Development of sulfasalazine resistance in human T cells induces expression of the multidrug resistance transporter ABCG2 (BCRP) and augmented production of TNFalpha

ATP-Binding Cassette Subfamily G Member 2 in Acute Myeloid Leukemia: A New Molecular Target?

Despite the progress in the knowledge of disease pathogenesis and the identification of many molecular markers as potential targets of new therapies, the cure of acute myeloid leukemia remains challenging. Disease recurrence after an initial response and the development of resistance to old and new therapies account for the poor survival rate and still make allogeneic stem cell transplantation the only curative option. Multidrug resistance (MDR) is a multifactorial phenomenon resulting from host-related characteristics and leukemia factors. Among these, the overexpression of membrane drug transporter proteins belonging to the ABC (ATP-Binding Cassette)-protein superfamily, which diverts drugs from their cellular targets, plays an important role. Moreover, a better understanding of leukemia biology has highlighted that, at least in cancer, ABC protein's role goes beyond simple drug transport and affects many other cell functions. In this paper, we summarized the current knowledge of ABCG2 (formerly Breast Cancer Resistance Protein, BCRP) in acute myeloid leukemia and discuss the potential ways to overcome its efflux function and to revert its ability to confer stemness to leukemia cells, favoring the persistence of leukemia progenitors in the bone marrow niche and justifying relapse also after therapy intensification with allogeneic stem cell transplantation.

Toward Overcoming Treatment Failure in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by inflammation and bone erosion. The exact mechanism of RA is still unknown, but various immune cytokines, signaling pathways and effector cells are involved. Disease-modifying antirheumatic drugs (DMARDs) are commonly used in RA treatment and classified into different categories. Nevertheless, RA treatment is based on a "trial-and-error" approach, and a substantial proportion of patients show failed therapy for each DMARD. Over the past decades, great efforts have been made to overcome treatment failure, including identification of biomarkers, exploration of the reasons for loss of efficacy, development of sequential or combinational DMARDs strategies and approval of new DMARDs. Here, we summarize these efforts, which would provide valuable insights for accurate RA clinical medication. While gratifying, researchers realize that these efforts are still far from enough to recommend specific DMARDs for individual patients. Precision medicine is an emerging medical model that proposes a highly individualized and tailored approach for disease management. In this review, we also discuss the potential of precision medicine for overcoming RA treatment failure, with the introduction of various cutting-edge technologies and big data.

The effects of drug transporters on the efficacy of methotrexate in the treatment of rheumatoid arthritis

The ATP-binding cassette (ABC) and solute carrier (SLC) transporter families consist of common drug transporters that mediate the efflux and uptake of drugs, respectively, and play an important role in the absorption, distribution, metabolism and excretion of drugs in vivo. Rheumatoid arthritis (RA) is an autoimmune disease characterized by erosive arthritis, and there are many RA patients worldwide. Methotrexate (MTX), the first-choice treatment for RA, can reduce the level of inflammation, prevent joint erosion and functional damage, and greatly reduce pain in RA patients. However, many patients show resistance to MTX, greatly affecting the efficacy of MTX. Many factors, such as irrational drug use and heredity, are associated with drug resistance. Considering the effect of drug transporters on drugs, many studies have compared the expression of drug transporters in drug-resistant and drug-sensitive patients, and abnormal transporter expression and transport activity have been found in patients with MTX resistance. Thus, drug transporters are involved in drug resistance. This article reviews the effects of transporters on the efficacy of MTX in the treatment of RA.

Beta1 integrin blockade overcomes doxorubicin resistance in human T-cell acute lymphoblastic leukemia

Growing evidence indicates that cell adhesion to extracellular matrix (ECM) plays an important role in cancer chemoresistance. Leukemic T cells express several adhesion receptors of the β1 integrin subfamily with which they interact with ECM. However, the role of β1 integrins in chemoresistance of T-cell acute lymphoblastic leukemia (T-ALL) is still ill defined. In this study, we demonstrate that interactions of human T-ALL cell lines and primary blasts with three-dimensional matrices including Matrigel and collagen type I gel promote their resistance to doxorubicin via β1 integrin. The blockade of β1 integrin with a specific neutralizing antibody sensitized xenografted CEM leukemic cells to doxorubicin, diminished the leukemic burden in the bone marrow and resulted in the extension of animal survival. Mechanistically, Matrigel/β1 integrin interaction enhanced T-ALL chemoresistance by promoting doxorubicin efflux through the activation of the ABCC1 drug transporter. Finally, our findings showed that Matrigel/β1 interaction enhanced doxorubicin efflux and chemoresistance by activating the FAK-related proline-rich tyrosine kinase 2 (PYK2) as both PYK2 inhibitor and siRNA diminished the effect of Matrigel. Collectively, these results support the role of β1 integrin in T-ALL chemoresistance and suggest that the β1 integrin pathway can constitute a therapeutic target to avoid chemoresistance and relapsed-disease in human T-ALL.

BCRP/ABCG2 and high-alert medications: Biochemical, pharmacokinetic, pharmacogenetic, and clinical implications

The human breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette efflux transporter that uses ATP hydrolysis to expel xenobiotics from cells, including anti-cancer medications. It is expressed in the gastrointestinal tract, liver, kidney, and brain endothelium. Thus, ABCG2 functions as a tissue barrier to drug transport that strongly influences the pharmacokinetics of substrate medications. Genetic polymorphisms of ABCG2 are closely related to inter-individual variations in therapeutic performance. The common single nucleotide polymorphism c.421C>A, p.Q141K reduces cell surface expression of ABCG2 protein, resulting in lower efflux of substrates. Consequently, a higher plasma concentration of substrate is observed in patients carrying an ABCG2 c.421C>A allele. Detailed pharmacokinetic analyses have revealed that altered intestinal absorption is responsible for the distinct pharmacokinetics of ABCG2 substrates in genetic carriers of the ABCG2 c.421C>A polymorphism. Recent studies have focused on the high-alert medications among ABCG2 substrates (defined as those with high risk of adverse events), such as tyrosine kinase inhibitors (TKIs) and direct oral anti-coagulants (DOACs). For these high-alert medications, inter-individual variation may be closely related to the severity of side effects. In addition, ethnic differences in the frequency of ABCG2 c.421C>A have been reported, with markedly higher frequency in East Asian (∼30-60%) than Caucasian and African-American populations (∼5-10%). Therefore, ABCG2 polymorphisms must be considered not only in the drug development phase, but also in clinical practice. In the present review, we provide an update of basic and clinical knowledge on genetic polymorphisms of ABCG2.

