On the phosphorylation of 2-chlorodeoxyadenosine (CdA) and its correlation with clinical response in leukemia treatment

Human Deoxycytidine Kinase Is a Valuable Biocatalyst for the Synthesis of Nucleotide Analogues

Natural ribonucleoside-5’-monophosphates are building blocks for nucleic acids which are used for a number of purposes, including food additives. Their analogues, additionally, are used in pharmaceutical applications. Fludarabine-5´-monophosphate, for example, is effective in treating hematological malignancies. To date, ribonucleoside-5’-monophosphates are mainly produced by chemical synthesis, but the inherent drawbacks of this approach have led to the development of enzymatic synthesis routes. In this study, we evaluated the potential of human deoxycytidine kinase (HsdCK) as suitable biocatalyst for the synthesis of natural and modified ribonucleoside-5’-monophosphates from their corresponding nucleosides. Human dCK was heterologously expressed in E. coli and immobilized onto Nickel-nitrilotriacetic acid (Ni-NTA) superflow. A screening of the substrate spectrum of soluble and immobilized biocatalyst revealed that HsdCK accepts a wide range of natural and modified nucleosides, except for thymidine and uridine derivatives. Upon optimization of the reaction conditions, HsdCK was used for the synthesis of fludarabine-5´-monophosphate using increasing substrate concentrations. While the soluble biocatalyst revealed highest product formation with the lowest substrate concentration of 0.3 mM, the product yield increased with increasing substrate concentrations in the presence of the immobilized HsdCK. Hence, the application of immobilized HsdCK is advantageous upon using high substrate concentration which is relevant in industrial applications.

Control of fluxes in metabolic networks

Understanding the control of large-scale metabolic networks is central to biology and medicine. However, existing approaches either require specifying a cellular objective or can only be used for small networks. We introduce new coupling types describing the relations between reaction activities, and develop an efficient computational framework, which does not require any cellular objective for systematic studies of large-scale metabolism. We identify the driver reactions facilitating control of 23 metabolic networks from all kingdoms of life. We find that unicellular organisms require a smaller degree of control than multicellular organisms. Driver reactions are under complex cellular regulation in Escherichia coli, indicating their preeminent role in facilitating cellular control. In human cancer cells, driver reactions play pivotal roles in malignancy and represent potential therapeutic targets. The developed framework helps us gain insights into regulatory principles of diseases and facilitates design of engineering strategies at the interface of gene regulation, signaling, and metabolism.

The many isoforms of human adenylate kinases

Adenine nucleotides are involved in a variety of cellular metabolic processes, including nucleic acid synthesis and repair, formation of coenzymes, energy transfer, cell and ciliary motility, hormone secretion, gene expression regulation and ion-channel control. Adenylate kinases are abundant phosphotransferases that catalyze the interconversion of adenine nucleotides and thus regulate the adenine nucleotide ratios in different intracellular compartments. Nine different adenylate kinase isoenzymes have been identified and characterized so far in human tissues, named AK1 to AK9 according to their order of discovery. Adenylate kinases differ in molecular weight, tissue distribution, subcellular localization, substrate and phosphate donor specificity and kinetic properties. The preferred substrate and phosphate donor of all adenylate kinases are AMP and ATP respectively, but some members of the family can phosphorylate other substrates and use other phosphate donors. In addition to their nucleoside monophosphate kinase activity, adenylate kinases were found to possess nucleoside diphosphate kinase activity as they are able to phosphorylate both ribonucleoside and deoxyribonucleoside diphosphates to their corresponding triphosphates. Nucleoside analogues are structural analogues of natural nucleosides, used in the treatment of cancer and viral infections. They are inactive prodrugs that are dependent on intracellular phosphorylation to their pharmacologically active triphosphate form. Novel data presented in this review confirm the role of adenylate kinases in the activation of deoxyadenosine and deoxycytidine nucleoside analogues.

