Safety and Pharmacokinetics of an Intravitreal Biodegradable Implant of Dexamethasone Acetate in Rabbit Eyes

Acetazolamide-loaded intravitreal implants for the treatment of glaucoma: formulation, physicochemical characterization and assessment of in vitro and in vivo safety

Glaucoma is a leading cause of permanent blindness, currently affecting over 75 million individuals worldwide. Treatments primarily aim to lower intraocular pressure (IOP), with acetazolamide being a common pharmacotherapy. However, systemic side effects and poor bioavailability limit its clinical effectiveness. This study presents the development and physicochemical and safety evaluation of biodegradable intravitreal implants loaded with acetazolamide using polylactic-co-glycolic acid (PLGA), designed to enhance targeted drug delivery directly to the vitreous humor. The acetazolamide-loaded PLGA implants were successfully fabricated via hot molding, yielding cylindrical rods measuring 5.00 ± 0.02 mm in diameter. Incorporation efficiency reached approximately 92 %, with a uniform drug dose of 230 μg/g. Comprehensive physicochemical characterization confirmed compatibility between the drug and polymer, with stability demonstrated for at least 6 months. The implants exhibited a biphasic drug release profile, sustaining approximately 50 % of the drug over 42 days without an initial burst, indicating their potential for effective and prolonged glaucoma management. Safety assessments revealed that the implants did not induce retinal toxicity at doses up to 0.020 mmol/L. In vitro cytotoxicity assays with ARPE-19 cells and in vivo tests - electroretinography, Goldmann applanation tonometry, optical coherence tomography, and histological analysis - showed no adverse effects on retinal function or structure. Importantly, the implants significantly reduced IOP in normotensive rabbits, confirming their therapeutic potential. This innovative approach represents a promising alternative for glaucoma treatment, addressing the limitations of current modalities while providing sustained drug delivery and enhanced safety.

Comprehensive Ocular and Systemic Safety Evaluation of Polysialic Acid-Decorated Immune Modulating Therapeutic Nanoparticles (PolySia-NPs) to Support Entry into First-in-Human Clinical Trials

An inflammation-resolving polysialic acid-decorated PLGA nanoparticle (PolySia-NP) has been developed to treat geographic atrophy/age-related macular degeneration and other conditions caused by macrophage and complement over-activation. While PolySia-NPs have demonstrated pre-clinical efficacy, this study evaluated its systemic and intraocular safety. PolySia-NPs were evaluated in vitro for mutagenic activity using Salmonella strains and E. coli, with and without metabolic activation; cytotoxicity was evaluated based on its interference with normal mitosis. PolySia-NPs were administered intravenously in CD-1 mice and Sprague Dawley rats and assessed for survival and toxicity. Intravitreal (IVT) administration in Dutch Belted rabbits and non-human primates was assessed for ocular or systemic toxicity. In vitro results indicate that PolySia-NPs did not induce mutagenicity or cytotoxicity. Intravenous administration did not show clastogenic activity, effects on survival, or toxicity. A single intravitreal (IVT) injection and two elevated repeat IVT doses of PolySia-NPs separated by 7 days in rabbits showed no signs of systemic or ocular toxicity. A single IVT inoculation of PolySia-NPs in non-human primates demonstrated no adverse clinical or ophthalmological effects. The demonstration of systemic and ocular safety of PolySia-NPs supports its advancement into human clinical trials as a promising therapeutic approach for systemic and retinal degenerative diseases caused by chronic immune activation.

Nanofiber-coated implants: Development and safety after intravitreal application in rabbits

Intravitreal injections are the preferred choice for drug administration to the posterior segment of the eye. However, the required frequent injections may cause complications to the patient and low adherence to the treatment. Intravitreal implants are able to maintain therapeutic levels for a long period. Biodegradable nanofibers can modulate drug release and allow the incorporation of fragile bioactive drugs. Age-related macular degeneration is one of the world major causes of blindness and irreversible vision loss. It involves the interaction between VEGF and inflammatory cells. In this work we developed nanofiber-coated intravitreal implants containing dexamethasone and bevacizumab for simultaneously delivery of these drugs. The implant was successfully prepared and the efficiency of the coating process was confirmed by scanning electron microscopy. Around 68% of dexamethasone was released in 35 days and 88% of bevacizumab in 48hs. The formulation presented activity in the reduction of vessels and was safe to the retina. It was not observed any clinical or histopathological change, neither alteration in retina function or thickness by electroretinogram and optical coherence tomography during 28 days. The nanofiber-coated implants of dexamethasone and bevacizumab may be considered as a new delivery system that can be effective for the treatment of AMD.

