Drug and gene delivery to the back of the eye: from bench to bedside

Safety Assessment of Formulation Vehicles Following Intravitreal Administration in Rabbits

PURPOSE

Evaluate 21 formulation vehicles administered to rabbits after intravitreal injection for tolerability and safety.

METHODS

Forty-two Dutch Belted rabbits were anesthetized, and the eyes received a single intravitreal injection of the excipient formulation. Clinical signs and ocular irritation responses were recorded twice daily for 7 days and microscopic evaluation of the eyes, optic nerve, and eyelids was completed at 1-week post treatment.

RESULTS

Saline (≥ 300 mOsm and ≤ 592 mOsm at pH 7.0 or 300 mOsm at pH 8.0) and 10 formulation excipients; (10% w/v PEG 3350 at pH 7.4, 1% polysorbate 21 at pH 7.4, PVA at pH 7.0, 0.2% polysorbate 80 at pH 7.2, 0.2% Pluronic F108® at pH 7.3, 2%, 100 mM sodium sulfate at pH 3.2, 2 mM sodium glycocholate at pH 7.4, and 275 mM D-mannitol pH 7.0 in sterile water, and 100 mM sodium phosphate in combination with 0.9% NaCl 300 mOsm and 0.01% or 0.05% polysorbate 80 at pH 7.4) considered as formulation vehicles for intravitreal injectables, were well-tolerated in rabbits. Clinical signs were transient and microscopic changes were not observed.

CONCLUSIONS

Of the 21 formulation vehicles evaluated, 10 formulation vehicles were well-tolerated in rabbits and feasible candidates for future investigations.

In vitro and in vivo assessment of delivery of hydrophobic molecules and plasmid DNAs with PEO–PPO–PEO polymeric micelles on cornea

The stability and bio-distribution of genes or drug complexes with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO–PPO–PEO, Pluronic F-68) polymeric micelles (PM) are essential for an effective nanosized PM delivery system. We used Förster resonance energy transfer (FRET) pairs with PM and measured the FRET ratio to assess the stability of PM in vitro and in vivo on the cornea. The FRET ratio reached a plateau at 0.8 with 3% PM. Differential scanning calorimetry measurement confirmed the complex formation of FRET pairs with PM. Confocal imaging with the fluorophores fluorescein isothiocyanate isomer I (FITC) and rhodamine B base (RhB) also showed the occurrence of FRET pairs in vitro. The fluorophores were mixed with 3% PM solution or the FITC-labeled PEO–PPO–PEO polymers (FITC-P) were mixed with RhB-labeled plasmids (RhB–DNA). In addition, the in vitro corneal permeation of FRET pair complexes with PM reached a 0.8 FRET ratio. One hour after eye drop administration, FRET pairs colocalized in the cytoplasm, and surrounded and entered the nuclei of cells in the cornea, and the polymers were located in the corneal epithelial layers, as detected through anti-PEG immunohistochemistry. Furthermore, fluorescence colocalization in the cytoplasm and cell nucleus of the corneal epithelium was confirmed in tissues where RhB or RhB–DNA complexed with FITC-P was found to accumulate. We demonstrate that at a concentration of 3%, PM can encapsulate FRET pairs or RhB–DNA and retain their integrity within the cornea 1 h after administration, suggesting the feasibility and stability of PEO–PPO–PEO polymers as a vehicle for drug delivery.

Ocular drug delivery targeted by iontophoresis in the suprachoroidal space using a microneedle

Treatment of many posterior-segment ocular indications would benefit from improved targeting of drug delivery to the back of the eye. Here, we propose the use of iontophoresis to direct delivery of negatively charged nanoparticles through the suprachoroidal space (SCS) toward the posterior pole of the eye. Injection of nanoparticles into the SCS of the rabbit eye ex vivo without iontophoresis led to a nanoparticle distribution mostly localized at the site of injection near the limbus and <15% of nanoparticles delivered to the most posterior region of SCS (>9 mm from the limbus). Iontophoresis using a novel microneedle-based device increased posterior targeting with >30% of nanoparticles in the most posterior region of SCS. Posterior targeting increased with increasing iontophoresis current and increasing application time up to 3 min, but further increasing to 5 min was not better, probably due to the observed collapse of the SCS within 5 min after injection ex vivo. Reversing the direction of iontophoretic flow inhibited posterior targeting, with just ~5% of nanoparticles reaching the most posterior region of SCS. In the rabbit eye in vivo, iontophoresis at 0.14 mA for 3 min after injection of a 100 μL suspension of nanoparticles resulted in ~30% of nanoparticles delivered to the most posterior region of the SCS, which was consistent with ex vivo findings. The procedure was well tolerated, with only mild, transient tissue effects at the site of injection. We conclude that iontophoresis in the SCS using a microneedle has promise as a method to target ocular drug delivery within the eye, especially toward the posterior pole.

