Corneal and conjunctival/scleral penetration of p-aminoclonidine, AGN 190342, and clonidine in rabbit eyes

Lipid nanoparticle technology-mediated therapeutic gene manipulation in the eyes

Millions of people worldwide have hereditary genetic disorders, trauma, infectious diseases, or cancer of the eyes, and many of these eye diseases lead to irreversible blindness, which is a major public health burden. The eye is a relatively small and immune-privileged organ. The use of nucleic acid-based drugs to manipulate malfunctioning genes that target the root of ocular diseases is regarded as a therapeutic approach with great promise. However, there are still some challenges for utilizing nucleic acid therapeutics in vivo because of certain unfavorable characteristics, such as instability, biological carrier-dependent cellular uptake, short pharmacokinetic profiles in vivo (RNA), and on-target and off-target side effects (DNA). The development of lipid nanoparticles (LNPs) as gene vehicles is revolutionary progress that has contributed the clinical application of nucleic acid therapeutics. LNPs have the capability to entrap and transport various genetic materials such as small interfering RNA, mRNA, DNA, and gene editing complexes. This opens up avenues for addressing ocular diseases through the suppression of pathogenic genes, the expression of therapeutic proteins, or the correction of genetic defects. Here, we delve into the cutting-edge LNP technology for ocular gene therapy, encompassing formulation designs, preclinical development, and clinical translation.

Fostering the unleashing potential of nanocarriers-mediated delivery of ocular therapeutics

Ocular delivery is the most challenging aspect in the field of pharmaceutical research. The major hurdle for the controlled delivery of drugs to the eye includes the physiological static barriers such as the complex layers of the cornea, sclera and retina which restrict the drug from permeating into the anterior and posterior segments of the eye. Recent years have witnessed inventions in the field of conventional and nanocarrier drug delivery which have shown considerable enhancement in delivering small to large molecules across the eye. The dynamic challenges associated with conventional systems include limited drug contact time and inadequate ocular bioavailability resulting from solution drainage, tear turnover, and dilution or lacrimation. To this end, various bioactive-based nanosized carriers including liposomes, ethosomes, niosomes, dendrimer, nanogel, nanofibers, contact lenses, nanoprobes, selenium nanobells, nanosponge, polymeric micelles, silver nanoparticles, and gold nanoparticles among others have been developed to circumvent the limitations associated with the conventional dosage forms. These nanocarriers have been shown to achieve enhanced drug permeation or retention and prolong drug release in the ocular tissue due to their better tissue adherence. The surface charge and the size of nanocarriers (10-1000 nm) are the important key factors to overcome ocular barriers. Various nanocarriers have been shown to deliver active therapeutic molecules including timolol maleate, ampicillin, natamycin, voriconazole, cyclosporine A, dexamethasone, moxifloxacin, and fluconazole among others for the treatment of anterior and posterior eye diseases. Taken together, in a nutshell, this extensive review provides a comprehensive perspective on the numerous facets of ocular drug delivery with a special focus on bioactive nanocarrier-based approaches, including the difficulties and constraints involved in the fabrication of nanocarriers. This also provides the detailed invention, applications, biodistribution and safety-toxicity of nanocarriers-based therapeutcis for the ophthalmic delivery.

Chemical Insights into Topical Agents in Intraocular Pressure Management: From Glaucoma Etiopathology to Therapeutic Approaches

Glaucoma encompasses a group of optic neuropathies characterized by complex and often elusive etiopathology, involvihttng neurodegeneration of the optic nerve in conjunction with abnormal intraocular pressure (IOP). Currently, there is no cure for glaucoma, and treatment strategies primarily aim to halt disease progression by managing IOP. This review delves into the etiopathology, diagnostic methods, and treatment approaches for glaucoma, with a special focus on IOP management. We discuss a range of active pharmaceutical ingredients used in glaucoma therapy, emphasizing their chemical structure, pharmacological action, therapeutic effectiveness, and safety/tolerability profiles. Notably, most of these therapeutic agents are administered as topical formulations, a critical aspect considering patient compliance and drug delivery efficiency. The classes of glaucoma therapeutics covered in this review include prostaglandin analogs, beta blockers, alpha agonists, carbonic anhydrase inhibitors, Rho kinase inhibitors, and miotic (cholinergic) agents. This comprehensive overview highlights the importance of topical administration in glaucoma treatment, offering insights into the current state and future directions of pharmacological management in glaucoma.

Patient Factors Influencing Intraocular Penetration of Brimonidine-Related Eye Drops in Adults: A Post Hoc Pooled Analysis

INTRODUCTION

The factors related to the ocular penetration of drugs after the administration of eye drops in humans have not been examined in detail. Therefore, this study assessed the influence of patient factors on the intraocular penetration of eye drops.

METHODS

A pooled analysis was performed on the data of 42 participants from three studies to evaluate the ocular pharmacokinetics in humans after the topical application of brimonidine-related eye drops. The patients were scheduled for vitrectomy and received brimonidine-related eye drops (0.1% brimonidine tartrate ophthalmic solution, 0.1% brimonidine tartrate and 0.5% timolol fixed-combination ophthalmic solution, or 0.1% brimonidine tartrate and 1% brinzolamide fixed-combination suspension) twice daily for 1 week. We analyzed the effects of patient factors (sex, the presence or absence of lens, age, corneal thickness, corneal endothelial cell density, tear secretion, eye axial length, height, weight and body mass index [BMI]) on brimonidine, timolol and brinzolamide concentrations in the aqueous and vitreous humor after topical application.

RESULTS

The drug concentrations in the aqueous and vitreous humor were not significantly different, regardless of sex or the presence or absence of lens. Age correlated positively with brimonidine (r = 0.3948, p = 0.012) and brinzolamide (r = 0.6809, p = 0.030) concentrations in the aqueous humor; the correlation with timolol showed a trend towards significance (r = 0.6425, p = 0.086). Corneal thickness, corneal endothelial cell density, tear secretion, eye axial length, height and BMI did not correlate with the drug concentrations in the aqueous or vitreous humor. Timolol concentration in the vitreous humor was negatively correlated with weight (r =  - 0.8333, p = 0.010).

CONCLUSION

The findings of this study emphasize the necessity of considering individual differences in ocular pharmacokinetics during drug therapy (formulation design of the eye drops and dose regimen).

Brimonidine tartrate ophthalmic solution 0.025% for redness relief: an overview of safety and efficacy

INTRODUCTION

Ocular redness, or conjunctival hyperemia, is a common ophthalmic sign associated with reduced quality of life. For redness without apparent underlying pathology, topical ophthalmic decongestants have been widely used.

AREAS COVERED

Brimonidine tartrate was approved in 2017 as a topical vasoconstrictor at a 0.025% concentration for relief of ocular redness. Since then, investigators have reported on efficacy and safety findings from studies evaluating low-dose brimonidine for reducing ocular redness.

