A proposed mechanism for the inhibitory effect of the anticancer agent titanocene dichloride on tumour gelatinases and other proteolytic enzymes

Titanocene dichloride, the most studied metallocene, exhibits antiproliferative activity in a wide spectrum of murine and human tumours. In this article it is demonstrated that titanocene dichloride inhibits tumour gelatinases in a dose-dependent manner. Substrate saturation experiments and the fact that the IC₅₀ values were increased in correlation with collagen substrate concentrations indicate that the titanocene dichloride induced inhibition is of a competitive type. Titanocene dichloride also specifically inhibits clostridium collagenase and trypsin, particularly when collagens are used as substrates. Binding experiments demonstrate that cyclopentadiene–Ti(IV) moieties, resulting from titanocene dichloride at physiological pH, are bound mainly to different types of collagens and to a lesser extent to casein or bovine serum albumin, forming soluble and stable adducts. These results indicate that titanocene dichloride behaves as a competitive inhibitor against various proteolytic enzymes by binding to the substrate rather than to the enzyme active site. This property may be responsible for the antiangiogenic effect of titanocene dichloride and additionally contributes to its anticancer action.     

Synthesis, characterization and structure of metallocene dicyanamide complexes

The bis(cyclopentadienyl) complexes [Cp₂Ti(dca)]₂O and Cp₂V(dca)₂ (dca = dicyanamide) have been prepared by reaction of sodium dicyanamide with aqueous solution of titanocene dichloride and vanadocene dichloride, respectively. The X-ray structure analyses of both complexes confirmed monodentate coordination of dicyanamide ligand through the terminal nitrogen atom of cyano group.     

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

Functionalization of cyclopentadienyl (Cp) ligands and incorporation of these into a Ti(IV) center require careful design and selection of the appropriate synthetic routes to obtain the desired product in reasonably good yields. As part of our research efforts in the area of titanocene antitumor agents, we have revisited the synthesis of Cp rings with electron-withdrawing groups and their corresponding titanocene dichlorides, (Cp-R)₂TiCl₂ and (Cp-R)CpTiCl₂, where R is CO₂CH₃ and CO₂CH₂CH₃. These complexes were characterized by elemental analysis and ¹H and ¹³C NMR and IR spectroscopies. This report presents the first detailed synthetic route for (Cp-CO₂CH₂CH₃)CpTiCl₂ and provides an alternate route for synthesis of (Cp-R)₂TiCl₂ complexes. The ability of these complexes to deliver Ti(IV) to apotransferrin was investigated to elucidate how the functionalized Cp ligands affect the titanium intake by apotransferrin. The subject complexes transfer Ti(IV) to human apotransferrin, loading both N- and C-lobes. The antitumor activity of these complexes against HT-29 cancer colon cells was determined using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Carboethoxy Cp functionalization results in complexes with a toxicity comparable to that of titanocene dichloride. The carbomethoxy-functionalized complexes proved to be nonactive at the time intervals studied here, regardless of their ability to donate the titanium atom to human apotransferrin.     

Theoretical study on the interaction of titanocene dichloride with deoxyguanosine monophosphate

The completely hydrolyzed titanocene dichloride, [Cp₂Ti(H₂O)₂]²⁺ binding to guanine (G) and phosphate group sites of DNA were investigated by DFT method, with using deoxyguanosine monophosphate (dGMP) as incoming ligand. In the first substitutions, the calculations reveal that the diaquated titanocene binding to O6 shows the lowest activation free energy with 17.9 kcal/mol, closely followed by N7 is 20.5 kcal/mol and the O of phosphate group is 26.3 kcal/mol, respectively. It was also found that all the titanation processes are mildly endothermic. In addition, for the Ti-B(dGMP) in all separated products, the bond dissociation free energies (BDFE) of Ti-O(P, P = phosphate) is higher than those of Ti-N7/O6. In the second substitutions, the reactions leading to the didentate adducts are considered. For bidentate-bridging N7, O6 binding mode, the path of the metal Ti binding to O6 has the lower activation free energy (11.3 kcal/mol) than that of the metal Ti binding to N7 (15.3 kcal/mol). For the bidentate-bridging N7, O(P) binding mode, the path of the metal Ti binding to O(P) has the lower activation free energies (25.3 kcal/mol) than that of the metal Ti binding to N7 (26.2 kcal/mol).     

