Comparison of conjunctival and corneal surface areas in rabbit and human

Dry Eye Management: Targeting the Ocular Surface Microenvironment

Dry eye can damage the ocular surface and result in mild corneal epithelial defect to blinding corneal pannus formation and squamous metaplasia. Significant progress in the treatment of dry eye has been made in the last two decades; progressing from lubricating and hydrating the ocular surface with artificial tear to stimulating tear secretion; anti-inflammation and immune regulation. With the increase in knowledge regarding the pathophysiology of dry eye, we propose in this review the concept of ocular surface microenvironment. Various components of the microenvironment contribute to the homeostasis of ocular surface. Compromise in one or more components can result in homeostasis disruption of ocular surface leading to dry eye disease. Complete evaluation of the microenvironment component changes in dry eye patients will not only lead to appropriate diagnosis, but also guide in timely and effective clinical management. Successful treatment of dry eye should be aimed to restore the homeostasis of the ocular surface microenvironment.

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.

Mechanistic modeling of ophthalmic drug delivery to the anterior chamber by eye drops and contact lenses

Ophthalmic drug for the anterior chamber diseases are delivered into tears by either eye drops or by extended release devices placed in the eyes. The instilled drug exits the eye through various routes including tear drainage into the nose through the canaliculi and transport across various ocular membranes. Understanding the mechanisms relevant to each route can be useful in predicting the dependency of ocular bioavailability on various formulation parameters, such as drug concentration, salinity, viscosity, etc. Mathematical modeling has been developed for each of the routes and validated by comparison with experiments. The individual models can be combined into a system model to predict the fraction of the instilled drug that reaches the target. This review summarizes the individual models for the transport of drugs across the cornea and conjunctiva and the canaliculi tear drainage. It also summarizes the combined tear dynamics model that can predict the ocular bioavailability of drugs instilled as eye drops. The predictions from the individual models and the combined model are in good agreement with experimental data. Both experiments and models predict that the corneal bioavailability for drugs delivered through eye drops is less than 5% due to the small area of the cornea in comparison to the conjunctiva, and the rapid clearance of the instilled solution by tear drainage. A contact lens is a natural choice for delivering drugs to the cornea due to the placement of the contact in the immediate vicinity of the cornea. The drug released by the contact towards the cornea surface is trapped in the post lens tear film for extended duration of at least 30min allowing transport of a large portion into the cornea. The model predictions backed by in vivo animal and clinical data show that the bioavailability increases to about 50% with contact lenses. This realization has encouraged considerable research towards delivering ocular drugs by contact lenses. Commercial contacts are, however, not ideal for drug delivery due to the short release durations which may necessitate wearing multiple lenses each day, reducing the viability of this approach. Recent research has focused on designing contacts that retain all critical properties while increasing the release durations to a few hours or a few days. Beagle dog studies with contact lenses containing vitamin E nanobarriers to attenuate drug transport have shown promising results. Human studies using contacts for drug delivery have also been conducted for allergy therapy but drug eluting contacts are not available in the market for any therapy.

Evaluating the Functionality of Conjunctiva Using a Rabbit Dry Eye Model

Purpose. To assess the conjunctival functionality in a rabbit dry eye (DE) model. Methods. Nictitating membrane, lacrimal and Harderian glands were surgically excised from male New Zealand white rabbits using minimally invasive surgery. Fluorescein/rose Bengal staining of ocular surface (OS) and Schirmer test were done before (BE) and after excision (AE). The expression of interleukin- (IL-) 1β, tumor necrosis factor- (TNF-) α, and MUC5AC proteins were estimated by immunoblotting from conjunctival impression cytology specimens. MUC5AC mRNA was quantified as well. The effect of epithelial sodium channel (ENaC) blockers on tear production and potential differences (PD) of OS were assessed under anesthesia in rabbits with and without surgery. Results. Increase in corneal and conjunctival staining was observed 1 month AE compared to BE. Schirmer tests failed to show decrease in tear production. Elevated IL-1β, and TNF-α, 1 month AE indicated inflammation. MUC5AC expression was elevated 1 month AE. ENaC blockers did not improve tear production in rabbit eyes AE but characteristic changes in PD were observed in rabbits with surgery. Conclusions. DE biomarkers are important tools for OS assessment and MUC5AC expression is elevated in rabbit DE. PD measurement revealed significant electrophysiological changes in rabbits with surgery.

Is the main lacrimal gland indispensable? Contributions of the corneal and conjunctival epithelia

The ocular surface system is responsible for ensuring that the precorneal tear film is sufficient in both quality and quantity to preserve optimal vision. Tear secretion is a complex, multifactorial process, and dysfunction of any component of the ocular surface system can result in tear film instability and hyperosmolarity with resultant dry eye disease. The tear film is primarily composed of lipids, aqueous, and mucins, with aqueous accounting for most of its thickness. The aqueous is produced by the main lacrimal gland, accessory lacrimal glands, and corneal and conjunctival epithelia. Although the main lacrimal gland has long been considered an indispensable source of the aqueous component of tears, there is evidence that adequate tear secretion can exist in the absence of the main lacrimal gland. We review and discuss the basics of tear secretion, the tear secretory capacity of the ocular surface, and emerging treatments for dry eye disease.

Small-molecule CFTR activators increase tear secretion and prevent experimental dry eye disease

Dry eye disorders, including Sjögren's syndrome, constitute a common problem in the aging population, with limited effective therapeutic options available. The cAMP-activated Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR) is a major prosecretory channel at the ocular surface. We investigated whether compounds that target CFTR can correct the abnormal tear film in dry eye. Small-molecule activators of human wild-type CFTR identified by high-throughput screening were evaluated in cell culture and in vivo assays, to select compounds that stimulate Cl(-)-driven fluid secretion across the ocular surface in mice. An aminophenyl-1,3,5-triazine, CFTRact-K089, fully activated CFTR in cell cultures with EC50 ∼250 nM and produced an ∼8.5 mV hyperpolarization in ocular surface potential difference. When delivered topically, CFTRact-K089 doubled basal tear volume for 4 h and had no effect in CF mice. CFTRact-K089 showed sustained tear film bioavailability without detectable systemic absorption. In a mouse model of aqueous-deficient dry eye produced by lacrimal ablation, topical administration of 0.1 nmol CFTRact-K089 3 times daily restored tear volume to basal levels, preventing corneal epithelial disruption when initiated at the time of surgery and reversing it when started after development of dry eye. Our results support the potential utility of CFTR-targeted activators as a novel prosecretory treatment for dry eye.-Flores, A. M., Casey, S. D., Felix, C. M., Phuan, P. W., Verkman, A. S., Levin, M. H. Small-molecule CFTR activators increase tear secretion and prevent experimental dry eye disease.