Effects of Rolapitant Administered Intravenously or Orally on the Pharmacokinetics of Digoxin (P-glycoprotein Substrate) and Sulfasalazine (Breast Cancer Resistance Protein Substrate) in Healthy Volunteers

Rolapitant is a selective and long-acting neurokinin-1 receptor antagonist approved in an oral formulation in combination with other antiemetic agents for the prevention of delayed chemotherapy-induced nausea and vomiting in adults. Four open-label phase 1 studies evaluated the safety and drug-drug interactions of a single dose of rolapitant given intravenously (166.5 mg) or orally (180 mg) with oral digoxin (0.5 mg) or sulfasalazine (500 mg), probe substrates for the P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), respectively. Administration of intravenous rolapitant with the substrates did not result in clinically significant effects on digoxin and sulfasalazine pharmacokinetics. In contrast, peak concentration and area under the curve for last quantifiable plasma concentrations increased by 71% (geometric mean ratio [GMR], 1.71; 90% confidence interval [CI], 1.49-1.95) and 30% (GMR, 1.30; 90%CI, 1.19-1.42), respectively, when rolapitant was coadministered orally with digoxin compared with digoxin alone; they increased by 140% (GMR, 2.40; 90%CI, 2.02-2.86) and 127% (GMR, 2.27; 90%CI, 1.94-2.65), respectively, when rolapitant was given orally with sulfasalazine compared with sulfasalazine alone. Adverse events were mild to moderate in severity in the absence or presence of rolapitant. There were no abnormal clinical laboratory or electrocardiogram findings. Thus, whether administered orally or intravenously, rolapitant was safe and well tolerated. Patients taking oral rolapitant with P-gp and BCRP substrates with a narrow therapeutic index should be monitored for potential adverse events; although increased plasma concentrations of these substrates may raise the risk of toxicity, they are not contraindicated.

Rheumatoid Arthritis Disease Activity Is Determinant for ABCB1 and ABCG2 Drug-Efflux Transporters Function

OBJECTIVE

To compare drug efflux function of ABCB1 and ABCG2 transporters in rheumatoid arthritis (RA) patients with active disease and in remission.

METHODS

Twenty two active RA patients (DAS28 ≥3.2) and 22 patients in remission (DAS28<2.6) were selected from an early RA clinic. All patients were evaluated at study inclusion and six months later. ABCB1 and ABCG2 functional activity was measured in peripheral lymphocytes by flow cytometry. The percentage of cells able to extrude substrates for ABCB1 and ABCG2 was recorded.

RESULTS

Active patients had higher ABCB1 and ABCG2 activity compared with patients in remission (median [interquartile range]): 3.9% (1.4-22.2) vs (1.3% (0.6-3.2), p = 0.003 and 3.9% (1.1-13.3) vs 0.9% (0.5-1.9) p = 0.006 respectively. Both transporters correlated with disease activity assessed by DAS28, rho = 0.45, p = 0.002 and rho = 0.47, p = 0.001 respectively. Correlation was observed between the time from the beginning of treatment and transporter activity: rho = 0.34, p = 0.025 for ABCB1 and rho = 0.35, p = 0.018 for ABCG2. The linear regression model showed that DAS28 and the time from the onset of treatment are predictors of ABCB1 and ABCG2 functional activity, even after adjustment for treatment. After six months we calculated the correlation between change in DAS28 and change in the functional activity in both transporters and found a moderate and significant correlation for ABCG2 (rho = 0.28, p = 0.04) and a non-significant correlation for ABCB1 (rho = 0.22, p = 0.11).

CONCLUSIONS

Patients with active RA have an increased function of ABCB1 and ABCG2, and disease activity is the main determinant of this phenomena.

Novel understanding of ABC transporters ABCB1/MDR/P-glycoprotein, ABCC2/MRP2, and ABCG2/BCRP in colorectal pathophysiology

AIM: To evaluate ATP-binding cassette (ABC) transporters in colonic pathophysiology as they had recently been related to colorectal cancer (CRC) development.

METHODS: Literature search was conducted on PubMed using combinations of the following terms: ABC transporters, ATP binding cassette transporter proteins, inflammatory bowel disease, ulcerative, colitis, Crohns disease, colorectal cancer, colitis, intestinal inflammation, intestinal carcinogenesis, ABCB1/P-glycoprotein (P-gp/CD243/MDR1), ABCC2/multidrug resistance protein 2 (MRP2) and ABCG2/breast cancer resistance protein (BCRP), Abcb1/Mdr1a, abcc2/Mrp2, abcg2/Bcrp, knock-out mice, tight junction, membrane lipid function.

RESULTS: Recently, human studies reported that changes in the levels of ABC transporters were early events in the adenoma-carcinoma sequence leading to CRC. A link between ABCB1, high fat diet and gut microbes in relation to colitis was suggested by the animal studies. The finding that colitis was preceded by altered gut bacterial composition suggests that deletion of Abcb1 leads to fundamental changes of host-microbiota interaction. Also, high fat diet increases the frequency and severity of colitis in specific pathogen-free Abcb1 KO mice. The Abcb1 KO mice might thus serve as a model in which diet/environmental factors and microbes may be controlled and investigated in relation to intestinal inflammation. Potential molecular mechanisms include defective transport of inflammatory mediators and/or phospholipid translocation from one side to the other of the cell membrane lipid bilayer by ABC transporters affecting inflammatory response and/or function of tight junctions, phagocytosis and vesicle trafficking. Also, diet and microbes give rise to molecules which are potential substrates for the ABC transporters and which may additionally affect ABC transporter function through nuclear receptors and transcriptional regulation. Another critical role of ABCB1 was suggested by the finding that ABCB1 expression identifies a subpopulation of pro-inflammatory Th17 cells which were resistant to treatment with glucocorticoids. The evidence for the involvement of ABCC2 and ABCG2 in colonic pathophysiology was weak.

CONCLUSION: ABCB1, diet, and gut microbes mutually interact in colonic inflammation, a well-known risk factor for CRC. Further insight may be translated into preventive and treatment strategies.

Multidrug resistance protein-1 expression, function and polymorphisms in patients with rheumatoid arthritis not responding to methotrexate

OBJECTIVE

To study the expression, function and polymorphism of MDR-1 protein on the peripheral blood lymphocytes in patients with RA following treatment with MTX and its relationship with response to therapy.

METHODS

RA patients naïve to MTX/DMARD- and glucocorticoid were enrolled. Expression and function of MDR-1 was carried out by flow cytometry at baseline and after 4 months of therapy. MDR-1 expression was measured by relative fluorescence intensities and percentage of positive cells. MDR-1 function was assessed by Rhodamine efflux in presence or absence of verapamil. Patients with reduction in disease activity score 28 ≥1.2 were defined as responders and <1.2 as non-responders. Three single nucleotide polymorphisms in MDR-1 gene, 3435T, 1236T and 2677T/A were studied.

RESULTS

Fifty-two patients of RA were grouped into responders (n = 41), and non-responders (n = 11) as per the defined criteria. There was no difference between the groups in terms of age, sex ratio or duration of illness, MTX dose and follow-up duration. The expression and function of the MDR-1 protein reduced significantly in the responder group after the treatment with MTX when compared to the baseline evaluation. The decrease was significant when compared to the non-responders at the fourth month. MDR-1 expression and function either increased or remained the same in the non-responder group after treatment with MTX. MTX unresponsiveness was not related to any of the three polymorphisms studied.

