A Mathematical Model for Ocular Tear and Solute Balance

Mitigation of pesticide-mediated ocular toxicity via nanotechnology-based contact lenses: a review

The xenobiotic stress exerted by pesticides leads to the deterioration of human and animal health including ocular health. Acute or prolonged exposure to these agricultural toxicants has been implicated in a number of pathological conditions of the eye such as irritation, epiphora or hyper-lacrimation, abrasions on the ocular surface, and decreased visual acuity. The issue is compounded by the fact that tissues of the eye absorb pesticides faster than other organs of the body and are more susceptible to damage as well. However, there is a lacuna in our knowledge regarding the ways by which pesticide exposure-mediated ocular insult might be counteracted. Topical instillation of drugs known to combat the pesticide induced toxicity has been explored to mitigate the detrimental impact of pesticide exposure. However, topical eye drop solutions exhibit very low bioavailability and limited drug residence duration in the tear film decreasing their efficacy. Contact lenses have been explored in this respect to increase bioavailability of ocular drugs, while nanoparticles have lately been utilized to increase drug bioavailability and increase drug residence duration in different tissues. The current review focuses on drug delivery and futuristic aspects of corneal protection from ocular toxicity using contact lenses.

How can machine learning and multiscale modeling benefit ocular drug development?

The eyes possess sophisticated physiological structures, diverse disease targets, limited drug delivery space, distinctive barriers, and complicated biomechanical processes, requiring a more in-depth understanding of the interactions between drug delivery systems and biological systems for ocular formulation development. However, the tiny size of the eyes makes sampling difficult and invasive studies costly and ethically constrained. Developing ocular formulations following conventional trial-and-error formulation and manufacturing process screening procedures is inefficient. Along with the popularity of computational pharmaceutics, non-invasive in silico modeling & simulation offer new opportunities for the paradigm shift of ocular formulation development. The current work first systematically reviews the theoretical underpinnings, advanced applications, and unique advantages of data-driven machine learning and multiscale simulation approaches represented by molecular simulation, mathematical modeling, and pharmacokinetic (PK)/pharmacodynamic (PD) modeling for ocular drug development. Following this, a new computer-driven framework for rational pharmaceutical formulation design is proposed, inspired by the potential of in silico explorations in understanding drug delivery details and facilitating drug formulation design. Lastly, to promote the paradigm shift, integrated in silico methodologies were highlighted, and discussions on data challenges, model practicality, personalized modeling, regulatory science, interdisciplinary collaboration, and talent training were conducted in detail with a view to achieving more efficient objective-oriented pharmaceutical formulation design.

Viscoelastic properties of the human tear film

When considering eye blinking from a tribological perspective, tear viscosity is expected to play a fundamental role. The application of rheological techniques to describe the tear film dates back to the late 1980s, but there has been a continuous need of reappraisal due to new findings in tear film biochemistry and to the development of new methods of physico-chemical characterization. This review provides an overview on tear rheological behavior by analyzing the peer-reviewed literature on this topic. Specifically, examples of in-vitro and in-vivo viscosity measurements are detailed, highlighting experimental criticalities and the need of a standard convention for rheological techniques to compare data across different studies, of analyses on tears of single individuals even within the limits of the low volume available, and of a further development of in-vivo techniques. Then, the controversial role of specific tear components on viscosity is discussed, together with the alterations associated to dry eye disease and contact lens wear. Finally, an updated focus is reported on the viscosity of artificial tears formulations.

Investigating key cell types and molecules dynamics in PyMT mice model of breast cancer through a mathematical model

The most common kind of cancer among women is breast cancer. Understanding the tumor microenvironment and the interactions between individual cells and cytokines assists us in arriving at more effective treatments. Here, we develop a data-driven mathematical model to investigate the dynamics of key cell types and cytokines involved in breast cancer development. We use time-course gene expression profiles of a mouse model to estimate the relative abundance of cells and cytokines. We then employ a least-squares optimization method to evaluate the model’s parameters based on the mice data. The resulting dynamics of the cells and cytokines obtained from the optimal set of parameters exhibit a decent agreement between the data and predictions. We perform a sensitivity analysis to identify the crucial parameters of the model and then perform a local bifurcation on them. The results reveal a strong connection between adipocytes, IL6, and the cancer population, suggesting them as potential targets for therapies.

