Induction of Human P-Glycoprotein in Caco-2 cells: Development of a Highly Sensitive Assay System for P-Glycoprotein-Mediated Drug Transport

Rebamipide nanocrystal with improved physicomechanical properties and its assessment through bio-mimicking 3D intestinal permeability model

This study investigated the formulation and characterization of rebamipide nanocrystals (REB-NCs) to enhance the solubility and permeability of rebamipide, an anti-ulcer medication known for its low aqueous solubility and permeability, classified as BCS class IV. Employing high-pressure homogenization and wet milling techniques, we successfully achieved nanonization of rebamipide, resulting in stable nanosuspensions that were subsequently freeze-dried to produce REB-NCs with an average particle size of 223 nm. Comprehensive characterization techniques, including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) confirmed the crystalline nature of the nanocrystals and their compatibility with the selected excipients. The saturation solubility study revealed a remarkable three-fold enhancement in PBS pH 7.4 compared to rebamipide API, indicating the effectiveness of the nanocrystal formulation in improving drug solubility. Furthermore, 3D in-vitro permeability assessments conducted on Caco-2 cell monolayers demonstrated an noticeable increase in the permeability of REB-NCs relative to the pure active pharmaceutical ingredient (API), highlighting the promise of this formulation to enhance drug absorption. The dissolution profile of the nanocrystal tablets exhibited immediate release characteristics, significantly outperforming conventional formulations in terms of the dissolution rate. This research underscores the potential of nanomilling as a scalable, environment-friendly, and less toxic approach to significantly enhance the bioavailability of rebamipide. By addressing the challenges associated with the solubility and permeability of poorly water-soluble drugs, our outcome offers insightful information into developing efficient nanomedicine strategies for enhancing therapeutic outcomes.

In vitro effects of European and Latin-American medicinal plants in CYP3A4 gene expression, glutathione levels, and P-glycoprotein activity

Many medicinal plants species from European -such as Artemisia absinthium, Equisetum arvense, Lamium album, Malva sylvestris, Morus nigra, Passiflora incarnata, Frangula purshiana, and Salix alba- as well as Latin American traditions -such as Libidibia ferrea, Bidens pilosa, Casearia sylvestris, Costus spicatus, Monteverdia ilicifolia, Persea americana, Schinus terebinthifolia, Solidago chilensis, Syzygium cumini, Handroanthus impetiginosus, and Vernonanthura phosphorica- are shortlisted by the Brazilian National Health System for future clinical use. However, they lack many data on their action upon some key ADME targets. In this study, we assess non-toxic concentrations (up to100 μg/ml) of their infusions for in vitro ability to modulate CYP3A4 mRNA gene expression and intracellular glutathione levels in HepG2 cells, as well as P-glycoprotein (P-gp) activity in vincristine-resistant Caco-2 cells (Caco-2 VCR). We further investigated the activation of human pregnane X receptor (hPXR) in transiently co-transfected HeLa cells and the inhibition of Gamma-glutamyl transferase (GGT) in HepG2 cells. Our results demonstrate L. ferrea, C. sylvestris, M. ilicifolia, P. americana, S. terebinthifolia, S. cumini, V. phosphorica, E. arvense, P. incarnata, F. purshiana, and S. alba can significantly increase CYP3A4 mRNA gene expression in HepG2 cells. Only F. purshiana shown to do so likely via hPXR activation. P-gp activity was affected by L. ferrea, F. purshiana, S. terebinthifolia, and S. cumini. Total intracellular glutathione levels were significantly depleted by exposure to all extracts except S. alba and S. cumini This was accompanied by a lower GGT activity in the case of C. spicatus, P. americana, S. alba, and S. terebinthifolia, whilst L. ferrea, P. incarnata and F. purshiana increased it. Surprisingly, S. cumini aqueous extract drastically decreased GGT activity (−48%, p < 0.01). In conclusion, this preclinical study shows that the administration of some of these herbal medicines causes in vitro disturbances to key drug metabolism mechanisms. We recommend active pharmacovigilance for Libidibia ferrea (Mart.) L. P. Queiroz, Frangula purshiana Cooper, Schinus terebinthifolia Raddi, and Salix alba L. which were able to alter all targets in our preclinical study.

