Transscleral drug delivery to the posterior eye: prospects of pharmacokinetic modeling

Application of Hydrogel Template Strategy in Ocular Drug Delivery

The hydrogel template strategy was previously developed to fabricate homogeneous polymeric microparticles. Here, we demonstrate the versatility of the hydrogel template strategy for the development of nanowafer-based ocular drug delivery systems. We describe the fabrication of dexamethasone-loaded nanowafers using polyvinyl alcohol and the instillation of a nanowafer on a mouse eye. The nanowafer, a small circular disk, is placed on the ocular surface, and it releases a drug as it slowly dissolves over time, thus increasing ocular bioavailability and enhancing efficiency to treat eye injuries.

Applications of polymers in intraocular drug delivery systems

We are entering a new era of ophthalmic pharmacology where new drugs are rapidly being developed for the treatment of anterior and posterior segment of the eye disease. The pharmacokinetics of drug delivery to the eye remains a very active area of ophthalmic research. Intraocular drug delivery systems allow the release of the drug, bypassing the blood-ocular barrier. The main advantage of these preparations is that they can release the drug over a long time with one single administration. These pharmaceutical systems are of great important in the treatment of the posterior segment diseases, and they can be prepared from biodegradable or nonbiodegradable polymers. Biodegradable polymers have the advantage of disappearing from the site of action after releasing the drug. The majority of intraocular devices are prepared from nonbiodegradable polymers, and they can release controlled amounts of drugs for months. Nonbiodegradable polymers include silicone, polyvinyl alcohol, and ethylene-vinyl acetate. The polymers usually employed to prepare nanoparticles for the topical ophthalmic route are poly (acrylic acid) derivatives (polyalquilcyanocrylates), albumin, poly-ε-caprolactone, and chitosan. Dendrimers are a recent class of polymeric materials with unique nanostructure which has been studied to discover their role in the delivery of therapeutics and imaging agents. Hydrogels are polymers that can swell in aqueous solvent system, and they hold the solvents in a swollen cross-linked gel for delivery. This review exhibits the current literature regarding applications of polymers in ophthalmic drug delivery systems including pharmacokinetics, advantages, disadvantages, and indications aimed to obtain successful eye therapy.

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.

Chitosan-coated PLGA nanoparticles of bevacizumab as novel drug delivery to target retina: optimization, characterization, and in vitro toxicity evaluation

In several ocular diseases, the vascular endothelial growth factor (VEGF) level has been found to be upregulated. Bevacizumab, an anti-VEGF drug, is the most commonly used off level drug for these conditions. Delivery of drug to the posterior site is desired for the effective management of these diseases. The present study was to develop and optimize the chitosan (CS)-coated poly(lactide-co-glycolic acid) (PLGA) nanoparticles (NPs) of bevacizumab for sustained and effective delivery to posterior ocular tissues. NPs were prepared by double emulsion solvent evaporation method and optimized for various variables (i.e., CS concentration, PLGA content, polyvinyl alcohol (PVA) concentration, and sonication time) by employing a 4-factor 3-level Box-Behnken statistical design. NPs were characterized for particle size, polydispersity index (PDI), entrapment efficiency (EE), and in vitro release. Transscleral flux was determined through goat sclera, and ocular tolerance assay was done by Hen's Egg Test chorioallantoic membrane method. The particle size and PDI of the optimized NPs were 222.28 ± 7.45 nm and 0.19 ± 0.08, respectively. The developed NPs showed an EE of 69.26 ± 1.31% with an extended release profile. The flux was significantly higher that is, 0.3204 ± 0.026 μg/cm²/h for the NPs compared to drug solution. Thus, CS-coated PLGA NPs can be potentially useful as ocular drug carriers to target retina.

Protective effects of sustained transscleral unoprostone delivery against retinal degeneration in S334ter rhodopsin mutant rats

It has been suggested that unoprostone isopropyl (UNO) has potent neuroprotective activity in the retina. The effect of sustained transscleral UNO delivery to the posterior segment of the eye on photoreceptor degeneration was evaluated. UNO was loaded into a device made of poly(ethyleneglycol) dimethacrylate by polydimethylsiloxane mold-based UV-curing. The amount of UNO diffusing from these devices was measured using high-performance liquid chromatography. The polymeric devices that released UNO at 1.8 μg/day were implanted on the sclerae of S334ter rats at postnatal 21 days, and electroretinograms (ERGs) were compared with those of topical application and placebo devices. Retinal thickness was evaluated by histological examination. Western blots of specimens 4 weeks after implantation were performed. ERGs showed that the UNO-loaded device prevented the reduction of ERG amplitudes 2 and 4 weeks after implantation, compared with results using a placebo device or topical application. Histological examination showed that the UNO-loaded device prevented the reduction of retinal thickness, and Western blots of specimens indicated that the UNO-loaded device decreased expression of ERK1/2, phosphorylated ERK1/2, and caspase-3. A device that provided sustained UNO administration protected against retinal degeneration in rhodopsin mutant rats, and thus, may have translational potential as a sustainable method to administer drugs to treat retinitis pigmentosa. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1730-1737, 2016.

