Recent advances in ophthalmic drug delivery

Ultrasound-enhanced ocular delivery of dexamethasone sodium phosphate: an in vivo study

BACKGROUND

The eye's unique anatomy and its physiological and anatomical barriers can limit effective drug delivery into the eye.

METHODS

An in vivo study was designed to determine the effectiveness and safety of ultrasound application in enhancing drug delivery in a rabbit model. Permeability of a steroid ophthalmic drug, dexamethasone sodium phosphate, was investigated in ultrasound- and sham-treated cases. For this study, an eye cup filled with dexamethasone sodium phosphate was placed on the cornea. Ultrasound was applied at intensity of 0.8 W/cm(2) and frequency of 400 or 600 kHz for 5 min. The drug concentration in aqueous humor samples, collected 90 min after the treatment, was determined using chromatography methods. Light microscopy observations were done to determine the structural changes in the cornea as a result of ultrasound application.

RESULTS

An increase in drug concentration in aqueous humor samples of 2.8 times (p < 0.05) with ultrasound application at 400 kHz and 2.4 times (p < 0.01) with ultrasound application at 600 kHz was observed as compared to sham-treated samples. Histological analysis showed that the structural changes in the corneas exposed to ultrasound predominantly consisted of minor epithelial disorganization.

CONCLUSIONS

Ultrasound application enhanced the delivery of an anti-inflammatory ocular drug, dexamethasone sodium phosphate, through the cornea in vivo. Ultrasound-enhanced ocular drug delivery appears to be a promising area of research with a potential future application in a clinical setting.

Thiolated hyaluronan-based hydrogels crosslinked using oxidized glutathione: an injectable matrix designed for ophthalmic applications

Future ophthalmic therapeutics will require the sustained delivery of bioactive proteins and nucleic acid-based macromolecules and/or provide a suitable microenvironment for the localization and sustenance of reparative progenitor cells after transplantation into or onto the eye. Water-rich hydrogels are ideal vehicles for such cargo, but few have all the qualities desired for novel ophthalmic use, namely in situ gelation speed, cytocompatibility, biocompatibility and capacity to functionalize. We describe here the development of an ophthalmic-compatible crosslinking system using oxidized glutathione (GSSG), a physiologically relevant molecule with a history of safe use in humans. When GSSG is used in conjunction with an existing hyaluronate-based, in situ crosslinkable hydrogel platform, gels form in less than 5 min using the thiol-disulfide exchange reaction. This GSSG hydrogel supports the 3-D culture of adipose-derived stem cells in vitro and shows biocompatibility in preliminary intracutaneous and subconjunctival experiments in vivo. In addition, the thiol-disulfide exchange reaction can also be used in conjunction with other thiol-compatible chemistries to covalently link peptides for more complex formulations. These data suggest that this hydrogel could be well suited for local ocular delivery, focusing initially on front of the eye therapies. Subsequent uses of the hydrogel include delivery of back of the eye treatments and eventually into other soft, hyaluronan-rich tissues such as those from the liver and brain.

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

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

Inflammation and pharmacological treatment in diabetic retinopathy

Diabetic retinopathy (DR), the most common microvascular complication of diabetes mellitus, is estimated to be the leading cause of new blindness in the working population of developed countries. Primary interventions such as intensive glycemic control, strict blood pressure regulation, and lipid-modifying therapy as well as local ocular treatment (laser photocoagulation and pars plana vitrectomy) can significantly reduce the risk of retinopathy occurrence and progression. Considering the limitations of current DR treatments development of new therapeutic strategies, it becomes necessary to focus on pharmacological treatment. Currently, there is increasing evidence that inflammatory processes have a considerable role in the pathogenesis of DR with multiple studies showing an association of various systemic as well as local (vitreous and aqueous fluid) inflammatory factors and the progression of DR. Since inflammation is identified as a relevant mechanism, significant effort has been directed to the development of new concepts for the prevention and treatment of DR acting on the inflammatory processes and the use of pharmacological agents with anti-inflammatory effect. Inhibiting the inflammatory pathway could be an appealing treatment option for DR in future practices, and as further prospective randomized clinical trials accumulate data, the role and guidelines of anti-inflammatory pharmacologic treatments will become clearer.

