Nature and uses of fluorescent dyes for drug transporter studies

A rapid and simple non-radioactive assay for measuring uptake by solute carrier transporters

Introduction: Solute carrier (SLC) transport proteins play a crucial role in maintaining cellular nutrient and metabolite homeostasis and are implicated in various human diseases, making them potential targets for therapeutic interventions. However, the study of SLCs has been limited due to the lack of suitable tools, particularly cell-based substrate uptake assays, necessary for understanding their biological functions and for drug discovery purposes. Methods: In this study, a cell-based uptake assay was developed using a stable isotope-labeled compound as the substrate for SLCs, with detection facilitated by liquid chromatography-tandem mass spectrometry (LC-MS/MS). This assay aimed to address the limitations of existing assays, such as reliance on hazardous radiolabeled substrates and limited availability of fluorescent biosensors. Results: The developed assay was successfully applied to detect substrate uptakes by two specific SLCs: L-type amino acid transporter 1 (LAT1) and sodium taurocholate co-transporting polypeptide (NTCP). Importantly, the assay demonstrated comparable results to the radioactive method, indicating its reliability and accuracy. Furthermore, the assay was utilized to screen for novel inhibitors of NTCP, leading to the identification of a potential NTCP inhibitor compound. Discussion: The findings highlight the utility of the developed cell-based uptake assay as a rapid, simple, and environmentally friendly tool for investigating SLCs' biological roles and for drug discovery purposes. This assay offers a safer alternative to traditional methods and has the potential to contribute significantly to advancing our understanding of SLC function and identifying therapeutic agents targeting SLC-mediated pathways.

Solid-Supported Membrane (SSM)-Based Electrophysiology Assays Using Surface Electrogenic Event Reader Technology (SURFE²R) in Early Drug Discovery

This article presents detailed descriptions of procedures and troubleshooting tips for solid-supported membrane (SSM)-based electrophysiology assays (SURFE²R) to measure electrogenic solute carrier transporter proteins (SLCs) and assess the effects of compounds that modulate their activity. SURFE²R allows the use of the standard 96-well format, making it an ideal platform for tertiary assays in a drug-discovery campaign. The assays are performed with cell-line-derived membrane fractions or proteoliposomes containing the transporter of interest. Three main protocols are described for the isolation of membrane fractions from cell culture and the generation of proteoliposomes containing the transporter of interest. Additionally, detailed protocols for SURFE²R single concentration and dose-response experiments are included to measure the potencies of test compounds in stimulating or inhibiting transporter function (EC₅₀ or IC₅₀ values, respectively) and kinetic functional assays to calculate apparent affinity (k_M ) and maximal velocity (V_max ) of substrate uptake. © 2023 Sanofi. Current Protocols published by Wiley Periodicals LLC. PROTOCOL GROUP 1: Sample preparation for SSM-based electrophysiology assays Support Protocol 1: Production of cell batches Support Protocol 2: Simple isolation of cell membranes Alternate Protocol 1: Isolation of cell membranes with sucrose gradient pre-purification Support Protocol 3: Production and isolation of liposomes Support Protocol 4: Preparation of sensor with isolated cell membranes Alternate Protocol 2: Preparation of sensor with isolated proteoliposomes PROTOCOL GROUP 2: Determination of assay parameters for SSM-based electrophysiology assay Support Protocol 5: Assay with stable buffer Alternate Protocol 3: Assay with ion gradient Support Protocol 6: Determination of membrane/liposome concentration Support Protocol 7: Determination of substrate dependency k_M PROTOCOL GROUP 3: Determination of advanced assay parameters for SSM-based electrophysiology assays Support Protocol 8: Assessment of ion concentration dependency Support Protocol 9: Assessment of pH dependency Support Protocol 10: Assessment of DMSO dependency Support Protocol 11: Assessment of signal stability with multiple activations PROTOCOL GROUP 4: Compound testing through SSM-based electrophysiology assays using SURFE²R apparatus Support Protocol 12: Assessment of signal specificity of a published inhibitor or unknown compound(s) Support Protocol 13: Compound wash-out Support Protocol 14: Statistical analysis.

