Iron binding capacity of didox (3,4-dihydroxybenzohydroxamic acid) and amidox (3,4-dihydroxybenzamidoxime) new inhibitors of the enzyme ribonucleotide reductase

Ribonucleotide reductase is the rate limiting enzyme of deoxynucleoside triphosphate synthesis and is considered to be an excellent target of cancer chemotherapy. Didox and amidox are newly synthesized compounds, which inhibit this enzyme and have in vitro and in vivo antitumor activity. We have now investigated the capability of didox and amidox to interfere with the iron metabolism. We show by photometric and polarographic methods, that didox and amidox are capable of forming an iron complex. However, their cytotoxic action cannot be completely circumvented by addition of Fe-ammoniumcitrate, indicating that the iron complexing capacity may not be responsible for the mechanism of action of these compounds. When L1210 leukemia cells were incubated with the didox-iron or amidox-iron complex itself, changes of the 50% growth inhibitory capacity of the complex in comparison with didox or amidox alone could be shown. We conclude, that didox and amidox are capable of forming iron complexes, but in contrast to other agents, the anticancer activity cannot be contributed to this effect alone. Future studies will have to elucidate the molecular mechanism of action of these new and promising anticancer agents.

DNA-protective activity of new ribonucleotide reductase inhibitors

The DNA-protective activity of hydroxyurea (HU) and novel ribonucleotide reductase (RR) inhibitors amidox (AX), didox (DX) and trimidox (TX) was examined using hydrogen peroxide as the DNA-damaging agent. The exposure of superspiralized plasmid DNA molecules (pBR 322) to H2O2 under precisely defined in vitro conditions initiates a change in DNA topology (DNA from I relaxes to DNA form II). This electrophoretically monitored change in the plasmid DNA topology is related to the induction of ss-DNA breaks and corresponds with DNA exposition to free radicals. The inhibition of DNA relaxation (the prevention of DNA damage induced by hydrogen peroxide) depended on the free radical scavenging capacity of the drugs investigated. HU exerted DNA protective activity at a concentration of 4 mM, AX at concentration of 1 microM, TX at a concentration of 5 microM and DX at a concentration of 25 microM (the free radical scavenging activity increases from HU to AX in following manner: HU << DX < TX < AX). It can be concluded that the new synthetic RR-inhibitor AX which is being investigated at the preclinical level as a potential anti-cancer drug possess the highest capacity for scavenging of free radicals.

Simultaneous determination of the new anticancer agent amidox and its metabolites in rat bile and plasma by high-performance liquid chromatography

A new reversed-phase ion-pair high-performance liquid chromatography method was developed to study the first-pass hepatic metabolism of the anti cancer drug amidox in bile. Separation of the metabolites was achieved on a Spherisorb C18 column after liquid-liquid extraction using a linear gradient system of heptanesulfonic acid in potassium phosphate monobasic (pH 4.0) with increasing amounts of methanol (0-40%). The method was further applied to a pharmacokinetic study of amidox in rats after 200 mg kg-1 intraperitoneal administration. Using 50 microliters of rat bile and 300 microliters of rat plasma the limit of detection for amidox was 60 ng and 85 ng, respectively, and the assay was linear from 0.1 to 150 micrograms ml-1. This method appears to be sensitive enough to be used in further pharmacokinetic studies of amidox in human volunteers.

The new inhibitors of ribonucleotide reductase--comparison of some physico-chemical properties

Amidox (AX), didox (DX) and trimidox (TX), compounds synthetized as new ribonucleotide reductase inhibitors, have been investigated by ultraviolet (UV) spectrophotometry, polarography and high performance liquid chromatography (HPLC). The experiments have been performed at various pH values. The changes in UV absorption of the compounds studied were recorded and it was demonstrated that these changes are related to the pH and to structural features of the investigated molecules. From the compounds included in our series of experiments, only amidox and trimidox are reduced during polarographic experiments in Britton-Robinson buffer. The reduction of both compounds proceeded in two one-electron steps in acidic pH. One two-electron diffuse irreversible wave was observed at basic pH. The values of the half-wave potential became more negative in accordance with the increasing pH. HPLC assay also showed changes in the retention of compounds investigated, particularly when the pH of the mobile phase was close to the dissociation constant of the particular drug. The changes of physico-chemical properties detected by the all used methods are related to different chemical structures (the most significant changes were observed in alkaline pH).

