Role of retinal pigment epithelium permeability in drug transfer between posterior eye segment and systemic blood circulation

A First-Passage Model of Intravitreal Drug Delivery and Residence Time-Influence of Ocular Geometry, Individual Variability, and Injection Location

Purpose

Standard of care for various retinal diseases involves recurrent intravitreal injections. This motivates mathematical modeling efforts to identify influential factors for ocular drug residence time, aiming to minimize administration frequency. We sought to describe the vitreal diffusion of therapeutics in nonclinical species frequently used during drug development assessments. In human eyes, we investigated the impact of variability in vitreous cavity size and eccentricity, and in injection location, on drug disposition.

Methods

Using a first-passage time approach, we modeled the transport-controlled distribution of two standard therapeutic protein formats (Fab and IgG) and elimination through anterior and posterior pathways. Anatomical three-dimensional geometries of mouse, rat, rabbit, cynomolgus monkey, and human eyes were constructed using ocular images and biometry datasets. A scaling relationship was derived for comparison with experimental ocular half-lives.

Results

Model simulations revealed a dependence of residence time on ocular size and injection location. Delivery to the posterior vitreous resulted in increased vitreal half-life and retinal permeation. Interindividual variability in human eyes had a significant influence on residence time (half-life range of 5-7 days), showing a strong correlation to axial length and vitreal volume. Anterior exit was the predominant route of drug elimination. Contribution of the posterior pathway displayed a 3% difference between protein formats but varied between species (10%-30%).

Conclusions

The modeling results suggest that experimental variability in ocular half-life is partially attributed to anatomical differences and injection site location. Simulations further suggest a potential role of the posterior pathway permeability in determining species differences in ocular pharmacokinetics.

Translational physiologically-based pharmacokinetic model for ocular disposition of monoclonal antibodies

We have previously published a PBPK model comprising the ocular compartment to characterize the disposition of monoclonal antibodies (mAbs) in rabbits. While rabbits are commonly used preclinical species in ocular research, non-human primates (NHPs) have the most phylogenetic resemblance to humans including the presence of macula in the eyes as well as higher sequence homology. However, their use in ocular research is limited due to the strict ethical guidelines. Similarly, in humans the ocular samples cannot be collected except for the tapping of aqueous humor (AH). Therefore, we have translated this rabbit model to monkeys and human species using literature-reported datasets. Parameters describing the tissue volumes, physiological flows, and FcRn-binding were obtained from the literature, or estimated by fitting the model to the data. In the monkey model, the values for the rate of lysosomal degradation for antibodies (Kdeg), intraocular reflection coefficients (σaq, σret, σcho), bidirectional rate of fluid circulation between the vitreous chamber and the aqueous chamber (QVA), and permeability-surface area product of lens (PSlens) were estimated; and were found to be 31.5 h-1, 0.7629, 0.6982, 0.9999, 1.64 × 10-5 L/h, and 4.62 × 10-7 L/h, respectively. The monkey model could capture the data in plasma, aqueous humor, vitreous humor and retina reasonably well with the predictions being within twofold of the observed values. For the human model, only the value of Kdeg was estimated to fit the model to the plasma pharmacokinetics (PK) of mAbs and was found to be 24.4 h-1 (4.14%). The human model could also capture the ocular PK data reasonably well with the predictions being within two- to threefold of observed values for the plasma, aqueous and vitreous humor. Thus, the proposed framework can be used to characterize and predict the PK of mAbs in the eye of monkey and human species following systemic and intravitreal administration. The model can also facilitate the development of new antibody-based therapeutics for the treatment of ocular diseases as well as predict ocular toxicities of such molecules following systemic administration.

Mathematical Models of Ocular Drug Delivery

Drug delivery is an important factor for the success of ocular drug treatment. However, several physical, biochemical, and flow-related barriers limit drug exposure of anterior and posterior ocular target tissues during drug treatment via topical, subconjunctival, intravitreal, or systemic routes. Mathematical models encompass various barriers so that their joint influence on pharmacokinetics (PKs) can be simulated in an integrated fashion. The models are useful in predicting PKs and even pharmacodynamics (PDs) of administered drugs thereby fostering development of new drug molecules and drug delivery systems. Furthermore, the models are potentially useful in interspecies translation and probing of disease effects on PKs. In this review article, we introduce current modeling methods (noncompartmental analyses, compartmental and physiologically based PK models, and finite element models) in ocular PKs and related drug delivery. The roles of top-down models and bottom-up simulations are discussed. Furthermore, we present some future challenges, such as modeling of intra-tissue distribution, prediction of drug responses, quantitative systems pharmacology, and possibilities of artificial intelligence.

