Molecular evidence and functional expression of multidrug resistance associated protein (MRP) in rabbit corneal epithelial cells

Revolutionizing eye care: the game-changing applications of nano-antioxidants in ophthalmology

Since the theory of free radical-induced aging was proposed in 1956, it has been constantly proven that reactive oxygen species (ROS) produced by oxidative stress play a vital role in the occurrence and progression of eye diseases. However, the inherent limitations of traditional drug therapy hindered the development of ophthalmic disease treatment. In recent years, great achievements have been made in the research of nanomedicine, which promotes the rapid development of safe theranostics in ophthalmology. In this review, we focus on the applications of antioxidant nanomedicine in the treatment of ophthalmology. The eye diseases were mainly classified into two categories: ocular surface diseases and posterior eye diseases. In each part, we first introduced the pathology of specific diseases about oxidative stress, and then presented the representative application examples of nano-antioxidants in eye disease therapy. Meanwhile, the nanocarriers that were used, the mechanism of function, and the therapeutic effect were also presented. Finally, we summarized the latest research progress and limitations of antioxidant nanomedicine for eye disease treatment and put forward the prospects of future development.

Prodrugs and nanomicelles to overcome ocular barriers for drug penetration

INTRODUCTION

Ocular barriers hinder drug delivery and reduce drug bioavailability. This article focuses on enhancing drug absorption across the corneal and conjunctival epithelium. Both, transporter targeted prodrug formulations and nanomicellar strategy is proven to enhance the drug permeation of therapeutic agents across various ocular barriers. These strategies can increase aqueous drug solubility and stability of many hydrophobic drugs for topical ophthalmic formulations.

AREAS COVERED

The article discusses various ocular barriers, ocular influx, and efflux transporters. It elaborates various prodrug strategies used for enhancing drug absorption. Along with this, the article also describes nanomicellar formulation, its characteristic and advantages, and applications in for anterior and posterior segment drug delivery.

EXPERT OPINION

Prodrugs and nanomicellar formulations provide an effective strategy for improving drug absorption and drug bioavailability across various ocular barriers. It will be exciting to see the efficacy of nanomicelles for treating back of the eye disorders after their topical application. This is considered as a holy grail of ocular drug delivery due to the dynamic and static ocular barriers, restricting posterior entry of topically applied drug formulations.

Intracameral dexamethasone injection in the treatment of cataract surgery induced inflammation: design, development, and place in therapy

Cataract surgery is one of the most commonly performed surgeries worldwide, with nearly 20 million cases annually. Appropriate prophylaxis after cataract surgery can contribute to a safe and quick visual recovery with high patient satisfaction. Despite being the current standard of care, the use of multiple postoperative eye drops can create a significant burden on these patients, contributing to documented and significant non-adherence to the postoperative regimen. Over the past 25 years, there have been a few studies analyzing the use of intracameral dexamethasone (DXM) in controlling inflammation following cataract surgery. This review explores various drug delivery approaches for managing intraocular inflammation after cataract surgery, documenting the strengths and weaknesses of these options and examining the role of intracameral DXM (among these other strategies) in controlling postoperative intraocular inflammation. Intracameral DXM has a particular advantage over topical steroids in possibly decreasing postoperative inflammatory symptoms and objective anterior cell and flare scores. Compared to topical steroids, there may be a slightly less theoretical risk of significant intraocular pressure spikes and systemic absorption. In addition, surveys indicate patients prefer an intraoperative intracameral injection over a self-administered postoperative eye drop regimen. However, there are several adverse effects associated with intracameral DXM delivery that are not seen with the noninvasive topical approach. Although it is unlikely that intracameral DXM will replace topical medications as the standard management for postoperative inflammation, it is seemingly another safe and effective strategy for controlling postoperative inflammation after routine cataract surgery.