CONCLUSION

Persistent expression and function of MDR-1 identifies a subset of RA patients not responding to MTX. Its early recognition may help in appropriately modulating therapy.

Role of the breast cancer resistance protein (BCRP/ABCG2) in drug transport--an update

The human breast cancer resistance protein (BCRP, gene symbol ABCG2) is an ATP-binding cassette (ABC) efflux transporter. It was so named because it was initially cloned from a multidrug-resistant breast cancer cell line where it was found to confer resistance to chemotherapeutic agents such as mitoxantrone and topotecan. Since its discovery in 1998, the substrates of BCRP have been rapidly expanding to include not only therapeutic agents but also physiological substances such as estrone-3-sulfate, 17β-estradiol 17-(β-D-glucuronide) and uric acid. Likewise, at least hundreds of BCRP inhibitors have been identified. Among normal human tissues, BCRP is highly expressed on the apical membranes of the placental syncytiotrophoblasts, the intestinal epithelium, the liver hepatocytes, the endothelial cells of brain microvessels, and the renal proximal tubular cells, contributing to the absorption, distribution, and elimination of drugs and endogenous compounds as well as tissue protection against xenobiotic exposure. As a result, BCRP has now been recognized by the FDA to be one of the key drug transporters involved in clinically relevant drug disposition. We published a highly-accessed review article on BCRP in 2005, and much progress has been made since then. In this review, we provide an update of current knowledge on basic biochemistry and pharmacological functions of BCRP as well as its relevance to drug resistance and drug disposition.

PTEN loss-mediated Akt activation increases the properties of cancer stem-like cell populations in prostate cancer

OBJECTIVE

To demonstrate that the PTEN/PI3K/Akt/NF-κB pathway plays an important role in regulating the prostate cancer stem-like cell population by upregulating ABCG2.

METHODS

Targeted PTEN knockdown in human prostate DU145 and 22Rv1 cells using a small interfering RNA were confirmed by immunoblot analysis using antibodies of PTEN, phospho-Akt, Akt, and α-tubulin. Knockdown PTEN DU145 and 22Rv1 cells were augmented, and the stem cell-like properties were examined by cell viability and tumor sphere formation and treated by Akt IV inhibitor to provide the signal transduction pathway. Luciferase activity assays were performed.

RESULTS

The knockdown of PTEN in prostate cancer cell lines increased the stem-like properties of the cells, including their sphere-forming ability, stem cell population number, epithelial-mesenchymal transition-related gene expression, and ABCG2 expression. Additionally, PTEN expression was highly associated with elevated expression of phospho-Akt. Treatment with an Akt inhibitor suppressed the PTEN-mediated effects on the properties of these stem-like cells as well as drug resistance, ABCG2 expression, and the NF-κB pathway.

CONCLUSION

The loss of PTEN in prostate cancer cells resulted in an increased PI3K/Akt pathway. Due to the Akt activation, PTEN loss may play an important role in prostate cancer by promoting cancer stemness through a mechanism that involves enhanced NF-κB signaling.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

Treat-to-target: a tailored treatment approach to rheumatoid arthritis

In contrast to articles and books written about rheumatoid arthritis (RA) two or more decades ago that largely focused on 'coping' with the disease, there have been significant developments in the treatment of RA over the past 10 years. Patients can now be diagnosed and treated and expect to live functional lives, with less likelihood of experiencing the associated joint damage and disability. An important goal of RA therapy has shifted to initiate treatment early and aggressively, with frequent assessment and a target to achieve remission as quickly as possible. This 'treat-to-target' concept has been endorsed to maximise long-term health-related quality of life through control of symptoms, prevention of structural damage, normalisation of function and social participation. This article will look at therapies and strategies for the treatment of RA. It will also discuss a treatment algorithm for rheumatoid arthritis tailored to the individual patient's disease activity status.

Inhibition of human MDR1 and BCRP transporter ATPase activity by organochlorine and pyrethroid insecticides

Despite the growing evidence suggesting that pesticides contribute to chronic diseases, there is a limited understanding of how these chemicals are removed from cells and whether pesticides can alter the disposition of drugs. The present study examined the effects of two classes of insecticides (organochlorine and pyrethroid) on the ATPase activity of the human multidrug resistance protein 1 (MDR1) and breast cancer resistance protein (BCRP) efflux transporters. Using plasma membranes from cells overexpressing MDR1 and BCRP, it was demonstrated that the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) (o,p'-DDT and p,p'-DDT isomers) as well as its metabolite (p,p'-dichlorodiphenyldichloroethane), inhibit both MDR1 and BCRP ATPase activity. In addition, p,p'-dichlorodiphenyldichloroethylene, and two pyrethroid pesticides inhibited BCRP ATPase activity between 4 and 7 μM. Additional research is necessary to further characterize the functional inhibition of MDR1 and BCRP activity and determine whether pesticides alter the transporter-mediated disposition of other chemicals.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

Collagen/β1 integrin signaling up-regulates the ABCC1/MRP-1 transporter in an ERK/MAPK-dependent manner

Collagen/β1 integrin/extracellular signal-regulated kinase signaling up-regulates the expression and function of ABCC1 transporter. This suggests that its activation could represent an important pathway in cancer chemoresistance.

The mechanisms by which β1 integrins regulate chemoresistance of cancer cells are still poorly understood. In this study, we report that collagen/β1 integrin signaling inhibits doxorubicin-induced apoptosis of Jurkat and HSB2 leukemic T-cells by up-regulating the expression and function of the ATP-binding cassette C 1 (ABCC1) transporter, also known as multidrug resistance–associated protein 1. We find that collagen but not fibronectin reduces intracellular doxorubicin content and up-regulates the expression levels of ABCC1. Inhibition and knockdown studies show that up-regulation of ABCC1 is necessary for collagen-mediated reduction of intracellular doxorubicin content and collagen-mediated inhibition of doxorubicin-induced apoptosis. We also demonstrate that activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase signaling pathway is involved in collagen-induced reduction of intracellular doxorubicin accumulation, collagen-induced up-regulation of ABCC1 expression levels, and collagen-mediated cell survival. Finally, collagen-mediated up-regulation of ABCC1 expression and function also requires actin polymerization. Taken together, our results indicate for the first time that collagen/β1 integrin/ERK signaling up-regulates the expression and function of ABCC1 and suggest that its activation could represent an important pathway in cancer chemoresistance. Thus simultaneous targeting of collagen/β1 integrin and ABCC1 may be more efficient in preventing drug resistance than targeting each pathway alone.