Tissue Engineering of Outer Blood Retina Barrier for Therapeutic Development

Age related macular degeneration and other retinal degenerative disorders are characterized by disruption of the outer blood retinal barrier (oBRB) with subsequent ischemia, neovascularization, and atrophy. Despite the treatment advances, there remains no curative therapy, and no treatment targeted at regenerating native-like tissue for patients with late stages of the disease. Here we present advances in tissue engineering, focusing on bioprinting methods of generating tissue allowing for safe and reliable production of oBRB as well as tissue reprogramming with induced pluripotent stem cells for transplantation. We compare these approaches to organ-on-a-chip models for studying the dynamic nature of physiologic conditions. Highlighted within this review are studies that employ good manufacturing practices and use clinical grade methods that minimize potential risk to patients. Lastly, we illustrate recent clinical applications demonstrating both safety and efficacy for direct patient use. These advances provide an avenue for drug discovery and ultimately transplantation.

Natural products for the treatment of age-related macular degeneration

BACKGROUND

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

PURPOSE

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

STUDY DESIGN AND METHODS

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

RESULTS

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

CONCLUSION

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

Transporter Protein Expression of Corneal Epithelium in Rabbit and Porcine: Evaluation of Models for Ocular Drug Transport Study

The transcorneal route is the main entry route for drugs to the intraocular parts, after topical administration. The outer surface, the corneal epithelium (CE), forms the rate-limiting barrier for drug permeability. Information about the role and protein expression of drug and amino acid transporter proteins in the CE is sparse and lacking. The aim of our study was to characterize transporter protein expression in rabbit and porcine CE to better understand potential drug and nutrient absorption after topical administration. Proteins, mainly Abc and Slc transporters, were characterized with quantitative targeted absolute proteomics and global untargeted proteomics methods. In the rabbit CE, 24 of 48 proteins were detected in the targeted approach, and 21 of these were quantified. In the porcine CE, 26 of 58 proteins were detected in the targeted approach, and 20 of these were quantified. Among these, 15 proteins were quantified in both animals: 4f2hc (Slc3a2), Aqp0, Asct1 (Slc1a4), Asct2 (Slc1a5), Glut1 (Slc2a1), Hmit (Slc2a13), Insr, Lat1 (Slc7a5), Mct1 (Slc16a1), Mct2 (Slc16a7), Mct4 (Slc16a3), Mrp 4 (Abcc4), Na+/K+-ATPase, Oatp3a1 (Slco3a1), and Snat2 (Slc38a2). Overall, the global proteomics results supported the targeted proteomics results. Organic anion transporting polypeptide Oatp3a1 was detected and quantified for the first time in both rabbit (1.4 ± 0.4 fmol/cm2) and porcine (11.1 ± 5.3 fmol/cm2) CE. High expression levels were observed for L-type amino acid transporter, Lat1, which was quantified with newly selected extracellular domain peptides in rabbit (48.9 ± 11.8 fmol/cm2) and porcine (37.6 ± 11.5 fmol/cm2) CE. The knowledge of transporter protein expression in ocular barriers is a key factor in the successful design of new ocular drugs, pharmacokinetic modeling, understanding ocular diseases, and the translation to human.

Recent Advances in Nanomedicine for Ocular Fundus Neovascularization Disease Management

As an indispensable part of the human sensory system, visual acuity may be impaired and even develop into irreversible blindness due to various ocular pathologies. Among ocular diseases, fundus neovascularization diseases (FNDs) are prominent etiologies of visual impairment worldwide. Intravitreal injection of anti-vascular endothelial growth factor drugs remains the primary therapy but is hurdled by common complications and incomplete potency. To renovate the current therapeutic modalities, nanomedicine emerged as the times required, which is endowed with advanced capabilities, able to fulfill the effective ocular fundus drug delivery and achieve precise drug release control, thus further improving the therapeutic effect. This review provides a comprehensive summary of advances in nanomedicine for FND management from state-of-the-art studies. First, the current therapeutic modalities for FNDs are thoroughly introduced, focusing on the key challenges of ocular fundus drug delivery. Second, nanocarriers are comprehensively reviewed for ocular posterior drug delivery based on the nanostructures: polymer-based nanocarriers, lipid-based nanocarriers, and inorganic nanoparticles. Thirdly, the characteristics of the fundus microenvironment, their pathological changes during FNDs, and corresponding strategies for constructing smart nanocarriers are elaborated. Furthermore, the challenges and prospects of nanomedicine for FND management are thoroughly discussed.

