The diffusion dynamics of PEGylated liposomes in the intact vitreous of the ex vivo porcine eye: A fluorescence correlation spectroscopy and biodistribution study

Active Transport and Ocular Distribution of Intravitreally Injected Liposomes

Purpose

Drug delivery to the retina remains a challenge due to ocular barriers and fast clearing mechanisms. Nanocarrier drug delivery systems (NDDSs) hold the promise of prolonging intraocular retention times and increasing drug concentrations in the retina.

Methods

Anionic and cationic PEGylated liposomes, loaded with oxaliplatin (OxPt) to be used as trace element, were prepared from dry lipid powders. The differently charged liposomes were intravitreally injected in C57BL/6JrJ mice; eyes were harvested 2 hours and 24 hours post-injection. To investigate active transport mechanisms in the eye, a subset of mice were pre-injected with chloroquine before injection with cationic liposomes. Eyes were dissected and the distribution of OxPt in different tissues were quantified by inductively coupled plasma mass spectrometry (ICP-MS).

Results

Both liposome formulations enhanced the retention time of OxPt in the vitreous over free OxPt. Surprisingly, when formulated in cationic liposomes, OxPt translocated through the retina and accumulated in the RPE-sclera. Pre-injection with chloroquine inhibited the transport of liposomal OxPt from the vitreous to the RPE-sclera.

Conclusions

We show that liposomes can enhance the retention time of small molecular drugs in the vitreous and that active transport mechanisms are involved in the trans retinal transport of NDDS after intravitreal injections.

Translational Relevance

These results highlight the need for understanding the dynamics of ocular transport mechanisms in living eyes when designing NDDS with the back of the eye as the target. Active transport of nanocarriers through the retina will limit the drug concentration in the neuronal retina but might be exploited for targeting the RPE.

Pyruvate-conjugation of PEGylated liposomes for targeted drug delivery to retinal photoreceptors

Despite several promising candidates, there is a paucity of drug treatments available for patients suffering from retinal diseases. An important reason for this is the lack of suitable delivery systems that can achieve sufficiently high drug uptake in the retina and its photoreceptors. A promising and versatile method for drug delivery to specific cell types involves transporter-targeted liposomes, i.e., liposomes surface-coated with substrates for transporter proteins highly expressed on the target cell. We identified strong lactate transporter (monocarboxylate transporter, MCT) expression on photoreceptors as a potential target for drug delivery vehicles. To evaluate MCT suitability for drug targeting, we used PEG-coated liposomes and conjugated these with different monocarboxylates, including lactate, pyruvate, and cysteine. Monocarboxylate-conjugated and dye-loaded liposomes were tested on both human-derived cell-lines and murine retinal explant cultures. We found that liposomes conjugated with pyruvate consistently displayed higher cell uptake than unconjugated liposomes or liposomes conjugated with lactate or cysteine. Pharmacological inhibition of MCT1 and MCT2 reduced internalization, suggesting an MCT-dependent uptake mechanism. Notably, pyruvate-conjugated liposomes loaded with the drug candidate CN04 reduced photoreceptor cell death in the murine rd1 retinal degeneration model while free drug solutions could not achieve the same therapeutic effect. Our study thus highlights pyruvate-conjugated liposomes as a promising system for drug delivery to retinal photoreceptors, as well as other neuronal cell types displaying high expression of MCT-type proteins.

Ophthalmic nano-bioconjugates: critical challenges and technological advances

Ophthalmic disease can cause permanent loss of vision and blindness. Easy-to-administer topical and systemic treatments are preferred for treating sight-threatening disorders. Typical ocular anatomy makes topical and systemic ophthalmic drug delivery challenging. Various novel nano-drug delivery approaches are developed to attain the desired bioavailability in the eye by increasing residence time and improved permeability across the cornea. The review focuses on novel methods that are biocompatible, safe and highly therapeutic. Novelty in nanocarrier design and modification can overcome their drawbacks and make them potential drug carriers for eye disorders in both the anterior and posterior eye segments. This review briefly discussed technologies, patented developments, and clinical trial data to support nanocarriers' use in ocular drug delivery.