Cytotoxic effects of a doxorubicin-transferrin conjugate in multidrug-resistant KB cells

Cancer chemotherapy is often limited by the emergence of multidrug-resistant tumor cells. Multidrug resistance (MDR) can be caused by amplification of the MDR genes and overexpression of the P-glycoprotein, which is capable of lowering intracellular drug concentrations. A doxorubicin-transferrin conjugate has been synthesized and exerts its cytotoxic effects through a transmembrane mechanism. We have examined the cytotoxicity of this conjugate and compared it with doxorubicin in sensitive (KB-3-1) and in multidrug-resistant KB cell lines (KB-8-5, KB-C1, and KB-V1). In the clonogenic assay, doxorubicin exhibited IC50 concentrations of 0.03 and 0.12 microM in the sensitive (KB-3-1) and resistant (KB-8-5) cell lines, respectively, whereas, doxorubicin-transferrin conjugate was more effective with IC50 concentrations of 0.006 and 0.028 microM, respectively. In highly multidrug-resistant KB-C1 and KB-V1 cells, doxorubicin up to 1 microM did not cause any cytotoxic effects, while the doxorubicin-transferrin conjugate inhibited colony formation of these cells with IC50 levels of 0.2 and 0.025 microM, respectively. These results demonstrate that doxorubicin-transferrin is effective against multidrug-resistant tumor cells.

Genes related to growth and invasiveness are repressed by sodium butyrate in ovarian carcinoma cells

Down-regulation of oncogene expression is one of the hallmarks of the process whereby transformed cells are forced into differentiation and/or growth arrest by potent inducers and therefore can represent an interim end point in cancer treatment. The differentiation inducer sodium butyrate (NaB) arrested growth of N.1 ovarian carcinoma cells and repressed expression of cyclin D1/prad1 and the invasiveness-related protease plasminogen activator-urokinase (plau). This was accompanied by the acquisition of a differentiated morphology, all of which characteristics were maintained as long as N.1 cells were exposed to the inducer. In accordance with a differentiated phenotype was the finding that fibronectin expression was increased significantly. Recently, it was shown that NaB represses the transcription factor c-myc by blocking Ca2+ signals and modulating serine threonine kinase activity. We wanted to investigate NaB-mediated interference on signals contributing to the expression on prad1, plau and growth arrest-specific 6 (gas6). Protein kinase A (PKA) inactivation de-repressed prad1 and plau transcript levels. NaB had onlygeneral but no specific influence on PKA-modulated prad1 and plau expression however. Protein kinase C activation up-regulated plau transcript levels, but not that of prad1. Prad1 expression seemed to depend on Ca2+-triggered signals. Constitutive plau expression was insensitive to additional Ca2+-mediated signals, but it became responsive upon NaB treatment.

Iron binding capacity of trimidox (3,4,5-trihydroxybenzamidoxime), a new inhibitor of the enzyme ribonucleotide reductase

Ribonucleotide reductase is the rate limiting enzyme of deoxynucleoside triphosphate synthesis and is considered to be an excellent target of cancer chemotherapy. Trimidox, a newly synthesized compound, inhibits this enzyme and has in vitro and in vivo antitumour activity. As trimidox was able to upregulate the expression of the transferrin receptor in HL-60 human promyelocytic leukaemia cells, we have now investigated the capability of trimidox to interfere with iron metabolism. We show by photometric and polarographic methods that trimidox is able for form an iron complex. However, its cytotoxic action cannot be circumvented by addition of iron-saturated transferrin or iron-ammonium citrate, indicating that the iron complexing capacity is not responsible for the mechanism of action of this compound. When HL-60, K562 or L1210 leukaemia cells were incubated with the trimidox-iron complex itself, we could observe increases of the 50% growth inhibitory capacity of the complex in comparison with trimidox alone. We conclude that trimidox is able to form an iron complex, but in contrast to other agents, the anticancer activity cannot be contributed to this effect alone. Further studies will have to elucidate the molecular mechanism of action of this new and promising anticancer agent.