The Role of Natural Products in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the most severe complications of diabetes mellitus and potentially leads to significant visual impairment and blindness. The complex mechanisms involved in the pathological changes in DR make it challenging to achieve satisfactory outcomes with existing treatments. Diets conducive to glycemic control have been shown to improve outcomes in diabetic patients, thus positioning dietary interventions as promising avenues for DR treatment. Investigations have demonstrated that natural products (NPs) may effectively manage DR. Many types of natural compounds, including saponins, phenols, terpenoids, flavonoids, saccharides, alkaloids, and vitamins, have been shown to exert anti-inflammatory, antioxidant, anti-neovascular, and antiapoptotic effects in vivo and in vitro. Nevertheless, the clinical application of NPs still faces challenges, such as suboptimal specificity, poor bioavailability, and a risk of toxicity. Prospective clinical studies are imperative to validate the therapeutic potential of NPs in delaying or preventing DR.

The prophylactic value of TNF-α inhibitors against retinal cell apoptosis and optic nerve axon loss after corneal surgery or trauma

BACKGROUND AND PURPOSE

Late secondary glaucoma is an often-severe complication after acute events like anterior segment surgery, trauma and infection. TNF-α is a major mediator that is rapidly upregulated, diffusing also to the retina and causes apoptosis of the ganglion cells and degeneration of their optic nerve axons (mediating steps to glaucomatous damage). Anti-TNF-α antibodies are in animals very effective in protecting the retinal cells and the optic nerve-and might therefore be useful prophylactically against secondary glaucoma in future such patients. Here we evaluate (1) toxicity and (2) efficacy of two TNF-α inhibitors (adalimumab and infliximab), in rabbits by subconjunctival administration.

METHODS

For drug toxicity, animals with normal, unburned corneas were injected with adalimumab (0.4, 4, or 40 mg), or infliximab (1, 10, or 100 mg). For drug efficacy, other animals were subjected to alkali burn before such injection, or steroids (for control). The rabbits were evaluated clinically with slit lamp and photography, electroretinography, optical coherence tomography, and intraocular pressure manometry. A sub-set of eyes were stained ex vivo after 3 days for retinal cell apoptosis (TUNEL). In other experiments the optic nerves were evaluated by paraphenylenediamine staining after 50 or 90 days. Loss of retinal cells and optic nerve degeneration were quantified.

RESULTS

Subconjunctival administration of 0.4 mg or 4.0 mg adalimumab were well tolerated, whereas 40.0 mg was toxic to the retina. 1, 10, or 100 mg infliximab were also well tolerated. Analysis of the optic nerve axons after 50 days confirmed the safety of 4.0 mg adalimumab and of 100 mg infliximab. For efficacy, 4.0 mg adalimumab subconjunctivally in 0.08 mL provided practically full protection against retinal cell apoptosis 3 days following alkali burn, and infliximab 100 mg only slightly less. At 90 days following burn injury, control optic nerves showed about 50% axon loss as compared to 8% in the adalimumab treatment group.

CONCLUSIONS

Subconjunctival injection of 4.0 mg adalimumab in rabbits shows no eye toxicity and provides excellent neuroprotection, both short (3 days) and long-term (90 days). Our total. accumulated data from several of our studies, combined with the present paper, suggest that corneal injuries, including surgery, might benefit from routine administration of anti-TNF-α biologics to reduce inflammation and future secondary glaucoma.

Quantitative intravitreal pharmacokinetics in mouse as a step towards inter-species translation

Although mouse models are widely used in retinal drug development, pharmacokinetics in mouse eye is poorly understood. In this study, we applied non-invasive in vivo fluorophotometry to study pharmacokinetics of intravitreal fluorescein sodium (molecular weight 0.38 kDa) and fluorescein isothiocyanate-dextran (FD-150; molecular weight 150 kDa) in mice. Intravitreal half-lives of fluorescein and FD-150 in mouse eyes were 0.53 ± 0.06 h and 2.61 ± 0.86 h, respectively. These values are 8-230 times shorter than the elimination half-lives of similar compounds in the human vitreous. The apparent volumes of distribution in the mouse vitreous were close to the anatomical volume of the mouse vitreous (FD-150, 5.1 μl; fluorescein, 9.6 μl). Dose scaling factors were calculated from mouse to rat, rabbit, monkey and human translation. Based on pharmacokinetic modelling and compound concentrations in the vitreous and anterior chamber, fluorescein is mainly eliminated posteriorly across blood-retina barrier, but FD-150 is cleared via aqueous humour outflow. The results of this study improve the knowledge of intravitreal pharmacokinetics in mouse and facilitate inter-species scaling in ocular drug development.

