Es nucleoside transporter content of acute leukemia cells: role in cell sensitivity to cytarabine (araC)

Synthesis, anticancer activity and potential application of diosgenin modified cancer chemotherapeutic agent cytarabine

Diosgenin (DG), a steroidal saponin, is mainly found in yam tubers. DG and its derivatives displayed significant pharmacological activities against inflammatory, hyperlipidemia, and various cancers. DG was selected to modify the cancer chemotherapeutic agent cytarabine (Ara-C) due to its anti-tumor activities as well as lipophilicity. After characterization, the biomembrane affinity and the kinetic thermal processes of the obtained DG-Ara-C conjugate were evaluated by differential scanning calorimetry (DSC). Thin hydration method with sonication was applied to prepare the DG-Ara-C liposomes without cholesterol since the DG moiety has the similar basic structure with cholesterol with more advantages. Dynamic Light Scattering (DLS) analysis and cytotoxic analysis were employed to characterize the DG-Ara-C liposomes and investigate their biological activities, respectively. The results indicated that DG changed the biomembrane affinity of Ara-C and successfully replaced the cholesterol during the liposome preparation. The DG-Ara-C liposomes have an average particle size of around 116 nm with a narrow size distribution and revealed better anti-cancer activity against leukemia cells and solid tumor cells than that of free DG or Ara-C. Therefore, it can be concluded that DG displayed the potential application as an anti-cancer drug carrier to improve the bio-activities, since DG counted for a critical component in modulating the biomembrane affinity, preparation of liposome, and release of hydrophilic Ara-C from lipid vesicles.

Apoptotic and anti-proliferative effect of guanosine and guanosine derivatives in HuT-78 T lymphoma cells

The effects of 100 μM of 3',5'-cGMP, cAMP, cCMP, and cUMP as well as of the corresponding membrane-permeant acetoxymethyl esters on anti-CD3-antibody (OKT3)-induced IL-2 production of HuT-78 cutaneous T cell lymphoma (Sézary lymphoma) cells were analyzed. Only 3',5'-cGMP significantly reduced IL-2 production. Flow cytometric analysis of apoptotic (propidium iodide/annexin V staining) and anti-proliferative (CFSE staining) effects revealed that 3',5'-cGMP concentrations > 50 μM strongly inhibited proliferation and promoted apoptosis of HuT-78 cells (cultured in the presence of αCD3 antibody). Similar effects were observed for the positional isomer 2',3'-cGMP and for 2',-GMP, 3'-GMP, 5'-GMP, and guanosine. By contrast, guanosine and guanosine-derived nucleotides had no cytotoxic effect on peripheral blood mononuclear cells (PBMCs) or acute lymphocytic leukemia (ALL) xenograft cells. The anti-proliferative and apoptotic effects of guanosine and guanosine-derived compounds on HuT-78 cells were completely eliminated by the nucleoside transport inhibitor NBMPR (S-(4-Nitrobenzyl)-6-thioinosine). By contrast, the ecto-phosphodiesterase inhibitor DPSPX (1,3-dipropyl-8-sulfophenylxanthine) and the CD73 ecto-5'-nucleotidase inhibitor AMP-CP (adenosine 5'-(α,β-methylene)diphosphate) were not protective. We hypothesize that HuT-78 cells metabolize guanosine-derived nucleotides to guanosine by yet unknown mechanisms. Guanosine then enters the cells by an NBMPR-sensitive nucleoside transporter and exerts cytotoxic effects. This transporter may be ENT1 because NBMPR counteracted guanosine cytotoxicity in HuT-78 cells with nanomolar efficacy (IC50 of 25-30 nM). Future studies should further clarify the mechanism of the observed effects and address the question, whether guanosine or guanosine-derived nucleotides may serve as adjuvants in the therapy of cancers that express appropriate nucleoside transporters and are sensitive to established nucleoside-derived cytostatic drugs.

Troxacitabine in acute leukemia

Troxacitabine (Troxatyl; BCH-4556; (-)-2'-deoxy-3'-oxacytadine) is the first synthetic l-nucleoside enantiomer to demonstrate broad spectrum cytotoxic activity. It was obtained by exchanging the sulphur endocyclic atom with oxygen in the structure of lamivudine, following the discovery that this agent had cytotoxic, as well as anti-viral activity. The unique "unnatural" stereochemistry of troxacitabine has produced impressive cytotoxic potency against a wide range of malignancies in the laboratory which led to its selection for clinical development. The initial trials with troxacitabine have established its efficacy in both solid and haematological malignancies, including those resistant to ara-C (cytarabine). This review will consider troxacitabine in terms of its pharmacology, mode of action, pharmacokinetics, tolerability and clinical efficacy.

Application of activated nucleoside analogs for the treatment of drug-resistant tumors by oral delivery of nanogel-drug conjugates

A majority of nanoencapsulated drugs that have shown promise in cancer chemotherapy are administered intravenously. Development of effective oral nanoformulations presents a very challenging medical goal. Here, we describe successful applications of innovative polymeric nanogels in the form of conjugates with activated nucleoside analogs for oral administration in cancer chemotherapy. Previously, we reported the synthesis of amphiphilic polyvinyl alcohol and dextrin-based nanogel conjugates with the phosphorylated 5-FU nucleoside Floxuridine and demonstrated their enhanced activity against regular and drug-resistant cancers (T.H. Senanayake, G. Warren, S.V. Vinogradov, Novel anticancer polymeric conjugates of activated nucleoside analogs, Bioconjug. Chem. 22 (2011) 1983-1993). In this study, we synthesized and evaluated oral applications of nanogel conjugates of a protected Gemcitabine, the drug never used in oral therapies. These conjugates were able to quickly release an active form of the drug (Gemcitabine 5'-mono-, di- and triphosphates) by specific enzymatic activities, or slowly during hydrolysis. Gemcitabine conjugates demonstrated up to 127 times higher in vitro efficacy than the free drug against various cancer cells, including the lines resistant to nucleoside analogs. Surprisingly, these nanogel-drug conjugates were relatively stable in gastric conditions and able to actively penetrate through the gastrointestinal barrier based on permeability studies in Caco-2 cell model. In tumor xenograft models of several drug-resistant human cancers, we observed an efficient inhibition of tumor growth and extended the life-span of the animals by 3 times that of the control with orally treated Gemcitabine- or Floxuridine-nanogel conjugates. Thus, we have demonstrated a potential of therapeutic nanogel conjugates with the activated and stabilized Gemcitabine as a successful oral drug form against Gemcitabine-resistant and other drug-resistant tumors.

