Drug Efflux Transport Properties of 2′,7′-Bis(2-carboxyethyl)-5(6)-carboxyfluorescein Acetoxymethyl Ester (BCECF-AM) and Its Fluorescent Free Acid, BCECF

Differential Action of Silver Nanoparticles on ABCB1 (MDR1) and ABCC1 (MRP1) Activity in Mammalian Cell Lines

Silver nanoparticles (AgNPs), due to their unique properties have been receiving immense attention in recent years. In addition to their antibacterial and antifungal activities, AgNPs also cause apoptosis, mitochondria disfunction, nucleic acid damage and show potent anticancer properties in both multidrug resistance (MDR) and sensitive tumors. The MDR phenomenon, caused by the presence of ATP-binding cassette (ABC) proteins, is responsible for the failure of chemotherapy. Thus, investigating the influence of widely used AgNPs on ABC transporters is crucial. In the present study, we have examined the cytotoxicity of silver nanoparticles of a nominal size of 20 nm (Ag20) on the cell lines of different tissue origins. In addition, we have checked the ATP-binding cassette transporters’ activity and expression under AgNP exposure. The results indicate that Ag20 shows a toxic effect on tested cells, as well as modulating the expression and transport activity of ABC proteins.

Cost-Effective Real-Time Metabolic Profiling of Cancer Cell Lines for Plate-Based Assays

A fundamental phenotype of cancer cells is their metabolic profile, which is routinely described in terms of glycolytic and respiratory rates. Various devices and protocols have been designed to quantify glycolysis and respiration from the rates of acid production and oxygen utilization, respectively, but many of these approaches have limitations, including concerns about their cost-ineffectiveness, inadequate normalization procedures, or short probing time-frames. As a result, many methods for measuring metabolism are incompatible with cell culture conditions, particularly in the context of high-throughput applications. Here, we present a simple plate-based approach for real-time measurements of acid production and oxygen depletion under typical culture conditions that enable metabolic monitoring for extended periods of time. Using this approach, it is possible to calculate metabolic fluxes and, uniquely, describe the system at steady-state. By controlling the conditions with respect to pH buffering, O2 diffusion, medium volume, and cell numbers, our workflow can accurately describe the metabolic phenotype of cells in terms of molar fluxes. This direct measure of glycolysis and respiration is conducive for between-runs and even between-laboratory comparisons. To illustrate the utility of this approach, we characterize the phenotype of pancreatic ductal adenocarcinoma cell lines and measure their response to a switch of metabolic substrate and the presence of metabolic inhibitors. In summary, the method can deliver a robust appraisal of metabolism in cell lines, with applications in drug screening and in quantitative studies of metabolic regulation.

Using a qPCR device to screen for modulators of ABC transporter activity: A step-by-step protocol

INTRODUCTION

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are transmembrane proteins which actively transport a large variety of substrates across biological membranes. ABC transporter overexpression can be the underlying cause of multidrug resistance in oncology. Moreover, it has been revealed that increased ABCC1 transporter activity can ameliorate behavioural changes and Aβ pathology in a rodent model of Alzheimer's disease and it is currently tested in AD patients.

METHODS

Finding substances that modulate ABC transporter activity (inhibitors and activators) is of high relevance and thus, different methods have been developed to screen for potential modulators. For this purpose, we have developed a cell-based assay to measure the kinetics of ABCC1-mediated efflux of a fluorescent dye using a common qPCR device (Agilent AriaMx).

RESULTS

We validated the specificity of our method with vanadate and benzbromarone controls. Furthermore, we provide a step-by-step protocol including statistical analysis of the resulting data and suggestions how to modify the protocol specifically to screen for activators of ABCC1.

DISCUSSION

Our approach is biologically more relevant than cell-free assays. The continuous detection of kinetics allows for a more precise quantification compared with assays with single end-point measurements.

