Transporters/receptors in the anterior chamber: pathways to explore ocular drug delivery strategies

Drug delivery to the anterior segment of the eye: A review of current and future treatment strategies

Research in the development of ophthalmic drug formulations and innovative technologies over the past few decades has been directed at improving the penetration of medications delivered to the eye. Currently, approximately 90% of all ophthalmic drug formulations (e.g. liposomes, micelles) are applied as eye drops. The major challenge of topical eye drops is low bioavailability, need for frequent instillation due to the short half-life, poor drug solubility, and potential side effects. Recent research has been focused on improving topical drug delivery devices by increasing ocular residence time, overcoming physiological and anatomical barriers, and developing medical devices and drug formulations to increase the duration of action of the active drugs. Researchers have developed innovative technologies and formulations ranging from sub-micron to macroscopic size such as prodrugs, enhancers, mucus-penetrating particles (MPPs), therapeutic contact lenses, and collagen corneal shields. Another approach towards the development of effective topical drug delivery is embedding therapeutic formulations in microdevices designed for sustained release of the active drugs. The goal is to optimize the delivery of ophthalmic medications by achieving high drug concentration with prolonged duration of action that is convenient for patients to administer.

Micelles: Promising Ocular Drug Carriers for Anterior and Posterior Segment Diseases

Micelles have been studied in the targeting of drug substances to different tissues as a nano-sized delivery system for many years. Sustained drug release, ease of production, increased solubility, and bioavailability of drugs with low water solubility are the most important superiorites of micellar carriers. These advantages paved the way for the use of micelles as a drug delivery system in the ocular tissues. The unique anatomical structure of the eye as well as its natural barriers and physiology affect ocular bioavailability of the drugs negatively. Conventional dosage forms can only reach the anterior segment of the eye and are used for the treatment of diseases of this segment. In the treatment of posterior segment diseases, conventional dosage forms are administered sclerally, via an intravitreal injection, or systemically. However, ocular irritation, low patient compliance, and high side effects are also observed. Micellar ocular drug delivery systems have significant promise for the treatment of ocular diseases. The potential of micellar systems ocular drug delivery has been demonstrated by in vivo animal experiments and clinical studies, and they are continuing extensively. In this review, the recent research studies, in which the positive outcomes of micelles for ocular targeting of drugs for both anterior and posterior segment diseases as well as glaucoma has been demonstrated by in vitro, ex vivo, or in vivo studies, are highlighted.

Ocular Drug Delivery

Although the eye is an accessible organ for direct drug application, ocular drug delivery remains a major challenge due to multiple barriers within the eye. Key barriers include static barriers imposed by the cornea, conjunctiva, and retinal pigment epithelium and dynamic barriers including tear turnover and blood and lymphatic clearance mechanisms. Systemic administration by oral and parenteral routes is limited by static blood-tissue barriers that include epithelial and endothelial layers, in addition to rapid vascular clearance mechanisms. Together, the static and dynamic barriers limit the rate and extent of drug delivery to the eye. Thus, there is an ongoing need to identify novel delivery systems and approaches to enhance and sustain ocular drug delivery. This chapter summarizes current and recent experimental approaches for drug delivery to the anterior and posterior segments of the eye.

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.

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

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

Recent advances in ophthalmic drug delivery

Topical ocular drug bioavailability is notoriously poor, in the order of 5% or less. This is a consequence of effective multiple barriers to drug entry, comprising nasolacrimal drainage, epithelial drug transport barriers and clearance from the vasculature in the conjunctiva. While sustained drug delivery to the back of the eye is now feasible with intravitreal implants such as Vitrasert (-6 months), Retisert (-3 years) and Iluvien (-3 years), currently there are no marketed delivery systems for long-term drug delivery to the anterior segment of the eye. The purpose of this article is to summarize the resurgence in interest to prolong and improve drug entry from topical administration. These approaches include mucoadhesives, viscous polymer vehicles, transporter-targeted prodrug design, receptor-targeted functionalized nanoparticles, iontophoresis, punctal plug and contact lens delivery systems. A few of these delivery systems might be useful in treating diseases affecting the back of the eye. Their effectiveness will be compared against intravitreal implants (upper bound of effectiveness) and trans-scleral systems (lower bound of effectiveness). Refining the animal model by incorporating the latest advances in microdialysis and imaging technology is key to expanding the knowledge central to the design, testing and evaluation of the next generation of innovative ocular drug delivery systems.

