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Metal complexes in medicine with a focus on enzyme inhibition
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Metal complexes in medicine with a focus on enzyme inhibition Chi-Ming Che and Fung-Ming Siu Since the clinical success of cisplatin and its derivatives, considerable effort has been expended by academics and pharmacological companies to the development of novel metal-based drugs. DNA is believed to be the main target of cisplatin, and there have been extensive studies on the binding between metal complexes and DNA targets. Recently, new light has been shed on the discovery of metal-based drugs that inhibit enzymatic activities or even target proteins directly. This review highlights some exciting results published recently on the development of platinum, gold, and ruthenium complexes as enzyme inhibitors for potential therapeutic applications. Addresses Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China Corresponding author: Che, Chi-Ming ([email protected])
Current Opinion in Chemical Biology 2010, 14:255–261 This review comes from a themed issue on Bioinorganic chemistry Edited by Kathy Franz and Chuan He Available online 16th December 2009 1367-5931/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cbpa.2009.11.015
Introduction The therapeutic value of metal-based drugs has long been established. Cisplatin is one of the first metal-based anticancer drugs discovered in the early 19th century. Cisplatin mediates its anticancer effect by covalently binding to DNA so as to form adducts that interferes with transcription and DNA replication, and thereby triggers programmed cell death. As a consequence of cisplatin, there have been extensive studies in the field of metal-based drugs focusing on platinum complexes binding to DNA. A survey on the ISI Web of KnowledgeSM database (until September, 2009) for publications with topics on ‘platinum drug’, ‘platinum drug + DNA’, ‘platinum drug + enzyme’ returns 3628, 1479, and 126 hits, respectively. Comprehensive reviews on the uses and development of metal-based drugs can be found elsewhere [1,2,3,4]. Sadler and his co-workers have reviewed the recent development in the field [3,4]. In this review, we focus on the promise and the future perspectives for developing platinum-based, gold-based, www.sciencedirect.com
and ruthenium-based complexes as novel protein or enzyme inhibitors, with an emphasis on substitution– inert complexes.
Metal complexes inhibit enzymes through ligand-substitution Metal complexes with labile ligands have long been known to undergo ligand-substitution reactions with biomolecular targets. Metal ions can bind to nitrogen, sulfur or selenium atoms of the histidine, cysteine, or selenocysteine residues in proteins leading to therapeutics effects. Some of the accumulated evidence is described in what follows. Gold: Auranofin (Figure 1), a gold(I) phosphine complex, is a drug for treatment of rheumatoid arthritis. Recently, it was found that gold is transferred from auranofin to seleno-protein thioredoxin glutathione reductase, resulting in a therapeutic effect in parasitic diseases [5]. Auranofin was found to inhibit the growth of tumor cells in vitro, but its high reactivity toward protein thiols limits its antitumor activity in vivo [6]. A gold(I) phosphine complex containing a naphthalimide ligand was recently found to function as a thioredoxin reductase inhibitor with promising antiproliferation and angiogenesis activities [7]. In vitro and in vivo study of a gold(III) dithiocarbamate complex (Figure 1) revealed the proteasome to be its primary target [8]. Platinum: Selenoenzyme thioredoxin reductase was reported to be efficiently inhibited by (2,20 :60 ,200 -terpyridine)platinum(II) complexes (Figure 1) with IC50 values at the nanomolar level [9]. Recently, Lo et al. used X-ray crystallographic and mass spectrometric methods to reveal that the aromatic thiolato platinum(II)–terpyridine complexes inhibited the activities of human thioredoxin reductase 1 by blocking its C-terminal active-site selenocysteine [10]. A series of platinum(II)–terpyridine complexes were found to inhibit topoisomerase II (top2) [10]. There are multiple mechanisms for top2 poisoning, including DNA intercalation, enzyme binding, and enzyme thiol group modification. As such, the authors have pointed out that platinum(II)–terpyridine complexes may also inhibit topoisomerase II activity by a ligand exchange reaction with the thiol groups of enzymes [10]. A series of platinum(II) complexes (PtCl2(smp), Figure 1) with two to three labile ligands were found to inhibit matrix metalloproteinase (MMP-3) [11]. The adduct between MMP-3 and PtCl2(smp) was characterized by ESIMS, CD, and NMR spectroscopy. The studies revealed that the chloride ligands of these platinum(II) complexes are replaced by an imidazole nitrogen of His224 from Current Opinion in Chemical Biology 2010, 14:255–261
256 Bioinorganic chemistry
Figure 1
Examples of metal-based drugs with enzyme inhibitory effects.
MMP-3 and a hydroxyl group modified from the smp ligand [11]. Ruthenium: By coupling ethacrynic acid (EA), an effective inhibitor of glutathione transferase, to a ruthenium complex, a new series of glutathione transferase inhibitors (Ru–EA, Figure 1) were developed [12]. Further mass spectrometry and X-ray crystallographic analyses revealed that the Ru–EA complex lost two chloride ligands and then was further cleaved to release the Current Opinion in Chemical Biology 2010, 14:255–261
ruthenium fragment (Figure 1). Collectively, metal complexes with labile ligands mainly target proteins with selenocysteine or cysteine at the active site, such as thioredoxin glutathione reductase, thioredoxin reductase, and glutathione transferase. The lack of selectivity is a potential drawback of the aforementioned metal complexes. These metal complexes may bind to human serum albumin or other proteins having potential metal binding sites (histidine, www.sciencedirect.com
Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 257
cysteine, or selenocysteine). All of these render the delivery of metal complexes to specific biomolecular targets difficult. Using auranofin as an example, this complex was found to inhibit thioredoxin glutathione reductase and the growth of tumor cells in vitro [5], but its high reactivity toward protein thiols limits its antitumor activity in vivo [6]. As such, Berners-Price and Filipovska designed a series of Au(I) complexes that selectively target proteins with selenocysteine, but not cysteine, by fine-tuning the ligand exchange reactions at the Au(I) center [13]. The effect of the labile ligand or leaving group in metal complexes has also been extensively studied [14]. We have been studying the thera-
peutic potential of metal complexes with enhanced stability in solutions under physiological conditions with the objective of discovering metal complexes with unique anticancer properties.
