Mechanism of multidrug resistance

Inconstant association between 27-kDa heat-shock protein (Hsp27) content and doxorubicin resistance in human colon cancer cells. The doxorubicin-protecting effect of Hsp27

To investigate the role of the small 27-kDa heat-shock protein (Hsp27) in the intrinsic resistance of colon cancer cells to doxorubicin, we modified Hsp27 expression either genetically by transfection or pharmacologically by cisplatin treatment. HT-29 cells were transfected with a full-length Hsp27 construct in the sense or antisense orientation. We found a good correlation between cell survival after doxorubicin treatment and Hsp27 content. A similar correlation was found for the thermoresistance of the Hsp27-transfected cells. In contrast, the sensitivity of the different transfected cells to 5-fluorouracil was not modified. cis-Platinum(II)diammine dichloride (cisplatin) treatment of HT-29 or Caco2 cells dramatically increased their Hsp27 mRNA and protein content. Accordingly, the cells became thermoresistant. Contrary to what has been previously assumed, however, cell resistance to doxorubicin was reduced. Our data suggest that the decreased resistance of the cells to doxorubicin may be due to a concomitant increase of topoisomerase II expression, the main target of anthracyclines. In conclusion, although Hsp27 seems to participate in the natural resistance of colon cancer cells to anthracyclines, its increase after cisplatin treatment is not associated with a decreased cytotoxicity to doxorubicin.

Cancer cell progression and chemoimmunotherapy--dual effects in the induction of resistance to therapy

To determine whether resistance to chemoimmunotherapy is acquired during therapy, we investigated the effects of chemotherapeutic agents and anti-tumour polysaccharide, lentinan, on the progression of Rous sarcoma virus-induced S908.D2 fibrosarcomas. The chemoimmunotherapy was effective against the parental S908.D2-bearing mice. Nearly all the mice that were treated with cyclophosphamide (CY) and lentinan achieved complete tumour regression. Only a few of the mice that achieved complete regression of the primary tumours showed a recurrence of the tumour in regional lymph nodes. S908.D2-vp.1 was established from metastatic tumours that developed in the regional lymph nodes of parental S908.D2-bearing mice during therapy. S908.D2-vp.2-or vp.3 cells were sequentially derived in a similar way from S908.D2-vp.1-or-vp.2-bearing mice respectively, in which complete tumour regression at each primary site was achieved during therapy. These lines acquired resistance to CY and lentinan and also to 5-fluorouracil (5-FU)/5'-deoxy-5-fluorouracil and lentinan. No significant difference in either the sensitivity to 5-FU or 4-deoxycyclophosphamide in vitro or in the susceptibility to immune effector cells was observed between the parental and progressed lines (S908.D2-vp1 -vp3). There was an increase in the level of prostaglandin E2 (PGE2) in the progressed lines during repeated therapy (parental, 1171 pg ml(-1); vp.1, 2199 pg ml(-1); vp.2, 5500pg ml(-1); vp3, 16187 pg ml(-1)). There was no significant increase in the production of transforming growth factor beta (TGF-beta). The amount of interleukin-2 (IL-2) produced by spleen cells isolated from the S908.D2-vp.2-bearing mice was decreased compared with the amount produced by the parental S908.D2- bearing mice. Furthermore, combination therapy with lentinan and IL-2 achieved complete tumour regression in all the mice transplanted with S908.D2 progressed tumour lines, although IL-2 alone did not show any anti-tumour effects in either the S908.D2 parental or progressed lines. The findings suggest that the reduced production of IL-2 induced an increase in the production of the PGE2 by progressed tumour lines is involved in the acquisition of resistance.

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.

Functional detection of MDR1/P170 and MRP/P190-mediated multidrug resistance in tumour cells by flow cytometry

Multidrug resistance (MDR) in tumour cells is often caused by the overexpression of the plasma membrane drug transporter P-glycoprotein (P-gp) or the recently discovered multidrug resistance-associated protein (MRP). In this study we investigated the specificity and sensitivity of the fluorescent probes rhodamine 123 (R123), daunorubicin (DNR) and calcein acetoxymethyl ester (calcein-AM) in order to detect the function of the drug transporters P-gp and MRP, using flow cytometry. The effects of modulators on the accumulation and retention of these probes were compared in several pairs of sensitive and P-gp- as well as MRP-overexpressing cell lines. R123, in combination with the modulator PSC833, provided the most sensitive test for detecting P-gp-mediated resistance. Moreover, in a 60 min drug accumulation assay R123 can be regarded as a P-gp-specific probe, since R123 is not very efficiently effluxed by MRP. In contrast to R123, a 60 min DNR or calcein-AM accumulation test could be used to detect MRP-mediated resistance. The MRP-specific modulator genistein could be used in combination with DNR, but not with calcein-AM. Vincristine (VCR) can be used to increase the cellular uptake of calcein-AM in MDR cells, but is not specific for MRP. Thus, although the combination of DNR with genistein appeared to be as sensitive as the combination of calcein-AM with VCR, the former may be used to probe specific MRP activity whereas the latter provides a combined (P-gp + MRP) functional MDR parameter. With these functional assays the role and relative importance of P-gp and MRP can be studied in, for example, haematological malignancies.

Chemosensitisation of spontaneous multidrug resistance by a 1,4-dihydropyridine analogue and verapamil in human glioma cell lines overexpressing MRP or MDR1

Multidrug resistance phenotypes in human tumours are associated with the overexpression of the 170 kDa P-glycoprotein encoded by the multidrug resistance 1 (MDR1) gene, and also with that of the non-P-glycoprotein-mediated multidrug resistance gene, MRP, which encodes a 190 kDa membrane ATP-binding protein. We have previously reported that overexpression of MRP appears to be responsible for spontaneous multidrug resistance in some human glioma cell lines (Abe et al., Int. J. Cancer, 58, 860-864, 1994). In this study, we investigated whether chemosensitising agents of P-glycoprotein-mediated multidrug resistance such as verapamil, a biscoclaurine alkaloid (cepharanthine), and a dihydropyridine analogue (NIK250) could also reverse multidrug resistance in human glioma cells. The glioma cell lines were the two MRP-expressing cell lines, T98G and IN500, an MDR1-expressing cell line, CCF-STTG1, and the MRP1 MDR1-non-expressing cell line, IN157. Verapamil and NIK250 almost completely reversed drug resistance to vincristine, etoposide and doxorubicin in T98G cells, while they also reversed drug resistance to vincristine and etoposide, but only partially to doxorubicin in IN500 cells. Cepharanthine as well as verapamil and NIK250 reversed vincristine resistance in CCF-STTG1 cells, but cepharanthine only partially reversed drug resistance in T98G and IN500 cells. The cellular accumulation of [3H]etoposide increased about 2- and 3-fold compared with control in T98G cells in the presence of verapamil and NIK250 respectively. Furthermore, the release of doxorubicin from the nuclei of T98G cells was blocked by NIK250. However, NIK250 and verapamil caused no apparent increase in vincristine accumulation in T98G cells. NIK250 or verapamil might exert inhibitory effects upon MRP function, resulting in a reversal of MRP-mediated spontaneous multidrug resistance in cultured human glioma cells.

The role of methoxymorpholino anthracycline and cyanomorpholino anthracycline in a sensitive small-cell lung-cancer cell line and its multidrug-resistant but P-glycoprotein-negative and cisplatin-resistant counterparts

The cytotoxic action of two morpholino anthracyclines, methoxymorpholino anthracycline (MRA-MT, FCE 23,762) and cyanomorpholino anthracycline (MRA-CN), was compared with the cytotoxicity of doxorubicin (DOX), the topoisomerase II inhibitor etoposide (VP-16), the topoisomerase I inhibitor camptothecin, methotrexate, and cisplatin in GLC4, a human small-cell lung-cancer cell line, in GLC4-Adr, its P-glycoprotein (Pgp)-negative, multidrug-resistant (MDR; 100-fold DOX-resistant) subline with overexpression of the MDR-associated protein (MRP) and a lowered topoisomerase II activity, and in GLC4-CDDP, its cisplatin-resistant subline. GLC4-Adr was about 2-fold cross-resistant for the morpholino anthracyclines and GLC4-CDDP was, relative to GLC4, more resistant for the morpholino anthracyclines than for DOX. Overall, MRA-CN was about 2.5-fold more cytotoxic than MRA-MT. The cytotoxicity profile of the morpholino anthracyclines in these cell lines mimicked that of camptothecin.