Sodium butyrate inhibits c-myc splicing and interferes with signal transduction in ovarian carcinoma cells

Sodium butyrate (NaB), a physiologically produced short chain fatty acid, dramatically changes the growth rate and also the morphology of a fast growing subclone (N.1) derived from the heterogenous human ovarian carcinoma HOC-7. The mRNA of the growth related proto-oncogene c-myc, constitutively expressed in N.1 cells decreased significantly within 24 h of NaB treatment and remained suppressed until the NaB block was released. Down-regulation was accomplished partially by accelerating degradation of c-myc mRNA and by inhibiting splicing of c-myc transcripts. We demonstrated that NaB blocked general mechanisms in signal transduction, such as the release of Ca2+ from intracellular stores, and modulated the activity of serine/threonine kinases. The multiple effects of sodium butyrate on HOC-7 derivatives, as well as on a variety of other cell types investigated by others, may be due to interference with general mechanisms of signal transduction.

Synergistic growth inhibitory and differentiating effects of trimidox and tiazofurin in human promyelocytic leukemia HL-60 cells

Increased ribonucleotide reductase (RR) activity has been linked with malignant transformation and tumor cell growth. Therefore, this enzyme is considered to be an excellent target for cancer chemotherapy. We have examined the effects of a newly patented RR inhibitor, trimidox (3,4,5-trihydroxybenzohydroxamidoxime). Trimidox inhibited the growth of human promyelocytic leukemia HL-60 cells with an IC50 of 35 mumol/L. Incubation of HL-60 cells with 50 mumol/L trimidox for 24 hours decreased deoxyguanosine triphosphate (dGTP) and deoxycytidine triphosphate (dCTP) pools to 24% and 39% of control values, respectively. Incubation of HL-60 cells with 20 to 80 mumol/L trimidox even up to a period of 4 days did not alter the distribution of cells in different phases of cell cycle. Sequential incubation of HL-60 cells with trimidox (25 mumol/L) for 24 hours and then with 10 mumol/L tiazofurin (an inhibitor of inosine monophosphate dehydrogenase) for 4 days produced synergistic growth inhibitory activity, and the cell number decreased to 16% of untreated controls. When differentiation-linked cell surface marker expressions were determined in cells treated with trimidox and tiazofurin, a significantly increased fluorescence intensity was observed for the CD 11b (2.9-fold). CD 33 (1.9-fold), and HLA-D cell surface antigens. Expression of the transferrin receptor (CD71) increased 7.3-fold in cells treated with both agents, compared with untreated controls. Our results suggest that trimidox in combination with tiazofurin might be useful in the treatment of leukemia.

Comparison of biochemical parameters of benzamide riboside, a new inhibitor of IMP dehydrogenase, with tiazofurin and selenazofurin