Antiangiogenic effects of topically administered multiple kinase inhibitor, motesanib (AMG 706), on experimental choroidal neovascularization in mice

PURPOSE

To investigate the effect of topical motesanib, an inhibitor of receptor tyrosine kinase, on experimental choroidal neovascularization (CNV).

METHODS

CNV was induced in 46 nine-week-old male C57BL/6 mice using fundus laser photocoagulation. The right eye of each mouse was treated with motesanib eye drop (4 times daily) and the left eye with vehicle eye drop (4 times daily) for 14 days. To evaluate changes in the CNV lesions, fluorescein angiography, immunofluorescence staining with CD34, and histological examinations were performed 14 days after CNV induction. The expression of phosphorylated extracellular signal-regulated kinase (ERK1/2) in choroidal tissues was determined using western blot analysis to demonstrate the inhibitory effect of topically administered motesanib on intracellular signaling pathways involved in CNV development.

RESULTS

Fluorescein angiography showed that fluorescence leakage in eyes treated with topical motesanib was significantly less than in mice treated with vehicle (P=0.01). On immunofluorescence staining, the CD34-labeled area was smaller in topical motesanib-treated eyes (P<0.001). The expression level of phosphorylated ERK1/2 relative to that of total ERK1/2 decreased in eyes treated with topical motesanib compared with eyes treated with vehicle.

CONCLUSION

Topical motesanib significantly reduced laser-induced CNV in the experimental mouse model.

Permeation of proteins, oligonucleotide and dextrans across ocular tissues: experimental studies and a literature update

Proteins and oligonucleotides represent powerful tools for the treatment of several ocular diseases, affecting both anterior and posterior eye segments. Despite the potential of these compounds, their administration remains a challenge. The last years have seen a growing interest for the noninvasive administration of macromolecular drugs, but still there is only little information of their permeability across the different ocular barriers. The aim of this work was to evaluate the permeation of macromolecules of different size, shape and charge across porcine ocular tissues such as the isolated sclera, the choroid Bruch's membrane and the cornea, both intact and de-epitelialized. Permeants used were two proteins (albumin and cytochrome C), an oligonucleotide, two dextrans (4 and 40 kDa) and a monoclonal antibody (bevacizumab). Obtained data and its comparison with the literature highlight the difficulties in predicting the behavior of macromolecules based on their physicochemical properties, because the interplay between the charge, molecular radius and conformation prevent their analysis separately. However, the data can be of great help for a rough evaluation of the feasibility of a noninvasive administration and for building computational models to improve understanding of the interplay among static, dynamic and metabolic barriers in the delivery of macromolecules to the eye.

A platform for controlled dual-drug delivery to the retina: protective effects against light-induced retinal damage in rats

Controlled transscleral co-delivery of two drugs, edaravone (EDV) and unoprostone (UNO), using a platform that comprises a microfabricated reservoir, controlled-release cover, and drug formulations, which are made of photopolymerized poly(ethyleneglycol) dimethacrylates, shows synergistic retinal neuroprotection against light injury in rats when compared with single-drug-loaded devices. The device would offer a safer therapeutic method than intravitreal injections for retinal disease treatments.

In vitro and in vivo release characteristics of Tacrolimus (FK506) from an episcleral drug-delivery implant

PURPOSE

To investigate the in vitro and in vivo release characteristics of Tacrolimus (FK506) from an episcleral drug-delivery implant.

METHODS

For in vitro experiments, Tacrolimus-loaded implants (0.5 mL; at concentrations of 0.25, 0.5, and 1.0 mg/mL) were immersed in a balanced salt solution. Samples of the surrounding liquid were aspirated at different times over a 96-h period. For in vivo experiments, the experimental group received an implant loaded with Tacrolimus (0.5 mg/mL; 0.5 mL); the control group was given a subconjunctival injection of 0.5 mL Tacrolimus (0.5 mg/mL). On postoperative days 3, 7, 14, 28, and 56, 3 animals were sacrificed, and their eyes were enucleated. Tacrolimus concentrations were determined by liquid chromatographic-tandem mass spectrometry. Ocular toxicity was evaluated by slit-lamp photography, fundus photography, intraocular pressure (IOP), and histology.

RESULTS

The implants released Tacrolimus in a biphasic pattern for 96 h in the in vitro study. The release kinetics were not dependent on the drug concentrations. The in vivo study showed statistically significant differences between the 2 treatment groups. Tacrolimus levels were particularly high in the conjunctiva, iris, ciliary body, cornea, sclera, choroid, and retina in the experimental group, while concentrations were low and only lasted for 1 week in the controls. Slit-lamp photography, fundus photography, IOP, and histology showed no evidence of toxic effects.

CONCLUSIONS

The episcleral drug-delivery implant mechanically released Tacrolimus through the apertures of capsules and, consequently, may be a promising drug vehicle for the treatment of immune-mediated ocular disorders.

