Bromocriptine Reverses P-Glycoprotein-mediated Multidrug Resistance in Tumor Cells

One of the most important causes of anticancer treatment failure is the development of multidrug resistance (MDR). The main characteristics of tumor cells displaying the MDR phenomena are cross-resistance to structurally unrelated cytotoxic drugs having different mechanisms of action and the overexpression of the MDR1 gene, which encodes a transmembrane glycoprotein named P- glycoprotein (P-gp). This study evaluated whether bromocriptine, a D₂ dopaminergic receptor agonist, influenced anticancer drug cytotoxicity and P-gp activity in a P-gp-expressing cell line compared to a non-expressing subline. The K_i values for P-gp of cyclosporine and verapamil were 1.09 and 540 μ M , respectively, and that of bromocriptine was 6.52 μ M in a calcein-AM efflux assay using porcine kidney epithelial LLC-PK1 and L-MDR1 cells, overexpressing human P-gp. Bromocriptine at 10 μ M reduced the IC₅₀ of doxorubicin (DXR) in K562-DXR from 9000 to 270 ng/ml and that of vincristine (VCR) in K562-VCR from 700 to 0.30 ng/ml, whereas the IC₅₀ values of DXR and VCR in the K562 subline were only marginally affected by these drugs. Bromocriptine restored the anticancer effect of DXR, VCR, vinblastine, vinorelbine and etoposide on MDR-tumor cells overexpressing P-gp. These observations suggest that bromocriptine has the potential to reverse tumor MDR involving the efflux protein P-gp in the clinical situation.     

Cross-resistance and collateral sensitivity to natural product drugs in cisplatin-sensitive and -resistant rat lymphoma and human ovarian carcinoma cells

 The cytotoxicity of mitotic spindle poisons, vinca alkaloids and the anthracycline, adriamycin, against cisplatin-sensitive and -resistant rat lymphoma and human ovarian carcinoma cell lines was investigated. Interestingly, it was found that all cell lines were more sensitive to the mitotic spindle poisons, vincristine and vinblastine. Adriamycin was the least effective and taxol had intermediate activity. The Walker rat lymphoma cell line resistant to cisplatin (WR) exhibited the multiple drug resistance phenotype since it showed collateral resistance to all drugs (ranging from twofold to taxol, colcemid and colchicine and sixfold to the vinca alkaloids). Verapamil potentiated the cytotoxic activity of adriamycin and vincristine in a striking fashion with the Walker cells. P-glycoprotein was found to be present in the plasma membranes of the Walker cells with approximately a 2.5-fold increase in the WR as compared to the sensitive (WS) cells. Glutathione levels were elevated in all of the cisplatin-resistant cell lines when compared to the cisplatin-sensitive parental cell lines. A profound effect of buthionine sulfoximine pretreatment on adriamycin cytotoxicity was observed. Glutathione S-transferase (π) was present in all the human cell lines but the WS cells had markedly lower levels (almost negligible) when compared to the WR cells. These observations imply that cisplatin-resistant cells may be more sensitive to mitotic spindle poisons and vinca alkaloids, irrespective of the mechanism of platinum resistance, and that the cytotoxicity of vinca alkaloids could be further modulated by verapamil, irrespective of the presence or absence of P-glycoprotein. Received: 8 November 1994/Accepted: 22 May 1995     

Induction of multiple-drug resistance during anti-neoplastic chemotherapy in vitro