IL-13 Stimulates Proliferation and Expression of Mucin and Immunomodulatory Genes in Cultured Conjunctival Goblet Cells

PURPOSE

To investigate the effects of IL-13 on goblet cell proliferation, differentiation, and expression of mucin and immunomodulatory genes.

METHODS

Explants were excised from the conjunctiva of young C57BL/6 mice. Cultures received 200 μL per week of either Keratinocyte media (KSFM) or KSFM supplemented with 10 ng/mL IL-13 and were incubated for 3 (D3), 7 (D7), or 14 (D14) days. Subsequently, cell proliferation was assessed or cultures were immunostained, collected for dot blot, or for reverse transcription (RT) and quantitative real-time PCR (qPCR) or for RT-PCR gene array.

RESULTS

The cultured conjunctival epithelium expressed goblet cell associated keratin 7 and mucins MUC5AC and MUC2 and when stimulated with IL-13 showed increased proliferation at D3 and D7 (P < 0.05) compared with control. MUC5AC expression was increased in the IL-13-treated group at D3 and D14 (P < 0.05). IL-13-treated cultures showed increased chemokine ligand 26 (CCL26), chloride channel calcium activated channel 3 (CLCA3), fas ligand (FasL), and Relm-β at D7. All conjunctival cultures expressed MUC2, and its expression was decreased at D3 (P < 0.05) and increased at D14 (P < 0.05) with IL-13 treatment.

CONCLUSIONS

This study demonstrated that conjunctival goblet cells are IL-13 responsive cells that produce factors known to maintain epithelial barrier, stimulate mucin production, and modulate immune response in nonocular mucosa when treated with IL-13. The functional significance of IL-13-stimulated factors remains to be determined.

Subconjunctival sirolimus in the treatment of autoimmune non-necrotizing anterior scleritis: results of a phase I/II clinical trial

PURPOSE

To investigate the safety, tolerability and efficacy of subconjunctival sirolimus injections as a treatment for active, autoimmune, non-necrotizing anterior scleritis.

DESIGN

Phase I/II, single-center, open-label, nonrandomized, prospective pilot study.

METHODS

Five participants with active, autoimmune, non-necrotizing anterior scleritis with scleral inflammatory grade of ≥1+ in at least 1 quadrant with a history of flares were enrolled. A baseline injection was given, with the primary outcome measure of at least a 2-step reduction or reduction to grade zero in the study eye by 8 weeks. Secondary outcomes included changes in visual acuity and intraocular pressure, ability to taper concomitant immunosuppressive regimen, and number of participants who experienced a disease flare requiring reinjection. Safety outcomes included the number and severity of systemic and ocular toxicities, and vision loss ≥15 ETDRS letters. The study included 6 visits over 4 months with an extension phase to 1 year for participants who met the primary outcome.

RESULTS

All participants (N = 5, 100%; 95% CI [0.60, 1.00]) met the primary outcome in the study eye by the week 8 visit. There was no significant change in mean visual acuity or intraocular pressure. Three out of 5 patients (60%) experienced flares requiring reinjection. No systemic toxicities were observed. Two participants (40%) experienced a localized sterile inflammatory reaction at the site of the injection, which resolved without complication.

CONCLUSIONS

Subconjunctival sirolimus leads to a short-term reduction in scleral inflammation, though relapses requiring reinjection do occur. There were no serious adverse events, though a local sterile conjunctival inflammatory reaction was observed.

Influence of drug lipophilicity on drug release from sclera after iontophoretic delivery of mixed micellar carrier system to human sclera

Mixed micelles prepared using sodium taurocholate (TA) and egg lecithin (LE) were previously found to be an effective carrier for sustained release of a poorly water-soluble drug in transscleral iontophoretic delivery. The objectives of the present study were to investigate the effects of drug lipophilicity upon micellar carrier solubilization potential and drug release profiles from the sclera after iontophoretic delivery of model lipophilic drugs dexamethasone (DEX), triamcinolone acetonide (TRIAM), and β-estradiol (E2β) with a mixed micellar carrier system of TA-LE (1:1 mole ratio). In this study, the micellar carrier system was characterized for drug solubilization. The micelles encapsulating these drugs were evaluated for transscleral passive and 2-mA iontophoretic delivery (both cathodal and anodal) and drug release from excised human sclera in vitro. The results show that drug solubility enhancement of the micellar carrier system increased with increasing drug lipophilicity. The more lipophilic drugs E2β and TRIAM displayed slower drug release from the sclera compared with the less lipophilic drug DEX after iontophoretic drug delivery with the mixed micelles. These results suggest that the combination of transscleral iontophoresis and micellar carriers is more effective in sustaining transscleral delivery of the more lipophilic drugs studied in this investigation.

