Induced multidrug resistance in murine leukemia L1210 and associated changes in a surface-membrane glycoprotein

In vivo model systems in P-glycoprotein-mediated multidrug resistance

In this article we review the in vivo model systems that have been developed for studying P-glycoprotein-mediated multidrug resistance (MDR) in the preclinical setting. Rodents have two mdr genes, both of which confer the MDR phenotype: mdr 1a and mdr 1b. At gene level they show strong homology to the human MDR1 gene and the tissue distribution of their gene product is very similar to P-glycoprotein expression in humans. In vivo studies have shown the physiological roles of P-glycoprotein, including protection of the organism from damage by xenobiotics. Tumors with intrinsic P-glycoprotein expression, induced MDR or transfected with an mdr gene, can be used as syngeneic or xenogenic tumor models. Ascites, leukemia, and solid MDR tumor models have been developed. Molecular engineering has resulted in transgenic mice that express the human MDR1 gene in their bone marrow and in knockout mice missing a murine mdr gene. The data on pharmacokinetics, efficacy, and toxicity of chemosensitizers of P-glycoprotein in vivo are described. Results from studies using monoclonal antibodies directed against P-glycoprotein and other miscellaneous approaches for modulation of MDR are mentioned. The importance of in vivo studies prior to clinical trials is being stressed and potential pitfalls due to differences between species are discussed.

Modulation of doxorubicin-toxicity by tamoxifen in multidrug-resistant tumor cells in vitro and in vivo

Modulation of the resistance of tumors offers new strategies to improve the therapeutical treatment of cancer. In this report, the anti-oestrogen tamoxifen was investigated in multidrug-resistant tumor cells in vitro and in vivo. The doxorubicin-resistance of L 1210/DOX-tumor cells, which express the multidrug-resistance phenotype, could be completely circumvented by addition of 1 microgram/ml tamoxifen. In contrast, no increased effect could be observed in the parental L 1210 tumor cells or in cytosine arabinoside-resistant L 1210 cells not expressing the multidrug-resistance phenotype. Thus, the enhancing effect of tamoxifen was restricted only to the multidrug-resistant L 1210/DOX tumor cells. Similar to the in vitro experiments, a significant reduction in the growth in solid tumors of mice by the combined treatment of doxorubicin and tamoxifen was again observed only in the multidrug-resistant L 1210/DOX tumors.

Differing patterns of cross-resistance resulting from exposures to specific antitumour drugs or to radiation in vitro

This article reviews the patterns of cross-resistance identified in various P-glycoprotein-mediated and non-P-glycoprotein-mediated drug resistant mammalian tumour cell lines. The differing patterns of cross-resistance and the variable levels of resistance expressed are summarised and discussed. Although the mechanism by which P-glycoprotein can recognise and transport a large group of structurally-unrelated substrates remains to be defined, the recent evidence indicating that membrane associated domains participate in substrate recognition and binding is summarised, and other possible explanations for these variable cross-resistance patterns are considered. Amongst the non-P-glycoprotein-overexpressing multidrug resistant cell lines, two subsets are clearly identifiable, one lacking and the other expressing cross-resistance to the Vinca alkaloids. Resistance mechanisms implicated in these various sublines and possible explanations for their differing levels and patterns of cross-resistance are summarised. Clinical resistance is identified in patients following treatment not only with antitumour drugs, but also after radiotherapy. Experimental data providing a biological basis for this observation are summarised. A distinctive multiple drug resistance phenotype has been identified in tumour cells following exposure in vitro to fractionated X-irradiation characterised by: the expression of resistance to the Vinca alkaloids and the epipodophyllotoxins but not the anthracyclines and overexpression of P-glycoprotein which is post-translationally regulated, but without any concomitant overexpression of P-glycoprotein mRNA. Finally, the possible clinical relevance of these variable patterns of cross-resistance to the antitumour drugs commonly used in the clinic is considered.

Influence of sequential exposure to R-verapamil or B8509-035 on rhodamine 123 accumulation in human lymphoblastoid cell lines

Modulators for the reversal of multidrug resistance such as R-verapamil and B8509-035, a dihydropyridine, effectively overcome multidrug resistance in vitro and are currently undergoing clinical trial. One problem with their use is the application protocol; the question as to whether they should be given by continuous administration or in sequential doses in combination with the cytotoxic drugs has to be addressed. Therefore, we examined the influence of the exposure time and the sequence of modulator administration on the active transport of the fluorescent dye rhodamine 123 (R123), a substrate for the P-glycoprotein, in the resistant lymphoblastoid cell line VCR1000 and the parental nonresistant cell line CCRF-CEM. Our results demonstrate the importance of coadministration of R-verapamil and the cytotoxic agent for the modulation of multidrug resistance, whereas the exposure sequence does not seem to be such an essential parameter in the case of B8509-035. This observation should be considered for the further design of clinical studies.