Design, development, and characterization of an idebenone-loaded poly-ε-caprolactone intravitreal implant as a new therapeutic approach for LHON treatment

Leber's Hereditary Optic Neuropathy (LHON) is a hereditary mitochondrial neurodegenerative disease of unclear etiology and lack of available therapeutic alternatives. The main goal of the current pilot study was based on the evaluation of the feasibility and characteristics of prolonged and controlled idebenone release from a PCL intravitreal implant. The design, development, and characterization of idebenone-loaded PCL implants prepared by an homogenization/extrusion/solvent evaporation method allowed the obtention of high PY, EE and LC values. In vitro characterization was completed by the assessment of mechanical and instrumental properties. The in vitro release of idebenone from the PCL implants was assessed and the implant erosion was monitored by the mass loss and surface morphology changes. DSC was used to estimate stability and interaction among implant's components. The present work demonstrated the controlled and prolonged idebenone delivery from the PCL implants in an in vitro model. A consistent preclinical base was established, supporting the idea of idebenone-loaded PCL implants as a new strategy of long-term sustained intraocular delivery for the LHON treatment.

Dexamethasone PLGA Microspheres for Sub-Tenon Administration: Influence of Sterilization and Tolerance Studies

Many diseases affecting the posterior segment of the eye require repeated intravitreal injections with corticosteroids in chronic treatments. The periocular administration is a less invasive route attracting considerable attention for long-term therapies. In the present work, dexamethasone-loaded poly(lactic-co-glycolic) acid (PLGA) microspheres (Dx-MS) were prepared using the oil-in-water (O/W) emulsion solvent evaporation technique. MS were characterized in terms of mean particle size and particle size distribution, external morphology, polymer integrity, drug content, and in vitro release profiles. MS were sterilized by gamma irradiation (25 kGy), and dexamethasone release profiles from sterilized and non-sterilized microspheres were compared by means of the similarity factor (f₂). The mechanism of drug release before and after irradiation exposure of Dx-MS was identified using appropriate mathematical models. Dexamethasone release was sustained in vitro for 9 weeks. The evaluation of the in vivo tolerance was carried out in rabbit eyes, which received a sub-Tenon injection of 5 mg of sterilized Dx-MS (20–53 µm size containing 165.6 ± 3.6 µg Dx/mg MS) equivalent to 828 µg of Dx. No detectable increase in intraocular pressure was reported, and clinical and histological analysis of the ocular tissues showed no adverse events up to 6 weeks after the administration. According to the data presented in this work, the sub-Tenon administration of Dx-MS could be a promising alternative to successive intravitreal injections for the treatment of chronic diseases of the back of the eye.

Rosmarinic Acid Intravitreal Implants: A New Therapeutic Approach for Ocular Neovascularization

Rosmarinic acid, a plant-derived compound with antiangiogenic activity, can be applied for the treatment of ocular diseases related to neovascularization, such as diabetic retinopathy, macular edema, and age-related macular degeneration. These diseases represent the leading causes of blindness worldwide if they are not properly treated. Intravitreal devices allow for localized drug delivery to the posterior segment, increasing the drug bioavailability and promoting extended release, thus, reducing side effects and enhancing the patient's compliance to the treatment. In this work, rosmarinic acid-loaded poly lactic-co-glycolic acid intraocular implants were developed with a view for the treatment of ocular neovascularization. Physical-chemical, biocompatibility, and safety studies of the implants were carried out in vitro and in vivo as well as an evaluation of the antiangiogenic activity in a chorioallantoic membrane assay. Data obtained showed that rosmarinic acid released from the implants was quantified in the vitreous for 6 weeks, while when it was in the solution formulation, after 24 h, no drug was found in the vitreous. The delivery device did not show any sign of toxicity after clinical evaluation and in electroretinographic findings. Histological analysis showed normal eye tissue. Rosmarinic acid released from implants reduced 30% of new vessel's formation. The intravitreal implant successfully allowed for the prolonged release of rosmarinic acid, was safe to rabbits eyes, and demonstrated activity in vessel reduction, thus demonstrating potential in preventing neovascularization in ophthalmic diseases.

