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 IC50 of 25 microM. In a soft
agar colony forming assay,
amidox yielded a 50% inhibition of colony formation at 13 microM. We also investigated the effects of
amidox treatment on the formation of deoxynucleosidetriphosphates.
Amidox (50 and 75 microM 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 microM
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.