Novel potent inhibitors of deoxycytidine kinase identified and compared by multiple assays

Deoxycytidine kinase (dCK) phosphorylates deoxycytidine, deoxyguanosine, and deoxyadenosine and plays an important role in the salvage pathway of nucleoside metabolism. dCK is also required for the phosphorylation of several antiviral and anticancer nucleoside drugs, with resistance to these agents often being associated with a loss or decrease in dCK activity. Data also indicate a role for dCK in immune function, and dCK inhibitors may provide treatment for immune disorders. To identify novel dCK inhibitors, the authors evaluated 2 existing biochemical assays, adapted both to high-throughput screening, and identified several series of hits. They also compared the potency of the hits between purified recombinant and endogenous enzyme. Meanwhile, they also developed a novel cell-based assay that rests on the rescue of cells from dCK-dependent cytotoxic agents such as AraC. A large number of compounds were tested using the 3 assays, and a strong correlation in potency was observed between the biochemical assay using endogenous enzyme and the cell-based assay. The hits identified in these screens have proved to be good starting points for the synthesis of much more potent tool compounds to further investigate the physiological functions of dCK and potentially lead to the development of therapeutic agents.

Development and validation of an assay for the quantitative determination of cladribine nucleotides in MDCKII cells and culture medium using weak anion-exchange liquid chromatography coupled with tandem mass spectrometry

The development and validation of an assay for the quantitative analysis of cladribine mono-, di- and triphosphate (2-chloro, 2'-deoxyadenosine 5'-mono-, di- and triphosphate or 2CdAMP, 2CdADP and 2CdATP) in culture medium (Optimem) and cell lysate is described. Cladribine mono- and diphosphate reference compounds were obtained by thermal degradation of cladribine triphosphate. The reference compounds were characterized using ion-pairing reversed-phase high-performance liquid chromatography with ultraviolet detection. The bioanalytical assay for 2CdAMP, 2CdADP and 2CdATP is based on weak anion-exchange liquid chromatography coupled with tandem mass spectrometry in the positive ion mode (WAXLC/MS/MS). A fused-silica electrospray capillary was used instead of a stainless steel electrospray capillary to minimize adsorption of analytes and thus decrease variation in the analyte signals. Dynamic ranges of 1.11-27.7, 0.550-55.0 and 1.31-52.3 nM for 2CdAMP, 2CdADP and 2CdATP, respectively, were validated in culture medium and cell lysate. Optimem samples required stabilization with 30% methanol to prevent conversion of 2CdATP into 2CdAMP and 2CdADP. All intra- and interday accuracies and precisions were within +/-20%. The stability of the compounds was assessed under various analytically relevant conditions. The method was successfully used to investigate cladribine nucleotide transport in vitro in Madin-Darby canine kidney II (MDCKII) cells.

Identification of in vivo phosphorylation sites on human deoxycytidine kinase. Role of Ser-74 in the control of enzyme activity

Deoxycytidine kinase (dCK) catalyzes the rate-limiting step of the deoxyribonucleoside salvage pathway in mammalian cells and plays a key role in the activation of numerous nucleoside analogues used in anti-cancer and antiviral chemotherapy. Although compelling evidence indicated that dCK activity might be regulated by phosphorylation/dephosphorylation, direct demonstration was lacking. Here we showed that dCK overexpressed in HEK 293T cells was labeled after incubating the cells with [32P]orthophosphate. Sorbitol, which was reported to decrease dCK activity, also decreased the labeling of dCK. These results indicated that dCK may exist as a phosphoprotein in vivo and that its activity can be correlated with its phosphorylation level. After purification of 32P-labeled dCK, digestion by trypsin, and analysis of the radioactive peptides by tandem mass spectrometry, the following four in vivo phosphorylation sites were identified: Thr-3, Ser-11, Ser-15, and Ser-74, the latter being the major phosphorylation site. Site-directed mutagenesis and use of an anti-phospho-Ser-74 antibody demonstrated that Ser-74 phosphorylation was crucial for dCK activity in HEK 293T cells, whereas phosphorylation of other identified sites did not seem essential. Phosphorylation of Ser-74 was also detected on endogenous dCK in leukemic cells, in which the Ser-74 phosphorylation state was increased by agents that enhanced dCK activity. Our study provided direct evidence that dCK activity can be controlled by phosphorylation in intact cells and highlights the importance of Ser-74 for dCK activity.