Role of human nucleoside transporters in the uptake and cytotoxicity of azacitidine and decitabine

The nucleoside analogs 5-azacytidine (azacitidine) and 5-aza-2'-deoxycytidine (decitabine) are active against acute myeloid leukemia and myelodysplastic syndromes. Cellular transport across membranes is crucial for uptake of these highly polar hydrophilic molecules. We assessed the ability of azacitidine, decitabine, and, for comparison, gemcitabine, to interact with human nucleoside transporters (hNTs) in Saccharomyces cerevisiae cells (hENT1/2, hCNT1/2/3) or Xenopus laevis oocytes (hENT3/4). All three drugs inhibited hCNT1/3 potently (K (i) values, 3-26 μM), hENT1/2 and hCNT2 weakly (K (i) values, 0.5-3.1 mM), and hENT3/4 poorly if at all. Rates of transport of [(3)H]gemcitabine, [(14)C]azacitidine, and [(3)H]decitabine observed in Xenopus oocytes expressing individual recombinant hNTs differed substantially. Cytotoxicity of azacitidine and decitabine was assessed in hNT-expressing or hNT-deficient cultured human cell lines in the absence or presence of transport inhibitors where available. The rank order of cytotoxic sensitivities (IC (50) values, μM) conferred by hNTs were hCNT1 (0.1) > hENT1 (0.3) ≫ hCNT2 (8.3), hENT2 (9.0) for azacitidine and hENT1 (0.3) > hCNT1 (0.8) ⋙ hENT2, hCNT2 (>100) for decitabine. Protection against cytotoxicity was observed for both drugs in the presence of inhibitors of nucleoside transport, thus suggesting the importance of hNTs in manifestation of toxicity. In summary, all seven hNTs transported azacitidine, with hCNT3 showing the highest rates, whereas hENT1 and hENT2 showed modest transport and hCNT1 and hCNT3 poor transport of decitabine. Our results show for the first time that azacitidine and decitabine exhibit different human nucleoside transportability profiles and their cytotoxicities are dependent on the presence of hNTs, which could serve as potential biomarkers of clinical response.

PPARα and PPARγ regulate the nucleoside transporter hENT1

Human equilibrative nucleoside transporter 1 (hENT1) is an important determinant for nucleoside analog based chemotherapy success. Preliminary data suggest hENT1 regulation by PPARs. Using A2780 cells, we investigated the role of PPARs on hENT1 expression and activity. PPARα and PPARγ agonists, Wy14,643 and RGZ, increased hENT1 expression, but only PPARα activation or overexpression resulted in higher hENT1 transport activity. On the other hand, promoter analysis showed two putative PPRE in hENT1 promoter and luciferase-coupled promoter constructs were generated and analyzed. Our results suggest that PPARα-but not PPARγ-mediated expression regulation of hENT1 is PPRE-dependent. In conclusion, PPARα and PPARγ activation modulate hENT1 expression.

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.

Prognostic role of human equilibrative transporter 1 (hENT1) in patients with resected gastric cancer

Nucleoside transporter proteins are specialized proteins that mediate the transport of nucleosides and nucleoside analog drugs across the plasma membrane. The human equilibrative nucleoside transporter 1 (hENT1) is a member of these proteins and mediates cellular entry of gemcitabine, cytarabine, and fludarabine. The hENT1 expression has been demonstrated to be related with prognosis and activity of gemcitabine-based therapy in breast, ampullary, lung, and pancreatic cancer. We investigated the immunohistochemical expression of hENT in tumor samples from 111 patients with resected gastric adenocarcinoma, correlating these data with clinical parameters and disease outcomes. None of the patients received chemotherapy or radiation therapy before or after surgery as a part of an adjuvant or neoadjuvant program. On univariate survival analysis, the hENT1 expression was associated with overall survival (OS) and disease free survival (DFS). Specifically, those patients with overexpression of hENT1 showed a shorter OS (P = 0.021) and a shorter DFS (P = 0.033). Considering only the node positive patients, higher hENT levels were associated with significantly shorter median DFS (21.7 months; 95% CI 11.1-32.4) compared with patients with low expression of hENT1. The hENT1 expression was defined, in the lymph-node positive patients, as an independent prognostic factor (P = 0.019). Furthermore, considering only patients with diffuse or mixed tumors and lymph-node positive, the expression of hENT1 was strongly related with DFS and OS. Immunohistochemistry for the hENT1 protein carries prognostic information in patients with resected gastric cancer and holds promise as a predictive factor in chemotherapy decisions.

Genetic factors influencing cytarabine therapy

The mainstay of acute myeloid leukemia chemotherapy is the nucleoside analog cytarabine (ara-C). Numerous studies suggest that the intracellular concentrations of the ara-C active metabolite, ara-CTP, vary widely among patients and, in turn, are associated with variability in clinical response to acute myeloid leukemia treatment. Thus, genetic variation in key genes in the ara-C metabolic pathway--specifically, deoxycytidine kinase (a rate-limiting activating enzyme), 5 nucleotidase, cytidine deaminase and deoxycytidylate deaminase (all three are inactivating enzymes), human equilibrative nucleoside transporter (ara-C uptake transporter) and ribonucleotide reductase (RRM1 and RRM2--enzymes regulating intracellular deoxycytidine triphosphate pools)--form the molecular basis of the interpatient variability observed in intracellular ara-CTP concentrations and response to ara-C. Understanding genetic variants in the key candidate genes involved in the metabolic activation of ara-C, as well as the pharmacodynamic targets of ara-C, will provide an opportunity to identify patients at an increased risk of adverse reactions or decreased likelihood of response, based upon their genetic profile, which in future could help in dose optimization to reduce drug toxicity without compromising efficacy. The pharmacogenetic studies on ara-C would also be equally applicable to other nucleoside analogs, such as gemcitabine, decitabine, clofarabine and so on, which are metabolized by the same pathway.

