Drug transport in corneal epithelium and blood-retina barrier: emerging role of transporters in ocular pharmacokinetics

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 This review is based on the literature and unpublished data from the authors' laboratory.

Nanostructure-based platforms-current prospective in ophthalmic drug delivery

The topically applied drugs as drops are washed off from the eye in very short period, resulting in low ocular bioavailability of drugs. Number of approaches have been attempted to increase the bioavailability and the duration of action of ocular drugs. This review provides an insight into various novel approaches; hydrophilic nanogels, solid lipid nanoparticles, and nanosponges applied very recently in the delivery of insoluble drugs, prolonging the ocular residence time, minimize pre-corneal drug loss and, therefore, bioavailability and therapeutic efficacy of the drugs. Despite various scientific approaches, efficient ocular drug delivery remains a challenge for pharmaceutical scientists.

Nanostructured materials for ocular delivery: nanodesign for enhanced bioadhesion, transepithelial permeability and sustained delivery

Effective drug delivery to the eye is an ongoing challenge due to poor patient compliance coupled with numerous physiological barriers. Eye drops for the front of the eye and ocular injections for the back of the eye are the most prevalent delivery methods, both of which require relatively frequent administration and are burdensome to the patient. Novel drug delivery techniques stand to drastically improve safety, efficacy and patient compliance for ocular therapeutics. Remarkable advances in nanofabrication technologies make the application of nanostructured materials to ocular drug delivery possible. This article focuses on the use of nanostructured materials with nanoporosity or nanotopography for ocular delivery. Specifically, we discuss nanotopography for enhanced bioadhesion and permeation and nanoporous materials for controlled release drug delivery. As examples, application of polymeric nanostructures for greater transepithelial permeability, nanostructured microparticles for enhanced preocular retention time and nanoporous membranes for tuning drug release profile are covered.

Synergistic Cysteamine Delivery Nanowafer as an Efficacious Treatment Modality for Corneal Cystinosis

A synergy between the polymer biomaterial and drug plays an important role in enhancing the therapeutic efficacy, improving the drug stability, and minimizing the local immune responses in the development of drug delivery systems. Particularly, in the case of ocular drug delivery, the need for the development of synergistic drug delivery system becomes more pronounced because of the wet ocular mucosal surface and highly innervated cornea, which elicit a strong inflammatory response to the instilled drug formulations. This article presents the development of a synergistic cysteamine delivery nanowafer to treat corneal cystinosis. Corneal cystinosis is a rare metabolic disease that causes the accumulation of cystine crystals in the cornea resulting in corneal opacity and loss of vision. It is treated with topical cysteamine (Cys) eye drops that need to be instilled 6-12 times a day throughout the patient's life, which causes side effects such as eye pain, redness, and ocular inflammation. As a result, compliance and treatment outcomes are severely compromised. To surmount these issues, we have developed a clinically translatable Cys nanowafer (Cys-NW) that can be simply applied on the eye with a fingertip. During the course of the drug release, Cys-NW slowly dissolves and fades away. The in vivo studies in cystinosin knockout mice demonstrated twice the therapeutic efficacy of Cys-NW containing 10 μg of Cys administered once a day, compared to 44 μg of Cys as topical eye drops administered twice a day. Furthermore, Cys-NW stabilizes Cys for up to four months at room temperature compared to topical Cys eye drops that need to be frozen or refrigerated and still remain active for only 1 week. The Cys-NW, because of its enhanced therapeutic efficacy, safety profile, and extended drug stability at room temperature, can be rapidly translated to the clinic for human trials.

In Vitro and In Vivo Models to Study Corneal Endothelial-mesenchymal Transition

Corneal endothelial cells (CECs) play a crucial role in maintaining corneal clarity through active pumping. A reduced CEC count may lead to corneal edema and diminished visual acuity. However, human CECs are prone to compromised proliferative potential. Furthermore, stimulation of cell growth is often complicated by gradual endothelial-mesenchymal transition (EnMT). Therefore, understanding the mechanism of EnMT is necessary for facilitating the regeneration of CECs with competent function. In this study, we prepared a primary culture of bovine CECs by peeling the CECs with Descemet's membrane from the corneal button and demonstrated that bovine CECs exhibited the EnMT process, including phenotypic change, nuclear translocation of β-catenin, and EMT regulators snail and slug, in the in vitro culture. Furthermore, we used a rat corneal endothelium cryoinjury model to demonstrate the EnMT process in vivo. Collectively, the in vitro primary culture of bovine CECs and in vivo rat corneal endothelium cryoinjury models offers useful platforms for investigating the mechanism of EnMT.

Facile Noninvasive Retinal Gene Delivery Enabled by Penetratin

Gene delivery to the posterior segment of the eye is severely hindered by the impermeability of defensive barriers; therefore, in clinical settings, genomic medicines are mainly administered by intravitreal injection. We previously found that penetratin could transport the covalently conjugated fluorophore to the fundus oculi by topical instillation. In this study, gene delivery systems enabled by penetratin were designed based on electrostatic binding to target the retina via a noninvasive administration route and prepared with red fluorescent protein plasmid (pRFP) and/or poly(amidoamine) dendrimer of low molecular weight (G3 PAMAM). Formulation optimization, structure confirmation, and characterization were subsequently conducted. Penetratin alone showed limited ability to condense the plasmid but had powerful uptake and transfection by corneal and conjunctival cells. G3 PAMAM was nontoxic to the ocular cells, and when introduced into the penetratin-incorporated complex, the plasmid was condensed more compactly. Therefore, further improved cellular uptake and transfection were observed. After being instilled in the conjunctival sac of rats, the intact complexes penetrated rapidly from the ocular surface into the fundus and resided in the retina for more than 8 h, which resulted in efficient expression of RFP in the posterior segment. Intraocular distribution of the complexes suggested that the plasmids were absorbed into the eyes through a noncorneal pathway during which penetratin played a crucial role. This study provides a facile and friendly approach for intraocular gene delivery and is an important step toward the development of noninvasive gene therapy for posterior segment diseases.