Expression of multidrug resistance-associated protein (MRP), MDR1 and DNA topoisomerase II in human multidrug-resistant bladder cancer cell lines

The acquisition of the multidrug resistance phenotype in human tumours is associated with an overexpression of the 170 kDa P-glycoprotein encoded by the multidrug resistance 1 (MDR1) gene, and also with a 190 kDa membrane ATP-binding protein encoded by a multidrug resistance-associated protein (MRP) gene. Human bladder cancer is a highly malignant neoplasm which is refractory to anti-cancer chemotherapy. In order to understand the mechanism underlying multidrug resistance in bladder cancer, we established three doxorubicin-resistant cell lines, T24/ADM-1, T24/ADM-2 and KK47/ADM, and one vincristine-resistant cell line, T24/VCR, from human bladder cancer T24 and KK47 cells respectively. Both T24/ADM-1 and T24/ADM-2 cells which had elevated MRP mRNA levels showed both a cross-resistance to etoposide and a decreased intracellular accumulation of etoposide. T24/VCR cells which had elevated levels of MDR1 mRNA and P-glycoprotein but not of MRP mRNA, showed cross-resistance to doxorubicin. On the other hand, KK47/ADM cells, which had elevated levels of both MRP and MDR1 mRNA and a decreased level of topoisomerase II mRNA, were found to be cross-resistant to etoposide, vincristine and a camptothecin derivative, CPT-11. Our present study demonstrates a concomitant induction of increased levels of MRP mRNA, decreased levels of topoisomerase II mRNA and decreased drug accumulation during development of multidrug resistance in human bladder cancer cells. The enhanced expression of the MRP gene is herein discussed in a possible correlation with the decreased expression of the topoisomerase II gene.

Inhibition of the ATPase activity of P-glycoprotein by porphyrin photosensitization of multidrug-resistant cells in vitro

The effectiveness of photodynamic therapy against P-glycoprotein ATPase activity in multidrug-resistant cells was studied. Chinese hamster ovary AUXB1 (drug-sensitive) and CR1R12 (multidrug-resistant) cell lines were compared with respect to uptake of 14C-polyhematoporphyrin and porphyrin photosensitization. Phototoxicity of Photofrin was similar in both cell lines, and no major differences in uptake or efflux of 14C-polyhematoporphyrin were observed. Porphyrin photosensitization in vitro of CR1R12 cells or isolated plasma membranes from these cells caused inhibition of P-glycoprotein ATPase activity. Application of porphyrin photosensitization at a sublethal level to CR1R12 cells resulted in a small but significant increase in adriamycin-induced cytotoxicity. The hydrophobic "picket-fence" porphyrin, meso-tetrakis-(o-propionamidophenyl)porphyrin, alpha,alpha,alpha,beta-isomer, was more inhibitory toward P-glycoprotein ATPase activity than the two less hydrophobic porphyrins tetraphenylporphine tetrasulfonate and Photofrin.

Zidovudine induces the expression of cellular resistance affecting its antiviral activity

We have previously shown that multidrug-resistant cells expressing the multidrug transporter P-glycoprotein are less sensitive to the antiviral activity of AZT. Subsequently, we addressed the question whether AZT itself is able to induce cellular resistance to the drug. Indeed, CEM cells propagated in the presence of increasing concentrations of AZT become resistant to the antigrowth and antiviral activity of AZT but do not express detectable level of P-glycoprotein. Sensitivity of these cells to other compounds, such as vinblastine, vincristine, ddI, and ddC remained unchanged, indicating that, in contrast to P-glycoprotein-positive cells, AZT-induced resistance is specific for AZT. Interestingly, in AZT-induced resistant cells the intracellular accumulation of AZT and exogenous deoxythymidine, as well as thymidine kinase activity, are significantly reduced when compared with the parental cell line. Our findings show that AZT itself may directly induce the expression of cellular mechanisms leading to the acquisition of specific cellular resistance that can affect its antiviral activity.

Synergism between gamma interferon and doxorubicin in a human MDR colon adenocarcinoma cell line

Recent interest in cancer therapy derives from the ability of interferons to synergistically increase the activity of chemotherapeutic agents. To understand the biological basis of this synergism we evaluated the effects of human recombinant IFN-gamma on the expression of the mdr1 gene and on the cellular growth of a human colon adenocarcinoma cell line (LoVo) and its MDR subline (LoVo/Dx) after coincubation with doxorubicin. Treatment with IFN-gamma showed unchanged levels of MDR1-glycoprotein, no perturbation on cell cycle distribution and a significant reduction of colony formation in both lines (P < 0.05) starting from 100 U/ml. A synergistic effect was observed in the LoVo/Dx cell line when doxorubicin was added after exposure to 0.1-10 U/ml of IFN-gamma. Our data indicate that the effects of IFN-gamma, independent from action on cell proliferation and from modulation of p-glycoprotein expression, are a cause of the synergistic activity between this lymphokine and conventional chemotherapeutic agents such as doxorubicin.

Retrovirus-mediated transfer of the multidrug resistance gene into human haemopoietic progenitor cells

We report the utilization of cord blood (CB) or bone marrow (BM) derived low density or purified CD34+ cells as a target for human multidrug resistance (MDR1) gene transfer. Cells were cocultivated for 48 h with an irradiated MDR1 retroviral producer line. Since some degree of MDR1 gene expression has been reported to occur in haemopoietic progenitor cells and in peripheral blood cells, efficiency of MDR1 gene transfer was assessed by: (1) Drug selection and culture in presence of 50 ng/ml doxorubicin, 10 ng/ml colchicine and 0.85 micrograms/ml taxol. In uninfected control, 1-2% of CFU-GM and CFU-GEMM were found to be drug-resistant, while 14-31% of original clonogenic activity was found after 2 weeks of culture of transduced cells. Efficiency of MDR1 transfer was significantly enhanced by prestimulation with cytokines, and found to be significantly superior in CB-derived compared to BM-derived progenitors. (2) Analysis of MDR1 gene expression by evaluating MDR1 mRNA through polymerase chain reaction. MDR1 expression was very low in cultures of uninfected controls, whereas, after drug selection, MDR1 mRNA levels in transduced cells was as high as in the MDR1 retroviral producer line (positive controls). (3) Flow cytometric analysis of the expression of CD34 and P-glycoprotein, the product of the MDR1 gene. After MDR1 transduction and 2 weeks of culture, membrane expression of P-glycoprotein was found on 17-25% of viable CD34+ cells. (4) Cytochemical localization by APAAP staining of P-glycoprotein. No specific localization was found in untransduced controls, whereas transduced and cultured CB-cells expressed P-glycoprotein on plasma and nuclei membrane. In conclusion, MDR1 gene transfer into CB- and BM-derived progenitor cells seems a feasible and attractive approach to generate a drug-resistant haemopoiesis.

Daunorubicin pharmacokinetics and the correlation with P-glycoprotein and response in patients with acute leukaemia

The aim of this study was to examine the relationship between the pharmacokinetics of daunorubicin (DNR), overexpression of P-glycoprotein (Pgp) and treatment response in acute leukaemia. Twenty-seven patients with acute leukaemia received DNR as part of induction therapy. The plasma and cellular levels of DNR and its metabolite daunorubicinol (DOL) were determined using high-performance liquid chromatography. There were no significant differences between patients who went into complete remission (12/23) compared with those who did not respond for the following pharmacokinetic parameters: DNR and DOL plasma AUC (area under the curve) and DNR plasma half-life and clearance. There was a significant difference in the cellular DNR and DOL AUC between responders and non-responders (P < 0.02). Seven patients were Pgp positive and 18 Pgp negative. There was no correlation between patient response and the presence of Pgp (P > 0.1), nor was there any correlation between the cellular concentration of DNR or DOL and Pgp (P > 0.3). To our knowledge this is the first report examining the relationship between DNR pharmacokinetics, patient response and Pgp expression. Our data indicated that acute leukaemia patients responding to chemotherapy had higher cellular DNR and DOL than non-responders; also, overexpression of Pgp appeared not to be the sole explanation for the lower cellular DNR levels as expected from in vitro studies.

