Study of the anticancer properties of methyl- and phenyl-substituted carbon- and silicon-bridged ansa-titanocene complexes
Graphical abstract
Carbon- and silicon-bridged ansa-titanocene complexes were synthesized and tested on a wide variety of cancer cell lines and primary normal mouse keratinocytes and lung fibroblasts. Titanocene compounds triggered blockage in division in cisplatin-resistant HCT116 and SW620 cells and underwent to mitochondrial form of apoptotic cell death.
Introduction
Titanocene derivatives are together with ferrocene complexes the most studied metallocenes in preclinical trials in the treatment of cancer [1]. The starting point of this research field was the work of Köpf and Köpf-Maier which showed the cytotoxic properties of titanocene dichloride, [Ti(η5-C5H5)2Cl2] and some of its analogs in the early 1980's [2]. Titanocene dichloride was studied in phase I clinical trials in 1993 [3], [4], [5] and some water soluble formulations were subsequently developed by Medac GmbH (Germany) [6]. Phase I clinical trials were not as satisfactory as expected, but the research in the topic continued with some phase II clinical trials in patients with breast metastatic carcinoma [7] and advanced renal cell carcinoma [8], which showed a low activity discouraging further studies.
However, after the recent work of many different groups worldwide, the interest in this topic has been renewed [9], [10], [11], [12], [13]. Thus, aminoacid-functionalized titanocene derivatives [14], [15], benzyl-substituted titanocene or ansa-titanocene complexes [13], ionic titanocene compounds [16], [17], alkylammonium-substituted titanocene derivatives [18], [19], [20], steroid-functionalized titanocenes [21], and alkyl-, aryl- and/or alkenyl-substituted titanocene or ansa-titanocene derivatives [22], [23], [24], [25] have been synthesized, characterized and studied in preclinical trials against different types of human cancer cell lines and primary cells.
In general, the cytotoxic activity of titanocene complexes has been correlated to their structure, and the study of the anticancer mechanism of titanocene derivatives is currently a very active research field. Several studies have led to the proposal that after hydrolysis of titanocene complexes, titanium ions reach the cell cytoplasm assisted by the major iron transport protein “transferrin” [26], [27], [28], [29]. Inside the cell, titanium ions are transported to the nucleus (probably assisted by ATP) and then bind to DNA and lead to cell death [30], [31], [32]. However, recent experiments have also reported on the potential interaction of a ligand-bound Ti(IV) complex to other proteins [33], [34], [35], which may also be implicated in the induction of cell death.
There are several reports on the cytotoxicity of titanocene derivatives with different substituents attached to Cp rings [1], [9], [13], but not many studies on the mechanistic properties of alkyl- or alkenyl-substituted silicon- or carbon-bridged ansa-titanocene derivatives [22], [23]. With the aim to contribute to the understanding of the mechanism of cancer cell death promoted by silicon- or carbon-bridged ansa-titanocene derivatives with simple alkyl or aryl substituents, a complete study which consists of the synthesis, characterization, cell cycle analysis, AnnexinV-FITC/PI staining and caspase detection has been carried out and is reported here.
Section snippets
General manipulations
All reactions were performed using standard Schlenk tube techniques in an atmosphere of dry argon. Solvents were distilled from the appropriate drying agents and degassed before use. [Ti{(Me2CMe2C)(η5-C5H4)2}Cl2] (1) [36], [Ti{Me2C(η5-C5H4)2}Cl2] (2) [37], [Ti{Me2Si(η5-C5H4)2}Cl2] (4) [38], [Ti{MePhSi(η5-C5H4)2}Cl2] (5) [39] and [Ti{MePhSi(η5-C5Me4)2}Cl2] (6) [39] were prepared according to previously described synthetic methods. Reference compound titanocene dichloride ([Ti(η5-C5H5)2}Cl2] (Ref
Synthesis and characterization
[Ti{(Me2CMe2C)(η5-C5H4)2}Cl2] (1) [36], [Ti{Me2C(η5-C5H4)2}Cl2] (2) [37], [Ti{Me2Si(η5-C5H4)2}Cl2] (4) [38], [Ti{MePhSi(η5-C5H4)2}Cl2] (5) [39] and [Ti{MePhSi(η5-C5Me4)2}Cl2] (6) [39] (Fig. 1) were prepared according to previously described synthetic methods.
For the preparation of the carbon-bridged ansa-titanocene [Ti{MePhC(η5-C5H4)2}Cl2] (3), the synthesis of the corresponding ligand MePhC(C5H5)2 was carried out according to literature procedures (Scheme 1). This ligand was di-lithiated by
Conclusions
The novel complex [Ti{MePhC(η5-C5H4)2}Cl2] (3) has been synthesized and characterized by traditional methods. The cytotoxicity of 3 and the known complexes [Ti{(Me2CMe2C)(η5-C5H4)2}Cl2] (1), [Ti{Me2C(η5-C5H4)2}Cl2] (2), [Ti{Me2Si(η5-C5H4)2}Cl2] (4), [Ti{MePhSi(η5-C5H4)2}Cl2] (5) and [Ti{MePhSi(η5-C5Me4)2}Cl2] (6) and the reference compound [Ti(η5-C5H5)2Cl2] (Ref) has been tested against seven different cancer cell lines.
An increase in the cytotoxic activity has been observed by the
Acknowledgments
The authors would like to acknowledge financial support from the Ministerio de Educación y Ciencia, Spain (Grant nos. CTQ-2011-24346 and CTQ-2012-30762) and from the Ministry of Science and Technological Development of the Republic of Serbia (Grant No. 173013). J.P. and M.H. thank to Grant Agency of the Czech Republic (Project No. P207/12/2368). We would also like to thank Karel Mach for being the starting point of this collaborative work.
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