Chiara Gabbiani

Gold compounds as anticancer agents: chemistry, cellular pharmacology, and preclinical studies

Gold compounds are a class of metallodrugs with great potential for cancer treatment. During the last two decades, a large variety of gold(I) and gold(III) compounds are reported to possess relevant antiproliferative properties in vitro against selected human tumor cell lines, qualifying themselves as excellent candidates for further pharmacological evaluation. The unique chemical properties of the gold center confer very interesting and innovative pharmacological profiles to gold‐based metallodrugs. The primary goal of this review is to define the state of the art of preclinical studies on anticancer gold compounds, carried out either in vitro or in vivo. The available investigations of anticancer gold compounds are analyzed in detail, and particular attention is devoted to underlying molecular mechanisms. Notably, a few biophysical studies reveal that the interactions of cytotoxic gold compounds with DNA are generally far weaker than those of platinum drugs, implying the occurrence of a substantially different mode of action. A variety of alternative mechanisms were thus proposed, of which those involving either direct mitochondrial damage or proteasome inhibition or modulation of specific kinases are now highly credited. The overall perspectives on the development of gold compounds as effective anticancer drugs with an innovative mechanism of action are critically discussed on the basis of the available experimental evidence.  © 2009 Wiley Periodicals, Inc. Med Res Rev, 30, No. 3, 550–580, 2010

Emerging Protein Targets for Anticancer Metallodrugs: Inhibition of Thioredoxin Reductase and Cathepsin B by Antitumor Ruthenium(II)−Arene Compounds

A series of ruthenium(II)-arene (RAPTA) compounds were evaluated for their ability to inhibit thioredoxin reductase (either cytosolic or mitochondrial) and cathepsin B, two possible targets for anticancer metallodrugs. In general, inhibition of the thioredoxin reductases was lower than that of cathepsin B, although selected compounds were excellent inhibitors of both classes of enzymes in comparison to other metal-based drugs. Some initial structure-activity relationships could be established. On the basis of the obtained data, different mechanisms of binding/inhibition appear to be operative; remarkably the selectivity of the ruthenium compounds toward solid metastatic tumors also correlates to the observed trends. Notably, docking studies of the interactions of representative RAPTA compounds with cathepsin B were performed that provided realistic structures for the resulting protein-metallodrug adducts. Good agreement was generally found between the inhibiting potency of the RAPTA compounds and the computed stability of the corresponding cat B/RAPTA adducts.

ESI mass spectrometry and X-ray diffraction studies of adducts between anticancer platinum drugs and hen egg white lysozyme

The interactions of cisplatin and its analogues, transplatin, carboplatin and oxaliplatin, with hen egg white lysozyme were analysed through ESI mass spectrometry, and the resulting metallodrug-protein adducts identified; the X-ray crystal structure of the cisplatin lysozyme derivative, solved at 1.9 A resolution, reveals selective platination of imidazole Nepsilon of His15.

Chemistry, antiproliferative properties, tumor selectivity, and molecular mechanisms of novel gold(III) compounds for cancer treatment: a systematic study

AbstractThe antiproliferative properties of a group of 13 structurally diverse gold(III) compounds, including six mononuclear gold(III) complexes, five dinuclear oxo-bridged gold(III) complexes, and two organogold(III) compounds, toward several human tumor cell lines were evaluated in vitro using a systematic screening strategy. Initially all compounds were tested against a panel of 12 human tumor cell lines, and the best performers were tested against a larger 36-cell-line panel. Very pronounced antiproliferative properties were highlighted in most cases, with cytotoxic potencies commonly falling in the low micromolar—and even nanomolar—range. Overall, good-to-excellent tumor selectivity was established for at least seven compounds, making them particularly attractive for further pharmacological evaluation. Compare analysis suggested that the observed antiproliferative effects are caused by a variety of molecular mechanisms, in most cases “DNA-independent,” and completely different from those of platinum drugs. Remarkably, some new biomolecular systems such as histone deacetylase, protein kinase C/staurosporine, mammalian target of rapamycin/rapamycin, and cyclin-dependent kinases were proposed for the first time as likely biochemical targets for the gold(III) species investigated. The results conclusively qualify gold(III) compounds as a promising class of cytotoxic agents, of outstanding interest for cancer treatment, while providing initial insight into their modes of action.Graphical AbstractA series of gold(III) compounds showed cytotoxic properties and tumor selectivity toward a panel of cancer cell lines. Compare analysis provided insight into their possible mechanisms of action.

Gold(III) compounds as anticancer drugs

Gold(III) complexes constitute a new class of metallodrugs, of potential interest for cancer treatment. During the past decade different kinds of gold(III) complexes have been reported to be appreciably stable under physiological-like conditions and to manifest relevant antiproliferative properties against selected human tumor cell lines. Some relevant examples are presented. Recent investigations point out that the interactions of cytotoxic gold(III) complexes with DNA are significantly different and weaker than those of platinum analogues; important interactions with model proteins and target proteins have been reported as well. Accordingly, the mechanisms of action of cytotoxic gold(III) complexes seem to be innovative and substantially different from that of cisplatin. Relevant antimitochondrial effects were demonstrated in some cases, eventually leading to cell apoptosis.

