Cotransport of Macrolide and Fluoroquinolones, a Beneficial Interaction Reversing P-glycoprotein Efflux

A Unique In Vitro Assay to Investigate ABCB4 Transport Function

ABCB4 is almost exclusively expressed in the liver, where it plays an essential role in bile formation by transporting phospholipids into the bile. ABCB4 polymorphisms and deficiencies in humans are associated with a wide spectrum of hepatobiliary disorders, attesting to its crucial physiological function. Inhibition of ABCB4 by drugs may lead to cholestasis and drug-induced liver injury (DILI), although compared with other drug transporters, there are only a few identified substrates and inhibitors of ABCB4. Since ABCB4 shares up to 76% identity and 86% similarity in the amino acid sequence with ABCB1, also known to have common drug substrates and inhibitors, we aimed to develop an ABCB4 expressing Abcb1-knockout MDCKII cell line for transcellular transport assays. This in vitro system allows the screening of ABCB4-specific drug substrates and inhibitors independently of ABCB1 activity. Abcb1KO-MDCKII-ABCB4 cells constitute a reproducible, conclusive, and easy to use assay to study drug interactions with digoxin as a substrate. Screening a set of drugs with different DILI outcomes proved that this assay is applicable to test ABCB4 inhibitory potency. Our results are consistent with prior findings concerning hepatotoxicity causality and provide new insights for identifying drugs as potential ABCB4 inhibitors and substrates.

Pharmacokinetic evaluation of roxithromycin and ciprofloxacin in treating complicated avian mycoplasmosis in broiler chickens

The use of roxithromycin along with ciprofloxacin is having potential to be a promising antimicrobial therapy to treat complicated avian mycoplasmosis in broiler chickens. The present research was undertaken to study the influence of roxithromycin (20 mg/kg body weight) and ciprofloxacin (10 mg/kg body weight) on the oral pharmacokinetics of each other, when both drugs are concomitantly administered in eight healthy male broiler chickens (n=8) and to establish their therapeutic dosage regimens. Their plasma concentrations were assayed by validated ultra high performance liquid chromatography (UHPLC) methods using UV detector. Oral pharmacokinetic parameters were calculated from plasma concentration versus time data based on non-compartmental analysis. Statistically, plasma roxithromycin concentration was significantly higher at one time point only (0.5 h) and plasma ciprofloxacin concentration was significantly lower at the time point of 2 h only when used in combination, in comparison to their respective values obtained after their alone administrations. The pharmacokinetic parameters of roxithromycin and ciprofloxacin showed no significant effect on values of either drug when given in combination and there was a lack of pharmacokinetic interaction between the two antimicrobials. The predicted effective oral dose rate of roxithromycin was 20 mg/kg body weight, every 12 h, and that of ciprofloxacin was 10 mg/kg body weight, every 24 h, to treat complicated avian mycoplasmosis in broiler chickens.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Potential pharmacokinetic effect of rifampicin on enrofloxacin in broilers: Roles of P-glycoprotein and BCRP induction by rifampicin

P-glycoprotein ( P-GP: , encoding gene Abcb1) and Breast Cancer Resistance Protein ( BCRP: , encoding gene Abcg2) are transport proteins that play a major role in modulating the bioavailability of oral drugs in humans and rodents. It has been shown that rifampicin is the typical inducer of P-gp in rodents by activating the nuclear receptor. However, its effect on Abcb1, Abcg2, CYP3A, and chicken xenobiotic-sensing orphan nuclear receptor ( CXR: ) mRNA expression in broilers is poorly understood. This study explored the effect of rifampicin on mRNA expression of Abcb1, Abcg2, CYP3A37, CXR as well as its effect on the pharmacokinetics of enrofloxacin in broilers. The mRNA levels of Abcb1, Abcg2, CYP3A37, and CXR were significantly increased in the liver (except Abcg2), kidney, jejunum, and ileum (P < 0.05) but not significantly changed in the duodenum (P > 0.05) after treated with rifampicin. Further analysis revealed that the variation tendencies of Abcb1, Abcg2, and CYP3A37 expression levels were significantly correlated with CXR mRNA expression levels in liver, kidney, jejunum, and ileum. Coadministration of rifampicin significantly changed the pharmacokinetic behavior of enrofloxacin orally administered by showing clearly lower AUC0-∞, AUC0-t, and Cmax as well as longer Tmax. The bioavailability of orally administered enrofloxacin was decreased from 72.5% to 24.8% by rifampicin. However, rifampicin did not significantly change the pharmacokinetics of enrofloxacin following intravenous administration. Our study shows that rifampicin up-regulated the small intestinal level of P-gp and BCRP and suggests that P-gp and BCRP are key factors that affected pharmacokinetic behavior of orally administered enrofloxacin by limiting its absorption from the intestine in broilers.