Binding and hydrolysis studies of antitumoural titanocene dichloride and Titanocene Y with phosphate diesters

The interaction of the antitumoural metallocene dihalides, titanocene dichloride (Cp₂TiCl₂) and Titanocene Y (bis-[(p-methoxybenzyl)cyclopentadienyl]titanium(IV) chloride), with bis(4-nitrophenyl) phosphate (BNPP), which is a widely used model for the phosphate diester linkages in DNA, has been studied. Cp₂TiCl₂ has been shown to promote the cleavage of the phosphate diester in weakly acidic solution. At pH 4, 37 °C, a 10⁶-fold rate acceleration over the uncatalysed reaction was observed under pseudo-first-order conditions, when freshly prepared solutions of Cp₂TiCl₂ were applied. The activity of aged solutions dropped significantly due to the formation of insoluble precipitates of hydrolysed Ti species. The precipitates isolated from aged solutions were shown to act as moderately active, heterogeneous catalysts for BNPP cleavage. By contrast, no hydrolysis of the phosphate diester could be observed in the presence of Titanocene Y. Implications for the mode of action of the apoptosis-inducing metallocene dihalides are discussed.     

Interactions of dichloro-bis(η5-cyclopentadienyl)titanium(IV) with nucleosides

The interactions of dichloro-bis(η⁵-cyclopentadienyl)titanium(IV) (titanocene dichloride, Cp₂TiCl₂) with nucleosides have been studied in methanolic solutions. Complexes of the general formula [Cp₂Ti(Nucl)MeOH]Cl₂ were isolated. The nucleoside complexes with one N(1)H ionizable imino proton (i.e. inosine and guanosine) undergo ionization in alkaline solution and complexes of the formula [Cp₂Ti(NuclH⁺)] Cl were isolated. All complexes have been characterized by elemental analyses and various spectroscopic techniques. In the first series of complexes, [Cp₂Ti(Nucl)MeOH]Cl₂, the nucleosides act as monodentate ligands with an intramolecular hydrogen bond between the coordinated methanol and the C6O group, while in the second, [Cp₂Ti(NuclH⁺)] Cl, they coordinate through both their N7 and O6 atoms.     

Antitumor properties of some titanocene chalcogenates

Five chalcogen-coordinated bis(η⁵-cyclopentadienyl)titanium(IV) chalcogenolates were tested against fluid Ehrlich ascites tumor for antitumor properties: the titanocene phenolates (C₅H₅)₂TiCl(2,4,6-OC₆H₂Cl₃) (I) and (C₅H₅)₂Ti(OC₆F₅)₂ (II); the titanocene thiophenolate (C₅H₅)₂Ti(SC₆F₅)₂ (III); the titanocene dithiolene chelate (C₅H₅)₂Ti[cis-1,2-S₂C₂ (CN)₂] (IV); and the titanocene selenophenolate (C₅H₅)₂TiCl(SeC₆H₅) (V). The best antitumor activity and an optimum cure rate of 100% were observed in the case of the pentafluorophenyl derivatives II and III, followed by IV and V which induced cure rates of 90 and 80% respectively. These results confirm that bis(η⁵-cyclopentadienyl)titanium(IV) diacido complexes can be widely varied at the position of the acido ligands without loss of antitumor potency. The titanocene derivatives described in the present study are the first neutral mercapto and seleno titanocene derivatives for which strong antiproliferative properties have been shown.     

