Ni(II), Cu(II), and Zn(II) diethyldithiocarbamate complexes show various activities against the proteasome in breast cancer cells

Novel metals and metal complexes as platforms for cancer therapy

Metals are essential cellular components selected by nature to function in several indispensable biochemical processes for living organisms. Metals are endowed with unique characteristics that include redox activity, variable coordination modes, and reactivity towards organic substrates. Due to their reactivity, metals are tightly regulated under normal conditions and aberrant metal ion concentrations are associated with various pathological disorders, including cancer. For these reasons, coordination complexes, either as drugs or prodrugs, become very attractive probes as potential anticancer agents. The use of metals and their salts for medicinal purposes, from iatrochemistry to modern day, has been present throughout human history. The discovery of cisplatin, cis-[Pt(II) (NH(3))(2)Cl(2)], was a defining moment which triggered the interest in platinum(II)- and other metal-containing complexes as potential novel anticancer drugs. Other interests in this field address concerns for uptake, toxicity, and resistance to metallodrugs. This review article highlights selected metals that have gained considerable interest in both the development and the treatment of cancer. For example, copper is enriched in various human cancer tissues and is a co-factor essential for tumor angiogenesis processes. However the use of copper-binding ligands to target tumor copper could provide a novel strategy for cancer selective treatment. The use of nonessential metals as probes to target molecular pathways as anticancer agents is also emphasized. Finally, based on the interface between molecular biology and bioinorganic chemistry the design of coordination complexes for cancer treatment is reviewed and design strategies and mechanisms of action are discussed.

Copper in diseases and treatments, and copper-based anticancer strategies

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.

Comparative activities of nickel(II) and zinc(II) complexes of asymmetric [NN'O] ligands as 26S proteasome inhibitors

In this study, we compare the proteasome inhibition capabilities of two anticancer candidates, [Ni(L(IA))(2)] (1) and [Zn(L(IA))(2)] (2), where L(IA-) is the deprotonated form of the ligand 2,4-diiodo-6-(((2-pyridinylmethyl)amino)methyl)phenol. Species 1 contains nickel(II), a considerably inert ion that favors covalency, whereas 2 contains zinc(II), a labile transition metal ion that favors predominantly ionic bonds. We report on the synthesis and characterization of 1 and 2 using various spectroscopic, spectrometric, and structural methods. Furthermore, the pharmacological effects of 1 and 2, along with those of the salts NiCl(2) and ZnCl(2), were evaluated in vitro and in cultured human cancer cells in terms of their proteasome-inhibitory and apoptotic cell-death-inducing capabilities. It is shown that neither NiCl(2) nor 1 have the ability to inhibit the proteasome activity at any sustained levels. However, ZnCl(2) and 2 showed superior inhibitory activity versus the chymotrypsin-like activity of both the 26S proteasome (IC(50) = 5.7 and 4.4 micromol/L, respectively) and the purified 20S proteasome (IC(50) = 16.6 and 11.7 micromol/L, respectively) under cell-free conditions. Additionally, inhibition of proteasomal activity in cultured prostate cancer cells by 2 was associated with higher levels of ubiquitinated proteins and apoptosis. Treatment with either the metal complex or the salt was relatively nontoxic toward human normal cells. These results strengthen the current working hypothesis that fast ligand dissociation is required to generate an [ML(IA)](+) pharmacophore, capable of interaction with the proteasome. This interaction, possibly via N-terminal threonine amino acids present in the active sites, renders the proteasome inactive. Our results present a compelling rationale for 2 along with its gallium(III) and copper(II) congeners to be further investigated as potential anticancer drugs that act as proteasome inhibitiors.

Proteasome inhibitors: Dozens of molecules and still counting

The discovery of the proteasome in the late 80's as the core protease of what will be then called the ubiquitin-proteasome system, rapidly followed by the development of specific inhibitors of this enzyme, opened up a new era in biology in the 90's. Indeed, the first proteasome inhibitors were instrumental for understanding that the proteasome is a key actor in most, if not all, cellular processes. The recognition of the central role of this complex in intracellular proteolysis in turn fuelled an intense quest for novel compounds with both increased selectivity towards the proteasome and better bioavailability that could be used in fundamental research or in the clinic. To date, a plethora of molecules that target the proteasome have been identified or designed. The success of the proteasome inhibitor bortezomib (Velcade(®)) as a new drug for the treatment of Multiple Myeloma, and the ongoing clinical trials to evaluate the effect of several other proteasome inhibitors in various human pathologies, illustrate the interest for human health of these compounds.

Metals in anticancer therapy: copper(II) complexes as inhibitors of the 20S proteasome

Selective 20S proteasomal inhibition and apoptosis induction were observed when several lines of cancer cells were treated with a series of copper complexes described as [Cu(L(I))Cl] (1), [Cu(L(I))OAc] (2), and [Cu(HL(I))(L(I))]OAc (3), where HL(I) is the ligand 2,4-diiodo-6-((pyridine-2-ylmethylamino)methyl)phenol. These complexes were synthesized, characterized by means of ESI spectrometry, infrared, UV-visible and EPR spectroscopies, and X-ray diffraction when possible. After full characterization species 1-3 were evaluated for their ability to function as proteasome inhibitors and apoptosis inducers in C4-2B and PC-3 human prostate cancer cells and MCF-10A normal cells. With distinct stoichiometries and protonation states, this series suggests the assignment of species [CuL(I)](+) as the minimal pharmacophore needed for proteasomal chymotryspin-like activity inhibition and permits some initial inference of mechanistic information.

Tumor cellular proteasome inhibition and growth suppression by 8-hydroxyquinoline and clioquinol requires their capabilities to bind copper and transport copper into cells

We have previously reported that when mixed with copper, 8-hydroxyquinoline (8-OHQ) and its analog clioquinol (CQ) inhibited the proteasomal activity and proliferation in cultured human cancer cells. CQ treatment of high-copper-containing human tumor xenografts also caused cancer suppression, associated with proteasome inhibition in vivo. However, the nature of the copper dependence of these events has not been elucidated experimentally. In the current study, using chemical probe molecules that mimic the structures of 8-OHQ and CQ, but have no copper-binding capability, we dissected the complex cellular processes elicited by 8-OHQ-Cu and CQ-Cu mixtures and revealed that copper binding to 8-OHQ or CQ is required for transportation of the copper complex into human breast cancer cells and the consequent proteasome-inhibitory, growth-suppressive, and apoptosis-inducing activities. In contrast, the non-copper-binding analogs of 8-OHQ or CQ blocked the very first step-copper binding-in this chain of events mediated by 8-OHQ-Cu or CQ-Cu.