Role of P-glycoprotein in distribution of rhodamine 123 into aqueous humor in rabbits

Recent Advances of Ocular Drug Delivery Systems: Prominence of Ocular Implants for Chronic Eye Diseases

Chronic ocular diseases can seriously impact the eyes and could potentially result in blindness or serious vision loss. According to the most recent data from the WHO, there are more than 2 billion visually impaired people in the world. Therefore, it is pivotal to develop more sophisticated, long-acting drug delivery systems/devices to treat chronic eye conditions. This review covers several drug delivery nanocarriers that can control chronic eye disorders non-invasively. However, most of the developed nanocarriers are still in preclinical or clinical stages. Long-acting drug delivery systems, such as inserts and implants, constitute the majority of the clinically used methods for the treatment of chronic eye diseases due to their steady state release, persistent therapeutic activity, and ability to bypass most ocular barriers. However, implants are considered invasive drug delivery technologies, especially those that are nonbiodegradable. Furthermore, in vitro characterization approaches, although useful, are limited in mimicking or truly representing the in vivo environment. This review focuses on long-acting drug delivery systems (LADDS), particularly implantable drug delivery systems (IDDS), their formulation, methods of characterization, and clinical application for the treatment of eye diseases.

P-glycoprotein Restricts Ocular Penetration of Loperamide across the Blood-Ocular Barriers: a Comparative Study in Mdr1a Knock-out and Wild Type Sprague Dawley Rats

The current research was undertaken to determine the existence and magnitude of P-glycoprotein (P-gp) expression on the blood-ocular barriers by studying the ocular penetration of loperamide, a specific P-gp substrate, in P-gp (Mdr1a) knock-out (KO) and wild type (WT) Sprague Dawley rats. A clear, stable, sterile solution of loperamide (1 mg/mL), for intravenous administration, was formulated and evaluated. Ocular distribution was studied in P-gp KO and WT rats following intravenous administration of loperamide (at two doses). The drug levels in plasma, aqueous humor (AH), and vitreous humor (VH) samples were determined with the aid of UHPLC-Q-TOF-MS/MS, and the AH/plasma (D _AH ) and VH/plasma (D _VH ) distribution ratios were estimated. Electroretinography (ERG), ultrastructural analyses, and histology studies were carried out, in both KO and WT rats, to detect any drug-induced functional and/or structural alterations in the retina. Dose-related loperamide levels were observed in the plasma of both WT and KO rats. The loperamide concentrations in the AH and VH of KO rats were significantly higher compared to that observed in the WT rats, at the lower dose. However, a marked increase in the D _AH and D _VH was noted in the KO rats. ERG, ultrastructure, and histology studies did not indicate any drug-induced toxic effects in the retina under the test conditions. The results from these studies demonstrate that P-gp blocks the penetration of loperamide into the ocular tissues from the systemic circulation and that the effect is more pronounced at lower plasma loperamide concentrations.

Retina Compatible Interactions and Effective Modulation of Blood Ocular Barrier P-gp Activity by Third-Generation Inhibitors Improve the Ocular Penetration of Loperamide

Effective drug delivery to the deeper ocular tissues remains an unresolved conundrum mainly due to the expression of multidrug resistance efflux proteins, besides tight junction proteins, in the blood ocular barriers (BOBs). Hence, the purpose of the current research was to investigate the ability of the third-generation efflux protein inhibitors, elacridar (EQ), and tariquidar (TQ), to diminish P-glycoprotein (P-gp) mediated efflux transport of loperamide (LOP), a P-gp substrate, across the BOB in Sprague Dawley rats. Initially, Western blot analysis confirmed the expression of P-gp in the iris-ciliary bodies and the retina choroid in the wild type rats. Next, the ocular distribution of LOP, in the presence and absence of EQ/TQ (at 2 doses), was evaluated. The significantly higher aqueous humor/plasma (D_AH) and vitreous humor (VH)/plasma (D_VH) distribution ratios of LOP in the rats pretreated with EQ or TQ demonstrated effective inhibition of P-gp activity in the BOB. Interestingly, the modulation of P-gp activity by EQ/TQ was more pronounced at the lower dose. The normal functioning and architecture of the retina, as indicated by electroretinography studies, confirmed the cytocompatibility of LOP and EQ/TQ interactions at the doses tested.

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.