Mobile ionophores are a novel class of P-glycoprotein inhibitors. The effects of ionophores on 4'-O-tetrahydropyranyl-adriamycin incorporation in K562 drug-resistant cells

The decrease of the intracellular concentration of drug in resistant cells compared to sensitive cells is, in most cases, correlated with the presence, in the membrane of resistant cells, of a 170-kDa P-glycoprotein responsible for an active efflux of the drug. In an attempt to identify mechanism(s) by which multidrug resistance can be circumvented, we have examined the cellular accumulation of 4'-O-tetrahydropyranyl-adriamycin, alone and in conjunction with various ionophores on the one hand and with cyclosporin A on the other hand. The present study was performed using a spectrofluorometric method with which it is possible to follow continuously the uptake and release of fluorescent molecules by living cells, as the incubation of the cells with the drug proceeds. Erythroleukemia K562 cell lines were used. Using experimental conditions in which these ionophores were unable to modify either the intracellular pH, or the transmembrane potential, or to induce an intracellular ATP depletion, we have shown that mobile ionophores as well as cyclosporin inhibit the P-glycoprotein-mediated efflux of 4'-O-tetrahydropyranyl-adriamycin in K562 resistant cells, whereas gramicidin, a channel-forming ionophore, does not. The concentration that must be used to inhibit 50% of the efflux was 0.7 microM for valinomycin, 0.4 microM for nonactin, 0.2 microM for nigericin, 1.1 microM for monensin, 0.4 microM for lasalocid, 1.2 microM for calcimycin and 0.4 microM for cyclosporin. Due to the high toxicity of the ionophores, the observation that they increased 4'-O-tetrahydropyranyl-adriamycin accumulation in the multidrug-resistant cells is not correlated with an effect of these compounds on drug resistance. However, the correlation exists in the case of cyclosporin. From our data showing that lipophilic neutral complexes, formed between carboxylic ionophores and metal ions, are both able to inhibit the P-glycoprotein-mediated efflux of anthracycline we can infer that the lipophilicity but not the cationic charge is an important physical property.

Cooperative P-glycoprotein mediated daunorubicin transport into DNA-loaded plasma membrane vesicles

Most of the multidrug resistant human tumor cell lines overexpress the MDR1 gene product P-glycoprotein (P-gp) which is believed to function as an energy-dependent drug efflux pump. Here we describe a novel method that allows the kinetic characterization of P-gp-mediated active drug transport. This method is based on the fluorescence quenching of anthracyclines transported into DNA-loaded plasma membrane vesicles. The uptake of daunorubicin (DNR) into the plasma membrane vesicles was saturable in terms of the extravesicular DNR concentration with a Km of 1.5 +/- 0.1 microM. This transport occurred by a cooperative process with a Hill coefficient close to 2 for DNR. A model is discussed in which P-gp pumps two molecules of drug per turnover.

Modulation of tumor cell response to chemotherapy by the organ environment

The outcome of cancer metastasis depends on the interaction of metastatic cells with various host factors. The implantation of human cancer cells into anatomically correct (orthotopic) sites in nude mice can be used to ascertain their metastatic potential. While it is clear that vascularity and local immunity can retard or facilitate tumor growth, we have found that the organ environment also influences tumor cell functions such as production of degradative enzymes. The organ microenvironment can also influence the response of metastases to chemotherapy. It is not uncommon to observe the regression of cancer metastases in one organ and their continued growth in other sites after systemic chemotherapy. We demonstrated this effect in a series of experiments using a murine fibrosarcoma, a murine colon carcinoma, and a human colon carcinoma. The tumor cells were implanted subcutaneously or into different visceral organs. Subcutaneous tumors were sensitive to doxorubicin (DXR), whereas lung or liver metastases were not. In contrast, sensitivity to 5-FU did not differ between these sites of growth. The differences in response to DXR between s.c. tumors (sensitive) and lung or liver tumors (resistant) were not due to variations in DXR potency or DXR distribution. The expression of the multidrug resistance-associated P-glycoprotein as determined by flow cytometric analysis of tumor cells harvested from lesions in different organs correlated inversely with their sensitivity to DXR: increased P-glycoprotein was associated with overexpression of mdr1 mRNA. However, the organ-specific mechanism for upregulating mdr1 and P-glycoprotein has yet to be elucidated.

Malignant phenotype correlating with drug resistance in two human neuroblastoma cell lines

The main cause for the failure of chemotherapeutic treatment of advanced cancer probably lies in the emergence of drug resistance clones. In the present study we compared the sensitivity to adriamycin (ADR) and the capacity of ADR uptake in two human neuroblastoma cell lines differing in properties relevant to metastatic potential, the GP2 and MB, of low- and high-malignancy phenotype, respectively. Examination of the ADR effect on in vitro proliferative capacity of the two cell lines revealed a higher sensitivity of GP2 as compared to the MB variant. Intracellular ADR accumulation was determined by fluorocytometry, spectrofluorometry and fluorescence microscopy. According to the three methods, the GP2 line cells, representing a low-malignancy phenotype, had a higher uptake ability than the MB cells, possessing a phenotype of higher aggressiveness. The quantitative determination revealed that over a broad range of ADR concentrations, the GP2 cells accumulated 2-3.5 folds the amount of cytotoxic agent penetrating the MB cells. The FACS analysis showed that the cell population of each of the variants consisted of two subpopulations varying in their ability to accumulate ADR. In the GP2 line the high permeability subpopulation represented nearly half of the total cell population, whereas in the MB line this subpopulation represented a minority. The correlation observed between ADR uptake capacity and sensitivity to the cytotoxic agent, as evidenced by its effect on proliferative capacity, suggests that the resistance of the MB cells is due to a P-G-P modification-related mechanism.

The multidrug-resistance-reverser verapamil interferes with cellular P-glycoprotein-mediated pumping of daunorubicin as a non-competing substrate

We examined P-glycoprotein-mediated verapamil transport, using two drug-sensitive and multi-drug resistant cell-line couples, i.e. A2780, 2780AD and SW-1573, SW-1573/1R500. The interaction of 3H-labeled verapamil with cells was measured using a flow-through system. The verapamil-containing medium was pumped over the cells and monitored on-line for radioactivity. In the P-glycoprotein-expressing cells, verapamil accumulation was increased by vinblastine and some known multidrug resistant (MDR) modifiers. Subsequent removal of these modifiers caused release of verapamil into the medium against a verapamil concentration gradient. In this manner, we obtained evidence that verapamil is actively transported by the MDR-related P-glycoprotein. Using the flow-through system, we also exposed the cells to flowing culture medium containing daunorubicin, and measured the inhibition of daunorubicin efflux by verapamil. We found that, although the active efflux of daunorubicin was maximally blocked by verapamil short-term, longer-term active efflux of daunorubicin resumed. At a daunorubicin concentration in the flowing medium of 5 microM, increasing the verapamil concentration resulted in the same short-term effects, but in a significantly longer period of a maximal inhibition of daunorubicin efflux from the cells. At a daunorubicin concentration of 20 microM, increasing the verapamil concentration affected neither the short-term nor the long-term effects. These and other observations are in agreement with a model in which daunorubicin and verapamil are non-competing substrates for P-glycoprotein. In conclusion, we obtained evidence that verapamil is actively transported by the MDR-related P-glycoprotein and that verapamil and daunorubicin are non-competing substrates for P-glycoprotein. Consequently, the effectiveness of verapamil as an MDR antagonist may be compromised because it is extruded by P-glycoprotein.

The effect of MDR-1 gene expression on outcome in acute myeloblastic leukaemia

Resistance to cytotoxic agents may be encountered during the treatment of acute myeloblastic leukaemia (AML). P-glycoprotein encoded by the MDR-1 gene has been implicated as a potential drug resistance mechanism in leukaemic cells. In recent years, many data have been accrued concerning the expression of P-glycoprotein in leukaemia, and several studies have been published which have related MDR status to outcome in AML. Conclusions as to the effect of P-glycoprotein expression on prognosis in AML have varied widely. The studies are not immediately comparable, since they differ in methodology, treatment regimens, demographic profile and, perhaps most importantly, criteria for positivity of MDR status. The technique of statistical overview (meta-analysis) can be used to pool observational studies. Application of this statistical method to existing studies suggests an estimated relative risk of 0.68 for P-glycoprotein expression with respect to complete remission in AML. Further large studies are required to determine fully the role of P-glycoprotein in AML.

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 microM, 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 drug-resistant 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.

Dominant expression of multidrug resistance in intraspecific murine lymphoma hybrid cells

Cultured P388/VCR mouse lymphoma cells resistant to vincristine (VCR) and to 5-bromodeoxyuridine (BUdR) and deficient in thymidine kinase (TK-) were fused with P388/DAG cells resistant to 1.2:5,6-dianhydrogalactitol (DAG), an anticancer alkylating agent, and to 6-thioguanine (6-TG) and deficient in hypoxanthine phosphoribosyl-transferase (HPRT-). The hybrid cells expressed multidrug resistance (MDR), i.e., resistance to VCR and cross-resistance to Adriamycin (ADM) and actinomycin D (Act. D), in a dominant manner. The presence of glycoprotein p170, the MDR gene product, was detected in the hybrid cells. Resistance to DAG was also expressed dominantly, whereas cross-resistance to dibromodulcitol (DBD), a chemically related anticancer drug, was slight.

