Iron is critical for cell growth and proliferation. Iron chelators are being explored for a number of clinical applications, including the treatment of neurodegenerative disorders, heart disease, and cancer. To uncover mechanisms of action of tachpyridine, a chelator currently undergoing preclinical evaluation as an anticancer agent, cell-cycle analysis was performed. Tachpyridine arrested cells at G 2 , a radiosensitive phase of the cell cycle, and enhanced the sensitivity of cancer cells but not nontransformed cells to ionizing radiation. G 2 arrest was p53 independent and was accompanied by activation of the checkpoint kinases CHK1 and CHK2. G 2 arrest was blocked by UCN-01, a CHK1 inhibitor, but proceeded in CHK2 knock-out cells, indicating a critical role for CHK1 in G 2 arrest. Tachpyridine
IntroductionIron is a metal critically important for cell proliferation and survival. It is an essential constituent of hemoglobin and myoglobin, mitochondrial electron transport proteins, hydroxylating enzymes, lipoxygenases, and cyclooxygenases, as well as ribonucleotide reductase, the enzyme that catalyzes the rate-limiting step in DNA synthesis. Tumor cells frequently exhibit increased uptake and utilization of iron, as evidenced by an increase in transferrin receptors at the cell surface. 1,2 This has led to the suggestion that agents that deplete intracellular iron may be useful in cancer therapy. [3][4][5] A variety of approaches, including the use of antisense technology 6,7 and antibodies 8 to target the transferrin receptor, have borne out this prediction. Indeed, gallium, a metal used in combination chemotherapy for the treatment of metastatic urothelial carcinoma 9 and non-Hodgkin lymphoma, 10 exerts its effects in part by interfering with iron metabolism.High-affinity, low-molecular-weight metal chelators offer a direct method of achieving intracellular iron deprivation, and the antitumor potential of iron chelators has also been the subject of active investigation. The preponderance of these studies has focused on desferrioxamine (deferoxamine, Desferal, DFO), a hexadentate iron chelator and the drug of choice for the treatment of chronic iron overload. Although desferrioxamine has been shown to retard tumor growth substantially in animal studies 11 and some clinical trials, 12 results have been mixed 13,14 and have led to the identification of chelators with potentially improved antitumor activity. For example, Triapine, a ribonucleotide reductase inhibitor and iron chelator, is currently in phase 1 clinical trials. 15 Pyridoxal isonicotinoyl hydrazone, 16 17 desferrithiocin,18,19 desferriexochelin, 20 di-2-pyridyl thiosemicarbazones, 21 and tachpyridine [22][23][24][25][26][27] are chelators under investigation as antitumor agents.Tachpyridine is a hexadentate metal chelator. 23 It is a member of a chelator family currently in preclinical development at the National Cancer Institute (NCI). Tachpyridine weakly binds divalent cations such as calcium, magnesium, and manganese, but it strongly binds iron,...