Fluorescence-based methods for studying activity and drug-drug interactions of hepatic solute carrier and ATP binding cassette proteins involved in ADME-Tox

In humans, approximately 70% of drugs are eliminated through the liver. This process is governed by the concerted action of membrane transporters and metabolic enzymes. Transporters mediating hepatocellular uptake of drugs belong to the SLC (Solute carrier) superfamily of transporters. Drug efflux either toward the portal vein or into the bile is mainly mediated by active transporters of the ABC (ATP Binding Cassette) family. Alteration in the function and/or expression of liver transporters due to mutations, disease conditions, or co-administration of drugs or food components can result in altered pharmacokinetics. On the other hand, drugs or food components interacting with liver transporters may also interfere with liver function (e.g., bile acid homeostasis) and may even cause liver toxicity. Accordingly, certain transporters of the liver should be investigated already at an early stage of drug development. Most frequently radioactive probes are applied in these drug-transporter interaction tests. However, fluorescent probes are cost-effective and sensitive alternatives to radioligands, and are gaining wider application in drug-transporter interaction tests. In our review, we summarize our current understanding about hepatocyte ABC and SLC transporters affected by drug interactions. We provide an update of the available fluorescent and fluorogenic/activable probes applicable in in vitro or in vivo testing of these ABC and SLC transporters, including near-infrared transporter probes especially suitable for in vivo imaging. Furthermore, our review gives a comprehensive overview of the available fluorescence-based methods, not directly relying on the transport of the probe, suitable for the investigation of hepatic ABC or SLC-type drug transporters.

Identification of 5-Carboxyfluorescein as a Probe Substrate of SLC46A3 and Its Application in a Fluorescence-Based In Vitro Assay Evaluating the Interaction with SLC46A3

The therapeutic modalities that involve the endocytosis pathway, including antibody-drug conjugates (ADCs), have recently been developed. Since the drug escape from endosomes/lysosomes is a determinant of their efficacy, it is important to optimize the escape, and the cellular evaluation system is needed. SLC46A3, a lysosomal membrane protein, has been implicated in the pharmacological efficacy of trastuzumab emtansine (T-DM1), a noncleavable ADC used for the treatment of breast cancer, and the cellular uptake efficacy of lipid-based nanoparticles. Recently, we identified the SLC46A3 function as a proton-coupled steroid conjugate and bile acid transporter, which can directly transport active catabolites of T-DM1. Thus, the rapid and convenient assay systems for evaluating the SLC46A3 function may help to facilitate ADC development and to clarify the physiological roles in endocytosis. Here, we show that SLC46A3 dC, which localizes to the plasma membrane owing to lacking a lysosomal-sorting motif, has a great ability to transport 5-carboxyfluorescein (5-CF), a fluorescent probe, in a pH-dependent manner. 5-CF uptake mediated by SLC46A3 was significantly inhibited by compounds reported to be SLC46A3 substrates/inhibitors and competitively inhibited by estrone 3-sulfate, a typical SLC46A3 substrate. The inhibition assays followed by uptake studies revealed that SG3199, a pyrrolobenzodiazepine dimer, which has been used as an ADC payload, is a substrate of SLC46A3. Accordingly, the fluorescence-based assay system for the SLC46A3 function using 5-CF can provide a valuable tool to evaluate the interaction of drugs/drug candidates with SLC46A3.

Is P-Glycoprotein Functionally Expressed in the Limiting Membrane of Endolysosomes? A Biochemical and Ultrastructural Study in the Rat Liver