Cladribine: not just another purine analogue?

Cladribine was synthesized as a purine analogue drug that inhibited adenosine deaminase. It received FDA approval in the 1980s for treatment of hairy cell leukemia. Given its toxicity towards lymphocytes and its corresponding immunosuppressive effects, it has been studied and found efficacious in a variety of hematologic malignancies and autoimmune conditions, most recently multiple sclerosis. This review highlights pharmacological, toxicological and clinical data for the use of cladribine. It also discusses existing and new mechanisms that may contribute to its unique clinical activity. Emerging data show that in addition to its known purine nucleoside analogue activity, cladribine possesses epigenetic properties, inhibiting S-adenosylhomocysteine hydrolase and DNA methylation. This may contribute to its efficacy and highlights the importance of studying combination therapy with other epigenetic or targeted agents. Clinical trials are underway in a variety of malignant and nonmalignant conditions.

Problems Related to Resistance to Cytarabine in Acute Myeloid Leukemia

First-line chemotherapy treatment in acute-myeloid leukemia patients usually consists of a combination of cytarabine (ara-C) and an anthracycline. These regimens induce complete response (CR) rates in 65-80% of newly diagnosed AML patients. However, clinical outcome is unsatisfactory, as most of the patients who achieve a CR will relapse within 2 years from diagnosis, often with resistant disease and poor response to subsequent therapy. Thus, understanding the factors which contribute to the emergence of chemoresistant leukemic cells is essential to improve outcome in patients suffering from this disease. In this review, we highlight the current knowledge concerning the cellular mechanisms of resistance to ara-C. We also discuss possible strategies that may be used to overcome such resistance. Efforts to increase intracellular levels and DNA incorporation of phosphorylated ara-C using pronucleotides of ara-C are very promising. Ara-C combined with agents modulating apototic responses are expected to provide additional benefit. In the same way that combination chemotherapy has provided curative treatment of AML, a multifactorial approach of ara-C resistance should allow significant progress in the treatment of currently chemoresistant disease.

Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.

Fine tuning of rabbit equilibrative nucleoside transporter activity by an alternatively spliced variant

The full-length cDNA encoding an equilibrative nucleoside transporter (rbENT2) and its novel C-terminal variant, rbENT2A, were isolated from rabbit trachea. Rabbit ENT2 protein consists of 456 amino acid residues; rbENT2A is shorter by 41 residues. Both rbENT2 and rbENT2A transcripts are found in rabbit tissues including intestine, kidney cortex, kidney, and trachea, at varying levels of expression. When transfected in a heterologous expression system-Madin Darby canine kidney (MDCK) epithelial cell line-both rbENT2 and rbENT2A were expressed. rbENT2 had a molecular mass of 49 kDa; rbENT2A had a molecular mass of 44 kDa. Clones of both transporters yielded functional proteins that were capable of mediating uridine uptake and efflux without the needing to be coupled to a secondary ion (e.g. Na(+)). Remarkably, rbENT2A displayed a higher affinity (K(m) = 41 microM) and a lower capacity (V(max) = 0.6 nmol/mg protein/5 min) towards substrates than rbENT2 (K(m) = 272.8 microM, V(max) = 1.26 nmol/mg protein/5 min). Pharmacological profiles showed that nitro-benzyl-mercapto-purine-ribose (NBMPR) potently inhibited (3)H-uridine uptake mediated by rbENT2A, but not uptake mediated by rbENT2. The constitutive splicing, broad expression, markedly different kinetics, and distinct pharmacological characteristics of rbENT2A appear to act in conjunction with the wild type, rbENT2, to fine-tune basolateral nucleoside transport function in rabbit trachea.

Troxacitabine in leukemia

Troxacitabine (Troxatyl; BCH-4556; (-)-2'-deoxy-3'-oxacytadine) is the first synthetic l-nucleoside enantiomer to demonstrate broad spectrum cytotoxic activity. It was obtained by exchanging the sulphur endocyclic atom with oxygen in the structure of lamivudine, following the discovery that this agent had cytotoxic, as well as anti-viral activity. The unique "unnatural" stereochemistry of troxacitabine has produced impressive cytotoxic potency against a wide range of malignancies in the laboratory which led to its selection for clinical development. The initial trials with troxacitabine have established its efficacy in both solid and haematological malignancies, including those resistant to ara-C (cytarabine). This review will consider troxacitabine in terms of its pharmacology, mode of action, pharmacokinetics, toxicities, and clinical efficacy.

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.

Discovery and development of clofarabine: a nucleoside analogue for treating cancer

The treatment of acute leukaemias, which are the most common paediatric cancers, has improved considerably in recent decades, with complete response rates approaching approximately 90% in some cases. However, there remains a major need for treatments for patients who do not achieve or maintain complete remission, for whom the prognosis is very poor. In this article, we describe the challenges involved in the discovery and development of clofarabine, a second-generation nucleoside analogue that received accelerated approval from the US FDA at the end of 2004 for the treatment of paediatric patients 1-21 years old with relapsed or refractory acute lymphoblastic leukaemia after at least two prior regimens. It is the first such drug to be approved for paediatric leukaemia in more than a decade, and the first to receive approval for paediatric use before adult use.