Computational modeling of drug distribution in the posterior segment of the eye: effects of device variables and positions

A computational model was developed to simulate drug distribution in the posterior segment of the eye after intravitreal injection and ocular implantation. The effects of important factors in intravitreal injection such as injection time, needle gauge and needle angle on the ocular drug distribution were studied. Also, the influences of the position and the type of implant on the concentration profile in the posterior segment were investigated. Computational Fluid Dynamics (CFD) calculations were conducted to describe the 3D convective-diffusive transport. The geometrical model was constructed based on the human eye dimensions. To simulate intravitreal injection, unlike previous studies which considered the initial shape of the injected drug solution as a sphere or cylinder, the more accurate shape was obtained by level-set method in COMSOL. The results showed that in intravitreal injection the drug concentration profile and its maximum value depended on the injection time, needle gauge and penetration angle of the needle. Considering the actual shape of the injected solution was found necessary to obtain the real concentration profile. In implant insertion, the vitreous cavity received more drugs after intraocular implantation, but this method was more invasive compared to the periocular delivery. Locating the implant in posterior or anterior regions had a significant effect on local drug concentrations. Also, the shape of implant influenced on concentration profile inside the eye. The presented model is useful for optimizing the administration variables to ensure optimum therapeutic benefits. Predicting and quantifying different factors help to reduce the possibility of tissue toxicity and to improve the treatment efficiency.

Nanotherapy for posterior eye diseases

It is assumed that more than 50% of the most enfeebling ocular diseases have their origin in the posterior segment. Furthermore, most of these diseases lead to partial or complete blindness, if left untreated. After cancer, blindness is the second most dreaded disease world over. However, treatment of posterior eye diseases is more challenging than the anterior segment ailments due to a series of anatomical barriers and physiological constraints confronted for delivery to this segment. In this regard, nanostructured drug delivery systems are proposed to defy ocular barriers, target retina, and act as permeation enhancers in addition to providing a controlled release. Since an important step towards developing effective treatment strategies is to understand the course or a route a drug molecule needs to follow to reach the target site, the first part of the present review discusses various pathways available for effective delivery to and clearance from the posterior eye. Promise held by nanocarrier systems, viz. liposomes, nanoparticles, and nanoemulsion, for effective delivery and selective targeting is also discussed with illustrative examples, tables, and flowcharts. However, the applicability of these nanocarrier systems as self-administration ocular drops is still an unrealized dream which is in itself a huge technological challenge.

Virtual pharmacokinetic model of human eye

A virtual pharmacokinetic 3D model of the human eye is built using Comsol Multiphysics® software, which is based on the Finite Element Method (FEM). The model considers drug release from a polymer patch placed on sclera. The model concentrates on the posterior part of the eye, retina being the target tissue, and comprises the choroidal blood flow, partitioning of the drug between different tissues and active transport at the retina pigment epithelium (RPE)-choroid boundary. Although most straightforward, in order to check the mass balance, no protein binding or metabolism is yet included. It appeared that the most important issue in obtaining reliable simulation results is the finite element mesh, while time stepping has hardly any significance. Simulations were extended to 100,000 s. The concentration of a drug is shown as a function of time at various points of retina, as well as its average value, varying several parameters in the model. This work demonstrates how anybody with basic knowledge of calculus is able to build physically meaningful models of quite complex biological systems.

Vitreous pharmacokinetics and bioavailability of memantine after subtenon, intravenous, and intravitreal administration in rabbits

PURPOSE

This study evaluated the vitreous pharmacokinetics and vitreous bioavailability of memantine following posterior-subtenon administration (PST) compared to intravitreal (INT) and intravenous routes (INV) in rabbits.

METHODS

Vitreous pharmacokinetic analysis was performed on female New Zealand (NZ) albino rabbits after PST, INT, and INV administration and calculating the pharmacokinetic parameters that describe memantine vitreous distribution. The vitreous bioavailability (F) and the relative vitreous bioavailability of memantine was estimated after posterior-subtenon administration (Frel (pst/int)) and after intravenous route (Frel (inv/int)) compared with intravitreal administration. Relative vitreous bioavailability of memantine was also estimated following PST administration compared with vitreous concentrations after intravenous administration (Frel (pst/inv)).

RESULTS

Memantine kinetics in the vitreous of NZ albino rabbits after PST administration can be explained by a one-compartment model, which was characterized by a fast absorption process, and a short terminal half-life. Vitreous pharmacokinetics following INV administration was also characterized by a fast absorption process, a terminal half-life significantly longer than the subtenon route, and low area under the curve values. High vitreous bioavailability after PST was observed, and the relative vitreous bioavailability of memantine following PST administration (0.53%) was greater than for intravenous administration (0.02%).

CONCLUSIONS

Our results suggest that memantine reaches the vitreous after PST administration by local diffusion. These data also show that local diffusion of the drug is responsible for greater vitreous availability of memantine following PST administration compared with INV administration.