Quantitation of synergism of arabinosylcytosine and cladribine against the growth of arabinosylcytosine-resistant human lymphoid cells

This report presents a quantitative analysis of the synergistic interaction of arabinosylcytosine (araC) and cladribine (CdA) in human H9-lymphoid cell lines sensitive and resistant to araC (H9-araC cells). H9-araC cells obtained by cultivation of H9 cells in the presence of 0.5 microM arabinosylcytosine (araC) had lower deoxycytidine kinase (dCK) than the parental cell line. The IC50 values of araC and CdA calculated by using median-effect analysis and CalcuSyn software were: 0.55 microM and 1.16 microM for CdA and 0.0058 microM and 3.5 microM for araC in H9 and H9-araC cells, respectively. These values were reduced to 0.10 microM and 0.38 microM for CdA and to 0.004 microM and to 0.77 microM for araC when the drugs were used in combination. Computerized simulation of dose reduction index (DRI) indicated that at 50-99% growth inhibition levels, the doses of araC could be reduced by 2.0 to 11.9-fold and 2.9 to 5.3-fold and the doses of CdA by 5.9 and 183.7-fold and 3.1 to 164.8-fold in H9 and H9-araC cells, respectively, when the drugs are used in combination. Assessment by combination index (CI) analysis showed that the combination exhibited moderate to strong synergistic lympho-cytotoxic effects. CdA metabolic studies (influx and activation) in the presence of deoxyadenosine, deoxycytidine, or araC suggested that CdA enters cells by a deoxyadenosine-inhibitable transport system, which is different than that of araC and deoxycytidine transport system. Thus, in addition to the known mechanisms, other mechanisms might be involved in the metabolism of CdA. The demonstration that araC and CdA combinations exert synergistic cytotoxicity even in the resistant cells raises hope that such a combination may be useful in tumors that were found resistant to these drugs.

Hairy cell leukemia: survival and relapse. Long-term follow-up of purine analog-based therapy and approach for relapsed disease

Since its initial description approximately 50 years ago, there has been an impressive evolution in therapies for hairy cell leukemia. Long-term follow-up with 2-cholorodeoxyadenosine and 2'-deoxycoformycin demonstrates that both agents result in a high complete remission rate and overall survival. Relapse rates appear to be between 20% and 30%. Therapeutic strategies for patients with relapsed disease include retreatment with a purine analog, rituximab, or the anti-CD22 pseudomonas exotoxin A immunoconjugate, BL-22.

Activation of deoxycytidine kinase by protein kinase inhibitors and okadaic acid in leukemic cells

Deoxycytidine kinase (dCK) is a key enzyme in the deoxynucleoside salvage pathway and in the activation of numerous nucleoside analogues used in cancer and antiviral chemotherapy. Recent studies indicate that dCK activity might be regulated through reversible phosphorylation. Here, we report the effects of a large panel of protein kinase inhibitors on dCK activity in the B-leukemia cell line EHEB, both in basal conditions and in the presence of the nucleoside analogue 2-chloro-2'-deoxyadenosine (CdA) which induces activation of dCK. Except staurosporine and H-7 that significantly reduced the activation of dCK by CdA, no specific protein kinase inhibitor diminished basal dCK activity or its activation by CdA. In contrast, genistein, a general protein tyrosine kinase inhibitor, and AG-490, an inhibitor of JAK2 and JAK3, increased basal dCK activity more than two-fold. Two specific inhibitors of the MAPK/ERK pathway, PD-98059 and U-0126, also enhanced dCK activity. These data suggest that the JAK/MAPK pathway could be involved in the regulation of dCK. Moreover, we show that the activity of dCK, raised by CdA, can return to its initial level by treatment with protein phosphatase-2A (PP2A). Accordingly, dCK activity in intact cells increased upon incubation with okadaic acid (OA) at concentrations that should inhibit PP2A, but not protein phosphatase-1. Activation of dCK by protein kinase inhibitors and OA was also observed in CCRF-CEM cells and in chronic lymphocytic leukemia B-lymphocytes, suggesting a general mechanism of post-translational regulation of dCK, which could be exploited to enhance the activation of antileukemic nucleoside analogues.