Substitution–inert metal complexes for anticancer uses Substitution–inert metal complexes provide a promising scaffold for the design of enzyme inhibitors for two reasons: (1) these metal complexes are stable in cells and (2) by mimicking known inhibitors of a specific target, the design of metal complexes for specific biomolecular targets may be achievable.
Figure 2
Rational design of metal complexes to stabilize G-quadruplex DNA. www.sciencedirect.com
Current Opinion in Chemical Biology 2010, 14:255–261
258 Bioinorganic chemistry
Gold(III) complexes as novel histone deacetylase inhibitors In contrast to these complexes, which easily undergo ligand-substitution reactions in solution, we have been exploring the anticancer activities of gold(III) complexes containing strongly chelating multi-anionic ligands. In the year 2003, a gold(III) porphyrin complex (gold-1a, Figure 1) was discovered to be more potent than cisplatin [15] under the program ‘Institute of Molecular Technology for Drug Discovery and Synthesis’. The promising properties of gold-1a include (i) gold-1a is substitutioninert under physiological conditions, (ii) gold-1a remains sensitive to cisplatin-resistant nasopharyngeal carcinoma cell lines; which implies that gold-1a has a unique mechanism of action, (iii) the IC50 value of gold-1a is ca. 240fold lower than that of cisplatin [15]. (iv) Mechanistic studies by transcriptomic and proteomic approaches revealed that gold-1a induced apoptosis through mitochondrial death and mitogen-activated protein kinase signaling pathways [16,17] and also via the up-regulation of Gadd34 and Gadd153 gene expression [18]. (v) Further in vitro and in vivo studies revealed that gold-1a is a promising anticancer agent for the treatment of colon and liver cancer [18,19]. Recently, a new gold(III) porphyrin complex (gold-2a, Figure 1) with improved lipophilicity and aqueous stability was synthesized, and its cytotoxicity was evaluated [20]. The IC50 values of gold-2a against a panel of human breast carcinoma cells were found to be in the nanomolar range, and gold-2a significantly suppressed mammary tumor growth in nude mice [20]. Gold-2a was found to regulate the association of class I histone deacetylase (HDAC) and histone acetylation at the promoter of Wnt signaling molecules. Molecular docking calculations were used to evaluate the binding energy and binding mode of this complex with HDAC8. Gold-2a might share a similar mechanism with another HDACi, cyclic tetrapeptide, which inhibits the activity of HDAC8 by blocking the entry of the 11 A˚ channel and the exit of the 14 A˚ internal cavity [20] (Figure 4a). This result is
unexpected as no HDACi has been reported to have a porphyrin-like structure or to be metal-containing. Thus, gold-2a may represent a novel lead structure of HDACi.
Pt(II) complexes as G-quadruplex stabilizers and telomerase inhibitors Planar aromatic and cationic molecules such as porphyrins, perylenes, telomestatin, quinolines, and berbine have all been demonstrated to bind to and stabilize G-quadruplex DNA. Upon stabilization, the enzymatic activity of telomerase is impaired. It is conceivable that substitution–inert metal complexes with a planar coordination geometry and cationic charge may also stabilize G-quadruplex DNA (Figure 2). As such, platinum(II) complexes with different ligand systems, such as dipyridophenazine [21] or a Schiff base [22], have recently been developed in our laboratory (Figure 1). These platinum(II) complexes can be structurally modified with ease. A series of platinum(II) complexes containing dipyridophenazine (dppz) and C-deprotonated 2-phenylpyridine (N^CH) ligands were synthesized. These complexes were found to bind G-quadruplex DNA through an external endstacking mode (Figure 4b) with one complex displaying a binding affinity of 107 dm3 mol1 and an IC50 value for the effect on human telomerase activity at 760 nM [21]. Recently, a series of platinum(II) Schiff base complexes that bind to c-myc G-quadruplex DNA have also been discovered [22]. The Pt(II) Schiff base complexes can moderately inhibit c-myc gene promoter activity in a cellfree system through stabilizing the G-quadruplex structure and can inhibit c-myc oncogene expression in cultured cells. Using an in silico G-quadruplex DNA model, new platinum(II) Schiff base complexes with improved inhibitory activities were discovered. We conceive that the rational design of platinum complexes with a planar geometry and which are cationic is a promising approach to develop novel class of G-quadruplex stabilizers and telomerase inhibitors (Figure 2).
Figure 3
Ruthenium-based enzyme inhibitors. Current Opinion in Chemical Biology 2010, 14:255–261
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Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 259
Pt(II) complexes as topoisomerases inhibitors
Figure 4
Most of the topoisomerase I inhibitors (poisons, e.g., camptothecin) are planar molecules. Camptothecin mediates cytotoxic effect by binding to the DNA cleavage site of the topoisomerase–DNA complexes through p–p interactions. We therefore envision platinum complexes with planar structures, such as platinum(II)–terpyridine complexes, also forming stable p–p interactions with the DNA cleavage site, subsequently leading to inhibitory effect on topoisomerases. In this regard, (2,20 :60 ,200 -terpyridine)platinum(II) complexes have recently been found to display inhibitory effect on topoisomerase I [10], although the exact binding mode remains to be elucidated.
Ruthenium complexes as protein kinases and HIV-1 reverse transcriptase inhibitors Staurosporine is a nonspecific protein kinase inhibitor. By replacing the carbohydrate unit of staurosporine with various ruthenium fragments (Figure 3), a series of selective inhibitors of glycogen synthase kinase 3, GSK3/b, Pim1, PAK1 (Figure 4c), PI3Ks, mammalian sterile 20 kinase, and BRAF serine/threonine kinase were discovered [23,24–27]. Coupling a fragment of an organic inhibitor to the metal center (Figure 3) may represent a promising approach for the rational design of novel metal-based enzyme inhibitors. Our group identified a mixed-valent ruthenium-oxo oxalate cluster to have potent anti-HIV activities [28]. The compound is stable under physiologically relevant condition. It could reduce the HIV-1 reverse transcriptase activity by half at nanomolar concentration, with IC50 = 1.9 nM [28].