P-glycoprotein as multidrug transporter: a critical review of current multidrug resistant cell lines

MDR has been studied extensively in mammalian cell lines. According to usual practice, the MDR phenotype is characterized by the following features: cross resistance to multiple chemotherapeutic agents (lipophilic cations), defective intracellular drug accumulation and retention, overexpression of P-gp (often accompanied by gene amplification), and reversal of the phenotype by addition of calcium channel blockers. An hypothesis for the function of P-gp has been proposed in which P-gp acts as a carrier protein that actively extrudes MDR compounds out of the cells. However, basic questions, such as what defines the specificity of the pump and how is energy for active efflux transduced, remain to be answered. Furthermore, assuming that P-gp acts as a drug transporter, one will expect a relationship between P-gp expression and accumulation defects in MDR cell lines. A review of papers reporting 97 cell lines selected for resistance to the classical MDR compounds has revealed that a connection exists in most of the reported cell lines. However, several exceptions can be pointed out. Furthermore, only a limited number of well characterized series of sublines with different degrees of resistance to a single agent have been reported. In many of these, a correlation between P-gp expression and transport properties can not be established. Co-amplification of genes adjacent to the mdr1 gene, mutations [122], splicing of mdr1 RNA [123], modulation of P-gp by phosphorylation [124] or glycosylation [127], or experimental conditions [26,78] could account for some of the complexity of the phenotype and the absence of correlation in some of the cell lines. However, both cell lines with overexpression of P-gp without increased efflux [i.e., 67,75] and cell lines without P-gp expression and accumulation defects/increased efflux [i.e., 25,107] have been reported. Thus, current results from MDR cell lines contradict--but do not exclude--that P-gp acts as multidrug transporter. Other models for the mechanism of resistance have been proposed: (1) An energy-dependent permeability barrier working with greater efficacy in resistant cells. This hypothesis is supported by studies of influx which, although few, all except one demonstrate decreased influx in resistant cells; (2) Resistant cells have a greater endosomal volume, and a greater exocytotic activity accounts for the efflux.(ABSTRACT TRUNCATED AT 400 WORDS)

Stereoisomers of calcium antagonists which differ markedly in their potencies as calcium blockers are equally effective in modulating drug transport by P-glycoprotein

The (-)-isomer of verapamil is 10-fold more potent as a calcium antagonist than the (+)-isomer. However, both enantiomers are equally effective in increasing cellular accumulation of anticancer drugs [Gruber et al., Int J Cancer 41: 224-226, 1988]. In addition to verapamil, there exists a wide variety of stereoisomers with phenylalkylamines and dihydropyridine structures which markedly differ in their potency as calcium antagonists. We have tested these drugs for their ability to increase intracellular accumulation of [3H]vinblastine ([3H]VBL) in a doxorubicin-resistant cell line (F4-6RADR) derived from the Friend mouse leukemia cell line (F4-6P) and in COS-7 monkey kidney cells. Both cell types express substantial amounts of multidrug resistance gene 1 mRNA and P-glycoprotein as revealed by RNA and immuno blot analysis. The enantiomers with phenylalkylamine structures [(+/-)-verapamil; (+/-)-devapamil; (+/-)-emopamil)] and with dihydropyridine structures [(+/-)-isradipine; (+/-)-nimodipine; (+/-)-felodipine; (+/-)-nitrendipine; (+/-)-niguldipine] increased [3H]VBL accumulation in both cell lines at micromolar concentrations. Although the stereoisomers of these drugs differ markedly in their potency as calcium channel blockers they were about equally effective in increasing VBL levels in the cells. There was no substantial difference in the potencies of the phenylalkylamine drugs in affecting cellular [3H]VBL transport. Major potency differences, however, were observed in the dihydropyridine drug series with the niguldipine isomers as the most effective drugs. Moreover, the niguldipine enantiomers were equally as effective in reversing VBL resistance in F4-6RADR cells as were the verapamil enantiomers. Since (-)-niguldipine (B859-35) displays a 45-fold lower affinity for calcium channel binding sites than (+)-niguldipine, but is equally potent in inhibiting drug transport by P-glycoprotein and in reversing drug resistance, it may be, in addition to (+)-verapamil, another useful candidate drug for the treatment of multidrug resistance in cancer patients.

