Multidrug resistance-associated protein (MRP) in haematological malignancies

LY103, a pomalidomide derivative, alleviates taxol resistance in NSCLC via energy metabolism crosstalk and tumor microenvironment intervention

In this study, we identified HIF 1α as a potential target for reversing taxol resistance in lung cancer by combining bioinformatics analysis with pharmacological analysis. Furthermore, pomalidomide derivative LY103 was also be synthesized by introducing an isatin analogue into the amino terminal ofpomalidomide, and it has a broad antitumor spectrum and showed excellent activity against A549/Taxol cells (IC50 = 6.33 ± 0.51 μM). The results of molecular docking showed that not only LY103 was inclined to bind to HIF 1α stably, it could also form multiple hydrogen bonds with VAL376, ASP256, ILE454, and GLU455 of HIF 1α even was reduced to LY103-NH2 by nitroreductase, which was further stabilized the complex formed by them, thereby inhibiting the activity of HIF 1α. LY103 was able to significantly induce DNA damage and inhibit angiogenesis. Concurrently, LY103 activated the immune response, reduced the expression of cytokines TNF-α, IL-6, and IL-1β, thus might be inhibit the proliferation and metastasis of tumor cells. Pharmacological analysis proved that LY103 led to cell apoptosis through the mitochondrial pathway, and its combination with taxol significantly promoted this process. In general, the consumption of glutathione, the crosstalk of energy metabolism, and the improvement of the tumor microenvironment caused by LY103 eventually led to the decrease of ABCC1 protein expression and the drug resistance was reversed. The rational design of LY103 provided a basis for the application of nitro compounds in the treatment of hypoxic tumors and the reversal of taxol resistance.

MC70 potentiates doxorubicin efficacy in colon and breast cancer in vitro treatment

A major limitation of cancer treatment is the ability of cancer cells to develop resistance to chemotherapeutic drugs, by the establishment of multidrug resistance. Here, we characterize MC70 as ABC transporters inhibitor and anticancer agent, alone or with chemotherapy. MC70 was analyzed for its interaction with ABCB1, ABCG2 and ABCC1 by specific transport assays. In breast and colon cancer cell lines, cell growth and apoptosis were measured by MTT assay and DNA laddering Elisa kit, respectively. Cell cycle perturbation and cellular targets modulation were analyzed by Flow-cytometry and Western blotting, respectively. MC70 interacted with ABC transporters. In breast cancer cells, MC70 slightly inhibited cell proliferation strongly enhancing doxorubicin effectiveness. By contrast, MC70 was found to inhibit cell growth in colon cancer cells without affecting doxorubicin efficacy and in combination with topoisomerase I inhibitors it could be a promising therapeutic approach. What is more, it was also observed that MC70 induced apoptosis, canceled in favor of necrosis when given in combination with high doses of doxorubicin. MC70 inhibited cell migration probably through its interaction with sigma-1 receptor. Modulations of i) cell cycle, ii) pAkt and the phosphorylation of the three MAPKs were highlighted, while any activity was excluded at transcription level, thus accounting for the phenotypic effects observed. MC70 might be considered as a new potential anticancer agent capable to i) enhance chemotherapy effectiveness and ii) to play a contributory role in the treatment of chemotherapy resistant tumors.

Multidrug-resistant protein-3 gene regulation by the transcription factor Nrf2 in human bronchial epithelial and non-small-cell lung carcinoma

Multidrug-resistant proteins (MRPs) are members of the ATP-binding cassette superfamily that facilitate detoxification by transporting toxic compounds, including chemotherapeutic drugs, out of cells. Chemotherapy, radiation, and other xenobiotic stresses have been shown to increase levels of select MRPs, although the underlying mechanism remains largely unknown. Additionally, MRP3 is suspected of playing a role in the drug resistance of non-small-cell lung carcinoma (NSCLC). Analysis of the MRP3 promoter revealed the presence of multiple putative electrophile-responsive elements (EpREs), sequences that suggest possible regulation of this gene by Nrf2, the key transcription factor that binds to EpRE. The goal of this investigation was to determine whether MRP3 induction was dependent upon the transcription factor Nrf2. Keap1, a key regulator of Nrf2, sequesters Nrf2 in the cytoplasm, preventing entry into the nucleus. The electrophilic lipid peroxidation product 4-hydroxy-2-nonenal (HNE) has been shown to modify Keap1, allowing Nrf2 to enter the nucleus. We found that HNE up-regulated MRP3 mRNA and protein levels in cell lines with wild-type Keap1 (the human bronchial epithelial cell line HBE1 and the NSCLC cell line H358), but not in the Keap1-mutant NSCLC cell lines (A549 and H460). Cell lines with mutant Keap1 had constitutively higher MRP3 that was not increased by HNE treatment. In HBE1 cells, silencing of Nrf2 with siRNA inhibited induction of MRP3 by HNE. Finally, we found that silencing Nrf2 also increased the toxicity of cisplatin in H358 cells. The combined results therefore support the hypothesis that MRP3 induction by HNE involves Nrf2 activation.