2-Chloro-2'-deoxyadenosine synergistically enhances azidothymidine cytotoxicity in azidothymidine resistant T-lymphoid cells

This report presents quantitative analysis of the synergistic interaction of azidothymidine (AZT) and cladribine (CdA) in human H9-lymphoid cell lines sensitive and resistant to AZT (H9-araC cells). H9-araC cells obtained by cultivation of H9 cells in the presence of 0.5 microM arabinosyl-cytosine (araC) had lower deoxycytidine kinase and thymidine kinase (TK) activities and expressed cross-resistance to araC and AZT. The IC(50) values of AZT and CdA were calculated by using median-effect analysis and CalcuSyn software. The IC(50) values were 0.44 and 0.82 microM for CdA and 67.8 and 30,310 microM for AZT in H9 and H9-araC cells, respectively. However, when the drugs were used in combination the IC(50) values of CdA and AZT were reduced to 0.12 and 15.5 microM in H9 cells and to 0.19 and 24.9 microM in H9-araC cells, respectively. Calculation of dose reduction index (DRI) indicated that at 50-90% growth inhibition level, the combination of the drugs caused 3.6-5.8- and 4.1-11.5-fold reduction in the dose of CdA and 4.4-37.6- and > 1000-fold reduction in the dose of AZT in H9 and H9-araC cells, respectively. The combination index (CI) values simulated from these data suggested synergistic to very strong synergistic lymphocytotoxic effects of AZT combined with CdA. These findings suggest the potential usefulness of a double-targeted approach for designing efficacious therapeutics for the kinase deficient drug resistant tumors.

Expression of deoxycytidine kinase in leukaemic cells compared with solid tumour cell lines, liver metastases and normal liver

Deoxycytidine kinase (dCK) is required for the phosphorylation of several deoxyribonucleoside analogues that are widely employed as chemotherapeutic agents. Examples include cytosine arabinoside (Ara-C) and 2-chlorodeoxyadenosine (CdA) in the treatment of acute myeloid leukaemia (AML) and gemcitabine to treat solid tumours. In this study, expression of dCK mRNA was measured by a competitive template reverse transcriptase polymerase chain reaction (CT RT-PCR) in seven cell lines of different histological origin, 16 childhood and adult AML samples, 10 human liver samples and 11 human liver metastases of colorectal cancer origin. The enzyme activity and protein expression levels of dCK in the cell lines were closely related to the mRNA expression levels (r=0.75, P=0.026 and r=0.86, P=0.007). In AML samples, dCK mRNA expression ranged from 1.16 to 35.25 (x10(-3)xdCK/beta-actin). In the cell line panel, the range was 2.97-56.9 (x10(-3)xdCK/beta-actin) of dCK mRNA expression. The enzyme activity in liver metastases was correlated to dCK mRNA expression (r=0.497, P=0.05). In the liver samples, these were not correlated. dCK mRNA expression showed only a 36-fold range in liver while a 150-fold range was observed in the liver metastases. In addition, dCK activity and mean mRNA levels were 2.5-fold higher in the metastases than in the liver samples. Since dCK is associated with the sensitivity to deoxynucleoside analogues and because of the good correlation between the different dCK measurements in malignant cells and tumours, the CT-RT PCR assay will be useful in the selection of patients that can be treated with deoxycytidine analogues.

