Role of the breast cancer resistance protein (ABCG2) in drug transport

Tumor necrosis factor alpha induces stronger cytotoxicity in ABCG2-overexpressing resistant breast cancer cells compared with their drug-sensitive parental line

Tumor necrosis factor alpha (TNF-α) has been reported to modulate the multidrug resistance (MDR) phenotype in vitro and in vivo. Multidrug-resistant cells overexpressing the ABCB1 transporter are more susceptible to inhibition of proliferation and induction of apoptosis by TNF-α than their drug-sensitive counterparts. This study was aimed to investigate TNF-α modulatory and antiproliferative effects on drug-resistant cells overexpressing ABCG2. The effects of TNF-α on viability and proliferation rate of MCF-7 breast cancer cells and their ABCG2-overexpressing sublines MCF-7/mitoxantrone (MX) cells were studied using dye exclusion assay, dimethylthiazolyl-2,5-diphenyl tetrazolium bromide technique, and flow cytometric analysis of cell cycle. TNF-α influence on MX accumulation was investigated by flow cytometry. ABCG2-overexpressing cells were more susceptible to antiproliferative and cytotoxic effects of TNF-α than their parental cells. TNF-α increased accumulation of MX in both parental and resistant cells. Higher sensitivity of MDR cells to TNF-α cytotoxicity would help in characterization of its complex modulatory effects on cancer cells and benefit us in designing new approaches to overcome MDR.

5,7-Dimethoxyflavone and multiple flavonoids in combination alter the ABCG2-mediated tissue distribution of mitoxantrone in mice

PURPOSE

The objective of our study was to investigate the effect of 5,7-DMF on the accumulation of mitoxantrone (MX) in BCRP-expressing normal cells and to investigate its impact on the PK and tissue distribution of MX in mice.

METHODS

The in vitro effect of 5,7-DMF on MX accumulation was examined in MDCK cells transfected with BCRP. The pharmacokinetic and tissue distribution of mitoxantrone, with and without co-administration of 5,7-DMF or multiple flavonoid combinations, were determined in mice.

RESULTS

In the presence of 2.5 μM or 25 μM of 5,7-DMF, the intracellular concentration of MX was significantly increased in MDCK/Bcrp1 and MDCK/BCRP cells, but not in MDCK/Mock cells. The AUC values of MX in several tissues were significantly increased when MX was co-administered with 5,7-DMF. The most substantial elevations of MX AUC in the presence of 5,7-DMF occurred in the liver (94.5%) and kidneys (61.9%), which is in apparent agreement with the relatively high levels of mouse Bcrp1 expression in these two tissues.

CONCLUSIONS

Bcrp1-mediated DMF-MX interactions occur both in vitro and in vivo. 5,7-DMF represents a novel and very promising chemosensitizing agent for the BCRP-mediated MDR due to its low toxicity and potent BCRP inhibition.

The ABCG2 transporter and its relations with the pharmacokinetics, drug interaction and lipid-lowering effects of statins

IMPORTANCE OF THE FIELD

The ABCG2 efflux transporter is expressed in multiple tissues and plays an important role in the disposition of many statins. The functional 421C>A polymorphism in ABCG2 that reduces transporter activity has been found to be associated with increased systemic exposures to certain statins.

AREAS COVERED IN THIS REVIEW

We review and evaluate the associations of the ABCG2 polymorphism on the pharmacokinetics and clinical efficacy of statins.

WHAT THE READER WILL GAIN

This article gives a detailed overview of the ABCG2 transporter and extensively reviews its relations with the pharmacokinetics and lipid-lowering effects of statins. This review also discusses the potential role of the ABCG2 polymorphism in the clinical outcomes in statin-treated patients and statin-drug interactions.

TAKE HOME MESSAGE

The impact of the ABCG2 421C>A polymorphism on the disposition of the statins varies between different drugs and the effect on systemic exposure was greater in the case of rosuvastatin than other statins. This genetic variant was associated with greater low-density lipoprotein cholesterol response to rosuvastatin in Chinese and caucasian patients. The effect of the ABCG2 421C>A polymorphism on the lipid response to other substrate statins and clinical outcomes need to be evaluated in future studies.

Vascular Recruitment of Human Retinoblastoma Cells by Multi-Cellular Adhesive Interactions with Circulating Leukocytes

Retinoblastoma (RB) is a retinal cancer of childhood. RB survivors tend to develop additional tumors later in life, although the physical mechanisms of RB metastatic spread are largely unknown. One step in metastasis through the blood stream is tumor cell adherence to the blood vessel wall through specific receptor:ligand interactions. Yet, human RB cell lines RB143 and WERI-Rb27 do not express selectin ligands or beta-2 integrins and cannot directly interact with inflamed endothelium. In this study, we show that RB cells express ICAM-1, a beta-2 integrin ligand that correlates with metastasis and is preferentially co-expressed on RB cells that also express ABCG2, a stem cell marker associated with chemoresistance and metastasis. Based on the presence of ICAM-1+ RB cells, we tested the hypothesis that RB cells could be recruited to an E-selectin surface via attachment to activated polymorphonuclear cells (PMNs). We characterized the dynamic adhesion between RB cells and PMNs within E-selectin coated microtubes under a physiological range of wall shear stress values (0.2-5 dyn/cm²). We show that activated PMNs are necessary for the recruitment of RB cells through ICAM-1:LFA-1 binding. Results from this work may lead to new strategies that target the metastatic spread of tumor cells.

Knockdown of caveolin-1 decreases activity of breast cancer resistance protein (BCRP/ABCG2) and increases chemotherapeutic sensitivity

The ATP-binding cassette transporter breast cancer resistance protein (BCRP/ABCG2) is supposed to be a major determinant of the multidrug resistance phenotype of tumors by extruding chemically diverse cytostatic drugs out of tumor cells. BCRP physically and possibly also functionally interacts with caveolin-1 (CAV1, encoded by Cav1), an integral membrane protein of lipid rafts important for signal transduction and membrane trafficking. Moreover, Cav1 is linked to an aggressive phenotype of cancer cells in various tumors. We therefore investigated whether Cav1 plays a functional role in the regulation of BCRP transport activity and in the resistance against chemotherapeutics that are BCRP substrates. As a cell model, we used the BCRP overexpressing cell line MDCKII-BCRP and the corresponding parental cell line MDCKII as a control. Cav1 expression was down-regulated using retrovirus-mediated RNA interference technology. BCRP activity was assessed by pheophorbide A efflux assay and the resistance towards cytostatic drugs was measured by proliferation assays. Efficient knockdown of Cav1 reduced Cav1 expression by 85-95% and BCRP activity by 35%. Concurrently, it reduced resistance towards the BCRP substrate mitoxantrone but not towards vincristine, a chemotherapeutic that is not extruded by BCRP. Western blot analysis of gradient ultracentrifugation fractions and immunofluorescence demonstrates that BCRP localization within the plasma membrane was largely unaltered in Cav1-deficient cells compared to controls. The diminished BCRP function after Cav1 knockdown is, thus, likely mediated by alterations in protein-protein interactions and suggests a positive regulation of BCRP function by CAV1.

