Feasibility of drug targeting to the retinal pigment epithelium with biodegradable microspheres

Polymers and their engineered analogues for ocular drug delivery: Enhancing therapeutic precision

Ocular drug delivery is constrained by anatomical and physiological barriers, necessitating innovative solutions for effective therapy. Natural polymers like hyaluronic acid, chitosan, and gelatin, alongside synthetic counterparts such as PLGA and PEG, have gained prominence for their biocompatibility and controlled release profiles. Recent strides in polymer conjugation strategies have enabled targeted delivery through ligand integration, facilitating tissue specificity and cellular uptake. This versatility accommodates combined drug delivery, addressing diverse anterior (e.g., glaucoma, dry eye) and posterior segment (e.g., macular degeneration, diabetic retinopathy) afflictions. The review encompasses an in-depth exploration of each natural and synthetic polymer, detailing their individual advantages and disadvantages for ocular drug delivery. By transcending ocular barriers and refining therapeutic precision, these innovations promise to reshape the management of anterior and posterior segment eye diseases.

Ocular barriers as a double-edged sword: preventing and facilitating drug delivery to the retina

In recent decades, the growing of the aging population in the world brings increasingly heavy burden of vision-threatening retinal diseases. One of the biggest challenges in the treatment of retinal diseases is the effective drug delivery to the diseased area. Due to the existence of multiple anatomical and physiological barriers of the eye, commonly used oral drugs or topical eye drops cannot effectively reach the retinal lesions. Innovations in new drug formulations and delivery routes have been continuously applied to improve current drug delivery to the back of the eye. Unique ocular anatomical structures or physiological activities on these ocular barriers, in turn, can facilitate drug delivery to the retina if compatible formulations or delivery routes are properly designed or selected. This paper focuses on key barrier structures of the eye and summarizes advances of corresponding drug delivery means to the retina, including various local drug delivery routes by invasive approaches, as well as systemic eye drug delivery by non-invasive approaches.

Long-acting formulation strategies for protein and peptide delivery in the treatment of PSED

The invigoration of protein and peptides in serious eye disease includes age-related macular degeneration, choroidal neovascularization, retinal neovascularization, and diabetic retinopathy. The transportation of macromolecules like aptamers, recombinant proteins, and monoclonal antibodies to the posterior segment of the eye is challenging due to their high molecular weight, rapid degradation, and low solubility. Moreover, it requires frequent administration for prolonged therapy. The long-acting novel formulation strategies are helpful to overcome these issues and provide superior therapy. It avoids frequent administration, improves stability, high retention time, and avoids burst release. This review briefly enlightens posterior segments of eye diseases with their diagnosis techniques and treatments. This article mainly focuses on recent advanced approaches like intravitreal implants and injectables, electrospun injectables, 3D printed drug-loaded implants, nanostructure thin-film polymer devices encapsulated cell technology-based intravitreal implants, injectable and depots, microneedles, PDS with ranibizumab, polymer nanoparticles, inorganic nanoparticles, hydrogels and microparticles for delivering macromolecules in the eye for intended therapy. Furthermore, novel techniques like aptamer, small Interference RNA, and stem cell therapy were also discussed. It is predicted that these systems will make revolutionary changes in treating posterior segment eye diseases in future.

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.

Drug delivery to the posterior segment of the eye for pharmacologic therapy

Treatment of diseases of the posterior segment of the eye, such as age-related macular degeneration, cytomegalovirus retinitis, diabetic retinopathy, posterior uveitis and retinitis pigmentosa, requires novel drug delivery systems that can overcome the many barriers for efficacious delivery of therapeutic drug concentrations. This challenge has prompted the development of biodegradable and nonbiodegradable sustained-release systems for injection or transplantation into the vitreous as well as drug-loaded nanoparticles, microspheres and liposomes. These drug delivery systems utilize topical, systemic, subconjunctival, intravitreal, transscleral and iontophoretic routes of administration. The focus of research has been the development of methods that will increase the efficacy of spatiotemporal drug application, resulting in more successful therapy for patients with posterior segment diseases. This article summarizes recent advances in the research and development of drug delivery methods of the posterior chamber of the eye, with an emphasis on the use of implantable devices as well as micro- and nanoparticles.