Increased membrane permeation and blood concentration of 6-carboxyfluorescein associated with dysfunction of paracellular route barrier in the small intestine of ulcerative colitis model rats

In the colon of patients with ulcerative colitis (UC), decreased function of the paracellular barrier, especially hypofunction of the tight junction, is associated with pathological conditions. However, there has been no report to date on the function of tight junctions in the small intestine. Here, we focused on the barrier function of the small intestine, especially in tight junctions, and compared it with that of the colon. Dextran sulfate sodium (DSS) was used to induce ulcerative colitis in rats in order to evaluate the function of the paracellular barrier in the jejunum, ileum, and colon. An in vitro diffusion chamber method was used to evaluate membrane resistance, which is an index of tight junction function and mucosal permeability, using 6-carboxyfluorescein (6-CF), a paracellular marker. In the jejunum and colon, with decrease of membrane resistance in the DSS group, mucosal permeability increased, whereas no marked difference was observed in the ileum. In the in situ closed-loop method, absorption of 6-CF from the jejunum was higher than that from the ileum. Immunohistochemical staining of claudin-4 showed heterogeneous attenuation of claudin-4 in the jejunum. Pharmacokinetic parameters were calculated from the blood concentration after intravenous injection and oral administration of 6-CF. In the DSS group, there was a delay in the elimination phase, suggesting a decrease in renal function, and an increase in maximum blood concentration, associated with an increased absorption rate constant. The increased absorption and decreased renal function due to decreased paracellular barrier function in the small intestine and colon may cause fluctuations in drug efficacy and side effects.

Assessment of HBEC-5i endothelial cell line cultivated in astrocyte conditioned medium as a human blood-brain barrier model for ABC drug transport studies

Endothelial cells are main components of the Blood-Brain Barrier (BBB) and form a tight monolayer that regulates the passage of molecules, with the ATP-Binding Cassette (ABC) transporters efflux pumps. We have developed a human in vitro model of HBEC-5i endothelial cells cultivated alone or with human astrocytes conditioned medium on insert. HBEC-5i cells showed a tight monolayer within 14 days, expressing ZO-1 and claudin 5, a low apparent permeability to small molecules, with a TEER stability during five days. The P-gp, BCRP, MRPs transporters were well expressed and functional. Accumulation and efflux ratio measurement with different ABC transporters substrates (Rhodamine 123, BCECF AM, Hoechst 33342) and inhibitors (verapamil, Ko143, probenecid and cyclosporin A) were conducted. At barrier level, the functionality of ABC transporters was three-fold enhanced in astrocyte conditioned medium. We validated our model by the transport of pharmacological substrates: caffeine, rivaroxaban, and methotrexate. The rivaroxaban and methotrexate were released with an efflux ratio >3 and were decreased by more than half with inhibitors. HBEC-5i model could be used as relevant tool in preclinical studies for assessing the permeability of therapeutic molecules to cross human BBB.

Pharmacological Characterization of the RPMI 2650 Model as a Relevant Tool for Assessing the Permeability of Intranasal Drugs

The RPMI 2650 cell line has been described as a potent model of the human nasal mucosa. Nevertheless, pharmacological data are still insufficient, and the role of drug efflux transporters has not been fully elucidated. We therefore pursued the pharmacological characterization of this model, initially investigating the expression of four well-known adenosine triphosphate [ATP]-binding cassette (ABC) transporters (P-glycoprotein (P-gp), multidrug resistance associated protein (MRP)1, MRP2, and breast cancer resistance protein (BCRP)) by means of ELISA and immunofluorescence staining. The functional activity of the selected transporters was assessed by accumulation studies based on specific substrates and inhibitors. We then performed standardized bidirectional transport experiments under air-liquid interface (ALI) culture conditions, using four therapeutic compounds of local intranasal relevance in upper airway diseases. Protein expression of P-gp, MRP1, MRP2, and BCRP was detected at the membrane of the RPMI 2650 cells. In addition, all four transporters exhibited functional activity at the cellular level. In the bidirectional transport experiments, the RPMI 2650 model was able to accurately discriminate the four therapeutic compounds according to their physicochemical properties. The ABC transporters tested did not play a major role in the efflux of these compounds at the barrier level. In conclusion, the RPMI 2650 model represents a promising tool for assessing the nasal absorption of drugs on the basis of preclinical pharmacological data.

Neuroprotective efficacy of a new brain-penetrating C-Abl inhibitor in a murine Parkinson's disease model