Artificial plasma membrane models based on lipidomic profiling

Phospholipid monolayers are often described as membrane models for analyzing drug-lipid interactions. In many works, a single phosphatidylcholine is chosen, sometimes with one or two additional components. Drug penetration is studied at 30mN/m, a surface pressure considered as corresponding to the pressure in bilayers, independently of the density of lipid molecular packing. In this work, we have extracted, identified, and quantified the major lipids constituting the lipidome of plasma and mitochondrial membranes of retinoblastoma (Y79) and retinal pigment epithelium cells (ARPE-19), using liquid chromatography coupled to high-resolution mass spectrometry (LC-MS/MS). The results obtained from this lipidomic analysis were used in an attempt to build an artificial lipid monolayer with a composition mimicking that of the plasma membrane of Y79 cells, better than a single phospholipid. The variety and number of lipid classes and species in cell extracts monolayers exceeding by far those of the phospholipids chosen to mimic them, the π-A isotherms of model monolayers differed from those of lipid extracts in shape and apparent packing density. We propose a model monolayer based on the most abundant species identified in the extracts, with a surface compressional modulus at 30mN/m close to the one of the lipid extracts.

Advances in the use of prodrugs for drug delivery to the eye

INTRODUCTION

Ocular drug delivery is presented with many challenges, taking into account the distinctive structure of the eye. The prodrug approach has been, and is being, employed to overcome such barriers for some drug molecules, utilizing a chemical modification approach rather than a formulation-based approach. A prodrug strategy involves modification of the active moiety into various derivatives in a fashion that imparts some advantage, such as membrane permeability, site specificity, transporter targeting and improved aqueous solubility, over the parent compound. Areas covered: The following review is a comprehensive summary of various novel methodologies and strategies reported over the past few years in the area of ocular drug delivery. Some of the strategies discussed involve polymer and lipid conjugation with the drug moiety to impart hydrophilicity or lipophilicity, or to target nutrient transporters by conjugation with transporter-specific moieties and retrometabolic drug design. Expert opinion: The application of prodrug strategies provides an option for enhancing drug penetration into the ocular tissues, and overall ocular bioavailability, with minimum disruption of the ocular diffusion barriers. Although success of the prodrug strategy is contingent on various factors, such as the chemical structure of the parent molecule, aqueous solubility and solution stability, capacity of targeted transporters and bioreversion characteristics, this approach has been successfully utilized, commercially and therapeutically, in several cases.

Mechanistic modeling of ophthalmic drug delivery to the anterior chamber by eye drops and contact lenses

Ophthalmic drug for the anterior chamber diseases are delivered into tears by either eye drops or by extended release devices placed in the eyes. The instilled drug exits the eye through various routes including tear drainage into the nose through the canaliculi and transport across various ocular membranes. Understanding the mechanisms relevant to each route can be useful in predicting the dependency of ocular bioavailability on various formulation parameters, such as drug concentration, salinity, viscosity, etc. Mathematical modeling has been developed for each of the routes and validated by comparison with experiments. The individual models can be combined into a system model to predict the fraction of the instilled drug that reaches the target. This review summarizes the individual models for the transport of drugs across the cornea and conjunctiva and the canaliculi tear drainage. It also summarizes the combined tear dynamics model that can predict the ocular bioavailability of drugs instilled as eye drops. The predictions from the individual models and the combined model are in good agreement with experimental data. Both experiments and models predict that the corneal bioavailability for drugs delivered through eye drops is less than 5% due to the small area of the cornea in comparison to the conjunctiva, and the rapid clearance of the instilled solution by tear drainage. A contact lens is a natural choice for delivering drugs to the cornea due to the placement of the contact in the immediate vicinity of the cornea. The drug released by the contact towards the cornea surface is trapped in the post lens tear film for extended duration of at least 30min allowing transport of a large portion into the cornea. The model predictions backed by in vivo animal and clinical data show that the bioavailability increases to about 50% with contact lenses. This realization has encouraged considerable research towards delivering ocular drugs by contact lenses. Commercial contacts are, however, not ideal for drug delivery due to the short release durations which may necessitate wearing multiple lenses each day, reducing the viability of this approach. Recent research has focused on designing contacts that retain all critical properties while increasing the release durations to a few hours or a few days. Beagle dog studies with contact lenses containing vitamin E nanobarriers to attenuate drug transport have shown promising results. Human studies using contacts for drug delivery have also been conducted for allergy therapy but drug eluting contacts are not available in the market for any therapy.

Editor's Highlight: Plasma miR-183/96/182 Cluster and miR-124 are Promising Biomarkers of Rat Retinal Toxicity

Retinal toxicity is one of the leading causes of attrition in drug development, and drug-induced retinal toxicity remains an issue in both drug discovery and postmarketed drugs. Derisking strategies to help with early identification of retinal injury utilizing a predictive retinal miRNA biomarker would greatly benefit decision-making in drug discovery programs, ultimately reducing attrition due to retinal toxicity. Our previous work demonstrated elevation of circulating retina-enriched miRNAs in a retinal toxicity model. To further validate our previous observation, 2 additional rat retinal injury models were utilized in this study: NaIO3-induced retinal injury and laser-induced choroidal neovascularization (CNV) injury model. Following induction of retina tissue injuries, circulating miR-183/96/182 cluster (miR-183 cluster), and miR-124 was investigated, as well as evaluations using an electroretinogram (ERG) and histopathology analysis. Data revealed that circulating miR-183/96/182 cluster was significantly increased (2- to 15-fold) compared with baseline/control in both laser-induced CNV and NaIO3-induced retinal injury models. Moreover, the severity of the retinal injury evaluated by ERG and histopathology correlated highly with elevation of these retina-enriched miRNAs in plasma. MiR-124 was also significantly increased in comparison with baseline/control by ∼25-fold postrepeat-doses of 30 mg/kg NaIO3 treatment. Increased level of these plasma miRNA biomarkers appeared to be dose- and time-dependent upon NaIO3 or laser treatment. The results suggest that the retina-enriched miRNAs (miR-183/96/182 cluster and miR-124) could serve as convenient and predictive biomarkers of retinal toxicity in drug development.

