopy, in custom designed PMMA wells. To improve on current DIB technology, the platform was designed to be reusable, with a high sampling rate and a limit of UV detection in the low μM regime. We demonstrate the use of our system to quantify passive diffusion in a reproducible and rapid manner where the system was validated by investigating multiple permeants of varying physicochemical properties across a range of lipid interfaces, each demonstrating differing kinetics. Our system permits the interrogation of structural dependence on the permeation rate of a given compound. We present this ability from two structural perspectives, that of the membrane, and the permeant. We observed a reduction in permeability between pure DOPC and DPhPC interfaces, concurring with literature and demonstrating our ability to study the effects of lipid composition on permeability. In relation to the effects of permeant structure, our device facilitated the rank ordering of various compounds from the xanthine class of compounds, where the structure of each permeant differed by a single group alteration. We found that DIBs were stable up to 5% DMSO, a molecule often used to aid solubilisation of pharmaceutical and agrochemical compounds. The ability of our device to rank-order compounds with such minor structural differences provides a level of precision that is rarely seen in current, industrially applied technologies.

Scientific and Regulatory Policy Committee Points to Consider: Fixation, Trimming, and Sectioning of Nonrodent Eyes and Ocular Tissues for Examination in Ocular and General Toxicity Studies

A Working Group of the Society of Toxicologic Pathology's Scientific and Regulatory Policy Committee conducted a technical and scientific review of current practices relating to the fixation, trimming, and sectioning of the nonrodent eye to identify key points and species-specific anatomical landmarks to consider when preparing and evaluating eyes of rabbits, dogs, minipigs, and nonhuman primates from ocular and general toxicity studies. The topics addressed in this Points to Consider article include determination of situations when more comprehensive evaluation of the globe and/or associated extraocular tissues should be implemented (expanded ocular sampling), and what constitutes expanded ocular sampling. In addition, this manuscript highlights the practical aspects of fixing, trimming, and sectioning the eye to ensure adequate histopathological evaluation of all major ocular structures, including the cone-dense areas (visual streak/macula/fovea) of the retina for rabbits, dogs, minipigs, and nonhuman primates, which is a current regulatory expectation for ocular toxicity studies.[Box: see text].

Immune responses to injury and their links to eye disease

The eye is regarded as an immune privileged site. Since the presence of a vasculature would impair vision, the vasculature of the eye is located outside of the central light path. As a result, many regions of the eye evolved mechanisms to deliver immune cells to sites of dysgenesis, injury, or in response to the many age-related pathologies. While the purpose of these immune responses is reparative or protective, cytokines released by immune cells compromise visual acuity by inducing inflammation and fibrosis. The response to traumatic or pathological injury is distinct in different regions of the eye. Age-related diseases impact both the anterior and posterior segment and lead to reduced quality of life and blindness. Here we focus attention on the role that inflammation and fibrosis play in the progression of age-related pathologies of the cornea and the lens as well as in glaucoma, the formation of epiretinal membranes, and in proliferative vitreoretinopathy.

Co-Injection of Sulfotyrosine Facilitates Retinal Uptake of Hyaluronic Acid Nanospheres Following Intravitreal Injection

Gene and drug delivery to the retina is a critical therapeutic goal. While the majority of inherited forms of retinal degeneration affect the outer retina, specifically the photoreceptors and retinal pigment epithelium, effective targeted delivery to this region requires invasive subretinal delivery. Our goal in this work was to evaluate two innovative approaches for increasing both the persistence of delivered nanospheres and their penetration into the outer retina while using the much less invasive intravitreal delivery method. We formulated novel hyaluronic acid nanospheres (HA-NS, 250 nm and 500 nm in diameter) conjugated to fluorescent reporters and delivered them intravitreally to the adult Balb/C mouse retina. They exhibited persistence in the vitreous and along the inner limiting membrane (ILM) for up to 30 days (longest timepoint examined) but little retinal penetration. We thus evaluated the ability of the small molecule, sulfotyrosine, to disrupt the ILM, and found that 3.2 µg/µL sulfotyrosine led to significant improvement in delivery to the outer retina following intravitreal injections without causing retinal inflammation, degeneration, or loss of function. Co-delivery of sulfotyrosine and HA-NS led to robust improvements in penetration of HA-NS into the retina and accumulation along the interface between the photoreceptors and the retinal pigment epithelium. These exciting findings suggest that sulfotyrosine and HA-NS may be an effective strategy for outer retinal targeting after intravitreal injection.