Intracellular mapping of the distribution of metals derived from the antitumor metallocenes

The intracellular distribution of transition metals in V79 Chinese hamster lung cells treated with subtoxic doses of the organometallic anticancer complexes Cp₂MCl₂, where Cp is η ⁵ -cyclopentadienyl and M is Mo, Nb, Ti, or V, has been studied by synchrotron-based X-ray fluorescence (XRF). While significantly higher concentrations of Mo and Nb were found in treated cells compared with control cells, distinct differences in the cellular distribution of each metal were observed. Analysis of thin sections of cells was consistent with some localization of Mo in the nucleus. Studies with a noncytotoxic thiol derivative of molybdocene dichloride showed an uneven distribution of Mo in the cells. For comparison, the low levels of Ti and V in cells treated with the more toxic titanocene and vanadocene complexes, respectively, resulted in metal concentrations at the detection limit of XRF. The results agree with independent chemical studies that have concluded that the biological chemistry of each of the metallocene dihalides is unique.Electronic Supplementary Material Supplementary material is available for this article at .     

Novel benzyl-substituted molybdocene anticancer drugs

From the reaction of 6-(p-methoxyphenyl) fulvene (1a), 6-(3,4-dimethoxyphenyl) fulvene (1b) and 6-(3,4,5-trimethoxyphenyl) fulvene (1c) with LiBEt₃H, lithiated cyclopentadienide intermediates (2a-c) were synthesised. These intermediates were then transmetallated to molybdocene using MoCl₄ (synthesized in situ) to yield the benzyl-substituted molybdocenes bis-[(p-methoxybenzyl)cyclopentadienyl] molybdenum (IV) dichloride (3a), bis-[(3,4-dimethoxybenzyl)cyclopentadienyl] molybdenum (IV) dichloride (3b), and bis-[(3,4,5-trimethoxybenzyl)cyclopentadienyl] molybdenum (IV) dichloride (3c). The molybdocene 3a was characterised by single crystal X-ray diffraction. All three molybdocenes had their cytotoxicity investigated through MTT based preliminary in vitro testing on the human renal cell line Caki-1 in order to determine their IC50 values and compare them with the corresponding titanocene and vanadocene dichloride derivatives. Molybdocenes 3b-c were found to have the same IC50 values of 290 μM, while 3a yielded a value of 84 μM, respectively     

Novel derivatives of ansa-titanocenes procured from 6-phenylfulvene: a combined experimental and theoretical study

The previously prepared trans-[(1,2-diphenyl-1,2-dicyclopentadienyl)ethanediyl] titanium(IV) dichloride, [1,2-(Ph)₂C₂H₂{η⁵-C₅H₄}₂]Ti(Cl)₂, was synthesised using an alternative procedure, from which its crystal structure was determined. Using this compound, a variety of other ansa-titanocene derivatives were synthesised by replacement of the chloride ligands with selected substituents. Thus RTi(X)(Y) systems where R=1,2-(Ph)₂C₂H₂η⁵-C₅H₄₂; X=Y=CH₃; X=CH₃, Y=Cl; X=Y=NCS; X=Y=NCO; X=Y=OPh and (X/Y)=O have been synthesised and characterised. DFT calculations were performed on the complexes trans-[(1,2-diphenyl-1,2-dicyclopentadienyl)-ethanediyl] titanium(IV) dichloride, bis-(6,6-diphenylfulvene)titanium and bis-(6,6-diphenylfulvene)iron. This demonstrated the role that the metal centre plays in their formation, generating either an ansa-metallocene, a ‘tucked in’ fulvene complex or a metallocene coordinating fulvene anions.     

Study of the cytotoxicity and particle action in human cancer cells of titanocene-functionalized materials with potential application against tumors