Multifunctional properties of organic-inorganic hybrid nanocomposites based on chitosan derivatives and layered double hydroxides for ocular drug delivery

To improve the ocular bioavailability of the model drug of pirenoxine sodium (PRN), organic-inorganic hybrid nanocomposites including layered double hydroxides (LDH) and chitosan derivatives (chitosan-glutathione (CG), chitosan-glutathione-valine (CG-V) and chitosan-glutathione-valine-valine (CG-VV)) were designed and characterized. In vivo precorneal retention study on rabbits showed that mean residence time (MRT) and area under the curve (AUC0-6h) of CG-PRN-LDH nanocomposite eye drop was up to 2.1-fold and 6.3-fold higher than those of commercial product, respectively. In vitro corneal penetration on rabbits demonstrated that the cumulative permeation of CG-VV-PRN-LDH nanocomposite dispersion was increased by 5.2 folds compared to that of commercial product, which may be due to the active transport effect of the nanocomposites by peptide transporter-1 (PepT-1). In addition, the ex vivo fluorescence imaging showed that fluorescent intensity of crystalline lens in rabbits was increased after the administration of PRN-LDH, CG-PRN-LDH, CG-V-PRN-LDH and CG-VV-PRN-LDH (in increasing order) nanocomposite eye drop. Finally, in vivo distribution evaluation in ocular tissues of rabbits revealed that AUC0-8h and MRT in crystalline lens exhibited 14.7-fold and 2.2-fold increase in CG-VV-PRN-LDH nanocomposite eye drop group than those of commercial group, respectively. In summary, the organic-inorganic hybrid nanocomposites with multifunctional properties may be a promising ocular drug delivery system to achieve prolonged precorneal retention, better corneal permeability and enhanced ocular bioavailability.

STATEMENT OF SIGNIFICANCE

Due to several structural and physiological intraocular barriers, drug delivery to the ocular mid-posterior segments still faces great challenges. In this manuscript, organic-inorganic hybrid nanocomposites based on chitosan derivatives and layered double hydroxides (LDH) were designed and constructed. Multifunctional properties of these hybrid nanocomposites were due to the possible active targeting to the peptide transporter-1 on the corneal epithelial cells, the bioadhesive ability and permeation enhancement of chitosan derivatives, and the electrostatic adsorption of LDH. Prolonged precorneal retention, better corneal permeability and enhanced ocular bioavailability of the model drug pirenoxine sodium were observed. Chitosan derivatives-LDH hybrid nanocomposites may be a promising ophthalmic drug system for the treatment of ocular diseases of mid-posterior segments.

Advanced drug delivery and targeting technologies for the ocular diseases

INTRODUCTION

Ocular targeted therapy has enormously been advanced by implementation of new methods of drug delivery and targeting using implantable drug delivery systems (DDSs) or devices (DDDs), stimuli-responsive advanced biomaterials, multimodal nanomedicines, cell therapy modalities and medical bioMEMs. These technologies tackle several ocular diseases such as inflammation-based diseases (e.g., scleritis, keratitis, uveitis, iritis, conjunctivitis, chorioretinitis, choroiditis, retinitis, retinochoroiditis), ocular hypertension and neuropathy, age-related macular degeneration and mucopolysaccharidosis (MPS) due to accumulation of glycosaminoglycans (GAGs). Such therapies appear to provide ultimate treatments, even though much more effective, yet biocompatible, noninvasive therapies are needed to control some disabling ocular diseases/disorders.

METHODS

In the current study, we have reviewed and discussed recent advancements on ocular targeted therapies.

RESULTS

On the ground that the pharmacokinetic and pharmacodynamic analyses of ophthalmic drugs need special techniques, most of ocular DDSs/devices developments have been designed to localized therapy within the eye. Application of advanced DDSs such as Subconjunctival insert/implants (e.g., latanoprost implant, Gamunex-C), episcleral implant (e.g., LX201), cationic emulsions (e.g., Cationorm™, Vekacia™, Cyclokat™), intac/punctal plug DDSs (latanoprost punctal plug delivery system, L-PPDS), and intravitreal implants (I-vitaion™, NT-501, NT- 503, MicroPump, Thethadur, IB-20089 Verisome™, Cortiject, DE-102, Retisert™, Iluvein™ and Ozurdex™) have significantly improved the treatment of ocular diseases. However, most of these DDSs/devices are applied invasively and even need surgical procedures. Of these, use of de novo technologies such as advanced stimuli-responsive nanomaterials, multimodal nanosystems (NSs)/nanoconjugates (NCs), biomacromolecualr scaffolds, and bioengineered cell therapies need to be further advanced to get better compliance and higher clinical impacts.

CONCLUSION

Despite mankind successful battle on ocular diseases, our challenge will continue to battle the ocular disease that happen with aging. Yet, we need to understand the molecular aspects of eye diseases in a holistic way and develop ultimate treatment protocols preferably as non-invasive systems.