Metal complexes that displayed inhibitory effects on enzymes are potent in vivo While there have been many examples of metal complexes reported to inhibit enzymes in vitro, the next challenge is to develop their therapeutic potential in vivo. Indeed, successful stories have been reported. The in vivo study of a gold(III) dithiocarbamate complex (Figure 1) revealed that during the 29-day treatment of nude mice with MDA-MB-231 xenografts, no toxicity was observed and mice did not display signs of weight loss, decreased activity, or anorexia [2]. Our gold(III) complexes, gold-1a and gold-2a (Figure 1), were found to show promising in vivo cytotoxic activities against liver [18]/colon [19] and breast cancers [20], respectively. Intraductal delivery of gold-2a effectively attenuates mammary MDA-MB-231 tumor growth in nude mice. Under the same experimental condition, 40% of the mice died after cisplatin-treatment, while the gold-2a treatment groups remained alive throughout the experimental period [20]. Collectively, the findings from these in vivo studies are particularly encouraging in the development of novel anticancer drugs with little toxic side effects. www.sciencedirect.com
(a) Molecular model of the gold-2a-bound HDAC8 adducts, adapted with permission from the Cancer Research, supplementary Figure 6 of Ref. [20], (b) molecular modeling of the binding interaction between a Pt(II)-dipyridophenazine complex and the human intramolecular telomeric repeat G4A2-quadruplex, adapted with permission from the J. Am. Chem. Soc., Figure 9 of Ref. [21], copyright 2009 American Chemical Society, and (c) cocrystal structure of (R)-DW12 with PAK1, adapted with permission from the J. Am. Chem. Soc., Figure 4 of Ref. [25], copyright 2008 American Chemical Society.
Current Opinion in Chemical Biology 2010, 14:255–261
260 Bioinorganic chemistry
Future perspectives In the recent past, the pharmaceutical industry and chemical biology as a field have been dependent on organic chemistry. Inorganic compounds, except cisplatin and its derivatives, have only played a minor role. In 2008, the US FDA approved a total of 21 new drugs, but not one of them is a metal-based drug [29]. This can be attributed to the lack of knowledge on the mechanisms and binding modes of metal-based drugs with biomolecular targets other than DNA. Traditionally, it was considered that the majority of metal-based drugs targeted DNA. Nowadays, considerable evidence has appeared to show that metal-based drugs target selenocysteine or cysteine residues of proteins through ligand-substitution reactions under physiological conditions. Important findings have been reported in literature revealing that substitution–inert metal-based drugs utilize additional protein targets, such as histone deacetylase, telomerase, topoisomerase, and protein kinases. Numerous metal complexes, which mediate cytotoxic activities through enzyme inhibition, have been found to show anticancer activities in vivo (both our own work and that done by others). These findings are expected to motivate inorganic chemists to develop and look into novel drug targets or the potential applications of metal-based drugs via their effects on enzyme inhibition. We envision clinical use of these metal-based drugs (enzyme inhibitors) will become available in the near future.
Acknowledgements
5.
Angelucci F, Sayed AA, Williams DL, Boumis G, Brunori M, Dimastrogiovanni D, Miele AE, Pauly F, Bellelli A: Inhibition of Schistosoma mansoni thioredoxin glutathione reductase by auranofin: structural and kinetic aspects. J Biol Chem 2009, 284:28977-28985.
6.
Mirabelli CK, Johnson RK, Sung CM, Faucette L, Muirhead K, Crooke ST: Evaluation of the in vivo antitumor activity and in vitro cytotoxic properties of auranofin, a coordinated gold compound, in murine tumor models. Cancer Res 1985, 45:32-39.
7.
Ott I, Qian X, Xu Y, Vlecken DH, Marques IJ, Kubutat D, Will J, Sheldrick WS, Jesse P, Prokop A et al.: A gold(I) phosphine complex containing a naphthalimide ligand functions as a TrxR inhibiting antiproliferative agent and angiogenesis inhibitor. J Med Chem 2009, 52:763-770.
8.
Milacic V, Chen D, Ronconi L, Landis-Piwowar KR, Fregona D, Dou QP: A novel anticancer gold(III) dithiocarbamate compound inhibits the activity of a purified 20S proteasome and 26S proteasome in human breast cancer cell cultures and xenografts. Cancer Res 2006, 66:10478-10486. An in vivo study revealed that gold(III) dithiocarbamate compound is a promising anticancer agent that inhibits the activity of 20S proteasome and 26S proteasome. 9.
Becker K, Herold-Mende C, Park JJ, Lowe G, Schirmer RH: Human thioredoxin reductase is efficiently inhibited by (2,20 :60 ,200 -terpyridine)platinum(II) complexes. Possible implications for a novel antitumor strategy. J Med Chem 2001, 44:2784-2792.
10. Lo YC, Ko TP, Su WC, Su TL, Wang AHJ: Terpyridine-platinum(II) complexes are effective inhibitors of mammalian topoisomerases and human thioredoxin reductase 1. J Inorg Biochem 2009, 103:1082-1092. 11. Arnesano F, Boccarelli A, Cornacchia D, Nushi F, Sasanelli R, Coluccia M, Natile G: Mechanistic insight into the inhibition of matrix metalloproteinases by platinum substrates. J Med Chem 2009, in press. 12. Ang WH, Parker LJ, De Luca A, Juillerat-Jeanneret L, Morton CJ, Lo Bello M, Parker MW, Dyson PJ: Rational design of an organometallic glutathione transferase inhibitor. Angew Chem Int Ed Engl 2009, 48:3854-3857.
This work was supported by the Areas of Excellence Scheme established under the University Grants Committee of the Hong Kong Special Administrative Region, China (Grant AoE/P-10/01), and the University of Hong Kong (University Development Fund). Pacific Edit reviewed the manuscript before submission. We acknowledge the permissions from the Journal of the American Chemical Society and the Cancer Research; the adapted figures are shown in Figure 4 of this article.
13. Hickey JL, Ruhayel RA, Barnard PJ, Baker MV, Berners-Price SJ, Filipovska A: Mitochondria-targeted chemotherapeutics: the rational design of gold(I) N-heterocyclic carbene complexes that are selectively toxic to cancer cells and target protein selenols in preference to thiols. J Am Chem Soc 2008, 130:12570-12571.
