Cytosine arabinoside phosphorylation and deamination in acute myeloblastic leukemia cells

Cytidine deaminase genotype and toxicity of cytosine arabinoside therapy in children with acute myeloid leukemia

Cytosine arabinoside (ara-C) is irreversibly deaminated to a non-toxic metabolite by cytidine deaminase (CDA). A common polymorphism, A79C, in the gene encoding cytidine deaminase (CDA) changes a lysine residue to glutamine resulting in decreased enzyme activity. CDA A79C genotypes were determined in 457 children with acute myeloid leukaemia (AML) treated on the Children's Cancer Group (CCG) 2941 and 2961 protocols and analyzed the impact of CDA genotype on therapy outcomes. Postinduction treatment-related mortality (TRM) was significantly elevated in children with the CC genotype (5-year TRM 17 +/- 13% CC vs. 7 +/- 4% AA, 5 +/- 4% AC, P = 0.05). This was more notable in children who received idarubicin, fludarabine, ara-C, and granulocyte colony-stimulating factor (IDA-FLAG; ara-C = 7590 mg/m2) (5-year TRM 24 +/- 21% CC vs. 6 +/- 6% AA, 6 +/- 7% AC, P = 0.07) as consolidation therapy compared to idarubicin, dexamethasone, cytarabine, thioguanine, etoposide and daunomycin (IDA-DCTER; ara-C = 800 mg/m2) (5-year TRM 15 +/- 20% CC vs. 8 +/- 6% AA, 4 +/- 6% AC; P = 0.29). Relapse-free survival was non-significantly increased in children with the CC genotype treated with IDA-FLAG (76 +/- 20% CC vs. 59 +/- 12% AA and 55 +/- 14% AC; P = 0.40). These data indicate that children with a low activity CDA genotype are at increased risk of TRM with ara-C based therapy for AML.

Delayed addition of deoxycytidine protects normal CD34+ cells against cytotoxicity of gemcitabine without compromising its activity against human leukemic cells

In phase I and II clinical trials, the deoxycytidine analogue 2',2' difluorodeoxycytidine (dFdC, gemcitabine) has shown promising antitumor activity in leukemia as well as in solid tumors. Preclinical and clinical studies of gemcitabine suggested that myelosuppression was the dose-limiting toxicity. The present investigations were designed to test the effect of continuously administered gemcitabine on the in vitro clonal growth of normal CD34(+) cells isolated from peripheral blood and the promyelocytic cell line, HL-60. For this purpose, CD34(+) and HL-60 cells were cultured in methylcellulose in the continuous presence of 0.1-16 nM of gemcitabine. The results show a dose-dependent inhibition of colony growth of normal as well as leukemic cells. However, HL-60 cells were up to 12-fold more sensitive towards gemcitabine than normal progenitors. For rescue experiments, the natural pyrimidine deoxycytidine (dCyd) was added to CD34(+) and HL-60 cells simultaneously or with delay. Coadministration of 1mM dCyd to separate cultures resulted in complete restoration of colony formation capacity of CD34(+) and HL-60 cells. Delayed addition of 1 mM dCyd after 48 and 72 hours recovered up to 90% and 40%, respectively, of stem cell proliferation, whereas HL-60 cells remained substantially inhibited (4.5% +/- 3.5% versus 0%). Delayed addition after 48 and 72 hours protected about 80% and 50%, respectively, of myelopoietic and erythropoetic colony formation, whereas colony formation obtained from HL-60 cells remained significantly inhibited (9.6% +/- 4.17% versus 0%). These in vitro data suggest that there is a marked difference in the susceptibility of leukemic and normal CD34(+) cells to gemcitabine and that delayed administration of dCyd may further reduce the bone marrow cytotoxicity of gemcitabine without impairing its antitumor effect.

