Enhanced effects of adriamycin by combination with a new ribonucleotide reductase inhibitor, trimidox, in murine leukemia.

Ribonucleotide reductase is the rate limiting enzyme of de novo DNA synthesis; its activity is significantly increased in tumor cells related to the proliferation rate. Therefore the enzyme is considered to be an excellent target for cancer chemotherapy. In the present study we tested the in vitro and in vivo antitumor effects of a drug combination using trimidox (3,4,5-trihydroxybenzamidoxime), a novel inhibitor of ribonucleotide reductase with adriamycin, a widely used anticancer drug. This combination was selected because adriamycin generates free radicals being responsible for cardiotoxic side effects; trimidox has been shown to be a good free radical scavenger. The in vitro cytotoxic effect of the drug combination was examined in L1210 mouse leukemia cells employing a MTT chemosensitivity assay. Incubation of these cells with adriamycin and trimidox together yielded less than additive cytotoxic effects compared to either drug alone. These effects were not caused by the involvement of p-glycoprotein mediated drug efflux. However, when the effect of trimidox and adriamycin in combination was examined in L1210 leukemia bearing mice antitumor effects of adriamycin could be enhanced by the presence of trimidox. Our data indicate, that the in vivo combination of adriamycin together with trimidox might be beneficial for the treatment of malignancies.

Iron binding capacity of didox (3,4 dihydroxybenzohydroxamic acid) and amidox (3,4 dihydroxybenzamidoxime) two 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 circumvented by addition of Fe-ammoniumcitrate, indicating the iron complexing capacity not to 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, only slight 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 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 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.

[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.

Effect of cell growth and cell differentiation on 1-beta-D-arabinofuranosylcytosine metabolism in myeloid cells.

Resistance of leukaemic blasts to 1-beta-D-arabinofuranosylcytosine (ara-C) has been shown to be associated with changes in the metabolism of this drug. However, effects of cell growth and maturation stage on ara-C metabolizing enzymes have to be excluded as a possible cause of different enzyme activities in leukaemic blasts between nonresponders and patients achieving complete remission. We evaluated the effects of cell cycle phase and cell differentiation on the activity of cytidine deaminase, deoxycytidylate deaminase and deoxycytidine kinase in myeloid cell lines. Our data indicate that different enzyme profiles in nonresponders might not only be caused by the emergence of mutator phenotypes but may also reflect the growth and maturation stage of leukaemic blasts.