RETAINED, NONDISSOLVING, TUBULAR FOREIGN BODIES IN THE VITREOUS CAVITY AFTER INTRAVITREAL DEXAMETHASONE (OZURDEX) IMPLANTATION

PURPOSE

To describe the retention of large, tubular, nondissolving foreign bodies because of a complication of the intravitreal dexamethasone implant (Ozurdex).

METHODS

This is a single-center, retrospective chart review of patients who were found to have retained, nondissolvable tubular foreign bodies in the vitreous cavity for more than 6 months (the expected dissolution time of the implants) after Ozurdex injections. Ocular symptomatology and multimodal imaging were reviewed.

RESULTS

Five patients had retained, nondissolvable tubular foreign bodies in the vitreous that persisted for months (mean 28.2 months, range 9-67 months) after intravitreal injection of Ozurdex. Two patients were symptomatic due to the foreign bodies and chose alternate local therapy, but none of the patients opted for surgical explantation.

CONCLUSION

Persistent, nondissolving, tubular foreign bodies can be seen in the vitreous cavity for years after injection of the Ozurdex implant. Clinicians should be aware of this complication that has the potential to cause visual symptoms and ocular morbidity.

Toxicity and in vivo release profile of sirolimus from implants into the vitreous of rabbits' eyes

PURPOSE

To assess the in vivo release profile and the retinal toxicity of a poly (lactic-co-glycolic acid) (PLGA) sustained-release sirolimus (SRL) intravitreal implant in normal rabbit eyes.

METHODS

PLGA intravitreal implants containing or not SRL were prepared, and the viability of ARPE-19 and hES-RPE human retinal cell lines was examined after 24 and 72 h of exposure to implants. New Zealand rabbits were randomly divided into two groups that received intravitreal implants containing or not SRL. At each time point (1-8 weeks), four animals from the SRL group were euthanized, the vitreous was collected, and drug concentration was calculated. Clinical evaluation of the eyes was performed weekly for 8 weeks after administration. Electroretinography (ERG) was recorded in other eight animals, four for each group, at baseline and at 24 h, 1, 4, 6, and 8 weeks after the injection. ERG was carried out using scotopic and photopic protocols. The safety of the implants was assessed using statistical analysis of the ERG parameters (a and b waves, a and b implicit time, B/A ratio, oscillatory potential, and Naka-Rushton analysis) comparing the functional integrity of the retina between the PLGA and SRL-PLGA groups. After the last electrophysiological assessment, the rabbits were euthanized and retinal histopathology was realized.

RESULTS

After 24 and 72 h of incubation with PLGA or SRL-PLGA implants, ARPE-19 and hES-RPE cells showed viability over 70%. The maximum concentration of SRL (199.8 ng/mL) released from the device occurred within 4 weeks. No toxic effects of the implants or increase in the intraocular pressure was observed through clinical evaluation of the eye. ERG responses showed no significant difference between the eyes that received PLGA or SRL-PLGA implants at baseline and throughout the 8 weeks of follow-up. No remarkable difference in retinal histopathology was detected in rabbit eyes treated with PLGA or SRL-PLGA implants.

CONCLUSIONS

Intravitreal PLGA or SRL-PLGA implants caused no significant reduction in cell viability and showed no evident toxic effect on the function or structure of the retina of the animals. SRL was released from PLGA implant after application in the vitreous of rabbits during 8 weeks.

Dexamethasone implant in silicone oil: in vitro behavior

Background

To determine the effect of the silicone on the dexamethasone intravitreal implant.