Selective drug delivery to the retinal cells: Biological barriers and avenues

Retinal drug delivery is a challenging, but important task, because most retinal diseases are still without any proper therapy. Drug delivery to the retina is hampered by the anatomical and physiological barriers resulting in minimal bioavailability after topical ocular and systemic administrations. Intravitreal injections are current method-of-choice in retinal delivery, but these injections show short duration of action for small molecules and low target bioavailability for many protein, gene based drugs and nanomedicines. State-of-art delivery systems are based on prolonged retention, controlled drug release and physical features (e.g. size and charge). However, drug delivery to the retina is not cell-specific and these approaches do not facilitate intracellular delivery of modern biological drugs (e.g. intracellular proteins, RNA based medicines, gene editing). In this focused review we highlight biological factors and mechanisms that form the basis for the selective retinal drug delivery systems in the future. Therefore, we are presenting current knowledge related to retinal membrane transporters, receptors and targeting ligands in relation to nanomedicines, conjugates, extracellular vesicles, and melanin binding. These issues are discussed in the light of retinal structure and cell types as well as future prospects in the field. Unlike in some other fields of targeted drug delivery (e.g. cancer research), selective delivery technologies have been rarely studied, even though cell targeted delivery may be even more feasible after local administration into the eye.

Comparison of barrier properties of outer blood-retinal barrier models - Human stem cell-based models as a novel tool for ocular drug discovery

The retinal pigment epithelial (RPE) cell monolayer forms the outer blood-retinal barrier and has a crucial role in ocular pharmacokinetics. Although several RPE cell models are available, there have been no systematic comparisons of their barrier properties with respect to drug permeability. We compared the barrier properties of RPE secondary cell lines (ARPE19, and ARPE19mel) and both primary (hfRPE) and stem-cell derived RPE (hESC-RPE) cells by investigating the permeability of nine drugs (aztreonam, ciprofloxacin, dexamethasone, fluconazole, ganciclovir, ketorolac, methotrexate, voriconazole, and quinidine) across cell monolayers. ARPE19, ARPE19mel, and hfRPE cells displayed a narrow P_app value range, with relatively high permeation rates (5.2-26 × 10^-6 cm/s). In contrast, hESC-RPE cells efficiently restricted the drug flux, and displayed even lower P_app values than those reported for bovine RPE-choroid, with the range of 0.4-32 cm^-6/s. Therefore, ARPE19, ARPE19mel, and hfRPE cells failed to form a tight barrier, whereas hESC-RPE cells restricted the drug flux to a similar extent as bovine RPE-choroid. Therefore, hESC-RPE cells are valuable tools in ocular drug discovery.

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

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

Ocular Barriers and Their Influence on Gene Therapy Products Delivery

The eye is formed by tissues and cavities that contain liquids whose compositions are highly regulated to ensure their optical properties and their immune and metabolic functions. The integrity of the ocular barriers, composed of different elements that work in a coordinated fashion, is essential to maintain the ocular homeostasis. Specialized junctions between the cells of different tissues have specific features which guarantee sealing properties and selectively control the passage of drugs from the circulation or the outside into the tissues and within the different ocular compartments. Tissues structure also constitute selective obstacles and pathways for various molecules. Specific transporters control the passage of water, ions, and macromolecules, whilst efflux pumps reject and eliminate toxins, metabolites, or drugs. Ocular barriers, thus, limit the bioavailability of gene therapy products in ocular tissues and cells depending on the route chosen for their administration. On the other hand, ocular barriers allow a real local treatment, with limited systemic side-effects. Understanding the different barriers that limit the accessibility of different types of gene therapy products to the different target cells is a prerequisite for the development of efficient gene delivery systems. This review summarizes actual knowledge on the different ocular barriers that limit the penetration and distribution of gene therapy products using different routes of administration, and it provides a general overview of various methods used to bypass the ocular barriers.

Brain and Retinal Organoids for Disease Modeling: The Importance of In Vitro Blood–Brain and Retinal Barriers Studies

Brain and retinal organoids are functional and dynamic in vitro three-dimensional (3D) structures derived from pluripotent stem cells that spontaneously organize themselves to their in vivo counterparts. Here, we review the main literature data of how these organoids have been developed through different protocols and how they have been technically analyzed. Moreover, this paper reviews recent advances in using organoids to model neurological and retinal diseases, considering their potential for translational applications but also pointing out their limitations. Since the blood–brain barrier (BBB) and blood–retinal barrier (BRB) are understood to play a fundamental role respectively in brain and eye functions, both in health and in disease, we provide an overview of the progress in the development techniques of in vitro models as reliable and predictive screening tools for BBB and BRB-penetrating compounds. Furthermore, we propose potential future directions for brain and retinal organoids, in which dedicated biobanks will represent a novel tool for neuroscience and ophthalmology research.

