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.

Ocular drug delivery for glaucoma management.

Current glaucoma management modalities are hindered by low patient compliance and adherence. This can be due to highly complex treatment strategies or poor patient understanding. Treatments focus on the management or reduction of intraocular pressure. This is most commonly done through the use of daily topical eye drops. Unfortunately, despite effective therapies, glaucoma continues to progress, possibly due to patients not adhering to their treatments. In order to mitigate these patient compliance issues, many sustained release treatments are being researched and are entering the clinic. Conjunctival, subconjunctival, and intravitreal inserts, punctal plugs, and drug depots are currently in clinical development. Each delivery system has hurdles, yet shows promise and could potentially mitigate the current problems associated with poor patient compliance.

The capsule drug device: novel approach for drug delivery to the eye.

Treatment of age-macular degeneration requires monthly intravitreal injections, which are costly and have serious risks. The objective of this study was to develop a novel intraocular implant for drug delivery. The capsule drug ring is a reservoir inserted in the lens capsule during cataract surgery, refillable and capable of delivering multiple drugs. Avastin was the drug of interest in this study. Prototypes were manufactured using polymethylmethacrylate sheets as the reservoir material, a semi-permeable membrane for controlled delivery and silicone check valves for refilling. The device showed near zero-order release kinetics and Avastin stability was investigated with accelerated temperature studies.

Transscleral iontophoretic and intravitreal delivery of a macromolecule: study of ocular distribution in vivo and postmortem with MRI.

The distribution and clearance of macromolecules in ocular delivery are not well understood. It has been hypothesized that iontophoresis can enhance transscleral delivery of macromolecules. The objective of this study was to investigate the ocular distribution of a macromolecule after transscleral iontophoretic delivery and intravitreal injection in vivo using nuclear magnetic resonance imaging (MRI) and to compare these results. Experiments of constant current transscleral iontophoresis of 4mA or intravitreal injection were performed on New Zealand white rabbits in vivo. Iontophoresis experiments were also performed on rabbits postmortem. Galbumin (Gd-labeled albumin) was the model permeant surrogate to clinical therapeutic agents. MRI was used to monitor the distribution of the molecule in the eye after ocular iontophoresis and intravitreal injection. In addition, the conjunctiva, sclera, choroid, and retina were extracted in the transscleral iontophoresis study to determine the amounts of Galbumin in these tissues using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results show that iontophoresis enhanced the ocular delivery of Galbumin. The macromolecule was mainly delivered into the conjunctiva and sclera in microgram quantities and then diffused towards the posterior section in the upper hemisphere of the eye in vivo. Both in vivo and postmortem studies show that the iontophoretic delivery of Galbumin into the vitreous was below the detection limit. In the intravitreal injection study, the diffusion coefficient of Galbumin in the vitreous humor was estimated to be close to that of free aqueous diffusion.

Iontophoretic transport across a multiple membrane system.

The objective of the present study was to investigate the iontophoretic transport behavior across multiple membranes of different barrier properties. Spectra/Por(R) (SP) and Ionac membranes were the synthetic membranes and sclera was the biomembrane in this model study. The barrier properties of SP membranes were determined individually in passive and iontophoresis transport experiments with tetraethylammonium ion (TEA), chloride ion (Cl), and mannitol as the model permeants. Passive and iontophoretic transport experiments were then conducted with an assembly of SP membranes. The contribution of electroosmosis to iontophoresis was assessed using the mannitol data. Model analysis was performed to study the contribution of diffusion and electromigration to electrotransport across the multiple membrane system. The effects of membrane barrier thickness upon ion-exchange membrane-enhanced iontophoresis were examined with Ionac, SP, and sclera. The present study shows that iontophoretic transport of TEA across the membrane system was related to the thicknesses and permeability coefficients of the membranes and the electromobilities of the permeant across the individual membranes in the assembly. Model analysis suggests significant contribution of diffusion within the membranes across the membrane system, and this mechanism is relatively independent of the current density applied across the system in iontophoresis dominant transport.

Examination of barriers and barrier alteration in transscleral iontophoresis.

The flux enhancing mechanisms of transscleral iontophoresis are not well understood. The objective of the present study was to investigate the ocular barrier and barrier alterations in transscleral iontophoretic delivery with magnetic resonance imaging (MRI). Experiments involving constant current transscleral iontophoresis of 2 mA (current density 10 mA/cm(2)) and subconjunctival injection were conducted with rabbits in vivo and postmortem and with excised sclera in side-by-side diffusion cells in vitro. The postmortem and in vitro experiments were expected to be helpful in clarifying the importance of vascular clearance and other transport barriers in transscleral iontophoresis. Manganese ion (Mn(2+)) and manganese ethylenediaminetetraacetic acid complex (MnEDTA(2-)) were the model permeants. The results show that pretreatment of the eye with an electric field by iontophoresis enhanced subconjunctival delivery of the permeants to the anterior segment of the eye in vivo. This suggests that electric field-induced barrier alterations can be an important absorption enhancing mechanism of ocular iontophoresis. Penetration enhancement was magnified in the postmortem experiments with larger amounts of the permeants delivered into the eye and to the back of the eye. The different results observed in the in vivo and postmortem studies can be attributed to ocular clearance in ocular delivery.