Methods

Basic, experimental, prospective and transversal study performed at the hospital “Nuestra Señora de la Luz” in Mexico City. One dexamethasone implant was placed in a test tube with 4 mL of each tamponade medium: 1000cS, 5000cS and heavy silicone oil; basic saline solution was used as the control medium. Photographs were taken weekly for 12 months. 200 µL samples were taken from each medium at 24 h, 1, 2 weeks and monthly for 12 months. ELISA test was performed to quantify dexamethasone release in every sample. An inflammatory stimulus was created and later exposed it to every sample in order to test their anti-inflammatory capacity by cytokine analysis using cytometric bead array. Statistically significant results were obtained with p < 0.05.

Results

Photographic follow-up showed disintegration of the implant in control medium. Implants in silicone oil suffered no changes during follow-up. Dexamethasone levels in control medium showed stability from month 2 to 12. Silicone oil mediums showed irregular dexamethasone release during the 1 year period. Dexamethasone in control medium had inhibitory effects on TNF-α starting at 24 h (p < 0.001) and remained stable. Dexamethasone in 1000cS silicone oil showed inhibitory effects from month 2 (p < 0.001) until month 6 (p < 0.001). Implants in denser silicone oils showed no inhibitory effects in any of the samples.

Conclusions

Denser mediums altered the implant pharmacokinetics and showed no anti-inflammatory effects even when concentrations were quantified at levels similar to control medium in vitro.

Etoposide-Loaded Poly(Lactic-co-Glycolic Acid) Intravitreal Implants: In Vitro and In Vivo Evaluation

Etoposide-loaded poly(lactic-co-glycolic acid) implants were developed for intravitreal application. Implants were prepared by a solvent-casting method and characterized in terms of content uniformity, morphology, drug-polymer interaction, stability, and sterility. In vitro drug release was investigated and the implant degradation was monitored by the percent of mass loss. Implants were inserted into the vitreous cavity of rabbits' eye and the in vivo etoposide release profile was determined. Clinical examination and the Hen Egg Test-Chorioallantoic Membrane (HET-CAM) method were performed to evaluate the implant tolerance. The original chemical structure of the etoposide was preserved after incorporation in the polymeric matrix, which the drug was dispersed uniformly. In vitro, implants promoted sustained release of the drug and approximately 57% of the etoposide was released in 50 days. In vivo, devices released approximately 63% of the loaded drug in 42 days. Ophthalmic examination and HET-CAM assay revealed no evidence of toxic effects of implants. These results tend to show that etoposide-loaded implants could be potentially useful as an intraocular etoposide delivery system in the future.

SAFETY AND FEASIBILITY OF A NOVEL 25-GAUGE BIODEGRADABLE IMPLANT OF DEXAMETHASONE FOR TREATMENT OF MACULAR EDEMA ASSOCIATED WITH RETINAL VEIN OCCLUSION: A PHASE I CLINICAL TRIAL

PURPOSE

To evaluate the safety and feasibility of a 25-gauge biodegradable implant containing 350 μg of dexamethasone (DDS-25) for the treatment of decreased vision due to macular edema associated with central or branch retinal vein occlusion.

METHODS

Prospective, nonrandomized, open-label, Phase I clinical trial, including 10 patients with decreased vision (best-corrected early treatment diabetic retinopathy study visual acuity of 20/40 or worse) due to macular edema associated with central retinal vein occlusion (n = 4) or branch retinal vein occlusion (n = 6) for more than 4 months. Comprehensive ophthalmic evaluation, including best-corrected visual acuity, spectral domain optical coherence tomography (Spectralis Heidelberg Engineering) for determination of central subfield thickness, full-field electroretinography (ISCEV standard ERG), and fluorescein angiography, was performed at baseline, and 1, 4, 12, and 24 weeks after intravitreal DDS-25 insertion.