Topotecan Delivery to the Optic Nerve after Ophthalmic Artery Chemosurgery

Extraocular retinoblastoma is a major challenge worldwide, especially in developing countries. Current treatment involves the administration of systemic chemotherapy combined with radiation, but there is a clear need for improvement of chemotherapy bioavailability in the optic nerve. Our aim was to study the ophthalmic artery chemosurgery (OAC) local route for drug delivery assessing ocular and optic nerve exposure to chemotherapy and to compare it to exposure after intravenous infusion (IV) of the same dose in an animal model. Topotecan was used as a prototype drug that is active in retinoblastoma and based on the extensive knowledge of its pharmacokinetics in preclinical and clinical settings. Five Landrace pigs received 4mg of topotecan via OAC as performed in retinoblastoma patients. At the end of the infusion, the eyes were enucleated, the optic nerve and retina were dissected, and the vitreous and plasma were separated. After recovery and a wash-out period, the animals received a 30-min IV infusion of topotecan (4 mg). The remaining eye was enucleated and tissues and fluids were separated. All samples were stored until quantitation using HPLC. A significantly higher concentration of topotecan in the optic nerve, vitreous, and retina was obtained in eyes after OAC compared to IV infusion (p<0.05). The median (range) ratio between topotecan concentration attained after OAC to IV infusion in the optic nerve, retina and vitreous was 84(54-668), 143(49-200) and 246(56-687), respectively. However, topotecan systemic exposure after OAC and IV infusion remained comparable (p>0.05). The median optic nerve-to-plasma ratio after OAC and IV was 44 and 0.35, respectively. Topotecan OAC delivery attained an 80-fold higher concentration in the optic nerve compared to the systemic infusion of the same dose with similar plasma concentrations in a swine model. Patients with retinoblastoma extension into the optic nerve may benefit from OAC for tumor burden by increased chemotherapy bioavailability in the optic nerve without increasing systemic exposure or toxicity.

Neuroprotective and restorative effects of the brain-derived neurotrophic factor in retinal diseases

Brain-derived neurotrophic factor (BDNF) is a neurotrophin proposed to be implicated in ameliorating the course of some neurodegenerative disorders. Given the fact that retina is considered as an out-pouching of the central nervous system, its related diseases have long been suggested to receive protective influence from this signaling molecule. The role of BDNF in retinal neurorestoration, neuroprotection and oxidative stress has extensively been tested over the past two decades. Nonetheless, almost the entire related literature root in animal studies and clinical research on this topic is lacking. Although much of the evidence have validated the protective properties of BDNF against various retinal cell diseases, bringing such insights into clinical context would depend on further well-designed research. The present review is an attempt to categorize and discuss the available evidence with regard to the BDNF and retinal diseases.

Collagen cross-linking: when and how? A review of the state of the art of the technique and new perspectives

Since the late 1990s corneal crosslinking (CXL) has been proposed as a new possibility to stop progression of keratoconus or secondary corneal ectasia, with the promising aim to prevent progressive visual loss due to the evolution of the pathology and to delay or avoid invasive surgical procedures such as corneal transplantation. The possibility of strengthening corneal tissue by means of a photochemical reaction of corneal collagen by the combined action of Riboflavin and ultraviolet A irradiation (UVA), radically modified the conservative management of progressive corneal ectasia. This is a review of the state of the art of CXL, reporting basic and clinical evidence. The paper describes basic principles, advantages and limits of different CXL techniques and possible future evolution of the procedure.

Vitreous humor analysis for the detection of xenobiotics in forensic toxicology: a review

Vitreous humor (VH) is a gelatinous substance contained in the posterior chamber of the eye, playing a mechanical role in the eyeball. It has been the subject of numerous studies in various forensic applications, primarily for the assessment of postmortem interval and for postmortem chemical analysis. Since most of the xenobiotics present in the bloodstream are detected in VH after crossing the selective blood-retinal barrier, VH is an alternative matrix useful for forensic toxicology. VH analysis offers particular advantages over other biological matrices: it is less prone to postmortem redistribution, is easy to collect, has relatively few interfering compounds for the analytical process, and shows sample stability over time after death. The present study is an overview of VH physiology, drug transport and elimination. Collection, storage, analytical techniques and interpretation of results from qualitative and quantitative points of view are dealt with. The distribution of xenobiotics in VH samples is thus discussed and illustrated by a table reporting the concentrations of 106 drugs from more than 300 case reports. For this purpose, a survey was conducted of publications found in the MEDLINE database from 1969 through April 30, 2015.

Positively charged micelles based on a triblock copolymer demonstrate enhanced corneal penetration

PURPOSE

The cornea is a main barrier to drug penetration after topical application. The aim of this study was to evaluate the abilities of micelles generated from a positively charged triblock copolymer to penetrate the cornea after topical application.

METHODS

The triblock copolymer poly(ethylene glycol)-poly(ε-caprolactone)-g-polyethyleneimine was synthesized, and the physicochemical properties of the self-assembled polymeric micelles were investigated, including hydrodynamic size, zeta potential, morphology, drug-loading content, drug-loading efficiency, and in vitro drug release. Using fluorescein diacetate as a model drug, the penetration capabilities of the polymeric micelles were monitored in vivo using a two-photon scanning fluorescence microscopy on murine corneas after topical application.

RESULTS

The polymer was successfully synthesized and confirmed using nuclear magnetic resonance and Fourier transform infrared. The polymeric micelles had an average particle size of 28 nm, a zeta potential of approximately +12 mV, and a spherical morphology. The drug-loading efficiency and drug-loading content were 75.37% and 3.47%, respectively, which indicates that the polymeric micelles possess a high drug-loading capacity. The polymeric micelles also exhibited controlled-release behavior in vitro. Compared to the control, the positively charged polymeric micelles significantly penetrated through the cornea.