One of the outstanding challenges of the mathematical modeling of cancer progression is the existence of many unknown parameters. In this work, we develop a data-driven mathematical model of breast cancer progression by deriving a system of ordinary differential equations for the interaction networks of key cell types and molecules in breast tumors. To overcome the limitations of unknown parameters, we utilize a time course data of a PyMT mice model of breast cancer and estimate parameters using an optimization method. Although the predicted dynamics of cancer and necrotic cells using the obtained values of parameters are in good agreement with the data, the predicted values for a few other variables do not match the data. This might indicate that there are some other key interactions that have not been modeled, and/or there is a noise in the data. The sensitivity and bifurcation analyses show that the most important parameters in controlling the cancer cells population are the proliferation and death rates of cancer cells and adipocytes. These results are in agreement with some biological and clinical studies of breast cancer, which have reported a link between adipocytes and breast cancer progression.

Ocular Fluid Mechanics and Drug Delivery: A Review of Mathematical and Computational Models

The human eye is a complex biomechanical structure with a range of biomechanical processes involved in various physiological as well as pathological conditions. Fluid flow inside different domains of the eye is one of the most significant biomechanical processes that tend to perform a wide variety of functions and when combined with other biophysical processes play a crucial role in ocular drug delivery. However, it is quite difficult to comprehend the effect of these processes on drug transport and associated treatment experimentally because of ethical constraints and economic feasibility. Computational modeling on the other hand is an excellent means to understand the associated complexity between these aforementioned processes and drug delivery. A wide range of computational models specific to different types of fluids present in different domains of the eye as well as varying drug delivery modes has been established to understand the fluid flow behavior and drug transport phenomenon in an insilico manner. These computational models have been used as a non-invasive tool to aid ophthalmologists in identifying the challenges associated with a particular drug delivery mode while treating particular eye diseases and to advance the understanding of the biomechanical behavior of the eye. In this regard, the author attempts to summarize the existing computational and mathematical approaches proposed in the last two decades for understanding the fluid mechanics and drug transport associated with different domains of the eye, together with their application to modify the existing treatment processes.

Eyedrop vaccination: an immunization route with promises for effective responses to pandemics

INTRODUCTION

Mucosal vaccines have several advantages over parenteral vaccines. They induce both systemic and mucosal antigen-specific immune responses, allow easy administration, and bypass the need for trained medical personnel.

AREAS COVERED

Eye mucosa is a novel route of mucosal vaccine administration. Eyedrop vaccination induces systemic and mucosal immune responses similar to other forms of mucosal vaccines such as oral and intranasal vaccines.

EXPERT OPINION

Eyedrop vaccines are free of serious adverse side effects like the infiltration of CNS by pathogens. Studies over the years have shown promising results for eye drop vaccines against infectious agents like the influenza virus, Salmonella typhi, and Escherichia coli in animal models. Such efficacy and safety of eyedrop vaccination enable the application of eyedrop vaccines against other infectious diseases as well as chronic diseases. In this review of published literature, we examine the mechanism, efficacy, and safety of eyedrop vaccines and contemplate their role in times of a pandemic.