Involvement of the concentrative nucleoside transporter 3 and equilibrative nucleoside transporter 2 in the resistance of T-lymphoblastic cell lines to thiopurines

Mechanisms of resistance to thiopurines, 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) were investigated in human leukemia cell lines. We developed two 6-MP- and 6-TG-resistant cell lines from the human T-lymphoblastic cell line (MOLT-4) by prolonged exposure to these drugs. The resistant cells were highly cross resistant to 6-MP and 6-TG, and exhibited marked reduction in cellular uptake of 6-MP (70% and 80%, respectively). No significant modification of the activities of hypoxanthine-guanine phosphoribosyl transferase, thiopurine methyltransferase or inosine monophosphate dehydrogenase was observed. Real-time PCR of concentrative nucleoside transporter 3 (CNT3) and equilibrative nucleoside transporter 2 (ENT2) of resistant cells showed substantial reductions in expression of messenger RNAs. Small interfering RNA designed to silence the CNT3 and ENT2 genes down-regulated the expression of these genes in leukemia cells. These decreases were accompanied by reduction of transport of 6-MP (47% and 21%, respectively) as well as its cytocidal effect (30% and 21%, respectively). Taken together these results show that CNT3 and ENT2 play a key role in the transport of 6-MP and 6-TG by leukemia cells. From a clinical point of view determination of CNT3 and ENT2 levels in leukemia cells may be useful in predicting the efficacy of thiopurine treatment.

Cytosine arabinoside affects multiple cellular factors and induces drug resistance in human lymphoid cells

Continuous in vitro cultivation of human lymphoid H9 cells in the presence of 0.5microM arabinosyl-cytosine (araC) resulted in cell variant, H9-araC cells, that was >600-fold resistant to the drug and cross resistant to its analogs and other unrelated nucleosides, e.g. dideoxycytidine (5-fold), thiacytidine (2-fold), 2-fluoro-adenine arabinoside (8.3-fold), and 2-chloro-deoxyadenosine (2.1-fold). Compared to the parental cell line, the resistant cells accumulated <1% araCTP, and had reduced deoxycytidine kinase (dCK) activity (31.4%) and equilibrative nucleoside transporter 1 (ENT1) protein. The expression of the dCK gene in araC resistant cells was reduced to 60% of H9 cells, which correlated with lower dCK protein and activity. Whereas, there was no difference in the expression of ENT1 mRNA between the cell lines, ENT1 protein content was much lower in the resistant cells than in H9 cells. The concentrative nucleoside transporter (CNT3) was slightly increased in H9-araC cells, but CNT2, and MDR1 remained unaffected. Although a definitive correlation remains to be established, the amount of Sp1 protein, a transcription factor, that regulates the expressions of dCK, nucleoside transporters and other cellular proteins, was found reduced in H9-araC cells. Like ENT1, the Sp1 mRNA levels remained unaffected in H9-araC whereas protein contents were reduced. These observations are indicative of differences in the production and/or turnover of ENT1 and Sp1 proteins in H9-araC cells. Since nucleoside transporters and dCK play an important role in the activity of potential antiviral and anticancer deoxynucleoside analogs, understanding of their regulation is important. These studies show that the exposure of cells to araC, in vitro, is capable of simultaneously affecting more than one target site to confer resistance. The importance of this observation in the clinical use of araC remains to be determined.

Analysis of human equilibrative nucleoside transporter 1 (hENT1) protein in non-Hodgkin's lymphoma by immunohistochemistry

The human equilibrative nucleoside transporter 1 (hENT1) is a member of the equilibrative nucleoside transporter family that mediates cellular entry of gemcitabine, cytarabine, and fludarabine. Deficiency in hENT1 confers resistance to toxicity of these drugs in a variety of model systems. Since some nucleoside analogs have a role in treating patients with non-Hodgkin's lymphoma (NHL), this study was undertaken to assess hENT1 abundance in NHL. A total of 115 cases of NHL of various subtypes and 15 reactive lymph nodes were evaluated for the presence of hENT1 protein using immunohistochemistry applied to frozen tissues. Samples were considered positive when >or=50% of neoplastic cells showed immunostaining. In reactive lymph nodes, hENT1 was confined to the germinal centers, whereas mantle zone B-cells and interfollicular T-cells were negative. In NHL, a relatively high frequency of hENT1 positivity was found in Burkitt lymphoma/leukemia (63%), diffuse large B-cell lymphoma (DLCL; 45%), and follicular lymphoma (40%). In DLCL, 26% of cases were positive for CD10, and CD10-positive DLCL cases were more likely to be hENT1 positive than CD10-negative cases (P=0.025). A lower frequency of hENT1 positivity was found in mantle cell lymphoma (13%) and peripheral T-cell lymphomas (37%). All marginal zone lymphomas (n=5), chronic lymphocytic leukemia small lymphocytic lymphomas (n=10), plasmacytoma (n=3), acute lymphoblastic lymphoma/leukemia, and anaplastic large-cell lymphomas (n=5) were negative. In conclusion, hENT1 was most frequently found in benign and malignant follicular center cells. Prospective studies to assess the value of hENT1 immunostaining in predicting resistance to nucleoside chemotherapy for NHL are warranted.

The absence of human equilibrative nucleoside transporter 1 is associated with reduced survival in patients with gemcitabine-treated pancreas adenocarcinoma

PURPOSE

Gemcitabine monotherapy is the standard palliative chemotherapy for pancreatic adenocarcinoma. Gemcitabine requires plasma membrane nucleoside transporter proteins to efficiently enter cells and exert it cytotoxicity. In vitro studies have demonstrated that deficiency of human equilibrative nucleoside transporter 1 (hENT1), the most widely abundant and distributed nucleoside transporter in human cells, confers resistance to gemcitabine toxicity, but the distribution and abundance of nucleoside transporters in normal and malignant pancreatic tissue is unknown.

EXPERIMENTAL DESIGN

We studied tumor blocks from normal pancreas and 21 Alberta patients with gemcitabine-treated pancreatic cancer. Immunohistochemistry on the formalin-fixed, paraffin-embedded tissues was performed with specific hENT1 and human Concentrative Nucleoside Transporter 3 monoclonal antibodies and scored by a pathologist blinded to clinical outcomes.