Direct Quantification of Serum Protein Interactions with PEGylated Micelle Nanocarriers

A large repertoire of nanocarrier (NC) technologies exists, each with highly specified advantages in terms of targetability, stability, and immunological inertness. The characterization of such NC properties within physiological conditions is essential for the development of optimized drug delivery systems. One method that is well established for reducing premature elimination by avoiding protein adsorption on NCs is surface functionalization with poly(ethylene glycol) (PEG), aptly called PEGylation. However, recent studies revealed that some PEGylated NCs have a delayed immune response, indicating the occurrence of protein-NC interactions. Obvious protein-NC interactions, especially in micellar systems, may have been overlooked as many early studies relied on techniques less sensitive to molecular level interactions. More sensitive techniques have been developed, but a major challenge is the direct measurement of interactions, which must be done in situ, as micelle assemblies are dynamic. Here, we report the use of pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS) to interrogate the interactions between two PEG-based micelle models and serum albumin protein to compare protein adsorption differences based on linear or cyclic PEG architectures. First, by measuring micelle diffusion in isolated and mixed solutions, we confirmed the thermal stability of diblock and triblock copolymer micelle assemblies. Further, we measured the co-diffusion of micelles and serum proteins, the magnitudes of which increased with concentration and continued incubation. The results demonstrate that PIE-FCCS is capable of measuring direct interactions between fluorescently labeled NC and serum proteins, even at concentrations 500 times lower than those observed physiologically. This capability showcases the potential utility of PIE-FCCS in the characterization of drug delivery systems in biomimetic conditions.

Ocular permeability, intraocular biodistribution of lipid nanocapsule formulation intended for retinal drug delivery

Recently, cGMP analogues have been investigated for the treatment of inherited retinal degenerations (IRD) using intravitreal injections. However, higher vitreous elimination rates limit the possibility to treat the retina with small molecule drugs. Here, we investigated the potential of lipid nanocapsules (LNCs) as vehicles to reduce clearance and prolong the delivery of cGMP analogue, CN03 to the retinal photoreceptors. Initially LNCs were investigated for both topical/periocular and intravitreal administration routes. While LNC-mediated drug permeation through the cornea proved to be too low for clinical applications, intravitreal application showed significant promise. Intravitreally administered LNCs containing fluorescent tracer in ex vivo porcine eyes showed complete intravitreal dispersal within 24 h. Ocular bio-distribution on histological sections showed that around 10 % of the LNCs had reached the retina, and 40 % accumulated in the ciliary body. For comparison, we used fluorescently labeled liposomes and these showed a different intraocular distribution with 48 % accumulated in the retina, and almost none were in the ciliary body. LNCs were then tested in retinal explants prepared from wild-type (WT) and rd1 mouse. In WT retina LNCs showed no significant toxic effects up to a concentration of 5 mg/mL. In rd1 retina, the LNC/CN03 formulation protected rd1 photoreceptors with similar efficacy to that of free CN03, demonstrating the usefulness of LNC/CN03 formulation in the treatment of IRD. Overall, our results indicate the suitability of LNCs for intraocular administration and drug delivery to both the retina and the ciliary body.

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.

Optimization of an Injectable Hydrogel Depot System for the Controlled Release of Retinal-Targeted Hybrid Nanoparticles

A drawback in the development of treatments that can reach the retina is the presence of barriers in the eye that restrain compounds from reaching the target. Intravitreal injections hold promise for retinal delivery, but the natural defenses in the vitreous can rapidly degrade or eliminate therapeutic molecules. Injectable hydrogel implants, which act as a reservoir, can allow for long-term drug delivery with a single injection into the eye, but still suffer due to the fast clearance of the released drugs when traversing the vitreous and random diffusion that leads to lower pharmaceutic efficacy. A combination with HA-covered nanoparticles, which can be released from the gel and more readily pass through the vitreous to increase the delivery of therapeutic agents to the retina, represents an advanced and elegant way to overcome some of the limitations in eye drug delivery. In this article, we developed hybrid PLGA-Dotap NPs that, due to their hyaluronic acid coating, can improve in vivo distribution throughout the vitreous and delivery to retinal cells. Moreover, a hydrogel implant was developed to act as a depot for the hybrid NPs to better control and slow their release. These results are a first step to improve the treatment of retinal diseases by protecting and transporting the therapeutic treatment across the vitreous and to improve treatment options by creating a depot system for long-term treatments.

