Copper is a transition metal that can exist in oxidized (Cu(II)) and reduced (Cu(I)) states. This allows it to participate in redox and catalytic chemistry, making it a suitable cofactor for a diverse range of enzymes and molecules. Copper complexes have been investigated for their therapeutic or diagnostic potential showing effectiveness in cancer treatment due to their cytotoxic action on tumour cells. In this review, the most remarkable achievements in the design and development of copper(I, II) complexes as antitumor agents are discussed. Special emphasis has been focused on the identification of structure-activity relationships for the different classes of complexes. Up to now, despite the enormous efforts in synthesizing different classes of copper complexes, very few data concerning the molecular basis of the mechanisms underlying their antitumor activity are available. The current overview, collecting the most significant strategies adopted in the last four years to design promising anticancer copper(I,II) compounds, aims to provide a useful reference for researchers working in this field

Copper is found in all living organisms and is a crucial trace element in redox chemistry, growth and development. It is important for the function of several enzymes and proteins involved in energy metabolism, respiration, and DNA synthesis, notably cytochrome oxidase, superoxide dismutase, ascorbate oxidase, and tyrosinase. The major functions of copper-biological molecules involve oxidation-reduction reactions in which they react directly with molecular oxygen to produce free radicals. Therefore, copper requires tightly regulated homeostatic mechanisms to ensure adequate supplies without any toxic effects. Overload or deficiency of copper is associated, respectively, with Wilson disease (WD) and Menkes disease (MD), which are of genetic origin. Researches on Menkes and Wilson disorders have provided useful insights in the field of copper homeostasis and in particular into the understanding of intracellular trafficking and distribution of copper at molecular levels. Therapies based on metal supplementation with copper histidine or removal of copper excess by means of specific copper chelators are currently effective in treating MD and WD, respectively. Copper chelation therapy is now attracting much attention for the investigation and treatment of various neurodegenerative disorders such as Alzheimer, Parkinson and CreutzfeldtJakob. An excess of copper appears to be an essential co-factor for angiogenesis. Moreover, elevated levels of copper have been found in many types of human cancers, including prostate, breast, colon, lung, and brain. On these basis, the employment of copper chelators has been reported to be of therapeutic value in the treatment of several types of cancers as anti-angiogenic molecules. More recently, mixtures of copper chelators with copper salts have been found to act as efficient proteasome inhibitors and apoptosis inducers, specifically in cancer cells. Moreover, following the worldwide success of platinum(II) compounds in cancer chemotherapy, several families of individual copper complexes have been studied as potential antitumor agents. These investigations, revealing the occurrence of mechanisms of action quite different from platinum drugs, head toward the development of new anticancer metallodrugs with improved specificity and decreased toxic side effects.

Metal-based antitumor drugs play a relevant role in antiblastic chemotherapy. Cisplatin is regarded as one of the most effective drugs, even if severe toxicities and drug resistance phenomena limit its clinical use. Therefore, in recent years there has been a rapid expansion in research and development of novel metal-based anticancer drugs to improve clinical effectiveness, to reduce general toxicity and to broaden the spectrum of activity. The variety of metal ion functions in biology has stimulated the development of new metallodrugs other than Pt drugs with the aim to obtain compounds acting via alternative mechanisms of action. Among non-Pt compounds, copper complexes are potentially attractive as anticancer agents. Actually, since many years a lot of researches have actively investigated copper compounds based on the assumption proposal that endogenous metals may be less toxic. It has been established that the properties of copper-coordinated compounds are largely determined by the nature of ligands and donor atoms bound to the metal ion. In this review, the most remarkable achievements in the design and development of copper(I, II) complexes as antitumor agents are discussed. Special emphasis has been focused on the identification of structure-activity relationships for the different classes of copper(I,II) complexes. This work was motivated by the observation that no comprehensive surveys of copper complexes as anticancer agents were available in the literature. Moreover, up to now, despite the enormous efforts in synthesizing different classes of copper complexes, very few data concerning the molecular basis of the mechanisms underlying their antitumor activity are available. This overview, collecting the most significant strategies adopted in the last ten years to design promising anticancer copper(I,II) compounds, would be a help to the researchers working in this field.