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.

Dendrimeric systems and their applications in ocular drug delivery

Ophthalmic drug delivery is one of the most attractive and challenging research area for pharmaceutical scientists and ophthalmologists. Absorption of an ophthalmic drug in conventional dosage forms is seriously limited by physiological conditions. The use of nonionic or ionic biodegradable polymers in aqueous solutions and colloidal dosage forms such as liposomes, nanoparticles, nanocapsules, microspheres, microcapsules, microemulsions, and dendrimers has been studied to overcome the problems mentioned above. Dendrimers are a new class of polymeric materials. The unique nanostructured architecture of dendrimers has been studied to examine their role in delivery of therapeutics and imaging agents. Dendrimers can enhance drug's water solubility, bioavailability, and biocompatibility and can be applied for different routes of drug administration successfully. Permeability enhancer properties of dendrimers were also reported. The use of dendrimers can also reduce toxicity versus activity and following an appropriate application route they allow the delivery of the drug to the targeted site and provide desired pharmacokinetic parameters. Therefore, dendrimeric drug delivery systems are of interest in ocular drug delivery. In this review, the limitations related to eye's unique structure, the advantages of dendrimers, and the potential applications of dendrimeric systems to ophthalmology including imaging, drug, peptide, and gene delivery will be discussed.

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.

Nanomedicines for back of the eye drug delivery, gene delivery, and imaging

Treatment and management of diseases of the posterior segment of the eye such as diabetic retinopathy, retinoblastoma, retinitis pigmentosa, and choroidal neovascularization is a challenging task due to the anatomy and physiology of ocular barriers. For instance, traditional routes of drug delivery for therapeutic treatment are hindered by poor intraocular penetration and/or rapid ocular elimination. One possible approach to improve ocular therapy is to employ nanotechnology. Nanomedicines, products of nanotechnology, having at least one dimension in the nanoscale include nanoparticles, micelles, nanotubes, and dendrimers, with and without targeting ligands. Nanomedicines are making a significant impact in the fields of ocular drug delivery, gene delivery, and imaging, the focus of this review. Key applications of nanotechnology discussed in this review include a) bioadhesive nanomedicines; b) functionalized nanomedicines that enhance target recognition and/or cell entry; c) nanomedicines capable of controlled release of the payload; d) nanomedicines capable of enhancing gene transfection and duration of transfection; f) nanomedicines responsive to stimuli including light, heat, ultrasound, electrical signals, pH, and oxidative stress; g) diversely sized and colored nanoparticles for imaging, and h) nanowires for retinal prostheses. Additionally, nanofabricated delivery systems including implants, films, microparticles, and nanoparticles are described. Although the above nanomedicines may be administered by various routes including topical, intravitreal, intravenous, transscleral, suprachoroidal, and subretinal routes, each nanomedicine should be tailored for the disease, drug, and site of administration. In addition to the nature of materials used in nanomedicine design, depending on the site of nanomedicine administration, clearance and toxicity are expected to differ.

Inhibition of corneal neovascularization with the combination of bevacizumab and plasmid pigment epithelium-derived factor-synthetic amphiphile INTeraction-18 (p-PEDF-SAINT-18) vector in a rat corneal experimental angiogenesis model

Bevacizumab, a 149-kDa protein, is a recombinant humanized monoclonal antibody to VEGF. PEDF, a 50-kDa glycoprotein, has demonstrated anti-vasopermeability properties. In this study, we demonstrated that the combination of bevacizumab and plasmid pigment epithelium-derived factor-synthetic amphiphile INTeraction-18 (p-PEDF-SAINT-18) has a favorable antiangiogenic effect on corneal NV. Four groups (Group A: 0 μg + 0 μg, B: 0.1 μg + 0.1 μg, C: 1 μg + 1 μg, and D: 10 μg + 10 μg) of bevacizumab + p-PEDF-SAINT-18 were prepared and implanted into the rat subconjunctival substantia propria 1.5 mm from the limbus on the temporal side. Then, 1 μg of p-bFGF-SAINT-18 was prepared and implanted into the rat corneal stroma 1.5 mm from the limbus on the same side. The inhibition of NV was observed and quantified from days 1 to 60. Biomicroscopic examination, western blot analysis and immunohistochemistry were used to analyze the 18-kDa bFGF, 50-kDa PEDF and VEGF protein expression. No inhibition activity for normal limbal vessels was noted. Subconjunctival injection with the combination of bevacizumab and p-PEDF-SAINT-18 successfully inhibited corneal NV. The bFGF and PEDF genes were successfully expressed as shown by western blot analysis, and a mild immune response to HLA-DR was shown by immunohistochemistry. We concluded that the combination of bevacizumab and p-PEDF-SAINT-18 may have more potent and prolonged antiangiogenic effects, making it possible to reduce the frequency of subconjunctival bevacizumab administration combined with a relatively safe profile and low toxicity.