Soft contact lenses for controlled ocular delivery: 50 years in the making

The use of contact lenses as ocular bandages for drug delivery was envisioned nearly 50 years ago by Wichterle and co-workers. Despite the therapeutic advantages that can be obtained, this application has to face up to the poor affinity shown by commercially available contact lenses for most ophthalmic drugs, resulting in small amounts of drug being loaded and short time of therapeutic levels in the eye structures. Novel strategies that appeared in the beginning of 21st century, for example coating lenses with vitamin E, incorporation of drug nanocarriers or application of molecular imprinting technology, are becoming relevant tools for development of true drug/contact lens combination products that may be available for ocular therapy in the foreseeable future.

Topical delivery of ocular therapeutics: carrier systems and physical methods

OBJECTIVE

The basic concepts, major mechanisms, technological developments and advantages of the topical application of lipid-based systems (microemulsions, nanoemulsions, liposomes and solid lipid nanoparticles), polymeric systems (hydrogels, contact lenses, polymeric nanoparticles and dendrimers) and physical methods (iontophoresis and sonophoresis) will be reviewed.

KEY FINDINGS

Although very convenient for patients, topical administration of conventional drug formulations for the treatment of eye diseases requires high drug doses, frequent administration and rarely provides high drug bioavailability. Thus, strategies to improve the efficacy of topical treatments have been extensively investigated. In general, the majority of the successful delivery systems are present on the ocular surface over an extended period of time, and these systems typically improve drug bioavailability in the anterior chamber whereas the physical methods facilitate drug penetration over a very short period of time through ocular barriers, such as the cornea and sclera.

SUMMARY

Although in the early stages, the combination of these delivery systems with physical methods would appear to be a promising tool to decrease the dose and frequency of administration; thereby, patient compliance and treatment efficacy will be improved.

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

OBJECTIVES

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

KEY FINDINGS

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

SUMMARY

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

Modulation of ocular surface glycocalyx barrier function by a galectin-3 N-terminal deletion mutant and membrane-anchored synthetic glycopolymers

Background

Interaction of transmembrane mucins with the multivalent carbohydrate-binding protein galectin-3 is critical to maintaining the integrity of the ocular surface epithelial glycocalyx. This study aimed to determine whether disruption of galectin-3 multimerization and insertion of synthetic glycopolymers in the plasma membrane could be used to modulate glycocalyx barrier function in corneal epithelial cells.

Methodology/Principal Findings

Abrogation of galectin-3 biosynthesis in multilayered cultures of human corneal epithelial cells using siRNA, and in galectin-3 null mice, resulted in significant loss of corneal barrier function, as indicated by increased permeability to the rose bengal diagnostic dye. Addition of β-lactose, a competitive carbohydrate inhibitor of galectin-3 binding activity, to the cell culture system, transiently disrupted barrier function. In these experiments, treatment with a dominant negative inhibitor of galectin-3 polymerization lacking the N-terminal domain, but not full-length galectin-3, prevented the recovery of barrier function to basal levels. As determined by fluorescence microscopy, both cellobiose- and lactose-containing glycopolymers incorporated into apical membranes of corneal epithelial cells, independently of the chain length distribution of the densely glycosylated, polymeric backbones. Membrane incorporation of cellobiose glycopolymers impaired barrier function in corneal epithelial cells, contrary to their lactose-containing counterparts, which bound to galectin-3 in pull-down assays.

Conclusions/Significance

These results indicate that galectin-3 multimerization and surface recognition of lactosyl residues is required to maintain glycocalyx barrier function at the ocular surface. Transient modification of galectin-3 binding could be therapeutically used to enhance the efficiency of topical drug delivery.

Lipogels: surface-adherent composite hydrogels assembled from poly(vinyl alcohol) and liposomes

Drug-eluting engineered surface coatings are of paramount importance for many biomedical applications from implantable devices to tissue engineering. Herein, we present the assembly of lipogels, composite physical hydrogels assembled from poly(vinyl alcohol) and liposomes using thiol-disulfide exchange between end group modified PVA and thiocholesterol containing liposomes, and the response of adhering cells to these coatings. We demonstrate the controlled loading of liposomes into the polymer matrix and the preserved mechanical properties of the lipogels. Furthermore, the lipogels are successfully rendered cell adhesive by incorporation of poly(l-lysine) into the PVA polymer matrix or by poly(dopamine) coating of the lipogels. The successful lipid uptake from the lipogels by macrophages, hepatocytes, and myoblasts was monitored by flow cytometry. Finally, the delivery of active cargo, paclitaxel, to adherent myoblasts is shown, thus illustrating the potential of the lipogels as a drug eluting interface for biomedical applications.