Induction of P-glycoprotein-related multi-drug-resistance (MDR) has been shown in normal and malignant tissues to result from environmental stresses such as heat shock, exposure to carcinogens or X-ray irradiation. To identify conditions under which MDR is enhanced during anti-neoplastic chemotherapy, a cell line showing low-level intrinsic MDR was investigated. In the pleural mesothelioma cell line, PXF1118, less than 1% of cells expressed P-glycoprotein (P-gp), as shown by immunocytochemical staining with monoclonal antibody (MAb) MRK16. Exposure of PXF1118 to vincristine, vindesine, vinblastine or doxorubicin for 2-3 weeks led to an increase in the MDR cell fraction of up to 15-28% during 2 to 3 weeks. For doxorubicin and vindesine, dose-dependence was observed: drug concentrations not capable of eliciting cytotoxicity failed to induce significant P-gp expression. Nutrient starvation in aging medium, exposure to activated cyclophosphamide (even at high concentrations) or cisplatin caused only negligible MDR induction. After exposure to vindesine for 6 weeks, tumor colonies exhibited highly enhanced resistance to Vinca alkaloids, doxorubicin, etoposide and dacarbacine, whereas their sensitivity to mitomycin, activated cyclophosphamide or cisplatin remained unchanged. As determined by [3H]-thymidine uptake and proliferation antigen expression, induction of MDR phenotype was observed at minimal proliferative activity with no change in cell count during exposure to anti-cancer drugs, thus suggesting that the drug treatments changed the phenotype of the cells rather than selecting for a resistant sub-population. In addition, changes in cell differentiation were observed during MDR induction. Induction of P-gp during exposure to anti-cancer drugs thus provides a model for MDR development during initially successful chemotherapy. of P-gp during exposure to anti-cancer drugs thus provides     

Bromocriptine reverses P-glycoprotein-mediated multidrug resistance in tumor cells.

One of the most important causes of anticancer treatment failure is the development of multidrug resistance (MDR). The main characteristics of tumor cells displaying the MDR phenomena are cross-resistance to structurally unrelated cytotoxic drugs having different mechanisms of action and the overexpression of the MDR1 gene, which encodes a transmembrane glycoprotein named P-glycoprotein (P-gp). This study evaluated whether bromocriptine, a D2 dopaminergic receptor agonist, influenced anticancer drug cytotoxicity and P-gp activity in a P-gp-expressing cell line compared to a non-expressing subline. The K(i) values for P-gp of cyclosporine and verapamil were 1.09 and 540 microM, respectively, and that of bromocriptine was 6.52 microM in a calcein-AM efflux assay using porcine kidney epithelial LLC-PK1 and L-MDR1 cells, overexpressing human P-gp. Bromocriptine at 10 microM reduced the IC50 of doxorubicin (DXR) in K562-DXR from 9000 to 270 ng/ml and that of vincristine (VCR) in K562-VCR from 700 to 0.30 ng/ml, whereas the IC50 values of DXR and VCR in the K562 subline were only marginally affected by these drugs. Bromocriptine restored the anticancer effect of DXR, VCR, vinblastine, vinorelbine and etoposide on MDR-tumor cells overexpressing P-gp. These observations suggest that bromocriptine has the potential to reverse tumor MDR involving the efflux protein P-gp in the clinical situation.     

Modulation of vinblastine cytotoxicity by dilantin (phenytoin) or the protein phosphatase inhibitor okadaic acid involves the potentiation of anti-mitotic effects and induction of apoptosis in human tumour cells.

Cellular insensitivity to vinca alkaloids is suggested to be primarily due to drug efflux by P-glycoprotein (P-gp). The anti-epileptic phenytoin (DPH), which does not bind to P-gp, can selectively enhance vincristine (VCR) cytotoxicity in wild-type (WT) or multidrug-resistant (MDR) cells. We now demonstrate that the protein phosphatase inhibitor okadaic acid (OKA) can mimic the effect of DPH by selectively enhancing cytotoxicity of vinblastine (VBL), but not taxol and doxorubicin, in human leukaemia HL-60 cells. Both DPH and OKA potentiate the anti-mitotic effects of VBL by enhanced damage to the mitotic spindle, resulting in prolonged growth arrest. Also, unlike VBL alone, in human leukaemia or non-small-cell lung carcinoma cells treated with VBL plus DPH, recovery from damage to the mitotic spindle is compromised in drug-free medium and cell death by apoptosis in interphase ensues. Since protein phosphatases are involved with the regulation of metaphase to anaphase transit of cells during the mitotic cycle, enhanced VBL cytotoxicity in the presence of DPH or OKA may involve effects during metaphase on the mitotic spindle tubulin leading to growth arrest and apoptosis in interphase. These novel results suggest that DPH or OKA could be powerful tools to study cellular effects of vinca alkaloids and possibly for the development of novel therapeutic strategies.     

