Ida Landini

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

Pharmacological Strategies for Overcoming Multidrug Resistance

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. One of the underlying mechanisms of MDR is cellular overproduction of P-glycoprotein (P-gp) which acts as an efflux pump for various anticancer drugs. P-gp is encoded by the MDR1 gene and its overexpression in cancer cells has become a therapeutic target for circumventing multidrug resistance. A potential strategy is to co-administer efflux pump inhibitors, although such reversal agents might actually increase the side effects of chemotherapy by blocking physiological anticancer drug efflux from normal cells. Although many efforts to overcome MDR have been made using first and second generation reversal agents comprising drugs already in current clinical use for other indications (e.g. verapamil, cyclosporine A, quinidine) or analogues of the first-generation drugs (e.g. dexverapamil, valspodar, cinchonine), few significant advances have been made. Clinical trials with third generation modulators (e.g. biricodar, zosuquidar, and laniquidar) specifically developed for MDR reversal are ongoing. The results however are not encouraging and it may be that the perfect reverser does not exist. Other approaches to multidrug resistance reversal have also been considered: encapsulation of anthracyclines in liposomes or other carriers which deliver these drugs selectively to tumor tissues, the use of P-gp targeted antibodies such as UIC2 or the use of antisense strategies targeting the MDR1 messenger RNA. More recently, the development of transcriptional regulators appears promising. Also anticancer drugs that belong structurally to classes of drugs extruded from cells by P-gp but that are not substrates of this drug transporter may act as potent inhibitors of MDR tumors (e.g. epothilones, second generation taxanes). Taking advantage of MDR has also been studied. Bone marrow suppression, one of the major side effects of cancer chemotherapy, can compromise the potential of curative and palliative chemotherapy. It is conceivable that drug resistance gene transfer into bone marrow stem cells may be able to reduce or abolish chemotherapy-induced myelosuppression and facilitate the use of high dose chemotherapy. Clinical trials of retroviral vectors containing drug resistance genes have established that the approach is safe and are now being designed to address the therapeutically relevant issues.

Overcoming tumor multidrug resistance using drugs able to evade P-glycoprotein or to exploit its expression

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. Cellular overproduction of P‐glycoprotein (P‐gp), which acts as an efflux pump for various anticancer drugs (e.g. anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and some of the newer antitumor drugs) is one of the more relevant mechanisms underlying MDR. P‐gp belongs to the superfamily of ATP‐binding cassette transporters and is encoded by the ABCB1 gene. Its overexpression in cancer cells has become a therapeutic target for circumventing MDR. As an alternative to the classical pharmacological strategy of the coadministration of pump inhibitors and cytotoxic substrates of P‐gp and to other approaches applied in experimental tumor models (e.g. P‐gp‐targeting antibodies, ABCB1 gene silencing strategies, and transcriptional modulators) and in the clinical setting (e.g. incapsulation of P‐gp substrate anticancer drugs into liposomes or nanoparticles), a more intriguing strategy for circumventing MDR is represented by the development of new anticancer drugs which are not substrates of P‐gp (e.g. epothilones, second‐ and third‐generation taxanes and other microtubule modulators, topoisomerase inhibitors). Some of these drugs have already been tested in clinical trials and, in most of cases, show relevant activity in patients previously treated with anticancer agents which are substrates of P‐gp. Of these drugs, ixabepilone, an epothilone, was approved in the United States for the treatment of breast cancer patients pretreated with an anthracycline and a taxane. Another innovative approach is the use of molecules whose activity takes advantage of the overexpression of P‐gp. The possibility of overcoming MDR using the latter two approaches is reviewed herein.  © 2011 Wiley Periodicals, Inc. Med Res Rev

Genomic and genetic alterations influence the progression of gastric cancer

Gastric cancer is one of the leading causes of cancer-related deaths worldwide, although the incidence has gradually decreased in many Western countries. Two main gastric cancer histotypes, intestinal and diffuse, are recognised. Although most of the described genetic alterations have been observed in both types, different genetic pathways have been hypothesized. Genetic and epigenetic events, including 1q loss of heterozygosity (LOH), microsatellite instability and hypermethylation, have mostly been reported in intestinal-type gastric carcinoma and its precursor lesions, whereas 17p LOH, mutation or loss of E-cadherin are more often implicated in the development of diffuse-type gastric cancer. In this review, we summarize the sometimes contradictory findings regarding those markers which influence the progression of gastric adenocarcinoma.