BCR-ABL tyrosine kinase inhibitors in the treatment of Philadelphia chromosome positive chronic myeloid leukemia: a review

Chronic Myeloid Leukemia (CML) is a clonal disease characterized by the presence of the Philadelphia (Ph+) chromosome and its oncogenic product, BCR-ABL, a constitutively active tyrosine kinase, that is present in >90% of the patients. Epidemiologic data indicates that almost 5000 new cases are reported every year and 10% of these patients eventually succumb to the disease. The treatment of CML was revolutionized by the introduction of imatinib mesylate (IM, Gleevec), a BCR-ABL tyrosine kinase inhibitor (TKI). The clinical use of specific BCR-ABL inhibitors has resulted in a significantly improved prognosis, response rate, overall survival, and patient outcome in CML patients compared to previous therapeutic regimens. However, the complete eradication of CML in patients receiving imatinib was limited by the emergence of resistance mostly due to mutations in the ABL kinase domain and to a lesser extent by molecular residual disease after treatment. The second-generation BCR-ABL TKIs nilotinib (Tasigna) and dasatinib (Sprycel), showed significant activity in clinical trials in patients intolerant or resistant to imatinib therapy, except in those patients with the T315I BCR-ABL mutation. Identifying key components involved in the CML pathogenesis may lead to the exploration of new approaches that might eventually overcome resistance mediated to the BCR-ABL TKIs. Here, we present an overview about the current treatment of Ph+ CML patients with the TKIs and the obstacles to successful treatment with these drugs.

Structure and function of the human breast cancer resistance protein (BCRP/ABCG2)

The human breast cancer resistance protein (BCRP/ABCG2) is the second member of the G subfamily of the large ATP-binding cassette (ABC) transporter superfamily. BCRP was initially discovered in multidrug resistant breast cancer cell lines where it confers resistance to chemotherapeutic agents such as mitoxantrone, topotecan and methotrexate by extruding these compounds out of the cell. BCRP is capable of transporting non-chemotherapy drugs and xenobiotiocs as well, including nitrofurantoin, prazosin, glyburide, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. BCRP is frequently detected at high levels in stem cells, likely providing xenobiotic protection. BCRP is also highly expressed in normal human tissues including the small intestine, liver, brain endothelium, and placenta. Therefore, BCRP has been increasingly recognized for its important role in the absorption, elimination, and tissue distribution of drugs and xenobiotics. At present, little is known about the transport mechanism of BCRP, particularly how it recognizes and transports a large number of structurally and chemically unrelated drugs and xenobiotics. Here, we review current knowledge of structure and function of this medically important ABC efflux drug transporter.

Transmembrane helices 1 and 6 of the human breast cancer resistance protein (BCRP/ABCG2): identification of polar residues important for drug transport

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and xenobiotics. In this study, we investigated the role of polar residues within or near the predicted transmembrane α-helices 1 and 6 of BCRP in drug transport. We substituted Asn(387), Gln(398), Asn(629), and Thr(642) with Ala, Thr(402) with Ala and Arg, and Tyr(645) with Phe, and the mutants were stably expressed in human embryonic kidney-293 or Flp-In-293 cells. Immunoblotting and confocal microscopy analysis revealed that all of the mutants were well expressed and predominantly targeted to the plasma membrane. While T402A and T402R showed a significant global reduction in the efflux of mitoxantrone, Hoechst 33342, and BODIPY-prazosin, N629A exhibited significantly increased efflux activities for all of the substrates. N387A and Q398A displayed significantly impaired efflux for mitoxantrone and Hoechst 33342, but not for BODIPY-prazosin. In contrast, T642A and Y645F showed a moderate reduction in Hoechst 33342 efflux only. Drug resistance profiles of human embryonic kidney-293 cells expressing the mutants generally correlated with the efflux data. Furthermore, N629A was associated with a marked increase, and N387A and T402A with a significant reduction, in BCRP ATPase activity. Mutations of some of the polar residues may cause conformational changes, as manifested by the altered binding of the 5D3 antibody to BCRP in the presence of prazosin. The inward-facing homology model of BCRP indicated that Thr(402) within transmembrane 1 may be important for helical interactions, and Asn(629) may be involved in BCRP-substrate interaction. In conclusion, we have demonstrated the functional importance of some of these polar residues in BCRP activity.

ABC transporters in human lymphocytes: expression, activity and role, modulating factors and consequences for antiretroviral therapies

IMPORTANCE OF THE FIELD

ATP-binding cassette (ABC) transporters are a superfamily of efflux pumps that transport numerous compounds across cell membranes. These transporters are located in various human tissues including peripheral blood cells, in particular lymphocytes, and present a high variability of expression and activity. This variability may affect the intracellular concentrations and efficacy of drugs acting within lymphocytes, such as antiretroviral drugs.

AREAS COVERED IN THIS REVIEW

This review focuses on the current knowledge about the expression, activity, roles and variability of ABC drug transporters in human lymphocytes. The identified modulating factors and their impact on the intracellular pharmacokinetics and efficacy of antiretroviral drugs are also detailed.

WHAT THE READER WILL GAIN

Controversial data regarding the expression, activity and sources of variability of ABC transporters in lymphocytes are discussed. The modulating factors and their pharmacological consequences regarding antiretroviral therapies are also provided.

TAKE HOME MESSAGE

Numerous studies have reported conflicting results regarding the expression and activity of ABC drug transporters in lymphocytes. Despite these discrepancies, which may partly result from heterogeneous analytical methods, ABCC1 appears to have the highest expression in lymphocytes and may thus play a predominant role in the resistance to antiretroviral drugs, particularly to protease inhibitors.

N-glycosylation of ABC transporters is associated with functional activity in sandwich-cultured rat hepatocytes

Hepatobiliary elimination via canalicular efflux transport proteins plays a key role in the clearance of endo- and xenobiotics. Correct membrane localization and coordinated action of the transport systems are essential for vectorial transport of drugs from blood into the bile. While basolaterally localized uptake transporters are responsible for the inward transport of substances from the blood into the hepatocyte, apically expressed ATP-dependent transport proteins such as P-glycoprotein (P-gp), multidrug resistance-associated protein (Mrp2) and breast cancer resistance protein (Bcrp) mediate the outward efflux into the bile canaliculus. Using sandwich-cultured rat hepatocytes we have characterized the expression and maturation of P-gp, Mrp2 and Bcrp transport proteins as well as their transport function over several days. The re-differentiation of the hepatocytes, which only occurs in sandwich configuration involves de novo synthesis and subsequent posttranslational N-glycosylation of all three transport proteins. Only fully N-glycosylated isoforms of the transporters were associated with functional activity as visualized by excretion of specific fluorescent substrates into the canalicular network. However, in what way N-glycosylation affects the functional activity of the ABC transporters investigated remains to be determined.

Potential role of cyclooxygenase-2 on the regulation of the drug efflux transporter ABCG2 in breast cancer cell lines

PURPOSE

ABCG2 (BCRP) implicated as a member of the multidrug resistance (MDR) proteins in tumors, mediating efflux of a wide spectrum of anticancer drugs. In recent years, there has been an increasing tendency toward the exploring of the potential link between cyclooxygenase-2 (COX-2) expression and development of multidrug resistance phenotype in patients with cancer. The aim of this study was to investigate the role of the COX-2 in modulating drug efflux by ABCG2 in a group of breast cancer cell lines.