Megestrol acetate reverses multidrug resistance and interacts with P-glycoprotein

Summary We evaluated the multidrug resistance (MDR)-modulating effects of progesterone (PRG) and an orally active, structurally related compound, megestrol acetate (MA), in several MDR human cell lines. At 100 m, both steroids inhibited the binding of aVinca alkaloid photoaffinity analog to P-glycoprotein (P-gp) in MDR human neuroblastic SH-SY5Y/VCR cells [which show >1500-fold resistance to vincristine (VCR) in the tetrazolium dye (MTT) assay]. However, 100 m MA markedly enhanced the binding of [³H]-azidopine to P-gp in both SH-SY5Y/VCR cells and the MDR human epidermoid KB-GSV2 cell line (which displays 250-fold resistance to VCR in the MTT assay). PRG had little effect on the binding of [³H]-azidopine to P-gp. MA at low doses was more effective than PRG in sensitizing cells to VCR and enhancing their accumulation of [³H]-VCR. The highly resistant SH-SY5Y/VCR subline exhibited significant collateral sensitivity to both steroids. These data suggest that MA may be a clinically useful modulator of MDR.This work was supported by NIH grant CA-47652. The first author (G. E. F.) was supported by PHS grant DK07134-15     

MDR1 (ABCB1) G1199A (Ser400Asn) polymorphism alters transepithelial permeability and sensitivity to anticancer agents

Purpose  P-glycoprotein (P-gp), encoded by MDR1 (or ABCB1), is important in anticancer drug delivery and resistance. We evaluated alterations in P-gp-mediated transport of anticancer agents due to the MDR1 G1199A polymorphism. Methods  Using stable recombinant epithelial cells expressing wild-type (MDR1 _wt ) or G1199A (MDR1 _1199A ), anticancer drug sensitivity and transepithelial permeability were evaluated. Results  The recombinant cells MDR1 _wt and MDR1 _1199A displayed comparable doxorubicin resistance. However, MDR1 _1199A cells displayed greater resistance to vinblastine, vincristine, paclitaxel, and VP-16 (11-, 2.9-, 1.9-, and 2.9-fold, respectively). Alterations in transepithelial permeability paralleled these changes. Efflux of doxorubicin was similar between MDR1 _wt - and MDR1 _1199A -expressing cells, while P-gp-mediated transport was greater for vinblastine and vincristine in MDR1 _1199A cells (2.9- and 2.0-fold, respectively). Conclusions  The occurrence and magnitude of the MDR1 G1199A effect is drug specific. Overall, the MDR1 G1199A polymorphism may impact anticancer efficacy through modulation of drug distribution and delivery to target tumor cells.     

Rapid up-regulation of mdr1 expression by anthracyclines in a classical multidrug-resistant cell line.

Studies were carried out in a variant human multidrug-resistant (MDR) cell line CEM/A7R, which expresses very low levels of mdr1 mRNA and P-glycoprotein (P-gp). The induction of mdr1 RNA expression by three anthracyclines, (doxorubicin, daunorubicin, epirubicin), VP-16 and two vinca alkaloids (vincristine, vinblastine) was semiquantitatively assessed by scanning Northern blots on a phosphorimager. The relative level of mdr1 expression was expressed as ratio of mdr1 to the internal RNA (actin). A significant increase (P < 0.02) in expression of mdr1 was noted within 4 hrs of exposure to 1.5 micrograms ml-1 daunorubicin or epirubicin. Neither vinblastine nor vincristine had any effect on mdr1 levels after an 8 h exposure. With increasing concentrations of daunorubicin or epirubicin in a fixed 24 h time period, mdr1 expression increased, although a biphasic response was seen. Based on MRK 16 binding, an increase in P-gp levels was seen in the CEM/A7R line after a 24 h exposure to 1 microgram ml-1 daunorubicin or epirubicin. The rapid increase in mdr1 expression after a short period of exposure to doxorubicin, daunorubicin or epirubicin suggests that induction of mdr1 expression may have an important role in the development of drug-resistant tumours.     