PXR and NF-κB correlate with the inducing effects of IL-1β and TNF-α on ABCG2 expression in breast cancer cell lines

In this study we aimed to evaluate PXR and ABCG2 gene expression patterns and NF-κB activity induced by proinflammatory cytokines in different breast normal and carcinoma cells. The effects of proinflammatory cytokines on ABCG2 and PXR mRNA expression were studied using real-time PCR. Western blot analysis used for evaluating the protein levels of ABCG2, PXR and the active form of NF-κB (p65 in nuclear protein extract). Significant inductions in the ABCG2 and PXR mRNA and protein levels and NF-κB activity, were observed in MCF7, BT-474, CAL51, 184A1 and HBL100 cells, upon treatment with 50 ng/ml of IL-1β and TNF-α. On the contrary significant reduction of the ABCG2 and PXR mRNA and protein levels and NF-κB activity, were observed in MDA-MB-435 cell line. In conclusion, IL-1β and TNF-α induced ABCG2 and PXR expression and NF-κB activity in some breast cancer and normal cell lines. Similar patterns of induction and reduction in PXR and ABCG2 genes and NF-κB activity suggest a probable relationship between ABCG2, PXR and NF-κB.

Inactivating PSMB5 mutations and P-glycoprotein (multidrug resistance-associated protein/ATP-binding cassette B1) mediate resistance to proteasome inhibitors: ex vivo efficacy of (immuno)proteasome inhibitors in mononuclear blood cells from patients with rheumatoid arthritis

Bortezomib (BTZ), a registered proteasome inhibitor (PI) for multiple myeloma, has also been proposed as a potential antirheumatic agent. Its reported side effects, however, make it unappealing for long-term administration, and resistance may also develop. To overcome this, second-generation PIs became available. Here, we investigated whether a novel class of peptide epoxyketone-based PIs, including carfilzomib, N-((S)-3-methoxy-1-(((S)-3-methoxy-1-(((S)-1-((R)-2-methyloxiran-2-yl)-1-oxo-3-phenylpropan-2-yl)amino)-1-oxopropan-2-yl)amino)-1-oxopropan-2-yl)-2-methylthiazole-5-carboxamide (ONX0912), and (S)-3-(4-methoxyphenyl)-N-((S)-1-((S)-2-methyloxiran-2-yl)-1-oxo-3-phenylpropan-2-yl)-2-((S)-2-(2-morpholinoacetamido)propanamido)propanamide (ONX0914), might escape two established BTZ-resistance mechanisms: 1) mutations in the proteasome β5 subunit (PSMB5) targeted by these PIs, and 2) drug efflux mediated by ATP-binding cassette transporters. THP1 myeloid sublines with acquired resistance to BTZ (54- to 235-fold) caused by mutations in the PSMB5 gene displayed marked cross-resistance but less pronounced cross-resistance to carfilzomib (9- to 32-fold), ONX0912 (39- to 62-fold), and ONX0914 (27- to 97-fold). As for ATP-binding cassette transporter-mediated efflux, lymphoid CEM/VLB cells with P-glycoprotein (Pgp)/multidrug resistance 1 overexpression exhibited substantial resistance to carfilzomib (114-fold), ONX0912 (23-fold), and ONX0914 (162-fold), whereas less resistance to BTZ (4.5-fold) was observed. Consistently, β5 subunit-associated chymotrypsin-like proteasome activity was significantly less inhibited in these CEM/VLB cells. Ex vivo analysis of peripheral blood mononuclear cells from therapy-naive patients with rheumatoid arthritis revealed that, although basal Pgp levels were low, P-glycoprotein expression compromised the inhibitory effect of carfilzomib and ONX0914. However, the use of P121 (reversin 121), a Pgp transport inhibitor, restored parental cell inhibitory levels in both CEM/VLB cells and peripheral blood mononuclear cells. These results indicate that the pharmacologic activity of these PIs may be hindered by drug resistance mechanisms involving PSMB5 mutations and PI extrusion via Pgp.

Interaction of natural products with cell survival and signaling pathways in the biochemical elucidation of drug targets in cancer

The use of natural products with therapeutic properties is as ancient as human civilization and for a long time mineral, plant and animal products were the main sources of drugs. Worldwide sales of medicinal plants, crude extracts and finished products amounted to US$15 billion in 1999 and it increased to $23 billion in 2002. More interestingly, the influence of natural products upon anticancer drug discovery and design cannot be underestimated. Approximately 60% of all drugs in clinical trials are either a natural product, compounds derived from natural products or contain pharmacophores derived from active natural products. Thus, even today, in the presence of massive numbers of agents from combinatorial libraries, compounds from natural sources are still in the forefront of cancer chemotherapeutics as sources of active drug types, as well as being involved in drug discovery in diseases such as microbial and parasitic infections and the control of cholesterol/lipids, among other functions.

Quantitative evaluation of the impact of active efflux by p-glycoprotein and breast cancer resistance protein at the blood-brain barrier on the predictability of the unbound concentrations of drugs in the brain using cerebrospinal fluid concentration as a surrogate

This study investigated the impact of the active efflux mediated by P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) at the blood-brain barrier (BBB) on the predictability of the unbound brain concentration (C(u,brain)) by the concentration in the cerebrospinal fluid (CSF) (C(u,CSF)) in rats. C(u,brain) is obtained as the product of the total brain concentration and unbound fraction in the brain (f(u,brain)) determined in vitro in brain slices. Twenty-five compounds, including P-gp and/or Bcrp substrates, were given a constant intravenous infusion, and their plasma, brain, and CSF concentrations were determined. P-gp and/or Bcrp substrates, such as verapamil, loperamide, flavopiridol, genistein, quinidine, dantrolene, daidzein, cimetidine, and pefloxacin, showed a higher CSF-to-brain unbound concentration ratio (K(p,uu,CSF/brain)) compared with non-P-gp and non-Bcrp substrates. K(p,uu,CSF/brain) values of P-gp-specific (quinidine and verapamil) and Bcrp-specific (daidzein and genistein) substrates were significantly decreased in Mdr1a/1b(-/-) and Bcrp(-/-) mice, respectively. Furthermore, consistent with the contribution of P-gp and Bcrp to the net efflux at the BBB, K(p,uu,CSF/brain) values of the common substrates (flavopiridol and erlotinib) were markedly decreased in Mdr1a/1b(-/-)/Bcrp(-/-) mice, but only moderately or weakly in Mdr1a/1b(-/-) mice and negligibly in Bcrp(-/-) mice. In conclusion, predictability of C(u,brain) by C(u,CSF) decreases along with the net transport activities by P-gp and Bcrp at the BBB. C(u,CSF) of non-P-gp and non-Bcrp substrates can be a reliable surrogate of C(u,brain) for lipophilic compounds.