Experimental evidence suggests that oxidative and nitrative mechanisms account for much of the dopaminergic neuronal injury in Parkinson’s disease (PD). The ubiquitously expressed non-receptor tyrosine kinase c-Abl is activated by oxidative stress and thus, may play a role in redox-mediated neurodegeneration. Recently, we reported that c-Abl is activated in PD and that a c-Abl inhibitor mitigated neuronal damage in a PD animal model, suggesting a novel neuroprotective therapeutic approach. In the studies presented here, we evaluated the efficacy of a potent and clinically relevant second-generation irreversible Abl kinase inhibitor, INNO-406, as a therapeutic agent for PD. Our studies reveal that INNO-406 is capable of preventing the progression of dopaminergic neuronal damage in a toxin-induced C57 mouse model of PD. Using bovine brain microvessel endothelium as an in vitro blood-brain barrier (BBB) model, we detected rapid and significant transfer of INNO-406. Additionally, pharmacokinetic analyses demonstrated significant nanomolar concentrations of INNO-406 in brain in the presence or absence of MPTP administration, however, INNO-406 did not alter the brain levels of MPP+ in MPTP-treated mice. Finally, we showed that 10 mg/kg of INNO-406 given to C57 mice for one week before MPTP treatment (4×20 mg/kg i.p., every 2 h) and then for one week after MPTP treatment decreased the loss of dopamine in the striatum by 45% and the loss of TH+ neurons in substantia nigra pars compacts by 40%. This treatment regimen also abrogated activation of c-Abl, tyrosine phosphorylation of the Abl substrate and E3-ubiquitin ligase parkin, and accumulation of the toxic parkin substrate AIMP2. We propose that compounds of the INNO-406 class of Abl inhibitors will be useful new neuroprotective drugs for the treatment of PD-like pathology in preclinical systems that should be easily translated to the clinic.

A prediction model of substrates and non-substrates of breast cancer resistance protein (BCRP) developed by GA-CG-SVM method

Breast cancer resistance protein (BCRP) is one of the key multi-drug resistance proteins, which significantly influences the therapeutic effects of many drugs, particularly anti-cancer drugs. Thus, distinguishing between substrates and non-substrates of BCRP is important not only for clinical use but also for drug discovery and development. In this study, a prediction model of the substrates and non-substrates of BCRP was developed using a modified support vector machine (SVM) method, namely GA-CG-SVM. The overall prediction accuracy of the established GA-CG-SVM model is 91.3% for the training set and 85.0% for an independent validation set. For comparison, two other machine learning methods, namely, C4.5 DT and k-NN, were also adopted to build prediction models. The results show that the GA-CG-SVM model is significantly superior to C4.5 DT and k-NN models in terms of the prediction accuracy. To sum up, the prediction model of BCRP substrates and non-substrates generated by the GA-CG-SVM method is sufficiently good and could be used as a screening tool for identifying the substrates and non-substrates of BCRP.

Silver nanoparticle induced blood-brain barrier inflammation and increased permeability in primary rat brain microvessel endothelial cells

The current report examines the interactions of silver nanoparticles (Ag-NPs) with the cerebral microvasculature to identify the involvement of proinflammatory mediators that can increase blood-brain barrier (BBB) permeability. Primary rat brain microvessel endothelial cells (rBMEC) were isolated from adult Sprague-Dawley rats for an in vitro BBB model. The Ag-NPs were characterized by transmission electron microscopy (TEM), dynamic light scattering, and laser Doppler velocimetry. The cellular accumulation, cytotoxicity (6.25-50 μg/cm(3)) and potential proinflammatory mediators (interleukin [IL]-1β, IL-2, tumor necrosis factor [TNF] α, and prostaglandin E(2) [PGE(2)]) of Ag-NPs (25, 40, or 80 nm) were determined spectrophotometrically, cell proliferation assay (2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide) and ELISA. The results show Ag-NPs-induced cytotoxic responses at lower concentrations for 25 and 40 nm when compared with 80-nm Ag-NPs. The proinflammatory responses in this study demonstrate both Ag-NPs size and time-dependent profiles, with IL-1B preceding both TNF and PGE(2) for 25 nm. However, larger Ag-NPs (40 and 80 nm) induced significant TNF responses at 4 and 8 h, with no observable PGE(2) response. The increased fluorescein transport observed in this study clearly indicates size-dependent increases in BBB permeability correlated with the severity of immunotoxicity. Together, these data clearly demonstrate that larger Ag-NPs (80 nm) had significantly less effect on rBMEC, whereas the smaller particles induced significant effects on all the end points at lower concentrations and/or shorter times. Further, this study suggests that Ag-NPs may interact with the cerebral microvasculature producing a proinflammatory cascade, if left unchecked; these events may further induce brain inflammation and neurotoxicity.