The drug efflux transporter P-glycoprotein (Pgp; ABCB1) plays an important role in drug absorption, disposition, and elimination. There is an ongoing debate whether, in addition to its localization at the plasma membrane, Pgp may also be expressed at the limiting membrane of endolysosomes (ELs), mediating active EL drug sequestration. If true, this would be an important mechanism to prevent drugs from reaching their intracellular targets. However, direct evidence demonstrating the functional expression of Pgp at the limiting membrane of ELs is lacking. This prompted us to perform a biochemical and ultrastructural study on the intracellular localization of Pgp in native rat liver. For this purpose, we established an improved subcellular fractionation procedure for the enrichment of ELs and employed different biochemical and ultrastructural methods to characterize the Pgp localization and function in the enriched EL fractions. Whereas the biochemical methods seemed to indicate that Pgp is functionally expressed at EL limiting membranes, transmission electron microscopy (TEM) indicated that this only occurs rarely, if at all. Instead, Pgp was found in the limiting membrane of early endosomes and intraluminal vesicles. In additional TEM experiments, using a Pgp-overexpressing brain microvessel endothelial cell line (hCMEC/D3-MDR1-EGFP), we examined whether Pgp is expressed at the limiting membrane of ELs when cells are exposed to high levels of the Pgp substrate doxorubicin. Pgp was seen in early endosomes but only rarely in endolysosomes, whereas Pgp immunogold labeling was detected in large autophagosomes. In summary, our data demonstrate the importance of combining biochemical and ultrastructural methods to investigate the relationship between Pgp localization and function.

Substrate-Dependent Trans-Stimulation of Organic Cation Transporter 2 Activity

The search of substrates for solute carriers (SLCs) constitutes a major issue, owing notably to the role played by some SLCs, such as the renal electrogenic organic cation transporter (OCT) 2 (SLC22A2), in pharmacokinetics, drug-drug interactions and drug toxicity. For this purpose, substrates have been proposed to be identified by their cis-inhibition and trans-stimulation properties towards transporter activity. To get insights on the sensitivity of this approach for identifying SLC substrates, 15 various exogenous and endogenous OCT2 substrates were analysed in the present study, using 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (DiASP) as a fluorescent OCT2 tracer substrate. All OCT2 substrates cis-inhibited DiASP uptake in OCT2-overexpressing HEK293 cells, with IC₅₀ values ranging from 0.24 µM (for ipratropium) to 2.39 mM (for dopamine). By contrast, only 4/15 substrates, i.e., acetylcholine, agmatine, choline and metformin, trans-stimulated DiASP uptake, with a full suppression of the trans-stimulating effect of metformin by the reference OCT2 inhibitor amitriptyline. An analysis of molecular descriptors next indicated that trans-stimulating OCT2 substrates exhibit lower molecular weight, volume, polarizability and lipophilicity than non-trans-stimulating counterparts. Overall, these data indicated a rather low sensitivity (26.7%) of the trans-stimulation assay for identifying OCT2 substrates, and caution with respect to the use of such assay may therefore be considered.

An Overview of Cell-Based Assay Platforms for the Solute Carrier Family of Transporters

The solute carrier (SLC) superfamily represents the biggest family of transporters with important roles in health and disease. Despite being attractive and druggable targets, the majority of SLCs remains understudied. One major hurdle in research on SLCs is the lack of tools, such as cell-based assays to investigate their biological role and for drug discovery. Another challenge is the disperse and anecdotal information on assay strategies that are suitable for SLCs. This review provides a comprehensive overview of state-of-the-art cellular assay technologies for SLC research and discusses relevant SLC characteristics enabling the choice of an optimal assay technology. The Innovative Medicines Initiative consortium RESOLUTE intends to accelerate research on SLCs by providing the scientific community with high-quality reagents, assay technologies and data sets, and to ultimately unlock SLCs for drug discovery.

