Synthesis, Ti(IV) intake by apotransferrin and cytotoxic properties of functionalized titanocene dichlorides

A Comprehensive Review on the Development of Titanium Complexes as Cytotoxic Agents

After the discovery of cis-platin, the first metal-based anticancer drugs, budotitane, and titanocene dichloride entered clinical trials. These two classes of complexes were effective against those cell lines that are resistant to cis-platin and other platinum-based drugs. However, the main limitation of these complexes is their low hydrolytic stability. After these two classes, a third generation titanium based complex, i.e. diaminebis(phenolato)bis(alkoxo) titanium(IV), was invented, which showed more hydrolytic stability and high cytotoxicity than budotitane and titanocene dichloride. The Hydrolytic stability of complexes plays an important role in cytotoxicity. Earlier research showed that hydrolytically less stable complexes decompose rapidly into non-bioavailable moiety and become inactive. The mechanism of Ti(IV) complexes of diaminebis(phenolato) bis(alkoxo) is under investigation and is presumed to involve Endoplasmic Reticulum (ER) stress, which leads to apoptosis. The proposed mechanism involves the removal of ligands from the titanium complex and the binding of the Ti center to transferrin protein and its release inside the cell. Also, the structure of the ligand plays a key role in the cytotoxicity of complexes; as the bulkiness of the ligand increased, the cytotoxic nature of complexes decreased.

Substance-Based Bibliometrics: Identifying Research Gaps by Counting and Analyzing Substances

Identifying research gaps and generating research questions are often a first step in developing ideas for writing a research paper or grant proposal. The concept of substance-based bibliometrics uses the counts of substances in the scientific literature to better understand, assess, and clarify the state and impact of information in the chemical sciences. Connecting substances indexed to specific bioactivity or target indicators can lead to assessing the biochemical, biological, and medicinal relevance of substances as well as developing ideas for expanding drug design and discovery through identifying and modifying the structural features of molecules. This study uses Chemical Abstracts through the SciFinder database to count for the occurrence of substances in the scientific literature. The study sets out search strategies for discovering potential research gaps and new ideas through visualization of chemical structures with known bioactivity and target indicators. The author recommends that subject librarians integrate research gap training in their bibliographic instruction classes, particularly to upper-level undergraduate and graduate chemistry students.

A molecular docking study of the interactions between human transferrin and seven metallocene dichlorides

Human Transferrin (hTf) is a metal-binding protein found in blood plasma and is well known for its role in iron delivery. With only a 30% of its capacity for Fe⁺³ binding, this protein has the potential ability to transport other metal ions or organometallic compounds from the blood stream to all cell tissues. In this perspective, recent studies have described seven metallocene dichlorides (Cp₂M(IV)Cl₂, M(IV)=V, Mo, W, Nb, Ti, Zr, Hf) suitable as anticancer drugs and less secondary effects than cisplatin. However, these studies have not provided enough data to clearly explain how hTf binds and transports these organometallic compounds into the cells. Thus, a computational docking study with native apo-hTf using Sybyl-X 2.0 program was conducted to explore the binding modes of these seven Cp₂M(IV)Cl₂ after their optimization and minimization using Gaussian 09. Our model showed that the first three Cp₂M(IV)Cl₂ (M(IV)=V, Mo, W) can interact with apo-hTf on a common binding site with the amino acid residues Leu-46, Ile-49, Arg-50, Leu-66, Asp-69, Ala-70, Leu-72, Ala-73, Pro-74 and Asn-75, while the next four Cp₂M(IV)Cl₂ (M(IV)=Nb, Ti, Zr, Hf) showed different binding sites, unknown until now. A decreasing order in the total score (equal to -log Kd) was observed from these docking studies: W (5.4356), Mo (5.2692), Nb (5.1672), V (4.5973), Ti (3.6529), Zr (2.0054) and Hf (1.8811). High and significant correlation between the affinity of these seven ligands (metallocenes) for apo-hTf and their bond angles CpMCp (r=0.94, p<0.01) and Cl-M-Cl (r=0.95, p<0.01) were observed, thus indicating the important role that these bond angles can play in ligand-protein interactions. Fluorescence spectra of apo-hTf, measured at pH 7.4, had a decrease in the fluorescence emission spectrum with increasing concentration of Cp₂M(IV)Cl₂. Experimental data has a good correlation between K_A (r=0.84, p=0.027) and K_d (r=0.94, p=0.0014) values and the calculated total scores obtained from our docking experiments. In conclusion, these results suggest that the seven Cp₂M(IV)Cl₂ used for this study can interact with apo-hTf, and their affinity was directly and inversely proportional to their bond angles CpMCp and ClMCl, respectively. Our docking studies also suggest that the binding of the first three Cp₂M(IV)Cl₂ (M(IV)=V, Mo, W) to hTf could abrogate the formation of the hTf-receptor complex, and as a consequence the metallocene-hTf complex might require another transport mechanism in order to get into the cell.