EXPERT OPINION

Brimonidine is highly selective for α₂-adrenergic receptors. Clinical trials have so far shown that the drug in low doses significantly reduces ocular redness in comparison to vehicle for up to 8 hours. Brimonidine-treated eyes did not present side effects of other vasoconstrictors, such as hypotension, cardiac arrhythmia or drowsiness. Ocular adverse events such as allergic reactions and redness rebound were also minimal. In this review, we examine in detail published literature on the mechanism of brimonidine tartrate and its efficacy and safety in relieving conjunctival hyperemia.

Targeted drug delivery vehicles mediated by nanocarriers and aptamers for posterior eye disease therapeutics: barriers, recent advances and potential opportunities

Nanomedicine and aptamer have excellent potential in giving play to passive and active targeting respectively, which are considered to be effective strategies in the retro-ocular drug delivery system. The presence of closely adjoined tissue structures in the eye makes it difficult to administer the drug in the posterior segment of the eye. The application of nanomedicine could represent a new avenue for the treatment, since it could improve penetration, achieve targeted release, and improve bioavailability. Additionally, a novel type of targeted molecule aptamer with identical objective was proposed. As an emerging molecule, aptamer shows the advantages of penetration, non-toxicity, and high biocompatibility, which make it suitable for ocular drug administration. The purpose of this paper is to summarize the recent studies on the effectiveness of nanoparticles as a drug delivery to the posterior segment of the eye. This paper also creatively looks forward to the possibility of the combined application of nanocarriers and aptamers as a new method of targeted drug delivery system in the field of post-ophthalmic therapy.

Topical tacrolimus nanocapsules eye drops for therapeutic effect enhancement in both anterior and posterior ocular inflammation models

Tacrolimus has shown efficacy in eye inflammatory diseases. However, due to the drug lability, its formulation into a stable ophthalmic product remains a challenge. Tacrolimus-loaded nanocapsules (NCs) were designed for ocular instillation. Further, the stability and effects of the formulation were analyzed under different experimental conditions. Physicochemical characterization of the NCs revealed suitable homogeneous size and high encapsulation efficiency. Moreover, the lyophilized formulation was stable at ICH long term and accelerated storage conditions, for at least 18 and 3 months, respectively. The tacrolimus NCs did not elicit any eye irritation in rabbits after single- and multiple-dose applications. Additionally, ex vivo penetration assays on isolated porcine cornea and pharmacokinetics analyses in various rabbit eye compartments demonstrated the superiority of the NCs in retention and permeation into the anterior chamber of the eye compared to the free drug dissolved in oil. Moreover, multiple dose ocular instillation of the NCs in rats allowed high tacrolimus levels in the eye with very low plasma concentrations. Finally, the developed delivery system achieved a significant decrease in four typical inflammatory markers in a murine model of keratitis, an anterior chamber inflammation. Furthermore, these NCs, applied as eye drops, displayed clinical and histological efficacy in the mainly posterior chamber inflammation model of murine, experimental auto-immune uveitis.

Peptidomimetics Therapeutics for Retinal Disease

Ocular disorders originating in the retina can result in a partial or total loss of vision, making drug delivery to the retina of vital importance. However, effectively delivering drugs to the retina remains a challenge for ophthalmologists due to various anatomical and physicochemical barriers in the eye. This review introduces diverse administration routes and the accordant pharmacokinetic profiles of ocular drugs to aid in the development of safe and efficient drug delivery systems to the retina with a focus on peptidomimetics as a growing class of retinal drugs, which have great therapeutic potential and a high degree of specificity. We also discuss the pharmacokinetic profiles of small molecule drugs due to their structural similarity to small peptidomimetics. Lastly, various formulation strategies are suggested to overcome pharmacokinetic hurdles such as solubility, retention time, enzymatic degradation, tissue targeting, and membrane permeability. This knowledge can be used to help design ocular delivery platforms for peptidomimetics, not only for the treatment of various retinal diseases, but also for the selection of potential peptidomimetic drug targets.

Topical ocular pharmacokinetics and bioavailability for a cocktail of atenolol, timolol and betaxolol in rabbits

Ocular bioavailability after eye drops administration is an important, but rarely determined, pharmacokinetic parameter. In this study, we measured the pharmacokinetics of a cocktail of three beta blockers after their topical administration into the albino rabbit eye. Samples from aqueous humour were analysed with LC-MS/MS. The pharmacokinetic parameters were estimated using compartmental and non-compartmental analyses. The ocular bioavailability was covering broad range of values: atenolol (0.07 %), timolol (1.22%, 1.51%) and betaxolol (3.82%, 4.31%). Absolute ocular bioavailability presented a positive trend with lipophilicity and the values showed approximately 60-fold range. The generated data enhances our understanding for ocular pharmacokinetics of drugs and may be utilized in pharmacokinetic model building in ophthalmic drug development.

Ocular and systemic pharmacokinetics of brimonidine and brinzolamide after topical administration in rabbits: comparison between fixed-combination and single-drug formulations

AIM

The aim of this study was to compare the ocular and systemic absorption of brimonidine (BMD) and brinzolamide (BZM) in rabbits after the topical administration of a fixed-combination ophthalmic suspension of 0.1% BMD tartrate and 1% BZM (FCBB) with that after the administration of the respective single-drug formulations.

MATERIALS AND METHODS

Ocular and systemic drug absorption was estimated by determining BMD and BZM concentrations in the aqueous humor, retina/choroid, vitreous body, and blood/plasma by liquid chromatography/tandem mass spectrometry after the administration of FCBB, 0.1% BMD tartrate ophthalmic solution (0.1% BMD), or 1% BZM ophthalmic suspension (1% BZM) to rabbits.

RESULTS

In concomitant administration, instilling 0.1% BMD and 1% BZM successively without interval lowered aqueous humor concentrations of both drugs compared to those observed with a 5-min interval. After FCBB administration, BMD and BZM concentrations in the aqueous humor were comparable with those observed after the administration of 0.1% BMD and 1% BZM, whereas BMD concentrations in posterior ocular tissues were equal to or higher than those observed after 0.1% BMD. Plasma BMD concentrations following the administration of FCBB were 0.8-fold lower than those after 0.1% BMD; no remarkable differences were observed in blood BZM concentrations for both formulations.

CONCLUSIONS

FCBB achieved drug distribution in the aqueous humor and systemic exposure that were comparable to those for the single-drug formulations. The viscosity of FCBB may increase BMD distribution in the retina/choroid. The administration interval affects ocular drug absorption with the concomitant administration of 0.1% BMD and 1% BZM, which can be overcome by using the fixed-combination of both drugs.

Efficiency and safety of phenylephrine and tropicamide used in premature retinopathy: a prospective observational study

Background

To determine effects and side effects of topical application of phenylephrine 2.5% and tropicamide 0.5% combination in preterm infants.

Methods

In this prospective observational study, 60 infants undergoing retinopathy of prematurity (ROP) screening were prospectively observed. Pupillary diameter, blood pressure, heart rate, and oxygen saturation were monitored before and after up to 24 h during ROP screening examinations.