Ion-sensitive in situ hydrogels of natamycin bilosomes for enhanced and prolonged ocular pharmacotherapy: in vitro permeability, cytotoxicity and in vivo evaluation

Delivery of therapeutic molecules into the deeper ocular compartments is mainly hampered by short precorneal residence and limited transmembrane permeability of topically administered drugs. Hence, the current study was undertaken to fabricate the ion-sensitive in situ gels of natamycin (NT) bilosomes (NB) for efficient ocular delivery. The effect of cholesterol and sodium taurocholate proportion on the properties of the bilosomes were studied and the formulation with better physicochemical properties was optimized and utilized to derive the in situ gelling system (IG). The impact of type/composition of gelling agent on the formation and characteristics of the hydrogel was investigated. The hydrogel formed from IG with 0.3% w/v gellan gum showed optimal viscoelastic and adhesive characteristics. The ocular safety and cytocompatibility of NB and its IG was confirmed by corneal histology and in vitro cytotoxicity evaluation. A 6- to 9-fold enhancement in the transcorneal flux of NB demonstrated efficient ocular penetration of bilosomes. Moreover, the superior mean dose normalized NT levels in the ocular tissues of rabbits treated with optimized NB and IG illustrated the effectiveness of bilosomes loaded ion-sensitive in situ hydrogels as a potential platform for the improved and prolonged ocular pharmacotherapy.

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

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

The suprachoroidal space as a route of administration to the posterior segment of the eye

The suprachoroidal space (SCS) is a potential space between the sclera and choroid that traverses the circumference of the posterior segment of the eye. The SCS is an attractive site for drug delivery because it targets the choroid, retinal pigment epithelium, and retina with high bioavailability, while maintaining low levels elsewhere in the eye. Indeed, phase III clinical trials are investigating the safety and efficacy of SCS drug delivery. Here, we review the anatomy and physiology of the SCS; methods to access the SCS; kinetics of SCS drug delivery; strategies to target within the SCS; current and potential clinical indications; and the safety and efficacy of this approach in preclinical animal studies and clinical trials.

Microspheres as intraocular therapeutic tools in chronic diseases of the optic nerve and retina

Pathologies affecting the optic nerve and the retina are one of the major causes of blindness. These diseases include age-related macular degeneration (AMD), diabetic retinopathy (DR) and glaucoma, among others. Also, there are genetic disorders that affect the retina causing visual impairment. The prevalence of neurodegenerative diseases of the posterior segment is increased as most of them are related with the elderly. Even with the access to different treatments, there are some challenges in managing patients suffering retinal diseases. One of them is the need for frequent interventions. Also, an unpredictable response to therapy has suggested that different pathways may be playing a role in the development of these diseases. The management of these pathologies requires the development of controlled drug delivery systems able to slow the progression of the disease without the need of frequent invasive interventions, typically related with endophthalmitis, retinal detachment, ocular hypertension, cataract, inflammation, and floaters, among other. Biodegradable microspheres are able to encapsulate low molecular weight substances and large molecules such as biotechnological products. Over the last years, a large variety of active substances has been encapsulated in microspheres with the intention of providing neuroprotection of the optic nerve and the retina. The purpose of the present review is to describe the use of microspheres in chronic neurodegenerative diseases affecting the retina and the optic nerve. The advantage of microencapsulation of low molecular weight drugs as well as therapeutic peptides and proteins to be used as neuroprotective strategy is discussed. Also, a new use of the microspheres in the development of animal models of neurodegeneration of the posterior segment is described.

Principles of pharmacology in the eye

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

Discerning the composition of penetratin for safe penetration from cornea to retina

Delivery of biomacromolecules into the eye is greatly hindered by several protective barriers. The cell-penetrating peptide, penetratin, has been found to be an effective absorption enhancer for noninvasive intraocular gene delivery. To discern the composition of penetratin for safe penetration from cornea to retina, we designed a series of penetratin derivatives by varying the hydrophobicity and evaluated their potency for retina-targeted delivery. The hydrophilic amino acids of penetratin, excluding the conserved basic amino acid residues, were respectively replaced with tryptophan. Secondary structure of the resultant derivatives was analyzed by computer simulation and circular dichroism, exhibiting that the hydrophobic derivatives had a propensity to form high content of helix and entered corneal and conjunctival cells more easily than did penetratin. As expected, the hydrophobic derivatives showed improved permeability in excised rabbit cornea and sclera, and kept intact after penetration. When instilled topically in the conjunctival sac of mice eyes, the hydrophobic derivatives distributed safely and rapidly into both cornea and retina, with increased amount and prolonged retention time in comparison to penetratin. In conclusion, we demonstrated that the ocular permeability of penetratin derivatives closely correlated with their hydrophobicity, and introducing hydrophobic amino acids in penetratin was a feasible approach to develop more powerful ocular absorption enhancers.

STATEMENT OF SIGNIFICANCE

Due to the defensive barriers of the eye, efficient and safe absorption enhancers are indispensable for noninvasive delivery of exogenous biomacromolecules to the posterior segment. In this manuscript, we designed a series of penetratin derivatives and validated they had significantly improved penetration ability from cornea to retina than wild-type penetratin, without increasing toxicity. More importantly, we provided a sequence of solid evidences that the ocular permeability of penetratin derivatives closely correlated with their hydrophobicity, and introducing hydrophobic amino acids in penetratin was a feasible approach to develop more powerful ocular absorption enhancers. We also demonstrated that the penetratin derivatives permeated through cornea and sclera with intact structure, and might enter the eye by non-corneal pathway.