Breast cancer resistance protein (BCRP/ABCG2): its role in multidrug resistance and regulation of its gene expression

Breast cancer resistance protein (BCRP)/ATP-binding cassette subfamily G member 2 (ABCG2) is an ATP-binding cassette (ABC) transporter identified as a molecular cause of multidrug resistance (MDR) in diverse cancer cells. BCRP physiologically functions as a part of a self-defense mechanism for the organism; it enhances elimination of toxic xenobiotic substances and harmful agents in the gut and biliary tract, as well as through the blood-brain, placental, and possibly blood-testis barriers. BCRP recognizes and transports numerous anticancer drugs including conventional chemotherapeutic and targeted small therapeutic molecules relatively new in clinical use. Thus, BCRP expression in cancer cells directly causes MDR by active efflux of anticancer drugs. Because BCRP is also known to be a stem cell marker, its expression in cancer cells could be a manifestation of metabolic and signaling pathways that confer multiple mechanisms of drug resistance, self-renewal (sternness), and invasiveness (aggressiveness), and thereby impart a poor prognosis. Therefore, blocking BCRP-mediated active efflux may provide a therapeutic benefit for cancers. Delineating the precise molecular mechanisms for BCRP gene expression may lead to identification of a novel molecular target to modulate BCRP-mediated MDR. Current evidence suggests that BCRP gene transcription is regulated by a number of trans-acting elements including hypoxia inducible factor 1α, estrogen receptor, and peroxisome proliferator-activated receptor. Furthermore, alternative promoter usage, demethylation of the BCRP promoter, and histone modification are likely associated with drug-induced BCRP overexpression in cancer cells. Finally, PI3K/AKT signaling may play a critical role in modulating BCRP function under a variety of conditions. These biological events seem involved in a complicated manner. Untangling the events would be an essential first step to developing a method to modulate BCRP function to aid patients with cancer. This review will present a synopsis of the impact of BCRP-mediated MDR in cancer cells, and the molecular mechanisms of acquired MDR currently postulated in a variety of human cancers.

The challenge of exploiting ABCG2 in the clinic

ABCG2, or breast cancer resistance protein (BCRP), is an ATP-binding cassette half transporter that has been shown to transport a wide range of substrates including chemotherapeutics, antivirals, antibiotics and flavonoids. Given its wide range of substrates, much work has been dedicated to developing ABCG2 as a clinical target. But where can we intervene clinically and how can we avoid the mistakes made in past clinical trials targeting P-glycoprotein? This review will summarize the normal tissue distribution, cancer tissue expression, substrates and inhibitors of ABCG2, and highlight the challenges presented in exploiting ABCG2 in the clinic. We discuss the possibility of inhibiting ABCG2, so as to increase oral bioavailability or increase drug penetration into sanctuary sites, especially the central nervous system; and at the other end of the spectrum, the possibility of improving ABCG2 function, in the case of gout caused by a single nucleotide polymphism. Together, these aspects of ABCG2/BCRP make the protein a target of continuing interest for oncologists, biologists, and pharmacologists.

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.

Drug transport by breast cancer resistance protein

IMPORTANCE OF THE FIELD

The ATP-binding cassette transporter ABCG2 is a well-known major mediator of multi-drug resistance in cancers. Beyond multi-drug resistance, experimental and recent clinical studies demonstrate a role for ABCG2 as a determinant of drug pharmacokinetic, safety and efficacy profiles.

AREAS COVERED IN THIS REVIEW

The clinical evidence of the role of ABCG2 in pharmacokinetics and pharmacodynamics is reviewed. Key questions that arise from the perspective of preclinical drug evaluation are addressed, including the structure of ABCG2 and mechanisms of drug-transporter interactions, mechanisms responsible for the polyspecificity of ABCG2, methods suitable for studying drug-ABCG2 interactions in vitro and in silico prediction of ABCG2 substrates and inhibitors.

WHAT THE READER WILL GAIN

An update on current knowledge of the importance of ABCG2 in drug disposition with special emphasis on drug development.

TAKE HOME MESSAGE

The field of drug-ABCG2 interaction is rapidly advancing and beginning to expand into clinical practice. However, the structural understanding of drug binding and transport by ABCG2 is still incomplete. Incorporation of novel concepts of drug-transporter interactions such as electrostatic funneling might help explain the multispecificity of ABCG2 and enable in silico predictions.

Effect of noncompetitive proteasome inhibition on bortezomib resistance

BACKGROUND

Bortezomib and the other proteasome inhibitors that are currently under clinical investigation bind to the catalytic sites of proteasomes and are competitive inhibitors. We hypothesized that proteasome inhibitors that act through a noncompetitive mechanism might overcome some forms of bortezomib resistance.

METHODS

5-amino-8-hydroxyquinoline (5AHQ) was identified through a screen of a 27-compound chemical library based on the quinoline pharmacophore to identify proteasome inhibitors. Inhibition of proteasome activity by 5AHQ was tested by measuring 7-amino-4-methylcoumarin (AMC) release from the proteasome substrate Suc-LLVY-AMC in intact human and mouse leukemia and myeloma cells and in tumor cell protein extracts. Cytotoxicity was assessed in 5AHQ-treated cell lines and primary cells from myeloma and leukemia patients using AlamarBlue fluorescence and MTS assays, trypan blue staining, and annexin V staining. 5AHQ-proteasome interaction was assessed by nuclear magnetic resonance. 5AHQ efficacy was evaluated in three leukemia xenograft mouse models (9-10 mice per group per model). All statistical tests were two-sided.

RESULTS

5AHQ inhibited the proteasome when added to cell extracts and intact cells (the mean concentration inhibiting 50% [IC(50)] of AMC release in intact cells ranged from 0.57 to 5.03 microM), induced cell death in intact cells from leukemia and myeloma cell lines (mean IC(50) values for cell growth ranged from 0.94 to 3.85 microM), and preferentially induced cell death in primary myeloma and leukemia cells compared with normal hematopoietic cells. 5AHQ was equally cytotoxic to human myelomonocytic THP1 cells and to THP1/BTZ500 cells, which are 237-fold more resistant to bortezomib than wild-type THP1 cells because of their overexpression and mutation of the bortezomib-binding beta5 proteasome subunit (mean IC(50) for cell death in the absence of bortezomib, wild-type THP1: 3.7 microM, 95% confidence interval = 3.4 to 4.0 microM; THP1/BTZ500: 6.6 microM, 95% confidence interval = 5.9 to 7.5 microM). 5AHQ interacted with the alpha subunits of the 20S proteasome at noncatalytic sites. Orally administered 5AHQ inhibited tumor growth in all three mouse models of leukemia without overt toxicity (eg, OCI-AML2 model, median tumor weight [interquartile range], 5AHQ vs control: 95.7 mg [61.4-163.5 mg] vs 247.2 mg [189.4-296.2 mg], P = .002).

CONCLUSIONS

5AHQ is a noncompetitive proteasome inhibitor that is cytotoxic to myeloma and leukemia cells in vitro and inhibits xenograft tumor growth in vivo. 5AHQ can overcome some forms of bortezomib resistance in vitro.