Features of Retinal Neurogenesis as a Key Factor of Age-Related Neurodegeneration: Myth or Reality?

Age-related macular degeneration (AMD) is a complex multifactorial neurodegenerative disease that constitutes the most common cause of irreversible blindness in the elderly in the developed countries. Incomplete knowledge about its pathogenesis prevents the search for effective methods of prevention and treatment of AMD, primarily of its "dry" type which is by far the most common (90% of all AMD cases). In the recent years, AMD has become "younger": late stages of the disease are now detected in relatively young people. It is known that AMD pathogenesis-according to the age-related structural and functional changes in the retina-is linked with inflammation, hypoxia, oxidative stress, mitochondrial dysfunction, and an impairment of neurotrophic support, but the mechanisms that trigger the conversion of normal age-related changes to the pathological process as well as the reason for early AMD development remain unclear. In the adult mammalian retina, de novo neurogenesis is very limited. Therefore, the structural and functional features that arise during its maturation and formation can exert long-term effects on further ontogenesis of this tissue. The aim of this review was to discuss possible contributions of the changes/disturbances in retinal neurogenesis to the early development of AMD.

The internal limiting membrane: Roles in retinal development and implications for emerging ocular therapies

Basement membranes help to establish, maintain, and separate their associated tissues. They also provide growth and signaling substrates for nearby resident cells. The internal limiting membrane (ILM) is the basement membrane at the ocular vitreoretinal interface. While the ILM is essential for normal retinal development, it is dispensable in adulthood. Moreover, the ILM may constitute a significant barrier to emerging ocular therapeutics, such as viral gene therapy or stem cell transplantation. Here we take a neurodevelopmental perspective in examining how retinal neurons, glia, and vasculature interact with individual extracellular matrix constituents at the ILM. In addition, we review evidence that the ILM may impede novel ocular therapies and discuss approaches for achieving retinal parenchymal targeting of gene vectors and cell transplants delivered into the vitreous cavity by manipulating interactions with the ILM.

Looking into the future: Gene and cell therapies for glaucoma

Glaucoma is a complex group of optic neuropathies that affects both humans and animals. Intraocular pressure (IOP) elevation is a major risk factor that results in the loss of retinal ganglion cells (RGCs) and their axons. Currently, lowering IOP by medical and surgical methods is the only approved treatment for primary glaucoma, but there is no cure, and vision loss often progresses despite therapy. Recent technologic advances provide us with a better understanding of disease mechanisms and risk factors; this will permit earlier diagnosis of glaucoma and initiation of therapy sooner and more effectively. Gene and cell therapies are well suited to target these mechanisms specifically with the potential to achieve a lasting therapeutic effect. Much progress has been made in laboratory settings to develop these novel therapies for the eye. Gene and cell therapies have already been translated into clinical application for some inherited retinal dystrophies and age-related macular degeneration (AMD). Except for the intravitreal application of ciliary neurotrophic factor (CNTF) by encapsulated cell technology for RGC neuroprotection, there has been no other clinical translation of gene and cell therapies for glaucoma so far. Possible application of gene and cell therapies consists of long-term IOP control via increased aqueous humor drainage, including inhibition of fibrosis following filtration surgery, RGC neuroprotection and neuroregeneration, modification of ocular biomechanics for improved IOP tolerance, and inhibition of inflammation and neovascularization to prevent the development of some forms of secondary glaucoma.

