Deoxynucleoside phosphorylating enzymes in monkey and human tissues show great similarities, while mouse deoxycytidine kinase has a different substrate specificity

Cladribine modifies functional properties of microglia

Cladribine (CdA), an oral prodrug approved for the treatment of relapsing multiple sclerosis, selectively depletes lymphocytes. CdA passes the blood–brain barrier, suggesting a potential effect on central nervous system (CNS) resident cells. We examined if CdA modifies the phenotype and function of naive and activated primary mouse microglia, when applied in the concentrations 0·1–1 μM that putatively overlap human cerebrospinal fluid (CSF) concentrations. Primary microglia cultures without stimulation or in the presence of proinflammatory lipopolysaccharide (LPS) or anti‐inflammatory interleukin (IL)‐4 were treated with different concentrations of CdA for 24 h. Viability was assessed by MTT [3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide] assay. Phagocytotic ability and morphology were examined by flow cytometry and random migration using IncuCyte Zoom and TrackMate. Change in gene expression was examined by quantitative polymerase chain reaction (qPCR) and protein secretion by Meso Scale Discovery. We found that LPS and IL‐4 up‐regulated deoxycytidine kinase (DCK) expression. Only activated microglia were affected by CdA, and this was unrelated to viability. CdA 0·1–1 μM significantly reduced granularity, phagocytotic ability and random migration of activated microglia. CdA 10 μM increased the IL‐4‐induced gene expression of arginase 1 (Arg1) and LPS‐induced expression of IL‐1β, tumor necrosis factor (TNF), inducible nitric oxide synthase (iNOS) and Arg1, but protein secretion remained unaffected. CdA 10 μM potentiated the increased expression of anti‐inflammatory TNF receptor 2 (TNF‐R2) but not TNF‐R1 induced by LPS. This suggests that microglia acquire a less activated phenotype when treated with 0·1–1 μM CdA that putatively overlaps human CSF concentrations. This may be related to the up‐regulated gene expression of DCK upon activation, and suggests a potential alternative mechanism of CdA with direct effect on CNS resident cells.

Treatment of Acute SIV infection in cynomolgus Monkeys with 2′,3′-dideoxyinosine (ddl) and 2′,3′-dideoxythymidiene (d4T)

2′,3′-dideoxyinosine (ddl) and 2′,3′-dideoxythymidiene (d4T) have been reported to inhibit the multiplication of human immunodeficiency virus (HIV) in patients, ddl and d4T were administered subcutaneously to cynomolgus monkeys starting 8 h prior to inoculation with 10–50 infectious doses of simian immunodeficiency virus (SIVSM). ddl, at a daily dose of 3 × 3.6 mg kg−1 for 10 days significantly delayed the appearance of SIVSM p24/26 antigen. A significant delay in the appearance of SIVSM p24/26 antigen was also seen when using a daily dose of 3 × 1 mg kg −1 of d4T for 10 days. Neither ddl nor d4T prevented SIVSM infection despite administration prior to virus inoculation.

Species Differences in Troxacitabine Pharmacokinetics and Pharmacodynamics

PURPOSE

Troxacitabine is the first unnatural L-nucleoside analog to show potent preclinical antitumor activity and is currently under clinical investigation. Significant differences in troxacitabine toxicity between mice, rats, monkeys, and humans were observed during preclinical and clinical evaluations. To better understand the different toxicity and efficacy results observed between the human xenograft mouse tumor models used for preclinical assessment and the clinical study results, the pharmacodynamics and pharmacokinetics of troxacitabine were reassessed in murine and human models.

EXPERIMENTAL DESIGN

Clonal and thymidine incorporation assays were used to investigate the in vitro antiproliferative activity of troxacitabine on a selected panel of mouse and human tumor cell lines and normal hemapoietic cells. Analysis of the intracellular metabolites of [14C]troxacitabine was determined in mouse and human T-lymphocytes obtained from peripheral blood. The antitumor efficacy of troxacitabine administered either as single or repeated high-dose bolus administrations or as low-dose continuous infusions was evaluated in the human colon HT-29 xenograft model. We also determined plasma concentrations of troxacitabine using the different administration schedules.

RESULTS

Five to nine hundred-fold lower concentrations of troxacitabine were required to inhibit cell growth in human compared with murine tumor and normal hemapoietic cell lines. Furthermore, the sensitivity of cells of both species to troxacitabine was strongly time dependent, requiring >24 hours exposure for maximum activity. Analysis of the intracellular metabolites of [14C]troxacitabine in T-lymphocytes obtained from peripheral blood revealed subsequently higher levels of mono-, di-, and triphosphates in human compared with mouse. Antitumor efficacy studies revealed that prolonged exposure schedules (up to 6 days) showed equivalent efficacy to repeated high-dose bolus administrations. Five-day continuous infusion of 20 mg/mL troxacitabine via subcutaneous implanted mini-osmotic pump maintained systemic concentrations of 262 ng/mL (1.2 micromol/L) for the duration of administration, which are clinically achievable plasma concentrations, and led to significant antitumor activity [treated versus control (T/C) of 27% and tumor regression during treatment].