Retrospective investigation of combination therapy with clarithromycin and levofloxacin for pulmonary Mycobacterium avium complex disease

Background

Fluoroquinolones are often used for the treatment of refractory Mycobacterium avium complex (MAC) disease when the clinical efficacy of the recommended regimen, which includes clarithromycin (CAM), rifampicin (RFP), and ethambutol (EB), is insufficient. However, recent in vitro and in vivo studies have suggested that fluoroquinolones decreased the antibacterial activity of CAM when they were administered in combination. In this study, we retrospectively investigated the influence of the combination of CAM and levofloxacin (LVFX) on clinical outcomes for pulmonary MAC disease patients.

Methods

Pulmonary MAC disease patients from 2010 to 2012 were divided into two groups, those who received LVFX together with CAM (LVFX group) and those who received CAM without LVFX (control group). The number of patients who showed improvement was evaluated at 1, 3, 6 and 12 months after the start of therapy based on bacteriological examination (culture and smear examination) and the bacilli negative conversion rate.

Results

There were no significant differences between the LVFX group (n = 18, 64.5 ± 6.5 years old) and the control group (n = 57, 71.0 ± 7.0 years old) in terms of gender, age, etiologic agent, baseline culture examination score, concomitant medication, and dosage of each drug. The clinical outcomes in the LVFX group were inferior to those in the control group at all endpoints and observational periods, and we found a significant difference in the percent improvement of the smear examination by fluorescence microscopy method (38 % vs. 83 %) and the bacilli negative conversion rate (38 % vs. 79 %) at 3 months. Our study suggests that the combination of CAM and LVFX causes unfavorable clinical outcomes for pulmonary MAC disease treatment. There was no significant difference between both groups in terms of frequency of adverse events.

Conclusion

The possibility that combined administration of CAM and LVFX causes unfavorable clinical outcomes for pulmonary MAC disease treatment was suggested.

Interaction Studies of Resolvin E1 Analog (RX-10045) with Efflux Transporters

PURPOSE

Screening interactions of a resolvin E1 analog (RX-10045) with efflux transporters (P-glycoprotein [P-gp], multidrug resistance-associated protein [MRP2], and breast cancer-resistant protein [BCRP]).

METHODS

Madin-Darby canine kidney cells transfected with P-gp, MRP2, and BCRP genes were selected for this study. [3H]-Digoxin, [3H]-vinblastine and [3H]-abacavir were selected as model substrates for P-gp, MRP2, and BCRP. Uptake and permeability studies across cell monolayer in both apical to basal (AP-BL) and BL-AP of these substrates were conducted in the presence of specific efflux pump inhibitors and RX-10045. Cell viability studies were conducted with increasing concentrations of RX-10045.

RESULTS

Uptake studies showed a higher accumulation in the presence of inhibitors (GF120918 and ketoconazole for P-gp; MK571 for MRP2; and β-estradiol for BCRP) as well as RX-10045. Similarly, dose-dependent inhibition studies demonstrated higher accumulation of various substrates ([3H]-digoxin, [3H]-vinblastine, and [3H]-abacavir) in the presence of RX-10045. IC50 values of dose-dependent inhibition of RX-10045 for P-gp, MRP2, and BCRP were 239±11.2, 291±79.2, and 300±42 μM, respectively. Cell viability assay indicated no apparent toxicity up to 350 μM concentration. Enhanced permeability for model substrates was observed in the presence of RX-10045. Uptake studies in human corneal epithelial cells suggest that RX-10045 is a strong inhibitor of organic cation transporter-1 (OCT-1).