A ubiquitous metal, difficult to track: towards an understanding of the regulation of titanium(iv) in humans

Despite the ubiquitous nature of titanium(iv) and several examples of its beneficial behavior in different organisms, the metal remains underappreciated in biology. There is little understanding of how the metal might play an important function in the human body. Nonetheless, a new insight is obtained regarding the molecular mechanisms that regulate the blood speciation of the metal to maintain it in a nontoxic and potentially bioavailable form for use in the body. This review surveys the literature on Ti(iv) application in prosthetics and in the development of anticancer therapeutics to gain an insight into soluble Ti(iv) influx in the body and its long-term impact. The limitation in analytical tools makes it difficult to depict the full picture of how Ti(iv) is transported and distributed throughout the body. An improved understanding of Ti function and its interaction with biomolecules will be helpful in developing future technologies for its imaging in the body.

Cytotoxicity of a Ti(IV) compound is independent of serum proteins

Titanium(IV) compounds are excellent anticancer drug candidates, but they have yet to find success in clinical applications. A major limitation in developing further compounds has been a general lack of understanding of the mechanism governing their bioactivity. To determine factors necessary for bioactivity, we tested the cytotoxicity of different ligand compounds in conjunction with speciation studies and mass spectrometry bioavailability measurements. These studies demonstrated that the Ti(IV) compound of N,N'-di(o-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED) is cytotoxic to A549 lung cancer cells, unlike those of citrate and naphthalene-2,3-diolate. Although serum proteins are implicated in the activity of Ti(IV) compounds, we found that these interactions do not play a role in [TiO(HBED)](-) activity. Subsequent compound characterization revealed ligand properties necessary for activity. These findings establish the importance of the ligand in the bioactivity of Ti(IV) compounds, provides insights for developing next-generation Ti(IV) anticancer compounds, and reveal [TiO(HBED)](-) as a unique candidate anticancer compound.

Pharmaceutical formulation affects titanocene transferrin interactions

Since the discovery of the anticancer activity of titanocene dichloride (TDC), many derivatives have been developed and evaluated. MKT4, a soluble, water-stable formulation of TDC, was used for both Phase I and Phase II human clinical trials. This formulation is investigated here by using (1)H and (13)C NMR, FT-ICR mass spectrometry, and UV/vis-detected pH-dependent speciation. DFT calculations are also utilized to assess the likelihood of proposed species. Human serum transferrin has been identified as a potential vehicle for the Ti anticancer drugs; these studies examine whether and how formulation of TDC as MKT4 may influence its interactions, both thermodynamic and kinetic, with human serum transferrin by using UV/vis absorption and fluorescence quenching. MKT4 binds differently than TDC to transferrin, showing different kinetics of binding as well as a different molar absorptivity of binding (7500 M(-1) cm(-1) per site). Malate, used in the buffer for MKT4 administration, acts as a synergistic anion for Ti binding, shifting the tyrosine to Ti charge transfer energy and decreasing the molar absorptivity to 5000 M(-1) cm(-1) per site. These differences may have had consequences after the change from TDC to MKT4 in human clinical trials.

Host-Guest Interactions between Calixarenes and Cp(2)NbCl(2)

The possible inclusion complexes of Cp(2)NbCl(2) into calixarenes hosts have been investigated. The existence of a true inclusion complex in the solid state was confirmed by a combination of NMR, ab-initio calculations, thermogravimetric analysis, FTIR, Raman and PXRD. Ab-initio calculations, (1)H NMR solution and solid state (13)C CP MAS NMR results demonstrated that p-sulfonic calix[6]arene does form an inclusion complex with Cp(2)NbCl(2). Raman spectroscopy showed, for the inclusion compound of p-sulfonic calix[6]arene-Cp(2)NbCl(2), a band between 500-850 cm(-1) characteristic of Nb-O vibration. This result suggests that Nb(V) may engage in coordination with the oxygen of the sulfonate group, as part of the host-guest interaction. However, it is important to mention that the niobocene dichloride (Cp(2)NbCl(2)) dissolves in water and undergoes oxidation and hydrolysis processes to yield Cp(2)NbCl(2)(OH) species. For that reason this band does not exclude that the Nb-O band belongs to Cp(2)NbCl(2)(OH). Solid State (13)C CP MAS NMR and solution (1)H NMR spectroscopies together with ab-initio results showed that Cp(2)NbCl(2) is included in the p-sulfonic calix[6]arene cavity, with both Cp rings inside the cavity. In contrast, the solution (1)H NMR results demonstrated that calix[6]arene does not form inclusion complex with Cp(2)NbCl(2) in CDCl(3) solution. Cp(2)NbCl(2) is not included in the calix[6]arene cavity, possibly due to the lack of sulfonate heads which promote Nb-O interactions and assist the inclusion of Cp(2)NbCl(2) into the cavity.