Design of an implantable device for ocular drug delivery

Ocular diseases, such as, glaucoma, age-related macular degeneration (AMD), diabetic retinopathy, and retinitis pigmentosa require drug management in order to prevent blindness and affecting million of adults in USA and worldwide. There is an increasing need to develop devices for drug delivery to address ocular diseases. This study focuses on the design, simulation, and development of an implantable ocular drug delivery device consisting of micro-/nanochannels embedded between top and bottom covers with a drug reservoir made from polydimethylsiloxane (PDMS) which is silicon-based organic and biodegradable polymer. Several simulations were carried out with six different micro-channel configurations in order to see the feasibility for ocular drug delivery applications. Based on the results obtained, channel design of osmotic I and osmotic II satisfied the diffusion rates required for ocular drug delivery. Finally, a prototype illustrating the three components of the drug delivery design is presented. In the future, the device will be tested for its functionality and diffusion characteristics.

Intravitreal versus subtenon triamcinolone acetonide injection for diabetic macular edema: a systematic review and meta-analysis

OBJECTIVE

To compare the efficacy of intravitreal (IV) triamcinolone acetonide (IVTA) versus subtenon (ST) triamcinolone acetonide (STTA) injection for the treatment of diabetic macular edema (DME).

METHODS

Searches for randomized clinical trials published between 1 January 1950 and 15 March 2011 were conducted using PubMed, MEDLINE, EMBASE, and the Cochrane Library included in the present meta-analysis are five randomized controlled trials, each with a minimum follow-up of 3 mo. All included studies evaluated the efficacy of TA for the treatment of refractory DME, and compared IVTA with STTA by measuring visual acuity (VA), central macular thickness (CMT), and intraocular pressure (IOP).

RESULTS

One mo post-injection, treatment with IVTA had significantly improved VA (MD, -0.14 logMAR; 95% CI = -0.16 to -0.13) and reduced CMT (MD = -174.02 μm; 95% CI = -249.97 to -98.08) compared with STTA. At 3 mo post-injection, treatment with IVTA had significantly improved VA (MD = -0.07 logMAR; 95% CI = -0.09 to -0.05) and reduced CMT (MD = -119.46 μm; 95% CI = -176.55 to -62.36) compared with STTA. The benefits of either treatment were no longer significant at 6 mo, and patients had to be retreated. Compared with STTA, IVTA injections produced no difference in IOPs at 1 mo, higher IOPs at 3 mo, and lower IOP values at 6 months

CONCLUSIONS

Within 3 mo, IVTA is more effective than is STTA in improving VA and reducing CMT in patients with refractory DME. However, the benefits of either regimen were no longer evident at 6 mo.

Trans-scleral delivery of macromolecules

Non-invasive drug delivery to the posterior segment of the eye represents an important unmet medical need, and trans-scleral delivery could be an interesting solution. This review analyses the possibility of trans-scleral drug delivery for high molecular weight compounds, such as proteins and genetic material, which currently represent the most innovative and efficacious molecules for the treatment of many diseases of the posterior segment of the eye. The paper reviews all the barriers, both static and dynamic, involved in trans-scleral administration of drugs, trying to elucidate the role of each of them in the specific case of macromolecules. Delivery systems to sustain drug release and enhancing strategies to improve trans-scleral penetration are also described. Finally, the review approaches the use of computational models as a screening tool to evaluate the feasibility of trans-scleral administration for macromolecules.

In vivo ocular fluorophotometry: delivery of fluoresceinated dextrans via transscleral diffusion in rabbits

PURPOSE

To evaluate the transscleral delivery of fluoresceinated dextrans (FITC-D) with molecular mass up to 70 kDa to the rabbit posterior segment using sub-Tenon injections.

METHODS

Eighteen NZW rabbits received a unilateral 200-μL injection of 2 mg/mL sodium fluorescein (NaF), 25 mg/mL 40-kDa FITC-D, or 25 mg/mL 70-kDa FITC-D, with (n = 9) or without (n = 9) immediate euthanatization. In live animals, fluorescence was measured in the retina/choroid and mid-vitreous by fluorophotometry, immediately after injection and after 4, 24, 48, and 72 hours. Euthanatized animals were examined hourly through 5 or 6 hours.

RESULTS

In live animals, the average peak NaF concentration in the retina/choroid was 310.2 ng/mL, measured 3 hours after injection. Average 40- and 70-kDa FITC-D concentrations in the retina/choroid peaked at 5409.6 and 2375.6 ng/mL, respectively, 24 hours after injection. Fluorescence returned to baseline levels 6 hours after NaF injection, and 48 and 72 hours after 40- and 70-kDa FITC-D injections, respectively. Rabbits that received NaF followed by euthanatization exhibited a continuous increase in retina/choroid and mid-vitreous fluorescence, beginning 1 hour after injection, whereas FITC-D-injected eyes did not show elevated retina/choroid or mid-vitreous fluorescence through 6 hours.