Reversal activity of cyclosporin A and its metabolites M1, M17 and M21 in multidrug-resistant cells

Cyclosporin A (CSA) is an effective inhibitor of the P-glycoprotein (P-gp) activity and has been shown to modulate multidrug resistance (MDR) in in vitro experimental models. During degradation of CSA, the metabolites arising from the parental compound reach high levels in the serum of patients, and it is not clear whether these metabolites maintain the reversal activity of the parental compound, like the metabolites of verapamil. In an in vitro experimental model, we compared the reversal activity of CSA and 3 CSA metabolites (M1, M17, and M21) in the range of concentrations obtained in whole blood during a clinical trial with CSA used as a revertant agent. As experimental model we used LoVo-resistant cells. Our in vitro studies indicated that the metabolic hydroxylation and demethylation of CSA lead to molecules that greatly differ from the parent drug in their reversal activity. In the range of concentration detected in the whole blood of the patients (1–3 μM), CSA had a significant reversal activity. It decreased the IC₅₀ of anti-neoplastic drugs involved in MDR (vincristine, taxol, doxorubicin and etoposide) but not the IC₅₀ of platinum or methotrexate. CSA increased intracellular doxorubicin content and inhibited P-gp ³[H]azidopine photolabeling. Conversely, CSA metabolite concentrations superimposable to those observed in the patients (0.5–2.2 μM) had no sensitizing effects on the cytotoxicity of MDR-related anti-neoplastic drugs, nor did they affect ³[H]azidopine photolabeling or doxorubicin uptake. This study demonstrates that, during degradation of CSA, metabolite derivatives arise that have a very different reversal activity from that of the parental compound. Int. J. Cancer 71: 900-906, 1997. © 1997 Wiley-Liss Inc.     

Reversal of the human and murine multidrug-resistance phenotype with megestrol acetate

MA is an orally active PG derivative with an excellent safety profile that is used primarily for the treatment of carcinomas of the breast and endometrium. We investigated the potential application of MA as an MDR-reversal agent using cell culture and human tumor xenograft models. The reversing activity of MA in vitro was compared with that of PG and VER in two human MDR cell lines, the colon carcinoma HCT-116/VM46 and the breast carcinoma MCF-7/ADR, and in a murine cell line, J774.2. At concentrations as low as 3 M, MA was capable of partially restoring sensitivity to Act D in the HCT-116/VM46 cells and sensitivity to DOX in the MCF-7/ADR cells. Although less effective than VER, MA was about 2.5 times more potent than PG in reversing MDR at equimolar concentrations. Increased accumulation of DOX in drugresistant cells that were treated simultaneously with MA was observed by flow cytometry. In vivo, using established human colon and breast carcinoma xenografts implanted s.c. in athymic mice, the combined therapy with MA and DOX resulted in enhanced antitumor activity relative to that of DOX alone in the MDR sublines. These results suggest that MA may be a promising clinical MDR-reversing agent.Abbreviations Act D actinomycin D - DOX doxorubicin - PG progesterone - MA megestrol acetate - VER verapamil - VBL vinblastine - MDR multidrug resistance - P-gp P-glycoprotein - MAP medroxyprogesterone acetate - MTD maximum tolerated dose - q4dx3 every 4 days for 3 treatments - TVDD tumor volume-doubling time - IC₅₀ drug concentration producing 50% cell kill in treatéd cultures as compared with controls - PBS phosphate-buffered saline - VM-26 teniposide The part of this work performed at Albert Einstein College of Medicine was supported by USPHS grant CA39821     

Overexpression of the ABC transporter TAP in multidrug-resistant human cancer cell lines.