Peptidomimetics Therapeutics for Retinal Disease

Ocular disorders originating in the retina can result in a partial or total loss of vision, making drug delivery to the retina of vital importance. However, effectively delivering drugs to the retina remains a challenge for ophthalmologists due to various anatomical and physicochemical barriers in the eye. This review introduces diverse administration routes and the accordant pharmacokinetic profiles of ocular drugs to aid in the development of safe and efficient drug delivery systems to the retina with a focus on peptidomimetics as a growing class of retinal drugs, which have great therapeutic potential and a high degree of specificity. We also discuss the pharmacokinetic profiles of small molecule drugs due to their structural similarity to small peptidomimetics. Lastly, various formulation strategies are suggested to overcome pharmacokinetic hurdles such as solubility, retention time, enzymatic degradation, tissue targeting, and membrane permeability. This knowledge can be used to help design ocular delivery platforms for peptidomimetics, not only for the treatment of various retinal diseases, but also for the selection of potential peptidomimetic drug targets.

Role of the Internal Limiting Membrane in Structural Engraftment and Topographic Spacing of Transplanted Human Stem Cell-Derived Retinal Ganglion Cells

Retinal ganglion cell (RGC) replacement holds potential for restoring vision lost to optic neuropathy. Transplanted RGCs must undergo neuroretinal integration to receive afferent visual signals for processing and efferent transmission. To date, retinal integration following RGC transplantation has been limited. We sought to overcome key barriers to transplanted human stem cell-derived RGC integration. Following co-culture ex vivo on organotypic mouse retinal explants, human RGCs cluster and extend bundled neurites that remain superficial to the neuroretina, hindering afferent synaptogenesis. To enhance integration, we increased the cellular permeability of the internal limiting membrane (ILM). Extracellular matrix digestion using proteolytic enzymes achieved ILM disruption while minimizing retinal toxicity and preserving glial reactivity. ILM disruption is associated with dispersion rather than clustering of co-cultured RGC bodies and neurites, and increased parenchymal neurite ingrowth. The ILM represents a significant obstacle to transplanted RGC connectivity and its circumvention may be necessary for functional RGC replacement.

Ocular barriers to retinal delivery of intravitreal liposomes: Impact of vitreoretinal interface

Drug delivery to the posterior segment of the eye is challenging due to several anatomical and physiological barriers. Thus, there is a need for prolonged action and targeted drug delivery to treat retinal diseases. Intravitreal injections avoid anterior eye barriers, but the vitreoretinal interface and inner limiting membrane (ILM) may prevent access of drug delivery systems to the retina. Existing data on retinal permeation of intravitreal nanoparticles are sparse and probably misleading due to the inter-species differences of retinal structures in rodents and humans. To bridge this gap, retinal permeation of light-activated liposomes was studied in an ex vivo bovine explant system that simulates the structure of vitreoretinal interface and intact ILM. Our findings indicate that the particle size plays a significant role in determining the retinal penetration as the liposomes of >100 nm sized failed to overcome the ILM and could not permeate into the retina. In addition, our results demonstrate the impact of surface charge and PEG-coating on retinal penetration. Small (≈ 50 nm) anionic liposomes with PEG coating showed the most extensive distribution and cellular localization in the retina. In summary, this study extends understanding of ocular barriers, and provides valuable information to augment design of retinal drug delivery systems.

Quantification of vascular and neuronal changes in the peripapillary retinal area secondary to diabetic retinopathy

PURPOSE

To investigate and quantify peripapillary vascular and neuronal changes secondary to diabetic retinopathy, using spectral-domain optical coherence tomography (OCT) and OCT angiography (OCTA).

DESIGN

This was a cross-sectional study.

METHODS

51 eyes of 51 patients affected by non-proliferative diabetic retinopathy (NPDR) and 19 age-matched healthy control eyes underwent full ophthalmic examination, including OCT and OCTA in the peripapillary area. Vessel area density (VAD), vessel length fraction (VLF) and vessel diameter index (VDI) were quantified in a ring-shaped region of interest of each OCTA image. Capillaries and larger vessels were separately analysed. The thickness of the peripapillary retinal nerve fibre layer (pRNFL) and macular ganglion cell complex (GCC) was also analysed.

RESULTS

VAD and VLF of peripapillary capillaries were significantly reduced in NPDR eyes, along with the progression of NPDR (p<0.05). VDI was significantly reduced in mild (p=0.0093) and moderate (p=0.0190) NPDR eyes, but not in severe NPDR (p=0.0841). Larger peripapillary vessels showed a significant increase of both VAD and VDI in NPDR eyes. pRNFL and GCC thickness decreased in NPDR eyes, reaching statistical significance only for GCC. No statistically significant correlation was found between perfusion parameters and pRNFL and GCC thickness.