Chemotherapeutic drug-induced ABCG2 promoter demethylation as a novel mechanism of acquired multidrug resistance

ABCG2 is an efflux transporter conferring multidrug resistance (MDR) on cancer cells. However, the initial molecular events leading to its up-regulation in MDR tumor cells are poorly understood. Herein, we explored the impact of drug treatment on the methylation status of the ABCG2 promoter and consequent reactivation of ABCG2 gene expression in parental tumor cell lines and their MDR sublines. We demonstrate that ABCG2 promoter methylation is common in T-cell acute lymphoblastic leukemia (T-ALL) lines, also present in primary T-ALL lymphoblast specimens. Furthermore, drug selection with sulfasalazine and topotecan induced a complete demethylation of the ABCG2 promoter in the T-ALL and ovarian carcinoma model cell lines CCRF-CEM and IGROV1, respectively. This resulted in a dramatic induction of ABCG2 messenger RNA levels (235- and 743-fold, respectively) and consequent acquisition of an ABCG2-dependent MDR phenotype. Quantitative genomic polymerase chain reaction and ABCG2 promoter-luciferase reporter assay did not reveal ABCG2 gene amplification or differential transcriptional trans-activation, which could account for ABCG2 up-regulation in these MDR cells. Remarkably, mimicking cytotoxic bolus drug treatment through 12- to 24-hour pulse exposure of ABCG2-silenced leukemia cells, to clinically relevant concentrations of the chemotherapeutic agents daunorubicin and mitoxantrone, resulted in a marked transcriptional up-regulation of ABCG2. Our findings establish that antitumor drug-induced epigenetic reactivation of ABCG2 gene expression in cancer cells is an early molecular event leading to MDR. These findings have important implications for the emergence, clonal selection, and expansion of malignant cells with the MDR phenotype during chemotherapy.

Effects of oral administration of S-1 on the pharmacokinetics of SN-38, irinotecan active metabolite, in patients with advanced colorectal cancer

Previous studies have assessed the efficacy and safety of combined treatment with irinotecan (7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycamptothecin, CPT-11) and S-1, containing tegafur, a prodrug of 5-fluorouracil, in the treatment of colorectal and gastric cancer. The objective of this study was to describe the interaction between CPT-11 and S-1 in 4 patients with colorectal cancer. Coadministration of S-1 changed the pharmacokinetic behavior of CPT-11 and its metabolites. In particular, maximum plasma concentration (Cmax) and area under the plasma concentration curve (AUC) of 7-ethyl-10-hydroxycampothecin (SN-38) was markedly decreased by coadministration of S-1. For SN-38, the median ratio of Cmax and AUC with S-1 to those without S-1 was median 0.34 (range 0.24-0.78) and 0.56 (range 0.23-0.68), respectively. A markedly difference in drug interaction among individual patients was observed. We conclude that the plasma concentration of SN-38 was decreased by oral administration of S-1 in patients with colorectal cancer. This observation might be important for clinical decisions regarding combination therapy.

ABC drug transporters and immunity: novel therapeutic targets in autoimmunity and cancer

ABC transporters were identified originally for their contribution to clinical MDR as a result of their capacity to extrude various unrelated cytotoxic drugs. More recent reports have shown that ABC transporters can play important roles in the development, differentiation, and maturation of immune cells and are involved in migration of immune effector cells to sites of inflammation. Many of the currently identified, endogenous ABC transporter substrates have immunostimulating effects. Increasing the expression of ABC transporters on immune cells and thereby enhancing immune cell development or functionality may be beneficial to immunotherapy in the field of oncology. On the contrary, in the treatment of autoimmune diseases, blockade of these transporters may prove beneficial, as it could dampen disease activity by compromising immune effector cell functions. This review will focus on the expression, regulation, and substrate specificity of ABC transporters in relation to functional activities of immune effector cells and discusses implications for the treatment of cancer on the one hand and autoimmune diseases on the other.

Leflunomide and its metabolite A771726 are high affinity substrates of BCRP: implications for drug resistance

BACKGROUND

Earlier publications have suggested a possible role for the efflux transporter breast cancer resistance protein (BCRP) in acquired resistance to disease-modifying antirheumatic drugs (DMARDs) such as leflunomide and its metabolite A771726 (teriflunomide). However, there is no direct evidence that BCRP interacts with these drugs.

OBJECTIVES

To characterise the interaction between BCRP transporter and leflunomide and its active metabolite A771726, with emphasis on the nature of the interaction (substrate or inhibitor) and the kinetic characterisation of the interactions.

METHODS

Different in vitro membrane-based methods (ATPase and vesicular transport assay) using BCRP-HAM-Sf9 membrane preparations and cellular assays (Hoechst assay and cytotoxicity assay) were performed on PLB985-BCRP and HEK293-BCRP cell lines overexpressing BCRP.

RESULTS

In all assays used, an interaction between the investigated drugs and BCRP was detected. In the vesicular transport assay, both leflunomide and its metabolite inhibited BCRP-mediated methotrexate transport. Both compounds are likely substrates of BCRP as shown by the vanadate-sensitive ATPase assay. In line with the membrane assays, leflunomide and A771726 inhibited BCRP-mediated Hoechst efflux from PLB985-BCRP cells. In the cytotoxicity assay, overexpression of BCRP conferred 20.6-fold and 7.5-fold resistance to HEK293 cells against leflunomide and A771726, respectively. The resistance could be reversed by Ko134, a specific inhibitor of BCRP.

CONCLUSION

Based on these results, BCRP could play an important role in the resistance to leflunomide and A771726 via interactions with these drugs. BCRP may also mediate drug-drug interactions when leflunomide is administered with other BCRP substrate drugs such as methotrexate.

Kinetic characterization of sulfasalazine transport by human ATP-binding cassette G2

The pharmacokinetics of sulfasalazine, an anti-inflammatory drug is influenced by ATP-binding cassette G2 (ABCG2) (breast cancer resistance protein (BCRP), mitoxantrone resistance protein (MXR)) both in vitro and clinically. Due to its low passive permeability, the intracellular concentration of sulfasalazine is dependent on uptake transporters, rendering the characterization of transporter specific interactions in cell based experimental systems difficult. Applying membrane assays a detailed kinetic analysis of sulfasalazine ABCG2 interaction was conducted and Km values of 0.70 +/- 0.03 microM and 0.66 +/- 0.08 microM were obtained at pH 7.0 and pH 5.5, respectively.

Involvement of breast cancer resistance protein expression on rheumatoid arthritis synovial tissue macrophages in resistance to methotrexate and leflunomide

OBJECTIVE

To determine whether multidrug-resistance efflux transporters are expressed on immune effector cells in synovial tissue from patients with rheumatoid arthritis (RA) and compromise the efficacy of methotrexate (MTX) and leflunomide (LEF).

METHODS

Synovial tissue biopsy samples obtained from RA patients before treatment and 4 months after starting treatment with MTX (n = 17) or LEF (n = 13) were examined by immunohistochemical staining and digital image analysis for the expression of the drug efflux transporters P-glycoprotein, multidrug resistance-associated protein 1 (MRP-1) through MRP-5, MRP-8, MRP-9, and breast cancer resistance protein (BCRP), and the relationship to clinical efficacy of MTX and LEF was assessed.