RESULTS

hENT1 was detected in normal Langerhan cells and lymphocytes but not in normal glandular elements. Patients in whom all adenocarcinoma cells had detectable hENT1 had significantly longer median survivals from gemcitabine initiation than those for whom hENT1 was absent in a proportion (10 to 100%) of adenocarcinoma cells (median survival, 13 versus 4 months, P = 0.01). Immunohistochemistry for human Concentrative Nucleoside Transporter 3 revealed moderate to high-intensity staining in all adenocarcinoma tissue samples.

CONCLUSIONS

Patients with pancreatic adenocarcinoma with uniformly detectable hENT1 immunostaining have a significantly longer survival after gemcitabine chemotherapy than tumors without detectable hENT1. Immunohistochemistry for hENT1 shows promise as a molecular predictive assay to appropriately select patients for palliative gemcitabine chemotherapy but requires formal validation in prospective, randomized trials.

Quantitative analysis of nucleoside transporter and metabolism gene expression in chronic lymphocytic leukemia (CLL): identification of fludarabine-sensitive and -insensitive populations

Resistance to fludarabine is observed in the clinic, and molecular predictive assays for benefit from chemotherapy are required. Our objective was to determine if expression of nucleoside transport and metabolism genes was associated with response to fludarabine therapy in patients with chronic lymphocytic leukemia (CLL). CLL cells from 56 patients were collected prior to treatment with fludarabine. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was performed on sample RNA to determine the relative levels of mRNA of 3 nucleoside transporters that mediate fludarabine uptake (human equilibrative nucleoside transporter 1 [hENT1], human equilibrative nucleoside transporter 2 [hENT2], and human concentrative nucleoside transporter 3 [hCNT3]), deoxycytidine kinase (dCK), and 3 5'-nucleotidases (ecto-5'nucleotidase [CD73], deoxynucleotidase-1 [dNT-1], and cytoplasmic high-Km 5-nucleotidase [CN-II]). Two-dimensional hierarchical cluster analysis of gene expression identified 2 distinct populations of CLL. Cluster 2 patients experienced a 3.4-fold higher risk of disease progression than cluster 1 patients (P = .0058, log-rank analysis). Furthermore, independent analysis of the individual genes of interest revealed statistically significant differences for risk of disease progression (adjusted hazard ratios [HRs]) with underexpression of dNT-1 (HR = 0.45; P = .042), CD73 (HR = 0.40; P = .022), and dCK (HR = 0.0.48; P = .035), and overexpression of hCNT3 (HR = 4.7; P = .0007) genes. Subjects with elevated hCNT3 expression experienced a lower complete response rate to fludarabine therapy (11% vs 69%; P = .002). No hCNT3-mediated plasma membrane nucleoside transport was detected in CLL samples expressing hCNT3 message, and hCNT3 protein was localized to the cytoplasm with immunohistochemical and confocal microscopy.

Determinants of sensitivity and resistance to gemcitabine: the roles of human equilibrative nucleoside transporter 1 and deoxycytidine kinase in non-small cell lung cancer

Gemcitabine is one of the most commonly used agents for lung cancer chemotherapy, but the determinants of sensitivity and/or resistance to this agent are not yet fully understood. In this study we used quantitative RT-PCR to examine the expression levels of human equilibrative nucleoside transporter 1 (hENT1) and deoxycytidine kinase (dCK) genes in non-small cell lung cancer (NSCLC) cell lines in relation to sensitivity and resistance to gemcitabine. The basal expression levels of hENT1 were significantly correlated with the IC50 values for gemcitabine (r =-0.6769, P = 0.0005), whereas dCK expression levels were not. In a highly gemcitabine-sensitive cell line, NCI-H23, the sensitivity to gemcitabine was inhibited by nitrobenzylmercaptopurine ribonucleoside (NBMPR), an inhibitor of hENT1, without significant modulation of hENT1 expression. These data suggest that hENT1 is associated with gemcitabine sensitivity in lung cancer. We also continuously exposed NCI-H23 cells to gemcitabine and subsequently established the drug-resistant clone H23/GEM-R, which showed a significant decrease of dCK expression; however, hENT1 expression was not altered in the continuously exposed sublines or in the resistant clone. We conclude that increased hENT1 expression is a determinant of gemcitabine sensitivity, while decreased dCK expression is associated with acquired resistance to gemcitabine in NSCLC cells. Thus, hENT1 and dCK might be useful as predictive markers for efficacy of gemcitabine therapy in NSCLC.

Nucleoside transporters in chronic lymphocytic leukaemia

Nucleoside derivatives have important therapeutic activity in chronic lymphocytic leukaemia (CLL). Experimental evidence indicates that in CLL cells most of these drugs induce apoptosis ex vivo, suggesting that programmed cell death is the mechanism of their therapeutic action, relying upon previous uptake and metabolic activation. Although defective apoptosis and poor metabolism often cause resistance to treatment, differential uptake and/or export of nucleosides and nucleotides may significantly modulate intracellular drug bioavailability and, consequently, responsiveness to therapy. Two gene families, SLC28 and SLC29, encode transporter proteins responsible for concentrative and equilibrative nucleoside uptake (CNT and ENT, respectively). Furthermore, selected members of the expanding ATP-binding cassette (ABC) protein family have recently been identified as putative efflux pumps for the phosphorylated forms of these nucleoside-derived drugs, ABCC11 (MRP8) being a good candidate to modulate cell sensitivity to fluoropyrimidines. Sensitivity of CLL cells to fludarabine has also been recently correlated with ENT-type transport function, suggesting that, besides the integrity of apoptotic pathways and appropriate intracellular metabolism, transport across the plasma membrane is also a relevant event during CLL treatment. As long as nucleoside transporter expression in leukaemia cells is not constitutive, the possibility of regulating nucleoside transporter function by pharmacological means may also contribute to improve therapy.