Clinicopathological parameters and expression of P-glycoprotein and MRP-1 in retinoblastoma

PURPOSE

The immunohistochemical expressions of two multidrug resistance proteins, P-glycoprotein (P-gp) and multidrug resistance-related protein-1 (MRP-1), were studied in retinoblastoma and the correlations with the clinicopathological parameters were assessed.

METHOD

Sixty-five enucleated eyes containing retinoblastoma were included in the study. Following hematoxylin-eosin staining, tumor differentiation, presence of choroidal invasion, optic nerve invasion, retinal invasion, necrosis and presence of calcification were evaluated with the light microscope. P-gp and MRP-1 expressions were evaluated immunohistochemically.

RESULTS

Fifty-three eyes were enucleated primarily and 12 eyes were operated after failure of chemotherapy. P-gp and MRP-1 expressions were positive in 69.3 and 73.4% of specimens, respectively. There was no statistically significant relationship between the expressions of P-gp and MRP-1, and tumor differentiation, presence of tumor invasion or treatment with chemotherapy.

CONCLUSION

Retinoblastoma intrinsically expresses both P-gp and MRP-1 and their expressions are not related to tumor differentiation. The expressions of P-gp and MRP-1 do not seem to be induced by chemotherapy and are not related to the degree of tumor invasion.

Multidrug resistance mediated by ABC transporters in osteosarcoma cell lines: mRNA analysis and functional radiotracer studies

Drug resistance remains a significant impediment to successful chemotherapy and constitutes a major prognostic factor in osteosarcoma (OS) patients. This study was designed to identify the role and prognostic significance of multidrug-resistance (MDR)-related transporters, such as multidrug resistance protein 1 (MDR1), multidrug-resistance-associated protein (MRP1) and breast-cancer-related protein (BCRP), in OS using cationic lipophilic radiotracers. We evaluated the chemosensitivity of four OS cell lines (Saos-2, 143B, MNNG/HOS and U-2OS) to doxorubicin (DOX), cisplatin (CIS) and methotrexate. The expression of MDR-related transporters was analyzed at mRNA level by quantitative polymerase chain reaction and at functional level by 99mTc sestamibi and 99mTc tetrofosmin. The effectiveness of MDR modulators [cyclosporin A (CsA) and imatinib] on transporter inhibition and on the reversal of resistance was also assessed. MNNG/HOS and U-2OS cells expressing high levels of MDR1 were highly resistant to DOX and showed reduced accumulation and higher efflux for radiotracers. Although MRP1 was uniformly expressed in all cells, only U-2OS was resistant to CIS. CsA restored sensitivity to DOX and CIS, and enhanced the accumulation and efflux half-life of radiotracers in MDR1-expressing cell lines. The chemosensitivity of OS cells to DOX was strongly dependent on mRNA MDR1 expression and could be circumvented by adding CsA. The kinetic parameters of radiotracers correlated with MDR1 expression levels, hence predicting DOX resistance. We concluded that sensitivity to chemotherapy is strongly dependent on the expression of MDR1 transporter and that radiotracer studies could prove clinically useful in predicting chemotherapy response and in evaluating the efficacy of MDR-reversing agents.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

The role of ABC transporters in clinical practice

Drug resistance remains one of the primary causes of suboptimal outcomes in cancer therapy. ATP-binding cassette (ABC) transporters are a family of transporter proteins that contribute to drug resistance via ATP-dependent drug efflux pumps. P-glycoprotein (P-gp), encoded by the MDR1 gene, is an ABC transporter normally involved in the excretion of toxins from cells. It also confers resistance to certain chemotherapeutic agents. P-gp is overexpressed at baseline in chemotherapy-resistant tumors, such as colon and kidney cancers, and is upregulated after disease progression following chemotherapy in malignancies such as leukemia and breast cancer. Other transporter proteins mediating drug resistance include those in the multidrug-resistance-associated protein (MRP) family, notably MRP1, and ABCG2. These transporters are also involved in normal physiologic functions. The expressions of MRP family members and ABCG2 have not been well worked out in cancer. Increased drug accumulation and drug resistance reversal with P-gp inhibitors have been well documented in vitro, but only suggested in clinical trials. Limitations in the design of early resistance reversal trials contributed to disappointing results. Despite this, three randomized trials have shown statistically significant benefits with the use of a P-gp inhibitor in combination with chemotherapy. Improved diagnostic techniques aimed at the selection of patients with tumors that express P-gp should result in more successful outcomes. Further optimism is warranted with the advent of potent, nontoxic inhibitors and new treatment strategies, including the combination of new targeted therapies with therapies aimed at the prevention of drug resistance.