In vitro Model Systems for Studies Into Retinal Neuroprotection

Therapy development for neurodegenerative diseases of the retina constitutes a major unmet medical need, and this may be particularly relevant for inherited diseases of the retina, which are largely untreatable to this day. Therapy development necessitates appropriate models to improve the understanding of the underlying degenerative mechanisms, as well as for the testing and evaluation of novel treatment approaches. This review provides an overview of various in vitro model systems used to study retinal neuroprotection. The in vitro methods and technologies discussed range from primary retinal cell cultures and cell lines, to retinal organoids and organotypic retinal explants, to the cultivation of whole eyeballs. The advantages and disadvantages of these methods are compared and evaluated, also in view of the 3R principles (i.e., the refinement, reduction, and replacement of live animal testing), to identify suitable in vitro alternatives for in vivo experimentation. The article further expands on the use of in vitro models to test and evaluate neuroprotective treatments and to aid the development of retinal drug delivery systems. Among the pharmacological agents tested and characterized in vitro are such that interfere with aberrant cyclic guanosine monophosphate (cGMP) -signaling or such that inhibit the activities of poly (ADP-ribose) polymerase (PARP), histone deacetylases (HDAC), calpain-type proteases, as well as unfolded protein response-related stress. We then introduce nanoparticle-based drug delivery systems and discuss how different in vitro systems may be used to assess their efficacy in the treatment of retinal diseases. The summary provides a brief comparison of available in vitro models and relates their advantages and limitations to the various experimental requirements, for instance, for studies into disease mechanisms, novel treatments, or retinal toxicity. In many cases, combinations of different in vitro models may be required to obtain a comprehensive view of the efficacy of a given retinal neuroprotection approach.

Lipid Nanoparticles as a Promising Drug Delivery Carrier for Topical Ocular Therapy-An Overview on Recent Advances

Due to complicated anatomical and physical properties, targeted drug delivery to ocular tissues continues to be a key challenge for formulation scientists. Various attempts are currently being made to improve the in vivo performance of therapeutic molecules by encapsulating them in various nanocarrier systems or devices and administering them via invasive/non-invasive or minimally invasive drug administration methods. Biocompatible and biodegradable lipid nanoparticles have emerged as a potential alternative to conventional ocular drug delivery systems to overcome various ocular barriers. Lipid-based nanocarrier systems led to major technological advancements and therapeutic advantages during the last few decades of ocular therapy, such as high precorneal residence time, sustained drug release profile, minimum dosing frequency, decreased drug toxicity, targeted site delivery, and, therefore, an improvement in ocular bioavailability. In addition, such formulations can be given as fine dispersion in patient-friendly droppable preparation without causing blurred vision and ocular sensitivity reactions. The unique advantages of lipid nanoparticles, namely, solid lipid nanoparticles, nanostructured lipid carriers, nanoemulsions, and liposomes in intraocular targeted administration of various therapeutic drugs are extensively discussed. Ongoing and completed clinical trials of various liposome-based formulations and various characterization techniques designed for nanoemulsion in ocular delivery are tabulated. This review also describes diverse solid lipid nanoparticle preparation methods, procedures, advantages, and limitations. Functionalization approaches to overcome the drawbacks of lipid nanoparticles, as well as the exploration of new functional additives with the potential to improve the penetration of macromolecular pharmaceuticals, would quickly progress the challenging field of ocular drug delivery systems.

Quantifying the heterogeneity of enzymatic dePEGyaltion of liposomal nanocarrier systems

Lipid nanoparticles have been clinically successful in particular recently within the vaccine field, but better tools are needed to analyze heterogeneities at the single particle level to progress drug delivery designs to the next level. Especially, liposomal nanocarriers are becoming increasingly complex e.g. by employing environmental cues for shedding their protective PEG layer, however a detailed mechanistic understanding of how the dePEGylation varies from liposome-to-liposome is still missing. Here we present the development of a fluorescence microscopy based assay capable of detecting the enzyme mediated dePEGylation of individual liposomes. We employ this methodology to understand how enzyme type-, concentration- and incubation time, in addition to liposome size, affects the dePEGylation at the single particle level.