Topical Ocular Delivery of TGF-β1 to the Back of the Eye: Implications in Age-Related Neurodegenerative Diseases

Dysregulation of the transforming growth factor-β1 (TGF-β1)/selected small mother against decapentaplegic (SMAD) pathway can be implicated in development of age-related macular degeneration (AMD), and the delivery of TGF-β1 could be beneficial for AMD. We developed a new ophthalmic formulation of TGF-β1 assessing the ocular pharmacokinetic profile of TGF-β1 in the rabbit eye. Small unilamellar vesicles (SUV) loaded with TGF-β1 were complemented with Annexin V and Ca²⁺, and the vitreous bioavailability of TGF-β1 was assessed after topical ocular administration by a commercial ELISA kit. We detected high levels of TGF-β1 (C_max 114.7 ± 12.40 pg/mL) in the vitreous after 60 min (T_max) from the topical application of the liposomal suspension. Ocular tolerability was also assessed by a modified Draize’s test. The new formulation was well tolerated. In conclusion, we demonstrated that the novel formulation was able to deliver remarkable levels of TGF-β1 into the back of the eye after topical administration. Indeed, this TGF-β1 delivery system may be useful in clinical practice to manage ophthalmic conditions such as age-related macular degeneration, skipping invasive intraocular injections.

Infliximab exerts a dose-dependent effect on retinal safety in the albino rabbit

PURPOSE

To assess the retinal toxicity of an intravitreal injection of infliximab, a monoclonal antibody to tumor necrosis factor α, in a rabbit model.

MATERIALS AND METHODS

Two groups of adult albino rabbits (n = 5) received intravitreal injections of infliximab (0.1 ml) in the study eye and balanced salt solution (BSS, 0.1 ml) in the control eye at baseline. Group 1 was administered with 1.5 mg/0.1 ml, and group 2 was injected with 7.5 mg/0.1 ml of infliximab solution. Electroretinography (ERG) was performed at baseline and at 1, 7, 30, and 45 days after the injection. Visual evoked potentials (VEPs) were recorded at 7 and 45 days after the injection. After the last electrophysiological assessment, the rabbits were euthanized and retinal histopathology and immunhistochemistry for glial fibrillary acidic protein (GFAP) were performed.

RESULTS

ERG responses demonstrated no significant deficit in retinal function in eyes injected with infliximab. Mean dark-adapted a-wave and b-wave maximal amplitude and semi-saturation constant values at baseline and throughout the 45 days of follow-up after the injection indicated no remarkable difference in outer retinal function between the control and experimental eyes. VEP responses were similar at each time point (7 and 45 days). No difference was seen in retinal histopathology and immunocytochemistry sections in eyes receiving the 1.5 mg/0.1 ml dose compared to the control eyes. However, increased GFAP labeling in retinal Müller cells was detected in rabbit eyes treated with the 7.5 mg/0.1 ml dose.

CONCLUSIONS

Intravitreal injection of 1.5 mg/0.1 ml infliximab dose has no toxic effect on the integrity (functional or structural) of the retina in rabbits. A higher dose of 7.5 mg/0.1 ml may be slightly toxic as suggested by positive Müller cell GFAP expression. Additional studies of retinal toxicity at higher doses and after multiple injections are needed to establish the retinal safety of intravitreal infliximab therapy in humans.

Advancements in Understanding Immunogenicity of Biotherapeutics in the Intraocular Space

Therapeutic breakthroughs in a number of retinal degenerative diseases have come about through the development of biotherapeutics administered directly into the eye. As a consequence of their use, we have gained more insight into the immune privileged status of the eye and the various considerations that development, manufacturing, and use of these drugs require. It has been observed that therapeutic proteins injected into the vitreous can elicit an immune response resulting in the production of anti-drug antibodies (ADAs) which can have clinical consequences. This review includes discussion of the anatomy, physiology, and specific area of the eye that are targeted for drug administration. The various immunologic mechanisms involved in the immune responses to intraocularly administered protein are discussed. This review entails discussion on chemistry, manufacturing, and control (CMC) and formulation-related issues that may influence the risk of immunogenicity. Based on the available immunogenicity profile of the marketed intraocular drugs and their reported adverse events, the animal models and the translational gap from animals to human are discussed. Thus, the objective of this review article is to assess the factors that influence immunogenicity in relation to intraocular administration and the steps taken for mitigating immunogenicity risks.

Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration

Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and metabolism. Recent studies have implicated TH signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we demonstrated that antithyroid drug treatment and targeting iodothyronine deiodinases (DIOs) to suppress cellular tri-iodothyronine (T3) production or increase T3 degradation preserves cones. In this work, we investigated the effectiveness of inhibition of the TH receptor (TR). Two genes, THRA and THRB, encode TRs; THRB2 has been associated with cone viability. Using TR antagonists and Thrb2 deletion, we examined the effects of TR inhibition. Systemic and ocular treatment with the TR antagonists NH-3 and 1-850 increased cone density by 30-40% in the Rpe65-/- mouse model of Leber congenital amaurosis and reduced the number of TUNEL+ cells. Cone survival was significantly improved in Rpe65-/- and Cpfl1 (a model of achromatopsia with Pde6c defect) mice with Thrb2 deletion. Ventral cone density in Cpfl1/Thrb2-/- and Rpe65-/- /Thrb2-/- mice was increased by 1- to 4-fold, compared with age-matched controls. Moreover, the expression levels of TR were significantly higher in the cone-degeneration retinas, suggesting locally elevated TR signaling. This work shows that the effects of antithyroid treatment or targeting DIOs were likely mediated by TRs and that suppressing TR protects cones. Our findings support the view that inhibition of TR locally in the retina is a therapeutic strategy for retinal degeneration management.-Ma, H., Yang, F., Butler, M. R., Belcher, J., Redmond, T. M., Placzek, A. T., Scanlan, T. S., Ding, X.-Q. Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration.