Expression of functional markers in acute lymphoblastic leukemia

We analyzed surface antigens, multidrug resistance (MDR) parameters (PGP, MRP, LRP), tissue infiltration parameters (CD18, CD44, VCAM, MMP2), receptors for colony stimulating factors (G-CSFr, GM-CSFr) and cell cycle parameters (Ki-67, topoisomerase IIalpha) in 86 patients with acute lymphoblastic leukemia (ALL). LRP, PGP and CD18 were associated with poor clinical outcome, and LRP expression was related with CD18, CD44 and G-CSFr. Of the cell cycle parameters, Ki-67 (+) fraction was increased in ALL with hepato-splenomegaly and extramedullary involvement. In conclusion, analysis of LRP, PGP, CD18 and Ki-67 could be helpful to predict the clinical behavior of ALL.

Expression of MDR1/P-glycoprotein, the multidrug resistance protein MRP, and the lung-resistance protein LRP in multiple myeloma

The purpose of this study was to determine the incidence of three genes associated with multidrug resistance (MDR) in multiple myeloma in relation to treatment status. MDR1/Pgp (P-glycoprotein) expression was detected in 41% of 93 myeloma samples. Generally, the incidence of MDR1/Pgp expression was higher in pretreated samples, and treatments with doxorubicin and/or vincristine were more effective in MDR1/Pgp expression than with alkylating agents. A significant association was observed between MDR1 /Pgp-positiveness and the ability of verapmil to increase doxorubicin sensitivity, suggesting functional relevance of MDR1/Pgp expression. MRP (multidrug resistance protein) expression was detected in 20.5% of 88 myeloma samples, in 26% at the mRNA level analyzed by quantitative reverse transriptase-polymerase chain reaction, and in only 3 of 79 samples by immunohistochemistry. LRP (lung-resistance protein) protein expression was observed in 12.5% of 72 myeloma samples. MRP and LRP expression was similar in samples with and without prior therapy. Approximately 80% of the myeloma samples with detectable mRNA expression of MDRI and MRP exhibited low expression levels corresponding to < 10% of the Pgp- and MRP-overexpressing multidrug-resistant human myeloma cell lines 8226/Dox6 and 8226/DOXint40c, respectively. Some normal bone marrow samples showed higher levels of MRP mRNA as compared to myeloma specimens, whereas MDRI mRNA expression in normal bone marrow was much lower (< or = 5%) than that in 8226/Dox6. These findings indicate a requirement to develop single-cell assays for MRP detection in multiple myeloma that are more sensitive than immunohistochemistry and might be useful to evaluate the incidence of genes associated with MDR.

Noninvasive detection of multidrug resistance in patients with hematological malignancies: are we there yet?

The success of chemotherapy in the treatment of malignancies may be limited by cellular mechanisms leading to drug resistance. In hematological malignancies, mechanisms leading to the development of multidrug resistance (MDR) include overexpression of the membrane-based export pump P-glycoprotein (Pgp) and the MDR-associated protein (MRP). Recently, the overexpression of the lung-resistance protein (LRP) has also been associated with reduced intracellular drug accumulation. A major problem in assessing the significance of the expression of these resistance proteins in clinical MDR has been the variability of detection techniques either at the mRNA or protein level. Currently, the detection of resistance proteins relies heavily on antibody and cDNA probes, and these methods may not be informative about the in vivo function of Pgp, MRP, or LRP. Nuclear medicine imaging techniques such as single-photon emission tomography (SPECT) and positron emission tomography (PET) have been evaluated for noninvasive determination of the presence and the function of Pgp- and MRP-mediated transport systems. Technetium 99m ((99m)Tc)-sestamibi, an agent in clinical use for myocardial perfusion and tumor imaging, is recognized as a substrate for Pgp and MRP, and has been used to visualize Pgp expression. (99m)Tc-tetrofosmin is also a substrate for the Pgp efflux pump mechanism and is used to evaluate Pgp function in in vitro and in vivo studies. Recently, radiopharmaceuticals including carbon 11-labeled colchicine, verapamil, and daunorubicin have been used in cell line and animal studies for the evaluation of Pgp-mediated transport functions using PET technology. Preliminary results suggest that the potential to detect MDR in tumors prior to or after exposure to chemotherapeutic agents exists in imaging using either (99m)Tc-labeled compounds and SPECT or positron emitting compounds and PET.