CONCLUSIONS

These studies support the hypothesis that troxacitabine infusions might be the administration regimen with the greatest likelihood of fully exploiting clinically the potent preclinical antitumor activity of troxacitabine.

Identification of residues involved in the substrate specificity of human and murine dCK

Deoxycytidine kinase (dCK) is a salvage pathway enzyme that can phosphorylate both pyrimidine and purine deoxynucleosides, including important antiviral and cytostatic agents. Earlier studies showed that there are differences in kinetic properties between human and murine dCK, which may explain differences in toxic effects of nucleoside analogs. To determine if certain substitutions in amino acid sequences between human and mouse dCK give these differences in substrate specificity the 14 mutants and hybrid forms of human dCK were studied. All variants were characterised with dCyd, dAdo and dGuo as phosphate acceptors and ATP and UTP as phosphate donor. The relative activities with dCyd, dAdo and dGuo were about 70, 20, 30%, respectively, with UTP as compared to ATP for human dCK and 40, 60, 70% for mouse dCK. Among all tested mutants only the triple combination of substitutions Q179R-T184K-H187N (RKN) had a kinetic behaviour very similar to mouse dCK. The kinetic patterns with several important nucleoside analogs, such as AraC, CdA, ddC and AraG have also been studied. Results demonstrated 50-70% low relative capacities of the recombinant mouse and triple mutant RKN to phosphorylate this nucleoside analogs compare with human dCK. A model for dCK was used to try to explain the functional role of these amino acid substitutions. According to this model the triple mutant RKN have altered amino acids in a region necessary for conformational changes during catalyses. This may affects the substrate selectivity both for the nucleosides and the phosphate donors.

Activation of deoxycytidine kinase by inhibition of DNA synthesis in human lymphocytes

Deoxycytidine kinase (dCK, EC.2.7.1.74) is a key enzyme in the intracellular metabolism of 2-chlorodeoxyadenosine, 1-beta-D-arabinofuranosylcytosine, difluorodeoxycytidine, and other drugs used in chemotherapy of different leukaemias and solid tumours. Recently, stimulation of dCK activity was shown by these analogues and by other genotoxic agents such as etoposide and NaF, all of which cause severe inhibition of DNA synthesis in cell cultures. Here we describe that direct inhibition of DNA polymerases by aphidicolin stimulated dCK activity in normal lymphocytes and acute myeloid leukaemic cells, as well as in HL 60 promyelocytic cell cultures. Increased dCK activity was not due to new protein synthesis under our conditions, as measured by immunoblotting. Partial purification by diethylaminoethyl-Sephadex chromatography revealed that the activated form of dCK survived purification procedure. Moreover, it was possible to inactivate purified dCK preparations by recombinant protein phosphatase with Ser/Thr/Tyr dephosphorylating activity. These data suggest that the activation of dCK may be due to phosphorylation, and that deoxynucleoside salvage is promoted during inhibition of DNA synthesis in human lymphocytes.

Safety assessment, in vitro and in vivo, and pharmacokinetics of emivirine, a potent and selective nonnucleoside reverse transcriptase inhibitor of human immunodeficiency virus type 1

Emivirine (EMV), formerly known as MKC-442, is 6-benzyl-1-(ethoxymethyl)-5-isopropyl-uracil, a novel nonnucleoside reverse transcriptase inhibitor that displays potent and selective anti-human immunodeficiency virus type 1 (HIV-1) activity in vivo. EMV showed little or no toxicity towards human mitochondria or human bone marrow progenitor cells. Pharmacokinetics were linear for both rats and monkeys, and oral absorption was 68% in rats. Whole-body autoradiography showed widespread distribution in tissue 30 min after rats were given an oral dose of [(14)C]EMV at 10 mg/kg of body weight. In rats given an oral dose of 250 mg/kg, there were equal levels of EMV in the plasma and the brain. In vitro experiments using liver microsomes demonstrated that the metabolism of EMV by human microsomes is approximately a third of that encountered with rat and monkey microsomes. In 1-month, 3-month, and chronic toxicology experiments (6 months with rats and 1 year with cynomolgus monkeys), toxicity was limited to readily reversible effects on the kidney consisting of vacuolation of kidney tubular epithelial cells and mild increases in blood urea nitrogen. Liver weights increased at the higher doses in rats and monkeys and were attributed to the induction of drug-metabolizing enzymes. EMV tested negative for genotoxic activity, and except for decreased feed consumption at the high dose (160 mg/kg/day), with resultant decreases in maternal and fetal body weights, EMV produced no adverse effects in a complete range of reproductive toxicology experiments performed on rats and rabbits. These results support the clinical development of EMV as a treatment for HIV-1 infection in adult and pediatric patient populations.