Exploration of the Plausible Mechanism of Ethambutol Induced Ocular Toxicity by Using Proteomics Informed Physiologically Based Pharmacokinetic (PBPK) Modeling

PURPOSE

Ethambutol (EMB) is a first-line anti-tubercular drug that is known to cause optic neuropathy. The exact mechanism of its eye toxicity is unknown; however, proposition is metal chelating effect of both EMB and its metabolite 2,2'-(ethylenediamino)-dibutyric acid (EDBA). The latter is formed by sequential metabolism of EMB by alcohol dehydrogenases (ADHs) and aldehyde dehydrogenases (ALDHs). The purpose of this study was to predict the levels of drug and EDBA in the eye using physiologically based pharmacokinetic (PBPK) modeling.

METHODS

The PBPK model of EMB was developed using GastroPlus. The intrinsic hepatic clearance of ALDH, calculated by the model, was scaled down using proteomics data to estimate the rate of formation of EDBA in the eye. Additionally, the comparative permeability of EMB and EDBA was assessed by employing in silico and in vitro approaches. The rate of formation of EDBA in the eye and permeability data were then incorporated in a compartmental model to predict the ocular levels of EMB and EDBA.

RESULTS

The simulation results of compartmental model highlighted that there was an on-site formation of EDBA upon metabolism of EMB. Furthermore, in silico and in vitro studies revealed that EDBA possessed much lower permeability than EMB. These observations meant that once EDBA was formed in the eye, it was not permeated out and hence achieved higher ocular concentration.

CONCLUSION

The on-site formation of EDBA in the eye, its higher local concentration due to lower ocular clearance and its pre-known characteristic to chelate metal species better explains the ocular toxicity shown by EMB.

Aqueous eye drops containing drug/cyclodextrin nanoparticles deliver therapeutic drug concentrations to both anterior and posterior segment

Using topical application to deliver therapeutic concentrations of drugs to the posterior segment of the eye remains very challenging. As a result, posterior segment diseases are usually treated by intravitreal injection or implant. While topical treatments are commonly used for anterior segment conditions, they sometimes require frequent applications. Eye drop formulations based on γ-cyclodextrin (γCD)-based nanoparticle aggregates were developed, which in animal models and clinical studies deliver therapeutic concentrations of drugs (dorzolamide and dexamethasone) to both anterior and posterior segments of the eye. An early study in humans showed dorzolamide/γCD eye drops could achieve comparable intraocular pressure decreases to commercial dorzolamide eye drops, but with less frequent application. Pilot studies with dexamethasone/γCD eye drops suggested that they could be effective in a range of conditions, including diabetic macular oedema, cystoid macular oedema and vitritis secondary to uveitis, postcataract surgery inflammation and postoperative treatment in trabeculectomy. Phase II studies with similar dexamethasone/γCD nanoparticle eye drops in diabetic macular oedema and postcataract surgery inflammation have recently been completed. This technology has the potential to be used with other classes of drug molecules and to replace or complement invasive treatments, providing safer, non-invasive therapies, particularly for posterior segment conditions, that can be self-administered as eye drops by patients.

Topical drug delivery to the retina: obstacles and routes to success

INTRODUCTION

Retinal diseases are one of the main reasons for vision loss where all available drug treatments are based on invasive drug administration such as intravitreal injections. Despite huge efforts and some promising results in animal models, almost all delivery technologies tested have failed in human trials. There are however examples of clinically effective topical delivery systems such as fast dissolving aqueous eye drop suspensions.

AREAS COVERED

Six obstacles to topical drug delivery to the eye have been identified and discussed in some details. These obstacles consist of static membrane barriers to drug permeation into the eye, dynamic barriers such as the lacrimal drainage and physiochemical barriers such as low thermodynamic activity. It is explained how and why these obstacles hamper drug permeation and how different technologies, both those that are applied in marketed drug products and those that are under investigation, have addressed these obstacles.

EXPERT OPINION

The reason that most topical drug delivery systems have failed to deliver therapeutic drug concentrations to the retina is that they do not address physiochemical barriers such as the thermodynamic activity of the permeating drug molecules. Topical drug delivery to the retina has only been successful when the static, dynamic, and physiochemical barriers are addressed simultaneously.

Pro-inflammatory activation changes intracellular transport of bevacizumab in the retinal pigment epithelium in vitro

Purpose

Bevacizumab is taken up and transported through the retinal pigment epithelium. Inflammatory signaling may influence this interaction. In the present study, we have investigated the effect of pro-inflammatory stimuli on the uptake, intracellular localization, and transepithelial transport of bevacizumab.