Analysis of protein composition of rabbit aqueous humor following two different cataract surgery incision procedures using 2-DE and LC-MS/MS

BACKGROUND

The aqueous humor (AH), a liquid of the anterior and posterior chamber of the eye, comprises many proteins with various roles and important biological functions. Many of these proteins have not been identified yet and their functions in AH are still unknown. Recently, our laboratory published the protein database of AH obtained from healthy rabbits which expanded known protein identifications by 65%. Our present study extends our previous work and analyses AH following two types of cataract surgery incision procedures (clear corneal and limbal incisions) by using two dimensional gel electrophoresis (2-DE) and liquid chromatography tandem mass spectrometry (LC-MS/MS). Although both incision protocols are commonly used during cataract surgeries, the difference in protein composition and their release into AH following each surgery has never been systematically compared and remains unclear. The first step, which is the focus of this work, is to assess the scale of the protein change, at which time does maximum release occurs and when possible, to identify protein changes.

RESULTS

Samples of AH obtained prior to surgery and at different time points (0.5, 2, 12, 24 and 48 hours) following surgery (n = 3/protocol) underwent protein concentration determination, 2-DE and LC-MS/MS. There was a large (9.7 to 31.2 mg/mL) and rapid (~0.5 hour) influx of proteins into AH following either incision with a return to baseline quantities after 12 hours and 24 hours for clear corneal and limbal incision, respectively. We identified 80 non-redundant proteins, and compared to our previous study on healthy AH, 67.5% of proteins were found to be surgery-specific. In addition, 51% of those proteins have been found either in clear corneal (20%) or limbal incision (31%) samples.

CONCLUSIONS

Our results imply that a mechanism of protein release into AH after surgery is a global response to the surgery rather than increase in amount of protective proteins found in healthy AH and a mechanism of protein release for each type of incision procedure could be different. Although the total protein concentration was increased (at 0.5 and 2 hour time points and between types of surgery) many of 2-DE protein spots were similar based on 2-DE and MS analyses, and only a small number of protein spots changed with either the time points or surgical conditions (0.4 -1.9%). This suggests that the high protein content is due to an increase in the concentration of the same proteins with only a few unique proteins being altered per time point and with the different surgery type. This is the first report on the comparison of AH protein composition following two different cataract surgery procedures and it establishes the basis for better understanding of protein release into AH during events such as cataract surgery or other possible intervention to the eyes.

Function and expression of ATP-binding cassette transporters in cultured human Y79 retinoblastoma cells

The aim of this study was to reveal the expression and function of P-glycoprotein and multidrug resistance-associated proteins (MRP), members of the ATP-binding cassette (ABC) superfamily of drug transporters, in cultured human Y79 retinoblastoma cells. ABC transporter mRNA expression was evaluated by conventional reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR analyses. Cellular accumulation of rhodamine 123 (P-glycoprotein substrate), calcein (MRP substrate), and doxorubicin (P-glycoprotein/MRP substrate) was analyzed by fluorometry. Conventional RT-PCR analysis showed the expression of multidrug resistance 1 (MDR1), MRP1, MRP2 and lung resistance-related protein (LRP) mRNAs. Real-time RT-PCR analysis revealed that the expression levels of the MDR1 and MRP2 genes in Y79 cells were much lower than those in human intestinal cell line Caco-2, while the expression level of MRP1 was higher than that in Caco-2 cells. The accumulation of rhodamine 123 was not enhanced by verapamil or reversin 205, inhibitors of P-glycoprotein, indicating no function of P-glycoprotein in Y79 cells. The accumulation of calcein was significantly increased by various MRP inhibitors including probenecid, indicating that MRP functions in Y79 cells. The accumulation of doxorubicin was increased in the presence of metabolic inhibitors (10 mM 2-deoxyglucose and 5 mM sodium azide). However, most MRP inhibitors such as probenecid and indomethacin did not affect doxorubicin accumulation, while cyclosporin A and taclorimus significantly increased doxorubicin accumulation. These results suggest that MRP, but not P-glycoprotein, functions in Y79 cells, and that the efflux of doxorubicin from Y79 cells may be due to an ATP-dependent transporter, which has not been identified yet.