A plasma membrane 'vacuum cleaner' for daunorubicin in non-P-glycoprotein multidrug-resistant SW-1573 human non-small cell lung carcinoma cells. A study using fluorescence resonance energy transfer

A multidrug resistant (MDR) human non-small cell lung carcinoma cell line, SW-1573/2R120 (2R120), not containing the drug-efflux pump P-glycoprotein (PgP), has been studied for the transport of daunorubicin (DN) across the cellular plasma membrane. Earlier, reduced initial DN-uptake rates and lower cellular DN steady-state concentrations were found for this cell line, when it was compared to the SW-1573 wild-type cell line. This finding was an indication for the presence of another cellular drug-efflux pump. However, we found similar DN-efflux rates in drug-free medium for the two cell lines, while for Pgp-containing MDR SW-1573/2R160 (2R160) cells the efflux rate was increased compared to wild-type cells. In order to elucidate differences in DN transport across the cellular plasma membrane, the association of DN with plasma membranes of intact cells was investigated, using fluorescence-resonance-energy transfer. For this purpose, the plasma-membrane probe 1-(4-trimethyl-ammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was chosen since, because of the overlap between the emission spectrum of TMA-DPH and the excitation spectrum of DN, transfer of energy can be achieved from TMA-DPH to DN. Cells were loaded with TMA-DPH and, after addition of 10 microM DN, the TMA-DPH fluorescence was quenched. Rapid initial quenching proved to be similar in the MDR 2R160 (Pgp-containing) cells and in the SW-1573 wild-type cells (21 +/- 1% and 20 +/- 2%, respectively), but was less in the MDR 2R120 cells not containing Pgp (14 +/- 1%). This finding correlated with a lowered amount of DN dissolved in the plasma membrane of 2R120 cells. We interpret these data to be the result of a 'vacuum-cleaner' pumping system other than Pgp which removes DN from a plasma membrane compartment and equilibrates relatively slowly with the interior of the cell.

A drug-resistant variant of topoisomerase II alpha in human HL-60 cells exhibits alterations in catalytic pH optimum, DNA binding and sub-nuclear distribution

Anion-exchange chromatography of partially purified human HL-60 topoisomerase II resolves the known alpha (170 kDa) and beta (180 kDa) isoenzymes at 150 mM NaCl and 230 mM NaCl, respectively. An additional topoisomerase II fraction was eluted by > 300 mM NaCl. It could be identified by Western blotting as a late-eluting variant of topoisomerase II alpha, which is functionally altered as compared to the early-eluting form, having the following properties: a shift in the catalytic optimum to pH 9; increased stability in DNA complex formation; approximately 100-fold resistance to orthovanadate; approximately 1000-fold resistance to the cytostatic substances N-[4-(9-acridinylamino)-3-methoxyphenyl]-methanesulphonamide (amsacrine) and the podophyllotoxin etoposide (VP 16). 80% of the late-eluting topoisomerase II alpha could be captured by SDS on calf thymus DNA without further enhancement by drugs. In contrast, the early-eluting topoisomerase II alpha exhibits 10% complex formation with SDS alone, and an increase to 90% complex formation in the presence of drugs. A HL-60 subline (HL-60/R), approximately 1000-fold resistant to etoposide and amsacrine, has equivalent proportions of topoisomerase II alpha and topoisomerase II beta and similar levels of both isoenzymes, as compared to the drug-sensitive HL-60/WT cells. However, determination of the cellular levels of the early-eluting and late-eluting forms of topoisomerase II alpha revealed that the HL-60/R cell line contains approximately 80% of the late-eluting topoisomerase II alpha, whereas the sensitive HL-60/WT cell line contains only 15-20% of this form. The nuclear distribution of the two forms also differs. Sensitive HL-60/WT cells show a diffuse nuclear distribution but in resistant cells the distribution is localized in the nucleoli. Apparently two functionally distinct subforms of topoisomerase II alpha coexist in drug-sensitive and drug-resistant HL-60 cells and changes in their relative levels affect the cellular sensitivity to topoisomerase-II-targeting drugs.

Changes in subcellular doxorubicin distribution and cellular accumulation alone can largely account for doxorubicin resistance in SW-1573 lung cancer and MCF-7 breast cancer multidrug resistant tumour cells

Doxorubicin accumulation defects in multidrug resistant tumour cells are generally small in comparison to the resistance factors. Therefore additional mechanisms must be operative. In this paper we show by a quantitative approach that doxorubicin resistance in several P-glycoprotein-positive non-small cell lung cancer and breast cancer multidrug resistant cell lines can be explained by a summation of accumulation defect and alterations in the efficacy of the drug once present in the cell. This alteration of efficacy was partly due to changes in intracellular drug localisation, characterised by decreased nuclear/cytoplasmic doxorubicin fluorescence ratios (N/C-ratios). N/C-ratios were 2.8-3.6 in sensitive cells, 0.1-0.4 in cells with high (> 70-fold) levels of doxorubicin resistance and 1.2 and 1.9 in cells with low or intermediate (7.5 and 24-fold, respectively) levels of doxorubicin resistance. The change of drug efficacy was reflected by an increase in the total amount of doxorubicin present in the cell at equitoxic (IC50) concentrations. N/C ratios in highly resistant P-glycoprotein-containing cells could be increased with the resistance modifier verapamil to values of 1.3-2.7, a process that was paralleled by a decrease of the cellular doxorubicin amounts present at IC50. At the low to moderate residual levels of resistance, obtained with different concentrations of verapamil, a linear relationship between IC50 and cellular doxorubicin amounts determined at IC50 was found. This shows that at this stage of residual resistance, extra reversal by verapamil should be explained by further increase of drug efficacy rather than by increase of cellular drug accumulation. A similar relationship was found for P-glycoprotein-negative MDR cells with low levels of resistance. Since in these cells N/C ratios could not be altered, verapamil-induced decrease of IC50 must be due to increased drug efficacy by action on as yet unidentified targets. Although the IC50 of sensitive human cells cannot be reached with resistance modifiers, when using these relationships it can be shown by extrapolation that cellular and nuclear doxorubicin amounts at IC50 at complete reversal of resistance were the same as in sensitive cells. It is concluded that doxorubicin resistance factors for multidrug resistant cells can for a large part, and in the case of P-glycoprotein-containing cells probably fully, be accounted for by decreased amounts of drug at nuclear targets, which in turn is characterised by two processes only: decreased cellular accumulation and a shift in the ratio nuclear drug/cytoplasmic drug.

Itraconazole and multidrug resistance: possible effects on remission rate and disease-free survival in acute leukemia

Itraconazole, a triazole antifungal agent, has been reported to reverse drug resistance against daunorubicin in acute leukemia. In a subanalysis from a double-blind, placebo-controlled trial examining the effects of itraconazole on the prevention of fungal infections in neutropenic patients, we studied the effects of itraconazole on remission rate and disease-free survival in patients with acute lymphoblastic (ALL) and acute myelogenous leukemia (AML) receiving remission induction treatment schedules containing daunorubicin (DNR). Eleven ALL and 17 AML patients received itraconazole and 12 ALL and 25 AML patients were given placebo. Among AML patients the remission rate was slightly higher in the itraconazole group, whereas the disease-free survival was higher among ALL patients given itraconazole. In AML patients DFS was comparable in both groups but the number of high-risk patients in the itraconazole group was higher. These preliminary results may suggest a role for itraconazole in reversing multidrug resistance. Larger trials, however, are required to substantiate these findings and to correlate them with its in vitro effects on multidrug resistance.

Do glutathione and related enzymes play a role in drug resistance in small cell lung cancer cell lines?

Small cell lung cancer (SCLC) is treated primarily with combination chemotherapy. Despite high initial response rates, most patients eventually die with drug resistant disease. In some tumours, resistance to multiple chemotherapeutic agents is attributed to overexpression of P-glycoprotein (P-gp). However, this does not appear to be a frequent occurrence in drug resistant SCLC. Increased levels of glutathione (GSH) and related enzymes may play a role in resistance to alkylating agents as well as natural product drugs. We measured levels of GSH, glutathione S-transferase (GST), glutathione reductase (GSH Red), glutathione peroxidase (GSH Px), and gamma-glutamyl transpeptidase (gamma-GT) in a panel of 20 SCLC cell lines. Most of these lines were established from patients treated at this centre. Each cell line had a characteristic and reproducible profile of GSH and related enzyme levels. Immunoblot analysis indicated that the predominant GST in the cell lines was the anionic pi isoenzyme. The relative sensitivity of each of these cell lines to 16 different chemotherapeutic agents was measured using a modified MTT assay. Spearman rank correlation analysis was used to determine the relationships between the relative chemosensitivity of these cell lines and the levels of GSH and related enzymes. The number of positive correlations was no greater than expected by chance alone. Furthermore, there was no correlation with the treatment history of the patients from whom the cell lines were derived. These data suggest that alterations in glutathione metabolism do not play a major role in resistance to chemotherapeutic agents in these human SCLC cell lines.