Methods

ARPE-19 cell line or primary porcine RPE cells were treated with clinical relevant concentrations of bevacizumab (250 µg/ml). Pro-inflammatory signaling was induced by TLR-3 agonist polyinosinic:polycytidylic acid (Poly I:C). Viability was investigated with MTT and trypan-blue exclusion assay, and cell number, uptake, and intracellular localization were investigated with immunofluorescence, investigating also actin filaments, the motor protein myosin 7a and lysosomes. Immunofluorescence signals were quantified. Intracellular bevacizumab was additionally detected in Western blot. Barrier function was investigated with transepithelial resistant measurements (TER). The transepithelial transport of bevacizumab and its influence on cytokine (IL-6, IL-8, IL-1β, TNFα) secretion was investigated with ELISA.

Results

Poly I:C in combination with bevacizumab reduced the viability of the cells. Treatment with Poly I:C reduced the uptake of bevacizumab, changed the intensity of the actin filaments, and reduced the colocalization with myosin 7a. In addition, Poly I:C reduced the capacity of RPE cells to transport bevacizumab over the barrier. In addition, bevacizumab reduced the secretion of IL-8 and TNFα after Poly I:C stimulation at selected time points.

Conclusions

Pro-inflammatory activation of RPE cells with TLR-3 agonist Poly I:C changes the interaction of RPE cells with the anti-VEGF compound bevacizumab, reducing its uptake and transport. On the other hand, bevacizumab might influence pro-inflammatory cytokine release. Our data indicate that inflammation may influence the pharmacokinetic of bevacizumab in the retina.

Diabetic eye: associated diseases, drugs in clinic, and role of self-assembled carriers in topical treatment

Introduction: Diabetes is a pandemic disease that causes relevant ocular pathologies. Diabetic retinopathy, macular edema, cataracts, glaucoma, or keratopathy strongly impact the quality of life of the patients. In addition to glycemic control, intense research is devoted to finding more efficient ocular drugs and improved delivery systems that can overcome eye barriers. Areas covered: The aim of this review is to revisit first the role of diabetes in the development of chronic eye diseases. Then, commercially available drugs and new candidates in clinical trials are tackled together with the pros and cons of their administration routes. Subsequent sections deal with self-assembled drug carriers suitable for eye instillation combining patient-friendly administration with high ocular bioavailability. Performance of topically administered polymeric micelles, liposomes, and niosomes for the management of diabetic eye diseases is analyzed in the light of ex vivo and in vivo results and outcomes of clinical trials. Expert opinion: Self-assembled carriers are being shown useful for efficient delivery of not only a variety of small drugs but also macromolecules (e.g. antibodies) and genes. Successful design of drug carriers may offer alternatives to intraocular injections and improve the treatment of both anterior and posterior segments diabetic eye diseases.

Primary cilia in retinal pigment epithelium development and diseases

Retinal pigment epithelium (RPE) is a highly polarized epithelial monolayer lying between the photoreceptor layer and the Bruch membrane. It is essential for vision through participating in many critical activities, including phagocytosis of photoreceptor outer segments, recycling the visual cycle‐related compounds, forming a barrier to control the transport of nutrients, ions, and water, and the removal of waste. Primary cilia are conservatively present in almost all the vertebrate cells and acts as a sensory organelle to control tissue development and homeostasis maintenance. Numerous studies reveal that abnormalities in RPE lead to various retinal diseases, such as age‐related macular degeneration and diabetic macular oedema, but the mechanism of primary cilia in these physiological and pathological activities remains to be elucidated. Herein, we summarize the functions of primary cilia in the RPE development and the mutations of ciliary genes identified in RPE‐related diseases. By highlighting the significance of primary cilia in regulating the physiological and pathological processes of RPE, we aim to provide novel insights for the treatment of RPE‐related retinal diseases.

Mechanistic Model for the Prediction of Small-Molecule Vitreal Clearance Combining Diffusion-Limited and Permeability-Limited Clearance

The discovery of new small-molecule drugs for intravitreal administration would benefit from simple models to predict vitreal clearance (CL). The current models available have limitations in their applicability to small-molecule drugs and translatability to humans. We developed a mechanistic model combining the diffusion rate of the molecule in the vitreous and permeability across posterior segment tissues and applied it to 30 small molecules with observed CL available mostly from literature. We used Caco-2 permeability as a surrogate for ocular tissue permeability. The model predicted rabbit vitreal CL well, with 80% of the predictions being within a 2-fold range of the observed CL. For an accurate prediction, it was crucial to consider the anterior diffusion CL from the vitreous to the aqueous and a limiting diffusion CL for the whole eye. We observed no major differences in model accuracy when using literature permeability values from retinal pigment epithelial cell models. Importantly, by adopting the specific dimensions of the human eye, the model was able to accurately predict vitreal CL of four compounds for which human vitreal CL data are available. In summary, this mechanistic model enables a simple, accurate, and translatable estimation of small-molecule vitreal CL.