Brimonidine-montmorillonite hybrid formulation for topical drug delivery to the eye

Brimonidine (BMD) is often prescribed as an eye drop to reduce the intraocular pressure (IOP) for glaucoma treatment. However, eye drops are limited by rapid clearance from the preocular surface, and hence a low ocular drug bioavailability. Therefore, in this study, we propose montmorillonite (MMT), as a delivery carrier, hybridized with BMD (BMD-MMT) for topical drug delivery to the eye. The BMD-MMT hybrid was prepared by intercalating the BMD molecules in the interlayer space of the MMT lattice via ion-exchange reaction; it was then formulated with polyvinyl alcohol (PVA) to produce a dry tablet (i.e., BMD-MMT@PVA). The BMD-MMT@PVA hybrid drug released BMD in a sustained manner for more than 5 h under in vitro conditions. When the hybrid drug was administered to rabbit eyes in vivo, 43% and 18.5% BMD-MMT still remained on the preocular surface for 10 and 60 min after administration, respectively. Thus, the BMD-MMT@PVA hybrid drug exhibited a prolonged decrease in IOP, that is, for 12 h, which was approximately two times longer than that observed with the commercially available BMD eye drop, Alphagan® P.

Dry Tablet Formulation of PLGA Nanoparticles with a Preocular Applicator for Topical Drug Delivery to the Eye

To enhance ocular drug bioavailability, a rapidly dissolving dry tablet containing alginate and drug-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles was proposed. For hygienic and easy administration of an accurate drug-dose with this tablet, the use of a preocular applicator was suggested. Herein, a dry tablet was prepared by embedding dexamethasone-loaded PLGA nanoparticles in alginate, which was deposited on the tip of the applicator. The nanoparticles were loaded with 85.45 μg/mg drug and exhibited sustained drug release for 10 h. To evaluate in vivo efficacy, dexamethasone concentration in the aqueous humor was measured after topical administration of the dry tablet, with the applicator, to rabbit eyes and was compared to that achieved with Maxidex^®, a commercially-available dexamethasone eye drops. When applied with the preocular applicator, the dry tablet containing alginate could be fully detached and delivered to the eye surface. In fact, it showed up to 2 h of nanoparticle retention on the preocular surface due to tear viscosity enhancement, causing an estimated 2.6-fold increase in ocular drug bioavailability compared to Maxidex^®. Therefore, the preocular applicator combined with a dry alginate tablet containing PLGA nanoparticles can be a promising system for aseptically delivering an accurate dose of ophthalmic drug with enhanced bioavailability.

Metal-organic frameworks, NH2-MIL-88(Fe), as carriers for ophthalmic delivery of brimonidine

We have proposed a metal-organic framework (MOF), NH₂-MIL-88(Fe), as a novel carrier for topical drug delivery to the eye. The NH₂-MIL-88(Fe) particles were prepared via a solvothermal synthesis method and their structure was confirmed by powder X-ray diffraction, Fourier transform infrared analysis, thermogravimetric analysis, electron microscopy, and N₂ adsorption-desorption measurements. When brimonidine, an anti-glaucoma medicine, was encapsulated into NH₂-MIL(Fe)-88 (i.e., NH₂-MIL-88(Fe)/Br), the drug was loaded at 121.3 µg/mg and released in a sustained manner for up to 12 h. The NH₂-MIL-88(Fe)/Br exhibited mucoadhesive properties and remained on rabbit eyes for a period of up to 4 h. Consequently, a high concentration of brimonidine was found in tears for a prolonged period after the administration of NH₂-MIL-88(Fe)/Br, which resulted in a greater than two-fold increase in drug bioavailability and activity period compared with those of Alphagan P, which are brimonidine eye drops already approved for clinical use. Hence, NH₂-MIL-88(Fe) is suggested to be a promising carrier for topical delivery to the eye that provides enhanced bioavailability of ocular drugs.

STATEMENT OF SIGNIFICANCE

We suggest NH₂-MIL(Fe)-88, a type of metal-organic frameworks (MOFs), as delivery carriers of an ophthalmic drug, brimonidine. The NH₂-MIL(Fe)-88 particles possess a mucoadhesive property, hence prolonged retention in the preocular space when topically administered to the eye. The particles can also encapsulate the drug in their micro-pores, through which the drug can be released in a sustained manner. Therefore, when tested to rabbit eyes in vivo, the drug-loaded NH₂-MIL(Fe)-88 particles were shown to enhance the ocular drug bioavailability, as compared with Alphagan P, the marketed eye drops of brimonidine.