Pharmacological and molecular characterization of intrinsic and acquired doxorubicin resistance in murine tumor cell lines

We have studied the pharmacological parameters of doxorubicin resistance in three lines of murine cells selected by long-term culture in the presence of this drug or vincristine. A line originating from rat hepatoma spontaneously presented an intrinsic doxorubicin resistance as compared to the other lines, originating from a rat glioblastoma and from simian-virus-40-transformed mouse hepatocytes. This intrinsic resistance, as well as the doxorubicin resistance exhibited by the vincristine-selected glioblastoma variant, could be entirely attribute to decreased drug accumulation due to drug efflux. In contrast, the doxorubicin-selected variants of the three lines exhibited an intracellular tolerance to this drug. Despite a reduction in drug accumulation when exposed to the same amount of doxorubicin, they accumulated 6-12 times more doxorubicin than wild lines when submitted to equitoxic exposures. Verapamil could restore in these lines the doxorubicin accumulation observed in sensitive lines but could not restore doxorubicin cytotoxicity. Quantitative evaluation of P-glycoprotein expression by Western blotting with the C219 antibody indicated that the wild hepatoma line overexpressed P-glycoprotein by a factor of 5 in comparison with the other wild lines, and that the vincristine-selected glioblastoma variant overexpressed this protein almost as much as the doxorubicin-selected variants. These observations favor the existence of P-glycoprotein-independent mechanisms of doxorubicin resistance, which are added to the classical multidrug-resistant phenotype in doxorubicin-selected highly resistant variant cell lines.

Sequence and structural homology among membrane-associated domains of CFTR and certain transporter proteins

A sequence comparison of the two membrane-associated (MA) domains of the cystic fibrosis transmembrane conductance regulator (CFTR), multidrug resistance transporter (MDR), and alpha-factor pheromone export system (STE6) proteins, each of which are believed to contain a total of 12 transmembrane (TM) segments, reveals significant amino acid homology and length conservation in the loop regions that connect individual TM sequences. Similar structural homology is observed between these proteins, hemolysin B (HLYB) and the major histocompatibility-linked peptide transporter, HAM1, the latter two which contain a single MA domain composed of six TM segments. In addition, there are specific sequences that are conserved within the TM segments of the five different membrane proteins. This observation suggests that the folding topologies of the MA domains of MDR, STE6, and CFTR in the plasma membrane are likely to be very similar. The sequence analysis also reveals that there are three characteristic motifs (a pair of aromatic residues, LTLXXXXXXP and GXXL) that are conserved in MDR, STE6, HLYB, HAM1, but not in CFTR. We propose that although CFTR may be evolutionarily related to these other membrane proteins, it belongs to a separate subclass.

Enhanced response to daunomycin of normal, tumor and metastatic cell lines via drug photoactivation

We have compared the cytotoxicity of daunomycin in vitro to highly differentiated normal epithelial cells (Fisher rat thyroid cells, FRTL-5) and to two neoplastic cell lines, a thyroid carcinoma (TK-6) and its lung metastasis (MPTK-6). Whereas the cell lines are equally sensitive to the drug in the dark, if irradiated during incubation with daunomycin (86 J/cm2 at 488 nm), they become more and differently sensitive. Namely, the drug doses producing 50% mortality decrease by factors of about 22, 28 and 16 for FRTL-5, TK-6 and MPTK-6 cell lines, respectively. This result correlates with differences in drug uptake and resistance observed in the normal and neoplastic cell lines.

The interaction of daunomycin with model membranes. Effect of the lipid physical state and the lipid composition

The sensitivity of the keto and carbonyl infrared bands of daunomycin (DNM) to hydrogen bonding with the solvent, has been used to study the effect of the physical state and lipid composition of the bilayer on drug location. Our results show that penetration of daunomycin into dihexadecylphosphatidylcholine (Hxd2GroPCho) or dipalmitoylphosphatidylcholine bilayers, is dependent on the molecular packing of the lipid. DNM incorporates into the bilayer once the interdigitation of the gel phase of Hxd2GroPCho has been removed, above the pretransition temperature. Melting of the hydrocarbon chains of both lipids, at the main transition temperature, allows a similar and deeper drug penetration into the bilayers. Experiments using liposomes with different lipid compositions suggest that the relative concentration of certain lipids may modulate the location of DNM within the bilayer. Cholesterol, in a concentration-dependent manner, inhibits incorporation of anthracycline into apolar regions of the bilayer, while the presence of the negatively charged lipid dihexadecylphosphatidic acid is able to prevent the inhibitory effect of the steroid, allowing deeper penetration of the drug. Due to the importance of drug-membrane interactions in anthracycline cytotoxicity, the relevance of the observed differences in daunomycin location, caused by physical and/or chemical changes in the biological membranes, is discussed.

Acquired multicellular-mediated resistance to alkylating agents in cancer

EMT-6 murine mammary tumor sublines highly resistant to cyclophosphamide, cis-diamminedichloro-platinum(II), or N,N',N"-triethylenethiophosphoramide were generated in vivo by sequential treatment of tumor-bearing mice with the respective drugs. Previous studies demonstrated the drug-resistant phenotypes of the sublines were not expressed in vitro when the cells were grown as monolayer cultures. We now show that expression of drug resistance--including patterns of cross-drug resistance observed in vivo--can be fully recapitulated in vitro when the cells are grown under in vivo-like, three-dimensional conditions--namely, as multicellular tumor spheroids. Moreover, the spheroids generated from all of the drug-resistant sublines manifested a much more compact structure. Immediate drug-sensitivity testing of single cells released by trypsin treatment from compact drug-resistant spheroids revealed that such cells lost much of their drug-resistant properties. The results suggest a possible mechanism of acquired drug resistance in tumors based on the response of a cell population (i.e., multicellular or tissue resistance) as opposed to classic (uni)cellular resistance mechanisms.

Purification and characterization of NF-R2 that regulates the expression of the human multidrug resistance (MDR1) gene

NF-R2 is a DNA-binding protein that interacts with the MDR1 gene proximal promoter sequence. We previously reported that NF-R2 binds within the promoter's -126 and -102 regions, which contain the ATTCAGTCA motif. In the present study, we have purified NF-R2 from the nuclear extract of K562/ADM cells, a multidrug-resistant cell line derived from human myelogenous leukemia K562 cells, using sequential chromatography on Sephacryl S-300, DEAE-Sepharose, heparin-Sepharose and a DNA affinity column consisting of a repetitive synthetic ATTCAGTCA motif coupled to Sepharose. NF-R2 runs as a single protein of 75 kDa on SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). CAT (chloramphenicol acetyltransferase) expression assay and gel mobility shift competition assay with mutated promoters revealed that the ATTCAGTCA motif is a positive regulatory element of MDR1 gene and that the motif is important for NF-R2 binding. These results suggest that NF-R2 may be involved in the positive regulation of the MDR1 gene transcription.

Multiple drug-resistance in variant of a human non-small cell lung carcinoma cell line, DLKP-A

A 300-fold adriamycin resistant variant (DLKP-A) of the human lung squamous cell carcinoma line DLKP was established by stepwise selection in increasing concentrations of adriamycin. Different levels of cross-resistance were observed towards VP-16, VM-26, colchicine, vincristine and, somewhat unexpectedly, cis-platin. Resistance was stable for at least 3 months in culture in the absence of drug. P-glycoprotein overexpression was detected by immunofluorescence and Western Blotting, and a direct causal role for P-glycoprotein overexpression in the resistant phenotype was established by transfection with an mdr1 specific antisense oligonucleotide. A modified cryopreservation procedure was necessary for the resistant variant line. The resistant population displays clonal heterogeneity with respect to resistance level. A higher frequency of double minute chromosomes was observed in DLKP-A when compared with the parental cell line.