Up-regulation of P-glycoprotein by HIV protease inhibitors in a human brain microvessel endothelial cell line

A major concern regarding the chronic administration of antiretroviral drugs is the potential for induction of drug efflux transporter expression (i.e., P-glycoprotein, P-gp) at tissue sites that can significantly affect drug distribution and treatment efficacy. Previous data have shown that the inductive effect of human immunodeficiency virus protease inhibitors (PIs) is mediated through the human orphan nuclear receptor, steroid xenobiotic receptor (SXR or hPXR). The objectives of this study were to investigate transport and inductive properties on efflux drug transporters of two PIs, atazanavir and ritonavir, at the blood-brain barrier by using a human brain microvessel endothelial cell line, hCMEC/D3. Transport properties of PIs by the drug efflux transporters P-gp and multidrug resistance protein 1 (MRP1) were assessed by measuring the cellular uptake of (3)H-atazanavir or (3)H-ritonavir in P-gp and MRP1 overexpressing cells as well as hCMEC/D3. Whereas the P-gp inhibitor, PSC833, increased atazanavir and ritonavir accumulation in hCMEC/D3 cells by 2-fold, the MRP inhibitor MK571 had no effect. P-gp, MRP1, and hPXR expression and localization were examined by Western blot analysis and immunogold cytochemistry at the electron microscope level. Treatment of hCMEC/D3 cells for 72 hr with rifampin or SR12813 (two well-established hPXR ligands) or PIs (atazanavir or ritonavir) resulted in an increase in P-gp expression by 1.8-, 6-, and 2-fold, respectively, with no effect observed for MRP1 expression. In hCMEC/D3 cells, cellular accumulation of these PIs appears to be primarily limited by P-gp efflux activity. Long-term exposure of atazanavir or ritonavir to brain microvessel endothelium may result in further limitations in brain drug permeability as a result of the up-regulation of P-gp expression and function.

The pH sensitive probe 5-(and-6)-carboxyl seminaphthorhodafluor is a substrate for the multidrug resistance-related protein MRP1

Cellular function is dependent on tight regulation of intracellular pH and numerous reports show cancer cells have abnormal pH values in the cytosol and organelles, such as lysosomes. 5-(and-6)-carboxyl seminaphthorhodafluor (SNARF-1) is a commonly used pH sensitive probe and was used here to determine cytosolic pH of HL-60 leukemia cells and a drug-resistant variant overexpressing multidrug-resistance related protein 1 (MRP1). Resistant cells accumulated significantly less SNARF-1 compared to parental cells but near control levels of probe accumulation were observed by preincubating cells with the specific MRP1 inhibitor MK571. Two new drug-resistant cell lines were generated following exposure to doxorubicin or daunorubicin and these upregulated MRP1 or P-glycoprotein expression, respectively. Experiments in these cells showed that reduced SNARF-1 accumulation was specific to MRP1 overexpression, as cells upregulating P-glycoprotein accumulated control levels of the probe. Confirmation that SNARF-1 is a MRP1 substrate was obtained using K562 and KG1a cells that have been shown to, respectively, constitutively express MRP1 and P-glycoprotein. Together, the data suggest that SNARF-1 is a substrate for MRP1 but not P-glycoprotein, and could therefore be used as a probe to distinguish between expression and activity of these 2 efflux proteins. Finally, we confirm that doxorubicin but not daunorubicin challenged MRP1 overexpressing HL-60 cells have elevated cytosolic pH.

HIV-1 viral envelope glycoprotein gp120 produces oxidative stress and regulates the functional expression of multidrug resistance protein-1 (Mrp1) in glial cells

Brain human immunodeficiency virus type-1 (HIV-1) infection is associated with oxidative stress, which may lead to HIV-1 encephalitis, a chronic neurodegenerative condition. In vitro, oxidative stress can be induced in glial cells by exposure to HIV-1 envelope protein glycoprotein (gp120). Multidrug resistance proteins (Mrps) are known to efflux endogenous substrates (i.e. GSH and GSSG) involved in cellular defense against oxidative stress. Altered GSH/GSSG export may contribute to oxidative damage during HIV-1 encephalitis. At present, it is unknown if gp120 exposure can alter the functional expression of Mrp isoforms. Heat-shock protein 70, inducible nitric oxide synthase, intracellular GSSG, 2',7'-dichlorofluorescein fluorescence, and extracellular nitrite were increased in primary cultures of rat astrocytes triggered with gp120, suggesting an oxidative stress response. RT-PCR and immunoblot analysis demonstrated increased Mrp1 mRNA (2.3-fold) and protein (2.2-fold), respectively, in gp120 treated astrocytes while Mrp4 mRNA or protein expression was not changed. Cellular retention of 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, an established Mrp substrate, was reduced (twofold) in gp120-treated astrocytes, suggesting increased Mrp-mediated transport. In addition, GSH and GSSG export were enhanced in gp120-triggered cells. These data suggest that gp120 can up-regulate Mrp1, but not Mrp4, functional expression in cultured astrocytes. Our observation of increased GSH/GSSG efflux in response to gp120 treatment implies that Mrp isoforms may be involved in regulating the oxidative stress response in glial cells.