Titanocene dichloride [Ti(η⁵-C₅H₅)₂Cl₂] (1), has been grafted onto dehydrated hydroxyapatite (HAP), Al₂O₃ and two mesoporous silicas MSU-2 (Michigan State University Silica type 2) and HMS (Hexagonal Mesoporous Silica), to give the novel materials HAP/[Ti(η⁵-C₅H₅)₂Cl₂] (S1) (1.01 wt.% Ti), Al₂O₃/[Ti(η⁵-C₅H₅)₂Cl₂] (S2) (2.36 wt.% Ti), HMS/[Ti(η⁵-C₅H₅)₂Cl₂] (S3) (0.75 wt.% Ti) and MSU-2/[Ti(η⁵-C₅H₅)₂Cl₂] (S4) (0.74 wt.% Ti), which have been characterized by powder X-ray diffraction, X-ray fluorescence, nitrogen gas sorption, multinuclear magic angle spinning NMR spectroscopy, IR spectroscopy, thermogravimetry analysis, UV spectroscopy, scanning electronic microscopy and transmission electronic microscopy. The cytotoxicity of the titanocene-functionalized materials toward human cancer cell lines from five different histogenic origins: 8505 C (anaplastic thyroid cancer), A253 (head and neck cancer), A549 (lung carcinoma), A2780 (ovarian cancer) and DLD-1 (colon cancer) has been determined. M₅₀ values (quantity of material needed to inhibit normal cell growth by 50%) and Ti-M₅₀ values (quantity of anchored titanium needed to inhibit normal cell growth by 50%) indicate that the activity of S1-S4 against studied human cancer cells depended on the surface type as well as on the cell line. In addition, studies on the titanocene release and the interaction of the materials S1-S4 with DNA show that the cytotoxic activity may be due to particle action, because no release of titanium complexes has been observed in physiological conditions, while electrostatic interactions of titanocene-functionalized particles with DNA have been observed.     

Antitumor activity of titanocene amino acid complexes

Seven ionic titanocene -amino acid (aa) complexes [(C₅H₅)₂Ti(aa)₂]²⁺[X]₂ ^– with aa = glycine,l-alanine, 2-methylalanine,d-l-phenylalanine,d,l-4-fluorophenylalanine and X = Cl or AsF₆, were investigated for antitumor activity against fluid Ehrlich ascites tumor growing in CF1 mice. These complexes are the first stable model compounds of titanocene units with protein components, synthesized from a water-like, methanolic medium. All titanocene amino acid complexes induced antitumor activity which was manifested by maximum cure rates ranging from 30 to 70% and increases in life span from 78 to 276% in comparison with untreated control animals. The complexes containing chloride as anion X were more effective than the hexafluoroarsenate derivatives, which surprisingly showed a low substance toxicity. In all cases, the antitumor activity of the ionic titanocene amino acid complexes tested was less pronounced than that of the neutral parent compound [(C₅H₅)₂TiCl₂].     

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, ¹H, and ¹³C 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.     

A (1)H NMR study of the interaction of antitumor metallocenes with glutathione

The interaction of the antitumor active metallocene dihalides Cp(2)MCl(2) (M=Ti, Nb, Mo) and 1 equiv. of glutathione was studied by (1)H NMR spectroscopy at pD 2-7 in 4 mM NaCl solutions. No interaction between glutathione and titanocene dichloride was detected at pD 2, while at pD 5-7 competitive hydrolysis of the cyclopentadienyl ligands occurred. With niobocene dichloride formation of approximately 20% of an adduct was observed at pD 2 and 5, but hydrolysis of the Cp ligands in the adduct occurred over 24 h. Molybdocene dichloride formed two stable adducts at pD 6 which were tentatively assigned as a Cp(2)Mo-glutathione chelate involving coordination of the cysteine thiol and glycine carboxylate to the metal centre, and a thiol centred 1:2 Cp(2)Mo-glutathione complex. The implications for the mechanism of antitumor action of the metallocene dihalides is discussed.     

Titanium(IV) targets phosphoesters on nucleotides: implications for the mechanism of action of the anticancer drug titanocene dichloride