Structural and functional analysis of the cytidine deaminase gene in patients with acute myeloid leukaemia

Gene transfer of the cytidine deaminase (CDD) cDNA has recently been shown to induce cellular resistance to cytarabine (AraC) in vitro. To investigate the role for CDD in acute myeloid leukaemia (AML) we analysed the CDD activity and CDD gene structure in blast material from well-defined patients with untreated and AraC refractory (RF) AML. Median CDD activity in previously untreated AML was significantly lower than in RF-AML blasts (P=0.015) and was significantly lower in patients with complete remission than with blast persistence following induction chemotherapy (P=0.043). Structural investigation of the CDD gene by Southern analyses and RT-PCR showed no detectable aberrations. Sequence analysis of the CDD cDNA from nine RF-AML patients showed inconsistent aberrations in three patients. Semiquantitative assessment of CDD mRNA expression revealed a significant correlation with CDD activity. In conclusion, concordant with another recent study our data suggest a correlation of pretherapeutic CDD activity with induction treatment response. Besides the previously described prognostic impact of mdrl expression, this result could be useful for the development of risk-adapted AML treatment strategies and warrants further studies of CDD activity in well-defined cohorts of AML patients and of the mechanisms involved in the regulation of CDD activity.

Increased ratio between deoxycytidine kinase and thymidine kinase 2 in CLL lymphocytes compared to normal lymphocytes

Deoxycytidine kinase (dCK) is important in the 5'-phosphorylation of deoxynucleoside analogs. Like dCK, thymidine kinase 2 (TK2) catalyzes the initial step of the phosphorylation of dcyd to dCTP. Thymidine is a strong inhibitor of the dCK activity of TK2. We examined the ratio of the dcyd phosphorylation carried out by dCK and by TK2 (dCK/TK2-dcyd) in lymphocytes from CLL patients and from donors. In the CLL lymphocytes we found a 3.5-fold average increase. Therefore, we conclude that addition of thymidine in the treatment of CLL with deoxynucleoside analogs will not be of any advantage. Furthermore, our results can explain earlier findings in CML and AML lymphocytes where the ara-C phosphorylation was twice the dcyd phosphorylation.

Human thymidine kinase 1. Regulation in normal and malignant cells

In mammalian cells, salvage pathway phosphorylation of thymidine is catalyzed by two thymidine kinases: the cell-cycle regulated cytoplasmic TK1 and the constitutively expressed mitochondrial TK2. Since TK1 is virtually absent in non-dividing cells, TK2 is probably the only thymidine kinase present in these cells. In cellular metabolism, TK1 and TK2 presumably serve to maintain sufficient dTTP for DNA replication and repair. TK1 purified from phytohemagglutinin-stimulated human lymphocytes is a dimer in the absence and a tetramer in the presence of ATP. In addition to the molecular weight transition, incubation with ATP at 4 degrees C or storage with ATP induces a reversible, enzyme concentration-dependent, kinetically slow transition from a low to a high affinity form of TK1, with Km values of 14 microM and 0.5 microM, respectively. This affinity difference implies that at cellular thymidine concentrations, the difference in catalytic activity between the two TK1 forms will be 3-5-fold. Calculations of cellular TK1 concentration suggested that the low affinity dimer form was dominant in G0/G1 cells and the high affinity tetramer form in S-phase cells. Hence, the transition may serve to fine-tune the cell-cycle regulation of thymidine kinase activity on the post-translational level. To study the ATP effect on the molecular level, an IPTG inducible T7 RNA polymerase-dependent expression system for the entire human TK1 polypeptide in E. coli was established. The recombinant TK1 has the same subunit mass and specific activity as the native enzyme. However, the recombinant TK1 solely displayed the kinetics of the high affinity form, with Km values of 0.3-0.4 microM regardless of pre-exposure to ATP, indicating that the ATP effect may be dependent on post-translational modifications absent in E. coli. Surprisingly, we did not observe any effect of ATP on TK1 purified from bone-marrow cells from a patient with acute monocytic leukemia (AMOL). Furthermore, the Km values of TK1 from these cells were 45 microM for the ATP-free enzyme and 65 microM for the ATP-incubated enzyme. With TK1 purified from HL-60 cells, we obtained the same pattern and kinetic values as for TK1 from lymphocytes. In the light of the results with the recombinant TK1, we presume that the lack of ATP effect and very high Km values observed for the AMOL TK1 may be due to changes in post-translational regulatory mechanisms in acute monocytic cells.