Chlorpyrifos, permethrin and cyfluthrin effect on cell survival, permeability, and tight junction in an in-vitro model of the human blood-brain barrier (BBB)

The blood-brain barrier (BBB) is a structural and functional interface between the plasma and the human brain. Predictive BBB in-vitro models like immortalized human capillary microvascular endothelial cells (HCMEC/D3) can be used to explore the BBB disruption potential of daily exposed chemicals. The present study was focused on investigating the human BBB permeation potential of one organophosphate pesticide, chlorpyrifos (CPF), and two pyrethroids, permethrin (PMT) and cyfluthrin (CFT). HCMEC/D3 cells were exposed to the chemical and the time-dependent pass across BBB along with permeation coefficient (Papp) was calculated. Transendothelial electrical resistance (TEER) was measured for the cells to check the monolayer formation and later to check the reduction in integrity after chemical exposure. Real time PCR was conducted to investigate the effect of chemicals on the expression BBB´s tight and adherens junction proteins. Calculated Papp value for three chemicals was in the following order: CPF>CFT>PMT, where CPF showed the highest permeation coefficient. TEER calculation showed that the integrity decreased after CPF exposure which was in concordance with Papp value whereas for other chemicals, no change in TEER after exposure was observed. In addition, the transwell study showed a higher efflux ratio (ER) (>2) of CFT indicating that CFT could be a substrate for active transport. For CPF and PMT, ER was less than 2, so no active transport seems to be involved. The evaluation of the mRNA expression analysis revealed a statistically significant decrease in Occludin (OCLN) gene expression for CPF, VE-Cadherin (CDH5) for PMT and Zonula Occludens (ZO1) expression for CFT. Our study showed that CPF has the highest potential for inducing cell death, higher permeation, and capability to induce BBB dysfunction than among the above-mentioned chemicals. Additionally, the results of the permeation study could be useful to build a human PBPK model using in vitro-to-in vivo extrapolation approach.

Rifampicin Induces Gene, Protein, and Activity of P-Glycoprotein (ABCB1) in Human Precision-Cut Intestinal Slices

P-glycoprotein (ABCB1), an ATP-binding cassette efflux transporter, limits intestinal absorption of its substrates and is a common site of drug–drug interactions. Drug-mediated induction of intestinal ABCB1 is a clinically relevant phenomenon associated with significantly decreased drug bioavailability. Currently, there are no well-established human models for evaluating its induction, so drug regulatory authorities provide no recommendations for in vitro/ex vivo testing drugs’ ABCB1-inducing activity. Human precision-cut intestinal slices (hPCISs) contain cells in their natural environment and express physiological levels of nuclear factors required for ABCB1 induction. We found that hPCISs incubated in William’s Medium E for 48 h maintained intact morphology, ATP content, and ABCB1 efflux activity. Here, we asked whether rifampicin (a model ligand of pregnane X receptor, PXR), at 30 μM, induces functional expression of ABCB1 in hPCISs over 24- and 48-h incubation (the time to allow complete induction to occur). Rifampicin significantly increased gene expression, protein levels, and efflux activity of ABCB1. Moreover, we described dynamic changes in ABCB1 transcript levels in hPCISs over 48 h incubation. We also observed that peaks of induction are achieved among donors at different times, and the extent of ABCB1 gene induction is proportional to PXR mRNA levels in the intestine. In conclusion, we showed that hPCISs incubated in conditions comparable to those used for inhibition studies can be used to evaluate drugs’ ABCB1-inducing potency in the human intestine. Thus, hPCISs may be valuable experimental tools that can be prospectively used in complex experimental evaluation of drug–drug interactions.

Vinblastine treatment decreases the undifferentiated cell contamination of human iPSC-derived intestinal epithelial-like cells