CONCLUSION

Positively charged micelles generated from a triblock copolymer are a promising vehicle for the topical delivery of hydrophobic agents in ocular applications.

Tear-mediated delivery of nanoparticles through transcytosis of the lacrimal gland

Rapid clearance from the tears presents a formidable obstacle to the delivery of peptide drugs to the eye surface. This impedes therapies for ocular infections, wound healing, and dry-eye disease that affect the vision of millions worldwide. To overcome this challenge, this manuscript explores a novel strategy to reach the ocular surface via receptor-mediated transcytosis across the lacrimal gland (LG), which produces the bulk of human tears. The LG abundantly expresses the coxsackievirus and adenovirus receptor (CAR); furthermore, we recently reported a peptide-based nanoparticle (KSI) that targets CAR on liver cells. This manuscript reports the unexpected finding that KSI both targets and transcytoses into the LG acinar lumen, which drains to tear ducts. When followed using ex vivo live cell imaging KSI rapidly accumulates in lumen formed by LG acinar cells. LG transduction with a myosin Vb tail, which is dominant negative towards transcytosis, inhibits lumenal accumulation. Transcytosis of KSI was confirmed in vivo by confocal and TEM imaging of LG tissue following administration of KSI nanoparticles. These findings suggest that it is possible to target nanomaterials to the tears by targeting certain receptors on the LG. This design strategy represents a new opportunity to overcome barriers to ocular delivery.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Ocular drug delivery nanowafer with enhanced therapeutic efficacy

Presently, eye injuries are treated by topical eye drop therapy. Because of the ocular surface barriers, topical eye drops must be applied several times in a day, causing side effects such as glaucoma, cataract, and poor patient compliance. This article presents the development of a nanowafer drug delivery system in which the polymer and the drug work synergistically to elicit an enhanced therapeutic efficacy with negligible adverse immune responses. The nanowafer is a small transparent circular disc that contains arrays of drug-loaded nanoreservoirs. The slow drug release from the nanowafer increases the drug residence time on the ocular surface and its subsequent absorption into the surrounding ocular tissue. At the end of the stipulated period of drug release, the nanowafer will dissolve and fade away. The in vivo efficacy of the axitinib-loaded nanowafer was demonstrated in treating corneal neovascularization (CNV) in a murine ocular burn model. The laser scanning confocal imaging and RT-PCR study revealed that once a day administered axitinib nanowafer was therapeutically twice as effective, compared to axitinib delivered twice a day by topical eye drop therapy. The axitinib nanowafer is nontoxic and did not affect the wound healing and epithelial recovery of the ocular burn induced corneas. These results confirmed that drug release from the axitinib nanowafer is more effective in inhibiting CNV compared to the topical eye drop treatment even at a lower dosing frequency.

Upregulation of P-glycoprotein expression by ophthalmic drugs in different corneal in-vitro models

OBJECTIVES

The purpose of this study was to analyse P-glycoprotein (P-gp) expression in different human in-vitro cornea models (HCE-T epithelial model and Hemicornea construct) after stimulation with P-gp substrates (rhodamine 123, levofloxacin and acebutolol).

METHODS

The influence of P-gp substrates on mRNA expression was analysed using reverse transcriptase polymerase chain reaction (PCR) and real-time PCR. The effect of stimulation on the transporter functionality was estimated with a digoxin efflux assay. The Caco-2 cell line was used as positive control.

KEY FINDINGS

The reverse transcriptase PCR results showed an increase in band intensity compared with the control medium for all substrates. The real-time PCR for the Caco-2 and HCE-T epithelial model yielded a similar outcome, in which all tested substrates upregulated P-gp. In contrast, the Hemicornea construct showed no significant increase in the mRNA expression after stimulation. Both in-vitro models possessed similar drug transport profiles after stimulation. A significantly increased efflux of digoxin was measured after 24 and 72 h of stimulation with levofloxacin and acebutolol.

CONCLUSIONS

The expression and functionality of the P-gp in corneal tissue can be influenced through time exposure with specific substrates. However, the exact mechanism still requires further elucidation.

Preparation and ocular pharmacokinetics of hyaluronan acid-modified mucoadhesive liposomes

The aim of this research was to formulate a liposomal preparation of DOX to be applied topically, and to investigate the in vitro and in vivo performance of the prepared liposomes. DOX liposomes were prepared by the solvent evaporation method, and then modified with bioadhesive material HA. Through MTT assay, we found that the safe concentration of liposomes delivered would hit 1 mg/mL. Cellular uptake studies showed that DOX liposomes coated with HA are much more targetable to cell nucleus. Their ocular pharmacokinetics in rabbits were investigated through the comparison with those obtained after dosing with non-modified liposomes and DOX solution. The in vitro transcorneal permeability of DOX in both kinds of liposomes was found to be slower than that of the solution because of sustained release. After in vivo instillation in rabbits, HA-modified liposomes had the longest retention time, following with naked liposomes. Significantly, the area under the curve of the aqueous humor concentration-time profiles of DOX liposomes was found to be 1.7-fold higher than that of DOX solution. The confocal experiment confirmed that HA-modified liposomes were able to maintain a higher DOX concentration and residence time than that of non-modified liposomes and free DOX. These results suggest that our liposomal preparation was of great help to improve the bioavailability of DOX.