Results

The mean pupillary diameter 1 h after the instillation of drops was 5.58 ± 0.75 mm for both eyes. The mean systolic and diastolic pressure and oxygen saturation of infants did not change statistically until the end of the study. The average heart rate decreased by a mean of 4.96 beats/minute from the baseline following eye drops instillation. General condition deterioration, fall in oxygen saturation and bradycardia were observed in 4 infants that already had respiratory distress syndrome.

Conclusion

The phenylephrine 2.5% plus tropicamide 0.5% drop is effective and safe as mydriatic combination for retinopathy of prematurity screening. In infants with an additional systemic disease such as respiratory distress syndrome, the side effects of mydriatic drops may be more common. Such babies should be kept under close observation.

Trial registration

The trial was retrospectively registered on 28 February 2018. The ClinicalTrials.gov Identifier is NCT03448640.

Ocular biopharmaceutics: impact of modeling and simulation on topical ophthalmic formulation development

The estimation of ocular pharmacokinetics (PK) in various eye tissues is limited because of sampling challenges. Computational modeling and simulation (M&S) tools underpinning the elucidation of drug access routes and prediction of ocular exposure are essential for the mechanistic assessment of biopharmaceutics in the eye. Therefore, theoretical and experimental evaluation of ocular absorption and transit models is necessary. Biopharmaceutical parameter sensitivity analysis based on permeability and drug dose illustrates utility in ocular drug delivery assessment, which could have innovative and cost-saving impacts on ophthalmic product development and therapeutic bioequivalence (BE) evaluations.

Recent advance of nanoparticle-based topical drug delivery to the posterior segment of the eye

INTRODUCTION

Considering that the number of patients afflicted by posterior eye diseases is increasing, effective drug delivery is currently in high clinical demand. Topical administration has been identified as the preferred option, while sufferingfrom multiple barriers. The development of nanoparticle-based drug delivery system provides an option, which would enhance the drug permeability across the barriers and achieve the desired drug level in the targeted tissue.

AREAS COVERED

This review highlights the barrier to the posterior segment of the eye via topical administration. The up-to-date development of lipid nanoparticles, liposomes, emulsions, spanlastics, micelles, polymeric nanoparticles, layered double hydroxides (LDH), dendrimers, cyclodextrins(CDs), and prodrugs are summarized. Moreover, nanocarriers currently in clinical trials for posterior segment diseases have been discussed.

EXPERT OPINION

Topical nanoparticle-based drug delivery systems have demonstrated significant progress. An ideal formulation should prolong retention time on the surface, enhance drug permeability through the ocular tissues, and efficiently deliver drugs to the targeted site. To design the rational targeting nanoparticle-based drug delivery system, a better understanding of the distribution of transporters and receptors on the eye is required. Ultimately, there is an urgent need to develop targeting hybrid drug delivery systems with the combination of the advantages of several nanocarriers.

Corneal and conjunctival drug permeability: Systematic comparison and pharmacokinetic impact in the eye

On the surface of the eye, both the cornea and conjunctiva are restricting ocular absorption of topically applied drugs, but barrier contributions of these two membranes have not been systemically compared. Herein, we studied permeability of 32 small molecular drug compounds across an isolated porcine cornea and built a quantitative structure-property relationship (QSPR) model for the permeability. Corneal drug permeability (data obtained for 25 drug molecules) showed a 52-fold range in permeability (0.09-4.70 × 10^-6 cm/s) and the most important molecular descriptors in predicting the permeability were hydrogen bond donor, polar surface area and halogen ratio. Corneal permeability values were compared to their conjunctival drug permeability values. Ocular drug bioavailability and systemic absorption via conjunctiva were predicted for this drug set with pharmacokinetic calculations. Drug bioavailability in the aqueous humour was simulated to be <5% and trans-conjunctival systemic absorption was 34-79% of the dose. Loss of drug across the conjunctiva to the blood circulation restricts significantly ocular drug bioavailability and, therefore, ocular absorption does not increase proportionally with the increasing corneal drug permeability.

Impact of Chemical Structure on Conjunctival Drug Permeability: Adopting Porcine Conjunctiva and Cassette Dosing for Construction of In Silico Model

Conjunctiva occupies most of the ocular surface area, and conjunctival permeability affects ocular and systemic drug absorption of topical ocular medications. Therefore, the aim of this study was to obtain a computational in silico model for structure-based prediction of conjunctival drug permeability. This was done by employing cassette dosing and quantitative structure-property relationship (QSPR) approach. Permeability studies were performed ex vivo across fresh porcine conjunctiva and simultaneous dosing of a cassette mixture composed of 32 clinically relevant drug molecules with wide chemical space. The apparent permeability values were obtained using drug concentrations that were quantified with liquid chromatography tandem-mass spectrometry. The experimental data were utilized for building a QSPR model for conjunctival permeability predictions. The conjunctival permeability values presented a 17-fold range (0.63-10.74 × 10^-6 cm/s). The final QSPR had a Q² value of 0.62 and predicted the external test set with a mean fold error of 1.34. The polar surface area, hydrogen bond donor, and halogen ratio were the most relevant descriptors for defining conjunctival permeability. This work presents for the first time a predictive QSPR model of conjunctival drug permeability and a comprehensive description on conjunctival isolation from the porcine eye. The model can be used for developing new ocular drugs.

Ocular Pharmacokinetics

Although the fundamental concepts of pharmacokinetics remain the same, ocular pharmacokinetics has its own challenges due to the uniqueness of barrier properties posed by various ocular tissues and its growing complexity with different routes of ocular administration. A thorough understanding of the barrier nature will aid in tailoring a drug or its carrier's physicochemical properties to its advantage. In order to deliver the right payload of a drug at the target site, various approaches can be taken to leverage the pharmacokinetics that includes molecular design based on desirable physicochemical properties, formulation approaches, and alternative routes of administration. In this chapter, a brief overview of the barrier properties with respect to various routes of administration is presented along with the physicochemical properties that influence the pharmacokinetics of ocular drugs. Recent advances in ocular pharmacokinetics are discussed in addition to new perspectives in interpreting existing data.

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

Physicochemical properties of core-shell type nanoparticles govern their spatiotemporal biodistribution in the eye

Due to the inherent barrier properties of eye tissues, a major challenge in treating eye diseases is to provide a therapeutic agent to the desired tissue in quantities and durations that are favorable. This study aimed at understanding the influence of physicochemical properties of nanoparticles on their spatiotemporal biodistribution in mouse eye. For this, core-shell nanoparticles with different properties were designed by varying either core or shell and administered as eye-drops to mice. The results demonstrated that all nanoparticles irrespective of type of core or shell followed the conjunctival-scleral pathway. The bioavailability of cores followed the order polylactide-co-glycolide≥polylactide≥polycaprolactone for all tissues and time-points. The bioavailability for all shell types was greater in conjunctiva, sclera, choroid and retina when compared to other eye tissues. Therefore, modulating physicochemical properties of nanoparticles can be used as a design strategy to devise drug carriers that target specific tissues of the eye.

