## 4 Genomic Instability, Fe-S Clusters and Iron Chelators
### Ribonucleotide Reductases  
Ribonucleotide reductase (RnR) is an essential enzyme in DNA replication and repair, reducing ribonucleotide 5\({}^{\prime}\) diphosphates to deoxyribonucleoside 5\({}^{\prime}\) diphosphates [106]. In cancer, there is an increased requirement for RnR highlighted by an increased DNA synthesis rate compared to non-neoplastic cells [107]. There are three classes of RnR: the predominant one in eukaryotes is class 1 Ia RNR, which contains a di-iron centre that maintains a diferic-tyrosyl radical (Fe\({}_{2}\)IIIY-) [108]. Class 1 Ia Rn has a large R1 and small R2 subunit; the small R2 subunit contains this di-iron centre required for catalysis [100]. RNR is able to redox cycle between the two iron states; this gives it its function by allowing RnR to function as an electron donor or acceptor [109]. An important part of retaining genomic integrity is ensuring there are normal levels of dNTPs, therefore deregulation of RnR is mutagenic. As RNR is directly dependent on the labile iron pool and intercellular iron, the dysregulation of iron levels can thus be mutagenic [110]. RnR also produces uridine diphosphate (dUDP), used in double-strand break repair. The disruption of RNR in tumours was shown to prevent thymidylate kinase from stopping dUDP from repairing DNA [111]. Therefore, an imbalance in RNR and dNTPs has resulted in DNA mutations and cell death [112; 113]. This imbalance could lead to genomic instability in cancer cells and potentially increase survival by increasing DNA repair [5]. Iron Chelators and Their Activity against RNR  
One hallmark of cancer is its ability to divide rapidly; for this to occur, neoplastic cells would need dNTPs in much greater volumes [4; 5]. Thus, the deficiency of RNR function, which could result from an iron chelator, would result in cell cycle arrest in the G\({}_{1}\)/S phase [114]. To that end, several iron chelators have been designed for cancer therapy to inhibit RnR: hydroxyurea, desferrioxamine, thiogenicarbazone derivates and pyridoxal isoniconyl hydrazone (PIH) have all been tested against cancer [115; 116; 117; 118]. Hydroxyurea was the first metal chelator to be used to inhibit RNR. This is achieved by attacking the di-iron centre and reducing the tyrosyl radical via one-electron transfer, thus depleting the dNTPs pools and causing DNA damage [119; 110]. This chelator is not iron specific, attacking other metallocentres, which produced varied results, with its anti-tumour capabilities being somewhat controversial [120]. This led to more iron-specific chelators, the most notable being desferrioxamine which is used clinically in iron overload diseases and is also researched for its anti-cancerous properties [121]. Desferrioxamine is a bacterial siderophore that binds iron in the labile iron pool, decreasing the available iron content to the cell [122]. High doses of Desferrioxamine can also disrupt iron homeostasis by removing iron, causing cell cycle arrest and enhancing apoptosis in cancer cells [123; 124; 125]. RnR, as described earlier, relies on the labile iron pool for the biosynthesis of its cofactors. Desferrioxamine complexes are very stable, leading to a full co-ordination sphere 1:1 complex. As previously stated, they do not have an unbonded co-ordination site for Fenton chemistry [126]. Thus, desferrioxamine inhibits RnR's function because it removes the available iron from the labile iron pool. This reduction in labile iron stops the tyrosyl radical forming in the di-iron centre of RnR [121]. However, desferrioxamine has several drawbacks as an RnR inhibitor: poor membrane permeability, a high cost and a short life in the bloodstream [127]. Nonetheless, its limited success encouraged the design of new iron chelators for RnR inhibition, such as PIH and triapine (3-AP). PIH is a tridentate chelator with hard oxygen donor atoms; as stated above, it prefers stronger cations as it's a strong donor and thus prefers Fe\({}^{\text{III}}\) over Fe\({}^{\text{II}}\)[128].