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Two forms of ribonucleotide reductase (RR), consisting of M1 with M2 subunits and M1 with p53R2 subunits, are involved in DNA replication and damage repair, respectively. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3AP), one of the heterocyclic carboxaldehyde thiosemicabazones (HCTs), is a potent RR inhibitor in clinical trial for cancer treatment. In this study, 3AP and its 7 derivatives showed 100–1000-fold higher inhibitory potency on KB nasopharyngeal carcinoma cells than hydroxyurea and were fully active against hydroxyurea- and gemcitabine-resistant KB cells. In vitro RR assays using two recombinant RRs showed that all 8 HCTs decreased the activity of both RRs in a dose-dependent manner and the efficiency was compatible with that on cell proliferation inhibition. Iron has different impact on the behavior of the compounds toward RRs. In the absence of iron, the HCTs showed more selective inhibition for p53R2-M1 than M2-M1, while addition of iron increased their activity but reduced their selectivity for two RRs. Radioligand binding assays showed that [3H]3AP directly bounded to the small subunits. Electron paramagnetic resonance measurements demonstrated that these HCTs generated reactive oxygen species with ferrous iron, which quenched the diiron-tyrosyl radical co-factor of the small subunits and hence the enzyme activity. While the ROS may be a common mediator responsible for the potent activity of the HCTs, the different characteristics of the small subunit proteins are probably associated with the subunit-selectivity of inhibition. Better understanding of the mechanism of action of RR inhibition may improve design of new potent and subunit-selective RR inhibitors for cancer therapy.