Distribution of 2-chloro-2'-deoxyadenosine, 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine, fludarabine and cytarabine in mice: a whole-body autoradiography study

The distribution characteristics of tritiated nucleoside analogs, 2-chloro-2'-deoxyadeonosine (CdA), 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine (CAFdA), 2-fluoroarabinosyladenine (F-ara-A) and cytosine arabinoside (ara-C) were compared in mice using whole-body autoradiography. CdA, CAFdA and F-ara-A have quite similar molecular structures, but they differ substantially in clinical activity as well as the side effects. Eight mice were injected intravenously in couples. One mouse from each pair was killed 20 min postinjection and the other mouse from each pair 4 h after the injection. The distribution of the label was then analyzed by whole-body autoradiography. The distribution of the nucleoside analogs was rapid and uniform. High concentrations were found in highly perfused organs. After 4 h the overall concentration had decreased but relatively high activities were found in the skin for CdA and CAFdA, in the thymus for ara-C and the bone marrow for CdA. Both CdA and CAFdA were found in the brain, but the concentration was surprisingly lower after 4 h for CAFdA, a lipophilic and more stable analog as compared to CdA. There was an uptake of CdA, F-ara-A and CAFdA in the skin. There were signs of retention of ara-C in parts of the thymus. The present investigations indicate that the nucleoside analog transport to the brain in mice is not primarily dependent upon passive diffusion over a lipophilic barrier, but suggestive of a specific transport mechanism.

Cloning and characterization of mouse deoxyguanosine kinase. Evidence for a cytoplasmic isoform

Deoxyguanosine kinase (dGK) is a nuclear gene product that catalyzes the phosphorylation of purine deoxyribonucleosides and their analogues. The human enzyme is located predominantly in the mitochondria, as shown by biochemical fractionation studies and in situ localization of the overexpressed recombinant protein. Here we describe the cloning of mouse dGK cDNA and the identification of a novel amino-terminally truncated isoform that corresponds to about 14% of the total dGK mRNA population in mouse spleen. In situ fluorescence assays suggest that the new isoform cannot translocate into the mitochondria and thus may represent a cytoplasmic enzyme. Expression of mouse dGK mRNA was highly tissue-specific and differed from the tissue distribution observed in humans. Recombinant mouse dGK showed similar specific activity and substrate specificity as compared with the human enzyme. The broad specificity, restricted tissue distribution, and location of mouse dGK in multiple cellular compartments raise new considerations with respect to the role of the individual deoxynucleoside kinases in nucleotide metabolism.

Efficacy of the carbocyclic 2'-deoxyguanosine nucleoside BMS-200475 in the woodchuck model of hepatitis B virus infection

Daily oral treatment with the cyclopentyl 2'-deoxyguanosine nucleoside BMS-200475 at doses ranging from 0.02 to 0.5 mg/kg of body weight for 1 to 3 months effectively reduced the level of woodchuck hepatitis virus (WHV) viremia in chronically infected woodchucks as measured by reductions in serum WHV DNA levels and endogenous hepadnaviral polymerase activity. Within 4 weeks of daily therapy with 0.5 or 0.1 mg of BMS-200475 per kg, endogenous viral polymerase levels in serum were reduced about 1,000-fold compared to pretreatment levels. Serum WHV DNA levels determined by a dot blot hybridization technique were comparably decreased in these treated animals. In the 3-month study, the sera of animals that had undetectable levels of WHV DNA by the dot blot technique were further analyzed by a highly sensitive semiquantitative PCR assay. The results indicate that BMS-200475 therapy reduced mean WHV titers by 10(7)- to 10(8)-fold, down to levels as low as 10(2) to 10(3) virions/ml of serum. Southern blot hybridization analysis of liver biopsy samples taken from animals during and after BMS-200475 treatment showed remarkable reductions in the levels of WHV DNA replicative intermediates and in the levels of covalently closed circular viral DNA. WHV viremia in BMS-200475-treated WHV carriers eventually returned to pretreatment levels after therapy was stopped. These results indicate that BMS-200475 should be evaluated in clinical trials for the therapy of chronic human hepatitis B virus infections.

Activation of deoxycytidine kinase during inhibition of DNA synthesis by 2-chloro-2'-deoxyadenosine (Cladribine) in human lymphocytes

Deoxycytidine kinase (dCK, EC.2.7.1.74), a key enzyme in intracellular metabolism of many antileukemic drugs, was shown to be activated during treatment of lymphocytes by 2-chloro-2'-deoxyadenosine (Cl-dAdo, cladribine), a potent inhibitor of DNA synthesis. While 5-[3H]-thymidine (TdR) incorporation into DNA was decreased by 80-90%, dCK activity was doubled as a consequence of incubating the cells with 1 microM 2-chloro-2'-deoxyadenosine. Thymidine kinase (dTK, EC.2.7.1.21) activity was slightly decreased under the same conditions, similarly to 5-[3H]-thymidine incorporation. dCK activation could not be prevented by cycloheximide, and neither the amount of dCK protein nor its mRNA level was increased after 2-chloro-2'-deoxyadenosine treatment. These results suggest a post-translational activation of dCK protein during inhibition of DNA synthesis.