Extended latanoprost release from commercial contact lenses: in vitro studies using corneal models

In this study, we compared, for the first time, the release of a 432 kDa prostaglandin F2a analogue drug, Latanoprost, from commercially available contact lenses using in vitro models with corneal epithelial cells. Conventional polyHEMA-based and silicone hydrogel soft contact lenses were soaked in drug solution (131 μg = ml solution in phosphate buffered saline). The drug release from the contact lens material and its diffusion through three in vitro models was studied. The three in vitro models consisted of a polyethylene terephthalate (PET) membrane without corneal epithelial cells, a PET membrane with a monolayer of human corneal epithelial cells (HCEC), and a PET membrane with stratified HCEC. In the cell-based in vitro corneal epithelium models, a zero order release was obtained with the silicone hydrogel materials (linear for the duration of the experiment) whereby, after 48 hours, between 4 to 6 μg of latanoprost (an amount well within the range of the prescribed daily dose for glaucoma patients) was released. In the absence of cells, a significantly lower amount of drug, between 0.3 to 0.5 μg, was released, (p <0:001). The difference observed in release from the hydrogel lens materials in the presence and absence of cells emphasizes the importance of using an in vitro corneal model that is more representative of the physiological conditions in the eye to more adequately characterize ophthalmic drug delivery materials. Our results demonstrate how in vitro models with corneal epithelial cells may allow better prediction of in vivo release. It also highlights the potential of drug-soaked silicone hydrogel contact lens materials for drug delivery purposes.

Pharmacokinetics of intravenously administered tigecycline in eye compartments: an experimental study

PURPOSE

The purpose of this study was to evaluate the ocular distribution of intravenously administered tigecycline in a rabbit uveitis model.

METHODS

Tigecycline, which has a broad spectrum of activity against many gram-positive, gram-negative, and anaerobic organisms, was given intravenously to rabbits at 7 mg/kg of body weight starting 24 h after induction of uveitis by intravitreal endotoxin injection. Tigecycline concentrations were determined by high performance liquid chromatography-mass spectrometry (LC-MS/MS) assay in the aqueous humor, vitreous humor, and plasma 1, 3, 6, and 24 h after administration of a single dose.

RESULTS

The maximum concentrations were found within 1 h after the end of the intravenously given tigecycline, and were 1,308.60 ± 301.76 ng/mL in plasma, 181.40 ± 51.32 ng/mL in vitreous humor and 145.00 ± 55.29 ng/mL in aqueous humor of the inflamed eye. After 24 h, no drug was detectable in the aqueous and vitreous of the normal eyes, whereas small amounts of drug were detectable in inflamed eyes and in plasma.

CONCLUSIONS

Tigecycline did not reach therapeutically significant levels in the aqueous and the vitreous humor of rabbit eyes. The findings suggest a limited role for intravenously administered tigecycline in the treatment of bacterial endophthalmitis.

Ocular cytochrome P450s and transporters: roles in disease and endobiotic and xenobiotic disposition

Drug metabolism and transport processes in the liver, intestine and kidney that affect the pharmacokinetics and pharmacodynamics of therapeutic agents have been studied extensively. In contrast, comparatively little research has been conducted on these topics as they pertain to the eye. Recently, however, catalytic functions of ocular cytochrome P450 enzymes have gained increasing attention, in large part due to the roles of CYP1B1 and CYP4V2 variants in primary congenital glaucoma and Bietti's corneoretinal crystalline dystrophy, respectively. In this review, we discuss challenges to ophthalmic drug delivery, including Phase I drug metabolism and transport in the eye, and the role of three specific P450s, CYP4B1, CYP1B1 and CYP4V2 in ocular inflammation and genetically determined ocular disease.

Planar microdevices enhance transport of large molecular weight molecules across retinal pigment epithelial cells

Large molecular weight drug delivery to the posterior eye is challenging due to cellular barriers that hinder drug transport. Understanding how to enhance transport across the retinal barrier is important for the design of new drug delivery systems. A novel mechanism to enhance drug transport is the use of geometric properties, which has not been extensively explored in the retina. Planar SU-8/Poly(ethyleneglycol)dimethacrylate microdevices were constructed using photolithography to deliver FITC dextran across an in vitro retinal model. The model consists of retinal pigment epithelial (RPE) cells grown to confluence on transwell inserts, which provides an environment to investigate the influence of geometry on paracellular and transcellular delivery of encapsulated large molecules. Planar microdevices enhanced transport of large molecular weight dextrans across different models of RPE in a size dependent fashion. Increased drug permeation across the RPE was observed with the addition of microdevices as compared to a traditional bolus of FITC dextran. This phenomena was initiated by a non-toxic interaction between the microdevices and the retinal tight junction proteins. Suggesting that increased drug transport occurs via a paracellular pathway. These experiments provide evidence to support the future use of planar unidirectional microdevices for delivery of biologics in ocular applications.

Nanocarriers of nanotechnology in retinal diseases

We are approaching a new era of retinal pharmacotherapy where new drugs are rapidly being worked out for the treatment of posterior-segment disease. Recent development in ocular drug delivery systems research has provided new insights into drug development, and the use of nanoparticles for drug delivery is thus a promising excellent approach for advanced therapy of ocular diseases. The primary goal is to develop a variety of drug delivery systems to complement and further enhance the efficacy of the available new medications. The ideal sustained release technology will provide a high level of safety with continuous release over an extended period of time while maintaining almost total drug bioactivity. The use of nanocarriers, such as cyclodextrin nanoparticle suspension, liposomes, nanospheres and, nanoemulsions for gene therapy of retinal diseases has been highlighted in this review.