Differential recognition of mdr1a and mdr1b gene products in multidrug resistant mouse tumour cell lines by different monoclonal antibodies

An immunocytochemical method was used to test the reactivity of the anti-P-glycoprotein antibodies, C219, MRK 16, JSB-1 and 265/F4 against multidrug resistant (MDR) variants derived from the human small cell lung carcinoma line, NCI-H69, the mouse fibrosarcoma line, RIF-1 and the mouse mammary tumour cell line, EMT6. C219 produced positive staining in MDR variants of both human and mouse tumour cell lines. MRK 16 and JSB-1 however recognised P-glycoprotein only in the human MDR cell lines and not in the mouse MDR cells. 265/F4 appeared the most selective of the monoclonal antibodies used, producing positive staining of MDR variants derived from the RIF-1 line, but not of MDR variants derived from the EMT6 line. Total RNA was prepared from the mouse cell lines and, following reverse transcription, cDNA sequences were amplified by the polymerase chain reaction with primers specific for either the murine mdr1a or the mdr1b genes. From this it was possible to show that only the mdr1a gene is overexpressed in the resistant EMT6 lines that do not stain with 265/F4 whereas both mdr1a and mdr1b are overexpressed in the positively staining resistant fibrosarcoma line, RIF/1.0. Low level expression of mdr1b was detected in the sensitive parent RIF-1 cells and increasing levels of expression correlated with increasing resistance in the lines, RIF/0.1, 0.2, 0.4 and 1.0. Expression of mdr1a was found only in the more resistant fibrosarcoma cell lines. It seems that 265/F4 recognises only the mdr1b P-glycoprotein. Western blotting confirmed that this antibody detects a 170 kDa protein only in membranes derived from the resistant fibrosarcoma cells. 265/F4 may thus be used to distinguish between the murine P-glycoprotein isoforms so revealing differences between tumour cell lines in cellular localisation and in the time of appearance of mdr1a and mdr1b P-glycoprotein following drug exposure.

The multidrug-resistance gene MDR1 is expressed in human glial tumors

The most consistantly reported alteration of multidrug-resistant carcinoma cells is the overexpression of a membrane glycoprotein, termed P-glycoprotein. In this study we examined whether the strong intrinsic chemotherapy resistance of glial tumors might be related to the expression of the MDR1 gene which codes for P-glycoprotein. Fourteen glial tumors were examined immunohistochemically using the monoclonal antibody C219. In addition, RNA samples of 11 of these tumors were analysed using a sensitive Northern blot assay. P-glycoprotein is expressed in all 14 glial tumors; the number of stained tumor cells, however, varied considerably ranging from 0.3% to 15%. There was no correlation between the number of MDR1-positive cells and the histological malignancy. Varying amounts of MDR1 mRNA were detectable in 7 from 11 examined tumors. The results of our study show that the MDR1 gene is expressed in human glial tumors and suggest that the multidrug transporter may contribute to the clinical non-responsiveness of these tumors to chemotherapy.

Selective growth-inhibition of multidrug-resistant CHO-cells by the monoclonal antibody 265/F4

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Circumvention of multi-drug resistance in human kidney and kidney carcinoma in vitro

This report investigated whether the calmodulin inhibitor, trifluoperazine, can circumvent multi-drug resistance both in primary tissue cultures of human kidney and kidney carcinoma. For detection of inherent multi-drug resistance, the expression of P-glycoprotein was determined by immunofluorescence and immunocytochemistry using the monoclonal antibody C219. For detection of doxorubicin resistance and reversal of this resistance by trifluoperazine, the incorporation of nucleic acid precursor was measured after addition of doxorubicin and trifluoperazine, respectively. Both P-glycoprotein expressing resistant normal and malignant kidney tissue cultures could be modified by trifluoperazine. However, sensitive normal kidney and kidney carcinoma cultures were little affected by trifluoperazine. Thus, circumvention of primary resistance to doxorubicin is not limited to tumor cells. This might have important implications for the use of resistance modifiers in the clinical setting.

