Breast cancer resistance protein is localized at the plasma membrane in mitoxantrone- and topotecan-resistant cell lines

Tumor cells may display a multidrug resistant phenotype by overexpression of ATP-binding cassette transporters such as multidrug resistance (MDRI) P-glycoprotein, multidrug resistance protein 1 (MRP1), and breast cancer resistance protein (BCRP). The presence of BCRP has thus far been reported solely using mRNA data. In this study, we describe a BCRP-specific monoclonal antibody, BXP-34, obtained from mice, immunized with mitoxantrone-resistant, BCRP mRNA-positive MCF-7 MR human breast cancer cells. BCRP was detected in BCRP-transfected cells and in several mitoxantrone- and topotecan-selected tumor cell sublines. Pronounced staining of the cell membranes showed that the transporter is mainly present at the plasma membrane. In a panel of human tumors, including primary tumors as well as drug-treated breast cancer and acute myeloid leukemia samples, BCRP was low or undetectable. Extended studies will be required to analyze the possible contribution of BCRP to clinical multidrug resistance.     

乳腺癌耐药蛋白--肿瘤多药耐药研究新进展

乳腺癌耐药蛋白（breast cancer resistance protein,BCRP)是近年发现的与肿瘤多药耐药有关的新的药物排出泵。BCRP是含655个氨基酸残基的跨膜蛋白,属于ABC转运蛋白超家族的成员,BCRP仅有6个跨膜区和1个ATP的结合位点,故被称为不完整转运分子,推测BCRP通过组成同二聚体或异二聚体构成跨膜通道而发挥功能,过表达BCRP的肿瘤细胞株对米托蒽醌,阿霉素,柔红霉素,鬼臼乙叉甙,拓扑替康,CPT－11等产生交叉耐药,而对长春新碱,紫杉醇无交叉耐药,GF120918和Fumitremorgin C能有效逆转过表达BCRP的肿瘤细胞株的耐药性,并与细胞内药物蓄积量呈正相关,人体正常组织中胎盘合体滋养层细胞,小肠和结肠粘膜上皮细胞,胆小管膜,乳腺小叶及血管内皮细胞和干细胞均能检测到BCRP的表达,推测BCRP具有抑制消化道吸收某些外源性物质（包括抗癌药和有毒物质）,参与形成胎盘屏障等生理功能,BCRP与急性髓性白血病,非小细胞肺癌,乳腺癌等多种肿瘤的临床化疗敏感性有关。     

The role of drug efflux pumps in acute myeloid leukemia

A major problem in the treatment of patients with acute myeloid leukemia (AML) is the occurrence of resistance to structurally and functionally unrelated chemotherapeutic agents, called multidrug resistance (MDR). One of the known MDR mechanisms is the overexpression of adenosine triphosphate (ATP)-dependent efflux pumps. Permeability-glycoprotein (P-gp), the best characterized of the human drug efflux pumps, has been shown to be associated with poor treatment outcome in AML patients. Besides P-gp, in addition the multidrug resistance protein 1 (MRP1) appeared to contribute to the observed resistance in AML. Alternative transporter proteins, such as the MRP1 homologues MRP2, MRP3, MRP5 and MRP6, and the breast cancer resistance protein (BCRP), have been shown to be expressed at variable levels in AML patient cells. The latter proteins have been described to confer resistance to chemotherapeutic agents, such as daunorubicin, mitoxantrone, etoposide and 6-mercaptopurine, which are generally used in the treatment of AML patients; however, theyhave not yet proven to play a role in drug resistance in AML. The present review gives an overview of the current knowledge concerning these drug transporters, with a focus on the role of the transporter proteins in AML.     

Atypical multidrug resistance: breast cancer resistance protein messenger RNA expression in mitoxantrone-selected cell lines