Human induced pluripotent stem cell-derived intestinal epithelial cells (hiPSC-IECs) are expected to be utilized in regenerative medicine. To perform a safe transplantation without the risk of tumor formation, residual undifferentiated hiPSCs must be removed from hiPSC-IECs. In this study, we examined whether vinblastine (a multiple drug resistance 1 [MDR1] substrate) could remove residual undifferentiated hiPSCs in hiPSC-IECs and attempted to generate hiPSC-IECs applicable to transplantation medicine. We found that the expression levels of pluripotent markers were largely decreased and those of intestinal markers were increased by vinblastine treatment. The treatment of undifferentiated hiPSCs with vinblastine significantly decreased their viability. These results suggested that undifferentiated hiPSCs can be eliminated from hiPSC-IECs by vinblastine treatment. We hypothesized that MDR1-negative cells (such as undifferentiated hiPSCs) die upon vinblastine treatment because they are unable to excrete vinblastine. As expected, the cell viability of MDR1-knockout hiPSC-IECs was significantly decreased by vinblastine treatment. Furthermore, teratomas were formed by subcutaneous transplantation of hiPSC-IECs mixed with undifferentiated hiPSCs into mice, but they were not observed when the transplanted cells were pre-treated with vinblastine. Vinblastine-treated hiPSC-IECs would be an effective cell source for safe regenerative medicine.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Efficient Generation of Small Intestinal Epithelial-like Cells from Human iPSCs for Drug Absorption and Metabolism Studies

The small intestine plays an important role in the absorption and metabolism of oral drugs. In the current evaluation system, it is difficult to predict the precise absorption and metabolism of oral drugs. In this study, we generated small intestinal epithelial-like cells from human induced pluripotent stem cells (hiPS-SIECs), which could be applied to drug absorption and metabolism studies. The small intestinal epithelial-like cells were efficiently generated from human induced pluripotent stem cell by treatment with WNT3A, R-spondin 3, Noggin, EGF, IGF-1, SB202190, and dexamethasone. The gene expression levels of small intestinal epithelial cell (SIEC) markers were similar between the hiPS-SIECs and human adult small intestine. Importantly, the gene expression levels of colonic epithelial cell markers in the hiPS-SIECs were much lower than those in human adult colon. The hiPS-SIECs generated by our protocol exerted various SIEC functions. In conclusion, the hiPS-SIECs can be utilized for evaluation of drug absorption and metabolism.

Efflux transporters and tight junction expression changes in human gastrointestinal cell lines cultured in defined medium vs serum supplemented medium

AIMS

Many gastrointestinal cell lines including Caco-2, LS174T and RKO require foetal calf serum (FCS) in culture medium. However, when isolating secreted product from conditioned medium (CM), after cell exposure to a trigger, it is better to remove FCS in the culture medium for identification of secreted products of interest. However, it is unknown whether defined medium adversely affects active efflux protein expression and tight junction formation.

MATERIALS AND METHODS

Using different gastrointestinal cell lines chosen with different levels of efflux transporter expression, fully defined components, such as using transferrin, insulin, selenium and ethanolamine without FCS or with a reduced percentage of FCS (2%) were tested as an optimal choice for cell growth. In addition to morphological characteristics, the expression of the ABC efflux transporters, ABCB1 (P-glycoprotein [P-gp]), ABCC2 (multidrug resistance associated protein 2), ABCG2 (breast cancer resistance protein) and occludin was determined.

KEY FINDINGS

The cells required a minimum of 2% FCS for expression of transporters. Fully defined medium with no serum adversely affected the expression of transporters, especially P-gp. An important characteristic of Caco-2 cells is its ability to form tight junctions. Caco-2 did not form adequate tight junctions without 10% FCS added in the medium, as evidenced by low TEER values and reduced occluding immunohistochemistry.

SIGNIFICANCE

FCS is required for efflux protein expression and tight junction generation. Nevertheless, it is possible to use 5 fold less FCS which assists with low molecular weight secretion isolation. Passage number also contributes significantly to the presence of these transporters.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

Permeability of plumbagin across human intestinal cell in vitro

Plumbagin is the active compound isolated from plants used in traditional medicine for treatment of various diseases such as activities malaria, leishmaniasis, viral infections and cancers. The aim of the study was to investigate the permeability of plumbagin across Caco-2 (human epithelial colorectal adenocarcinoma) cell monolayer and its effects on the expression and function of P-glycoprotein. The integrity of Caco-2 cell monolayer was evaluated by measuring trans-epithelial electrical resistance and permeation (Papp) of Lucifer yellow across the cell monolayer. The effect of plumbagin on P-glycoprotein was detected by measuring its interference with the transport of the P-glycoprotein substrate (R123) and the effect on MDR-1 mRNA expression was detected by RT-PCR. The Papp of plumbagin (2-8 µM) for the apical to basolateral and basolateral to apical directions were 10.29-15.96 × 10(-6) and 7.40-9.02 × 10(-6) cm/s, respectively, with the efflux ratios of 0.57-0.73. Plumbagin is not either a substrate or inhibitor of P-glycoprotein. It did not interfere with the P-glycoprotein-mediated R123 transport across Caco-2 cell monolayer, as well as the function of P-glycoprotein and the expression of MDR-1 mRNA. Results suggest moderate permeability of plumbagin across the Caco-2 cell monolayer in both directions. The transport mechanism is likely to be a passive transport.