Application of an organotypic ocular perfusion model to assess intravitreal drug distribution in human and animal eyes

Intravitreal (ITV) drug delivery is a new cornerstone for retinal therapeutics. Yet, predicting the disposition of formulations in the human eye remains a major translational hurdle. A prominent, but poorly understood, issue in pre-clinical ITV toxicity studies is unintended particle movements to the anterior chamber (AC). These particles can accumulate in the AC to dangerously raise intraocular pressure. Yet, anatomical differences, and the inability to obtain equivalent human data, make investigating this issue extremely challenging. We have developed an organotypic perfusion strategy to re-establish intraocular fluid flow, while maintaining homeostatic pressure and pH. Here, we used this approach with suitably sized microbeads to profile anterior and posterior ITV particle movements in live versus perfused porcine eyes, and in human donor eyes. Small-molecule suspensions were then tested with the system after exhibiting differing behaviours in vivo. Aggregate particle size is supported as an important determinant of particle movements in the human eye, and we note these data are consistent with a poroelastic model of bidirectional vitreous transport. Together, this approach uses ocular fluid dynamics to permit, to our knowledge, the first direct comparisons between particle behaviours from human ITV injections and animal models, with potential to speed pre-clinical development of retinal therapeutics.

Investigating Ex Vivo Animal Models to Test the Performance of Intravitreal Liposomal Drug Delivery Systems

There is a strong need for innovative and efficient drug delivery systems for ocular therapy development. However, testing intravitreal drug delivery systems without using live animals is challenging. Ex vivo animal models offer an interesting alternative. We analyzed the potential of using fresh porcine eyes obtained from the local slaughterhouse as a model for testing the intravitreal biodistribution and retention of liposomes with or without polyethylene glycol (PEG) conjugation and with different surface charges. The histology of the eyes was analyzed to localize the liposomes, and it was found that liposomes with PEG absorbed rapidly on the retina (within 1 h), with positively charged and PEG-coated liposomes being retained for at least 24 h. In parallel, fluorophotometry was employed on intact eyes, to determine the pharmacokinetics of the fluorophore calcein, as a substitute for a small hydrophilic therapeutic compound. We found a 4.5-fold increase in the vitreous half-life of calcein loaded in liposomes, compared with the free solution. Retinal toxicity was addressed using murine-derived retinal explant cultures. Liposomes were non-toxic up to 500 µg/mL. Toxicity was observed at 5 mg/mL for anionic and cationic liposomes, with 2-fold and 2.5-fold increased photoreceptor cell death, respectively. Overall, we could show that important ocular drug delivery considerations such as pharmacokinetics and biodistribution can be estimated in ex vivo porcine eyes, and may guide subsequent in vivo experiments.

siRNA Therapeutics in Ocular Diseases

The field of RNAi therapeutics has quickly adapted to the treatment of ocular diseases. Although the eye provides a unique system for the delivery of siRNAs, its complex structure and composition fostered the development of novel strategies for efficient gene silencing in the target compartment. Moreover, anterior and posterior segments differ in their multiple drug barriers and clearance mechanisms. This chapter summarizes the recent achievements in terms of routes of administration, chemical modifications, and delivery systems for siRNAs that specifically apply to eye disorders. Methods employed for siRNA detection/quantitation in ocular tissues are also described, together with safety concerns that need to be addressed to fulfill regulatory requirements of new drug approval. Even though RNAi therapies for ocular diseases have not yet translated into patient care, we document herein the rising number of candidate drugs currently under preclinical or clinical development.

Advanced Static and Dynamic Fluorescence Microscopy Techniques to Investigate Drug Delivery Systems

In the past decade(s), fluorescence microscopy and laser scanning confocal microscopy (LSCM) have been widely employed to investigate biological and biomimetic systems for pharmaceutical applications, to determine the localization of drugs in tissues or entire organisms or the extent of their cellular uptake (in vitro). However, the diffraction limit of light, which limits the resolution to hundreds of nanometers, has for long time restricted the extent and quality of information and insight achievable through these techniques. The advent of super-resolution microscopic techniques, recognized with the 2014 Nobel prize in Chemistry, revolutionized the field thanks to the possibility to achieve nanometric resolution, i.e., the typical scale length of chemical and biological phenomena. Since then, fluorescence microscopy-related techniques have acquired renewed interest for the scientific community, both from the perspective of instrument/techniques development and from the perspective of the advanced scientific applications. In this contribution we will review the application of these techniques to the field of drug delivery, discussing how the latest advancements of static and dynamic methodologies have tremendously expanded the experimental opportunities for the characterization of drug delivery systems and for the understanding of their behaviour in biologically relevant environments.