Expression, activity and pharmacokinetic impact of ocular transporters

The eye is protected by several tissues that limit the permeability and entry of potentially harmful substances, but also hamper the delivery of drugs in the treatment of ocular diseases. Active transport across the ocular barriers may affect drug distribution, but the impact of drug transporters on ocular drug delivery is not well known. We have collected and critically reviewed the literature for ocular expression and activity of known drug transporters. The review concentrates on drug transporters that have been functionally characterized in ocular tissues or primary cells and on transporters for which there is available expression data at the protein level. Species differences are highlighted, since these may explain observed inconsistencies in the influence of specific transporters on drug disposition. There is variable evidence about the pharmacokinetic role of transporters in ocular tissues. The strongest evidence for the role of active transport is available for the blood-retinal barrier. We explored the role of active transport in the cornea and blood retinal barrier with pharmacokinetic simulations. The simulations show that the active transport is important only in the case of specific parameter combinations.

Stereoisomeric Prodrugs to Improve Corneal Absorption of Prednisolone: Synthesis and In Vitro Evaluation

A series of stereoisomeric prodrugs have been designed to examine efficacy in generating higher corneal absorption relative to prednisolone. Prodrugs have been studied and identified with LC/MS/MS and NMR analyses. Prodrugs have been characterized for aqueous solubility, buffer stability, and cytotoxicity. Cellular uptake and permeability studies have been conducted across MDCK-MDR1 cells to determine prodrug affinity towards P-glycoprotein (P-gp) and peptide transporters. Enzyme-mediated degradation of prodrugs has been determined using Statens Seruminstitut rabbit cornea (SIRC) cell homogenates. Prodrugs exhibited higher aqueous solubility relative to prednisolone. Prodrugs circumvented P-gp-mediated cellular efflux and were recognized by peptide transporters. Prodrugs (DP, DDP) produced with D-isomers (D-valine) were significantly stable against both chemical and enzymatic hydrolyses. The order of degradation rate constants observed in chemical and enzymatic hydrolyses were in the same order, i.e., L-valine-L-valine-prednisolone (LLP) > L-valine-D-valine-prednisolone (LDP) > D-valine-L-valine-prednisolone (DLP) > D-valine-D-valine-prednisolone (DDP). Results obtained from this study clearly suggest that stereoisomeric prodrug approach is an effective strategy to overcome P-gp-mediated efflux and improve transcorneal permeability of prednisolone following topical administration.

Prodrug approach to improve absorption of prednisolone

Amino acid and dipeptide prodrugs have been developed to examine their potential in enhancing aqueous solubility and permeability as well as to bypass P-glycoprotein (P-gp) mediated cellular efflux of prednisolone. Prodrugs have been synthesized and identified with LC/MS/MS and NMR. Prodrugs displayed significantly higher aqueous solubility relative to prednisolone. These compounds also exhibited higher stability under acidic conditions relative to basic medium. [14]-Erythromycin uptake remained unaltered in the presence of valine-valine-prednisolone (VVP) indicating lower affinity toward P-gp. Moreover, VVP generated significantly higher transepithelial permeability across MDCK-MDR1 cells compared to prednisolone. Importantly, [3H]-GlySar uptake diminished significantly in the presence of VVP indicating high affinity toward peptide transporters. Moreover, prednisolone was regenerated from VVP due to enzymatic hydrolysis in SIRC cell homogenate. Results obtained from these studies clearly suggest that peptide transporter targeted prodrugs is a viable strategy to improve aqueous solubility and overcome P-gp mediated cellular efflux of prednisolone.

Ocular cytochrome P450s and transporters: roles in disease and endobiotic and xenobiotic disposition

Drug metabolism and transport processes in the liver, intestine and kidney that affect the pharmacokinetics and pharmacodynamics of therapeutic agents have been studied extensively. In contrast, comparatively little research has been conducted on these topics as they pertain to the eye. Recently, however, catalytic functions of ocular cytochrome P450 enzymes have gained increasing attention, in large part due to the roles of CYP1B1 and CYP4V2 variants in primary congenital glaucoma and Bietti's corneoretinal crystalline dystrophy, respectively. In this review, we discuss challenges to ophthalmic drug delivery, including Phase I drug metabolism and transport in the eye, and the role of three specific P450s, CYP4B1, CYP1B1 and CYP4V2 in ocular inflammation and genetically determined ocular disease.

