Constitutive overexpression of P-glycoprotein, rather than breast cancer resistance protein or organic cation transporter 1, contributes to acquisition of imatinib-resistance in K562 cells

Contribution of the α1-Acid Glycoprotein in Drug Pharmacokinetics: The Usefulness of α1-Acid Glycoprotein-Knockout Mice

α1-Acid glycoprotein (AGP) is a primary binding protein for many basic drugs in plasma. The number of drugs that bind to AGP, such as molecular target anticancer drugs, has been continuously increasing. Since the plasma level of AGP fluctuates under various pathological conditions such as inflammation, it is important to evaluate the contribution of AGP to drug pharmacokinetics. Here, we generated conventional AGP-knockout (AGP-KO) mice and used them to evaluate the contribution of AGP. The pharmacokinetics of drugs that bind to two AGP variants (F1*S or A variants) or albumin were evaluated. Imatinib (a F1*S-binding drug) and disopyramide (an A-binding drug) or ibuprofen (an albumin-binding drug) were administered to wild-type (WT) and AGP-KO. The plasma level of imatinib and disopyramide decreased rapidly in AGP-KO as compared to WT. In AGP-KO, AUC and t1/2 were decreased, then CLtot was increased. Compared with disopyramide, imatinib pharmacokinetics showed more marked changes in AGP-KO as compared to WT. The results seemed to be due to the difference in plasma level of each AGP variant (F1*S:A = 2-3:1). No differences were observed in ibuprofen pharmacokinetics between the WT and AGP-KO mice. In vitro experiments using plasma from WT and AGP-KO showed that unbound fractions of imatinib and disopyramide were higher in AGP-KO. These results suggest that the rapid elimination of imatinib and disopyramide in AGP-KO could be due to decreased protein binding to AGP. Taken together, the AGP-KO mouse could be a potential animal model for evaluating the contribution of AGP to the pharmacokinetics of various drugs.

Transporter-Mediated Cellular Distribution of Tyrosine Kinase Inhibitors as a Potential Resistance Mechanism in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a hematologic neoplasm characterized by the expression of the BCR::ABL1 oncoprotein, a constitutively active tyrosine kinase, resulting in uncontrolled growth and proliferation of cells in the myeloid lineage. Targeted therapy using tyrosine kinase inhibitors (TKIs) such as imatinib, nilotinib, dasatinib, bosutinib, ponatinib and asciminib has drastically improved the life expectancy of CML patients. However, treatment resistance occurs in 10-20% of CML patients, which is a multifactorial problem that is only partially clarified by the presence of TKI inactivating BCR::ABL1 mutations. It may also be a consequence of a reduction in cytosolic TKI concentrations in the target cells due to transporter-mediated cellular distribution. This review focuses on drug-transporting proteins in stem cells and progenitor cells involved in the distribution of TKIs approved for the treatment of CML. Special attention will be given to ATP-binding cassette transporters expressed in lysosomes, which may facilitate the extracytosolic sequestration of these compounds.

Genetic and Epigenetic Modulation of Drug Resistance in Cancer: Challenges and Opportunities

Cancer is a complex disease that has the ability to develop resistance to traditional therapies. The current chemotherapeutic treatment has become increasingly sophisticated, yet it is not 100% effective against disseminated tumours. Anticancer drugs resistance is an intricate process that ascends from modifications in the drug targets suggesting the need for better targeted therapies in the therapeutic arsenal. Advances in the modern techniques such as DNA microarray, proteomics along with the development of newer targeted drug therapies might provide better strategies to overcome drug resistance. This drug resistance in tumours can be attributed to an individual's genetic differences, especially in tumoral somatic cells but acquired drug resistance is due to different mechanisms, such as cell death inhibition (apoptosis suppression) altered expression of drug transporters, alteration in drug metabolism epigenetic and drug targets, enhancing DNA repair and gene amplification. This review also focusses on the epigenetic modifications and microRNAs, which induce drug resistance and contributes to the formation of tumour progenitor cells that are not destroyed by conventional cancer therapies. Lastly, this review highlights different means to prevent the formation of drug resistant tumours and provides future directions for better treatment of these resistant tumours.

