Clinical application of a rapid, functional assay for multidrug resistance based on accumulation of the fluorescent dye, fluo-3

Understanding the molecular mechanism of host-based statin resistance in hepatitis C virus replicon containing cells

A number of statins, the cholesterol-lowering drugs, inhibit the in vitro replication of hepatitis C virus (HCV). In HCV-infected patients, addition of statins to the earlier standard of care therapy (pegIFN-α and ribavirin) resulted in increased sustained virological response rates. The mechanism by which statins inhibit HCV replication has not yet been elucidated. In an attempt to gain insight in the underlying mechanism, hepatoma cells carrying an HCV replicon were passaged in the presence of increasing concentrations of fluvastatin. Fluvastatin-resistant replicon containing cells could be generated and proved ∼8-fold less susceptible to fluvastatin than wild-type cultures. The growth efficiency of the resistant replicon containing cells was comparable to that of wild-type replicon cells. The fluvastatin-resistant phenotype was not conferred by mutations in the viral genome but is caused by cellular changes. The resistant cell line had a markedly increased HMG-CoA reductase expression upon statin treatment. Furthermore, the expression of the efflux transporter P-gp was increased in fluvastatin-resistant replicon cells (determined by qRT-PCR and flow cytometry). This increased expression resulted also in an increased functional transport activity as measured by the P-gp mediated efflux of calcein AM. In conclusion, we demonstrate that statin resistance in HCV replicon containing hepatoma cells is conferred by changes in the cellular environment.

Functional evaluation of multidrug resistance transporter activity in surgical samples of solid tumors

Determination of multidrug resistance (MDR) activity of tumor cells could provide important information for the personalized therapy of cancer patients. The functional calcein assay (MultiDrug Quant Assay, Solvo Biotechnology, Budaörs, Hungary) has been proven to be clinically valuable in hematological malignancies by determining the transporter activity of MDR protein 1 (MDR1, ATP-binding cassette protein [ABC] B1, P-glycoprotein-170) and MDR-related protein 1 (MRP1, ABCC1). In this study, we evaluated if the same functional test was adaptable for the analysis of MDR activity in solid tumors. For this purpose, tissue specimens of human colorectal cancer samples were subjected to limited enzymatic digestion by collagenase to provide a single-cell suspension; dead cells were excluded by 7-aminoactinomycin D staining, and epithelial cancer cells were detected by Cy5-conjugated anti-BerEP4 monoclonal antibody. The transporter functions of MDR1 and MRP1 in viable epithelial cells were assessed by flow cytometry detecting the intracellular accumulation of calcein dye after exposing cells to various MDR inhibitors. Collagenase disintegration preserved the MDR activity and the antigenicity of tumor cells. Thus using the extended calcein assay provided sufficient viable and functionally active tumor cells from surgical biopsies to determine the functional MDR activity. In conclusion, the newly described modified calcein assay may be applicable for evaluating the MDR phenotype in solid tissue specimens from colorectal forceps biopsy to surgical samples.

A continuous fluorescence assay for the study of P-glycoprotein-mediated drug efflux using inside-out membrane vesicles

A fluorimetric procedure for assaying the transport activity of P-glycoprotein (P-gp) using a membrane vesicle model has been developed. In this assay methylene blue is incorporated into inside-out vesicles prepared from human acute lymphoblastic leukemic cells resistant to 100 ng. ml-1 vinblastine (VBL100) and their sensitive controls. The fluorescence of a fluorescent derivative of colchicine (fluorescein-colchicine) is quenched as the probe is transported across the vesicle membrane. The fluorescein-colchicine transport was found to be dependent on the presence of P-glycoprotein, required ATP, and was inhibited by vanadate and the reversal agent, verapamil, in a dose-dependent manner. Furthermore, the transport was competed against by the P-gp substrates, vinblastine and methotrexate. The transport of fluorescein-colchicine by P-gp was found to be cooperative (n = 1. 23). The assay is rapid, requires small amounts of sample, and removes the need for the radioactive procedures used in the past. The assay should find use in characterizing the transport kinetics of P-gp, for examining and optimizing combinations of chemotherapeutics, and for examining the effects of reversal agents and substrates which potentially compete for transport with the fluorescent substrate probe. Other possible applications include examining P-gp-mediated transport properties of purified P-gp in reconstituted systems.