Degradation of Drug Delivery Nanocarriers and Payload Release: A Review of Physical Methods for Tracing Nanocarrier Biological Fate

Nanoformulations offer multiple advantages over conventional drug delivery, enhancing solubility, biocompatibility, and bioavailability of drugs. Nanocarriers can be engineered with targeting ligands for reaching specific tissue or cells, thus reducing the side effects of payloads. Following systemic delivery, nanocarriers must deliver encapsulated drugs, usually through nanocarrier degradation. A premature degradation, or the loss of the nanocarrier coating, may prevent the drug's delivery to the targeted tissue. Despite their importance, stability and degradation of nanocarriers in biological environments are largely not studied in the literature. Here we review techniques for tracing the fate of nanocarriers, focusing on nanocarrier degradation and drug release both intracellularly and in vivo. Intracellularly, we will discuss different fluorescence techniques: confocal laser scanning microscopy, fluorescence correlation spectroscopy, lifetime imaging, flow cytometry, etc. We also consider confocal Raman microscopy as a label-free technique to trace colocalization of nanocarriers and drugs. In vivo we will consider fluorescence and nuclear imaging for tracing nanocarriers. Positron emission tomography and single-photon emission computed tomography are used for a quantitative assessment of nanocarrier and payload biodistribution. Strategies for dual radiolabelling of the nanocarriers and the payload for tracing carrier degradation, as well as the efficacy of the payload delivery in vivo, are also discussed.

The role of cGMP-signalling and calcium-signalling in photoreceptor cell death: perspectives for therapy development

The second messengers, cGMP and Ca²⁺, have both been implicated in retinal degeneration; however, it is still unclear which of the two is most relevant for photoreceptor cell death. This problem is exacerbated by the close connections and crosstalk between cGMP-signalling and calcium (Ca²⁺)-signalling in photoreceptors. In this review, we summarize key aspects of cGMP-signalling and Ca²⁺-signalling relevant for hereditary photoreceptor degeneration. The topics covered include cGMP-signalling targets, the role of Ca²⁺ permeable channels, relation to energy metabolism, calpain-type proteases, and how the related metabolic processes may trigger and execute photoreceptor cell death. A focus is then put on cGMP-dependent mechanisms and how exceedingly high photoreceptor cGMP levels set in motion cascades of Ca²⁺-dependent and independent processes that eventually bring about photoreceptor cell death. Finally, an outlook is given into mutation-independent therapeutic approaches that exploit specific features of cGMP-signalling. Such approaches might be combined with suitable drug delivery systems for translation into clinical applications.

Titanium dioxide nanoparticles impair the inner blood-retinal barrier and retinal electrophysiology through rapid ADAM17 activation and claudin-5 degradation

Background

Depending on their distinct properties, titanium dioxide nanoparticles (TiO₂-NPs) are manufactured extensively and widely present in our daily necessities, with growing environmental release and public concerns. In sunscreen formulations, supplementation of TiO₂-NPs may reach up to 25% (w/w). Ocular contact with TiO₂-NPs may occur accidentally in certain cases, allowing undesirable risks to human vision. This study aimed to understand the barrier integrity of retinal endothelial cells in response to TiO₂-NP exposure. bEnd.3 cells and human retinal endothelial cells (HRECs) were exposed to TiO₂-NP, followed by examination of their tight junction components and functions.

Results

TiO2-NP treatment apparently induced a broken structure of the junctional plaques, conferring decreased transendothelial electrical resistance, a permeable paracellular cleft, and improved cell migration in vitro. This might involve rapid activation of metalloproteinase, a disintegrin and metalloproteinase 17 (ADAM17), and ADAM17-mediated claudin-5 degradation. For the in vivo study, C57BL/6 mice were administered a single dose of TiO2-NP intravitreally and then subjected to a complete ophthalmology examination. Fluorescein leakage and reduced blood flow at the optical disc indicated a damaged inner blood-retinal barrier induced by TiO₂-NPs. Inappreciable change in the thickness of retinal sublayers and alleviated electroretinography amplitude were observed in the TiO₂-NP-treated eyes.

Conclusions

Overall, our data demonstrate that TiO2-NP can damage endothelial cell function, thereby affecting retinal electrophysiology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-020-00395-7.