RESULTS

BCRP expression was observed in all RA synovial biopsy samples, both pretreatment and posttreatment, but not in control noninflammatory synovial tissue samples from orthopedic patients. BCRP expression was found both in the intimal lining layer and on macrophages and endothelial cells in the synovial sublining. Total numbers of macrophages in RA patients decreased upon treatment; in biopsy samples with persistently high macrophage counts, 2-fold higher BCRP expression was observed. Furthermore, median BCRP expression was significantly increased (3-fold) in nonresponders to disease-modifying antirheumatic drugs (DMARDs) compared with responders to DMARDs (P = 0.048). Low expression of MRP-1 was found on synovial macrophages, along with moderate expression in T cell areas of synovial biopsy specimens from one-third of the RA patients.

CONCLUSION

These findings show that the drug resistance-related proteins BCRP and MRP-1 are expressed on inflammatory cells in RA synovial tissue. Since MTX is a substrate for both BCRP and MRP-1, and LEF is a high-affinity substrate for BCRP, these transporters may contribute to reduced therapeutic efficacy of these DMARDs.

Expression of multidrug resistance-1 protein correlates with disease activity rather than the refractoriness to methotrexate therapy in rheumatoid arthritis

Disease-modifying antirheumatic drugs (DMARDs) improve the disability and slow the progression of the joint damage in rheumatoid arthritis (RA). However, a large proportion of patients experience inefficacy by the end of 2 years. This loss of efficacy may be due to expression of multidrug resistance (MDR) proteins on lymphocytes. The objective is to study the expression of MDR protein on the peripheral blood lymphocytes in patients with RA and correlate it with the disease status and response to treatment. Twenty-eight patients were enrolled. Expression of MDR-1 by flow cytometry was carried out on lymphocytes at baseline and after 4 months of therapy. This expression was correlated with disease activity scores (DAS 28). There were 25 females with mean age of 48.13 years and median disease duration of 48 months. Eighteen patients were DMARD naive and ten were refractory to DMARD (methotrexate). The percentage of cells expressing MDR-1 in the DMARD-naive (p<0.O5) and DMARD-refractory (p<0.05) groups were significantly higher than the healthy controls at the baseline. The relative fluorescence intensity was significantly higher in the DMARD-refractory group (p<0.05) as compared to the DMARD-naive group. After 4 months of therapy, there was significant improvement in the D value (p<0.01) in the DMARD-naive group (treated with methotrexate only) and DMARD-refractory group (p<0.05). A significant correlation (r=0.563) between the DAS 28 scores and the D value (p=0.003) was observed. Expression of MDR-1 in RA correlated with disease activity status and improved with DMARD therapy. It is not related to the refractoriness to therapy with methotrexate.

Interleukin-1 beta and tumor necrosis factor-alpha increase ABCG2 expression in MCF-7 breast carcinoma cell line and its mitoxantrone-resistant derivative, MCF-7/MX

OBJECTIVE

In this study, we aimed to evaluate the influence of proinflammatory cytokines on ABCG2 expression and function in human MCF-7 breast cancer cell line and its mitoxantrone-resistant derivative MCF-7/MX.

METHODS

The effects of proinflammatory cytokines on ABCG2 mRNA expression were studied using real-time PCR method. Cytokine-mediated modification of ABCG2 protein expression and function was investigated by means of flow cytometry.

RESULTS

Significant inductions in the ABCG2 mRNA levels, protein expression, and activity were observed in IL-1 beta and TNF-alpha-treated MCF-7 cells. IL-6 increased ABCG2 protein, but had no effects on ABCG2 mRNA and function in MCF-7 cells. Although IL-1 beta did not alter mRNA and protein levels of the transporter in MCF-7/MX cells, ABCG2-mediated efflux was significantly increased in IL-1 beta-treated MCF-7/MX cells. TNF-alpha-treated MCF-7/MX cells also demonstrated greater ABCG2 protein expression and function without any changes in mRNA levels of the transporter. Neither ABCG2 mRNA nor its protein expression and function were affected by IL-6 in MCF-7/MX cells.

CONCLUSION

IL-1 beta and TNF-alpha induce ABCG2 mRNA and protein expression and increase its activity in breast cancer cell line MCF-7. In MCF-7/MX cells these cytokines modulate ABCG2 protein expression and/or function, but they have no influence on the transporter mRNA levels.

Cellular folate status modulates the expression of BCRP and MRP multidrug transporters in cancer cell lines from different origins

As cellular folate levels seem to have a different effect on cancer cells from different origins, we extended our initial study to a broader panel of cancer cells. BCRP and MRP1-5 expression was determined in KB, OVCAR-3, IGROV-1, ZR75-1/R/MTX, SCC-11B, SCC-22B, and WiDr either grown in standard RPMI 1640 containing 2.3 micromol/L supraphysiologic concentration of folic acid [high folate (HF)] or adapted to more physiologic concentrations [1-5 nmol/L folic acid or leucovorin; low folate (LF)]. Compared with the HF counterparts, KB LF cells displayed 16.1-fold increased MRP3 and OVCAR-3 LF cells showed 4.8-fold increased MRP4 mRNA levels along with increased MRP3 and MRP4 protein expression, respectively. A marked increase on BCRP protein and mRNA expression was observed in WiDr LF cells. These cells acquired approximately 2-fold resistance to mitoxantrone compared with the HF cell line, a phenotype that could be reverted by the BCRP inhibitor Ko143. Of note, WiDr cells expressed BCRP in the intracellular compartment, similarly to what we have described for Caco-2 cells. Our results provide further evidence for an important role of cellular folate status in the modulation of the expression of multidrug resistance transporters in cancer cells. We show that up-regulation of intracellularly localized BCRP in response to adaptation to LF conditions may be a common feature within a panel of colon cancer cell lines. Under these circumstances, folate supplementation might improve the efficacy of chemotherapeutic drugs by decreasing BCRP expression.

Breast cancer resistance protein (ABCG2) and drug disposition: intestinal expression, polymorphisms and sulfasalazine as an in vivo probe

Breast cancer resistance protein (BCRP) is an efflux transporter expressed in tissues that act as barriers to drug entry. Given that single nucleotide polymorphisms (SNPs) in the ABCG2 gene encoding BCRP are common, the possibility exists that these genetic variants may be a determinant of interindividual variability in drug response. The objective of this study is to confirm the human BCRP-mediated transport of sulfasalazine in vitro, evaluate interindividual variation in BCRP expression in human intestine and to determine the role of ABCG2 SNPs to drug disposition in healthy patients using sulfasalazine as a novel in vivo probe. To evaluate these objectives, pinch biopsies were obtained from 18 patients undergoing esophagogastro-duodenoscopy or colonoscopy for determination of BCRP expression in relation to genotype. Wild-type and variant BCRP were expressed in a heterologous expression system to evaluate the effect of SNPs on cell-surface trafficking. A total of 17 healthy individuals participated in a clinical investigation to determine the effect of BCRP SNPs on sulfasalazine pharmacokinetics. In vitro, the cell surface protein expression of the common BCRP 421 C>A variant was reduced in comparison with the wild-type control. Intestinal biopsy samples revealed that BCRP protein and mRNA expression did not significantly differ between patients with 34GG/421CC versus patients with 34GG/421CA genotypes. Remarkably, in subjects with 34GG/421CA genotype, sulfasalazine area under the concentration-time curve was 2.4-fold greater compared with 34GG/421CC subjects (P<0.05). This study links commonly occurring SNPs in BCRP with significantly increased oral sulfasalazine plasma exposure in humans. Accordingly, sulfasalazine may prove to have utility as in vivo probe for assessing the clinical impact of BCRP for the disposition and efficacy of drugs.

