Heptacoordinate Heteroleptic Salan (ONNO) and Thiosalan (OSSO) Titanium(IV) Complexes: Investigation of Stability and Cytotoxicity

From the Design of Innovative Ti-Pt Heterometallic Complexes to the Development of Highly Anti-Proliferative Water-Soluble Cationic Titanocenes

Two innovative early/late Ti-Pt-heterobimetallic complexes were synthesized, characterized, and screened in cell-based assays using several human (SW480 and MDA-MB-231) and murine cancer cell lines (CT26 and EMT6) as well as a non-cancerous cell line (HMEC). The combination of the two metals - titanium(IV) and platinum (IV) - in a single molecule led to a synergistic biological activity (higher anti-proliferative properties than a mixture of each of the corresponding monometallic complexes). This study also investigated the benefits of associating a metal-free terpyridine moiety (with intrinsic biological activity) with a water-soluble titanocene fragment. The present work reveals that these combinations results in water-soluble titanocene compounds displaying an anti-proliferative activity down to the submicromolar level. One of these complexes induced an antitumor effect in vivo in CT26 tumor bearing BALB/C mice. The terpyridine moiety was also used to track the complex in vitro by multiphoton microscopy imaging.

Novel Titanocene Y derivative with albumin affinity exhibits improved anticancer activity against platinum resistant cells

The antitumor activity of Ti(IV)-based compounds put them in the spotlight for cancer treatment in the past, but their lack of stability in vivo due to a high rate of hydrolysis has hindered their development as antitumor drugs. As a possible solution for this problem, we have reported a synthesis strategy through which we combined a titanocene fragment, a tridentate ligand, and a long aliphatic chain. This strategy allowed us to generate a titanium compound (Myr-Ti) capable of interacting with albumin, highly stable in water and with cytotoxic activity in tumor cells[1]. Following a similar strategy, now we report the synthesis of a new compound (Myr-TiY) derived from titanocene Y that shows antitumoral activity in a cisplatin resistant model with a 50% inhibitory concentration (IC50) of 41-76 μM. This new compound shows high stability and a strong interaction with human serum albumin. Myr-TiY has a significant antiproliferative and proapoptotic effect on the tested cancer cells and shows potential tumor selectivity when assayed in non-tumor human epithelial cells being more selective (1.3-3.8 times) for tumor cells than cisplatin. These results lead us to think that the described synthesis strategy could be useful to generate compounds for the treatment of both cisplatin-sensitive and cisplatin-resistant cancers.

Bioactive O^N^O^ Schiff base appended homoleptic titanium(iv) complexes: DFT, BSA/CT-DNA interactions, molecular docking and antitumor activity against HeLa and A549 cell lines

Five new homoleptic derivatives of titanium(iv) have been developed and characterized by physicochemical techniques. Metal complexes, TiH2L1 [(C38H26N6O4)Ti], TiH2L2 [(C38H24F2N6O4)Ti], TiH2L3 [(C38H24Cl2N6O4)Ti], TiH2L4 [(C38H24Br2N6O4)Ti] and TiH2L5 [(C38H24N8O8)Ti], were obtained by treating Ti(OPri)4 with appropriate ONO ligands (H2L1-H2L5) in anhydrous THF as solvent. The electronic structures and properties of titanium(iv) complexes (TiH2L1-TiH2L5) and ligands (H2L1-H2L5) were examined by DFT studies. The stability of all synthesized derivatives was assessed by a UV-visible technique using 10% DMSO, GSH medium and n-octanol/water systems. The binding interactions of BSA and CT-DNA with respective titanium(iv) complexes were successfully evaluated by employing UV-visible absorption, fluorescence, circular dichroism (CD) techniques and docking studies. The in vitro cytotoxicity of TiH2L2, TiH2L3 and TiH2L4 complexes was assessed against HeLa (human epithelioid cervical cancer cells) and A549 (lung carcinoma) cell lines. The IC50 values of TiH2L2, TiH2L3 and TiH2L4 were observed to be 28.8, 14.7 and 31.2 μg mL-1 for the HeLa cell line and 38.2, 32.9 and 67.78 μg mL-1 for A549 cells, respectively. Complex TiH2L3 exhibited remarkably induced cell cycle arrest in the G1 phase and 77.99% ROS production selectivity in the HeLa cell line.