Filter-cultured ARPE-19 cells as outer blood-retinal barrier model

Retinal pigment epithelium (RPE) regulates drug transfer between posterior eye segment and blood circulation, but there is no established RPE cell model for drug delivery studies. We evaluated ARPE-19 filter culture model for this purpose. Passive permeability of 6-carboxyfluorescein, betaxolol and FITC-dextran (40kDa) and active transport of 6-carboxyfluorescein, sodium fluorescein, rhodamine 123, cyclosporine A and digoxin in ARPE-19 model were investigated and compared with isolated bovine RPE-choroid tissue. In addition, barrier properties, and mRNA expression of RPE-specific and melanogenesis-related genes (RPE65, VMD2, CRALBP, OTX-2, MITF-A, TRP-1, tyrosinase) were measured in various culture conditions. The filter grown ARPE-19 cell model showed reasonable barrier properties (TER close to 100Omegacm(2)), but its permeability was slightly higher than that of isolated bovine RPE/choroid specimens. In active transport studies the ARPE-19 model mimics qualitatively the permeability profile of bovine RPE-choroid, but ARPE-19 model underestimates the importance of active transport relative to passive diffusion. Long-term filter-cultured ARPE-19 cells expressed various RPE-specific and melanogenesis-related genes at higher levels than the ARPE-19 cells cultured short-term in flasks. ARPE-19 model can be used to study drug permeation processes in the RPE.

Evaluation of the impact of P-glycoprotein (P-gp) drug efflux transporter blockade on the systemic and ocular disposition of P-gp substrate

PURPOSE

The impact of P-glycoprotein (P-gp) blockade on the intravenous (i.v.) pharmacokinetics of rhodamine-123 (Rho-123), and the subsequent effect on its disposition in ocular and nonocular tissues, was studied by using rabbits.

METHODS

Three (3) control rabbits received only an i.v. bolus dose of Rho-123 (1.52 mg/kg). Three (3) blocker-pretreated rabbits received an i.v. dose of GF120918 (3.5 mg/kg) 30 min before the i.v. bolus of Rho-123. The plasma concentration of Rho-123 at different time points was subjected to a pharmacokinetic compartmental analysis, using WinNonlin (Scientific Consultants, Lexington, KY). For tissue-distribution study, a drug treatment similar to the i.v. kinetic study was followed by having 5 rabbits in each group. The animals were sacrificed at 30 min with an excess of anesthesia. Plasma and tissues samples were analyzed by using a validated high-performance liquid chromatographic IV method with a fluorescent detector.

RESULTS

The method validated was sensitive enough to estimate Rho-123 up to 1.94 ng/mL in plasma. I.v. Rho-123 data fitted well into the three-compartment model, and P-gp blocker treatment changed it into a two-compartment model. The P-gp blockade significantly increased the mean tissue concentrations in the lungs and spleen, whereas the rise in mean tissue levels in the heart, liver, and kidney and in all ocular tissues were found to be statistically insignificant.

CONCLUSIONS

Increasing the ocular concentration of systemically given drugs may not be possible with the degree of P-gp blockade achieved when using GF120918 at the studied concentration after an i.v. administration.

Evaluation of the modulation of P-glycoprotein (P-gp) on the intraocular disposition of its substrate in rabbits

PURPOSE

To evaluate the functional role of P-gp and ocular tissue distribution of intravitreally injected Rhodamine-123 (Rho-123) in the presence of P-gp specific blocker (GF 120918) in normal as well as rifampicin-fed rabbits using microdialysis and direct sampling technique.

METHODS

Intravitreal pharmacokinetics of Rho-123 were conducted in male New Zealand albino rabbits. Direct sampling and microdialysis were employed to study the disposition of Rho-123 in normal as well as rifampicin-fed conditions. Control animals received Rho-123 at the concentration of 350 ng in PBS (0.05 ml) intravitreally, and the blocker-treated group received GF 120918 intravenously at the dose of 3.5 mg/kg 30 min prior to intravitreal injection of Rho-123. In case of direct sampling, four eyes were enucleated at different time points, and ocular tissues and humors were stored at -86 degrees C until analysis by HPLC with fluorescence detection.

RESULTS

In direct sampling, the blocker group showed significant increase (2.6 fold) in the mean vitreous concentration of Rho-123. Other tissues like ret-choroid, iris, and cornea also showed significant increase in their mean concentration. Microdialysis did not significantly predict the changes observed with direct sampling. Rifampicin-fed rabbits showed a vitreous pharmacokinetic profile comparable with non-fed (control) animals, and the pharmacokinetic parameters were unaffected by the blocker pretreatment.