Ocular disposition of ciprofloxacin from topical, PEGylated nanostructured lipid carriers: Effect of molecular weight and density of poly (ethylene) glycol

Ciprofloxacin (CIP) is an antibacterial agent prescribed for the treatment of ocular infections. The objective of the present project is to investigate the effect of surface PEG functionalization of the Nano structured lipid carriers (NLCs) on formulation stability, ocular penetration and distribution. CIP NLCs were tested with different molecular weight (poly ethylene glycol) PEGs ranging from (2K to 20K) grafted onto the phospholipid and with different chain lengths (14-18 carbons) of phospholipids derivatized with PEG-2K. Drug load in the formulations was maintained at 0.3%w/v. Formulations prepared were evaluated with respect to in vitro release, transcorneal permeation, autoclavability, morphological characteristics and in vivo ocular tissue distribution. Scanning Transmission electron microscopy (STEM) studies revealed that the PEG-CIP-NLCs were spherical in shape. Transcorneal penetration of CIP was optimum with PEG molecular weight in between 2K-10K. Carbon chain length of the phospholipid, however, did not affect transcorneal penetration of CIP. In vivo ocular tissue CIP concentrations attained from the various formulations was consistent with the in vitro data obtained. The results suggest that surface functionalization of PEGs, within a specified range of molecular weight and surface packing density, significantly enhance trans-ocular penetration and impart sterilization-stabilization characteristics into the formulations.

Photopharmacological control of bipolar cells restores visual function in blind mice

Photopharmacological control of neuronal activity using synthetic photochromic ligands, or photoswitches, is a promising approach for restoring visual function in patients suffering from degenerative retinal diseases. Azobenzene photoswitches, such as AAQ and DENAQ, have been shown to restore the responses of retinal ganglion cells to light in mouse models of retinal degeneration but do not recapitulate native retinal signal processing. Here, we describe diethylamino-azo-diethylamino (DAD), a third-generation photoswitch that is capable of restoring retinal ganglion cell light responses to blue or white light. In acute brain slices of murine layer 2/3 cortical neurons, we determined that the photoswitch quickly relaxes to its inactive form in the dark. DAD is not permanently charged, and the uncharged form enables the photoswitch to rapidly and effectively cross biological barriers and thereby access and photosensitize retinal neurons. Intravitreal injection of DAD restored retinal light responses and light-driven behavior to blind mice. Unlike DENAQ, DAD acts upstream of retinal ganglion cells, primarily conferring light sensitivity to bipolar cells. Moreover, DAD was capable of generating ON and OFF visual responses in the blind retina by utilizing intrinsic retinal circuitry, which may be advantageous for restoring visual function.

Topical Formulation of Self-Assembled Antiviral Prodrug Nanomicelles for Targeted Retinal Delivery

Topical drug administration for back of the eye delivery is extremely challenging due to the presence of protection mechanisms and physiological barriers. Self-assembled polymeric nanomicelles have emerged as promising vehicles for drug delivery. Apart from serving as an inert nanocarrier for therapeutic agents, polymeric nanomicelles are known to bypass mononuclear phagocytic system (MPS) and efflux transporters thereby improving drug bioavailability. In this investigation, a highly efficacious biotinylated lipid prodrug of cyclic cidofovir (B-C12-cCDF) was formulated within polymeric nanomicelles as a carrier for targeted retinal delivery. Polymeric nanomicelles were prepared from polyoxyethylene hydrogenated castor oil 40 (HCO-40) and octoxynol 40 (OC-40). In vitro release studies revealed that B-C12-cCDF-loaded nanomicelles released B-C12-cCDF at a faster rate in stimulated tear fluid (STF) in comparison to PBST. MTT and LDH assays demonstrated negligible cytotoxicity of B-C12-cCDF-loaded nanomicelles relative to CDF and B-C12-cCDF in HRPE (human retinal pigment epithelial, D407), HCE-T (human corneal epithelial), and CCL 20.2 (human conjunctival epithelial) cells. Confocal laser scanning microscopy and flow cytometry analyses indicated that B-C12-cCDF-loaded nanomicelles were efficiently internalized into D407 and HCE-T cells in contrast to CDF and B-C12-cCDF. Moreover, little B-C12-cCDF was also observed in the nuclei after 24 h of incubation. Polymeric nanomicelles carrying the transporter targeted prodrug did not produce any cytotoxic effects and were internalized into the cells effectively. Permeability experiments across HCE-T cells further confirmed significant transport of prodrug loaded nanomicelles and their subsequent uptake into D407 cells. These findings indicate that HCO-40/OC-40 based polymeric nanomicelles could become a promising topical delivery system for ocular administration of antiviral agents.

A minimally invasive adjustable-depth blunt injector for delivery of pharmaceuticals into the posterior pole

PURPOSE

To investigate the feasibility and safety of a novel minimally invasive adjustable-depth blunt injector for pharmaceuticals delivery into the posterior segment.

METHODS

Indocyanine green (ICG), sodium fluorescein and iron oxide nanoparticles (IONPs) were injected using the new injector into the extravascular spaces of the choroid (EVSC) compartment of rabbits and cadaver pig eyes. Spectral domain optical coherence tomography (SD-OCT), fundus imaging and histology analysis were performed for assessment of injection safety and efficacy.

RESULTS

Indocyanine green, fluorescein and IONPs were detected across the EVSC in rabbit eyes, covering over 80 per cent of the posterior eye surface. Injected IONPs were retained in the EVSC for at least 2 weeks following injection. No retinal detachment, choroidal haemorrhage or inflammation was detected in any of the injected eyes. In cadaver pig eyes, ICG was detected across the EVSC.