The biochemical and cytotoxic activities of the IMP dehydrogenase (IMPDH) inhibitors benzamide riboside, tiazofurin, and selenazofurin were compared. These three C-nucleosides exert their cytotoxicity by forming an analogue of NAD, wherein nicotinamide is replaced by the C-nucleoside base. The antiproliferative activities of these three agents were compared in a panel of 60 human cancer cell lines. To examine the relationship of benzamide riboside and selenazofurin to tiazofurin, COMPARE computer analysis was performed, and correlation coefficients of 0.761 and 0.815 were obtained for benzamide riboside and selenazofurin, respectively. The biochemical activities of these agents were examined in human myelogenous leukemia K562 cells. Incubation of K562 cells for 4 hr with 10 microM each of benzamide riboside, selenazofurin and tiazofurin resulted in a 49, 71, and 26% decrease in IMPDH activity with a concurrent increase in intracellular IMP pools. As a consequence of IMPDH inhibition, GTP and dGTP concentrations were curtailed. These studies demonstrated that selenazofurin was the most potent of the three agents. To compare the cellular synthesis of NAD analogues of these agents, K562 cells were incubated with 10 microM each of benzamide riboside, tiazofurin and selenazofurin after prelabeling the cells with [2,8-3H]adenosine. The results demonstrated that benzamide riboside produced 2- and 3-fold more of NAD analogue (BAD) than tiazofurin and selenazofurin did. To elucidate the effects of the three compounds on other NAD-utilizing enzymes, the inhibitory activities of purified benzamide adenine dinucleotide (BAD), thiazole-4-carboxamide adenine dinucleotide (TAD) and selenazole-4-carboxamide adenine dinucleotide (SAD) were studied in commercially available purified preparations of lactate dehydrogenase, glutamate dehydrogenase and malate dehydrogenase. TAD and SAD did not inhibit these three dehydrogenases. Although BAD did not influence lactate and glutamate dehydrogenases, it selectively inhibited 50% of malate dehydrogenase activity at a 3.2 microM concentration. These studies demonstrate similarities and differences in the biochemical actions of the three C-nucleosides, even though they share similar mechanisms of action.

Biochemical and antitumor activity of trimidox, a new inhibitor of ribonucleotide reductase

Trimidox (3,4,5-trihydroxybenzamidoxime), a newly synthesized analog of didox (N,3,4-trihydroxybenzamide) reduced the activity of ribonucleotide reductase (EC 1.17.4.1) in extracts of L1210 cells by 50% (50% growth-inhibitory concentration, IC50) at 5 microM, whereas hydroxyurea, the only ribonucleotide reductase inhibitor in clinical use, exhibited an IC50 of 500 microM. Ribonucleotide reductase activity was also measured in situ by incubating L1210 cells for 24 h with trimidox at 7.5 microM, a concentration that inhibits cell proliferation by 50% (IC50) or at 100 microM for 2 h; these concentrations resulted in a decrease in enzyme activity to 22% and 50% of the control value, respectively. Trimidox and hydroxyurea were cytotoxic to L1210 cells with IC50 values of 7.5 and 50 microM, respectively. Versus ribonucleotide reductase, trimidox and hydroxyurea yielded IC50 values of 12 and 87 microM, respectively. A dose-dependent increase in life span was observed in mice bearing intraperitoneally transplanted L1210 tumors. Trimidox treatment (200 mg/kg; q1dx9) significantly increased the life span of mice bearing L1210 leukemia (by 82% in male mice and 112% in female mice). The anti-tumor activity appeared more pronounced in female mice than in male mice. Viewed in concert, these findings suggest that trimidox is a new and potent inhibitor of ribonucleotide reductase and that it is a promising candidate for the chemotherapy of cancer in humans.

Synergistic action of tiazofurin with hypoxanthine and allopurinol in human neuroectodermal tumor cell lines

The activity of IMP dehydrogenase (EC 1.2.1.14), the key enzyme of de novo guanylate biosynthesis, was shown to be increased in tumor cells. Tiazofurin (TR), a potent and specific inhibitor of this enzyme, proved to be effective in the treatment of refractory granulocytic leukemia in blast crisis. We examined the effects of tiazofurin as a single agent and in combination with hypoxanthine and allopurinol in six different neuroectodermal tumor cell lines, the STA-BT-3 and 146-18 human glioblastoma cell lines, the SK-N-SH, LA-N-1 and LA-N-5 human neuroblastoma cell lines, and the STA-ET-1 Ewing tumor cell line. Tiazofurin inhibited tumor cell growth with IC50 values between 2.2 microM (LA-N-1 cell line) and 550 microM (LA-N-5 cells) and caused a significant decrease of intracellular GTP pools (GTP concentrations decreased to 39-79% of control). Incorporation of [8-14C]guanine into GTP pools was determined as a measure of guanylate salvage activity; incubation with 100 microM hypoxanthine caused a 62-96% inhibition of the salvage pathway. Incubation with tiazofurin (100 microM) and hypoxanthine (100 microM) synergistically inhibited tumor cell growth, and the addition of allopurinol (100 microM) strengthened these effects. Therefore, this drug combination, inhibiting guanylate de novo and salvage pathways, may prove useful in the treatment of human neuroectodermal tumors.