Alterations in the properties of mouse leukemia L1210 cell lines selected by different methods for resistance to deoxyguanosine

Mouse leukemia L1210 cells were generated for resistance to deoxyguanosine by two different methods. In one case the L1210 cells were subjected to gradual increases in deoxyguanosine (dGuo-R); in the second approach, the cells were subjected to deoxyguanosine at a concentration ten times the IC50 value and plated out on soft agar (D-92). The dGuo-R and D-92 cell lines had different phenotypic expressions. The dGuo-R cells showed a higher degree of resistance to dGuo than the D-92 cells. The levels of resistance to other cytotoxic drugs such as araC or 2-chloro-2'-deoxyadenosine did not necessarily correlate with the degree of resistance to dGuo. Deoxycytidine kinase activity was decreased in both of the cell lines, although there was a larger decrease in the dGuo-R cell line. The levels of kinase activities toward the other substrates were not all coordinately decreased in these cell lines. The degree of resistance of these cell lines to dGuo cannot be ascribed solely to an alteration at the deoxycytidine kinase site.

In vivo antiviral activity and pharmacokinetics of (-)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine in woodchuck hepatitis virus-infected woodchucks

The (-) enantiomer of cis-5-fluoro-1l-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine [(-)-FTC)], a substituted oxathiolane compound with anti-hepatitis B virus activity in vitro, was assessed for its efficacy in woodchucks with naturally acquired woodchuck hepatitis virus (WHV) infection. Pharmacokinetics and in vitro anabolism were also determined. (-)-FTC was anabolized to the 5'-triphosphate in a dose-related fashion, reaching a maximum concentration at about 24 h in cultured woodchuck hepatocytes. Following administration of a dose of 10 mg/kg of body weight intraperitoneally (i.p.), the clearance of (-)-FTC from plasma was monoexponential, the terminal half-life was 3.76 +/- 1.4 h, and the systemic clearance was 0.12 +/- 0.06 liters/h/kg. The antiviral efficacy of (-)-FTC in the woodchuck model was assessed by quantitation of serum WHV DNA levels and by WHV particle-associated DNA polymerase activity at two dosages, 30 and 20 mg/kg given i.p. twice daily (b.i.d.), respectively. The level of WHV DNA in serum was reduced 20- to 150-fold (average, 56-fold) in the 30-mg/kg-b.i.d. treatment group and 6- to 49-fold (average, 27-fold) in the 20-mg/kg-b.i.d. treatment group. Viral DNA polymerase levels diminished accordingly. One week after treatment was discontinued, WHV levels returned to pretreatment levels in both studies. These animals were biopsied before and following treatment with 30 mg of (-)-FTC per kg. Their livers were characterized by a mild increase in cytoplasmic lipid levels, but this change was not associated with altered liver enzyme levels. Serum chemistry and hematology results were within the normal ranges for all treated animals. We conclude that (-)-FTC is a potent antihepadnaviral agent and that it has no detectable toxic effects in woodchucks when given for up to 25 days. Further development of (-)-FTC as an anti-hepatitis B virus therapy for patients is warranted.

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.

Regulation of phosphorylation of deoxycytidine and 2',2'-difluorodeoxycytidine (gemcitabine); effects of cytidine 5'-triphosphate and uridine 5'-triphosphate in relation to chemosensitivity for 2',2'-difluorodeoxycytidine