Multidrug resistance (MDR) to anti-cancer drugs has been associated with the overexpression of P-glycoprotein (P-gp) and the multidrug resistance-associated protein (MRP), both being members of the ATP-binding cassette (ABC) superfamily of transporters. We investigated whether in addition to P-gp and MRP, another ABC transporter, the transporter associated with antigen processing (TAP), is associated with MDR. TAP plays a major role in MHC class I-restricted antigen presentation by mediating peptide translocation over the endoplasmic reticulum membrane. TAP1 and P-gp share a significant degree of homology among their transmembrane domains, which are thought to be the primary determinants of substrate specificity, and both can apparently mediate the translocation of peptides. Using immunocytochemistry and Western blot, TAP was overexpressed in parallel with MHC class I in several MDR human cancer cell lines. TAP was overexpressed more frequently in MRP-positive MDR cell lines (three out of three) than in P-gp positive MDR cells (two out of five). Reversal of resistance resulted in a decrease in TAP levels. Transfection of the TAP genes into TAP-deficient lymphoblastoid T2 cells conferred mild resistance to etoposide, vincristine and doxorubicin (2- to 2.5-fold). Furthermore, etoposide and vincristine inhibited TAP-dependent peptide translocation to the endoplasmic reticulum. Collectively, our results suggest that TAP may modestly contribute to the MDR phenotype, in particular in MRP- overexpressing MDR cells. Further insight into the role of TAP in MDR will require the study of other transfectants, as well as the investigation of TAP expression in P-gp and MRP-negative MDR cancer cell lines.     

Dibenzocyclooctadiene lingnans: a class of novel inhibitors of P-glycoprotein

Purpose: To determine if five dibenzocyclooctadiene lingnans, a class of naturally occurring compounds from Schisandra chinensis (Turcz.) Baill, have the activities to reverse P-glycoprotein (P-gp) mediated multidrug resistance (MDR). Methods: The IC₅₀s of four MDR cell lines (K562/Adr, MCF-7/Adr, KBv200, and Bcap37/Adr) toward daunorubicin, vincristine, and paclitaxel in the presence or absence of one of the dibenzocyclooctadiene lingnans were determined by a FACscan assay. The intracellular daunorubicin accumulation in the four MDR cell lines was determined by incubation of cells with daunorubicin (2 μg/ml) in the presence or absence of one of the dibenzocyclooctadiene lingnans by a FACscan assay. The interaction of the five dibenzocyclooctadiene lingnans with P-gp was assayed by their inhibition of ³H-azidopine photoaffinity labeling of P-gp. Results: Among the five lingnans, while schisandrin A and B, and schisantherin A demonstrated strong and comparable activities to reverse the drug resistance and the intracellular drug accumulation in four MDR cell lines, schisandrol A and B showed very limited activities. The poor activities of schisandrol A and B are possibly caused by the hydroxyl groups on the cyclooctadiene ring, because the activities of the molecules resumed when the hydroxyl group was esterified to form a benzoate. Further studies demonstrated that these compounds physically interacted with P-gp. Conclusion: Schisandrin A and B, and schisantherin A are potent P-gp inhibitor and is of potential for future clinical application.     

Failure of taxol-based combination chemotherapy for malignant glioma cannot be overcome by G2/M checkpoint abrogators or altering the p53 status