Permeability of the Retina and RPE-Choroid-Sclera to Three Ophthalmic Drugs and the Associated Factors

In this study, Retina-RPE-Choroid-Sclera (RCS) and RPE-Choroid-Sclera (CS) were prepared by scraping them off neural retina, and using the Ussing chamber we measured the average time-concentration values in the acceptor chamber across five isolated rabbit tissues for each drug molecule. We determined the outward direction permeability of the RCS and CS and calculated the neural retina permeability. The permeability coefficients of RCS and CS were as follows: ganciclovir, 13.78 ± 5.82 and 23.22 ± 9.74; brimonidine, 15.34 ± 7.64 and 31.56 ± 12.46; bevacizumab, 0.0136 ± 0.0059 and 0.0612 ± 0.0264 (×10^-6 cm/s). The calculated permeability coefficients of the neural retina were as follows: ganciclovir, 33.89 ± 12.64; brimonidine, 29.83 ± 11.58; bevacizumab, 0.0205 ± 0.0074 (×10^-6 cm/s). Between brimonidine and ganciclovir, lipophilic brimonidine presented better RCS and CS permeability, whereas ganciclovir showed better calculated neural retinal permeability. The large molecular weight drug bevacizumab demonstrated a much lower permeability than brimonidine and ganciclovir. In conclusion, the ophthalmic drug permeability of RCS and CS is affected by the molecular weight and lipophilicity, and influences the intravitreal half-life.

Partitioning and Spatial Distribution of Drugs in Ocular Surface Tissues

Ocular drug absorption after eye drop instillation has been widely studied, but partitioning phenomena and spatial drug distribution are poorly understood. We investigated partitioning of seven beta-blocking drugs in corneal epithelium, corneal stroma, including endothelium and conjunctiva, using isolated porcine tissues and cultured human corneal epithelial cells. The chosen beta-blocking drugs had a wide range (-1.76-0.79) of n-octanol/buffer solution distribution coefficients at pH 7.4 (Log D_7.4). In addition, the ocular surface distribution of three beta-blocking drugs was determined by matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) after their simultaneous application in an eye drop to the rabbits in vivo. Studies with isolated porcine corneas revealed that the distribution coefficient (K_p) between the corneal epithelium and donor solution showed a positive relationship and good correlation with Log D_7.4 and about a 50-fold range of K_p values (0.1-5). On the contrary, K_p between corneal stroma and epithelium showed an inverse (negative) relationship and correlation with Log D_7.4 based on a seven-fold range of K_p values. In vitro corneal cell uptake showed a high correlation with the ex vivo corneal epithelium/donor K_p values. Partitioning of the drugs into the porcine conjunctiva also showed a positive relationship with lipophilicity, but the range of K_p values was less than with the corneal epithelium. MALDI-IMS allowed simultaneous detection of three compounds in the cornea, showed data in line with other experiments, and revealed uneven spatial drug distribution in the cornea. Our data indicate the importance of lipophilicity in defining the corneal pharmacokinetics and the K_p values are a useful building block in the kinetic simulation models for topical ocular drug administration.

Topical drug delivery to the posterior segment of the eye: Thermodynamic considerations

Almost all studies on non-invasive topical drug delivery to the eye have emphasized the importance of biological barriers, static membrane barriers such as the cornea and the conjunctiva/sclera and dynamic barriers such as the lacrimal drainage. Hardly any have discussed the importance of the thermodynamic activity of the permeating drug molecules. Most drugs permeate from the eye surface into the eye by passive diffusion where, according to Fick's first law, the drug concentration gradient over the various permeation barriers (e.g., the tear fluid and the lipophilic membrane barriers) is the driving force. At the barrier interphases the dissolved drug molecules must partition from one barrier to another. For example, at the tear-cornea interphase the drug molecules must partition from the aqueous exterior into the lipophilic membrane. The drug partition coefficient between two phases is commonly defined as the equilibrium concentration ratio. However, these are only approximations. The actual driving force in Fick's first law is the gradient of the chemical potential and the equilibrium between two phases is attained when the chemical potential of the drug in one phase is equal to that in the other phase. Here the importance of thermodynamic considerations in topical drug delivery to the eye is reviewed.