Expression of efflux transporters in human ocular tissues

To investigate the expression profiles of efflux transporters in human ocular tissues, quantitative real-time polymerase chain reaction, Western blotting, and immunohistochemistry were used to obtain the relative mRNA and protein expressions of various efflux transporters in human ocular tissues. The cornea, conjunctiva, iris-ciliary body (ICB), retina and choroid, human corneal epithelial cell line (HCEC), and human retinal pigment epithelial cell line (ARPE-19) were examined for the expressions of multidrug resistance-associated proteins 1-7 (MRP1-7), multidrug resistance 1 (MDR1) P-glycoprotein, lung resistance protein (LRP), and breast cancer-resistance protein (BCRP). The expression sites and patterns of efflux transporters were significantly different in ocular tissues, HCEC, and ARPE-19, as well as the expression profiles of efflux transporters in mRNA and protein levels in ocular tissues. At the protein level, MRP1-7, MDR1, and LRP were expressed in the corneal epithelium; MRP1-7, MDR1, LRP, and BCRP were expressed in the conjunctival epithelium; MRP1-2, MRP6-7, MDR1, and LRP were expressed in the ICB; MRP1-3, MRP6-7, MDR1, and LRP were expressed in the retina; MRP1-3, MRP6-7, MDR1, and LRP were expressed in the HCEC; and MRP7, MDR1, LRP, and BCRP were expressed in the ARPE-19. This quantitative and systematic study of efflux transporters in normal ocular tissues and cell lines provides evidence of cross-ocular tissue transporter expression differences, implying that efflux transporter expression variability should be taken into consideration for better understanding of ocular pharmacokinetic and pharmacodynamic data.

Utility of transporter/receptor(s) in drug delivery to the eye

The eye is a highly protected organ, and designing an effective therapy is often considered a challenging task. The anatomical and physiological barriers result in low ocular bioavailability of drugs. Due to these constraints, less than 5% of the administered dose is absorbed from the conventional ophthalmic dosage forms. Further, physicochemical properties such as lipophilicity, molecular weight and charge modulate the permeability of drug molecules. Vision-threatening diseases such as glaucoma, diabetic macular edema, cataract, wet and dry age-related macular degeneration, proliferative vitreoretinopathy, uveitis, and cytomegalovirus retinitis alter the pathophysiological and molecular mechanisms. Understanding these mechanisms may result in the development of novel treatment modalities. Recently, transporter/receptor targeted prodrug approach has generated significant interest in ocular drug delivery. These transporters and receptors are involved in the transport of essential nutrients, vitamins, and xenobiotics across biological membranes. Several influx transporters (peptides, amino acids, glucose, lactate and nucleosides/nucleobases) and receptors (folate and biotin) have been identified on conjunctiva, cornea, and retina. Structural and functional delineation of these transporters will enable more drugs targeting the posterior segment to be successfully delivered topically. Prodrug derivatization targeting transporters and receptors expressed on ocular tissues has been the subject of intense research. Several prodrugs have been designed to target these transporters and enhance the absorption of poorly permeating parent drug. Moreover, this approach might be used in gene delivery to modify cellular function and membrane receptors. This review provides comprehensive information on ocular drug delivery, with special emphasis on the use of transporters and receptors to improve drug bioavailability.

Novel strategies for anterior segment ocular drug delivery

Research advancements in pharmaceutical sciences have led to the development of new strategies in drug delivery to anterior segment. Designing a new delivery system that can efficiently target the diseased anterior ocular tissue, generate high drug levels, and maintain prolonged and effective concentrations with no or minimal side effects is the major focus of current research. Drug delivery by traditional method of administration via topical dosing is impeded by ocular static and dynamic barriers. Various products have been introduced into the market that prolong drug retention in the precorneal pocket and to improve bioavailability. However, there is a need of a delivery system that can provide controlled release to treat chronic ocular diseases with a reduced dosing frequency without causing any visual disturbances. This review provides an overview of anterior ocular barriers along with strategies to overcome these ocular barriers and deliver therapeutic agents to the affected anterior ocular tissue with a special emphasis on nanotechnology-based drug delivery approaches.