Contributions of MET activation to BCR-ABL1 tyrosine kinase inhibitor resistance in chronic myeloid leukemia cells

Resistance to the breakpoint cluster region-abelson 1 (BCR-ABL1) tyrosine kinase inhibitor (TKI) imatinib poses a major problem when treating chronic myeloid leukemia (CML). Imatinib resistance often results from a secondary mutation in BCR-ABL1. However, in the absence of a mutation in BCR-ABL1, the basis of BCR-ABL1-independent resistance must be elucidated. To gain insight into the mechanisms of BCR-ABL1-independent imatinib resistance, we performed an array-based comparative genomic hybridization. We identified various resistance-related genes, and focused on MET. Treatment with a MET inhibitor resensitized K562/IR cells to BCR-ABL1 TKIs. Combined treatment of K562/IR cells with imatinib and a MET inhibitor suppressed extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) activation, but did not affect AKT activation. Our findings implicate the MET/ERK and MET/JNK pathways in conferring resistance to imatinib, providing new insights into the mechanisms of BCR-ABL1 TKI resistance in CML.

Altered intracellular signaling by imatinib increases the anti-cancer effects of tyrosine kinase inhibitors in chronic myelogenous leukemia cells

Tyrosine kinase inhibitors (TKI), including imatinib (IM), improve the outcome of CML therapy. However, TKI treatment is long‐term and can induce resistance to TKI, which often leads to a poor clinical outcome in CML patients. Here, we examined the effect of continuous IM exposure on intracellular energy metabolism in K562 cells, a human Philadelphia chromosome‐positive CML cell line, and its subsequent sensitivity to anti‐cancer agents. Contrary to our expectations, we found that continuous IM exposure increased sensitivity to TKI. Cancer energy metabolism, characterized by abnormal glycolysis, is linked to cancer cell survival. Interestingly, glycolytic activity was suppressed by continuous exposure to IM, and autophagy increased to maintain cell viability by compensating for glycolytic suppression. Notably, increased sensitivity to TKI was not caused by glycolytic inhibition but by altered intracellular signaling, causing glycolytic suppression and increased autophagy, as evidenced by suppression of p70 S6 kinase 1 (S6K1) and activation of AMP‐activated protein kinase (AMPK). Using another human CML cell line (KCL22 cells) and BCR/ABL+ Ba/F3 cells (mimicking Philadelphia chromosome‐positive CML cells) confirmed that suppressing S6K1 and activating AMPK increased sensitivity to TKI. Furthermore, suppressing S6K1 and activating AMPK had a synergistic anti‐cancer effect by inhibiting autophagy in the presence of TKI. The present study provides new insight into the importance of signaling pathways that affect cellular energy metabolism, and suggests that co‐treatment with agents that disrupt energy metabolic signaling (using S6K1 suppressors and AMPK activators) plus blockade of autophagy may be strategies for TKI‐based CML therapy.

Reversal of ABCB1 mediated efflux by imatinib and nilotinib in cells expressing various transporter levels

Recently, it has been suggested that imatinib (IM) and nilotinib (NIL) could be studied beyond their original application, as inhibitors of the drug efflux pump ABCB1 (P-glycoprotein, MDR1). Since the reversal of ABCB1-mediated resistance has never been successfully demonstrated in the clinic, we addressed the question of whether IM and NIL may actually serve as efficient inhibitors of ABCB1. Here we define an efficient inhibitor as a compound that achieves full (90-100%) reversal of drug efflux at a concentration that does not exhibit significant off-target toxicity in vitro. In this study, human leukemia K562 cells expressing various levels of ABCB1 were used. We observed that cells expressing higher ABCB1 levels required higher concentrations of IM and NIL to achieve full reversal of drug efflux. Among the well-known ABCB1 inhibitors, a similar effect was found for cyclosporin A (CsA) but not for zosuquidar. IM was efficient only in cells with the low and moderate ABCB1 expression at high concentrations that were cytotoxic in the absence of Bcr-Abl. In contrast, NIL was as efficient an inhibitor of ABCB1 as CsA. Low and moderate expression levels of ABCB1 could be efficiently inhibited by NIL concentrations without cytotoxic effects in the absence of Bcr-Abl. However, high expression levels of ABCB1 required higher NIL concentrations with off-target cytotoxic effects. In conclusion, application of NIL, but not of IM, in clinics is promising, however, only in cells with low ABCB1 expression levels. We hypothesize that some patients may benefit from an inhibitor exhibiting an ABCB1 expression-dependent effect.

Pharmacogenetics of drug transporters in modulating imatinib disposition and treatment outcomes in chronic myeloid leukemia & gastrointestinal stromal tumor patients

The use of imatinib in the treatment of BCR-ABL-positive chronic myeloid leukemia and gastrointestinal stromal tumors has significantly improved survival outcomes in patients afflicted by these malignancies. However, a substantial proportion of imatinib-treated patients still experience treatment failure. Suboptimal concentrations of imatinib have been postulated to contribute at least partially to the development of resistance against imatinib. Indeed, variations in the genes encoding drug transporters have been reported to markedly influence imatinib disposition and treatment outcomes in various populations. This review aims to consolidate and critically assess the studies conducted to date which have investigated the influence of pharmacogenetic variants in drug transporters on the disposition of imatinib and treatment outcomes in Asians and other populations.