Intravitreal Corticosteroids in the Management of Diabetic Macular Edema

Diabetic macular edema (DME) remains an important worldwide cause of visual loss. Corticosteroids have a role in the treatment of some patients with advanced or recurrent DME. The best studied steroids for this indication are triamcinolone acetonide, dexamethasone, and fluocinolone acetonide. All steroids are associated with risks of cataract and intraocular pressure elevation. In addition, intravitreal injection of any medication is associated with risks of infectious endophthalmitis, which has led to the investigation of various extended-release steroid implants. At this time, no steroid is approved by the United States Food and Drug Administration (FDA) for the treatment of DME.

Suprachoroidal delivery in a rabbit ex vivo eye model: influence of drug properties, regional differences in delivery, and comparison with intravitreal and intracameral routes

PURPOSE

First, to determine the influence of drug lipophilicity (using eight beta-blockers) and molecular weight (using 4 kDa and 40 kDa fluoroscein isothiocyanate [FITC]-dextrans) on suprachoroidal delivery to the posterior segment of the eye by using a rabbit ex vivo eye model. Second, to determine whether drug distribution differs between the dosed and undosed side of the eye following suprachoroidal delivery. Third, to compare the suprachoroidal delivery of sodium fluorescein (NaF) with the intracameral and intravitreal routes by using noninvasive fluorophotometry.

METHODS

Using a small hypodermic 26G needle (3/8") with a short bevel (250 µm), location of the suprachoroidal injection in an ex vivo New Zealand white rabbit eye model was confirmed with India ink. Ocular tissue distribution of NaF (25 µl of 1.5 µg/ml) at 37 °C was monitored noninvasively using the Fluorotron Master(TM) at 0, 1, and 3 h following suprachoroidal, intravitreal, or intracameral injections in ex vivo rabbit eyes. For assessing the influence of lipophilicity and molecular size, 25 µl of a mixture of eight beta-blockers (250 µg/ml each) or FITC-dextran (4 kDa and 40 kDa, 30 mg/ml) was injected into the suprachoroidal space of excised rabbit eyes and incubated at 37 °C. Eyes were incubated for 1 and 3 h, and frozen at the end of incubation. Ocular tissues were isolated in frozen condition. Beta-blocker and FITC-dextran levels in excised ocular tissue were measured by liquid chromatography-tandem mass spectrometry and spectrofluorometry, respectively.

RESULTS

Histological sections of India ink-injected albino rabbit eye showed the localization of dye as a black line in the suprachoroidal space. Suprachoroidal injection of NaF showed signal localization to the choroid and retina at 1 and 3 h post injection when compared with intravitreal and intracameral injections. Drug delivery to the vitreous after suprachoroidal injection decreased with an increase in solute lipophilicity and molecular weight. With an increase in drug lipophilicity, drug levels in the choroid-retinal pigment epithelium (RPE) and retina generally increased with some exceptions. Beta-blockers and FITC-dextrans were localized more to the dosed side when compared to the opposite side of the sclera, choroid-RPE, retina, and vitreous. These differences were greater for FITC-dextrans as compared to the beta-blockers.

CONCLUSIONS

The suprachoroidal route of injection allows localized delivery to the choroid-RPE and retina for small as well as large molecules. Suprachoroidal drug delivery to the vitreous declines with an increase in drug lipophilicity and molecular weight. Drug delivery differs between the dosed and opposite sides following suprachoroidal injection, at least up to 3 h.

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.

Antioxidants and cataract

The major causes for cataract formation are free radicals, and these free radicals are neutralized by the presence of endogenous antioxidants in the eye. Using xenobiotics, it has been confirmed that free radicals mediate the formation of cataract. Two cataract model-selenite model and the diabetic cataract model-have been developed to study the pathophysiology of cataract formation due to free radicals and the role of antioxidants during the process of cataractogenesis. This review focuses on natural compounds with antioxidant properties that could actually be applied as an interventional strategy on a large scale and are also relatively inexpensive. A brief overview of plants with antioxidant properties that in addition possess potential anti-cataract properties has been discussed. In addition to plants, three natural compounds (curcumin, vitamin C and vitamin E), on which a lot of data exist showing anti-cataract and antioxidant activities, have also been discussed. These antioxidants can be supplemented in the diet for a better defence against free radicals. Studies on vitamin C and vitamin E have proved that they are capable of preventing lipid peroxidation, thereby preventing the generation of free radicals, but their efficacy as anti-cataract agent is questionable. Unlike vitamins C and E, curcumin is well established as an anti-cataract agent, but the issue of curcumin bioavailability is yet to be addressed. Nanotechnology proves to be a promising area in increasing the curcumin bioavailability, but still a lot more research needs to be done before the use of curcumin as an effective anti-cataract agent for humans.