Prodrug Approach for Posterior Eye Drug Delivery: Synthesis of Novel Ganciclovir Prodrugs and in Vitro Screening with Cassette Dosing

Because of poor ocular drug bioavailability, intravitreal injections have become the gold standard for drug delivery to the posterior eye. The prodrug approach can be used for optimizing the biopharmaceutical properties of intravitreal drugs. The preclinical screening of prodrugs' properties, such as hydrolysis and bioconversion, should be conducted in a resource-efficient way for an extensive set of synthesized compounds with validated methods. Our objective was to explore cassette dosing in in vitro prodrug hydrolysis and bioconversion studies in buffer, vitreous, and retinal pigment epithelium (RPE) homogenate for rapid medium-throughput screening. Moreover, our aim was to correlate the prodrug structure with hydrolytic behavior. We synthesized 18 novel ganciclovir prodrugs and first studied their hydrolysis in aqueous buffer and porcine vitreous in vitro with cassette dosing for 35 h. A method for vitreous homogenate pH equilibration to a physiological level by using buffer and incubation under 5% carbon dioxide was validated. The hydrolysis of the prodrugs was evaluated in porcine RPE homogenate in vitro with cassette dosing, and five prodrugs were assayed individually to examine their bioconversion into ganciclovir in RPE after 2 h. Lastly, the prodrugs' binding to melanin was studied in vitro. The prodrugs showed a wide spectrum of hydrolysis rates, ranging from a few percentages to 100% in the vitreous and RPE; in general, hydrolysis in RPE was faster than in vitreous. Prodrugs with long carbon chains and disubstitution showed lability in the tissue homogenates, whereas prodrugs with branched carbon chains and aromatic groups were stable. All five prodrugs chosen for the bioconversion study in RPE were hydrolyzed into ganciclovir, and their hydrolytic behavior matched results from the cassette mix experiment, supporting the cassette mix approach for hydrolysis and bioconversion studies. None of the prodrugs bound highly to melanin (<50% bound). In conclusion, cassette dosing proved useful for the rapid screening of prodrug hydrolysis and bioconversion properties. Analyzing several compounds simultaneously can complicate the analytics, and thus, choosing the compounds of the cassette mix should be done carefully to avoid mutual interference of the compounds with the results. The methodology and results of the work are applicable in ocular drug research and prodrug design.

Drug Flux Across RPE Cell Models: The Hunt for An Appropriate Outer Blood-Retinal Barrier Model for Use in Early Drug Discovery

The retinal pigment epithelial (RPE) cell monolayer forms the outer blood–retinal barrier and has a crucial role in ocular pharmacokinetics. Although several RPE cell models are available, there have been no systematic comparisons of their barrier properties with respect to drug permeability. We compared the barrier properties of several RPE secondary cell lines (ARPE19, ARPE19mel, and LEPI) and both primary (hfRPE) and stem-cell derived RPE (hESC-RPE) cells by investigating the permeability of nine drugs (aztreonam, ciprofloxacin, dexamethasone, fluconazole, ganciclovir, ketorolac, methotrexate, voriconazole, and quinidine) across cell monolayers. ARPE19, ARPE19mel, and hfRPE cells displayed a narrow P_app value range, with relatively high permeation rates (5.2–26 × 10⁻⁶ cm/s. In contrast, hESC-RPE and LEPI cells efficiently restricted the drug flux, and displayed even lower P_app values than those reported for bovine RPE-choroid, with the range of 0.4–32 cm⁻⁶/s (hESC-RPE cells) and 0.4–29 × 10⁻⁶ cm/s, (LEPI cells). Therefore, ARPE19, ARPE19mel, and hfRPE cells failed to form a tight barrier, whereas hESC-RPE and LEPI cells restricted the drug flux to a similar extent as bovine RPE-choroid. Therefore, LEPI and hESC-RPE cells are valuable tools in ocular drug discovery.

Polysaccharides in Ocular Drug Delivery

Polysaccharides, such as cellulose, hyaluronic acid, alginic acid, and chitosan, as well as polysaccharide derivatives, have been successfully used to augment drug delivery in the treatment of ocular pathologies. The properties of polysaccharides can be extensively modified to optimize ocular drug formulations and to obtain biocompatible and biodegradable drugs with improved bioavailability and tailored pharmacological effects. This review discusses the available polysaccharide choices for overcoming the difficulties associated with ocular drug delivery, and it explores the reasons for the dependence between the physicochemical properties of polysaccharide-based drug carriers and their efficiency in different formulations and applications. Polysaccharides will continue to be of great interest to researchers endeavoring to develop ophthalmic drugs with improved effectiveness and safety.