Intraocular sustained-release delivery systems for triamcinolone acetonide

Recently, the use of triamcinolone acetonide (TA) injection has increased dramatically in treatment for several ocular diseases. Among them, macular diseases such as macular edema due to diabetic retinopathy, venous occlusive diseases, ocular inflammation and age-related macular degeneration (AMD) are very common vision threatening disorders and are great challenges to treat. In these types of chronic retinal diseases, repeated intraocular injections of TA are often required which increases the likelihood of complications. In order to achieve sustained-release, maintain therapeutic levels of TA over longer times and reduce frequency of intravitreal injections, researchers are investigating different implantable devices or injectable systems. However, as of yet, there is no sustained-release product for TA available on the commercial market. This review discusses and compares different sustained-release devices or injectable systems that are currently being developed.

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.

Drug delivery systems for vitreoretinal diseases

The eye has an environment that is specific unto itself in terms of pharmacokinetics: the inner and outer blood-retinal barriers separate the retina and the vitreous from the systemic circulation and vitreous body, which physiologically has no cellular components, occupies the vitreous cavity, an inner space of the eye, and reduces practical convection of molecules. Considering this, development of a drug delivery system (DDS) is becoming increasingly important in the treatment of vitreoretinal diseases not only to facilitate drug efficacy but also to attenuate adverse effects. The DDS has three major goals: enhances drug permeation (e.g., iontophoresis and transscleral DDS), controls release of drugs (e.g., microspheres, liposomes, and intraocular implants), and targets drugs (e.g., prodrugs with high molecular weight and immunoconjugates). Comprehensive knowledge of these should lead to development of innovative treatment modalities.

Starch acetate microparticles for drug delivery into retinal pigment epithelium—in vitro study

Starch acetates are novel biodegradable polymers which undergo slower degradation and swelling than native starch. Retinal pigment epithelium (RPE) is an important target tissue in ocular treatment. The cellular uptake of starch acetate microparticles and degradation of starch acetate by cultured human RPE-cell line (D407) was examined. Calcein-containing starch acetate microparticles were prepared by a modified water-in-oil-in-water double-emulsion technique. The cellular uptake of the starch acetate microparticles was analysed using flow cytometry and confocal microscopy. Degradation of starch acetate films by the homogenate of lysed RPE cells was determined by gel permeation chromatography. The effect of the microparticles on RPE cell viability was determined by the MTT colorimetric assay. The mean diameter (D50%) of microparticles was 11 microm. During 3-h incubation in RPE-cell culture, 8.1 +/- 0.8% of D407 cells took up starch acetate microparticles. Confocal microscopy confirmed the internalisation of microparticles. Incubation of the starch acetate film in the RPE-cell homogenate considerably decreased the molecular weight of starch acetate in the film during 24 h. The viability of cultured RPE cells was at least 82% after 24-h incubation with the microparticles. The present results show that the starch acetate microparticles are taken up by the RPE cells and the polymer can be degraded by the enzymes in these cells. Starch acetate microparticles may be suitable for drug delivery to the RPE.

Polyhydroxyalkanonate derivatives in current clinical applications and trials

Polyhydroxyalkanonate is a typical biodegradable material, which is permitted for use in the medical and pharmaceutical fields. For its biodegradability, biocompatibility, and toxicological safety, the majority of products practically used are composed of homo-polymers of poly(lactic acid), poly(glycolic acid), and poly(epsilon-caprolactone) and their co-polymers. On the market, suture strings are still the main usage. The needs of biodegradable materials have been being gradually increased by the development of drug delivery systems, tissue engineering, and regenerative medicine. Some types of formulation, that is, mono-fibers, twisted fibers, films, fabrics, sponges, and injectable particles are developed to match each purpose. This article reviews the current clinical applications and trials of polyhydroxyalcanonate products.

Retinal pigment epithelium engineering using synthetic biodegradable polymers

Retinal pigment epithelium (RPE) plays a key role in the maintenance of the normal functions of the retina, especially photoreceptors. Alteration in RPE structure and function is implicated in a variety of ocular disorders. Tissue engineering strategies using synthetic biodegradable polymers as temporary substrates for RPE cell culture and subsequent transplantation may provide a promising new therapy. In this review article, the manufacture of thin biodegradable poly(DL-lactic-co-glycolic acid) (PLGA) films and their degradation behavior in vitro are discussed. RPE cell proliferation and differentiation on these PLGA films are reviewed. The fabrication of model substrates with desired chemical micropatterns in the micrometer scale is discussed and the effects of surface patterning on RPE morphology and function are assessed. Finally. the preparation of biodegradable micropatterns with adhesive PLGA and non-adhesive poly(ethylene glycol)/PLA domains to modulate RPE cell adhesion is presented.