Intraocular distribution of topically applied hydrophilic and lipophilic substances in rat eyes

PURPOSE

Topical administration is the preferred route of drug delivery for ophthalmic ailments. However, poor permeation through ocular surface and significant systemic absorption, makes the topical drug delivery challenging. Furthermore, distribution of topically delivered drugs varies with their physicochemical properties and the type of formulation used. Hence, this study was done to understand the pattern of ocular drug distribution of topically applied hydrophilic and lipophilic substances in two different formulations.

METHODS

5-Carboxyfluorescein and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were used as representative candidates for hydrophilic and lipophilic substances, respectively. They were formulated in solution and liposomes. Single drop of either formulation containing hydrophilic or lipophilic substance was instilled topically, unilaterally to rat eyes. Subsequently, rats were sacrificed at 10, 30 and 120 min post-instillation. Eyes were cryosectioned and examined under confocal microscope to determine the fluorescence intensity in ocular tissues.

RESULTS

Corneal permeation of hydrophilic and lipophilic substances in both formulations peaked at 30 min post-instillation. Liposomal-lipophilic dye and non-liposomal-hydrophilic dye showed better corneal distribution. Fluorescence was absent in contralateral eyes of non-liposomal-hydrophilic dye-treated animals but was present in contralateral eyes of liposomal-hydrophilic dye-treated animals. Fluorescence in contralateral eyes of liposomal-lipophilic dye-treated animals was significantly higher compared to non-liposomal-lipophilic dye-treated animals.

CONCLUSIONS

Topically applied liposomal formulation of lipophilic substance provides higher corneal concentration of drug with lesser systemic absorption compared to its solution. For hydrophilic substance, topical use of solution provides greater corneal concentration compared to liposomes which is more likely to be absorbed systemically.

Effects of two commonly used mydriatics on choroidal thickness: direct and crossover effects

PURPOSE

To investigate the effect of 2 commonly used mydriatics on choroidal thickness using enhanced depth imaging optical coherence tomography (EDI-OCT).

METHODS

In this prospective study, 90 healthy subjects were enrolled. The participants were randomly divided into 3 groups based on the application of drops. One eye of each subject received a drop of tropicamide 1% in the tropicamide group (n=30), a drop of phenylephrine 2.5% in the phenylephrine group (n=30), and a drop of artificial tear in the control group (n=30). Drops were given thrice at 5 min intervals. Subfoveal choroidal thickness (SFCT) was measured using EDI-OCT before and at 45 min after drop application in both the dilated eye and nondilated contralateral eye.

RESULTS

The SFCT was significantly decreased after drop instillation in both the dilated eye and contralateral eye in the tropicamide group (P<0.001 and P=0.001, respectively) and in the phenylephrine group (P<0.001 and P=0.001, respectively). However, the SFCT did not significantly differ after drop instillation in either the dropped eye or contralateral eye in the control group (P=0.108 and P=0.695, respectively).

CONCLUSION

The findings of the present study revealed that tropicamide and phenylephrine cause a decrease in choroidal thickness.

Distribution of cyclosporine A in ocular tissues after topical administration of cyclosporine A cationic emulsions to pigmented rabbits

PURPOSE

The aim of this study was to compare the ocular and systemic distribution of cyclosporine A (CsA) in rabbits after the instillation of preservative-free CsA cationic and anionic emulsions.

METHODS

For the single-dose pharmacokinetic (PK) study, rabbits were instilled with 50 μL of the test material. For the multiple-dose PK study, rabbits were instilled twice daily with Restasis or once daily with NOVA22007 for 10 days. At each time point, the cornea, conjunctiva, and whole blood were harvested for CsA quantification. Ocular and systemic distribution were determined after 4 times daily instillations with 50 μL of 3H-CsA cationic and anionic emulsions for 7 days. Restasis was used as a reference in all studies.

RESULTS

Single-dose PK data demonstrated that NOVA22007 0.1% and 0.05% delivered higher CsA concentrations to the cornea than Restasis [concentration maximum (C max): 2692, 1372, and 748 ng/g, respectively] and have a better exposition (area under the curve). Conjunctival Cmax values were 1914, 696, and 849 ng/g and area under the curve values were 3984, 2796, and 2515 ng/g · h, for either dose of the cationic emulsions and Restasis, respectively. The multiple-dose PK and the 3H-CsA distribution data demonstrated that the systemic distribution after repeated instillations was low and comparable for all emulsions.

CONCLUSIONS

These data demonstrate that the CsA cationic emulsions were more effective than Restasis at delivering CsA to target tissues, thus confirming the potential advantage of cationic emulsions over anionic emulsions as vehicle for ocular drug delivery for the treatment of ocular surface diseases.

Hybrid dendrimer hydrogel/PLGA nanoparticle platform sustains drug delivery for one week and antiglaucoma effects for four days following one-time topical administration

We report a novel hybrid polyamidoamine (PAMAM) dendrimer hydrogel/poly(lactic-co-glycolic acid) (PLGA) nanoparticle platform (HDNP) for codelivery of two antiglaucoma drugs, brimonidine and timolol maleate. This platform was not cytotoxic to human corneal epithelial cells. Cellular uptake of Nile red-encapsulating PLGA nanoparticles was significantly increased by dendrimer hydrogel. A prolonged residence time of nanoparticles was demonstrated through investigation of FluoSpheres loaded into dendrimer hydrogel. Both brimonidine and timolol maleate were slowly released in vitro over a period of 28-35 days. Following topical administration of one eye drop (30 μL of 0.7% w/v brimonidine and 3.5% w/v timolol maleate) in normotensive adult Dutch-belted male rabbits, the HDNP formulation resulted in a sustained and effective IOP reduction (18% or higher) for 4 days. Furthermore, the HDNP maintained significantly higher concentrations of brimonidine in aqueous humor and cornea as well as timolol maleate in the aqueous humor, cornea, and conjunctiva up to 7 days as compared to saline, DH, and PLGA nanoparticle dosage forms, without inducing ocular inflammation or discomfort. Histological analysis of the cornea and conjunctiva did not reveal any morphological or structural changes. Our work demonstrated that this new platform is capable of enhancing drug bioavailability and sustaining effective IOP reduction over an extended period of time. This newly developed platform can greatly reduce dosing frequency of topical formulations, thus, improving long-term patient compliance and reducing enormous societal and economic costs. Given its high structural adaptability, many other chronic ocular diseases would benefit from long-lasting drug delivery of this new platform.

Comparison of human ocular distribution of bimatoprost and latanoprost

PURPOSE

This study investigated the ocular distribution of bimatoprost, latanoprost, and their acid hydrolysis products in the aqueous humor, cornea, sclera, iris, and ciliary body of patients treated with a single topical dose of 0.03% bimatoprost or 0.005% latanoprost for understanding concentration-activity relationships.

METHODS

Thirty-one patients undergoing enucleation for an intraocular tumor not affecting the anterior part of the globe were randomized to treatment with bimatoprost or latanoprost at 1, 3, 6 or 12 h prior to surgery. Concentrations of bimatoprost, bimatoprost acid, latanoprost, and latanoprost acid in the human aqueous and ocular tissues were measured using liquid chromatography tandem mass spectrometry.