Dietary flavonoid derivatives enhance chemotherapeutic effect by inhibiting the DNA damage response pathway

Flavonoids are the most common group of polyphenolic compounds and abundant in dietary fruits and vegetables. Diet high in vegetables or dietary flavonoid supplements is associated with reduced mortality rate for patients with breast cancer. Many studies have been proposed for mechanisms linking flavonoids to improving chemotherapy efficacy in many types of cancers, but data on this issue is still limited. Herein, we report on a new mechanism through which dietary flavonoids inhibit DNA damage checkpoints and repair pathways. We found that dietary flavonoids could inhibit Chk1 phosphorylation and decrease clonogenic cell growth once breast cancer cells receive ultraviolet irradiation, cisplatin, or etoposide treatment. Since the ATR-Chk1 pathway mainly involves response to DNA replication stress, we propose that flavonoid derivatives reduce the side effect of chemotherapy by improving the sensitivity of cycling cells. Therefore, we propose that increasing intake of common dietary flavonoids is beneficial to breast cancer patients who are receiving DNA-damaging chemotherapy, such as cisplatin or etoposide-based therapy.

ABCB1 Overexpression Is a Key Initiator of Resistance to Tyrosine Kinase Inhibitors in CML Cell Lines

The tyrosine kinase inhibitor (TKI) imatinib has resulted in excellent responses in the majority of Chronic Myeloid Leukaemia (CML) patients; however, resistance is observed in 20–30% of patients. More recently, resistance to the second generation TKIs, nilotinib and dasatinib, has also been observed albeit at a lower incidence. ABCB1 has previously been implicated in TKI export and its overexpression linked to TKI resistance. In this study the dynamics of nilotinib resistance was studied in CML cell lines with particular focus on ABCB1 expression levels during development of resistance. Results revealed ABCB1 overexpression is likely an important initiator of nilotinib resistance in vitro. ABCB1 overexpression was also observed in cell lines as an intermediate step during development of resistance to imatinib and dasatinib in vitro. We conclude that ABCB1 overexpression may provide an initial platform to facilitate development of additional mechanisms for resistance to TKIs. This provides a rationale for investigating this phenomenon in patients undergoing TKI therapy.

Overexpression of glypican-1 implicates poor prognosis and their chemoresistance in oesophageal squamous cell carcinoma

Background:

Despite the recent improvements in multimodal therapies for oesophageal squamous cell carcinoma (ESCC), the prognosis remains poor. The identification of suitable biomarkers for predicting the prognosis and chemo-sensitivity is required to develop targeted treatments and improve treatment results.

Methods:

Proteins highly expressed in ESCC cell lines compared with normal oesophageal cell lines were screened by isobaric tag for relative and absolute quantitation (iTRAQ). We identified glypican-1 (GPC1) as a novel molecule. The clinicopathological characteristics of GPC1 were evaluated by immunohistochemistry using ESCC specimens, and clinical parameters were assessed. The correlation between GPC1 expression levels and chemo-sensitivity were analysed in vitro.

Results:

In the immunohistochemical assessment of 175 ESCC patients, 98.8% expressed GPC1. These patients demonstrated significantly poorer prognosis compared with patients with low-GPC1 expression by survival assay (P<0.001). Higher chemoresistance was observed in the GPC1 high-expression group. GPC1 expression levels positively correlated with chemo-sensitivity against cis-Diammineplatinum (II) dichloride (CDDP), and are potentially associated with anti-apoptotic function based on alterations in the MAPK downstream signalling pathway and Bcl-2 family member proteins.

Conclusions:

GPC1 is an independent prognostic factor in ESCC and is a critical molecule for altering the threshold of chemoresistance to CDDP.

Pharmacokinetics and tolerability of NSC23925b, a novel P-glycoprotein inhibitor: preclinical study in mice and rats