Biodegradable microspheres for vitreoretinal drug delivery

Vitreoretinal disorders are one of the major causes of blindness in the developed world. Treatments of these pathologies often include repeated intravitreous injections to achieve intraocular drug levels within the therapeutical range. However, the risks of complications increase with the frequency of intravitreous injections. Controlled drug delivery formulations, offer an excellent alternative to multiple administrations. These systems are capable of delivering drugs over longer time periods than conventional formulations. Currently, several kinds of polymer devices for drug delivery to the posterior segment of the eye are under clinical use, or under investigation. Among these devices, microparticulates, such as microspheres, provide an alternative to multiple injections to obtain sustained release of the drug with a single administration. Among the polymers used to make the injectable microparticles, the most commonly used are poly(lactic acid), poly(glycolic acid) and copolymers of lactic and glycolic acids because they are biocompatible and degrade to metabolic products that are easily eliminated from the body. This article reviews the literature of biodegradable polymeric microspheres loaded with drugs, that have been investigated for delivery by intravitreous injection to treat diverse vitreoretinal diseases.

Biodegradable scleral implant for intravitreal controlled release of fluconazole

PURPOSE

To evaluate the feasibility of using a biodegradable polymeric scleral implant containing fluconazole (FLCZ), a bis-triazole antifungal agent, as a potential intravitreal-controlled drug delivery system.

METHODS

The scleral implants, loaded with 10, 20, 30, and 50% FLCZ, were prepared with biodegradable polymers of poly (DL-lactide-co-glycolide). Those with all loading doses were used for the in vitro release studies; those with 30% FLCZ were used for the intravitreal release studies in pigmented rabbits. The in vitro and in vivo release rates of FLCZ from the implants were measured periodically with spectrophotometry and high performance liquid chromatography, respectively. The effects of the implants on ocular tissues were evaluated ophthalmoscopically, histologically, and electrophysiologically.

RESULTS

The scleral implants loaded with 10, 20, and 30% doses gradually released FLCZ over 4 weeks in vitro; those with 50% FLCZ released most of the drug in one week. FLCZ concentration in the rabbit vitreous remained within the 99% inhibitory concentration for Candida albicans for 3 weeks after implantation. The scleral implant gradually biodegraded, and it disappeared by 4 months after implantation. The electrophysiologic and histopathologic findings demonstrated no substantial toxic reactions in the ocular tissues.

CONCLUSION

The current study suggests that a biodegradable, polymeric scleral implant containing FLCZ may be a promising intravitreal drug delivery system to treat fungal endophthalmitis.

In vitro phagocytosis of polylactide microspheres by retinal pigment epithelial cells and intracellular drug release

We investigated phagocytosis of biodegradable microspheres containing a drug by retinal pigment epithelial (RPE) cells and drug release within the cells to evaluate the potential usefulness of microspheres for intracellular drug delivery. The biodegradable polymers used were L-lactic acid, and DL-lactic acid with different molecular weights or the copolymers of different monomer compositions. The microspheres containing a non-bioactive fluorescent dye (rhodamine 6GX) as a model drug, were prepared by a solvent evaporation method. The in vitro release of the dye from the microspheres was examined. Phagocytosis of the microspheres by RPE cells was conducted to evaluate the extent of phagocytosis by phase-contrast microscopy and transmission electron microscopy. The RPE cells ingesting the microspheres at different stages were examined by fluorescent microscopy to estimate the intracellular release of the dye. The dye was released with time from every microsphere and the release was controlled by changing the type of polymers constituting microspheres. The microspheres containing the dye were phagocytosed by RPE cells and the dye was released intracellularly with time. The present study indicates that the drug incorporated in the microspheres was delivered into RPE cells by way of phagocytosis and released within the cells. It is concluded that this microsphere system is a promising delivery form capable of drug targeting to RPE cells.