Equilibrative nucleoside transporter-2 (hENT2) protein expression correlates with ex vivo sensitivity to fludarabine in chronic lymphocytic leukemia (CLL) cells

Solute Carrier Nucleoside Transporters in Hematopoiesis and Hematological Drug Toxicities: A Perspective

Simple Summary

Anticancer nucleoside analogs are promising treatments that often result in damaging toxicities and therefore ineffective treatment. Mechanisms of this are not well-researched, but cellular nucleoside transport research in mice might provide additional insight given transport’s role in mammalian hematopoiesis. Cellular nucleoside transport is a notable component of mammalian hematopoiesis due to how mutations within it relate to hematological abnormities. This review encompasses nucleoside transporters, focusing on their inherent properties, hematopoietic role, and their interplay in nucleoside drug treatment side effects. We then propose potential mechanisms to explain nucleoside transport involvement in blood disorders. Finally, we point out and advocate for future research areas that would improve therapeutic outcomes for patients taking nucleoside analog therapies.

Abstract

Anticancer nucleoside analogs produce adverse, and at times, dose-limiting hematological toxicities that can compromise treatment efficacy, yet the mechanisms of such toxicities are poorly understood. Recently, cellular nucleoside transport has been implicated in normal blood cell formation with studies from nucleoside transporter-deficient mice providing additional insights into the regulation of mammalian hematopoiesis. Furthermore, several idiopathic human genetic disorders have revealed nucleoside transport as an important component of mammalian hematopoiesis because mutations in individual nucleoside transporter genes are linked to various hematological abnormalities, including anemia. Here, we review recent developments in nucleoside transporters, including their transport characteristics, their role in the regulation of hematopoiesis, and their potential involvement in the occurrence of adverse hematological side effects due to nucleoside drug treatment. Furthermore, we discuss the putative mechanisms by which aberrant nucleoside transport may contribute to hematological abnormalities and identify the knowledge gaps where future research may positively impact treatment outcomes for patients undergoing various nucleoside analog therapies.

Equilibrative Nucleoside Transporter 2: Properties and Physiological Roles

Equilibrative nucleoside transporter 2 (ENT2) is a bidirectional transporter embedded in the biological membrane and is ubiquitously found in most tissue and cell types. ENT2 mediates the uptake of purine and pyrimidine nucleosides and nucleobase besides transporting a variety of nucleoside-derived drugs, mostly in anticancer therapy. Since high expression of ENT2 has been correlated with advanced stages of different types of cancers, consequently, this has gained significant interest in the role of ENT2 as a potential therapeutic target. Furthermore, ENT2 plays critical roles in signaling pathway and cell cycle progression. Therefore, elucidating the physiological roles of ENT2 and its properties may contribute to a better understanding of ENT2 roles beyond their transportation mechanism. This review is aimed at highlighting the main roles of ENT2 and at providing a brief update on the recent research.

Role of equilibrative nucleoside transporter 1 (ENT1) in the disposition of cytarabine in mice

Cytarabine (Ara-C) is a nucleoside analog used in the treatment of acute myeloid leukemia (AML). Despite the many years of clinical use, the identity of the transporter(s) involved in the disposition of Ara-C remains poorly studied. Previous work demonstrated that concurrent administration of Ara-C with nitrobenzylmercaptopurine ribonucleoside (NBMPR) causes an increase in Ara-C plasma levels, suggesting involvement of one or more nucleoside transporters. Here, we confirmed the presence of an NMBPR-mediated interaction with Ara-C resulting in a 2.5-fold increased exposure. The interaction was unrelated to altered blood cell distribution, and subsequent studies indicated that the disposition of Ara-C was unaffected in mice with a deficiency of postulated candidate transporters, including ENT1, OCTN1, OATP1B2, and MATE1. These studies indicate the involvement of an unknown NBMPR-sensitive Ara-C transporter that impacts the pharmacokinetic properties of this clinically important agent.

CPX-351 exhibits hENT-independent uptake and can be potentiated by fludarabine in leukaemic cells lines and primary refractory AML

CPX-351, a liposomal formulation co-encapsulating cytarabine and daunorubicin (DNR) in a synergistic 5:1 M ratio, has shown favourable response in newly diagnosed elderly high-risk AML. This study assessed intracellular ara-CTP levels following in vitro exposure of human immortalised leukaemic cell lines and primary AML blasts to CPX-351, and investigated fludarabine potentiation of intracellular ara-CTP formation from CPX-351. Comparison of intracellular handling of CPX-351 to cytarabine in HL-60 cells indicated slower conversion to ara-CTP for CPX-351, but equivalent cytotoxicity to cytarabine and combined DNR/cytarabine (DA) at 48 h, mostly likely reflecting the need for intracellular liposome processing to release encapsulated drugs. Further assessment demonstrated cytotoxicity of CPX-351 to be superior to DA at 48 and 72 h in cytarabine-resistant THP-1 cells (p < 0.001), and this effect could not be inhibited upon blockade of human equilibrative nucleoside transporter (hENT) function with dipyridamole. Assessment of Flu-CPX in primary blasts from presentation AML patients (n = 5) demonstrated a more rapid and pronounced potentiation of ara-CTP from CPX-351 than in immortalised cell lines, with 4/5 patients showing significant increases in ara-CTP, notably for those that went on to fail induction and relapse treatment in vivo (n = 3). This suggests a favourable impact on patient outcome from Flu-CPX.

Targeting the Adenosinergic Axis in Chronic Lymphocytic Leukemia: A Way to Disrupt the Tumor Niche?

Targeting adenosine triphosphate (ATP) metabolism and adenosinergic signaling in cancer is gaining momentum, as increasing evidence is showing their relevance in tumor immunology and biology. Chronic lymphocytic leukemia (CLL) results from the expansion of a population of mature B cells that progressively occupies the bone marrow (BM), the blood, and peripheral lymphoid organs. Notwithstanding significant progress in the treatment of these patients, the cure remains an unmet clinical need, suggesting that novel drugs or drug combinations are needed. A unique feature of CLL is its reliance on micro-environmental signals for proliferation and cell survival. We and others have shown that the lymphoid niche, an area of intense interactions between leukemic and bystander non-tumor cells, is a typically hypoxic environment. Here adenosine is generated by leukemic cells, as well as by cells of myeloid origin, acting through autocrine and paracrine mechanisms, ultimately affecting tumor growth, limiting drug responses, and skewing the immune cells towards a tolerant phenotype. Hence, understanding the mechanisms through which this complex network of enzymes, receptors, and metabolites functions in CLL, will pave the way to the use of pharmacological agents targeting the system, which, in combination with drugs targeting leukemic cells, may get us one step closer to curing these patients.