Circumvention of drug resistance in cisplatin-resistant sublines of the human squamous carcinoma cell line HLac 79 in vitro and in vivo

In a previous report we have characterized cisplatin (CDDP)-resistant sublines (HLac 79-DDP1 to DDP4) of the recloned squamous cell head and neck cancer (SCHNC) line HLac 79-ML revealing significant alterations of glutathione (GSH) metabolism and drug accumulation. In order to overcome CDDP-resistance in HLac 79 cells we now investigated the effect of buthionine sulfoximine (BSO), a specific inhibitor of GSH synthesis, verapamil (VRP), a calcium channel blocker that has been found to modulate resistance towards a broad spectrum of antineoplastic drugs, cyclosporin A (CSA), an immunosuppressive agent probably affecting drug pharmacokinetics, and aphidicolin (APC), a fungal metabolite interfering with DNA repair through inhibition of DNA polymerase alpha, on HLac 79 CDDP-sensitivity. Using the colorimetric MTT assay, GSH depletion with BSO led to a significant decrease of the 50% inhibitory drug concentration (IC50) in all HLac 79 sublines by dose modifying factors (IC50 CDDP/IC50 BSO + CDDP) ranging from 1.8 to 3.3. VRP, CSA or APC were not effective to overcome CDDP resistance in HLac 79 cells. The potential of BSO to modulate CDDP resistance in vitro was tested in vivo in HLac 79 tumor bearing NMRI nu-nu mice subsequently. Oral administration of BSO 7 days prior and during (days -7 to 8) CDDP treatment (3 mg/kg bw i.p. days 0, 4, 8) produced a significant prolongation of mean survival time mean as compared to chemotherapy alone. This held true for both the maternal line ML in terms of chemosensitization (CDDP: mean = 40.2 +/- 15.9 days vs. CDDP + BSO: mean = 80.3 +/- 30.4 days, p less than 0.001) and the CDDP resistant subline DDP4 in terms of partially overcoming secondary drug resistance (CDDP: mean = 56.5 +/- 13.6 days vs. CDDP + BSO: mean = 72.5 +/- 15.8 days, p less than 0.001). Enhanced toxicity of combined BSO and CDDP treatment manifested by transient 10% reduction of animal mean body weight.

Activity of various amphiphilic agents in reversing multidrug resistance of L 1210 cells

Several compounds (bamipine, chlorphenoxamine, estracyt, hycanthone, quinidine, quinine, tamoxifen, trifluoperazine and verapamil) have a common basic structure with the following features: lipophilic aromatic ring system; linked chain hydrophilic N-alkyl group. They are used medically for varying diseases. Their activity in reversing multidrug-resistance (MDR) with other compounds (diethylstilbestrol, beta-estradiol, methylbiguanide, methylpiperazine, testosterone) lacking one of these chemical features is compared. The in vitro test system we used was the nucleoside incorporation assay using parental L 1210 ascites tumor cells and a doxorubicin resistant subline, which expresses the MDR phenotype. The substances lacking one of these features were not effective in reversing the MDR whereas all other tested substances demonstrated modulating potential in the MDR resistant L 1210 cells.

Correlations between natural resistance to doxorubicin, proliferative activity, and expression of P-glycoprotein 170 in human kidney tumor cell lines

The natural resistance to doxorubicin of 15 human renal carcinoma cell lines was analyzed and compared to proliferative activity and expression of P-glycoprotein. We found a significant negative correlation between proliferative activity and natural resistance to doxorubicin, as well as between proliferative activity and the expression of P-glycoprotein. A positive correlation between resistance and expression of P-glycoprotein was found.

Relationship of DNA ploidy to chemoresistance of tumors as measured by in vitro tests

To examine whether patients with aneuploid tumors might derive more benefit from chemotherapy than would patients with diploid tumors, predictive tests for determining resistance in human tumors were carried out and the test results compared with the DNA ploidy of the corresponding tumors. Multidrug-resistance in 15 kidney carcinomas grown as primary cultures was determined by immunofluorescence by Mab C219, which is specific for the plasma membrane glyco-protein P-170, and by the use of tritiated nucleotide incorporation after addition of doxorubicin. Aneuploid tumors had a higher tendency to be more sensitive than diploid tumors, but the correlation was not significant. This was confirmed by reanalyzing our earlier data on ovarian and lung cancers. In conclusion, DNA measurement using flow cytometry does not appear to be a suitable tool for prediction of resistance of human tumors to chemotherapy.

Detection of P-glycoprotein in human leukemias using monoclonal antibodies

Overexpression of a Mr 170,000 membrane glycoprotein (P-glycoprotein) is consistently associated with multidrug resistance in cell lines. Two monoclonal antibodies (Mab) against P-glycoprotein (265/F4 and C 219) were used to examine tumour samples from patients with leukemias for evidence of P-glycoprotein overexpression. High levels of P-glycoprotein (greater than 5% positive cells) were detected with both antibodies in samples from 3 out of 18 patients suggesting that a multidrug resistant phenotype may also occur in human leukemias.