References and recommended reading
14. Montana AM, Batalla C: The rational design of anticancer platinum complexes: the importance of the structure–activity relationship. Curr Med Chem 2009, 16:2235-2260.
Papers of particular interest were highlighted as: of special interest of outstanding interest 1.
Sun RWY, Ma DL, Wong ELM, Che CM: Some uses of transition metal complexes as anti-cancer and anti-HIV agents. Dalton Trans 2007:4884-4892. This review focuses the uses of metal-based drugs as anticancer and anti-HIV agents.
2.
Chen D, Milacic V, Frezza M, Dou QP: Metal complexes, their cellular targets and potential for cancer therapy. Curr Pharm Des 2009, 15:777-791. This review focuses on the development of non-platinum metal complexes as anticancer drugs.
3. Bruijnincx PCA, Sadler PJ: New trends for metal complexes with anticancer activity. Curr Opin Chem Biol 2008, 12:197-206. The use of metal as scaffold rather than reactive center and the departure from cisplatin paradigm of activity toward a more targeted, cancer cellspecific approach were discussed in this review. 4.
Van Rijt SH, Sadler PJ: Current applications and future potential for bioinorganic chemistry in the development of anticancer drugs. Drug Discov Today 2009, 14:1089-1097.
Current Opinion in Chemical Biology 2010, 14:255–261
15. Che CM, Sun RWY, Yu WY, Ko CB, Zhu N, Sun H: Gold(III) porphyrins as a new class of anticancer drugs: cytotoxicity. DNA binding and induction of apoptosis in human cervix epitheloid cancer cells. Chem Commun (Camb) 2003:1718-1719. A substitution–inert gold(III) porphyrin complex (gold-1a) is found to be stable against GSH reduction. 16. Wang Y, He QY, Sun RWY, Che CM, Chiu JF: Gold(III) porphyrin 1a induced apoptosis by mitochondrial death pathways related to reactive oxygen species. Cancer Res 2005, 65:11553-11564. 17. Wang Y, He QY, Che CM, Tsao SW, Sun RWY, Chiu JF: Modulation of gold(III) porphyrin 1a-induced apoptosis by mitogen-activated protein kinase signaling pathways. Biochem Pharmacol 2008, 75:1282-1291. 18. Lum CT, Yang ZF, Li HY, Sun RWY, Fan ST, Poon RTP, Lin MCM, Che CM, Kung HF: Gold(III) compound is a novel chemocytotoxic agent for hepatocellular carcinoma. Int J Cancer 2006, 118:1527-1538. 19. Tu S, Sun RWY, Lin MCM, Cui JT, Zou B, Gu Q, Kung HF, Che CM, Wong BCY: Gold (III) porphyrin complexes induce apoptosis www.sciencedirect.com
Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 261
and cell cycle arrest and inhibit tumor growth in colon cancer. Cancer 2009, 115:4459-4469. 20. Chow KHM, Sun RWY, Lam JBB, Li CKL, Xu A, MA DL, Abagyan R, Wang Y, Che CM: Anti-cancer complex gold(III) porphyrin 2a as a novel histone deacetylase inhibitor targeting Wnt/b-catenin pathway. Cancer Res 2009, in press. An in vivo study revealed that gold(III) porphyrin complex (gold-2a) is a potent anticancer agent and a novel histone deacetylase inhibitor. 21. Ma DL, Che CM, Yan SC: Platinum(II) complexes with dipyridophenazine ligands as human telomerase inhibitors and luminescent probes for G-quadruplex DNA. J Am Chem Soc 2009, 131:1835-1846. 22. Wu P, Ma DL, Leung CH, Yan CH, Zhu N, Abagyan R, Che CM: Stabilization of G-Quadruplex DNA with platinum(II) Schiffbase complexes: luminescent probe and down regulation of cmyc oncogene expression. Chemistry 2009, 15:13008-13021. A series of platinum(II) complexes, mimic organic stabilizers, were found to stabilize c-myc G-quadruplex DNA. 23. Anand R, Maksimoska J, Pagano N, Wong EY, Gimotty PA, Diamond SL, Meggers E, Marmorstein R: Toward the development of a potent and selective organoruthenium mammalian sterile 20 kinase inhibitor. J Med Chem 2009, 52:1602-1611. A series of organoruthenium complexes, mimic staurosporine, were found to be potent protein kinase inhibitors.
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24. Atilla-Gokcumen GE, Pagano N, Streu C, Maksimoska J, Filippakopoulos P, Knapp S, Meggers E: Extremely tight binding of a ruthenium complex to glycogen synthase kinase 3. Chembiochem 2008, 9:2933-2936. 25. Maksimoska J, Feng L, Harms K, Yi C, Kissil J, Marmorstein R, Meggers E: Targeting large kinase active site with rigid, bulky octahedral ruthenium complexes. J Am Chem Soc 2008, 130:15764-15765. 26. Xie P, Streu C, Qin J, Bregman H, Pagano N, Meggers E, Marmorstein R: The crystal structure of BRAF in complex with an organoruthenium inhibitor reveals a mechanism for inhibition of an active form of BRAF kinase. Biochemistry 2009, 48:5187-5198. 27. Xie P, Williams DS, Atilla-Gokcumen GE, Milk L, Xiao M, Smalley KSM, Herlyn M, Meggers E, Marmorstein R: Structurebased design of an organoruthenium phosphatidyl-inositol-3kinase inhibitor reveals a switch governing lipid kinase potency and selectivity. ACS Chem Biol 2008, 3:305-316. 28. Wong EL, Sun RWY, Chung NPY, Lin CLS, Zhu N, Che CM: A mixed-valent ruthenium-oxo oxalato cluster Na7[Ru4(m3O)4(C2O4)6] with potent anti-HIV activities. J Am Chem Soc 2006, 128:4938-4939. 29. Hughes B: 2008 FDA drug approvals. Nat Rev Drug Discov 2009, 8:93-96.