Effect of anthracycline analogs on photolabelling of p-glycoprotein by [125I]iodomycin and [3H]azidopine: relation to lipophilicity and inhibition of daunorubicin transport in multidrug resistant cells

Eight anthracycline analogs that have been shown to modulate multidrug resistance (Friche et al., Biochem. Pharmacol., 39, 1721-1726; 1990) were tested for their inhibitory effect on the photolabelling of P-glycoprotein. We photoaffinity labelled P-glycoprotein in daunorubicin (DNR) resistant Ehrlich ascites tumour cells (EHR2/DNR +) with a [125I]iodinated Bolton-Hunter derivative of daunorubicin ([125I]iodomycin) and with [3H]azidopine. The photolabelling of P-glycoprotein by [125I]iodomycin was inhibited more than 50% by 10 microM (1000-fold molar excess) of DNR (52%), N,N-dibenzyl-DNR (52%), and N-benzyladriamycin-14-valerate (AD-198) (85%). Vincristine at 10 microM inhibited [125I]iodomycin labelling of P-glycoprotein by 95%. Thus vincristine was more potent than any of the eight anthracyclines tested, despite its relatively low lipophilicity. Increasing the concentration of DNR, AD-198 and N,N-dibenzyl-DNR to 40 microM resulted in 90, 99.5 and 99.5% inhibition of P-glycoprotein labelling by [125I]iodomycin, respectively. In comparison with the other anthracycline analogs, N,N-dibenzyl-DNR and Ad-198 were also found to exert the greatest inhibition of [3H]azidopine labelling of P-glycoprotein (about 90% at 100-fold molar excess). The solvents Cremophor EL and Tween 80 (30 micrograms ml-1; 0.003% v/v), which are modulators of multidrug resistance in EHR2/DNR + cells, also inhibited [125I]iodomycin labelling > 90%. We showed earlier that there is a correlation between the lipid solubility within the anthracycline group of MDR-associated drugs and their ability to enhance DNR accumulation in EHR2/DNR + cells but a corresponding correlation to lipophilicity when it comes to the inhibitory effect on the specific photolabelling of Pgp ligand binding sites could not be demonstrated. Neither could a correlation between the modulating effect of the analogs on DNR accumulation and inhibition on the labelling of Pgp be demonstrated. With increasing lipophilicity of the analogs it seems that the chemical structure plays a lesser role, and the degree of lipophilicity becomes a more important feature.

Doxorubicin-induced lipid peroxidation and glutathione peroxidase activity in tumor cell lines selected for resistance to doxorubicin

Doxorubicin-induced lipid peroxidation was evaluated in four human or murine cell strains in culture and in their doxorubicin-resistant variants, by the quantification of malondialdehyde produced after a 2-h incubation of cells with the drug. Significantly increased malondialdehyde levels were obtained 24 h after doxorubicin treatment in three of the wild-type cell lines with doses as low as 0.05-0.1 micrograms/ml, which is within an order of magnitude of the concentration of the drug which inhibits cell growth by 50%. This production of malondialdehyde was abolished in two doxorubicin-resistant strains, even with high doses of drug (100-300 micrograms/ml), but was maintained in the third resistant line. No malondialdehyde production was observed in the fourth cell line, sensitive or resistant. It is remarkable that an enhancement of selenium-dependent and non-selenium-dependent glutathione peroxidase activities was exhibited during the acquisition of resistance to doxorubicin in the two first lines, but not in the third, whereas a constitutively high non-selenium-dependent glutathione peroxidase activity existed in the doxorubicin-sensitive and doxorubicin-resistant variants of the fourth cell line. Gene expression of selenium-dependent glutathione peroxidase and of glutathione S-transferase pi, which is known partially to bear a non-selenium-dependent glutathione peroxidase activity, were correlated with the corresponding enzyme activities. It appears, therefore, that the already known enhancement of glutathione peroxidase activity and expression in doxorubicin-resistant cell lines has a quantifiable consequence upon doxorubicin-induced lipid peroxidation and may have consequences in the mechanism of resistance to this drug.

Sensitization of P388 murine leukemia cells to epirubicin cytotoxicity by reserpine

Reserpine, the crystalline active substance isolated from the Rauvolfia plant, produces a characteristic vasodepressor effect in hypertensive patients. Apart from its antihypertensive property, reserpine also possesses transquillising and vasodepressor action, hence it is employed as supportive therapy in the treatment of cardiac disorders. Doxorubicin is a potent anticancer agent, the use of which is limited by its cumulative dose-dependent cardiotoxicity. Epirubicin is a derivative of doxorubicin having more favourable therapeutic index than doxorubicin and possessing less hematologic and cardiac toxicity at comparable doses. The data presented in this paper show the effect of reserpine as a chemosensitizer, when used in combination with epirubicin on P388 murine leukemia cells sensitive (P388/S) and resistant to doxorubicin (P388/DOX) cells. Inhibition of 3H-TdR incorporation into DNA was used as an index of the cytotoxic effects of drug when used alone or in combination. The combination of reserpine (1 microM) and epirubicin (1.7, 8.6 and 17.2 microM) indicated a significant enhancement in the DNA biosynthesis inhibition in P388/S and P388/DOX cell lines. The most prominent feature of the multidrug-resistant cell is the reduced accumulation of the drug intracellularly. P388/DOX cells showed less accumulation of epirubicin in the cell as compared to that of the parental cell line. Further studies demonstrated that reserpine significantly enhanced the intracellular accumulation of epirubicin in both the cell lines. The nature of DNA damage caused by the combination of reserpine and epirubicin was irreversible when studied in P388/DOX cell line. The combination of reserpine (5mg/kg) and epirubicin (1mg/kg) significantly potentiated the antitumor activity of epirubicin in P388/DOX tumor bearing mice. These studies suggest that reserpine can be used as an adjuvant in the cancer chemotherapy to potentiate the antiproliferative activity of anticancer drugs.

Human cell lines as models for multidrug resistance in solid tumours

In spite of our expanding knowledge on the molecular biology of cancer, relatively little progress has been made in improving therapy for the solid tumours which are major killers, e.g., lung, colon, breast. Significant advances over the past 10-15 years in chemotherapy of some tumours such as testicular cancer and some leukaemias indicates that, in spite of the undesirable side-effects, chemotherapy has the potential to effect cure in the majority of patients with certain types of cancer. Multidrug resistance, inherent or acquired, is one important limiting factor in extending this success to most solid tumours. In vitro studies described in this review are now uncovering a diversity of possible mechanisms of cross-resistance to different types of drug. Sensitive methods such as immunocytochemistry, RT-PCR or in situ RNA hybridisation may be necessary to identify corresponding changes in clinical material. Only by classifying individual tumours according to their specific resistance mechanisms will it be possible to define the multidrug resistance problem properly. Such rigorous definition is a prerequisite to design (and choice on an individual basis) of specific therapies suited to individual patients. Since a much larger proportion of cancer biopsies should be susceptible to accurate analysis by the immunochemical and molecular biological techniques described above than to direct assessment of drug response, it seems reasonable to hope that this approach will succeed in improving results for cancer chemotherapy of solid tumours where other approaches such as individualised in vitro chemosensitivity testing have essentially failed. Results from clinical trials using cyclosporin A or verapamil are encouraging, but these agents are far from ideal, and reverse resistance in only a subset of resistant tumours. Proper definition of the other mechanisms of MDR, and how to antagonize them, is an urgent research priority.

Interleukin 6: a fibroblast-derived growth inhibitor of human melanoma cells from early but not advanced stages of tumor progression

Recently we reported that human dermal fibroblasts, or conditioned media obtained from such cells, affect the growth of human melanoma cells as a direct function of tumor progression: melanoma cells obtained from early-stage (metastatically incompetent) primary lesions were growth inhibited, whereas cells obtained from more advanced (metastatically competent) primary lesions, or metastases, were growth stimulated. Ion-exchange and gel-filtration chromatography of fibroblast conditioned medium revealed the inhibitor to be a protein of molecular mass between 20 and 30 kDa and distinct from the stimulator. This is the approximate molecular mass of interleukin 6 (IL-6), a ubiquitous multifunctional cytokine known to affect in particular many kinds of hemopoietic and lymphoid cells. Since this cytokine is known to be made by fibroblasts, we attempted to determine if the human fibroblast-derived growth inhibitor (hFDGI) was identical to IL-6. Neutralizing antibodies specific for IL-6 completely eliminated the inhibitory activity of hFDGI. Moreover, exposure to human recombinant IL-6 was found to inhibit the growth of early-stage melanoma cells obtained from radial growth phase (RGP) or early vertical growth phase (VGP) primary lesions in three of four cases. In contrast, melanoma cells from a number of more advanced VGP primary lesions, or from distant metastases, were completely resistant to this IL-6-mediated growth inhibition. Acquisition of an "IL-6-resistant" phenotype by metastatically competent melanoma cell variants may provide such cells with a proliferative advantage within the dermal mesenchyme (a hallmark of melanoma cells that are malignant), helping them eventually to dominate advanced primary lesions and to establish secondary growths elsewhere.

