Amidox, an Inhibitor of Ribonucleotide Reductase, Potentiates the Action of Ara‐C in HL‐60 Human Promyelocytic Leukemia Cells

Amidox (3,4‐dihydroxybenzamidoxime), a new polyhydroxy‐substituted benzoic acid derivative, is a potent inhibitor of the enzyme ribonucleotide reductase (RR), which catalyses the de novo synthesis of DNA. RR is considered to be an excellent target for cancer chemotherapy. In the present study we investigated the antineoplastic effects of Amidox alone and in combination with Arabinofuranosylcytosine (Ara‐C) in HL‐60 human promyelocytic leukemia cells. In growth inhibition experiments Amidox yielded an IC50 of 30 µM, colony formation was inhibited at an IC50 of 20 µM as determined by a soft agar assay. Exposure of the cells to 75 and 100 µM Amidox for 24 hours was shown to significantly decrease intracellular dCTP, dGTP and dATP pools, whereas dTTP concentration increased, as determined by HPLC. The combination of Amidox with Ara‐C yielded more than additive cytotoxic effects both in growth inhibition assays and in soft agar assays. We could show that—after preincubating the cells with 75 and 100 µM Amidox and subsequent exposure to Ara‐C—intracellular Ara‐CTP levels increased by 576% and 1143%, respectively. In conclusion, Amidox might offer an additional option for the treatment of leukemia and thus be further investigated in vitro and in vivo.