Pharmacokinetics and Model-Based Dosing to Optimize Fludarabine Therapy in Pediatric Hematopoietic Cell Transplant Recipients

A prospective multicenter study was conducted to characterize the pharmacokinetics (PK) and pharmacodynamics (PD) of fludarabine plasma (f-ara-a) and intracellular triphosphate (f-ara-ATP) in children undergoing hematopoietic cell transplantation (HCT) and receiving fludarabine with conditioning. Plasma and peripheral blood mononuclear cells (PBMCs) were collected over the course of therapy for quantitation of f-ara-a and f-ara-ATP. Nonlinear mixed-effects modeling was used to develop the PK model, including identification of covariates impacting drug disposition. Data from a total of 133 children (median age, 5 years; range, .2 to 17.9) undergoing HCT for a variety of malignant and nonmalignant disorders were available for PK-PD modeling. The implementation of allometric scaling of PK parameters alone was insufficient to describe drug clearance, particularly in very young children. Renal impairment was predicted to increase drug exposure across all ages. The rate of f-ara-a entry into PBMCs (expressed in pmoles per million cells) decreased over the course of therapy, resulting in 78% lower f-ara-ATP after the fourth dose (1.7 pmoles/million cells [range, .2 to 7.2]) compared with first dose (7.9 pmoles/million cells [range, .7 to 18.2]). The overall incidence of treatment-related mortality (TRM) was low at 3% and 8% at days 60 and 360, respectively, and no association with f-ara-a exposure and TRM was found. In the setting of malignancy, disease-free survival was highest at 1 year after HCT in subjects achieving a systemic f-ara-a cumulative area under the curve (cAUC) greater than 15 mg*hour/L compared to patients with a cAUC less than 15 mg*hour/L (82.6% versus 52.8% P = .04). These results suggest that individualized model-based dosing of fludarabine in infants and young children may reduce morbidity and mortality through improved rates of disease-free survival and limiting drug-related toxicity. ClinicalTrials.gov Identifier: NCT01316549.

Relationship between Azathioprine metabolites and therapeutic efficacy in Chinese patients with neuromyelitis optica spectrum disorders

BACKGROUND

Neuromyelitis optica spectrum disorders (NMOSD) are demyelinating autoimmune diseases in the central nervous system (CNS) that are characterized by a high relapse rate and the presence of anti-aquaporin 4 antibodies (AQP4-IgG) in the serum. Azathioprine (AZA) is a first-line immunomodulatory drug that is widely used for the treatment of patients with NMOSD. However, the efficacy and safety of AZA vary in different individuals.

METHOD

Thirty-two patients with NMOSD who regularly took AZA were enrolled in the study at Beijing Tiantan Hospital, Capital Medical University. The efficacy of AZA was evaluated using the expanded disability status scale (EDSS) and the annual relapse rate (ARR). The erythrocyte concentrations of AZA metabolites were detected using an LC-MS/MS method.

RESULTS

The erythrocyte concentrations of 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine nucleotides (6-MMPNs) were 202.03 ± 63.35 pmol/8*10⁸ RBC and 1618.90 ± 1607.06 pmol/8*10⁸ RBC, respectively. After the patients had received AZA therapy for more than one year, the EDSS score decreased from 5.21 ± 0.24 to 2.57 ± 0.33 (p < 0.0001), and the ARR decreased from 1.41 ± 0.23 to 0.36 ± 0.09 (p < 0.0001). The 6-TGN and 6-MMPN levels were significantly different between the non-relapsed and relapsed groups (p < 0.0001, p = 0.006, respectively). A higher ARR was significantly correlated with higher erythrocyte concentrations of 6-TGNs (p < 0.0001) and 6-MMPNs (p = 0.004).

CONCLUSION

AZA can reduce the EDSS score and ARR in NMOSD patients. Additionally, the efficacy of AZA is significantly related to the erythrocyte concentrations of 6-TGNs and 6-MMPNs. Within the safe upper limits, a higher concentration of 6-TGNs is associated with better efficacy of AZA.

TRIAL REGISTRATION NUMBER

ISRCTN16551495 , retrospectively registered on May 22, 2017.

The Adenosinergic System as a Therapeutic Target in the Vasculature: New Ligands and Challenges

Adenosine is an adenine base purine with actions as a modulator of neurotransmission, smooth muscle contraction, and immune response in several systems of the human body, including the cardiovascular system. In the vasculature, four P1-receptors or adenosine receptors—A₁, A_2A, A_2B and A₃—have been identified. Adenosine receptors are membrane G-protein receptors that trigger their actions through several signaling pathways and present differential affinity requirements. Adenosine is an endogenous ligand whose extracellular levels can reach concentrations high enough to activate the adenosine receptors. This nucleoside is a product of enzymatic breakdown of extra and intracellular adenine nucleotides and also of S-adenosylhomocysteine. Adenosine availability is also dependent on the activity of nucleoside transporters (NTs). The interplay between NTs and adenosine receptors’ activities are debated and a particular attention is given to the paramount importance of the disruption of this interplay in vascular pathophysiology, namely in hypertension., The integration of important functional aspects of individual adenosine receptor pharmacology (such as in vasoconstriction/vasodilation) and morphological features (within the three vascular layers) in vessels will be discussed, hopefully clarifying the importance of adenosine receptors/NTs for modulating peripheral mesenteric vascular resistance. In recent years, an increase interest in purine physiology/pharmacology has led to the development of new ligands for adenosine receptors. Some of them have been patented as having promising therapeutic activities and some have been chosen to undergo on clinical trials. Increased levels of endogenous adenosine near a specific subtype can lead to its activation, constituting an indirect receptor targeting approach either by inhibition of NT or, alternatively, by increasing the activity of enzymes responsible for ATP breakdown. These findings highlight the putative role of adenosinergic players as attractive therapeutic targets for cardiovascular pathologies, namely hypertension, heart failure or stroke. Nevertheless, several aspects are still to be explored, creating new challenges to be addressed in future studies, particularly the development of strategies able to circumvent the predicted side effects of these therapies.