CONCLUSIONS

This novel minimally invasive delivery system may be used to safely deliver large volumes of pharmaceuticals into a new treatment reservoir compartment - the EVSC which can serve as a depot, in close proximity to the retina, covering most of the surface of the back of the eye without insertion of surgical instruments under the central retina. This system is predicted to enhance the therapeutic effect of treatments for posterior eye disorders.

Recent perspectives on the delivery of biologics to back of the eye

INTRODUCTION

Biologics are generally macromolecules, large in size with poor stability in biological environments. Delivery of biologics to tissues at the back of the eye remains a challenge. To overcome these challenges and treat posterior ocular diseases, several novel approaches have been developed. Nanotechnology-based delivery systems, like drug encapsulation technology, macromolecule implants and gene delivery are under investigation. We provide an overview of emerging technologies for biologics delivery to back of the eye tissues. Moreover, new biologic drugs currently in clinical trials for ocular neovascular diseases have been discussed. Areas covered: Anatomy of the eye, posterior segment disease and diagnosis, barriers to biologic delivery, ocular pharmacokinetic, novel biologic delivery system Expert opinion: Anti-VEGF therapy represents a significant advance in developing biologics for the treatment of ocular neovascular diseases. Various strategies for biologic delivery to posterior ocular tissues are under development with some in early or late stages of clinical trials. Despite significant progress in the delivery of biologics, there is unmet need to develop sustained delivery of biologics with nearly zero-order release kinetics to the back of the eye tissues. In addition, elevated intraocular pressure associated with frequent intravitreal injections of macromolecules is another concern that needs to be addressed.

Non-viral strategies for ocular gene delivery

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

Thrombospondin-derived peptide attenuates Sjögren's syndrome-associated ocular surface inflammation in mice

Sjögren's syndrome is the second most common rheumatic disease in which autoimmune response targets exocrine glands (salivary and lacrimal glands) result in clinical symptoms of dry mouth and dry eye. Inflammation of the lacrimal gland induces tear abnormalities that contribute to the inflammation of the ocular surface, which includes ocular mucosa. Thrombospondin-1 (TSP-1) plays a critical regulatory role in the ocular mucosa and as such TSP-1^-/- mice develop spontaneously chronic ocular surface inflammation associated with Sjögren's syndrome. The autoimmune pathology is also accompanied by a peripheral imbalance in regulatory (T_reg ) and inflammatory Th17 effectors. In this study, we demonstrate an in-vitro effect of a CD47-binding TSP-derived peptide in the induction of transforming growth factor (TGF)-β1-secreting forkhead box protein 2 (Foxp3⁺ ) T_regs from activated CD4⁺ CD25^- T cells and the inhibition of pathogenic T helper type 17 (Th17)-promoting interleukin (IL)-23 derived from antigen-presenting cells. The in-vivo administration of this peptide promotes Foxp3⁺ T_reg induction and inhibition of Th17 development. Consistent with these results, topical administration of CD47-binding TSP peptide, both before and after the onset of the disease, attenuates clinical symptoms of SS-associated dry eye in TSP-1^-/- mice. Augmented expression of Foxp3 detected in the draining lymph nodes of TSP peptide -treated mice compared to those treated with control peptide suggests the ability of TSP peptide to restore peripheral immune imbalance. Thus, our results suggest that TSP-derived peptide attenuates Sjögren's syndrome-associated dry eye and autoimmune inflammation by preventing Th17 development while promoting the induction of T_regs . Collectively, our data identify TSP-derived peptide as a novel therapeutic option to treat autoimmune diseases.

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

Visual tracing of diffusion and biodistribution for amphiphilic cationic nanoparticles using photoacoustic imaging after ex vivo intravitreal injections

To visually trace the diffusion and biodistribution of amphiphilic cation micelles after vitreous injection, various triblock copolymers of monomethoxy poly(ethylene glycol)-poly(ε-caprolactone)-polyethylenimine were synthesized with different structures of hydrophilic and hydrophobic segments, followed by labeling with near-infrared fluorescent dye Cyanine5 or Cyanine7. The micellar size, polydispersity index, and surface charge were measured by dynamic light scattering. The diffusion was monitored using photoacoustic imaging in real time after intravitreal injections. Moreover, the labeled nanoparticle distribution in the posterior segment of the eye was imaged histologically by confocal microscopy. The results showed that the hydrophilic segment increased vitreous diffusion, while a positive charge on the particle surface hindered diffusion. In addition, the particles diffused through the retinal layers and were enriched in the retinal pigment epithelial layer. This work tried to study the diffusion rate via a simple method by using visible images, and then provided basic data for the development of intraocular drug carriers.

Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration

Recent studies have implicated thyroid hormone (TH) signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we found that antithyroid treatment preserves cones. This work investigates the significance of targeting intracellular TH components locally in the retina. The cellular TH level is mainly regulated by deiodinase iodothyronine (DIO)-2 and -3. DIO2 converts thyroxine (T4) to triiodothyronine (T3), which binds to the TH receptor, whereas DIO3 degrades T3 and T4. We examined cone survival after overexpression of DIO3 and inhibition of DIO2 and demonstrated the benefits of these manipulations. Subretinal delivery of AAV5-IRBP/GNAT2-DIO3, which directs expression of human DIO3 specifically in cones, increased cone density by 30-40% in a Rpe65^-/- mouse model of Lebers congenital amaurosis (LCA) and in a Cpfl1 mouse with Pde6c defect model of achromatopsia, compared with their respective untreated controls. Intravitreal and topical delivery of the DIO2 inhibitor iopanoic acid also significantly improved cone survival in the LCA model mice. Moreover, the expression levels of DIO2 and Slc16a2 were significantly higher in the diseased retinas, suggesting locally elevated TH signaling. We show that targeting DIOs protects cones, and intracellular inhibition of TH components locally in the retina may represent a novel strategy for retinal degeneration management.-Yang, F., Ma, H., Belcher, J., Butler, M. R., Redmond, T. M., Boye, S. L., Hauswirth, W. W., Ding, X.-Q. Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration.

Nanomedicine in the application of uveal melanoma

Rapid advances in nanomedicine have significantly changed many aspects of nanoparticle application to the eye including areas of diagnosis, imaging and more importantly drug delivery. The nanoparticle-based drug delivery systems has provided a solution to various drug solubility-related problems in ophthalmology treatment. Nanostructured compounds could be used to achieve local ocular delivery with minimal unwanted systematic side effects produced by taking advantage of the phagocyte system. In addition, the in vivo control release by nanomaterials encapsulated drugs provides prolong exposure of the compound in the body. Furthermore, certain nanoparticles can overcome important body barriers including the blood-retinal barrier as well as the corneal-retinal barrier of the eye for effective delivery of the drug. In summary, the nanotechnology based drug delivery system may serve as an important tool for uveal melanoma treatment.

In vitro and in vivo rescue of aberrant splicing in CEP290-associated LCA by antisense oligonucleotide delivery

Leber congenital amaurosis (LCA) is a severe disorder resulting in visual impairment usually starting in the first year of life. The most frequent genetic cause of LCA is an intronic mutation in CEP290 (c.2991 + 1655A > G) that creates a cryptic splice donor site resulting in the insertion of a pseudoexon (exon X) into CEP290 mRNA. Previously, we showed that naked antisense oligonucleotides (AONs) effectively restored normal CEP290 splicing in patient-derived lymphoblastoid cells. We here explore the therapeutic potential of naked and adeno-associated virus (AAV)-packaged AONs in vitro and in vivo In both cases, AON delivery fully restored CEP290 pre-mRNA splicing, significantly increased CEP290 protein levels and rescued a ciliary phenotype present in patient-derived fibroblast cells. Moreover, administration of naked and AAV-packaged AONs to the retina of a humanized mutant Cep290 mouse model, carrying the intronic mutation, showed a statistically significant reduction of exon X-containing Cep290 transcripts, without compromising the retinal structure. Together, our data highlight the tremendous therapeutic prospective of AONs for the treatment of not only CEP290-associated LCA but potentially many other subtypes of retinal dystrophy caused by splicing mutations.

Repurposing an orally available drug for the treatment of geographic atrophy

PURPOSE

Chronic oxidative stress and subacute inflammation have been implicated as causes of age-related macular degeneration (AMD). In this study, we tested whether an orally available 5-OH-tryptamine (5HT) 1a receptor agonist, xaliproden, could protect against retinal pigment epithelium (RPE) cell damage in culture and in a mouse model of geographic atrophy.

METHODS

Paraquat was used to create mitochondrial oxidative stress in ARPE-19 cells, and tumor necrosis factor-α (TNF-α) was used to stimulate the production of inflammatory cytokines in these cells. The production of antioxidant proteins, metallothionein, and inflammatory cytokines was assayed with quantitative real-time PCR. Cell survival was analyzed with microscopy and a cell titer assay. Integrity of the RPE monolayer was determined by measuring the transepithelial electrical resistance (TEER) and with immunocytochemistry with zona occludens protein 1 (ZO-1) antibody. RPE atrophy was studied in mice deleted for Sod2 (the gene for mitochondrial superoxide dismutase) specifically in the RPE. The mice were treated orally with daily doses of xaliproden at 0.5 and 3 mg/kg for 4 months. The retinal structure was analyzed with spectral domain optical coherence tomography (SD-OCT) and with light and electron microscopy. Retinal function was assessed with full-field electroretinography (ERG) and with optokinetic measurements.

RESULTS

Xaliproden led to a dose-dependent increase in cell survival following treatment with paraquat. Synthesis of the antioxidant response genes NqO1, GSTM1, CAT, HO-1, and Nrf2 was increased in response to the drug, as was the zinc chaperone metallothionein. Treatment of cells with TNF-α led to increased production of IL-1β, IL-6, chemokine (C-C motif) ligand 20 (CCL20), and vascular endothelial growth factor (VEGF) by ARPE-19 cells, and this response was attenuated by treatment with xaliproden. TNF-α also led to a decrease in the TEER that was prevented by treatment with the 5HT1a agonist. Daily gavage with xaliproden at either dose induced the production of protective enzymes in the mouse retina, and treatment of the Sod2-deleted mice with the drug showed improved thickness of the outer nuclear layer and improved visual acuity relative to the control-treated mice. There was no significant difference in full-field scotopic ERG among the treatment groups, however. Vacuolization of the RPE and disorganization of the photoreceptor outer segments were reduced at both dose levels of xaliproden.

CONCLUSIONS

Xaliproden protected RPE cells from oxidative and inflammatory insults and protected the mouse RPE and retina from RPE atrophy in the face of excess mitochondrial oxidative stress. These results suggest that this drug, which had a reasonable safety profile in clinical trials, may be used to prevent the progression of geographic atrophy in humans.