Induction of surface antigen recognized by new monoclonal antibody, YU311, on 1-beta-D-arabinofuranosylcytosine-resistant human leukemic cell line

A new monoclonal antibody, YU311, against an antigen expressed on 1-beta-D-arabinofuranosylcytosine(ara-C)-resistant human leukemic cell line with decreased deoxycytidine kinase activity was generated. YU311 reacted with ara-C-resistant human leukemic cell line (KY-Ra), but not with its parental cell line (KY-821) which was sensitive to ara-C. YU311 recognized the 92-kDa membrane protein. Furthermore, YU311 inhibited the growth of KY-Ra in suspension medium with and without ara-C. In immunocytochemistry, there was no difference in expression of usual differentiation antigens between KY-Ra and KY-821. These findings indicate that antigenic change could occur in ara-C-resistant human leukemic cells with stable expression of differentiation antigens and that the 92-kDa membrane protein may be one of the membrane proteins which regulate cell growth of KY-Ra.

Effect of tetrahydrouridine and deoxytetrahydrouridine on the interaction between 2'-deoxycytidine and 1-beta-D-arabinofuranosylcytosine in human leukemia cells

The interaction between 2'-deoxycytidine (dCyd) and 1-beta-D-arabinofuranosylcytosine (ara-C), administered at pharmacologically achievable concentrations, was examined in four continuously cultured human leukemia cell lines, HL-60, KG-1, K-562, and CCRF-CEM. In three of the cell lines (HL-60, K-562, and CCRF-CEM), co-administration of 20 or 50 microM dCyd with 10 microM ara-C reduced ara-CTP formation by at least 90% and incorporation of ara-C into DNA by at least 80%. In contrast, KG-1 cells exhibited substantially smaller reductions in both ara-CTP formation and incorporation of ara-C into DNA under identical conditions. KG-1 cells were distinguished by the highest activity of the enzyme cytidine deaminase of the four lines assayed, and exhibited the smallest increments in the intracellular accumulation of both dCyd and deoxycytidine triphosphate (dCTP) in response to exogenous dCyd. Co-administration of 1 mM tetrahydrouridine (THU) or 0.5 mM deoxy-tetrahydrouridine (dTHU) had little effect on the ability of dCyd to antagonize ara-C metabolism in HL-60, KG-1 and K-562 cells. In contrast, these deaminase inhibitors substantially increased the intracellular accumulation of dCTP as well as the ability of dCyd to antagonize ara-CTP formation and incorporation of ara-C into DNA in KG-1 cells. THU and dTHU also permitted dCyd to antagonize ara-C growth inhibitory effects in KG-1 cells to the extent observed in the other leukemic cell lines. These studies suggest that the intracellular deamination of exogenous deoxycytidine may influence the degree to which this nucleoside antagonizes ara-C metabolism and toxicity in some leukemic cells. They also raise the possibility that deaminase inhibitors may be employed to modulate, and perhaps to improve, the therapeutic selectivity of pharmacologically relevant concentrations of ara-C and dCyd in the treatment of acute leukemia in man.

Modulation of the effect of 1-beta-D-arabinofuranosylcytosine based on changes of cytidine deaminase activity in HL60 cells

The effect of changes in cytidine deaminase activity on the growth inhibitory effect of 1-beta-D-arabinofuranosylcytosine (Ara-C) has been investigated in the human promyelocytic cell line, HL60. 1,25 Dihydroxy vitamin D3 (vit. D3), which is an inducer of differentiation, caused an increase of cytidine deaminase in HL60 cells simultaneous with an inhibition of the effect of Ara-C. On the other hand, retinoic acid, which also induces differentiation had no influence on either the activity of cytidine deaminase or the effect of Ara-C. An atoxic inhibitor of cytidine deaminase tetrahydrouridine, THU, enhanced the effect of Ara-C. THU also enhanced the effect of Ara-C, even if the Ara-C effect was inhibited by deoxycytidine. These results show that it is possible to modulate the effect of Ara-C by changing the activity of cytidine deaminase. Furthermore, changes in the activity of cytidine deaminase and the effect on the Ara-C growth inhibition vary dependent on the differentiating inducer used.