METHODS

The cytotoxicity of COX-2 inducer (TPA, tetradecanoyl phorbol acetate) and its inhibitor (celecoxib) was determined by an MTT assay. ABCG2 activity was measured by flow cytometric mitoxantrone efflux assay.

RESULTS

TPA exhibited very little inhibitory activity in all cell lines, while long-term treatment with celecoxib significantly inhibited the growth of all cell lines. Furthermore, using mitoxantrone efflux assay was shown that TPA could increase ABCG2 activity in all the cell lines with the greatest stimulatory effects in MCF7-MX (more than 6 times the control level). It seemed that celecoxib inverted the effects of TPA on ABCG2 activity. This was more obvious in MCF7-MX.

CONCLUSION

The results suggest a probable causal link between COX-2 and ABCG2 activity. The use of celecoxib for adjuvant therapy in cancer treatment may contribute to decreased resistance to chemotherapeutic drugs transported by ABCG2.

Effects of single and multiple flavonoids on BCRP-mediated accumulation, cytotoxicity and transport of mitoxantrone in vitro

PURPOSE

The objective of our study was to investigate the effect of single and multiple flavonoids on the accumulation and cytotoxicity of mitoxantrone in BCRP-overexpressing breast cancer cells and on the transport of mitoxantrone in BCRP-expressing normal cells.

METHODS

The effect of flavonoids on mitoxantrone accumulation and cytotoxicity was studied in the human breast cancer MCF-7 MX100 cell line. Mitoxantrone transport in the presence of flavonoids was studied in human and murine BCRP-transfected MDCK cell lines, and mitoxantrone concentrations were determined by HPLC.

RESULTS

Our results demonstrated that multiple flavonoid combinations act additively and exhibit strong BCRP inhibition for increasing mitoxantrone accumulation in breast cancer cells. Kaempferide, biochanin A, 5,7-dimethoxyflavone, and 8-methylflavone greatly increased the cytotoxicity of mitoxantrone in BCRP-overexpressing breast cancer cells. Additionally, the basolateral-to-apical membrane-directed transport of mitoxantrone in murine Bcrp1- and human BCRP-expressing MDCK cells, in the presence of 2.5 microM of these flavonoids, was also significantly decreased.

CONCLUSION

The results indicate that flavonoids are potent BCRP inhibitors and that they exert additive effects when used in combination. Flavonoids demonstrate MDR-reversing effects, but also may influence the disposition of mitoxantrone and cause pharmacokinetic interactions.

Regulation of gene expression in brain tissues of rats repeatedly treated by the highly abused opioid agonist, oxycodone: microarray profiling and gene mapping analysis

Although oxycodone is the most often used opioid agonist, it remains one of the most understudied drugs. We used microarray analysis to better understand the global changes in gene expression in brain tissues of rats repeatedly treated with oxycodone. Many genes were significantly regulated by oxycodone (e.g., Fkbp5, Per2, Rt1.Dalpha, Slc16a1, and Abcg2). Validation of the microarray data by quantitative real-time-polymerase chain reaction (Q-PCR) indicated that there was a strong significant correlation (r = 0.979, p < 0.0000001) between the Q-PCR and the microarray data. Using MetaCore (a computational platform), many biological processes were identified [e.g., organic anion transport (p = 7.251 x 10(-4)) and regulation of immune response (p = 5.090 x 10(-4))]. Among the regulated genes, Abcg2 mRNA was up-regulated by 2.1-fold, which was further confirmed by immunoblotting (1.8-fold up-regulation). Testing the Abcg2 affinity status of oxycodone using an Abcg2 ATPase assay suggests that oxycodone behaves as an Abcg2 substrate only at higher concentrations (> or = 500 microM). Furthermore, brain uptake studies demonstrated that oxycodone-induced Abcg2 up-regulation resulted in a significant (p < 0.05) decrease (approximately 2-fold) in brain/plasma ratios of mitoxantrone. These results highlight markers/mediators of neuronal responses and identify regulatory pathways involved in the pharmacological action of oxycodone. These results also identify genes that potentially modulate tolerance, dependence, immune response, and drug-drug interactions. Finally, our findings suggest that oxycodone-induced up-regulation of Abcg2 enhanced the efflux of the Abcg2 substrate, mitoxantrone, limiting its brain accumulation and resulting in an undesirable drug-drug interaction. Extrapolating these results to other Abcg2 substrates (e.g., daunorubicin and doxorubicin) indicates that the brain uptake of these agents may be affected if they are administered concomitantly with oxycodone.

Impact of ATP-binding cassette transporters on human immunodeficiency virus therapy

Even though potent antiretrovirals are available against human immunodeficiency virus (HIV)-1 infection, therapy fails in a significant fraction of patients. Among the most relevant reasons for treatment failure are drug toxicity and side effects, but also the development of viral resistance towards the drugs applied. Efflux by ATP-binding cassette (ABC-) transporters represents one major mechanism influencing the pharmacokinetics of antiretroviral drugs and particularly their distribution, thus modifiying the concentration within the infected cells, that is, at the site of action. Moreover, drug-drug interactions may occur at the level of these transporters and modulate their activity or expression thus influencing the efficacy and toxicity of the substrate drugs. This review summarizes current knowledge on the interaction of antiretrovirals used for HIV-1 therapy with ABC-transporters and highlights the impact of ABC-transporters for cellular resistance and therapeutic success. Moreover, the suitability of different cell models for studying the interaction of antiretrovirals with ABC-transporters is discussed.

The human breast cancer resistance protein (BCRP/ABCG2) shows conformational changes with mitoxantrone

BCRP/ABCG2 mediates efflux of drugs and xenobiotics. BCRP was expressed in Pichia pastoris, purified to > 90% homogeneity, and subjected to two-dimensional (2D) crystallization. The 2D crystals showed a p12(1) symmetry and projection maps were determined at 5 A resolution by cryo-electron microscopy. Two crystal forms with and without mitoxantrone were observed with unit cell dimensions of a = 55.4 A, b = 81.4 A, gamma = 89.8 degrees , and a = 57.3 A, b = 88.0 A, gamma = 89.7 degrees , respectively. The projection map without mitoxantrone revealed an asymmetric structure with ring-shaped density features probably corresponding to a bundle of transmembrane alpha helices, and appeared more open and less symmetric than the map with mitroxantrone. The open and closed inward-facing forms of BCRP were generated by homology modeling, representing the substrate-free and substrate-bound conformations in the absence of nucleotide, respectively. These models are consistent with the experimentally observed conformational change upon substrate binding.

Berry anthocyanins and anthocyanidins exhibit distinct affinities for the efflux transporters BCRP and MDR1

BACKGROUND AND PURPOSE

Dietary anthocyanins hold great promise in the prevention of chronic disease but factors affecting their bioavailability remain poorly defined. Specifically, the role played by transport mechanisms at the intestinal and blood-brain barriers (BBB) is currently unknown.

EXPERIMENTAL APPROACH

In the present study, 16 anthocyanins and anthocyanidins were exposed to the human efflux transporters multidrug resistance protein 1 (MDR1) and breast cancer resistance protein (BCRP), using dye efflux, ATPase and, for BCRP, vesicular transport assays.