BACKGROUND: Human cancer cell lines grown in the presence of the cytotoxic agent mitoxantrone frequently develop resistance associated with a reduction in intracellular drug accumulation without increased expression of the known drug resistance transporters P-glycoprotein and multidrug resistance protein (also known as multidrug resistance-associated protein). Breast cancer resistance protein (BCRP) is a recently described adenosine triphosphate-binding cassette transporter associated with resistance to mitoxantrone and anthracyclines. This study was undertaken to test the prevalence of BCRP overexpression in cell lines selected for growth in the presence of mitoxantrone. METHODS: Total cellular RNA or poly A+ RNA and genomic DNA were isolated from parental and drug-selected cell lines. Expression of BCRP messenger RNA (mRNA) and amplification of the BCRP gene were analyzed by northern and Southern blot hybridization, respectively. RESULTS: A variety of drug-resistant human cancer cell lines derived by selection with mitoxantrone markedly overexpressed BCRP mRNA; these cell lines included sublines of human breast carcinoma (MCF-7), colon carcinoma (S1 and HT29), gastric carcinoma (EPG85-257), fibrosarcoma (EPF86-079), and myeloma (8226) origins. Analysis of genomic DNA from BCRP-overexpressing MCF-7/MX cells demonstrated that the BCRP gene was also amplified in these cells. CONCLUSIONS: Overexpression of BCRP mRNA is frequently observed in multidrug-resistant cell lines selected with mitoxantrone, suggesting that BCRP is likely to be a major cellular defense mechanism elicited in response to exposure to this drug. It is likely that BCRP is the putative "mitoxantrone transporter" hypothesized to be present in these cell lines.     

Cyclosporin A is a broad-spectrum multidrug resistance modulator.

PURPOSE: Overexpression of the multidrug resistance proteins P-glycoprotein (Pgp), multidrug resistance protein (MRP-1), breast cancer resistance protein (BCRP), and lung resistance protein (LRP) is associated with treatment failure in acute myeloid leukemia (AML) and other malignancies. The Pgp modulator cyclosporin A has shown clinical efficacy in AML, whereas its analogue PSC-833 has not. Cyclosporin A is known to also modulate MRP-1, and we hypothesized that broad-spectrum multidrug resistance modulation might contribute to its clinical efficacy. EXPERIMENTAL DESIGN: We studied the effects of cyclosporin A and PSC-833 on in vitro drug retention and cytotoxicity in resistant cell lines overexpressing Pgp, MRP-1, and BCRP and on nuclear-cytoplasmic drug distribution and cytotoxicity in cells overexpressing LRP. Cellular drug content was assessed by flow cytometry and nuclear-cytoplasmic drug distribution by confocal microscopy. RESULTS: Cyclosporin A enhanced retention of the substrate drug mitoxantrone in cells overexpressing Pgp (HL60/VCR), MRP-1 (HL60/ADR), and BCRP (8226/MR20, HEK-293 482R) and increased cytotoxicity 6-, 4-, 4-, and 3-fold, respectively. Moreover, cyclosporin A enhanced nuclear distribution of doxorubicin in 8226/MR20 cells, which also express LRP, and increased doxorubicin cytotoxicity 12-fold without an effect on cellular doxorubicin content, consistent with expression of wild-type BCRP, which does not efflux doxorubicin. Cyclosporin A also enhanced nuclear doxorubicin distribution in a second cell line with LRP overexpression, HT1080/DR4. PSC-833 enhanced mitoxantrone retention and cytotoxicity in cells overexpressing Pgp, but had no effect in cells overexpressing MRP-1, BCRP, or LRP. CONCLUSIONS: Cyclosporin A modulates Pgp, MRP-1, BCRP, and LRP, and this broad-spectrum activity may contribute to its clinical efficacy.     

VX-710 (biricodar) increases drug retention and enhances chemosensitivity in resistant cells overexpressing P-glycoprotein, multidrug resistance protein, and breast cancer resistance protein.

PURPOSE: The pipecolinate derivative VX-710 (biricodar; Incel) is a clinically applicable modulator of P-glycoprotein (Pgp) and multidrug resistance protein (MRP-1); we studied its activity against the third multidrug resistance (MDR)-associated drug efflux protein, breast cancer resistance protein (BCRP). EXPERIMENTAL DESIGN: VX-710 modulation of uptake, retention, and cytotoxicity of mitoxantrone, daunorubicin, doxorubicin, topotecan, and SN38 was studied in cell lines overexpressing Pgp, MRP-1 and wild-type (BCRP(R482)) and mutant (BCRP(R482T)) BCRP. RESULTS: In 8226/Dox6 cells (Pgp), VX-710 increased mitoxantrone and daunorubicin uptake by 55 and 100%, respectively, increased their retention by 100 and 60%, respectively, and increased their cytotoxicity 3.1- and 6.9-fold, respectively. In HL60/Adr cells (MRP-1), VX-710 increased mitoxantrone and daunorubicin uptake by 43 and 130%, increased their retention by 90 and 60%, and increased their cytotoxicity 2.4- and 3.3-fold. In 8226/MR20 cells (BCRP(R482)), VX-710 increased mitoxantrone uptake and retention by 60 and 40%, respectively, and increased cytotoxicity 2.4-fold. VX-710 increased daunorubicin uptake and retention by only 10% in 8226/MR20 cells, consistent with the fact that daunorubicin is not a substrate for BCRP(R482), but, nevertheless, it increased daunorubicin cytotoxicity 3.6-fold, and this increase was not associated with intracellular drug redistribution. VX-710 had little effect on uptake, retention, or cytotoxicity of mitoxantrone, daunorubicin, doxorubicin, topotecan, or SN38 in MCF7 AdVP3000 cells (BCRP(R482T)). CONCLUSIONS: VX-710 modulates Pgp, MRP-1, and BCRP(R482), and has potential as a clinical broad-spectrum MDR modulator in malignancies such as the acute leukemias in which these proteins are expressed.     