Stem Cell-Based Human Blood-Brain Barrier Models for Drug Discovery and Delivery

The development of novel neuropharmaceuticals requires the evaluation of blood-brain barrier (BBB) permeability and toxicity. Recent studies have highlighted differences in the BBB among different species, with the most important differences involving the expression of P-glycoprotein (P-gp), multidrug resistance-associated proteins, transporters, and claudins. In addition, functional studies have shown that brain pharmacokinetics of P-glycoprotein substrates are different in humans and rodents. Therefore, human BBB models may be an important platform for initial drug screening before in vivo studies. This strategy might help to reduce costs in drug development and failures in clinical studies. We review the differences in the BBB among species, recent advances in the generation of human BBB models, and their applications in drug discovery and delivery.

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.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

Human organic cation transporter 1 (hOCT1) as a mediator of bendamustine uptake and cytotoxicity in chronic lymphocytic leukemia (CLL) cells

Bendamustine is used in the treatment of chronic lymphocytic leukemia (CLL). Routes for bendamustine entry into target cells are unknown. This study aimed at identifying transporter proteins implicated in bendamustine uptake. Our results showed that hOCT1 is a bendamustine transporter, as bendamustine could cis-inhibit the uptake of a canonical hOCT1 substrate, with a Ki in the micromolar range, consistent with the EC50 values of the cytotoxicity triggered by this drug in HEK293 cells expressing hOCT1. hOCT1 polymorphic variants determining impaired bendamustine-transporter interaction, consistently reduced bendamustine cytotoxicity in HEK293 cells stably expressing them. Exome genotyping of the SLC22A1 gene, encoding hOCT1, was undertaken in a cohort of 241 CLL patients. Ex vivo cytotoxicity to bendamustine was measured in a subset of cases and shown to correlate with SLC22A1 polymorphic variants. In conclusion, hOCT1 is a suitable bendamustine transporter, thereby contributing to its cytotoxic effect depending upon the hOCT1 genetic variants expressed.

Localized delivery of chemotherapy to the cervix for radiosensitization

OBJECTIVE

Chemoradiation is the mainstay of therapy for advanced cervical cancer, with the most effective treatment regimens involving combinations of radiosensitizing agents. However, administration of radiosensitizing chemotherapeutics concurrently with pelvic radiation is not without side effects. The aim of this study was to examine the utility of localized drug delivery as a means of improving drug targeting of radiosensitizing chemotherapeutics to the cervix while limiting systemic toxicities.

METHODS

An initial proof-of-concept study was performed in 14 healthy women following local administration of diazepam utilizing a novel cervical delivery device (CerviPrep™). Uterine vein and peripheral blood samples were collected and diazepam was measured using a GC-MS method. In the follow-up study, gemcitabine was applied to the cervix in 17 women undergoing hysterectomy for various gynecological malignancies. Cervical tissue, uterine vein blood samples, and peripheral plasma were collected, and gemcitabine and its deaminated metabolite 2',2'-difluorodeoxyuridine (dFdU) were measured using HPLC-UV and LC/MS methods.

RESULTS

Targeted delivery of diazepam to the cervix was consistent with parent drug detectable in the uterine vein of 13 of 14 women. In the second study, pharmacologically relevant concentrations of gemcitabine (0.01-6.6 nmol/g tissue) were detected in the cervical tissue of 11 of 16 available specimens with dFdU measureable in 15 samples (0.04-8.8 nmol/g tissue). Neither gemcitabine nor its metabolites were detected in the peripheral plasma of any subject.

CONCLUSIONS

Localized drug delivery to the cervix is possible and may be useful in limiting toxicity associated with intravenous administration of chemotherapeutics for radiosensitization.

Nucleoside transporters: biological insights and therapeutic applications

Nucleoside transporters play important physiological roles by regulating intra- and extra-cellular concentrations of purine and pyrimidine (deoxy)nucleosides. This review describes the biological function and activity of the two major families of membrane nucleoside transporters that exist in mammalian cells. These include equilibrative nucleoside transporters that transport nucleosides in a gradient-dependent fashion and concentrative nucleoside transporters that import nucleosides against a gradient by coupling movement with sodium transport. Particular emphasis is placed on describing the roles of nucleoside transport in normal physiological processes, including inflammation, cardiovascular function and nutrient transport across the blood–brain barrier. In addition, the role of nucleoside transport in pathological conditions such as cardiovascular disease and cancer are discussed. The potential therapeutic applications of manipulating nucleoside transport activities are discussed, focusing on nucleoside analogs as anti-neoplastic agents. Finally, we discuss future directions for the development of novel chemical entities to measure nucleoside transport activity at the cellular and organismal level.

Bone marrow stromal cells modulate mouse ENT1 activity and protect leukemia cells from cytarabine induced apoptosis

Background

Despite a high response rate to chemotherapy, the majority of patients with acute myeloid leukemia (AML) are destined to relapse due to residual disease in the bone marrow (BM). The tumor microenvironment is increasingly being recognized as a critical factor in mediating cancer cell survival and drug resistance. In this study, we propose to identify mechanisms involved in the chemoprotection conferred by the BM stroma to leukemia cells.

Methods

Using a leukemia mouse model and a human leukemia cell line, we studied the interaction of leukemia cells with the BM microenvironment. We evaluated in vivo and in vitro leukemia cell chemoprotection to different cytotoxic agents mediated by the BM stroma. Leukemia cell apoptosis was assessed by flow cytometry and western blotting. The activity of the equilibrative nucleoside transporter 1 (ENT1), responsible for cytarabine cell incorporation, was investigated by measuring transport and intracellular accumulation of ³H-adenosine.

Results

Leukemia cell mobilization from the bone marrow into peripheral blood in vivo using a CXCR4 inhibitor induced chemo-sensitization of leukemia cells to cytarabine, which translated into a prolonged survival advantage in our mouse leukemia model. In vitro, the BM stromal cells secreted a soluble factor that mediated significant chemoprotection to leukemia cells from cytarabine induced apoptosis. Furthermore, the BM stromal cell supernatant induced a 50% reduction of the ENT1 activity in leukemia cells, reducing the incorporation of cytarabine. No protection was observed when radiation or other cytotoxic agents such as etoposide, cisplatin and 5-fluorouracil were used.