Modulation of the function of human MDR1 P-glycoprotein by the antimalarial drug mefloquine

MDR1 P-glycoprotein in membranes of human tumor cells of the CEM/VBL100 line was selectively labelled using photoreactive analogs of verapamil, N-(p-azido-3-[125I]salicyl)amino-verapamil ([125I]ASA-V) and prazosin, 2-[4-(4-azido-3-[125I]iodobenzoyl)piperazin-1-yl]4 -amino-6,7-dimethoxyyquinazoline ([125I]ASA-P). Mefloquine, a quinolinemethanol antimalarial drug, was shown to inhibit the labelling of P-glycoprotein with an efficiency similar to that for verapamil, a known chemosensitizer. By contrast, chloroquine competed poorly for the binding site on P-glycoprotein. Mefloquine also inhibited the functional activity of P-glycoprotein. It decreased the rates of extrusion of [3H]vinblastine and the fluorescent dyes, fluo-3 acetomethoxy ester and rhodamine 123, from drug-resistant cells and decreased the level of resistance of these cells to vinblastine. The ability of mefloquine to inhibit P-glycoprotein function may be involved in the neurotoxic side-effects occasionally associated with the use of mefloquine as an antimalarial drug.

Functional study of multidrug resistance with fluorescent dyes. Limits of the assay for low levels of resistance and application in clinical samples

Fluorescent dyes such as rhodamine 123 (R123) and Hoechst 33342 (Ho342) have been widely used to characterize multidrug-resistance (MDR) phenotype cells in cell populations, on the basis of their reduced accumulation in resistant cells. Taking advantage of the high fluorescence quantum yield of R123 and Ho342 compared with that of anthracyclines, we investigated the limits of fluorescence image cytometry in detecting MDR by the level of R123 and Ho342 accumulation and efflux. We were able to separate with this technique a cell line with a level of resistance as low as 3. We then studied the presence of MDR cells in lymphocytes isolated from patients with hematological malignancies.

A new method for a quantitative assessment of P-glycoprotein-related multidrug resistance in tumour cells

A rapid, functional and quantitative diagnostic method for the estimation of the P-glycoprotein (P-gp)-dependent multidrug resistance is required in the clinical treatment of human tumours, as chemotherapy protocols and resistance-reversing agents could be applied accordingly. In the present work, by using a calcein accumulation method in combination with immunorecognition and drug-resistance studies, a new method is described for the quantitative estimation of the expression and function of the multidrug transporter. MDR1-transfected and drug-selected tumour cell lines with various levels of drug resistance were examined. The expression of P-gp and its cell-surface appearance were assessed by quantitative immunoblotting and by immunofluorescence cytometry. The transport function of the P-gp was assessed by measuring the extrusion of calcein acetoxymethyl ester (AM) with fluorometry and flow cytometry, while in parallel experiments drug resistance was directly examined in cell survival assays. The MDR1 activity factor (MAF), calculated from the calcein AM extrusion assay, is demonstrated to provide a reliable quantitative measure for MDR1 specific activity, reflecting cellular drug resistance. This relatively simple and rapid new functional P-gp assay surpasses the formerly used techniques in both sensitivity and reproducibility.

Inheritance of chromosome 7 is associated with a drug-resistant phenotype in somatic cell hybrids

A major form of drug resistance in tumour cells known as classical multidrug resistance (MDR) is associated with the overexpression of the mdr1 gene product, the membrane protein P-glycoprotein (P-gp), which acts as an energy-dependent drug efflux pump. In this study the inheritance of P-gp expression was examined using hybrids formed after somatic cell fusion between a drug-sensitive human T-cell leukaemia cell line, CEM/CCRF, and a drug-resistant derivative, CEM/A7, which is characterized by a clonal chromosomal duplication dup(7)(q11.23q31.2). Fourteen hybrids, chosen at random, were analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) and by binding studies involving the monoclonal antibody MRK16, which recognises an external P-gp epitope. Only two hybrids were positive for both MRK16 antibody labelling and mdr1 mRNA. Partial karyotypic analysis of all hybrids revealed that only the MRK16-positive hybrids contained the duplication in chromosome 7 seen in the CEM/A7 parental MDR line. Therefore, P-gp overexpression in the MRK16-positive hybrids may be linked to the inheritance of chromosome 7 from CEM/A7 and possibly associated with the chromosome 7 abnormality.