Synthesis, in vitro antitumor evaluation and structure activity relationship of heptacoordinated amino-bis(Phenolato) Ti(IV) complexes stabilized by 2,6-dipicolinic acid

Eighteen novel Ti(IV) complexes stabilized by different chelating amino-bis(phenolato) (ONNO, ONON, ONOO) ligands and 2,6-dipicolinic acid as a second chelator were synthesized with isolated yields ranging from 79 to 93%. Complexes were characterized by 1H and 13C-NMR spectroscopy, as well as by HRMS and X-Ray diffraction analysis. The good to excellent aqueous stability of these Ti(IV) complexes can be modulated by the substitutions on the 2-position of the phenolato ligands. Most of the synthesized Ti(IV) complexes demonstrated potent inhibitory activity against Hela S3 and Hep G2 tumor cells. Among them, the naphthalenyl based Salan type 2j, 2-picolylamine based [ONON] type 2n and N-(2-hydroxyethyl) based [ONOO] type 2p demonstrated up to 40 folds enhanced cytotoxicity compared to cisplatin together with a significantly reduced activity against healthy AML12 cells. The three Ti(IV) complexes exhibited fast cellular uptake by Hela S3 cells and induced almost exclusively apoptosis. 2j could trigger higher level of ROS generation than 2p and 2n.

An anticancer Ti(IV) complex increases mitochondrial reactive oxygen species levels in relation with hypoxia and endoplasmic-reticulum stress: A distinct non DNA-related mechanism

PhenolaTi is a promising Ti(IV) anticancer complex, with high stability and cytotoxicity, without notable toxic side-effects. Its cellular mechanism was proposed to relate to ER stress. Herein, we investigated the downstream effects of this mode of action in two cancer cell lines: ovarian carcinoma A2780 and cervical adenocarcinoma HeLa. First, although Ti(IV) is a non-redox metal, the formation of mitochondrial reactive oxygen species (ROS) was detected with live-cell imaging. Then, we inspected the effect of the mitochondrial ROS on cytotoxicity, using two methods: (a) addition of compounds that either elevate or reduce the mitochondrial glutathione concentration, thus affecting the oxidative state of the cells; and (b) scavenging mitochondrial ROS. Unlike the results observed for cisplatin, neither method influenced the cytotoxicity of phenolaTi, implying that ROS formation was a mere side effect of its activity. Additionally, live cell imaging displayed the hypoxia induced by phenolaTi, which can be associated with ROS formation. Overall, the results support the notion that ER-stress is the main cellular mechanism of phenolaTi, leading to hypoxia and mitochondrial ROS. The distinct mechanism of phenolaTi, which is different from that of cisplatin, combined with its stability and favorable anticancer properties, altogether make it a strong chemotherapeutic drug candidate.

Synthesis and X-ray structure analysis of cytotoxic heptacoordinated Salan hafnium(IV) complexes stabilized with 2,6-dipicolinic acid

Four novel Salan Hf^(IV) complexes stabilized by 2,6-dipicolinic acid (Dipic) were synthesized and characterized by ¹H, ¹³C NMR and X-ray diffraction spectroscopy. These Hf^(IV)bis-chelates could be obtained in good to excellent yields (88%-91%) and demonstrated rather good stability in aqueous media and on silica gel. [L₂Hf^(IV)Dipic^4-H,Cl] containing steric bulk L₂ were stable in about 10% H₂O (H₂O/THF (v/v)), however, [L₁Hf^(IV)Dipic^4-H,Cl] with non-steric L₁ could slowly dissociate and release nontoxic L₁. [L_1-2Hf^(IV)Dipic^4-Cl] showed excellent anti-tumoral activity in the range of cisplatin (Hela S3: IC₅₀ = 3.5 ± 0.4 μM, Hep G2: IC₅₀ = 11.2 ± 2.1 μM). In addition, the cellular uptake and apoptosis investigation of [L₁Hf^(IV)Dipic^4-Cl] suggested a fast cellular uptake process against Hela S3 cells with an almost exclusive induced apoptosis cell death path.