Current Opinion in Chemical Biology 2010, 14:255–261
Metal complexes in medicine with a focus on enzyme inhibition Chi-Ming Che and Fung-Ming Siu Since the clinical success of cisplatin and its derivatives, considerable effort has been expended by academics and pharmacological companies to the development of novel metal-based drugs. DNA is believed to be the main target of cisplatin, and there have been extensive studies on the binding between metal complexes and DNA targets. Recently, new light has been shed on the discovery of metal-based drugs that inhibit enzymatic activities or even target proteins directly. This review highlights some exciting results published recently on the development of platinum, gold, and ruthenium complexes as enzyme inhibitors for potential therapeutic applications. Addresses Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China Corresponding author: Che, Chi-Ming ([email protected])
Current Opinion in Chemical Biology 2010, 14:255–261 This review comes from a themed issue on Bioinorganic chemistry Edited by Kathy Franz and Chuan He Available online 16th December 2009 1367-5931/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cbpa.2009.11.015
Introduction The therapeutic value of metal-based drugs has long been established. Cisplatin is one of the first metal-based anticancer drugs discovered in the early 19th century. Cisplatin mediates its anticancer effect by covalently binding to DNA so as to form adducts that interferes with transcription and DNA replication, and thereby triggers programmed cell death. As a consequence of cisplatin, there have been extensive studies in the field of metal-based drugs focusing on platinum complexes binding to DNA. A survey on the ISI Web of KnowledgeSM database (until September, 2009) for publications with topics on ‘platinum drug’, ‘platinum drug + DNA’, ‘platinum drug + enzyme’ returns 3628, 1479, and 126 hits, respectively. Comprehensive reviews on the uses and development of metal-based drugs can be found elsewhere [1,2,3,4]. Sadler and his co-workers have reviewed the recent development in the field [3,4]. In this review, we focus on the promise and the future perspectives for developing platinum-based, gold-based, www.sciencedirect.com
and ruthenium-based complexes as novel protein or enzyme inhibitors, with an emphasis on substitution– inert complexes.
Metal complexes inhibit enzymes through ligand-substitution Metal complexes with labile ligands have long been known to undergo ligand-substitution reactions with biomolecular targets. Metal ions can bind to nitrogen, sulfur or selenium atoms of the histidine, cysteine, or selenocysteine residues in proteins leading to therapeutics effects. Some of the accumulated evidence is described in what follows. Gold: Auranofin (Figure 1), a gold(I) phosphine complex, is a drug for treatment of rheumatoid arthritis. Recently, it was found that gold is transferred from auranofin to seleno-protein thioredoxin glutathione reductase, resulting in a therapeutic effect in parasitic diseases [5]. Auranofin was found to inhibit the growth of tumor cells in vitro, but its high reactivity toward protein thiols limits its antitumor activity in vivo [6]. A gold(I) phosphine complex containing a naphthalimide ligand was recently found to function as a thioredoxin reductase inhibitor with promising antiproliferation and angiogenesis activities [7]. In vitro and in vivo study of a gold(III) dithiocarbamate complex (Figure 1) revealed the proteasome to be its primary target [8]. Platinum: Selenoenzyme thioredoxin reductase was reported to be efficiently inhibited by (2,20 :60 ,200 -terpyridine)platinum(II) complexes (Figure 1) with IC50 values at the nanomolar level [9]. Recently, Lo et al. used X-ray crystallographic and mass spectrometric methods to reveal that the aromatic thiolato platinum(II)–terpyridine complexes inhibited the activities of human thioredoxin reductase 1 by blocking its C-terminal active-site selenocysteine [10]. A series of platinum(II)–terpyridine complexes were found to inhibit topoisomerase II (top2) [10]. There are multiple mechanisms for top2 poisoning, including DNA intercalation, enzyme binding, and enzyme thiol group modification. As such, the authors have pointed out that platinum(II)–terpyridine complexes may also inhibit topoisomerase II activity by a ligand exchange reaction with the thiol groups of enzymes [10]. A series of platinum(II) complexes (PtCl2(smp), Figure 1) with two to three labile ligands were found to inhibit matrix metalloproteinase (MMP-3) [11]. The adduct between MMP-3 and PtCl2(smp) was characterized by ESIMS, CD, and NMR spectroscopy. The studies revealed that the chloride ligands of these platinum(II) complexes are replaced by an imidazole nitrogen of His224 from Current Opinion in Chemical Biology 2010, 14:255–261
256 Bioinorganic chemistry
Figure 1
Examples of metal-based drugs with enzyme inhibitory effects.
MMP-3 and a hydroxyl group modified from the smp ligand [11]. Ruthenium: By coupling ethacrynic acid (EA), an effective inhibitor of glutathione transferase, to a ruthenium complex, a new series of glutathione transferase inhibitors (Ru–EA, Figure 1) were developed [12]. Further mass spectrometry and X-ray crystallographic analyses revealed that the Ru–EA complex lost two chloride ligands and then was further cleaved to release the Current Opinion in Chemical Biology 2010, 14:255–261
ruthenium fragment (Figure 1). Collectively, metal complexes with labile ligands mainly target proteins with selenocysteine or cysteine at the active site, such as thioredoxin glutathione reductase, thioredoxin reductase, and glutathione transferase. The lack of selectivity is a potential drawback of the aforementioned metal complexes. These metal complexes may bind to human serum albumin or other proteins having potential metal binding sites (histidine, www.sciencedirect.com
Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 257
cysteine, or selenocysteine). All of these render the delivery of metal complexes to specific biomolecular targets difficult. Using auranofin as an example, this complex was found to inhibit thioredoxin glutathione reductase and the growth of tumor cells in vitro [5], but its high reactivity toward protein thiols limits its antitumor activity in vivo [6]. As such, Berners-Price and Filipovska designed a series of Au(I) complexes that selectively target proteins with selenocysteine, but not cysteine, by fine-tuning the ligand exchange reactions at the Au(I) center [13]. The effect of the labile ligand or leaving group in metal complexes has also been extensively studied [14]. We have been studying the thera-
peutic potential of metal complexes with enhanced stability in solutions under physiological conditions with the objective of discovering metal complexes with unique anticancer properties.