KEY RESULTS

All test compounds interacted with the BCRP transporter in vitro. Of these, seven emerged as potential BCRP substrates (malvidin, petunidin, malvidin-3-galactoside, malvidin-3,5-diglucoside, cyanidin-3-galactoside, peonidin-3-glucoside, cyanidin-3-glucoside) and 12 as potential inhibitors of BCRP (cyanidin, peonidin, cyanidin-3,5-diglucoside, malvidin, pelargonidin, delphinidin, petunidin, delphinidin-3-glucoside, cyanidin-3-rutinoside, malvidin-3-glucoside, pelargonidin-3,5-diglucoside, malvidin-3-galactoside). Malvidin, malvidin-3-galactoside and petunidin exhibited bimodal activities serving as BCRP substrates at low concentrations and, at higher concentrations, as BCRP inhibitors. Effects on MDR1, in contrast, were weak. Only aglycones exerted mild inhibitory activity.

CONCLUSIONS AND IMPLICATIONS

Although the anthocyanidins under study may alter pharmacokinetics of drugs that are BCRP substrates, they are less likely to interfere with activities of MDR1 substrates. The present data suggest that several anthocyanins and anthocyanidins may be actively transported out of intestinal tissues and endothelia, limiting their bioavailability in plasma and brain.

Role of basic residues within or near the predicted transmembrane helix 2 of the human breast cancer resistance protein in drug transport

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and xenobiotics out of cells. In this study, we investigated the role of five basic residues within or near transmembrane (TM) 2 of BCRP in transport activity. Lys(452), Lys(453), His(457), Arg(465), and Lys(473) were replaced with Ala or Asp. K452A, K453D, H457A, R465A, and K473A were stably expressed in human embryonic kidney (HEK) cells, and their plasma membrane expression and transport activities were examined. All of the mutants were expressed predominantly on the plasma membrane of HEK cells. After normalization to BCRP levels, the activities of K452A and H457A in effluxing mitoxantrone, boron-dipyrromethene-prazosin, and Hoechst33342 were increased approximately 2- to 6-fold compared with those of wild-type BCRP, whereas the activities of K453D and R465A were decreased by 40 to 60%. Likewise, K452A and H457A conferred increased resistance to mitoxantrone and 7-ethyl-10-hydroxy-camptothecin (SN-38), and K453D and R465A exhibited lower resistance. The transport activities and drug-resistance profiles of K473A were not changed. These mutations also differentially affected BCRP ATPase activities with a 2- to 4-fold increase in V(max)/K(m) for K452A and H457A and a 40 to 70% decrease for K453D and R465A. These mutations may induce conformational changes as manifested by the altered binding of the 5D3 antibody to BCRP in the presence of prazosin and altered trypsin digestion. Molecular modeling and docking calculations indicated that His(457) and Arg(465) might be directly involved in substrate binding. In conclusion, we have identified several basic residues within or near TM2 that may be important for interaction of substrates with BCRP.

N-(3,4-dimethoxyphenethyl)-4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2[1H]-yl)-6,7-dimethoxyquinazolin-2-amine (CP-100,356) as a "chemical knock-out equivalent" to assess the impact of efflux transporters on oral drug absorption in the rat

The utility of the diaminoquinazoline derivative CP-100,356 as an in vivo probe to selectively assess MDR1/BCRP-mediated drug efflux was examined in the rat. CP-100,356 was devoid of inhibition (IC(50) >50 microM) against major human P450 enzymes including P4503A4. In human MDR1-transfected MDCKII cells, CP-100,356 inhibited acetoxymethyl calcein (calcein-AM) uptake (IC(50) approximately 0.5 +/- 0.07 microM) and digoxin transport (IC(50) approximately 1.2 +/- 0.1 microM). Inhibition of prazosin transport (IC(50) approximately 1.5 +/- 0.3 microM) in human BCRP-transfected MDCKII cells by CP-100,356 confirmed the dual MDR1/BCRP inhibitory properties. CP-100,356 was a weak inhibitor of OATP1B1 (IC(50) approximately 66 +/- 1.1 microM) and was devoid of MRP2 inhibition (IC(50) >15 microM). In vivo inhibitory effects of CP-100,356 in rats were examined after coadministration with MDR1 substrate fexofenadine and dual MDR1/BCRP substrate prazosin. Coadministration with increasing doses of CP-100,356 resulted in dramatic increases in systemic exposure of fexofenadine (36- and 80-fold increase in C(max) and AUC at a CP-100,356 dose of 24 mg/kg). Significant differences in prazosin pharmacokinetics were also discernible in CP-100,356-pretreated rats as reflected from a 2.6-fold increase in AUC. Coadministration of CP-100,356 and P4503A substrate midazolam did not result in elevations in systemic exposure of midazolam in the rat. The in vivo methodology should have utility in drug discovery in selective and facile assessment of the role of MDR1 and BCRP efflux transporters in oral absorption of new drug candidates.

ABC transporters and isothiocyanates: potential for pharmacokinetic diet-drug interactions

Isothiocyanates, a class of anti-cancer agents, are derived from cruciferous vegetables such as broccoli, cabbage and watercress, and have demonstrated chemopreventive activity in a number of cancer models and epidemiologic studies. Due to public interest in cancer prevention and alternative therapies in cancer, the consumption of herbal supplements and vegetables containing these compounds is widespread and increasing. Isothiocyanates interact with ATP-binding cassette (ABC) efflux transporters such as P-glycoprotein, MRP1, MRP2 and BCRP, and may influence the pharmacokinetics of substrates of these transporters. This review discusses the pharmacokinetic properties of isothiocyanates, their interactions with ABC transporters, and presents some data describing the potential for isothiocyanate-mediated diet-drug interactions.

Effect of pregnancy on nitrofurantoin disposition in mice

We investigated the effect of pregnancy on nitrofurantoin (NFT) disposition in wild-type and Bcrp1(-/-) mice. Pregnant and non-pregnant mice were administered NFT intravenously (5 mg/kg) or orally (10 mg/kg). Blood samples were collected at various times (5-60 min) after drug administration, plasma NFT concentrations determined by HPLC/UV, and pharmacokinetic parameters estimated. Dose-normalized area under the plasma concentration-time curve (AUC), terminal plasma half-life (T(1/2)), total plasma clearance (CL), and steady-state volume of distribution (V(ss)) of intravenous NFT in wild-type or Bcrp1(-/-) mice were not altered by pregnancy. After oral administration, pregnancy did not affect dose-normalized AUC of NFT in wild-type mice; however, dose-normalized AUC in Bcrp1(-/-) mice was decreased by approximately 70% by pregnancy. In conclusion, since Bcrp1 plays a minor role in the systemic clearance of NFT in female mice, pregnancy did not affect disposition of intravenous NFT despite the fact that Bcrp1 expression in the liver and kidney of mice is significantly induced by pregnancy. On the other hand, pregnancy may affect expression and activity of certain intestinal efflux transporters and/or metabolic enzymes in Bcrp1(-/-) mice, resulting in a drastic decrease in the systemic exposure of oral NFT in pregnant Bcrp1(-/-) mice.