Biochemical modulation of cytosine arabinoside

1-beta-D arabinofuranosylcytosine (ara-C) is an analog of the naturally occurring nucleoside 2'-deoxycytidine which is a potent antileukemic agent in man. Because the metabolism (and, ultimately, the effectiveness) of this agent is regulated by multiple processes involved in pyrimidine biosynthesis, attempts to improve its efficacy through biochemical modulation have been the focus of intense interest. These approaches have included combination of ara-C with inhibitors of de novo pyrimidine biosynthesis, deaminase inhibitors, nucleoside transport blockers, nucleosides, and more recently, hematopoietic growth factors. Although potentiation of ara-C metabolism and cytotoxicity has been documented in multiple experimental in vitro and in vivo experimental systems, clinical studies in humans have thus far failed to document definitive improvements in ara-C selectivity and efficacy through biochemical modulation. It is likely that such improvements will require the identification of more optimal schedules, sequences and dose relationships, and possibly combined modality approaches.

Changes in the activities of cytidine deaminase during differentiation of HL60 cells induced by 1,25 dihydroxy D3

The activities of the enzymes cytidine deaminase (CDD), deoxycytidine kinase (dCK), adenosine deaminase (ADA), and purine nucleoside phosphorylase (PNP), have been investigated in the promyelocytic leukemia cell line HL60. The activities of the enzymes corresponded well with that seen in acute myeloid leukemia cells except, that the CDD activity was very low in the HL60 cells. Induction of differentiation in HL60 cells by 1,25 dihydroxy D3 resulted in an increase in CDD from 12 to 247 nmol/h/mg and a decrease in ADA from 1326 to 896 nmol/h/mg, while the activities of dCK, and PNP were unchanged. Retinoic acid, another used inducer of differentiation, gave no changes of the enzyme activities. The increase in CDD activity induced by 1,25 dihydroxy D3 was prevented by inhibition of protein synthesis, whereas inhibition of proliferation of the cells did not abolish the increase of CDD. The changes correspond well with the differences seen between immature and mature myeloid cells. The results may have consequences for the interpretation of results obtained with cytostatics, which are metabolized by the enzymes.

Low cytidine deaminase levels in human hematopoietic cell lines

Purine and pyrimidine enzyme profiles of human cell lines have been investigated. A novel observation was the finding that most of the cell lines showed very low or undetectable levels of cytidine (deoxycytidine) deaminase, while they possessed pyrimidine 5'-nucleotidase, cytidine and deoxycytidine kinase activities. Most cell lines showed high levels of adenosine deaminase and purine nucleoside phosphorylase activities and low levels of purine 5'-nucleotidase. We propose that high adenosine deaminase and purine nucleoside phosphorylase activities and low cytidine deaminase activity may be of importance for immature hematopoietic cells in order to ensure a balanced synthesis of the DNA precursors.

In-vitro studies on phosphorylation and dephosphorylation of cytosine arabinoside in human leukemic cells

The cytotoxic effect of cytosine arabinoside (ara-C) depends on the capacity of cells to form and retain intracellularly the phosphorylated metabolite cytosine arabinoside triphosphate (ara-CTP). In this study accumulation and cellular retention of ara-CTP have been measured in vitro in the bone marrow cells of 69 patients with acute leukemia. Cells were incubated with 3H-ara-C and the amount of ara-CTP formed was determined after separation of the nucleotides by thin-layer chromatography. Phosphorylation of ara-C to ara-CTP appeared to be a saturable process. The Km-equivalents varied between 1.1 and 16.2 microM ara-C. Maximal ara-CTP formation ranged from 12 to 125 pmol ara-CTP/10(6) cells in 30 min. The phosphorylation activity did not correlate with the percentage of S-phase cells. The intracellular half-life time of ara-CTP measured in vitro ranged from 53 to 210 min. Phosphorylation of ara-C was comparable in patients with acute myeloid leukemia (n = 51) and in patients with acute lymphoblastic leukemia (n = 18). Ara-CTP elimination appeared slower in lymphoblasts than in myeloblasts. The average intracellular ara-CTP level in relapsed patients (n = 34) appeared higher than in previously untreated patients (n = 52). The less favourable outcome of second remission induction therapy with conventional doses of ara-C compared to the first remission induction treatment is not explained by an alteration in the intracellular metabolism of ara-C.