RESULTS

Following topical administration, intact bimatoprost was distributed in human eyes with a rank order of cornea/sclera >iris/ciliary body >aqueous humor. Bimatoprost acid was also detected in these tissues, where its low levels in the cornea relative to that of latanoprost acid indicated that bimatoprost hydrolysis was limited. Latanoprost behaved as a prodrug that entered eyes predominantly via the corneal route. Levels of latanoprost acid were distributed as cornea >>aqueous humor>iris>sclera>ciliary body.

CONCLUSIONS

Our study provided experimental evidence that levels of bimatoprost in relevant ocular tissues, and not only aqueous humor, are needed to understand the mechanisms by which bimatoprost lowers intraocular pressure (IOP) in human subjects. The data suggest that bimatoprost reached the target tissues favoring the conjunctival/scleral absorption route. Findings of intact bimatoprost in the target ciliary body indicated its direct involvement in reducing IOP. However, bimatoprost acid may have only a limited contribution on the basis that bimatoprost has greater/similar IOP-lowering efficacy than latanoprost, yet bimatoprost acid levels were a fraction of latanoprost acid levels in the aqueous humor and cornea and only sporadically detectable in the ciliary body. In this report, human ocular tissues were examined concurrently with aqueous humor for the in vivo distribution of bimatoprost, bimatoprost acid, latanoprost, and latanoprost acid.

Ocular pharmacokinetics/pharmacodynamics of besifloxacin, moxifloxacin, and gatifloxacin following topical administration to pigmented rabbits

PURPOSE

The purpose of this investigation was to evaluate the ocular pharmacokinetic/pharmacodynamic (PK/PD) relationship for besifloxacin, moxifloxacin, and gatifloxacin using rabbit ocular PK data, along with in vitro minimum inhibitory concentration (MIC90) values against methicillin- and ciprofloxacin-resistant Staphylococcus aureus (MRSA-CR) and Staphylococcus epidermidis (MRSE-CR).

METHODS

Rabbits received a topical instillation of Besivance™ (besifloxacin ophthalmic suspension, 0.6%), Vigamox (moxifloxacin hydrochloride ophthalmic solution, 0.5% as base), or Zymar (gatifloxacin ophthalmic solution, 0.3%), and ocular tissues and plasma were collected from 4 animals/treatment/collection time at 8 predetermined time intervals during the 24h after dosing. Ocular levels of each agent were measured by LC/MS/MS, and PK parameters (Cmax, Tmax, and AUC₀₋₂₄) were determined. AUC₀₋₂₄/MIC₉₀ ratios were calculated for tears, conjunctiva, cornea, and aqueous humor using previously reported MIC₉₀values for MRSA-CR and MRSE-CR.

RESULTS

All of the fluoroquinolones tested demonstrated rapid penetration into ocular tissues after a single instillation. Besifloxacin demonstrated the highest exposure in tear fluid, while exposure in conjunctiva was comparable for all 3 compounds. Peak concentrations of all fluoroquinolones in aqueous humor were at or below ~1g/mL. In comparison with their MIC₉₀values against MRSE-CR and MRSA-CR, besifloxacin achieved an AUC₀₋₂₄/MIC₉₀ ratio of ~800 in tears, compared with values of ≤10 for moxifloxacin and gatifloxacin. In cornea, conjunctiva, and aqueous humor, the AUC₀₋₂₄/MIC₉₀ ratios were <10 for all compounds. However, in these tissues AUC₀₋₂₄/MIC₉₀ ratios for besifloxacin were 1.5- to 38-fold higher than moxifloxacin and gatifloxacin.

CONCLUSIONS

In rabbits, besifloxacin demonstrates a nonclinical ocular PK profile characterized by high and sustained concentrations in tear fluid, resulting in AUC₀₋₂₄/MIC₉₀ ratios of ~800 for ciprofloxacin-resistant MRSE and MRSA after a single administration. Although besifloxacin had the highest AUC₀₋₂₄/MIC₉₀ratios for intraocular tissues, the ratios for all of the drugs were below the target values needed for effective bacterial killing of ciprofloxacin-resistant MRSE and MRSA. Taken together, these nonclinical data indicate that besifloxacin has a favorable ocular PK/PD profile, consistent with the reported clinical efficacy of besifloxacin in the treatment of bacterial conjunctivitis, and consistent with the profile needed for ocular surface sterilization.

Trans-Scleral Diffusion of Triamcinolone Acetonide

PURPOSE

To assess ex vivo human scleral permeability to triamcinolone acetonide (TA).

METHODS

The experiments were carried out using scleral samples and a Franz-type vertical diffusion cell. A suspension containing TA was prepared and placed in the donor chamber. The concentration of TA in the receptor chamber was measured by high-performance liquid chromatography (HPLC) assay and expressed as a percentage relative to TA concentration dissolved in the donor chamber. Control experiments using a commercial TA suspension were performed.

RESULTS

TA (+/-SEM) dissolved in the donor suspension was 10.69 +/- 1.28 microg/ml. The diffusion rate of TA varied from 30% after 1 day to 72% after 4 days, after which equilibrium was reached. The human scleral permeability coefficient (P(s) +/- SEM) was 1.47+/- 0.17 x 10(- 5) cm/s.

CONCLUSIONS

TA crossed human sclera. The mean amount of drug retained in the sclera increased with time, 4 days being necessary to equilibrate the unidirectional flux. The TA permeability coefficient was comparable to that of other corticosteroids.

Molecular design for enhancement of ocular penetration

Over the past two decades, many oral drugs have been designed in consideration of physicochemical properties to attain optimal pharmacokinetic properties. This strategy significantly reduced attrition in drug development owing to inadequate pharmacokinetics during the last decade. On the other hand, most ophthalmic drugs are generated from reformulation of other therapeutic dosage forms. Therefore, the modification of formulations has been used mainly as the approach to improve ocular pharmacokinetics. However, to maximize ocular pharmacokinetic properties, a specific molecular design for ocular drug is preferable. Passive diffusion of drugs across the cornea membranes requires appropriate lipophilicity and aqueous solubility. Improvement of such physicochemical properties has been achieved by structure optimization or prodrug approaches. This review discusses the current knowledge about ophthalmic drugs adapted from systemic drugs and molecular design for ocular drugs. I propose the approaches for molecular design to obtain the optimal ocular penetration into anterior segment based on published studies to date.