Fluorescence resonance energy transfer (FRET) analysis demonstrates dimer/oligomer formation of the human breast cancer resistance protein (BCRP/ABCG2) in intact cells

The human breast cancer resistance protein (BCRP/ABCG2) is a half ATP-binding cassette (ABC) efflux transporter that plays an important role in drug resistance and disposition. Although BCRP is believed to function as a homodimer or homooligomer, this has not been demonstrated in vivo in intact cells. Therefore, in the present study, we investigated dimer/oligmer formation of BCRP in intact cells. Wild-type BCRP and the mutant C603A were attached to cyan or yellow fluorescence protein and expressed in HEK293 cells by transient transfection. Protein levels, cell surface expression, and efflux activities of wild-type and mutant BCRP were determined by immunoblotting, 5D3 antibody binding, and flow cytometric efflux assay, respectively. Dimer/oligomer formation of BCRP in intact cells was analyzed using fluorescence resonance energy transfer (FRET) microscopy. Wild-type BCRP and C603A were expressed in HEK293 cells at comparable levels. C603A was predominantly expressed in the plasma membrane as was wild-type protein. Furthermore, C603A retained the same mitoxantrone efflux activity and the ability of dimer/oligmer formation as wild-type BCRP. Finally, cross-linking experiments yielded data consistent with the FRET analysis. In conclusion, we have, for the first time, demonstrated that BCRP can form a dimer/oligomer in vivo in intact cells using the FRET technique. We have also shown that Cys(603) alone does not seem to be essential for dimer/oligomer formation of BCRP.

What lies behind serum urate concentration? Insights from genetic and genomic studies

Many factors, including genetic components and acquired factors such as obesity and alcohol consumption, influence serum uric acid (urate) concentrations. Since serum urate concentrations are determined by the balance between renal urate excretion and the volume of urate produced via purine metabolism, urate transporter genes as well as genes coding for enzymes involved in purine metabolism affect serum urate concentrations. URAT1 was the first transporter affecting serum urate concentrations to be identified. Using the characterization of this transporter as an indicator, several transporters have been shown to transport urate, allowing the construction of a synoptic renal urate transport model. Notable re-absorptive urate transporters are URAT1 at apical membranes and GLUT9 at basolateral membranes, while ABCG2, MRP4 (multidrug resistance protein 4) and NPT1 are secretive transporters at apical membranes. Recent genome-wide association studies have led to validation of the in vitro model constructed from each functional analysis of urate transporters, and identification of novel candidate genes related to urate metabolism and transport proteins, such as glucokinase regulatory protein (GKRP), PDZK1 and MCT9. However, the function and physiologic roles of several candidates, as well as the influence of acquired factors such as obesity, foods, or alcoholic beverages, remain unclear.

Cyclo-oxygenase 2 up-regulates the effect of multidrug resistance

COX-2 (cyclo-oxygenase 2), an inducible form of the enzyme that catalyses the first step in the synthesis of prostanoids, is associated with inflammatory diseases and carcinogenesis, which is suspected to promote angiogenesis and tissue invasion of tumours and resistance to apoptosis. COX-2 is also involved in drug resistance and poor prognosis of many neoplastic diseases or cancers. The activation of the COX-2/PGE2 (prostaglandin E2)/prostaglandin E receptor signal pathway can up-regulate the expression of all three ABC (ATP-binding-cassette) transporters, MDR1/P-gp (multidrug resistance/P-glycoprotein), MRP1 (multidrug-resistance protein 1) and BCRP (breast-cancer-resistance protein), which encode efflux pumps, playing important roles in the development of multidrug resistance. In addition, COX inhibitors inhibit the expression of MDR1/P-gp, MRP1 and BCRP and enhance the cytotoxicity of anticancer drugs. Therefore we can use the COX inhibitors to potentialize the effects of chemotherapeutic agents and reverse multidrug resistance to facilitate the patient who may benefit from addition of COX inhibitors to standard cytotoxic therapy.

Dofequidar fumarate sensitizes cancer stem-like side population cells to chemotherapeutic drugs by inhibiting ABCG2/BCRP-mediated drug export

The ATP-binding cassette (ABC) transporters (ABC-T) actively efflux structurally and mechanistically unrelated anticancer drugs from cells. As a consequence, they can confer multidrug resistance (MDR) to cancer cells. ABC-T are also reported to be phenotypic markers and functional regulators of cancer stem/initiating cells (CSC) and believed to be associated with tumor initiation, progression, and relapse. Dofequidar fumarate, an orally active quinoline compound, has been reported to overcome MDR by inhibiting ABCB1/P-gp, ABCC1/MDR-associated protein 1, or both. Phase III clinical trials suggested that dofequidar had efficacy in patients who had not received prior therapy. Here we show that dofequidar inhibits the efflux of chemotherapeutic drugs and increases the sensitivity to anticancer drugs in CSC-like side population (SP) cells isolated from various cancer cell lines. Dofequidar treatment greatly reduced the cell number in the SP fraction. Estimation of ABC-T expression revealed that ABCG2/breast cancer resistance protein (BCRP) mRNA level, but not the ABCB1/P-gp or ABCC1/MDR-associated protein 1 mRNA level, in all the tested SP cells was higher than that in non-SP cells. The in vitro vesicle transporter assay clarified that dofequidar had the ability to suppress ABCG2/BCRP function. Dofequidar treatment sensitized SP cells to anticancer agents in vitro. We compared the antitumor efficacy of irinotecan (CPT-11) alone with that of CPT-11 plus dofequidar in xenografted SP cells. Although xenografted SP tumors showed resistance to CPT-11, treatment with CPT-11 plus dofequidar greatly reduced the SP-derived tumor growth in vivo. Our results suggest the possibility of selective eradication of CSC by inhibiting ABCG2/BCRP.

Identification and functional characterization of breast cancer resistance protein in human bronchial epithelial cells (Calu-3)

Breast cancer resistance protein (BCRP), a 72 kDa protein belongs to the subfamily G of the human ATP-binding cassette transporter superfamily. Overexpression of BCRP was found to play a major role in the development of resistance against various chemotherapeutic agents. BCRP plays an important role in absorption, distribution and elimination of several therapeutic agents. BCRP expression and functional activity across human bronchial epithelium and its impact on pulmonary drug accumulation has not been established. The objective of this study was to identify and characterize the BCRP efflux transporter across human bronchial epithelium. Calu-3, a human bronchial epithelial cell line was employed as a model for this study. Reverse transcription-polymerase chain reaction (RT-PCR), Western blot and immunocytochemical studies were performed to identify and characterize the expression of BCRP. RT-PCR studies detected ABCG2 mRNA levels in Calu-3 cells. A strong band for BCRP with a molecular weight of approximately 72 kDa was observed in Western blot analysis. Immunocytochemical studies confirmed the presence of BCRP on the apical membrane of human bronchial epithelium. Functional activity of BCRP was determined by performing uptake of radioactive substrate [3H]-mitoxantrone in the presence and absence of BCRP inhibitors. Uptake of [3H]-mitoxantrone was elevated significantly in the presence of GF120918 and fumitremorgin C. An increase in the accumulation of Hoechst 33342, a fluorescent dye was also detected in the presence of BCRP inhibitors when compared to control. In summary, this study provides evidence for the presence of an ATP dependent, membrane bound efflux transporter BCRP across human bronchial epithelial cell line, Calu-3.