Microengineered biomimetic ocular models for ophthalmological drug development

Current ophthalmological drug discovery and testing methods have limitations and concerns regarding reliability, ethicality, and applicability. These drawbacks can be mitigated by developing biomimetic eye models through mathematical and experimental methods which are often referred to as "eye-on-a-chip" or "eye chip". These eye chip technologies emulate ocular physiology, anatomy, and microenvironmental conditions. Such models enable understanding of the fundamental biology, pharmacology, and toxicology mechanisms by investigating the pharmacokinetics and pharmacodynamics of various candidate drugs under ocular anatomical and physiological conditions without animal models. This review provides a comprehensive overview of the latest advances in theoretical and in vitro experimental models of the anterior segment of the eye and its microenvironment, including eye motions and tear film dynamics. The current state of ocular modeling and simulation from predictive models to experimental models is discussed in detail with their advantages and limitations. The potential for future eye chip models to expedite new ophthalmic drug discoveries is also discussed.

The Pre-Sac Reflux Test (PSRT): a new diagnostic test for presaccal stenosis/obstruction

PURPOSE

To introduce a new procedure, the Pre-Sac Reflux Test (PSRT), to screen for epiphora caused by presaccal stenosis/obstruction, and to determine the reliability of this new test in the diagnosis of presaccal stenosis/obstruction.

METHODS

A nonrandomized, prospective, comparative case series of 280 eyes of 140 patients with unilateral presaccal stenosis/obstruction was performed. The PSRT was performed by a masked examiner on both eyes of each patient. After fluorescein staining, the lower lid was everted and lower punctum was observed using a cobalt-blue filter under slit-lamp magnification. After blotting excess fluorescein on the punbtum, the area of lacrimal ampulla was massaged with mild pressure with an index finger. The reflux (billowing of the fluorescein-stained tear) from each lower punctum were compared, and the bilateral symmetry was measured. In the same way, reflux from each upper punctum were compared for bilateral symmetry. The PSRT was considered positive if reflux from punctum was absent, or reduced asymmetrically, compared with that from contralateral punctum. If the test of either upper or lower punctum showed positive, the eye was considered positive. The validity of PSRT was measured by sensitivity and specificity, as well as positive- and negative-predictive values.

RESULTS

The PSRT used to diagnose presaccal stenosis/obstruction has a sensitivity of 92.9 % and a specificity of 97.1 %. It has a positive-predictive value of 97.0 % and a negative-predictive value of 93.2 %.

CONCLUSIONS

The PSRT is a reliable screening test for diagnosis of presaccal stenosis/obstruction.

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.

Eyedrop Vaccination Induced Systemic and Mucosal Immunity against Influenza Virus in Ferrets

We investigated eyedrop vaccination (EDV) in pre-clinical development for immunological protection against influenza and for potential side effects involving ocular inflammation and the central nervous system (CNS). Live attenuated influenza EDV, CA07 (H1N1), PZ-4 (H1N2) and Uruguay (H3N2), induced both systemic and mucosal virus-specific antibody responses in ferrets. In addition, EDV resulted in a clinically significant protection against viral challenge, and suppression of viral replication in nasal secretion and lung tissue. Regarding safety, we found that administered EDV flow through the tear duct to reach the base of nasal cavity, and thus do not contact the olfactory bulb. All analyses for potential adverse effects due to EDV, including histological and functional examinations, did not reveal significant side effects. On the basis of these findings, we propose that EDV as effective, while being a safe administration route with minimum local side effects, CNS invasion, or visual function disturbance.