Expression of the nucleoside-derived drug transporters hCNT1, hENT1 and hENT2 in gynecologic tumors

Deoxynucleoside analogs are used in the treatment of a variety of solid tumors. Their transport across the plasma membrane may determine their cytotoxicity and thus nucleoside transporter (NT) expression patterns may be of clinical relevance. Lack of appropriate antibodies for use in paraffin-embedded biopsies has been a bottleneck to undertake high-throughput analysis of NT expression in solid tumors. Here we report the characterization of 2 new antibodies raised against the low-affinity equilibrative NTs, hENT1 and hENT2, suitable for that purpose. These 2 antisera, along with a previously characterized antibody that specifically recognizes the high-affinity Na-dependent concentrative NT, hCNT1, have been used to analyze, using a tissue array approach, NT expression in gynecologic cancers (90 ovarian, 80 endometrial and 118 uterine cervix carcinomas). Human CNT1 was not detected in 33% and 39% of the ovarian and uterine cervix carcinomas, respectively, whereas hENT1 and hENT2 expression was significantly retained in a high percentage of tumors (91% and 96% for hENT1, 84% and 98% for hENT2, in ovarian and cervix carcinomas, respectively). Only a few endometrial carcinomas (15%) were found to be negative for hCNT1, but they all retained hENT1 and hENT2 expression. In ovarian cancer, the loss of all 3 NT proteins was a more common event in the clear cell histologic subtype than in the serous, mucinous and endometrioid histotypes. In uterine cervix tumors, the loss of expression of hCNT1 was significantly associated with the adenocarcinoma subtype. In summary, hCNT1 was by far the isoform whose expression was most frequently reduced or lost in the 3 types of gynecologic tumors analyzed. Moreover, NT expression is related to the type of gynecologic tumor and its specific subtype, hCNT1 protein loss being highly correlated with poor prognosis histotypes. Since hCNT1, hENT1 and hENT2 recognize fluoropyrimidines as substrates, but with different affinities, this study anticipates high variability in drug uptake efficiency in solid tumors.

Phase II study of troxacitabine, a novel dioxolane nucleoside analog, in patients with untreated or imatinib mesylate-resistant chronic myelogenous leukemia in blastic phase

A phase II study of troxacitabine, a non-natural dioxolane nucleoside L-enantiomer, was conducted in patients with chronic myelogenous leukemia in blastic phase (CML-BP). Patients were untreated for BP, or treated with imatinib mesylate (IM) as sole prior therapy for BP. Troxacitabine was given as an intravenous infusion over 30 min daily for 5 days at a dose of 8.0 mg/m(2) per day. Thirty-one patients, 29 (93%) of whom had failed prior IM therapy, received 51 courses of therapy. Grade 3 or 4 toxicities included stomatitis (4%), hand-foot syndrome (18%), and skin rash (12%). Four patients (13%) responded. Troxacitabine-based combinations merit study in IM-resistant CML.

Nucleoside anticancer drugs: the role of nucleoside transporters in resistance to cancer chemotherapy

The clinical efficacy of anticancer nucleoside drugs depends on a complex interplay of transporters mediating entry of nucleoside drugs into cells, efflux mechanisms that remove drugs from intracellular compartments and cellular metabolism to active metabolites. Nucleoside transporters (NTs) are important determinants for salvage of preformed nucleosides and mediated uptake of antimetabolite nucleoside drugs into target cells. The focus of this review is the two families of human nucleoside transporters (hENTs, hCNTs) and their role in transport of cytotoxic chemotherapeutic nucleoside drugs. Resistance to anticancer nucleoside drugs is a major clinical problem in which NTs have been implicated. Single nucleotide polymorphisms (SNPs) in drug transporters may contribute to interindividual variation in response to nucleoside drugs. In this review, we give an overview of the functional and molecular characteristics of human NTs and their potential role in resistance to nucleoside drugs and discuss the potential use of genetic polymorphism analyses for NTs to address drug resistance.

Nucleoside transporters in the disposition and targeting of nucleoside analogs in the kidney

Systemic disposition of nucleosides and nucleoside analogs is dependent on renal handling of these compounds. There are five known, functionally characterized nucleoside transporters with varying substrate specificities for nucleosides: concentrative nucleoside transporters (CNT1-CNT3; Solute Carrier (SLC) 28A1-28A3), which mediate the intracellular flux of nucleosides, and equilibrative nucleoside transporters (ENT1-ENT2; SLC29A1-SLC29A2), which mediate bi-directional facilitated diffusion of nucleosides. All five of these transporters are expressed in the kidney. Concentrative nucleoside transporters primarily localize to the apical membrane of renal epithelial cells while equilibrative nucleoside transporters primarily localize to the basolateral membrane. These transporters work in concert to mediate reabsorptive flux of naturally occurring nucleosides and nucleoside analogs. In addition, equilibrative transporters also participate in secretory flux of some nucleoside analogs. Nucleoside transporters also serve in the targeting of nucleoside analog therapies to renal tumors. This review examines the role that these transporters play in renal disposition of nucleosides and nucleoside analogs in both systemic and kidney-specific therapies.

Randomized phase I/II study of troxacitabine combined with cytarabine, idarubicin, or topotecan in patients with refractory myeloid leukemias

PURPOSE

Troxacitabine has significant single-agent activity. This study was conducted to define the dose-limiting toxicities (DLTs) of its combination with cytarabine (ara-C), idarubicin, or topotecan.

PATIENTS AND METHODS

Patients with refractory acute myeloid leukemia (AML), advanced myelodysplastic syndromes (MDS), or chronic myelogenous leukemia in blastic phase (CML-BP) were initially randomly assigned to receive troxacitabine 5.0 mg/m(2) by intravenous (IV) infusion over 30 minutes on days 1 to 5 with ara-C 1.0 g/m(2)/d IV [DOSAGE ERROR CORRECTED] over 2 hours on days 1 to 5, idarubicin 12 mg/m(2) by 5 minute IV infusion on days 1 to 3, or topotecan 1.0 mg/m(2) as an continuous IV infusion on days 1 to 5. Doses were then adjusted to define DLT for each combination.