Mitochondrial expression and functional activity of breast cancer resistance protein in different multiple drug-resistant cell lines

The multidrug resistance (MDR) phenotype is characterized by the overexpression of a few transport proteins at the plasma membrane level, one of which is the breast cancer resistance protein (BCRP). These proteins are expressed in excretory organs, in the placenta and blood-brain barrier, and are involved in the transport of drugs and endogenous compounds. Because some of these proteins are expressed in the mitochondria, this study was designed to determine whether BCRP is expressed at a mitochondrial level and to investigate its function in various MDR and parental drug-sensitive cell lines. By using Western blot analysis, immunofluorescence confocal and electron microscopy, flow cytometry analysis, and the BCRP (ABCG-2) small interfering RNA, these experiments showed that BCRP is expressed in the mitochondrial cristae, in which it is functionally active. Mitoxantrone accumulation was significantly reduced in mitochondria and in cells that overexpress BCRP, in comparison to parental drug-sensitive cells. The specific inhibitor of BCRP, fumitremorgin c, increased the accumulation of mitoxantrone significantly in comparison with basal conditions in both whole cells and in mitochondria of BCRP-overexpressing cell lines. In conclusion, this study shows that BCRP is overexpressed and functionally active in the mitochondria of MDR-positive cancer cell lines. However, its presence in the mitochondria of parental drug-sensitive cells suggests that BCRP can be involved in the physiology of cancer cells.

Pim-1 expression and monoclonal antibody targeting in human leukemia cell lines

OBJECTIVE

Based on our previous findings that Pim-1 was expressed on the cell surface and could be targeted with a highly specific anti-Pim-1 monoclonal antibody (P9), this study aims to evaluate the possibility that Pim-1 could be targeted for the treatment of human leukemia.

MATERIALS AND METHODS

Pim-1 expression was investigated in a series of human leukemia cell lines with immunohistochemistry and flow cytometry. The inhibitory effect of P9 on cell proliferation was evaluated with (3)H-thymidine incorporation assay. Cell apoptosis was assayed with Annexin-V/propidium iodide dual staining. The in vivo effect of P9 was evaluated with xenograft tumor models in severe combined immunodeficient mice.

RESULTS

Pim-1 expression varied depending on the cell lines and correlated with the inhibitory effects mediated by P9. An association between Pim-1 expression and drug resistance was observed. Although the drug-resistant CEM/A7R cells were highly resistant to cytotoxic P-glycoprotein substrates, their growth was inhibited by P9 as demonstrated by in vitro proliferation assay and in vivo inhibition of xenograft tumors. P9 had little effect on P-glycoprotein expression and intracellular Rhodamine 123 accumulation, but it inhibited the phosphorylation of Bad and induced apoptosis.

CONCLUSIONS

Pim-1 is variably expressed in leukemia cell lines and associated with drug resistance. Targeting Pim-1 with monoclonal antibody could be explored for the treatment of leukemia and may represent a novel strategy to overcome drug resistance.

Role of placental ATP-binding cassette (ABC) transporters in antiretroviral therapy during pregnancy

Highly active anti-retroviral therapy (HAART) is used to treat HIV-infected patients and involves administration of multiple antiretroviral drugs acting at different steps of the HIV life cycle. In treating HIV-infected pregnant patients, the aim of therapy is not only to treat the mother but also to prevent the transmission of the virus to the fetus. Among the antiretroviral drugs used, there are differences in the extent of transfer of these drugs across the placenta; HIV protease inhibitors are particularly poorly transferred. Activities of ABC transporters expressed in the human placenta as well as differences in plasma protein binding may account for the poor transplacental transfer of certain drugs. This review discusses factors affecting the extent of placental transfer of antiretroviral drugs during pregnancy. These issues may also apply to drugs in other therapeutic categories.

In vivo evaluation of P-glycoprotein and breast cancer resistance protein modulation in the brain using [(11)C]gefitinib

Gefitinib (Iressa) is a selective inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase. Recent studies confirmed that gefitinib interacted with the breast cancer resistance protein (BCRP) at submicromolar concentrations, whereas other multidrug transporters, including P-glycoprotein (P-gp), showed much lower reactivity toward gefitinib. Recently, many tracers for positron emission tomography (PET) have been prepared to study P-gp function in vivo; however, PET tracers had not been evaluated for both P-gp and BCRP modulation in the brain. Therefore, we evaluated in vivo brain penetration-mediated P-gp and BCRP in mice using [(11)C]gefitinib. Co-injection with gefitinib (over 50 mg/kg), a nonspecific P-gp modulator cyclosporin A (50 mg/kg), and the dual P-gp and BCRP modulator GF120918 (over 5 mg/kg) induced an increase in the brain uptake of [(11)C]gefitinib in mice 30 min after injection. In the PET study of mice, the radioactivity level in the brain with co-injection of GF120918 (5 mg/kg) was three- to fourfold higher than that in control after initial uptake. The radioactivity level in the brain in P-gp and Bcrp knockout mice was approximately eightfold higher than that in wild-type mice 60 min after injection. In conclusion, [(11)C]gefitinib is a promising PET tracer to evaluate the penetration of gefitinib into the brain by combined therapy with P-gp or BCRP modulators, and into brain tumors. Furthermore, PET study with GF120918 is a promising approach for evaluating brain penetration-mediated P-gp and BCRP.

Up-regulation of hepatic ABCC2, ABCG2, CYP1A1 and GST in multixenobiotic-resistant killifish (Fundulus heteroclitus) from the Sydney Tar Ponds, Nova Scotia, Canada

Cellular defence against accumulation of toxic xenobiotics includes metabolism by phase I and II enzymes and export of toxicants and their metabolites via ATP-binding cassette (ABC) transporters. Liver gene expression of representatives of these three protein groups was examined in a population of multixenobiotic-resistant killifish (Fundulus heteroclitus) from the Sydney Tar Ponds, Nova Scotia, Canada. The Tar Ponds are heavily polluted with polycyclic aromatic hydrocarbons, polychlorinated biphenyls and heavy metals. The relationship among ABC transporters ABCB1, ABCB11, ABCC2, ABCG2, phase I enzyme cytochrome P4501A1 (CYP1A1) and phase II enzyme glutathione-S-transferase (GST-mu) was investigated by quantifying hepatic transcript abundance. In Tar Pond killifish, hepatic mRNA expression levels of ABCC2, ABCG2, CYP1A1 and GST-mu were elevated compared to reference sites, suggesting that hydrophobic contaminants undergo phase I and II metabolism and are then excreted into the bile of these fish. Hepatic ABCB1 and ABCB11 mRNA were not up-regulated in Tar Pond fish compared to two reference sites, indicating that these two proteins are not involved in conferring multixenobiotic resistance to Tar Pond killifish. The results suggest instead that liver up-regulation of phase I and II enzymes and complementary ABC transporters ABCC2 and ABCG2 may confer contaminant resistance to Tar Pond fish.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

MicroRNA-328 negatively regulates the expression of breast cancer resistance protein (BCRP/ABCG2) in human cancer cells