Immunohistochemical and functional characterization of peptide, organic cation, neutral and basic amino acid, and monocarboxylate drug transporters in human ocular tissues

Since there is paucity of information on solute transporters in human ocular tissues, the aim of this study was immunohistochemical and functional characterization of peptide transporters (PEPT), organic cation transporters (OCTs), neutral and basic amino acid transporters (ATB(0,+)), and monocarboxylate transporters (MCTs) in human ocular barriers. Immunohistochemical localization of transporters was achieved using 5-µm-thick paraffin-embedded sections of whole human eyes. In vitro transport studies were carried out across human cornea and sclera-choroid-retinal pigment epithelium (SCRPE) using a cassette of specific substrates in the presence and absence of inhibitors to determine the role of transporters in transtissue solute delivery. Immunohistochemistry showed the expression of PEPT-1, PEPT-2, ATB(0,+), OCT-1, OCT-2, MCT-1, and MCT-3 in human ocular tissues. PEPT-1, PEPT-2, OCT-1, MCT-1, and ATB(0,+) expression was evident in the cornea, conjunctiva, ciliary epithelium, and neural retina. Expression of PEPT-1, PEPT-2, and OCT-1 was evident in choroid tissue as well. OCT-2 expression could be seen in the corneal and conjunctival epithelia, whereas MCT-3 expression was confined to the RPE layer. OCT-2 expression was evident in conjunctival blood vessel walls, whereas PEPT-1, PEPT-2, and OCT-1 were expressed in the choroid. Preliminary transport studies indicated inward transport of Gly-Sar (PEPT substrate), 1-methyl-4-phenylpyridinium (MPP+) (OCT substrate), and l-tryptophan (ATB(0,+) substrate) across cornea as well as SCRPE. For phenylacetic acid (MCT substrate), transporter-mediated inward transport across the cornea and outward transport across SCRPE were evident. Thus, PEPT, OCT, and ATB(0,+) are influx transporters present in human ocular barriers, and they can potentially be used for transporter-guided retinal drug delivery after topical, transscleral, and systemic administrations.

Mucoadhesive chitosan-dextran sulfate nanoparticles for sustained drug delivery to the ocular surface

PURPOSE

To characterize nanoparticles produced by self-assembly of oppositely charged polymers, cationic chitosan (CS), and anionic dextran sulfate (DS), for drug delivery to the ocular surface. The goal is to overcome the short residence time of topical drugs through their sustained release from mucoadhesive nanoparticles.

METHODS

Chitosan-dextran sulfate nanoparticles (CDNs) were produced by mixing CS and DS; polyethylene glycol-400 was used as a surface stabilizing agent. Fourier transform infrared spectroscopy (FTIR) spectra of CS, DS, and CDNs were determined in the wavenumber range of 4,000-700 cm(-1) to assess the ionic interactions in the formation of CDNs. The physicochemical properties, entrapment efficacy, and dissolution profile of CDNs were investigated using Rhodamine B (RhB) and Nile Red (NR) as drug analogs. The mucoadhesiveness of the CDNs was assessed by imaging the retention of the fluorescein isothiocyanate-labeled CDNs on the cornea ex vivo, which was subjected to shear stress by a steady stream of saline solution.

RESULTS

CDNs were obtained by the polyelectrolyte complexation technique. The FTIR spectra of CDNs showed spectral shifts in the amine and sulfate regions, confirming an involvement of electrostatic interactions between cationic CS and anionic DS. The CDNs were spherical in shape and segregated. They possessed a particle size of ~400 nm with a polydispersity index of 0.3 and exhibited a zeta potential of ~40 mV. A high entrapment efficacy of up to 80% was observed with both RhB and NR. In the dissolution experiments, NR was released from CDNs within 60 min, but RhB was not released. This indicates that the release of drugs could depend on their molecular interactions with the particle. Exposure of CDNs to lysozyme, which is found in tears, had no effect on the mean particle size or the surface charge. Instillation of NR, RhB, and FITC in the presence of saline irrigation resulted in their rapid disappearance (<5 min) from the corneal surface. In contrast, fluorescent CDNs showed retention on the cornea even after 60 min.