Substitution–inert metal complexes for anticancer uses Substitution–inert metal complexes provide a promising scaffold for the design of enzyme inhibitors for two reasons: (1) these metal complexes are stable in cells and (2) by mimicking known inhibitors of a specific target, the design of metal complexes for specific biomolecular targets may be achievable.
Figure 2
Rational design of metal complexes to stabilize G-quadruplex DNA. www.sciencedirect.com
Current Opinion in Chemical Biology 2010, 14:255–261
258 Bioinorganic chemistry
Gold(III) complexes as novel histone deacetylase inhibitors In contrast to these complexes, which easily undergo ligand-substitution reactions in solution, we have been exploring the anticancer activities of gold(III) complexes containing strongly chelating multi-anionic ligands. In the year 2003, a gold(III) porphyrin complex (gold-1a, Figure 1) was discovered to be more potent than cisplatin [15] under the program ‘Institute of Molecular Technology for Drug Discovery and Synthesis’. The promising properties of gold-1a include (i) gold-1a is substitutioninert under physiological conditions, (ii) gold-1a remains sensitive to cisplatin-resistant nasopharyngeal carcinoma cell lines; which implies that gold-1a has a unique mechanism of action, (iii) the IC50 value of gold-1a is ca. 240fold lower than that of cisplatin [15]. (iv) Mechanistic studies by transcriptomic and proteomic approaches revealed that gold-1a induced apoptosis through mitochondrial death and mitogen-activated protein kinase signaling pathways [16,17] and also via the up-regulation of Gadd34 and Gadd153 gene expression [18]. (v) Further in vitro and in vivo studies revealed that gold-1a is a promising anticancer agent for the treatment of colon and liver cancer [18,19]. Recently, a new gold(III) porphyrin complex (gold-2a, Figure 1) with improved lipophilicity and aqueous stability was synthesized, and its cytotoxicity was evaluated [20]. The IC50 values of gold-2a against a panel of human breast carcinoma cells were found to be in the nanomolar range, and gold-2a significantly suppressed mammary tumor growth in nude mice [20]. Gold-2a was found to regulate the association of class I histone deacetylase (HDAC) and histone acetylation at the promoter of Wnt signaling molecules. Molecular docking calculations were used to evaluate the binding energy and binding mode of this complex with HDAC8. Gold-2a might share a similar mechanism with another HDACi, cyclic tetrapeptide, which inhibits the activity of HDAC8 by blocking the entry of the 11 A˚ channel and the exit of the 14 A˚ internal cavity [20] (Figure 4a). This result is
unexpected as no HDACi has been reported to have a porphyrin-like structure or to be metal-containing. Thus, gold-2a may represent a novel lead structure of HDACi.
Pt(II) complexes as G-quadruplex stabilizers and telomerase inhibitors Planar aromatic and cationic molecules such as porphyrins, perylenes, telomestatin, quinolines, and berbine have all been demonstrated to bind to and stabilize G-quadruplex DNA. Upon stabilization, the enzymatic activity of telomerase is impaired. It is conceivable that substitution–inert metal complexes with a planar coordination geometry and cationic charge may also stabilize G-quadruplex DNA (Figure 2). As such, platinum(II) complexes with different ligand systems, such as dipyridophenazine [21] or a Schiff base [22], have recently been developed in our laboratory (Figure 1). These platinum(II) complexes can be structurally modified with ease. A series of platinum(II) complexes containing dipyridophenazine (dppz) and C-deprotonated 2-phenylpyridine (N^CH) ligands were synthesized. These complexes were found to bind G-quadruplex DNA through an external endstacking mode (Figure 4b) with one complex displaying a binding affinity of 107 dm3 mol1 and an IC50 value for the effect on human telomerase activity at 760 nM [21]. Recently, a series of platinum(II) Schiff base complexes that bind to c-myc G-quadruplex DNA have also been discovered [22]. The Pt(II) Schiff base complexes can moderately inhibit c-myc gene promoter activity in a cellfree system through stabilizing the G-quadruplex structure and can inhibit c-myc oncogene expression in cultured cells. Using an in silico G-quadruplex DNA model, new platinum(II) Schiff base complexes with improved inhibitory activities were discovered. We conceive that the rational design of platinum complexes with a planar geometry and which are cationic is a promising approach to develop novel class of G-quadruplex stabilizers and telomerase inhibitors (Figure 2).
Figure 3
Ruthenium-based enzyme inhibitors. Current Opinion in Chemical Biology 2010, 14:255–261
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Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 259
Pt(II) complexes as topoisomerases inhibitors
Figure 4
Most of the topoisomerase I inhibitors (poisons, e.g., camptothecin) are planar molecules. Camptothecin mediates cytotoxic effect by binding to the DNA cleavage site of the topoisomerase–DNA complexes through p–p interactions. We therefore envision platinum complexes with planar structures, such as platinum(II)–terpyridine complexes, also forming stable p–p interactions with the DNA cleavage site, subsequently leading to inhibitory effect on topoisomerases. In this regard, (2,20 :60 ,200 -terpyridine)platinum(II) complexes have recently been found to display inhibitory effect on topoisomerase I [10], although the exact binding mode remains to be elucidated.
Ruthenium complexes as protein kinases and HIV-1 reverse transcriptase inhibitors Staurosporine is a nonspecific protein kinase inhibitor. By replacing the carbohydrate unit of staurosporine with various ruthenium fragments (Figure 3), a series of selective inhibitors of glycogen synthase kinase 3, GSK3/b, Pim1, PAK1 (Figure 4c), PI3Ks, mammalian sterile 20 kinase, and BRAF serine/threonine kinase were discovered [23,24–27]. Coupling a fragment of an organic inhibitor to the metal center (Figure 3) may represent a promising approach for the rational design of novel metal-based enzyme inhibitors. Our group identified a mixed-valent ruthenium-oxo oxalate cluster to have potent anti-HIV activities [28]. The compound is stable under physiologically relevant condition. It could reduce the HIV-1 reverse transcriptase activity by half at nanomolar concentration, with IC50 = 1.9 nM [28].