Cytotoxicity, differentiating activity and metabolism of tiazofurin in human neuroblastoma cells

The IMP dehydrogenase inhibitor, tiazofurin (TR)-2-beta-D-ribofuranosylthiazole-4-carboxamide, which exhibited oncolytic activity in patients with chronic myelogenous leukaemia (CML) in blast crisis was found to inhibit the growth of human neuroblastoma SK-N-SH cells with an IC50 of 4.2 microM. TR treatment of cells perturbed nucleic acid and catecholamine pathways. As biochemical markers of TR action decreased cellular GTP pools, increased inosine and hypoxanthine concentrations and depleted dopamine content were found. Incubation of tumour specimens obtained from paediatric patients with grade-IV neuroblastoma with TR resulted in the formation of the active metabolite, thiazole-4-carboxamide adenine dinucleotide, in concentrations sufficient to inhibit tumour growth. Cytotoxic and biochemical effects of TR were enhanced by combining it with allopurinol (an inhibitor of xanthine dehydrogenase), and hypoxanthine (an alternate substrate for hypoxanthine-guanine phosphoribosyltransferase). Induction of transdifferentiation of SK-N-SH cells from a neuroblast to an epitheloid, substrate-adherent phenotype was more pronounced with TR than with all-trans-retinoic acid. Transdifferentiating treatment with TR resulted in a 2-fold-enhanced sensitivity towards adriamycin. However, differentiation with all-trans-retinoic acid rendered the cells more resistant to adriamycin. Our results suggest that TR might be a promising agent for the treatment of children suffering from neuroblastoma.

GM-CSF: modulation of biochemical and cytotoxic effects of tiazofurin in HL-60 cells

Cytokines, such as granulocyte macrophage colony stimulating factor (GM-CSF) or interleukin-3 (IL-3) recruit quiescent cells into the cell cycle and sensitize these cells towards cell cycle specific chemotherapeutic agents. We examined the in vitro effects of GM-CSF on HL-60 cells and tested its modulatory influence on biochemical and cytotoxic effects seen with tiazofurin, a potent and specific inhibitor of IMP dehydrogenase. Incubation of HL-60 cells with 500 U/ml GM-CSF for 4 d enhanced cell proliferation, which was accompanied by a significant increase in IMP dehydrogenase activity (from 2.22 in control cells to 3.70 nmol/mg/h in cells pretreated with GM-CSF). When HL-60 cells were incubated with 100 microM tiazofurin for 2 h, intracellular GTP decreased to 46% of untreated control cells. In HL-60 cells pretreated with GM-CSF, GTP pools decreased to 38% of control after incubation with tiazofurin which is 69% of the predicted value for additive effect. The MTT chemosensitivity assay yielded significantly decreased IC50 values for tiazofurin in HL-60 cells, preincubated with GM-CSF (IC50 decreased from 13 microM to 10 microM). Therefore our results suggest that combination therapy with GM-CSF and tiazofurin may be beneficial for the treatment of refractory leukaemia patients.

