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.

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.

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.