Real-time quantitative PCR assays for deoxycytidine kinase, deoxyguanosine kinase and 5'-nucleotidase mRNA measurement in cell lines and in patients with leukemia

The relative levels of the deoxycytidine kinase (dCK), deoxyguanosine kinase (dGK), and the 5'-nucleotidase (5'-NT) are of importance for the effect of many nucleoside analogues used in the treatment of hematological malignancies. To elucidate dCK, dGK and 5'-NT gene expressions in cell lines and in samples from patients with leukemia, we have established a real-time quantitative PCR (RQ-PCR) method. From the available dCK, dGK and 5'-NT cDNA sequences we designed specific primers and fluorogenic probes for the respective genes. The mRNA of dCK, dGK and 5'-NT was also measured by semi-quantitative RT-PCR, the enzyme activities by a radioactive substrate-based technique and Western blot was used to measure the amount of dCK and dGK protein. A MOLT-4 wild-type and its 9-beta-D-arabinofuranosylguanine (Ara-G)-resistant subline was used for the methods comparisons and the RQ-PCR assay was used in 35 samples from pediatric patients with ALL and AML. The results from RQ-PCR for the cell lines were in agreement with the semi-quantitative RT-PCR. The mRNA expression for dCK, dGK and 5'-NT (expressed as the ratio of the respective gene and the reference gene) in pediatric ALL and AML patients showed a large interindividual variability from 0.06 to 2.34, non-detectable to 0.06 and 0.04 to 0.30, respectively. These results show that the quantitative evaluation by RQ-PCR is a valuable tool in the determination of dCK, dGK and 5'-NT mRNA levels in cell lines and in clinical samples which were expressed at various levels. This rapid, convenient and specific method is suitable for further studies of these genes in clinical samples.

Expression of high Km 5'-nucleotidase in leukemic blasts is an independent prognostic factor in adults with acute myeloid leukemia

Cytarabine (ara-C) requires activation into its triphosphorylated form, ara-CTP, to exert cytotoxic activity. Cytoplasmic 5'-nucleotidase (5NT) dephosphorylates ara-CMP, a key intermediate, preventing accumulation of ara-CTP and may reduce cellular sensitivity to the cytotoxic activity of ara-C. To determine whether the level of expression of 5NT is correlated with clinical outcome in patients with acute myeloid leukemia (AML) treated with ara-C, this study analyzed the levels of messenger RNA expression of high Km 5NT by real-time polymerase chain reaction at diagnosis in blast cells of 108 patients with AML. High Km 5NT was expressed at diagnosis in the blast cells of 54% of patients. In univariate analysis, (1) patients whose blast cells contained high levels (values greater than the median value for total population) of high Km 5NT at diagnosis had significantly shorter disease-free survival (DFS) than patients with low levels of high Km 5NT (11 months versus 17.5 months, P =.02) and (2) high levels of high Km 5NT also predicted significantly shorter overall survival (15.7 months versus 39 months, P = .01) in young patients (< or = 57 years; median value for the entire population). In a multivariate analysis taking into account age, karyotype risk, and other factors found to have prognostic significance in univariate analysis, (1) high Km 5NT expression was an independent prognostic factor for DFS and (2) high levels of high Km 5NT also predicted significantly shorter overall survival in young patients. These results demonstrate that the expression of high levels of high Km 5NT in blast cells is correlated with outcome in patients with AML.

In vitro inhibition of cytidine triphosphate synthetase activity by cyclopentenyl cytosine in paediatric acute lymphocytic leukaemia

Cytidine triphosphate (CTP) synthetase is a key enzyme for the synthesis of cytosine (deoxy)ribonucleotides, catalysing the conversion of uridine triphosphate (UTP) into CTP, and has a high activity in several malignancies. In this preclinical study, the enzyme activity and mRNA expression of the enzyme and (deoxy)ribonucleotide concentrations were analysed in leukaemic cells of 57 children suffering from acute lymphocytic leukaemia (ALL). In addition, in vitro experiments were performed with the CTP synthetase inhibitor cyclopentenyl cytosine (CPEC). A significantly higher activity of CTP synthetase (6.5 +/- 3.9 nmol CTP/mg/h) was detected in ALL cells than in lymphocytes of healthy controls (1.8 +/- 0.9 nmol CTP/mg/h, P < 0.001) that was independent of white blood cell (WBC) count, blast percentage, age, gender or type of ALL. The enzyme activity was not correlated with the CTP synthetase mRNA expression. The activity of CTP synthetase in ALL cells compared with non-malignant CD34+ bone marrow controls (5.6 +/- 2.4 nmol CTP/mg/h) was not statistically different. In vitro treatment of ALL cells with CPEC induced a dose-dependent decrease of the CTP concentration. The lowest concentration of CPEC (0.63 microM) induced a depletion of CTP of 41 +/- 20% and a depletion of dCTP of 27 +/- 21%. The degree of CTP depletion of ALL cells after treatment with CPEC was positively correlated with the activity of CTP synthetase. The inhibition of CTP synthetase in situ was confirmed by flux studies using radiolabelled uridine. From these results, it can be expected that CPEC has a cytostatic effect on lymphoblasts of children with ALL.