Identification of novel inhibitors of rat Mrp3

Multidrug resistance-associated protein (MRP; ABCC gene family) mediated efflux transport plays an important role in the systemic and tissue exposure profiles of many drugs and their metabolites, and also of endogenous compounds like bile acids and bilirubin conjugates. However, potent and isoform-selective inhibitors of the MRP subfamily are currently lacking. Therefore, the purpose of the present work was to identify novel rat Mrp3 inhibitors. Using 5(6)-carboxy-2',7'-dichlorofluorescein diacetate (CDFDA) as a model-(pro)substrate for Mrp3 in an oil-spin assay with primary rat hepatocytes, the extent of inhibition of CDF efflux was determined for 1584 compounds, yielding 59 hits (excluding the reference inhibitor) that were identified as new Mrp3 inhibitors. A naive Bayesian prediction model was constructed in Pipeline Pilot to elucidate physicochemical and structural features of compounds causing Mrp3 inhibition. The final Bayesian model generated common physicochemical properties of Mrp3 inhibitors. For instance, more than half of the hits contain a phenolic structure. The identified compounds have an AlogP between 2 and 4.5, between 5 to 8 hydrogen bond acceptor atoms, a molecular weight between 260 and 400, and 2 or more aromatic rings. Compared to the depleted dataset (i.e. 90% remaining compounds), the Mrp3 hit rate in the enriched set was 7.5-fold higher (i.e. 17.2% versus 2.3%). Several hits from this first screening approach were confirmed in an additional study using Mrp3 transfected inside-out membrane vesicles. In conclusion, several new and potent inhibitors of Mrp3 mediated efflux were identified in an optimized in vitro rat hepatocyte assay and confirmed using Mrp3 transfected inside-out membrane vesicles. A final naive Bayesian model was developed in an iterative way to reveal common physicochemical and structural features for Mrp3 inhibitors. The final Bayesian model will enable in silico screening of larger libraries and in vitro identification of more potent Mrp3 inhibitors.

Differential in vitro interactions of the Janus kinase inhibitor ruxolitinib with human SLC drug transporters

Interactions of the Janus kinase (JAK) inhibitor ruxolitinib with solute carriers (SLCs) remain incompletely characterised. The present study was therefore designed to investigate this issue.The interactions of ruxolitinib with SLCs were analysed using transporter-overexpressing human embryonic kidney HEK293 cells. Substrate accumulation was detected by spectrofluorimetry, liquid chromatography coupled to tandem mass spectrometry or scintillation counting.Ruxolitinib was found to potently inhibit the activities of organic anion transporter 3 (OAT3), organic cation transporter 2 (OCT2), multidrug and toxin extrusion 1 (MATE1) and MATE2-K (half maximal inhibitory concentration (IC₅₀) < 10 µM). It blocked OAT1, OAT4, OATP1B1, OATP1B3, OATP2B1 and OCT3, but in a weaker manner (IC₅₀ > 10 µM), whereas OCT1 was not impacted. No time-dependent inhibition was highlighted. When applying the US Food and Drug Administration (FDA) criteria for transporters-related drug-drug interaction risk, OCT2 and MATE2-K, unlike MATE1 and OAT3, were predicted to be in vivo inhibited by ruxolitinib. Cellular uptake studies additionally indicated that ruxolitinib is a substrate for MATE1 and MATE2-K, but not for OAT3 and OCT2.Ruxolitinib in vitro blocked activities of most of SLC transporters. Only OCT2 and MATE-2K may be however clinically inhibited by the JAK inhibitor, with the caution for OCT2 that in vitro inhibition data were generated with an FDA-non recommended fluorescent substrate. Ruxolitinib MATEs-mediated transport may additionally deserve attention for its possible pharmacological consequences in MATE-positive cells.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

Structural Similarities between Some Common Fluorophores Used in Biology, Marketed Drugs, Endogenous Metabolites, and Natural Products

It is known that at least some fluorophores can act as 'surrogate' substrates for solute carriers (SLCs) involved in pharmaceutical drug uptake, and this promiscuity is taken to reflect at least a certain structural similarity. As part of a comprehensive study seeking the 'natural' substrates of 'orphan' transporters that also serve to take up pharmaceutical drugs into cells, we have noted that many drugs bear structural similarities to natural products. A cursory inspection of common fluorophores indicates that they too are surprisingly 'drug-like', and they also enter at least some cells. Some are also known to be substrates of efflux transporters. Consequently, we sought to assess the structural similarity of common fluorophores to marketed drugs, endogenous mammalian metabolites, and natural products. We used a set of some 150 fluorophores along with standard fingerprinting methods and the Tanimoto similarity metric. Results: The great majority of fluorophores tested exhibited significant similarity (Tanimoto similarity > 0.75) to at least one drug, as judged via descriptor properties (especially their aromaticity, for identifiable reasons that we explain), by molecular fingerprints, by visual inspection, and via the "quantitative estimate of drug likeness" technique. It is concluded that this set of fluorophores does overlap with a significant part of both the drug space and natural products space. Consequently, fluorophores do indeed offer a much wider opportunity than had possibly been realised to be used as surrogate uptake molecules in the competitive or trans-stimulation assay of membrane transporter activities.