Mitochondrial localization of P-glycoprotein and peptide transporters in corneal epithelial cells--novel strategies for intracellular drug targeting

This study was designed to investigate functional localization of both efflux (P-glycoprotein, P-gp) and influx (peptide) transporters in the mitochondrial membrane of cultured rabbit primary corneal epithelial cells (rPCECs). Isolation and purification of mitochondria was performed by optimized cell fractionation method. Mitochondrial integrity was measured by JC-1 uptake experiment. The efflux activity of P-gp was assessed by performing in vitro uptake studies on isolated mitochondria with Rhodamine 123 (Rho-123) alone and in the presence of P-gp inhibitors (quinidine and cyclosporine A) using fluorimetry and flow cytometry analysis. Functional activity of peptide transporter was assessed by performing in vitro uptake studies of [3H] Gly-sar on isolated mitochondria in the presence or absence of peptide transporter substrate (Val-Val). Molecular characterization of P-gp and peptide transporter was assessed by western blot and confocal analysis. Enhanced JC-1 accumulation in the isolated fraction confirmed mitochondrial membrane integrity. Significantly higher uptake of Rho-123 on isolated mitochondria was observed in the presence of quinidine (75 and 100 μM) and cyclosporine A (10 μM). Significantly lower uptake of [3H] Gly-sar was observed in the presence of val-val due to competitive inhibition of peptide transporter on isolated mitochondria. Western blot and confocal analysis further confirmed the presence of P-gp and peptide transporter on the mitochondrial membrane of rPCECs. The present study demonstrates the functional and molecular characterization of P-gp and peptide transporters in the mitochondrial membranes of rPCECs. This knowledge of mitochondrial existence of P-gp and peptide transporter will aid in the development of subcellular ocular drug delivery strategies.

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.

Ocular distribution of difluprednate ophthalmic emulsion 0.05% in rabbits

PURPOSE

To evaluate ocular distribution and excretion of tritium-labeled difluprednate ((3)H-DFBA) ophthalmic emulsion 0.05% after a single or repeated instillation to pigmented rabbit eyes.

METHODS

(3)H-DFBA ophthalmic emulsion 0.05% was instilled in the right eyes of pigmented rabbits, in either a single or repeated quater in die (QID) for 3 days or 7 days) dose of 25  μg/50  μL. The radioactivity in right and left ocular tissues, urine, blood, plasma, and feces were measured with a liquid scintillation counter. Additionally, the distribution of radioactivity around ocular tissues was investigated with autoradiography.

RESULTS

After a single instillation, the highest maximum radioactive concentrations were found in the cornea (2,081  ng eq./g), followed by the iris/ciliary body (926  ng eq./g), conjunctiva (330  ng eq./g), anterior retina/choroid (273  ng eq./g), sclera (222  ng eq./g), and aqueous humor (144  ng eq./mL) of treated eyes. The maximum radioactivity concentration of the posterior retina/choroid was 59  ng eq./g, and difluprednate delivered as a topical ophthalmic emulsion reached the back of the eye. However, radioactivity in untreated eyes was very low. Total radioactivity excreted in urine and feces 168  h after a single instillation was 99.5% of the total dose. Radioactivity concentration levels measured 24  h after 28 instillations were no more than twice those measured 24  h after 12 instillations. Radioactive concentrations in ocular and periocular tissues were highest at 0.5 or 1  h after a single instillation, and were mostly eliminated from these tissues by the end of the study. Radioactivity was barely detectable in the blood, with very little accumulation even after multiple doses.

CONCLUSIONS

After instillation of (3)H-DFBA ophthalmic emulsion 0.05% in rabbit eyes, radioactivity was distributed at the anterior segment and cleared rapidly. Some radioactivity was detected in the posterior retina/choroid, suggesting that difluprednate and its metabolites reach these tissues. These results suggest that difluprednate delivered as a topical ophthalmic emulsion reached the anterior and posterior segments of the eye quickly and may be a potential treatment for ocular inflammation in these areas.

