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Cisplatin [(cis-diamino-dichloroplatin (II)], which is a yellow solid with a square planar geometry, was the first inorganic drug to be used in the clinical setting, individually or in combination with other drugs, for the treatment of various neoplasms including those of the lungs, ovaries, testicles, prostate and neck. However, despite the appearance of second and third generation analogues that have overtime replaced its use, it remains a standard compound with which to compare new potentials inorganic antitumor species [52]. Despite its effectiveness, especially for testicular cancer, where success of the cure spans between 90 and 100%, its clinical use is limited by dose-dependent side effects on the brain, liver and kidneys and by intrinsic or acquired resistance by malignant cells [53].

Cisplatin is administered intravenously into the bloodstream and due to the high concentration of chlorides present (about 100 mM), remains in the neutral form without giving rise to ligands dissociation. When the complex permeates the cell membrane, the lower concentration of chlorides (about 4-20 mM) promotes the slow release of one or both of the coordinated chlorides, with consequent formation of mono- and di-aqua cationic species. The entry into the cell occurs mainly by passive transport because of the neutral nature of the complex. Once inside the cellular environment, the aquaction process and the consequent formation of cationic species, prevents the expulsion of the drug. Unfortunately, a similar mechanism probably contributes to the high cytotoxicity of the compound.

On the other hand, it has been discovered that cellular uptake can also occur according to active transport mechanisms. The trans-membrane protein CRT1 and in general the proteins involved in copper transport seem to play an important role in controlling the accumulation and the expulsion of substances from the cell [16, 53]. The cationic mono- and di-aquo species formed inside the cell are very electrophilic and can react with the numerous biological ligands present in the cellular environment. The soft nature of Pt (II) allows the interaction of its complexes mainly with the -SH groups present in metallothioneins and cysteines, although their primary target seems to be DNA.

The nucleophilicity of N7 nitrogen of adenine and guanine present in the DNA, form with cisplatin monoadducts or bifunctional adducts, mainly by intrastrand bonds with two contiguous guanines, a guanine and an adjacent adenine or with two guanines spaced from a third intermediate base.

Remarkably, these interactions are not possible with transplatin for geometric reasons. In addition, interstrand bonds are possible with two bases belonging to different filaments and between DNA and proteins (Fig. 1.12).

Fig. 1.12 Adducts between DNA and platinated agents.

The DNA platination, in the case of the formation of the bifunctional adduct between the two adjacent guanine, induces a distortion of the dihedral angle between the two bases, a bending and a partial unwinding of the double helix near the involved sites [16]. Conformational modifications can be recognized by specific proteins capable of triggering a series of events aimed at repairing the damage or, hopefully promoting the cell death. As mentioned earlier, an important limitation in the use of cisplatin is the intrinsic or acquired resistance manifested by the malignant cells.

A series of mechanisms responsible for this characteristic have been identified [16, 54]:

• Decreased cisplatin level within the cell caused by a minor uptake and a greater efflux of the complex which is regulated by the trans-membrane proteins and by the specific proteins related to copper leakage (ATP7A and ATP7B).

• Increased concentration of glutathione (GSH) and metallothioneins with consequent increase their competitiveness toward DNA. The electrophilic character of the cationic complexes formed by the cisplatin inside the cell and the soft nature of Pt (II), promote an interaction between the metal and the thiol groups of these species. Specific pumps are then able to expel the complexes formed.

• Increased DNA repair capacity, which is mainly due the efficiency of the NER (Nucleotides Excision Repair), a system able to recognize, cut, remove and synthesize the altered site. An inefficient NER system is thought to cause the effectiveness of cisplatin in the treatment of testicular tumours.

• Increased tolerance of damaged DNA due to deficiencies in the MMR (Mismatch Repair), which is a system involving various proteins and enzymes responsible for DNA damage repair.

• Bypass of apoptotic responses through changes in signal chains and activation of survival pathways.