Transporter targeted gatifloxacin prodrugs: synthesis, permeability, and topical ocular delivery

In this work, we aim to design and synthesize prodrugs of gatifloxacin targeting organic cation transporter (OCT), monocarboxylate transporter (MCT), and ATB (0, +) transporters and to identify a prodrug with enhanced delivery to the back of the eye. Dimethylamino-propyl, carboxy-propyl, and amino-propyl(2-methyl) derivatives of gatifloxacin (GFX), DMAP-GFX, CP-GFX, and APM-GFX, were designed and synthesized to target OCT, MCT, and ATB (0, +) transporters, respectively. An LC-MS method was developed to analyze drug and prodrug levels in various studies. Solubility and log D (pH 7.4) were measured for prodrugs and the parent drug. The permeability of the prodrugs was determined in the cornea, conjunctiva, and sclera-choroid-retinal pigment epitheluim (SCRPE) and compared with gatifloxacin using an Ussing chamber assembly. Permeability mechanisms were elucidated by determining the transport in the presence of transporter specific inhibitors. 1-Methyl-4-phenylpyridinium iodide (MPP+), nicotinic acid sodium salt, and α-methyl-DL-tryptophan were used to inhibit OCT, MCT, and ATB (0, +) transporters, respectively. A prodrug selected based on in vitro studies was administered as an eye drop to pigmented rabbits, and the delivery to various eye tissues including vitreous humor was compared with gatifloxacin dosing. DMAP-GFX exhibited 12.8-fold greater solubility than GFX. All prodrugs were more lipophilic, with the measured log D (pH 7.4) values ranging from 0.05 to 1.04, when compared to GFX (log D: -1.15). DMAP-GFX showed 1.4-, 1.8-, and 1.9-fold improvement in permeability across the cornea, conjunctiva, and SCRPE when compared to GFX. Moreover, it exhibited reduced permeability in the presence of MPP+ (competitive inhibitor of OCT), indicating OCT-mediated transport. CP-GFX showed 1.2-, 2.3-, and 2.5-fold improvement in permeability across the cornea, conjunctiva, and SCRPE, respectively. In the presence of nicotinic acid (competitive inhibitor of MCT), the permeability of CP-GFX was reduced across the conjunctiva. However, the cornea and SCRPE permeability of CP-GFX was not affected by nicotinic acid. APM-GFX did not show any improvement in permeability when compared to GFX across the cornea, conjunctiva, and SCRPE. Based on solubility and permeability, DMAP-GFX was selected for in vivo studies. DMAP-GFX showed 3.6- and 1.95-fold higher levels in vitreous humor and CRPE compared to that of GFX at 1 h after topical dosing. In vivo conversion of DMAP-GFX prodrug to GFX was quantified in tissues isolated at 1 h after dosing. The parent drug-to-prodrug ratio was 8, 70, 24, 21, 29, 13, 55, and 60% in the cornea, conjunctiva, iris-ciliary body, aqueous humor, sclera, CRPE, retina, and vitreous humor, respectively. In conclusion, DMAP-GFX prodrug enhanced solubility, log D, as well as OCT mediated delivery of gatifloxacin to the back of the eye.

Regional differences in rat conjunctival ion transport activities

Active ion transport and coupled osmotic water flow are essential to maintain ocular surface health. We investigated regional differences in the ion transport activities of the rat conjunctivas and compared these activities with those of cornea and lacrimal gland. The epithelial sodium channel (ENaC), sodium/glucose cotransporter 1 (Slc5a1), transmembrane protein 16 (Tmem16a, b, f, and g), cystic fibrosis transmembrane conductance regulator (Cftr), and mucin (Muc4, 5ac, and 5b) mRNA expression was characterized by RT-PCR. ENaC proteins were measured by Western blot. Prespecified regions (palpebral, fornical, and bulbar) of freshly isolated conjunctival tissues and cell cultures were studied electrophysiologically with Ussing chambers. The transepithelial electrical potential difference (PD) of the ocular surface was also measured in vivo. The effect of amiloride and UTP on the tear volume was evaluated in lacrimal gland excised rats. All selected genes were detected but with different expression patterns. We detected αENaC protein in all tissues, βENaC in palpebral and fornical conjunctiva, and γENaC in all tissues except lacrimal glands. Electrophysiological studies of conjunctival tissues and cell cultures identified functional ENaC, SLC5A1, CFTR, and TMEM16. Fornical conjunctiva exhibited the most active ion transport under basal conditions amongst conjunctival regions. PD measurements confirmed functional ENaC-mediated Na(+) transport on the ocular surface. Amiloride and UTP increased tear volume in lacrimal gland excised rats. This study demonstrated that the different regions of the conjunctiva exhibited a spectrum of ion transport activities. Understanding the specific functions of distinct regions of the conjunctiva may foster a better understanding of the physiology maintaining hydration of the ocular surface.

Drug metabolism and pharmacokinetics

In this article, aspects of absorption, distribution, metabolism, and excretion have been described bearing in mind the pathogenesis of allergic diseases and their possible therapeutic opportunities. The importance of the routes of administration of the different therapeutic groups has been emphasized. The classical aspects of drug metabolism and disposition related to oral administration have been reviewed, but special emphasis has been given to intranasal, cutaneous, transdermal, and ocular administration as well as to the absorption and the subsequent bioavailability of drugs. Drug-metabolizing enzymes and transporters present in extrahepatic tissues, such as nasal mucosa and the respiratory tract, have been particularly discussed. As marketed antiallergic drugs include both racemates and enantiomers, aspects of stereoselective absorption, distribution, metabolism, and excretion have been discussed. Finally, a new and promising methodology, microdosing, has been presented, although it has not yet been applied to drugs used in the treatment of allergic diseases.

Tear Dynamics Model

PURPOSE

Quantitative understanding of tear dynamics may help in developing better ophthalmic drug delivery vehicles and dry eye treatments. This paper attempts to develop a comprehensive model that can predict the effect of physiological parameters on various issues related to tear dynamics.

METHODS

The model is based on mass balances of water and solutes such as glucose, sodium, potassium, and chloride. The mass balances require models for the drainage of fluid through the canaliculi and the transport of tears and solutes through the conjunctiva. The model parameters are obtained by simulating Ussing-chamber experiments. The differential equations for the unsteady balances are solved numerically.

RESULTS

The model predicts that under normal conditions, the tear volume, tear osmolarity and potential are 7.1 microl, 297.6 mM, and -15.1 mV, respectively. The model also predicts that the conjunctiva is secretory and contributes about 25% of the total tear production. We also predict the effect of evaporation on tear physiology and show that an increase in evaporation increases osmolarity, reduces tear volume, and increases conjuctival secretion. Additionally, the new tear dynamics model helps assess the effect of osmolarity of the instilled drops, insertion of punctum plugs and use of moisture chamber as treatments for dry eyes. Furthermore, the model is used to predict the effect of modulation of specific transport pathways, which is proposed as a potential remedy for dry eyes, on conjuctival secretion and total tear volume.