Facile synthesis of [ONON] type titanium(IV)bis-chelated complexes in alcoholic solvents and evaluation of anti-tumor activity

Novel anti-tumoral diamino-bis-(phenolato) [ONON] type titanium^(IV) complexes stabilized by 2,6-dipicolinic acid were synthesis via an efficient protocol using n-propanol as solvent and H₂O for isolation. In total 20 [ONON] type and 2 Salan Ti^(IV)bis-chelated complexes were synthesized with yields ranging from 68% to 96%. All reactions could reach to completion in 1.5 min at 80 °C either using Ti(OⁱPr)₄ or TiCl₄ as starting materials. Most [ONON] type Ti^(IV) complexes exhibit selectively enhanced inhibition activity against Hep G2 cells in comparison with Salan Ti^(IV) complexes. Among which, the inhibitory activity of 2 t (IC₅₀: 0.15 ± 0.1 μM) against Hep G2 cells is about 80 times enhanced than that of cisplatin (IC₅₀: 12.4 ± 1.2 μM). The [ONON] type Ti^(IV) complexes slowly released nontoxic phenolato ligands in presence of large amount of aqueous media, and a fast cellular uptake process was proposed for these Ti^(IV) complexes based on metal uptake analysis. Decagram scale synthesis indicates this facile synthetic methodology can be applied to large scale synthesis.

The crystal structure of 6,6′-(((2-(dimethylamino)ethyl)azanediyl)bis(methylene))bis(benzo[d][1,3]dioxol-5-ol ato-κ4 N,N′,O,O′)-(pyridine-2,6-dicarboxylato-N,O,O′)-titanium(IV)-dichloromethane(1/1), C27H25N3O10Ti

C27H25N3O10Ti, monoclinic, P21/n (no. 14), a = 11.3366(11) Å, b = 11.0253(11) Å, c = 23.7160(2) Å, β = 100.839(2)°, V = 2911.4(5) Å3, Z = 4, R gt (F) = 0.0615, wR ref (F 2) = 0.1411, T = 296(2) K.

The crystal structure of 1,1′-(((2 (dimethylamino)ethyl)azanediyl)bis(methylene)) bis(naphthalen-2-olato-κ4 N,N′,O,O′)-(pyridine-2,6-dicarboxylato-N,O,O′)- titanium(IV) ─ dichloromethane (2/1), C33H29N3O6Ti

C33H29N3O6Ti, monoclinic, C2/c (no. 15), a = 27.4022(15) Å, b = 14.0345(7) Å, c = 17.0047(10) Å, β = 116.009(3)°, V = 5877.3(6) Å3, Z = 8, R gt (F) = 0.0369, wRref (F 2) = 0.1125, T = 100(2) K.

New titanocene derivative with improved stability and binding ability to albumin exhibits high anticancer activity

Titanium-based therapies have emerged as a promising alternative for the treatment of cancer patients, particularly those with cisplatin resistant tumors. Unfortunately, some titanium compounds show stability and solubility problems that have hindered their use in clinical practice. Here, we designed and synthesized a new titanium complex containing a titanocene fragment, a tridentate ligand to improve its stability in water, and a long aliphatic chain, designed to facilitate a non-covalent interaction with albumin, the most abundant protein in human serum. The stability and human serum albumin affinity of the resulting titanium complex was investigated by UV-Vis absorption and fluorescence spectroscopy techniques. Complex [TiCp₂{(OOC)₂py-O-myr}] (3) (myr = C₁₄H₂₉, py = pyridine) and its analogous [TiCp₂{(OOC)₂py-OH}] (4), lacking the aliphatic chain, showed improved stability in phosphate saline buffer compared with [TiCp₂Cl₂] (1). 3 showed a strong interaction with human serum albumin in a 1:1 stoichiometry. The cytotoxic effect of 3 was higher compared to [TiCp₂Cl₂] in tumor cell lines and showed potential tumor selectivity when assayed in non-tumor human epithelial cells. Finally, 3 showed an antiproliferative effect on cancer cells, decreasing the population in the S phase, and increasing apoptotic cells in a significant manner. All this makes the novel Ti(IV) compound 3 a firm candidate to continue further studies of its therapeutic potential in vitro and in vivo.

A metal-free salalen ligand with anti-tumor and synergistic activity in resistant leukemia and solid tumor cells via mitochondrial pathway

PURPOSE

Since the discovery of the well-known cis-platin, transition metal complexes are highly recognized as cytostatic agents. However, toxic side effects of the metal ions present in the complexes may pose significant problems for their future development. Therefore, we investigated the metal-free salalen ligand WQF 044.

METHODS

DNA fragmentations in leukemia (Nalm6) and solid tumor cells (BJAB, MelHO, MCF-7, RM82) proved the apoptotic effects of WQF 044, its overcoming of resistances and the cellular pathways that are affected by the substance. The apoptotic mechanisms finding were supported by western blot analysis, measurement of the mitochondrial membrane potential and polymerase chain reactions.