Non-systemic delivery of topical brimonidine to the brain: A neuro-ocular tissue distribution study

To date, the risks of central nervous system (CNS) side effects of topically administered ophthalmic therapeutic agents are thought to be the consequence of systemic absorption of these drugs. This paper envisions the possibility of drug delivery to the CNS following ocular application through non-systemic routes. After single instillation of 50 microl of 3H-radiolabeled Alphagan solution (0.2%) in the cul de sac of the right eye, three male albino rabbits (2-2.5 kg) were sacrificed at each time point (5, 15, 30 and 60 min). Both sides (eyes) specimens of aqueous humor, cornea, iris, lens, vitreous, conjunctiva, sclera, ciliary body, choroid, retina, optic nerve, optic tract and olfactory bulb were weighed, and blood samples were measured, before combustion in tissue oxidizer and radioactive liquid scintillation counting. Significant 3H-brimonidine levels were found in right and left optic nerves and tracts with extremely low corresponding drug levels in blood. Uveal tract (ciliary body, iris and choroid tissues) brimonidine levels were relatively high in the treated eye, and the highest among contralateral eye tissues. Our data provide the first case of good CNS availability after ocular application of conventional ophthalmic therapeutic agent, through non-systemic routes. Similar neuro-ocular pharmacokinetic studies should be adopted as a routine ocular therapeutics evaluation study.

Effects of the new ethacrynic acid oxime derivative SA12590 on intraocular pressure in cats and monkeys

To evaluate the pharmacological characteristics of SA12590, a new oxime-derivative of the ethacrynic acid (ECA) derivative SA9000, we examined both its ocular hypotensive effects (in ocular normotensive cats and cynomolgus monkeys) and its potential corneal toxicity (in rats). A 50 microl topical administration of 3% SA12590 significantly reduced intraocular pressure (IOP) (by 3.5 mmHg) in anesthetized cats (p<0.05). Twenty-four hours after 3 drops (5-min intervals) of 20 microl 3% SA12590, IOP was reduced by 8 mmHg (p<0.05, n=4) in conscious monkeys without evidence of corneal toxicity. Three days' daily single 20 microl dosing with 3% SA12590 reduced IOP by 4 mmHg (p<0.01, n=3) at 72 h after the first administration in conscious monkeys. The toxicity of topically administered 20 microl 3% SA9000 or SA12590 (3 drops with 5-min intervals) on rat corneal epithelium was assessed using a photo-slit lamp. In this study, 3% SA12590, unlike 3% SA9000, exhibited no corneal toxicity. In a glutathione assay for sulfhydryl (SH) reactivity, SA12590, unlike SA9000, displayed no in vitro SH reactivity. Thus, oxime-modification may both improve efficacy towards IOP upon topical administration and improve the safety profile, probably by enhancing corneal penetration and minimizing SH reactivity-related toxicity. These findings indicate that SA12590 has potential as a new ocular hypotensive drug.

In situ-forming hydrogels for sustained ophthalmic drug delivery

Ophthalmic drug delivery is one of the most interesting and challenging endeavors facing the pharmaceutical scientist. The conventional ocular drug delivery systems like solutions, suspensions, and ointments show drawbacks such as increased precorneal elimination, high variability in efficiency, and blurred vision respectively. In situ-forming hydrogels are liquid upon instillation and undergo phase transition in the ocular cul-de-sac to form visco-elastic gel and this provides a response to environmental changes. In the past few years, an impressive number of novel temperature, pH, and ion induced in situ-forming systems have been reported for sustain ophthalmic drug delivery. Each system has its own advantages and drawbacks. The choice of a particular hydrogel depends on its intrinsic properties and envisaged therapeutic use. This review includes various temperature, pH, and ion induced in situ-forming polymeric systems used to achieve prolonged contact time of drugs with the cornea and increase their bioavailability.

Ocular Powder: Dry Topical Formulations of Timolol Are Well Tolerated in Rabbits

PURPOSE

Although eye drops are the most common form of ocular drugs, they have several limitations. Drug absorption into the eye is, in general, less than 5%, addition of preservatives is often necessary, and many drugs cannot be formulated as eye drops. Formulating ocular drugs as powder may solve these problems. The aim of this study was to investigate ocular irritation in rabbits following powder administration.

METHODS

Timolol maleate (TM) powder was administered to pigmented lop rabbits. Both pure TM powder and freeze-dried with PVP-polymer (2.4% of mass) were tested in 1.0- and 0.1-mg doses. Additionally, 4 rabbits received 0.1 mg of the pure powder 3 times a day for 8 d. Redness of the bulbar conjunctiva and the amount of discharge was rated from photographs (0-3 points, randomized and masked evaluation). The 8-d experiment additionally included examination with a slit lamp and examination of hematoxylin-eosin stained sections of eyes with light microscopy.

RESULTS

No serious or irreversible signs of irritation were noted. Doses of 1.0 mg were more irritating than 0.1-mg doses. There was no detectable difference in irritation between pure or freeze-dried powder. Slit-lamp examination, surface photographs and histology showed a negligible difference between drug and control eyes following the 8-d experiment.

CONCLUSIONS

The results suggest that 0.1 mg of timolol powder does not irritate the eye and that testing topical timolol powder in humans is feasible.

Influence of format on in vitro penetration of antibody fragments through porcine cornea

AIM

Antibody fragments, appropriately formulated, can penetrate through the ocular surface and thus have potential as therapeutic agents. The aim was to investigate the influence of protein fragment format on the kinetics and extent of ocular penetration in vitro.

METHODS

Immunoglobulin single chain variable domain fragments of a murine monoclonal antibody with specificity for rat CD4 were engineered with a 20 or 11 amino acid linker by assembly polymerase chain reaction, expressed in Escherichia coli and purified by chromatography. Fab fragments of the parental antibody were prepared by papain digestion. Antibody fragments were formulated with a penetration and a viscosity enhancer and were applied to the surface of perfused pig corneas for up to 10 hours in vitro. Penetration was quantified by flow cytometry on rat thymocytes.

RESULTS

20-mer antibody fragments formed natural monomers and dimers following purification that could be separately isolated, while 11-mer fragments were dimeric. All formats of fragment (20-mer monomers and dimers, 11-mer dimers, Fab) showed penetration through the pig cornea after 6 hours of intermittent topical administration.

CONCLUSION

Antibody fragments of different shapes and sizes can penetrate the cornea after topical administration, thereby increasing the potential of this class of proteins for topical ophthalmic use.

Effects of a topical alpha2 adrenergic agonist on ciliary blood flow and aqueous production in rabbits

The relationship between ciliary perfusion and aqueous production is poorly understood. It was recently reported that aqueous production decreases when ciliary blood flow is reduced by lowering the ocular perfusion pressure, and hypothesized that drug-induced reduction of ciliary blood flow would also decrease aqueous production. In the present study, we test this hypothesis with an alpha2 adrenergic agonist (brimonidine) formulated for topical application. When used acutely, brimonidine decreases intraocular pressure (IOP) by suppressing aqueous production, although its mechanism of action is unclear. The experiments were performed in four groups of anesthetized rabbits (n=33) in which the following variables were measured: ocular mean arterial pressure (OMAP), IOP, orbital venous pressure (OVP), aqueous flow, ciliary blood flow, ciliary oxygen tension (PO2), episcleral venous pressure (EVP), carotid blood flow and heart rate. The measurements were made before and after brimonidine (0.15%, 40 microl) was applied to the cornea. Brimonidine decreased IOP (-33%, p<0.01), aqueous flow (-39%, p<0.01), ciliary blood flow (-37%, p<0.01), EVP (-42%, p<0.01) and ciliary PO2 (-32%, p<0.05). We conclude that topical brimonidine is a ciliary vasoconstrictor, and that alpha2 adrenergic agonist-induced decreases in ciliary blood flow decrease aqueous production.