Examination of penetration routes and distribution of ionic permeants during and after transscleral iontophoresis with magnetic resonance imaging.

Previously, transscleral and transcorneal iontophoretic delivery was studied and compared to passive delivery and intravitreal injection using nuclear magnetic resonance imaging (MRI). The objective of the present study was to employ MRI to further investigate the factors affecting transscleral iontophoretic delivery. In the present study, anodal and cathodal constant current transscleral iontophoresis were conducted with excised sclera in side-by-side diffusion cells in vitro and with rabbits in vivo. The total current and duration of application were 2 and 4mA (current density 10 and 20mA/cm(2)) and 20-60min, respectively. The delivery and distribution of the model permeants manganese ion (Mn(2+)) and manganese ethylenediaminetetraacetic acid complex (MnEDTA(2-)) into the eye during iontophoresis were determined with MRI and compared with the results obtained in previous studies of subconjunctival injection and passive delivery. Both anodal and cathodal iontophoresis provided significant enhancement in ocular delivery compared to passive transport in the in vitro and in vivo experiments. Transscleral iontophoretic delivery was related to the position and duration of the iontophoresis application in vivo. Permeants were observed to be delivered primarily into the anterior segment of the eye when the pars plana was the application site. Extending the duration of iontophoresis at this site allowed the permeants to be delivered into the vitreous more deeply and to a greater extent than when the application site was at the back of the eye near the fornix. The present results show that electrode placement was an important factor in transscleral iontophoresis, and the ciliary body (pars plana) was determined to be the pathway of least resistance for iontophoretic transport. These new findings continue to support the utility of MRI as a noninvasive technique in ocular drug delivery research and testing.

Assessment of subconjunctival delivery with model ionic permeants and magnetic resonance imaging.

PURPOSE: The objective was to assess the permeation and clearance of model ionic permeants after subconjunctival injection with nuclear magnetic resonance imaging (MRI). METHODS: New Zealand white rabbit was the animal model and manganese ion (Mn2+) and manganese ethylenediaminetetraacetic acid complex (MnEDTA2-) were the model permeants. The current study was divided into three parts: in vitro, postmortem, and in vivo. Transscleral passive permeation experiments were conducted with excised sclera in side-by-side diffusion cells in vitro. Subconjunctival delivery experiments were conducted with rabbits postmortem and in vivo. The distribution and elimination of the probe permeants from the subconjunctival space after subconjunctival injections were determined by MRI. RESULTS: The data of excised sclera in vitro suggest large effective pore size for transscleral transport and negligible pore charge effects upon the permeation of the ionic permeants. The permeability coefficients of Mn2+ and MnEDTA2- across the sclera in vitro were 3.6 x 10(-5) cm/s and 2.4 x 10(-5) cm/s, respectively. Although relatively high sclera permeability was observed in vitro, subconjunctival injections in vivo did not provide significant penetration of Mn2+ and MnEDTA2- into the globe; permeant concentrations in the eye were below the detection limit, which corresponds to less than 0.05% of the concentration of the injection solution (e.g., less than 0.02 mM when 40 mM injection solution was used). The volume of the subconjunctival pocket and the concentration of the permeants in the pocket were observed to decrease with time after the injection, and this could contribute to the lower than expected subconjunctival absorption in vivo. Different from the results in vivo, experiments with rabbits postmortem show significant penetration of Mn2+ and MnEDTA2- into the globe with the permeants primarily delivered into the anterior segment of the eye. This difference suggests blood vasculature clearance as a main barrier for passive transscleral transport. The data also show that the pars plicata/pars plana is the least resistance pathway for passive transscleral drug delivery of the polar permeants, and there are indications of the presence of another barrier, possibly the retinal epithelium and/or Bruch's membrane, at the back of the eye. CONCLUSIONS: Subconjunctival delivery of the ionic permeants in vivo cannot be quantitatively predicted by the in vitro results. MRI is a noninvasive complementary technique to traditional pharmacokinetic methods. It can provide insights into ocular pharmacokinetics without permeant redistribution that can occur in surgical procedure postmortem in traditional pharmacokinetic studies when the blood vasculature barrier is absent.