Ascorbic acid enhances arsenic trioxide-induced cytotoxicity in multiple myeloma cells

Multiple myeloma (MM) is a clonal B-cell malignancy characterized by slow-growing plasma cells in the bone marrow (BM). Patients with MM typically respond to initial chemotherapies; however, essentially all progress to a chemoresistant state. Factors that contribute to the chemorefractory phenotype include modulation of free radical scavenging, increased expression of drug efflux pumps, and changes in gene expression that allow escape from apoptotic signaling. Recent data indicate that arsenic trioxide (As(2)O(3)) induces remission of refractory acute promyelocytic leukemia and apoptosis of cell lines overexpressing Bcl-2 family members; therefore, it was hypothesized that chemorefractory MM cells would be sensitive to As(2)O(3). As(2)O(3) induced apoptosis in 4 human MM cell lines: 8226/S, 8226/Dox40, U266, and U266/Bcl-x(L). The addition of interleukin-6 had no effect on cell death. Glutathione (GSH) has been implicated as an inhibitor of As(2)O(3)-induced cell death either through conjugating As(2)O(3) or by sequestering reactive oxygen induced by As(2)O(3). Consistent with this possibility, increasing GSH levels with N-acetylcysteine attenuated As(2)O(3) cytotoxicity. Decreases in GSH have been associated with ascorbic acid (AA) metabolism. Clinically relevant doses of AA decreased GSH levels and potentiated As(2)O(3)-mediated cell death of all 4 MM cell lines. Similar results were obtained in freshly isolated human MM cells. In contrast, normal BM cells displayed little sensitivity to As(2)O(3) alone or in combination with AA. Together, these data suggest that As(2)O(3) and AA may be effective antineoplastic agents in refractory MM and that AA might be a useful adjuvant in GSH-sensitive therapies. (Blood. 2001;98:805-813)

Intracellular P-gp contributes to functional drug efflux and resistance in acute myeloid leukaemia

Drug compartmentalization as well as drug efflux can contribute to drug resistance. We demonstrate the presence of P-gp in intracellular vesicles in certain AML cell lines and show localization of DNR to a similar subcellular compartment(s) that can be altered in the presence of P-gp inhibitors. Analysis of leukaemic cell lines and 50 AML patient samples showed that the level of P-gp mRNA or total P-gp protein correlated better with drug efflux than surface P-gp protein, suggesting that intracellular P-gp may contribute to MDR in AML. Therefore, the level of total P-gp protein or mRNA may be a better indicator of MDR than surface P-gp protein. In addition, we provide evidence for a novel mechanism of drug sequestration in K562 myeloid leukaemic cells.

Calcein assay for multidrug resistance reliably predicts therapy response and survival rate in acute myeloid leukaemia

In this study, we evaluated the suitability of the calcein assay as a routine clinical laboratory method for the identification of multidrug-resistant phenotype in acute leukaemia. This study presents the results of the calcein tests obtained in two large haematological centres in Hungary. Assays were performed with blast cells of 93 de novo acute leukaemia patients, including 65 patients with acute myeloid leukaemia (AML). Results were expressed as multidrug resistance activity factor (MAF) values. AML patients were divided into responders and non-responders and MAF values were calculated for each group. In both centres, responder patients displayed significantly lower MAF values than non-responders (P = 0.0045 and P = 0.0454). Cut-off values were established between the MAFR + SEM and MAFNR - SEM values. On the basis of these cut-off levels, multidrug resistance (MDR) negativity showed a 72% predictive value for the response to chemotherapy, whereas MDR positivity was found to have an average predictive value of 69% for therapy failure. MDR activity was a prognostic factor for survival rate and the test was suitable for detecting patients at relapse. The calcein assay can be used as a quantitative, standardized, inexpensive screening test in a routine clinical laboratory setting. The assay detects both P-glycoprotein and multidrug resistance-associated protein activities, and identifies AML patients with unfavourable therapy responses.