Overexpression of P-glycoprotein (Pgp) increases multidrug resistance (MDR) in cancer, which greatly impedes satisfactory clinical treatment and outcomes of cancer patients. Due to unknown pharmacokinetics, the use of Pgp inhibitors to overcome MDR in the clinical setting remains elusive despite promising in vitro results. The purpose of our current preclinical study is to investigate the pharmacokinetics and tolerability of NSC23925b, a novel and potent P-glycoprotein inhibitor, in rodents. Plasma pharmacokinetic studies of single-dose NSC23925b alone or in combination with paclitaxel or doxorubicin were conducted in male BALB/c mice and Sprague-Dawley rats. Additionally, inhibition of human cytochrome P450 (CYP450) by NSC23925b was examined in vitro. Finally, the maximum tolerated dose (MTD) of NSC23925b was determined. NSC23925b displayed favorable pharmacokinetic profiles after intraperitoneal/intravenous (I.P./I.V.) injection alone or combined with chemotherapeutic drugs. The plasma pharmacokinetic characteristics of the chemotherapy drugs were not affected when co-administered with NSC23925b. All the animals tolerated the I.P./I.V. administration of NSC23925b. Moreover, the enzymatic activity of human CYP450 was not inhibited by NSC23925b. Our results demonstrated that Pgp inhibitor NSC23925b exhibits encouraging preclinical pharmacokinetic characteristics and limited toxicity in vivo. NSC23925b has the potential to treat cancer patients with MDR in the future.

Role of drug transport and metabolism in the chemoresistance of acute myeloid leukemia

Acute myeloid leukemia is a clonal but heterogeneous disease differing in molecular pathogenesis, clinical features and response to chemotherapy. This latter frequently consists of a combination of cytarabine and anthracyclines, although etoposide, demethylating agents, and other drugs are also used. Unfortunately, chemoresistance is a common and serious problem. Multiple mechanisms account for impaired effectiveness of drugs and reduced levels of active agents in target cells. The latter can be due to lower drug uptake, increased export or decreased intracellular proportion of active/inactive agent due to changes in the expression/function of enzymes responsible for the activation of pro-drugs and the inactivation of active agents. Characterization of the "resistome", or profile of expressed genes accounting for multi-drug resistance (MDR) phenotype, would permit to predict the lack of response to chemotherapy and would help in the selection of the best pharmacological regime for each patient and moment, and to develop strategies of chemosensitization.

Drug resistance in cancer: an overview

Cancers have the ability to develop resistance to traditional therapies, and the increasing prevalence of these drug resistant cancers necessitates further research and treatment development. This paper outlines the current knowledge of mechanisms that promote or enable drug resistance, such as drug inactivation, drug target alteration, drug efflux, DNA damage repair, cell death inhibition, and the epithelial-mesenchymal transition, as well as how inherent tumor cell heterogeneity plays a role in drug resistance. It also describes the epigenetic modifications that can induce drug resistance and considers how such epigenetic factors may contribute to the development of cancer progenitor cells, which are not killed by conventional cancer therapies. Lastly, this review concludes with a discussion on the best treatment options for existing drug resistant cancers, ways to prevent the formation of drug resistant cancers and cancer progenitor cells, and future directions of study.

Resistance to daunorubicin, imatinib, or nilotinib depends on expression levels of ABCB1 and ABCG2 in human leukemia cells

The effect of ABCB1 (P-gp, (P-glycoprotein), MDR1) and ABCG2 (BCRP1, (breast cancer resistance protein 1)) expressions on cell resistance to daunorubicin (DRN), imatinib, and nilotinib was studied in human leukemia cells. We used a set of cells derived from a parental K562 cell line, expressing various levels of ABCB1 and ABCG2, respectively. The function of ABCB1 and ABCG2 was confirmed using calcein AM and pheophorbide A accumulation assays, respectively. These assays indicated distinct differences in activities of ABCB1 and ABCG2 which corresponded to their expression levels. We observed that the resistance to DRN and imatinib was proportional to the expression level of ABCB1. Similarly, the resistance to nilotinib and imatinib was proportional to the expression level of ABCG2. Importantly, K562/DoxDR05 and K562/ABCG2-Z cells with the lowest expressions of ABCB1 and ABCG2, respectively, failed to reduce the intracellular levels of imatinib to provide a significant resistance to this drug. However, the K562/DoxDR05 and K562/ABCG2-Z cells significantly decreased the intracellular levels of DRN and nilotinib, respectively, thereby mediating significant resistances to these drugs. Only cells which expression of ABCB1 or ABCG2 exceeded a certain level exhibited a significantly decreased intracellular level of imatinib, and this effect was accompanied by a significantly increased resistance to this drug. Our results clearly indicated that resistance to anticancer drugs mediated by main ABC transporters, ABCB1 and ABCG2, strongly depends on their expressions at protein levels. Importantly, resistance for one drug might be maintained while resistance for other ones might become undetectable at low transporter expression levels.