CONCLUSIONS

Retinal capillary remodelling in NPDR involves the peripapillary vascularisation too, as confirmed by OCTA quantitative parameters. The peripapillary macrovasculature and microvasculature need to be separately evaluated. The lack of direct correlation between peripapillary capillaries changes and the loss of retinal nerve fibres suggests that neuronal damage cannot be simply considered secondary to the microvascular one.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Tuning surface functionalities of sub-10 nm-sized nanocarriers to target outer retina in designing drug delivery agents for intravitreal administration

Age-related macular degeneration (AMD) is one of the leading causes of irreversible blindness, generally affecting people over 50 years of age in industrialized countries. Despite the effectiveness of anti-vascular endothelial growth factor (VEGF) therapy in attenuating the growth of new blood vessels, substantial visual improvements are rare with this complex disease. Furthermore, the current regimen of repeated monthly intravitreal injections of drugs can result in serious side effects. Combination therapies-to complement anti-VEGF alone-with a prolonged therapeutic effect and efficient delivery to the intended site are urgently needed, which could be realized through the use of carefully designed nanocarriers. To understand the physicochemical effects (e.g., size, charge, geometry) of intravitreally administered nanocarriers on their bioavailability, distribution, and targeting efficiency across multiple layers of the retina, here we prepared seven different types of surface-functionalized water-soluble dendritic nanocarriers with hydrodynamic sizes mostly under 5 nm. A similar stoichiometric amount of fluorophore was covalently attached to each of these biocompatible nanocarriers for quantitative analyses by confocal microscopy of cryosectioned healthy mouse eyes. Interestingly, at 24 h post-injection, the nanocarrier with multiple copies of glucosamine on the surface (D_NSG) accumulated predominantly in the photoreceptor layer and the retinal pigment epithelium (RPE), which are speculated to be associated with AMD pathogenesis (i.e., target sites). Furthermore, extended residence at these outer retinal layers was demonstrated by D_NSG, which appeared to gradually turn into micron-scale particles potentially through aggregation. Our systematic findings may provide useful guidelines for the rational design of intravitreal nanocarriers to treat vision-threatening retinal diseases, including AMD.

PET study of ocular and blood pharmacokinetics of intravitreal bevacizumab and aflibercept in rats

Intravitreal injections are the standard procedure in the treatment of retinal pathologies, such as the administration of the anti-VEGF antibodies in age-related macular degeneration. The aim of this study is to evaluate the intraocular and blood pharmacokinetics after an intravitreal injection of ⁸⁹Zr-labelled bevacizumab and ⁸⁹Zr-labelled aflibercept in Sprague-Dawley rats using Positron Emission Tomography. First, both antibodies were radiolabelled to zirconium-89 with a maximum specific activity of 15 Mbq/mg for bevacizumab and 10 Mbq/mg for aflibercept. Four µL containing 1-1.2 Mq of ⁸⁹Zr-labelled compound were injected into the vitreous through a 35 G needle. A microPET acquisition was carried out immediately after the injection and at different time points through a 12-day study and blood samples were obtained through the tail vein. Radiolabelling was successfully performed with a radiochemical purity after ultrafiltration above 95% for both agents. Both antibodies ocular curves followed a two-compartment model in which an intraocular elimination half-life of 16.44 h was found for ⁸⁹Zr-bevacizumab and 4.51 h for ⁸⁹Zr-aflibercept, considering the alpha phase as the elimination phase. Regarding the beta phase, a half-life of 3.23 days for ⁸⁹Zr-bevacizumab and 4.69 days for ⁸⁹Zr-aflibercept were observed. With regards to blood concentration, ⁸⁹Zr-bevacizumab showed a blood half-life of 7.08 days, whereas ⁸⁹Zr-aflibercept's was 3.18 days, by a one-compartment model with first-order absorption kinetics. In conclusion, this study shows for the first time the ocular and blood pharmacokinetic analysis after intravitreal injection of aflibercept and bevacizumab in rats.