Cyclosporin increases cellular idarubicin and idarubicinol concentrations in relapsed or refractory AML mainly due to reduced systemic clearance

The feasibility of adding both the multidrug resistance modulator cyclosporin (CsA) and granulocyte colony-stimulating factor (G-CSF) to a standard salvage regimen of idarubicin (IDA) and cytarabine was evaluated in patients with resistant or relapsed acute myeloid leukemia and myelodysplastic syndrome. Three patients received IDA 12 mg/m2/day, the next four patients 9 mg/m2/day. The dose of CsA was 16 mg/kg/day. Six patients showed Pgp expression and none MRP1 expression. Grade III or IV toxicity (CTC-NCIC criteria) was registered in six patients for gastrointestinal, two patients for cardiovascular and one patient for neurological complications. Three patients died in hypoplasia and three patients showed leukemic regrowth. Three control patients were treated with IDA 12 mg/m2/day and cytarabine, but no CsA and G-CSF. The plasma IDA and idarubicinol (ida-ol) area under the curve's of patients treated with IDA 12 mg/m2 plus CsA were higher (P< 0.05) than in controls. Cellular IDA concentrations were almost similar, but cellular ida-ol concentrations were significantly higher (P < 0.05) in the presence of CsA than in controls. We conclude that the toxicity either with IDA 12 or 9 mg/m2/day was too high. The modulating effect of CsA was mainly based on changes in plasma kinetics of IDA and ida-ol, although ida-ol cellular clearance was delayed in the presence of CsA.

Accumulation of sestamibi in lymphoma cell lines in vitro

Although some authors have suggested that sestamibi imaging is useful in evaluation of patients with lymphoma, others have obtained equivocal results. This discrepancy has been further investigated in vitro using two patient-derived non-Hodgkin's lymphoma cell lines, OCI-Ly3 and OCI-Ly18. Sestamibi (0.2 MBq/ml) was added to a suspension of OCI-Ly3 or OCI-Ly18 cells and aliquots were removed over 1 h and centrifuged to determine cell-associated radioactivity. Further experiments studied the effect of addition of a P-glycoprotein (Pgp) modulator or alteration in plasma and/or mitochondrial membrane potentials. Accumulation of sestamibi reached plateau values within 30 min, but these values were 6-fold higher in OCI-Ly3 than in OCI-Ly18. Inhibition of Pgp function with GG918 or PSC833 did not affect OCI-Ly3 cells but increased accumulation in OCI-Ly18 cells 3-fold, indicating a moderate level of Pgp. However, both cell lines responded similarly to membrane potential alterations: hyperpolarization of the mitochondrial membrane with nigericin had little effect on accumulation: in contrast, depolarization of the plasma membrane with an isotonic high potassium buffer reduced accumulation of sestamibi to 52% of control and additional depolarization of the mitochondrial membrane with valinomycin further reduced accumulation to 12% of control levels. These studies suggest that there can be wide differences in accumulation between cell lines, in part due to Pgp-mediated efflux, but that both of these cell lines have highly polarized mitochondria with little further capacity for hyperpolarization.

P-glycoprotein and multidrug resistance-associated protein, but not lung resistance protein, lower the intracellular daunorubicin accumulation in acute myeloid leukaemic cells

The in vitro intracellular daunorubicin accumulation (IDA) of blast cells from 69 patients with newly diagnosed acute myeloid leukaemia (AML) was correlated with the expression and functional activity of the multidrug resistance (MDR) proteins, P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP) and lung-resistance protein (LRP). An inverse and significant association was found between IDA and Pgp-related efflux activity (r = -0.31, P = 0.01) and also MRP (r = -0.25, P = 0.04) but not with LRP (r = -0.13, P = 0.28). Coexpression of the MDR proteins had an additive effect in further lowering of IDA levels, suggesting that the clinical MDR phenotype is dependent on the sum of multiple MDR factors available to the leukaemic cell. Thus, the median IDA of leukaemic cells without any MDR proteins was significantly higher than that of blasts carrying two MDR proteins (0.466 vs. 0.296, P = 0.046). Seven patients with no expression of Pgp, MRP and LRP still had low IDA levels, suggesting the presence of efflux MDR mechanisms other than those studied. The relation of IDA to clinical parameters known to be associated with poor prognosis, such as age, secondary AML, karyotype, peripheral blood blast and CD34 counts, was also studied, but no significance was found on multifactorial analysis. There was a non-significant trend for earlier relapse in patients with low IDA levels (leukaemia-free survival of 16.3 months compared with 21.1 months in patients with high IDA levels). Our data suggest that, while the IDA assay is a quick and relatively easy test for the combined efflux MDR phenotype, it is unable to detect other MDR mechanisms, such as LRP, which may be important to the clinical outcome of patients with AML.

