Modulation of 5-fluorouracil resistance in human colon tumor cell lines by azidothymidine.

5-fluorouracil (5-FU), an inhibitor of thymidylate synthase (EC 2.1.1.45), is clinically used in the treatment of several solid tumors, including colorectal, head and neck, gastric, and pancreatic cancer. The drug effectively inhibits deoxynucleoside triphosphate de novo synthesis. However, this inhibition can be circumvented by increased thymidine kinase (EC 2.7.1.21) activity. In the present study we examined the effects of 5-FU combined with azidothymidine (AZT), a competitive inhibitor of thymidine kinase in human colon tumor cells in vitro, including three 5-FU resistant cell lines. The cells were simultaneously incubated with various concentrations of 5-FU (0.015 to 150 microM) and AZT (20 to 300 microM) for 6 days. 5-FU alone yielded an IC50 of 18 microM in the parental CCL 227 cell line and IC50s of 470 and 1100 microM in the 5-FU resistant cell lines as determined by a MTT chemosensitivity assay. Addition of 100 microM AZT alone, a drug concentration that can be achieved in patients, had no effect on the growth of the cell lines examined. However, when added simultaneously with 5-FU, the IC50s of 5-FU synergistically decreased to 10 microM in the sensitive and to 360 or 760 microM in the resistant cell lines, respectively. Our results demonstrate that the combination of 5-FU with AZT synergistically inhibited the growth of 5-FU resistant cells, suggesting the use of 5-FU in combination with AZT for the treatment of 5-FU sensitive as well as resistant human colon tumors.

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.

[Synergistic cytotoxic effects of chemotherapy in colon tumor cells by simultaneous inhibition of de novo and salvage energy metabolism pathways]. / Synergistische zytotoxische Effekte einer Chemotherapie an Kolontumorzellen durch gleichzeitige Hemmung von de novo und salvage Stoffwechselwegen.

The success of chemotherapy of colon tumours is currently limited. We have therefore used the human colon tumour cell line HT-29 to evaluate the cytotoxic effects of various drug combinations. Trimidox (3,4,5-trihydroxybenzamidoxime), a recently patented inhibitor of ribonucleotide reductase was combined with cytosinearabinoside (Ara-C) or 2',2'-difluorodeoxycytidine (DFDC) in order to inhibit both pyrimidine de novo and salvage pathways. Synergistic cytotoxic effects were observed. When HT-29 cells were sequentially treated with trimidox (20 microM for 24 h) and Ara-C (2 microM for 2 h), colony numbers decreased to 71% of the value calculated for additive cytotoxicity. When cells were simultaneously treated with trimidox (10 microM and 15 microM) and DFDC (0.2 nM), synergistic inhibition of colony formation was likewise noted (colony numbers decreased to values as low as 73% or 71% of the values calculated for additive cytotoxicity). On the other hand, we combined tiazofurin, an inhibitor of the guanylate de novo pathway, with allopurinol, which inhibits the guanylate salvage pathway by increasing intracellular hypoxanthine concentrations, leading to inhibition of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Synergistic cytotoxic effects were observed under these conditions too. When cells were treated with 10 microM tiazofurin and 400 microM or 800 microM allopurinol the number of colonies decreased to 69% and 27%, respectively, of the values calculated for additive effects. Our data suggest these drug combinations to be promising options in the treatment of human colon cancer.

[The ribonucleotide reductase enzyme as a target for enzyme-directed chemotherapy effects of trimidox (3,4,5-trihydroxybenzohydroxamidoxim), a new inhibitor of ribonucleotide reductases]. / Das Enzym Ribonukleotid-Reduktase als Target für enzymgerichtete Chemotherapie-Wirkungen von Trimidox (3,4,5-Trihydroxybenzohydroxamidoxim), einem neuen Inhibitor der Ribonukleotid-Reduktase.

Inhibition of the enzyme ribonucleotide reductase by polyhydroxy-substituted benzohydroxamide derivates is an example for the effects of antimetabolites. We present an overview of the effects of antimetabolites, in particular regarding their action on leukemia cells. Trimidox is one of the most effective inhibitors of ribonucleotide reductase. It inhibits the enzyme in cell extracts as well as in the in situ assay and causes decreased dGTP and dCTP pools in HL-60 cells. We describe combinations with other antimetabolites, as well as biochemical, morphological and cytotoxic effects of trimidox. This manuscript gives an overview of our results with trimidox and describes selection criteria, effects and combinations used in enzyme-targeted chemotherapy.

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.

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.