Deoxycytidine kinase(dCK) and deoxycytidine deaminase (dCDA) are two key enzymes in the activation and inactivation, respectively, of deoxycytidine and its antiviral and anticancer analogues. One purpose of this study was to determine whether or not the deoxycytidine-converting activity of both enzymes would correlate with growth inhibition by 2',2'-difluorodeoxycytidine (dFdC), a deoxycytidine analogue with established antitumour activity in solid tumours. Another aim of this work was to determine the effects of normal nucleotides on dCK. dCK and dCDA activities were measured with both deoxycytidine and dFdC as substrates in 5 solid tumour cell lines, but no correlation with cellular sensitivity to dFdC was found with either substrate. The normal dCK activities with deoxycytidine as substrate varied between 0.8 and 13 nmol/hr/10(6) cells. The activities determined with dFdC as substrate were remarkably similar in all 5 cell lines (1.1-1.6 nmol/hr/10(6) cells). dCDA activities varied considerably with both substrates (20-30 fold). Because dFdC markedly affected intracellular concentrations of cytidine 5'-triphosphate (CTP) and uridine 5'-triphosphate (UTP), we studied their effect on deoxycytidine-and dFdC-phosphorylating activities in 3 cell lines (i.e., A2780, WiDr and C26-10) with similar dCK activity but major differences in dFdC sensitivity, 1 mM CTP inhibited deoxycytidine phosphorylation (at 230 muM) by 20-30% in A2780 and C26-10 cells, but increased that of WiDr cells by approximately 70%. CTP did not++ affect dFdC phosphorylation (at 230 muM) in A2780 cells, but did increase it by 40% in WiDr cells. At 1 and 10 muM of deoxycytidine the effects of CTP on dCK activity in A2780, C26-10 and WiDr cells were less pronounced. 1mM UTP enhanced deoxycytidine phosphorylation at 230 muM in WiDr cells by approximately 40%, whereas dFdC phosphorylation was increased 40% by UTP in C26-10 cells but decreased by 70-80% in WiDr cells. UTP caused a more pronounced increase in dCK activity at 1 and 10 muM deoxycytidine in C26-10 cells, but provoked a higher inhibition in A2780 and WiDr Cells at 10 muM. Because of these complex results, dCK kinetics were studied in greater detail. Biphasic kinetics for deoxycytidine were observed in all 3 cell lines, with Km values of 23.2 and 0.4 muM for A2780 cells, 15.9 and 1.5 muM for C26-10 cells, and 27.2 and 0.9 muM for WiDr cells. In all 3 cell lines, adenosine 5'-triphosphate (ATP) was the optimal phosphate donor, as compared to CTP and UTP. In conclusion, the efficiency of dCK (Vmax/Km ratio) seems to correlate with accumulation of dFdCTP, the active metabolite of dFdC, and with cellular sensitivity. UTP and CTP, which are seriously affected in cells exposed to dFdC, display varying effects in these solid tumour cell lines. Both activation and inhibition have been observed; the physiologically low CTP pools and the relatively minor effect on dCK in A2780 cells seem to favour dFdC phosphorylation in these cells, which are the most sensitive.

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

The nucleoside analog 2-chlorodeoxyadenosine (CdA, Cladribine) is a chemotherapeutic agent for treatment of leukemias and lymphomas, most successfully used in hairy cell leukemia and B-cell chronic lymphocytic leukemia. CdA is phosphorylated intracellularly to its monophosphate derivative by the enzymes deoxycytidine kinase and deoxyguanosine kinase. Cell lines deficient in deoxycytidine kinase were shown to be resistant to CdA and a high deoxycytidine kinase level in combination with low 5'-nucleotidase has been proposed to partly explain the selectivity in CdA toxicity for lymphoid cells. In this report biochemical properties in CdA phosphorylation mediated by deoxycytidine kinase and deoxyguanosine kinase are reviewed and discussed in relation to the further metabolism of CdA 5'-monophosphate, the different possible mechanisms of action and the correlation with clinical response. It is concluded that much is known about the metabolism and mechanisms of action of CdA, but that the remarkable therapeutic effect in hairy cell leukemia has yet to be explicitly explained.

Development of a novel mouse tk+/- embryonic stem cell line for use in mutagenicity studies

A tk+/- mouse embryonic stem (ES) cell line, designated 1G2, has been created in which one allele of the thymidine kinase (tk) gene was inactivated by targeted homologous recombination. This line is an analog of the mouse lymphoma tk+/- L5178Y cell line, which is used widely to assess the mutagenicity of chemical agents. Treatment of 1G2 cells with the alkylating agent N-ethyl-N-nitrosourea (ENU) resulted in a dose-related increase in trifluorothymidine-resistant colonies. Mutant frequencies of 152 and 296 per 10(6) cells were determined for 0.1 and 0.3 mg/ml doses of ENU, compared with a spontaneous mutant frequency of 15 per 10(6) cells. The data indicate that tk+/- 1G2 ES cells may be useful for the creation of a transgenic mouse model for assessing in vivo mutation using an endogenous autosomal gene.

Hepatic purine and pyrimidine metabolism: implications for antiviral chemotherapy of viral hepatitis

The use of nucleoside analogues as antiviral agents is expanding. For most nucleoside analogues, intracellular phosphorylation is the major prerequisite for activity. Antiviral activity may be limited by poor uptake, absence of appropriate activating enzymes, catabolism, and competition from endogenous nucleotides. Appreciation of these factors, which are species-, tissue- and cell-specific is important in the understanding of the pharmacology and toxicology of nucleoside analogues. The use of nucleoside analogues against the agents of viral hepatitis is inherently problematic for many reasons including active hepatic nucleoside catabolism, probable absence of virus-specific activating enzymes, competition from endogenous nucleotides synthesised de novo or derived from RNA turnover, and factors related to mitochondrial toxicity. Despite these drawbacks, some nucleoside analogues have been found efficacious against hepatitis B virus and it is likely that as knowledge of their mechanism of action accumulates, their efficacy can be improved both by rational drug design and by use in combination with other drugs, including interferon.