Conclusion

The BM stroma secretes a soluble factor that significantly protects leukemia cells from cytarabine-induced apoptosis and blocks ENT1 activity. Strategies that modify the chemo-protective effects mediated by the BM microenvironment may enhance the benefit of conventional chemotherapy for patients with AML.

Translocation of nucleoside analogs across the plasma membrane in hematologic malignancies

Nucleoside analogs are currently used in the treatment of various hematologic malignancies due to their ability to induce apoptosis of lymphoid cells. For nucleoside-derived drugs to exert their action, they must enter cells via nucleoside transporters from two gene families, SLC28 and SLC29 (CNT and ENT, respectively). Once inside the cell, these drugs must be phosphorylated to their active forms. In contrast, some members of the ATP-binding cassette (ABC) protein family have been identified as responsible for the efflux of the phosphorylated forms of these nucleoside-derived drugs. Here, we review the main nucleoside analogs used in hematologic malignancies and focus especially on those that are currently used in chronic lymphocytic leukemia (CLL). Moreover, we discuss the pharmacological profile of the nucleoside transporters, which determines the bioavailability of and cell sensitivity to these nucleoside-derived drugs. We also discuss the expression of nucleoside transporters and their activities in CLL as well as the possibility of modulating these transporter activities as a means of modulating intracellular drug availability and, consequently, responsiveness to therapy.

Impacts of blood-brain barrier in drug delivery and targeting of brain tumors

INTRODUCTION

Entry of blood circulating agents into the brain is highly selectively con-trolled by specific transport machineries at the blood brain barrier (BBB), whose excellent barrier restrictiveness make brain drug delivery and targeting very challenging.

METHODS

Essential information on BBB cellular microenvironment were reviewed and discussed towards impacts of BBB on brain drug delivery and targeting.

RESULTS

Brain capillary endothelial cells (BCECs) form unique biological structure and architecture in association with astrocytes and pericytes, in which microenvironment the BCECs express restrictive tight junctional complexes that block the paracellular inward/outward traverse of biomolecules/compounds. These cells selectively/specifically control the transportation process through carrier and/or receptor mediated transport machineries that can also be exploited for the delivery of pharmaceuticals into the brain. Intelligent molecular therapies should be designed using such transport machineries for the efficient delivery of designated drugs into the brain. For better clinical outcomes, these smart pharmaceuticals should be engineered as seamless nanosystems to provide simultaneous imaging and therapy (multimodal theranostics).

CONCLUSION

The exceptional functional presence of BBB selectively controls inward and outward transportation mechanisms, thus advanced smart multifunctional nanomedicines are needed for the effective brain drug delivery and targeting. Fully understanding the biofunctions of BBB appears to be a central step for engineering of intelligent seamless therapeutics consisting of homing device for targeting, imaging moiety for detecting, and stimuli responsive device for on-demand liberation of therapeutic agent.

Phase 2 trial of clofarabine in combination with etoposide and cyclophosphamide in pediatric patients with refractory or relapsed acute lymphoblastic leukemia

The outcomes in children with refractory/relapsed (R/R) acute lymphoblastic leukemia (ALL) are dismal. The efficacy and safety of intravenous clofarabine 40 mg/m(2) per day, cyclophosphamide 440 mg/m(2) per day, and etoposide 100 mg/m(2) per day for 5 consecutive days in pediatric patients with R/R ALL was evaluated in this phase 2 study. The primary endpoint was overall response rate (complete remission [CR] plus CR without platelet recovery [CRp]). Among the 25 patients (median age, 14 years; pre-B cell ALL, 84%; ≥ 2 prior regimens: 84%; refractory to previous regimen: 60%), the overall response rate was 44% (7 CR, 4 CRp) with a 67.3-week median duration or remission censored at last follow-up. Most patients proceeded to alternative therapy, and 10 patients (40%) received hematopoietic stem cell transplantation. Six patients (24%) died because of treatment-related adverse events associated with infection, hepatotoxicity, and/or multiorgan failure. The study protocol was amended to exclude patients with prior hematopoietic stem cell transplantation after 4 of the first 8 patients developed severe hepatotoxicity suggestive of veno-occlusive disease. No additional cases of veno-occlusive disease occurred. The regimen offered encouraging response rates and sustained remission in R/R patients. Future investigation should include exploration of patient selection, dosing, and supportive care. This trial was registered at www.clinicaltrials.gov as #NCT00315705.

Holoclone forming cells from pancreatic cancer cells enrich tumor initiating cells and represent a novel model for study of cancer stem cells

Background

Pancreatic cancer is one of the direct causes of cancer-related death. High level of chemoresistance is one of the major obstacles of clinical treatment. In recent years, cancer stem cells have been widely identified and indicated as the origin of chemoresistance in multi-types of solid tumors. Increasing evidences suggest that cancer stem cells reside in the cells capable of forming holoclones continuously. However, in pancreatic cancer, holoclone-forming cells have not been characterized yet. Therefore, the goal of our present study was to indentify the holoclone-forming pancreatic cancer stem cells and develop an in vitro continuous colony formation system, which will greatly facilitate the study of pancreatic cancer stem cells.

Methodology/Principal Findings

Pancreatic cancer cell line BxPC3 was submitted to monoclonal cultivation to generate colonies. Based on the morphologies, colonies were classified and analyzed for their capacities of secondary colony formation, long-term survival in vitro, tumor formation in vivo, and drug resistance. Flowcytometry and quantitative RT-PCR were performed to detect the expression level of cancer stem cells associated cell surface markers, regulatory genes and microRNAs in distinct types of colonies. Three types of colonies with distinct morphologies were identified and termed as holo-, mero-, and paraclones, in which only holoclones generated descendant colonies of all three types in further passages. Compared to mero- and paraclones, holoclones possessed higher capacities of long-term survival, tumor initiation, and chemoresistance. The preferential expression of cancer stem cells related marker (CXCR4), regulatory genes (BMI1, GLI1, and GLI2) and microRNAs (miR-214, miR-21, miR-221, miR-222 and miR-155) in holoclones were also highlighted.

Conclusions/Significance

Our results indicate that the pancreatic tumor-initiating cells with high level of chemoresistance were enriched in holoclones derived from BxPC3 cell line. Generation of holoclones can serve as a novel model for studying cancer stem cells, and attribute to developing new anti-cancer drugs.