A review on therapeutic contact lenses for ocular drug delivery

Contact lenses for ophthalmic drug delivery have become very popular, due to their unique advantages like extended wear and more than 50% bioavailability. To achieve controlled and sustained drug delivery from contact lenses, researchers are working on various systems like polymeric nanoparticles, microemulsion, micelle, liposomes, use of vitamin E, etc. Numerous scientists are working on different areas of therapeutic contact lenses to treat ocular diseases by implementing techniques like soaking method, molecular imprinting, entrapment of drug-laden colloidal nanoparticles, drug plate/film, ion ligand polymeric systems, supercritical fluid technology, etc. Though sustained drug delivery was achieved using contact lens, the critical properties such as water content, tensile strength (mechanical properties), ion permeability, transparency and oxygen permeability were altered, which limit the commercialization of therapeutic contact lenses. Also issues like drug stability during processing/fabrication (drug integrity test), zero order release kinetics (prevent burst release), drug release during monomer extraction step after fabrication (to remove un-reacted monomers), protein adherence, drug release during storage in packaging solution, shelf life study, cost-benefit analysis, etc. are still to be addressed. This review provides an expert opinion on different methodology to develop therapeutic contact lenses with special remark of their advantages and limitations.

Tear dynamics in healthy and dry eyes

PURPOSE

Dry-eye disease, an increasingly prevalent ocular-surface disorder, significantly alters tear physiology. Understanding the basic physics of tear dynamics in healthy and dry eyes benefits both diagnosis and treatment of dry eye. We present a physiological-based model to describe tear dynamics during blinking.

MATERIALS AND METHODS

Tears are compartmentalized over the ocular surface; the blink cycle is divided into three repeating phases. Conservation laws quantify the tear volume and tear osmolarity of each compartment during each blink phase. Lacrimal-supply and tear-evaporation rates are varied to reveal the dependence of tear dynamics on dry-eye conditions, specifically tear osmolarity, tear volume, tear-turnover rate (TTR), and osmotic water flow.

RESULTS

Predicted periodic-steady tear-meniscus osmolarity is 309 and 321 mOsM in normal and dry eyes, respectively. Tear osmolarity, volume, and TTR all match available clinical measurements. Osmotic water flow through the cornea and conjunctiva contribute 10 and 50% to the total tear supply in healthy and dry-eye conditions, respectively. TTR in aqueous-deficient dry eye (ADDE) is only half that in evaporative dry eye (EDE).

CONCLUSIONS

The compartmental periodic-steady tear-dynamics model accurately predicts tear behavior in normal and dry eyes. Inclusion of osmotic water flow is crucial to match measured tear osmolarity. Tear-dynamics predictions corroborate the use of TTR as a clinical discriminator between ADDE and EDE. The proposed model is readily extended to predict the dynamics of aqueous solutes such as drugs or fluorescent tags.

Nanostructured mucoadhesive microparticles for enhanced preocular retention

We describe nanostructured microparticles (NMs) containing a mucoadhesive polymer for enhanced preocular retention and consider them as potential carriers of drugs to the eye. These NMs are each composed of entangled nanofibers to give an enlarged specific surface area, and thus can better adhere to the preocular mucus surface. This physical design allows the microparticles still to be composed mainly of a wall material, poly(lactic-co-glycolic acid), as required for controlled drug delivery, while the effects of an additive, mucoadhesive material, polyethylene glycol, can be synergistically improved via the nanostructured morphology. Thus, when formulated in a dry tablet dosage form, the NMs in this work show more than a 10-fold increase in preocular retention in vivo compared to conventional spherical microparticles. Therefore, we conclude that these mucoadhesive NMs can reside on the preocular surface for a prolonged period, and thus appear to be a promising system for topical drug delivery to the eye.