Differences in kinetic properties of pure recombinant human and mouse deoxycytidine kinase

Human and mouse deoxycytidine kinase (dCK) (EC 2.7.1.74) were cloned and expressed in Escherichia coli. Michaelis-Menten kinetics were determined for the purified enzymes with 2'-deoxycytidine (dCyd), 2'-deoxyadenosine (dAdo), 2-chloro-2'-deoxyadenosine (CdA), 2',3'-dideoxycytidine (ddCyd) and 9-beta-D-arabinofuranosylguanine (araG) as substrates and ATP and UTP as phosphate donors. Both human and mouse dCK showed highest affinity to dCyd with Km values of 0.05-0.2 microM. The anti-leukaemic compound CdA was the superior substrate of the nucleoside analogues tested. Both enzymes were able to efficiently utilize ATP and UTP as phosphate donors. However, the use of UTP instead of ATP as phosphate donor decreased Km values for all substrates investigated. The kinetic properties of mouse and human dCK differed in that the human enzyme showed higher affinity for the substrates dAdo, CdA, ddCyd and araG. The human enzyme also showed higher affinity for ATP and UTP. The ability to phosphorylate dCyd was, however, similar for both human and mouse dCK. At physiological concentration of the feedback inhibitor dCTP, mouse dCK showed lower activity than human dCK for all substrates investigated.

Expression of deoxycytidine kinase and phosphorylation of 2-chlorodeoxyadenosine in human normal and tumour cells and tissues

Deoxycytidine kinase (dCK) activates several clinically important drugs, including the recently developed antileukaemic compound 2-chlorodeoxyadenosine (CdA). The distribution of dCK in cells and tissues has previously been determined by activity measurements, which may be unreliable because of the presence of other enzymes with overlapping substrate specificities. Therefore we have measured dCK polypeptide levels in extracts of normal and malignant human peripheral blood mononuclear cells, gastrointestinal tissues and sarcomas, using a specific immunoblotting technique, as well as the phosphorylation of CdA in the same extracts. High levels of dCK were found in all major subpopulations of normal mononuclear leucocytes (120 +/- 19 ng dCK/mg protein) and in B-cell chronic lymphocytic leukaemia (81 +/- 30 ng/mg, n = 23). Hairy-cell leukaemia contained lower levels (28 +/- 23 ng/mg, n = 7), as did three samples of T-cell chronic lymphocytic leukaemia (18 +/- 14 ng/mg). Phytohaemagglutinin stimulation of normal lymphocytes did not lead to any substantial increase in either dCK activity or protein expression (less than 2.5-fold). The human CEM wt T-lymphoblastoid cell line contained 56 +/- 1 ng/dCK/mg protein, while in the CEM ddC50 and AraC8D mutants that lack dCK activity, no dCK polypeptide could be detected. In colon adenocarcinomas, the dCK content was significantly higher (20 +/- 9 ng/mg, n = 20) than in normal colon mucosa (8 +/- 3.5 ng/mg, n = 19, P < 0.05). A similar pattern of dCK expression was found in gastric adenocarcinomas (21 +/- 13 ng/mg, n = 5) and normal stomach mucosa (6 +/- 5 ng/mg, n = 5, P < 0.15). One leiomyosarcoma and one extra-skeletal osteosarcoma showed dCK levels comparable with those found in normal lymphocytes (84 +/- 6 and 109 +/- 4 ng/mg, respectively), while other sarcoma samples contained lower levels, comparable to the gastrointestinal adenocarcinomas (20 +/- 7 ng/mg, n = 12). Thus, dCK is expressed constitutively and predominantly in lymphoid cells, but it is also found in solid non-lymphoid tissues, with increased levels in malignant cells. The phosphorylation of CdA in crude extracts showed a close correlation to the dCK polypeptide level.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.

Interspecies differences in the kinetic properties of deoxycytidine kinase elucidate the poor utility of a phase I pharmacologically directed dose-escalation concept for 2-chloro-2'-deoxyadenosine

2-Chloro-2'-deoxyadenosine (CdA, Cladribine), is a purine antimetabolite currently under investigation in phase II clinical trials for the treatment of lymphoid malignancies. Significant differences in CdA toxicity between mice and humans were observed during phase I clinical evaluation. For the elucidation of interspecies differences in drug toxicity the pharmacokinetics of CdA after subcutaneous injection and the kinetic properties of the CdA-phosphorylating enzyme, deoxycytidine kinase (dCK), were compared in mice and humans. The ratio of the dose lethal to 10% of mice (LD10) to the maximum tolerated dose (MTD) in humans was 50 and the ratio of the area under the curve obtained at approximately one-half the LD10 (AUCapprox. one-half the LD10)/AUC(MTD) was 49. A significant interspecies difference was observed in the kinetic properties of dCK, the main CdA-activating enzyme. With CdA as a substrate, the Michaelis constant (Km) of dCK in crude extracts of mouse thymus was 10 times higher than that in human thymus. An approximately 9-fold interspecies difference in maximum velocity (Vmax)/Km indicated a higher efficiency of dCK for CdA in humans than in mice. The peak plasma concentration was 210 times higher and exceeded the Km in mice. Initial and terminal half-lives were approximately 7 times shorter in mice and trough levels were similar in mice and humans. Thus, the differences in AUCs at equitoxic doses are largely explained by differences in the target enzyme properties and the pharmacokinetic pattern. The observed lower tolerance for CdA in humans as compared with mice confirms the view that antimetabolites may not be good candidates for pharmacokinetically guided dose-escalation schemes unless detailed information on interspecies variability in drug bioactivation is available.