CONCLUSIONS

Most of the predicted results agree with the reported experimental results, at least qualitatively. However, some predictions disagree with experiments suggesting that further improvements in the model are needed. The model developed in this paper can improve our understanding of tear dynamics and also serve as a tool to evaluate the efficacy of various modalities at treating dry eyes.

A Mathematical Model for Ocular Tear and Solute Balance

PURPOSE

In this paper we develop a mathematical model that can predict the steady-state tear film thickness and the dynamic tear film thickness and the solute concentration after instillation of a solute-laden fluid in the eye.

METHODS

The mathematical model developed in this paper is based on a balance between the inflow and outflow of tears into the tear film. It incorporates a tear drainage model and a model that relates the tear film thickness to the meniscus radius of curvature. To predict the solute concentrations, the tear balance is coupled with the solute balance. The differential equations for the unsteady balances are solved numerically.

RESULTS

The model predicts that the tear film thickness depends on a number of physiological factors, such as rates of tear production and evaporation, geometry and modulus of the canaliculi, and surface tension and viscosity of tears, and varies from about 3 to 15 microm. The model also predicts that the drainage time for an instilled volume of 15 microl is 1283 s. Additionally, the time required for the tracer concentration to decay to 1% of the value immediately after instillation of a drug-laden 40 microl drop is about 2480 s. Similarly, the time for intensity decay for a radioactive tracer after 25 microl instillation is about 1566 s. Also, the model predicts that the fraction of the instilled drug that reaches the cornea is about 1.3% for topical application of timolol.

CONCLUSIONS

The predicted results agree reasonably with the reported experimental results, at least qualitatively. The model developed here can serve as a useful tool to develop a more precise understanding of various issues related to tears and also evaluate the effect of various parameters on the tear volume.

Effect of viscosity on tear drainage and ocular residence time

PURPOSE

An increase in residence time of dry eye medications including artificial tears will likely enhance therapeutic benefits. The drainage rates and the residence time of eye drops depend on the viscosity of the instilled fluids. However, a quantitative understanding of the dependence of drainage rates and the residence time on viscosity is lacking. The current study aims to develop a mathematical model for the drainage of Newtonian fluids and also for power-law non-Newtonian fluids of different viscosities.

METHODS

This study is an extension of our previous study on the mathematical model of tear drainage. The tear drainage model is modified to describe the drainage of Newtonian fluids with viscosities higher than the tear viscosity and power-law non-Newtonian fluids with rheological parameters obtained from fitting experimental data in literature. The drainage rate through canaliculi was derived from the modified drainage model and was incorporated into a tear mass balance to calculate the transients of total solute quantity in ocular fluids and the bioavailability of instilled drugs.

RESULTS

For Newtonian fluids, increasing the viscosity does not affect the drainage rate unless the viscosity exceeds a critical value of about 4.4 cp. The viscosity has a maximum impact on drainage rate around a value of about 100 cp. The trends are similar for shear thinning power law fluids. The transients of total solute quantity, and the residence time agrees at least qualitatively with experimental studies.

CONCLUSIONS

A mathematical model has been developed for the drainage of Newtonian fluids and power-law fluids through canaliculi. The model can quantitatively explain different experimental observations on the effect of viscosity on the residence of instilled fluids on the ocular surface. The current study is helpful for understanding the mechanism of fluid drainage from the ocular surface and for improving the design of dry eye treatments.

Mucoadhesive microdiscs engineered for ophthalmic drug delivery: effect of particle geometry and formulation on preocular residence time

PURPOSE

To test the hypothesis that mucoadhesive microdiscs formulated in a rapidly dissolving tablet can increase preocular residence time.

METHODS

Microparticles smaller than 10 mum in diameter were fabricated by emulsification with poly(lactic-co-glycolic acid) as a core material and, in some cases, poly(ethylene glycol) as a mucoadhesion promoter. To examine the effect of particle geometry, microparticles were also cut to have flat surfaces (i.e., microdiscs) and were compared with spherical particles (i.e., microspheres). In vitro mucoadhesion of microparticles was tested on a mucous layer under shear stress, mimicking the human blink. The resultant microparticles were also formulated in two dosage forms, an aqueous suspension and a dry tablet, to test the effect of formulation on the retention capacity of microparticles on the preocular space of rabbits in vivo.

RESULTS

Mucoadhesive microdiscs adhered better to the simulated ocular surface than did other types of microparticles. When a dry tablet embedded with mucoadhesive microdiscs was administered in the cul-de-sac of the rabbit eye in vivo, these microdiscs exhibited longer retention than the other formulations tested in this study. More than 40% and 17% of mucoadhesive microdiscs remained on the preocular surface at 10 minutes and 30 minutes after administration, respectively. Fluorescence images from the eye surface showed that mucoadhesive microdiscs remain for at least 1 hour in the lower fornix.

CONCLUSIONS

This study demonstrated that mucoadhesive microdiscs formulated in a dry tablet can achieve a prolonged residence time on the preocular surface and thus are a promising drug delivery system for ophthalmic applications.

Barriers to glaucoma drug delivery

Topical medications remain the mainstay of glaucoma treatment. This review will aim to cover the pharmacokinetics of topically applied drops, the ocular barriers to drug delivery, and the role of ophthalmic drug formulation in enhancing drug delivery to the target tissue while minimizing side effects and increasing patient compliance. Recent advances in surgical techniques, therapeutic approaches, and material sciences have produced exciting new therapies for ocular diseases. The development of new vehicles and drug formulations that require less patient compliance is also discussed, as are the routes of drug delivery for neuroprotection.

Digital photography as a novel technique of measuring ocular surface dimensions

PURPOSE

To introduce a novel technique for measuring ocular surface dimensions using digital photography and computerized image analysis in the context of ptosis repair surgery.