CONCLUSIONS

In summary, the resolvin analog (RX-10045) was identified as a substrate/inhibitor for efflux transporters (MRP2 and BCRP). Also, RX-10045 appears to be a strong inhibitor/substrate of OCT-1. Novel formulation strategies such as nanoparticles, nanomicelles, and liposomes for circumventing efflux barriers and delivering higher drug concentrations leading to a higher therapeutic efficacy may be employed.

Innovative in vitro method to predict rate and extent of drug delivery to the brain across the blood-brain barrier

The relevant parameters for predicting rate and extent of access across the blood-brain barrier (BBB) are fu,plasma (unbound fraction in plasma), Vu,brain (distribution volume in brain) and Kp,uu,brain (ratio of free concentrations in plasma and brain). Their estimation still requires animal studies and in vitro low throughput experiments which make difficult the screening of new CNS candidates. The aim of the present work was to develop a new whole in vitro high throughput method to predict drug rate and extent of access across the BBB. The system permits estimation of fu,plasma, Vu,brain and Kp,uu,brain in a single experimental system, using in vitro cell monolayers in different conditions. From the ratios of the apparent permeability values (Papp) with the adequate mathematical analysis the relevant parameters can be estimated. Papp of ten model compounds has been obtained in MDCKII and MDCK-Mdr1cell monolayers in the absence and presence of albumin and brain homogenate. The ratio of Papp in the absence and presence of albumin allows estimation of in vitro fu,plasma. Papp in the presence of brain homogenate is used to estimate fu,brain and Vu,brain. Kp,uu,brain is estimated from the apical to basal versus basal to apical clearances. The BBB parameters obtained with the new method were predictive of the in vivo behavior of candidates. In vitro fu,plasma, Kp,uu,brain and Vu,brain (calculated with Papp from MDCKII cell line) presented a good correlation with in vivo fu,plasma, Kp,uu,CSF and Vu,brain published values (r=0.92; r=0.85; and r=0.99 respectively). Despite its simplicity the predictive performance is fairly good considering the reduced number of tested compounds with different physicochemical and transport properties. Further experimental modifications could be checked to optimize the method, but the present data support its feasibility. As other in vitro cell culture models, the system is suitable for miniaturization and robotization to allow high throughput screening of CNS candidates.

Differential effect of P-gp and MRP2 on cellular translocation of gemifloxacin

Fluoroquinolones are broad spectrum antibiotics widely indicated in the treatment of both human and animal diseases. The primary objective of this study was to assess short and long term affinities of gemifloxacin towards efflux transporters (P-gp, MRP2) and nuclear hormone receptor (PXR). Uptake and dose dependent inhibition studies were performed with [(14)C] erythromycin (0.25 μCi/ml) on MDCKII-MDR1 and MDCKII-MRP2 cells. Cellular accumulation of calcein-AM was further determined to confirm the affinity of gemifloxacin towards P-gp and MRP2. Transport studies were conducted to determine bi-directional permeability and to assess efflux ratio of gemifloxacin. LS-180 cells were treated with three different concentrations of gemifloxacin for 72 h and real-time PCR analysis was performed to study the quantitative gene expression levels of PXR, MDR1 and MRP2. Further, [(14)C] erythromycin uptake was also performed on LS-180 treated cells to better delineate the functional activity of efflux transporters. Results from our study suggest that gemifloxacin may be a substrate of both the efflux transporters studied. This compound inhibited both P-gp and MRP2 mediated efflux of [(14)C] erythromycin in a dose dependent manner with IC(50) values of 123 ± 2 μM and 16 ± 2 μM, respectively. The efflux ratio of [(14)C] erythromycin lowered from 3.56 to 1.63 on MDCKII-MDR1 cells and 4.93 to 1.26 on MDCKII-MRP2 cells. This significant reduction in efflux ratio further confirmed the substrate specificity of gemifloxacin towards P-gp and MRP2. Long term exposure significantly induced the expression of PXR (18 fold), MDR1 (6 fold) and MRP2 (6 fold). A decrease (20%) in [(14)C] erythromycin uptake further confirmed the elevated functional activity of P-gp and MRP2. In conclusion, our studies demonstrated that gemifloxacin is effluxed by both P-gp and MRP2. Long term exposure induced their gene expression and functional activity. This substrate specificity of gemifloxacin towards these efflux transporters may be one of the major factors accounting for low oral bioavailability (71%). Better understanding of these mechanistic interactions may aid in the development of newer strategies to achieve adequate therapeutic levels and higher bioavailability.