The potential of lipid emulsion for ocular delivery of lipophilic drugs

For nearly a decade, oil-in-water lipid emulsions containing either anionic or cationic droplets have been recognized as an interesting and promising ocular topical delivery vehicle for lipophilic drugs. The aim of this review is to present the potential of lipid emulsions for ocular delivery of lipophilic drugs. The review covers an update on the state of the art of incorporating the lipophilic drugs, a brief description concerning the components and the classification of lipid emulsions. The ocular fate following topical instillation, safety evaluation experiments and the applications of lipid emulsions are thoroughly discussed.

Cationic vectors in ocular drug delivery

Despite extensive research in the field, the major problem in the ocular drug delivery domain still is rapid precorneal drug loss and poor corneal permeability. One of the approaches recently developed is the drug incorporation into cationic submicronic vectors which exploit the negative charges present at the corneal surface for increased residence time and penetration. This review will focus on the formulation of three main representative cationic colloids developed for ophthalmic delivery: liposomes, emulsions and nanoparticles (NP). Parameters such as choice of the vector type and size, nature of the cationic molecule, pH and ionic strength of the external phase and characteristics of the encapsulated drug will be discussed with accent on the relevance of the positive charge.

Ocular pharmacokinetic modeling using corneal absorption and desorption rates from in vitro permeation experiments with cultured corneal epithelial cells

PURPOSE

To determine corneal absorption and desorption rate constants in a corneal epithelial cell culture model and to apply them to predict ocular pharmacokinetics after topical ocular drug application.

METHOD

In vitro permeation experiments were performed with a mixture of six beta-blockers using an immortalized human corneal epithelial cell culture model. Disappearance of the compounds from the apical donor solution and their appearance in the basolateral receiver solution were determined and used to calculate the corneal absorption and desorption rate constants. An ocular pharmacokinetic simulation model was constructed for timolol with the Stella program using the absorption and desorption rate constants and previously published in vivo pharmacokinetic parameters.

RESULTS

The corneal absorption rates of beta-blockers increased significantly with the lipophilicity of the compounds. The pharmacokinetic simulation model gave a realistic mean residence time for timolol in the cornea (57 min) and the aqueous humor (90 min). The simulated timolol concentration in the aqueous humor was about 1.8 times higher than the previously published experimental values.

CONCLUSIONS

The simulation model gave a reasonable estimate of the aqueous humor concentration profile of timolol. This was the first attempt to combine cell culture methods and pharmacokinetic modeling for prediction of ocular pharmacokinetics. The wider applicability of this approach remains to be seen.

The potential of antibody-based immunosuppressive agents for corneal transplantation

Corneal transplantation is a sight-restorative procedure but its success is limited by irreversible graft rejection, which accounts for up to 50 per cent of failures. The normal eye is an immune-privileged site. Multiple mechanisms maintain ocular privilege, including the blood-eye barrier, the lack of blood vessels and lymphatics in the normal cornea, the relative paucity of mature antigen-presenting cells in the central cornea, the presence of immunomodulatory factors in ocular fluids, and the constitutive expressive of CD95L (Fas ligand) within the eye. However, privilege can be eroded by the sequelae of inflammation and neovascularization. Corneal graft rejection in humans is currently suppressed with topical glucocorticosteroids, which are moderately effective. Systemically administered immunosuppressive therapy is of limited efficacy and may be accompanied by unacceptable morbidity. Alternative therapies are needed to improve outcomes. Corneal graft rejection is primarily a cell-mediated response controlled by the CD4+ T cell, and thus CD4 and costimulatory molecule blockade are appealing targets for new therapeutic interventions. A number of monoclonal antibodies have shown promise as immunosuppressants to prolong corneal graft survival in experimental animal models, and may eventually prove to be useful adjuncts to corticosteroids.

Formulation effects on ocular absorption of brimonidine in rabbit eyes

Purite (stabilized oxychloro complex) and benzalkonium chloride (BAK) are preservatives. We investigated formulation effects on ocular absorption of brimonidine in rabbit eyes. The formulations compared were: Alphagan (0.2% brimonidine tartrate/0.005% BAK, pH 6.4), Brimonidine-Purite (0.2% brimonidine tartrate/0.005% Purite, pH 7.2), and Brimonidine-PF (0.2% brimonidine tartrate, preservative-free (PF), pH 6.4) solutions. The study was conducted in a cross-over fashion; albino rabbits (n = 18) were given a single 35 microl drop of each test formulation in each eye. Aqueous humor samples were collected at selected times post-dose from subgroups of 2 rabbits per timepoint and analyzed for brimonidine concentrations by LC-MS/MS. The AUC and Cmax were calculated. The results showed rapid ocular absorption of brimonidine, with peak concentrations at 0.33-1 hr. The AUC(0-5hr) values were 3.78 +/- 0.38, 2.77 +/- 0.22, and 2.49 +/- 0.22 microg-hr/ml (mean +/- SEM) for Brimonidine-Purite, Alphagan and Brimonidine-PF, respectively. The corresponding Cmax values were 2.69 +/- 0.72, 1.74 +/- 0.13, and 1.24 +/- 0.22 microg/ml (mean +/- SEM). Brimonidine-Purite provided significantly higher AUC(0-5hr) than Alphagan (p < 0.05). No statistical significant difference in AUC(0-5hr) was found between Alphagan and Brimonidine-PF. In conclusion, 0.2% Brimonidine-Purite was 1.4 and 1.5 times more ocularly bioavailable in rabbits than 0.2% Alphagan and 0.2% Brimonidine-PF, respectively.

New glaucoma medications in the geriatric population: efficacy and safety

Glaucoma can be considered a disease of the aging eye. Most medications used to treat glaucoma are in topical eyedrop form and may cause numerous untoward systemic effects in older persons. In recent years, several new ocular hypotensive medications have become available. These medications are being used more commonly because there is a growing trend by ophthalmologists to aggressively lower intraocular pressure. Therefore, geriatricians require a comprehensive knowledge of medications used to treat glaucoma, in addition to an understanding of their mechanism of action profiles of untoward effects and possible interactions with other diseases or medications. Therefore, we performed a review of the medications recently introduced into clinical practice. We selected drugs approved by the U.S. Food and Drug Administration between 1996 and September 2001. The safety profiles of these agents and their untoward side effects were reviewed by class: topical carbonic anhydrase inhibitors (brinzolamide: ocular tolerance, taste perversion), beta-adrenoceptor antagonists (timolol: bradycardia and bronchospasm), alpha-adrenergic agonists (brimonidine: oral dryness, headache, and fatigue), and prostaglandin analogs (latanoprost, bimatoprost, travoprost, and unoprostone isopropyl: ocular hyperemia, iris color changes). The function of this review is to make geriatricians more aware of the efficacy and untoward effects of medications recently introduced into clinical practice. We recommend that geriatricians perform a medication review on all medications their patients use, including eye drops.