Effects of polyethylene glycols on intestinal efflux pump expression and activity in Caco-2 cells

The present study was planned to investigate the influence of polyethylene glycols (PEGs) on the activity and expression of P-glycoprotein (P-gp). Sub-toxic concentrations of PEGs in Caco-2 cells were determined using the MTT test assay. Then the measurement of Rhodamine-123 (Rho-123) uptake, a P-gp fluorescence substrate, in Caco-2 cells confronting PEG 400 (1% and 2% w/v), PEG 4000 (2% and 4% w/v), PEG 6000 (2% and 4% w/v), PEG 10000 (2% and 4% w/v), PEG 15000 (1% and 2% w/v), and PEG 35000 (2% and 4% w/v) overnight was taken to elucidate whether non-toxic concentrations of PEGs are able to impact P-gp activity. Furthermore, western blotting was carried out to investigate P-gp protein expression. The results showed that PEG 400 at concentrations of 1% (w/v) and 2% (w/v) and PEG 6000 at the concentration of 4% (w/v) are notably capable of blocking P-gp. Based on the obtained results it is concluded that the mentioned excipients could be used to obstruct P-gp efflux transporter in order to increase the bioavailability of co-administered substrate drug.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Sinapine as an active compound for inhibiting the proliferation of Caco-2 cells via downregulation of P-glycoprotein

Sinapine, an alkaloid from seeds of the cruciferous species, shows favorable biological activities such as antioxidant and radio-protective activities. However, the inhibitory effect of sinapine on tumors, and the molecular mechanisms have not been completely understood thus far. In this study, we determined anti-proliferative effects of sinapine. We examined the anti-tumor effects of the combination of sinapine and doxorubicin. The results of the MTT assay and apoptosis showed that sinapine increased the sensitivity of Caco-2 cells to doxorubicin in a dose-dependent manner, whereas no or less effect was observed in the cells treated with doxorubicin alone. The combination of sinapine and doxorubicin had a synergistic effect and increased the cytotoxicity of doxorubicin against Caco-2 cells. Doxorubicin accumulation assay showed that sinapine increased the intracellular accumulation of doxorubicin in dose-dependent manner. Immunoblotting and QT-PCR analysis showed that sinapine suppressed P-glycoprotein (P-gp) expression via ubiquitination. A significant correlation was observed between the expression of p-ERK1/2 and P-gp. These results indicated that sinapine played an important role in the down-regulation of P-gp expression through suppression of FGFR4-FRS2α-ERK1/2 signaling pathway. To our knowledge, this is the first study to show that sinapine can be used as an effective natural compound for chemo-resistance.

Involvement of AP-1 and NF-κB in the up-regulation of P-gp in vinblastine resistant Caco-2 cells

Caco-2 is a widely used cell model in drug absorption and P-glycoprotein (P-gp, MDR1) substrate identification. Long-term vinblastine treatment of Caco-2 cells could increase the expression of P-gp; thus, the vinblastine resistant Caco-2 (Caco-2 vbl) cells can be used as a rapid and sensitive alternative model in identifying P-gp substrates. The mechanism of P-gp induction in this model is not clear; this study was therefore intended to clarify the possible factors involved in P-gp up-regulation in Caco-2 vbl cells. Since vinblastine is the inducer of both activator protein-1 (AP-1) and nuclear factor kappa B (NF-κB), we investigated the role of AP-1 and NF-κB in the regulation of MDR1 gene expression. Our results indicated that the AP-1 and NF-κB luciferase activity was higher in Caco-2 vbl cells than that in Caco-2 cells according to reporter gene assay. The mRNA expression of AP-1 subunit c-Jun and NF-κB was increased in Caco-2 vbl cells. The c-Jun inhibitor SP600125 and NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC) suppressed the expression of MDR1 mRNA in Caco-2 vbl cells. In conclusion, this study provides the evidence that AP-1 and NF-κB are involved in the P-gp induction in Caco-2 vbl cells.