Abstract Reactions between the anticancer drug titanocene dichloride (Cp2TiCl2) and various nucleotides and their constituents in aqueous solution or N,N-dimethylformamide (DMF) have been investigated by 1H and 31P NMR spectroscopy and in the solid state by IR spectroscopy. In aqueous solution over the pH* (pH meter reading in D2O) range 2.3-6.5, CMP forms one new species with Ti(IV) bound only to the phosphate group. In acidic media at pH*<4.6, three species containing titanocene bound to the phosphate group of dGMP, AMP, dTMP and UMP are formed rapidly. The bases also appear to influence titanocene binding. Only one of these Ti(IV)-bound species can be detected in the pH* range of 4.6-6.5 in each case. The order of reactivity towards Cp2TiCl2(aq) at pH* ca. 3 is GMP>TMP approximately AMP > CMP. At pH* > 7.0, hydrolysis of Cp2TiCl2 predominated and little reaction with the nucleotides was observed. Binding of deoxyribose 5'-phosphate and 4-nitrophenyl phosphate to Cp2TiCl2(aq) via their phosphate groups was detected by 31P NMR spectroscopy, but no reaction between Cp2TiCl2(aq) and deoxyguanosine, 9-ethylguanine or deoxy-D-ribose was observed in aqueous solution. The nucleoside phosphodiesters 3',5'-cyclic GMP and 2',3'-cyclic CMP did not react with Cp2TiCl2(aq) in aqueous solution; however, in the less polar solvent DMF, 3',5'-cyclic GMP coordination to [Cp2Ti]2+ via its phosphodiester group was readily observed. Binding of titanocene to the phosphodiester group of the dinucleotide GpC was also observed in DMF by 31P NMR. The nucleoside triphosphates ATP and GTP reacted more extensively with Cp2TiCl2(aq) than their monophosphates; complexes with bound phosphate groups were formed in acidic media and to a lesser extent at neutral pH. Cleavage of phosphate bonds in ATP (and GTP) by Cp2TiCl2(aq) to form inorganic phosphate, AMP (or GMP) and ADP (or GDP) was observed in aqueous solutions. In addition, titanocene binding to ATP was not inhibited by Mg(II), but the ternary complex titanocene-ATP-Mg appeared to form. These reactions contrast markedly with those of the drug cisplatin, which binds predominantly to the base nitrogen atoms of nucleotides and only weakly to the phosphate groups. The high affinity of Ti(IV) for phosphate groups may be important for its biological activity.     

Comparative in vitro study regarding the biocompatibility of titanium-base composites infiltrated with hydroxyapatite or silicatitanate

Background

The development of novel biomaterials able to control cell activities and direct their fate is warranted for engineering functional bone tissues. Adding bioactive materials can improve new bone formation and better osseointegration. Three types of titanium (Ti) implants were tested for in vitro biocompatibility in this comparative study: Ti6Al7Nb implants with 25% total porosity used as controls, implants infiltrated using a sol–gel method with hydroxyapatite (Ti HA) and silicatitanate (Ti SiO₂). The behavior of human osteoblasts was observed in terms of adhesion, cell growth and differentiation.

Results

The two coating methods have provided different morphological and chemical properties (SEM and EDX analysis). Cell attachment in the first hour was slower on the Ti HA scaffolds when compared to Ti SiO₂ and porous uncoated Ti implants. The Alamar blue test and the assessment of total protein content uncovered a peak of metabolic activity at day 8–9 with an advantage for Ti SiO₂ implants. Osteoblast differentiation and de novo mineralization, evaluated by osteopontin (OP) expression (ELISA and immnocytochemistry), alkaline phosphatase (ALP) activity, calcium deposition (alizarin red), collagen synthesis (SIRCOL test and immnocytochemical staining) and osteocalcin (OC) expression, highlighted the higher osteoconductive ability of Ti HA implants. Higher soluble collagen levels were found for cells cultured in simple osteogenic differentiation medium on control Ti and Ti SiO₂ implants. Osteocalcin (OC), a marker of terminal osteoblastic differentiation, was most strongly expressed in osteoblasts cultivated on Ti SiO₂ implants.

Conclusions

The behavior of osteoblasts depends on the type of implant and culture conditions. Ti SiO₂ scaffolds sustain osteoblast adhesion and promote differentiation with increased collagen and non-collagenic proteins (OP and OC) production. Ti HA implants have a lower ability to induce cell adhesion and proliferation but an increased capacity to induce early mineralization. Addition of growth factors BMP-2 and TGFβ1 in differentiation medium did not improve the mineralization process. Both types of infiltrates have their advantages and limitations, which can be exploited depending on local conditions of bone lesions that have to be repaired. These limitations can also be offset through methods of functionalization with biomolecules involved in osteogenesis.