Sustained reduction of intraocular pressure by supraciliary delivery of brimonidine-loaded poly(lactic acid) microspheres for the treatment of glaucoma

Although effective drugs that lower intraocular pressure (IOP) in the management of glaucoma exist, their efficacy is limited by poor patient adherence to the prescribed eye drop regimen. To replace the need for eye drops, in this study we tested the hypothesis that IOP can be reduced for one month after a single targeted injection using a microneedle for administration of a glaucoma medication (i.e., brimonidine) formulated for sustained release in the supraciliary space of the eye adjacent to the drug's site of action at the ciliary body. To test this hypothesis, brimonidine-loaded microspheres were formulated using poly(lactic acid) (PLA) to release brimonidine at a constant rate for 35 days and microneedles were designed to penetrate through the sclera, without penetrating into the choroid/retina, in order to target injection into the supraciliary space. A single administration of these microspheres using a hollow microneedle was performed in the eye of New Zealand White rabbits and was found to reduce IOP initially by 6 mmHg and then by progressively smaller amounts for more than one month. All administrations were well tolerated without significant adverse events, although histological examination showed a foreign-body reaction to the microspheres. This study demonstrates, for the first time, that the highly-targeted delivery of brimonidine-loaded microspheres into the supraciliary space using a microneedle is able to reduce IOP for one month as an alternative to daily eye drops.

Photokinetic Drug Delivery: Light-Enhanced Permeation in an In Vitro Eye Model

PURPOSE

To investigate light-enhanced molecular movement as a potential technology for drug delivery. To do this, we developed an in vitro eye model while representing similar concentration gradient conditions and compositions found in the eye.

METHODS

The eye model unit was fabricated by inserting a cross-linked type I collagen membrane in a spectrophotometer cuvette with 1% hyaluronic acid as the drug recipient medium. Photokinetic delivery was studied by illuminating 1 mg/mL methotrexate (MTX) placed in the drug donor compartment on top of the membrane, with noncoherent 450 nm light at 8.2 mW from an LED source pulsed at 25 cycles per second, placed in contact with the solution. A modified UV-visual spectrophotometer was employed to rapidly determine the concentration of MTX, at progressive 1 mm distances away from the membrane, within the viscous recipient medium of the model eye after 1 h.

RESULTS

A defined, progressive concentration gradient was observed within the nonagitated drug recipient media, diminishing with greater distances from the membrane. Transport of MTX through the membrane was significantly enhanced (ranging from 2 to 3 times, P < 0.05 to P ≤ 0.001) by photokinetic methods compared with control conditions by determining drug concentrations at 4 defined distances from the membrane. According to scanning electron microscopy images, no structural damage or shunts were created on the surface of the cross-linked gelatin membrane.

CONCLUSION

The application of pulsed noncoherent visible light significantly enhances the permeation of MTX through a cross-linked collagen membrane and hyaluronic acid recipient medium without causing structural damage to the membrane.

Systemic treatment with a 5HT1a agonist induces anti-oxidant protection and preserves the retina from mitochondrial oxidative stress

Chronic oxidative stress contributes to age related diseases including age related macular degeneration (AMD). Earlier work showed that the 5-hydroxy-tryptamine 1a (5HT1a) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) protects retinal pigment epithelium (RPE) cells from hydrogen peroxide treatment and mouse retinas from oxidative insults including light injury. In our current experiments, RPE derived cells subjected to mitochondrial oxidative stress were protected from cell death by the up-regulation of anti-oxidant enzymes and of the metal ion chaperone metallothionein. Differentiated RPE cells were resistant to oxidative stress, and the expression of genes for protective proteins was highly increased by oxidative stress plus drug treatment. In mice treated with 8-OH-DPAT, the same genes (MT1, HO1, NqO1, Cat, Sod1) were induced in the neural retina, but the drug did not affect the expression of Sod2, the gene for manganese superoxide dismutase. We used a mouse strain deleted for Sod2 in the RPE to accelerate age-related oxidative stress in the retina and to test the impact of 8-OH-DPAT on the photoreceptor and RPE degeneration developed in these mice. Treatment of mice with daily injections of the drug led to increased electroretinogram (ERG) amplitudes in dark-adapted mice and to a slight improvement in visual acuity. Most strikingly, in mice treated with a high dose of the drug (5 mg/kg) the structure of the RPE and Bruch's membrane and the normal architecture of photoreceptor outer segments were preserved. These results suggest that systemic treatment with this class of drugs may be useful in preventing geographic atrophy, the advanced form of dry AMD, which is characterized by RPE degeneration.

Lipid Nanoparticles for Ocular Gene Delivery

Lipids contain hydrocarbons and are the building blocks of cells. Lipids can naturally form themselves into nano-films and nano-structures, micelles, reverse micelles, and liposomes. Micelles or reverse micelles are monolayer structures, whereas liposomes are bilayer structures. Liposomes have been recognized as carriers for drug delivery. Solid lipid nanoparticles and lipoplex (liposome-polycation-DNA complex), also called lipid nanoparticles, are currently used to deliver drugs and genes to ocular tissues. A solid lipid nanoparticle (SLN) is typically spherical, and possesses a solid lipid core matrix that can solubilize lipophilic molecules. The lipid nanoparticle, called the liposome protamine/DNA lipoplex (LPD), is electrostatically assembled from cationic liposomes and an anionic protamine-DNA complex. The LPD nanoparticles contain a highly condensed DNA core surrounded by lipid bilayers. SLNs are extensively used to deliver drugs to the cornea. LPD nanoparticles are used to target the retina. Age-related macular degeneration, retinitis pigmentosa, and diabetic retinopathy are the most common retinal diseases in humans. There have also been promising results achieved recently with LPD nanoparticles to deliver functional genes and micro RNA to treat retinal diseases. Here, we review recent advances in ocular drug and gene delivery employing lipid nanoparticles.