The effects of media on pharmaceutically relevant transporters in the human HT-29 adenocarcinoma cell line: does culture media need to be controlled?

The HT-29 cell line forms a confluent monolayer with tight junctions, but displays different phenotypes when cultured for 21 days in galactose-supplemented media (differentiated) versus glucose-supplemented media (dedifferentiated). This study is aimed at elucidating how media differences might affect selected drug transporter expression and peptide-based substrate transport toward reducing this variability. A vial of HT-29 cells was amplified and cultured over several passages in four different mediums (American Type Culture Collection recommended McCoy's 5A versus Dulbecco's modified Eagle's media containing glucose, galactose, or neither carbohydrate) with normal supplementation. Transporter mRNA expression was characterized at days 5 and 21 postseeding utilizing SABiosciences quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) drug transporter arrays. Transport studies using [H]histidine, [(3) H]glycylsarcosine, [(3) H]valacyclovir, and [(3) H]carnosine were performed to assess the functional effects of oligopeptide transporter expression changes in HT-29 cells grown in each media. qRT-PCR arrays illustrated variable, media-dependent transporter expression between both the initial and differentiated time points. Permeability studies illustrated considerable media-dependent differences in both paracellular and transcellular substrate fluxes. The results demonstrate that these cells exhibit differing monolayer characteristics and genotypic/phenotypic profile properties when cultured under different media. The results suggest a need for standardization of culture methodologies for reducing inter- and intralaboratory variability.

The impact of P-glycoprotein mediated efflux on absorption of 11 sedating and less-sedating antihistamines using Caco-2 monolayers

Caco-2 cells were used to compare P-gp mediated efflux and passive permeability using bidirectional transport of 11 antihistamines. An efflux ratio >2 indicated active efflux, with PSC833 and GF120918 used as functional P-gp inhibitors. Antihistamines were measured directly by HPLC or LC/MS. Fexofenadine had an efflux ratio of 37, yet had negligible passive permeability, even in the presence of a pH gradient (0.1 × 10(-6) cm/sec). Its precursor, terfenadine, had an efflux ratio of 2.5, while cetirizine, desloratadine and hydroxyzine were 4, 7 and 14, respectively. After incubation with P-gp inhibitors, these ratios dropped significantly. Loratadine, by contrast, had equivalent transport in both directions and passive permeability was high (24 × 10(-6) cm/sec). Dimenhydrinate was the only other sedating antihistamine to exhibit efflux, with a ratio of 10. Gradient conditions of pH (6/7.4) increased efflux of terfenadine and desloratadine to over 31 and 38 fold respectively, yet this increased efflux was not associated with P-gp. Altering functional P-gp in the gut is likely to influence absorption of some sedating antihistamines such as dimenhydrinate and hydroxyzine and most less-sedating antihistamines except loratadine. In addition, desloratadine exhibits pH dependent efflux which could further induce variable absorption of this antihistamine.

The physiological performance of a three-dimensional model that mimics the microenvironment of the small intestine

Our focus was to develop a three-dimensional (3D) human dynamic in vitro tissue model that mimics the natural microenvironment of the small intestine. We co-cultured human Caco-2 cells with primary-isolated human microvascular endothelial cells (hMECs) on decellularized porcine jejunal segments within a custom-made dynamic bioreactor system that resembles the apical and basolateral side of the intestine for up to 14 days. The obtained data were compared to results generated using routine static Caco-2 assays. We performed histology and immunohistochemistry. Permeability was measured using directed transport studies. Histological analyses revealed that in tissue-engineered segments, which had been cultured under dynamic conditions, the Caco-2 cells showed a high-prismatic morphology, resembling normal primary enterocytes within their native environment. We further identified that the transport of low permeable substances, such as fluorescein and desmopressin increased within the dynamic bioreactor cultures. Immunohistochemical staining showed a significantly higher expression of the efflux transport p-glycoprotein (p-gp) under dynamic culture conditions when compared to the static cultures. We conclude that the integration of physiological parameters is crucial for the establishment of a reliable 3D intestinal in vitro model, which enables the simulation of drug transport over the gut-blood-barrier in a simplified way.