Breast cancer resistance protein (BCRP/ABCG2) is a molecular determinant of pharmacokinetic properties of many drugs in humans. To understand post-transcriptional regulation of ABCG2 and the role of microRNAs (miRNAs) in drug disposition, we found that microRNA-328 (miR-328) might readily target the 3'-untranslated region (3'-UTR) of ABCG2 when considering target-site accessibility. We then noted 1) an inverse relation between the levels of miR-328 and ABCG2 in MCF-7 and MCF-7/MX100 breast cancer cells and 2) that miR-328 levels could be rescued in MCF-7/MX100 cells by transfection with miR-328 plasmid. Luciferase reporter assays showed that ABCG2 3'-UTR-luciferase activity was decreased more than 50% in MCF-7/MX100 cells after transfection with miR-328 plasmid, the activity was increased over 100% in MCF-7 cells transfected with a miR-328 antagomir, and disruption of miR-328 response element within ABCG2 3'-UTR led to a 3-fold increase in luciferase activity. Furthermore, the level of ABCG2 protein was down-regulated when miR-328 was over-expressed, and the level was up-regulated when miR-328 was inhibited by selective antagomir. Altered ABCG2 protein expression was associated with significantly declined or elevated levels of ABCG2 3'-UTR and coding sequence mRNAs, suggesting possible involvement of the mechanism of mRNA cleavage. Finally, miR-328-directed down-regulation of ABCG2 expression in MCF-7/MX100 cells resulted in an increased mitoxantrone sensitivity, as manifested by a significantly lower IC(50) value (2.46 +/- 1.64 microM) compared with the control (151 +/- 32 microM). Together, these findings suggest that miR-328 targets ABCG2 3'-UTR and, consequently, controls ABCG2 protein expression and influences drug disposition in human breast cancer cells.

Molecular expression and functional evidence of a drug efflux pump (BCRP) in human corneal epithelial cells

PURPOSE

Breast Cancer Resistance Protein (BCRP) belongs to the family of efflux transporters involved in drug efflux leading to drug resistance. The objective of this study was to explore physical barriers for ocular drug absorption and to verify the presence and possible role of BCRP as a barrier for ocular drug resistance.

METHODS

Transfected human corneal epithelial cells (SV40-HCEC) were selected as an in vitro model for corneal epithelium with MDCKII-BCRP as positive control. [(3)H]-Mitoxantrone ([(3)H]-MTX), which is a proven substrate for organic anion transporter like BCRP, was selected as a model drug for functional expression studies. Fumetremorgin C (FTC), a known specific inhibitor for BCRP and GF120918, an inhibitor for BCRP and P-gp, were added to inhibit BCRP-mediated efflux. PGP-4008, a specific inhibitor of P-gp was used to delineate the contribution of P-gp. The mRNA extracted from cells was used for RT-PCR analysis and gene expression. Membrane fractions of SV40-HCEC and MDCKII-BCRP were used for immunoprecipitation followed by Western blot analysis.

RESULTS

Efflux was inhibited significantly in the presence of FTC and GF120918. Dose-dependent inhibition of efflux by BCRP was noticed in SV40-HCEC and MDCKII-BCRP in the presence of FTC and GF120918, and the efflux was ATP-dependent. The metabolic inhibitor, 2,4-DNP, significantly inhibited efflux. No pH-dependent efflux was noticed except at pH 5.5. RT-PCR analysis indicated a unique and distinct band at approximately 429 bp, corresponding to BCRP in SV40-HCEC and MDCKII-BCRP cells. Western Blot analysis indicated a specific band at approximately 70 kDa in the membrane fraction of SV40-HCEC and MDCKII-BCRP cells.

CONCLUSIONS

We have demonstrated the expression of BCRP in human corneal epithelial cells and, for the first time, demonstrated its functional activity leading to drug efflux. RT-PCR and Western blot analysis further confirmed this finding.

Pharmacokinetics and bioavailability of the flavonoid 7,8-benzoflavone in rats

The flavonoid 7,8-benzoflavone was recently identified as one of the most potent inhibitors of breast cancer resistance protein (BCRP); however, little is known of the in vivo disposition of 7,8-benzoflavone. The objective of this study was to investigate the pharmacokinetics and bioavailability of 7,8-benzoflavone in rats. Three intravenous (5, 10, and 25 mg/kg) and three oral (12.5, 25, and 50 mg/kg) doses were administered to female Sprague-Dawley rats. Plasma samples were analyzed by high-performance liquid chromatography. Pharmacokinetic analysis was conducted by WinNonlin and ADAPT II. The dose-normalized plasma concentration versus time curves did not superimpose with each other, indicating the nonlinear pharmacokinetics of 7,8-benzoflavone. 7,8-benzoflavone exhibited a large volume of distribution (V(ss) approximately 1.5 L/kg) and rapid oral absorption (t(max) < 30 min). The bioavailability of 7,8-benzoflavone was low (0.61-13.2%) and dose-dependent. A pharmacokinetic model with dose-dependent bioavailability, linear absorption and nonlinear elimination best described the pharmacokinetic profiles of 7,8-benzoflavone. Using a 50 mg/kg oral dose of 7,8-benzoflavone, we could significantly increase the AUC for the BCRP substrate nitrofurantoin, demonstrating the potential for BCRP-mediated drug interactions.

Membrane topology of the human breast cancer resistance protein (BCRP/ABCG2) determined by epitope insertion and immunofluorescence

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and organic anions across the plasma membrane. Hydropathy analysis suggests that BCRP consists of a nucleotide-binding domain (residues approximately 1-395) and a membrane-spanning domain (MSD) (residues approximately 396-655); however, its exact topology structure remains unknown. In this study, we determined the topology structure of BCRP by inserting hemagglutinin (HA) tags in its predicted hydrophilic regions of the MSD. HA-tagged BCRP mutants were expressed in HEK cells and tested for their ability to efflux mitoxantrone and BODIPY-prazosin. Polarity of the inserted tags with respect to the plasma membrane was determined by immunofluorescence. All of the mutants were expressed at levels comparable to wild-type BCRP as revealed by immunoblotting with specific antibodies against BCRP and the HA tag. Insertions at residues 423, 454, 462, 499, 529, 532, and 651 produced functional mutants, whereas insertions at residues 560, 594, and 623 resulted in mutants with significantly reduced activity and insertions at residues 387, 420, 474, and 502 completely abrogated the activity. HA tags inserted at residues 387, 474, 529, 532, 560, and 651 were localized intracellularly, whereas those inserted at residues 420, 423, 454, 499, 502, 594, and 623 revealed an extracellular location. Residue 462 was localized in a transmembrane (TM) segment. These results provide the first direct experimental evidence in support of a 6-TM model for BCRP with the amino and carboxyl termini of the MSD located intracellularly. These data may have important implications for understanding the transport mechanism of BCRP.

