Competing pathways of iron chelation: Angiogenesis or anti‐tumor activity: Targeting different molecules to induce specific effects

Dear Sir, In a recent letter published in the International Journal of Cancer, Linden and Wenger1 commented on the anti-proliferative and anti-angiogenic activity of ciclopirox (CPX) that was reported in a previous article.2 Linden and Wenger1 point out that CPX is an anti-fungal agent that binds iron (Fe) and inhibits tumor growth in vitro by this mechanism.3 Moreover, the Fe-depletion induced by this compound can also encourage angiogenesis by upregulating the vascular endothelial growth factor-1 (VEGF-1). 1 This effect is mediated through the stabilization of hypoxia inducible factor-1 (HIF-1 ) via inhibiting proline hydroxylation of this protein by an Fe-containing prolyl-4-hydroxylase.1 Indeed, Linden and Wenger1 cite work by Simonart et al.4 who implemented the clinically used Fe chelator, desferrioxamine (DFO), to treat immunodeficient mice implanted with the endothelial-derived tumor, Kaposi’s sarcoma. In this latter investigation, DFO did not inhibit growth of the cancer and resulted in slightly larger tumors in vivo compared to animals treated with vehicle alone.4 Based on these studies, Linden and Wenger1 suggest that CPX may not be suited toward the treatment of cancer in vivo but could possibly be used to increase angiogenesis for wound healing. Although the point of Linden and Wenger1 is clearly of interest, we would like to suggest that the chelator-mediated response obtained, that is either angiogenesis or anti-proliferative activity, are dependent on the extent of Fe chelation and the intracellular pathways targeted by these ligands. Indeed, as discussed below, both of these competing pathways can be induced by Fe chelation and it may be possible to pharmacologically promote either pathway depending upon the design of the ligand. A critical consideration in terms of the balance between inducing angiogenesis or anti-proliferative activity is the efficacy of Fe chelation. For example, DFO is a relatively poor anti-proliferative agent due to its short plasma half-life and its inability to readily permeate cell membranes to bind intracellular Fe pools.5,6 In fact, the cited study of Simonart et al.3 clearly demonstrated that DFO did not induce Fe-depletion, as reflected by the lack of alteration in serum ferritin levels, serum Fe levels, total Fe-binding capacity, transferrin-Fe saturation or the hematocrit. Hence, the lack of effect on tumor growth was not surprising, as the compound was ineffective at inducing Fe-depletion. Indeed, the concentration of DFO used in the Simonart et al.3 study may be insufficient to result in Fe deprivation and local toxicity. As a consequence of the poor anti-proliferative activity and Fe chelation efficacy of DFO, several laboratories have developed novel Fe chelators that have high membrane permeability and marked anti-proliferative activity compared to DFO.5,6 For example, there are multiple Fe chelators that cause marked cell cycle arrest and apoptosis in vitro and in vivo, including desferrithiocin analogs, tachpyridine, O-trensox, pyridoxal isonicotinoyl hydrazone (PIH) analogs and Triapine . 5,6 In fact, Triapine is currently in clinical trials for the treatment of a variety of cancers.7,8 Importantly, although the treatment of tumors with potent Fe chelators results in HIF-1 stabilization and VEGF-1 expression to possibly promote angiogenesis,1,9 it must be pointed out that there are also counteracting pathways induced by Fedepletion that inhibit growth. In fact, effective Fe chelation is well known to cause cell cycle arrest and apoptosis.5,6,9 For instance, without a constant supply of Fe, the conversion of ribonucleotides to deoxyribonucleotides by ribonucleotide reductase is inhibited and DNA synthesis prevented.5,6 In addition, recent studies have demonstrated that Fe chelators decrease the expression of molecules required for cell cycle progression such as the cyclins (e.g., cyclin A, B, D) and cdk2.10 To further enforce cell cycle arrest and programmed cell death, Fe chelators inhibit the translation of p21.11 Concerning all Fe chelation studies, it is important to distinguish between the toxicity results obtained in vitro and in vivo. In vitro, there is a concentration window where Fe chelators show low toxicity against normal cells, but high cytotoxicity toward tumors. In contrast, the situation is more complex in vivo, and as a consequence, extrapolations from in vitro results can be misleading. This is especially relevant for chelation therapy using potent Fe chelators because the majority of toxicity studies have been carried out in vitro. It is important to note, however, that several investigations have demonstrated the effectiveness of Fe chelation in vivo using DFO and Triapine,5–8 thus encouraging further studies. The multiple mechanisms by which potent Fe chelators cause cell cycle arrest and programmed cell death may overwhelm the angiogenic effects of VEGF-1 and account for the