Algorithms for immunochromatographic assay: review and impact on future application

Lateral flow immunoassay (LFIA) is a critical choice for applications of point-of-care testing (POCT) in clinical and laboratory environments because of its excellent features and versatility. To obtain authentic values of analyte concentrations and reliable detection results, the relevant research has featured the application of a diversity of methods of mathematical analysis to technical analysis to allow for use with a small quantity of data. Accordingly, a number of signal and image processing strategies have also emerged for the application of gold immunochromatographic and fluorescent strips to improve sensitivity and overcome the limitations of correlative hardware systems. Instead of traditional methods to solve the problem, researchers nowadays are interested in machine learning and its more powerful variant, deep learning technology, for LFIA detection. This review emphasizes different models for the POCT of accurate labels as well as signal processing strategies that use artificial intelligence and machine learning. We focus on the analytical mechanism, procedural flow, and the results of the assay, and conclude by summarizing the advantages and limitations of each algorithm. We also discuss the potential for application of and directions of future research on LFIA technology when combined with Artificial Intelligence and deep learning.

Implication of human drug transporters to toxicokinetics and toxicity of pesticides

Human membrane drug transporters are recognized as major actors of pharmacokinetics. Pesticides also interact with human drug transporters, which may have consequences for pesticide toxicokinetics and toxicity. The present review summarizes key findings about this topic. In vitro assays have demonstrated that some pesticides, belonging to various chemical classes, modulate drug transporter activity, regulate transporter expression and/or are substrates, thus bringing the proof of concept for pesticide-transporter relationships. The expected low human concentration of pesticides in response to environmental exposure constitutes a key-parameter to be kept in mind for judging the in vivo relevance of such pesticide-transporter interactions and their consequences for human health. Existing data about interactions of pesticides with drug transporters remain, however, rather sparse; more extensive and systematic characterization of pesticide-transporter relationships, through the use of high throughput in vitro assays and/or in silico methods, is, therefore, required. In addition, consideration of transporter polymorphisms, pesticide mixture effects and physiological and pathological factors governing drug transporter expression may help to better define the in vivo relevance of pesticide-transporter interactions in terms of toxicokinetics and toxicity for humans. © 2019 Society of Chemical Industry.

Holding-Oriented versus Gating-Oriented Live-Cell Distinction: Highlighting the Role of Transporters in Cell Imaging Probe Development

Small molecule imaging probes are powerful tools to understand complex biological systems. The mainstreams of imaging probe developments have been focused on the target holding of the probes; the holding targets are often cell-type-specific biomarkers. This type of the probe mechanism can be designated as holding-oriented live-cell distinction (HOLD). Our group has worked on the development of cell-type-selective probes using a diversity-oriented fluorescence library approach (DOFLA), where unbiased phenotypic screening is employed using fluorescent library compounds. Through the conventional target identification methods such as an affinity-based analysis, we elucidated that some of the probe mechanisms are HOLD. However, we also realized that sometimes there is no specific holding target for probes or the holding targets are ubiquitous. The observation led us to test an alternative mechanism of cell-type-specific probes as gating-oriented live-cell distinction (GOLD). We started to examine the gating mechanism of probes, which is mainly based on transporters but which does not necessarily require probe holding to cellular targets. Transporters can control the in and out movement of various nutrients and chemicals. Different expression levels of transporters in various cell types could provide the molecular mechanism of differential staining of cells by regulating the intracellular accumulation of a certain specific probe. A number of GOLD probes have been developed by modifying or mimicking endogenous substrates of transporters such as inorganic ions, glucose, amino acids, or neurotransmitters, utilizing broad substrate specificity of transporters. The radiolabeled or fluorophore-conjugated substrate mimetics have been widely used for live cell distinction and various applications such as disease-related cell or tissue imaging. In humans, there are about 400 solute carrier (SLC) transporters and 50 ATP-binding cassette (ABC) transporters. Since some transporters have broad substrate specificity, they can transport not only derivatives of endogenous natural substrates but also totally synthetic diverse imaging probes, such as DOFLA probes. Without preconsidering the structure of endogenous substrates, we recently demonstrated a series of live-cell imaging probes and elucidated their molecular mechanism as a gating one, either by SLC or ABC transporters. Transporter inhibitor panel and CRISPR-based transporter libraries could provide a systematic gating target elucidation platform. Considering the generality of DOFLA and the CRISPR-based genomic tool for transporter systems (>450 in humans), the GOLD approach will offer new insight and promise for unprecedented levels of novel cell imaging probe development.