Modulation of drug block of the cardiac potassium channel KCNA5 by the drug transporters OCTN1 and MDR1

BACKGROUND AND PURPOSE

A common site for drug binding on voltage-gated ion channels is at the interior face of the channel pore. In this study, we tested the hypothesis that the extent of drug block of the human cardiac KCNA5 (K(v) 1.5) channel underlying the atrial-specific, ultra-rapidly activating, delayed K(+) current (I(Kur) ) is modulated by the drug uptake and efflux transporters encoded by organic cation transporter 1 (OCTN1) and multiple drug-resistant gene 1 (MDR1) and expressed in human heart.

EXPERIMENTAL APPROACH

Drug block of KCNA5 was assessed in Chinese hamster ovary cells transiently transfected with KCNA5 alone or in combination with the OCTN1 or MDR1 transporter construct, as well as in an MDR1 stably expressed cell line.

KEY RESULTS

Co-expression of OCTN1 significantly facilitated block by quinidine (10 µM), verapamil (20 µM), propafenone (5 µM) and clofilium (30 µM). Further evidence of drug transport modulating drug block was the finding that with OCTN1, block developed faster and only partially washed-out, and that block potentiation was prevented by cimetidine, an inhibitor of OCTN1. MDR1 expression attenuated KCNA5 block by erythromycin (an MDR1 substrate). Block was restored by reversin-205 (10 µM, an MDR1 inhibitor). MDR1 did not affect KCNA5 inhibition by KN-93 (1 µM), a blocker acting on the outer mouth of the channel pore.

CONCLUSIONS AND IMPLICATIONS

The extent of drug block of KCNA5 can be modulated by drug uptake and efflux transporters. These data provide further support for the idea that modifying intracellular drug concentrations could modulate the effects of blocking ion channels in patients.

Effect of HEPES buffer on the uptake and transport of P-glycoprotein substrates and large neutral amino acids

HEPES has been widely employed as an organic buffer agent in cell culture medium as well as uptake and transport experiments in vitro. However, concentrations of HEPES used in such studies vary from one laboratory to another. In this study, we investigated the effect of HEPES on the uptake and bidirectional transport of P-gp substrates employing both Caco-2 and MDCK-MDR1 cells. ATP-dependent uptake of glutamic acid was also examined. ATP production was further quantified applying ATP Determination Kit. An addition of HEPES to the growth and incubation media significantly altered the uptake and transport of P-gp substrates in both Caco-2 and MDCK-MDR1 cells. Uptake of P-gp substrates substantially diminished as the HEPES concentration was raised to 25 mM. Bidirectional (A-B and B-A) transport studies revealed that permeability ratio of P(appB-A) to P(appA-B) in the presence of 25 mM HEPES was significantly higher than control. The uptake of phenylalanine is an ATP-independent process, whereas the accumulation of glutamic acid is ATP-dependent. While phenylalanine uptake remained unchanged, glutamic acid uptake was elevated with the addition of HEPES. Verapamil is an inhibitor of P-gp mediated uptake; elevation of cyclosporine uptake in the presence of 5 muM verapamil was compromised by the presence of 25 mM HEPES. The results of ATP assay indicated that HEPES stimulated the production of ATP. This study suggests that the addition of HEPES in the medium modulated the energy dependent efflux and uptake processes. The effect of HEPES on P-gp mediated drug efflux and transport may provide some mechanistic insight into possible reasons for inconsistencies in the results reported from various laboratories.

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.