Metabolism and cytotoxic effects of 2-chloroadenine, the major catabolite of 2-chloro-2'-deoxyadenosine

EHEB cells, a continuous cell line derived from a patient with B cell chronic lymphocytic leukemia (B-CLL), synthesized, when incubated with tritiated 2-chloro-2'-deoxyadenosine (CdA), labeled mono-, di-, and triphosphate ribonucleosides at a much higher rate than CdA deoxyribonucleotides. Further analysis revealed that these ribonucleotides were formed from labeled 2-chloroadenine (CAde), which contaminated commercial tritiated CdA at a proportion of 2-3%. Since CAde is the major catabolite of CdA measured in plasma after oral or intravenous administration of CdA to patients, its metabolism and in particular its potential cytotoxicity were investigated both in EHEB cells and in B-CLL lymphocytes. Phosphorylation of CAde was inhibited by adenine, indicating that its initial metabolism most probably proceeds via adenine phosphoribosyltransferase (EC 2.4.2.7). In both cell types, chloro-ATP was the major metabolite formed from CAde and its concentration increased proportionally at least up to 50 microM CAde. At high concentration, CAde metabolism was accompanied by a decrease in intracellular ATP. Cytotoxicity of CAde, evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, showed an IC(50) of 16 microM in EHEB cells and 5 microM in B-CLL lymphocytes. At cytotoxic concentrations, apopain/caspase-3 activation and high molecular weight DNA fragmentation were observed, indicating that CAde cytotoxicity results from induction of apoptosis. However, since CAde cytotoxicity requires higher concentrations than CdA, it probably does not play a role in the therapeutic effect of CdA in the treatment of hematologic malignancies.

Induction of apoptosis by 2-chloro-2'deoxyadenosine (2-CdA) alone and in combination with other cytotoxic drugs: synergistic effects on normal and neoplastic lymphocytes by addition of doxorubicin and mitoxantrone

2-CdA is active as a single agent in the treatment of low-grade lymphomas. We analyzed the induction of apoptosis by 2-CdA alone (n=5) and in combination with other drugs in peripheral lymphocytes from 25 patients with leukemic low-grade lymphomas and from 25 healthy volunteers. 2-CdA was tested in 4 escalating concentrations (0.05 microg/ml to 0.4 microg/ml). Linear regressions showed a dose dependent apoptosis rate of 0.29 x microg 2-CdA/ml + 0.11 (r2=0.88, p=0.006) in normal cells and 0.41 x microg 2-CdA/ml + 0.15 (r2=0.88, p=0.005) in leukemic cells. Intracellular metabolization of 2-CdA into 2-CdA-5'mono-, -di- and the active metabolite -triphosphate was analyzed by HPLC and paralleled the dose dependent increase of apoptosis. The combination of 2-CdA with doxorubicin or mitoxantrone had a synergistic effect on the induction of apoptosis (p<0.001) in both normal and neoplastic lymphocytes, whereas 2-CdA plus etoposide or cytosine arabinoside were only additive. Due to the flat slope of the dose response of 2-CdA concentration on apoptosis we assume that higher in vivo dosages of 2-CdA in the treatment of low-grade lymphomas may not result in a higher clinical efficacy. The synergistic lymphocytotoxic effect of 2-CdA combined with doxorubicin or mitoxantrone may be relevant for new treatment approaches.

Nucleoside analogues in the therapy of Langerhans cell histiocytosis: a survey of members of the histiocyte society and review of the literature

BACKGROUND

Previous reports have suggested activity of the nucleoside analogues 2-chlorodeoxyadenosine (2-CdA) and 2'-deoxycoformycin (2'-DCF) in Langerhans cell histiocytosis (LCH).