Toward the practical implementation of eye-related bioavailability prediction models

The development and registration of reformulated ophthalmic products (OPs) requires eye-related bioavailability (BA) assessments. Common BA algorithms associated with other routes of application, such as the oral route, cannot be easily applied to eye-related BA testing. Here, we provide an analysis of the current literature and suggestions for further directions in the development of high-capacity, cost-effective, and highly predictive nonclinical models of eye-related drug BA. One, or a combination of these models, has the potential for routine use in research laboratories and/or the pharmaceutical industry to overcome various obstacles in reformulated OP development and registration.

Extended release of timolol from nanoparticle-loaded fornix insert for glaucoma therapy

We have developed a cylindrical insert that can be inserted in the fornix for extended release of glaucoma drug timolol. The insert is prepared by incorporating timolol-loaded nanoparticles into a poly hydroxyl ethyl methacrylate (p-HEMA) matrix. A 1-mm diameter, 7.5-mm long insert with 25% (w/w) particles can release timolol for about 10 days at an average rate of about 15 μg/day, which may be therapeutically effective. The increase in particle fraction increases drug loading, but also increases the release duration. The net effect of increasing the particle fraction is a significant increase in release duration, but a decrease in daily drug release rates, in the first few weeks. The release duration increases to about 1 and 3 months on increasing the particle fraction to 50% and 75%, respectively. The average daily release rates in the first 3 weeks are 15, 9, and 3 μg/day for the inserts with 50%, 75%, and 100% (w/w) particles, respectively. The mechanism of release is hydrolysis of the ester bond that links timolol to the propoxylated glyceryl triacrylate matrix, and thus the release profiles fit a first order reaction model. The water content of the inserts decreases from 31% to almost zero on increasing the particle loading from 25% to 100%. The rate constant for the hydrolysis decreases with an increase in particle loading in the insert most likely due to the reduction in the water content. The inserts can be packaged in wet conditions and stored in a refrigerator, but the inserts will exhibit a burst release caused by release of the drug from the particles into the p-HEMA matrix during the shelf life. Also, the magnitude of drug release after the initial burst is reduced due to the storage. The burst effect could potentially be avoided by packaging the inserts in a dry state, with hydration before insertion.

Mucoadhesive microparticles in a rapidly dissolving tablet for sustained drug delivery to the eye

PURPOSE

To test the hypothesis that mucoadhesive microparticles formulated in a rapidly dissolving tablet can achieve sustained drug delivery to the eye.

METHODS

Mucoadhesive microparticles, smaller than 5 μm were fabricated with poly(lactic-co-glycolic acid) and poly(ethylene glycol) as a core material and mucoadhesion promoter, respectively, and encapsulated pilocarpine as a model drug. These microparticles were embedded in a poly(vinyl alcohol) matrix to form a dry tablet designed to reduce rapid clearance of the microparticles on initial application to the eye.

RESULTS

This in vitro drug release study exhibited that for all formulations, approximately 90% of pilocarpine was released during the first 10 minutes, and the remaining 10% was released slowly for 3 hours. In vivo mucoadhesion test on the rabbit eye indicated that mucoadhesive microparticles adhered significantly better to the preocular surface than other formulations. To assess the pharmacodynamics, the most prolonged pilocarpine-induced pupil constriction was observed in rabbit eyes in vivo using a tablet with mucoadhesive microparticles; it lasted up to 330 minutes.

CONCLUSIONS

The authors conclude that mucoadhesive microparticles formulated into a dry dosage form is a promising system for sustained drug delivery to the eye.