Cloning of the Dck gene encoding rat deoxycytidine kinase

In order to study the mutational inactivation of deoxycytidine kinase (Dck) in a rat model for acute myeloid leukemia we have cloned the complete coding region of the rat Dck gene. Using primers chosen from the human Dck cDNA sequence, we obtained a rat-specific probe via PCR and used it to isolate two clones from a rat lymphocyte cDNA library. The ORF showed 89.7 and 92.2% nucleotide identity with the human and mouse Dck, respectively, and encodes a 260-amino-acid protein, that is 91.9 and 94.6% homologous to human and mouse Dck, respectively. Northern blot analysis of rat tissues revealed high expression of a 4.1-kb Dck transcript in the thymus, whereas spleen, liver and lung samples showed weak expression of the gene. This tissue-specific expression pattern was confirmed by cDNA-PCR analysis.

New targets for pyrimidine antimetabolites for the treatment of solid tumours. 2: Deoxycytidine kinase

Deoxycytidine kinase is an enzyme required for the activation of, for example, cytarabine, the most widely used agent for the chemotherapy of haematological malignancies. However, deoxycytidine kinase also plays an important role in the activation of several new agents used in the treatment of leukaemia, such as cladribine. Recently, a new cytidine analogue, gemcitabine, has shown impressive activity as a single agent against several solid malignancies (ovarian cancer, non-small cell lung cancer), demonstrating that in solid tumours deoxycytidine kinase can be an important target for the activation of antimetabolites. Studies on the regulation of deoxycytidine kinase have shown that the enzyme has a complicated regulation (feedback inhibition by the product and regulation by ribonucleotides). Modulation of deoxycytidine kinase activity has already been shown to be an effective way to improve the effect of cytarabine and will probably be a target for new therapies.

Use of nucleoside kinase-deficient mouse leukemia L1210 cell lines to determine metabolic routes of activation of antitumor nucleoside analogs

Mouse leukemia L1210 cell lines that were selected for resistance to deoxyguanosine (dGuo-R) or lacked adenosine kinase activity (ED2) were used to evaluate the nature of the nucleoside kinase that was required to phosphorylate nucleoside analogs to their respective active nucleotide form. The dGuo-R cells had reduced levels of kinase activity toward araC, dGuo and 2-CldAdo as substrates with essentially no loss of activity toward dCyd. This cell line showed resistance to dGuo, araC, araG, FdAdo, and Fara A but not to dAdo or araA. The ED2 cell line was resistant to pyrazofurin and 6-methylmercaptopurine riboside and to araA/EHNA but not to 2-CldAdo or 2-Cl-2'-FaraA. The study of the effects of newer nucleoside analogs such as dFdCyd, MdAdo, MdCyd and MdGuo in these cell lines showed that some of these agents are primarily phosphorylated by deoxyribonucleoside kinase (dFdCyd) or by adenosine kinase (MdAdo) or in some instances by multiple kinases (FaraA). These cell lines will be useful in defining the nature of the kinase(s) responsible for activating new nucleoside analogs and defining cross-resistance patterns.

Properties and levels of deoxynucleoside kinases in normal and tumor cells; implications for chemotherapy