METHODS

Digital photographs and patient questionnaires on dry eye symptoms were obtained from 31 patients before and after ptosis repair. Patients were examined preoperatively and at 1 and 6 weeks postoperatively. Adobe Photoshop 7.0 (Adobe Systems Incorporated, 345 Parkl Avenue, San Jose, CA 95110-2704, USA) was used to digitally measure palpebral fissure height, fissure width, and ocular surface area (OSA). Similar digital measurements were obtained on 30 control subjects as well. Digital calculations of OSA for control, preoperative, and postoperative groups were compared with other published techniques.

RESULTS

Graphical comparison between our method of measuring OSA and the mathematical estimations proposed by previous studies suggests that our method is more precise in measuring OSA, and that it is also better able to identify individual variations of OSA from general population trends.

CONCLUSION

Digital ocular photography combined with computerized image analysis is a fast, easy to use, and reliable method of measuring ocular surface dimensions. In addition to ptosis surgery, this method can be used in other ocular surface studies.

In vitro alternatives to the use of animals in ocular toxicology testing

Chemical substances, including household products, industrial chemicals, and cosmetics, must be tested for ocular toxicity or irritancy so that the public can be assured of their safety or warned of dangers associated with their use. The in vivo Draize test is the standard method used to meet this requirement; however, this test is coming under increasing criticism on scientific and ethical grounds. This has led to the development of a large number of proposed in vitro tests, some of which are routinely used to screen chemicals in toxicology laboratories. This review addresses regulations governing ocular irritancy testing and the current status of the movement toward use of alternative methods. Such methods include the use of cultured cells, hen's eggs, isolated animal eyes and corneas, human corneal epithelial cell lines, and the recently developed in vitro corneal equivalent models. The protocols for these methods are outlined, and their endpoints are described with respect to prediction of in vivo responses. The tests are evaluated in the context of the outcomes of validation studies and acceptance by regulatory agencies. While several of these tests yield useful information concerning ocular irritancy, to date, no in vitro alternative test has been validated as a replacement for the Draize test. If the goal of replacing the in vivo test while protecting the public from chemical eye injury is to be achieved, further development and improvement of alternative tests, as well as establishment of a human ocular toxicity data base, are required.

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.

Corneal epithelium as a platform for secretion of transgene products after transfection with liposomal gene eyedrops

BACKGROUND

The first objective of the study was to evaluate the transfection of corneal epithelium with non-viral vectors to secrete transgene products into the tear fluid and aqueous humor. The second goal was to evaluate the differentiated corneal epithelial cell culture for transfection studies.

METHODS

The human corneal epithelial (HCE) cell line was cultured to different stages of differentiation and transfected with complexes of pCMV-SEAP2 with DOTAP/DOPE, DOTAP/DOPE/protamine sulfate (PS) and polyethylenimine (PEI). The complexes of DOTAP/DOPE with plasmid (CMV-SEAP2 or pCMV-Luc4) were subsequently applied topically to the rabbit eyes. Secreted alkaline phosphatase (SEAP) was analyzed using chemiluminescent assay. Luciferase (Luc) was detected at the mRNA level in cornea and conjunctiva using a qRT-PCR.

RESULTS

The transfection levels decreased with differentiation of HCE cells. PEI was effective in transfecting both the dividing and partly differentiated cells, but ineffective in differentiated cells. DOTAP/DOPE showed high activity in differentiated cell cultures, while added PS did not improve transfection. Significant SEAP expression was observed for three days after in vivo transfection in the tear fluid and aqueous humor. The luciferase mRNA was found both in the cornea and conjunctiva. The rates of SEAP secretion from both the basolateral side of differentiated HCE cells and cornea in vivo were within the same range.

CONCLUSIONS

Corneal epithelium can be transfected topically to secrete gene products to the tear fluid and aqueous humor. The differentiated HCE model is a useful tool in the evaluation of non-viral carriers for corneal transfection.

Aquaporins and CFTR in ocular epithelial fluid transport

Aquaporins (AQPs) and the cystic fibrosis transmembrane conductance regulator (CFTR) provide the molecular routes for transport of water and chloride, respectively, through many epithelial tissues. In ocular epithelia, fluid transport generally involves secondary active chloride transport, which creates the osmotic gradient to drive transepithelial water transport. This review is focused on the role of AQPs and CFTR in water and ion transport across corneal/conjunctival epithelia, corneal endothelium, ciliary epithelium, and retinal pigment epithelium. The potential relevance of water and chloride transport to common disorders of ocular fluid balance is also considered. Recent data suggest AQPs and CFTR as attractive targets for drug development for therapy of keratoconjunctivitis sicca, recurrent corneal erosions, corneal edema, glaucoma, retinal detachment, and retinal ischemia.

Ocular drug delivery

Drug delivery to the eye is hampered by anatomical factors, including the corneal epithelium, the blood-aqueous barrier and the blood-retinal barrier. This review aims to outline the major routes of ocular drug delivery, including systemic, topical, periocular and intravitreal. The pharmacokinetics, the disadvantages and the clinical relevance of these drug delivery routes have been emphasised. Recent advances in surgical techniques, therapeutic approaches and material sciences have produced exciting new therapies for ocular diseases. The role of ophthalmic drug formulation in targeting the desired ocular tissue and enhancing drug delivery by the chosen route whilst minimising side effects is also discussed.