In vitro cell culture models to study the corneal drug absorption

INTRODUCTION

Many diseases of the anterior eye segment are treated using topically applied ophthalmic drugs. For these drugs, the cornea is the main barrier to reaching the interior of the eye. In vitro studies regarding transcorneal drug absorption are commonly performed using excised corneas from experimental animals. Due to several disadvantages and limitations of these animal experiments, establishing corneal cell culture models has been attempted as an alternative.

AREAS COVERED

This review summarizes the development of in vitro models based on corneal cell cultures for permeation studies during the last 20 years, starting with simple epithelial models and moving toward complex organotypical 3D corneal equivalents.

EXPERT OPINION

Current human 3D corneal cell culture models have the potential to replace excised animal corneas in drug absorption studies. However, for widespread use, the contemporary validation of existent systems is required.

Small Neutral Amino Acid Ester Prodrugs of Acyclovir Targeting Amino Acid Transporters on the Cornea: Possible Antiviral Agents against Ocular HSV-1 Infections

The aim of this study was to characterize the affinity and permeability patterns of the amino acid ester prodrugs of acyclovir (ACV), L-alanine-ACV (AACV), L-serine-ACV (SACV), L-serine-succinate-ACV (SSACV) and L-cysteine-ACV (CACV) on rabbit primary corneal epithelial cell culture (rPCEC) and on rabbit cornea. Amino acid prodrugs of acyclovir, AACV, SACV, SSACV and CACV were synthesized in our laboratory. Chemical hydrolysis in aqueous buffer, enzymatic hydrolysis in corneal homogenates and transport across freshly excised rabbit cornea of these prodrugs were studied. SSACV inhibited the uptake of [3H] L-alanine on rPCEC and across the intact rabbit cornea. Lineweaver-Burk plot transformation revealed competitive inhibition between L-alanine and SSACV. In corneal tissue homogenate, the half lives of SSACV, SACV and CACV (t1/2) were observed to be 3.5 ± 0.4, 9.2 ± 0.6 and 1.8 ± 0.1 hr respectively, whereas AACV was readily converted to the active parent drug acyclovir exhibiting complete degradation before 5 min. Interestingly translocation of SACV across cornea was inhibited in the presence of 5 mM arginine (~51%), a specific substrate for cationic transport system and in presence of BCH (~38%), a substrate specific for large neutral amino acid transport system (LAT) or cationic and neutral amino acid transport system (B0,+). SACV exhibited higher permeability across cornea along with excellent antiviral activity against herpes simplex virus (HSV-1) and varicella-zoster virus (VZV) in comparison to ACV. Recognition by multiple transporters, stability in corneal homogenate and changes in physico-chemical properties contributed to the increased permeability of SACV across cornea.

Interaction between topically and systemically coadministered P-glycoprotein substrates/inhibitors: effect on vitreal kinetics

The objective of the present study was to investigate the effect of topically coadministered P-glycoprotein (P-gp) substrates/inhibitors on the vitreal kinetics of a systemically administered P-gp substrate. Anesthetized male rabbits were used in these studies. The concentration-time profile of quinidine in the vitreous humor, after intravenous administration, was determined alone and in the presence of topically coadministered verapamil, prednisolone sodium phosphate (PP), and erythromycin. The vitreal pharmacokinetic parameters of quinidine in the presence of verapamil [apparent elimination rate constant (λ(z)), 0.0027 ± 0.0002 min(-1); clearance (CL_F), 131 ± 21 ml/min; area under the curve (AUC(0-∞)), 39 ± 7.0 μg · min/ml; and mean residence time, 435 ± 20 min] were significantly different from those of the control (0.0058 ± 0.0006 min(-1), 296 ± 46 ml/min, 17 ± 3 μg · min/ml, and 232 ± 20 min, respectively). A 1.7-fold decrease in the vitreal λ(z) and a 1.5-fold increase in the vitreal AUC of quinidine were observed in the presence of topical PP. Statistically significant differences between the vitreal profiles of the control and erythromycin-treated group were also observed. Plasma concentration-time profiles of quinidine, alone or in the presence of the topically instilled compounds, remained unchanged, indicating uniform systemic quinidine exposure across groups. This study demonstrates an interaction between topically and systemically coadministered P-gp substrates, probably through the modulation of P-gp on the basolateral membrane of the retinal pigmented epithelium, leading to changes in the vitreal kinetics of the systemically administered agent.