Intestinal oxidative state can alter nutrient and drug bioavailability

Organic cations (OCs) are substances of endogenous (e.g. dopamine, choline) or exogenous (e.g. drugs like cimetidine) origin that are positively charged at physiological pH. Since many of these compounds can not pass the cell membrane freely, their transport in our out of cells must be mediated by specific transport systems. Transport by organic cation transporters (OCTs) can be regulated rapidly by altering their trafficking and/or affinities in response to a stimuli. However, for example, a specific disease could lead to modifications in the expression of OCTs. Chronic exposure to oxidative stress has been suggested to alter regulation and functional activity of proteins through several pathways. According to results from a previous work, oxidation-reduction pathways were thought to be involved in intestinal organic cation uptake modulation. The present work was performed in order to evaluate the influence of oxidative stressors, especially glutathione, on the intestinal organic cation absorption. For this purpose, the effect of compounds with different redox potential (glutathione, an endogenous antioxidant, and procyanidins, diet antioxidants) was assessed on MPP+ (1-methyl-4-phenylpyridinium iodide) uptake in an enterocyte cell line (Caco-2). Caco-2 cells were subcultured with two different media conditions (physiological: 5 mM glucose, referred as control cells; and high-glucose: 25 mM glucose, referred as HG cells). In HG cells, the uptake was significantly lower than in control cells. Redox changing interventions affected MPP+ uptake, both in control and in high-glucose Caco-2 cells. Cellular glutathione levels could have an important impact on membrane transporters activity. The results indicate that modifications in the cellular oxidative state modulate MPP+ uptake by Caco-2 cells. Such modifications may reflect in changes of nutrient and drug bioavailability.

In vivo probes of drug transport: commonly used probe drugs to assess function of intestinal P-glycoprotein (ABCB1) in humans

Intestinal P-glycoprotein (P-gp, ABCB1) may significantly influence drug absorption and elimination. Its expression and function is highly variable, regio-selective and influenced by genetic polymorphisms, drug interactions and intestinal diseases. An in vivo probe drug for intestinal P-gp should a registered, safe and well tolerated nonmetabolized selective substrate with low protein binding for which P-gp is rate-limiting during absorption. Other P-gp dependent processes should be of minor influence. The mechanism(s) and kinetics of intestinal uptake must be identified and quantified. Moreover, the release properties of the dosage form should be known. So far, the cardiac glycoside digoxin and the ß₁-selective blocker talinolol have been used in mechanistic clinical studies, because they meet most of these criteria. Digoxin and talinolol are suitable in vivo probe drugs for intestinal P-gp under the precondition, that they are used as tools in carefully designed pharmacokinetic studies with adequate biometrically planning of the sample size and that several limitations are considered in interpreting and discussion of the study results.

Intestinal absorption of HMG-CoA reductase inhibitor pitavastatin mediated by organic anion transporting polypeptide and P-glycoprotein/multidrug resistance 1

The purpose of this study was to investigate the involvement of organic anion transporting polypeptide (OATP/Oatp) and P-glycoprotein (P-gp)/multidrug resistance 1 (MDR1/Mdr1) in intestinal absorption of pitavastatin. Pitavastatin was found to be a substrate for human OATP1A2, OATP2B1, and MDR1 and rat Oatp1a5, Oatp2b1, and Mdr1a in experiments using transporter-expressing Xenopus oocytes and LLC-PK1 cell systems. Naringin inhibited Oatp1a5- and Mdr1a-mediated transport of pitavastatin with IC(50) values of 18.5 and 541 µM, respectively. The difference in the IC(50) values of naringin for Oatp1a5 and Mdr1a-mediated pitavastatin transport may account for the complex concentration-dependent effect of naringin on the intestinal absorption of pitavastatin. Rat intestinal permeability of pitavastatin measured by the in situ closed-loop perfusion method was indeed significantly reduced by 200 µM naringin, but was significantly increased by 1000 µM naringin. Furthermore, the permeability was significantly increased by elacridar, a potent inhibitor of Mdr1, while the permeability was significantly decreased in the presence of both elacridar and naringin, suggesting that Oatp1a5 and Mdr1a are both involved in intestinal absorption of pitavastatin. Our present results indicate that OATP/Oatp and MDR1/Mdr1 play roles in the intestinal absorption of pitavastatin as influx and efflux transporters, respectively.