Roles of the conjunctiva in ocular drug delivery: a review of conjunctival transport mechanisms and their regulation

Conjunctiva plays many roles including protection of ocular surface, production of tear film, and a conduit for drug clearance (depending on drug properties) into the systemic circulation or for drug transport to the deep tissues of the eye. The conjunctiva, which is a moderately tight epithelium, endowed with various transport processes for the homeostasis of ions, solutes, and water in the conjunctival surface and tear film. Modulation of ion transport in the conjunctiva leads to alterations in transconjunctival fluid flow that may become useful for treatment of dry-eye state in the eye. As a possible drug delivery route to the posterior portion of the eye, conjunctiva is an attractive route due to both larger surface area than that of cornea and expression of several key transport processes. Tear contains D-glucose and many amino acids, in addition to the usual ions in the body fluids. Several ion-coupled solute transport processes for absorption of amino acids, D-glucose, monocarboxylate, nucleosides, and dipeptides are expressed in the conjunctiva. Thanks to the rich endowment of these transport processes, drug transport across the conjunctiva into the intraocular tissues may become quite feasible. Subconjunctival injection of microparticles and matrix materials (which allows sustained release of drugs) is shown to maintain reasonable levels of various drugs in the vitreous, perhaps attesting to the fact that conjunctiva per se may contribute as a part of multiple transport barrier(s) in ocular drug delivery. In addition, several conjunctival approaches have been investigated to optimize treatment of dry-eye syndrome and intraocular diseases, and more can be accomplished in the coming years.

Model of transient drug diffusion across cornea

A mathematical model of solute transient diffusion across the cornea to the anterior chamber of the eye was developed for topical drug delivery. Solute bioavailability was predicted given solute molecular radius and octanol-to-water distribution coefficient (Phi), ocular membrane ultrastructural parameters, tear fluid hydrodynamics, as well as solute distribution volume (Vd) and clearance rate (Cla) in the anterior chamber. The results suggest that drug bioavailability is primarily determined by solute lipophilicity. In human eyes, bioavailability is predicted to range between 1% and 5% for lipophilic molecules (Phi>1), and to be less than 0.5% for hydrophilic molecules (Phi<0.01). The simulations indicate that the distribution coefficient that maximizes bioavailability is on the order of 10. It was also found that the maximum solute concentration in the anterior chamber (Cmax) and the time needed to reach Cmax significantly depend on Phi, Vd, and Cla. Consistent with experimental findings, model predictions suggest that drug bioavailability can be increased by lowering the conjunctival-to-corneal permeability ratio and reducing precorneal solute drainage. Because of its mechanistic basis, this model will be useful to predict drug transport kinetics and bioavailability for new compounds and in diseased eyes.

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.

Vesicular systems in ocular drug delivery: an overview

The main aim of pharmacotherapeutics is the attainment of effective drug concentration at the intended site of action for a sufficient period of time to elicit a response. Poor bioavailability of drugs from ocular dosage form is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. Though the topical and localized application are still an acceptable and preferred way to achieve therapeutic level of drugs used to treat ocular disorders but the primitive ophthalmic solution, suspension, and ointment dosage form are no longer sufficient to combat various ocular diseases. This article reviews the constraints with conventional ocular therapy and explores various novel approaches, in general, to improve ocular bioavailability of the drugs, advantages of vesicular approach over these and the future challenges to render the vesicular system more effective.

Control of mucin production by ocular surface epithelial cells

Multiple species of mucins are synthesized and secreted by corneal and conjunctival epithelial cells. These mucins are vital components of the tear film protecting the ocular surface from the external environment by providing a physical and chemical barrier. The release of mucins must be tightly regulated as both mucin overproduction and underproduction cause ocular surface disorders. Mucin production can be regulated by controlling mucin synthesis, mucin release, or proliferation of the cells that produce the mucin. This review will focus on the evidence demonstrating the control of the mechanisms responsible for production of mucins, their secretion, and corneal and conjunctival epithelia cell proliferation. By understanding these mechanisms under normal conditions, treatments can be designed for diseases of the mucous production of the ocular surface.

Regulation of mucin and fluid secretion by conjunctival epithelial cells

Tears play a vital role in the health and protection of the cornea and conjunctiva. The tear film consists of multiple layers and different glands secrete each layer. Because of many and varied requirements of the ocular surface cells, the volume, composition and structure of the tear film must be exquisitely controlled. If any layer of the tear film is disrupted or altered, the entire tear film is affected, often with deleterious effects. This chapter reviews the current knowledge of the neural and growth factor regulation of electrolyte, water and protein secretion from the goblet and stratified squamous cells of the conjunctiva as well as the mechanisms used for fluid secretion. The evidence presented in this review suggests that parasympathetic nerves stimulate goblet, but not stratified squamous, cell secretion. Sympathetic nerves stimulate stratified squamous, but not goblet, cell secretion, while P2Y(2) agonists stimulate secretion from both cell types. Growth factors regulate goblet cell secretion, but their effects on stratified squamous cell secretion are unknown.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

Penetration enhancers and ocular bioadhesives: two new avenues for ophthalmic drug delivery

This review is focused on the two avenues of development that promise a major impact on future ocular drug therapeutics: bioadhesives, including hydrogels and other agents like carbopols, polyacrylic acids, chitosan, etc., and penetration enhancers, including different surfactants, calcium chelators, etc. The capacity of some polymers to adhere to the mucin coat covering the conjunctiva and the corneal surface of the eye forms the basis for ocular mucoadhesion. These systems markedly prolong the residence time of a drug in the conjunctival sac, since clearence is now controlled by the much slower rate of mucus turnover rather than the tear turnover rate. But improving the corneal drug retention alone is inadequate in bringing about a significant improvement of drug bioavailability. Another approach consists of transiently increasing the pentration characteristics of the cornea with appropriate substances, known as penetration enhancers or absorption promoters. The main aim of this article is to give an insight into the potential application of mucoadhesives and corneal penetration enhancers for the conception of innovative opthalmic delivery appraoches, to decrease the systemic side effects, and create a more focused effect, which may be achieved with lower doses of the drug. Ophthalmic formulations based on these mucoadhesives and penetration enhancers are simple to manufacture and exhibit an excellent tolerance when administered into the cornea. The use of the former considerably prolongs the corneal contact time and the use of the latter increases the rate and amount of drug transport. The various corneal epithelial barriers along with the major routes of transport of drugs are discussed. The article includes a list of the various substances in use or under investigation for the aforementioned properties, along with their mechanisms of action. A fair appraisal of the subject with regard to these two therapeutic approaches and any expected ill effects has been made.