A polymeric device for controlled transscleral multi-drug delivery to the posterior segment of the eye

The design of drug delivery systems that can deliver multiple drugs to the posterior segment of the eye is a challenging task in retinal disease treatments. We report a polymeric device for multi-drug transscleral delivery at independently controlled release rates. The device comprises a microfabricated reservoir, controlled-release cover and three different fluorescent formulations, which were made of photopolymeized tri(ethyleneglycol)dimethacrylate (TEGDM) and poly(ethyleneglycol)dimethacrylate (PEGDM). The release rate of each fluorescent is controlled by varying the PEGDM/TEGDM ratio in its formulation and the cover. The release kinetics appeared to be related to the swelling ratio of the PEGDM/TEGDM polymers. When the devices were implanted onto rat sclerae, fluorescence was observable in the ocular tissues during 4 weeks' implantation and distributed locally around the implantation site. Our polymeric system, which can administer multiple compounds with distinct kinetics, provides prolonged action and less invasive transscleral administration, and is expected to provide new tools for the treatment of posterior eye diseases with new therapeutic modalities.

Anterior eye segment drug delivery systems: current treatments and future challenges

New technologies for delivery of drugs, such as small molecules and biologics, are of growing interest among clinical and pharmaceutical researchers for use in treating anterior segment eye disease. The challenge is to deliver effective drugs at therapeutic concentrations to the targeted ocular tissue with minimal side effects. To achieve this, a better understanding of the unmet needs, what is required of the various methods of delivery to achieve successful delivery, and the potential challenges of anterior segment drug delivery is necessary and the primarily aim of this review. This review covers the various physiological and anatomical barriers that exist for effective delivery to the targeted tissue of the eye, the pathological conditions of the anterior segment, and the unmet needs for treatment of these ocular diseases. Second, it reviews the novel delivery technologies that have the potential to maintain and/or improve the drug's therapeutic index and improving both patient adherence for chronic therapy and potential patient outcomes. This review bridges the pharmaceutical and clinical research/challenges and provides a detailed overview of anterior segment drug delivery accomplishments thus far, for researchers and clinicians.

Impaired hepatic and intestinal ATP-binding cassette transporter G5/8 was associated with high exposure of β-sitosterol and the potential risks to blood-brain barrier integrity in diabetic rats

OBJECTIVES

Plant sterols are thought to treat hypercholesterolemia via inhibiting intestinal cholesterol absorption. The aim of this study was to evaluate the contribution of impaired ATP-binding cassette transporter G5/8 (ABCG5/8) expression by diabetes to the increased β-sitosterol (BS) exposure and impact of increased BS on integrity of blood-brain barrier (BBB).

METHODS

Basal BS level in tissues of streptozotocin-inducted rats and ABCG5/8 protein levels in liver and intestine were investigated; pharmacokinetics of BS was studied following oral dose; and primarily cultured rat brain microvessel endothelial cells (rBMECs) were used to study BS transportation across BBB and effect of BS on BBB integrity.

KEY FINDINGS

Diabetic rats showed greatly upgraded basal levels of BS in plasma, intestine, cerebral and hippocampus, accompanied by impairment of ABCG5/8 protein expression in liver and intestine. Pharmacokinetics studies demonstrated higher AUC0-48 and Cmax , and lower faecal recoveries of BS after oral administration, indicating enhancement of absorption or efflux impairment. In-vitro data showed increased ratio of BS/cholesterol in high levels BS-treated rBMECs was associated with increased BBB permeability of some biomarkers including BS itself.

CONCLUSIONS

Impaired ABCG5/8 protein expression by diabetes led to increase in BS exposure, which may be harmful to BBB function.

A review of topically administered mini-tablets for drug delivery to the anterior segment of the eye

OBJECTIVES

The human eye is a unique and intricate structure which has made drug delivery to the eye a formidable undertaking. Anterior-segment eye diseases are ubiquitous, especially among elderly patients, and conventional eye drops, although a first-choice dosage form, are not always an efficient treatment option. The development of novel drug delivery systems for improved treatment is therefore imperative.

KEY FINDINGS

In an attempt to circumvent the obstacles presented by the structure of the eye, advanced systems such as ocular mini-tablets have been developed. In this review, a concerted effort has been made to provide a detailed overview of topically administered ocular mini-tablets and other solid devices for drug delivery to the anterior segment of the eye. These mini-tablets have been shown in vitro and in vivo to have significant advantages in comparison with liquid preparations. This is a step toward attaining better patient convenience and compliance, which are critical factors.

SUMMARY

Solid ophthalmic dosage forms have several advantages that can contribute to assisting with patient compliance and, ultimately, effective disease treatment. In addition to the challenges associated with topical ocular drug delivery, the shortcomings of conventional eye drops, advantages of mini-tablets, and improvements to date to these systems are discussed. The requirement for further advancements in the ocular field is also emphasized.

Toward the practical implementation of eye-related bioavailability prediction models

The development and registration of reformulated ophthalmic products (OPs) requires eye-related bioavailability (BA) assessments. Common BA algorithms associated with other routes of application, such as the oral route, cannot be easily applied to eye-related BA testing. Here, we provide an analysis of the current literature and suggestions for further directions in the development of high-capacity, cost-effective, and highly predictive nonclinical models of eye-related drug BA. One, or a combination of these models, has the potential for routine use in research laboratories and/or the pharmaceutical industry to overcome various obstacles in reformulated OP development and registration.