RESULTS

Eighty-seven patients (68 AML, eight MDS, 11 CML-BP) were treated. DLTs were hepatic transaminitis, hyperbilirubinemia, and hand foot syndrome (HFS) on the troxacitabine plus ara-C combination. The recommended phase II doses were 6 mg/m(2) once a day for 5 days and 1.0g/m(2) once a day for 5 days, respectively. DLTs were diarrhea, rash, and mucositis on the troxacitabine plus topotecan combination. The recommended phase II doses were 4 mg/m(2) once a day for 5 days and 0.75 mg/m(2) once a day for 5 days, respectively. DLTs were HFS, rash, and mucositis on the troxacitabine plus idarubicin combination. The recommended phase II doses were 4 mg/m(2) once a day for 5 days and 9 mg/m(2) once a day for 3 days, respectively. Among 74 evaluable patients with AML or MDS, 10 (13%) achieved complete remission and four (5%) had hematologic improvement. Two of 11 (18%) evaluable patients with CML-BP returned to chronic phase.

CONCLUSION

Troxacitabine-based combinations had significant antileukemic activity.

Potential mechanisms of resistance to cytarabine in AML patients

To determine whether the human equilibrative nucleoside transporter 1 (hENT1), deoxycytidine kinase (dCK), cytoplasmic 5'-nucleotidase (5NT), cytidine deaminase (CDD), topoisomerase I (TOPO I) and topoisomerase II alpha (TOPO II) are involved in clinical resistance to cytarabine (ara-C), we analyzed the level of expression of these parameters by reverse transcriptase polymerase chain reaction (rt-PCR), at diagnosis in the blast cells of 77 acute myeloid leukemia (AML) patients treated with ara-C, including 31 for whom samples were collected at first relapse. By univariate and/or multivariate analyses, patients with expression of 5NT or hENT1 deficiency at diagnosis had significantly shorter disease-free survival (DFS) and overall survival (OS). These results suggest that expression of 5NT and reduced hENT1 in leukemic blasts at diagnosis are correlated with clinical outcome and may play a role in resistance mechanisms to ara-C in patients with AML.

Nucleoside analogues and nucleobases in cancer treatment

Cytotoxic nucleoside analogues and nucleobases were among the first chemotherapeutic agents to be introduced for the medical treatment of cancer. This family of compounds has grown to include a variety of purine and pyrimidine nucleoside derivatives with activity in both solid tumours and malignant disorders of the blood. These agents behave as antimetabolites, compete with physiological nucleosides, and interact with a large number of intracellular targets to induce cytotoxicity. Progress has recently been made in the identification and characterisation of nucleoside transporters and the enzymes of nucleoside metabolism. In addition, there is now greater understanding of the molecular mechanisms of anticancer nucleoside activity, which provides opportunities for potentiating their antitumour effects. Strategies to optimise intracellular analogue accumulation and to enhance cancer-cell selectivity are proving beneficial in clinical trials.

Correlation of nucleoside and nucleobase transporter gene expression with antimetabolite drug cytotoxicity

Antimetabolite drugs that inhibit nucleic acid metabolism are widely used in cancer chemotherapy. Nucleoside and nucleobase transporters are important for the cellular uptake of nucleic acids and their corresponding anticancer analogue drugs. Thus, these transporters may play a role both in antimetabolite drug sensitivity, by mediating the uptake of nucleoside analogues, and in antimetabolite drug resistance, by mediating the uptake of endogenous nucleosides that may rescue cells from toxicity. Therefore, we examined the relation of the expression of nucleoside and nucleobase transporters to antimetabolite cytotoxicity. We measured the RNA levels of all eight known nucleoside and nucleobase transporters in 50 cell lines included in the National Cancer Institute's Anticancer Drug Screen panel. RNA levels of concentrative nucleoside transporters (CNTs), equilibrative nucleoside transporters (ENTs) and nucleobase transporters (NCBTs) were determined by quantitative RT-PCR using real-time fluorescence acquisition. This method was validated by measuring the expression of the MDR1 gene, and correlating our results with independently determined measurements of MDR1 RNA levels and protein function in these cell lines. We then correlated the pattern of RNA levels to the pattern of cytotoxicity of anticancer drugs in the NCI drug screen database using the COMPARE analysis. Several hypothesized relations between transporter gene expression and cytotoxicity, based upon known interactions between certain nucleoside analogues and transporter proteins, were not observed, suggesting that expression of individual transporters may not be a significant determinant of the cytotoxicity of these drugs. The most closely correlated drug cytotoxicity patterns to transporter gene expression patterns (where increased expression corresponds to increase sensitivity) included those between CNT1 and O6-methylguanine and between ENT2 and hydroxyurea. We also observed that p53 status influenced correlations between ENT1 transporter gene RNA levels and sensitivity to the drugs tiazafurin, AZQ and 3-deazauridine. One of three drugs identified by correlation of cytotoxicity patterns with ENT1 RNA levels, 3-deazauridine, inhibited uptake of the classic ENT1 substrate uridine, demonstrating a physical interaction between an identified drug and the transporter. These studies demonstrate that it is possible to correlate genetic information to functional databases to determine the influence of transport gene expression on drug sensitivity and to identify transporter-drug interactions.