PROCEDURE

To assess the efficacy of 2-CdA and 2'-DCF as salvage therapy for LCH, a survey of members of the Histiocyte Society and a literature review were undertaken. Twenty-three patients treated with 2-CdA and 4 treated with 2'-DCF were found, age range 2 months to 49 years.

RESULTS

All 15 survey patients had multiorgan involvement, and 14 were heavily pretreated. Doses of 2-CdA ranged from 0.1 mg/kg/day continuous infusion for 5-7 days (majority of patients) to 13 mg/m(2)/day for 5 days, for 1-6 courses. One of the 15 patients had an early death, 5 had no response (NR), 3 had partial response (PR), and 6 achieved complete response (CR). Among 8 published patients, 7 achieved stable CR and 1 NR. Among 4 patients treated with 2'-DCF (4 mg/m(2)/week for 8 weeks then q 2 weekly), 2 achieved CR for 16+ and 18+ months and 2 PR for 2 and 5 months. Toxicity consisted mainly of combined myelo- and immunosuppression but no significant infections occurred and there were no toxic deaths. A cumulative thrombocytopenia was noted, which in 1 case took up to 6 months to resolve. Transient gastrointestinal toxicity and elevation of liver enzymes was seen, and 2 patients developed renal tubular acidosis. The peripheral neuropathy reported in adult patients receiving high doses was not seen.

CONCLUSIONS

2-CdA and 2'-DCF appear to have a useful role in LCH and are worthy of prospective trial in patients unresponsive to routine therapy.

Phosphorylation of anticancer nucleoside analogs by human mitochondrial deoxyguanosine kinase

The kinetic properties of recombinant human mitochondrial deoxyguanosine kinase (dGK, EC 2.7.1.113) for 2'-deoxyguanosine and the clinically important nucleoside analogs 2-chloro-2'-deoxyadenosine (CdA), 9-beta-D-arabinofuranosylguanine (araG) and 2',2',-difluorodeoxyguanosine (dFdG) were determined. The Michaelis-Menten kinetic parameters, comparing ATP and UTP as phosphate donors, demonstrated a marked increase in phosphorylation efficiency (VmaxKm) with UTP in comparison with ATP for both CdA and araG. The difluoro analog dFdG was an efficient substrate for recombinant dGK with an apparent Km of 16 microM with ATP as phosphate donor. We compared the kinetic properties of dGK with those of the related enzyme deoxycytidine kinase (dCK, EC 2.7.1.74). Although the purines 2'-deoxyguanosine (dGuo) and 2'-deoxyadenosine are substrates for both dGK and dCK, only CdA among the purine nucleoside analogs tested was an efficient substrate for both dCK and dGK. In competition with dGuo, the most efficient analog for phosphorylation by dGK was araG, as indicated by a lower Ki value than for CdA and dFdG. Of the purine analogs tested as substrates for dCK, only CdA could compete with 2'-deoxycytidine (dCyd). No inhibition of dCK-mediated dCyd phosphorylation was found by either araG or dFdG. In crude cell extract of HeLa and Capan 2 cells, the major CdA phosphorylation was contributed by dCK, while most araG phosphorylation was a result of dGK activity. Our study with pure recombinant enzymes confirms that dGK is mainly responsible for araG and dFdG phosphorylation, whereas dCK is the most important enzyme for activation of CdA and 2',2'-difluorodeoxycytidine (dFdC).