Correlation of atrial natriuretic peptide and cyclic guanosine monophosphate plasma concentrations in patients with heart disorders during rest and exercise

The concentration of atrial natriuretic peptide was measured in order to evaluate its importance in patients suffering from a variety of cardiac diseases. There was a correlation between plasma concentrations of atrial natriuretic peptide and its "second messenger" cyclic guanosine monophosphate (cGMP) in all of the cases examined. We investigated the relationship between atrial natriuretic peptide and cGMP plasma concentrations during rest and exercise in comparison with the scintigraphically assessed left- and right-ventricular ejection fraction in patients with chronic heart disease (n = 20), and after orthotopic heart transplantation (n = 16); plasma concentrations were also measured in healthy controls (n = 14). Atrial natriuretic peptide and cGMP concentrations showed a similar correlation during rest and exercise with r = 0.74 and r = 0.81, respectively. With the exception of patients after heart transplantation, a significant negative correlation was seen between the left ventricular ejection fraction and atrial natriuretic peptide or cGMP plasma concentrations during rest conditions (r = 0.76 or 0.58, respectively). No correlation was apparent between plasma concentrations of atrial natriuretic peptide or cGMP and the left- or right ventricular ejection fraction during exercise. The concentrations of atrial natriuretic peptide and cGMP in plasma differed significantly between healthy controls and patients during rest and exercise. It is noteworthy that atrial natriuretic peptide and cGMP concentrations were markedly higher in patients after heart transplantation than in patients suffering from chronic heart disease. Our results indicate that plasma atrial natriuretic peptide and cGMP concentrations are sensitive markers of cardiac impairment.

Regulation of deoxycytidine kinase activity and inhibition by DFDC

(1) Deoxycytidine kinase activity increased in a transformation- and progression-linked fashion in rat hepatomas of different proliferation rates. The activity also increased and was growth rate-linked in a series of tissue culture cell lines of human and animal tumors. (2) Deoxycytidine kinase activity was stringently linked with expression of the neoplastic proliferative program as it sharply increased in log phase in tissue culture cells of hepatoma 3924A and several human carcinoma strains. (3) Deoxycytidine kinase is subject to nutritional and hormonal regulation. On starvation the activity in liver decreased and on refeeding it returned to normal. Steroid hormone increased liver enzymic activity. Deoxycytidine kinase is substrate-inducible, since deoxycytidine injections in rat led to a 2- to 3-fold increase in hepatic enzyme activity. (4) Actinomycin or cycloheximide treatment blocked the increase in liver deoxycytidine kinase activity induced by steroid or deoxycytidine treatment. Therefore, it is assumed that the rise in deoxycytidine kinase activity requires new RNA and protein synthesis. (5) Cycloheximide treatment of rats carrying hepatomas yielded a t1/2 = 3.4 hr in the tumor for deoxycytidine kinase activity which was the shortest among the examined enzymes of purine and pyrimidine biosynthesis. (6) Actinomycin treatment of rats carrying hepatomas yielded a t1/2 of 5.8 hr for deoxycytidine kinase activity in the tumor which was one of the shortest in the examined enzymes of purine and pyrimidine biosynthesis. (7) Difluorodeoxycytidine (DFDC) is a competitive inhibitor (Ki = 7-28 microM) of deoxycytidine kinase from rat hepatoma and from human pancreatic carcinoma and ovarian carcinoma cells in culture.

Cell cycle dependent regulation of IMP dehydrogenase activity and effect of tiazofurin

The activity of IMP dehydrogenase (IMP DH), the rate-limiting enzyme of de novo GTP biosynthesis, was shown to be increased in cancer cells. Tiazofurin, an inhibitor of IMP dehydrogenase, proved to be an effective agent in the treatment of refractory granulocytic leukemia. To examine the cell cycle dependent alterations of GTP synthesis and sensitivities to tiazofurin, we measured IMP DH activities and GTP pools, as well as the effects of tiazofurin on cell cycle phase enriched HL-60 cells. We now show that IMP DH activities and GTP concentrations are increased in S-phase enriched fractions of HL-60 cells. Moreover, the depletion of GTP concentrations by tiazofurin is most effective in S-phase enriched HL-60 cells. These results may be utilized in cancer chemotherapy to combine tiazofurin with biologic response modifiers which recruit quiescent leukemic cells into the cell cycle.