RESULTS

A complex intervention in the mitochondrial pathway of apoptosis with a Bcl-2 and caspase dependence was observed. Additionally, a wide range of tumors were affected by the ligand in a low micromolar range in-vitro. The compound overcame multidrug resistances in P-gp over-expressed acute lymphoblastic leukemia and CD95-downregulated Ewing's sarcoma cells. Quite remarkable synergistic effects with vincristine were observed in Burkitt-like lymphoma cells.

CONCLUSION

The investigation of a metal-free salalen ligand as a potential anti-cancer drug revealed in promising results for a future clinical use.

Effective Oral Administration of an Antitumorigenic Nanoformulated Titanium Complex

Orally administered anticancer drugs facilitate treatment, but the acidic conditions in the stomach often challenge their availability. PhenolaTi is a Ti^IV -based nontoxic anticancer drug with marked in-vivo efficacy. We report that nanoformulation protects phenolaTi from decomposition in stomach-like conditions. This is evidenced by similar NMR characteristics and similar in-vitro cytotoxicity toward murine (CT-26) and human (HT-29) colon cancer cells before and after incubation of nanoformulated phenolaTi (phenolaTi-F) at pH 2, unlike results with the unformulated form of the complex. Furthermore, administration of phenolaTi-F in animal drinking water revealed a notable inhibition of tumor growth in Balb/c and immune-deficient (Nude) mice inoculated with CT-26 and HT-29 cells, respectively. In-vivo efficacy was at least similar to that of the corresponding intraperitoneal treatment with phenolaTi-F and the clinically employed oral drug, capecitabine. No body weight loss or clinical signs of toxicity were evident in the phenolaTi-F-treated animals. These findings demonstrate a new convenient mode of cancer treatment through oral administration by safe titanium-based drugs.

Anticancer Potential of Diiron Vinyliminium Complexes

Although ferrocene derivatives have attracted considerable attention as possible anticancer agents, the medicinal potential of diiron complexes has remained largely unexplored. Herein, we describe the straightforward multigram-scale synthesis and the antiproliferative activity of a series of diiron cyclopentadienyl complexes containing bridging vinyliminium ligands. IC₅₀ values in the low-to-mid micromolar range were determined against cisplatin sensitive and resistant human ovarian carcinoma (A2780 and A2780cisR) cell lines. Notable selectivity towards the cancerous cells lines compared to the non-tumoral human embryonic kidney (HEK-293) cell line was observed for selected compounds. The activity seems to be multimodal, involving reactive oxygen species (ROS) generation and, in some cases, a fragmentation process to afford monoiron derivatives. The large structural variability, amphiphilic character and good stability in aqueous media of the diiron vinyliminium complexes provide favorable properties compared to other widely studied classes of iron-based anticancer candidates.

Cytotoxic homoleptic Ti(iv) compounds of ONO-type ligands: synthesis, structures and anti-cancer activity

Reacting variously substituted dianionic tridentate ONO-type acylhydrazone ligands with titanium(iv) tetra(isopropoxide) gave a new class of eight homoleptic titanium(iv) compounds showing exceptional stability and promising cytotoxicity.

Cytotoxic heteroleptic heptacoordinate salan zirconium(iv)-bis-chelates - synthesis, aqueous stability and X-ray structure analysis

Herein we report the synthesis and structural characterization of a series of novel Zr(iv)salan complexes. The initial metalation product [(L1)2Zr] is highly water sensitive while ligand exchanged [L1Zr(dipic)] hydrolyses slowly with a bis-hydroxo Zr(iv) species identified by MS as an intermediate. [L1Zr(dipic)] is cytotoxic in the range of cisplatin against two human carcinoma cell lines.

Metal-free salan-type compound induces apoptosis and overcomes multidrug resistance in leukemic and lymphoma cells in vitro

PURPOSE

We report on our preclinical findings of a simple salicylic diamine compound (THG 1213) which has yielded exceptional results as a potential chemotherapeutic drug. THG 1213 is an easy to synthesize chiral and metal-free salan compound.

METHODS

THG 1213 was tested on several leukemia, lymphoma and solid tumor cell lines in vitro. The effects have been studied by LDH release essay, FACS flow cytometry, photometric cell count, immunoblotting, and NMR spectroscopy.

