Original articleIn vitro anticancer activity of gold(III) complexes with some esters of (S,S)-ethylenediamine-N,N′-di-2-propanoic acid
Graphical abstract
Synthesis, characterization and biological activity of five novel gold(III) complexes with some R2edda-type esters, [AuCl2{(S,S)-R2eddip}]PF6, R = n-Bu, n-Pe, i-Bu, i-Am, cPe, 1–5, is described. [AuCl2{(S,S)-(i-Bu)2eddip}]PF6 forms metal-protein adducts with BSA. Antitumoral activity was tested in vitro on four tumor cell lines. Complex 4 showed highest activity and selectivity, 4 times more active and 28 times more selective than cisplatin. Apoptosis is the main mode of cell death induced by these compounds in HeLa cells.
Introduction
The progress of medicinal inorganic chemistry includes several applications of a variety of metal complexes in medicine [1], [2], [3], [4]. To design a metal-based applicable anticancer drug is challenging. Any candidate for an antitumor agent needs to demonstrate its positive reactions with target biomolecules and favorable physiological responses to tumors before entering clinical trials. Cisplatin has made an impressive impact on cancer chemotherapy, and nowadays is frequently used in treatment of various types of cancers [5], [6], [7], [8], [9]. Its activity, as well as activity of new-generation platinum compounds, is compromised due to inevitable serious side effects such as nephrotoxicity and neurotoxicity, hair and hear loss and many others [10], [11], [12], [13]. Interest in medicinal chemistry of gold has been growing with the successful use of auranofin for treatment of rheumatoid arthritis [14], [15]. Gold(III) complexes have greatly attracted researchers' attention in the last decade for their outstanding cytotoxic actions against different tumor cells [16], [17], even against the cisplatin-resistant cell lines [18], [19], [20]. With square-planar geometry (d8 system), gold(III) complexes are isoelectronic and isostructural to platinum(II) complexes, thus they could show a model of binding to the biomolecules similarly to cisplatin [21], [22]. The strict relationship to platinum(II) compounds makes gold(III) complexes good candidates for development as anticancer drugs, although gold(III) complexes are not very stable under physiological conditions because of their high reduction potential and fast hydrolysis rate. These problems can possibly be circumvented by forming gold(III) compounds with one or more multidentate nitrogen-donor ligands to enhance their stability [23], [24], [25]. Recent findings by Messori et al. showed that most of the cytotoxic gold(III) complexes have a weak binding affinity to DNA, which is the primary target for platinum(II) antitumor drugs [22]. Also, it was found that cytotoxic gold(III) complexes have shown high reactivity toward different protein models [26]. Most of the known active gold(III) complexes might react through gold(I) species produced by gold(III) reduction in vivo [27]. However, there are exceptions where e.g. porphyrinato ligand markedly stabilizes the gold(III) ion against reduction diminishing the possibility to be reduced by biological reductants such as glutathione and ascorbic acid [28].
Generally, very few gold(III) compounds demonstrate anticancer activity in vivo [29]. Since the early investigations on [AuX2(damp)] (damp = 2-[(dimethylamino)methyl]-phenyl, X2/X = malonato/acetato) [30], [31], in vivo anticancer activity has been reported for only four other types of gold(III) compounds, gold(III) dithiocarbamate [32], gold(III) porphyrins [33], [34], cyclometallated gold(III) NHC [35] and gold(III) phosphine complex [36].
Recently, synthesis and characterization of gold(III) complexes with esters of cyclohexyl-functionalized ethylenediamine-N,N′-diacetate was reported (Fig. 1) [37]. The in vitro cytotoxic evaluation of the investigated complexes against tumor cell lines: human adenocarcinoma (HeLa cells), human myelogenuos leukemia (K562 cells) and against normal peripheral blood mononuclear cells (PBMC), showed that the cytotoxic action of gold(III) complexes with cyclohexyl-functionalized ethylenediamine-N,N′-diacetate esters, (R = i-Bu, i-Am), is fairly comparable to that of cisplatin [37].
Inspired by these promising results, five novel gold(III) complexes of N,N′ bidentate (S,S)-R2eddip ligands with general formulae [AuCl2{(S,S)-R2eddip}]PF6: ((S,S)-eddip = (S,S)-ethelendiamine-N,N’-di-2-propanoate; R = n-Bu, n-Pe, i-Bu, i-Am, cPe, 1–5, respectively) were synthesized. Density functional theory (DFT) analyses were performed for indication of the preferred configuration of nitrogen atoms. Stability of 3 in DMSO and in physiological medium (PBS) was examined, as well as possibility of reduction by ascorbic acid, by time-dependent UV/Vis spectrometry and 13C NMR spectroscopy. Interaction of a selected complex, 3 with bovine serum albumin (BSA) is monitored by UV/Vis spectrometry over time. All compounds were tested against cervix adenocarcnoma cell line (HeLa), human chronic myelogenous leukemia (K562), human melanoma (Fem-x) and non-cancerous cell line, human embryonic lung fibroblast (MRC-5) with the aim of assessing in vitro activity and selectivity. The mode of HeLa cell death induced by 1–5 was also studied, as well as cell cycle distribution of HeLa cells upon treatment with these complexes.
Section snippets
Chemistry
In the reaction of Na[AuCl4]·2H2O and an equimolar amount of corresponding (S,S)-R2eddip ligand, previously deprotonated with LiOH in methanol, and after addition of ammonium hexafluorophosphate to the reaction mixture, desired complexes 1–5 (Scheme 1) were obtained as yellow powders. The complexes are soluble in methanol, ethanol, acetone, dichloromethane, chloroform, dimethyl sulfoxide and acetonitrile.
Spectroscopic characterizations
IR spectra showed strong ν(CO) absorption stretching bands from 1732 to 1738 cm−1 (1–5) [38]
Conclusions
Synthesis of five novel gold(III) complexes, [AuCl2{(S,S)-R2eddip}]PF6, R = n-Bu, n-Pe, i-Bu, i-Am, cPe, 1–5, respectively, was described. The compounds were characterized by elemental analysis, UV/Vis, IR, NMR spectroscopy and mass spectrometry. Spectroscopic data suggest ligand chelation via nitrogen donor atoms. NMR spectra show presence of one isomer. DFT calculations indicate that (R,R)-N,N′- diastereoisomer is the most stable one. Complex 3 was stable in DMSO but in physiological medium
Materials and methods
The n-butyl, n-pentyl, isobutyl, isoamyl and cyclopentyl esters of (S,S)-ethylenediamine-N,N′-di-3-propanoic acid were synthesized according to described method [38], [40], [41], [42]. Na[AuCl4] was synthesized by the standard procedure [61].
Elemental analyses were performed on an Elemental Vario EL III microanalyzer. A Nicolet 6700 FT–IR spectrometer and ATR technique were used for recording mid-infrared spectra (4000–400 cm−1) for all complexes. Far-IR spectra were recorded at room
Acknowledgments
This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, grant numbers 172035 and 175011. The authors thank D. Vučetić, Department of Chemistry, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, for his help in NMR spectroscopy.
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