Involvement of multiple influx and efflux transporters in the accumulation of cationic fluorescent dyes by Escherichia coli

Background

It is widely believed that most xenobiotics cross biomembranes by diffusing through the phospholipid bilayer, and that the use of protein transporters is an occasional adjunct. According to an alternative view, phospholipid bilayer transport is negligible, and several different transporters may be involved in the uptake of an individual molecular type. We recognise here that the availability of gene knockout collections allows one to assess the contributions of all potential transporters, and flow cytometry based on fluorescence provides a convenient high-throughput assay for xenobiotic uptake in individual cells.

Results

We used high-throughput flow cytometry to assess the ability of individual gene knockout strains of E coli to take up two membrane-permeable, cationic fluorescent dyes, namely the carbocyanine diS-C3(5) and the DNA dye SYBR Green. Individual strains showed a large range of distributions of uptake. The range of modal steady-state uptakes for the carbocyanine between the different strains was 36-fold. Knockouts of the ATP synthase α- and β-subunits greatly inhibited uptake, implying that most uptake was ATP-driven rather than being driven by a membrane potential. Dozens of transporters changed the steady-state uptake of the dye by more than 50% with respect to that of the wild type, in either direction (increased or decreased); knockouts of known influx and efflux transporters behaved as expected, giving credence to the general strategy. Many of the knockouts with the most reduced uptake were transporter genes of unknown function (‘y-genes’). Similarly, several overexpression variants in the ‘ASKA’ collection had the anticipated, opposite effects. Similar results were obtained with SYBR Green (the range being approximately 69-fold). Although it too contains a benzothiazole motif there was negligible correlation between its uptake and that of the carbocyanine when compared across the various strains (although the membrane potential is presumably the same in each case).

Conclusions

Overall, we conclude that the uptake of these dyes may be catalysed by a great many transporters of putatively broad and presently unknown specificity, and that the very large range between the ‘lowest’ and the ‘highest’ levels of uptake, even in knockouts of just single genes, implies strongly that phospholipid bilayer transport is indeed negligible. This work also casts serious doubt upon the use of such dyes as quantitative stains for representing either bioenergetic parameters or the amount of cellular DNA in unfixed cells (in vivo). By contrast, it opens up their potential use as transporter assay substrates in high-throughput screening.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1561-0) contains supplementary material, which is available to authorized users.

Cytometry of Reaction Rate Constant: Measuring Reaction Rate Constant in Individual Cells To Facilitate Robust and Accurate Analysis of Cell-Population Heterogeneity