Enhanced Delivery of siRNA to Retinal Ganglion Cells by Intravitreal Lipid Nanoparticles of Positive Charge

RNAi therapy has been developed and explored for treating retinal conditions since last decades. The progression of retinal diseases including the age-related macular degeneration and glaucoma is associated with the malfunction of specific retinal cells. Therefore, to deliver therapeutic RNAi to selective retinal tissues with desired gene downregulation is crucial for the treatment of retinal diseases via RNAi therapy. Lipid-based nanoparticles are potent delivery vectors for RNAi therapeutics to achieve high gene silencing efficiency. The surface charge has been demonstrated to affect the intraocular behaviors and retinal distribution of intravitreally administered lipid nanoparticles (LNPs), which could subsequently affect the gene knockdown efficiency in specific retinal layers. Here, we evaluated three charged LNPs for their ability to deliver siRNA and facilitate gene downregulation both in vitro and in vivo. LNPs with different surface charges ranging from neutral to positive (5-34 mV) were successfully formulated. All types of charged LNPs managed gene knockdown in both mammalian cell line and primary neurons. At 48 h post intravitreal injection, neutral LNPs (6.2 mV) and mildly positive LNPs (15.9 mV) mediated limited retinal gene suppression (<10%) and the more positive LNPs (31.2 mV) led to ∼25% gene suppression in the retinal ganglion cell (RGC) layer. No gene silencing in the retinal pigmented epithelium layer was facilitated by any LNPs independent of the charges. In summary, this study has shown that positive LNPs with an optimized charge managed specific gene downregulation in the RGC layer. These RNAi carriers hold potential for the treatment of RGC-associated retinal diseases.

Preparation and in vivo evaluation of nanoliposomes containing vancomycin after intravitreal injection in albino rabbits

Objective(s):

The in vivo efficacy of nanoliposomal formulation of vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) assessed.

Materials and Methods:

Nanoliposomal formulations were prepared and characterized. The in vivo study was carried out on rabbits which received liquid culture medium containing MRSA under anesthesia. After 48 hr, the eyes treated with the liposomal and free form of vancomycin. The rabbits were euthanized at predesignate intervals at 12, 24, 48, 96, 144 hr intervals injection. The antibacterial activity of different vancomycin formulations was assayed by the time killing method.

Results:

The zeta potential, mean sizes and encapsulation efficacy of liposomal vancomycin were 29.7 mV, 381.93±30.13 nm and 47%, respectively. The results of time–killing studies indicated that the liposomal formula was more effective than the free form of vancomycin.

Conclusion:

The results of this study revealed that liposomal vancomycin formulation is a powerful nano-antibacterial agent to combat infectious endophthalmitis.

Triamcinolone Acetonide-Loaded PEGylated Microemulsion for the Posterior Segment of Eye

Present work investigates the possibility of a polyethyleneglycolylated (PEGylated) microemulsion (ME) to deliver drug to the posterior segment of eye. Triamcinolone acetonide (TA), a widely used drug in intraocular diseases, was selected as the model drug. Based on solubility and emulsification capacity, components of microemulsion were selected and optimum formulation was obtained using a pseudoternary phase diagram. The optimized ratio of Capmul MCM C8 (oil): AccononMC8-2 (surfactant): Transcutol (cosurfactant): deionized water was 5:35.5:4.5:55. This was further PEGylated using 1,2-distearoylphosphatylethanolamine-polyethyleneglycol 2000 (DSPE-PEG 2000). This PEGylated ME loaded with TA was characterized and evaluated in vitro, ex vivo, and in vivo for topical ocular use. The developed PEGylated ME loaded with TA was homogenous, stable, and nonirritable to eye and had the ability to reach the posterior segment of eye on topical instillation.

PEGylated microemulsion for dexamethasone delivery to posterior segment of eye

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

No flow through the vitreous humor: How strong is the evidence?

When analyzing vitreal drug delivery, or the pharmacological effects of drugs on intraocular pressure, or when interpreting outflow facility measurements, it is generally accepted that the fluid in the vitreous humor is stagnant. It is accepted that for all practical purposes, the aqueous fluid exits the eye via anterior pathways only, and so there is negligible if any posteriorly directed flow of aqueous through the vitreous humor. This assumption is largely based on the interpretation of experimental data from key sources including Maurice (1957), Moseley (1984), Gaul and Brubaker (1986), Maurice (1987) and Araie et al. (1991). However, there is strong independent evidence suggesting there is a substantial fluid flow across the retinal pigment epithelium from key sources including Cantrill and Pederson (1984), Chihara and Nao-i, Tsuboi (1985), Dahrouj et al. (2014), Smith and Gardiner (2017) and Smith et al. (2019). The conflicting evidence creates a conundrum-how can both interpretations be true? This leads us to re-evaluate the evidence. We demonstrate that the data believed to be supporting no aqueous flow through the vitreous are in fact compatible with a significant normal aqueous flow. We identify strong and independent lines of evidence supporting fluid flow across the RPE, including our new outflow model for the eye. On balance it appears the current evidence favors the view that there is normally a significant aqueous flow across the RPE in vivo. This finding suggests that past and future analyses of outflow facility, interpretations of some drug distributions and the interpretation of some drug effects on eye tissues, may need to be revised.