Epigenetics and chemoresistance in colorectal cancer: an opportunity for treatment tailoring and novel therapeutic strategies.

Colorectal cancer is the second leading cause of cancer-related deaths in the world. Despite many therapeutic opportunities, prognosis remains dismal for patients with metastatic disease, and a significant portion of early-stage patients develop recurrence after chemotherapy. Epigenetic gene regulation is a major mechanism of cancer initiation and progression, through the inactivation of several tumor suppressor genes. Emerging evidence indicates that epigenetics may also play a key role in the development of chemoresistance. In the present review, we summarize epigenetic mechanisms triggering resistance to three commonly used agents in colorectal cancer: 5-fluorouracil, irinotecan and oxaliplatin. Those epigenetic biomarkers may help stratify colorectal cancer patients and develop a tailored therapeutic approach. In addition, epigenetic modifications are reversible through specific drugs: histone-deacetylase and DNA-methyl-transferase inhibitors. Preclinical studies suggest that these drugs may reverse chemoresistance in colorectal tumors. In conclusion, an epigenetic approach to colorectal cancer chemoresistance may pave the way to personalized treatment and to innovative therapeutic strategies.

Chemistry and Biology of Two Novel Gold(I) Carbene Complexes as Prospective Anticancer Agents

Two novel gold carbene compounds, namely, chlorido (1-butyl-3-methyl-imidazole-2-ylidene) gold(I) (1) and bis(1-butyl-3-methyl-imidazole-2-ylidene) gold(I) (2), were prepared and characterized as prospective anticancer drug candidates. These compounds consist of a gold(I) center linearly coordinated either to one N-heterocyclic carbene (NHC) and one chloride ligand (1) or to two identical NHC ligands (2). Crystal structures were solved for both compounds, the resulting structural data being in good agreement with expectations. We wondered whether the presence of two tight carbene ligands in 2 might lead to biological properties distinct from those of the monocarbene complex 1. Notably, in spite of their appreciable structural differences, these two compounds manifested similarly potent cytotoxic actions in vitro when challenged against A2780 human ovarian carcinoma cells. In addition, both were able to overcome resistance to cisplatin in the A2780R line. Solution studies revealed that these gold carbene complexes are highly stable in aqueous buffers at physiological pH. Their reactivity with proteins was explored: no adduct formation was detected even upon a long incubation with the model proteins cytochrome c and lysozyme; in contrast, both compounds were able to metalate, to a large extent, the copper chaperone Atox-1, bearing a characteristic CXXC motif. The precise nature of the resulting gold-Atox-1 adducts was elucidated through ESI-MS analysis. On the basis of these findings, it is proposed that the investigated gold(I) carbene compounds are promising antiproliferative agents warranting a wider pharmacological evaluation. Most likely these gold compounds produce their potent biological effects through selective metalation and impairment of a few crucial cellular proteins.

Exploring the biochemical mechanisms of cytotoxic gold compounds: a proteomic study

We have recently shown that a group of structurally diverse gold compounds are highly cytotoxic toward a panel of 36 human tumor cell lines through a variety of biochemical mechanisms. A classic proteomic approach is exploited here to gain deeper insight into those mechanisms. This investigation is focused on Auoxo6, a novel binuclear gold(III) complex, and auranofin, a clinically established gold(I) antiarthritic drug. First, the 72-h cytotoxicity profiles of Auoxo6 and auranofin were determined against A2780 human ovarian carcinoma cells. Subsequently, protein extraction from gold-treated A2780 cells sensitive to cisplatin and 2D gel electrophoresis separation were carried out according to established procedures. Notably, both metallodrugs caused relatively modest changes in protein expression in comparison with controls as only 11 out of approximately 1,300 monitored spots showed appreciable quantitative changes. Very remarkably, six altered proteins were in common between the two treatments. Eight altered proteins were identified by mass spectrometry; among them was ezrin, a protein associated with the cytoskeleton and involved in apoptosis. Interestingly, two altered proteins, i.e., peroxiredoxins 1 and 6, are known to play crucial roles in the cell redox metabolism. Increased cleavage of heterogeneous ribonucleoprotein H was also evidenced, consistent with caspase 3 activation. Overall, the results of the present proteomic study point out that the mode of action of Auoxo6 is strictly related to that of auranofin, that the induced changes in protein expression are limited and selective, that both gold compounds trigger caspase 3 activation and apoptosis, and that a few affected proteins are primarily involved in cell redox homeostasis.