In an effort to develop more effective forms of adjuvant chemotherapy for malignant brain tumors, we sought to develop a taxol-based combination chemotherapy regimen for glioma cell lines in vitro. Here, we report that coexposure of LN-229 or T98G glioma cells to taxol and either doxorubicin, camptothecin, cytarabine, or VM26 resulted in antagonistic effects rather than additive or synergistic cytotoxicity. There were no interactions of taxol with the effects of carmustine (BCNU) or vincristine. Antagonism was more prominent in cytotoxic cell death assays than in clonogenic cell death assays and was not overcome by G₂/M checkpoint abrogators such as caffeine or pentoxyfilline. Ectopic expression of mutant and wild-type p53val¹³⁵ attenuated taxol cytotoxicity in both T98G cells, which are mutant for p53, and LN-229 cells, which exhibit functional wild-type p53 activity. Interestingly, wild-type p53val¹³⁵ abrogated the taxol-imposed G₂/M arrest in both cell lines. However, wild-type p53val¹³⁵ did not promote glioma cell killing by combinations of taxol and any of the other drugs. Further, an analysis of a panel of 12 human glioma cell lines revealed no relationship between genetic or functional p53 status and taxol sensitivity. In summary, combination either with other chemotherapy drugs, with abrogators of the G₂/M checkpoint, with wild-type p53 gene transfer was not a promising approach for a taxol-based combination chemotherapy regimen in malignant glioma. Received: 14 December 1998 / Accepted: 15 February 1999     

Sipholenol A, a marine-derived sipholane triterpene, potently reverses P-glycoprotein (ABCB1)-mediated multidrug resistance in cancer cells

Through extensive screening of marine sponge compounds, the authors have found that sipholenol A, a sipholane triterpene isolated from the Red Sea sponge, Callyspongia siphonella, potently reversed multidrug resistance (MDR) in cancer cells that overexpressed P-glycoprotein (P-gp). In experiments, sipholenol A potentiated the cytotoxicity of several P-gp substrate anticancer drugs, including colchicine, vinblastine, and paclitaxel, but not the non-P-gp substrate cisplatin, and significantly reversed the MDR of cancer cells KB-C2 and KB-V1 in a concentration-dependent manner. Furthermore, sipholenol A had no effect on the response to cytotoxic agents in cells lacking P-gp expression or expressing MDR protein 1 or breast cancer resistance protein. Sipholenol A (IC(50) > 50 microM) is not toxic to all the cell lines that were used, regardless of their membrane transporter status. Accumulation and efflux studies with the P-gp substrate [(3)H]-paclitaxel demonstrated that sipholenol A time-dependently increased the intracellular accumulation of [(3)H]-paclitaxel by directly inhibiting P-gp-mediated drug efflux. In addition, sipholenol A did not alter the expression of P-gp after treating KB-C2 and KB-V1 cells for 36 h and 72 h. However, it efficaciously stimulated the activity of ATPase of P-gp and inhibited the photolabeling of this transporter with its transport substrate [(125)I]-iodoarylazidoprazosin. Overall, the present results indicate that sipholenol A efficiently inhibits the function of P-gp through direct interactions, and sipholane triterpenes are a new class of potential reversing agents for treatment of MDR in P-gp-overexpressing tumors.     

Cellular and biochemical characterization of thioxanthenes for reversal of multidrug resistance in human and murine cell lines

We have previously shown that phenothiazines sensitize multidrug resistant (MDR) cells to chemotherapeutic drugs in a manner related to specific structural features, and have identified structurally related thioxanthenes with increased anti-MDR activity. We have now studied the structure-activity relationships of 16 thioxanthenes in the human breast cancer line MCF-7 AdrR. trans-Thioxanthene stereoisomers were 2- to 7-fold more potent than cis-thioxanthenes for antagonizing MDR. The most potent thioxanthenes possessed a halogenated tricyclic ring connected by a 3-carbon alkyl bridge to a piperazinyl or piperadinyl side group. The chemosensitizing effects of the lead compound, trans-flupenthixol, its stereoisomer cis-flupenthixol, its phenothiazine homologue fluphenazine, and the calcium channel blocker verapamil, were further studied in a series of sensitive and MDR cell lines. trans-Flupenthixol caused a greater reversal of cellular resistance to doxorubicin, vinblastine, vincristine, and colchicine in MCF-7 AdrR, KB-V1, and P388/DOX MDR cells than the other chemosensitizers. In particular, trans-flupenthixol was 2- to 3-fold more potent for reversing MDR than equimolar concentrations of verapamil. Furthermore, trans-flupenthixol fully reversed resistance to doxorubicin, vincristine, and colchicine in MDR MCF-7 and NIH 3T3 cells transfected with the mdr1 gene. None of these agents altered MDR in a non-P-glycoprotein expressing MCF-7 cell line selected with mitoxantrone, nor in any of the parental cell lines. The stereoselective antagonism of the flupenthixol isomers on several putative cellular targets was studied to explore the mechanism of their chemosensitizing activity. cis- and trans-flupenthixol were equally active inhibitors of protein kinase C and calmodulin. Both cis- and trans-flupenthixol were also potent inhibitors of [3H]azidopine binding to P-glycoprotein. The apparent lack of clinical toxicity of trans-flupenthixol makes it an attractive drug for possible use in the modulation of tumor resistance in vivo if appropriate tissue concentrations can be achieved.     