The mouse Bcrp1/Mxr/Abcp gene: amplification and overexpression in cell lines selected for resistance to topotecan, mitoxantrone, or doxorubicin.

Mouse fibroblast cell lines lacking functional Mdr1a, Mdr1b, and Mrp1 genes were selected for resistance to topotecan, mitoxantrone, or doxorubicin. Each of the resulting drug-resistant lines showed marked gene amplification of Bcrp1, the mouse homologue of the human ATP-binding cassette transporter gene BCRP/MXR/ABCP, and greatly elevated expression of Bcrp1 mRNA. All three of the resistant cell lines were highly cross-resistant to topotecan and mitoxantrone and, to a variable extent, doxorubicin. All showed greatly reduced cellular accumulation and greatly increased efflux of mitoxantrone that was dependent on cellular ATP and efficiently reversed by the compound GF120918. The mouse Bcrp1 cDNA encodes a 657-amino-acid protein with 81% identity (86% similarity) to the human breast cancer resistance protein (BCRP) and a virtually superimposable hydrophobicity profile. Our data argue strongly that mouse Bcrp1 is functionally comparable with human BCRP, conferring multidrug resistance to topotecan, mitoxantrone, doxorubicin, and related compounds. Mouse models and cell lines should, therefore, be highly informative in understanding the clinical, pharmacological, and physiological roles of BCRP.     

多药耐药相关蛋白7及其抑制剂

多药耐药(MDR)的机制与转运蛋白有关,现在研究最多的为P-糖蛋白(P-gp)、多药耐药相关蛋白(MRP)1、乳腺癌耐药相关蛋白(BCRP)等的抑制剂.MRP7可介导对紫杉醇、长春新碱和长春碱等的耐药.MRP7抑制剂近年研究主要包括千斤藤素、酪氨酸酶抑制剂、环孢素A等,MDR是多种机制共同作用的结果,对其他转运体的研究会提供更全面、更广泛的MDR逆转途径.     

Modulation of P-glycoprotein but not MRP1- or BCRP-mediated drug resistance by LY335979

Our study examines the ability of LY335979 (Zosuquidar trihydrochloride) to modulate 3 distinct ABC transporters that are mechanisms of drug resistance: P-glycoprotein (Pgp, ABCB1), multidrug resistance associated protein (MRP1, ABCC2) and breast cancer resistance protein (BCRP, ABCG2). Pgp-mediated resistance can be modulated by coadministration with the highly potent, selective inhibitor, LY335979. Modulation of resistance by mitoxantrone and vinorelbine, 2 drugs used to treat certain solid tumors, was examined in a 3-day cytotoxicity assay using a panel of HL60 leukemia cell lines or MCF-7 breast cancer transfectants. LY335979, at 0.5 microM, substantially reversed mitoxantrone resistance and fully reversed vinorelbine resistance of Pgp-expressing HL60/Vinc cells. However, LY335979 did not modulate drug resistance in the MRP1-expressing HL60/ADR or drug-sensitive parental HL60 cells. To ascertain if LY335979 modulates BCRP-mediated drug resistance, the sensitivity of 26-fold mitoxantrone resistant, BCRP-transfected MCF-7 cells was evaluated. Addition of 5 microM LY335979, a concentration approximately 100-fold higher than the affinity of Pgp, had little to no effect on the BCRP transfectant. [(125)I]Iodomycin photolabeled Pgp in CEM/VLB(100) membranes and was inhibited by 5 microM LY335979 and GF120918. No photolabeling of MRP or BCRP occurred in H69AR or MCF-7/BCRP membranes, respectively. These results further demonstrate that LY335979 is highly specific for Pgp and does not modulate MRP1- or BCRP-mediated resistance and can be used in combination with mitoxantrone and vinorelbine in tumor cells.     