Distribution of brimonidine into anterior and posterior tissues of monkey, rabbit, and rat eyes

The objectives of the study were to evaluate the distribution of brimonidine (alpha2-adrenergic agonist) into anterior and posterior ocular tissues. Single or multiple doses of a 0.2 or 0.5% brimonidine tartrate solution were administered to one or both eyes of monkeys or to one eye of rabbits. Brimonidine was administered intraperitoneally to rats. After topical administration, [14C]brimonidine was rapidly absorbed into the cornea and conjunctiva and distributed throughout the eye. [14C]Radioactivity was higher and cleared more slowly in pigmented tissues (iris/ciliary body, choroid/retina, and optic nerve) than in nonpigmented tissues. Single and multiple dosing led to a similar drug distribution, with higher levels of brimonidine measured in pigmented tissues after multiple dosing. Most of the radioactivity extracted from ocular tissues represented unchanged brimonidine. In the rabbits and the monkey treated in only one eye, levels of radioactivity in the untreated eye were low, consistent with the low systemic levels and rapid drug clearance. Posterior ocular tissue concentrations of radioactivity exceeded systemic blood concentrations. The vitreous humor brimonidine concentrations in monkeys treated topically with 0.2% brimonidine tartrate was 82 +/- 45 nM. Vitreous levels in rabbits confirmed the penetration of brimonidine to the posterior segment. Similar concentrations of brimonidine (22 to 390 nM) were measured in the vitreous and retina of rats injected intraperitoneally with brimonidine. Both topically applied and systemically administered brimonidine reach the back of the eye at nanomolar concentrations sufficient to activate alpha2-adrenergic receptors. The brimonidine levels achieved at the retina are relevant for neuroprotection models.

Transscleral drug delivery to the retina and choroid

Rapid advances in the molecular pathogenesis of retinal and choroidal disorders have highlighted the urgent need for innovative drug delivery modalities to the loci of pathology. In vitro and in vivo studies suggest that the transscleral route may offer a means to achieve the goal of sustained, targeted drug delivery to the posterior segment. Potentially therapeutic concentrations of macromolecules with retention of bioactivity can be attained in the choroid and retina via minimally invasive transscleral delivery.

Neuroprotective effects of brimonidine against transient ischemia-induced retinal ganglion cell death: a dose response in vivo study

The purpose of this study was to investigate the dose-response effects of topically administered brimonidine (BMD) on retinal ganglion cell (RGC) survival, short and long periods of time after transient retinal ischemia. In adult Sprague-Dawley rats, RGCs were retrogradely labeled with the fluorescent tracer fluorogold (FG) applied to both superior colliculi. Seven days later, the left ophthalmic vessels were ligated for 90 min. One hr prior to retinal ischemia, two 5 microl drops of saline alone or saline containing 0.0001, 0.001, 0.01 or 0.1% BMD were instilled on the left eye. Rats were processed 7, 14 or 21 days later and densities of surviving RGCs were estimated by counting FG-labeled RGCs in 12 standard regions of each retina. The following have been found. (1) Seven days after 90 min of transient ischemia there is loss of approximately 46% of the RGC population. (2) topical pre-treatment with BMD prevents ischemia-induced RGC death in a dose-dependent manner. Administration of 0.0001% BMD resulted in the loss of approximately 37% of the RGC population and had no significant neuroprotective effects. Administration of higher concentrations of BMD (0.001 or 0.01%) resulted in the survival of 76 or 90%, respectively, of the RGC population, and 0.1% BMD fully prevented RGC death in the first 7 days after ischemia. (3) Between 7 and 21 days after ischemia there was an additional slow cell loss of approximately 25% of the RGC population. Pre-treatment with 0.1% BMD also reduced significantly this slow cell death. These results indicate that the neuroprotective effects of BMD, when administered topically, are dose-dependent and that the 0.1% concentration achieves optimal neuroprotective effects against the early loss of RGCs. Furthermore, this concentration is also effective to diminish the protracted loss of RGCs that occurs with time after transient ischemia.

Topically applied clonidine protects the rat retina from ischaemia/reperfusion by stimulating alpha(2)-adrenoceptors and not by an action on imidazoline receptors

Ischaemia was induced to the rat retina by raising the intraocular pressure above the systolic blood pressure for 45 min. After a reperfusion period of 5 days, alterations in the localisation of choline acetyltransferase (ChAT) and calretinin immunoreactivities, a reduction in the thickness of the inner retinal layers and a decline in the b-wave amplitude of the electroretinogram were recorded. These changes were blunted when clonidine was injected intraperitoneally before or after ischaemia or when applied topically by a specific regime. Other alpha(2)-adrenoceptor agonists, brimonidine and apraclonidine, acted in a similar way to clonidine when applied topically but because of the number of experiments carried out a comparison between the effectiveness of the different alpha(2)-adrenoceptor agonists was not possible. The protective effect of clonidine was attenuated when the alpha(2)-adrenoceptor antagonists yohimbine or rauwolscine were co-administered, suggesting that the mechanism of action of the drug is to stimulate alpha(2)-adrenoceptors. In addition, the imidazoline receptor ligands, BU-226 and AGN-192403 did not blunt the effect of ischaemia/reperfusion, supporting the notion that the protective action of the alpha(2)-adrenoceptor agonists does not involve imidazoline sites but rather the activation of alpha(2)-adrenoceptors. The protective effect of 0.5% clonidine appeared to be greater when topically applied to the eye that received ischaemia than when applied by the same regime to the contralateral eye. These studies suggest that while most of topically applied clonidine reaches the retina by a systemic route one cannot rule out additional pathways.

Effect of brimonidine tartrate on ocular hemodynamics in healthy volunteers

While alpha2-adrenergic agonists, such as brimonidine tartrate, significantly reduce the intraocular pressure (IOP), the presence of vasoconstrictor postsynaptic alpha2 receptors on vascular smooth muscle raise the possibility that brimonidine could potentially compromise ocular blood flow. Consequently, the ocular hemodynamic effects of brimonidine were studied in normal subjects. Twelve healthy volunteers were included in this prospective, double-masked, placebo controlled, crossover-designed clinical trial. They received either brimonidine tartrate 0.2% or placebo b.i.d. for 2 weeks. Goldmann tonometry and color Doppler imaging (CDI) were performed at baseline, at 2 hr, 1 week, and 2 weeks after the treatment. Fundus angiography using a scanning laser ophthalmoscope was performed at baseline and 2 weeks after treatment to determine retinal arteriovenous passage time. Brimonidine lowered IOP at 2 hr, 1 week, and 2 weeks (p = 0.058, p = 0.031, and p = 0.022, respectively). Brimonidine did not affect the retrobulbar arterial velocities measured by CDI, nor retinal arteriovenous passage time. In conclusion, two-week treatment with brimonidine reduces IOP and does not reduce the bulk retinal or retrobulbar arterial perfusion in young healthy volunteers.