RESULTS

Mean best-corrected visual acuity was 0.72 ± 0.1 logMAR (20/100) at baseline and improved by 7 early treatment diabetic retinopathy study letters to 0.58 ± 0.08 logMAR (20/80 + 1) at 24 weeks (P = 0.049), with 3 central retinal vein occlusion and 3 branch retinal vein occlusion patients improving between 1 and 4 early treatment diabetic retinopathy study lines. Significant central subfield thickness reduction was observed at 24 weeks compared with baseline (P = 0.011); mean ± standard error (range) central subfield thickness (μm) was 461.2 ± 41.3 (288-701) at baseline, and 439.6 ± 40.4 (259-631), 442.5 ± 44.6 (255-632), 354.6 ± 31.2 (228-537), and 316.5 ± 26.4 (226-441) at 1, 4, 12, and 24 weeks, respectively. No significant changes in electroretinography responses or area of retinal nonperfusion were observed during 24 weeks of follow-up. There was no significant change in mean intraocular pressure at any of the study visits compared with baseline. One patient had mild anterior chamber inflammation (1-5 cells) at one week after DDS-25 insertion.

CONCLUSION

In this Phase I study demonstrating the feasibility of intravitreal DDS-25 insertion for the treatment of decreased vision due to macular edema associated with retinal vein occlusion, no safety concerns were observed. A larger prospective randomized study with longer follow-up is warranted to confirm these findings.

Overview of biopolymers as carriers of antiphlogistic agents for treatment of diverse ocular inflammations

Inflammation of the eye is a usual clinical condition that can implicate any part of the eye. The nomenclature of variety of such inflammations is based on the ocular part involved. These diseases may jeopardize normal functioning of the eye on progression. In general, corticosteroids, antihistamines, mast cell stabilizers and non-steroidal anti-inflammatory drugs (NSAIDs) are used to treat inflammatory diseases/disorders of the eye. There have been several attempts via different approaches of drug delivery to overcome the low ocular bioavailability resulting from shorter ocular residence time. The features like safety, ease of elimination and ability to sustain drug release have led to application of biopolymers in ocular therapeutics. Numerous polymers of natural origin such as gelatin, collagen, chitosan, albumin, hyaluronic acid, alginates etc. have been successfully employed for preparation of different ocular dosage forms. Chitosan is the most explored biopolymer amongst natural biopolymers because of its inherent characteristics. The emergence of synthetic biopolymers (like PVP, PACA, PCL, POE, polyanhydrides, PLA, PGA and PLGA) has also added new dimensions to the drug delivery strategies meant for treatment of ophthalmic inflammations. The current review is an endeavor to describe the utility of a variety of biomaterials/polymers based drug delivery systems as carrier for anti-inflammatory drugs in ophthalmic therapeutics.

In vivo release and retinal toxicity of cyclosporine-loaded intravitreal device

PURPOSE

To determine the in vivo release profile and retinal safety of cyclosporine A (CsA) delivered from a biodegradable poly-lactide-co-glycolide (PLGA) device in the vitreous cavity of rabbits' eyes.

METHODS

A total of 60 animals (60 eyes) divided into two groups were used. For the in vivo release study, 32 eyes received PLGA implants containing 350 µg of CsA, and 16 eyes received the implants without drug (control). Four animals of CsA group and two of the control group were killed weekly until 8 weeks; the vitreous was removed, and CsA concentration was evaluated. Ophthalmological examination was performed in the animals prior to implant placement and weekly during the study period. Electroretinography (ERG) was performed in other six animals for each group, treated and control, at the beginning and at the end of the study (8 weeks) when they were killed and had their eyes processed for histology.

RESULTS

No sign of inflammation was noticed on slit lamp examinations and the IOP maintained stable during the study period in CsA and control groups. CsA concentration in the vitreous (ng/ml) was 257.07 ± 117.23, 271.15 ± 98.96, 296.66 ± 86.25, 256.27 ± 99.22, 304.50 ± 88.18, 326.35 ± 105.24, 491.25 ± 119.90 and 589.93 ± 132.55 after 1, 2, 3, 4, 5, 6, 7 and 8 weeks of implantation, respectively. At the end of the study, 21.67 % of mass loss was found. The retina did not show any histological alteration in either group, but a significant reduction in dark-adapted b-wave amplitude was observed in the CsA group, with no changes in a-wave amplitude.

CONCLUSIONS

These data show that the PLGA system is safe, but the selective reduction in ERG b-wave amplitude indicates that the PLGA with 350 µg CsA causes retinal function impairment, specifically on the rod postreceptor pathway, 8 weeks after implantation. These ERG changes were not associated with any histological damage as seen at the light microscopy level.