ABCG2: structure, function and role in drug response

ABCG2 was discovered in multi-drug-resistant cancer cells, with the identification of chemotherapeutic agents, such as mitoxantrone, flavopiridol, methotrexate and irinotecan as substrates. Later, drugs from other therapeutic groups were also described as substrates, including antibiotics, antivirals, HMG-CoA reductase inhibitors and flavonoids. An expanding list of compounds inhibiting ABCG2 has also been generated. The wide variety of drugs transported by ABCG2 and its normal tissue distribution with highest levels in the placenta, intestine and liver, suggest a role in protection against xenobiotics. ABCG2 also has an important role in the pharmacokinetics of its substrates. Single nucleotide polymorphisms of the gene were shown to alter either plasma concentrations of substrate drugs or levels of resistance against chemotherapeutic agents in cell lines. ABCG2 was also described as the determinant of the side population of stem cells. All these aspects of the transporter warrant further research aimed at understanding ABCG2's structure, function and regulation of expression.

Real-time quantitative polymerase chain reaction for BCRP, MDR1, and MRP1 mRNA levels in lymphocytes and monocytes

The expression levels of mRNAs for MDR1 (P-glycoprotein), multidrug resistance-associated proteins (MRP1), and BCRP (breast cancer resistance protein; mitoxantrone resistance protein) were quantitatively determined in lymphocytes and monocytes. Monocytes and lymphocytes were obtained from 3 healthy male and 2 healthy female volunteers. BCRP, MDR1, and MRP1 mRNA levels were determined by real-time quantitative reverse transcriptase polymerase chain reaction. In lymphocytes, relative concentrations of mRNAs for target proteins (BCRP, MDR1, and MRP1) were 1.00 +/- 0.478, 9.67 +/- 5.53, and 0.116 +/- 0.0613 respectively, and in monocytes 1.00 +/- 0.854, 0.821 +/- 0.263, and 0.090 +/- 0.052, respectively. The MDR1 mRNA level was cell type dependent, whereas there was no difference in BCRP and MRP1 expression levels between lymphocytes and monocytes. Comparison of mRNA levels for the three major multidrug-resistant efflux pumps reveals that MDR1 is the predominant form in lymphocytes and BCRP is the predominant form in monocytes.

Sulfasalazine sensitises human monocytic/macrophage cells for glucocorticoids by upregulation of glucocorticoid receptor alpha and glucocorticoid induced apoptosis

BACKGROUND

Glucocorticoids (GCs) are commonly used in the treatment of (chronic) inflammatory diseases and cancer, but inherent or acquired resistance to these drugs limits their optimal efficacy. The availability of drugs that could modulate GC resistance is therefore of potential clinical interest.

OBJECTIVE

To explore the molecular basis of GC sensitisation of GC resistant monocytic/macrophage cells after chronic exposure to sulfasalazine.

METHODS

Human monocytic/macrophage THP1 and U937 cells represent a cell line model system characterised by inherent resistance to the GCs dexamethasone and prednisolone. Both cell lines were chronically exposed in vitro to 0.3-0.6 mM sulfasalazine (SSZ) for approximately 3 months, after which they were characterised for GC sensitivity, expression levels of GC receptor and components of the nuclear factor kappa B (NFkappaB) signalling pathway, and their ability to undergo GC induced apoptosis.

RESULTS

Chronic exposure to SSZ markedly sensitised both U937 and THP1 cells to dexamethasone (781-fold and 1389-fold, respectively) and prednisolone (562-fold and 1220-fold, respectively). Restoration of GC sensitivity in cells exposed to SSZ was provoked via GC induced apoptosis, coinciding with inhibition of NFkappaB activation. Moreover, western blot analysis revealed a markedly increased expression of glucocorticoid receptor alpha (GRalpha) in cells exposed to SSZ. Since GRalpha mRNA levels were only marginally increased, these results suggest that an altered post-transcriptional mechanism was operable which conferred a stable GRalpha protein on SSZ exposed cells.

CONCLUSION

These results suggest that chronic targeting of the NFkappaB signalling pathway by SSZ may be exploited as a novel strategy to stabilise GRalpha expression and thereby sensitise primary resistant cells to GCs.

Involvement of up-regulation of hepatic breast cancer resistance protein in decreased plasma concentration of 7-ethyl-10-hydroxycamptothecin (SN-38) by coadministration of S-1 in rats

The safety and efficacy of combination therapy with 7-ethyl-10-[4-[1-piperidino]-1-piperidino]carbonyloxycamptothecin (CPT-11, irinotecan) and S-1 composed of tegafur, a prodrug of 5-fluorouracil, gimeracil, and potassium oxonate, have been confirmed in patients with colorectal cancer. Previously, we showed that p.o. coadministration of S-1 decreased the plasma concentration of both CPT-11 and its metabolites in a patient with advanced colorectal cancer. The aim of this study was to clarify the mechanism of drug interaction using both in vivo and in vitro methods. Rats were administered S-1 p.o. (10 mg/kg) once a day for 7 consecutive days. On day 7, CPT-11 (10 mg/kg) was administered by i.v. injection. Coadministration of S-1 affected the pharmacokinetic behavior of CPT-11 and its metabolites, with a decrease in the C(max) and area under the plasma concentration curve (AUC) of the active metabolite 7-ethyl-10-hydroxycampothecin (SN-38) lactone form. Furthermore, the rate of biliary excretion of the SN-38 carboxylate form increased on coadministration of S-1. In the liver, the level of breast cancer resistance protein (BCRP), but not P-glycoprotein and multidrug resistance-associated protein 2, was elevated after administration of S-1. Enzymatic conversion of CPT-11 to SN-38 by carboxylesterase (CES) was unaffected by the liver microsomes of rats treated with S-1. In addition, components of S-1 did not inhibit the hydrolysis of p-nitrophenylacetate, a substrate of CES, in the S9 fraction of HepG2 cells. Therefore, the mechanism of drug interaction between CPT-11 and S-1 appears to involve up-regulation of BCRP in the liver, resulting in an increased rate of biliary excretion of SN-38 accompanied by a decrease in the C(max) and AUC of SN-38.