Deoxynucleoside kinases are key enzymes in deoxyribonucleoside salvage, activating several clinically important chemotherapeutic drugs. The four known kinases, cytosolic thymidine kinase (TK1) and deoxycytidine kinase (dCK) and the mitochondrial thymidine kinase (TK2) and deoxyguanosine kinase (dGK), have been purified and characterized as to the subunit structure as well as specificity with a large number of analogs. These results are summarized and used to establish selective assays for the four enzymes in crude extracts of normal and malignant human peripheral blood mononuclear cells, gastrointestinal tissues and sarcomas. TK2 and dGK activities were found at low levels in all tissues, possibly correlated to the content of mitochondria. TK1 activity was detected only in samples containing a significant number of S phase cells. We have measured dCK activity as well as dCK polypeptide level by immuno blotting in these extracts. High levels of dCK were found in normal mononuclear leukocytes (91-145 ng dCK/mg protein) and in B-cell chronic lymphocytic leukemia (80 +/- 30 ng/mg, n = 23). Hairy cell leukemia contained lower levels (28 +/- 23 ng/mg, n = 7), as did unexpectedly three samples of T-cell chronic lymphocytic leukemia (18 +/- 14 ng/mg). Phytohemaglutinine stimulation of normal lymphocytes did not lead to any substantial increase in either dCK activity or expression (less than 2.5-fold). In colon adenocarcinomas, the dCK content was significantly higher (21 +/- 9.3 ng/mg, n = 20) than in normal colon mucosa (8.2 +/- 3.7 ng/mg, n = 19, p < 0.05). A similar pattern of dCK expression was found in gastric adenocarcinomas (21 +/- 13 ng/mg, n = 5) and normal ventricular mucosa (6.2 +/- 5.4 ng/mg, n = 5, p < 0.15). One leiomyosarcoma and one extra-skeletal osteosarcoma showed a dCK levels comparable to those found in normal lymphocytes (84 +/- 6 and 109 +/- 4 ng/mg), while other sarcoma samples contained levels comparable to the gastrointestinal adenocarcinomas (20 +/- 7 ng/mg, n = 12). We confirm that dCK is expressed constitutively and predominantly in lymphoid cells, but conclude that a significant expression may be found in non-lymphoid tissues as well, with increased levels in the corresponding tumor tissue. 2-Chlorodeoxyadenosine (CdA), an antileukemic agent used in treatment of hairy cell leukemia and chronic lymphocytic leukemias (B-CLL), is phosphorylated by dCK which was used as the selective substrate for this enzyme. A study was performed to investigate if there was a correlation between the dCK levels and the response to CdA treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Deoxycytidine kinase and deoxycytidine deaminase activities in human tumour xenografts

Deoxycytidine kinase (dCK) and deaminase (dCDA) are both key enzymes in the activation and inactivation, respectively, of several deoxycytidine antimetabolites. We determined the total dCK and dCDA activities using standard assays, in 28 human solid tumours grown as xenografts in nude mice, and four corresponding cell lines. dCK activities in colon tumours varied from 11 to 12 nmol/h/mg protein, in ovarian tumours from 3 to 10 nmol/h/mg protein, in soft tissue sarcomas from 2 to 7 nmol/h/mg protein and in squamous cell carcinomas of the head and neck about 45-fold, between 0.4 and 18 nmol/h/mg protein. The dCDA activities showed a larger variation, from 243 to 483, 14 to 1231, 3 to 7 and 1 to 222 nmol/h/mg protein, respectively. The ratios of dCK vs. dCDA activities in these tumours varied from 0.025 to 0.046, 0.004 to 0.240, 0.581 to 1.123 and from 0.012 to 4.227, respectively. In four cell lines (A2780, OVCAR-3, WiDr and UM-SCC-14C), sources for some of the above mentioned tumours, a different pattern in dCK and dCDA was observed than in the corresponding tumours. The variation in dCDA activities was in a smaller range (20-fold) than in the tumours (40-fold). In all cell lines dCK activity was higher than dCDA activity, in contrast to the corresponding tumours, in which the reverse pattern was observed. Previously, some of the tumours were tested for sensitivity to the deoxycytidine analogues 5-aza-deoxycytidine and 2',2'-difluorodeoxycytidine. In the sensitive tumours, both the highest and lowest dCK activity was observed, indicating that dCK activity in solid tumours is high enough to activate deoxycytidine analogues.

Mammalian thymidine kinase 2. Direct photoaffinity labeling with [32P]dTTP of the enzyme from spleen, liver, heart and brain

Thymidine kinase 2 (TK2), also called mitochondrial thymidine kinase, is a pyrimidine deoxyribonucleoside kinase expressed in all cells and tissues. It was recently purified to apparent homogeneity from human leukemic spleen and the active enzyme was shown to be a monomer of a 29-kDa polypeptide. The enzyme is feedback-inhibited by both end products, dCTP and dTTP. Here we show that TK2 purified from several different sources, including purified beef heart mitochondria, could be directly photoaffinity labeled with radioactive dTTP (approximately 18% of all TK2 molecules were cross-linked to dTTP after 20 min of ultraviolet irradiation) or to a lower extent with dCTP. Photo-incorporation was inhibited by the presence of the other effector but also the phosphate donor ATP blocked photolabeling, with dTTP. Addition of nucleoside substrates gave only a marginal inhibition of photo-incorporation. There were no detectable difference in the molecular size of photolabeled TK2 isolated from human spleen, brain or placenta, monkey liver, beef heart and beef heart mitochondria. Nor was there any significant differences in the enzyme kinetic properties of these enzymes. Cleavage of labeled TK2 with cyanogen bromide showed that dTTP was incorporated into a single 3-kDa peptide. TK2 was the only pyrimidine deoxynucleoside kinase expressed in liver, heart and brain. A detailed characterization of the subunit structure and substrate specificity of this enzyme is of importance for the design of new antiviral and cytostatic therapies based on nucleoside analogs.