Cytotoxicity of a transferrin-adriamycin conjugate to anthracycline-resistant cells

Conjugates of adriamycin coupled to transferrin by glutaraldehyde are cytotoxic to human promyelocytic (HL-60) and erythroleukemic (K562) cells. Growth inhibition of adriamycin-sensitive cells, as evaluated by thymidine incorporation and the MTT-assay, was higher for conjugates than for free adriamycin. The cytotoxicity toward adriamycin-resistant K562 and HL-60 cells was 3-fold and more than 10-fold higher, respectively, for the transferrin-adriamycin conjugate than for the free drug. The effect of the conjugate was dependent on its adriamycin content, i.e., on its conjugation number.

Regulation of GTP biosynthesis

In the regulation of GTP biosynthesis, complex interactions are observed. A major factor is the behavior of the activity of IMPDH, the rate-limiting enzyme of de novo GTP biosynthesis, and the activity of GPRT, the salvage enzyme of guanylate production. The activities of GMP synthase, GMP kinase and nucleoside-diphosphate kinase are also relevant. In neoplastic transformation, the activities and amounts of all these biosynthetic enzymes are elevated as shown by kinetic assays and by immunotitration for IMPDH. In cancer cells, the up-regulation of guanylate biosynthesis is amplified by the concurrent decrease in activities of the catabolic enzymes, nucleotidase, nucleoside phosphorylase, and the rate-limiting purine catabolic enzyme, xanthine oxidase. The up-regulation of the capacity for GTP biosynthesis is also manifested in the stepped-up capacity of the overall pathways of de novo and salvage guanylate production. The linking with neoplasia is also seen in the elevation of the activities of IMPDH and GMP synthase and de novo and salvage pathways as the proliferative program is expressed as cancer cells enter log phase in tissue culture. The activity of GMP reductase showed no linkage with neoplastic or normal cell proliferation; however, in induced differentiation in HL-60 cells the activity increased concurrently with the decline in the activity of IMPDH. This reciprocal regulation of the two enzymes is observed in differentiation induced by retinoic acid, DMSO or TPA in HL-60 cells. In support of enzyme-pattern-targeted chemotherapy, evidence was provided for synergistic chemotherapy with tiazofurin (inhibitor of IMPDH) and hypoxanthine (competitive inhibitor of GPRT and guanine salvage activity) in patients and in tissue culture cell lines. These investigations should contribute to the clarification of the controlling factors of GMP biosynthesis, the role of the various enzymes, the behavior of GMP reductase in mammalian cells and the application of the approaches of enzyme-pattern-targeted chemotherapy in patients.

Determination of pyrimidine deoxynucleoside triphosphates in leukaemia cell extracts containing 1-beta-D-arabinofuranosylcytosine triphosphate

Deoxynucleoside 5'-triphosphates (dNTPs) can be determined in cell extracts by high performance liquid chromatography after prior selective degradation of ribonucleoside 5'-triphosphates with sodium periodate and methylamine. When the method is used for the evaluation of deoxynucleoside triphosphates in 1-beta-D-arabinofuranosylcytosine triphosphate (ara-CTP)-containing cell extracts, an additional peak coeluting with thymidine triphosphate (dTTP) is observed. This peak is due to the formation of a carboxylic acid derivative of ara-CTP by periodate oxidation, and it can lead to considerable overestimation of dTTP. Formation of this peak can be avoided by using alkaline reaction conditions (pH 7.5) and by changing the sequence of addition of the reagents used in the periodation procedure. By employing this modified protocol, cellular dNTP and ara-CTP levels can be monitored in extracts of leukaemic blasts during cytosine arabinoside treatment in two separate HPLC runs.