Neuroprotective effects of intravitreal triamcinolone acetonide and dexamethasone implant in rabbit retinas after pars plana vitrectomy and silicone oil injection

PURPOSE

To investigate potential retinal neuroprotective effects of intravitreal triamcinolone acetonide and dexamethasone implant in rabbits after pars plana vitrectomy and intravitreal silicone oil injection.

METHODS

The right eyes of 84 rabbits, divided into 3 groups of 28 rabbits each, underwent standard 3-port pars plana vitrectomy with silicone oil (SO group), silicone oil and intravitreal dexamethasone implant (SO/DEX group), or silicone oil and triamcinolone acetonide (SO/TA group). The retina from the left eye of each rabbit served as a control. The animals were killed at 4 weeks after surgery. Qualitative and quantitative histopathologic analyses were performed 4 weeks after surgery, and investigation for apoptosis was performed using the Tunel assay.

RESULTS

Intravitreal triamcinolone acetonide and dexamethasone implant were associated with increased retinal neuronal survival, primarily in the outer nuclear layer, inner nuclear layer, and ganglion cell layer. In the SO group, the cell density in eyes that underwent PPV/SO was 31% lower in the outer nuclear layer, 33% lower in the inner nuclear layer, and 45% lower in the ganglion cell layer compared to control eyes (p < 0.05 for all PPV/SO versus control comparisons). Compared to eyes that underwent PPV/SO, the cell density in eyes treated with triamcinolone was 27% higher in the outer nuclear layer, 66% higher in the inner nuclear layer, and 100% higher in the ganglion cell layer (p < 0.05 for all triamcinolone versus PPV/SO comparisons). Compared to eyes that underwent PPV/SO, the cell density in eyes treated with dexamethasone was 46% higher in the outer nuclear layer, 62% higher in the inner nuclear layer, and 77% higher in the ganglion cell layer (p < 0.05 for all dexamethasone versus PPV/SO comparisons). Analyses using the Tunnel assay demonstrated apoptotic bodies in all eyes in the SO group, compared with none of the eyes in the SO/TA and SO/DEX groups. The presence of cell nuclei stained with 49,6-diamidino-2-phenylindole (DAPI) was demonstrated in all groups.

CONCLUSION

In this experimental model of neuroprotection, increased retinal neuronal survival was seen in the steroid-treated groups compared with the controls.

Assessing the maxillary sinus mucosa of rabbits in the presence of biodegradable implants

In an attempt to improve the quality of life of patients with vitreous humor disease, ophthalmologists began offering steroid-eluting biodegradable implants to their patients. These implants can be used as an alternative treatment for CRS and this is why this experimental study was carried out on rabbit maxillary sinuses.

Objective

This study aims to assess the histology of the mucosa of the maxillary sinuses of rabbits after the placement of a prednisolone-eluting biodegradable implant.

Method

Eighteen rabbits were randomly divided into two groups: group 1 - subjects had drug-eluting implants placed on their left maxillary sinuses; group 2 - subjects had non-drug-eluting implants placed on their left maxillary sinuses. The right maxillary sinuses served as the controls. After seven, 14, and 28 days three rabbits in each group were randomly picked to have their tissue inflammatory response assessed.

Results

Levels of mucosal inflammation were not significantly different between the groups with and without drug-eluting implants and the control group, or when the groups with drug-eluting implants and non-drug-eluting implants were compared.

Conclusion

Signs of toxicity or mucosal inflammation were not observed in the maxillary sinuses of rabbits given prednisolone-eluting implants or non-drug-eluting implants.

Experimental uveitis can be maintained in rabbits for a period of six weeks after a safe sensitization method

PURPOSE

New treatments against long-lasting uveitis need to be tested. Our aim was to develop a six-week model of uveitis in rabbits.

METHODS

Rabbits were presensitized with an s.c. injection of Mycobacterium tuberculosis H37RA emulsified with TiterMax Gold adjuvant. Uveitis was induced at day 28 and 50, by intravitreal challenges of antigen suspension. Ocular inflammation was assessed till euthanasia at day 71 after s.c. injection of M. tuberculosis H37RA by: (a) the number of inflammatory cells in aqueous humor (AH); (b) the protein concentration in AH; (c) the clinical score (mean of conjunctival hyperaemia, conjunctival chemosis, oedema and secretion); (d) the microscopical score (mean presence of fibrin and synechiae, aqueous cell density and aqueous flare grade, as scored by slit lamp).