Reduced ABCG2 and increased SLC22A1 mRNA expression are associated with imatinib response in chronic myeloid leukemia

Imatinib mesylate (IM) has become a standard of care in chronic myeloid leukemia (CML) therapy. Single nucleotide polymorphisms (SNPs) and altered expression in drug transporter genes may influence IM response. In order to investigate whether mRNA expression and SNPs in drug transporters are associated with IM resistance, we studied 118 chronic-phase CML patients receiving the standard dose of IM (400 mg/day). They were assigned as responders and non-responders according to European LeukemiaNet criteria (2009). mRNA expression in samples at diagnosis (without IM therapy) and outcomes after IM failure were also evaluated in subgroups of patients. Major molecular response (MMR), complete molecular response and primary and secondary resistance were all assessed. BCR-ABL1, ABCB1, ABCG2, SLC22A1 and SLCO1A2 mRNA expression and SNPs in ABCG2 and SLC22A1 genes were analyzed. ABCG2 mRNA expression in the non-responders was higher before and during IM therapy. Furthermore, ABCG2 was overexpressed in those who did not achieve MMR (P=0.027). In a subgroup of patients who switched to second-generation tyrosine kinase inhibitors, high mRNA expression of ABCG2 was associated with a risk of 24 times that of not achieving complete cytogenetic response (OR 24.00, 95% CI 1.74-330.80; P=0.018). In the responder group, patients who achieved MMR (P=0.009) presented higher mRNA levels of SLC22A1. The SNPs were not associated with mRNA expression of ABCG2 and SLC22A1. Our data suggest that elevated ABCG2 expression (an efflux transporter) could be associated with IM resistance and could impact on second-generation TKI response, whereas high SLC22A1 expression (an influx transporter) may be associated with a successful IM therapy in CML patients.

Multidrug resistance in chronic myeloid leukaemia: how much can we learn from MDR-CML cell lines?

The hallmark of CML (chronic myeloid leukaemia) is the BCR (breakpoint cluster region)-ABL fusion gene. CML evolves through three phases, based on both clinical and pathological features: a chronic phase, an accelerated phase and blast crisis. TKI (tyrosine kinase inhibitors) are the treatment modality for patients with chronic phase CML. The therapeutic potential of the TKI imatinib is affected by BCR-ABL dependent an independent mechanisms. Development of MDR (multidrug resistance) contributes to the overall clinical resistance. MDR involves overexpression of ABC -transporters (ATP-binding-cassette transporter) among other features. MDR studies include the analysis of cancer cell lines selected for resistance. CML blast crisis is accompanied by increased resistance to apoptosis. This work reviews the role played by the influx transporter OCT1 (organic cation transporter 1), by efflux ABC transporters, molecules involved in the modulation of apoptosis (p53, Bcl-2 family, CD95, IAPs (inhibitors of apoptosis protein)], Hh and Wnt/β-catenin pathways, cytoskeleton abnormalities and other features described in leukaemic cells of clinical samples and CML cell lines. An MDR cell line, Lucena-1, generated from K562 by stepwise exposure to vincristine, was used as our model and some potential anticancer drugs effective against the MDR cell line and patients' samples are presented.

Interaction of the efflux transporters ABCB1 and ABCG2 with imatinib, nilotinib, and dasatinib

The efflux transporters adenosine triphosphate (ATP)-binding cassette (ABC)B1 and ABCG2 have been demonstrated to interact with the tyrosine kinase inhibitors (TKIs) imatinib, nilotinib, and dasatinib. However, although some studies conclude that TKIs are substrates of one or both transporters, other studies demonstrate only an inhibitory function. This variation is probably due to differences in the concentration of TKIs assayed and the experimental systems used. This article examines the evidence for clinically relevant interactions between three currently approved TKIs and ABCB1/ABCG2.

Phosphorylated Crkl reduction levels are associated with the lowest P-glycoprotein activity levels in cells from chronic myeloid leukemia patients

ABCB1/P-glycoprotein (Pgp) and ABCG2/BCRP overexpression have been described as related to imatinib resistance in chronic myeloid leukemia (CML). We showed in CML cells from 55 patients that Pgp activity was more frequently detected than BCRP activity (p=0.0074). Imatinib-induced Crkl phosphorylated protein (pCrkl) reduction was more pronounced in K562 (Pgp-negative) than in K562-Lucena (Pgp-positive) CML cell line. Expressive pCrkl reduction levels after in vitro imatinib treatment was observed in samples from patients exhibiting lower Pgp activity levels compared with patients exhibiting higher Pgp activity levels (p=0.0045). Pgp activity in association with pCrkl reduction levels might help to distinguish between imatinib-resistant and imatinib-sensitive CML cells.