Differential expression of human equilibrative nucleoside transporter 1 (hENT1) protein in the Reed-Sternberg cells of Hodgkin's disease

Gemcitabine is a cytotoxic nucleoside analog with activity in relapsing/refractory Hodgkin's disease (HD). Because gemcitabine is hydrophilic, it requires plasma membrane nucleoside transporter proteins to access intracellular targets. The most abundant and widely distributed transporter in human cells is human equilibrative nucleoside transporter 1 (hENT1). Because our prior studies showed that a deficiency in hENT1 confers high-level resistance to gemcitabine toxicity in vitro, we developed an immunohistochemical method to assess the hENT1 abundance of cells in tumor tissue. We now report the application of this method for visualizing the hENT1 protein abundance in the plasma membranes of Reed-Sternberg cells in lymph nodes of HD patients. Frozen sections of 30 lymph nodes were stained with monoclonal antibodies (mAb 10D7G2) raised against a synthetic peptide comprised of residues 254-271 from the large intracellular loop of hENT1 and staining intensity was scored on a 0-4 + scale. hENT1-staining intensity varied among HD lymph node samples (score/n; 0/8; 1/10; 2/9; 3/3; 4/0) and suggested that at least 60% of the tumors appeared hENT1 deficient. Because Epstein-Barr virus (EBV) is often associated with HD, staining for Epstein-Barr early RNA was also examined. Although 9/30 patients tested positive for EBV, there was no correlation with hENT1 staining. hENT1-staining intensities were positively correlated with age of the patient but were independent of other clinical, laboratory or pathology features (tumor stage, histologic subtype, presence of B symptoms, staining for CD15 or CD30, serum biochemistry, disease free survival, and overall survival). We conclude that, because hENT1 deficiency has been previously related to nucleoside-drug resistance, immunohistochemical staining for hENT1 warrants evaluation as a predictive tool for guiding the appropriate use of gemcitabine in the treatment of HD.

Troxacitabine-based therapy of refractory leukemia

Unique among currently approved or in-development nucleoside analogs, troxacitabine (Troxatyl) is an L-nucleoside with significant cytotoxic activity. Its stereochemistry and cellular transport characteristics render it insensitive to some tumor cell mechanisms of resistance to D-nucleosides, such as cytarabine and fludarabine. Troxacitabine's dose-limiting toxicities were mucositis and hand-foot syndrome in patients with refractory leukemia. Three complete and one partial remissions were observed in 30 patients with refractory acute myeloid leukemia on a Phase I study. Significant activity in blastic phase of chronic myeloid leukemia was seen on a Phase II study. Combinations of troxacitabine with ara-C, topotecan and idarubicin are active in patients with refractory acute myeloid leukemia (AML). Phase II studies in patients with refractory lymphoproliferative diseases are ongoing. Troxacitabine merits further study in patients with hematological malignancies.

Phase II study of troxacitabine, a novel dioxolane nucleoside analog, in patients with refractory leukemia

PURPOSE

To investigate the activity of a novel dioxolane L-nucleoside analog, troxacitabine (L-(-)-OddC, BCH-4556), in patients with refractory leukemia.

PATIENTS AND METHODS

Study participants were patients with refractory or relapsed acute myeloid (AML) or lymphocytic (ALL) leukemia, myelodysplastic syndromes (MDS), or chronic myelogenous leukemia in blastic phase (CML-BP). Troxacitabine was provided as an intravenous infusion for more than 30 minutes daily for 5 days at a dose of 8.0 mg/m(2)/d (40 mg/m(2) per course). Courses were given every 3 to 4 weeks according to antileukemic efficacy.

RESULTS

Forty-two patients (AML, 18 patients; MDS, one patient; ALL, six patients; CML-BP, 17 patients) were treated. Median age was 51 years (range, 23 to 80 years); 22 patients were male. Stomatitis was the most significant adverse event, with three patients (7%) and two patients (5%), respectively, experiencing grade 3 or 4 toxicity. Ten patients (24%) had grade 3 hand-foot syndrome, and two patients (5%) had grade 3 skin rash. One patient (2%) had grade 3 fatigue and anorexia. Marrow hypoplasia occurred between days 14 and 28 in 12 (75%) of 16 assessable patients with AML. Two complete remissions and one partial remission (18%) were observed in 16 assessable patients with AML. None of six patients with ALL responded. Six (37%) of 16 assessable patients with CML-BP experienced a return to chronic-phase disease.

CONCLUSION

Troxacitabine has significant antileukemic activity in patients with AML and CML-BP.

Dual mechanisms of 9-beta-D-arabinofuranosylguanine resistance in CEM T-lymphoblast leukemia cells

The guanine nucleoside analog araG is selectively toxic to T-lymphoblasts and has recently shown promise in treatment of lymphoid malignancies of T-cell origin. The molecular mechanism of this tissue-selective cytotoxicity is, however, yet unclear. AraG is phosphorylated, and thereby pharmacologically activated, by the mitochondrial deoxguanosine kinase and the cytosolic/nuclear deoxycytidine kinase. We have recently shown that araG is predominantly incorporated into mitochondrial DNA of cancer cell lines, which suggests a role of mitochondria as its pharmacological target. In the present study, we have generated araG-resistant CEM T-lymphoblast cell lines and show that araG resistance may occur by two separate molecular mechanisms that can occur sequentially. The first mechanism is associated with a decrease of araG incorporation into mitochondrial DNA, and the second event is associated with loss of dCK activity.

Nucleoside analogues: mechanisms of drug resistance and reversal strategies

Nucleoside analogues (NA) are essential components of AML induction therapy (cytosine arabinoside), effective treatments of lymphoproliferative disorders (fludarabine, cladribine) and are also used in the treatment of some solid tumors (gemcitabine). These important compounds share some general common characteristics, namely in terms of requiring transport by specific membrane transporters, metabolism and interaction with intracellular targets. However, these compounds differ in regard to the types of transporters that most efficiently transport a given compound, and their preferential interaction with certain targets which may explain why some compounds are more effective against rapidly proliferating tumors and others on neoplasia with a more protracted evolution. In this review, we analyze the available data concerning mechanisms of action of and resistance to NA, with particular emphasis on recent advances in the characterization of nucleoside transporters and on the potential role of activating or inactivating enzymes in the induction of clinical resistance to these compounds. We performed an extensive search of published in vitro and clinical data in which the levels of expression of nucleoside-activating or inactivating enzymes have been correlated with tumor response or patient outcome. Strategies aiming to increase the intracellular concentrations of active compounds are presented.