Quantitative targeted absolute proteomics of human blood-brain barrier transporters and receptors

We have obtained, for the first time, a quantitative protein expression profile of membrane transporters and receptors in human brain microvessels, that is, the blood-brain barrier (BBB). Brain microvessels were isolated from brain cortexes of seven males (16-77 years old) and protein expression of 114 membrane proteins was determined by means of a liquid chromatography-tandem mass spectrometric quantification method using recently established in-silico peptide selection criteria. Among drug transporters, breast cancer resistance protein showed the most abundant protein expression (8.14 fmol/μg protein), and its expression level was 1.85-fold greater in humans than in mice. By contrast, the expression level of P-glycoprotein in humans (6.06 fmol/μg protein) was 2.33-fold smaller than that of mdr1a in mice. The organic anion transporters reported in rodent BBB, that is, multidrug resistance-associated protein, organic anion transporter and organic anion-transporting polypeptide family members, were under limit of quantification in humans, except multidrug resistance-associated protein 4 (0.195 fmol/μg protein). Among detected transporters and receptors for endogenous substances, the glucose transporter 1 level was similar to that of mouse, while the L-type amino acid transporter 1 level was fivefold smaller than that of mouse. These findings should be useful for understanding human BBB function and its differences from that in mouse.

Southwest Oncology Group Study S0530: a phase 2 trial of clofarabine and cytarabine for relapsed or refractory acute lymphocytic leukaemia

Clofarabine and cytarabine target different steps in DNA synthesis and replication, are synergistic in vivo, and have non-overlapping toxicities, making this combination a potentially promising treatment for acute lymphocytic leukaemia. Thirty-seven patients were treated. The median age was 41 years, 44% of patients were either in ≥2nd relapse or had refractory disease and 59% of patients had poor risk cytogenetics. Six out of 36 patients (17%) achieved a complete remission with or without complete count recovery; median overall survival was 3 months. Nucleoside transporter expression did not predict outcome. This regimen lacked sufficient activity to warrant further testing.

Drug transporter pharmacogenetics in nucleoside-based therapies

This article focuses on the different types of transporter proteins that have been implicated in the influx and efflux of nucleoside-derived drugs currently used in the treatment of cancer, viral infections (i.e., AIDS) and other conditions, including autoimmune and inflammatory diseases. Genetic variations in nucleoside-derived drug transporter proteins encoded by the gene families SLC15, SLC22, SLC28, SLC29, ABCB, ABCC and ABCG will be specifically considered. Variants known to affect biological function are summarized, with a particular emphasis on those for which clinical correlations have already been established. Given that relatively little is known regarding the genetic variability of the players involved in determining nucleoside-derived drug bioavailability, it is anticipated that major challenges will be faced in this area of research.

Human nucleoside transporters: biomarkers for response to nucleoside drugs

This review describes recent advances in developing human nucleoside transporters (hNTs) as biomarkers to predict response to nucleoside analog drugs with clinical activity. Understanding processes that contribute to drug response or lack thereof will provide strategies to potentiate efficacy or avoid toxicities of nucleoside analog drugs. hNT abundance, evaluated by immunohistochemical methods, has shown promise as a predictive marker to assess clinical drug response that could be used to identify patients who would most likely benefit from nucleoside analog drug treatment.

High fludarabine exposure and relationship with treatment-related mortality after nonmyeloablative hematopoietic cell transplantation

Despite its common use in nonmyeloablative preparative regimens, the pharmacokinetics of fludarabine are poorly characterized in hematopoietic cell transplantation (HCT) recipients and exposure-response relationships remain undefined. Our objective of this study was to evaluate the association between plasma F-ara-A exposure, the systemically circulating moiety of fludarabine, and engraftment, acute graft vs host disease (GVHD), treatment-related mortality (TRM) and survival after HCT. The preparative regimen consisted of cyclophosphamide 50 mg/kg/day i.v. day −6; plus fludarabine 30-40 mg/m²/day i.v. on days −6 to −2 and TBI 200 cGy on day −1. F-ara-A pharmacokinetics were performed with the first dose of fludarabine in 87 adult patients. Median (range) F-ara-A AUC _(0-∞) was 5.0 ug*hr/mL (2.0-11.0), clearance 15.3 L/hour (6.2-36.6), C_min 55 ng/mL (17-166), and concentration on day_zero 16.0 ng/mL (0.1-144.1). Despite dose reductions, patients with renal insufficiency had higher F-ara-A exposures. There was strong association between high plasma concentrations of F-ara-A and increased risk of TRM and reduced overall survival. Patients with an AUC_(0-∞) greater than 6.5 ug*hr/mL had 4.56 greater risk of TRM and significantly lower survival. These data suggest that clinical strategies are needed to optimize dosing of fludarabine to prevent overexposure and toxicity in HCT.

Molecular outcome prediction in mantle cell lymphoma

Mantle cell lymphoma (MCL), a subtype of B-cell non-Hodgkin lymphoma, is usually incurable with current therapeutic approaches and the clinical course displays considerable variability. Objective assessment of the efficacy of new and more tailored treatment strategies requires deeper molecular insights into the disease and more individual risk assessment. The molecular feature of tumor cell proliferation as measured by Ki-67 immunohistochemistry or, more precisely, by microarray-based gene-expression profiling, has been shown to be of strong predictive value in MCL. The recently proposed quantitative reverse transcription-PCR based five-gene model survival predictor is applicable to fresh-frozen and routinely obtained formalin-fixed and paraffin-embedded tumor tissues, and provides the potential to investigate its prognostic value in prospective clinical trials of MCL patients.

p65 activity and ZAP-70 status predict the sensitivity of chronic lymphocytic leukemia cells to the selective IkappaB kinase inhibitor BMS-345541

PURPOSE

Constitutive nuclear factor-kappaB (NF-kappaB) activation has been implicated in the pathogenesis of chronic lymphocytic leukemia (CLL). Our purpose was to characterize the molecular mechanisms underlying for the selective IkappaB kinase inhibitor BMS-345541 in CLL cells together with the analysis of its combination with several antineoplasic drugs.