Nanoscale phase dynamics of the normal tear film

The tear film is a dynamic multilayered structure. The interactions and the interfacial dynamics between the layers that occur during a blink cycle must be such that they allow for maintenance of a stable tear film. Attempts to understand these dynamics have been limited by the techniques and biomarkers used. Quantum dots (qdots) offer a new potential to monitor the dynamics of the tear film layers in vivo without the drawbacks of previously used methodologies. Indium phosphide-gallium qdots were used to differentially assess the dynamics of the lipid and aqueous layers of the tear film in real time. In the aqueous, qdots dispersed to form a stable local region that was swept away into the upper and lower menisci during a blink. They did not redisperse onto the ocular surface but were progressively removed from the menisci through the puncta. Some of these qdots adhered to the mucin layer on the ocular surface in a meshlike pattern and remained there for five to six blinks before they were removed. The organic qdots dispersed quickly but patchily over the whole outer surface of the tear film. They also strongly marked both eyelid margins and slowly dispersed onto the skin and eyelashes and not through the puncta. Some were trapped in the menisci as blobs that rolled along the meniscus. These data support the view of a distinct three-layered tear film: an inner mucin layer attached to the epithelial cells, a fluid aqueous layer, and an outer viscoelastic lipid layer.

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.

Effect of blinking on tear volume after instillation of midviscosity artificial tears

PURPOSE

To determine by optical coherence tomography (OCT) the effect of blinking on ocular surface tear volume after instillation of artificial tears.

DESIGN

Experimental study.

METHODS

Normal human eyes (n = 21) were imaged to obtain dimensions of the tear film and menisci during blinking. The imaging was carried out immediately and five minutes after the instillation of 35 microl midviscosity artificial tears (1.0% carboxymethylcellulose; Refresh Liquigel; Allergan, Irvine, California, USA). The exposed ocular surface area and the lid lengths were used to calculate the volumes.

RESULTS

Immediately after the instillation, total tear volume was increased (P < .001) compared with five minutes after the instillation, with the major increases in the lower tear meniscus volume and tear film volume. After the instillation, blinking caused tear loss in total tear volume because of the decrease of the lower tear meniscus volume (P < .05). In contrast, blinking increased the tear film volume (P < .05). At the end of the eye-opening period, tear film volume decreased and lower tear meniscus volume increased significantly (P < .05), with no significant changes in total tear volume (P > .05). During the blink cycle immediately after the instillation, net loss was evident in tear film volume, lower tear meniscus volume, and total tear volume (P < .05).

CONCLUSIONS

Blinking plays a crucial role in distribution and removal of instilled tears. When the tear system is overloaded, the increase in blink output helps restore balance.

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.

Timolol transport from microemulsions trapped in HEMA gels

Approximately 90% of all ophthalmic drug formulations are now applied as eye-drops. While eye-drops are convenient and well accepted by patients, about 95% of the drug contained in the drops is lost due to absorption through the conjunctiva or through the tear drainage. A major fraction of the drug eventually enters the blood stream and may cause side effects [J.C. Lang, Adv. Drug Delivery Rev. 16 (1995) 39-43; C. Bourlais, L. Acar, H. Zia, P.A. Sado, T. Needham, R. Leverge, Prog. Retinal Eye Res. 17 (1998) 33-58; M.P. Segal, FDA Consumer Mag. (1991)]. The drug loss and the side effects can be minimized by using microemulsion-laden soft contact lenses for ophthalmic drug delivery [D. Gulsen, A. Chauhan, Invest. Ophthalmol. Vis. Sci. 45 (2004) 2342-2347; D. Gulsen, A. Chauhan, Abstr. Pap. Am. Chem. Soc. 227 (2004) U875]. In order for microemulsion-laden gels to be effective, these should load sufficient quantities of drug and should release it a controlled manner. The presence of a tightly packed surfactant at the oil-water interface of microemulsions may provide barrier to drug transport, and this could be used to control the drug delivery rates. In this paper we focus on trapping ethyl butyrate in water microemulsions stabilized by Pluronic F127 surfactant in 2-hydroxyethyl methacrylate (HEMA) gels and measuring the transport rates of timolol, which is a beta-blocker drug that is used for treating a variety of diseases including glaucoma. The results described here show that microemulsion-laden gels could have high drug loadings, particularly for drugs such as timolol base which can either be dissolved in the oil phase or form the oil phase of the microemulsions. However, the surfactant covered interface of the Pluronic microemulsions does not provide sufficient barrier to impede the transport of timolol, perhaps due to the small size of this drug.