Metal complexes that displayed inhibitory effects on enzymes are potent in vivo While there have been many examples of metal complexes reported to inhibit enzymes in vitro, the next challenge is to develop their therapeutic potential in vivo. Indeed, successful stories have been reported. The in vivo study of a gold(III) dithiocarbamate complex (Figure 1) revealed that during the 29-day treatment of nude mice with MDA-MB-231 xenografts, no toxicity was observed and mice did not display signs of weight loss, decreased activity, or anorexia [2]. Our gold(III) complexes, gold-1a and gold-2a (Figure 1), were found to show promising in vivo cytotoxic activities against liver [18]/colon [19] and breast cancers [20], respectively. Intraductal delivery of gold-2a effectively attenuates mammary MDA-MB-231 tumor growth in nude mice. Under the same experimental condition, 40% of the mice died after cisplatin-treatment, while the gold-2a treatment groups remained alive throughout the experimental period [20]. Collectively, the findings from these in vivo studies are particularly encouraging in the development of novel anticancer drugs with little toxic side effects. www.sciencedirect.com
(a) Molecular model of the gold-2a-bound HDAC8 adducts, adapted with permission from the Cancer Research, supplementary Figure 6 of Ref. [20], (b) molecular modeling of the binding interaction between a Pt(II)-dipyridophenazine complex and the human intramolecular telomeric repeat G4A2-quadruplex, adapted with permission from the J. Am. Chem. Soc., Figure 9 of Ref. [21], copyright 2009 American Chemical Society, and (c) cocrystal structure of (R)-DW12 with PAK1, adapted with permission from the J. Am. Chem. Soc., Figure 4 of Ref. [25], copyright 2008 American Chemical Society.
Current Opinion in Chemical Biology 2010, 14:255–261
260 Bioinorganic chemistry
Future perspectives In the recent past, the pharmaceutical industry and chemical biology as a field have been dependent on organic chemistry. Inorganic compounds, except cisplatin and its derivatives, have only played a minor role. In 2008, the US FDA approved a total of 21 new drugs, but not one of them is a metal-based drug [29]. This can be attributed to the lack of knowledge on the mechanisms and binding modes of metal-based drugs with biomolecular targets other than DNA. Traditionally, it was considered that the majority of metal-based drugs targeted DNA. Nowadays, considerable evidence has appeared to show that metal-based drugs target selenocysteine or cysteine residues of proteins through ligand-substitution reactions under physiological conditions. Important findings have been reported in literature revealing that substitution–inert metal-based drugs utilize additional protein targets, such as histone deacetylase, telomerase, topoisomerase, and protein kinases. Numerous metal complexes, which mediate cytotoxic activities through enzyme inhibition, have been found to show anticancer activities in vivo (both our own work and that done by others). These findings are expected to motivate inorganic chemists to develop and look into novel drug targets or the potential applications of metal-based drugs via their effects on enzyme inhibition. We envision clinical use of these metal-based drugs (enzyme inhibitors) will become available in the near future.
Acknowledgements
5.
Angelucci F, Sayed AA, Williams DL, Boumis G, Brunori M, Dimastrogiovanni D, Miele AE, Pauly F, Bellelli A: Inhibition of Schistosoma mansoni thioredoxin glutathione reductase by auranofin: structural and kinetic aspects. J Biol Chem 2009, 284:28977-28985.
6.
Mirabelli CK, Johnson RK, Sung CM, Faucette L, Muirhead K, Crooke ST: Evaluation of the in vivo antitumor activity and in vitro cytotoxic properties of auranofin, a coordinated gold compound, in murine tumor models. Cancer Res 1985, 45:32-39.
7.
Ott I, Qian X, Xu Y, Vlecken DH, Marques IJ, Kubutat D, Will J, Sheldrick WS, Jesse P, Prokop A et al.: A gold(I) phosphine complex containing a naphthalimide ligand functions as a TrxR inhibiting antiproliferative agent and angiogenesis inhibitor. J Med Chem 2009, 52:763-770.
8.
Milacic V, Chen D, Ronconi L, Landis-Piwowar KR, Fregona D, Dou QP: A novel anticancer gold(III) dithiocarbamate compound inhibits the activity of a purified 20S proteasome and 26S proteasome in human breast cancer cell cultures and xenografts. Cancer Res 2006, 66:10478-10486. An in vivo study revealed that gold(III) dithiocarbamate compound is a promising anticancer agent that inhibits the activity of 20S proteasome and 26S proteasome. 9.
Becker K, Herold-Mende C, Park JJ, Lowe G, Schirmer RH: Human thioredoxin reductase is efficiently inhibited by (2,20 :60 ,200 -terpyridine)platinum(II) complexes. Possible implications for a novel antitumor strategy. J Med Chem 2001, 44:2784-2792.
10. Lo YC, Ko TP, Su WC, Su TL, Wang AHJ: Terpyridine-platinum(II) complexes are effective inhibitors of mammalian topoisomerases and human thioredoxin reductase 1. J Inorg Biochem 2009, 103:1082-1092. 11. Arnesano F, Boccarelli A, Cornacchia D, Nushi F, Sasanelli R, Coluccia M, Natile G: Mechanistic insight into the inhibition of matrix metalloproteinases by platinum substrates. J Med Chem 2009, in press. 12. Ang WH, Parker LJ, De Luca A, Juillerat-Jeanneret L, Morton CJ, Lo Bello M, Parker MW, Dyson PJ: Rational design of an organometallic glutathione transferase inhibitor. Angew Chem Int Ed Engl 2009, 48:3854-3857.
This work was supported by the Areas of Excellence Scheme established under the University Grants Committee of the Hong Kong Special Administrative Region, China (Grant AoE/P-10/01), and the University of Hong Kong (University Development Fund). Pacific Edit reviewed the manuscript before submission. We acknowledge the permissions from the Journal of the American Chemical Society and the Cancer Research; the adapted figures are shown in Figure 4 of this article.
13. Hickey JL, Ruhayel RA, Barnard PJ, Baker MV, Berners-Price SJ, Filipovska A: Mitochondria-targeted chemotherapeutics: the rational design of gold(I) N-heterocyclic carbene complexes that are selectively toxic to cancer cells and target protein selenols in preference to thiols. J Am Chem Soc 2008, 130:12570-12571.