Impact of BCRP/MXR, MRP1 and MDR1/P-Glycoprotein on thermoresistant variants of atypical and classical multidrug resistant cancer cells

The impact of the ABC transporters breast cancer resistance protein/mitoxantrone resistance associated transporter (BCRP/MXR), multidrug resistance-associated protein 1 (MRP1) and multidrug resistance gene-1/P-glycoprotein (MDR1/PGP) on the multidrug resistance (MDR) phenotype in chemoresistance and thermoresistance was investigated in the parental human gastric carcinoma cell line EPG85-257P, the atypical MDR subline EPG85-257RNOV, the classical MDR subline EPG85-257RDB and their thermoresistant counterparts EPG85-257P-TR, EPG85-257RNOV-TR and EPG85-257RDB-TR. Within the atypical MDR subline EPG85-257RNOV expression of BCRP/MXR and of MRP1 were clearly enhanced (vs. parental and classical MDR lines). MDR1/PGP expression was distinctly elevated in the classical MDR subline EPG85-257RDB (vs. parental and atypical MDR sublines). In all thermoresistant counterparts basal expression of BCRP/MXR, MRP1 and MDR1/PGP was increased relative to thermosensitive sublines. Although it could be shown that the overexpressed ABC transporters were functionally active, however, no decreased drug accumulations of doxorubicin, mitoxantrone and rhodamine 123 were observed. Thus, expression of BCRP/MXR, MRP1 and MDR1/PGP was found to be dependent on the appropriate type of chemoresistance; correlating with a classical or atypical MDR phenotype. Within the thermoresistant variants, however, the increase in ABC transporter expression did obviously not influence the MDR phenotype.     

Fumitremorgin C reverses multidrug resistance in cells transfected with the breast cancer resistance protein

Fumitremorgin C (FTC) is a potent and specific chemosensitizing agent in cell lines selected for resistance to mitoxantrone that do not overexpress P-glycoprotein or multidrug resistance protein. The gene encoding a novel transporter, the breast cancer resistance protein (BCRP), was recently found to be overexpressed in a mitoxantrone-selected human colon cell line, S1-M1-3.2, which was used to identify FTC. Because the drug-selected cell line may contain multiple alterations contributing to the multidrug resistance phenotype, we examined the effect of FTC on MCF-7 cells transfected with the BCRP gene. We report that FTC almost completely reverses resistance mediated by BCRP in vitro and is a pharmacological probe for the expression and molecular action of this transporter.     

Prognostic significance of multidrug resistance-related proteins in childhood acute lymphoblastic leukaemia

An important problem in the treatment of children with acute lymphoblastic leukaemia (ALL) is pre-existent or acquired resistance to structurally and functionally unrelated chemotherapeutic compounds. Various cellular mechanisms can give rise to multidrug resistance (MDR). Best studied is the transmembrane protein-mediated efflux of cytotoxic compounds that leads to decreased cellular drug accumulation and toxicity. Several MDR-related efflux pumps have been characterised, including P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1), breast cancer resistance protein (BCRP) and lung resistance protein (LRP). P-gp expression and/or activity has been associated with unfavourable outcome in paediatric ALL patients, whereas MRP1 and BCRP do not seem to play a major role. LRP might contribute to drug resistance in B-lineage ALL, but larger studies are needed to confirm these results. The present review summarises the current knowledge concerning multidrug resistance-related proteins and focuses on the clinical relevance and prognostic value of these efflux pumps in childhood ALL.     

Phytoestrogens/flavonoids reverse breast cancer resistance protein/ABCG2-mediated multidrug resistance