Interaction of gatifloxacin with efflux transporters: a possible mechanism for drug resistance

The purpose of the study is to screen the interactions of fourth generation fluoroquinolone-gatifloxacin with efflux pumps, i.e., P-gp, MRP2 and BCRP. Mechanism of gatifloxacin interaction with efflux transporters may explain its acquired resistance. Such clarification may lead to the development of strategies to overcome efflux and enhance its bioavailability at target site. This process will aid in the reduction of dose volume, further eliminating the chances of systemic toxicity from topical gatifloxacin eye drops. MDCK cell lines transfected with the targeted efflux transporters were used for this study. [(14)C] Erythromycin was selected as a model substrate for P-gp and MRP2 whereas Hoechst 33342 was employed as a substrate for BCRP. Uptake and transport studies of these substrates were performed in the presence of gatifloxacin to delineate its interaction with efflux transporters. Further the efflux ratio in the presence of gatifloxacin was calculated from bidirectional transport studies. The concentration of [(14)C] erythromycin and Hoechst 33342 was measured using scintillation counter and fluorescence plate reader, respectively. A concentration dependent inhibition effect in the presence of gatifloxacin was revealed on [(14)C] erythromycin uptake. The efflux ratio (BL-AP/AP-BL) of substrates was found to approach unity at higher gatifloxacin concentrations. Increased concentration of gatifloxacin did not elevate uptake of Hoechst 33342. All these studies were validated with known inhibitors as positive control. Uptake and transport studies support the hypothesis that gatifloxacin is a substrate for P-gp, MRP2 but not for BCRP. Possible interactions of gatifloxacin with P-gp and MRP2 may be a possible mechanism for acquired resistance of gatifloxacin. This information can be further extended to design prodrugs or formulations in order to prevent development of acquired resistance and improve therapeutic efficacy with its reduction in side effects.

Effluxing ABC transporters in human corneal epithelium

ATP-binding cassette (ABC) transporters are able to efflux their substrate drugs from the cells. We compared expression of efflux proteins in normal human corneal epithelial tissue, primary human corneal epithelial cells (HCEpiC), and corneal epithelial cell culture model (HCE model) based on human immortal cell line. Expression of multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 1-6 (MRP1-6) and breast cancer resistance protein (BCRP) was studied using quantitative RT-PCR, Western blot, and immunohistochemistry. Only MRP1, MRP5, and BCRP were expressed in the freshly excised human corneal epithelial tissue. Expression of MRP1 and MRP5 was localized predominantly in the basal cells of the central cornea and limbus. Functional efflux activity was shown in the cell models, but they showed over-expression of most efflux transporters compared to that of normal corneal epithelium. In conclusion, MRP1, MRP5, and BCRP are expressed in the corneal epithelium, but MDR1, MRP2, MRP3, MRP4, and MRP6 are not significantly expressed. HCE cell model and commercially available primary cells deviate from this expression profile.

Interaction of ocular hypotensive agents (PGF2 alpha analogs-bimatoprost, latanoprost, and travoprost) with MDR efflux pumps on the rabbit cornea

PURPOSE

The objectives of this work were (i) to screen ocular hypotensive prostaglandin (PGF2 alpha) analogs--bimatoprost, latanoprost, and travoprost as well as their free acid forms--for interaction with efflux pumps on the cornea and (ii) to assess the modulation of efflux upon co-administration of these prostaglandin analogs.

METHODS

Cultured rabbit primary corneal epithelial cells (rPCEC) were employed as an in vitro model for rabbit cornea. Transporter-specific interaction studies were carried out using Madin-Darby canine kidney (MDCK) cells overexpressing MDR1, MRP1, MRP2, MRP5, and BCRP. Freshly excised rabbit cornea was used as an ex vivo model to determine transcorneal permeability.

RESULTS

Cellular accumulation studies clearly showed that all prostaglandin analogs and their free acid forms are substrates of MRP1, MRP2, and MRP5. Bimatoprost was the only prostaglandin analog in this study to interact with P-gp. In addition, none of these molecules showed any affinity for BCRP. K (i) values of these prostaglandin analogs obtained from dose-dependent inhibition of erythromycin efflux in rPCEC showed bimatoprost (82.54 microM) and travoprost (94.77 microM) to have similar but higher affinity to efflux pumps than latanoprost (163.20 microM). Ex vivo studies showed that the permeation of these molecules across cornea was significantly elevated in the presence of specific efflux modulators. Finally, both in vitro and ex vivo experiments demonstrated that the efflux of these prostaglandin analogs could be modulated by co-administering them together.