COMPARISON OF DRUG EFFLUX TRANSPORT KINETICS IN VARIOUS BLOOD-BRAIN BARRIER MODELS

The present study quantitatively compared the drug efflux transport kinetics of 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and its fluorescent metabolite 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) in various blood-brain barrier (BBB) models. BCECF-AM was exposed to freshly isolated bovine brain microvessels (BBM), primary cultured bovine brain microvessel endothelial cells (BBMEC), and MDCK-MDR1 cells for 30 min in the presence or absence of the P-glycoprotein (P-gp) inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). P-gp transport kinetics were determined indirectly by calculating the difference in BCECF accumulation when P-gp was functional and completely inhibited by GF120918 (3.2 microM). Multidrug resistance-associated protein (MRP) transport kinetics were determined by measuring the amount of BCECF transported out of the cell over time. For P-gp-related transport, Km values for BCECF-AM were approximately the same in all three models (around 2 microM), whereas the Vmax was 4-fold greater in the BBM than in the BBMEC or MDCKII-MDR1 cells. For MRP-related transport, Km values for BCECF varied widely among the three BBB models with a rank order of MDCKII-MDR1 < BBMEC < BBM. Like P-gp, the Vmax of BCECF for MRP-related transport was overwhelmingly higher in the BBM compared with the cultured cells. Because differences in the expression of P-gp, MRP5, and MRP6 were observed in the various BBB models using reverse transcription-polymerase chain reaction techniques, the disparity in transport kinetics between the BBB models may be linked to variations in the amount or type of drug efflux transporters expressed in each model. The present study introduces a method of quantitatively evaluating drug efflux transport kinetics in the BBB.

Interaction of ivermectin with multidrug resistance proteins (MRP1, 2 and 3)

Ivermectin is a potent antiparasitic drug from macrocyclic lactone (ML) family, which interacts with the ABC multidrug transporter P-glycoprotein (Pgp). We studied the interactions of ivermectin with the multidrug resistance proteins (MRPs) by combining cellular and subcellular approaches. The inhibition by ivermectin of substrate transport was measured in A549 cells (calcein or 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, BCECF) and in HL60-MRP1 (calcein). Ivermectin induced calcein and BCECF retention in A549 cells (IC(50) at 1 and 2.5microM, respectively) and inhibited calcein efflux in HL60-MRP1 (IC(50)=3.8microM). The action of ivermectin on the transporters ATPase activity was followed on membranes from Sf9 cells overexpressing human Pgp, MRP1, 2 or 3. Ivermectin inhibited the Pgp, MRP1, 2 and 3 ATPase activities after stimulation by their respective activators. Ivermectin showed a rather good affinity for MRPs, mainly MRP1, in the micromolar range, although it was lower than that for Pgp. The transport of BODIPY-ivermectin was followed in cells overexpressing selectively Pgp or MRP1. In both cell lines, inhibition of the transporter activity induced intracellular retention of BODIPY-ivermectin. Our data revealed the specific interaction of ivermectin with MRP proteins, and its transport by MRP1. Although Pgp has been considered until now as the sole active transporter for this drug, the MRPs should be taken into account for the transport of ivermectin across cell membrane, modulating its disposition in addition to Pgp. This could be of importance for optimizing clinical efficacy of ML-based antiparasitic treatments. This offers fair perspectives for the use of ivermectin or non-toxic derivatives as multidrug resistance-reversing agents.

Inhibition of e-cadherin-mediated homotypic adhesion of Caco-2 cells: a novel evaluation assay for peptide activities in modulating cell-cell adhesion