Monocationic hydrophilic complexes [Cu(thp)4](+) 3 and [Cu(bhpe)2](+) 4 were synthesized by ligand exchange reactions starting from the labile [Cu(CH3CN)4][PF6] precursor in the presence of an excess of the relevant hydrophilic phosphine. Complexes 3 and 4 were tested against a panel of several human tumor cell lines. Complex 3 has been shown to be about 1 order of magnitude more cytotoxic than cisplatin. Chemosensitivity tests performed on cisplatin and multidrug resistance phenotypes suggested that complex 3 acts via a different mechanism of action than the reference drug. Different short-term proliferation assays suggested that lysosomal damage is an early cellular event associated with complex 3 cytotoxicity, probably mediated by an increased production of reactive oxygen species. Cytological stains and flow cytometric analyses indicated that the phosphine copper(I) complex is able to inhibit the growth of tumor cells via G2/M cell cycle arrest and paraptosis accompanied with the loss of mitochondrial transmembrane potential.

Platinum anticancer drugs have been used for three decades despite their serious side effects and the emerging of resistance phenomena. Recently, a phosphine copper(I) complex, [Cu(thp)4][PF6] (CP), gained special attention because of its strong antiproliferative effects. CP killed human colon cancer cells more efficiently than cisplatin and oxaliplatin and it overcame platinum drug resistance. CP preferentially reduced cancer cell viability whereas non-tumour cells were poorly affected. Colon cancer cells died via a programmed cell death whose transduction pathways were characterized by the absence of hallmarks of apoptosis. The inhibition of 26S proteasome activities induced by CP caused intracellular accumulation of polyubiquitinated proteins and the functional suppression of the ubiquitin–proteasome pathway thus triggering endoplasmic reticulum stress. These data, providing a mechanistic characterization of CP-induced cancer cell death, shed light on the signaling pathways involved in paraptosis thus offering a new tool to overcome apoptosis-resistance in colon cancer cells.

The new sodium bis(1,2,4-triazol-1-yl)acetate ligand, Na[HC(CO(2))(tz)(2)], has been prepared in methanol solution by using 1,2,4-triazole, dibromoacetic acid, and NaOH. Treatment of the [Cu(CH(3)CN)(4)][PF(6)] acceptor with Na[HC(CO(2))(tz)(2)] or Na[HC(CO(2))[(pz(Me2))(2)] in the presence of the tris(hydroxymethyl)phosphine coligand in methanol/acetonitrile solutions produced unprecedented mononuclear copper(I) complexes of the [L(n)]Cu[P(CH(2)OH)(3)](2) (L(1), 2; L(2), 3) [(CH(3)CN)(2)Cu(P(CH(2)OH)(3))(2)]PF(6), 4. These compounds have been characterized by elemental analyses, FTIR, ESI-MS, and multinuclear (1H and 31P) NMR spectral data. The new copper(I) complexes were tested for their cytotoxic properties against a panel of several human tumor cell lines. The results reported here indicate that all the complexes showed in vitro antitumor activity similar or better than that of cisplatin, the most used metal-based antitumor drug. In particular, [HC(CO(2))(pz(Me2))(2)]Cu[P(CH(2)OH)(3)](2), 3 showed IC(50) values markedly lower than the reference compound against all tumor cell lines. Chemosensitivity tests performed on cisplatin sensitive and resistant cell lines have demonstrated that all these Cu(I) complexes were able to overcome cisplatin resistance, supporting the hypothesis of a different mechanism of action compared to that exhibited by the reference drug. Flow cytometric analysis on 2008 human ovarian carcinoma cells revealed that complex 3, chosen as the best candidate, induced a marked enlargement of both cell size and granularity, and a significant increase in the fraction of G2/M cells that, differently from cisplatin, was not accompanied by the appearance of a relevant sub-G1 fraction. Besides, no evidence of caspase-3 activation was detected in cells treated with complex 3. We hypothesize that the cytotoxic activity of the new copper(I) complex may be correlated to its ability to trigger paraptosis, a nonapoptotic mechanism of cell death.

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