     

Ethanol catalyzed synthesis of titanocene aryl carboxylate complexes and crystal structure of (η-C5H5)2Ti(2-OH-5-S-O2CC6H3)2

A method for the synthesis of titanocene (IV) aryl carboxylate complexes is presented in this paper. It is based on the fact that alcohol can catalyze the reaction between Cp₂TiCl₂ and aryl carboxylate ligands in the presence of sodium hydroxide (NaOH). The effects of the catalyst on the reaction system were studied and the possible reaction mechanism was proposed. This method was used to prepare a series of titanocene (IV) aryl carboxylate complexes and a macrocyclic titanocene (5,5′-dithiodisalicylato titanocene), whose structure was determined by X-ray diffraction analysis.     

New antitumor titanocene derivatives containing hydrophilic ligands

Four titanocene derivatives containing hydrophilic ligands were tested for antiproliferative activity against Ehrlich ascites tumor in mice. The new compounds (C₅H₅)₂TiCl(p-SC₆H₄NH₃⁺Cl^?) (I) and (C₅H₅)₂Ti(p-SC₆H₄NH₃⁺Cl^?)₂ (II), containing hydrochlorinated p-aminothiophenolate ligands, and the known compounds (C₅H₅)₂Ti(cis-OOCCHCHCOOH)₂ (III) and (C₅H₅)₂Ti(OOCCCl₃)₂ (IV) containing the carboxylic acid anions hydrogen- maleinate and trichloroacetate as acido ligands, induced maximum cure rates of 100%. The T.I. values amounted to 4.4–4.6 (I), 3.5–4.1 (II), 3.7– 3.8 (III) and 5.5 (IV), and were slightly increased in comparison to (C₅H₅)₂TiCl₂ (T.I. = 3.3). The complexes I–III were rather soluble in water and equally active in a DMSO/saline (1/9, v/v) mixture, in pure saline and in buffered solutions. In the case of IV, the toxicity was considerably low (LD₅₀,440 mg/kg; LD₁₀₀, 500 mg/kg) in relation to (C₅H₅)₂TiCl₂ (LD₅₀, 100 mg/kg; LD₁₀₀, 140 mg/kg).     

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.     

Cytotoxic studies of substituted titanocene and ansa-titanocene anticancer drugs

A variety of substituted titanocene and ansa-titanocene complexes have been synthesized and characterized using traditional methods. The cytotoxic activity of the different titanocene complexes was tested against tumour cell lines human adenocarcinoma HeLa, human myelogenous leukemia K562, human malignant melanoma Fem-x and normal immunocompetent cells, peripheral blood mononuclear cells PBMC. Alkenyl substitution, either on the cyclopentadienyl ring or on the silicon-atom ansa-bridge of the titanocene compounds [Ti{Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃{CMe₂CH₂CH₂CHCH₂})}Cl₂] (8), [Ti{Me(CH₂CH)Si(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂] (9) and [Ti(η⁵-C₅H₄{CMe₂CH₂CH₂CHCH₂})₂Cl₂] (12) showed higher cytotoxic activities (IC₅₀ values from 24 ± 3 to 151 ± 10 μM) relative to complexes bearing an additional alkenyl-substituted silyl substituent on the silicon bridge [Ti{Me{(CH₂CH)Me₂SiCH₂CH₂}Si(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂] (10) and [Ti{Me{(CH₂CH)₃SiCH₂CH₂}Si(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂] (11) which causes a dramatic decrease of the cytotoxicity (IC₅₀ values from 155 ± 9 to >200 μM). In addition, the synthesis of the analogous niobocene complex [Nb(η⁵-C₅H₄{CMe₂CH₂CH₂CH=CH₂})₂Cl₂] (13), is described. Structural studies based on DFT calculations of the most active complexes 8, 9 and 12 and the X-ray crystal structure of 13 are reported.