CONCLUSIONS

Cationic and biocompatible mucoadhesive CDNs have been developed for sustained drug delivery to the ocular surface. The CDNs were stable to lysozyme and showed prolonged adherence to the corneal surface.

Development of sustained-release microspheres for the delivery of SAR 1118, an LFA-1 antagonist intended for the treatment of vascular complications of the eye

The objective of this study was to design 1, 3, and 6 month sustained-release poly (lactide-co-glycolide) (PLGA) microspheres of SAR 1118, a lymphocyte function-associated antigen-1 antagonist, using Design of Experiments. A full-factorial design was used to identify the polymers suitable for degradation in 1, 3, and 6 months and the Box-Behnken design was used to study the influence of the polymer type, polymer concentration, and drug to polymer ratio on drug loading, burst release, and particle size. From the full-factorial design, PLGA (50:50), PLGA (75:25), and PLGA (85:15) with an inherent viscosity of 0.3-0.5 dL/g were identified as polymers suitable for degradation in 1, 3, and 6 months, respectively. From the Box-Behnken design, the optimized polymer concentration (12% w/v) and drug to polymer ratio (0.15) were identified and used to prepare the SAR 1118-encapsulated microspheres with the above 3 polymers and evaluated for drug loading, burst release, and sustained drug release. The burst release in these 3 batches was less than 20% and the drug loading ranged from 15%-18%. More than 90% of SAR 1118 release from PLGA (50:50), PLGA (75:25), and PLGA (85:15) microspheres occurred in 1, 3, and 6 months, respectively. Thus, the in vitro cumulative release data are remarkably close to the predicted values. The results demonstrated the potential of the Design of Experiments in designing the SAR 1118 microspheres with a high loading efficiency, low burst release, and sustained release for a desired duration.

Suprachoroidal injection of ketorolac tromethamine does not cause retinal damage

Rabbit right eyes were injected with 3 or 6 mg ketorolac tromethamine into the suprachoroidal space. Electroretinography results demonstrated no abnormal changes in rod cell response, maximum rod cell or cone cell mixing reaction, oscillation potential, cone cell response, waveform, amplitude, and potential of 30 Hz scintillation response in right eyes before injection, and at 1, 2, and 4 weeks after injection. There was no difference between left (control) and right eyes. Under light microscopy, the histomorphology of cells in each retinal layer was normal at 4 weeks following 6 mg ketorolac tromethamine administration. These results indicate that a single suprachoroidal injection of 3 or 6 mg ketorolac tromethamine into rabbits was safe. Suprachoroidal space injection appears to be safe.

Recent advances in ocular drug delivery

Amongst the various routes of drug delivery, the field of ocular drug delivery is one of the most interesting and challenging endeavors facing the pharmaceutical scientist. Recent research has focused on the characteristic advantages and limitations of the various drug delivery systems, and further research will be required before the ideal system can be developed. Administration of drugs to the ocular region with conventional delivery systems leads to short contact time of the formulations on the epithelium and fast elimination of drugs. This transient residence time involves poor bioavailability of drugs which can be explained by the tear production, non-productive absorption and impermeability of corneal epithelium. Anatomy of the eye is shortly presented and is connected with ophthalmic delivery and bioavailability of drugs. In the present update on ocular dosage forms, chemical delivery systems such as prodrugs, the use of cyclodextrins to increase solubility of various drugs, the concept of penetration enhancers and other ocular drug delivery systems such as polymeric gels, bioadhesive hydrogels, in-situ forming gels with temperature-, pH-, or osmotically induced gelation, combination of polymers and colloidal systems such as liposomes, niosomes, cubosomes, microemulsions, nanoemulsions and nanoparticles are discussed. Novel ophthalmic delivery systems propose the use of many excipients to increase the viscosity or the bioadhesion of the product. New formulations like gels or colloidal systems have been tested with numerous active substances by in vitro and in vivo studies. Sustained drug release and increase in drug bioavailability have been obtained, offering the promise of innovation in drug delivery systems for ocular administration. Combining different properties of pharmaceutical formulations appears to offer a genuine synergy in bioavailability and sustained release. Promising results are obtained with colloidal systems which present very comfortable conditions of use and prolonged action.