AZT: a biochemical response modifier of methotrexate and 5-fluorouracil cytotoxicity in human ovarian and pancreatic carcinoma cells

In ovarian and pancreatic carcinoma cell lines, the activity of the salvage enzyme, thymidine kinase (EC 2.7.1.21), was 2- to 13-fold higher than that of the key enzyme of thymidylate de novo biosynthesis, thymidylate synthase (dTMP synthase, EC 2.1.1.45). AZT (3'-azido-3'-deoxythymidine, zidovudine) competitively inhibited thymidine kinase activity in extracts of human ovarian and pancreatic carcinoma cells, with Dixon plots yielding Ki = 1.1 microM in both cell lines. AZT (20 microM) yielded synergistic cytotoxicity with methotrexate (0.4 microM) in human pancreatic carcinoma cells in clonogenic assay and also with methotrexate (0.02 microM) in human ovarian carcinoma cells, as measured by cell counts. Thymidine (10 microM) and hypoxanthine (100 microM) reversed these inhibitions. AZT (20 or 40 microM) also provided synergistic cytotoxicity with 5-fluorouracil (0.5 and 1.0 microM) in human pancreatic carcinoma cells in clonogenic assay. These studies suggest a new role for AZT, which, as an inhibitor of thymidine salvage, should be useful as a biochemical response modifier to provide a synergistic clinical anticancer impact on de novo biosynthesis of thymidylates in conjunction with methotrexate or 5-fluorouracil.

Concordant changes of pyrimidine metabolism in blasts of two cases of acute myeloid leukemia after repeated treatment with ara-C in vivo

Though data from cell lines are abundant, the reason for the development of resistance to 1-beta-D arabinofuranosylcytosine (ara-C) in vivo remains unresolved. A broad interpatient variation of metabolic parameters has further complicated interpretation of the results. The present study compares ara-C metabolism in leukemic blasts of two patients with newly diagnosed disease, before and after repeated treatment with ara-C containing chemotherapy regimens in vivo. Membrane transport of ara-C was unchanged after treatment. In addition, cell-free extracts of blasts obtained after treatment failure showed an unchanged cytidine deaminase activity. Though deoxycytidine kinase activity in cell extracts was unaltered or increased after treatment failure, the activity in situ, measured as the rate of 1-beta-D-arabinofuranosylcytosine triphosphate (ara-CTP) formation, was decreased. This could be shown to be due to an expansion of the deoxycytidine triphosphate (dCTP) pool. The severalfold increase in dCTP pool was accompanied by a decrease in thymidine triphosphate (dTTP) pool and correlated with a decrease in deoxycytidylate deaminase (dCMP-deaminase) activity in cell free extracts. Low dCMP-deaminase activity had been shown to confer an ara-C resistant phenotype to cell lines in vitro. Data presented in this paper show that a selection for leukemic blasts with low dCMP-deaminase activity can also be favored by ara-C containing treatment regimens in vivo. Our data suggest that this mechanism might contribute to treatment failure.

Correlation between heart disorders and concentrations of directly measured atrial natriuretic peptide in plasma

We used new commercially available direct radioimmunoassay to measure human atrial natriuretic peptide (h-ANP) in plasma from 48 individuals who were being evaluated for left and right ventricular function. For 13 healthy individuals with normal ventricular function these concentrations ranged up to 54 ng/L. Measurements of h-ANP clearly differentiated between normal subjects, patients with coronary artery disease, and patients who had undergone orthotopic heart transplantation (ANOVA P less than 0.0001, significant differences between all groups)--all showing normal ventricular function at rest. There was a strong negative correlation (r = -0.64, P less than 0.001) between left ventricular ejection fraction and h-ANP concentrations in plasma of patients with proven coronary artery disease, patients with cardiomyopathy, and healthy individuals. Results by the present method and methods involving extraction of the sample correlated well. Evidently the direct assay of h-ANP in plasma yields information that could be used to help evaluate heart disorders and other pathophysiological conditions causing increased h-ANP concentrations in plasma.