Synthesis and cytotoxicity studies of steroid-functionalized titanocenes as potential anticancer drugs: sex steroids as potential vectors for titanocenes

Six titanocenyls functionalized with steroidal esters have been synthesized and characterized by infrared, (1)H, and (13)C NMR spectroscopy and elemental analysis. Among those steroids, dehydroepiandrosterone, trans-androsterone, and androsterone are androgens and pregnenolone is a progesterone precursor. Clionasterol is a natural steroid compound. These steroid-functionalized titanocenyls were tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay for in vitro cytotoxicity for MCF-7 breast cancer and HT-29 colon cancer cells. All complexes exhibited more cytotoxicity than titanocene dichloride. The titanocenyls containing androgen and progesterone derivatives as pendant groups had higher antiproliferative activities than those with cholesterol steroid compounds. Of particular significance is titanocenyl-dehydroepiandrosterone complex, which is 2 orders of magnitude more cytotoxic than titanocene dichloride and also shows much more sensitivity and selectivity for the MCF-7 cell line.

Titanium(IV) complexes: cytotoxicity and cellular uptake of titanium(IV) complexes on caco-2 cell line

Replacement of the ancillary ligand in titanocene dichloride by amino acids provides titanocene species with high water solubility. As part of our research efforts in the area of titanium-based antitumor agents, we have investigated the cytotoxic activity of Cp(2)TiCl(2) and three water soluble titanocene-amino acid complexes - [Cp(2)Ti(aa)(2)]Cl(2) (aa=L-cysteine, L-methionine, and D-penicillamine) and one water soluble coordination compound, [Ti(4)(maltolato)(8)(micro-O)(4)] on the human colon adenocarcinoma cell line, Caco-2. At pH of 7.4 all titanocene species decompose extensively while [Ti(4)(maltolato)(8)(micro-O)(4)] is stable for over seven days. In terms of cytotoxicity, the [Cp(2)Ti(aa)(2)]Cl(2) and [Ti(4)(maltolato)(8)(micro-O)(4)] complexes exhibited slightly higher toxicity than titanocene dichloride at 24h, but at 72h titanocene dichloride and [Ti(4)(maltolato)(8)(micro-O)(4)] have higher cytotoxic activity. Cellular titanium uptake was quantified at various time intervals to investigate the possible relationship between Ti uptake and cellular toxicity. Results indicated that there was not a clear relationship between Ti uptake and cytotoxicity. A structure-activity relationship is discussed.

Synthesis, Structure and Biological Activity of Amide-Functionalized Titanocenyls: Improving their Cytotoxic Properties

Nine amide-functionalized titanocenyls have been synthesized and characterized by spectroscopic and analytical methods and the solid state structure of Cp(CpCO-NH-C(6)H(4)-OCF(3))TiCl(2) was determined by single crystal X-ray diffraction. X-ray analysis of Cp(CpCO-NH-C(6)H(4)-OCF(3))TiCl(2) showed that titanium is in a pseudo tetrahedral geometry and contains a Ti-O(amide) coordination. In principle, Ti-O coordination should provide more hydrolytic stability to the corresponding titanocenyls than titanocene dichloride. The cytotoxic activities of these amide-functionalized titanocenyls on HT-29 colon cancer cell line were determined by MTT assay to elucidate structure-activity relationship. All complexes were more cytotoxic than titanocene dichloride and there is no correlation between the para substituents on the phenyl ring and their cytotoxicities.

Water-soluble molybdenocene complexes with both proliferative and antiproliferative effects on cancer cell lines and their binding interactions with human serum albumin

Two water-soluble molybdenocene complexes containing oxygen chelating ligands, maltolato and malonate, have been synthesized to elucidate the role of the ancillary ligands in the molybdenocene cytotoxic activity. The structural characterizations of these species by 1H NMR and IR spectroscopies suggest that both molybdenocene complexes contain the ligands in a bidentate fashion and elemental analysis and mass spectrometry corroborate the proposed formula for the species to be Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl (Cp is cyclopentadienyl). Metal-albumin binding studies were pursued using UV-vis spectroscopy and cyclic voltammetric techniques. Whereas metal-albumin binding studies using UV-vis spectroscopy did not show any evidence of interaction, cyclic voltammetry experiments showed that molybdenocene complexes may be involved in weak binding interactions with albumin, most likely in hydrophobic interactions. The cytotoxic activities of Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl alone with Cp2MoCl2 were investigated in HT-29 colon cancer and MCF-7 breast cancer cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability assay. Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl showed slight improvement in terms of cytotoxic activity as compared with Cp2MoCl2 in the HT-29 colon cancer cell line, whereas for MCF-7 all the molybdenocene species exhibited a proliferative profile. The molybdenocene-containing chelating ligands showed stronger proliferative effects than Cp2MoCl2. There is no correlation between the binding affinity of molybdenocenes for human serum albumin and cytotoxic activity toward HT-29 and MCF-7 cancer cells.