Therapeutic applications of ultrasound in ophthalmology

Therapeutic ultrasound, although less well known than ultrasound for diagnostic imaging, has become a topic of growing interest in ophthalmology. High intensity focused ultrasound (HIFU) for the treatment of glaucoma and ultrasonic drug delivery are the two main areas of research and potential clinical applications. For the treatment of glaucoma, the specific advantage of HIFU, particularly when compared to the laser, is that the energy can be focused through optically opaque media, especially through the sclera which is a strongly light-scattering medium. HIFU is therefore a possible method for partial coagulation of the ciliary body (an anatomical structure responsible for the production of the liquid filling the eye) and, hence, reducing intraocular pressure and the risk of glaucoma. Ocular drug bioavailability also remains a challenge, being limited by multiple barriers to drug entry and lacrimal drainage, and making it difficult to achieve a sufficient drug concentration for numerous diseases of the front and back of the eye. As the front wall of the eye (cornea and anterior sclera) is a pathway for topically applied drugs, locally applied ultrasound has been proposed as a way of enhancing the delivery and activity of drugs and genes. Despite the fact that experimental studies seem to confirm the potential benefit of ultrasound ocular drug delivery, there is still a lack of clinical evidence. The aim of this contribution is to provide an update on recent advances in the field of therapeutic ultrasound in ophthalmology.

Pigmented-MDCK (P-MDCK) cell line with tunable melanin expression: an in vitro model for the outer blood-retinal barrier

Retinal pigment epithelium, which forms the outer blood-retinal barrier, is a critical barrier for transport of drugs to the retina. The purpose of this study was to develop a pigmented MDCK (P-MDCK) cell line as a rapidly established in vitro model for the outer blood-retinal barrier to assess the influence of melanin pigment on solute permeability. A melanin synthesizing P-MDCK cell line was developed by lentiviral transduction of human tyrosinase and p-protein genes in MDCK (NBL-2) cells. Melanin content, tyrosinase activity (conversion of L-dopa to dopachrome), and transepithelial electrical resistance (TEER) were measured. Expression of tyrosinase protein and p-protein in P-MDCK cells was confirmed by confocal microscopy. Effect of l-tyrosine (0 to 2 mM) in culture medium on melanin synthesis in P-MDCK cells was evaluated. Cell uptake and transepithelial transport of pigment-binding chloroquine (Log D = 1.59) and a negative control salicylic acid (Log D = -1.14) were investigated. P-MDCK cells expressed tyrosinase and p-protein. Tyrosinase activity was 4.5-fold higher in P-MDCK cells compared to wild type MDCK cells. The transepithelial electrical resistance stabilized by day 4 in both cell types, with the TEER being 958 ± 33 and 964 ± 58 Ω·cm(2) for P-MDCK and wild type cells, respectively. Melanin content in P-MDCK cells depended on the concentration of l-tyrosine in culture medium, and increased from 3 to 54 μg/mg protein with an increase in l-tyrosine content from 0 to 2 mM. When the cells were grown in 2 mM l-tyrosine, uptake of chloroquine was 2.3-fold higher and the transepithelial transport was 2.2-fold lower in P-MDCK cells when compared to wild type MDCK cells. No significant difference was observed for both cell uptake and transport of salicylic acid. We developed a P-MDCK cell line with tunable melanin synthesis as a rapidly developing surrogate for retinal pigment epithelium.

RETRACTED: Influence of choroidal neovascularization and biodegradable polymeric particle size on transscleral sustained delivery of triamcinolone acetonide

PURPOSE

One objective of this study was to determine whether polymeric nanoparticles and/or microparticles sustain transscleral choroidal and retinal delivery of triamcinolone acetonide (TA) for two months in therapeutically effective concentrations after single periocular administration. Another objective of this study was to assess the influence of choroidal neovascularization on transscleral delivery of TA.