Review on the systemic delivery of insulin via the ocular route

Systemic drug absorption from the ocular route is well known. Although there is some absorption from the conjunctival sac, the nasal meatus is the site where the majority of systemic absorption of instilled drug takes place. This article reviews the principles of systemic absorption of insulin applied topically to the eye. The physiological and pharmaceutical considerations for formulation development and the strategy of improving the systemic absorption and bioavailability of insulin are also discussed.

Plasma concentration of topically applied betaxolol in elderly glaucoma patients

Our aim was to study the concentration of betaxolol in plasma after its topical ocular use during the normal 12 hr dosing interval. Twenty microliters of betaxolol 0.5% solution were applied into both eyes of nine glaucoma patients, and the plasma concentrations of the drug were measured 12 hr thereafter using a radioreceptor assay. The same amount of betaxolol was then applied ocularly, and its concentration in plasma was measured at 5, 10, 15, 30 min and 1, 2, 4 and 8 hr thereafter. The mean (SD) concentration of betaxolol in plasma twelve hr after the first dose was 0.4 (0.2) ng/ml. After the second dose, the patients showed a biphasic concentration vs. time curve, the first peak occurring at 8 (4) min, and the second peak at 210 (132) min; the mean (SD) peak concentrations being 1.1 (0.3) and 2.0 (1.1) ng/ml, respectively. The area under the concentration vs. time curve showed a 4-fold variation among our patients. Topically applied betaxolol was rapidly absorbed into systemic circulation, and concentrations were detectable even at 12 hr. The interindividual variation in the systemic absorption of betaxolol was large.

Serotonin-elicited inhibition of Cl(-) secretion in the rabbit conjunctival epithelium

The effects of serotonin [5-hydroxytryptamine (5-HT)] on the transepithelial electrical properties of the short-circuited rabbit conjunctiva were examined. With this epithelium, the short-circuit current (I(sc)) measures Cl(-) secretion plus an amiloride-resistant Na(+) absorptive process. Apical addition of 5-HT (10 microM) elicited a prompt I(sc) reduction from 14.2 +/- 1.2 to 10.9 +/- 1.2 microA/cm(2) and increased transepithelial resistance from 0.89 +/- 0.05 to 1.03 +/- 0.06 kOmega. cm(2) (means +/- SE, n = 21, P<0.05). Similar changes were obtained with conjunctivae bathed without Na(+) in the apical bath, as well as with conjunctivae preexposed to bumetanide with the Cl(-)-dependent I(sc) sustained by the parallel activities of basolateral Na(+)/H(+) and Cl(-)/HCO(3)(-) exchangers. In contrast, the 5-HT-evoked effects were attenuated by the absence of Cl(-) (DeltaI(sc) = -0.5 +/- 0.2, n = 5), suggesting that reduced Cl(-) conductance(s) is an effect of 5-HT exposure. In amphotericin B-treated conjunctiva and in the presence of a transepithelial K(+) gradient, 5-HT addition reduced K(+) diffusion across the preparation by 13% and increased transepithelial resistance by 4% (n = 6, P < 0.05), indicating that an inhibition in K(+) conductance(s) was also detectable. Significant electrical responses also occurred under physiological conditions when 5-HT was introduced to epithelia pretreated with adrenergic agonists or protein kinase C, phospholipase C, phosphodiesterase, or adenylyl cyclase inhibitors or after perturbation of Ca(2+) homeostasis. Briefly, the conjunctiva harbors the only known Cl(-)-secreting epithelium in which 5-HT evokes Cl(-) transport inhibition; receptor subtype and signal transduction mechanism were not determined.

Biopharmaceutical considerations in topical ocular drug delivery

1. Despite the accessibility of the front of the eye, efficient delivery of drug to treat various ocular disorders is a challenge to the formulation scientist. The majority of ophthalmic medications are formulated as eye drops. Due to anatomical constraints, the volume that can be administered is limited to approximately 30 microL. This, together with the efficient clearance system that exists in the front of the eye, makes it difficult to maintain an effective pre-ocular drug concentration for a desired length of time. Various formulation strategies have been used to increase pre-ocular retention of eye drops. The most successful of these has been the inclusion of viscosity enhancing polymers, particularly those able to interact with the mucous layer on the eye surface or those that can undergo a transition from a solution to a gel under the conditions of the pre-ocular area. 2. When the target site is intra-ocular, drug must be absorbed from the pre-ocular region into the eye. The main route for absorption is across the cornea. However, absorption of drug across the cornea is inefficient due to its impermeable nature and small surface area. Thus, the intra-ocular bioavailability of topically administered medications is typically less than 10%. 3. Corneal permeability favours moderately lipophilic compounds. These compounds often have a low aqueous solubility. Problems in ocular drug delivery and formulation are compounded for poorly soluble drugs that must be formulated as suspensions. 4. Reformulation of ophthalmic suspensions as solutions has many advantages. This may be achieved by complexation using cyclodextrins. Solubilization using cyclodextrins can overcome many of the formulation problems. However, it is unclear as to their potential for improving ocular bioavailability, which is seemingly drug dependent and may be influenced by both the physicochemical properties of the drug and the complex formed.

Periocular accumulation of timolol and betaxolol in glaucoma patients under long-term therapy

PURPOSE

To determine if betaxolol or timolol is present in measurable concentration in the Tenon capsule in patients under long-term topical therapy.

METHODS

Small (1-cc) specimens of Tenon capsule were removed at the time of filtering surgery from 15 glaucoma patients under long-term preoperative topical therapy, nine of whom had been treated with timolol and six of whom had been receiving betaxolol. Methanol extracts of these tissue samples were analyzed quantitatively for the presence of either beta-adrenergic antagonist by high-performance liquid chromatography.

RESULTS

Drug was detected in every specimen. A mean total of 2.6 (range, 0.1-30.0) microg of betaxolol was detected per 1-cc specimen.

CONCLUSION

Timolol and betaxolol penetrate the conjunctiva and accumulate in the Tenon capsule. In patients under long-term therapy, the periocular tissue can accumulate a greater quantity of beta-antagonist than is present in a daily dosage of applied eyedrops, manyfold higher than the maximal intraocular concentration.