Breast cancer resistance protein (BCRP), also called ABCG2, confers resistance to anticancer agents such as 7-ethyl-10-hydroxycamptothecin (SN-38), mitoxantrone, and topotecan. We found previously that sulfated estrogens are physiologic substrates of BCRP. Flavonoids with weak estrogenic activities are called phytoestrogens. In this study, we show that phytoestrogens/flavonoids, such as genistein, naringenin, acacetin, and kaempferol, potentiated the cytotoxicity of SN-38 and mitoxantrone in BCRP-transduced K562 (K562/BCRP) cells. Some glycosylated flavonoids, such as naringenin-7-glucoside, also effectively inhibited BCRP. These flavonoids showed marginal effect on the drug sensitivity of K562 cells. Genistein and naringenin reversed neither P-glycoprotein-mediated vincristine resistance nor multidrug resistance-related protein 1-mediated VP-16 resistance. Genistein and naringenin increased cellular accumulation of topotecan in K562/BCRP cells. K562/BCRP cells also accumulated less [(3)H]genistein than K562 cells. [(3)H]genistein transport in the basal-to-apical direction was greater in BCRP-transduced LLC-PK1 (LLC/BCRP) cells, which express exogenous BCRP in the apical membrane, than in parental cells. Fumitremorgin C abolished the increased transport of [(3)H]genistein in LLC/BCRP cells compared with parental cells. TLC analysis revealed that genistein was transported in its native form but not in its metabolized form. These results suggest that genistein is among the natural substrates of BCRP and competitively inhibits BCRP-mediated drug efflux. The results have two important clinical implications: (a) flavonoids and glycosylated flavonoids may be useful in overcoming BCRP-mediated drug resistance in tumor cells; and (b) coadministration of flavonoids with BCRP-substrate antitumor agents may alter the pharmacokinetics and consequently increase the toxicity of specific antitumor agents in cancer patients.     

表达BCRP的胚肾细胞系HEK293/BCRP的建立及生物学特征

目的:建立HEK293/BCRP多药耐药细胞系并研究其生物学功能。方法:构建有BCRP基因的表达载体,利用脂质体转染方法将载体转入HEK293细胞,并用RT-PCR、间接免疫荧光染色和流式细胞术分析检测转染细胞系BCRP的表达,体外细胞毒试验检测其对米托蒽醌等的耐药指数,流式细胞术和激光共聚焦分别检测其对罗丹明123和阿霉素的外排作用。结果:HEK293/BCRP细胞系在BCRP mRNA和蛋白表达均明显升高(P<0.05),对米托蒽醌的耐药指数达112.07倍,经1.5 h和3 h外排,细胞内罗丹明123浓度分别降低了42.25%和69.01%,激光共聚焦显示细胞内阿霉素浓度降低。结论:成功构建了表达BCRP的胚肾细胞系HEK293/BCRP,并对多种抗肿瘤药物具有耐受性。     

Novel mechanisms of drug resistance in leukemia.

A key issue in the treatment of acute leukemia is the development of resistance to chemotherapeutic drugs. Several mechanisms may account for this phenomenon, including failure of the cell to undergo apoptosis in response to chemotherapy, or failure of the drug to reach and/or affect its intracellular target. This review focuses on the latter mechanism, and on intracellular drug transport resistance mechanisms in particular. Expression of the ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp) has generally been reported to correlate with prognosis in acute myeloid leukemia (AML). Additionally, but more controversial, expression of the ABC transporter multidrug resistance protein (MRP) and the vault-transporter lung resistance protein (LRP) have been correlated with outcome in AML. Despite these findings, functional efflux assays indicate the presence of non-Pgp, non-MRP transporters in AML. Recently, a novel ABC transporter, breast cancer resistance protein (BCRP) was cloned and sequenced in our laboratory. Transfection and overexpression of BCRP in drug-sensitive cells confers drug-resistance to the cells. BCRP is a half-transporter, and may homodimerize or form heterodimers (with a yet unknown half-transporter) to produce an active transport complex. Relatively high expression of BCRP mRNA is observed in approximately 30% of AML cases, suggesting a potential role for this new transporter in drug resistance in leukemia.     

Increased expression of the breast cancer resistance protein (BCRP) in relapsed or refractory acute myeloid leukemia (AML).

Expression of the multidrug resistance proteins P-glycoprotein, encoded by the MDR1 gene, multidrug resistance-associated protein (MRP1) and the lung resistance-related protein or major vault protein (LRP/MVP) is associated with clinical resistance to chemotherapy in acute myeloid leukemia (AML). Recently, the breast cancer-resistant protein (BCRP), the equivalent of mitoxantrone-resistant protein (MXR) or placental ABC transporter (ABCP), was described in AML. We investigated MDR1, MRP1, LRP/MVP and BCRP mRNA expression simultaneously in 20 paired clinical AML samples from diagnosis and relapse or refractory disease, using quantitative Taqman analysis. In addition, standard assays for P-glycoprotein expression and function were performed. BCRP was the only resistance protein that was expressed at a significantly higher RNA level (median 1.7-fold, P = 0.04) at relapsed/refractory state as compared to diagnosis. In contrast, LRP/MVP mRNA expression decreased as disease evolved (P = 0.02), whereas MDR1 and MRP1 mRNA levels were not different at relapse as compared to diagnosis. Also, at the protein level no difference of MDR1 between diagnosis and relapse was found. A significant co-expression of BCRP and MDR1 was found at diagnosis (r = 0.47, P = 0.04). The present results suggest that BCRP, but not MDR1, MRP1 or LRP/MVP is associated with clinical resistant disease in AML.     