METHODS

Polymeric nano- and micro-particles of TA were prepared by o/w emulsion-solvent evaporation method using poly-l-lactide (PLA). Particles were characterized for drug loading, size, surface morphology, and the in vitro drug release profile. Choroidal neovascularization (CNV) was induced in brown Norway (BN) rats using a 532 nm diode argon laser and the CNV induction was assessed using fluorescein angiography. In vivo delivery was assessed in control and CNV induced rats at 2 months after periocular injection of TA loaded nano- or micro-particle suspension, or plain TA suspension in PBS (pH 7.4). Ocular tissue levels of TA were estimated using LC-MS/MS following liquid-liquid extraction of drug from tissue samples. Nile red loaded microparticles entrapped in periocular tissue at the end of the study was visualized using scanning electron microscopy and confocal microscopy. Inhibitory effect of TA on VEGF secretion was evaluated in ARPE-19 cells.

RESULTS

Triamcinolone acetonide-PLA nano- (551 nm) and micro-particles (2090 nm), with 14.7 and 29.5% drug loading, respectively, sustained in vitro TA release for about 45 and 120 days. After subconjunctival injection, microparticles were able to sustain the delivery in all intraocular tissues for 2 months; whereas no drug levels were detected for TA loaded nanoparticles and plain suspension of TA. Intraocular delivery of TA from microparticles was higher in CNV induced rats when compared to control rats. Significant amount of microparticles remained in periocular tissue at 2 months after injection, and maintained spherical shape. TA decreased VEGF secretion by 50% at 0.07 μM. At the end of the in vivo study, choroid-RPE and retina TA levels in CNV induced rats were 16- and 5-fold higher than the IC(50) for VEGF secretion.

CONCLUSIONS

Single periocular injection of polymeric microparticles but not nanoparticles sustained effective levels of TA in choroid-RPE and retina for 2 months, with the TA delivery being greater in CNV induced rats than the control rats.

Silibinin, dexamethasone, and doxycycline as potential therapeutic agents for treating vesicant-inflicted ocular injuries

There are no effective and approved therapies against devastating ocular injuries caused by vesicating chemical agents sulfur mustard (SM) and nitrogen mustard (NM). Herein, studies were carried out in rabbit corneal cultures to establish relevant ocular injury biomarkers with NM for screening potential efficacious agents in laboratory settings. NM (100nmol) exposure of the corneas for 2h (cultured for 24h), showed increases in epithelial thickness, ulceration, apoptotic cell death, epithelial detachment microbullae formation, and the levels of VEGF, cyclooxygenase-2 (COX-2) and matrix metalloproteinase-9 (MMP-9). Employing these biomarkers, efficacy studies were performed with agent treatments 2h and every 4h thereafter, for 24h following NM exposure. Three agents were evaluated, including prescription drugs dexamethasone (0.1%; anti-inflammatory steroid) and doxycycline (100nmol; antibiotic and MMP inhibitor) that have been studied earlier for treating vesicant-induced eye injuries. We also examined silibinin (100μg), a non-toxic natural flavanone found to be effective in treating SM analog-induced skin injuries in our earlier studies. Treatments of doxycycline+dexamethasone, and silibinin were more effective than doxycycline or dexamethasone alone in reversing NM-induced epithelial thickening, microbullae formation, apoptotic cell death, and MMP-9 elevation. However, dexamethasone and silibinin alone were more effective in reversing NM-induced VEGF levels. Doxycycline, dexamethasone and silibinin were all effective in reversing NM-induced COX-2 levels. Apart from therapeutic efficacy of doxycycline and dexamethasone, these results show strong multifunctional efficacy of silibinin in reversing NM-induced ocular injuries, which could help develop effective and safe therapeutics against ocular injuries by vesicants.

Hydrophilic prodrug approach for reduced pigment binding and enhanced transscleral retinal delivery of celecoxib