Ocular drug delivery

Ocular drug delivery has been a major challenge to pharmacologists and drug delivery scientists due to its unique anatomy and physiology. Static barriers (different layers of cornea, sclera, and retina including blood aqueous and blood-retinal barriers), dynamic barriers (choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution), and efflux pumps in conjunction pose a significant challenge for delivery of a drug alone or in a dosage form, especially to the posterior segment. Identification of influx transporters on various ocular tissues and designing a transporter-targeted delivery of a parent drug has gathered momentum in recent years. Parallelly, colloidal dosage forms such as nanoparticles, nanomicelles, liposomes, and microemulsions have been widely explored to overcome various static and dynamic barriers. Novel drug delivery strategies such as bioadhesive gels and fibrin sealant-based approaches were developed to sustain drug levels at the target site. Designing noninvasive sustained drug delivery systems and exploring the feasibility of topical application to deliver drugs to the posterior segment may drastically improve drug delivery in the years to come. Current developments in the field of ophthalmic drug delivery promise a significant improvement in overcoming the challenges posed by various anterior and posterior segment diseases.

Identification and Characterization of a Na+-Dependent Neutral Amino Acid Transporter, ASCT1, in Rabbit Corneal Epithelial Cell Culture and Rabbit Cornea

PURPOSE

The aim of this study was to investigate the presence of a Na+-dependent neutral amino acid transporter, ASCT1, in rabbit primary corneal epithelial cell culture and rabbit cornea.

METHODS

Uptake studies were carried out on rabbit primary corneal epithelial culture (rPCEC) cells using 12-well plates. Transport studies were conducted with isolated rabbit corneas at 34 degrees C. Uptake and transport of L-alanine was determined at various concentrations. Inhibition studies were conducted in presence of various L- and D-amino acids, metabolic inhibitors like ouabain and sodium azide, and in the absence of sodium to delineate the functional characteristics of L-alanine uptake and transport. Reverse transcription-polymerase chain reaction (RT-PCR) was performed on total RNA harvested from rabbit cornea and rPCEC cells for identification of ASCT1.

RESULTS

Uptake of L-Ala was found to be saturable with a Km of 0.71 mM and a Vmax value of 0.84 micromoles min(-1) mg(-1) protein. Uptake was independent of pH and energy but depends on sodium. It was inhibited by serine, threonine, cysteine, and glutamine but did not respond to BCH (2-aminobicyclo [2,2,1] heptane-2-carboxylic acid) and MeAIB (alpha -methylaminoisobutyric acid). Transport of L-Ala across rabbit cornea was also saturable (Km 6.52 mM and Vmax 1.09 x 10(-2) micromoles min(-1) cm(-2)), energy independent, and subject to similar competitive inhibition. Presence of ASCT1 on rPCEC and on rabbit cornea was identified by RT-PCR.

CONCLUSIONS

L-Alanine, the chosen model substrate, was actively transported by Na+-dependent, neutral amino acid exchanger ASCT1, which was identified and functionally characterized on rPCEC cells and rabbit cornea.

Amino acid prodrugs of acyclovir as possible antiviral agents against ocular HSV-1 infections: Interactions with the neutral and cationic amino acid transporter on the corneal epithelium

PURPOSE

The aim of this study was to explore the feasibility of improvement of ocular bioavailability of the antiviral agent acyclovir by designing amino acid prodrugs targeted to the amino acid transporters on the rabbit cornea.

MATERIALS AND METHODS

Transcorneal flux of two water-soluble amino acid ester prodrugs of acyclovir (ACV), gamma-glutamate-ACV (EACV) and L-tyrosine-ACV (YACV), was studied across freshly excised rabbit cornea. Chemical and enzymatic hydrolysis studies of the two prodrugs were also conducted.

RESULTS

EACV inhibited the uptake of [(3)H]L-Arg in rabbit primary corneal epithelial cells (rPCECs). The compound also exhibited longer half-life (t(1/2)) in cornea in comparison to YACV. Transcorneal flux of EACV was observed to be concentration-, energy-, and sodium-dependent and independent of pH within the range studied. EACV transport was inhibited by neutral and cationic amino acids, L-ornithine (specific for cationic amino acids), and BCH (2-aminobicyclo-[2,2,1]-heptane-2-carboxylic-acid) (specific inhibitor for L-type system and B(0,+) system). On the other hand, YACV was not recognized by this amino acid transporter as it failed to inhibit the uptake of [(3)H]Arg, and also its transport across cornea was not inhibited by arginine. YACV and EACV exhibited excellent antiviral activity against HSV-1 and 2 and Varicella-Zoster Virus (VZV) in comparison to ACV.