Synthesis, structure, electrochemistry, and cytotoxic properties of ferrocenyl ester derivatives

A series of ferrocenyl ester complexes, varying the lipophilic character of the pendant groups, was prepared and characterized by spectroscopic and analytical methods. The syntheses of Fe(C(5)H(4)CO(2)CH(3))(2), Fe(CpCOOCH(3)) (CpCOO CH(2)CH(3)), and Fe(CpCOOCH(2)CH(3))(2) are reported. The solid-state structure of Fe(C(5)H(4)CO(2)CH(3))(2) has been determined by X-ray crystallography. Fe(C(5)H(4)CO(2)CH(3))(2) has the cyclopentadienyl rings virtually in an eclipsed conformation with the pendant groups not completely opposite to each other. Cyclic voltammetry characterization showed that the functionalized ferrocenes oxidize at potentials, E(pa), higher than ferrocene as a result of the electro withdrawing effect of the pendant groups on the cyclopentadienyl ligand. The cytotoxicities of Fe(C(5)H(4)CO(2)CH(2)CH(2)OH)(2), Fe(C(5)H(4)CO(2)CH(2)CH=CH(2))(2), Fe(C(5)H(4)CO(2)CH(3))(2), Fe(CpCOOCH(3))(CpCOOCH(2)CH(3)), and Fe(CpCOOCH(2)CH(3))(2) in colon cancer HT-29 and breast cancer MCF-7 cell lines were measured by the MTT biological viability assay and compared to ferrocene and ferrocenium. Fe(C(5)H(4)CO(2)CH(2)CH=CH(2))(2) showed the best IC(50) values, 180(10) muM for HT-29 and 190(30) muM for MCF-7 cell lines, with cytotoxicities similar to ferrocenium. The cytotoxic data suggest that as we increase the lipophilic character of the functionalized ferrocene, the cytotoxicity improves approaching to the cytotoxic activity of ferrocenium.

Structure-activity studies of Ti(IV) complexes: aqueous stability and cytotoxic properties in colon cancer HT-29 cells

As part of our research efforts in the area of titanium-based antitumor agents, we have investigated the cytotoxic activity of [Ti(4)(maltolato)(8)(mu-O)(4)], (Cp-R)(2)TiCl(2) and (Cp-R)CpTiCl(2) (R = CO(2)CH(3) and CO(2)CH(2)CH(3)), and three water-soluble titanocene-amino acid complexes-[Cp(2)Ti(aa)(2)]Cl(2) (aa = L: -cysteine, L: -methionine, and D: -penicillamine)-on the human colon adenocarcinoma cell line, HT-29. The capacity of [Ti(4)(maltolato)(8)(mu-O)(4)] to donate Ti(IV) to human apo-transferrin and its hydrolytic stability have been investigated and compared to the previously reported data on modified titanocenes with either hydrophilic ancillary ligands or the functionalized cyclopentadienyl ligands. Notably, the titanium-maltolato complex does not transfer Ti(VI) to human apo-transferrin at any time within the first seven days of its interaction, demonstrating the inert character of this species. Stability studies on these complexes have shown that titanocene complexes decompose at physiological pH while the [Ti(4)(maltolato)(8)(mu-O)(4)] complex is stable at this pH without any notable decomposition for a period of ten days. The antitumor activity of these complexes against colon cancer HT-29 cells was determined using an MTT cell viability assay at 72 and 96 h. The titanocene-amino acid and the (Cp-R)(2)TiCl(2)/(Cp-R)CpTiCl(2) (R = CO(2)CH(3)) complexes were not biologically active when human transferrin was absent; they also were inactive when human transferrin was present at dose-equivalent concentrations. (Cp-R)(2)TiCl(2) and (Cp-R)CpTiCl(2) (R = CO(2)CH(2)CH(3)) showed cytotoxic activity in HT-29 cells comparable to that which is displayed by titanocene dichloride. The titanium-maltolato complex had higher levels of cytotoxic activity than any other titanocene complex investigated. Transferrin may be important in protecting the titanium center from hydrolysis, but this may be achieved by selecting ligands that could result in hydrolytically stable, yet active, complexes.