Analysis of 2-chloro-2'-deoxyadenosine incorporation into cellular DNA by quantitative polymerase chain reaction

Thermus aquaticus (Taq) DNA polymerase elongation is blocked by several DNA adducts. This property has been exploited in polymerase chain reaction (PCR) methods to analyze cellular DNA damage and repair after exposure to damaging agents. Such methods have not been applied previously to detect nucleoside analog incorporation into cellular DNA. 2-Chloro-2'-deoxyadenosine (CldAdo), a deoxyadenosine analog, is clinically effective for hairy cell leukemia. CldAdo is taken up by cells, converted to the triphosphate, and incorporated into cellular DNA. Here, we measured by primer extension the ability of CldAMP residues in 98-base single-stranded DNA to block Taq elongation. In contrast to control DNA, no full-length 98-mers were produced on CldAMP-containing templates, and Taq polymerase was halted at the first CldAMP site. We then examined the possibility of using quantitative PCR to measure CldATP incorporation into the N-ras gene after incubation of cultured human leukemia cells with CldAdo or with cisplatin as a positive control for DNA damage. Treatment with either drug resulted in reduced amounts of amplified DNA product compared to untreated cells. CldAMP residues within cellular DNA inhibited PCR amplification in a dose-dependent manner; 100 nM CldAdo produced approximately 0.4 CldAMP sites within a 523-bp region of the N-ras sequence. Thus, PCR analysis with Taq polymerase provides a sensitive assessment of nucleoside analog incorporation after cellular exposure to antileukemic drugs.

2-Chloro-2'-deoxyadenosine, an antileukemic drug, has an early effect on cellular mitochondrial function

2-Chloro-2'-deoxyadenosine [CldAdo (cladribine)], a novel effective antileukemic agent, was examined for its effects on cellular mitochondrial function and DNA content after long term (< or = 7-day) incubation of cultured CCRF-CEM human leukemia cells. Dideoxycytidine (ddC), which is known to have a delayed effect on mitochondrial DNA content, was used as a positive control to monitor mitochondrial dysfunction. CldAdo at 6-16 nM was toxic to cells within 24 hr, which is in contrast to 300 nM ddC, which had no effect on cell growth for the first 4 days of treatment. Cellular lactic acid production was used to monitor concomitant perturbations in oxidative phosphorylation during drug treatment. Unlike the delayed increase in lactate observed with ddC exposure, CldAdo-treated cells exhibited a 2-2.4-fold increase in lactate levels after 2 days of exposure to 16 nM CldAdo. By days 4 and 7, however, lactate production returned to control levels. Shorter incubations with CldAdo revealed that lactate levels began to increase within 12 hr of drug exposure, paralleling cytotoxicity. We also examined mitochondrial DNA content during drug treatment by competitive polymerase chain reaction. ddC (300 nM) reduced mitochondrial DNA levels from approximately 1000 copies/untreated cell to approximately 130 copies/cell after 7 days of exposure. In contrast, cytotoxic doses of CldAdo had little or no effect on mitochondrial DNA content during the 1-week incubation. Thus, the early CldAdo-induced perturbation of mitochondrial function was not associated with a loss of mitochondrial DNA per cell. In addition, no evidence of DNA laddering, indicative of cellular apoptosis, was detected at these dosage levels and treatment times.

Cloning and expression of human deoxyguanosine kinase cDNA

A human cDNA sequence homologous to human deoxycytidine kinase (dCK; EC 2.7.1.74) was identified in the GenBank sequence data base. The longest open reading frame encoded a protein that was 48% identical to dCK at the amino acid level. The cDNA was expressed in Escherichia coli and shown to encode a protein with the same substrate specificity as described for the mitochondrial deoxyguanosine kinase (dGK; EC 2.7.1.113). The N terminus of the deduced amino acid sequence had properties characteristic for a mitochondrial translocation signal, and cleavage at a putative mitochondrial peptidase cleavage site would give a mature protein size of 28 kDa. Northern blot analysis determined the length of dGK mRNA to 1.3 kbp with no cross-hybridization to the 2.8-kbp dCK mRNA. dGK mRNA was detected in all tissues investigated with the highest expression levels in muscle, brain, liver, and lymphoid tissues. Alignment of the dGK and herpes simplex virus type 1 thymidine kinase amino acid sequences showed that five regions, including the substrate-binding pocket and the ATP-binding glycine loop, were also conserved in dGK. To our knowledge, this is the first report of a cloned mitochondrial nucleoside kinase and the first demonstration of a general sequence homology between two mammalian deoxyribonucleoside kinases. Our findings suggest that dCK and dGK are evolutionarily related, as well as related to the family of herpes virus thymidine kinases.