Interactions between riluzole and ABCG2/BCRP transporter

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative fatal disease. Drugs used in this disease need to cross the blood-brain barrier (BBB). Only riluzole is approved for ALS treatment. We have investigated riluzole as a breast cancer resistance protein (BCRP) substrate by studying its brain transport in CF1 mdr1a (-/-) mice and its intracellular uptake on BeWo cells (human placental choriocarcinoma cell line). We have also investigated the effect of riluzole on BCRP expression level and on its activity using the prazocin as a test probe for brain transport and intracellular uptake. Assays on mdr1a (-/-) mice and BeWo cells showed a higher uptake of riluzole when pretreated with a BCRP inhibitor. After repeated doses of riluzole, BCRP activity was increased in CF1 mdr1a (-/-) mice, riluzole uptake was decrease and both BCRP expression and activity were increased in BeWo cells. In conclusion, we report in this study that riluzole is transported by BCRP at the BBB level and can enhance its function. These results taken with our previous studies on riluzole and P-glycoprotein show that drug-drug interactions between riluzole and efflux transporters substrates may occur at the BBB level and should be taken into account in future clinical trial design in ALS.

Expression of breast cancer resistance protein is associated with a poor clinical outcome in patients with small-cell lung cancer

BACKGROUND

ATP-binding cassette (ABC) transporter and DNA excision repair proteins play a pivotal role in the mechanisms of drug resistance. The aim of this study was to investigate the expression of ABC transporter and DNA excision repair proteins, and to elucidate the clinical significance of their expression in biopsy specimens from patients with small-cell lung cancer (SCLC).

METHODS

We investigated expression of the ABC transporter proteins, P-glycoprotein (Pgp), multidrug resistance associated-protein 1 (MRP1), MRP2, MRP3, and breast cancer resistance protein (BCRP), and the DNA excision repair proteins, excision repair cross-complementation group 1 (ERCC1) protein and breast cancer susceptibility gene 1 (BRCA1) protein, in tumor biopsy specimens obtained before chemotherapy from 130 SCLC patients who later received platinum-based combination chemotherapy, and investigated the relationship between their expression and both response and survival.

RESULTS

No significant associations were found between expression of Pgp, MRP1, MRP2, MRP3, ERCC1, or BRCA1 and either response or survival. However, there was a significant association between BCRP expression and both response (p=0.026) and progression-free survival (PFS; p=0.0103).

CONCLUSIONS

BCRP expression was significantly predictive of both response and progression-free survival (PFS) in SCLC patients receiving chemotherapy. These findings suggest that BCRP may play a crucial role in drug resistance mechanisms, and that it may serve as an ideal molecular target for the treatment of SCLC.

Effects of atazanavir/ritonavir or fosamprenavir/ritonavir on the pharmacokinetics of rosuvastatin

BACKGROUND

Rosuvastatin (RSV) is a potent statin with a lower potential for drug interactions. However, recent data have revealed unexpected increases in RSV concentrations with lopinavir/ritonavir. The objective is to study the pharmacokinetic interaction of RSV with atazanavir/ritonavir (ATV/RTV) or fosamprenavir/ritonavir (FPV/RTV).

METHODS

In a prospective pharmacokinetic drug interaction study, six HIV-seronegative, healthy adult volunteers received single 10-mg doses of RSV at baseline and after 6 days of ATV/RTV and FPV/RTV, with 6-day washout periods. Plasma concentrations of RSV and its metabolites, N-desmethyl-RSV and RSV-lactone, were measured by using a internally validated tandem mass spectrometric (LC-MS/MS) method over 24 hours.

RESULTS

Compared to baseline, the area under the plasma concentration-time curve (AUC 0-24h) and maximum plasma concentration (Cmax) of RSV increased by 213% and 600%, respectively, and the time to reach Cmax was shorter (1.75 h vs. 2.91 h) when given with ATV/RTV (P < 0.05). However, coadministration with FPV/RTV did not significantly affect the pharmacokinetics of RSV. The AUC 0-24h of N-desmethyl-RSV was not significantly affected by either combinations, but that of RSV-lactone increased (P < 0.05) by 61% and 76% after coadministration with ATV/RTV and FPV/RTV, respectively.

CONCLUSION

ATV/RTV significantly increases the plasma concentrations of rosuvastatin, most likely by increasing rosuvastatin's oral bioavailability. Dose limitations of RSV with ATV/RTV may be needed.

N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918) as a chemical ATP-binding cassette transporter family G member 2 (Abcg2) knockout model to study nitrofurantoin transfer into milk

Genetic knockout mice studies suggested ATP-binding cassette transporter family G member 2 (ABCG2)/Abcg2 translocates nitrofurantoin at the mammary-blood barrier, resulting in drug accumulation in milk. The purpose of this study was to establish the role of Abcg2 in nitrofurantoin accumulation in rat milk using N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918) as a "chemical knockout" equivalent. The inhibitory effect of GF120918 was verified in Madin-Darby canine kidney II cells stably expressing rat Abcg2 with Hoechst 33342 and nitrofurantoin flux in Transwells. Nitrofurantoin was infused (0.5 mg/h) in the absence and presence of GF120918 (10 mg/kg in dimethyl sulfoxide) to Sprague-Dawley lactating female rats using a balanced crossover design. Administration of GF120918 increased nitrofurantoin concentration in serum (from 443 +/- 51 to 650 +/- 120 ng/ml) and decreased concentration in milk (from 18.1 +/- 0.9 to 1.9 +/- 1.2 microg/ml), resulting in corresponding mean values for milk to serum concentration ratio (M/S) of 41.4 +/- 19.1 versus 3.04 +/- 2.27 in the absence and presence of GF120918 (p < 0.05), respectively. There was a decrease in systemic clearance with GF120918 (2.8 +/- 0.5 l/h/kg) compared with vehicle controls (4.1 +/- 0.5 l/h/kg; p < 0.05). Western blot analysis revealed good expression of Abcg2 and no P-glycoprotein (P-gp) expression in mammary gland, whereas immunohistochemistry confirmed the apical expression of Abcg2 in lactating mammary gland epithelia. Nitrofurantoin active transport into rat milk can be inhibited by GF120918 resulting in a 10-fold lower M/S. Although GF120918 inhibits both Abcg2 and P-gp, the high expression of Abcg2 and the absence of detectable P-gp expression in lactating mammary gland validate an important role for Abcg2 in nitrofurantoin accumulation in rat milk. GF120918 is particularly useful as a rat chemical knockout model to establish ABCG2's role in drug transfer into milk during breastfeeding.

BCRP/ABCG2 in the placenta: expression, function and regulation

Knowledge concerning transport of maternally administered drugs across the placental barrier is essential for determining potential toxicity of drugs to the fetus and the value of drug therapy during pregnancy. An important determinant for fetal drug exposure is the expression of efflux transporters in the placenta. Among human tissues, the ATP-binding cassette efflux transporter BCRP (gene symbol ABCG2) is most abundantly expressed in the apical membrane of placental syncytiotrophoblasts. Although the precise physiological role of BCRP in the placenta is still unclear, existing data strongly suggest that BCRP plays an important role in protecting the fetus against the potential toxicity of drugs, xenobiotics, and metabolites by expelling them across the placental barrier. In this review, we summarize the current knowledge with respect to the expression, function, and polymorphisms of BCRP, as well as transcriptional and posttranscriptional regulation of the transporter in the placenta. Finally, clinical significance of BCRP in the placenta for drug therapy in pregnant women is discussed.