Serum TRPM1 autoantibodies from melanoma associated retinopathy patients enter retinal on-bipolar cells and attenuate the electroretinogram in mice

Melanoma-associated retinopathy (MAR) is a paraneoplastic syndrome associated with cutaneous malignant melanoma and the presence of autoantibodies that label neurons in the inner retina. The visual symptoms and electroretinogram (ERG) phenotype characteristic of MAR resemble the congenital visual disease caused by mutations in TRPM1, a cation channel expressed by both melanocytes and retinal bipolar cells. Four serum samples from MAR patients were identified as TRPM1 immunoreactive by 1. Labeling of ON-bipolar cells in TRPM1+/+ but not TRPM1−/− mouse retina, 2. Labeling of TRPM1-transfected CHO cells; and 3. Attenuation of the ERG b-wave following intravitreal injection of TRPM1-positive MAR IgG into wild-type mouse eyes, and the appearance of the IgG in the retinal bipolar cells at the conclusion of the experiment. Furthermore, the epitope targeted by the MAR autoantibodies was localized within the amino-terminal cytoplasmic domain of TRPM1. Incubation of live retinal neurons with TRPM1-positive MAR serum resulted in the selective accumulation of IgG in ON-bipolar cells from TRPM1+/+ mice, but not TRPM1−/− mice, suggesting that the visual deficits in MAR are caused by the uptake of TRPM1 autoantibodies into ON-bipolar cells, where they bind to an intracellular epitope of the channel and reduce the ON-bipolar cell response to light.

Potential pharmacokinetic role of organic cation transporters in modulating the transcorneal penetration of its substrates administered topically

PURPOSE

We hypothesize organic cation transporters (OCT) may have a potential role in determining the pharmacokinetics and toxicity of organic cation drugs applied topically. Hence, in the present in vivo study, we attempted to evaluate the role of OCT in modulating the transport of its substrates after topical application.

METHODS

New Zealand albino rabbits of either sex were used. Transcorneal penetration of OCT substrates tetraethylammonium and metformin after single instillation was evaluated in the absence and presence of OCT blockers (quinidine and atropine). Aqueous humor (AH) samples were collected through paracentesis amounting to 70-100 μl under topical anesthesia at various time intervals. The samples were subjected for estimation of both substrate as well as blocker concentrations using liquid chromatography mass spectrometry.

RESULTS

Topical pre-treatment (30 min before substrate) of OCT blockers significantly decreased the transcorneal penetration of OCT substrates after single topical administration. The levels of blockers reaching AH in the presence of substrates were also modulated at 60 min after its administration as compared with its control.

CONCLUSION

OCT are functionally active in the uptake of their substrates from tear to AH. Therefore, OCT in the corneal epithelium may be positioned from apical to basolateral. When administering their substrates/blockers topically, both may be competing for OCT for their uptake across the cornea, thereby decreasing the corneal penetration. Hence OCT can have a potential pharmacokinetic role in modulating the ocular bioavailability of their substrates administered topically, which are used as ocular therapeutics.

Molecular expression and functional activity of efflux and influx transporters in hypoxia induced retinal pigment epithelial cells

A decrease in tissue oxygen levels (aka hypoxia) mediates a number of vascular retinal diseases. Despite introduction of novel therapeutics, treatment of retinal disorders remains challenging, possibly due to complex nature of hypoxia signaling. To date, the differential effect of hypoxia on expression of efflux and influx transporters in retinal cells has not been studied. Therefore, the objective of this study was to delineate molecular and functional expression of membrane transporters in human retinal pigment epithelial (RPE) cells cultured under normoxic and hypoxic conditions. Quantitative real time polymerase chain reaction (qPCR), ELISA and immunoblot analysis were performed to examine the RNA and protein expression levels of transporters. Further, functional activity was evaluated by performing the uptake of various substrates in both normoxic and hypoxic conditions. qPCR analysis showed elevated expression of efflux transporters (P-glycoprotein, multidrug resistant protein 2, breast cancer resistant protein) and influx transporters (folate receptor-α, cationic and neutral amino acid transporter, sodium dependent multivitamin transporter) in a time dependent manner. Immunoblot analysis further confirmed elevated expression of breast cancer resistant protein and sodium dependent multivitamin transporter. A decrease in the uptake of efflux transporter substrates (digoxin, lopinavir and abacavir) and enhanced uptake of influx transporter substrates (arginine, folic acid and biotin) in hypoxia relative to normoxia further confirmed elevated expression of transporters, respectively. This study demonstrates for the first time that hypoxic conditions may alter expression of efflux and influx transporters in RPE cells. These findings suggest that hypoxia may further alter disposition of ophthalmic drugs.

Ocular drug delivery systems: An overview

The major challenge faced by today’s pharmacologist and formulation scientist is ocular drug delivery. Topical eye drop is the most convenient and patient compliant route of drug administration, especially for the treatment of anterior segment diseases. Delivery of drugs to the targeted ocular tissues is restricted by various precorneal, dynamic and static ocular barriers. Also, therapeutic drug levels are not maintained for longer duration in target tissues. In the past two decades, ocular drug delivery research acceleratedly advanced towards developing a novel, safe and patient compliant formulation and drug delivery devices/techniques, which may surpass these barriers and maintain drug levels in tissues. Anterior segment drug delivery advances are witnessed by modulation of conventional topical solutions with permeation and viscosity enhancers. Also, it includes development of conventional topical formulations such as suspensions, emulsions and ointments. Various nanoformulations have also been introduced for anterior segment ocular drug delivery. On the other hand, for posterior ocular delivery, research has been immensely focused towards development of drug releasing devices and nanoformulations for treating chronic vitreo-retinal diseases. These novel devices and/or formulations may help to surpass ocular barriers and associated side effects with conventional topical drops. Also, these novel devices and/or formulations are easy to formulate, no/negligibly irritating, possess high precorneal residence time, sustain the drug release, and enhance ocular bioavailability of therapeutics. An update of current research advancement in ocular drug delivery necessitates and helps drug delivery scientists to modulate their think process and develop novel and safe drug delivery strategies. Current review intends to summarize the existing conventional formulations for ocular delivery and their advancements followed by current nanotechnology based formulation developments. Also, recent developments with other ocular drug delivery strategies employing in situ gels, implants, contact lens and microneedles have been discussed.