Multidrug resistance mediated by the breast cancer resistance protein BCRP (ABCG2)

Observations of functional adenosine triphosphate (ATP)-dependent drug efflux in certain multidrug-resistant cancer cell lines without overexpression of P-glycoprotein or multidrug resistance protein (MRP) family members suggested the existence of another ATP-binding cassette (ABC) transporter capable of causing cancer drug resistance. In one such cell line (MCF-7/AdrVp), the overexpression of a novel member of the G subfamily of ABC transporters was found. The new transporter was termed the breast cancer resistance protein (BCRP), because of its identification in MCF-7 human breast carcinoma cells. BCRP is a 655 amino-acid polypeptide, formally designated as ABCG2. Like all members of the ABC G (white) subfamily, BCRP is a half transporter. Transfection and enforced overexpression of BCRP in drug-sensitive MCF-7 or MDA-MB-231 cells recapitulates the drug-resistance phenotype of MCF-7/AdrVp cells, consistent with current evidence suggesting that functional BCRP is a homodimer. BCRP maps to chromosome 4q22, downstream from a TATA-less promoter. The spectrum of anticancer drugs effluxed by BCRP includes mitoxantrone, camptothecin-derived and indolocarbazole topoisomerase I inhibitors, methotrexate, flavopiridol, and quinazoline ErbB1 inhibitors. Transport of anthracyclines is variable and appears to depend on the presence of a BCRP mutation at codon 482. Potent and specific inhibitors of BCRP are now being developed, opening the door to clinical applications of BCRP inhibition. Owing to tissue localization in the placenta, bile canaliculi, colon, small bowel, and brain microvessel endothelium, BCRP may play a role in protecting the organism from potentially harmful xenobiotics. BCRP expression has also been demonstrated in pluripotential "side population" stem cells, responsible for the characteristic ability of these cells to exclude Hoechst 33342 dye, and possibly for the maintenance of the stem cell phenotype. Studies are emerging on the role of BCRP expression in drug resistance in clinical cancers. More prospective studies are needed, preferably combining BCRP protein or mRNA quantification with functional assays, in order to determine the contribution of BCRP to drug resistance in human cancers.     

Breast cancer resistance protein and P-glycoprotein in 149 adult acute myeloid leukemias.

PURPOSE: Recently, a new ABC protein, breast cancer resistance protein (BCRP), was described. But its prognosis is not known in acute myeloid leukemia (AML). In addition, the prognosis of P-glycoprotein (Pgp) and BCRP in patients treated homogeneously by the same anthracycline (daunorubicin, idarubicin, or mitoxantrone) during all of the treatment with aracytine is not known. Therefore, we have evaluated the relationship between drug resistance phenotype, in vitro anthracene sensitivity, and the relation to treatment outcome. EXPERIMENTAL DESIGN: We have analyzed 149 AML treated according to protocol of the European Organization for Research and Treatment of Cancer group. The prognostic value of BCRP and Pgp were analyzed in the whole population and according to intercalating agent. RESULTS: BCRP was a prognostic factor, for achievement of complete remission (43% in positive patients and 69% in negative patients, P = 0.005), the 4-year disease-free survival (12% versus 33%, P = 0.03), and the 4-year overall survival (19% versus 38%, P = 0.003). When BCRP expression and Pgp function were categorized in three groups, +/+, +/- or -/+, and -/-, the achievement of complete remission was 45%, 66%, and 90% (P = 0.0003), the 4-year disease-free survival was 8%, 26%, and 40% (P = 0.01), and the 4-year overall survival was 16%, 37%, and 48% (P = 0.001), respectively. Pgp function was a prognostic factor in patients treated by daunorubicin and idarubicin but not by mitoxantrone. In contrast, BCRP expression was a prognostic factor in patients treated by daunorubicin and mitoxantrone but not by idarubicin. CONCLUSIONS: BCRP would be implicated in the resistance to chemotherapies in AML. But these are the patients expressing both BCRP and Pgp who have the poorest prognosis.