Transient modulation of E-cadherin-mediated cell-cell adhesion may improve paracellular drug delivery through biological barriers. Therefore, there is a need to develop an efficient method to evaluate cadherin peptides that can modulate the intercellular junctions. The objective of this study was to establish a novel assay to evaluate peptide activity in modulating E-cadherin-mediated homophilic interactions, based on the homotypic adhesion of Caco-2 cells. Fluorescence-labeled Caco-2 single cells were incubated with Caco-2 monolayers that were treated beforehand with Ca(2+)-free medium. The homotypic adhesion in the presence or absence of peptide and antibody was determined fluorometrically. The Ca(2+)-deficient pretreatment dramatically increased the number of single cells bound to the monolayers. Immunofluorescence staining showed that some of E-cadherins became accessible without surfactant-induced permeabilization of Caco-2 cell monolayers after the Ca(2+)-deficient pretreatment. The homotypic adhesion was largely dependent on extracellular Ca(2+) concentrations and significantly inhibited by the presence of anti-E-cadherin monoclonal antibody DECMA-1. In contrast, DECMA-1 did not inhibit E-cadherin-independent adhesion, such as the homotypic adhesion of Caco-2 cells in the absence of Ca(2+) or the heterotypic adhesion of Molt-3 T cells to Caco-2 monolayers. These results indicate the predominant involvement of E-cadherin-mediated cell-cell adhesion in this assay. E-cadherin-derived peptides, which had been shown in our previous studies to inhibit E-cadherin-mediated cell-cell adhesion, significantly inhibited homotypic adhesion in a dose-dependent manner. These results, taken together, suggest that the present assay can be used for evaluation of peptide, protein, or antibody activity in modulating the E-cadherin-mediated homophilic interactions in the context of whole live cells.

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Rapid endothelial cell-selective loading of connexin 40 antibody blocks endothelium-derived hyperpolarizing factor dilation in rat small mesenteric arteries

In resistance arteries, spread of hyperpolarization from the endothelium to the adjacent smooth muscle is suggested to be a crucial component of dilation resulting from endothelium-derived hyperpolarizing factor (EDHF). To probe the role of endothelial gap junctions in EDHF-mediated dilation, we developed a method, which was originally used to load membrane impermeant molecules into cells in culture, to load connexin (Cx)-specific inhibitory molecules rapidly (approximately 15 minutes) into endothelial cells within isolated, pressurized mesenteric arteries of the rat. Validation was achieved by luminally loading cell-impermeant fluorescent dyes selectively into virtually all the arterial endothelial cells, without affecting either tissue morphology or function. The endothelial monolayer served as an effective barrier, preventing macromolecules from entering the underlying smooth muscle cells. Using this technique, endothelial cell loading either with antibodies to the intracellular carboxyl-terminal region of Cx40 (residues 340 to 358) or mimetic peptide for the cytoplasmic loop (Cx40; residues 130 to 140) each markedly depressed EDHF-mediated dilation. In contrast, multiple antibodies directed against different intracellular regions of Cx37 and Cx43, and mimetic peptide for the intracellular loop region of Cx37, were each without effect. Furthermore, simultaneous intra- and extraluminal incubation of pressurized arteries with inhibitory peptides targeted against extracellular regions of endothelial cell Cxs (43Gap 26, 40Gap 27, and (37,43)Gap 27; 300 micromol/L each) for 2 hours also failed to modify the EDHF response. High-resolution immunohistochemistry localized Cx40 to the end of endothelial cell projections at myoendothelial gap junctions. These data directly demonstrate a critical role for Cx40 in EDHF-mediated dilation of rat mesenteric arteries.

A Fluorometric Screening Assay for Drug Efflux Transporter Activity in the Blood-Brain Barrier

PURPOSE

To examine the capability of a fluorometric assay to identify and characterize the drug efflux interactions of a broad spectrum of drug agents in an in vitro model of the blood-brain barrier (BBB).

METHODS

Various concentrations of drug agent (1, 10, and 100 microM) were evaluated for their effect on the cellular accumulation of the P-glycoprotein (P-gp) probe R123 (3.2 microM), and the mixed P-gp and multidrug resistance-associated protein (MRP) probe, BCECF (1 microM), in bovine brain microvessel endothelial cell (BBMEC) monolayers. Drugs demonstrating a significant effect were further quantitated using an expanded concentration range and a nonlinear regression curve fit to determine the potency (IC50) and efficacy (Imax) of the drug for P-gp and/or MRP.

RESULTS

Several of the 36 therapeutic agents examined showed drug efflux transporter interactions in BBMEC monlayers. Melphalan and risperidone significantly enhanced the accumulation of R123 over control (1.47- and 1.82-fold, respectively) with resulting IC50s of 1.4 and 14.6 microM, respectively. Chlorambucil and valproic acid significantly enhanced the accumulation of BCECF compared to control monolayers (2.02- and 4.01-fold, respectively) with resulting IC50s of 146.1 and 768.5 microM, respectively.

CONCLUSIONS

The current study demonstrates the feasibility of a fluorometric assay consisting of R123 and BCECF in assessing the drug efflux interactions of a variety of drugs in the BBB.