Liposomal delivery of CRISPR/Cas9

Liposomes are one of the most widely investigated carriers for CRISPR/Cas9 delivery. The surface properties of liposomal carriers, including the surface charge, PEGylation, and ligand modification can significantly affect the gene silencing efficiency. Three barriers of systemic CRISPR/Cas9 delivery (long blood circulation, efficient tumor penetration, and efficient cellular uptake/endosomal escape) are analyzed on liposomal carriers with different surface charges, PEGylations, and ligand modifications. Cationic formulations dominate CRISPR/Cas9 delivery and neutral formulations also have good performance while anionic formulations are generally not proper for CRISPR/Cas9 delivery. The PEG dilemma (prolonged blood circulation vs. reduced cellular uptake/endosomal escape) and the side effect of repeated PEGylated formulation (accelerated blood clearance) were discussed. Effects of ligand modification on cationic and neutral formulations were analyzed. Finally, we summarized the achievements in liposomal CRISPR/Cas9 delivery, outlined existing problems, and provided some future perspectives. Liposomes are one of the most widely investigated carriers for CRISPR/Cas9 delivery. The surface properties of liposomal carriers, including the surface charge, PEGylation, and ligand modification can significantly affect the gene silencing efficiency. Three barriers of systemic siRNA delivery (long blood circulation, efficient tumor penetration, and efficient cellular uptake/endosomal escape) are analyzed on liposomal carriers with different surface charges, PEGylations, and ligand modifications. Cationic formulations dominate CRISPR/Cas9 delivery and neutral formulations also have good performance while anionic formulations are generally not proper for CRISPR/Cas9 delivery. The PEG dilemma (prolonged blood circulation vs. reduced cellular uptake/endosomal escape) and the side effect of repeated PEGylated formulation (accelerated blood clearance) were discussed. Effects of ligand modification on cationic and neutral formulations were analyzed. Finally, we summarized the achievements in liposomal CRISPR/Cas9 delivery, outlined existing problems, and provided some future perspectives.

Human vitreous in proliferative diabetic retinopathy: Characterization and translational implications

Diabetic retinopathy (DR) is one of the leading causes of visual impairment in the working-age population. DR is a progressive eye disease caused by long-term accumulation of hyperglycaemia-mediated pathological alterations in the retina of diabetic patients. DR begins with asymptomatic retinal abnormalities and may progress to advanced-stage proliferative diabetic retinopathy (PDR), characterized by neovascularization or preretinal/vitreous haemorrhages. The vitreous, a transparent gel that fills the posterior cavity of the eye, plays a vital role in maintaining ocular function. Structural and molecular alterations of the vitreous, observed during DR progression, are consequences of metabolic and functional modifications of the retinal tissue. Thus, vitreal alterations reflect the pathological events occurring at the vitreoretinal interface. These events are caused by hypoxic, oxidative, inflammatory, neurodegenerative, and leukostatic conditions that occur during diabetes. Conversely, PDR vitreous can exert pathological effects on the diabetic retina, resulting in activation of a vicious cycle that contributes to disease progression. In this review, we recapitulate the major pathological features of DR/PDR, and focus on the structural and molecular changes that characterize the vitreal structure and composition during DR and progression to PDR. In PDR, vitreous represents a reservoir of pathological signalling molecules. Therefore, in this review we discuss how studying the biological activity of the vitreous in different in vitro, ex vivo, and in vivo experimental models can provide insights into the pathogenesis of PDR. In addition, the vitreous from PDR patients can represent a novel tool to obtain preclinical experimental evidences for the development and characterization of new therapeutic drug candidates for PDR therapy.