Robust and accurate analysis of cell-population heterogeneity is challenging but required in many areas of biology and medicine. In particular, it is pivotal to the development of reliable cancer biomarkers. Here, we prove that cytometry of reaction rate constant (CRRC) can facilitate such analysis when the kinetic mechanism of a reaction associated with the heterogeneity is known. In CRRC, the cells are loaded with a reaction substrate, and its conversion into a product is followed by time-lapse fluorescence microscopy at the single-cell level. A reaction rate constant is determined for every cell, and a kinetic histogram "number of cells versus the rate constant" is used to determine quantitative parameters of reaction-based cell-population heterogeneity. Such parameters include, for example, the number and sizes of subpopulations. In this work, we applied CRRC to a reaction of substrate extrusion from cells by ATP-binding cassette (ABC) transporters. This reaction is viewed as a potential basis for predictive biomarkers of chemoresistance in cancer. CRRC proved to be robust (insensitive to variations in experimental settings) and accurate for finding quantitative parameters of cell-population heterogeneity. In contrast, a typical nonkinetic analysis, performed on the same data sets, proved to be both nonrobust and inaccurate. Our results suggest that CRRC can potentially facilitate the development of reliable cancer biomarkers on the basis of quantitative parameters of cell-population heterogeneity. A plausible implementation scenario of CRRC-based development, validation, and clinical use of a predictor of ovarian cancer chemoresistance to its frontline therapy is presented.

Inhibition of Human Drug Transporter Activities by the Pyrethroid Pesticides Allethrin and Tetramethrin

Pyrethroids are widely-used chemical insecticides, to which humans are commonly exposed, and known to alter functional expression of drug metabolizing enzymes. Limited data have additionally suggested that drug transporters, that constitute key-actors of the drug detoxification system, may also be targeted by pyrethroids. The present study was therefore designed to analyze the potential regulatory effects of these pesticides towards activities of main ATP-binding cassette (ABC) and solute carrier (SLC) drug transporters, using transporter-overexpressing cells. The pyrethroids allethrin and tetramethrin were found to inhibit various ABC and SLC drug transporters, including multidrug resistance-associated protein (MRP) 2, breast cancer resistance protein (BCRP), organic anion transporter polypeptide (OATP) 1B1, organic anion transporter (OAT) 3, multidrug and toxin extrusion transporter (MATE) 1, organic cation transporter (OCT) 1 and OCT2, with IC50 values however ranging from 2.6 μM (OCT1 inhibition by allethrin) to 77.6 μM (OAT3 inhibition by tetramethrin) and thus much higher than pyrethroid concentrations (in the nM range) reached in environmentally pyrethroid-exposed humans. By contrast, allethrin and tetramethrin cis-stimulated OATP2B1 activity and failed to alter activities of OATP1B3, OAT1 and MATE2-K, whereas P-glycoprotein activity was additionally moderately inhibited. Twelve other pyrethoids used at 100 μM did not block activities of the various investigated transporters, or only moderately inhibited some of them (inhibition by less than 50%). In silico analysis of structure-activity relationships next revealed that molecular parameters, including molecular weight and lipophilicity, are associated with transporter inhibition by allethrin/tetramethrin and successfully predicted transporter inhibition by the pyrethroids imiprothrin and prallethrin. Taken together, these data fully demonstrated that two pyrethoids, i.e., allethrin and tetramethrin, can act as regulators of the activity of various ABC and SLC drug transporters, but only when used at high and non-relevant concentrations, making unlikely any contribution of these transporter activity alterations to pyrethroid toxicity in environmentally exposed humans.

A review of fluorescent signal-based lateral flow immunochromatographic strips

Fluorescent signal-based lateral flow immunochromatographic strips (FLFICS) have received great expectations since they combine the quantitative sensitivity of fluorescence analysis and the simplicity, rapidness, and portability of a common lateral flow immunochromatographic strip (LFICS).

A Fluorescent Probe for Neural Stem/Progenitor Cells with High Differentiation Capability into Neurons

Selection of a specific neural stem/progenitor cells (NSPCs) has attracted broad attention in regenerative medicine for neurological disorders. Here, we report a fluorescent probe, CDg13, and its application for isolating strong neurogenic NSPCs. In comparison to the NSPCs isolated by other biomarkers, CDg13-stained NSPCs showed higher capability to differentiate into neurons. Target identification revealed that the fluorescence intensity of the probe within cells is inversely proportional to the expression levels of mouse and human Abcg2 transporters. These findings suggest that low Abcg2 expression is a biomarker for neurogenic NSPCs in mouse brain. Furthermore, CDg13 can be used to isolate Abcg2^low cells from heterogeneous cell populations.