Development of novel rifampicin-derived P-glycoprotein activators/inducers. synthesis, in silico analysis and application in the RBE4 cell model, using paraquat as substrate

P-glycoprotein (P-gp) is a 170 kDa transmembrane protein involved in the outward transport of many structurally unrelated substrates. P-gp activation/induction may function as an antidotal pathway to prevent the cytotoxicity of these substrates. In the present study we aimed at testing rifampicin (Rif) and three newly synthesized Rif derivatives (a mono-methoxylated derivative, MeORif, a peracetylated derivative, PerAcRif, and a reduced derivative, RedRif) to establish their ability to modulate P-gp expression and activity in a cellular model of the rat’s blood–brain barrier, the RBE4 cell line P-gp expression was assessed by western blot using C219 anti-P-gp antibody. P-gp function was evaluated by flow cytometry measuring the accumulation of rhodamine123. Whenever P-gp activation/induction ability was detected in a tested compound, its antidotal effect was further tested using paraquat as cytotoxicity model. Interactions between Rif or its derivatives and P-gp were also investigated by computational analysis. Rif led to a significant increase in P-gp expression at 72 h and RedRif significantly increased both P-gp expression and activity. No significant differences were observed for the other derivatives. Pre- or simultaneous treatment with RedRif protected cells against paraquat-induced cytotoxicity, an effect reverted by GF120918, a P-gp inhibitor, corroborating the observed P-gp activation ability. Interaction of RedRif with P-gp drug-binding pocket was consistent with an activation mechanism of action, which was confirmed with docking studies. Therefore, RedRif protection against paraquat-induced cytotoxicity in RBE4 cells, through P-gp activation/induction, suggests that it may be useful as an antidote for cytotoxic substrates of P-gp.

Mechanisms of P-gp inhibition and effects on membrane fluidity of a new rifampicin derivative, 1,8-dibenzoyl-rifampicin

PURPOSE

To assess P-glycoprotein (P-gp)-modulation ability and the mechanisms of P-gp inhibition mediated by a new synthetic rifampicin derivative, 1,8-dibenzoyl-rifampicin (DiBenzRif), in an in vitro model of the blood-brain barrier (BBB), RBE4 cells, and in membrane mimetic models (liposomes).

METHODS

P-gp expression (western blot) and activity [rhodamine 123 accumulation studies] were assessed until 72h of exposure to DiBenzRif. The effects on intracellular ATP levels and on P-gp ATPase activity were studied using luciferin-luciferase bioluminescence assay. Membrane fluidity changes were tracked by steady-state anisotropy measurements. Non-P-gp-related rhodamine 123 accumulation was evaluated using liposomes prepared with the main lipids present in RBE4 cell membranes.

RESULTS

A significant increase in intracellular rhodamine 123 content was observed in DiBenzRif-treated cells at all tested time-points. This effect was associated with a significant reduction in ATP intracellular levels, the inhibition of P-gp ATPase activity and a significant increase in membrane fluidity. DiBenzRif also favoured rhodamine 123 accumulation in a liposomal model of RBE4 cells, suggesting that it may be useful in increasing intracellular levels of substances that passively diffuse into the cells.

CONCLUSION

DiBenzRif-induced inhibitory effect on P-gp increases xenobiotic accumulation in BBB cells, which may contribute to the development of therapeutic adjuvants to enhance brain penetration of drugs.

OCT1 identity crisis

"OCT1" is used in the literature to describe two distinct genes and their resulting proteins, namely organic cation transporter 1 (encoded by SLC22A1) and octamer binding transcription factor 1 (encoded by POU2F1). The interchangeable use of "OCT1" is misleading and could result in erroneous research data.

The Interface between BCR-ABL-Dependent and -Independent Resistance Signaling Pathways in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a clonal hematopoietic disorder characterized by the presence of the Philadelphia chromosome which resulted from the reciprocal translocation between chromosomes 9 and 22. The pathogenesis of CML involves the constitutive activation of the BCR-ABL tyrosine kinase, which governs malignant disease by activating multiple signal transduction pathways. The BCR-ABL kinase inhibitor, imatinib, is the front-line treatment for CML, but the emergence of imatinib resistance and other tyrosine kinase inhibitors (TKIs) has called attention for additional resistance mechanisms and has led to the search for alternative drug treatments. In this paper, we discuss our current understanding of mechanisms, related or unrelated to BCR-ABL, which have been shown to account for chemoresistance and treatment failure. We focus on the potential role of the influx and efflux transporters, the inhibitor of apoptosis proteins, and transcription factor-mediated signals as feasible molecular targets to overcome the development of TKIs resistance in CML.