CONCLUSION

Intravenously injected blocker significantly altered the ocular disposition of intravitreally injected P-gp substrate. Rifampicin pretreatment did not upregulate P-gp transporters of the retina to the extent to affect the intravitreal kinetics of Rho-123 significantly.

Effect of benzo[a]pyrene on P-glycoprotein-mediated transport in Caco-2 cell monolayer

The main exposure pathway of benzo[a]pyrene (Bap) for humans is considered to be via the daily diet. The purpose of this study was to investigate the effect of BaP on the intestinal transport of chemicals mediated by P-glycoprotein (P-gp). The intestinal epithelial membrane transport of rhodamine-123 (Rho-123), a substrate of P-gp, was examined using a monolayer of the human Caco-2 cell line grown in transwells. In the monolayer exposed to Bap for 72 h before transport experiments, the ratio of the apparent permeability coefficients (P(app)) of Rho-123 efflux increased compared to that of the control. The permeability of rhodamine-B (Rho-B), not a substrate of P-gp, showed no difference between the monolayers. Treatment with quinidine or cyclosporine A, which are P-gp inhibitors, decreased the P(app) of Rho-123 to the same degree in both monolayers. The transport of Rho-123 was not influenced by the presence of Bap. Thus, Bap seemed not to act directly on the efflux activity of P-gp and be a binding site competitor of Rho-123. In the Caco-2 cells that enhanced the efflux of Rho-123 by the treatment with Bap, an increase in mRNA expression of MDR 1 (P-gp) was confirmed compared to that of control by RT-PCR. Furthermore, Western blot analysis using a monoclonal antibody, C219, demonstrated the increase of P-gp in Caco-2 cells exposed to Bap, compared with controls. It was inferred that Bap exposure induced the expression of P-gp, which led to the observed increase in efflux transport of Rho-123. The possibility was suggested that Bap might affect the disposition of medicines by increasing P-gp expression.

Effect of P-glycoprotein on the ocular disposition of a model substrate, quinidine

PURPOSE

The objective of this study was to determine the effect of the multi-drug efflux transport protein, P-glycoprotein (P-gp), on the ocular distribution of a model substrate, quinidine.

METHODS

Male New Zealand albino rabbits (2-2.5 kg) were employed in these studies. Animals were kept under anesthesia and a concentric microdialysis probe was implanted in the vitreous humor and a linear probe in the anterior chamber. Isotonic phosphate buffered saline was perfused through the probes, and samples were collected every 20 minutes over a period of 10 hours. Quinidine was administered both systemically (5 mg/kg bodyweight) and intravitreally (5.68 microg and 0.568 microg). Inhibition experiments were performed in vivo in the presence of verapamil, which is a known P-gp inhibitor.

RESULTS

Vitreal pharmacokinetic parameters of quinidine in the presence of verapamil, i.e., Area under the curve (AUC) (39.27 +/- 6.47 min. microg/ml), maximum concentration achieved (Cmax) (0.095 +/- 0.011 microg/ml), vitreal elimination half-life (231.96 +/- 10.77 min), vitreal permeation half-life (16.57 +/- 6.96 min) were significantly different from the control values (19.21 +/- 3.73 min. microg/ml, 0.05 +/- 0.008 microg/ml, 165.08 +/- 31.5 min, 43.29 +/- 12.5 min respectively). A significant elevation in anterior chamber Cmax and AUC was also observed in the presence of verapamil. Verapamil had no significant effect on vitreal kinetics of quinidine following intravitreal dose of 5.68 micro g, but a significant difference was observed at a lower dose of quinidine (0.568 microg). A decrease in vitreal elimination half-life and AUC was observed in the presence of verapamil relative to control. Ocular kinetics of fluorescein was studied to ascertain ocular barrier integrity in the presence of verapamil. Western-blot analysis of retina-choroid sections indicates expression of P-gp on rabbit retina-choroid.

CONCLUSION

Results suggest the involvement of a multi drug efflux transporter on the retinal pigment epithelium and neural retina affecting the intraocular kinetics of its substrates following systemic and intravitreal administrations.

