Biochemical modulation of Ara-C effects by amidox, an inhibitor of ribonucleotide reductase in HL-60 promyelocytic human leukemia cells

Abstract

Amidox, 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 anti cancer chemotherapy. We investigated the biochemical and antineoplastic effects of amidox as a single agent and in combination with Ara-C in human HL-60 promyelocytic leukemia cells. Amidox inhibited the growth of HL-60 cells in a growth inhibition assay with an IC 50 of 25 μM. In a soft agar colony forming assay, amidox yielded a 50% inhibition of colony formation at 13 μM. We also investigated the effects of amidox treatment on the formation of deoxynucleosidetriphosphates. Amidox (50 and 75 μM for 24 hours) could significantly decrease intracellular concentrations of dCTP, dATP and dGTP pools, whereas dTTP levels increased. We then tested the combination effects of amidox with Ara-C; this combination yielded additive cytotoxic effects both in growth inhibition and in soft agar colony formation assays. This effect was due to the increased formation of Ara-CTP, the active metabolite of Ara-C, after preincubation with amidox. Preincubation of HL-60 cells with 75 and 100 μM amidox for 24 hours caused an increase in the intracellular Ara-CTP concentrations by 576% and 1143%, respectively. Therefore amidox might offer an additional option for the treatment of leukemia and thus be further investigated in in vivo studies as a single agent and in combination with Ara-C.