Tyrosine kinase inhibitors as modulators of ABC transporter-mediated drug resistance

Tyrosine kinases (TKs) are involved in key signaling events/pathways that regulate cancer cell proliferation, apoptosis, angiogenesis and metastasis. Deregulated activity of TKs has been implicated in several types of cancers. In recent years, tyrosine kinase inhibitors (TKIs) have been developed to inhibit specific kinases whose constitutive activity results in specific cancer types. These TKIs have been found to demonstrate effective anticancer activity and some of them have been approved by the Food and Drug Administration for clinical use or are in clinical trials. However, these targeted therapeutic agents are also transported by ATP-binding cassette (ABC) transporters, resulting in altered pharmacokinetics or development of resistance to these drugs in cancer patients. This review covers the recent findings on the interactions of clinically important TKIs with ABC drug transporters. Future research efforts in the development of novel TKIs with specific targets, seeking improved activity, should consider these underlying causes of resistance to TKIs in cancer cells.

Overexpression of P-glycoprotein induces acquired resistance to imatinib in chronic myelogenous leukemia cells

Imatinib, a breakpoint cluster region (BCR)-Abelson murine leukemia(ABL) tyrosine kinase inhibitor (TKI), has revolutionized the treatment of chronic myelogenous leukemia (CML). However, development of multidrug resistance(MDR) limits the use of imatinib. In the present study, we aimed to investigate the mechanisms of cellular resistance to imatinib in CML. Therefore, we established an imatinib-resistant human CML cell line(K562-imatinib) through a stepwise selection process. While characterizing the phenotype of these cells, we found that K562-imatinib cells were 124.6-fold more resistant to imatinib than parental K562 cells. In addition, these cells were cross-resistant to second- and third-generation BCR-ABL TKIs. Western blot analysis and reverse transcription-polymerase chain reaction(RT-PCR) demonstrated that P-glycoprotein(P-gp) and MDR1 mRNA levels were increased in K562-imatinib cells. In addition, accumulation of [14C]6-mercaptopurine (6-MP) was decreased, whereas the ATP-dependent efflux of [14C]6-MP and [3H]methotrexate transport were increased in K562-imatinib cells. These data suggest that the overexpression of P-gp may play a crucial role in acquired resistance to imatinib in CML K562-imatinib cells.

Development of imatinib and dasatinib resistance: dynamics of expression of drug transporters ABCB1, ABCC1, ABCG2, MVP, and SLC22A1

About 20% of patients with chronic myeloid leukemia (CML) do not respond to treatment with imatinib either initially or because of acquired resistance. To study the development of CML drug resistance, an in vitro experimental system comprising cell lines with different resistance levels was established by exposing K562 cells to increasing concentrations of imatinib and dasatinib anticancer agents. The mRNA levels of BCR- ABL1 and of genes involved in drug transport or redistribution (ABCB1, ABCC1, ABCC3, ABCG2, MVP, and SLC22A1) were measured and the ABL1 kinase domain sequenced. Results excluded BCR- ABL1 overexpression and mutations as relevant resistance mechanisms. Most studied transporters were overexpressed in the majority of resistant cell lines. Their expression pattern was dynamic: varying with resistance level and chronic drug exposure. Studied efflux transporters may have an important role at the initial stages of resistance, but after prolonged exposure and for higher doses of drugs other mechanisms might take place.

Association of SLCO1B3 polymorphism with intracellular accumulation of imatinib in leukocytes in patients with chronic myeloid leukemia

Intracellular concentration of imatinib in leukemic cells is thought to affect the clinical efficacy of this drug in patients with chronic myeloid leukemia (CML); however, there is no report that directly indicates the relationship between intracellular concentration and clinical outcome and/or, plasma concentration. In addition, the impacts of genetic variations of drug transporters, which mediate leukocyte concentration of imatinib, are unknown. In the present study, we investigated the correlation between intracellular imatinib concentrations in leukocytes, plasma imatinib levels, and genotypes of drug transporters, including ATP binding cassette B1 (ABCB), ABCG2, solute carrier 22A1 (SLC22A1), solute carrier organic anion transporter family members 1B1 (SLCO1B1) and SLCO1B3. The imatinib levels in leukocytes were determined using HPLC in 15 patients with chronic phase CML. No significant correlation between intracellular and plasma concentrations of imatinib was observed. The intracellular concentration was comparable in both patients with or without complete cytogenetic response. The intracellular imatinib concentration was significantly higher in patients with SLCO1B3 334TT than in those with 334TG/GG (p=0.0188). Plasma concentrations were similar in both SLCO1B3 genotypes (p=0.860), thereby resulting in the intracellular to plasma concentration ratio being higher in patients with SLCO1B3 334TT than those with 334 TG/GG (p=0.0502). These results suggested that the SLCO1B3 334T>G polymorphism could have a significant impact on the intracellular concentration of imatinib in leukocytes as a promising biomarker for personalized treatment of CML patients.

