## 2 Zinc Binding Groups in Chelation Therapy
### Treatment of Alzheimer's Disease  
Among other causes, the abnormal aggregation of the \(\beta\)-amyloid (A\(\beta\)) protein in the brain is involved in Alzheimer's disease (AD) [19; 20]. It is demonstrated that A\(\beta\) chelates Cu\({}^{2+}\), Zn\({}^{2+}\), therefore these metals play an important role in protein precipitation and plaque formation as well as Fe\({}^{3+}\) and Al\({}^{3+}\)[21]. Some authors associated the formation of these aggregates to the production of reactive oxygen species (ROS) that lead to an increase in intracellular calcium levels, with consequent lipid peroxidation, mitochondrial dysfunctions, and neuronal inflammation [22]. The use of chelating agents capable of mainly crossing the blood-brain barrier (BBB) is a feasible strategy for the treatment of AD, since they could bind zinc, copper and iron ions (both free and accumulated within A\(\beta\)) without interfering with the activity of metalloenzymes [23]. Another important feature of chelators is the selectivity towards the different cations. Since only Cu\({}^{+/2+}\) directly catalyze the overproduction of ROSs (Zn\({}^{2+}\) is a redox inert), it is, of course, the preferred target in chelation therapy. Moreover, Cu\({}^{2+}\) and Zn\({}^{2+}\) bind to the same amino acid residues of the A\(\beta\) peptide, so they are in competition in the inter-synaptic space for both proteins and chelators. Considering that Zn\({}^{2+}\) is involved in neurotransmission and its significant removal can cause damages, Cu\({}^{2+}\) can be identified again as the main target for potential anti-AD drugs. Starting from these considerations, it seems crucial that the chelator must have a higher Cu\({}^{2+}\) over Zn\({}^{2+}\) selectivity than those of A\(\beta\) peptide (calculated as ca. 4 orders of magnitude) [24; 25]. The most commonly used moiety in the design of chelating agents for the treatment of AD is 8-hydroxyquinoline, a monobasic group that contains both nitrogen and oxygen atoms: it is a monobasic bidentate ligand and forms neutral 3:1 complexes with Fe\({}^{3+}\) and 2:1 complexes with Zn\({}^{2+}\) and Cu\({}^{2+}\), removing these metals from cells [26]. Clioquinol (CQ) is an 8-hydroxyquinoline analog that chelates Zn\({}^{2+}\) and Cu\({}^{2+}\) with higher affinity than Mg\({}^{2+}\) and Ca\({}^{2+}\) and with the suitable selectivity [27], which makes it capable of solubilizing Zn/Cu-assembled deposits in A\(\beta\) and inhibit A\(\beta\) redox chemistry. Moreover, CQ is hydrophobic and crosses the BBB easily, making it a candidate drug for oral treatment of Alzheimer's disease [28]. The structural characteristics of the Zn\({}^{2+}\) and Cu\({}^{2+}\) complexes of CQ have been known since 2004: after deprotonation of its phenolic oxygen, it forms neutral 2:1 complexes with both ions, where the pyridine nitrogen and the phenolate oxygen are metal donors of the bidentate ligand [29]. The structure of CQ is reported in Figure 1, as well as the supposed chelation mechanism. Unfortunately, studies show that CQ can be neurotoxic, probably because it removes metal ions not only from A\(\beta\) but also from other proteins, resulting in a mobilization of ions that causes oxidant effects if the CQ-metal complex is redox-active [30]. Figure 1: Clioquinol. Metal chelators could be useful as a radiopharmaceutical for the determination of metal distribution and concentration in the brain by positron emission tomography (PET) or single-photon emission computed tomography (SPECT) [31]. A possible radiolabeled compound is \({}^{123}\)I-CQ, but unfortunately, it cannot be useful because of its radio-deiodination in vivo and low brain uptake. For this reason, CQ analogs were developed as potential drug candidates and PET ligands [31].