CONCLUSIONS

FITC-D weighing up to 70-kDa, as well as NaF, reached the posterior retina/choroid after sub-Tenon injections in live rabbits. NaF and 40-kDa FITC-D reached higher peak concentrations and were cleared from the eye more rapidly than was 70-kDa FITC-D. There was minimal penetration of NaF and FITC-D into the mid-vitreous in the in vivo experiments.

Drug delivery to the posterior segment of the eye

Delivery of drugs to the posterior eye is challenging, owing to anatomical and physiological constrains of the eye. There is an increasing need for managing rapidly progressing posterior eye diseases, such as age-related macular degeneration, diabetic retinopathy and retinitis pigmentosa. Drug delivery to the posterior segment of the eye is therefore compounded by the increasing number of new therapeutic entities (e.g. oligonucleotides, aptamers and antibodies) and the need for chronic therapy. Currently, the intravitreal route is widely used to deliver therapeutic entities to the retina. However, frequent administration of drugs via this route can lead to retinal detachment, endophthalmitis and increased intraocular pressure. Various controlled delivery systems, such as biodegradable and non-biodegradable implants, liposomes and nanoparticles, have been developed to overcome such adverse effects, with some success. The periocular route is a promising alternative, owing to the large surface area and the relatively high permeability of the sclera. Yet, the blood-retinal barrier and efflux transporters hamper the transport of therapeutic entities to the retina. As such, the efficient delivery of drugs to the posterior eye remains a major challenge facing the pharmaceutical scientist. In this review, we discuss the barriers of the posterior eye drug delivery and the various drug-delivery strategies used to overcome these barriers.

A scalable controlled-release device for transscleral drug delivery to the retina

A transscleral drug-delivery device, designed for the administration of protein-type drugs, that consists of a drug reservoir covered with a controlled-release membrane was manufactured and tested. The controlled-release membrane is made of photopolymerized polyethylene glycol dimethacrylate (PEGDM) that contains interconnected collagen microparticles (COLs), which are the routes for drug permeation. The results showed that the release of 40-kDa FITC-dextran (FD40) was dependent on the COL concentration, which indicated that FD40 travelled through the membrane-embedded COLs. Additionally, the sustained-release drug formulations, FD40-loaded COLs and FD40-loaded COLs pelletized with PEGDM, fine-tuned the release of FD40. Capsules filled with COLs that contained recombinant human brain-derived neurotrophic factor (rhBDNF) released bioactive rhBDNF in a manner dependent on the membrane COL concentration, as was found for FD40 release. When capsules were sutured onto sclerae of rabbit eyes, FD40 was found to spread to the retinal pigment epithelium. Implantation of the device was easy, and it did not damage the eye tissues. In conclusion, our capsule is easily modified to accommodate different release rates for protein-type drugs by altering the membrane COL composition and/or drug formulation and can be implanted and removed with minor surgery. The device thus has great potential as a conduit for continuous, controlled drug release.

Barrier analysis of periocular drug delivery to the posterior segment

Periocular administration is a potential way of delivering drugs to their targets in posterior eye segment (vitreous, neural retina, retinal pigment epithelium (RPE), choroid). Purpose of this study was to evaluate the role of the barriers in periocular drug delivery. Permeation of FITC-dextrans and oligonucleotides in the bovine sclera was assessed with and without Pluronic gel in the donor compartment. Computational model for subconjunctival drug delivery to the choroid and neural retina/vitreous was built based on clearance concept. Kinetic parameters for small hydrophilic and lipophilic drug molecules, and a macromolecule were obtained from published ex vivo and in vivo animal experiments. High negative charge field of oligonucleotides slows down their permeation in the sclera. Pluronic does not provide adequate rate control to modify posterior segment drug delivery. Theoretical calculations for subconjunctival drug administration indicated that local clearance by the blood flow and lymphatics removes most of the drug dose which is in accordance with experimental results. Calculations suggested that choroidal blood flow removes most of the drug that has reached the choroid, but this requires experimental verification. Calculations at steady state using the same subconconjunctival input rate showed that the choroidal and vitreal concentrations of the macromolecule is 2-3 orders of magnitude higher than that of small molecules. The evaluation of the roles of the barriers augments to design new drug delivery strategies for posterior segment of the eye.

Novel nanoparticulate gel formulations of steroids for the treatment of macular edema

PURPOSE

This article describes the development and characterization of PLGA nanoparticles of dexamethasone (DEX), hydrocortisone acetate (HA), and prednisolone acetate (PA) suspended in thermosensitive gels indicated for the treatment of macular edema (ME).

METHODS

Nanoparticles were prepared by oil-in-water (O/W) emulsion and dialysis methods using PLGA 50:50 and PLGA 65:35. These particles were characterized for entrapment efficiency, size distribution, surface morphology, crystallinity, and in vitro release. Further, ex vivo permeation studies of DEX in suspension and nanoparticulate formulations were carried out across the rabbit sclera.