RESULTS

THG 1213 selectively inhibits proliferation and induces apoptosis in leukemia, lymphoma and solid tumor cell lines. Necrosis or effects on healthy leucocytes could not be detected. Apoptosis is induced via the intrinsic and extrinsic pathways. The salan THG 1213 overcomes multidrug resistance in tumor cells and acts synergistically with vincristine and daunorubicin.

CONCLUSIONS

THG 1213 displays remarkable antitumor properties. In particular, the lack of metallic components of THG 1213 could prove to be beneficial in future clinical trials, as metal-containing drugs are known to show severe side effects.

Synthesis, structure and cytotoxicity of a series of Dioxidomolybdenum(VI) complexes featuring Salan ligands

Seven hexacoordinated cis-dioxidomolybdenum(VI) complexes [MoO₂L^1-7] (1-7) derived from various tetradentate diamino bis(phenolato) "salan" ligands, N,N'-dimethyl-N,N'-bis-(2-hydroxy-3-X-5-Y-6-Z-benzyl)-1,2-diaminoethane {(X=Br, Y=Me, Z=H (H₂L¹); X=Me, YCl, Z=H (H₂L²); X=ⁱPr, Y=Cl, Z=Me (H₂L³)} and N,N'-bis-(2-hydroxy-3-X-5-Y-6-Z-benzyl)-1,2-diaminopropane {(X=Y=^tBu, Z=H (H₂L⁴); X=Y=Me, Z=H (H₂L⁵); X=ⁱPr, YCl, Z=Me (H₂L⁶); X=Y=Br, Z=H (H₂L⁷)} containing O-N donor atoms, have been isolated and structurally characterized. The formation of cis-dioxidomolybdenum(VI) complexes was confirmed by elemental analysis, IR, UV-vis and NMR spectroscopy, ESI-MS and cyclic voltammetry. X-ray crystallography showed the O₂N₂ donor set to define an octahedral geometry in each case. The complexes (1-7) were tested for their in vitro antiproliferative activity against HT-29 and HeLa cancer cell line. IC₅₀ values of the complexes in HT-29 follow the order 6<7<<1<2<5<<3<4 while the order was 6<7<5<1<<3<4<2 in HeLa cells. Some of the complexes proved to be as active as the clinical referred drugs, and the greater potency of 6 and 7 (IC₅₀ values of 6 are 2.62 and 10.74μM and that of 7 is 11.79 and 30.48μM in HT-29 and HeLa cells, respectively) may be dependent on the substituents in the salan ligand environment coordinated to the metal.

Synthesis and X-ray structure analysis of cytotoxic heptacoordinate sulfonamide salan titanium(IV)-bis-chelates

A series of novel sulfonamide substituted heteroleptic salan titanium(IV)-bis-chelates complexed to 2,6-pyridinedicarboxylic acid were synthesized, structurally characterized and evaluated for their anticancer activity against two human carcinoma cell lines. All cytotoxic complexes showed complete inhibition of cell growth at active concentration, two complexes based on pyrrolidine and azepane substituted sulfonamides displayed IC50 values below 1.7 μM and are more cytotoxic than cisplatin in both tested cell lines. The azepane substituted complex [L3Ti(dipic)] exhibited excellent activity with an IC50 value of 0.5 ± 0.1 μM in Hela S3 and 1.0 ± 0.1 μM in Hep G2.

Titanium Salan/Salalen Complexes: The Twofaced Janus of Asymmetric Oxidation Catalysis

Optically pure chiral epoxides and sulfoxides are ubiquitous building blocks in fine organic synthesis, employed in the pharmaceutical, agrochemical, and cosmetic industries. On the road to chiral epoxides and sulfoxides, efficient and stereoselective transition metal-based catalysts are the most promising guides. Among transition metals, we favor titanium for its cheapness and availability, nontoxicity, and well-known ability to catalyze a variety of stereoselective transformations, including oxidations with environmentally benign H2O2. In this personal account, we summarize the state-of-the-art of rational design of chiral titanium(IV) salan and salalen catalysts, and investigations of their catalytic reactivities and stereoselectivities in the epoxidations of olefins and oxidations of thioethers, unraveling the peculiarities and mechanisms of their catalytic action.

Monomeric Ti(iv)-based complexes incorporating luminescent nitrogen ligands: synthesis, structural characterization, emission spectroscopy and cytotoxic activities

Novel photoluminescent 2,2′-bipyrimidine ligands and their titanium(iv) complexes are cytotoxic.