CONCLUSIONS

Analyses of the inhibition pattern of EACV transport suggests the involvement of a single transport system; namely, B(0,+). Design of amino acid prodrugs seems to be an attractive strategy to enhance the solubility of the otherwise poorly aqueous soluble compounds and also to afford a targeted and possibly enhanced delivery of the active drug.

Vitreal kinetics of quinidine in rabbits in the presence of topically coadministered P-glycoprotein substrates/modulators

The purpose of this study was to investigate whether topically administered P-glycoprotein (P-gp) substrates/modulators can alter vitreal kinetics of intravitreally administered quinidine. Male New Zealand rabbits were used under anesthesia. Vitreal kinetics of intravitreally administered quinidine (0.75-microg dose) was determined alone and in the presence of verapamil (coadministered topically/intravitreally) or prednisolone hemisuccinate sodium (PHS) (coadministered topically). In the presence of topically instilled verapamil (1% w/v), elimination half-life (t(1/2)) (176 +/- 7 min), apparent elimination rate constant (lambda(z)) (0.0039 +/- 0.0001 min(-1)), and mean retention time (MRT) (143 +/- 30 min) of intravitreally administered quinidine were significantly different from those of the control (105 +/- 11 min, 0.0066 +/- 0.0007 min(-1), and 83 +/- 13 min, respectively). A 2-fold increase in the t(1/2) with a corresponding decrease in lambda(z) and a 1.5-fold increase in the MRT of quinidine were observed in the presence of topically coadministered 2% w/v PHS. Intravitreal coadministration of quinidine and verapamil resulted in a significant increase in t(1/2) (159 +/- 9 min) and a decrease in lambda(z) (0.0043 +/- 0.0002 min(-1)) of quinidine. The vitreal pharmacokinetic parameters of sodium fluorescein, alone or in the presence of topically instilled verapamil, did not show any statistically significant difference, indicating that ocular barrier integrity was not affected by topical verapamil administration. Results from this study suggest that topically applied P-gp substrates/modulators can alter vitreal pharmacokinetics of intravitreally administered P-gp substrates, possibly through the inhibition of P-gp expressed on the basolateral membrane of the retinal pigmented epithelium.

Ocular pharmacokinetics of acyclovir amino acid ester prodrugs in the anterior chamber: evaluation of their utility in treating ocular HSV infections

PURPOSE

To evaluate in vivo corneal absorption of the amino acid prodrugs of acyclovir (ACV) using a topical well model and microdialysis in rabbits.

METHODS

Stability of L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) prodrugs was evaluated in various ocular tissues. Dose-dependent toxicity of these prodrugs was also examined in rabbit primary corneal epithelial cell culture (rPCEC) using 96-well based cell proliferation assay. In vivo ocular bioavailability of these compounds was also evaluated with a combination of topical well infusion and aqueous humor microdialysis techniques.

RESULTS

Among the amino acid ester prodrugs, SACV was most stable in aqueous humor. Enzymatic degradation of EACV was the least compared to all other prodrugs. Cellular toxicity of all the prodrugs was significantly less compared to trifluorothymidine (TFT) at 5mM. Absorption rate constants of all the compounds were found to be lower than the elimination rate constants. All the prodrugs showed similar terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately two-fold increase in area under the curve (AUC) relative to ACV (p<0.05). Clast (concentration at the last time point) of SACV was observed to be 8+/-2.6microM in aqueous humor which is two and three times higher than VACV and ACV, respectively.

CONCLUSIONS

Amino acid ester prodrugs of ACV were absorbed through the cornea at varying rates (ka) thereby leading to varying extents (AUC). The amino acid ester prodrug, SACV owing to its enhanced stability, comparable AUC and high concentration at last time point (Clast) seems to be a promising candidate for the treatment of ocular HSV infections.

Ophthalmic drug delivery considerations at the cellular level: drug-metabolising enzymes and transporters

Ophthalmic drugs typically achieve < 10% ocular bioavailability. A drug applied to the surface of the eye may cross ocular-blood barriers where it may encounter metabolising enzymes and cellular transporters before it distributes to the site of action. Characterisation of ocular enzyme systems and cellular transporters and their respective substrate selectivity have provided new insight into the roles these proteins may play in ocular drug delivery and distribution. Altered metabolism and transport have been proposed to contribute to a number of ocular disease processes including inflammation, glaucoma, cataract, dry eye and neurodegeneration. As ocular enzyme and transport systems are better characterised, their properties become an integral consideration in drug design and development.

The potential of lipid emulsion for ocular delivery of lipophilic drugs

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