RESULTS

Entrapment efficiencies of DEX, HA, and PA were found to be lower with the dialysis method. O/W emulsion/solvent evaporation technique resulted in higher entrapment efficiencies, that is, 77.3%, 91.3%, 92.3% for DEX, HA, and PA, respectively. Release from nanoparticles suspended in thermosensitive gels followed zero-order kinetics with no apparent burst effect. Ex vivo permeability studies further confirmed sustained release of DEX from nanoparticles suspended in thermosensitive gels.

CONCLUSIONS

These novel nanoparticulate systems containing particles suspended in thermosensitive gels may provide sustained retina/choroid delivery of steroids following episcleral administration.

Filter-cultured ARPE-19 cells as outer blood-retinal barrier model

Retinal pigment epithelium (RPE) regulates drug transfer between posterior eye segment and blood circulation, but there is no established RPE cell model for drug delivery studies. We evaluated ARPE-19 filter culture model for this purpose. Passive permeability of 6-carboxyfluorescein, betaxolol and FITC-dextran (40kDa) and active transport of 6-carboxyfluorescein, sodium fluorescein, rhodamine 123, cyclosporine A and digoxin in ARPE-19 model were investigated and compared with isolated bovine RPE-choroid tissue. In addition, barrier properties, and mRNA expression of RPE-specific and melanogenesis-related genes (RPE65, VMD2, CRALBP, OTX-2, MITF-A, TRP-1, tyrosinase) were measured in various culture conditions. The filter grown ARPE-19 cell model showed reasonable barrier properties (TER close to 100Omegacm(2)), but its permeability was slightly higher than that of isolated bovine RPE/choroid specimens. In active transport studies the ARPE-19 model mimics qualitatively the permeability profile of bovine RPE-choroid, but ARPE-19 model underestimates the importance of active transport relative to passive diffusion. Long-term filter-cultured ARPE-19 cells expressed various RPE-specific and melanogenesis-related genes at higher levels than the ARPE-19 cells cultured short-term in flasks. ARPE-19 model can be used to study drug permeation processes in the RPE.

New techniques for drug delivery to the posterior eye segment

Ocular drug delivery has become an increasingly important field of research especially when treating posterior segment diseases of the eye, such as age-related macular degeneration, diabetic retinopathy, posterior uveitis and retinitis. These diseases are the leading causes of vision loss in developed countries which require repeated long-term administration of therapeutic agents. New drugs for the medication of the posterior ocular segment have emerged, but most drugs are delivered by repeated intravitreal injections associated with ocular complications. Advances in ocular drug delivery system research are expected to provide new tools for the treatment of the posterior segment diseases, providing improved drug penetration, prolonged action, higher efficacy, improved safety and less invasive administration, resulting in higher patient compliance. This review provides an insight into the recent progress and trends in ocular drug delivery systems for treating posterior eye segment diseases, with an emphasis on transscleral iontophoresis.

Episcleral clearance of sodium fluorescein from a bioerodible sub-tenon's implant in the rat

We quantified episcleral drug clearance of sodium fluorescein (NaFl) in rats to examine the hypothesis that there is rapid clearance of episcleral water soluble drugs, and that this rapid clearance may limit the amount of drug that is able to reach the posterior segment from an episcleral location. 2 mm implants containing either 12 or 22 microg of NaFl were manufactured and in vitro release rates were determined. Implants were placed in the sub-Tenon's space and the amount of drug remaining in the conjunctiva/sclera/choroid complex (CSCC) at various time points was quantified following tissue solubilization and fluorescence quantification using a spectrofluorometer. Kinetics of NaFl clearance was determined in live animals, following euthanasia and in animals in which choroidal non-perfusion had been achieved with indocyanine green-enhanced 810 nm diode laser thrombosis of the choroidal vasculature. Choroidal non-perfusion in these laser-treated rats was verified with Concavalin-A staining of choroidal flatmounts. In vitro, >99% of drug was released by 25 min for the low dose implants, and by 60 min for the high dose implants. In vivo, both implant doses were >99% cleared from the episcleral tissue by 3 h. By 7 h, an average of only 0.14 +/- 0.131 ng of NaFl per mg of wet tissue weight (mean +/- SD) remained in the CSCC with the low dose implant, and 0.29 +/- 0.428 ng of NaFl per mg of wet tissue weight remained in animals with the high dose implant. By comparison, in euthanized animals at 7 h following sub-Tenon's implantation, 432.0 +/- 181.40 ng of NaFl per mg of wet tissue weight was in the episcleral tissue of animals with the low dose implant, and of 787.8 +/- 409.89 ng of NaFl per mg of wet tissue weight remained in the animals with the high dose implant. In live animals with selective thrombosis of the choroidal vasculature, the difference in the amount of drug remaining in the episcleral tissue as compared to control live animals was not significant at all time points for both implant doses. In conclusion, there is rapid clearance of episcleral NaFl delivered from a bioerodible sub-tenon's implant. The clearance mechanisms are dramatically reduced following euthanasia, suggesting that elimination is occurring via active physiologic mechanisms, rather than by passive diffusion clearance (CL(diff)) (Pfister et al., 2003). Interestingly, the choroid does not appear to play a prominent role as clearance of episcleral NaFl was not affected by elimination of choroidal blood flow. Further work is needed to delineate the pathways of episcleral drug clearance.