Cytosine arabinoside transport by human leukaemic cells

The membrane transport of cytosine arabinoside (araC) has been studied in blasts freshly isolated from a variety of acute leukaemias. The major fraction of araC influx was facilitated and this fraction was 80-87% at l microM araC and 68-80% at 200 microM araC. Competitive kinetics were observed between araC and deoxycytidine for entry into leukaemic blasts and, moreover, araC influx was blocked by phloretin, a broad-spectrum inhibitor of facilitated transport systems. Kinetic analysis of facilitated araC influx gave KmS which varied over a 10-fold range between patients and which were positively correlated to the Vmax. Nucleoside influx Vmax also varied over an 80-fold range between individuals, although the mean araC transport was 4-fold greater in myeloblasts than in lymphoblasts. Larger transport of araC may explain the greater sensitivity of acute myeloid leukaemia to this drug.

Biochemical and biophysical approaches to improving the anticancer effectiveness of Ara-adenine

Ara-C at very low dosage has been reported to decrease the host toxicity of ara-AMP or ara-A in combination with 2'-deoxycoformycin, a potent adenosine deaminase inhibitor, while increasing the toxicity to intracerebral L1210 leukemia. The possibility of increasing the selectivity of ara-A by prior administration of ara-C is explored. The importance of deoxynucleoside kinases, some of which may be cancer-induced, in obtaining selective anticancer effects is discussed. The possibility of a conformational basis for the differing degrees of selectivity and activity of various novel arabinosyl nucleosides is evaluated. The levels of cyclic nucleotides, which have opposing effects on leukemia, may possibly be manipulated to interfere with the growth of cancer cells. Approaches to minimizing major metabolic distortions, such as the progressive accumulation of dATP associated with the use of potent adenosine deaminase inhibitors and which limit the therapeutic effects of ara-A, are proposed.

DNA synthesis in thymic-acute lymphoblastic leukaemia

Deoxyribonucleoside triphosphate (dNTP) concentrations were measured in bone marrow and peripheral blood leucocytes from seven patients with acute Thy-lymphoblastic leukaemia (Thy-ALL), 12 patients with acute myeloblastic leukaemia and 15 patients with acute non-T, non-B lymphoblastic leukaemia (c-ALL), and in thymocytes from patients with myasthenia gravis. Labelled thymidine and deoxycytidine incorporation into DNA was also studied. In Thy-ALL, dNTP concentrations were markedly increased compared with those in the other acute leukaemias. The dNTP concentrations in thymocytes were, however, similar to those in Thy-ALL. 3H-nucleoside incorporation studies showed a marked difference in labelled deoxycytidine incorporation and particularly in the deoxycytidine/thymidine DNA labelling ratio between Thy-ALL and the other cell types. We conclude that the pathways of DNA synthesis in Thy-ALL blasts are different from those in the cells from acute acute leukaemias and some but not all these differences may correspond to differences between normal cortical thymocytes and bone marrow cells.

Enzymatic studies on possible improvement of cytosine arabinoside treatment

Initial phosphorylation and deamination of cytosine arabinoside (Ara-C) and the natural metabolite deoxycytidine (CdR) were estimated in cell free extracts of leucocytes from patients with CML and controls, Phosphorylation of CdR had been increased while deamination of Cdr in extracts of CML cells from peripheral blood had been decreased compared with normal leucocytes. Comparing the ratios between Ara-C and CdR phosphorylation it was revealed that these were twice as high in CML cells as in normal leucocytes, whereas no difference was found when comparing ratios between Ara-C and CdR deamination. From these discoveries it is proposed that Ara-C can be combined with CdR with advantage, because apparently CdR protects the normal cells more than the malignant ones.