CONCLUSION

Bimatoprost, latanoprost, travoprost, and their free acid forms are substrates of multiple drug efflux pumps on the cornea. Co-administration of these molecules together is a viable strategy to overcome efflux, which could simultaneously elicit a synergistic pharmacological effect, since these molecules have been shown to activate different receptor population for the reduction of intraocular pressure (IOP).

Probenecid treatment enhances retinal and brain delivery of N-4-benzoylaminophenylsulfonylglycine: an anionic aldose reductase inhibitor

Anion efflux transporters are expected to minimize target tissue delivery of N-[4-(benzoylaminophenyl)sulfonyl]glycine (BAPSG), a novel carboxylic acid aldose reductase inhibitor, which exists as a monocarboxylate anion at physiological conditions. Therefore, the objective of this study was to determine whether BAPSG delivery to various eye tissues including the retina and the brain can be enhanced by probenecid, a competitive inhibitor of anion transporters. To determine the influence of probenecid on eye and brain distribution of BAPSG, probenecid was administered intraperitoneally (120 mg/kg body weight; i.p.) 20 min prior to BAPSG (50 mg/kg; i.p.) administration. Drug disposition in various eye tissues including the retina and the brain was determined at 15 min, 1, 2 and 4h after BAPSG dose in male Sprauge-Dawley rats. To determine whether probenecid alters plasma clearance of BAPSG, influence of probenecid (120 mg/kg; i.p.) on the plasma pharmacokinetics of intravenously administered BAPSG (15 mg/kg) was studied as well. Finally, the effect of probenecid co-administration on the ocular tissue distribution of BAPSG was assessed in rabbits following topical (eye drop) administration. Following pretreatment with probenecid in the rat study, retinal delivery at 1h was increased by about 11-fold (2580 ng/g vs. 244 ng/g; p<0.05). Further, following probenecid pretreatment, significant BAPSG levels were detectable in the brain (45 + or - 20 ng/g) at 1h, unlike controls where the drug was not detectable. Plasma concentrations, plasma elimination half-life, and total body clearance of intravenously administered BAPSG were not altered by i.p. probenecid pretreatment. In the topical dosing study, a significant decline in BAPSG delivery was observed in the iris-ciliary body but no significant changes were observed in other tissues of the anterior segment of the eye including tears. Thus, inhibition of anion transporters is a useful approach to elevate retinal and brain delivery of BAPSG.

Expression of multidrug resistance associated protein 5 (MRP5) on cornea and its role in drug efflux

PURPOSE

The purpose of this manuscript is to investigate the presence of nucleoside/nucleotide efflux transporter in cornea and to evaluate the role in ocular drug efflux.

METHODS

RT-PCR, immunoprecipitation followed by Western blot analysis and immunostaining were employed to establish molecular presence of multidrug resistance associated protein 5 (MRP5) on cornea. Corneal efflux by MRP5 was studied with bis(POM)-PMEA and acyclovir using rabbit and human corneal epithelial cells along with MRP5 over expressing cells (MDCKII-MRP5). Ex vivo studies using excised rabbit cornea and in vivo ocular microdialysis in male New Zealand white rabbits were used to further evaluate the role of MRP5 in conferring ocular drug resistance.

RESULTS

RT-PCR confirms the expression of MRP5 in both rabbit and human corneal epithelial cells along with MDCKII-MRP5 cells. Immunoprecipitation followed by Western blot analysis using a rat (M511-54) monoclonal antibody that reacts with human epitope confirms the expression of MRP5 protein in human corneal epithelial cells and MDCKII-MRP5 cells. Immunostaining performed on human cornea indicates the localization of this efflux pump on both epithelium and endothelium. Efflux studies reveal that depletion of ATP decreased PMEA efflux significantly. MRP5 inhibitors also diminished PMEA and acyclovir efflux. However, depletion of glutathione did not alter efflux. MDR1 and MRP2 did not contribute to PMEA efflux. However, MRP2 is involved in acyclovir efflux while MDR1 do not participate in this process. TLC/autoradiography suggested the conversion of bis(POM)-PMEA to PMEA in rabbit and human corneal epithelial cells. Two well known antiglaucoma drugs, bimatoprost and latanoprost were rapidly effluxed by MRP5. Ex vivo study on intact rabbit corneas demonstrated accumulation of PMEA in cornea in the presence of ATP-depleting medium. In vivo ocular pharmacokinetics also revealed a significant increase in maximum aqueous humor concentration (C(max)) and area under the aqueous humor time curve (AUC) of acyclovir in the presence of MK-571, a specific MRP inhibitor.