Development and validation of RP-HPLC-fluorescence method for quantitative determination of quinidine, a probe substrate for P-glycoprotein inhibition assay using Caco-2 cell monolayer

A simple, sensitive and specific reverse-phase high-performance liquid chromatographic (RP-HPLC) method with fluorescence detection was developed for quantitation of quinidine from HBSS buffer. The method was applicable in the bi-directional transport assay for evaluation of the inhibitory effect of test compounds on P-glycoprotein-mediated quinidine transport; quinidine was used as a probe P-glycoprotein substrate. The calibration curve was linear (correlation coefficient >/=99) in the range 0.30-100.00 nm. The method was validated and is specific and sensitive with limit of quantitation of 300 pm for quinidine. The method was found to be accurate and precise in the working calibration range. Stability studies were carried out at different storage conditions where the analyte was found to be stable. The applicability and reliability of the analytical method was evaluated by successful demonstration of efflux ratio (P(app)B --> A/P(app)A --> B) in the Caco-2 cell monolayer efflux assay. The efflux ratio for quinidine (100 nm) alone was 10.8, which reduced to less than 2 in the presence of the classical P-gp inhibitors verapamil and ketoconazole (100 mum each).

Species difference in the effect of grapefruit juice on intestinal absorption of talinolol between human and rat

Bioavailability of talinolol, a beta(1)-adrenergic receptor antagonist, was enhanced by coadministration with grapefruit juice (GFJ) in rats, whereas GFJ ingestion markedly reduced the absorption of talinolol in humans. Because our recent study indicated that the inhibitory effect of GFJ on organic anion-transporting polypeptide (Oatp)- and P-gp-mediated talinolol absorption depends on the concentration of naringin in ingested GFJ, the apparent inconsistent findings may be explained by the species difference in the affinity of naringin for OATP/Oatp and P-gp multidrug resistance 1 (MDR1/Mdr1) between humans and rats. Although human MDR1-mediated talinolol transport was not inhibited by 2000 microM naringin, naringin inhibited human OATP1A2-, rat Oatp1a5-, and rat Mdr1a-mediated talinolol transport with IC(50) values of 343, 12.7, and 604 microM, respectively, in LLC-PK1 cell and Xenopus laevis oocyte systems. Because the naringin concentration in commercially prepared GFJ was found to be approximately 1200 microM, these results suggested that GFJ would reduce the intestinal absorption of talinolol through inhibition of OATP1A2-mediated talinolol uptake in humans, whereas an increase of talinolol absorption is mainly through inhibition of Mdr1a-mediated efflux in rats. The rat intestinal permeability of talinolol measured by the in situ closed loop method was indeed significantly increased in the presence of GFJ, whereas a significant decrease was observed with 6-fold diluted GFJ, in which the naringin concentration was approximately 200 microM. The present study indicated that the species difference in the effect of GFJ on intestinal absorption of talinolol between humans and rats may be due to differences in the affinity of naringin for OATP/Oatp and MDR1/Mdr1 transporters between the two species.

The transmembrane transport of metformin by osteoblasts from rat mandible

Previous studies have demonstrated that metformin, one of systemic antihyperglycemic drugs, can slow bone loss caused by diabetes mellitus and has an osteogenic action on osteoblasts in vitro. It is tempting to speculate that metformin would be transported into bone tissues around dental implant by topical administration to improve the bone-implant contact in diabetic patients. In this study, the osteoblasts from rat mandible were cultured with 5.5 mM (control) or 16.5 mM d-glucose, then the uptake of metformin by osteoblasts was detected with high performance liquid chromatography (HPLC). Rat organic cation transporter (rOct) expression was characterized by immunocytochemistry, RT-PCR and Western blotting. It was found that, the uptake of metformin was saturable, Na(+)-dependent, affected by extracellular pH and inhibited by both phenformin and cimetidine (an inhibitor of Octs). rOct1 but no rOct2 was expressed extensively in osteoblasts and the protein level of rOct1 could be up-regulated by metformin. The uptake of metformin and phosphorylated-rOct1 at hyperglycaemic cell culture (16.5 mM d-glucose) significantly increased versus 5.5 mM control (p < 0.05). In conclusion, rat osteoblasts have the ability to transport the metformin intra-cellularly, the uptake of metformin by osteoblasts is a secondary active transportation mediated by rOct1 and high-glucose can improve the uptake of metformin by osteoblasts through phosphorylation of rOct1. The current results suggest that metformin could be used for dental implant topically in type 2 diabetic patients to increase the bone formation, therefore, to enhance the success rate of dental implants clinically.