Ocular penetration of acyclovir and its peptide prodrugs valacyclovir and val-valacyclovir following systemic administration in rabbits: An evaluation using ocular microdialysis and LC-MS

PURPOSE

To investigate the ocular penetration of acyclovir and its prodrugs following systemic administration and to elucidate the mechanism of penetration.

METHODS

Hydrophilic peptide prodrugs of acyclovir were infused intravenously in New Zealand albino rabbits over 45 min at a dose equivalent to 30 mmoles/kg acyclovir. Aqueous and vitreous humor samples were obtained utilizing ocular microdialysis and blood samples were obtained from the mid ear vein using a cannula.

RESULTS

The plasma bioavailability for acyclovir, valacyclovir and val-valacyclovir were similar with area under curve values being 896.24 (+/-143.58), 776.54 (+/-197.52), 824.69 (+/-217.43) min x micromoles/L respectively. Anterior segment area under curve values were 53.70 (+/-35.58), 139.85 (+/-9.43) and 291.05 (+/-88.13) min x micromoles/L respectively while the mean residence time values were 46.47 (+/-24.94), 76.30 (+/-7.24) and 188.39 (+/-80.73) min respectively. Vitreous levels of the prodrugs were not measurable.

CONCLUSIONS

The valine and valine-valine ester prodrugs of ACV penetrated the anterior segment of the eye much better than acyclovir alone, probably via a carrier mediated transport mechanism.

Expression and function of P-glycoprotein in rats with carbon tetrachloride-induced acute hepatic failure

Acute hepatic failure was induced experimentally in rats by intraperitoneal injection of 2.5 mL kg(-1) carbon tetrachloride (CCl4), and the effects on the expression and function of P-glycoprotein in the liver, kidney and brain were evaluated. The CCl4 injection significantly increased the indicators of hepatic function (glutamate oxaloacetate transaminase, glutamate pyruvate transaminase), but not of renal function (blood urea nitrogen, glomerular filtration rate). In rats with acute hepatic failure, the hepatic P-glycoprotein concentration increased 1.5-fold and the ATP concentration decreased to approximately 40% that in control rats. In contrast, P-glycoprotein concentrations in the kidney and brain and ATP concentrations in the kidney remained unchanged. The in-vivo P-glycoprotein function in these tissues was suppressed as evaluated by biliary and renal secretory clearances and brain distribution of rhodamine 123, a P-glycoprotein substrate. These findings suggest that factors other than P-glycoprotein concentration are involved in the systemic suppression of P-glycoprotein function in diseased rats. In Caco-2 cells, plasma collected from CCl4-treated rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of rhodamine 123 than that from control rats, suggesting the accumulation of an endogenous P-glycoprotein substrate/inhibitor in the plasma of diseased rats. In fact, the plasma concentration of corticosterone, an endogenous P-glycoprotein substrate, increased 2-fold in CCl4-treated rats compared with control rats. It was demonstrated that P-glycoprotein function is systemically suppressed in rats with CCl4-induced acute hepatic failure, not only in the target organ (liver), but also in other organs (kidney and brain), although the P-glycoprotein concentration remained unchanged in the kidney and brain, and increased in the liver. In the systemic suppression of the P-glycoprotein function in the diseased state, the alteration of plasma concentrations or components of endogenous P-glycoprotein-related compounds, such as corticosterone, would likely be involved.

Expression and function of P-glycoprotein in rats with glycerol-induced acute renal failure

The effect of glycerol-induced acute renal failure on P-glycoprotein expression and function was evaluated in rats. The in vivo function of P-glycoprotein was evaluated by measuring renal secretory and biliary clearance and brain distribution of rhodamine 123 (Rho-123), a P-glycoprotein substrate, under a steady-state plasma concentration. In acute renal failure rats, the P-glycoprotein level increased 2.5-fold in the kidney, but not in the liver and brain. In contrast, P-glycoprotein function in these tissues was suppressed. Interestingly, not only the renal but also the biliary clearance of Rho-123 was correlated with the glomerular filtration rate. In Caco-2 cells, plasma from renal failure rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of Rho-123 than did plasma from control rats. In conclusion, P-glycoprotein function was systemically suppressed in acute renal failure, even though the level of P-glycoprotein remained unchanged or rather increased. This may be due to the accumulation of some endogenous P-glycoprotein substrates/modulators in the plasma in disease states.