Structural and solution chemistry, protein binding and antiproliferative profiles of gold(I)/(III) complexes bearing the saccharinato ligand

A series of new gold(I) and gold(III) complexes based on the saccharinate (sac) ligand, namely M[Au(sac)(2)] (with M being Na(+), K(+) or NH(4)(+)), [(PTA)Au(sac)], K[Au(sac)(3)Cl] and Na[Au(sac)(4)], were synthesized and characterized, and some aspects of their biological profile investigated. Spectrophotometric analysis revealed that these gold compounds, upon dissolution in aqueous media, at physiological pH, manifest a rather favourable balance between stability and reactivity. Their reactions with the model proteins cytochrome c and lysozyme were monitored by mass spectrometry to predict their likely interactions with protein targets. In the case of disaccharinato gold(I) complexes, cytochrome c adducts bearing four coordinated gold(I) ions were preferentially formed in high yield. In contrast, [(PTA)Au(sac)] (PTA=1,3,5-triaza-7-phosphaadamantane) turned out to be poorly effective, only producing a mono-metalated adduct in very low amount. In turn, the gold(III) saccharinate derivatives were less reactive than their gold(I) analogues: K[Au(sac)(3)Cl] and Na[Au(sac)(4)] caused moderate protein metalation, again with evidence of formation of tetragold adducts. Finally, the above mentioned gold compounds were challenged against the reference human tumor cell line A2780S and its cisplatin resistant subline A2780R and their respective cytotoxic profiles determined. [(PTA)Au(sac)] turned out to be highly cytotoxic whereas moderate cytotoxicities were observed for the gold(III) complexes and only modest activities for disaccharinato gold(I) complexes. The implications of these results are thoroughly discussed in the light of current knowledge on gold based drugs.

Size dependent biological profiles of PEGylated gold nanorods

The perspective of introducing plasmonic particles for applications in biomedical optics is receiving much interest. However, their translation into clinical practices is delayed by various factors, which include a poor definition of their biological interactions. Here, we describe the preparation and the biological profiles of gold nanorods belonging to five different size classes with average effective radii between ∼5 and 20 nm and coated with polyethylene glycol (PEG). All these particles exhibit decent stability in the presence of representative proteins, low cytotoxicity and satisfactory compatibility with intravenous administration, in terms of their interference with blood tissue. However, the suspension begins to become unstable after a few days of exposure to blood proteins. Moreover, the cytotoxicity is a little worse for smaller particles, probably because their purification is more critical, while undesirable interactions with the mononuclear phagocyte system are minimal in the intermediate size range. Overall, these findings hold implications of practical relevance and suggest that PEGylated gold nanorods may be a versatile platform for a variety of biomedical applications.

Proteomic analysis of ovarian cancer cell responses to cytotoxic gold compounds

Platinum-based chemotherapy is the primary treatment for human ovarian cancer. Overcoming platinum resistance has become a critical issue in the current chemotherapeutic strategies of ovarian cancer as drug resistance is the main reason for treatment failure. Cytotoxic gold compounds hold great promise to reach this goal; however, their modes of action are still largely unknown. To shed light on the underlying molecular mechanisms, we performed 2-DE and MS analysis to identify differential protein expression in a cisplatin-resistant human ovarian cancer cell line (A2780/R) following treatment with two representative gold compounds, namely Auranofin and Auoxo6. It is shown that Auranofin mainly acts by altering the expression of Proteasome proteins while Auoxo6 mostly modifies proteins related to mRNA splicing, trafficking and stability. We also found that Thioredoxin-like protein 1 expression is greatly reduced after treatment with both gold compounds. These results are highly indicative of the likely sites of action of the two tested gold drugs and of the affected cellular functions. The implications of the obtained results are thoroughly discussed in the frame of current knowledge on cytotoxic gold agents.