EXPERIMENTAL DESIGN

Primary cells from 34 CLL patients were incubated with different doses of BMS-345541. NF-kappaB DNA-binding activity was analyzed by ELISA-based kits and the characterization of the apoptotic pathway was done by flow cytometry, immunoblotting, quantitative reverse transcription-PCR, and immunofluorescence techniques.

RESULTS

BMS-345541 selectively induced apoptosis in CLL cells in the low micromolar range irrespective of p53 status. Noteworthy, the high ZAP-70 group was significantly more sensitive to BMS-345541 than the low ZAP-70 group, in correlation with high levels of p65 phosphorylation and DNA-binding activity. Following NF-kappaB inhibition, BMS-345541 led to induction of the mitochondrial apoptotic pathway and activation of both caspase-dependent and caspase-independent factors. Moreover, BMS-345541-induced apoptosis was accompanied by down-regulation of several antiapoptotic NF-kappaB-target genes, including both BCL2 family members and apoptotic endogenous inhibitors. In addition, we showed a strong synergism between BMS-345541 and conventional chemotherapeutics such as mitoxantrone and dexamethasone as well as with new promising drugs such as the BH3-mimetic GX15-070/Obatoclax or the anti-TRAIL-R1 monoclonal antibody mapatumumab.

CONCLUSIONS

These data confirm that NF-kappaB is a relevant target in CLL and indicate that inhibitors of IkappaB kinase, alone or in combination, represent a novel therapeutic strategy for the treatment of CLL patients, especially for the group with high ZAP-70.

Development of gene therapy in association with clinically used cytotoxic deoxynucleoside analogues

The clinical use of cytotoxic deoxynucleoside analogues is often limited by resistance mechanisms due to enzymatic deficiency, or high toxicity in nontumor tissues. To improve the use of these drugs, gene therapy approaches have been proposed and studied, associating clinically used deoxynucleoside analogues such as araC and gemcitabine and suicide genes or myeloprotective genes. In this review, we provide an update of recent results in this area, with particular emphasis on human deoxycytidine kinase, the deoxyribonucleoside kinase from Drosophila melanogaster, purine nucleoside phosphorylase from Escherichia coli, and human cytidine deaminase. Data from literature clearly show the feasibility of these systems, and clinical trials are warranted to conclude on their use in the treatment of cancer patients.

Current status of older and new purine nucleoside analogues in the treatment of lymphoproliferative diseases

For the past few years more and more new cytotoxic agents active in the treatment of hematological malignancies have been synthesized and become available for either in vitro studies or clinical trials. Among them the class of antineoplastic drugs belonging to the purine nucleoside analogues group (PNAs) plays an important role. Three of them: pentostatin (DCF), cladribine (2-CdA) and fludarabine (FA) were approved by Food and Drug Administration (FDA) for the treatment of hematological malignancies. Recently three novel PNAs: clofarabine (CAFdA), nelarabine (ara-G) and forodesine (immucillin H, BCX-1777) have been synthesized and introduced into preclinical studies and clinical trials. These agents seem to be useful mainly for the treatment of human T-cell proliferative disorders and they are currently undergoing clinical trials in lymphoid malignancies. However, there are also several studies suggesting the role of these drugs in B-cell malignancies. This review will summarize current knowledge concerning the mechanism of action, pharmacologic properties, clinical activity and toxicity of PNAs accepted for use in clinical practice, as well as new agents available for clinical trials.

Identification of a novel point mutation in ENT1 that confers resistance to Ara-C in human T cell leukemia CCRF-CEM cells

The genetic basis for the Ara-C resistance of CCRF-CEM Ara-C/8C leukemia cells was investigated. DNA sequencing revealed that these cells expressed an equilibrative nucleoside transporter 1 (ENT1) with a single missense mutation resulting in glycine to arginine replacement (G24R). To test the importance of this residue, additional G24 mutants were created and examined for [3H]-uridine and [3H]-Ara-C uptake. Both a G24E and G24A mutant showed reduced ENT1-dependent activity. An EGFP-tagged G24R ENT1 displayed plasma membrane localization even though it was unable to bind [3H]-NBMPR, an ENT1-specific inhibitor. These results define G24 as critical amino acid for ENT1 nucleoside uptake and suggest that mutations in TM1 may provide a mechanism for Ara-C resistance in CCRF-CEM Ara-C/8C cells.

Physiological and pharmacological roles of nucleoside transporter proteins

Nucleoside transporter proteins, CNT and ENT, encoded by gene families SLC28 and SLC29, respectively, mediate the uptake of natural nucleosides (among them adenosine) and are major routes of entry for a variety of nucleoside analogs used in anticancer and antiviral therapies. Expression of NT proteins is apparently redundant in most cell types, and the elucidation of their particular physiological roles still remains elusive. Moreover, transporter-mediated uptake of nucleoside-derived anticancer drugs is crucial for the pharmacogenomic response triggered by these molecules in tumor cells. This review focuses on recent data demonstrating that nucleoside transporters, particularly CNTs, can play physiological roles other than salvage, whereas particular NT isoforms can significantly contribute to the transcriptomic response triggered by nucleoside analogs in cancer cells.

Five-gene model to predict survival in mantle-cell lymphoma using frozen or formalin-fixed, paraffin-embedded tissue

PURPOSE

Despite the common underlying translocation t(11;14) involving cyclin D1 that is present in nearly all cases of mantle-cell lymphoma (MCL), the clinical course of the disease is highly variable. The aim of the present study was to develop a quantitative gene expression-based model to predict survival in newly diagnosed patients with MCL that involves a minimum number of genes and is applicable to fresh-frozen and formalin-fixed, paraffin-embedded (FFPE) tumor samples.

PATIENTS AND METHODS

The expression of 33 genes with potential prognostic and pathogenetic impact in MCL was analyzed using quantitative reverse-transcription polymerase chain reactions (qRT-PCR) in a low-density array format in frozen tumor samples from 73 patients with MCL. Multivariate Cox methods and stepwise algorithms were applied to build gene expression-based survival predictors. An optimized five-gene model was subsequently applied to FFPE tumor samples from 13 patients with MCL from the initial series and to 42 independent MCL samples.

RESULTS

The optimized survival predictor was composed of the five genes RAN, MYC, TNFRSF10B, POLE2, and SLC29A2 and was validated for application in FFPE tissue samples. It allowed the survival prediction of patients with MCL with widely disparate clinical outcome and was superior to the immunohistochemical marker Ki-67, an established prognostic factor in MCL.