The exposed ocular surface and its relationship to spontaneous eyeblink rate in elderly caucasians

The purpose of this study was to assess spontaneous eyeblink activity in the primary eye position, and its relationship to the exposed ocular surface area in elderly individuals without significant ocular surface disease. Spontaneous eyeblink frequency (SEBF) was assessed for a group of Caucasians aged 61 to 81 years, by videography in silence over a 5 min period; inter-blink interval (IBI), the maximum inter-blink interval (mIBI) and the modal eyeblink frequency (mEBF) were derived from video recordings. The ocular surface area was calculated from video-planimetry using different formulae. The average palpebral aperture was 8.3 +/- 2.3 mm (n = 45, +/- S.D.), resulting in an average exposed area of 1.13-2.13 cm2 depending on the formula used; the preferred estimate was 1.25 cm2. A highly significant relationship (P < 0.001, r2 = 0.835) was found between time-averaged palpebral aperture (p) and exposed ocular surface (A = 0.244p-0.763). An average SEBF of 11.9 blinks min-1 was measured (range 0.8-43.6). No correlation (P > 0.35, r2 < or = 0.01) was found between SEBF, IBI, mIBI, or mEBF and the palpebral aperture or the exposed surface area, regardless of the formulae used. The exposed ocular surface area per se does not appear to be an important determinant of spontaneous eyeblink activity in elderly Caucasians.

Modulation of chloride secretion across the pigmented rabbit conjunctiva

The purpose of the present study was to investigate whether active Cl- secretion in the pigmented rabbit conjunctiva was subject to cAMP, Ca2+ and protein kinase C (PKC) modulation. The excised pigmented rabbit conjunctivas were mounted in the modified Ussing-type chambers for measurement of unidirectional 36Cl fluxes under the open-circuit condition and of the short-circuit current (Isc), potential difference, and transconjunctival electrical resistance. The results indicate that Cl- secretion across the conjunctiva was abolished by mucosal application of 1 mM N-phenylanthranilic acid and was reduced by 40% by serosal application of 10 microM bumetanide. Net Cl- flux was stimulated by 133% by 1 mM 8-Br cAMP, 107% by 10 microM A23187, and 87% by 1 microM phorbol 12-myristate-13-acetate (PMA), suggesting that cAMP, Ca2+, and PKC all modulated active Cl- secretion, respectively. There existed a linear correlation between measured changes in net Cl- flux and observed changes in Isc (r2=0.99). The serial treatment of the conjunctiva with (a) 1 mM 8-Br cAMP and 10 microM A23187 and (b) 10 microM A23187 and 1 microM PMA resulted in sequence-independent, additive stimulation of Isc. In the case of 1 mM 8-Br cAMP and 1 microM PMA, additive stimulation of Isc was observed only when 1 mM 8-Br cAMP was added prior to 1 microM PMA. These results suggest that a given pharmacological agent may affect more than one channel type and that there might be a possible connection among the channels at the signal transduction level. In summary, Cl- appears to enter the pigmented rabbit conjunctiva from the serosal fluid via Na+-(K+)-2Cl- cotransport process and exit to the mucosal fluid via channels, resulting in active Cl- secretion. Active Cl- secretion in the pigmented rabbit conjunctiva appears to be modulated by cAMP, Ca2+, and PKC.

Human sclera: thickness and surface area

PURPOSE

To assess the mean thickness and surface area of human sclera.

METHODS

Fifty-five formalin-fixed eye bank eyes were hemisected from anterior to posterior. Cross-sectional slides were taken to include a millimeter scale ruler in each photograph. Slide photographs were projected and the scleral silhouette sketched. Mean scleral thickness measurements with standard deviation were obtained. Twenty-five human eye bank eyes were used to determine total scleral surface area by either a computerized tracing method (17 globes) or volumetric calculations (eight globes) using fluid displacement.

RESULTS

Mean scleral thickness +/- SD was 0.53 +/- 0.14 mm at the corneoscleral limbus, significantly decreasing to 0.39 +/- 0.17 mm near the equator, and increasing to 0.9 to 1.0 mm near the optic nerve. The mean total scleral surface area by surface area computerized tracings was 16.3 +/- 1.8 cm2 and, by the volume displacement method, was 17.0 +/- 1.5 cm2.

CONCLUSIONS

Scleral thickness and surface area measurements from cadaver eyes are important for ophthalmic surgeons and have implications for transscleral diffusion.

Arginine vasopressin transport and metabolism in the pigmented rabbit conjunctiva

The purpose of this study was to evaluate the transepithelial transport and metabolism of arginine vasopressin (AVP) in the pigmented rabbit conjunctiva, both in the absence and presence of protease inhibitors. The apparent permeability coefficient, P(app), for 3H-AVP was determined in the modified Ussing chamber, and AVP metabolites were monitored by reversed phase HPLC using a C18 column. At 50 nM donor 3H-AVP, the P(app) in the mucosal-to-serosal (ms) direction was about five times higher than that in the opposite direction. Excess (0.1 mM) AVP decreased the P(app) for labelled AVP in the mucosal-to-serosal (ms) direction by about 50%. However, intact AVP transport showed neither concentration nor direction dependence. HPLC analysis revealed two subspecies of 3H-AVP in the receiver fluid and virtually no degradation products in the donor fluid following 3 h flux experiments. 3H-AVP transported in the ms direction underwent extensive hydrolysis (73%), which was decreased by 33% with mucosal application of 2 mM camostat mesylate (an aminopeptidase inhibitor) or by 27% with 0.5 mM leupeptin (a serine protease inhibitor). By contrast, 3H-AVP transported in the serosal-to-mucosal (sm) direction resulted in only 37% hydrolysis, and mucosal application of either inhibitor did not significantly affect the P(app) for intact AVP. These data suggest that intact AVP transport in the conjunctiva may be mediated mostly by passive diffusion and enzymatic degradation of AVP may be mediated by proteolytic enzymes present on the mucosal side of the conjunctiva.