Probably is a combination of these mechanisms that reduces the therapeutic efficacy of the drug [16, 54].

Fig. 1.13 Cisplatin mechanism of action [55].

Over the years, numerous platinum-based compounds have been synthesized, with the aim of obtaining new therapeutic agents characterized by less toxicity and less resistance developed by malignant cells. Despite the efforts, only carboplatin and oxaliplatin are currently used in the therapeutic field for the treatment of different types of cancer all over the world. However, it is noteworthy that nedaplatin and lobaplatin have been approved for clinical use in Japan, China and Korea, respectively [56].

Fig. 1.14 Cisplatin and commercial analogues [16].

Carboplatin differs from cisplatin for the presence of the 1,1-cyclobutandicarboxylate group, which is hydrolyzed more slowly than chlorides. It is used against the same type of neoplasia treated with cisplatin but, owing to its reduced cytotoxicity, it requires a higher dosage (about four times higher) [16, 57].

Oxaliplatin has the oxalate as the displaceable group and the 1R,2R-diaminocyclohexane (DACH) as spectator ligand. This compound forms adducts with DNA analogous to those observed in the case of cisplatin and carboplatin, but the presence of a non-labile ligand more bulky and hydrophobic than ammonia, contrasts the action of the proteins involved in the repair of DNA and the altered site. These considerations confirm the hypothesis that structural modifications of the inert ligand may affect the activity of the compound [16].

Pt (IV) octahedral complexes have also been tested. These complexes show some advantages compared to square planar Pt (II) derivatives since they are inert toward substitution reactions therefore allowing the minimization of the collateral interactions with biological agents before reaching DNA [16]. The activation of these complexes inside the cell occurs as a consequence of the reduction of Pt (IV) to Pt (II) by reducing agents such as glutathione. Moreover, the presence of two further coordination sites renders more easy the modulation of their reactivity and activity, intervening on properties such as redox potential, kinetic stability, lipophilicity and selectivity toward specific sites.

In this respect, Dyson and coworkers have proposed that the intracellular reduction of the pro-drug of Pt (IV), leads to the formation of cisplatin and promotes the release of two equivalents of ethacrynic acid, an inhibitor of glutathione S-transferase, enzyme responsible for drug resistance in some cancers (Figure 1.15) [41, 58].

Fig. 1.15 Schematic approach of target delivery [41].

The numerous derivatives of Pt (II) and Pt (IV) tested over the years as possible anti-tumour agents, allowed the identification of correlations between the structure of the complexes and their activity [16]:

• The square planar complexes of Pt (II) must exchange only some of their ligands and the activation occurs as a result of the hydrolysis of the anionic leaving groups. The leaving groups must also be cis each other, since the trans isomers are in fact generally inactive.

• The rate of hydrolysis must be moderate, to avoid the indiscriminate interaction of the complex with the biological components before reaching the tumour cells. However, the rate of hydrolysis cannot be too low otherwise the complex will be inert and therefore almost ineffective.

• The ligand trans to the leaving groups must be firmly coordinated to the metal. The presence of hydrogen atoms in these groups can promote the formation of hydrogen bonds with the target molecule and influence the nature of the adducts between the complex and the DNA.

• The complexes, although the active form may be charged as a consequence of the solvolysis, should initially be neutral.

• In the case of Pt (IV) complexes, the two additional ligands in axial position may confer particular properties to the compound or act as targeting ligands once released as a consequence of the reduction of the metal.

As previously reported, cisplatin and its currently used analogues present a series of limitations in their clinical utilization owing to their toxicity and the intrinsic or induced mechanisms of resistance of some tumour cells. Furthermore, the applicability of a limited type of tumours and the cytotoxicity extended to healthy cells, encuoraged the researchers to turn their attention also toward metal derivatives different from those of platinum. Their different nature and reactivity might open new and interesting perspectives in the therapeutic field.