Expression of multidrug resistance gene mdr1 mRNA in a subset of normal bone marrow cells

The multidrug resistance gene mdr1, encoding P-glycoprotein (P-gp), can be expressed at high levels in tumour cells derived from normal tissues with constitutive high expression of this gene. In myelogenous leukaemia, the incidence of increased expression of mdr1 gene contrasts with the low expression of this gene in normal bone marrow (b.m.). To detect cells expressing mdr1 gene in normal and post-chemotherapy b.m., we used in situ RNA hybridization and RNA phenotyping by the polymerase chain reaction for mdr1 mRNA detection. The presence of P-gp was evaluated by immunocytochemistry with MRK16. Fifteen b.m. (eight normal and seven post chemotherapy) were tested by in situ hybridization and either PCR (three b.m.) or immunocytochemistry (11 b.m.) or both (one b.m.). With in situ mRNA hybridization, a subset (7.7% +/- 3.1%) of b.m. cells expressed mdr1 mRNA in all cases tested, with no significant differences between normal b.m. and post chemotherapy b.m. 18% of myeloid recognizable cells and 7% of the cells with lymphoid morphology expressed mdr1 mRNA. By RNA phenotyping, the four samples tested for in situ hybridization and two additional post chemotherapy b.m. expressed mdr1. MRK16 was unable to detect a significant number of cells expressing P-gp either by immunocytochemistry in the 12 b.m. tested for in situ hybridization (0% in nine cases; 0.4%, 1% and 3% of positive cells in three cases), or by flow cytometry in six additional normal b.m. (0-1.4% positive cells).

Kinetics of daunorubicin transport by P-glycoprotein of intact cancer cells

Drug permeation across the plasma membrane of multidrug-resistant cells depends on the kinetics of the P-glycoprotein-mediated pump activity as well as on the passive permeation of the drug. We here demonstrate a method to characterize kinetically the pump in intact cells. To this purpose, we examined the membrane-transport properties of daunorubicin in various sensitive cancer cell lines and in their multidrug resistant (MDR) counterparts. First, we determined the passive permeability coefficient for daunorubicin. Then, using a flow-through system, the drug flux into the cell was measured after inhibition of the P-glycoprotein-mediated efflux pump. Combining the two results allowed us to calculate the intracellular free concentration of the drug. In the steady-state, the pump rate must equal the net rate of passive diffusion of the drug and, therefore, the same experiments gave us the pumping rate of daunorubicin. These experiments were then repeated at various extracellular drug concentrations. By plotting the pumping rate versus the intracellular drug concentration, we then characterized the P-glycoprotein kinetically. Four independent methods were used to measure the passive permeability coefficient for the cell line A2780. Similar values were obtained. Maximal pump rates (Vmax) showed a good correlation with the amount of P-glycoprotein in the cell lines used. We obtained saturation curves for the variation of the pump rates with the intracellular daunorubicin concentrations. These curves were typical for positive cooperativity, which provides evidence that at least two binding sites for daunorubicin are present on the active transport system of daunorubicin. The apparent Km values for P-glycoprotein-mediated transport, the intracellular free cytosolic daunorubicin concentrations at half-maximal velocity for the cell lines used, were approximately 1.5 microM. Except for the cell lines with the highest amount of P-glycoprotein, the passive efflux rate of daunorubicin proved to be a substantial part of the total daunorubicin efflux rate for the cell lines used. In cell lines with relatively low levels of P-glycoprotein, passive daunorubicin efflux was even the main route of daunorubicin transport from the cells, determining the intracellular steady-state concentrations of daunorubicin.

Fluorescent dye rhodamine 6G as a molecular probe to study drug resistance of C6 rat glioma cells

A study was made of the membrane transport of cytoplasm and mitochondria stained fluorescence dye Rhodamine 6G (R6G). In rat glioma C6 cells and 1-(4-amino-2-methyl-5-pyrymidinyl)-methyl-3-(2-chloroethyl) -3-nitrosourea hydrochloride (ACNU) and vincristine (VCR) resistant cell lines (C6/ACNU, C6/VCR), the rate of uptake of R6G decreased in C6/VCR cells, but verapamil increased the intracellular accumulation of R6G in C6/VCR. The intracellular accumulation of R6G of C6/ACNU cells was essentially the same as that of wild-type cells. C6/ACNU cells did not show cross resistance and were sensitive to VCR and cisplatin. C6/VCR cells showed cross resistance to ACNU and CDDP, but C6/VCR cells in the presence of verapamil were more sensitive to drugs than C6/VCR cells in the absence of verapamil. We conclude that the reduction of R6G fluorescence staining intensity in C6/VCR cells compared to wild-type cells may be associated with the mechanism of multidrug resistance (MDR) but does not reflect the mechanism of resistance to ACNU. Verapamil increased the accumulation of R6G in C6/VCR cells and overcame MDR, suggesting that there is a correlation between the MDR overcoming effect and enhancement of R6G accumulation, and that this correlation validates the use of the R6G staining test for clinical and laboratory investigation of MDR.

Reversal of multidrug resistance by surfactants

Cremophor EL, a pharmacologically inactive solubilising agent, has been shown to reverse multidrug resistance (MDR). Using flow cytometric evaluation of equilibrium intracellular levels of daunorubicin (DNR), we found that eight other surface active agents will also reverse MDR. All the active detergents contain polyethoxylated moieties but have no similarities in their hydrophobic components. The properties of three polyethoxylated surfactants that showed the lowest toxicities, Cremophor, Tween 80 and Solutol HS15, were examined in more detail. The concentrations of Tween 80 and Solutol required to reverse DNR exclusion were 10-fold lower than for Cremophor. However while concentrations greater than or equal to 1:10(2) of the former two surfactants resulted in breakdown of cells, even 1:10 of Cremophor did not lyse cells. Studies of the effects of Cremophor on the uptake and efflux of DNR in normal and MDR cell types showed that Cremophor increases intracellular DNR primarily by locking the rapid efflux from the cells. This blockage of drug efflux may be mediated by a substantial alteration in the fluidity of cell membranes induced by Cremophor, as shown by decreased fluorescence anisotropy of a membrane probe. Consistent with these data, coinjection of adriamycin plus Cremophor into mice carrying a multidrug resistant P388 transplantable tumour significantly increased the survival time of the mice compared with adriamycin treatment alone.

P-glycoprotein overexpression cannot explain the complete doxorubicin-resistance phenotype in rat glioblastoma cell lines

We have associated pharmacological studies to a semi-quantitative evaluation of P-glycoprotein(s) expression, to establish if classical multidrug resistance (MDR) could account for the complete resistance phenotype exhibited by progressively doxorubicin-resistant rat glioblastoma cells. Three resistant variants (C6 0.001, C6 0.1 and C6 0.5) of the C6 glioblastoma cell line (C6 S) were selected by long-term culture in the presence of three concentrations of doxorubicin (0.001, 0.1 and 0.5 microgram.ml-1 respectively). The degree of doxorubicin resistance was respectively 7, 33 and 400, and all the cell variants were cross-resistant to m-AMSA, etoposide and vincristine. Doxorubicin incorporation was reduced similarly in all resistant cells, irrespective of the level of resistance. When exposed to their respective doxorubicin IC50, the 7-fold resistant cells had the same intracellular drug incorporation as the sensitive cells, whereas the 33-fold and 400-fold resistant cells could incorporate respectively 3.7 and 17 times more drug. The ratio of doxorubicin exposures required for 50% DNA synthesis inhibition and 50% growth inhibition was dependent on the degree of resistance; this ratio was 12.8 in C6 S, 11.6 in C6 0.001, 6.3 in C6 0.1 and 1.8 in C6 0.5. P-glycoprotein(s) overexpression was of the same magnitude as the resistance factor in variants C6 0.001 and C6 0.1, but was lower than resistance factor in variant C6 0.5. Reversal of drug incorporation by verapamil was complete in all resistant cell lines; however, reversal of doxorubicin cytotoxicity was complete only in the 7-fold resistant line and was only partial in the most resistant lines, which remained 10-fold and 20-fold resistant to doxorubicin. These results suggest that classical MDR was the first phenotype selected by doxorubicin in C6 0.001, whereas mechanism(s) of doxorubicin resistance other than classical MDR are added in the most resistant lines.

Levels of expression of the mdr1 gene and glutathione S-transferase genes 2 and 3 and response to chemotherapy in multiple myeloma

We have quantitated the levels of mRNAs in bone marrow samples from patients with multiple myeloma of the mdr1 gene (responsible for the Multidrug Resistance phenotype) and for two of the glutathione S-transferase gene, GST-2 and GST-3 (which can also inactivate a wide variety of cytotoxic drugs) and examined the relationship between the levels of expression of these genes and response to subsequent chemotherapy. From a total of 47 patients, 37 were treated with chemotherapy with 34 evaluable for response. Twenty-nine of the patients treated had not received any treatment prior to the marrow sampling while eight had previously received chemotherapy. Patients who failed to respond to initial chemotherapy had significantly higher levels of mdr1 than patients who responded (P = 0.01). In the total myeloma patient data set, mRNA levels for mdr1 and GST-2 were significantly correlated (Spearman rank correlation coefficient (r) = 0.54, P = 0.0004) as were expression levels of GST-2 with GST-3 (r = 0.43, P = 0.017). GST-3 and mdr1 levels were more weekly associated (r = 0.16, P = 0.4). These data would suggest a significant relationship between failure of chemotherapy in multiple myeloma patients and increases in expression of the mdr1 gene together with other genes whose products will generate additional mechanisms of resistance to chemotherapeutic agents.