References and recommended reading
14. Montana AM, Batalla C: The rational design of anticancer platinum complexes: the importance of the structure–activity relationship. Curr Med Chem 2009, 16:2235-2260.
Papers of particular interest were highlighted as: of special interest of outstanding interest 1.
Sun RWY, Ma DL, Wong ELM, Che CM: Some uses of transition metal complexes as anti-cancer and anti-HIV agents. Dalton Trans 2007:4884-4892. This review focuses the uses of metal-based drugs as anticancer and anti-HIV agents.
2.
Chen D, Milacic V, Frezza M, Dou QP: Metal complexes, their cellular targets and potential for cancer therapy. Curr Pharm Des 2009, 15:777-791. This review focuses on the development of non-platinum metal complexes as anticancer drugs.
3. Bruijnincx PCA, Sadler PJ: New trends for metal complexes with anticancer activity. Curr Opin Chem Biol 2008, 12:197-206. The use of metal as scaffold rather than reactive center and the departure from cisplatin paradigm of activity toward a more targeted, cancer cellspecific approach were discussed in this review. 4.
Van Rijt SH, Sadler PJ: Current applications and future potential for bioinorganic chemistry in the development of anticancer drugs. Drug Discov Today 2009, 14:1089-1097.
Current Opinion in Chemical Biology 2010, 14:255–261
15. Che CM, Sun RWY, Yu WY, Ko CB, Zhu N, Sun H: Gold(III) porphyrins as a new class of anticancer drugs: cytotoxicity. DNA binding and induction of apoptosis in human cervix epitheloid cancer cells. Chem Commun (Camb) 2003:1718-1719. A substitution–inert gold(III) porphyrin complex (gold-1a) is found to be stable against GSH reduction. 16. Wang Y, He QY, Sun RWY, Che CM, Chiu JF: Gold(III) porphyrin 1a induced apoptosis by mitochondrial death pathways related to reactive oxygen species. Cancer Res 2005, 65:11553-11564. 17. Wang Y, He QY, Che CM, Tsao SW, Sun RWY, Chiu JF: Modulation of gold(III) porphyrin 1a-induced apoptosis by mitogen-activated protein kinase signaling pathways. Biochem Pharmacol 2008, 75:1282-1291. 18. Lum CT, Yang ZF, Li HY, Sun RWY, Fan ST, Poon RTP, Lin MCM, Che CM, Kung HF: Gold(III) compound is a novel chemocytotoxic agent for hepatocellular carcinoma. Int J Cancer 2006, 118:1527-1538. 19. Tu S, Sun RWY, Lin MCM, Cui JT, Zou B, Gu Q, Kung HF, Che CM, Wong BCY: Gold (III) porphyrin complexes induce apoptosis www.sciencedirect.com
Metal complexes in medicine with a focus on enzyme inhibition Che and Siu 261
and cell cycle arrest and inhibit tumor growth in colon cancer. Cancer 2009, 115:4459-4469. 20. Chow KHM, Sun RWY, Lam JBB, Li CKL, Xu A, MA DL, Abagyan R, Wang Y, Che CM: Anti-cancer complex gold(III) porphyrin 2a as a novel histone deacetylase inhibitor targeting Wnt/b-catenin pathway. Cancer Res 2009, in press. An in vivo study revealed that gold(III) porphyrin complex (gold-2a) is a potent anticancer agent and a novel histone deacetylase inhibitor. 21. Ma DL, Che CM, Yan SC: Platinum(II) complexes with dipyridophenazine ligands as human telomerase inhibitors and luminescent probes for G-quadruplex DNA. J Am Chem Soc 2009, 131:1835-1846. 22. Wu P, Ma DL, Leung CH, Yan CH, Zhu N, Abagyan R, Che CM: Stabilization of G-Quadruplex DNA with platinum(II) Schiffbase complexes: luminescent probe and down regulation of cmyc oncogene expression. Chemistry 2009, 15:13008-13021. A series of platinum(II) complexes, mimic organic stabilizers, were found to stabilize c-myc G-quadruplex DNA. 23. Anand R, Maksimoska J, Pagano N, Wong EY, Gimotty PA, Diamond SL, Meggers E, Marmorstein R: Toward the development of a potent and selective organoruthenium mammalian sterile 20 kinase inhibitor. J Med Chem 2009, 52:1602-1611. A series of organoruthenium complexes, mimic staurosporine, were found to be potent protein kinase inhibitors.
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24. Atilla-Gokcumen GE, Pagano N, Streu C, Maksimoska J, Filippakopoulos P, Knapp S, Meggers E: Extremely tight binding of a ruthenium complex to glycogen synthase kinase 3. Chembiochem 2008, 9:2933-2936. 25. Maksimoska J, Feng L, Harms K, Yi C, Kissil J, Marmorstein R, Meggers E: Targeting large kinase active site with rigid, bulky octahedral ruthenium complexes. J Am Chem Soc 2008, 130:15764-15765. 26. Xie P, Streu C, Qin J, Bregman H, Pagano N, Meggers E, Marmorstein R: The crystal structure of BRAF in complex with an organoruthenium inhibitor reveals a mechanism for inhibition of an active form of BRAF kinase. Biochemistry 2009, 48:5187-5198. 27. Xie P, Williams DS, Atilla-Gokcumen GE, Milk L, Xiao M, Smalley KSM, Herlyn M, Meggers E, Marmorstein R: Structurebased design of an organoruthenium phosphatidyl-inositol-3kinase inhibitor reveals a switch governing lipid kinase potency and selectivity. ACS Chem Biol 2008, 3:305-316. 28. Wong EL, Sun RWY, Chung NPY, Lin CLS, Zhu N, Che CM: A mixed-valent ruthenium-oxo oxalato cluster Na7[Ru4(m3O)4(C2O4)6] with potent anti-HIV activities. J Am Chem Soc 2006, 128:4938-4939. 29. Hughes B: 2008 FDA drug approvals. Nat Rev Drug Discov 2009, 8:93-96.
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