CONCLUSIONS

Taken together immunolocalization on human cornea, in vitro efflux in human, rabbit corneal and MRP5 over expressing cells, ex vivo and in vivo studies in intact rabbit cornea suggest that MRP5 on cornea can significantly lower the permeability of antiviral and glaucoma drugs. These findings may be valuable in developing formulation strategies to optimize ocular bioavailability of topically administered ocular agents.

Molecular expression and functional evidence of a drug efflux pump (BCRP) in human corneal epithelial cells

PURPOSE

Breast Cancer Resistance Protein (BCRP) belongs to the family of efflux transporters involved in drug efflux leading to drug resistance. The objective of this study was to explore physical barriers for ocular drug absorption and to verify the presence and possible role of BCRP as a barrier for ocular drug resistance.

METHODS

Transfected human corneal epithelial cells (SV40-HCEC) were selected as an in vitro model for corneal epithelium with MDCKII-BCRP as positive control. [(3)H]-Mitoxantrone ([(3)H]-MTX), which is a proven substrate for organic anion transporter like BCRP, was selected as a model drug for functional expression studies. Fumetremorgin C (FTC), a known specific inhibitor for BCRP and GF120918, an inhibitor for BCRP and P-gp, were added to inhibit BCRP-mediated efflux. PGP-4008, a specific inhibitor of P-gp was used to delineate the contribution of P-gp. The mRNA extracted from cells was used for RT-PCR analysis and gene expression. Membrane fractions of SV40-HCEC and MDCKII-BCRP were used for immunoprecipitation followed by Western blot analysis.

RESULTS

Efflux was inhibited significantly in the presence of FTC and GF120918. Dose-dependent inhibition of efflux by BCRP was noticed in SV40-HCEC and MDCKII-BCRP in the presence of FTC and GF120918, and the efflux was ATP-dependent. The metabolic inhibitor, 2,4-DNP, significantly inhibited efflux. No pH-dependent efflux was noticed except at pH 5.5. RT-PCR analysis indicated a unique and distinct band at approximately 429 bp, corresponding to BCRP in SV40-HCEC and MDCKII-BCRP cells. Western Blot analysis indicated a specific band at approximately 70 kDa in the membrane fraction of SV40-HCEC and MDCKII-BCRP cells.

CONCLUSIONS

We have demonstrated the expression of BCRP in human corneal epithelial cells and, for the first time, demonstrated its functional activity leading to drug efflux. RT-PCR and Western blot analysis further confirmed this finding.

Enhanced corneal absorption of erythromycin by modulating P-glycoprotein and MRP mediated efflux with corticosteroids

PURPOSE

The objectives were (i) to test in vivo functional activity of MRP2 on rabbit corneal epithelium and (ii) to evaluate modulation of P-gp and MRP2 mediated efflux of erythromycin when co-administered with corticosteroids.

METHODS

Cultured rabbit primary corneal epithelial cells (rPCECs) was employed as an in vitro model for rabbit cornea. Cellular accumulation and bi-directional transport studies were conducted across Madin-Darby Canine Kidney (MDCK) cells overexpressing MDR1 and MRP2 proteins to delineate transporter specific interaction of steroids. Ocular pharmacokinetic studies were conducted in rabbits following a single-dose infusion of erythromycin in the presence of specific inhibitors and steroids.