The Caco-2 cell monolayer: usefulness and limitations

BACKGROUND

The Caco-2 monolayer has been used extensively for the high-throughput screening of drug permeability and identification of substrates, inhibitors, and inducers of intestinal transporters, especially P-glycoprotein (P-gp). Traditionally, the Caco-2 monolayer is viewed as a single barrier rather than a polarized cell monolayer consisting of metabolic enzymes that are sandwiched between two membrane barriers with distinctly different transporters.

OBJECTIVE

This review addressed the usefulness and limitations of the Caco-2 cell monolayer in drug discovery and mechanistic studies.

METHODS

This mini-review covered applications of the Caco-2 monolayer, clarified misconceptions, and critically addressed issues on data interpretation.

CONCLUSION

The catenary model extends the usefulness of Caco-2 monolayer and provides proper mechanistic insight and data interpretation.

Effect of P‐Glycoprotein Expression Levels on the Concentration‐Dependent Permeability of Drugs to the Cell Membrane

The purpose of this study is to develop a kinetic model that can predict the in vivo absorption of P-glycoprotein (P-gp) substrates from in vitro data. Apical (AP) to basal (BL) absorptive permeability of typical P-gp substrate drugs including quinidine, verapamil, vinblastine, and digoxin, were measured in several cell monolayers with different levels of P-gp expression, normal, P-gp induced, P-gp highly induced and MDR1-knockdown Caco-2 cells and MDR1-MDCKII cells. In all cell monolayers, AP to BL permeability of P-gp substrates increased when their AP concentration was increased, showing a sigmoid-type relationship to donor (AP) concentrations. At the higher concentration range, permeability reached a maximum value, suggesting saturation of P-gp-mediated efflux, and at the lower concentration range, permeability decreased depending on P-gp expression level. A simple kinetic model was applied to the permeability-concentration curve of each drug to obtain the fundamental parameters for P-gp-mediated transport, K(m(app)) and V(max). Both K(m(app)) and V(max) of each drug were found to show linear correlations with expression level of P-gp. This study clearly demonstrated the possibility to estimate the permeability of P-gp substrate drugs in human intestine from the expression level of P-gp, and thus the possibility to predict oral absorption of those drugs.

Effect of cell differentiation and passage number on the expression of efflux proteins in wild type and vinblastine-induced Caco-2 cell lines

The mRNA level expression of MDR1, MRP1-6, BCRP and CYP3A4 was determined by quantitative PCR in wild type (Caco-2WT) and vinblastine-treated (Caco-2VBL) Caco-2 cells at different passage levels (32-53). Differentiation increased the mRNA levels of MDR1, BCRP and all the MRPs except MRP4. Corresponding mRNA levels were observed in Caco-2WT and Caco-2VBL, except that the expression of MRD1 was higher in Caco-2VBL than in Caco-2WT cells. CYP3A4 was barely detected in either cell line. MDR1 functionality was studied using rhodamine123 and verapamil as a substrate-inhibitor pair. Corresponding to the observed differences in mRNA levels, MDR1 activity was higher in the Caco-2VBL cells. In Caco-2WT, MDR1 functionality was elevated at low passage numbers (32-35) compared to higher ones (49-53). Verapamil inhibited MDR1 efflux except at higher passage Caco-2WT cells, where no MDR1 activity could be observed. The results support the use of Caco-2VBL cells in MDR1 screening. The functional expression is higher than in Caco-2WT and remains consistent across the studied passages without major differences in mRNA levels of other efflux proteins. As both the passage number and the level of cell differentiation affect the expression profile of efflux proteins, short-term cell growth protocols should be evaluated accordingly.