Iontophoretic transport of charged macromolecules across human sclera

The mechanisms of transscleral iontophoresis have been investigated previously with small molecules in rabbit sclera. The objective of the present study was to examine transscleral iontophoretic transport of charged macromolecules across excised human sclera. Passive and 2mA iontophoretic transport experiments were conducted in side-by-side diffusion cells with human sclera. The effects of iontophoresis upon transscleral transport of model permeants bovine serum albumin (BSA) and polystyrene sulfonic acid (PSS) as well as a model drug bevacizumab (BEV) were determined. Passive and iontophoretic transport experiments of tetraethylammonium (TEA) and salicylic acid (SA) and passive transport experiments of the macromolecules served as the controls. The results of iontophoresis enhanced transport of TEA and SA across human sclera were consistent with those in a previous rabbit sclera study. For the iontophoretic transport of macromolecules BSA and BEV, higher iontophoretic fluxes were observed in anodal iontophoresis as compared to passive and cathodal iontophoresis. This suggests the importance of electroosmosis. For the polyelectrolyte PSS, higher iontophoretic flux was observed in cathodal iontophoresis compared to anodal iontophoresis. Both electroosmosis and electrophoresis affected iontophoretic fluxes of the macromolecules; the relative contributions of electroosmosis and electrophoresis were a function of molecular size and charge of the macromolecules.

Fibrin sealant for retinoblastoma: where are we?

Chemoreduction is currently the most popular treatment strategy for intraocular retinoblastoma worldwide. Despite the dramatic clinical responses obtained with multiagent systemic chemotherapy regimens, enthusiasm for this treatment approach has been tempered by the potential toxicities of these drugs in the pediatric population. As a response to these concerns, novel approaches for the local delivery of chemotherapeutic agents to ocular structures have been investigated by both clinicians and scientists. Ocular oncologists have developed the periocular injection of carboplatin as a method for controlling intraocular tumor growth of retinoblastoma while minimizing systemic drug exposure. In parallel, the pharmaceutical industry has introduced drug-delivery systems to the posterior segment of the globe for a variety of ocular diseases. One example of the collaborative work by ophthalmologists and biopharmaceutical scientists is the use of fibrin sealants as a targeted drug-administration device, formulated to deliver sustained concentrations of chemotherapy at the site of application. This review integrates the recent ophthalmology and pharmaceutics literature on the potential role of fibrin sealants for periocular chemotherapy administration in the treatment of retinoblastoma.

MRI in ocular drug delivery

Conventional pharmacokinetic methods for studying ocular drug delivery are invasive and cannot be conveniently applied to humans. The advancement of MRI technology has provided new opportunities in ocular drug-delivery research. MRI provides a means to non-invasively and continuously monitor ocular drug-delivery systems with a contrast agent or compound labeled with a contrast agent. It is a useful technique in pharmacokinetic studies, evaluation of drug-delivery methods, and drug-delivery device testing. Although the current status of the technology presents some major challenges to pharmaceutical research using MRI, it has a lot of potential. In the past decade, MRI has been used to examine ocular drug delivery via the subconjunctival route, intravitreal injection, intrascleral injection to the suprachoroidal space, episcleral and intravitreal implants, periocular injections, and ocular iontophoresis. In this review, the advantages and limitations of MRI in the study of ocular drug delivery are discussed. Different MR contrast agents and MRI techniques for ocular drug-delivery research are compared. Ocular drug-delivery studies using MRI are reviewed.

Computer modeling of drug delivery to the posterior eye: effect of active transport and loss to choroidal blood flow

PURPOSE

The direct penetration route following transscleral drug administration presents several barrier and clearance mechanisms-including loss to choroidal blood flow, active transport by the retinal pigment epithelium (RPE), and loss to the conjunctival lymphatics and episcleral blood vessels. The objective of this research was to quantify the role of choroidal and episcleral losses.

MATERIALS AND METHODS

A finite element model was created for drug distribution in the posterior human eye. The volumetric choroidal loss constant, active transport component and mass transfer from the scleral surface were unknown parameters in the model. The model was used to simulate drug distribution from a systemic source, and the results were compared to existing experimental results to obtain values for the parameters.

RESULTS

The volumetric choroidal loss constant, mass transfer coefficient from the scleral surface and active transport component were evaluated to be (2.0 +/- 0.6) x 10(-5) s(-1), (2.0 +/- 0.35) x 10(-5) cm/s and 8.54 x 10(-6) cm/s respectively.

CONCLUSION

Loss to the choroidal circulation was small compared to loss from the scleral surface. Active transport was predicted to induce periscleral movement of the drug, resulting in more rapid distribution and elevated drug concentrations in the choroid and sclera.