Absolute levels of MDR-1, MRP, and BCL-2 MRNA and tumor remission in acute leukemia

Mononuclear cells prepared from peripheral blood or bone marrow of 119 AML and 28 ALL patients prior and following therapy were analyzed for absolute transcript levels of the chemoresistance genes mdr-1 and MRP, and the proto-oncogene bcl-2, by validated contamination-protected quantitative RT-PCR. In newly diagnosed AML mainly tumors of the granulocytic lineage (FAB M1-M2) expressed increased mdr-1 mRNA amounts. The MRP gene was expressed in all investigated samples without relation to a particular FAB class. High initial expression of both genes did not confer a poor prognosis even at high number of CD34+ cells. Data compared prior to and after therapy start (paired samples) revealed that AML patients who did not respond to therapy (NR) expressed increased levels of mdr-1 mRNA, as well as MRP and bcl-2 cDNA normalized to GAPDH reference transcripts, when compared to patients achieving complete remission (CR; p = 0.003, 0.008 and 0.0005, respectively). In ALL-NR the mdr-1 and bcl-2 genes were entirely more active after induction chemotherapy. Arbitrary cut-off values were established in order to delimit pathological from non-pathological gene expression. 59% of studied AML and 33% of ALL-NR exceeded the arbitrary values (mdr-1: > 2 amol/microgram RNA, MRP: > 10 zmol/amol GAPDH, bcl-2: > 5 zmol/amol GAPDH) for one and 11% of AML-NR for two parameters. Only 17% of the AML-CR and none of the ALL-CR group were above these limits. The results indicate that high individual activity of usually one, rarely two of the investigated genes might be associated with poor clinical outcome in treated acute leukemia.

The prognostic significance of expression of the multidrug resistance-associated protein (MRP) in primary breast cancer

In the present study, we determined the frequency and intensity of MRP protein expression by monoclonal antibody immunohistochemistry in a series of 259 resected invasive primary breast carcinomas, and we evaluated MRP immunoreactivity in relation to patient and tumour characteristics, relapse-free (RFS) and overall survival (OS). The immunostaining was graded on a semiquantitative scale that ranged from (-) to ( ). Overall, 34% of the tumours were positive for anti-MRP antibody: 19% showed weak cytoplasmic staining (+), 14% had clear cytoplasmic staining (++) and only 1% of the tumours had a strong cytoplasmic as well as membranous staining ( ). MRP expression was not related to patient's age, menopausal status, tumour size, differentiation grade, oestrogen and progesterone receptor level or lymph node involvement. In an exploratory univariate analysis of all patients, only primary tumour size and number of lymph nodes involved were significantly associated with shortened RFS (P < 0.001 and P < 0.001 respectively) and OS (P = 0.02 and P < 0.001 respectively). In Cox univariate analysis for RFS in subgroups of patients stratified by menopausal status, tumour size, nodal status, adjuvant systemic therapy and oestrogen and progesterone receptor status, MRP expression was associated with increased risk for failure in patients with small tumours (T1), in node-negative patients and in node-positive patients who received adjuvant systemic chemotherapy with cyclophosphamide, methotrexate and 5-fluorouracil (CMF); the relative hazard rate (RHR) for relapse was increased in the presence of MRP, with RHR values with 95% confidence limits (CL) of 2.8 (1.2-6.9), 2.1 (1.0-4.2) and 2.8 (0.8-9.9) respectively. In analysis for OS, expression of MRP was also associated with increased risk for failure in patients with small tumours (T1) [RHR (95% CL) 2.3 (0.9-6.0)] and in node-positive patients who received adjuvant systemic chemotherapy with CMF [RHR (95% CL) 3.7 (0.8-17.1)] but not in node-negative patients [RHR (95% CL) 1.1 (0.4-2.6)]. In conclusion, our results show that MRP is frequently overexpressed in primary breast cancer and suggest that MRP expression might be of prognostic significance in the subgroups of patients with the more favourable prognosis, i.e. patients with small tumours and node-negative patients, as well as in the setting of adjuvant systemic chemotherapy. In primary breast cancer, MRP might be related to altered cell biological behaviour, including a more aggressive phenotype, and resistance to adjuvant systemic chemotherapy.