Flow cytometric functional analysis of multidrug resistance by Fluo-3: a comparison with rhodamine-123

Using four cell lines including drug-sensitive K562/Parent cells, P-glycoprotein (Pgp)-mediated multidrug resistant (MDR) K562/VCR, K562/ADR and revertant K562/ADR-R cells, two fluorescent agents, Fluo-3 and rhodamine-123 (Rh-123), were compared as indicators in a functional assay of MDR. Cells were incubated with 4 microM Fluo-3 or 1 microM Rh-123 for 45 min and then the intracellular accumulation of the agent was measured using a flow cytometer. Verapamil (20 microM) or cepharanthine (biscoclaurine alkaloid, 10 microM) was added just before the fluorescent agents. Efflux patterns were also studied 60 min after incubation with or without verapamil and cepharanthine. Increased intracellular accumulation and a delayed efflux pattern of Fluo-3 by verapamil and cepharanthine were demonstrated in multidrug resistant K562/VCR and K562/ADR cells, indicating that Fluo-3 is another good indicator of MDR. However, a similar, but lower, increase in uptake and a delayed efflux pattern of Fluo-3 by verapamil and cepharanthine were also demonstrated even in Pgp-non-overexpressed K562/Parent cells. In contrast, accumulation of Rh-123 was not affected by verapamil and cepharanthine. To further study the Pgp dependency of Fluo-3, another cell line, K562/NC16 expressing minimum MDR1 mRNA, was cloned. Increased uptake and a delayed efflux pattern of Fluo-3, but not Rh-123, with verapamil or cepharanthine were again demonstrated in K562/NC16 cells, indicating that intracellular accumulation of Fluo-3 may be non-specifically influenced by verapamil and cepharanthine at very low levels of Pgp-related MDR, while the influx and efflux patterns of Rh-123 may be specifically affected by Pgp overexpression.

Dyes providing increased sensitivity in flow-cytometric dye-efflux assays for multidrug resistance

In an effort to improve detection of P-glycoprotein-mediated multidrug resistance (mdr), several dyes were compared to rhodamine 123 (R123) in efflux assays. Two dyes (SY-38 and SY-3150) were found that provided better sensitivity. These dyes were effluxed by a cell line known to be mdr-positive (P388/R84) but not by an mdr-negative cell line (P388). Efflux was blocked by both verapamil and cyclosporine A. Efflux from KG1a cells was less than from P388/R84, just as has been seen with R123 and daunomycin. In further experiments, a model system was used to demonstrate two-color immunofluorescence plus efflux measurements. This was done using a sequential staining method. A procedure was devised that, at least for this model system, allowed single-step staining with both dye and antibody. The sensitivity of the efflux measurement was slightly compromised by using this one-step staining method.

Calcein accumulation as a fluorometric functional assay of the multidrug transporter

Acetoxymethyl ester (AM) derivatives of various fluorescent indicators (fura-2, fluo-3, indo-1, BCECF, calcein) are actively extruded by the multidrug transporter (MDR1, P-glycoprotein-Homolya, L. et al. (1993) J. Biol. Chem. 268, 21493-21496). In the present paper we show that the measurement of the accumulation of a fluorescent cell viability marker, calcein, can be effectively used as a rapid and sensitive fluorometric and flow cytometric assay for studying P-glycoprotein function. The rate of calcein accumulation in human MDR1-expressing cells is significantly lower than in the control cells, while various drug-resistance reversing agents (verapamil, vinblastine, oligomycin, cyclosporin A and UIC2 monoclonal antibody) greatly increase calcein trapping only in the MDR1-expressing cells. Since calcein-AM is not fluorescent and free calcein is not a substrate of the multidrug transporter, the assay is readily applicable for rapid kinetic studies of the MDR1 function. Calcein has a high fluorescence intensity in the visible range, thus changes in calcein uptake can be easily visualised and MDR1-expressing and control cells separated by conventional flow cytometry.

Common expression of the multidrug resistance marker P-glycoprotein in B-cell chronic lymphocytic leukaemia and correlation with in vitro drug resistance

The expression of P-glycoprotein (Pgp), which is associated with multidrug resistance (MDR), was investigated in 20 B-cell chronic lymphocytic leukaemia (B-CLL) patients by flow cytometry using two Pgp-specific monoclonal antibodies (mAb), MRK-16 which recognizes an extracellular epitope, and JSB-1 which recognizes an intracellular epitope. Sixteen (80%) patients were positive with MRK-16 whereas all patients were positive with JSB-1. The proportion of Pgp-positive lymphocytes from each patient sample varied from 2-94% for MRK-16 and 20-93% for JSB-1. There was no correlation between the level of positivity and disease stage or treatment history. In vitro drug resistance to vincristine (VCR) and doxorubicin (DOX) was determined by the colorimetric MTT assay. All patients were resistant to one or both drugs being consistent with the expression of Pgp. There was no correlation between the level of resistance and disease stage or drug treatment. We investigated the expression of Pgp in the normal counterpart of the B-CLL cells, CD5+CD19+ B-lymphocytes. A minor subpopulation (3%) of CD5+CD19+ lymphocytes isolated from normal controls expressed Pgp suggesting that these cells may be the potential precursors to the B-CLL cell. We conclude that Pgp expression and drug resistance are inherent characteristics of the B-CLL lymphocyte.