The efflux of a fluorescent probe is catalyzed by an ATP-driven extrusion system in Lactococcus lactis

Many bacteria, both gram positive and gram negative, extrude in an energy-dependent manner the fluorescent pH indicator 2',7'-bis-(2-carboxyethyl)-5[and -6]-carboxyfluorescein (BCECF) (D. Molenaar, T. Abee, and W. N. Konings, Biochim. Biophys. Acta 1115:75-83, 1991). This efflux was studied in detail in Lactococcus lactis, and several indications that a transport system is involved were found. This transport system is most likely driven by ATP or a related compound. The evidence is that BCECF extrusion (i) occurs against a BCECF gradient, (ii) is strictly correlated with ATP concentration and not with the proton motive force, and (iii) is inhibited by vanadate and to a lesser extent by N,N'-dicyclohexylcarbodiimide. Most convincingly, a UV mutant with a strongly reduced efflux rate was isolated. Such a mutant was isolated from a BCECF-loaded and lactose-energized population by selection of highly fluorescent cells in a flow cytometer-cell sorter. The physiological function of this extrusion system is unknown, but its characteristics classify it among the traffic ATPases.

SDZ 280-446, a novel semi-synthetic cyclopeptolide: in vitro and in vivo circumvention of the P-glycoprotein-mediated tumour cell multidrug resistance

SDZ 280-446 is a semi-synthetic derivative of a natural cyclic peptolide. Its ability to sensitise in vitro tumour cells whose resistance is due to P-glycoprotein-mediated anticancer-drug efflux was shown using four different pairs of parental drug-sensitive (Par-) and multidrug-resistant (MDR-) cell lines, from three different species (mouse, human, Chinese hamster) representing four different cell lineages (monocytic leukaemia, nasopharyngeal epithelial carcinoma, colon epithelial carcinoma, ovary fibroblastoid carcinoma), and using four different drug classes (colchicine, vincristine, daunomycin/doxorubicin and etoposide). By measuring its capacity to restore normal drug sensitivity of MDR-cells in culture in vitro, it appeared that SDZ 280-446 belongs to the same class of very potent chemosensitisers as the cyclosporin derivative SDZ PSC 833: both are about one order of magnitude more active than cyclosporin A (CsA), which is itself about one order of magnitude more active than other known chemosensitisers (including verapamil, quinidine and amiodarone which have already entered clinical trials in MDR reversal). Low concentrations of SDZ 280-446 could also restore cellular daunomycin retention in MDR-P388 cells to the levels found in the Par-P388 cells. SDZ 280-446 was also effective as a chemosensitiser when given orally in vivo. In a syngeneic mouse model, combined therapy with vinca alkaloids given i.p. and SDZ 280-446 given per os for 5 consecutive days significantly prolonged the survival of MDR-P388 tumour-bearing mice, when compared with mice receiving vinca alkaloids alone. Another protocol, using three cycles of i.p. doxorubicin at 4 day intervals, could also not increase MDR-P388 tumour-bearing mouse survival unless the mice received SDZ 280-446 orally 4 h before each doxorubicin injection. Though only very few combined therapy treatment protocols have been tested so far, clear increases in survival time of MDR-tumour-bearing mice were regularly obtained, leaving hope for major improvement of the therapy using other dosing schedules.

Antibody-directed therapy of multidrug-resistant tumor cells

The major obstacle to effective cancer chemotherapy is the resistance of tumor cells to cytostatic agents. Tumor cells frequently express a multidrug-resistance (MDR) phenotype. In an effort to devise new strategies to overcome MDR, antibody-directed approaches for the eradication of MDR cells have been developed. Experimental data have shown that multidrug-resistant tumor cells can be efficiently killed by monoclonal antibodies, immunotoxins, or bispecific antibodies. The current experimental results indicate that an antibody-directed therapeutic approach to eradication of MDR cells might be a promising adjunct to conventional chemotherapy of cancer patients.

Dose dependence of the cytokinetic and cytotoxic effects of epirubicin in vitro

CHO cells were exposed in vitro for 1 h to concentrations of 0.1-20 micrograms/ml of the cytostatic drug epirubicin. Population growth, survival fractions, cell-cycle-phase distribution, and BrdU incorporation were analyzed. A fraction of the cells showed a transitory cytostatic reaction at 1 microgram/ml, and greater than 99% of the cells were killed at 10 micrograms/ml. The survival curve was biphasic with a steep slope at concentrations of up to 5 micrograms/ml. Approximately 0.1% of the cells were resistant to higher concentrations of epirubicin. Bivariate DNA/BrdU flow cytometry revealed that the sensitive cells were blocked and probably killed in the G2M phase of the cell cycle.

Doxorubicin selected multidrug-resistant small cell lung cancer cell lines characterised by elevated cytoplasmic Ca2+ and resistance modulation by verapamil in absence of P-glycoprotein overexpression

Sublines from the small cell lung cancer (SCLC) cell lines U1285 and U1690, denoted U1285-100, U1285-250, U1690-40 and U1690-150, were adapted to grow in the continuous presence of 100, 250, 40 and 150 ng ml-1 doxorubicin (Dox), respectively. The Dox resistance was accompanied by cross-resistance to vincristine (Vcr), Vp-16 and for U1285-100 also to cisplatinum. Sublines of U1690-40 and U1285-100, cultured in absence of Dox for 4 months were only partially reversed with respect to Dox resistance. Neither the parental nor the most Dox resistance sublines had detectable levels of mdr 1 RNA but a small fraction of cells in all cell lines stained weakly positive for P-glycoprotein (P-gp). Verapamil (Ver) at 5 microM reversed the Dox resistance completely and partly in the U1690 and U1285 sublines, respectively, but did not increase the cellular accumulation of Dox. The cytoplasmic free Ca2+ concentration (Ca2+i) was close to 100 nM in both parental cell lines but elevated in the U1285-100 and U1690-40 sublines by 21 and 44%, respectively, and in U1285-250 and U1690-150 by 51 and 91%, respectively. The partly reverted sublines still showed significant but smaller elevations in Ca2+i of 10-30%. Ver was without acute or long term effects of Ca2+i in the U1285-100 and U1690-40 sublines. Selection for Dox resistance in SCLC may thus result in atypical multidrug-resistance characterised by absence of P-gp overexpression and atypical cross-resistance. Although Ver did not seem to affect Dox accumulation it may still work as a resistance modulator.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia-induced drug resistance: comparison to P-glycoprotein-associated drug resistance

In this report, we investigate several examples of hypoxia-induced drug resistance and compare them with P-glycoprotein associated multidrug resistance (MDR). EMT6/Ro cells exposed to drugs in air immediately after hypoxic treatment developed resistance to adriamycin, 5-fluorouracil, and actinomycin D. However, these cells did not develop resistance to colchicine, vincristine or cisplatin. When the cells were returned to a normal oxygen environment, they lost resistance. There was no correlation between the content of adriamycin and the development of adriamycin resistance induced by hypoxia. There was no difference between the efflux of adriamycin from aerobic cells and that from hypoxia-treated cells. The mRNA for P-glycoprotein was not detected in the hypoxia-treated cells. These results suggest that hypoxia-induced drug resistance is different from P-glycoprotein associated multidrug resistance.

Early multidrug resistance, defined by changes in intracellular doxorubicin distribution, independent of P-glycoprotein

Resistance to multiple antitumour drugs, mostly antibiotics or alkaloids, has been associated with a cellular plasma membrane P-glycoprotein (Pgp), causing energy-dependent transport of drugs out of cells. However, in many common chemotherapy resistant human cancers there is no overexpression of Pgp, which could explain drug resistance. In order to characterise early steps in multidrug resistance we have derived a series of P-glycoprotein-positive (Pgp/+) and P-glycoprotein-negative (Pgp/-) multidrug resistant cell lines, from a human non-small cell lung cancer cell line, SW-1573, by stepwise selection with increasing concentrations of doxorubicin. These cells were exposed to doxorubicin and its fluorescence in nucleus (N) and cytoplasm (C) was quantified with laserscan microscopy and image analysis. The fluorescence N/C ratio in parent cells was 3.8 and decreased both in Pgp/+ and Pgp/- cells with increasing selection pressure to 1.2-2.6 for cells with a resistance factor of 7-17. N/C ratios could be restored partly with verapamil only in Pgp/+ cells. N/C ratio measurements may define a general Pgp-independent type of defense of mammalian cells against certain anticancer agents which may precede Pgp expression in early doxorubicin resistance.