In vitro prevention of the emergence of multidrug resistance in a pediatric rhabdomyosarcoma cell line.

We have established preclinical models for the development of drug resistance to vincristine (a major drug used in the treatment of pediatric rhabdomyosarcoma) using cell lines. The RD cell line has a mutant P53 phenotype and does not have detectable P-glycoprotein (P-gp) or multidrug resistance-related protein (MRP) despite expressing low levels of mdr-1 mRNA, which encodes P-gp and mrp1 mRNA. Resistant variants of RD were derived by exposure to increasing concentrations of vincristine. This was repeated on six occasions, resulting in three cell lines which could tolerate 64 x the IC(50) concentration. Six independent agents were tested for their ability to prevent the development of resistance in this model. Despite at least 10 attempts, resistance did not develop in the presence of the multidrug resistance (MDR) modulators PSC833, VX710, and XR9576. This strongly suggests that these agents may delay or even prevent the development of resistance to vincristine. This was also confirmed in a second rhabdomyosarcoma cell line, Rh30. In contrast, the agents indomethacin (MRP1 modulator), CGP41251 (protein kinase C inhibitor), and dexrazoxane (putative MDR prevention agent) did not affect the development of resistance in the RD model. Characterization of the resistant cell lines indicated the presence of increased mdr-1 and P-gp expression, which resulted in resistance to the agents doxorubicin, etoposide, and vincristine but not cisplatin. The resistance could be modulated using PSC833 or VX710, confirming that functional P-gp is present. No apparent differences were seen between the resistant cell lines derived in the absence and presence of the various agents. These experiments strongly suggest that the development of MDR may be preventable using modulators of MDR and merit clinical studies to test this hypothesis.     

Circumvention of multidrug resistance by a quinoline derivative, MS-209, in multidrug-resistant human small-cell lung cancer cells and its synergistic interaction with cyclosporin A or verapamil

Purpose and methods: To develop a clinically useful approach to circumvent P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) in MDR human small-cell lung cancer (SCLC), we examined the ability of a novel quinoline compound, MS-209, to reverse MDR by inhibition of P-gp function in combination with other MDR-reversing drugs using a cytotoxicity assay. Results: We established MDR human SCLC cells by culture in medium with gradually increasing concentrations of adriamycin (ADM). Compared with the parental human SCLC cells, SBC-3, the MDR variant SBC-3 cells obtained (SBC-3/ADM) were highly resistant to various chemotherapeutic agents due to P-gp expression. MS-209 reversed the resistance to ADM and vincristine (VCR) of SBC-3/ADM and H69/VP cells in a dose-dependent manner. Moreover, MS-209 in combination with cyclosporin A (CsA) or verapamil (VER) synergistically enhanced the antitumor effects of ADM and VCR on SBC-3/ADM cells. MS-209 restored ADM incorporation and this effect was enhanced by CsA and VER, suggesting that these synergistic effects were due to competitive inhibition of P-gp function. Conclusion: MS-209 in combination with CsA or VER might increase the efficacy of these chemotherapeutic agents against MDR human SCLC cells. Received: 10 December 1997 / Accepted: 16 April 1998