RESULTS

At the sites of presensitization injection, rabbits presented flat nodules which progressively vanished. The first challenge induced a significant increase in the four parameters (p < 0.05 the Wilcoxon/Kruskal-Wallis test). The AH contained 764 ± 82 cells/µl and 32 ± 0.77 mg protein/ml. During the following days, inflammatory parameters decreased slightly. The second intravitreal challenge increased inflammation (3564 ± 228 cells/µl AH and 31 ± 1 mg protein/ml), which remained at a high level for a longer period of time.

CONCLUSION

We developed a model of long-term uveitis, which could be maintained in rabbits for at least six weeks. Such a model could be used to test the efficacy of either new drugs or various drug delivery systems intended to deliver active agents during a few months.

Critical appraisal of the clinical utility of the dexamethasone intravitreal implant (Ozurdex) for the treatment of macular edema related to branch retinal vein occlusion or central retinal vein occlusion

Macular edema is a common cause of visual loss in patients with retinal vein occlusions. Ozurdex^®, a dexamethasone intravitreal implant, has been shown in randomized controlled trials to reduce macular edema and improve visual acuity in patients with either branch retinal vein occlusions or central retinal vein occlusions. It was approved in the United States in 2009. Since then, new therapeutic agents and clinical data have emerged. The purpose of this review is to critically evaluate the clinical utility of Ozurdex^® in the current treatment strategy of macular edema related to retinal vein occlusion.

Implants as drug delivery devices for the treatment of eye diseases

The treatment of diseases affecting the posterior segment of the eye is limited by the difficulty in transporting effective doses of drugs to the vitreous, retina, and choroid. Topically applied drugs are poorly absorbed due to the low permeability of the external ocular tissues and tearing. The blood-retina barrier limits drug diffusion from the systemic blood to the posterior segment, thus high doses of drug are needed to maintain therapeutic levels. In addition, systemic side effects are common. Intraocular injections could be an alternative, but the fast flowing blood supply in this region, associated with rapid clearance rates, causes drug concentration to quickly fall below therapeutic levels. To obtain therapeutic levels over longer time periods, polymeric sustained-drug release systems implanted within the vitreous are being studied for the treatment of vitreoretinal disorders. These systems are prepared using different kinds of biodegradable or non-biodegradable polymers. This review aims to demonstrate the main characteristics of these drug delivery implants and their potential for clinical application.

Sustained-release ophthalmic drug delivery systems for treatment of macular disorders: present and future applications

Macular disease currently poses the greatest threat to vision in aging populations. Historically, most of this pathology could only be dealt with surgically, and then only after much damage to the macula had already occurred. Current pathophysiological insights into macular diseases have allowed the development of effective new pharmacotherapies. The field of drug delivery systems has advanced over the last several years with emphasis placed on controlled release of drug to specific areas of the eye. Its unique location and tendency toward chronic disease make the macula an important and attractive target for drug delivery systems, especially sustained-release systems. This review evaluates the current literature on the research and development of sustained-release posterior segment drug delivery systems that are primarily intended for macular disease with an emphasis on age-related macular degeneration.Current effective therapies include corticosteroids and anti-vascular endothelial growth factor compounds. Recent successes have been reported using anti-angiogenic drugs for therapy of age-related macular degeneration. This review also includes information on implantable devices (biodegradable and non-biodegradable), the use of injected particles (microspheres and liposomes) and future enhanced drug delivery systems, such as ultrasound drug delivery. The devices reviewed show significant drug release over a period of days or weeks. However, macular disorders are chronic diseases requiring years of treatment. Currently, there is no 'gold standard' for therapy and/or drug delivery. Future studies will focus on improving the efficiency and effectiveness of drug delivery to the posterior chamber. If successful, therapeutic modalities will significantly delay loss of vision and improve the quality of life for patients with chronic macular disorders.