RESULTS

Bi-directional transport of erythromycin across MDCK-MDR1 and MDCK-MRP2 cells showed significant difference between BL-AP and AP-BL permeability, suggesting that erythromycin is a substrate for P-gp and MRP2. Cellular accumulation of erythromycin in rPCEC was inhibited by steroids in a dose dependent manner. MK571, a specific MRP inhibitor, modulated the aqueous humor concentration of erythromycin in vivo. Even, steroids inhibited P-gp and MRP2 mediated efflux with maximum increase in k(a), AUC(0-infinity), C(max) and C(last) values of erythromycin, observed with 6alpha-methyl prednisolone.

CONCLUSION

MRP2 is functionally active along with P-gp in effluxing drug molecules out of corneal epithelium. Steroids were able to significantly inhibit both P-gp and MRP2 mediated efflux of erythromycin.

Molecular design for enhancement of ocular penetration

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

Cell culture models of the human cornea - a comparative evaluation of their usefulness to determine ocular drug absorption in-vitro

Cell culture models of the cornea are continually developed to replace the isolated animal cornea for transcorneal drug absorption studies. The aim of this study was to determine and compare epithelial tightness and permeability of currently available corneal cell culture models to avoid interlaboratory variability and to assess their usefulness for in-vitro permeation studies. Pure epithelial cell culture models (CEPI, SIRC and HCE-T cell lines), primary cultures of human corneal epithelium (HCEpiC) and the two commercially available models (RHC and Epiocular), as well as organotypic human cornea constructs (HCC, HCC-HCE-T), were investigated and data were compared with those obtained from the excised bovine cornea. Barrier properties were assessed by measurements of transepithelial electrical resistance (TEER) and permeability of three passively absorbed substances (mannitol, testosterone and timolol maleate) with different physico-chemical properties. TEER experiments revealed weak barrier functions for all of the investigated epithelial models (<or=100-200 Omega cm2), except the HCE-T cell line. Transport studies confirmed TEER results insofar that models showing low TEER values also had higher permeation rates in comparison with the excised bovine cornea. However, models based on HCE-T cells demonstrated similar barrier properties to isolated corneal tissue. The corneal models investigated in our laboratory show clear differences in epithelial barrier function. In-vitro systems comprising the HCE-T cell line seem to be most appropriate to replace excised animal cornea for assessing corneal permeability.

Molecular evidence and functional expression of a novel drug efflux pump (ABCC2) in human corneal epithelium and rabbit cornea and its role in ocular drug efflux

Cornea is considered as a major barrier for ocular drug delivery. Low ocular bioavailability of drugs has been attributed primarily to low permeability across corneal epithelium, thus leading to sub-therapeutic concentrations of drug in the eye and treatment failure. The role of drug efflux proteins, particularly the P-glycoprotein (P-gp) in ocular drug bioavailability has been reported. The objective of this research was to determine whether human corneal epithelium expresses multidrug resistance associated proteins (MRPs) contributing to drug efflux by employing both cultured corneal cells and freshly excised rabbit cornea. SV40-HCEC and rPCEC were selected for in vitro testing. SV40-HCEC and freshly excised rabbit corneas were utilized for transport studies. [(3)H]-cyclosporine-A and [(14)C]-erythromycin, which are known substrates for ABCC2 and MK-571, a specific inhibitor for MRP were applied in this study. RT-PCR indicated a unique and distinct band at approximately 272 bp corresponding to ABCC2 in HCEC, SV40-HCEC, rabbit cornea, rPCEC, and MDCKII-MRP2 cells. Also RT-PCR indicated a unique band approximately 181 bp for HCEC and SV40-HCEC. Immunoprecipitation followed by Western Blot analysis revealed a specific band at approximately 190 kDa in membrane fraction of SV40-HCEC, MDCKII-MRP2 and no band with isotype control. Uptake of [(3)H]-cyclosporine-A and [(14)C]-erythromycin in the presence of MK-571 was significantly enhanced than control in both SV40-HCEC and rPCEC. Similarly a significant elevation in (A-->B) permeability of [(3)H]-cyclosporine-A and [(14)C]-erythromycin was observed in the presence of MK-571 in SV40-HCEC. A-->B transport of [(3)H]-cyclosporine-A was elevated in the presence of MK-571 in freshly excised rabbit cornea indicating potential role of this efflux transporter and high clinical significance of this finding.