CONCLUSION

We here present a validated qRT-PCR-based test for survival prediction in patients with MCL that is applicable to fresh frozen as well as to FFPE tissue specimens. This test may prove useful to guide individualized treatment approaches for patients with MCL.

Adenoviral-mediated overexpression of human equilibrative nucleoside transporter 1 (hENT1) enhances gemcitabine response in human pancreatic cancer

Nucleoside-derived anticancer agents must be transported across the plasma membrane as a preliminary step to their conversion into active drugs. Hence, modulation of a specific nucleoside transporter may affect bioavailability and contribute significantly to sensitizing tumor cells to these anticancer agents. We have generated and functionally characterized a new recombinant adenovirus (Ad-hENT1) that has allowed us to overexpress the equilibrative nucleoside transporter hENT1 and to analyze its effects in human pancreatic tumor cells. Overexpression of hENT1 is associated with changes in cell cycle profile, in a variable manner depending on the particular cell type, thus suggesting a metabolic link between hENT1-mediated transport processes and the enzymatic machinery responsible for intracellular nucleoside metabolism. When assayed in vivo in a human pancreatic adenocarcinoma xenograft, intratumoral Ad-hENT1 injection improved the therapeutic response to gemcitabine. In summary, hENT1 overexpression is associated with alterations in nucleoside enzymatic machinery and cell cycle progression in cultured cells and enhances gemcitabine action in vivo.

A novel phenotypic method to determine fludarabine triphosphate accumulation in T-lymphocytes from hematopoietic cell transplantation patients

PURPOSE

Fludarabine is an integral anticancer agent for patients with chronic lymphocytic leukemia (CLL) and those receiving conditioning regimens prior to allogeneic hematopoietic cell transplantation (HCT). An individual's response to fludarabine may be influenced by the amount of CD4(+) and CD8(+) T-lymphocyte suppression. Fludarabine undergoes cellular uptake and activation to form the cytotoxic metabolite, fludarabine triphosphate (F-ara-ATP).

METHODS

We have previously developed a highly sensitive LC-MS method to quantitate intracellular F-ara-ATP concentrations in a leukemic cell line. However, quantitation of F-ara-ATP concentrations within CD4(+) and CD8(+) T-lymphocytes from pharmacokinetic blood samples obtained from patients receiving fludarabine therapy is not feasible because of the limited number of T-lymphocytes that can be isolated from each blood sample. Thus, we sought to determine F-ara-ATP accumulation after ex vivo exposure of freshly isolated human CD4(+) or CD8(+) T-lymphocytes to fludarabine. The method was optimized in T-lymphocytes obtained from healthy volunteers, and proved to be a feasible method to determine F-ara-ATP accumulation in patients undergoing HCT.

RESULTS

Considerable variability was observed in F-ara-ATP accumulation in HCT patients (10.5- and 12.5-fold in CD4(+) and CD8(+) cells, respectively), compared to healthy volunteers (1.6- and 1.9-fold in CD4(+) and CD8(+) cells, respectively). Larger variability was also observed in gene expression of transporters and enzymes involved in F-ara-ATP accumulation in HCT patients; however, F-ara-ATP accumulation was not correlated with gene expression, which is in agreement with previous studies.

CONCLUSIONS

The quantitation of F-ara-ATP accumulation in T-lymphocytes provides a novel tool to evaluate patient sensitivity to fludarabine. This tool can be used in future studies to evaluate whether intracellular F-ara-ATP accumulation is associated with efficacy and/or toxicity in patients receiving fludarabine.

Expression and functionality of anti-human immunodeficiency virus and anticancer drug uptake transporters in immune cells

Almost all drugs used in anti-human immunodeficiency virus (HIV)-1 and anticancer therapies require membrane proteins to get into the cell to develop their proper activity. Nevertheless, little is known regarding the expression and activity of specific carriers involved in the uptake of these drugs in immune cells. Here, we assessed the mRNA levels, protein expression profile, and activity of the gene families SLC28 (coding for concentrative nucleoside transporters, hCNT1-3), SLC29 (equilibrative nucleoside transporters, hENT1-2), and SLC22 (organic cation transporters, hOCT1-3 and hOCTN1-2). Both hENTs and hCNT2 were abundant in primary lymphocytes, with a preferential activity of hENT1. A significant up-regulation in hENTs expression (100-fold) and activity (30-fold) was seen under stimulation of primary T lymphocytes. In contrast, monocytes, monocyte-derived macrophages (MDMs), and immature monocyte-derived dendritic cells predominantly expressed hCNT3, a functional transporter in MDMs. Finally, in immune cells, hOCTs showed a more heterogeneous expression profile and a lower activity than human nucleoside transporters (hNTs), although up-regulation of hOCTs also occurred upon lymphocyte activation. Overall, the expression and activity of most of the studied transporters emphasize their relevance in relation to anti-HIV and anticancer therapies. The identification of the transporter involved in each specific drug uptake in immune cells could help to optimize pharmacological therapeutic responses.

The role of mitochondrial and plasma membrane nucleoside transporters in drug toxicity

Many anticancer and antiviral drugs are nucleoside analogues, which interfere with nucleotide metabolism and DNA replication to produce pharmacological effects. Clinical efficacy and toxicity of nucleoside drugs are closely associated with nucleoside transporters because they mediate the transport of nucleoside drugs across biological membranes. Two families of human nucleoside transporters (equilibrative nucleoside transporters and concentrative nucleoside transporters) have been extensively studied for several decades. They are widely distributed, from the plasma membrane to membranes of organelles such as mitochondria, and the distribution differs in different tissues. In addition, they have different specificities to nucleoside drugs. The characteristics of equilibrative and concentrative nucleoside transporters affect the therapeutic outcomes achieved with anticancer and antiviral nucleoside drugs. In this review, an overview of the role of mitochondrial and plasma membrane nucleoside transporters in nucleoside drug toxicity is provided. Rational design and therapeutic application of nucleoside analogues are also discussed.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

Human equilibrative nucleoside transporter-1 (hENT1) is required for the transcriptomic response of the nucleoside-derived drug 5′-DFUR in breast cancer MCF7 cells