Multifarious Biologic Loaded Liposomes that Stimulate the Mammalian Target of Rapamycin Signaling Pathway Show Retina Neuroprotection after Retina Damage

A common event in optic neuropathies is the loss of axons and death of retinal ganglion cells (RGCs) resulting in irreversible blindness. Mammalian target of rapamycin (mTOR) signaling pathway agonists have been shown to foster axon regeneration and RGC survival in animal models of optic nerve damage. However, many challenges remain in developing therapies that exploit cell growth and tissue remodeling including (i) activating/inhibiting cell pathways synergistically, (ii) avoiding tumorigenesis, and (iii) ensuring appropriate physiological tissue function. These challenges are further exacerbated by the need to overcome ocular physiological barriers and clearance mechanisms. Here we present liposomes loaded with multiple mTOR pathway stimulating biologics designed to enhance neuroprotection after retina damage. Liposomes were loaded with ciliary neurotrophic factor, insulin-like growth factor 1, a lipopeptide N-fragment osteopontin mimic, and lipopeptide phosphatase tension homologue inhibitors for either the ATP domain or the c-terminal tail. In a mouse model of N-methyl-d-aspartic acid induced RGC death, a single intravitreal administration of liposomes reduced both RGC death and loss of retina electrophysiological function. Furthermore, combining liposomes with transplantation of induced pluripotent stem cell derived RGCs led to an improved electrophysiological outcome in mice. The results presented here show that liposomes carrying multiple signaling pathway modulators can facilitate neuroprotection and transplant electrophysiological outcome.

An in Vivo Mouse Model to Investigate the Effect of Local Anesthetic Nanomedicines on Axonal Conduction and Excitability

Peripheral nerve blocks (PNBs) using local anesthetic (LA) are superior to systemic analgesia for management of post-operative pain. An insufficiently short PNB duration following single-shot LA can be optimized by development of extended release formulations among which liposomes have been shown to be the least toxic. In vivo rodent models for PNB have focused primarily on assessing behavioral responses following LA. In a previous study in human volunteers, we found that it is feasible to monitor the effect of LA in vivo by combining conventional conduction studies with nerve excitability studies. Here, we aimed to develop a mouse model where the same neurophysiological techniques can be used to investigate liposomal formulations of LA in vivo. To challenge the validity of the model, we tested the motor PNB following an unilamellar liposomal formulation, filled with the intermediate-duration LA lidocaine. Experiments were carried out in adult transgenic mice with fluorescent axons and with fluorescent tagged liposomes to allow in vivo imaging by probe-based confocal laser endomicroscopy. Recovery of conduction following LA injection at the ankle was monitored by stimulation of the tibial nerve fibers at the sciatic notch and recording of the plantar compound motor action potential (CMAP). We detected a delayed recovery in CMAP amplitude following liposomal lidocaine, without detrimental systemic effects. Furthermore, CMAP threshold-tracking studies of the distal tibial nerve showed that the increased rheobase was associated with a sequence of excitability changes similar to those found following non-encapsulated lidocaine PNB in humans, further supporting the translational value of the model.

Nanoliposome-Encapsulated Brinzolamide-hydropropyl-β-cyclodextrin Inclusion Complex: A Potential Therapeutic Ocular Drug-Delivery System

Novel ocular drug delivery systems (NODDSs) remain to be explored to overcome the anatomical and physiological barriers of the eyes. This study was to encapsulate brinzolamide (BRZ)-hydropropyl-β-cyclodextrin (HP-β-CD) inclusion complex (HP-β-CD/BRZ) into nanoliposomes and investigate its potential as one of NODDS to improve BRZ local glaucomatous therapeutic effect. HP-β-CD/BRZ was firstly prepared to enhance the solubility of poorly water-soluble BRZ. The HP-β-CD/BRZ loaded nanoliposomes (BCL) were subsequently constructed by thin-film dispersion method. After the optimization of the ratio of BRZ to HP-β-CD, the optimal BCL showed an average size of 82.29 ± 6.20 nm, ζ potential of -3.57 ± 0.46 mV and entrapment efficiency (EE) of 92.50 ± 2.10% with nearly spherical in shape. The X-ray diffraction (XRD) confirmed the formation of HP-β-CD/BRZ and BCL. The in vitro release study of BCL was evaluated using the dialysis technique, and BCL showed moderate sustained release. BCL (1 mg/mL BRZ) showed a 9.36-fold increase in the apparent permeability coefficient and had a sustained and enhanced intraocular pressure reduction efficacy when compared with the commercially available formulation (BRZ-Sus) (10 mg/mL BRZ). In conclusion, BCL might have a promising future as a NODDS for glaucoma treatment.