Variation of MDR proteins expression and activity levels according to clinical status and evolution of CML patients

The involvement of the multidrug resistance (MDR) mediated by ABC transporter proteins P-glycoprotein (Pgp) and multidrug resistance-associated protein-1 (MRP1) overexpressions in patients with chronic myeloid leukemia (CML) are not completely understood. Pgp and MRP1 expressions and activity were analyzed in samples from 158 patients with chronic myeloid leukemia (CML). Using flow cytometry, Pgp expression was more frequently observed in early chronic (P = 0.00) and in advanced (P = 0.02) CML phases when it was compared to MRP1 expression. Variation of MDR expression and activity were observed during the CML evolution in patients previously treated with interferon and imatinib. In the K562-Lucena cell line, Pgp positive, imatinib caused an enhancing in Pgp expression at protein and mRNA levels, whereas in the Pgp negative cell line, this drug was capable of decreasing MDR1/Pgp mRNA levels. Our result emphasizes the importance of understanding the different aspects of MDR status in patients with CML when they are under investigation in determining imatinib resistance.

Contribution of BCR-ABL-independent activation of ERK1/2 to acquired imatinib resistance in K562 chronic myeloid leukemia cells

BCR-ABL tyrosine kinase, generated from the reciprocal chromosomal translocation t(9;22), causes chronic myeloid leukemia (CML). BCR-ABL is inhibited by imatinib; however, several mechanisms of imatinib resistance have been proposed that account for loss of imatinib efficacy in patients with CML. Previously, we showed that overexpression of the efflux drug transporter P-glycoprotein partially contributed to imatinib resistance in imatinib-resistant K562 CML cells having no BCR-ABL mutations. To explain an additional mechanism of drug resistance, we established a subclone (K562/R) of the cells and examined the BCR-ABL signaling pathway in these and wild-type K562 (K562/W) cells. We found the K562/R cells were 15 times more resistant to imatinib than their wild-type counterparts. In both cell lines, BCR-ABL and its downstream signaling molecules, such as ERK1/2, ERK5, STAT5, and AKT, were phosphorylated in the absence of imatinib. In both cell lines, imatinib effectively reduced the phosphorylation of all the above, except ERK1/2, whose phosphorylation was, interestingly, only inhibited in the wild-type cells. We then observed that phospho-ERK1/2 levels decreased in the presence of siRNA targeting BCR-ABL, again, only in the K562/W cells. However, using an ERK1/2 inhibitor, U0126, we found that we could reduce phospho-ERK1/2 levels in K562/R cells and restore their sensitivity to imatinib. Taken together, we conclude that the BCR-ABL-independent activation of ERK1/2 contributes to imatinib resistance in K562/R cells, and that ERK1/2 could be a target for the treatment of CML patients whose imatinib resistance is due to this mechanism.

Imatinib resistance in multidrug-resistant K562 human leukemic cells

The multidrug resistance phenotype (MDR) is one of the major causes of failure in cancer chemotherapy and it is associated with the over-expression of P-glycoprotein (P-gp or MDR1) in tumor cell membranes. A constitutive NF-kappaB activity has been observed in several haematological malignancies and this is associated with its anti-apoptotic role. In the present work, the relationship between NF-kappaB and MDR phenotype was evaluated in wild type K562 human leukemic cells (K562-WT) and in its vincristine-resistant counterpart, K562-Vinc cells. These data showed that K562-Vinc cells, which express an active P-gp, exhibited MDR phenotype. The resistant indexes (IC(50)(K562-Vinc)/IC(50)(K562-WT)) for structurally unrelated drugs like imatinib, doxorubicin and colchicine were 8.0+/-0.3, 2.8+/-0.4 and 44.8+/-8.8, respectively. The imatinib resistance was reversed by P-gp blockade suggesting the involvement of P-gp in imatinib transport. We observed that NF-kappaB was constitutively activated in both cell lines but in a lesser extent in K562-Vinc. The inhibition of NF-kappaB with BAY 11-7082 increased the cytotoxicity of imatinib in K562-Vinc cells but not in K562-WT. Further, the co-administration of imatinib and BAY 11-7082 sensitized multidrug-resistant K562 cells to cell death as detected by increased percentage of annexin V positive cells. The induced cell death in K562-Vinc cells was associated with activation of caspases 9 and 3. Finally, we provide data showing that BAY 11-7082 down-regulates the expression of P-gp suggesting that the activity of NF-kappaB could be functionally associated to this protein in K562 cells. Our results indicate that the vincristine-resistant K562 cells which developed MDR phenotype, exhibited resistance to imatinib associated with a functional P-gp over-expression. This resistance could be partially overcome by the inhibition of NF-kappaB pathway.