Nucleoside analogues are broadly used in cancer treatment. Although nucleoside metabolism is a necessary step in the development of their cytotoxicity, mediated transport across the plasma membrane might be needed for nucleoside-derived drugs to exert their pharmacological action. In this study, we have addressed the question of whether particular plasma membrane transporters contribute to the transcriptomic response associated with nucleoside-derived drug therapy. Firstly, we have characterized the nucleoside transporters responsible for 5'-DFUR uptake into the breast cancer cell line MCF7. 5'-DFUR is the immediate precursor of 5-FU and a metabolite of the orally administered pro-drug capecitabine, currently used in the treatment of breast cancer and other solid tumors. Although 5'-DFUR is a substrate for both plasma membrane equilibrative nucleoside carriers, hENT1 shows higher affinity for this molecule than hENT2. Inhibition of hENT1 function partially protected MCF7 cells from 5'-DFUR-induced cytotoxicity. Secondly, we have used a pharmacogenomic approach to determine how inhibition of hENT1 function contributes to the transcriptomic response associated to 5'-DFUR treatment. Under hENT1 inhibition most of the transcriptional targets of 5'-DFUR action, which were genes associated with apoptosis and cell cycle progression were blocked. This study demonstrates that although 5'-DFUR is substrate for both equilibrative nucleoside carriers, hENT1 function is essential for the full transcriptional response to 5'-DFUR treatment.

Nucleoside transporter expression and activity is regulated during granulocytic differentiation of NB4 cells in response to all-trans-retinoic acid

NB4 cells express multiple nucleoside transporters (NTs), including: hENT1 (es), and hENT2 (ei), and the CNT subtype referred to as, csg; a concentrative sensitive guanosine specific transporter. csg activity is a distinguishing feature of the NB4 cell line and its presence suggests a particular requirement of these cells for guanosine salvage. Proliferation and differentiation pathways determine, in part, the number of NTs in cells and tissues. In this study, all-trans-retinoic acid (ATRA)-induced granulocytic differentiation of NB4 cells resulted in biphasic changes in guanosine transport. Transient increases in csg and es activity, the result of an increase in V(max) (pmol/muls) of both transporter systems, served as early markers of differentiation while expression of a fully differentiated phenotype was accompanied by a selective loss of csg activity and the return of es activity to that of proliferating cells. Intracellular incorporation of [(3)H]-guanosine decreased as cells matured despite increased transport rates and suggested a reduced intracellular requirement of NB4-granulocytes compared to their proliferating counterparts. Whether a loss of csg activity could serve to assess clinical response to differentiation therapies is not known. Nitrobenzylthioinosine (NBMPR) binding sites within nuclear membrane (NM) preparations, suggested the presence of functional intracellular NTs. An increase in plasma membrane (PM) associated transporters coincided with the early increase in guanosine transport and a decrease in NBMPR binding to NM fractions and suggests that intracellular NTs may serve as a reserve pool for translocation to the (PM) when additional transport capacity is required. The modulation of transporters during differentiation could potentially regulate drug bioavailability and cytotoxicity and should be evaluated prior to combining differentiating agents with traditional nucleoside analogs in the treatment of APL.

Functional characterization and haplotype analysis of polymorphisms in the human equilibrative nucleoside transporter, ENT2

The equilibrative nucleoside transporter 2 (ENT2; SLC29A2) is a bidirectional transporter that is involved in the disposition of naturally occurring nucleosides as well as a variety of anticancer and antiviral nucleoside analogs. The goal of the current study was to evaluate the function of genetic variants in ENT2 in cellular assays and to determine the haplotype structure of the coding and flanking intronic region of the gene. As part of a large study focused on genetic variation in membrane transporters (Leabman et al., 2003), DNA samples from ethnically diverse populations (100 African-Americans, 100 European-Americans, 30 Asians, 10 Mexicans, and 7 Pacific Islanders) were screened for variants in membrane transporters, including SLC29A2. Fourteen polymorphic sites in SLC29A2 were found, including 11 in the coding region. Five protein-altering variants were identified: three nonsynonymous variants, and two deletions. Each of the protein-altering variants was found at a very low frequency, occurring only once in the sample population. The nonsynonymous variants and the deletions were constructed via site-directed mutagenesis and were subsequently characterized in Xenopus laevis oocytes. All variants were able to take up inosine with the exception of ENT2-Delta845-846, which resulted in a frameshift mutation that prematurely truncated the protein. ENT2 showed very infrequent variation compared with most other transporter proteins studied, and it was found that five haplotypes were sufficient to describe the entire sample set. The low overall genetic diversity in SLC29A2 makes it unlikely that variation in the coding region contributes significantly to clinically observed differences in drug response.

Therapy of chronic lymphocytic leukaemia with purine nucleoside analogues: facts and controversies

Chronic lymphocytic leukaemia (CLL) is a neoplastic disease of unknown aetiology characterised by an absolute lymphocytosis in peripheral blood and bone marrow. The disease is diagnosed most commonly in the elderly with the median age at diagnosis being about 65 years. The purine nucleoside analogues (PNAs) fludarabine, cladribine (2-chlorodeoxyadenosine) and pentostatin (2'-deoxycoformycin) are highly active in CLL, both in previously treated and in refractory or relapsed patients. These three agents share similar chemical structures and mechanisms of action such as induction of apoptosis. However, they also exhibit significant differences, especially in their interactions with enzymes involved in adenosine and deoxyadenosine metabolism. Recent randomised studies suggest that fludarabine and cladribine have similar activity in CLL. However, clinical observations indicate the existence of cross-resistance between fludarabine and cladribine. Patients who received PNAs as their initial therapy and achieved long-lasting response can be successfully retreated with the same agent. PNAs administered in combination with other chemotherapeutic agents and/or monoclonal antibodies may produce higher response rates, including complete response (CR) or molecular CR, compared with PNAs alone or other treatment regimens. Management decisions are more difficult in elderly patients because of the apparent increase in toxicity of PNAs in this population. In elderly patients, we recommend chlorambucil as the first-line treatment, with PNAs in lower doses in refractory or relapsed patients. Myelosuppression and infections, including opportunistic varieties, are the most frequent adverse effects in patients with CLL treated with PNAs. Therefore, some investigators recommend routine antibacterial and antiviral prophylaxis during and after PNA treatment. This review presents current results and treatment strategies with the use of PNAs in CLL, especially in elderly patients.