Transscleral retinal delivery of celecoxib, an anti-inflammatory and anti-VEGF agent, is restricted by its poor solubility and binding to the melanin pigment in choroid-RPE. The purpose of this study was to develop soluble prodrugs of celecoxib with reduced pigment binding and enhanced retinal delivery. Three hydrophilic amide prodrugs of celecoxib, celecoxib succinamidic acid (CSA), celecoxib maleamidic acid (CMA), and celecoxib acetamide (CAA) were synthesized and characterized for solubility and lipophilicity. In vitro melanin binding to natural melanin (Sepia officinalis) was estimated for all three prodrugs. In vitro transport studies across isolated bovine sclera and sclera-choroid-RPE (SCRPE) were performed. Prodrug with the highest permeability across SCRPE was characterized for metabolism and cytotoxicity and its in vivo transscleral delivery in pigmented rats. Aqueous solubilities of CSA, CMA, and CAA were 300-, 182-, and 76-fold higher, respectively, than celecoxib. Melanin binding affinity and capacity were significantly lower than for celecoxib for all three prodrugs. Rank order for the % in vitro transport across bovine sclera and SCRPE was CSA > CMA ~ CAA ~ celecoxib, with the transport being 8-fold higher for CSA than celecoxib. CSA was further assessed for its metabolic stability and in vivo delivery. CSA showed optimum metabolic stability in all eye tissues with only 10-20% conversion to parent celecoxib in 30 min. Metabolic enzymes responsible for bioconversion included amidases, esterase, and cytochrome P-450. In vivo delivery in pigmented BN rats showed that CSA had 4.7-, 1.4-, 3.3-, 6.0-, and 4.5-fold higher delivery to sclera, choroid-RPE, retina, vitreous, and lens than celecoxib. CSA has no cytotoxicity in ARPE-19 cells in the concentration range of 0.1 to 1000 μM. Celecoxib succinamidic acid, a soluble prodrug of celecoxib with reduced melanin binding, enhances transscleral retinal delivery of celecoxib.

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.

Effects of poly(amidoamine) dendrimers on ocular absorption of puerarin using microdialysis

PURPOSE

The purpose of this study was to investigate the effects of poly(amidoamine) (PAMAM) dendrimers on ocular absorption of puerarin.

METHODS

The samples of rabbits' aqueous humor were collected by in vivo microdialysis. The ocular pharmacokinetic properties of puerarin were measured to evaluate the effects of PAMAM dendrimers on ocular absorption of puerarin.

RESULTS

In the pharmacokinetic studies, the AUC(0∼∞) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers (G3.0, G4.0, G5.0) were greater than those in the control group by 2.3-, 3.5-, and 2.1-folds, respectively. The C(max) values of puerarin solution with 0.2% (w/v) PAMAM dendrimers were 1.5, 2.5, and 1.3 times the values of the control group and the t(1/2) values were significantly longer than that of control group.

CONCLUSIONS

Using PAMAM dendrimers might be a promising strategy to enhance the absorption of puerarin. The generation of PAMAM dendrimers may play a key role in increasing the ocular absorption of puerarin.

Influence of drug solubility and lipophilicity on transscleral retinal delivery of six corticosteroids

The influence of drug properties including solubility, lipophilicity, tissue partition coefficients, and in vitro transscleral permeability on ex vivo and in vivo transscleral delivery from corticosteroid suspensions was determined. Solubility, tissue/buffer partition coefficients for bovine sclera and choroid-retinal pigment epithelium (CRPE), and in vitro bovine sclera and sclera-choroid-retinal pigment epithelium (SCRPE) transscleral transport were determined at pH 7.4 for triamcinolone, prednisolone, dexamethasone, fluocinolone acetonide, triamcinolone acetonide, and budesonide in solution. Ex vivo and in vivo transscleral delivery was assessed in Brown Norway rats after posterior subconjunctival injection of a 1 mg/ml suspension of each corticosteroid. Corticosteroid solubility and partition coefficients ranged from ∼ 17 to 300 μg/ml and 3.0 to 11.4 for sclera and from 7.1 to 35.8 for CRPE, respectively, with the more lipophilic molecules partitioning more into both tissues. Transport across sclera and SCRPE was in the range of 3.9 to 10.7% and 0.3 to 1.8%, respectively, with the transport declining with an increase in lipophilicity. Ex vivo and in vivo transscleral delivery indicated tissue distribution in the order CRPE ≥ sclera > retina > vitreous. Tissue partitioning showed a positive correlation with drug lipophilicity (R(2) = 0.66-0.96). Ex vivo and in vivo sclera, CRPE, retina, and vitreous tissue levels of all corticosteroids showed strong positive correlation with drug solubility (R(2) = 0.91-1.0) but not lipophilicity (R(2) = 0.24-0.41) or tissue partitioning (R(2) = 0.24-0.46) when delivered as suspensions. In vivo delivery was lower in all eye tissues assessed than ex vivo delivery, with the in vivo/ex vivo ratios being the lowest in the vitreous (0.085-0.212). Upon exposure to corticosteroid suspensions ex vivo or in vivo, transscleral intraocular tissue distribution was primarily driven by the drug solubility.