Evaluation of P-Glycoprotein Inhibitory Potential Using a Rhodamine 123 Accumulation Assay

In vitro evaluation of P-glycoprotein (P-gp) inhibitory potential is now a regulatory issue during drug development, in order to predict clinical inhibition of P-gp and subsequent drug-drug interactions. Assays for this purpose, commonly based on P-gp-expressing cell lines and digoxin as a reference P-gp substrate probe, unfortunately exhibit high variability, raising thus the question of developing alternative or complementary tests for measuring inhibition of P-gp activity. In this context, the present study was designed to investigate the use of the fluorescent dye rhodamine 123 as a reference P-gp substrate probe for characterizing P-gp inhibitory potential of 16 structurally-unrelated drugs known to interact with P-gp. 14/16 of these P-gp inhibitors were found to increase rhodamine 123 accumulation in P-gp-overexpressing MCF7R cells, thus allowing the determination of their P-gp inhibitory potential, i.e., their half maximal inhibitor concentration (IC50) value towards P-gp-mediated transport of the dye. These IC50 values were in the range of variability of previously reported IC50 for P-gp and can be used for the prediction of clinical P-gp inhibition according to Food and Drug Administration (FDA) criteria, with notable sensitivity (80%). Therefore, the data demonstrated the feasibility of the use of rhodamine 123 for evaluating the P-gp inhibitory potential of drugs.

The biological significance of brain barrier mechanisms: help or hindrance in drug delivery to the central nervous system?

Barrier mechanisms in the brain are important for its normal functioning and development. Stability of the brain's internal environment, particularly with respect to its ionic composition, is a prerequisite for the fundamental basis of its function, namely transmission of nerve impulses. In addition, the appropriate and controlled supply of a wide range of nutrients such as glucose, amino acids, monocarboxylates, and vitamins is also essential for normal development and function. These are all cellular functions across the interfaces that separate the brain from the rest of the internal environment of the body. An essential morphological component of all but one of the barriers is the presence of specialized intercellular tight junctions between the cells comprising the interface: endothelial cells in the blood-brain barrier itself, cells of the arachnoid membrane, choroid plexus epithelial cells, and tanycytes (specialized glial cells) in the circumventricular organs. In the ependyma lining the cerebral ventricles in the adult brain, the cells are joined by gap junctions, which are not restrictive for intercellular movement of molecules. But in the developing brain, the forerunners of these cells form the neuroepithelium, which restricts exchange of all but the smallest molecules between cerebrospinal fluid and brain interstitial fluid because of the presence of strap junctions between the cells. The intercellular junctions in all these interfaces are the physical basis for their barrier properties. In the blood-brain barrier proper, this is combined with a paucity of vesicular transport that is a characteristic of other vascular beds. Without such a diffusional restrain, the cellular transport mechanisms in the barrier interfaces would be ineffective. Superimposed on these physical structures are physiological mechanisms as the cells of the interfaces contain various metabolic transporters and efflux pumps, often ATP-binding cassette (ABC) transporters, that provide an important component of the barrier functions by either preventing entry of or expelling numerous molecules including toxins, drugs, and other xenobiotics. In this review, we summarize these influx and efflux mechanisms in normal developing and adult brain, as well as indicating their likely involvement in a wide range of neuropathologies. There have been extensive attempts to overcome the barrier mechanisms that prevent the entry of many drugs of therapeutic potential into the brain. We outline those that have been tried and discuss why they may so far have been largely unsuccessful. Currently, a promising approach appears to be focal, reversible disruption of the blood-brain barrier using focused ultrasound, but more work is required to evaluate the method before it can be tried in patients. Overall, our view is that much more fundamental knowledge of barrier mechanisms and development of new experimental methods will be required before drug targeting to the brain is likely to be a successful endeavor. In addition, such studies, if applied to brain pathologies such as stroke, trauma, or multiple sclerosis, will aid in defining the contribution of brain barrier pathology to these conditions, either causative or secondary.