Rabbit as an animal model for intravitreal pharmacokinetics: Clinical predictability and quality of the published data

Intravitreal administration is the method of choice in drug delivery to the retina and/or choroid. Rabbit is the most commonly used animal species in intravitreal pharmacokinetics, but it has been criticized as being a poor model of human eye. The critique is based on some anatomical differences, properties of the vitreous humor, and observed differences in drug concentrations in the anterior chamber after intravitreal injections. We have systematically analyzed all published information on intravitreal pharmacokinetics in the rabbit and human eye. The analysis revealed major problems in the design of the pharmacokinetic studies. In this review we provide advice for study design. Overall, the pharmacokinetic parameters (clearance, volume of distribution, half-life) in the human and rabbit eye have good correlation and comparable absolute values. Therefore, reliable rabbit-to-man translation of intravitreal pharmacokinetics should be feasible. The relevant anatomical and physiological parameters in rabbit and man show only small differences. Furthermore, the claimed discrepancy between drug concentrations in the human and rabbit aqueous humor is not supported by the data analysis. Based on the available and properly conducted pharmacokinetic studies, the differences in the vitreous structure in rabbits and human patients do not lead to significant pharmacokinetic differences. This review is the first step towards inter-species translation of intravitreal pharmacokinetics. More information is still needed to dissect the roles of drug delivery systems, disease states, age and ocular manipulation on the intravitreal pharmacokinetics in rabbit and man. Anyway, the published data and the derived pharmacokinetic parameters indicate that the rabbit is a useful animal model in intravitreal pharmacokinetics.

Vision from next generation sequencing: multi-dimensional genome-wide analysis for producing gene regulatory networks underlying retinal development, aging and disease

Genomics and genetics have invaded all aspects of biology and medicine, opening uncharted territory for scientific exploration. The definition of "gene" itself has become ambiguous, and the central dogma is continuously being revised and expanded. Computational biology and computational medicine are no longer intellectual domains of the chosen few. Next generation sequencing (NGS) technology, together with novel methods of pattern recognition and network analyses, has revolutionized the way we think about fundamental biological mechanisms and cellular pathways. In this review, we discuss NGS-based genome-wide approaches that can provide deeper insights into retinal development, aging and disease pathogenesis. We first focus on gene regulatory networks (GRNs) that govern the differentiation of retinal photoreceptors and modulate adaptive response during aging. Then, we discuss NGS technology in the context of retinal disease and develop a vision for therapies based on network biology. We should emphasize that basic strategies for network construction and analyses can be transported to any tissue or cell type. We believe that specific and uniform guidelines are required for generation of genome, transcriptome and epigenome data to facilitate comparative analysis and integration of multi-dimensional data sets, and for constructing networks underlying complex biological processes. As cellular homeostasis and organismal survival are dependent on gene-gene and gene-environment interactions, we believe that network-based biology will provide the foundation for deciphering disease mechanisms and discovering novel drug targets for retinal neurodegenerative diseases.

Recent advances in topical delivery of proteins and peptides mediated by soft matter nanocarriers

Proteins and peptides are increasingly important therapeutics for the treatment of severe and complex diseases like cancer or autoimmune diseases due to their high specificity and potency. Their unique structure and labile physicochemical properties, however, require special attention in the production and formulation process as well as during administration. Aside from conventional systemic injections, the topical application of proteins and peptides is an appealing alternative due to its non-invasive nature and thus high acceptance by patients. For this approach, soft matter nanocarriers are interesting delivery systems which offer beneficial properties such as high biocompatibility, easiness of modifications, as well as targeted drug delivery and release. This review aims to highlight and discuss technological developments in the field of soft matter nanocarriers for the delivery of proteins and peptides via the skin, the eye, the nose, and the lung, and to provide insights in advantages, limitations, and practicability of recent advances.

Nanoparticle-assisted targeted delivery of eye-specific genes to eyes significantly improves the vision of blind mice in vivo

Application of viruses as a carrier, though not safe, to deliver genes to eye tissue was successful. However, a safer, nonviral, biocompatible lipid-based nanoparticle has never been tested to treat blinding eye diseases. We created an artificial virus using a nanoparticle, liposome-protamine-DNA complex (LPD), modified with a cell permeable peptide and a nuclear localization signaling (NLS) peptide, to deliver a functional gene for eye disease treatment. In the eye, a photochemical, 11-cis-retinal, allows the visual pigment rhodopsin to absorb light in the visible range. Without the photochemical, we lose the ability to see light. Retinal pigment epithelium protein 65 (Rpe65) is the key enzyme in regulating the availability of photochemical; deficiency of this gene results in a blinding eye disease. Here we show for the first time that LPD promotes efficient delivery in a cell specific-manner, and a long-term expression of Rpe65 gene to mice lacking Rpe65 gene, leading to in vivo correction of blindness. Thus, LPD nanoparticles could provide a promising, efficient, nonviral method of gene delivery with clinical applications in eye disease treatment.