Fluoroquinolone efflux mediated by ABC transporters

Quinolones and fluoroquinolones are broad spectrum bactericidal drugs, which are widely used in both human and veterinary medicine. These drugs can quite easily enter cells and are often used to treat intracellular pathogens. Some fluoroquinolones have been reported to undergo efflux, which could explain their low bioavailability. There is a growing need to understand resistance mechanisms to quinolones, involving for instance mutations or the action of efflux pumps. Several members of the ATP-binding cassette (ABC) drug efflux transporter family (MDR, MRP, ABCG2) significantly affect the pharmacokinetic disposition of quinolones. Active secretory mechanisms common to all fluoroquinolones have been suggested, as well as competition between fluoroquinolones at transporter sites. For grepafloxacin and its metabolites, MRP2 has been demonstrated to mediate biliary excretion. However, MDR1 is responsible for grepafloxacin intestinal secretion. Recently it has been shown that ciprofloxacin and enrofloxacin are efficiently transported ABCG2 substrates which are actively secreted into milk. It appears that multiple ABC transporters contribute to the overall secretion of fluoroquinolones. The objective of this work is to review the recent advances in insights into ABC transporters and their effects on fluoroquinolone disposition and resistance including data on drug secretion into milk.

Modulation of the cellular accumulation and intracellular activity of daptomycin towards phagocytized Staphylococcus aureus by the P-glycoprotein (MDR1) efflux transporter in human THP-1 macrophages and madin-darby canine kidney cells

P-glycoprotein (P-gp; MDR1), a major efflux transporter, recognizes various antibiotics and is present in macrophages. We have examined its effect on the modulation of the intracellular accumulation and activity of daptomycin towards phagocytized Staphylococcus aureus (ATCC 25923) in human THP-1 macrophages, in comparison with MDCK epithelial cells (wild type and MDCK-MDR1 overexpressing P-gp; the bulk of the protein was immunodetected at the surface of all three cell types). Daptomycin displayed concentration-dependent intracellular activity (Hill equation pattern) in THP-1 and MDCK cells with (i) 50% effective drug extracellular concentration (EC(50); relative potency) and static concentrations at 9 to 10 times the MIC and (ii) maximal efficacy (E(max); CFU decrease at infinite extracellular drug concentration) at 1.6 to 2 log compared to that of the postphagocytosis inoculum. Verapamil (100 microM) and elacridar (GF 120918; 0.5 microM), two known inhibitors of P-gp, decreased daptomycin EC(50) (about threefold) in THP-1 and MDCK cells without affecting E(max). Daptomycin EC(50) was about three- to fourfold higher and accumulation in MDCK-MDR1 commensurately lower than in wild-type cells. In THP-1 macrophages, (i) verapamil and ATP depletion increased, and ouabain (an inducer of mdr1 [the gene encoding P-gp] expression) decreased the accumulation of daptomycin in parallel with that of DiOC(2) (a known substrate of P-gp); (ii) silencing mdr1 with duplex human mdr1 siRNAs reduced the cell content in immunoreactive P-gp to 15 to 30% of controls and caused an eight- to 13-fold increase in daptomycin accumulation. We conclude that daptomycin is subject to efflux from THP-1 macrophages and MDCK cells by P-gp, which reduces its intracellular activity against phagocytized S. aureus.

Functional studies on the activity of efflux transporters in an ex vivo model with chicken splenocytes and evaluation of selected fluoroquinolones in this model

The efflux proteins P-glycoprotein (P-gp), BCRP and members of the MRP-family (MRPs) are increasingly recognized as determinants of the absorption, tissue distribution and excretion of numerous drugs. A widely applied in vitro screening method, to assess the effect of these efflux transporters in transmembrane transport of drugs is based on the use of peripheral blood mononuclear cells (PBMC), in which the efflux of fluorescent dye Rhodamine 123 (Rh-123) can be easily measured. In avian species, the isolation of PBMCs is compromised by the presence of thrombocytes having approximately the same size. As an alternative, we validated the use of isolated splenocytes to assess Rhodamine 123 transport in the presence and absence of specific inhibitors for P-gp, MRPs and BCRP. Rh-123 efflux was concentration-dependent with the percentage of efflux that decreased with increasing concentrations. P-gp inhibitors, PSC833 and GF120918, significantly inhibit Rh-123 efflux, whereas inhibitors for MRPs and BCRP, MK571 and Ko-143, respectively, have a limited inhibitory effect. However, the effect of GF120918 was more pronounced as compared to PSC833, suggesting an additional role for BCRP next to P-gp in Rh-123 efflux. Moreover, fluoroquinolones were selected to test the applicability of the described model. None of these fluoroquinolones significantly inhibit P-gp function at concentrations up to 50 microM, with exception of danofloxacin and danofloxacin mesylate that were found to reduce Rh-123 efflux by approximately 15%.

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.