New uses for old drugs. Auranofin, a clinically established antiarthritic metallodrug, exhibits potent antimalarial effects in vitro: Mechanistic and pharmacological implications

The clinically established gold‐based antiarthritic drug auranofin (AF) manifests a pronounced reactivity toward thiol and selenol groups of proteins. In particular, AF behaves as a potent inhibitor of mammalian thioredoxin reductases causing severe intracellular oxidative stress. Given the high sensitivity of Plasmodium falciparum to oxidative stress, we thought that auranofin might act as an effective antimalarial agent. Thus, we report here new experimental results showing that auranofin and a few related gold complexes strongly inhibit P. falciparum growth in vitro. The observed antiplasmodial effects probably arise from direct inhibition of P. falciparum thioredoxin reductase. The above findings and the safe toxicity profile of auranofin warrant rapid evaluation of AF for malaria treatment in animal models.

Exploring metallodrug-protein interactions by mass spectrometry: comparisons between platinum coordination complexes and an organometallic ruthenium compound.

Electrospray ionisation mass spectrometry was used to analyse the reactions of metal compounds with mixtures of selected proteins. Three representative medicinally relevant compounds, cisplatin, transplatin and the organometallic ruthenium compound RAPTA-C, were reacted with a pool of three proteins, ubiquitin, cytochrome c and superoxide dismutase, and the reaction products were analysed using high-resolution mass spectrometry. Highly informative electrospray ionisation mass spectra were acquired following careful optimisation of the experimental conditions. The formation of metal–protein adducts was clearly observed for the three proteins. In addition, valuable information was obtained on the nature of the protein-bound metallofragments, on their distribution among the three different proteins and on the binding kinetics. The platinum compounds were less reactive and considerably less selective in protein binding than RAPTA-C, which showed a high affinity towards ubiquitin and cytochrome c, but not superoxide dismutase. In addition, competition studies between cisplatin and RAPTA-C showed that the two metallodrugs have affinities for the same amino acid residues on protein binding.

Exploring Metallodrug–Protein Interactions by ESI Mass Spectrometry: The Reaction of Anticancer Platinum Drugs with Horse Heart Cytochrome c

Since DNA is commonly believed to be the primary target for platinum metallodrugs, researchers’ interest has mainly focused on the characterisation of platinum–nucleic acid adducts while devoting much less attention to platinum–protein adducts. However, protein-bound platinum fragments probably represent truly active anticancer species—rather than mere drug-inactivation products—provided that metal transfer among distinct binding sites is kinetically allowed. Moreover, platination of specific side chains, which can affect the function of biologically crucial proteins or enzymes through the formation of tight coordinative bonds, might play a relevant role in the overall mechanism and toxicity of platinum drugs. The state of the art of platinum–protein interactions is described in a few articles and reviews; in any case, this issue warrants further experimental work. Thanks to the latest improvements, electrospray ionisation mass spectrometry (ESI-MS) today represents a very powerful method for exploring metallodrug–protein interactions. Owing to the introduction of “soft” ionisation methods, it is possible to transfer the intact metal–protein adduct—whole, in the gas phase—to determine its molecular mass with high accuracy and, thus, obtain its full molecular characterisation. However, much work is still required for the optimisation and the standardisation of experimental ESI-MS procedures directed at these systems. A great variability in ESI-MS responses is generally found in the current literature that depends on many factors, such as the nature of the protein, the nature of the metal, the specific solution conditions, the nature of the metalbound ligands, pH and the kind of buffer. Apparently, the intrinsic “fragility” of the metal–protein coordination bonds represents a major obstacle, often leading to extensive bond cleavage during ionisation and to loss of chemical information. Some pioneering ESI-MS studies of platinum–protein interactions were reported a few years ago by Gibson and co-workers, who used either ubiquitin or myoglobin as model proteins. A number of platinum–protein adducts were identified and characterised in detail. Afterwards, a few additional ESI-MS studies of various metallodrug–protein adducts were reported by other research groups. Cytochrome c is a small electron-carrier heme protein, localised in the mitochondria, that plays a crucial role in apoptotic pathways. Cytochrome c is also known to be an excellent ESI-MS probe and has been the subject of a number of investigations. This led us to choose cytochrome c as the model protein for our study. The following classical platinum drugs were selected: cisplatin, transplatin, carboplatin and oxaliplatin (Scheme 1).

ESI–MS Characterisation of Protein Adducts of Anticancer Ruthenium(II)‐Arene PTA (RAPTA) Complexes

Electrospray ionization mass spectrometry allows a rapid characterisation of the adducts formed between three novel anticancer ruthenium(II)-arene PTA compounds and horse heart cytochrome c or hen egg white lysozyme. Specific information on the nature of the protein-bound metallic fragments and the extent of protein metallation was readily obtained.

[Au2(phen2Me)2(μ-O)2](PF6)2, a Novel Dinuclear Gold(III) Complex Showing Excellent Antiproliferative Properties

A novel dioxo-bridged dinuclear gold(III) complex with two 2,9-dimethylphenanthroline ligands was synthesized and thoroughly characterized. Its crystal structure was solved, and its solution behavior assessed. Remarkably, this compound revealed excellent antiproliferative properties in vitro against a wide panel of 36 cancer cell lines, combining a high cytotoxic potency to pronounced tumor selectivity. Very likely, these properties arise from an innovative mode of action (possibly involving histone deacetylase inhibition), as suggested by COMPARE analysis. In turn, electrospray ionization-mass spectrometry studies provided valuable insight into its molecular mechanisms of activation and of interaction with protein targets. Gold(III) reduction, dioxo bridge disruption, coordinative gold(I) binding to the protein, and concomitant release of the phenanthroline ligand were proposed to occur upon interaction with superoxide dismutase, used here as a model protein. Because of the reported results, this new gold(III) compound qualifies itself as an optimal candidate for further pharmacological testing.